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Oxygen deficient SnO2 (110) and TiO2 (110): A comparative study by photoemission
Solid State Communications, Vol. 60, No. 10, pp. 835-838, 1986. Printed in Great Britain.
0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110): A COMPARATIVE STUDY BY PHOTOEMISSION R.G. Egdell*t, S. Eriksen and W.R. Flavellt Department of Chemistry, Imperial College, South Kensington, London SW7 2AZ, UK
(Received 30July 1986 by R.A. Cowley) Oxygen deficient SnO2 (110) and TiO2 (110) surfaces prepared by argon ion bombardment of stoichiometric crystals have been studied by He(I) photoemission spectroscopy. On TiO2 (110), 3d I polaronic states localised at crystallographic shear planes give rise to a band peaking 1 eV below the Fermi level. By contrast for SnO2 (110) there is strong 5s-5p hybridisation that pushed the Sn z+ lone-pair-like defect state down toward the valence band edge. TIN (IV) OXIDE (SnO2) FINDS appfication in a number of areas where surface properties are important, including for example oxidation catalysis [1 ] and gas monitoring [2]. The material adopts the rutile structure, but in contrast to TiO2 [3, 4] itself there have been few studies of SnO2 single crystal surfaces. In particular there has been no attempt to apply photoemission to study the influence of oxygen deficiency on the surface electronic structure of SnO2. Surface conductivity measurements suggest that oxygen vacancies created by
ion bombardment of SnO2 do not act as donor states [5]. This contrasts with the behaviour of bulk oxygen vacancies which act as shallow donor levels [6] and with sputtered TiO2 surfaces where oxygen deficiency introduces new states into the upper part of the bulk bandgap [3, 4]. In the present Communication we compare photoemission spectra of oxygen deficient SnO2 (110) and TiO2 (110) surface prepared by ion sputtering and confirm that the states introduced by oxygen deficiency in SnO2 lie toward the bottom of the bulk bandgap. At the same time we consider in detail the nature of the TiO2 bandgap emission. The differences between the
(a)
(b) I
o
t
,'o
Binding EnergyleV
Fig. 1. He(I) photoelectron spectra of TiO2 (1113). (a) clean, oxygen annealed surface (b) following 1 k e y argon ion bombardment to a total dose of 0.005 Ccrn -2 onto crystal surface. Binding energies are given relative to Fermi energy of Ta sample holder. *To whom correspondence and proofs should be addressed. tPreviously at Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK. 835
Striding EnergyleV
Fig. 2. He(I)photoelectron specUa of SnO 2 (110). (a) clean, oxygen annealed surface (b) following 1 keY argon ion bombardment to a total dose of 0.001 C era -2 onto crystal surface. The defect induced feature is shown by hatching. Binding energies relative to Fermi energy of Pt ~zsnple holder.
836
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110)
two oxides reflect fundamental differences in bandstructure and defect chemistry. Single crystal SnO2 (110) and TiO2 (110) were studied in two separate UHV electron spectrometers (base pressures less than 10-1° torr). Both were equipped with gas discharge lamps, MgKa X-ray sources, lOOmm mean radius spherical sector energy analysers and identical Penning ion guns. Details of sample cleaning and characterisation of defect-free surfaces are given elsewhere [7, 8]. He(1) photoemission spectra from clean, oxygen annealed surfaces are shown in Figs. l(a) and 2(a) respectively, in each case primary emission is dominated by a peak arising from 0:2p valence band states: the peak at higher apparent binding energy arises from secondaries. For neither surface are states visible at the Fermi energy but there is sufficient n-type conductivity to prevent sample charging. Nearly flat band