The enhancement of response speed by loading the noble metal into the sensing layer for FET-Type N O₂ sensors

In order to enhance the response speed, noble metal (Ru) loading into the W O₃-NaN O₂ sensing layer for the field effect transistor (FET)-based N O₂ sensors was investigated. The sensing layer was prepared by heat-treating the mixture powder of NaN O₂ and noble metal (Ru)-loaded W O₃ at 300°C. The effects of loading amounts of the noble metal on N O₂ sensing performances as well as the microstructure of the sensing layer have been examined. It was found that the optimization of the noble metal loading amount into the sensing layer was important for the improvement of 90% response-and recovery-times. Field emission scanning electron microscopy observations revealed that the sensing layer became porous with increasing noble metal loading amounts. However, extensive addition of Ru (above 0.2 wt % to W O₃) into the sensing layer resulted in the degradation of sensing characteristics, although the sensing layer showed a highly porous structure. According to theoretical analysis based on Knudsen gas diffusion reported by Matsunaga, the diffusion of gas molecules into such a porous sensing layer is very fast. Therefore the obtained results suggested that the improvement of the response-and recovery-speeds is owing to the large contribution of noble metal nanoparticle to the electrochemical promotion rather than the diffusion of N O₂ gas into the sensing layer.