Novel one-step fabrication of highly ordered Mo-doped TiO₂ nanotubes arrays with enhanced visible light catalytic activity

Doping with transition metals has been established as a promising approach for extending the absorption spectrum of TiO₂ to the visible light region for enhancing its photocatalytic (PC) activity under visible light illumination. In this work, highly ordered Mo-doped TiO₂ nanotubes arrays (Mo-TNTs) were fabricated by a facile in situ anodization of Ti in ethylene glycol/fluoride electrolyte using sodium molybdate as the molybdenum source. The doping levels (0.13–1.51 at.%) of Mo were conveniently adjusted by varying the initial concentrations of sodium molybdate and ammonium fluoride in the electrolyte. Various characterizations were performed to investigate the morphologies, crystal phases, elements’ valence states of the as-prepared samples. Results indicated that Mo⁶⁺ ions were successfully introduced into the lattice of anatase TiO₂, forming isostructural substitution of Ti⁴⁺. Compared with undoped TNTs, the Mo-TNTs exhibited the obvious extension of absorption spectra to the visible light region, higher photoelectric conversion efficiency and lower recombination of photogenerated carriers. Meanwhile, their photoelectrochemical properties were also significantly affected by the doping level of Mo. Of all samples, 6Mo-TNTs (0.82 at.% Mo) showed the best photoelectrochemical performance and highest PC activity under visible light. This was mainly attributed to the synergetic contributions of the enhanced visible light absorption and suppressed recombination.