1,721,073 research outputs found
Simultaneous Electrochemical Reduction and Delamination of Graphene Oxide Films
No full textHere we report an electrochemical method to simultaneously reduce and delaminate graphene oxide (G-0) thin films deposited on metal (Al and Au) substrates. During the electrochemical reaction, interface charge transfer between the G-0 thin film and the electrode surface was found to be important in eliminating oxygen-containing groups, yielding highly reduced graphene oxide (rG-0). In the meantime, hydrogen bubbles were electrochemically generated at the rG-0 film/electrode interface, propagating the film delamination. Unlike other metal-based G-0 reduction methods, the metal used here was either not etched at all (for Au) or etched a small amount (for Al), thus making it possible to reuse the substrate and lower production costs. The delaminated rG-0 film exhibits a thickness-dependent degree of reduction: greater reduction is achieved in thinner films. The thin rG-0 films having an optical transmittance of 90% (lambda=550 nm) had a sheet resistance of 6390 +/- 447 Omega/square (ohms per square). rG-0-based stretchable transparent conducting films were also demonstratedclose
Continuous Carbon Nanotube-Ultrathin Graphite Hybrid Foams for Increased Thermal Conductivity and Suppressed Subcooling in Composite Phase Change Materials
No full textContinuous ultrathin graphite foams (UGFs) have been actively researched recently to obtain composite materials with increased thermal conductivities. However, the large pore size of these graphitic foams has resulted in large thermal resistance values for heat conduction from inside the pore to the high thermal conductivity graphitic struts. Here, we demonstrate that the effective thermal conductivity of these UGF composites can be increased further by growing long CNT networks directly from the graphite struts of UGFs into the pore space. When erythritol, a phase change material for thermal energy storage, is used to fill the pores of UGF-CNT hybrids, the thermal conductivity of the UGF-CNT/erythritol composite was found to increase by as much as a factor of 1.8 compared to that of a UGF/erythritol composite, whereas breaking the UGF-CNT bonding in the hybrid composite resulted in a drop in the effective room-temperature thermal conductivity from about 4.1 ?? 0.3 W m-1 K-1 to about 2.9 ?? 0.2 W m-1 K-1 for the same UGF and CNT loadings of about 1.8 and 0.8 wt %, respectively. Moreover, we discovered that the hybrid structure strongly suppresses subcooling of erythritol due to the heterogeneous nucleation of erythritol at interfaces with the graphitic structures.close
Rapid Identification of the Layer Number of Large-Area Graphene on Copper
No full textChemical
vapor deposition (CVD) on Cu foils emerged as an important
method for preparing high-quality and large-area graphene films for
practical applications. However, to date it remains challenging to
rapidly identify the structural features, especially the layer numbers,
of CVD-graphene directly on Cu substrate. Herein, we report an O2-plasma-assisted approach for identifying the coverage, wrinkles,
domain size, and layer number of large-area graphene films on Cu foils
by optical microscopy. The wrinkles and grain boundaries of five-layer
graphene can be observed with a grayscale increment of ∼23.4%
per one graphene layer after O2-plasma treatment for only
15 s, which allows for checking graphene on Cu foils with a sample
size of 17 cm × 20 cm in a few minutes. The Raman spectroscopy
and X-ray photoelectron spectroscopy presents a strong layer number
dependence of both the plasma induced graphene defects and Cu oxides,
which, as indicated by molecular dynamic simulation, is responsible
for the improved image contrast as a result of the interaction between
O-ions and graphene with different layer numbers. We expect that this
O2-plasma-assisted method would be applied to meter-scale
samples if atmospheric-pressure plasma is used and therefore will
be beneficial for the fast evaluation of CVD-graphene in both laboratory
and industry
Selective mechanical transfer of graphene from seed copper foil using rate effects
No full textA very fast, dry transfer process based on mechanical delamination successfully effected the transfer of large-area, CVD grown graphene on copper foil to silicon. This has been achieved by bonding silicon backing layers to both sides of the graphene-coated copper foil with epoxy and applying a suitably high separation rate to the backing layers. At the highest separation rate considered (254.0 ??m/s), monolayer graphene was completely transferred from the copper foil to the target silicon substrate. On the other hand, the lowest rate (25.4 ??m/s) caused the epoxy to be completely separated from the graphene. Fracture mechanics analyses were used to determine the adhesion energy between graphene and its seed copper foil (6.0 J/m2) and between graphene and the epoxy (3.4 J/m2) at the respective loading rates. Control experiments for the epoxy/silicon interface established a rate dependent adhesion, which supports the hypothesis that the adhesion of the graphene/epoxy interface was higher than that of the graphene/copper interface at the higher separation rate, thereby providing a controllable mechanism for selective transfer of graphene in future nanofabrication systems such as roll-to-roll transferclose5
Poly(vinyl alcohol) Reinforced and Toughened with Poly(dopamine)-Treated Graphene Oxide, and Its Use for Humidity Sensing
No full textPoly(dopamine)-treated graphene oxide/poly(vinyl alcohol) ("dG-O/PVA") composite films were made and characterized. G-O was modified with poly(dopamine) in aqueous solution and then chemically reduced to yield poly(dopamine)-treated reduced G-O. A combination of hydrogen bonding, strong adhesion of poly(dopamine) at the interface of PVA and G-O sheets, and reinforcement by G-O resulted in increases in tensile modulus, ultimate tensile strength, and strain-to-failure by 39, 100, and 89%, respectively, at 0.5 wt % dG-O loading of the PVA. The dG-O serves as a moisture barrier for water-soluble PVA, and the dG-O/PVA composite films were shown to be effective humidity sensors over the relative humidity range 40-100%.close2
Oxygen-Promoted Chemical Vapor Deposition of Graphene on Copper: A Combined Modeling and Experimental Study
