Showing papers by "Rodney S. Ruoff published in 2011"

Carbon-based Supercapacitors Produced by Activation of Graphene

Abstract: Supercapacitors, also called ultracapacitors or electrochemical capacitors, store electrical charge on high-surface-area conducting materials. Their widespread use is limited by their low energy storage density and relatively high effective series resistance. Using chemical activation of exfoliated graphite oxide, we synthesized a porous carbon with a Brunauer-Emmett-Teller surface area of up to 3100 square meters per gram, a high electrical conductivity, and a low oxygen and hydrogen content. This sp 2 -bonded carbon has a continuous three-dimensional network of highly curved, atom-thick walls that form primarily 0.6- to 5-nanometer-width pores. Two-electrode supercapacitor cells constructed with this carbon yielded high values of gravimetric capacitance and energy density with organic and ionic liquid electrolytes. The processes used to make this carbon are readily scalable to industrial levels.

Transfer of CVD-Grown Monolayer Graphene onto Arbitrary Substrates

Abstract: Reproducible dry and wet transfer techniques were developed to improve the transfer of large-area monolayer graphene grown on copper foils by chemical vapor deposition (CVD). The techniques reported here allow transfer onto three different classes of substrates: substrates covered with shallow depressions, perforated substrates, and flat substrates. A novel dry transfer technique was used to make graphene-sealed microchambers without trapping liquid inside. The dry transfer technique utilizes a polydimethylsiloxane frame that attaches to the poly(methyl methacrylate) spun over the graphene film, and the monolayer graphene was transferred onto shallow depressions with 300 nm depth. The improved wet transfer onto perforated substrates with 2.7 μm diameter holes yields 98% coverage of holes covered with continuous films, allowing the ready use of Raman spectroscopy and transmission electron microscopy to study the intrinsic properties of CVD-grown monolayer graphene. Additionally, monolayer graphene transfer...

Large-Area Graphene Single Crystals Grown by Low-Pressure Chemical Vapor Deposition of Methane on Copper

Abstract: Graphene single crystals with dimensions of up to 0.5 mm on a side were grown by low-pressure chemical vapor deposition in copper-foil enclosures using methane as a precursor. Low-energy electron microscopy analysis showed that the large graphene domains had a single crystallographic orientation, with an occasional domain having two orientations. Raman spectroscopy revealed the graphene single crystals to be uniform monolayers with a low D-band intensity. The electron mobility of graphene films extracted from field-effect transistor measurements was found to be higher than 4000 cm2 V−1 s−1 at room temperature.

Nanostructured reduced graphene oxide/Fe2O3 composite as a high-performance anode material for lithium ion batteries.

Abstract: Reduced graphene oxide/Fe2O3 composite was prepared using a facile two-step synthesis by homogeneous precipitation and subsequent reduction of the G-O with hydrazine under microwave irradiation to yield reduced graphene oxide (RG-O) platelets decorated with Fe2O3 nanoparticles. As an anode material for Li-ion batteries, the RG-O/Fe2O3 composite exhibited discharge and charge capacities of 1693 and 1227 mAh/g, respectively, normalized to the mass of Fe2O3 in the composite (and ∼1355 and 982 mAh/g, respectively, based on the total mass of the composite), with good cycling performance and rate capability. Characterization shows that the Fe2O3 nanoparticles are uniformly distributed on the surface of the RG-O platelets in the composite. The total specific capacity of RG-O/Fe2O3 is higher than the sum of pure RG-O and nanoparticle Fe2O3, indicating a positive synergistic effect of RG-O and Fe2O3 on the improvement of electrochemical performance. The synthesis approach presents a promising route for a large-sca...

Oxidation resistance of graphene-coated Cu and Cu/Ni alloy

Abstract: The ability to protect refined metals from reactive environments is vital to many industrial and academic applications. Current solutions, however, typically introduce several negative effects, including increased thickness and changes in the metal physical properties. In this paper, we demonstrate for the first time the ability of graphene films grown by chemical vapor deposition to protect the surface of the metallic growth substrates of Cu and Cu/Ni alloy from air oxidation. In particular, graphene prevents the formation of any oxide on the protected metal surfaces, thus allowing pure metal surfaces only one atom away from reactive environments. SEM, Raman spectroscopy, and XPS studies show that the metal surface is well protected from oxidation even after heating at 200 °C in air for up to 4 h. Our work further shows that graphene provides effective resistance against hydrogen peroxide. This protection method offers significant advantages and can be used on any metal that catalyzes graphene growth.

