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Electrical conductivity and chemical bond of graphite intercalation compound with fluorine and metal fluoride
Solid State Ionics 11 (1983) 65-69 North-Holland Publishing Company
ELECTRICAL CONDUCTIVITY AND CHEMICAL BOND OF GRAPHITE INTERCALATION COMPOUND WITH FLUORINE AND METAL FLUORIDE Tsuyoshi NAKAJIMA, Masayuki KAWAGUCHI and Nobuatsu WATANABE Departmentof IndustricllChemistry, Faculty of Engineering, Kyoto University, Yoshidn, Sakyo-ku, Kyoto, 606 Japan Received 24 March 1983
Graphite intercalation compounds with fluorine and metal fluoride (MgF2 or CuF2) were prepared from petroleum coke and pyrolytic graphite. With progress in the intercalation reaction, the first stage compound with identity period 9.4 A changed to another structure of identity period 10.7 A. It was found from ESCA measurements that the chemical interaction between intercalated fluorine and carbon was similar to the covalent bond around the surface and slightly ionic in the bulk. The maximum electrical conductivities in the direction of the ah-axis were (1.9-2.0) x lo5 (a cm)-‘, which were lo-13 times that of the original pyrolytic graphite.
1, Introduction
Since it was reported that Cl&F5 had a high electrical conductivity [ 11, much attention has been concentrated on the electrical conductivity of graphite intercalation compounds. However, most of the known compounds are hygroscopic and easily decompose in air. The ternary graphite intercalation compounds of fluorine with AlF,, MgF2 or CuF2 have been prepared from natural graphite [2-51. They . were relatively stable in air. In this study, the graphite intercalation compounds of fluorine and MgF2 or CuF2 were prepared from petroleum coke and pyrolytic graphite, and the chemical interaction between intercalated fluorine and graphite layer and the electrical conductivity in the direction of the &axis were reported.
5 mm in width, 0.5-l .Omm in thickness). Anhy drous CuF2 (Ozark-Mahoning Co. Ltd.) was dried under vacuum for 14 h with the temperature increasing from room temperaturti to 115’C, and was treated in a fluorine gas flow for 12 h at this temperature . Commercially available MgF2 (purity >98%) was used without further purification. Fluorine gas (purity 99.4%) was purified by the same method as in a previous paper [2]. Graphite (0.2 g) and metal fluoride (0.2 g) were placed in a Ni reaction tube and the system was evacuated by a rotary pump for about 12 h. Fluorine gas was then introduced into the system at a temperature between 20 and 300°C. Being kept at that temperature for 0.5-69 h, the system was cooled to room temperature. After 5-l 20 h from the start of cooling, the products were obtained. 2.2. Analyses of the products
2. Experimental 2.1. Preparation of intercalation compounds The host graphites of the intercalation compounds were petroleum coke (heat-treated at 28OO’C) and pyrolytic graphite (heat-treated at 3000°C; 5 X 0 167-2738/83/0000-0000/$03.00
0 1983 North-Holland
X-ray diffractometry was made using DX-GOS goniometer (JEOL) with Cu Ka! radiation. The ESCA measurements were carried out using a Du Pont 650B electron spectrometer with Mg Kar radiation. The C and F contents in the intercalation compounds were determined by conventional elemental
66
T. NakajYmaet al. f Graphite intercalationcompound with fluorine and metal fluoride
analyses. Mg and Cu contained in them were analysed by the atomic absorption method for a solution obtained by oxidizing an intercalation compound in a mixture of HC104 and HNO, (3 : 1) at about 150°C and by dissolving the residue in water. The chemical compositions were calculated in the same way as shown in previous papers [4,5]. When the composition of an intercalation compound is expressed as C,F(MF&, (M = Mg or Cu), the average value ofy was 0.04 for both compounds. In the case of the pyrolytic graphite host, the composition was estimated by fixingy at 0.04 and calculatingx from the weight increase. 2.3. Electrical conductivify measurement The electrical conductivity measurement was made by a contactless Wien bridge in air at 22 f 2’C [6].
