Green Synthesis of ZnO Nanoparticles Using Ocimum basilicum var. purpurascens: As-Synthesized Phase Formation and Thermal Evolution of Optical Properties

This study reports a facile, phyto-mediated synthesis of ZnO nanoparticles utilizing the Ocimum basilicum var. purpurascens extract. The high phenolic (1807.28 ± 57.38 µmol GAE/g) and flavonoid (33.17 ± 3.50 µmol QE/g) contents of the extract successfully induced the formation of the crystalline hexagonal wurtzite phase under mild reaction conditions, circumventing the conventional requirement for the initial high-temperature calcination. The structural, morphological, and optical evolution of the nanoparticles was systematically evaluated from their as-synthesized state to the thermal annealing temperatures of 700 °C to 900 °C. X-ray diffraction analysis confirmed an increase in crystallite size from 21.5 nm to 55.6 nm, while scanning electron microscopy revealed a corresponding growth in average particle size from 143.33 nm to 261.50 nm due to thermal sintering. Furthermore, FTIR and EDS verified the degradation of organic capping agents and a progressive stoichiometric refinement, with the high-temperature samples approaching theoretical elemental purity (18.22% normalized oxygen mass). Optical characterization demonstrated a red-shift in the band gap energy from 3.28 eV to 3.21 eV, alongside a significant increase in visible diffuse reflectance progressing from a baseline of 30–60% to values exceeding 95% upon the removal of the biochemical components. These findings validate the OBPE-mediated protocol as a sustainable, thermodynamically advantageous route for producing structurally and optically tunable ZnO nanomaterials for advanced applications.