Electronic Structure of Epitaxial Sn-Doped Anatase Grown on SrTiO3

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Electronic Structure of Epitaxial Sn-Doped Anatase Grown on SrTiO(001) by Dip Coating 3

Freddy E Oropeza, Kelvin Hong Liang Zhang, Robert G. Palgrave, Anna Regoutz, and Russell G. Egdell J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp405054t • Publication Date (Web): 27 Jun 2013 Downloaded from http://pubs.acs.org on July 4, 2013

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The Journal of Physical Chemistry

Electronic Structure of Epitaxial Sn-Doped Anatase Grown on SrTiO3(001) by Dip Coating F. E. Oropeza¶, K.H.L. Zhang$, R.G. Palgrave†, A. Regoutz and R. G. Egdell* University of Oxford, Department of Chemistry, Inorganic Chemistry Laboratory, South Parks Road, Oxford OX1 3QR, United Kingdom. J. P. Allen, N. M. Galea, and G. W. Watson School of Chemistry and CRANN, Trinity College Dublin, Dublin 2, Ireland

* Corresponding author. Phone: 0044-1865-285157 Email: [email protected] ¶

Present address: Department of Materials, Imperial College London, Exhibition Road,

London SW7 2AZ, UK $

Present address: Materials Science Division, Pacific Northwest National Laboratory , PO

Box 999, Richland, WA 99352, USA †

Present address: University College London, Kathleen Lonsdale Materials Chemistry,

Department of Chemistry, 20 Gordon Street, London WC1H 0AJ, UK.

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Abstract Sn doping in the anatase polymorph of TiO2 promotes transformation to the thermodynamically stable rutile phase at much lower temperature than found for the undoped material. Here it is shown that the anatase-to-rutile phase transition in Sn-doped TiO2 is inhibited in epitaxial (001) oriented films grown on SrTiO3(001) by a dip coating procedure. X-ray photoemission spectroscopy demonstrates that there is pronounced segregation of Sn to the anatase (001) surface and that the bandgap increases with Sn doping level. This behaviour is in contrast to that found in the rutile phase of Sn-doped TiO2, where the bandgap initially decreases for low levels of Sn doping. Pure SnO2 cannot be stabilised by epitaxy in the anatase phase even though good matching between anatase SnO2(001) and SrTiO3(001) is predicted by density functional theory calculations.

Keywords Titania. Thin film. X-ray photoemission. Band gap.


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1. Introduction. There is a growing interest in the exploitation of wide gap oxide semiconductors with the ability to harness sunlight to bring about photocatalytic reactions such as the degradation of organic pollutants or the production of hydrogen from water or hydrocarbons. Photocatalytic processes involve the excitation of a valence electron into the conduction band followed by reduction of surface adsorbed species by the excited electrons and oxidation by the holes. Efficient photocatalysis therefore depends on separation of the photogenerated electrons and holes and their subsequent migration to the surface before recombination. Titanium dioxide (TiO2) has proved to be one of the most valuable photocatalytic materials.1-6 The two most important polymorphs of TiO2 are anatase and rutile. Anatase belongs to the space group I4/amd with a = 3.7845 Å, c = 9.5143 Å and Z = 4; and rutile to the space group P42/mnm with a = 4.5941 Å, c = 2.9589 Å and Z = 2.7-9 Both materials have crystal structures based on a framework of distorted TiO6 octahedra linked via 3-coordinate O ions. The volume per formula unit is 34.067 Å3 for anatase and 31.225 Å3 for rutile, so that the volume per formula unit is 9.1% greater for anatase than for rutile. Rutile has a bandgap of 3.06 eV while anatase has a bandgap of 3.20 eV at room temperature.10 These gaps are both in excess of 3 eV and thus lie within the near ultraviolet region of the electromagnetic spectrum. It follows that both oxides have limited ability to bring about photocatalytic reactions under visible light irradiation. However under UV irradiation catalytic activity is observed and despite its wider bandgap anatase is generally found to be the better photocatalyst. This is probably a consequence of the fact that the bandgap of anatase is indirect whereas rutile has a direct but forbidden bandgap11,12, which coupled with higher electron mobility in anatase enables more efficient separation of the photoexcited electron hole pairs. Following the observation of Asahi et al.1 that nitrogen doping in anatase TiO2 promotes visible region photocatalytic activity there has been a huge upsurge in interest in development



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of strategies for preparation of material with improved visible region photocatalytic performance. Nitrogen doping remains the most popular approach2,3,13-19. In addition, metal dopants including the group 5 elements V 20, Nb 21 and Ta21 and co-doping with Rh and Sb22 have been shown to promote a visible light response by introduction of states into the bulk bandgap23. Following earlier work which showed that Sn-doped rutile TiO2 exhibits superior photocatalytic activity to N-doped anatase24 we here explore the influence of Sn doping on the electronic structure of TiO2 in the anatase phase. SnO2 itself exists as a tetragonal rutile polymorph7 with lattice parameters a = 4.737 Å, c = 3.188 Å25 that are somewhat bigger than those for TiO226. However in contrast to TiO2, SnO2 does not have an anatase polymorph. The rutile phase of SnO2 has a direct but dipole forbidden band gap27 with a value of 3.596 eV28 that is bigger than for either of the polymorphs of TiO2 discussed above. It follows that SnO2 in itself is of little interest as a material for visible region photocatalysis. Substitutional solid solutions SnxTi1-xO2 which retain the rutile structure of the end members are obtained by reaction between SnO2 and TiO2 at elevated temperatures.29-31 Above 1450 °C solid solutions across the complete composition range 0.0 < x < 1.0 are thermodynamically stable.32 Below 1450 °C there is a roughly symmetrical miscibility gap that increases in width with decreasing temperature, within which solid solution undergo spinodal decomposition into Sn and Ti rich phases.32 Because the bandgap of SnO2 is bigger than that of both the anatase and rutile polymorphs of TiO2 it would be expected that Sn doping in TiO2 should lead to widening of the bandgap in both phases. However it has recently been found that there is a pronounced band bowing effect such that the bandgap in the rutile phase of Ti1-xSnxO2 initially decreases with increasing doping level x in ceramic samples prepared by a high temperature route, the bandgap reaching a minimum value for x = 0.02.33 This finding is in agreement with density functional theory calculations performed by Long et al.,34 who further predicted that Sn


