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ACS Nano www.acsnano.org Article distinguishes the number of layers in graphene is the shape, size, and intensity of 2D peak. As an example, a single-layer graphene displays a sharp and symmetric 2D peak, while multilayers with Bernal (AB) stacking shows complex 2D shape fitted by multiple Lorentzian lines.33 The Raman spectrum of the monolayer graphene more closely resembles that of liquid exfoliated graphene rather than broad and highly disordered spectrum of thermally reduced rGO.20,21 However, as the Raman beam spot size (<2 μm) is smaller than most of the exfoliated graphene, it is safe to assume that the D peak might be dominated by the edge effect. Yet, we cannot completely rule out any contribution from basal plane defects induced during the exfoliation process. We therefore performed Raman mapping and in situ AFM of the graphene flakes. Raman map of G and D band (Figure 3b,c) indicates higher defect density along the edges of the graphene. In situ Raman analysis of the same flake demonstrated no defect in Figure 4a,b illustrates the temperature and velocity of the graphite particle captured by the in-flight particle diagnostic sensor. With an increasing plasma power from 10 to 40 kW and primary gas flow rate from 80 to 140 SCFH, both temperature and velocity of graphite particle increases. Now, referring the schematic in Figure 4c and magnified view in Figure 4d, as soon as the graphite particles are injected into the laminar region of hot plasma, it experiences thermal shock at high temperature (3430 °C). Note that at such high temperature, any damage in graphite was prevented by the use of an inert argon gas shroud and the short residence time (in μs) of graphite particle in hot plasma due to the technique’s high cooling rate 106 K/s.35 In addition, the heat transfer to the graphite particle could have an important role in preventing the particle from damage. Assuming the boundary condition of the graphite powder to be symmetric, the heat transfer within the particle could be approximated by the conduction equation (eq 1). the midportion of the flake, while an intense D band appears at ∂H 1∂i ∂Ty38 jz the edge (Figure 3d,e). This suggests that D band measured here is associated with the flake edges. We note that the I /I DG k2{ ratio along the edge (∼0.2) is similar to that of LPEG and significantly lower than that of ball milled graphene, indicating the defect-free nature of our exfoliated graphene.21−23 It is crucial to ascertain the quality of graphene after exfoliation. XPS was used to characterize the surface chemistry of our exfoliated graphene. The XPS results (Figure 3f) indicate that exfoliated graphene consists of in-plane oxygen concentration of ∼4.5 at. %, which is lower than 6.1 at. % of noncovalently bonded adsorbed oxygen in graphite. The atomic ratio of carbon and oxygen (C/O) is ∼21.2, which is higher than previously reported value of GO, rGO, and EcG and is close to LPEG and CVD grown graphene.14,18,34 Increase in the C/O ratio is an indication of removal of some oxygen containing functional groups during plasma spraying. The high resolution of the C 1s peak of exfoliated graphene can be fitted into three peaks at 284.6, 287.1, and 290.9 eV. The first two correspond to the CC and CO bonds, respectively (Figure 3g).17,28,35 The 290.9 eV peak related to the π−π* transition is clearly visible after exfoliation, indicating that the conjugated aromatic structure is preserved after 36 where r is the radial distance from the center of the particle. H is the enthalpy of the graphite particle, having a thermal conductivity of Kp, and T is the temperature at the given radius. Hence, for a graphite particle with similar dimension, heat transfer will be more at higher temperature, which will certainly avoid the localized heating and further prevent any damage of the graphite. The thermal shock will lead to an increase in graphitic interlayer spacing. The graphitic interlayer spacing (d) follows a nonlinear dependency with temperature (t) up to 2600 °C has already been demonstrated, as per the equation.39 −6 −9 2 (2) d=3.357+91.9×10 t+5.3×10 t Since van der Waals attraction force is inversely proportional to r6 (r is the distance between the molecules),40 increased interlayer spacing upon thermal shock will lower the van der Waals force and results to that of a weakened graphite particle. In parallel to thermal shock, weakened graphite particles also experience a shear force (stage-2) due to the combination of viscosity and velocity of the plume simultaneously in the laminar region.41 By increasing the primary gas flow rate to 120 SCFH, the plume velocity increases, and subsequently, the velocity of the graphite particle reaches 350 m/s. Conversely, the viscosity of the plasma gas exponentially increases at higher temperature, and it will decelerate the forward moving graphite particle (for details, refer to Supporting Information, S15). Hence, this opposing force could help in shearing the graphitic layer and aid the exfoliation of the expanded graphite layers. The already weakened and partially sheared graphite particle transcends downstream toward the turbulent region (Figure 4c,e) where it encounters the large-scale eddies of cold ambient gas (i.e., shroud argon gas). Since these entrained cold eddies will have the higher density compare to their high temperature counterparts, entrapped graphite particle will experience greater inertia.41 Consequently, the graphite particle will try to travel along the axial direction at much lower velocity, while the hot plasma plume travels with very high velocity due to the higher primary gas flow rate (120 SCFH). This is exactly the competing phenomena where the already weakened and initially sheared graphite particle again encounters strong shear between the graphitic layers resulting the autoexfoliation of graphene sheet. https://dx.doi.org/10.1021/acsnano.0c09451 exposure to very high temperature. Additionally, FTIR revealed nearly featureless spectra for exfoliated graphene which is in contrast to the spectra of GO (Supporting Information S14).37 This indicates that we produce graphene rather than some form of derivatized graphene. Figure 3h compares our exfoliated graphene with graphene obtained from various methods/sources. Comparison is made based on two important factors (carbon content, C and % of sp2) which define the quality of graphene. We observe that with C = 95.5% and sp2 % = 95% the quality of our exfoliated graphene is much higher than that of GO and rGO and are comparable with that of EcG, LEG, and CVD graphene. We then try to explore the mechanism which could have aided the exfoliation of graphite to graphene. The entire exfoliation step can mainly be divided into three stages: thermal shock of graphite by hot plasma (stage-1), and two- stage shearing of the graphite particle in laminar (stage-2) as well as in turbulent region (stage-3) of the plasma plume. Since the thermal and kinetic history of the in-flight particle has an important role in thermal shock and subsequent shearing of graphite particle, the temperature and velocity of the sprayed graphite are scrutinized with respect to exfoliation efficiency. 1780 = 2 jKpr z ∂t r∂r ∂r (1) ACS Nano 2021, 15, 1775−1784PDF Image | Production of High Quality Exfoliated Graphene
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