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Effect of Temperature and pH on Phase Transformations in Citric Acid Mediated Hydrothermal Growth of Tungsten Oxide
Author(s) -
Wenderich Kasper,
Noack Johannes,
Kärgel Anne,
Trunschke Annette,
Mul Guido
Publication year - 2018
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/ejic.201701156
Subject(s) - chemistry , raman spectroscopy , hydrothermal circulation , crystallization , tungstate , monoclinic crystal system , hydrothermal synthesis , citric acid , inorganic chemistry , sodium tungstate , hydrate , phase (matter) , tungsten trioxide , nuclear chemistry , tungsten , chemical engineering , crystal structure , crystallography , physics , organic chemistry , optics , engineering
The temperature‐dependent composition of suspension during citric acid mediated crystallization of tungsten trioxide (WO 3 ) 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 WO 3 · 2H 2 O precipitates from the tungstate solution upon acidification to pH 0.5 at room temperature. This is first transformed to WO 3 · H 2 O initiating at T = 70 °C. At temperatures above 170 °C, the crystallization of phase‐pure monoclinic WO 3 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 WO 3 , respectively. Comparing the activity of selected samples in photocatalytic oxidation of propane using visible light, demonstrates the presence of hydrate phases or hexagonal WO 3 is detrimental to performance.