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doping within the anatase phase would lead to a monotonic increase in the bandgap with Sn doping level. It needs to be emphasised however that at the relatively low level of density functional theory employed in this work the calculated absolute value of the bandgaps for both rutile and anatase were much too low. Anatase is thermodynamically unstable with respect to rutile, albeit with a very small enthalpy difference of about 3.26 kJ/mole (0.034 eV/formula unit) between the two phases.35,36 However, the anatase phase is usually obtained when TiO2 is precipitated from solution in sol-gel and related preparation processes. Anatase is metastable at room temperature and converts exothermically and irreversibly to rutile at elevated temperatures, with an activation barrier of around 20 kJ/mol37. The rate of transformation of high purity commercial anatase powder is immeasurably slow below 610 °C but rapid above 730 °C38,39. Sn doping promotes the “rutilisation” of free-standing TiO2 anatase powder at lower temperatures than found for undoped material: thus transformation of 5% and 10% Sn-doped TiO2 into the rutile phase is essentially complete after annealing at 600 °C for four hours.24 Here we show that the Sn doped TiO2 is stabilised in the anatase phase to much higher temperatures when grown as an epitaxial layer on SrTiO3(001). Epitaxial thin films of pure and transition metal doped anatase have been grown by a range of techniques including magnetron sputtering40, pulsed laser deposition41, magnetron and O-plasma assisted molecular beam epitaxy42,43. Both SrTiO3(001) and LaAlO3(001) substrates have been employed, the cubic lattice parameter of the latter (a = 3.793 Å) being better matched with anatase. However the mismatch between SrTiO3 (a = 3.905 Å) and anatase (a = 3.7845 Å) is still only -3.1% and the match is expected to improve as Sn is doped into the anatase phase, which will lead to lattice expansion. We have recently shown that (001) oriented anatase films can be grown on SrTiO3(001) by a very simple dip coating process involving a titanium isopropoxide precursor. The extension of this approach reported



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here has allowed preparation of Sn-doped anatase thin films with Sn doping levels extending to 15% that are stable up to 900 °C. The high quality of these epitaxial samples has in turn facilitated detailed investigation of the electronic structure of Sn-doped anatase by high resolution X-ray photoemission spectroscopy.

2. Experimental Samples were prepared by a dip coating method. Titanium (IV) isopropoxide (Aldrich, 97%) and an appropriate amount of tin (IV) tert-butoxide (Aldrich 99.99%) were dissolved in isopropanol (Fischer Scientific, analytical grade) to give 25 cm3 of a solution with 0.33 M Ti concentration, which was acidified using HNO3 (HNO3/Ti molar ratio 0.15). The resulting solution was used to dip coat SrTiO3(001) substrates in a 25 cm3 wide neck flask. The substrates were cleaned by rinsing in acetone followed by sonication in isopropanol. After immersion, substrates were withdrawn from the alkoxide solution using a motor drive at a pulling rate of 3.5 mm/min. The wet substrates were immediately placed in flowing N2 gas saturated with H2O at room temperature, so that the alkoxide was hydrolysed on the SrTiO3(001) surface. Samples were prepared using between 2 and 8 dip/hydrolysis cycles, although the results presented here all relate to 8-dip samples. The amorphous film products of the hydrolysis were calcined in air for 4 hours at temperatures ranging from 700 °C up to 1000 °C. Following alignment of the crystal, specular θ-2θ X-ray diffraction profiles were measured using a PANalytical X’Pert Pro diffractometer incorporating a monochromated Cu Kα source (λ = 1.5406 Å). Atomic force microscopy (AFM) images were recorded in a Digital Instruments Multimode Scanning Probe Microscopy instrument with a Nanoscope IIIa controller operating in a contact mode, as described in detail elsewhere.44 Film thicknesses were estimated using a Beaglehole Instruments Picometer Ellipsometer. 6 ACS Paragon Plus Environment

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High resolution X-ray photoemission spectra (XPS) were recorded on a Scienta ESCA 300 spectrometer located in the NCESS facility at Daresbury Laboratory, UK. This incorporated a rotating anode Al Kα (hν =1486.6 eV) X-ray source, a seven crystal X-ray monochromator, a 300 mm mean radius spherical sector electron energy analyser and a parallel electron detection system. The X-ray source was run with 200 mA emission current and 14 kV anode bias, while the analyzer operated at 150 eV pass energy with 0.8 mm slits. Gaussian convolution of the analyzer resolution with a linewidth of 260 meV for the monochromated X-ray source gives an effective instrument resolution of about 400 meV. Photoelectron spectra were charge calibrated relative to the weak C 1s contaminant peak which was assigned a binding energy of 285.0 eV.

