Thermal annealing-controlled ZnO photocatalysis for solar-driven methylene blue degradation

• Surfactant-free ZnO photocatalyst tuned by post-synthesis annealing. • Annealing temperature controls crystallinity, defects, and reaction kinetics. • ZnO-500 achieves 99% MB removal under solar irradiation. • Defect-related states correlate with improved solar photocatalysis. • ZnO shows stable performance over multiple degradation cycles. Water contamination by synthetic dyes represents a significant environmental challenge, driving the demand for sustainable and energy-efficient remediation technologies. In this work, zinc oxide (ZnO) nanoparticles were synthesized via a simple, surfactant-free colloidal method and thermally treated at different temperatures to tailor their structural, morphological, and optical properties. X-ray diffraction confirmed the formation of a hexagonal wurtzite-type structure with crystallite sizes ranging from 5 to 26 nm. High-resolution scanning electron microscopy showed aggregated nanoparticles with a coral-like hierarchical morphology. Ultraviolet-visible diffuse reflectance analysis indicated band gap energies ranging from 3.21 to 3.31 eV, and photoluminescence measurements evidenced visible emissions associated with oxygen vacancy (Vo). Photocatalytic performance was assessed by methylene blue degradation under direct solar irradiation. Among the evaluated samples, ZnO nanoparticles annealed at 500°C exhibited the highest activity, achieving 99% degradation within 60 min, following pseudo-first-order kinetics, and demonstrating excellent stability over three consecutive cycles. These results demonstrate that controlled thermal treatment effectively tunes the kinetics of photocatalytic reactions under solar irradiation, providing quantitative insights for process optimization. The simplicity and stability of the ZnO system highlight its engineering relevance and potential scalability for wastewater treatment applications.