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Syntheses and studies of some graphite and graphite flouride intercalation compounds Xia, Jun 1991

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SYNTHESES AND STUDIES OF SOME GRAPHITE AND GRAPHITE FLUORIDE INTERCALATION COMPOUNDS BY JUN XIA B.Sc, Wuhan University, Hubei, China, 1982 A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THE REQUIREMENT FOR THE DEGREE OF MASTER OF SCIENCE IN THE FACULTY OF GRADUATE STUDIES (Department of Chemistry) We accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA © J. XIA, August 1990 In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written remission. Department of chemistry The University of British Columbia Vancouver, B.C. Canada Date October 11,1991 ABSTRACT The oxidative intercalation of S2O6F2 into graphite ( SP-1 powder) in the gas phase has been studied. X-ray powder diffraction spectra shows that stage-1, stage-2 and stage-3 graphite fluorosulfates have been obtained. The compositions of these three compounds are C7.8SO3F, C15.5SO3F and C24S03F according to gravimetry with the repeat space distances of 7.8lA, 11.08A and 14.33A respectively. X-ray photoelectron spectroscopy (XPS) has been extensively used to study some graphite acceptor intercalation compounds. Two effects have been observed, charge transfer leads to higher binding energy and Fermi Level shift to the lower binding energy relative to that of graphite. Graphite fluoride intercalation compounds of S2O6F2, HSO3F and HSO3CF3 have been synthesized by direct reactions. X-ray powder diffraction, XPS, IR, Raman and microanalysis have been employed to study these compounds. The results show that: A. There are some graphite islands in graphite fluorides, CFX t when x<l. B. The islands can be intercalated by S206F2 to form stage-1 intercalation compounds and by HSO3F and HSO3CF3 to form higher stage intercalation compounds. C. The intercalates for the intercalants of S2O6F2, HS03F andHS03CF3 are SO3F-, HSO3F and HS03CF3 respectively. ii TABLE OF CONTENTS Page ABSTRACT ii LIST OF TABLES viii LIST OF FIGURES ix GLOSSARY xii ACKNOWLEDGEMENT xiv CHAPTER 1 INTRODUCTION 1 1.1 General Introduction 2 1.2 Graphite 3 1.3 Graphite Intercalation Compounds 7 1.3.1 Intercalation of Graphite 7 1.3.2 Staging Phenomenon 10 1.3.3 Donor Intercalation Compounds 14 1.3.4 Acceptor Intercalation Compounds 17 1.3.5 Neither Donor nor Acceptor Compounds 18 1.3.6 Covalent Graphite Compounds 19 1.3.7 Synthesis of Graphite Acceptor Intercalation Compounds 19 m 1.4 Graphite Fluoride 26 1.4.1 Introduction 26 1.4.2 Preparation and Characterization 27 1.4.3 Structure 29 1.4.4 Properties 34 1.4.5 Applications 36 1.4.6 Graphite Intercalation Compounds of Fluorine 38 1.5 Purpose of This Work 39 CHAPTER 2 EXPERIMENTAL 41 2.1 General Comments 42 2.2 Apparatus 42 2.2.1 Glass Vacuum Line 42 2.2.2 Dry Atmosphere Box 43 2.2.3 Reaction Vessels 43 2.3 Instrumentation 50 2.3.1 I.R. Spectroscopy 50 2.3.2 Raman Spectroscopy 50 2.3.3 X-ray Powder Diffractometry 50 IV 2.3.4 X-ray Photoelectron Spectroscopy 51 2.3.5 Elemental Analyses 51 2.4 Preparation and Purification of Reagents and Chemicals 52 2.4.1 Graphite 52 2.4.2 Graphite Fluoride 52 2.4.3 S206F2 52 2.4.4 H S O 3 F 53 2.4.5 HSO3CF3 56 2.4.6 K S O 3 F and KSO3CF3 56 2.5 Synthetic Reactions 57 2.5.1 Graphite Fluorosulfate 57 2.5.2 Ci 2 S0 3 CF 3 58 2.5.3 C 1 4 S O 3 F H S O 3 F 58 2.5.4 C 8SbF 6 59 2.5.5 (CFx)nS03F 59 2.5.6 (CFx)nHS03F and (CFx)nHS03CF3 59 CHAPTER 3 GRAPHITE FLUOROSULFATES 61 3.1 Introduction 62 V 3.2 Stage One Compounds 64 3.3 Stage Two Compounds 67 3.4 Stage Three Compounds 73 3.5 Conclusion 75 CHAPTER 4 XPS OF SOME ACCEPTOR GRAPHITE INTERCALATION COMPOUNDS 76 4.1 Introduction 77 4.2 Graphite Fluorosulfates 80 4.3 C12SO3CF3 89 4.4 CF X * 92 4.5 Other Graphite Acceptor Intercalation Compounds 97 4.6 Conclusion 101 CHAPTER 5 GRAPHITE FLUORIDE AND THEIR INTERCALATION COMPOUNDS 102 5.1 Graphite Fluorides 103 5.2 Graphite Fluoride-S206F2 Intercalation Compounds 109 5.3 Graphite Huoride-HSC^F Intercalation Compounds 119 5.4 Graphite Fluoride-HS03CF3 Intercalation Compound 133 vi 5.5 Conclusion 141 SUMMARY AND CONCLUSIONS 142 REFERENCES 143 vii LIST OF TABLES Table Page 1.1 Anisotropy Factor for Various Types of Graphite 8 1.2 Intercalate Sandwich Thickness Is for Several Graphite Intercalation Compounds 16 1.3 Resistivity of CFX 35 3.1 X-ray Diffraction Data of C7.8SO3F 66 3.2 X-ray Diffraction Data of C15SO3F 72 3.3 X-ray Diffraction Data of C24SO3F 74 4.1 Binding Energies of Elements in SO3F" for Different Compounds 88 4.2 Binding Energies of SO3CF3" in Some Compounds 90 4.3 Binding Energies of CFx's 95 4.4 Cis Electron Binding Energies of graphite and Acceptor GICs 97 5.1 Composition of CFX-S206F2 Compounds 110 5.2 X-ray Diffraction Data of (CFo.5)7.9S03F 113 5.3 F Is 1/2 Binding Energy of the Compounds 117 5.4 Composition of CFX-HS03F 120 5.5 X-ray Diffraction Data of (CFo.25)45.9HS03F 125 5.6 X-ray Diffraction Data of (CF0.33)25.6HSO3F 126 5.7 X-ray Diffraction Data of (CFo.5)i5.5HS03F 127 5.8 X-ray Diffraction Data of (CF0.83)23.lHSO3F 128 5.9 Composition of CFX-HS03CF3 Intercalation Compounds 134 viii LIST OF FIGURES Figures page 1.1 Molecular Structure of Hexagonal Graphite 4 1.2 Molecular Structure of Rhombohedral Graphite 6 1.3 Staging in Graphite Intercalation 11 1.4 The Daumas-Herold Model for Staging 13 1.5 Interchange of Domains of Stage-3 and Stage-4 Regions 15 as Might Occur During a Stage Transformation 1.6 Apparatus for the Electrochemical Synthesis of Intercalation Compounds 22 1.7 Absorption Derivatives for 1 9 F NMR in Graphite Fluorides 30 1.8 Possible Layer Structures for Poly( carbon monofluoride) 32 1.9 Bond Lengths and c-axis Length of (C2F)n 33 2.1 One Part Reactor and One Part Storage Vessel 44 2.2 Two Part Reaction Vessels 46 2.3 One Part Two Chamber Reactor 47 2.4 S2O6F2 Addition Trap 49 2.5 Reactor for Deintercalation Studies 49 2.6 Apparatus for the Preparation of S206F2 54 2.7 Fluorosulfuric Acid Distillation Apparatus 55 3.1 X-ray Powder Diffraction Spectra of Graphite and C7SO3F 65 3.2 X-ray Powder Diffraction Spectrum of C7.8SO3F 66 ix 3.3 XPS Spectra of C 7. 8S0 3F 68 3.4 X-ray Powder Diffraction Spectra of C7SO3F After Exposing to Moist Air and Water 69 3.5 X-ray Powder Diffraction Spectrum of C12.5SO3F 70 3.6 X-ray Powder Diffraction Spectrum of C15.5SO3F 72 3.7 X-ray Powder Diffraction Spectrum of C24SO3F 74 4.1 Density of Occupied and Empty State in the Materials and the Definition of the Fermi Level Ep and the Work Function <|>s 79 4.2 XPS Spectra of Graphite and C 7S0 3F 82 4.3 Two Effects on Chemical Shift of Cis Electron Binding Energy, Fermi Level Motion and Electrostatic Attraction 83 4.4 XPS Spectra of C7SO3F-AU 85 4.5 XPS Spectra of KS03F and C7S03F 87 4.6 Structural Model of C7SO3F 88 4.7 XPS Spectrum of C12SO3CF3 90 4.8 Electrostatic Effect and Fermi Level Motion in XPS of C i 2 S 0 3 C F 3 91 4.9 XPS Spectra of CF0.5 93 4.10 XPS Spectra of CF1.0 94 4.11 XPS Spectra of CF 2 96 4.12 XPS Spectra of Ci 4S0 3HS0 3F 98 4.13 XPS Spectra of C8SbF6 99 4.14 XPS Spectra of CgAsF6 100 X 5.1 IR Spectra of Graphite Fluorides 104 5.2 X-ray Powder Diffraction Spectra of CF X 105 5.3 XPS Spectra of CF X 108 5.4 X-ray powder Diffraction Spectra of CFX-S206F2 111 5.5 XPS Spectra of CFX-S206F2 Intercalation Compounds 114 5.6 XPS Spectra of (CF0.83)l6.lSO3F and K S O 3 F 116 5.7 XPS Spectra of Residue of (CF0.83)i6.lSO3F 118 5.8 Intercalation Amounts of S206F2 and HSO3F into CF X 122 5.9 X-ray Powder Diffraction Spectra of CF x -HS0 3 F 123 5.10 XPS Spectra of (CF0.83)23.lHSO3F 130 5.11 X-ray Powder Diffraction Spectrum of Residue of (CFo.83)23. iHS0 3F Deintercalation Compound 131 5.12 XPS Spectrum of Residue of (CF0.83)23.lHSO3F 131 5.13 Raman Spectrum of Collected Volatile of (CF0.5)i5.5HSO3F 132 5.14 IR Spectrum of (CFo.5)l7.75HS03CF3 135 5.15 X-ray Powder Diffraction Spectra of CFX-HS03CF3 Intercalation Compounds 137 5.16 XPS Spectra of (CF0.83)2l.78HSO3CF3 140 xi GLOSSARY 1. ACCEPTOR INTERCALATION COMPOUND: Compounds formed from electron acceptor intercalants in oxidative intercalation 2. BOUNDING LAYERS: The carbon layers adjacent to an intercalate layer 3. DEINTERCALATION: The process that occurs at elevated temperatures which produces the initial intercalants as well as other volatile decomposition products 4. DONOR INTERCALATION COMPOUND: Compounds formed from electron donor intercalants in reductive intercalation 5. INTERCALANT: The bulk reagent that provides atoms, molecules or ions for intercalation 6. INTERCALATE: Atoms, molecules or ions that are intercalated in the vacant sites of a host material 7. INTERCALATION: Insertion of atoms, molecules or ions into a host material having suitable vacant sites 8. LIMITING COMPOSITION: In some intercalation reactions, the intercalation will not proceed after a limiting composition of the product is reached. This limiting composition need not be that of stage one compound, e.g. the hmiting composition of CsBr refers to a stage two compound 9. RESIDUAL COMPOUND: The solid material left after deintercalation 10. SANDWICH THICKNESS: The distance between two graphite layers with an intercalate layer sandwiched between them Xll 11. STAGE INDEX: The number of carbon layers separating two nearest intercalate layers 12. STAGING: The regular alternation of intercalate layers and empty lamellar spaces along the vertical or c-axis direction xiii ACKNOWLEDGEMENT I would like to express my thanks and gratitude to my research supervisor, Professor F. Aubke, for his guidance, encouragement and time which he expended on my graduate work. Thanks are also due to Roshan Cader, whose help has been an asset throughout this research. The late Professor J.G. Hooley is thanked for supplying the graphite. I would also like to thank Professor D.C. Frost and Mr. H. Hu for their measurement of X-ray photoelectron spectra and many fruitful discussions. Thanks are also extended to Dr. N. Osborne who kindly allowed and showed the use of X-ray powder diffractometer in the physics department of this university and to Mr. P. Borda for his microanalysis. Fred Mistry, Dingliang Zhang and Germaine Hwang in our group are also thanked for their help throughout my research. Finally, I would like to thank my wife, Mingming Yang, who gave me a lot of encouragement and support during this work. X I V CHAPTER 1 INTRODUCTION 1 1.1 General Introduction The term intercalation, in chemistry, describes the reversible insertion of a guest species into a lamellar host structure with maintenance of the general structural features of the host. Graphite has a layer structure and carbon occupies a middle position in the order of electronegativity of the elements in the periodic table. In elemental form, graphite is moderately reactive. Therefore, it welcomes many molecules or atoms as guests or intercalants, making it possible to produce hundreds of intercalation compounds. Graphite intercalation compounds are sometimes termed synthetic metals. They are formed by insertion of atoms or molecules of a guest species termed intercalants between the layers of carbon atoms that make up graphite to form an "intercalate layer". Research on graphite intercalation compounds (GICs) has undergone rapid growth during the last 25 years, and the synthesis and characterization of GIC's have drawn many specialists from varying fields such as inorganic, physical and solid state chemistry, solid state physics, material science and electrical and chemical engineering to work together, and to reach a better understanding of the chemical, structural, bonding, and electronic properties of this potentially very useful group of compounds. The use of graphite intercalation compounds as electrical storage systems and planar two-dimensional conductors 1 - 3 has made them prime candidates in industrial applications. The utilization of graphite as the host material in heterogeneous catalysis 4 - 1 0 has also contributed to the popularity of intercalated compounds in basic and industrial research. In addition, the extensive use of graphite as inexpensive versatile electrode materials in industrial electrolysis has spawned research into intercalation phenomena, viewed as a deterioration problem in electrolysis. 2 The first graphite intercalation compound, graphite sulfate, was obtained by Pelisov-Schafhautl in 1841 1 1 by treating graphite with oleum resulting in oxidative intercalation. The presently known intercalation agents are numerous. They range from strong oxidants to powerful reductants 1 2 . Research on graphite intercalation compounds first reached a peak during 1959-1960 with the vast majority of the effort directed towards their synthesis . There have been several comprehensive reviews devoted to graphite intercalation compounds, of which those by Henning 1 3 (1959), Rudorff 1 4 (1959), Herold et al 1 5 (1965), Ebert 1 6 (1976), Herold 1 7 (1979), Fischer 18(1979), Selig and Ebert 19(1980), Bartlett et a l 2 0 (1982), Forsman et al 21(1983), deal with structural, physical and chemical aspects of the compounds and more recendy, Hiroshi 22(1987), Dresselhaus 23(1988) and Inagaki 2 4 (1989) have described some new trends and applications in intercalation chemistry. Since the principal features of graphite intercalation compounds are best viewed as either extensions, or as suitable modifications of the corresponding properties of graphite itself, a detailed summary of the structural and bonding aspects and physical properties of graphite is attempted in the following section. 1.2 Graphite Graphite has a typical layer structure. Natural graphite exists in two allotropic forms: hexagonal and rhombohedral. The more common hexagonal form is characterized by a ABAB- layer arrangement along the vertical c-axis, while the layer stacking sequence in rhombohedral graphite in ABC, ABC- where A,B as well as C indicate positionally different layers. The molecular structure of hexagonal graphite is shown in Fig. 1.1. Infinite planar sheets of hexagons, formed by carbon atoms, are stacked together to give a layer structure to the graphite lattice. The intralayer C-C bond distance of 1.42A, is 3 Rg. 1.1 Molecular structure of hexagonal graphite (from reference 25) 4 less than the C-C distance of 1.54A in diamond. Planarity within the layers and bond angles of 120° suggest the involvement of carbon sp2 hybrid orbitals to form o-bonds. The remaining valence electrons reside in atomic p orbitals perpendicular to the infinite sheets and hence contribute to delocalized 7t-bonds to produce a polyaromatic system. It is this Tt-electron density in the valence band which is responsible for the two dimensional metallic conductance in the basal plane. The interlayer spacing in the lattice is 3.35A, much too long for covalent bond formation but consistent with the view that these carbon planes are held together by relatively weak van der Waals forces. The rhombohedral form with its carbon layers arranged in an ABCABC sequence along the vertical axis, as shown in Fig 1.2., is rarely observed, and has not been used extensively as host material for intercalation 2 5 . Therefore the term "graphite" used throughout this thesis refers to the hexagonal form of graphite. Graphite used in intercalation reactions differs from graphite used as electrode material in industrial electrolysis, in nuclear reactors as moderators or as combustion material. It must meet the requirements of high chemical purity and structural order. For practical purposes, graphite may be classified into two broad groups: natural graphite and synthetic or pyrolytic graphite. Both have been used extensively in graphite intercalation. Natural graphites contain impurities such as Fe, Ca and silicates, and in addition, many natural graphites have crystal defects, which could interfere with both the intercalation process 2 6 and the reproducibility of intercalation products. Nevertheless natural graphite which has been subjected to extensive purification processes finds use in intercalation reactions. Pyrolytic graphite is a monolithic graphite material with a high degree of preferred crystallographic orientation of the c-axis. It is made by pyrolysis of small hydrocarbons such as methane and subsequent heat treatment at temperatures above 2100K or by the chemical vapor deposition-method. Hot-press of pyrolytic graphite at 5 Fig. 1.2 Molecular structure of rhombohedral graphite (from reference 27) 6 temperatures above 2800K results in another synthetic graphite, known as highly oriented pyrolytic graphite (HOPG) 2 7 " 2 9 . HOPG, which is available in plates, exhibits a highly ordered structure with nearly parallel carbon layers in the basal plane. This form has been used extensively in intercalation reactions. As can be seen from table 1.1, natural and synthetic graphite exhibit quite different physical properties in a- and c-axes. The anisotropy ratio (aa / a c) is fairly low for natural graphite as compared to synthetic graphite. SP-1 graphite (spectroscopic grade), which was used in all our synthetic reactions, is a highly purified natural graphite of grain size about 50-100 microns. It is made from Madagascar graphite, treated with HCl and HF to remove basic impurities. A CI2 stream at high temperature is then applied to remove any metallic components present. 