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Synthesis and Characterisation of Carbide Derived

Sigita Urbonaite

Department of Physical, Inorganic and Structural Stockholm University 2008

Doctoral Thesis 2008 Department of Physical, Inorganic and Structural Chemistry Stockholm University

Cover: Some artefacts found during TEM investigation of CDCs.

Faculty opponent:

Prof. Rik Brydson Department of Nanoscale Materials Characterisation Institute for Materials Research University of Leeds, UK

Evaluation committee:

Prof. Bertil Sundqvist, Nanofysik och material, UmU Prof. Margareta Sundberg, Strukturkemi, SU Prof. Kristina Edström, Strukturkemi, UU Docent Lioubov Belova, Teknisk materialfysik, KTH

© Sigita Urbonaite, Stockholm 2008 ISBN 978-91-7155-589-2 pp. 1-82.

Printed in Sweden by US-AB, Stockholm 2008 Distributor: FOOS/Structurkemi

ii ABSTRACT

Carbide derived carbons (CDCs) have been synthesised through chlorina- tion of VC, TiC, WC, TaC, NbC, HfC and ZrC at different temperatures. The aim of the investigation was to systematically study changes of struc- tural and adsorption properties depending on the synthesis conditions. CDCs were characterised using nitrogen and dioxide adsorption, Raman spectroscopy, scanning electron microscopy, transmission electron micros- copy, and electron energy loss spectroscopy. The studies revealed the CDCs structures to range from amorphous to ordered, from microporous to mesoporous. It was found that structural ordering and porosity can be modi- fied by: i) synthesis temperature, ii) precursor, iii) density and volume of precursor, iv) catalysts, v) incorporation of nitrogen in to carbide structure, and CDCs can be tuned up to the demanded quality. They also exhibited a high potential for storage.

iii iv LIST OF PUBLICATIONS

Paper I. Porosity development along the synthesis of carbons from metal carbides S. Urbonaite; J. M. Juárez-Galán; J. Leis; F. Rodríguez-Reinoso; G. Svensson MICROPOROUS AND MESOPOROUS MATERIALS: 2007, in Press.

Paper II. EELS studies of carbide derived carbons S. Urbonaite; S. Wachtmeister; C. Mirguet; E. Coronel; W.Y. Zou; S. Csillag; G. Svensson CARBON: 45 (2007) 2047-2053.

Paper III. Raman spectroscopy studies of carbide derived carbons S. Urbonaite; L. Hälldahl; G. Svensson 2008 MANUSCRIPT

Paper IV. Reverse Monte Carlo studies of nanoporous carbon from TiC P. Zetterström; S. Urbonaite; F. Lindberg; R.G. Deleplane; J. Leis; G. Svensson JOURNAL OF PHYSICS: CONDENSED MATTER: 17 (2005) 3509-3524.

Paper V. Barrel-like carbon from carbides by catalyst assisted chlori- nation A. Perkson; J. Leis; M. Arulepp; S. Urbonaite; G. Svensson CARBON: 41 (2003) 1729-1735.

Paper VI. Carbon hollow nanospheres from chlorination of ferrocene N.A. Katcho; E. Urones-Garrote; D. Ávila-Brande; A. Gómez-Herrero; S. Urbonaite; S. Csillag; E. Lomba; F. Agulló-Rueda; A.R. Landa-Cánovas; L. C. Otero-Diáz CHEMISTRY OF MATERIALS: 19 (2007) 2304-2309.

v ADDITIONAL PAPERS NOT INCLUDED IN THESIS

Paper VII. Synthesis of TiC1-xNx and TaC1-xNx by spark plasma S. Urbonaite; M. Johnsson; G. Svensson JOURNAL OF MATERIALS SCIENCE: 39 (2004) 1907-1911.

Paper VIII Thermoanalytical study on oxidation of TaC1-xNx powders by simultaneous TG-DTA-MS technique S. Shimada; M. Johnsson; S. Urbonaite THERMOCHIMICA ACTA: 419 (2004) 143-148.

Paper IX Characterization of unburned carbon in bagasse fly ash V.S. Batra; S. Urbonaite; G. Svensson FUEL: 2008, SUBMITTED.

Paper X Sol-gel synthesis and characterization of garnet A. Leleckaite; S. Urbonaite; K. Tõnsuaadu; A. Kareiva SPIE PROCEEDINGS (OPTICAL MATERIALS AND APPLICATIONS; Ed. A. Rosental): 5946 (2005) 47-54.

CONTRIBUTIONS TO THE NON-FIRST AUTHOR PAPERS

Paper IV: Synthesis, N2 adsorption and TEM studies. Paper V: TEM studies. Paper VI: N2 adsorption studies. Paper VIII: Synthesis. Paper IX: N2 adsorption studies. Paper X: SEM studies.

vi CONTENTS

CHAPTER 1. INTRODUCTION 1

1.1. BONDS IN CARBON 1 1.1.1. SINGLE BONDS 2 1.1.2. DOUBLE BONDS 2 1.1.3. TRIPLE BONDS 3 1.2 TYPES OF CARBON 3 1.3 CDCS 5 1.4 CHARACTERISATION OF CDCS 5 1.5 AIMS OF THE WORK AND OUTLINE OF THE THESIS 7

CHAPTER 2. CDC CHEMISTRY 9

2.1 THERMODYNAMICAL CALCULATION 9 2.2 EXPERIMENTAL 12 2.3 RESULTS AND DISCUSSION 12 2.4 SUMMARY OF CDC CHEMISTRY 14

CHAPTER 3. GAS ADSORPTION 17

3.1 ADSORPTION ISOTHERMS 17 3.2 INTERPRETATION OF ISOTHERMS 19 3.2.1 BET METHOD 19 3.2.2 D-R METHOD 20

3.2.3 αS-PLOT 20 3.3 PORE STRUCTURE 21 3.3.1 DFT METHOD 21 3.4 COMPARISON OF N2 AND CO2 ADSORPTION 22 3.5 METHANE CAPACITY 22 3.6 EXPERIMENTAL 23 3.6.1 N2 ADSORPTION 23 3.6.2 CO2 ADSORPTION 23 3.6.3 CH4 ADSORPTION 23

vii 3.7 RESULTS AND DISCUSSION 24 3.7.1 N2 ADSORPTION 24 3.7.2 CO2 ADSORPTION 25 3.7.3 CH4 ADSORPTION 26 3.8 SUMMARY OF GAS ADSORPTION RESULTS 28

CHAPTER 4. ELECTRON MICROSCOPY 33

4.1 BASICS BEHIND ELECTRON INTERACTION WITH THE MATTER 34 4.1.1 ELASTIC SCATTERING 34 4.1.2 INELASTIC SCATTERING 34 4.2 SCANNING ELECTRON MICROSCOPY – SEM 36 4.3 TRANSMISSION ELECTRON MICROSCOPY – TEM 36 4.4 ELECTRON ENERGY LOSS SPECTROSCOPY – EELS 36 4.4.1 BASIC COMPONENTS OF AN EEL SPECTRUM 37 4.4.2 MAGIC ANGLE 42 4.5 EXPERIMENTAL 42 4.5.1 SEM 42 4.5.2 TEM 42 4.5.3 EELS 42 4.6 RESULTS AND DISCUSSION 43 4.6.1 SEM 43 4.6.2 TEM 45 4.6.3 EELS 48 4.7 SUMMARY OF ELECTRON MICROSCOPY RESULTS 50

CHAPTER 5. RAMAN SPECTROSCOPY 53

5.1 SPECTROMETER 54 5.2. RAMAN SPECTROSCOPY OF CARBON MATERIALS 55 5.2.1 SPECTRA FITTING 57 5.2.2 IN-PLANE CORRELATION LENGTH 57 5.2.3 WAVELENGTH DEPENDENT DISPERSION 58 5.3 EXPERIMENTAL 58 5.4 RESULTS AND DISCUSSION 59 5.5 SUMMARY OF RAMAN SPECTROSCOPY RESULTS 63

CHAPTER 6. SIDETRACKS 67

6.1 SIDETRACK 1: EFFECT OF DENSITY AND NITROCARBIDES 67 6.1.1 DENSE TIC PELLETS 67

6.1.2 DENSE TIC1-XNX PELLETS 70 6.1.3 CONCLUSIONS 72 6.2 SIDETRACK 2. EFFECT OF CATALYSTS 73 viii 6.2.1 CONCLUSIONS 74 6.3 SIDETRACK 3. METALLOCENE DERIVED CARBON 75 6.4. SUMMARY OF SIDETRACKS 76

CHAPTER 7. CONCLUDING REMARKS 79

7.1. CONTROL OF CDCS PROPERTIES 79 7.2. FUTURE ASPECTS 80

ACKNOWLEDGEMENTS 81

ix

x CHAPTER 1. INTRODUCTION CHAPTER 1. INTRODUCTION

Pure elemental carbon has been one of the most important inorganic materials for a long time. Carbide derived carbon (CDC) is considered to be real elemental carbon, free of surface compounds, such as hydrogen and oxygen [1]. It distinguishes them from organic chars, given that in CDCs there are no free radicals trapped during carbonisation, which can be de- tected using paramagnetic electron spin resonance [2]. Common facts for all types of elemental carbon are presented in Table 1.

Table 1.1. Basic facts about elemental carbon.

Name Carbon Symbol C Atomic number 6 Group number 14 Period number 2 Isotopes 3 (12C, 13C, 14C) Molar mass 12.0107 (12C) Electronic shell [He]2s22p2 Phase Classification Non-metal

1.1. BONDS IN CARBON Bonding in carbon takes place with only the valence electrons. In ground state, carbon has two unpaired electrons. In such configuration, car- bon would have the possibility to form at most two bonds. As the bond for- mation releases energy, it is favourable to form as many bonds as possible. Therefore, the carbon atom will form an excited state by transferring one 2s electron into the empty 2p orbital and then four electrons are unpaired and available for bond formation. Therefore, an isolated carbon atom has four valence orbitals: three perpendicular p orbitals and one spherically symmet- rical s orbital; see Figure 1.1.1.

1 CHAPTER 1. INTRODUCTION

Figure 1.1.1. Carbon atomic orbitals [3].

One popular way to describe bonding among light elements is the hy- bridisation concept. The s and p orbitals of carbon may hybridise in three ways and form three types of bonds: single, double or triple. In elemental carbon, the bonds are covalent and can be of σ or π type. A σ bond is a cova- lent bond formed by an overlap between two atomic orbitals oriented along the bond. A π bond is a covalent bond formed by the overlap between two p orbitals perpendicular to the bonding direction belonging to the different atoms.

1.1.1. SINGLE BONDS The carbon single bond is a σ bond between two atomic orbitals. The hybridisation of atomic orbitals is of sp3 type, which is a combination of all three p orbitals with one s orbital forming four equivalent atomic orbitals; see Figure 1.1.1.1. The geometry of hybridised atoms is tetrahedral with 109.5o angle. Each of the four sp3 orbitals may form a σ bond with a length of 153 pm.

Figure 1.1.1.1. sp3 hybridisation and formation of a single bond [3].

1.1.2. DOUBLE BONDS The carbon double bond is a combination of a σ bond and a π bond. The hybridisation of atomic orbitals is of sp2 type, which is reached by com- bining two of three p orbitals with one s orbital into three equivalent orbitals, 2 CHAPTER 1. INTRODUCTION separated by a 120o angle and one remaining p orbital, normal to the plane of the sp2 orbitals, see Figure 1.1.2.1. The carbon – carbon bond length is 134 pm.

Figure 1.1.2.1. sp2 hybridisation and formation of a double bond [3].

1.1.3. TRIPLE BONDS The carbon triple bond is a combination of a single σ bond and two π bonds, perpendicular to each other, see Figure 1.1.3.1. The hybridisation of atomic orbitals is of sp type, which is achieved by combining one of three p orbitals with one s orbital into two atomic orbitals, which may form two equivalent σ bonds. The geometry is linear with 180o angle and bond length of 120 pm.

Figure 1.1.3.1. sp hybridisation and formation of a triple bond [3].

1.2 TYPES OF CARBON Elemental carbon has several allotropes. Allotropes are defined as dif- ferent physical forms of the same element. Allotropes differ in the way the atoms bond with each other and arrange themselves into a structure. Due to their different structures, allotropes have different physical and chemical properties.

3 CHAPTER 1. INTRODUCTION Two naturally occurring crystalline allotropes of carbon are and graphite. Diamond is one of the hardest known materials, while graphite is one of the softest. Those properties are the result of differences in the bonding of the carbon atom. Diamond consists of tetrahedrally bonded car- bon atoms and forms a three-dimensional lattice. Carbon atoms in diamond are 100% sp3 bonded. It is hard, colourless, highly abrasive and a good elec- trical insulator. Graphite is an allotropic form of the element carbon, consist- ing of layers of hexagonally arranged carbon atoms in a planar condensed ring system. The layers are stacked parallel to each other. The bonds be- tween the layers are of van der Waals type. Graphite is considered to be 100% sp2 bonded and is black, soft, slippery and a good electrical conductor. A third allotrope of carbon – fullerene, was first postulated in 1985 [4] and in 1990 its existence was confirmed and methods of synthesis were in- vented [5]. Unlike diamond and graphite, which can have unending crystal structure, the fullerene forms molecules. According to IUPAC, fullerenes are compounds composed solely of an even number of carbon atoms, which form a cage-like fused-ring polycyclic system with twelve five-membered rings while the rest are six-membered rings. The term has been broadened to include any closed cage structure consisting entirely of three-coordinate carbon atoms. There are many other types of carbons, with more disordered struc- tures than the ones mentioned above: amorphous carbon, carbon black, glass-like carbon, carbon fibres, activated carbons, etc. Amorphous carbon is considered to be a form of graphite as it consists of randomly arranged graphitic crystallites, although sometimes it is referred to as a naturally occurring allotrope. There are two types of amorphous car- bons: graphitising and non-graphitising. Graphitising carbons are relatively dense and soft, whereas non-graphitising carbons are hard, low-density ma- terials [6]. Carbon black is an industrially manufactured carbon material in the form of spheres and their fused aggregates with a size of below 1000 nm. Carbon black is a commercial product manufactured by thermal decomposi- tion, including detonation, or by incomplete combustion of hydrocarbons. It has a well-defined morphology with a minimum content of tars or other ex- traneous materials. Carbon fibres are fine filaments consisting of at least 92% (mass frac- tion) carbon, usually in a non-graphitic state. They are made by pyrolysis of organic precursor fibres or by growth from gaseous hydrocarbons [7]. Activated carbon is a porous carbon material, a char, which has been subjected to a reaction with gases, sometimes adding chemicals, during or after carbonisation, in order to increase porosity. Activated carbons have a large adsorption capacity and are extensively used for liquid-phase adsorb- ents in water treatment, decolourisation of solutions, and collection and re- covery of solutes. They are also used for gas purification and catalyst sup- ports. The form in which activated carbon is produced depends on the in-

4 CHAPTER 1. INTRODUCTION tended application. It is mainly used in granular form, though textile-like activated carbon can also be made.

1.3 CDCS One of the disordered porous carbon production routes is selective etching of carbides resulting in different carbon structures [8-17]. Such car- bons have had many names, such as nanostructured or nanoporous carbon, but lately for the carbon resulting from metal carbide chlorination, Gogotsi et al. [11] have introduced the term Carbide Derived Carbons (CDC). CDCs are very interesting future materials with a huge potential for diverse appli- cations as they exhibit a broad spectra of commercially required properties. CDCs have similar, though more distinct properties (narrow pore distribu- tion, possibility to tune pore size) than activated carbons and can be consid- ered as their substitute in more demanding applications. It has been claimed that CDCs can even compete with carbon nanotubes in their field of applica- tions [11]. CDCs are predicted to have a high impact on future technological ap- plications, e.g. in adsorption, electronics or as energy storage. They are men- tioned as promising materials for [18], hydrogen storage [17, 19], and as other porous , they may possibly be used for gas storage, molecular sieves, gas separation, purification of gases and liquids, catalyst support, adsorbents for soil and air decontamination, gas chromatography, etc [2].

1.4 CHARACTERISATION OF CDCS Many methods may be used for the characterisation of CDCs. For their adsorption properties all methods used in the characterisation of porous solids can be applied. The properties of interest for porous solids are surface area, pore volume, pore structure, and pore size distribution (PSD). These properties are mainly determined by means of gas adsorption. The most commonly used gases in the characterisation of porous carbon materials are nitrogen, argon, and . The latter is considered to be a compli- mentary technique to nitrogen adsorption when narrow microporosity is present. Because of such excellent properties as high surface area and large pore volume, together with low density of carbon materials, they are often considered as prominent candidates for energy storage. Therefore, methane storage capacity is often investigated by performing high-pressure methane adsorption studies. One of the important parameters for methane storage is the density. Three types of density are of interest when talking about porous carbons (bulk, He pycnometric and apparent density). Bulk density, which is important for methane storage, includes the porosity and packing. The den-

5 CHAPTER 1. INTRODUCTION sity of the carbon network, excluding packing and porosity, can be measured by He pycnometry. The third type is apparent density, which neglects pack- ing influence, but accounts for porosity, is hard to obtain, as powder particles are small and the use of unconventional techniques is required. One way to solve this problem is the use of Electron Energy Loss Spectroscopy (EELS). This technique gives information about the electronic structure of carbon materials. From recorded and evaluated spectra, knowl- edge about particle density and bonding e.g. sp2/sp3 ratio can be obtained on a semi-quantitative basis. Ordering of carbon materials is also reflected in EELS data. Structural ordering can also be obtained by XRD and Raman spectros- copy. The latter can also be used to quantify hybridisation of the bonds, though it requires the UV Raman spectroscopy. For morphology studies, SEM and TEM instruments are of great help. SEM studies give information about morphology of the particles, typically in a scale down to 10 nm. TEM provides a very clear picture of structural changes at subnanometer level, such as the degree of ordering of the graph- itic layers.

6 CHAPTER 1. INTRODUCTION

1.5 AIMS OF THE WORK AND OUTLINE OF THE THESIS The aim of the present work was to systematically study the depend- ence of CDCs properties on the synthesis conditions, such as precursor and synthesis temperature. Chapter 1 gives general information about the carbon materials and the CDCs place among them. This chapter also briefly intro- duces techniques applied in this work for characterisation of CDCs. Chapter 2 places CDCs in a historical perspective, giving a detailed description of the synthesis procedure and synthesis results. The following chapters (3 – 5) are devoted to experimental techniques used in carrying out this project. Each chapter starts with a brief technique introduction, its application for carbon materials, followed by experimental and brief description of results, which are discussed further and in more detail in the papers. Chapter 6, as is obvi- ous from the title, sidetracks from the main line of the thesis, involving as well some unfinished investigations, though still interesting and carrying information complementing the results of the main work. Chapter 7 summa- rises the results of this study, and gives an insight into future research possi- bilities.

