Differential charging effects from impurities in pyrolytic graphite

Abstract

This work reports differential charging effects in samples of pyrolytic graphite. Despite being considered a nuisance in XPS analyses, differential charging effects can be exploited for the identification and quantification of chemical species in heterogeneous materials. In this case, charging effects were useful to identify impurities at the surface of pyrolytic graphite. For these impurities, the analysis of the O 1s, Si 2p, and Al 2p core levels allowed distinguishing the contributions of SiO2 and Al2O3 from those of silicone/alumino-silicate oxides. Though both groups of compounds are insulators, a splitting of the above peaks was induced by the use of the flood gun device of the spectrometer when the samples were mounted on a conductive metallic sample holder. The shape and relative concentration of the chemical species ascribed to the split peaks served as a basis for the decomposition of the corresponding O 1s, Si 2p, and Al 2p peaks registered over a ceramic sample holder. The latter allowed for inducing a well controlled differential charging of the samples that led to the identification of true chemical shifts, i.e. those ascribed to variations on the chemical environment of the elements. A qualitative model for describing the observed charging effects was postulated. The model was based on an analogy between electrical circuits and the geometrical configuration of the impurity particles intercalated within the graphite particles.