Mass production of low-boiling point solvent- and water-soluble graphene by simple salt-assisted ball milling.

Large-scale production of high-quality graphene is very critical for practical applications of graphene materials and devices. Exfoliation of graphite in an aqueous solution of surfactants is one of the most promising approaches to produce graphene. In this study, a novel anionic surfactant [sulfonated used engine oil (SUEO)], which was prepared from used engine oil, was employed to exfoliate the graphite nanoplatelets into graphene sheets in an aqueous solution under sonication to form a stable dispersion. The efficiency of SUEO for exfoliating and dispersing graphene was investigated and compared with that of traditional surfactants, such as sodium dodecyl sulfate, sodium dodecyl benzene sulfate, cetyl trimethyl ammonium bromide, and polyvinylpyrrolidone. Result showed that the graphene dispersion with excellent stability had a higher concentration (0.477 mg/mL) than others at 0.5 g/L optimal SUEO dosage in 4 h sonication time. The superior performance of SUEO can be attributed to its special molecular structures, whose hydrophobic moieties contain cycloalkanes/aromatics with different molecular weights and/or side chain –R with different lengths. Structural diversities are very helpful to the “jigsaw-puzzle” process on the graphene surface, where the total interfacial energy of the mixture system was minimized. Microscopic (SEM, TEM, and AFM) and spectroscopic (XRD, XPS, and Raman) measurements revealed that the dispersion consisted of few-layer graphene sheets with lower levels of defects or oxidation. This study presents a new class of dispersing agents for graphene that assists in the exfoliation process in water with high concentration and the stabilization of the graphene sheets against reaggregation.

A novel, simple and efficient method by the combination of hydroxyl radicals and electrical exfoliation of graphite for the green production of high-quality graphene from graphite was first developed in our self-manufactured exfoliation apparatus. In this work, we focused on the investigation of the roles of various electrolyte systems for the exfoliation of graphite. Sodium chloride, sodium hydroxide, poly vinyl pyrrolidone (PVP), dodecyl trimethyl ammonium bromide (DTAB) and sodium dodecyl benzene sulfonate (SDBS) were tested as the electrolyte. The yields of the graphene product in sodium hydroxide, PVP, DTAB, sodium chloride and SDBS electrolyte system were 32.9%, 34.0%, 45.2%, 77.5% and 83.5%, respectively. The experimental result demonstrated that graphite can be successfully exfoliated to graphene in these electrolytes, with SDBS showing the best exfoliation effect. We further investigated the effects of process parameters on the graphite exfoliation in the SDBS system by single factor experiments. The obtained optimal process parameters were as follows: graphite dosage, 5.0 g; SDBS solution concentration, 10.0% (w/v); applied current strength, 10 mA; air flow, 1.0 L h−1; and exfoliation time, 3 h. At these conditions, the yield of the graphene product was 89.7%. TEM results revealed that the graphene product possessed the characteristic features of a thin-layer graphene sheet. XRD results showed that the graphene products still maintained the structures of carbon atoms or molecules. FT-IR and Raman results indicated that the products exhibited the characteristic peaks and the absorption peaks of graphene. AFM test results revealed that the layer number of graphene product obtained was about 2, while the layer numbers of the graphene products obtained from sodium hydroxide, PVP, DTAB and sodium chloride systems were 30, 20, 4 and 3, respectively, at the same experimental conditions. The observed exfoliation effect in the SDBS system was due to its good electrical conductivity, which was favorable for the formation of hydroxyl radicals in exfoliation. Furthermore, SDBS has good hydrophilic properties and can enable even dispersion of graphite in the system. These two effects facilitated the exfoliation of graphite to form good-quality graphene. SDBS as the electrolyte did not corrode the electrode, and it could be recycled; also, it does not pollute the environment and reduces the production cost, which is favorable for mass production.

