ELECTROCHROMIC BEHAVIOR OF MOLYBDENUM TRIOXIDE THIN-FILMS, PREPARED BY THERMAL-OXIDATION OF ELECTRODEPOSITED MOLYBDENUM TRISULFIDE, IN MIXTURES OF NONAQUEOUS AND AQUEOUS-ELECTROLYTES

Electrochromic molybdenum trioxide thin films were prepared by thermal oxidation in air of electrodeposited molybdenum trisulfide. MoO3 prep ared at temperatures ranging from 400 to 550 degrees C were found to b e polycrystalline. The best electrochromic properties (coloration effi ciency and coloration time) evaluated in 1 M LiClO4/propylene carbonat e were observed for films prepared by annealing at 425 degrees C for 1 0 min. The coloration efficiency (35 cm(2)/C) for these optimum films compare well with those reported in the literature. The intensity of c oloration of the film (absorbance at 634 nm) was found to scale linear ly with film thickness. The electrochemical reduction of MoO3 involves the transformation of Mo6+ species to Mo5+, evidenced by x-ray photoe lectron spectroscopy. The stability of molybdenum trioxide-coated tin oxide electrodes upon cycling and switching between oxidized and reduc ed states was investigated by cyclic voltammetry, potential-step exper iments, and in situ spectroelectrochemistry. The results indicate that the stability of the molybdenum trioxide electrode is improved when i t is used in a 0.1 M LiClO4, 14 mM HClO4, 2% H2O/PC solution rather th an in a pure, nonaqueous electrolyte such as 0.1 M LiClO4/PC. The impr oved stability is related to the fact that the chemical disorder induc ed by cycling the MoO3 electrode in the protic media is much less seve re than in the aprotic media, primarily because while Li+ are intercal ated in the latter, only protons are intercalated in the aqueous/nonaq ueous mixture.