Kinetic Studies of WO₃-Based Photochromism in Polyvinyl Alcohol Film

Abstract

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.

Cyclodextrin-Assisted Surface-Enhanced Photochromic Phenomena of Tungsten(VI) Oxide Nanoparticles for Label-Free Colorimetric Detection of Phenylalanine

“…Based on several literatures on WO 3 photochromism − and the results obtained in this study, we propose the relationship between the adsorption behavior of Phe molecules on the WO 3 surface and the photochromic behavior of WO 3 nanocolloid particles in the acidic Phe/TMβCDx/WO 3 ternary aqueous system, as shown in Scheme . In the bulk aqueous solution, cationic Phe molecules form inclusion complexes with TMβCDxs with 1:1 stoichiometric ratio.…”

Effect of Dynamic and Preferential Decoration of Pt Catalyst Surfaces by WOx on Hydrodeoxygenation Reactions

“…58,59 Recent work has suggested that the absorption stems from intervalence charge transfer of mixed-valent W− O−W pairs (e.g., W 5+ −O−W 6+ ). 60,61 Deconvolution of this absorbance is suggestive of the presence of predominantly W 5+ species after this step (Figure S15), though the relative cross sections of these two absorption bands have not been reported, preventing quantification of the two species through UV−vis spectroscopy. The second oxidation step eliminates this feature, and the sample absorption spectrum becomes essentially identical to the initial spectrum assigned to W 6+ , highlighting the reversibility of oxidation state changes for W in this catalyst.…”