Structural and Spectroelectrochemical Investigations of Sol‐Gel Derived Electrochromic Spinel Co3 O 4 Films

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Abstract
Thin films of the cubic spinel were prepared by the sol‐gel route from a Co(II) sulfate precursor using a dip‐coating technique. The films (50 to 60 nm thick) were deposited on indium‐tin oxide glass substrates with a single dipping. The electrochromic properties of the films were studied in an aqueous alkaline electrolyte (0.1 M LiOH) using cyclic voltammetry (CV) and in situ UV‐vis spectroelectrochemistry. The intermediate oxide products that appeared during cycling at current peak potentials were analyzed by x‐ray diffraction (XRD) and ex situ Fourier transform infrared (FTIR) reflection‐absorption spectroscopy performed at near‐grazing incidence angle conditions (NGIA). High hydration of the spinel structure enables complex redox processes to take place inside the structure or on the surface of the film when potential cycling is performed. Vibrational bands characteristic of the formation of CoO, , and hydrated phases have been assigned and discussed according to the results obtained from ex situ NGIA FTIR, CV, and XRD studies. The change in transmittance (λ = 634 nm) between bleached and colored states for single‐dipped films was 25%, and for electrochromic efficiency (η) was approximately .

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