title: In situ study of water-induced segregation of bromide in bromide-doped sodium chloride by scanning polarization force microscopy.
abstract: The adsorption of water on Br-doped NaCl crystals has been studied in situ using scanning polarization force microscopy, a noncontact electrostatic atomic force microscopy operation mode. Both topography and contact potential images were acquired as a function of relative humidity at room temperature, from 0% to more than 55%. It was found that the surface of the freshly cleaved crystal has an inhomogeneous electrical surface potential distribution with the steps more negative than the terraces below 40% relative humidity. This difference disappears when the humidity reaches 40% and higher. Below 40% the step morphology experiences only small changes due to water adsorption; however, above 40% major changes take place due to solvation, segregation, and redistribution of lattice ions. Bromide-rich islands and crystallites segregate to the surface above 40% relative humidity followed by drying. These islands and crystallites have a negative surface potential relative to the rest of the surface. These effects are attributed to the preferential solvation and segregation of Br- ions.
authors.affiliation: Department of Chemistry, University of California, Irvine, California 92697, USA.
authors.forename: Sutapa
authors.initials: S
authors.lastname: Ghosal
authors.forename: Albert
authors.initials: A
authors.lastname: Verdaguer
authors.forename: John C
authors.initials: JC
authors.lastname: Hemminger
authors.forename: Miquel
authors.initials: M
authors.lastname: Salmeron