WO3 is an important raw material for production of all-solid-state electrochromic device, in which tungsten trioxide is first formed into a tungsten trioxide electrochromic film. The discoloration of WO3 film can be explained by inter-valency charge transfer model. The inter-valency charge transfer model, also known as Fanghnan model or dual injection/extraction model, was proposed by Fanghnan based on the basic theory of ITA. They believed that:
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Under the action of electric field, the cations M+ (such as H+ and Li+) and the electron e- are co-injected. The electron e- is confined to a certain W5+ ion and enters the 5d channel. In order to maintain the electric neutrality, M+ must also reside in this region, and tungsten bronze compound MxWO3 is formed. The equation is:
WO3+x(M++e-)=MxWO3
Here, M+=H+, Li+, Na+; e- is an electron. When a tungsten bronze compound is formed in an electron and small ion injected transparent tungsten oxide film, the film turns blue. And when ions and electrons are extracted, the film is reduced to the original transparent state. It has been found that in this reaction, some of the W6+ is reduced to W5+. It is generally believed that the formation of W5+ produces an electrochromic effect, in which the injection of ions and electrons plays a key role. Therefore, the more precise reaction formula is as follows:
WO3+x(M++e-)=MxW(1-x)^6+Wx^5+O3
