CO sensing properties of novel nanostructured La2O3 microspheres

La2O3 has outstanding dielectric and catalytic properties; however, little is known about its gas sensing properties. In this work, nanostructured La2O3 hollow microspheres were prepared through a coprecipitation method. At room temperature, the reaction between lanthanum nitrate and formic acid produced lanthanum formate (La(HCOO)(3)) microspheres. An aqueous pectin solution was added to preserve the spherical shape during further calcination. Annealing La(HCOO)(3) produced several crystalline phases, but single-phase hexagonal La2O3 was obtained up to 650 degrees C. At this calcination temperature La2O3 microspheres were hollow and had diameters between 0.5 and 6 mu m. Their surface was permeable, possessing extensive nanoporosity. The effects on the surface morphology of calcination temperature and amount of lanthanum nitrate in solution were investigated. The gas sensing characterization was performed on thick films, which were prepared by depositing the as-prepared La2O3 on alumina substrates. The response to carbon monoxide was evaluated by impedance and direct current measurements. The effects of the applied frequency and operating temperature were evaluated. The results revealed a reproducible and reliable detection of CO at 400 degrees C and 100 kHz. The gas sensing behavior corresponded to a p-type semiconductor material, differing from other rare earth oxides (Nd2O3, Gd2O3 and CeO2), in which an n-type semiconductor response was previously measured. (C) 2014 Elsevier B.V. All rights reserved.