3. Computational Methods All calculations within this study were performed using the periodic density functional theory (DFT) code VASP45,46, which employs a plane-wave basis set to describe the valence electronic states. Interactions between the cores (Sn:[Kr] and O:[He]) and the valence electrons were described using the projector-augmented wave (PAW) method47,48, with the Sn valence shell including the 4d states. Previous theoretical examinations have been hampered by the self-interaction error (SIE)49 and band gap problems50,51 associated with “standard” DFT functionals. Hybrid functionals, which include a percentage of exact exchange, are now becoming the method of choice in the solid state community, especially as these functionals become more computationally tractable. These functionals include a component of exact exchange from Hartree-Fock theory which counteracts the SIE and band gap problems of purely local functionals such as the local density approximtion (LDA) or the generalised gradient approximation (GGA) improving the prediction of structure52,53, band gaps54,55 and defects 7 ACS Paragon Plus Environment


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properties53,56. We have utilized the PBE0 hybrid functional which has been shown to yield a band gap of 3.60 eV for rutile structured SnO254, which is very close to the experimental band gap of 3.596 eV27,28. The proportion of exact exchange in PBE0 is constant at 25%. Structural optimisation were performed at a series of volumes in order to calculate the equilibrium lattice parameters. In each case, the atomic positions, lattice vectors and cell angles were allowed to relax, while the total volume was held constant. The resulting energyvolume curves were fitted to the Murnaghan equation of state to obtain the equilibrium bulk cell volume. This approach avoids the isotropic problems of Pulay stress and changes in basis set which can accompany volume changes in plane wave calculations57. Convergence with respect to the plane-wave energy cut-off and k-point sampling were checked, with a cut-off of 400 eV and Monkhorst-Pack grids of 4 × 4 × 6 for rutile structured SnO2 and 4 × 4 × 2 for anatase structure SnO2 were found to be sufficient. Structural optimizations were deemed to be converged when the forces on each ion were less than 0.005 eV Å−1.

4. Results and discussion. 4.1 Computational results The lattice parameters and energies and volumes per formula unit derived from the VASP calculations for the rutile and anatase polymorphs of SnO2 are given in table 1, along with experimental value for the lattice parameters for the rutile polymorph. The calculated a and c parameters are very close to the experimental values for the rutile polymorph, being respectively 0.34% and 0.28% bigger than the values reported by Yamanaka et al.25 Assuming a similar discrepancy of around 0.3% applies to the anatase polymorph we can apply a small correction to the computed values to obtain “corrected” computed lattice parameters which can then be used to construct a Végard plot for Sn-doped TiO2 in the


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anatase phase (see below). A simple interpolation shows that the lattice match between SrTiO3 and SnxTi1-xO2 improves with increasing Sn doping and that there should be perfect lattice matching between SrTiO3 and anatase-Ti1-xSnxO2 for x = 0.62. However this composition falls within the miscibility gap. For the hypothetical anatase phase of SnO2 with a = 3.978 Å the mismatch with a for SrTiO3 is only +1.9%, which is numerically less than the mismatch of -3.1% for anatase-TiO2.

The rutile polymorph of SnO2 is calculated to be more stable than the anatase polymorph by 0.204 eV per formula unit. This difference is much bigger than the experimental difference of only 0.034 eV per formula unit between the anatase and rutile polymorphs of TiO2, thus helping to explain the fact that it has not yet been possible to prepare SnO2 in the anatase phase. It should however be noted that it is rather difficult for theory to reproduce the very small differences in energy between different phases of TiO2 and the PBE0 functional incorrectly predicts that the anatase polymorph of TiO2 is 0.065 eV per formula unit more stable than the rutile polymorph. It is also notable that the volume difference between anatase and rutile phases of SnO2 is 12.6% and therefore bigger than the difference of 9.1% for TiO2. Moreover the bandgap of anatase SnO2 is predicted to be 4.17 eV so that the difference between the two polymorphs (0.57 eV) is also bigger than when dealing with TiO2.

Table 1. Lattice parameters, energies, unit cell volumes and bandgaps for rutile and anatase polymorphs of SnO2 derived from DFT calculations

a (Å) c (Å) Energy (eV per formula unit) Volume (Å3 per formula unit) Bandgap (eV)

Rutile SnO2 Rutile SnO2 Anatase SnO2 Experimental PBE0 PBE0 4.740 4.756 3.990 3.188 3.197 10.235 N/A -29.351358 -29.1471325 35.813

36.162

40.733

3.596 eV

3.60 eV

4.17 eV


Anatase SnO2 Corrected 3.978 10.204 N/A 40.368


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4.2 X-ray diffraction and the range of phase stability in epitaxial thin films We explored a range of doping levels, differing annealing temperatures and deposition parameters, especially the number of dips, at each stage characterising the deposited films with simple θ-2θ X-ray diffraction. Since the ultimate objective was to prepare continuous films without pinholes for photoemission studies, most attention was devoted to films prepared with 8 dip-hydrolysis cycles: coverage of the substrate without pinholes and attenuation of substrate photoemission features was not complete for thinner samples. Some typical results for an undoped film and a film with a nominal doping level of 5 atomic % Sn (Sn0.05Ti0.95O2), both annealed at 800 °C for four hours, are shown in figure 1. Aside from the (001) and (002) reflections of SrTiO3, the only peak found in the diffraction patterns is the (004) reflection of anatase. This demonstrates an epitaxial relationship SrTiO3(001) anatase-TiO2(001). The (004) peak was found to move to low angle with Sn doping, as expected from the fact that incorporation of Sn on Ti lattice sites within the anatase structure will lead to an expansion of the unit cell. Further experiments showed that the anatase phase was stable for Sn doping levels up to 15% and for annealing temperatures up to 900 °C at the highest doping level: following a 4 hour anneal at 1000 °C for 10% and 15% Sn-doped films diffraction peaks associated with the rutile phase were observed (figure 2). Thus Sn doping at these concentrations compromises the kinetic stability of the anatase phase: undoped films on SrTiO3(001) are stable to about 1100 °C. There is nonetheless pronounced epitaxial stabilisation since free-standing 10% Sn-doped anatase powder begins to transform to the rutile phase at temperatures as low as 400 °C and transformation is complete after 4 hours at 600 °C.24 The variation in lattice parameter a with nominal Sn doping level (as defined by the Sn/Ti ratio in the solution used for dip coating) is shown in figure 3, where comparison is 10 ACS Paragon Plus Environment

