Chloro-Alkoxide Route to Transition Metal Oxides. Synthesis of WO₃ Thin Films and Powders from a Tungsten Chloro-Methoxide

A tungsten chloromethoxide was prepared by the methanolysis of WCl 6 . The resulting solutions were modified by addition of acetylacetone to improve their stability. The resulting deep blue solutions were studied by infrared and Raman spectroscopy. The structure of the solutions was characterized by polynuclear tungsten species containing W—O—W bonds. The solutions were used for preparing thin films, by spin-coating onto oxidized silicon substrates, and powders, by evaporation at reduced pressure. The as-dried thin films and powders were heat-treated at various temperatures up to 700 °C and characterized by infrared and Raman spectroscopy and X-ray diffraction. The investigations showed that the precursor structure follows a continuous evolution to the final WO 3 structure, by further condensation and thermally activated reorganization of the initial species. The final crystalline structure was a mixture of the most stable δ-triclinic and γ-monoclinic phases. The absence of W═O groups both in the solutions and the final materials and the weakness of the W—Cl bond explained the lower stability of the solutions, if compared to oxychloride precursors like WOCl 4 . The structure and morphology of the films, characterized by scanning and transmission electron microscopy, was constituted by discrete spheroidal grains, elongated along the substrate surface. Chemoresistive gas sensing devices were prepared with the WO 3 powders heat-treated at 500 °C and tested for the detection of ammonia and nitrogen dioxide. The results indicated the capability of the sensors to detect the target gases over a broad range of concentrations, with a stable signal.