Insights into Photocatalytic Reduction Activities of Different Well-defined Single Bulk Crystal TiO2 Surfaces in Liquid

The photocatalytic performance of titanium dioxide (TiO 2 ) is widely recognized to depend on its crystal phases and surface structures. The reactivity of well-defined, bulk single-crystal surfaces has been studied in the gas phase and in aqueous solutions to elucidate their relationship, yet most studies have focused on oxidative reactions. This work provides a systematic facet-resolved investigation of reductive photocatalytic reactivity on four well-defined TiO 2 single-crystal surfaces-anatase (001), anatase (101), rutile (001), and rutile (110)-under aqueous-phase conditions using resazurin-toresorufin conversion as a probe reaction. By comparing polished and unpolished surfaces, intrinsic facet effects were decoupled from surface roughness-induced activation. Interestingly, rutile consistently outperforms anatase across both surface types: polished with a root-mean-square (RMS) roughness of less than 1 nm and unpolished with an RMS roughness in the tens of nm range. Anatase remains largely inactive on polished facets, including both the highly stable anatase (101) facet and the less stable anatase (001) facet. Unpolished surfaces show enhanced activity across all tested rutile and anatase facets, with defects and roughness enabling measurable reactivity on anatase. These results challenge the conventional view of anatase as the superior polymorph. Systematic comparison of bulk crystals with well-defined sample conditions provides a clearer view of the intrinsic reactivity of each phase and facet. The results indicate that surface-specific parameters, including defectmediated molecule adsorption and charge transfer, dominate photocatalytic performance,