Green Synthesis and Phase Stabilization of Bi2O3 Thin Films via Chemical Spray Pyrolysis

Abstract Bismuth oxide (Bi2O3) is a vital semiconductor whose complex polymorphism demands precise structural classification benchmarked against global databases. This study investigates the green synthesis of Bi2O3 thin films via chemical spray pyrolysis (CSP) using an aqueous Mentha pulegium leaf extract, systematically quantifying the effects of substrate temperature (300–400°C) and precursor volume (10–30 spray pulses). Experimental X-ray diffraction (XRD) data was rigorously evaluated against Crystallography Open Database (COD) reference cards to validate phase transitions. At 300°C with 20 sprays, the films crystallized into the thermodynamically stable monoclinic α-Bi2O3 phase (COD 96-901-2755), exhibiting a large crystallite size of 653 Å and a remarkably low lattice microstrain of 0.287%. By optimizing the deposition temperature strictly to 350°C, the study successfully stabilized the high-energy, metastable tetragonal β-Bi2O3 phase (COD 96-152-4111) at room temperature. This stabilization is primarily driven by the internal microstrain and substrate interface kinetics. This transition induced a substantial structural contraction, yielding a crystallite size of 52 Å and an elevated microstrain of 3.515%. Volumetric variance at 350°C proved critical; a restricted 10-spray volume formed an α-sub-phase (139 Å), whereas extending deposition to 30 sprays triggered in-situ self-annealing, generating a mixed β/α intergrowth with expanded crystallites up to 724 Å and a relaxed microstrain down to 0.148%. These quantitative findings establish an optimized, chemical-free blueprint for engineering phase-stabilized optoelectronic materials.