Publications by Year: 2011

Pan, S. Y. ; Aksay, I. A. Factors Controlling the Size of Graphene Oxide Sheets Produced via the Graphite Oxide Route. ACS Nano 2011, 5 4073-4083.Abstract
We have studied the effect of the oxidation path and the mechanical energy input on the size of graphene oxide sheets derived from graphite oxide. The cross-planar oxidation of graphite from the (0002) plane results in periodic cracking of the uppermost graphene oxide layer, limiting its lateral dimension to less than 30 mu m. We use an energy balance between the elastic strain energy associated with the undulation of graphene oxide sheets at the hydroxyl and epoxy sites, the crack formation energy, and the interaction energy between graphene layers to determine the cell size of the cracks. As the effective crack propagation rate in the cross-planar direction is an order of magnitude smaller than the edge-to-center oxidation rate, graphene oxide single sheets larger than those defined by the periodic cracking cell size are produced depending on the aspect ratio of the graphite particles. We also demonstrate that external energy input from hydrodynamic drag created by fluid motion or sonication, further reduces the size of the graphene oxide sheets through tensile stress buildup in the sheets.
Aksay, I. A. ; Milius, D. L. ; Korkut, S. ; Prud'homme, R. K. Functionalized graphene sheets having high carbon to oxygen ratios, 2011.Abstract
Functionalized graphene sheets having a C to O molar ratio of at least ∼23:1 and method of prepg. the same. [on SciFinder(R)]
Zhang, S. ; Shao, Y. Y. ; Liao, H. G. ; Liu, J. ; Aksay, I. A. ; Yin, G. P. ; Lin, Y. H. Graphene Decorated with PtAu Alloy Nanoparticles: Facile Synthesis and Promising Application for Formic Acid Oxidation. Chemistry of Materials 2011, 23, 1079-1081.
Zhang, S. ; Shao, Y. Y. ; Liu, J. ; Aksay, I. A. ; Lin, Y. H. Graphene-Polypyrrole Nanocomposite as a Highly Efficient and Low Cost Electrically Switched Ion Exchanger for Removing ClO4- from Wastewater. ACS Applied Materials & Interfaces 2011, 3 3633-3637.Abstract
Perchlorate (ClO4-) contamination is a widespread concern affecting water utilities. In the present study, functionalized graphene sheets were employed as the scaffold to synthesize a novel graphene-polypyrrole (Ppy) nanocomposite, which served as an excellent electrically switched ion exchanger for perchlorate removal. Scanning electron microscopy and electrochemical measurements showed that the 3D nanostructured graphene-Ppy nanocomposite exhibited a significantly improved uptake capacity for ClO4- compared with Ppy film alone. X-ray photoelectron spectroscopy confirmed the uptake and release process of ClO4- in graphene-Ppy nanocomposite. In addition, the presence of graphene substrate resulted in high stability of graphene-Ppy nanocomposite during potential cycling. The present work provides a promising method for large scale water treatment.
Xiao, J. ; Mei, D. H. ; Li, X. L. ; Xu, W. ; Wang, D. Y. ; Graff, G. L. ; Bennett, W. D. ; Nie, Z. M. ; Saraf, L. V. ; Aksay, I. A. ; et al. Hierarchically Porous Graphene as a Lithium-Air Battery Electrode. Nano Letters 2011, 11, 5071-5078.Abstract
The lithium-ir battery is one of the most promising technologies among various electrochemical energy storage systems. We demonstrate that a novel air electrode consisting of an unusual hierarchical arrangement of functionalized graphene sheets (with no catalyst) delivers an exceptionally high capacity of 15000 mAh/g in lithium-O-2 batteries which is the highest value ever reported in this field. This excellent performance is attributed to the unique bimodal porous structure of the electrode which consists of microporous channels facilitating rapid O-2 diffusion while the highly connected nanoscale pores provide a high density of reactive sites for Li-O-2 reactions. Further, we show that the defects and functional groups on graphene favor the formation of isolated nanosized Li2O2 particles and help prevent air blocking in the air electrode. The hierarchically ordered porous structure in bulk graphene enables its practical applications by promoting accessibility to most graphene sheets in this structure.
Korkut, S. ; Roy-Mayhew, J. D. ; Dabbs, D. M. ; Milius, D. L. ; Aksay, I. A. High Surface Area Tapes Produced with Functionalized Graphene. ACS Nano 2011, 5 5214-5222.Abstract
We describe a scalable method for producing continuous graphene networks by tape casting surfactant-stabilized aqueous suspensions of functionalized graphene sheets. Similar to all other highly connected graphene-containing networks, the degree of overlap between the sheets controls the tapes' electrical and mechanical properties. However, unlike other graphene-containing networks, the specific surface area of the cast tapes remains high (>400 m(2).g(-1)). Exhibiting apparent densities between 0.15 and 0.51, with electrical conductivities up to 24 kS.m(-1) and tensile strengths over 10 MPa, these tapes exhibit the best combination of properties with respect to density heretofore observed for carbon-based papers, membranes, or films.
