CO-sensing properties of In2O3-doped SnO2 thick-film sensors: effect of doping concentration and grain size

In₂O₃-doped SnO₂ nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl₄ 5H₂O) and indium nitrate. The doping concentration was varied from 7.718×10^-5 to 3.859×10^-4 moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300°C-900°C. Thick-film samples with a thickness of ∼15 μm, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220°C-240°C. A blue shift in the sensing temperature and increase in sensitivity factor (S_f) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of ∼5 is found for the highest doping concentration (3.859×10^-4 moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, and EDAX techniques. The results are discussed based on elemental analyses and available theories.