Showing papers by "Rodney S. Ruoff published in 2018"
TL;DR: The as-prepared core-shell nanostructures enhance the exposed surface area of birnessite and achieve high electrochemical performances and excellent rate capability over a potential window of up to 1.2 V.
Abstract: Two-dimensional birnessite has attracted attention for electrochemical energy storage because of the presence of redox active Mn4+/Mn3+ ions and spacious interlayer channels available for ions diffusion. However, current strategies are largely limited to enhancing the electrical conductivity of birnessite. One key limitation affecting the electrochemical properties of birnessite is the poor utilization of the MnO6 unit. Here, we assemble β-MnO2/birnessite core–shell structure that exploits the exposed crystal face of β-MnO2 as the core and ultrathin birnessite sheets that have the structure advantage to enhance the utilization efficiency of the Mn from the bulk. Our birnessite that has sheets parallel to each other is found to have unusual crystal structure with interlayer spacing, Mn(III)/Mn(IV) ratio and the content of the balancing cations differing from that of the common birnessite. The substrate directed growth mechanism is carefully investigated. The as-prepared core–shell nanostructures enhance th...
408 citations
TL;DR: The structure-performance relationship of these materials is subsequently rationalized and perspectives on developing more efficient and sustainable electrocatalysts from carbon nanomaterials are also suggested.
Abstract: N-doped carbon nanomaterials have rapidly grown as the most important metal-free catalysts in a wide range of chemical and electrochemical reactions. This current report summarizes the latest advances in N-doped carbon electrocatalysts prepared by N mono-doping and co-doping with other heteroatoms. The structure-performance relationship of these materials is subsequently rationalized and perspectives on developing more efficient and sustainable electrocatalysts from carbon nanomaterials are also suggested.
297 citations
TL;DR: After chemical reduction of G-O, the aerogel exhibited elasticity and absorption capacity superior to that of graphene aerogels with "traditional" pore structures made by conventional freeze-casting, and can be expanded to many other configurations.
Abstract: We 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 G-O 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 el...
247 citations
Journal Article•
245 citations
TL;DR: An amino functionalized boron nitride (FBN) based filtration membrane with a nanochannel network for molecular separation and permeation and shows high transport performance for solvents depending on the membranes’ thickness is reported.
Abstract: Pressure-driven, superfast organic solvent filtration membranes have significant practical applications. An excellent filtration membrane should exhibit high selectivity and permeation in aqueous and organic solvents to meet increasing industrial demand. Here, we report an amino functionalized boron nitride (FBN) based filtration membrane with a nanochannel network for molecular separation and permeation. This membrane is highly stable in water and in several organic solvents and shows high transport performance for solvents depending on the membranes' thickness. In addition, the FBN membrane is applicable for solute screening in water as well as in organic solvents. More importantly, the FBN membranes are very stable in acidic, alkaline and oxidative media for up to one month. The fast-flow rate and good separation performance of the FBN membranes can be attributed to their stable networks of nanochannels and thin laminar structure, which provide the membranes with beneficial properties for practical separation and purification processes.
229 citations
TL;DR: Graphene is considered a promising material for industrial application based on the intensive laboratory-scale research in the fields of physics, chemistry, materials science and engineering, and biology over the last decade as discussed by the authors.
Abstract: Graphene is considered a promising material for industrial application based on the intensive laboratory-scale research in the fields of physics, chemistry, materials science and engineering, and biology over the last decade. Many companies have thus started to pursue graphene materials on a scale of tons (for the flake material) or hundreds of thousands of square meters (for the film material) for industrial applications. Though the graphene industry is still in its early stages, very significant progress in mass production and certain industrial applications has become obvious. In this report, we aim to give a brief review of the mass production of graphene materials for some industrial applications and summarize some features or challenges for graphene in the marketplace.
212 citations
TL;DR: The colossal grain growth is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by “consumption” of neighboring grains.
Abstract: Single-crystal metals have distinctive properties owing to the absence of grain boundaries and strong anisotropy. Commercial single-crystal metals are usually synthesized by bulk crystal growth or by deposition of thin films onto substrates, and they are expensive and small. We prepared extremely large single-crystal metal foils by "contact-free annealing" from commercial polycrystalline foils. The colossal grain growth (up to 32 square centimeters) is achieved by minimizing contact stresses, resulting in a preferred in-plane and out-of-plane crystal orientation, and is driven by surface energy minimization during the rotation of the crystal lattice followed by "consumption" of neighboring grains. Industrial-scale production of single-crystal metal foils is possible as a result of this discovery.
