Nano Tungsten Trioxide Preparation by Microemulsion Method

Because of its large specific surface area and remarkable surface effect, nano-tungsten trioxide has been widely used in various gas-sensing materials, and has become one of the indispensable materials for many high-quality gas-sensing materials. At present, the preparation methods of nano tungsten trioxide are mainly sol gel method, co precipitation method, microemulsion method, physical deposition method and so on. Microemulsion method has been widely applied in recent years because of its simple process.

Nano sized tungsten trioxide was prepared by microemulsion. The non aqueous components of the microemulsion system were selected: the oil phase was n-octane, the surfactant was sixteen alkyl three methyl ammonium bromide (CTAB), and the cosurfactant was n-butanol. The specific methods and steps for the formation of tungsten trioxide powder are as follows:

(1) preparation of microemulsion non aqueous component system
At 10~60 degree temperature, the non-aqueous components of the system were mixed and stirred evenly according to the ratio of CTAB surfactants: octane oil phase: butanol cosurfactant to 1: 3~7: 1~4, and the nonaqueous component system of microemulsion was prepared.

(2) preparation of sodium tungstate microemulsion
Sodium tungstate was dissolved in water phase and stirred evenly to prepare sodium tungstate aqueous solution with concentration of 0.05-0.10g/ml. The mass ratio of CTAB surfactant, n-octane oil phase, n-butanol cosurfactant and sodium tungstate solution is 1:3-7:1-4:0.45-0.7. Then the phacoemulsification was carried out at a temperature of 20~60 degrees Celsius. The ultrasonic frequency was 15 to 50KHZ and the emulsification time was 2 to 30min. Transparent sodium tungstate microemulsion was prepared.

(3) preparation of concentrated hydrochloric microemulsion
In the first step (1), concentrated hydrochloric acid was added into the microemulsion non-aqueous component system and stirred evenly. After mixing, the mass ratio of each component was CTAB surfactant: n-octane oil phase: n-butanol cosurfactant: concentrated hydrochloric acid was 1:3-7:1-4:0.4-1. Then phacoemulsification was performed at 20~60 degree temperature, the ultrasonic frequency was 15 ~ 50KHZ, and the emulsification time was 2 ~ 30min, and then concentrated hydrochloric acid microemulsion was prepared.

(4) preparation of tungstic acid microemulsion
After the completion of step (2) and (3), the concentrated hydrochloric acid microemulsion prepared by step (3) steps: the mass ratio of sodium tungstate microemulsion prepared by step (2) steps is 1: 2.5 to 4, and the reaction is carried out by mixing and stirring, the reaction temperature is 20~80 degrees, the stirring speed is 500 ~ 1500r/min, and the reaction time is 5 to 50h, and yellow tungstic acid microemulsion is prepared.

(5) demulsification
After completion of step (4), in the tungstic acid microemulsion prepared by step (4), 100 to 240% water and 5 ~ 125% ethanol equivalent to the total mass of the tungstic acid microemulsion were added, and then stirred at a temperature of 60~90 degrees. After demulsification and precipitation, the precipitate was filtered out and the filtrate was discarded.

(6) washing
The precipitation filtered out after (5) step demulsification is washed 2-10 times with anhydrous acetone equivalent to 5-100 times the volume of the precipitate and precipitated. The precipitation of WO₃ after washing is collected, which is conducive to environmental protection and cost saving.

(7) calcination
The WO₃ precipitate collected and washed in step (6) was calcined at a temperature of 200-600 C and a calcination time of 2-10 H. The calcined powders are nano-sized tungsten trioxide powders.
The preparation of nano-tungsten trioxide by microemulsion method has the advantages of strong universality, mild reaction conditions, simple equipment, easy operation. The prepared nano-tungsten trioxide powder can be widely used to prepare high-quality gas-sensitive materials such as NO_x, H₂S, NH₃, etc. and can also be widely used as solar energy absorption materials.

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