Deciphering the Anomalous Acidic Tendency of Terminal Water at Rutile(110)–Water Interfaces

Understanding the mechanism of the oxygen evolution reaction (OER) is essential to improve the efficiency of photocatalysis for TiO₂. Previous studies have highlighted the importance of terminal hydroxide radical (TiOH•) in the OER. Ab initio molecular dynamics simulations (AIMD) with hybrid functional have revealed that this radical readily loses its proton, creating the key intermediate, oxygen radical anion (Ti₅cO•⁻). Herein, we combine machine-learning-accelerated molecular dynamics with density functional theory calculations to demonstrate that the Ti₅cO•⁻ can alternatively be generated through the trapping of a hole in a terminal oxygen anion (Ti₅cO²⁻) at rutile(110)–water interfaces. Further examination reveals that the Ti₅cO₂– results from the deprotonation of Ti₅cOH⁻ and remains stable at the charge-neutral interfaces for a transient time period of ca. 100 ps. The AIMD-based free energy perturbation method predicts that the acidity constant of Ti₅cOH⁻ is even smaller than that of Ti₅cOH₂, thereby rationalizing the stability of Ti₅cO²⁻. Structural analyses show that this anomalous acidic tendency of terminal water originates from the decrease of Ti–O bond length and the transition of Titanium`s coordination from octahedral to pyramidal in Ti₅cO²⁻. Our findings provide valuable insights into the surface acid–base chemistry and a potential explanation for the pH-dependent behavior of photogenerated holes for TiO₂.