Organometallic Synthesis of Intermetallic Transition Metal Aluminide and ‐Zincide Nanoparticles

Nanometallurgy in Organic Solution: Organometallic Synthesis of Intermetallic Transition Metal Aluminide and ‐Zincide Nanoparticles

Dissertation

By Mirza Cokoja

Nanometallurgy in Organic Solution: Organometallic Synthesis of Intermetallic Transition Metal Aluminide and ‐Zincide Nanoparticles

DISSERTATION

Zur Erlangung der Doktorwürde der Fakultät für Chemie und Biochemie an der Ruhr-Universität Bochum

Vorgelegt von Diplom-Chemiker Mirza Cokoja 2007

This work has been performed in the time between January 2004 and March 2007 at the Chair of Inorganic Chemistry II, Organometallics & Materials Chemistry of the Ruhr University of Bochum.

Herein I declare that I have written this thesis independently and without unauthorised help. Further, I assure that I have used no other sources, auxiliary means or quotes than those stated. I further declare that I have not submitted this thesis in this or in a similar form to any other university of college. Besides, I declare that I have not already undertaken an unsuccessful attempt to obtain a doctorate from another university of college.

Mirza Cokoja, May 2007

Day of examination:

1st referee: Prof. Dr. Roland A. Fischer 2nd referee: Prof. Dr. Christof Wöll

III

I am grateful to my supervisor,

Prof. Dr. Roland A. Fischer, for giving me absolute scientific freedom in my work. Your trust has always been a motivation to me and I am thankful for your great support.

Acknowledgements

First, I want to thank Dr. Christian Gemel and Dr. Tobias Steinke for their support over the past five years. At the very beginning, it was only the three of us, and it was a great pleasure to work with you, and I have always enjoyed the scientific discussions with you.

I also want to thank Dr. Harish Parala for patiently introducing me to XRD, UV-Vis, TEM and for always being there in case of necessity.

Of course, I would not miss to thank the other members of the Organometallics group, with whom I was directly working with: Andreas Kempter, Thomas Cadenbach and Beatrice Buchin.

I sincerely thank Prof. Dr. B. Chaudret and Prof. Dr. C. Amiens, from the Laboratoire de Chimie de Coordination, CNRS, Toulouse, for introducing me into their nanochemistry and for fruitful discussions, as well as Dr. Olivier Margeat and Diana Ciuculescu for a successful and pleasant time in Toulouse.

Sincere thanks are given to all group members of AC II:

Saeed Amirjalayer, Daniela Bekermann, Rolf Deibert, Jun. Prof. Dr. Anjana Devi, Daniel Esken, Markus Halbherr, Malte Hellwig, Ursula Herrmann, Todor Hikov, Heike Gronau, Dr. Emmanuel Lamouroux, Sabine Masukowitz, Mikhail Meilikhov, Andrian Milanov, Maike Müller, Daniel Rische, Dr. Rochus Schmid, Nana Schröter, Dr. Jelena Sekulić, Stephan Spöllmann, Dr. Maxim Tafipolski, Tobias Thiede, Manuela Winter, Denise Zacher and Xiaoning Zhang.

Also, I want to give my warmest regards to the following former members of the group:

Dr. Arne Baunemann, Dr. Ralf Becker, Dr. Raghunandan Bhakta, Dr. Stephan Hermes, Dr. Frank Hipler, Dr. Jayaprakash Khanderi, Dr. Eva Maile, Dr. Urmila Patil and Dr. Jurij Weiß.

Further, I want to thank the Deutsche Forschungsgemeinschaft (DFG) for financial support in the scope of the Sonderforschungsbereich 558 – Metall-Substrat Wechselwirkungen in der heterogenen Katalyse.

As well, I have to thank the Deutsche Akademische Austauschdienst (DAAD) for generous funding within the PROCOPE project, which allowed me to spend an incredible time in Toulouse.

Besides, I would also like to thank the following persons without whom it would not have been possible to realise this work:

- Prof. Dr. Wolfgang Grünert, Dr .Maurits van den Berg and Dr. Konstantin Klementiev, Department of Technical Chemistry at the Ruhr-University Bochum, for the XPS measurements and the XAS measurements at Hasylab in Hamburg. - Prof. Dr. Yuri Grin, Dr. Frank Haarmann and Dr. Marc Armbrüster from the Max-Planck Institute for Chemical Physics of Solids, Dresden, for inspiring discussions. - Dr. Pierre Lecante, Centre d’Elaboration des Matériaux et d’Etudes Structurales, CNRS, Toulouse, for the kind introduction into WAXS and the numerous measurements. - Dr. Osama Shekkhah and Dr. Thomas Strunskus, Department of Physical Chemistry 1 at the Ruhr-University Bochum, for their kindly help with XPS measurements. - Dr. Alexander Birkner, Dr. Harish Parala and Dr. Marie Katrin Schröter for with TEM measurements. As well, kind regards to Todor Hikov and Dr. Abdelkrim Chemseddine (Hahn-Meitner Institute, Berlin) for High-resolution TEM studies. - The NMR department at the Ruhr-Universität Bochum - Jun. Prof. Dr. Raphael Stoll, Gregor Barchan, Martin Gartmann, and especially Hans-Jochen Hauswald for the numerous 27Al-MAS-NMR measurements. - Andrea Ewald from the Chair of Inorganic Chemistry 1 at the Ruhr-Universität Bochum for the GC-MS measurements. - Dr. Andreas Trautwein (Südchemie AG, Heufeld), and Karin Bartholomäus (Ruhr- Universität Bochum) for elemental analysis measurements. - The entire staff from the chemical lager, the glasblowing and the fine mechanics factories of the Faculty of Chemistry and Biochemistry of the Ruhr-University Bochum.

Finally, I want to thank Felicitas Schröder for her love and support.

For my parents and Felicitas

VII

Table of Contents

1. Introduction...... 1 1.1. Intermetallics...... 1 1.2. Synthetic strategies for colloidal binary intermetallic nanoparticles ...... 2 1.2.1. State of the art ...... 3 1.2.2. Reductive methods ...... 3 1.2.2.1. Synthesis of bimetallic nanoparticles via the polyol process...... 3 1.2.2.2. Borohydride reduction...... 5 1.2.2.3. Reduction with trialkylaluminium compounds...... 6 1.2.2.4. Reduction by hydrazine...... 7 1.2.2.5. Reduction by citric acid/sodium citrate...... 8 1.2.2.6. Reduction by alkalides/electrides...... 8 1.2.2.7. Synthesis of alloy nanoparticles by electrochemical reduction ...... 8 1.2.2.8. Transmetallation reactions...... 9 1.2.2.9. Alloying with “active magnesium”...... 9 1.2.2.10. Soft hydrogenolysis of all-hydrocarbon complexes...... 10 1.2.3. Thermolytic decomposition...... 10 1.2.4. Alternative synthetic methods...... 11 1.3. Access to transition metal aluminides and -zincides...... 12

2. Motivation and objectives...... 15 2.1. Objectives of this work ...... 15 2.2. Soft chemical synthesis of Cu-Zn and M-Al phases (M = Cu, Ni, Co) ...... 16 2.2.1. Synthesis of Cu-Zn brass nanoparticles ...... 16 2.2.2. Concept of a wet chemical approach to metal aluminides ...... 16

2.3. Oxidation of M-E nanoparticles to core-shell EOx@M structures...... 18

3. Categories of intermetallic compounds...... 22 3.1. Laves phases...... 23 3.2. Zintl phases...... 23 3.3. Hume-Rothery phases...... 24

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3.3.1. Intermetallic Cu-Zn phases ...... 25 3.3.2. Intermetallic Cu-Al phases...... 26 3.3.3. Intermetallic Ni-Al phases ...... 29 3.3.4. Intermetallic Co-Al phases...... 31

4. Synthesis of intermetallic α‐ and β‐CuZn phases...... 32

4.1. Hydrogenolysis of [CpCu(PMe3)] to copper nanoparticles ...... 32 4.1.1. Synthesis and characterisation of nano-Cu powder ...... 32 4.1.1.1. 1H- and 31P-NMR spectroscopic measurements ...... 33 4.1.1.2. X-ray powder diffraction analysis...... 34 4.1.2. Synthesis and characterisation of colloidal Cu nanoparticles from

[CpCu(PMe3)] ...... 35 4.1.2.1. X-ray diffraction analysis...... 35 4.1.2.2. Transmission electron microscopy...... 37 4.1.2.3. IR spectroscopy of the Cu/PPO particles...... 37 4.1.2.4. UV-Vis spectroscopy...... 38 4.1.2.5. X-ray absorption spectroscopic studies on precipitated Cu/PPO colloids. 40

4.2. Hydrogenolysis of [ZnCp*]2 to Zn particles...... 42 4.2.1. Synthesis...... 42 4.2.2. Characterisation...... 43 4.2.2.1. X-ray powder diffraction...... 43 4.2.2.2. 1H-NMR and GC-MS of the filtrate...... 43 4.3. Soft chemical synthesis of β-CuZn nanoparticles...... 45

4.3.1. Synthesis and characterisation of β-Cu0.50Zn0.50 nanopowder ...... 45 4.3.1.1. X-ray powder diffraction...... 46

4.4. Colloidal β-Cu0.50Zn0.50/PPO nanoparticles ...... 47 4.4.1. Synthesis...... 47 4.4.2. Characterisation...... 47

4.4.2.1. UV-Vis spectroscopy of β-Cu0.50Zn0.50/PPO colloids...... 47 4.4.2.2. X-ray powder diffraction...... 48 4.4.2.3. High resolution ransmission electron microscopy ...... 49 4.4.2.4. X-ray absorption spectroscopy studies ...... 50

4.5. Colloidal solutions of α-Cu1-xZnx/PPO nanoparticles...... 54

4.5.1. Synthesis of α-Cu1-xZnx/PPO colloids (0.09 ≤ x ≤ 0.33) ...... 54

4.5.2. Characterisation of α-Cu1-xZnx colloids...... 54 4.5.2.1. X-ray powder diffraction of precipitated α-CuZn/PPO particles ...... 54 4.5.2.2. TEM measurements ...... 56 4.6. Oxidation behaviour of α-CuZn/PPO colloids ...... 56

4.6.1. Intentional oxidation of colloidal α-Cu1-xZnx/PPO nanoparticles ...... 56

4.6.1.1. X-ray diffraction measurements on oxidised α-Cu1-xZnx/PPO colloids .... 57

4.6.1.2. UV-Vis spectroscopy of α-Cu1-xZnx/PPO colloids...... 58 4.7. α-/β-CuZn colloids - Potential model systems for the Cu/ZnO methanol catalyst.. 62 4.7.1. State of the art ...... 62

4.7.2. Air-oxidised (ZnO)δ@Cu1-xZnx-δ/PPO colloids ...... 64 4.7.3. Synthesis of a soluble Cu-ZnO/PPO nanocomposite from

β-Cu0.50Zn0.50/PPO colloids...... 65 4.7.3.1. X-ray diffraction studies ...... 65 4.7.3.2. UV-Vis spectroscopy...... 66 4.7.3.3. Transmission electron microscopy...... 67 4.7.3.4. Catalytic activity of Cu/ZnO/PPO particles...... 68 4.8. Conclusion...... 69

5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles ...... 70

5.1. Synthesis of Al nanoparticles by hydrogenolysis of [(AlCp*)4]...... 70 5.1.1. Structural characterisation of Al ...... 71 5.1.1.1. X-ray powder diffraction...... 71 5.1.1.2. Transmission electron microscopy...... 72 5.1.2. Spectroscopic characterisation...... 72 5.1.2.1. 1H-NMR and GC-MS of the filtrate...... 72 5.1.2.2. 27Al-MAS-NMR of the Al powder ...... 73 5.1.2.3. XPS measurements of the Al-powder...... 74 5.1.3. Mechanistic insight into the decomposition pathway ...... 75

5.2. Synthesis of Al powder from [(Me3N)AlH3] ...... 77 5.2.1. NMR spectroscopic analysis of the decomposition reaction ...... 77

5.2.1.1. Decomposition of [(Me3N)AlH3] in a NMR tube...... 77 5.2.1.2. 27Al-MAS-NMR of the Al powder ...... 79

5.2.2. Structural characterisation of the Al powder...... 79 5.2.2.1. X-ray powder diffraction...... 79

5.3. Synthesis of copper particles from [{Cu(mesityl)}5] ...... 80 5.3.1. Synthesis...... 80 5.3.1.1. X-ray powder diffraction...... 81

5.3.2. Synthesis of Cu/PPO colloids from [{Cu(mesityl)}5]...... 82 5.3.2.1. X-ray powder diffraction...... 83 5.3.2.2. Transmission electron microscopy...... 83 5.3.2.3. UV-Vis spectroscopy...... 84 5.4. Synthesis of intermetallic copper aluminide phases ...... 85

5.4.1. Wet chemical synthesis of the intermetallic θ-CuAl2 phase ...... 86 5.4.1.1. NMR spectroscopic studies of the reaction solution...... 88 5.4.1.2. X-ray powder diffraction...... 92 5.4.1.3. 27Al-MAS-NMR spectroscopy...... 96 5.4.1.4. Transmission electron microscopy...... 98

5.4.2. Synthesis of the intermetallic Cu0.50Al0.50 phase...... 99

5.4.2.1. Synthesis attempts by co-hydrogenolysis of [CpCu(PMe3)]

with [(AlCp*)4] and [(Me3N)AlH3] ...... 99

5.4.2.2. Synthesis from [{Cu(mesityl)}5] and [(Me3N)AlH3]...... 100

5.4.3. Synthesis of the intermetallic γ-Cu9Al4 phase...... 103

5.4.3.1. Synthesis attempts by co-hydrogenolysis of [CpCu(PMe3)]

with [(AlCp*)4] and [(Me3N)AlH3] ...... 103

5.4.3.2. Synthesis from [{Cu(mesityl)}5] and [(Me3N)AlH3]...... 104

5.5. Intermetallic Cu-Ga phases from [{Cu(mesityl)}5] and [(quinuclidine)GaH3] ..... 107

5.5.1. Synthesis of the θ-CuGa2 phase ...... 108 5.5.1.1. 1H-NMR spectroscopy ...... 108 5.5.1.2. X-ray powder diffraction...... 109

5.6. Preparation of Cu1-xAlx colloids (0.10 ≤ x ≤ 0.50)...... 109 5.6.1. Synthesis and characterisation ...... 109 5.6.1.1. X-ray diffraction studies ...... 111 5.6.1.2. Transmission electron microscopy...... 112

5.6.2. Oxidation behaviour of Cu1-xAlx/PPO colloids (0.10 ≤ x ≤ 0.50)...... 113

5.6.2.1. X-ray powder diffraction of air-oxidised Cu1-xAlx/PPO colloids ...... 113

5.6.2.2. UV-Vis spectroscopy of Cu1-xAlx/PPO colloids (0.10 ≤ x ≤ 0.50) ...... 114 5.7. Conclusion...... 116

6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles...... 118 6.1. Intermetallic β-NiAl nanoparticles...... 118

6.1.1. Synthesis of β-NiAl nanopowder NP1 from [Ni(cod)2] and [(AlCp*)4] ...... 118 6.1.1.1. X-ray powder diffraction of NP1 ...... 118 6.1.1.2. TEM analysis of the particle morphology...... 119 6.1.1.3. IR-spectroscopy ...... 120 6.1.1.4. Examination of the organic byproducts by NMR and GC-MS spectroscopy ...... 121

6.1.2. Synthesis of β-NiAl nanoparticles NP2 from [Ni(cod)2] and

[(Me3N)AlH3]...... 122 6.2. Hydrocarbon-stabilised colloidal β-NiAl nanoparticles ...... 123 6.2.1. Synthesis of colloidal β-NiAl nanoparticles NP3 ...... 123 6.2.2. Characterisation...... 123 6.2.2.1. X-ray powder diffraction...... 123 6.2.2.2. IR-spectroscopic study of NP3 ...... 124 6.2.2.3. NMR characterisation of NP3...... 126 6.2.2.4. Transmission electron microscopy...... 126 6.2.3. Olefin hydrogenation catalysed by colloidal β-NiAl nanoparticles NP3...... 127 6.3. Carboxylic acid-stabilised Ni/Al colloids NP4 ...... 128 6.3.1. Synthesis and characterisation of 17O-enriched 1-adamantanecarboxylic acid (ACA)...... 128 6.3.1.1. Synthesis of ACA...... 129 6.3.1.2. Characterisation by 1H- and 13C-NMR spectroscopy...... 129 6.3.1.3. 17O-NMR spectroscopy...... 130 6.3.1.4. Mass spectroscopic determination of the 17O enrichment grade in ACA 131 6.3.2. Post-synthetic stabilisation of colloidal NiAl particles by addition of ACA . 131 6.3.2.1. X-ray powder diffraction of ACA@NiAl nanoparticles NP4...... 132 6.3.2.2. Transmission electron microscopy...... 133 6.3.2.3. NMR spectroscopy...... 134 6.3.2.4. IR spectroscopy...... 135

XII

6.4. Nanocrystalline intermetallic α-NiAl powder ...... 136

6.4.1. Synthesis of α-Ni1-xAlx phases (0.09 ≤ x ≤ 0.33)...... 136 6.4.2. Characterisation...... 137 6.4.2.1. X-ray powder diffraction studies ...... 137 6.4.2.2. Transmission electron microscopy...... 139

6.5. Oxidation behaviour of α-and β-Ni1-xAlx powder...... 140

6.5.1. Intentional oxidation of Ni1-xAlx nanoparticles (0.09 ≤ x ≤ 0.50)...... 140

6.5.1.1. X-ray diffraction studies of the oxidised Ni1-xAlx samples ...... 141

6.5.1.2. X-ray photoelectron spectroscopy of oxidised β-Ni0.50Al0.50 nanoparticles ...... 142 6.5.1.3. X-ray absorption spectroscopy of oxidised α-/β-NiAl nanopowder ...... 144 6.6. Conclusion...... 146

7. Synthesis of colloidal intermetallic β‐CoAl nanoparticles...... 148 7.1. Synthesis of colloidal β-CoAl nanoparticles...... 149 7.2. Characterisation...... 149 7.2.1. 1H-NMR and GC-MS spectroscopy...... 150 7.2.2. Wide angle X-ray scattering (WAXS) ...... 150 7.2.3. Transmission electron microscopy...... 151 7.2.4. Magnetic measurements...... 152 7.3. Conclusion...... 153

8. Summary and Outlook ...... 155

9. Experimental...... 160 9.1. General considerations...... 160 9.1.1. Handling techniques under inert gas atmosphere...... 160 9.1.2. Purchased materials...... 161 9.2. Syntheses of the organometallic precursors...... 161 9.3. Instrumental details...... 161 9.3.1. Methods of characterisation of binary alloy nanoparticles ...... 161 9.3.1.1. X-ray powder diffraction (XRD) and wide angle X-ray scattering (WAXS) ...... 161

XIII

9.3.1.2. Transmission electron microscopy (TEM) ...... 164 9.3.1.3. X-ray absorption spectroscopy (XAS)...... 165 9.3.1.4. X-ray photoelectron spectroscopy (XPS)...... 167 9.3.1.5. UV-Vis spectroscopy...... 169 9.3.1.6. Nuclear magnetic resonance (NMR)...... 170 9.3.2. IR spectroscopy...... 171 9.3.3. Mass spectrometry...... 172 9.3.4. Gas chromatography-mass spectrometry (GC-MS)...... 172 9.3.5. Elemental analysis...... 172 9.3.6. Magnetism of nanoparticles...... 173 9.4. Syntheses of the materials...... 175 9.4.1. Intermetallic Cu-Zn nanoparticles...... 175

9.4.1.1. Synthesis of nano-Cu powder from [CpCu(PMe3)]...... 175

9.4.1.2. Cu colloids from [CpCu(PMe3)] and PPO as surfactant...... 175 9.4.1.3. Synthesis of nano-Zn powder ...... 175 9.4.1.4. Synthesis of nano β-CuZn powder...... 176

9.4.1.5. Synthesis of PPO stabilised Cu1-xZnx colloids (0.09 ≤ x ≤ 0.50)...... 176

9.4.1.6. Oxidation of PPO stabilised Cu1-xEx colloids (E = Zn, Al; 0.09 ≤ x ≤ 0.50) ...... 177 9.4.2. Intermetallic Cu-Al nanoparticles ...... 177

9.4.2.1. Nano-aluminium from [(AlCp*)4] ...... 177 i 9.4.2.2. Reaction of [(AlCp*)4] with Bu2AlH ...... 178

9.4.2.3. Synthesis of Al nanoparticles from [(Me3N)AlH3]...... 178

9.4.2.4. Synthesis of Cu powder from [{Cu(mesityl)}5]...... 178

9.4.2.5. Synthesis of θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4] ...... 179

9.4.2.6. Synthesis of Cu1-xAlx powder from [CpCu(PMe3)] and [(Me3N)AlH3] (x = 0.67, 0.50, 0.31)...... 179

9.4.2.7. Synthesis of Cu1-xAlx powder from [{Cu(mesityl)}5] and [(Me3N)AlH3] (x = 0.67, 0.50, 0.31)...... 180

9.4.2.8. Synthesis of Cu1-xAlx colloids from [CpCu(PMe3)] and [(AlCp*)4] (0.10 ≤ x ≤ 0.50)...... 181

9.4.2.9. Synthesis of the θ-CuGa2 phase from [{Cu(mesityl)}5] and

[(quinuclidine)GaH3]...... 181

XIV

9.4.3. Intermetallic Ni-Al nanoparticles...... 182

9.4.3.1. Synthesis of Ni1-xAlx nanoparticles (0.09 ≤ x ≤ 0.50)...... 182

9.4.3.2. Synthesis of β-NiAl nanoparticles from [Ni(cod)2] and

[(Me3N)AlH3] (NP2)...... 182 9.4.3.3. Synthesis of β-NiAl colloids (NP3) ...... 183 9.4.3.4. Synthesis of 17O-enriched 1-adamantanecarboxylic acid (ACA) ...... 183 9.4.3.5. Synthesis of ACA-stabilised Ni/Al colloids (NP4)...... 183 9.4.3.6. Hydrogenation of cyclohexene with β-NiAl colloids (NP3) as catalyst.. 184

9.4.3.7. General procedure for the oxidation of Ni1-xAlx nanoparticles on air...... 184 9.4.4. Intermetallic Co-Al nanoparticles ...... 184 9.4.4.1. Synthesis of colloidal β-CoAl nanoparticles...... 184

10. References ...... 186

11. Appendix...... 206 11.1. List of publications...... 206 11.2. Oral presentations...... 207 11.3. Poster presentations...... 208 11.4. Curriculum vitae...... 209

Abbreviations

AAS Atom absorption spectrometry ACA 1-Adamantanecarboxylic acid (1-tricyclo[3.3.1.1]decanecarboxylic acid) acac Acetylacetonate (2,4-pentanedionate) AOT Sodium bis-(2-ethylhexyl)sulfosuccinate at.% Atomic percent bcc Body centred cubic bct Body centred tetragonal cod cis,cis-1,5-cyclooctadiene cot cis,cis,cis-1,3,5-cyclooctatriene Cp Cyclopentadienyl-anion CpH 1,3-Cyclopentadiene Cp* 1,2,3,4,5-Pentamethylcyclopentadienyl-anion Cp*H 1,2,3,4,5-Pentamethylcyclopentadiene CTAB Cetyltrimethylammonium bromide Cy Cyclohexyl d Doublet EA Elemental analysis EDX Energy dispersive spectroscopy Equiv. (Molar) equivalent Et Ethyl EXAFS Extended X-ray absorption fine structure fcc Face centred cubic fct Face centred tetragonal FT-IR Fourier transform infrared spectroscopy FWHM Full width at half maximum GC-MS Gas chromatography coupled with mass spectrometry hcp Hexagonal close packed HDA Hexadecylamine hfac 1,1,1,5,5,5-Hexafluoroacetylacetonate iBu isobutyl iBuH Isobutane ICP-OES Inductively coupled plasma optical emission spectrometer

XVI

ICSD Inorganic crystal structure database IR Infrared spectroscopy JCPDS Joint committee on powder diffraction standards m Multiplet MAS-NMR Magic-angle-spinning nuclear magnetic resonance Me Methyl

Me2HDA N,N-Dimethylhexadecylamine Mesitylene 1,3,5-Trimethylbenzene MS Mass spectrometry NMR Nuclear magnetic resonance OAc Acetate OLEA Oleic acid (octadec-9-ene-1-carboxylic acid) Ph Phenyl PPO Poly(2,6-dimethyl)(1,4-phenylene)oxide PVA Polyvinyl alcohol PVP Polyvinylpyrrolidone Quinuclidine 1-Azabicyclo[2.2.2]octane s Singlet SAED Selected area electron diffraction sept Septet SPR Surface plasmon resonance TBP Tri-n-butylphosphine TEM Transmission electron microscopy THF Tetrahydrofuran TMPD N,N,N’,N’-tetramethyl-p-phenylenediamine TOAB Tetraoctylammonium bromide TOP Tri-n-octylphosphine TOPO Tri-n-octylphosphine oxide VE Valence electrons wt.% Weight percent XANES X-ray absorption near-edge structure XAS X-ray absorption spectroscopy XPS X-ray photoelectron spectroscopy XRD X-ray powder diffraction

XVII 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1. Introduction

1.1. Intermetallics

Intermetallic materials, in the colloquial language established as metal ‘alloys’, are materials that are known since the ancient times. The discovery of the first alloys - the Cu bronzes - around 3000 B.C. marked the epoch of the Bronze Age and ever since is the archetype of today’s metallurgy. The invention of weapons, tools, coins or jewellery made from metal and metal alloys, represents a milestone in the technical and social development of mankind of that era. The modern industry and technology require materials with various physical and mechanical aspects. Alloys of low-price metals, particularly the long-known Cu- bronzes and aluminides, exhibit properties which are customised to their purpose as materials for the fabrication of machines or turbine blades, namely high melting points, low density, ductility, or oxidation stability, creep resistance and high temperature stability against deformation and wear.[1] During the second half of the last century, metal alloys were discovered as functional materials with applications that go far beyond the classic functions in metal industry, i.e. as magnetic devices in microelectronics. For instance, the magnetic heads [2] of the first tape recorders consisted of the Fe3Al alloy. These developments led to a rising need of distinct intermetallic phases, for a fundamental examination and understanding of the structure and binding types in comparison to the separate metal components. The physical properties of one metal can be suitably modulated by the addition of a second appropriate metal component. In particular, structurally well-ordered binary alloy particles have found a large use as magnetic information storage materials.[3-5] Over the last two decades, there has been an immense research interest in the preparation of nanostructured mono- and bimetallic nanoparticles, which mainly focused on the size reduction of existing magnetic and electronic devices, such as high-density information storage systems, hard disks, processor chips or high-quality permanent magnets.[6] In the past few years, magnetic nanoparticles have also been used as contrast agents in the magnetic resonance imaging (MRI) of biological events, e.g. the in vivo detection of cancer cells.[7] Nanoparticles of magnetic metals, such as Fe, Co or Ni are promising materials for this purpose, since they are very well explored, and there is a large knowledge and several protocols describing how to modify the size, shape and

1 1. Synthetic strategies for colloidal binary intermetallic nanoparticles assembly of these particles.[8] Moreover, the free variation of the composition of bimetallic particles and the influence on the phase structure is also of fundamental interest.[9] As well, some binary alloys and oxide supported metals allow chemical processes on the particle surface, including gas adsorption, oxidation, reduction, and catalysis.[10] For example, Raney-nickel, which is the archetypical catalyst of olefin hydrogenation, is a nanoporous Ni material prepared from the NiAl alloy, formed by basic leaching of Al.[11] Metal oxide supported transition metal nanoparticles are also well known catalysts, e.g. Alumina@Ni [12] nanoparticles are known CO2 reforming catalysts, and Cu/ZnO/Al2O3 nanoparticles are the state of the art catalysts of the methanol production from syngas, which is an industrially very important raw chemical.[13] Nanosized RuPt particles were found to be efficient fuel cell anode catalysts of methanol electrooxidation, which are resistant to CO poisoning, as is the case of unalloyed Pt catalysts.[14] The requirements for size reduction of transistors and data storage devices demand the size decrease of mono- and bimetallic crystallites. The common top-down strategies, such as (high temperature) ball milling, do not comply with the required particle size, and the formation of a specific or metastable alloy phase. In the last two decades, synthetic bottom-up methods were developed, and much effort has been made to develop low-cost alloying techniques. Especially the fabrication of thin alloy films by Chemical Vapour Deposition (CVD) techniques from organometallic precursors is a meanwhile well-established method in the industrial production of microprocessors and hard disks.[15] The today’s state of the art chemical bottom-up approach to alloys in the nanoscale is the synthesis of free standing alloy particles dispersed as colloids in aqueous of organic solution, reducing the crystallite size to few nanometres.

1.2. Synthetic strategies for colloidal binary intermetallic nanoparticles

Most of the state-of-the-art colloidal mono- and bimetallic magnetic nanoparticles are prepared by an organometallic route, which is based on the decomposition of metal precursors in presence of surfactants binding at the particle surface, which do not only prevent the particles from agglomeration, but also have a great influence on the particle shape and anisotropy.[8]

2 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1.2.1. State of the art Suitable organometallic precursors and the adaptation of colloid chemistry to non- aqueous media have pushed metal nanoparticle research beyond the limitations of the classical techniques for bulk alloy particle synthesis: (arc plasma) melting,[16] ball milling,[17] metal salt reduction and vapour condensation,[18] which most often give large, micrometer sized crystallites of the thermodynamically stable intermetallic phase. Colloidal nanoalloys are prepared by co-reduction or co-decomposition of two organometallic precursors. The syntheses are carried out in organic as well as in aqueous media, and recently, also in ionic liquids.[19] The conditions of precursor decomposition allows an access to metastable structural alloy phases, and a morphology design, as well as size control in the nanometre regime.

1.2.2. Reductive methods

1.2.2.1. Synthesis of bimetallic nanoparticles via the polyol process In 1976, Toshima et al. reported on the synthesis of monodisperse colloidal Rh [20] nanoparticles, obtained by reduction of RhCl3 with polyvinyl alcohol in methanol solution. Later, Fiévet et al. presented a general approach to metallic nanoparticles by reduction of commercially available metal salts, usually bearing acetylacetonate (acac) ligands, in refluxing ethylene glycol, which acts as a solvent and surfactant, which prevents the small particles from agglomeration.[21] This concept relies on the dehydration of the diol reagent to an aldehyde, which reduces the metals to the zerovalent state, itself being further oxidised to a diketone (Scheme 1.1).

O H ∆T + (1-x) ML + x NL` R CH C CH n m 2 R CH 2 R CH2 C M1-xNx colloids + - H O 2 ∆T, surfactant R CH C OH OH O 2 - n(1-x) L, - mx L` O

Scheme 1.1. Access to colloidal mono‐ and bimetallic nanoparticles via the polyol process.

Meanwhile, the polyol process has become the most common synthesis protocol for the preparation of transition metal nanoparticles. In the last years, the best studied bimetallic systems have been transition metal-platinum alloys. Table 1.2 gives a summary of all colloidal M-Pt and M1-M2 nanoparticles, synthesised by the polyol process. The most cited work here is the report of Sun and Murray on the synthesis of monodisperse, spherical

3 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

OLEA/oleylamine capped colloids of the face-centred-cubic Fe0.50Pt0.50 phase (L12 structure, AuCu-type, Chapter 3.3., Figure 3.1, p. 25) of 6 nm in diameter, obtained by simultaneous [22] thermal decomposition of [Fe(CO)5] and reduction of [Pt(acac)2] with 1,2-hexadecanediol. A variation of the amount of capping ligands allows the tuning of the particle size and shape.[23] However, the fcc phase is chemically disordered, i.e. the Fe and Pt atoms are rather randomly distributed in the cubic lattice, and thus the obtained particles exhibit rather soft magnetisation. The well-ordered tetragonal (fct) FePt phase (L10 structure, AuCu3-type, Figure 3.1, p. 25), instead, shows a significantly higher magnetisation, due to the high anisotropy of the particles.[5] The desired fct phase is mainly obtained after high-temperature annealing of the particles, which leads to particles growth and decreasing solubility. Subsequently, several groups reported on modified syntheses of fcc-FePt nanoparticles, which however, are all based on Sun’s fundamental publication.[24-30] Other groups have reported on the direct synthesis of fct-FePt (vide infra).

Table 1.2. Summary of bimetallic M‐Pt and M1‐M2 colloids synthesised via the polyol process. M‐Pt phase M‐precursor Pt‐precursor Surfactant Reference

Fe(CO)5 [22‐29] FePt Pt(acac)2 OLEA/oleylamine Fe(acac)3 [30]

RuCl3 H2PtCl6 or PtCl4 PVP [31‐33] RuPt Ru(acac)3 Pt(acac)2 OLEA/oleylamine [34]

CoPt3 Co2(CO)8 Pt(acac)2 ACA/HDA [26,35]

CoPt CoCl2·6 H2O H2PtCl6·6 H2O PVP [36]

MnPt Mn2(CO)10 Pt(acac)2 OLEA/oleylamine [37]

Ni1‐xPtx Ni(OAc)2 Pt(acac)2 OLEA/oleylamine [38]

Cu(acac)2 Pt(acac)2 PVP or OLEA/oleylamine [39] CuPt CuSO4·5 H2O Pt(Oac)2 or H2PtCl6 PVP [40]

AuPt HAuCl4 H2PtCl6 PVP [41]

PdPt PdCl2 H2PtCl6 PVP [42]

M1‐M2 phase M1‐precursor M2‐precursor Surfactant Reference

NiCo Ni(OAc)2·4 H2O Co(OAc)2·4 H2O 1,2‐propanediol [43]

FeCo Fe(acac)3 Co(acac)2 OLEA/oleylamine [44]

Ni(OAc)2·4 H2O H2PdCl4 [45] NiPd PVP Ni(SO4)2·7 H2O Pd(OAc)2 [46]

FePd Fe(CO)5 Pd(acac)2 ACA/TBP [47]

CuPd Cu(OAc)2· H2O Pd(OAc)2 PVP [48]

CoPd Co(acac)2·2 H2O Pd(acac)2 1‐octadecanethiol [49]

4 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

In fact, all other M-Pt alloy nanoparticles were prepared by similar routes and thus derive from the report of Sun and Murray. Other colloidal magnetic nanocrystalline phases, e.g. [35,38] CoPt3 and Ni1-xPtx phases, synthesised by the group of Weller, are also referencing to this original work. Another industrially relevant phase is the binary Ru-Pt system, which is a promising fuel cell catalyst of methanol oxidation.[31-34] Besides the Pt-alloys, several other groups reported on the polyol synthesis of colloidal nanoparticles of CoNi,[43] CoFe[44] and NiPd,[45,46] from the respective acetate salts, and CuPd[48] and CoPd[49] obtained from the Pd- and Co-acac, or Cu-acac complexes, respectively.

1.2.2.2. Borohydride reduction Besides the polyol process, a convenient method to synthesise mono- and bimetallic nanoparticles is the co-reduction of simple metal salts with sodium tetraborohydride (NaBH4). The first work dates back to the report of Schlesinger et al. on the reduction of transition [50] metal salts with NaBH4. This concept is based on a salt metathesis reaction of a metal halide and NaBH4, giving M(0), as well as sodium halide, diborane and dihydrogen as byproducts (Scheme 1.2).

MXn + n NaBH4 M(0) + n NaX + n/2 B2H6 + n/2 H2

‐ Scheme 1.2. General procedure of metal particle synthesis by BH4 reduction.

Van Wonterghem and co-workers synthesised amorphous Fe1-xCox particles by co-reduction [51] of FeSO4 and CoCl2 with KBH4 in aqueous solution. Klabunde et al. described the synthesis of cobalt nanopowder, obtained by reduction of CoCl2 with NaBH4 in aqueous solution.[52] However, the reduction is often accompanied by the formation of metal borides.[52,63] Subsequently, Chow et al. synthesised the NiCu alloy under the same [53] conditions, using NiCl2 and CuCl2 as precursors O’Connor et al. have published a synthesis protocol for Fe/Pt alloys by reduction of aqueous iron and platinum salts with NaBH4 in presence of hexadecyltrimethylammonium bromide (CTAB).[54] Colloidal nanocrystals of [55] [56] [57] PtSn, AuCuSn2, AuCu and AuCu3 were synthesised Schaak et al. by borohydride co- reduction of K2[PtCl6] and SnCl2, and [Cu(acac)2] and HAuCl4·3 H2O, respectively, in tetraethylene glycol and PVP as surfactant. In the same group, a series of colloidal intermetallic M1-xSnx (Fe, Co, Ni, Cu, Ag, Au; 0 ≤ x ≤ 1), M-Pt (Sn, Pb, Sb, Bi) and CoSbx (x = 1, 3) phases was synthesised as well.[58] The precursors were again simple salts, like acetates (Co, Cu, Pb), acetylacetonates (Fe), halides (Pt, Au, Sn, Sb), or nitrates (Ag, Bi),

5 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

which were dissolved in tetraethylene glycol and reduced by NaBH4, followed by heat treatment, in order to anneal the particles. Zhong et al. reported on the synthesis of colloidal solutions of 2 nm sized spherical tetraoctyldecanethiolate capped AuAg nanoparticles in [59] toluene, by co-reduction of HAuCl4·3 H2O and AgNO3 with NaBH4. CoPt and FePt3 nanowires were synthesised by Yamashita et al., using K2PtCl4 and [(NH4)2Co][SO4]2 or [60] [(NH4)2Fe][SO4]2, respectively, using the tobacco mosaic virus as template. An aqueous solution of PVP stabilised AuZn nanoparticles was synthesised by Ascencio et al. from co- [61] reduction of HAuCl4·3 H2O and zinc chloride monohydrate. A sophisticated route to prepare bimetallic alloy nanoparticles is the decomposition of single-source precursors, which already contain both alloy components in a defined stoichiometry. Hur et al. synthesised fct-FePt nanoparticles of 3 nm in diameter by decomposition of [(OC)3Fe(µ-CO)(µ-dppm)PtCl2] (dppm = Bis(diphenylphosphino)methane) [62] with Li[BEt3H].

Bönnemann et al. presented a modified reduction method, employing K[Et3BH] as reducing agent in organic solution (Scheme 1.3). The main advantage is the absence of boride [63] impurities, as is the case using NaBH4. A vast number of metal and alloy powder nanoparticles (e.g. CoNi, CoFe, CoPt, RhPt, PdPt, RhIr, PtIr and CuSn) was systematically [64] synthesised by co-reduction of metal halides with M[Et3BH] (M = Li, Na, K). A cation exchange of M by N(octyl)4 leads to stabilisation of colloidal solutions of the bimetallic particles.[65,66]

1 2 1 2 M Xn + M Xm + (n+m) E[Et3BH] M M + (n+m) EX + (n+m) BEt3 + (n+m)/2 H2

1 2 [65,66] M = Rh; M = Pt; X = Cl; E = N(octyl)4 1 2 [64] M = Co; M = Ni; X = OH; E = Na 1 2 [64] M = Co; M = Fe, Pt; X = Cl; E = Li M1 = Rh; M2 = Pt, Ir; X = Cl; E = Li [64] 1 2 [64] M = Pt; M = Pd, Ir; X = Cl; E = Li 1 2 [64] M = Cu; M = Sn; X = Cl; E = Li

‐ Scheme 1.3. Bönnemann route to colloidal and powder alloy nanoparticles via [Et3BH] reduction.

1.2.2.3. Reduction with trialkylaluminium compounds

Besides the metal salt reduction with the superhydrides E[Et3BH] (E = Li, Na, K,

N(octyl)4), the group of H. Bönnemann has also used aluminium triorganyl compounds AlR3 (R = Me, Et, iBu, octyl). Consequently, M-Pt alloy particles (M = Ru, Pd, Sn) were prepared

6 1. Synthetic strategies for colloidal binary intermetallic nanoparticles by reduction of the corresponding metal-acetylacetonate complexes with the Al-species.[67] The entire reaction is driven by the formation of stable Al-O bonds. The coordinatively unsaturated Al-organyl both cleaves the acac-ligands and reduces the metal centres via alkylation and subsequent alkane elimination. However, the nature of the obtained

[Al(acac)R]x-species, which stabilises the particles, is still not fully elucidated. The Bönnemann concept of stabilising mono- and bimetallic nanoparticles by Al-ligands was the starting point for the preparation of metal aluminide alloys (vide infra). The reaction of a toluene solution of [M(cod)2] (M = Pd, Pt) with AlEt3 under 50-100 bar H2 pressure at room temperature affords a black, unidentified suspension. The subsequent annealing at 200 °C [68,69] under H2 pressure gives nanocrystalline MAl powder.

1.2.2.4. Reduction by hydrazine The co-reduction of two metal halide salts by hydrazine is also a very common and convenient method to synthesise alloyed nanoparticles in aqueous solution. The driving force for the reduction is the formation of dinitrogen (Scheme 1.4). The syntheses are usually carried out under basic conditions, in order to bind hydrochloric acid, which is always produced as byproduct.

MXn + n N2H4 M + n X-H + n N2 + n/2 H2

Scheme 1.4. Synthesis of nano‐alloys by co‐reduction of metal salts (X = Cl, NO3, SO4).

Baglioni reported on the synthesis of water soluble, spherical fcc-AuCu3 (L12 structure) nanocrystals by co-reduction of CuCl2 and HAuCl4, using sodium bis-(2- ethylhexyl)sulfosuccinate (AOT) as surfactant.[70] Analogously, Chen et al. have prepared [71] AOT-capped colloidal AuPd colloids (3-5 nm) size from HAuCl4 and H2PdCl4. Lee et al. synthesised NiPt nanoparticles from hydrazine co-reduction of NiCl2·6 H2O and K2PtCl4 in water/PVP, which were dispersed on a carbon support, and found to catalyse the oxidation of methanol to formaldehyde.[72] Gengyu et al. prepared CTAB-stabilised nanocrystalline CuPt [73] and fct-Cu3Pt particles from CuCl2 and H2PtCl6 in isooctane/n-butanol solution. Zhang et al. studied the formation of colloidal NiCu phases by treatment of NiCl2·6 H2O and CuSO4·5

H2O with hydrazine in a n-heptane/n-butanol solution, and sodium dodecylsulfate as [74] surfactant. Hwang et al. observed the alloying of Cu and Pt by co-reduction of Cu(NO3)2·2

H2O and H2PtCl6 with hydrazine in presence of AOT, by means of X-ray absorption spectroscopic studies.[75]

7 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1.2.2.5. Reduction by citric acid/sodium citrate The synthesis of metal nanoparticles by reduction of metal salts with a reducing agent dates back to the early work of Faraday[76] and Turkevich[77] on the preparation of Au colloids - from [AuCl4] by reduction with phosphorus or sodium citrate, respectively. Liu et al. synthesised colloidal AuPt nanoparticles by co-reduction of an aqueous solution of HAuCl4 [78] and H2PtCl6 by sodium citrate, in presence of tannic acid (=gallotannin; C76H52O46). Liz- Marzán et al. prepared AgAu nanoparticles by treating an aqueous solution of CTAB, [79] HAuCl4·3 H2O and AgNO3 with sodium citrate. Multishell Ag(Au)@AgAu particles were obtained by sequential reduction of the Ag- or Au-salt in presence of the AgAu nanoparticles.

1.2.2.6. Reduction by alkalides/electrides Alkalides and electrides are salt like compounds, exhibiting negatively charged alkali metals or free electrons, which are stabilised by electrostatic forces with alkali-cations being inside multidentate ligands (e.g. crown ethers). Thus, these species are very prone to oxidise transition metals. Dye et al. have demonstrated the potential of these reducing agents by synthesising a series of metal and alloy colloids, e.g. fcc-AuZn, AuCu, AuTe and CuTe from

ZnI2, AuCl3, CuCl2 and TeBr4, respectively. The strength of the reducing agent was shown on the co-reduction of AuCl3 and FeCl2 with strong Lewis-acids such as TiCl4 or TaCl5 to the AuTi and FeTa phases.[80]

1.2.2.7. Synthesis of alloy nanoparticles by electrochemical reduction Reetz et al. described a facile route to nano-alloy powder by electrolysis of simple metal halide salts in THF solutions and in presence of tetraalkylammonium salts.[81] By this method, the intermetallic CuPd, NiPd, PtPd, PtSn, FeNi and FeCo alloys could be synthesised. It was found that an increase of the current density leads to a reduction of the particle size. The particles were characterised as alloys by detection of both metals in the regime of the expected ratio by EDX spectroscopy of a selected area of a single particle in the TEM micrograph. Hempelmann et al. reported on the electrochemical deposition of microcrystalline thin films of MnAl And FeAl on carbon electrodes by decomposition of

MnCl2 and FeCl3, respectively, with AlCl3 in an ionic liquid (1-ethyl-3-methylimidazolium chloride) as solvent.[82]

8 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1.2.2.8. Transmetallation reactions The concept of transmetallation reactions was transferred to small metal particles by [83] Hampden-Smith et al. It was observed that the treatment of the Cu-precursor [Cu(hfac)2] (hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate) reacts with the cobalt carbonyl complex

[Co2(CO)8] (0.5 equiv.) to Cu powder, [Co(hfac)2] and carbon monoxide. Along these lines,

Cheon et al. synthesised colloidal OLEA-capped Co1Pt1 and CoPt3 nanoparticles from [84] [Pt(hfac)2] and [Co2(CO)8] in the molar ratios of 1:1 and 1:2, respectively. Colloidal Pt@Co core-shell type particles (vide infra) could be synthesised by treatment of cobalt nanoparticles with [Pt(hfac)2], which react at the particle surface to Pt@Co and [85] [Co(hfac)2]. 2 Bogdanović et al. studied the transmetallation reactions of metal halides M Cly with the 1 [86] Grignard-like compound [M (MgCl)x], which was in situ formed from MgH2 or Mg- [87] 1 anthracene and M Clx (Scheme 1.5). Later, it was shown that the reactive intermediate 1 1 [88,89] [M (MgX)y] is also accessible via M Cly and [MgEt2]. However, it was not possible to isolate free standing colloidal alloy nanoparticles, due to the extreme reactivity of the Mg- compounds.

MgH2 1 2 + M Clx 1 + M Cly 1 2 Mg-anthracene [M (MgCl)x] M xM y + (x+y) MgCl2 THF ∆T

MgEt2 M1 = Ti; M2 = Cr, Mn, Fe, Co, Ni, Sn [88,89] M1 = Ni; M2 = Fe, Cu [89] M1 = Pt; M2 = Fe, Cu, Sn [88,89] M1 = Ru, Rh; M2 = Pt [89] M1 = Pd; M2 = Fe, Sn, Te [88] 1 2 [88] M = Ir; M = Sn 1 Scheme 1.5. Transmetallation reactions of metal halides with in situ formed Grignard‐like [M (MgCl)x] 1 2 compounds to M xM y alloys and MgCl2.

