Liquid phase method is the most widely used method among many methods for synthesizing nano tungsten trioxide. Its advantage is that the nano structured WO3 particles are fine and evenly distributed, the content of the chemical components can be precisely controlled, the morphology of the nanostructured WO3 can be controlled by adjusting the morphology control agent, the synthetic process is simple, economical and good to mass production. There are four main methods including hydrothermal synthesis, sol-gel method, precipitation method and microemulsion method.
1.0 Hydrothermal synthesis method
Hydrothermal synthesis method also called hydrothermal method is one of the most typical liquid phase chemical methods. It reacts in a specific closed container, use the solvent of water solution or other organic solvent, heat the closed container to a set temperature to get high temperature and high pressure, make the mixed solution to do chemical reaction under this condition, enable the substance which is difficult to dissolve or insoluble to dissolve and recrystallize. Its biggest advantage is that it not needs to calcinate the precursor, which effectively avoids impurity doping and powder agglomeration.
2 Sol gel method
The sol-gel method mainly is to make initial sol through the hydrolysis and polymerization reaction of metal alkoxides or metal non-alkoxides, the sol is concentrated to form a gel, the gel has three-dimensional network structure, is dried and heated, metal oxide nano powder is obtained after cooling and grinding. Sol-gel method has the advantages of low reaction temperature, easily controlling product composition, small particle size, narrow particle size distribution and high purity of powder etc. There are still many problems existing such as the expensive price of raw materials, easily reuniting products in the drying process, and the long cycle of synthesis.
3.0 Precipitation method
The precipitation method is to add precipitant into metal salt solution to precipitate, then heat and dry or calcine the precipitating product to obtain nanoparticles finally. The precipitation method is simple in process flow and low in cost, but still has some disadvantages such as some impurity substances difficultly removing and particles easily agglomerating.
4.0 Microemulsion method
Generally the microemulsion method system is the liquid dispersion system composed of surfactant, cosurfactant, solvent and water. It has the characteristic of transparent or translucent, isotropic and thermodynamically stable. The microemulsion method has the advantages of simple reaction equipment, good dispersibility of powder, low energy consumption, easy operation, less agglomeration of powder and smaller particle size etc. However, there are still many shortcomings for the microemulsion method, for example, the dosage of surfactants and solvents required is very large, is not easy to remove later and the reaction cost is high.
With the development and improvement of technology, various synthetic methods are generally combined to prepare better nanostructured WO3.