No full textMass production of large, high-quality single-crystalline graphene is dependent on a complex coupling of factors including substrate material, temperature, pressure, gas flow, and the concentration of carbon and hydrogen species. Recent studies have shown that the oxidation of the substrate surface such as Cu before the introduction of the C precursor, methane, results in a significant increase in the growth rate of graphene while the number of nuclei on the surface of the Cu substrate decreases. We report on a phase-field model, where we include the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu. Our calculations reproduce the experimental observations, thus validating the proposed model. Finally, and more importantly, we present growth rate from our model as a function of O concentration and precursor flux to guide the efficient growth of large single-crystal graphene of high qualit
Ultrafast-Charging Silicon-Based Coral-Like Network Anodes for Lithium-Ion Batteries with High Energy and Power Densities
No full textFast charging rate and large energy storage are becoming key elements for the development of next-generation batteries, targeting high-performance electric vehicles. Developing electrodes with high volumetric and gravimetric capacity that could be operated at a high rate is the most challenging part of this process. Using silicon as the anode material, which exhibits the highest theoretical capacity as a lithium-ion battery anode, we report a binder-free electrode that interconnects carbon-sheathed porous silicon nanowires into a coral-like network and shows fast charging performance coupled to high energy and power densities when integrated into a full cell with a high areal capacity loading. The combination of interconnected nanowires, porous structure, and a highly conformal carbon coating in a single system strongly promotes the reaction kinetics of the electrode. This leads to fast-charging capability while maintaining the integrity of the electrode without structural collapse and, thus, stable cycling performance without using binder and conductive additives. Specifically, this anode shows high specific capacities (over 1200 mAh g(-1)) at an ultrahigh charging rate of 7 C over 500 charge-discharge cycles. When coupled with a commercial LiCoO2 or LiFePO4 cathode in a full cell, it delivers a volumetric energy density of 1621 Wh L-1 with a LiCoO2 cathode and a power density of 7762 W L-1 with a LiFePO4 cathode.11Nsciescopu
Using Single-Crystal Graphene to Form Arrays of Nanocapsules Enabling the Observation of Light Elements in Liquid Cell Transmission Electron Microscopy
No full textWe have designed and fabricated a TEM (transmission electron
microscopy)
liquid cell with hundreds of graphene nanocapsules arranged in a stack
of two Si3N4–x membranes.
These graphene nanocapsules are formed on arrays of nanoholes patterned
on the Si3N4–x membrane
by focused ion beam milling, allowing for better resolution than for
the conventional graphene liquid cells, which enables the observation
of light elements, such as atomic structures of silicon. We suggest
that multiple nanocapsules provide opportunities for consecutive imaging
under the same conditions in a single liquid cell. The use of single-crystal
graphene windows offers an excellent signal-to-noise ratio and high
spatial resolution. The motion of silicon nanoparticles (a low atomic
number (Z) material) interacting with nanobubbles was observed, and
analyzed, in detail. Our approach will help advance liquid-phase TEM
observations by providing a straightforward method to encapsulate
liquid between monolayers of various 2-dimensional materials
Do-It-Yourself Transfer of Large-Area Graphene Using an Office Laminator and Water
Full text linkWe demonstrate a simple method for transferring large areas (up to A4-size sheets) of CVD graphene from copper foils onto a target substrate using a commercially available polyvinyl alcohol polymer foil as a carrier substrate and commercial hot-roll office laminator. Through the use of terahertz time-domain spectroscopy and Raman spectroscopy, large-area quantitative optical contrast mapping, and the fabrication and electrical characterization of ∼50 individual centimeter-scale van der Pauw field effect devices, we show a nondestructive technique to transfer large-area graphene with low residual doping that is scalable, economical, reproducible, and easy to use and that results in less doping and transfer-induced damage than etching or electrochemical delamination transfers. We show that the copper substrate can be used multiple times with minimal loss of material and no observable reduction in graphene quality. We have additionally demonstrated the transfer of multilayer hexagonal boron nitride from copper and iron foils. Finally, we note that this approach allows graphene to be supplied on stand-alone polymer supports by CVD graphene manufacturers to end users, with the only equipment and consumables required to transfer graphene onto target substrates being a commercial office laminator and water.</p
Freeze-Casting Produces a Graphene Oxide Aerogel with a Radial and Centrosymmetric Structure
No full textWe report the assembly of graphene oxide (G-O) building blocks into a vertical and radially aligned structure by a bidirectional freeze-casting approach. The crystallization of water to ice assembles the G-O sheets into a structure, a GO aerogel whose local structure mimics turbine blades. The centimeter-scale radiating structure in this aerogel has many channels whose width increases with distance from the center. This was achieved by controlling the formation of the ice crystals in the aqueous G-O dispersion that grew radially in the shape of lamellae during freezing. Because the shape and size of ice crystals is influenced by the G-O sheets, different additives (ethanol, cellulose nanofibers, and chitosan) that can form hydrogen bonds with H2O were tested and found to affect the interaction between the G-O and formation of ice crystals, producing ice crystals with different shapes. A G-O/chitosan aerogel with a spiral pattern was also obtained. After chemical reduction of G-O, our aerogel exhibited elasticity and absorption capacity superior to that of graphene aerogels with "traditional" pore structures made by conventional freeze-casting. This methodology can be expanded to many other configurations and should widen the use of G-O (and reduced G-O and "graphenic") aerogels
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