Adsorption of Zn2+ on the (110) Surface of TiO2 (Rutile): A Density Functional Molecular Dynamics Study

Adsorption of Zn2+ at the rutile TiO2 (110)-aqueous interface was studied with Born-Oppenheimer molecular dynamics at 300 K. Simulations were carried out using the periodically repeated slab model with vacuum gap of 15 angstrom filled with 72 H2O molecules. Two possible adsorption sites, monodentate above bridging oxygen (Ti-O-Ti or O-br) and bidentate above terminal oxygens (Ti-O), were investigated. Sites with different coordination environment for adsorbed Zn2+ differ from each other by the position of Zn2+ above surface level and by characteristic Zn-O distances. Obtained results gave evidence that 4-fold coordination of adsorbed Zn2+ is more probable than the 6-fold coordination found for aqueous species. The hydrolysis of H2O molecules was observed in the first coordination shell of adsorbed ion, resulting in formation of OH- groups attached to Zn2+. Calculated energies favor the tetrahedral bidentate structure of hydrated Zn2+ on the rutile surface. The model structures are compared to observed positions of Zn2+ above the ruffle (110) surface using X-ray scattering techniques.