Nano yellow tungsten oxide may be applied for the preparation of thermal insulation film. And this type of heat-insulating film may show its color by light irradiation with suitable energy. The photochromic mechanism of yellow tungsten oxide film can be expressed by the following equation:
WO3+hv→WO3*+e-+h+
h++1/2H2O→H++1/4O2
WO3+xe-+xH+→HxWO3
When the yellow tungsten oxide film is irradiated with light, electrons e- and holes h+ are generated, and the photogenerated electrons enter the conduction band of the yellow tungsten oxide. The photogenerated holes react with the adsorbed water on the surface of the film to generate H+, which then diffuses into the interior of the yellow tungsten oxide, resulting in color changing. The pure yellow tungsten oxide film only inducts ultraviolet light. Therefore, for the un-pretreated initial film of yellow tungsten oxide, electrons and holes can only be excited by ultraviolet light. After cathodic polarization, trap levels are generated in the crystal lattice. These energy levels are in the band gap region and can be excited by visible light, resulting in discoloration of the visible light.
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