Molecular metal oxide cluster-surface modified titanium(iv) dioxide photocatalysts

The surface modification of TiO ₂ with molecular sized metal oxide clusters has recently been shown to be a promising approach for providing TiO ₂ with visible-light activity and/or improved UV activity. This short review summarizes the effects of the surface modification of TiO ₂ with the oxides of iron and tin selected from d- and p-blocks, respectively, on the photocatalytic activity. Fe(acac)3 and [Sn(acac)2]Cl2 chemisorption on the TiO ₂ surface occurs by ligand-exchange and ion-exchange, respectively. Taking advantage of the strong adsorption, we formed extremely small metal oxide clusters on TiO ₂ by the chemisorptioncalcination cycle (CCC) technique with their loading amount strictly controlled. The iron oxide surface modification of P-25 (anatase/rutile≤4:1, w/w, Degussa) gives rise to a high level of visible-light activity and a concomitant increase in the UV-light activity for the degradation of model organic pollutants. On the other hand, only the UV-light activity is increased by the tin oxide surface modification of ST-01 (anatase, Ishihara Sangyo). This striking difference can be rationalized on the basis of the material characterization and DFT calculations, which show that FeO _x surface modification of rutile leads to visible-light activity, while SnO ₂-modified anatase enhances only the UV-light activity. We propose the mechanisms behind the FeO _x and SnO ₂ surface modification, where the surface-to-bulk and bulk-to-surface interfacial electron transfer are taken into account in the former and the latter, respectively.