Adsorption and oxide formation on aluminium films

Type I aluminium films deposited at 0 $^\circ$C were of relatively low area, ca. 1000 cm$^2$, stable and with a certain porosity since CO uptake was proportional to film weight. Thus a film area could be determined from its weight, since at saturation CO molecules occupy 0.75 of the total surface area given by the B.E.T. method (established for higher area type II films). It was found that H$_2$ and N$_2$ were not chemisorbed, N$_2$O and NO reacted giving surface oxide, C$_2$H$_4$ gave a slow reversible adsorption, and CO gave a slow irreversible adsorption, rate proportional to p/g, where p is the gas pressure, g the quantity of gas taken up by the film. The kinetics of oxygen uptake were followed in the range 10$^{-2}$ to 10$^{-3}$ mm Hg pressure. The first two monolayers were taken up too rapidly for kinetic measurement, and thereafter gas uptake followed the direct logarithmic law, until about 7 A of film had been formed $dg/dt = a e^{-\gamma g/RT} p^0.6,$ where a and $\gamma$ are constants. The rate-determining step is considered to be a place exchange of surface oxygen with underlying aluminium atoms, stimulated by reversibly chemisorbed oxygen atoms. As the film thickens it changes from covalent amorphous to ionic spinel character, and the activation energy for place exchange increases. The process of film growth in the ionic film (thickness > ca. 10 A) involves electron tunnelling and cation migration.