Reactivity of 2D Materials Induced via Out-of-Plane Distortions

Abstract

Due to the increasing scarcity of raw materials and the great extent of waste produced from solvent-based reactions, the scientific community has developed an interest in pursuing solid-state methods of driving chemical reactions. Mechanochemistry- inducing chemical reactions by the direct absorption of mechanical energy, can lead to increased product yield and increased selectivity in a solvent-free manner. While research on mechanochemistry has been rapidly expanding, the complexity of reaction conditions in macroscale methods such as ball-milling and twin-screw extrusion highlight the need for reaction platforms able to explore fundamental questions such as the influence of directionality on selectivity and reactivity. Two-dimensional (2D) materials offer an ideal platform for the investigation of mechanochemical reactivity. Their well-defined structures and relatively inert basal planes lead to their use as model systems for elucidating the influence of force and directionality on their susceptibility to undergo mechanochemical transitions. This dissertation includes research into the susceptibility of 2D materials such as graphene and molybdenum disulfide (MoS2) to react upon the application of out-of-plane lattice distortions, leading to the formation of out-of-plane covalent bonds. Using Raman spectroscopy and first-principles calculations, graphene was shown to have a higher degree of reactivity with aryl radicals upon the application of out-of-plane distortions induced through lattice conformation to a rough underlaying surface. Instrumentation has also been developed to apply tailorable amounts of out-of-plane distortion to 2D membranes through a gas or liquid backing pressure, while performing in situ spectral (Raman spectroscopy or X-ray photoelectron spectroscopy) and morphological (atomic force microscopy) characterization of the 2D lattice. This has allowed for the ability to monitor in real time the increased susceptibility of graphene distorted out-of-plane to oxidation via water exposure, a process that does not occur when the lattice maintains its planar character. Additionally, Raman spectroscopy was used to monitor the susceptibility of MoS2, a common solid lubricant, to oxidation through UV-ozone as a function of underlaying surface roughness. The fundamental information gained from these studies help to fill in the knowledge gap on the influence of directionality in mechanochemical processes and connect the atomistic tenets of mechanochemistry to macroscale processes.