AN ABSTRACT OF THE DISSERTATION OF Watcharee Katinonkul for the degree of Doctor of Philosophy in Chemistry presented on August 27, 2007. Title: Graphite Intercalation Compounds containing Fluoroanions. Abstract approved: Michael M. Lerner New chemical synthesis strategies have been investigated for the preparation of acceptor-type graphite intercalation compounds (GIC’s) with fluoroanions in order to obtain new materials and to develop a better understanding of the synthetic approaches and properties of the products. New GIC’s containing borate chelate anion such as bis(oxalato)borate, CxB[OC(O)C(O)O]2·δF, are prepared by the chemical oxidation of graphite with fluorine gas in the presence of a solution containing the intercalate anion in anhydrous hydrofluoric acid. Due to the chemical method employed (F2/AHF as the oxidizing agent), fluoride co-intercalates are present in the galleries. The GIC’s compositions are determined using a newly-developed digestion method with an ion-selective electrode and potentiometer. The composition parameters x and δ are obtained using both elemental and thermogravimetric analyses (TGA). Elemental analyses of B and F for a stage 1 GIC indicate x ≅ 41 and δ ≅ 4. Powder XRD data and the structural refinement of these GIC’s show that the intercalate anions “stand up” in the galleries, with the longer anion dimension oriented perpendicular to the graphene sheets, leading to the relatively large gallery height of di ≅ 1.42 nm. The same chemical synthesis strategy is also used for chemical preparations of other two borate chelate CxB[OC(CF3)2C(CF3)2O]2·δF, and GIC’s; bis(perfluoropinacolato)borate, bis(hexafluorohydroxyisobutyrato)borate, CxB[OC(CF3)2C(O)O]2·δF. Stage 1 GIC’s of these anions are obtained with the gallery height, di, of 1.43-1.45 nm. The fluoride co-intercalate contents in these GIC samples are also determined. The composition parameters x and δ are obtained using elemental analyses for stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF and for stage 1 CxB[OC(CF3)2C(O)O]2·δF. TGA cannot be used to confirm these compositions due to the incomplete degradation of intercalates. Most likely, anions and their degradation products are trapped in the bulk of the large flaky graphite particles. The detailed compositions of GIC’s containing bis(trifluoromethanesulfonyl) imide, CxN(SO2CF3)2·δF, and perfluorooctanesulfonate, CxC8F17SO3·δF and fluoride co-intercalates are determined for the first time. These GIC’s are prepared in aqueous HF, using K2MnF6 as the oxidizing agent. The fluoride co-intercalate content is approximately equimolar to anion content in both GIC’s. For CxN(SO2CF3)2·δF, the fluoride content increases steadily with reaction time while the anion content is relatively constant. In contrast, CxC8F17SO3·δF shows little change in the fluoride content over time. © Copyright by Watcharee Katinonkul August 27, 2007 All Rights Reserved Graphite Intercalation Compounds containing Fluoroanions by Watcharee Katinonkul A DISSERTATION submitted to Oregon State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy Presented August 27, 2007 Commencement June 2008 Doctor of Philosophy dissertation of Watcharee Katinonkul presented on August 27, 2007 APPROVED: Major Professor, representing Chemistry Chair of the Department of Chemistry Dean of the Graduate School I understand that my dissertation will become part of the permanent collection of Oregon State University libraries. My signature below authorizes release of my dissertation to any reader upon request. Watcharee Katinonkul, Author ACKNOWLEDGMENTS I would like to thank Drs. Kevin Gable, Douglas Keszler, Philip Watson, and Jim McManus for serving on my doctoral dissertation committee and for their helpful suggestions and comments. I thank to Chris Pastorek, Joe Magner, and Ted Hinke for their help with equipment throughout many years, to Alan Richardson for his suggestions about structural model calculation, and past members of the Lerner group; Wei Yan and Lioubov Kabalnova, for their help and guidance. I thank to my parents for giving me an opportunity to get the highest education mankind can acquire, especially my beloved mother, Sa-ngium Katinonkul, for her inspiration. I also would like to acknowledge the Royal Thai Government for financial support and thank all the people at the Royal Thai Embassy in Washington DC for taking care of me during my stay in the United States of America. Finally, this thesis would never be successfully completed without the excellent advice from my thesis advisor, Dr. Michael Lerner, who always provides me useful guidance, suggestions, encouragement, understanding and also patiently practices my technical skill during the whole research. CONTRIBUTION OF AUTHORS Dr. Michael M. Lerner was involved in the design, analysis, and writing of each manuscript. TABLE OF CONTENTS Page 1. INTRODUCTION.......................................................................................... 1 1.1 GENERAL INTRODUCTION……..................................................... 1 1.2 GRAPHITE INTERCALATION COMPOUNDS…............................ 8 1.2.1 Types of GIC’s…………………………….. ……………….... 13 1.2.1.1 Donor-type compounds………………………. ……. 13 1.2.1.2 Acceptor-type compounds.....………………………. 15 1.2.2 Preparation of acceptor-type GIC’s…………………….…….. 21 1.2.2.1 Electrochemical oxidation method…………………. 21 1.2.2.2 Chemical oxidation method………………………… 23 1.3 GENERAL APPARATUS AND PROCEDURES…………...……..... 26 1.3.1 Stainless steel vacuum line……………….…………………… 26 1.3.1.1 Handling anhydrous HF (AHF)…………………...... 28 1.3.1.2 Assembled reaction apparatus for use with AHF....... 28 1.3.2 Graphite……………………..…………..……….…………… 29 1.3.3 Elemental analyses …………………………………………… 29 1.3.4 Powder X-ray diffraction (Powder XRD) ……………………. 30 1.3.5 Thermal gravimetric analysis (TGA) ………………………… 30 TABLE OF CONTENTS (Continued) Page 1.4 FLUORINE ANALYSIS IN GIC’s ………………………….............. 1.4.1 31 Control of solution parameters……………………………….. 31 1.4.2 Test of digestion procedure ………………………………….. 31 1.4.3 Test for container reactions …………………………………... 34 POWDER X-RAY DIFFRACTION ANALYSIS OF GIC’S ……...... 36 1.6 STRUCTURAL REFINEMENT MODELING……… ……………… 39 1.5 1.6.1 Overview……………………………………………………… 39 1.6.2 Structural model calculations………………………….. …….. 40 1.6.3 One-dimensional electron density map…………………..…… 41 1.6.3.1 Optimization of intercalate gallery structure………… 46 THESIS OVERVIEW..………………………………………………. 47 1.8 REFERENCES ………………………………………………………. 49 2. EFFECT OF REACTION TIME ON COMPOSITION OF GRAPHITE BIS(TRIFLUOROMETHANESULFONYL)IMIDE …………………….... 55 2.1 ABSTRACT………………………………………………………….. 56 2.2 INTRODUCTION……………………………………………………. 57 2.3 EXPERIMENTAL ………………………………………………........ 61 2.4 RESULTS AND DISCUSSION ……………………………………... 64 2.5 REFERENCES……………………………………………………….. 75 1.7 TABLE OF CONTENTS (Continued) Page 3. A STUDY OF FLUORIDE CONTENT IN GRAPHITE PERFLUOROOCTANESULFONATE INTERCALATION COMPOUNDS……………………………………………………………… 77 3.1 ABSTRACT………………………………………………………….. 78 3.2 INTRODUCTION……………………………………………………. 79 3.3 EXPERIMENTAL ………………………………………………........ 82 3.4 RESULTS AND DISCUSSION ……………………………………... 85 3.5 REFERENCES……………………………………………………….. 94 4. THE FIRST SYNTHESIS OF A GRAPHITE BIS(OXALATO)BORATE INTERCALATION COMPOUND………………………………………… 96 4.1 ABSTRACT………………………………………………………….. 97 4.2 INTRODUCTION……………………………………………………. 98 4.3 EXPERIMENTAL ………………………………………………........ 102 4.4 RESULTS AND DISCUSSION ……………………………………... 107 4.5 REFERENCES……………………………………………………….. 118 5. CHEMICAL SYNTHESIS OF GRAPHITE BIS(PERFLUOROPINACOLATO)BORATE AND GRAPHITE BIS(HEXAFLUOROHYDROXYISOBUTYRATO)BORATE INTERCALATION COMPOUNDS……………………………………………………………... 120 5.1 ABSTRACT………………………………………………………….. 121 5.2 INTRODUCTION……………………………………………………. 122 5.3 EXPERIMENTAL ………………………………………………........ 124 TABLE OF CONTENTS (Continued) Page 5.4 RESULTS AND DISCUSSION ……………………………………... 128 5.5 REFERENCES……………………………………………………….. 140 6. CONCLUSION …………………………………………………………….. 142 BIBLIOGRAPHY …………………………………………………………........ 144 LIST OF FIGURES Figure Page 1.1 Dimensional classification of intercalation hosts………………………… 4 1.2 Schematic drawing of a layered intercalation compound………………… 5 1.3 Schematic view of the graphite structure, formed by the ABAB stacking of graphene sheets………………………………………………………… 7 1.4 Schematic view of GIC’s showing staging, or ordering, along the stacking direction……………………………………………………..…... 10 1.5 Daumas-Hérold model ……………………………………………............ 11 1.6 Galvanostatic potential-charge curve of LiN(SO2CF3)2 in nitromethane… 22 1.7 Schematic of stainless steel vacuum line ……………………………........ 27 1.8 Powder XRD pattern of stage 1 products of CxB[OC(O)C(O)O]2·δF. Assigned (00l) indices are indicated. …………………………………… 37 (a) A geometry-optimized structural model of isolated bis(oxalato)borate anion, (b) structural model of stage 2 CxB[OC(O)C(O)O]2·δF, with four fluoride co-intercalates per borate anion. Axes and some atomic labels are included…………………………………………………………. …… 43 1.10 One-dimensional electron density profiles of (a) graphene sheets and (b) bis(oxalato)borate anion intercalate using (i) 9, (ii) 25, and (iii) 50 terms in the calculation…………………………………………… 44 1.11 One-dimensional electron density profiles of stage 2 CxB[OC(O)C(O)O]2·δF derived from calculated structural model using (a) 9 terms, (b) 25 terms, and (c) derived from observed powder XRD data using 9 terms………………………………………………………… 45 1.9 2.1 Powder XRD patterns of the products obtained, with reaction times indicated. For each powder XRD trace, there is a break in intensity data as indicated by hashes on the y-axis……………………………………… 66 LIST OF FIGURES (Continued) Figure 2.2 Page Structural model for CxN(SO2CF3)2·δF. The fluoride positions are partially occupied ………………………………………………………… 68 2.3 TGA data for products described in Table 2.1 …………………………… 69 2.4 Increase in fluoride content, δ, in CxN(SO2CF3)2·δF vs. reaction time …... 71 2.5 One-dimensional electron density profiles derived from calculated structural model (i) and observed powder XRD data (ii)………………… 74 Powder XRD patterns of the products obtained in a mixed acid solution (HF/HNO3:66/33), using K2MnF6 as an oxidizing reagent, with reaction times indicated. Impurity peaks of CxF are marked *. …………………… 88 Structural model for CxC8F17SO3·δF. The fluoride positions are partially occupied.………………………………………………………………….. 90 3.3 TGA data for products described in Table 3.1.…………………………… 91 4.1 Scheme of GIC showing staging, or ordering along the stacking direction, for different products… ……….…………................................. 100 General design for reactor employed in syntheses using anhydrous HF And F2.........………………………………………………………............. 105 Powder XRD pattern of stage 1-3 products of CxB[OC(O)C(O)O]2·δF. Assigned (00l) indices are indicated.……………………………………... 111 TGA mass loss profiles for LiBOB salt and stage 2 GIC CxB[OC(O)C(O)O]2·δF. …………………………………………….. 116 (a) Energy-minimized BOB anion conformation, (b) GIC structural model, and (c) one-dimensional electron density profiles derived from calculated structural model (i) and observed powder XRD data (ii).…….. 117 3.1 3.2 4.2 4.3 4.4 4.5 LIST OF FIGURES (Continued) Figure 5.1 5.2 5.3 Page Powder XRD pattern of stage 1 products of (a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF. Assigned (00l) indices are indicated; impurity peaks are marked*.……... 131 Structural models of stage 1 GIC’s of (a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF. ……………………………………… 133 TGA mass loss data for (a) NaB[OC(CF3)2C(CF3)2O]2 salt (⋅⋅⋅) and stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF (⎯), (b) NaB[OC(CF3)2C(O)O]2 salt (⋅⋅⋅) and stage 1 CxB[OC(CF3)2C(O)O]2·δF (⎯)…………… ……….. 139 LIST OF TABLES Table Page 1.1 Donor-type GIC’s with gallery height (di) and n (stage)…………...…….. 14 1.2 Fluoride GIC’s with gallery height (di) ………………………………….. 18 1.3 Stage-1 acid GIC’s with c-axis repeat distances (Ic) …………………….. 19 1.4 Synthetic reactions and results using different oxidizing reagents …….… 25 1.5 Fluoride content of digested control samples ……………………………. 33 1.6 Comparison of fluoride content between digested and undigested control samples…….……………………………………………………... 34 Comparison of fluoride analyses between digested samples using glass and plastic labware ……………………………………………………… 35 Data obtained from powder XRD of stage 1 CxB[OC(O)C(O)O]2·δF…… 38 1.7 1.8 2.1 Gallery height (di) and composition for stage 2 graphite bis(trifluoromethanesulfonyl)imide, CxN(SO2CF3)2·δF, at different reaction times as determined by powder XRD, elemental analyses, and thermogravimetry... 3.1 67 Gallery height (di) and composition for stage 2 graphite perfluorooctanesulfonate, CxC8F17SO3·δF, at different reaction times as determined by powder XRD, elemental analyses, and thermogravimetry.………………. 89 4.1 Powder XRD data for CxB[OC(O)C(O)O]2·δF products…………………. 112 4.2 Compositional data for CxB[OC(O)C(O)O]2·δF from elemental analyses.. 113 5.1 Powder XRD structural data for (a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF products obtained after 24 h reaction..…… 132 Compositional data for CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF products….…………………………............... 135 5.2 GRAPHITE INTERCALATION COMPOUNDS CONTAINING FLUOROANIONS CHAPTER 1 INTRODUCTION 1.1 GENERAL INTRODUCTION The term intercalation is derived from the Latin verb intercalare and refers to (in the original Latin) the insertion of an additional day in special years to synchronize the calendar with the solar year. [1] In host-guest chemistry, intercalation is defined as the insertion of guest species (atoms, ions or molecules) into a crystalline host lattice generally with an increase in some dimensions but also retention of major host structure. The term intercalation, however, does not indicate anything about the specific nature of the chemical reaction underlying this process. It describes only the topological relationship between the host and the final product of the reaction. The particular requirement for forming intercalation compounds is anisotropic bonding in the host leading to unchangeable structural elements such as layers, chains, or three-dimensional frameworks as illustrated in Figure 1.1. If electrically neutral structural elements are formed, they are held together by weak van der Waals forces or they contain empty one-dimensional channels (running parallel through the structure, or intersecting each other) or interconnected cavities. If these structural elements carry 2 electrical charges, these are compensated by small counterions occupying the channels and cavities. [2, 3-8] In most of these compounds there are many more available sites than can be occupied by additional atoms or molecules. Therefore the mobility of the interstitial species can be very high if their interaction with the host structure is not too strong. [5] Consequently, this can lead to the reversible uptake of atoms, ions, or molecules while the structure of the host lattice is conserved. Whereas in framework structures only those compounds can be taken up that fit into the channels or cavities, in layered or chain host lattices expansion can occur in one or two dimensions without serious restriction. This allows the accommodation of guest species other than simple ions or atoms, such as large metal complexes, or organic/ inorganic cations or anions. For two dimensional hosts, layer expansions up to 5 nm have been observed (Figures 1.1 and 1.2). In channel compounds, defects can completely block diffusion and it can therefore be difficult to achieve homogeneous filling of all possible lattice sites. In chain and layer compounds, the species to be intercalated has access from all, or at least two, dimensions. However, in chain structures the insertion of additional species, especially if they are bulky, weakens the interaction between the chains, thus leading to the easy disintegration of the solid. In layered hosts, the two-dimensional structural element provides enough stability to prevent host disintegration, at least for moderate layer separations. This ideal combination of easy access to the interlayer galleries and the stability of the final products is the reason that layered compounds are preferred 3 for reversible and facile intercalation reactions, and why most of the known intercalation hosts have layered structures. [2, 3-8, 10-16] Layered hosts may be neutral, positively-charged, or negatively-charged, and range in electrical properties from metallic conductors to semiconductors or insulators. [2] Clay materials, such as montmorillonite or hectorite, are probably the most widely studied hosts in this group because they are naturally-occurring and readily form a wide range of intercalation compounds. Some other important layered hosts that have been studied include layered silicates, MoO3, MPS3 (M=Mn, Cd, Fe, Co, Ni, Zn, V), protonic layered titanate (HxTi2-x x/4O4·H2O), protonic layered tetratitanate (H2Ti4O9·xH2O), and also a number of layered dioxides and layered dichalcogenides. Layered host surfaces can also be modified with organic groups, so that they become compatible with hydrophobic intercalates. For instance, a variety of molten styrenederivative polymers can intercalate into alkylammonium-functionalized layered silicates. [17,18] Another example is a nanocomposite of poly(ether imide) and an organically modified layered silicate, which is prepared by cation exchange between Li fluorohectorite or Na montmorillonite and protonated primary amines. [19] The intercalated polymer shows improved fire retardancy relative to the pure polymer. 4 Type of lattice frame work structures schematic representation model compound dimensions of accessibility Zeolite 3 other examples Mo6S8, WoO3 a) 3-dimensional network of channels Nb3S4 b) uni-directional channels layer structures + 1 Ti3S4, WO3 2 graphite, + TaS2 FeOCl, MoO3 layered dichalcogenides chain structures 3 NbSe3 KFeS2, AMo3X3 Se Nb Figure 1.1 Dimensional classification of intercalation hosts. [1] (A= alkali metal, In, Tl; X=S, Se) 5 the unit repeat along the stacking direction gallery expansion host layer thickness Figure 1.2 Schematic drawing of a layered intercalation compound. 