1.3. Graphite Intercalation Compounds (GICs) 1.3.1 Intercalation of Graphite Intercalation refers to the insertion of ions, atoms, or molecules generally into a layer structure and specifically between carbon layers of the graphite lattice19'21. The planarity of the layers in the host lattice is retained in the intercalation process. The stacking order, interlayer separation, and to a far lesser extent intralayer bond distance may all be changed due to intercalation. Molecules, ions or atoms capable of such insertion are termed intercalants. The species actually present inside the lattice after intercalation has taken place is called the intercalate. Depending on the nature of the "guest-host", intercalant and intercalate may differ in their electronic structure, electric charge, molecular structure and 7 Table 1.1 Anisotropy factor for various types of graphite (from reference 44) Material T(K) O c (ohm"1 nr1) Oa<5c Natural graphite 300 8.3 x 104 100 (Ceylon.Mexico) Natural graphite 300 104 100 (Ceylon) Natural graphite 300 1.5-2.3 x 104 100-170 (Ticonderoga) Natural graphite 300 2 X 104 130 (Ticonderoga) Natural graphite 300 3.3 x 104 80 (Ticonderoga) Kish graphite 300 1.3-1.5 x 104 Pyrolytic carbon 300 125 5500 l(Td= 2200:C) Pyrolyoc carbon 300 83 5000 (Td = 2500°C) Pvrolytic carbon 300 385 5200 2(HTT = 3000:C) H O P G 300 590 3800 (annealed 3500°C) l. Td = Deposition temperature 2. HIT + Heat treatment temperature 8 even in their chemical identity. The renewed interest in these materials during the last decade has contributed largely to the syntheses of several novel intercalation compounds. These compounds can be divided into four major groups according to the nature of the "guest-host" interaction involved during their formation: 1. Donor compounds— compounds formed by electron donor intercalants, e.g. Li, K, resulting in a net reduction of graphite. 2. Acceptor compounds— compounds formed by electron acceptor intercalants, e.g. AsFs, SbFs or S2O6F2, resulting in a net oxidation of graphite. 3. Intercalation compounds where the intercalates are apparently neither donors nor acceptors, e.g. KrF2BrF3. 4. Covalent compounds of graphite in which the bond between the carbon and intercalate is covalent, e.g. (C-F)n. A final comment is concerned with the two contrasting types of intercalation reactions, i.e. oxidation and reduction. Oxidation will generate usually Cn+-positively charged graphite layers. The reduced electron density in the valence band (carrier density) results in a lowering of the Fermi level. Filling interstitial space with oppositely charged, generally molecular intercalate anions, will increase the anisotropy of electrical conductance, with enhanced conductance in the ab-plane and reduced conductance in the direction of c-axis. Conversely, reducing agents are commonly atoms (e.g. Li, K) which will donate electrons into the empty conductance band causing a rise in the Fermi level with reduced anisotropy of conductance due to the atomic nature of the intercalate metal atoms or ions. 9 Interestingly, donor GIC's have found extensive use as model reducing agents, whereas research on acceptor GIC's has focussed more on the resulting electronic features. 1.3.2. Staging Phenomenon The presence of weak interlayer attractive forces in graphite permits the formation of intercalation compounds with the layers of intercalate stacked in between carbon layers. The basic reason why the intercalation compounds are especially well suited to study as two-dimensional conductors relates to the phenomenon of staging. The filling of intercalants proceeds in an ordered sequential manner, with one layer filled before the next one begins to be filled. The interlayer separation on intercalation will increase from 3.35 A to values between 5A and 10A approximately, depending on the size and nature (molecular or atomic) of the intercalates. Subsequently, a constant number of layers of a host material are sandwiched between sequential intercalate layers, as shown in Fig. 1.3 . This type of periodic arrangement for various compositions are supported by X-ray diffractograms of these samples, which are dominated by ( 001 ) diffractions. Due to their ordered arrangement along the c-axis, the intercalation compounds can be characterized by their layer stoichiometry, called the "stage index" of the sample. The stage index, n, of a GIC is defined as the number of carbon layers separating any two intercalate layers in that particular compound (see Fig. 1.3. ). Also, the X-ray diffractogram of that compound would show the highest intensity for the diffraction due to the [00(n+l)] plane. The stage index does not depend on the arrangement of the intercalate molecules within the intercalate layer, or the extent of charge transfer involved. It simply reflects the extent to which the galleries are occupied. 10 — ooo ooo = OOO = QQQ ooo OOO OOO OOO First Second Third Carbon Lever Intercolcte Layer Fig. 1.3 Staging in graphite intercalation The intercalation process leads to some ordering in the carbon layers as well. The carbon layers adjacent to an intercalate layer are termed "bounding layers" and the others are called "interior layers". These interior layers are found in compounds of stage >2 stacked in the hexagonal (ABAB •layer sequence) arrangement, just like in pristine hexagonal graphite. The internuclear distance between interior layers also remain essentially the same as in graphite30. Closely related to staging is the repeat distance Ic of the intercalate layers along the c-axis. The repeat distance Ic in a stage n intercalation compound can be expressed by Ic = Is + (n-l)Co d-1) where Is = separation of two bounding carbon layers with an intercalate layer sandwiched between them and Cn= interlayer separation in pristine graphite. Theoretically one should be able to prepare compounds with an ideal structure as shown in Fig. 1.3. However, in practice a sample of a particular stage n may not have all the intercalates arranged in layers next to every nth carbon layer along the c-axis. Instead it may also have some of the intercalate molecules spread outside these layers. This situation seems to be more likely in samples made from graphite plates, where the microscopic distribution of the intercalate may easily constitute a mixture of several stages. To accommodate the macroscopic distribution of the intercalate, Daumas and Herold31 proposed a domain (or plated layer) model for the stacking sequence in GICs. Fig. 1.4 illustrates the arrangement of carbon and intercalate layers in compounds of stage 1,2 and 3 as proposed by this model. This model is based on the assumption that all lamellar spaces are filled equally but not continuously during intercalation . This type of non-continuous occupation of all galleries leads to formation of intercalate islands instead of continuous 12 STAGE ONE STAGE TWO Z7 STAGE THREE i mmm m • • • m y — - M w^^^^m—m 7 CARBON LAYER IfiTERCALATE LAYER Fig. 1.4 The Daumas-Herold model for staging. layers. The model can be used to explain the mixed arrangement of carbon layers present in some of the GICs and can also explain the conversion of a stage n compound to stage (n-1) during continuous intercalation, or to stage (n+1) by thermal decomposition. In both cases, the stage transformation can easily be explained by considering the interchange of intercalate domains as shown in Fig. 1.5. Rather convincing evidence for Daumas and Herold model was found in the electron microscope photographs32 of the graphite-FeCl3 system, which clearly shows the existence of twisted carbon layers in these compounds. 1.3.3. Donor Intercalation Compounds Donor intercalation compounds are formed by reacting graphite with electropositive elements with low ionization potentials such as Li, K, Rb, Cs and Ba which will donate electrons into the empty conductance band causing a rise in the Fermi level and n-type conductance enhancement. The properties of donor intercalation compounds strongly reflect the interactions between carbon and intercalate layers. The c-axis conductivity of donor compounds is greater than that of graphite and the separation between two carbon layers with an intercalate layer sandwiched between them, termed I s, is smaller (about 3 A) than that of acceptor intercalation compounds due to the fact that donor species are commonly mono-atomic while acceptors are molecular aggregates as shown in Table 1.2. Fischer 3 3 has shown that the distance Is for donor compounds is much shorter than in the ideal separation expected assuming a completely ionic model for these compounds. Raman studies 2 7 of in-plane C-C vibrations of donor compounds have shown that the charge transferred from the donor atoms to the carbon layers should largely be confined to the adjacent layers. All evidences indicate that donor intercalates not only reduce adjacent carbon layers, but also interact with these layers by orbital mixing, resulting possibly in weak metal-carbon bonds. 14 STAGE 3 STAGE 4 Fig. 1.5 Interchange of domains of stage 3 and stage 4 regions as might occur during a stage transformation (stage transformation is complete when all the intercalate layers have moved to the opposite sides as shown in the diagram) 15 Table 1.2 Intercalate sandwich thickness l s for several graphite intercaoon compounds (from reference 23) Intercalant I s (A) Li 3.71 PT 4.57 Sm 4.72 K 5.35 Cs 5.94 Br 7.04 HNO3 7.84 SO3 7.96 AsF 5 8.15 SbF5 8.46 MnCl: 9.30 FeCl 3 9.37 AICI3 9.54 16 1.3.4. Acceptor Intercalation Compounds Intercalation of graphite by electron acceptors with sufficient electron affinity to withdraw rc-electron density from the carbon layers results in the formation of acceptor GICs. In contrast to donor compounds, the acceptor molecules which participate in oxidation reactions remain ionized and the electronic charge gained is localized in the intercalate layers. The oxidation leads to p-type conductance with a decrease of the Fermi level. As a consequence, the conductance in acceptor GICs is more anisotropic than in graphite due to reduced c-axis conductivity. A wide variety of systems, ranging from protonic acids such as H2SO4 or Lewis acids such as AsFs to radicals like «S03F, halogen molecules like Br2 and ionic compounds such as N02SbF6 in a suitable solvent have been found to form acceptor GICs on intercalation. The intercalate in acceptor GICs may or may not differ from the intercalant in its molecular structure. Two examples are given below to illustrate this difference: 1. S206F2> bis(fluorosulfuryl) peroxide dissociates to «S0 3F radical, which in turn acts as a one electron oxidizer and produces the SO3F" ion on intercalation. 2. AsFs, arsenic(V) fluoride intercalates into graphite to give a compound of limiting composition CsAsF6 3 4 . However, the intercalate appears to be described by the equilibrium: 3 AsFs + 2e- ^ = 2 AsF6~ + ASF3 Here the nature of the intercalate is chemically and structurally different from the intercalant 17 Therefore , in this system the extent of oxidation, the equilibrium position and the exact concentrations of the various intercalate species are subject to much controversy, and are not easily deduced from the stoichiometric composition. The major groups of these compounds were reviewed by Selig19 and Dresselhaus27. 1.3.5. Neither Donor Nor Acceptor Compounds In some GICs, there is no apparent charge transfer and the intercalates remain as neutral molecules. A group of compounds in this category are the noble gas fluorides. Although they are powerful oxidizers, it appears unlikely that they are electron-oxidizing graphites. Thus, KrF2 intercalates spontaneously and appears to have an enhanced stability when intercalated. The graphite/KrF2 material is also a less extreme fluorinating and oxidizing agent than KrF2 itself. Yet reduced species such as KrF2" can not be formed . Nor are such species as KrF3" very probable. It appears that KrF2 must be present as the molecular species. A related group of compounds that appear to behave similarly to the noble gas fluorides are halogen fluorides. Those that intercalate spontaneously include BrF3 f IF5 and IOF5. Most of the work on these fluorides and the noble gas fluorides was initiated by Selig and co-workers, and a full report of the chemical and analytical aspects of these graphite intercalation compounds is given in the comprehensive review by Selig and Ebert19. 18 1.3.6. Covalent Graphite Compounds Covalent compounds of graphite are formed when graphite is reacted with strong oxidizing agents such as fluorine and Mn(VII)19. The chemical nature of graphite oxide is uncertain. Some strongly oxidizing systems such as nitric acid-alkali chlorate, or sulfuric acid-sodium nitrate-permanganate can react with graphite to form a compound with a formula of C802(OH)2 or C2O4H2 1 9 . In a somewhat broader sense, they can also be regarded as intercalation compounds because of the layer structure. The formation of covalent bonds at the carbon atoms of the host results in loss of planarity of the carbon layers to form a puckered arrangement of sp3 hybridized carbon atoms. A typical example for this type of compounds is poly (carbon monofluoride), (CF)n. This compound will be discussed in the following sections in detail. The covalent bond formation is irreversible and the covalent compounds of graphite are stable in a vacuum even at elevated temperatures. 1.3.7. Synthesis of Graphite Acceptor Intercalation Compounds The principal methods used in graphite acceptor intercalation synthesis can be classified as: (A ) direct intercalation; (B ) oxidation by an external chemical species which will not itself intercalate; (C) electrochemical or anodic oxidation; (D ) intercalate exchange and substitution; (E) intercalate oxidation or reduction and (F) dissolved solute intercalation. A brief summary of each technique is give below. 19 A. Direct Intercalation The principal route to synthesize GICs is the direct intercalation by exposure of graphite to gaseous, liquid or dissolved intercalants according to nC(s) + X(i.)g.)SOiution) —> CnX(s) Direct intercalation using bulk reagents, which exist as gases or low boiling liquids at ambient temperature and atmospheric pressure, can be carried out conveniently at room i temperature. Following intercalation by weighing the sample from time to time is the simplest method of product analysis. The intercalation of solid material such as FeCl3 and AICI3, or viscous liquids like SbFs involves reactions at elevated temperature. After the sample has reached the required composition, excess of the intercalant can be removed by filtration or vacuum distillation. An example is the formation of graphite fluorosulfate according to nC + S2 06 F2 (excess) -» C11SO3F where n = 7 to 8. B. Oxidation by External Chemical Species Some neutral molecules and anions can not be intercalated into graphite by themselves. An oxidizing agent has to be be added in the reaction. For example, AICI3 20 vapor can react with graphite only in the presence of Cl 2 gas 3 6 and a stage one compound C+3oAlCl4-2AlCl3 is obtained. The role of CI2 as the oxidizer is evident from the following probable mechanism 3 7 «38: Cl2(g) CI 2 (ads) l/2Al2Cl6(g) AlCl3(g) AlCl3(ads) AlCl3(ads) + Cl2(ads) a+AlCl4-(ads) C n + Cl+AlCLf +mAlCl3(g) -> Cn+AlCl4"«rnAlCl3 + Cl-(a d s ) 2Cl-(ads)-> Cl2(ads)-» G2(g) In the intercalation of H2SC>4, CTO3 reacts initially with graphite and compounds of general formula C+24nHS04-2H2S04are synthesized 3 9 . Strong acid such as HCIO4 and CF3COOH can be intercalated in a similar manner, using C1O3 or other oxidants such as KMnO-4, Mn02 and PbC>2. C. Electrochemical Methods (Anodic Oxidation of Graphite) The electrolysis of non-aqueous electrolytes using a graphite anode 40,41,42 has been adopted to the synthesis of some GICs. This method is primarily used for the intercalation of protonic acids. A typical electrolysis cell is shown in Fig. 1.6, where Ci denotes the graphite anode. The voltage drop between the anodes Ci and C2 ( an electrode made of pyrolytic graphite ) is recorded continuously. As the intercalation 21 H voltc;e Recorder \-qny Electrometer Electrolyte C^s C2 ore Grophite Electrodes R s^ R2 ore Resistances Fig. 1.6 Apparatus for the electrochemical synthesis of intercalation compounds 22 proceeds, anions and neutral acid molecules will insert into the graphite anode, and the voltage builds up with time. For the two protonic acids CF3SO3H and H2SO4 the following reactions can be written43: 26C + (x+1) CF3SO3H —> C+26CF3S03--xCF3S03H + 0.5 H 2 ( x=1.63 ) 24C + (x+1)H2SO4 -» C+24HS04-xH2S04 + 0.5H 2 ( x = 2.42) In practice one can observe the voltage remaining constant for a few minutes when a stage two compound is formed, then it will increase again until stage one composition is reached. The method can be easily adopted to the syntheses of samples of various stages, but the cointercalation of neutral molecules from the bulk intercalant, or the solvent molecules, together with the anion cannot be avoided. The variable amount of neutral molecules present in the final samples causes discrepancies in the determination of the actual stoichiometry of the intercalation compound formed D. Intercalate Exchange and Substitution This method was first used in the conversion of C24HSO4 to the corresponding perchlorate GIC according to 4 0 23 C+24HSG-4" + excess HCIO4 -> C+24C104- + H2SO4 Recent work done in our group has provided some additional examples of intercalate exchange reaction 4 4 : C7SO3F + excess HSO3CF3 —> Ci2S03CF3 + C7SO3F + excess SbF5 —> C8SbF6 + Ci2BrSC»3F + excess HSO3CF3 —> Ci 2 S03CF 3 + Substitution reactions are generally rare and even when they occur, interpretation of results can be difficult since equilibrium mixtures can form at certain stages of the reaction. For example, when CsAsFs was treated with NO^SbFg, the resulting product showed the following intercalates: SbF^, AsF6~, SbFs, AsFs and A S F 3 4 5 . E. Intercalate Oxidation or Reduction This method is illustrated by studying the redox reaction of the graphite-FeCl3 system. The intercalated FeCl3 can be reduced either to FeCl2 by treating the intercalated product with H2 4 6 or to FeO by heating in an oxygen stream47. Attempts to reduce the intercalated metal halides to pure metal, where the resulting product in rum could be used as a potential catalyst, have not been successful so far48. 