Influence of crystalline perfection and surface species on the X-ray photoelectron spectra of natural and synthetic graphites

Abstract

Valence band and core level photoelectron spectra have been measured for a variety of natural and synthetic graphites. The valence band spectra confirm previous work, an influence of contamination by chemisorbed species was demonstrated, however. A linear correlation between the C 1 s binding energy (b.e.) and the c 0 lattice constant as a measure of crystalline perfection was found. The line width is larger the more perfectly the graphite is crystallized, indicative of a relaxation effect. A characteristic satellite structure on the high b.e. side of the C 1 s peak is due to the plasmon spectrum as well as to a large number of surface compounds. By comparison with b.e.'s for discrete molecules, the satellites are associated with alcoholic or etheric, carbonylic and carboxylic surface oxides, and aliphatic and/or aromatic surface chlorides. The assignments are confirmed by variable-temperature XPS and by the products of thermal decomposition. Trace surface impurities of sulfur and nitrogen were found in many graphites. The relevance of these findings for XPS on graphite intercalation compounds is briefly discussed.