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Acceptor site in metal halide intercalated graphite
Solid State Communications, Vol. 33, pp. 809—811. Pergamon Press Ltd. 1980. Printed in Great Britain. ACCEPTOR SITE IN METAL HALIDE INTERCALATED GRAPHITE G.K. Wertheim, P.M.Th.M. van Attekum,* H.J. Guggenheim and K.E. Clements~ Bell Laboratories, Murray Hill, N.J. 07974, U.S.A. We present evidence suggesting that metal ion vacancies in the intercalant layer produce acceptor sites responsible for the charge transfer in C6~6FeCl3and related intercalation compounds.
RECENT ELECTRON ENERGY LOSS [1] measurements on FeCl3 intercalated graphite have shown a lowering of the Fermi level by 0.9 eV. This corresponds to a charge transfer of 0.015 electrons per carbon atom according to the density of states shown in [2]. If this charge were localized on iron atoms 10% of them would be divalent. The available evidence firmly contradicts this proposition. Mössbauer measurements [3,4] set a limit of less than 3%, the same as the limit given for pure FeC13. With Raman scattering [5] no evidence was found for FeC!2 vibrations. The question of where the charge goes thus becomes a serious one, We here present evidence which points to cation vacancies in the intercalated layer as the acceptor site. The samples studied were prepared by the standard technique in which a small slab of highly oriented pyrolytic graphite (HOPG) is exposed to the vapor of the intercalant in a closed quartz tube. The graphite is always kept at a temperature higher that that of the intercalant to avoid liquid phase reactions. Data were taken in a HP 5950A ESCA spectrometer, using surfaces prepared by cleaving in the spectrometer vacuum, Typcial data for the C66FeC13 compound are shown in Fig. I. The composition of the sample was checked using theoretical cross-section ratios [6] and experimental X-ray photoemission line intensity ratios, corrected for differences in the spectrometer sensitivity and in the mean free path. The results verify the expected composition to within 10%. The Fe 2p spectrum yields no evidence for the existence of divalent iron which would appear at smaller binding energy. The binding energy of the C ls line relative to E~is reduced by 0.8 eV compared to HOPG, in agreement with the energy loss measurements [1]. The Cl 2p line is of unusual shape, but can be well represented by the superposition of signals from the —
___________
* Fulbright-Hays research scholar. tSummer Research Student, Auburn University, Auburn, Alabama.
two inequivalent chlorine sites indicated in the figure. For confirmation of this unusual behavior we turn to CuC12 intercalated graphite, Fig. 2. The Cu 2p spectrum exhibits only lines with binding energy and satellite structure typical of CuCl2. The C !s line closely resembles that of pure HOPG, but is shifted 0.2 eV toward smaller binding energy. The shift is smaller than in C66FeC13 probably because of the fact that the sample was not a stage 1 compound. The Cl 2p doublet is again poorly resolved. The dashed line indicates the normal shape of this doublet, here obtained from an evaporated layer of CuCl on a graphite substrate. These results show that there is no valence change in the copper, although there is definitely a lowering of the Fermi energy in the graphite conduction band. The charge which leaves the graphite again dominantly affects the Cl atoms resulting in the unusual shape. We propose the following model for the intercalated layers, based on the structure of [7]. They consist of metal atoms octahedrally coordinated by Cl, as shown in Fig. 3, with occasional metal ion vacancies which act as acceptor sites. The mechanism which gives rise to the defects in the intercalated layer is found in the composition of the vapor during the intercalation process. Heating FeC!3 in vacuum in a closed system results in the dissociation reaction: 2FeCl3 ~ 2FeCl2 + Cl2.
(I)
The vapor pressure of FeC12 is small, so that it remains at the cold end of the tube. The graphite is therefore exposed to a mixture of FeCI3 vapor and Cl2. Crystallization in the presence of excess chlorine can result in an intercalated layer with metal ion vacancies. This point of view also explains the well-known fact that the deliberate addition of excess halogen to the reaction volume promotes metal halide intercalation [8] Furthermore, it makes clear why intercalation is not restricted to metal ions with multiple valency, and explains how the excess halogen [81 is incorporated into the lattice.