24 Intercalate oxidation has also been observed in graphite-fluorosulfate compounds. When Ci2BrS03F was reacted with S2O6F2, a compound of composition Ci6Br(SC>3F)3 was obtained 4 4 . This product may then undergo intercalate reduction, to yield the initial compound, i.e. Ci2BrSO*3F, in the following manner: 3 Ci6Br(SO-3F)3 + 3 Br2 —» 4 Ci2BrS03F + 5 BrS03F F. Dissolved Solute Intercalation Intercalation of solid solutes dissolved in non-aqueous solvents into graphite is a very versatile method which has resulted in the synthesis of several acceptor compounds. In these reactions, a suitable solute may function as an oxidizing agent, e.g. Cr03 dissolved in protonic acids such as HSO3F facilitates the intercalation of solvent molecules, according to49: C1O3 n C + HSO3F > C nHS0 3F n=5±l There were no indication given in the report, to what extent graphite is oxidized and what the counter anion is. More often the solute acts as oxidant as well as an intercalant, resulting in the formation of graphite salts as shown below 5 0 , 5 1 : HSO3F 22 C + I(S03F)3 > C22I(S03F)3 nC + NG-2+X- + yCH 3 N0 2 > C n +X-yCH 3N02 + N0 2 25 where X = SbFg, PF6 and BF4. However, this method has many disadvantages. Removal of excess intercalants is commonly accomplished by repeated washing with excess solvent, which often results in partial loss of the intercalate as well, and affords materials of uncertain composition, requiring detailed and complete chemical analysis. The cointercalation of solvent molecules often can not be avoided, and binary compounds of the form C n + X " are not obtained by this method, and nitronium salts such as NO2BF4 and NO2PF6 yield only stage two compounds at the highest concentration51. In addition, great care in solvent purification and drying is required to avoid complications. 1.4. Graphite Fluoride 1.4.1. Introduction Graphite fluoride is a non-stoichiometric solid layered compound obtained by direct fluorination of graphite or carbon at high temperature according to 300-600 °C C + F 2 > CFX (x<1.3) The value of x strongly depends on the fluorination temperature of the carbon material. At present, two crystal forms, poly(carbon monofluoride) (CF)n, and poly(dicarbon monofluoride) ( C 2 F ) n are known. The (CF)n is a gray-white compound prepared at 300-600 °C. However, ( C 2 F ) n is a black compound obtained by fluorinating high-quality graphite, such as natural graphite, at 350-400 °C. They differ in structure. The chemical 26 bonds between fluorine and carbon are covalent in both of these compounds, hence they are electrical insulators. Poly (carbon monofluoride) has been known since 1934, when Ruff et a l . 5 3 prepared a gray compound of composition CFo.92- Later, Rudorff et al. 5 4 > 5 5 published several papers on graphite fluorides whose composition ranged from CFo.67 to CFo.98. These compounds were obtained by reaction of elementary fluorine with graphite at temperatures between 410 °C to 550 °C. Later, Palin and Wadsworth 5 6 prepared CF1.04 by a similar method. From the 1960s to the 1970s, Margrave and co-workers were also active in this field. No practical use of graphite fluoride was found until the end of the 1960s, when it was found, that graphite fluoride is not only an excellent solid lubricant but can be used also as a cathode material when properly modified in lithium batteries. Since that time, interest in these materials has led to systematic research on the fluorination reaction of carbon materials, crystal structure, physicochemical properties, and appUcations of graphite fluorides. Some comprehensive reviews on these materials have been published including Watanabe et al. (1974)57, Kammarchik and Margrave (1978)58, Selig and Ebert (1980)19, Watanabe and Nakajima (1982)59, and Watanabe et al. (1985,1988)60-61. We will discuss the preparation, structural characterization, physical and chemical properties as well as the applications of graphite fluoride in the following sections. 1.4.2. Preparation and Characterization Preparative methods involve the direct fluorination of graphite 5 8 > 5 9 using a fluidized bed or a rotating reactor. The chemical composition, crystallinity and crystal 27 structure of graphite fluoride depend primarily on the reaction temperature. The sample's crystallinity also depends on that of the starting graphite. The fluorination reaction is strongly exothermic 6 2 , so that careful temperature control and positioning of the sample are required to achieve reproducible results without combustion or thermal decomposition. The composition and color of graphite fluoride are a function of the reaction temperature. The value of x in (CFx)n increases with rising temperature, the color changes from black through gray to white above 600 °C under 1 atm of fluorine pressure. It should be noted that the x value of graphite fluoride prepared at 375 °C no longer increases even after further treatment at 600 °C in a fluorine atmosphere, while its color changes from black to white. On the basis of these results, together with X-ray and XPS analyses, the formation reaction and the characterization of the two well defined graphite fluorides, (CF)n and (C2F) n can be summarized as follows: 1. Only (CF)n is formed in the temperature range 600-640 °C by the following reaction: 2. Graphite fluoride prepared between 350 and 400 °C is essentially (C2F) n , formed according to the following reaction: 3. Non-stoichiometric graphite fluoride (CF x) n (0.5< x <1) is formed in the temperature range 400-600 °C; the material is actually a mixture of (CF)n and ( C 2 F V 2n C(S) + nF2(g) » 2 (CF)n(S) 4n C(s) + nF2(g) * 2 (C2F)n ( S) 28 4. When x<0.5 in (CFx)n, the material is a mixture of (C2F)n and graphite. Some graphite intercalation compounds of fluorine may also be present in this compound. 1.4.3. Structure Graphite fluoride is a covalent compound. The electronic structure of an isolated two-dimensional infinite layer of (CF)n has been calculated by a modified extended Huckel model &>M. The XPS and solid state 1 3C-NMR patterns confirm the presence of covalent C-F bond in graphite fluorides ^,66 The planarity of the layer in pristine graphite disappears because of the formation of the covalent C-F bonds, and buckled sheets are created. In the case of (C2F)n, in addition to covalent C-F bonds, covalent C-C bonds are also found between adjacent carbon layers . Concerning the nature of the bonding, the only difference between (CF)n and (C2F)nis observed by broad-line 1 9 F-NMR spectroscopy as shown in Fig. 1.7. fluorine resonance due to (C2F)n consists of both broad and narrow lines with linewidths of 8.5 and 2.0 G, respectively. The latter does not disappear even after heat-treatment at 600°C. This shows that a small number of fluorine atoms are rather weakly combined with carbon atoms in (C2F)n. The narrow line is due to fluorine atoms which are trapped in the defects of the crystal-bulk. These fluorine atoms have a relatively large degree of freedom compared with those covalently bonded with carbon. The C-F bond energy of (CF)n is reported to be ~ 480 KJ moH 6 2 29 A H (Gauss) Fig. 1.7 Absorption derivatives for 1 9 F resonance in graphite fluorides (from reference 61) A.(CF)n B. (C2F)n 30 Although buckled, the layer structure of graphite remains after fluorination according to a scanning electron micrograph67. The hexagonal lattice parameters for (CF)n prepared from natural graphite at 600 °C are a=b=2.57A and c=5.85A, in which c depends slighdy on the crystallinity of the original carbon material and the fluorination temperature. Rudorff 5 4 proposed a linked cyclohexane chair structure for (CF)n (Fig. 1.8A) and later Ebert 6 9 suggested the cis/rrans-linked cyclohexane boat structurefFig 1.8 ). Both structures have been examined by means of solid state 1 9F-NMR 65,70 The structure of (C2F)n is thought to be of hexagonal symmetry, according to transmission Laue photographs with a=2.5A and c=8.2A obtained from the interpretation of structural data of X-ray diffraction65'66. On the basis of the above considerations and the similarity between (C2F)n and (CF) n , the most plausible structural model of (C2F)nis shown in Fig. 1.9. In this structure, the C-F and C-C bond lengths are 1.41 A and 1.53A respectively. The van der Waals radius for F is 1.36A and the C-C-C bond angle is 109 °28', the tetrahedral angle. The most significant feature which distinguishes (C2F)n from the (CF)n structure is, that successive layers along the c-axis consist of trans-linked cyclohexane rings, in which three carbons are combined with those of adjacent cyclohexane by diamond-like sp3 bonds perpendicular to the a-axis and three others are combined with fluorine atoms. Graphite fluoride, (CF)n can be loosely considered as a first-stage compound from the structural viewpoint of graphite intercalation compounds, whereas (C2F)n would be a second stage compound. However the principle of staging is normally not applied to graphite fluorides. 31 (A) • = C O - F Fig. 1.8 Possible layer structures for poly(carbon monofluoride) (A) linked cyclohexane "chairs" (from reference 54) (B) cis/trans-linked cyclohexane "boats" (from reference 69) 32 Fig. 1.9 Bond lengths and c-axis length of C 2 F (from reference 61) 33 1.4.4 Properties White graphite fluoride is prepared in an oxidative atmosphere of fluorine gas and is more stable in an oxidative environment than in a reductive one. (CFx)n shows remarkable inertness towards strong protonic acids such as sulfuric , nitric , hydroiodic as well as towards alkalies. It does not react with hydrogen even at 400 °C. With increasing fluorination temperature, some amorphous carbon is lost as volatile fluorocarbons such as CF4 and C2F6. (CFx)n is hydrophobic and insoluble in organic solvents such as acetone, ether, benzene, alcohol, etc. The resistivity of (CFx)n will increase as the x value increases because the formation of covalent C-F bonds block the movement of TU-electrons in the carbon layer. When x>l, it is almost an insulator or a non-conductor as shown in Table 1.3. Graphite fluoride prepared from natural graphite is stable in room temperature, but decomposes at temperatures above 500 °C to form fluorocarbons such as CF4 and C2F6, and amorphous carbon. When heated with alkali metal chloride, bromide, and iodide, graphite fluoride produces alkali metal fluorides and halogens. For example, with KI, the following reaction is observed: KI + CF -> KF + 1/212 + C Differential thermal analysis (DTA) and thermogravimetry (TG) show this reaction starts at 400 °C 61. 34 Table 1.3. Resistivity of CF X (at room temperature) CF X %F Resistivity (Q cm) C F U 63.5% 1 x 109 CFo.85 57.3% 9x 104 CF0.55 46.5% 246 CFo.49 43.6% 1.93 CFo.4 38.8% 1.559 Graphite 0% 4.0 x 10-2 (From Reference 71) 35 The process in primary lithium batteries with CF cathodes is described as anode: Li (s) -» Li+(solv.) + e~ cathode: Li+ + CF + e" > C + LiF overall: Li (s) + CF •» LiF + C Reaction of graphite fluoride with potassium carbonate will start at 300 ° C , producing KF andCQ2. When heated in hydrogen gas, reduction starts at temperatures above 400 °C, producing carbon and hydrogen fluoride. CF + 1/2 H 2 —> C + HF 1.4.5. Applications The applications of graphite fluoride are based on lubricant properties combined with its high thermal stability and use in lithium-graphite fluoride batteries. Graphite can be used as a solid lubricant. When graphite is directly fluorinated at high temperature, a strong covalent C-F bond is formed, resulting in the loss of aromaticity. The average bond CF + 1/2 K2CO3 —> KF + 3/4 CO2 + 3/4 C 36 energy of a C-F bond is so high that the bond will not be easily ruptured even at high temperature and pressure. It has low surface energy, low coefficients of friction and high bond energy. In graphite fluoride, the chemical species facing other monolayers is fluorine bonded to tertiary carbons, resulting in extensive F-F repulsion between layers. It is therefore expected that graphite fluorides will exhibit lubricity in the same way as graphite itself. For a solid lubricant, desirable characteristics are thermal stability, low shear strength, lamellar structure, surface protection and surface adherence. Graphite fluoride has all these properties except the last, and lubricates as well in dry as in moist air. Some patents for lubrication by graphite fluoride have appeared since 1961 7 2 " 7 4 . After these, several papers were published on the lubricity of graphite fluoride. Karnmarchik and Margrave58 have examined the lubricant properties of graphite fluoride in detail. Its use as an additive to conventional grease, graphite or Teflon and related composites suppresses the surface temperature due to friction, which allows use at high pressures and temperatures. There is however a drawback to the use of CF in combustion engines as an additiveto motoroil. CF will not mix with hydrocarbon based oil and will separate out when an engine stops running resulting in clogging problems. Lithium batteries based on (CF)n/Li and (C2F)n/Li system have been studied since 1968 5 9 , 7 5 - 7 8' 6 8. A primary battery based on fluorine as cathode and lithium as anode is theoretically the best high-energy-density power source. However, it is very difficult to use fluorine gas itself as a cathode because of its high reactivity. Many kinds of fluorides were tested for use as a cathode material instead, but none gave satisfactory results. Eventually, the application of (CF)n as a cathode material for a primary lithium battery with an organic electrolyte was found to be successful. A conductive additive such as polyacetylene or graphite must be mixed with graphite fluoride when (CF)n is used as a 37 cathode. The discharge reaction mechanism for (CF)n/Li and (C2F)n /Li cells has been investigated 7 9 « 8 0 . 1.4.6. Graphite Intercalation Compounds of Fluorine It is generally accepted that no reaction occurs between elemental fluorine and graphite at ordinary temperature and pressure, but that reaction at high temperature leads to covalent compounds such as graphite fluoride. However, fluorine is reportedly intercalated into graphite along with hydrogen fluoride (HF) at room temperature 8 1 " 8 3 : x C + 1/2 F 2 + y HF —» CXF (HF)y where the y values depend on whether liquid or gaseous HF is employed, and on the vapor pressure of HF in a fluorine atmosphere when gaseous HF is used. Recently, Mallouk and Bartlett 8 4 reported the reaction of graphite with fluorine and determined the resulting structure of CxF(HF)y. When liquid HF is used, Stage two C12HF2 and stage three C18HF2 are obtained. The electrical conductivities of C12HF2 and C18HF2 are 1.5 and 4-5 times that of HOPG (highly oriented pyrolytic graphite). X-ray photoelectron spectroscopy (XPS)86 shows the binding energy of carbon in these intercalation compounds to be 284.3 eV, interpreted as carbon forming an ionic bond with the intercalates. The binding energy of fluorine is intermediate between that of fluorine in LiF (an ionic compound) and fluorine in Teflon (a covalent compound). This suggests that the fluorine ligand in CxF(HF)y bears a partial negative charge and that the C-F bond has semi-ionic character 8 4 . It is noteworthy that the rate of the fluorine intercalation is greatly accelerated by addition of a trace of a covalent fluoride such as AsFs, IF5 or OsF6 into the graphite. 