Aqueous Synthesis of Tailored ZnO Nanocrystals, Nanocrystal Assemblies, and Nanostructured Films by Physical Means Enabled by a Continuous Flow Microreactor

摘要

ZnO nanofilms with four distinctly different morphologies were fabricated by adjusting physical parameters including the flow rate of the solution and rpm of the rotating disk in the continuous flow mircroreactor system while keeping the same chemical precursors, precursor solution concentration, and reaction temperature. Controlled reactive species including colloidal ZnO nanocrystal, ZnO assemblies, and molecular and ionic precursors were synthesized from a microreactor at 70 °C reaction temperature by changing the flow rate of the solution. These reactive species were directly delivered onto a thermally grown oxide layer (100 nm) of Si substrate (1.5 cm × 1.5 cm) at 80 °C deposition temperature to densely form a flowerlike film, amorphous film, vertical nanowire (NW) arrays, and crystalline film. ZnO assemblies were delivered onto the substrate and served as seed layers to form a flowerlike film. Amorphous films were formed after depositing Zn(OH)_3~? ionic species on the substrate. For the vertical ZnO NW arrays, colloidal ZnO nanoparticles, delivered onto the amorphous layer, formed a polycrystalline ZnO layer, and NWs were grown subsequently after Zn(OH)_3~? deposition onto the polycrystalline ZnO layer. Lastly, crystalline film was fabricated by delivering ZnO assemblies onto the amorphous layer followed by Zn(OH)_3~? deposition. Thickness and size of all the films were found to vary by the deposition period. In contrast, only powdery flowerlike ZnO could be obtained using a batch reactor at the same reaction conditions. ZnO nanofilms were also deposited on a stainless steel substrate (SS304) for the two-phase pool boiling experiment to investigate the structural impact of the nanofilms on boiling heat transfer performances. The boiling performance was significantly impacted by the structure of the nanofilms. The highest boiling performance was achieved by the crystalline film, exhibiting a heat transfer coefficient of 4.2 W/cm~2 K along with a critical heat flux of about 100 W/cm~2. This work demonstrates a highly controlled and scalable process for the fabrication of tailored metal oxide nanofilms.