7 CHAPTER 1. INTRODUCTION REFERENCES

[1] Mohun WA. A novel amorphous carbon. Proc Conf Carbon, Buffalo, 4th. 1960:443-453. [2] Nikitin A, Gogotsi Y. Nanostructured carbide-derived carbon. Encyclopedia of Nanoscience and Nanotechnology. 2004;7:553-574. [3] Reusch W. Virtual textbook of organic chemistry. 1999 [cited 2008 01 14]; Available from: http://www.cem.msu.edu/~reusch/VirtualText/intro1.htm#contnt [4] Kroto HW, Heath JR, O'Brien SC, Curl RF, Smalley RE. C60: Buckminster- fullerene. Nature. 1985;318:162-163. [5] Krätschmer W, Lamb LD, Fostiropoulos K, Huffman DR. Solid c60: A new form of carbon. Nature. 1990;347:354-358. [6] Radovic LR. Chemistry and physics of carbon: Marcel Dekker, Inc. 2003. [7] IUPAC. Compendium of chemical terminology (“gold book”). Blackwell, 1997. [8] Leis J, Perkson A, Arulepp M, Kaarik M, Svensson G. Carbon nanostructures produced by chlorinating carbide. Carbon. 2001;39(13):2043-2048. [9] Leis J, Perkson A, Arulepp M, Nigu P, Svensson G. Catalytic effects of met- als of the iron subgroup on the chlorination of carbide to form nanostruc- tural carbon. Carbon. 2002;40(9):1559-1564. [10] Shanina BD, Konchits AA, Kolesnik SP, Veynger AI, Danishevskii AM, Popov VV, Gordeev SK, Grechinskaya AV. A study of nanoporous carbon obtained from ZC powders (z= Si, Ti, and B). Carbon. 2003;41(15):3027-3036. [11] Gogotsi Y, Nikitin A, Ye H, Zhou W, Fischer JE, Yi B, Foley HC, Barsoum MW. Nanoporous carbide-derived carbon with tunable pore size. Nature Materials. 2003;2(9):591-594. [12] Perkson A, Leis J, Arulepp M, Kaarik M, Urbonaite S, Svensson G. Barrel- like carbon nanoparticles from carbide by catalyst assisted chlorination. Carbon. 2003;41(9):1729-1735. [13] Jacob M, Palmqvist U, Alberius PCA, Ekstrom T, Nygren M, Lidin S. Syn- thesis of structurally controlled nanocarbons in particular the nanobarrel carbon. Solid State Sciences. 2003;5(1):133-137. [14] Arulepp M, Permann L, Leis J, Perkson A, Rumma K, Janes A, Lust E. In- fluence of the solvent properties on the characteristics of a double layer capacitor. J Power Sources. 2004;133(2):320-328. [15] Dash RK, Yushin G, Gogotsi Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from carbide. Microporous Mesoporous Mater. 2005;86:50-57. [16] Yushin GN, Hoffman EN, Nikitin A, Ye H, Barsoum M, Gogotsi Y. Synthe- sis of nanoporous carbide-derived carbon by chlorination of titanium carbide. Chem Mater. 2005;43(10):2075–2082. [17] Dash R, Chmiola J, Yushin G, Gogotsi Y, Laudisio G, Singer J, Fischer J, Kucheyev S. derived nanoporous carbon for energy-related appli- cations. Carbon. 2006;44(12):2489-2497. [18] Arulepp M, Leis J, Lätt M, Miller F, Rumma K, Lust E, Burke AF. The ad- vanced carbide-derived carbon based . J Power Sources. 2006;162:1460-1466. [19] Johansson E, Hjörtvarsson B, Ekstrom T, Jacob M. Hydrogen in carbon nanostructures. J Alloys Compd. 2002;330-332:670-675.

8 CHAPTER 2. CDC CHEMISTRY CHAPTER 2. CDC CHEMISTRY

The first metal carbide chlorinations were done in the early XX cen- tury to obtain metal chlorides, not carbon [1]. The synthesis can be described by the general chemical reaction:

MeCx(s) +y/2 Cl2(g) ⇒ MeCly(g) + xC(s). During the reaction metal (Me) is transported away from the reactor as metal chloride (MeCly(g)) together with an excess of chlorine, leaving pure carbon. To remove residual chlorine and metal chlorides trapped in the car- bon structure, heating in the gas (air, N2, H2O, CO2, H2, He/H2, Ar) stream [2-7] is used. Other varying parameters are precursor, Cl2 flow rate and chlorination time, which sums up in to molar ratio of Cl2:MeC [3, 4]. Similar types of synthesis were done in the Soviet Union in the 1980’s for the production of carbons as novel materials for technological and indus- trial applications [8]. In recent decades, CDCs have become a subject of interest again, and many articles devoted to the synthesis, characterisation, and applications of CDCs have been published [2, 4-7, 9-19], [Papers I, II, III, IV, V].

2.1 THERMODYNAMICAL CALCULATION Thermodynamical calculations were made by HSC Chemistry for Windows, using databases available in the program [20]. Thermodynamical calculations are a guide for choosing synthesis parameters. It is important to find how the amount of chlorine influences the yield of the reaction as well as the synthesis temperature impact. Figure 2.1.1 shows the equilibrium species vs. synthesis temperature for chlorination of TiC, where it can clearly be seen that if synthesis tem- o perature is below 700 C, a significant amount of CCl4 is produced. As a re- sult, the carbon yield may be significantly lowered.

9 CHAPTER 2. CDC CHEMISTRY

2.0 4:1 (Cl2:TiC) Cl2(g)

l 1.5 o

es, m TiCl (g)

eci 4 1.0 sp C(s) rium

ilib 0.5

Equ CCl4(g) Cl(g)

0.0 0 200 400 600 800 1000 1200 o Synthesis temperature, C

Figure 2.1.1. The equilibrium of species close to optimal molar ratio be- tween TiC and Cl2.

The calculations of Cl2 amount vs. equilibrium species show that the molar ratio between Cl2 and TiC 2:1 is sufficient to fully convert TiC to car- bon. A further increase in molar ratio to 4:1 does not change equilibrium at 1000oC, though at 700oC the amount of carbon starts to decrease as a slightly larger amount of CCl4 is produced; see Figures 2.1.2 and 2.1.3.

l o 1.0 C(s) 0.8 TiCl4(g) 0.6 Cl (g)

species, m TiC(s) 0.4 2 CCl (g) Cl(g) 0.2 4 brium

li 0.0 0246810

Equi Amount of Cl , mol 2 1000oC 0.10 mol 0.08 0.06 Cl(g) ecies, p 0.04 0.02 CCl (g) 0.00 4 246810 Equilibrium s Amount of Cl , mol 2

o Figure 2.1.2. Equilibrium species vs. Cl2 amount at 1000 C.

10 CHAPTER 2. CDC CHEMISTRY

1.0 TiC(s) TiCl (g) 0.8 4 C(s) Cl (g) 0.6 2 species, mol 0.4 CCl (g) 0.2 4 0.0 0246810

Equilibrium Amount of Cl , mol 2 700oC 0.10 0.08 0.06 CCl (g) 0.04 4 0.02 0.00 246810

Equilibrium species, mol Amount of Cl , mol 2

o Figure 2.1.3. Equilibrium species vs. Cl2 amount at 700 C.

For the heavier metal carbides, if the synthesis conditions were not to be changed (amount of precursor, amount of chlorine), the molar ratio of Cl2:MeC would increase and as a result more CCl4 would be produced, as could be expected from Figures 2.1.2 and 2.1.3.

10 13:1 (Cl :TaC) 2 Cl (g) 8 2

ecies, mol 6 sp 4 m TaCl (g) CCl (g) 2 5 4 C(s) Cl(g)

ilibriu 0 0 200 400 600 800 1000 1200 o Equ l Synthesis temperature, C 1.0 TaCl (g) 0.8 5 C(s) ecies, mo p

s 0.6 0.4 CCl (g) 0.2 4 Cl(g)

uilibrium 0.0

Eq 200 400 600 800 1000 1200 o Synthesis temperature, C

Figure 2.1.4. The equilibrium of species close to the experimental molar ratio between TaC and Cl2.

However, as can be seen from Figure 2.1.4, the amount of CCl4 is not significantly increased with Cl2:TaC molar ratio 13:1 (TaC is one of the 11 CHAPTER 2. CDC CHEMISTRY metal carbides with the greatest molar mass used in CDCs synthesis), there- fore the same synthesis conditions (amount of carbide, chlorine flow, chlori- nation time) as used for TiC was followed during the entire synthesis work.

2.2 EXPERIMENTAL The chlorination treatment of the carbides was performed in a silica tube in a tube furnace (Carbolite HST 12/25/200, Aston Lane, England). The gases used in the experiments were Ar (Detector, ADR Klass 2.1 A, AGA Gas AB, Sweden) and Cl2 (2.8, ADR Klass 2.2 TC, AGA Gas AB, Sweden). Ar gas was cleaned from remains of water and oxygen prior to entering the reactor.

Table 2.2.1. Specifications of precursors.

Manufac- Purity, Crystal Space Unit Cell Precursor turer % System group Volume/Z* VC ACBR 99 cubic Fm-3m 72.62/4 TiC H.C. Starck 99.5 cubic Fm-3m 81.97/4 WC ACBR 99.5 hexagonal P-6m2 20.75/1 TaC ACBR 99.5 cubic Fm-3m 88.49/4 NbC ACBR 98 cubic Fm-3m 89.23/4 HfC ACBR 99 cubic Fm-3m 99.96/4 ZrC ACBR 99.8 cubic Fm-3m 102.69/4 *Arrangement of carbides in tables and figures is according unit cell vol- ume per carbon atom. Z – number of carbon atoms in the unit cell.

The reaction tube was flushed with Ar (50 ml/min) for 0.5 h, whereaf- ter the quartz vessel with ~0.5 g precursor, see Table 2.2.1, was inserted and flushed with Ar (20 ml/min) for the next hour. When the set temperature was reached, the Ar gas was changed to Cl2 gas (20 ml/min). After the chosen reaction time, typically 40 min, Cl2 gas was changed back to Ar gas (20 ml/min). For removal of residual Cl2 and MeCly, the temperature was ad- justed to 1000oC and kept for 1 h, whereafter it was decreased to room tem- perature.

2.3 RESULTS AND DISCUSSION A number of CDCs were synthesised, see Table 2.3.1, and the residual yield of the successful reaction was 89 – 98 %, except for VC chlorination, where carbon was “eaten up”, and the yield was as low as 20%. The reason for this is unclear, as it was not expected from the thermodynamical calcula- tions. As a consequence the reaction time for VC was shortened to the 30 min, and yield was increased, see Table 2.3.1. The yield is defined by:

12 CHAPTER 2. CDC CHEMISTRY m(product) ω%1=⋅00%; mC()theoretical

where mp(roduct) is mass of residue after chlorination, mC()theoretical is the mass of C in the carbide. A yield lower than 100% indicates, that all metal has been removed, as well as the loss of some amount of carbon. The complete removal of metal for some samples has been checked by the ash content after combustion, and it was less than 1% for carbons with the yield of less than 100%.

Table 2.3.1. Yield in % of CDCs from synthesis at different temperatures.

Carbide VC TiC WC TaC NbC HfC ZrC Temperature 700 89 98 132 98 98 98 98 800 91 97 - 89 90 96 95 900 67 98 - 98 94 96 97 1000 54 93 96 90 89 91 99 1100 - 94 - - - - - 1200 - * - - - - - *Provided by Tartu Technologies Ltd.

Another anomaly is WC, which had a very low reactivity at 700oC in comparison with other carbides. It was not possible to improve the results by increasing the reaction time. Each chlorination of WC at 700oC resulted in the carbon containing some unreacted carbide and a yield of higher than 100%. The CDC samples shadowed in the Table 2.3.1 were further used for analysis and characterisation of the CDCs.

13 CHAPTER 2. CDC CHEMISTRY

2.4 SUMMARY OF CDC CHEMISTRY

• Thermodynamical calculations showed that if the synthesis tempera- o ture is lower than 700 C, a significant amount of CCl4 may be pro- duced, lowering carbon yield. Yield sensitivity to the amount of chlorine was found to be much lower. • The adjustments of synthesis conditions led to the carbon yield of 89-98%, with the exceptions of VC and WC chlorinations. • The quality of synthesised CDCs was checked by the ash content af- ter combustion, and it was lower than 1%.

14 CHAPTER 2. CDC CHEMISTRY REFERENCES

[1] Hutchins O, inventor. Method of the production of silicon tetrachlorid. USA patent 1271713. 1918. [2] Nikitin A, Gogotsi Y. Nanostructured carbide-derived carbon. Encyclopedia of Nanoscience and Nanotechnology. 2004;7:553-574. [3] Jänes A, Thomberg T, Lust E. Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of carbide. Carbon. 2007;45:2717- 2722. [4] Hoffman EN, Yushin G, Barsoum MW, Gogotsi Y. Synthesis of Carbide- Derived Carbon by Chlorination of Ti2AlC Chem Mater. 2005;17(9):2317-2322. [5] Leis J, Perkson A, Arulepp M, Kaarik M, Svensson G. Carbon nanostructures produced by chlorinating . Carbon. 2001;39(13):2043-2048. [6] Leis J, Perkson A, Arulepp M, Nigu P, Svensson G. Catalytic effects of met- als of the iron subgroup on the chlorination of titanium carbide to form nanostruc- tural carbon. Carbon. 2002;40(9):1559-1564. [7] Zheng J, Ekstrom TC, Gordeev SK, Jacob M. Carbon with an onion-like structure obtained by chlorinating titanium carbide. J Mater Chem. 2000;10(5):1039- 1041. [8] Fedorov NF, Ivakhnyuk GK, Gavrilov DN. Carbon adsorbents from group IV-VI carbides. Zh Prikl Khim. 1982;55(2):272-275. [9] Arulepp M, Leis J, Lätt M, Miller F, Rumma K, Lust E, Burke AF. The ad- vanced carbide-derived carbon based supercapacitor. J Power Sources. 2006;162:1460-1466. [10] Arulepp M, Permann L, Leis J, Perkson A, Rumma K, Janes A, Lust E. In- fluence of the solvent properties on the characteristics of a double layer capacitor. J Power Sources. 2004;133(2):320-328. [11] Chmiola J, Yushin G, Dash RK, Gogotsi Y. Effect of pore size and surface area of carbide derived carbons on specific capacitance. J Power Sources. 2005;158:765-772. [12] Dash R, Chmiola J, Yushin G, Gogotsi Y, Laudisio G, Singer J, Fischer J, Kucheyev S. Titanium carbide derived nanoporous carbon for energy-related appli- cations. Carbon. 2006;44(12):2489-2497. [13] Dash RK, Yushin G, Gogotsi Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from zirconium carbide. Microporous Mesoporous Mater. 2005;86:50-57. [14] Gogotsi Y, Dash RK, Yushin G, Yildirim T, Laudisio G, Fischer JE. Tailor- ing of Nanoscale Porosity in Carbide-Derived Carbons for Hydrogen Storage. J Am Chem Soc. 2005;127(46):16006-16007. [15] Gogotsi Y, Nikitin A, Ye H, Zhou W, Fischer JE, Yi B, Foley HC, Barsoum MW. Nanoporous carbide-derived carbon with tunable pore size. Nature Materials. 2003;2(9):591-594. [16] Yushin GN, Hoffman EN, Nikitin A, Ye H, Barsoum M, Gogotsi Y. Synthe- sis of nanoporous carbide-derived carbon by chlorination of titanium . Chem Mater. 2005;43(10):2075–2082. [17] Jacob M, Palmqvist U, Alberius PCA, Ekstrom T, Nygren M, Lidin S. Syn- thesis of structurally controlled nanocarbons in particular the nanobarrel carbon. Solid State Sciences. 2003;5(1):133-137. [18] Johansson E, Hjörtvarsson B, Ekstrom T, Jacob M. Hydrogen in carbon nanostructures. J Alloys Compd. 2002;330-332:670-675.

15 CHAPTER 2. CDC CHEMISTRY [19] Shanina BD, Konchits AA, Kolesnik SP, Veynger AI, Danishevskii AM, Popov VV, Gordeev SK, Grechinskaya AV. A study of nanoporous carbon obtained from ZC powders (Z= Si, Ti, and B). Carbon. 2003;41(15):3027-3036. [20] Outokumpu HSC Chemistry® for Windows. 5.11 ed: Outokumpu Research Oy 2002.

16 CHAPTER 3. GAS ADSORPTION CHAPTER 3. GAS ADSORPTION

Gas adsorption is the phenomenon, when the concentration of the gas at the surface has a higher concentration than in the bulk of the gas [1]. There are two adsorption types: physisorption and chemisorption. Physisorp- tion, which does not involve chemical bonding, occurs for all gases and solid surfaces. It increases with increasing pressure and, as adsorption is always an exothermic process, with decreasing temperature. It must be distinguished from chemisorption, which involves the formation of direct chemical bonds between gas and solid surface. Several terms must be defined before any further description of gas adsorption: a) Adsorptive is the gas or vapour prior to being adsorbed. b) Adsorbent is the solid material on which adsorption occurs. c) Adsorbate is the gas or vapour when adsorbed.

3.1 ADSORPTION ISOTHERMS Adsorption isotherm for a single gaseous adsorptive on an adsorbent, is a function which relates (at constant temperature) the amount of substance adsorbed at equilibrium, to the pressure of the adsorptive in the gas phase [2]. There are six principal types of isotherm shapes [3], see Fig. 3.1.1. Type I is the characteristic reversible isotherm of highly microporous materials, having relatively small external surfaces; it saturates at low relative pres- sures. The reversible Type II isotherm is typical for non-porous or macro- porous materials and it represents unrestricted monolayer-multilayer adsorp- tion. The reversible Type III isotherm is not common and generally associ- ated with weak adsorbent-adsorbate but strong adsorbate-adsorbate interac- tions. It is characteristic for adsorption on surfaces with low adsorption potential, like organic polymers. The Type IV isotherms resemble Type II, the difference is its hysteresis loop (see below) which is associated with cap- illary condensation taking place in mesopores and limiting uptake at high relative pressures. It is a typical isotherm for mesoporous materials. The Type V isotherm is uncommon, but obtained for some porous adsorbents, where the adsorbent-adsorbate interactions are exceptionally weak in com- parison with adsorbate-adsorbate interactions. It is typical for water vapour adsorption on microporous carbons. The last Type VI isotherm represents stepwise multilayer adsorption on uniform non-porous surfaces. The step height represents the monolayer capacity and usually remains constant for two-three adsorbed layers. 17 CHAPTER 3. GAS ADSORPTION

Fig. 3.1.1. Classification of adsorption-desorption isotherms.

