Electrochemical and in situ STM studies of anomalous phosphate adsorption induced on Zn UPD at Au( [formula omitted]) in the presence of halide ions in aqueous phosphate solutions

Abstract

Underpotential deposition (UPD) of Zn 2+ at Au(1 1 1) was studied in the phosphate solutions with and without halide ions (Cl −, Br −, and I −) by cyclic voltammetry and in situ STM for the evaluation of the anion adsorption induced on the Zn UPD. The order of the strength of anion adsorption induced on Zn UPD was found to be different from the order of specific adsorption strength of I −>Br −>Cl −>PO 4 3−⩾SO 4 2− at an Au substrate electrode. This anomalous tendency of anion adsorption induced on UPD Zn at Au seems to be identical with the cases of sulfate and phosphate adsorption induced on UPD Zn at a Pt electrode in the presence of chloride, being confirmed by radiotracer method. Such anomalous phosphate adsorption induced on Zn UPD was found to take place at Au(1 1 1) not only in the presence of chloride or bromide, but also even in the presence of iodide in the phosphate solution. The in situ STM images of Zn UPD were observed at Au(1 1 1) in the phosphate solution with iodide ions. At −200 mV (saturated calomel electrode; SCE) where the iodide adsorbs at substrate Au(1 1 1), the STM image was c(p× 3 R30°) , and in the Zn UPD region, the STM image became ( 3 × 3 )R30° . The latter image is discussed to be assigned to the image in the absence of iodide, i.e., that the adsorbed anions induced at Zn UPD are phosphate species even in the solution containing iodide ions.