1.2.2.9. Alloying with “active magnesium” The group of B. Bogdanović has extensively studied the reduction of transition metal complexes with extremely reactive Mg-hydride species.[86] In particular focus was the highly pyrophoric compound MgH2, synthesised by treating a THF solution of Mg-anthracene with [86a] 80 bar H2 at 60 °C. Bimetallic NiMg2 nanoparticles were synthesised in a two-step 3 synthesis from [Ni(η -allyl)2] and MgH2 and subsequent annealing at 400 °C, with propene as the only byproduct.[90]

9 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1.2.2.10. Soft hydrogenolysis of all-hydrocarbon complexes Over the last 15 years, the group of B. Chaudret has developed a strategy to synthesise colloidal mono- and bimetallic nanoparticles using complexes of transition metals in a low oxidation state (usually 0 or +1) having only hydrocarbon ligands (olefin or (cyclo)alkyl), which can easily be cleaved off by hydrogen pressure at rather low temperatures and do not bind at the surface of the particles.[91] This concept derives from the coordination chemistry of 4 polyhydride complexes of ruthenium, e.g. [RuH2(H2)2(PCy3)2], synthesised from [Ru(η - 6 [92] 4 6 cod)(η -cot)] and PCy3 under 3 bar H2 in n-pentane. The Ru(0) compound [Ru(η -cod)(η - cot)] was found to decompose under mild conditions (1 bar H2, 25 °C) to Ru colloids in presence of suitable surfactants, and cyclooctane as the only byproduct.[93] Soon after, this concept of ‘la chimie douce’ (fr.: soft chemistry), namely the decomposition of organometallic complexes to metals by soft hydrogenolysis (3 bar H2), was transferred to [94] other metals, e.g. cubic superlattices of Fe nanoparticles from [Fe{N(SiMe3)2}2] or Co 4 3 [95] nanorods from [Co(η -cod)(η -C8H13)], rigorously avoiding ionic salt-like and oxygen- containing acetate or acetylacetonate precursor compounds. As well, co-hydrogenolysis of two metal precursors under hydrogen pressure led to colloidal alloy nanoparticles, e.g. Co1- [96,97] [97] [98] [99] [100] xRhx, Co1-xRux, CoFe, NiFe and Ru1-xPtx.

1.2.3. Thermolytic decomposition The probably most intuitive way of decomposition of compounds, namely via thermolysis, has not been in particular focus of research, since many employed metal sources are either too stable (e.g. ionic salts, such as halides), or do not decompose in a “clean” way, i.e. without undesired side products. An obvious route, however, is the co-decomposition of homoleptic carbonyl complexes, as shown by Lui et al. by co-decomposition of [Fe(CO)5] and [Mo(CO)6] in presence of octanoic acid/2-ethylhexylamine in refluxing octylether, giving monodisperse spherical FeMo nanoparticles of 4 nm in diameter.[101] Other methods include the co-thermolysis of [Fe(CO)5] and [Pt(acac)2] in presence of 1-adamantanecarboxylic acid [102] (ACA) in boiling HDA, and a similar method, using [Fe(acac)3] and [Pt(acac)2] and an OLEA/oleyalamine mixture in refluxing paraffine, to give the tetragonal fct-FePt phase [103] colloids (L10 structure). O’Brien described the synthesis of CoPt3 nanocrystals by injection of a 1,3-dichlorobenzene solution of [Co2(CO)8] to a hot diphenyl ether solution of [104] [Pt(acac)2] and ACA/HDA.

10 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

Lee et al. prepared solid NiPt alloy nanoparticles (Pt structure) on a carbon support by ball milling of the ionic precursor [Ni(bipy)3][PtCl6] (bipy = 2,2’-bipyridine) and graphite in a [105] H2/argon stream at 550 °C. Nuzzo et al. synthesised Ru-Pt phases of different stoichiometries on carbon supports by decomposition of [Pt2Ru4C(CO)18] and [PtRu5C(CO)16] in presence of graphite at 400 °C in a hydrogen atmosphere.[106]

1.2.4. Alternative synthetic methods Binary intermetallic nanoparticles can also be accessed in a variety of alternative methods, which are, however in most cases designed for one particular system and cannot be used for a general synthesis procedure, as it is the case with the polyol process. Evans et al., for example, reduced [Pt(acac)2] with Collman’s reagent, Na2[Fe(CO)4] in high-boiling hydrocarbons, and selectively obtained the fct-FePt phase, in dependence of the surfactant and solvent used.[107] Several groups have reported on the microwave-assisted synthesis of bimetallic particles. Bensebaa et al. synthesised RuPt colloids by a modified polyol process from [31] ethylene glycol reduction of a mixture of RuCl3·3 H2O, H2PtCl6 and PVP. Microwaves were used for efficient heating/energy transfer and to decrease the reaction time. Bocarsly et al. presented a general route to nanopowder of intermetallic M-Co (M = Ru, Ir, Pd, Pt), M-Pd (M = Fe, Ni) and M-Pt (M = Ru, Ni) phases by treatment of aqueous solutions of metal chlorides and metal cyanides with microwaves.[108] Other methods include preparation protocols for intermetallic phases by treatment of metal chloride salts with UV-light (AuPt),[109] γ-radiation (AgPd and AgPt),[110] or laser [111] [112] irradiation (AgAu), as well as with ultrasound (RuPt), and reduction in a H2 stream (AuPt),[113] or by vanadanocene (RhPd).[114] Also, bimetallic particles were prepared by co- condensation of metal vapour in organic media and in presence of stabilising agents, as shown by Cardenas et al. on the example of AuCu colloids.[115]

11 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

1.3. Access to transition metal aluminides and -zincides

Classical bulk Hume-Rothery bronzes and aluminides are of great importance in material sciences due to their physical properties, such as high creep resistance, ductility and wear stability (Chapter 3.3., p. 24).[1] The relevant reports on the synthesis of nanoscaled metal aluminides cited here appear to be more or less restricted to the study of mechanical alloying processes (ball milling or welding),[118-131] or other physical techniques such as arc plasma melting,[132-136] combustion synthesis[137-140] or laser evaporation.[141-145] One of the obvious reasons may be, that typical alloy components for Hume-Rothery phases are the Group 2, 12 and 13 metals E (E = Mg, Zn or Al), which are particularly difficult to be set free as atoms or small clusters in solution by simple chemical reduction of common salt precursors in contrast to late/noble transition metals, unlike the synthetic methods of preparation described above.[3-5,8,20,21,63] However, little is known on free standing metal aluminide or more general brass-type phase nanoparticles and materials in solution. They have apparently not been in the focus of interest in nanoparticle chemistry so far, despite their ubiquitous importance in metallurgy and materials science of intermetallics.[116] For the synthesis of metal aluminide or -zincide alloys in solution, it must be considered that free Al and Zn atoms are highly oxophilic and immediately react to metal oxides, hydroxides or alkoxides, respectively, in presence of the precursors mentioned above, and cannot be reduced by chemical methods any longer. Aluminide nanoparticles and colloids are particularly challenging targets, since the discovery of quasicrystal phases in the Al(rich)-Mn system has generally revived the interest in structure and bonding of classic Hume-Rothery phases including their materials properties.[117] The entry to a wet chemical nanometallurgy of aluminides in general relies on a clean molecular source for bare aluminium atoms or particles in solution allowing for moderate conditions and reasonable quantities and particle concentrations. The synthesis protocols for binary nanoparticles of noble metals presented above, involve simple metal halide (hydrate) salts, or oxygen containing ligands, such as acetylacetonates or acetates. The reaction media (e.g. polyglycol) and surfactants (e.g. fatty acids) also contain loads of oxygen or water, which however, do not prevent alloying by formation of metal oxides. The soft Lewis acidic metal centres are more or less inert towards O-sources. In literature, there are just few reports on the solution synthesis of brass powder and colloids to date, including the co-decomposition of the copper(II)-aminoalkoxide

12 1. Synthetic strategies for colloidal binary intermetallic nanoparticles

[Cu{OC(Me)CH2NMe2}2] and diethylzinc. Following a certain protocol of controlled co- pyrolysis of these two precursors in HDA as both solvent and surfactant at 200-250 °C, alloyed α-CuZn and γ-CuZn (but not β-CuZn) nanoparticles were obtained, which were stabilised in HDA solution, as revealed by XRD and TEM-SAED studies.[146] Very recently, this synthesis protocol was extended to PdZn and AuZn, prepared by a sequential pyrolysis of [147] [M(acac)2] (M = Pd, Cu, Au) and [ZnEt2] in hot HDA. Ito et al. reported on the selective synthesis of the β-CuZn phase by co-reduction of [Zn(acac)2] and bis(2,2,6,6-trimethyl-3,5- heptanedionato)copper with Na[(MeO)(EtO)AlH2] in presence of oleic acid and hot diphenylether.[148] However, both synthesis protocols include loads of oxygen, either from the precursors or from the solvents, which may lead to the formation of ZnO during decomposition, instead of Zn(0). The synthesis procedures for M-Al alloys (M = Fe, Co, Ni, Cu) existing to date are not convenient and require extreme reaction conditions. In addition, contamination with halogen containing side products cannot be fully excluded. Buhro et al. reported on the reaction of

NiCl2 and LiAlH4 in boiling mesitylene to a black, pyrophoric powder, which gave the desired β-NiAl phase upon annealing at 550 °C (particle size: 35-40 nm).[149] Alternative soft chemical strategies were developed in particular aiming at other nanocrystalline aluminides, e.g. TiAl[150] and FeAl.[151] The concept is based on the reaction of metal halides with lithium tetrahydridoaluminate in non aqueous media which is thought to generate intermediate species such as [M(AlH4)n], which are thermally labile and decompose into MAl at temperatures even below 200 °C. Withers et al. synthesised Ni3Al powder (particle size: 1.5 [152] µm) from NiCl2 and Al at 750 °C in a molten eutectic salt mixture of NaCl and KCl. Abe et al. prepared a mixture of NiAl and Ni3Al from NiCl2 and AlCl3 in presence of ammonium [153] carboxlyates. The subsequent annealing at 1400 °C yielded a mixture of NiAl and Ni3Al. Tillement et al. claimed a successful approach to NiAl powder by reduction of nickel acetate and [Al(acac)3] (acac = acetylacetonate) with an excess of NaH and traces of tert-butanol in boiling THF.[154] However, this work lacks substantial analytical evidences for the presence of an intermetallic phase. Recently, a comparably facile synthetic concept was presented by the [68,69] above introduced Bönnemann-route. The reaction of [M(cod)2] (M = Ni, Pt; cod = 1,5- cyclooctadiene) with AlEt3 in toluene under 50-100 bar H2 pressure at room temperature gave an undefined black material, which was isolated, treated with H2 pressure at 200 °C and subsequently annealed in an argon stream to give phase pure MAl powder with an average particle size of 2-4 nm. However, summing up, there is no report in the literature on a

13 1. Synthetic strategies for colloidal binary intermetallic nanoparticles straightforward, one-step wet chemical synthesis of nanocrystalline M-Al phases in solution to date. Besides, colloidal dispersions of transition metal aluminide nanoparticles in non aqueous media are still terra incognita, making the synthesis of metal aluminide phases a veritable synthetic challenge.

14 2. Motivation and objectives

2. Motivation and objectives

2.1. Objectives of this work

The main objective of this work was the organometallic synthesis of transition metal aluminide and -zincide phases in organic media. This work focused on the preparation of M-E alloy nanoparticles (E = Zn; M = Cu, E = Al; M = Cu, Ni, Co) by means of hydrogenolysis, using [(AlCp*)4] and [(Me3N)AlH3] as Al-sources, and [ZnCp*2] as Zn-precursor. The initial synthetic target was the phase pure synthesis of Cu1-xZnx, Cu1-xAlx Ni1-xAlx and Cu1-xAlx phases (0 ≤ x ≤ 0.50). These binary systems exhibit several distinct α- and β-phases (see

Chapter 3.3. below, p. 24) e.g. β-CuZn, as well as θ-CuAl2, γ-Cu9Al4 and Cu0.50Al0.50, which appeared to be suitable for a systematic study. Following Chaudret’s concept of clean precursor decomposition with hydrogen pressure,[91-100] the transition metal source should preferably contain hydrocarbons only, readily yield the naked metal under moderate conditions of hydrogen pressure (~ 3 bar) and temperatures in the range of 100-150 °C. Besides, the decomposition path of the metal precursor and its byproducts must not interfere with the decomposition of the Al- or Zn-source. As well, only hydrocarbon solvents should be used, avoiding high-boiling ethers of poly glycols. In this work, the precursors [155] [156] [157] 4 3 [95] [CpCu(PMe3)], [{Cu(mesityl)}5], [Ni(cod)2] and [Co(η -C8H12)(η -C8H13)] were selected as transition metal sources, since all of them have already been used before as precursor materials for the synthesis of metal nanoparticles by means of pyrolysis or hydrogenolysis, respectively. Another aspect of this work was the synthesis of free standing nanoparticles of intermetallic M1-xEx phases as a colloidal solution. The second part of this thesis was orientated towards oxidation of metal aluminide and - zincide nanoparticles and potential applications. All M-E nanoparticles were examined on their oxidation behaviour under realistic conditions, i.e. direct exposure to air, with the aim to answer the question, if the particles form a core-shell type (EOy)δ/2@M1-xEx-δ nanoparticles alumina shell covering the particle core and protecting it from oxygen. The synthesis of α-/β-

Ni1-xAlx phases as well as of the β-CoAl phase (Chapters 3.3.3., p. 29 and 3.3.4, p. 30) was object of investigation, with a special regard to retain the physical properties of the Ni- and Co core.

15 2. Motivation and objectives

2.2. Soft chemical synthesis of Cu-Zn and M-Al phases (M = Cu, Ni, Co)

2.2.1. Synthesis of Cu-Zn brass nanoparticles Despite much fundamental work has been done on the nano chemistry of classical alloy systems such as the colloidal alloys of the coinage metals Au-Ag[59,79,158] or Au-Cu,[57,70] almost nothing similar is available for Cu-Zn phases or M-Al at present (vide supra). This warrants the development of a new and rigorously oxygen-free precursor combination as well as definitely aprotic conditions, i.e. avoiding HDA and similar surfactants which may interfere with highly reactive metal alkyls like [ZnEt2]. The organometallic Cu and Zn precursors should be chemically inert against each other. They should thermally decompose under mild conditions under hydrogen at about the same rate to liberate the Cu and Zn atoms and the free ligands, which themselves should be inert as well and do not cause side reactions, i.e. do not strongly adsorb at the alloy particle surface. Thus, the oxygen-free compounds

[CpCu(PMe3)] and decamethylzincocene [ZnCp*2] were selected as the precursors of choice for the nanometallurgy of brass materials in organic solution.

2.2.2. Concept of a wet chemical approach to metal aluminides The main obstacle for the solution synthesis of M-Al phases is the Al-source. The previous reports, cited above in Chapter 1.3. (p. 12), show the intricacy of the Al-reduction, which requires high temperatures and pressure. Al(III)-halides must be strictly avoided, and Al(III)-organyls are difficult to decompose under moderate conditions, as shown by Bönnemann.[68-69] There are few other Al-complexes which fulfil these requirements. Yet, metastable subvalent Al(I)-compounds should be prone to be reduced to the zero state easier than Al(III). Meanwhile, low valent Group 13 organyls ECp* (E = Al,[159] Ga,[160] In[161]) are quite well accessible on the labscale. The coordination chemistry of ECp* at late d10 transition metals has been intensively investigated for a number of years.[162] The ECp* compounds are Lewis bases and isolobal to CO or phosphines (Scheme 2.1), i.e. they exhibit both σ-donating and π-backbonding properties,[163] so that they can be regarded as carbenoid. The synthesised [164] [165] cluster complexes [Pt2(GaCp*)5] or [Pd3(AlCp*)6] could be interpreted as small metal clusters, stabilised by ECp* ligands, which are as bulky as typical colloid particle capping surfactants, having an average cone angle of around 110°.[166] But it was Schnöckel’s report [167] on the synthesis of a fascinating Al38 cluster, stabilised by 12 AlCp* ligands (Figure 2.1),

16 2. Motivation and objectives

which raised the question, whether larger metal clusters [Ma(AlCp*)b] (a ≤ b), or metal nanoparticles can be stabilised by AlCp*, similarly to the role of PPh3 in Schmid’s [168] [Au55(PPh3)12Cl6] cluster.

∗ px π

py E OC

Scheme 2.1. Illustration of the isolobal analogy between ECp* and CO.

Al(I)-compounds are metastable and thus intrinsically prone to disproportionate to Al(III) and Al(0) at ambient conditions.[169] Schnöckel et al. have used this feature to synthesise a number [170] 2- [171] of metalloid Al-clusters, e.g. [SiAl8(AlCp*)6], or [Al77{N(SiMe3)2}20] . Also, a variety of smaller Al-clusters was obtained by controlled disproportion of the Al(I)-Cp* or -halide species.[172]

Figure 2.1. Crystal structure of Schnöckel’s [Al38(AlCp*)12] cluster on the front cover of Angewandte Chemie (2004, 116, 3248).

Consequently, the compound [(AlCp*)4] is supposed to be a feasible material for both particle stabilisation, and as a precursor for Al nanoparticles, and metal aluminide alloys in solution. However, whereas the coordination chemistry of AlCp* as a ligand on transition metal centres

17 2. Motivation and objectives is well explored,[162] little is known about the decomposition of this compound, particularly in terms of hydrogen treatment, aiming at intermetallic M-Al phases. It is reported that

[(AlCp*)4] decomposes at 150 °C to a stoichiometrically poor defined mixture of Al(0) and [169c] AlCp*3, without further details. A hot toluene solution of AlCp* is stable towards irradiation with UV-light.[173] Another aspect of the investigation of the decomposition of

[(AlCp*)4] under hydrogen pressure is the general question, whether [MCp*2], where M is an oxophilic main group metal (e.g. Mg,[174] Si[175]), or a late transition metal, such as Zn,[176] also serve as precursors for the respective metals and alloys under the same conditions, according to Chaudret’s method. Solution syntheses of alloys of these metals are as poorly known in literature reports as aluminides. Another Al-complex is in the focus of this work, namely the alane compound

[(Me3N)AlH3], which has widely been used in the chemical vapour deposition (CVD) of thin Al films.[177] The use of this compound as precursor for naked Al atoms dates back to the work of Buhro[178) and others[179] on the thermal and catalytically induced decomposition to

Al-nanoparticles. This and related [(R3N)AlH3] complexes as sources for Al particles are well known to cleanly decompose upon thermolysis in solution at 150 °C to Al(0), NMe3 and H2. Thus, the synthesis of transition metal aluminides with the more accessible Al-precursor

[(Me3N)AlH3] appears to be a convenient general wet chemical route to metal aluminides.

The combination of labile all hydrocarbon metal precursors MLn such as [Ni(cod)2] (with cod

= L; n = 2) with [(Me3N)AlH3] can be regarded as an extension of the work of Bogdanović, who used solid or solubilised magnesium hydride (MgH2) to prepare amorphous binary magnesium intermetallics.[86] Yet, it also shows similarities to Buhro’s route of treating metal halides MXn with LiAlH4 cited above. However, the use of the alane avoids the formation of LiCl as only partly soluble (in organic solvents) solid-state byproduct. Noteworthy, neither i AlR3 (R = Me, Et), nor Bu2AlH as most obvious alternative Al-precursors decompose to yield aluminium particles under rather mild conditions of 3 bar H2 at 150 °C in mesitylene over a period of several hours of treatment, according to own experiments.

2.3. Oxidation of M-E nanoparticles to core-shell EOx@M structures

The chemistry of colloidal, free standing particles of alloyed character with a well- defined metal composition and structure has mainly focused on magnetic materials, such as Fe, Co, or Ni, and their alloys with Pt, as shown in recent reviews by Schüth[4] and Cheon.[5]

18 2. Motivation and objectives

However, many of them are either sensitive towards oxygen, which leads to a dramatic loss of magnetisation, or ‘poisoned’ by a considerable amount of a non-magnetic metal in the lattice. In order to protect small metal clusters from oxidation, several approaches have been investigated, among others, the post synthetic coating with silica[180] or other metal oxides,[181,185] or graphitic carbon.[182] The coating of an air-sensitive material by a noble, corrosion stable metal (e.g. Pd, Pt, Au), is a convenient method of particle protection. Usually, the metal@metal nanoparticles are prepared by sequential decomposition of the precursors. For example, Cheon et al. synthesised air stable Pt@Co nanoparticles by transmetallation reaction of Co nanoparticles [84,85] with [Pt(acac)2], giving [Co(acac)2] as byproduct. Yang et al. reported on the synthesis of

Fe3O4@Pt nanoparticles by sequential decomposition of [Pt(acac)2] and [Fe(CO)5] in boiling octyl ether/1,2-hexadecanediol, with OLEA and oleylamine as capping ligand.[183] Lu et al. added a solution of Co colloids to a solution of K[AuCl4] and obtained Au@Co nanoparticles - [184] by reduction of [AuCl4] at the particle surface. Early works of Klabunde et al. synthesised Mg coated Fe nanoparticles by sequentional metal vapour co-condensation. The following air oxidation led to the passivation of Fe by a MgO@Mg@Fe core-shell nanoparticle.[185] However, the full coating of a particles surface with a metal oxide is difficult, if the surface structures of the colloid and the oxide species are too different.[186] Yu et al. have demonstrated that the decomposition of [Fe(CO)5] in presence of Au colloids (diameters between 2 and 8 nm) and subsequent air exposure led to dumbbell-structured Au-Fe3O4 nanoparticles (Figure 2.2, left image),[187] highlighting the intricacy of the preparation of a full shell around a metal core, which suggests that not all metals are prone to form such structures, at least not by sequential decomposition of metal complexes. Thus, it is important to establish a concept of the preparation of a well-ordered alloy particle, which already contains both the metal, which is to be passivated (such as Fe, Co, or Ni), as well as the sacrificial metal (e.g. Mg, Al, Si, or Zn), which has the role of an oxygen-getter. Yet, there are few reports on the preparation of metal oxide@metal core-shell particles. Baldi et al. presented an approach to

CoFe2O4 core-shell particles by oxidation of CoFe nanoparticles; however, this structural [188] motif is not fully elucidated. Sun et al. obtained Fe3O4@FePt nanoparticles by direct exposure of FePt colloids to ambient air.[189] Schaak et al. have shown by HRTEM (Figure

2.2, right image), that the ternary AuCuSn2 alloy quickly forms a 2 nm thick SnO2 shell around the particles.[56] Besides the question, whether M-Al alloy nanoparticles can be synthesised in organic solution by means of hydrogenolysis, it is also interesting to investigate the behaviour of these alloys upon oxidation. It is mentioned above that many groups are

19 2. Motivation and objectives working on the passivation of air sensitive magnetic nanoparticles and post-synthetic coating of the sensitive core does not always give a desired full shell.[186]

[187] Figure 2.2. HRTEM images of Yu’s dumbbell‐like Fe3O4‐Au nanoparticles (left) and (SnO2)x@AuCuSn2‐x nanoparticles, synthesised by Schaak et al.[56] (right image).

An alternative approach to oxide coated metal particles is the selective oxidation of one metal component of alloyed nanoparticles, particularly aluminides and -zincides. Several groups have extensively studied the oxidation behaviour of thin films of crystalline NiAl with [190-192] oxygen. It was found that upon O2 treatment, Al atoms diffuse onto the bulk surface, forming a passivating Al2O3 layer which prevent the core, or Ni, respectively, from corrosion, as schematically illustrated in Figure 2.3.

Figure 2.3. Schematic illustration of the surface formation of crystalline γ‐Al2O3 upon Al‐oxidation of thin films of NiAl, according to ref. [190].

For this reason, transferring this concept to small, freestanding β-phase metal aluminide and - zincide M-E nanoparticles (M = Fe, Co, Ni, Cu; E = Al, Zn), should lead to a full coverage of the particle surface by alumina or zinc oxide, respectively, and consequently, to a protection of the M-rich core from oxygen (Figure 2.4a). However, the core will certainly still contain the metal E. The incorporation of E into the lattice of a magnetic nanoparticle is equivalent to a poisoning of the nano-magnet. Thus, it is important to dope the metal M with as little E as it is necessary to form a full alumina shell on the particle surface, so that the core is more or less

20 2. Motivation and objectives

E-free. Hence, a reduction of the E-content in the alloy leads to α-M-E phase with a structural motif of the metal M. If the E-content in the alloy is so low, that only a partial shell of alumina will be formed upon oxidation, the metal core is exposed to oxygen and becomes oxidised, too, as illustrated in Figure 2.4b. Thus, the determination of the minimal E amount in a nanosized Hume-Rothery M-E alloy phase is of much significance regarding magnetic materials.

Figure 2.4. Schematic presentation of the conceptual phase segregation and surface oxide formation in a) a β‐ phase M1‐xEx nanoparticle by diffusion of the oxophilic metal E onto the surface and following formation of a full EOx shell, protecting the metal M core (or an E‐deficient M1‐xEx α‐phase) from oxidation, and b) a α‐phase

M1‐xEx nanoparticle, with an incomplete EOx shell, exhibiting free metal surface, which is exposed to oxygen.

21 3. Categories of intermetallic compounds

3. Categories of intermetallic compounds

Nearly 80 % of the existing elements are metals, and virtually all of them form alloys with other metals. At a closer look, the above introduced terms ‘alloy’, ‘intermetallic phase’ and “intermetallic compound’ appear blurry and may be misleading. Whereas intermetallic compounds are defined as substances of a well defined stoichiometry, e.g. MgCu2 or Na4Si4, an intermetallic phase (e.g. β-NiAl or θ-CuAl2) describes a (broad or narrow) section in the binary M-N phase diagram, within all M1-xNx compositions exhibit the same crystal structure. Although the sum formulae imply a chemical relationship, the structures, oxidation states and chemical reactivities, the physical properties are, however, very different. Among the plethora of possible phase compositions, it is difficult to overview a trend or structure in alloy formation. Yet, it is few factors, which influence the nature of an intermetallic compound, i.e. its atomic structure, ordering and stoichiometry: 1) the number of valence electrons (VE) per atom, 2) the electronegativity of the element and 3) the atomic radii. The categorisation of all metals into different subgroups assembled by these criteria, allows a division of intermetallic substances into combinations of the respective metals into several classes.[193] - A1 metals: The alkali and earth alkali elements (Li-Cs and Be-Ba), as well as lanthanoids or actinoids, which are very electropositive and have rather large radii. - A2 metals: All transition metals, apart from the main group metal-like Zn, Cd and Hg. They exhibit similar atomic radii and do not show large divergences in electronegativity, but in the number of valence electrons. - B1 metals: Zn, Cd, Hg, as well as the metals of Groups 13 and the heavier group 14 metals Sn and Pb. These metals exhibit a higher electronegativity than A1 and A2. - B2 metals: Heavier elements of the Groups 14 (Si, Ge), 15 (As, Sb, Bi) and 16 (Se, Te, Po), which lie in the transition region to non-metals. In this paragraph, the three most prominent classes of intermetallic phases will be briefly summarised, namely the Laves- and Zintl phases, and - in the context of this work most interesting - the Hume-Rothery phases. The ‘solid solutions’, i.e. alloys of two metals which exhibit the same crystal structure, have similar radii (deviation less than 15 %) and small differences of electronegativities, will not be considered here in much detail. The metals A and B exhibit full solubility in any A/B ratio to give mixed crystals, where the atoms are randomly distributed in the crystal lattice. Typical representatives of this class are A1-A1, B2-

22 3. Categories of intermetallic compounds

B2 (NiAs-type) and especially A2-A2 combinations (e.g. Cu/Au and Ag/Au),

3.1. Laves phases

Laves phases are a series of binary intermetallic compounds of the general sharply defined stoichiometric composition MN2, consisting of a) A1 and A2 metals (alkali-/earth alkali metals and transition metals), as well as b) A1 and B1 metals (Group 12, 13 and heavy group 14 metals).[193] This class of substances is defined by the large difference of atomic radii (M >> N; rM/rN ≈ 1.23), and is independent from the electron configuration or electronegativity of the metals. All MN2 Laves phases exhibit closely related structures of the types MgX2 (X = Ni, Cu, Zn), where the Mg atoms are arranged in a closed packed diamond structure mode (X = Cu; cubic diamond structure, X = Zn; hexagonal diamond structure, X =

Ni; combination of MgCu2 and MgZn2). The metals N are filling the tetrahedral cavities in the closed-packed lattice.

3.2. Zintl phases

The Zintl phases - or Zintl anions - are defined as binary intermetallic compounds of metals having a large electronegativity difference (A1-B2).[193] They are semiconducting materials, which however, have no industrial relevance, due to the poor synthetic accessibility. The structural model can best be described with an ionic concept, i.e. an electropositive cation (A1) and the electronegative anion (B2). The simplest Zintl phases crystallise as stoichiometric structures in the antifluorite-, Na3As-, or Li3Bi-type. More complex Zintl-phases underlie the Zintl-Klemm-Busmann concept of an electropositive cation (A1), which donates its VE to the electronegative anion (B2). The Zintl-anions are isoelectronic to the elements of the next group of elements in the periodic system of elements, and thus called ‘pseudo-elements’ (e.g. Tl- Æ C, Ge- Æ P, Si2- Æ S). The anions are tending to form two- and threedimensional networks. For instance, in the NaTl phase, the Tl atoms exhibit diamond structure, and the Na cations take in the octahedral voids. Other Zintl anions, 4- 6- e.g. the tetrahedral nido-[Si4] (isolobal to P4) or arachno-[Si4] are defined as polyhedral cages, whose structures can be regarded as isolobal to the corresponding borane-clusters (e.g. 4- 6- [Si4] [B4H8], or [Si4] [B4H10]) and thus obey the Wade-Mingos-Rudolph rules.

23 3. Categories of intermetallic compounds

3.3. Hume-Rothery phases

Hume-Rothery phases are a class of binary intermetallic phases M1-xNx, where 0 ≤ x ≤ 1. In general, this category of bimetallic systems involves late transition metals and main group metals (A2-B1), having similar atomic radii and electronegativities (deviation < 15 %). The main characteristic of this alloy type is the influence of the number of valence electrons per metal atom to the crystal structure of the respective phase. Hume-Rothery phases are not representing stoichiometrically well defined intermetallic compounds with an exact atomic ratio of two metals, but rather general structure types spanning a narrow or broad atomic composition. William Hume-Rothery studied the phase diagrams of numerous binary alloys and found a general order of structural phase changes for a series of systems.[194] Regarding a typical phase diagram of a binary M-N alloy, several main phase regions are distinguishable, being representative for all A2-B1 alloys. The so-called α-phase is a region of metal M-rich alloys, up to around 40 at.% of the metal N. All phases of the composition M1-xNx (0 ≤ x ≤ 0.40) exhibit the face-centred-cubic (fcc) structure (Figure 3.1). The N atoms are randomly distributed (‘dissolved’) in the lattice of M, and the lattice distortion gradually increases with a rise of the N content in the alloy, which is visible by means of X-ray diffraction measurements, by the deviation of the reflection position from the ideal fcc reflections. In general, the lattice constant is in linear dependence to the atomic content of metal N in the α- [195] M1-xNx phase, which is known as Vegard’s Law. The β-phase region dominates between 40 and 58 at.% of the metal N and features a body-centred-cubic (bcc) CsCl defect structure (Figure 3.1). The N-rich region (above around 60 at.% N) is known as the γ-phase, which exhibits a very complicated cubic Cu5Zn8 defect structure type. The δ- and ε-phases are hexagonal-closed-packed. Typically, each structural domain describes an evolution of the lattice versus the VE per atom ratio (Table 3.1). However, each binary phase diagram exhibits more or less pronounced deviations from this rough classification.

Table 3.1. General influence of the ratio VE/atom at the crystal structure of the phase region of a binary Hume‐ Rothery alloy of the composition M1‐xNx (0 ≤ x ≤ 1). α‐phase β‐phase γ,δ‐phase ε‐phase

Crystal structure fcc (A1) bcc (B2) Cu5Zn8 structures hcp (A3) Ratio VE/atom 1 – 1.38 1.38 – 1.48 1.48 – 1.62 ~ 1.75 Typical range x ≤ 0.3, 0.4 0.4 ≤ x ≤ 0.6 x > 0.6 0.6 < x < 1.0

24 3. Categories of intermetallic compounds

Figure 3.1. Schematic illustration of the most common mono‐ and binary crystal structures.

With respect to the aim of this work to selectively synthesise Hume-Rothery aluminide and - zincide phases by a soft organometallic route in solution, in the following section, the intermetallic Cu-Zn, Cu-Al, Ni-Al and Co-Al phase diagrams are described.

3.3.1. Intermetallic Cu-Zn phases Copper-zinc (brass) alloys are the most archetypical examples of Hume-Rothery phases.

Brass materials have been used since the very beginning of metallurgy. Especially the α-Cu1- [196] xZnx alloys (x < 0.40, see the Cu-Zn phase diagram in Figure 3.2) are known to have an increased heat transfer, strength and hardness versus pure copper, as well as an enhanced corrosion resistance. Thus, these alloys have a widespread application as electric conductor materials and transformer coils, but also as machine parts, water pipes and fittings.[1,197] This alloy class is industrially produced by traditional metallurgical methods. The access to bulk

25 3. Categories of intermetallic compounds brass materials to be more or less limited to traditional metallurgical processes, such as mechanical alloying processes (ball milling)[198] or other physical techniques such as laser [199] evaporation synthesis. The α-Cu1-xZnx phases (x < 0.40) exhibit fcc Cu-type structure. The β-phase region (B2, CsCl-type structure) spans a range between 40 and 58 at.% Zn. The Zn- rich region (above 58 at.% Zn) is known as the γ-phase, which exhibits a cubic-complicated

Cu5Zn8 structure type. The following δ- and ε-phases exhibit a structural relationship to hexagonal-closed-packed (hcp) Zn.

Figure 3.2. Binary phase diagram of the intermetallic Cu‐Zn phase.[196]

3.3.2. Intermetallic Cu-Al phases Cu-Al bronzes belong to a class of materials with the best chemical and corrosion resistance, due to a dense passivation layer of alumina on the alloy surface. Therefore, these materials are mainly used for the production of steam fittings and valves.[1,197] This work focuses on a clean preparation protocol for the Al-rich θ-CuAl2 phase, and on the question, if distinct intermetallic α-phases, namely γ-Cu9Al4 and Cu0.50Al0.50 are accessible, with respect to the intricate situation in the α-region of the binary Cu-Al system. The Cu-Al phase diagram

26 3. Categories of intermetallic compounds

(Figure 3.3)[200] is more complex than the Cu-Zn brass system, yet containing the typical regions of the α-, β- and γ-phases at the Cu rich side, but exhibiting several intermetallic phases with a plethora of distinct solid state structures and crystallographic properties at specific conditions of existence. Whereas the formation of the clearly defined body centred tetragonal (bct) θ-CuAl2 phase (Figure 3.4) does not compete with the formation of potentially neighbouring Cu-Al phases, the synthesis of phase pure Cu-rich intermetallic Cu- Al phases, which do not exhibit Cu fcc structure, is a quite ambitious goal. As mentioned above, between 20 and 50 at.% Al numerous phases occur with distinct crystal structures, e.g. [200] η1,2-Cu1Al1, δ-Cu3Al2, Cu2Al1, γ-Cu9Al4 and the orthorhombic β'-Cu3Al phase. The α-Cu structure appears underneath 20 at.% Al.

Figure 3.3. Binary phase diagram of the intermetallic Cu‐Al phase.[200]

Therefore, the kinetics of the formation of one single phase plays an important role and is intrinsically difficult to control. A slight deviation of the metal content can already have an enormous effect of the product distribution. Thus, regarding the Cu-rich side of the phase diagram between the α-phases that have Cu-structure, and θ-CuAl2, it is obvious that it is difficult to prepare a phase pure copper aluminide compound Cu1-xAlx, with 0.25 ≤ x ≤ 0.50 by conventional metallurgical melting/cooling processes.

27 3. Categories of intermetallic compounds

Figure 3.4. Elementary cell of the intermetallic θ‐CuAl2 phase (left, Cu: dark red, Al: grey) and views at the (001) lattice plane (middle) and at the (010) plane (right). Structural data are taken from refs. [132] and [133], and from the ICSD database No. 42518.

To date, there are several processes to obtain Cu1-xAlx alloys, including furnace- or arc melting. Havinga et al. first published the synthesis of the θ-CuAl2 phase by arc melting, characterised by X-ray diffraction, as well as an entire class of other intermetallic phases, [133] having the bct θ-CuAl2-structure (Figure 3.4). Recently, Grin et al. presented a full [132] structural characterisation of θ-CuAl2, including single crystal X-ray structural analysis.

El-Boragy et al. synthesised the hexagonal Cu0.58Al0.42 phase, as well as the monoclinic [201] Cu0.51Al0.49 phase (Figure 3.5) by heat treatment. Harbrecht et al. investigated the phase changes in single-crystalline phase-pure α-CuAl structures.[202] In the same group, the [203] orthorhombic ζ1- and ζ2-Cu4Al3 phases were synthesised. It was shown that already low temperatures have an effect at the atom arrangement and the crystal space group, e.g. the phase transformations in the ζ2-Cu3Al4-δ phase, which decomposed above 400 °C to the η2- [204] CuAl and the ζ1-Cu4Al3 phase.

Figure 3.5. Elementary cell of the intermetallic Cu0.51Al0.49 phase. Crystal structure data taken from ref. [201] and the ICSD database No. 40332.

28 3. Categories of intermetallic compounds

[205] Westman et al. synthesised single crystals of the cubic γ-Cu9Al4 phase (Figure 3.6). The formation of γ-Cu9Al4 and θ-CuAl2 was also observed in typical mechanical alloying processes, such as ball milling[126-128] and interfacial reactions.[142,206] Depending on the conditions, other intermetallic Cu/Al compounds, i.e. β'-Cu3Al, Cu4Al2 and CuAl, mentioned above are detected, too. The solid-state synthesis between Al and Cu powders during high energy ball milling showed the formation of only γ-Cu9Al4 compound for the Cu1-xAlx (0.3 ≤ x ≤ 0.7) alloys because this compound has the largest driving force under such conditions, as noted by Xi and co-workers.[128] Jiang et al. studied thin film reactions of Cu/Al multilayers which had been investigated by using differential scanning calorimetry and transmission electron microscopy. Sequential intermetallic compound formation was found in the temperature range from 25 to 350 °C. The activation energies for the formation of θ-CuAl2 and γ-Cu9Al4 were determined to 0.78 (±0.11) and 0.83 (±0.2) eV, respectively. The θ-CuAl2 [206] phase was formed prior to the γ-Cu9Al4 phase even in the presence of excess copper.

However, Lima et al. obtained the γ-Cu9Al4 intermediate phase by ball milling for a sufficiently long time together a Cu and Al powder mixture consisting of 33 at. % Al.[207]

Figure 3.6. Illustration of the elementary cell of the γ‐Cu9Al4 phase. Crystal structure data taken from ref. [205] and ICSD database No. 1625.

3.3.3. Intermetallic Ni-Al phases Nickel aluminide phases are presumably the most prominent representatives of binary intermetallic Hume-Rothery phases, being an attractive material for many industrial products, e.g. car and machine parts or turbine blades,[208] due to their bulk physical properties such as low density, high melting point and creep resistance.[122,209] The binary Ni-Al phase diagram,[210] presented in Figure 2.7, exhibits the typical α-, β-, and γ-phase regions, and

29 3. Categories of intermetallic compounds

additionally, there are several distinct intermetallic phases, such as Ni2Al3 and γ’-Ni3Al.

Particularly, the β-NiAl phase (B2 structure, Figure 3.1) and the γ’-Ni3Al phase (L12 structure, AuCu3 type, Figure 3.1) are preferable target phases in industry, since they exhibit [208] an excellent corrosion stability. The formation of a crystalline γ-Al2O3 layer on thin films of Ni1-xAlx by preferential Al-oxidation and the diffusion of Al onto the surface of NiAl phases upon oxidation was studied in detail.[190-192]

Figure 3.7. Binary phase diagram of the intermetallic Ni‐Al phase.[210]

The metallurgy and chemistry of ultrafine and nanocrystalline nickel aluminide powders including the surface oxidation has more recently been studied largely because of improving the materials properties of aluminide based alloys as the Hall-Petch effect predicts an increase of hardness or strength associated with decreasing grain size.[211] Nickel aluminide materials are typically prepared by metallurgical processes, such as high energy mechanical milling or welding,[121-125] arc plasma,[134,135] and laser evaporation/condensation.[137-140]

30 3. Categories of intermetallic compounds

3.3.4. Intermetallic Co-Al phases

The most prominent, B2 structured bulk β-CoAl phase (CsCl type, Figure 3.1) exhibits similar physical properties as β-NiAl. Likewise the phases described above, β-CoAl is also being synthesised by conventional metallurgical methods, such as mechanical alloying,[119,120] laser evaporation.[137] As well, several groups reported on the combustion synthesis from Co [212] [213] and Al powder. In the Al-rich region (xCo < 0.5) of the Co-Al phase-diagram (Figure

3.8), there are several intermetallic Co1-xAlx phases, having distinct crystal structures, e.g. [214] [215] [216] Co2Al5 (hexagonal), Co4Al13 (orthorhombic) and Co2Al9 (monoclinic).

Figure 3.8. Binary phase diagram of the intermetallic Co‐Al phase.[213]

31 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.1. Hydrogenolysis of [CpCu(PMe3)] to copper nanoparticles

Over the past 20 years, the research on colloidal copper particles has been extensively studied, and there is a plethora of reports on the synthesis and physical properties, including several different preparation techniques and stabilising agents, such as long chained alcohols, thiols, amines, phosphine oxides, fatty acids, or long-chained polymers (Table 4.2, p. 39). However, most Cu compounds used for reduction are simple Cu salts, such as nitrates, chlorides, sulfates or acetates, which cannot be co-decomposed with any Al- or Zn-source without giving Al2O3, or ZnO, respectively. Also, in most cases, the decomposition conditions are not compatible with the Al- or Zn-precursors. The complex [CpCu(PMe3)] has already been used as precursor for the chemical vapour deposition of elemental copper above 260 °C and 10-3 mbar, with trimethylphosphine and 1,5-dihydrofulvalene as side products.[217] Recently, this precursor was embedded in the gas phase into the cavities of the ZnO-based metal-organic framework MOF-5.[155] The precursor decomposes within the cavities under hydrogen as well as upon irradiation with UV-light to Cu nanoparticles with a size of 3.2 nm.

Therefore, [CpCu(PMe3)] appears to be a suitable starting material for the soft and clean chemical reduction with hydrogen in solution.

4.1.1. Synthesis and characterisation of nano-Cu powder

The treatment of a pale yellow mesitylene solution of [CpCu(PMe3)] (c = 0.05 mol/L) with 3 bar H2 pressure at 150 °C led to the formation of a red-brown deposit within 5 minutes (Scheme 4.1), which was identified as elemental copper (see below). In order to ensure the complete decomposition, the red-brown suspension was stirred for 1 h at 150 °C.

P 3 bar H2, 150 °C Cu Cu(0) ++H C CH3 Mesitylene, 1 h 3 CH3 PMe3

Scheme 4.1. Decomposition of [CpCu(PMe3)] under H2 pressure in mesitylene solution to Cu particles.

32 4. Synthesis of intermetallic α‐ and β‐CuZn phases

After precipitation of the metallic powder, the supernatant was decanted. Free PMe3 was detected in the filtrate by means of GC-MS, 1H- and 31P-NMR, however, the Cp ligand could not be traced by any means of analysis.

4.1.1.1. 1H- and 31P-NMR spectroscopic measurements

The decomposition of [CpCu(PMe3)] was repeated in a pressure stable NMR tube in d12-mesitylene, since the organic byproducts could not be clearly traced in the supernatant, as described before. The original 1H-NMR spectrum of the precursor, shown in Figure 4.1 (top left), exhibited a singlet of the Cp moiety at 6.08 ppm (5 H) and a doublet at 0.67 ppm of the 2 31 PMe3 ligand, caused by P-H coupling ( JP-H = 7.687 Hz). The P-H decoupled P-NMR spectrum showed a very broad signal of the PMe3 phosphorus atom at -31.4 ppm (Figure 4.1, top right). Most presumably, the line broadening derives from the interaction with the Cu nucleus, which has a quadrupolar momentum.

Figure 4.1. Hydrogenolysis of [CpCu(PMe3)] in a pressure stable NMR tube in d12‐mesitylene (4 bar H2, 150 °C, 2 h). Top: 1H‐ (left) and 31P‐NMR spectra (right) before decomposition. Bottom: 1H‐ (left) and 31P‐NMR spectra (right) after full decomposition. Asterisks mark the residual ring and methyl protons in d12‐mesitylene.

33 4. Synthesis of intermetallic α‐ and β‐CuZn phases

After 2 h of treatment with 4 bar H2 pressure at 150 °C, a red-brown solid precipitated from the initially colourless solution. The corresponding 1H-NMR spectrum (Figure 4.1, bottom left) did not exhibit the precursor signals any longer. Instead, multiplet resonances of 2,4- cyclopentadiene at 6.49 and 6.32 ppm (ortho- and meta-H’s) and at 2.75 ppm (ipso-H) became visible (see box inserts). The integral ratio of 1.7:1.9:2.0 quite matched to that of 2 CpH. As well, the doublet of free PMe3 was located at 0.83 ppm ( JP-H = 2.634 Hz). 31 Correspondingly, in the P-NMR spectrum (Figure 4.1, bottom right), the signal of free PMe3 was detected at -62.9 ppm and the signal of coordinated PMe3 disappeared expectedly. Thus, it can be concluded that the Cu-precursor [CpCu(PMe3)] quantitatively decomposes under hydrogen pressure to Cu metal, releasing CpH and PMe3, according to Scheme 4.1. Thus, it can also be employed for the preparation of a colloidal solution of Cu nanoparticles by addition of a suitable surfactant as well for alloying upon co-decomposition with another suitable metal precursor. Obviously, PMe3 is not able to stabilise the Cu particles as colloids, which is not surpsising, since it is gaseous at 150 °C, and thus not prone to coordinate at the particle surface in significant amounts.

4.1.1.2. X-ray powder diffraction analysis The red-brown powder was identified as analytically pure face-centred cubic (fcc) copper (100 at.% Cu, according to AAS), showing the expected reflections at 2θ = 43.31° (111), 50.34° (200), 74.24° (220) and 90.05° (220), as shown in Figure 4.2.

Figure 4.2. XRD pattern of Cu powder, synthesised by hydrogenolysis of [CpCu(PMe3)] (reference taken from the JCPDS database, No.: 4‐0836).

34 4. Synthesis of intermetallic α‐ and β‐CuZn phases

The size of the primary particles was estimated to 25 ± 4 nm, based on calculations using the Scherrer formula (see Chapter 9.3.1.1., p. 162).

4.1.2. Synthesis and characterisation of colloidal Cu nanoparticles from [CpCu(PMe3)] It was attempted to prepare Cu colloids by hydrogenolysis of the above precursor in presence of a surfactant which is commonly used. However, it was not possible to obtain Cu colloids by hydrogenolysis of [CpCu(PMe3)] in presence of HDA, Me2HDA, TOPO or OLEA. In each case, Cu powder precipitated instead. The polyether poly(2,6-dimethyl-1,4- phenylene oxide) (PPO), is a well known alternative stabilising agent of transition metal colloids, e.g. Fe,[218] and Co[95d] nanoparticles. PPO has an average molecular weight of 244,000 g·mol-1, which roughly corresponds to 2,000 phenoxy chains. In this work, PPO was used for the preparation of Cu colloids, as well as for α- and β-CuZn and CuAl colloids, since it is inert towards the oxophilic compounds [ZnCp*2] and [(AlCp*)4] which exhibit a high reactivity with all other surfactants mentioned above (see Chapter 5).

