Impacts of oxygen vacancies on the electrocatalytic activity of AuTiO 2 nanocomposites towards oxygen reduction

Nanocomposites based on metal nanoparticles supported on oxide surfaces have been used extensively as effective catalysts for fuel cell electrochemistry. In this study, functional nanocomposites based on gold nanoparticles deposited onto TiO2 colloids were prepared by a simple wet chemistry method, and subject to hydrothermal treatment at a controlled temperature in the presence of ascorbic acid. Transmission electron microscopic measurements showed that the gold nanoparticles (10–30 nm in diameter) were embedded within the TiO2 matrix consisting of colloids of 5–10 nm in diameter and anatase crystalline structures, as evidenced in x-ray diffraction studies. Interestingly, electron paramagnetic resonance measurements showed the formation of oxygen vacancies after hydrothermal treatment and the concentrations of oxygen vacancies increased with the amount of ascorbic acid added. Consistent results were obtained in x-ray photoelectron spectroscopic measurements, which suggested partial charge transfer from gold to oxygen-deficient TiO2. The Au:Ti atomic ratio in the nanocomposites was estimated to be ca. 11% and consistent among the series of samples. Electrochemically, the nanocomposites exhibited apparent electrocatalytic activity towards oxygen reduction reactions in alkaline media, which showed a peak-shaped variation with the concentration of the oxygen vacancies. This was accounted for by the deliberate manipulation of the binding energy of oxygen species onto the nanocomposite surfaces. In addition, the AuTiO2 nanocomposites exhibited markedly enhanced tolerance against methanol crossover, as compared to commercial Pt/C catalysts.