behaviour is indicated by the fact that the energy separation between the Fermi energies (determined from separate experiments on metal sample holders in contact with the oxide crystals) and the valence band edges (which are admittedly difficult to locate accurately) correspond reasonably well to the respective bulk bandgaps of TiO2 (3.1 eV) [9] and SnO2 (3.6eV) [10]. In both cases we assume that the Fermi energy of the crystal is pinned by donor states lying close to the bottom of the conduction band [9, 11]. The influence of argon ion bombardment on the spectra is shown in Figs. l(b) and 2(b) where new features are found above the valence band maxima. For T i O 2 there is a distinct peak some I eV below the Fermi energy but for SnO2 we find only a shoulder on the upper edge of the valence band with no new features closer to the Fermi energy. It is well known that ion bombardment leads to preferential sputtering of oxygen,
producing substoichiometric surfaces. Under our conditions the oxygen deficiency must extend well below the selvedge over a depth range at least equal to the electron escape depth in He(I) photoemission experiments. The photoemission spectra of TiO2 in Fig. l are very similar to those obtained previously by Henrich [3] and Somorjai [4]. We took particular care to measure the energy of the defect level relative to the Fermi energy (Fig. 3) and consider the nature of the state in some detail. Of course ion bombarded surfaces are highly disordered so that it is unreasonable to discuss the electronic states in terms of highly specific atomic arrangements. However one can make some general comments. It is known from extensive theoretical [12] and experimental [13] work on reduced futile that the crystal does not tolerate point defects for deviations from stoichiometry, x in TiO2_x, greater than x ~ lO -a . Instead point defects are eliminated by formation of crystallographic shear planes comprising face sharing TiO6 octahedra similar to those found in Ti2Oa. (121) shear planes found at large x eventually order to give the Magneli phases TinO2n-1. The deviations from stoichiometry that can be probed by photoemission typically correspond to x values in the range 0.05-0.5. Although we do not expect formation of well-defined Magneli phases, argon bombardment probably provides sufficient energy to make local elimination of point defects kinetically allowed and we envisage a final atomic arrangement where titanium ions below the selvedge retain sixfold coordination by octahedral face sharing. The energies of defect states are then determined in large part by polaronic self-trapping. Using a simple continuum model we estimate that the Ti:3d defect states should lie E a below the edge of the Ti:3d(t2g) conduction band where: Ea = ~
o
~
~.
Binding EnergyleV
Fig. 3. The defect-induced feature in the He(I)vhotoelectron spectrum of argon-ion bombarded TiO2 (110). Binding energy relative to Fermi energy of the Ta sample holder.
Vol. 60, No. l0
[ 1 / e ( ~ ) - l/e(0)l -
IV~
.
Here e(~) and e(0) are the high and low frequency dielectric constants, R is the effective radius of the small polaron state and W is the width of the Ti:3d(t2g) conduction band. With l / e ( ~ ) - - l/e(0) = 1/7 [9], R = 0.74A [14] and W= 1.4eV [15] we estimate that Ea = 0.7eV. However, Catlow and co-workers have calculated that trapping an electron at a shear plane rather than at a regular rutlle titanium ion, as presumed in our simple calculation, further lowers the defect energy by 0.3eV [12]. This gives a defect energy Ea = 1.0eV in perfect agreement with the observed value. Note that surface defect states 1 eV below the Fermi energy have been postulated to play an important role in the photoelectrolysis of water in cells with TiO2 oxygen electrodes [161.
