Yellow tungsten oxide, or tungsten trioxide, is an electrochromic material that and can be used to produce smart window. And the prepared smart windows have been commercialized, which are increasingly used in architectural glass, especially on glass walls. The smart windows of tungsten trioxide nanoparticles have excellent heat insulation performance and heat preservation effect.

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Since the electrochromic mechanism of nano grain size yellow tungsten oxide is extremely complex, the real cause of discoloration has been controversial. Scientists have proposed various models to explain the mechanism of color change, including the color center model proposed by Deb in 1973, the small polaron model proposed by Schirmer, and the inter-valent charge transfer model proposed by Faughnan. Currently, what is commonly accepted by electrochromic researchers is the inter-valency charge transfer model, as shown in the following equation:
WO3|Transparent + xM+ + xe- ? MxWO3|Blue
Wherein, M+ is a positive ion, and may be H+, Li+, Na+, etc., and 0