The effects of anions on the underpotential deposition of Hg on Au(111) An electrochemical and in situ surface X-ray diffraction study

Abstract

We report on in situ surface X-ray diffraction studies of mercury underpotential deposition (UPD) on Au(111) electrodes in four different supporting electrolyte solutions including 0.10 M sulfuric acid, 0.10 M perchloric acid, 0.10 M perchloric acid with 1.0 mM sodium chloride, and 0.10 M acetic acid with 0.10 M sodium acetate. The anions have been found to play important roles in determining the Hg overlayer structure. Three ordered phases were found in sulfuric acid solutions. The first one was observed at the initial stage corresponding to a Hg 2SO 4 co-adsorbed bilayer structure with a distorted honeycomb lattice. Further deposition at more negative potentials results in two successive hexagonal Hg monolayers. The entire process is consistent with the multistep mechanism proposed by electrochemical studies. The overlayer structure in 0.10 M perchloric acid was dominated by trace amounts of chloride ions in solution and was similar to that observed in a solution containing 1.0 mM chloride ions. No in-plane diffraction from the deposited overlayer was observed in either 0.10 M perchloric acid or 0.10 M perchloric acid containing 1.0 mM chloride ions. In acetate solutions, an incommensurate hexagonal lattice was found with a bilayer structure likely in the form of [Hg(CH 3COO)] complexes. The lattice constant varied dramatically with the electrode potential over a large potential range, suggesting that the charge state of the deposited Hg atoms changes with potential. In all four electrolytes, the mercury underpotential deposition appears to follow a common mechanism in which the initial stage consists in the desorption of pre-adsorbed anions and the deposition of a co-adsorbed layer consisting of a mercury-anion neutral species. The integrated charge under the cyclic voltammetric waves is consistent with the overlayer densities derived from specular crystal truncation rod (CTR) measurements.