Visible-light-activated oxygen-rich TiO₂ as next generation photocatalyst: importance of annealing temperature on the photoactivity toward reduction of carbon dioxide

Herein, visible-light-active oxygen-rich TiO₂ (O₂-TiO₂) photocatalysts were synthesized via a facile and dopant-free strategy using different annealing temperatures ranging from 200 to 800°C. The annealing treatment had important effects on the anatase-rutile phase composition, textural property and photocatalytic performance of O₂-TiO₂. The photoactivity of the samples were evaluated through the photocatalytic reduction of CO₂ under the irradiation of low-powered energy-saving daylight bulbs. O₂-TiO₂ calcined at an optimum temperature of 300°C (O₂-TiO₂-300) displayed the highest photocatalytic performance, achieving an enhancement factor of 12.2 and 11.2 over pure anatase TiO₂ and the commercial Degussa P25, respectively. Although the improvement in photoactivity could be attributed to the combined effect of several factors such as enhanced optical properties and high surface area of O₂-TiO₂-300, the findings from this work provided evidence that the optimum anatase-rutile phase composition in the sample played a dominant role in the enhancement of photocatalytic performance. Based on the experimental results obtained, a plausible mechanism for the charge transfer and separation process in O₂-TiO₂ was also proposed. The present work could provide new insights into the architecture of oxygen-modified TiO₂ photocatalysts with markedly enhanced photocatalytic activities in energy- and environmental-related applications.