Hydrogen Bonds and H₃O⁺Formation at the Water Interface with Formic Acid Covered Anatase TiO₂

Carboxylic acid-modified TiO2 surfaces in aqueous environment are of widespread interest, yet atomic-scale understanding of their structure is limited. We here investigate formic acid (FA) on anatase TiO2 (101) (A-101) in contact with water using density functional theory (DFT) and ab initio molecular dynamics (AIMD). Isolated FA molecules adsorbed in a deprotonated bridging bidentate (BD) form on A-101 are found to remain stable at the interface with water, with the acid proton transferred to a surface oxygen to form a surface bridging hydroxyl (ObrH). With increasing FA coverage, adsorbed monolayers of only BD and successively of alternating monodentate (MD) and BD species give rise to a higher concentration of surface ObrH's. Simulations of these adsorbed monolayers in water environment show that some protons are released from the surface ObrH's to water resulting in a negatively charged surface with nearby solvated H3O+ ions. These results provide insight into the complex acid-base equilibrium between an oxide surface, adsorbates and water and can also help obtain a better understanding of the wetting properties of chemically modified TiO2 surfaces.