Tailoring TiO₂ thin films with Fe₂O₃ for rapid and sensitive CO₂ gas detection

In this work, TiO₂ thin films and Fe₂O₃/TiO₂ bilayer heterostructures were successfully fabricated using chemical spray pyrolysis for enhanced CO₂ gas sensing applications. Structural analysis confirmed the formation of polycrystalline anatase TiO₂ along with hematite Fe₂O₃ phases after deposition of the Fe₂O₃ overlayer. XRD results revealed that the inclusion of Fe and heat treatment increased the crystallite size from 16 nm to 25 nm, accompanied by slight peak shifts attributed to localized lattice distortion. AFM and FE-SEM analyses demonstrated that Fe₂O₃ incorporation increased surface roughness, grain size, and intergranular porosity, thereby providing additional active adsorption sites and improved gas diffusion pathways. Photoluminescence measurements showed a redshift and gradual narrowing of the optical band gap from 3.38 to 3.30 eV, indicating the formation of defect states and enhanced charge-transfer capability within the TiO₂/Fe₂O₃ heterostructure. Gas-sensing measurements revealed that the modified films exhibited significantly enhanced CO₂ responsivity compared with pristine TiO₂, with the TF5 sample achieving the best sensing performance at an operating temperature of 50 °C. The improved sensing behavior is attributed to the synergistic effects of heterojunction formation, increased oxygen-vacancy-related defect states, enhanced surface roughness, and facilitated charge transport. These findings demonstrate that Fe₂O₃ surface modification is an effective strategy for developing low-temperature, highly responsive TiO₂-based CO₂ gas sensors using a simple and scalable fabrication route.