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Metallization of initially insulating VO2 via ionic liquid electrolytes(electrolyte gating) has recently been a topic of much interest. It is clearthat the metallization takes place electrochemically and there has previouslybeen extensive evidence for the removal of small amounts of oxygen during ionicliquid gating. Hydrogen intercalation has also been proposed, but the source ofthe hydrogen has remained unclear. In this work, solid-state magic anglespinning NMR spectroscopy is used to investigate the thermal metal-insulatortransition in VO2, before progressing to catalytically hydrogenated andelectrochemically metallized VO2. Electrochemical metallization of bulk VO2particles is shown to be associated with intercalation of hydrogen, the degreeof which can be measured with quantitative 1H NMR spectroscopy. Possiblesources of the hydrogen are explored, and by using a selectively deuteratedionic liquid, it is revealed that the hydrogenation is due to deprotonation ofthe ionic liquid; specifically, for the commonly used dialkylimidazolium basedionic liquids, it is the "carbene" proton that is responsible. Increasing thetemperature of the electrochemistry is shown to increase the degree ofhydrogenation, forming first a less hydrogenated metallic phase then a morehydrogenated insulating Curie-Weiss paramagnetic phase, both of which were alsoobserved for catalytically hydrogenated VO2. The NMR results are supported bymagnetic susceptibility measurements, which corroborate the degree of Pauli andCurie-Weiss paramagnetism. NMR spectroscopy is used to identify the presence ofhydrogen in an electrolyte gated thin film of VO2, suggesting that electrolytebreakdown, proton intercalation and reactions with decomposition productswithin the electrolyte should not be ignored when interpreting the electronicand structural changes observed in electrochemical gating experiments.

The role of ionic liquid breakdown in the electrochemical metallization of VO2: An NMR study of gating mechanisms and VO2 reduction