Article pubs.acs.org/accounts
Structural Studies of Hydrographenes Elena Vishnyakova,† Gaowei Chen,† Bruce E. Brinson,† Lawrence B. Alemany,†,‡ and W. Edward Billups*,† †
Department of Chemistry and The Smalley-Curl Institute, Rice University, 6100 Main Street, Houston, Texas 77005 United States ‡ Department of Chemistry and Shared Equipment Authority, Rice University, 6100 Main Street, Houston, Texas 77005 United States
CONSPECTUS: As a result of the unique physical and electrical properties, graphene continues to attract the interest of a large segment of the scientific community. Since graphene does not occur naturally, the ability to exfoliate and isolate individual layers of graphene from graphite is an important and challenging process. The interlayer cohesive energy of graphite that results from van der Waals attractions has been determined experimentally to be 61 meV per carbon atom (61 meV/C atom). This requires the development of a method to overcome the strong attractive forces associated with graphite. The exfoliation process that we, and others, have investigated involves electron transfer into bulk graphite from intercalated lithium to yield lithium graphenide. The resulting graphenide can be reacted with various reagents to yield functionalized graphene. As a part of our interest in the functionalization of graphene, we have explored the Birch reduction as a route to hydrographenes. The addition of hydrogen transforms graphene into an insulator, leading to the prediction that important applications will emerge. This Account focuses mainly on the characterization of the hydrographenes that are obtained from different types of graphite including synthetic graphite powder, natural flake graphite, and annealed graphite powder. Analysis by solid state 13C NMR spectroscopy proved to be important since it was shown that the hydrographenes are composed of interior, isolated aromatic (predominantly fully substituted benzene) rings surrounded by saturated rings. The expected clusters of benzene rings were not found. NMR spectroscopy also offers strong evidence for the presence of tert-butyl alcohol and ethanol (workup solvent) that could not be removed in vacuo from the samples. These compounds could be observed to move freely within the layers of the hydrographene. High-resolution transmission electron microscopy images revealed a remarkable change in morphology that results when hydrogen is added to the graphenide. The appearance of edge and circular dislocations and increased distances between graphitic layers are most visible in the case of the hydrographenes that are formed from annealed graphite. The repetitive hydrogenation of synthetic graphite powder leads to an increase in the distances between the graphitic layers in the (002) direction from 3.4 Å for the initial graphite to 4.11 Å after the first reduction and to 4.29 Å after a third reduction of the same material. Defect-free graphite is formed when the hydrographenes are heated. The distance between carbon layers decreases from 4.11 to 3.44 Å after heating the samples to 1200 °C. This trend toward the spacing of graphite confirms the reversibility of the functionalization process. The C−H bonds have been broken yielding hydrogen, and the exposed carbon orbitals are in close enough proximity to have reverted to graphite. This Account introduces only a narrow area of materials chemistry, and many applications of graphene and its derivatives can be expected as researchers exploit this burgeoning field. dots,3,11 and others.10,17−21 Graphene is defined as stacked sheets of sp2-hybridized carbon where the number of sheets does not
1. INTRODUCTION Graphene is among the group of carbon nanomaterials that promises to occupy a significant position in nanoscience and nanotechnology.1−21 Potential applications include composite materials,4,7,8,12−14,22,23 liquid crystals,15 electrical circuits,16 quantum © 2017 American Chemical Society
Received: November 23, 2016 Published: May 9, 2017 1351
DOI: 10.1021/acs.accounts.6b00588 Acc. Chem. Res. 2017, 50, 1351−1358
Article
Accounts of Chemical Research exceed 10.9,24 It occurs in a stacked hexagonal structure of graphite with an interlayer spacing of 3.35 Å, the van der Waals distance for sp2-bonded carbon. Exfoliation has been achieved by either peeling the top surface of pyrolytic graphite25 or by exfoliation of the oxidation products of graphite (graphite oxide)23,26−28 where various reagents have been used to remove the oxygen functionality. Under these conditions, the original crystalline structure of graphene is not restored as the graphene is heavily disordered by defects that are introduced into the basal plane. The use of intercalating agents29−32 provides another method to exfoliate graphite. These intercalants are usually classified according to whether they form donor or acceptor compounds. The most common and most widely studied of the donor compounds are the alkali metals.33 The exfoliation process that we, and others, have investigated involves electron transfer into bulk graphite from intercalated lithium to yield lithium graphenide. Recent studies have also focused on the nature of the starting graphite34,35 and the type of intercalating agents.36 As part of our interest in the functionalization of carbon nanomaterials, we have explored the Birch reduction of graphite as a route to the hydrographenes, materials that have potential applications in numerous areas.37−39 The addition of hydrogen transforms graphene from a zero-band gap semiconductor into a wide band gap insulator,40,41 depending on the extent of hydrogenation.42,43 Other potential applications include ferromagnetism,44−46 hydrogen storage,42,44,47 and high-performance anode material for lithium ion batteries.48 Hirsch and his co-workers reported the first wet-chemical method to synthesize hydrographene.49 A salient feature of their work involved the use of water as the hydrogen source. In this way, competing hydrogen formation was avoided when the water was added slowly. Under these conditions, the hydrogenation is accompanied by very strong fluorescence of the reaction product. In this Account, we describe the conditions that we have used to synthesize and characterize hydrographene. Structural characterization relied mainly on high resolution transmission electron microscopy and solid state 13C nuclear magnetic resonance spectroscopy. Different sources of graphite were used as starting materials including graphite powders purchased from Aldrich (99.99% powder, synthetic,
