Adsorption and oxidation of K deposited on graphite

The bonding state of K intercalated in highly oriented pyrolitic graphite (HOPG) has been studied by X-ray photoelectron spectroscopy (XPS) and X-ray absorption fine structure/near-edge X-ray absorption fine structure (SEXAFS/NEXAFS) spectroscopies. Intercalation of potassium into HOPG occurs at room temperature after deposition on its surface by evaporation from a getter source. In this state XPS and angle-resolved NEXAFS at the K 1s edge show the existence of a transfer of charge from the potassium to the graphite layers and provide some clues about the hybridization of the p states of K with the sigma* and pi* empty states of the graphite layers. Meanwhile, SEXAFS shows that the K atoms are placed at the centre of the hexagon units of an expanded and reordered graphite structure. Adsorption of 80 L of oxygen at 298 K on K-HOPG produces changes in the shape and intensity of the K 2p photoelectron peak, while the width and asymmetry of the C 1s peak decrease. The progressive incorporation of oxygen into the system is monitored by the development of an O 1s peak with a binding energy between 531.3 and 531.8 eV. In all cases, a rough estimation of the O/K ratio calculated from the areas of the O 1s and K 2p peaks gives values around one. Further changes in the same direction at the K2p and C 1s photoelectron peaks and an additional increase in the amount of incorporated oxygen occur after adsorption of oxygen at 373 K. Similarly, under these conditions the calculated O/K ratio remains close to one. The SEXAFS spectra after adsorption of oxygen are similar to those obtained before adsorption, except for a general and small decrease of the intensity of the EXAFS oscillations. Fitting analysis of the spectra provides the same C-K distance and N-30/N-70 ratio (N-30,N-70, apparent coordination numbers measured at incident angles of the radiation of 30 degrees and 70 degrees in respect to the surface normal) as before adsorption of oxygen. However, there is a decrease of similar to 45% in the absolute values of N in respect to the original sample. NEXAFS spectra at grazing incidence are also similar but depict a new feature when recorded at normal incidence. The intensity of this feature increases with the amount of incorporated oxygen. The model proposed to explain these results assumes that adsorption of oxygen produces a depletion of the negative charge in the graphite layer and the formation of O-2(delta-) species at the surface (delta likely to be two). Simultaneously, the adsorption of oxygen at 298 and 373 K induces the partial segregation of the intercalated potassium to the external surface of the HOPG crystal where it becomes associated to the adsorbed oxygen. In this state SEXAFS spectra are dominated by the potassium still intercalated within the layers, while XPS is more sensitive to the atoms segregated to the surface.