Comprehensive understanding on the formation of highly ordered mesoporous tungsten oxides by X-ray diffraction and Raman spectroscopy

Highly ordered mesoporous tungsten oxides (WO3) have been synthesized via the hard templating method using mesoporous silica as a hard template and phosphotungstic acid as a precursor. The influences of the calcination temperature, the impregnation ratio of the precursor to the silica hard template and the hydrothermal treatment time of the silica template on the structure of products have been systematically investigated. Both X-ray diffraction and Raman spectroscopy were performed to study the crystalline phases of the resultant mesoporous WO3 as a function of the calcination temperature. The ideal calcination temperature for the synthesis of highly ordered mesoporous WO3 is found to be between 823 and 1073 K. Only a small fraction of crystalline WO3 can be detected in the composites when the calcination temperature is below 773 K. When the calcination temperature is increased from 773 to 1173 K, the monoclinic and triclinic phases of WO3 coexist in the products. Moreover, the ratio of the monoclinic to triclinic phase changes with the variation of the calcination temperature, and it reaches a maximum at about 923 K. Increasing the hydrothermal treatment time of the silica template KIT-6 to 5 days at 373 K favors the synthesis of highly ordered mesoporous WO3. When KIT-6 prepared without hydrothermal treatment is used as the hard template, a symmetry transformation from Ia3¯d to I41/a can be observed. The present work provides a convenient and reproducible method for the synthesis of highly ordered mesoporous WO3 materials with crystalline walls, which may find potential applications in different areas.