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

A hybrid based on graphene nanoplatelets and carbon nanotubes (G‐CNT) is obtained through direct exfoliation of graphite in a liquid medium, and tested as reinforcement material in epoxy resin. CNT act as exfoliation material in contact with graphite under ultrasound irradiation, and the thickness of graphite decreases to produce graphene nanoplatelets in an easy one‐step method. The morphology of the hybrid, is observed through transmission electron microscopy micrographs exhibiting delamination of graphite. XRD, UV–vis, and atomic force microscopy (AFM) results provide evidence of the exfoliation of graphite by interaction with CNT. Ultrasonic process does not generate additional oxygenated groups on carbon structures, as it is supported by XPS spectra; and the disorder in the carbon structures detected in Raman spectra is attributed to defects originated by some fragmentations. Epoxy nanocomposites incorporating 0.3 wt% of G‐CNT display the highest enhancement in storage modulus (~2706 MPa), glass transition region increases meaning higher thermal stability, and glass transition temperature increases until 119°C. Impact resistance also enhances with 0.3 wt% of G‐CNT, obtaining ~95% of increment. The hybrid obtained by exfoliating graphite with CNT in one‐step improves the performance of nanocomposites, and it offer an easy and viable alternative to obtain hybrid systems.

Nanofibrillated cellulose (NFC) is a natural fibrillar material with exceptionally high mechanical properties. It has, however, been exceedingly difficult to achieve nanocomposites with drastically improved mechanical properties by dispersing NFC as random networks to polymer matrices, even using compatibilization. We show nanocomposites consisting of aligned assemblies of multilayered graphene and NFC with excellent tensile mechanical properties without any surface treatments. The optimum composition was found at 1.25 wt % graphene multilayers, giving a Young's modulus of 16.9 GPa, ultimate strength of 351 MPa, strain of 12%, and work-of-fracture of 22.3 MJ m(-3). This combines high strength with relatively high toughness and is obtained by direct exfoliation of graphite within aqueous hydrogels of NFC where an optimum sonication power is described. The results suggest the existence of an attractive interaction between multilayered graphene flakes and cellulose. Aligned assemblies are obtained by removal of water by filtration. The concept can be beneficial for applications because it results in high mechanical properties by a simple and environmentally green process.

Although graphene has been developed and widely used in many fields over the past decades, the effect of oxygen (O2) on the exfoliation of graphite is still unclear. Here, a simple bubble‐triggered exfoliation method is employed for controllable synthesis of graphene nanoplatelets (GNPs) with low‐oxidation level and high electrical conductivity. The exfoliation process is studied and a formation mechanism for the synthesis of the low‐defect GNPs is proposed. The results show that the synergistic effect of H2SO4 and H2O2 plays an important role in the procession of exfoliation of graphite. Moreover, density functional theory (DFT) simulations exhibit that H2O2 would preferentially adsorb on the edge of graphite, indicating the possibility to initiate the exfoliation by O2. In the whole process, some oxygen free radicals can be released from H2O2, resulting in some O2 and oxygen functional groups formed on the graphite edge, and which can finally lead to the fast exfoliation of graphite. In addition, a time‐dependent temperature profile under an applied voltage of 5 V exhibits a steady‐state with temperature up to 275 °C. Overall, this work elucidates the selectivity of gas bubble absorbed on the side edge of graphite and presents a potential application for high‐electrical conductivity graphene.

