Satoshi Sugaya

Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO₃, Bi₂O₃ and V₂O₅, while the use of WO_3, Nb₂O₅ and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO₃ > Bi₂O₃ > V₂O₅, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified.

Direct observation of WO₃/ZrO₂ by using high-resolution TEM images revealed the formation of polytungstate cluster with a narrow size distribution ranging in 0.4–0.7 nm, and the agglomeration into monolayer tungstate after the aging at 1073 K for 100 h.