Metal Chalcogenides Syntheses Using Reactions of Ionic Liquids

Metal chalcogenides syntheses using reactions of ionic liquids

Dissertation

zur Erlangung des akademischen Grades

Doctor rerum naturaluim (Dr. rer. nat.)

vorgelegt

dem Bereich Mathematik und Naturwissenschaften der Technischen Universität Dresden

von

M.Eng. Tao Zhang

geboren am 12. April 1989 in Shandong (China)

Eingereicht am 29. März 2018

Die Dissertation wurde in der Zeit von Oktober 2014 bis März 2018 an der Professur für Anorganische Chemie II angefertigt.

Gutachter: Prof. Dr. Michael Ruck (TU Dresden) Prof. Dr. Claus Feldmann (KIT)

Tag der Verteidigung: 30. Mai 2018

Contents

1. Background and motivation ...... 1 1.1. Properties of ILs and DESs ...... 3 1.2. Reactions of ionic liquids and deep eutectic solvents ...... 5 1.2.1 Reactions of metal-containing ionic liquids ...... 5 1.2.2. Reactions of fluorine-containing ionic liquids ...... 6 1.2.3. Reactions of hydroxide-based ionic liquids ...... 7 1.2.4. Reactions of chalcogen-containing ionic liquids ...... 8 1.2.5. Reactions of deep eutectic solvents ...... 10 1.3. Motivation ...... 12 1.4. References ...... 13 2. Solvothermal synthesis and enhanced photoelectrochemical performance of hierarchically structured strontium titanate particles ...... 21 2.1. Background ...... 23 2.2. Experimental section ...... 24 2.2.1. Chemicals ...... 24

2.2.2. Preparation of SrTiO3 ...... 24

2.2.3. Characterization of SrTiO3 ...... 25 2.2.4. Photo-electrochemical measurement ...... 25 2.3. Results and discussion ...... 26

2.3.1. Structural characterization of SrTiO3 particles ...... 26

2.3.2. Growth mechanism of SrTiO3 particles ...... 27

2.3.3. Nitrogen physisorption of SrTiO3 particles ...... 34

2.3.4. Optical and photoelectrochemical properties of SrTiO3 particles ...... 35 2.4. Conclusions ...... 37 2.5. References ...... 37 3. Synthesis of metal sulfides from a deep eutectic solvent precursor .. 43 3.1. Background ...... 45 3.2. Experimental section ...... 45 3.2.1. Chemicals ...... 45

Metal chalcogenides syntheses using reactions of ionic liquids

3.2.2. Synthesis of the DESs ...... 46 3.2.3. Synthesis of metal sulfides ...... 46 3.2.4. Materials’ characterization ...... 46 3.3. Results and discussion ...... 47 3.3.1. Structural and morphological analysis of metal sulfides ...... 47 3.3.2. Influence of the DESP composition ...... 55 3.3.3. Comparison with DES synthesis at ambient pressure ...... 56 3.3.4. Mechanism of formation of final products ...... 58 3.4. Conclusions ...... 60 3.5. References ...... 60 4. Dissolution behavior and activation of selenium in phosphonium based ionic liquids ...... 65 4.1. Background ...... 67 4.2. Experimental section ...... 68 4.2.1. Chemicals ...... 68

4.2.2. Dissolution of Se in [P6 6 6 14]Cl ...... 68

4.2.3. Dissolution of Se in [P6 6 6 14][decanoate] ...... 69

4.2.4. Dissolution of Se in [P4 4 4 4]Cl ...... 69 4.2.5. Preparation of trioctylphosphane selenide solution...... 69 4.2.6. Synthesis of nickel diselenides ...... 69 4.2.7. Synthesis of zinc diselenides ...... 70 4.3. Results and discussion ...... 70 4.3.1. Dissolution tests of selenium in phosphonium ionic liquids ...... 70 4.3.2. Synthesis of metal selenides in phosphonium ionic liquids ...... 74 4.4. Conclusions ...... 77 4.5. References ...... 77 5. Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures ...... 81 5.1. Background ...... 83 5.2. Experimental section ...... 84 5.2.1. Chemicals ...... 84

5.2.2. Dissolution of tellurium in [P6 6 6 14]Cl ...... 84

5.2.3. Dissolution of tellurium in [P6 6 6 14][N(CN)2] ...... 85

5.2.4. Dissolution of tellurium in [P6 6 6 14][decanoate] ...... 85

5.2.5. Dissolution of tellurium in [P4 4 4 4][decanoate] ...... 85

5.2.6. Synthesis of Bi2Te3 in [P6 6 6 14]Cl ...... 86

Contents

5.2.7. Synthesis of Bi-decanoate and Bi2Te3 in [P6 6 6 14][decanoate] ...... 86

5.2.8. Synthesis of Ag2Te in [P6 6 6 14][N(CN)2] ...... 87 5.2.9. Materials characterization...... 87 5.3. Results and discussion ...... 87 5.3.1. Dissolution tests in phosphonium based ionic liquids ...... 88

5.3.2. Synthesis of Bi2Te3 and Ag2Te from phosphonium ionic liquids ...... 99 5.4. Conclusions ...... 102 5.5. References ...... 103 6. Conclusions and perspectives ...... 107 Abbreviations ...... 111 List of Publications ...... 113 Acknowledgements ...... 115 Collaborations ...... 117 Versicherung ...... 119 Erklärung ...... 119

Metal chalcogenides syntheses using reactions of ionic liquids

Chapter 1

Background and motivation

Metal chalcogenides syntheses using reactions of ionic liquids

Background and motivation

Conventional inorganic materials synthesis following the high temperature solid- state reaction route is quite time- and energy-consuming. The low temperature synthesis relies heavily on water and traditional organic solvents. Alternatively, the fabrication of inorganic materials using, or in presence of, ionic liquids or deep eutectic solvents provides a promising direction in materials chemistry. Ionic liquids and deep eutectic solvents offer many unique properties and therefore provide great opportunities to discover new compounds or new phases which are not accessible in conventional organic or aqueous solvents or with a solid-state method.[1-3]

1.1. Properties of ionic liquids and deep eutectic solvents

Figure 1.1. Popular cations and anions of ionic liquids.

Ionic liquids (ILs), first reported by Walden in 1914,[4] are nowadays a large and widely explored class of ionic compounds that melt below 100 °C.[5] Figure 1.1 presents a few popularly used cations and anions of ILs. Although the use of ILs for organic chemistry has widely and extensively studied since the 1980s, their application in inorganic materials synthesis just began in the early 2000s, and having a rapid advance soon afterwards.[2, 6] ILs display several particular

Metal chalcogenides syntheses using reactions of ionic liquids properties for inorganic materials preparation. For examples, ILs are good solvents to dissolve versatile precursors including both inorganic and organic compounds, which is fundamental for the preparation of most materials.[7] The wide electrochemical windows (–4 to 4 V), wide liquid range (–96 to 400 °C), and high conductivity (up to 0.1 S cm–1) allow ILs to be used as ideal electrolytes for nanoparticles electrodeposition, and aprotic ILs can solve the problem of hydrogen evolution, thus a number of strategically important materials with very negative redox potentials can be obtained.[8] The ionic nature of ILs renders their low vapor pressure and many ILs have good thermal stability, making them suitable for ionothermal synthesis.[9] ILs are excellent microwave adsorbers due to their ionic character and high polarizability, thus creating new inorganic compounds by microwave heating.[10] Furthermore, ILs have been held to be “designer solvents” by varying the anion-cation combinations for a specific application.[11]

Figure 1.2. Typical halide salts and hydrogen bond donors used for deep eutectic solvents synthesis.

Background and motivation

In 2003, Abbott coined a new class of IL analog solvents, called deep eutectic solvents (DESs).[12] Although DESs share many characteristics of traditional ILs (e.g. high polarity, low vapor pressure, and tailorable chemical properties), some distinctions between these two are obvious. Particularly, DESs are more synthetically accessible, because in most cases they are prepared just by mixing a quaternary ammonium or phosphonium salt with a hydrogen-bond donor (HBD) (Figure 1.2) to form a transparent solution at a relatively low temperature, and require no purification. Moreover, DESs are quite inexpensive as they generally consist of two or three cheap constituents such as choline chloride and urea.[3, 13, 14] These easier and cheaper preparation properties of DESs make them possible for the large quantity use in the wet chemical synthesis and for the facilitating scale-up.

1.2. Reactions of ionic liquids and deep eutectic solvents

Due to their attractive properties, ILs and DESs are now of growing interests in a variety of inorganic materials preparation, including metals, metal oxides, metal chalcogenides, carbon materials, and open frameworks.[13-20] However, in many cases ILs and DESs are not chemically stable in the reactions and they can undergo decompositions under certain conditions. As any decomposition may change the chemical and physical properties of ILs and DESs, such as conductivity, viscosity, and dissolving capacity, it may influence the mechanism of the reactions and bring detrimental results for the final products.[21] Nevertheless, decomposition of ILs does not always make negative impact on the products, which may also introduce some new features to the compounds. Within the scope of this thesis, those inorganic reactions in which ILs and DESs take part as reactants are of primary interest.

1.2.1. Reactions of metal-containing ionic liquids

Metal-containing ILs have recently gained increasing research attention due to their combination of the properties of ILs and the catalytic, optical, or magnetic properties of the incorporated metal salts.[22] Thus, their possible applications in catalysis, optical devices, and magnetic components are promising.[17, 23-25] In

Metal chalcogenides syntheses using reactions of ionic liquids contrast, the use of metal-containing ILs as metal sources for nanomaterials synthesis has been less studied. In 2004, Taubert prepared CuCl nanoplatelets from a Cu-containing IL (1) in the presence of 6-O-palmitoyl ascorbic acid (2) (Figure 1.3).[26] It was found that the mixture of 1 and 2 forms thermotropic liquid crystals with layered structures, which therefore templated the formation of CuCl nanoplatelets at elevated temperatures.[27] In this reaction, the used Cu- containing IL served as solvent-reactant-template combination and can, thus, be viewed as an “all-in-one” IL. After this study, a series of metal-containing “all-in- one” ILs were designed and synthesized to be used as metal sources for nanoparticles synthesis.

Figure 1.3. Components used for CuCl nanoplatelets synthesis. 1, Cu-containing IL; 2, 6-O-palmitoyl ascorbic acid.[26]

Dai and co-workers designed a zinc-containing IL Zn(L)4(NTf2)2 (L = alkylamine,

– NTf2 = N(SO2CF3)2) as the zinc source for the ionothermal synthesis of hierarchical ZnO nanostructures.[28] The morphologies of ZnO were dependent on the nature of IL (ligands).

Zheng and co-workers synthesized three-dimensional hierarchical CuS microspheres from a Cu-based ionic liquid precursor [BMIm]2Cu2Cl6 (BMIm = 1- butyl-3-methylimidazolium) via a solvothermal method.[29] It has been shown that the IL plays a crucial role for the formation and self-assembly of CuS nanosheets. The imidazolium rings ([BMIm]+) prefer to adsorb onto the (001) facets of CuS crystals, inhibiting the crystal growth along the [001] direction. The alkyl chain of IL has an influence on the assembly of CuS nanosheets and the short chain IL tends to form a tight hierarchical structure.

1.2.2. Reactions of fluorine-containing ionic liquids

Background and motivation

– – ILs with fluorine-containing anions ([BF4] or [PF6] ) have received much interest in recent years because they can be used as fluoride sources in the synthesis of metal fluorides. Fluorides are an important class of inorganic materials due to their wide bandgap, low energy phonons, and high transparency.[7] Typically, fluorides are synthesized using HF, NaF, or NH4F as fluoride sources, which are usually toxic and additional templates or structure-directing agents are often needed. Thus, synthesis in fluorine-based ILs opens a new and safe path to obtain fluorides with novel functions and morphologies. Rui et al. synthesized pure phase tetragonal

MnF2 nanoparticles using Mn(CH3COO)2·4H2O as manganese source and

[30] [BMIm][BF4] as fluorine source via an one-step solvothermal synthesis method.

The as-synthesized MnF2 can be utilized as the anode material for rechargeable lithium batteries, showing better cycling performance than conventional conversion reaction electrodes. Li et al. prepared a series of hydrated iron-based fluoride nanostructures for lithium and sodium batteries using Fe(NO3)3·9H2O and

[31-33] [BMIm][BF4] as starting materials. The [BMIm][BF4] IL served as a stabilization medium and the fluoride source in the reaction process. Chen et al. synthesized spherical NaYF4 nanoclusters in [BMIm][BF4] with the assistance of microwave radiation.[34] It was found that the IL played key roles, which were used as the reaction solvent, the fluoride source, and the absorbent of microwave irradiation. The as-obtained NaYF4 nanoclusters in [BMIm][BF4] could further be doped with lanthanide ions and exhibited excellent luminescence properties. Schütte et al. fabricated several fluoride nanoparticles using metal amidinates in

[35] fluorous [BMIm] ILs, including the synthesis of MF2 (M=Mn, Fe, and Co) nanoparticles in [BMIm][BF4] and synthesis of FeF2 nanoparticles in the fluorine- containing 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIm][PF6]) and 1-butyl-3-methylimidazolium trifluoro-methanesulfonate ([BMIm][TfO]) ILs. Thus,

− − − the [BF4] , [PF6] , and [TfO] anions acted as fluoride sources for the formation of these MF2 nanoparticles. Very recently, Terraschke et al. reported a facile IL- assisted synthesis strategy for direct precipitation of Eu2+-doped nanoparticles.[36]

The used IL [BMIm][BF4] fulfills multiple functions, serving as fluoride source as well as coordinating the Eu2+ ions and stabilizing their oxidation state.

1.2.3. Reactions of hydroxide-based ionic liquids

Metal chalcogenides syntheses using reactions of ionic liquids

The basic ILs do not simply provide an alkaline environment like the commonly used NaOH and KOH but the organic cations of ILs may play a significant role in the crystal growth process.[37] Li et. al. developed a new type of basic IL tetrabutylammonium hydroxide (TBAH) for the synthesis of ZnO nanoparticles with different sizes and shapes.[37, 38] Interestingly, it was found that not only regular ZnO nanocrystals,[38] but also special hollow ZnO mesocrystals,[37] can be fabricated in TBAH by adjusting zinc acetate concentrations. The resulting tubular microparticles were composed of nanometer-sized ZnO primary particles, and showed a high degree of order and can therefore be classified as mesocrystals. The large tetrabutylammonium cation prevents further growth of these small primary particles and aids their aggregation into larger structure by reversing the polarity of the negatively charged surfaces of the primary particles. In their following studies, a variety of metal (hydr)oxides were successfully prepared from the water/TBAH mixtures by changing zinc acetate to other metal acetates

[39, 40] (M(OAc)2, M = Fe, Co, Mn, Ni, Cu, etc.). Additionally, some other types of basic ILs like tetraethylammonium hydroxide (TEAH) and benzyltrimethylammonium hydroxide (BTMAH) were also tested for the synthesis of ZnO.[41, 42]

1.2.4. Reactions of chalcogen-containing ionic liquids (including reactions of ionic liquids with chalcogens)

Zheng and co-workers designed a new selenium-containing IL 1-n-butyl-3- methylimidazolium methylselenite ([BMIm][SeO2(OCH3)]) as the selenium

[43] − precursor. In the previously used Na2SeO3 systems, SeO3 could react with

− metal ions to form precipitates. The utilization of the [SeO2(OCH3)] anion avoids this precipitation problem because of its weaker polarizing capability, and thus the metal ions are able to exist as free ions in the solution. Furthermore, this IL can serve as a stabilizer. Various metal selenides with special morphologies, including

[44] [45] ZnSe hollow nanospheres, Cu2−xSe nanocrystals and CuSe nanoflakes, and CdSe nanospheres and nanodendrites,[46] have been successfully synthesized. To some extent the use of this selenium-containing IL as the selenium source is similar to that of the metal-containing ILs as metal sources mentioned above, since both selenium and metal species are included in the anions of ILs.

Imidazolium ILs are the most investigated group. The C2 atom (in between the N atoms) of the imidazolium cation is one of its active sites due to the weak acidity

Background and motivation of the C2 proton. A variety of compounds such as biomass, oxidants, organic compounds, and inorganic compounds are able to react with imidazolium ILs at their C2 atoms, especially in the presence of bases.[21] Rogers and co-workers found that imidazolium acetates could directly react with elemental chalcogens (e.g. sulfur and selenium) to yield corresponding imidazole-2-chalcogenones (Scheme 1.1).[47] 1-Ethyl-3-methylimidazole-2-thione was obtained with a yield of around 50% upon stirring stoichiometric amounts of S8 and 1-ethyl-3- methylimidazolium acetate ([EMIm][OAc]) at 25 °C for 24 h. Similarly, the reaction of selenium powder and [EMIm][OAc] led to the corresponding 1-ethyl- 3-methylimidazole-2-selone in 90% yield at 75 °C. However, when other anion-

− − − − − − − based (e.g. Cl , HSO4 , SCN , CH3SO4 , CH3C6H4SO3 , CF3SO3 , and CF3COO ) imidazolium ILs were used, no reactions were observed with sulfur. The results indicate that the imidazolium acetate IL can act as both the base and the carbene source, producing carbene in situ and afterwards reacting with sulfur to give the thione.

Scheme 1.1. Reactions of imidazolium acetate ionic liquid with chalcogens.[47]

− − On the other hand, these imidazolium salts with neutral anions (e.g. Cl , [NTf2] ,

− and [BF4] ) can also undergo reactions with chalcogens in the presence of strong bases or under ultrasound irradiation conditions. Ansell and co-workers first reported the preparation of 1,3-dimethylimidazole-2-thione by reaction of 1,3-

[48] dimethylimidazolium iodide with sulfur in the presence of K2CO3 in methanol. Later, a variety of chalcogenones starting from the corresponding imidazolium salts and chalcogens, including 1,3-dialkylimidazole-2-thione/selone,[49, 50] bridged bis(imidazoline-2-thione/selone),[51-53] bridged mixed bidentate N- heterocyclic carbene (NHC)/sulfur ligands,[54] were synthesized by this popular

“MeOH/K2CO3” method.

Recently, other “base/solvent” combination methods were also explored. For instance, Streubel and co-workers reported the reaction of imidazolium salts with

Metal chalcogenides syntheses using reactions of ionic liquids sulfur in the presence of NaH and KO(t-Bu) in THF in high yields.[55] Wasserscheid and co-workers also found that 1-alkyl-3-methylimidazole-2-thiones were obtained by reacting the respective 1,3-dialkylimidazolium halide salts with sulfur in the presence of NaOMe in refluxing methanol.[56] Laus and co-workers used a

“pyridine/Et3N” method to introduce sulfur into the C2 position of a series imidazolium salts to yield corresponding imidazole-2-thiones.[57] Tian et al. synthesized a series of 1,3-dialkylimidazole-2-selenones by reactions of 1,3- dialkylimidazolium salts with selenium using Na2CO3 as a base in water under refluxing conditions.[58]

In addition, Inesi and co-workers combined the electrochemical and ultrasound methods for the synthesis of imidazole-2-thiones, which is first electrochemical reduction of 1,3-dialkylimidazolium ILs to the corresponding NHCs and afterwards reaction with elemental sulfur under ultrasound irradiation conditions to the final thiones.[59] Lei and co-workers found that 1,3-disubstituted imidazole-2-thiones could be formed by reacting 1,3-disubstituted imidazolium salts with potassium thioacetate or potassium thiocyanate as sulfur sources under microwave conditions.[60]

The formed imidazole-2-chalcogenones were mainly used as potential antioxidants and antithyroid drugs,[61] as catalysts for organic reactions,[62] and as valuable intermediates for metal complexes syntheses,[51-53, 63] but there were few reports about the utilization of imidazole-2-chalcogenones as chalcogen precursors for metal chalcogenides syntheses. Shi and co-workers successfully synthesized various metal selenides (e.g. CdSe, Bi2Se3, ZnSe, and PbSe) using an organochalcogenone compounds 1,5-bis(3-methylimidazole-2-selone)pentane (Pbis) as a novel selenium precursor.[64] Pbis is air stable and can easily be stored. Furthermore, the 1,5-bis(3-methylimidazole)pentane part of Pbis is significantly positively charged and, thus, the valence state of Se in Pbis was negative, leading to the easy and rapid reaction of Pbis with metal ions. More cases of using imidazole-2-chalcogenones as chalcogen sources for metal chalcogenides syntheses are expected.

1.2.5. Reactions of deep eutectic solvents

The choline chloride (ChCl)/urea based DES is the most widely investigated system. However, the decomposition of urea usually happens upon heating. Gu and co-

Background and motivation workers synthesized a variety of materials such as transition metal complex and layered transitions metal hydroxides, and their derived compounds by utilization of the decomposition feature of urea at a relatively high temperature (120–

210 °C). For example, Ni[NH3]6Cl2 crystals with an open octahedral morphology were obtained from a ChCl/urea mixture with NiCl2·6H2O dissolved under

[65] solvothermal conditions. In the sealed autoclave, the NH3 was released by the urea portion in DES at elevated temperature, to precipitate the nickel ion to form the final product. This nickel amine chloride complex has been potentially used as an ammonia storage material and also for the indirect hydrogen storage.[66] Furthermore, when injecting a small amount of water into the hot ChCl/urea

2+ solution dissolved with Ni in an open reaction system, -Ni(OH)2 nanoflowers with a uniform size of about 100 nm formed. It has been found that heating the ChCl/urea mixture leads to the accumulation of alkaline species.[67] The add of water would introduce a large amount of OH– and then precipitate the metal ions.

