Effects of Co ion addition and annealing conditions on nickel ferrite gas response

Pure and cobalt-ion-containing nickel ferrite samples Ni1-xCoxFe2O4 (x = 0, x = 0.01, x = 0.05 and x = 0.1) were prepared by using the sol-gel auto-combustion method. To form the desired structure after the auto-combustion process, the samples were annealed in air at 800 °C for 1 h, followed by slow cooling in air (2.5 °C/min) or rapid cooling by quenching in water. The influences of cobalt ion addition and the cooling rate on the electronic structure, morphology and electrical properties of Ni-Co ferrite were studied using X-ray diffraction (XRD), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The gas-sensing properties of pure and cobalt-ion-containing nickel ferrites were studied by using acetone as a test gas. The gas-response of nickel ferrite samples was strongly influenced by the cooling rate and cobalt addition. The effect was attributed to a change in the Ni3+ concentration and a restriction of the transduction of the chemical signal into an output electrical signal. It was demonstrated that the gas response of Ni1-xCoxFe2O4 tends to increase with decreasing cobalt ion content and with decreasing concentration of the point defects such as cation vacancies and Ni3+ ions.