3. Results and discussion 3.1. Variation of X-ray dgfiaction patterns with intercalation reaction
Fig. 1 shows the X-ray diffraction patterns of intercalation compounds prepared by the reaction of several carbon materials with MgF2 and fluorine. From the observed (001) diffraction lines, the identity period in the direction of the c-axis was 9.4 A. The intercalation compounds prepared from pyrolytic graphite or petroleum coke contained more intercalant than that of natural graphite, showing the Xray diffraction pattern of a typical first stage compound whose (002) diffraction line had the highest intensity. This is consistent with many examples that a graphite intercalation compound whose identity period is near the integer times 3.35 Whas the most strongest (OO(n t 1)) line (n: stage number). With progress in the intercalation reaction, an intercalation compound containing much more intercalant was formed. This intercalation compound has a larger identity period of 10.7 A. The stage number of this compound is shown as 1’ in figs. 1 and 2. It was found that the unknown peak (28 = 16.44’) in a previous paper [2] was the (002) diffraction line of this compound. It has been proposed for the formation of these intercalation compounds that metal
I
10
I
20 Diffraction
1
I
I
30 angle
40 28
I0
50 (Cu
I
60 Kv.)
Fig. 1. X-ray diffraction patterns of graphite intercalation compound with fluorine and MgFa. (a) Ca.aF(MgFa)o.os prepared from natural graphite [ 31. Zc = 9.37 f 0.06 A, stage 1; (b) Ce.,F(MgFa)o.o4 prepared from pyrolytic graphite. Zc = 9.43 f 0.06 A, stage: 1; (c) (28.1 F(MgFa)c.ee prepared from petroleum coke. I, = 9.4 * 0.1 A, stage:1 ; (d) Cs.sF(MgF2)o.04 prepared from pyrolytic graphite.Zc = 9.39 ?: 0.05 A, stage: 1, *I, = 10.7 A, stage: 1’; (e) C4.tF(MgFa)o.o4 prepared from pyrolytic graphite. I, = 9.41 * 0.03 A, stage: 1, *Zc = 10.71 f 0.06 -4, stage: 1’.
fluoride and fluorine first react to form a gaseous complex (eq. (l)), then followed by the intercalation of the gaseous complex with fluorine into graphite:
61
T. Nakajima et al. / Graphite intercaIation compound with fluorine and metal fltloride
imF2(g)+MFn(s)+MFn+m(d
(1)
2
graphite t MF,,,
+ F2 -, C,F(MF,),,
,
(a)
(2)
where M = Al, Mg or Cu, n = 3 or 2. The first stage compound would be formed due to the change in ionicity of intercalated fluorine with progress in the reaction. The chemical analyses show that these intercalation compounds have many fluorine atoms and a small amount of metal fluoride. This is probably because of the low vapor pressure of MF,+, in eq. (1). As the intercalation reaction proceeds, the amount of intercalant tends to increase from C,.,F(MgF,),.,, to C,.,F(MgF,),.,, (fig. 1). Cg.lF(“gFZ)o.09 (fig. lc) and C,,F(CuF,),.,, (fig. 2b) prepared from petroleum coke would have had less amount of intercalant than those just after preparation because the compounds with the identity period, 9.4 A were unstable and analysed after storage in glass ampoule for 2 weeks. On the other hand, as the X-ray diffractograms were taken just after the preparation, the diffraction patterns would reflect the structure undecomposed in air. Their (003) diffraction lines were very broad, having peak positions between 26 and 28” in 28 probably due to the overlapping with (002) diffraction line of graphite. Except for the first stage, the compounds of petroleum coke were less stable than those of natural graphite. A higher stage compound was so unstable that the peak corresponding to graphite appeared in a few days after preparation. However, the first stage compound was fairly stable in air. It was observed after 70 d that the (002) diffraction line only shifted to a higher angle, that is, the identity period became smaller by about 0.5 A. Regarding the kind of metal fluoride, C,F(CuF,),, is more stable than CxF(MgF2),, in all the hosts. Fig. 2 shows the X-ray diffraction pattern of an intercalation compound of fluorine with CuF2. In the same way as the MgF2 system, the reaction was faster in the petroleum coke compound, and the relative intensity of (002) diffraction line of the product was much stronger than that of natural graphite. C4.2F(CuF& -03 shown in fig. 2 is a first stage compound whose identity period is 11.07 A.
