Doped-ZnO nanoparticles for selective gas sensors

Pure ZnO, Al-3%- and Ca-5%-doped ZnO nanoparticles have been synthesized by the sol-gel process under supercritical dry conditions of ethanol. The obtained nanopowders were annealed at 400 degrees C for 2 h in air in order to stabilize their microstructure and characterized by X-ray diffraction analysis (XRD) and transmission electron microscopy (TEM). XRD demonstrated that the nanoparticles of pure and doped samples are constituted by ZnO as the primary phase. TEM images showed particles of irregular form having a broad particles size distribution. Conductometric ZnO sensors were fabricated by depositing the synthesized nanopowders on alumina substrates by screen printing technique and tested in the monitoring of environmental toxic carbon oxides, i.e. CO and CO2. Al-doped ZnO sensor showed high response towards carbon monoxide CO, while the Ca-doped sensor showed a good response to CO2, operating at the temperature of 300 and 450 degrees C, respectively. Both sensors are characterized by a good selectivity towards the target gas and fast response/recovery time. In order to decrease the operating temperature of Ca-5%-doped sensor, co-doping by adding Al-3% was also proposed.