Scheme 4.2. Polymeric structure of poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO).

The addition of PPO to a mesitylene solution of [CpCu(PMe3)] and the subsequent hydrogenolysis led to the characteristic wine red solution of Cu colloids, denoted in the following as Cu/PPO. A PPO:Cu mass ratio of 10:1 was used, relating to the mass of Cu metal in the precursor. This roughly corresponds to an atomic ratio of one PPO chain to 400 Cu atoms. The particles were precipitated upon adding n-pentane or n-hexane to the solution. Also, the particles could be deposited upon cooling the colloidal solution at -30 °C overnight. The red-brown residue was washed with pentane and dried. Noteworthy, the powder can easily be re-dissolved in toluene or mesitylene by heating or by ultrasound.

4.1.2.1. X-ray diffraction analysis The powder X-ray diffractogram of the PPO-stabilised Cu particles, precipitated from the colloidal solution, revealed reflections of metallic fcc Cu, which are broadened in comparison to Figure 4.1. The average particle size of 10 ± 5 nm as estimated via the Scherrer

35 4. Synthesis of intermetallic α‐ and β‐CuZn phases equation (Table 4.1). The reduction of the PPO concentration does neither have a large influence on the size of the Cu particles nor on the stability of the colloids in solution. If the PPO content is reduced to a PPO:Cu mass ratio of 5:1, the XRD pattern does hardly exhibit any sharpening of the reflection, i.e. the particles are not significantly growing if less PPO is used (Figure 4.3). If the mass ratio PPO:Cu is 1:1, the reflections are becoming sharper. This is also reflected in the particle size calculation (Table 4.1). However, in the co- decompositions of [CpCu(PMe3)] with [ZnCp*2] and [(AlCp*)4], it was shown that a decrease of a tenfold mass excess of PPO leads to particle precipitation. With increasing incorporation of Zn or Al, the particle size of the Cu1-xEx particles (E = Zn, Al) increases (see TEM images below), and analogously, the solubility decreases. Above 50 at.% E, for example in the case of θ-CuAl2 (Chapter 5.6., p. 110), the particles cannot be prevented from precipitation using a tenfold mass content of PPO, whereas at 50 at.% and below, colloidal solutions were obtained.

Figure 4.3. XRD pattern of precipitated Cu/PPO particles with a PPO:Cu mass ratio of a) 10:1, b) 5:1 and c) 1:1.

36 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Table 4.1. Comparison of the FWHM and calculated average particles sizes of Cu/PPO particles with different PPO concentration. FWHM [°] Particle size [nm] PPO:Cu PPO:Cu h k l 2θ [°] PPO:Cu 5:1 PPO:Cu 1:1 PPO:Cu 5:1 PPO:Cu 1:1 10:1 10:1 1 1 1 43.332 1.003 0.696 0.506 10.2 14.7 20.2 2 0 0 50.439 1.525 1.098 0.754 6.9 9.6 13.9 2 2 0 74.222 1.022 0.907 0.761 11.6 13.1 15.6 3 1 1 90.030 1.222 1.023 0.828 11.0 13.1 16.2

4.1.2.2. Transmission electron microscopy The TEM image of Cu/PPO colloids (Figure 4.4, left), synthesised in a Cu:PPO mass ratio of 1:10, shows free-standing spherical particles with a rather broad size distribution of around 15 ± 5 nm, matching the XRD data. Selected area electron diffraction and energy- dispersive X-ray spectroscopy (right) expectedly give additional proof for pure, crystalline fcc copper. The less uniform particle size distribution and the considerably larger typical particle size of Cu/PPO compared with Cu/HDA is likely to be a consequence of the weaker particle surface-surfactant interaction in the case of PPO.

Figure 4.4. Left: TEM image of Cu/PPO colloids and HRTEM of a single Cu particle (top), right: SAED pattern (top) and EDX spectrum (bottom).

4.1.2.3. IR spectroscopy of the Cu/PPO particles The FT-IR spectrum of the Cu/PPO material, diluted with KBr, showed the presence of

37 4. Synthesis of intermetallic α‐ and β‐CuZn phases hydrocarbon components of the PPO protecting shell, as shown by a comparison with the IR spectrum of PPO in Figure 4.5. The indicated by characteristic vibration absorptions of the aromatic C-C, C-H and C-O bonds at 1602, 1465, 1304, 1186, 1016 and 859 cm-1. The broad absorption at around 3400 cm-1 stems from the O-H vibration of residual water in KBr.

Figure 4.5. IR spectra of PPO (top) and precipitated Cu/PPO colloids (bottom). The samples were prepared as KBR pellets.

4.1.2.4. UV-Vis spectroscopy The surface plasmon resonance (SPR) of nano Cu/PPO is clearly visible at 573 nm (Figure 4.6). This value slightly deviates from our previous data of 566 nm obtained for Cu/HDA colloids.[219] However, the SPR of Cu depends on the particle size and red-shifts to higher wavelengths with increasing particle size as investigated by Pileni et al. for a series of Cu colloids of different size regime, e.g. prepared in reverse micelles by reduction of copper salts.[220] Also, different surfactants present at the surface and the dielectric properties of the

38 4. Synthesis of intermetallic α‐ and β‐CuZn phases solvent may cause subtle changes in the exact position of the SPR.[219-233] Table 4.2 comprises the preparation methods of colloidal solutions of copper nanoparticles known to date.

Figure 4.6. Time resolved UV‐Vis spectra of Cu/PPO colloids after oxidation on air.

Table 4.2. Comparison of the preparation methods of copper colloids reported to date, particle sizes, surface plasmon resonances and surfactants used. Cu precursor Reducing agent Surfactant Particle size [nm] SPR [nm] Ref. UV light Cu(acac) ‐‐‐‐‐ ‐‐‐‐‐ 574 [221] 2 (in ethanol)

[Cu(mesityl)]5 Thermal decomp. HDA 9 568 [156] Intramolecular [Cu(OCH(Me)CH NMe ) ] HDA 8 566 [219] 2 2 2 reduction /ΔT t [CpCu(CN Bu)] CO PPh3 2 550 [222]

Cu(NO3)2, Cu(CH3COO)2 1‐Hexanethiol 1‐Hexanethiol 7 ‐‐‐‐‐ [223] TOAB /1‐ NaBH 2 ‐‐‐‐‐ [224] 4 Hexanethiol Cu(NO ) 3 2 Na[BH CN] or 3 AOT ‐‐‐‐‐ 557 [225] TMPD

[N(octyl)4][CuCl2Br2] Li[BEt3H] TOAB 5‐10 560 [226]

CuCl2 NaBH4 Octanethiol 1‐7 ‐‐‐‐‐ [227] UV CuSO ‐‐‐‐‐ 4.5 570 [228] 4 radiation/ferritin

Cu(ClO4)2 γ‐radiation Poly(ethyleneimine) 7.5 570 [229] CuO Ethylene glycol D‐Sorbitol 2000 ‐‐‐‐‐ [230] Laser ablation 2‐Propanol 29 580 [231]

Cu(CH3COO)2 Hydrazine PVP 23 575 [232] L‐Cysteine 11‐15 487 Cu(Stearate)2 NaBH4 [233] Dodecanethiol 9 458 NaBH or Cu(AOT) 4 AOT 2‐10 570 [220] 2 hydrazine

39 4. Synthesis of intermetallic α‐ and β‐CuZn phases

A common feature of all Cu colloids prepared to date is the fast surface oxidation upon air contact. The Cu/PPO particles also rapidly oxidise when exposed to the ambient air, [219] presumably transforming into a Cu2O@Cu core-shell system as it is the case for Cu-HDA. The surface oxidation is accompanied by the colour change of the solution from wine red to [220d] green (Figure 4.7), indicating the surface oxidation of Cu(0) to Cu2O. Accordingly, the UV-Vis absorption shifts gradually within 1 h from the initial position at 573 nm to the final maximum at 610 nm, which remains constant, then (Figure 4.6). Reduction of the Cu2O@Cu system back to the original wine red state of Cu/PPO is possible by treatment of the sample with H2 (3 bar, 7 h, 150 °C).

Figure 4.7. Vials containing diluted solutions of Cu/PPO colloids. Left: Cu colloids under argon, right: Cu2O@Cu colloids after 1 h of exposure to air.

Previously, the reversible redox chemistry of HDA-stabilised Cu colloids was examined by FT-IR spectroscopic studies of the CO adsorption at the particle surface.[234] If the Cu/PPO particles are oxidised by short exposure to the air and then precipitated, the XRD and SAED still displays the characteristic Cu signature, without any indications for copper oxides. The XRD peaks of copper oxides were only be detected after annealing of the sample overnight at 150 °C (under inert conditions in a sealed capillary). In addition, the related surface oxidation of Cu particles embedded into MOF-5 was characterised by XRD and XAS.[235]

4.1.2.5. X-ray absorption spectroscopic studies on precipitated Cu/PPO colloids The Cu/PPO colloids revealed a typical XAS spectrum which resembled metallic copper (Figure 4.8). A closer inspection and simulation of the data revealed the presence of a small amount of a light element, which could not be unambiguously identified on the basis of XANES and EXAFS. Traces of oxygen are a likely guess, because rigorous exclusion of any

40 4. Synthesis of intermetallic α‐ and β‐CuZn phases leak during sample preparation for the measurement is just not possible. The calculated Cu- Cu distance of 2.549(4) Å is comparable to the interatomic Cu-Cu distance of 2.556 Å in the Cu bulk,[236] thus showing only a slight deviation. The Cu-Cu coordination number was found to be ~8 (Table 4.3). Thus, due to the spherical nature of the particles, a particle size of roughly 2 nm can be deduced, according to a method discussed by Borowski.[237]

Table 4.3. Model parameters of the measurement of the Cu‐edge in Cu/PPO colloids.

‐3 2 Shell Coord. No. r [Å] σ [10 Å ] E0 [eV] R‐factor [%]

Cu‐O 1.1 ± 0.2 1.88 ± 0.03 10 ± 5.3 7.2 4.5 (3.13 < k < 12.85) Cu‐Cu 7.8 ± 0.4 2.549 ± 0.004 6.7 ± 0.5 2.2 4.4 (3.13 < k < 12.85) r = atomic distance, σ = Debye‐Waller factor, E0 = edge correction

There is a discrepancy of that size estimation and the data from XRD and TEM, pointing to a typical particle size of 10-20 nm which may be related to a rather broad size distribution, or to agglomeration of small primary particles, respectively. Intentional oxidation of the Cu/PPO colloids resulted in little change in the spectrum and model parameters, resembling the spectrum of Cu/PPO colloids measured under air exclusion, shown in Figure 4.8, which may be explained by the development of a thin copper oxide shell.

Figure 4.8. XANES (left) and EXAFS (right) of the Cu‐edge of Cu/PPO particles (red lines). For comparison, reference spectra of CuO and Cu2O are included. The FT reference spectrum of Cu foil is shown as grey background underneath the Cu/PPO XANES and EXAFS spectra. The spectra are scaled artificially, in order to highlight similarities with measured samples.

41 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.9. k2‐weighed absorption spectra of Cu/PPO colloids. Measured and fitted χ(k) function (left) and Fourier transformed EXAFS spectrum (right).

4.2. Hydrogenolysis of [ZnCp*]2 to Zn particles

4.2.1. Synthesis

A yellow solution of [ZnCp*2] in mesitylene (c = 30 mol/L) was set to 3 bar H2 and heated to 150 °C (Scheme 4.3). Within 15 minutes, a grey precipitate of analytically pure zinc was formed (100 %, AAS). The supernatant was filtered, and the residue washed with n- pentane. Noteworthy, [ZnCp*2] remains stable below 150 °C and 3 bar H2 pressure. The byproduct (Cp*H) was found in the filtrate by 1H-NMR and GC-MS.

3 bar H , 150 °C Zn 2 Zn(0) + 2 Cp*H Mesitylene, 2 h

Scheme 4.3. Quantitative formation of Zn(0) by hydrogenolysis of [ZnCp*2].

In contrast to the copper case and in accordance to the previous report on the synthesis of brass colloids,[146] a solution of colloidal Zn nanoparticles cannot be obtained by

42 4. Synthesis of intermetallic α‐ and β‐CuZn phases

decomposition of [ZnCp*2] under H2 in presence of PPO or HDA. An insoluble precipitate of Zn is formed even in the presence of large amounts of common surfactants. It should also be noted here, that other Zn-alkyls such as ZnEt2, do not decompose by means of hydrogenolysis, at least not under the conditions described.

4.2.2. Characterisation

4.2.2.1. X-ray powder diffraction In the XRD diagram of the isolated powder, shown in Scheme 4.10, the detected reflections were assigned to hexagonal-closed packed (hcp) Zn (2θ = 36.31° (002), 38.99° (100), 43.24° (101), 54.41° (102), 70.16° (103), 70.68° (110), 82.14° (112) and 86.61° (201). The particle size was estimated via the Scherrer equation to around 34 ± 4 nm.

Figure 4.10. XRD pattern of Zn, obtained by decomposition of [ZnCp*2] under hydrogen pressure (reference data taken from the JCPDS database, No. 4‐0831).

4.2.2.2. 1H-NMR and GC-MS of the filtrate In the 1H-NMR of the supernatant, shown in Figure 4.11, besides the signals of mesitylene at 6.67 and 2.16 ppm (marked with asterisks), the signals of Cp*H at 1.81, 1.73 and 0.99 ppm

43 4. Synthesis of intermetallic α‐ and β‐CuZn phases

(integral ratio of 6:6:3) could be observed. No other signals of significant intensities were detected. A sample of the supernatant was taken for compound separation via gas chromatography, coupled with a mass spectrometer. In the GC, only the signals of mesitylene and Cp*H were detected. The corresponding mass spectrum of Cp*H is presented in Figure + 4.12. The molecular peak of Cp*H was found at M = 136 m/z (Int: 84 %, Cp*H: C10H16, M = -1 + 136 g·mol ). Other prominent signals were found at m/z = 121 (100 %, M -CH3), and 105 (98 + %, M -H, -CH3). The other, less intense peaks match to the systematic loss of C- and CH2- groups: 93; 105-C (61 %, cyclo-C4Me3), 91; 105-CH2 (60 %, cyclo-C5Me2H), 79; 93-CH2 (50 + %, cyclo-C4Me2H), 65; 79-CH2 (16 %, C4ÆC5 ring rearrangement, cyclo-C5H5 ), 53; 65-C + + (15 %, ring opening, 1,3-butadienyl-cation C4H5 ), 41; 53-C (28 %, 2-propenyl-cation C3H5 ), + 27; 41-CH2 (15 %, ethenyl-cation C2H3 ).

1 Figure 4.11. H‐NMR of the supernatant of the decomposition of [ZnCp*2] under H2 pressure.

44 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.12. Mass spectrum of Cp*H, derived from the GC separation of the supernatant of the decomposition of [ZnCp*2].

4.3. Soft chemical synthesis of β-CuZn nanoparticles

Since [ZnCp*2] and [CpCu(PMe)3] decompose on a similar timescale during co- hydrogenolysis, the in situ formed Cu and Zn atoms are likely to form a phase-pure alloy, which does not contain Cu- or Zn-rich regions, which would presumably form upon different decomposition time.

4.3.1. Synthesis and characterisation of β-Cu0.50Zn0.50 nanopowder

At the conditions of 150 °C and 3 bar H2, the compounds [CpCu(PMe3)] and [ZnCp*2] release Cu and Zn at a comparable rate. After just a few minutes of stirring an exactly equimolar mixture of the above complexes, a nicely golden coloured and metallic shiny precipitate formed (Scheme 4.4). The supernatant was worked up by means of filtration, and the brown precipitate was washed with n-pentane and dried. According to elemental analysis, the powder contained 49.6 wt.% Zn and 50.4 wt.% Cu, traces of hydrocarbons were not present.

45 4. Synthesis of intermetallic α‐ and β‐CuZn phases

3 bar H2, 150 °C Cu + Zn β-CuZn + 2 Cp*H + CpH + PMe3 Mesitylene, 2 h PMe3 - 2 Cp*H, - CpH, - PMe3

Scheme 4.4. Synthesis of β‐CuZn brass by hydrogenolysis of [CpCu(PMe3)] and [ZnCp*2].

4.3.1.1. X-ray powder diffraction Elemental analysis, and XRD confirmed the quantitative formation of the analytically pure intermetallic B2 structured bcc β-CuZn phase (Zhanghengite, cubic Pm3m, Figure 4.13), exhibiting reflections at 2θ = 43.30° (110), 62.97° (200) and 79.49° (211). The average size of the primary particles lay at about 29 ± 4 nm according to XRD line broadening analysis. The observed reflection pattern is thus in good agreement to the XRD diagram of OLEA-capped β-CuZn nanoparticles, synthesised by Ito et al.[148] There are no indications for the presence of other Cu-Zn phases or segregated Cu or zn metal.

Figure 4.13. X‐ray powder diffraction pattern of the β‐Cu0.50Zn0.50 sample (Zhanghengite), obtained from the hydrogenolysis of a 1:1 stoichiometric combination of [CpCu(PMe3)] and [ZnCp*2] in mesitylene at 150 °C and 3 bar H2 (reference data taken from JCPDS No.: 2‐1231).

46 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.4. Colloidal β-Cu0.50Zn0.50/PPO nanoparticles

4.4.1. Synthesis

The smooth and clean decomposition of [CpCu(PMe3)] and [ZnCp*2] via hydrogenolysis at moderate temperatures around 150 °C in mesitylene afforded “naked” agglomerated metal particles of β-CuZn forming a precipitate, which cannot be redispersed post-synthesis by just stirring and employing ultra sound and the addition of surfactants. In order to keep the in situ formed alloy particles in solution and prevent agglomeration and Ostwald ripening to some extent, poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) was chosen as a comparably weak surfactant, as discussed above in the case of Cu/PPO nanoparticles.

When [CpCu(PMe3)] and [ZnCp*2] are combined in a precisely 1:1 molar ratio in mesitylene and are treated with hydrogen in the presence of PPO (Scheme 4.5), a dark violet solution of β-CuZn/PPO particles formed within 20 minutes without any precipitation.

3 bar H2, 150 °C Cu + Zn β-Cu0.50Zn0.50/PPO colloids Mesitylene, PPO PMe3

Scheme 4.5. Synthesis of a colloidal solution of the β‐Cu0.50Zn0.50 phase in mesitylene and PPO as surfactant.

The solution was concentrated, whereupon a dark violet precipitate formed, which was separated from the solution by means of filtration with a cannula, washed with n-pentane and dried. The powder could easily be re-dispersed in common aromatic solvents, such as toluene, benzene, and mesitylene.

4.4.2. Characterisation

4.4.2.1. UV-Vis spectroscopy of β-Cu0.50Zn0.50/PPO colloids The SPR of Cu is expected to shift upon alloying with other metals and the shift can be modelled within the frame of Mie’s theory[238] based on the dielectric function of the bulk

Cu1-xZnx material. Using the real and imaginary part of the known dielectric function of bulk

47 4. Synthesis of intermetallic α‐ and β‐CuZn phases

β-CuZn,[239] a value of 520 nm for the SPR of β-CuZn nanoparticles was estimated, matching literature data.[148] However, the whole situation is complex and there is no simple correlation of the SPR with the composition as has been investigated with AuCu nano colloids in some detail.[57,70] The UV-absorption of the β-CuZn/PPO colloid is significantly blue shifted to 535 nm from the characteristic surface plasmon resonance (SPR) at 573 nm of pure Cu nanoparticles, i.e. the Cu-PPO colloid (Figure 4.14). The value of 535 nm for our β-CuZn colloid is in good agreement to the absorption of OLEA-stabilised β-CuZn particles at 520 nm recently reported by Ito et al.[148] The SPR does not shift or disappear upon exposure to ambient air over several days, giving a clear hint on surface passivation of the particle core by formation of ZnO. In contrast, pure Cu colloids rapidly oxidise on air, instead (Figure 4.6, Table 4.2 and references therein).

Figure 4.14. β‐CuZn/PPO colloids exposed to air over time, monitored by UV‐Vis spectroscopy.

4.4.2.2. X-ray powder diffraction

The XRD measurement of the violet powder of precipitated β-CuZn/PPO particles gave a clear evidence for the β-Cu0.50Zn0.50 phase (Figure 4.15, top) with a crystallite domain size around 25 ± 4 nm, calculated with the Scherrer equation. The XRD pattern of a sample of β-

Cu0.50Zn0.50/PPO after exposure to ambient air (Figure 4.15, bottom), still exhibits reflections of β-Cu0.50Zn0.50, which are, however, broadened in comparison to the sample under argon. This indicates a reduction of the average crystallite domain size. One may speculate, that this

48 4. Synthesis of intermetallic α‐ and β‐CuZn phases observation points to the suggested formation of a ZnO shell around the primary particle, and/or to the rising structural defects of the particle core upon Zn-migration to the surface.

Figure 4.15. XRD pattern of precipitated β‐CuZn/PPO nanoparticles under argon (top), and after air oxidation

(bottom). Reference reflections for β‐Cu0.50Zn0.50 are taken from the JCPDS database No. 2‐1231.

4.4.2.3. High resolution ransmission electron microscopy The HRTEM image of the redispersed powder (Figure 4.16) reveals individual particles with a more or less spherical shape of varying sizes around 30-40 nm. By focussing on single particles (resolution: 150 nm2), the EDX analysis clearly reveals the presence of both Cu and Zn with 49.4 at.% Cu and 50.6 at.% Zn, resembling the expected 1:1 composition within the accuracy of the method and with only slight variations from particle to particle below 0.5 at.% within the standard deviation. It should explicitly be noted that particles containing only Cu or Zn could not be found. The obtained selected area electron diffraction data from that sample were surprisingly poor (SAED, Figure 4.16), but at least sustained the assignment of the β- CuZn phase, showing a single spot matching the reflection of the (110) lattice plane (d = 2.07

Å; dLit = 2.08 Å). The (200), (211) and (220) reflections of β-Cu0.50Zn0.50 (XRD, Figures 4.13 and 4.15) could, however, not be detected by SAED.

49 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.16. TEM image of β‐CuZn/PPO colloids (left), the SAED pattern (upper right) and the EDX spectrum (lower right).

4.4.2.4. X-ray absorption spectroscopy studies

The β-CuZn/PPO colloids were further examined by extended X-ray absorption fine structure analysis (EXAFS) before and after intentional oxidation. One aliquot of the as- synthesised sample, kept under argon, was deliberately exposed to ambient conditions by opening the vessel and allowing contact to the air for several hours. In contrast to XRD and UV-Vis data, which give information of the volume properties of the samples, EXAFS provides information about the local environment of Cu and Zn in our case. In particular, the Cu-Cu, Cu-Zn and Zn-Zn contacts and as well Cu-O and Zn-O contacts can be analysed by EXAFS.

Measurement at the Cu-edge

The near-edge (XANES) and EXAFS spectra of the precipitated β-CuZn/PPO material are presented in Figure 4.17. The composition of the as-synthesised β-Cu0.50Zn0.50 sample was calculated as β-Cu0.51Zn0.49 from the ratio of the heights of the Cu and Zn absorption edges and using the tabulated scattering factors.[240] The copper XANES (Figure 4.17, left) of the β- CuZn/PPO sample measured under argon (red line) did not allow a straightforward identification of the phase present. The comparison showed some similarity with both Cu foil

(brown line) and Cu2O (green line). The EXAFS however (Figure 4.17, right), clearly

50 4. Synthesis of intermetallic α‐ and β‐CuZn phases

revealed that there is no Cu2O phase. In addition, the spectrum could not be modelled using scattering parameters from copper foil only. However, a simulated EXAFS spectrum of the

Cu0.50Zn0.50 Zhanghengite phase (Figure 4.17, shown as grey background) nicely matched the experimental data. The copper edge data could be satisfactorily fitted with the parameters extracted from the structural data of Zhanghengite. The calculated Cu coordination number of 7.6 ± 0.4, as well as the number and distance of the second Cu neighbours of 1.5 ± 0.7, and 4.149 ± 0.02 Å, respectively (Table 4.4), confirmed the B2 (CsCl) bcc structure model, which is typical for β-CuZn.[196]

Figure 4.17. XANES (left) and EXAFS (right) of the Cu‐edge of β‐Cu0.50Zn0.50/PPO particles. For comparison, the spectra of several reference compounds are included. The FT reference spectrum of Zhanghengite is shown as grey background underneath the CuZn EXAFS spectrum and was simulated using the FEFF code[241] and literature data.[242] The spectra are scaled artificially, in order to highlight similarities with measured samples.

Table 4.4. Model parameters of the measurement of the Cu‐edge in β‐Cu0.50Zn0.50/PPO particles. ‐3 2 Shell Coord. No. r [Å] σ [10 Å ] E0 [eV] R‐factor [%]

Cu‐Zn 7.6 ± 0.4 2.548 ± 0.003 7.8 ± 0.4 2.4 Cu‐Cu 1.6 ± 0.7 2.924 ± 0.03 10.0 ± 3.7 ‐1.3 Cu‐Cu 1.5 ± 0.7 4.149 ± 0.02 2.7 ± 2.4 ‐1.0 11.0 (3.13 < k < 12.85) Cu‐Zn 17.0 ± 0.5 4.926 ± 0.009 8.4 ± 1.0 1.9 Cu‐Zn‐Cu 27.9 ± 11.8 4.026 ± 0.04 10.0 ± 0.0 6.8 r = atomic distance, σ = Debye‐Waller factor, E0 = edge correction

51 4. Synthesis of intermetallic α‐ and β‐CuZn phases

2 Figure 4.18. k ‐weighed absorption spectra of the Cu‐edge of β‐Cu0.50Zn0.50/PPO particles. Measured and fitted χ(k) function (left) and Fourier transformed EXAFS spectrum (right).

Measurement at the Zn-edge

The EXAFS of the Zn edge (red curve in Figure 4.19, right) revealed the presence of a small Zn-O shell. The weak shoulder at 1.8 Å could be attributed to a Zn-O distance (see ZnO EXAFS, black curve), and thus indicates a slight (surface) oxidation of the Zn component. The same argument that was mentioned above about the light elements next to the Cu also holds for the Zn-edge data. The largest peak in the FT spectrum (due to Zn-Zn) was found to be in agreement with a spectrum of Zn foil, however, there are some discrepancies, as becomes clear from the XANES (Figure 4.19, left) as well. Again, the simulated EXAFS data of the Zn edge of Zhanghengite agreed well (on a qualitative basis) with the Cu0.50Zn0.50 data (shown as grey background in Figure 4.19, right), even at higher distances from the central Zn atom. Thus, it can be concluded that in line with the X-ray diffraction results, this colloid can be described with the term “nano-brass”. The intentional oxidation of the β-CuZn sample (violet curve) caused a noticeable decrease of the Zn-Zn contribution and a relative increase in the Zn-O contribution in the EXAFS. The copper however, remained unaffected by the oxidation treatment (vide supra). This finding of preferential Zn-oxidation, presumably at the surface of the particles, agrees well with the XRD and UV-Vis study on the oxidation behaviour of α-CuZn-PPO particles discussed below.

52 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Table 4.5. Model parameters of the measurement of the Zn‐edge in β‐Cu0.50Zn0.50/PPO particles. ‐3 2 Shell Coord. No. r [Å] σ [10 Å ] E0 [eV] R‐factor [%]

Zn‐O 1.3 ± 0.2 1.97 ± 0.03 5.8 ± 3.9 10.0 8.2 (2.93 < k < 12.79) Zn‐Cu 5.5 ± 0.6 2.549 ± 0.004 0.7 ± 0.1 3.8 r = atomic distance, σ = Debye‐Waller factor, E0 = edge correction

Figure 4.19. XANES (left) and EXAFS (right) of the Zn‐edge of β‐Cu0.50Zn0.50/PPO particles. For comparison, the spectra of several reference compounds are included. The FT reference spectrum of Zhanghengite is shown as grey background underneath the CuZn EXAFS spectrum and was simulated using the FEFF code[241] and literature data.[242] The spectra are scaled artificially, in order to highlight similarities with measured samples.

2 Figure 4.20. k ‐weighed absorption spectra of the Zn‐edge of β‐Cu0.50Zn0.50/PPO particles. Measured and fitted χ(k) function (left) and Fourier transformed EXAFS spectrum (right).

53 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.5. Colloidal solutions of α-Cu1-xZnx/PPO nanoparticles

4.5.1. Synthesis of α-Cu1-xZnx/PPO colloids (0.09 ≤ x ≤ 0.33) In order to systematically study the oxidation behaviour of nano-brass colloids as a function of the composition, a series of samples of the type Cu1-xZnx/PPO (0.09 ≤ x ≤ 0.33) were synthesised, namely Cu0.91Zn0.09, Cu0.83Zn0.17, Cu0.67Zn0.33, according to Scheme 4.6. In the following section, the properties of these as-synthesised samples will be discussed first, followed by an examination of the structural changes caused by oxidation.

x 3 bar H2, PPO 1 Cu + Zn Cu1-xZnx/PPO colloids (1-x) Mesitylene, 150 °C (1-x) PMe3 0.09 < x < 0.33

Scheme 4.6. Synthesis of colloidal solutions of α‐Cu1‐xZnx phases (0.09 ≤ x ≤ 0.33) in mesitylene and PPO as surfactant.

4.5.2. Characterisation of α-Cu1-xZnx colloids

4.5.2.1. X-ray powder diffraction of precipitated α-CuZn/PPO particles

Likewise Cu/PPO, the Cu1-xZnx/PPO samples were isolated from the mesitylene solution by precipitation/washing with n-pentane. The XRD diagrams of all as-synthesised samples α-

Cu1-xZnx (0.09 ≤ x ≤ 0.33) only exhibited the characteristic reflections of fcc Cu (Figures 4.21a-c). According to the Cu-Zn phase diagram,[196] α-CuZn phases, i.e. “solid solutions” of Zn in Cu, are thermodynamically stable at ambient conditions below a Zn content of 40 at.%. The lattice constant of the fcc α-CuZn defect structure, e. g. the d-spacing of the (111) reflection gradually increases with rising Zn molar fraction (Vegard´s law).[195] Table 4.6 compiles the measured variation of the 2θ values and the corresponding lattice constants for the series of α-Cu1-xZnx samples.

54 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.21. XRD diagrams of precipitated PPO‐stabilised colloidal particles of a) Cu0.67Zn0.33, b) Cu0.83Zn0.17 and c) Cu0.91Zn0.09, measured under argon. Reference JCPDS data: Cu: 4‐0836. The poor signal to noise ratio of diagram a) is caused by the short measurement time.

This observation unambiguously gave proof of the alloying of Cu and Zn in the α-Cu1-xZnx- PPO samples. The average particle sizes, calculated via the Scherrer equation, were around 21 ± 5 nm for x = 0.33, 23 ± 6 nm for x = 0.17 and 21 ± 9 nm for x = 0.09, respectively, being in good agreement with the data obtained by TEM (Figure 4.22, vide infra).

Table 4.6. Deviation of the 2θ XRD reflections (in deg) and the lattice plane distances (in Å) of α‐Cu1‐xZnx/PPO phases (determined by XRD and SAED data) from the Cu fcc structure (in 2θ/°), in dependence of the Zn content. Cu x = 0.09 x = 0.17 x = 0.33 Cu x = 0.09 x = 0.17 x = 0.33

h k l 2θLit* ∆2θ dLit* dXRD dSAED dXRD dSAED dXRD dSAED

(1 1 1) 43.298 ‐0.018 ‐0.038 ‐0.456 2.088 2.090 2.090(3) 2.091 2.090(7) 2.111 2.109(5)

(2 0 0) 50.434 ‐0.024 ‐0.057 ‐0.583 1.808 1.810 1.808(3) 1.811 1.801(7) 1.829 1.819(5)

(2 2 0) 74.132 ‐0.007 ‐0.008 ‐0.954 1.278 1.279 1.280(4) 1.279 1.271(8) 1.293 1.299(6)

(3 1 1) 89.934 ‐0.063 +0.016 ‐1.122 1.090 1.091 1.091(4) 1.091 1.107(8) 1.102 1.096(6)

(2 2 2) 95.143 ‐0.017 +0.057 ‐0.895 1.044 1.045 ‐‐‐‐ 1.044 ‐‐‐‐ 1.052 ‐‐‐‐ * Data taken from the JCPDS database (No. 4‐0836).

55 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.5.2.2. TEM measurements

Similarly to the case of β-Cu0.50Zn0.50/PPO, the TEM images of all α-CuZn/PPO colloids (Figure 4.22) reveal that the particles are quite well dispersed, though with a rather broad size regime. In all cases, the particles exhibited a more or less spherical shape, yet not having a uniform size distribution. The particle morphology did not appear to change significantly by changing reaction parameters, such as an increase of the surfactant content, higher temperature or longer reaction times. The SAED pattern clearly exhibited Cu reflections. The calculated lattice constants d of each α-Cu1-xZnx-PPO sample showed a linear dependence of the Zn content in the respective phase, which is described by Vegard’s Law,[195] and corresponded to the XRD data of these samples.

Figure 4.22. TEM images (top) and SAED pattern (bottom) of colloidal a) Cu0.67Zn0.33‐, b) Cu0.83Zn0.17‐ and c)

Cu0.91Zn0.09/PPO nanoparticles. The hkl reflections of Cu are inserted into the SAED images.

4.6. Oxidation behaviour of α-CuZn/PPO colloids

4.6.1. Intentional oxidation of colloidal α-Cu1-xZnx/PPO nanoparticles

A common feature of the previously reported[146,148] and the new β-CuZn alloy particles is that their XRD pattern and UV-Vis spectra do not change at all upon prolonged exposure to the ambient, i.e. upon oxidation even over several days. That finding is in sharp contrast to both types of pure copper colloids Cu/PPO and Cu/HDA, which exhibit a fast surface

56 4. Synthesis of intermetallic α‐ and β‐CuZn phases oxidation, visible on the shift of the SPR. This evidently different behaviour of the Cu-Zn nanoparticles is supposed to be a consequence of preferential oxidation of the Zn component to ZnO, which covers the Cu atoms, preventing them from corrosion. Hence, if the Zn content is reduced, the ZnO layer should become thinner, and a further decrease of Zn in the α-CuZn alloy would lead to an incomplete ZnO shell, so that in this case, Cu would oxidise, too. Thus, the synthesised colloidal solutions of α-Cu1-xZnx nanoparticles (0.09 ≤ x ≤ 0.33) were exposed to air for 24 h at room temperature (for experimental details see Chapter 9.4.1.6., p. 177), in order to determine, how much Zn is required to form a ZnO shell, which can fully passivate the Cu core.

4.6.1.1. X-ray diffraction measurements on oxidised α-Cu1-xZnx/PPO colloids

The XRD diagrams of the oxidised, precipitated, washed and dried samples Cu0.67Zn0.33 and Cu0.83Zn0.17 (Figure 4.23a and b) only reveal the more or less unchanged fcc Cu signature. However, in case of the Zn content of 9 at.%, the particles again rapidly oxidise, in analogy to pure nano Cu, as seen by the significant red shift to about 600 nm in the UV-Vis spectrum (Figure 4.24c, vide infra). Accordingly, the XRD diagram of the sample (Figure 4.23c) changes. Correspondingly, the signature of the fcc Cu type structure disappears and new reflections of Cu2O and CuO rise up.

Figure 4.23. XRD patterns of precipitated PPO‐stabilised nanoparticles: a) Cu0.67Zn0.33‐, b) Cu0.83Zn0.17‐ and c)

Cu0.91Zn0.09 after air oxidation. Reference JCPDS data: Cu: 4‐0836, CuO: 5‐0661, Cu2O: 5‐0667.

57 4. Synthesis of intermetallic α‐ and β‐CuZn phases

From this observation together with the above discussed EXAFS study on β-CuZn/PPO it can be concluded, that the copper component in the particles Cu1-xZnx/PPO is largely unaffected by oxidation as long as x ≥ 0.17. The Zn component is preferentially oxidised involving Zn atoms at the surface and/or Zn atoms segregating to the particle surface during oxidation. Some sort of ZnO species may thus result at the particle surface, forming a protective shell around the Cu enriched core and passivating it against further oxidation. Multiphase particles of the type (ZnO)δ@Cu1-xZnx-δ may be formed. However, below x = 0.17, rapid oxidation of both Cu and Zn takes place, eventually leading to the complete oxidation of the particle. Quite similar observations were made in the case of α/β-CuAl particles and colloids, which is discussed in Chapter 5. Preferential Al oxidation and Al2O3 surface layer formation on single crystal aluminide thin films of the more electronegative transition metals is well studied, e.g. [190-192] the surface oxidation of NiAl, which yields highly crystalline layers of γ-Al2O3@NiAl

(see Chapter 6.5, p. 140). Consequently, it is quite likely that especially the Cu1-xZnx-PPO particles (x ≥ 0.17) are quickly passivated by formation of a full ZnO shell around a core of the type (ZnO)δ@Cu1-xZnx-δ. In comparison, claaical metallurgical studies have shown that bulk α-Cu1-xZnx alloys, which are used as corrosion resistant materials (see Chapter 3.3.1.) develop a passivating ZnO layer down to 7 at.% Zn.[243] The colloidal particles require a higher Zn content, which is most presumably a consequence of the larger surface area, and the increased number of surface atoms, respectively.

4.6.1.2. UV-Vis spectroscopy of α-Cu1-xZnx/PPO colloids

The UV-Vis spectra of the samples Cu1-xZnx-PPO with 0.17 ≤ x ≤ 0.50 do not change when the colloids are exposed to air (Figure 4.24). The UV-Vis spectrum of the Cu0.67Zn0.33 (x = 0.33) colloid shows a very broad absorption in the range of 500 nm (Figure 4.24a), which does not change upon air oxidation. For x = 0.17, the UV-Vis absorption becomes similar to that of pure Cu colloids; it gradually red-shifts and the SPR is clearly visible. After exposure to air, the SPR does not shift even after 2 days, in contrast to pure Cu colloids (Figure 4.24b). A further decrease of the Zn content (x = 0.09) shows a rapid oxidation of Cu (Figure 4.24c), indicating that Cu is not protected by ZnO any longer, which confirms the results of the XRD measurements of the oxidised α-Cu1-xZnx samples. Regarding this general trend, the broad absorption of Cu0.67Zn0.33 at 500 nm (Figure 4.24a) appears to be unusual in the series of Cu1- xZnx colloids, since with rising Cu content, a red-shift to wavelengths longer than 535 nm

58 4. Synthesis of intermetallic α‐ and β‐CuZn phases

observed for the β-Cu0.50Zn0.50/PPO colloids would be expected at a first glance. Therefore,

Mie´s theory was used to calculate the UV-Vis spectrum of colloidal Cu0.67Zn0.33 from the [239] known dielectric function of bulk Cu0.65Zn0.35, which was experimentally determined by

Sasovskaya and Korabel and shown in Figure 4.25 and the structural data of the Cu0.67Zn0.33- PPO particles obtained from TEM. The optical absorption spectra of metallic nanoparticles can be described according Mie´s theory,[238] which gives the total extinction coefficient κ according to Equation 4.1 (also see Chapter 9.3.1.5., p. 173):

18πNVε 3/ 2 ε κ = m 2 2 2 Equation 4.1 λ [ε1 + 2ε m ] + ε 2

Indeed, a value of 500 nm was obtained for the absorption maximum (Figure 4.24a, red line). The observed broadening of the absorption is probably due to the rather inhomogeneous particle size distribution. The remaining UV-Vis spectra of the α-brass colloids with 17 at.% and 9 at.% Zn were not modeled because of lacking the data of the dielectric functions of the respective bulk phases.

Figure 4.24. Time‐resolved air oxidation of colloidal a) Cu0.67Zn0.33‐, b) Cu0.83Zn0.17‐ and c) Cu0.91Zn0.09/PPO nanoparticles upon exposure to air, monitored by UV‐Vis spectroscopy.

59 4. Synthesis of intermetallic α‐ and β‐CuZn phases

The blue-shift of the SPR of the β-Cu0.50Zn0.50/PPO colloids was not observed in the previous work on CuZn/HDA colloids prepared from the copper(II)aminoalkoxide precursor and diethylzinc in neat HDA without additional hydrogen.[146] In fact, the Cu/HDA showed absorption at 558 nm and that value expectedly remained constant upon alloying with small amounts of Zn (5-10 %) but surprisingly was only slightly red shifted to somewhat longer wavelengths of about 564 nm rather than significantly blue shifted to 520 nm upon alloying Cu with Zn in a 1:1 ratio. Thus, it may be questionable, if this previous work correctly reported and discussed the data. Below 15 at.% Zn, the developing ZnO shell cannot cover the whole particle surface rapidly enough, so that Cu atoms are also oxidised. This model would explain the independence of the blue shifted SPR of 535 nm observed for the β-CuZn/PPO material upon exposure to the air in contrast to Cu-PPO. Having now this study in hand and the UV-Vis reference data reported by Ito et al. on β-CuZn/OLEA[148] for comparison, it is doubtful that the sample β-CuZn/HDA[146] of the previous work was correctly assigned. In a recent study, it was shown that the previously synthesised CuZn/HDA particles, which exhibited a UV-Vis absorption of 564 nm, are in fact not alloyed, but rather consist of a core of small Cu clusters, which are surface decorated with ZnO islands, according to XAS measurements.[244]

2 2 Figure 4.25. The real part ε1 (upper curves) and imaginary part ε2/(ε1 +ε2 ) (lower curves) of the dielectric function for bulk (1) Cu0.50Zn0.50, (2) Cu0.53Zn0.47, (3) Cu0.61Zn0.39 and (4) β‐Cu0.65Zn0.35 brass materials (Graph taken from ref.[239]).

60 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Table 4.7. Calculation of the UV‐Vis absorption spectrum of a colloidal solution of β‐Cu0.65Zn0.35 nanoparticles [a] with Mie’s equation from the dielectric function of the bulk intermetallic Cu0.65Zn0.35 phase. ε [b] 2 [b] [b] [c] 2 [d] ε1 2 2 hω [eV] λ [nm] (x axis) κ [nm /mol] (y axis) ()ε 1 + ε 2 ‐23.793 0.0046 1.544 952.67 2.584 ‐20.172 0.0038 1.667 911.09 2.693 ‐16.897 0.0045 1.772 883.38 2.736 ‐13.814 0.0076 1.912 848.69 2.837 ‐12.288 0.0103 2.000 817.49 2.879 ‐10.932 0.0132 2.070 789.85 2.968 ‐9.224 0.0184 2.179 786.29 2.982 ‐7.845 0.0298 2.268 758.06 3.093 ‐5.948 0.0417 2.357 722.85 3.243 ‐4.224 0.0524 2.474 698.16 3.371 ‐2.931 0.0999 2.614 680.45 3.488 ‐2.586 0.1689 2.754 662.88 3.570 ‐2.500 0.1648 2.825 641.75 3.693 ‐2.586 0.1504 2,912 624.18 3.824 ‐2.759 0.1465 3.000 609.96 3.943 ‐2.845 0.1406 3.089 592.38 4.065 ‐3.017 0.1361 3.161 575.07 4.218 ‐3.190 0.1359 3.250 557.74 4.365 ‐3.276 0.1345 3.357 543.89 4.460 ‐3.190 0.1348 3.446 516.16 4.630 ‐3.362 0.1377 3.536 488.46 4.596 ‐3.276 0.1436 3.661 465.44 4.455 ‐3.276 0.1465 3.839 447.82 4.231 ‐3.276 0.1470 3.982 430.19 4.074 ‐3.276 0.1536 4.000 408.75 3.871 ‐3.190 0.1578 4.140 394.92 3.606 ‐2.759 0.1800 4.298 377.61 3.345 ‐2.586 0.1936 4.474 349.90 2.720 ‐2.441 0.2162 4.614 329.17 1.837 ‐2.155 0.2304 4.825 304.86 1.077 [a] Premise: spherical particles with a diameter of 20 nm (≈ 457,000 atoms, 65% Cu, 35% Zn). [b] Data extracted from Figure 4.25. [c] Calculated from ħω (column 3). [d] Calculated from the given wavelengths (column 4) with Mie’s equation.

61 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.7. α-/β-CuZn colloids - Potential model systems for the Cu/ZnO methanol catalyst

4.7.1. State of the art Methanol is one of the most important organic raw products with a production scale of 40 million tons in 2006.[245] It has mainly been used in fuel industry for the production of tert- butylmethyl ether, which still is an important fuel additive (antiknock agent). Besides, methanol is used for the large-scale synthesis of other organic compounds, such as formaldehyde, acetic acid, or methylmethacrylate, which are also raw materials for further large-scale syntheses of various organic products, synthetic materials and plastics. It is well known, that binary Cu/ZnO and ternary Cu/ZnO/Al2O3 powder materials represent commercial, well-established heterogeneous catalysts of the large-scale industrial production [13] of methanol from syngas (CO, CO2 and H2). The conversion of syngas to methanol is described by the following equilibrium reactions (Scheme 4.7), which describe both the methanol formation (Eq. 1 and 2), and the reverse water gas shift reaction (Eq. 3).

-1 1) CO2 + 3 H2 CH3OH + H2O -∆H298 K, 50 bar = 40.9 kJ mol

-1 2) CO + 2 H2 CH3OH -∆H298 K, 50 bar = 90.7 kJ mol

-1 3) CO2 + H2 CO + H2O -∆H298 K, 50 bar = -49.8 kJ mol

Scheme 4.7. Equilibrium schemes of the reactions of CO and CO2 with H2 to methanol.

The Faculty of Chemistry of the Ruhr-University Bochum hosts the Research Centre (Sonderforschungsbereich) 558, which is financially supported by the German Research Society (DFG), in order to investigate the mechanism of methanol formation on the catalyst surface and the interaction of Cu and its oxide supports. From a fundamental point of view, the Cu/ZnO system represents a prototype of the so-called strong metal support interaction [13] (SMSI). The industrial Cu/ZnO/Al2O3 catalysts are prepared from cheap metal nitrate salts by sophisticated stepwise co-precipitation-annealing-reduction procedures.[13] It was shown that each of these steps, e.g. the pH value of the stock solution, the temperature, and the ageing period are crucial for the efficiency of the prepared catalyst. Several groups reported that CO and H2 adsorb on the Cu surface and that the intimate interfacial contact between nanoparticles of copper and zinc oxide influences the surface, as well as the bulk structure

62 4. Synthesis of intermetallic α‐ and β‐CuZn phases

(strain) of the Cu particle, which affects the methanol formation at these sites (Figure 4.26).[246-248]

Figure 4.26. HRTEM images of catalytically active nanocrystalline Cu‐ZnO composites by Topsøe et al.[247] (left) and Ressler et al.[248] (right), showing an intimate Cu‐ZnO contact.