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made with a Végard plot derived from the experimental lattice parameter for anatase-TiO2 and the corrected theoretical a parameter for anatase-SnO2. The experimental points are quite close to the linear Végard plot, confirming the incorporation of Sn into the anatase lattice. It proved to be impossible to grow pure SnO2 films with the anatase structure despite the good predicted lattice match discussed above. Using pure tin (IV) tert-butoxide in the precursor solution and annealing at 800 °C after 8 dip-hydrolysis cycles, an (004) reflection characteristic of the anatase phase was found, but with a lattice parameter c = 9.583 Å very much lower than the value c = 10.204 Å expected for SnO2 itself and in fact very close to the experimental value for Sn0.15Ti0.85O3 in figure 3. This suggests that use of the pure Sncontaining precursor promotes segregation of TiO2 to the SrTiO3 surface and reaction between SnO2 and TiO2 to give Sn-doped TiO2.

4.3 Sample morphology Atomic force microscopy images (AFM) of Sn-doped anatase thin films annealed at 800 °C for three different doping levels are shown in figure 4. The film morphology is seen to involve an array of interlinked square islands with relatively few deep pinholes. Ellipsometry revealed that each dip-hydrolysis cycle added just under 3 nm to the film thickness and that 8-dip films had a mean thickness of around 20 nm, which is probably best quoted as 20 ± 5 nm given the z range of about 10 nm seen in the AFM images. The edges of the protruding islands are aligned parallel to the [010] and [100] directions of the substrate. Somewhat surprisingly the size of the islands increases with Sn doping level. This contrasts with behaviour seen in nanocrystalline Sn-doped rutile-TiO2 where Sn doping appears to inhibit particle sintering and the mean grain size inferred from XRD line broadening decreases with increasing Sn doping level.24 We may speculate that the increasing island size found here is 11 ACS Paragon Plus Environment


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related to the lattice expansion and corresponding reduced strain associated with Sn substitution into the anatase lattice.

4.4 Core level X-ray photoemission spectra and surface composition A typical core level photoemission spectrum measured at normal emission from a thin-film sample of 3% Sn-doped anatase is shown in figure 5(a). Figure 5(b) shows the corresponding spectrum measured at more grazing (20°) emission angle. The surface Sn concentrations [Sn]/([Sn]+[Ti]) derived from spectra of this sort after correction with atomic sensitivity factors are shown in figure 6. It can be seen that the surface Sn concentrations are greater than the nominal values in all cases and that the apparent Sn doping level increases in going from normal to grazing emission. This suggests that there is pronounced segregation of the Sn dopant to the anatase surface. Similar segregation has been observed in Sn-doped rutileTiO2 prepared by both high temperature solid state methods33 and by a sol-gel technique.24 Dopant surface segregation in systems where a post transition metal ion dopant is introduced into an oxide host lattice has been seen in a number of systems including Sb-doped SnO258-60, Sb-doped TiO261 and Sn-doped In2O362. Segregation in these systems has been attributed to the preference of post transition metal ions in the N-2 oxidation state to occupy surface sites, where N refers to the group oxidation state. Free ions in the N-2 oxidation state such as Sn2+ and Sb3+ have a configuration 5s25p0. In solid state oxides, the 5s electrons hybridise strongly with O 2p states to give antibonding states of mixed metal 5s–O 2p character at the top of the valence band. These states can further interact with nominally empty metal 5p states to give a directional electron lone pair provided that the cation occupies a site which lacks inversion symmetry. Bulk sites within the anatse structure are almost centrosymmetric but the lowered coordination at surface sites removes the inversion symmetry, thereby allowing 5s–5p


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hybridisation and a lowering of the internal electronic energy of the dopant ion. Thus segregation and reduction of bulk ions in the N oxidation state to give surface ions in the N-2 oxidation state lowers the surface energy and provides a thermodynamic driving force for segregation.

4.5 Valence band X-ray photoemission. Valence band and shallow core photoemission spectra of Sn-doped samples for a range of compositions between TiO2 and Sn0.10Ti0.90O2 (i.e. 0 % and 10 % doping) are shown in figure 6. The most obvious change in the spectra with Sn doping is the progressive growth in intensity of the shallow core Sn 4d peak, paralleling the growth in intensity of the deeper lying Sn 3d peak. At the same time a shoulder grows on the low binding energy side of the O 2s peak. This feature is associated with hybridisation between Sn 4d and O 2s states (which have similar energies) leading to an antibonding state at the top of the O 2s band. A weak peak at the bottom of the main O 2p valence band also grows in intensity with Sn doping, as seen more clearly in figure 7 which shows expanded scans across a narrower range including only the valence band itself. This peak is attributed to bonding states arising from hybridisation between Sn 5s and O 2p states: photoemission spectra of SnO2 itself show a peak at similar binding energy58,59,63. Finally there is a small but well-defined shift to higher binding energy in the position of the valence band edge, as shown in figure 8. Assuming that the Fermi energy is pinned close to the bottom of the conduction band at about the same energy for all samples, this shift must reflect progressive widening of the bandgap with Sn doping, as predicted by Long et al. 34

. The simple linear extrapolation of the band edge used in figure 8 does not provide a

reliable means of estimating the absolute value of the bandgap64, but the shift observed using identical extrapolation procedures is of physical significance. The progressive widening of



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the gap with Sn-doping in the anatase phase differs from the behaviour found in the rutile phase where the gap initially decreases with Sn doping before increasing once the doping level exceeds 2%33. However even at 10% Sn-doping in the rutile phase the gap is still smaller than for undoped rutile-TiO2.

