Structure and Mobility of Acetic Acid at the Anatase (101)/Acetonitrile Interface

Acetic acid is one of the simplest molecules containing a carboxylic moiety, a common anchoring groups used to functionalize TiO2-based devices. The behavior of acetic acid in proximity of the anatase (101) surface has been investigated by means of first-principles density functional theory (DFT) calculations, including explicit liquid solvent in the simulations. A novel acetic acid binding mode, characterized by proton insertion below the first layer of oxide atoms, has been found employing a sufficiently thick anatase slab model. Hybrid DFT calculations show that the subsurface proton insertion leads to a trap-state for excess electrons, favoring localization below the surface edge. Proton deintercalation represents the largest barrier for acid mobility and desorption. However, if the proton is adsorbed on top of the surface, the acid molecule can partially detach from the surface and easily move toward a thermodynamically more stable state. A series of consecutive changes in the adsorption mode can lead to long-range diffusion of the molecule along the [010] direction of the surface, with a barrier of only E-act = 20.3 kJ/mol. Similarly, the free energy barrier to completely detach an acetic acid molecule from the surface into the solvent has been computed to be E-act = 51.0 kJ/mol if the proton is adsorbed on top of the anatase slab. Where significant, a comparison between the "explicit liquid" environment and the more often employed "solvent monolayer" environment has been carried out, highlighting the importance of solvent interactions.