Quasi in situ XPS study of anion intercalation into HOPG from the ionic liquid [EMIM][BF4]

Highly oriented pyrolytic graphite (HOPG) electrodes were electrochemically oxidized in the ionic liquid [EMIM][BF4]. Both, the electrolyte and the electrode surface were investigated using X-ray photoelectron spectroscopy (XPS) after electrochemical treatment. For that purpose an electrochemical preparation chamber was attached to the ultra high vacuum system allowing for preparation of electrodes in non-aqueous electrolyte and subsequent sample transfer under inert nitrogen atmosphere. The XP-spectra of all species detected on the oxidized HOPG surface show core level shifts towards lower binding energies referring to a Fermi level shift and proving that a graphite intercalation compound was formed. Anion intercalation occurs together with co-intercalation of cations at 2 V vs. carbon quasi-reference electrode and is found to be irreversible. XPS analysis of the ionic liquid prior to and after electrochemical treatment indicates a change in electrolyte composition.