Superstructures and Order-Disorder Transition of Sulfate Adlayers on Pt(111) in Sulfuric Acid Solution

The surface structure of Pt(111) in a 0.1 M H2SO4 electrolyte was investigated in the potential range of sulfate adsorption with electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry. Two ordered anion structures were observed coexisting in the potential range between 0.49 and 0.79 V (vs RHE): the well-known (root 3 x root 7)-R19.1 degrees superstructure with an anion coverage of 0.20 monolayer and a new, high-density (3 x 1) superstructure with a coverage of 0.33 monolayer. Both superstructures undergo a reversible order-disorder transition at 0.8 V. Simultaneous imaging of the adsorbed ions and of topographic details of the Pt substrate lattice allows us to study the local adsorption geometry of the sulfate. In the (root 3 x root 7)R19.1 degrees, structure the sulfate ions are adsorbed close to depressions in the STM image of the Pt substrate which may be identified with face-centered cubic (fcc) hollow sites. In addition to the sulfate ions, a coadsorbed species, possibly water molecules, is observed in the unit cell of the (root 3 x root 7)R19.1 degrees superstructure. Preliminary potentiodynamic STM data indicate that the transformation of the ordered sulfate adlayer into a disordered structure at 0.8 V is not directly related to adsorption/desorption features in the voltammogram commonly attributed to the adsorption/desorption of OH, and that the sulfate adlayer remains oil the surface for potentials well above the adsorption potentials of OH.