Molecular Metal Oxide Cluster-Surface Modified Titanium(IV) Dioxide Photocatalysts

Abstract

The surface modification of TiO2 with molecular sized metal oxide clusters has recently been shown to be a promising approach for providing TiO2 with visible-light activity and/or improved UV activity. This short review summarizes the effects of the surface modification of TiO2 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 TiO2 surface occurs by ligand-exchange and ion-exchange, respectively. Taking advantage of the strong adsorption, we formed extremely small metal oxide clusters on TiO2 by the chemisorption–calcination 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 FeOx surface modification of rutile leads to visible-light activity, while SnO2-modified anatase enhances only the UV-light activity. We propose the mechanisms behind the FeOx and SnO2 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.