809
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-
810
ACCEPTOR SITE IN METAL HALIDE INTERCALATED GRAPHITE
Vol. 33, No.7
9JC~Je
Cu 2P (50
740
720
730
25
710
20 15
-J
910
I 0
950
940
930
1.1 U,
JL_ I2
15
Q
‘b
2-
I
300
I
295
290
285
_________________________________________________________________
5-
12
9 6
Cl 2P/~
/
3.
(di
Cl2p
2 I
___________________
210
I
205 BINDING ENERGY 200 (eV)
I
20~ 15
195
Fig. 1. X-ray photoemission spectra of 3~ C6,6FeCl3. spectrum.(a) (b) The CFeIs2pregion. regionThe showing line isa shifted typical —0.8 Fe eV from the 284.5 eV position in HOPG. (c) The Cl 2p region. The two spin—orbit spectra which reproduce the experimental data are indicated. B denotes bridging, T denotes terminal chlorines. It is also worth noting that FeC!3 and CuC12 as well as many of the other’halides which intercalate readily into graphite are themselves layered compounds, i.e. they exist as sheets without dangling bonds. In C6,6FeCl3 each metal ion vacancy converts six briding Cl ions into terminal-like ones with dangling bonds, see Fig. 3. Similar dangling bonds are also found on the periphery of islands of intercalant, but not in sufficient number to account for the charge transfer.
lo._______________________________________ ItO 205 200 115
0.5
•IPID9IO (NEROY (SV)
Fig. 2. X-ray photoemission data of CuC1 2 intercalated into graphite and CuCI deposited on graphite. (a) The Cu 2p region of CuCl2 graphite showing a spectrum typical of divalent copper including the normal satellite structure. (b) For comparison the spectrum of a thin film of Cud deposited on graphite. (c) The C ls spectrum of Cud2 graphite. (d) The Cl 2p region. The Cl 2p spectrum of CuC! is shown as a dashed line. Analysis of the Cl 2p line in Fig. 1 suggests that 1/3 of the Cl ions are affected, i.e. that 1118 of the iron sites are empty. The binding energy of the terminallike chiorines (those surrounding the vacancy) is found to be I eV larger than that of the bridging ones. If we
11
Vol. 33, No.7
ACCEPTOR SITE IN METAL HALIDE INTERCALATED GRAPHITE
Cu Cl2 LAYER (W IT H VA C A N C Y)
•
811
from the graphite according to the above estimate of charge transfer. A better model postulates that the six chiorines at the metal ion vacancy share one electron. The net positive charge in the chionnes explains the increase in core electron binding energy of the terminal chlorine acceptor. In the case of CuC12 graphite, a metal ion vacancy reduces the metal coordination of the chlorine from three to two with similar results. In summary, we have presented evidence from XPS that the charge removed from the graphite ir-band by metal halide intercalation is localized on chlorine rather that metal ions. This chlorine acceptor action is made possible by metal-ion vacancies produced during the intercalation process by the presence of free chlorine. The latter derives from the dissociation of the metal chloride. This proposal has the additional merit of acchalides ountingthat for acceptor have onlyaction one stable and intercalation valence. of metal Acknowledgement
—
One of the authors (PvA) wants
Educational Exchange for a Fuibright—Hays research scholarship.
o FeC!3
LAYER
Fig. 3. The structure of layers of CuCl2 and FeC13. A cation vacancy is shown in the CuC12 layer. The structure of the FeC13 layer is similar to that of CuCl2 except that 1/3 of the metal sites are always empty. The additional vacancies discussed in the text are not show in this case. imagine the iron removed as a neutral atom to preserve charge neutrality in the layer, we can readily understand both the change in binding energy and the fact that the chlorine acts as an acceptor. Each Cl ion at a vacancy site then looses 0.5 electrons when the neutral Fe atom is removed, but gains only 0.1 electrons
to thank the Netherlands America Commission for REFERENCES I E.J. Mele & J.J. Ritsko,Phys. Rev. Lett. 43,68 (1979). 2. 3. 4. 5. 6. 7. 8.
B.R. Weinberger, J. Kaufer, A.J. Heeger, i.E. Fischer, M. Moran & N.A.W. Holzwarth, Phys. Rev. Lett. 41, 1417 (1978) K. Ohhashi & I. Tsujikawa, J. Phys. Soc. Japan. 36, 422 (1974). J.G. Hooley, M.W. Bartlett, B.V. Liengme & J.R. Sams, Carbon 6, 681 (1968). N. Caswell & S.A. Solin, Solid State Commun. 27,961 (1978). J.H. Scofield,J. Electr. Spectrosc. Relat. Phenom. 8, 129 (1976). J.M. Cowley & J.A. Ibers, Acta Cryst. 9, 421 (1956). E. Stumpp,Mat. Sci. Eng. 31, 53 (1977).
