Characterization and enhanced carbon dioxide sensing performance of spin-coated Na- and Li-doped and Co-doped cobalt oxide thin films†

Recognizing the substantial effects of carbon dioxide on human health and the environment, monitoring CO₂ levels has become increasingly vital. Owing to energy constraints and the widespread application of CO₂ gas sensors, it is important to design cost-effective, more efficient, and faster response CO₂ gas sensors that operate at room temperature and involve a low-cost technique. This study aims to develop a cost-effective and efficient CO₂ gas detector that functions at room temperature and uses less power than traditional high-temperature CO₂ sensors. In this study, we achieved this by employing innovative Co₃O₄ thin films with optimized spinel-structured p-type semiconductors through spin-coating, facilitated by Li and Na doping as well as Li/Na codoping. Doping with 3% Li/Na reduced the crystallite size from 92.4 to 8.03 nm and increased the band gap from 3.31 to 3.69 eV. At room temperature (30 °C), the sensor response improved significantly, increasing from 50% to 345.01% for 3% Li-Co₃O₄ upon the addition of 3% Na at a concentration of 9990 ppm. This performance surpasses that of most metal-oxide-based CO₂ sensors reported in the literature. Additionally, this optimized sensor demonstrated a very short response time of 18.8 s and a recovery time of 16.4 s at a CO₂ concentration of 9990 ppm diluted with air. It outperformed other films in terms of sensitivity, stability, response and recovery times, and performance across a wide range of relative humidity levels (43–90%). The sensor exhibited superior selectivity for CO₂ than for N₂, H₂, and NH₃. Overall, the 3% Li, Na-Co₃O₄ sensor is well-suited for climate change mitigation and industrial applications.