Quantifying structural disorder in bulk hydrogenated graphene using x-ray and neutron diffraction

Abstract

Functionalization of graphene's two-dimensional sheets can be used to modify graphene's physical properties, such as changing its conductivity or inducing ferromagnetism, which is of broad interest for a myriad of applications. However, functionalized graphene nano-powders can exhibit considerable structural disorder that is often overlooked and it is difficult to quantitatively characterize. Hydrogenation of graphene is one such pathway to engineer graphene's properties that has shown remarkable modifications like p-orbital magnetism as well as converting graphene to an insulator, demonstrating a promise for both electronic and spintronic applications. While a plethora of studies have demonstrated various interesting property modifications upon synthesis of H-Gr, the fully hydrogenated graphane structure has yet to be determined experimentally. In this thesis, tools of x-ray and neutron diffraction were used in order to quantify the atomic structure of bulk H-Gr. The electrochemical synthesis of H-Gr was first pursued with the consideration of its accessbility for non-chemists. Attempts to electrochemically exfoliate and subsequently hydrogenate a graphite rod starting material using an ionic-liquid based electrolyte resulted in a highly heterogeneous structure revealed by x-ray diffraction suggesting that graphite domains were intercalated by electrolyte molecules with the relative degree of intercalation in graphite domains being proportional to the electrochemical treatment time. Attempts of electrochemical hydrogenation of a graphene nanopowder starting material with the ionic liquid electrolyte were shown to exfoliate and expand the interlayer distance between few-layer graphene, however no corroborating measurements were performed to elucidate this interlayer functionality. While there was no confirmation of the successful synthesis of H-Gr, future attempts could be guided by the work laid out here. The second part of this work examined the structure of H-Gr synthesized by Birch reduction of graphite-oxide. By combining x-ray diffraction (XRD) and neutron diffraction (ND), a method used to quantitatively characterize the H-Gr nano-structure is shown, including the determination of the H content. It's shown that a significant portion of the H-Gr contains highly disordered carbon while a small portion of the sample contains 2-3 layers of graphene with a significantly expanded interlayer spacing due to the H. Modeling the coherent diffuse scattering in XRD and ND, and comparing it with the incoherent ND, H to carbon ratio is determined and it is concluded that the structure is quite different from idealized two-dimensional sheets of H-Gr. The approach and methods used herein have broad relevance to many types of graphitic carbons.