Additive-Assisted Crystal Growth and Nanostructure Design of Hydrothermally Synthesized ZnO Nanoparticles

Additive-assisted nanostructure design is an effective strategy for regulating crystal growth and functionality in oxide nanomaterials. In this study, we systematically investigate the roles of molecular additives with different functional groups (amine and sulfonic acid) and alkyl chain lengths on the crystal growth behavior and nanostructure morphology of ZnO nanoparticles synthesized via a hydrothermal method. Structural characterization using X-ray diffraction (XRD), scanning electron microscopy (SEM), and scanning transmission electron microscopy combined with energy-dispersive X-ray spectroscopy (STEM–EDS) reveals distinct additive-assisted growth mechanisms under identical synthesis conditions. Amine-based additives suppress crystal growth along the polar c-plane through electrostatic surface-capping effects governed by the pH-dependent surface potential of ZnO. In contrast, sulfonic-acid-based additives promote the formation of mesoporous ZnO nanostructures via an etching-dominated growth modification mechanism rather than simple surface capping. Functional evaluation further indicates that these additive-controlled nanostructures significantly influence the accessible surface characteristics of ZnO nanoparticles, including polar surface exposure and effective surface area. These findings provide mechanistic insight into additive-assisted crystal growth regulation and demonstrate that the morphology and growth behavior of ZnO nanoparticles can be systematically controlled through a clear bifurcation between surface-capping-dominated growth inhibition and etching-driven structural modification, enabled by the rational selection of molecular additives and synthesis pH.