Photocatalytic reactivity of {121} and {211} facets of brookite TiO2 crystals

Abstract

For the facet engineering of brookite TiO₂, the surface atomic structure is known but the electronic structure has been rarely studied to date. Herein, we investigated both the surface atomic and electronic structure of brookite TiO₂ with various facets exposed. Theoretical calculations reveal that the {121} surface contains more undercoordinated Ti atoms and a higher surface energy than that of the {211} surface, and the experimental results show that brookite TiO₂ nanorods exposed with majority {121} facets (T₁₂₁) have a lower valence band (VB) potential; those above-mentioned superior properties enable T₁₂₁ to show excellent performance in RhB photodegradation. Nevertheless, the electronic structure analyzed from the Density of State (DOS) plots revealed that the electron density is dispersed in the bulk for TiO₂ covered with a {121} surface, indicating that the electrons might be more reluctant to migrate from bulk to surface, which might be the reason for the poor H₂ productivity of T₁₂₁. In contrast, brookite TiO₂ nanosheets exposed with dominant {211} facets (T₂₁₁) exhibited a much higher conduction band (CB) potential resulting in a much higher H₂ evolution rate (801 μmol h⁻¹) in photocatalytic water splitting. Accordingly, combining the analyses of the surface atomic structure and electronic band structure, it is suggested that, for brookite TiO₂, the {121} surface is beneficial for photocatalytic oxidation reactions while the {211} surface can facilitate the photocatalytic reduction process.