Electrochemical Preparation of Molybdenum Trioxide Thin Films:  Effect of Sintering on Electrochromic and Electroinsertion Properties

Molybdenum trioxide (MoO 3 ) 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 MoO 3 thin films in 1 M LiClO 4 /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 films heated to 250 °C exist as disordered, mixed-phase materials comprising monoclinic β-MoO 3 and orthorhombic α-MoO 3 . Crystallization to the more thermodynamically stable orthorhombic α-MoO 3 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.