The effect of chemical residues on the physical and electrical properties of chemical vapor deposited graphene transferred to SiO2

Abstract: The effects of residues introduced during the transfer of chemical vapor deposited graphene from a Cu substrate to an insulating (SiO2) substrate on the physical and electrical of the transferred graphene are studied X-ray photoelectron spectroscopy and atomic force microscopy show that this residue can be substantially reduced by annealing in vacuum The impact of the removal of poly(methyl methacrylate) residue on the electrical properties of graphene field effect devices is demonstrated, including a nearly 2 × increase in average mobility from 1400 to 2700 cm2/Vs The electrical results are compared with graphene doping measurements by Raman spectroscopy

Thermal Properties of Isotopically Engineered Graphene

Abstract: was shown to be substantially different in two-dimensional (2D) crystals, such as graphene, than in three-dimensional (3D) graphite [7-10]. Here, we report the first experimental study of the isotope effects on the thermal properties of graphene. Isotopically modified graphene containing various percentages of

High-performance supercapacitors based on poly(ionic liquid)-modified graphene electrodes.

Abstract: We report a high-performance supercapacitor incorporating a poly(ionic liquid)-modified reduced graphene oxide (PIL:RG-O) electrode and an ionic liquid (IL) electrolyte (specifically, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide or EMIM-NTf2). PIL:RG-O provides enhanced compatibility with the IL electrolyte, thereby increasing the effective electrode surface area accessible to electrolyte ions. The supercapacitor assembled with PIL:RG-O electrode and EMIM-NTf2 electrolyte showed a stable electrochemical response up to 3.5 V operating voltage and was capable of yielding a maximum energy density of 6.5 W·h/kg with a power density of 2.4 kW/kg. These results demonstrate the potential of the PIL:RG-O material as an electrode in high-performance supercapacitors.

Raman Measurements of Thermal Transport in Suspended Monolayer Graphene of Variable Sizes in Vacuum and Gaseous Environments

Abstract: Using micro-Raman spectroscopy, the thermal conductivity of a graphene monolayer grown by chemical vapor deposition and suspended over holes with different diameters ranging from 2.9 to 9.7 μm was measured in vacuum, thereby eliminating errors caused by heat loss to the surrounding gas. The obtained thermal conductivity values of the suspended graphene range from (2.6 ± 0.9) to (3.1 ± 1.0) × 103 Wm−1K−1 near 350 K without showing the sample size dependence predicted for suspended, clean, and flat graphene crystal. The lack of sample size dependence is attributed to the relatively large measurement uncertainty as well as grain boundaries, wrinkles, defects, or polymeric residue that are possibly present in the measured samples. Moreover, from Raman measurements performed in air and CO2 gas environments near atmospheric pressure, the heat transfer coefficient for air and CO2 was determined and found to be (2.9 +5.1/−2.9) and (1.5 +4.2/−1.5) × 104 Wm−2K−1, respectively, when the graphene temperature was heat...

Simultaneous Reduction and Surface Functionalization of Graphene Oxide by Mussel‐Inspired Chemistry

Abstract: This study presents a method of simultaneous reduction and surface functionalization of graphene oxide by a one-step poly(norepinephrine) functionalization. The pH-induced aqueous functionalization of graphene oxide by poly(norepinephrine), a catecholamine polymer inspired by the robust adhesion of marine mussels, chemically reduced and functionalized graphene oxide. Moreover, the polymerized norepinephrine (pNor) layer provided multifunctionality on the reduced graphene oxide that includes surface-initiated polymerization and spontaneous metallic nanoparticle formation. This facile surface modifi cation strategy can be a useful platform for graphene-based nano-composites.

Reduction of graphite oxide using alcohols

Abstract: A method for reducing graphite oxide using a variety of commercially available alcohols is described. The carbon products were found to exhibit high C : O ratios (up to 30 : 1, as determined by elemental combustion analysis), high conductivities (up to 4600 S m−1), and good specific capacitances (up to 35 F g−1) when tested as electrode materials in ultracapacitors.

Toward Practical Gas Sensing with Highly Reduced Graphene Oxide: A New Signal Processing Method To Circumvent Run-to-Run and Device-to-Device Variations

Abstract: Graphene is worth evaluating for chemical sensing and biosensing due to its outstanding physical and chemical properties. We first report on the fabrication and characterization of gas sensors using a back-gated field-effect transistor platform with chemically reduced graphene oxide (R-GO) as the conducting channel. These sensors exhibited a 360% increase in response when exposed to 100 ppm NO2 in air, compared with thermally reduced graphene oxide sensors we reported earlier. We then present a new method of signal processing/data interpretation that addresses (i) sensing devices with long recovery periods (such as required for sensing gases with these R-GO sensors) as well as (ii) device-to-device variations. A theoretical analysis is used to illuminate the importance of using the new signal processing method when the sensing device suffers from slow recovery and non-negligible contact resistance. We suggest that the work reported here (including the sensor signal processing method and the inherent simpl...