5. Concluding remarks. Sn-doped anatase thin films have been grown epitaxially on SrTiO3(001) by a simple dip coating procedure using Ti and Sn alkoxide precursors. Films containing 15 atomic % of the cationic Sn dopant (Sn0.15Ti0.85O2) are stable to around 900 °C, in contrast to free-standing Sn-doped anatase powders which begin to convert to rutile at temperatures as low as 400 °C24. We have been unable to grow SnO2 on SrTiO3 as an anatase thin film, even though the lattice match is predicted by DFT to be better than for TiO2. However the energy difference between the two polymorphs is predicted to be bigger for SnO2 than TiO2. In contrast to Sn-doped rutile, the bandgap of Sn-doped anatase increases monotonically with Sn doping. Qualitatively the behaviour within the rutile phase may be understood in terms of two competing effects. Sn-doping leads to expansion of the unit cell and internal strain of this sort is expected to lead to a reduction in the bandgap, in a way similar to the bandgap reduction associated with tensile strain in thin epitaxial films65,66. On the other hand, the basic differences in band edge positions between TiO2 and SnO2 is expected to lead to bandgap widening as SnO2 is alloyed with TiO2 and this effect must eventually become dominant We may further speculate that the anatase phase of TiO2 is less influenced by internal strain as it is less dense than the rutile polymorph, thus accounting for the difference between the two phases. Further computational work is required to explore these ideas.


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Acknowledgements. FEO was supported by Pembroke College Oxford and FUNDAYACUCHO (Caracas, Venezuela), while KHLZ is grateful to the University of Oxford for the award of a Clarendon scholarship. RGP was supported by the European Union grant NATAMA (NMP3-CT-2006032583). The XPS facility at Daresbury Laboratory was supported by EPSRC grant EP/E025722/1.



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References. (1)

Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Visible-Light

Photocatalysis in Nitrogen-Doped Titanium Oxides. Science 2001, 293, 269-271. (2)

Di Valentin, C.; Finazzi, E.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Paganini,

M. C.; Giamello, E. N-doped TiO2: Theory and Experiment. Chem. Phys. 2007, 339, 44-56. (3)

Qiu, X. F.; Burda, C. Chemically Synthesized Nitrogen-Doped Metal Oxide

Nanoparticles. Chem. Phys. 2007, 339, 1-10. (4)

Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Photocatalysis on TiO2 Surfaces -

Principles, Mechanisms, and Selected Results. Chem. Rev. 1995, 95, 735-758. (5)

Wong, J. C. S.; Linsebigler, A.; Lu, G. Q.; Fan, J. F.; Yates, J. T.

Photooxidation of CH3Cl on TiO2(110) Single-Crystal and Powdered TiO2 Surfaces. J. Phys. Chemistry 1995, 99, 335-344. (6)

Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity ff Titanium

Dioxide. Prog. Solid State Chem. 2004, 32, 33-177. (7)

Clendenen, R. L.; Drickamer, H. G. Lattice Parameters of 9 Oxides and

Sulfides as a Function of Pressure. J. Chem. Phys. 1966, 44, 4223-4228. (8)

Rao, K. V. K.; Naidu, S. V. N.; Iyengar, L. Thermal Expansion of Rutile and

Anatase. J. Amer. Ceram. Soc. 1970, 53, 124-126. (9)

Semiconductors: Physics of Non-Tetrahedrally Bonded Binary Compounds II;

Springer Verlag: Berlin, 1984; Vol. 17, Pt. f. (10)

Tang, H.; Levy, F.; Berger, H.; Schmid, P. E. Urbach Tail of Anatase TiO2.

Phys. Rev. B 1995, 52, 7771-7774. (11)

Mo, S. D.; Ching, W. Y. Electronic and Optical-Properties of 3 Phases of

Titanium-Dioxide - Rutile, Anatase, and Brookite. Phys. Rev. B 1995, 51, 13023-13032.


Page 16 of 33

Page 17 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(12)

Labat, F.; Baranek, P.; Domain, C.; Minot, C.; Adamo, C. Density Functional

Theory Analysis of the Structural and Electronic Properties of TiO2 Rutile and Anatase Polytypes: Performances of Different Exchange-Correlation Functionals. J. Chem. Phys. 2007, 126, 154703/1-12. (13)

Di Valentin, C.; Pacchioni, G.; Selloni, A.; Livraghi, S.; Giamello, E.

Characterization of Paramagnetic Species in N-Doped TiO2 Powders by EPR Spectroscopy and DFT Calculations. J. Phy. Chem. B 2005, 109, 11414-11419. (14)

Livraghi, S.; Paganini, M. C.; Giamello, E.; Selloni, A.; Di Valentin, C.;

Pacchioni, G. Origin of Photoactivity of Nitrogen-Doped Titanium Dioxide Under Visible Light. J. Amer. Chem. Soc. 2006, 128, 15666-15671. (15)

Napoli, F.; Chiesa, M.; Livraghi, S.; Giamello, E.; Agnoli, S.; Granozzi, G.;

Pacchioni, G.; Di Valentin, C. The Nitrogen Photoactive Centre in N-Doped Titanium Dioxide Formed via Interaction of N Atoms with the Solid. Nature and Energy Level of the Species. Chem. Phys. Lett. 2009, 477, 135-138. (16)

Livraghi, S.; Votta, A.; Paganini, M. C.; Giamello, E. The Nature of

Paramagnetic Species in Nitrogen Doped TiO2 Active in Visible Light Photocatalysis. Chem. Commun. 2005, 498-500. (17)

Livraghi, S.; Chierotti, M. R.; Giamello, E.; Magnacca, G.; Paganini, M. C.;

Cappelletti, G.; Bianchi, C. L. Nitrogen-Doped Titanium Dioxide Active in Photocatalytic Reactions with Visible Light: a Multi-Technique Characterization of Differently Prepared Materials. J. Phys. Chem. C 2008, 112, 17244-17252. (18)

