The roles of surface-doped metal ions (V, Mn, Fe, Cu, Ce, and W) in the interfacial behavior of TiO2 photocatalysts

Six types of transition metal ions, including V, Mn, Fe, Cu, Ce, and W, are doped into the surface lattice of TiO2 powders, and their roles in the charge trapping, recombination, interfacial transfer, and photocatalytic activity are systematically studied. The surface-doped TiO2 powders exhibit photocatalytic activity in the order of Fe/TiO2 > Cu/TiO2 > V/TiO2 > W/TiO2 > Ce/TiO2 > Mn/TiO2. While the Fe, Cu and V ions improve the activity, the W, Ce, and Mn ions cause detrimental effects. The different influences are associated with their energy levels, coordination numbers and electronegativity. The surface-doped ions trap charge carriers and interact with adsorbates to provide alternative pathways for interfacial charge transfer. The Fe and Cu ions inhibit defect-mediated annihilation, facilitating interfacial charge transfer in terms of d-d transitions and thermally induced de-trapping. The Mn ions, which introduce both occupied and unoccupied states in the mid-band-gap region, in contrast, trap holes and electrons to severely consume charge carriers via intra-atomic relaxation. The Ce and W ions, which have high coordination numbers and electronegativity, strongly bond the O-2(-) radicals, thus limiting charge utilization as well as photocatalytic performance. (C) 2014 Elsevier B.V. All rights reserved.