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

The paper examines formation of a monolayer and submonolayers of S-chem on Pt(111) electrodes accomplished through its immersion in aqueous Na2S solut ion. The Schem monolayer, having the (1 x 1) structure at theta(Schem) = 1, can be gradually removed by oxidative desorption at E greater than or equa l to 0.95 V, RHE, and the S-chem coverage can be precisely controlled. The Schem 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 w ell-defined structures on Pt(111) for 0.50 greater than or equal to theta(S chem) greater than or equal to 0.25: c(2 x 2) at theta(Schem) = 1/2, (root 3 x root 3)R30 degrees at theta(Schem) = 1/3, and p(2 x 2) at theta(Schem) = 1/4; when theta(Schem) less than or equal to 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 inc omplete negative charge due to partial charge transfer between Schem and th e Pt(111) substrate. Presence of S-chem influences thermodynamics of the H- UPD on Pt(111) resulting in less-negative values of Delta G(ads(S))(H-UPD). In the absence of S-chem, the H-UPD is enthalpy-driven whereas in the pres ence of S-chem it becomes entropy-driven. The Pt(111)-HUPD bond energy is w eaker in the presence of Schem than in its absence, and this bond energy di minution may be assigned to local electronic effects arising from presence of s(chem). (C) 1998 Elsevier Science Ltd. All rights reserved.