SULFATE ADSORPTION ON A AU(111) ELECTRODE STUDIED BY AES, CEELS, LEEDAND CYCLIC VOLTAMMETRY

We have studied interactions of sulfate anions with a Au(111) electrod e using Auger electron spectroscopy (AES), low energy electron diffrac tion (LEED), core electron energy loss spectroscopy (CEELS), and elect rochemistry, and have employed a nonstandard method of quantitative an alysis 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 u pon exposure to ultra-high vacuum (UHV). Since the AES ratio of oxygen -to-sulfur in the sulfate adlattice is 4, and the S(LMM) Anger electro n transitions and S(L(2,3)) core electron energy loss spectra show a c haracteristic S6+ surface valency, our results indicate that no decomp osition, e.g., dehydration, of the anion occurs in UHV. In a narrow po tential range around E=1.12 V on the potential scale in use, the adsor bate forms an ordered Au(111) (root 3 X root 3)R30 degrees adlattice t hat gives rise to a clear, but diffuse LEED pattern. This structure is discussed vis-a-vis recent scanning tunneling microscopy results with the same Au(111)/solution interface. In addition to the structural co nsiderations, our companion spectroscopic analyses suggest that surfac e 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-surf ace bonding via a back-donation into empty electronic orbitals. Above 1.12 V, the electron loss spectra are dominated by the final state eff ect induced by the decrease in the number of sulfate oxygen atoms that coordinate the electrode surface.