An X-ray photoelectron spectroscopic study of the interaction of oxygen and nitric oxide with aluminium

Abstract

The interaction of oxygen and nitric oxide with clean aluminium surfaces has been investigated by X-ray photoelectron spectroscopy. Studies have been in the main confined to the temperature range 80–290 K and pressure range 10 -6 –10 -2 Pa. The Al(2p) level is shown to exhibit a shifted component at a binding energy of ~ 74.5 eV after oxygen interaction at 80 K. A curve-fitting and deconvolution analysis of the shifted peak indicates that it is made up of two components, one at a binding energy of 74.0 eV and the other at 75.3 eV. The lower binding energy component (designated α ) develops preferentially at 80 K while the higher binding energy one ( β ), assigned to Al 2 O 3 , is dominant at 290 K. We suggest that α is an incorporated oxygen structure which is a precursor to the formation of Al 2 O 3 . The initial sticking probability of oxygen at 80 K is 0.07 while at 290 K it is 0.02. The plasmon-loss features associated with the Al(2s) peak are shown to be sensitive to oxygen adsorption and therefore useful in confirming the surface monolayer. At 290 K and an oxygen pressure of 10 4 Pa the oxide thickness is estimated to be about 0.9 nm. When NO was adsorbed at 80 K three distinct N(1s) peaks were observed at binding energies of about 397, 403 and 407 eV. We assign the 397 eV peak to N δ- ads arising from dissociation of the molecule and the two higher binding energy peaks to N δ- ads and N 2 O ads . The N(1s) peak characteristic of NO δ- ads is close to the lower binding energy peak of the two N(1s) peaks associated with N 2 O ads while the O(1s) of NO δ- ads overlaps with the O(1s) peak of the surface oxide at a binding energy of about 532 eV. Mass spectroscopic analysis of the gas phase indicated that on warming to 85 K, the adlayer formed at 80 K, N 2 O was desorbed, confirming our assignment of the core-level spectra. The NO δ- and N 2 O species are not observed at 290 K while at 80 K exposure of the adlayer to water vapour results in the complete removal of weakly adsorbed N 2 O. By monitoring the intensities of the Al(2p), N(1s) and O(1s) peaks, estimates were made of the absolute and relative concentrations of the various species and various molecular processes delineated.