Synthesis, characterization and applications of graphene architectures

Abstract

In Chapter 4 we focus on graphene oxide papers produced by top-down exfoliation methods. More specifically, controlled instabilities or wrinkles created on graphene oxide thin films were developed as a tunable optical transmission layer for use in dynamic glazing systems. Graphene oxide thin films, prepared using the top-down synthesis, were subjected to compressive strains in the uni-axial and bi-axial direction to create uniform wrinkling of the films. Scanning electron, optical and atomic force microscopy was used to image the wrinkling morphology to qualitatively understand the behavior of the films and delaminated buckling of the graphene oxide films was determined to be the cause of the wrinkling. UV-VIS-NIR transmission measurements were carried out to determine the average transmission of the films with uni-axial and bi-axial wrinkling. Maximization of the dynamic range of optical transmission in the visible wavelength region was achieved with bi-axial wrinkling and this method was studied in depth to understand the role of applied strain, substrate pre-strain, graphene oxide film thickness and cycling stability.

In Chapter 3, we explore the assembly of graphene papers by top down methods (i.e. exfoliation of bulk graphite). We then explore the use of such graphene papers as an anode material in Lithium-ion batteries. The morphologically novel electrode fabrication and its exceptional performance as a lithium ion battery anode were explored and an in-depth investigation was carried out to determine the precise reason for the enhanced anode performance. A modified thermal reduction technique of a stable graphene oxide paper was developed to create a novel, free standing, binder free, reduced graphene oxide architecture using the top-down synthesis approach. The process was optimized to maximize the capacity by varying temperature and time as the critical parameters for reduction. An in-depth study was undertaken using raman spectroscopy, computational modelling, scanning electron microscopy, x-ray diffraction and x-ray photoelectron spectroscopy to show that lithium metal was plated into the nano-pores of the anode and the defective nature of the graphene sheets acted as seed points for this plating.

In Chapter 2, we focus on bottom-up synthesis of graphene sheets by chemical vapor deposition. We then studied the wetting properties of graphene coated surfaces. More specifically the wetting properties of single and multilayer graphene films on flat and nanoscale rough surfaces are explored and the insights gained are used in improving heat transfer performance of copper surfaces. Single layer graphene, on certain flat surfaces, was shown to exhibit `wetting transparency' as a result of its sheer thinness and this property is of interest in various wetting related applications. Surface protection from corrosion and/or oxidation without change in wetting properties is tremendously useful in multiple fields and we looked to apply this property to dehumidification of copper surfaces. The short time scales results demonstrated that graphene indeed served to prevent oxidation of the surface which in turn promoted increased heat transfer co-efficients with respect to the oxidized copper surfaces. Closer inspection of the surface over long time scales however revealed that the oxide layer changed the wetting properties and this was detrimental to the heat transfer process.

Graphene, a two-dimensional sheet of sp2 hybridized carbon atoms arranged in a honeycomb lattice structure, has garnered tremendous interest from the scientific community for its unique combination of properties. It has interesting electrical, thermal, optical and mechanical properties that scientists and engineers are trying to understand and harness to improve current products as well as focus on disruptive technologies that can be made possible by this next generation material. In this thesis the synthesis, characterization and applications of various graphene architectures were explored from the context of a bottom-up and top-down synthesis approach. The work is divided into three main chapters and each one deals with a unique architecture of graphene as well as its properties and an application to a real world problem.