The graphene oxide could be exist in the sample even though the synthesis process did not involve the oxidation reactions directly. It is possibly due to the C + from the graphene sheet edge (when deffects occured) captures the electron from hydroxyl (OH - ) forming the graphene oxide. The shoulder at the higher wavenumber of G band peak as seen in Fig. 1, it indicated that the presence of graphene oxide (C-OH) bonds in the EMLE graphene produced . However the C-OH bonds are minority in the sample as indicated by the lower intensity of D band peaks than G band peak and the absence of D’ and D+G extra bands . From the analyses either from band position, band shapes, comparisons or from the calculations performed, it can be concluded that the graphene produced in this research has defects with the number of layers are ~ 3-10 layer (multi-layer) and with minor impurity of graphene oxide. We call this product as electrochemical and mechanical liquid exfoliation (EMLE) graphene or EMLE graphene.
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few-layer flakes(8). Although exfoliation can be achieved mechanically on a small scale(9, 10), liquid phase exfoliation methods are required for many applications(11). Critically, a simple liquid exfoliation method would allow the formation of novel hybrid and composite materials. While TMDs can be chemically exfoliated in liquids(12-14), this method is time consuming, extremely sensitive to the environment and incompatible with most solvents.
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The exfoliation of graphite demonstrated by Geim and Novolosov was achieved essentially by rubbing graphite on a surface.(1) Such mechanical exfoliation remains the source of the highest quality graphene samples available and has resulted in some major advances.(1) However, it suffers from low yield and a production rate that is not technologically scalable in its current form. One possible solution is the exfoliation of layered compounds in liquids to give large quantities of dispersed nanosheets. Liquid exfoliation techniques are versatile methods for obtaining sizable quantities of 2D materials which are cheap, easy and scalable and will allow exfoliated nanosheets to be processed using existing industrial techniques such as reel-to-reel manufacturing, etc. This is a critical advantage that cannot be understated and as a result, we focus this review on liquid exfoliation. Here we briefly outline the four main liquid exfoliation techniques for layered materials (see figure 2 for schematics and figure 3 for examples of exfoliated nanosheets).
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impurity and defects in samples, while the CVD costs highly and is applied to prepare continuous film instead of disperse nanosheets that are more popular in practical applications. Recently, liquid exfoliation of BNNSs from hexagonal boron nitride (hBN) powder has received much attention because it is easy to use, economical, free of defect, etc. The driving forces were ascribed dynamically to sonic vibration [19, 20] or liquid shear [21, 22], and thermodynamically to minimization of Gibbs mixing free energy [23, 24] or interfacial energy  between the nanosheets and solvents. According to the latter, the composition and properties of used solvent play an important role in liquid exfoliation. Much research has shown that hBN can be exfoliated preferentially in few pure solvents such as N-methyl-2- pyrrolidone (NMP), dimethylformanmide (DMF), and iso- propanol (IPA) [9, 19–24, 26] and some mixed solvents [25, 27–29]. However, high-efficient and cheap solvents for hBN liquid exfoliation have been rarely reported, limit- ing the large-scale preparation and applications of BNNSs. In the present paper, monoethanolamine (MEA) aque- ous solution was attempted for the first time for liquid exfoliation of BNNSs. It was found to exfoliate hBN more efficiently than the other solvents with very high yield. Moreover, this solution has higher specific surface tension (SST) than that of known solvents. The obtained BNNSs were characterized by X-ray diffraction (XRD),
Bright-ﬁeld TEM imaging was performed using a JEOL 2100, operated at 200 kV, while HRTEM was conducted on a FEI Titan TEM (300 kV). High resolution TEM images (Fig. 5f) were taken using an FEI Titan 60–300 Ultimate Microscope operated at 300 kV. The FL-BP was dropped on grids using a drop casting method and excess ﬂuid was absorbed by an underlying ﬁlter membrane. It was then baked in vacuum at 120 °C for several hours. The samples were imaged on the day they were received which is termed Day 1. The same ﬂake was then imaged on Day 3 and 16. No changes in nanosheet structure of morphology were observed between Day 1 and Day 3, but by Day 16 a combination of reaction products and water adsorption results in a liquid layer on the ﬂake. This layer can be removed with the beam, and comparison of the shape of the ﬂakes between Day 1 and Day 16 shows that the overall shape and size of the ﬂake has not changed. There is also lattice apparent in the ﬂake on Day 16 when using a high magniﬁcation ( 300k).