Similarly, -Co(OH)2 with stacked nanosheets morphology was also obtained by this water-injection method.[68] Their corresponding metal oxides can be obtained by calcination of these metal hydroxides at higher temperatures. However, Fe2O3 nanospindles assembled by nanoparticles were directly formed using the “hot- injection” method without further heat treatment.[69] In addition, a “two-stage” water injection method was developed to synthesize CoFe based layered double hydroxide (LDH) with expanded interlayer spacing (11.3 Å), exhibiting improved electrochemical activity.[70] The formed CoFe LDH can be further employed as a precursor to transform to its corresponding CoFe oxide with lattice contraction but reserve its original two dimensional (2D) morphology.[71]

In 2004, Morris and co-workers first reported the synthesis of a novel zeolitic framework (SIZ-2, Al2(PO4)3·3NH4) in the ChCl/urea eutectic mixture by an ionothermal method.[72] In this case, the ammonium cation, released by partial decomposition of urea at elevated temperature, acted as the template for the formation of this new structure. Later, the same group investigated several other urea derivatives (including 1,3-dimethyl urea, ethylene urea, and N,N′- trimethylene urea) and quaternary ammonium halides (including choline chloride and tetraethylammonium bromide) based eutectic mixtures to produce new types of zeolitic materials.[73] As expected, the corresponding organic cations (e.g. methylammonium, ethylene diammonium, and propylene diammonium) served as

Metal chalcogenides syntheses using reactions of ionic liquids the templates to be delivered to the reaction by the breakdown of one of the components of DESs, leading to some new aluminophosphate materials.

1.3. Motivation

Although preparations of inorganic materials in ILs and DESs have advanced significantly in recent years and therefore a large number of interesting materials have been obtained, most cases focus on the demonstration of synthesis protocols. The reactivity of ILs and DESs during the inorganic syntheses has not been widely and intensively studied. More fundamental investigations should be done to well understand the chemical reactivity of ILs and DESs with inorganic or organic compounds in the reactions, in order to make better use of ILs and DESs for inorganic syntheses. Moreover, the designer nature of ILs and EDSs, by combination different cations and anions or quaternary ammonium/phosphonium salts and hydrogen bond donors, has endowed their ability to produce a huge number of solvents with various properties. However, the most extensively studied groups are focusing on imidazolium-based ILs and [ChCl/urea]-type DES. More IL and DES families have to be explored and studied.

Nanostructured transition-metal chalcogenides are of great interest in diverse areas, especially in energy-related applications such as fuel cells, solar cells, sensors, and light-emitting diodes.[74] Thus, they are attracting more and more researchers to explore new synthetic methods and examine their structures and properties. IL-/DES-based methods may provide a new strategy for metal chalcogenides synthesis, but the fact is that syntheses of metal chalcogenides using ILs or DESs are still limited and the majority of them focus on the binary compounds fabrication.[2] In this thesis, we aim to explore more complex reactions (e.g. ternary materials preparation), new types of IL or DES systems (e.g. phosphonium-based ILs), and at the same time, the role or chemical reactivity of ILs or DESs in the reactions is thoroughly demonstrated.

In chapter 2, we successfully extended the use of TBAH, which has been proved to be a powerful IL for binary oxides (e.g. ZnO, CuO, and Fe2O3) synthesis as mentioned in 1.2.3, for the preparation of a perovskite-type ternary oxide SrTiO3. The role of TBAH was investigated in detail.

Background and motivation

In chapter 3, we designed a new choline chloride (ChCl) and thioacetamide (TAA) based DES for the synthesis of a variety of metal sulfide nanoparticles including

Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS. The ChCl/TAA based DES was used as both the reaction medium and the sulfur precursor.

In chapter 4 and 5, we systematically investigated the interplay between phosphonium ILs and elemental selenium and tellurium at high temperatures, respectively. The dissolution behaviors of selenium and tellurium in ILs were studied by a series of nuclear magnetic resonance (NMR) experiments. In addition, various metal selenides and tellurides were further synthesized using the formed solutions in ILs as selenium and tellurium precursors.

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[51] W.-G. Jia, Y.-B. Huang, Y.-J. Lin, G.-X. Jin, Syntheses and structures of half- sandwich iridium(iii) and rhodium(iii) complexes with organochalcogen (S, Se) ligands bearing N-methylimidazole and their use as catalysts for norbornene polymerization. Dalton Trans. 2008, 5612–5620.

[52] W.-G. Jia, Y.-B. Huang, Y.-J. Lin, G.-L. Wang, G.-X. Jin, Nickel Complexes and Cobalt Coordination Polymers with Organochalcogen (S, Se) Ligands Bearing an N-Methylimidazole Moiety: Syntheses, Structures, and Properties. Eur. J. Inorg. Chem. 2008, 2008, 4063–4073.

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[54] M. Slivarichova, R. Ahmad, Y.-Y. Kuo, J. Nunn, M. F. Haddow, H. Othman, G. R. Owen, New Mixed-Donor Bidentate Ligands Based on N-Heterocyclic Carbene and Thione Donors. Organometallics 2011, 30, 4779–4787.

[55] S. Sauerbrey, P. K. Majhi, G. Schnakenburg, A. J. Arduengo Iii, R. Streubel, Synthesis, structure and reactivity of 4-phosphanylated 1,3-dialkylimidazole-2-thiones. Dalton Trans. 2012, 41, 5368–5376.

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[57] G. Laus, V. Kahlenberg, K. Wurst, T. Muller, H. Kopacka, H. Schottenberger, Synthesis and Crystal Structures of New 1,3-Disubstituted Imidazoline-2- thiones. Z. Naturforsch. B 2013, 68b, 1239–1252.

[58] F. Tian, Y. Chen, P. Li, S. Lu, A Novel Method for Synthesis of 1, 3- Dialkylimidazole-2-Selenones in Water. Phosphorus, Sulfur Silicon Relat. Elem. 2014, 189, 1391–1395.

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Metal chalcogenides syntheses using reactions of ionic liquids

Chapter 2

Solvothermal synthesis and enhanced photo- electrochemical performance of hierarchically structured strontium titanate particles*

------*T.Zhang, T. Doert, M. Ruck, Dalton Trans., 2017, 46, 14219–14225.

Metal chalcogenides syntheses using reactions of ionic liquids

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles

2.1. Background

Over the past decades, three dimensional (3D) hierarchical micro-/nanostructures have received much attention because of their particular structures and properties.[1-3] These 3D hierarchical architectures are generally composed of nanometer-sized building units via interactions such as hydrogen bonds, van der Waals forces, and covalent and ionic bonds, while the total size is in micrometer or submicrometer scale. It has been demonstrated that such hierarchical structures with a combination of microstructure and nanostructure often exhibit several unique features including the prevention of the agglomeration of nanosized particles, more active sites, short ionic diffusion lengths, and enhanced light- scattering capability.[2] Thus, they can be promising candidates for many applications like catalysts or electrodes in supercapacitors, rechargeable batteries, and solar cells.[4-6] To date, significant efforts have been made in the design and synthesis of 3D hierarchical architectures. These efforts were successful for e.g.

[7-11] MgO, TiO2, Fe2O3, CeO2, and ZnCo2O4. On the other hand, there are many functional oxide materials that are not yet available with such morphologies, among them strontium titanate.

SrTiO3 as an important perovskite-type ternary oxide has been widely investigated

[12-14] for its potential applications in optics and electronics. SrTiO3 has been considered as one of the most promising photocatalysts for the degradation of

[15-17] organic pollutants and for water splitting. Generally, SrTiO3 can be synthesized by solid-state reaction,[18, 19] chemical vapor deposition (CVD),[20, 21] and the sol-gel method.[22, 23] Compared with these relatively high temperature synthesis methods, the wet-chemistry routes at relatively low temperature, such as the one-pot hydro-/solvothermal process, are more favorable to control the

[15-17, 24, 25] size and morphology of the final SrTiO3 product. Over the past years,

[16, 26] various SrTiO3 micro-/submicro-/nanostructures, including hollow spheres, nanoparticles,[27, 28] polyhedral submicro-/nanocrystals,[25, 29, 30] microscale superstructures,[31] and nanosheets,[32] have been prepared successfully.

In this chapter, we reported a new IL-assisted solvothermal synthetic route to synthesize hierarchically structured SrTiO3 particles with large surface areas. The

Metal chalcogenides syntheses using reactions of ionic liquids role of the used basic IL TBAH was investigated in detail. Finally, the photoelectrochemical properties of as-synthesized SrTiO3 particles were studied.

2.2. Experimental section

2.2.1. Chemicals

Titanium isopropoxide [Ti(OC3H7)4, 97%], strontium chloride hexahydrate

(SrCl2·6H2O, 99%), tetrabutylammonium hydroxide 30-hydrate (TBAH,

(C4H9)4NOH, 98%), ethylene glycol (C2H6O2, ≥99.5%) were purchased from

Sigma-Aldrich, Germany. Tetrabutylphosphonium hydroxide (TBPH, (C4H9)4POH, 40% w/w aqueous solution) was purchased from Alfa-Aesar, Germany. All chemicals were used without further purification.

2.2.2. Preparation of SrTiO3

SrTiO3 particles were synthesized via a one-step solvothermal process. 1.1 mmol

(0.293 g) SrCl2·6H2O were directly dissolved in 15 mL ethylene glycol, and then 2 g (2.5 mmol) TBAH were added to the solution under stirring. A transparent solution formed within seconds of ultrasonic treatment. Then 0.33 mL Ti(OC3H7)4 were added dropwise under stirring and an ultrasonic treatment was performed to obtain a translucent solution. The solution was immediately transferred into a 25 mL Teflon-lined stainless steel autoclave. The sealed autoclave was heated at 180 °C for 20 h. After the reaction, the autoclave was removed from the furnace and allowed to cool down to room temperature. The solid products were collected by centrifugation and washed with distilled water, 0.05 mol L–1 acetic acid, and finally with ethanol. The final precipitates were dried under vacuum at room temperature overnight. Two further samples were prepared at 150 °C and 210 °C.

For comparison, SrTiO3 particles were also synthesized by a high temperature solid state reaction according to a previous report.[18] For this, equimolar amounts of

SrCO3 and TiO2 were thoroughly mixed and reacted at 900 °C for 12 h, followed by calcination at 1100 °C for 24 h. With respect to the highest temperature applied in synthesis, the SrTiO3 samples are denoted as STO-150, STO-180, STO-210, and STO-1100.

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles

2.2.3. Characterization of SrTiO3

Powder X-ray diffraction (PXRD) patterns were typically recorded using a

PANalytical X’Pert Pro diffractometer with Cu-K1 radiation (λ = 1.54 Å). The morphologies of the samples were investigated by scanning electron microscopy (SEM; Hitachi SU8020). Transmission electron microscopy (TEM) and high- resolution transmission electron microscopy (HRTEM) images were recorded with a Titan Themis TEM operating at an accelerating voltage of 200 kV. The ultraviolet- visible (UV-Vis) absorbance spectra were collected using a Varian Cary 4000 UV- Vis spectrophotometer equipped with a Praying Mantis diffuse reflectance accessory (Harrick Scientific Products) between 200 and 800 nm. Nitrogen adsorption isotherms were measured on a Quadrasorb Adsorption Instrument at 77 K. The Brunauer-Emmett-Teller (BET) method was used to estimate specific surface areas (SBET) based on adsorption points at relative pressures (p/p0) between 0.05 and 0.25. Pore size distributions (PSDs) were derived from the adsorption branches of the isotherms based on the Barrett-Joyner-Halenda (BJH) method. Thermogravimetric analysis (TGA) was performed on a Netzsch STA 409 C/CD instrument at a heating rate of 5 K min–1 from room temperature to 600 °C under air flow.

2.2.4. Photo-electrochemical measurement

Electrochemical measurements were performed in a standard three-electrode configuration using a CHI 760E electrochemical workstation (CH Instruments, USA) with an Ag/AgCl electrode as the reference electrode and a platinum wire as the counter electrode. A titanium foil modified with the SrTiO3 samples was employed as the photoanode (the preparation details see below).[33] The effective area was 2.0 cm2. A 1.0 M KOH aqueous solution was used as electrolyte and a 200 W xenon lamp coupled with an AM 1.5G filter (Newport) was utilized as the simulated sunlight source (100 mW cm–2).

The SrTiO3 photoelectrode was prepared by an electrophoretic deposition method.

Typically, 50 mg of ground SrTiO3 powder were dispersed in 100 mL isopropanol.

–3 A small amount of Mg(NO3)2·6H2O (10 M; ≈ 25 mg) was added into the suspension in order to generate positive surfaces charges on the perovskite (by absorption of Mg2+ ions) and to facilitate electro-migration. The suspension was continuously stirred for one hour and sonicated for 30 minutes at room

Metal chalcogenides syntheses using reactions of ionic liquids temperature. For the electrophoretic deposition, the titanium foil was used as working electrode and a platinum foil was used as the counter electrode. A constant working voltage was set to 50 V and the electrophoretic deposition process was performed for 10 minutes. The final SrTiO3-coated titanium foil was washed with distilled water to remove residual isopropanol and Mg(NO3)2 salt and dried at room temperature in the air before using.

2.3. Results and discussion

2.3.1. Structural characterization of SrTiO3 particles

Figure 2.1. (a) Low-magnification SEM image of STO-180 particles. (b) SEM image of a single STO-180 particle. (c) PXRD pattern of STO-180 and simulated pattern of SrTiO3 based on ICSD database entry no. 56092. (d) Low-magnification TEM image of a single STO-180 particle. (e) HRTEM image and fast Fourier transform (FFT) pattern (inset) of STO-180 particles. (f) STEM-EDS element mapping of a randomly selected STO-180 particle.

As shown in Figure 2.1c, all reflections of the STO-180 sample can be indexed to the standard SrTiO3 pattern with the cubic structure in space group Pm3̅m (ICSD no. 56092). No signs of impurities are observed in the XRD diagram. The

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles morphological feature of as-synthesized SrTiO3 particles was observed by SEM and

TEM images. Figure 2.1a, b show typical SEM images of the SrTiO3 particles from sample STO-180. The low magnification image, Figure 2.1a, shows ellipsoidal but irregularly shaped particles with diameters of approximately 0.5–1.5 μm. The high magnification SEM image, Figure 2.1b, clearly reveals that the SrTiO3 particle consists of intergrown nanosheets with an average thickness of about 10 nm that are arranged in various directions resembling a desert rose. Such a nanosheet is formed by even smaller platelets, which also differ in orientation. The TEM image in Figure 2.1d shows such an agglomerate of platelets in transmission at slightly higher magnification. In the HRTEM image of Figure 2.1e, densely intergrown crystalline domains of only a few nanometers can be identified. The lattice fringe spacings, e.g. 0.28 nm for the d spacing of the (110) planes, and the corresponding fast Fourier transform pattern match cubic SrTiO3. The STEM–EDS measurement, Figure 1f, confirms the homogeneous distribution of Sr, Ti, and O across the SrTiO3 particle.

2.3.2. Growth mechanism of SrTiO3 particles

Figure 2.2. PXRD patterns of SrTiO3 particles obtained at 180 °C with reaction times of 2.5 h, 5 h, 10 h, 15 h, and 20 h, respectively.

To understand the formation mechanism of the SrTiO3 particles, a series of time- and temperature-dependent synthetic experiments were carried out, and the

Metal chalcogenides syntheses using reactions of ionic liquids structures and morphologies of the products were analyzed. As can be seen from Figure 2.2, the PXRD pattern of the products synthesized at 180 °C for 2.5 h shows no diffraction maxima, indicating that the corresponding sample is highly amorphous. Very weak and broad reflections of SrTiO3 are visible for the sample obtained from a reaction of 5 h, suggesting the initial formation of the crystalline phase. Extending the reaction time to 10 h, the intensities of diffractions of SrTiO3 phase increase. When extending the reaction duration to 15 h and 20 h, the intensities of the reflections increase continuously, showing a continuous growth of the SrTiO3 phase. However, the width of the reflections (FWHM) remains essentially the same, which indicates that the domain structure does not change much at these comparatively low temperatures. Since there are no additional reflections in any PXRD pattern, a direct phase transformation from the amorphous titanium precursor (see below) to crystalline SrTiO3 can be assumed. The domain structure seen in the HRTEM suggests either the formation of numerous seeds that grow independently or a distracted crystal growth.

Figure 2.3. SEM images of SrTiO3 particles synthesized at 180 °C with reaction times of 2.5 h (a), 5 h (b), 10 h (c), and 15 h (d), respectively.

The SEM images (Figure 2.3) show the corresponding morphological evolution. We can see that the particle size is about 500 nm and the shape is slightly ellipsoidal at the initial reaction stage (Figure 2.3a, b). It becomes more spherical and the size increases to around 1 μm in course of the growth of the SrTiO3 particles with increasing solvothermal reaction time (Figure 2.3c, d). It is worth noting that the

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles desert-flower morphology was successfully maintained after continuous solvothermal transformation from the titanium precursor to SrTiO3, except that the sheets become a little isolated from each other with increasing reaction time. From these observations, one may infer that either (A) the uncorrelated heterogeneous, i.e. on surface, the formation of seeds is at least in the initial stages preferred over the growth of existing crystal domains or (B) the growth mechanism includes branching.

Figure 2.4. PXRD patterns of SrTiO3 samples synthesized at 180 °C with different reaction times after calcination at 450 °C in air for 2h.

Figure 2.5. The TGA curve of the SrTiO3 sample synthesized at 180 °C for 2.5 h.

Metal chalcogenides syntheses using reactions of ionic liquids

In order to clarify the structure of the products formed at the initial reaction stage, the obtained samples were calcined at 450 °C in air for 2 h. The PXRD patterns after calcination are shown in Figure 2.4. From the PXRD diagram, we can see that the sample reacted at 180 °C for 2.5 h was converted to TiO2 in its anatase modification upon calcination. Referring to similar investigations,[34] we assume that amorphous “TiO2 hydrates”, i.e. sols and gels containing Ti–O–Ti and Ti–

(OH)–Ti bridges, form in the initial stage of the reaction by hydrolysis of Ti(OC3H7)4

(formal reaction Ti(OC3H7)4 + (2+n)H2O  TiO2·nH2O + 4C3H7OH). They serve as the titanium source for the following reaction process, which was also previously reported.[35] TGA was further utilized to study the transformation of this titanium precursor upon heating (Figure 2.5). According to the TGA curve of as-synthesized STO-180 with the reaction time of 2.5 h, there is a total weight loss of about

17.2% between 100 and 500 °C, which is mainly due to the loss of H2O as well as some organic species. The main weight loss ends at around 450 °C, which is indicative of the formation of crystalline anatase.

Figure 2.6. PXRD patterns of the as-synthesized STO-150 sample, STO-210 sample, and STO-150 sample after calcination at 450 °C in air for 2h.

The influence of the reaction temperature on the crystallization process and the morphology of the SrTiO3 products was also investigated. We found that the product obtained after 20 h at 150 °C is also amorphous according to PXRD (Figure 2.6, blue). The PXRD pattern (Figure 2.6, green) of STO-150 after calcination at

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles

450 °C in air for 2 h confirmed the formation of anatase, indicating that the reaction of amorphous TiO2 to crystalline SrTiO3 is hindered when the reaction temperature remains at 150 °C. The SEM images (Figure 2.7a, b) represent the generation of desert-rose structures of TiO2 hydrates composed of more compact nanosheets compared to STO-180 and the surface area can be as high as 298 m2 g–1. When the reaction temperature increases to 210 °C, the hierarchical structure of the SrTiO3 particles is destroyed to some extent as shown in Figure 2.7c, d. However, the particle size of STO-210 remains almost the same as for STO-180, which contradicts a potential Ostwald ripening.

Figure 2.7. SEM images for as-synthesized STO-150 (a, b) and STO-210 (c, d) samples.

Furthermore, different amounts of the basic IL TBAH were employed to investigate concentration effects. The transformation from the titanium precursor to SrTiO3 remains incomplete when 1.5 g (1.9 mmol) of TBAH were used in the reaction as shown in the XRD pattern of Figure 2.8 (green). This demonstrates the essential role of TBAH as the basic component in the reaction. Additionally, it is clear from

2+ – the chemical reaction equation TiO2·nH2O + Sr + 2OH → SrTiO3 + (1+n)H2O that a high OH– concentration shifts the equilibrium towards the right side of the equation. When 4 g (5 mmol) of TBAH were used for the reaction, no obvious morphological changes were observed according to the SEM images (Figure 2.9).

Metal chalcogenides syntheses using reactions of ionic liquids

Figure 2.8. PXRD patterns of as-obtained SrTiO3 samples using a different amount of TBAH as reactants.

Figure 2.9. SEM images of as-obtained SrTiO3 samples using 4 g TBAH as the reactant.

To further study the role of TBAH in the reaction, an equal molar amount of sodium hydroxide and water were used to replace TBAH while all other conditions were the same as for STO-180. The PXRD pattern in Figure 2.10 (black) confirms the formation of pure SrTiO3 with a seemingly identical size of coherently scattering crystalline domains as for STO-180. No obvious morphological differences were observed in the SEM (Figure 2.11a). In another experiment, TBAH was substituted by an aqueous solution of tetrabutylphosphonium hydroxide (TBPH). Again no influence of the (organic) cation on the morphology was observed (Figure 2.11b). Hence we conclude that EG plays the dominant role in the formation of the hierarchical desert-rose particles.

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles

Figure 2.10. PXRD patterns of as-obtained SrTiO3 samples using NaOH and tetrabutylphosphonium hydroxide (TBPH) as reactants, and TBAH as the sole solvent.

