Plasma-induced photoresponse in few-layer graphene

A device consisting of a few layers of graphene (FLG) sheets was exposed to atmospheric plasma, resulting in the generation of significant number of defects, oxygen absorption, and doping. The plasma-induced electrical transformation and photoconducting properties of pristine FLG and plasma-irradiated FLG (p-FLG) were compared under visible and ultraviolet (UV) light illumination. The visible light photoresponsivity of p-FLG was 0.47 AW⁻¹ at 535 nm, comparatively higher than that observed for pristine FLG (10 m AW⁻¹); this result was attributed to the formation of defect midgap states band by plasma irradiation. Photoinduced molecular desorption causes the responsivity of the higher energy (UV) photons. Our results suggest that plasma irradiation is a simple, novel way to tailor the optoelectronic properties of graphene layers.     

Effects of processing and material parameters on synthesis of monolayer ultralarge graphene oxide sheets

Chemically-derived ultralarge graphene oxide (UL-GO) sheets are synthesized from natural graphite (NG) flakes based on the modified Hummers method. Three different approaches are adopted and the effects of ultrasonication, thermal shock expansion, degree of oxidation and precursor NG flake size are specifically studied on the quality and size of GO sheets produced. Results show that the use of large-size NG flakes as precursors does not necessarily produce large GO sheets. Optimal processing conditions are identified to be thermal shock exfoliation with the addition of moderate oxidation, i.e. with an expanded graphite to KMnO₄ weight ratio = 1:7 for 24 h, and avoiding ultrasonication during the oxidation process. The resulting UL-GO sheets have a maximum area over 10,000 μm² with a mean area 3400 μm² at a yield of 39.8% for GO sheets larger than 2500 μm², which are considered quite sufficient as precursors for many multifunctional applications, including transparent conductive films, optoelectronic devices and aligned graphene composites.     

One-step metal electroplating and patterning on a plastic substrate using an electrically-conductive layer of few-layer graphene

Few-layer graphene (FLG) was investigated as an electrically-conductive interleaf layer for one-step electroplating and patterning of metal on nonconductive polymer substrates without using multiple and toxic pretreatment processes in traditional electroplating. An individual FLG (5–10 nm of thickness with 6.4% of oxygen content) was obtained by expanding graphite with microwave followed by exfoliating the expanded graphite with sonication in N-methyl-pyrrolidone. Stacking FLG in the in-plane direction, a robust FLG film was obtained by the vacuum-assisted filtering and drying methods, and transferred to a polyethylene terephthalate (PET) substrate via an intermediate transfer to the water surface. The sheet resistance of the FLG film on the PET substrate was 0.9 kΩ/sq with a thickness of 80 nm and the root-mean-square roughness of 29 nm. In the electroplating of nickel on the FLG film, hemisphere-shape metal seeds appeared in the early stage of electroplating and they subsequently grew up to 200–480 nm, which became connected to form a continuous nickel layer. The thickness of the continuous nickel layer increased linearly with electroplating time. The developed electroplating method demonstrated its capability of selective patterning on nonconductive substrates using a simple masking technique.     

Kitchen blender for producing high-quality few-layer graphene

The application of a kitchen blender for producing high-quality few-layer graphene (FLG) is demonstrated. The obtained FLG flakes, with an average thickness of ∼1.5 nm (∼20% ⩽ 1 nm), are high-quality and free of oxidation and basal-plane defects. With a rotating impeller, the kitchen blender can induce multiple fluid dynamics events which are featured by shear, turbulence, and collisions. These fluid dynamics events and their cooperative effects are responsible for the exfoliation mechanism, resulting in a gentle lateral-force-dominated way for graphite self-exfoliation through its lateral self-lubricating ability.     

Graphite foam from pitch and expandable graphite

Graphite foams were prepared from a coal tar pitch that was partially converted into mesophase. Expandable graphite was used instead of an inert gas to “foam” the pitch. The resulting foam was subjected to a series of heat treatments with the objective of first crosslinking the pitch, and thereafter carbonizing and graphitizing the resulting foam. XRD confirmed that the graphitization at 2600 °C resulted in a highly graphitic material. The porosity of this foam derives from the loose packing of the vermicular exfoliated graphite particles together with their internal porosity. During the foaming process the pitch tends to coat the outside surface of the expanding graphite flakes. It also bonds them together. The graphite foam prepared with 5 wt.% expandable graphite had a bulk density of 0.249 g cm⁻³, a compressive strength of 0.46 MPa and a thermal conductivity of 21 W m⁻¹ K⁻¹. The specific thermal conductivity (thermal conductivity divided by the bulk density) of this low-density carbon foam was 0.084 W m² kg⁻¹ K⁻¹ which is considerably higher than that of copper metal (0.045 W m² kg⁻¹ K⁻¹) traditionally used in thermal management applications.     