The adsorption-desorption process on materials possessing mesopores is accompanied by the appearance of hysteresis loops. Adsorption hysteresis is said to occur when adsorption and desorption values deviate from each other. This phenomenon has been extensively studied but its origin is still not fully understood and is usually attributed to thermodynamic and network effects: capillary condensation or evaporation in pores and existence of a network with different pore sizes which are emptied at different relative pressures [4]. Low pressure hysteresis loops (non-closing hysteresis loop) arise from the swelling of the adsorbent during the adsorption process or partial chemisorption [3]. IUPAC distinguish between four types of hystere- sis loops, see Fig. 3.1.1. The Type H1 and Type H4 are the two extreme cases. In the Type H1 hysteresis, the adsorption and desorption lines are almost vertical and parallel to each other over quite a narrow range of partial pressures. This is associated with materials with good pore connectivity and narrow pore size distribution [5, 6]. In the Type H4 hysteresis loop, adsorp- tion and desorption lines are almost horizontal and parallel over a wide range of partial pressures. It was attributed to materials with narrow slit like pores, though recent experiments suggest that Type H4 hysteresis arises due to large mesopores embedded in the matrix of smaller pores [7]. The Types H2 and H3 are to some extent intermediate between these two extremes. It is necessary to note that the closure point of hysteresis loops at lower relative pressures is dependent on the nature of adsorptive, not the nature of adsorb- ent (e.g. for nitrogen at 77K the hysteresis loop is closing at p/po ≈0.42) [7].

18 CHAPTER 3. GAS ADSORPTION

3.2 INTERPRETATION OF ISOTHERMS

Quantitative isotherm interpretation is needed to enable a comparison of different materials. However, an interpretation of isotherms is not straightforward; especially adsorption processes in micropores are difficult to describe accurately. The adsorption in mesopores is better understood and macroporosity acts in a similar way to open surfaces, though it accounts for less than 1% of the adsorption processes within microporous carbons [8]. The extraction of information is usually done by applying mathematical models to the isotherm data. Some of the most popular methods are Brun- auer-Emmett-Teller (BET), Dubinin-Radushkevich (D-R) together with its modifications, t- and α- plots, and Density Functional Theory (DFT).

3.2.1 BET METHOD

BET pc1−1 =+ p / po equation o Vp()− p VcmmVc V amount adsorbed in volume STP (cm3/g) or in molar amount (mmol/g); 3 Vm monolayer capacity in volume STP (cm /g) or in molar amount (mmol/g); p equilibrium vapour pressure (Pa); po saturation vapour pressure (Pa); c average energy of adsorption (c = exp [(∆HA - ∆HL)/RT]); ∆HA heat of adsorption; ∆HL heat of liquefaction.

The BET method is based on several assumptions: a) the surface is flat; b) all the surface sites have uniform energy, molecules in the first layer act as sites for molecules in the second and higher layers; c) the condensa- tion and evaporation properties of all layers above the first are similar to the liquid adsorptive; d) there is no interaction between adsorbed molecules; e) the number of layers is unlimited [7]. The BET equation is widely used to interpret isotherms obtained using nitrogen at 77K [8], but this method has many limitations [7], therefore the use of a term apparent surface area is suggested. The best fit for BET equa- tions is normally restricted to the low relative pressure values (<0.15) for microporous carbons. The c parameter for non-porous carbons is less than 150, while for microporous carbons much higher than 200 [9].

19 CHAPTER 3. GAS ADSORPTION

3.2.2 D-R METHOD

D-R ⎡ BT ⎛⎞po ⎤ equation WW=−exp log2 o ⎢ ⎜⎟⎥ ⎣⎢ β ⎝⎠p ⎦⎥ W volume of adsorbate filling the micropores (cm3/g) at relative pressure p/p0 and temperature T(K); 3 Wo total volume of micropores (cm /g); p equilibrium vapour pressure (Pa); po saturation vapour pressure (Pa);

β adsorbate affinity coefficient (C6H6 is a standard vapour β = 1); B constant related to the adsorption potential of the micropores.

The D-R equation is based on considerations of energies of adsorption [8], involving characteristic energy of a given solid and the change of poten- tial energy between gas and adsorbed phases. For most activated carbons the relative pressure limit for D-R equation applicability is < 0.15 [9]. The advantage of the D-R method is that it can be used to evaluate isotherms over low ranges of relative pressure, and obtain values of microporous filling. It is usually applied for CO2 adsorption at 273 – 295 K. If narrow micropores are present, use of the D-R method for ana- lysing N2 isotherms is limited due to its diffusion restrictions at 77 K.

3.2.3 αS-PLOT

αS-PLOT ax()= amicro,max + kstd Sextαs (x) equation a(x) amount adsorbed (cm3/g STP); x relative pressure p/po; 3 amicro,max amount adsorbed in saturated micropores (cm /g STP); αs(x) = astd(x)/astd(0.4), dimensionless; 3 astd(x) amount adsorbed by the standard used (cm /g STP); o 3 astd(0.4) amount adsorbed by the standard at p/p =0.4 for N2 gas (cm /g STP); kstd = astd(x=0.4)/ Sstd, (cm); 2 Sstd is specific surface area of the standard in use (cm /g); 2 Sext external surface area (cm /g).

The αs-plot is based on the assumption that the adsorption in micro- pores is complete, before the adsorption on the remaining surface. It was introduced by K.S.W. Sing, who pointed out that when compar- ing isotherms it is sufficient to normalise them by taking the amount ad- sorbed at some fixed relative pressure [9]. The standard isotherm in αs-plot is usually some experimental isotherm on a non-porous adsorbent selected specifically for its chemical and structural similarity to the adsorbent under study. αs-plot allows determination of the content not only microporous sur-

20 CHAPTER 3. GAS ADSORPTION face areas and volumes, but also the mesoporous and external (macropor- ous).

3.3 PORE STRUCTURE The phenomena of adsorption and porosity are inseparable. Porosity is usually characterised by pore volume and pore size distribution (PSD) [10]. The pore size is generally specified as a pore width. The single value of pore width is meaningless without definition of the pore shape [11]. In practice, it is usually an effective pore width: an available inner diameter of tubular pores or a distance between the opposite walls in slit-shaped pores. The clas- sification of pores given by IUPAC is: micropores < 20 Å, mesopores from 20 to 500 Å, macropores > 500 Å [2]. Micropores can be further divided into narrow (< 0.7 nm) and wide (0.7-2.0 nm) [9]. In mesoporous and macropor- ous solids, the effective pore width is consistent with the thermodynamical requirements of capillarity, while in microporous solids it is a dimension controlling the entry of molecules of particular size. Different pore sizes (micropores and mesopores) were traditionally treated separately, mainly due to a lack of mathematical models. Micropore PSD were traditionally evaluated using the D-R method (described above) and the Horvath- Kawazoe (H-K) method. The mesopore PSD was obtained using the BJH (Barrett-Joyner-Halenda) method. The DFT method is now a well- established and frequently used method, advantageous over the previously mentioned methods as it describes both microporous and mesoporous PSDs without separating them. A detailed description of the pore structure in terms of pore size dis- tribution is important as it may be used for the correlation and prediction of the performance of carbon materials in industrial applications, as well as in the development of new porous materials [12].

3.3.1 DFT METHOD The free energy of an inhomogeneous system, such as fluid in the po- rous material, can be expressed as a function of a density profile. In DFT, variational methods are used to minimise the free energy function and de- termine the density profile. The modified non-local density functional theory (NLDFT) is often used to describe all pore distributions. The calculation of pore volume per pore width is done by interpolation of measured pressure levels with the given ones. The main advantage is that the PSD can be calcu- lated over a broad range of pore sizes. The disadvantages are due to a num- ber of idealised assumptions such as: smoothness of adsorbent surface, no connections between pores, exclusion of chemical functional groups on sur- face and the shape of pores [13].

21 CHAPTER 3. GAS ADSORPTION

3.4 COMPARISON OF N2 AND CO2 ADSORPTION Gas adsorption is frequently used in the characterisation of porous car- bons, among them carbide derived carbons [14-25]. The most commonly used gases for sorption experiments are nitrogen (N2) at 77 K and carbon dioxide (CO2) at 273 K or 298 K [12, 26-37]. Both gases have their advan- tages and limitations. Nowadays nitrogen has an established place in the modern characterisation of porous carbons [9]. The limitations of N2 sorp- tion experiments are obvious when dealing with carbons possessing narrow microporosity, especially pores smaller than 5 Å, due to restricted diffusion of N2 present at 77 K [9, 26, 27, 38]. The CO2 adsorption is often referred to as a method for characterisation of narrow microporosity as it operates at much higher temperatures. On the other hand, CO2 adsorption measured at 273 K up to atmospheric pressure does not provide information about pores larger than ~10 Å [30]. Consequently, both methods are needed to obtain a complete picture of the porosity present in the carbons.

3.5 METHANE CAPACITY Methane is a gas that can be compressed at room temperature without condensation and is usually stored in metal cylinders at around 20 - 25 MPa. In recent years there has been an increasing interest in using CH4 as fuel for vehicles with a need to store CH4 at much lower pressures than 20 MPa. The physisorption of methane on porous carbon is one way to store CH4 at lower pressure, as the density of CH4 molecules on a carbon surface is higher than in the gas phase [39]. The methane storage at 3.4 MPa has been intensively studied with a goal of obtaining recovery around or more than 150 v/v (vol- ume of gas per volume of adsorbent) [9, 40, 41]. The ideal pore size for methane adsorption has been discussed in lit- erature and a number of different pore sizes have been suggested. The small- est reported pore size considered to be ideal is 6.3 Å [42]. However, theo- retical studies suggest that an ideal adsorbent with a maximum methane stor- age should have micropores with size corresponding to two layers of meth- ane molecules, ~8 Å [37, 40, 43]. If the deliverability is taken into account, then the pore size should correspond to at least three layers of methane molecules, and theoretical studies which include mass and heat transfer ef- fects suggest ideal pore size to be ~11.4 Å [40, 44]. Nevertheless some au- thors suggest other optimal pore sizes: 9.4 Å [45], 20 Å [46]. The ideal pore size of ~20 Å is supported by the presence of the commercially available adsorbent Maxsorb, which has so far the highest methane capacity reported and an average pore size of 22 Å [46]. Still, other experimental investiga- tions of pore size influence on methane capacity gives 14 Å [47, 48].

22 CHAPTER 3. GAS ADSORPTION

3.6 EXPERIMENTAL Physisorption is one of the key techniques used for the characterisa- tion of CDCs and very few studies are done without at least reporting their surface areas. The standard methods used to characterise microporous mate- rials are N2 and CO2 adsorptions. The CDCs have been characterised by N2 [14-17, 49-51], [Papers I, III, IV and V] and Ar [20, 22, 23]. Ar has been used as it gives more information about the narrow micropores than nitro- gen, though it is disadvantageous if comparison with other porous materials is wanted, since most of the adsorption studies are done using N2 gas. As mentioned above, CO2 adsorption gives an ability to study narrow micropo- rosity, though it has not been used for CDCs characterisation before Paper I. However, it is frequently used for the characterisation of various activated carbons [9, 12, 26, 27, 29-31, 36, 47]. Methane capacity has been reported twice for CDCs [19], [Paper I], though many studies of other carbons have been conducted in the search for materials suitable for methane storage [19, 34, 37, 39, 42-46, 48, 52-55].

3.6.1 N2 ADSORPTION The nitrogen adsorption studies were carried out in an ASAP 2020 (Micromeritics) instrument. Samples were degassed at 300oC for 180 min, prior to the analysis. The sample amount in the adsorption studies was ~0.1 g. The isotherms were evaluated according to BET, D-R and αs-plot methods using the ASAP 2020 software. The pore size distribution, pore volume, and pore sized were also evaluated according to the DFT method using DFT plus software (Micromeritics).

3.6.2 CO2 ADSORPTION

The CO2 isotherms were recorded by subatmospheric CO2 adsorption in an Autosorb 6 apparatus at 0oC. The microporous volume of the carbons according to the CO2 adsorption data was determined using the D-R method.

3.6.3 CH4 ADSORPTION The methane high-pressure adsorption was measured by adsorption of o CH4 at T = 25 C up to a pressure of 30 bars in a volumetric system HPA 100, VTI, after previous degassing at 250oC for 4 h in high vacuum. Meth- ane capacity as volume fraction (cm3/cm3) and mass fraction (mg/g) were calculated using CH4 density at STP.

23 CHAPTER 3. GAS ADSORPTION

3.7 RESULTS AND DISCUSSION

3.7.1 N2 ADSORPTION The use of methods traditionally employed in the characterisation of the activated carbons for evaluation of CDCs isotherms simplifies compari- son between materials, reveals their differences, and may to faster com- mercialisation of CDCs. Therefore, in Paper I, the full N2 isotherms were recorded and evaluated using BET, D-R, α-plot, and DFT methods.

500 P T S

/g 400 3

300 o rbed, cm 700 C o 200 800 C Adso 900oC ty 1000oC anti 100 o u 1100 C Q 1200oC 0 0.00.20.40.60.81.0 o p/p

Figure 3.7.1.1. N2 adsorption isotherms of TiC based CDCs synthesised at different temperatures.

The synthesis temperature influences many parameters, such as appar- ent surface area, total pore volume, microporous pore volume and PSD of CDCs. The temperature influence is already apparent in the recorded iso- therms of CDCs, see Figure 3.7.1.1. At lower temperatures, the isotherms are of type I, indicating a high content of microporosity, while at higher tem- peratures isotherms exhibit type H4 hysteresis loops, associated with capil- lary condensation and mesoporosity [16, 19, 20, 23], [Paper I]. The tempera- ture effect is also reflected in pore size distribution and pore sizes. The pores get larger the higher the synthesis temperature, so temperature is a tool for pore tuning [14, 15, 20, 22, 23], [Paper I]. However, the pore sizes are not the only parameter that can be tuned [Paper I]:

24 CHAPTER 3. GAS ADSORPTION

• The apparent surface area (SBET) and total pore volume increases with temperature, though there is a certain synthesis temperature at which it starts to decrease again. • Microporous volume and microporous surface area decrease with increasing temperature, while the pore volume of larger pores as well as external surface area are increasing and com- pensates for the loss in microporosity. • However, at some temperatures, specific to each of the starting carbides, the growth of larger pores does not compensate any- more for microporosity loss. Consequently the apparent surface area and total pore volume starts to decrease as the highly po- rous structure is collapsing.

The pore size distributions from N2 sorption data showed that all CDCs synthesised at 700oC (except for WC based CDC due to low reactivity of WC at 700oC) were purely microporous. All CDCs made at 1000oC con- tained mesopores, except for ZrC based CDC, which PSD stayed in the mi- croporous region. The critical synthesis temperature, at which TiC based CDCs change from purely microporous to mesoporous, is between 900 and 1000oC. Even if the influence of nature of starting carbide is obvious (different apparent surface areas, pore volumes, pore size distribution, different behav- iour with changed synthesis temperature), which gives an additional means for tuning adsorption properties, its origin is not completely understood. In Paper I, CDCs based on VC, TiC, WC, TaC, NbC, HfC and ZrC were stud- ied and no clear correlation in adsorption behaviour, depending on any start- ing carbide properties (such as, for example volume per C atom in starting carbide), was observed.

3.7.2 CO2 ADSORPTION

The utilization of CO2 adsorption for characterisation of CDCs seems to be very low, in fact Paper I seems to be the first. CO2 studies of TiC based CDCs made at 700 – 1000oC and CDCs based on NbC, HfC and ZrC made at 1000oC confirmed the presence of high amounts of narrow micropores. The difference between Vmicro (D-R) from N2 and CO2 can be used to estimate the relative amount of narrow and wide micropores present in the samples [26]. According to adsorption behaviour for N2 and CO2, carbons can be divided into three groups [9, 26]: • Group A: Nitrogen is adsorbed to a lesser extent than carbon dioxide. Vmicro (D-R)N2 < Vmicro (D-R)CO2 • Group B: Nitrogen and carbon dioxide are adsorbed more or

less in equal amounts. Vmicro (D-R)N2 ≈ Vmicro (D-R)CO2 • Group C: Nitrogen is adsorbed to a much greater extent than carbon dioxide. Vmicro (D-R)N2 > Vmicro (D-R)CO2.

25 CHAPTER 3. GAS ADSORPTION The studied CDCs belong to groups A or C depending on the synthe- sis temperature and precursor. The CO2 accessibility is better than N2 for TiC based CDCs made at 700 and 800oC, and they belong to group A. All other studied CDCs belong to group C, where wider microporosity is domi- nant. It is a good example of the usefulness of CO2 adsorption as a comple- mentary technique to the N2, because without the CO2 measurement it would be impossible to even detect the full microporous volume in the TiC based o CDCs made at 700 and 800 C. Therefore, the combination of both N2 and CO2 adsorption is recommended to use as a standard set of techniques for complete characterisation of carbons containing narrow microporosity [9, 28], such as CDCs.

3.7.3 CH4 ADSORPTION Methane adsorption was performed on three CDCs based on TiC syn- thesised at 800, 900 and 1000oC and three CDCs based on NbC, HfC and ZrC made at 1000oC. As mentioned in section 3.5, the CDCs possess pore sizes close to the ones considered to be ideal for methane storage. Methane capacity is often given as volumetric (gas volume per vol- ume of adsorbent v/v (cm3/cm3)), and gravimetric (weight of gas adsorbed per weight of adsorbent (mg/g)) fractions. The methane capacity for the studied CDCs is presented in Table 3.7.3.1 together with other characteris- tics including pore size.

