Nanocrystalline W3O3 polymorphs. Surfactant assisted growth steps to tailor microstructure and NO2 response

Nanocrystalline WO3 samples are synthesized by different procedures. The first series of samples are obtained by sol-gel reaction, starting from WCI6, followed by thermal treatments in the range 300-750 degrees C. To improve the oxide microstructure, a second series of samples is obtained by submitting the xerogels, obtained from the sol-gel reaction, to prolonged (170 h) hydrothermal (HT) growth steps in the presence of a surfactant, either non-ionic (Lutensol ON70) or ionic (cetylpyridinium chloride), and to a final firing. The HT treatment, in the presence of cetylpyridinium chloride is also combined with Ag promotion (1% Ag). The phase composition of all samples is characterized jointly by XRD Rietveld refinement and Raman spectroscopy. The observed different temperature domains of the nanocrystalline WO3 polymorphs with respect to bulk systems are attributed to the occurrence of surface relaxation phenomena. TEM and SEM images show that the samples submitted to the surfactant HT treatment present a generally improved microstructure while the presence of Ag induces crystallite growth and sintering between the particles. The NO2 sensing measurements show for all samples that the film response decreases with the operating temperatures and is promoted by the presence of humidity. The samples obtained by the surfactant HT treatment show a much better sensor performance with respect to the other samples, the more so in the case of the cationic molecules. The role played by the HT treatment in promoting the features of the WO3 samples is discussed also on grounds of Raman analyses in the water-OH stretching region.