Tuning the water-splitting mechanism on titanium dioxide surfaces through hydroxylation

Abstract

Experimental research demonstrates that surface hydroxyl groups can boost TiO₂'s ability to split water but the water splitting mechanism and roles of hydroxyl groups are still not clear. The hydroxyl groups formed by H₂O or H₂ cracking on pure TiO₂ surfaces are represented by types I (OH1) and II (OH2), respectively. Six types of hydroxylated TiO₂ surfaces of anatase (101), rutile (110), and brookite (210) with OH1 and OH2 hydroxyl groups were constructed. The mechanism of the water oxidation process on the hydroxylated TiO₂ surfaces was systematically investigated through density functional theory calculations. The variation and significant roles of hydroxyl groups in the mechanism of the oxygen evolution reaction (OER) and product selectivity were discussed. All hydroxylated TiO₂ surfaces eventually tend to produce oxygen through a four-electron/proton process, which is fundamentally different from the OER process on pure Ti₂O surfaces from a thermodynamic standpoint. The lowest surface overpotential of R-110-OH1 is 0.53 V, the highest surface overpotential of B-210-OH2 is 1.49 V, and the surface overpotentials of other hydroxylated TiO₂ are between 0.5 and 1.5 V. Rutile (110) and brookite (210) have hydroxyl groups of the OH1-type that are more conducive to the OER process. This study investigates the mechanism of water splitting on the surface of hydroxylated TiO₂, allowing for a deeper understanding of the function of surface hydroxyl groups in the OER process as well as providing instructions for future research into the development of effective water-splitting catalysts based on hydroxylated TiO₂ surfaces.