ACID-BASE BEHAVIOR OF ALUMINUM AND SILICON-OXIDES - A COMBINATION OF 2 APPROACHES - XPS AND LEWIS ACIDO-BASICITY - REST POTENTIAL AND BRONSTED ACIDO-BASICITY

In the frame of an investigation on adhesion phenomena between elastomers and oxide substrates, X-ray photoelectron spectroscopy (XPS) and rest electrochemical potential measurements have been used in order to attempt a characterization of the reactivity of various oxides. The first step of the study is a definition of donor/acceptor properties for a solid surface in an interactive model such as the Lewis one. In this case, the Fermi level is an appropriate parameter. An experimental approach is developed to estimate this parameter through photoelectronical data obtained by XPS. As proposed by Mullins and Averbach in 1988 in the case of Al and Si oxides, XPS allows an estimation of the Fermi energy E(F), defined as the energy difference between the Fermi level and the upper occupied valence band. A classification of E(F) values is obtained for different samples of aluminum and silicon oxides; the donor (acceptor) behavior increases for decreasing (increasing) E(F). The second step is the validation of the E(F) classification obtained by XPS. A potentiometric method is used to estimate the rest potential E(o) in aqueous's solution. The relationship between the E(o) and the solution pH is a Nernst law for a large range of pH values, which means that the system is governed by acid-base interactions in Bronsted definition in the interfacial area. When the nature of the oxide substrate is modified, the translation of the E(o) versus pH line can be correlated with the E(F) classification obtained previously. This confirms the relationship already reported between point of zero charge and E(F). It can also be considered as a validation of the idea that XPS allows the classification of E(F) values for those oxides.