38 It has been found that fluorine-graphite intercalation compounds can be prepared in the presence of metal fluorides such as LiF, C11F2 or AgF 85,86 These materials have high electrical conductivities and thermal stabilities. MF n x C + 1/2 F 2 » CXF 120 °C 1.5. Purpose of this work (A) . S2O6F2, bis (fluorosulfuryl) peroxide, has found frequent use as an intercalant into various forms of graphite. Stage one and stage two graphite fluorosulfates have been synthesized by using liquid phase reactions and gas phase reactions with a controlled pressure of S206F2 . There is no report on the synthesis of a stage three graphite fluorosulfate. In our work , an attempt is made to synthesize stage two and stage three graphite fluorosulfates via a gas phase reaction by carefully controlling the amount of S2O6F2. X-ray powder diffraction and XPS are employed to characterize these compounds. (B) . Some graphite acceptor intercalation compounds have been synthesized in our group and their structures and properties have been studied by means of IR, Raman spectra, solid state NMR, ESR and electrical conductivity 5 0 . In the present work on GIC's, X-ray photoelectron spectroscopy (XPS) is used extensively to provide additional information on bonding and structure of GIC's Unlike other techniques, XPS enables the investigator to look at both the graphite lattice and the intercalate. 39 (C). As discussed above, graphite fluoride has a layer structure and may form intercalation compounds just as graphite does. The use of S2O6F2 and strong protonic acids such as HSO3CF3 and HSO3F as intercalants towards graphite fluoride CFX (x <, 1) to form new acceptor intercalation compounds is attempted in this work. IR and Raman spectra, XPS, X-ray powder diffraction and microanalysis are used to characterize these materials. 40 CHAPTER 2 EXPERIMENTAL 41 2.1 General Comments This chapter will deal with general experimental techniques and the sources of starting materials used in this study. The synthetic reactions were carried out in well ventilated fumehoods and solids were handled in a dry box. All volatile compounds were handled in glass vacuum lines mounted on metal frameworks inside fumehoods. 2.2 Apparatus 2.2.1. Glass Vacuum Line Standard high vacuum techniques were used in all synthetic reactions. A general purpose glass vacuum line of about 60 cm length with five outlets fitted with Kontes Teflon stem stopcocks were used to manipulate volatile liquids and gases. Other apparatuses were attached to the vacuum line through B-10 ground glass cone and socket joints. The vacuum line was connected to a Welch Duo-seal mechanical pump (model 1405) via a liquid nitrogen cold trap to prevent any corrosive volatile materials being drawn through the pump. Pressures measured in the manifold using a mercury manometer showed values from 0.5 torr to 1 atmosphere. Transfer of liquids and other volatile materials from one reaction vessel to another was carried out using a T-connecting bridge, which was attached to the vacuum line via a B-10 cone. 42 2.2.2. Dry Atmosphere Box The manipulation of all air sensitive compounds was carried out in a Vacuum Atmosphere Corporation "Dri-Lab", model HE-493, filled with dry and purified nitrogen. phospherus pentaoxide was kept in an open container inside the dry box to remove any residual moisture and to act as an indicator. The dryness of the atmosphere was ensured by circulation over molecular sieves, which were regenerated about once per month by heating over a Cu catalyst contained within the HE-493 "Dri-Train" purifier. A Mettler P160 top loading balance was used inside the dry box in order to weigh hygroscopic materials. 2.3. Reaction Vessels A. One-Part Reactor One-part reactor (Fig.2.1 (A)) and one-part storage vessel (Fig. 2.1 (B)) were used in the synthetic reactions. The reactor of about 30 ml capacity was mainly used to carry out liquid phase reactions. After adding the solid reactants (graphite or graphite fluorides), the liquid reagent would then be distilled into the reactor in vacuo and the mixture after warming up to room temperature magnetically stirred using Teflon coated stirring bars and an external magnetic stirrer. Storage vessel was primarily used to store liquid reagents, such as S2O6F2 and HSO3F. It was made up of a round bottom flask (100ml) with a constriction and a B-19 cone. It can also be used to carry out large scale liquid phase reactions. 43 B 10 inner, ground-glass joint (A) (B) Fig. 2.1 (A) One part reactor (B) One pan storage vessel (two versions: normal or thick wall glass) 44 B. Two-Part Glass Reactor Two part glass reactors were made up of either a round bottom flask (25ml-100ml) or an Erlenmeyer flask with a standard B-19 ground glass cone. The reactor top consisted of an adapter with a Kontes Teflon stem stopcock sandwiched between a B-19 socket and a BIO ground glass cone for attachment to the glass vacuum line (Fig. 2.2). This reactor was used for HOPG plates which were more easily placed into a two-part reactor. The obvious disadvantage was the possible contamination of products due to fluorocarbon grease (or its reaction products) which had to be applied at the ground glass joints to maintain leakproof connections under vacuum. Flat bottom reactors are rather useful in solid-liquid reactions, because they allow more extensive mixing between liquid and solid reactants. However, the flat bottom reactors may implode in a high vacuum. C. One-Part -Two-Chamber Reactor This reactor is shown in Fig 2.3. Two reaction vials were connected via a glass tube and solid powder reactant like SP1 graphite was loaded into the left vial through the stopcock using a glass funnel and a liquid reagent such as S2O6F2 was distilled into the left vial from the right through the side arm in vacuo. When the nitrogen bath was removed, or replaced by another warmer bath, the volatile liquid reagent would have sufficient vapor pressure to start the reaction. As the liquid reagent is consumed , more would evaporate continuously till the completion of the reaction. 45 (A) Fig. 22 Two part reaction vessels (A) Flat bottom reactor (B) Round bottom reactor 46 Fig. 2.3 One-part-rwo-chambcr reactor 47 D. S2O6F2 Addition Trap When stoichiometric amounts of S2O6F2 were needed for a reaction, a calibrated addition trap was used (Fig. 2.4). Exact volumes of up to 0.50 ml could be distilled using this device. The trap was made up of a pipette fitted with a 20 mm long (10 mm O.D.) Pyrex bulb to which a Kontes Teflon stem stopcock was attached. A side arm extension ended with a BIO ground glass cone. The compact nature of the trap made it easy to weigh inside an analytical balance, providing a second more accurate method to check the amounts added. For addition of larger amounts of S2O6F2, a similar type of trap, with a 4.00ml capacity pipette, was used. E. Reactor for Deintercalation This reactor was a one-part reactor with a glass tube attached to one side (Fig. 2.5). The intercalation compound kept at the bottom of the vial was heated using an oil bath and the volatiles were collected in the glass tube by means of an external liquid nitrogen bath. The collected volatiles in the glass tube can then be analysized by Raman spectroscopy. 48 49 2.3 Instrumentation 2.3.1. I.R. Spectroscopy Infrared spectra were recorded at room temperature using a Perkin-Elmer 598 grating spectrophotometer, with a spectral range of 4000 - 200 cm-1. KBr and KRS-5 (thallium bromide-iodide) windows with transmission range down to 250 cm-1 were used. Since the samples were hygroscopic and often reacted with conventional mulling agents like Nujol and hexachlorobutadiene, a solid film of the sample between the windows was used and was protected from atmospheric moisture by electrical insulation tape wrapped around the edges of the windows. Samples were prepared inside the dry box and spectra were recorded as soon as the samples were taken out of the dry box, to prevent attack on the windows by the samples. All infrared spectra were calibrated with a polystyrene film as reference at 1601cm-1. 2.3.2. Raman Spectroscopy Raman spectra were recorded using a Spex Ramalog-5 spectrometer equipped with a Spectra physics 164 Argon ion laser. The green line at 514.5 nm was used for excitation. 2.3.3. X-ray Powder Diffractometry X-ray powder diffraction patterns were recorded on a high brilliancy, Ru 200B Series Rigaku Rotating Anode X-ray Powder Diffractometer operating in line focus with 50 12kw maximum operating power. The diffractometer detected Cu K a target radiation through a 20 u.m Ni filter with a Horizontal-Type Nal scintillator probe (SC-30). The parameters are: 20 = 0.002, 6 = 0.60. A horizontal goniometer was used for the rotating anode. The diffractometer was interfaced with a DMax/B computer system driven by an IBM PS/2. The peak-finding program was provided by Rigaku. Powdered samples were put on glass plates with a two sided scotch tape and a plastic film was employed to seal samples from moisture. All operations were carried out in the dry box and samples were analyzed as soon as possible after they were taken out of the dry box. 2.3.4. X-ray Photoelectron Spectroscopy (XPS) X-ray photoelectron spectra were recorded with a Varian IEE-15 XPS spectrometer using Al Ka X-rays of 1486.6 eV energy. Samples were carefully thinly dusted onto 2 cm length 3M Scotch tape, wrapped around the sample slug 8 7 in the dry box. The carbon Is 1/2 binding energy at 284.0 eV was used as a reference for all measurements and peak heights were used to assess relative abundances. The chemical shift is the difference between the measured binding energy of a peak and that of the chosen standard atomic peak. It is important to emphasize that all XPS values in this work were obtained by using Varian IEE-15 instrument only with an accuracy of ±0.1 eV. 2.3.5. Elemental Analyses Carbon, hydrogen and sulfur analyses were carried out by Mr. P. Borda of the Chemistry Department, The University of British Columbia. A Carlo Elba Model 1106 57 analyzer employing a flash oxidation technique was used. The samples were first treated with pure powdered CuO. Details of this method have been published 8 8 . Samples for microanalyses were transferred into glass tubes in the dry box and flame sealed. 2.4. Preparation and Purification of Reagents and Chemicals 2.4.1. Graphite SP-1 graphite (spectroscopic grade, purified natural graphite ) of 50-lOOp. grain size was obtained from Union Carbide Ltd., Parma, Ohio., USA. It was dried in a dynamic vacuum for 24 hrs. at room temperature before use. 2.4.2. Graphite Fluorides Powder graphite fluorides CFX ( 0.25 <x < 1.3) were obtained from ATO Chem., formerly Pennwalt -Ozark Mahoning Company, Oklahoma, USA. Small amounts of free samples were obtained from Dr. T. Mamaud of ATO Chem. They were dried in a dynamic vacuum for 24 hrs. at room temperature before use. 2.4.3. S206F2 Bis(fluorosulfuryl)peroxide, S206F2, was prepared (in one to two kilogram quantities) by the reaction of fluorine and sulfur trioxide using an AgF2 catalyst at a temperature of ~180 °C 8 9 »90. The experimental set-up for this preparation is shown in 52 Fig.2.6. This method is a modified version of the general synthetic route as described in the literature. Pressure regulated fluorine was passed through a stainless steel cylinder containing NaF. This trap is necessary to remove any HF from the fluorine gas. HF free fluorine was then allowed to react with sulfur trioxide, which was carried to the AgF2 catalytic reactor by a stream of dry nitrogen. Excess fluorine was detected using a fluorolube oil bubble counter and destroyed by using a soda lime trap. To improve the yield of the reaction, sulfur trioxide was heated using a heating mantle to 50 °C, and the overall reaction temperature was maintained at ~ 180 °C. The products generated were collected by condensation in the traps A, B and C, the latter two kept at -78 °C by dry ice. The condensed colorless liquid was extracted with 96-98% H2SO4 to remove any unreacted sulfur trioxide once flakes of this compound became apparent. FSO3F dissoved in crude S2O6F2 was removed by pumping on the trap held at -78 °C (dry ice). The purified S2O6F2 was then vacuum distilled into Pyrex storage vessels. Gas phase IR and liquid 1 9 F NMR were used to confirm the purity of the S2O6F2 obtained. 2.4.4. H S O 3 F Technical grade fluorosulfuric acid was obtained from Orange County Chemicals, USA. and was purified by a double distillation technique described by Thompson91 and Barr et a l 9 2 . The apparatus used for this method is shown in Fig. 2.7. The entire system was first flame dried to remove any moisture, and nitrogen was flushed through for about 15 h. Distillation was carried out under a blanket of dry nitrogen at atmospheric pressure, 53 To Flowmeter! SOOmt Pyre* Flash Reoctor (J) Whifey Volve •G" Hofce 413 Volve rr i Au toe love Engineering LJ Voives 180 C N o F Trop —— -^ j ~~T~~~j rTl 23 0*t>y P r c w u r « G u o q « F 2 Outlet Copper B34 V B34 a 34 w w w A B C 25 - C -78 #C -78 'C COLLECTION VESSELS 1 lb Cylinder •0-To Soda - lime Trop -Fluorolube Oil Tube Fig. 2.6 Apparatus for the preparation of S 2 O 6 F 2 thermometer B-19 ground glass joint Fig. 17 Huorosulfuric acid distillatioo apparatus and the first fraction collected was made HF free by a counterflow of nitrogen. The constant boiling second fraction was then collected at 163 °C. The storage container was evacuated and the acid stored in the dry box to protect it from any contamination. 2.4.5. H S O 3 C F 3 HSO3CF3 was obtained from Alfa Chemicals, Morton Thiokol Inc. USA. It was distilled by trap to trap vacuum distillation before use. 2.4.6. KS0 3 F and K S 0 3 C F 3 K S O 3 F and KSO3CF3 were synthesized according to: KCl + excess HSO3F > KSO3F + HCl T KCl + excess HSO3CF3 > KSO3CF3 + HCl T The HCl and excess HSO3F as well as HSO3CF3 were removed in dynamic vacuum with heating and decomposing solvates of the type K S O 3 X H S O 3 X , X=F or CF3initially formed until the product kept constant expected weight. 56 2.5. Synthetic Reactions 2.5.1. Graphite Fluorosulfate A. Liquid Phase Reaction Liquid phase reactions were only used to synthesize stage one graphite fluorosulfates. In a typical reaction, 100-200 mg of SP1 graphite was put in a one-part reactor and an excess (about 10ml) of liquid S2O6F7 was distilled into the reactor via the T bridge. After the mixture of solid and liquid were stirred magnetically for 48 hrs, excess of S2O6F2 was removed in vacuo and the product was kept in a dynamic vacuum until constant weight was reached. B. Gas Phase Reaction This method could be used to synthesize three types of graphite fluorosulfates, i.e. stage one, stage two and stage three. A one-part two-chamber reactor was used in these reactions. A known weight of graphite was put in the left chamber and a stoichiometric amount of S206F2 (in case of stage one compound, an excess S2O6F2 was needed) was distilled into the right chamber (refer to Fig.2.3) from the S2O6F2 addition trap through the vacuum line by using an external liquid nitrogen bath. After removal of liquid nitrogen, the volatile S2O6F2 vaporized and reacted with the graphite. The reaction mixture was held in the reactor for one week and the products were then maintained in a dynamic vacuum until constant weight was obtained. 57 2.5.2. C12SO3CF3 C 1 2 S O 3 C F 3 was prepared by the solvolysis of C 7 S O 3 F in a large excess of trifluoromethyl sulfuric acid as discribed in reference 93. In a typical reaction, about 10ml of freshly distilled HSO3CF3 was distilled into a reactor containing 200-300mg of C7SO3F. The mixture was magnetically stirred at room temperature for 48 hrs. The excess acid was removed in vacuo and the sample exposed to a dynamic vacuum. When the sample reached constant weight, a blue powdery solid was obtained. 