Tungsten-Tin solid solutions: synthesis, morphological, structural, electronic, and gas sensing properties

Today there is a variety of structures, materials and working principles exploitable to realize gas sensors. Among them, semiconductor gas sensors using metal oxides (MOX) as gas sensing element are a suitable choice to detect a wide range of gases both in scientific research and in industrial and environmental applications. Many actions can be made to improve the sensing performances by focusing on the sensing material, such as the synthesis of nanostructures with high specific surface area, the loading with noble metals. In particular, the synthesis of mixed metal oxides and solid solutions have been usually considered for the superior performances shown with respect to the single oxides. , Pure oxides, SnO2 and WO3, were prepared through sol-gel route. Tungsten-tin (W-Sn) solid solutions at increasing nominal Sn molar fraction (0.1, 0.3, 0.5 and 0.9) were synthesized by sol-gel co-precipitation with the aim to join advantages of high sensitivity toward oxidizing gases for WO3 and Sn addition to reduce WO3 grain growth with temperature. The obtained powders were characterized by electron microscopy (SEM and TEM), X-ray diffraction, specific surface area measurements (BET), UV-Vis-NIR and IR spectroscopies and ICP-OES analysis. It turned out that the mixed materials achieve the goal to reduce the WO3 grain growth with temperature. The XRD patterns of the pure WO3 and W-Sn(0.1, 0.3 and 0.5) powders correspond to the monoclinic crystal structure (space group P21/n) of polycrystalline WO3 without any other phases. No change in these W-Sn materials unit cell parameters and unit cell volume was observed with respect to pure WO3. The XRD pattern of WS(0.9) corresponds to the tetragonal crystalline structure (space group P42/mnm) of SnO2 without any other phase segregation. No appreciable change in the unit cell parameters and unit cell volume was observed with respect to pure SnO2. The prepared powders were then deposited to realize gas sensors in form of thick films through screen-printing technology. The corresponding sensing layers were tested in a sealed test chamber using the flow-through technique and test gases like NOx and CO. All studied materials, SnO2, WO3, and the W-Sn solid solutions behave as n-type semiconductors because of lattice defects. In SnO2 and WS(0.9), oxygen vacancies act as electron donor levels, while WO3 and W-Sn(0.1, 0.3 and 0.5) are characterized by polarons as evidenced by spectroscopic analysis. The gas sensing measurements toward NOx and CO highlighted that SnO2 is more performant in CO detection. The W-Sn(0.1, 0.3 and 0.5) sensors offer a better response with respect to pure WO3 toward NOx, at the same time maintaining the characteristics of almost complete insensitivity to carbon monoxide.