title: Coordination structure of adsorbed Zn(II) at water-TiO2 interfaces.
abstract: The local structure of aqueous metal ions on solid surfaces is central to understanding many chemical and biological processes in soil and aquatic environments. Here, the local coordination structure of hydrated Zn(II) at water-TiO(2) interfaces was identified by extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) spectroscopy combined with density functional theory (DFT) calculations. A nonintegral coordination number of average ∼4.5 O atoms around a central Zn atom was obtained by EXAFS analysis. DFT calculations indicated that this coordination structure was consistent with the mixture of 4-coordinated bidentate binuclear (BB) and 5-coordinated bidentate mononuclear (BM) metastable equilibrium adsorption (MEA) states. The BB complex has 4-coordinated Zn, while the monodentate mononuclear (MM) complex has 6-coordinated Zn, and a 5-coordinated adsorbed Zn was found in the BM adsorption mode. DFT calculated energies showed that the lower-coordinated BB and BM modes were thermodynamically more favorable than the higher-coordinated MM MEA state. The experimentally observed XANES fingerprinting provided additional direct spectral evidence of 4- and 5-coordinated Zn-O modes. The overall spectral and computational evidence indicated that Zn(II) can occur in 4-, 5-, and 6-oxygen coordinated sites in different MEA states due to steric hindrance effects, and the coexistence of different MEA states formed the multiple coordination environments.
authors.affiliation: State Key Laboratory of Environmental Aquatic Chemistry, Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences, Beijing 100085, China.
authors.forename: Guangzhi
authors.initials: G
authors.lastname: He
authors.forename: Gang
authors.initials: G
authors.lastname: Pan
authors.forename: Meiyi
authors.initials: M
authors.lastname: Zhang
authors.forename: Glenn A
authors.initials: GA
authors.lastname: Waychunas
mesh_terms: D000327:Adsorption; D008956:Models, Chemical; D015394:Molecular Structure; D014025:Titanium / Q000737:chemistry*; D056928:X-Ray Absorption Spectroscopy; D015032:Zinc / Q000737:chemistry*
chemical_list: C009495:titanium dioxide; D014025:Titanium; D015032:Zinc