It is for photocatalyst application.
Synthesized WO3 is still yellow after annealing and cooled afterwards. But for the commercial WO3, it turned whitish green like a light cabbage colour and the photocurrent is low. Also it is more difficult to dispersed and grind. This is really confusing. Sometimes, it sometimes turned yellow after annealing.
The possible reason for the colour and properties changes with the commercial WO3 could be as a result of the method of synthesis. The WO crystals undergo structural and morphology changes, and the degree of changes depends on the preparation method and annealing treatment. For instance, WO3 prepared via hydrothermal route may contain the monoclinic structure and can be transformed successively to WO3. Y H2O after annealing between 350- 500oC. So this could be the reason for changes in the colour you observed and will eventually affect the properties of the WO3.
i agree with Mr Akeem Oladipo, The possible reason for the colour and properties changes with the commercial WO3 could be as a result of the method of synthesis
Dear Soong Leong Sim
Greetings of the day!!
I strongly concur with Prof. Artur Braun. Tungsten oxide WO3, is a chemical compound containing oxygen and the transition metal tungsten. Tungsten oxide occurs naturally in the form of hydrates, which include minerals: tungstite WO3·H2O, meymacite WO3·2H2O and hydrotungstite (of the same composition as meymacite, however sometimes written as H2WO4). The appearance of WO3 depends on the stoichiometry degree of this oxide. If it is a stoichiometry, then its color is usually canary yellow powder, but if isn't stoichiometric, it may takes different colors begins from yellowish wet color, greenish yellow and blue, as illustrated in the appended images. This may be attributed to crystallinity degree. In the crystalline tungsten trioxide there are a changing in the electronic density of states, which is responsible for the electrochromic effect, the electronic conductivity and the structural transformations are strongly correlated to each other. The crystal structure of WO3 can be described as derivatives of an ideal cubic structure based on corner-sharing [WO6] octahedra, known as the ReO3-type structure. The different degrees of distortion with respect to the ideal cubic structure are caused by the second order Jahn-Teller effect, reflecting an interplay between the lattice phonons and the electronic bands.
Good luck
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