Charge carrier dynamics and photocatalytic behavior of TiO2 nanopowders submitted to hydrothermal or conventional heat treatment

Abstract

The sol–gel technique followed by conventional (TiO₂-1) and hydrothermal (TiO₂-2) thermal treatment was employed to prepare TiO₂-based photocatalysts with distinct particle sizes and crystalline structures. The as prepared metal oxides were evaluated as photocatalysts for gaseous HCHO degradation, methanol, and dye oxidation reactions. Additionally, metallic platinum was deposited on the TiO₂ surfaces and H₂ evolution measurements were performed. The photocatalytic activities were rationalized in terms of morphologic parameters along with the electron/hole dynamics obtained from transient absorption spectroscopy (TAS). TiO₂-2 exhibits smaller particle size, poorer crystallinity, and higher surface area than TiO₂-1. Moreover the hydrothermal treatment leads to formation of the metastable brookite phase, while TiO₂-1 exhibits only the anatase phase. TAS measurements show that the electron/hole recombination of TiO₂-2 is faster than that of the latter. Despite that, TiO₂-2 exhibits higher photonic efficiencies for photocatalytic oxidation reactions, which is attributed to its larger surface area that compensates for the decrease of the surface charge carrier concentration. For H₂ evolution, it was found that the surface area has only a minor effect and the photocatalyst performance is controlled by the efficiency of the electron transfer to the platinum islands. This process is facilitated by the higher crystallinity of TiO₂-1, which exhibits higher photonic efficiency for H₂ evolution than that observed for TiO₂-2. The results found here provide new insights into the correlations between thermal treatment conditions and photocatalytic activity and will be useful for the design of high performance photocatalysts.