Table 3.7.3.1. Summary of CH4 capacity and adsorption properties of CDCs.

f o

n

3 /g /g 3

3 ) ) /g R tity ,

- m /cm g 3 T dth, , 3 D ( /g T an

F mperature , c 2 E o 2 r cm , m B D c i 4 packing qu S , cm g/ cm /N 4 m rbed, cm 4 ρ Sample 2 Å ( o V CH Pore wi metric fractio CH imetric fraction of CH CO u v ads l Carbide/Te Vo Gra TiC/800 1422 0.57/0.54 8.3/13.6 0.28 152 99 43 TiC/900 1599 0.58/0.61 8.4/13.6 0.26 158 103 41 TiC/1000 1570 0.52/0.58 8.6/21.4 0.22 148 96 33 NbC/1000 1721 0.51/0.57 8.4/21.7 0.17 148 96 25 HfC/1000 1883 0.58/0.67 8.6/20.4 0.35 145 94 50 ZrC/1000 1692 0.53/0.62 7.9/19.8 0.24 162 105 38

The results do not reveal any clear correlation between PSD and meth- ane adsorption capacity, despite the fact that all the pore sizes are close to the ones considered to be ideal, see above section 3.5. Similar methane ca- pacity for TiC based CDC (made at 800oC) has been reported by Dash et al. (45 v/v) [19]. In the present study, the ZrC based CDC has the highest gra-

26 CHAPTER 3. GAS ADSORPTION vimetric capacity, which is above average, though the volumetric fraction of methane is highest for HfC based CDC. Overall, the methane gravimetric storage capacity is quite good, though the volumetric is low, and far from the commercial goal of 150 cm3/cm3. The reason for this is that the powder packing density of CDCs is low.

27 CHAPTER 3. GAS ADSORPTION

3.8 SUMMARY OF GAS ADSORPTION RESULTS • The pore size of CDCs gets larger with higher synthesis tempera- tures. Temperature can be used as a pore tuning parameter. • The apparent surface area and total pore volume increase with tem- perature up to a certain point whereafter they start to diminish. Mi- croporous volume and microporous surface area decrease with an increase in temperature, while the pore volume of larger pores and external surface area increase, compensating for loss of microporos- ity. However, at a certain temperature, which is specific to each of the starting carbides, the growth of the larger pores does not com- pensate any longer for the microporosity loss. Above this tempera- ture, the apparent surface area and total pore volume starts to de- crease and the highly porous structure collapses. • There is no clear correlation between the unit cell volume per carbon atom in the starting carbide and the adsorption properties of their de- rived CDCs. However, the nature of the precursor carbide has an impact on the adsorption properties and the PSD of the resulting carbons. Thus, properties of the CDCs can be adjusted by selecting a suitable precursor.

• The CO2 studies showed that at higher synthesis temperatures the narrow micropores are diminished. • The methane capacity studies revealed that all CDCs have the poten- tial for methane storage, though the problem of low packing density must be overcome.

28 CHAPTER 3. GAS ADSORPTION REFERENCES

[1] De Boer JH. The Dynamical Character of Adsorption. Second edition ed. Oxford: Oxford University Press 1968. [2] IUPAC. Compendium of Chemical Terminology (“Gold Book”). Blackwell 1997. [3] Sing KSW, Everett DH, Haul RAW, Moscou L, Pierotti RA, Rouquerol J, Siemieniewska T. Reporting physisorption data for gas/solid systems, with special reference to the determination of surface area and porosity (recommendations 1984). Pure Appl Chem. 1985;57(4):603-619. [4] Rouquerol F, Rouquerol J, Sing K. Adsorption by Powders and Porous Sol- ids: Principles, Methodology and Applications. London: Academic Press 1999. [5] Huo Q, Margolese DI, Stucky GD. Surfactant Control of Phases in the Syn- thesis of Mesoporous Silica-Based Materials. Chem Mater. 1996;8(5):1147-1160. [6] Kruk M, Jaroniec M, Sayari A. Application of Large Pore MCM-41 Molecu- lar Sieves To Improve Pore Size Analysis Using Nitrogen Adsorption Measure- ments. Langmuir. 1997;13(23):6267-6273. [7] Kruk M, Jaroniec M. Gas Adsorption Characterization of Ordered Organic- Inorganic Nanocomposite Materials. Chem Mater. 2001;13(10):3169-3183. [8] Byrne JF, Marsh H. Introductory Overview. In: Patrick JW, ed. Porosity in Carbons: Characterization and Applications. London Edward Arnold 1995. [9] Marsh H, Rodriguez-Reinoso F. Activated Carbon. Oxford: Elsevier 2006. [10] Do DD, Nguyen C, Do HD. Characterization of micro-mesoporous carbon media. Colloids and Surfaces A: Physicochemical and Engineering Aspects. 2001;187-188:51-71. [11] Schuth F, Sing KSW, Weitkamp J, eds. Handbook of Porous Solids. Wein- heim: Wiley-VCH Verlag GmbH 2002. [12] Jagiello J, Thommes M. Comparison of DFT characterization methods based on N2, Ar, CO2, and H2 adsorption applied to carbons with various pore size distri- butions. Carbon. 2004;42(7):1227-1232. [13] El-Merraoui M, Aoshima M, Kaneko K. Micropore Size Distribution of Ac- tivated Carbon Fiber Using the Density Functional Theory and Other Methods. Langmuir. 2000;16(9):4300-4304. [14] Arulepp M, Leis J, Lätt M, Miller F, Rumma K, Lust E, Burke AF. The ad- vanced carbide-derived carbon based supercapacitor. J Power Sources. 2006;162:1460-1466. [15] Arulepp M, Permann L, Leis J, Perkson A, Rumma K, Janes A, Lust E. In- fluence of the solvent properties on the characteristics of a double layer capacitor. J Power Sources. 2004;133(2):320-328. [16] Leis J, Perkson A, Arulepp M, Kaarik M, Svensson G. Carbon nanostructures produced by chlorinating aluminium carbide. Carbon. 2001;39(13):2043-2048. [17] Leis J, Perkson A, Arulepp M, Nigu P, Svensson G. Catalytic effects of met- als of the iron subgroup on the chlorination of titanium carbide to form nanostruc- tural carbon. Carbon. 2002;40(9):1559-1564. [18] Chmiola J, Yushin G, Dash RK, Gogotsi Y. Effect of pore size and surface area of carbide derived carbons on specific capacitance. J Power Sources. 2005;158:765-772. [19] Dash R, Chmiola J, Yushin G, Gogotsi Y, Laudisio G, Singer J, Fischer J, Kucheyev S. Titanium carbide derived nanoporous carbon for energy-related appli- cations. Carbon. 2006;44(12):2489-2497.

29 CHAPTER 3. GAS ADSORPTION [20] Dash RK, Yushin G, Gogotsi Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from zirconium carbide. Microporous Mesoporous Mater. 2005;86:50-57. [21] Gogotsi Y, Dash RK, Yushin G, Yildirim T, Laudisio G, Fischer JE. Tailor- ing of Nanoscale Porosity in Carbide-Derived Carbons for Hydrogen Storage. J Am Chem Soc. 2005;127(46):16006-16007. [22] Gogotsi Y, Nikitin A, Ye H, Zhou W, Fischer JE, Yi B, Foley HC, Barsoum MW. Nanoporous carbide-derived carbon with tunable pore size. Nature Materials. 2003;2(9):591-594. [23] Hoffman EN, Yushin G, Barsoum MW, Gogotsi Y. Synthesis of Carbide- Derived Carbon by Chlorination of Ti2AlC Chem Mater. 2005;17(9):2317-2322. [24] Yushin GN, Hoffman EN, Nikitin A, Ye H, Barsoum M, Gogotsi Y. Synthe- sis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide. Chem Mater. 2005;43(10):2075–2082. [25] Nikitin A, Gogotsi Y. Nanostructured carbide-derived carbon. Encyclopedia of Nanoscience and Nanotechnology. 2004;7:553-574. [26] Garrido J, Linares-Solano A, Martin-Martinez JM, Molina-Sabio M, Rodri- guez-Reinoso F, Torregrosa R. Use of nitrogen vs. carbon dioxide in the characteri- zation of activated carbons. Langmuir. 1987;3(1):76-81. [27] Rodriguez-Reinoso F, Garrido J, Martin-Martinez JM, Molina-Sabio M, Torregrosa R. The combined use of different approaches in the characterization of microporous carbons. Carbon. 1989;27(1):23-32. [28] Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A. Usefulness of CO2 adsorption at 273 K for the characterization of porous carbons. Carbon. 2004;42(7):1233-1242. [29] Pittman CU, Jiang W, Yue ZR, Leon y Leon CA. Surface area and pore size distribution of microporous carbon fibers prepared by electrochemical oxidation. Carbon. 1999;37(1):85-96. [30] Ravikovitch PI, Vishnyakov A, Russo R, Neimark AV. Unified approach to pore size characterization of microporous carbonaceous materials from N-2, Ar, and CO2 adsorption isotherms. Langmuir. 2000;16(5):2311-2320. [31] Bello G, Garcia R, Arriagada R, Sepulveda-Escribano A, Rodriguez-Reinoso F. Carbon molecular sieves from Eucalyptus globulus charcoal. Microporous Mesoporous Mater. 2002;56(2):139-145. [32] Brouwer S, Groen JC, Peffer LAA. The impact of mesoporosity on micropo- rosity assesment by CO2 adsorption, revisited. In: Llewellyn PL, Rodriguez-Reinoso F, Roqerol J, Seaton N, eds. Characterization of porous solids VII. Amsterdam: Elsevier 2007:145 -152. [33] Duda JT, Jagiello L, Jagiello J, Milewska-Duda J. Complementary study of microporous adsorbents with DFT and LBET. Appl Surf Sci. 2007;253(13):5616- 5621. [34] Inomata K, Kanazawa K, Urabe Y, Hosono H, Araki T. Natural gas storage in activated carbon pellets without a binder. Carbon. 2002;40(1):87-93. [35] Yu C, Yu Y, Zhao D. Highly ordered large caged cubic mesoporous silica structures templated by triblock PEO-PBO-PEO copolymer. Chemical Communica- tions (Cambridge). 2000(7):575-576. [36] Rodriguez-Reinoso F, Martin-Martinez JM, Prado-Burguete C, McEnaney B. A standard adsorption isotherm for the characterization of activated carbons. J Phys Chem. 1987;91(3):515-516.

30 CHAPTER 3. GAS ADSORPTION [37] Sun J, Chen S, Rood MJ, Rostam-Abadi M. Correlating N2 and CH4 Adsorp- tion on Microporous Carbon Using a New Analytical Model. Energy Fuels. 1998;12(6):1071-1078. [38] Rios R, Silvertre-Albero J, Sepulveda-Escribano A, Molina-Sabio M, Rodri- guez-Reinoso F. Kinetic restrictions in the characterization of narrow microporosity in carbon materials. J Phys Chem C. 2007 Mar;111(10):3803-3805. [39] Rodriguez-Reinoso F, Almansa C, Molina-Sabio M. Contribution to the Evaluation of Density of Methane Adsorbed on Activated Carbon. J Phys Chem. 2005;109(43):20227-20231. [40] Menon VC, Komarneni S. Porous Adsorbents for Vehicular Natural Gas Storage: A Review. J Porous Mater. 1998;5(1):43-58. [41] Vasiliev LL, Kanonchik LE, Mishkinis DA, Rabetsky MI. Adsorbed natural gas storage and transportation vessels. International Journal of Thermal Sciences. 2000;39(9-11):1047-1055. [42] Duren T, Sarkisov L, Yaghi OM, Snurr RQ. Design of New Materials for Methane Storage. Langmuir. 2004 March 30, 2004;20(7):2683-2689. [43] Cracknell RF, Gordon P, Gubbins KE. Influence of pore geometry on the design of microporous materials for methane storage. J Phys Chem. 1993;97(2):494- 499. [44] Matranga KR, Myers AL, Glandt ED. Storage of natural gas by adsorption on activated carbon. Chem Eng Sci. 1992;47(7):1569-1579. [45] Seki K. Design of an adsorbent with an ideal pore structure for methane ad- sorption using metal complexes. Chem Commun. 2001(16):1496-1497. [46] Biloe S, Goetz V, Guillot A. Optimal design of an activated carbon for an adsorbed natural gas storage system. Carbon. 2002;40(8):1295-1308. [47] Lozano-Castello D, Alcaniz-Monge J, de la Casa-Lillo MA, Cazorla-Amoros D, Linares-Solano A. Advances in the study of methane storage in porous carbona- ceous materials. Fuel. 2002;81(14):1777-1803. [48] Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A, Quinn DF. Influ- ence of pore size distribution on methane storage at relatively low pressure: prepara- tion of activated carbon with optimum pore size. Carbon. 2002;40(7):989-1002. [49] Zheng J, Ekstrom TC, Gordeev SK, Jacob M. Carbon with an onion-like structure obtained by chlorinating titanium carbide. J Mater Chem. 2000;10(5):1039- 1041. [50] Kravchik AE, Kukushkina JA, Sokolov VV, Tereshchenko GF. Structure of nanoporous carbon produced from carbide. Carbon. 2006;44:3263-3268. [51] Avila-Brande D, Katcho NA, Urones-Garrote E, Gomez-Herrero A, Landa- Canovas AR, Otero-Diaz LC. Nano-structured carbon obtained by chlorination of NbC. Carbon. 2006;44:753-761. [52] Lozano-Castello D, Cazorla-Amoros D, Linares-Solano A. Powdered acti- vated carbons and activated carbon fibers for methane storage: a comparative study. Energy Fuels. 2002;16(5):1321-1328. [53] Sircar S, Golden TC, Rao MB. Activated carbon for gas separation and stor- age. Carbon. 1996;34(1):1-12. [54] Sosin KA, Quinn DF. Using the high pressure methane isotherm for determi- nation of pore size distribution of carbon adsorbents. J Porous Mater. 1995;V1(1):111-119. [55] Sweatman MB, Quirke N. Gas Adsorption in Active Carbons and the Slit- Pore Model 1: Pure Gas Adsorption. J Phys Chem B. 2005;109(20):10381-10388.

31 CHAPTER 3. GAS ADSORPTION

32 CHAPTER 4. ELECTRON MICROSCOPY CHAPTER 4. ELECTRON MICROSCOPY

Electron microscopy (EM) is an analysis technique which uses an electron beam as a probe to investigate various specimens and their proper- ties. The wavelength of electrons used is in order of pikometers which makes it possible to obtain a very high spatial resolution in EM. The electron is a charged particle and it can strongly interact with matter in various ways, raising different signals which may be detected and used to provide informa- tion about the specimen. There are a variety of signals that can be detected after electron inter- action with the specimen, see Figure 4.1.

Incident Backscattered electrons X-ray beam Auger electrons

Secondary electrons Cathodo-luminescence

Specimen

Secondary electrons

Elastically scattered electrons (Bragg scattering) Inelastically scattered electrons

Figure 4.1. Signals arising from electron interaction with specimen.

33 CHAPTER 4. ELECTRON MICROSCOPY

4.1 BASICS BEHIND ELECTRON INTERACTION WITH THE MATTER

4.1.1 ELASTIC SCATTERING Elastic scattering generally involves no change in the kinetic energy of the incident electrons, although electrons may change the trajectory. Elastic scattering is a principal mechanism by which electrons are affected in elec- tron microscopy and has a major contribution to diffraction patterns and images.

Figure 4.1.1.1. Wavevector diagram for elastic scattering.

It is common to represent the scattering of a primary electron by at- oms in the sample in terms of a vector diagram, see Fig. 4.1.1.1. The initial and final wavevectors for elastic scattering are equal and q depends solely on the scattering angle θ, which is related to the diffraction angle θd, θ = 2θd.

4.1.2 INELASTIC SCATTERING Inelastic scattering is incoherent and involves a loss of energy from the primary electron. It is the basis for electron energy loss spectroscopy. There are four general types of inelastic scattering mechanisms: phonon excitation, plasmon excitation, single electron excitation and direct radia- tion losses [1, 2]. Phonon excitation is a lattice vibration caused by incident electrons raising the local temperature. The primary electron loses a small amount of energy, less than 0.02 eV, and is not detected in transmission EELS. Plasmon excitation is a collective excitation of the valence electrons. The energy loss for primary electrons is between 5 and 30 eV; and the mean free path is about 100 nm. The rest of the inelastic processes can be explained by the excitation of single atomic electron to the higher energy level. Both the inner shell and

34 CHAPTER 4. ELECTRON MICROSCOPY the outer shell electrons can undergo single electron excitation. Inner shell electrons can make an upward transition only if it absorbs an amount of en- ergy greater than its binding energy. With each collision, fast electrons lose an equal amount of energy, which is required for inner shell electrons to be excited. In de-excitation, the energy is transformed to x-rays or Auger emis- sion. Outer shell electrons make an interband transition across an energy gap or, as in the case of metals, to a higher state within the same energy band. If the valence electrons escape from the specimen they can be used for secondary electron imaging. De-excitation is in the form of visible light or heat. Direct radiation losses occurs when primary electrons decelerate in the solid and energy is emitted directly as photons. The energy loss magni- tude in this case may be large, up to the energy of the primary electron. This radiation is known as bremstrahlung or braking radiation. The wavevector diagram for inelastic scattering is presented in Fig. 4.1.2.1. In the case of inelastic scattering, the wavevector kfo will change to kf, undergoing a change in direction as well as in magnitude. The characteris- E tic scattering angle is given by formula: θE = 2 ; where γ mv0 1 − ⎛⎞v2 2 γ =−⎜1 2 ⎟ – the relativistic factor for a particle of velocity v, c is veloc- ⎝⎠c ity of light; m0 is the electron rest mass; E is the mean energy loss [1, 3]. The E formula can be approximated to:θE ≈ [1, 3-6] for electrons with en- 2E0 ergy at or above 100 keV.

Figure 4.1.2.1 Wavevector diagram for inelastic scattering.

35 CHAPTER 4. ELECTRON MICROSCOPY

4.2 SCANNING ELECTRON MICROSCOPY – SEM SEM is designed to study the surface or near surface structure of the specimen. The image is obtained in a SEM by scanning electron beam across the specimen surface and detecting signals, which arise from the electron beam – specimen interactions. Secondary electrons are related to the topography due to their low en- ergy. Only those secondary electrons, which are close to the surface, will escape specimen and be detected. Any changes in topography of the speci- men surface that are larger than sampling depth will change the yield of sec- ondary electrons. Backscattered electrons are primary beam electrons that have under- gone one or more elastic collisions within the sample and re-emerged from the surface they came in. Signal varies strongly with the atomic number; the heavier the atom, the more electrons will be backscattered. Other detectable features are grain and crystal orientations. The backscattered electrons have a high energy and are from larger specimen volume than secondary elec- trons. The x-rays emitted from each atom have a characteristic energy, which is unique for the element it came from and therefore called character- istic x-rays. It enables the qualitative and quantitative (relative ratio) elemen- tal analysis using EDX (energy dispersive x-ray spectroscopy) system.

