Kinetic Studies of WO₃-Based Photochromism in Polyvinyl Alcohol Film

Tungsten oxide (WO₃) has been extensively studied for various photochromic applications. Blue coloration of WO₃ is explained in terms of the intervalence charge transfer (IVCT) transition of electrons between W⁶⁺ and W⁵⁺. However, various absorption spectra with different shapes have been reported. Herein, a transparent film was prepared by drying aqueous solutions containing polyvinyl alcohol, WO₃ nanoparticles and ethylene glycol (EG). For comparison, the photochromic behavior of an aqueous WO₃ 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 (r₀) 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, r₀ evaluated at 640 or 984 nm was independent of the water amounts but increased proportionally to the EG amounts and the light intensity, although r₀ 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 W⁶⁺ and W⁵⁺, which is stabilized with water in the bulk and the absorption peaks at 640 and 984 nm are attributable to IVCT on the WO₃ surface.