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Continuous Production of Graphite Nanosheets by Bubbling Chemical Vapor Deposition Using Molten Copper Yongliang Tang,†,‡ Peng Peng,§ Shuangyue Wang,†,‡ Zhihong Liu,‡ Xiaotao Zu,*,† and Qingkai Yu*,‡ †

Institute of Fundamental and Frontier Sciences, University of Electronic Science and Technology of China, Chengdu 610054, P. R. China ‡ Ingram School of Engineering and MSEC, Texas State University, San Marcos, Texas 78666, United States § 2D Carbon Tech Inc. Ltd. and Jiangnan Graphene Institute, Changzhou, Jiangsu 213161, China S Supporting Information *

ABSTRACT: We report a bubbling chemical vapor deposition method for mass production of high-quality graphite nanosheets using molten copper as the catalyst for continuous growth. Bubbles containing precursor gas (CH4 or natural gas) are produced by inserting an aerator into molten copper. Highquality graphite nanosheets with a thickness ranging from a few to 40 graphitic layers are grown on bubble surfaces and carried to the copper surface. The production rate can be as high as 9.4 g/h using a crucible with a volume of 3 L. The high quality of the graphite nanosheets is demonstrated by composites with very high conductivity. The highly conductive composite shows excellent performance in an electromagnetic interference (EMI) shielding application with an EMI effectiveness of >70 dB at X band. Moreover, except for precursor gases, the lack of other chemicals in the growth process makes it an environmentally friendly approach. Natural gas can also be used as the precursor, making it a low-cost production. In addition, the naturally crumpled feature of the graphite nanosheets should allow them to ber used in multiple applications, because restacking can be prevented.

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INTRODUCTION The mass production of high-quality graphene represents one of the bottlenecks for the applications of the material.1−4 More than a decade after graphene was isolated from graphite, two approaches to manufacture it in large volumes have been developed. One approach is to use graphite powders as the starting material followed by oxidization, exfoliation, and reduction, specifically termed reduced graphene oxide (rGO).5−8 The advantage of this approach is the large volume of graphene produced (e.g., in tons). However, the graphene produced by this approach has a high defect density, which is significantly detrimental to its performance in applications. For example, compared to that of graphene produced by chemical vapor deposition (CVD), the conductivity of r-GO is several orders of magnitude lower, because of the high defect density.9,10 Recent progress on the microwave reduction graphene oxide (MWrGO) process reveals that high-quality graphene can be produced via microwave reduction.11 Because of the severe chemical reactions in the overall process, though, the quality of graphene from MWrGO (Raman ID/IG ∼ 0.1) is still worse than the quality of the graphene produced by CVD (Raman ID/IG ∼ 0). In addition, because of the high-volume usage of strong oxidants during oxidation, such as sulfur acid, potassium permanganate, and hydrogen peroxide, this approach has a significant impact on the environment.6,7 The other approach is CVD, which uses carbon-containing gases as its precursor.10,12−14 At high temperatures, graphene film forms on © 2017 American Chemical Society

the surface of metal catalysts, such as Cu and Ni. Although high-quality and large-area graphene can be produced (e.g., in hundreds of square meters), the volume of the graphene is still extremely low, which limits its application in many areas. In this research, we attempt to mass produce graphene powder by bubbling chemical vapor deposition (B-CVD). Using this novel approach, both the high quality and the large quantity of graphene (a few layers) and graphite nanosheets (GNs, >10 layers) are achieved simultaneously. The graphene and GNs have thicknesses in the range of a few to 40 layers. So far, we still cannot grow the whole batch of the product with 30 s to reach the equilibrium state for absorbing all chemicals. The changes in κ are dependent on the density of oils and organic solvents. To remove the oil and organic solvent absorbed, the saturated GNs were heated to the boiling points of oil and organic solvent. Thus, the oil and organic solvent can be thermally evaporated and collected with a condensing unit.

