- A unique microreactor-assisted nanomaterial synthesis and printing process was studied for the fabrication of patterned metal oxide nanostructured thin films. The process uses a continuous flow microreactor to control and generate a reactive chemical flux that was transported to a patterned microfluidic channel. The microreactor-assisted nanomaterial synthesis process can generate reactive building blocks, ranging from molecules, clusters to nanoparticles with constant concentration and compositions. The reactive solution was then delivered to the substrate surface, guided by a patterned microchannel. The growth kinetics of the nanostructured ZnO on silicon wafers were studied by measuring the thickness of the film using the stylus profiler and scanning electron microscope under various growth conditions controlled by using the microreactor and the microchannel made of Polydimethylsiloxane. The process contains three zones, a mixing zone, a reactant generation zone, and a deposition zone. This process allows a separation of the homogeneous reaction in the solution and the heterogeneous reaction on the substrate surface, resulting in a well-controlled growth of the ZnO nanostructures on the substrate surface so that the growth mechanisms and kinetics can be studied. Different growth parameters, including flow rate, residence time, the temperature of the chemical solution and temperature of the substrate, are varied to study the growth kinetics.
A key process parameter is the flow rate; a higher flow rate can result in faster growth of ZnO nanostructures. A lower flow rate will result in a longer residence time of the chemical solution within zone 2, which can lead to the homogenous formation of solid nanoparticles, thus reduce the concentration of reactive Zn(OH)₂ (aq). This finding is further supported by growth experiments carried out using reactors with two different reaction channel lengths within zone 2. Besides, the thickness of rectangular patterned ZnO nanostructured films show a saddle-shaped profile, which is thinner near the center. This thickness profile is a result of combined heat and mass transfer of the reactive solution within the patterned channel, as qualitatively supported by COMSOL simulations. By measuring the growth rates as a function of substrate temperatures, the activation energy of the rate constant is obtained at 22.65kJ/mol. This process not only provides better control to fabricate patterned metal oxide nanostructures but also offers the unique capability to study the growth mechanisms.
The improved understandings were applied to demonstrate a novel, scalable process to fabricate ZnO nanostructures with multiscale 3D geometric shapes. In particular, the precursor solutions were firstly mixed and heated in a microreactor to control solution temperature and to generate reactive species. The reacting solution was then delivered to the substrate surface guided by a patterned PDMS channel with different designs, including the spiral pattern, the parallel pattern, and the split-and-recombine pattern. ZnO nanostructures with multiscale 3D geometric shapes were formed guided by the patterned channel. It is found that geometry is controlled by the channel geometry, flow rate, and substrate temperature. The stylus profiler measures film thickness, and the result shows that the unique characteristics of each pattern type. With the aid of the Comsol simulation, the parameters that control the growth are studied: in the spiral pattern, the consumption of the reactant can result in a thinner film as the solution flows through the channel. In the parallel-design pattern, the film thickness is determined by the flow rate of the solution in each channel in parallel. With the channel narrowed and the flow rate reduced, a thinner ZnO nanostructured film is obtained. In the split-and-recombine design pattern, the film growth rate is halved as the channel split and doubled as the channels re-combined. The temperature profile within the channel is another critical parameter of controlling the growth of ZnO nanostructures in all dimensions. This scalable process, aided with new understandings, will provide a unique capability to fabricate metal oxide nanostructures of controlled multiscale 3D geometric shapes.
Besides ZnO, the microreactor-assisted nanomaterial synthesis and printing process was used to deposit patterned CuO and Cu₂(OH)₃NO₃ nanostructures on surface, including dense nanocrystalline CuO film, CuO nanorods, Cu₂(OH)₃NO₃ nanorods, and Cu₂(OH)₃NO₃ nanoplates using the same reactants, Cu(NO3)2 and Hexamethylenetetramine. The critical process parameter that controls the formation of different products is the concentration of the OH- in the solution, which can be controlled by the ratio of the reactants and the temperature of the microreactor. The high concentration of OH- leads to the formation of Cu(OH)₂, which is then converted to CuO on the heated substrate surface. In contrast, the low concentration of OH- leads to the formation of Cu₂(OH)₃NO₃. These results show the applicability of the microreactor-assisted nanomaterial synthesis and printing process to deposit metal oxide nanostructures with controlled structure and composition.
The utility of microreactor-assisted nanomaterial synthesis and printing process was demonstrated via the fabrication of heterojunction ZnO/CuO bi-layer film. The bilayer film was built by depositing a patterned nanocrystalline CuO film on a gold-coated glass surface using the microreactor-assisted nanomaterial synthesis and printing process, followed by the deposition of ZnO nanostructured film with a smaller-size pattern using the same process. The heterojunction ZnO/CuO bi-layer film shows rectifying behavior; it allows currents to flow when forward biased and passes only low leakage currents with reverse bias; the p-n diode has a rectification ratio around 10⁴, which is comparable to the values among the best solution-processed p-n junction diodes. The results of this study demonstrate the capability of our microreactor-assisted nanomaterial synthesis and printing process to fabricate structured thin films for functional devices.