Formation of cesium carbonate in ion-implanted graphite, examined with dual-source x-ray photoelectron spectroscopy, density functional theory calculations and thermodynamic modelling

A sample of highly orientated pyrolytic graphite (HOPG) was implanted with Cs+ ions in order to study the effect on the graphite matrix and the chemical form of Cs once embedded. The mean implantation depth was calculated to be-22 nm, allowing for investigation with X-ray Photoelectron Spectroscopy, (XPS), where both a traditional Al K alpha X-ray source and a higher energy Ag L alpha source was used for an increased sampling depth deeper into the surface. Analysis found that there was a reduction of sp2 C-C bonding with increasing implantation dose, resulting in a total loss of sp2 character at-6 atomic % Cs+. A striking increase in higher binding energy components (285.8-289.4 eV) in the C 1s spectra cannot be attributed solely to the presence of C-O species, instead indicating a dramatic re-ordering of the graphitic lattice to accommodate and neutralize the embedded Cs. XPS analysis indicates the formation of cesium carbonate within the graphitic material; this is reinforced by analysis of a Cs2CO3 reference sample. Additionally, thermodynamics equilibrium simulations, supported by simplified density functional theory (DFT) calculations, are performed, leading to strong agreement with the XPS findings; that is, the most stable form of Cs within the graphite matrix is expected to be cesium carbonate (Cs2CO3), which is in thermodynamic equilibrium with degraphitised HOPG and oxygenated degraphitised HOPG.