Zhao, Y. X.; Qiu, X. F.; Burda, C. The Effects of Sintering on the

Photocatalytic Activity of N-Doped TiO2 Nanoparticles. Chem. Mat. 2008, 20, 2629-2636. (19)

Irie, H.; Watanabe, Y.; Hashimoto, K. Nitrogen-Concentration Dependence on

Photocatalytic Activity of TiO2-xNx Powders. J. Phys. Chem. B 2003, 107, 5483-5486.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(20)

Klosek, S.; Raftery, D. Visible Light Driven V-Doped TiO2 Photocatalyst and

its Photooxidation of Ethanol. J. Phys. Chem. B 2001, 105, 2815-2819. (21)

Niishiro, R.; Kato, H.; Kudo, A. Nickel and either Tantalum or Niobium-

Codoped TiO2 and SrTiO3 Photocatalysts with Visible-Light Response for H2 or O2 Evolution from Aqueous Solutions. Phys. Chem. Chem. Phys. 2005, 7, 2241-2245. (22)

Niishiro, R.; Konta, R.; Kato, H.; Chun, W. J.; Asakura, K.; Kudo, A.

Photocatalytic O2 Evolution of Rhodium and Antimony-Codoped Rutile-Type TiO2 under Visible Light Irradiationj. Phys. Chem. B 2007, 111, 17420-17426. (23)

Oropeza, F. E.; Egdell, R. G. Control of Valence States in Rh-Doped TiO2 by

Sb Co-Doping: a Study by High Resolution X-Ray Photoemission Spectroscopy. Chem. Phys. Lett. 2011, 515, 249-253. (24)

Oropeza, F. E.; Davies, B.; Palgrave, R. G.; Egdell, R. G. Electronic Basis of

Visible Region Activity in High Area Sn-Doped Rutile TiO2 Photocatalysts. Phys. Chem. Chem. Phys. 2011, 13, 7882-7891. (25)

Yamanaka, T.; Kurashima, R.; Mimaki, J. X-ray Diffraction Study of Bond

Character of Rutile-Type SiO2, GeO2 and SnO2. Z. Krist. 2000, 215, 424-428. (26)

Taverner, A. E.; Rayden, C.; Warren, S.; Gulino, A.; Cox, P. A.; Egdell, R. G.

A Comparison of the Energies of Vanadium Donor Levels in Doped SnO2 and TiO2. Phys. Rev. B 1995, 51, 6833-6837. (27)

Frohlich, D.; Kenklies, R.; Helbig, R. Band-Gap Assignment in SnO2 by 2-

Photon Spectroscopy. Phys. Rev. Lett. 1978, 41, 1750-1751. (28)

Reimann, K.; Steube, M. Experimental Determination of the Electronic Band

Structure of SnO2. Solid State Commun. 1998, 105, 649-652. (29)

Sinclair, W. R.; Loomis, T. C. Measurements of Diffusion in the System

TiO2/SnO2. Angewandte Chemie-Int. Ed. 1958, 70, 603.


Page 18 of 33

Page 19 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(30)

Drobeck, D. L.; Virkar, A. V.; Cohen, R. M. The Effect of Aliovalent Dopants

on Cation Diffusion in TiO2 -SnO 2. J. Phys. Chem. Solids 1990, 51, 977-988. (31)

Radecka, M.; Zakrzewska, K.; Rekas, M. SnO2-TiO2 Solid Solutions for Gas

Sensors. Sensors and Actuators B-Chemical 1998, 47, 194-204. (32)

Naidu, H. P.; Virkar, A. V. Low-Temperature TiO2-SnO2 Phase Diagram

using the Molten-Salt Method. J. Amer. Ceram. Soc. 1998, 81, 2176-2180. (33)

Harunsani, M. H.; Oropeza, F. E.; Palgrave, R. G.; Egdell, R. G. Electronic

and Structural Properties of SnxTi1-xO2 (0.0 < x < 0.1) Solid Solutions. Chem. Mater. 2010, 22, 1551-1558. (34)

Long, R.; Dai, Y.; Huang, B. Geometric and Electronic Properties of Sn-

Doped TiO2 from First-Principles Calculations. J. Phys. Chem. C 2009, 113, 650-653. (35)

Mitsuhashi, T.; Kleppa, O. J. Transformation Enthalpies of the TiO2

Polymorphs. J. Amer. Ceram. Soc. 1979, 62, 356-357. (36)

Zhang, H. Z.; Banfield, J. F. Understanding Polymorphic Phase

Transformation Behavior During Growth of Nanocrystalline Aggregates: Insights from TiO2. J. Phys. Chem. B 2000, 104, 3481. (37)

Shannon, R. D.; Pask, J. A. Kinetics of Anatase-Rutile Transformation

J. Amer. Ceram. Soc. 1965, 48, 391-398. (38)

Czanderna, A. W.; Rao, C. N. R.; Honig, J. M. The Anatase-Rutile Transition

.1. Kinetics of the Transformation of Pure Anatase. Trans. Faraday Soc. 1958, 54, 10691073. (39)

Czanderna, A. W.; Clifford, A. F.; Honig, J. M. Preparation of Highly Purified

TiO2 (Anatase). J. Amer. Chem. Soc. 1957, 79, 5407-5409.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(40)

Jellison, G. E.; Boatner, L. A.; Budai, J. D.; Jeong, B. S.; Norton, D. P.

Spectroscopic Ellipsometry of Thin Film and Bulk Anatase (TiO2). J. Appl. Phys. 2003, 93, 9537-9541. (41)

Murakami, M.; Matsumoto, Y.; Nakajima, K.; Makino, T.; Segawa, Y.;

Chikyow, T.; Ahmet, P.; Kawasaki, M.; Koinuma, H. Anatase TiO2 Thin Films Grown on Lattice-Matched LaAlO3 Substrate by Laser Molecular-Beam Epitaxy. Appl. Phys. Lett. 2001, 78, 2664-2666. (42)

Weng, X.; Fisher, P.; Skowronski, M.; Salvador, P. A.; Maksimovc, O.