Alumina was found to be an important additive, acting as a structural promoter and thus preventing Cu particles from sintering during the reaction process.[249] Hence, the

Cu/ZnO/Al2O3 exhibits to a significantly higher turn-over number in the catalytic process than alumina-free Cu/ZnO.[250] The absence of ZnO, would however lead to a dramatic loss of catalytic activity. In particular, the presence of ZnO and/or CuZnOx species at the surface of Cu particles supported by ZnO are discussed to play a crucial role in the active state of the catalyst.[251] Thus, numerous studies concentrated on the elucidation of the mechanism of methanol formation from CO and H2 at the Cu/ZnO surface, and the structural nature of the catalytically active sites. Under extreme reducing conditions, the formation of Cu/Zn surface alloying was also observed.[252]

Motivated by the still not solved questions around the SMSI effect of the Cu/ZnO/Al2O3 system, the synthesis of nanophase model catalysts was investigated by several groups. The impregnation of the channels of periodic mesoporous silica networks (PMS, e.g. MCM-41 and -48, SBA-15) and -zeolithes with Cu- and Zn-salts was reported.[253] However, in most cases, the embedded Cu2+ ions were strongly bound to the Si-OH walls of the channels, so that the reduction required extreme temperature conditions. Consequently, none of these Cu/ZnO@PMS materials exhibited a significant catalytic activity in the methanol synthesis from syngas. Alternatively, Cu/ZnO@PMS materials were obtained by embedding the organometallic compounds [Cu{OC(Me)CH2CH2NMe2}2] and [ZnEt2] into the pores of MCM-41 and subsequent heat treatment (~350°C).[254] Yet, the obtained Cu/ZnO

63 4. Synthesis of intermetallic α‐ and β‐CuZn phases nanoparticles exhibited a poor catalytic efficiency of 10 %, compared to the state-of-the art industrial catalyst. Recently, the organometallic heterocubane complex [(MeZn-

OCH2CH2OMe)4] was embedded inside the pores of MCM-48 and decomposed via thermolysis to ZnO. The subsequent impregnation with Cu (classic aqueous deposition starting from Cu(II)nitrate) gave a Cu/ZnO nanocomposite, which was found to be a highly active catalyst at the level of standard industrial methanol catalysts.[255] Recent investigations, which focused on the nanochemistry of the Cu/ZnO system, were largely motivated by the goal to achieve a quasi homogeneous, non-aqueous colloidal model of the Cu/ZnO heterogeneous solid state catalyst for the methanol synthesis. As connecting link between this research on Cu/ZnO heterogeneous catalysts and the metal colloid chemistry appears the recent publication by Schüth et al. on the observation of catalytically highly active copper colloids (derived by the Bönnemann method) for methanol synthesis from synthesis gas at temperatures as low as 170 °C in tetrahydrofurane (THF) at 260 bar.[256] Productivities of about 25 mol methanol per mol of colloidal copper and hour were measured in a batch reactor set-up, which is about 25 % of the continuous productivity of the industrial catalyst at 220-250 °C and 20-50 bar. In a parallel work, a stable colloidal solution of ZnO decorated Cu particles in squalane, denoted as ZnO@Cu, of a size between 1-3 nm were synthesised, which also proved quite active in methanol synthesis under continuous conditions and reached a level of above 300 % of the activity of a slurry of a powdered Cu/ZnO/Al2O3 industrial reference catalyst.[257] It was also shown that the catalytic activity of colloidal ZnO@Cu nanoparticles decreases if the ZnO content is lower the 50 at.% in the active catalyst.

4.7.2. Air-oxidised (ZnO)δ@Cu1-xZnx-δ/PPO colloids According to reports of Nakamura et al., alloying of Cu and Zn could be observed at the Cu-ZnO interface under reducing conditions (e.g. syngas treatment at elevated temperatures), which causes O-deficient ZnO1-x sites on the ZnO surface, which migrate to the Cu surface.[252] These sites are supposed to be the actual catalytically active centres. In this chapter, it was shown that colloidal β-CuZn/PPO nanoparticles form a stable ZnO shell around the Cu-rich core, giving (ZnO)δ@Cu1-xZnx-δ/PPO composite particles. This can be regarded as a model of Nakamura’s above cited reduced Cu/ZnO catalysts. Therefore, colloidal solutions of Cu1-xZnx/PPO nanoparticles (x = 0.50 and 0.10) in mesitylene were synthesised according to the procedures described in Chapter 4.5. The obtained solutions were exposed to air for 24 h, and afterwards precipitated by addition of n-pentane. The powders

64 4. Synthesis of intermetallic α‐ and β‐CuZn phases were thoroughly washed. For catalytic test runs, the particles were suspended in 160 mL squalane (2,6,10,15,19,23-hexamethyltetracosane), which is known to exhibit a high solubility of gases. The suspension (c = 0.5 mol·L-1) was transferred into a continuously operating steel autoclave, pressurised with 26 bar of a mixture of 72 % H2, 10 % CO, 4 %

CO2 and 14 % N2 and heated to 220 °C. The methanol production was detected by periodic GC-measurements (each 20 min) over 1 day. However, for both x = 0.50 and x = 0.10, no catalytic activity could be observed at all. The reasons for this are still unclear. A possible explanation for the inactivity could be the dense ZnO shell, and an insufficient Cu-ZnO contact area, respectively. By a full segregation of Cu and Zn, the contact area could be increased, e.g. by full oxidation to Cu(I)-oxide and ZnO particles, and subsequent reduction of Cu2O to Cu(0).

4.7.3. Synthesis of a soluble Cu-ZnO/PPO nanocomposite from β-Cu0.50Zn0.50/PPO colloids

By treating a deep violet solution of β-Cu0.50Zn0.50/PPO nanoparticles with 3 bar O2 at 150 °C, the solution colour changed to pale yellow within 1 h. After removal of residual oxygen in vacuo by several freeze-pump-thaw cycles, the solution was pressurised with 3 bar

H2 and stirred at 150 °C for 7 h, whereupon the colour changed to wine red (Scheme 4.8), indicating the formation of copper colloids. The drastic oxidation of the brass colloids led to a phase segregation and to a full oxidation of both alloy components. The reduction of copper oxide with H2 or CO gave a mixture of soluble Cu and ZnO nanoparticles.

3 bar H2, 150 °C 3 bar O2, 150 °C Cu + Zn β-Cu0.50Zn0.50/PPO Cu2O/ZnO/PPO Mesitylene, PPO 1 h PMe3 16 h 3 bar H2, 7 h 150 °C

Cu/ZnO/PPO

Scheme 4.8. Synthesis of a PPO stabilised solution of a Cu/ZnO nanocomposite by oxidation of β‐

Cu0.50Zn0.50/PPO particles and subsequent re‐reduction of Cu(I) to Cu(0).

4.7.3.1. X-ray diffraction studies

After oxidation with oxygen pressure, the Cu2O/ZnO/PPO nanoparticles were precipitated by addition of n-pentane. The XRD diagram of the brownish powder, shown in

65 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.27b, only exhibited two very broad reflections at 2θ = 36.5° and around 43°, which could be attributed to the (111) reflections of Cu2O and Cu, respectively. Obviously, the oxidation led to full segregation of Cu and Zn, giving ZnO, which could not be detected by

XRD (even upon annealing at 300 °C), and Cu2O@Cu particles core-shell particles. The powder could be redispersed in mesitylene and treated with H2 pressure to give a wine red solution. The Cu/ZnO/PPO particles were precipitated as described above. The XRD pattern of the dark red powder (Figure 4.27c) perfectly matched to fcc Cu reflections, which suggests that no Zn atoms are incorporated in the lattice any longer, s it was the case in β-

Cu0.50Zn0.50/PPO (Figure 4.27b), or Cu-rich α-Cu1-xZnx/PPO particles.

Figure 4.27. XRD diagrams of precipitated a) β‐Cu0.50Zn0.50/PPO, b) Cu2O/ZnO/PPO and c) Cu/ZnO/PPO particles.

4.7.3.2. UV-Vis spectroscopy The UV-Vis spectrum in Figure 4.28 (left) of a diluted aliquot of the yellow

66 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Cu2O/ZnO/PPO colloid solution did not exhibit any absorption in the visible range. In contrast, the UV-Vis spectrum of the wine red solution of re-reduced Cu/ZnO/PPO colloids (Figure 4.28, right) revealed the typical Cu SPR at 573 nm, which was also observed for other Cu/PPO colloids synthesised in this work (see Chapter 4.1.2.4., p. 38, and Chapter 5.3.2.3., p.

84). In sharp contrast to the initial β-Cu0.50Zn0.50/PPO colloids, the Cu/ZnO/PPO colloids rapidly oxidise on air, which is visible on the SPR shift to 588 nm. This clearly indicates a free Cu surface, which is being oxidised.

Figure 4.28. UV‐Vis spectra of colloidal solutions of Cu2O/ZnO/PPO (left) and re‐reduced Cu/ZnO‐PPO (right), under argon and after air oxidation.

4.7.3.3. Transmission electron microscopy The TEM image of a sample of Cu/ZnO/PPO colloids diluted in toluene, exhibited polydisperse particles in the broad size regime of 10-30 nm. Although it could not be shown that the ZnO particles are in immediate contact with the Cu particles, EDX analyses of selected particles showed a Cu:Zn ratio of nearly 1:1.

67 4. Synthesis of intermetallic α‐ and β‐CuZn phases

Figure 4.29. TEM image of Cu/ZnO/PPO colloids.

4.7.3.4. Catalytic activity of Cu/ZnO/PPO particles

In analogy to the above described catalytic tests of (ZnO)δ@Cu1-xZnx-δ/PPO particles, the synthesised Cu/ZnO/PPO nanoparticles were tested for catalytic activity of the conversion of syngas to methanol. However, these particles did also not show any catalytic activity. The reason for this is unknown. It can be speculated, that the particles are too large, or that the Cu and ZnO components are separated. Pure Cu colloids do catalyse the methanol formation from syngas. Though, catalytic test runs of any Cu/ZnO/PPO nanoparticles, prepared by the methods presented in this chapter, also suffer from several drawbacks. The major problem is the polymeric surfactant, which is a dense matrix, where the metal particles are embedded in.

Although it was shown that gases (e.g. O2) can penetrate trough and reach the particle surface, it is unclear, which effect large amounts of the polymer could have on the adsorption of CO and H2 onto the particle surface and the catalytic conversion to MeOH. PPO might inhibit gas adsorption on the surface of precipitated particles. Secondly, PPO is insoluble in squalane. Also, the presence of residual traces of the phosphine ligand, which is well known to poison the particle’s surfaces, cannot be completely excluded.

68 4. Synthesis of intermetallic α‐ and β‐CuZn phases

4.8. Conclusion

In summary, a novel, very clean and reliable non-aqueous organometallic preparation of colloidal copper/zinc (brass) nanoparticles is presented, which is based on the co- hydrogenolysis of [CpCu(PMe3)] together with [ZnCp*2] in mesitylene solution at 150 °C and

3 bar H2. A series of colloidal CuZn samples with free variation of the Cu:Zn molar ratio from the copper rich α-CuZn to the stoichiometric 1:1 β-CuZn phase were obtained by this soft- metallurgical preparation concept. Deep red colloids of nano-Cu as well as red to violet colloids of α- and β-CuZn, revealing free-standing nanoparticles dispersed in mesitylene were obtained in the presence of PPO as surfactant during hydrogenolysis. However, this surfactant obviously has a weak interaction with the particle surface and thus, the particle assembly, as well as the shape was rather poor, and the size of the particles is quite large. It was not possible to exchange PPO by addition of another surfactant to the colloidal solution. Neither the stability of the particles, nor the size and shape could be improved. Evidence for alloyed α/β-CuZn particles was given by XRD and XAS studies. Quite interestingly, preferential oxidation of the Zn-component was observed down to threshold of about 15 at.% of Zn. Below that level, the Cu-component was significantly oxidised, too. Based on XRD, XAS and

UV-Vis, a core-shell type particle structure (ZnO)δ@Cu1-xZn1-δ was suggested to explain the oxidation behaviour of the colloids. With respect to the motivation of this study outlined in the introduction, a link between chemical nanometallurgy in organic solvents and quasi- homogeneous catalysis with nanoparticles [244,256,257] was established. Preliminary data suggest the oxidation of CuZn alloy colloids to Cu/CuO/ZnO nano-composites, which can be reduced back by H2/CO to Cu/ZnO particles, now having a free Cu surface. The properties of those particles as quasi-homogeneous methanol catalysts are to be investigated in future.

69 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.1. Synthesis of Al nanoparticles by hydrogenolysis of [(AlCp*)4]

-1 Treatment of 100 mg [(AlCp*)4] dissolved in mesitylene (c=0.15 mol·L ) with 3 bar H2 at 150 °C yields 160 mg of a pyrophoric black precipitate already after 15 minutes, which was identified as analytically pure aluminium (elemental analysis: 99.8 wt.% Al). The powder was isolated by filtration, and washed with toluene and n-pentane. The reaction is clean and quantitative according to 1H-NMR and GC-MS studies of the supernatant (Scheme 5.1). Since the tetrameric cluster [(AlCp*)4] is nearly insoluble in any organic solvent at room temperature, the hydrogenation has to be performed at a relatively high temperature of 150 °C, which is required to dissolve the Al-precursor and thus to give the highly reactive monomeric species [:AlCp*].[258] It is noteworthy that it was not possible to stabilise Al nanoparticles as colloids, by use of various surfactants. The decomposition of [(AlCp*)4] in presence of 4 equiv. HDA, which is one of the most frequently used stabilising capping ligands, resulted in the formation of a solution of a Al(III)-species, according to 27Al-NMR measurements. The use of PVP as colloid matrix resulted in the formation of Al2O3, presumably due to the reaction of AlCp* with the ring carbonyl group. Understoichiometric amounts of bulky carboxylic acids (OLEA, ACA) did not prevent the Al particles from precipitation, nor did long-chained phosphines and phosphine oxides.

CH3 H3C 3 bar H2, 150 °C CH3 1/4 Al(s) + Al Mesitylene, 30 min H 4 H3C CH3

Scheme 5.1. Hydrogenolysis of [(AlCp*)4] to Al nanoparticles and Cp*H.

70 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.1.1. Structural characterisation of Al

5.1.1.1. X-ray powder diffraction

The XRD pattern of the obtained black powder revealed reflections at 2θ = 38.56°, 44.78°, 65.23°, 78.39° and 82.60°, which could be assigned to the (111), (200), (220), (311) and (222) lattice planes of fcc aluminium (cubic Fm3m, Figure 5.1). The average particle size of 22 ± 1 nm was estimated from the FWHM (Table 5.1) of the observed reflections using the Scherrer equation.

Figure 5.1. X‐ray diffraction pattern of the nano‐Al powder obtained according to Scheme 5.1. (lines: JCPDS reference data No. 4‐0787).

Table 5.1. Calculation of the average size of Al nanoparticles from XRD data. h k l 2θ [°] cos(θ) FWHM [°] FWHM [rad] Particle size [nm]

1 1 1 38.56 0.944 0.380 6.63·10‐3 22 2 0 0 44.78 0.924 0.457 7.98·10‐3 21 2 2 0 65.23 0.842 0.464 8.10·10‐3 23 3 1 1 78.39 0.774 0.521 9.09·10‐3 22 2 2 2 82.60 0.751 0.520 9.07·10‐3 23

71 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.1.1.2. Transmission electron microscopy

The TEM images of the Al-powder obtained by hydrogenolysis of [(AlCp*)4] show surprisingly uniform and discrete, in (111)-direction elongated particles with a diameter of 23 ± 2 nm, matching the estimation from X-ray powder diffraction reflection broadening based on the Scherrer equation (Figure 5.2). A close-up of a selected particle points to a 3 nm thick amorphous coating around the nanocrystalline Al-core. The shell, which possibly consists of

Al2O3, was most presumably formed upon exposure of the TEM grid to air, during the specimen transfer to the TEM chamber.

Figure 5.2. TEM images of the nano‐Al powder obtained according to Scheme 5.1. Bottom left: particle size distribution. Right: close‐up of a single particle (top) and HRTEM image, showing the Al2O3 shell covering the particle (bottom).

5.1.2. Spectroscopic characterisation

5.1.2.1. 1H-NMR and GC-MS of the filtrate

1 The H-NMR spectrum of the filtrate of the reaction mixture (recorded in C6D6) exhibits the expected three signals of the methyl protons of Cp*H, namely two singlets at 1.81 and 1.74 ppm, as well as a doublet at 0.99 ppm (J = 7.64 Hz) in a ratio of 6:6:3 (Figure 5.3). The signal of the proton at the aliphatic ring carbon was not detected due to the low concentration

72 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles of the sample in the original filtrate. Additionally, signals of residual toluene from the work up (between 7.14 and 6.97 and at 2.15 ppm) were visible. Correspondingly, the 27Al-NMR spectrum of the filtrate did not show any signals. Hence, a formation of any Al(III) species during the hydrogenolysis of [(AlCp*)4] can be excluded, as well as the presence of 27 [258] unchanged [(AlCp*)4] ( Al-NMR shift: -80 ppm ) or other Al(I)-species.

1 Figure 5.3. H‐NMR spectrum of the supernatant of the decomposition of [(AlCp*)4] under H2 pressure. The asterisks mark the ring (left) and methyl protons (right) of mesitylene.

5.1.2.2. 27Al-MAS-NMR of the Al powder Bare nano-Al is, of course, a getter for oxygen. The 27Al-MAS-NMR spectrum (Figure

5.4) of a freshly synthesised sample (glovebox, argon atmosphere, solvents with O2, H2O < 1 ppm), diluted with SiO2 and immediately transferred into a ZrO2 rotor (4 mm) and sealed, [259] exhibited only traces of the Al2O3 signals at 1.0 ppm (AlO6) and 57.0 ppm (AlO4) besides the easily observed Knight shift resonance at 1639 ppm, which perfectly matches to Al metal shifts reported in the literature as well as to own reference measurements.[260] Interestingly, the two alumina signals are quite distinguishable, pointing to quite ordered Al2O3. No other 27Al signals were detected and similarly, hydrocarbon impurities were absent according to 1H- MAS-NMR and IR data (KBr pellet).

73 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.4. 27Al‐MAS‐NMR spectrum of Al nanoparticles.

5.1.2.3. XPS measurements of the Al-powder X-ray photoelectron spectroscopic measurements, shown in Figure 5.5, unambiguously proved the presence of Al(0) (2p3/2 peak at 71.4 eV) and Al(III) (2p1/2 peak at 74.4 eV) pointing to Al2O3.

Figure 5.5. XPS survey spectrum (left) and spectrum of the Al‐region (right) of Al nanoparticles.

Supporting the observation of the particle morphology by TEM, this indicates a very thin

Al2O3 shell (~ 2-3 nm) on a core of Al(0). A presence of ‘naked’ Al(0) without a surface

74 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles oxide layer is very unlikely, even if any traces of oxygen are strictly avoided.

5.1.3. Mechanistic insight into the decomposition pathway Matrix isolation FT-IR studies and quantum chemical calculations reported by Himmel et al. revealed that the photo induced reaction of monomeric [(AlCp*)4] with H2 in an argon [261] matrix at 12 K yields the dimer [Cp*Al(µ-H)]2. That species may also play a role in the hydrogenolysis described above, since the tetrahedral cluster of [(AlCp*)4] undergoes a dissociation/association equilibrium with monomeric AlCp* at elevated temperatures in solution.[258] Monomeric AlCp* can - as a strong Lewis base - undergo an oxidative addition reaction with dihydrogen to the dimeric [Cp*AlH]2 complex, which is then supposed to rapidly release Al(0) and Cp*H (Scheme 5.2).

H Al Al Al Al 2 Al(0) + HH H HH CH3 H C 3 CH3 Al 2 H

H3C CH3

Scheme 5.2. Proposed mechanism for the decomposition of [(AlCp*)4] under hydrogen pressure.

In order to shine some light into the hydrogenolysis mechanism of [(AlCp*)4] and because of the so far synthetic inaccessibility of [Cp*AlH2] as possible intermediate, [(AlCp*)4] was i treated with Bu2AlH at 150 °C in mesitylene. Immediate precipitation of aluminium occurred i and the observed byproducts [Cp*Al Bu2], Cp*H and isobutane are likely to stem from the i III I decomposition of the intermediate adduct [ Bu2(H)Al -Al Cp*] (Scheme 5.3). The yield of Al(0) of 66 % corresponds to the stoichiometric reaction (Scheme 5.3). For a quantitative i detection of the organic species, [(AlCp*)4] and 4 equivalents of Bu2AlH were suspended in d8-toluene and heated in a pressure stable NMR tube at 120 °C for 2 h. After few minutes, Al(0) precipitated. In the recorded 1H-NMR spectrum (Figure 5.6, left), the signals of i [Cp*Al Bu2] were visible at 1.84 ppm (s, 15 H, Cp*), 0.97 ppm (m, 12 H, -CH2-CH-(CH3)2) and -0.16 ppm (m, 4 H, -CH2-CH-(CH3)2). The signals of the rest of Cp*H appeared at 1.78 i ppm (s, 3 H) and 1.71 ppm (s, 3 H) matched to the expected ratio [Cp*Al Bu2]:Cp*H of

75 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

1.0:0.5. The proton signals at the aliphatic carbon of the Cp* ring and the corresponding ipso- i methyl group could not be detected due to overlapping signals of [Cp*Al Bu2] and isobutane, respectively. The methyl proton signal of isobutane appeared at 1.06 ppm (m, 9 H, CH(CH3)3) i and also matched to the expected ratio [Cp*Al Bu2]:isobutane of 1:1. The proton at the tertiary carbon, which should appear at around 1.8 ppm, was not visible due to other signals lying above. The 27Al-NMR spectrum, shown in Figure 5.6 (right), exhibited a signal at -86.9 i ppm, which can be assigned to [Cp*Al Bu2], as the only Al-species in solution. This is in good agreement with 1H-NMR data, i.e. the Cp*Al:isobutyl ratio. Although there are no comparable complexes reported to date, this shift lies in the range of other Cp*Al(III)- [262] [263] compounds, such as [(Cp*AlBr2)2] (-46 ppm), the complex [Cp*3Al5I6] (-83 ppm) or + [264] the ionic complex [AlCp*2] [Cp*AlCl3] (-115.2 ppm).

H Mesitylene, 150 °C 3 AlCp*+ 3 iBu AlH 3 *CpAl AliBu 3 [AliBu] 2 30 min 2 2

i 3 Cp*H + 2 Al Bu3 + 4 Al(0)

i i 2 [Cp*Al Bu2] + 1 Cp*H + 2 BuH

i Scheme 5.3. Reaction of [(AlCp*)4] and Bu2AlH via the proposed mechanism.

1 27 i Figure 5.6. H‐ and Al‐NMR spectra of the reaction between [(AlCp*)4] and Bu2AlH. The large broad signal at around 68 ppm represents the static signal of the probe head of the spectrometer. The signal at ‐86.9 ppm i corresponds to the complex [Cp*Al Bu2].

76 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

According to theoretical studies by G. Frenking et al., the Al-Al bond dissociation energies [265] for R3Al-AlR’ compounds (R = H, Me; R’ = H, Cl) are very low. Thus, it is reasonable to i III I assume, that [ Bu2(H)Al -Al Cp*] is very likely to be rather instable at 150 °C and is supposed to rapidly decompose according to Scheme 5.3. At least, the stoichiometry of the hydrogenolysis of [(AlCp*)4] (Scheme 5.1) being based on a mass-balance and identification (NMR, GC-MS) of all products substantiates this suggestion.

5.2. Synthesis of Al powder from [(Me3N)AlH3]

The compound [(Me3N)AlH3] quantitatively decomposes to elemental aluminium in mesitylene solution under 3 bar H2 at 150 °C within 1 h, which is slightly longer than in the case of [(AlCp*)4], which decomposes after 15 min. It should be stated here, that it is unclear, whether the compound has decomposed thermally, according to Buhro’s report,[178] or the decomposition has been invoked by hydrogenolysis. It is well known that above 100 °C,

[(Me3N)AlH3] decomposes to NMe3 and polymeric [AlH3]x, which further decomposes to [266] Al(0) and H2, so that it can be assumed that under the conditions described, the complex decomposed by thermolysis. The supernatant was decanted, and the obtained, highly pyrophoric grey powder washed with n-pentane and dried. The yield on the gaseous byproducts NMe3 and H2 could however not be quantified, thus, the reaction was repeated in a pressure stable NMR-tube.

5.2.1. NMR spectroscopic analysis of the decomposition reaction

5.2.1.1. Decomposition of [(Me3N)AlH3] in a NMR tube In order to shine some light into the fate of the ligands, the reaction was followed by NMR spectroscopy (decomposition in a NMR tube in d12-mesitylene). In the 1H-NMR spectrum of [(Me3N)AlH3] (Figure 5.7, top left), a broad signal at 3.81 ppm, assigned to the

Al-H atoms, and a sharp singlet at 1.93 ppm of the protons of coordinated NMe3 were observed. The corresponding 27Al-NMR spectrum (Figure 5.7, top right) exhibited a broad resonance shift of Al at 136 ppm, which corresponds to the signal of the starting material. 27Al-NMR spectroscopy is a very sensitive method to determine the coordination number of

77 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Al centres. Whereas octahedral complexes typically appear in the range of 0 ppm, the low- field shifting of the Al-signal correlates with a decreasing coordination number. For tetrahedral complexes the shift usually appears between 125 and 180 ppm.[267] In the 1H-NMR spectrum recorded after treatment with 3 bar H2 at 150 °C for 24 h (Figure 5.7, bottom left), the signal of the hydrides disappeared, and the NMe3 signal shifted from 1.93 ppm

(coordination at Al) to 2.10 ppm (free amine). Further, a weak signal of H2 at 4.61 ppm was observed, however, it is not clear, whether this signal stemmed from the H2 as a side product 27 of the decomposition, or from H2 which was added. Accordingly, in the Al-NMR spectrum (Figure 5.7, bottom right), the signal at 136 ppm vanished, and no other signals apart from the static signal of the probe head at 68 ppm could be observed. This speaks in favour of the absence of any Al-species in solution, i.e. the entire amount of Al was in the precipitate, respectively.

1 27 Figure 5.7. H‐ and Al‐NMR spectra of [(Me3N)AlH3] in d12‐mesitylene before decomposition (upper spectra) and after decomposition in a pressure stable NMR tube with 3 bar H2 at 150 °C (lower spectra). The large broad signal at around 68 ppm represents the static signal of the probe head of the spectrometer. The asterisks mark the residual proton signals of the methyl groups of d12‐mesitylene.

78 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.2.1.2. 27Al-MAS-NMR of the Al powder Solid state 27Al-MAS-NMR measurements of the grey precipitate (Figure 5.8), confirmed the zerovalent state of aluminium, exhibiting a Knight shift of 1640 ppm, which is matching to literature data,[260] as well as our previous reference measurements, and the

Knight shift of Al-powder, obtained from hydrogenolysis of [(AlCp*)4] (see Chapter 5.1.2.2., p. 73) Besides, two signals of Al2O3 were observed at 56 ppm (AlO4) and 1 ppm (AlO6), [259] according to the nature of coordination geometry of the AlOx species.

27 Figure 5.8. Al‐MAS‐NMR of Al particles obtained by hydrogenolysis of [(Me3N)AlH3], diluted with SiO2.

5.2.2. Structural characterisation of the Al powder

5.2.2.1. X-ray powder diffraction The XRD pattern of the powder, presented in Figure 5.9, confirms the presence of aluminium. The Al reflections are sharper in comparison to Al from [(AlCp*)4]. The average particle size is calculated to be around 50 nm, which is in quite good accordance to Buhro’s results.[178]

79 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.9. XRD diagram of Al nanopowder obtained by hydrogenolysis of [(Me3N)AlH3].

5.3. Synthesis of copper particles from [{Cu(mesityl)}5]

In the previous chapter, it was shown that the compound [CpCu(PMe3)] is a valuable precursor for the synthesis of both Cu nanoparticles, as well as for nanoscaled intermetallic α- and β-CuZn particles. Further, it was investigated, whether an alternative Cu-precursor,

[{Cu(mesityl)}5], would release Cu upon H2 decomposition. Boyle et al. have shown that pyrolysis of this compound in 300 °C hot hexadecylamine (HDA) gives Cu colloids of a very narrow size regime.[156] However, its reactivity and behaviour under hydrogen pressure as well as ability to serve as precursor for Cu-alloys in solution is so far unknown. The versatility and advantage of this precursor compared with [CpCu(PMe3)] is reflected in the supposed conversion of the organic ligand to mesitylene, which would not be distinguished from the solvent mesitylene.

5.3.1. Synthesis

Indeed, the treatment of a mesitylene solution of [{Cu(mesityl)}5] with 3 bar H2 pressure at 150 °C lead to the formation of metallic copper grains of 0.5-1 mm in diameter

80 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

(Scheme 5.4). The decomposition occurred after 15-20 minutes. According to elemental analysis, the Cu content is 99 wt.%, with C and H as the only other elements present. Expectedly, in the 1H NMR and GC-MS of the colourless supernatant, no evidence for byproducts could be found, leading to the conclusion that mesitylene was formed as the only side product. As well, in the IR spectrum of the isolated metallic shiny material, no hydrocarbons were found. This complex is thus a very elegant source for naked Cu, which allows its use for the wet chemical preparation of Cu-alloys.

1 3 bar H2, 150 °C Cu Cu(0) + 5 Mesitylene, 30 min 5

Scheme 5.4. Quantitative decomposition of [{Cu(mesityl)}5] to Cu powder under hydrogen pressure in mesitylene solution.

5.3.1.1. X-ray powder diffraction The metallic shiny copper grains were examined by X-ray powder diffraction. The pattern, shown in Figure 5.10, exhibits sharp reflections at 2θ = 43.30° (111), 50.48° (200), 74.14° (220), 90.06° (311) and 95.21° (222), which perfectly match to the Cu reference reflections (reference JCPDS No. 4-0836). Interestingly, the intensity ratios of the 220 (56 %), 311 (79 %) and 222 (31 %) reflections to the strongest (111) reflection significantly deviate from literature data for Cu bulk (220: 20 %, 311: 17 %, 222: 5 %), pointing to anisotropic particles. However, it cannot be excluded that the Cu grains were moving inside the capillary during the measurement, which may have caused a distorted result. The average primary crystallite size was estimated to 25 ± 5 nm with the Scherrer equation from the FWHM of all reflections.

81 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.10. XRD from Cu powder obtained by decomposition of [{Cu(mesityl)}5] under 3 bar H2 pressure/150 °C in mesitylene.

5.3.2. Synthesis of Cu/PPO colloids from [{Cu(mesityl)}5]

Since it was shown that the compound [{Cu(mesityl)}5] readily decomposes under H2 pressure to Cu powder and mesitylene, it was investigated whether Cu colloids can be prepared by hydrogenolysis of this precursor in presence of a stabilising agent. In analogy to the case of [CpCu(PMe3)], it was not possible to produce stable colloidal solutions of Cu with 1 equiv. of HDA (in contrast to Boyle’s Cu colloids[156] using an excess of HDA), TOP or

TOPO at decomposition conditions of 3 bar H2 at 150 °C. Addition of OLEA to a solution of

[{Cu(mesityl)}5] in C6D6 led to the formation of a colourless solution of Cu(I)-oleate, according to preliminary 1H-NMR studies. The subsequent hydrogenolysis of this complex gave a wine red solution of Cu colloids. However, the particles are prone to precipitate at elevated temperature and upon longer storage under argon. Hence, upon addition of PPO

(Cu:PPO mass ratio of 1:10) to a solution of [{Cu(mesityl)}5] in mesitylene and subsequent hydrogenolysis (3 bar H2/150 °C), a colloidal solution of Cu/PPO nanoparticles was obtained, which was stable for weeks. The particles easily precipitated upon addition of n-pentane and could be redissolved in toluene, or other aromatic solvents.

82 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.3.2.1. X-ray powder diffraction The XRD diagram (Figure 5.11) of precipitated and washed (with 3 x 50 mL n-pentane) Cu/PPO particles reveals the typical Cu reflections at 2θ = 43.33° (111), 50.45° (200), 74.20° (220), 90.01° (311) and 95.23° (222).

Figure 5.11. XRD diagram of precipitated Cu/PPO colloids, obtained from decomposition of [{Cu(mesityl)}5] with PPO as surfactant.

The reflections are significantly broader from those of Cu powder, as a consequence of the particle stabilisation by PPO, which indicates size reduction, i.e. loss of long-range crystallinity in comparison to Cu powder (Figure 5.10). Hence, the average particle size is 12 ± 2 nm, calculated using the Scherrer equation.

5.3.2.2. Transmission electron microscopy In the transmission electron microscopy images of a diluted sample of Cu/PPO colloids deposited onto a carbon coated Au grid, nearly spherical Cu particles are visible (Figure 5.12, left). Similarly to Cu/PPO colloids, obtained from the [CpCu(PMe3)] precursor, the particles exhibit a very broad size regime (18 ± 14 nm, Figure 5.12, right), which is in a slight mismatch to the average size calculated from XRD data (12 ± 2 nm). An agglomeration of small primary particles is thus a likely guess.

83 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.12. TEM image (left) of Cu/PPO colloids from [{Cu(mesityl)}5] and particle size distribution (right).

5.3.2.3. UV-Vis spectroscopy The UV-Vis spectrum of a diluted solution of Cu/PPO colloids, obtained by hydrogenolysis of [{Cu(mesityl)}5] in presence of the polymer PPO, displays a characteristic surface plasmon resonance of small Cu particles at 573 nm (Figure 5.13), which is in congruence to the SPR of Cu/PPO colloids, prepared from [CpCu(PMe3)], and also lies in the range of the SPR shifts of Cu colloids known in literature (Chapter 4.1.2.4. and Table 4.2, p. 39). The sensivity of the plasmon electrons at the particle surface is reflected on the rapid shift of the SPR upon air exposure. Already after few minutes, the colloidal solution changes the wine red colour over blue to green. This is substantiated by the recorded time-resolved UV-Vis spectra. After 1 h, the SPR reach the final wavelength of 617 nm, indicating

Cu2O@Cu core-shell type particles.

Figure 5.13. Time resolved UV‐Vis spectra of Cu/PPO colloids after air oxidation.

84 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.4. Synthesis of intermetallic copper aluminide phases

Aiming at the development of a generally applicable soft chemical synthesis of transition metal aluminide colloids in non aqueous solution, the binary Cu-Al system was selected as the first test case. In the previous chapters, it was shown that the organometallic compounds [CpCu(PMe3)] and [{Cu(mesityl)}5] are suitable deliverers of ‘naked’ Cu atoms under hydrogenolysis. The organic ligands were found to be easily cleaved from the Cu(I) centres, releasing Cu atoms, without undergoing undesired side reactions. Furthermore, they are not supposed to be transferred to the Al side in contrast to acetylacetonate or halide as the typical ligands for example used in the Bönnemann synthesis of copper colloids, which typically uses [M(acac)2] as metal source and AlR3 as both reducing agent and [67,256,268] surfactant. As well, the two Al-compounds [(AlCp*)4] and [(Me3N)AlH3] are both promising candidates for the co-decomposition with a transition metal complex to a metal aluminide alloy. Hence, in the following chapter, the synthesis of different α- and β-copper aluminide alloys from the available Cu- and Al-precursors will be described. Besides, the efficiency of each precursor combination, i.e. the yield and purity of the obtained Cu-Al phases will be discussed. All precursor combinations, and the successful syntheses of selected Cu-Al phases is shown it Table 5.2. Furthermore, it was studied in detail, whether colloidal solutions of Cu-Al alloy nanoparticles can be prepared. It should be noted here that the synthesis of Cu1-xZnx particles using the compound [{Cu(mesityl)}5] was not considered in this work due to a lack on a Zn-hydride complex, which would allow a systematic study and comparison to the related synthesis of this Cu-compound with [(Me3N)AlH3].

+ x [(AlCp*) ] + x [(AlCp*) ] 4 4 4 4 - x Cp*H, - (1-x) CpH, - (1-x) PMe 3 1-x (1-x) Cu Cu Al Cu 1-x x 5

PMe3 5

+ x [(Me3N)AlH3] + x [(Me3N)AlH3] - x NMe3, - (1-x) CpH, - x NMe3, - (1-x) C9H12 - (1-x) PMe3

3 bar H2, 150 °C, mesitylene x = 0.67, 0.50, 0.31

Scheme 5.5. Wet chemical access to the intermetallic Cu1‐xAlx phases by co‐hydrogenolyses of the combinations of the Cu‐ and Al‐precursors [CpCu(PMe3)], [{Cu(mesityl)}5], [(AlCp*)4] and [(Me3N)AlH3]. The dotted arrow indicates that the co‐hydrogenolysis of [{Cu(mesityl)}5] and [(AlCp*)4] does not lead to Cu1‐xAlx phases.

85 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Table 5.2. Illustration of the selective syntheses of the θ‐CuAl2‐, Cu0.50Al0.50‐ and γ‐Cu9Al4 phases, obtained by combinations of the presented Cu‐ and Al‐precursors.

Cu‐Al phase [CpCu(PMe3)] [{Cu(mesityl)}5] Al‐precursor

9 X [(AlCp*)4] θ‐CuAl2 9 9 [(Me3N)AlH3]

X X [(AlCp*)4] Cu0.50Al0.50 X 9 [(Me3N)AlH3]

X X [(AlCp*)4] γ‐Cu9Al4 X 9 [(Me3N)AlH3] 9 = Successful phase‐pure synthesis; X = No single phase obtained.

5.4.1. Wet chemical synthesis of the intermetallic θ-CuAl2 phase

θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4]

The co-hydrogenolysis of a mixture of [CpCu(PMe3)] and [(AlCp*)4] with a molar Cu:Al ratio of 1:2 yields a grey, surprisingly air stable, non pyrophoric precipitate of the θ-

CuAl2 phase according to Scheme 5.5, and a clear colourless supernatant, which was filtrated. The grey residue was washed with 3 x 50 mL n-pentane and dried. As a result of the elemental analysis, a sample of the obtained powder contained 42.7 wt.% Cu and 38.6 % Al, which corresponds to a Cu:Al molar ratio of 1.00:2.13. The residual 18.7 wt.% presumably stemmed from a byproduct of the reaction. This assumption was supported by an XRD measurement of the grey θ-CuAl2 product (Figure 5.17a, see below), which revealed the presence of another crystalline compound. However, the IR-spectrum of the θ-CuAl2 powder only showed weak absorption bands in the range of aliphatic C-H bonds and aromatic C-C bonds. Thus, the powder was washed with 3 x 50 mL 1,4-dioxane, in order to remove eventual phosphine- and phosphine oxide byproducts. Thereafter, it was washed with n- pentane and dried. The subsequently performed elemental analysis revealed a Cu and Al content of 45.1 and 41.7 wt.%, respectively, roughly confirming the Cu:Al ratio of 2:1. The increased Cu and Al contents give indication to a mass loss, presumably caused by extraction of the byproduct from the θ-CuAl2 powder. Yet, the remaining 13.2 wt.% can either be attributed to an incomplete removal of the impurity or hydrocarbon residues. The organic byproducts (Scheme 5.5) were found by 1H-NMR und 31P-NMR. In the 1H-NMR spectrum of the filtrate of the reaction mixture, the three typical resonances of Cp*H methyl protons are observed again, similar to the pure Al case above (1.81 (s), 1.74 (s) and 0.99 ppm (d) in a

86 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles ratio of 6:6:3). The olefinic protons of cyclopentadiene (CpH) were however not observed.

Besides, the resonances at 0.81 ppm (d, JP-H (Hz): 2.585) are assigned to the free PMe3 ligand and in the 31P-NMR spectrum, the signal of the P atom expectedly appears at -62.4 ppm. In order to identify the byproducts of this reaction more clearly, a quantitative NMR experiment in a sealed tube was performed under similar conditions as described above (vide infra).

θ-CuAl2 from [CpCu(PMe3)] and [(Me3N)AlH3]

Upon addition of a mesitylene solution of [(Me3N)AlH3] (2 equiv.) to a mesitylene solution of freshly sublimed [CpCu(PMe3)] (1 equiv.) in a Fischer-Porter bottle, the solution gradually turns from bright yellow over orange to dark red, soon after (< 30 s), a greyish powder precipitates, accompanied by vigorous gas formation (Scheme 5.5). In order to complete the reaction, the solution was set to 3 bar H2, placed into a 150 °C hot oil bath and stirred for 24 h. The precipitate was allowed to settle, and the solution was decanted by cannulation. The residue was washed with 3 x 50 mL 1,4-dioxane, in order to remove residual phosphine- and phosphine oxide byproducts, and thereafter it was washed with n-pentane and dried. The results of elemental analysis match to the expected molar ratio. In the supernatant, only the free phosphine ligand was found, but not the Cp moiety. The byproducts NMe3 and

H2 of the Al-precursor could not be traced, since both are gaseous at ambient working conditions. In the IR spectrum of the isolated powder, no significant absorptions were observed.

θ-CuAl2 from [{Cu(mesityl)}5] and [(Me3N)AlH3]

The treatment of a mesitylene solution of [{Cu(mesityl)}5] (0.2 equiv.) with a mesitylene solution of [(Me3N)AlH3] (2 equiv.) gave a red-brown solution, which was set to 3 bar H2 and then heated to 150 °C. Already after few minutes, a grey, metallic shiny precipitate formed. The reaction was stirred for 3 days. The colourless solution was decanted via a syringe and the remaining powder was washed with n-pentane and dried. The mass yield of the powder matches quite well to the expected theoretical mass of CuAl2, suggesting a quantitative decomposition of both precursors, according to Scheme 5.5. According to elemental analysis (AAS), a sample of the powder contained 46.9 wt.% Cu and 42.0 wt.% Al (molar ratio Cu:Al of 1.00:2.11), quite matching the expected Cu:Al ratio of 1:2. 1H-NMR and GC-MS examinations of the supernatant did not give any evidence of a special fate of the

87 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles mesityl group, which is a hint on the conversion of Cu(mesityl) to Cu(0) and mesitylene, which cannot be distinguished from the mesitylene used as solvent in the reaction.

θ-CuAl2 from [{Cu(mesityl)}5] and [(AlCp*)4)]

Unlike the reaction of [CpCu(PMe3)] and 0.5 equiv. [(AlCp*)4)] under H2 pressure, which quantitatively gives the desired θ-CuAl2 phase, the analogous co-decomposition of 0.2 equiv. [{Cu(mesityl)}5] with 0.5 equiv. [(AlCp*)4] (corresponds to a molar Cu:Al ratio of 1:2) does not result in the formation of CuAl2. After 1 week of treatment with 3 bar H2 at 150 °C, a dark precipitate formed, which was isolated by removal of the solvent in vacuo. The resulting powder was analysed by means of XRD.

5.4.1.1. NMR spectroscopic studies of the reaction solution

θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4]

1 In the H-NMR spectrum (Figure 5.14, left) of the co-hydrogenolysis of [(AlCp*)4] and 2 [CpCu(PMe3)] in d12-mesitylene, the signal of free PMe3 at 0.82 ppm was visible ( JP-H =

2.634 Hz), as well as those of Cp*H at 1.84 and 1.75 ppm (CH3 moieties at olefinic ring carbons), and at 1.00 ppm (J = 7.646 Hz). The signals of the cyclopentadienyl ligand stemming from the Cu-precursor appeared as multiplets at 6.49 (2 H) and 6.32 ppm (2 H). In 31 the recorded P-NMR spectrum (Figure 5.14, right), besides the signal of free PMe3 at -62 ppm, another resonance at -37 ppm appeared, whose origin is not clear. The presence of 31 Me3P=O ( P-NMR shift: 30 ppm), as a result of unintentional oxidation, can be excluded. The 27Al-NMR did not display any signals, allowing the conclusion that the observed byproduct does not contain Al. In conclusion, the NMR reaction of [(AlCp*)4] and

[CpCu(PMe3)] with H2 yielded Cp*H, CpH and PMe3, according to Scheme 5.5. A full reaction of AlCp* to Al(0) can be expected, since the 27Al-NMR spectrum does not give any 27 hint on soluble Al-species, and the Al-MAS-NMR spectrum of the CuAl2-powder (vide infra) supports this argumentation.

88 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

1 31 Figure 5.14. H‐NMR (left) and P‐NMR (right) spectra of the hydrogenolysis of [CpCu(PMe3)] and 0.5 equiv.

[(AlCp*)4] in a pressure stable NMR tube in d12‐mesitylene.

31 However, PMe3 was not the only observed P-containing species in the P-NMR spectrum, which is a strong indication of a side reaction of PMe3. The fate of the Cp ligand and the origin of the second phosphorus signal in the 31P-NMR are not fully elucidated. On the other hand, looking at the phase purity of the obtained crystallographic data (see XRD, vide infra), there are no indications for a side reaction involving the Cu centre.

θ-CuAl2 from [CpCu(PMe3)] and [(Me3N)AlH3]

In order to shine some light into the nature of the organic products of the decomposition of [CpCu(PMe3)] and [(Me3N)AlH3] under H2 pressure, an reaction in a pressure stable NMR tube was carried out. Thus, the two compounds (Cu:Al molar ratio of 1:2) were suspended in d12-mesitylene, whereupon the colour became darker, and soon after, a grey metallic precipitate formed. In the following, the NMR tube was pressurised with 4 bar H2 and heated 1 to 150 °C for 2 h. In the recorded H-NMR spectrum, the signals of CpH and free PMe3 appeared at 6.49 and 6.32 ppm, and 0.82 ppm, respectively. The signal of free NMe3 was found at 2.11 ppm. In the 31P-NMR spectrum, only the expected resonance of the free phosphine at -62.2 ppm was detected, confirming the byproduct distribution according to Scheme 5.5. The 27Al-NMR spectrum did not exhibit any signals.

89 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

θ-CuAl2 from [{Cu(mesityl)}5] and [(Me3N)AlH3]

An NMR reaction in a pressure stable NMR tube was carried out to investigate the 1 decomposition mechanism. The initial H-NMR spectrum of [{Cu(mesityl)}5] in Figure 5.15a shows two sets of signals from the pentameric cluster [{Cu(mesityl)}5] at 6.64 ppm (ring-H,

3H), 2.96 ppm (ortho-CH3, 6H) and 2.02 ppm (para-CH3, 3H), which is, in solution, in an equilibrium with the dimeric species [{Cu(mesityl)}2] (6.56, 2.90 and 1.93 ppm), according to the literature report.[269]

1 Figure 5.15. Left: H‐NMR spectra (in d12‐mesitylene) of a) [{Cu(mesityl)}5], b) after co‐hydrogenolysis of

[{Cu(mesityl)}5] and [(Me3N)AlH3] for 24 h at 4 bar H2/150 °C. The asterisks mark the solvent signals (ring protons: 6.67 ppm, methyl protons: 2.16 ppm).

