Effect of Cations on the TiO₂/Acetonitrile Interface Structure: A Molecular Dynamics Study

Abstract

The TiO2/electrolyte interface was investigated as a key element of dye-sensitized solar cells (DSSCs). The influence of cations such as lithium (Li+), 1,2-dimethyl-3-propylimidazolium (DMPIM+), and tetrabutylammonium (TBA+) on the double-layer structure in acetonitrile-based electrolyte at the anatase (101) surface was studied by molecular dynamics (MD) simulation. The calculations were performed for the uncharged surface as well as for the negatively charged one, which imitated TiO2 nanocrystals with excess electron density under light irradiation. It was shown that acetonitrile molecules form a self-assembled monolayer on the anatase (101) surface, inhibiting adsorption/desorption of both cations and anions. The effective dipole moments of acetonitrile molecules in the monolayer are directed away from the surface and produce a potential drop across the interface of ∼1.3 V. The characteristic time for the reversible adsorption/desorption process of Li+ cations occurs in the second time range. The MD simulations show that cationic species have a strong effect on the electrolyte structure both in bulk and at the interface. In particular, a significant influence of cations on the density distribution of I– at the interface was found. This result suggests that the mechanism, by which cations affect the dye regeneration kinetics, consists of the local change of I– density in the region where the −NCS groups of ruthenium dyes (involved in the regeneration process) are located.