Advances in Understanding Graphene Hybrids and MXenes

Abstract

The overall goal of this research was to contribute to understanding the effects of additional components on the microstructures and properties of two-dimensional (2D) systems, namely graphene hybrids and MXenes. The first part of this research involved synthesizing, characterizing, and electrochemical testing of graphene/manganese oxide hybrids. Combining 2D ultra-large reduced graphene oxide (RGO) sheets with an aspect, or length to diameter, ratio of ~40,000 with 1D manganese oxide (MnO2) nanowires consisting of an aspect ratio of ~50 produced a hybrid (MnO2/RGO) material. Testing two different compositions of hybrids were studied to understand the effects of combining different geometries and orders of magnitude different aspect ratios on the electrochemical properties. The individual components provided insights into the microstructures and electrochemical behavior of the hybrids. The 3:1 ratio of MnO2/RGO (75/25 wt%) showed a more uniform distribution of nanowires along the sheets and outstanding electrochemical performance compared to the 10:1 ratio of MnO2/RGO (90/10 wt%). The very large specific surface area of the RGO sheets in this research enabled faster ion and electron diffusion compared to smaller sheets used in the prior literature. Also, combining 2D/1D geometries with very different aspect ratios in the hybrid showed enhanced electrochemical performance than either component alone. This highlights the potential advantages of multicomponent systems that can be extended to other applications, such as fibers and sensors. The second part of this dissertation was performed in collaboration with Dr. Majid Beidaghi in Materials Engineering at Auburn University. It involved understanding the rheological properties of aqueous Ti3C2Tx MXene dispersions with two different objectives. The first objective was to provide insights into direct ink writing design parameters for a highly concentrated unfractionated MXene dispersion. Since MXenes are a recently developed class of 2D materials, particle interactions for processing are not well understood. The rheological behavior provided insights into viscoelastic properties and flow behavior necessary for determining optimal printability. The second objective was to investigate the effects of varying ionic strength, through sodium chloride (NaCl) addition, on the phase behavior and dispersion microstructures of large MXene flake dispersions. Polarized optical microscopy was performed to investigate different structures and phases with varying flake and salt concentrations. Rheology experiments were performed to better understand the thermodynamic interactions and augment the microscopy results. A simplified theoretical approach based on DLVO theory was also performed to provide insights into the phase behavior. A phase diagram was proposed that consists of liquid crystalline phases, gel, and flocculation. Graphene and MXenes are charged systems, similar to nanoclays, that exhibit unique optical, mechanical, and electrical properties. Understanding the fundamental particle and solvent interactions provided a basis for fluid-phase processing and fabrication into electrochemical devices. Dispersion properties such as Debye screening length, aspect ratio, viscosity, and the presence of additional components were studied to increase understanding of the graphene hybrids and MXenes. In summary, results from this research contribute to bridging the gap in understanding the effects of tuning 2D nanomaterial dispersions on processing and final properties for improving microscale and macroscale applications.