Pope, M. A. ; Punckt, C. ; Aksay, I. A. Intrinsic Capacitance and Redox Activity of Functionalized Graphene Sheets. Journal of Physical Chemistry C 2011, 115, 20326-20334.Abstract
We present a general method for characterizing the intrinsic electrochemical properties of graphene sheets, such as the specific double-layer capacitance, in the absence of porosity-related artifacts and uncertainties. By assembling densely tiled monolayers of electrically insulating or conductive functionalized graphene sheets onto electrode substrates (gold and highly oriented pyrolytic graphite), we demonstrate our ability to isolate their intrinsic electrochemical response in terms of surface-specific double-layer capacitance and redox behavior. Using this system, the electrochemical properties of various types of graphene can be directly compared without the need to take into account changes in electrode morphology and electrolyte accessibility arising because of varying material properties.
Yan, L. ; Punckt, C. ; Aksay, I. A. ; Mertin, W. ; Bacher, G. Local Voltage Drop in a Single Functionalized Graphene Sheet Characterized by Kelvin Probe Force Microscopy. Nano Letters 2011, 11, 3543-3549.Abstract
We studied the local voltage drop in functionalized graphene sheets of sub mu m size under external bias conditions by Kelvin probe force microscopy. Using this noninvasive experimental approach, we measured ohmic current-voltage characteristics and an intrinsic conductivity of about 3.7 x 10(5) S/m corresponding to a sheet resistance of 2.7 k Omega/sq under ambient conditions for graphene produced via thermal reduction of graphite oxide. The contact resistivity between functionalized graphene and metal electrode was found. to be <6.3 x 10(-7) Omega cm(2).
Pan, S. ; Aksay, I. A. ; Prudhomme, R. K. Multifunctional graphene-silicone elastomer nanocomposite, method of making the same, and uses thereof, 2011.Abstract
In a nanocomposite compn. having a silicone elastomer matrix having therein a filler loading of greater than 0.05 wt %, based on total nanocomposite wt., the filler is functional graphene sheets (FGS) having a surface area of from 300 m2/g to 2630 m2/g; and a method for producing the nanocomposite and uses thereof. [on SciFinder(R)]
Yan, L. ; Punckt, C. ; Aksay, I. A. ; Mertin, W. ; Bacher, G. Potential Distribution in Functionalized Graphene Devices Probed by Kelvin Probe Force Microscopy. In Physics of Semiconductors: 30th International Conference on the Physics of Semiconductors; Ihm, J. ; Cheong, H., Ed. American Institute of Physics, 2011; Vol. 1399. Publisher's VersionAbstract
Kelvin probe force microscopy was used to study the impact of contacts and topography on the local potential distribution on contacted, individual functionalized graphene sheets (FGS) deposited on a SiO2/Si substrate. Negligible contact resistance is found at the graphene/Ti interface and a graphene resistance of 2.3 k Omega is extracted for a single sheet with sub-mu m size. Pronounced steps in the topography, which we attribute to a variation of the spacing between graphene and substrate, result in a significant change of the local resistivity.
Cao, Y. L. ; Li, X. L. ; Aksay, I. A. ; Lemmon, J. ; Nie, Z. M. ; Yang, Z. G. ; Liu, J. Sandwich-type functionalized graphene sheet-sulfur nanocomposite for rechargeable lithium batteries. Physical Chemistry Chemical Physics 2011, 13, 7660-7665.Abstract
A functionalized graphene sheet-sulfur (FGSS) nanocomposite was synthesized as the cathode material for lithium-sulfur batteries. The structure has a layer of functionalized graphene sheets/stacks (FGS) and a layer of sulfur nanoparticles creating a three-dimensional sandwich-type architecture. This unique FGSS nanoscale layered composite has a high loading (70 wt%) of active material (S), a high tap density of similar to 0.92 g cm(-3), and a reversible capacity of similar to 505 mAh g(-1) (similar to 464 mAh cm(-3)) at a current density of 1680 mA g(-1) (1C). When coated with a thin layer of cation exchange Nafion film, the migration of dissolved polysulfide anions from the FGSS nanocomposite was effectively reduced, leading to a good cycling stability of 75% capacity retention over 100 cycles. This sandwich-structured composite conceptually provides a new strategy for designing electrodes in energy storage applications.
