Photothermal Boosting of Photocatalytic Hydrogen Evolution Induced by Defects and Cocatalysts on TiO2

In the development of photocatalytic materials, electronic defect states in semiconductor photocatalysts function as recombination centers and are thus considered to have an adverse effect on the photocatalytic activity. Meanwhile, surface modification with cocatalysts such as Pt nanoparticles is indispensable for reducing the surface reaction overpotential for the hydrogen evolution reaction. The optimal amounts of cocatalysts have been considered to be governed by the balance between the cocatalyst-induced acceleration of the surface reaction kinetics and the modifier-induced light-shielding effect. However, electronic defect states and metal nanoparticles absorb light at wavelengths longer than the absorption edge of photocatalysts and should thus exhibit photothermal effects that increase the temperature in the immediate vicinity of the active sites. In this study, we elucidate the photothermal effects of defect states and cocatalyst nanoparticles on the acceleration of the photocatalytic hydrogen evolution reaction using reduced TiO2 particles modified with Pt cocatalysts as a model. Visible-light absorption by the defect states and cocatalysts does not generate active charge carriers for photocatalysis but contributes to the acceleration of photocatalysis induced by the band gap photoexcitation of TiO2. The present study provides new insights for considering photothermal effects when designing photocatalysts intended for future artificial photosynthetic systems that operate under solar radiation.