6 Graphite is one of many layered materials that can form intercalation compounds. Graphite is composed of planar sheets (graphene sheets) with carbon atoms bonded to three others by a trigonal sp2 interaction to form a hexagonal network. The remaining non-hybridized 2p electrons are delocalized in an extended π bonding interaction. Adjacent sheets are thus bound only by van der Waals forces. Figure 1.3 shows the structure of graphite indicating the in-plane carbon-carbon bond length (0.142 nm) and the layer separation (0.335 nm). Graphite has two types of stacking structures, known as the hexagonal and rhombohedral phases. The planes are stacked ABAB for the hexagonal phase as shown in Figure 1.3 and ABCABC for the rhombohedral phase. The rhombohedral structure is metastable and transforms into hexagonal graphite at high temperature or by the formation of an intercalation compound followed by its dissociation. [4] The direction parallel to the graphene sheets exhibits metallic properties, but in the perpendicular direction, graphite is a semiconductor. Graphite is a unique layered material that has excellent thermal and electrical conductivity. Additionally, graphite has distinctive chemical and physical properties, including a high sheet flexibility that leads to the staging phenomenon, which will be explained in Section 1.2. 7 0.142 nm A B 0.335 nm A Figure 1.3 Schematic view of the graphite structure, formed by the ABAB stacking of graphene sheets. 8 1.2 GRAPHITE INTERCALATION COMPOUNDS Graphite can undergo intercalation reactions because of the relatively weak bonding between graphene sheets. The intercalation reaction always involves a redox process, electrons can be removed from the graphite valence band by oxidation, and anions inserted between sheets to satisfy the neutrality condition. This produces acceptor-type graphite intercalation compounds (GIC’s). Alternatively, donor-type compounds with cationic intercalates can be synthesized using strong reducing agents that donate electrons into the graphite conduction band. These reactions are illustrated in equations 1.1 and 1.2. xC + An¯ → CxAn + e¯ (1.1) xC + M+ + e¯ → MCx (1.2) where C represents graphene sheets and An¯ and M+ represent intercalate anions and cations, respectively. In a GIC, not every pair of adjacent sheets are necessarily separated to form a gallery occupied by intercalate guests. The appearance of an ordered sequence of occupied galleries and adjacent graphene sheets, the so-called staging phenomenon, is one of the many fascinating properties of GIC’s. Stage n refers to n graphene sheets between intercalate galleries. Therefore, in stage 1 compounds, graphene sheets and 9 intercalate alternate, and in stage 2 compounds two adjacent graphene sheets alternate with intercalate as is illustrated in Figure 1.4. The symbol Ic denotes the unit cell repeat distance along the stacking direction, and di is the gallery height or distance along the c axis between graphene sheet centers encasing an intercalate gallery. The relationship between Ic, di and n is indicated by equation 1.3 Ic = di + (n-1) 0.335 nm (1.3) The actual compositions vary with synthetic conditions and therefore GIC’s are examples of non-stoichiometric compounds. It is common to specify the composition together with the stage, for example stage-4 NaC24 or NaC6n (n = 4-8). [20] To describe staging in graphite intercalation, the Daumas-Hérold model has been employed. This model describes intercalate volumes as domains of intercalate islands. GIC’s are composed of a number of domains with regularly stacked islands. Stage disorder is common in GIC’s and exists not only between domains but also within each domain as shown in Figure 1.5. [21] This model was derived from the flexibility and presence of defects in graphene sheets. The movement of islands over domain boundaries allows facile staging transitions. [21] The Daumas-Hérold model has been supported by a large number of experiments, and the domains have been observed directly in CxFeCl4·δFeCl3 by lattice imaging with high resolution TEM. [22] 10 di Ic di Ic Ic = di 0.335 nm graphite stage 3 stage 2 stage 1 graphene sheet intercalates Figure 1.4 Schematic view of GIC’s showing staging, or ordering, along the stacking direction. 10 11 stage 1 stage 2 graphene sheet intercalate Figure 1.5 Daumas-Hérold model. stage 3 12 In some GIC’s, there are neutral molecules present in the galleries along with intercalate ions. There is no convincing evidence that graphite ever intercalates neutral molecules without simultaneously being oxidized or reduced. However, GIC’s commonly take up neutral molecules along with intercalate ions, this is also observed with other layered hosts, such as transition metal chalcogenides. These neutral molecules generally do not require significant additional separation of the graphene sheets, and their uptake is reversible. Repulsive forces between the intercalate ions contribute to decreased stability of the lattice. Neutral co-intercalates are usually positioned between the ions, thereby acting as dielectric spacers and stabilizing the GIC lattice. Sources of neutral co-intercalates can include (1) excess neutral reactants, (2) byproducts of the intercalation reaction, and (3) solvents used in the intercalation reaction. For examples, FeCl3 dissolved in nitromethane (CH3NO2) yields the cointercalation compound C24FeCl3·0.73CH3NO2 through direct reaction with graphite [23], the electrochemical intercalation of PF6¯ and AsF6¯ anions dissolved in nitromethane provides stage-2 C48PF6·2-4CH3NO2 with Ic = 1.1 nm and stage-1 C24AsF6·2CH3NO2 with Ic = 0.80 nm, respectively. [24,25] Similarly, an electrochemical oxidation in solutions of complex anion salts in dry propylene carbonate (PC) yields C24An·δPC with An = BF4¯, PF6¯, or SbF6¯. [26] 13 1.2.1 Types of GIC’s 1.2.1.1 Donor-type compounds The most common cationic intercalates are alkali metal cations, and these result in relatively small intercalate gallery dimensions (di ≤ 0.7 nm). These GIC’s are readily prepared and their brilliant gold color and high conductivities for stage 1 has attracted research interest for decades. There are several intercalation methods for these GIC’s, including direct reactions of graphite with the alkali metal, high pressure reactions, electrochemical reactions, and solution-phase reactions. The most common intercalation method used is the direct reaction of graphite with the metal in the temperature range 200-550°C. A Pyrex glass tube can be used as a reaction chamber, and alkali metal GIC’s with welldefined single stage phases can be obtained. An example is shown in the following reaction. xC + K(m) 150-400°C KCx (1.4) The most studied and technologically important donor-type GIC is LiCx, which is used as the active anodic material in commercial Li-ion batteries. [5] KCx has also been widely studied for its interesting properties, including low resistivity, 14 magnetoresistivity, and high hydrogen sorption capacity. [27] Other donor-type GIC’s are shown in Table 1.1. Alkali metal cations can also intercalate into graphite when the alkali metal is solvated in liquid ammonia or an organic solvent such as dimethylsulfoxide (DMSO), (CH3)2SO, dimethylformamide (DMF), C3H7NO, or furan, C4H4O. In this case, the solvent is often included within galleries. Electrochemical reduction can also be used for alkali metal intercalation. Using KI dissolved in DMSO solution, CxK·(CH3)2SO compounds have been prepared by electrochemical reduction. [28] Table 1.1 Donor-type GIC’s with gallery height (di) and n (stage). compound di (nm) Reference NaC6n (n = 4-8) 0.46 [20] LiC6n (n ≥ 1) 0.37 [29] KC8 0.54 [29] RbC8 0.56 [30] CaC6 0.46 [31] SrC6 0.49 [31] BaC6 0.52 [31] NaC2-3 0.70 [32, 33] RbC24·6(CH3)2SO 1.2 [34] KC24·1.44C4H4O 0.88 [35] 15 1.2.1.2 Acceptor-type compounds A wide range of anions have been intercalated into graphite, including tetrahedral or octahedral fluoro-, chloro-, bromo- or oxo-metallates, and perfluorinated organic anions. The interlayer spacing of acceptor-type GIC’s ranges from 0.67 nm (for CxHF2) to 3.4 nm (for the bilayer gallery structure observed in CxC10F21SO3). [36,37] Following early work by Rüdorff et al. since 1963 [38], transition metal chloride GIC’s have been extensively investigated, both in the synthetic chemistry and structure as well as materials properties, including electrical and magnetic properties, and application in batteries and catalysts. [20, 39-41] Either vapor-phase or solutionphase syntheses can be used. Metal chlorides can be reacted with graphite in the presence of chlorine gas in the appropriate temperature range. [20] Cl2 is involved in the oxidation process as shown in the following reaction. xC + MClm + (δ/2) Cl2 CxMClm+δ (1.5) However, the presence of Cl2 is not required when using oxidizing metal chlorides such as FeCl3, AuCl3, and SbCl5. In these cases, auto-dissociation is responsible for graphite oxidation, as shown in the following reaction: e¯ + 2FeCl3 FeCl2 + FeCl4¯ (1.6) 16 A large number of fluoride intercalation compounds have been prepared, including metal fluorides, halogen fluorides, boron fluorides, hydrogen fluoride, and rare gas fluorides as summarized in Table 1.2. [42-49] The intercalation of hexafluorides or pentafluorides has been studied largely because of their high vapor pressure and the strong covalent character of the metal-fluorine bond. Among these species, AsF5 and to a lesser extent SbF5, have been particularly well investigated. [45, 48, 50-51] The catalytic activity of metal fluoride-GIC’s has also been widely investigated for organic fluorine chemistry and is now of great importance for perchlorofluorocarbon substitution reactions. [52-54] Various preparation procedures have been described. Some fluorometallate anions are intercalated spontaneously by direct reaction of graphite with the metal fluoride. For example, intercalation of some hexafluorides, MF6, into graphite occurs as shown in the following reaction scheme: [55-56] xC + MF6 CxMF6 (1.7) where M = Re, Os, Ir, Pt, U Spontaneous interaction of some metal pentafluorides, MF5, into graphite also occurs, involving the disproportionation: 2e¯ + 3MF5 2MF6¯ + MF3 (1.8) 17 where M = As [57], Sb [58], Ru [59], Os [59] Non-oxidizing metal fluorides require the addition of oxidizing reagents such as fluorine, chlorine, CrO3, or chromyl fluoride [56-57]. For example, some pentafluorides MF5 intercalate into graphite only in the presence of F2 [58] or Cl2. [60] e¯ + PF5 + 1/2 Cl2 + HF PF6¯ + HCl (1.9) The intercalation of fluorides in graphite can be performed in liquid anhydrous HF (AHF). [61] The intercalation rate is generally increased by the addition of elemental fluorine which can be added by bubbling the gas through the liquid. The intercalation of HF2¯ under these conditions is rapid, a phase-pure stage-2 CxHF2 can be produced within minutes at ambient temperature. [62] Following this, a slower exchange between intercalated HF2¯ and dissolved fluorometallate species takes place, [42] as shown in the following reaction scheme: (i) (ii) xC + (1+δ)HF + (1/2)F2 CxHF2·δHF + MFn¯ CxHF2·HF CxMFn + HF2¯ + δHF (1.10) (1.11) A wide range of alternate chemical and electrochemical oxidation reactions have also been studied. [63-64] For example, the oxidant K2MnF6 has been used in aqueous HF. 18 [63] MF6¯ anions (M = P,As,Sb) can be intercalated into graphite by an electrochemical method in oxidatively-stable solvents such as propylene carbonate (PC) [64] or nitromethane. [65-67] Table 1.2 Fluoride GIC’s with gallery height (di) anion (An) composition x as CxAn di (nm) reference HF2¯ C2-3(HF2)0.2-0.5 0.55-0.64 [42] RhF3¯ C28RhF3.3·1.3HF2 0.79 [42] SiF6 ¯ C12SiF6 0.79 [43] PF6¯ C12PF6 0.76 [43] MoF6¯ C11MoF6 0.84 [43] BF4¯ C7BF4 0.77 [44] AsF6¯ C8AsF6 0.81 [45] WF6¯ C14WF6·δHF 0.84 [46] OsF6¯ C8OsF6 0.81 [46] PtF6¯ C12PtF6 0.76 [46] TaF6¯ C5-8.8TaF 6 0.84 [47] SbF6¯ C6SbF6 0.85 [48] NbF6¯ C8.3-8.6NbF6 0.84 [49] 2 19 The conjugate anions of strong oxo-acids can also form acceptor-type GIC’s, as summarized in Table 1.3. HNO3 and H2SO4 GIC’s have been most intensively investigated among the acid GIC’s. [68-71] These compounds are prepared by oxidant-assisted intercalation or electrochemical methods. Several oxidants have been used in the intercalation reactions, including HNO3 [72], CrO3 [73], KMnO4 [74], (NH4)2S2O8 [75], PbO2 [76], HClO4 [76], MnO2 [77], Na2O2 [78], H2O2 [79], and N2O5 [80]. In highly oxidative conditions with acid, the graphite host is itself converted to graphite oxide, where oxygen bonds covalently to graphite, resulting in the loss of π-bonding and formation of an oxygen-bridged corrugated sheet structure. [81] Table 1.3 Stage-1 acid GIC’s with c-axis repeat distances (Ic). Acid composition Ic (nm) Reference HNO3 C24 NO3·3HNO3 0.78 [82] H2SO4 C24 HSO4·2H2SO4 0.80 [83] HClO4 C24 ClO4·2HClO4 0.77-0.80 [84] HSO3CF3 C26 CF3SO3·1.63HSO3CF3 0.80 [85] CF3COOH C26 CF3COO·δCF3COOH 0.82 [86] Graphite forms intercalation compounds with all the halides except iodine. In addition, mixed halogen species such as ICl and IBr can be intercalated. Some specific features in the synthesis of halogen GIC’s are that the reaction temperature is quite 20 low, ranging around room temperature, and it is difficult to obtain stage-1 GIC’s even at high reactant vapor pressures. The intercalation of fluorine is considerably different from that of the other halogens, however. When graphite reacts with fluorine gas, two types of product can be obtained, classified as fluorinated and fluorine-intercalated graphite. The former is produced at high temperature ~ 380-600°C, comprises only sp3 hybridized carbon, and is gray or white in color. [43, 47, 87-88] In the latter type, fluoride or bifluoride ions are intercalated in planar-sheet graphitic galleries under milder conditions. [43,89] Many other acceptor-type GIC’s have been reported. Among these, CrO3 GIC’s which obtained from graphite and CrO3 in refluxing glacial acetic acid have been studied extensively. [90] When GIC’s are heated rapidly, a dramatic thermal expansion can occur along the c-axis, resulting in extremely large volume increases, a process called exfoliation. Exfoliation is generally irreversible and leads to a spongy or foamy, low-density, highsurface area carbon, which is advantageous for some applications because of its flexibility, chemical inertness, low transverse thermal conductivity, and ability to withstand high temperatures. [91] Examples of applications are in gas or oil adsorption or, when exfoliated graphite is pressed into sheets, the product (Grafoil®) is widely used as a high-temperature gasket or packing material. 21 1.2.2 Preparation of acceptor-type GIC’s 1.2.2.1 Electrochemical oxidation method By evaluation of galvanostatic potential-charge curves and coulometry when oxidizing a graphite electrode, intercalation reactions can be continuously monitored and controlled. Intermediate phases (higher stages) can therefore be readily obtained. The oxidation potential required for graphite intercalation is greater than 4.0 V vs. Li/Li+, requiring very oxidatively-stable anions for intercalation. This requirement also limits the list of appropriate solvents, current collectors, and binders. As the applied potential is increased, higher intercalate content (lower stage) GIC’s are obtained as illustrated in Figure 1.6. The curve displayed was obtained with graphite electrode oxidized in the LiN(SO2CF3)2/CH3NO2 electrolyte and shows a series of transition points (Figure 1.6, labeled a-e) that indicate intermediate stages formed by intercalation of the graphite electrode. A stage-3 GIC is obtained at point a and a lower stage obtained at higher potential (stage 1 at point e). Beyond point d, the overpotential increases rapidly and the final plateau at 5.5 V corresponds to the oxidative stability limit for the electrolyte. 22 6.0 d 5.0 E, V b e c a 4.0 3.0 0 50 100 150 200 250 Charge, mAh/g Figure 1.6 Galvanostatic potential-charge curve of LiN(SO2CF3)2 in nitromethane. 23 1.2.2.2 Chemical oxidation method This method allows the preparation of bulk quantities of the GIC in a shorter time, and requires no binders or additives. The key step in this method is the selection of an appropriate oxidizing agent for graphite and, if required, a suitable oxidatively-stable solvent. In aqueous HF solution, our research group has found K2MnF6 to be a convenient and effective oxidizing agent. [63] It is highly soluble, stable at room temperature, and can be easily prepared. However, due to the limited oxidative stability of aqueous HF, only stage-2 or higher GIC’s can be formed. [92] KMnO4 and K2Cr2O7 are both strong oxidizing agents that are used in preparation of low-stage graphite sulfate and graphite nitrate in concentrated acids, [74] but they are not stable in hydrofluoric acid. PbO2 and NaBiO3 can oxidize graphite and form lowstage GIC’s of perfluorooctanesulfonate (PFOS) in aqueous HF, but are relatively insoluble, resulting in slow reactions and an admixture of solid reagents and products. The oxidation of graphite in anhydrous hydrogen fluoride (AHF) using F2 gas in a metal-vacuum line is another approach. F2 is a powerful oxidizing reagent and can generate stage-1 GIC’s. However, the handling of F2 or AHF is relatively difficult, and the scale for such reactions in the laboratory is typically limited to < 1 g. Also, anions must dissolve in AHF and be stable to the strong oxidizing reagent F2. However, in cases where no route can be found in aqueous acid, the anhydrous method can be successful. For example, the GIC containing bis(oxalato)borate anion was first prepared this way, as will be described in Chapter 3. [93] However, in many 24 cases, the large fluoroanion GIC’s that have been prepared using other methods cannot be obtained using F2/AHF (Table 1.4). Most likely, these anions are either not stable or sufficiently soluble under these conditions for the desired reaction to proceed. Some GIC compounds described later in this thesis were prepared using electrochemical intercalation when chemical methods failed. For examples, GIC containing bis(perfluoropinacolato)borate anion, and GIC containing bis(hexafluorohydroxyisobutyrato)borate anion cannot be obtained using K2MnF6 in aqueous HF, but can be prepared using electrochemical method in nitromethane. 