Preparation, characterization and oxygen gas sensitivity of nanosolid superacid SO₄ ^2-/ LaCoFeO₄

Metal oxides have been applied in gas sensor technology while suffering poor sensitivity and high working temperature. In this work, a novel method has been adopted to improve these problems of the sensors. A mixture of Fe ³⁺, La³⁺, Co2+ (Fe³⁺: La³⁺: Co ²⁺ =10:3:1) and dispersing agent in water solution was precipitated by adding NH4 +(pH=9) under agitation. The precipitate was filtered, washed and drying, then immersed with 0.5M (NH4)₂SO₄ in 12hr. Superacid SO₄ ^2-/ LaCoFeO4 was prepared after calcined at 673K for 1h. The thick film units for oxygen sensor were dintered at 673K for 3h.The structure of this catalyst has been characterized by means of powder X-ray diffraction (XRD), transmission electron microscope (TEM), IR, X-ray photoelectron spectroscopy (XPS) and specific surface area measurements. The result indicated that the SO₄ ^2-/ LaCoFeO₄ grain sizes (≤10nm) were smaller and the specific surface area (126M ²/g ) was higher than the pure metal oxide LaCoFeO₄. The acid strength was measured by the model reaction of ethyl acetate synthesis and attributed as superacid. In addition, the SO₄ ^2- was combined with metal ions in a bidentate fashion. The binding energy of SO ₄ ^2-/LaCoFeO₄ was larger than that of LaCoFeO₄. The sensitivity as well as the working temperature is improved by comparison with the pure LaCoFeO₄ phase. The relationship between the oxygen sensing properties and structure has also been discussed.