Effect of Annealing on the Nb-Doped TiO[sub 2] Films Prepared by DC/RF Cosputtering

Nb-doped titanium oxide ͑TiO 2 ͒ thin films were deposited on glass by dc/radio-frequency ͑dc/rf͒ magnetron cosputtering, in which dc and rf were utilized for Ti and Nb targets, respectively.The coated samples were postannealed at temperatures ranging from 473 to 773 K for 1 h in ambient air.Glancing incidence X-ray diffraction revealed a polycrystalline phase for the Nb-doped films postannealed at 523 K, in contrast to the undoped one that has to be annealed at 723 K, indicating that Nb dopant can enhance the crystallization of amorphous TiO 2 .Furthermore, the as-deposited Nb-doped film postannealed at 673 K was found to have an anatase-dominated phase with a fine-grain microstructure observed by transmission electron microscopy.Heat-treatment also induces a change in the surface morphology of the TiO 2 films examined by field-emission scanning electron microscopy.The optical properties of the TiO 2 films were characterized by UV/visible spectrophotometry.The average transmittance of the films is higher than 85%, and a small absorbance zone occurs in the visible region.Under visible light irradiation, all the Nb-doped TiO 2 films exhibit better photocatalytic activity than that of undoped ones.Among them, the Nb-doped TiO 2 film annealed at 673 K shows the best photocatalytic performance.