Multistep photochromism by using photoinduced redox reaction in tungsten oxide colloidal solution containing Cu(II) ion

Abstract

Tungsten oxide is one of the most studied inorganic photochromic materials and its color changes to blue under ultraviolet (UV) irradiation. In this paper, we reported multistep photochromism from colorless and transparent to green via yellow in WO3 colloidal solution containing Cu2+ and ethylene glycol under UV irradiation. In the absence of Cu2+, absorbance at 775 nm (Abs775) due to the intervalence charge transfer (IVCT) transition between W5+ and W6+ increased with an increase in the irradiation time and the color of the colloidal solution turned blue. The Abs775 value increased with increasing the EG concentration and reached a constant level at molar ratio of [EG]/[WO3] ≥ 3.0. On the other hand, the addition of 0.61 or 1.22 mM CuCl2 to the colloidal solution increased absorption at ca. 450 nm at first and then the absorbance at 775 nm increased, changing the transparent solution to yellow and then green during the irradiation for 120 s. In the case of [Cu2+] ≥ 3.6 mM, only a small increase in the absorbance at 450 nm without IVCT transition was observed. Electron spin resonance measurements revealed that the reduction of W6+ to W5+ occurred after all Cu2+ were reduced to Cu+ under UV irradiation. Bleaching from green to colorless via yellow occurred by purging air in the colloidal solution. The multistep photochromism showed good stability in the repeated experiments. Based on the obtained data, we proposed a possible mechanism for the multistep photochromism.