High-capacity graphene oxide/graphite/carbon nanotube composites for use in Li-ion battery anodes

A composite of graphene oxide sheets, carbon nanotubes (CNTs), and commercial graphite particles was prepared. The composite’s use as a high-capacity and binder-free anode material for Li-ion batteries was examined. Results showed that this novel composite had a very high reversible Li-storage capacity of 1172.5 mA h g⁻¹ at 0.5C (1C = 372 mA g⁻¹), which is thrice that of commercial graphite anode. The composite also exceeded the theoretical sum of capacities of the three ingredients. More importantly, its reversible capacity below 0.25 V can reach up to 600 mA h g⁻¹. In summary, the graphene oxide/graphite/CNT composite had higher reversible capacity, better cycling performance, and similar rate capability compared with the graphene oxide/graphite composite.     

Comparison of the homemade and commercial graphene in heightening mechanical properties of Al2O3 ceramic

Graphene has been successfully fabricated by a novel method, using graphite powder and NMP (N-Methyl Pyrrolidone) as the raw materials based on the principles of liquidoid exfoliation and mechanical milling. SEM, TEM and Raman spectrum were utilized to characterize the morphology of the homemade graphene, illustrating the few defects and rare layers were endowed in this study. Afterwards, the homemade and commercial graphene were doped into Al₂O₃ powder with the mass ratio of 0%, 1%, 2%, and 3% to reinforce the mechanical properties of the matrix. The composites were processed at 1600 °C, pressure of 30 MPa and soaking time of 1 h by vacuum hot pressing. The test results illustrated the bending strength and fracture toughness tended to be intensive at first and subdued afterwards, achieving the optimal performance of 625.4±18.2 MPa and 6.07±0.22 MPa m^1/2 at 2 wt% prepared graphene additive, and the commercial grapheme owned the best heighten effect in 3 wt% graphene/Al₂O₃ composites. Compared to the blank Al₂O₃ sintered samples, the graphene/Al₂O₃ specimens (both prepared and commercial additive) behaved evident increase in mechanical properties, even upon 30% enhanced in fracture toughness and bending strength generally by the prepared grapheme. Moreover, the prepared graphene had better improvement effect than commercial graphene in enhancing mechanical properties of Al₂O₃ ceramic.     

A comparative study on different aqueous-phase graphite exfoliation methods for few-layer graphene production and its application in alumina matrix composites

Four different methods (attrition milling, shear mixing, low-power bath sonication and tip sonication) used for the aqueous-phase, surfactant-assisted exfoliation of graphite were compared. Few-layer graphene (FLG) concentration, yield and production rate were measured for each method at different production times and the quality of the as produced FLG was determined using Raman spectroscopy and X-ray diffraction. It was inferred from the results that a combined method comprising tip sonication and shear exfoliation would offer the best balance between quality and quantity of FLG for relatively short processing times (<6 h). FLG dispersions produced with this method were used to fabricate 1 wt.% FLG/Al₂O₃ nanocomposites by ball milling and extrusion, followed by pressure-less sintering. The influence of the FLG addition on the microstructure and mechanical properties was studied, with observed increases of 26.4% and 67.6% in flexural strength and fracture toughness,respectively, and a 25.3% decrease in average grain size.     

Porous structures in stacked,crumpled and pillared graphene-based 3D materials

Graphene, an atomically thin material with the theoretical surface area of 2600 m² g⁻¹, has great potential in the fields of catalysis, separation, and gas storage if properly assembled into functional 3D materials at large scale. In ideal non-interacting ensembles of non-porous multilayer graphene plates, the surface area can be adequately estimated using the simple geometric law ∼2600 m² g⁻¹/N, where N is the number of graphene sheets per plate. Some processing operations, however, lead to secondary plate–plate stacking, folding, crumpling or pillaring, which give rise to more complex structures. Here we show that bulk samples of multilayer graphene plates stack in an irregular fashion that preserves the 2600/N surface area and creates regular slot-like pores with sizes that are multiples of the unit plate thickness. In contrast, graphene oxide deposits into films with massive area loss (2600–40 m² g⁻¹) due to nearly perfect alignment and stacking during the drying process. Pillaring graphene oxide sheets by co-deposition of colloidal-phase particle-based spacers has the potential to partially restore the large monolayer surface. Surface areas as high as 1000 m² g⁻¹ are demonstrated here through colloidal-phase deposition of graphene oxide with water-dispersible aryl-sulfonated ultrafine carbon black as a pillaring agent.     