Thermal transport across twin grain boundaries in polycrystalline graphene from nonequilibrium molecular dynamics simulations.

Abstract: We have studied the thermal conductance of tilt grain boundaries in graphene using nonequilibrium molecular dynamics simulations. When a constant heat flux is allowed to flow, we observe sharp jumps in temperature at the boundaries, characteristic of interfaces between materials of differing thermal properties. On the basis of the magnitude of these jumps, we have computed the boundary conductance of twin grain boundaries as a function of their misorientation angles. We find the boundary conductance to be in the range 1.5 × 1010 to 4.5 × 1010 W/(m2 K), which is significantly higher than that of any other thermoelectric interfaces reported in the literature. Using the computed values of boundary conductances, we have identified a critical grain size of 0.1 μm below which the contribution of the tilt boundaries to the conductivity becomes comparable to that of the contribution from the grains themselves. Experiments to test the predictions of our simulations are proposed.

Synthesis and Characterization of Large-Area Graphene and Graphite Films on Commercial Cu–Ni Alloy Foils

Abstract: Controlling the thickness and uniformity during growth of multilayer graphene is an important goal. Here we report the synthesis of large-area monolayer and multilayer, particularly bilayer, graphene films on Cu–Ni alloy foils by chemical vapor deposition with methane and hydrogen gas as precursors. The dependence of the initial stages of graphene growth rate on the substrate grain orientation was observed for the first time by electron backscattered diffraction and scanning electron microscopy. The thickness and quality of the graphene and graphite films obtained on such Cu–Ni alloy foils could be controlled by varying the deposition temperature and cooling rate and were studied by optical microscopy, scanning electron microscopy, atomic force microscopy, and micro-Raman imaging spectroscopy. The optical and electrical properties of the graphene and graphite films were studied as a function of thickness.

CoMn2O4 Spinel Nanoparticles Grown on Graphene as Bifunctional Catalyst for Lithium-Air Batteries

Abstract: Positive electrodes for the oxygen reduction reaction (ORR) and the oxygen-evolution reaction (OER) play a critical role in fuel cells and metal-air batteries. Tetragonal CoMn 2 O 4 spinel nanoparticles have been grown on the surface of graphene sheets (CMOG) via a two-step synthesis. The ORR/OER catalytic characteristics of CMOG were studied with a rotating-disk electrode. Also a lithium-air primary cell having a non-aqueous electrolyte and a rechargeable lithium-air cell with a Li-ion solid electrolyte separating a non-aqueous anode electrolyte from an alkaline aqueous cathode electrolyte were assembled with a CMOG cathode and tested. The results indicate that a CMOG cathode can provide a catalytic platform of considerable activity for the ORR in both electrolytes and also for the OER in the aqueous electrolyte.

Interfacial capacitance of single layer graphene

Abstract: The interfacial capacitance of large area, single layer graphene was directly measured with electrolyte accessing both sides of the graphene sheet. PMMA and photoresist patterns were used as supports to suspend the CVD grown graphene in electrolyte during electrochemical testing. Both one and two sides of single layer graphene films were measured and compared. The results show that the area normalized charge that can be stored simultaneously on both sides is significantly lower than could be stored on just one side of single layer graphene, consistent with charge storage having a quantum capacitance component. These measurements are also consistent with the specific capacitance of graphene materials as previously measured in supercapacitor cells and provide a basis for the further understanding and development of graphene based materials for electrical energy storage.

Domain (grain) boundaries and evidence of "twinlike" structures in chemically vapor deposited grown graphene.

Abstract: Understanding and engineering the domain boundaries in chemically vapor deposited monolayer graphene will be critical for improving its properties. In this study, a combination of transmission electron microscopy (TEM) techniques including selected area electron diffraction, high resolution transmission electron microscopy (HR-TEM), and dark field (DF) TEM was used to study the boundary orientation angle distribution and the nature of the carbon bonds at the domain boundaries. This report provides an important first step toward a fundamental understanding of these domain boundaries. The results show that, for the graphene grown in this study, the 46 measured misorientation angles are all between 11° and 30° (with the exception of one at 7°). HR-TEM images show the presence of adsorbates in almost all of the boundary areas. When a boundary was imaged, defects were seen (dangling bonds) at the boundaries that likely contribute to adsorbates binding at these boundaries. DF-TEM images also showed the presence...

