Effect of Sn surface states on the photocataiytic activity of anatase TiO(2)

The influence of surface Sn-doping on the photocataiytic properties of anatase TiO(2) has been investigated in samples prepared by a grafting route using Sn(IV) tert-butoxide as Sn precursor. The grafting procedure leads to the formation of isolated Sn(IV) sites on the surface of anatase TiO(2) powders as gauged by structural characterisation based on XRD, Raman spectroscopy and XAS. Studies of the surface reduction based on TPR experiments and XPS provide the conditions for a selective reduction of surface Sn(IV) to the divalent oxidation state. Electronic structure characterisation based on valence band XPS and DRS shows that there is a slight widening of the band gap upon Sn(IV)-grafting, but Sn(II) related states emerge at the top of the main valence band upon reduction at temperatures up to 350 C, and this induces visible light absorption. Crafting of TiO(2) with Sn(IV) increases the formation rate of 'OH radicals on the surface of the material. Reduction of the Sn(IV)-grafted TiO(2) to form surface Sn(II) brings about substantial increase of the photocataiytic efficiency for the methylene blue degradation under irradiation with lambda greater than or equal to 320 nm compared with Sn(IV)-grafted and pure anatase TiO(2). This observation is explained based on a surface hole trapping by the Sn(II)-related surface states which lie above the top of the main valence band and can therefore act as trapping sites for holes produced under photoexcitation.