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.