Vol. 60, No. I0
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110)
837
SnO2 clearly differs from TiO2 and other transition Acknowledgements - The equipment used was funded metal oxides such as WO3 [17] in that no well-defined by the SERC. We are grateful to Professor R. Helbig for states arise in the upper part of the bulk bandgap as a the SnO2 crystal. WRF is an SERC Postdoctoral Fellow. result of ion bombardment. The new structure appearing Note added in proof as a shoulder on the valence band edge corresponds Munnix and Schmeits have recently reported results of instead to a "deep" state which we associate with calculations on oxygen deficient SnO2 (110)[25] and hybridized 5s-5p Sn 2+ lone pair electrons for the TiO2 (110)[26] surfaces using the so-called scattering theoretic method. For SnO2 (110), unrelaxed oxygen following reasons. Unlike TiO2, SnO2 shows no tendency toward vacancies in a variety of sites close to the surface failed accommodation of gross nonstoichiometry by formation to produce states in the bulk bandgap. This contrasts with the present experimental data, where gap states are of shear planes [18]. On the other hand oxygen vacancy clearly observed. The only simple way of reconciling this point defects act as shallow donors [6] and small discrepancy is to suppose that structural rearrangement polarons are not expected in SnO2 owing to its greater accompanying oxygen loss under iort bombardment conduction band width. Moreover SnO2 becomes metallic plays a crucial role in promoting the s-p mixing that for carrier concentration greater than around 1019 cm -3 pushes states down into the gap. For TiO2 (110), vacancies in the oxygen plane immediately below the and if there were a significant concentration of unrecon- surface did produce gap states although vacancies elsestructed vacancies we should expect to observe emission where at the surface or in the bulk [27] did not. The from the Sn:5s conduction band. Certainly conduction model of Muunix and Schmeits requires that electrons electrons can be observed directly as a result of antimony associated with oxygen vacancies that do not produce doping [19-21]. Accompanying oxygen loss under gap states must occupy the t2g conduction band of TiO2, producing a quasimetallic state. This is again at argon ion bombardment we thus envisage structural variance with the experimental data and we must conreorganisation to produce local coordination for Sn ions clude that ionic relaxations not properly incorporated in similar to that in SnO. The crucial feature of both red the calculations are crucial in pushing states down into and black modifications of SnO [22] relevant to the the gap. present discussion is that Sn ions occupy non-centresymmetric sites (unlike the effectively D4h sites in REFERENCES SnO2 ) where the local electric field gradient induces 5s1. F.J. Berry, Advances in Catalysis 30, 97 (1982). 5p hybridisation leading to a lone-pair like state lying 2. M. Nitta, S. Ohtani & M. Hardane, J. Electron well below the bottom of the Sn:5s conduction band of Materials 9,727 (1980). SnO2: this can become doubly occupied to produce 3. V. Henrich, G. Dresselhaus & H.J. Zeiger, Phys. Sn 2÷ ions. Rev. Lett. 36, 1335 (1976). Two observations support the idea of local SnO-like 4. W.J. Lo, Y.W. Chung & G.A. Somorjai, Surf. Sci. 71,199 (1978). coordination. Firstly, the valence band X-ray photo5. E. De Fresart, J. Darville & J.M. Gilles, Appl. Surf. electron spectrum of red SnO contains a peak 3 eV Sci. 11/12,637 (1982). below the Fermi energy [23] which must be associated 6. J. Robertson, Phys. Rev. B 30, 3520 (1984). with the tin lone pair states: the ion beam induced 7. P.A. Cox, R.G. Egdell, W.R. Flavell & R. Helbig, feature in our photoemission spectrum occurs at the Vacuum 33, 835 (1983). 8. P.A. Cox, R.G. EgdeU, S. Eriksen & W.R. Flavell, same energy. Secondly, the Sn:3d core lines in XPS of J. Electron Spectrosc. 39, 117 (1986). SnO and SnO2 occur at essentially the same binding 9. A. Harnnett, in Landolt-Bornstein Numerical Data energy, implying a cancellation between free ion potenand Functional Relationships in Science and tiais and site potentials [23]. Our own XPS experiments Technology (Edited by K.F. Hellwege and O. confirmed that core levels arising from ion beam reducMadelung), Vol. III/17g, Springer Verlag, Berlin, (1984). tion of SnO2 overlapped the original Sn4÷ peaks, so that the site potential for the reduced Sn 2+ species must be 10. J. Robertson, J. Phys. C. 12, 4767 (1979). 11. Z.M. Jarzebski & J.P. Marten, J. Electrochem. Soc. similar to that in red SnO [24]. 123, 299C (1976). In summary then ion beam reduction of TiO2 and 12. C.R.A. Catlow & R. James, RSC Specialist PeriodSnO2 surfaces produces Ti 3+ and Sn 2+ states in the bulk ical Report on Chemical Physics o f Solids and Surface, p. 108, Vol. 8 (Edited by M.W. Roberts bandgaps of the parent oxides. These are strongly and J_M. Thomas), The Chemical Society, London reminiscent of corresponding states in the bulk oxides (1980). TinO2n-1 and SnO respectively. Further work is in hand 13. R.J.D. Tilley, RSC Specialist Periodical Report on to explore the influence of the relatively "deep" Sn 2+ Chemical Physics o f Solids and Surfaces, p. 121, state on defective SnO2 upon the surface chemistry of Vol. 8 (Edited by M.W. Roberts and J.M. Thomas), The Chemical Society, London (1980). this oxide.
