Mo doped TiO2: impact on oxygen vacancies, anatase phase stability and photocatalytic activity

Abstract

This work outlines an experimental and theoretical investigation of the effect of molybdenum (Mo) doping on the oxygen vacancy formation and photocatalytic activity of TiO₂. Analytical techniques such as x-ray diffraction (XRD), Raman, x-ray photoelectron spectroscopy (XPS) and photoluminescence (PL) were used to probe the anatase to rutile transition (ART), surface features and optical characteristics of Mo doped TiO₂ (Mo–TiO₂). XRD results showed that the ART was effectively impeded by 2 mol% Mo doping up to 750 °C, producing 67% anatase and 33% rutile. Moreover, the crystal growth of TiO₂ was affected by Mo doping via its interaction with oxygen vacancies and the Ti–O bond. The formation of Ti–O–Mo and Mo–Ti–O bonds were confirmed by XPS results. Phonon confinement, lattice strain and non-stoichiometric defects were validated through the Raman analysis. DFT results showed that, after substitutional doping of Mo at a Ti site in anatase, the Mo oxidation state is Mo⁶⁺ and empty Mo-s states emerge at the titania conduction band minimum. The empty Mo-d states overlap the anatase conduction band in the DOS plot. A large energy cost, comparable to that computed for pristine anatase, is required to reduce Mo–TiO₂ through oxygen vacancy formation. Mo⁵⁺ and Ti³⁺ are present after the oxygen vacancy formation and occupied states due to these reduced cations emerge in the energy gap of the titania host. PL studies revealed that the electron–hole recombination process in Mo–TiO₂ was exceptionally lower than that of TiO₂ anatase and rutile. This was ascribed to introduction of 5s gap states below the CB of TiO₂ by the Mo dopant. Moreover, the photo-generated charge carriers could easily be trapped and localised on the TiO₂ surface by Mo⁶⁺ and Mo⁵⁺ ions to improve the photocatalytic activity.