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This Account describes recent work to develop such a processing route inspired by previous theoretical and experimental studies on the solvent-dispersion of carbon nanotubes. That work had shown that nanotubes could be effectively dispersed in solvents whose surface energy matched that of the nanotubes. We describe the application of the same approach to the exfoliation of graphite to give graphene in a range of solvents. When graphite powder is exposed to ultrasonication in the presence of a suitable solvent, the powder fragments into nanosheets, which are stabilized against aggregation by the solvent. The enthalpy of mixing is minimized for solvents with surface energies close to that of graphene (~68 mJ/m 2 ). The exfoliated nanosheets are free of defects and oxides and can be produced in large quantities. Once solvent exfoliation is possible, the process can be optimized and the nanosheets can be separated by size. The use of surfactants can also stabilize exfoliated graphene in water, where the zeta potential of the surfactant-coated graphene nanosheets controls the dispersed concentration.
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Since XPS shows that oxides are present in both GaS powder and exfoliated nanosheets (Fig. 1I and S2) and virtually nothing is known about the long term stability of GaS in the exfoliated state, it is important to track where oxides species reside. This is particularly crucial in light of recent investigations on exfoliated black phosphorus nanosheets which have shown significant degradation under exposure to ambient conditions. 81-83 To gain insights into potential oxidation of GaS, we have analyzed liquid exfoliated nanosheets by STEM imaging and electron energy loss spectroscopy (EELS). A representative STEM image of a nanosheet at the edge region is shown in figure 3A. EEL spectra were recorded from the same sample region to form a map, with the intensity of each pixel in the map (ranging from black to yellow/white) corresponding to the integrated intensity of the oxygen K-edge in the pixel location. From this data, an oxygen content map could be constructed from the same sample region (Fig. 3B) as shown in STEM image (Fig.3A). Hence, the map is color-coded and shows increasing oxygen content from black/blue (no/low oxygen) to green to red to yellow to white. It is clear that the oxides reside mostly near the edges of the nanosheets and at step edges throughout the nanosheets. The EEL spectra corresponding to regions with different oxygen content are displayed in figure 3C.
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production an equally wide range of 2D systems with a very broad pallet of properties. While 2D materials can be produced by micromechanical cleavage of layered crystals 11, 12 or bottom-up 13, 14 growth methods such as chemical vapour deposition, these methods are unsuitable for many practical applications. A number of areas, especially those involving the formation of thin films or composites, require exfoliation methods which give relatively large quantities of materials in a processable form. In such cases, the most appropriate method for producing 2D materials is liquid exfoliation. 4 This technique is extremely useful because it gives liquid-suspended nanosheets which can then be processed into films, composites or other structures. It has proven very successful in exfoliating a wide range of layered crystals including graphite, metal chalcogenides and metal oxides. 4
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Due to its inherent impermeability to gases, graphene has been utilized as a barrier enhancer in polymer composites. 49,50 Compared to nanoclays, graphene nanosheets tend to have larger aspect ratios, giving them a distinct advantage in areas such as permeability reduction. Hexagonal BN is isostructural with graphene and can be exfoliated in a number of ways, 51,52 including by liquid exfoliation, 12,14,24 to give high aspect ratio nanosheets which are impermeable to gases. 53 Recently, it has been reported that solution processing can be used to mix h-BN with a range of materials, including gelatin, cellulose, chit- osan and soy protein, 54–57 resulting in improvements in their barrier properties. In fact, exfoliated h-BN nanosheets (BNNS) are a very attractive additive for barrier composites because their wide bandgap and insulating nature. Because of these properties, it may be possible to decrease the permeability without aﬀecting the optical or electrical properties of the plastic, factors which will be important in certain applications. In addition, the exceptional mechanical properties of h-BN mean that the resultant composites may even have enhanced strength and sti ﬀ ness compared to the base polymer. 12,58 Importantly for real applications, the achievable production rate for BNNS has been steadily increasing with batches of up to 20 g produced recently. 52
these methods, liquid exfoliation not only produces novel materials with the same composition yet dra- matically changed electrical properties but also pro- vides a facile way to prepare thin-layer nanosheets, which offers novel opportunities in the optoelectron- ics applications [17, 31–34].