25 Table 1.4 Synthetic reactions and results using different oxidizing reagents. anion oxidizing reagent / solution product N(SO2CF3)2¯ K2MnF6 / aq.HF F2 / AHF stage 2 GIC NR C(SO2CF3)3¯ K2MnF6 / aq.HF F2 / AHF high stage GIC NR N(SO2CF2CF3)2¯ K2MnF6 / aq.HF F2 / AHF high stage GIC NR N(SO2CF3)(SO2(CF2)3CF3)¯ K2MnF6 / aq.HF F2 / AHF high stage GIC NR Al(OR)4¯ * (R=C(CF3)3) (Ag salt) K2MnF6 / aq.HF PbO2 / aq.HF NaBiO3 / aq.HF F2 / AHF NR NR NR NR Al(OR)4¯ * (R=C(CF3)3) (Li salt) K2MnF6 / aq.HF PbO2 / aq.HF NaBiO3 / aq.HF F2 / AHF NR NR NR NR B[OC(O)C(O)O]2¯ K2MnF6 / aq.HF F2 / AHF NR stage 1/2 GIC B[OC(CF3)2C(O)O]2¯ K2MnF6 / aq.HF F2 / AHF NR stage 1 GIC B[OC(CF3)2C(CF3)2O]2¯ K2MnF6 / aq.HF F2 / AHF NR stage 1 GIC * salt provided courtesy of Dr.Ingo Krossing, Anorganische Chemie, Universität Karlsruhe, Institut Für Anorganische Chemie, Karlsruhe. 26 1.3 GENERAL APPARATUS AND PROCEDURES 1.3.1 Stainless steel vacuum line Anhydrous hydrogen fluoride (AHF) (Matheson, Pure grade) and F2 (Air Products and Chemicals, Inc., > 97 %) were handled on a stainless steel vacuum line equipped with high-pressure Autoclave Engineering SW-4074 valves and Whitey 1KS4 valves. The vacuum line was fitted with a Kel-F trap (approximately 20 mL volume) into which AHF could be dynamically condensed (Figure 1.7). Un-reacted volatile fluorides were destroyed by passing through a soda-lime tower placed before a N2 cold trap. A Helicoid gauge (0-1500 torr) and Varian thermocouple gauge (0-2000 μm) were attached to the vacuum line in order to monitor the pressure. Reaction vessels were constructed with 316 stainless steel Swagelok compression fittings equipped with Teflon ferrules and connected to the line. The vacuum line and reaction vessels were periodically passivated with F2 under appropriate conditions. 27 autoclave valves Helicoid gauge F2 cylinder to vacuum pump to reactors AHF trap soda lime trap AHF cylinder autoclave valves Whitey valves Figure 1.7 Schematic of stainless steel vacuum line. N2 (liq) trap 28 1.3.1.1 Handling anhydrous HF (AHF) AHF (Matheson, pure grade) was condensed into a Kel-F trap, and then subjected to two freeze/pump/thaw cycles at 0°C to remove H2 before use. The trap was fitted with Whitey valves at both the inlet and outlet ends of the stainless steel head. The inlet of the trap was connected to the metal vacuum line while the outlet was connected to the HF reagent cylinder. The trap was held at 77 K while HF was condensed in under dynamic vacuum. After collection, the trap was slowly warmed to room temperature. Very dry AHF may be transferred into reaction vessels when the temperature is held at 0°C to reduce the vapor pressure of any H2O impurity. The trap may contain a small amount of SbF5 (to react with water and form involatile oxides). 1.3.1.2 Assembled reaction apparatus for use with AHF Reactions utilizing AHF were carried out in 3/8 ״FEP Teflon tubing with one end heat sealed and the other connected via a 316 stainless steel tee to accommodate two Teflon reaction tubes (as illustrated in Figure 3.1). The reaction tubes were preloaded with involatile reactants in an Ar/He glove box and then attached to the metal vacuum line via a Swagelok fitting with Teflon ferrules. 29 1.3.2 Graphite Graphite flake (Graftech Inc., GTA grade, average particle diameter ≅ 250 μm) was employed and used as received in reactions except where otherwise noted. SP-1 grade powder (Union Carbide, average particle diameter ≅ 100 μm) and natural graphite flake (Aldrich, 1-2 μm) were also used in some reactions. 1.3.3 Elemental analyses GIC samples to be analyzed for F and B content were first digested using microwave heating (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each in Teflon advanced composite vessels containing excess HNO3 (70%, Fisher Scientific) in order to form an acid solution. The solutions were then diluted with milli-Q water. Elemental analyses for F in GIC products were performed using an ionselective fluoride combination electrode (VWR International, Inc.) with a standard single-junction sleeve-type reference electrode and pH meter with an expanded mV scale (VWR 8005). Elemental analyses for B in GIC product were performed using inductively coupled plasma - atomic emission spectroscopy, ICP-AES (S.A. Inc, JY2000U analyzer). 30 1.3.4 Powder X-ray diffraction (Powder XRD) Powder XRD data were collected on a Siemens D5000 powder diffractometer with Ni-filtered Cu Kα radiation (1.5418 Å) using a variable slit mode and a detector slit width of 0.2 mm. Data were collected at 0.02° 2Ө steps, between 2° and 65°. Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data used in structural modeling. The materials dealt with mostly were layered structures, and the platy particles tend to lie flat on the sample holder (the preferred orientation effect). Therefore, the structural information obtained often relates primarily to the direction perpendicular to the layers. The diffracted peak positions directly provide the repeat distances of layers (basal spacing) and the change of interlayer distance due to an intercalation reaction can be readily obtained. 1.3.5 Thermal gravimetric analysis (TGA) Thermal analyses were used to observe physical and chemical changes as a function of temperature. TGA data shows the mass of samples as the temperature increases linearly. In this research, TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed with nitrogen. Thermal scans from ambient to 800°C were performed. 31 1.4 FLUORINE ANALYSIS IN GIC’s 1.4.1 Control of solution parameters In this thesis research, a new sample digestion protocol/method was developed to allow the determination of free fluoride for GIC products using analyses based on ionselective fluoride electrodes. HF is a weak acid in aqueous solutions (pKa = 3.18). For pH < 5, the formation of HF or HF2¯ (which cannot be detected by the fluoride electrode) interferes with accurate F analyses. In addition, the total ionic strength of sample solutions must be maintained with the specification range of the electrode. In order to address these issues, we used a combination of both samples and standards with an equal volume of low level–total ionic strength adjustment buffer (LL-TISAB) to provide control over both pH and ionic strength. 1.4.2 Test of digestion procedure In order to determine the effectiveness of our digestion protocol, and eliminate the possibility of the presence of fluoride from other sources, control experiments were performed. The other sources of fluoride could be from HNO3 or other reagents used in digestion, the anions, graphite flake, or even milli-Q water used in dilution of samples. All dilutions were performed using standard silica glassware at ambient 32 temperature. Sodium fluoride was used as a standard fluoride sample to prepare stock and standard fluoride solutions. Control samples of milli-Q water, graphite flake, 70%HNO3, and salts were dissolved with 70%HNO3 using the microwave digester and diluted to 0.2%v/v acid solutions due to the limited pH range of fluoride electrode. Equal volumes of LL-TISAB were added into sample solutions prior to analysis in order to adjust total ionic strength. Less than 0.1 ppm F of all samples was detected as shown in Table 1.5. It was therefore shown that no other significant source of fluoride is introduced in this digestion method. Additionally, in order to check the accuracy of fluoride measurement determined by this method, a comparison study between digested and undigested samples with known fluoride content was performed. Sodium fluoride was used as fluoride source. One set of samples was digested according to the previously described procedure, and another was only dissolved into milli-Q water. Results are shown in Table 1.6. 33 Table 1.5 Fluoride content of digested control samples. Control samples fluoride (ppm) milli-Q water 0.009 graphite flake 0.011 HNO3 0.024 lithium(trifluoromethanesulfonyl)imide, LiN(SO2CF3) 0.021 potassium perfluorooctanesulfonate, KC8F17SO3 0.029 lithium(oxalato)borate, LiB[OC(O)C(O)O]2 0.026 sodium bis(perfluoropinacolato) borate, NaB[OC(CF3)2C(CF3)2O]2 0.027 sodium bis(hexafluorohydroxyisobutyrato)borate, NaB[OC(CF3)2C(O)O]2 0.023 34 Table 1.6 Comparison of fluoride content between digested and undigested control samples. glassware fluoride (ppm) detected calculated %error digested 1 2 3 3.277 3.408 3.042 3.225 3.507 3.037 mean 1.6 -2.8 -0.2 -0.5 mean 1.9 1.2 1.4 1.5 undigested 1 2 3 3.767 4.613 3.521 3.696 4.556 3.472 Both sample sets gave average errors <2%. Results obtained from digested samples are slightly lower than those from undigested samples; this might be explained by the sample loss during the additional transfer step. 1.4.3 Test for container reactions One concern in handling hydrofluoric acid for quantitative analysis is the error associated with HF reaction with silica. Because it is convenient to perform dilutions using standard silica glassware at ambient temperature, control experiments were performed using only Nalgene® plastic labware in order to evaluate any effect on 35 fluoride content resulting from HF reaction with silica. Data obtained are in Table 1.7. As can be seen, there is no significant difference between the sample sets. In both cases, detected fluoride is consistently lower than the calculated value, by approximately 2%. This is most likely explained by a slight loss of sample during transfer from microwave vessel to volumetric flask. Table 1.7 Comparison of fluoride analyses between digested samples using glass and plastic labware. glassware fluoride (ppm) detected calculated %error Pyrex® 1 2 3 Nalgene 1 2 3 2.084 1.769 1.441 2.114 1.802 1.472 ® 1.630 1.468 1.843 mean -1.4 -1.8 -2.1 -1.8 mean -1.7 -2.2 -1.8 -1.9 1.658 1.502 1.877 As can be seen from all control experiments, the digestion method developed with standard glassware can be used for quantitative F analyses with errors of < 2%. Additional work in the future should be performed to reduce the small systematic error found in these control experiments. 36 1.5 POWDER X-RAY DIFFRACTION ANALYSIS OF GIC’S Powder XRD data provided a series of (00l) reflections, as shown in the Figure 1.8 for stage 1 CxB[OC(O)C(O)O]2·δF, the data from this Figure are summarized in Table 1.8. Integrated peak areas are determined by DIFFRAC AT software from intensities in counts per second (cps) multiplied by the step time (s); a linear background was assumed in all cases. [94] As can be seen, the major reflections of the powder XRD patterns in Figure 1.8 agree well for stage 1 GIC in both observed and calculated gallery heights, di. However, there is some evidence for a mixture of stage 1 and 2 GIC’s, as observed from asymmetrical peaks and a shoulder on the (005) peak. In the case of flaky graphite, which has a very large particle diameter (≅ 250 μm), it is often difficult to obtain homogeneous single-stage GIC products. It is likely that the higher stage product is located near the particle centers, and lower stage GIC’s are near the edges. 0 10 30 40 50 (009) (006) 20 (008) (005) (002) (001) intensity (004) 37 60 2 Ө / degree Figure 1.8 Powder XRD pattern of stage 1 products of CxB[OC(O)C(O)O]2·δF. Assigned (00l) indices are indicated. 38 Table 1.8 Data obtained from powder XRD of stage 1 CxB[OC(O)C(O)O]2·δF peak # 2Ө dobs / nm dcalc / nm index (hkl) integrated peak area 1 6.163 1.43 1.42 (001) 9.9 2 12.43 0.713 0.712 (002) 11.3 3 25.19 0.354 0.350 (004) 385 4 31.43 0.284 0.285 (005) 51 5 37.94 0.237 0.237 (006) 3.5 6 51.46 0.177 0.178 (008) 10.8 7 58.32 0.158 0.158 (009) 8.8 39 1.6 STRUCTURAL REFINEMENT MODELING 1.6.1 Overview The energy-minimized structure of large anions such as C8F17SO3¯ [95], N(SO2CF3)2¯ [92], B[OC(CF3)2C(CF3)2O]2¯ B[OC(O)C(O)O]2¯ [93], B[OC(O)C(CF3)2O]2¯ [96], [97] are calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis set and GAUSSIAN software. One-dimensional electron density maps are calculated from a structural model and also obtained from observed powder XRD reflection intensities. Observed intensity data are used to generate structure factors after correction by a Lorentz-polarization factor: Fobs ⎡ ⎛ sin 2 θ cosθ = ± ⎢ I × ⎜⎜ 2 ⎣ ⎝ 1 + cos 2θ ⎞⎤ ⎟⎟⎥ ⎠⎦ 1/ 2 (1.12) where I is the integrated peak intensity. Calculated structure factors, F00l, are obtained using the angle-dependent atomic scattering factors. [98] Electron density maps are then generated for z = 0-1, at increments of 0.004, using: 1⎡ ⎛ 2π ρ ( z ) = ⎢ F0 + 2∑ F00l cos⎜ c⎣ ⎝ z ⎞⎤ ⎟⎥ ⎠⎦ (1.13) 40 where z is the fractional coordinate of atoms along the c axis, c is the unit cell dimension, and Fo is the zero-order structure factor. Structures are refined by minimizing the crystallographic R factor: R= ∑ k F (00l ) − F (00l ) ∑ k F (00l ) 00 l cal (1.14) obs In order to illustrate the method used to fit structural models from a onedimensional electron density map, the GIC containing bis(oxalato)borate anion (CxB[OC(O)C(O)O]2·δF) with the powder XRD data shown in the previous section will be used as an example below. 1.6.2 Structural model calculations All of the molecular structures generated from this research are determined using Gaussian 03W software. The standard orientation geometry displays atomic coordinates for atoms internally in Cartesian coordinates. This orientation is chosen for maximum calculation efficiency, and corresponds to placing the center of nuclear charge for the molecule at the origin. Figure 1.9 (a), shows the optimized geometry determined for an isolated bis(oxalato)borate anion. Figure 1.9 (b) indicates that this relatively large anion is oriented with the long axis perpendicular to the graphene surfaces, having the B atom 41 at the gallery center, with four O atoms coordinated in tetrahedral geometry. This orientation is also employed for the other borate chelate anions. [96,97] In the structural model shown in Figure 1.9 (b), the GIC containing bis(oxalato)borate is prepared using F2/AHF, leading to additional fluoride ions being co-intercalated into the galleries. The position of these fluoride sites is indicated; these sites are only partially filled. 1.6.3 One-dimensional electron density map One-dimensional electron density maps of graphene sheets and bis(oxalato) borate anion intercalate for the structural model were calculated and are shown in Figure 1.10 (a) and (b), respectively. The number of the terms used in the calculation the one-dimensional electron density map affects the intensity and width of calculated peaks, and also the presence higher-order (ringing) peaks. The dimensions of the bis(oxalato)borate anion in Figure 1.9 (a) indicate that this relatively large anion must be oriented with the long axis perpendicular to the graphene surfaces. As illustrated in Figure 1.10 (b), this arrangement results in the electron density peak due to the B atom at the center (z = 0.5) with two adjacent O atom peaks (z ≅ 0.45 and 0.55), two C atom peaks at z ≅ 0.35 and 0.65, and another two O atom peaks appear at z ≅ 0.25 and 0.75. When fluoride co-intercalates are 42 included in the galleries by positioning on the graphene surfaces, these electron density peaks appear at z ≅ 0.28 and 0.72. One-dimensional electron density profiles of stage 2 CxB[OC(O)C(O)O]2·δF derived from the calculated structural model using 9 terms, 25 terms, and derived from observed powder XRD data using 9 terms are shown in Figure 1.11 (a), (b), and (c), respectively. As can be seen, using more terms in the one-dimensional electron density profile calculation provides a more detailed and sharper calculated density map. However, the number of terms for calculated maps should not be much greater than the number of available diffraction peak intensities in observed data. Due to the limited diffraction data, the correspondence of these maps is sufficient to show the orientation of the chelate borate anions; however, they cannot be used to quantify the compositional parameters x and δ in CxAn·δF. 43 B C O F x z (a) y (b) Figure 1.9 (a) A geometry-optimized structural model of isolated bis(oxalato)borate anion, (b) structural model of stage 2 CxB[OC(O)C(O)O]2·δF, with four fluoride cointercalates per borate anion. Axes and some atomic labels are included. 44 (a) e¯ density (i) (ii) (iii) 0.0 0.2 0.4 O C O 0.6 B O 0.8 1.0 C O (b) e¯ density (i) (ii) (iii) 0.0 0.2 0.4 0.6 0.8 1.0 z Figure 1.10 One-dimensional electron density profiles of (a) graphene sheets and (b) bis(oxalato)borate anion intercalate using (i) 9, (ii) 25, and (iii) 50 terms in the calculation. 45 density (a) 0.0 0.2 0.4 0.6 0.8 1.0 z density (b) 0.0 0.2 0.4 0.6 0.8 1.0 z density (c) 0.0 0.2 0.4 0.6 0.8 1.0 z Figure 1.11 One-dimensional electron density profiles of stage 2 CxB[OC(O)C(O)O]2·δF derived from calculated structural model using (a) 9 terms, (b) 25 terms, and (c) derived from observed powder XRD data using 9 terms. 