Electrochimica Acta, Vol.111, 642-655, 2013
Formation of zinc-oxianion complex adlayer by underpotential deposition of Zn on Au(111) electrode: Preferential formation of zinc monohydrogen phosphate complex in weakly acidic solutions
The underpotential deposition (upd) of zinc was studied on Au(1 1 1) electrode in weakly acidic solutions by voltammetry. The difference of anion co-adsorption strength was investigated on upd Zn in the solutions containing phosphate, sulfate, perchlorate, and halides. The order of anion co-adsorption strength was found to be phosphate > sulfate, phosphate >> perchlorate, and phosphate >> halides. We present the electrochemical evidence that no co-adsorption of halide progresses on upd Zn at E> -0.7V (vs. SCE) in spite of the relatively high adsorbability of halide on Au(1 1 1). In 0.1 M KH2PO4 (pH =4.4), the amount of charge density of the upd Zn stripping corresponded to that of 1/3 monolayer of the Zn adlayer formed as a result of mono-electron transfer between -0.35 V and -0.6 V. The coverage was coincident with that inferred from the (root 3 x root 3)R30 degrees STM image reported by us in the same condition. The formation of zinc-oxianion (phosphate and sulfate) surface complexes by Zn upd was proposed base on the difference of complex formation constants among the related zinc-anion complexes. Identification of the zinc-phosphate surface complex was thermodynamically carried out around pH=4 with a Nernst equation, where the shifts of Zn upd peak potential were investigated with the concentration changes of Zn2+, H2PO4-, and H+ under an imaginarily reversible condition. The formation of ZnHPO4 surface complex was concluded in the Zn upd with mono-electron transfer and explained a recent XAS analysis, which demonstrate each phosphate anion bridging three upd Zn atoms. (C) 2013 Elsevier Ltd. All rights reserved.