Electrochemically Induced Transformations of Vanadium Dioxide Nanocrystals

Vanadium dioxide (VO 2 ) 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 2, including electrochemical gating. Here, we prepare and electrochemically reduce mesoporous films of VO 2 nanocrystals, prepared from colloidally synthesized V 2 O 3 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 2 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 2 films and is attributed to improved oxygen vacancy formation kinetics and diffusion due to the mesoporous nanocrystal film structure.