46 1.6.3.1 Optimization of intercalate gallery structure In order to obtain the best fit of one-dimensional electron density maps for the structural model with the observed data, the orientation and position of borate chelate anion and fluoride co-intercalates can be optimized. In these studies, the borate chelate anions are oriented with long z-axis fixed to be perpendicular to the graphene sheets, with the anion centered between graphene sheets. The following parameters were considered in fitting the one-dimensional electron density: The distance of graphene sheet to fluoride co-intercalate plane fixed at 0.31 nm Mole ratio of graphene sheet and intercalate, x optimized parameter Mole ratio of borate chelate anion to fluoride co-intercalate, δ optimized parameter The distance of graphene sheet to fluoride co-intercalate plane was at first set to be 0.32 nm from the value known from stage 2 CxHF2. Starting with this value, a better fit was obtained by slightly adjusting the value to 0.31 nm. The fluoride co-intercalates are fixed to lie in equal concentration on both encasing graphene surfaces. The optimized values obtained for x and δ by this method can subsequently be compared to those determined by elemental analyses. 47 1.7 THESIS OVERVIEW Further investigation into new graphite intercalation compounds can provide new materials and a better understanding of their properties. In this research, the goal has been to develop new chemical synthesis strategies, and to prepare acceptor-type GIC’s with new anions. Chapter 2 and 3 are studies on the fluoride contents of GIC’s containing both large fluoroanions (bis(trifluoromethanesulfonyl)imide and perfluoroctanesulfonate) and fluoride co-intercalate. A new digestion method is developed using an ionselective electrode and potentiometer that can quantitatively determine the fluoride cointercalate content in the solid sample. Before these studies, these GIC’s had been reported without detailed analyses of fluoride co-intercalate content. Chapter 4 describes the preparation of a new GIC containing bis(oxalato) borate by chemical intercalation using F2(g)/AHF. This is the first time that this GIC was synthesized. GIC’s up to stage 1 have been prepared, with a gallery height of 1.42 nm. The composition of this GIC is determined from thermogravimetric and elemental analyses. Chapter 5 describes the preparation of GIC’s containing the borate chelate anions bis(perfluoropinacolato)borate and bis(hexafluorohydroxyisobutyrato)borate by chemical intercalation using F2(g)/AHF. This is the first time that these GIC’s were prepared by chemical intercalation; previously they had been prepared using electrochemical oxidation. By these chemical syntheses, GIC’s up to stage 1 can be 48 obtained, with gallery heights of 1.45 and 1.43 nm, respectively. The composition of these GIC’s are determined using thermogravimetric and elemental analyses. 49 1.8 REFERENCES 1. Lerf, A. 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Zhang, X.; Lerner, M. M. Phys. Chem. Chem. Phys. 1999, 1, 5065. 96. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, 1. 97. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169. 98. Su, Z.; Coppens, P. Acta Crystallogr. 1997, A53, 749. 55 CHAPTER 2 EFFECT OF REACTION TIME ON THE COMPOSITION OF GRAPHITE BIS(TRIFLUOROMETHANESULFONYL)IMIDE Watcharee Katinonkul and Michael M. Lerner Department of Chemistry Oregon State University, Corvallis, OR 97331-4003 USA Carbon 2007, 45(3), 499-504. 56 2.1 ABSTRACT Graphite intercalation compounds (GIC’s) of composition CxN(SO2CF3)2·δF are prepared under ambient conditions in 48% hydrofluoric acid, using K2MnF6 as an oxidizing reagent. The stage 2 GIC product structures are determined using powder XRD and modeled by fitting one dimensional electron density profiles. A new digestion method followed by selective fluoride electrode elemental analyses allows the determination of free fluoride within products, and the compositional x and δ parameters are determined for reaction times from 0.25 to 500 h. Keywords: Graphite; Intercalation; X-ray diffraction; Modeling 57 2.2 INTRODUCTION Transition metal fluoroanions can be used as oxidants in the formation of graphite intercalation compounds (GIC’s) in anhydrous HF (AHF) or hydrofluoric acid. An initial study reported the formation of graphite bifluoride and planar-sheet graphite fluoride by this approach: [1] xC where M = AMFn / AHF 25 °C CxHF2·δHF (x > 28) or CxF (x > 2) (2.1) Mn (IV), Ni (IV), Co (III), or Fe (III). In these GIC products, x corresponds to the ratio of graphene carbon to anion, and δ to the ratio of neutral intercalate to anion intercalate. Reactions using these same oxidants in combination with fluoroanion salts have also produced a range of new GIC’s. The oxidant Mn(IV), in the form of MnF62-, has been particularly suitable for obtaining new GIC’s because the fluoromanganate anion does not itself intercalate into graphite under these conditions. For example, the addition of soluble perfluoroalkanesulfonate salts leads to intercalation of these anions, such as in the following reaction where a stage 2 GIC is obtained with x≅18 and δ≅4 [2]: aqu. HF xC + KC8F17SO3 K2MnF6 CxC8F17SO3·δF (2.2) 58 Stage 2 GIC products of the intercalates C10F21SO3¯, C2F5OC2F4SO3¯, and C2F5(C6F10)SO3¯ have been similarly prepared in hydrofluoric / nitric acid solutions, [3] and mixed-stage GIC’s with large fluoroanions such as bis(pentafluoroethanesulfonyl)imide, sulfonylimide, (CF3CF2SO2)2N¯, trifluoromethanesulfonyl-n-nonafluorobutane- (CF3SO2)(CF3(CF2)3SO2)N¯, and tris(trifluoromethanesulfonyl) methide, (CF3SO2)3C ¯, have also been obtained by similar reactions. [4] GIC’s containing the bis(trifluoromethanesulfonyl)imide (TMSI) anion, CxN(SO2CF3)2, were prepared for the first time under ambient conditions in 48% hydrofluoric acid containing K2MnF6. Within 15 minutes, a stable, stage 2 GIC product with x=37 and gallery height, di, of 0.81 nm was obtained. The anion monolayers are oriented within the galleries such that the pseudo-cylindrical anions have their long axes parallel to the graphene sheets, with the N oriented towards either encasing sheet. [4] It is well known that the oxidation of graphite and intercalation of anions can also lead to reversible co-intercalation of neutral species. These can act as dielectric spacers (i.e. solvents) within galleries. Examples include the co-intercalation of parent acid with conjugate anions: xC + HSO4¯ xC + HF2¯ H2SO4 HF CxHSO4·δH2SO4 CxHF2·δHF + + e¯ e¯ (2.3) (2.4) 59 where typical x and δ values are 24 and 2-3, respectively [5]. A chemical method to obtain a similar product to that from reaction (2.4) used elemental fluorine in the presence of liquid or gaseous HF at 20°C to produce a stage 2 salt, CxF·δHF (stage 2: x=16; 1≤δ≤4.7). This GIC evolves HF and increases in stage (from stage 2 to 3, or 1 to 2) after evacuation to remove neutrals. [6-8] Another common co-intercalation involves organic solvent molecules, for example: xC + LiAsF6 - e¯ / CH3NO2 CxLiAsF6·δ CH3NO2 + e¯ (2.5) where typical x and δ values are 24 and 2, respectively. [9] Aside from the co-intercalation of neutral species, the intercalation reaction can be complicated by the incorporation of more than one species of anionic intercalate. Important examples of this complexity are the presence of oxide or hydroxide in highly-oxidized GIC’s containing oxoanion intercalates, and the presence of fluoride in GIC’s containing fluoroanion intercalates. The presence of oxide or fluoride intercalate within graphene galleries can have significant impact on physical or chemical properties. [10] During graphite oxidation in an aqueous H2SO4 electrolyte, graphite sulfate salts are formed initially. Graphite oxide, which, unlike the graphite salts, involves directed C-O bonding, is produced when oxidation is continued on the 1st-stage salt beyond a charge on carbon corresponding to C21+. [11] This is reinforced by longknown chemical preparations of graphite oxide which involve the action of strong 60 oxidizing agent (KMnO4, KClO3) on the graphite salts of oxoanions (ClO4¯, HSO4¯, NO3¯). [12] The mechanism for the conversion of the graphite salt to graphite oxide is not completely understood. Indeed, the structure for graphite oxide itself is still being explored: the oxide or hydroxide within galleries may result from conversion of oxoanions of neutral co-intercalate species. [13] Similar reactions are known for fluoroanions to generate fluoride intercalate within graphene galleries. For example, the electrochemical conversion of the graphite bifluoride salt, CxHF2(HF)δ (x=16), to the planar-sheet graphite fluoride, CxF(HF)δ (x=5.5), by oxidation in anhydrous HF/1M NaF has been demonstrated. [6] In this study, we explore the detailed composition of the GIC obtained by oxidation of graphite in hydrofluoric acid containing the TMSI anion. After evacuation of the product to remove any neutral HF co-intercalate, a quantitative approach to determining the fluoride intercalate content is developed and employed. Several quantitative analytical methods for fluoride are known, including spectrophotometry [14] and chromatography; [15] however, the most common and convenient approach is the electroanalytical method based on potentiometry with ionselective electrodes. [16, 17] Aqueous matrices are generally the easiest to analyze, and typical applications include the determination of fluoride ion in drinking water, [18] rainwater, [19] serum, [20] etc. The accuracy of the analyses depends on the calibration procedure (direct reading potentiometry, single or double standard addition, bracketing solutions, Gran’s method). [21] In this work, a new digestion method is developed for GIC samples containing both fluoroanions and fluoride co- 61 intercalate that can quantitatively determine the fluoride intercalate content in the solid sample. 2.3 EXPERIMENTAL Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter), hydrofluoric acid (Mallinckrodt AR, 48% w/o), hexane (Fischer Chem, certified grade), lithium trifluoromethanesulfonimide, LiN(SO2CF3)2, (3M Corp), nitric acid (Fischer Scientific, certified ACS PLUS, 68.0-70.0%), glacial acetic acid (EM Science, 99.7%), and sodium chloride (Mallinckrodt, AR grade) were used as received. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h before use. Potassium hexafluoromanganate, K2MnF6, was synthesized as a bright yellow powder according to a literature method. [22] Ultrapure water (resistivity = 18 MΩ·cm at 25°C) from a Milli-Q Labo system (Millipore, Milford, MA), was used throughout the experiments. Low Level Total Ionic Strength Adjustment Buffer (LLTISAB) was prepared by addition of glacial acetic acid (57 mL) and reagent-grade NaCl (58 g) to 500 mL water, slow addition of 5 M NaOH until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [23] In these reactions, LiN(SO2CF3)2 (200 mg, 0.70 mmol) and K2MnF6 (180 mg, 0.70 mmol) were first dissolved into hydrofluoric acid (15 mL) at ambient temperature, and then graphite (100 mg, 8 mmol) was added to provide mole ratios of 62 12:1:1 for graphene C : Mn (IV) : N(SO2CF3)2¯ anion. The solutions were stirred in air for the specified reaction times, then filtered and rinsed with 15 mL hydrofluoric acid. The filtrate was briefly rinsed with 5 mL hexane and dried overnight under vacuum. Products were subsequently stored under an inert atmosphere. Powder X-ray diffraction (powder XRD) data were collected on a Siemens D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a detector slit width of 0.2 mm. Data were collected at 0.02 °2Ө steps, between 2˚ and 65˚. Collection times varied from 0.1 s / step for routine analyses to 1 s / step for data used in structural modeling. TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed with nitrogen. Thermal scans from ambient to 800 °C were performed under flowing N2 at 5°C / min. A new sample digestion protocol was developed for the GIC products. Samples (30 mg) were digested with 70% HNO3 (1 mL) using a microwave digester (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this method completely decomposed the solid product and generates a homogeneous solution. The solutions were diluted to 50.0 mL and stored in Nalgene® plastic bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis. Fluoride analyses on the products were performed using an ion-selective fluoride combination electrode with a standard single-junction sleeve-type reference electrode and pH meter with an expanded mV scale (VWR International, Inc.). The 63 mean of three replicate data are reported. A calibration curve was obtained using fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F / L. All dilutions were performed using standard silica glassware at ambient temperature. Below pH 5, the formation of HF or HF2¯, which cannot be detected by the fluoride electrode, can interfere with accurate F analyses. [23]. In addition, the total ionic strength of the samples solutions must be maintained with the specification range of the electrode. Finally, under some conditions, hydrofluoric acid can react with silica. In order to address these issues, combination of both samples and standards with an equal volume of LL-TISAB, provided the required control over both pH and ionic strength. In addition, control experiments performed using only Nalgene® labware showed less than 1% difference in detected fluoride content from data obtained using standard glassware. The energy-minimized structure of the N(SO2CF3)2¯ anion was initially calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis set and GAUSSIAN 03W software. One-dimensional electron density maps were calculated from a structural model and also obtained from observed powder XRD (00l) reflection intensities (corrected for Lorentz-polarization). The mole ratio of graphene carbon to N(SO2CF3)2¯ anions (x) and to intercalated fluoride (δ) were refined to minimize the crystallographic R factor for observed and calculated maps. Additional details of this method have been described previously. [2,4] 64 2.4 RESULTS AND DISCUSSION As reported previously, intercalation of the bis(trifluoromethanesulfonyl)imide anion (TMSI) by chemical oxidation in hydrofluoric acid is remarkably rapid and forms a stage 2 GIC product, which is obtained within 0.25 h at ambient temperature. However, due to the limited chemical oxidation potential in 48% hydrofluoric acid, stage 1 GIC’s cannot be obtained under these conditions. [4] Over the range of reaction times explored here (0.25–500 h), all powder XRD reflections observed can be indexed with (00l) indices for stage 2 GICs with gallery heights, di, close to 0.8 nm. Powder XRD and compositional data for products are provided in Figure 2.1 and Table 2.1. After the longest reaction time (500 h), a shoulder on the largest diffraction peak (at approximately 24 °2θ) indicates a minor impurity phase that can be ascribed to decomposition of K2MnF6 in aqueous HF to form an insoluble fluoromanganate salt. The gallery heights of these GIC products are similar to those for graphite sulfate [24] or graphite nitrate [25] anion intercalate. This places a steric requirement that the long axis of the TMSI anion be parallel to the encasing graphene sheets. Figure 2.2 indicates the structure model employed for the graphite TMSI product; this will be discussed in greater detail below. Because the GIC’s were dried in vacuo prior to analyses, low temperature losses that can be ascribed to surface-adsorbed or intercalated H2O or HF are almost entirely absent (less than 0.5% mass loss to 100°C). TGA data shows three mass loss 65 major regions from ambient to 800°C (Figure 2.3). The first loss occurs at approximately 100-120°C, the next at 150-300°C and the third above 550°C. Using DSC and powder XRD on thermolyzed samples, the first two mass loss regions have been ascribed to intercalate anion decomposition and volatilization. [4] Powder XRD patterns of the GIC products heated briefly at 300°C showed an amorphous structure, with only a broad reflection at 0.34 nm. After annealing under an inert atmosphere at 300°C, a crystalline graphite structure was reformed, indicating that the graphene sheets remain largely intact after heating at this temperature. The final mass loss above 550°C is ascribed to decomposition of the graphene sheets. Under the same conditions, graphite itself starts to decompose at 700°C. In these analyses, mass loss below 500°C is therefore ascribed to intercalate loss only. It is notable that the TMSI anion is far less thermally stable in the GIC product than in the lithium salt, where decomposition occurs as a single sharp event with an onset at 400-500°C. (Figure 2.3) In order to evaluate the fluoride ion intercalate content in these products separately from that of the TMSI anion, a new digestion procedure was developed. Using control experiments, conditions were developed that allowed quantitative dissolution of the graphene sheets and dissolution of the fluoride co-intercalate without significant decomposition of the TMSI anion, which would itself form additional fluoride co-intercalate. Control experiments using LiN(SO3CF3)2 and NaF were run and indicated that under these conditions < 1% of fluoride intercalate content determined by analysis were due to TMSI anion decomposition. (006) (007) 500 h (004) (002) (003) 66 intensity 190 h 100 h 50 h 25 h 0.25 h 0 10 20 30 40 50 60 2Ө / degree Figure 2.1 Powder XRD patterns of the products obtained, with reaction times indicated. For each powder XRD trace, there is a break in intensity data as indicated by hashes on the y-axis. 67 Table 2.1 Gallery height (di) and composition for stage 2 graphite bis(trifluoromethanesulfonyl)imide, CxN(SO2CF3)2·δF, at different reaction times as determined by powder XRD, elemental analyses, and thermogravimetry. reaction time /h di / nm mass loss / pct x δ 0.25 0.793 29.4 56 0.08 25 0.799 31.6 51 0.14 50 0.799 32.3 50 0.27 100 0.796 32.4 50 0.30 190 0.793 31.5 53 0.54 500 0.802 8.5 62 0.84 68 y z x N B S C C O O F F Figure 2.2 Structural model for CxN(SO2CF3)2·δF. The fluoride positions are partially occupied. 69 100 mass loss / pct 80 25 h 60 40 20 0 LiN(SO2CF3)2 0 200 400 600 800 temperature / °C 100 0.25 h mass loss/ pct 25 h 100 h 80 60 40 0 200 400 600 temperature /°C Figure 2.3 TGA data for products described in Table 2.1. 