Figure 2.11. SEM image of as-obtained SrTiO3 samples using NaOH (a) and TBPH (b) as the reactants, and TBAH as the sole solvent (c, d).

In a controlled experiment, TBAH was further employed as the sole solvent for the synthesis of SrTiO3 particles, i.e. EG was omitted. 4 g (5 mmol) TBAH were molten

Metal chalcogenides syntheses using reactions of ionic liquids

at 40 °C and 0.33 mL Ti(OC3H7)4 and 1.1 mmol (0.293 g) SrCl2·6H2O were added to the liquid. Upon ultrasound irradiation for a few minutes, a homogeneous mixture formed. The mixture was reacted at 180 °C for 20 h. Dense, polyhedral

SrTiO3 particles with diameters in the range of 50–150 nm, in Figure 2.11c, d, were obtained. In conclusion, TBAH can be used as “all-in-one” solvent and

[36] reactant for the synthesis of SrTiO3 particles, however, without creating the special 3D hierarchical morphology.

On the basis of the above findings, the formation of desert-rose-like SrTiO3 hierarchical structures mainly involves two steps. In the first stage, Ti(OC3H7)4 hydrolyzes in an alkaline medium and forms an amorphous titanium(IV) intermediate (TiO2 hydrate) with 3D hierarchical morphology by polycondensation. The latter acts as a template for the subsequent reaction with Sr2+ cations, including further condensation, to the final SrTiO3 particles. The transformation preserves the morphology of the intermediate (pseudomorphosis). Sufficiently high temperature and hydroxide concentration are necessary to ensure the complete transformation.

EG plays a crucial role in the formation of the hierarchical morphology. In this mechanism, the twofold deprotonated EG anion forms in an alkaline medium under the solvothermal conditions and acts as a chelating strong ligand to transition metal cations.[2] In the titanium-containing intermediate, the substitution of two hydroxide groups by an EG dianion will block the condensation reaction in the respective directions. We assume this to be the reason for the tiny nano-domains in the dense parts and the branching growth of crystallites that results in the desert-rose morphology.

2.3.3. Nitrogen physisorption of SrTiO3 particles

The porous structure of SrTiO3 products can be observed from the TEM image (Figure 2.1d), and is further confirmed by nitrogen adsorption-desorption isotherms (Figure 2.12). The adsorption-desorption isotherms for STO-180 and STO-210 appear to be intermediate between type II and IV with the H3 hysteresis loops at high relative pressures, indicating the existence of mesopores constructed by the packed platelets of the nanosheets in STO-180 and STO-210.[16] The BET specific surface areas of as-prepared STO-180 and STO-210 are calculated to be 186 and 164 m2 g–1, respectively, i.e. about two orders of magnitude higher than

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles that of SrTiO3 samples obtained by solid-state reactions or by the sol-gel method followed by calcination.[18, 37] The BET data indicate that higher temperature destroys the porous structure to some extent, in good agreement with SEM images of STO-210 (Figure 2.7c, d), which leads to a slight decrease of BET surface area. The Barrett-Joyner-Halenda (BJH) pore-size distribution analysis (inset of Figure 2.12) exhibits a size distribution of 3–10 nm for samples STO-180 and STO-210, revealing the mesoporous characteristics of the obtained SrTiO3 samples.

Figure 2.12. Nitrogen adsorption-desorption isotherms and pore-size distribution (inset) of the STO-180 and STO-210 samples.

2.3.4. Optical and photoelectrochemical properties of SrTiO3 particles

The optical properties of obtained SrTiO3 particles were studied using UV-Vis diffuse reflectance spectra (Figure 2.13a). According to the Kubelka-Munk function,

1/2 the indirect band gap values (Eg) were estimated by plotting (F(R)h) versus photon energy (h) as shown in the inset of Figure 2.13a. The estimated Eg values of STO-180, STO-210, and STO-1100 are found to be 3.62, 3.57, and 3.21 eV, respectively. Compared to the bulk SrTiO3 (STO-1100), the absorption edges of STO-180 and STO-210 were blue shifted by about 0.41 and 0.36 eV, respectively, which is probably attributed to the size quantization effect of as-prepared SrTiO3 particles with smaller crystallite size.[38, 39]

Metal chalcogenides syntheses using reactions of ionic liquids

Figure 2.13. The UV-Vis absorption spectra (a) and photocurrent densities vs. time response (b) of as-synthesized SrTiO3 samples.

Table 2.1. Comparison of structural information and photocurrent densities between the hierarchically structured SrTiO3 products synthesized by the solvothermal method and bulk SrTiO3 samples obtained by the solid-state reaction.

reaction BET surface photocurrent particle size Eg values sample temperature area density (μm) (eV) (°C) (m2 g–1) (μA cm–2) STO-180 180 0.8–1.5 186 3.62 8.3

STO-210 210 0.8–1.5 164 3.57 9.6

STO-1100 1100 — 2.4 3.21 4.3

To investigate the photoresponse of the STO-180, STO-210, and STO-1100 samples, the transient photocurrents of the samples were carried out during repeated ON/OFF illumination cycles at –0.6 V (vs. Ag/AgCl) in a 1.0 M KOH aqueous solution (100 mW cm–2, AM 1.5G). As shown in Figure 2.13b, all of the

SrTiO3 samples delivered prompt and reproducible photocurrent response upon each light irradiation. The STO-180, STO-210, and STO-1100 photoelectrodes exhibited average photocurrent densities of 8.3, 9.6, and 4.3 μA cm–2, respectively. The STO-180 and STO-210 samples have similar photocurrent density values due to their similar particle sizes, crystallinities, and band gaps (Table 2.1).[40] Although STO-1100 has much better crystallinity than samples STO-180 and STO- 210, the photocurrent densities of STO-180 and STO-210 are 1.9 and 2.2 times higher as compared to that of STO-1100, respectively, obviously benefitting from

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles their special 3D hierarchical microstructures. As shown in the SEM and TEM images, the SrTiO3 microstructures are built up by numerous nanoplatelets. The hierarchical microarchitecture prevents aggregation of the platelets and maintains a large accessible and active surface area, being significantly higher than in bulk perovskites.[2, 24] The small particle size ensures that, photogenerated carriers are transferred quickly to the sample surface without recombination.[41] Moreover, the widened band gaps, due to the quantum size effect, may improve the redox activity of the photogenerated electrons and holes in the microstructures.[42]

2.4. Conclusions

In this chapter, the hierarchical desert-rose-like SrTiO3 particles have been successfully prepared through a facile solvothermal reaction. The formation mechanism study of such SrTiO3 hierarchical structures indicates a morphology- conserving process from an amorphous titanium-containing intermediate to the crystalline SrTiO3 phase. Moreover, the crystallization and morphology of

SrTiO3 particles can easily be controlled by varying the synthetic parameters such as reaction time, temperatures, and concentration of the used basic ILs. The as- synthesized 3D hierarchical SrTiO3 microstructures comprise intergrown nanosheets which themselves consist of even smaller nanoplatelets, resulting in a large surface area. The interesting textural characteristics, along with the intrinsically favorable band structure results in an enhanced photo-electrochemical performance of the so-prepared material. The photocurrent density values for

SrTiO3 samples obtained at 180 and 210 °C are 1.9 and 2.2 times higher than that of their corresponding bulk counterparts obtained by solid-state reaction, respectively. Our proposed facile synthetic strategy can potentially be used to generate other perovskite-type titanates with similar 3D hierarchical architectures, and thus to trigger enhanced photoelectrochemical properties.

2.5. References

Metal chalcogenides syntheses using reactions of ionic liquids

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[2] M. Chen, Y. Zhang, L. Xing, Y. Liao, Y. Qiu, S. Yang, W. Li, Morphology- Conserved Transformations of Metal-Based Precursors to Hierarchically Porous Micro-/Nanostructures for Electrochemical Energy Conversion and Storage. Adv. Mater. 2017, 29, 1607015.

[3] D. Fattakhova-Rohlfing, A. Zaleska, T. Bein, Three-Dimensional Titanium Dioxide Nanomaterials. Chem. Rev. 2014, 114, 9487–9558.

[4] F. Jiao, J. Bao, A. H. Hill, P. G. Bruce, Synthesis of Ordered Mesoporous Li– Mn–O Spinel as a Positive Electrode for Rechargeable Lithium Batteries. Angew. Chem. Int. Ed. 2008, 47, 9711–9716.

[5] D. Chen, F. Huang, Y.-B. Cheng, R. A. Caruso, Mesoporous Anatase TiO2 Beads with High Surface Areas and Controllable Pore Sizes: A Superior Candidate for High-Performance Dye-Sensitized Solar Cells. Adv. Mater. 2009, 21, 2206–2210.

[6] J. Liu, J. Jiang, C. Cheng, H. Li, J. Zhang, H. Gong, H. J. Fan, Co3O4

Nanowire@MnO2 Ultrathin Nanosheet Core/Shell Arrays: A New Class of High-Performance Pseudocapacitive Materials. Adv. Mater. 2011, 23, 2076– 2081.

[7] S. Yang, P. Huang, L. Peng, C. Cao, Y. Zhu, F. Wei, Y. Sun, W. Song, Hierarchical flowerlike magnesium oxide hollow spheres with extremely high surface area for adsorption and catalysis. J. Mater. Chem. A 2016, 4, 400– 406.

[8] L.-S. Zhong, J.-S. Hu, A.-M. Cao, Q. Liu, W.-G. Song, L.-J. Wan, 3D Flowerlike Ceria Micro/Nanocomposite Structure and Its Application for Water Treatment and CO Removal. Chem. Mater. 2007, 19, 1648–1655.

[9] H. B. Wu, X. W. Lou, H. H. Hng, Synthesis of Uniform Layered Protonated

Titanate Hierarchical Spheres and Their Transformation to Anatase TiO2 for Lithium-Ion Batteries. Chem. Eur. J. 2012, 18, 2094–2099.

[10] L. S. Zhong, J. S. Hu, H. P. Liang, A. M. Cao, W. G. Song, L. J. Wan, Self- Assembled 3D Flowerlike Iron Oxide Nanostructures and Their Application in Water Treatment. Adv. Mater. 2006, 18, 2426–2431.

[11] H. Yu, C. Guan, X. Rui, B. Ouyang, B. Yadian, Y. Huang, H. Zhang, H. E. Hoster, H. J. Fan, Q. Yan, Hierarchically porous three-dimensional electrodes

of CoMoO4 and ZnCo2O4 and their high anode performance for lithium ion batteries. Nanoscale 2014, 6, 10556–10561.

Solvothermal synthesis and enhanced photo-electrochemical performance of hierarchically structured strontium titanate particles

[12] T. Sakudo, H. Unoki, Dielectric Properties of SrTiO3 at Low Temperatures. Phys. Rev. Lett. 1971, 26, 851–853.

[13] J. H. Haeni, P. Irvin, W. Chang, R. Uecker, P. Reiche, Y. L. Li, S. Choudhury, W. Tian, M. E. Hawley, B. Craigo, A. K. Tagantsev, X. Q. Pan, S. K. Streiffer, L. Q. Chen, S. W. Kirchoefer, J. Levy, D. G. Schlom, Room-temperature

ferroelectricity in strained SrTiO3. Nature 2004, 430, 758–761.

[14] J. Sun, T. Gao, X. Song, Y. Zhao, Y. Lin, H. Wang, D. Ma, Y. Chen, W. Xiang, J. Wang, Y. Zhang, Z. Liu, Direct Growth of High-Quality Graphene on High-

κ Dielectric SrTiO3 Substrates. J. Am. Chem. Soc. 2014, 136, 6574–6577.

[15] Q. Kuang, S. Yang, Template Synthesis of Single-Crystal-Like Porous SrTiO3 Nanocube Assemblies and Their Enhanced Photocatalytic Hydrogen Evolution. ACS Appl. Mater. Interfaces 2013, 5, 3683–3690.

[16] J. H. Pan, C. Shen, I. Ivanova, N. Zhou, X. Wang, W. C. Tan, Q.-H. Xu, D. W. Bahnemann, Q. Wang, Self-Template Synthesis of Porous Perovskite Titanate Solid and Hollow Submicrospheres for Photocatalytic Oxygen Evolution and Mesoscopic Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 14859–14869.

[17] T. K. Townsend, N. D. Browning, F. E. Osterloh, Nanoscale Strontium Titanate Photocatalysts for Overall Water Splitting. ACS Nano 2012, 6, 7420–7426.

[18] D. Wang, J. Ye, T. Kako, T. Kimura, Photophysical and Photocatalytic

Properties of SrTiO3 Doped with Cr Cations on Different Sites. J. Phys. Chem. B 2006, 110, 15824–15830.

[19] X. Li, H. Zhao, W. Shen, F. Gao, X. Huang, Y. Li, Z. Zhu, Synthesis and

properties of Y-doped SrTiO3 as an anode material for SOFCs. J. Power Sources 2007, 166, 47–52.

[20] L. A. Wills, B. W. Wessels, D. S. Richeson, T. J. Marks, Epitaxial growth of

BaTiO3 thin films by organometallic chemical vapor deposition. Appl. Phys. Lett. 1992, 60, 41–43.

[21] M. Kiyotoshi, K. Eguchi, Ultrathin SrTiO3 films prepared by chemical vapor

deposition on Nb-doped SrTiO3 substrates. Appl. Phys. Lett. 1995, 67, 2468–2470.

[22] G. Pfaff, Sol-gel synthesis of strontium titanate powders of various compositions. J. Mater. Chem. 1993, 3, 721–724.

[23] M. N. Kamalasanan, N. Deepak Kumar, S. Chandra, Structural, optical, and

dielectric properties of sol-gel derived SrTiO3 thin films. J. Appl. Phys. 1993, 74, 679–686.

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[24] H. Zhan, Z.-G. Chen, J. Zhuang, X. Yang, Q. Wu, X. Jiang, C. Liang, M. Wu, J. Zou, Correlation between Multiple Growth Stages and Photocatalysis of

SrTiO3 Nanocrystals. J. Phys. Chem. C 2015, 119, 3530–3537.

[25] L. Dong, H. Shi, K. Cheng, Q. Wang, W. Weng, W. Han, Shape-controlled

growth of SrTiO3 polyhedral submicro/nanocrystals. Nano Res. 2014, 7, 1311–1318.

[26] C. Ye, J. Xu, S. Zhou, M. Chen, L. Wu, Facile Synthesis, Self-Assembly, and

Photoelectrical Performance of SrTiO3 Hollow Spheres with Open Holes. Langmuir 2013, 29, 13502–13508.

[27] T. Alammar, I. Hamm, M. Wark, A.-V. Mudring, Low-temperature route to metal titanate perovskite nanoparticles for photocatalytic applications. Appl. Catal., B 2015, 178, 20–28.

[28] S. Tonda, S. Kumar, O. Anjaneyulu, V. Shanker, Synthesis of Cr and La-

codoped SrTiO3 nanoparticles for enhanced photocatalytic performance under sunlight irradiation. Phys. Chem. Chem. Phys. 2014, 16, 23819– 23828.

[29] F. Dang, K.-i. Mimura, K. Kato, H. Imai, S. Wada, H. Haneda, M. Kuwabara,

Growth of monodispersed SrTiO3 nanocubes by thermohydrolysis method. CrystEngComm 2011, 13, 3878–3883.

[30] L. Hu, C. Wang, S. Lee, R. E. Winans, L. D. Marks, K. R. Poeppelmeier, SrTiO3 Nanocuboids from a Lamellar Microemulsion. Chem. Mater. 2013, 25, 378– 384.

[31] X. Yuan, M. Zheng, Y. Zhang, T. Zhou, C. Li, X. Fang, L. Ma, W. Shen, Self-

Assembly of Three-Dimensional SrTiO3 Microscale Superstructures and Their Photonic Effect. Inorg. Chem. 2013, 52, 2581–2587.

[32] G. Xu, X. Huang, Y. Zhang, S. Deng, X. Wei, G. Shen, G. Han, Self-assembly

and formation mechanism of single-crystal SrTiO3 nanosheets via solvothermal route with ethylene glycol as reaction medium. CrystEngComm 2013, 15, 7206–7211.

[33] J. Liu, S. Wen, Y. Hou, F. Zuo, G. J. O. Beran, P. Feng, Boron Carbides as Efficient, Metal-Free, Visible-Light-Responsive Photocatalysts. Angew. Chem. Int. Ed. 2013, 52, 3241–3245.

[34] P. D. Moran, J. R. Bartlett, G. A. Bowmaker, J. L. Woolfrey, R. P. Cooney,

Formation of TiO2 Sols, Gels and Nanopowders from Hydrolysis of Ti(OiPr)4 in AOT Reverse Micelles. J. Sol-Gel Sci. Technol. 1999, 15, 251–262.

[35] Y. Wang, H. Xu, X. Wang, X. Zhang, H. Jia, L. Zhang, J. Qiu, A General

Approach to Porous Crystalline TiO2, SrTiO3, and BaTiO3 Spheres. J. Phys. Chem. B 2006, 110, 13835–13840.

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[36] A. Taubert, Z. Li, Inorganic materials from ionic liquids. Dalton Trans. 2007, 723–727.

[37] A. F. Demirörs, A. Imhof, BaTiO3, SrTiO3, CaTiO3, and BaxSr1−xTiO3 Particles: A General Approach for Monodisperse Colloidal Perovskites. Chem. Mater. 2009, 21, 3002–3007.

[38] L. E. Brus, Electron-electron and electron-hole interactions in small semiconductor crystallites: The size dependence of the lowest excited electronic state. J. Chem. Phys. 1984, 80, 4403–4409.

[39] L. Brus, Electronic wave functions in semiconductor clusters: experiment and theory. J. Phys. Chem. 1986, 90, 2555–2560.

[40] J. Hou, Y. Qu, D. Krsmanovic, C. Ducati, D. Eder, R. V. Kumar, Hierarchical assemblies of bismuth titanate complex architectures and their visible-light photocatalytic activities. J. Mater. Chem. 2010, 20, 2418–2423.

[41] S. P. Adhikari, A. Lachgar, Effect of particle size on the photocatalytic activity

of BiNbO4 under visible light irradiation. J. Phys. Conf. Ser. 2016, 758, 012017.

[42] F. Liu, X. Li, Q. Zhao, Y. Hou, X. Quan, G. Chen, Structural and photovoltaic

properties of highly ordered ZnFe2O4 nanotube arrays fabricated by a facile sol–gel template method. Acta Mater. 2009, 57, 2684–2690.

Metal chalcogenides syntheses using reactions of ionic liquids

Chapter 3

Synthesis of metal sulfides from a deep eutectic solvent precursor*

------*T.Zhang, T. Doert, M. Ruck, Z. Anorg. Allg. Chem., 2017, 643, 1913–1919.

Metal chalcogenides syntheses using reactions of ionic liquids

Synthesis of metal sulfides from a deep eutectic solvent precursor

3.1. Background

Metal sulfides, emerged as a diverse and important class of materials, have attracted particular attention due to their unique properties and promising applications in a variety of energy devices such as Li-ion batteries, supercapacitors,

[1-4] solar cells, and fuel cells. For instance, Sb2S3 rods and hierarchical CuS microspheres were tested as electrode materials in sodium-ion batteries and asymmetric supercapacitors, respectively.[5, 6] In addition, metal sulfides (such as ZnS, CdS, and PbS) are among the quintessential quantum dot materials. Their unique optical properties make them applicable for biological imaging or in light- emitting and photovoltaic devices.[7]

In this chapter, we designed a new DES based on choline chloride and thioacetamide (ChCl/TAA) for the synthesis of a series of binary metal sulfides including Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS. Similar to the “all-in-one” ILs, the ChCl/TAA-based DES serves as solvent, reactant, and template during the reaction process. This deep eutectic solvent precursor (DESP) makes the reaction system much simpler. Additional surfactants, such as 1-oleylamine, cetyltrimethylammonium bromide (CTAB), or 1-dodecanethiol (DDT), which are often needed in conventional synthetic approaches for the formation of nanoparticles or to prevent particle agglomeration, are not necessary as the DES plays the role of a polar capping agent.[8-10]

3.2. Experimental section

3.2.1. Chemicals

Choline chloride ((CH3)3N(CH2CH2(OH)Cl, 98%), lead acetate trihydrate

(Pb(CH3COO)2·3H2O, 98%), cadmium acetate dihydrate (Cd(CH3COO)2·2H2O,

99%), silver nitrate (AgNO3, 98%), bismuth nitrate pentahydrate (Bi(NO3)3·5H2O,

99%), antimony acetate (Sb(CH3COO)3, 98%) were purchased from Sigma Aldrich

GmbH, Germany. Thioacetamide (CH3CSNH2, 98%), zinc nitrate hexahydrate

(Zn(NO3)2·6H2O, 98%), copper(II) nitrate trihydrate (Cu(NO3)2·3H2O, 98%) were

Metal chalcogenides syntheses using reactions of ionic liquids purchased from ABCR GmbH, Germany. All these chemicals are used as received without further purification.

3.2.2. Synthesis of the DESs

The ChCl/TAA based eutectic mixtures were prepared by stirring the two components in a molar ratio of 1 : 2 (25 mmol ChCl (3.49 g) and 50 mmol TAA (3.77 g)) at 70 °C until a homogeneous, yellow liquid formed. Note that the above- prepared DES was used for one experiment mentioned below.

3.2.3. Synthesis of metal sulfides

The respective amounts of the metal salts (1 mmol for Cd, Cu, and Zn, 0.5 mmol for Bi and Sb, and 2 mmol for Ag) were dissolved or homogeneously dispersed in the previously prepared DES at 70 °C. The DES reaction mixtures were then transferred into a 15 mL Teflon-lined stainless steel autoclave for solvothermal reaction at 150 °C or 180 °C for 15 h. After cooling down to room temperature, the precipitate was centrifuged and washed with distilled water and ethanol. The products were dried under vacuum at ambient temperature overnight. Note that for the synthesis of Sb2S3 at 150 °C, the dried sample needed an additional annealing process at 350 °C for 2 h to obtain the final products.

