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 W5+ 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.Keywords: dispersing agents; methylcellulose; photochromism; transparent films; tungsten oxide

Kinetic studies on the photocatalytic degradation of trichloroethylene (TCE) were performed in TiO2 or WO3 aqueous sol which consisted of anatase nanocrystalline TiO2 of ca. 3.9 nm or WO3 spherical particles of ca. 17.6 nm. Photocatalytic activity of TiO2 colloid is higher than that of WO3 by a factor of 2.1. Kinetic analyses for both sols give a similar rate law containing γ value which represents a ratio of the reaction rate of the photogenerated electrons with O2 against the total disappearance rate of the photogenerated electrons. Reaction temperature did not affect the TiO2 photocatalysis whereas higher temperature increased the oxidation rate of TCE in the WO3 sol. Such a difference in the temperature effect was explained in terms of the γ values. The photocatalytic degradation of TCE was enhanced by mixing the WO3 and the TiO2 sol and the best performance was obtained by using the mixed sol with WO3:TiO2 molar ratio of 1:2. In the mixed sol, the similar temperature dependence as in the WO3 sol was obtained. Reaction mechanism and the rate law are proposed to account for the obtained kinetic data.Graphical abstractHighlights► TCE was most effectively degraded in the mixed sol of TiO2/WO3 (2:1). ► Temperature-effect on the reaction rate in the WO3 was similar to the TiO2/WO3 sol. ► Mechanism and rate law were proposed to account for the obtained kinetic data.

Photochromism of a WO3 aqueous sol has been investigated in a nitrogen atmosphere under controlled temperature. Effects of ageing of the WO3 sols, concentrations of WO3 sols or Cl− ion and temperature on the coloring rate were examined. The coloring rate was the first-order with respect of the WO3 concentration. The coloring process was accelerated by an addition of TiO2 aqueous sols. Spectral changes were measured using the mixing sol with various molar ratios (γ) of WO3 and TiO2. The absorption spectra changed from those having the single peak at 775 nm to those with two peaks at 640 and 980 nm. Such spectral transformation was ascribed to the structural change of the WO3 nanoclusters, depending on the γ value and the concentration of Cl− ion.Graphical abstractHighlights► Photochromism of WO3 and WO3/TiO2 aqueous sol. ► Effect of various reaction conditions on the coloring rate. ► The coloring rate was the first-order with respect of the WO3 concentration. ► Mixing of WO3 with TiO2 changed the absorption spectra of the photochromism. ► Spectral transformation was ascribed to the structural change of WO3 in the presence of Cl−.

Cu-deposited TiO2 films were prepared by photoreduction of Cu(II) in the presence of sodium formate. With the initial Cu(II) concentrations more than 100 mg L−1, induction periods were observed before the transmittance decreased. Scanning electron microscopy indicated that Cu particles of 2.6 ± 0.5 μm were deposited isolatedly with much open space in the induction periods. The films prepared by changing the irradiation time within the induction periods showed a higher photocatalytic activity than a pure TiO2 for the degradation of methylene blue under the reaction condition without purging air.

The photocatalytic degradation of chlorinated ethanes was studied in a tubular photoreactor packed with TiO2 pellets prepared by sol−gel method. The steady-state condition was not obtained, but the deterioration in the photocatalytic activity was observed during the irradiation. Effects of mole fractions of water vapor, O2, and C2H5Cl or C2H4Cl2 and reaction temperature on the photodegradation of C2H5Cl or C2H4Cl2 were examined, and these data were compared with those obtained by the photodegradation of chlorinated ethylenes. On the basis of the products detected with and without oxygen in the reactant’s gas stream, we proposed the degradation mechanism. Measurement of diffuse reflectance infrared Fourier transform spectroscopy of pyridine adsorbed on the catalysts showed that decrease in the conversion for the photodegradation of C2H5Cl was attributable to the formation of Brønsted acid sites. Comparison of the data obtained with the TiO2 and the sulfated TiO2 (SO42−/TiO2) pellets indicated that the photodegradation of C2H5Cl was suppressed by the presence of the Brønsted sites, but that of trichloroethylene was not affected. Such a difference is attributable to the adsorption process of these reactants on the acid sites on the catalyst surface.

The thermal treatment of TiO2 pellets prepared by the sol–gel method decreased the photocatalytic activity. The activity divided by the specific surface area of the pellets for the complete mineralization of ethylene or chloroform was maximized at the firing temperature of 400°C. For the photocatalytic degradation of trichloroethylene (TCE), most of them were converted to chlorinated by-products, such as dichloroacetic acid, chloroform, and phosgene, and the stoichiometric ratio of [CO2]formed/[TCE]degraded showed a maximal value at 400°C. The electron spin resonance (ESR) spin-trapping with 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) in the flow injection system indicated that firing at 400°C gave the highest signal intensity of DMPO–OH adducts. These findings indicated that the OH radical was produced most effectively on the TiO2 fired at 400°C, which would be related to the content of anatase and rutile. Concerning the formation of chlorinated by-products from TCE, more dichloroacetic acid (DCAA) were detected and less CHCl3 and COCl2 were formed at lower firing temperatures, suggesting that the branching ratio of chloroethoxy radicals to the formation of DCAA or CHCl3 and COCl2 by C–C bond scission depended on the firing temperature.

1,4-Dioxane in hexane as a solvent was adsorbed on TiO2 due to an electrostatic interaction. The porous TiO2 pellets (SG) prepared by sol–gel method were superior adsorbent to ST-B21 and Degussa P-25. Effects of firing temperature of the pellets and the initial concentrations of 1,4-dioxane on the adsorption percents were examined. Photocatalytic degradation of aqueous 1,4-dioxane gave 1,2-ethanediol diformate and formic acid as intermediates. Analysis of total organic carbon indicated that 1,4-dioxane was mineralized effectively in the following order: P-25 > ST-B21 > SG. The photocatalytic degradation of 1,4-dioxane adsorbed on the TiO2 pellets in air showed that ST-B21 had a higher activity than SG. These facts indicate that SG pellet acts as a good adsorbent because of its high specific surface area but the internal region of the pores is not illuminated and acts only as a support.

The photocatalytic degradation of chloroform was studied in a tubular photoreactor packed with TiO2 pellets prepared by sol–gel method. Concentrations of chloroform and CO2 in the gas stream and Cl− ion which was dissolved from the pellets into water after the photodegradation experiments were detected quantitatively. Chloroform was completely mineralized to CO2 and HCl but the conversion decreased drastically with an increase in the irradiation time. Such a decrease in the catalytic activity was attributable to the accumulation of Cl− on the TiO2 surface and was recovered by washing the pellets with water. The presence of Cu and Ca(OH)2 increased the concentration of Cl− accumulated on the surface. The deterioration of the catalytic activity was suppressed with Ca(OH)2 whereas no appreciable effect was observed with Cu. The TiO2 prepared by sol–gel method exhibited a higher catalytic activity than commercially available Degussa P-25.