Chemical vapor deposition is widely utilized to synthesize graphene with controlled properties for many applications. And it is one of the most important methods for the preparation of graphene with high quality in large area. Cu substrate is most commonly used for the preparation of graphene in chemical vapor deposition. As is well known, the properties of graphene are greatly affected by the number of layers. However, the syntheses and mechanisms of bi-layer and multi-layer graphene on Cu substrates are still under debate. And how to make a breakthrough in realizing the controllable syntheses of bi-layer and multi-layer graphene on Cu substrates has become a direction for many researchers. In this work, we report bi-layer and multi-layer graphene on Cu substrates prepared by atmospheric pressure chemical vapor deposition. Firstly, the Cu foil is placed on the quartz slides of the tube furnace and heated to 1000℃ with a rate of 15℃/min. After reaching 1000℃, the Cu foilis annealed for 2 h in a gas mixture of hydrogen (20 sccm) and argon (380 sccm). After that, the graphene growth is carried out at 1000℃ under an 80 sccm gas mixture of argon and ethanol. Then, the samples are cooled down to the room temperature with a rate of 100℃/min in a protection gas of hydrogen and argon, and then taken out of the furnace. The graphene is prepared on the Cu foils and finally transferred onto the SiO2/Si substrates. The quality and number of layers of the as-produced graphene are assessed by field emission scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, and optical microscopy. By tuning the graphene growth, the monolayer, bi-layer and multi-layer graphene with higer quality and better continuity are obtained. And the growth times of monolayer, bi-layer, and four-layers graphene are respectively 25, 40, and 60 s. And wefind that the graphene layer will be increased in the process of insulation. The growth mechanisms of bi-layer and multi-layer graphene on copper substrates by atmospheric pressure chemical vapor deposition are also discussed. There will be some indiffusible carbon atoms or radicals near the copper foil surface due to the small molecular diffusion mean free path under normal pressure. We suggeste that the bi-layer and multi-layer graphene grown on copper substrates by atmospheric pressure chemical vapor deposition is dominated by van der Waals epitaxial mechanism. This work provides a reference for improving the quality of chemical vapor deposition monolayer, bi-layer and multi-layer graphene.

The carrier transport properties of a multilayer graphene with the turbostratic stacking structure are evaluated by transfer characteristics in a field‐effect transistor and Hall‐effect measurement with van der Pauw geometry. Herein, the multilayer graphene with the turbostratic stacking structure is synthesized by graphene overlayer growth on a monolayer graphene template using a chemical vapor deposition method. The sheet resistance of the inner layer in the grown multilayer graphene is lower than that of the monolayer graphene used as a growth template. This is caused by a screening effect due to the multistacking. Moreover, the value of carrier mobility in the grown multilayer graphene is higher than that of the monolayer graphene template. The enhancement of carrier mobility is caused by a turbostratic stacking effect where the band structure of the multilayer graphene is similar to that of the monolayer graphene. These results indicate that the synthesis technique of the multilayer graphene with the turbostratic stacking structure using the graphene template is effective to improve the carrier transport properties in the multilayer graphene sheet.

The infrared transmitting electromagnetic interference shielding (Infrared-EMI) filters were fabricated through transferring multilayer graphene on ZnS and As 40 Se 60 infrared substrates. The monolayer graphene was grown by chemical vapor deposition (CVD) catalyzed by copper foil. The carrier mobility of monolayer graphene transferred on quartz glass is 1318.9 cm 2 /V·s, sheet resistance (R s ) is 483 Ω/ and I(2D)/I(G) of Raman result is 2.27. The monolayer graphene on copper foil was transferred on ZnS and As 40 Se 60 substrates using wet chemical etching method, the multilayer graphene were overlaid on ZnS and As 40 Se 60 substrates by repeating the same transferring process. The Rs of monolayer graphene film on ZnS substrate is 1100 Ω/ and decreases to 65 Ω/ as the number of graphene layer rises to 13. The R s of monolayer graphene on As 40 Se 60 substrate is 1414 Ω/ and decreases to 68 Ω/ as the number of graphene layer is up to 17. ZnS substrate transferred with 13 layers of graphene shows relative 94.7 % infrared transmittance in 8~12 μm waveband. As 40 Se 6 substrate transferred with 17 layers of graphene on As 40 Se 6 substrate show relative 81.3 % infrared transmittance in 8~18 μm waveband. OTA300 - 104 V. The average EMI shielding effectiveness of multilayer graphene on ZnS (13 layers) and on As 40 Se 60 substrates (17 layers) in a frequency range of 30 MHz~1.5 GHz is 15.6 dB and 13.3 dB, respectively. ZnS-graphene and As 40 Se 60 -graphene Infrared-EMI filter realize the coexistence of infrared transmitting and EMI shielding function.

