Tungsten trioxide may be applied for manufacturing thermal insulation coating. And the widely accepted mechanism of discoloration of WO3 is the dual charge injection/extraction model (inter-band transition of electrons) proposed in 1975.
The dual charge injection/extraction model was first discovered and proposed in the electrochromic study of WO3 and was later widely used to explain the photochromic performance of WO3.
When tungsten trioxide is irradiated by ultraviolet light, electrons in the valence band are excited into the conduction band, leaving electron holes in the valence band, generating electron-hole pairs, and photo-generated electrons being captured by W6+ to form W5+. And simultaneously, the reducing species inside or on the surface of photo-generated hole oxidation film is injected into the interior of the film, and combines with the reduced oxide to form a blue tungsten bronze HxWO3. And blue color is due to the transition of electrons in the W5+ valence band to the W6+ conduction band.
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