AVS2010 Session GR+NS-MoA: Graphene: Chemical Reactions

Graphene/graphite oxide (GO), with its non-stoichiometric chemical structure and highly hygroscopic functionalized graphene backbone, has been of interest for chemical energy storage applications such as supercapacitors and ultracapacitors as well as its reduced form for bioelectronic applications such as sensor networks and for printed electronics. For all these applications, it is important to characterize and control the chemical nature of GO at various stages of thermal reduction. In particular, the formation and evolution of defect structures within GO can greatly impact the resulting properties. Similarly, the properties of species intercalated between individual planes can alter the reduction chemistry. The interplay between intercalated species and defect sites can lead to interesting new chemistry.

To understand these chemical interactions, we have studied the deoxygenation process of both single- and multi-layer(s) of GO and focused on comparing the removal of oxygen upon heating both from the basal plane and the edges, by bringing to bear a number of methods. In particular, we have carried out a series of experiments using in-situ IR absorption spectroscopy (FTIR), x-ray diffraction technique (XRD), ex-situ raman scattering, thermogravimetric analysis (TGA), transmission electron microscopy (TEM), and x-ray photoelectron spectroscopy (XPS), together with density functional (DFT) calculations and molecular dynamic (MD) simulations of various oxygen structures and resulting electronic states. The IR absorption spectra reveal the formation of ketones, ethers and sp²-hybridized C=C as well as a loss of hydroxyls, carboxyls and epoxides upon gradual annealing. They also confirm that defect formation is a direct result of CO₂ and CO formation, and highlight the role of intercalated species, such as water. At higher temperatures (~850⁰C), oxygen is found to remain at the edges of defective sites in a very stable configuration, involving the alignment of edge ether. Overall, the structure of GO upon annealing exhibits interesting, and in some cases unexpected features, which could have a crucial role and applicability to various systems such as vibration-powered energy scavenging, night vision tracking systems and opto-electronics.

Understanding the interface interaction between metal nano-clusters and carbon supports composed of graphite materials is one of the most important needs in the development of carbon related-catalysts. We have studied the carbon support effects on the electrocatalysts for fuel cell as well as the interface interaction using model catalysts of Pt-deposited highly oriented pyrolytic graphite (HOPG) by IETS-STS-STM, TPD, and molecular/atomic beam technique.　Here, we report the formation of non-bonding state in the vicinity of defects on the HOPG surface. We have measured STS spectra at many different points in the vicinity of the point defect. STS spectra were very different depending on the point of the measurement. In the directions perpendicular to short zigzag edges at the point defect, STS peaks appear near the Fermi level, suggesting the edge state or non-bonding state. The non-bonding state propagates 3-4 nanometers away from the defect with three-fold symmetry. No such a STS peak was observed near the point defect in the other directions. We thus constructed a STS map showing the distribution of the non-bonding state. We also observed spatial “oscillations” in the dI/dV peak position and the intensity. The oscillations have been explained as electron-electron interaction or electron-hole interaction. We currently consider that the non-bonding state is responsible for the chemical reactivity such as adsorption of atom, molecules, and clusters.

References

1. E. Yoo, J. Nakamura, et al., Nano Letters 9 (2009) 2255.

2. T. Kondo, J. Nakamura, et al., Phys. Rev. B 80 (2009) 233408.

3. J. Oh, T. Kondo, J. Nakamura, et al., J. Phys. Chem. Lett. 1 (2010) 463.

4. T. Kondo, J. Nakamura, et al.,J. Phys. Chem. C 112 (2008) 15607.

A solution based method that allows uniform and controllable deposition of reduced graphene oxide thin films with thicknesses ranging from a single monolayer up to several layers over large areas will be described. Transport of carriers in reduced GO is limited by the structural disorder. However, conductivity of 10⁵ S/m and mobilities of ~ 10 cm²/V-s are sufficiently large for applications where inexpensive and moderate performance electronics are required. The oxidation treatment during synthesis of GO creates sp³ C-O sites where oxygen atoms are bonded in the form of various functional groups. GO is therefore a two dimensional network of sp² and sp³ bonded atoms, in contrast to an ideal graphene sheet which consists of 100% sp² carbon atoms. This unique atomic and electronic structure of GO, consisting of variable sp²/sp³ fraction, opens up possibilities for new functionalities. The most notable difference between GO and mechanically exfoliated graphene is the opto-electronic properties arising from the presence of finite band gap. In particular, the photoluminescence can be tuned from blue to green emission. The atomic and electronic structure along with tunable photoluminescence of graphene oxide at various degrees of reduction will be described.

