Comparing the Surface Activity of Compounds Using an Elemental Adsorbate

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It has been proposed that the interaction of a substrate with a suitable elemental adsorbate can be used to monitor its surface chemical activity. If the extent of adsorbate–substrate interaction can be measured by a surface chemical probe such as x-ray photoelectron spectroscopy, one could compare the surface activities of a variety of solids by studying how the same adsorbate reacts with each surface. The scope of this technique has been investigated by using an electropositive metal (Ni) as an adsorbate on a range of ceramic and polymeric compounds. This adsorbate is expected to probe the electronegativity or anionic activity of the surface which, for simple compounds, is directly related to overall anionic concentrations. A model of Ni adsorption in single crystal oxides has been developed which links the extent of adsorbate–oxide interaction to anionic point defects. Experimental data on Mg-doped Al2O3 are in quantitative agreement with this model. It has been seen that even non-ideal surfaces such as polymers and ion-damaged materials show a definite qualitative correlation between the overall anionic concentration and the extent of Ni–substrate interaction. However, in the case of complex compounds, especially those involving transition elements, there may be several cationic defects influencing the anionic lattice which should be recognized in interpreting the overall anionic activity observed. This technique is a simple, yet unique way of comparing the surface chemistries of different substrates.