Unraveling the origin of facet-dependent photocatalytic H2O2 production over anatase TiO2

Abstract

A typical photocatalytic H2O2 production reaction primarily comprises three processes: photoabsorption, electron–hole separation and transfer, and surface oxygen reduction reaction. Previous researches have focused on revealing the origin of the facet effect in photocatalysis. However, a comprehensive understanding of the structure–activity relationship for facet effect at atomic scale remains limited. Herein, we provide fundamental insights into photocatalytic H2O2 production performance of three types of TiO2 with predominantly exposed (101), (100), or (001) facets. The TiO2 nanosheets with exposed (001) facet exhibit the highest H2O2 yield rate of 649.2 μmol g-1 h-1, which is 29.2 and 5.2 times higher than those of TiO2 with (101) facet and (100) facet, respectively. Band alignment, charge carrier behavior, and surface reaction are studied systematically and correlated with unique surface atom arrangement. Even with narrowed light-response range, short charge carrier lifetime, inferior charge separation, and inconspicuous specific surface area, the (001) facet exposed TiO2 still performs best. The experimental results and density functional theory (DFT) calculations reveal that the under-coordinated Ti atoms on (001) surface serve as active sites and have strong interaction with oxygen molecule, while the Ti atoms on (101) and (100) surface can hardly adsorb and activate oxygen molecule. Hence, we suggest that the significantly lowered barrier for oxygen molecule adsorption and activation results from intrinsic surface sites rather than band-alignment, and the charge carrier behavior plays the principal role in this case. This work may provide semiquantitative insights into the origin of facet effect in photocatalysis.