Currently, methods to synthesize tungsten bronze compounds mainly are the wet chemical method, heat reduction method and thermal decomposition method. Wet chemical method to synthesize ammonium tungsten bronze is to put the starting material in the reducing solvent refluxing for several days, size of the sample obtained by this method is too large, it’s usually between a few to ten micron, and the preparation process requires a long time and high energy consumption. Thermal reduction method is to uniformly mix tungsten oxide, tungsten powder and metal tungstates in proper proportions, then heated in a vacuum or under an inert atmosphere, the reaction temperature is usually about 1000 ℃, and remove unreacted impurities after the reaction is completed. Since the thermal stability of ammonium tungsten bronze difference is poor and decomposition temperature (300 ℃) is lower than the synthesis temperature, the thermal reduction method can not be used to synthesize ammonium tungsten bronze. The thermal decomposition method to synthesize ammonium tungsten bronze is to heat and decompose ammonium paratungstate in a reducing atmosphere (H2 or a mixed gas of H2, Ar, etc.), the size of the resulting sample is too large, and this method can not obtain completely pure phase ammonium tungsten bronze, ammonium content in sample is too low and easy to excessive decomposed into tungsten oxide.
The pure phase ammonium tungsten bronze nano-powder can not be directly obtained in current study, so usually break the large micron-sized particles obtained into small particles by milling, but these compounds are easily to be oxidized and lost live and decompose in the milling process, also accompanied by crystallization performance degradation. For the above problems, some scholars have proposed a synthesis method to directly synthesize ammonium tungsten bronze powder with controllable particle size.
Preparation of reduced state ammonium tungsten bronze nanoparticles: dissolve 0.01~1g tungsten hexachloride or tungsten tetrachloride in 20~40mL oleic acid solution, and stirred to obtain homogeneous solution, then added 4~30mL oleylamine, and mix evenly, move to supercritical reaction kettle, crystallization reaction at 150~350 ℃ for 0.5~48 hours, the powder samples were centrifuged and washed after reaction, dry under vacuum at 40~250 ℃ for 1~12 hours, and the reduced state ammonium tungsten bronze nanoparticles are obtained, the mole fraction of ammonium group in the composition is between 0.2~0.3. In addition, samples obtained by this method have strong near-infrared absorption ability, the film containing nanoparticles can effectively shield the near infrared rays of 780 ~ 2500nm and maintain high visible light transmittance.
It was proposed a synthesis method to directly synthesize ammonium tungsten bronze powder with controllable particle size since the pure ammonium tungsten bronze nano-powder can not be directly obtained in current study.
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