Sustainable strategy for controlled phase formation of ZnO and ε-Zn(OH)2 nanoparticles for methylene blue degradation

This study reports a facile, one-step, and additive-free ion-exchange synthesis for the controlled formation of zinc oxide (ZnO) and zinc hydroxide (3-Zn(OH)2) nanoparticles under room-temperature conditions with remarkably fast phase formation (within 2 minutes). The primary novelty lies in utilizing ordinary tap water as a green solvent, and an active chemical parameter to dictate phase evolution, thereby eliminating distilled water usage and energy-intensive thermal calcination. Structural and morphological characterization via XRD and TEM revealed a precise temperature-driven (15–45 °C) phase selection, yielding elongated 3Zn(OH)2 structures at 15 °C, pure ZnO nanorods at 24–32 °C, and pseudo-spherical ZnO (9 nm) with a specific surface area of 35.76 m2 g−1 at 45 °C. Application of these nanomaterials in photocatalysis demonstrated exceptional efficiency, achieving over 99.2% methylene blue degradation under natural sunlight and 77% under indoor UV-A light (6 W lamp) within 150 minutes. Tauc plot analysis revealed a defect-induced bandgap narrowing, (Eg z 3.21–3.27 eV vs. 3.27 eV for bulk ZnO), expanding its lightharvesting capacity into the solar spectrum. Crucially, kinetic and thermodynamic analyses correlated this performance with an anomalous negative apparent activation energy (Eap), revealing a non-Arrhenius regime governed by exothermic adsorption–desorption equilibria, typical of a Langmuir–Hinshelwood mechanism. Furthermore, post-reaction FTIR analysis confirmed the high structural stability and photocorrosion resistance of the catalysts. Overall, these findings highlight a highly sustainable, energyefficient, and industrially scalable pathway for fabricating high-efficiency photocatalysts for environmental remediation.