3.2.4. Materials’ characterization

Powder X-ray diffraction (XRD) patterns were typically recorded using a

PANalytical X’Pert Pro diffractometer with Cu-K1 radiation (λ = 1.54 Å). The single crystal XRD studies were performed at room temperature on a four-circle diffractometer Bruker-Nonius APEX II CCD using graphite-monochromated Mo-K1 radiation (λ = 0.71 Å). Structure solutions and refinements were performed with SHELXS-2014 and SHELXL-2014.[11] The morphologies of as-prepared samples were characterized using a Hitachi SU8020 field emission scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS; Oxford X-MaxN). The microsphere size distribution for ZnS was calculated by the software (Nano Measurer System 1.2.5) based on SEM image analysis.

Synthesis of metal sulfides from a deep eutectic solvent precursor

3.3. Results and discussion

3.3.1. Structural and morphological analysis of metal sulfides

Figure 3.1. PXRD patterns of ZnS microspheres synthesized at different

Zn(NO3)2·6H2O concentrations.

Figure 3.2. SEM images of ZnS obtained using 0.5 mmol (a–c) and 1 mmol (d– f) Zn(NO3)2·6H2O as starting materials.

Metal chalcogenides syntheses using reactions of ionic liquids

Figure 3.3. Size distribution of ZnS microspheres obtained by using 0.5 mmol (a) and 1 mmol (b) Zn(NO3)2·6H2O as starting materials, respectively, and SEM image of ZnS microspheres obtained by using 2 mmol Zn(NO3)2·6H2O as starting materials (c).

As is well known, ZnS exists in two modifications, the cubic zinc blende (sphalerite) and the hexagonal wurtzite structure type.[12] Both forms comprise Zn and S atoms in tetrahedral coordination while the stacking sequence of atomic layers is different. Zinc blende is the stable phase at low temperature while wurtzite is a high temperature phase and can be obtained from sphalerite at T  1020 °C.[13, 14] In the PXRD patterns (Figure 3.1) of the products, only reflections that are common to both stacking sequences (cubic and hexagonal) are sharp and intense. It is, thus, hard to point towards the formation of only one crystalline phase.

Figure 3.2 represents typical SEM images of the ZnS microspheres obtained from different Zn(NO3)2·6H2O starting concentrations. The morphology and the size of ZnS microspheres are uniform according to the low-magnification images (Figure 3.2a, d). Detailed morphology studies indicate that the surfaces of the ZnS particles are relatively rough (Figure 3.2b, e) and high-magnification SEM images (Figure 3.2c, f) evidence a hierarchical structure. The microspheres consist of smaller ZnS platelets with lengths of about 200 nm and thicknesses of roughly 40 nm. Figure 3.2c, f propose that the ZnS microspheres contain larger ZnS sheets if a lower amount of Zn(NO3)2·6H2O (0.5 mmol) was used as starting material.

The sizes of the microspheres follow the opposite trend. Higher Zn(NO3)2·6H2O amounts (1 mmol) lead to larger microsphere size with average diameters of 4.5

μm (Figure 3.3b), lower amounts (0.5 mmol Zn(NO3)2·6H2O) end up with average diameters of 2.2 μm (Figure 3.3a). If the amount of Zn(NO3)2·6H2O increases up to 2 mmol, the product contains inhomogeneous ZnS microspheres with two main particle diameters of about 1 μm and about 4 μm (Figure 3.3c). This might be

Synthesis of metal sulfides from a deep eutectic solvent precursor indicative of the precipitation of both ZnS modifications, zinc blende and wurtzite, under this reaction conditions. This observation is in accordance with the XRD results and with previous reports pointing out, that ZnS nanoparticles in sphalerite and wurtzite structure types can be obtained simultaneously in a low temperature synthesis process.[12, 15]

Figure 3.4. Structure of CuS (covellite). (a) Hexagonal unit cell with two distinct Cu sites, Cu(1) (light blue) and Cu(2) (dark blue), and two distinct S sites, S(1)

(rose) and S(2) (red). (b) The network of Cu(2)S4 tetrahedra and Cu(1)S3 triangles. (c) PXRD pattern of CuS single crystals.

Table 3.1. Crystallographic and refinement data for hexagonal CuS single crystals.

Chemical formula CuS

Formula weight 95.60 g/mol

Temperature 296(2) K

Wavelength 0.71073 Å

Crystal size 0.009 x 0.037 x 0.045 mm

Crystal habit metallic-black platelet

Crystal system hexagonal

Space group P63/mmc

Unit cell dimensions a = 3.780(1) Å, c = 16.316(8) Å

Volume 201.87(2) Å3

Z 6

Density (calculated) 4.72 g cm–3

Absorption coefficient 17.0 mm–1

F(000) 270

Metal chalcogenides syntheses using reactions of ionic liquids

Theta range for data collection 2.50 to 35.18°

Index ranges –6<=h<=6, –6<=k<=6, –26<=l<=26

Reflections collected 4240

Independent reflections 215 [R(int) = 0.0258]

Coverage of independent reflections 99.5 %

Absorption correction numerical

Max. and min. transmission 0.86 and 0.52

Refinement method Full-matrix least-squares on F2

Refinement program SHELXL-2014/7 (Sheldrick, 2014)

2 2 2 Function minimized Σ w(Fo – Fc )

Data / restraints / parameters 215 / 0 / 11

Goodness-of-fit on F2 1.11

183 data; I > 2σ(I) R1 = 0.016, wR2 = 0.043

Final R indices all data R1 = 0.023, wR2 = 0.045

2 2 2 w = 1/[σ (Fo ) + (0.0262 P) + 0.0672 P] 2 2 Weighting scheme where P = (Fo + 2Fc )/3

Largest diff. peak and hole 0.52 and –0.42 eÅ–3

R.M.S. deviation from mean 0.101 eÅ–3

Starting from copper(II) nitrate, covellite type CuS single crystals of 20–70 μm lateral size and thickness of about 5–10 μm were obtained from the DESP solution at a relatively low temperature. The formation of the covellite has been confirmed by powder and single crystal X-ray diffraction experiments (Figure 3.4). The crystallographic and refinement data for CuS are listed in Table 3.1. In accordance

[16] with literature data, covellite crystallizes in the hexagonal space group P63/mmc, with lattice parameters a = 3.780(1) Å and c = 16.316(8) Å and six formula units per unit cell (Figure 3.4a). The covellite structure (Figure 3.4b) is a three- dimensional network which consists of double layers of [Cu(2)S(1)S(2)3] tetrahedra that share all of their vertices and Cu(1) in trigonal coordination by S(1) on the central plane of the double layer. Covalent bonds S(2)–S(2) with a bond length of 207 pm connect adjacent double layers.[16-18]

The as-prepared CuS products have the morphology of hexagonal plates with one smooth and one rough side (Figure 3.5). With a high TAA concentration, the polar

Synthesis of metal sulfides from a deep eutectic solvent precursor

DES is preferentially adsorbed onto the [001] face of CuS, which retards the crystal growth along the c-axis and facilitate the growth of thin lamellar CuS crystals (Figure 3.5c) instead. The formation of this specific morphology and a considerable surface roughening was also observed for hexagonal ZnO crystals if polyelectrolytes (PES) were added in the mineralization process.[19] The surface roughening of the ZnO plates was attributed to the PES grafting and interference of the adsorbed polymers with the growth spirals.[20]

Figure 3.5. Low-magnification (a), smooth side (b), and rough side (c) SEM images of the hexagonal CuS plates.

Figure 3.6. PXRD patterns of Bi2S3 (a) and Sb2S3 (d) particles from syntheses at

180 and 150 °C. SEM images of Bi2S3 nanowires (b, c) and structured Sb2S3 plates (e, f).

Metal chalcogenides syntheses using reactions of ionic liquids

Bi2S3 and Sb2S3 as significant semiconductors have a direct band gap of 1.3 eV and 1.78 eV, respectively. They belong to the A2B3 family of heavy atom compounds (A = Sb, Bi, As, and B = S, Se, Te), which attract increasing attention

[4, 21] due to their potential application as thermoelectric materials. Bi2S3 nanowires and Sb2S3 plates were successfully synthesized in a DESP based reaction. Figure

3.6a shows the PXRD patterns of Bi2S3. All reflections can be indexed according to orthorhombic Bi2S3 (bismuthinite structure type, space group Pnma; ICSD no.

171863). The morphologies of the Bi2S3 particles precipitated from DESP are shown in Figure 3.6b, c. Bi2S3 forms relatively uniform flowers composed of nanowires with a diameter of about 30 nm. Increasing the synthesis temperature to 180 °C reduces the agglomeration. Instead, isolated Bi2S3 nanowires of the same size are found (Figure 3.7a). The preferential formation of Bi2S3 nanowires or nanorods as a consequence of its strongly anisotropic crystal structure has been reported previously.[22, 23]

Figure 3.7. SEM images of the Bi2S3 nanowires (a) and Sb2S3 submicron- /microrods (b) obtained at 180 °C.

Synthesis attempts to form antimony sulfide were undertaken at 150 and 180 °C.

Unlike the case of Bi2S3, no Sb2S3 was obtained at 150 °C. The X-ray powder diagrams of the reaction product (Figure 3.8a) point towards an Sb-S precursor complex, but its structure could not be elucidated yet. SEM images of the precursor, as shown in Figure 3.8b, c, indicate that the plate-like particles are composed of stacked sheets with thicknesses of about 50 nm. Annealing this precursor at 350 °C for 2 h results in the formation of Sb2S3 plates, the corresponding XRD pattern is shown in Figure 3.6d (red). All reflections of the diagram can be indexed according to orthorhombic Sb2S3 (stibnite type) with space group Pnma (ICSD no. 22176). Moreover, the sharp and narrow reflections

Synthesis of metal sulfides from a deep eutectic solvent precursor

indicate a highly crystalline product. Figure 3.6e, f show the SEM images of Sb2S3 samples after the annealing process at 350 °C. It seems that the stacked nanosheets are destroyed by the annealing process. The annealed samples consist of rod-like particles, agglomerated to plates with rough surfaces, in contrast to the precursor. When the initial reaction is conducted at 180 °C, phase pure Sb2S3 is obtained in a one-step synthesis. Figure 3.6d (blue) shows the respective PXRD pattern. However, all h0l reflections show considerably higher relative intensities as compared to the reference pattern (black) and the sample after the annealing process (red). This indicates a preferential crystal growth along the [010]

[24] direction, in agreement with the observed shape of the Sb2S3 submicron- /micro-rods (Figure 3.7b).

Figure 3.8. PXRD pattern (a) and SEM images (b, c) of the Sb2S3 precursor.

In order to test the universality of this synthetic protocol, we extended the study towards the synthesis of PbS, CdS, and Ag2S. PbS was obtained from the DESP solution at 150 °C. The corresponding PXRD pattern (Figure 3.9a) indicates the presence of cubic PbS (galena) as the only crystalline product. Figure 3.9b, c show

Metal chalcogenides syntheses using reactions of ionic liquids the evolutions of PbS submicron-sized crystals obtained at different temperatures. At 150 °C, PbS tends to form truncated cubes as revealed by SEM images, Figure 3.9b. However, Figure 3.9c shows PbS crystallites in shapes of truncated octahedra obtained at 180 °C. It is widely accepted that the different growth rates of the [100] and [111] faces of seeds are responsible for the different particle morphologies.[25] The growth of the uncharged [100] faces is obviously preferred at higher temperatures resulting in octahedrally shaped crystals, the growth of the polar [111] faces, favored at lower temperatures, yields cubes.[25, 26] Presumably, a change in surface coordination takes place upon heating.

Figure 3.9. PXRD patterns of the PbS particles (a), and SEM images of PbS nano- /submicron-crystals obtained at 150 °C (b) and 180 °C (c).

CdS nanoparticles were also synthesized. The PXRD patterns of the respective products (Figure 3.10a) show that the obtained CdS adopts the greenockite type

Synthesis of metal sulfides from a deep eutectic solvent precursor

and crystallizes with the hexagonal space group P63mc (ICSD no. 60629). SEM images, shown in Figure 3.10b, c, indicate an average spherical particle size of 30 nm for this product.

Finally, Ag2S particles were obtained. Ag2S crystallizes in the monoclinic low- temperature form in space group P21/n (acanthite structure type, ICSD no. 44507) under these conditions as shown in Figure 3.10d. The SEM images are shown in

Figure 3.10e, f, indicating the prepared Ag2S particles having polyhedral shapes with smooth surfaces and a 2–8 μm size distribution.

Figure 3.10. PXRD patterns of CdS (a) and Ag2S (d) particles, and SEM images of CdS nanoparticle (b, c) and Ag2S particle (e, f).

3.3.2. Influence of the DESP composition

In order to check whether the composition of the DESP has any influence on product formation, purity, or morphology, we performed a series of experiments to synthesize CuS in ChCl : TAA mixtures with 1 : 1, 1 : 2, and 1 : 3 molar ratios, respectively. The main influence was found in the morphology of the final material. As can be seen in the SEM image (Figure 3.11b), the CuS particles are mainly microspheres with a diameter of 5–15 μm when using an equimolar amount of

Metal chalcogenides syntheses using reactions of ionic liquids

ChCl/TAA mixture as the reaction medium. With increasing amount of TAA (ChCl : TAA = 1 : 2 and 1 : 3), the shapes of the CuS particles change to hexagonal plates with 20–70 μm lateral size (Figure 3.11c, d). The plates obtained with the highest TAA amount appear to be thinner than that of a 1 : 2 mixture. This can be attributed to an increased adsorption of the DES on the [001] plane inhibiting the crystal growth in this direction.[19] The PXRD patterns of as-prepared samples indicate the formation of CuS as the sole crystalline product; no composition changes of the products upon changing the DES composition were observed, Figure 3.11a.

Figure 3.11. PXRD patterns of as-synthesized CuS using ChCl/TAA mixtures of different molar ratios (a), and SEM images of obtained CuS particles in ChCl:TAA mixtures with 1:1 (b), 1:2 (c), and 1:3 (d) molar ratios, respectively.

3.3.3. Comparison with DES synthesis at ambient pressure

Synthesis of metal sulfides from a deep eutectic solvent precursor

Figure 3.12. PXRD patterns (a) and SEM images (b–c) of as-synthesized PbS using an autoclave and a flask, all other reaction parameters (ChCl/TAA-ratio, amount of precursor, reaction temperature (T = 150 °C), and reaction time) were kept identical.

As shown above, DES-based synthesis using the solvothermal approach provides access to highly crystalline material with good control over particles size and shape. For comparison, we also prepared PbS particles under ambient pressure condition, i.e. by replacing the autoclave with a common glass flask as the reaction vessel. All other reaction parameters (1 : 2 molar ratio of ChCl/TAA, amount of precursor, reaction temperature (T = 150 °C), and reaction time) were kept identical. As can clearly be seen from the comparison of the PXRD patterns of the respective products (Figure 3.12a), PbS particles from the autoclave reaction have a better crystallinity compared to those obtained in the flask reaction. Moreover, the

Metal chalcogenides syntheses using reactions of ionic liquids respective SEM images evidence smaller particle sizes and more regular crystallite shapes for the PbS particles obtained via the solvothermal route (Figure 3.12b, c).

3.3.4. Mechanism of formation of final products

Although the individual reaction mechanism may vary with different metal precursors, the general reaction process of metal sulfides in DESP can be proposed as illustrated in Scheme 3.1.

Scheme 3.1. Illustration of the formation mechanism of metal sulfides in DESP.

In our case, the ChCl/TAA based DES is formed by mixing ChCl and TAA in a ratio of 1 : 2 at 70 °C according to Scheme 3.2.

Scheme 3.2. Formation of DES from choline chloride and thioacetamide.

In a second step metal salts are added and metal-DES complexes are formed at the same temperature. Thioacetamide complexes of transition metal cations have been studied for quite a long time. However, most of the investigations were performed in classical (organic) solvents. For M[TAA]2Cl2 complexes (M = Zn and Cd), e.g., the thiocarbonyl sulfur atoms are believed to act as the respective donor

[28, 29] atoms for metal coordination. The formation of Cu3(TAA)3Cl3 in distilled water is discussed by Yao et. al.[27] Here, the trimeric molecule shown in Figure 3.13 was evidenced by PXRD data. The molecular precursor, thus, combines the desired composition on the atomic scale with preformed Cu–S bonds. This is believed to foster the product formation at moderate temperatures.

Synthesis of metal sulfides from a deep eutectic solvent precursor

[27] Figure 3.13. Cu3(TAA)3Cl3 complex as suggested by Yao.

At higher temperatures (e.g. 150 °C), the metal-DES complex decomposes to form the final metal sulfide products. In contrast to the classical formation of merely crystalline metal sulfides by thermal or aqueous decomposition of TAA, our investigations suggest that TAA is able to act as sulfide donor for various metals via the formation of (accessible) intermediate M(TAA)n+-complexes in the DES. The decomposition of these complexes takes place at moderate temperatures and leads to a controlled synthesis and crystallization. The accessibility of a variety of metal sulfide nano- or microstructures in ChCl/TAA based DES is probably due to its good solvent and complexing properties, as well as to its role as polar dynamic

[25] capping agent. Yao et al. state the decomposition of the Cu3(TAA)3Cl3 complex in water as a combined oxidation/hydrolysis process: [27]

Cu3(TAA)3Cl3 + O2 + H2O → CuS + NH4Cl + CH3COOH

However, the process may be different for the reactions in dry organic solvents, ILs or DES; detailed decomposition studies for most of the TAA-metal complexes are not known yet and require further studies. Choline chloride is generally considered as stable in these reactions.

Similar to traditional ILs,[30] DESs can be regarded as supramolecular liquids with expanded hydrogen bond systems between the halide anions and hydrogen bond donor components. For the ChCl/TAA based DES system, the hydrogen bonds form between the –NH2 groups of TAA and the chloride anions (Scheme 3.2). As a DES is usually prepared by mixing a quaternary ammonium salt and a molecular (organic) HBD, it provides a combination of properties of both inorganic and organic solvents as well as of different interactions such as electrostatic interactions and hydrogen bonding, allowing to solve and to stabilize a variety of

Metal chalcogenides syntheses using reactions of ionic liquids chemically different starting materials. The use of DESs does also not require the addition of capping agents or surfactants, such as 1-oleyamine and CTAB, in nanoparticles synthesis. DESs seem also to act as templating materials and to promote a directed structure and particle formation.[25]

3.4. Conclusions

In this chapter, a series of metal sulfides such as Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS have been obtained from a DESP solution by a simple and general synthetic method. The reaction mainly proceeds in two steps: i) the dispersion of metal salts in the DES and the formation of a metal-DES complex, and ii) the decomposition of the metal-DES complex and formation of the final products. The sizes and morphologies of the products can be addressed by different reaction conditions such as temperature and concentrations. Some unique shaped products, namely rod-like particles agglomerated Sb2S3 plates, hexagonal CuS single crystals, and three-dimensional hierarchical ZnS microspheres, were prepared via this route. The designed ChCl/TAA based DES proved to be an efficient “all-in-one” reaction medium or DESP for the synthesis of metal sulfides nanoparticles. To prove, whether this route also allows an easy access to ternary sulfides or to other metal chalcogenide nanoparticles will, however, need further investigations. For the latter case, the identification of Se and Te containing DESPs of a suitable reactivity are essential.

3.5. References

[1] P. W. Dunne, C. L. Starkey, M. Gimeno-Fabra, E. H. Lester, The rapid size- and shape-controlled continuous hydrothermal synthesis of metal sulphide nanomaterials. Nanoscale 2014, 6, 2406–2418.

[2] M.-R. Gao, Y.-F. Xu, J. Jiang, S.-H. Yu, Nanostructured metal chalcogenides: synthesis, modification, and applications in energy conversion and storage devices. Chem. Soc. Rev. 2013, 42, 2986–3017.

[3] X. Rui, H. Tan, Q. Yan, Nanostructured metal sulfides for energy storage. Nanoscale 2014, 6, 9889–9924.

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[4] C.-H. Lai, M.-Y. Lu, L.-J. Chen, Metal sulfide nanostructures: synthesis, properties and applications in energy conversion and storage. J. Mater. Chem. 2012, 22, 19–30.

[5] J. Zhang, H. Feng, J. Yang, Q. Qin, H. Fan, C. Wei, W. Zheng, Solvothermal Synthesis of Three-Dimensional Hierarchical CuS Microspheres from a Cu- Based Ionic Liquid Precursor for High-Performance Asymmetric Supercapacitors. ACS Appl. Mater. Interfaces 2015, 7, 21735–21744.

[6] H. Hou, M. Jing, Z. Huang, Y. Yang, Y. Zhang, J. Chen, Z. Wu, X. Ji, One-

Dimensional Rod-Like Sb2S3-Based Anode for High-Performance Sodium-Ion Batteries. ACS Appl. Mater. Interfaces 2015, 7, 19362–19369.

[7] S. Shen, Q. Wang, Rational Tuning the Optical Properties of Metal Sulfide Nanocrystals and Their Applications. Chem. Mater. 2013, 25, 1166–1178.

[8] Y. Wang, A. Tang, K. Li, C. Yang, M. Wang, H. Ye, Y. Hou, F. Teng, Shape- Controlled Synthesis of PbS Nanocrystals via a Simple One-Step Process. Langmuir 2012, 28, 16436–16443.