4.3 TRANSMISSION ELECTRON MICROSCOPY – TEM Transmission electron microscopy (TEM) is a powerful analysis tool to see structural ordering of a material. The electrons with high accelerating voltage pass through the sample and are scattered in different degree, loosing or prevailing initial energy. The elastically scattered electrons form an imag- ing contrast. The image resolution is determined by spherical and chromatic aberra- tions in the objective lens. The important parameter for TEM imaging is the sample thickness and the sample preparation should assure that the thickness of the sample is less than 100 nm. The two main modes in which TEM can be used are image mode and diffraction mode. Elastically scattered electrons are used in TEM imaging and electron diffraction, while inelastically scat- tered electrons are employed in electron energy loss spectroscopy (EELS), described in the next section. The secondary effects are also widely used in TEM, with EDX technique being the most popular.

4.4 ELECTRON ENERGY LOSS SPECTROSCOPY – EELS In EELS the primary processes of electron excitation are directly stud- ied. When passing through the sample, fast electrons losing a characteristic

36 CHAPTER 4. ELECTRON MICROSCOPY amount of energy are directed into the spectrometer, which separates them according to their kinetic energy and produces an electron energy loss spec- trum. So, in the EEL spectrum the scattered intensity is a function of de- crease in kinetic energy of the primary electron [3]. EELS can be applied in transmission and reflection mode. The first one can be termed as transmission EELS and is performed in a TEM, where a focused electron beam goes through a thin solid sample and the transmitted electron beam is spectroscopically analysed. The second one is an alternative technique also known as surface EELS and based on electron beam interac- tion with a surface of material with the backscattered electrons being spec- troscopically analysed [1].

4.4.1 BASIC COMPONENTS OF AN EEL SPECTRUM

4.4.1.1 ZERO-LOSS PEAK The zero-loss peak (ZLP) is located 0 eV of energy loss and it is the most intense feature in the EEL spectrum [1-3]. It contains signals from elas- tically and quasi-elastically scattered electrons. The electrons with small energy losses, which arise from excitation of phonons in the specimen, are present in ZLP, though in transmission EELS they are not resolved. The full width half-maximum (FWHM) of the ZLP determines the overall resolution of the recorded spectrum.

4.4.1.2 LOW-LOSS REGION The low-loss spectrum is part of the EEL spectrum involving signals from outer-shell electrons [1-3]. The low-loss region extends to the 50 eV and the three features are recognised: ZLP, bulk plasmons and interband transitions. The most prominent feature of the low-loss spectra, besides ZLP, is plasmon peak. Plasmons are characterised by the peak energy, which de- pend on the density of the valence electrons, and the peak width, which is defined by the decay rate of the resonant mode. One parameter that can be obtained from low-loss spectra is specimen I thickness t: t =Λln T ; where Λ is a mean free path for inelastically scat- IZL tered electrons, IT is the integral of total intensity in the spectrum, while IZL is the integrated intensity of the zero-loss peak. The modern software at- tached to the EEL spectrometer enables calculation of the sample thickness by several methods such as Log-ratio and Kramers-Kronig sum rule. The Log-ratio method can give relative or absolute thickness of the specimen, while Kramers-Kronig sum rule gives an absolute value of the sample thick- ness. With an increased sample thickness, the probability of single plasmon excitation and plural inelastic scattering increases. Effects of plural scatter-

37 CHAPTER 4. ELECTRON MICROSCOPY ing can be removed from the low-loss spectra by Fourier transform deconvo- lution, so called Fourier-Log method, see Figure 4.4.1.2.1.

Figure 4.4.1.2.1. An effect of plural scattering and ZLP removal.

The plasmon position is related to the density of the valence electrons, so that any changes due to structural rearrangement or alloying in metals can be detected by the plasmon energy shift. Insulators and semiconductors give broader plasmon peaks than metals. The relation of plasmon position to the density of valence electrons gives the opportunity to obtain the mass density of specimen [7-9]. For carbon materials, the mass density can be determined using formula [10]:

1 M C m *ε 0 2 ρ = 2 2 E p ; 4 N A = e where MC is the carbon molar mass; NA is the Avogadro number; e is the electron charge; ħ is Planck’s constant; ε0 is the vacuum dielectric function; and m* is the effective electron mass, defined as 0.87m, where m is the free electron mass. Alternatively, the simple method for mass density determination of dense carbon materials with high sp3 content, can be used [11]:

ρ =+1.92 1.37(sp3 fraction) .

This method is applicable when density is high and its variations oc- cur only due to hybridization of the bonds, e.g. high content of sp2 bonds gives density values close to those of graphite. Another feature of low-loss spectrum is interband transitions defined as single electron transition from valence band to the unoccupied states in

38 CHAPTER 4. ELECTRON MICROSCOPY the conduction band. They can appear as peaks superimposed on the main plasmon peak and may shift the plasmon peak to higher energies, since elec- trons causing interband transition can contribute to the resonant oscillations. If the interband transition is higher in energy than the plasmon peak, then it may shift this peak to lower energy. In the case of carbon materials, the oc- currence of π-π* interband transition and its intensity are related to the pres- ence and number of unsaturated bonds.

4.4.1.3 HIGH-LOSS REGION High-loss region of the spectra extends from 40 - 50 eV up to several thousand eV. As the energy loss is increasing, the edges appear on the mono- tonically decreasing background. These edges are due to excitation of inner- shell electrons and are termed as ionization or core edges; they are character- istic to each element [1-3]. The energy of the edge threshold is determined by the electron bind- ing energy in the atomic subshell, which is unique to each element. Classifi- cation of the edges is done according to the standard spectroscopic nomen- clature: K is ionization of 1s electrons, L1, L2 and L3 are ionization of 2s, 2p1/2 and 2p2/3 electron, respectively, M1 to the ionization of 3s. The excitations of the K-edges are sharp saw-tooth shaped steps dis- playing no other features. Other core-loss excitations occurring in interaction with the free atoms have a variety of edge shapes, though chemical bonding can modify the unoccupied electronic states near the Fermi level. This would change the density of states and influence the basic shape of the edge within 30-40 eV after the edge threshold. The modified edge structure is termed energy-loss near-edge structure (ELNES) and gives information about the local structure and bonding. After the ELNES there are found weaker, ex- tended oscillations, known as extended energy-loss fine structure (EXELFS), attributed to the short-range order in the material. Besides the qualitative identification of elements present in the speci- men, EELS technique gives the ability to quantify the amounts. The quanti- fication of core-loss region requires determination of the edge intensities entirely due to the ionization events. There are several procedures that need to be applied to the raw EELS spectra before proceeding with analysis. As the core-loss edges lie on the background composed of tails from the plasmon excitations and the core-loss edges occurring at lower energies, the first step is to fit and remove background from the core-loss edge of in- terest. The background is extrapolated and removed using provided software, by fitting it to the 15 – 20 eV broad region before the edge threshold, see Figure 4.4.3.2.1.

39 CHAPTER 4. ELECTRON MICROSCOPY

Figure 4.4.3.2.1. An effect of background removal.

The next step is a removal of multiple scattering from core-loss spec- trum by the Fourier-Ratio deconvolution provided in the software using cor- responding low-loss spectrum from the same specimen area, see Figure 4.4.3.2.2.

Figure 4.4.3.2.2. An effect of multiple scattering removal.

The exact form of ELNES in some core-loss edges is dependent on the local environment around the atom undergoing excitation and it allows qualitative determination of the relative atomic environment in the solid. The ELNES shapes are coordination fingerprints, giving information about the number of atoms and their geometry as well as the types of atoms within the first coordination shell. ELNES is used extensively for the determination of sp2/sp3 bond ratio within carbon materials [11-14].

40 CHAPTER 4. ELECTRON MICROSCOPY The C-K edge has two features: a π* peak, which corresponds to the 1s - π* transition, and a σ* peak, which corresponds to the 1s - σ* transition, see Figure 4.4.3.3.1.

Figure 4.4.3.3.1. Typical features of carbon spectra.

The sp2/sp3 bond ratio of carbon materials is quantified by comparing the characteristic features of carbon with unknown ratio to the standard car- bon material, such as graphite, which is considered to be 100% sp2 bonded. It involves recording spectra of graphite and carbon under investiga- tion, estimating the intensities of the π* peaks and the σ* peaks, and calcu- lating sp2/sp3 ratio by formula:

u Iπ * 2 u u sp Iπ * + Iσ * 2 3 = g ; sp + sp Iπ * g g Iπ * + Iσ * where the I:s represent the integrated intensities of the 1s→π* and 1s→σ* peaks respectively. The reference sample of graphite is denoted as g, while u refers to an unknown sample. In this formula it is assumed that I π * and Iσ * are entirely due to 1s→π* and 1s→σ* transitions and correspond to sp2 and g 3 Iπ * sp hybridised C atoms. The term g g is not constant, but varies with Iπ **+ Iσ the experimental conditions, therefore graphite spectra needs to be recorded at the same experimental conditions as the carbon under investigation [13]. One of the problems involved in this kind of quantification is the anisotropy

41 CHAPTER 4. ELECTRON MICROSCOPY of graphite. The most convenient and common solution to this problem is the use of the magic angle at the spectrum recording stage.

4.4.2 MAGIC ANGLE Anisotropy of specimen has a very significant effect on the ELNES structure, therefore use of the correct magic angle is important. The magic angle is the collection angle at which the ELNES is independent of the specimen’s orientation. The magic collection angle is usually expressed as a multiple of the characteristic scattering angle, and the magnitude of that mul- tiple has raised discussions since 1998 [4-6, 15-19].

The experimental value of βmagic = 2θE was found by Daniels et al. [15] and confirmed independently by Hebert et al. [16]. Despite that, the theoretical prediction in the same work by Daniels et al. was confronted by

Hebert et al. and proved the magic angle to be 4θE, when calculated using quantum mechanics. Later Jouffrey et al. [4] demonstrated that if relativistic correction is taken in to account, βmagic = 1.46θE at 200keV which was closer to the experimental. However, the discussion is continuing as more detailed calculation and experiments are performed [17, 20-22].

4.5 EXPERIMENTAL

4.5.1 SEM The SEM studies were done using a JEOL JSM7000 microscope op- erated at 5 and 1 kV in secondary electron detection mode. Samples were prepared by placing the powder on the carbon stickers attached to an alumin- ium holder.

4.5.2 TEM The TEM studies were done using a JEOL UHR3010 microscope op- erated at 300 kV. During sample preparation, CDCs powders were prepared by dispersing them in n-butanol using an ultrasonic bath. Several drops of the dispersion were then placed onto a copper grid coated with holey carbon film.

4.5.3 EELS Samples prepared for the TEM were used for the EELS studies. All measurements were made under the magic angle conditions. EELS studies were performed in two TEMs: FEI Tecnai F30 ST with FEG, equipped with

42 CHAPTER 4. ELECTRON MICROSCOPY a Gatan Imaging Filter; and a Philips CM20, with LaB6 filament, equipped with Digipeels Gatan 766. The microscopes were operated at 300 kV and

100 kV, respectively. The theoretical magic angle (βmagic) for 300 kV is 0.95 mrad, and for 100 kV - 2.85 mrad. The attainable collection angles closest to these values were used for both microscopes. For the 300 kV instrument, the experimental conditions were: con- denser aperture 70 µm, spectrometer entrance aperture 2 µm and collection angle 1.09 mrad, Gatan Digital Micrograph software was used for the re- cording and evaluation of data. For the 100 kV instrument, the experimental conditions were: condenser aperture 100 µm, spectrometer entrance aperture 1 µm, convergence angle 2.7 mrad and collection angle 3.12 mrad, Gatan ELP software was used for recording and Gatan Digital Micrograph software for evaluation of the data. Highly oriented pyrolitic graphite (HOPG) was used as a standard for quantification of sp2 content.

4.6 RESULTS AND DISCUSSION

4.6.1 SEM SEM studies are rarely used for the investigation of CDCs, as the resolution is too low to provide information about porosity or microstructure of carbon materials. However, it can visualise particle morphology and in comparison with precursor carbide micrographs can give information about chlorination effect. SEM investigation has been done on CDCs from VC, TiC, NbC, HfC, TaC and ZrC. The micrographs of precursor carbides, TaC and ZrC, and their derived carbons are presented in Figure 4.6.1.1. Carbides and CDCs look alike, displaying preservation of particle shape with slight shrinkage and some signs of etching. SEM micrographs of other CDCs are presented in Figure 4.6.1.2. VC based CDC exhibit disintegration of particle into layers of sheets and can be related to the layered structure seen in TEM, presented in Figure 4.6.2.4, section 4.6.2. The SEM micrograph of NbC based CDC displays a dendritic structure, which can be seen closer in the low magnification TEM images in the next section, Figure 4.6.2.1. In the micrographs, HfC and TiC based CDCs display faceted surfaces; see Figure 4.6.1.2. After chlorination, each of carbides results in morphologically differ- ent and original CDC particles. From studied carbide – CDCs pairs it seems that morphology is preserved during chlorination.

43 CHAPTER 4. ELECTRON MICROSCOPY

Figure 4.6.1.1. SEM micrographs of CDCs and their corresponding precur- sor carbides as indicated on the images.

Figure 4.6.1.2. SEM micrographs of CDCs from carbides as indicated on the images.

44 CHAPTER 4. ELECTRON MICROSCOPY 4.6.2 TEM The TEM images accompany many investigations of CDCs [23-32], [Papers I, II, III, IV]. Some of them focus on various features which can be found in CDCs [23, 26], such as nano-onions or nano-barrels; others are presented in a series and used in exhibiting temperature or precursor effects.

Figure 4.6.2.1. Low magnification TEM images of CDCs based on various carbides.

Figure 4.6.2.1 shows the low magnification micrographs of CDCs from various precursors. Some of them confirm the particle shape preserva- tion exhibited in SEM micrographs. The shape of the particle can be clearly seen in images of VC, TiC, NbC and TaC based CDCs. The CDCs from HfC are peculiar as they exhibit two types of particles at the same synthesis temperature. At 700oC the more amorphous structure (right micrograph in Figure 4.6.2.2, where grain boundaries may be seen) prevails; while at 1000oC the more graphitic, “rose” shape particles of differ-

45 CHAPTER 4. ELECTRON MICROSCOPY ent size are dominant (left micrograph in Figure 4.6.2.2). A closer look at structures of the different CDCs is presented in Figures 4.6.2.3 and 4.6.2.4.

Figure 4.6.2.2. Low magnification images of HfC based CDCs.

Figure 4.6.2.3. TEM micrographs of TiC based CDCs synthesised at differ- ent temperatures, as indicated on the images.

46 CHAPTER 4. ELECTRON MICROSCOPY In Figure 4.6.2.3, TiC based CDCs change from amorphous to or- dered, depending on their synthesis temperature, which varies from 700 to 1200oC with 100oC step. Figure 4.6.2.4 shows the structure change for the CDCs made from various carbides at two temperatures: 700oC and 1000oC. Both figures confirm the earlier stated fact that with an increase in synthesis temperature more structural features are reflected in images of CDCs. Even higher magnification images can be found in Papers I and II.

Figure 4.6.2.4. TEM micrographs of CDCs from various carbides as indi- cated on the images.

47 CHAPTER 4. ELECTRON MICROSCOPY 4.6.3 EELS The electronic structure of CDCs has not been reported in literature as extensively as other properties. Only two publications are devoted to this kind of investigation i.e. of the XANES qualitative study of TiC based CDCs [31] and quantitative EELS investigation presented in Paper II. Both of them are in agreement that CDCs are mostly sp2 bonded. In fact, Dash et al. [31] claim CDCs have no sp3 bonded carbon.

1.0

0.8 ) 3 p

+ s 0.6 2 sp ( / 2

sp 0.4

0.2

0.0 700 800 900 1000 1100 1200 Graphite VC TiC WC TaC NbC HfC ZrC o Synthesis temperature, C Precursor

Figure 4.6.3.1. sp2/(sp2+sp3) dependence on synthesis temperature (TiC based CDCs) and precursor (T = 1000oC).

The CDCs studied in Paper II were found to possess small amounts of sp3 bonded carbon (2-17%), which varied depending on carbon precursor (various metal carbides) and was found to be dependent to some extent on the volume per carbon atom in carbide structure, see Figure 4.6.3.1. How- ever, the temperature effect on sp2/(sp2+sp3) ratio is negligible, even if the spectra of temperature series, recorded from TiC based CDCs, exhibits sharper and more narrow peaks with an increase in temperature; see Figure 4.6.3.2. It is a sign of increased ordering seen in TEM images; see previous section, Figure 4.6.2.3. Thus, constant sp2 content and increased ordering to the conclusion that synthesis temperature does not influence the type of bonds but only their arrangement, while selection of precursor seems to affect both parameters.

48 CHAPTER 4. ELECTRON MICROSCOPY

3.0

2.5

Graphite 2.0 o u.

. 1200 C a

y, 1.5 o

t

i 1100 C ns e

t o

n 1000 C I 1.0 900oC 0.5 800oC 700oC 0.0 280 290 300 310 320 330 Energy loss, eV

Figure 4.6.3.2. EELS core-loss spectra of TiC derived carbons.

However, as Paper II was dedicated to the EELS studies, the informa- tion not only from core-loss but also from the low-loss data was analysed. Low-loss spectra allow finding apparent density, see Figure 4.6.3.3, which accounts for porosity, but neglects packing effects. A choice of precursor also appears to have stronger effect than a change in the synthesis tempera- ture.

2.2

2.1

2.0

1.9

3 1.8 m c /

, g 1.7 ρ 1.6

1.5

1.4

1.3

1.2

1.1 700 800 900 1000 1100 1200 VC TiC WC TaC NbC HfC ZrC o Synthesis temperature, C Precursor

Figure 4.6.3.3. Densities of CDCs obtained from plasmon positions vs. syn- thesis temperature (TiC based CDCs) and precursor (T = 1000oC).

49 CHAPTER 4. ELECTRON MICROSCOPY

4.7 SUMMARY OF ELECTRON MICROSCOPY RESULTS • SEM shows preserved morphology of the particles before and after chlorination: carbide and carbon particles look alike. • TEM shows some of the shape preservation found during SEM stud- ies. • TEM studies show an increased ordering with increasing tempera- ture, which could be interpreted as graphitisation (increase in the amount of sp2 bonds). However, the EELS studies contradict the ob- servations and suggest that the ordering is caused by rearrangement of existing sp2 bonds, not the creation of the new ones. • EELS studies demonstrate the apparent density of the CDCs to be in between pycnometric and bulk; the majority of studied CDCs have an apparent density of ~1.5 cm3/g. • EELS studies also suggest that bonding and density of the resulting carbon is more influenced by the precursor than by the synthesis temperature.