L
I
10
I
I
20
I
Diffraction
I
I
11
30 40 angle 28 P
11
11
50 (Cu WC?
1
I
70
Fig. 2. X-ray diffraction patterns of graphite intercalation compound with fluorine and CuF2. (a) C~_OF(CUF~)OJ,~ prepared from natural graphite [ 41. Zc = 9.42 j: 0.05 A, stage.: 1; @) C14F(CuF2)0.14 prepared from petroleum coke. Zc = 9.3 A, stage: 1; (c) C4_2F(CuF2)0.03 prepared from petroleum coke. Zc = 11.07 A, stage: 1’.
3.2. Chemical bond between carbon and jkorine Figs. 3 and 4 are ESCA spectra of CxF(MgF& and C,F(CuF&, . The spectra of(b), (c) and (d) in figs. 3 and 4 show the chemical bond around the surface of the compounds and (a) are those of a sample obtained by cleavage of pyrolytic graphite intercalated by fluorine and metal fluoride. As ESCA analyses only the surface to a depth of about 30 A for carbon materials, the different spectra were obtained by observation of the surface and bulk of the intercalation compounds. In the C 1s spectra of the intercalation compounds prepared by introduction of fluorine gas into the reactor at 200-300°C (figs. 3d, 4d), a peak shifted to 289 eV was observed with a strong peak at 284 eV. The former is near the position correspond-
T. Nakajimaet al. / Graphite intercalationcompound withfluorine and metal jluoride
“’;“I (a)
I
290
I
I
690 260 Binding energy (eV)
/“i”
I
680
Fig. 3. ESCA spectra of graphite intercalation compound with fluorine and MgF2. (a) Cs.eF(MgF2)ove4 prepared from pyrolytic graphite at 2OT; (b) Ce.1F(MgF~)o.c9 prepared from petroleum coke, introduction of fluorine at 300°C and temperature decrease after 0.5 h;(c) C1eF(MgF2)o.os prepared from natural graphite at 2O’C [ 31; (d) C 5. e F(MgF2)o _05 prepared from natural graphite. After introduction of fluorine at ~O’C, thetemperature wasincreased to 300°C and decreased after 48 h [3].
ing to C-F covalent bond (290 eV) and the latter corresponds to C-C covalent bond of graphite. The F 1s peak observed at 688 eV was also near that of graphite fluoride (689 ev). As Cg.1F(MgF2)0 o9 (fig. 3b) was prepared from petroleum coke maint&ed in a fluorine atmosphere for 0.5 h at 300°C, the intensity of a shifted C 1s peak at 289 eV was not so strong as that Of C5.&MgFz)o_05 (fig. 3d). In the case of C,4F(CuF2)o_14 (fig. 4b) prepared from petroleum coke held in fluorine atmosphere for 2 h at 200°C, only a shifted shoulder for C 1s electron was observed and the F 1s peak was found at a lower binding energy (686.3 eV) than that of C7.-,F(C~F2)0_02 (688 eV) held in fluorine atmosphere for 48 h at 200°C. C16F(MgF2)O_OS(fig. 3c) and C5.4F(CuF2)o_02 (fig. 4c)prepared from natural graphite at room temperature showed no shifted C 1s peak and F 1s shoulder at lower binding energies than 688 eV. It was found that a chemical bond similar to the C-F covalent bond was formed around the sur-
-Tkc--&
Binding
--Air-A
energy
/eV
Fig. 4. ESCA spectra of graphite intercalation compound with fluorine and CuF2. (a) Cl oF(CuF2)0.o4 prepared from pyrolytic graphite at 2O’C; (b) C~~F(CUF~)O.~~ prepared from petroleum coke, introduction of fluorine at 200°C and temperature decrease after 2 h; (c) Cg.4F(CuFz)o.o2 prepared from natural graphite at 20°C [4] ; (d) C,_oF(CuF2)c.e2 prepared from natural graphite, introduction of fluorine at 200” C and temperature decrease after 48 h [ 41.