[9] Y. Du, Z. Yin, J. Zhu, X. Huang, X.-J. Wu, Z. Zeng, Q. Yan, H. Zhang, A general method for the large-scale synthesis of uniform ultrathin metal sulphide nanocrystals. Nat. Commun. 2012, 3, 1177.

[10] Z. Deng, H. Yan, Y. Liu, Controlled Colloidal Growth of Ultrathin Single- Crystal ZnS Nanowires with a Magic-Size Diameter. Angew. Chem. Int. Ed. 2010, 49, 8695–8698.

[11] G. M. Sheldrick, SHELX2014, Programs for crystal structure determination, Universität Göttingen, Germany, 2014.

[12] S. A. Acharya, N. Maheshwari, L. Tatikondewar, A. Kshirsagar, S. K. Kulkarni, Ethylenediamine-Mediated Wurtzite Phase Formation in ZnS. Cryst. Growth Des. 2013, 13, 1369–1376.

[13] F. A. La Porta, J. Andres, M. S. Li, J. R. Sambrano, J. A. Varela, E. Longo, Zinc blende versus wurtzite ZnS nanoparticles: control of the phase and optical properties by tetrabutylammonium hydroxide. Phys. Chem. Chem. Phys. 2014, 16, 20127–20137.

[14] Y. Zhao, Y. Zhang, H. Zhu, G. C. Hadjipanayis, J. Q. Xiao, Low-Temperature Synthesis of Hexagonal (Wurtzite) ZnS Nanocrystals. J. Am. Chem. Soc. 2004, 126, 6874–6875.

[15] W. Vogel, J. Urban, M. Kundu, S. K. Kulkarni, Sphalerite−Wurtzite Intermediates in Nanocrystalline CdS. Langmuir 1997, 13, 827–832.

Metal chalcogenides syntheses using reactions of ionic liquids

[16] H. J. Gotsis, A. C. Barnes, P. Strange, Experimental and theoretical investigation of the crystal structure of CuS. J. Phys.: Condens. Matter 1992, 4, 10461–10468.

[17] W. Liang, M. H. Whangbo, Conductivity anisotropy and structural phase transition in Covellite CuS. Solid State Commun. 1993, 85, 405–408.

[18] H. Nozaki, K. Shibata, N. Ohhashi, Metallic hole conduction in CuS. J. Solid State Chem. 1991, 91, 306–311.

[19] R. Muñoz-Espí, Y. Qi, I. Lieberwirth, C. M. Gómez, G. Wegner, Surface- Functionalized Latex Particles as Controlling Agents for the Mineralization of Zinc Oxide in Aqueous Medium. Chem. Eur. J. 2006, 12, 118–129.

[20] G. Wegner, M. M. Demir, M. Faatz, K. Gorna, R. Munoz-Espi, B. Guillemet, F. Gröhn, Polymers and Inorganics: A Happy Marriage? Macromol. Res. 2007, 15, 95–99.

[21] T. Pandey, A. K. Singh, Simultaneous enhancement of electrical conductivity

and thermopower in Bi2S3 under hydrostatic pressure. J. Mater. Chem. C 2016, 4, 1979–1987.

[22] J. Jiang, S.-H. Yu, W.-T. Yao, H. Ge, G.-Z. Zhang, Morphogenesis and

Crystallization of Bi2S3 Nanostructures by an Ionic Liquid-Assisted Templating Route: Synthesis, Formation Mechanism, and Properties. Chem. Mater. 2005, 17, 6094–6100.

[23] J. W. Thomson, L. Cademartiri, M. MacDonald, S. Petrov, G. Calestani, P.

Zhang, G. A. Ozin, Ultrathin Bi2S3 Nanowires: Surface and Core Structure at the Cluster-Nanocrystal Transition. J. Am. Chem. Soc. 2010, 132, 9058– 9068.

[24] J. Ma, X. Duan, J. Lian, T. Kim, P. Peng, X. Liu, Z. Liu, H. Li, W. Zheng, Sb2S3 with Various Nanostructures: Controllable Synthesis, Formation Mechanism, and Electrochemical Performance toward Lithium Storage. Chem. Eur. J. 2010, 16, 13210–13217.

[25] A. Querejeta-Fernández, J. C. Hernández-Garrido, H. Yang, Y. Zhou, A. Varela, M. Parras, J. J. Calvino-Gámez, J. M. González-Calbet, P. F. Green, N. A. Kotov, Unknown Aspects of Self-Assembly of PbS Microscale Superstructures. ACS Nano 2012, 6, 3800–3812.

[26] Z. Zhuang, X. Lu, Q. Peng, Y. Li, A Facile “Dispersion–Decomposition” Route to Metal Sulfide Nanocrystals. Chem. Eur. J. 2011, 17, 10445–10452.

[27] Z. Yao, X. Zhu, C. Wu, X. Zhang, Y. Xie, Fabrication of Micrometer-Scaled Hierarchical Tubular Structures of CuS Assembled by Nanoflake-built Microspheres Using an In Situ Formed Cu(I) Complex as a Self-Sacrificed Template. Cryst. Growth Des. 2007, 7, 1256–1261.

Synthesis of metal sulfides from a deep eutectic solvent precursor

[28] C. D. Flint, M. Goodgame, Spectral studies of some transition-metal- thioacetamide complexes. J. Chem. Soc. A 1968, 750–752.

[29] R. R. Iyengar, D. N. Sathyanarayana, C. C. Patel, Thioacetamide complexes of nickel(II) and copper(I) chlorides. J. Inorg. Nucl. Chem. 1972, 34, 1088– 1091.

[30] C. S. Consorti, P. A. Z. Suarez, R. F. de Souza, R. A. Burrow, D. H. Farrar, A. J. Lough, W. Loh, L. H. M. da Silva, J. Dupont, Identification of 1,3- Dialkylimidazolium Salt Supramolecular Aggregates in Solution. J. Phys. Chem. B 2005, 109, 4341–4349.

Metal chalcogenides syntheses using reactions of ionic liquids

Chapter 4

Dissolution behavior and activation of selenium in phosphonium based ionic liquids*

------*T.Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Chem. Commun., 2017, 53, 7588–7591.

Metal chalcogenides syntheses using reactions of ionic liquids

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

4.1. Background

Elemental selenium exists in several allotropic forms: crystalline trigonal selenium, crystalline monoclinic selenium and amorphous selenium. The thermodynamically stable form under ambient conditions is the grey trigonal selenium which consists of infinite helical chains of two-bonded selenium atoms.[1, 2] This polymeric structure makes grey selenium difficult to be dissolved in most common solvents. While red monoclinic and amorphous selenium exhibit a certain solubility in carbon

−5 disulphide (CS2), grey trigonal selenium is negligibly soluble in CS2 (4.5 × 10 wt% at 25 °C).[3, 4] The low solubility of selenium in common solvents limits its activation in wet-chemical synthesis. Recently, an increasing number of studies reported about the dissolution behavior of selenium in molecular solvents. Selenium is moderately soluble in strongly basic amines to form selenium complex ions.[5] Hydrazine dissolves selenium to a certain extent and can thus be used as co-solvent for the synthesis of metal selenides or to remove selenium impurities from the final products.[6-8] The binary thiol–amine mixture can also give a solution of grey selenium by reduction of the thiol in the presence of the amine, such as a mixture of ethanethiol and ethylenediamine or a mixture of dodecanethiol and oleylamine.[4, 9] Trioctylphosphane and tributylphosphane are often used for the preparation of soluble selenium precursors to synthesize metal selenides based quantum dots.[10, 11] Another method is directly dissolving selenium powder in high-boiling-point solvents like olive oil, silicone oil, and oleylamine (OLA).[12-14] However, these examples also show some drawbacks. Hydrazine has the disadvantage of being highly toxic and volatile, and thiols may lead to sulphur contamination of the final products. Trioctylphosphane and tributylphosphane are air sensitive and require inert reaction conditions. For the direct dissolving approach, the reactivity of selenium precursors is difficult to control.

In contrast, studies about the dissolution behavior of selenium in ILs are rare. Rodríguez et al. found that selenium can react with imidazolium acetate ILs to yield imidazole-2-selone. In most other cases, the addition of a base is needed for deprotonation of the imidazolium cation at the C2 position.[15] Boros et al. gave a very simple description that a dilute solution of selenium (< 1 wt%) in trihexyltetradecylphosphonium decanoate displays an orange color, which could

Metal chalcogenides syntheses using reactions of ionic liquids

•− [16] be due to the formation of [Se3] radical anions. Ding et al. found that 2 mmol selenium can be dissolved in 4 g tetrabutylphosphonium chloride by microwave heating at 240 °C to form a dark brown solution. Upon cooling, needle-like selenium microcrystals precipitate from the solution.[17]

In our case, however, we found that a relatively high amount of selenium powder can be dissolved in some phosphonium ILs at a high temperature (e.g. 5 mmol selenium was soluble in 10 mmol trihexyltetradecylphosphonium chloride ([P6 6 6

14]Cl) after a continuous heating at 220 °C under Ar). The formed clear solutions upon cooling to room temperature are stable under air for a relatively long time.

In this chapter, we give a mechanism study responsible to the dissolution process. A series of 31P and 77Se nuclear magnetic resonance (NMR) experiments were used to investigate the dissolution behavior of selenium in phosphonium based ILs. Furthermore, these selenium solutions in ILs can be used as selenium precursors for metal selenides (e.g. NiSe2 and ZnSe) synthesis.

4.2. Experimental section

4.2.1. Chemicals

Tetrabutylphosphonium chloride ([P4 4 4 4]Cl, >95 %), trihexyltetradecylphosphonium chloride ([P6 6 6 14]Cl, >95 %), trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide ([P6 6 6 14][NTf2], >98 %), trihexyltetradecylphosphonium decanoate ([P6 6 6 14][decanoate], >95 %) were purchased from IO-LI-TEC, Germany. Selenium powder (99.99 %) was purchased from ChemPur, Germany.

Nickel(II) nitrate hexahydrate (Ni(NO3)2·6H2O, 99 %) and trioctylphosphane (TOP, 97 %) were purchased from ABCR GmbH, Germany. The ionic liquids (ILs) are dried at 110 °C overnight before using. The other chemicals are used as received without further purification.

4.2.2. Dissolution of Se in [P6 6 6 14]Cl

2 mmol (157.92 mg) selenium powder was added to 5 g (9.63 mmol) [P6 6 6 14]Cl and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min–1. After about 5 h of heating, the final reaction mixture was found to be clear

Dissolution behavior and activation of selenium in phosphonium based ionic liquids light yellow. In addition, a dissolution reaction was also performed by heating stoichiometric amounts of selenium and [P6 6 6 14]Cl mixture at 220 °C under Ar overnight. The supernatant was used for the following separation.

Separation of P6 6 6Se and P6 6 14Se from the reaction solution via flash chromatography: silica gel, diameter 1 cm, length 10 cm, Et2O.

31 31 1 77 P NMR (CDCl3, 25 °C): δ( P) = 36.5 ppm, JPSe = 680 Hz; Se NMR (CDCl3,

77 1 25 °C): δ( Se) = –383.1 ppm, –383.3 ppm, JSeP = 680 Hz.

4.2.3. Dissolution of Se in [P6 6 6 14][decanoate]

2 mmol (157.92 mg) selenium powder was added to 5 g (7.63 mmol) [P6 6 6

14][decanoate] and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min–1. After about 10 min of heating, the final reaction mixture was found to be clear orange and to give an orange gel after cooling to room temperature. In addition, a dissolution reaction was also performed by heating stoichiometric amounts of selenium and [P6 6 6 14][decanoate] mixture at 220 °C under Ar for 1 h. The selenium powder was completely dissolved and an orange gel was obtained after cooling down which was used for the following separation.

Separation of P6 6 6Se and P6 6 14Se from the reaction solution via flash chromatography: silica gel, diameter 1 cm, length 10 cm, Et2O.

4.2.4. Dissolution of Se in [P4 4 4 4]Cl

2 mmol (157.92 mg) selenium powder was added to 5 g (16.96 mmol) [P4 4 4 4]Cl and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min–1. After about 4 h of heating, the final reaction mixture was found to be clear light yellow and to give a white solid after cooling to room temperature.

4.2.5. Preparation of trioctylphosphane selenide solution

A mixture of stoichiometric amounts of selenium powder and trioctylphosphane was stirred at room temperature under Ar overnight. A clear colorless solution was obtained and used for the NMR experiment.

4.2.6. Synthesis of nickel diselenide

Metal chalcogenides syntheses using reactions of ionic liquids

1 mmol (78.96 mg) selenium powder was added to 4 g [P6 6 6 14]Cl and heated up to 220 °C under Ar at a heating rate of 5 °C min–1. When the selenium solution turned colorless, it was cooled down to 100 °C under constant stirring. 0.5 mmol

(145.40 mg) Ni(NO3)2·6H2O was added to 4 g [P6 6 6 14]Cl and stirred at 120 °C under Ar until a transparent solution formed. These two solutions were mixed and stirred at 100 °C for 5 min. The final mixture was transferred into a 25 mL Teflon- lined autoclave. Then the sealed autoclave was placed in the oven at 200 °C. After 24 h, the autoclave was removed from the furnace and cooled down. The precipitates were washed with water and ethanol several times and carefully collected by centrifugation. The final products were dried in vacuum at room temperature overnight. The synthesis of NiSe2 was also performed using different amount of Ni(NO3)2·6H2O and different temperatures with the same procedure as described above.

4.2.7. Synthesis of zinc selenide

0.3 mmol (23.69 mg) selenium powder was added to 4 g [P6 6 6 14][decanoate] and heated up to 200 °C under Ar at a heating rate of 5 °C min–1. When the selenium was dissolved completely, it was kept at 200 °C for another 30 min and then cooled down to 100 °C under constant stirring. 0.3 mmol (189.70 mg) zinc stearate was added to 4 g [P6 6 6 14][decanoate] and stirred at 130 °C under Ar until a transparent solution formed. These two solutions were mixed and stirred at 100 °C for 5 min. Then the mixture was heated to 250 °C at a heating rate of 5 °C min–1 and kept at 250 °C for 1 h to get a light yellow colloidal solution. After cooling down the reaction solution, the ZnSe nanocrystals were precipitated by adding 15 mL methanol into the colloidal solution, centrifuged, washed repeatedly, and dispersed in toluene for following characterizations.

4.3. Results and discussion

4.3.1. Dissolution tests of selenium in phosphonium ionic liquids

In the experiments, four phosphonium based ILs with different cations and anions were chosen for the study of dissolution behavior of selenium using NMR experiments, namely, tetrabutylphosphonium chloride ([P4 4 4 4]Cl), trihexyltetra-

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

decylphosphonium chloride ([P6 6 6 14]Cl), trihexyltetradecylphosphonium decanoate ([P6 6 6 14][decanoate]), and trihexyltetradecylphosphonium bis(trifluoromethylsulfonyl)amide ([P6 6 6 14][NTf2]). All ILs used in our experiments were dried at 110 °C in dynamic vacuum overnight prior to use. [P6 6 6 14]Cl was taken as a first example to test the dissolution behavior of selenium. 2 mmol (157.92 mg) selenium powder were added to 5 g (9.63 mmol) [P6 6 6 14]Cl and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min−1. The color of the mixture changed from dark brown to black and finally to light yellow after about 5 h of heating.

31 Figure 4.1. P NMR spectra of [P6 6 6 14]Cl (a), [P6 6 6 14]Cl after heating at 220 °C for 5 h under Ar (b), and reaction solution of 2 mmol selenium in 5 g [P6 6 6 14]Cl at 220 °C for 5 h under Ar (c); (C6D6-capillary, 80 °C).

The resulting clear solution was characterized by 31P and 77Se NMR spectroscopy. For comparison, we also measured the 31P NMR spectra of the IL (δ(31P) = 33.2 ppm) after drying and after heating at 220 °C for 5 h as shown in Figure 4.1a, b,

Metal chalcogenides syntheses using reactions of ionic liquids

respectively. It indicates the relatively high stability of pure IL [P6 6 6 14]Cl after long-time heat treatment. The NMR investigation of the reaction mixture of [P6 6 6

31 14]Cl with selenium reveals the formation of a phosphane selenide instead. The P NMR spectrum shows, compared to the starting material, a downfield shifted

31 1 resonance at δ( P) = 38.2 ppm with a JPSe coupling constant of 702 Hz (Figure 4.1c). The corresponding 77Se NMR spectrum (Figure 4.2) shows two doublet resonances at δ(77Se) = −389.5 ppm and δ(77Se) = −389.8 ppm which exhibit the

1 same coupling constant of JSeP = 702 Hz, evidencing the formation of two compounds (P6 6 6Se and P6 6 14Se). The separation of the formed trialkylphosphane selenides from the IL [P6 6 6 14]Cl succeeds via column chromatography (see Experimental section for details).

Figure 4.2. 77Se NMR spectrum of the reaction solution of 2 mmol selenium in 5 g [P6 6 6 14]Cl at 220 °C for 5 h under Ar (C6D6-capillary, 80 °C).

To support the assumption of the formed phosphane selenides, we also reacted TOP with elemental selenium to give trioctylphosphane selenide (TOPSe) (see Experimental section for reaction details). The corresponding NMR spectroscopic

31 1 77 data (δ( P) = 36.2 ppm, JPSe = 710 Hz; δ( Se) = −387.4 ppm) compare well with those of the phosphane selenide P6 6 6Se and P6 6 14Se. Mechanistically, we suppose that one of the alkyl substituents, R14 or R6, in [P6 6 6 14]Cl is abstracted to form an alkyl chloride along with the corresponding phosphane via nucleophilic

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

[18] attack of the anion of ILs at higher temperature (SN2 attack). This intermediately formed trialkylphosphane P6 6 6 or P6 6 14 reacts with selenium to the respective phosphane selenide.

Similarly, the dissolution of selenium was also tested in [P4 4 4 4]Cl, showing the same result as for the aforementioned reaction of [P6 6 6 14]Cl with selenium (see Experimental section for reaction details). The NMR spectroscopic investigation, however, reveals no formation of two compounds, due to four equivalent tetra-n-

+ butyl chains of [P4 4 4 4] cation (Figure 4.3).

31 Figure 4.3. P NMR spectra of [P4 4 4 4]Cl after drying (a), reaction solution of 2 77 mmol selenium in 5 g [P4 4 4 4]Cl at 220 °C for 4 h under Ar (b), and Se NMR spectrum of the reaction solution of 2 mmol selenium in 5 g [P4 4 4 4]Cl at 220 °C for 4 h under Ar (c); (C6D6-capillary, 80 °C).

Figure 4.4. 31P (a) and 77Se NMR (b) spectra of the separated phosphane selenides from the Se-[P6 6 6 14][decanoate] reaction solution (CDCl3, 25 °C).

Metal chalcogenides syntheses using reactions of ionic liquids

In a second series of experiments, we changed the anions of the phosphonium based ILs to investigate their influence on the dissolution behavior of selenium.

For example, 5 g (7.63 mmol) [P6 6 6 14][decanoate] can dissolve 2 mmol (157.92 mg) grey selenium powder in 10 min at 220 °C under Ar to give an orange gel upon cooling to room temperature. The dissolution of selenium in [P6 6 6

14][decanoate] is, thus, significantly faster compared to the ILs with chloride anions. The separation of the formed phosphane selenides from the IL also succeeds via column chromatography. The NMR spectroscopic investigation reveals the formation of the respective phosphane selenides (Figure 4.4).

In contrast, selenium shows a very low solubility in [P6 6 6 14][NTf2]. In our experiments, it was even impossible to dissolve 1 mmol (78.96 mg) selenium in

5 g (6.55 mmol) [P6 6 6 14][NTf2] at 220 °C for 7 h under Ar. This is because of the quite inert, weakly coordinating bis(trifluoromethylsulfonyl)amide anion of the IL.[19] It is, thus, more difficult to form alkyl bis(trifluoromethylsulfonyl)amide.

Scheme 4.1. I) Reactions of [P6 6 6 14]Cl and [P6 6 6 14][decanoate] with selenium to give the corresponding phosphane selenides; II) Reaction of [P4 4 4 4]Cl with selenium to give tributylphosphane selenide.

On the basis of above investigations, we can conclude that the dissolution of selenium in phosphonium based ILs is due to the chemical reactions of ILs with selenium at high temperature forming the respective trialkylphosphane selenides (Scheme 4.1). The main factor influencing the dissolution behavior of selenium seems to be the anion of ILs. The reactivity of anions of phosphonium based ILs

– – – interacted with selenium decreases in the order C9H19COO > Cl > NTf2 .

4.3.2. Synthesis of metal selenides in phosphonium ionic liquids

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

To test the reactivity of the pre-formed phosphane selenides as selenium precursors, a chemical synthesis of metal selenides in [P6 6 6 14]Cl was performed.

Nickel diselenide (NiSe2) which crystallizes in the pyrite crystal structure has been proven as a promising hydrogen evolution reaction (HER) catalyst[20] and was thus chosen as one target compound. Ni(NO3)2·6H2O was dissolved in [P6 6 6 14]Cl and well mixed with the pre-formed phosphane selenides solution at 100 °C. The combined solution was then heated to 200 °C for 24 h, to give the final products (see details in Experimental section).

Figure 4.5. PXRD patterns of NiSe2 particles synthesized with different amount of

Ni(NO3)2·6H2O at 200 °C (a), and at 220 °C and 250 °C (b).