50 CHAPTER 4. ELECTRON MICROSCOPY REFERENCES

[1] Brydson R. Electron Energy Loss Spectroscopy. Oxford: BIOS Scientific Publishers Ltd 2001. [2] Shindo D, Oikawa T. Analytical Electron Microscopy for Materials Science. Tokyo: Sringer-Verlag 2002. [3] Egerton RF. Electron Energy-Loss Spectroscopy in the Electron Microscope. New York: Plenum Press 1989. [4] Jouffrey B, Schattschneider P, Hebert C. The magic angle: a solved mystery. Ultramicroscopy. 2004;102:61-66. [5] Sun YK, Yuan J. Determination of fast electron energy dependence of magic angles in electron energy loss spectroscopy for anisotropic systems. Los Alamos National Laboratory, Preprint Archive, Condensed Matter. 2004:1-16, arXiv:cond- mat/0412739. [6] Menon NK, Yuan J. Towards atomic resolution EELS of anisotropic materi- als. Ultramicroscopy. 1999;78:185-205. [7] Pharr GM, Callahan DL. Hardness, elastic modulus, and structure of very hard carbon films produced by cathodic-arc. Appl Phys Lett. 1996;68(6):779. [8] Fallon PJ, Veerasamy VS, Davis CA, Robertson J, Amaratunga GAJ, Milne WI, et al. Properties of filtered--beam-deposited diamondlike carbon as a func- tion of ion energy. Physical Review B: Condensed Matter and Materials Physics. 1993;48(7):4777-4782. [9] Ferrari AC, Kleinsorge B, Morrison NA, Hart A, Stolojan V, Robertson J. Stress reduction and bond stability during thermal annealing of tetrahedral amor- phous carbon. J Appl Phys. 1999;85(10):7191-7197. [10] Calliari L, Filippi M, Laidani N, Anderle M. The electronic structure of car- bon films deposited in rf argon-hydrogen plasma. J Electron Spectrosc Relat Phe- nom. 2006;150(1):40-46. [11] Ferrari AC, Libassi A, Tanner BK, Stolojan V, Yuan J, Brown LM, et al. Density, sp3 fraction, and cross-sectional structure of amorphous carbon films de- termined by x-ray reflectivity and electron energy-loss spectroscopy. Physical Re- view B: Condensed Matter and Materials Physics. 2000;62(16):11089-11103. [12] Jeanne-Rose V, Golabkan V, Mansot JL, Largitte L, Césaire T, Ouensanga A. An EELS-based study of the effects of pyrolysis on natural carbonaceous materi- als used for activated charcoal preparation. Journal of Microscopy. 2003;210(1):53- 59. [13] Stolojan V. Nanochemistry of Grain Boundaries in Iron [PhD thesis]. Cam- bridge: University of Cambridge; 2000. [14] Bruley J, Williams DB, Cuomo JJ, Pappas DP. Quantitative near-edge struc- ture analysis of diamond-like carbon in the electron microscope using a two-window method. Journal of Microscopy. 1995;180(1):22-32. [15] Daniels H, Brown A, Scott A, Nichells T, Rand B, Brydson R. Experimental and theoretical evidence for the magic angle in transmission electron energy loss spectroscopy. Ultramicroscopy. 2003;96(3-4):523-234. [16] Hebert C, Jouffrey B, Schattschneider P. Comment on "Experimental and theoretical evidence for the magic angle in transmission electron energy loss spec- troscopy" by H. Daniels, A. Brown, A. Scott, T. Nichells, B. Rand and R. Brydson. Ultramicroscopy. 2004;101(2-4):271-273. [17] Schattschneider P, Hebert C, Franco H, Jouffrey B. Anisotropic relativistic cross sections for inelastic electron scattering, and the magic angle. Physical Review B: Condensed Matter and Materials Physics. 2005;72(4):045142. 51 CHAPTER 4. ELECTRON MICROSCOPY [18] Sun YK, Yuan J. Magic angle electron energy loss spectroscopy (MAEELS) of core electron excitation in anisotropic systems. Los Alamos National Laboratory, Preprint Archive, Condensed Matter. 2004:1-16, arXiv:cond-mat/0405105. [19] Menon NK, Yuan J. Quantitative analysis of the effect of probe convergence on electron energy loss spectra of anisotropic materials. Ultramicroscopy. 1998;74:83-94. [20] Hebert C, Schattschneider P, Franco H, Jouffrey B. ELNES at magic angle conditions. Ultramicroscopy. 2006;106(11-12):1139-1143. [21] Sun Y, Yuan J. Electron energy loss spectroscopy of core-electron excitation in anisotropic systems: Magic angle, magic orientation, and dichroism. Physical Review B: Condensed Matter and Materials Physics. 2005;71(12):125109. [22] Jorissen K, Rehr JJ. Real space multiple scattering calculations of relativistic electron energy loss spectra. Los Alamos National Laboratory, Preprint Archive, Condensed Matter. 2007:1-14, arXiv:0709 0020v0701 [cond-mat mtrl-sci]. [23] Zheng J, Ekstrom TC, Gordeev SK, Jacob M. Carbon with an onion-like structure obtained by chlorinating titanium carbide. J Mater Chem. 2000;10(5):1039- 1041. [24] Leis J, Perkson A, Arulepp M, Kaarik M, Svensson G. Carbon nanostructures produced by chlorinating aluminium carbide. Carbon 2001;39(13):2043-2048. [25] Leis J, Perkson A, Arulepp M, Nigu P, Svensson G. Catalytic effects of met- als of the iron subgroup on the chlorination of titanium carbide to form nanostruc- tural carbon. Carbon 2002;40(9):1559-1564. [26] Johansson E, Hjörtvarsson B, Ekstrom T, Jacob M. Hydrogen in carbon nanostructures. J Alloys Compd. 2002;330-332:670-675. [27] Dash RK, Yushin G, Gogotsi Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from zirconium carbide. Microporous Mesoporous Mater. 2005;86:50-57. [28] Gogotsi Y, Nikitin A, Ye H, Zhou W, Fischer JE, Yi B, et al. Nanoporous carbide-derived carbon with tunable pore size. Nature Materials. 2003;2(9):591-594. [29] Hoffman EN, Yushin G, Barsoum MW, Gogotsi Y. Synthesis of Carbide- Derived Carbon by Chlorination of Ti2AlC Chem Mater. 2005;17(9):2317-2322. [30] Yushin GN, Hoffman EN, Nikitin A, Ye H, Barsoum M, Gogotsi Y. Synthe- sis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide. Chem Mater. 2005;43(10):2075–2082. [31] Dash R, Chmiola J, Yushin G, Gogotsi Y, Laudisio G, Singer J, et al. Tita- nium carbide derived nanoporous carbon for energy-related applications. Carbon. 2006;44(12):2489-2497. [32] Chmiola J, Yushin G, Dash RK, Gogotsi Y. Effect of pore size and surface area of carbide derived carbons on specific capacitance. J Power Sources. 2005;158:765-772.

52 CHAPTER 5. RAMAN SPECTROSCOPY CHAPTER 5. RAMAN SPECTROSCOPY

Raman spectroscopy is an inelastic scattering technique suitable for the analysis of many types of chemical compounds in liquid, solid, and even gaseous form. It has applications in chemistry, material science, biology and, due to its non-destructive character and fast spectra recording, in industry. Raman spectra have their origin in the electronic polarisation caused by ultraviolet, visible, and near infrared light. The Raman scattering is very weak, only 1 of 1010 incident photons will undergo Raman scattering. Al- though Raman scattering is much weaker than the Rayleigh scattering, it is still possible to observe the former by using a strong exciting source. Raman scattering can occur with a change in vibrational, rotational or electronic energy of a molecule. The rotational transitions are lower in energy and about three orders of magnitude slower than vibrational transitions, and therefore rotational Raman spectroscopy is usually carried out only on gases at low pressure to assure that the time between molecular collisions is longer than the time of rotational transition. Raman spectra of solids are presented as shifts by the magnitude of vibrational energy from the incident frequency of primary light.

Virtual state E'

hν hν hν hν 0 0 0 0

ν = 3 ν = 2 ν = 1 Ground state ν =0 E STOKES RAYLEIGH ANTI-STOKES 0 E = h ( ν - ν ) E = hν E = h ( ν + ν ) 0 1 0 0 1

Figure 5.1. Schematic representation of photon interaction with matter.

The main light and matter interaction effects are presented in Figure 5.1. The photons, which are elastically (Rayleigh) scattered, enter and leave the molecule with no change in energy, ∆E = 0, as E = E0 = hv0; where E is 53 CHAPTER 5. RAMAN SPECTROSCOPY the energy of scattered light, E0 is energy of the incident light, h is Planck’s constant, v0 is the frequency of incident light. Some of the incident light energy is absorbed, which increases the vi- brational energy. The exciting photon energy is decreased by the amount which is absorbed by the molecule bond, ∆E = ERaman shift = E0 – E1= h(v0-v1); where, besides the terms defined above, ERaman shift is the difference between incident and scattered light energies, E1 is the energy of Raman scattered light, v1 is frequency of scattered light. In this process, called Stokes scatter- ing, light is red shifted. The Stokes scattering is temperature independent. Absorbed energy can raise the ground state vibrational energy and when light is scattered, photons can exit with increased energy, ∆E = ERaman shift = E0 + E1= h(v0+v1). In this process, called anti-Stokes scattering, light is blue shifted. The anti-Stokes is dependent upon the thermal energy of material and hence temperature. Both Stokes and anti-Stokes lines are equally spaced with regards to the Rayleigh line. However, the anti-Stokes signal is much weaker than the Stokes, and normally only the more intense Stokes lines are measured.

5.1 SPECTROMETER In Raman spectrometer, see Figure 5.1.1., a sample is illuminated by monochromatic light and the resulting Raman scattered light is analysed.

Prism

PC Detector

Lens Lens Sample

Laser

Figure 5.1.1. Sketch of basic components of a Raman spectrometer with near back scattering geometry.

As the Raman effect is weak, the intensity of illuminating light should be high. Lasers are therefore ideal excitation sources in Raman spectroscopy

54 CHAPTER 5. RAMAN SPECTROSCOPY as they emit intense, highly collimated, monochromatic light. The choice of the laser or more exactly the choice of the wavelength may be very impor- tant for resulting information. Many different types of lasers used in Raman spectroscopy enable a wavelength range from near infrared to deep ultravio- let.

5.2. RAMAN SPECTROSCOPY OF CARBON MATERIALS Raman spectra have been shown to be sensitive to many forms of car- bon, even when the material is present in small quantities or as thin films. Main parameters for characterisation of carbon materials by Raman spectroscopy are presence and positions of so called D- (disordered) and G- (graphitic) peaks, their intensity ratio (ID/IG), and full width at half maximum (FWHM). The Raman spectrum depends on: a) clustering of the sp2 phase; b) bond disorder; c) presence of sp2 rings or chains; d) the sp2/sp3 ratio. In the Raman spectra, recorded using laser with wavelength in visible light range, the sp2 sites are dominating since the cross section of the π states is much greater than for the σ states [1, 2]. The latter become more active at wavelengths in the UV-regime [3]. The vibrations giving rise to G-peak are due to simple stretching modes of sp2 bonded carbons, which in crystalline graphite has symmetry -1 E2g, giving a peak at 1580 cm . The G-peak appears due to the in-plane bond-stretching motion of pairs of C sp2 atoms. This mode does not require the presence of sixfold rings, so, it occurs at all sp2 sites, not only those in rings. When the rings in the graphite structure break up and linear chains form, the position of the G-peak shifts towards higher energy. However, in some cases, e.g. if crystalline graphite is broken up into nanocrystalline graphite keeping the ring structure, the observed shift is due to the appear- ance of a D´ disorder induced band at 1620 cm-1 as no chains of sp2 bonded carbon are formed [4]. A broadening of the G-peak can be interpreted as an increase in bond angle disorder [5]. The G-peak width is a measure of qual- ity of graphene planes. -1 The D-band at ~1340 cm has the symmetry A1g and is intense due to the breathing mode of six-fold rings. In crystalline graphite it is absent in Raman spectra due to symmetry restrictions. However, it appears when the graphitic layers break up into the smaller units. Therefore the intensity of the D-band can be used as an indicator to determine the degree of order in disor- dered carbons [3, 6]. However, its intensity is directly dependent on the pres- ence of sixfold aromatic rings. Ring order other than six tends to decrease the peak height and increase their width. Thus, the information about the less distorted aromatic rings is in the intensity maximum and not in the width [7]. In several articles, Robertson and Ferrari [3, 7-9] have classified the variations of main parameters for carbon spectra characterisation (ID/IG ratio, positions of G- and D- peaks and their FWHM) discussed above in to three stages, depending on the sp2 ratio and the degree of disorder:

55 CHAPTER 5. RAMAN SPECTROSCOPY Stage 1: graphite → nanocrystalline graphite; Stage 2: nanocrystalline graphite → amorphous carbon; Stage 3: amorphous carbon → tetrahedrally sp3 bonded carbon.

Graphite nc-graphite a-C ta-C

2.0 G /I

D 1.5 I

1.0

0.5

-1 0.0 1600 Stage 1 Stage 2 Stage 3 cm

,

n 1580

1560 sitio

1540

eak po 1520 p -

G 1500 100% 100% 80% 0% 2 sp content

Figure 5.2.1. The three stage model.

Stage 1 is characterised by transition from perfect sp2 bonded graphite sheets down to 20 Å of size crystallites. The G-peak shifts from 1583 to- wards 1600 cm-1 due to the appearance of a D´-peak at 1620 cm-1, the inten- sity of the D-peak which is absent in graphite increases with the disorder. The intensity of the G-peak is more or less unaffected as the number of sp2 bonded carbon does not change, the FWHM of the peaks increases as the disorder increases. Stage 2 is described by transition from the topological disordering of a graphite layer and loss of aromatic bonding with a purely sp2 network to the introduction of up to 20 % of sp3 bonds. The G-peak position shifts down- ward from 1600 cm-1 to 1510 cm-1 due to the introduction of odd membered (five and seven) rings, that weaken the bonding [10]. The D-peak position increases as smaller aromatic clusters have higher energies [11]. The inten- sity of G-peak does not change as the number of sp2 bonds remains the same, while D-peak intensity decreases due to a decrease in the amount of six membered rings, which leads to the loss of aromaticity and order. If the dis- order leads to the introduction of sp3 bonds, it gives a downward shift in both the G- and D-peak positions and the G-peak intensity is decreased due to a smaller amount of the sp2 bonds. The FWHM increases as the disorder in- creases.

56 CHAPTER 5. RAMAN SPECTROSCOPY Stage 3 is described by transition from 20% of sp3 bonded to the purely sp3 bonded carbon. The G-peak shifts upwards from 1510 cm-1 to- wards 1630 cm-1 due to the formation of olefinic sp2 chains and the intensity of the already very weak D-peak continues to decrease towards zero for tet- rahedral carbon. The FWHM increases.

5.2.1 SPECTRA FITTING The fitting of the Raman spectra of disordered carbons is not always straightforward as the D- and G- peaks are normally distorted. Often 2 + 2 Gaussian profiles are used to fit the peaks since the two main D- and G- peaks at 1300 - 1360 cm and 1580 - 1600 cm-1 are often accompanied by two minor peaks at 1150 and 1450 cm-1 [12]. When the Gaussian fitting is used, the ID/IG ratio is taken as the ratio of integrated intensities of the main peaks. A second method, which is used in this work, fits an asymmetry shifted Breit-Wigner-Fano (BWF) line for the G-peak and a symmetric Lor- entzian for the D-peak [7]. The BWF-line shape is given by:

2 I0[1 + 2(ω −ω0 )/QΓ] I(w) = 2 ; (5.1) 1+ [2(ω −ω0 )/Γ]

where I0 is the peak intensity at ω0, ω0 is the peak position, Γ corresponds to the full width at half maximum (FWHM) and Q-1 is the BWF coupling coef- ficient. The Lorentzian line shape is obtained when Q-1 → 0. A positive re- spectively negative sign of Q gives the relative position of the asymmetry.

The maximum of a BWF peak shape is not at ω0 since the peak profile is asymmetric. Therefore, in this study we are using ωmax, which is shifted ac- cording to:

Γ ω = ω + . (5.2) max 0 2⋅ Q

As Q < 0, all G - peaks will be shifted towards lower ω although this effect is largest when FWHM is large, i.e. more disordered carbons. When the peak fitting is done using the combination of BWF and Lorentzian func- tions, the ID/IG ratio is taken as the ratio of peak heights.

5.2.2 IN-PLANE CORRELATION LENGTH

As mentioned before, intensity ratio of D- and G- peaks (ID/IG) is pro- portional to the in plane correlation length. ID/IG increases with the decay of perfect graphite sheets, as the disorder is growing and D-mode is becoming more active, as predicted by the T-K relationship [6]:

57 CHAPTER 5. RAMAN SPECTROSCOPY IC(λ) D = ; (5.3) IGaL where ID and IG are height or integrated intensity of the D- and G- peaks, respectively, C is parameter dependent on the laser wavelength, La is in plane correlation length. Parameter C is wavelength dependent and can be ascribed by the following equation:

CC≈ 0+ λC1; (5.4) where C0 = -12.6 nm and C1 = 0.033, parameters valid in laser wavelength region 400 nm < λ < 700 nm [13]. It must be taken into account that T-K model is developed for disor- dered graphite, i.e. 100 % sp2 bonded carbon, and therefore it is valid only for carbons belonging to stage 1, see above. Carbons containing a small 3 amount of sp belong to the stage 2, and the dependence between ID/IG and La is described by Ferrari-Robertson (F-R) relationship [7]:

ID 2 = C'(λ) ⋅ La . (5.5) IG Parameter C’ is also wavelength dependent and is derived from the combination of equations 5.3 and 5.5, when La = 2 nm, which is considered to be a transition region for validity of the aforementioned equations: 3 CC'/=La =C/8. (5.6)

5.2.3 WAVELENGTH DEPENDENT DISPERSION Raman modes for sp2 carbons can be dispersive, i.e. their positions vary with the photon excitation energy. The G-mode of graphite or glassy carbon does not disperse, but for carbons from stages 2 and 3 (see above) it does. Its dispersion increases with disorder and it is highest for ta-C (tetrahedral amorphous carbon). For sp2 bonded a-C (amorphous carbon), the G-peak position rises with photon en- ergy and saturates at 1600 cm-1. The G-peak dispersion is roughly linear with excitation wavelength. G-peak position disperses less with an increase in excitation energy [14]. The dispersion of D-peak position is strongest in well ordered car- bons, contrary to the G-mode, i.e. proportional to order [14]. The dispersion of ID/IG ratio is also more pronounced for ordered carbons, and hardly no- ticeable for very disordered. A shorter wavelength also reduces the intensity of the D-peak, causing dispersion of the ID/IG ratio [9]. The FWHM de- creases with increasing excitation energy [14].

