Structure and gas sensitivity of WO ₃–In ₂O ₃ and WO ₃–Co ₃O ₄ oxide compositions

Using oxide compositions is a promising method of increasing the sensitivity and selectivity of semiconductor gas sensors on the basis of SnO₂, In₂O₃, WO₃ and other oxides. We have studied nanocrystalline tungsten oxide (WO₃), indium oxide (In₂O₃), cobalt oxide (Co₃O₄) and mixed oxide compositions with different WO₃/In₂O₃ and WO₃/Co₃O₄ ratios synthesized using the sol-gel method after xerogel annealing at 400–600 °C. The morphology, phase composition and structure of the materials have been studied using X-ray diffraction, infrared spectroscopy, scanning electron microscopy and transmission electron microscopy. We showed that stable structures can be produced in WO₃–In₂O₃ and WO₃–Со₃O₄ nanoheterogeneous compositions. The growth of grain size in WO₃ and In₂O₃, WO₃ and Co₃O₄ during heat treatment of mixed compositions occurs slower than in simple oxides. An increase in the gas sensitivity of the compositions in comparison with simple oxides can be accounted for by smaller grain sizes and hence larger specific surface area, as well as by the dependence of grain surface state on material composition. Both compositions exhibit the greatest nitrogen dioxide response at 130–150 °C and the greatest carbon oxide response at above 230 °C. We have produced low-power nitrogen dioxide sensors with a sensitivity of << 1 ppm and power consumption of ≤ 85 mW.