Synthesis of tungsten oxide nanoparticles using a hydrothermal method at ambient pressure

Tungsten oxide (WO3) nanostructures receive sustained interest for a wide variety of applications, and especially for its usage as a photocatalyst. It is therefore important to find suitable methods allowing for its easy and inexpensive large scale production. Tungstite (WO3·H2O) nanoparticles were synthesized using a simple and inexpensive low temperature and low pressure hydrothermal (HT) method. The precursor solution used for the HT process was prepared by adding hydrochloric acid to diluted sodium tungstate solutions (Na2WO4·2H2O) at temperatures below 5 °C and then dissolved using oxalic acid. This HT process yielded tungstite (WO3·H2O) nanoparticles with the orthorhombic structure. A heat treatment at temperatures at or above 300 °C resulted in a phase transformation to monoclinic WO3, while preserving the nanoparticles morphology. The production of WO3nanoparticles using this method is therefore a three step process: protonation of tungstate ions, crystallization of tungstite, and phase transformation to WO3. Furthermore, this process can be tailored. For example, we show that WO3can be doped with cesium and that nanorods can also be obtained. The products were characterized using powder x-ray diffraction, transmission electron microscopy (including electron energy-loss spectroscopy and electron diffraction), and x-ray photoelectron spectroscopy.