Tungsten oxide-based photochromic films which changed reversibly in air between colorless- transparent in the dark and dark blue under UV irradiation were prepared by using methylcellulose as a film matrix and polyols such as ethylene glycol (EG), propylene glycol (PG), and glycerin (Gly) as dispersing agents. Influence of the dispersing agents and water in the films on the photochromic behavior was systematically studied. Under UV irradiation, absorption bands around 640 and 980 nm increased and the coloring rate was the following order: Gly > EG > PG. An increase in the amounts of dispersing agents or water accelerated the coloring rate. By increasing the water content of the film, a new absorption peak appeared at ca. 775 nm and the Raman spectra indicated a shift of W-O-W stretching vibration to lower wavenumber which was due to the formation of hydrogen bonding. All absorption spectra were fit by three Lorentz functions, whose bands were ascribed to various packing of WO6 octahedra. After the light was turned off, the formation of W(5+) was stopped and bleaching occurred by the reaction with O2 in air to recover its original transparent state. We anticipate that the biodegradable photochromic films developed in this study can be applied in recyclable display medium and especially in detachable films for glass windows whose light transmission properties are changed by sunlight, i.e., for usage as an alternative of smart windows without applying voltage.
Green Chemistry
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The photoassisted catalytic degradation of chloroethylene was studied in a tubular photoreactor packed with TiO 2 pellets prepared by a sol-gel method. The experiments were performed in a noncirculating mode. Kinetic data and the reaction products were compared with those for the photodegradation of ethylene, trichloroethylene, and tetrachloroethylene. The theoretical calculations at the MP4/6-31G**//B3LYP/6-31G** level indicated that the addition of OH radicals to chlorinated ethylenes is more exothermic than that of Cl radicals by 14.6-29.5 kcal mol -1 . Examination of Cl mass balance indicated that the concentration of Clcollected from the TiO 2 surface was higher than that from the product gas stream. When the photodegradation of ethylene was performed on the TiO 2 pellets which had been used for that of TCE or which were pretreated with HCl, the formation of chloroacetaldehyde was confirmed by the GC/MS. We proposed that during the photodegradation of the chlorinated ethylenes, the Cl -, as one of the reaction products, accumulated and was oxidized to Cl radical on the TiO 2 surface, which might be due to the oxidation by OH radical. Then, the Cl radical reacted with chlorinated ethylenes, leading to the formation of undesirable chlorinated byproducts.
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