Characterisation of Au(111) and Au(210) aqueous solution interfaces by electrochemical immittance spectroscopy

Gold single crystals oriented to expose the (111) and (210) faces were investigated using electrochemical immittance spectroscopy (EIS) in aqueous solutions of HClO4 and KF in the double layer region with the aim of identifying and explaining the frequency dispersion of interfacial capacitance known as constant phase angle (CPA) dispersion. Au(111) and Au(210) were chosen as representing the whole range of variance of electrochemical properties of Au(hkl) electrodes. Au(111) as the most uniform, microscopically smooth surface behaved with almost ideal capacitance in HClO4 solutions in the whole potential range and also in KF solutions, in that case with the exception of potentials well positive to the potential of zero charge (pzc). Au(210) being microscopically the most corrugated surface displayed significant CPA dispersion in both electrolytes. In HClO4, dispersion on Au(210) occurred mostly in the potential region slightly positive to the potential of zero charge where the capacitance hump of the Helmholtz layer appeared. Analogous dispersion occurred on Au(210) in KF solutions only at high concentrations. The behaviour closest to ideal, dispersionless behaviour was always observed at sufficiently negative potentials. In KF solutions at positive potentials dispersion on both electrodes may be attributed to the adsorption (OH-, HF). In the intermediate potential range, close to the Helmholtz capacitance hump it can be attributed to solvent-metal interactions. Dispersion was lower in well conducting (concentrated) electrolytes and this suggests its geometrical nature is related possibly to the fractal pattern of the structured solvent. (C) 1998 Elsevier Science S.A. All rights reserved.