838 14. 15. 16. 17. 18. 19. 20.
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110) D.M. Adams, Inorganic Solids, John Wiley, London (1974). K. Vos, J. Phys. C, 10,3893(1977). J.G. Mavroides, D.I. Tchemev, J.A. Kafalas & D.F. Kolesar, Mater Res. Bull. 10, 1023 (1975). R.D. Bfingans, H. Hochst & H.R. Shanks, Surf. Sci. 111, 80 (1981). D. Pyke, R. Reid & RJ.D. TiUey, Solid State Chem. 25,231 (1978). P.A. Cox, R.G. Egdeil, C. Harding, A.F. Orchard, W.R. Patterson & P.J. Tavener, Solid State Commun. 44, 837 (1982). P.A. Cox, R.G. Egdell, C. Harding, W.R. Patterson & P.J. Tavener, Surf Sci. 123, 179 (1982).
Vol. 60, No. 10
21.
R.G. Egdell, W.R. Flavell & PJ. Tavener, J. Solid
22.
State Chem. 51,345 (1984). A.F. Wells, Structural Inorganic
23. 24. 25. 26. 27.
Chemistry, Clarendon Press, Oxford (1975). C. Lau & G.K. Wertheim,J. Vac. ScL Technol. 15, 622 (1978). S. Eriksen, R.G. Egdell & W.R. FlaveU, in preparation. S. Munnix & R. Schmeits, Phys. Rev. B. 27, 7624 (1986). S. Munnix & R. Schmeits, Phys. Rev. B. 31 3369 (1985). S. Munnix & R. Schmeits, Solid State Commun. 50, 1087 (1984).
0038-1098/86 $3.00 + .00 Pergamon Journals Ltd.
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110): A COMPARATIVE STUDY BY PHOTOEMISSION R.G. Egdell*t, S. Eriksen and W.R. Flavellt Department of Chemistry, Imperial College, South Kensington, London SW7 2AZ, UK
(Received 30July 1986 by R.A. Cowley) Oxygen deficient SnO2 (110) and TiO2 (110) surfaces prepared by argon ion bombardment of stoichiometric crystals have been studied by He(I) photoemission spectroscopy. On TiO2 (110), 3d I polaronic states localised at crystallographic shear planes give rise to a band peaking 1 eV below the Fermi level. By contrast for SnO2 (110) there is strong 5s-5p hybridisation that pushed the Sn z+ lone-pair-like defect state down toward the valence band edge. TIN (IV) OXIDE (SnO2) FINDS appfication in a number of areas where surface properties are important, including for example oxidation catalysis [1 ] and gas monitoring [2]. The material adopts the rutile structure, but in contrast to TiO2 [3, 4] itself there have been few studies of SnO2 single crystal surfaces. In particular there has been no attempt to apply photoemission to study the influence of oxygen deficiency on the surface electronic structure of SnO2. Surface conductivity measurements suggest that oxygen vacancies created by
ion bombardment of SnO2 do not act as donor states [5]. This contrasts with the behaviour of bulk oxygen vacancies which act as shallow donor levels [6] and with sputtered TiO2 surfaces where oxygen deficiency introduces new states into the upper part of the bulk bandgap [3, 4]. In the present Communication we compare photoemission spectra of oxygen deficient SnO2 (110) and TiO2 (110) surface prepared by ion sputtering and confirm that the states introduced by oxygen deficiency in SnO2 lie toward the bottom of the bulk bandgap. At the same time we consider in detail the nature of the TiO2 bandgap emission. The differences between the
(a)
(b) I
o
t
,'o
Binding EnergyleV
Fig. 1. He(I) photoelectron spectra of TiO2 (1113). (a) clean, oxygen annealed surface (b) following 1 k e y argon ion bombardment to a total dose of 0.005 Ccrn -2 onto crystal surface. Binding energies are given relative to Fermi energy of Ta sample holder. *To whom correspondence and proofs should be addressed. tPreviously at Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, UK. 835
Striding EnergyleV
Fig. 2. He(I)photoelectron specUa of SnO 2 (110). (a) clean, oxygen annealed surface (b) following 1 keY argon ion bombardment to a total dose of 0.001 C era -2 onto crystal surface. The defect induced feature is shown by hatching. Binding energies relative to Fermi energy of Pt ~zsnple holder.
