COMPARING THE SURFACE-ACTIVITIES OF COMPOUNDS USING AN ELEMENTAL ADSORBATE

It has been proposed that the interaction of a substrate with a suitab le elemental adsorbate can be used to monitor its surface chemical act ivity. If the extent of adsorbate-substrate interaction can be measure d by a surface chemical probe such as x-ray photoelectron spectroscopy , one could compare the surface activities of a variety of solids by s tudying 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. T his adsorbate is expected to probe the electronegativity or anionic ac tivity 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-d oped Al2O3 are in quantitative agreement with this model. It has been seen that even non-ideal surfaces such as polymers and ion-damaged mat erials show a definite qualitative correlation between the overall ani onic concentration and the extent of Ni-substrate interaction. However , in the case of complex compounds, especially those involving transit ion elements, there may be several cationic defects influencing the an ionic lattice which should be recognized in interpreting the overall a nionic activity observed. This technique is a simple, yet unique way o f comparing the surface chemistries of different substrates.