Yellow tungsten oxide, which is known as tungsten trioxide (WO3), is a transition metal oxide with excellent photochromic property. For example, WO3 film turns blue when irradiated by ultraviolet light (in the sunlight). And the research on the photochromic property of yellow tungsten oxide can be explained by the dual charge injection - extraction model.
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The dual charge injection - extraction model points out that after WO3 is irradiated with ultraviolet rays, the electrons in the valence band are excited into the conduction band, leaving holes in the valence band, and the photogenerated electrons are captured by W (Ⅵ) to generate W (Ⅴ). At the same time, the reducing substances inside or on the surface of the photo-generated hole oxidizing film (such as water) generate H+, which is injected into the film and combines with the reduced oxide to produce blue tungsten bronze HxWO3. Wherein, the blue color is the result of the transition from the electrons in the W (Ⅴ) valence band to the W (Ⅶ) conduction band.
