Effect of Temperature and pH on Phase Transformations in Citric Acid Mediated Hydrothermal Growth of Tungsten Oxide

The temperature-dependent composition of suspension during citric acid mediated crystallization of tungsten trioxide (WO₃) from sodium tungstate was studied by in situ Raman spectroscopy. Additionally, microwave-assisted hydrothermal synthesis experiments combined with ex situ analysis by X-ray diffraction and SEM were performed to analyze the effect of pH on the eventually, isothermally, obtained crystal phase and morphology. The Raman results suggest that WO₃·2H₂O precipitates from the tungstate solution upon acidification to pH 0.5 at room temperature. This is first transformed to WO₃·H₂O initiating at T = 70 °C. At temperatures above 170 °C, the crystallization of phase-pure monoclinic WO₃ with well-defined plate-like morphology was observed at pH 0.5. Using the microwave-assisted hydrothermal synthesis procedure shows that increasing the pH to values of 1.5 or 2 results in significant or dominant formation of hexagonal WO₃, respectively. Comparing the activity of selected samples in photocatalytic oxidation of propane using visible light, demonstrates the presence of hydrate phases or hexagonal WO₃ is detrimental to performance.