Grafting silica species on anatase surface for visible light photocatalytic activity

The grafting of optimal trace silica on TiO2 (anatase) substrates possessing a large specific surface area can significantly enhance their photocatalytic activity for decomposing organic contaminants, such as chlorophenols and phenol in water under visible-light illumination. The silica grafting produces surface electronic states within the TiO2 band gap, which are responsible for the visible-light activity. Different from the bulk doping, SiO2 was grafted on the anatase surface in two possible modes: (i) formation of SiO2 clusters on the surface; and (ii) substitution of Si atoms for Ti atoms of the outmost layer at the surface. Thus, the surface electronic states originate mainly from the 2p orbitals of coordination-unsaturated oxygen atoms bound to Si atoms. The location of these states depends on the forms by which the introduced silica species exist and the type of the grafted facet. Nonetheless, the attempt of grafting the same substrate with alumina failed. Also a greater enhancement in the visible-light activity was achieved when TiO2 surfaces with higher surface energy, such as the surfaces of anatase (010) and (116) planes, were grafted. Advantages of the grafted approach include: (i) different from bulk-doping, the surface-grafting does not change the crystalline and electronic structure in the bulk of the photocatalysts; (ii) and consequently the electron mobility in the photocatalysts and their photocatalytic activity under UV irradiation are not affected, (iii) the grafted photocatalysts have high chemical stability and repeatable photocatalytic activity.