5.3 EXPERIMENTAL The Raman spectra were collected with a Renishaw inVia Raman mi- croscope instrument equipped with an Argon ion laser (λ = 514 nm) and a 58 CHAPTER 5. RAMAN SPECTROSCOPY diode laser (λ = 785 nm). Before each session, the instrument was calibrated against a Si crystal. The measurements were performed at 10 % of the Ar- ion and 1 % of the diode laser effects to avoid damaging the sample. Typical measuring times were 60 s per scan. As mentioned above, G- and D- peaks in the recorded spectra were fit- ted using BWF in combination with a symmetrical Lorentzian. The peak heights were used for ID/IG ratio determination. The F-R method was applied for the calculation of in-plane correlation length La.

5.4 RESULTS AND DISCUSSION In recent years many articles concerning CDCs investigations include results from Raman spectroscopy. In most of the reported Raman studies made on CDCs, the integrated intensities of the peaks without referring to the integration method or a simple fitting of two Lorentzian or two Gaussian peaks were used [15-19]. The in-plane correlation length is usually calcu- lated using the T-K relationship [15-19], neglecting the fact that the T-K relationship is based on the assumption that carbon under study consists of perfect graphitic units, and consequently is 100% sp2 bonded.

λ = 514.5 nm λ = 785 nm L L 2.0 u. . 1200oC 1200oC 1.5 o 1100oC

ity, a 1100 C

o o ns 1.0 1000 C 1000 C o 900oC 900 C Inte 0.5 o o 800 C 800 C o 700oC 700 C 0.0 1000 1200 1400 1600 1000 1200 1400 1600 λ = 514.5 nm λ = 785 nm L L 2.5

ZrC ZrC a.u. 2.0 HfC HfC

ity, 1.5

NbC NbC 1.0 TaC TaC WC Intens WC 0.5 TiC TiC 0.0 VC VC 1000 1200 1400 1600 1000 1200 1400 1600 -1 -1 Raman shift, cm Raman shift, cm

Figure 5.4.1. Raman spectra of studied CDCs recorded by laser with differ- ent wavelength.

59 CHAPTER 5. RAMAN SPECTROSCOPY In Paper III a different approach is used. Given that in the EELS stud- ies [Paper II] described in Chapter IV, it was found that all CDCs possess a small amount of sp3, it is clear that the T-K relationship is not valid and the studied CDCs fall into the stage 2 of the amorphisation trajectory proposed by Ferrari and Robertson [7], described above. Raman spectra of studied CDCs are presented in Figure 5.4.1. All spectra exhibit two distinct bands, one at ~1580 – 1600 cm-1, the G-band, and another at ~1300 – 1350 cm-1, the D-band. Peaks of the spectra recorded using Ar-ion laser (λL = 514.5 nm) are more narrow than those from diode laser (λL = 785 nm). FWHM of both bands and ID/IG ratio are shifted to higher values with an increase in excitation wavelength. It is also obvious that CDCs appearing most disordered in TEM images (TiC based CDCs made at 700oC or NbC based CDC, see Figures 4.6.2.3 and 4.6.2.4) have the largest FWHM values, see Figures 5.4.2 and 5.4.3. The switch between wavelengths (from 514.5 nm to 785 nm) also affects the band positions. D- band position changes from ~1340 – 1350 cm-1 to ~1300 cm-1, while the G- band is less affected.

λ =514.5 nm λ =785 nm 2 L L G /I D

I 1

-1 2000

m D-peak c , 150 G-peak M H

100 FW 50

-1 0

m

c 1600 1590 on, i t

i 1580 s

po 1340

nd 1320 a

B 1300 700 800 900 1000 1100 1200 700 800 900 1000 1100 1200 Temperature, oC Temperature, oC

Figure 5.4.2. Variation of Raman spectra parameters with increasing synthe- sis temperature for TiC based CDCs.

TiC based CDCs made at different temperatures have a constant amount (close to 90%) of sp2 bonded carbon according to the EELS studies, see Chapter 4. Therefore, all changes in Raman spectra, i.e. FWHM, band positions, and ID/IG ratio, with increasing synthesis temperature are due to increased ordering, see Figure 5.4.2. The G-band position increases at a higher wavelength as it becomes more sensitive to the structural ordering [14] achieved by increasing synthesis temperature in the temperature series

60 CHAPTER 5. RAMAN SPECTROSCOPY of TiC based CDCs. It is also confirmed by a temperature dependent de- crease in FWHM at both used wavelengths. In the case of CDCs synthesised from different carbides at 1000oC, there is a slight change in sp2 amount, according EELS studies (see Chapter 4), ranging from ~80 to 98%, and therefore changes in the parameters de- rived from those spectra are due to changes in both hybridisation and order- ing, see Figure 5.4.3.

λ =514.5 nm λ =785 nm 2 L L G /I D

I 1

-1 2000 VC TiCWCTaCNbCHfCZrCVC TiCWCTaCNbCHfCZrC

m D-peak

, c 150 G-peak M H

100 FW 50

-1 0 VC T iC WC T aC NbC Hf C Zr C VC TiCWCTaCNbCHfCZrC 1600 , cm

n 1590 o ti

i 1580 s

po 1340 1320

Band 1300 VC TiC WC TaC NbC HfC ZrC VC TiC WC TaC NbC HfC ZrC Precursor Precursor

Figure 5.4.3. Variation of Raman spectra parameters with choice of precur- sor for synthesis of CDCs at 1000oC.

The calculated in-plane correlation length is presented in Figure 5.4.4 as a function of synthesis temperature and precursor. The discrepancies be- tween values determined by different lasers can be explained by the assign- ment of C value (equations 5.4, 5.5, 5.6), as in this study an assumption was made that equation 5.4 is applicable even for wavelength of 785 nm, while the actual upper application limit is indicated as 700 nm. With an increase in synthesis temperature, in-plane correlation length increases slightly for excitation wavelength of 514.5 nm, while for wave- length of 785 nm it is almost constant (difference of 0.2-0.3 nm), see Figure 5.4.4. In CDCs from various carbides, the difference between the lowest and highest value of an in-plane correlation length is ~0.4 nm. An increase in 0.2 – 0.4 nm is insignificant as it barely accounts for an increase of an in-plane correlation length by one ring. Thus, it suggests that while ordering increases with temperature, the in-plane correlation length stays constant. The slight change in sp2 amount (EELS studies, see Chapter 4) in CDCs from various carbides does not significantly affect the La values, though there is an obvi- ous correlation, especially with in-plane correlation length calculated from Raman spectra measured by laser with excitation wavelength of 785 nm.

61 CHAPTER 5. RAMAN SPECTROSCOPY

1.0 1.8 1.0 1.8

0.8 1.6 0.8 1.6 ) 3 0.6 1.4 0.6 1.4 +sp L 2 a 2 2 3 , n /(sp

sp /(sp +sp ) m 2

sp 0.4 L (λ = 514.5 nm) 1.2 0.4 1.2 a L L (λ = 785 nm) a L

0.2 1.0 0.2 1.0

0.0 0.8 0.0 0.8 700 800 900 1000 1100 1200 VC TiC WC TaC NbC HfC ZrC o Synthesis temperature, C Precursor

2 Figure 5.4.4. La (Raman) and sp amount (EELS) dependence on synthe- sis temperature of TiC based CDCs and precursor of CDCs made at 1000oC.

62 CHAPTER 5. RAMAN SPECTROSCOPY

5.5 SUMMARY OF RAMAN SPECTROSCOPY RESULTS • When the sp2 content is constant according to EELS studies, in- creased ordering can be detected by a decrease of FWHM values as well by G-peak shift to the higher wavenumbers using laser with lower excitation energy, i.e. λL= 785 nm. • ID/IG ratio showed to be less sensitive to the temperature effect, e.g. ordering of CDCs, though it better detects the influence of precursor, in this case, it is also reflected in sp2 amount determined by EELS. • In-plane correlation length for CDCs was found to be ~1 – 1.5 nm.

63 CHAPTER 5. RAMAN SPECTROSCOPY REFERENCES

[1] Wada N, Gaczi PJ, Solin SA. "Diamond-like" 3-fold coordinated amorphous carbon. Journal of Non-Crystalline Solids 1980;35-36(1):543-548. [2] Sails SR, Gardiner DJ, Bowden M, Savage J, Rodway D. Monitoring the quality of diamond films using Raman spectra excited at 514.5nm and 633nm. Dia- mond and Related Materials 1996;5(6-8):589-591. [3] Ferrari AC, Robertson J. Resonant Raman spectroscopy of disordered, amor- phous, and diamondlike carbon. Physical Review B: Condensed Matter and Materi- als Physics 2001;64(7):075414/075411-075414/075413. [4] Roy D, Chhowalla M, Wang H, Sano N, Alexandrou I, Clyne TW, Amaratunga GAJ. Characterisation of carbon nano-onions using Raman spectros- copy. Chem Phys Lett. 2003;373(1,2):52-56. [5] McCulloch DG, Prawer S, Hoffman A. Structural investigation of xenon-ion- beam-irradiated glassy carbon. Physical Review B: Condensed Matter and Materials Physics 1994;50(9):5905-5917. [6] Tuinstra F, Koenig JL. Raman spectrum of graphite. Journal of Chemical Physics 1970;53(3):1126-1130. [7] Ferrari AC, Robertson J. Interpretation of Raman spectra of disordered and amorphous carbon. Physical Review B: Condensed Matter and Materials Physics. 2000;61(20):14095-14107. [8] Ferrari AC. Determination of bonding in diamond-like carbon by Raman spectroscopy. Diamond Relat Mater. 2002;11:1053-1061. [9] Robertson J. Diamond-like amorphous carbon. Materials Science and Engi- neering: R: Reports 2002;37(4-6):129-281. [10] Beeman D, Silverman J, Lynds R, Anderson MR. Modelling studies of amorphous carbon. Physical Review B: Condensed Matter and Materials Physics 1984;30(2):870-875. [11] Mapelli C, Castiglioni C, Zerbi G, Mullen K. Common force field for graph- ite and polycyclic aromatic hydrocarbons. Physical Review B: Condensed Matter and Materials Physics 1999;60(18):12710-12725. [12] Schwan J, Ulrich S, Batori V, Ehrhardt H, Silva SRP. Raman spectroscopy on amorphous carbon films. Journal of Applied Physics 1996;80(1):440-447. [13] Matthews MJ, Pimenta MA, Dresselhaus G, Dresselhaus MS, Endo M. Ori- gin of dispersive effects of the Raman D band in carbon materials. Physical Review B. 1999;59(10):R6585-R6588. [14] Ferrari AC, Robertson J. Resonant Raman spectroscopy of disordered, amor- phous, and diamondlike carbon. Physical Review B: Condensed Matter and Materi- als Physics. 2001;64(7):075414/075411-075414/075413. [15] Portet C, Yushin G, Gogotsi Y. Electrochemical performance of carbon on- , nanodiamonds, carbon black and multiwalled nanotubes in electrical double layer capacitors. Carbon. 2007;45:2511-2518. [16] Yushin GN, Hoffman EN, Nikitin A, Ye H, Barsoum M, Gogotsi Y. Synthe- sis of nanoporous carbide-derived carbon by chlorination of titanium silicon carbide. Chem Mater. 2005;43(10):2075–2082. [17] Jänes A, Thomberg T, Lust E. Synthesis and characterisation of nanoporous carbide-derived carbon by chlorination of vanadium carbide. Carbon. 2007;45:2717- 2722. [18] Jänes A, Kurig H, Lust E. Characterisation of activated nanoporous carbon for supercapacitor electrode materials. Carbon. 2007;45:1226-1233.

64 CHAPTER 5. RAMAN SPECTROSCOPY [19] Dash RK, Yushin G, Gogotsi Y. Synthesis, structure and porosity analysis of microporous and mesoporous carbon derived from zirconium carbide. Microporous Mesoporous Mater. 2005;86:50-57.

65 CHAPTER 5. RAMAN SPECTROSCOPY

66 CHAPTER 6. SIDETRACKS CHAPTER 6. SIDETRACKS

6.1 SIDETRACK 1: EFFECT OF DENSITY AND NITROCARBIDES Sintered pellets of TiC and pellets of mixtures between TiC and with nominal composition TiC1-xNx (x = 0.2, 0.4, 0.6, 0.8) to be used as pre- cursors in the chlorination experiments were prepared using SPS equipment. Synthesis procedure and detailed characterisation of precursor pellets may be found in Papers VII and VIII. This investigation was carried out to find the influence of precursor density and nitrogen content in relation to the structure of CDCs. It also pro- vides information about the possibility of producing preshaped CDCs.

6.1.1 DENSE TIC PELLETS After successful chlorination of TiC powder and an established syn- thesis route resulting in a high yield of pure CDCs, an attempt was made to synthesise preshaped CDCs from dense TiC pellets. However, the chlorina- tion reaction of sintered TiC pellets was very sluggish. The synthesis time suitable for the TiC powders at 1000oC resulted in only ~ 20% of reacted pellet (TiC-1); see Table 6.1.1.1.

Table 6.1.1.1. Synthesis parameters and sample reactivity.

Sample m (TiC), g t, min / r, ml/min Reacted % of Yield, % sample TiC-1 0.3997 40 / 20 19 402 TiC-2 0.0712 40 / 20 60 200 TiC-3 0.4161 180 / 20 38 307 TiC-4 0.0803 180 / 20 82 93 TiC-5 0.4263 180 / 40 35 321 TiC-6 0.5654 300 / 20 41 296

As can be seen from table 6.1.1.1., different parameters were changed when trying to produce CDCs without the residual TiC: a) reduced amount of the sample, b) increased reaction time and c) increased gas flow rate. An increase in reaction time by 5 times (3 h) resulted in double the amount of 67 CHAPTER 6. SIDETRACKS reacted sample, (TiC-3); 5 h chlorination gave 41% of reacted sample (TiC- 6); see Table 6.1.1.1. Decrease of the sample amount by ~5 times resulted in an increase in the reacted amount of the sample to 60 % (TiC-2). Increase in Cl2 gas-flow rate did not improve the yield, i.e. the amount of reacted pellet was the same (TiC-5). It became obvious that the reaction is strongly diffusion controlled. After chlorination, all samples were covered in a crust of pure carbon while the inner parts consisted of unreacted TiC. The resulting carbon crust had the same shape as the starting pellet and could easily be peeled off as solid pieces.

Figure 6.1.1.1. SEM image of chlorinated TiC pellet (TiC-3): TiC and CDC parts are indicated on the image.

The EDX elemental analysis of sample TiC-3 showed that the inner part of the pellet contained Ti, while only a weak signal from chlorine was observed from the outer carbon crust (EDS detector limit Z ≥ Ne). It was an indication that the inner part of the pellet contained TiC while the crust con- sisted of carbon and some trapped chlorine. The TEM images of the same CDC sample (TiC-3) did not have any structural features, see Figure 6.1.2.1. It appears to be more similar to TiC powder based CDCs made at 800 or even 700oC than at 1000oC; for com- parison see TEM images in Figure 4.6.2.3, Chapter 4. This indicates that the structural rearrangement is inhibited by the compact nature of starting car- bide.

68 CHAPTER 6. SIDETRACKS

Figure 6.1.1.2. TEM image of CDC from dense TiC pellet.

As the chlorination of compact TiC pellets seemed to be diffusion re- stricted, one way to overcome this is by decreasing the length of the gas diffusion path. Dense TiC pellet was cut into thin (~1 mm) slices (See Table 6.1.1.1.; Samples 2 and 4). However, after 3h of chlorination, even if the yield was 93.2 %, it re- sulted in only a thin layer of carbon on the unreacted TiC, see Figure 6.1.1.3.

Figure 6.1.1.3 SEM images of sample TiC-4: to the left – part of TiC pellet after reaction, to the right – region 2 in higher magnification.

In the regions 2, 3, 4 marked on the image to the left, see Figure 6.1.1.3, unreacted TiC was found while region 1 consisted of carbon. A magnification of the TiC in region 2 is shown in the image to the right, see Figure 6.1.1.3. The reason why chlorination was not complete is still not clear and requires more experimental work.

69 CHAPTER 6. SIDETRACKS

6.1.2 DENSE TIC1-XNX PELLETS Another way to overcome diffusion restrictions is to introduce, for ex- ample, nitrogen into the precursor structure. It would leave as N2(g) during chlorination and would allow better access to the volume of a compact pel- let. The synthesis of carbon using TiC1-xNx pellets as precursors was per- o formed at T = 1000 C, Cl2 gas-flow – 20 ml/min, time dependent on amount and composition of sample (See Table 6.1.2.1).

Table 6.1.2.1. Synthesis conditions of carbon from TiC1-xNx pellets

Sample (TiC1-xNx) m (TiC1-xNx), g t, min w, % TiCN-1 (x=0.6) 0.3420(2) 30 13,0(1) TiCN-2 (x=0.4) 0.4665(2) 25 203,2(1) TiCN-3 (x=0.4) 0.2672(2) 35 19,9(1) TiCN-4 (x=0.4) 0.4659(2) 60 59,1(1) TiCN-5 (x=0.2) 0.5179(2) 30 292,0(1) TiCN-6 (x=0.2) 0.3877(2) 90 107,6(1)

The pellets with composition x = 0.2 reacted as sluggishly as the TiC pellets. The pellets in the nitrogen rich part of the series (x = 0.8), even after a very short reaction time, resulted in an empty vessel. The simplest chemi- cal description of the chlorination reaction would be:

TiC1-xNx(s) + 2 Cl2(g) → TiCl4(g) + (½) xN2(g) + (1-x) C(s) The increased chlorination rate of nitrogen rich pellets could be ex- plained by formation of pores (increase in diffusion rate) with nitrogen re- lease. Only the two syntheses with pellets of composition x = 0.4 resulted in pure CDCs (See Table 6.1.2.1., Samples TiCN-3 and TiCN-4). However, the carbon yield in these experiments was low. The obtained CDCs kept the same shape as the precursor pellets and grains, see Figure 6.1.2.1, but were quite brittle.

Figure 6.1.2.1. SEM images of CDCs from samples with nominal composi- tion TiC0.6N0.4 at different magnifications.

70 CHAPTER 6. SIDETRACKS The TEM studies showed that carbon from titanium nitrocarbide pellet has a much more open structure than carbon from TiC pellet. This is clearly seen by comparing the TEM images of carbon from TiC0.4N0.6 and TiC0.6N0.4, shown in Figure 6.1.2.2, with an image of TiC based dense CDCs presented in Figure 6.1.1.2.

