Hydrogenation of monolayer graphene in the diamond anvil cell and supercritical phenomena in methane
Functionalisation with hydrogen could allow exploitation of the remarkable electronic properties of graphene by creating tuneable electronic band gaps as well as offering access to its incredibly high surface area-to-volume ratio for uses in advanced materials by opening pathways to conventional organic chemistry on the material. While partially-hydrogenated graphene is regularly produced and its properties studied, the current methods of producing the material – which typically employ bombarding graphene with atomised hydrogen – have not yet shown the potential to synthesise fully-hydrogenated graphene, termed graphane.This thesis describes an alternative method of hydrogenating graphene by heating the material in an atmosphere of molecular hydrogen under high pressure (2.6 – 6.5 GPa) in a diamond anvil cell. The hydrogen content of functionalised samples can be estimated by observing the Raman spectrum and such analysis suggests that the diamond anvil cell method currently hydrogenates samples to an extent that is competitive with existing methods. By tailoring the sample architecture to allow hydrogen direct access to both sides of a graphene crystal, it is feasible that the diamond anvil cell method of hydrogenation could be used to synthesise the first graphane crystal.Also presented in this thesis is a series of experiments probing methane in the supercritical region of its pressure-temperature phase diagram, combining Raman spectroscopy and direct structural measurements through X-ray diffraction. At 298K, we observe discontinuous changes in the vibrational Raman spectrum of methane which are not accompanied by a change in density. This phenomena may be explained by a crossing of the recently-theorised Frenkel line or by critical point proximity effects described by Widom in the 1960s. The Raman discontinuity is not observed at 523 K, suggesting that alternative methods must be employed to conclusively determine the presence (or absence) of the Frenkel line.
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