Water Oxidation on TiO₂: A Comparative DFT Study of 1e^-, 2e^-, and 4e^- Processes on Rutile, Anatase, and Brookite

Experimental studies of the surface reactions of photocatalyzed or photoelectrocatalyzed water oxidation on rutile, anatase, and brookite TiO₂ show significant differences between the three polymorphs. Yet a fundamental understanding of the differences is still lacking. In this work, we carried out a systematic comparative density functional theory (DFT) investigation of the mechanisms and energetics of water oxidation on rutile TiO₂ (110), anatase TiO₂ (101), and brookite TiO₂ (210) model surfaces. Our results indicate that for all three phases, the most facile mechanism of water oxidation proceeds as a two-electron/proton process toward H₂O₂ formation via surface peroxo O∗ intermediates. The calculated overall overpotentials toward H₂O₂ formation are âˆ0.27, 0.51, and 0.62 V on rutile, anatase, and brookite, respectively. The rate-limiting steps toward H₂O₂ formation are the OH∗ formation step for all three phases. We studied also the effect of pH. pH alters the binding energies of the reaction intermediates and affects the threshold values for the 1-electron, 2-electron, and 4-electron processes but does not affect the selectivity. Overpotentials for the 4-electron O₂ evolution range from 0.8, 1.04, and âˆ1.15 V on rutile, anatase, and brookite, respectively, with the same rate-determining steps as for the 2-electron process. Under photocatalytic conditions of light irradiation corresponding to the redox potential versus NHE of photogenerated holes in the valence band of the materials (âˆ3.0 V for rutile, âˆ3.2 V for anatase, and âˆ3.3 V for brookite), there is enough energy to drive the 4-electron O₂ evolution spontaneously as well. Under these conditions, product selectivity (H₂O₂ vs O₂) may require characterizing the reaction kinetics rather than coming out from the thermodynamic overpotentials.