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ACS Nano www.acsnano.org Article plasma gases, electricity charge, labor cost, etc.). Hence, this may be taken as reference. Although not comprehensive, we note that our production cost is lower than most of the commercially available single-layer or few-layer graphene. A rigorous comparison was drawn against the literature available for gauging our exfoliated graphene in terms of a few essential factors including number of layers, quality of graphene, yield, and production rate of the protocol (Supporting Information S19). This indicates that no work portrays production rates above 48 g/h coupled with ID/IG intensity ratios below 0.20 (overall). In reality, 80% of the manuscripts visited had production rates below 1 g/h, far below that required for commercial production. Our C/O ratio and sp2 are high enough to compete with high quality graphene. Another important metric to realize in graphene commerci- alization is reproducibility. We produced graphene using the same optimized parameter (P: 40 kW, G: 120 SCFH) in a total of five (5) batches (Supporting Information S20). All batches were independently inspected for structural integrity and selectivity toward the number of layers. Raman spectra of all five batches demonstrated intense G and 2D bands at ∼1580 and 2719 cm−1, respectively. A minor D band was also observed in all batches and is consistent with previous results. Thickness inspection using AFM shows that 75−85% of flakes are single layer. The superlative properties of graphene make it ideal for all- encompassing applications both as standalone and as an additive. To date, more than 40 application areas have been identified where graphene has the potential to make a significant commercial impact.1 Hence, in order to gauge the viability of our exfoliated graphene, we tested it in some applications that require high quality graphene in large quantities (details about the experiments have been discussed in Supporting Information S21). One major application area where graphene has already been utilized to its full spectrum is as strengthening additives, thanks to its exceptionally high Young’s modulus (∼1 TPa).43 We performed indentation using atomic force microscope to measure the Young’s modulus of the exfoliated graphene (experimental details in Supporting Information S21a). The solid lines in Figure 5a shows the respective force−indentation depth curves of single, bi, and trilayer exfoliated graphene performed at constant incremental load of 10 nN. For each layer, a total of 40 loading/unloading curves were collected and the distribution of derived moduli were plotted as histograms (Figure 5b). The single-layer exfoliated graphene shows an effective Young’s modulus of 850 ± 65 GPa, which reduces to 665 ± 55 and 550 ± 52 GPa for bi- and trilayer, respectively. Notably, the modulus of our exfoliated graphene is closer to the modulus of mechanically cleaved single layer graphene (1 TPa) measured using similar technique.43 However, it is much higher than that of the GO (∼207 GPa), which is an indicator of defect-free graphene.44 The bilayer and trilayer graphene show lower Young’s modulus compared to that of the single-layer graphene. This is due to the weaker interlayer interaction in adjacent graphene layers which slide relative to each other upon indentation.45 We performed macroscale lubricity test of the exfoliated graphene. Graphene deposited over Si wafer showed a very low coefficient of friction (COF = 0.03 ± 0.01) against the uncoated Si surface (0.42), while sliding with bare steel ball at 1 N loading (Figure 5c). Note that this ∼90% reduction in COF is encouraging, as a mere reduction of 20% friction can impact on cost in view of energy reserves and environmental benefits.46 The COF remains consistent even for higher load (Supporting Information S21b). Layer-wise electrical conductivity of the graphene was confirmed using a conductive AFM (cAFM). The linear I−V curve (Figure 5d) for an individual single, bi, and trilayer graphene is representative of its excellent conductivity.47 We also produced a thin film of the exfoliated graphene for potential transparent conductive coatings applications. Shown in Figure 5e is the digital image of the thin graphene film with thickness of ∼50 nm (Supporting Information S21d). The film is visually transparent with transmittance up to 73% at λ = 550 nm (Figure 5f). Note that this transmittance is little lower than commercially available ITO and FTO.48,49 However, we believe that higher transmittance could be achieved by optimizing the thickness of the film and its post-processing. The film also has low sheet resistance of ∼30 Ω/sq (Figure 5g), which is much lower than films prepared using graphene obtained from other exfoliation techniques.50 We performed electrochemical test to analyze its potential as energy storage devices like supercapacitors. The cyclic voltammetry (CV) curves of exfoliated graphene demonstrate near-perfect rectangular shape at different scan rate (Figure 5h), indicating pseudocapacitance behavior.51 The curve follows the pseudo- capacitive nature even at lower scan rate and is higher than that of graphite (Supporting Information S21e). The maximum specific capacitance obtained for exfoliated graphene is 375 F/ g at 5 mV/s, comparable to 3D graphene architecture used for energy storage devices.52,53 Furthermore, the galvanometric charge−discharge (GCD) curves at various current densities (Figure 5i) are highly symmetrical, indicating ideal capacitive behavior and excellent electrochemical reversibility. The specific capacitance obtained from GCD curves are com- parable to that obtained by CV at scan rate 5 mV/s. Thus, we anticipate that this work will results in ultrafast production of the high quality and defect-free graphene powder in kilograms that can be used in diversified applications. CONCLUSIONS We demonstrated an ultrafast strategy to exfoliate graphite into high quality graphene with ∼85% selectivity of single layer without the use of any intercalants, chemicals, or solvent. This was achieved by the instantaneous temperature surge and two- stage shear at laminar and turbulent region of plume using a technology that is well established and highly scalable. Raman and XPS characterization (ID/IG ≈ 0, C/O ≈ 21.2, sp2 %: ∼95%) reveals high quality graphene with minimalistic defects. Laboratory tests demonstrated ultrahigh production capacity (48 g/h) indicating that scale-up of continuous graphene synthesis can be achieved without losing yield or quality. On top of that, the quality of graphene remained same for different batches demonstrating the reproducibility of the process. This straightforward method has the potential to provide sub- kilogram scale low-cost graphene (our laboratory-scale production cost: USD $1.12 per gram) that holds great promise for a large number of applications such as reinforce- ments, superlubric coatings, energy devices, and transparent conducting films. We believe that this work could be a game changer in the production of pristine graphene in large scale for numerous applications. https://dx.doi.org/10.1021/acsnano.0c09451 1782 ACS Nano 2021, 15, 1775−1784PDF Image | Production of High Quality Exfoliated Graphene
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