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    Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals

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    Vanadium dioxide (VO<sub>2</sub>) undergoes significant optical, electronic, and structural changes as it transforms between the low-temperature monoclinic and high-temperature rutile phases. Recently, alternative stimuli have been utilized to trigger insulator-to-metal transformations in VO<sub>2</sub>, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO<sub>2</sub> nanocrystals, prepared from colloidally synthesized V<sub>2</sub>O<sub>3</sub> nanocrystals that have been oxidatively annealed, in a three-electrode electrochemical cell. We observe a reversible transition between infrared transparent insulating phases and a darkened metallic phase by in situ visible–near-infrared spectroelectrochemistry and correlate these observations with structural and electronic changes monitored by X-ray absorption spectroscopy, X-ray diffraction, Raman spectroscopy, and conductivity measurements. An unexpected reversible transition from conductive, reduced monoclinic VO<sub>2</sub> to an infrared-transparent insulating phase upon progressive electrochemical reduction is observed. This insulator–metal–insulator transition has not been reported in previous studies of electrochemically gated epitaxial VO<sub>2</sub> films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure
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