Evaluation of XPS for the quantitative determination of surface coverages: fluoride adsorption on hydrous ferric oxide particles

Abstract

Quantitative XPS approaches for homogeneous materials can be adapted for use with heterogeneous substrates having chemically modified surfaces. Several methods have been proposed for the XPS determination of adsorbate surface coverages on non-planar samples such as high surface area particles with overlayers. This paper evaluates various quantitative approaches through a study of the pH-dependent adsorption of fluoride on high surface area hydrous ferric oxide (HFO) particles. A fluoride ion selective electrode (ISE) determines fluoride concentration remaining in solution, allowing for the estimation of fluoride surface coverages in the monolayer range on the HFO particles. Analysis of XPS measurements of fluoride adsorbed on the HFO surface are compared with the ISE results. This comparison shows that if the substrate and overlayer photopeaks are close together in binding energy, several different quantitative methods provide reasonable results. However, if there is a large binding energy difference between the photopeaks, only a three-layer exponential attenuation model taking adventitious carbon into account provides results that agree well with ISE-based calculations.