There is a growing interest in the use of coloured complex metal oxides as visible light-active photocatalysts and photoelectrodes. In contrast to simple oxides (AOx), complex oxides (ABOx) possess two ordered metal sub lattices for the large A and smaller B cations. In order to afford visible light activity, elements other than d0 and d10 metals must be incorporated. Conventional doping of simple oxides like TiO2 or WO3 for inducing visible light activity often leads to the formation of localised defects that detrimentally affect the charge mobility. On the other hand, incorporation of an extended and ordered lattice facilitates the formation of energy bands that extend throughout the structure, rather than localised defect sites, and thus does not hinder charge migration through the semiconductor particle1.Mixed oxides that contain post-transitional metals in the N-2 oxidation state (where N is the group oxidation state) as the A cation and a d0/d10 as the cation B are particularly interesting. Free ions in the N-2 oxidation state such as Sn2+ and Bi3+ have a configuration ns2np0. The ns electrons hybridise strongly with O 2p states to give anti-bonding states of mixed metal ns�O 2p character at the top of the valence band. These states can further interact with nominally empty metal np states to give a directional electron lone pair provided that the cation occupies a site, which lacks inversion symmetry2. This mixing of electronic states at the valence band induces visible light absorption in this type of materials, and therefore makes them good candidates as visible light photocatalysts and photodevices. Another important consequence of the metal ns�O 2p interaction is the enhanced potential for hole conductivity as suggested by theoretical calculations3.In this contribution, we will report an experimental study of the electronic structure of series of single crystalline bismuth titaniate (Bi2Ti2O7 and Bi12TiO20) thin films based on hard X-ray photoelectron spectra (HAXPES) and Hall effect measurements. Results show that there is a strong Bi 6s charatere at the top of the valence band and that the materials contain p-type and n-type carries. n-type and p-type doping is possible, which gives these type of materials great potential for photovoltaic devices as well as photocatalysts. The characteristics of pn junctions is currently being studied.So far, we have carried out photoemission experiment in core and valence band levels, which have given us interesting hints regarding the interaction between post-transitional metal ns electrons and oxygen 2p at the surface of the materials. Valence band XPS (Figure 1 (left)) shows well defined states related to Bi 6s and O 2p close in energy. An interaction between these two electronic states similar to that reported for Bi2O3 is expected.4 Figure 1.  (Left) Valence and conduction band photoelectron spectrum of Bi2Ti2O7 measured at h? = 1486.6 eV (b) .(Right) One electron photoionisation cross-sections of Bi 6s (circles), O 2p (triangles) orbitals as a function of photon energy. The HAXPES experiment enable us to access bulk electronic states and also give us means to explore the composition of the valence band of the material. The strong interaction between post-transition metal ns and oxygen 2p orbitals results in a mixed (top-of) valence band. Provided the photoionisation cross section of the O 2p orbital decreases much faster than the metal ns orbital upon increasing the excitation photon energy (Figure 1 (right)), analysis of the relative changes in spectral intensity within the valence band states upon increasing the ionisation photon energy could reveal which region of the valence band has more metal ns character and in which region the O 2p character dominates. Theoretical calculations suggest that the top and bottom of the valence bands of this type of complex oxides have strong metal ns character. Accordingly, there is an increased spectral intensity in these two regions of the spectra as the excitation photon energy increases.The metal ns character at the top of the valence band may be a key element for improving hole conductivity, and therefore post-transition metal-based complex oxides could show potential p-type semiconducting behaviour as well as n-type. This type of oxides can therefore be important material for electronic device applications as well as photocatalysis. We are therefore currently studying the p-type and n-type doping of bismuth titanates and np junctions based on theses materials.________1 J. H. Clark, M. S. Dyer, R. G. Palgrave, C. P. Ireland, J. R. Darwent, J. B. Claridge, and M. J. Rosseinsky, J. Am.Chem. Soc. 133 (4), 1016 (2011).2A. Walsh, D. J. Payne, R. G. Egdell, and G. W. Watson, Chem. Soc. Rev. 40 (9), 4455 (2011).3L. A. Burton and A. Walsh, J. Solid State Chem. 196 , 157 (2012).4(a) D. J. Payne, R. G. Egdell, A. Walsh, G. W. Watson, J. Guo, P. A. Glans, T. Learmonth, and K. E. Smith, Phys. Rev. Lett. 96  (15) (2006); (b) A. Walsh, G. W. Watson, D. J. Payne, R. G. Edgell, J. H. Guo, P. A. Glans, T. Learmonth, and K. E. Smith, Phys. Rev. B 73  (23) (2006).

Electronic structure of bismuth titanates thin films and their application in solar energy conversion

247th National Meeting (2014)

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