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 °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 °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 °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.

► Incorporation of vanadium in TiO2 thin film reduces its optical band gap.
► Amorphous V-doped TiO2 and TiO2-V2O5 composite films were air-annealed up to 550ºC.
► Annealing of the films up to 350 °C leads to slight increase of the band gap.