X-ray absorption spectroscopy characterization of Zn underpotential deposition on Au(1 1 1) from phosphate supporting electrolyte

Zn K-edge X-ray absorption spectroscopy (XAS) has been used to investigate the structure of Zn monolayers prepared on Au(1 1 1) electrodes via underpotential deposition (UPD) from phosphate supporting electrolyte. Theoretical modeling of the XAS data indicates that the Zn adatoms adopt a commensurate (√3 x √3)R30o (θsc = 0.33) adlayer structure and reside within the 3-fold hollow sites of the Au(1 1 1) surface. Meanwhile, phosphate counter-ions co-adsorb on the UPD adlayer and bridge between the Zn adatoms in a (√3 x √3)R30o (θsc = 0.33) configuration, with each phosphorous atom residing above a vacant 3-fold hollow site of the Au(1 1 1). Significantly, this surface structure is invariant between the electrochemical potential for UPD adlayer formation and the onset of bulk Zn electrodeposition. Analysis of the Zn K-edge absorption onset also presents the possibility that the Zn adatoms do not fully discharge during the process of UPD, which had been proposed in prior voltammetric studies of the phosphate/Zn(UPD)/Au(1 1 1) system.