Influence of Iron Oxide and Lanthanum Oxide on the Structural and Antimicrobial Properties of Bismuth Oxide-Based Nanocomposites Synthesized via Plant Extract-Assisted Microwave–Hydrothermal Method

Bismuth oxide (Bi2O3) nanoparticles are attractive for biomedical and radiation-shielding technologies and can be further tailored through the addition of other metal oxides to address emerging needs such as antimicrobial resistance. This study investigated the effects of incorporating Fe2O3 and La2O3 on the structure, morphology, and antimicrobial performance of Bi2O3-based nanocomposites synthesized via a plant extract-assisted microwave–hydrothermal route using soapnut extract. XRD indicated that pure Bi2O3 (100B) comprised predominantly monoclinic α-Bi2O3 with coexisting metastable tetragonal β-Bi2O3. The addition of Fe (3F; Fe:Bi = 30:70) promoted β- Bi2O3 and formed BiFeO3, while increasing La substitution (3L–20L) reduced the BiFeO3 intensity and, beyond a threshold (≥7L), yielded distinct La2O3 peaks consistent with a La2O3–BiFeO3–Bi2O3 composite. Crystallite size decreased from ~46 nm (100B) to ~25 nm (3F), varying with La between 33 and 25 nm. SEM/TEM revealed a reflection in morphology and size with composition from disk-like particles to petal-like spherical aggregates. Antimicrobial screening revealed composition-dependent inhibition: against S. aureus, 20 L was the most potent (~94%). Overall, La/Fe tuning under a plant extract-assisted microwave–hydrothermal route enabled phase- and morphology-controlled Bi2O3-based nanocomposites with enhanced antimicrobial activity, with ultrafine, high-surface-area architectures emerging as promising antibacterial candidates.