Visible photoresponse of TiO2 nanotubes in comparison to that of nanoparticles and anodic thin film

Abstract

Reports on the defective anodic titanium dioxide (TiO2) that harvests solar energy to perform photochemical reactions generating value-added chemicals and/or energy are scarce. In this study, we synthesize TiO2 with two different morphologies (i.e., nanotube and compact layer) on a Ti substrate in a fluoride- and sulfide-containing electrolyte by using the electrochemical anodization method. The effect of high-temperature annealing on various TiO2 structures and substrates in comparison with commercial TiO2 nanoparticles coated on fluorine-doped tin oxide substrate under different gaseous environments (i.e., Ar, air, and O2) is investigated herein by virtue of crystallinity, optoelectronic-physical properties, and photoelectrochemical performances. Unlike traditional TiO2 nanoparticles, which are only active in ultraviolet illumination and with severe aggregation, the anodic TiO2 nanotubes endow the effect of the oxygen vacancy and the self-doping of F− and S− ions upon annealing under the Ar environment, which is crucial to extending the photoresponse to the visible region. The gray anodic TiO2 obtained through thermal annealing at 450 °C crystallizes in the anatase phase and maintains an intact surface morphology under different gaseous environments. The electron paramagnetic spectra are used to confirm the presence of Ti3+ or oxygen vacancy. The entrapment of the F− and S− ions within TiO2 is determined by X-ray photoelectron spectroscopy and energy-dispersive spectroscopy introducing intrinsic defect states, which can resolve the bottlenecks in materials and encourages new paradigms beyond solar light-induced photocatalysis.