The vital role of surface Bronsted acid/base sites for the photocatalytic formation of free •OH radicals

Hydroxyl radicals (center dot OH) have been considered as one of the most important active species for many photocatalytic processes. However, the formation mechanism of free center dot OH radicals is not well understood. Herein, we present our experimental evidence that the surface Bronsted acid/base sites play a vital role in the photocatalytic formation of these free center dot OH radicals. In situ diffuse reflectance infrared Fourier transform (DRIFT) spectra show that the surface hydroxyl groups (Bronsted acid/base sites) on TiO2 is responsible for the high formation of free center dot OH radicals. Due to the lack of exposure of Bronsted acid/base sites, BiOCl is unfavorable for the formation of free center dot OH radicals. Inspired by these findings, we further constructed Bronsted acid/base sites by grafting phosphate ions onto the BiOCl surface, which greatly enhanced the formation of free center dot OH radicals. This work highlights the importance of surface Bronsted acid/base sites for the design of photocatalysts.