836
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110)
two oxides reflect fundamental differences in bandstructure and defect chemistry. Single crystal SnO2 (110) and TiO2 (110) were studied in two separate UHV electron spectrometers (base pressures less than 10-1° torr). Both were equipped with gas discharge lamps, MgKa X-ray sources, lOOmm mean radius spherical sector energy analysers and identical Penning ion guns. Details of sample cleaning and characterisation of defect-free surfaces are given elsewhere [7, 8]. He(1) photoemission spectra from clean, oxygen annealed surfaces are shown in Figs. l(a) and 2(a) respectively, in each case primary emission is dominated by a peak arising from 0:2p valence band states: the peak at higher apparent binding energy arises from secondaries. For neither surface are states visible at the Fermi energy but there is sufficient n-type conductivity to prevent sample charging. Nearly flat band behaviour is indicated by the fact that the energy separation between the Fermi energies (determined from separate experiments on metal sample holders in contact with the oxide crystals) and the valence band edges (which are admittedly difficult to locate accurately) correspond reasonably well to the respective bulk bandgaps of TiO2 (3.1 eV) [9] and SnO2 (3.6eV) [10]. In both cases we assume that the Fermi energy of the crystal is pinned by donor states lying close to the bottom of the conduction band [9, 11]. The influence of argon ion bombardment on the spectra is shown in Figs. l(b) and 2(b) where new features are found above the valence band maxima. For T i O 2 there is a distinct peak some I eV below the Fermi energy but for SnO2 we find only a shoulder on the upper edge of the valence band with no new features closer to the Fermi energy. It is well known that ion bombardment leads to preferential sputtering of oxygen,
producing substoichiometric surfaces. Under our conditions the oxygen deficiency must extend well below the selvedge over a depth range at least equal to the electron escape depth in He(I) photoemission experiments. The photoemission spectra of TiO2 in Fig. l are very similar to those obtained previously by Henrich [3] and Somorjai [4]. We took particular care to measure the energy of the defect level relative to the Fermi energy (Fig. 3) and consider the nature of the state in some detail. Of course ion bombarded surfaces are highly disordered so that it is unreasonable to discuss the electronic states in terms of highly specific atomic arrangements. However one can make some general comments. It is known from extensive theoretical [12] and experimental [13] work on reduced futile that the crystal does not tolerate point defects for deviations from stoichiometry, x in TiO2_x, greater than x ~ lO -a . Instead point defects are eliminated by formation of crystallographic shear planes comprising face sharing TiO6 octahedra similar to those found in Ti2Oa. (121) shear planes found at large x eventually order to give the Magneli phases TinO2n-1. The deviations from stoichiometry that can be probed by photoemission typically correspond to x values in the range 0.05-0.5. Although we do not expect formation of well-defined Magneli phases, argon bombardment probably provides sufficient energy to make local elimination of point defects kinetically allowed and we envisage a final atomic arrangement where titanium ions below the selvedge retain sixfold coordination by octahedral face sharing. The energies of defect states are then determined in large part by polaronic self-trapping. Using a simple continuum model we estimate that the Ti:3d defect states should lie E a below the edge of the Ti:3d(t2g) conduction band where: Ea = ~
o
~
~.
Binding EnergyleV
Fig. 3. The defect-induced feature in the He(I)vhotoelectron spectrum of argon-ion bombarded TiO2 (110). Binding energy relative to Fermi energy of the Ta sample holder.