145 citations
TL;DR: Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on "homemade" single crystal Cu/Ni(111) alloy foils over large area.
Abstract: Fast-growth of single crystal monolayer graphene by CVD using methane and hydrogen has been achieved on “homemade” single crystal Cu/Ni(111) alloy foils over large area. Full coverage was achieved in 5 min or less for a particular range of composition (1.3 at.% to 8.6 at.% Ni), as compared to 60 min for a pure Cu(111) foil under identical growth conditions. These are the bulk atomic percentages of Ni, as a superstructure at the surface of these foils with stoichiometry Cu6Ni1 (for 1.3 to 7.8 bulk at.% Ni in the Cu/Ni(111) foil) was discovered by low energy electron diffraction (LEED). Complete large area monolayer graphene films are either single crystal or close to single crystal, and include folded regions that are essentially parallel and that were likely wrinkles that “fell over” to bind to the surface; these folds are separated by large, wrinkle-free regions. The folds occur due to the buildup of interfacial compressive stress (and its release) during cooling of the foils from 1075 °C to room tempera...
124 citations
TL;DR: It is shown that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors using a folded graphene composite electrode designed for a high areal capacity anode.
Abstract: We show that a high energy density can be achieved in a practical manner with freestanding electrodes without using conductive carbon, binders, and current collectors. We made and used a folded graphene composite electrode designed for a high areal capacity anode. The traditional thick graphene composite electrode, such as made by filtering graphene oxide to create a thin film and reducing it such as through chemical or thermal methods, has sluggish reaction kinetics. Instead, we have made and tested a thin composite film electrode that was folded several times using a water-assisted method; it provides a continuous electron transport path in the fold regions and introduces more channels between the folded layers, which significantly enhances the electron/ion transport kinetics. A fold electrode consisting of SnO2/graphene with high areal loading of 5 mg cm–2 has a high areal capacity of 4.15 mAh cm–2, well above commercial graphite anodes (2.50–3.50 mAh cm–2), while the thickness is maintained as low as ...
105 citations
TL;DR: In this paper, pressure-assisted thermal decomposition of graphene oxide films by hot pressing at different temperatures (i.e., 300°C, 1000°C or 2000°C) was discussed.
77 citations
TL;DR: Epitaxial graphene grown on single crystal Cu(111) foils by chemical vapor deposition is found to be free of wrinkles and under biaxial compressive strain.
Abstract: Epitaxial graphene grown on single crystal Cu(111) foils by chemical vapor deposition is found to be free of wrinkles and under biaxial compressive strain. The compressive strain in the epitaxial regions (0.25-0.40%) is higher than regions where the graphene is not epitaxial with the underlying surface (0.20-0.25%). This orientation-dependent strain relaxation is through the loss of local adhesion and the generation of graphene wrinkles. Density functional theory calculations suggest a large frictional force between the epitaxial graphene and the Cu(111) substrate, and this is therefore an energy barrier to the formation of wrinkles in the graphene. Enhanced chemical reactivity is found in epitaxial graphene on Cu(111) foils as compared to graphene on polycrystalline Cu foils for certain chemical reactions. A higher compressive strain possibly favors lowering the formation energy and/or the energy gap between the initial and transition states, either of which can lead to an increase in chemical reactivity.
TL;DR: It is proved that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps.
Abstract: Compressive strain relaxation of a chemical vapor deposition (CVD) grown graphene overlayer has been considered to be the main driving force behind metal surface step bunching (SB) in CVD graphene growth. Here, by combining theoretical studies with experimental observations, we prove that the SB can occur even in the absence of a compressive strain, is enabled by the rapid diffusion of metal adatoms beneath the graphene and is driven by the release of the bending energy of the graphene overlayer in the vicinity of steps. Based on this new understanding, we explain a number of experimental observations such as the temperature dependence of SB, and how SB depends on the thickness of the graphene film. This study also shows that SB is a general phenomenon that can occur in all substrates covered by films of two-dimensional (2D) materials.