Effective mobility of single-layer graphene transistors as a function of channel dimensions

Abstract: A detailed analysis of the extracted back gated FET mobility as a function of channel length, channel width, and underlying oxide thickness for both exfoliated and chemical vapor deposited (CVD) graphene is presented. The mobility increases with increasing channel length eventually saturating at a constant value for channel lengths of several micrometers. The length dependence is consistent with the transition from a ballistic to diffusive transport regime. The mobility as a function of channel width first increases and then decreases. The increase in mobility for very small channel widths is consistent with a reduction in edge scattering. The decrease in mobility for larger channel widths is observed to be strongly dependent on the oxide thickness suggesting that electrostatics associated with fringing fields is an important effect. This effect is further confirmed by a comparative analysis of the measured mobility of graphene devices with similar channel dimensions on oxides of different thicknesses. The observed electrical measurements are in excellent agreement with theoretical studies predicting the width dependence of conductivity and mobility. The mobility of CVD grown graphene is slightly lower than that of exfoliated graphene but shows similar trends with length and width. The mobility values reported in the literature are in agreement with the trend reported here.

Growth of carbon nanowalls at atmospheric pressure for one-step gas sensor fabrication

Abstract: Carbon nanowalls (CNWs), two-dimensional "graphitic" platelets that are typically oriented vertically on a substrate, can exhibit similar properties as graphene. Growth of CNWs reported to date was exclusively carried out at a low pressure. Here, we report on the synthesis of CNWs at atmosphere pressure using "direct current plasma-enhanced chemical vapor deposition" by taking advantage of the high electric field generated in a pin-plate dc glow discharge. CNWs were grown on silicon, stainless steel, and copper substrates without deliberate introduction of catalysts. The as-grown CNW material was mainly mono- and few-layer graphene having patches of O-containing functional groups. However, Raman and X-ray photoelectron spectroscopies confirmed that most of the oxygen groups could be removed by thermal annealing. A gas-sensing device based on such CNWs was fabricated on metal electrodes through direct growth. The sensor responded to relatively low concentrations of NO2 (g) and NH3 (g), thus suggesting high-quality CNWs that are useful for room temperature gas sensors.

Large-Area Graphene Films by Simple Solution Casting of Edge-Selectively Functionalized Graphite

Abstract: We report edge-selective functionalization of graphite (EFG) for the production of large-areauniformgraphene filmsbysimplysolution-castingEFGdispersionsindichloromethaneon silicon oxide substrates, followed by annealing. The resultant graphene films show ambipolar transport properties with sheet resistances of 0.523.11 kΩ/sq at 6390% optical transmittance. EFG allows solution processing methods for the scalable production of electrically conductive, optically transparent, and mechanically robust flexible graphene films for use in practice.

Microwave-Exfoliated Graphite Oxide/Polycarbonate Composites

Abstract: We present the first report of polymer composites using microwave-exfoliated graphite oxide (MEGO) as filler, a high surface area carbon material that resembles graphene on a local scale. MEGO has a “wormlike” layered structure which can be sheared apart during melt mixing with a polymer host. In this study, we produced MEGO/polycarbonate (PC) composites at various loadings and evaluated their morphology and properties. Transmission electron microscopy and X-ray scattering studies suggested an exfoliated morphology, with wrinkled platelets of approximately 4–5 nm thickness evenly dispersed throughout the PC matrix. Frequency scans of composite melts using shear rheology showed an onset of frequency-independent terminal behavior around 2.1 wt %, suggesting an effective aspect ratio of nearly 50 for the dispersed platelets, in agreement with TEM analysis. The composites showed significant increases in electrical conductivity, with an onset of electrical percolation around 1.3 wt %, but only exhibited modest...

Graphene Growth Using a Solid Carbon Feedstock and Hydrogen

Abstract: Graphene has been grown on Cu at elevated temperatures with different carbon sources (gaseous hydrocarbons and solids such as polymers); however the detailed chemistry occurring at the Cu surface is not yet known. Here, we explored the possibility of obtaining graphene using amorphous-carbon thin films, without and with hydrogen gas added. Graphene is formed only in the presence of H2(g), which strongly suggests that gaseous hydrocarbons and/or their intermediates are what yield graphene on Cu through the reaction of H2(g) and the amorphous carbon. The large area, uniform monolayer graphene obtained had electron and hole mobilities of 2520 and 2050 cm2 V–1 s–1, respectively.

One-Pot Synthesis of Platinum Nanoparticles Embedded on Reduced Graphene Oxide for Oxygen Reduction in Methanol Fuel Cells

Abstract: A simple approach has been developed for the synthesis of Pt nanoparticles with uniform diameters of approximately 2.9 nm supported on reduced graphene oxide (RG-O) platelets via a modified polyol method. Compared to Johnson Matthey (JM) Pt/C (75 wt % Pt) catalyst, the Pt/RG-O (70 wt % Pt) composite showed much higher electrochemical surface area, greater catalytic activity towards the oxygen reduction reaction (ORR), and significantly better single cell polarization performance. The maximum power density of the Pt/RG-O composite was about 128 mW cm−2, an 11% greater than the JM Pt/C commercial catalyst.