The reactivity of copper toward liquid anhydrous hydrofluoric acid (AHF) has been examined using ex situ X-ray photoelectron spectroscopy (XPS). Exposure of either oxide-free or air-oxidized Cu surfaces to AHF yielded XPS spectra consistent with the presence of a CuF2 layer devoid of oxygen. Cyclic voltammetric experiments involving oxide-free Cu electrodes in hexafluorobutanol (HFB)+AHF displayed a prominent oxidation peak during the first few cycles. XPS analysis of Cu electrodes emersed at potentials positive to this feature provided evidence for the presence of a CuF2 layer much thicker than that found by simple immersion in AHF. Furthermore, the onset potential for both oxidation and subsequent reduction of the film was 0.0 V versus a Cu electrode in the same solution, which suggests that the Cu/CuF2 redox couple controls the potential of the reference electrode.
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