2.5.3. C 1 4 S O 3 F H S O 3 F This compound was prepared previously in our group according to 4 4 14 C + 1/2S206F2 > C14SO3F C14SO3F + excessHSO3F > C 1 4 S O 3 F H S O 3 F In a typical reaction ~300mg of SP-1 graphite was exposed to S2O6F2 vapor for 1 hour. To reduce the vapor pressure of S2O6F2 in order to moderate the intercalation reaction, the reservoir of liquid S2O6F2 was maintained at 0 °C. After the reaction had been stopped and a brief pumping procedure carried out, a blue powder was obtained suggesting the composition C14SO3 F according to weight increase. The product thus obtained was 58 reacted with H S O 3 F for 24 hrs at room temperature. After removal of all excess acid in vacuo, the product became a deep blue free-flowing powder of constant weight. The composition, determined by gravimetry, was C14SO3FHSO3F. 2.5.4. C 8 SbF 6 An excess of SbFs was distilled into a one-part reactor containing a known amount of C7SO3F. The mixture was warmed to 45 °C to facilitate mixing of the reactants and maintained for one week. All the volatile products were removed in vacuo and the solid was exposed to a dynamic vacuum until constant weight was reached41. 2.5.5. (CFx)nS03F (CFx)nS03F was synthesized by the reaction of graphite fluoride CFX t where 0.25 < x < 1.3, with S2O6F2. 200-300mg powdery CFX was put into a one-part reactor and S2 O6F2 was distilled into the reactor (~10ml). After stirring the mixture magnetically for 48 hrs. at room temperature, excess S2O6 F 2 was removed by exposing the product to a dynamic vacuum until constant weight. 2.5.6. (CFx)nHS03F and (CFx)nHS03CF3 These two compounds were obtained by the direct reaction of graphite fluorides with HSO3F and HSO3CF3 respectively. A known weight of powdery CFX was put into a 59 one-part reactor and H S O 3 F or H S O 3 C F 3 was distilled into the reactor through the T-shaped distillation bridge. The reaction was carried out by stirring the mixture of solid (CFX) and liquid (acids) magnetically at room temperature for one week. Excess acid was removed by exposing the reactor to a dynamic vacuum (at least 48 hrs.) until the product reached a constant weight. Because of the high boiling point and low vapor pressure of both acids, extended time was needed when they were distilled or removed. In order to compare the products of intercalated graphite fluorides same reaction conditions were employed for all graphite fluorides and each reaction was repeated at least three times. 60 CHAPTER 3 GRAPHITE FLUOROSULFATES 61 3.1. Introduction Bis(fluorosulfuryl) peroxide, S2O6F2, has recently found use as an intercalant into various forms of graphite. The usefulness of S2O6F2 as an oxidizing synthetic reagent will be illustrated by the following brief summary of its physical and chemical properties. A. Chemical properties of S2O6F2: Bis(fluorosulfuryl)peroxide is versatile. Its molecular structure, presumed to be of C2 symmetry 9 6 , is best viewed as a symmetrical combination of two fluorosulfate radicals linked via a weak 0-0 bond as shown below: ff ff F - S - O - O - S - F o ft in good analogy with the dihalogen molecules. The rupture of the relatively weak peroxy linkage into two free radicals is easily accomplished and the existence of the fluorosulfate radical in equilibrium with its dimer was first suggested by the reversible appearance of a yellow coloration when S2O6F2 is heated. S2O6F2 = " 2S0 3 F» Dudley and Cady 9 7 first synthesized S2O6F2 from SO3 and F2 catalyzed by AgF2 at 180 °C. The evidence for the reversible dissociation equilibrium was shown by measuring the temperature dependence of the pressure at constant volume between 450K and 600K, and the temperature dependence of the absorption of the fluorosulfate radical at 447 nm. The enthalpy of dissociation of S2O6F2 into two free radicals has been estimated to be 92.1 kJ/mol and 97.6kJ/mol by these two methods respectively. A kinetic study 9 8 on this equilibrium has determined the enthalpy of dissociation to be 91.3kJ/mol and a close value is found from temperature dependent ESR measurements on the S03F* radical 9 9 . The electron affinity of the free radical appears to be rather high 1 0 ° . This rather high electron 62 affinity of the free radical makes S2O6F2 a favorable oxidative intercalant which could abstract electrons from the rc-electron cloud of graphite according to: S2O6F2 + 2e" > 2SO3F-forming a very stable fluorosulfate anion which could be intercalated into graphite in the oxidative process. Some of the rather extensive chemistry of S2O6F2 has been summarized by De Marco and Shreeve101. B. Physical properties of S2O6F2: The low boiling, volatile liquid (b.p. =67.1°C and m.p.= -55.4 °C 1 0 2 ) is easily transferred into a reactor containing graphite. Likewise, the removal of excess reagent after the reaction, using standard vacuum line techniques is easily accomplished. The reagent, when kept apart from moisture and oxidizable material, is safe and non-corrosive, which permits the use of glass apparatus. The synthesis of binary fluorosulfates of graphite by the oxidation of graphite by S2O6F2 was first undertaken by Bardett et a l . 1 0 3 and a first stage compound formulated as C8+S03F" was obtained. Hooley104 synthesized a stage one compound with a limiting composition of C7SO3F by using the gas phase reaction and more recently, Karunanithy and Aubke 4 3 , 1 0 5 obtained a stage one graphite fluorosulfate from powdery graphite, HOPG plates and carbon fibers with a limiting composition of C7SO3F by using a direct liquid phase reaction. A stage two graphite fluorosulfate was also synthesized by the same authors 4 3 with a composition of C14SO3F via gas phase intercalation by controlling the vapor pressure of S2O6F2. The bonding structure and properties were studied by using X-ray powder diffraction, 1 9 F solid state NMR, conductivity and Raman spectra 106-108 An objective of our work was to find new methods to synthesize graphite fluorosulfates, especially stage two and stage three compounds and to characterize them by using X-ray powder diffraction and X-ray photoelectron spectroscopy to a greater extent than before. 63 3.2. Stage One Compounds Stage one graphite fluorosulfates have been obtained by using liquid and gas phase reactions as described in sec. 2.5.1. A. Direct Intercalation of Liquid S2O6F2 This is the most frequently used method. Only stage-one graphite fluorosulfate can be obtained by using an excess of liquid S2O6F2 since no suitable solvent, which will not intercalate itself, for S2O6F2 can be found. Intercalation of S2O6F2 in known amounts from an inert solution can not be used. A limiting composition of C7SO3F was obtained in our synthesis according to the weight increase. X-ray diffraction patterns (Fig.3.1. a,b.) indicates that the peak at 29 = 26.58 due to the diffraction of 002 planes in graphite has been shifted to 22.85 . The 002 diffraction peak has the highest intensity and indicates C7SO3F is a stage one GIC because the X-ray diffractogram of stage-n compounds would show the highest intensity for the diffraction due to the [00(n+l)] planes44. B. Gas Phase Reaction In a gas phase reaction, graphite was exposed to S2O6F2 vapor at room temperature. A limiting composition of C7.8SO3F was obtained after reacting for one week and this composition did not change even at prolonged-reaction times. Fig.3.2 shows that the X-ray powder diffraction patterns is same as that of C7SO3F made using liquid intercalation.The highest intensity of the peak at 29=22.15 is due to the diffraction of 002 planes. X-ray diffraction data are shown in Table 3.1. The intercalation separation, Ic, which is the average value of L x d, is 7.8lA compared to 3.35A of graphite. Ir is the relative peak intensity. 64 002 i > i11 • i • i11 • i • i • i • I • i1111111 rrri ' i" 1 ' 1 ' 1 I'1'1'1 1 I 1 " ' I ' ' 1 1 I ' 5. 10. 15. 20. 25. 30. 26 'T' I' 1' |' I' 111' 1' j ' 11111111 11 I 1111 ' 35. 40. 45. 50. 55. Pig. 3.1 x-ray powder diffraction patterns of (a) Graphite (b) C7SO3F 65 002 001 003 004 005 .0 10 15 20 25 30 35 40 45 50- 55 60 2 6 Fig. 3 . 2 X-ray powder diffraction patterns ofC7.eSC»3F Table 3.1 X-ray powder diffraction data of C 7 . R S 0 3 F 00L 0 0 1 0 0 2 0 0 3 0 0 4 0 0 5 2 6 11.15 22.15 34.65 4 6 . 8 5 5 9 . 2 0 d 7 .93 3 . 8 9 2.59 1.9 1.56 Ld 7 .93 7.78 7.77 7.76 7 . 8 0 I , 10.64 100.00 10.58 3 . 0 4 2 . 2 2 66 X-ray photoelectron spectroscopy (see Fig. 3.3) indicates stage one graphite fluorosulfate for a C Is core electron binding energy of 283.1 eV which is ~leV less than graphite (284.0 eV). It is attributed to the effects of Fermi level shift and charge transfer from carbon atom to intercalate group. We will discuss this chemical shift in detail in the next chapter. C7SO3F is very sensitive to moisture. When it is exposed to air, the compound is dissociated to HSO3F, H2S04t HF and graphite. X-ray powder diffraction data (Fig.3.4) show that C7SO3F has been decomposed after exposing it to air for 4 hrs. or treating it with water. The characteristic peak of C7SO3F at 20=22.15 is missing and a broad peak at 20=26.2 which is close to that of graphite is observed. 3.3 Stage Two Graphite Fluorosulfates S2O6F2 is very reactive and heterogeneous reactions with graphite proceed rapidly. It is therefore difficult to obtain a stage two graphite fluorosulfate by means of a liquid phase reaction. Attempt to synthesize this compound by reducing the reaction temperature and time resulted in the form of mixture of stage one and stage two GIC's. For example, when the solid graphite and liquid S2O6F2 were stirred together for only a few minutes at -40 °C the composition obtained after removal of excess S2O6F2 was C12.5SO3F, which is a mixture of stage one and stage two graphite fluorosulfates. The X-ray powder diffraction (Fig. 3.5) of C12.5SO3F indicates that this compound is primarily stage two graphite fluorosulfate because the peak with the highest intensity is due to the diffraction of 003 planes. However, the peak at 20=11.4 and a shoulder peak at 20=22.2 due to the diffraction of 001 and 002 planes of the stage one compound are also observed. 67 lU 11% H* 21 i } M t»« 1*8 H 4 H S 170 Binding Energy (cV) • Binding Energy (eV) •00: »42 Binding Energy (eV) Binding Energy (eV) Fig. 3.3 X-ray photoelectron spectra of C7.8SO3F (Error Limit of BE «± 0.1 cV) I ' l M M ' i ' i ' i M M ' j ' i ' i ' i ' i ' i ' i ' i ' i ' i ' i ' i ' i ' i M ' i ' i ' i ' i ' r i ' i ' i ' i ' i ' j ' i ' i ' i ' i ' i ' r r i ' i ' i ' i ' i ' i ' i ' i ' i M ' i ' i ' i 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60. 26 p'lMM'p't'Vl'I'IMMM'IM'I'I'I'I'I'I'I'I'j'I'I'I'I'I'I'I'I'I'j'I'I'I'I'I'I'I'l'l'I'I'I'I'I'I'I'I'I'I'J 5 . SO. 15. 2 0 . 2 5 . * 3 0 . 3 5 . 4 0 . 4 5 . SO. 5 5 . 6 0 . 26 Fig- 3.4 X-ray powder diffraction patterns of C7SO3F after (a). Exposing to air for 4 hours (b). Treating by water 69 Counts Karunanithy 4 4 obtained a stage two compound with a composition of C14SO3F by using a gas phase reaction, controlling the pressure of S2O6F2. The difficulty with this approach is the determination of a suitable pressure to get the desired compound. Deintercalation may be a useful method to obtain a stage two compound, but it is not easy to control the pyrolysis temperature and time. Sometimes, a mixture of stage one, stage two and stage three GICs may be obtained if heating is not uniform. Furthermore, intercalation is not always and not entirely reversible. Residue compounds obtained during deintercalation at elevated temperature may be chemically different from graphite used as starting material and formation of either graphite fluoride or graphite oxide as impurities from C7SO3F is probable. We have obtained stage two graphite fluorosulfates by means of a gas phase reaction, where controlled amounts of S2O6F2 were reacted with graphite. A stoichiometric amount of S2O6F2 can be transfered into the one-part-two-chamber reactor (see Fig. 2.3) from the addition trap (see Fig. 2.4). The graphite in the left chamber was exposed to S2O6F2 vapor and as the reaction proceeds, the S206F2 in the right chamber would go into the vapor phase continuously until all S2O6F2 was consumed. X-ray powder diffraction (Fig. 3.6), where the highest intensity peak at 26=24.00 is due to the diffraction of 003 planes indicates a stage two compound of composition C15.5SO3F. The diffraction data (Table 3.2) indicates the repeat space distance along the c-axis Ic to be 11.08A. All data of peaks in the patterns due to the diffractions of 001 to 007 planes are in agreement with a stage two compound (Ld values are very close) and shows C15 .5SO3F is a pure stage two compound. 71 003 001 002 004 005 006 007 ' I I I [ I I | ' | ' | I I'l I | I'I'I I | M I I I ' lT I ' l | I I I I | I'l I'l | I'I'I'I j I I I I | I I I I 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60 26 Fig. 3.6 X-ray powder diffraction patterns C15.5SO3F Table 3.2 X-ray Diffraction data of C15.5SO3F OOL 001 002 003 004 005 006 007 26 7.95 16.00 24.00 32.20 41.70 49.50 58.60 d 11.11 5.54 3.71 2.78 2.22 1.84 1.57 Ld 11.11 11.07 11.11 11.11 11.08 11.04 11.02 Ir 18.97 10.74 100.00 15.91 2.00 4.62 1.64 72 3.4. Stage Three Compound A stage three graphite fluorosulfate compound can be obtained using the same approach as used in obtaining stage two compound. This type of compound has not been reported in the literature up to now. We have synthesized a stage three graphite fluorosulfate of the composition of C24SO3F. X-ray powder diffraction patterns (Fig. 3.7) indicate that the peak at 26=24.80 is due to the diffraction of 004 planes and has the highest intensity. X-ray diffraction data (Table 3.3) shows that all the peaks in the patterns due to the diffraction from 001 to 009 planes (the intensity of 003 and 007 reflection are too small to be observed) are responsible for a stage three GIC. The repeat space distance Ic along the C-axis is 14.33A. Both stage two and stage three as well as stage one graphite fluorosulfates follow the relationship: Ic = Is+ (n-l)Co where Is is separation of two bounding carbon layers with an intercalate layer sandwiched between them (~7.8A); Co is interlayer separation in pristine graphite (-3.35 A) and n is the stage index. XPS of stage two and stage three graphite fluorosulfates are very similar to that of stage one compound. There is about leV chemical shift of the C isi/2 core electron binding energy in these graphite fluorosulfates relative to pure graphite. It is also attributed to the Fermi Level shift and charge transfer. Both effects have opposite signs and comparable magnitudes(see next chapter). A small chemical shift in the core level binding energy of graphite fluorosulfates relative to graphite indicates that the bond between the carbon atoms 73 004 • 002 005 008 009 .0 10 15 20 25 30 35 40 45 50 55 BO 26 Fig. 3.7 X-ray powder diffraction patterns ofC24SC»3F Table 3.3. X-ray diffraction data of C24SO3F 00L 001 002 004 005 006 008 009 29 6.15 12.30 24.8 31.30 37.7 51.2 58.1 d 14.35 7.19 3.59 2.86 2.39 1.78 1.59 Ld 14.53 14.38 14.36 14.30 14.33 14.24 14.31 It 12.81 9.86 100.00 17.45 5.27 5.19 2.17 74 and S O 3 F is ionic since the formation of a covalent bond would normally lead to greater chemical shift than that of ionic one. 3.5. Conclusion Graphite fluorosulfates of different stages can be synthesized via a gas phase intercalation by controlling the amount of S2O6F2 reacting with graphite in a two chamber reactor developed for this purpose. X-ray powder diffraction analysis shows C7.8SO3F, C15.5SO3F and C24SO3F are stage one, stage two and stage three graphite intercalation compounds respectively. The highest intensity of peaks for stage "n" compound is due to the diffraction of 00(n+l) planes and the repeat space distance along the C-axis follows the equation Ic = Is + (n-1) C 0 XPS indicates ~leV decrease of the Ciscore electron binding energy in these compounds as compared to that of pure graphite. 75 CHAPTER. 4 THE STUDY OF SOME GRAPHITE COMPOUNDS BY X-RAY PHOTOELECTRON SPECTROSCOPY 76 4.1. Introduction X-ray photoelectron spectroscopy (XPS, also known as electron spectroscopy for chemical analysis or ESCA ) is a widely-used technique for the study of electronic properties of atoms, molecules, solids and surfaces. The first XPS experiment was performed by Robinson and Rawlinson 1 0 9 in 1914. From the first observations by Steinhardt and Serfass110' that core photoelectron peak intensities could be used for quantitative analysis and core electron binding energies exhibited chemically-induced shifts depending on the chemical environment of that element111. The number of publications dealing with XPS is quite large, including several reviews 1 1 2 - H 5 . >ye will outline the basic principles of this technique and discuss some applications of XPS relevant to the study of graphite intercalation compounds. The basic process in XPS is the absorption of a quantum of energy fro such that A + hi) > A+ + e- (4 -- 1) where Einstein's relation predicts that T v = ha) -E B v ( i ) (4-2) Here, V1 is the measured kinetic energy of the ejected photoelectron, referenced to vacuum level and EB v(I) is the binding energy of the electron in the ith orbital of atom A, referenced to the same vacuum level. For most commercial XPS instruments hi) is either 1256.3 eV generated from a Mg K a anode or 1486.7 eV from an Al Ka source (we use this source), although other energies are also available. 