Developing High-Performance Low-Temperature CO₂ Gas Sensors Based on Nanostructured Co₃O₄ Thin Films: A Sol-Gel Approach and the Role of Annealing

In this study, we synthesized Co 3 O 4 thin films using a sol–gel spin coating method and investigated the effect of heat treatment on their carbon dioxide (CO 2 ) gas sensing properties. To optimize their crystallinity and enhance their CO 2 sensitivity, the thin films were annealed at temperatures ranging from 400 ∘ C to 550 ∘ C. We characterized the morphological, structural, electrical, and optical properties of the Co 3 O 4 thin films to gain insights into their behavior in the low operating temperature range (OTR). Our X-ray diffraction (XRD) analysis revealed that all the thin films exhibited a cubic structure, with improved crystallinity observed at higher annealing temperatures. We observed a decrease in band edges in the optical transmittance spectra of the thin films as the annealing temperature increased, indicating a redshift in the absorption edge. Notably, the highest electrical conductivity was observed for the sample annealed at 550 ∘ C, suggesting enhanced crystallinity and improved CO 2 gas sensitivity. These findings provide valuable insights into the optimization of Co 3 O 4 thin films for CO 2 gas sensing applications.