Crystalline structure, defects and gas sensor response to NO₂ and H₂S of tungsten trioxide nanopowders

Structural and NO₂ and H₂S gas-sensing properties of nanocrystalline WO₃ powders are analysed in this work. Sensor response of thick-film gas sensors was studied in dry and humid air. Interesting differences were found on the sensor response between sensors based on 400 and 700 °C-annealed WO₃, what motivated a structural study of these materials. Crystalline structure and defects were characterised by X-ray diffraction (XRD), Raman spectroscopy and transmission electron microscopy (TEM). Experimental results showed that both triclinic and monoclinic structures are present in the analysed materials, although their amount depends on the annealing treatment. Crystalline shear planes, which are defects associated to oxygen deficient tungsten trioxide, were found in 400 °C-annealed WO₃ and their influence on XRD spectra was analysed by XRD simulations. Moreover, XRD and Raman spectra were also acquired at normal metal oxide-based gas sensor working temperatures in order to relate both crystalline structure and sensor response.