Effect of the Surface Structure of Gold Electrodes on the Coadsorption of Water and Anions

The potential-dependent water adsorption on gold surfaces in perchloric and sulfuric acid solutions has been studied by surface-enhanced infrared absorption spectroscopy (SEIRAS). It is found that the surface structure of the gold electrodes has a major impact on the SEIRAS spectra. When the gold films are composed of nanoparticles of 47 +/- 11 nm, the SEIRAS spectra are in agreement with previous reports. However, when the size of the gold nanoparticles is decreased to 27 +/- 8 nm by depositing the gold at 1 angstrom/s instead of 0.1 angstrom/s, it is found that the SEIRAS bands associated with water molecules coordinated to coadsorbed anions are absent. The combination of both types of gold electrodes allows a detailed study of the properties of the adsorbed water molecules. It is found that water molecules coadsorbed with sulfate and perchlorate anions appear to belong to the hydration shell of the anions because (i) the intensity of the SEIRAS bands of these water molecules increase with potential in the same way as the SEIRAS bands of the adsorbed anions and (ii) the frequencies of the O-H stretch resemble those of the water molecules in the hydration shell of the anions in solution.