Solvent or surfactant exfoliated graphene gives defect-free flakes but with a relatively low monolayer content. Each method results in dispersions with concentrations of up to a few mg/ml, and can be produced up to liter quantities. [ 69 ] However both methods have one very serious weakness; they tend to produce small flakes. Dispersed GO is generally produced as flakes with lateral size ~ 100 s of nm, while solvent or surfactant exfoliated graphene is usually characterised by flake size ~ 1 micron. This is a significant problem. Liquid exfoliation of graphene is usually presented as a method to produce graphene in large quantities for applications such as in composites or films. However, many of these applications require flakes which are considerably larger than those currently available. For example, Gong et al recently showed that in order to produce effectively reinforced graphene-polymethylmethacrylate composites, the flake length would have to be a few microns or greater. [ 4 ] Currently available exfoliated graphene is usually significantly smaller than this, which partly explains why most graphene composite papers describe reinforcement values much lower than the theoretical limit [ 154 ] of dY/dV f ~ 1 TPa where Y is the composite modulus and V f is the graphene volume fraction. [ 155 – 164 ]
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ABSTRACT: Liquid phase exfoliation (LPE) is a commonly-used method to produce 2D nanosheets from a range of layered crystals. However, such nanosheets display broad size and thickness distributions and correlations between area and thickness, issues which limit nanosheet application-potential. To understand the factors controlling the exfoliation process, we have liquid-exfoliated 11 different layered materials, size-selecting each into fractions before using AFM to measure the nanosheet length, width and thickness distributions for each fraction. The resultant data shows a clear power-law scaling of nanosheet area with thickness for each material. We have developed a simple non-equilibrium thermodynamics-based model predicting that the power-law pre-factor is proportional to both the ratios of in-plane- tearing/out-of-plane-peeling energies and in-plane/out-of-plane moduli. By comparing the experimental data with the modulus ratio calculated from first principles, we find close agreement between experiment and theory. This supports our hypothesis that energy equipartition holds between nanosheet tearing and peeling during sonication-assisted exfoliation.
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The current study has some limitations. First, as in all asso- ciation studies, the described relationship between ET-1 and hemodynamic dysfunction in eyes with XFS or XFG does not necessarily imply causation. In other words, both hemodynamic alterations and ET-1 levels might be related to the presence of exfoliation material, or to the effects of other factors, unaccounted for in the present study. Second, our participants with XFG had been treated with antiglaucoma medications. It remains unknown if, and to what extent, IOP-lowering agents may have influenced ET-1 levels. Third, the determination of short PCA flow using CDI is inherently problematic. This happens because several of these arteries and the flow parameters we used in this study were based on the values derived from the PCAs that were technically easier to assess. Perhaps, a larger sample size would have limited measurement variability in these particular vessels.
Guo et al.  reported a basic NMP‐based LPE method for the fabrication of phosphorene with an excellent water stability, a controllable size, a number of layers and high yield. The schematic of the synthesis process of basic NMP‐exfoliated phosphorene is given in Figure 18.104.22.168.b.The basic NMP‐exfoliation process can be detailed as follows: bulk BP (15 mg) was added to a saturated NaOH NMP solution (30 mL) and sonicated for 4 hours at 40 kHz frequency and 80% power, and the phosphorene in NMP was separated and transferred to water by centrifugation (at 3000 rpm for 10 min) in order to remove the unexfoliated bulk BP. Guo et al.  adopted different centrifugation speeds to control the phosphorene thickness. The supernatant solution was centrifuged at 12 000 rpm for 20 min to obtain 5.3 ± 2.0 nm thick (5 to 12 L) phosphorene samples with an average diameter of ≈670 nm (referred to as 12000 phosphorene) and further centrifuged at 18 000 rpm for 20 min to obtain 2.8 ± 1.5 nm thick (1 to 7 L) phosphorene samples with average diameter of ≈210 nm (referred to as 18000 phosphorene). The results are also demonstrated for the two conditions with and without the addition of NaOH to NMP, which confirms that a thorough exfoliation of BP was achieved by the basic NMP process (with NaOH) than by the NMP‐only exfoliation (without NaOH). The negative charge of phosphorene corresponds to the OH– ions, obtained by adding NaOH, that are absorbed on the surface of phosphorene leading to an excellent stability in water.