Multilayer graphene was synthesized by overlayer growth of graphene on a monolayer graphene template using a chemical vapor deposition method under a high process temperature of 1300 °C. Structural analysis using Raman spectra revealed that the synthesized multilayer graphene forms highly crystalline graphene layers with a turbostratic stacking structure. Atomic force microscope images indicated that the step edges of the grown graphene layer proceed via lateral growth mode. The electrical transport properties of the synthesized multilayer graphene showed higher conductivity and carrier mobility than those of the monolayer graphene template. The improvement of the electrical transport properties is caused by the turbostratic stacking structure that has the electronic band dispersion similar to that of monolayer graphene. This result means that the synthesis of graphene layers grown on the graphene template is effective to improve the carrier transport properties in multilayer graphene sheets.

Graphene was obtained by direct exfoliation of graphite in o-dichlorobenzene (oDCB) or benzylamine, and further functionalized with 4,4′-[(1,3-dioxo-1,3-propanediyl)bis(oxy)]bis[2,2,6,6-tetramethyl-1-piperidinyloxy] (1-TEMPO) organic radicals by using the Bingel–Hirsch cyclopropanation reaction. Here, the use of different solvents permits variation of the density of radicals anchored to the carbon layers. Covalent grafting is unambiguously demonstrated by TGA, μ-Raman, XPS and EPR measurements, which also rule out spurious physisorption. Our transport measurements indicate that the conduction mechanism varies as a function of the density of radicals grafted to the carbon layers. Moreover, the presence of paramagnetic moieties influences the magnetoresistive behaviour of the system by the appearance of a low field magnetoresistance (LFMR) effect. Finally, the derivatization of graphene with diamagnetic ethylmalonate molecules, by following an analogous route, permits us to discard the chemical derivatization, which is responsible for the observed differences in the MR response that must be rather ascribed to the grafting of organic spin carriers.

A high-yielding dispersion of graphene at high concentration in solvent is critical for practical applications. Herein, we demonstrate the formation of a stable dispersion of pristine graphene in ethanol by exfoliating graphite flakes into individual graphene layers using a non-ionic polymer surfactant under bath-type sonication. Oligothiophene-terminated poly(ethylene glycol) was synthesized and used as a non-ionic and amphiphilic surfactant for exfoliating graphite into graphene. A high-quality graphene film was fabricated from the exfoliated graphene solution by the vacuum filtration method. TEM and SEM reveal that the size of exfoliated graphene flakes is larger than 1 μm. When the graphene film was treated with nitric acid and thionyl chloride after washing with solvent, the film showed high performance with a sheet resistance of 0.3 kΩ sq−1 and a transparency of 74% at 550 nm.

Preparation of PVP–PVA–exfoliated graphite cross-linked composite hydrogels for the incorporation of small tin nanoparticles

Nanocomposites based on PtPd nanoparticles with chemical ordering like disordered solid solution on surface of multilayer graphene have been prepared through thermal shock of mechanically obtained mixture of double complex salt [Pd(NH3)4][PtCl6] and different carbon materials–exfoliated graphite, graphite oxide and graphite fluoride. An effect of original carbon precursors on formation of PtPd bimetallic nanoparticles was studied using X-ray absorption spectroscopy (XAFS), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). It was shown that the distribution of bimetallic nanoparticles over the multilayer graphene surface as well as the particles size distribution is controlled by the graphene precursors. For all nanocomposites, the surface of the nanoparticles was found to be Pd-enriched. In case when the thermal exfoliated graphite and graphite oxide were used as the graphene precursors a thin graphitized layer covered the nanoparticles surface. Such a graphitized layer was not observed in the nanocomposite, which used the fluorinated graphite as the precursor.

A brief review is given on exotic electronic properties of monolayer, bilayer, and multilayer graphenes from a theoretical point of view. In multi-layer graphenes, the Hamiltonian is decomposed into those of monolayer and bilayer graphenes.