Hydrogen sulfide (H₂S) gas sensor based on cobalt–manganese spinel oxide MnCo₂O₄ thin films that were prepared by chemical precipitation and green synthesis methods. Structural, morphological, and compositional characterizations of MnCo₂O₄ thin films were performed using XRD, FE-SEM, EDS, and AFM. It was found through XRD patterns that MnCo₂O₄ was formed in a cubic spinel structure for both methods; however, the chemically precipitated thin films possessed better crystallinity and larger crystallite sizes, whereas the green-synthesized films exhibited slightly lower crystallinities and smaller sizes, as evidenced by the broader peaks observed. In addition, FE-SEM and AFM images revealed that the MnCo₂O₄ films prepared by chemical precipitation have a porous nanostructure with increased roughness and uniformity, while the MnCo₂O₄ films prepared by the green method possessed smoother, less rough surfaces. EDS confirmed that manganese, cobalt, and oxygen are the only contributors to the elemental composition of the thin films in both methods. Gas sensing characteristics were determined to be strongly dependent on operating temperature as well. The chemically precipitated films had the highest sensitivity to H₂S with 26.9% at 100 °C, response time (4 s), and recovery time (79 s), while the green processed films had low sensitivity, slight response at 200 °C, and only at higher temperature ranges, and a detectable presence. This demonstrates how the different routes of preparation provided crystal-clear differences in crystallization, which affected surface characteristics and overall characteristics. Ultimately, this study shows that chemically precipitated MnCo₂O₄ films are proper candidates for low-temperature, selective, and rapid H₂S gas detection.