90 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

1 After 24 h of decomposition at 4 bar H2 and 150 °C, the H-NMR spectrum (Figure 5.15b), no other signals than that a singlet from free trimethylamine at 2.11 ppm could be observed, 27 which also hints on a full decomposition of [(Me3N)AlH3]. The Al-NMR spectrum did not exhibit any signals. The initial signal of the alane complex at 136 ppm disappeared, so that undesired side reactions, which would lower the yield on Al(0) can be ruled out. The signals of mesitylene, which is supposed to be a decomposition product of [{Cu(mesityl)}5], could however not be found. Presumably, the signals overlap with the signals of the residual protons of d12-mesitylene. In order to answer the question, whether the very first reaction step is a nucleophilic attack of Al-hydride at the mesityl moiety at the Cu centre, [{Cu(mesityl)}5] was 2 treated with [(Me3N)AlD3] in C6H6 in a pressure stable NMR tube. The H-NMR spectrum, which was recorded immediately after the addition of [(Me3N)AlD3], exhibited the signal of the AlD3-species was found at 3.67 ppm. Besides, a weak resonance at 6.64 ppm appeared, giving rise to deuteration of the mesityl ring (C6H2DMe3), i.e. cleavage of one mesityl group of the Cu centre. This observation indicates that in principle, hydrogen gas is not necessary to reduce the Cu-complex, if an excess of a reductive H-donor is present.

θ-CuAl2 from [{Cu(mesityl)}5] and [(AlCp*)4]

In the 1H-NMR spectrum of the reaction (Figure 5.16, left), which was carried out over 1 week at 150 °C in d12-mesitylene, the proton signals of the Cp*H methyl groups appeared as singlets at 1.83 and 1.75 ppm and a doublet of the CH3-ipso group, matching the expected intensity ratio of 6:6:3. However, other signal at 1.60 and 1.55 ppm were observed, which also most presumably stem from protons of a CH3 group at an olefin ring carbon. Additionally, several other signals in the range of 1.10 and 0.85 ppm are observed, pointing to the presence of other ring protons, e.g. resulting from partial of full hydrogenation of Cp*H. Surprisingly, in the 27Al-NMR (Figure 5.16, right), a very weak signal at -82 ppm was detected, which indicates that some intact Cp*Al-species are still present, even after 1 week 1 of decomposition time at 3 bar H2/150 °C. The signal at 1.96 ppm in the H-NMR could theoretically arise from such a species.

91 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

1 27 Figure 5.16. H‐ (left) and Al‐NMR spectra (right) of the co‐hydrogenolysis of [{Cu(mesityl)}5] and [(AlCp*)4)] in an NMR tube in d12‐mesitylene. The asterisk marks the signal of the residual methyl protons (2.16 ppm) in d12‐mesitylene. The large broad signal at around 68 ppm marks the static signal of the probe head of the spectrometer.

It is thus reasonable to assume that the two precursors did not decompose at a similar time scale, i.e. the Cu-complex decomposed before AlCp*, which then stabilised small Cu particles, which themselves, as a consequence of that, were not visible by means of XRD. However, the hypothesis, that Al@Cu core-shell particles were obtained is under these circumstances just a vague guess.

5.4.1.2. X-ray powder diffraction

θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4]

X-ray powder diffraction (XRD) of the powder as-synthesised proved the formation of

θ-CuAl2 (Khatyrkite, tetragonal body-centred I4/mcm, Figure 5.17a). However, several other reflections were observed at 2θ = 17.95°, 29.76°, 32.33°, 33.98°, 52.92°, 58.76°, 63.40° and 64.47°, which could not be assigned to any Cu-Al phase available in the powder diffraction database, nor to elemental Cu or Al, or to Cu- or Al-oxide. Thus, and as a consequence of the results of the NMR decomposition (vide supra), it is reasonable to assume that the unidentified reflections stem from the phosphorus-containing side product. Hence, the θ-

CuAl2 powder was washed with several solvents, in order to remove the impurity. The best results were performed when the powder was washed with polar solvents, e.g. 1,4-dioxane, which suggests that the nature of the impurity is also polar or salt-like. However, after all

92 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles efforts to work up the dioxane-filtrate in order to isolate this byproduct, it was not possible to identify it to date. The XRD diagram of the θ-CuAl2 powder after several washing cycles with boiling hot 1,4-dioxane is presented in Figure 5.17b. The absence of the contamination reflections is nicely demonstrated. The XRD pattern perfectly matched reference data (Table 5.3), showing a well-crystalline product. The average particle size was calculated to be 35 ± 5 nm via the Scherrer equation. A hint on the rather small grain size of the obtained powder was the diffraction pattern, which exhibited relatively broad reflections (FWHM of the (110) reflection: 2θ = 0.289°), compared to a highly crystalline, annealed reference sample (FWHM of the (110) reflection: 2θ = 0.143°[270]). The peak broadening was demonstrated in six reflections, that were expected to appear as three pairs at 2θ = 61.03°, 61.43°, 80.68°, 81.01°, 81.84° and 82.18°, but could not be fully resolved due to overlapping and emerged as three broadened reflections at 2θ = 61.65°, 81.16° and 82.44°, respectively. The reaction was performed at 150 °C and further annealing of the sample was not intended giving rise to further particle growth, peak sharpening and crystallisation. So the co-formation of any other amorphous Cu/Al phases can certainly not be excluded.

Figure 5.17. XRD diagrams of the θ‐CuAl2 phase, obtained by co‐hydrogenolysis of a) [CpCu(PMe3)] and 0.5 equiv. [(AlCp*)4] before purification, b) after washing the powder of a) with 1,4‐dioxane, c) [CpCu(PMe3)] and 2 equiv. [(Me3N)AlH3] and d) 0.2 equiv. [{Cu(mesityl)}5] and 2 equiv. [(Me3N)AlH3]. θ‐CuAl2 reference data: JCPDS No. 25‐0012. The asterisks in diagram a) mark the reflections of an impurity.

93 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Table 5.3. Comparison of XRD reflections (2θ/°) of synthesised θ‐CuAl2 and reference data (JCPDS No. 25‐0012). h k l CuAl sample b) CuAl sample c) CuAl sample d) CuAl reference (Int. in %) 2 2 2 2 1 1 0 (100) 20.68 20.74 20.70 20.620 2 0 0 (35) 29.47 29.54 29.45 29.386

2 1 1 (70) 38.01 38.03 37.98 37.867 2 2 0 (35) 42.20 42.24 42.21 42.071 1 1 2 (90) 42.68 42.73 42.69 42.591 3 1 0 (70) 47.44 47.51 47.45 47.332 2 0 2 (60) 47.91 47.95 47.85 47.808 2 2 2 (13) 57.26 57.32 57.27 57.129 4 0 0 (2) 61.032 61.65 61.63 61.58 3 1 2 (6) 61.435 4 1 1 (6) 66.40 66.51 66.46 66.335 2 1 3 (9) 67.21 67.28 67.21 67.034 4 2 0 (11) 69.33 69.42 69.38 69.173 4 0 2 (21) 73.62 73.73 73.64 73.462 3 3 2 (20) 77.48 77.59 77.49 77.251

0 0 4 (8) 78.41 78.51 78.37 78.382 5 1 0 (11) 80.678 81.16 81.38 81.15 4 2 2 (11) 81.007 4 3 1 (9) 81.842 82.44 82.43 82.42 1 1 4 (9) 82.181 2 0 4 (1) 86.12 86.04 86.04 85.950 5 2 1 (5) 89.43 89.50 89.41 89.103 4 1 3 (3) 90.10 90.03 90.04 89.829 4 4 0 (3) 92.06 92.21 91.872 92.37 5 1 2 (5) 93.28 92.51 92.205 3 2 4 (3) 93.50 93.64 93.47 93.220 5 3 0 (2) 95.78 95.91 95.82 95.577 3 1 4 (17) 97.18 97.29 97.19 97.063

6 0 0 (5) 99.54 99.66 99.50 99.267

θ-CuAl2 from [CpCu(PMe3)] and [(Me3N)AlH3]

The XRD clearly exhibits signals of the θ-CuAl2 phase (Figure 5.17c). Here, the impurity signals, also observed in the CuAl2 powder prepared from [CpCu(PMe3)] and

[(AlCp*)4], would be present again, if the powder was not washed with hot 1,4-dioxane. The calculated particle size was estimated to be around 75 nm. Although there is no indication for another phase present, it cannot be fully excluded by XRD, that another, amorphous Cu-Al phase is being formed under the conditions described.

94 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

θ-CuAl2 from [{Cu(mesityl)}5] and [(Me3N)AlH3]

The XRD powder pattern of the synthesised sample clearly shows all reflections of the

θ-CuAl2 phase (Khatyrkite, body-centred tetragonal I4/mcm, JCPDS No. 25-0012, Figure 5.17d). The reflections appear to be slightly broadened in comparison to the reflections of the

θ-CuAl2 samples, which was prepared from [CpCu(PMe3)] and [(AlCp*)4], or [(Me3N)AlH3], respectively. Accordingly, the particle size was calculated to 25 ± 5 nm with the Scherrer equation.

θ-CuAl2 from [{Cu(mesityl)}5] and [(AlCp*)4]

The XRD diagram in Figure 5.18 shows a broad reflection at 44° as well as sharp reflections of Al metal at 2θ = 38.46° (100), 65.20° (220), 78.33° (311) and 82.48° (222). The (200) reflection, which appears at 2θ = 44.85°, is lying underneath the broad signal. From this, it is obvious that alloying of the two metals did not take place. Although the broad reflection at 2θ = 44° suggests the presence of Cu metal or Cu-containing alloys, there are no other indications for this hypothesis.

Figure 5.18. XRD pattern of the product of the co‐hydrogenolysis of 0.2 equiv. [{Cu(mesityl)}5] and 0.5 equiv.

[(AlCp*)4].

95 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.4.1.3. 27Al-MAS-NMR spectroscopy

θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4]

27 The Al Knight Shift resonance at 1486 ppm of the CuAl2 material is clearly shifted to high field from pure aluminium at 1639 ppm (Figure 5.19a). The observed signal exactly corresponds to the Al-shift observed by the groups of Bastow,[271] Torgeson[272] and Grin,[273] [270] as well as to own θ-CuAl2 reference measurements (Table 5.4). The Knight Shift is a sensitive probe for the local electronic density of states, reflecting different crystallographic aluminium sites and thus, the qualitative change of the isotropic value of the Knight Shift resonance unambiguously indicates alloying of Al with Cu in this case. Traces of oxygen at the particle surface, being certainly present (see the TEM of Al particles in Figure 5.2, p. 72), are no obstacle for alloying. The signals of Al2O3 were found at 56 and 2 ppm.

27 Table 5.4. Literature reports on the Knight shift of Al in the Al solid state NMR of θ‐CuAl2.

27Al‐NMR Knight shift [ppm] Reference

1590, 1530 and 1480 [272]

1480 [271] 1500 [273]

1486 This work – from [CpCu(PMe3)] and [(AlCp*)4]

1492 This work – from [CpCu(PMe3)] and [(Me3N)AlH3]

1495 This work – from [{Cu(mesityl)}5] and [(Me3N)AlH3]

θ-CuAl2 from [CpCu(PMe3)] and [(Me3N)AlH3]

The 27Al-MAS-NMR measurement, shown in Figure 5.19b, exhibits a Knight shift of Al at 1492 ppm. This value is in good agreement to earlier reports of Bastow et al., Torgeson et al., Haarmann et al. and our own measurements of different θ-CuAl2 samples, prepared from

[CpCu(PMe3)] and [(AlCp*)4], and from [{Cu(mesityl)}5] and [(Me3N)AlH3]. A summary of

Al Knight shifts in the CuAl2 phase reported to date is given in Table 5.4. In the recorded spectrum, a strong signal of Al2O3 at 31 ppm was also detected, indicating a partial oxidation of the sample. The signals of the two structurally different Al-sites in Al2O3 now appear as one broad resonance with a shoulder, indicating the other signal lying underneath. In comparison, in the θ-CuAl2 sample, synthesised from [CpCu(PMe3)] and [(AlCp*)4], the two signals of AlO4 and AlO6 could clearly be distinguished. A possible reason for the

96 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

superimposing signals can be the contribution of AlO5, suggesting an amorphous structure of

Al2O3 at the surface of CuAl2. Smith et al. modelled the contributions of AlO4 (65 ppm) AlO5 [259] (28 ppm) and AlO6 (7 ppm) to the signal of amorphous alumina.

27 Figure 5.19. Al‐MAS‐NMR spectra of the θ‐CuAl2 powder materials obtained by co‐hydrogenolysis of a)

[CpCu(PMe3)] and 0.5 equiv. [(AlCp*)4], b) [CpCu(PMe3)] and 2 equiv. [(Me3N)AlH3] and c) 0.2 equiv.

[{Cu(mesityl)}5] and 2 equiv. [(Me3N)AlH3].

97 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

θ-CuAl2 from [{Cu(mesityl)}5] and [(Me3N)AlH3]

27 The Al-MAS-NMR spectrum of the θ-CuAl2 powder (Figure 5.19c) exhibits a Knight shift of Al at 1494 ppm, which is in a good accordance to previously published data and own measurements on θ-CuAl2 samples synthesised in this work from different Cu- and Al- compounds. Interestingly, the spectrum does not show the signal of alumina, which usually exhibits two strong resonances in the range of 50 and 0 ppm.

5.4.1.4. Transmission electron microscopy

For TEM measurements, aliquots of all synthesised θ-CuAl2 samples were suspended in toluene and treated with ultrasound for better particle dispersion. The images revealed agglomerates of primary particles with a broad size distribution of around 100-200 nm (Figure 5.20, top). This finding is not surprising, as the precursors were decomposed without any surfactant present, which would suppress agglomeration. The EDX analysis of the selected area in image a) (synthesised from [CpCu(PMe3)] and [(AlCp*)4]) revealed an atomic Cu:Al ratio of around 1.0:2.1. EDX of image b) ([CpCu(PMe3)] and [(Me3N)AlH3]) exhibited a Cu:Al ratio of 1.0:2.3. The EDX analysis of image c), which shows θ-CuAl2 particles obtained from [{Cu(mesityl)}5] and [(Me3N)AlH3] revealed a Cu:Al ratio of 1.0:2.0.

Figure 5.20. TEM images of θ‐CuAl2 powders (top), synthesised by co‐hydrogenolysis of a) [CpCu(PMe3)] and 0.5 equiv. [(AlCp*)4], b) [CpCu(PMe3)] and 2 equiv. [(Me3N)AlH3] and c) 0.2 equiv. [{Cu(mesityl)}5] and 2 equiv.

[(Me3N)AlH3] and the EDX spectrum (right).

98 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.4.2. Synthesis of the intermetallic Cu0.50Al0.50 phase

5.4.2.1. Synthesis attempts by co-hydrogenolysis of [CpCu(PMe3)] with [(AlCp*)4] and

[(Me3N)AlH3]

The co-hydrogenolysis of a mesitylene suspension of 4 equiv. [CpCu(PMe3)] and 1 equiv. [(AlCp*)4] (Cu:Al molar ratio 1:1) under the standard conditions of 3 bar H2 and 150 °C, afforded a grey powder after 15 min of reaction time. The powder was isolated by means of filtration, and was washed first with hot 1,4-dioxane, and then with n-pentane, and dried in vacuo. An X-ray powder diffraction measurement of the isolated powder exhibited reflections which could not be assigned to the target Cu0.50Al0.50 phase (monoclinic C2/m, JCPDS No.

26-0016), but rather to a mixture of several Cu-rich phases, including γ-Cu6.108Al3.892 and γ-

Cu9Al4 (Figure 5.21a). However, not all reflections could be indexed to a CuAl phase available in the JCPDS and ICSD databases. Reflections of Cu-/Al-oxides, or Cu(0) and Al(0), respectively, were not observed.

The reaction of equimolar amounts of [CpCu(PMe3)] and [(Me3N)AlH3] is as vigorous as in the previous case of the synthesis of the θ-CuAl2 phase. The powder sample was worked up analogously to the preparation described in Chapter 5.4.1. (p. 87). Similarly to the experiment of synthesising a Cu0.50Al0.50 phase from [CpCu(PMe3)] and [(AlCp*)4], the XRD pattern of the prepared sample did also not give an indication for the presence of the desired Cu0.50Al0.50 phase. Instead, reflections of the Cu-rich γ-Cu6.108Al3.892 and γ-Cu9Al4 phases were found (Figure 5.21b). Yet, there were still several weak reflections at 2θ = 29.83°, 32.37°, 52.96° and 58.86°, which could not be indexed as a known phase or as any other possible metallic or oxide material. This suggests the presence of other, presumably poorly crystalline phases, which are supposed to be Al-rich, since only Cu-rich phases were detected by means of XRD. This example nicely demonstrates the problem of a selective, phase-pure synthesis of a Cu- rich α-CuAl phase.

99 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.21. XRD pattern of the powder obtained by co‐hydrogenolysis of a) 4 equiv. of [CpCu(PMe3)] and 1 equiv. of [(AlCp*)4] and b) equimolar amounts of [CpCu(PMe3)] and [(Me3N)AlH3]. Green lines: Cu6.108Al3.892

(JCPDS No. 19‐0010), red lines: Cu9Al4 (JCPDS No. 24‐0003).

5.4.2.2. Synthesis from [{Cu(mesityl)}5] and [(Me3N)AlH3]

In analogy to the synthesis protocol of θ-CuAl2 (vide supra), mesitylene solutions of

[{Cu(mesityl)}5] and 5 equiv. of [(Me3N)AlH3] (Cu:Al ratio of 1:1) were combined in a

Fischer-Porter bottle, while a deep red solution formed. Subsequent treatment with 3 bar H2 and 150 °C afforded a grey precipitate within 15 minutes.

100 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

X-ray powder diffraction

The XRD diagram of the obtained powder (sealed in a capillary), which is shown in Figure 5.22a, exhibits very broad reflections with low intensity, which were assigned to the

Cu0.51Al0.49 phase (monoclinic C2/m, JCPDS No. 26-0016). However, it could not be excluded that other amorphous or nanocrystalline phases were present. For this reason, the capillary containing the sample was annealed at 200 °C, which was considered to be sufficient to crystallise a material with a bulk melting point of around 650 °C (see Cu-Al phase diagram,[200] p. 27). Yet, this heat treatment did not lead to crystallisation of the sample.

Figure 5.22. XRD diagram of Cu0.51Al0.49, obtained by co‐hydrogenolysis of [{Cu(mesityl)}5] and [(Me3N)AlH3].

JCPDS reference lines: Cu0.51Al0.49: 26‐0016, γ‐Cu9Al4: 24‐0003.

101 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

New reflections did not appear, and the observed reflections did not sharpen (Figure 5.22b), presumably as a consequence of an alumina layer covering the particles, thus preventing agglomeration. Thus, a sample of the powder was set into a quartz ampoule, evacuated (10-3 mbar), sealed and subsequently annealed at 500 °C for 24 h. The resulting XRD (Figure

5.22c) exhibited a surprising pattern, which was assigned to the γ-Cu9Al4 phase. The reflections of the Cu0.51Al0.49 phase were no longer visible in the pattern. The reason for the reduction of the Al content from 49 to 30 at.% is unclear. Presumably, a phase transformation has taken plase, giving the γ-Cu9Al4 phase besides another Al-rich phase, which is not crystalline and thus not visible by means of XRD. However, it cannot be excluded that before annealing, the powder consisted of more than one phase, or that oxidation of the Al- component to Al2O3 has taken place.

27Al-MAS-NMR-spectroscopy

27 The Al-MAS-NMR spectrum of the Cu0.51Al0.49 powder revealed a very broad Knight shift of Al at 645 ppm and a signal of Al2O3 at 21 ppm (Figure 5.23). The equidistant weak signals (20 KHz), which extend over the entire spectrum, are the rotation side bands of the alumina signal.

27 Figure 5.23. Al‐MAS‐NMR spectrum of Cu0.51Al0.49, obtained by co‐hydrogenolysis of [{Cu(mesityl)}5] and

[(Me3N)AlH3].

102 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

It should be explicitly stated here that the broad signal at 645 ppm is not a phase-shifted 27 background signal. In comparison to the Al-MAS-NMR spectrum of the γ-Cu9Al4 powder (Figure 5.25, p. 106), the Knight shift is more intense than the broad background curves. The high-field movement of Al Knight shift in comparison to that of pure Al was also observed in the θ-CuAl2 samples (Table 5.4, p. 96). With decreasing Al concentration the Knight shift moves to high fields. Below a certain Al content in the alloy, e.g. for α-CuAl phases, the signal of metallic Al becomes not only too weak, but also too broad. XRD data showed that the long range order of crystallinity is very poor. Hence, the Al atoms in the Cu0.51Al0.49-phase are randomly distributed in the Cu lattice, rather than periodically. Thus, each atom occupies a different position in the solid material, so that the signal becomes a sum of many weak resonances. Hence, the observation of a Knight shift in this sample speaks - in accordance with the XRD data - in favour of a Cu-Al phase.

5.4.3. Synthesis of the intermetallic γ-Cu9Al4 phase

5.4.3.1. Synthesis attempts by co-hydrogenolysis of [CpCu(PMe3)] with [(AlCp*)4] and

[(Me3N)AlH3]

In analogy to the efforts to synthesise phase-pure θ-CuAl2 and Cu0.50Al0.50 alloys, the co-decomposition of 9 equiv. [CpCu(PMe3)] and 1 equiv. [(AlCp*)4] (molar Cu:Al ratio 9:4) was performed under 3 bar H2 at 150 °C. However, the reaction did not result in the formation of the γ-Cu9Al4 phase, but in a α-CuAl phase with distorted fcc Cu structure. In the XRD diagram of the isolated powder, the Cu reflections were found at 2θ = 42.92° (111), 49.96° (200), 73.35° (220), 88.84° (311) and 93.93° (222). The reflections exhibited a shift compared to the ideal Cu structure, which is caused by the presence of Al atoms in the Cu-lattice. The

∆2θ deviation of the position of the sample, denoted as “Cu0.69Al0.31”, from the ideal fcc Cu structure, is shown in Table 5.5 (p. 112, vide infra). A comparison of the ∆2θ deviation with those of the series of precipitated α-CuAl/PPO colloids shows a acceptable agreement with the sample containing 33 at.% Al. By annealing the isolated powder in a sealed quartz ampoule in vacuo to 500 °C for 5 days, the crystal lattice did not undergo a phase transformation. This comparison may explain why it is possible to obtain apparently phase- pure θ-CuAl2 particles rather than a more complex mixture together with γ-Cu9Al4 and other possible compounds by this organometallic route. At very low temperatures of 150 °C, the

103 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles alloying process is certainly kinetically controlled.

Likewise the reaction of [CpCu(PMe3)] and [(AlCp*)4)], upon combination of [CpCu(PMe3)] with [(Me3N)AlH3] in the molar Cu:Al ratio 9:4, a vigorous reaction took place, giving a grey precipitate. After stirring the reaction mixture under 3 bar H2 pressure at 150 °C, the powder was worked up as described above. The XRD diagram of the isolated powder showed α-Cu phase reflections (shifted Cu reflection due to lattice distortion caused by Al atoms) at 2θ = 42.93° (111), 49.92° (200), 73.34° (220), 88.87° (311) and 93.92° (222). The deviation of the reflection positions matches that of the above case of the Cu0.69Al0.31 powder, synthesised from [CpCu(PMe3)] and [(AlCp*)4] (Table 5.5, p. 112). Thus it can be concluded, that it is not possible to synthesise phase selective α-CuAl nanoparticles neither from the precursor combination [CpCu(PMe3)] and [(AlCp*)4], nor from [CpCu(PMe3)] and [(Me3N)AlH3], by hydrogenolysis under the conditions described.

5.4.3.2. Synthesis from [{Cu(mesityl)}5] and [(Me3N)AlH3]

A sample of [{Cu(mesityl)}5] (1 equiv.) and [(Me3N)AlH3] (2.2 equiv., corresponding to a molar Cu:Al ratio of 2.25:1, or 9:4, respectively) was suspended in mesitylene and decomposed under the standard conditions (3 bar H2/150 °C) for 3 days, whereupon a grey solid precipitated. The supernatant was filtered and the residue washed with n-pentane, and then dried in vacuo. According to elemental analysis (AAS), the obtained powder contained 70.3 wt.% Cu and 13.4 wt.% Al, which corresponds to a Cu:Al ratio of 2.15:1.00, or 8.6:4, as well as 12.6 wt.% of hydrocarbon content, presumably due to solvent residuals. The remaining 3.7 wt% can most likely be attributed to oxygen. Likewise the synthesis of the

Cu0.51Al0.49 phase described above, only NMe3 was found in the supernatant.

X-ray powder diffraction

According to the XRD pattern, shown in Figure 5.24a, the synthesised powder clearly revealed diffraction reflections of the targeted γ-Cu9Al4 phase. However, analogously to all other syntheses of α-CuAl phases, in this case, the reflections were very broad and weak, so that it could not be excluded, that the powder contained other phases, being amorphous or nanocrystalline and thus not detected by XRD. Therefore, the sealed XRD capillary containing the powder sample was annealed at 200 °C for 24 h. The recorded XRD diagram did not exhibit any change in the diffraction pattern (Figure 5.24b). After a further annealing

104 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles treatment in a sealed ampoule at 500 °C for 24 h in vacuo, the XRD pattern changed drastically (Figure 5.24c). The γ-Cu9Al4 reflections disappeared and a set of very sharp reflections became visible, pointing at a phase transformation. The rearrangement of Cu and Al atoms to differenct crystal lattices upon thermal treatment, has already been observed before, especially in the α-phase region (see Chapter 3.2.2., p. 28).[202-204] Hence, it was not possible to identify any Cu-Al phase by comparison of the observed reflections with the database references of available intermetallic Cu-Al phases. Thus, it is reasonable to assume, that in the case of the γ-Cu9Al4 phase, annealed at 500 °C, similar phase changes may have occurred, as in the case of the Cu0.51Al0.49 phase (Figure 5.22, p. 101), synthesised from the same precursors.

Figure 5.24. XRD diagram of a) γ‐Cu9Al4, obtained by co‐hydrogenolysis of [{Cu(mesityl)}5] and [(Me3N)AlH3], b) after annealing at 200 °C and c) after annealing at 500 °C in a sealed quartz ampoule in vacuo.

105 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

The observed reflections could not be assigned by hkl indices, and thus it was not possible to clarify, whether the sample consists of a single phase or a mixture of two or more phases.

27Al-MAS-NMR spectroscopy

In comparison with the 27Al-MAS-NMR Knight shifts of aluminium in the synthesised

θ-CuAl2 phase (1495 ppm, see Table 5.5, p. 112) and the Cu0.51Al0.49 sample (645 ppm), for the γ-Cu9Al4 sample, a further high-field shift of the Al resonance was expected. In the recorded spectrum, however, only the signal of alumina at 31 ppm and its rotational side bands could be observed (Figure 5.25). Presumably, the measured γ-Cu9Al4 sample did not exhibit a long-range order of crystallinity, as suggested from XRD. Consequently, the concentration of Al atoms having equivalent atomic positions in the crystal lattice is expected to be rather low. Also, an increasing effect of the quadrupolar Cu nuclei at the signal width in comparison to the Cu1-xAlx samples (x = 0.50 and 0.67) must be considered. Since there are no reports in the literature on the Al Knight shift in the γ-Cu9Al4 alloy, it is also likely that the

Al signal lies underneath the resonance of Al2O3, and thus might not be visible.

27 Figure 5.25. Al‐MAS‐NMR spectrum of the intermetallic γ‐Cu9Al4 phase, obtained by co‐hydrogenolysis of 27 [{Cu(mesityl)}5] and [(Me3N)AlH3]. The signal at 30.9 ppm refers to the Al‐shift of alumina.

106 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.5. Intermetallic Cu-Ga phases from [{Cu(mesityl)}5] and

[(quinuclidine)GaH3]

The successful use of the compound [(Me3N)AlH3] as a readily available starting material for the wet chemical synthesis of metal aluminide nanoalloys can be extended to the heavier congener, namely gallium. Hence, this chapter briefly presents the perspectives of this reaction type. In contrast to the widespread [(R3N)AlH3] compounds, it is difficult to isolate gallium trihydride species, and only few compounds, e.g. the complex [274] [(quinuclidine)GaH3], have been synthesised to date. Thus, the reactivity of this compound towards [{Cu(mesityl)}5] and [CpCu(PMe3)] was investigated. Similarly to the binary Cu-Al phase diagram, the Cu-Ga phase diagram,[275] shown in Figure 5.26, exhibits several Cu-rich phases (γ1,2,3-Cu3Ga1-x, β-Cu3Ga, ζ-Cu4Ga1-x), as well as the Ga-rich θ-CuGa2 phase. Taking into account the intricacy of phase purity and possible phase transformations, shown in this chapter on the example of α-CuAl phases, the θ-CuGa2 phase was selected as synthetic target.

Figure 5.26. Illustration of the binary Cu‐Ga phase diagram.[275]

107 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Noteworthy, [(quinuclidine)GaH3] is stable over weeks of treatment with 3 bar H2 and 150 °C. In contrast, the treatment of a mesitylene solution of the gallane at 150 °C in the absence of H2 leads to a rapid decomposition to Ga(0), leading to the assumption that under H2, the Ga-trihydride is in an equilibrium state. For this reason, this section describes the preliminary results of the reaction with the Cu-precursor was conducted under argon, without addition of

H2 pressure.

5.5.1. Synthesis of the θ-CuGa2 phase

Analogously to the reaction of [{Cu(mesityl)}5] with [(Me3N)AlH3], the treatment of a mesitylene solution of the Cu-precursor with a solution of [(quinuclidine)GaH3] in mesitylene immediately lead to a reaction (Scheme 5.6), while a dark red solution formed. The mixture was stirred under argon at 150 °C for 24 h, while a grey solid formed. The supernatant was decanted and the grey residue washed with n-pentane and dried.

Mesitylene, 150 °C, 30 min 1 + 5 Cu 2 NGaH3 θ-CuGa2 - C9H12, - 2 C(CH2)3N 5

Scheme 5.6. Synthesis of the intermetallic θ‐CuGa2 phase from [{Cu(mesityl)}5] and [(quinuclidine)GaH3].

5.5.1.1. 1H-NMR spectroscopy The reaction was performed in a pressure stable NMR tube, in order to characterise the organic byproducts. Thus, [{Cu(mesityl)}5] and [(quinuclidine)GaH3] were dissolved in d12- mesitylene. Shortly after, a grey solid already precipitated. The reaction mixture was heated at 150 °C for 1 h, to ensure that the reaction is complete. In the recorded 1H-NMR spectrum, the signals of the Cu-precursor were not observed. The protons of the free quinuclidine ligand were found at 2.75 ppm (multiplet, 6 H, N(CH2)3), 1.54 ppm (septet, 1 H, HC(CH2CH3)N) and 1.36 ppm (multiplet, 6 H, HC(CH2CH3)N). As well, a broad singlet of dihydrogen at 4.61 ppm was detected, showing the full decomposition of the Ga-complex. In comparison, the proton signals of the quinuclidine ligand in [(quinuclidine)GaH3] (in d12-mesitylene) appear at 2.50, 1.24 and 1.05 ppm, and the signal of the gallium hydrides exhibit a resonance shift at 4.51 ppm.

108 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

5.5.1.2. X-ray powder diffraction The X-ray powder diffraction pattern of the synthesised powder reveals all reflections of the θ-CuGa2 phase (tetragonal P4/mmm, Figure 5.27), and perfectly matches to the literature data found in the database. Other phases, as well as metallic Cu or Ga, were not present. Taking a look in the Cu-Ga phase diagram (vide supra), this reaction offers an approach to a low-temperature phase, which is stable up to ~250 °C. Thus, this phase cannot be accessed by conventional metallurgic melting of the two metals, and is so far only available by mechanical milling of the metal powders.[276]

Figure 5.27. XRD diagram of the θ‐CuGa2 powder (JCPDS No. 25‐0275), obtained by co‐hydrogenolysis of

[{Cu(mesityl)}5] and [(quinuclidine)GaH3].

5.6. Preparation of Cu1-xAlx colloids (0.10 ≤ x ≤ 0.50)

5.6.1. Synthesis and characterisation

The results described above, proved the suitability of [(AlCp*)4] and [(Me3N)AlH3] as Al sources for alloying with Cu. The interest was then turned towards the synthesis of α-

CuAl, respectively Cu1-xAlx alloy colloids (0.10 ≤ x ≤ 0.50). From the point of view of the selective Al-oxidation in Cu-Al alloys, the preparation of α-CuAl nanoparticles does not require phase-pure crystalline materials, and allows a rather random solution of Al in Cu. The

109 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles choice of an appropriate surfactant, weakly adsorbing at the particle surface and controlling Ostwald ripening is crucial for this purpose. However, the aluminium complexes appeared to be much too reactive towards all the usual choices of capping ligands (see Chapter 5.1., p.

70). Alkyl chain polyethers of the type RO(CH2CH2O)nR introduce the problem of traces of water, which are very difficult to rigorously reduce below the level of 1 ppm. Thus, the polymer poly(2,6-dimethyl-1,4-phenylene)oxide (PPO) was selected, which was already used to stabilise Cu-Zn colloids and proved to behave inert towards the selected Al-precursors.

However, it is noteworthy that it was not possible to obtain colloidal solutions of Cu1-xAlx nanoparticles using [(Me3N)AlH3] as Al-source. Even in the presence of an excess of the stabilising agent PPO, upon addition of diluted solutions of [(Me3N)AlH3] to stock solutions of both Cu-precursors, the Cu-Al particles would precipitate within minutes. In conclusion, colloidal intermetallic Cu-Al nanoparticles could not be synthesised using [(Me3N)AlH3] as Al-donor and PPO or HDA as stabilising agents, due to its extreme reactivity. The so far only precursor combination, which has proven to be suitable for the synthesis of colloidal Cu1-xAlx nanoparticles, was the mixture of [CpCu(PMe3)] and [(AlCp*)4] (Scheme 5.7).

x 3 bar H2, 150 °C 1 Cu + [(AlCp*)4] Cu Al /PPO colloids 4·(1-x) Mesitylene, PPO (1-x) 1-x x PMe3 0.10 < x < 0.50

Scheme 5.7. Co‐hydrogenolysis of [(AlCp*)4] and [CpCu(PMe3)] with PPO as surfactant to Cu1‐xAlx/PPO colloids.

Yet, even using PPO, it was so far not possible to derive a θ-CuAl2 nanomaterial being composed of non aggregated, individual θ-CuAl2 particles dispersed in colloidal solution. The same is true for all the efforts to obtain well defined pure nano-Al colloids in mesitylene, stabilised by PPO. But by reducing the Al content to x ≤ 0.50, the co-hydrogenolysis of

[(AlCp*)4] and [CpCu(PMe3)] in molar Al:Cu ratios from 1:1 to 1:9 in presence of PPO resulted in nicely deep red-coloured Cu1-xAlx colloids (0.10 ≤ x ≤ 0.50), that could be precipitated with n-pentane and thereafter, fully redispersed in toluene or mesitylene by ultrasound or heating treatment. Apparently, the solubility of alloyed Cu1-xAlx/PPO particles in mesitylene increases with a rising Cu content on moving away from intermetallic compounds such as CuAl2 and towards the solid solution α-CuAl phase. A similar behaviour was observed for related nanobrass α/β-CuZn particles stabilised by PPO, with the best solubility observed for α-CuZn particles rather than β-CuZn etc. (Chapters 4.4. and 4.5., p.

46). Noteworthy, the solubility of PPO and the Cu1-xAlx particles, respectively, improved

110 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles using anisole as solvent rather than mesitylene. Consequently, it is more difficult to precipitate the particles from an anisole solution than from mesitylene.

5.6.1.1. X-ray diffraction studies The XRD pattern of the PPO stabilised Cu-Al particles in a molar Cu:Al ratio of 1:1 could not be assigned to the expected Cu0.51Al0.49 phase (monoclinic C2/m, JCPDS No. 26-

0016), but rather to γ-Cu9Al4 (Figure 5.28a, cubic P-43m, JCPDS No. 24-0003). Surprisingly, only the high-temperature phase γ-Cu9Al4 was visible in the XRD of colloidal Cu0.50Al0.50 under these reaction conditions. A powder sample prepared by combining [(AlCp*)4] and

[CpCu(PMe3)] in the equimolar Cu:Al ratio without PPO exhibited reflections indexed for γ-

Cu6.108Al3.892 and γ-Cu9Al4, as well as some other unidentified phases (Figure 5.21, p. 100).

Hence, in the XRD of the colloidal Cu0.50Al0.50 sample, the other possible, typically Al-rich phases were presumably amorphous, at least this cannot be ruled out. Thus, the

“Cu0.50Al0.50/PPO” sample should be regarded as a possible mixture of several Cu-Al α- phases. Instead, the XRD diagrams of precipitated Cu1-xAlx colloids (0.10 ≤ x ≤ 0.33, Figures 5.28b-d) exhibited reflections that could clearly be assigned to the typical fcc Cu pattern with slightly shifted Cu reflections, caused by the random incorporation of Al atoms into the copper lattice, forming a solid solution and thus corresponding to the α-phase region.

Figure 5.28. XRD diagrams of precipitated PPO stabilised particles of a) Cu0.50Al0.50, b) Cu0.67Al0.33, c) Cu0.83Al0.17 and d) Cu0.90Al0.10, measured under argon. Reference JCPDS data: Cu: 4‐0836, Cu9Al4: 24‐0003.

111 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

The shift of the reflections is expectedly proportional to the Al content in the samples (Table

5.5). Noteworthy, in the XRD diagram of the α-Cu0.67Al0.33 sample, the (330) reflection of γ-

Cu9Al4 at 2θ = 44.07° (Int. 100 %) was observed besides the Cu reflections (Figure 5.28b), indicating the coexistence of α- and β-phases. Other reflections of γ-Cu9Al4 were not visible, due to the low crystallinity of this phase in the sample. Upon oxidation, the γ-Cu9Al4 reflection remained unchanged, most likely due to the formation of a passivating alumina layer, as it was the case with θ-CuAl2, see discussion below.

Table 5.5. Deviation of the XRD reflections of Cu0.69Al0.31 powder samples and Cu1‐xAlx/PPO particles from the Cu structure (in 2θ/°), in dependence of the Al content. h k l 1 1 1 2 0 0 2 2 0 3 1 1 2 2 2

Cu[a] 43.298 50.434 74.132 89.934 95.143

Cu0.90Al0.10/PPO ‐ 0.074 ‐ 0.094 ‐ 0.057 ‐ 0.093 ‐ 0.046

Cu0.83Al0.17/PPO ‐ 0.160 ‐ 0.208 ‐ 0.242 ‐ 0.250 ‐ 0.209

Cu0.67Al0.33/PPO ‐ 0.302 ‐ 0.365 ‐ 0.582 ‐ 0.662 ‐ 0.658 [b] „Cu0.69Al0.31“ ‐ 0.363 ‐ 0.478 ‐ 0.786 ‐ 1.099 ‐ 1.216 [c] „Cu0.69Al0.31“ ‐0.377 ‐0.511 ‐0.790 ‐1.062 ‐1.220 [a] Literature data taken from JCPDS reference No. 4‐0836. [b] Powder sample synthesised from [CpCu(PMe3)] and [(AlCp*)4] (Chapter 5.4.3.1., p. 100). [c] Powder sample synthesised from [CpCu(PMe3)] and [(Me3N)AlH3] (Chapter 5.4.3.1., p. 100).

5.6.1.2. Transmission electron microscopy

All Cu1-xAlx/PPO colloids exhibited a rather broad particle size distribution of around 16 ± 5 nm (for x = 0.50: 17 ± 5 nm; x = 0.33: 16 ± 3 nm; x = 0.17: 18 ± 4 nm; x = 0.10, 18 ± 5 nm; see Figure 5.29). Energy dispersive X-ray spectroscopy (EDX) of the precipitated, washed and redispersed Cu1-xAlx particles revealed the expected compositions of the particles close to the expected ratio of the metals within the accuracy of the not deliberately calibrated method (10 % rel. error).

112 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.29. TEM images of colloidal solutions of a) Cu0.50Al0.50‐, b) Cu0.67Al0.33‐, c) Cu0.83Al0.17‐ and d)

Cu0.90Al0.10/PPO nanoparticles.

5.6.2. Oxidation behaviour of Cu1-xAlx/PPO colloids (0.10 ≤ x ≤ 0.50)

5.6.2.1. X-ray powder diffraction of air-oxidised Cu1-xAlx/PPO colloids

The colloidal solutions of the Cu1-xAlx/PPO nanoparticles were exposed to air over a period of 24 h analogously to the oxidation of Cu1-xZnx/PPO particles (for experimental details see Chapter 9.4.1.6., p. 177), in order to examine the nature of the alumina layer on the particle surface. Thereafter, the particles were precipitated by addition of n-pentane, washed and dried in vacuo. The obtained powders were exposed for another 24 h to ambient air. The powder diffraction diagrams of Cu1-xAlx colloids (0.17 ≤ x ≤ 0.33), presented in Figures 5.30a-c, showed that in the samples down to 20 at.% Al Cu is only present in the zerovalent state, supporting the idea of an alumina layer which is entirely covering the core, thus protecting it from corrosion. Contrasting this, in the sample with 10 at.% Al, Cu2O was found besides Cu (Figure 5.30d). This finding points to incomplete oxidation of Cu, most presumably forming multiphase composite or core-shell particles with a mixed Cu2O/Al2O3 surface structure. That structural damage resulting from oxidation of those α-CuAl alloy particles is also indicated by the line broadening of the remaining fcc Cu reflections.

113 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.30. XRD diagrams of air oxidised, PPO stabilised precipitated particles of a) Cu0.50Al0.50, b) Cu0.67Al0.33, c)

Cu0.83Al0.17 and d) Cu0.90Al0.10, (oxidation of the particles before precipitation). Reference JCPDS data: Cu: 4‐

0836, Cu9Al4: 24‐0003, Cu2O: 5‐0667.

5.6.2.2. UV-Vis spectroscopy of Cu1-xAlx/PPO colloids (0.10 ≤ x ≤ 0.50) The alloying of Cu and Al was reflected by the transformation of the characteristic 573 nm surface plasmon resonance (SPR) of the pure Cu reference colloid (see Chapter 4.1.2.4., p.

39) into a very broad absorption structure centred at 520 nm for the Cu0.50Al0.50 sample (Figure 5.31a). With rising Cu content, the absorption became more distinct visible and for

Cu0.67Al0.33, a well developed absorption peak at 519 nm was observed (Figure 5.31b). The typical SPR of nano-Cu appeared again at 573 nm with reduction of the Al-mole fraction down to 10 % in Cu1-xAlx (0.10 ≤ x ≤ 0.33, Figures 5.31c and d). Interestingly, even after few days of exposure to ambient conditions (moist air), the UV-Vis absorption of these Cu1-xAlx colloids (0.17 ≤ x ≤ 0.50) did not change at all (Figures 5.31a-c), quite similar to related observations in the case of the Cu1-xZnx/PPO alloy particles (Chapter 4.6.1.2., p. 59). This observation again clearly indicates alloyed α-CuAl particles, protected by some kind of an oxidation inhibiting shell, which seems to be composed of alumina preferentially. In contrast, the absorption of a Cu/PPO reference colloid quickly shifts upon oxidation to 610 nm as the result of Cu2O@Cu core-shell particle formation (Chapter 4.1.2.4., p. 39).

114 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

Figure 5.31. UV‐Vis spectroscopic monitoring of the time resolved oxidation of a) Cu0.50Al0.50‐, b) Cu0.67Al0.33‐, c)

Cu0.83Al0.17‐ and d) Cu0.90Al0.10/PPO colloids.

While these Cu2O@Cu core-shell particles can be reduced in solution by addition of CO, syn- [234] gas or pure H2, the oxidised α-CuAl colloids discussed here, showed no change at all upon similar reductive treatment. Preferential Al oxidation and Al2O3 surface layer formation is known for single crystal aluminide thin films of the more electronegative transition metals, [190-192] with NiAl to yield γ-Al2O3@NiAl as a well studied example. Thus, it is very likely that the Cu or Cu/Al alloy core is not being substantially oxidised, as long as the Al content is above 15 at.%. Below a level of 15 at.% Al, the Cu component cannot be effectively passivated by the growth of a dense Al2O3 layer and appears to be oxidised as well, resulting in the typical colour change from red to green and the characteristic UV-Vis absorption at 604 nm occurs (Figure 5.31d). This observation is in accordance with the XRD measurements on the precipitated α-Cu1-xAlx/PPO particles (vide supra). The UV-Vis spectra of these samples clearly resembled those of pure oxidised, core-shell type Cu2O@Cu/PPO colloids. The optical absorption spectra of metallic nanoparticles can be described according to Mie´s theory.[238] However, due to the lack of suitable reference data - at least to the author’s knowledge - for the bulk dielectric function of the various Cu-Al phases being involved in

115 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles this study, the observed SPR spectra could so far not be directly modelled. However, for instance, a related simulation was possible for α- and β-CuZn (with 35 and 50 at.% Zn, respectively) based on the data of the bulk dielectric function of α- and β-brass[239] and revealed a matching of the observed absorption around 525-535 nm with the calculated maximum at 500 nm for α-CuZn (Chapter 4.6.1.2., p. 59) and 520 nm for β-CuZn.[148] Assuming that the bulk dielectric functions of α/β-CuAl and α/β-CuZn are not fundamentally different, it can be concluded, that the observed shift of the SPR as a function of the composition clearly indicates alloy Cu1-xAlx particles. In addition, it should be noted here, that the particle size distribution has some influence also and the SPR shifts to longer wavelengths with increasing particle size giving rise to substantial peak broadening, which may be especially important at higher Al contents.

5.7. Conclusion

In summary, it was shown that [(AlCp*)4] has significant potential as a very clean and controllable source for bare nano-Al particles in non-aqueous, aprotic solution. The alternative Al-source, [(Me3N)AlH3], is also a valuable precursor for the synthesis of nanoscaled metal aluminide alloys. The advantage of the alane over [(AlCp*)4] is the better synthetic availability. Whereas [(Me3N)AlH3] can easily be synthesised in mass scales of 20 g [179c] by a simple salt metathesis reaction from Me3NHCl + LiAlH4, the low valent Al(I)- compound is synthesised in 4 reaction steps, including the synthetically delicate reduction of Al(III) to Al(I) by the Na/K alloy.[262] However, the alane is highly reactive and the reactions are very vigorous and uncontrolled. Contrasting this, [(AlCp*)4] is a mild reagent, which first has to be dissolved, before it decomposes to Al(0). This causes an intrinsic delay of decomposition, so that the time scale of releasing Al(0) matches quite well to that of the metal precursors used here. Noteworthy, it was not possible to obtain a colloidal solution of Al nanoparticles, despite of all efforts.

