Defect-engineered ZnO nanoparticles synthesized via green routes for enhanced solar photocatalytic activity

In the present investigation, ZnO nanoparticles synthesized via a green route using Averrhoa bilimbi (ZnO B₀ and B₁) and Brassica oleracea (ZnO C₀ and C₁) extracts demonstrated distinct photocatalytic performance as a function of their structural and defect characteristics. X-ray diffraction and Williamson-Hall (W-H) analyses revealed that ZnO (B₁) (synthesized with Averrhoa bilimbi fruit extract and calcined) exhibited a crystallite size of 44.8 nm and a moderate lattice strain of 0.0011, while ZnO (C₁) (synthesized using Brassica oleracea var. botrytis leaf extract and calcined) exhibited a larger crystallite size of 58.6 nm and a relatively higher strain of 0.0015. These microstructural variations played a pivotal role in influencing the photocatalytic behavior under solar irradiation, as further supported by EPR and XPS analyses, which revealed a higher concentration of oxygen vacancies and defect states responsible for enhanced charge separation and activity. ZnO (B₁) achieved the highest methylene blue (MB) degradation efficiency, attributed to its balanced defect concentration and strain-induced surface activity. In contrast, despite improved crystallinity, ZnO (C₁) exhibited reduced activity, likely due to excessive lattice strain and oversaturation of defect sites, which promoted electron-hole recombination. Kinetic analysis based on the Langmuir-Hinshelwood model and thermodynamic evaluation further supported the structure-driven enhanced photocatalytic behavior of ZnO (B₁). Overall, this study establishes, for the first time, a systematic correlation between Williamson-Hall microstructural parameters and the photocatalytic kinetic and thermodynamic behavior of green-synthesized ZnO nanoparticles.