Structural Characterization of TiO2 Films Grown on LaAlO3 and SrTiO3 Substrates using Reactive Molecular Beam Epitaxy. J. Crystal Growth 2008, 310, 545-550. (43)

Chambers, S. A.; Thevuthasan, S.; Farrow, R. F. C.; Marks, R. F.; Thiele, J.

U.; Folks, L.; Samant, M. G.; Kellock, A. J.; Ruzycki, N.; Ederer, D. L.; Diebold, U. Epitaxial Growth and Properties of Ferromagnetic Co-Doped TiO2 Anatase. Appl. Phys. Lett. 2001, 79, 3467-3469. (44)

Zhang, K. H. L.; Payne, D. J.; Palgrave, R. G.; Lazarov, V. K.; Chen, W.;

Wee, A. T. S.; McConville, C. F.; King, P. D. C.; Veal, T. D.; Panaccione, G.; Lacovig, P.; Egdell, R. G. Surface Structure and Electronic Properties of In2O3(111) Single-Crystal Thin Films Grown on Y-Stabilized ZrO2(111). Chem. Mat. 2009, 21, 4353-4355. (45)

Kresse, G.; Furthmuller, J. Efficient Iterative Schemes for Ab Initio Total-

Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B 1996, 54, 11169-11186. (46)

Kresse, G.; Hafner, J. Ab-Initio Molecular-Dynamics Simulation of The

Liquid-Metal Amorphous-Semiconductor Transition in Germanium Phys. Rev. B 1994, 49, 14251-14269. (47)

Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50,

17953-17979.


Page 20 of 33

Page 21 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(48)

Kresse, G.; Joubert, D. From Ultrasoft Pseudopotentials to the Projector

Augmented-Wave Method. Phys. Rev. B 1999, 59, 1758-1775. (49)

Scanlon, D. O.; Morgan, B. J.; Watson, G. W. Modeling the Polaronic Nature

of P-Type Defects in Cu2O: the Failure of GGA and GGA Plus U. J. Chem. Phys. 2009, 131, 124703/1-8. (50)

Godinho, K. G.; Walsh, A.; Watson, G. W. Energetic and Electronic Structure

Analysis of Intrinsic Defects in SnO2. J. Phys. Chem. C 2009, 113, 439-448. (51)

Scanlon, D. O.; Watson, G. W. (Cu2S2)(Sr3Sc2O5) - a Layered, Direct Band

Gap, P-Type Transparent Conducting Oxychalcogenide: a Theoretical Analysis. Chem. Mat. 2009, 21, 5435-5442. (52)

Allen, J. P.; Scanlon, D. O.; Watson, G. W. Electronic Structures of Silver

Oxides. Phys. Rev. B 2011, 84, 115141/1-14. (53)

Heyd, J.; Scuseria, G. E. Efficient Hybrid Density Functional Calculations in

Solids: Assessment of the Heyd-Scuseria-Ernzerhof Screened Coulomb Hybrid Functional. J. Chem. Phys. 2004, 121, 1187-1192. (54)

Scanlon, D. O.; Watson, G. W. On the Possibility of P-Type SnO2. J. Mater.

Chem. 2012, 22, 25236-24245. (55)

Scanlon, D. O.; Walsh, A.; Watson, G. W. Understanding the P-Type

Conduction Properties of the Transparent Conducting Oxide CuBO2: a Density Functional Theory Analysis. Chem. Mat. 2009, 21, 4568-4576. (56)

Scanlon, D. O.; Watson, G. W. Understanding the P-Type Defect Chemistry

of CuCrO2. J. Mater. Chem. 2011, 21, 3655-3633. (57)

Allen, J. P.; Scanlon, D. O.; Parker, S. C.; Watson, G. W. Tin Monoxide:

Structural Prediction from First Principles Calculations with Van der Waals Corrections. J. Phys. Chem. C 2011, 115, 19916-19924.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(58)

Cox, P. A.; Egdell, R. G.; Harding, C.; Orchard, A. F.; Patterson, W. R.;

Tavener, P. J. Free Electron Behaviour of Carriers in Sb-Doped Tin Dioxide. Solid State Commun. 1982, 44, 837-839. (59)

Cox, P. A.; Egdell, R. G.; Harding, C.; Patterson, W. R.; Tavener, P. J. Surface

Properties of Antimony-Doped Tin Oxide. Surf. Sci. 1982, 123, 179-203. (60)

Egdell, R. G.; Flavell, W. R.; Tavener, P. Antimony Doped Tin(IV) Oxide:

Surface Composition and Electronic Structure J. Solid State Chem. 1984, 51, 345-354. (61)

Gulino, A.; Taverner, A. E.; Warren, S.; Harris, P.; Egdell, R. G. A

Photoemission Study of Sb-doped TiO2. Surf. Sci. 1994, 315, 351-361. (62)

Cox, P. A.; Flavell, W. R.; Egdell, R. G. Solid state and surface chemistry of

Sn-doped In2O3 ceramics. J. Solid State Chem. 1987, 68, 340-350. (63)

Themlin, J. M.; Sporken, R.; Darville, J.; Caudano, R.; Gilles, J. M.; Johnson,

R. L. Resonant-Photoemission Study of SnO2 - Cationic Origin of the Defect Band-Gap States. Phys. Rev. B 1990, 42, 11914-11925. (64)

King, P. D. C.; Veal, T. D.; Fuchs, F.; Wang, C. Y.; Payne, D. J.; Bourlange,

A.; Zhang, H.; Bell, G. R.; Cimalla, V.; Ambacher, O.; Egdell, R. G.; Bechstedt, F.; McConville, C. F. Band Gap, Electronic Structure and Surface Electron Accumulation of Cubic and Rhombohedral In2O3. Phys. Rev. B 2009, 79, 205211/1-10. (65)

Walsh, A.; Catlow, C. R. A.; Zhang, K. H. L.; Egdell, R. G. Band Gap Control

of Metal Oxides Through Epitaxial Strain: the Case of Indium Oxide. Phys. Rev. B 2011, 83, 161202/1-4. (66)

Zhang, K. H. L.; Lazarov, V. K.; Veal, T. D.; Oropeza, F. E.; McConville, C.

