XPS study of cobalt doped TiO2 films prepared by pulsed laser deposition

Five percent Co-doped TiO2 films have been prepared under an oxygen partial pressure from 10(-6) to 10(-4) Torr. The samples were stored for two weeks under ambient conditions prior to X-ray photoelectron spectroscopy investigation. The X-ray photoelectron spectra of Ti 2p and 3p, as well as X-ray induced Auger electron spectrum Ti L3M23V all show evidence for Ti3+ at the surface of the film grown in 10(-6) Torr O-2 partial pressure. The curve fitted Ti 2p(3/2) spectrum indicates a concentration of 6.5% Ti3+. The films grown in higher O-2 pressure do not show Ti3+ state clearly. Surface-enriched cobalt in Co2+ state has been noted. Such Co2+ enrichment can be removed by in-situ annealing at 650 degrees C for 45 min resulting in elemental state cobalt at the annealed surface. The annealed film still shows Ti3+ state in the surface region but the concentration becomes lower. The Ti3+ concentration was lowered to the detection limit after the annealed sample was exposed to atmosphere for 30 min. The surface enrichment of cobalt may play some role in preventing the Ti3+ species in the uppermost surface layer from being quickly oxidized in atmosphere. Copyright (C) 2014 John Wiley & Sons, Ltd.