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

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.