Nanostructured Cobalt Doped Barium Strontium Titanate Thin Films with Potential in CO₂ Detection

In this work, (Ba_0.75Sr_0.25) (Ti_0.95Co_0.05) O₃ perovskite nanostructured material, denoted subsequently as Co-doped BaSrTiO₃, was synthesized in a one-step process in hydrothermal conditions. The obtained powder was heat-treated at 800 °C and 1000 °C, respectively, in order to study nanostructured powder behavior during thermal treatment. The Co-doped BaSrTiO₃ powder was pressed into pellets of 5.08 cm (2 inches) then used for thin film deposition onto commercial Al₂O₃ substrates by RF sputtering method. The microstructural, thermal, and gas sensing properties were investigated. The electrical and thermodynamic characterization allowed the evaluation of thermodynamic stability and the correlation of structural features with the sensing properties revealed under real operating conditions. The sensing behavior with respect to the temperature range between 23 and 400 °C, for a fixed CO₂ concentration of 3000 ppm, highlighted specific differences between Co-doped BaSrTiO₃ treated at 800 °C compared to that treated at 1000 °C. The influence of the relative humidity level on the CO₂ concentrations and the other potential interfering gases was also analyzed. Two possible mechanisms for CO₂ interaction were then proposed. The simple and low-cost technology, together with the high sensitivity when operating at room temperature corresponding to low power consumption, suggests that Co-doped BaSrTiO₃ has a good potential for use in developing portable CO₂ detectors.