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Abstract: In this work we report the synthesis, microstructure, electric and spectroscopic properties, and sensing performances of Ti x Sn1− x O2 (x =0.1, 0.2, 0.3, 0.5, 0.7, 0.9) nano-powders and of SnO2 and TiO2 reference samples, prepared via sol-gel route starting from metal-organic precursors working in hydro-alcoholic media. Actually, the attention is particularly focused on properties of the sample with x =0.2, in comparison with ones of the other solid solutions and of the single oxides. Indeed, this solid solution showed a borderline behaviour between that of the solid solution with x =0.1 and that of the other solid solutions with x ≥0.3. An abrupt change in the structural, electrical and spectroscopic properties has been observed, passing from sample with x =0.1, showing a behaviour very similar to that of SnO2, to one x =0.3 showing a behaviour very similar to that of TiO2. The borderline properties of the mixed oxide with x =0.2 represent the expected continuous transition among the two behaviours. [Copyright &y& Elsevier]

Abstract: Tin–titanium solid solutions at increasing Ti molar fraction, x =0.1, 0.3, 0.5, 0.7 and 0.9, were prepared by symplectic gel co-precipitation (SGC) method from stoichiometric hydroalcoholic solutions of Ti and Sn metalalkoxides. Structural, spectroscopic and electrical analyses were performed and the results have been correlated. The peaks of the XRD patterns change from the positions of SnO2 cassiterite to those of TiO2 rutile, when increasing the Ti content in the solid solution. Photoluminescence (PL), FT-IR and UV–vis-NIR characterizations are in agreement with the formation of solid solutions: PL peak falls either in the visible or IR region, depending on the Ti content in the solid solution; FT-IR spectra reveal a progressive upward shift of the high-frequency absorption edge of the skeletal vibration modes from the value of SnO2 to one of TiO2 on increasing the Ti content; UV–vis-NIR spectra reveal a progressive downward shift of the VB–CB absorption edge on increasing Ti content. The changes induced by CO interaction at increasing temperature in the FT-IR and UV–vis-NIR spectra revealed a different behaviour of the materials following the Ti content: for the lowest Ti molar fraction (x =0.1) as well as for SnO2 the reduction gives rise to mono-ionized oxygen vacancies, for the other solid solution the loss of reticular oxygen gives rise to doubly ionized oxygen vacancies and to Ti3+ ions. From the electrical-conductance point of view, two groups of films can be identified: pure SnO2 and the solid solution with x =0.1, and all the other solutions including pure titania, in agreement with the results of FT-IR investigations. The best response to carbon monoxide was achieved by the solutions with x =0.3 and x =0.5. Opposite to electrical properties, exhibiting good responses to reducing gases, these agents did not appreciably modify the PL yield of the solid solutions. Conversely, PL emissions were sensitive to NO2, particularly for Ti0.5Sn0.5O2. [Copyright &y& Elsevier]

Abstract: Novel Ti x Sn1−x O2 (0.3< x <0.9) nanostructured oxides for gas sensing, have been synthesized and characterized through a great variety of investigation techniques, such as electron microscopy, X-ray diffraction, specific surface area measurements, UV–vis and IR spectroscopies, photoluminescence emission spectra and finally electrical measurements. All structural investigations showed solid solutions of the two single oxides; only the oxide with x =0.7 showed a beginning of spinodal decomposition revealed by XRD. Surprisingly, the samples with x =0.3 and 0.5 exhibited carbon monoxide response much higher than for pure SnO2. Investigations on this point, performed through spectroscopic measurements in the presence of CO, suggest a different behaviour of the solid solutions from the single oxides. [Copyright &y& Elsevier]

Abstract: Polycrystalline WO3 films were fabricated through thick film technology. Stimulated temperature measurements showed that the intergranular Schottky barrier is not high enough to apply the depletion approximation to evaluate the fundamental parameters related to the sensing properties. Therefore, a semi-analytical approach to the Poisson''s equation with the complete charge density was chosen. As a result, we could estimate the dependence on frequency of both apparent permittivity and depletion region width by impedance spectroscopy. [Copyright &y& Elsevier]

Nanopowders of pure and of Mn-, Ta- and Zr-loaded (5 wt.%) WO3 were prepared and printed as thick films. Investigation of the influence of the doping on morphology, structure and gas response versus NO2 has been performed. Pure nanometric WO3 was prepared by a modified sol–gel synthesis while loading was carried out by impregnation with Mn(II), Ta(V) and Zr(IV) chlorides. Addition of Ta resulted in grain coalescence and phase transition inhibitions in the layers with respect to pure WO3 films, being the effect strongly enhanced in the films fired at 850 °C. The Ta-doped films turned out to be the most sensitive films with a response extending down to the sub-ppm domain. [Copyright &y& Elsevier]

Different synthetical routes involving several solvents for obtaining WO3 powders, suitable for gas sensing have been studied. Aqueous and alcoholic solvents proved to be the most promising media to grow nanometric particles. Among these, a modified sol–gel route that involves ethanol and a di-chetone, highlighted a more homogeneous grain-size distribution. The structure and morphology evolution of pure WO3 powders with time and annealing temperature have been studied. The study of the energy barrier and conductance versus temperature allows to understand the reason for the high response of WO3 to oxidising gases and its lack of sensitivity to reducing agents. Films of WO3 responded to NO2, sensing as low a gas concentration as 200 ppb. [Copyright &y& Elsevier]