Exfoliated graphite with relative dielectric constant reaching 360, obtained by exfoliation of acid-intercalated graphite flakes without subsequent removal of the residual acidity

Exfoliated graphite (obtained by rapid heating of sulfuric-acid intercalated and subsequently deacidified graphite flakes) is optionally subjected to residual acidity removal, which involves repeated washing with water, such that the pH of the wash water increases from 2 to 7. Compared to washed exfoliated graphite, the unwashed material exhibits lower specific surface area (24 vs. 45 m²/g), a higher value (360 vs. 38 at 50 Hz) of the relative dielectric constant (real part), a similar value of the conductivity (50 S/m), a higher value of the specific carbon–contact interfacial capacitance (1.17 vs. 0.04 μF/m²), and a lower value of the carbon–contact interfacial resistivity (0.08 vs. 0.27 Ω cm²). The greater concentration of residual intercalate (containing sulfur and oxygen) present without washing contributes to the polarizability without interfering the conduction. The carbon–contact interface is superior when the exfoliated graphite has not been washed. At 2.0 MHz, the relative dielectric constant (real part) remains high (280) and the carbon–contact interfacial specific capacitance remains high (1.13 μF/m²). The imaginary part of the relative dielectric constant and the dielectric loss angle are not affected by the washing. The relative dielectric constant of 360 is even higher than the value of 121 for potassium-hydroxide-activated graphite nanoplatelet.     

Customized casting of unstacked graphene with high surface area (>1300 m2 g−1) and its application in oxygen reduction reaction

Introducing graphene protuberances covalently bonded with the graphene sheets is a straightforward strategy to avoid the stacking of graphene layers. The as-obtained unstacked double-layer templated graphene (DTG) is expected to fully demonstrate intrinsic properties of graphene assemblies if its detailed structure and component can be well controlled. Herein, both the lateral size and graphene protuberance size and areal density of DTG were well modulated through adjusting the template morphology and casting procedures. The lateral size of the as-obtained DTG was ranging from 0.4 to 2 μm. They exhibited a very high specific surface area ranging from 1336 to 1579 m² g⁻¹ and tailorable graphene protuberances with areal density from 5.3 × 10¹⁴ to 7.8 × 10¹⁴ m⁻². DTG flakes with more hydrophilic surface and well tunable reactivity were obtained by introducing nitrogen into DTG through in-situ deposition. The as-synthesized N-doped DTG afforded significantly improved reactivity on oxygen reduction reaction (nearly 50 mV positively shifted onset potential compared with that of pristine DTG and a current preservation of 92.8% after 16,000 s test).     

The synthesis of a hybrid graphene–nickel/manganese mixed oxide and its performance in lithium-ion batteries

Mixing of aqueous suspensions of delaminated NiMn layered double hydroxide (LDH) and graphene oxide leads to the instantaneous precipitation of a hybrid material that after calcination under inert atmosphere at 450 °C leads to Ni₆MnO₈ nanoparticles deposited on larger reconstituted graphene sheets. This material exhibits electrical conductivity similar to graphite, superparamagnetism and can be used as an anode for Li-ion batteries. A maximum capacity value of 1030 mA h g^?1 was found during the first discharge, and capacity values higher than 400 mA h g^?1 were still achieved after 10 cycles. The methodology used here should allow the preparation of a large variety of hybrid graphene-metal oxide materials starting from other LDHs in which the properties derived from both constituents coexist.     

Discovery of effective solvents for platelet-type graphite nanofibers

The dispersibility of platelet-type graphite nanofibers (PGNFs), an archetype of carbon material with a surface dominated by graphitic edge planes, has been measured in 28 solvents and rationalized on the basis of solvent surface tension and Hansen solubility parameters. Successful solvents possess surface tensions of ∼25–35 mJ m⁻² and substantial values of the hydrogen-bonding Hansen parameter (δ_H ∼ 14–16 MPa^1/2), and many of them are alcohols, such as 1-butanol, ethanol or cyclohexanol. Such result is mainly attributed to the fact that the PGNF edge planes are decorated with oxygen functional groups. The dispersion behavior of the nanofibers could be changed to that typically exhibited by carbon nanotubes and graphene by means of a high temperature annealing that converted their surface edge planes to curved basal planes.     

High-performance supercapacitor electrodes based on highly corrugated graphene sheets

Highly corrugated graphene sheets (HCGS) have been prepared by a rapid, low cost and scalable approach through the thermal reduction of graphite oxide at 900 °C followed by rapid cooling using liquid nitrogen. The wrinkling of the graphene sheets can significantly prevent them from agglomerating and restacking with one another face to face and thus increase the electrolyte-accessible surface area. The maximum specific capacitance of 349 F g^?1 at 2 mV s^?1 is obtained for the HCGS electrode in 6 M KOH aqueous solution. Additionally, the electrode shows excellent electrochemical stability along with an approximately 8.0% increase of the initial specific capacitance after 5000 cycle tests. These features make the present HCGS material a quite promising alternative for next generation of high-performance supercapacitors.     

