The structure and composition of oxidized and reduced tungsten oxide thin films |
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Authors: | Simon Penner,Xianjie Liu,Bernhard Klö tzer,Frederik Klauser,Bernd Jenewein,Erminald Bertel |
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Affiliation: | Institute of Physical Chemistry, University of Innsbruck, Innrain 52a, A-6020 Innsbruck, Austria |
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Abstract: | The structure, morphology and composition of pure WO3 thin films deposited onto vacuum-cleaved NaCl(001) single crystals have been studied at different substrate temperatures up to 580 K and under different oxidative and reductive treatments in the temperature range 373-873 K by Transmission Electron Microscopy, Selected-Area Electron Diffraction and X-ray Photoelectron Spectroscopy (XPS). A transition from an amorphous structure obtained after deposition at 298 K to a more porous structure with small crystallites at the highest substrate temperatures has been observed. XPS spectra reveal the presence of W6+ irrespective of the preparation procedure. Significant changes in the film structure were only observed after an oxidative treatment in 1 bar O2 at 673 K, which induces crystallization of a monoclinic WO3 structure. After raising the oxidation temperature to 773 K, the film shows additional reconstruction and a hexagonal WO3 structure becomes predominant. This hexagonal structure persists at least up to 873 K oxidation temperature. However, these structural transformations observed upon oxidation were almost completely suppressed by mixing the WO3 thin film with a second oxide, e.g. Ga2O3. Reduction of the WO3 films in 1 bar H2 at 723-773 K eventually induced the formation of the β-W metal structure, as evidenced by electron diffraction and XPS. |
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Keywords: | Electron microscopy Selected area electron diffraction X-ray photoelectron spectroscopy Tungsten oxide Oxidation Reduction β-W |
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