77 Measurement of core-level binding energies for gas phase molecules is generally accomplished by measuring the kinetic energy of the photoelectron relative to a standard species with a precisely known ionization potential116. Unfortunately, no such standards are available for solids. A convenient approach for solids, such as metals, is to reference E B to its Fermi level. In a metal at absolute zero, the Fermi level Ep is defined as the highest occupied level separating the valence from the conduction band as indicated in Fig. 4.1 (a). This interpretation of Epis also very nearly true for metals at normal experimental temperatures. For semiconductors and insulators, however, it is not so simple to locate the Fermi level, which lies somewhere between the filled valence bands and the empty conduction bands because both bands are now separated by an energy gap E g as indicated in Fig.4.1 (b). The work function <(>s for a solid is defined to be the energy separation between the vacuum level and Fermi level. In this case, a Fermi level referenced binding energy E B f can be calculated when both the work function (J>s and the energy of the exciting radiation ho are known according to E B F = hi) - T v - <|)S (4 -3) The feature of electron spectroscopy of interest to chemists is the effect of the chemical environment of a given atom in a solid on the binding energy E B f . It seems to be theoretically and experimentally well established, that the chemical shifts of core levels are relatively the same for all levels of a given atom in a given environment. The exact value of the binding energy measured for a given element depends on the chemical environment of that element. For example, 2p level of Al has a binding energy of 72.6 ±0.1 eV in Al metal relative to the Fermi level and an energy of 75.3 ±0.1 eV in AI2O3. This shift of 2.7 eV is typical for the chemical shifts encountered, when the oxidation state of an atom is changed by several formal charges. Therefore, XPS can be used to identify the chemical 78 (a) Metal (b) Semiconductor or Insulator Fig . 4.1 Density of Occupied and Empty State and the Defintions of Fermi Level Ep and Work Function <[)s 79 environment and the oxidation state of an element by comparison with the binding energies of a set of reference compounds involving the same element More important is a thorough understanding of the factors affecting the chemical shifts and the type of physical and chemical information contained in these shifts. XPS is viewed as a suitable technique for the study of graphite compounds for the following reasons: (1) all graphite compounds, ranging from intercalation compounds (GIC's) to covalent compounds such as CFX are solids. (2) the carbon Is core electron is sensitive to changes in electronic charge density near the carbon atoms, bonding structures and the Fermi level. There are a few previous studies of GIC's by XPS 1 1 7 - 1 2 1 , Most of these studies, published during the last decade, involve donor GIC's. We will show XPS spectra of some graphite acceptor intercalation compounds, made in our group, and use these spectra to describe bonding and structure in these compounds. 4.2. XPS of graphite fluorosulfates Details on XPS instrument, measurements and the sample preparation have all been described in the experimental section. Graphite fluorosulfates were made as described in 2.5.1. Stage one compounds made by reactions either in the liquid or in the gas phase give identical XPS spectra. A sample of C7SO3F, made in the liquid phase, is used to illustrate some properties and features of XPS spectra. Information is primarily derived from shifts in the C i s binding energy measured relative to Fermi level, E F . These shifts are due to both initial and final state effects. In the 80 initial state, charge transfer to the intercalate acting as an acceptor shifts the binding energy by direct electrostatic interaction between the intercalates and carbon atoms. On the other hand, charge or electron removal from the graphite lattice by acceptor intercalate decreases the density of states and causes the Fermi level to move relative to the core levels. These two initial state phenomena have effects of opposite direction and are of comparable magnitude. In the final state, the chemical shift is due to changes in the screening of the core hole produced by photoemission. In general, when the amount of mobile charge increases, the measured binding energy is reduced. The XPS spectra of pristine SP-1 graphite and its C7SO3F derivatives are shown in Fig. 4.2. The C i s core electron binding energies are 284.0±0.1 eV and 283.1±0.1eV for the graphite and C7SO3F respectively. The asymmetry and the long tail toward higher binding energies are due to electronic excitation in the final state 122,-123^  The line shape is more asymmetrical in the case of C7SO3F than that of the graphite, as a result of stronger final state excitation. The observed decrease in the C i s binding energy in the intercalation compound C7+S03F", compared to graphite, seems in conflict with general expectations because positive ions generally have a greater core electron binding energy than the corresponding neutral atoms as a result of the reduced mobile charge. In this case, the effect of the Fermi level motion due to a decrease of the density of states in the 2p orbital plays a major role and this motion has the opposite effect from the electrostatic effect as shown in Fig.4.3. The electrostatic effect contributes to an increase of about 0.6 eV (we will discuss this value later) in the C i s core electron binding energy. However, the Fermi level shift causes a decrease of about 1.5 eV and we hence observe a net decrease of 0.9 eV in the binding energy in C7SO3F relative to graphite. 81 8000 -I 0 1 i i i i i i i i i [ i i i i i i i i i i i i i i i i i i i i i \ i i i i i i i i 274 279 284 289 294 Binding Energy (eV) Fig. 4.2 XPS spectra of graphite and C7SO3F 82 284.6 y-_ C7SO3F chemical shift v by electrostatic effect, 0.6 eV 284.0 Graphite BE*284.0 eV 282.5 Final state, BE«283.1, chemical shift 0.9 eV Fermi level motion, 1.5 eV Fig.4.3 Two effects on the chemical shift of the C Is electron binding energy, Fermi level shift and electrostatic attraction In order to get an accurate estimate of the chemical shift due to intercalation, the value of Fermi Level shift has to be measured or estimated. One effective method used is to coat a gold layer on the fresh surface of the sample 1 2 4 and measure the binding energy of the Au 4f7/2 level. The shift on measured binding energy of the Au 4f7/2 level of the gold coating will give the value of the Fermi level shift, because the binding energy of Au will shift by an equivalent amount as that of samples due to an equilibrium between the gold level and the samples. This method was successfully used to obtain the Fermi level shift of Li-graphite intercalation compound by Wertheim et al. 1 1 9. They found a 1.6+0.2 eV shift on Fermi level toward higher binding energy for LiC6. However, there are problems with our samples when this method is used. C7SO3F is very sensitive to moisture, and it is difficult to obtain a very fresh surface when coating the gold on it by sputtering, additionally, a reaction between the C7SO3F and gold has been observed. This can been seen from Fig. 4.4. The binding energy of standard Au Afip, is 83.4 ±0.1 eV (Fig. 4.4. a) in our experiment. After gold was coated on a C7SO3F surface for 4 hrs. the binding energy shifts to 82.7± 0.1 eV (Fig. 4.4. b), a decrease of about 0.7 eV of the binding energy is apparendy the result of a Fermi level shift, but it is not a reliable value for the Fermi Level shift, since the binding energy of the same sample after 24 hours has shifted back to 84.3±0.1 eV (Fig.4.4 c ). The increase in binding energy is attributed to a reaction of the Au film with C7SO3F. A number of stable gold compounds such as Au(S03F)3 1 2 5 « 1 2 6 have been studied by our group. Therefore the measurement should be carried out as soon as possible after the coating is applied. Unfortunately, it takes at least 3 hrs. to achieve a suitable vacuum needed for the measurement after samples are inserted into the sample chamber. It is hence not possible to obtained a reliable value of the Fermi level shift by this method. No other methods appear to be recommended to obtain the Fermi level shift. 84 3*00 -3 •^oo i M i . i r*i 1111111 11111111111 73 76 83 88 93 Binding Energy (eV) (c) C7S03F(Au), 24 hours after coating Fig. 4.4 XPS spectra of C7SO3F-AU AS In order to estimate a value of the Fermi level shift, we have carried out a XPS experiment on K S O 3 F . Here sulfur (S) has a similar chemical environment as is in C 7 S O 3 F 1 0 8 . However, the reference compound K S O 3 F is according to its crystal structure ionic and viewed as consisting of K + and SO3F" ions. The electron transfer factor f reflecting the transfer from K to SO3F is assumed to be 1.0. In the case of C7SO3F, a fractional electron transfer factor f is expected since we have neither true Cj+ cations nor SO3F" anions. The fractional electron transfer from graphite to SO3F causes (a) a shift of the Fermi level for graphite relative to pristine graphite and (b) a shift of the Fermi level for the SO3F group relative to SO3F". Both shifts on first approximation are similar. Fig. 4.5 is the XPS spectra of the S 2p in C7SO3F and KSO3F. There is a 1.5 eV decrease of S 2p binding energy in C7SO3F relative to the KSO3F. We consider that this difference is close to the Fermi level shift value in C7SO3F. LiSC»3F and Sn(SC«3F)2 have similar results as shown in table 4.1. Besides S 2p3/2, both O Is 1/2 and F Is 1/2 binding energies in the KSO3F, LiSO^F and Sn(SC«3F)2 are all greater than that of C7SO3F, (see table 4.1). Obviously, this difference results from Fermi level shifts in C7SO3F vs these compounds as discussed in detail for sulfur. The XPS spectra reported here can also supply additional information on the bonding and structure of graphite fluorosulfates. A. Only a single peak for the O lsi/2 shows that the three O atoms in C7SO3F are equivalent and in the same chemical environment B. The difference in binding energy of O lsi/2 and F lsi/2 among the four compounds show both F and O atom are affected by different cations such as Oz8*, K + , Li+ and Sn 2 + . 86 Fig. 4.5 XPS of KSOsF and C7SO3F K7 Table 4.1. Binding Energy (±0.1 eV) of the Elements in SO3F for Different Compounds compound S2ps/2 OlSi/2 Flsi/2 C 7 S O 3 F 167.6 530.8 686.6 K S O 3 F 169.1 532.0 688.0 J J S O 3 F 169.1 532.1 687.0 Sn(S03F)2 169.1 532.2 687.9 f " " c; (V) Ie (cdc): 7,63 & 4.46^ Ic(cxp): 7.61 I k. •c; loyer — — Fig. 4.6 Structure model of C7SO3F (from reference 107) C. The chemical shift of C Is 1/2 core electron binding energy in C7SO3F relative to graphite is small (1.5 - 0.9 = 0.6 eV). This fact shows that the bonds between the carbon atoms and intercalate groups are not covalent because covalent C-0 or C-F bonds have a greater chemical shift. The results obtained support the structure model shown in Fig. 4.6, described by Karunanithy and Aubke 1 0 7 . This structure can be described as CXSXCXSXC, where, X=F or O. 4.3. C12 SO3CF3 The intercalation compound C12SO3CF3 was described in section 2.5.3. The XPS spectrum (Fig.4.7) of this compound shows two peaks for the C Is 1/2 core electron binding energy. The peak at BE (binding energy)= 283.7+0.1 eV results from carbon atoms of graphite and is in essence similar to the BE for C 7 S O 3 F . The peak of BE = 290.7±0.1 eV is attributed to the - C F 3 group of the intercalate. There is a 0.3 eV chemical shift of the binding energy for the first peak of the spectrum relative to that of graphite. This is also attributed to two opposite effects. Fermi level lowering leads to lower binding energies while electrostatics effects suggest higher binding energies. We have compared the binding energy of S 2p electrons in C12SO3CF3, K S O 3 C F 3 and AgS03CF3 as in the case of C7S03F(see table 4.2). There is about 0.7 eV difference in C12SO3CF3 from the other two compounds. This difference indicates that chemical shift of binding energy due to the charge transfer from carbon atoms into intercalates is ~0.4 eV as shown in Fig. 4.8. 89 Table 4.2. Binding energies (±0.1 eV) of SO3CF3 in some compounds Compound C12SO3CF3 AgS03CF3 KSO3CF3 S IS3/2 O lsi / 2 168.3 169.0 168.9 531.8 532.2 532.0 F l s 1/2 687.0 688.1 688.0 6D00-4000-2000-274 Binding Energy (eV) 1 11111111 1 1 1 294 Fig. 4.7 XPS spectrum of C12SO3CF3 90 •i j 0.7eV 284.0 eV 284.0 eV 283.7 eV Graphite BE* 284.0 eV "I To.4eV C12SO3CF3 electrostatic effect chemical shift: 0.4eV Fermi level motion: 0.7eV Final state, BE«283.7 eV Ev=vaccum level EF=Fermi level 4>s = work function Fig. 4.8 Electrostatic effect and Fermi level motion in XPS ofCi 2S03CF 3 91 4.4. CFX The bonding and structure of graphite fluorides have already been discussed in section 1.4. The bonds between carbon and fluorine atoms in these compounds are considered to be covalent. Carbon atoms become sp3 hybridized from sp2 after the formation of a C-F bond. XPS of CFX illustrate this change as seen in Fig.4.9. There are two peaks in the spectrum of CFn.5 (Fig. 4.9 a) for C Is core electron binding energy. BE=284.1 eV is attributed to those carbon atoms which have not reacted with fluorine and are viewed as sp2 hybridized. A second binding energy of 289.2 eV is due to carbon atoms which have reacted with fluorine resulting in a covalent C-F bond formation. Therefore it appears hence that there are both sp2 and sp3 carbons in CF0.5. There is approximately a 5 eV chemical shift in binding energy relative to graphite after the formation of the covalent bond or sp3 carbon. This value is much grater than that of C7SO3F or other GICs in which the bond between carbon and intercalate groups are ionic where carbon atoms are the sp2 hybrids119-120. Therefore it is possible to identify the bonding environment of carbon in graphite intercalation compounds by XPS analysis and differentiate clearly from sp3 carbon in CFX. A single peak at BE = 687.8 ±0.1eV has been observed in the XPS spectrum of F lsi/2 core level in CFX as shown in Fig. 4.9 b. This peak is attributed to the covalent fluorine atoms. In the XPS spectra of CFi, two peaks are observed for the C Is core electron binding energy as shown in Fig.4.10(a). The intensity of the higher energy peak appears to be much greater than that of the lower one. It indicates that most of the carbon atoms in CFi have reacted with fluorine but there is still a small amount of carbon atoms without change in binding energy relative to graphite. It is also possible that some -CF2 groups may have formed and exist in this compounds. However, the amount is too small to be analyzed. 92 eooo-i O | i i i 11 i i i i i I I i 1 1 i i i i i ' i i i i i i i ' i i i i i i i i i i i 274 279 284 289 294 Binding Energy (eV) Binding Energy (eV) Fig. 4.9 X P S spectra of CFQ.S 93 8000-1 6000' I 4000H 2000 H (a) CHi /2 0 1 1 I I I I I I I | I I 1 » I I I I I | I I I I M 1 1 I | I I I I I I I I I | 274 279 284 289 294 Binding Energy (eV) 2500-j 2000 H 1500H c o u 1000 500-3 3 0 1 i i i i i i i i i i I I i i i ' ' ' ' I ' ' » i » i i i ' i i I I I I i 11 » i 676 681 686 691 696 Binding Energy (eV) Fig. 4.10 XPS spectra of CF^ 94 A single peak at BE=688.3 ±0.1eV for F Is core level is also observed as shown in Fig.4.10 b. There is an increase of 0.5 eV in binding energy in CFi compared to CF0.5. This increase is due to the effect of neighboring carbon atoms which are linked covalently to fluorine atoms. Fig. 4.11. shows the spectra of (C2F4)n (Teflon). This compound is obtained by the polymerization of tetrafluoroethylene according to n F F F F There are two fluorine atoms bonded to each carbon atom, therefore, the Cis core electron binding energy is 292.1±0.1eV (Fig. 4.1 la), which is greater by 2.3eV than that of CFi. The XPS spectrum of F Is (Fig. 4.11b ) shows a single peak at BE = 689.1 eV. This is an increase of 0.8 eV in binding energy for (C2F4)n compared to CFi. This change is explained by the fact that each carbon atom is bonded to two highly electronegative, electron withdrawing fluorine atoms. As shown in table 4.3. the binding energies of both the Cis and Fls core electrons increase in compounds of the general composition CFX with increasing x. Binding energies of CFx's CFX BE of Cis (±0.1 eV) BE of Fls (±0.1 eV) CF0.5 289.2 687.8 CF1.0 289.8 688.3 (C2F4) 292.1 689.1 95 4.5. Other Graphite A c c e p t o r Intercalation Compounds W e have o b t a i n e d X P S s p e c t r a o f some o t h e r a c c e p t o r G I C s . T h e y are C i 4 S 0 3 F « H S 0 3 F , C 8 S b F 6 , and C g A s F g as s h o w n i n F i g . 