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 similar to 0.27, 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 O-2 evolution range from 0.8, 1.04, and similar to 1.15 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 (similar to 3.0 V for rutile, similar to 3.2 V for anatase, and similar to 3.3 V for brookite), there is enough energy to drive the 4-electron O-2 evolution spontaneously as well. Under these conditions, product selectivity (H2O2 vs O-2) may require characterizing the reaction kinetics rather than coming out from the thermodynamic overpotentials.