Nano tungsten trioxide for heat insulation coating can be synthesized under hydrothermal conditions using sodium tungstate and hydrochloric acid as the main reaction raw materials. The effects of different pH values and reaction temperatures on the structure, morphology and photochromic properties of tungsten trioxide were found to be:
1. If the pH value is 1.0-1.5 and the temperature is 80-140°C, the product is hexagonal tungsten trioxide. And the powder particles are short-spiral spheres with uniform particle size distribution.
2. If the pH value is 2.0-2.5 and the temperature is 160-180°C, the product is transformed from hexagonal phase into hydrated tungsten trioxide of cubic phase. And the particles of the synthesized powder are irregular in shape and the particle size distribution is extremely uneven.
It means that the pH values and hydrothermal temperatures of the system have a great influence on the structure, morphology and properties of the WO3 product. Compared with cubic phase tungsten trioxide and the hexagonal phase tungsten trioxide, the hexagonal phase provides a channel for the transmission of photogenerated protons (H+) required for photochromism due to its special pore structure.
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