Green Synthesis of Lanthanum Oxide Nanoparticles Using <i>Coleus Barbatus</i>: Annealing Effect on Gas Sensing and Antimicrobial Efficacy Applications

Lanthanum oxide (La₂O₃) nanoparticles (NPs) were synthesized via a green co-precipitation method using Coleus barbatus leaf extract as a reducing and capping agent, with CTAB as a stabilizer. X-ray diffraction (XRD) confirmed a hexagonal polycrystalline structure with an average crystallite size of 16 nm, further supported by Raman spectroscopy. Calcination temperature played a critical role: at 600 °C, incomplete precursor decomposition was observed, while 800 °C treatments led to oxygen vacancies and peak broadening. Optimum properties were achieved at 700 °C, with 73.80% oxygen content, sharp XRD peaks, and absence of secondary phases. UVvisible diffuse reflectance spectroscopy revealed bandgap narrowing from 4.6 to 4.28 eV with increasing temperature, attributed to reduced strain and improved crystallinity, and enhanced electronphonon coupling. Photoluminescence spectra (550 nm excitation) showed enhanced emission intensity at higher temperatures, indicating increased defect states and thermally assisted radiative transitions. High resolution transmission electron microscopy revealed spherical to hexagonal particles averaging 31.7 nm, while scanning electron microscopy indicated moderate agglomeration. The La₂O₃ NPs exhibited significant antibacterial activity and excellent H₂S gas sensing performance, with 81% sensitivity and rapid response (11 s) and recovery (35 s) times, demonstrating strong potential for environmental monitoring and biomedical applications.