F.; Egdell, R. G.; Walsh, A. Thickness Dependence of the Strain, Band Gap and Transport Properties of Epitaxial In2O3 Thin Films Grown on Y-stabilised ZrO2 (111). J. Phys. Condensed Matter 2011, 23, 334211/1-8.


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Captions for figures.

Figure 1. θ-2θ X-ray diffraction profiles for (a) undoped (b) 5% Sn-doped anatase thin films grown on SrTiO3 by 8 dip-hydrolysis cycles, followed by annealing at 800 °C for 4 hours. The intensity scale is logarithmic. Expanded views of the anatase (004) reflection are shown in (c).

Figure 2. θ-2θ X-ray diffraction profiles for Sn0.15Ti0.85O2 (15% Sn-doped TiO2) thin film grown on SrTiO3(001) by 8 dip-hydrolysis cycles followed by annealing for 4 hours at (a) 900 °C (b) 1000 °C. The intensity scale is logarithmic. Annealing at the higher temperature is seen to promote transformation of the anatase phase into the rutile phase.

Figure 3. Variation in lattice parameter a with Sn doping level in Sn-doped anatase thin films (SnxTi1x O2 )

grown on SrTiO3(001) by 8 dip-hydrolysis cycles, followed by annealing at 800 °C in

for 4 hours. The solid line shows the variation expected from interpolation between the experimental lattice parameter for bulk anatase TiO2 and the corrected computed lattice parameter for anatase SnO2.

Figure 4. 5 µm × 5 µm AFM images of anatase-SnxTi1-xO2 films for (a) x = 0.00 (b) x = 0.05 (c) x = 0.10. Samples were prepared by 8 dip-hydrolysis cycles, followed by annealing at 800 °C for 4 hours.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Figure 5. Core level XPS of 3% Sn-doped TiO2 in the region of the Sn 3d and Ti 2p core levels (a) at normal (90°) emission angle (b) at 20° emission angle. Sample prepared by 8 dip-hydrolysis cycles followed by annealing at 800 °C for 4 hours. (c) Open circles: apparent surface Sn fraction from core level XPS taken at normal emission as a function of nominal bulk doping level. The solid line is for guidance only. The dashed line shows the variation expected if XPS probed the nominal bulk composition. Open square: corresponding Sn fraction derived from XPS at 20° emission angle.

Figure 6. Valence band and shallow core level XPS of anatase SnxTi1-xO2 as a function of doping level x. Thin film samples prepared by 8 dip-hydrolysis cycles and annealing at 800 °C for 4 hours.

Figure 7. Expanded view of valence band region in core level photoemission of samples as in figure 6, showing growth of feature associated with Sn 5s states with increasing Sn doping.

Figure 8. Detail of valence band onset in photoemission spectra of anatase thin film samples SnxTi1-xO2 for x=0.00, 0.03 and 0.10.


Page 24 of 33

Page 25 of 33

undoped o 800 C anneal

(a)

SrTiO O3(002)

anatase (004)

SrTiO3(001)

Intensity (arbitary logarithm mic scale)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b) 5% Sn-doped o 800 C anneal

20

30

40

50

60

2 (degrees) undoped 5% Sn doped o 800 C anneal

36.0

36.5

37.0

(c) anatase (004)

37.5

38.0

38.5

2 (degrees)

ACS Paragon Plus Environment

39.0


o

SrTiO3(002)

anatase(004)

rutile e(111)

SrTiO3 (001)

(a) 900 C

Intensity (arbitary logarith hmic scale)

o

20

30

40 2 (degrees)

SrT TiO3 (002)

11) rutile(11

anatase e(004)

SrTiO3(001)

(b) 1000 C

rutile(110)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

50


60

Page 26 of 33

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Lattice parametter a (Angstroms)

Page 27 of 33

o

9.64 9.62 9.60 9.58 9.56 9.54 9.52 9.50 0.00

0.05

0.10

0.15

Sn doping level (x in SnxTi1-xO2)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(a) x = 0.00

[010] (b) x = 0.05

[010] (c) x = 0.10

[010]


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Page 29 of 33

o

90 emission x(a) = 0.055

Ti 2p

Sn 3d

o

(b) 20 emission

500

490

480

470

460

450

Binding energy (eV) Surface [Sn]/([Sn] + [Ti])

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.20

(c)

0 15 0.15 0.10 0.05 0.00 0.00

0.02

0.04

0.06

0.08

0.10

Bulk [Sn]/([Sn] + [Ti])


x x x x x

= 0.00 = 0.01 = 0.03 = 0.05 = 0.10

Sn 5s

VB

30

25

20

15

10

VB

O 2s

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Sn 4d


5

0

Binding energy / eV


Page 30 of 33

VB B

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


x x x x x

Sn 5s

Page 31 of 33

= 0.00 = 0.01 = 0.03 = 0.05 = 0.10

VB

12

10

8

6

4

2

0

Binding energy (eV)


-2


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 33

x = 0.00 x = 0,03 x = 0.10

4.5

4.0

3.5

3.0

2.5

2.0

Binding energy (eV)


1.5

Page 33 of 33

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Graphical abstract 35x20mm (300 x 300 DPI)


Electronic Structure of Epitaxial Sn-Doped Anatase Grown on SrTiO3

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