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A reversible color change of WO3 has been widely studied to develop new energy-saving technologies such as smart windows, rewritable paper, and information displays. A blue coloration arises from the intervalence charge transfer between W­(VI) and W­(V), which is partially formed by the reduction of WO3 under UV light or an applied voltage. This means that WO3 has a mixed-valence state of W­(V) and W­(VI) upon the reduction. However, despite many studies for various applications, how many W­(V) atoms are formed and contribute to the intervalence charge transfer (IVCT) transition remains unclear because W­(V) formed in WO3 cannot be determined quantitatively. We determined the amount of the photogenerated W­(V) in an aqueous WO3 colloidal solution containing ethylene glycol (EG) by observing the localized surface plasmon resonance (LSPR) peaks of Ag nanoparticles which were produced by a redox reaction between W­(V) and Ag+. EG acted as a hole scavenger to suppress the recombination between the photogenerated holes and electrons. First, we explored the reaction condition where only the IVCT transition was observed under UV irradiation, and then it decreased in response to the increase in the LSPR peak in the dark. Under such a condition, the absorbance at 775 nm (A 775) due to the IVCT transition was observed after the UV irradiation for 30 s, and the absorbance at 410 nm (A 410) due to the LSPR absorption was obtained when A 775 completely disappeared in the dark. Experiments were performed at various UV intensities to confirm a proportional relationship between A 775 and A 410. Electron spin resonance measurements revealed that A 775 was proportional to the amount of W­(V). Furthermore, Ag nanoparticles were synthesized by a polyol reduction method to obtain the relationship between the LSPR peak intensity and the Ag+ concentration, which was consumed for the formation of Ag. On the basis of all of these relationships, A 775 of 1.669 corresponded to 2.53 × 10–4 mol dm–3 W­(V), which was estimated to be only 0.21% of 0.12 mol dm–3 WO3 used in this study, and the molar absorption coefficient for the IVCT transition between W­(V) and W­(VI) was evaluated to be 6.85 × 103 dm3 mol–1 cm–1.

Tungsten oxide (WO 3 ) has been extensively studied for various photochromic applications. Blue coloration of WO 3 is explained in terms of the intervalence charge transfer (IVCT) transition of electrons between W 6+ and W 5+ . However, various absorption spectra with different shapes have been reported. Herein, a transparent film was prepared by drying aqueous solutions containing polyvinyl alcohol, WO 3 nanoparticles and ethylene glycol (EG). For comparison, the photochromic behavior of an aqueous WO 3 colloidal solution containing EG was also investigated. Under UV irradiation, a single intense peak was always observed at ca. 777 nm in the colloidal solution, but the absorption spectra of the film changed from a peak at 770 nm to two distinct peaks at 654 and 1003 nm. All absorption spectra observed with the film and the colloidal solution were deconvoluted into five peaks at 540, 640, 775, 984, and 1265 nm. Kinetic studies using the colloidal solution indicated that the coloration rates (r 0 ) estimated at the deconvoluted peaks of 640, 775, and 984 nm followed the same rate law. On the other hand, in the case of the film, r 0 evaluated at 640 or 984 nm was independent of the water amounts but increased proportionally to the EG amounts and the light intensity, although r 0 at 775 nm significantly increased with the increasing amounts of water and EG. Raman and electron spin resonance spectroscopic observations of the film revealed that the photogenerated electrons migrated toward the terminal W�O moiety to accumulate and then a small anisotropic electron spin resonance signal appeared. Our study demonstrates that the absorption at 775 nm is due to IVCT between W 6+ and W 5+ , which is stabilized with water in the bulk and the absorption peaks at 640 and 984 nm are attributable to IVCT on the WO 3 surface.