THE PROXIMITY EFFECT ON SEMICONDUCTING MINERAL SURFACES: A NEW ASPECT OF MINERAL SURFACE REACTIVITY AND SURFACE COMPLEXATION THEORY?

Abstract

The observation and description of surface proximity effects, whereby the chemical reaction of one surface site influences the electronic structure and reactivity of neighboring or nearby sites, is presented in this study for the semiconducting minerals galena (PbS) and pyrite (FeS2). The methods used to study this effect include ab initio molecular orbital calculations and scanning tunneling microscopy and spectroscopy. The surface proximity effect can be manifested in different ways, although the principle is the same. For example, we predict that electron transfer in redox reactions on galena surfaces can involve separated sites with specific and special locations. Another example is seen for pyrite where the oxidation of one site on a terrace influences next-nearest neighbor sites, making them far more susceptible to oxidative attack relative to sites further away. The range of potential applications of the surface proximity effect model is also outlined for a number of environmentally and industrially important examples. These findings, in combination with surface complexation theory, an important model for attachment/detachment reactions at mineral-water interfaces, may eventually lead to an extended model that will include the specific influence of semiconductor-type proximity effects.