We previously reported a novel photochromism of aqueous Mo 6+ -doped TiO2 (Mo-TiO2) colloidal solutions where Mo 5+ was formed by the reaction with photogenerated electrons under UV irradiation, resulting in the color change from a colorless-transparent to black via gray. However, one drawback of this photochromism was a slow bleaching process. In this study, we have investigated electrochromic properties of Mo-TiO2 to remove this drawback because the oxidation of Mo 5+ to Mo 6+ occurs quickly by switching potentials. As the amount of Mo 6+ increased from 1.1 to 5.0 mol%, it took more times to obtain transparent Mo-TiO2 colloidal solutions and 5.0 mol% Mo-TiO2 was the most appropriate to fabricate a dip-coating film on fluorine-doped tin oxide glass for electrochromic experiments. In tetrabutylammonium perchlorate/propylene carbonate (PC) electrolytes, the electrochromism of the Mo-TiO2 electrode was observed but no coloration occurred on the TiO2 electrode without doping. In LiClO4/PC electrolytes, the coloration of both electrodes remarkably increased at the applied potential (E) less than ca. -1.5 V vs. Ag/AgCl where the current was controlled by the diffusion of Li + . On the TiO2 electrode, no coloration was observed at E ≥ -1.4 V whereas the Mo-TiO2 electrode exhibited coloration even at -1.0 V whose transmittance was 72.7 ± 1.5 % in the visible region of 400 -800 nm. At -1.8 V, the transmittance of Mo-TiO2 and TiO2 electrodes were 6.7 ± 3.2 and 47.6 ± 7.7 %, respectively. These results indicate that the Mo-TiO2 electrode is superior to the TiO2 electrode for the electrochromic application.

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