The introduced Cu-precursors [CpCu(PMe3)] and [{Cu(mesityl)}5] have both proven to be clean and quantitative sources for Cu(0) without any Cu-containing byproducts. Thus, they both were suitable candidates for the synthesis of Cu-Al alloys by co-hydrogenolysis with

[(AlCp*)4] and [(Me3N)AlH3], respectively. However, although the co-hydrogenolysis of

[CpCu(PMe3)] with both [(AlCp*)4] and [(Me3N)AlH3] gives the apparently pure θ-CuAl2 phase, the products are contaminated with a so far unidentified phosphine-based side product,

116 5. Synthesis of intermetallic α‐ and β‐CuAl nanoparticles

which is quite difficult to remove. With [CpCu(PMe3)], it was also not possible to obtain other, Cu-rich α-CuAl phases in the same quality of phase-purity. Interestingly, colloidal Cu-

Al alloys could only be synthesised from the co-hydrogenolysis of [CpCu(PMe3)] with

[(AlCp*)4] in presence of the polyether PPO. Unfortunately, a major constraint in the synthesis of colloidal metal aluminide nanoparticles is the incompatibility of [(AlCp*)4] with all common surfactants, so that to date, the only suitable and inert surfactant appears to be the polymer used in this work. The results of the controlled Al-oxidation of these Cu1-xAlx-PPO nanoparticles correspond to that of the parent Cu-Zn colloids (including the particle size and shape), showing the formation of an alumina shell, which protects the Cu-core. Below 17 at.% Al, copper is being oxidised. All attempts to produce colloidal CuAl-nanoparticles using

[(Me3N)AlH3] have failed, due to the extreme reactivity of the alane, even at low temperatures (< 30 °C).

The compound [{Cu(mesityl)}5], on the other hand, is a very clean source for Cu(0) upon hydrogenation in mesitylene solution. However, it appears to react with [(AlCp*)4] before decomposition, so that no Cu-Al phase could be obtained from these two precursors. In contrast to the Cu-phosphine complex, it reacts with [(Me3N)AlH3] even before the addition of hydrogen to the θ-CuAl2, Cu1Al1 and γ-Cu9Al4 phases, respectively. Also, the low- temperature θ-CuGa2 phase was accessed by reaction of [{Cu(mesityl)}5] and

[(quinuclidine)GaH3]. This reaction offers novel perspectives in terms of a widespread synthesis of transition metal aluminide and -gallide nanoparticle powder in organic solution at very soft conditions, even without hydrogen.

117 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.1. Intermetallic β-NiAl nanoparticles

6.1.1. Synthesis of β-NiAl nanopowder NP1 from [Ni(cod)2] and [(AlCp*)4]

The co-hydrogenolysis of [Ni(cod)2] and 0.25 equiv. [(AlCp*)4] under 3 bar H2 pressure in mesitylene at 150 °C leads to the formation of a brown-black solution, which is stable for 8-10 h. After 10 h, the solution becomes colourless and a black powder of the intermetallic β- NiAl phase (in the following denoted as NP1) precipitates.

3 bar H2, 150 °C Ni + 1/4 ß-NiAl Mesitylene, 4 days Al 4

Scheme 6.1. Co‐hydrogenolysis of [Ni(cod)2] and [(AlCp*)4] to β‐NiAl nanoparticles (NP1).

The suspension is stirred for 4 days, while a complete loss of H2 pressure was observed (Scheme 6.1). The colourless supernatant is decanted and the powder washed with n-pentane and well dried in high vacuo at 100 °C.

6.1.1.1. X-ray powder diffraction of NP1 The extremely pyrophoric powder was characterised by means of XRD as apparently phase-pure β-Ni1.1Al0.9 (cubic, Pm3m, JCPDS No. 44-1187, Figure 6.1a). The average crystallite domain size was estimated to be 4 ± 1 nm via the Scherrer equation on the full widths at half maximum (FWHM) of the (111), (110), (200) and (211) reflections. The preparation of β-NiAl from [Ni(cod)2] and [(AlCp*)4] was repeated several times and the measured β-NiAl reflections exhibited slight shifts corresponding to a variation in compositions of Ni1.1Al0.9, Ni0.52Al0.48, Ni0.50Al0.50 and Ni0.9Al1.1. The random deviations of the lattice constants for each experiment in comparison to Ni0.50Al0.50 is pointing to slight variations of the molar Ni:Al ratio in the synthesis. The very accurate determination of the molar ratio of the particular organometallic precursors by weighing and the quantitative

118 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles transfer into the reaction vessel is more difficult than it is usually the case when highly pure elemental metals or simple inorganic compounds are the starting materials. Nevertheless the inaccuracy of the obtained composition of less than ± 5 % appears quite good. In order to exclude the presence of any other Ni/Al phases or Ni metal as a product of phase segregation, a sample of NP1 was annealed in a sealed quartz tube in vacuo for 24 h at 200 and 500 °C, respectively. Indeed, no other reflections than those of the observed Ni1.1Al0.9 phase could be detected (Figures 6.1b and c). In the case of the powder annealed at 200 °C, the average particle size of 4 nm does not differ from the particles as-synthesised. The particle size rises to around 8 nm with annealing at 500 °C. Presumably, during the synthesis of NiAl, a thin

Al2O3 layer, formed from traces of oxygen or water (e.g. in the solvent), inhibits particle growth.

Figure 6.1. XRD diagrams of a) β‐Ni1.1Al0.9 (NP1) as‐synthesised, b) annealed at 200 °C, and c) annealed at 500 °C.

6.1.1.2. TEM analysis of the particle morphology The TEM image reveals broad sections of agglomerates, consisting of small primary particles, which confirm the particle size calculation from XRD data (Figure 6.2, left image). In a HRTEM image, a faint shell around the particle is visible (Figure 6.2, right), however, it was not possible to determine whether it is an alumina shell, formed upon sample preparation on air, or (hydro)carbon residues from the synthesis. The results of the EDX analysis verify the Ni:Al ratio of around 1:1, within the accuracy of the method of measurement.

119 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.2. TEM images of agglomerates of nano β‐Ni1.1Al0.9 NP1 (left) and a zoom on a single particle (right).

6.1.1.3. IR-spectroscopy According to elemental analysis, of one representative sample, the NP1 particles contain 50.9 % Ni and 23.1 % Al, which corresponds to a Ni:Al stoichiometry of 1.013:1.000, or the sum formula Ni0.503Al0.497. The significant C- and H-contents rest contents of 23.4 % and 2.6

%, respectively (C:H molar ratio 1.00:1.32), suggest the presence of mesitylene (C9H12, C:H ratio 1.00:1.33). The IR spectrum of a sample of NP1 exhibits only quite weak absorptions. No significant C-H vibration bands apart from those in the region between 400 and 1000 cm-1 are visible (Figure 6.3). The reason for the extreme band broadening is unclear. However, this observation is in qualitative agreement with the mesitylene traces in the powder.

Figure 6.3. IR‐spectrum of NP1.

120 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.1.1.4. Examination of the organic byproducts by NMR and GC-MS spectroscopy The organic byproducts of the co-hydrogenolysis being dissolved in the mesitylene supernatant were characterised by GC-MS and NMR (1H, 27Al). The GC-MS analysis (Figure -1 6.4) shows cyclooctane (1, C8H16, M = 112 g·mol ), 1,2,3,4,5-pentamethylcyclopentane (2, -1 -1 C10H20, M = 140 g·mol ) and 1,3,5-trimethylcyclohexane (3, C9H18, M = 126 g·mol ) besides mesitylene. Thus, during the decomposition of the precursors, the ligands cyclooctene and 1,2,3,4,5-pentamethylcyclopentadiene and, to some extent, the solvent itself were hydrogenated. Signals of Cp*H or cyclooctadiene were not detected. In the 1H-NMR spectrum, the resonance of 1 appears at 1.51 ppm, which is in good accordance to literature reports. The signals of 2 and 3 appear as a variety of multiplets in the range between 1.2 and 0.3 ppm, since the methyl groups of 3 can adopt both axial and equatorial positions.[277] Most importantly, the 27Al-NMR does not exhibit any signals, so that it can be concluded that there are no Al-side products apart from Al(0), supporting Scheme 6.1.

Figure 6.4. GC spectrum of the supernatant of the co‐hydrogenolysis of [Ni(cod)2] and [(AlCp*)4] (top left) and GC separated mass spectra of 1) cyclooctane (top right), 2) 1,2,3,4,5‐pentamethylcyclopentane (bottom left) and 3) 1,3,5‐trimethylcyclohexane (bottom right).

121 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.1.2. Synthesis of β-NiAl nanoparticles NP2 from [Ni(cod)2] and [(Me3N)AlH3]

Alternatively, the synthesis of β-NiAl particles can also be performed with

[(Me3N)AlH3] as more readily Al-source, according to Scheme 6.2. Treating a solution of

[Ni(cod)2] in mesitylene with an equimolar amount of the highly reactive aluminium trihydride compound leads to an immediate reaction, upon which a black powder precipitates instantaneously. In order to complete the reaction, the suspension was pressurised with 3 bar

H2 pressure and heated at 150 °C for 1 day.

3 bar H2, 150 °C Ni + H3Al NMe3 ß-NiAl + NMe3 + 2 Mesitylene, 24 h

Scheme 6.2. Synthesis of β‐NiAl nanoparticles NP2 from co‐hydrogenolysis of [Ni(cod)2] and 1 equiv.

[(Me3N)AlH3].

The particles were isolated by means of filtration, washed with pentane and dried as described above. The organic products of the co-decomposition, cyclooctane and NMe3 were found in the filtrate. By elemental analysis, 54.2 wt.% Ni and 24.8 wt.% Al were found in the sample, which corresponds to a Ni:Al ratio of 1.004:1.000. The C and H content was 16.6 % and 3.2 % (C:H ratio 1.0:2.3). The XRD diagram of the obtained air sensitive powder unequivocally proves the presence of β-NiAl (Figure 6.5). The primary particles are very small, having a diameter of around 3 nm, as calculated from the FWHM of the (110) reflection.

Figure 6.5. XRD pattern of Ni1.1Al0.9 nanoparticles NP2, synthesised from co‐decomposition of [Ni(cod)2] and

[(Me3N)AlH3].

122 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.2. Hydrocarbon-stabilised colloidal β-NiAl nanoparticles

6.2.1. Synthesis of colloidal β-NiAl nanoparticles NP3

The co-hydrogenolysis of [Ni(cod)2] and [(AlCp*)4] leads to a brown-black solution, which is stable up to 8 h reaction time, and beyond 8 h, NiAl particles are precipitating. Noteworthy, after 8 h, the entire hydrogen pressure is consumed, as indicated by a drop of the pressure from 3 to 0 bar. In order to examine the content of the solution, the reaction is interrupted after 8 h of decomposition time. Subsequent removal of all liquid components in high vacuo affords a black, metallic shining residue (NP3). The reaction was followed by NMR, but the reaction mixture was strongly paramagnetic, so that no NMR spectrum could be recorded. This observation is a hint on the formation of Ni particles or clusters, since the Ni-precursor decomposes at a faster rate than AlCp*. In the context of our work on the coordination chemistry of low valent group 13 organyls ECp* (E = Al, Ga, In) on d10 metal [162a] centres, we found that in an inert gas atmosphere, [Ni(cod)2] reacts with an excess of

[(AlCp*)4] to the thermodynamically stable complex [Ni(AlCp*)4], which, however, is not [278] paramagnetic. Besides, the reaction of equimolar amounts of [Ni(cod)2] and GaCp* under [279] dihydrogen yields in the distorted cubic Ni8-cluster [Ni8(GaCp*)6]. As the two precursors decompose at different time scales, it is reasonable to assume the formation of AlCp*- stabilised Ni clusters at an early stage of the reaction, besides the parallel decomposition of

[Ni(cod)2], before splitting off the Cp*H from the Al centres.

6.2.2. Characterisation

6.2.2.1. X-ray powder diffraction

XRD analysis confirms that the obtained powder NP3 consists of β-NiAl nanoparticles (Figure 6.6). The reflections are very broad as compared with the samples of Figure 6.1 and thus imply a crystallite domain size below 4 nm. Based on the FWHM of the (110) reflection only, the particle size was estimated to about 2 nm.

123 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.6. XRD diagram of β‐NiAl nanoparticles (NP3), isolated from a colloidal solution.

6.2.2.2. IR-spectroscopic study of NP3 The reason for the unexpected stability of the as-synthesized NiAl nanoparticles as a colloidal solution without the presence of any surfactant added is surprising. The particles can easily be redispersed in aromatic solvents, such as benzene, toluene or mesitylene. The powder contains besides 42.1 wt.% Ni and 19.7 % Al (Ni:Al ratio of 1.000:1.018) around 36 % of hydrocarbons (32.0 % C, 4.2 % H, C:H ratio of 1.00:1.56). The obtained mass yield of NP3 is 117 % of the expected mass of the pure metals after quantitative decomposition. NMR- and IR measurements of a sample of NP3 were undertaken. In contrast to NP1, the IR spectrum, shown in Figure 6, reveals broad aliphatic C-H vibration bands at around 2900 and -1 -1 1300-1500 cm and a very strong Al-C vibration band arising at 477 cm . Thus, it can be concluded that the particles are accompanied by an undefined, presumably high molecular mass organic product, which has a weak interaction with the particle surface, but allows the dispersion of the particles in organic solvents. Few groups have reported on the phenomenon of the stability of colloidal metal nanoparticles in presence of non-functionalised surfactants, having no distinct covalent or ionic/electrostatic interaction with the particle surface.

Recently, it was shown that the thermal co-decomposition of [Cu(OCH(Me)CH2NMe2)2] and

[ZnEt2] in 200 °C hot squalane (2,6,10,15,19,23-hexamethyltetracosane) yields free standing ZnO@Cu colloids, which gradually precipitate after prolonged storage under argon.[244]

124 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.7. IR spectrum of β‐NiAl nanoparticles NP3.

Chaudret et al. synthesised Ru nanoparticles stabilised by perfluorohexacosane.[93f] In the case of NiAl colloids, the surfactant is presumably a product of ring opening of the Cp* ligand and subsequent oligomerisation, catalysed by Ni or NiAl. This hypothesis is for example substantiated by the work of Rabinovich et al. observed the ring opening of cyclopentadiene over a Pt/Al2O3 catalyst at 500 °C to 1,3-pentadiene, which can further form longer chains.[280] Besides, di- tri- and tetramerisation Diels-Alder products were also observed.

Noteworthy, if Cp*H is added to a solution of [Ni(cod)2] in mesitylene without [(AlCp*)4], subsequent hydrogenolysis (3 bar H2/150 °C) does not afford colloidal nickel, but Ni powder precipitates, instead, pointing at the relevance of AlCp*, i.e. the reaction of Cp* to a longer chained ‘surfactant’ is supposed to occur at the Al centres. Accordingly, in the IR spectrum of NP3, an Al-C vibration band was visible, indicating an interaction between the hydrocarbon shell and the Al atoms at the surface. Beyond 8 h of hydrogenation (Ni:Al 1:1), the particles precipitate, and in the IR-spectrum of the obtained material NP1, the Al-C band disappeared. This supports the hypothesis, that in the early stage of decomposition, small Ni clusters are stabilised by AlCp* ligands, which, in the next step decompose to Al(0) and Cp*H, which most presumably undergoes a ring opening or oligomerisation. C-H activation of the Cp* methyl groups, for example, was already observed for the 18 valence electron compounds [281] [M(AlCp*)5] (M = Fe, Ru). Also, aromatic solvents can be involved in bond activation,

125 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

e.g. the reaction of [Ni(cod)2] and [(AlCp*)4] (4 equiv.) in benzene leads to activation of a C- [278] H bond in the solvent, giving the complex [Ni(H)(AlCp*)3{AlCp*(Ph)}].

6.2.2.3. NMR characterisation of NP3

1 In the H-NMR of a sample of NP3 dissolved in C6D6, a very broad signal between 2.5 and 1.0 ppm is visible besides signals of remaining mesitylene at 6.67 (ring protons, 3 H) and 2.13 ppm (methyl protons, 9 H). The 13C-NMR spectrum reveals only one weak resonance at 27 21.3 ppm, pointing to aliphatic CH2 or CH3 groups. The Al-NMR spectrum exhibits no signals, so that so that undesired side reactions of [(AlCp*)4], e.g. oxidation of Al(I) can be ruled out.

6.2.2.4. Transmission electron microscopy The TEM image of the NiAl colloids NP3 (Figure 6.8, left image) shows a poor dispersion of a network of small particles, in spite of the good solubility and redispersability of the isolated NP3 material.

Figure 6.8. TEM images of NiAl colloids without surfactant (NP3, left), and with 1‐adamantanecarboxylic acid as surfactant (NP4, right).

126 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.2.3. Olefin hydrogenation catalysed by colloidal β-NiAl nanoparticles NP3

The loss of the excess hydrogen pressure occurs after the quantitative formation of β- NiAl nanoparticles, so that it seems that the supposedly initially formed Ni particles are not solely responsible for the hydrogenation, but rather the formed β-NiAl nanoparticles are quite active hydrogenation catalysts. Petró et al. reported on the catalytic activity of NiAl powder for hydrogenation on the examples of various organic compounds, e.g. nitrobenzene, acetophenone, eugenol and benzonitrile.[282] Hence, the catalytic activity of the β-NiAl nanoparticles NP3 for cyclohexane hydrogenation as a test case was studied. Thus, a sample of NP3 (0.35 mmol) was suspended in 5 mL cyclohexene (50 mmol) and treated with 3 bar H2 at 100 °C, whereupon a rapid pressure loss was observed. Hydrogen was added as long as the pressure would not decrease any longer. The total consumption of 14.6 bar H2 corresponds to 50 mmol, which hints of a complete hydrogenation of cyclohexene. The solvent was distilled off and an aliquot was taken for a 1H-NMR measurement. The spectrum, shown in Figure 6.9 (bottom), exhibits only one singlet at 1.40 ppm, which is in agreement with both literature data for cyclohexane. Signals of cyclohexene, shown for comparison in Figure 6.9 (top), were not present, giving rise to a completed hydrogenation.

1 Figure 6.9. H‐NMR spectra of cyclohexene before addition of NiAl and H2 pressure (top), and the resulting hydrogenation product cyclohexane (bottom).

127 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.3. Carboxylic acid-stabilised Ni/Al colloids NP4

Isolated β-NiAl nanoparticles NP3 can easily be redispersed in toluene and stored in a Schlenk vessel under argon for several days without any precipitation. However, the particles are strongly agglomerated and polydisperse, and apparently, the organic shell is too weakly bound to keep the particles in solution for a longer period of time. In the previous chapter, it was shown that the co-hydrogenolysis of [CpCu(PMe3)] and [(AlCp*)4] in presence of common surfactants like HDA, or trioctylphosphineoxide (TOPO) did not give stable colloids. Sterically demanding carboxylic acids, such as 1-adamantanecarboxylic acid (ACA) or fatty acids, such as oleic acid or stearic acid, represent well-known examples of efficient, yet chemically inert surfactants, which allow a control over the particle shape and assembly. However, in all cases, they stabilise rather oxidation stable metal and alloy colloids such as [35,104] [22-30] [38] [283] CoPt3, FePt and NiPt, but as well metal oxides, e.g. TiO2, when applied as additive during synthesis. The presence of a Brønsted acid immediately leads to side reactions with [(AlCp*)4] or [(Me3N)AlH3] to stable Al(III) species, which cannot be decomposed to Al(0), at least not under the conditions described. Hence, monolayer of the capping ligand as a post-synthesis additive should be sufficient to bind on the Al surface atoms to prevent the colloidal particles from precipitation. The bulky compound 1-adamantanecarboxylic acid (ACA), which is solid at room temperature (m.p. 172 °C), was the surfactant of choice preferred to long chained acids, such as OLEA or stearic acid. The latter acids are viscous liquids at ambient temperature and thus, particles capped with those ligands are difficult to precipitate for the sake of structural solid-state analyses.

6.3.1. Synthesis and characterisation of 17O-enriched 1-adamantanecarboxylic acid (ACA)

NMR and IR spectroscopy are convenient methods to determine the presence of a surfactant-particle bond as well as to characterise the nature of the binding mode of the surfactant to the metal particle surface.[284] Carboxylic acids bind onto metal surfaces in form of carboxylates R-COO-, and thus the surface-oxygen bond as well as any changes of the coordination geometry of the carboxylic O-C-O group should easily be traced by means of IR spectroscopy. Additionally, the oxygen atoms can be detected with 17O-NMR spectroscopy. However, for several reasons, it is difficult to obtain a valuable NMR signal of a molecule binding on a metal surface. First, the 17-oxygen nucleus has a quadrupole momentum of I =

128 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

5/2, which causes a significant line broadening due to the reverse directions of the magnetic momentum and the spin of the nucleus. Secondly, the concentration of the ligand molecules bound to a surface is very low and the natural abundance of 17O is only 0.04 %. For these reasons, the surfactant ACA was enriched with 17O in order to increase the concentration of NMR-active nuclei. However, it has to be taken into account, that a monolayer of a ligand being strongly bound to the surface of a small colloidal particle also exhibits very broad signals in NMR, or cannot be detected at all. Since the ligand is rigid, it behaves as a solid material, i.e. the atoms bound to the surface exhibit a chemical shift anisotropy of the dipole- dipole interactions, which are not averaged as in the case of molecules in solution. The anisotropy results from the slow tumbling of the particles in solution (in contrast to discrete molecules) and thus causes a strong line broadening. For example, Chaudret et al.[285] as well as Reven et al.[286] and Murray et al.[287] have observed, that the in the case of long-chained- thiolate- and HDA-capped ruthenium, platinum and gold nanoparticles, the protons and carbons in the α-, β- and γ-position are immobile and could thus not be detected by NMR spectroscopy, whereas the more distant C and H atoms were visible.

6.3.1.1. Synthesis of ACA A THF solution of 1-adamantylcarbonyl chloride was treated with a slight excess (1.3 equiv.) of 35-40 % 17O-enriched water at room temperature. After complete addition, a gas production was observed. The reaction was refluxed overnight, in order to ensure completeness of the reaction. Thereafter, the solvent was removed in vacuo. For complete removal of the excess water, the white raw product was dissolved in toluene, to form an azeotrope with water, which was removed in vacuo.

6.3.1.2. Characterisation by 1H- and 13C-NMR spectroscopy The 1H-NMR spectrum of ACA, shown in Figure 6.10 (left), exhibits resonances at 1.90 and 1.77 ppm, in a ratio of 6:3, which could be assigned to the adamantyl ring protons H1 and H3 (see insert in Figure 6.10), and the H2 protons (1.48 ppm, 6 H) of the methylene linker to the C-COOH group, respectively. Besides, the signal of the acid proton at 12.44 ppm (1 H) was detected. Correspondingly, in the 13C-NMR, the cyclohexyl carbons Cc and Cd give signals at 36.5 and 28.1 ppm, respectively (Figure 6.10, right). The methylene Cb and the quaternary Ca appear at 38.8 and 40.8 ppm, respectively. The carboxyl Ce signal is located at 184.9 ppm.

129 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.10. 1H‐ (left) and 13C‐NMR spectrum (right) of 17O‐enriched 1‐adamantanecarboxylic acid (ACA). The 1 asterisks mark the signal of the remaining protons of C6D6 in the H‐NMR spectrum, and the resonance of the carbons in the 13C‐NMR, respectively.

6.3.1.3. 17O-NMR spectroscopy

17 The high-resolution O-NMR spectrum of ACA in C6D6 reveals one signal of the two carboxylic O atoms at 247.5 ppm (Figure 6.11), which are chemically equivalent in solution, due to a fast proton exchange between the O atoms. The shift of the ACA oxygen atoms is in a typical range of carbonyl and carboxyl containing organic molecules,[288] as well as of organometallic complexes with such molecules as ligands.[289]

Figure 6.11. 17O‐NMR spectrum of 17O‐enriched ACA.

130 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.3.1.4. Mass spectroscopic determination of the 17O enrichment grade in ACA Mass spectrometry is a useful tool for the determination of atom isotopes and the concentration of a particular isotope, respectively. In the synthesis of 17O enriched ACA, one or both O atoms can be enriched. The molecular mass peak of non-enriched Ad-C16O16OH (Ad = adamantyl) is 220 m/z, whereas the one fold and twofold substituted ACA molecules have a mass of 221 and 222 m/z, respectively. In the mass spectrum of the synthesised 17O- enriched ACA (Figure 6.12), the absence of a peak at 222 m/z leads to the conclusion that the sample does not contain two 17O atoms in one molecule. The signal ratio of the non-enriched acid at 220 m/z and the one fold enriched acid (221 m/z) is around 5:1, so that the enrichment grade of ACA with 17-oxygen can be estimated to around 20 %. The deviation from the 17 original enrichment grade of 35-40 % O-enriched H2O used in the synthesis could presumably derive from the fact that a slight excess H2O (1.4 equiv.) was used in the synthesis. Hence, it is likely that marked 17OH groups of the acid exchanged with the 17 unmarked H2O molecules to give an unmarked acid and H2 O, which was later removed from the acid during workup.

Figure 6.12. Mass spectrum of 17O‐enriched 1‐adamantanecarboxylic acid (ACA).

6.3.2. Post-synthetic stabilisation of colloidal NiAl particles by addition of ACA Assuming a spherical shape of a colloidal NiAl particle with a diameter of around 2 nm

(estimated by XRD), 60 molecules of ACA (diameter ~ 5 Å) are necessary to entirely cover its surface, which roughly corresponds to a NiAl:ACA molar ratio of 1.0:0.5. The brown-

131 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles black solution of NiAl nanoparticles in mesitylene (c = 36 mmol·L-1) was treated with 0.5 equiv. of 17O-enriched ACA (Scheme 6.3), whereupon the colour changed to deep red. The solution was stirred for 16 h at 150 °C without further colour change or precipitation. Upon evaporation of the solvent vacuo at 100 °C, a black, shiny powder was isolated. The powder can easily be redispersed in aromatic solvents. In contrast to NP3, the ACA-stabilised sample (NP4) is stable under argon for weeks. Trace amounts of oxygen, however, lead to a rapid precipitation.

HO O O O O O Mesitylene, 150 °C O β-NiAl colloids O + O NiAl 16 h, argon O O O O O

ACA@NiAl colloids

Scheme 6.3. Stabilisation of NiAl‐colloids NP4 by addition of 1‐adamantanecarboxalic acid (ACA).

According to elemental analysis, the ACA@Ni/Al sample, denoted as NP4, contains 29.8 wt.% Ni and 14.1 % Al, as well as 39.1 % C and 4.6 % H. The residual weight percentage of 12.4 can be assigned to oxygen. This result corresponds to a sum formula

Ni1.00Al1.03C6.41H8.99O1.51. Taking into account that the sample contains traces of mesitylene and complete exclusion of air cannot be completely guaranteed, this formula is in quite good agreement to a molar ratio of NiAl to ACA (C11H16O2) of 2:1. Noteworthy, a surface plasmon resonance of ACA-capped NiAl colloids was not observed by UV/Vis spectroscopy. However, this is not surprising, since the absorption properties of the surface electrons of Ni colloids are poorly reported in literature to date.

6.3.2.1. X-ray powder diffraction of ACA@NiAl nanoparticles NP4 The XRD pattern of the particles NP4 reveals very broad reflections that can be assigned to Ni (Figure 6.13). Yet, the reflections exhibit small shifts in comparison to the fcc- Ni reference data, indicating the presence of another component in the Ni lattice. According to the binary Ni-Al phase diagram and quite typical for Hume-Rothery phases, the β-NiAl phase lies in a range of 40-55 at.% Al at ambient conditions, and the α-Ni phase region dominates below 40 % (see phase diagram,[210] p. 30). The deviation of the lattice constant

132 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

from the Ni reference allows an estimation of the molar fraction of Al(0) dissolved in the lattice of Ni which turns out to be around 38 ± 4 at.% (see Figure 6.17, p. 139). This finding is in good agreement with the line fit of the linear relationship of the lattice constant of Ni1-xAlx nanoparticles as a function of the molar concentration of the molar Al-content x in the Ni1- [195] xAlx alloy, described by Vegard´s Law. The treatment of β-NiAl colloids NP3 with ACA thus leads to a transformation into α-NiAl colloids. Obviously, upon treatment of the β-NiAl alloy particles NP3 with ACA, aluminium atoms diffuse onto the surface, forming an Al-rich surface layer, which is capped by ACA molecules. The average particle diameter is approximately 2 nm, calculated from the FWHM of the (111) reflection. The other reflections were too weak and broad to be used for reliable size estimation via the Scherrer equation.

Figure 6.13. XRD diagram of ACA stabilised Ni/Al colloids NP4.

6.3.2.2. Transmission electron microscopy

The TEM image of the ACA-capped α-NiAl colloids NP4, shown in Figure 6.8 (p. 126, right image), exhibits well dispersed rod-like particles of around 10 nm in diameter. However, in a close-up (insert in the right image of Figure 6.8), the displayed particles appear to be poorly shaped agglomerates of very small primary particles, being around 2 nm and below. In the EDX spectrum, the Ni:Al ratio is still around 1:1 (48 at.% Ni, 52 at.% Al).

133 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

6.3.2.3. NMR spectroscopy

1 The H-NMR spectrum of NP4 in C6D6 displayed numerous broad signals between 2.5 and 1.0 ppm. Thus, it is difficult to correctly assign the protons of the surfactant. The absence of the carboxyl proton indicates the conversion of the acid to a carboxylate. Besides, the 1H- NMR spectrum exhibited traces of mesitylene. In the 27Al-NMR spectrum, no signals could be detected. In the 13C-NMR spectrum (Figure 6.14, left) only the resonance shifts of the adamantyl carbons Cb at 39.5 ppm, Cc at 36.8 ppm and Cd at 28.4 ppm were visible (for the indexing see Figure 6.10, p. 130). The signals of the carboxylate carbon Ce and the neigboured Ca atom were not detected. This is not surprising, since it is well documented, that for colloidal ligand-stabilised nanoparticles, the ligand α- and β-carbons and -protons behave as ‘quasi-solid’ (see Chapter 6.3.1., p. 128) and thus exhibit very broad signals, which usually cannot be detected.[285-287] Correspondingly, the 17O-NMR spectrum (Figure 6.14, right) showed an extremely broad signal extending between 500 and -100 ppm. The signal of the free acid at 247.5 ppm was not visible, concluding that ACA must be quantitatively attached to the particle surface. Besides, two weak signals were visible at 354.2 and 148.5 ppm, whose origin is not clear to date. It may be speculated that these two signals stem from ACA ligands coordinated at particles of a smaller size, which would allow a faster tumbling of the nanoparticle and thus an averaging of the anisotropy of the dipolar interactions. At least, the TEM images suggest that the synthesised NP4 particles consist of primary particles of various sizes of 1-2 nm and below. It is reasonable to assume that the actual 17O-signal of the ACA ligands on the particle surface may well be the broad background signal, which spans a range of nearly 600 ppm, due to the above described reasons.

13 17 Figure 6.14. C‐NMR spectrum (left) and O‐NMR spectrum (right) of ACA@Ni/Al colloids (NP4) in C6D6 13 solution. The asterisk in the left spectrum marks the C signal of C6D6 at 128 ppm.

134 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Hence, in principle, surface coordinated ACA could be detected by MAS-NMR. However, in the case of the NP4 material, 17O-MAS-NMR did not reveal any signal apart from that of 17 Al2O3, most presumably due to several reasons: the quadrupolar momentum of O nucleus, the low concentration and the low abundance of 17O in the examined material.

As well, a strong resonance at 68.9 ppm was observed, which stems from Al2O3, according to literature reports on the 17O-MAS-NMR shift of alumina.[290] Thus, it is likely that the ACA ligands are bound to Al2O3-surface-coated α-NiAl particles, rather than to α-NiAl particles with a ‘clean’ oxide-free surface. The origin of the alumina signal is unclear. Most presumably, a surface alumina shell has already formed during synthesis/workup of the particles. Traces of water in ACA caould also lead to Al2O3 formation. There are no indications for an oxygen transfer from ACA to Al.

6.3.2.4. IR spectroscopy The absence of the IR carbonyl vibration band of the free acid (Figure 6.15, top) at 1693 cm-1 confirms the formation of carboxylate species adsorbed at the particle surface of NP4 (Figure 4.15, bottom). Tao et al. studied the coordination behaviour of monolayers of carboxylic acids on Cu, Ag and Al surfaces.[291]

Figure 6.15. IR spectrum of free ACA (top) and ACA coordinated at the surface of NiAl particles (bottom).

135 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Tao reported that all examined films formed oxide layers on air, and the basicity of the metal oxide seems to have an influence of the binding mode of the acid; whereas the carboxylate ligand is binding in bidentate fashion at Cu and Ag. Contrasting this, the acid is binding with one oxygen on an Al2O3@Al surface, as shown by reflection absorption IR spectrometry (RAIRS) measurements. Deacon et al. examined the carboxyl IR vibration bands of numerous transition metal carboxylate complexes, and showed that an asymmetric R-COO coordination, e.g. unidentate or bridging, gives rise to an increase of the difference of the wavenumbers of ~ ~ the symmetric (ν s ) and the asymmetric (ν a ) COO stretching band, whereas a rather low ~ ~ -1 [291] ν a (COO)-ν s (COO) difference (< 100 cm ) indicates a symmetric bidentate coordination.

In Tao’s work, the two vibration bands of COO coordinating at Al2O3@Al surfaces exhibit a wavenumber difference of 133 cm-1. In the case of the ACA-stabilised Ni/Al particles NP4, ~ ~ -1 ν a (COO)-ν s (COO) is 151 cm (Figure 6.15, bottom).

6.4. Nanocrystalline intermetallic α-NiAl powder

6.4.1. Synthesis of α-Ni1-xAlx phases (0.09 ≤ x ≤ 0.33)

The co-hydrogenolysis of various molar ratios of [Ni(cod)2] and [(AlCp*)4] to Ni1-xAlx phases (0.09 ≤ x ≤ 0.50) allows a free variation of the Al content in the aluminide alloy. Thus, the synthesis of α-Ni particles doped with Al was performed in analogy to the formation of β-

NiAl nanopowder NP1 (Scheme 6.4). In sharp contrast to the synthesis of Ni0.50Al0.50, the Ni1- xAlx particles with x ≤ 0.33 precipitate within minutes of H2 pressure treatment.

x 3 bar H2, 150 °C 1 1 [Ni(cod)2][(AlCp*)+ 4] Ni1-xAlx 4·(1-x) Mesitylene, 4 days (1-x) 0.09 < x < 0.33

Scheme 6.4. Wet chemical synthesis of α‐Ni1‐xAlx nanoparticles.

The initially bright yellow solution gradually changes the colour over orange to bright red and finally dark red, before a black powder precipitates. The particles were isolated by filtration, washed with n-pentane and dried. The actual atomic percentage of Al in the synthesised Ni1- xAlx compounds slightly deviates from the Al content which was calculated from the mass of used [(AlCp*)4], being 34.1 % for the sample denoted with x = 0.33, 25.3 % (x = 0.25), 17.0

136 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

(x = 0.17) and 8.2 % (x = 0.09), as shown by elemental analysis (Experimental Section,

Chapter 9.4.3.1., Table 9.4, p. 182). The evidently different agglomeration behavior of α-Ni1- xAlx particles is in congruence with the decrease of concentration of AlCp* or Cp, again suggesting that the Cp* moiety has a pronounced influence on the stabilisation of the primary particles formed during co-hydrogenolysis.

It was not possible to prepare alloyed α-Ni1-xAlx/PPO colloids by co-decomposition of

[Ni(cod)2] and [(AlCp*)4], in an analogous way to the preparation of Cu1-xZnx/PPO and Cu1- xAlx/PPO colloids, presented in the Chapters 4 and 5. XRD studies on the isolated Ni1- xAlx/PPO powders (0.09 ≤ x ≤ 0.50) only exhibited reflections, which perfectly matched to fcc-Ni, indicating that there was no Al in the Ni lattice, due to a full oxidation of Al. Presumably, a catalytically driven C-O bond activation is occurring in favour of the formation of stable Al-O bonds. It should be noted here, however, that precipitation of α-Ni1-xAlx particles can in fact be prevented when unsaturated hydrocarbon polymers, such as -1 polybutadiene (Mw = 5000 g·mol ) are added during co-hydrogenolysis [Ni(cod)2] and

[(AlCp*)4]. First experiments to synthesise colloidal α-NiAl alloy nanoparticles (< 40 at.% Al) were undertaken by addition of polybutadiene, which should compensate the deficiency on Cp*. Preliminary results suggest the formation of α-NiAl colloids which are stable over weeks inside the polyolefin matrix. The colloids could then be redispersed by post-synthetic addition of strongly coordinating ligands, such as carboxylic acids (ACA, OLEA).

6.4.2. Characterisation

6.4.2.1. X-ray powder diffraction studies

The XRD diagrams of the Ni1-xAlx particles (0.09 ≤ x ≤ 0.33), presented in Figure 6.16, all exhibit very broad reflections that were assigned to the fcc-Ni structure (JCPDS No. 4- 0850). Interestingly, in spite of the fast particle precipitation, the primary particle sizes of all measured Ni1-xAlx samples do not show large deviations from those of the β-NiAl samples NP1 and NP2. From the FWHM of all observed reflections (apart from the (200) reflections), the average particle sizes of the measured Ni1-xAlx powder samples were calculated to be 3 ±

1 nm for Ni0.67Al0.33, 7 ± 1 nm for Ni0.75Al0.25 and 8 ± 1 nm for Ni0.83Al0.17 and Ni0.91Al0.09, respectively. Yet, the trend to particle growth with decreasing Cp* content is visible. With rising atomic percentage of Al, the phase reflections exhibit an increasing shift difference from the Ni reference (Table 6.1), as a result of the growing distortion of the Ni-lattice, which

137 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles is caused by the incorporation of Al atoms. Noteworthy, it was not possible to obtain a phase pure γ’-Ni3Al phase from the decomposition of 3 equiv. [Ni(cod)2] with 1 equiv. of

[(AlCp*)4]. A α-Ni phase was formed, instead, indicating that the Al atoms are randomly distributed in the Ni lattice and thus do not form a well-ordered intermetallic phase of its own. This is not surprising, since the decomposition of the Ni-precursor occurs very fast, so that Ni particles are already formed, before the entire Al-precursor has decomposed. Several groups reported on the synthesis of bulk γ’-Ni3Al, however, in all cases, the samples were amorphous [292] and had to be annealed at > 500 °C to obtain the L12 γ’-Ni3Al structure (Figure 3.1, p. 25).

The lattice constants of the Ni1-xAlx nanoparticles were calculated via the Bragg equation from the (220) reflection, since the strongest (111) reflection did not show significant difference from the Ni reference in any case (Table 6.1). The second strongest reflection (200) overlapped with (111) for x = 0.33, so that an exact position of the reflection was not possible.

Figure 6.16. XRD diagrams of a) Ni0.67Al0.33, b) Ni0.75Al0.25, c) Ni0.83Al0.17 and d) Ni0.91Al0.09 α‐phase nanoparticles, measured under argon (fcc Ni reference reflections taken from the JCPDS database, No. 4‐0850).

138 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

According to Vegard´s Law,[195] the diagram displaying the lattice constant versus the Al content, shown in Figure 6.17, shows a quite good agreement between the experimental data and the fitted line. The data point marked with an arrow represents the lattice constant of the powder diffraction pattern of ACA-capped NiAl nanoparticles NP4. Thus, the content of Al(0) dissolved in the lattice of Ni colloids is 38 at.% Al, with deviation margins of ± 4 %, as calculated by the line fit equation (see box insert in Figure 6.17).

Figure 6.17. Variation of lattice constants of α‐Ni1‐xAlx phases as a linear function of the Al content (Vegard’s law). The lattice constant of NP4 was calculated from the (220) reflection (75.77° 2θ) using the Bragg equation.

Table 6.1. Deviation of the 2θ XRD reflections from the Ni fcc structure in dependence of the atomic Al content (determined by elemental analysis) and the lattice plane constants of the synthesised α‐Ni1‐xAlx phases. Ni x = 0.082 x = 0.170 x = 0.253 x = 0.341

[a] h k l 2θLit [°] Δ2θ [°] (1 1 1) 44.508 0.084 0.087 0.087 0.088 (2 0 0) 51.847 0.169 0.234 0.448 0.970 (2 2 0) 76.372 0.097 0.166 0.360 0.526 (3 1 1) 92.947 0.191 0.217 0.446 1.057 (2 2 2) 98.449 0.212 0.284 0.561 1.407 [b] ‐4 ‐4 ‐4 ‐4 d220 [Å] 2.4939 2.4968 ± 3·10 2.4996 ± 3·10 2.5049 ± 3·10 2.5096 ± 3·10 [a] Data taken from the JCPDS database No. 4‐0850. [b] Calculated with the Bragg equation (Cu‐Kα radiation; λ = 1.54178 Å).

6.4.2.2. Transmission electron microscopy

TEM measurements (Figure 6.18) of the α-Ni1-xAlx nanopowders (0.09 ≤ x ≤ 0.33) support the calculated average particle size from XRD. In analogy to β-NiAl, the

139 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles agglomerations consist of very small, polydisperse primary particles. The Ni:Al ratio, measured by EDX, was determined to be 2.6:1 for Ni0.67Al0.33, 3.5:1 for Ni0.75Al0.25, 4.5:1 for

Ni0.83Al0.17 and 9.3:1 for Ni0.91Al0.09, within the accuracy of the method.

Figure 6.18. TEM images of a) Ni0.67Al0.33, b) Ni0.75Al0.25, c) Ni0.83Al0.17 and d) Ni0.91Al0.09 α‐phase nanoparticles.

6.5. Oxidation behaviour of α-and β-Ni1-xAlx powder

6.5.1. Intentional oxidation of Ni1-xAlx nanoparticles (0.09 ≤ x ≤ 0.50)

In the case of the colloidal Cu1-xAlx nanoparticles (0.10 ≤ x ≤ 0.50), it was shown that intentional oxidation by air exposure leads to an alumina film, which prevents further oxidation of the Cu bulk core (Chapter 5.6., p. 109). From related metallurgical corrosion studies on NiAl, it is known that the particular oxidation conditions, i.e. temperature and oxygen pressure have a large influence on the kinetics of Al diffusion and the formation of alumina on the NiAl surface.[191a] Earlier works have shown that upon oxidation, the Al atoms diffuse onto the surface, and the Ni gradient exhibits the opposite direction, i.e. inwards the [191b] bulk core. Also, it was shown, that drastic oxidation with atmospheric O2 pressure leads [191c] to the formation of NiAl2O4 at the interface between Al2O3 and the Ni bulk. In order to

140 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles determine the minimum amount of Al which is necessary to prevent the oxidation of the Ni component, presumably by formation a fully developed shell around the remaining Ni/Al core of the particle, the above described α-/β-Ni1-xAlx powder samples (0.09 ≤ x ≤ 0.50) were oxidised by careful exposure to oxygen at room temperature. The vials containing the highly oxophilic samples were left open in the glovebox over night, then taken out and left on air for 24 h.

6.5.1.1. X-ray diffraction studies of the oxidised Ni1-xAlx samples

In the XRD diagram of Ni0.50Al0.50 after air oxidation at 25 °C (Figure 6.19a, top), the reflections became broader and a loss of intensity was observed, indicating a deformation of the NiAl structure. The XRD pattern of α-Ni1-xAlx powder samples (0.09 ≤ x ≤ 0.33) after air oxidation, (Figures 6.19c-d, top diagrams), exhibit no changes in comparison to the samples under argon. The oxidised powder samples a) - e) were annealed at 1000 °C in a sealed quartz ampoule in vacuo to determine the presence of NiO (m.p. 1985 °C), i.e. to find out which sample has an insufficient Al content for a full alumina shell. However, in all cases, only reflections of Ni were observed (Figure 6.19b-e, bottom diagrams). This indicates that even at an Al content of 9 at.% in the alloy, air stable Ni particles are obtained, which is in contrast to the results obtained from the oxidation of the Cu1-xAlx particles, which exhibit Cu oxidation below 17 at.% Al, as described in Chapter 5.6.2. (p. 113). The reflections of α-Al2O3 (Corundum, JCPDS No. 46-1212) appeared as very weak, broad signals at 26°, 36°, and 67° 2θ. With decreasing Al content (x < 0.17), the reflections were not visible any longer.

Presumably, the crystallinity and concentration of Al2O3 were too low in this sample. These findings show that X-ray diffraction does not give a sufficient proof for the absence of any NiO in the bulk particle core. In order to re-evaluate the XRD results and analyse the particle composition after oxidation in more detail, particularly the oxidation state of nickel, XPS measurements on the air-oxidised β-Ni0.50Al0.50 sample (25 °C) were carried out.

Additionally, the oxidation of all synthesised Ni1-xAlx (0.09 ≤ x ≤ 0.50) particles was investigated by X-ray absorption spectroscopy.

141 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.19. XRD diagrams of powder nanoparticles of a) Ni0.50Al0.50, b) Ni0.67Al0.33, c) Ni0.75Al0.25, d) Ni0.83Al0.17 and e) Ni0.91Al0.09 phases, after air oxidation (top diagrams) and after annealing in a sealed quartz ampoule at 1000

°C in vacuo (bottom diagrams). The asterisks mark the reflections of α‐Al2O3.

6.5.1.2. X-ray photoelectron spectroscopy of oxidised β-Ni0.50Al0.50 nanoparticles Numerous groups have studied the surface oxidation process in NiAl alloys with XPS,[190,294-296] however, in most cases, the samples were pre-heated (> 1000 °C) in ultra-high vacuum and then treated with oxygen gas, typically below 10-5 mbar. In all these reports, the selective Al-oxidation to alumina was observed, with Ni still being in the zerovalent oxidation state, which may give rise to softer oxidation processes. In the case of exposure to ambient air

142 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles at atmospheric pressure, the conditions of oxidation, especially of small particles with a high surface, are much different. The groups of Young et al.[295] and Venezia et al.[296] studied the oxidation behaviour of β-NiAl and γ’-Ni3Al, respectively, at more realistic atmospheric pressure conditions (> 700 °C). It was shown, that before oxidation, in the Al-region, a strong Ni 3p signals were observed at 66.8 eV, characterising the clean alloy. Accordingly, the Al

2p3/2 signal of alloyed Al at 72.9 eV was clearly visible. The Ni 2p3/2 signal was visible at 853.0 eV, accompanied by a d9-satelite at around 859 eV. It was found that upon oxidation the Al signal shifted to 76.0 eV, and the Ni 3p peak lost intensity and moved to 70.0 eV, thus arguing that the surface of oxidised NiAl consisted of a top layer of NiO, with a mixture of

Al2O3 and NiAl2O4 below. This work correlates with the earlier work of Kuenzly et al. on the air oxidation of Ni0.50Al0.50 at T > 900 °C, giving a mixture of NiO, NiAl2O4, and Al2O3, shown by XRD.[297] If the pressure was however lowered (e.g. 10-7 mbar), no Ni2+ species could be found.[296] The findings of Young and Venezia are in qualitative agreement to the observed X-ray photoelectron spectrum of the oxidised NP1 nanoparticles (Figure 6.20). In the close-up spectra of the Ni region, besides the peak of the Ni 2p3/2 photoelectrons at 856.7 eV, a shake-up satellite, characteristic for NiO, appeared at 862.0 eV, pointing at oxidised Ni on the particle surface.

Figure 6.20. XPS survey spectrum of oxidised Ni0.50Al0.50 nanoparticles. The Ni‐ (left) and Al‐region (right) spectra are shown as inserts.

143 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

The Al-region shows the signal of Al 2p1/2 at 74.5 eV, and a very weak Ni 3p peak at 68.1 eV.

For this reason, in all other Ni1-xAlx samples (x < 0.50), NiO would have been observed by XPS, and thus, there was no reason to perform further studies on those samples, too. Although NiO was detected at the surface, XPS only describes the situation at the particle surface, and it is still unclear, whether Ni is also oxidised in the bulk core.

