Structural, magnetic and optical properties of Ni-doped TiO2 thin films deposited on silicon(100) substrates by sol–gel process

•Ni doping decreases the activation energy for the anatase-to-rutile transformation.•With increasing Ni content, the reversible ferromagnetism is found.•Sharp decrease of the EOBG at low Ni content is mainly related to phase composition. Ni-doped TiO2 films were deposited on Si(100) substrates by sol–gel technique. X-ray photoelectron spectroscopy analysis suggests that the valence of Ni ion is in +2 and oxygen vacancies increase with increasing Ni content. X-ray diffraction measurements indicate that Ni doping catalyzes the anatase-to-rutile transformation (ART) of TiO2 films, which is due to the decrease of the ART activation energy. The reversible ferromagnetism of the samples with Ni fraction is found, which is due to an anatase-to-rutile junction destroying an F-center bound magnetic polaron. Optical properties of Ni-doped TiO2 films were studied by the ellipsometric spectra. With increasing Ni content, the optical band gap of TiO2 films is decreased from 3.56 to 3.34eV, which may be related to phase composition and impurity band.