EDITORIAL pubs.acs.org/JPCC
Virtual Issue: Graphene and Functionalized Graphene
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arbon nanostructures have dominated advances in nanoscience and nanotechnology since the latter part of the 20th century. The 1996 Nobel Prize in Chemistry awarded jointly to Robert F. Curl, Jr., Sir Harold W. Kroto, and Richard E. Smalley “for their discovery of fullerenes” and the 2010 Nobel Prize in Physics awarded jointly to Andre Geim and Konstantin Novoselov “for groundbreaking experiments regarding the twodimensional material graphene” recognize the potential impact of fullerenes, carbon nanotubes, graphene, and other carbon nanostructures in future nanotechnology-based discoveries. Fullerenes, carbon nanotubes and nanofibers, and carbon quantum dots all consist of similar carbon atom networks, but exhibit significantly different properties that are dictated by size, shape, and chirality. Having established many of their electronic and optical properties during the last two decades, scientists switched attention to the parent structure, graphene. Physical chemistry has been at the forefront of disseminating a fundamental understanding of graphene and functionalized graphene structures. Several recent perspective articles highlight current progress and emerging issues related to graphene research.1-6 In this virtual issue, we discuss representative papers published in the Journals of Physical Chemistry A/B/C and Letters to provide a physical chemistry perspective on graphene-based nanostructures (Scheme 1). The electronic structure of graphene (flat carbon) exhibits properties that are different from its one-dimensional analog, viz., carbon nanotube. Graphene is considered to be a zero-bandgap semiconductor material. By application of an electric field or with suitable functionalization, the Fermi level can be altered to obtain n-doped or p-doped material. The early work of de Heer and coworkers7 highlighted electronic device applications based on nanopatterned epitaxial graphene. Ultrathin epitaxial graphite films grown by thermal decomposition on the (0001) surface of 6H-SiC exhibited remarkable two-dimensional electron gas behavior. This landmark JPC-B paper has garnered nearly 700 citations and is included in the citation of the 2010 Nobel Prize in Physics (http://nobelprize.org/nobel_prizes/physics/laureates/ 2010/sciback_phy_10_2.pdf).
’ THEORY The combination of the unique physical properties with the extremely rich carbon chemistry makes graphene one of the most exciting and challenging materials of modern theoretical work. Theory focuses on the interaction of graphene with atoms and molecules, diffusion of the adsorbed species, graphene’s chemical derivatives such as graphane (hydrogenated graphene), graphene oxide, thiolate, and fluoride, defects, dopants, nanoribbons, and edges. The studies are motivated by the possibility of multiple applications in electronics, spintronics, hydrogen storage, biological detection, electrical batteries, capacitors, etc. The ability of graphene to adsorb hydrogen makes it an excellent candidate for hydrogen fuel storage. Theory is needed to guide experiments in the optimal design of graphene derivatives, since hydrogen molecules desorb from flat graphene at room r 2011 American Chemical Society
Scheme 1. Seeking Fundamental Information of GrapheneBased Materials through Physical Chemistry
temperature, while chemisorption of atomic hydrogen requires highly elevated temperatures.8,9 Theoretical studies of graphene interactions with oxygen, fluorine, sulfur, and other species establish structures and reaction mechanisms for the chemical modifications of graphene.10-12 Theory can predict how the adsorbed species can be detected spectroscopically.13 Graphitic carbon is considered the state-of-the-art material for the negative electrode in lithium ion batteries. Modeling of lithium diffusion in graphene assists in the rational design of carbonaceous electrodes with high charge/discharge rates.14 Graphene provides an ideal model for interaction of water with hydrophobic surfaces.15 Such interactions are very important in biology. Graphene has led to a number of other novel materials. Theory predicts that in contrast to graphene, which is a conductor, graphane16 and graphene oxide17 are wide-bandgap semiconductors. They can replace silicon in electronics applications. Doping with phosphorus or boron allows precise chemical interactions with graphene and its derivatives, which in turn provide extra flexibility for adjusting their electronic properties. Quantum confinement is another route for making graphene into a semiconductor. Graphene nanoribbons can be obtained both chemically and bottom-up and top-down using nanolithography. Calculations18 show that ribbon edges are chemically very active and generate additional properties, such as half-metallicity and spinpolarization. The latter forms the basis for spintronics—an alternative to electronics. Calculations also predict that graphene defects trap charges and that defects can be reconstructed by thermal activation.19 State-of-the-art time-domain simulations20 show that energy dissipation to heat is slower in graphene nanoribbons than carbon nanotubes, making the former a better material for electronic devices.
