The Physical and Chemical Properties of Tungsten Trioxide

Tungsten trioxide is both an n-type semiconductor material and a "d⁰" oxide. This dual identity allows tungsten oxide to have both catalytic, optical, and electrical properties. The formula of tungsten trioxide is quite simple, but its physical properties are very complicated. The structure of tungsten trioxide is not stable and unstable, this is closely related to its octahedral crystal structure and complex outer electronic structure, and also makes its electrical and optical properties exist in multiple states.

The molecular weight of WO₃ is 231.85, the density is 7.16g/cm³, insoluble in water, insoluble in acid, and slightly soluble in hydrofluoric acid, which can be dissolved in hot alkali and ammonia water. The melting point is 1473 degrees centigrade. The boiling point is above 1750 ℃, but it begins to sublimate at 1100 ℃, and the sublimation heat is 459.8K/mol. When the heat is heated, the color is deepened and the cooling is restored.

The outer electron configuration of tungsten is 5d⁴6s². Therefore, six tungsten valence states of +2, +3, +4, +5 and +6 can be present in the compound, and different valence states coexist. The content of oxygen in WO₃ does not meet the strict stoichiometric ratio, usually in the form of WO_3-y that satisfy the WO₃ stoichiometry and without any impurities, should be colorless and transparent insulator, its band width is 2.9eV at room temperature. The non stoichiometric WO_3-Y ceramics exhibit semiconductor behavior, and the change of color with oxygen content varies from bright yellow to yellow green.

For a single crystal, the electrical properties of a single crystal can be characterized by the change of its structure and oxygen content, and the behavior of metal or semiconductors can be presented respectively. And the changes of electrical properties after doping is also great, and also by the influence of atmosphere environment. As mentioned above, the oxygen content in the tungsten oxide is difficult to be strictly controlled. The oxygen content, to a large extent, depends on the electrical properties of the material, therefore, it is rather difficult to give definite electrical parameters for tungsten oxides, such as carrier concentration and mobility. In practical applications, approximate or assumed values are often taken.

The molecular structure of WO₃ catalyst can be used as the working principle is to use their changeable structure similar to similar effect is similar to the potential energy of the reactants, generate a number of potential energy similar to the intermediate state, the gradual transition into the product molecules. Therefore, the physical and chemical properties of tungsten trioxide is used as catalyst, photocatalyst with special skills.

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