Spherical aggregation of ZnO nanoparticles: a joint experimental and theoretical approach

Zinc oxide (ZnO) is of interest for many applications as an important nanomaterial with an excellent combination of optical, electrical and microstructural properties. The multifunctional behaviour of ZnO nanomaterials is partly determined by its electronic structure, which depends on the size and morphology. Among the various synthesis methods, the solvothermal route is attractive due to its simplicity and excellent control over particle size, shape and dispersibility. The effects of different organic additives, diethanolamine (DEA) and monoethanolamine (MEA), and the variation of ageing times between 4 and 72 hours on the mechanism of formation and stability of spherical aggregates of ZnO nanoparticles (NPs) were investigated. Among the various organic additives used so far, a versatile family of ethanolamines combining the properties of amines and alcohols has found important applications as they can induce a significant change in particle surface properties. From our recent publications as well as from the literature review [1, 2], it appears that the aggregation of primary ZnO NPs into a sphere was achieved in the second stage with a minimum of particle surface energy, confirming that the effect of the ethanolamine family on the morphology of ZnO NPs depends on the number of hydroxyl branches. Spherical aggregation of ZnO nanoparticles, which is fine-tuned by organic additives during solvothermal synthesis, is driven by various interactions, in particular strong O–H…O-hydrogen bonds as dominant interactions. The experimental results of structural, optical and morphological properties obtained by powder X-ray diffraction, UV-Vis spectroscopy and scanning electron microscopy (FESEM), were supported by quantum chemical calculations at the density functional theory level (DFT). This integrated experimental and theoretical study revealed that considerable stability of spherical aggregates of ZnO nanoparticles was achieved in DEA media even during the prolonged ageing of up to 72 hours, while it was lower in MEA media, as shown by the FESEM images in Figure 1. Understanding the mechanism of spherical ZnO aggregate formation and its stability by mimicking the processes at the computer level is achieved through theoretical simulations of the interactions between the ZnO surface and additives using a credible (ZnO)36–DEA and (ZnO)36–MEA model. The calculated negative free release energy ΔG*INT indicates that the surface additive/interaction in DEA media is a spontaneous exergonic process (ΔG*INT = -7.73 kcal mol^-1), whereas in MEA media it is an unfavourable slightly endergonic process (ΔG*INT> 0). This study highlights the stability and compactness of spherical ZnO aggregates of ZnO NPs prepared in the presence of diethanolamine, and provides a promising method and flexible design of ZnO material to improve its adsorptive and photocatalytic properties.