A Quantitative Study on the Relationship of Specific Surface Area and Carrier Lifetime to Photocatalytic Activity of Anatase TiO<sub>2</sub> Nanoparticles
Shimpei Naniwa, Kosaku Kato, Akira Yamakata, Akira Yamamoto, Hisao Yoshida
Abstract
Heterogeneous photocatalysis is a promising technology for green and sustainable chemical processes. Understanding the relationship between the structure, properties, and activity of photocatalysts is a key to improving the design of photocatalysis. Although the specific surface area ( S BET ) of photocatalysts and the lifetime of photoexcited carriers (τ) are known as determining factors of the photocatalytic activity, their quantitative relationship with the activity remains unclear. Here we show that the relationship of S BET and τ with the photocatalytic activity can be given by a simple rate equation derived from a general kinetic model of photocatalysis. We examined activities of 21 anatase TiO 2 photocatalysts with various S BET and τ values in the direct homocoupling of benzene by means of time-resolved infrared absorption spectroscopy (TRIR) with microsecond time resolution. We found that the activity of most of the photocatalysts increased with the increase of S BET from 9.0 to 265.9 m 2 g –1, while some of them showed different activity despite their similar S BET values. Carrier lifetimes (τ 1, τ 2, and τ 3 ) estimated by triple-exponential fit of decay curves in TRIR were also different when the photocatalysts have similar S BET values. The photocatalysts also showed different activity despite their similar τ values, suggesting that both S BET and τ affect the photocatalytic activity. The activities of the photocatalysts were explained by a rate equation derived from a simple kinetic model that considers three primary processes in photocatalysis. In the equation, relations of S BET and τ to the photocatalytic activity are equivalent in a mathematical sense, proposing the product of S BET and τ ( S BET τ) as a determining factor of the photocatalytic activity. The present study demonstrates that a statistical approach based on a kinetic model is effective to understand a quantitative structure–property–activity relationship in photocatalysis.