ZnO:Ca nanopowders with enhanced CO2 sensing properties

Calcium doped ZnO (CZO) nanopowders with [Ca]/[Zn] atomic ratios of 0, 0.01, 0.03 and 0.05 were prepared via a sol-gel route and characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction and Fourier transform infrared spectroscopy (FT-IR). Characterization data showed that undoped and Ca-doped ZnO samples have a hexagonal wurtzite structure with a slight distortion of the ZnO lattice and no extra secondary phases, suggesting the substitution of Ca ions in the ZnO structure. Chemo-resistive devices based on a thick layer of the synthesized CZO nanoparticles were fabricated and their electrical and sensing properties towards CO2 were investigated. Sensing tests have demonstrated that Ca loading is the key factor in modulating the electrical properties and strongly improving the response of ZnO matrix towards CO2. An increased CO2 adsorption with Ca loading has been also evidenced by FT-IR, providing the basis for the formulation of a plausible mechanism for CO2 sensing operating on these sensors.