Visible light sensitive photocatalysts In1-xMxTaO4 (M=3d transition-metal) and their activity controlling factors

A series of novel oxide semiconductors, 3d transition-metal doped In1-xMxTaO4 (M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn), were synthesized by the ceramic method, and their photocatalytic properties for H2O decomposition under visible light irradiation were examined. It was found that the photocatalytic activity of InTaO4 was significantly enhanced while In was substituted by Ni, but suppressed in most of other substitutions. Theoretical calculations indicated that under the octahedral crystal field, the M 3d electron orbitals split into the lower energy t(2g) and the higher energy e(g) parts. With increase in the atomic number of transition-metal M, the 3d energy states shift to the lower energy side. Most of these 3d orbitals hybridize with O-2p or Ta 5d orbitals, localizing the charge carriers. However, Ni 3d-e(g) states, which lie slightly above the O-2p orbitals, play a role in narrowing the band gap of the un-doped InTaO4 and hence significantly enhance the photocatalytic activity. The controlling factors of photocatalytic activity, such as co-catalysts, surface area, crystallinity of these oxide semiconductors were also studied. The results indicate that not only a suitable energy band structure, but also nano-structured bulk and surface properties are crucial for photocatalysis reaction. (c) 2004 Elsevier Ltd. All rights reserved.