(Invited) The Effect of Anion Adsorption on Surface-Alloying during Underpotential Deposition

Abstract

The spontaneous surface alloying during deposition is an unexplored route for developing new materials with a great potential for applications in catalysis, electronics, sensing and photonics. Although the surface-stress driven alloying has been theoretically predicted and explored on a fundamental level in the Ultra High Vacuum (UHV)1, very little of that knowledge has been employed in practice.The electrochemically controlled surface-alloying has not been understood well, although it was reported in many systems. The formation of UPD monolayer in the presence of specifically adsorbing ions is often accompanied by competitive adsorption, changes of the kinetics and energetics of nucleation and growth, and the co-adsorption that could stabilise new stages and phases of growth2,3. The effect of anion adsorption on surface alloying kinetics and energetics can significantly affect alloys’ properties and their (electro)chemical reactivity.Here in this work, we focused on systems, Pb UPD on Au(111) and Pb UPD on Cu(100), in which surface alloying has been reported in the past4-8. Pb UPD on Au(111) in perchlorate solution has been a well-studied system by a range of surface techniques4,5. However, very little is known about the effect of halide adsorption in this system. In the perchlorate solutions, in-situ surface stress relaxation4,5 and the in-situ STM4 surface changes unambiguously demonstrated changes associated with the surface alloying. Pb UPD on Cu(100) is another system where the formation of Pb3Cu4 ordered surface alloy is reported and shown to self-assemble in the nanoribbons pattern in the presence of chloride6-8. The studies on those UPD systems aimed to understand the effect of strong halides adsorption (chloride and bromide) on the electrochemical behaviour and surface alloying. Both chloride and bromide form well-ordered anion adlayers on Au and Cu while they differ in the strength of their interaction with the substrate and UPD metal layer. The studies included the electrochemical methods such as the polarisation at fixed sub-ML coverages of Pb over an extended period and following the changes in electrochemical behaviour. In addition to that, we investigated some of the structural and compositional changes by surface techniques such as X-ray Photoelectron Spectroscopy (XPS), Ultraviolet Photoelectron Spectroscopy (UPS) Energy-Filtered Photoemission Electron Microscopy (EF-PEEM) and Work Function (WF) mapping9.