4.12 4.14. The C i s B E o f these compounds are s h o w n i n table 4.4. T h e results show: (1) N o co v a l e n t bonds are f o r m e d between the graphite carbon and intercalates i n these compounds because there i s o n l y a sli g h t c h e m i c a l shift r e l a t i v e to that o f graphite. (2) F e r m i l e v e l shift plays a major r o l e i n the changes o f B E due to the f i n a l state c h e m i c a l s h i f t to l o w e r B E f o r a l l the compounds compared to p r i s t i n e graphite. (3) A l l GIC's except C i 4 S 0 3 F « H S 0 3 F have a slig h t h i g h e r B E than that o f C7SO3F as a result o f less F e r m i l e v e l shift i n these G I C s than that i n C7SO3F, i n d i c a t i n g that more charge has been removed f r o m graphite carbon i n C7SO3F than other G I C s . Tabl e 4.4. C i s b i n d i n g energies o f graphite and G I C s Compounds B E (±0.1 e V ) Graphite 284.0 C7SO3F 283.1 C14SO3FHSO3F 283.1 C12SO3CF3 283.7 C 8 S b F 6 283.6 C 8 A s F 6 283.7 97 i i , i i , , , i i . . i i i T n i i i | i i i i i l l li | ; i i i i i i i i • i i i • i i i i i i | • • 522 S27 532 537 542 6'6 M l 666 Fig. 4.12 XPS spectra of Ci4S03F-HS03F Counts 5 X Ol VI x> a o O O S (/> cr -n o to? 4 <D OJ 3' n. 3' «n to V r -M O O o o J to-J 4» Counts fn rT o • n to > V) c U t c TO c o i n CN to o • n m 111111H1111111111 n 111111 n 04 o o o • r t i n i n «N r» CN «o <r | i i i o o T T T I I I | I I I I O o i n i i i i i Ti i o o • n CN ai CN Ol CO -»\ o» ** t> CO c MUi o» c c in cn CN 1 1 1 1 1 1 1 1 1 1 1 r i I T I o o o 9 i n CN < O O u O s t* CO a. x S 8 S)UAOQ 4.6. Conclusion XPS is a suitable technique for the analysis of graphite intercalation compounds. It can be used to observe charge transfer between the carbon layer and intercalate layer and to identify the formation of covalent or ionic bonds. For graphite acceptor intercalation compounds, two effects have been observed, (a) Charge transfer leads to higher binding energy (b) Fermi level shift makes the binding energy lower relative to graphite. 101 CHAPTER 5 GRAPHITE FLUORIDES AND THEIR INTERCALATION COMPOUNDS 102 5.1 Graphite Fluorides Graphite fluorides are solid, layered compounds of variable composition ranging from CFX, x = 0.25 - 1.2, obtained by the direct reaction of graphite with elementary fluorine at high temperatures. With their composition, their color varies from black and dark gray materials of the composition CFo.25 to near white compounds of the approximate formula CF1.2 . Two crystalline forms, poly(carbon monofluoride), (CF)n and poly(dicarbon monofluoride), ( C 2 F ) n are known. The structure and properties of these compounds have been described in section 1.4. Graphite fluorides used in this study were obtained commercially and analysized by IR, X-ray powder diffraction and X-ray photoelectron spectroscopy. Fig. 5.1 shows the IR spectra of graphite fluorides. Only a single strong band at ~1220cm-1 is observed and assigned to the C-F stretching vibration. The intensity of the peaks decreases as the value of x decreases and the band is difficult to observe when x<0.5, because of the dark color. The X-ray powder diffraction patterns of some graphite fluorides are shown in Fig. 5.2. A peak at 26=26.58 is observed in all instances of CFX except for x=l .This line has the same position as it does in graphite and the relative intensity of the peak decreases as the x value increases. This observation indicates that there are still some graphitic islands or domains present in CFX when x<l and that the graphitic islands will decrease when the amount of fluorine increase. When x>l, in extensively fluorinated samples, this peak is missing. On the other hand, lines at 29 = ~ 13-18 and ~40-45 are observed , with the relative intensity increasing as the fluorine content increases. The two peaks are from the diffraction of 002 and 100 planes of graphite fluorides, indicating the presence of 103 wavenu mbe r/cnr1 Fig. 5.1 IR spectra of graphite fluorides I I ' l l I ' l ' I ' I ' I ' I ' I ' ITT 5. 10. 15. 20. ) ' ' ' ' ' l ' l ' j ' l ' l ' l ' l ' j ' r i ' I M ' j ' l ' l ' l ' l ' j ' l ' l I ' l ' j ' l ' i y j ' j I T I ' l ' j T I I'I'j IM'I'I | 40. 45. 50. 55. 60. 25. 30. 35. (a) CFg.25 c s 6 ['I'l'I'I'j'rriM'JM'IMM'j'IM'I'I'j'I'ITl'j'ITI'I'I'I'I'I'I'j'I'I'I'I'j'I'I'l I ^ 1111111' j' 11111' I' j 5. 10. 15.. .20. 25. 30. 35. 40. 45. 50. 55. 60. (b) CF 0 f Rg. 5.2 X-ray powder diffraction patterns of CF X 105 M M M - ] • I • I M M ' |-1 M M M • I • !• !• IM • I • IM • I • 1 • I • I' I • I • I ' l ' I - 1 ' I • !• I-1' I • I • !• I • I • I' !• I' !• I' !• I • 111 • I 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60. (d) CF 1 > 0 Fig. 5.2 continued 106 graphite fluoride islands in CFX. Obviously, when the fluorine content increases, the amount of graphite fluoride increases as well. The powder diagrams also indicate that the peaks at 20=~17 have shifted to ~15 beyond x>0.5. This difference is caused by the different structures of poly(monocarbon fluoride), (CF)n and poly(dicarbon fluoride), (C2F)n as described in section 1.4. Selected XPS spectra of the C Is level in graphite fluorides are shown in Fig. 5.3. Two peaks are observed in every spectrum. The peak at high energy with a Is core electron binding energy BE=~290 eV is assigned to covalent C-F groups, generally sp3 hybridized carbon and the low energy peaks at BE=~284 eV indicates the presence of sp2 carbon. As the fluorine content increases, the relative intensity of high energy peaks increase, indicating the increase of the amount of C-F bonds. This result is in agreement with that of X-ray powder diffraction analysis. The binding energies in Fig. 5.3 show a small increase as the fluorine content increases in these compounds eg. 282.8 eV -> 283.5 eV -» 284.7 eV for the sp2 carbon, as a result of increased amount of neighboring C-F bonds as shown below: BE of the middle C: The XPS spectrum of CF0.37 allows one additional comment: The composition would suggest an approximate ratio of sp2 : sp3 carbon of 2 : 1, however, the observed intensities are seemingly close to 1 : 1. This suggests: (a) peak intensities may not be a reliable clue for the composition, (b) other factors may contribute to relative intensity such as local effects, which refers to the exact position of the various carbon. It should be C F V 107 • 0 0 0 3 • 0 0 0 4 0 0 0 toco 274 2*9.7 eV C Hi/2 283.5 eV i U e 1 , 4 2 f i 9 > I 2B4 (a) CF0.25 CM CF0.37 Fig 5.3 XPS spectra of CF, m remembered that the CFX compounds are not homogeneous compounds of a unique and precise composition, (c). sp3 carbon may arise from the formation of C-F or C-C bonds. In summary, IR spectroscopy provides a simple, rough test for the presence of C-F groups in the graphite fluorides, while both X-ray powder diffraction and XPS allows structural conclusions depending on the fluorine contents. There are both graphitic and fluorocarbon zones of varying extent within the layer structures. It should hence be possible to intercalate suitable acceptors into these graphitic zones with the extent of intercalation depending on the extent of the graphitic zones originally present 5.2. Graphite Fluoride - S2O6F2 Intercalation Compounds As described before, S206F2 is a useful oxidizer towards graphite to form acceptor graphite intercalation compounds of various stages. It should be possible to react S2O6F2 with graphite fluorides because graphite fluoride is also a layered compound and there are graphitic islands in CFX when x<l, which should be better accessible due to initial fluorination. This conclusion is confirmed and intercalation appear to be demonstrated according to weight increase and microanalysis after the reaction of CFX with S2O6F2 as described in section 2.5.5. The result of microanalysis is very close to that of weight increase, especially S elemental analysis. For example, CFn.25 -S2O6F2 intercalation compound has S=14.20% according to the weight increase and 14.04% from elemental analysis. The compositions of CFX-S206F2 intercalation compounds are shown in table 5.1. 109 Table 5.1 Composition of CFX-S206F2 Compounds CF X W C F X A W CF0^5 0.1930 0.1496 CFo.5 0.1259 0.0783 CFo.83 0.1998 0.0443 CFi.oo 0 . 2 5 3 2 0 . 0 0 7 0 A W / W C F X Suggest Composition 0.7751 (CF0^5)7.6SO3F 0 3 8 6 2 (CF0^)7.9SO3F 0.2217 (CF0.83)16.1 SO3F 0.0276 (CF)ii5.5S03F where, W C F X = initial, weight of CF X ; A W = increased weight after reaction with S2O6F2 followed by the drying of the sample. The intercalated amount of S2O6F2 decreases as the fluorine content increases, obviously because both amount and size of graphitic islands decrease with increasing the fluorine content in CF X . Therefore, there is a maximum amount of reacted S2O6F2 for graphite in which x=0 and minimum amount for poly (carbon monofluoride) in which x=l. A small weight increase of the CF1.00 after stirring with S2O6F2 indicates that a reaction has occurred between the two compounds even though the composition CF1.00 does not suggest the presence of large or many graphitic zones. It should be however recalled that completely fluorinated graphite will reach a composition of CF1.2. Fig. 5.4 shows x-ray powder diffraction diagrams of graphite fluorides-S206F2 intercalation compounds. The peaks at 2 6 = 2 6 3 8 , which are responsible for graphite islands have shifted to 26=22.5, which is close to that of graphite fluorosulfate C7SO3F, indicating the formation of a stage one intercalation compound. The repeat space distance of 110 ' IMM M • IM M • IM' j M M M' I • I • I' I' I' I ' I' 1 ' 1M' I ' j • 1' I • I • 1 • 111' I' I • I 'j' I' I* I' I' IM • I' I' 111' I • I • I' PIM 1 1 ' I ' l ' 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60 (a) (CF 0 . 25)7 .6SO 3 F 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60 (b) (CF0.5)7.9SO3F Fig. 5.4 X-ray powder diffraction patterns of CFX-S2O6F2 intercalation compounds in (d) (CF1.0)1i5.5SO3F Fig. 5.4 continued 112 products has increased to 8.4 A, as calculated by the X-ray diffraction data of (CFo.5)7.9S03F which is shown in table 5.2 below: Table 5.2 X-ray Diffraction Data of (CF0.5 )7.9 SO3F 26 00L d Ld 11.2 001 8.4 8.4 22.5 002 4.2 8.4 33.9 003 2.8 8.4 46.0 004 2.1 8.4 The repeat distance Ic = 8.4 A ,which is the average value of Ld A slighdy longer repeat distance in CFx(S03F)n than in C7SO3F may be due to the effect of the C-F bond.The highest peak is due to diffraction from the 002 planes, indicating that the products are stage one intercalation compounds. (CFo.25)7.6 SO3F and (CFn.83)i6.1 SO3F have the same results as that of (CFn.5.)7.9 SO3F. Two small peaks at 26 =24.0 and 36.0 are observed in (CFi .00)115.5 SO3F attributed to the diffraction from the 002 and 003 plane respectively. The repeat space distance of 7.9 A, indicates that intercalation has occurred during the reaction of CF1.00 with S2O6F2. XPS spectra of the intercalation compounds CF0.37-S2O6F2 and CF0.83-S2O6F2 are shown in Fig. 5.5. A significant change has apparently taken place in regard to the relative intensity of the two peaks for Cis core electron binding energy. The intensity of 113 B O O O i 6000 H 8- CF0.37-S2O-6F2 4000 H 2000 H 0 1 • . . . M . | . . . . . . • I ' M • ' M " " " ' ' ' 274 279 264 259 6303 -I . Binding Energy (eV) 284.3 C IS1/2 b. CF 0.83-S2O6F: 289.7 Fig. 5.5 XPS spectra of CFx-S20eF2 intercalation compounds (Error Limit of BE = ± 0.1 eV) 114 the low energy peak at BE of ~284 eV has increased relative to the higher energy peak for both intercalation compounds compared to CFn.37 and CF0.83 respectively. This change may be attributed to the strong electron accepting ability of 'SO^F radical. Similar change is also noted in other CFX-S2O6F2 intercalation compounds. It is shown that the C-F covalent bond in CFX has been influenced by the intercalates. A possible interpretation is that the C-F bond has been more extensively polarized by S2O6F2 according to C-F + 1/2 S206F2 > C^-F5-(S03F) It is interesting to note that a similar binding energy of 284.3 eV is reported by Nakajima8^ for intercalated fluorine. Likewise a more extensive polarization of carbon-carbon bonds may have occurred. The carbon atomic hybrid orbital has transformed from sp3 to sp2. However, the proceeding discussion has cast some doubt on such a simple interpretation of the relative band intensity. It nevertheless seems safe to conclude that intercalation has resulted in a more enhanced band for the sp2 carbon region, even though the exact reason may not be entirely clear. It seems however unlikely that intercalation of S2O6F2 into CFX has resulted in intercalated fluorine. XPS spectra (Fig.5.6) of (CFo.83 )i6.lSC>3F and KSO3F for S 2p3/2 and O lsi/2 core electron binding energy show identical chemical shifts which indicates that the intercalate group in this intercalation compound is SO3F. Fermi level motion is not observable in these compounds. Fluorine Is core electron binding energy also indicates that in the above cases, the C-F bond may be polarized due to intercalation as shown in table 5.3. 115 Fig. 5.6 XPS spectra of (CFo.83) 16.1 S0 3 F and KSO3F (Error Limit of BE = ±0.1 eV) 116 Table 5.3 F Is 1/2 Binding energy of the Compounds Compound LiF CFn.83 CF1.00 CF2.o (CF0.83 )l6.1 SO3F BE(eV±0.1 ) 684.9 688.3 688.3 689.1 686.8 It is noted that the binding energy of the F Is core electron in (CFn.83 )i6.l SO3F is between that of the ionic LiF and covalent C F X . Only a single peak attributed to F i s is observed for (CFn.83 )i6.l SO3F and the peak position may well be influenced by the binding energy of F bonded to sulfur. Thermal deintercalation of (CFo.83)i6.lS03F was carried out as described in section 2.3E. When the compound was heated in a dynamic vacuum, some intercalates can be removed and a residue compound is obtained. XPS spectra of the residue compound of (CFrj.83)l6.1 SO3F is shown in Fig. 5.7. When the compound is heated at 90 °C for 4 hrs, the relative intensity of high energy peak at BE of -289.5 eV increases, indicating that some intercalates have been removed and when it is heated at 200 °C for 4 hrs, almost all SO3F intercalates are lost because XPS spectrum of this residue compound is similar to that of CFQ.83. 117 8000-i (a) heated at 90°C, 4hrs. 0 i i i i i i i i i i i i i i i i i i i i 'I i i i i i i i i i i i i i i i i i i 274 279 284 289 294 Binding Energy (eV) 8000 -i 6000 H CT) § 4000H o o 2000 H C1SV2 (b) heated at 200°C, 4hrs. 0 i i i i i i i i i i ' i i i i i i i i i i i i i i i i i i i i i i i i i i i i i 274 279 284 289 294 Binding Energy (eV) Fig. 5.7 XPS spectra of the residue of (CFo.83)i6.lS03F after deintercalation m 5.3 Graphite fluoride -HSO3F intercalation compounds It is generally accepted that most of the protonic acids have very limited oxidizing ability. In the case of HSO3F, the self dissociation equilibrium: HSO3F ^=r HF + S03 (5.1) lies far to the left at room temperature, where rigorous purification of the acid has removed residual amounts of SO3 which is often present in technical grade acid. Therefore, H S O 3 F does not intercalate into graphite itself to any significant extent, unless the graphite lattice is oxidized electrochemically or by an external chemical oxidizing agent such as Cr03 or S2O6F2. Electrochemical intercalation of H S O 3 F was first reported by Ubbelohde et al.4 0. The electrolysis of anhydrous H S O 3 F using graphite as an anode, apparently yielded a stage one compound. An attempt to intercalate pure H S O 3 F without any oxidizing agent present, resulted 4 9 only in a material claimed to be a fifth stage compound with a reported layer repeat distance Ic of 21.35 A. The same authors reported that the addition of Cr03 to H S O 3 F led to a stage one intercalation compound of the composition claimed to be C5+1HSO3F with a c-axis layer repeat distance of 8.04 A. It is, however, unclear, whether the intercalate is H S O 3 F as claimed or the SO3F" ion where only weight increase data are provided. Karunanithy 4 4 synthesized graphite acid fluorosulfate by successive intercalation of first S2O6F2 and then H S O 3 F or more precisely by the addition of H S O 3 F to C14SO3F with a composition of Ci4S03F«HS03F . The interlayer separation was calculated to be 7.83 A. We have tried to synthesize graphite fluoride-HS03F intercalation compounds based on the following: 119 A. There appear to be some graphite islands in graphite fluoride CFX when x<l. B. Some carbon atoms have been oxidized by fluorine in CFX, similar to the addition of an oxidizing intercalant to graphite. C. Some of the layer separations have increased due to fluorination and F--F repulsion. In this study it is noted that intercalation compounds are synthesized during the reaction of CFX with HSO3F as described in section 2.5.7. The compositions of different CFX-HS03F intercalation compounds calculated by the increased weight are shown in table 5.4. Table 5.4 Composition of CFX-HS03F intercalation compounds CFX WCFX A W AW/WCFX Composition CF0.25 0.2154 0.0280 0.130 (CF0.25)45.9HSO3F CFn.33 0.1709 0.0366 0.2142 (CF0.33)25.6HSO3F CF0.5 0.1527 0.0459 0.3005 (CFo.5)l5.5 H S O 3 F CFo.83 0.1857 0.0289 0.1556 (CF0.83)23.lHSO3F CF1.0 0.2947 0.0082 0.0278 (CFi.o)in H S O 3 F where, WCFX = weight of initial CFX A W = increased weight after reaction (weight of HSO3F in products) 120 The result shows that the ratio of AW/WCFX initially increases as the x value increases but when x=0.5 the ratio reaches a maximum value, and then decreases as the x value increases. Two reasons may be responsible for this result. First, graphite can be intercalated by a large amount of H S O 3 F only when it is oxidized by an oxidizing agent either during or prior to the intercalation reaction. As the x value increases , more carbon atoms in graphite have been oxidized by fluorine, and therefore more H S O 3 F can intercalate into graphite fluoride. However, when x>0.5 only small amounts of graphitic islands exist in graphite fluoride and the intercalation amount of H S O 3 F is limited. The second reason may be due to structural effects in CFX. The interlayer of CF0.5 is more open than that of CF0.25 so that the intercalant enters more easily. The behavior found here differs from the intercalation of S2O6F2 into CFX discussed and the difference is apparent from Fig. 5.8. We are in essence looking here at an intercalation reaction into a layer lattice which has been oxidized by fluorination. X-ray powder diffraction patterns (Fig. 5.9) of (CFX )nHS03F indicate graphitic islands in CFX have been intercalated by H S O 3 F . The peak at 20=26.58 due to the diffraction of graphite lattice is missing and some new peaks attributed to intercalation compounds are observed. X-ray diffraction data (Table 5.5) show that the (CFo.25)45.9HS03F is primarily a stage two compound because the highest peak intensity at 20 = 23.8 (see Fig. 5.9 a) is due to the diffraction of 003 planes. 