face of the compounds prepared by introduction of fluorine gas at high temperatures (200-300°C). On the other hand, when the bulk of the intercalation compounds was observed by cleavage of pyrolytic graphite, F 1s spectra were found at lower binding energies, 685.6 eV (fig. 3a) and 685.8 eV (fig. 4a), which are near that of the fluoride ion of ionic metal fluorides such as LiF (684.5 ev) or molecular fluorine (686 eV). These results show that the graphite intercalation compound prepared has a nearly covalent bond around the surface and a slightly ionic nature in the bulk. 3.3. Ekxtrical conductivity Figs. 5 and 6 show the relation between the electrical conductivity and the wt% of intercalant. At first the electrical conductivity increased rapidly with weight increase to about 10 wt%, where a compound
Z Nakajima etal. / Graphite intercalationcompound withfluotine and metal jluoride
10050
, ,
30
,
Composition 20 IS
(
)
(,
CxF(MgF2)0.0C 5
I
I
I
lX104 ;,
10 lntercalant
1
I 20 / Wt %
I 30
Fig. 5. Electrical conductivity of graphite intercalation compound with fluorine and MgFz .
higher than the third stage was formed, and reached a maximum value. For C, F(MgFz),, , the maximum conductivity was 1.9 X lo5 ($2cm)-l at 8-10 wt% of intercalant corresponding to a mixture of the firstfourth stage compounds. On the other hand, that of C,F(CuF.& was 2.0 X 10s (SI cm)-l at 12-15 v&% of inter&u-n corresponding to a mixture of the fnst and second stage. These values were higher by one order than the electrical conductivity of the original pyrolytic graphite, (1.7 + 0.2) X 104 (a cm)-l. The electrical conductivity of the first stage compound was
,1x&,,
5 i? w
10050
Composition/ 30 20 I5
11
I
I
I
b
, LQ
lower to some extent than the maximum value. As the carrier is generated by charge transfer between carbon and intercalant, the amount of carrier is proportional to that of intercalant; however, the excess of inter&ant reduces the mobility of carrier. For this reason, an electrical conductivity generally has a maximum point which corresponds to a second or third stage compound. In the graphite intercalation compounds of AsF, and SbF, , the second stage C1,AsF, and C, $bF5 have the maximum conductivities in their series. The chemical composition of C,F(MF& giving the maximum conductivities were C15-18F(MgF2)0.04 and CIO-I~F(C~FZ)O.~~. The numbers of carbon atoms per one fluorine atom are 15-18 and 10-15, respectively, which are near the ratios of C/AsF5 and C/SbF, of the above compounds. One fluorine atom is therefore almost equivalent to one AsF, or SbF5 molecule as an electron acceptor from graphite.
Acknowledgement Huorine gas used in this study was supplied by courtesy of Daikin K@yo Co. Ltd., which the authors heartily thank. They also wish to express their thanks to Mr. Noriyuki Tanaka of the Applied Science Research Institute for his helpful advise in making the contactless Wien bridge and to Dr. TEu Nagaoki of Nihon Carbon Co. Ltd. for his kindness.
C;o~(C~~2)ao4 5
1
I
--Original
1xto4
69
I lntercalant
20 / Wt %
I 30
Fig, 6. Electrical conductivity of graphite intercalation compound with fluorine and CuF2.
References [l] F.L. Vogel, G.M.T. Foley, C. Zeller, E.R. Falardeau and J. Gan, Mater. Sci. Eng. 31(1977) 261. [2] T. Nakajima, M. Kawaguchi and N. Watanabe, Chem. Letters (1981) 1045; Z. Naturforsch. 36 (1981) 1419. [3] T. Nakajima, M. Kawaguchi and N. Watanabe, Carbon 20 (1982) 287. [41T. Nakajima, M. Kawaguchi, T. Kawasaki and N. Watanabe, Nippon Kagaku K&i (1983) 283. [51T. Nakajima, M. Kawaguchi and N. Watanabe, Electrochim. Acta 27 (1982) 1535. VI S.C. Singhal and A. Kemick, Synthetic Metals 3 (1981) 247.
