Coverage evolution of sulfur on Pt(111) electrodes: From compressed overlayers to well-defined islands

Abstract

The paper examines formation of a monolayer and submonolayers of S chem on Pt(111) electrodes accomplished through its immersion in aqueous Na 2S solution. The S chem monolayer, having the (1×1) structure at θ S chem =1, can be gradually removed by oxidative desorption at E≥0.95 V, RHE, and the S chem coverage can be precisely controlled. The S chem layer suppresses the H UPD and anion adsorption as well as it affects the oxide growth behavior on Pt(111). LEED data reveal that S chem forms well-defined structures on Pt(111) for 0.50≥ θ S chem ≥0.25: c(2×2) at θ S chem =1/2, ( 3 × 3 )R30° at θ S chem =1/3, and p(2×2) at θ S chem =1/4; when θ S chem ≤0.20, structured islands of S chem are observed. AES and CEELS data indicate that the adsorbed S is not present in an oxidized state; it is almost of atomic character with an incomplete negative charge due to partial charge transfer between S chem and the Pt(111) substrate. Presence of S chem influences thermodynamics of the H UPD on Pt(111) resulting in less-negative values of Δ G ads(S)(H UPD). In the absence of S chem, the H UPD is enthalpy-driven whereas in the presence of S chem it becomes entropy-driven. The Pt(111)–H UPD bond energy is weaker in the presence of S chem than in its absence, and this bond energy diminution may be assigned to local electronic effects arising from presence of S chem.