Sulfate adsorption on a Au(111) electrode studied by AES, CEELS, LEED and cyclic voltammetry

We have studied interactions of sulfate anions with a Au(111) electrode using Auger electron spectroscopy (AES), low energy electron diffraction (LEED), core electron energy loss spectroscopy ( CEELS ), and electrochemistry, and have employed a nonstandard method of quantitative analysis by AES in which the anion coverage is determined using a thick Na₂SO₄ film as a reference. A favorable comparison of surface coverage results obtained in this manner with the radiochemical, in situ, data shows that the sulfate adsorbate formed in solution does not desorb upon exposure to ultra-high vacuum (UHV). Since the AES ratio of oxygen-to-sulfur in the sulfate adlattice is 4, and the S(LMM) Auger electron transitions and S(L_{2, 3}) core electron energy loss spectra show a characteristic S⁶⁺ surface valency, our results indicate that no decomposition, e.g., dehydration, of the anion occurs in UHV. In a narrow potential range around E = 1.12 V on the potential scale in use, the adsorbate forms an ordered Au(111)( 3× 3) R30°adlattice that gives rise to a clear, but diffuse LEED pattern. This structure is discussed vis-á-vis recent scanning tunneling microscopy results with the same Au(111)/solution interface. In addition to the structural considerations, our companion spectroscopic analyses suggest that surface gold electrons participate in the anion chemisorption. Namely, below 1.12 V, the down-shift in the core electron loss energy is indicative of increasing electron density on sulfur with increasing sulfate-surface bonding via a back-donation into empty electronic orbitals. Above 1.12 V, the electron loss spectra are dominated by the final state effect induced by the decrease in the number of sulfate oxygen atoms that coordinate the electrode surface.