Ligand-directed hydrothermal synthesis of ZnO nanostructures for enhanced photocatalysis

Zinc oxide (ZnO) is a promising photocatalyst for environmental remediation, but its efficiency strongly depends on its nanostructure. In this study, a series of ZnO nanoassemblies was synthesized via hydrothermal treatment by varying the zinc precursor, pH, temperature, and the nature of complexing agents, namely urea, HMTA, sucrose, and spermidine which, to our knowledge, has not previously been applied as a structure-directing ligand in ZnO synthesis. This approach enabled precise tuning of crystal growth, anisotropy, and facet exposure. Morphological and structural features were investigated by SEM and XRD. The photocatalytic performance of each material was evaluated by monitoring methylene blue degradation under UV irradiation. The results reveal a strong correlation between morphology and photocatalytic activity: well-aligned nanowires with high crystallinity and dominant polar facets, particularly those synthesized with spermidine, exhibited superior degradation kinetics ( k = 0.0739 min⁻¹), comparable to commercial ZnO. This high activity was maintained over five consecutive reuse cycles, demonstrating excellent photocatalytic stability. In contrast, compact or branched architectures with disordered growth and limited facet exposure showed reduced activity. Overall, this study highlights the critical role of morphology engineering — driven by precursor chemistry and ligand effects — in optimizing ZnO-based photocatalysts for environmental applications. • ZnO nanostructures synthesized via hydrothermal route under mild conditions. • Spermidine introduced for the first time as a bio-inspired structure-directing ligand. • Ligand identity dictates anisotropy, crystallinity, and nanoassembly growth. • Morphology–crystallinity–activity correlations establish design rules for ZnO photocatalysts. • ZnO–spermidine nanostructures show competitive methylene blue degradation efficiency under UV light.