Synthesis of V-doped TiO2 films by chemical bath deposition and the effect of post-annealing on their properties

Amorphous composite films, composed of a Ti1-xVxO2 solid-solution phase and a V2O5 phase, were produced by chemical bath deposition and subsequently air-annealed at various temperatures up to 550 degrees C. The microstructure and chemical composition of the as-prepared and annealed films were investigated by a combinatorial experimental approach using Scanning electron microscopy, X-ray powder diffraction and X-ray photoelectron spectroscopy. Ultraviolet-Visible Spectrometry was applied to determine the optical band gap of the as-prepared and annealed films. It followed that the incorporation of vanadium in the as-deposited films reduces the optical band gap of TiO2 from about 3.8 eV to 3.2 eV. Annealing of the films up to 350 degrees C leads to slight increase of band gap, as attributed to a reduction of the defect density in the initially amorphous oxide films due to the gradual development of long-range order and a concurrent reduction of the V4+-dopant concentration in the Ti1-xVxO2 solid-solution phase. The films crystallized upon annealing in air at 550 degrees C, which resulted in drastic changes of the phase constitution, optical absorbance and surface morphology. Due to the lower solubility of V4+ in crystalline TiO2, V4+ segregates out of the crystallizing Ti1-xVxO2 solid-solution phase, forming crystalline V2O5 at the film surface. (C) 2012 Elsevier B.V. All rights reserved.