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Kang et al.  synthesized electronic‐grade BP dispersions using sealed‐tip ultrasonication at a reduced sonication time by means of anhydrous oxygen‐free organic solvents, thus avoiding the chemical degradation pathways for BP. A sealed container lid was attached to an ultrasonicator tip/probe (0.125 in.) and driven at a higher power compared to conventional bath sonication to minimize the ultrasonication duration. Additionally, the interface between the tip and the lid was carefully sealed with PDMS, whereas Parafilm and Teflon tapes were used to seal the pathways between the lid and container to restrict O2 and H2O penetration. The synthesis was performed in an ice bath at ≈30 W power to obtain a BP concentration of ≈1 mg mL–1 in 1 h; on the other hand, bath sonication needs 15 to 24 h for the same exfoliation process .To optimize the solvent, BP crystals were ultrasonicated under identical preparation conditions in acetone, chloroform, hexane, ethanol, IPA, DMF, and NMP, and the samples were opened only in an Ar glovebox to minimize O2 and H2O contamination. The obtained dispersions were further centrifuged at different speeds (500 to 15000 rpm) for 10 min to tune the size distribution of the solvent‐exfoliated BP nanosheets, resulting in the solution color changing from brown to yellow depending on the centrifugation speed (see Fig.22.214.171.124.). They confirmed a monotonic increase in the BP concentration with an increase in boiling point and surface tension; which agrees well with graphene . According to their results, NMP was found to be the optimal solvent to achieve stable BP dispersions. The light yellow solution has the most dilute concentration (≈0.01 mg mL–1) of BP nanosheets, which confirms the correlation between the centrifugation speed and the BP concentration. Moreover, the flake thickness and lateral size were also observed to decrease with increasing centrifugation speeds, and the BP dispersions centrifuged at 500 rpm yield thick BP nanosheets (>50 nm thick). Conversely, centrifugation speeds of 10000 and 15 000 rpm minimize the lateral size of the BP nanosheet in comparison with the BP dispersions centrifuged at 5000 rpm, giving rise to a relatively lower lateral area for the higher centrifugation speeds. Although probe sonication and bath sonication are commonly used in the exfoliation of 2D layered materials, bath sonication was reported to be more efficient than probe sonication and the use of either one of them may result in the formation of irregular BP nanosheets . Therefore, highly dispersed suspensions of ultrasmall BP QDs with a lateral size of 2.6 nm and a thickness of ≈1.5
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We assess the hypothesis that exfoliation efficiency might be enhanced using whiskey as a stabiliser by comparing whiskey-exfoliated dispersions with con- trol samples exfoliated using a 46:54 blend of ethanol and water under identical conditions (see methods and materials). Figure 1(C) shows the obtained concentra- tions measured immediately after exfoliation, normal- ised to the concentration of the WPE sample. First, we note that the graphene sample in ethanol/water showed very low concentrations even when initially exfoliated in NMP and solvent-exchanged into the ethanol/water mixture. This is notable as nanosheets solvent-exchanged from NMP are usually stable at reasonable concentrations, likely due to adsorbed polymerised NMP [14, 27 – 29]. The inadequacy of both ethanol/water and whiskey as exfoliating solvents for graphite could be due to the evaporation of etha- nol during the sonication process as this may signifi- cantly alter the 46:54 ethanol/water ratio (a thermally induced Angel ’ s Share). The reports on graphene sta- bilised in an ethanol/water blend also show that the stability is sensitive to changes in this ratio  mean- ing an imbalance here could prevent efficient exfo- liation. In contrast, after solvent exchange from NMP, the whiskey-phase graphene sample yields an appre- ciable concentration (0.47 mg ml −1 ), roughly 100 times higher than that in ethanol/water sample. This provides some evidence that the whiskey compounds are stabilising the dispersion as the suboptimal 46:54 ethanol/water ratio can be overcome in whiskey but not ethanol/water alone. The WS 2 and BN are much
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tensions ranging from 22 to 50 mJ/m 2 . In detail, the OD and mass concentration of the GNP dispersions as a function of the surface tension of the solvent mixtures are presented in Fig. 1a. In addition, the relationship between the mass con- centration and OD of the GNP dispersions is shown in the Additional file 1. Figure 1b displays the relationship between the volume fraction of ethanol and surface tension of the solvent mixtures. The results indicated that the concentra- tion of the GNP dispersions strongly depended on the sur- face tension of the solvent mixture. All three flaked graphite samples dispersed the most effectively in the ethanol (45 vol%)-water (55 vol%) mixture with a surface tension of ~ 30 mJ/m 2 , which was in good agreement with previous literature . Therefore, the ethanol/water mixture with a surface tension of 30 mJ/m 2 was selected as the dispersing liquid medium to exfoliate the flaked graphite samples.
