Adsorption of halide anions at the Pt(111)-solution interfacestudied by in situ surface x-ray scattering

In this paper we present x-ray scattering results of iodide, bromide, and chloride adsorption onto the Pt(111) surface in solution. Iodide forms two commensurate adlayer structures, a ($\sqrt{7}$\ifmmode\times\else\texttimes\fi{}$\sqrt{7}$)R19.1\ifmmode^\circ\else\textdegree\fi{} phase and a hexagonal (3\ifmmode\times\else\texttimes\fi{}3) phase, which coexist on the Pt surface. Formation of the (3\ifmmode\times\else\texttimes\fi{}3) phase appears to be kinetically limited, whereas the $\sqrt{7}$ phase shows a hysteretic effect as a function of the electrode potential, associated with an order-disorder transition. Bromide forms a series of high-order commensurate structures on Pt(111) that are poorly ordered unless the size of the unit cell is small. No ordered structures for chloride adsorption are observed and specular x-ray reflectivity results suggest that the chloride coverage at low potential is too small to form a close-packed monolayer on the surface. The differences between the structures formed by the adsorbed anions, and the differences between results for Pt(111) and Au(111), are discussed in terms of the strength of the metal-halide interaction.