large-scale production, but established procedures (such as mechanical exfoliation or chemical vapour deposition) are not

ideal for the manufacture of processable graphene sheets. An alternative method is the reduction of graphene oxide, a

material that shares the same atomically thin structural framework as graphene, but bears oxygen-containing functional

groups. Here we use molecular dynamics simulations to study the atomistic structure of progressively reduced graphene

oxide. The chemical changes of oxygen-containing functional groups on the annealing of graphene oxide are elucidated and

the simulations reveal the formation of highly stable carbonyl and ether groups that hinder its complete reduction to

graphene. The calculations are supported by infrared and X-ray photoelectron spectroscopy measurements. Finally, more

effective reduction treatments to improve the reduction of graphene oxide are proposed [1].

[1] Akbar Bagri, Cecilia Mattevi, Muge Acik, Yves J. Chabal, Manish Chhowalla and

Vivek B. Shenoy, Nature Chemistry (in press, 2010).

Graphene was grown by CVD on a copper substrate. The as-grown samples and samples transferred to silicon on insulator (SOI) and SiO₂ substrates were then exposed to XeF₂ gas resulting in fluorination of the graphene sheets. The samples were characterized using XPS and Raman spectroscopies. XPS showed maximum fluorine content in the graphene of 20 % for the graphene on copper and 50 % on SOI. The marked difference is owed to the etching of the silicon layer on the SOI substrate, allowing exposure of the underside of the graphene sheet to the XeF₂ gas and fluorination of both sides of the graphene sheet. The fluorine was removed from the film using both thermal and chemical (hydrazine reduction) methods. Chemical, mechanical, and electrical properties of these materials will also be discussed.

Graphene is the most likely carbon-based successor material for CMOS electronics. Recently, interest in chemically modified graphene (CMG) has risen for producing large-scale flexible conductors and for its potential to open an electronic gap in graphene structures. We have developed a means to tune the topographical and electrical properties of several CMGs with nanoscopic resolution by local thermal processing with an AFM tip. Heating converts the CMG back towards graphene with nanoscale resolution. Nanostructures of one CMG, graphene oxide, show an increase in conductivity up to four orders of magnitude as compared to pristine material. Variably conductive graphene nanoribbons have been produced in a single step that is clean, rapid and reliable. Critically, the "carbon skeleton" is continuous across the CMG/graphene boundary. Recent work suggests that ribbons formed this way may be superior to ribbons that were cut.

We have used low-energy electron microscopy to investigate how graphene is removed from Ru(0001) and Ir(111) by reaction with oxygen. We find two mechanisms on Ru(0001). At short times, oxygen reacts with carbon monomers on the surrounding Ru surface, decreasing their concentration below the equilibrium value. This undersaturation causes a flux of carbon from graphene to the monomer gas. In this initial mechanism, graphene is etched at a rate that is given precisely by the same nonlinear dependence on carbon monomer concentration that governs growth. Thus, during both growth and etching, carbon attaches and detaches to graphene as clusters of several carbon atoms. At later times, etching accelerates. We present evidence that this process involves intercalated oxygen, which destabilizes graphene. On Ir, this mechanism creates observable holes. It also occurs most quickly near wrinkles in the graphene islands, depends on the orientation of the graphene with respect to the Ir substrate, and, in contrast to the first mechanism, can increase the density of carbon monomers. We also observe that both layers of bilayer graphene islands on Ir etch together, not sequentially. Work at Sandia was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, U. S. Department of Energy under Contract No. DE-AC04-94AL85000.