The band gap of tungsten trioxide (WO₃) is 2.8eV, and by doping and surface modification, the band gap can be further reduced, so that the electrons on the band gap can be excited to the conduction band under visible light irradiation, thereby having photocatalytic property. Thus, controlling the size and morphology of WO₃ particles in the manufacturing process is the key directions for the researchers, and the hollow mesoporous WO₃ sphere prepared from ammonium metatungstate (AMT) is one of the researching results. Because the level shift of conduction band electrons and valence band holes which caused by the quantum effect makes reducing and oxidizing of WO3 increased; when particle size of tungsten trioxide becomes smaller, the photo-generated electron and hole pairs will soon reach the surface of tungsten trioxide and react, thus to effectively reduce the recombination rate of photo-generated electrons and holes; at the same time of the particle size becomes smaller, the specific surface area and adsorption points will be increased, and effectively promoting the photocatalytic reaction.

The principle of preparing hollow mesoporous WO₃ sphere from the raw material of ammonium metatungstate is that during the heat treatment since the inconsistent position of each AMT hollow ball, each sub-grain will grow along crystal orientation preferentially to form the elongated particles, and thus to form mesoporous on the surface of the hollow sphere. The steps are as follows:

1. Select 20g of ammonium metatungstate with the WO₃ content of 85.0~89.0wt% (without further purification) as the raw material, and dissolve in deionized water to get a ammonium metatungstate solution with the concentration of 20g/L;

2. Spray drying to firstly get ammonium metatungstate hollow sphere;

3. Move the ammonium metatungstate hollow sphere into a high temperature tube furnace for heat treatment, heated to 700°C with the temperature elevation rate of 5°C/min and holding for 2 hours to obtain hollow mesoporous WO₃ sphere.

 

Research indicates that hollow mesoporous WO₃ sphere has good photocatalytic property, which the photocatalytic degradation of methylene blue solution reached to 65.9% within 15 minutes.