Vol. 60, No. l0
[ 1 / e ( ~ ) - l/e(0)l -
IV~
.
Here e(~) and e(0) are the high and low frequency dielectric constants, R is the effective radius of the small polaron state and W is the width of the Ti:3d(t2g) conduction band. With l / e ( ~ ) - - l/e(0) = 1/7 [9], R = 0.74A [14] and W= 1.4eV [15] we estimate that Ea = 0.7eV. However, Catlow and co-workers have calculated that trapping an electron at a shear plane rather than at a regular rutlle titanium ion, as presumed in our simple calculation, further lowers the defect energy by 0.3eV [12]. This gives a defect energy Ea = 1.0eV in perfect agreement with the observed value. Note that surface defect states 1 eV below the Fermi energy have been postulated to play an important role in the photoelectrolysis of water in cells with TiO2 oxygen electrodes [161.
Vol. 60, No. I0
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110)
837
SnO2 clearly differs from TiO2 and other transition Acknowledgements - The equipment used was funded metal oxides such as WO3 [17] in that no well-defined by the SERC. We are grateful to Professor R. Helbig for states arise in the upper part of the bulk bandgap as a the SnO2 crystal. WRF is an SERC Postdoctoral Fellow. result of ion bombardment. The new structure appearing Note added in proof as a shoulder on the valence band edge corresponds Munnix and Schmeits have recently reported results of instead to a "deep" state which we associate with calculations on oxygen deficient SnO2 (110)[25] and hybridized 5s-5p Sn 2+ lone pair electrons for the TiO2 (110)[26] surfaces using the so-called scattering theoretic method. For SnO2 (110), unrelaxed oxygen following reasons. Unlike TiO2, SnO2 shows no tendency toward vacancies in a variety of sites close to the surface failed accommodation of gross nonstoichiometry by formation to produce states in the bulk bandgap. This contrasts with the present experimental data, where gap states are of shear planes [18]. On the other hand oxygen vacancy clearly observed. The only simple way of reconciling this point defects act as shallow donors [6] and small discrepancy is to suppose that structural rearrangement polarons are not expected in SnO2 owing to its greater accompanying oxygen loss under iort bombardment conduction band width. Moreover SnO2 becomes metallic plays a crucial role in promoting the s-p mixing that for carrier concentration greater than around 1019 cm -3 pushes states down into the gap. For TiO2 (110), vacancies in the oxygen plane immediately below the and if there were a significant concentration of unrecon- surface did produce gap states although vacancies elsestructed vacancies we should expect to observe emission where at the surface or in the bulk [27] did not. The from the Sn:5s conduction band. Certainly conduction model of Muunix and Schmeits requires that electrons electrons can be observed directly as a result of antimony associated with oxygen vacancies that do not produce doping [19-21]. Accompanying oxygen loss under gap states must occupy the t2g conduction band of TiO2, producing a quasimetallic state. This is again at argon ion bombardment we thus envisage structural variance with the experimental data and we must conreorganisation to produce local coordination for Sn ions clude that ionic relaxations not properly incorporated in similar to that in SnO. The crucial feature of both red the calculations are crucial in pushing states down into and black modifications of SnO [22] relevant to the the gap. present discussion is that Sn ions occupy non-centresymmetric sites (unlike the effectively D4h sites in REFERENCES SnO2 ) where the local electric field gradient induces 5s1. F.J. Berry, Advances in Catalysis 30, 97 (1982). 