Impact of Synthesis Conditions on WO3 Powders from Ammonium Paratungstate

Transition element oxides are of great interest for both fundamental science and technological applications due to their multifunctional properties. Tungsten oxides WOx , where 2 ≤ x ≤ 3, are among the most important substances from this large group. Nowadays they are commonly used as photocatalysts, gas sensors, and electrochromic materials. There are also some new applications of WOx for dyesensitized solar cells, optical data storage and field-emission displays. Besides, cation-doped WOx is considered as high-Tc superconductor

Oxides WOx and their hydrates can be synthesized using various chemical and physical methods, such as wet chemical precipitation, sol-gel hydrolysis of tungsten alkoxide, ion exchange and thermal decomposition of tungsten salts. Films of WOx can be deposited on various substrates using various vacuum techniques. Different crystalline structures as well morphologies occur depending on the processing conditions. The WOx phase transitions are rather complex.

To study the effect of synthesis conditions on the crystal and local structures of WO3 powders prepared by wet chemical precipitation and thermal decomposition of tungsten salts, the following experiment has been conducted:
Ammonium paratungstate (APT), sodium tungstate (ST) Na2WO4∙2H2О (98.5%), precursors (tungsten trioxide hydrates or tungstic acids) were prepared by acidification of tungstates salts solutions with hydrochloric HCl (analytical grade) or nitric HNO3 (analytical grade) acids until the final pH ~ 0. Tungsten trioxide WO3 powders were prepared by calcination of synthesized precursors or APT at 300 - 1000оС for 2 - 6 h in air.

Finally, crystal structures of synthesized powders were studied by X-ray diffraction, X-ray absorption fine structure spectroscopy, IR- and Raman-spectroscopy, and scanning electron microscopy
In conclusion, the calcination of all initial compounds at temperatures above 500 °C led to formation of single phase of monoclinic I γ-WO3. It was concluded that the neutral octahedral complex [W=O(ОН)4(Н2О)] can be a structural unit of the precursors prepared in acidic suspensions. The local structure of synthesized tungsten oxides consists of edge-shared and corner-shared distorted octahedral WO6 species linked together.

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