Effect of Bismuth Incorporation on the Structural, Optical, and Gas-Sensing Characteristics of Fe₂O₃ Thin Films Synthesized by Spray Pyrolysis

Pure and bismuth-doped Fe₂O₃ thin films were fabricated using chemical spray pyrolysis, and the influence of Bi incorporation on their structural, morphological, optical, and gas-sensing characteristics was systematically analyzed. XRD results confirmed the monoclinic polycrystalline phase of Fe₂O₃, with Bi doping promoting larger crystallite size and improved crystallinity. AFM analysis showed that increasing Bi content reduced particle size and surface roughness, leading to more compact and uniform film surfaces. Optical studies revealed a decline in both transmittance and bandgap energy with higher Bi levels, attributed to the introduction of localized electronic states. Corresponding variations in the refractive index, extinction coefficient, and absorption coefficient further reflected dopant-induced structural changes. Gas-sensing measurements toward NO₂ demonstrated that undoped Fe₂O₃ achieved the highest sensitivity, whereas Bi-doped films exhibited reduced response due to enhanced charge-carrier recombination. Overall, Bi doping provides an effective route for tuning Fe₂O₃ film properties for optoelectronic and sensing applications.