RECENT ELECTRON ENERGY LOSS [1] measurements on FeCl3 intercalated graphite have shown a lowering of the Fermi level by 0.9 eV. This corresponds to a charge transfer of 0.015 electrons per carbon atom according to the density of states shown in [2]. If this charge were localized on iron atoms 10% of them would be divalent. The available evidence firmly contradicts this proposition. Mössbauer measurements [3,4] set a limit of less than 3%, the same as the limit given for pure FeC13. With Raman scattering [5] no evidence was found for FeC!2 vibrations. The question of where the charge goes thus becomes a serious one, We here present evidence which points to cation vacancies in the intercalated layer as the acceptor site. The samples studied were prepared by the standard technique in which a small slab of highly oriented pyrolytic graphite (HOPG) is exposed to the vapor of the intercalant in a closed quartz tube. The graphite is always kept at a temperature higher that that of the intercalant to avoid liquid phase reactions. Data were taken in a HP 5950A ESCA spectrometer, using surfaces prepared by cleaving in the spectrometer vacuum, Typcial data for the C66FeC13 compound are shown in Fig. I. The composition of the sample was checked using theoretical cross-section ratios [6] and experimental X-ray photoemission line intensity ratios, corrected for differences in the spectrometer sensitivity and in the mean free path. The results verify the expected composition to within 10%. The Fe 2p spectrum yields no evidence for the existence of divalent iron which would appear at smaller binding energy. The binding energy of the C ls line relative to E~is reduced by 0.8 eV compared to HOPG, in agreement with the energy loss measurements [1]. The Cl 2p line is of unusual shape, but can be well represented by the superposition of signals from the —
___________
* Fulbright-Hays research scholar. tSummer Research Student, Auburn University, Auburn, Alabama.
two inequivalent chlorine sites indicated in the figure. For confirmation of this unusual behavior we turn to CuC12 intercalated graphite, Fig. 2. The Cu 2p spectrum exhibits only lines with binding energy and satellite structure typical of CuCl2. The C !s line closely resembles that of pure HOPG, but is shifted 0.2 eV toward smaller binding energy. The shift is smaller than in C66FeC13 probably because of the fact that the sample was not a stage 1 compound. The Cl 2p doublet is again poorly resolved. The dashed line indicates the normal shape of this doublet, here obtained from an evaporated layer of CuCl on a graphite substrate. These results show that there is no valence change in the copper, although there is definitely a lowering of the Fermi energy in the graphite conduction band. The charge which leaves the graphite again dominantly affects the Cl atoms resulting in the unusual shape. We propose the following model for the intercalated layers, based on the structure of [7]. They consist of metal atoms octahedrally coordinated by Cl, as shown in Fig. 3, with occasional metal ion vacancies which act as acceptor sites. The mechanism which gives rise to the defects in the intercalated layer is found in the composition of the vapor during the intercalation process. Heating FeC!3 in vacuum in a closed system results in the dissociation reaction: 2FeCl3 ~ 2FeCl2 + Cl2.
(I)
The vapor pressure of FeC12 is small, so that it remains at the cold end of the tube. The graphite is therefore exposed to a mixture of FeCI3 vapor and Cl2. Crystallization in the presence of excess chlorine can result in an intercalated layer with metal ion vacancies. This point of view also explains the well-known fact that the deliberate addition of excess halogen to the reaction volume promotes metal halide intercalation [8] Furthermore, it makes clear why intercalation is not restricted to metal ions with multiple valency, and explains how the excess halogen [81 is incorporated into the lattice.
809
-
-
810
ACCEPTOR SITE IN METAL HALIDE INTERCALATED GRAPHITE
Vol. 33, No.7
9JC~Je
Cu 2P (50
740
720
730
25
710
20 15
-J
910
I 0
950
940
930
1.1 U,
JL_ I2
15
Q
‘b
2-
I
300
I
295
290
285
_________________________________________________________________
5-
12
9 6
Cl 2P/~
/
3.
