Synthesis of nano-zinc oxide by conventional chemical precipitation vs microwaves and its effect on the germination of cucumber (Cucumis sativus) seeds

Nanostructured ZnO (ZnO-NE) were successfully synthesized and seed germination and growth of cucumber seedlings in the laboratory were evaluated. The synthesis of ZnO-NE was done by direct precipitation (DP) and microwave-assisted heating with a domestic microwave (Domestic MW) vs a professional microwave (Professional MW). The crystalline phases, morphology, and absorption properties of the samples were characterized by XRD, FTIR, HR-SEM, and UV–Vis DRS respectively. The germination of cucumber seeds was evaluated through an experimental design completely randomized with five treatments and six repetitions. The number of roots, length and diameter of the main root, length and diameter of the hypocotyl were quantified. The germination percentage, germination indices and seedling vigor were calculated. All ZnO-NE synthesis methods presented wurtzite structure; smaller size (34 nm) of crystal with DP and larger (60 nm) with professional microwave. The synthesis with a Professional MW produced hexagonal prismatic particles while with a Domestic MW and DP there was no defined morphology, but with a tendency to form bars. All syntheses had a high percentage of Zn, O and a higher percentage of C in the ZnO-NE by Domestic MW, and in the ZnO-NE by DP and Professional MW Na was detected, both with a similar stoichiometric ratio, although the closest to one was with Domestic MW. The Eg values were 3.12, 3.14 and 3.16 eV for ZnO-NE synthesized by Professional MW, Domestic MW and DP, respectively. The different sources of ZnO-NE modified the length of the root and hypocotyl, the number of secondary roots, the vigor index and percentage of survival of seedlings in containers. Therefore, the preparation of ZnO-NE and its application in the germination of cucumber seeds contributes to improving the morphological characteristics of post-emergence seedlings.