TL;DR: This work introduces for the first time the synthesis of covalent quinazoline networks (CQNs) using an ionothermal synthesis protocol and finds them a promising CO2 adsorbent for the selective capture of CO2 from flue gas.
Abstract: The development of different classes of porous polymers by linking organic molecules using new chemistries still remains a great challenge. Herein, we introduce for the first time the synthesis of covalent quinazoline networks (CQNs) using an ionothermal synthesis protocol. Zinc chloride (ZnCl2 ) was used as the solvent and catalyst for the condensation of aromatic ortho-aminonitriles to produce tricycloquinazoline linkages. The resulting CQNs show a high porosity with a surface area up to 1870 m2 g-1 . Varying the temperature and the amount of catalyst enables us to control the surface area as well as the pore size distribution of the CQNs. Furthermore, their high nitrogen content and significant microporosity make them a promising CO2 adsorbent with a CO2 uptake capacity of 7.16 mmol g-1 (31.5 wt %) at 273 K and 1 bar. Because of their exceptional CO2 sorption properties, they are promising candidates as an adsorbent for the selective capture of CO2 from flue gas.
TL;DR: Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge.
Abstract: Raman spectra of large graphene bubbles showed size-dependent oscillations in spectral intensity and frequency, which originate from optical standing waves formed in the vicinity of the graphene surface. At a high laser power, local heating can lead to oscillations in the Raman frequency and also create a temperature gradient in the bubble. Based on Raman data, the temperature distribution within the graphene bubble was calculated, and it is shown that the heating effect of the laser is reduced when moving from the center of a bubble to its edge. By studying graphene bubbles, both the thermal conductivity and chemical reactivity of graphene were assessed. When exposed to hydrogen plasma, areas with bubbles are found to be more reactive than flat graphene.
TL;DR: The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties and the synergistic behavior between the reinforcement and folding to be quantified.
Abstract: A folding technique is reported to incorporate large-area monolayer graphene films in polymer composites for mechanical reinforcement. Compared with the classic stacking method, the folding strategy results in further stiffening, strengthening, and toughening of the composite. By using a water-air-interface-facilitated procedure, an A5-size 400 nm thin polycarbonate (PC) film is folded in half 10 times to a ≈0.4 mm thick material (1024 layers). A large PC/graphene film is also folded by the same process, resulting in a composite with graphene distributed uniformly. A three-point bending test is performed to study the mechanical performance of the composites. With a low volume fraction of graphene (0.085%), the Young's modulus, strength, and toughness modulus are enhanced in the folded composite by an average of 73.5%, 73.2%, and 59.1%, respectively, versus the pristine stacked polymer films, or 40.2%, 38.5%, and 37.3% versus the folded polymer film, proving a remarkable mechanical reinforcement from the combined folding and reinforcement of graphene. These results are rationalized with combined theoretical and computational analyses, which also allow the synergistic behavior between the reinforcement and folding to be quantified. The folding approach could be extended/applied to other 2D nanomaterials to design and make macroscale laminated composites with enhanced mechanical properties.
TL;DR: Tensile measurements are made on centimeter-scale 300 nm-thick graphene oxide film specimens, much thinner than the ≈2 μm minimum thickness of macroscale graphene-oxide films previously reported, demonstrating the mechanical performance of large-area graphene in a size scale relevant to many applications.
Abstract: Camphor is used to transfer centimeter-scale ultrathin films onto custom-designed substrates for mechanical (tensile) testing. Compared to traditional transfer methods using dissolving/peeling to remove the support-layers, camphor is sublimed away in air at low temperature, thereby avoiding additional stress on the as-transferred films. Large-area ultrathin films can be transferred onto hollow substrates without damage by this method. Tensile measurements are made on centimeter-scale 300 nm-thick graphene oxide film specimens, much thinner than the ≈2 μm minimum thickness of macroscale graphene-oxide films previously reported. Tensile tests were also done on two different types of large-area samples of adlayer free CVD-grown single-layer graphene supported by a ≈100 nm thick polycarbonate film; graphene stiffens this sample significantly, thus the intrinsic mechanical response of the graphene can be extracted. This is the first tensile measurement of centimeter-scale monolayer graphene films. The Young's modulus of polycrystalline graphene ranges from 637 to 793 GPa, while for near single-crystal graphene, it ranges from 728 to 908 GPa (folds parallel to the tensile loading direction) and from 683 to 775 GPa (folds orthogonal to the tensile loading direction), demonstrating the mechanical performance of large-area graphene in a size scale relevant to many applications.
