Adsorption of oxygen on Au(111) by exposure to ozone

Atomic oxygen coverages of up to 1.2 ML may be cleanly adsorbed on the Au(111) surface by exposure to O₃ at 300 K. We have studied the adsorbed oxygen layer by AES, XPS, HREELS, LEED, work function measurements and TPD. A plot of the O(519 eV)/Au(239 eV) AES ratio versus coverage is nearly linear, but a small change in slope occurs at H_O=0.9 ML. LEED observations show no ordered superlattice for the oxygen overlayer for any coverage studied. One-dimensional ordering of the adlayer occurs at low coverages, and disordering of the substrate occurs at higher coverages. Adsorption of 1.0 ML of oxygen on Au(111) increases the work function by +0.80 eV, indicating electron transfer from the Au substrate into an oxygen adlayer. The O(1s) peak in XPS has a binding energy of 530.1 eV, showing only a small (0.3 eV) shift to a higher binding energy with increasing oxygen coverage. No shift was detected for the Au 4f_7/2 peak due to adsorption. All oxygen is removed by thermal desorption of O₂ to leave a clean Au(111) surface after heating to 600 K. TPD spectra initially show an O₂ desorption peak at 520 K at low H_O and the peak shifts to higher temperatures for increasing oxygen coverages up to H_O=0.22 ML. Above this coverage, the peak shifts very slightly to higher temperatures, resulting in a peak at 550 K at H_O=1.2 ML. Analysis of the TPD data indicates that the desorption of O₂ from Au(111) can be described by first-order kinetics with an activation energy for O₂ desorption of 30 kcal mol^-1 near saturation coverage. We estimate a value for the Au-O bond dissociation energy D(Au-O) to be ∼56 kcal mol^-1.