6.5.1.3. X-ray absorption spectroscopy of oxidised α-/β-NiAl nanopowder

The local structure around the central nickel atom was examined by X-ray absorption spectroscopy at the Ni K-edge (8333.0 eV). The Ni1-xAlx samples (0.09 ≤ x ≤ 0.50) were deliberately oxidised prior to measurement, in order to establish the role of a protective layer around the core. In Figure 6.21, the XANES (left) and the EXAFS (right) of samples and reference materials are shown. The edge position (viz. XANES) of the samples is similar to that of metallic nickel (dashed curve) and clearly different to that of NiO (dash-dot curve). Thus, the XANES show that the nickel has not been oxidised as a result of the treatment, and is indeed protected by a closed shell. This observation correlates with the XRD results, proving the passivation of the Ni-core by alumina, which formed a full shell with even 9 at.% Al. Presumably, in the samples with less than 50 at.% Al, the Ni particles had formed before

[(AlCp*)4] was fully decomposed, so that a considerable amount of Al atoms was located at the surface. Also, the α-NiAl particles exhibit a particle size of around 8 nm in diameter (10 % surface atoms), which is two times longer than in the case of the β-NiAl particles (36 % surface atoms). Thus, the smaller particle surface compensates the lower Al content in the particle, so that upon oxidation, there are still enough Al atoms to diffuse onto the surface to form a stable shell to prevent the core from further corrosion. Although NiO was most likely partially oxidised at the surface (as seen in XPS), according to XAS, the surface oxidation to Ni2+ is negligible and not detected, even after months of exposure to ambient air. Alloy formation evidence obtained from XAS has been documented by several authors.[298-301] The XANES spectra for NiAl alloy reported in this literature appear to show rather substantial differences, which is likely due to the particle size of the samples used. Smaller, often disordered particles can lead to less pronounced features in the XANES.[301] The analysis of the Ni K-edge spectra is further complicated by the fact that similarity exists between spectra of Ni foil and Raney nickel.[298]

144 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.21. XANES (left) and EXAFS (right) of Ni1‐xAlx samples after oxidation. For comparison, the FT of a simulated Ni0.75Al0.25 FT has been added (EXAFS).

In the nomenclature of ref. [298], feature F in the XANES is indicative of a Ni-Ni resonance scattering found only in the metal, and could serve as a fingerprint to distinguish the alloy from the metal. In our samples, this feature was found in the spectra of Ni0.75Al0.25, Ni0.83Al0.17 and Ni0.91Al0.09, but it is absent in the case of Ni67Al33 and Ni50Al50. This suggests that for these samples, Ni is present in its fcc lattice. In the Fourier transformed spectrum, a substantial incorporation of aluminium into the Ni lattice (Al50Ni50) leads to a splitting of the first peak as reported.[298-300] In our samples, a splitting is only seen in the case of NiAl. Modelling of this spectrum with FEFF paths generated from model NiAl compounds proved unsuccessful due to the poor data quality, likely due to the coexistence of several Ni- containing phases. This observation is further substantiated by the absence of higher shells, suggesting the lack of long-range order. The argument of whether nickel or NiAl is present is apparently settled by the appearance of the χ-function (Figure 6.22). Clearly, the Ni0.75Al0.25 sample is similar to the foil, whereas the simulated spectra of Ni foil and Ni0.75Al0.25 show obvious differences. This matches the XRD data of the Ni0.75Al0.25 sample, which exhibited the fcc α-Ni structure, instead of the γ’-Ni3Al phase with L12 structure. The higher shells in the EXAFS (see peaks labelled 2 and 3) are visible for samples which contain more Ni (Ni1- xAlx, x < 0.33).

145 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles

Figure 6.22. Comparison of χ functions of a) measured and b) simulated fcc‐Ni and Ni0.75Al0.25.

These higher shells also coincide with the higher shells of fcc-Ni. The small intensity of the first peak in the FT for the Ni1-xAlx (x < 0.33) samples is not fully understood. Higher shells are evident in these samples (see Figure 6.21) which suggests more long-range order, and thus larger particles. A coexistence of smaller and larger particles is not excluded by the TEM images. Incorporation of Al into the Ni lattice without consequent splitting of the first peak is also considered. The first FT peak would then be broadened and reduced in height. In light of the presence of feature F in the XANES, the latter explanation can rather be excluded.

6.6. Conclusion

In summary, a soft, one-step chemical approach to NiAl nanoparticles with a freely adjustable Ni:Al ratio is presented. Two precursors for the Al component were successfully employed, namely [(Me3N)AlH3] and [(AlCp*)4], which were combined with [Ni(cod)2] to yield the respective α-/β-NiAl phases. The particles remain unaffected against exposure to ambient air, due to the formation of an alumina layer, which is formed in situ by diffusion of Al to the particle surface. The Ni rich core is thus protected against further oxidation as expected from classical metallurgical data on NiAl alloys and matching known thin film

146 6. Synthesis of intermetallic α‐ and β‐NiAl nanoparticles surface chemistry of single crystalline NiAl. However, a rigorous exclusion of any oxidation (oxygen getter effects) has not been possible with our experimental approach. The primary β-

NiAl nanoparticles derived using [(AlCp*)4] form metastable colloids in solution. In contrast, the use of [(Me3N)AlH3] does not allow the preparation of colloidal NiAl nanoparticles at all.

The reaction of the alane with [Ni(cod)2] is very vigorous and rapidly leads to the precipitation of a nanocrystalline NiAl powder material.

It appears that the rather unusual all-hydrocarbon precursor [(AlCp*)4] is an excellent choice for the wet nanometallurgy of aluminides, particularly when aiming at the stabilisation of free-standing surface protected metal aluminide nanoparticles. In order to further explain this, we speculate that at some intermediate stage of the formation of the NiAl title particles, still intact [(AlCp*)4] may bind to the particle surface, i.e. (Cp*Al)x@Ni1Al1-x in a way being comparable to the known molecular cluster chemistry of species such as [Ni(AlCp*)4] and other [Ma(AlCp*)b]. In that latter cases, the AlCp* moiety acting as very strong σ-donor ligand to the zero-valent transition metal centres.[278,281] Prolonged hydrogenolysis at elevated temperatures completely splits the remaining Cp* group from the Al centres and/or from the aluminide nanoparticles in the form of Cp*H. Due to the above described catalytic properties of the NiAl surface some hydrogenation of the Cp*H and further coupling reactions of still unsaturated products may take place which eventually leads to the above mentioned formation of oligomeric hydrocarbon species which act as primary protection against agglomeration. This ill-defined primary shell is however easily replaced by 1-adamantanecarboxylic acid (ACA). The treatment with ACA leads to a transformation of the β-NiAl nanoparticles into ACA-capped α-NiAl colloids by partial Al segregation towards the particle surface. As a consequence of that, the Al component in the alloy nanoparticle was partially oxidised giving a ACA@(Al2O3)δ/2@α-Ni1-xAlx-δ core-shell nanocomposite, but XRD measurements have shown that the particle core still consisted of around 40 at.% Al dissolved in the Ni lattice. The acid strongly binds to the - at this stage presumably only partly oxidised - particle surface and thus greatly enhances the stability of the colloidal solution. These results suggest that the ACA-stabilised NiAl nanoparticles could be an attractive air stable magnetic material, and may serve as a model case for related studies. Thus, extremely air sensitive magnetic nanoparticles, derived from the controlled hydrogenolysis of organometallic precursors, e.g. Fe, Co, or important binary alloy phases, such as CoRh, could be doped with small amounts of aluminium, to generate the alumina@metal core-shell structured particles by gentle oxidation.

147 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles

7. Synthesis of colloidal intermetallic β‐CoAl nanoparticles

Ferromagnetic nanoparticles of Fe, Co, or Ni, as well as their alloys are good candidates for many technological applications such as magnetic tunnel junctions, tunnelling magnetoresistances, MRAM, read heads and sensors.[3-5] However, they are highly air- sensitive and rapidly oxidise, so that they lose their magnetic properties. There are numerous reports on the coating of magnetic particles by layers, e.g. SiO2, carbon, or noble metals (see Chapter 2.3., p. 18). These layers act protecting shells, preventing the nanoparticles against air oxidation and sintering. Usually, the coating shell is introduced after the nanoparticle synthesis, and often, this method does not yield a full passivating shell.[187] Another convenient method is the coating with SiO2 by a sol-gel process, e.g. by hydrolysis of [180] [Si(OEt)4] in the presence of the nanoparticles. However, this method is constrained to air- [302] stable nanoparticles, e.g. Fe2O3 and cannot be employed for particles that are sensitive to any O-source. Several groups reported on the particle coating with graphitic carbon upon decomposition of orcanic surfactants.[303] Though, so far, this method lacks on the synthetic demand and a control of the particle size. It requires extremely high temperatures (> 400 °C) to form a dense carbon layer aroung the particles, which are prone to sinter at these temperatures and mostly exhibit poor dispersity. In the previous chapters, it was shown that the air oxidation of metal aluminide nanoparticles yields core-shell type alumina@metal particles. In this context, Al2O3@Co nanocomposites are ideal systems to test as they have been widely studied as thin films. Especially, it is well-known from surface science studies that in CoAl alloys, that Al selectively oxidises and forms an outer passivating layer of alumina.[304] It is therefore expected that upon exposure to air, CoAl nanoparticles should generate a corrosion stable cobalt core surrounded by an alumina shell, i.e. a Al2O3@Co nanocomposite. This phenomenon has been observed at the nanoscale for the Cu1-xAlx (see Chapter 5.6.2., p. 113) and Ni1-xAlx systems (see Chapter 6.5., p. 140), confirming the feasibility of this approach. This chapter gives the brief report on the synthesis of β-CoAl nanoparticles, unequivocally evidencing the alloy character. Besides, first experiments showed that the oxidation induced the segregation of a cobalt core inside an alumina shell. By comparison with the results reported on binary intermetallic Cu-Al and Ni-Al phases, the low valent Al- organyl complex [(AlCp*)4] was used as Al source. This work was performed in cooperation

148 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles with the group of B. Chaudret (Laboratoire de Chimie de Coordination, CNRS, Toulouse), 4 3 who have synthesised Co nanoparticles from the all-hydrocarbon complex [Co(η -C8H12)(η - [95] C8H13)]. They have demonstrated that this precursor is a good and versatile source of Co atoms under dihydrogen pressure producing cyclooctane as only byproduct, which will not interact with the Al precursor or Al atoms.

7.1. Synthesis of colloidal β-CoAl nanoparticles

The synthesis was carried out by hydrogenation of an equimolar mixture of both organometallic precursors under 3 bar dihydrogen pressure in mesitylene at 150 °C for 48 h. After 48 h of reaction time, a brown-black solution without any precipitate was obtained, even without addition of any capping ligand. Evaporation of the solvent in vacuo led to a black powder of β-CoAl nanoparticles (Scheme 7.1).

3 bar H2, 150 °C Co + 1/4 ß-CoAl Mesitylene, 48 h Al 4 - 2 C8H16

4 3 Scheme 7.1. Synthesis of colloidal β‐CoAl nanoparticles from [Co(η ‐C8H12)(η ‐C8H13)] and [(AlCp*)4].

4 3 Interestingly, [Co(η -C8H12)(η -C8H13)] and [(AlCp*)4] behaved inert against each other, and did not react to form mono- or multimetallic clusters, a behaviour previously observed between [(AlCp*)4] and olefinic complexes of zerovalent transition metals such as [278] 4 6 [281] [Ni(cod)2] or [Ru(η -cod)(η -cot)].

7.2. Characterisation

The alloyed character of the particles was stated by means of chemical analysis together with wide angle X-ray scattering (WAXS), transmission electron microscopy and superconducting quantum interference device (SQUID) magnetometry.

149 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles

7.2.1. 1H-NMR and GC-MS spectroscopy Although the synthesised CoAl-nanoparticles are very stable in solution and do not precipitate under inert gas atmosphere in the absence of any additional surfactant, it was not possible to conduct NMR measurements of the colloids diluted in a locking solvent (e.g. 1 C6D6), due to the magnetic nature of the Co-component in the alloy. H-NMR and GC-MS spectra of the evaporated reaction solution revealed only the signal of cyclooctane, which derives from the hydrated cod- and cyclooctyl ligands of the Co-precursor. The Cp* moiety of the Al-source could, however, could not be traced. In congruence to the NiAl-colloids NP3 (Chapter 6.2., p. 123), it is likely that during the hydrogenolysis, a ring opening or polymerisation of the Cp*H took place, giving a high molecular mass hydrocarbon, which may be responsible for the solubility of the particles in mesitylene, or toluene, respectively. Chemical analysis gives a total metal content of 41 wt.% (28.55 % Co; 12.48 % Al), which corresponds to a Al:Co ratio of 0.95:1.00, close to the expected Co1Al1 composition. The mass yield of 109 % gives additional hints on the presence of hydrocarbons in the β-CoAl material. The same effect was observed for colloidal β-NiAl nanoparticles NP3, which however do not exhibit the same stability in solution as in the case of β-CoAl particles, precipitating after 8 h of stirring at 150 °C.

7.2.2. Wide angle X-ray scattering (WAXS)

The isolated β-CoAl material did not exhibit any reflections which could be observed in the recorded X-ray powder diffraction pattern, possibly because of the very small size of the nanoparticles. Thus, a WAXS measurement on the β-CoAl sample was performed, in order to obtain information on the structure of the nanocrystalline material. Yet, the WAXS reflection pattern, shown as a red curve in Figure 7.1 (left image), clearly evidenced that the nanoparticles consist of the intermetallic β-Co1Al1 phase (ICSD No. 57596). Thus, after careful subtraction of the background contribution and subsequent Fourier transformation to real space, the radial distribution function (RDF) was generated (Figure 7.1, right image, red curve). From the obtained RDF, it is evident that the coherence length reaches 3 nm, so that the crystalline domain of the β-CoAl crystallites extends over the whole particle. The first local maximum at 0.249 nm (249 pm) corresponds to the first order metal-metal distance of the β-Co1Al1 reference (i.e. the Co-Al distance), which thus unambiguously proves the existence of the Co0.50Al0.50 alloy.

150 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles

The WAXS specimen was exposed to air for 2 h by careful opening the capillary and thereafter, a WAXS measurement was recorded again. The resulting WAXS pattern show a dramatic collapse of the β-CoAl structure, only exhibiting very broad reflections, which could not be assigned to a Co-Al phase any longer. Correspondingly, the RDF function (Figure 7.1, right image, blue curve) exhibited a huge loss of the intensity of the local maxima, which could be interpreted to a complete oxidation of the β-CoAl sample.

Figure 7.1. WAXS diagrams (left) of the synthesised β‐CoAl sample under argon (in reciprocal space with S‐ weighed reduced intensity) and after 2 h of exposure to air (blue curve) (CoAl reference: black lines, ICSD No.

57596, with Mo‐Kα radiation at λ = 0.071069 nm). Right: the radial distribution functions (RDF) of the β‐CoAl sample under argon (red curve) and after 2 h of exposure to air (blue curve).

7.2.3. Transmission electron microscopy

Analysis of the TEM images (Figure 7.2) of a solution of β-CoAl nanoparticles revealed agglomerates of very small primary particles (size circa 3 nm), embedded in a shadowy organic material which could correspond to the 59 wt.% of non-metal content in the sample. The energy dispersive X-ray spectrum of a selected area of the sample exhibited a molar Co:Al ratio of 1.09:1.00, confirming a nearly 1:1 stoichiometry of the nanoparticles.

151 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles

Figure 7.2. TEM image of hydrocarbon‐stabilised β‐CoAl colloids, diluted with THF.

7.2.4. Magnetic measurements The magnetic behaviour of the material, dispersed in vacuum grease and conditioned in a gelatine capsule, was investigated by SQUID. Figure 7.3 displays the hysteresis cycle recorded at 2 K. The magnetization saturates at 5 T at a value corresponding to 23 % of that of pure cobalt (horizontal line reported as a reference).[305] This points to a chemically disordered alloy, as magnetism in bulk CoAl is related to segregated or ‘out-of-lattice’ cobalt atoms.[306] After, exposure of the gelatine capsule to the open air for 1 h, the hysteresis cycle was recorded again (dotted line Figure 7.3). Surprisingly, the magnetisation measured at 5 T reaches nearly 70 % of the value expected for pure cobalt. To our knowledge it is the first time that air exposure of magnetic nanoparticles leads to an enhanced saturation magnetisation. This is in agreement with an increase in the ratio of segregated cobalt in the nanoparticles. Drastic exposure to air leads however to a complete collapse of the magnetisation, this time evidencing full oxidation of the sample. This was corroborated by WAXS experiments (see Figure 7.1), which show that direct exposure of the material to air leads to full oxidation of the sample. Noteworthy, for a better particle separation, the colloidal solution was treated post-synthesis with equimolar amounts of a) PPO, b) a 1:1 mixture of hexadecylamine (HDA) and oleic acid (OLEA) and c) 1-adamantanecarboxylic acid, over night at 150 °C under argon. Indeed, the TEM images show the improved particle dispersity. However, preliminary WAXS measurements of the isolated materials showed oxidation of cobalt.

152 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles

Figure 7.3. Magnetic measurements of the β‐CoAl powder sample under argon (black curve), after 1 h of exposition to air (red curve), and the reference magnetisation of bulk cobalt (dotted line).

7.3. Conclusion

In analogy to the work on nanoscaled intermetallic Cu-Al and Ni-Al phase particles, the β-CoAl phase was successfully synthesised by co-hydrogenolysis of the organometallic precursors in solution. As for the case of NiAl, the particles are soluble in aromatic solvents, without any surfactants present, which leads to the assumption that the Cp* moiety or its derivatives is involved in the stabilisation of the particles. Besides, it was shown that air oxidation, as it was performed in the case of β-NiAl and θ-CuAl2 particles, would not lead to a corrosion resistive Co core, but to CoO, instead. Alumina-passivated cobalt particles were obtained by a gentle oxidation of the sample in a porous capsule. The reason for the oxidation of cobalt upon air treatment could, of course, be the smaller diameter of the CoAl particles in comparison to Ni/Al and Cu/Al. From the average diameter of the nanoparticles it is possible to estimate the number of metal atoms. Taking into account the composition of the nanoparticles and assuming that the nanoparticles have the same capacity as the bulk, each nanoparticle should contain around 700 Co atoms and 700 Al atoms. In the ideal case, upon severe air oxidation, Al atoms would completely segregate to the surface and oxidise into an alumina shell. Assuming a spherical core particle with 700 Co atoms (d ~ 1 nm), the thickness of the Al2O3 shell would be around 0.6 nm (less than two full

153 7. Synthesis of colloidal intermetallic β−ΧοΑλ nanoparticles atomic layers of Al), which is clearly insufficient to insure the passivation of the cobalt core. However, the results reported here, clearly demonstrate the feasibility of the method. Preliminary studies show that the nanoparticles can be redispersed in organic solvents, in the presence of ligands such as oleic acid/oleylamine mixture. This opens a door to the deposition of this nanocomposite via spin-coating. However, more work is needed to control the core/shell type Al2O3@Co structure. In summary, this study demonstrates the possibility to form in organic solution small (3 nm) particles of CoAl alloy in the β-phase. These particles are very air sensitive, giving rise to a fast segregation upon exposure to air. The result of this process is a strong enhancement of the magnetic properties of the particles as a result of the formation of a cobalt core. However, the resulting protecting alumina shell is too thin for long term protection of the particles and full oxidation is then observed upon prolonged exposure to air. Further development of this work will have two main goals: i) to adapt the synthesis conditions in order to reach nanoparticles of larger size; in this case the Al content of each nanoparticle would be sufficient to ensure the formation of a thick alumina shell upon oxidation, hence be able to passivate the cobalt core from further air oxidation; ii) to produce a cobalt core embedded in an alumina shell at small size, which will require to increase the aluminum content in the initial alloy.

154 8. Summary and Outlook

8. Summary and Outlook

Synthesis of Cu1‐xEx nano‐powder and ‐colloids

This work presents a novel, clean approach to intermetallic Hume-Rothery Cu1-xZnx,

Cu1-xAlx, Ni1-xAlx and Co1-xAlx powder materials in organic solution by co-hydrogenolysis of organometallic all-hydrocarbon precursor complexes under rigorous oxygen-free conditions.

It was shown that the complexes [(AlCp*)4] and [ZnCp*2] readily give Al(0) and Zn(0) upon hydrogenolysis. These compounds were so far unknown as precursors for naked Al and Zn, respectively. In sharp contrast to all synthesis protocols for such classical Hume-Rothery phases in solution existing to date, the decomposition of the precursors quantitatively affords the metals under relatively soft reaction conditions. The organic byproducts are inert, and do not inhibit the alloy formation from the in situ released metals. This synthesis method allows a free variation of the Zn- and Al-content, respectively, and thus an access to both α- and β- phase materials (see below). The synthesis of colloidal alloys, however, depends on the transition metal and does not guarantee a general protocol using one particular surfactant so far. For the synthesis of (E = Al, Zn) alloy colloids, the polymer PPO was added as steric surfactant. However, the obtained nanoparticles were rather large and polydisperse. Other surfactants could not stabilise the alloy particles. In the case of β-MAl alloys (M = Co, Ni), the particles were stable as metastable colloidal solutions even without any surfactant present. The colloid stability was enhanced by post-synthetic addition of ACA, which however, partially oxidised the Al component. First experiments showed that the co-hydrogenolysis of a metal- and an Al-precursor in presence of an inert hydrocarbon polymer, such as polybutadiene (vide supra), gives - at least temporarily - stable colloidal solutions. The subsequent addition of a strongly binding containing capping ligand (e.g. carboxylic acids) is a promising perspective towards a widely applicable transition metal aluminide colloid procedure.

Oxidation of M1‐xEx alloys and core‐shell particle formation

The surface oxidation of M1-xEx alloy particles has been a particular investigation target of this work. The passivation of metal nanoparticles via Al oxidation of metal aluminide nanoparticles bears interesting perspectives, aiming at air-stable applied materials, e.g.

155 8. Summary and Outlook magnetic Ni- or Co-nanoparticles. In contrast to other post-synthetic particle coating methods, e.g. by silica coating by sol-gel processes or by noble metals, the in situ formation of an alumina (or ZnO) shell from a M1-xEx alloy is based the high oxophility of Al, and Zn, respectively, and thus appears to be a very elegant, non-aqueous method. It was shown that the synthesised M1-xAlx nanopowders indeed undergo a phase segregation upon air oxidation, which results in a core of the metal M, which is passivated by an alumina shell. This work offers first perspectives for the generation of air-stable metal nanoparticles from the respective alloys, as well as a study of their magnetic properties. It was shown that the reduction of the Al content in the alloy corresponds to a decrease of the concentration in the bulk core, which adopts the structural type of the pure metal. This is of special importance, since Al (or Zn) is ‘poisoning’ the metal in terms of magnetisation. Hence, this work also concentrated on the determination of the minimum Al content, which is required to form a full oxide shell upon oxidation of a α-phase alloy (Scheme 8.1).

Scheme 8.1. Access to air‐stable metal nanoparticles by in situ formation of an alumina shell upon selective Al‐ oxidation in α‐M1‐xAlx alloy nanoparticles. The cross section of the oxidised particle (right) indicates the full alumina shell.

The investigations of the oxidation behaviour of the synthesised M1-xEx nanoparticle powder and colloids have shown that there is so far no general method to produce core-shell type alumina@metal particles. The criteria for the formation of corrosion stable alumina@metal species appear to be the oxophility and the diffusion gradient of the transition metal, the particle size, and the oxidation procedure. In the case of Cu-Al alloys, the colloidal particles were rather large (~ 20 nm) and an interaction of the Cp* ligand with the particle surface was not observed. The oxidation produced a Cu core in an alumina shell, which is unaffected against oxidation down to 17 at.% Al. In contrast, the Ni1-xAlx powder particles exhibit a much smaller size of the primary particles (4-8 nm), and have an oxidation resistant Ni core at an even lower Al content than in the case of Cu-Al. In the β-CoAl nanoparticles (~2 nm), both Al and Co oxidise upon air exposure, even with a Al content of 50 at.%. Although the selective Al-oxidation of β-CoAl nanoparticles in a gelatine capsule does not affect the Co atoms, the observed oxidation sensivity of the cobalt component in the alloy is a high hurdle

156 8. Summary and Outlook to overcome towards a simple access to a dense protective layer of alumina@cobalt nanoparticles. A potential perspective might be the chemical oxidation by soft O-donors, such as trimethylamine oxide (Me3NO) or ortho-chlorperbenzoic acid, which have already been used for the synthesis of core-shell nanoparticles before.[309,310]

The oxidised Ni1-xAlx and β-CoAl nanoparticles show the potential of surface decorated metal nanoparticles, in terms of magnetisation and long-term air resistance, which is necessary for an industrial application of any of the nanoparticles mentioned here and in the introduction. First experiments have shown that Al2O3@Co nanoparticles exhibit a significantly higher magnetisation than non-oxidised β-CoAl, confirming the presented concept. A comprehensive investigation of the magnetisation of these materials, in dependence of the Al content and the comparison with bulk M reference data is certainly warranted. As well, the solubilisation of α-MAl alloys as colloids and the control over the particle shape and anisotropy, aiming at a general post-synthetic solubilisation procedure and subsequent oxidation studies represent promising future objects of study.

Novel perspectives in preparation of nano‐alloys

The obtained results suggest [(AlCp*)4] as an exotic, but quite interesting precursor to be generally applicable for wet-chemical nanometallurgy of late transition metal aluminides, for example extending the scope of B. Chaudret´s work[90-100] on metal colloids towards classical Hume-Rothery phases that have not been readily available so far. Following the work described above, an investigation of further transition metal aluminide nanoparticles is certainly warranted. Promising precursor candidates are [Pt(cod)2], [Pd2(dvds)3] (dvds = 1,3- divinyl-1,1,3,3-tetramethyldisiloxane), or [Ru(η4-cod)(η6-cot)], which all are bearing ‘innocent’ olefin ligands, which are supposed to be easily hydrogenated. First experiments have shown, that in the case of the decomposition of equimolar metal amounts of the above mentioned Pd- and Pt-precursors with [(AlCp*)4], binary α-phases of Pd-Al and Pt-Al, respectively, were obtained. Preliminary work on the synthesis of Ru-Al nanoparticles by 4 6 hydrogenolysis of [Ru(η -cod)(η -cot)] and [(AlCp*)4] in mesitylene led to the formation of 6 4 the complex [(η -mesitylene)Ru(η -cod)], which is quite stable under H2 pressure, which however, can be overcome by use of high boiling aliphatic solvents, which do not coordinate at the Ru centre. The synthesis of colloidal M-Al nanoparticles is on the borderline between molecular clusters of the type [Ma(AlCp*)b] (a ≥ b) and solid nanoalloys. Previous work has shown that

157 8. Summary and Outlook

[278] [Ni(cod)2] and [(AlCp*)4] readily react to the stable complex [Ni(AlCp*)4]. A decrease of the AlCp* concentration certainly leads to the formation of AlCp* stabilised Ni clusters of various sizes. It is reasonable to assume that during the co-hydrogenolysis of [Ni(cod)2] and

[(AlCp*)4], such clusters are formed, prior to the decomposition of the AlCp* ligands by H2. Thus, trapping of small clusters and the control over the cluster size is for sure of high interest for understanding the mechanisms of alloy formation by means of hydrogenolysis. Another convenient method to obtain M-E nanoparticles (M = late transition metal d8-d10, E = Al, Ga,

In) is the decomposition of known bimetallic [Ma(ECp*)b] clusters. First experiments have shown that some of these clusters can serve as precursors for a well-defined E-rich phase. For [164] example, the hydrogenolysis of the bimetallic Pt-cluster [Pt(µ-GaCp*)3(GaCp*)2] readily gives the intermetallic PtGa2 phase (Figure 6.1). The hydrogenolysis in presence of equimolar amounts of HDA gives a colloidal solution of monodisperse, spherical PtGa2 nanoparticles (diameter: 8 nm).

Figure 8.1. Synthesis of PtGa2 nanoparticles by hydrogenolysis of [Pt(μ‐GaCp*)3(GaCp*)2].

Regarding the successful use of [(AlCp*)4] and [ZnCp*2] as straightforward sources for the naked metals, it can be concluded, that the Cp* moiety is an advantageous leaving group being cleanly split off by hydrogenolysis. The question is raising, whether other important alloy components, which are also notoriously difficult to introduce by simple salt reduction, [175] can be accessed from other Cp* complexes of oxophilic metals, such as Jutzi’s [SiCp*2]

158 8. Summary and Outlook

The hydrogenation and subsequent cleavage of Cp*H, of course, depends on the nature of the polarity of the M-Cp* bond. Rather ionic bonds, e.g. in [MgCp*2] are too stable to be hydrogenated. Just to give an inspiring example, the unusual molecular SiAl14 cluster, [170] [(Cp*Al)6(SiAl8)] was obtained by employing [SiCp*2] as Si source.

The complex [(quinuclidine)GaH3] has been introduced as a precursor for the wet chemical synthesis of transition metal gallide nanoparticles. The synthesis of the θ-CuGa2 phase, which decomposes above 250 °C, nicely demonstrates the potential of the presented synthetic concept, namely the access to metastable alloy phases, which are inaccessible by other, traditional metallurgical routes. Another synthetic targets include the PtGa5 phase, [307] which is stable up to 180 °C, from [Pt(cod)2] and [(quinuclidine)GaH3]. Along these lines, [308] the complex [(pyridine)ZnH2] may also well be a promising candidate for the reaction with the metal complexes used in this work, as well as with the above mentioned noble metal hydrocarbon precursors, to M-Zn nanoparticles, extending the pioneering work of Bogdanović et al. on metal hydrides as precursors for alloys.

159 9. Experimental

9. Experimental

9.1. General considerations

9.1.1. Handling techniques under inert gas atmosphere All manipulations and chemical reactions were performed using Schlenk-line and glovebox techniques (argon atmosphere; H2O and O2 content < 1 ppm) and sealed Fischer- Porter vessels with and without a pressure gage, if not necessary (Andrews Glass, volume: 90 mL, see Figure 9.1). The Si-OH surfaces of all glass vessels, including NMR tubes, were silylated by thoroughly washing with boiling hot 1,1,1,3,3,3-hexamethyldisilaazane (99%, Acros) and subsequent washing with n-pentane, in order to remove the residual silylation reagent. Mesitylene, 1,4-dioxane and cyclohexene were dried by passing through a Schlenk frit filled with activated alumina (chromatography grade, Merck) and distilled subsequently. All other solvents used (toluene, n-hexane, n-pentane, THF) were dried, degassed and argon- saturated by using a continuous solvent purification system (MBraun; H2O content: ~ 1 ppm).

The NMR solvents (C6D6, d8-toluene, d12-mesitylene) were dried over activated molecular sieve (dried over weeks at 200 °C) and degassed by several freeze-pump-thaw cycles.

Figure 9.1. Fischer‐Porter vessels used for high pressure hydrogenolysis experiments (max. pressure ~ 10 bar).

160 9. Experimental

9.1.2. Purchased materials 17O-enriched water (enrichment grade 35-40 %) was purchased from Deutero GmbH. 1- adamantanecarbonyl chloride was purchased from Aldrich.

9.2. Syntheses of the organometallic precursors

The following precursors were synthesised according to literature: [262] • [(AlCp*)4] [179c] • [(Me3N)AlH3] [274] • [(quinuclidine)GaH3] [176] • [ZnCp*2] [311] • [CpCu(PMe3)] [269] • [{Cu(mesityl)}5] [312] • [Ni(cod)2] 4 3 [313] • [Co(η -C8H12)(η -C8H13)]

9.3. Instrumental details

9.3.1. Methods of characterisation of binary alloy nanoparticles

9.3.1.1. X-ray powder diffraction (XRD) and wide angle X-ray scattering (WAXS) X-ray powder diffraction is the most valuable method to characterise crystalline bulk materials with a crystallite size down to several nanometres. XRD allows a phase analysis, the lattice parameters and substance identification by comparison with compounds deposited in databases, such as JCPDS or ICSD. The principle of X-ray diffraction is based on the irradiation of a crystalline powder simple by monochromatic X-ray photons and the subsequent elastic scattering of the photons by atoms in a periodic lattice.[314,315] The scattered photons, which are in phase, exhibit a constructive interference (Figure 9.2). The angles, under which the photons leave the crystal, are characteristic for each crystal lattice plane. This dependence is described by Bragg’s Law (Equation 9.1):

λ = 2 ⋅ d ⋅ sin(θ ) Equation 9.1

161 9. Experimental where λ is the wavelength of the X-ray photons, d is the distance between two lattice planes of the same orientation in the crystal, and θ is the angle between the reflected X-ray and the lattice plane (Figure 9.2).

Figure 9.2. Schematic illustration of the diffraction of X‐rays on a lattice plane of a crystalline material (left), and illustration of the scattering vector in the reciprocal space (right).

The XRD pattern is measured with a stationary X-ray source and a movable detector, which scans the intensity of the diffracted radiation as a function of the angle 2θ between the incoming and the diffracted beams. The observed intensity maxima in the XRD pattern represent the lattice planes of the crystal geometry of the sample. The reflection width depends on the crystallinity, i.e. the long-range order of the sample, as well as on the size of the crystallites. The relation of the crystal size to reflection width is expressed by the Scherrer equation (Equation 9.2):

K ⋅ λ d = Equation 9.2 β ⋅ cos(θ )

where d is the average crystallite size, K is a constant, which varies for different particle shapes (for spherical particles K ~ 1), λ is the X-ray wavelength, β is the full width at half maximum of a reflection (in radian), and θ is the position of the reflection. However, with decreasing crystallite size, e.g. in nanoparticles, the reflections become broader, due to an incomplete destructive interference of scattered photons that are out of phase, and the FWHM cannot be accurately measured. Thus, for small particles (< 10 nm), the size determination via Scherrer’s equation is only a rough estimation. Alloyed materials or nanoparticles can be identified by their XRD reflections by comparison with reference data. If the crystallites become too small, they have no long-range order, and must be regarded as either amorphous or nanocrystalline. Consequently, the X-rays are not diffracted

162 9. Experimental in phase, and there is no constructive periodic interference of the reflected X-rays, but rather a diffuse scattering in all directions of space. Thus, the XRD pattern of such a sample would exhibit no reflections at all. For that reason, the wide angle X-ray scattering measurement technique can be employed to examine the structure of such materials with a crystallite size of less than 4 nm.[95d] The radius of the Ewald sphere of diffracted or scattered photons, respectively, is in reciprocal relationship to the irradiated X-ray wavelength (Figure 9.2, right). Thus, the samples are typically irradiated with photons of lower wavelengths, e.g. Mo-

Kα or synchrotron radiation, which causes a better resolution of scattering signals at low θ angles, which contain substantial structural information on the sample. In WAXS measurements, the scattered photons are detected in dependence of the scattering vector S (Equation 9.3).

4π sin(θ ) S = Equation 9.3 λ

The detector moves in equidistant S steps (unit: nm-1) in the reciprocal space to collect all scattered X-rays. The resulting scattering diagram in the reciprocal space, where the scattering vector is plotted against the intensity of the scattered X-rays, is equivalent to a diffraction pattern and can be converted to 2θ. If the particles are nanocrystalline, this pattern can already be used for a pattern structure search, as it is common in XRD data evaluation. For an extraction of structural information, such as particle size, the crystal lattice and atom-atom distances, the scattering contribution of the glass of the capillary, of other components within the sample (e.g. a polymer-, framework- or mesoporous matrix, or a colloid surfactant, in which the nanoparticle are embedded), and fluorescence cannot be neglected and must be subtracted from the total scattering intensity. After the data reduction steps, the reduced intensity function i(S) is obtained, which contains the interatomic distances in the sample. By Fourier transformation of i(S) (see Eq. 9.4), the Radial Distribution Function (RDF) F(r) is obtained, i.e. the occurrence diagram of the interatomic distances inside the nanoparticle.

Smax 2π F( r ) = S ⋅i( S )⋅ sin( r ⋅ S )ds Equation 9.4 r ∫ Smin

The obtained RDF exhibit a set of local maxima, which correspond to the metal-metal distances in the nano-crystallite. The coherence length of the RDF gives information of the particle size. The extracted RDF can be used for an identification of the sample by a

163 9. Experimental comparison with the calculated RDF of a reference sample.

Instrumental details All powder X-ray diffractograms were recorded by the author of this work on a D8-

Advance-Bruker-AXS-diffractometer (Cu-Kα-radiation: 1.54178 Å, accelerating voltage: 40 kV, heating current: 30 mA, scan step: 0.0141° 2θ) in Bragg-Brentano θ-2θ-geometry, using a Göbel mirror as monochromator and a position sensitive detector. The detector was calibrated to the reflections of crystalline α-Al2O3. The specimens (powder) were prepared in Lindemann-capillaries in the glovebox (diameter: 0.5, 0.7 or 1.0 mm). The capillaries were kept in the glovebox and flame-sealed prior to the measurements. The WAXS measurements were performed by Dr. Pierre Lecante at the Centre d’Elaboration des Matériaux et d’Etudes Structurales, CNRS, Toulouse. The samples were prepared in the glovebox in Lindemann-capillaries (diameter 1 mm) irradiated with graphite monochromated Mo-Kα-radiation (0.71069 Å), using a two-axis diffractometer. The data collection time was 20 h for a set of 457 measurements in the range of 0° < θ < 65°, for equidistant S values.

9.3.1.2. Transmission electron microscopy (TEM) Transmission electron microscopy is a straightforward technique to determine the size and shape of nanoparticles.[314,315] Beyond the visual observation of the size, geometry and dispersion of the primary particles, in case of multiphase materials, TEM can reveal information on the composition and further structural features, such as alloying or core-shell systems (contrast difference). Highly crystalline nanoparticles, which exhibit a long-range order of crystallinity, allow further structural characterisation. Today, the state-of-the art TEM instruments exhibit a level of magnitude in the interatomic range, so that the lattice planes of the material become visible, from which the lattice constants can be calculated. For example, in the case of crystalline β-phase alloy nanoparticles, the lattices constants are different from those of the pure metals (vide supra), so that the presence of alloyed particles can be evidenced by TEM. Besides, the TEM electron beam can be employed for diffraction on a selected particle area (SAED), which is, in principle analogous to X-ray diffraction. The obtained pattern appears as diffraction rings or distinct dots, which contain structural information of the measured sample. However, if the nanoparticles are amorphous or too small, no pattern will be observed.

164 9. Experimental

By energy dispersive X-ray spectrometry (EDX), the atoms in the specimen can be excited by the electron beam to release an X-ray photon upon relaxation, which is characteristic for all elements. The detected photon intensities of each element in the sample can be quantified, giving information on the quantitative composition of the sample.

Instrumental details TEM measurements were carried out by Dr. H. Parala on a Hitachi-H-8100 instrument

(Accelerating voltage up to 200 kV, LaB6-filament). High-resolution transmission electron microscopy studies were performed by Dr. A. Chemseddine and T. Hikov at the Hahn- Meitner Institute in Berlin on a Philips CM12 instrument with an accelerating voltage up to 120 kV. All TEM samples were prepared as diluted solutions or suspensions in toluene on carbon coated copper or gold grids (Plano).

9.3.1.3. X-ray absorption spectroscopy (XAS) X-ray absorption spectrometry (XAS) is a powerful tool for a structural characterisation of materials.[314,315] Whereas other X-ray diffraction and -scattering techniques require a long- range order of the specimen crystallites, XAS allows a determination of the local environment of an atom, which is, in the context of this work, of particular importance for the identification of alloyed materials in the nanoscale. The principle of XAS is based on the irradiation of atoms with highly monochromatic X-rays, which leads to absorption, as long as the binding energy Eb of the core electron is higher than the photon energy. In the case that the photon energy is sufficient to eject the electron from a core orbital, characteristic absorption edges appear, as shown in Figure 9.3. XAS studies are typically performed at 5-30 keV, which is for most atoms in the range of photoelectron emission of the K orbital. The scattered photoelectron itself can be scattered back from a neighbour atom. Since electrons have both particle and wave character, the emitted and backscattered photoelectrons interfere, which is observed in the fine structure oscillations, spanning a range of several hundred eV behind the edge. These oscillations contain structural information, since the photoelectrons are scattered back from either atoms of the same kind, but different lattice position, or different atoms, as it is the case in alloys or metal oxides. The resulting X-ray absorption spectrum exhibits two major areas (Figure 9.3), which are relevant in data evaluation: the X- ray absorption near edge structure (XANES), and the extended X-ray absorption fine structure (EXAFS).

165 9. Experimental

Figure 9.3. Illustration of a typical X‐ray absorption spectrum with an absorption edge for Eb = hν.

Each region contains important information on the local structure of the examined atom. The XANES gives information on the oxidation state of the measured atom and its coordination geometry. The EXAFS exhibits data on the type of nearest neighbour and the distance of nearest neighbour shells. Besides, for an identification of the measured material, EXAFS spectra can be compared with calculated EXAFS spectra of reference compounds. The raw data processing, however, is quite elaborate and first involves a number of pre-data reduction steps, e.g. removal of glitches or steps, energy calibration (by simultaneous measurement of a reference foil) and background subtraction. The EXAFS-function χ(k), which contains the structural information of the sample, is expressed by Equation 9.5:

2π χ( k ) = A ( k )⋅ sin(2kr + φ ( k )) , with k = 2m ()hν − E ∑ j j j h e b Equation 9.5 j

where k is the wavenumber of the photoelectron, h is the Planck constant, me is the mass of an electron, j is the label of the coordination shells around the electron-emitting atom, rj is the distance of the electron-emitting atom and the atoms in the jth shell, and φ(k) is the phase shift of the absorbing and the backscattering atoms. Aj(k) is the amplitude, i.e. the scattering intensity, caused from j coordination shells (Equation 9.6), which contains the most desirable information.

 − 2rj  exp   λ( k ) 2 2 2 Equation 9.6 Aj ( k ) = N j ⋅ ⋅ S0 ( k )⋅ Fj ( k )⋅ exp()− 2k ⋅σ j k ⋅ rj

th Nj is the coordination number of atoms in the j shell, S0 is the correction for relaxation

166 9. Experimental

th effects in the emitting atom, Fj is the backscattering factor of atoms in the j shell, λ is the mean free path of the photoelectron, and σ2 is the mean-squared displacement of atoms on the specimen. The radial distribution function of the EXAFS function χ(k) is obtained by a Fourier transformation, giving the information of the distance of the neighbour atoms of the different coordination shells.

Instrumental details The absorption edges of Ni (8333.0 eV), Cu (8979.0 eV) and Zn (9659.0 eV) were measured by Dr. M. W. E. van den Berg at Hasylab E4 station (DESY Hamburg, Germany). This beamline was equipped with a Si(111) double-crystal monochromator that was used to detune to 50% of the maximum intensity in order to exclude higher harmonics present in the X-ray beam. Samples were mixed with cellulose and pressed into wafers. Immediately preceding the recording of XAS spectra, samples were cooled rapidly to liquid nitrogen temperature. The spectra µ(k) were measured in transmission mode using ionisation chambers. A metal foil (between the second and the third ionisation chamber) was measured at the same time for energy calibration purposes. Data treatment was carried out using the software package VIPER.[316] For background subtraction a Victoreen polynomial was fitted to the pre-edge region. A smooth atomic background µ0(k), was evaluated using smoothed cubic splines. The radial distribution function FT[k2χ(k)] was obtained by Fourier 2 transformation of the k -weighted experimental function χ(k) = (µ(k)-µ0(k))/µ0(k) multiplied by a Bessel window. Duplicate spectra were recorded to ensure data reproducibility. The samples were diluted with polyethylene (Aldrich, spectrophotometric grade) or cellulose powder (Aldrich, grain size: ~ 20 microns).

9.3.1.4. X-ray photoelectron spectroscopy (XPS) XPS is a surface sensitive method of characterisation and yields information on the sample composition, the oxidation states of the elements in the specimen and, in some cases, mixtures of substances. The principle of XPS relies on the photoelectric effect, namely the irradiation of a sample by X-ray photons and the subsequent emission of electrons from core- near orbitals of an atom.[314,315] In spite of the penetration depth of the X-ray beam is in the range of several micrometres, only the photoelectrons of the first surface atomic layers can leave the solid into the continuum. If an atom absorbs a photon of the energy hν, a core

167 9. Experimental

electron with the binding energy Eb will be ejected. The irradiated photon energy is a sum of the binding and the kinetic energy of the emitted photoelectron. Thus, according to Eq. 9.7, the element-specific binding energy of a photoelectron can be determined by the measurement of its kinetic energy:

Ekin = hν − Eb −ϕ Equation 97.7

where φ is the work function of the spectrometer, i.e. the energy loss of the photoelectron between X-ray induced ejection and arrival at the detector of the spectrometer. The emitted photoelectrons have distinct binding energies, which are characteristic for each element, and thus serve as a fingerprint. Hence, XPS gives qualitative and quantitative information on the elemental composition of the measured sample. Binding energies are not only element specific, but also contain further information on the chemical environment and the oxidation state. For instance, alloyed metals exhibit a slight shift of the binding energy of the photoelectrons, compared to that of the pure metal. Also, the surface oxidation of one alloy component can be determined by the absence of the metal valence electrons and a resulting peak shift. Especially in the case of intentional oxidation of one alloy component, e.g. the oxidation of Al in M1-xAlx alloy particles to a core-shell type

(Al2O3)δ/2@M1-xAlx-δ, which aims at a protection of the metal M, XPS can give information on the oxidation state of the metal M. If it is still in the oxidation state zero, the alumina shell is dense enough to cover the entire particle.

Instrumental details X-ray photoelectron spectra of air sensitive samples were recorded by Dr. M. W. E. van den Berg on a SES 2002 Scienta spectrometer (monochromated Al-Kα radiation: 1486.6 eV) equipped with an inert gas sample transfer holder. The pass energies were 500 eV (survey) and 200 eV (region spectra). XPS measurements of air-oxidised samples were undertaken by Dr. O. Shekkhah on a modified ultrahigh vacuum Leybold MAX system, equipped with an

EA11 energy analyser and an X-ray twin anode (Al-Kα, Mg-Kα). The measurements were undertaken using Al-Kα radiation (1486.6 eV). The pass energy for the survey spectrum was set to 200 eV and for the region spectra to 46.1 eV. The spectra were calibrated to the O-KLL edge (binding energy 978 eV).

168 9. Experimental

9.3.1.5. UV-Vis spectroscopy Colloidal solutions of metals, such as Cu, Ag or Au exhibit a characteristic absorption of light in the visible range. This phenomenon is giving rise to the surface electrons (plasmons) in the valence band, which can best be described as an ‘electron gas’ at the particle surface. The irradiation with photons in the visible light spectrum leads to an excitation of collective modes of oscillation of the plasmon electrons. The absorption spectra of colloidal metals can be calculated using the classical Mie’s theory for colloidal particles (Equation 9.8),[238,317] where the absorption of a colloid is only dependent to the particle size and the dielectric properties of the particle and the surrounding medium.

