Electrochemical preparation of molybdenum trioxide thin films: Effect of sintering on electrochromic and electroinsertion properties

Molybdenum trioxide (MoO₃) thin films prepared by cathodic electrodeposition on indium-tin-oxide coated glass substrates from aqueous peroxo-polymolybdate solutions have been studied as a function of sintering temperature (25-450°C). Cyclic voltammetry, chronopotentiometry, chronoamperometry, and spectroelectrochemical measurements performed with MoO₃ thin films in 1 M LiClO₄/propylene carbonate demonstrate that the electrochemical behavior (Li⁺ insertion/extraction and coloration) is strongly dependent upon thermally induced changes in micro-/nanocrystallinity, which directly influence measured Li⁺ diffusion properties as well as electroinsertion and electrochromic reversibilities. Structural analysis using X-ray photoelectron spectroscopy, X-ray diffraction, and atomic force microscopy indicate that films heat treated at 100°C or less exist as amorphous oxide-hydrates of molybdenum; whereas film heated to 250°C exist as disordered, mixed-phase materials comprising monoclinic β-MoO₃ and orthorhombic α-MoO₃. Crystallization to the more thermodynamically stable orthorhombic α-MoO₃ occurs at 350°C and above. The mixed-phase material exhibits inhomogeneous electrochemical activity, evidenced by the existence of complicated voltammetric and chronoamperometric responses. The effects of sintering temperature on ion insertion and electrocoloration properties are discussed.