ELECTRICAL CONDUCTIVITY AND CHEMICAL BOND OF GRAPHITE INTERCALATION COMPOUND WITH FLUORINE AND METAL FLUORIDE Tsuyoshi NAKAJIMA, Masayuki KAWAGUCHI and Nobuatsu WATANABE Departmentof IndustricllChemistry, Faculty of Engineering, Kyoto University, Yoshidn, Sakyo-ku, Kyoto, 606 Japan Received 24 March 1983
Graphite intercalation compounds with fluorine and metal fluoride (MgF2 or CuF2) were prepared from petroleum coke and pyrolytic graphite. With progress in the intercalation reaction, the first stage compound with identity period 9.4 A changed to another structure of identity period 10.7 A. It was found from ESCA measurements that the chemical interaction between intercalated fluorine and carbon was similar to the covalent bond around the surface and slightly ionic in the bulk. The maximum electrical conductivities in the direction of the ah-axis were (1.9-2.0) x lo5 (a cm)-‘, which were lo-13 times that of the original pyrolytic graphite.
1, Introduction
Since it was reported that Cl&F5 had a high electrical conductivity [ 11, much attention has been concentrated on the electrical conductivity of graphite intercalation compounds. However, most of the known compounds are hygroscopic and easily decompose in air. The ternary graphite intercalation compounds of fluorine with AlF,, MgF2 or CuF2 have been prepared from natural graphite [2-51. They . were relatively stable in air. In this study, the graphite intercalation compounds of fluorine and MgF2 or CuF2 were prepared from petroleum coke and pyrolytic graphite, and the chemical interaction between intercalated fluorine and graphite layer and the electrical conductivity in the direction of the &axis were reported.
5 mm in width, 0.5-l .Omm in thickness). Anhy drous CuF2 (Ozark-Mahoning Co. Ltd.) was dried under vacuum for 14 h with the temperature increasing from room temperaturti to 115’C, and was treated in a fluorine gas flow for 12 h at this temperature . Commercially available MgF2 (purity >98%) was used without further purification. Fluorine gas (purity 99.4%) was purified by the same method as in a previous paper [2]. Graphite (0.2 g) and metal fluoride (0.2 g) were placed in a Ni reaction tube and the system was evacuated by a rotary pump for about 12 h. Fluorine gas was then introduced into the system at a temperature between 20 and 300°C. Being kept at that temperature for 0.5-69 h, the system was cooled to room temperature. After 5-l 20 h from the start of cooling, the products were obtained. 2.2. Analyses of the products
2. Experimental 2.1. Preparation of intercalation compounds The host graphites of the intercalation compounds were petroleum coke (heat-treated at 28OO’C) and pyrolytic graphite (heat-treated at 3000°C; 5 X 0 167-2738/83/0000-0000/$03.00
0 1983 North-Holland
X-ray diffractometry was made using DX-GOS goniometer (JEOL) with Cu Ka! radiation. The ESCA measurements were carried out using a Du Pont 650B electron spectrometer with Mg Kar radiation. The C and F contents in the intercalation compounds were determined by conventional elemental
66
T. NakajYmaet al. f Graphite intercalationcompound with fluorine and metal fluoride
analyses. Mg and Cu contained in them were analysed by the atomic absorption method for a solution obtained by oxidizing an intercalation compound in a mixture of HC104 and HNO, (3 : 1) at about 150°C and by dissolving the residue in water. The chemical compositions were calculated in the same way as shown in previous papers [4,5]. When the composition of an intercalation compound is expressed as C,F(MF&, (M = Mg or Cu), the average value ofy was 0.04 for both compounds. In the case of the pyrolytic graphite host, the composition was estimated by fixingy at 0.04 and calculatingx from the weight increase. 2.3. Electrical conductivify measurement The electrical conductivity measurement was made by a contactless Wien bridge in air at 22 f 2’C [6].
