In-situ STM study of phosphate adsorption on Cu(111), Au(111) and Cu/Au(111) electrodes

The interaction of Cu(111), Au(111) and Cu-covered Au(111) electrodes with a neutral phosphate buffer solution has been studied by means of cyclic voltammetry (CV) and in situ electrochemical scanning tunneling microscopy (EC-STM). Under low potential conditions, both the Cu(111) and the Au(111) surface appear apparently adsorbate free, indicated by the presence of a (4 x 4) structure and the herringbone surface reconstruction, respectively. Upon potential increase, phosphate anions adsorb on both surfaces and for Cu(111) the formation of a (root 3 x root 3) R30 degrees structure is found, whereas on Au(111) a "(root 3 x root 7)" structure is formed. For a Cu-submonolayer on Au(111), coadsorption of phosphate anions leads to the formation of a (2 x 2) vacancy structure within an assumed pseudomorphic structure of the Cu-submonolayer with the phosphate anions occupying the vacancies. When desorbing the phosphate anions at low potentials, the Cu-submonolayer first becomes mobile and eventually undergoes an irreversible transition to a coalescent non pseudomorphic structure. (C) 2012 Elsevier B.V. All rights reserved.