18πNVε 3/ 2 ε κ = m 2 2 2 Equation 9.8 λ [ε1 + 2ε m ] + ε 2

λ is the wavelength of the absorbing radiation, V is the particle volume, N is the number of atoms in the particle and εm is the dielectric constant of the surrounding medium, which is assumed to be frequency independent. The values ε1 and ε2 are the real and imaginary parts of the complex dielectric function ε(ω) = ε1(ω) + i·ε2(ω) of the material (ω = angular frequency of the light). The wavelength of the absorption maximum itself, as well as the number of maxima depends on the particle size and shape (e.g. spherical vs. rodlike).[318] Regarding colloidal nanoparticles of Cu-, Ag or Au-alloys, UV-Vis spectrometry is a fine tool to observe the shift of SPR, for two reasons. First, the (surface) oxidation of a metal particle has an influence on the SPR, since the electronic state at the surface is changing. Hence, the SPR is shifting and loses intensity. Second, the SPR of a metal is changing, if the metal lattice is doped with another metal component (alloying). The position of the SPR, in comparison to that of the pure metal) is then dependent on the content of the second alloy component. Thus, alloy formation as such can be shown by the shift of the observed SPR in comparison to calculated spectra of the bulk alloys, and those of the pure metal colloids. Several groups have [57,70] [146,148,244] reported on the SPR shift in Cu1-xMx colloids (M = Au, Zn ), in dependence of the atomic percentage of M in the alloys.

Instrumental details All UV-Vis-spectra were recorded by the author of this work on a Perkin-Elmer Lambda 9 UV/Vis/NIR spectrometer. The samples were diluted with dry mesitylene or toluene and placed into quartz glass vials (1 cm path length) using a glovebox.

169 9. Experimental

9.3.1.6. Nuclear magnetic resonance (NMR) NMR spectroscopy has become a widely used analytical method to examine small powder nanoparticles as well as colloidal nanoparticles.[284] Above all, the study of the binding nature of surfactants or gas adsorption on the surface of small colloidal particles is the main field of application.[319] NMR can also be used for an identification of the (metallic) components of a nanoparticle. The resonance shift of metallic samples, known as the Knight shift,[320] gives rise to the interaction of the metal nucleus with the conducting electrons and is measured as a fraction of the applied magnetic field. NMR is a feasible method to examine the qualitative structure of alloyed particles, or to verify the alloying as such. Alloyed samples, for instance metal aluminide alloys usually distribute a different 27Al-NMR Knight shift than that of the pure Al-metal or Al2O3. These solid materials can be measured by solid-state NMR.[321] However, solid samples consist of many crystallites that have random anisotropic orientations, and thus not equally orientated upon application of a magnetic field. Dipole-dipole- and quadruple-field-gradient interactions cannot be neglected. This leads to an extreme signal broadening, in contrast to high-resolution NMR in solution, where the resonances adopt an average frequency, due to a rapid tumbling of the molecules. In the solid state, the reorientation of nuclei can be simulated by magic-angle-spinning-NMR. The heteronuclear interaction Hamiltonian, which describes the dependence of the chemical shift and the dipolar coupling from the angle of orientation to the magnetic field, is given in Equation 9.9:

ˆ hetero  µ0  γ I ⋅γ S ⋅ h ˆ ˆ 2 H DD = −  ⋅ ⋅ I z ⋅ Sz ⋅ (3cos (θ ) −1) Equation 9.9  4π  r3

where m0 is the magnetic momentum of the dipole, γI and γS are the magnetogyric ratios of the ˆ ˆ spins I and S, IZ and SZ are the operators of the z component of each nuclear spin, r is the distance between the two dipoles, and θ is the orientation angle of the dipole upon application of a magnetic field. If the term [3cos2(θ)-1] = 0, then the contribution of the dipole-dipole interaction is zero, which means that the line broadening effect of dipolar coupling is removed. The angle θ, for which the above term is zero, is called the Magic Angle (θ = arcos(3-1/2) ≈ 74.74°). As a consequence, fast spinning of a solid sample (> 5 KHz) around an axis in a magic angle of 54.74° from the external magnetic field, thus averaging the anisotropy of the nuclei and the dipole-dipole coupling, and resulting in line sharpening, which is comparable to high resolution NMR in solution.

170 9. Experimental

Instrumental details

High resolution nuclear magnetic resonance in solution All high resolution NMR spectra in solution were recorded by the author of this work on a Bruker DPX 250 spectrometer or a Bruker DRX 400 spectrometer, using C6D6, d8-toluene and d12-mesitylene as locking solvents (T = 25 °C, 1H: 250.1 MHz, 13C: 62.9 MHz, 27Al: 31 17 65.2 MHz, P: 101.2 MHz, O: 33.9 MHz). The chemical shifts (in δ ppm) are referenced to 1 13 the residual proton signals of the deuterated solvent (C6D6: H: 7.15 ppm, C: 128 ppm; d8- toluene: 1H: 2.09 ppm, d12-mesitylene: 1H: 6.67 and 2.16 ppm) or the probe head (27Al: 68 17 27 31 ppm) The O-, Al- and P-NMR spectra are internally referenced to H2O (0 ppm), 3+ [Al(H2O)6] (0 ppm) and H3PO4 (0 ppm), respectively. In situ NMR reactions and high pressure hydrogenolysis reactions were done using pressure stable NMR tubes with Young PTFE screw caps (Wilmad-LabGlass, Figure 9.4).

Figure 9.4. NMR tubes used for experiments under atmospheric pressure under argon (top) and for high pressure hydrogenolysis reactions (bottom, max. pressure ~ 10 bar).

Solid state nuclear magnetic resonance The solid state MAS-NMR measurements were recorded by. H.-J. Hauswald on a Bruker DSX 400 spectrometer with a magnetic field of 9 T. The samples were prepared in 2.5 27 and 4 mm ZrO2 rotors at 104.2 MHz ( Al) with a rotation frequency of 20 kHz, unless otherwise stated. All samples were diluted with SiO2 to separate the metallic grains, in order to avoid magnetisation of the sample due to development of current of metal surface electrons as a consequence of the high rotation.

9.3.2. IR spectroscopy All IR samples were diluted with KBr (dried overnight at 10-3 mbar and 100 °C) and pressed as pellets (Ø = 13 mm). The measurements were carried out by the author of this work

171 9. Experimental on a Perkin-Elmer 1720 X Fourier-Transform-spectrometer, with a resolution of 4 cm-1.

9.3.3. Mass spectrometry The electron ionisation (EI) mass spectrum of 17O-enriched 1-adamantanecarboxylic acid was performed on a Varian MAT CH5 spectrometer. All detected peaks are positive ions.

9.3.4. Gas chromatography-mass spectrometry (GC-MS) GC-MS measurements were performed by A. Ewald on a Shimadzu GCMS-QP2010 with an automatic sample inlet AOC-20i. The sample separation was performed via a FS-OV- 1-CB-0.25 column (length: 25 m, inner diameter: 0.25 mm, outer diameter: 0.36 mm, film thickness of adsorbing material: 0.25 µm). The injection temperature was 290 °C and the column temperature was increased from 60 to 290 °C within 25 minutes. The separated substances were passed to a quadrupole mass spectrometer.

9.3.5. Elemental analysis The analysis for the metal content of the Ni-Al and Co-Al samples was undertaken by the Südchemie AG (Bruckmühl), using a Spectro Modula inductively coupled plasma optical emission spectrometer (ICP-OES) instrument. The samples were dissolved in 5 mL conc. HCl (37 wt.%) and diluted with water to 100 mL. The metal content (Al: 396.152 nm, Ni: 352,454 nm, Co: 228.616 nm) was determined by comparison with calibration samples (Al: 5, 10, 20 and 30 ppm, Ni: 20, 40, 80 and 160 ppm, Co: 10, 20 and 30 ppm). The C,H content was measured with an Elementar Vario 3 atom absorption spectrometer (AAS). The samples were combusted at 1200 °C with oxygen. The combustion gases are separated by first passing through a Cu-column in a He stream (filtration of O2 and NOx), then subsequent passing through three adsorption columns (N2, CO2, H2O then SO2), followed by a sequential heating of these columns to desorb the gases, which are detected by a heat conductivity detector and calibrated vs. sulfanilic acid to calculate the weight content. The analysis for the metal content of the Al, Cu, Cu-Zn and Cu-Al samples was undertaken at the Ruhr-University Bochum, using a Vario 6 AAS instrument. The samples were dissolved in aqua regia.

172 9. Experimental

9.3.6. Magnetism of nanoparticles Magnetism and magnetic phenomena describe the orientation of a magnetic dipole (e.g. the electronic spin) by application of an external magnetic field. In general, magnetic materials (e.g. metals) can be roughly classified into diamagnetic, ferromagnetic, paramagnetic, as well as ferrimagnetic and antiferromagnetic. Diamagnetic materials do not have magnetic dipoles in the absence of an external magnetic field. In presence of an external field, the dipoles are weakly repulsed. A paramagnet has randomly orientated dipoles, which are aligned upon application of an external field. Ferromagnetic materials (e.g. Fe, Co, Ni) always exhibit magnetic dipoles, even without an external magnetic field. In ferri- and antiferromagnetic materials, the adjacent dipoles are aligned antiparallel without an external field. Whereas the dipoles of an antiferromagnet completely cancel each other, in a ferrimagnet, one dipole is stronger than the adjacent.[3,5,10] Usually, the magnetisation M of a ferromagnet is measured as a function of an increasing and decreasing applied magnetic field H, giving a typical hysteresis loop (see Figure 9.4), which contains information of the magnetic properties of the material, such as the maximal saturation magnetisation Ms, the remanence magnetisation Mr (residual magnetisation at H = 0, indicator for the hardness of the magnet) and the external field Hc (coercivity), which is required to bring the magnetisation back to zero. A bulk ferromagnetic material is considered to consist of multiple oppositely aligned domains separated by domain walls, in order to minimise the magnetostatic energy, which is proportional to the material volume.

Scheme 9.4. Typical magnetisation curve of a ferromagnetic material. Graph taken from ref. [5b]

173 9. Experimental

Thus, restructuring the material into more and more domains would into infinite small domains, which would however, lead to an increase of the domain wall energy, which is proportional to the boundary surface between the domains. Thus, the material tends to build walls of finite thickness (i.e. a finite number of domains), which is known as the finite-size effect. Below a critical size (~ 10-20 nm), the particles change from a multiple magnetic domain of the bulk to a state where each nanoparticle is a single magnetic domain, where all spins are orientated to the same direction. Since there are no domain walls to overcome, magnetisation reversal occurs by spin rotation, which is the reason for a general high coercivity of nanoparticles. A decrease of the particle size, leads to an increase of the thermal energy versus the energy, which keeps all spins in a single domain orientated in one direction, resulting in a loss of spin orientation. If the orientation changes faster than it can be observed on the experimental timescale, the particles are considered as superparamagnetic and no hysteresis will be observed. Below a certain temperature, the blocking temperature (Tb), the thermal energy is not sufficient to flip the spins which are ‘blocked’ in one direction. Superparamagnetic nanoparticles find an increasing application in data storage devices, but also in biomedicine (MRI) or catalysis.[3,5]

Instrumental details The SQuID measurements were undertaken at the Laboratoire de Chimie de Coordination, CNRS, Toulouse on a MPMS 5 Quantum Design magnetometer. The hysteresis circles were recorded with a magnetisation up to 5 T. The CoAl samples, which were prepared in porous gelatine capsules in the glovebox, were transferred under inert gas to the magnetometer. After the measurement, the sample was exposed to air, which slowly penetrated through the capsule membrane and oxidising the sample. After 1 h of air oxidation, another measurement was carried out.

174 9. Experimental

9.4. Syntheses of the materials

9.4.1. Intermetallic Cu-Zn nanoparticles

9.4.1.1. Synthesis of nano-Cu powder from [CpCu(PMe3)]

In a Fischer-Porter-bottle, 0.100 g [CpCu(PMe3)] (0.488 mmol) were dissolved in mesitylene (10 mL). The colourless solution was degassed and set to 3 bar hydrogen pressure. The bottle was then placed into a 150 °C hot oil bath. After 5 min, the solution became darker and soon after, a red-brown precipitate of elemental copper formed. The mixture was stirred for 1 h to ensure full decomposition of the precursor. After cooling to room temperature, the light yellow solution was filtered, and the residue washed with 2 x 10 mL n-pentane and dried at 25 °C in dynamic vacuo (10-3 mbar). Yield: 31 mg (100 %). AAS: Cu, 100 %. XRD reflections (2θ): 43.31° (111), 50.34° (200), 74.24° (220), 90.05° (220).

This reaction was also performed in a pressure stable NMR tube. 0.005 g [CpCu(PMe3)]

(0.024 mmol) were dissolved in 0.7 mL d12-mesitylene, pressurised with 4 bar H2 (0.26 mmol) and heated to 150 °C. After 5 minutes, Cu(0) precipitated. NMR spectra were recorded after 1 h at 150 °C. 1H NMR (250.1 MHz, 25 °C): δ 6.49 (m, 2 H, meta-H), 6.32 (m, 2 H, 2 31 1 ortho-H), 2.75 (q, 2 H, ipso-H), 0.83 (d, 9 H, P(CH3)3, JP-H = 7.687 Hz). P{ H} NMR

(101.2 MHz, 25 °C): δ -62.9 (P(CH3)3).

9.4.1.2. Cu colloids from [CpCu(PMe3)] and PPO as surfactant

In a Fischer-Porter-bottle, 0.100 g [CpCu(PMe3)] (0.488 mmol) and 0.310 g PPO were dissolved in mesitylene (20 mL). The colourless solution was degassed and set to 3 bar H2. The bottle was then placed into an oil bath set to 150 °C. After 5 min the solution became light red, soon after, the characteristic wine red solution of Cu colloids was observed. The solution was stirred at 2 h at 150 °C. After cooling to room temperature, the solution was transferred into a Schlenk tube. XRD reflections (2θ): 43.33° (111), 50.44° (200), 74.22° (220), 90.03° (220).

9.4.1.3. Synthesis of nano-Zn powder

Analogously to the procedure above, 0.100 g [ZnCp*2] (2, 0.298 mmol) was dissolved

175 9. Experimental

in mesitylene (10 mL), set to 3 bar H2 pressure and placed into a 150 °C hot oil bath. After 10 min, the solution became darker, followed by a rapid formation of elemental zinc. The mixture was stirred for 1 h. After cooling to room temperature, the colourless solution was filtered, and the grey residue washed with 2 x 10 mL pentane and dried in vacuo. Yield: 19 mg (100 %). AAS: Zn, 100 %. XRD reflections (2θ): 36.31° (002), 38.99° (100), 43.24° (101), 54.41° (102), 70.16° (103), 70.68° (110), 82.14° (112), 86.61° (201). 1H-NMR

(filtrate): δ 1.81 (s, 6 H, Cp*H, meta-CH3), 1.73 (s, 6 H, Cp*H, ortho-CH3), 0.99 (d, 3 H,

Cp*H, ipso-CH3).

9.4.1.4. Synthesis of nano β-CuZn powder

In a Fischer-Porter-bottle, 0.100 g of [CpCu(PMe3)] (0.488 mmol) and 0.164 g

[ZnCp*2] (0.488 mmol) were dissolved in mesitylene (20 mL). The yellow solution was degassed and set to 3 bar H2. The bottle was then placed into a 150 °C hot oil bath. After 5 min the solution became light red. After 10 min, a metallic golden precipitate on the glass walls was visible. The mixture was stirred for 1 h. After cooling to room temperature, the yellow solution was decanted by means of cannulation, and the gold-brown solid was washed with 2 x 10 mL pentane and dried in vacuo. Yield: 63 mg (100 %). AAS: Cu, 49.6 %; Zn, 50.4 %. XRD reflections (2θ): 43.30° (110), 62.97° (200), 79.49° (211).

9.4.1.5. Synthesis of PPO stabilised Cu1-xZnx colloids (0.09 ≤ x ≤ 0.50) In a Fischer-Porter-bottle, a sample of PPO (Table 9.1) was dissolved in hot mesitylene

(40 mL) and thereafter, 0.100 g [CpCu(PMe3)] (0.488 mmol) was added. Then, [ZnCp*2] was added. The exact masses of PPO and [ZnCp*2] needed for the desired Cu1-xZnx material are given in Table 9.1. The bright yellow solution was degassed and set to 3 bar hydrogen pressure. The bottle was then placed into a 150 °C hot oil bath. After 5 min the solution became light red and soon after, a deep red solution formed, in the case of Cu0.50Zn0.50, the solution was deep violet. The solution was stirred for 16 h at 150 °C in order to ensure annealing. After cooling to room temperature, the solution was transferred into a Schlenk tube. EDX analysis (± 4 % relative error): x = 0.09: 91.4 at.% Cu, 8.6 at.% Zn, x = 0.17: 83.9 at.% Cu, 16.1 at.% Zn, x = 0.33: 55.7 at.% Cu, 34.3 at.% Zn, x = 0.50: 49.4 at.% Cu, 50.6 at.% Zn. XRD reflections or the deviations from the ideal fcc-Cu structure are given in Table 4.6 (p. 55).

176 9. Experimental

Table 9.1. Masses of [ZnCp*2] and PPO required for the target Cu1‐xZnx colloid.

Cu1‐xZnx m([CpCu(PMe3)] m(Cu) m([ZnCp*2]) m(Zn) m(PPO) 0.100 g 0.016 g x = 0.09 31 mg 3.1 mg 341 mg (0.488 mmol) (0.049 mmol) 0.100 g 0.033 g x = 0.17 31 mg 6.4 mg 375 mg (0.488 mmol) (0.098 mmol) 0.100 g 0.082 g x = 0.33 31 mg 16.0 mg 470 mg (0.488 mmol) (0.244 mmol) 0.100 g 0.164 g x = 0.50 31 mg 31.9 mg 620 mg (0.488 mmol) (0.488 mmol)

9.4.1.6. Oxidation of PPO stabilised Cu1-xEx colloids (E = Zn, Al; 0.09 ≤ x ≤ 0.50)

In order to oxidise Cu1-xEx/PPO colloids (0.09 ≤ x ≤ 0.50), the solutions, synthesised as described above, were transferred from the Fischer-Porter bottle into a Schlenk tube on air. The open Schlenk was left open over a period of 24 h. Thereafter, 100 mL n-pentane was added, whereupon the particles immediately precipitated. Alternatively, the particles can be precipitate by cooling at ~0° overnight in a closed Schlenk. After allowing the precipitate to settle, the supernatant was decanted and the particles washed with 3 x 100 mL n-pentane, in order to ensure the complete removal of mesitylene. Subsequently, the particles were dried in vacuo at room temperature.

9.4.2. Intermetallic Cu-Al nanoparticles

9.4.2.1. Nano-aluminium from [(AlCp*)4]

A sample of [(AlCp*)4] (1.000 g, 1.541 mmol) was suspended in 10 mL mesitylene at

25 °C, pressurised with 3 bar H2 and heated to 150 °C (oil bath). After few minutes,

[(AlCp*)4] was dissolved completely to give a yellow solution. After 15 minutes, a black finely dispersed suspension formed. The mixture was stirred for 1 h, whereupon the solution became colourless and the black solid precipitated. After cooling to 25 °C, the supernatant was decanted under argon and the highly pyrophoric Al powder was washed with pentane (3 x 10 mL) and dried in vacuo. Yield: 0.160 g (100 %). AAS: Al, 100 %. 27Al-MAS-NMR 0 (104.2 MHz, 25 °C, ν = 10 KHz): δ 1639 (Al ), 56 (AlO4), 1 (AlO6). XRD reflections (2θ): 38.53 (hkl 1 1 1; Int. 100 %), 44.81 (2 0 0, 39 %), 65.25 (2 2 0, 22 %), 78.36 (3 1 1, 20 %), 1 82.58 (2 2 2, 6 %). H-NMR (filtrate): δ 1.81 (s, 6 H, Cp*H, meta-CH3), 1.74 (s, 6 H, Cp*H, ortho-CH3), 0.99 (d, 3 H, Cp*H, ipso-CH3).

177 9. Experimental

i 9.4.2.2. Reaction of [(AlCp*)4] with Bu2AlH

[(AlCp*)4] (0.500 g, 0.771 mmol) was dissolved in mesitylene (30 mL) at 150 °C and i treated with a 1 M solution of Bu2AlH in THF (3.1 mL, 3.100 mmol). Immediately, a black solid precipitated. The mixture was stirred for 16 h at 150 °C. After cooling down to room temperature, the solution was filtered and the black solid was washed with pentane (2 x 20 mL), dried and identified as Al by XRD. Yield: 0.055 g (2.055 mmol, referring to 66 % yield i supporting the stochiometry of Scheme 3.3). For a quantitative detection of [Cp*Al Bu2],

Cp*H and isobutane, a NMR reaction was performed: [(AlCp*)4] (0.050 g, 0.077 mmol) and i Bu2AlH (0.308 mL, 1 M solution in toluene) were suspended in d8-toluene (1 mL) and heated at 120 °C for 2 h. After few minutes, Al(0) precipitated. 1H-NMR (250.1 MHz, 25 °C): i δ 1.86 ppm (s, 15 H, C5(CH3)5-Al( Bu)2), 0.98 (m, 12 H, Cp*Al{CH2-CH-(CH3)2}), -0.13 (m,

4 H, Cp*Al{CH2-CH-(CH3)2}) 1.78 ppm (d, 3 H, Cp*H, meta-CH3) and 1.71 ppm (d, 3 H, 27 Cp*H, ortho-CH3), 1.08 ppm (m, 9 H, CH(CH3)3). Al-NMR (65.2 MHz, 25 °C): δ -86.9 i ([Cp*Al Bu2]).

9.4.2.3. Synthesis of Al nanoparticles from [(Me3N)AlH3]

In a Fischer-Porter bottle, 0.100 g [(Me3N)AlH3] (1.122 mmol) were dissolved in 10 mL mesitylene. The bottle was degassed in vacuo, set to 3 bar H2 pressure and placed into a 150 °C hot oil bath. After 1 h, a grey precipitate formed, which was allowed to settle in the glovebox. The colourless supernatant was carefully removed with a syringe. The residue was washed with 3 x 20 mL n-pentane and dried in vacuo (10-3 mbar) Yield: 26 mg (86 %). AAS: 27 0 99.7 wt.% Al. Al-MAS-NMR (104.2 MHz, 25 °C, ν = 20 KHz): δ 1640 (Al ), 56 (AlO4), 1

(AlO6).

This reaction was also performed in a pressure stable NMR tube. 0.010 g [(Me3N)AlH3]

(0.112 mmol) were dissolved in 0.7 mL d12-mesitylene, pressurised with 3 bar H2 and heated to 150 °C. After 5 minutes, Al(0) precipitated. NMR spectra were recorded after 1 h at 150 1 °C. H NMR (250.1 MHz, 25 °C): δ 2.11 (s, 9 H, N(CH3)3).

9.4.2.4. Synthesis of Cu powder from [{Cu(mesityl)}5]

A yellow solution of 0.100 g [{Cu(mesityl)}5] (0.547 mmol) was treated with 3 bar H2 in a Fischer-Porter bottle and heated at 150 °C. After 15 min, a red-brown, metallic shiny solid formed. The reaction was kept stirring for 1 h. Thereafter, the reaction was worked up as

178 9. Experimental described above. Yield: 0.031 g (89 %). AAS: Cu, 99 wt.%. XRD reflections (2θ): 43.30° (111), 50.48° (200), 74.14° (220), 90.06° (311), 95.21° (222).

9.4.2.5. Synthesis of θ-CuAl2 from [CpCu(PMe3)] and [(AlCp*)4]

In a Fischer-Porter bottle, [CpCu(PMe3)] (0.159 g, 0.245 mmol) and [(AlCp*)4] (0.100 g, 0.488 mmol) were combined in 10 mL mesitylene, treated with 3 bar H2 and set into a 150 °C hot oil bath. After few minutes, the yellow solution became darker and after 15 min, a grey precipitate formed. After 3 h of stirring at 150 °C the precipitated grey CuAl2 powder was isolated as described above. Yield: 0.055 g (96 %, according to Scheme 5.4). AAS: 38.6 wt.% Cu, 42.7 wt.% Al. Since the product contained impurities, according to XRD, the powder was washed with 3 x 50 mL 1,4-dioxane and 3 x 50 mL n-pentane. AAS: 45.1 wt.% Cu, 41.7 wt.% Al. EDX analysis: Al, 69 %; Cu, 31 %. 27Al-MAS-NMR (104.2 MHz, 25 °C, ν = 10

KHz): δ 1486 (Knight shift of Al(0) in CuAl2), 56.1 and 1.9 (Al2O3). XRD data are listed in Table 5.3 (p. 94).

The reaction was repeated in a pressure stable NMR tube. 0.005 g [CpCu(PMe3)] (0.024 mmol) and 0.008 g [(AlCp*)4] (0.012 mmol) were dissolved in 0.7 mL d12-mesitylene, pressurised with 4 bar H2 and heated to 150 °C. After 5 minutes, CuAl2 precipitated. NMR spectra were recorded after 1 h at 150 °C. 1H-NMR (250.1 MHz, 25 °C): δ 6.49 (m, 2 H, CpH, meta-H), 6.32 (m, 2 H, CpH, ortho-H), 2.75 (m, 2 H, ipso-H), 1.81 (s, 6 H, Cp*H, meta-

CH3), 1.74 (s, 6 H, Cp*H, ortho-CH3), 0.99 (d, 3 H, Cp*H, ipso-CH3), 0.81 (d, 9 H, P(CH3)3, 2 31 1 JP-H = 2.634 Hz). P{ H} NMR (101.2 MHz, 25 °C): δ -62.9 (P(CH3)3), -37.7 (unknown byproduct).

9.4.2.6. Synthesis of Cu1-xAlx powder from [CpCu(PMe3)] and [(Me3N)AlH3] (x = 0.67, 0.50, 0.31)

In a Fischer-Porter bottle, 0.224 g [CpCu(PMe3)] (1.094 mmol) and [(Me3N)AlH3] were dissolved in 20 mL mesitylene. The mass amounts of Al-precursor required to synthesise the respective Cu-Al phase are given in Table 9.2. The precursors reacted immediately, whereupon a grey solid formed. In order to insure completition of the reaction, the bottle was set to 3 bar H2 pressure and placed into a 150 °C hot oil bath and stirred for 24 h. The residue was washed with 3 x 20 mL of hot 1,4-dioxane to remove the P-containing byproduct, and then washed with 3 x 20 mL n-pentane. Subsequently, the grey powder was dried in vacuo

179 9. Experimental

(10-3 mbar). The yields are listed in Table 9.2. Data for x = 0.67: AAS: 38.6 wt.% Cu, 42.7 wt.% Al. EDX analysis: 30 at.% Cu, 70 at.% Al. 27Al-MAS-NMR (104.2 MHz, 25 °C, ν = 20

KHz): δ 1492 (Al), 31 (Al2O3). XRD data are listed in Table 5.3 (p. 94).

Table 9.2. Required masses of [(Me3N)AlH3)] and [CpCu(PMe3)] for the synthesis of the Cu1‐xAlx phases.

Cu1‐xAlx m([(Me3N)AlH3)]) Elemental analysis Yield

x = 0.67 195 mg (2.819 mmol) Cu, 42.7; Al, 38.6 103 mg (80 %) x = 0.50 98 mg (1.095 mmol) Cu, 50.9; Al, 23.1 72 mg (73 %) x = 0.31 43 mg (0.482 mmol) Cu, 50.9; Al, 23.1 58 mg (70 %)

9.4.2.7. Synthesis of Cu1-xAlx powder from [{Cu(mesityl)}5] and [(Me3N)AlH3] (x = 0.67, 0.50, 0.31)

In a Fischer-Porter bottle, to a solution of 0.200 g [{Cu(mesityl)}5] (1.094 mmol) in 10 mL mesitylene, a solution of [(Me3N)AlH3] in 10 mL mesitylene was added via a syringe, whereupon the colour of the solution in all cases turned dark red. The required masses for the respective Cu1-xAlx phase are given in Table 9.3. The solution was degassed, set to 3 bar H2 and heated 3 days at 150 °C, whereupon a grey, metallic solid precipitated. The colourless supernatant and the residue were separated as described above. The yields are given in Table 9.3. Data for x = 0.67: EDX analysis: 33 at.% Cu, 67 at.% Al. 27Al-MAS-NMR (104.2 MHz, 25 °C, ν = 20 KHz): δ 1495 (Al). XRD reflections are given in Table 5.3 (p. 94). Data for x = 27 0.50: Al-MAS-NMR (104.2 MHz, 25 °C, ν = 20 KHz): δ 645 (Al), 21 (Al2O3). XRD reflections (2θ): 15.61° (001), 23.55° (-110), 25.20° (-111), 30.89° (-311), 44.62° (-511), 63.97° (-622), 76.69° (800), 82.12° (-333). Data for x = 0.31: EDX analysis: Al, 70 %; Cu, 30 %. XRD reflections (2θ): 22.72° (210), 25.08° (211), 30.66° (300), 35.85° (222), 44.14° (330), 49.00° (332), 51.36° (422), 64.26° (600), 73.61° (631), 75.74° (444), 77.64° (550), 81.29° (721), 88.40° (651), 92.06° (741), 97.50° (660).

Table 9.3. Required masses of [{Cu(mesityl)}5] and [(Me3N)AlH3)] for the synthesis of the Cu1‐xAlx phases.

Cu1‐xAlx m([(Me3N)AlH3)]) Elemental analysis [%] Yield

x = 0.67 195 mg (2.819 mmol) Cu, 46.9; Al, 42.0 113 mg (88 %) x = 0.50 98 mg (1.095 mmol) Cu, 50.9; Al, 23.1 86 mg (87 %) x = 0.31 43 mg (0.482 mmol) Cu, 70.3; Al, 13.4 76 mg (92 %)

180 9. Experimental

9.4.2.8. Synthesis of Cu1-xAlx colloids from [CpCu(PMe3)] and [(AlCp*)4] (0.10 ≤ x ≤ 0.50)

Samples of [CpCu(PMe3)] and [(AlCp*)4] with the desired molar ratio (0.10 ≤ x ≤ 0.50), e.g. x = 0.33 which refers to 0.025 mmol Cu and 0.244 mmol Al, were combined in mesitylene (40 mL). A sample of PPO (0.188 g) was added and the mixture was treated with stirring under 3 bar H2 at 150 °C for 16 h. From the deeply red coloured colloidal solutions, the PPO-stabilised Cu1-xAlx particles were quantitatively precipitated by addition of n-hexane (100 mL), washed with n-hexane and quantitatively redispersed in toluene or mesitylene again and characterized by TEM-EDX. EDX analysis for the composition Cu1-xAlx: x = 0.50, found: Al, 0.54; Cu, 0.46. x = 0.33, found: Al, 0.30; Cu, 0.70. x = 0.17, found: Al, 0.19; Cu, 0.81. x = 0.10, found: Al, 0.08, Cu, 0.92. XRD data: Table 5.5, p. 109.

9.4.2.9. Synthesis of the θ-CuGa2 phase from [{Cu(mesityl)}5] and [(quinuclidine)GaH3]

Samples of [{Cu(mesityl)}5] (0.050 g, 0.273 mmol) and [(quinuclidine)GaH3] (0.100 g, 0.547 mmol) were combined in a Fischer-Porter bottle. When the two solids were shaken together, the colorless to pale yellow mixture turned slightly brownish black indicating an apparent reaction. Addition of mesitylene (10 mL) to this mixture resulted in a reddish brown homogeneous solution with vigorous gas evolution. The bottle was placed in an oil bath at 150 °C and within 5-10 min, a black solid precipitated. After 4 h of heating, the solution was cooled, the supernatant was carefully decanted, and the black precipitate was washed with n- pentane (3 x 20 mL) and dried in vacuo. Yield: 0.047 g (87 %). XRD reflections (2θ): 15.14° (001), 30.62° (002), 31.54 (100), 35.17° (101), 44.54° (102), 45.25° (110), 46.67° (003), 47.98° (111), 55.65° (112), 57.49° (103), 63.78° (004), 65.98° (200), 67.06° (113), 68.14° (102), 72.94° (104), 74.47° (202), 75.01° (210), 77.04° (211).

The reaction was repeated in a pressure stable NMR tube. 0.010 g [{Cu(mesityl)}5] (0.055 mmol) and 0.020 g [(quinuclidine)GaH3] (0.110 mmol) were dissolved in 0.7 mL d12- mesitylene. After 5 minutes, θ-CuGa2 precipitated. In order to ensure a completition of the 1 reaction, the mixture was heated to 150 °C for 1 h. H-NMR (250.1 MHz, 25 °C): δ 4.61 (H2),

2.75 (m, 6 H, HC(CH2CH2)3N), 1.54 (sept, 1 H, HC(CH2CH2)3N), 1.36 (m, 6 H,

HC(CH2CH2)3N).

181 9. Experimental

9.4.3. Intermetallic Ni-Al nanoparticles

9.4.3.1. Synthesis of Ni1-xAlx nanoparticles (0.09 ≤ x ≤ 0.50)

For the synthesis of Ni1-xAlx nano-powder, in a Fischer-Porter bottle, 0.200 g [Ni(cod)2]

(0.727 mmol) and the corresponding amount of [(AlCp*)4] (see Table 9.4) were suspended in mesitylene (20 mL). The bright yellow suspension was degassed, set under 3 bar H2 pressure and placed into a 150 °C hot oil bath. The colour immediately turned to orange, then bright red. After one minute, a dark red solution formed, which rapidly turned red-brown. Whereas Ni-rich α-phase particles (0.09 ≤ x ≤ 0.33) precipitated within minutes, for x = 0.50 (NP1), the particles precipitated after 10 h. The suspension was stirred over 4 days, while hydrogen was completely consumed. The particles were allowed to settle and the colourless supernatant was carefully decanted inside the glovebox. The residue was washed with 3 x 50 mL n- pentane. Thereafter, the residual solvent and the hydrocarbon byproducts were removed in vacuo and the black residue was thoroughly dried (1 h at 10-3 mbar/100 °C). XRD reflections for x = 0.50 (2θ): 31.07° (100), 44.42° (110), 55.29° (111), 64.79° (200), 73.52° (210), 82.02° (211), 98.52° (220), 115.87° (310). For the XRD reflections of the samples x = 0.33, 0.25, 0.17 and 0.09 see Table 6.1 on page 139.

Table 9.4. Masses of [(AlCp*)4] required for the desired Ni1‐xAlx phases and the yields.

Ni1‐xAlx m([(AlCp*)4]) Elemental analysis [%] EDX analysis Yield

x = 0.50 0.118 g (0.182 mmol) Ni, 50.9; Al, 23.1 Ni, 50.9; Al, 23.1 58 mg (93 %) x = 0.33 0.059 g (0.091 mmol) Ni, 69.3; Al, 16.5 Ni, 72; Al, 28 48 mg (91 %) x = 0.25 0.039 g (0.060 mmol) Ni, 78.9; Al, 12.3 Ni, 78; Al, 22 47 mg (96 %) x = 0.17 0.024 g (0.037 mmol) Ni, 75.6; Al, 7.1 Ni, 82; Al, 18 41 mg (86 %) x = 0.09 0.012 g (0.018 mmol) Ni, 84.7; Al, 3.5 Ni, 90; Al, 10 38 mg (84 %)

9.4.3.2. Synthesis of β-NiAl nanoparticles from [Ni(cod)2] and [(Me3N)AlH3] (NP2)

A solution of 0.065 mg [(Me3N)AlH3] (0.727 mmol) in mesitylene (5 mL) was slowly added to a bright yellow mesitylene solution of 0.200 g [Ni(cod)2] in a Fischer-Porter bottle. The solution immediately turned to dark red, and after complete addition, a black powder precipitated, accompanied with gas production. The bottle was pressurised with 3 bar H2 and heated at 150 °C for 24 h. Thereafter, the bottle was transferred into the glovebox, where the powder was allowed to settle. Subsequently, the colourless solution was decanted, and the

182 9. Experimental residue was washed with 3 x 20 mL n-pentane and dried. Yield: 0.057 g (87 %). ICP-OES: 54.2 wt.% Ni, 24.8 % Al, 16.6 wt.% C, 3.2 wt.% H. XRD reflections (2θ): 30.98° (100), 44.70° (110), 55.42° (111), 65.44° (200), 82.78° (211), 99.13° (220).

9.4.3.3. Synthesis of β-NiAl colloids (NP3)

In a Fischer-Porter bottle, 0.200 g [Ni(cod)2] (0.727 mmol) and 0.118 g [(AlCp*)4] (0.182 mmol) were suspended in mesitylene (20 mL). The bright yellow suspension was degassed, set under 3 bar H2 pressure and stirred at 150 °C. The colour immediately turned to orange, then bright red. After one minute, a dark red solution formed, which rapidly turned red-brown. The solution was stirred at 150 °C for 8 h. The solvent and the hydrocarbon byproducts were removed in vacuo as described above. Yield: 0.072 g (116 %). ICP-OES: 42.1 wt.% Ni,19.7 % Al, 32.0 wt.% C, 4.2 wt.% H. XRD reflections (2θ): 44.56° (110), 64.79° (200), 82.02° (211), 98.40° (220).

9.4.3.4. Synthesis of 17O-enriched 1-adamantanecarboxylic acid (ACA) In a Schlenk tube, to a solution of 3.18 g 1-adamantanecarbonyl chloride (0.016 mol) 20 17 mL THF, 400 µL (0.022 mol) O-enriched H2O (enrichment grade 35-40 %) were added via an Eppendorf-pipette. The colourless solution was refluxed overnight. In order to remove HCl gas, argon was passed over the boiling solution from time to time. The solvent was removed in vacuo. For complete removal of water, the white residue was redissolved in toluene (30 mL) to form an azeotrope with water, which was removed in vacuo (10-3 mbar). The remaining white solid was used without further purification. Yield: 2.90 g (100 %). 1H-NMR

(C6D6, 25 °C): δ 12.44 (1 H, s, COOH), 1.90 (6 H, m, {cyclo-(CH)3(CH2)3}-(CH2)3C- 13 1 COOH), 1.77 (3 H, m, {cyclo-(CH)3(CH2)3}-), 1,48 (6 H, m, {cyclo-(CH)3(CH2)3}-). C{ H}

NMR: 184.9 (COOH), 40.8 ({cyclo-(CH)3(CH2)3}-(CH2)3C-COOH), 38.8 ({cyclo-

(CH)3(CH2)3}-(CH2)3C-COOH), 36.5 ({cyclo-(CH)3(CH2)3}-), 28.1 ({cyclo-(CH)3(CH2)3}-). 17O NMR: 247.5 (s).

9.4.3.5. Synthesis of ACA-stabilised Ni/Al colloids (NP4) NiAl colloids (0.072 g, 0.727 mmol) were synthesised in mesitylene (20 mL) as described above. After 8 h of hydrogenation, the Fischer-Porter bottle was transferred into the glovebox, where 0.065 g ACA (0.363 mmol, 0.5 equiv.) was added. The initially brown-black

183 9. Experimental coloured solution changed to dark red. The solution was stirred overnight at 150 °C in an argon atmosphere. The solvent was removed in vacuo and the black, metallic shiny solid was 13 dried as described above. Yield: 0.113 g (82 %). C-NMR (C6D6, 25 °C): δ 39.5 ({cyclo-

(CH)3(CH2)3}-(CH2)3C-COO), 36.8 ({cyclo-(CH)3(CH2)3}-), 28.4 ({cyclo-(CH)3(CH2)3}-). 17 O-NMR (C6D6, 25 °C): 354.2, 148.5, 68.9. ICP-OES: 29.8 wt.% Ni, 14.1 wt.% Al, 39.1 wt.% C, 4.6 wt.% H, 12.4 wt.% O. EDX: 48 at.% Ni, 52 at.% Al. XRD reflections (2θ): 44.42° (111), 75.77° (220), 91.51° (311).

9.4.3.6. Hydrogenation of cyclohexene with β-NiAl colloids (NP3) as catalyst

0.030 g (0.35 mmol) of β-NiAl colloids NP3, prepared as described above, were suspended in 5 mL cyclohexene (50 mmol), pressurised with 3 bar H2 and placed into a 100 °C hot oil bath. The pressure fell rapidly to vacuo, so that the bottle had to be pressurised until the pressure did not fall any more. The hydrogenation was completed after 4 h. The Fischer- Porter bottle was transferred into the glovebox. The NiAl particles were allowed to settle and the solvent was removed with a syringe for further analyses. The total pressure used for full 1 conversion to cyclohexane was 14.6 bar H2 (50 mmol). H-NMR (C6D6, 250.1 MHz, 25 °C):

δ 1.40 (s, 12 H, cyclo-(CH2)6).

9.4.3.7. General procedure for the oxidation of Ni1-xAlx nanoparticles on air

All Ni1-xAlx nanoparticles were synthesised and isolated in the glovebox as described above. The powder materials were transferred into glass vials inside the glovebox and were left open in the box over night. Subsequently, the vials were taken out of the glovebox and exposed to air for 24 h at room temperature.

9.4.4. Intermetallic Co-Al nanoparticles

9.4.4.1. Synthesis of colloidal β-CoAl nanoparticles

In a Fischer-Porter bottle, 0.300 g [Co(C8H12)(C8H13)] (1.084 mmol) and 0.176 g

[(AlCp*)4] (0.271 mmol) were suspended in 20 mL mesitylene, degassed, set to 3 bar H2 pressure (~ 10 mmol) and placed into a 150 °C hot oil bath. The mixture was stirred for 48 h, whereupon a brown-black solution formed. After removal of the solvent and all other volatile

184 9. Experimental components in vacuo (100 °C, 10-3 mbar), a black solid was obtained. Yield: 102 mg (109 %). Elemental analysis: 28.55 wt.% Co, 12.48 wt.% Al. EDX: 52 at.% Co, 48 at.% Al. WAXS reflections (2θ): 14.25° (100), 20.18° (110), 24.72° (111), 28.64° (200), 32.10° (210), 35.25° (211), 40.92° (220), 45.95° (310).

185 10. References

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205 11. Appendix

11. Appendix

11.1. List of publications

The results of this work are comprised in the following publications:

[1] Nanometallurgy of Colloidal Aluminides: Soft Chemical Synthesis of CuAl2 and

α/β-CuAl Colloids by Co-Hydrogenolysis of (AlCp*)4 with [CpCu(PMe3)]. M. Cokoja, H. Parala, M. K. Schröter, A. Birkner, M. W. E. van den Berg, W. Grünert, R. A. Fischer, Chem. Mater. 2006, 18, 1634-1642.

[2] Nano-Brass Colloids: Synthesis by Co-Hydrogenolysis of [CpCu(PMe3)] with

[ZnCp*2] and Investigation of the Oxidation Behaviour of α/β-CuZn Nanoparticles. M. Cokoja, H. Parala, M. K. Schröter, A. Birkner, M. W. E. van den Berg, K. V. Klementiev, W. Grünert, R. A. Fischer, J. Mater. Chem. 2006, 16, 2420-2428.

[3] Organometallic Synthesis of Colloidal α-/β-NiAl Nanoparticles and Selective Al

Oxidation in α-Ni1-xAlx Nanoalloys. M. Cokoja, H. Parala, A. Birkner, O. Shekhah, M. W. E. van den Berg, R. A. Fischer, submitted to Chemistry of Materials.

206 11. Appendix

Contributions to other projects:

[1] Transition Metal Chemistry of Low Valent Group 13 Organyls. C. Gemel, T. Steinke, M. Cokoja, A. Kempter, R. A. Fischer, Chem. Eur. J. 2004, 4161- 4176.

[2] AlCp* as a Directing Ligand: C-H and Si-H Bond Activation at the Reactive

Intermediate [Ni(AlCp*)3]. T. Steinke, C. Gemel, M. Cokoja, M. Winter, R. A. Fischer, Angew. Chem. 2004, 116, 2349-2352; Angew. Chem. Int. Ed. 2004, 43, 2299-2302.

[3] C-H Activated Isomers of [M(AlCp*)5] (M = Fe, Ru). T. Steinke, M. Cokoja, C. Gemel, A. Kempter, A. Krapp, G. Frenking, U. Zenneck, R. A. Fischer, Angew, Chem. 2005, 17, 3003-3007; Angew. Chem. Int. Ed. 2005, 44, 2943- 2947.

[4] A Colloidal ZnO/Cu Nanocatalyst for Methanol Synthesis. M.-K. Schröter, L. Khodeir, M. W. E. van den Berg, T. Hikov, M. Cokoja, S. Miao, W. Grünert, M. Muhler, R. A. Fischer, Chem. Commun. 2006, 2498-2500.

11.2. Oral presentations

• Synthesis and characterisation of air stable metal colloids by in situ Al2O3 formation on metal aluminide nanoparticles. Royal Society of Chemistry Conference “Nanoparticles: New Opportunities and Challenges for Colloid Scientists”, University of Warwick, UK, March 28-30, 2007.

• Nanometallurgy in aprotic media using the organometallic precursors (AlCp*)4

and ZnCp*2: Synthesis and properties of colloidal Cu/Al and Cu/Zn alloys. 231st National Spring Meeting of the American Chemical Society, Atlanta, USA, March 26-30, 2006.

207 11. Appendix

11.3. Poster presentations

• Synthese und Charakterisierung kolloidaler intermetallischer Cu/Al- und Cu/Zn Legierungsphasen. M. Cokoja, H. Parala, R. A. Fischer, A. Birkner, M. W. E. van den Berg, W. Grünert, Presentation in the frame of the expertise for the 3rd funding period of the SFB 558 at the Ruhr-University Bochum, Bochum, February 28, 2006.

• New Approaches to Nanostructured Heterogeneous Catalysts. T. Hikov, M.-K. Schröter, M. Cokoja, R. A. Fischer, 89th International Bunsen Discussion Meeting „Chemical Processes at Oxide Surfaces: From Experiment to Theory“, Meschede, Germany, June 15-17, 2005.

208 11. Appendix

11.4. Curriculum vitae

Personal data Name Mirza Cokoja Date of birth February, 2nd 1979 Place of birth Oberhausen (Germany) Nationality Bosnia and Hercegovina Family status Unmarried

Education 06/1998 Abitur at the Elsa‐Brändström Gymnasium, Oberhausen 10/1998 – 04/2003 Chemistry study at the Ruhr‐University Bochum 10/2002 – 04/2003 Diploma thesis in the Institute of Inorganic Chemistry 2 Title: “Insertion Reactions of Low Valent Group 13 Organyls Into Ruthenium Halide Bonds”, under the guidance of Prof. Dr. Roland A. Fischer (passed with distinction) 01/2004 – 06/2007 PhD thesis in the Institute of Inorganic Chemistry 2 Title:” Nanometallurgy in Organic Solution: Organometallic Synthesis of Intermetallic Transition Metal Aluminide and ‐Zincide Nanoparticles” under the guidance of Prof. Dr. Roland A. Fischer

Other data Languages German, Serbo‐Croatian, English, French

Awards Wilke‐price of the Verein zur Förderung der Chemie und Biochemie (VfCh) of the Ruhr‐University Bochum Stays abroad Research stay in the group of B. Chaudret, Laboratoire de Chimie de Coordination, CNRS, Toulouse (France) during January‐February 2004 and February‐March 2006

209