Liu, J. ; Aksay, I. A. ; Choi, D. ; Kou, R. ; Nie, Z. ; Wang, D. ; Yang, Z. Self assembled multi-layer nanocomposite of graphene and metal oxide materials, 2011.Abstract
Nanocomposite materials having at least two layers, each layer consisting of one metal oxide bonded to at least one graphene layer were developed. The nanocomposite materials will typically have many alternating layers of metal oxides and graphene layers, bonded in a sandwich type construction and will be incorporated into an electrochem. or energy storage device. [on SciFinder(R)]
Liu, J. ; Choi, D. ; Kou, R. ; Nie, Z. ; Wang, D. ; Yang, Z. ; Aksay, I. A. Self assembled multi-layer nanocomposite of graphene and metal oxide materials, 2011.Abstract
Nanocomposite materials having at least two layers, each layer consisting of one metal oxide bonded to at least one graphene layer were developed. The nanocomposite materials will typically have many alternating layers of metal oxides and graphene layers, bonded in a sandwich type construction and will be incorporated into an electrochem. or energy storage device. [on SciFinder(R)]
Roy-Mayhew, J. ; Aksay, I. Semiconductor coated microporous graphene scaffolds for solar cells, 2011.Abstract
The invention refers to a high surface area scaffold to be used for a solar cell, made of a three-dimensional percolated network of functionalized graphene sheets. It may be used in the prepn. of a high surface area electrode by coating with a semiconductive material. Electronic devices can be made therefrom, including solar cells such as dye-sensitized solar cells. [on SciFinder(R)]
Kou, R. ; Shao, Y. Y. ; Mei, D. H. ; Nie, Z. M. ; Wang, D. H. ; Wang, C. M. ; Viswanathan, V. V. ; Park, S. ; Aksay, I. A. ; Lin, Y. H. ; et al. Stabilization of Electrocatalytic Metal Nanoparticles at Metal-Metal Oxide-Graphene Triple Junction Points. Journal of the American Chemical Society 2011, 133, 2541-2547.Abstract
Carbon-supported precious metal catalysts are widely used in heterogeneous catalysis and electrocatalysis, and enhancement of catalyst dispersion and stability by controlling the interfacial structure is highly desired. Here we report a new method to deposit metal oxides and metal nanoparticles on graphene and form stable metal-metal oxide-graphene triple junctions for electrocatalysis applications. We first synthesize indium tin oxide (ITO) nanocrystals directly on functionalized graphene sheets, forming an ITO-graphene hybrid. Platinum nanoparticles are then deposited, forming a unique triple-junction structure (Pt-ITO-graphene). Our experimental work and periodic density functional theory (DFT) calculations show that the supported Pt nanoparticles are more stable at the Pt-ITO-graphene triple junctions. Furthermore, DFT calculations suggest that the defects and functional groups on graphene also play an important role in stabilizing the catalysts. These new catalyst materials were tested for oxygen reduction for potential applications in polymer electrolyte membrane fuel cells, and they exhibited greatly enhanced stability and activity.
Castrillon, S. R. V. ; Giovambattista, N. ; Aksay, I. A. ; Debenedetti, P. G. Structure and Energetics of Thin Film Water. Journal of Physical Chemistry C 2011, 115, 4624-4635.Abstract
We present a molecular dynamics (MD) simulation study of the structure and energetics of thin films of water adsorbed on solid substrates at 240 K. By considering crystalline silica as a model hydrophilic surface, we systematically investigate the effect of film thickness on the hydrogen bonding, density, molecular orientation, and energy of adsorbed water films over a broad surface coverage range (delta). At the lowest coverage investigated (delta = 1 monolayer, >90% of water molecules form three hydrogen bonds (H-bonds) with surface silanol groups and none with other water molecules; when delta = 1 ML, the most probable molecular orientation is characterized by both the molecular dipole and the OH vectors being parallel to the surface. As 6 increases, water-water and water-surface interactions compete, leading to the appearance of an orientational structure near the solid-liquid interface characterized by the dipole moment pointing toward the silica surface. We find that the water-surface H-bond connectivity and energetics of the molecular layer nearest to the solid liquid interface do not change as delta increases. Interfacial water molecules, therefore, are able to reorient and form water-water H-bonds without compromising water-surface interactions. The surface-induced modifications to the orientational structure of the adsorbed film propagate up to similar to 1.4 nm from the solid-liquid interface when delta = 15.1 ML (a film that is similar to 2.3 run thick). For the thinner adsorbed films (delta <= 4.3 ML, thickness <= 0.8 nm) orientational correlations imposed by the solid liquid and liquid-vapor interfaces are observed throughout.