800 70 d powder For the composition of CxN(SO2CF3)2·δF, both x and δ need to be determined. From the data obtained above, mass loss below 500°C was ascribed to total intercalate content, and the fluoride anion content was separately determined using F analyses as described above. Therefore, the compositional parameters were determined by simultaneously solving the following relations: mass loss pct = 280.1 + 19.0δ × 100 (2.6) 12.0x + 280.1 + 19.0δ mass fraction of fluoride = 19.0δ (2.7) 12.0x + 280.1 + 19.0δ Figure 2.4 and Table 2.1 indicate the values determined for x and δ vs. reaction time. These data represent an unusually detailed understanding of the intercalate composition during chemical synthesis in hydrofluoric acid, which is typically limited to estimate of the fluoroanion content. As can be seen, fluoride content increases steadily over long reaction times, but does not exceed a value of 1 (which would indicate equimolar content of fluoride and the imide anions). From the powder XRD data for the sample obtained at longest reaction time, the value obtained, δ=0.84, is likely to be slightly higher than actually present in the GIC phase due to a small impurity of a fluoromanganate salt byproduct in the sample . 71 1.0 0.8 δ 0.6 0.4 0.2 0.0 0 100 200 300 400 500 time / h Figure 2.4 Increase in fluoride content, δ, in CxN(SO2CF3)2·δF vs. reaction time. 72 This result contrasts the fluoride content determined for graphite perfluorooctanesulfonate from partial refinement of crystallographic data. In that study, δ was determined to be approximately 4 after a 72 h reaction. The large difference in the fluoride content is likely related to the very different gallery dimensions, the perfluorooctanesulfonate anion forms GIC’s containing intercalate bilayers with a gallery height of 3 nm. [2] Because the sulfonate oxygen and nitrogen bear the largest negative charge (Mulliken charges calculated are N = -0.71, O = -0.54), the anion orientation was restricted in our structure model to provide close contact of these atoms with the positively charged graphene sheets. This is accommodated by having the N and 2 sulfonate O’s atoms directed towards a single graphene sheet. This orientation allows for two possibilities, the anions may face towards either of the two encasing positive graphene surfaces. The structure model employed assumes equal populations of both “up” and “down” orientations. The graphene C to N / O anion atom separations were fixed at approximately 0.30 nm, which is comparable to the graphene plane-oxygen distance of 0.32 nm known for graphite nitrate. [25] The fluoride anions present in the intercalate galleries were fixed to lie in a plane 0.31 nm from the graphene carbon plane. This distance was determined by comparison to stage 1 planar-sheet graphite fluorides, CxF, where the distance is in the range of 0.28–0.30 nm (assuming a central position of the fluoride in the galleries). [26] Due to the bilayer arrangement of fluoride in CxF, and the semi-covalent nature of the C-F intercalation, these values are likely to represent a lower limit for this distance. [27,28] Stage 2 graphite bifluoride 73 exhibits Ic=0.98 nm, which gives a maximum graphene plane to fluoride plane distance of 0.32 nm. The structure model uses equal populations of fluoride intercalate on both encasing graphene sheets. Figure 2.5 shows one-dimensional electron density profiles derived from refined structural model and observed powder XRD data for the product obtained after 50 h reaction. The x value determined in Table 2.1 was used in the structural model. The agreement between profiles shows, at least qualitatively, that the structural model employed correctly describes the GIC products obtained. 74 1.0 0.8 (i) (ii) 0.6 z 0.4 0.2 0 electron density Figure 2.5 One-dimensional electron density profiles derived from calculated structural model (i) and observeXRD data (ii). 75 2.5 REFERENCES 1. Lemmon, J.; Lerner, M. M. Carbon 1993, 31, 437. 2. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100. 3. Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981. 4. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363. 5. Aronson, S.; Lemont, S.; Weiner, J. Inorg. Chem. 1971, 10, 1296. 6. Takenaka, H.; Kawaguchi, M.; Lerner, M. M.; Bartlett, N. J. Chem. Soc. Chem. Commun. 1987, 19, 1431. 7. Watanabe, N.; Touhara, H.; Nakajima, T.; Bartlett, N.; Mallouk, T.; Selig, H. Fluorine Intercalation Compounds of Graphite. In Inorganic Solid Fluorides; Hagenmuller, P., Ed.; Academic Press: New York, 1985; 332. 8. Hagiwara, R.; Bartlett, N. Thermodynamic Aspects of the Intercalation of Graphite by Fluoroanions. In Fluorine-Carbon and Fluoride Carbon Materials; Nakajima, T., Ed.; New York: Marcel Dekker, 1995; 67. 9. Billaud, D.; Chenite, A.; Metrot, A. Carbon 1982, 20, 493. 10. Dresselhaus, M. S.; Endo, M.; Issi, J. Physical Properties of Fluorine- and Fluoride-Graphite Intercalation Compounds. In Fluorine-Carbon and Fluoride Carbon Materials; Nakajima, T., Ed.; Marcel Dekker : New York, 1995; 95. 11. Metrot, A.; Fischer, J. E. Synth. Met. 1981, 3, 201. 12. Hummers, W. S., Jr.; Offeman, R. E. J. Am. Chem. Soc. 1958, 80, 1339. 13. Nakajima, T.; Mabuchi, A.; Hagiwara, R. Carbon 1988, 26, 357. 14. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination of Water and Waste Water 18th Edition. American Public Health Assn: Washington, DC, 1992; 4. 15. Michigami, Y.; Kuroda, Y.; Ueda, K.; Yamoto, Y. Anal. Chim. Acta. 1993, 274, 299. 16. Wen, M.; Li, Q.; Wang, C. Fluoride 1996, 29, 82. 76 17. Wen, M.; Shi, N.; Qin, Y.; Wang, C. Fluoride 1998, 31, 74. 18. Ortiz, D.; Castro, L.; Turrubiartes, F.; Milan, J.; Diaz-Barriga, F. Fluoride 1998, 31, 183. 19. Hara, H.; Huang, C. C. Anal. Chim. Acta. 1997, 338, 141. 20. Torra, M.; Rodamilans, M.; Corbella, J. Sci. Total Environ. 1998, 220, 81. 21. Commann, K. Application of Ion Selective Electrodes. Wydawnictwa NaukowoTechniczne. Scientific publishers: Warsaw, 1977. 22. Bode, H.; Jenssen, H.; Bandte, F. Angew. Chem. 1953, 65, 304. 23. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual. VWR International, Inc., Westchester, PA, USA, 2002. 24. Lopez-Gonzalez, J. D.; Rodriguez, A. M.; Vega, F. D. Carbon 1969, 7, 583. 25. Zhang, X.; Lerner, M. M.; Gotoh, H.; Kawaguchi, M. Carbon 2000, 38, 1775. 26. Mallouk, T.; Bartlett, N. J. Chem. Soc. Chem. Commun. 1983, 3, 103. 27. Mallouk, T.; Hawkins, B. L.; Conrad, M. P.; Zilm, K.; Maciel, G. E.; Bartlett, N. Phil. Trans. R. Soc. London 1985, A314, 179. 28. Matsuo, Y.; Segawa, M.; Mitani, J.; Sugie, M. J. Fluorine Chem. 1998, 87, 145. 77 CHAPTER 3 A STUDY OF FLUORIDE CONTENT IN GRAPHITE PERFLUOROOCTANESULFONATE INTERCALATION COMPOUNDS Watcharee Katinonkul and Michael M. Lerner Department of Chemistry Oregon State University, Corvallis, OR 97331-4003 USA 78 3.1 ABSTRACT Graphite intercalation compounds (GIC) containing perfluorooctanesulfonate, CxC8F17SO3·δF are prepared under ambient conditions in 48% hydrofluoric acid, using K2MnF6 as an oxidizing reagent. Powder X-ray diffraction indicates a gallery height, di, of 3.0 nm. The composition (x and δ parameters) is evaluated by thermogravimetric and elemental analyses. 79 3.2 INTRODUCTION The intercalation of perfluoroalkanesulfonate anions into graphite was first described by Boehm and co-workers. They prepared these compounds by the electrochemical oxidation of graphite in the corresponding acids at elevated temperature. [1] The first and smallest perfluoroalkylsulfonate intercalate, CF3SO3¯, shows a gallery expansion of 0.47 nm, which is consistent with a monolayer arrangement of intercalate anions. [2] These studies included the significant observation that GIC’s were formed with greatly expanded galleries when n > 4 in CnF2n+1SO3¯. In subsequent work on the electrochemical oxidation of graphite in neat CnF2n+1SO3H (n = 4, 6, 8) at elevated temperature, or at ambient temperature in an acid/propylene carbonate electrolyte, the first GIC’s with a bilayer arrangement of anion intercalates were obtained. [1,3-4]. The bilayer arrangement is a common intercalate orientation upon exchange of smectite clays [5] or other layered metal oxides or sulfides for surfactant-like intercalates. [6] This bilayer arrangement is favorable as it allows increased interaction between the charged sheet surfaces and oppositely-charged headgroups, while at the same time providing close interaction between nonpolar chains at the gallery centers. These reports demonstrated for the first time that graphite could form intercalation compounds with the galleries larger than 1.0 nm, and provided the first example of an amphoteric bilayer intercalate structure in a GIC. For example, the distance between graphene carbon planes encasing intercalated galleries, di, was found to be ~ 3.4 nm 80 for the perfluorooctanesulfonate compound, CxC8F17SO3 or CxPFOS, which agreed well with the expected dimensions for a bilayer intercalate structure. Previously in our group, CxPFOS was prepared by the electrochemical oxidation of graphite in LiC8F17SO3 (sat.)/CH3NO2 at ambient temperature. [7] Compounds of stages 3-7 were obtained, with di ≅ 2.5 nm for all stages, and x ≅ 17 for the stage 2 product with di ≅ 2.7 nm. However, the electrosynthetic reactions are inherently limited by the low charge density required to avoid a large overpotential at the carbon surface, which would result in electrolyte decomposition and passivation of the graphite surface. In addition, the high potential required for graphite oxidation limits the selection of binders, electrolytes, and current collectors. For these reasons, scalable chemical oxidation reactions are more likely to be realized. The first chemical route for the bulk preparation of this GIC was successfully found in 48% hydrofluoric acid, using the oxidant K2MnF6. Within 15 minutes, a stable, stage 2 GIC product with x=22 and gallery height, di, of 3.0 nm was obtained. [8] The same chemical method has also been employed to produce new GIC’s containing N(SO2CF3)2¯, N(SO2CF2CF3)2¯, N(SO2CF3)(SO2C4F9)¯, and C(SO2CF3)3¯. [9-10] It is well known that the oxidation of graphite and intercalation of anions can also lead to reversible co-intercalation of neutral species, a common case is the parent acid co-intercalates with the conjugate anions (H2SO4/HSO4¯ or HF/HF2¯). Another common case is the co-intercalation of organic solvent molecules. Also, the intercalation reaction can be complicated by the incorporation of more than one species of anionic intercalate. Important examples of this complexity are the presence 81 of oxide or hydroxide in highly-oxidized GIC’s containing oxoanion intercalates, and the presence of additional fluoride in GIC’s containing fluoroanion intercalates. The cointercalation of oxide or fluoride within graphene galleries can have significant impact on the products’ physical or chemical properties. [11] In previous studies, there are few, if any, detailed compositional analyses on fluoride co-intercalate in GIC’s. Recently, a GIC containing bis(trifluoromethanesulfonyl)imide, TMSI, was studied in this regard and it showed approximately equimolar content of fluoride and the imide anions. [12] For the larger anions such as borate chelate anions; bis(oxalato)borate, B[OC(O)C(O)O]2¯, the fluoride content tends to increase accordingly. [13] In this chapter, we determine the fluoride cointercate contents in GIC’s containing perfluorooctanesulfonate, CxPFOS, prepared in hydrofluoric acid with K2MnF6 as an oxidant. 82 3.3 EXPERIMENTAL SP-1 grade graphite (Union Carbide, 100 µm average particle diameter), hydrofluoric acid (Mallinckrodt AR, 48% w/o), hexane (Fischer Chem, certified grade), nitric acid (Fischer Scientific, certified ACS PLUS, 68.0-70.0%), glacial acetic acid (EM Science, 99.7%), and sodium chloride (Mallinckrodt, AR grade) were used as received. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h before use. C8F17SO2F (3M, experimental product) was treated with excess 20% KOH solution and refluxed at 120 °C overnight. The white precipitate potassium perfluorooctanesulfonate, KC8F17SO3, was washed with distilled water and dried under vacuum at room temperature. Potassium hexafluoromanganate, K2MnF6, was synthesized as a bright yellow powder according to a literature method. [14] Ultrapure water (resistivity = 18 MΩ·cm at 25°C) from a Milli-Q Labo system (Millipore, Milford, MA), was used throughout the experiments. Low Level Total Ionic Strength Adjustment Buffer (LL-TISAB) was prepared by addition of glacial acetic acid (57 mL) and reagent-grade NaCl (58 g) to 500 mL water, then slow addition of 5 M NaOH until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [15] To prepare the GIC’s, KC8F17SO3 (200 mg, 0.37 mmol) and K2MnF6 (200 mg, 0.81 mmol) were first dissolved into hydrofluoric acid (15 mL) at ambient temperature, and then graphite (50 mg, 4.2 mmol) was added with mole ratios of 11:2:1 for graphene C:Mn (IV):C8F17SO3¯ anion. The solutions were stirred in air for the specified reaction times, then filtered and rinsed with 15 mL hydrofluoric acid. 83 The filtrate was briefly rinsed with 5 mL hexane and dried overnight under vacuum. Products were subsequently stored under an inert atmosphere. Powder X-ray diffraction (powder XRD) data were collected on a Siemens D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a detector slit width of 0.2 mm. Data were collected at 0.02 °2Ө steps, between 7˚ and 60˚. Collection times varied from 0.1 s / step for routine analyses to 1 s / step for data used in structural modeling. TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed with nitrogen. Thermal scans from ambient to 800°C were performed under flowing N2 at 5°C / min. A new sample digestion protocol was developed for the GIC products. Samples (30 mg) were digested with 70% HNO3 (1 mL) using a microwave digester (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this method completely decomposed the solid product and generates a homogeneous solution. The solutions were diluted to 50.0 mL and stored in Nalgene® plastic bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis. Fluoride analyses on the products were performed using an ion-selective fluoride combination electrode with a standard single-junction sleeve-type reference electrode and pH meter with an expanded mV scale (VWR International, Inc.). The mean of three replicate data are reported. A calibration curve was obtained using 84 fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F / L. All dilutions were performed using standard silica glassware at ambient temperature. Below pH 5, the formation of HF or HF2¯, which cannot be detected by the fluoride electrode, can interfere with accurate F analyses. [15]. In addition, the total ionic strength of the sample solutions must be maintained with the specification range of the electrode. Finally, under some conditions, hydrofluoric acid can react with silica. In order to address these issues, combination of both samples and standards with an equal volume of LL-TISAB, provided the required control over both pH and ionic strength. In addition, control experiments performed using only Nalgene® labware showed less than 1% difference in detected fluoride content from data obtained using standard glassware. The energy-minimized structure of the C8F17SO3¯ anion was initially calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis set and GAUSSIAN 03W software. 85 3.4 RESULTS AND DISCUSSION Stage 2 GIC’s containing perfluorooctanesulfonate anion, CxPFOS, prepared by chemical oxidation in a mixture of hydrofluoric acid and nitric acid (66/33) can be obtained within 24 h at ambient temperature. Because of the limited chemical oxidation potential in 48% hydrofluoric acid, stage 1 GIC’s cannot be obtained under these conditions. [9] In Figure 3.1, powder XRD data are presented for various reaction times from 3 to 96 h. A relatively constant pattern is obtained after 12 h. The powder XRD patterns obtained are similar in general appearance to those previously reported. [8] Most reflections relate to an Ic ≅ 3.3-3.4 nm, with notable set of five strong peaks centered at d ≅ 0.34 nm. Figure 3.2 indicates the structural model employed for stage 2 CxPFOS product. As the anion length is ~ 1.5 nm, the gallery dimensions indicate that a bilayer intercalate structure is most reasonable. All the powder XRD patterns show additional reflections at d = 0.58 and 0.55 nm, which occur in the region typical of (001) reflections from layered graphite fluorides. These impurity peaks are somewhat variable in intensity, but are only a few percent of the total diffracted intensity after 12 h. The use of aqueous media for graphite intercalation is clearly restricted to acidic solutions due to the high potential required for graphite oxidation. Concentrated acids such as fuming H2SO4 and 97%HNO3, have long been used to prepare GIC’s of their conjugate anions. [16-18] 86 Table 3.1 shows compositional data for these products obtained from thermogravimetric and elemental analyses. TGA mass loss of KPFOS salt and GIC products at various reaction times are shown in Figure 3.3 a-b, respectively. The TGA curve of the KPFOS salt shows a sharp mass decrease beginning at 420°C. The remaining carbonaceous material is completely volatilized during the final mass loss beginning at 600°C. For GIC products, four mass loss regions between 25 and 900°C are shown in the TGA traces. The first loss, below 120°C, ranges from 0.5 to 3 mass percent, with longer reaction times associated with a greater mass loss. This mass loss is ascribed to intercalated or surface-adsorbed H2O or HF. The next two losses occur between 150-200°C and 400-500°C, which have been ascribed to intercalate anion decomposition and volatilization. The final loss above 600°C is ascribed to decomposition of the graphene sheets. Under the same thermolysis conditions, graphite itself starts to decompose at 700°C. It is notable that the PFOS anion is far less thermally stable in the GIC product than in the potassium salts, where decomposition occurs as a single sharp event with an onset at 420-600°C. (Figure 3.3 a) In order to evaluate the fluoride co-intercalate content in these products separately from that of the PFOS anion, a new digestion procedure was developed. Using control experiments, conditions were developed that allowed quantitative dissolution of the graphene sheets and dissolution of the fluoride anions but avoiding significant decomposition of the PFOS anion, which could produce additional fluoride. Control experiments using KPFOS and NaF were run and indicated that 87 under these conditions < 1% of fluoride content determined by analysis are due to PFOS anion decomposition. 