Impact of drying techniques on the structural properties and methyl orange degradation efficiency of Moringa oleifera mediated ZnO nanoparticles

This study aims to investigate the effects of oven-drying (OD) and freeze-drying (FD) on physicochemical properties and structural integrity of green-synthesized ZnO NPs using Moringa oleifera leaf extract. The physicochemical and morphological characteristics of the extracted ZnO NPs using the two drying techniques have been determined. Several tests, including TEM, FTIR, SEM, and XRD, have been used to assess the characterisation of the targeted NPs. The outcomes showed that the selected drying method significantly affected the ZnO NPs’ characteristics. XRD analysis provides clear evidence that the thermal nature of OD-ZnO (at 90 C) facilitates crystal growth with particles of (12.18 nm) while FD process preserving the smaller, as-synthesised crystallite size of (9.85 nm). FD-ZnO particles exhibit a significantly smaller mean diameter (∼84.5 nm) compared to the OD-ZnO sample, which displayed extensive agglomeration with sizes reaching up to ∼535 nm. EDX data quantitatively supported that FD-ZnO retained a higher oxygen-to-zinc ratio (0.37) compared to OD-ZnO (0.33). Overall particle morphology transitioned from porous 2D nanoflakes (mean ∼84.5 nm) in the FD-ZnO sample into significantly larger, denser 3D aggregates (∼535 nm) in the OD-ZnO control. To demonstrate the validity of the produced green-synthesized ZnO NPs, an evaluation has been done on three methyl orange samples with varying concentrations ranging from 10 to 30 ppm and varying durations ranging from 1 to 3 h. In this case, the initial concentration of 10 ppm and after 60 min under sunlight, achieved an average removal percentage of 97.25%. These results demonstrated that plant-based synthesis approaches can offer a more cost-effective and environmentally friendly method for removing dyes and can serve as a useful replacement for conventional chemical production methods. • Green synthesis of ZnO NPs was achieved using Moringa oleifera leaves. • Freeze-drying and oven-drying effects on NP properties were compared. • Biogenic FD-ZnO NPs exhibited a porous, nanoflake-like morphology with a mean particle size of ∼84.5 nm. • 94.3% methyl orange removal was achieved under sunlight in 60 min. • Plant-mediated ZnO NPs outperformed traditional chemical synthesis methods.