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 twoweeks 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 O₂ partial pressure. The curve fitted Ti 2p_3/2 spectrum indicates a concentration of 6.5% Ti³⁺. The films grown in higher O₂ 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°C for 45min 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 30min. 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.