88 96 h intensity / arb unit 24 h 12 h * 6h * 3h 10 20 30 40 50 2θ / deg Figure 3.1 Powder XRD patterns of the products obtained in a mixed acid solution (HF/HNO3:66/33), using K2MnF6 as an oxidizing reagent, with reaction times indicated. Impurity peaks of CxF are marked *. 89 Table 3.1 Gallery height (di) and composition for stage 2 graphite perfluorooctanesulfonate, CxC8F17SO3·δF, at different reaction times as determined by powder XRD, elemental analyses, and thermogravimetry. reaction time /h di / nm TGA mass mass fraction x loss/ pct of fluoride δ x/δ 3 3.04 54 0.021 37 1.1 33.6 6 3.04 60 0.021 29 0.93 31.2 12 3.05 66 0.016 22 0.66 33.3 24 3.02 65 0.010 23 0.42 54.8 96 3.05 73 0.016 16 0.58 27.6 90 N S C O F di ≅ 3.0 nm Figure 3.2 Structural model for CxC8F17SO3·δF. The fluoride positions are partially occupied. 91 100 KPFOS mass loss / pct 80 60 24 h 40 20 0 0 200 400 600 800 temperature / °C mass loss / pct 100 80 60 3h 6h 40 12 h 24 h 20 96 h 0 0 200 400 600 temperature / °C Figure 3.3 TGA data for products described in Table 3.1. 800 92 Table 3.1 shows compositional data of CxC8F17SO3·δF determined from thermogravimetric and elemental analyses. The compositional parameters x and δ were obtained by ascribing mass loss below 450°C to total intercalate content, along with the elemental analysis of fluoride anion content as described above. Therefore, the composition can be determined by simultaneously solving the following relations: mass loss pct = 280.1 + 19.0δ × 100 (3.1) 12.0x + 499.1 + 19.0δ mass fraction of fluoride 19.0 δ = (3.2) 12.0 x + 499.1 + 19.0 δ Table 3.1 indicates the values determined for x and δ vs. reaction time. These data represent a detailed understanding of the intercalate composition during chemical synthesis in hydrofluoric acid, which is typically limited to an estimate of the fluoroanion content. As can be seen, fluoride co-intercalate content decreases slightly over long reaction times, and δ does not exceed a value of which indicates approximately equimolar content of fluoride and the PFOS anions. As can be seen from the powder XRD data in Figure 3.1, the sample obtained at the shortest reaction time has the greatest content of impurity of fluorine-rich phases CxF (x ≅ 3-4), which may account in part for the higher δ value. 93 The result obtained in this study does not agree well with those obtained from previous study (δ≅4 after 72 h reaction), which was determined both from refinement of crystallographic data and elemental analyses. [8] One possible explanation lies in different methods of the elemental analysis used to determine fluoride content. The method used in this study is specific to fluoride co-intercalate and likely to be more accurate than a digestion method that counts all F in the GIC, because the anion has 17 fluoride atoms and small errors in determination of x lead to large errors in δ. Also, the previous refinement of crystallographic data, assumed that the entire additional electron density observed above the graphene sheets was due to fluoride only. It is also possible that an oxide cointercalate forms during synthesis in aqueous HF, to form CxC8F17SO3·δ(F or O) To test this theory, the synthesis was also attempted in anhydrous HF as indicated below. xC + C8F17SO3¯ (sol) F2/ AHF CxC8F17SO3·δF + e¯ (3.3) After 24 h reaction, the GIC product obtained was a mixture of stages, with values of δ and x are 1.9 and 91, respectively. It was therefore not possible to prepare comparable GIC’s of single phase to test the latter theory. 94 3.5 REFERENCES 1. Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met .1988, 23, 395. 2. Horn D.; Boehm, H. P. Mater. Sci. Eng. 1977, 31, 87. 3. Ruisinger, B.; Boehm, H. P. Angew. Chem. Int. Ed. Engl. 1987, 26, 253. 4. Ruisinger, B.; Boehm, H. P. Carbon 1993, 31, 1131. 5. Pinnavaia, T. Science 1983, 220, 365. 6. Jacobson, A. Organic and Organometallic Intercalation Compounds of the Transition Metal Dichalcogenides. In Intercalation Chemistry; Whittingham, S.; Jacobsen, A., Eds.; Academic Press: New York, 1982; Chapter 7. 7. Zhang, Z.; Lerner, M. M. Chem. Mater. 1996, 8, 257. 8. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100. 9. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363. 10. Zhang, X.; Lerner, M. M. Mol. Cryst. Liq. Cryst. Sci. Tech., Section A 2000, 340, 37. 11. Dresselhaus, M. S.; Endo, M.; Issi, J. Physical Properties of Fluorine- and Fluoride-Graphite Intercalation Compounds. In Fluorine-Carbon and Fluoride Carbon Materials; Nakajima, T., Ed.; Marcel Dekker : New York, 1995; 95. 12. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499. 13. Katinonkul, W.; Lerner, M. M. J. Phys. Chem. Sol. 2007, 68, 394. 14. Bode, H.; Jenssen, H.; Bandte, F. Angew. Chem. 1953, 65, 304. 15. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual. VWR International, Inc., Westchester, PA, USA, 2002. 16. Bartlett, N.; McQuillan, B. Graphite Chemistry. In Intercalation Chemistry; Whittingham, S.; Jacobson, A., Eds.; Academic Press: New York, 1982; Chapter 2. 95 17. Watanabe, N.; Touhara, H.; Nakajima, T.; Bartlett, N.; Mallouk, T.; Selig, H. Fluorine Intercalation Compounds of Graphite. In Inorganic Solid Fluorides; Hagenmuller, P., Ed.; Academic Press: New York, 1985; 332. 18. Ubbelohde, A. Proc. R. Soc. London A 1968, 304, 25. 96 CHAPTER 4 THE FIRST SYNTHESIS OF A GRAPHITE BIS(OXALATO)BORATE INTERCALATION COMPOUND Watcharee Katinonkul and Michael M. Lerner Department of Chemistry and Oregon State University, Corvallis, OR 97331-4003 USA J. Phys. Chem. Sol. 2007, 68, 394-399. 97 4.1 ABSTRACT A new graphite intercalation compound containing the bis(oxalato)borate anion, B[OC(O)C(O)O]2¯, is prepared for the first time by chemical oxidation of graphite with fluorine gas in the presence of a solution containing the intercalate anion in anhydrous hydrofluoric acid. The products of stage 1-3 compounds are characterized by powder X-ray diffraction, thermogravimetric analysis, and elemental analyses. X-ray diffraction data indicate a standing-up orientation for the borate anion, with long axis perpendicular to the graphene sheets. Elemental analyses provide x and δ for the nominal composition of CxB[OC(O)C(O)O]2·δF, where the chelate borate and fluoride are co-intercalates, and indicate a low borate, and high fluoride, intercalate content as compared to anion packing in other graphite intercalation compounds. Keywords: A. inorganic compounds, B. chemical synthesis, C. x-ray diffraction 98 4.2 INTRODUCTION Acceptor-type graphite intercalation compounds (GIC’s), in which the planar graphene sheets are oxidized and anions intercalate, have been prepared with a wide range of intercalate anions, including octahedral or tetrahedral chloro-, fluoro-, or oxometallates. GIC’s show staging, or ordering, in the introduction of intercalate, for example a stage 2 GIC has intercalate between alternate sheets, stage 3 has intercalate between every third set of sheets (see Figure 1). For GIC’s containing tetrahedral or octahedral anions, an x value (a mole ratio of graphene carbon / intercalate anions) of approximately 24n, where n=stage, is consistently observed at a very distinctive transition in potential-charge curves. [1] The relative independence of this transition on the nature of the intercalate anion suggests that it is the graphene-sheet charge density, rather than the intercalate distribution, that governs stage transitions. Anions such as MF4¯, MF6¯ or MCl6¯ require “footprints” (i.e., areas above the graphene surfaces) that are far less than the areas available within a C24n+ gallery. These GIC’s therefore have significant gallery void volumes. It is well known that neutral intercalates can be accommodated into these voids with little or no expansion in the gallery dimension. [2-3] For larger intercalates, it may be important to consider the effects of sterics in the packing within graphene galleries. This can helps to explain the “standing-up” orientation of larger anisotropic anions. This effect has also been described previously by Boehm and co-workers. [4] With the longer z-axis parallel to graphene sheets, i.e., 99 in a “lying down” orientation where di is minimized, the anions would require a larger footprint than that available for stage 2 with x=48. An example of this “standing-up” orientation is found for the long chain alkylsulfonate intercalate, CxC8F17SO3, which forms a bilayer intercalate structure with anionic sulfonate headgroups oriented towards the graphene sheets and the hydrophobic fluorocarbon chains oriented toward the gallery centers. The gallery height for this GIC is 3.3 nm. [5] The even longer anion, C10F21SO3¯, has a gallery height of 3.5 nm and a similar gallery structure [6]. The chelate borate anions, as will be discussed below, provide another example of this anion orientation. The most significant technological application to date has probably been their use in the preparation of exfoliated graphite microparticles that form self-supporting structures used in high-temperature seals. [7] In pillared clays and other pillared layered compounds, the generation of open gallery structures allows for catalytic activity, and the incorporation of organics and polymers within galleries has been widely studied as a means to prepare porous materials for various applications. [8-11] Similar materials prepared from GIC’s would offer interesting new properties because graphene sheets have very different electronic, acid/base, and electrochemical properties than clays or other layered inorganic hosts. A larger anion intercalate dimension, relatively isotropic geometry, and monolayer intercalate arrangement could lead to steric constraints for anion packing and therefore hold promise for the generation of novel properties, such as facile intragallery chemistry for GIC’s with lower graphene sheet charge densities. 100 di Ic di Ic Ic= di stage 1 stage 2 stage 3 Figure 4.1. Scheme of GIC showing staging, or ordering along the stacking direction, for different products. 101 Generally, either chemical or electrochemical oxidation methods can be used to prepare acceptor-type GIC’s. The electrochemical method may provide additional information on products and allow greater control over the reaction progress, but a chemical method can be more scalable for commercial production. Both methods have been applied to the intercalation of large fluoroalkylanions. Bis(trifluoromethanesulfonyl)imide, N(SO2CF3)2¯, was prepared for the first time by electrochemical oxidation. [12] Electrolytes with nitromethane and ethyl methyl sulfone were also used to prepare GIC’s of this intercalate up to stage 1 (with all galleries occupied). Larger imide or methide derivatives, including N(SO2CF2CF3)2¯ and N(SO2CF3)(SO2C4F9)¯, have also been prepared by a chemical method. [13] These GIC’s containing large perfluoroanions are remarkably more air stable than those containing smaller anions. [13] Acceptor-type GIC’s containing borate chelate anions have been prepared recently. [14,15] Aside from BF4¯, the electrochemical intercalation of the chelate borate anions B[OC(CF3)2C(CF3)2O]2¯ and B[OC(CF3)2C(O)O]2¯ provided the first GIC’s containing boroanions. GIC’s from stage n=1-4 were obtained with gallery heights, di, of 1.40-1.45 nm, indicating a single intercalate layer between graphene sheets with the intercalate anions orientated with long axis perpendicular to the graphene sheet surface. The large anion dimension and unusual anion orientation (with long axis perpendicular to graphene sheets) result in an unprecedented large gallery height for a single intercalate layer. [15] These GIC’s also demonstrated 102 comparatively high ambient stabilities; stage 2 is stable for several days and even the stage 1 product can persist for several hours in air. The B[OC(O)C(O)O]2¯ (BOB) anion has been the subject of several reports recently, principally due to the interest in new electrolyte salts for lithium-ion batteries. [16-19] It is reported to have an oxidative stability limit close to 4.5 V vs. Li/Li+ in organic electrolytes. Since this anion contains no labile fluorine and is thermally stable its electrolyte solutions demonstrate stable performance in cells even at 60°C, where LiPF6 based electrolytes would usually fail. [18] Its solubility (>1.0 M in some solvents), high ambient temperature ion conductivities (8-9 mS/cm), low cost, thermal stability, and the stability of its decomposition products all make this an attractive candidate for Li-ion battery electrolytes. The electrochemical reactions described above to prepare borate chelate anions, did not succeed in preparing any GIC’s containing the BOB anion. In this paper, the first preparation of GIC’s containing the BOB anion intercalate is described by the chemical oxidation of graphite with F2 (g) in an anhydrous HF /LiBOB solution. 4.3 EXPERIMENTAL Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter), graphite powder (Union Carbide, SP-1 grade, 100 µm), lithium hydroxide (Lancaster Synthesis, Inc., anhydrous 98+%), oxalic acid dihydrate (Sigma-Aldrich, 99%), and 103 boric acid (Aldrich, 99.999%) were used as received. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h before use. Lithium bis(oxalato)borate salt (LiBOB) was synthesized according to a previous literature method. [20] Anhydrous hydrofluoric acid (AHF) (Matheson, pure grade) was subjected to two freeze/pump/thaw cycles at 0°C to remove H2 before use. Fluorine gas (Air Products, > 97 %) was connected directly to a stainless steel vacuum line fitted with a Helicoid gauge (0-1500 torr) and used as received. Ultrapure water (resistivity = 18 MΩcm at 25°C) from a Milli-Q Labo system was used throughout the experiments. LL-TISAB (low level- total ionic strength adjustment buffer) was prepared by addition of glacial acetic acid (57 mL) and reagent-grade NaCl (58 g) to 500 mL water, slowly addition of 5 M NaOH until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [21] To prepare GIC’s, graphite (30 mg) and the desired mass of LiBOB were loaded into separate 3/8″ FEP Teflon tubes that were heat-sealed at one end. The tubes were connected via 316 stainless-steel Swagelok fittings to a 1KS4 Whitey valve (see Figure 4.2). The assembled reactor was connected to a metal vacuum line, evacuated, and AHF (approximately 2 mL) condensed onto the LiBOB salt to produce a paleyellow solution. The solution was poured onto the graphite powder in approximately 0.25 mL aliquots. Fluorine gas was then slowly and incrementally added into the reactor over a period of several minutes to an overpressure of approximately 1200 torr. A pressure of 1000-1200 torr was maintained by adding additional F2 for the entire 104 reaction. For reactions where the desired reaction stoichiometries were determined in advance, the graphite and LiBOB powders were loaded into the same tube. After the reaction, fluorine gas was removed by evacuation through a soda lime tower, and the AHF solution was carefully poured into the second reaction tube. The product was rinsed at least three times by re-condensing AHF onto the product, allowing soluble components to dissolve, and again pouring the solution to the second tube. In this way, soluble byproducts and unreacted LiBOB were removed from the GIC product. After rinsing, AHF was evacuated through a soda lime tower and the solid product dried under vacuum for several hours at ambient temperature. The GIC product was subsequently maintained under an inert atmosphere. Powder X-ray diffraction (PXRD) data were collected on a Siemens D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a detector slit width of 0.2 mm. Data were collected at 0.02°2Ө steps, between 2˚ and 65˚. Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data used in structural modeling. TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed with nitrogen. Thermal scans from ambient to 800°C were performed at 2°C/ min. A new sample digestion protocol was developed for the GIC products. Samples (30 mg) were digested into 70% HNO3 (1 mL) using a microwave digestor (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this 105 To metal vacuum line 1KS4 Whitey valves 3/8″ OD FEP Teflon tube heat seals Figure 4.2. General design for reactor employed in syntheses using anhydrous HF and F2. 