THE EFFECTS OF ANIONS ON THE UNDERPOTENTIAL DEPOSITION OF HG ON AU(111) - AN ELECTROCHEMICAL AND IN-SITU SURFACE X-RAY-DIFFRACTION STUDY

We report on in situ surface X-ray diffraction studies of mercury unde rpotential deposition (UPD) on Au(lll) electrodes in four different su pporting electrolyte solutions including 0.10 M sulfuric acid, 0.10 M perchloric acid, 0.10 M perchloric acid with 1.0 mM sodium chloride, a nd 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 structu re. Three ordered phases were found in sulfuric acid solutions. The fi rst one was observed at the initial stage corresponding to a Hg2SO4 co -adsorbed bilayer structure with a distorted honeycomb lattice. Furthe r deposition at more negative potentials results in two successive hex agonal Hg monolayers. The entire process is consistent with the multis tep mechanism proposed by electrochemical studies. The overlayer struc ture in 0.10 M perchloric acid was dominated by trace amounts of chlor ide ions in solution and was similar to that observed in a solution co ntaining 1.0 mM chloride ions. No in-plane diffraction from the deposi ted overlayer was observed in either 0.10 hi 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 structu re likely in the form of [Hg(CH3COO)] complexes. The lattice constant varied dramatically with the electrode potential over a large potentia l range, suggesting that the charge state of the deposited Hg atoms ch anges with potential. In all four electrolytes, the mercury underpoten tial deposition appears to follow a common mechanism in which the init ial stage consists in the desorption of pre-adsorbed anions and the de position 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. (C) 1998 Elsevier Science B.V.