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We report an effective method for producing graphene sheets using solvothermal-assisted exfoliation of graphite in a mixed solvent of toluene and oleylamine. The mixed solvent of toluene and oleylamine produces higher yield of graphene than its constituents, oleylamine and toluene. The oleylamine molecules with its long chain enwrap the graphene sheets efficiently, while toluene helps the oleylamine molecules become more flexible and easily intercalate into the edge of graphite. The prepared graphene sheets have a high quality, and the concentration of graphene in the dispersion is as high as 0.128 mg mL − 1 . The high-quality graphene sheets obtained in this work make them suitable for application in many fields such as energy-storage materials and polymer composites.
micrograph presented in Fig. 2b give a clear indication of the GNS lateral dimension. It is confirmed from AFM image that the size of the GNS is around 1 μm. GNS prepared via liquid phase exfoliation are poly- disperse in size but the centrifugation process makes possible to select a range of sizes. The number of layers per sheets are 3~10 in general via liquid phase exfoli- ation method which is reasonable for the mechanical characteristics of PNCs [6, 28]. Interestingly, Gong et al. showed that the mechanical properties are maxi- mized at N ~ 3 and monolayers are not essential for high degree of reinforcement . Here in this case, AFM analysis predicts GNS thickness in the range 10– 16 nm. It has been shown that for polymer-stabilized GNS, the monolayer thickness is ~2 nm . Consider- ing the polymer grafting on to the GNS, the number of GNS layers in our case is in the range N ~ 6–8 support- ing the Raman spectra estimation of N ~ 8. GNS-PVA composites are prepared via the solution processing technique. It is assumed that with solution casting, the GNS are aligned in-plane inside nanocomposites. The final composites are in the form of thin films with approximate dimensions 3 × 3 cm. The thickness of the composite films is around 0.10–0.15 mm. These films are cut into strips (punched) for tensile testing. The dispersion of the GNS in nanocomposites is assessed via scanning electron microscopy of fractured surfaces. The samples were freeze-fractured in liquid nitrogen for the cross-sectional analysis. The micrograph shown in Fig. 3b predicts a homogeneous dispersion and no evidence of large aggregates at the maximum GNS loading (0.006 V f ) while Fig. 3a is the neat polymer.
The process of preparing graphene consisted of three steps, as shown schematically in Figure 1: (a) aqueous phase intercalation of natural graphite to produce TEA-GIC, (b) microwave irradiation to obtain EG, and (c) sonication of EG in organic solvents such as NMP, DMF and GBL to obtain graphene. The TEA-GIC was produced by tip sonication (855 W) of graphite in aqueous solution containing thionin acetate salt, sodium hydroxide and TEA. Thionin cations entered graphite galleries after the intercalation of TEA to stabilize TEA-GIC while sodium hydroxide provided hydroxide anions for subsequent elimination of TEA. The TEA-GIC was further microwave irradiated in air. The release of gaseous species induced by the decomposition of TEA led to expansion of graphite. After mild sonication in organic solvent, this EG could be readily exfoliated into dispersive graphene sheets. The single-cycle yield of graphene was 5% determined by weighing the residue after filtration of the graphene dispersion. This value is four times higher than that obtained by liquid-phase exfoliation of graphite in NMP for half an hour  and even higher than that obtained by treatment of graphite using the same method for 460 hours . The yield of graphene can be further improved with unexfoliated graphite recycled to repeat the above process.
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