106 method completely decomposed the solid products and generated homogeneous solutions. The solutions were diluted to 50.0 mL and stored in Nalgene plastic bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis. Fluoride analyses on the products were performed using an ion-selective fluoride combination electrode (VWR International, Inc.) with a standard singlejunction sleeve-type reference electrode and pH meter with an expanded mV scale (VWR 8005). A calibration curve was obtained using fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F/L. All dilutions were performed using standard silica glassware at ambient temperature. Below pH 5, the presence of HF or HF2¯, which cannot be detected by the fluoride electrode, can interfere with accurate F analyses. [21] In addition, the total ionic strength of the samples solutions must be maintained with the specification range of the electrode. Finally, under some conditions, hydrofluoric acid can react with silica. In order to address these issues, the combination of both samples and standards with an equal volume of LL-TISAB provided the required control over both pH and ionic strength. In addition, control experiments were performed using only Nalgene labware and showed less than 1% difference in detected fluoride content from data obtained using standard glassware. Boron analyses used the same digestion procedure as described above. Samples were subsequently diluted to form a 2% v/v HNO3 solution, and B content determined using ICP-AES (S.A. Inc, JY2000U analyzer) calibrated using a boron 107 standard (1.0-10 mg B/L) that was prepared from anhydrous boric acid according to a literature procedure. [22] A 2 % v/v HNO3 acid solution was used as control blank solution for calibration. The energy-minimized structure of the B[OC(O)C(O)O]2¯ anion was calculated using the hybrid density functional method (B3LYP) with a 6-31G* basis set and GAUSSIAN 03W software. One-dimensional electron density maps were calculated from a structural model and also obtained from observed powder XRD (00l) reflection intensities (corrected for Lorentz-polarization). Additional details of this method have been described previously. [5,6] 4.4 RESULTS AND DISCUSSION In the synthesis of these graphite intercalation compounds (GIC’s), F2 serves as the oxidant and bis(oxalato)borate (BOB) and fluoride anions are co-intercalates according to the following overall reaction: xC + [(δ+1)/2]F2 + B[OC(O)C(O)O]2¯ CxB[OC(O)C(O)O]2·δF + F¯ (4.1) where is x is defined as the mole ratio of graphene carbon to borate anion and δ is the mole ratio of intercalate fluoride to BOB. To our knowledge, this work represents the first preparation of any graphite BOB intercalation compound. When the borate anion 108 is not included under these reaction conditions, graphite bifluoride (CxHF2·δHF) is formed within minutes of the introduction of fluorine gas. It is therefore likely the bifluoride anion also intercalates first in these reactions, but is subsequently replaced by BOB and also converted to fluoride. Initial experiments, using aliquot additions of the LiBOB solution to graphite, provided an estimate of reaction stoichiometries to within approximately 10%. It was determined that an initial ratio of graphene carbon to anion of 24:1 (mol:mol) was sufficient to produce the first-stage GIC at longer reaction times. After the initial tests, this stoichiometry was employed for all reactions. Graphite flake is a highly crystalline material with a large particle diameter, that results in higher diffraction intensity for (00l) reflections in the GIC’s obtained (about 10-fold greater diffraction peak intensities than for SP-1 graphite). Due to the strong preferred orientation of the platy particles, only (00l) reflections are observed in powder XRD data. The large particle diameter in graphite flake also results in slower intercalation reactions, these typically required 2-3 times longer to reach similar stages compared with the SP-1 graphite. Also, many GIC products using graphite flake show a mixture of stages, presumably arising from a lower intercalate content in the crystallite particle centers. Because higher quality diffraction data were desired in this study, graphite flake was employed exclusively after initial tests. As shown in Figure 4.3, the products obtained showed a series of reflections that can be ascribed to the GIC CxB[OC(O)C(O)O]2·δF. Table 4.1 provides the basal repeat, Ic, and gallery height di, obtained for the GIC products at different reaction 109 times. In all products, di is close to 1.4 nm. This gallery dimension is similar to that obtained previously for GIC’s with the bis(hexafluorohydroxyisobutyrato) and bis(perfluoropinacolato)borate chelate anions. [14,15] A stage 1 GIC (with a small shoulder on the main PXRD peak indicating a minor stage 2 component) is obtained within 24 h, while shorter reaction times resulted in higher-stage products. A simple structural model, based on the dimensions of the energy-minimized isolated anion, indicates that the BOB anion is oriented with long axis perpendicular to the graphene surfaces. This unusual intercalate orientation, which maximizes rather than minimizes the gallery height, di, can likely be ascribed to a packing effect for anions with large footprints. This orientation was observed for the other borate chelate anions [14,15] and allows all these large intercalate anions to pack more densely in the galleries. Elemental analyses for B and F in the GIC’s obtained (Table 4.2) were determined using ICP-AES and the digestion protocol described above. The observed mass percents for these elements were converted to the compositional parameters x and δ in CxB[OC(O)C(O)O]2·δF products by using the following relations: mass % B = 10.8 × 100 (4.2) × 100 (4.3) 12.0x + 186.8 + 19.0δ mass % F = 19.0 δ 12.0x + 186.8 + 19.0δ 110 Previous studies on borate chelate GIC’s prepared by electrochemical oxidation in nitromethane employed both coulometry and the modeling of diffraction data to determine the graphene carbon to anion ratio, x. Due to the unknown efficiency for the electrochemical reaction, coulometry could only provide a lower limit for x. Results obtained were as follows: for the stage 2 CxB[OC(CF3)2C(O)O]2, x>28 (coulometry) and x=44 (diffraction); [14] for the stage 2 CxB[OC(CF3)2C(CF3)2O]2, x>41 (coulometry) and x=49 (diffraction). [15] These x values are similar to those generally obtained with smaller intercalate anions, and approximately follow the relation of C24nAn where n=stage. [1] There are numerous examples of smaller anion intercalates that are close to x=48 for stage 2, including CxPF6 (x=48) [23], CxAsF6 (x=45) [23], CxSbF6 (x=48) [24], CxHSO4 (x=48) [25], and CxSO3CF3 (x=52) [26]. The present study found much larger x values for stages 1-3 than those for the compounds just described. For example,x=110 for the stage 2 CxB[OC(O)C(O)O]2·δF. These results indicate a much lower packing density for the BOB anions within galleries. On the other hand, relatively high concentrations of the fluoride cointercalate (δ≈3-4) were observed. (Table 4.2) These two results are likely to be related, because the fluoride anion requires space on graphene sheet surface, and also must be associated with some charge transfer with the host sheets. Thus the presence of relatively large concentration of fluoride co-intercalate will tend to decrease the packing density of the larger BOB anions within the galleries. In previous studies, intercalation of borate chelate anions was obtained using electrochemical oxidation in an organic solvent; therefore, no fluoride anions were present to co-intercalate. 111 (009) (006) (008) (005) (002) (001) (004) stage 1 10 30 40 50 (0011) (0010) (007) 20 (0011) (007) (006) (005) stage 3 (002) (001) 0 (0010) (006) (002) (001) intensity (005) stage 2 60 2 Ө / degree Figure 4.3 Powder XRD pattern of stage 1-3 products of CxB[OC(O)C(O)O]2·δF. Assigned (00l) indices are indicated. 112 Table 4.1 Powder XRD data for CxB[OC(O)C(O)O]2·δF products. Sample reaction time /h stage Ic / nm di / nm 1 3 3 2.10 1.43 2 12 2 1.77 1.43 3 24 1/2* 1.42 1.42 * the product is predominantly stage 1 with a small stage 2 component 113 Table 4.2 Compositional data for CxB[OC(O)C(O)O]2·δF from elemental analyses. Sample B / mass pct F / mass pct x δ 1 1.422 10.0 41 4.0 2 0.677 3.40 110 2.9 3 0.649 3.49 120 3.1 114 Another study has determined the composition of CxN(SO2CF3)2·δF prepared using chemical oxidation in hydrofluoric acid, and found for stage 2, x=50-60 and δ=0-1. [27] For this imide anion intercalate, the gallery height is only about 0.8 nm, similar to that in graphite sulfate or nitrate, and much less than for the borate chelate anions. The amount of fluoride co-intercalate appears to be highly dependent on both anion size and synthetic method. The composition obtained was also compared to TGA mass loss data (Figure 4.4). The decomposition of the BOB anion is known from common salts such as LiBOB, which completely decomposes to Li2CO3 and B2O3 by 650°C via reaction (4.4): LiB[OC(O)C(O)O]2 → ½Li2CO3 + ½B2O3 + volatiles (4.4) For reaction (4), the mass losses are: 63.0 % (calc), 68.7 % (obs), it may be that some Li2O is formed rather than complete conversion to Li2CO3. The loss of graphene carbon due to sheet degradation is not expected below 650°C, and planar-sheet graphite fluorides lose all fluoride content above 450°C, [17] therefore the thermolysis reaction for CxB[OC(O)C(O)O]2·δF to 650°C was ascribed to reaction (4.5): CxB[OC(O)C(O)O]2·δF → Cx + ½B2O3 + volatiles (4.5) 115 Using the compositional parameters x and δ from Table 1, the stage 2 GIC mass loss was 13.2% (calc) and 13.1% (obs). The TGA analysis for total intercalate content therefore agrees well with the compositional data presented. Due to the large gallery height, the structural model employed for CxB[OC(O)C(O)O]2·δF places the borate chelate anion with long axis oriented perpendicular to the graphene sheets. The known anion and gallery dimensions result in a graphene C to anion O distance (along the direction perpendicular to graphene sheets) of 0.39 nm, somewhat longer than that for graphite nitrate (0.32 nm). The greater separation of the intercalate anion from host sheet surface may be related to the high concentration of fluoride co-intercalate. Figure 4.5 provides one-dimensional electron density profiles derived from the refined structure model and from observed powder XRD data for a stage 2 product. The fluoride co-intercalate anions were fixed to lie in equal concentration on both encasing graphene surfaces at 0.31 nm above the surfaces (for comparison, stage 2 CxHF2 has a graphene C–F distance of 0.32 nm). Although the observed profile shows a general correspondence to the structural model, the electron density peaks for intercalates near z=0.3, 0.5, and 0.7 show differences from the model. It seems most likely that arises due to a minor phase impurity (the presence of a higher stage GIC or graphite) in the diffraction pattern. 116 mass loss / pct 100 80 60 LiBOB 40 0 200 400 600 800 temp / ° C Figure 4.4 TGA mass loss profiles for LiBOB salt and stage 2 GIC CxB[OC(O)C(O)O]2·δF. 117 B C O F 1.0 z x y 0.8 0.6 z (i) (ii) 0.4 0.2 (b) 0 (a) electron density (c) Figure 4.5 (a) Energy-minimized BOB anion conformation, (b) GIC structural model, and (c) one-dimensional electron density profiles derived from calculated structural model (i) and observed powder XRD data (ii). 118 4.5 REFERENCES 1. Besenhard, J.; Fritz, H. Angew. Chem. Int. Ed. Engl. 1983, 22, 950. 2. Billaud, D.; Pron, A.; Vogel, F. Synth. Met. 1980, 2, 177. 3. Billaud, D.; El Haouari, A.; Gerardin, R. Synth. Met. 1989, 29, F241. 4. Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met. 1988, 23, 395. 5. Zhang, X.; Lerner, M. M. Chem. Mater. 1999, 11, 1100. 6. Yan, W.; Lerner, M. M. Carbon 2004, 42, 2981. 7. Buckley, J.; Edie, D. Carbon-Carbon Materials and Composites; Noyes Publ.: Park Ridge, NJ, 1993. 8. Pinnavaia, T. Science 1983, 220, 365. 9. Rebbah, H.; Borel, M.; Raveau, B. Mater. Res. Bull. 1980, 15, 317. 10. Yang, D.; Sandoval, S.; Divigalpitiya, M.; Irwin, J.; Frindt, R. Phys. Rev. B 1991, 43, 12053. 11. Lemmon, J.; Wu, J.; Lerner, M. M. Preparation and Characterization of Nanocomposites of Poly(Ethylene Oxide) with Layered Solids. In Hybrid Organic/Inorganic Composites; Mark, J.; Lee, C.; Bianconi, P., Eds.; ACS Symp. Series 585; ACS: Washington DC, 1995; Chapter 5. 12. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2001, 148, D83. 13. Zhang, X.; Sukpirom, N.; Lerner, M. M. Mater. Res. Bull. 1999, 34, 363. 14. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2004, 151, J15. 15. Yan, W.; Lerner, M. M. J. Electrochem. Soc. 2003, 150, D169. 16. Ue, M.; Shima, K.; Mori, S. Electrochim. Acta. 1994, 39, 2751. 17. Xu, W.; Angell, C. Electrochim. Solid-State Lett. 2001, 4, E1. 18. Xu, K.; Zhang, S.; Jow, T.; Xu, W.; Angell, C. A. Electrochim. Solid-State Lett. 2002, 5, A26. 119 19. Xu, W.; Shusterman, A. J.; Videa, M.; Velikov, V.; Marzke, R.; Angell, C. J. Electrochem. Soc. 2003, 50, E74. 20. Lishka, U.; Wietelmann, U.; Wegner, M. German Pat. DE19829030 C1, 1999. 21. VWR International, Inc. Combination Fluoride Electrodes Instruction Manual. VWR International, Inc., Westchester, PA, USA, 2002. 22. Greenberg, A. E.; Clesceri, L. S.; Eaton, A. D. Standard Methods for the Examination of Water and Waste Water; 18th Edition; American Public Health Assn: Wash, DC, 1992. 23. Billaud, D.; Chenite, A. J. Power Sources 1984, 13, 1. 24. Jobert, A.; Touzain, P.; Bonnain, L. Carbon 1981, 19, 193. 25. Besenhard, J.; Wudy, E.; Moehwald, H.; Nickl, J.; Biberacher, W.; Foag, W. Synth. Met. 1983, 7, 185. 26. Horn, D.; Boehm, H. Mater. Sci. Eng. 1977, 31, 87. 27. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499. 120 CHAPTER 5 CHEMICAL SYNTHESIS OF GRAPHITE BIS(PERFLUOROPINACOLATO)BORATE AND GRAPHITE BIS(HEXAFLUOROHYDROXYISOBUTYRATO)BORATE INTERCALATION COMPOUNDS Watcharee Katinonkul and Michael M. Lerner Department of Chemistry Oregon State University, Corvallis, OR 97331-4003 USA Carbon 2007, in press. 121 5.1 ABSTRACT Graphite chelate borate intercalation compounds, CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF, are prepared for the first time by chemical oxidation of flaky graphite with fluorine gas in the presence of a solution of the intercalate anion in anhydrous hydrofluoric acid. Powder XRD data indicate that products up to stage 1 with gallery heights, di =1.43-1.45 nm are formed. The compositions of CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2 C(O)O]2·δF are determined using B and F elemental analyses. Thermogravimetric analyses indicate the graphene sheets begin to decompose before intercalate thermolysis is complete. 122 5.2 INTRODUCTION Graphite intercalation compounds (GIC’s) are layered compounds comprising graphene sheets and intercalate guests. Both chemical and electrochemical oxidation methods have been used to prepare GIC’s. In general, the electrochemical method may provide additional information on products and allow greater control over the reaction progress, but chemical methods can produce purer products and may be more readily scaled to commercial production. GIC’s are also unusual in the common occurrence of several well-defined stages, i.e., phases with ordered arrangements of occupied and unoccupied galleries, where the integral stage number (n) indicates the number of galleries contained in a repeat sequence where only the first is occupied by intercalate. The gallery height (di) indicates the separation of graphene sheet centers in an intercalated gallery, and the basal repeat length (Ic) is the repeating unit cell along the stacking direction. The relationship between these parameters can also be expressed as: Ic = di + (n-1) 0.335 nm (5.1) GIC’s with oxidized graphene sheet (acceptor-type) have been studied for a wide range of intercalate anions such as octahedral or tetrahedral chloro-, fluoro-, or oxo-metallates. GIC’s containing fluoroborate anions have been prepared using both chemical and electrochemical methods. [1-5] As an example, CxBF4 was prepared by 123 electrochemical oxidation in nitromethane, with a gallery height di ≅ 0.78 nm obtained: [1,2] xC + BF4¯(sol) CH3NO2 CxBF4·δCH3NO2 + e¯ (5.2) This electrochemical method produced materials with compositions near x = 24n, where n is the stage of the product. [5] The intercalation of oxidatively-stable larger anions such as perfluoroalkylsulfonates (CyF2y+1SO3¯, y = 1, 4, 6, 8, or 10) [6-12], trifluoroacetate sulfonylimide or (CF3COO¯) methide [13], anions and the perfluoroalkyl-substituted (N(SO2CF3)2¯, N(SO2CF2CF3)2¯, N(SO2CF3)(SO2C4F9)¯, and C(SO2CF3)3¯) [14-15] have also been reported. For the perfluoroalkylsulfonate anions, a bilayer intercalate arrangement of intercalate results in di as large as 3.4 nm. [12] Such highly expanded gallery in GIC’s may lead to a broader subsequent chemistry and new applications. By analogy to pillared clays, the generation of open gallery structures could allow for size-selective catalysis [16-18], and the incorporation of organics and polymers within galleries has been widely studied as a means to prepare nanocomposite materials for various applications. [1923] Analogous materials prepared from expanded GIC’s would offer interesting new properties because graphene sheets have very different electronic, acid/base, and electrochemical properties than clays or other layered inorganic hosts. Our group has previously reported the first synthesis of acceptor-type GIC’s containing relatively large borate chelate anions; bis(perfluoropinacolato)borate, 124 B[OC(CF3)2C(CF3)2O]2¯, and bis(hexafluorohydroxyisobutyrato)borate, B[OC(CF3)2C(O)O]2¯. These GIC’s were prepared by an electrochemical method. Both GIC’s have relatively large gallery heights, di, of 1.40-1.45 nm, that indicate an anion monolayer with the intercalate anions orientated with long axis perpendicular to the graphene sheet surface. [24,25] Recently the first chemical preparation of GIC containing the bis(oxalato) anion, B[OC(O)C(O)O]2¯ (BOB) was successfully accomplished by the chemical oxidation of graphite with F2 in an anhydrous HF solution. [26] In this chapter, the first chemical preparations of GIC’s containing B[OC(CF3)2C(CF3)2O]2¯ and B[OC(CF3)2C(O)O]2¯ are described. 