121 0.2 0.4 0.6 0.8 1-0 x (a) WS206F2/WcFx vs xofCF x Fig. 5.8 Intercalation amounts of S2O6F2 and H S O 3 F into CFX 122 ' IMM'I ' I ' l ' IM ' I ' I ' I ' I ' l J I ' J ' I ' I ' IM ' I ' IM' I ' I ' I ' I ' I ' I ' I ' I ' l I'j'I'I'I'I'I'I'ITI'I'IM'I'I'I'I'I'I'l 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60. (a) (CF 0. 25)45.9HSO 3F 'I'l'IM'I'I'I'I'l 'j'I'I'I'I'j'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'I'j'I'I'IM'I'I'I'I'l 'j'l'IM'I'j'I'I'I'l' 5. 10. 15. 20. 25. 30. ' 35. 40. 45. 50. 55. 60. (b) (CFo.33)2s.6HS03F Fig. 5.9 X-ray powder diffraction patterns of CFX-HSO3F intercalation compounds 123 |"T 1 ' I • I • 1*1^1 • | • 11111' 111 • I' | • 1' I • I' I' | • I' I • I' I • | • I • 1 11 ' I' | • I • I' I • I ' ) ' I • 1 1 1 ' I' | 1 1 • I' 1' 11111' 1 11 ' I' |' I • I • I' 111 5. 10. 15. 20. 25. 30. 35. 40. 45. -50. 55. 60. (c) (CF0.5)i5.5HSO3F 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55: 60. (d) (CF0.83)23.iHSO3F Fig. 5.9 continued 124 Table 5.5 X-ray diffraction data of (CFo.25)45.9HSC»3F 26 d(A) 00L Ld Stage index 23.8 3.7356 003 11.21 two 31.7 2.8204 004 11.28 two 48.6 1.8719 006 11.23 two 57.1 1.6118 007 11.28 two The repeat distance along the c-axis Ic= 11.25 A which is the average value of Ld. Therefore the separation of intercalate layer, Is =7.9A given according to (1.1) Ic = I s + (n-l)Co where n=2, Co = 3.35. A small amount of a stage three intercalation compound may exist because two small peaks at 26 = 36.1 and 42.7 are observed. These two peaks may be due to the diffractions of 005 and 006 planes of stage-3 compound with the Ic=14.82 A. (CFo.33)25.6HS03F is a mixture of stage one and stage two intercalation compounds according to the X-ray diffraction data shown in table 5.6. 125 Table 5.6 X-ray diffraction data of (CFo.33)25.6HSC>3F 28 d(A) 00L Ld Stage index 22.4 3.966 002 7.93 one 23.5 3.767 003 11.30 two 32.0 2.795 004 11.18 two 33.7 2.657 003 7.97 one The peaks at 29= 22.4 and 33.7 are due to the diffraction of 002 and 003 planes of stage one compound with Ic =7.95 A. The other two peaks at 26 = 23.5 and 32.0 are attributed to the diffraction of 003 and 004 planes of a stage two compound with the Ic = 11.24 A. Formation of stage one intercalation compounds in the CF0.33-HSO3F system should be attributed to the more extensively oxidized lattice due to fluorine addition. (CFo.5)i5.5HSC>3F is also a mixture of stage one and stage two compounds according to table 5.7. 126 Table 5.7 X-ray diffraction data of (CFo.5)i5.sHS03F 20 d(A) 00L Ld Stage index 22.4 3.966 002 7.93 one 23.5 3.767 003 11.30 two 26.5 3.36 002 6.72 graphite 32.0 2.795 004 11.18 two 33.6 2.665 003 7.99 one The percentage increase of stage one compound in the CF0.5-HSO3F system seems to be more than that in CF0.33-HSO3F because the relative intensity of peak at 20=22.4 due to the diffraction of 002 planes of stage one compound is greater in the CF0.5- HSO3F than in the CF0.33HSO3F (see Fig. 5.9 b,c ). However, (CFn.83)23.lHS03F is a stage two compound, according to table 5.8. 127 Table 5.8 X-ray diffraction data of (CFo.83)23.lHSC»3F 20 d 00L Ld 23.7 3.751 003 11.25 26.5 3.36 graphite 6.72 31.7 2.82 004 11.28 although there are more fluorine atoms in CFo.83 than in CFn.5, the structure of CFn.83 is different from that of CF0.5. CF0.5 is close to poly(dicarbon monofluoride) but CFo.83 is more like poly(carbon monofluoride). Both CF0.5 and CF have different structures as described in section 1.4.3. A possible reason for limited intercalation may be steric hindrance with many C-F groups around small graphitic islands, and their presence may obstruct the H S O 3 F when attempting to enter the graphite interlayer. That is why there is a small amount of graphite left over which has not been intercalated in the reaction. (a peak at 20 = 26.5 due to the diffraction of graphite is observed in Fig. 5.9 d) 128 XPS spectrum of (CFo.83)23.lHS03F is shown in Fig.5.10. A change of the relative intensity of two peaks is also observed for C Is core electron binding energy. The intensity at BE ~283.5 eV attributed to sp2 carbon has increased and that at BE ~288.5 eV due to C-F bond decreased relative to that of CFo.83. This fact shows that the covalent C-F bond in CFo.83 has been influenced by the intercalates similar to that of (CFo.83)i6.lSC«3F. In this case the C-F bond is polarized by HSO3F. A hydrogen bond may be formed as represented below: C-F + HSO3F > C5+F8--H5+-"SO3FS-The carbon atomic hybrid orbital has transformed from sp3 to sp2. F Is core electron binding energy shows a similar result. The peak at BE= 688.3 eV in CFo.83 has shifted to BE=687.5 eV indicating that the C-F bond has been polarized. Deintercalation of (CFo.83)23.lHS03F has supported the idea that the intercalate group is HSO3F. X-ray powder diffraction (Fig. 5.11) and XPS spectra (Fig. 5.12) of the residual material show the intercalation compound has almost returned to the original compound CFo.83-A Raman spectrum (Fig.5.13) shows that the volatile collected is HSO3F. The peaks at 1250 cnr 1 and 961 cm - 1 are attributed to SO3 stretching and O H bending vibrations. The 851 cm"1 peak is assigned to the vibration of S-F bond and the peaks at 550 cm"1 and 399 cm - 1 are attributed to the vibrational deformation modes of SO3F 1 2 7 . 129 3000 H 2500 H 2000 H 1500H 283.5 eV 288.5 eV C 161/2 1 00C I , i • • • • • • ' " " " 1 " 1 1 1 1 " " ^ 4 274 279 Binding Energy (eV) 687.5 eV Binding Energy (eV) 676 Fig. 5.10 XPS spectra of (CF0.e3)23.iHSO3F (Error limit of BE = ± 0.1 cV) 130 IMMM'I'lMMM'JIMMM'jMM'I'I'j'I'I'I'I'I'IMM'I'jM'I'IM'j'l'I'IM'I'I'I'I'I'p'IMM'j'I'I'ri' 5 . 1 0 . 15 . 2 0 . 2 5 . 3 0 . 3 5 . 4 0 . .45. 5 0 . 5 5 . 5 0 . Fig. 5.11 X-ray powder diffraction patterns of the residue of (CFoj3)23.iHS03F deintercalation compound 5500: 4500: £ 3500: o u 2500 i 1500: 500 C 1s1 / 2 i i i i i i I I M i I I I l i I I I I ' l I I I I » i i I i i I I I I I I i | 274 279 284 289 294 Binding Energy (eV) Fig. 5.12 XPS spectrum of Ac residue of (CFo.83)23.lHS03F deintercalation compound 231 651 » I * I » • « « I • * * > * 1 1 1 1 1 > i t 1 1000 500 100 wavenumber/cnr1 Fig. 5.13 Raman spectrum of collected volatile of (CFo.5)l5.5HS03F 5.4 Graphite fluorides-HS03CF3 intercalation compound Trifluoromethylsulfuric acid, H S O 3 C F 3 is one of the strongest simple protonic acids known. It may be slightly weaker than HSO3F. It exhibits high thermal stability and excellent ionizing ability. Self dissociation does not appear to occur thus reducing the chance for oxidative side reactions. Its anion SO3CF3" is very weakly basic. Intercalation reactions involving H S O 3 C F 3 should not involve graphite oxide or fluoride formation as possible side reactions. Again the ability of H S O 3 C F 3 to intercalate by itself is expected to be very poor, but no reports on this subject are found. On the other hand the electrochemical intercalation of H S O 3 C F 3 has been reported by Boehm et al. 4 3. Also the oxidation using K2Cr207 dissolved in the acid results in intercalation of H S O 3 C F 3 and its anion . The composition of the stage one compound was determine as C+26S03CF3~«1.63HS03CF343. Karunanithy44 synthesized graphite trifluoromethylsulfate of a composition C12SO3CF3 by the irreversible solvolysis of C8SO3F in a large excess of H S O 3 C F 3 . The salt is identified as a stage one intercalation compound with a repeat space distance Ic=8.12A. Since H S O 3 F and H S O 3 C F 3 are of comparable acidity and exhibit comparable volatility, a successful intercalation of H S O 3 C F 3 into the graphite fluorides should be possible. We have synthesized CFX-HSO3CF3 intercalation compounds by means of a direct intercalation reaction as described in sec. 2.5.7. The compositions of these compounds obtained by weight increase are shown in table 5.9. 133 Table 5.9 Composition of CFX - HSO3CF3 Intercalation compounds CFX WcFx(g) AW(g) AW/WcFx Composition CF0.25 0.2008 0.0520 0.2621 (CFo.25)34.58HS03CF3 CF0.33 0.1663 0.0484 0.2949 (CFo.33)28.2lHS03CF3 CF0.5 0.1763 0.0693 0.3931 (CFo.5)i7.75HS03CF3 CFQ.83 0.1839 0.0456 0.2489 (CF0.83)2i.78HSO3CF3 where, W C F x is initial weight of CFX. and A W is increased weight after the reaction. The composition shown in Table 5.9 is similar to those of the CFX-HSC»3F intercalation compounds shown in Table 5.4. CF0.5 can be intercalated by the largest amount of HSO3CF3. The IR spectrum of (CFo.5)i7.7sHS03CF3 is shown in Fig 5.14. . The result indicates that the intercalate group in this compound is HSO3CF3. The peak at -3560cm-1 is assigned to the stretching vibrations of the OH group; the bands at 1260 and 1035 cnr1 are attributed to asymmetric and symmetric vibration of SO3 group respectively. 1240cm"1 and the 1170cm"1 bands are due to the vibration of CF3 . However an exact assignment is problematic considering the close band spacing and the comparable symmetry of CF3 and 134 Fig. 5.14 IR spectrum of (CF0.5)i7.75HSO3CF3 SO3 stretching vibrations. It is however noteworthy that like in C12SO3CF3 4 4 vibrations due to the intercalation are clearly observed. The X-ray powder diffraction pattern of (CFo,25)34.58HS03CF3 (Fig.5.15 a) shows that most of the graphitic islands have not been intercalated by HSO3CF3 because a strong peak at 29=26.5 due to the diffraction of the graphite is still observed. Small peak at 29=23.7 and 25.5 are also observed. Both peaks are assigned to the diffraction of the zones of high stage intercalation compounds. This result is different from that of CF0.25-HSO3F in which all graphite islands seem to have been intercalated by HSO3F. The reason may be that HSO3F has a more pronounced ability to intercalate CF X than HSO3CF3 and in case of the size of the two acids, HSO3F is smaller than that of HSO3CF3 so that it easily enters the interlayers of graphite islands. In addition, in accordance with general physical properties of fluorocarbons such as low coefficients of friction, high volatility and good flow properties , CF--CF repulsion may play a role here. There are also some graphitic islands which have not been intercalated by HSO3CF3 in (CFo.33)28.2lHS03CF3 according to the X-ray powder diffraction pattern (Fig.5.15 b ). However, more graphite islands have been intercalated in (CFo.33)28.2lHSC>3CF3 than in (CFo.25)34.58HSC>3CF3 based on the relative intensities of peaks. The peak at 29=24.5 is due to the 005 diffraction and has the highest intensity. This indicates that it is primarily a stage four intercalation compound. X-ray diffraction data (table 5.10) show that the repeat space distance I c is 18.2A and the separation of the intercalation layer is 8.15A. The X-ray powder diffraction patten (Fig. 5.15 c) of (CFo.5)i7.75HSC>3CF3 shows that all graphite islands in CFn.5 have been intercalated by HSO3CF3. The peak at 29=26.5 136 TlM'l 'I ' I ' I ' I ' t 'J 1 I ' l 11 ' I ' j ' IT 1' I'I'I'I'I'I'I'I'I'l' I'j1 I T I'l' I'lM ' l 'Ep 1 I'l' I'j'I1 I'l11'j'I'I'I'l 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. BO. (a) (CF0.25)34.58HSO3F ' I T I ' I ' | ' I T I ' I ' | T I ' I T | ' I T I T | ' I ' I T I ' | ' I T I T ) T I T I ' | ' I T I T | T I I'I'I'I'I'I'I'I'I'l' 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60. (b) (CFo.33)28.2iS03CF3 Fig. 5.15 X-ray powder diffraction patterns of CF x -HS0 3 CF 3 intercalation compounds 157 5. 10. 15. 20. 25. 30. 35. 40. 45. 50. 55. 60. (d) (CFO.83)21.78HS0 3CF 3 Fig. 5.15 continued 138 due to the diffraction of graphite is missing. This result is in agreement with the composition of (CFo.5)i7.75HSC«3CF3 which contains the largest amount of H S O 3 C F 3 relative to other CFX-HS03CF3 intercalation compounds. Two peaks at 26=24.2 and 26=29.8 are due to the diffraction of 005 and 006 planes of stage four intercalation compound with the repeat space distance IC=18.17A. The separation of the intercalate layer is 8.12A calculated by using the equation: Ic = Is + (n-l)Co X-ray powder diffraction patter n of (CFo.83)2l.78HSC>3CF3 (Fig. 5.15.d) is similar to that of (CFo.5)i7.75HS03CF3. Two peaks observed at 26=24.1 and 29.9 are due to the diffraction of 005 and 006 planes of a stage four intercalation compound with a repeat space distance of 18.19A and intercalate separation of 8.14A. However, the relative intensity of these peaks are lower than that of (CFo.5)i7.7sHS03CF3 because of less amount of graphite islands in CF0.83 than in CF0.5. XPS spectra of (CFo.83)2i.78HS03CF3 (Fig. 5.16) also show that the C-F bond has been polarized by the intercalate group HSO3CF3 according to C-F + HSO3CF3 > C&+F8--H&+-(S03CF3)5-A hydrogen bond may be formed between the C-F and HSO3CF3. The intensity of peak at BE= 284 eV has increased after the reaction, indicating the transformation of sp3 hybrid carbon atoms into sp2. BE of Fls core electron has also decreased from 688.3 to 687.5 eV after the reaction but this value is higher than 686.8 eV of CF0.83-S2O6F2 and 687.4 eV of 139 Fig. 5.16 XPS spectra of (CFo.e3)2i.78HSC»3CF3 140 CFn.83~HS03F, indicating that the polarizability of H S O 3 C F 3 is poorer than that of S2O6F2 and HSO3F. 5.5 Conclusion Graphite fluorides CFX appear to contain graphitic islands when x<l. These islands can be intercalated by S2O6F2, H S O 3 F and HSO3CF3 involving both oxidative (S2O6F2) and non-oxidative intercalation. The covalent C-F bonds in CFX have been polarized by intercalate groups and some sp3 hybrid carbon atoms have been transformed into sp2 hybrid after intercalation . XPS, Raman, and IR spectra show the intercalate groups to be S O 3 F - , H S O 3 F and H S O 3 C F 3 for CFX-S206F2, CFX-HS03F and CFX-HS03CF3 intercalation compounds respectively. X-ray powder diffraction patterns show that the CFX-S206F2 system gives a stage one intercalation compound but CFX-HS03F and CFX-HS03CF3 give higher stage intercalation compounds. 141 Summary and Conclusions The following advances are reported in this thesis: (1) . A simple synthetic method is developed for the synthesis of higher stage graphite fluorosulfates. (2) . Powder diffraction and X-ray photoelectron spectroscopy are used widely to characterize graphite intercalation compounds. (3) . The intercalation into partly fluorinated graphite fluorides by an oxidizing (S2O6F2) and by non-oxidizing, or very weakly oxidizing intercalants (HSO3F, HSO3CF3) is reported for the first time. Remaining topics, in order to complete the study, are (1) . Detailed 1 9 F - and where applicable tH-NMR studies on graphite fluoride-fluorosurfates (2) . Electrical conductivity measurements on graphite fluoride fluorosulfates are to be conducted. 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