(di
Cl2p
2 I
___________________
210
I
205 BINDING ENERGY 200 (eV)
I
20~ 15
195
Fig. 1. X-ray photoemission spectra of 3~ C6,6FeCl3. spectrum.(a) (b) The CFeIs2pregion. regionThe showing line isa shifted typical —0.8 Fe eV from the 284.5 eV position in HOPG. (c) The Cl 2p region. The two spin—orbit spectra which reproduce the experimental data are indicated. B denotes bridging, T denotes terminal chlorines. It is also worth noting that FeC!3 and CuC12 as well as many of the other’halides which intercalate readily into graphite are themselves layered compounds, i.e. they exist as sheets without dangling bonds. In C6,6FeCl3 each metal ion vacancy converts six briding Cl ions into terminal-like ones with dangling bonds, see Fig. 3. Similar dangling bonds are also found on the periphery of islands of intercalant, but not in sufficient number to account for the charge transfer.
lo._______________________________________ ItO 205 200 115
0.5
•IPID9IO (NEROY (SV)
Fig. 2. X-ray photoemission data of CuC1 2 intercalated into graphite and CuCI deposited on graphite. (a) The Cu 2p region of CuCl2 graphite showing a spectrum typical of divalent copper including the normal satellite structure. (b) For comparison the spectrum of a thin film of Cud deposited on graphite. (c) The C ls spectrum of Cud2 graphite. (d) The Cl 2p region. The Cl 2p spectrum of CuC! is shown as a dashed line. Analysis of the Cl 2p line in Fig. 1 suggests that 1/3 of the Cl ions are affected, i.e. that 1118 of the iron sites are empty. The binding energy of the terminallike chiorines (those surrounding the vacancy) is found to be I eV larger than that of the bridging ones. If we
11
Vol. 33, No.7
ACCEPTOR SITE IN METAL HALIDE INTERCALATED GRAPHITE
Cu Cl2 LAYER (W IT H VA C A N C Y)
•
811
from the graphite according to the above estimate of charge transfer. A better model postulates that the six chiorines at the metal ion vacancy share one electron. The net positive charge in the chionnes explains the increase in core electron binding energy of the terminal chlorine acceptor. In the case of CuC12 graphite, a metal ion vacancy reduces the metal coordination of the chlorine from three to two with similar results. In summary, we have presented evidence from XPS that the charge removed from the graphite ir-band by metal halide intercalation is localized on chlorine rather that metal ions. This chlorine acceptor action is made possible by metal-ion vacancies produced during the intercalation process by the presence of free chlorine. The latter derives from the dissociation of the metal chloride. This proposal has the additional merit of acchalides ountingthat for acceptor have onlyaction one stable and intercalation valence. of metal Acknowledgement
—
One of the authors (PvA) wants
Educational Exchange for a Fuibright—Hays research scholarship.
o FeC!3
LAYER
Fig. 3. The structure of layers of CuCl2 and FeC13. A cation vacancy is shown in the CuC12 layer. The structure of the FeC13 layer is similar to that of CuCl2 except that 1/3 of the metal sites are always empty. The additional vacancies discussed in the text are not show in this case. imagine the iron removed as a neutral atom to preserve charge neutrality in the layer, we can readily understand both the change in binding energy and the fact that the chlorine acts as an acceptor. Each Cl ion at a vacancy site then looses 0.5 electrons when the neutral Fe atom is removed, but gains only 0.1 electrons
to thank the Netherlands America Commission for REFERENCES I E.J. Mele & J.J. Ritsko,Phys. Rev. Lett. 43,68 (1979). 2. 3. 4. 5. 6. 7. 8.
B.R. Weinberger, J. Kaufer, A.J. Heeger, i.E. Fischer, M. Moran & N.A.W. Holzwarth, Phys. Rev. Lett. 41, 1417 (1978) K. Ohhashi & I. Tsujikawa, J. Phys. Soc. Japan. 36, 422 (1974). J.G. Hooley, M.W. Bartlett, B.V. Liengme & J.R. Sams, Carbon 6, 681 (1968). N. Caswell & S.A. Solin, Solid State Commun. 27,961 (1978). J.H. Scofield,J. Electr. Spectrosc. Relat. Phenom. 8, 129 (1976). J.M. Cowley & J.A. Ibers, Acta Cryst. 9, 421 (1956). E. Stumpp,Mat. Sci. Eng. 31, 53 (1977).