3. Results and discussion 3.1. Variation of X-ray dgfiaction patterns with intercalation reaction
Fig. 1 shows the X-ray diffraction patterns of intercalation compounds prepared by the reaction of several carbon materials with MgF2 and fluorine. From the observed (001) diffraction lines, the identity period in the direction of the c-axis was 9.4 A. The intercalation compounds prepared from pyrolytic graphite or petroleum coke contained more intercalant than that of natural graphite, showing the Xray diffraction pattern of a typical first stage compound whose (002) diffraction line had the highest intensity. This is consistent with many examples that a graphite intercalation compound whose identity period is near the integer times 3.35 Whas the most strongest (OO(n t 1)) line (n: stage number). With progress in the intercalation reaction, an intercalation compound containing much more intercalant was formed. This intercalation compound has a larger identity period of 10.7 A. The stage number of this compound is shown as 1’ in figs. 1 and 2. It was found that the unknown peak (28 = 16.44’) in a previous paper [2] was the (002) diffraction line of this compound. It has been proposed for the formation of these intercalation compounds that metal
I
10
I
20 Diffraction
1
I
I
30 angle
40 28
I0
50 (Cu
I
60 Kv.)
Fig. 1. X-ray diffraction patterns of graphite intercalation compound with fluorine and MgFa. (a) Ca.aF(MgFa)o.os prepared from natural graphite [ 31. Zc = 9.37 f 0.06 A, stage 1; (b) Ce.,F(MgFa)o.o4 prepared from pyrolytic graphite. Zc = 9.43 f 0.06 A, stage: 1; (c) (28.1 F(MgFa)c.ee prepared from petroleum coke. I, = 9.4 * 0.1 A, stage:1 ; (d) Cs.sF(MgF2)o.04 prepared from pyrolytic graphite.Zc = 9.39 ?: 0.05 A, stage: 1, *I, = 10.7 A, stage: 1’; (e) C4.tF(MgFa)o.o4 prepared from pyrolytic graphite. I, = 9.41 * 0.03 A, stage: 1, *Zc = 10.71 f 0.06 -4, stage: 1’.
fluoride and fluorine first react to form a gaseous complex (eq. (l)), then followed by the intercalation of the gaseous complex with fluorine into graphite:
61
T. Nakajima et al. / Graphite intercaIation compound with fluorine and metal fltloride
imF2(g)+MFn(s)+MFn+m(d
(1)
2
graphite t MF,,,
+ F2 -, C,F(MF,),,
,
(a)
(2)
where M = Al, Mg or Cu, n = 3 or 2. The first stage compound would be formed due to the change in ionicity of intercalated fluorine with progress in the reaction. The chemical analyses show that these intercalation compounds have many fluorine atoms and a small amount of metal fluoride. This is probably because of the low vapor pressure of MF,+, in eq. (1). As the intercalation reaction proceeds, the amount of intercalant tends to increase from C,.,F(MgF,),.,, to C,.,F(MgF,),.,, (fig. 1). Cg.lF(“gFZ)o.09 (fig. lc) and C,,F(CuF,),.,, (fig. 2b) prepared from petroleum coke would have had less amount of intercalant than those just after preparation because the compounds with the identity period, 9.4 A were unstable and analysed after storage in glass ampoule for 2 weeks. On the other hand, as the X-ray diffractograms were taken just after the preparation, the diffraction patterns would reflect the structure undecomposed in air. Their (003) diffraction lines were very broad, having peak positions between 26 and 28” in 28 probably due to the overlapping with (002) diffraction line of graphite. Except for the first stage, the compounds of petroleum coke were less stable than those of natural graphite. A higher stage compound was so unstable that the peak corresponding to graphite appeared in a few days after preparation. However, the first stage compound was fairly stable in air. It was observed after 70 d that the (002) diffraction line only shifted to a higher angle, that is, the identity period became smaller by about 0.5 A. Regarding the kind of metal fluoride, C,F(CuF,),, is more stable than CxF(MgF2),, in all the hosts. Fig. 2 shows the X-ray diffraction pattern of an intercalation compound of fluorine with CuF2. In the same way as the MgF2 system, the reaction was faster in the petroleum coke compound, and the relative intensity of (002) diffraction line of the product was much stronger than that of natural graphite. C4.2F(CuF& -03 shown in fig. 2 is a first stage compound whose identity period is 11.07 A.