5.3 EXPERIMENTAL Graphite flake (Graftech Inc., GTA grade, 250 µm average particle diameter), perfluoropinacol (Indofine Chemical Company, Inc., 95%), sodium borate (EM Science, >99%), hexafluoro-2-hydroxyisobutyric acid (SynQuest Laboratories, Inc., 95+%), and boric acid (Aldrich, 99.999%) were used as supplied. Sodium fluoride (Mallinckrodt, AR grade) was baked at 105°C for 1 h before use. Anhydrous hydrofluoric acid (AHF) (Matheson, pure grade) was subjected to two freeze/pump/ thaw cycles at 0°C to remove H2 before use. Fluorine gas (Air Products, > 97%) was connected directly to a stainless steel vacuum line fitted with a Helicoid gauge (0-1500 torr) and used as received. Ultrapure water (resistivity = 18 MΩcm at 25°C) from a 125 Milli-Q Labo system was used throughout the experiments. LL-TISAB (low leveltotal ionic strength adjustment buffer) was prepared by dissolving glacial acetic acid (57 mL) and reagent-grade NaCl (58 g) in 500 mL water, slow addition of 5 M NaOH until the pH was between 5.0-5.5, and then dilution to 1.00 L total volume. [27] Sodium bis(perfluoropinacolato)borate, NaB[OC(CF3)2C(CF3)2O]2, was synthesized according to the following literature method. [28] Perfluoropinacol (12.026 g, 36.0 mmol) was added to 10 mL of a hot aqueous solution containing sodium borate (0.763 g, 2.0 mmol). After concentration, the colorless gel obtained was crystallized from ethanol to yield a white solid, then dried in vacuo at 100°C for 48 h. Sodium bis(hexafluorohydroxyisobutyrato)borate, NaB[OC(CF3)2C(O)O]2, was synthesized according to the following literature method. [25] Hexafluoro-2hydroxyisobutyric acid (4.44 g, 21.0 mmol) was added to 10.0 mL of a hot aqueous solution containing sodium borate (1.00 g, 2.62 mmol). After heating at 80°C for 10 min, the solution was concentrated and cooled to yield a white solid. The solid was recrystallized from ethanol and then dried in vacuo at 100°C for 48 h. To prepare GIC’s, graphite (30 mg) and the desired mass of borate salts were loaded into separate 3/8″ FEP Teflon tubes that were heat-sealed at one end. The tubes were connected via 316 stainless-steel Swagelok fittings to a 1KS4 Whitey valve. [26]. The assembled reactor was connected to a metal vacuum line, evacuated, and AHF (approximately 2 mL) condensed onto the borate chelate salt to produce a paleyellow solution. The solution was poured onto the graphite powder in approximately 0.25 mL aliquots. Fluorine gas was then slowly and incrementally added into the 126 reactor over a period of several minutes to an overpressure of approximately 1200 torr. A pressure of 1000-1200 torr was maintained by adding additional F2 during the entire reaction. For reactions where the desired reaction stoichiometries were determined in advance, the graphite and borate salt powders were loaded into the same Teflon tube. After the reaction, fluorine gas was removed by evacuation through a soda lime tower, and the AHF solution was carefully poured into the second reaction tube. The product was rinsed at least three times by re-condensing AHF onto the product, allowing soluble components to dissolve, and again pouring the solution into the second tube. In this way, soluble byproducts and unreacted borate salts were removed from the desired GIC product. After rinsing, AHF was evacuated through a soda lime tower and the solid product dried under vacuum for several hours at ambient temperature. The product was subsequently stored under an inert atmosphere. Powder X-ray diffraction (PXRD) data were collected on a Siemens D5000 diffractometer with Ni-filtered Cu Kα radiation using a variable slit mode and a detector slit width of 0.2 mm. Data were collected at 0.02°2Ө steps, between 2° and 65°. Collection times varied from 0.1 s/step for routine analyses to 1 s/step for data used in structural modeling. TGA data were obtained using a Shimadzu, Inc. TGA-50 thermogravimetric analyzer. Samples were loaded into a platinum pan; the sample chamber was flushed with nitrogen. Thermal scans from ambient to 800°C were performed at 2°C/min. A new sample digestion protocol was developed for the GIC products. Samples (30 mg) were digested into 70% HNO3 (1 mL) using a microwave digestor 127 (CEM Corporation, MDS 2000) at 100, 150, and then 200 psi for 5 min each; this method completely decomposed the solid products and generated homogeneous solutions. The solutions were diluted to 50.0 mL and stored in Nalgene plastic bottles. Sample solutions were added to equal volumes of LL-TISAB prior to analysis. Fluoride analyses on the products were performed using an ion-selective fluoride combination electrode (VWR# 14002-788 F) with a standard single-junction sleeve-type reference electrode and pH meter with an expanded mV scale (VWR 8005). A calibration curve was obtained using fluoride standards prepared by diluting a standard solution (10 µg F / 1.00 mL) to 0.0500, 0.100, 0.200, 0.400, 0.600, 1.00, 2.00, 3.00, and 4.00 mg F/L. All dilutions were performed using standard silica glassware at ambient temperature. Below a pH of ≈5, the presence of HF or HF2¯, which cannot be detected by the fluoride electrode, can interfere with accurate F analyses. [27] In addition, the total ionic strength of sample solutions must be maintained within the specification range of the electrode. Finally, hydrofluoric acid can react with silica. In order to address these issues, the combination of both samples and standards with an equal volume of LL-TISAB provided the required control over both pH and ionic strength. In addition, control experiments were performed using only Nalgene labware and these showed less than 1% difference in detected fluoride content from data obtained using standard glassware. Boron analyses used the same digestion procedure as described above. Samples were subsequently diluted to form a 2% v/v HNO3 solution, and the B 128 content determined using ICP-AES (S.A. Inc, JY2000U analyzer) calibrated using a boron standard (1.0-10 mg B/L) prepared from anhydrous boric acid according to a literature procedure. [29] A 2% v/v HNO3 acid solution was used as control blank solution for calibration. Energy-minimized structural models for the B[OC(CF3)2C(CF3)2O]2¯ and B[OC(CF3)2C(O)O]2¯ anions were calculated using Gaussian 03W software, and full geometry optimizations were carried out using density functional method (B3LYP) with a 6-31G* basis set. 5.4 RESULTS AND DISCUSSION In these syntheses, F2 serves as the oxidant, and B[OC(CF3)2C(CF3)2O]2¯, or B[OC(CF3)2C(O)O]2¯, and fluoride anions are co-intercalates according to the following overall reaction: xC + [(δ+1)/2]F2 (g) + An¯ (sol) CxAn·δF + F ¯ (sol) (5.3) where An¯ is the borate anion, x is the ratio of graphene carbon to borate intercalate, and fluoride is co-intercalated into the galleries. Although the GIC products obtained were placed in vacuo at ambient temperature for at least 8 h, the formulation of cointercalate as fluoride should also allow that F(HF)n¯ species such as bifluoride may 129 also be present. This work therefore represents the first chemical preparation of GIC’s containing borate chelate anions. No reaction occurs between graphite and elemental fluorine at room temperature even with a high fluorine pressure (5 kbar) and several days reaction time, unless a carrier such as gaseous or liquid HF is present. [30] The compositions, structures and physical properties of graphite fluoride (CxF) and graphite bifluoride (CxHF2·δHF) are strongly dependent on the preparative conditions employed. Under the conditions used in this study, CxHF2·δHF is formed within minutes. It is therefore likely that bifluoride anions intercalate first in these reactions, but are replaced over time by borate chelate anions and are also converted to fluoride. In initial reactions, additions of aliquots of AHF solutions containing NaB[OC(CF3)2C(CF3)2O]2 or NaB[OC(CF3)2C(O)O]2 indicated that a mole ratio of graphene carbon to anion of 24:1 were sufficient to produce first-stage GIC’s. For all subsequent reactions, this stoichiometry was employed. The stability of these borate chelate anion salts in AHF was confirmed separately. Powder XRD for the products obtained are shown in Figure 5.1 (a-b). All major reflections can be ascribed to a single phase of CxB[OC(CF3)2C(CF3)2O]2·δF or CxB[OC(CF3)2C(O)O]2·δF. Table 5.1 provides the list of indexed reflections and the gallery height, di, for the GIC products obtained after 24 h reaction. Both products have gallery heights close to 1.4 nm. This gallery dimension is similar to those obtained previously for borate chelate GIC’s using electrochemical oxidation. [24,25] A stage 1 CxB[OC(CF3)2C(O)O]2·δF can be obtained with this chemical synthesis, but 130 not with the electrochemical method employed previously, where the maximum extent of intercalation was stage 2. [25] However, some minor impurities are also present in the GIC products obtained from the chemical syntheses. As can be seen in Figure 5.1 (a), there is a peak ascribed to CxF at 14.9° 2θ (d = 0.60 nm), and very small peaks ascribed to residue anion salt. Small impurity peaks from residue anion salt are also seen in Figure 5.1 (b). Figure 5.2 (a-b) shows the structural models of stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF, respectively, including fluoride co-intercalates positioned on the graphene surfaces in the gallery. The observed gallery heights agree well with those calculated when the chelate borate anions adopt a “standing up” orientation in the intercalate galleries, with the long anion z-axis aligned perpendicular to the graphene sheets. * * 10 (009) CxF (008) (005) (007) a intensity (004) 131 20 30 40 50 60 70 60 70 (009) (006) (008) (005) intensity b (004) 2θ/ degree * 10 20 30 40 50 2θ/ degree Figure 5.1 Powder XRD pattern of stage 1 products of (a) CxB[OC(CF3)2C(CF3)2O]2 ·δF and (b) CxB[OC(CF3)2C(O)O]2·δF. Assigned (00l) indices are indicated; impurity peaks are marked *. 132 Table 5.1 Powder XRD structural data for (a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF products obtained after 24 h reaction. peak # 2Ө dobs / nm (a) CxB[OC(CF3)2C(CF3)2O]2·δF dcalc / nm index (hkl) (di = 1.45 nm) 1 14.86 0.607 0.600 2 24.55 0.362 0.362 (004) 3 31.16 0.287 0.290 (005) 4 45.38 0.200 0.207 (007) 5 50.87 0.180 0.181 (008) 6 58.28 0.159 0.161 (009) (b) CxB[OC(CF3)2C(O)O]2·δF * (di = 1.44 nm) 1 24.72 0.360 0.360 (004) 2 30.96 0.289 0.288 (005) 3 37.60 0.239 0.240 (006) 4 50.95 0.179 0.180 (008) 5 57.63 0.160 0.160 (009) * this peak belongs to graphite fluoride, CxF 133 z B C O x F y a 1.45 nm b 1.44 nm Figure 5.2 Structural models of stage 1 GIC’s of (a) CxB[OC(CF3)2C(CF3)2O]2·δF and (b) CxB[OC(CF3)2C(O)O]2·δF. 134 Table 5.2 provides the compositional data for these GIC products as determined by elemental analyses for B and F in the solid samples using ICP-AES and the digestion protocol described above. The observed mass fractions were converted to the compositional parameters x and δ in CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF using the following relations: mass fraction B = 10.8 (5.4) 12.0x + Mw of anion + 19.0δ mass fraction F = 19.0 δ (5.5) 12.0x + Mw of anion + 19.0δ Previous studies on borate chelate GIC’s prepared by electrochemical oxidation in nitromethane used both coulometric data and structural modeling of diffraction data to determine the graphene carbon to anion mol/mol ratio, x. However, due to the inefficient electrochemical oxidation, coulometry could determine only a lower limit for this parameter. Results obtained were as follows: for stage 2 CxB[OC(CF3)2C(O)O]2, x≥28 (coulometry) and x=44 (diffraction); [25] for stage 2 CxB[OC(CF3)2C(CF3)2O]2, x≥41 (coulometry) and x=49 (diffraction). [24] These x values are similar to those obtained with smaller intercalate anions, and approximately follow the general composition of C24nAn where n = stage. [31] Examples of smaller anion GIC’s that are close to x=48 for stage 2 include CxPF6·2CH3NO2 (x=48) [5], 135 Table 5.2 Compositional data for CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF products. Sample B F / mass fraction / mass fraction x δ CxB[OC(CF3)2C(CF3)2O]2·δF 0.149 0.0503 58 3.8 CxB[OC(CF3)2C(O)O]2·δF 0.193 0.0692 51 4.1 136 CxAsF6·2CH3NO2 (x=45) [5], CxSbF6·4C4H6O3 (x=48) [32], CxHSO4·2H2SO4 (x=48) [33], and CxSO3CF3·2CF3SO3H (x=52) [6]. The present study found x values for stage 1 that are much larger than those noted above (x=58 for the stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF and x=51 for the stage 1 CxB[OC(CF3)2C(O)O]2·δF). Note that these are for stage 1, not stage 2, products. The GIC for the smaller anion, CxB[OC(O)C(O)O]2·δF, prepared by chemical oxidation had x=41 for the mixed stage 1/2 CxB[OC(O)C(O)O]2·δF. [26] These results indicate a much lower packing density for the large borate chelate anions within GIC galleries. In addition, these products show relatively high concentrations of fluoride co-intercalate (δ≈3-4). Both the large size of these anions, and the steric and charge effect of the fluoride co-intercalate, can explain the low packing density of borate chelate anions within the galleries. In previous electrochemical syntheses studies, no fluoride co-intercalate was present because those borate chelate GIC’s were prepared using electrochemical oxidation in an organic solvent. Another previous study of fluoride co-intercalates in GIC’s described the preparation of CxN(SO2CF3)2·δF using chemical oxidation in hydrofluoric acid. It found a composition for stage 2 of x=50-60 and δ=0-1. [34] With this imide anion intercalate, the gallery height was about 0.8 nm, similar to that in GIC’s containing sulfate or nitrate, and much less than that for the GIC’s containing borate chelate anions. The relative concentration of fluoride co-intercalate therefore appears to be strongly dependent on anion and gallery dimensions; however, more examples are needed to understand this relationship in detail. 137 The thermal degradation of these borate chelate GIC’s was studied using thermogravimetric analysis (TGA). Mass loss data for the GIC’s and for NaB[OC(CF3)2C(CF3)2O]2 and NaB[OC(CF3)2C(O)O]2 salts are shown in Figure 5.3. The decomposition of borate chelate anions has been investigated for common salts such as lithium bis(oxalato)borate which completely decomposes to Li2CO3 and B2O3 by 650°C via reaction (5.6): LiB[OC(O)C(O)O]2 ½Li2CO3 + ½B2O3 + volatiles (5.6) However, the volatile decomposition products were not identified in this study. Therefore, for NaB[OC(CF3)2C(CF3)2O]2 and NaB[OC(CF3)2C(O)O]2 salts, the expected thermal decomposition reactions are: NaB[OC(CF3)2C(CF3)2O]2 NaB[OC(CF3)2C(O)O]2 ½Na2CO3 + ½B2O3 + volatiles (5.7) ½Na2CO3 + ½B2O3 + volatiles (5.8) Using reactions (5.7) and (5.8), the calculated mass losses agree reasonably well with the observed values: 95.0 % (calc), 99.0 % (obs), and 92.3 % (calc), 90.0 % (obs), respectively. The loss of graphene carbon due to sheet degradation is not generally observed below 650°C, and planar-sheet graphite fluorides have been shown to lose essentially 138 all fluoride content above 450°C [17], therefore the thermolysis reaction for CxB[OC(CF3)2C(CF3)2O]2·δF and CxB[OC(CF3)2C(O)O]2·δF might be expected to follow equations (5.9) and (5.10): CxB[OC(CF3)2C(CF3)2O]2·δF CxB[OC(CF3)2C(O)O]2·δF Cx + ½B2O3 + volatiles (5.9) Cx + ½B2O3 + volatiles (5.10) and therefore be useful in compositional analysis for x and δ. However, using the x and δ from Table 5.2, the stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF mass losses are 53.2 % (calc) and 21.5 % (obs), and 61.6 % (calc) and 15.0 % (obs) for stage 1 CxB[OC(CF3)2C(O)O]2·δF. Thus, TGA analysis for total intercalate content does not match with results from the elemental analysis. Powder XRD of the GIC’s after heating to 450°C (not shown) indicate that the GIC’s are not fully converted to graphite at this temperature. The product appears to be a mix of high-stage GIC’s, most likely, the anions are trapped in the bulk of the large flaky graphite particles. Therefore, observed mass losses are much less than those calculated assuming the full conversion back to graphite. 139 100 a mass pct 80 60 40 20 0 0 200 400 600 800 0 200 400 600 800 100 b mass pct 80 60 40 20 0 temperature, °C Figure 5.3 TGA mass loss data for (a) NaB[OC(CF3)2C(CF3)2O]2 salt (⋅⋅⋅) and stage 1 CxB[OC(CF3)2C(CF3)2O]2·δF (⎯), (b) NaB[OC(CF3)2C(O)O]2 salt (⋅⋅⋅) and stage 1 CxB[OC(CF3)2C(O)O]2·δF (⎯). 140 5.5 REFERENCES 1. Billaud, D.; Pron, A.; Vogel, F.; Herold, A. Mater. Res. Bull. 1980, 15, 1627. 2. Billaud, D.; Pron, A.; Vogel, F. Synth. Met. 1980, 2, 177. 3. Forsman, W.; Mertwoy, H. Carbon 1982, 20, 255. 4. Chenite, A.; Billaud, D. Carbon 1982, 20, 120. 5. Billaud, D.; Chenite, A. J. Power Sources 1984, 13, 1. 6. Horn, D.; Boehm, H. Mater. Sci. Eng. 1977, 31, 87. 7. Ruisinger, B.; Boehm, H. Carbon 1993, 31, 1131. 8. Boehm, H.; Helle, W.; Ruisinger, B. Synth. Met. 1988, 23, 395. 9. Ruisinger, B.; Boehm, H. Angew. Chem. Int. Ed. Engl. 1987, 26, 253. 10. 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Besenhard, J.; Wudy, E.; Moehwald, H.; Nickl, J.; Biberacher, W.; Foag, W. Synth. Met. 1983, 7, 185. 34. Katinonkul, W.; Lerner, M. M. Carbon 2007, 45, 499. 142 CHAPTER 6 CONCLUSION A new acceptor-type graphite intercalation compound (GIC) containing the borate chelate anion bis(oxalato)borate, CxB[OC(O)C(O)O]2·δF is obtained for the first time by chemical oxidation of graphite with fluorine gas in the presence of a solution containing the intercalate anion in anhydrous hydrofluoric acid. GIC’s up to stage 1 with a gallery height, di ≅ 1.42 nm, can be obtained. The structural model and one-dimensional electron density map indicates that the intercalate anions adopt a “standing up” orientation in the gallery. The composition (x and δ parameters) is determined using both thermogravimetric and elemental analyses. The concentration of co-intercalated fluoride anions from this chemical synthesis is studied quantitatively with a newly-developed digestion method using an ion-selective electrode and potentiometer. Results indicate that there are approximately four fluoride cointercalate per borate chelate anion in the galleries. Stage 1 GIC’s of two other borate chelate anions, bis(perfluoropinacolato) borate, CxB[OC(CF3)2C(CF3)2O]2·δF, and bis(hexafluorohydroxyisobutyrato)borate, CxB[OC(CF3)2C(O)O]2·δF, are obtained for the first time using chemical oxidation. The gallery heights, di, for these GIC’s are 1.43-1.45 nm. The chemical syntheses used to prepare these GIC’s are the same as described above. The fluoride co-intercalate content for both these GIC products is determined to be approximately four fluoride co-intercalates per borate chelate anion. 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