5p hybridisation leading to a lone-pair like state lying 2. M. Nitta, S. Ohtani & M. Hardane, J. Electron well below the bottom of the Sn:5s conduction band of Materials 9,727 (1980). SnO2: this can become doubly occupied to produce 3. V. Henrich, G. Dresselhaus & H.J. Zeiger, Phys. Sn 2÷ ions. Rev. Lett. 36, 1335 (1976). Two observations support the idea of local SnO-like 4. W.J. Lo, Y.W. Chung & G.A. Somorjai, Surf. Sci. 71,199 (1978). coordination. Firstly, the valence band X-ray photo5. E. De Fresart, J. Darville & J.M. Gilles, Appl. Surf. electron spectrum of red SnO contains a peak 3 eV Sci. 11/12,637 (1982). below the Fermi energy [23] which must be associated 6. J. Robertson, Phys. Rev. B 30, 3520 (1984). with the tin lone pair states: the ion beam induced 7. P.A. Cox, R.G. Egdell, W.R. Flavell & R. Helbig, feature in our photoemission spectrum occurs at the Vacuum 33, 835 (1983). 8. P.A. Cox, R.G. EgdeU, S. Eriksen & W.R. Flavell, same energy. Secondly, the Sn:3d core lines in XPS of J. Electron Spectrosc. 39, 117 (1986). SnO and SnO2 occur at essentially the same binding 9. A. Harnnett, in Landolt-Bornstein Numerical Data energy, implying a cancellation between free ion potenand Functional Relationships in Science and tiais and site potentials [23]. Our own XPS experiments Technology (Edited by K.F. Hellwege and O. confirmed that core levels arising from ion beam reducMadelung), Vol. III/17g, Springer Verlag, Berlin, (1984). tion of SnO2 overlapped the original Sn4÷ peaks, so that the site potential for the reduced Sn 2+ species must be 10. J. Robertson, J. Phys. C. 12, 4767 (1979). 11. Z.M. Jarzebski & J.P. Marten, J. Electrochem. Soc. similar to that in red SnO [24]. 123, 299C (1976). In summary then ion beam reduction of TiO2 and 12. C.R.A. Catlow & R. James, RSC Specialist PeriodSnO2 surfaces produces Ti 3+ and Sn 2+ states in the bulk ical Report on Chemical Physics o f Solids and Surface, p. 108, Vol. 8 (Edited by M.W. Roberts bandgaps of the parent oxides. These are strongly and J_M. Thomas), The Chemical Society, London reminiscent of corresponding states in the bulk oxides (1980). TinO2n-1 and SnO respectively. Further work is in hand 13. R.J.D. Tilley, RSC Specialist Periodical Report on to explore the influence of the relatively "deep" Sn 2+ Chemical Physics o f Solids and Surfaces, p. 121, state on defective SnO2 upon the surface chemistry of Vol. 8 (Edited by M.W. Roberts and J.M. Thomas), The Chemical Society, London (1980). this oxide.
838 14. 15. 16. 17. 18. 19. 20.
OXYGEN DEFICIENT SnO2 (110) AND TiO2 (110) D.M. Adams, Inorganic Solids, John Wiley, London (1974). K. Vos, J. Phys. C, 10,3893(1977). J.G. Mavroides, D.I. Tchemev, J.A. Kafalas & D.F. Kolesar, Mater Res. Bull. 10, 1023 (1975). R.D. Bfingans, H. Hochst & H.R. Shanks, Surf. Sci. 111, 80 (1981). D. Pyke, R. Reid & RJ.D. TiUey, Solid State Chem. 25,231 (1978). P.A. Cox, R.G. Egdeil, C. Harding, A.F. Orchard, W.R. Patterson & P.J. Tavener, Solid State Commun. 44, 837 (1982). P.A. Cox, R.G. Egdell, C. Harding, W.R. Patterson & P.J. Tavener, Surf Sci. 123, 179 (1982).
Vol. 60, No. 10
21.
R.G. Egdell, W.R. Flavell & PJ. Tavener, J. Solid
22.
State Chem. 51,345 (1984). A.F. Wells, Structural Inorganic
23. 24. 25. 26. 27.
Chemistry, Clarendon Press, Oxford (1975). C. Lau & G.K. Wertheim,J. Vac. ScL Technol. 15, 622 (1978). S. Eriksen, R.G. Egdell & W.R. FlaveU, in preparation. S. Munnix & R. Schmeits, Phys. Rev. B. 27, 7624 (1986). S. Munnix & R. Schmeits, Phys. Rev. B. 31 3369 (1985). S. Munnix & R. Schmeits, Solid State Commun. 50, 1087 (1984).