The PXRD patterns of NiSe2 particles obtained at different reaction conditions are shown in Figure 4.5. The powder patterns indicate that the as-prepared NiSe2 products obtained at 200 °C starting from different Ni(NO3)2·6H2O amounts are phase pure. However, NiSe impurities occur when using 1 mmol Ni(NO3)2·6H2O at

220 °C and 0.5 mmol Ni(NO3)2·6H2O at 250 °C, showing temperature is the main influence parameter for the phase formation. We also investigated the effect of the amount of Ni(NO3)2·6H2O on the morphologies of the final NiSe2 samples. SEM images of NiSe2 products are shown in Figure 4.6. Starting with 0.5 mmol

Ni(NO3)2·6H2O, the lowest amount in our reactions, the shapes of NiSe2 particles are regular octahedral and the surfaces are relatively smooth; the particles are largely isolated. With increasing amount of Ni(NO3)2·6H2O (1 mmol and 2 mmol), the octahedral shape of the crystals becomes more irregular and the faces are rougher. Obviously, the particle sizes are smaller and the crystals tend to

Metal chalcogenides syntheses using reactions of ionic liquids intergrow. This ILs based synthetic method simplifies the reaction system, which only needs a metal source and Se-ILs solution.

Figure 4.6. SEM images of NiSe2 samples obtained at 200 °C using 0.5 mmol (a– c), 1 mmol (d–f), and 2 mmol (g–i) Ni(NO3)2·6H2O as starting materials, respectively.

It should be noted that this synthetic strategy can also be extended to other metal selenides. We prepared, e.g., ZnSe nanocrystals using pre-formed phosphane selenides as selenium precursor and zinc stearates (ZnSt2) as zinc precursor in [P6

6 6 14][decanoate] (see Experimental section for reaction details). The PXRD patterns (Figure 4.7a) suggest the formation of crystalline ZnSe nanocrystals with the hexagonal wurtzite type structure. The TEM images (Figure 4.7b, c) show that the ZnSe nanoparticles (5–10 nm) self-assemble into larger aggregates (50–70 nm).

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

Figure 4.7. PXRD patterns (a) and TEM images (b, c) of as-prepared ZnSe nanocrystals at 250 °C.

4.4. Conclusions

In this chapter, a detailed study of the dissolution behavior of grey selenium in phosphonium based ILs has been carried out using NMR spectroscopy. Trialkylphosphane selenides were formed due to the chemical reactions of ILs at high temperatures in the presence of selenium by elimination of one alkyl substituent. Additionally, these trialkylphosphane selenides provide a new and potentially large family of selenium precursors by a reasonable design of phosphonium based ILs. As examples, octahedral NiSe2 microparticles and ZnSe nanoparticles were synthesized using these phosphane selenides as selenium precursors in [P6 6 6 14]Cl and [P6 6 6 14][decanoate], respectively. The shapes and sizes can be controlled easily by temperature and reactant concentrations, providing a simple and efficient reaction system for the synthesis of metal selenides.

4.5. References

Metal chalcogenides syntheses using reactions of ionic liquids

[1] X. Zhou, P. Gao, S. Sun, D. Bao, Y. Wang, X. Li, T. Wu, Y. Chen, P. Yang, Amorphous, Crystalline and Crystalline/Amorphous Selenium Nanowires and Their Different (De)Lithiation Mechanisms. Chem. Mater. 2015, 27, 6730– 6736.

[2] E. Wiberg, N. Wiberg, A. F. Holleman, Inorganic chemistry, Academic Press ; De Gruyter, San Diego; Berlin; New York, 2001.

[3] J. W. Moody, R. C. Himes, Monoclinic selenium crystal growth. Mater. Res. Bull. 1967, 2, 523–530.

[4] D. H. Webber, J. J. Buckley, P. D. Antunez, R. L. Brutchey, Facile dissolution of selenium and tellurium in a thiol-amine solvent mixture under ambient conditions. Chem. Sci. 2014, 5, 2498–2502.

[5] R. Arakawa, A. Sasao, N. Sonoda, Electrospray ionization mass spectrometric analysis of chemical reactions of dissolution of selenium in strongly basic amines. J. Mass Spectrom. 2005, 40, 66–69.

[6] C.-H. Chung, S.-H. Li, B. Lei, W. Yang, W. W. Hou, B. Bob, Y. Yang, Identification of the Molecular Precursors for Hydrazine Solution Processed

CuIn(Se,S)2 Films and Their Interactions. Chem. Mater. 2011, 23, 964–969.

[7] X. Liu, N. Zhang, R. Yi, G. Qiu, A. Yan, H. Wu, D. Meng, M. Tang, Hydrothermal synthesis and characterization of sea urchin-like nickel and cobalt selenides nanocrystals. Mater. Sci. Eng. B 2007, 140, 38–43.

[8] Z. Zhuang, Q. Peng, J. Zhuang, X. Wang, Y. Li, Controlled Hydrothermal Synthesis and Structural Characterization of a Nickel Selenide Series. Chem. Eur. J. 2006, 12, 211–217.

[9] Y. Liu, D. Yao, L. Shen, H. Zhang, X. Zhang, B. Yang, Alkylthiol-Enabled Se Powder Dissolution in Oleylamine at Room Temperature for the Phosphine- Free Synthesis of Copper-Based Quaternary Selenide Nanocrystals. J. Am. Chem. Soc. 2012, 134, 7207–7210.

[10] Z. A. Peng, X. Peng, Formation of High-Quality CdTe, CdSe, and CdS Nanocrystals Using CdO as Precursor. J. Am. Chem. Soc. 2001, 123, 183– 184.

[11] L. Qu, X. Peng, Control of Photoluminescence Properties of CdSe Nanocrystals in Growth. J. Am. Chem. Soc. 2002, 124, 2049–2055.

[12] Q. Guo, S. J. Kim, M. Kar, W. N. Shafarman, R. W. Birkmire, E. A. Stach, R.

Agrawal, H. W. Hillhouse, Development of CuInSe2 Nanocrystal and Nanoring Inks for Low-Cost Solar Cells. Nano Lett. 2008, 8, 2982–2987.

[13] Q. Dai, N. Xiao, J. Ning, C. Li, D. Li, B. Zou, W. W. Yu, S. Kan, H. Chen, B. Liu, G. Zou, Synthesis and Mechanism of Particle- and Flower-Shaped ZnSe

Dissolution behavior and activation of selenium in phosphonium based ionic liquids

Nanocrystals: Green Chemical Approaches toward Green Nanoproducts. J. Phys. Chem. C 2008, 112, 7567–7571.

[14] A. K. Sinha, A. K. Sasmal, S. K. Mehetor, M. Pradhan, T. Pal, Evolution of amorphous selenium nanoballs in silicone oil and their solvent induced morphological transformation. Chem. Commun. 2014, 50, 15733–15736.

[15] H. Rodriguez, G. Gurau, J. D. Holbrey, R. D. Rogers, Reaction of elemental chalcogens with imidazolium acetates to yield imidazole-2-chalcogenones: direct evidence for ionic liquids as proto-carbenes. Chem. Commun. 2011, 47, 3222–3224.

[16] E. Boros, M. J. Earle, M. A. Gilea, A. Metlen, A.-V. Mudring, F. Rieger, A. J. Robertson, K. R. Seddon, A. A. Tomaszowska, L. Trusov, J. S. Vyle, On the dissolution of non-metallic solid elements (sulfur, selenium, tellurium and phosphorus) in ionic liquids. Chem. Commun. 2010, 46, 716–718.

[17] K. Ding, H. Lu, Y. Zhang, M. L. Snedaker, D. Liu, J. A. Maciá-Agulló, G. D. Stucky, Microwave Synthesis of Microstructured and Nanostructured Metal Chalcogenides from Elemental Precursors in Phosphonium Ionic Liquids. J. Am. Chem. Soc. 2014, 136, 15465–15468.

[18] B. Wang, L. Qin, T. Mu, Z. Xue, G. Gao, Are Ionic Liquids Chemically Stable? Chem. Rev. 2017, 117, 7113–7131.

[19] S. Tyrrell, M. Swadzba-Kwasny, P. Nockemann, Ionothermal, microwave- assisted synthesis of indium(iii) selenide. J. Mater. Chem. A 2014, 2, 2616– 2622.

[20] J. Zhuo, M. Cabán-Acevedo, H. Liang, L. Samad, Q. Ding, Y. Fu, M. Li, S. Jin, High-Performance Electrocatalysis for Hydrogen Evolution Reaction Using Se-Doped Pyrite-Phase Nickel Diphosphide Nanostructures. ACS Catal. 2015, 5, 6355–6361.

Metal chalcogenides syntheses using reactions of ionic liquids

Chapter 5

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures*

------*T.Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, 2018, doi:10.1002/chem.201800320.

Metal chalcogenides syntheses using reactions of ionic liquids

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

5.1. Background

The element tellurium is a typical p-type semiconductor with a narrow band gap (0.35 eV) at room temperature, exhibiting attractive physical and chemical properties with potential applications in a variety of different areas such as gas sensing, ion detection and removal, photoconductivity, or lithium–tellurium batteries.[1-5] The crystal structure of tellurium consists of infinite helical chains of covalently bonded tellurium atoms. These helical chains are stacked parallel to each other and held together by van der Waals forces to form a trigonal lattice. The chain-like structure of tellurium is reflected in a needle-like crystal growth, where the needle axis coincides with the crystallographic c-direction.[5] Thus, typical one dimensional (1D) tellurium nanostructures, including nanowires, nanorods, nanotubes, and nanobelts have been prepared by different methods.[6- 9] In contrast, more complex hierarchical tellurium structures are scarce.[10, 11] Furthermore, the polymeric nature of tellurium hinders its dissolution in many of

[12] the common solvents, tellurium can be regarded as insoluble in CS2, for example.[13] More recently, tellurium has been reported to have 9.3 wt% solubility in a 1:4 mixture (vol/vol) of ethanethiol and ethylenediamine at room temperature.[13] Small portions of tellurium can also be dissolved in ethylenediamine or glycerol under solvothermal conditions.[14, 15] The limited range of solvents for the dissolution of tellurium restricts the use of wet-chemical routes for the preparation of metal tellurides. In addition, there are very few studies to investigate the dissolution behavior of tellurium in ILs.[16, 17] For example, Boros et al. found that the dilute solution of tellurium in trihexyltetradecylphosphonium decanoate displays a purple color.[16] Ding et al. observed the recrystallization behavior in tetrabutylphosphonium chloride upon microwave heating and subsequent cooling and synthesized a series of metal tellurides in ILs using tellurium powder as starting material.[17] However, the mechanisms occurring during dissolution and reaction remained unclear.

As a continuous study of chapter 4, in this chapter, we systematically investigated the dissolution behavior of tellurium in various phosphonium ILs, namely, tetrabutylphosphonium decanoate ([P4 4 4 4][decanoate]), trihexyltetradecylphosphonium chloride ([P6 6 6 14]Cl), trihexyltetradecylphosphonium dicyanamide

Metal chalcogenides syntheses using reactions of ionic liquids

([P6 6 6 14][N(CN)2]), and trihexyltetradecylphosphonium decanoate ([P6 6 6

14][decanoate]). It was found that the interplay between ILs and tellurium was much more complicated than that of selenium. The formed tellurium solutions were further utilized as tellurium feedstocks for the synthesis of nanostructured tellurium and metal tellurides such as Bi2Te3 and Ag2Te.

5.2. Experimental section

5.2.1. Chemicals

Trihexyltetradecylphosphonium chloride ([P6 6 6 14]Cl, >95 %), trihexyltetradecylphosphonium dicyanamide ([P6 6 6 14][N(CN)2], >95 %), trihexyltetradecylphosphonium decanoate ([P6 6 6 14][decanoate], >95 %) were purchased from io-li-tec,

Germany. Tetrabutylphosphonium hydroxide solution (40 wt. % in H2O) was purchased from Alfa Aesar, Germany. Decanoic acid (≥98.0 %), bismuth chloride

(BiCl3, ≥98.0 %), sodium dicyanamide (NaN(CN)2, 96 %), silver nitrate (AgNO3, 98 %) was purchased from Sigma Aldrich, Germany. Tellurium powder (Te, -325 mesh, 99.99 %), bismuth(III) acetate (Bi(CH3COO)3, 99 %) was purchased from ABCR GmbH, Germany. The ionic liquids (ILs) are dried at 110 °C overnight before using. The tellurium powder was reduced in H2 atmosphere at 400 °C for 7 h. The other chemicals are used as received without further purification.

5.2.2. Dissolution of tellurium in [P6 6 6 14]Cl

Preparation of tellurium solution of [P6 6 6 14]Cl. Tellurium does not dissolve well in

[P6 6 6 14]Cl under the condition used for the other ILs as described below. Thus, the tellurium solution of [P6 6 6 14]Cl was prepared using a solvothermal method.

12 mg tellurium powder was added to 4 g [P6 6 6 14]Cl in an Ar-filled glovebox. The mixture was then transferred into a 25 mL Teflon lined stainless steel autoclave. The autoclave was sealed in the glovebox and was then placed in the oven for 48 h at 250 °C. After the autoclave was cooled down to room temperature, a clear, light yellow tellurium solution of [P6 6 6 14]Cl was obtained.

Preparation of tellurium single crystals and 3D leaf-like tellurium microstructures. 18 mg, 24 mg, and 32 mg tellurium powder were also used for dissolution tests, respectively, by the same method and keeping the other reaction parameters

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures constant. In these cases, tellurium single crystals were obtained when the autoclave was cooled down to room temperature. The supernatant was separated from the tellurium single crystals and placed statically at room temperature for several days under air atmosphere. A black precipitate was obtained after the ILs was washed away by toluene several times and the product was dried under vacuum at room temperature overnight.

5.2.3. Dissolution of tellurium in [P6 6 6 14][N(CN)2]

The dissolution test of tellurium in [P6 6 6 14][N(CN)2] was carried out in a 25 mL three-neck flask. Typically, 1 mmol (127.6 mg) tellurium powder was added to

5.5 g (10 mmol) [P6 6 6 14][N(CN)2] and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min–1. The color of the mixture changed to black after a few minutes and finally turned into a red suspension with brown precipitate after about 6 h of heating. A red clear reaction solution was obtained by removal of this brown precipitate by centrifugation. In addition, the dissolution reaction was also performed by heating a tellurium and [P6 6 6 14][N(CN)2] mixture with molar ratios of 1 : 2 and 1 : 1, respectively, at 220 °C under Ar for about 24 h. Some unreacted tellurium powder (as evidenced by PXRD) and formed insoluble brown powder were observed after the respective reaction.

5.2.4. Dissolution of tellurium in [P6 6 6 14][decanoate]

The dissolution test of tellurium in [P6 6 6 14][decanoate] was carried out in a 25 mL three-neck flask. Typically, 1 mmol (127.6 mg) tellurium powder was added to 5 g (7.63 mmol) [P6 6 6 14][decanoate] and the reaction mixture was heated to 220 °C under Ar at a heating rate of 5 °C min–1. The color of the mixture changed to black after a few minutes and finally was found to be clear yellow after about 4 h of heating. In addition, the dissolution reaction was also performed by heating a tellurium and [P6 6 6 14][decanoate] mixture with a molar ratio of 1 : 2 and 1 : 1, respectively, at 220 °C under Ar for about 24 h. Unreacted tellurium powder (according to PXRD) was found after the respective reactions.

5.2.5. Dissolution of tellurium in [P4 4 4 4][decanoate]

[P4 4 4 4][decanoate] was obtained by neutralizing tetrabutylphosphonium

[18] hydroxide with decanoic acid. The dissolution test of tellurium in [P4 4 4

Metal chalcogenides syntheses using reactions of ionic liquids

4][decanoate] was carried out in a 25 mL three-neck flask. In a typical dissolution test, 1 mmol (127.6 mg) tellurium powder was added to 5 g (11.6 mmol) [P4 4 4

4][decanoate] and the reaction mixture was heated at 220 °C under Ar for about 5 h. The color of the mixture changed to black after a few minutes and finally was found to be clear yellow when cooling down to room temperature. In addition, the dissolution reaction was also performed by heating stoichiometric amounts of tellurium and [P4 4 4 4][decanoate] mixture at 220 °C under Ar for about 24 h. Unreacted tellurium powder was observed and identified by PXRD after the reaction.

5.2.6. Synthesis of Bi2Te3 in [P6 6 6 14]Cl

0.5 mmol (157.7 mg) BiCl3 was dissolved in 2 g [P6 6 6 14]Cl at 100 °C under Ar to form a yellow solution. The tellurium precursor solution was freshly prepared by the above-mentioned method. These two solutions were thoroughly mixed and then transferred into a Teflon lined stainless steel autoclave in an Ar-filled glovebox. The sealed autoclave was heated at 220 °C for 20 h. After the reaction, the autoclave was allowed to cool down to room temperature. The precipitates were washed by toluene several times and collected by centrifugation. The final products were dried under vacuum at room temperature overnight. Bi2Te3 samples were also synthesized using 0.063 mmol (20 mg) BiCl3, leaving all other reaction conditions unchanged.

5.2.7. Synthesis of bismuth-decanoate and Bi2Te3 in [P6 6 6 14][decanoate]

At first, 0.2 mmol (77.2 mg) bismuth acetate was added to 1 g decanoic acid and the mixture was heated to 100 °C under Ar atmosphere until a clear brown solution was obtained. The tellurium precursor solution was prepared by heating a mixture of 0.3 mmol (38.3 mg) tellurium powder and 4 g [P6 6 6 14][decanoate] at 220 °C under Ar atmosphere until a yellow solution formed. These two solutions were thoroughly mixed and then transferred into the Teflon lined stainless steel autoclave in an Ar-filled glovebox. The sealed autoclave was heated at 220 °C for 20 h. After the reaction, the autoclave was allowed to cool down to room temperature. The precipitates were washed by toluene several times and collected by centrifugation. The final products were dried under vacuum at room temperature overnight.

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

5.2.8. Synthesis of Ag2Te in [P6 6 6 14][N(CN)2]

Ag[N(CN)2] was freshly prepared by reacting equimolar amounts of silver nitrate and sodium dicyanamide in aqueous solution followed by filtration.[19] 0.2 mmol

(34.8 mg) Ag[N(CN)2] was added to 3 g [P6 6 6 14][N(CN)2] and was stirred at room temperature until a clear solution formed. The tellurium precursor solution was prepared by heating a mixture of 0.1 mmol (12.8 mg) tellurium powder and 4 g

[P6 6 6 14][N(CN)2] at 220 °C under Ar atmosphere for 2 h. A clear red solution was obtained after removing the small amount of brown powder by centrifugation. These two solutions were thoroughly mixed and then transferred into the Teflon lined stainless steel autoclave in an Ar-filled glovebox. The sealed autoclave was heated at 220 °C for 20 h. After the reaction, the autoclave was allowed to cool down to room temperature. The precipitates were washed by toluene several times and collected by centrifugation. The final products were dried under vacuum at room temperature overnight.

5.2.9. Materials characterization

Powder X-ray diffraction (PXRD) patterns were typically recorded using a

PANalytical X’Pert Pro diffractometer with Cu-K1 radiation (λ = 1.54 Å). The single crystal XRD studies were performed at room temperature on a four-circle diffractometer Bruker-Nonius APEX II CCD using graphite-monochromated Mo-K1 radiation (λ = 0.71 Å). The scanning electron microscopy (SEM) images were obtained at 3 kV using a Hitachi SU8020 field emission SEM coupled with energy dispersive X-ray spectroscopy (EDS; Oxford X-MaxN). NMR spectra were measured on a Bruker AVANCE III HD Nanobay, 400 MHz UltraSield (31P (161.98 MHz); 125Te (126.24 MHz)) or on a Bruker AVANCE III HDX, 500 MHz Ascend (31P (202.45 MHz; 125Te (157.78 MHz)) equipped with a Prodigy-Cryo Probe. Chemical

31 shifts were referenced to δH3PO4(85%) = 0.00 ppm ( P) and δMe2Te = 0.00 ppm (125Te). Chemical shifts (δ) are reported in ppm. Coupling constants (J) are reported in Hz. Low temperature NMR measurements have been performed on the 400 MHz system.

5.3. Results and discussion

Metal chalcogenides syntheses using reactions of ionic liquids

5.3.1. Dissolution tests of tellurium in phosphonium based ionic liquids

Figure 5.1. Photograph of solutions of tellurium in [P6 6 6 14]Cl (a), [P6 6 6

14][N(CN)2] (d), [P6 6 6 14][decanoate] (e), [P4 4 4 4][decanoate] (f), recrystallized tellurium single crystals in [P6 6 6 14]Cl (b) and precipitated 3D leaf-like tellurium powder in [P6 6 6 14]Cl (c); cf. text for details.