L
I
10
I
I
20
I
Diffraction
I
I
11
30 40 angle 28 P
11
11
50 (Cu WC?
1
I
70
Fig. 2. X-ray diffraction patterns of graphite intercalation compound with fluorine and CuF2. (a) C~_OF(CUF~)OJ,~ prepared from natural graphite [ 41. Zc = 9.42 j: 0.05 A, stage.: 1; @) C14F(CuF2)0.14 prepared from petroleum coke. Zc = 9.3 A, stage: 1; (c) C4_2F(CuF2)0.03 prepared from petroleum coke. Zc = 11.07 A, stage: 1’.
3.2. Chemical bond between carbon and jkorine Figs. 3 and 4 are ESCA spectra of CxF(MgF& and C,F(CuF&, . The spectra of(b), (c) and (d) in figs. 3 and 4 show the chemical bond around the surface of the compounds and (a) are those of a sample obtained by cleavage of pyrolytic graphite intercalated by fluorine and metal fluoride. As ESCA analyses only the surface to a depth of about 30 A for carbon materials, the different spectra were obtained by observation of the surface and bulk of the intercalation compounds. In the C 1s spectra of the intercalation compounds prepared by introduction of fluorine gas into the reactor at 200-300°C (figs. 3d, 4d), a peak shifted to 289 eV was observed with a strong peak at 284 eV. The former is near the position correspond-
T. Nakajimaet al. / Graphite intercalationcompound withfluorine and metal jluoride
“’;“I (a)
I
290
I
I
690 260 Binding energy (eV)
/“i”
I
680
Fig. 3. ESCA spectra of graphite intercalation compound with fluorine and MgF2. (a) Cs.eF(MgF2)ove4 prepared from pyrolytic graphite at 2OT; (b) Ce.1F(MgF~)o.c9 prepared from petroleum coke, introduction of fluorine at 300°C and temperature decrease after 0.5 h;(c) C1eF(MgF2)o.os prepared from natural graphite at 2O’C [ 31; (d) C 5. e F(MgF2)o _05 prepared from natural graphite. After introduction of fluorine at ~O’C, thetemperature wasincreased to 300°C and decreased after 48 h [3].
ing to C-F covalent bond (290 eV) and the latter corresponds to C-C covalent bond of graphite. The F 1s peak observed at 688 eV was also near that of graphite fluoride (689 ev). As Cg.1F(MgF2)0 o9 (fig. 3b) was prepared from petroleum coke maint&ed in a fluorine atmosphere for 0.5 h at 300°C, the intensity of a shifted C 1s peak at 289 eV was not so strong as that Of C5.&MgFz)o_05 (fig. 3d). In the case of C,4F(CuF2)o_14 (fig. 4b) prepared from petroleum coke held in fluorine atmosphere for 2 h at 200°C, only a shifted shoulder for C 1s electron was observed and the F 1s peak was found at a lower binding energy (686.3 eV) than that of C7.-,F(C~F2)0_02 (688 eV) held in fluorine atmosphere for 48 h at 200°C. C16F(MgF2)O_OS(fig. 3c) and C5.4F(CuF2)o_02 (fig. 4c)prepared from natural graphite at room temperature showed no shifted C 1s peak and F 1s shoulder at lower binding energies than 688 eV. It was found that a chemical bond similar to the C-F covalent bond was formed around the sur-
-Tkc--&
Binding
--Air-A
energy
/eV
Fig. 4. ESCA spectra of graphite intercalation compound with fluorine and CuF2. (a) Cl oF(CuF2)0.o4 prepared from pyrolytic graphite at 2O’C; (b) C~~F(CUF~)O.~~ prepared from petroleum coke, introduction of fluorine at 200°C and temperature decrease after 2 h; (c) Cg.4F(CuFz)o.o2 prepared from natural graphite at 20°C [4] ; (d) C,_oF(CuF2)c.e2 prepared from natural graphite, introduction of fluorine at 200” C and temperature decrease after 48 h [ 41.