TL;DR: A phase-field model is reported on, where the effects of oxygen on the number of nuclei, the energetics at the growth front, and the graphene island morphology on Cu are included, to guide the efficient growth of large single-crystal graphene of high quality.
Abstract: Mass 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 quality
TL;DR: In this article, 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 is presented.
Abstract: Chemical 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...
TL;DR: In this article, aqueous G-O dispersions in the concentration range of 0.004 < ϕ < 3.5 wt% have been studied, and the authors found that the rheology of G-oxide dispersions strongly depends on the g-O concentration ϕ.
Abstract: Even though the rheological behavior of aqueous graphene oxide (G-O) dispersions has been shown to be strongly time-dependent, only few transient measurements have been reported in the literature. In this work, we attempt to fill the gap between transient and steady shear rheological characterizations of aqueous G-O dispersions in the concentration range of 0.004 < ϕ < 3.5 wt%, by conducting comprehensive rheological measurements, including oscillatory shear flow, transient shear flow, and steady shear flow. Steady shear measurements have been performed after the evaluation of transient properties of the G-O dispersions, to assure steady-state conditions. We identify the critical concentration ϕ
c
= 0.08 wt% (where G-O sheets start to interact) from oscillatory shear experiments. We find that the rheology of G-O dispersions strongly depends on the G-O concentration ϕ. Transient measurements of shear viscosity and first normal stress difference suggest that G-O dispersions behave like nematic polymeric liquid crystals at ϕ/ϕ
c
= 25, in agreement with other work reported in the literature. G-O dispersions also display a transition from negative to positive values of the first normal stress difference with increasing shear rates. Experimental findings of aqueous graphene oxide dispersions are compared and discussed with models and experiments reported for nematic polymeric liquid crystals, laponite, and organoclay dispersions.
TL;DR: A facile H2O-assisted CVD process to grow bilayer graphene on Cu foils, where graphene growth is controlled by injecting intermittent pulses of H 2O vapor using a pulse valve is reported in this article.
Abstract: Bernal-stacked bilayer graphene is uniquely suited for application in electronic and photonic devices because of its tunable band structure. Even though chemical vapor deposition (CVD) is considered to be the method of choice to grow bilayer graphene, the direct synthesis of high-quality, large-area Bernal-stacked bilayer graphene on Cu foils is complicated by overcoming the self-limiting nature of graphene growth on Cu. Here, we report a facile H2O-assisted CVD process to grow bilayer graphene on Cu foils, where graphene growth is controlled by injecting intermittent pulses of H2O vapor using a pulse valve. By optimizing CVD process parameters fully covered large area graphene with bilayer coverage of 77 ± 3.6% and high AB stacking ratio of 93 ± 3% can be directly obtained on Cu foils, which presents a hole concentration and mobility of 4.5 × 1012 cm–2 and 1100 cm2 V–1 s–1, respectively, at room temperature. The H2O selectively etches graphene edges without damaging graphene facets, which slows down the ...
TL;DR: Long bundles of defect-free carbon nanotubes can exhibit exceptionally high strength.
Abstract: Long bundles of defect-free carbon nanotubes can exhibit exceptionally high strength.
06 Sep 2018
TL;DR: In this article, annealing in ultrahigh-vacuum (UHV)-annealed chemical-vapor-deposition-grown graphene samples is used to reduce surface contamination.
Abstract: The chemical interaction between Ti and graphene is of significant interest for engineering low-resistance electrical contacts To study the interface chemistry, sequential depositions of Ti are performed on both as-received and ultrahigh-vacuum (UHV)-annealed chemical-vapor-deposition-grown graphene samples In situ X-ray photoelectron spectroscopy (XPS) reveals no experimental evidence for the reaction of Ti with graphene at room temperature or after heating to 500 °C The presence of the TiC chemical state is instead attributed to reactions between Ti and background gases in the UHV chamber as well as adventitious C on the surface of the graphene sample We find that surface contamination can be substantially reduced by annealing in UHV The deposition of Ti on graphene results in n-type doping, which manifests in core-level shifts and broadening of the graphene C 1s peak Annealing the sample following the deposition of Ti reverses the n-type doping The Raman spectroscopy results are in agreement wit