[P6 6 6 14]Cl was taken as the first example to study the dissolution of tellurium using the autoclave. Tellurium showed a very limited solubility in [P6 6 6 14]Cl. At temperature T < 220 °C, no significantly increased amount of soluble tellurium was visible with a conventional synthesis setup using a flask. We therefore used a solvothermal setup, i.e. an autoclave to access higher temperatures (for details see Experimental Section). In these experiments, only 12 mg tellurium powder dissolved in 4 g [P6 6 6 14]Cl after a heat treatment at T = 250 °C for 48 h under Ar to form a light yellow solution (Figure 5.1a). When more tellurium powder (e.g. 24 mg or 32 mg, see details in Experimental section) was used, uniform needle- shaped micrometer-sized tellurium single-crystals with hexagonal cross-section were obtained after the autoclave was cooled down to room temperature (Figure 5.1b). This indicates the direct recrystallization of polycrystalline tellurium powder to tellurium single-crystals, in other words, by a solvothermal crystal growth, as evidenced in the SEM images of polycrystalline powder in Figure 5.2 and single- crystals in Figure 5.3a, b. The formation of tellurium single crystals has been confirmed by single X-ray diffraction. The refined trigonal lattice parameters of a = 4.474(5) Å and c = 5.955(1) Å are in very good agreement with the reported values.[20] These tellurium single crystals have dimensions of about 5–10 μm in diameter and 0.1–1 mm in length. Additionally, the PXRD pattern of the needle- like tellurium single crystals shown in Figure 5.4 exhibits much stronger intensities in h00/0k0 reflections compared to the polycrystalline tellurium powder diffraction patterns, which is a result of the orientated growth along the c axis. This transformation is assumed to follow a dissolution-recrystallization mechanism.[14,

15, 21] The solubility of tellurium in [P6 6 6 14]Cl increases with increasing

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures temperatures and mass transport is realized by convection and diffusion during the solvothermal treatment. Tellurium particles probably serve as nuclei for the crystal growth. If the crystals growth proceeds already at the reaction temperature or if the decrease in temperature is needed for the initiation is, however, not clear yet.

Figure 5.2. SEM images of commercial tellurium powder.

Figure 5.3. SEM images of tellurium single crystals recrystallized in [P6 6 6 14]Cl (a, b) and leaf-like tellurium particles precipitated in [P6 6 6 14]Cl (c, d, and e).

Metal chalcogenides syntheses using reactions of ionic liquids

Figure 5.4. PXRD patterns of commercial tellurium, tellurium powder precipitated from [P6 6 6 14]Cl, and tellurium single-crystals recrystallized in [P6 6 6 14]Cl.

Interestingly, microscale tellurium particles can also be obtained from the light yellow solution of tellurium in [P6 6 6 14]Cl if the solution is not stored under inert conditions. We observed a dark precipitate if the solution was placed statically under air at room temperature for several days (Figure 5.1c). These particles were identified as crystalline tellurium by EDX and PXRD (Figure 5.4). All reflections can be indexed with the trigonal unit cell of tellurium in space group P3121 (ICSD no. 76150). The low-magnification SEM image in Figure 5.3c shows that the precipitated tellurium particle has a 3D morphology resembling fern-leafs with lengths of 5–10 μm. From the high-magnification SEM image in Figure 5.3d, it is clear that the leaf-like particle is hierarchical and consists of two main branches which are made up by even more secondary branches. These secondary branches are stacked parallel to each other, and their lengths decrease towards the tips of the branches. The higher magnification SEM image in Figure 5.3e indicates that each secondary branch is composed of stacked nano-sized tellurium layers. In a controlled experiment, the tellurium solution of [P6 6 6 14]Cl was stored in an Ar- filled glove box at room temperature for above 30 days and no precipitates were observed, revealing that oxygen or moisture plays an important role to drive the precipitation of tellurium in [P6 6 6 14]Cl. The precise nature of this influence is currently under investigation. No significant oxygen content was found in EDX and PXRD analyses of the thus precipitated tellurium.

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

We then changed the anion of the phosphonium based ILs to investigate their influence on the dissolution behavior of tellurium. Thus, [P6 6 6 14][N(CN)2] was chosen as an example and tellurium was found to have a relatively high solubility at even lower temperatures. 1 mmol (127.6 mg) tellurium powder was completely dissolved in 5.5 g (10 mmol) [P6 6 6 14][N(CN)2] within 6 h at 220 °C under Ar. After cooling to room temperature, a red colored suspension with a brown precipitate was obtained. The brown precipitate turned out as a by-product, see below. A red clear reaction solution can be easily obtained by removal of the brown powder by centrifugation, Figure 5.1d. In addition, 5 mmol and 10 mmol of tellurium as starting amounts were also tested for the dissolution reactions keeping all other parameters unchanged. Some unreacted tellurium powder (based on PXRD) and an insoluble brown powder were observed after the respective reaction (see details in the Experimental section).

31 Figure 5.5. P NMR spectra of pure [P6 6 6 14][N(CN)2] (a) and reaction solutions with 1 mmol (b), 5 mmol (c), or 10 mmol (d) tellurium powder in 10 mmol [P6 6 6

14][N(CN)2] for the dissolution test at 220 °C under Ar.

Metal chalcogenides syntheses using reactions of ionic liquids

Figure 5.6. 125Te NMR spectra of reaction solutions with 1 mmol (a), 5 mmol (b), or 10 mmol (c) tellurium powder in 10 mmol [P6 6 6 14][N(CN)2] for the dissolution test at 220 °C under Ar.

Figure 5.5d and Figure 5.6c show the 31P and 125Te NMR spectra of the reaction solution after removing the brown powder and unreacted tellurium powder where an equimolar amount of tellurium and [P6 6 6 14][N(CN)2] were used. The NMR spectroscopic investigation reveals the formation of the respective trialkylphosphane tellurides. As shown in Figure 5.5d, an upfield shifted resonance

31 at δ( P) = –12.8 ppm, as compared to that of the pure [P6 6 6 14][N(CN)2]

31 1 (δ( P) = 33.4 ppm; Figure 5.5a), is visible. The large JPTe coupling constant of 1656 Hz is in the range typically observed for phosphane tellurides.[22] The corresponding 125Te NMR spectrum (Figure 5.6c) shows two sets of doublets (δ(125Te) = –778.3 ppm and –778.7 ppm) revealing the same coupling constant

1 JTeP = 1656 Hz, which evidences a P–Te bond formation and the formation of two products at the same time, most likely (C6H13)3PTe and (C6H13)2(C14H29)PTe as a consequence of the two different possibilities for elimination. The observed chemical shifts and the coupling constant values are both within the range of typical organophosphorus(V)-tellurium compounds,[22] indicating the oxidation of phosphorus by tellurium.

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

Figure 5.7. 31P and 125Te NMR spectra measured at low temperature (–53 °C) for the reaction solutions with 1 mmol (a, b) and 5 mmol (c, d) tellurium powder in

10 mmol [P6 6 6 14][N(CN)2] for the dissolution test at 220 °C under Ar.

When a minor amount of tellurium powder (Te : IL = 1 : 10) is used for the dissolution, the corresponding 31P and 125Te NMR spectra in Figure 5.5b and 5.6a

31 reveal only one single, broad resonance (δ( P) = 16.8 ppm, 1/2 = 669 Hz, and

125 δ( Te) = –795.3 ppm, 1/2 = 628 Hz), respectively. With a larger amount of tellurium (Te : IL = 1 : 2), in spite of the appearance of a broad doublet resonance in the 125Te NMR spectrum as shown in Figure 5.6b, the accompanied 125Te satellites in the corresponding 31P NMR spectrum (Figure 5.5c) are not observed. This can be explained by a dynamic process in which a rapid exchange of the free phosphane and the phosphane tellurides is present takes place at ambient temperature.[23] This fast exchange process leads to a collapse of the well-resolved doublet splitting into a broad resonance and thus, an absence of a 31P–125Te coupling. Thus, we also measured the 31P and 125Te NMR spectra of reaction solutions with Te : IL = 1 : 10 and 1 : 2 molar ratios at low temperature (–53 °C) (Figure 5.7). The 125Te NMR spectrum (Figure 5.7b) of Te : IL = 1 : 10 reveals a

1 broad doublet with a coupling constant JTeP = 1558 Hz, confirming the

Metal chalcogenides syntheses using reactions of ionic liquids deceleration of the dynamic process at low temperatures. However, the respective resonance in the 31P NMR spectrum remains a broad singlet (Figure 5.7a). The low temperature 125Te NMR spectrum (Figure 5.7d) for sample Te : IL = 1 : 2 shows, as expected, a significantly sharper doublet resonance with a better-resolved

125 1 coupling constant (δ( Te) = –791.0 ppm; JTeP = 1600 Hz) compared to the room temperature 125Te NMR spectrum (Figure 5.6b). The same is observed for the low temperature 31P NMR spectrum (Figure 5.7c) in which the 125Te satellites can be observed, although, they are not visible in the room temperature 31P NMR spectrum (Figure 5.5c).

Based on the identification of the formation of phosphane tellurides by NMR spectroscopy, one pathway for the decomposition of the quaternary phophonium

[24] ILs may take place via an SN2 attack of the anion of the ILs at high temperature. One of the alkyl substituents of quaternary phosphonium cations is abstracted to form the alkyl dicyanamide along with the corresponding trialkylphosphane. The intermediately formed trialkylphosphane (P6 6 6 or P6 6 14) reacts with tellurium to the respective trialkylphosphane telluride, which appears to be the reason for the dissolution of tellurium in phosphonium based ILs. In other words, the dissolution of tellurium is due to the chemical decomposition of the IL at higher temperatures.

Figure 5.8. PXRD pattern of the brown by-product obtained in the reaction of tellurium in [P6 6 6 14][N(CN)2].

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

Figure 5.9. SEM images of the brown by-product obtained in the reaction of tellurium in [P6 6 6 14][N(CN)2].

Figure 5.10. EDX spectrum of the brown by-product obtained in the reaction of tellurium in [P6 6 6 14][N(CN)2].

A brown powder as the by-product was simultaneously obtained during the reaction, and a rising amount of this by-product was observed with an increasing starting amount of tellurium. The PXRD pattern of the by-product is shown in Figure 5.8. The low-magnification image (Figure 5.9a) shows the uniform morphology and size of the particle, weakly pointing towards a single phase by- product. The high-magnification image (Figure 5.9b) reveals that the nanoplatelets with around 50 nm in thickness self-assemble into intergrown micrometer-sized aggregates (10–20 μm). The EDX analysis (Figure 5.10) shows the presence of carbon, nitrogen, and phosphorus, but no tellurium in this by- product, so that the formation of this by-product is probably due to the interaction of the cation and anion of [P6 6 6 14][N(CN)2]. Although the precise nature of the organophosphorous by-product could not be elucidated yet, we have to consider that the decomposition of [P6 6 6 14][N(CN)2] at a high temperature (in the presence of tellurium) follows different routes at the same time.

Metal chalcogenides syntheses using reactions of ionic liquids

31 Figure 5.11. P NMR spectra of pure [P6 6 6 14][decanoate] (a) and reaction solutions with a molar ratio of Te : [P6 6 6 14][decanoate] = 1 : 7.6 (b), 1 : 2(c), and 1 : 1 (d), respectively.

125 Figure 5.12. Te NMR spectra of reaction solutions with a molar ratio of Te : [P6

6 6 14][decanoate] = 1 : 7.6 (a), 1 : 2(b), and 1 : 1 (c), respectively.

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

The next IL used for the dissolution behavior test of tellurium was [P6 6 6

14][decanoate]. 1 mmol tellurium powder (127.6 mg) can be completely dissolved in 5 g (7.63 mmol) [P6 6 6 14][decanoate] in 4 h at 220 °C under Ar which results in a yellow gel upon cooling to room temperature as shown in Figure 5.1e. In addition, the dissolution of tellurium was also tested with Te: [P6 6 6 14][decanoate] molar ratios of 1 : 1 and 1 : 2 and unreacted tellurium powder was observed (see details in the Experimental section). The NMR spectra indicate similar results as for the aforementioned reaction of tellurium in [P6 6 6 14][N(CN)2] (Figure 5.11 and 5.12). Accordingly, the corresponding trialkylphosphane tellurides formed with stoichiometric amounts of tellurium and [P6 6 6 14][decanoate] (Figure 5.11d and 5.12c). Again, the 125Te resonance collapses from a well-resolved doublet of doublets in case of the 1 : 1 composition to a single line or a broad doublet when an excess of the IL is used for the dissolution tests (Figure 5.12). The 125Te satellites in the corresponding 31P NMR spectra also disappear, indicating the fast exchange between the free phosphane and the phosphane tellurides. Moreover, a signal at δ(31P) = 49.1 ppm in the 31P NMR spectra (Figure 5.11) indicates the formation of an organophosphorus by-product during the reaction, which probably stems from the decomposition of [P6 6 6 14][decanoate].

31 Figure 5.13. P NMR spectra of pure [P4 4 4 4][decanoate] (a) reaction solutions with a molar ratio of Te : [P4 4 4 4][decanoate] = 1 : 11.6 (b) and 1 : 1(c), respectively.

Metal chalcogenides syntheses using reactions of ionic liquids

125 Figure 5.14. Te NMR spectra of reaction solutions with a molar ratio of Te : [P4

4 4 4][decanoate] = 1 : 11.6 (a) and 1 : 1(b), respectively.

The last IL used for the dissolution tests of tellurium was [P4 4 4 4][decanoate]. This

IL shows similar results as observed for the reaction of [P6 6 6 14][decanoate] with tellurium (Figure 5.13 and 5.14). The resonance at δ(31P) = –13.0 ppm with a coupling constant of 1660 Hz in the 31P NMR spectrum (Figure 5.13c) and δ(125Te) = –778.3 ppm with the same coupling constant in the 125Te NMR spectrum (Figure 5.14b) confirm the formation of tributylphosphane telluride and consistent with a

[25] + previous report. As [P4 4 4 4] is a tetra-n-butyl substituted cation, the formation of only one phosphane telluride is possible, in accordance with the observation of only one doublet in the 125Te NMR spectrum (Figure 5.14b). Again, using a minor amount of tellurium powder resulted in only one broad single resonance in the 125Te NMR spectrum (Figure 5.14a) and no 125Te satellites were observed in the 31P NMR spectrum (Figure 5.13b).

According to the previous report,[26] the dissolution of elemental selenium in phosphonium based ILs could be explained by only one decomposition mode of

ILs, namely an SN2 type attack of the anion of the IL leading to an elimination of one alkyl chain. The so formed trialkylphosphane reacts with selenium to give the respective trialkylphosphane selenide. To this point, the selenium interaction with the phosphonium ILs and the tellurium interaction described here are exactly the same. However, in the case of tellurium, the existence of a competitive IL decomposition route besides the SN2 reaction must be considered as the formation of the P-organic by-product evidence. This may contribute, at least partially, to

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures the lower solubility of tellurium in phosphonium based ILs compared to that of selenium. The main reason for the much lower solubility and the markedly different reactivity of tellurium can, however, be attributed to the relatively weak P–Te bond,[22] limiting the formation of the trialkylphosphane tellurides.

Nevertheless, the low solubility of tellurium in [P6 6 6 14]Cl can be utilized in a variety of tellurium recrystallization processes, including the formation of needle-like tellurium single crystals or 3D leaf-like morphologies as shown above. The solubility of tellurium in [P6 6 6 14][N(CN)2], [P6 6 6 14][decanoate], and [P4 4 4

4][decanoate] is much higher as compared to [P6 6 6 14]Cl, due to the stronger Lewis

– – [27, 28] basicity of C9H19COO and N(CN)2 anions. NMR results clearly indicate different decomposition mechanisms for quaternary phosphonium ILs at a relatively high temperature in the presence of tellurium. One route of decomposition of the quaternary phosphonium cations should proceed by an elimination of one alkyl substituent via an SN2 reaction, forming the respective trialkylphosphane tellurides in the presence of tellurium, which is then responsible

1 for the dissolution of tellurium in phosphonium ILs. However, the JPTe coupling which indicates a P–Te bond formation is only observed in the NMR spectra when a sufficient amount of tellurium (e.g. Te : IL = 1 : 1) is provided. Using smaller amounts of tellurium, the 125Te satellites in the 31P NMR spectra disappear and the doublets in the 125Te NMR spectra collapse to one broad resonance line.

5.3.2. Synthesis of Bi2Te3 and Ag2Te from phosphonium ionic liquids

To test the reactivity of the intermediately formed tellurium-IL solutions as tellurium precursors, the synthesis of metal tellurides in the above stated ILs was investigated. As examples, we chose Bi2Te3 and Ag2Te as target phases. Good results were obtained, whenever ILs and metal salts contained the same anions.

We discuss the results of the formation of Bi2Te3 in [P6 6 6 14]Cl and [P6 6 6

14][decanoate] and of Ag2Te in [P6 6 6 14][N(CN)2] in the following.

Bi3Te2 was in the first experiment synthesized using a freshly prepared solution of tellurium in [P6 6 6 14]Cl as tellurium precursor and a solution of BiCl3 in [P6 6 6 14]Cl as bismuth precursor (see Experimental section for details). The PXRD pattern in

Figure 5.15a shows that Bi2Te3 particles and some tellurium were obtained when stoichiometric amounts of tellurium (0.094 mmol, 12 mg) and BiCl3 (0.063 mmol, 20 mg) were used as starting materials. The SEM images (Figure 5.15b) further

Metal chalcogenides syntheses using reactions of ionic liquids confirmed the formation of needle-shaped tellurium crystals as by-products.

Increasing the BiCl3 amount to 0.5 mmol while keeping all other reaction parameters constant, highly crystalline Bi2Te3 particles were obtained according to the PXRD pattern (Figure 5.16, red). All reflections can be indexed to trigonal

[20] Bi2Te3 (space group R-3m; ICSD no. 44983). The morphological features of as- obtained Bi2Te3 nanoplates are displayed in the SEM images of Figure 5.17a, b. The low magnification SEM image (Figure 5.17a) shows the relatively homogeneous particle size distribution of Bi2Te3 nanoplates with diameters of around 1–3 μm. The thickness of the Bi2Te3 plates is about 100 nm, see inset of Figure 5.17a.

Bi2Te3 was also prepared in [P6 6 6 14][decanoate] using Bi(C9H19COO)3 as the bismuth source. Figure 5.16 (blue) shows the corresponding PXRD pattern, indicating the formation of phase pure Bi2Te3. The low-magnification SEM image in Figure 5.17c demonstrates the overall morphology of three-dimensionally intergrown Bi2Te3 crystals with diameters of approximately 300–500 nm. The high-magnification SEM images, Figure 5.17d and the inset of Figure 5.17c, reveal that these Bi2Te3 particles are formed by assembling Bi2Te3 nanosheets with an average thickness of about 10 nm in various directions.

Figure 5.15. PXRD pattern (a) and SEM images (b) of as-prepared Bi2Te3 particles using stoichiometric amounts of tellurium and BiCl3 as starting materials.

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Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures

Figure 5.16. PXRD patterns of Bi2Te3 products synthesized in [P6 6 6 14]Cl (BiCl3 as the bismuth source) and [P6 6 6 14][decanoate] (Bi(C9H19COO)3 as the bismuth source), respectively.

Figure 5.17. SEM images of as-synthesized Bi2Te3 samples using BiCl3 (a, b) and

Bi(C9H19COO)3 (c, d) as bismuth sources, respectively.

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Metal chalcogenides syntheses using reactions of ionic liquids

The solution of tellurium in [P6 6 6 14][N(CN)2] was used to synthesize silver telluride,

Ag2Te. Silver dicyanamide, AgN(CN)2, was found to be soluble in [P6 6 6 14][N(CN)2] and the solution can therefore be used as the silver source (for details see

Experimental section). The formation of Ag2Te was confirmed by PXRD. As shown in Figure 5.18a, all reflections of the as-obtained sample can be indexed to the

[20] monoclinic Ag2Te pattern (space group P21/c; ICSD no. 73402). No signals of impurities can be observed in the XRD diagram. The SEM images, shown in Figure

5.18b, c, indicate that Ag2Te is formed in particles with 30–300 nm size range.

Figure 5.18. XRD pattern (a) and SEM images (b) of as-prepared Ag2Te particles.

Based on the above observations, we believe that tellurium solutions in ILs can act as efficient tellurium precursors or tellurium feedstock and react with respective metal salts to give phase pure and highly crystalline metal tellurides. The sizes and shapes of the particles can to some extent be controlled by choosing various ILs and metal sources.

5.4. Conclusions

In this chapter, phosphonium based ILs display a certain range of chemical reactivity with tellurium at a relatively high temperature corresponding to various

102

Dissolution behavior of tellurium in phosphonium based ionic liquids for syntheses of tellurium and tellurides nanostructures dissolution behaviors and solubilities of tellurium in ILs. The very weak reactivity of [P6 6 6 14]Cl with tellurium gives very low solubility of tellurium, allowing the recrystallization of needle-shaped tellurium single-crystals or 3D leaf-like microstructures. A remarkably increased solubility is observed when more Lewis-

– – basic anions like [N(CN)2] and C9H19COO are used. The NMR investigations reveal the formation of the respective trialkylphosphane tellurides by an elimination of one of the alkyl substituents of the IL cation. The formation of this intermediate can be influenced by the initial ratio of tellurium powder and IL. No 31P–125Te coupling is visible in the NMR spectra if an excess of the quaternary phosphonium IL is used. The intermediately formed trialkylphosphane tellurides can be used as tellurium precursors for metal tellurides synthesis. As examples, crystalline and phase pure Bi2Te3 nanoplates, flower-like Bi2Te3, and Ag2Te were successfully synthesized in [P6 6 6 14]Cl, [P6 6 6 14][decanoate], and [P6 6 6 14][N(CN)2], respectively, starting from corresponding metal salts. Thus, phosphonium based ILs provide a new and simple pathway for metal tellurides synthesis.

5.5. References

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[2] Y. Wang, Z. Tang, P. Podsiadlo, Y. Elkasabi, J. Lahann, N. A. Kotov, Mirror- Like Photoconductive Layer-by-Layer Thin Films of Te Nanowires: The Fusion of Semiconductor, Metal, and Insulator Properties. Adv. Mater. 2006, 18, 518–522.

[3] T.-Y. Wei, H.-Y. Chang, C.-C. Huang, Synthesis of tellurium nanotubes via a green approach for detection and removal of mercury ions. RSC Adv. 2013, 3, 13983–13989.

