Nano grain size tungsten trioxide may be used to manufacture thermal insulation coating, and therefore be applied in thermal insulation glass, such as building glass and car window glass. The glass will turn blue when sun shines on such glass.
The reason why tungsten trioxide turns blue is the result of the transition of electrons in the W(V) valence band to the W(VII) conduction band. The double charge injection/extraction model, which was proposed in 1975, is the widely accepted model used to explain photochromism today. In this model, after the tungsten trioxide is irradiated with ultraviolet rays, electrons in the valence band are excited into the conduction band and leave holes in the valence band. While photogenerated electrons are trapped by W(VI) to generate W(V). And the reducing substance inside or on the surface of the oxide film of photogenerated hole forms H+, which is injected into the interior of the film to combine with the reduced oxide to form blue tungsten bronze.
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