face of the compounds prepared by introduction of fluorine gas at high temperatures (200-300°C). On the other hand, when the bulk of the intercalation compounds was observed by cleavage of pyrolytic graphite, F 1s spectra were found at lower binding energies, 685.6 eV (fig. 3a) and 685.8 eV (fig. 4a), which are near that of the fluoride ion of ionic metal fluorides such as LiF (684.5 ev) or molecular fluorine (686 eV). These results show that the graphite intercalation compound prepared has a nearly covalent bond around the surface and a slightly ionic nature in the bulk. 3.3. Ekxtrical conductivity Figs. 5 and 6 show the relation between the electrical conductivity and the wt% of intercalant. At first the electrical conductivity increased rapidly with weight increase to about 10 wt%, where a compound
Z Nakajima etal. / Graphite intercalationcompound withfluotine and metal jluoride
10050
, ,
30
,
Composition 20 IS
(
)
(,
CxF(MgF2)0.0C 5
I
I
I
lX104 ;,
10 lntercalant
1
I 20 / Wt %
I 30
Fig. 5. Electrical conductivity of graphite intercalation compound with fluorine and MgFz .
higher than the third stage was formed, and reached a maximum value. For C, F(MgFz),, , the maximum conductivity was 1.9 X lo5 ($2cm)-l at 8-10 wt% of intercalant corresponding to a mixture of the firstfourth stage compounds. On the other hand, that of C,F(CuF.& was 2.0 X 10s (SI cm)-l at 12-15 v&% of inter&u-n corresponding to a mixture of the fnst and second stage. These values were higher by one order than the electrical conductivity of the original pyrolytic graphite, (1.7 + 0.2) X 104 (a cm)-l. The electrical conductivity of the first stage compound was
,1x&,,
5 i? w
10050
Composition/ 30 20 I5
11
I
I
I
b
, LQ
lower to some extent than the maximum value. As the carrier is generated by charge transfer between carbon and intercalant, the amount of carrier is proportional to that of intercalant; however, the excess of inter&ant reduces the mobility of carrier. For this reason, an electrical conductivity generally has a maximum point which corresponds to a second or third stage compound. In the graphite intercalation compounds of AsF, and SbF, , the second stage C1,AsF, and C, $bF5 have the maximum conductivities in their series. The chemical composition of C,F(MF& giving the maximum conductivities were C15-18F(MgF2)0.04 and CIO-I~F(C~FZ)O.~~. The numbers of carbon atoms per one fluorine atom are 15-18 and 10-15, respectively, which are near the ratios of C/AsF5 and C/SbF, of the above compounds. One fluorine atom is therefore almost equivalent to one AsF, or SbF5 molecule as an electron acceptor from graphite.
Acknowledgement Huorine gas used in this study was supplied by courtesy of Daikin K@yo Co. Ltd., which the authors heartily thank. They also wish to express their thanks to Mr. Noriyuki Tanaka of the Applied Science Research Institute for his helpful advise in making the contactless Wien bridge and to Dr. TEu Nagaoki of Nihon Carbon Co. Ltd. for his kindness.
C;o~(C~~2)ao4 5
1
I
--Original
1xto4
69
I lntercalant
20 / Wt %
I 30
Fig, 6. Electrical conductivity of graphite intercalation compound with fluorine and CuF2.
References [l] F.L. Vogel, G.M.T. Foley, C. Zeller, E.R. Falardeau and J. Gan, Mater. Sci. Eng. 31(1977) 261. [2] T. Nakajima, M. Kawaguchi and N. Watanabe, Chem. Letters (1981) 1045; Z. Naturforsch. 36 (1981) 1419. [3] T. Nakajima, M. Kawaguchi and N. Watanabe, Carbon 20 (1982) 287. [41T. Nakajima, M. Kawaguchi, T. Kawasaki and N. Watanabe, Nippon Kagaku K&i (1983) 283. [51T. Nakajima, M. Kawaguchi and N. Watanabe, Electrochim. Acta 27 (1982) 1535. VI S.C. Singhal and A. Kemick, Synthetic Metals 3 (1981) 247.