Figure 6.1.2.2. Images to the left are TiC0.4N0.6 based CDC and to the right from TiC0.6N0.4 based CDC, at different magnifications.

The TEM images of TiC0.4N0.6 exhibit a layered structure, where graphitic layers are intercalated by a more disordered structure. The lower nitrogen content precursor, TiC0.6N0.4, resulted in a less ordered material, which seems to consist of disordered carbon with hollow spherical features. The TEM images of these two CDCs also confirm the hypothesis mentioned above, that for carbon atoms to rearrange themselves into the ordered struc- ture a certain volume is needed. The higher the nitrogen content, the larger 71 CHAPTER 6. SIDETRACKS volume per carbon atom is available during the chlorination for rearrange- ment. Therefore, a CDC with higher nitrogen content is more ordered than with a lower one.

6.1.3 CONCLUSIONS Chlorination of dense TiC pellets was very sluggish due to diffusion restrictions. Reaction rate in dense pellets with low nitrogen content chlori- nation was also low. Dense pellets with high nitrogen content during chlorination most probably disintegrated to volatile species as no material was found even after short reaction times. However, chlorination of precursors with intermediate nitrogen content resulted in pure CDCs, even if with low yield. Comparison of TEM images a) between CDCs from TiC powder and TiC pellet synthesised at the same temperature, and b) between nitrocarbide pellets with intermediate nitrogen content, leads to the conclusion that there is some critical volume per carbon atom, which is needed for structural rear- rangement and ordering.

72 CHAPTER 6. SIDETRACKS

6.2 SIDETRACK 2. EFFECT OF CATALYSTS The influence of catalyst metals, such as Fe, Ni and Co, on the struc- ture of CDCs has been reported twice: for TiC based CDCs [1] and for Al4C3 based CDCs in Paper IV. The study of catalyst assisted synthesis of TiC based CDCs revealed that at lower synthesis temperatures (up to 800oC), the presence of catalysts has an effect on increasing surface area and pore vol- ume [1]. At higher temperatures, catalysts promote graphitisation, resulting in low surface areas and small pore volumes. In a previous study of Al4C3 based CDCs it was found that large amounts of nano-barrel like porous carbon could be produced using this precursor [2]. However, it was unclear whether the formation of this type of carbon was due to the inherent properties of Al4C3 or impurities in the start- ing carbide acting as catalysts. The main aim of Paper IV was to investigate the synthesis conditions suitable for formation of nano-barrel like carbon. The difference between precursors used in this study is presented in Table 6.2.1. Several hypotheses for the formation of nano-barrel like carbon were tested: a) The formation of nanoparticles was due to the nature of aluminium carbide, i.e. favourable configuration of carbon atoms. Aluminium carbides from different batches were tested and only some of them resulted in nano-barrel like carbon. b) Catalytic effect of aluminium promotes nano-barrel formation, i.e. the activation barrier on the reaction path of carbon production is surpassed via carbon – aluminium chloride active complexes. This was tested by chlo- rinating Al4C3 mixtures with carbides, which do not form nano-barrel struc- tures. Synthesis of CDCs from Al4C3, Mo2C, TiC, B4C (99 % purity) was carried out for the investigation of starting carbide influence. Different mix- tures of Al4C3 and TiC or B4C were chlorinated to study how the presence of aluminium influences the resulting carbon. This resulted in local carbon rearrangement and showed that the pres- ence of Al4C3 does not promote the formation of nano-barrel like carbon. c) Catalytic impurities left in carbide after their production may be responsible for a higher amount of uniform nano-barrels. This was tested by chlorinating Al4C3 with different purity grades. The last step was the investi- gation into how different amounts of added transition metal catalysts (CoCl2, NiCl2 and FeCl3) to the high purity Al4C3 (99.9 %) influences carbon struc- ture. The effect in both cases was obvious. Chlorination of aluminium car- bide with 99% purity without added catalysts as well as aluminium carbide of 99.9% purity with added catalysts (sample 16, see Table 6.2.1) resulted in nano-barrel like carbon. Besides this, synthesis temperature series of Al4C3 based CDCs with and without catalysts were performed and gave similar results to the study catalyst assisted chlorination of TiC based CDCs. However, in the case of

73 CHAPTER 6. SIDETRACKS

Al4C3 based CDCs, the surface areas and pore volumes started to decrease even at such low synthesis temperatures as 300oC, while for TiC based CDCs, this effect was achieved only at synthesis temperatures above 800oC [1].

Table 6.2.1. Precursor characteristics for CDCs synthesised at 700oC.

CDCs Precursor Catalysts added, mg/g Purity of carbides, sample carbide CoCl2/NiCl2/FeCl3 % 1 Mo2C - / - / - 99 2 TiC - / - / - 99 3 B4C - / - / - 99 4 Al4C3 - / - / - 99 5 25B4C/75Al4C3 - / - / - 99 / 99 6 50B4C/50Al4C3 - / - / - 99 / 99 7 75B4C/25Al4C3 - / - / - 99 / 99 8 25TiC/75Al4C3 - / - / - 99 / 99 9 50TiC/750Al4C3 - / - / - 99 / 99 10 75TiC/25 Al4C3 - / - / - 99 / 99 11 Al4C3 - / - / - 99.9 12 Al4C3 - / - / - 99.8 13 75TiC/25 Al4C3 30/ - / - 99 / 99 14 75TiC/25 Al4C3 30/30/30 99 / 99 15 Al4C3 0.13/0.13/ - 99.9 16 Al4C3 1.6/1.6/ - 99.9 17 Al4C3 - /1.6/1.2 99.9 18 Al4C3 3.3/3.3/ - 99.9 19 Al4C3 30/30/22.8 99.9

6.2.1 CONCLUSIONS The structure of CDCs is strongly influenced by the presence of cata- lytic transition metals (Co, Ni, Fe) even at low concentrations. Even trace levels, such as impurities in commercially available carbides left during their production, of catalysts can result in carbon nanostructures such as nano- barrel like carbon. High concentrations of catalysts promote the ordering of resulting CDCs at much lower temperatures if compared with synthesis from pure carbides. Catalysts proved to be an additional means to modify CDCs structure and properties.

74 CHAPTER 6. SIDETRACKS

6.3 SIDETRACK 3. METALLOCENE DERIVED CARBON Carbon derived from organometallic precursors, such as metallocenes, have very interesting structures, and in contrary to the CDCs, their structure is influenced more by the formed metal chloride than the atomic structure of precursor metallocene [3]. Synthesis of carbon from metallocenes, such as ferrocene and cobaltocene, was reported previously and resulted in nano- tubes, nanobags, nanolobes, and nanobranches [3-5]. Two samples, investigated in Paper V, were prepared by chlorination o of ferrocene, –Fe-(C5H5)2–, at 900 C for 30 min and 3 h. The reaction can be described as:

Fe-(C5H5)2 (s) + 13/2 Cl2 (g) → FeCl3 (g) + 10 HCl (g) + 10 C (s) Resulting carbon hollow carbon nanospheres were characterised using XRD, TEM, EELS, EFTEM, N2 adsorption and Raman spectroscopy. By combined interpretation of data, it was found that formed particles, after 30 min of chlorination, are hollow with walls, which are 12-25 nm thick. The walls consist of a disordered array of independent curved graphene nan- oflakes ~4 nm in diameter. The walls of resulting particles after 3 h of chlorination consist of nanocrystalline graphite. Despite increased ordering, N2 adsorption studies do not exhibit significant differences. EELS studies revealed similar sp2/sp3 ratios in both samples.

75 CHAPTER 6. SIDETRACKS

6.4. SUMMARY OF SIDETRACKS • High-density precursor carbide hinders the rearrangement of carbon atoms and leads to more disordered carbon structures. Chlorination reaction with highly dense precursors is diffusion controlled. CDCs made from dense TiC at 1000oC have a similar structure to TiC powder-based CDCs synthesised at lower temperatures. • Synthesis of CDCs, based on nitrocarbides with different nitrogen content, confirmed that larger C–C distance in precursor promotes the rearrangement of carbon atoms into a more ordered structure. The higher the nitrogen content, the more volume per carbon atom is obtained during the chlorination and, consequently, CDCs from pre- cursors with higher nitrogen content are more ordered. • Catalysts proved to be an additional means to modify CDCs struc- ture and properties. A higher amount of catalysts promotes ordering, yet may reduce pore volume and surface area of CDCs at higher temperatures. • Organometallic precursors can also be used to obtain porous car- bons. Catalytic metal containing precursor has a double impact; it acts as a carbon source and as a catalyst. In this study, the longer the synthesis time, the more ordered carbon is obtained.

76 CHAPTER 6. SIDETRACKS REFERENCES

[1] Leis J, Perkson A, Arulepp M, Nigu P, Svensson G. Catalytic effects of met- als of the iron subgroup on the chlorination of titanium carbide to form nanostruc- tural carbon. Carbon. 2002;40(9):1559-1564. [2] Leis J, Perkson A, Arulepp M, Kaarik M, Svensson G. Carbon nanostructures produced by chlorinating aluminium carbide. Carbon. 2001;39(13):2043-2048. [3] Avila-Brande D, Urones-Garrote E, Katcho NA, Lomba E, Gomez-Herrero A, Landa-Canovas AR, Carlos Otero-Diaz L. Electron microscopy characterization of nanostructured carbon obtained from chlorination of metallocenes and metal carbides. Micron. 2007;38(4):335-345. [4] Urones-Garrote E, Avila-Brande D, Ayape-Katcho N, Gomez-Herrero A, Landa-Canovas AR, Otero-Diaz LC. Amorphous carbon nanostructures from chlori- nation of ferrocene. Carbon. 2005;43:978-985. [5] Urones-Garrote E, Avila-Brande D, Katcho NA, Gomez-Herrero A, Landa- Canovas AR, Lomba E, Otero-Diaz LC. Spherical carbon nanoparticles produced by direct chlorination of cobaltocene. Carbon. 2007;45(8):1699-1701.

77 CHAPTER 6. SIDETRACKS

78 CHAPTER 7. CONCLUDING REMARKS CHAPTER 7. CONCLUDING REMARKS

7.1. CONTROL OF CDCS PROPERTIES Synthesis of carbide derived carbons from VC, TiC, WC, TaC, NbC, HfC, and ZrC were made using a chlorination reaction. A detailed study of their adsorption and structural properties was made using gas adsorption, electron microscopy, and Raman spectroscopy. Parameters influencing the properties of the resulting CDCs were found to be: • Synthesis temperature; • Precursor carbide; • Catalysts and impurities; • Density and volume of precursor (i.e. powders, pellets) • Incorporation of nitrogen in carbide structure (nitrocarbides). A change in any of the above-mentioned parameters has an impact on the sorption properties and the ordering of CDCs. Combining and using sets of these parameters enables the modification of CDCs for individual needs, therefore knowledge of what is the effect of each of them is necessary. Increasing synthesis temperature promotes formation of mesopores by reducing micropores volume and results in to the splitting of PSD into two segments. Further temperature increase leads to the formation of larger mesopores, keeping the average size of micropores intact. However, narrow micropores diminish at higher synthesis temperatures. TEM, EELS and Ra- man spectroscopy clearly display increased ordering, which surprisingly leaves sp2/sp3 ratio and apparent density unaffected. The effect of precursor will firstly influence size and morphology of the particles as confirmed by SEM studies. A change in sp2/sp3 ratio and apparent density is also more influenced by precursor carbide than tempera- ture. At fixed temperature by varying the precursor, it is possible to tune adsorption properties and PSD. For example PSD of CDCs from ZrC made at higher temperatures, at which all other carbides have a fraction of mesopores, is still in the microporous region. The same is valid for structural ordering, e.g. CDCs based on NbC do not exhibit big changes in ordering with synthesis temperature as can be seen from TEM studies, which is also confirmed by broad bands in Raman spectra. Catalyst influence is complex, depending on the synthesis temperature and the starting carbide. At lower temperatures, for some of carbides, the presence of a catalyst can enhance pore volume and surface areas, and at 79 CHAPTER 7. CONCLUDING REMARKS higher temperatures can reduce the same parameters, while for other car- bides the collapse of the porous network is expressed already at low synthe- sis temperatures. However, a larger amount of catalysts leads to the in- creased ordering and collapse of porous structures. Certain amounts of cata- lyst or impurities can result in the formation of specific nanoparticles in combination with the right precursor and temperature. The influence of bulk dense pellets hinders structural rearrangements and enables the production of more disordered CDCs at higher synthesis temperatures. The chlorination of nitrocarbides with different nominal com- positions demonstrated that larger volume per carbon atom in precursor has an influence on the CDCs ordering. The larger the volume, e.g. higher nitro- gen content in nitrocarbide, the more ordered structure is obtained.

7.2. FUTURE ASPECTS Further investigation of CDCs needs to proceed towards potential ap- plications since many control parameters are established. The adaptation of CDCs properties for hydrogen and methane storage is already started, as well as testing CDCs for use in supercapacitors. In the current project CDCs were tested as potential methane storage materials since their PSD is very close to the ideal as reported in theoretical and experimental studies. However, even if gravimetric methane capacity was as high as expected, the volumetric capacity, which is important for industrial applications, was too low due to low packing density of CDCs. This problem may be solved in several ways: • Simply press CDCs powder into pellets; • Use binders for the production of CDC granules; • Establish the production of bulk particles. The highest density should be achieved using a third of the above mentioned ways since then density can be expected to be as high as the ap- parent density of particles determined in the EELS studies. The expected volumetric methane capacity is above 200 v/v for all studied CDCs, which is much higher than the commercial goal of 150 v/v. However, this requires solving the diffusion problems encountered in CDCs synthesis from bulk carbide pellets. The hypothetical cure for this problem would be to use less dense carbide pellets, e.g. containing some porosity in the grains or their boundaries. Another aspect, which would be interesting to establish, is the size/volume range of particles vs. their density leading to successful CDCs production within a reasonable time span. It would lead to the prediction of a preshaped device size range and their potential functionality as it might be size controlled.

80

ACKNOWLEDGEMENTS

The greatest gratitude is to my supervisors, Professor Gunnar Svensson and Associate professor Mats Johnsson. Gunnar; your help, support, never ending patience and open mind for dis- cussions kept me looking forward, learning new techniques and not stopping when results seemed to be hopeless (we had a few of those! ☺). Mats; your guidance in the beginning of my scientific life made me un- derstand that everything that seems to be complicated can be made simple just by looking at it from another angle.

The person, who actually made me to come to Stockholm and to change my scientific field from analytical to structural inorganic chemistry, was Professor Aivaras Kareiva. I am very thankful for trusting me and giving this opportunity.

Special thanks to Professor Osamu Terasaki for being always there to answer any of my questions, and even more for his friendliness and interest- ing conversations.

A large part of my work has been done in collaboration, and I want to thank everyone who helped me to learn how to use new equipment, to un- derstand new methods, to formulate and solve scientific problems encoun- tered in my project, and discussions: Yolande Kihn, Claude Mirguet, Staf- fan Wachtmeister, Ernesto Coronel, Jaan Leis, Juan Manuel Juarez, Esteban Urones, Lars Hälldahl, Kjell Jansson, Mats Nygren, Magnus Sandström, Janos Mink and Jekabs Grins. Very special thanks for help and guidance in the field of EELS, and the possibility to meet world best experts during the IHP network meetings, is to Professor Stefan Csillag. I want to thank Professor Christian Colliex and Professor Gehan Amaratunga, for valuable discussions and an introduction to the “magic angle” ☺. I am very grateful for help in understanding the specifics of carbon gas adsorption from Professor Francisco Rodriguez-Reinoso.

I am very happy to have been given an opportunity to teach in several courses during my studies and I would like to thank everyone I have been working with during these courses: Arnold Maliniak, Dag Noreus, Marga- reta Sundberg, Aatto Laaksonen, Niklas Hedin, Lars Eriksson, Emiliana Risberg, Karin Söderberg, Andreas Flemström and Richard Becker. And of course to all my students! 81

I am very grateful to Caroline Wood for overcoming her boredom while revising English of my thesis ☺.

A huge thanks to our department’s technical and administrative staff, you make everyone’s life at the department less complicated: Ann-Britt Rönell, Jaroslava Östberg, Eva Pettersson, Agnes Laurin, Karin Häggbom Sandberg, Hans-Erik Ekström, Per Jansson, Lars Göthe, Rolf Eriksson, Per-Erik Persson, and our finest librarian Hillevi Isaksson!

My officemates deserve gratitude for being patient when I was interrupt- ing their work with questions. Thank you very much for all discussions we had: Keiichi Miyasaka and Juanfang Ruan. Thank you, all my friends at the department, for making the dark days lighter with jokes and for all the fun, I had at different parties with you! Ya- suhiro, Alfonso, Zuzana, Kristina, Ali, Petr, Sam, Norihiro, Baltzar, Vasco, Mikaela, Shahriar, Ehsan, Jovice, Zoltan, Jeppe, Alvaro, and Javier. Thank you, my friends, who are now far away. I still remember and miss you: Nathalie and Anders, Francesca and Nando, Abbas and Zubda, Guil- laume, Rachel, Sergei, Yulya with family, Oleg, Dmitriy, Nadja, Anita, Nick with family, Aleksandr, Misha and Andrey. And my friends, who are still in Stockholm, I am very happy to see you any time: Pasha and Liudmila, Misha, Jan ☺. Thanks to my smoking-room company: Martin, Peter, Joakim, Lina, and everyone I do not know names for nice conversations in different fields, even scientific! ☺

My dearest friends in Lithuania, thank you very much for keeping in touch with me even if I am not the best e-mail writer ☺: Egle, Ilona, Monika, Beata and Renata.

I would like to thank every lecturer at Faculty of Chemistry at Vilnius University for showing me that science can be fun, and especially Prof. Vida Vickackaite, Prof. Audrius Padarauskas, Prof. Rolandas Kazlauskas, Doc. Juozas Skadauskas, Prof. Stasys Tautkus, Prof. Jurgis Barkauskas, Doc. Rimantas Raudonis, Prof. Gintaras Baltrunas, Prof. Eugenijus Butkus and Doc. Petras Kadziauskas.

My dearest friends, Lena and Katia, thank you very much for being there for me at any time! I know I can always count on you! ☺

I want to thank my family in Stockholm - Sevald, Martin and Patrick for endless love! My extended family in Lithuania for not forgetting me and still inviting me to all the bigger family parties! Especially I want to thank my mum and dad, for their love and support! As well for sending Lithuanian delicacies to me at any opportunity they got! Ačiū visiems!

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