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

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.