Tunable Growth of ZnO Nanostructures on the Inner Wall of Capillary Tubes

Abstract

Structured and functional microfluidic channels make microfluidic chips and devices more powerful in a variety of applications. In situ decoration of nanostructures on the channel wall is a low-cost and high-efficiency method toward multifunctional microfluidic chips. However, well-controlled growth of nanostructures on a large curvature substrate (i.e., microtubes with a circular cross section) in a shear flow is still lacking. In this work, dip-coating, an easy and efficient coating method widely used in industry, has been developed to deposit the ZnO seed layers on the inner wall of the capillary tubes, which shortens the time needed for the deposition of seed layers from a few hours to minutes. Then, a continuous-flow chemical processing method was used for the growth of ZnO nanostructures in a dynamic way. The morphology and number density of nanostructures can be controlled by the coating speed during the dip-coating process. In addition, the experiments demonstrated a maximum growth rate of the nanostructures under a certain flow rate, which can be defined from the theoretical calculation. The optimum flow rate Qop captures well the dependence on the reaction–diffusion process under the Poiseuille flow. The findings here could promote the functionalized nanostructure decorations for the applications including the dynamical ultrapurification of water and gas and medical detection in microfluidic devices.