Photo-sensitive 2D Arrangement of -OH/H2O on Brookite TiO2(210)

The brookite phase of TiO2 is much less explored than the other two polymorphs, rutile and anatase, despite its potential applications in photo-catalytic CO2 reduction and water splitting. The first hydration layer and surface hydroxyl groups on the brookite (210) surface and their structural changes under photo-irradiation have been considered to play significant roles in such applications. Hence, in this work, we focus on studying them at the atomic scale using a combination of liquid-environment frequency modulation atomic force microscopy (FM-AFM) and density functional theory (DFT) calculations. The striped feature found in AFM images and its photo-switching behavior accompanied by photo-increased surface hydrophilicity are revealed by ab initio molecular dynamics simulations to originate from photo-sensitive two-dimensional arrangements of molecular and dissociative H2O and proton-hopping behavior. The formation mechanism of the arrangements is further clarified by DFT static and nudged-elastic-band calculations.