Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals.

Vanadium dioxide (VO) 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, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO nanocrystals, prepared from colloidally synthesized VO 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 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 films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure.