Architecture, optical absorption, and photocurrent response of oxygen-deficient mixed-phase titania nanostructures

The optical absorption and photoelectric properties of oxygen-deficient titania (TiO2) nanostructures consisting of anatase nanotubes and rutile film layer were investigated. The nanostructures were prepared by electrochemical anodization followed by long-time annealing at four temperatures – 450, 550, 650 and 750 °C. Various characterization techniques, including X-ray photoelectron spectroscopy depth profiling, revealed that elemental stable zones (structural regions in which the concentrations of O and Ti are stable) formed within the TiO2 nanostructures at high annealing temperatures (650 and 750 °C) have O/Ti atomic ratios significantly less than 2. A direct relationship between oxygen vacancy concentration and annealing temperature was established on the basis of this finding. Measurement of the optical absorption spectra of the TiO2 nanostructures revealed a blue-shift in the absorption edge along with a notable increase in the long-wavelength absorption due to the presence of oxygen vacancies. This observation is in agreement with the first-principles calculations of the absorption coefficients of anatase TinO2n−1 and TinO2n−2 structures, in which the oxygen vacancy concentration can be adjusted by varying the supercell size. The contrary photocurrent responses of the TiO2 nanostructures under ultraviolet and visible light were measured. A strong photocurrent response under filtered visible light (λ > 500 nm) was found for the TiO2 nanostructures annealed at 650 and 750 °C, which suggests that the dominant positive effect of oxygen vacancies exceeds the adverse impact of other features associated with thickening of the rutile film layer at high annealing temperatures, such as a reduction in the specific surface area and an increased charge recombination rate.