Effect of Photoexcited Electron Dynamics on Photocatalytic Efficiency of Bismuth Tungstate

摘要

Photoexcited carrier dynamics of bismuth tungstate (Bi2WO6) photocatalysts was investigated by time-resolved infrared (IR) absorption spectroscopy. Monotonic absorption at the mid-IR region, which is attributable to absorption by photoexcited electrons, was monitored as a function of time delay from the microsecond to millisecond range after photoexcitation. Bi2WO6 particles with different crystalline content were prepared by hydrothermal reaction at several temperatures and used to elucidate the relation between density of photoexcited carriers and steadystate photocatalytic efficiency. Photocatalytic efficiency was tested using two reactions: oxidative decomposition of acetic acid in an aqueous solution (reaction 1) and oxidative decomposition of acetaldehyde in air (reaction 2). Crystallization of Bi2WO6 particles suppressed the fast recombination of photoexcited electrons and holes within 1 μs. In the case of crystallized particles, the density of the photoexcited electron increased with an increase in the crystalline content, and the photocatalytic efficiency for reaction 1 strongly depended on the crystalline content, indicating that photoexcited electrons remaining in the submillisecond time range significantly affect the reaction rate. On the other hand, photocatalytic efficiency for reaction 2 showed a proportional relation with specific surface area rather than crystalline content. The difference in a decisive factor depending on reaction condition is considered to be the slower rate of reaction of photoexcited electrons with molecular oxygen, which might occur within a time range between 200 μs and 3 ms over Bi2WO6.