An AES and XPS study of the initial oxidation of polycrystalline magnesium with water vapour at room temperature

Abstract

The initial oxide formation on polycrystalline magnesium surfaces at room temperature has been quantitatively followed from the earliest stage using Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS). Clean surfaces were prepared by sputtering with a minimum dose of argon ions to avoid the creation of heavily disordered crystal planes. Crystallographic orientations of various grain faces were determined using the electron back scattering diffraction technique in order to allow AES measurements to be made on grains of known orientation. By using calibrated doses of deuterated water vapour, three stages of early oxide growth were recognised: (1) a chemisorption stage during doses up to approximately 0.7 langmuir (L) ( 1 L = 1.3 × 10 −4 Pa · s); (2) a rapid oxide nucleation and island growth stage which is complete by about 5 L at an average island height of four monolayers and (3) a slow, diffusion-controlled growth stage after coalescence of the islands. Modelling results suggest that the rate of initial oxygen uptake is faster on grain faces that have more open-packed, high index orientations, particularly in the second (island nucleation and growth) stage. In addition, the slow growth stage has been adequately described by a logarithmic type growth law for exposures up to 1 × 10 6 L, suggesting a limiting oxide thickness in water vapour of approximately 11 monolayers of MgO. The slow growth process was found to be controlled by the movement of metal cations through the oxide film from the metal/oxide interface to the oxide/gas interface. Finally, the role of hydrogen in the oxide film was studied using XPS and nuclear reaction analysis (NRA). The results indicated that hydrogen was present in the film only in small relative amounts, likely as hydroxyl groups trapped at the metal/oxide interface. Detailed XPS spectra showed two distinct types of oxygen: one of these is assigned to be oxygen in a normal MgO lattice position; the other is ascribed to oxygen in a “defective” chemical environment.