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.

Automated summary

The study investigates the structure of carboxylic acid-modified TiO2 surfaces in contact with water, using density functional theory and ab initio molecular dynamics. It was found that isolated formic acid molecules adsorbed on the TiO2 surface remained stable, with protons transferred to form surface bridging hydroxyls. Adsorbed monolayers of formic acid molecules in water resulted in a negatively charged surface with solvated H3O+ ions nearby.