Adlattice structure and hydrophobicity of Pt (111) in aqueous potassium iodide solutions: Influence of pH and electrode potential

Abstract

Measurements by means of Auger spectroscopy and LEED of the composition and structure of Pt (111) surfaces after immersion into aqueous iodide solutions are reported as a function of pH and electrode potential. Voltammetric current for reductive desorption of halogen was observed near −0.3 V (vs. Ag/AgCl reference). Comparison of the coulometric charge for halogen reductive desorption with the original packing density of iodine determined by Auger spectroscopy revealed that a one-electron reduction of the halogen had occurred: I(adsorbed) + e − → I −. The packing density of I atoms was a function of electrode potential, declining at the negative extreme of potential due to the reductive elimination process and also declining at the positive extreme of potential due to oxidation of the adsorbed halogen atoms and of the Pt surface. On the basis of LEED data, the halogen layer was found to be ordered in registry with the Pt (111) surface. The halogen adlattice structure was potential dependent: a (3 × 3) adlattice (Θ I = 4/9) at relatively positive potentials; a ▪ adlattice (Θ 1 = 3/7) at potentials in mid-range; a ▪ adlattice (Θ I = 1/3) at relatively negative potentials; and a virtually halogen-free surface at potentials approaching the negative limit. The pH dependence of iodine adsorption was relatively slight, evidently because strong adsorption of halogen suppressed adsorption of OH and related species. The Pt (111)(3 × 3)-I and Pt (111) ▪ surfaces were remarkably hydrophobic, while the Pt ▪ surface was distinctly hydrophilic. Ionic species were not retained to a measurable degree at the hydrophobic surface, although normal retention of solution films and ions occurred at the surface in its hydrophilic states.