The Role of Ionic Liquid Breakdown in the Electrochemical Metallization of VO2: An NMR Study of Gating Mechanisms and VO2 Reduction.


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Authors
Cui, Bin 
Gao, Fang 
Dutton, Siân E 
Abstract

Metallization of initially insulating VO2 via ionic liquid electrolytes, otherwise known as electrolyte gating, has recently been a topic of much interest for possible applications such as Mott transistors and memory devices. It is clear that the metallization takes place electrochemically, and, in particular, there has previously been extensive evidence for the removal of small amounts of oxygen during ionic liquid gating. Hydrogen intercalation has also been proposed, but the source of the hydrogen has remained unclear. In this work, solid-state magic angle spinning NMR spectroscopy (1H, 2H, 17O, and 51V) is used to investigate the thermal metal-insulator transition in VO2, before progressing to catalytically hydrogenated VO2 and electrochemically metallized VO2. In these experiments electrochemical metallization of bulk VO2 particles is shown to be associated with intercalation of hydrogen, the degree of which can be measured with quantitative 1H NMR spectroscopy. Possible sources of the hydrogen are explored, and by using a selectively deuterated ionic liquid, it is revealed that the hydrogenation is due to deprotonation of the ionic liquid; specifically, for the commonly used dialkylimidazolium-based ionic liquids, it is the "carbene" proton that is responsible. Increasing the temperature of the electrochemistry is shown to increase the degree of hydrogenation, forming first a less hydrogenated metallic orthorhombic phase then a more hydrogenated insulating Curie-Weiss paramagnetic orthorhombic phase, both of which were also observed for catalytically hydrogenated VO2. The NMR results are supported by magnetic susceptibility measurements, which corroborate the degree of Pauli and Curie-Weiss paramagnetism. Finally, NMR spectroscopy is used to identify the presence of hydrogen in an electrolyte gated thin film of VO2, suggesting that electrolyte breakdown, proton intercalation, and reactions with decomposition products within the electrolyte should not be ignored when interpreting the electronic and structural changes observed in electrochemical gating experiments.

Description
Keywords
cond-mat.str-el, cond-mat.str-el, cond-mat.mtrl-sci
Journal Title
J Am Chem Soc
Conference Name
Journal ISSN
0002-7863
1520-5126
Volume Title
140
Publisher
American Chemical Society (ACS)
Sponsorship
Engineering and Physical Sciences Research Council (EP/M009521/1)
Engineering and Physical Sciences Research Council (EP/P007767/1)
European Commission Horizon 2020 (H2020) Future and Emerging Technologies (FET) (737109)
Oppenheimer Foundation The Winston Churchill Foundation of the United States Herchel Smith Scholarship EPSRC (EP/MO09521/1) EU H2020 program “Phase Change Switch” Alexander von Humboldt Foundation
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