Photoelectroactivity and Raman spectroscopy of anodized titania (TiO2) photoactive water-splitting catalysts as a function of oxygen-annealing temperature

Abstract

The photoelectrochemical (PEC) activity of untreated (not anodized) and anodized nanotubular TiO₂ films (synthesized by the electrochemical anodization of Ti foil) was correlated with the phase composition of the film as a function of O₂-annealing temperature at 400, 500 and 600 °C. TiO₂ nanotubes have been shown to be more efficient than polycrystalline TiO₂ for the photocatalytic splitting of water. Raman spectroscopy was used to identify the amorphous and crystalline TiO₂ phases as well as the carbon species. The amorphous TiO₂ nanotubular array (unheated) exhibits a Raman spectrum consistent with TiO₆⁸⁻ octahedra having the same average structure as those present in the anatase and rutile phases of TiO₂. Ratios of integrated Raman peaks were used as a semi-quantitative measure of the degree of crystallinity for rutile and the rutile/anatase weight ratio in the films. Results show that the anatase-to-rutile transformation on Ti metal initiates at much lower temperatures compared to polycrystalline TiO₂ and this is attributed to oxygen vacancies located at the metal/oxide interface. For untreated films, the amorphous TiO₂ crystallizes directly to rutile, and the photocurrent density increases almost linearly with rutile crystallinity as the O₂-annealing temperature is increased; anatase does not form on untreated O₂-annealed Ti foil. By comparison, amorphous TiO₂ nanotubular arrays are converted to about three times as much anatase as rutile at 400 °C, where the photocurrent density is only slightly greater than the corresponding untreated film. At 500 °C, however, the photocurrent density increases to 2.3× that of the untreated-oxidized film, where ∼83% of the TiO₂ nanotubular film is rutile and ∼17% is anatase; this enhancement is attributed to the increase in surface area and photoactive sites of the rutile provided by the TiO₂ nanotubular array architecture acting as a support. At 600 °C the rutile transformation continues (∼92% rutile), but this is countered by the significant loss of surface area and surface photoactive sites due to degradation and collapse of the nanotubular structure as seen by SEM.