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[5] Z. He, Y. Yang, J.-W. Liu, S.-H. Yu, Emerging tellurium nanostructures: controllable synthesis and their applications. Chem. Soc. Rev. 2017, 46, 2732–2753.

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[6] H.-S. Qian, S.-H. Yu, J.-Y. Gong, L.-B. Luo, L.-f. Fei, High-Quality Luminescent Tellurium Nanowires of Several Nanometers in Diameter and High Aspect Ratio Synthesized by a Poly (Vinyl Pyrrolidone)-Assisted Hydrothermal Process. Langmuir 2006, 22, 3830–3835.

[7] J.-W. Liu, J. Xu, W. Hu, J.-L. Yang, S.-H. Yu, Systematic Synthesis of Tellurium Nanostructures and Their Optical Properties: From Nanoparticles to Nanorods, Nanowires, and Nanotubes. ChemNanoMat 2016, 2, 167–170.

[8] H. Zhu, H. Zhang, J. Liang, G. Rao, J. Li, G. Liu, Z. Du, H. Fan, J. Luo, Controlled Synthesis of Tellurium Nanostructures from Nanotubes to Nanorods and Nanowires and Their Template Applications. J. Phys. Chem. C 2011, 115, 6375–6380.

[9] M. S. Mo, J. H. Zeng, X. M. Liu, W. C. Yu, S. Y. Zhang, Y. T. Qian, Controlled hydrothermal synthesis of thin single-crystal tellurium nanobelts and nanotubes. Adv. Mater. 2002, 14, 1658–1662.

[10] D. Wang, Y. Zhao, H. Jin, J. Zhuang, W. Zhang, S. Wang, J. Wang, Synthesis of Au-Decorated Tripod-Shaped Te Hybrids for Applications in the Ultrasensitive Detection of Arsenic. ACS Appl. Mater. Interfaces 2013, 5, 5733–5740.

[11] X. Wu, Y. Wang, S. Zhou, X. Y. Yuan, T. Gao, K. Wang, S. Lou, Y. Liu, X. Shi, Morphology Control, Crystal Growth, and Growth Mechanism of Hierarchical Tellurium (Te) Microstructures. Cryst. Growth Des. 2013, 13, 136–142.

[12] P. Ghosh, J. Bhattacharjee, U. V. Waghmare, The Origin of Stability of Helical Structure of Tellurium. J. Phys. Chem. C 2008, 112, 983–989.

[13] D. H. Webber, J. J. Buckley, P. D. Antunez, R. L. Brutchey, Facile dissolution of selenium and tellurium in a thiol-amine solvent mixture under ambient conditions. Chem. Sci. 2014, 5, 2498–2502.

[14] J. Lu, Y. Xie, F. Xu, L. Zhu, Study of the dissolution behavior of selenium and tellurium in different solvents-a novel route to Se, Te tubular bulk single crystals. J. Mater. Chem. 2002, 12, 2755–2761.

[15] X. Chen, Z. Wang, X. Wang, J. Wan, Y. Qian, Polyol-mediated synthesis of single-crystal tellurium nanowires directly from polycrystalline powder. Appl. Phys. A 2005, 80, 1443–1445.

[17] K. Ding, H. Lu, Y. Zhang, M. L. Snedaker, D. Liu, J. A. Maciá-Agulló, G. D. Stucky, Microwave Synthesis of Microstructured and Nanostructured Metal

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Chalcogenides from Elemental Precursors in Phosphonium Ionic Liquids. J. Am. Chem. Soc. 2014, 136, 15465–15468.

[18] Q. Yang, D. Xu, J. Zhang, Y. Zhu, Z. Zhang, C. Qian, Q. Ren, H. Xing, Long- Chain Fatty Acid-Based Phosphonium Ionic Liquids with Strong Hydrogen- Bond Basicity and Good Lipophilicity: Synthesis, Characterization, and Application in Extraction. ACS Sustainable Chem. Eng. 2015, 3, 309–316.

[19] T. Zhang, L. Liu, C. Li, Y. Zhang, Z. Li, S. Zhang, Synthesis and characterization of energetic salts based on the new propan-2-ylidene methanetriamium cations. J. Mol. Struct. 2014, 1067, 195–204.

[20] Inorganic Crystal Structure Database, ICSD, Version 2018/1, FIZ Karlsruhe, Germany and NIST, Gaithersburg, USA, 2018.

[21] Z. Wang, L. Wang, J. Huang, H. Wang, L. Pan, X. Wei, Formation of single- crystal tellurium nanowires and nanotubes via hydrothermal recrystallization and their gas sensing properties at room temperature. J. Mater. Chem. 2010, 20, 2457–2463.

[22] A. Nordheider, J. D. Woollins, T. Chivers, Organophosphorus–Tellurium Chemistry: From Fundamentals to Applications. Chem. Rev. 2015, 115, 10378–10406.

[23] C. H. W. Jones, R. D. Sharma, 125Te NMR and Moessbauer spectroscopy of tellurium-phosphine complexes and the tellurocyanates. Organometallics 1987, 6, 1419–1423.

[24] B. Wang, L. Qin, T. Mu, Z. Xue, G. Gao, Are Ionic Liquids Chemically Stable? Chem. Rev. 2017, 117, 7113–7131.

[25] H. Liu, J. S. Owen, A. P. Alivisatos, Mechanistic Study of Precursor Evolution in Colloidal Group II−VI Semiconductor Nanocrystal Synthesis. J. Am. Chem. Soc. 2007, 129, 305–312.

[26] T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Dissolution behaviour and activation of selenium in phosphonium based ionic liquids. Chem. Commun. 2017, 53, 7588–7591.

[27] D. R. MacFarlane, S. A. Forsyth, J. Golding, G. B. Deacon, Ionic liquids based on imidazolium, ammonium and pyrrolidinium salts of the dicyanamide anion. Green Chem. 2002, 4, 444–448.

[28] D. R. MacFarlane, J. M. Pringle, K. M. Johansson, S. A. Forsyth, M. Forsyth, Lewis base ionic liquids. Chem. Commun. 2006, 1905–1917.

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Chapter 6

Conclusions and perspectives

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108

Conclusions and perspectives

Nowadays, great advances have been made in the application of ILs and DESs in inorganic materials syntheses. Although a variety of inorganic compounds with interesting properties were prepared, most studies have put much focus on the description of new synthetic strategies. The chemical reactivity of ILs or DESs in the reactions is often neglected. To be true, the reactivity of ILs or EDSs in inorganic synthesis limits their application to some extent because the decompositions are unexpected in many cases which may make the reaction system more complicated. However, these decompositions may also result in some materials with special structures or morphologies. In this dissertation, a series of metal chalcogenides were synthesized by utilization of the decompositions of ILs or DESs. The role or chemical reactivity of ILs and DESs in the reactions was demonstrated in detail.

Previous studies showed that TBAH was an effective IL for the binary oxides synthesis. We first extended the use of TBAH for the synthesis of a perovskite- type ternary oxide SrTiO3. The hierarchical desert-rose-like SrTiO3 particles with a high surface area up to 186 m2 g−1 were obtained based on an ethylene glycol (EG) mediated one-pot solvothermal synthetic route using TBAH as the alkali. The photocurrent density values for as-obtained hierarchical SrTiO3 samples can be 2.2 times higher than that of their corresponding bulk counterparts obtained by solid-state reaction. The mechanism studies show that EG plays a key role in the formation of this 3D hierarchical morphology. The formed EG anion in alkaline medium acts as a chelating strong ligand to titanium cations, which blocks the condensation reaction and results in the formation of nano-domains, followed by a branching growth of crystallites and finally resulting in a desert-rose morphology. On the other hand, the used basic IL TBAH can replace EG as the sole solvent to synthesize polyhedral SrTiO3, serving as both solvent and reactant.

A new DES based on choline chloride and thioacetamide (ChCl/TAA) was designed and used as both solvent and sulfur source to synthesize a series of binary metal sulfides including Sb2S3, Bi2S3, PbS, CuS, Ag2S, ZnS, and CdS. The sizes and shapes of obtained samples can to some extent be controlled by temperature or reactant concentrations. The reaction is supposed to proceed in two steps. A metal-DES complex is first generated by adding the respective metal salts to the DES solution at a relatively low temperature. Then the metal-DES complex decomposes to form the final metal sulfides products upon heating. This method

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Metal chalcogenides syntheses using reactions of ionic liquids provides a simple reaction system and a general strategy to synthesize metal sulfides.

Various phosphonium-base ILs were employed as the solutes to dissolve elemental selenium and tellurium and the obtained solutions were further used as the selenium and tellurium precursors for metal selenides (e.g. NiSe2 and ZnSe) and tellurides (e.g. Bi2Te3 and Ag2Te) preparation. The interplay between ILs and elemental selenium and tellurium was thoroughly investigated by a series of NMR experiments. NMR results show that the dissolution of selenium and tellurium is based on the reaction of the phosphonium cation with selenium and tellurium to the corresponding trialkylphosphane selenides and tellurides by an elimination of one alkyl substituent (SN2 route) upon heating to a relatively high temperature. However, for tellurium, the dissolution behavior is more complicated compared to selenium because of the weak P-Te bond. The decomposition of phosphonium ILs takes in different ways simultaneously in the presence of tellurium at a high temperature, which only SN2 decomposition route contributes to the dissolution of tellurium in ILs. This may at least partially explain the relatively lower solubility of tellurium in phosphonium ILs compared to that of selenium. In addition, the formation of the respective trialkylphosphane telluride intermediates can be influenced by the initial ratio of tellurium powder and ILs. No 31P–125Te coupling is visible in the NMR spectra if an excess of the quaternary phosphonium IL is used.

These investigations show that the stabilities of the ILs and DESs are limited and their decompositions or reactivity should not be underestimated, in spite of their many advantageous properties. On the other hand, the reactivity of the ILs and DESs may allow the formation of useful reactive intermediates and to establish new synthesis routes for inorganic materials. Either way, the reaction mechanisms need to be well understood for the directed use of ILs and DESs in materials preparation.

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Abbreviations

IL(s) ionic liquid(s)

DES(s) deep eutectic solvent(s)

DESP deep eutectic solvent precursor

HBD hydrogen-bond donor

EMIm 1-ethyl-3-methylimidazolium

BMIm 1-butyl-3-methylimidazolium

NTf2 bis(trifluoromethylsulfonyl)imide

TfO trifluoromethanesulfonate

KO(t-Bu) potassium tert-butoxide

OMe methoxide

Et ethyl group

EG ethylene glycol

THF tetrahydrofuran

OAc acetate

CTAB cetyltrimethylammonium bromide

DDT 1-dodecanethiol

OLA oleylamine

TOP trioctylphosphane

TBAH tetrabutylammonium hydroxide

TBPH tetrabutylphosphonium hydroxide

TEAH tetraethylammonium hydroxide

BTMAH benzyltrimethylammonium hydroxide

P6 6 6 14 trihexyltetradecylphosphonium

P4 4 4 4 tetrabutylphosphonium

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Metal chalcogenides syntheses using reactions of ionic liquids

PTFE polytetrafluoroethylene

ChCl choline chloride

ClChCl chlorocholine chloride

TMACl tetramethylammonium chloride

TEABr tetraethylammonium bromide

TBACl tetrabutylammonium chloride

Et2(EtOH)ACl N,N-diethyl-2-hydroxyethanaminium chloride

MeP(Ph)3Br methyltriphenylphosphonium bromide

TAA thioacetamide

LDH layered double hydroxide

NHC(s) N-heterocyclic carbene(s)

1D one dimensional

2D two dimensional

3D three dimensional

NMR nuclear magnetic resonance

PXRD powder X-ray diffraction

SEM scanning electron microscopy

TEM transmission electron microscopy

HRTEM high-resolution transmission electron microscopy

EDS energy dispersive X-ray spectroscopy

UV-Vis ultraviolet-visible

BET Brunauer-Emmett-Teller

TGA thermogravimetric analysis

PSD pore size distribution

BJH Barrett-Joyner-Halenda

CVD chemical vapor deposition

FWHM full width at half maximum

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List of Publications

Peer reviewed publications

1. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Understanding the chemical reactivity of phosphonium based ionic liquids with tellurium. Chem. Eur. J. 2018, doi:10.1002/chem.201800320.

2. T. Zhang, T. Doert, M. Ruck, Solvothermal synthesis and enhanced photo- electrochemical performance of hierarchically structured strontium titanate micro-particles. Dalton Trans. 2017, 46, 14219–14225.

3. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Dissolution behavior and activation of selenium in phosphonium based ionic liquids. Chem. Commun. 2017, 53, 7588–7591.

4. T. Zhang, T. Doert, M. Ruck, Synthesis of metal sulfides from a deep eutectic solvent precursor (DESP). Z. Anorg. Allg. Chem. 2017, 643, 1913–1919.

5. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Ultrafast access to complex three-dimensional tellurium microstructures based on an ionic liquid-assisted synthesis. in preparation.

6. T. Zhang, T. Doert, M. Ruck, Chemical reactivity of ionic liquids and deep eutectic solvents in inorganic synthesis (a review). in preparation.

Conference presentations

1. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Dissolution behavior and activation of selenium in phosphonium based ionic liquids, Workshop on Synthesis Strategies in Ionic Liquids (DFG Priority Program 1708). November 08–10, 2017, Nürnberg, Germany. (Oral)

2. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Dissolution behavior and activation of selenium in phosphonium based ionic liquids, 4th International Conference on Ionic Liquid-based Materials. October 24–27, 2017, Santiago de Compostela, Spain. (Poster)

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3. T. Zhang, T. Doert, M. Ruck, Solvothermal synthesis and enhanced photo-

electrochemical performance of SrTiO3 particles with hierarchical structures, Nanotage 2017. August 31–September 2, 2017, Bad Herrenalb, Germany. (Oral)

4. T. Zhang, K. Schwedtmann, J. J. Weigand, T. Doert, M. Ruck, Dissolution behavior of selenium in phosphonium based ionic liquids, Mitteldeutsches Anorganiker-Nachwuchssymposium (MANS-15). August 31, 2017, Leipzig, Germany. (Oral)

5. T. Zhang, T. Doert, M. Ruck, Synthesis of metal sulfides from a deep eutectic solvent precursor (DESP). 18th Conference of the GDCh Division: Solid State Chemistry and Materials Research. September 19–21, 2016, Innsbruck, Austria. (Poster)

Abstract: Z. Anorg. Allg. Chem. 2016, 642, 1031.

6. T. Zhang, T. Doert, M. Ruck, Synthesis of metal sulfides from a deep eutectic solvent precursor (DESP). 26th EUCHEM Conference on Molten Salts and Ionic Liquids. July 3–8, 2016, Vienna, Austria. (Poster)

7. T. Zhang, T. Doert, M. Ruck, Morphology and size-controlled SrTiO3 particles from an ionic liquid precursor (ILP), Workshop on Material Synthesis in Ionic Liquids and Interfacial Processes (DFG Priority Program 1708). April 13–15, 2016, Goslar, Germany. (Poster)

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Acknowledgements

When all the people are ready to celebrate Christmas, I complete my PhD thesis. I have expected the day coming soon on one hand. While I will lament the passing of time, marvel at how quickly my PhD has passed by, and have a desire to slow down time on the other hand. As I look back over the past three and a half years, I come to realize that there has been at least one significant change – me, not only the improvements in academic performance but also the ability to deal with stress, depression, and failure. I would like to thank all the people around me who make the best myself of today.

First of all, I am especially grateful to my supervisor Prof. Dr. Michael Ruck, for giving me a PhD opportunity to work at TU Dresden on this interesting theme. His constant supports and encouragements helped me make continuous progress during the past few years. He has fostered an enjoyable working environment within the whole group, enabling my efficient and productive work. I have benefited a lot from his broad chemistry knowledge and scientific experience, which will also make a positive influence on my future research career. The PhD time in Germany will definitely be very fond memories in my life.

I am also very grateful to Prof. Dr. Thomas Doert for the numerous discussions about my research, which is very helpful for the outcome of my thesis. He helped me a lot in understanding problems in various ways and also taught me a lot of writing techniques. I have learned a great deal from his suggestions and ideas. Without his kindness, patience, and support, my PhD journey wouldn’t have been so smooth and positive.

I am very fortunate to have the kind colleagues in our group. I would like to thank Alexander Wolff so much. We shared the thoughts and feelings about all aspects of the research topic, which was really important to overcome the barrier in the early stage of my research. He is also very kind enough to give me many valuable comments outside the lab. I am also grateful to Dr. Jen-Hui Chang, Dr. Matthias F. Groh, and Dr. Martin Kaiser, who also helped me a lot inside and outside the

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Metal chalcogenides syntheses using reactions of ionic liquids institute, especially at my beginning stage here. I am very appreciative to Ulrike Müller, Deming Tan, and Matthias Grasser. We are not only in the same office but also really work well together. I am grateful to Dr. Alexander Weiz, who taught me a lot of SEM and EDX skills. In addition, I would like to thank the kind assistance from the colleagues, Dr. Martin Heise, Dr. Bertold Rasche, Dr. Maria Roslova, Dr. Jens Hunger, Dr. Philipp Schlender, Dr. Anna Isaeva, Alexander Zeugner, Mai Lê Anh, Johannes Teichert, Haijiao Hu, Hagen Poddig, Maximilian Knies, and Paul Gebauer. It was a pleasant time to share the labs with all of you.

I am very thankful to Andrea Brünner and Michaela Münch, who not only taught me a lot of useful techniques in the lab but also help me solve many problems in my practical work. I express my appreciation to Alina Markova for her organizing the conferences and workshops, which gave me opportunities to share my own thoughts in the meetings. I am grateful to Ina Wittig and Ilona Salzmann for solving the administrative problems, and Rüdiger Kunschke for solving a lot of computer-related problems.

I would like to thank my collaborator Dr. Kai Schwedtmann for the NMR measurements and analysis. Thanks for your contribution of time and expertise to this dissertation, making it more complete and comprehensive.

I am grateful to Dr. Yang Hou for the photoelectrochemical property measurement, Dr. Junjie Li for the TEM and HRTEM measurements, and Dr. Guangping Hao for the discussion of nitrogen physisorption. The TG measurements by Ilka Kunert are also greatly appreciated.

I would like to thank my friends, Bin Cai, Pei Wang, Ye Liu, Lanfa Liu, Tianyi Li, Bao Chang, En Zhang, Long Zhang, Haichao Li, Yang Liu, Hanjun Sun, and Guangbo Chen, who brightened my life outside the lab in Dresden.

I would like to thank the Chinese Scholarship Council (CSC) and the DFG (Priority Program SPP 1708) for the financial support of my studies in TU Dresden.

Finally, I would like to express my deepest and sincerest gratitude to my family for their endless love. They taught me to be kind, independent, and to have my own thoughts and opinions. Their support gave me the greatest courage to proceed through each stage of my life.

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Collaborations

In order to ensure good scientific practice, all collaborations with other persons who contribute to this thesis are given below:

TEM measurements for ZnSe nanocrystals were performed by Dr. Bin Cai (TU

Dresden). (Chapter 4.3.2); The TEM, HRTEM, and STEM measurements for SrTiO3 microparticles were performed by Dr. Junjie Li (INL, Portugal). (Chapter 2.3.1)

The TG measurements for SrTiO3 hierarchical microparticles were carried out by Ilka Kunert (TU Dresden). (Chapter 2.3.2)

The BET measurement was performed with the help of Ulrike Polnick (TU Dresden). The results were discussed and analyzed with Dr. Guangping Hao (TU Dresden) and Dr. Yang Hou. (Chapter 2.3.3)

The UV-Vis absorption spectra of SrTiO3 hierarchical microparticles were measured with the help of Kai Eckhardt (TU Dresden). The photoelectrochemical property of

SrTiO3 hierarchical microparticles was measured by Dr. Yang Hou (TU Dresden). He also helped analyze the data. (Chapter 2.3.4)

Dr. Alexander Weiz (TU Dresden) performed the SEM measurements for the ZnO microparticles. (Chapter 3.3.1)

The single crystal X-ray diffraction measurements for CuS and Te single crystals were carried out by Dr. Martin Kaiser (TU Dresden). Structure solutions and refinements were also performed by him. Alexander Wolff and Johannes Teichert (both are from TU Dresden) assisted the measurements. (Chapter 3.3.1 and 5.3.1)

NMR spectra have been measured by Dr. Kai Schwedtmann (TU Dresden). He also assisted in the analyses of the spectra. Prof. Dr. Jan J. Weigand (TU Dresden) solved some NMR-related problems. (Chapter 4.3.1 and 5.3.1)

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Versicherung

Hiermit versichere ich, dass ich die vorliegende Arbeit ohne unzulässige Hilfe Dritter und ohne Benutzung anderer als der angegebenen Hilfsmittel angefertigt habe; die aus fremden Quellen direkt oder indirekt übernommenen Gedanken sind als solche kenntlich gemacht. Die Arbeit wurde bisher weder im Inland noch im Ausland in gleicher oder ähnlicher Form einer anderen Prüfungsbehörde vorgelegt.

Erklärung

Die vorliegende Arbeit wurde an der Professur für Anorganische Chemie II der Technischen Universität Dresden unter wissenschaftlicher Betreuung von Herrn Prof. Dr. Michael Ruck im Zeitraum von Oktober 2014 bis März 2018 angefertigt. Es haben keine früheren erfolglosen Promotionsverfahren stattgefunden. Hiermit erkenne ich die Promotionsordnung der Fakultät Mathematik und Naturwissen- schaften der Technischen Universität Dresden in der derzeit gültigen Fassung vom 23. Februar 2011, geändert am 15. Juni 2011 und 18. Juni 2014, an.

Dresden, März 2018

Tao Zhang

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