Unraveling the Water Oxidation Mechanism on a Stoichiometric and Reduced Rutile TiO₂ (100) Surface Using First-Principles Calculations

Photocatalytic water splitting provides a direct route to produce hydrogen and oxygen through the use of light and a water-splitting photocatalyst. While the photo-oxidation of water on rutile (110) surfaces has been extensively explored, only 64% of the surface of most rutile nanocrystals are (110). However, there have been only limited reports of the experimental and first-principles reactivity of the (100) and (101) surfaces, which make up the majority of the remaining surfaces. Herein, we report a systematic study of water oxidation on the rutile (100) surface under experimental conditions, including oxygen vacancies, surface adsorbates at pH = 0, and applied potential performed using density functional theory. Water oxidation mechanisms on oxygen-covered and reduced surfaces are characterized by their overpotential and rate-limiting steps. Maximal stability and activity were found for fully covered (100) surfaces and those with oxygen vacancies in the first two sublayers of the slab. The lowest overpotential for oxygen evolution was ∼0.86 V for a reduced rutile (100) surface with a vacancy in the second subsurface oxygen layer. The oxygen covered surface had an overpotential of ∼1.36 V. In both, the rate-limiting step was the transfer of a proton from a surface adsorbed OH to the electrolyte.