’ SYNTHESIS, FUNCTIONALIZATION, AND CHARACTERIZATION Among the synthetic approaches, the chemical vapor deposition technique on metal or Si surfaces has provided a means to Published: February 15, 2011 3195
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The Journal of Physical Chemistry C investigate physical, optical electronic, and spectroscopic properties.21 Another common route is exfoliation of graphene sheets by chemical oxidation using strong chemical oxidants, such as HNO3, KMnO4, and H2O2. The exfoliated 2-D sheets of distorted carbon networks carry carboxylate, hydroxyl, and epoxide functional groups and are commonly referred to as graphene oxide or graphitic oxide (typically abbreviated GO). These functional groups enable suspension of GO sheets in polar solvents and thus enable their study in solvent media. A partial restoration of the sp2 network of GO can be achieved using thermal,22 chemical,23 electrochemical,24 photothermal,25 photocatalytic,26 sonochemical,27 and microwave reduction28 methods. Covalent and noncovalent functionalization of graphene oxide with photochemically or electrochemically active groups and organic radicals has been the focus of recent investigations.29 For example, by covalently functionalizing graphene with chromophores, porphyrin, or fullerene, it is possible to enhance nonlinear optical performance in the nanosecond regime.30 Donoracceptor interactions greatly influence the optical properties of graphene.30 Covalent functionalization of epitaxial graphene by azidotrimethylsilane has enabled the tuning of bandgap31 π-π interactions between porphyrin and graphene and has also been exploited to prepare graphene films with superior conductive properties.32 The fluorescence dynamics of poly(3-hexylthiophene-2,5-diyl) (P3HT) and poly[2-methoxy-5-(20 -ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) blended with graphene microsheets have provided insight into the energy transfer dynamics.33 Electron microscopy (TEM and SEM) and atomic force microscopy have been used to characterize a single layer to several layers of graphene and graphene oxide sheets deposited on various substrates. The first layer of graphene has been found often to interact with the substrate and thus to exhibit properties that differ from a pristine graphene layer.34 Angle-resolved nearedge X-ray absorption fine structure (NEXAFS) spectroscopy of single- and bilayered graphene grown by chemical vapor deposition on Cu and Ni substrates indicated the presence of new electronic states in the conduction band derived from hybridization of the C-C network with Cu and Ni d-orbitals.35 Raman spectroscopy in particular has been found to be a valuable tool to elucidate the structural properties of graphene.34 The position and intensity ratio of D and G bands reveal the extent of defect sites, as well as the interaction of the graphene layer with surroundings. Deposition of Ag and Au nanoparticles on graphene sheets often enhances the intensity of Raman bands, possibly due to plasmon enhancement of the electromagnetic field.27
’ APPLICATIONS OF GRAPHENE-BASED NANOASSEMBLIES The two-dimensional carbon network serves as a simple platform to anchor metal and semiconductor nanoparticles.2 Such nanocomposites provide a simple way to disperse catalyst particles and are capable of influencing the chemical reactions with direct or indirect participation. Early efforts were focused on producing graphene oxide-metal27,28,36,37 and graphene oxidesemiconductor nanocomposites by solution processing methods.26,38 Suspended graphene composites can be cast as thin films on electrode surfaces by drop cast or electrophoretic deposition methods, thus allowing their use in different applications.39 Carbon nanostructures have played a significant role as supports for increasing the electrocatalytic area in fuel cells and other
EDITORIAL
energy conversion systems.6,40 Early studies of graphene-based electrocatalysts have shown promising applications in fuel cells and Li ion batteries.41,42 Poly(ethyleneimine)-modified graphene sheets with their interconnected network of carbon structures and well-defined nanopores have been found to be promising as supercapacitor electrodes.43,44 Similarly, dispersion of semiconductor nanoparticles on graphene oxide (GO) sheets has been found to be extremely useful in improving the performance of photoelectrochemical solar cells.26 The graphene sheets not only improve the charge separation in semiconductor nanostructures but also facilitate collection and transportation of electrons.26 Furthermore, composites of GO-TiO2 exhibit improved photocatalytic activity toward mineralization of organic contaminants. This improved performance arises from graphene oxide’s ability to adsorb organic molecules and thus increase its local concentration near the photocatalyst surface. Other applications of graphene are in the areas of sensing and developing conducting surfaces.45,46 The papers highlighted in this virtual issue (go to http://pubs. acs.org/page/vi/2011/graphene.html) provide a physical chemistry perspective of ongoing graphene research. The flat carbon geometry provides new opportunities to design two-dimensional hybrid assemblies composed of different catalyst particles and molecules with specific functionality. A fundamental understanding of the surface interactions and elucidation of the charge transfer and transport properties will be the key to the development of next-generation catalyst systems and sensing devices. Other developments in the design of graphene-based systems will continue to emerge as we venture into the 2-D domain of carbon nanostructures in the near future. Oleg V. Prezhdo Senior Editor University of Rochester
Prashant V. Kamat Deputy Editor University of Notre Dame
George C. Schatz Editor-in-Chief Northwestern University
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