Structural effects on water adsorption on gold electrodes

We study the molecular properties of the interface formed between aqueous sulfuric acid solutions and gold electrodes by means of surface-enhanced infrared absorption spectroscopy (SEIRAS). The shape of the SEIRAS spectra is observed to be strongly dependent on the deposition rate with which the gold electrodes are prepared. We find that the water molecules coordinating to coadsorbed sulfate anions become invisible in the SEIRAS spectra when the gold films are deposited at 1 Å/s, instead of the customary deposition rate of 0.1 Å/s employed in previous studies. Atomic force microscopy images of the gold deposits demonstrate that the increase of the gold deposition rate produces a decrease in the size of the nanoparticles composing the gold films. This suggests that water molecules coadsorbed with sulfate anions on small gold nanoparticles are oriented parallel to the surface. On the other hand, the fact that these water molecules are not detected by SEIRAS facilitates the study of the adsorption of hydronium cations, since these SEIRAS bands overlap. It is concluded that the adsorption of sulfate anions does not involve the coadsorption of any hydronium-water complex, since the SEIRAS band of the latter species exhibits a steady decrease with increasing potential.