From doping to phase transformation: Ammonia sensing performances of chloroalkoxide-derived WO3 powders modified with chromium

Abstract WO 3 precursor solutions were prepared by methanolysis of WCl 6 in presence of acetylacetone as a stabilizer. Chromium addition was achieved by mixing Cr 2-ethylhexanoate with the pure solutions, with Cr: W atomic concentrations ranging from 2% to 22%. Powders were prepared by drying the solutions and heat-treating the product up to 700 °C. After heat-treating at 400 °C, crystalline WO 3 was obtained, and X-ray diffraction and Transmission Electron Microscopy showed that the powders were constituted by a mixture of the WO 3 monoclinic and triclinic crystallographic phases. The Cr-modified samples, with a Cr concentration of at least 5%, presented the additional phase Cr 2 WO 6 . Structural investigations suggested that this phase was favored instead of chromium oxides due to the incorporation of Cr in the WO 3 lattice in interstitial position. The sensing tests towards ammonia gas, in concentrations ranging from 50 to 500 ppm, showed that, up to 5% concentration, Cr addition is beneficial in lowering the best operating temperatures and/or improving the response with respect to the pure powders. For higher Cr concentrations, the response severely decays. This result was interpreted in terms of the Cr 2 WO 6 grains and of the influence of lowered concentration of interstitial Cr on the oxygen vacancies.

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