Production of two-dimensional nanosheets using Taylor-Couette flow reactor and its application

Abstract

Two-dimensional (2D) nanosheets are anisotropic materials with an ultrathin thickness of sub-nanometer. Since the graphene discovery, the potential unique properties of various 2D nanosheets have been studied based on the synthesis and exfoliation processes. The 2D nanosheet through bottom-up methods has the advantage of the unique characteristics by uniform synthesis, but the production rate is very lower than top-down processes. The 2D nanosheets through top-down methods has a advantage of the high production rate, but the yield is low and it is difficult to produce and maintain the single-sheet. Although 2D single-sheets of unique characteristics have been studied and applied to various applications, it is essential to develop a production system of a 2D single-sheet having high yield and high production rate in order to be used in industry. In this study, we have studied the production systems of 2D nanosheets, the analysis of fluids and materials, and the application of nanosheets produced using a Taylor-Couette flow reactor.

First, we have developed a very fast, scalable, high yield 2D nanosheet production process through high shear and effective mass transfer using controlled wavy Taylor-Vortex flow in Taylor-Couette flow generator. A high yield (76.9%), a high concentration (20 mg/mL) and a high production rate (8.6 g/h) were obtained by selecting graphene, molybdenum disulfide ($MoS_2$) and boron nitride (BN). The origin of such a high exfoliation system was found by analyzing the flow characteristics in the reactor through computational fluid dynamics (CFD) and analyzing the correlation between material and fluid molecules through calculation of density functional theory (DFT). In addition, the 2D nanosheets produced through the high shear mixing process were dispersed very stably in water and used very efficiently in post-treatment processes for applications such as vacuum filtration and inkjet printing. Through this process, it could be showed as a film electrode and a circuit with high capacitive performances and high electric conductivity.

Secondly, the 2D nanosheets produced in the Taylor-Couette flow generator are exfoliated with very low physical damage, resulting in very few defects and very large sheet sizes. Ionic liquid functionalized nanosheets are also suitable for films of volumetric energy storage electrode materials. Graphene nanosheets with few defects show very high electrical conductivity and metallic phase $MoS_2$ has a high theoretical capacity of 1,000 F/g. In addition, even if the two nanosheets are filmed at high packing density, the functionalization of the ionic liquid allows high ions mobility due to the ionic liquid present between the inter-sheets. The hybrid 2D-2D film had a high density of 2.02 g/$cm^3$, a very high volumetric capacitance of 1,430.5 F/$cm^3$ at 1 A/g, and a high charge/discharge capacity retention of 80% at 1,000 A/g. We have fabricated a flexible supercapacitor device with a high energy density of 1.14 Wh/$cm^3$ as a hybrid film with excellent characteristics.

Finally, we have developed uniformly deposited metal/metal oxides on 2D nanosheets using a Taylor-Couette flow reactor in a high shear and mixing flow. Because of the various properties of each substance, applications and researches on composite materials that can be used in various industrial fields are being actively carried out. 2D nanosheets with large surface area have been functionalized with ionic liquids and can be applied to substrates with various metal/metal oxide deposition characteristics. In addition, the excellent mixing properties of the Taylor-Couette flow reactor have resulted in the uniform formation of various metal/metal oxides such as manganese dioxide ($MnO_2$), palladium (Pd), platinum (Pt), ruthenium (Ru) on graphene and BN in 5 min. The deposition time of metal/metal oxide was at least 6 times faster than the previously reported large scale reactors such as hydrothermal reactor, ball mill reactor, and continuous stirred tank reactor. These very high mixing characteristics can be identified through a study of the residence time distribution of the device. The synthesized $MnO_2$/graphene composite was applied as an electrode to a supercapacitor storage device, resulting in a high energy density of 744 F/g at 1 A/g, a fast charge/discharge capacity retention of 94.7% at 1,000 A/g. In addition, the Pd/BN composte was applied as a catalyst to the 4-nitrophenol reduction reaction and showed a conversion efficiency close to 99% in 16 minutes.

In conclusion, Taylor-Couette flow system offered opportunities for the mass production of 2D nanosheets. Produced 2D nanosheets can be easily applied to fabricate nanocomposites, for electronics, biosensors, catalyst, and energy-storage/conversion systems. Our approach would be useful to overcome the limitation of the conventional material synthesis processes.