Effects of fluid flow on the growth and assembly of ZnO nanocrystals in a continuous flow microreactor

The assembly of nanocrystals is considered to be one of the most promising approaches to design nano-, microstructures and complex mesoscopic architectures. A variety of strategies to induce nanocrystal assembly have been reported, including directed assembly methods that apply external forces to fabricate assembled structures. In this study, ZnO nanocrystals were synthesized in an aqueous solution using a continuous flow microreactor. The growth mechanism and stability of the ZnO nanocrystals were studied by varying the pH and flow conditions of the aqueous solution. It was found that convective fluid flowing from Dean vortices in a winding microcapillary tube could be used for the assembly of ZnO nanocrystals. The ZnO nanocrystal assemblies formed three-dimensional mesoporous structures of different shapes, including a tactoid and a sphere. The assembly results from a competing interaction between the electrostatic forces caused by the surface charge of the nanocrystals and the collision of the nanocrystals associated with Dean vortices. The dispersion behaviours of the ZnO assembly in some solvents were also studied. MeOH, a strong precipitant, led to the precipitation of the ZnO assembly. This study shows that the external forces from convective fluid flow could be applied to fabricate an assembly of functional metal oxides with complex architectures using a continuous flow microreactor.