Tailoring Crystal Structure, Optical Band and Wettability of Green-Synthesized BiOI Nanoparticles via pH-Controlled Hydrothermal Method

Abstract In this study, BiOI nanoparticles were synthesized through a green hydrothermal route using Murraya koenigii(curry leaf) extract, with the reaction pH systematically varied from 5 to 13. to investigate its influence on the structural, morphological, optical, surface-charge, and wettability characteristics. X-ray diffraction analysis showed that BiOI retained a tetragonal structure at pH 5–10, whereas a structural transformation toward an orthorhombic phase was observed under strongly alkaline conditions at pH 13. The crystallite size varies from 18.67±4.86 to 30.37±11.34 nm, accompanied by changes in crystallinity, lattice strain, and defect density. Scanning electron microscopy revealed a pronounced morphological evolution from cauliflower-like aggregates to hierarchical flower-like structures and subsequently to larger plate/rod-like features with increasing pH, indicating the influence of solution chemistry on crystal growth and aggregation. FTIR analysis identified characteristic vibrational features of Bi–O–I nanoparticle framework and surface functional groups. UV–Vis analysis revealed pH-dependent optical band gaps, with indirect values of 1.607 ± 0.005–2.202 ± 0.011 eV and direct values of 2.170 ± 0.012–2.523 ± 0.009 eV. The zeta potential became increasingly negative from −30.93 ± 1.61 to −40.83 ± 0.34 mV, while the water contact angle decreased from 53.57 ± 2.23° to 23.03 ± 2.29°, indicating systematic changes in surface charge and wettability. Thus, reaction pH provides an effective route to tune the structural, morphological, optical, and interfacial properties of BiOI-based materials, offering potential for photocatalytic, photo-electrochemical, optoelectronic, self-cleaning, and atmospheric water-harvesting applications.