DFT-D study of adsorption of diaminoethane and propylamine molecules on anatase (101) TiO2 surface

The adsorption on anatase (101) TiO2 surface of two model amines, diaminoethane (DAE) and propylamine (PPA), was investigated using Density Functional Theory-Dispersion included (DFT-D) calculations. The investigated coverage is ranging from 0.25 monolayer to full coverage (one amine molecule per surface Ti ion). Both interactions of the adsorbed layer with the anatase (101) TiO2 surface and intermolecular interactions are described. A structural transition from a bridge to a perpendicular structure is found for DAE when evolving from 0.25 monolayer to full coverage. At full coverage, a dense, ordered adhesive layer is formed. For DAE, at intermediate coverage, different isoenergetic configurations are found and structural transition from a bridge to a perpendicular structure is found. In contrast, the adsorption mode of PPA is more regular with only perpendicularly adsorbed molecules at all investigated coverages. Dispersion forces already account for 40% of the adsorption energy at low coverage (0.25 ML) and are the driving force for monolayer formation with a contribution of 60% up to 100% at high coverage. As revealed by molecular dynamics, the molecules can change their orientation towards the surface in a concerted way.