An efficient method of producing stable graphene suspensions with less toxicity using dimethyl ketoxime

A highly stable graphene suspension has been prepared using dimethyl ketoxime (DMKO) as reductant. Nitrogen was doped into the graphene plane at the same time as the graphene oxide (GO) sheets were reduced. X-ray photoelectron spectroscopy indicated that the C/O ratio of graphene was significantly increased after GO was treated with DMKO and the quantity of nitrogen incorporated into the graphene lattice was 3.67 at.%. The electrical conductivity of the graphene paper was found to be ～10² S m^?1, which was 5 orders of magnitude better than that of GO, and this demonstrated the effective chemical reduction of GO. The mechanism of the chemical reaction of GO with DMKO was also discussed. The as-produced graphene material showed good capacitive behavior and long cycle life with a specific capacitance of ～140 F g^?1.     

High-yield graphene exfoliation using sodium dodecyl sulfate accompanied by alcohols as surface-tension-reducing agents in aqueous solution

The exfoliation of graphite was investigated in aqueous solutions containing sodium dodecyl sulfate (SDS) as a surfactant. The exfoliation was greatly enhanced near the surface aggregation concentration (SAC) of SDS, 2.6 mM, and then decreased for higher SDS contents. However, the flakes exfoliated near the SAC were graphite, whereas graphene was obtained above the critical micelle concentration (CMC). The effect of the use of alcohols as surface-tension-reducing agents (STRAs) on the exfoliation was then investigated. With ethyl alcohol, a dispersion of 2.1 mg ml⁻¹ graphene was achieved from 2.6 mM SDS after only 1 h of sonication, whereas a dispersion of 0.2 mg ml⁻¹ was obtained above the CMC in the absence of STRAs. The results demonstrate that the SDS content near the SAC is highly beneficial for exfoliation as long as the surface tension is maintained near 41.0 mN ml⁻¹. This finding supports the notion that the structure of the adsorbed SDS, depending on its concentration, strongly affects the exfoliation of graphite into graphene.     

High-concentration aqueous dispersions of graphene produced by exfoliation of graphite using cellulose nanocrystals

Stable high-concentration aqueous dispersions (>1 mg ml⁻¹) of single and few-layer graphene flakes were produced by direct exfoliation of graphite using cellulose nanocrystals (CNC). Biodegradable and widely available from renewable sources, CNC have proven to be very efficient graphene stabilizers even at low concentrations (0.2 mg ml⁻¹), thus enabling remarkably high graphene/CNC ratios (up to 3.8).     

Influence of processing on fracture toughness of Si3N4 + graphene platelet composites

Silicon nitride + 1 wt% graphene platelet composites were prepared using various graphene platelets (GPL) and two processing routes; hot isostatic pressing (HIP) and gas pressure sintering (GPS). The influence of the processing route and graphene platelets’ addition on the fracture toughness has been investigated. The matrix of the composites prepared by GPS consists of Si₃N₄ grains with smaller diameter in comparison to the composites prepared by HIP. The indentation fracture toughness of the composites was in the range 6.1–9.9 MPa m^0.5, which is significantly higher compared to the monolithic silicon nitride 6.5 and 6.3 MPa m^0.5. The highest value of K_IC was 9.9 MPa m^0.5 in the case of composite reinforced by the smallest multilayer graphene nanosheets, prepared by HIP. The composites prepared by GPS exhibit lower fracture toughness, from 6.1 to 8.5 MPa m^0.5. The toughening mechanisms were similar in all composites in the form of crack deflection, crack branching and crack bridging.     

A simple process to prepare nitrogen-modified few-layer graphene for a supercapacitor electrode

We report a simple approach to prepare the nitrogen-modified few-layer graphene (FLG) directly from graphite flakes. With the aid of melamine, graphite flakes can be directly ultrasonicated into FLG in acetone. The subsequent annealing process further transforms the melamine absorbed on the surface of graphene into melon (C₆N₉H₃)_x, which is one type of condensation product of melamine, and simultaneously dopes the graphene with nitrogen. When tested as a supercapacitor electrode, the nitrogen-modified FLG shows a much higher specific capacitance (e.g., 227 F/g at 1A/g) than that of reduced graphene oxide (rGO) (e.g., 133 F/g at 1A/g).