the precursor gas can no longer access a fresh metal surface, so no new GNs form.10,13,15 In our strategy, as shown in Figure 1a, carbon-containing gas is fed into molten catalytic metal, where the gas bubbles rise and break at the top surface. Both the surface of bubbles inside the molten metal (process 1) and the top surface of the whole body of the molten metal (process 2) could be the sites for GN growth. With bubbles continuously taking form, the fresh metal surface can be generated for the formation of new GNs. Therefore, a continuous growth process can be applied. The as-grown GNs can be carried by gas flow and collected at the exhausted end of the system (Movie S1). In our typical growth, 1.5 L of copper is melted in a graphite crucible heated by an induction furnace (see details in the Experimental Section). When the temperature reaches 1450 °C, an aerator is inserted into the molten copper and methane with carrier gas nitrogen is fed through the aerator. The production rate can be as high as 9.4 g/h, and the efficiency of conversion of methane to GNs can be as high as 58%. Figure 1b shows ∼3 g of GNs collected at the exhausted end after growth for 20 min. In this research, natural gas also was used to replace methane (recipes C11−C13 in Table S2), and the growth results are similar to those of corresponding recipes using pure methane (recipes C6, C9, and C10 in Table S1). As shown by the optical microscopy (OM) and scanning electron microscopy (SEM) images in panels c and d of Figure 1, the GNs are large and highly crumpled. The size of the crumpled sheets is in the range of tens to hundreds of micrometers. A Raman investigation (Figure 1e) confirmed the high quality of the GNs,16,17 in which the ratio of ID and IG is ∼0.2 [the ID is mainly due to the crumpled feature, not real defects; the ID/IG of the flat GNs on the copper surface is 1250 °C (see further analysis in Sections S1 and S3)]. Copper nanoparticles can be found on the GNs (Figure S4a), which can be attributed to the splashing of molten copper driven by the breakage of bubbles and the vaporization of the copper at high temperatures and can be efficiently removed (Figure S4b,c). The TEM images in

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RESULTS AND DISCUSSION The incumbent CVD approach cannot produce large-volume GNs because after a graphitic layer fully covers a metal catalyst, 8406

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Figure 3. Evolution of GNs on a copper surface with growth time. (a−g) SEM images of GNs on copper surfaces obtained over different growth time (from 5 s to 1 h). (a−c and e−g) Copper surfaces after growth for 5 s, 10 s, 15 s, 20 s, 1 min, and 1 h, respectively. (d) Enlarged image of the area boxed in red in panel c. The white arrows indicate the standing wrinkles, and the black arrows indicate the folded wrinkles. Scale bars are 3 μm.

experimental results (Figure 2c) showed that (1) the wettability of an orifice had little influence on the growth of GNs, (2) the aerator with more orifices could produce more GNs, and (3) the diameter of an orifice has little influence on growth. Two types of surfaces, the surface of bubbles inside the molten copper and the outer surface of the whole body of the molten copper, could be the sites for GN growth. GN growth at these two types of surfaces represents two different growth modes. In the first mode, GNs could form at the bubble surface during the bubble rising and the as-grown GNs can be carried by the bubble to the top surface of the whole body of molten copper (here named the inner mode). After the bubble bursts at the top surface of copper, the GNs are left on the top copper surface. In the second mode, the bubble surface is just a site for precursor dissociation and diffusion of carbon into the copper body. Beyond the melting temperature, the diffusion of carbon in copper can greatly enhance the process25 (e.g., the magnitude of the diffusion coefficient of carbon in metals upon melting is 1−2 orders higher than that in solid metals). Because of the low energy at the surface, carbon could segregate at the top surface of the copper body (here named the top mode). In both modes, the formed GNs could then be separated from the surface of the copper body by gas bubbles and, subsequently, broken into smaller pieces and blown into the B-CVD chamber. Both the inner mode and the top mode should play a role in the growth of GNs, and clarifying the dominating mode definitely would be necessary for optimizing the growth for future progress. To elucidate the growth modes suggested above, we designed the following experiments. A graphite board was used to separate the crucible containing the molten copper into two parts. The board was drilled with an array of grids (Figure S2), through which the dissolved carbon atoms can be transported by diffusion. The bubbles would not, however, go through the holes on the board from one side to the other. As the scheme shows in panels a and b of Figure 4, in the first design, precursor gas was fed into the molten copper on the left side, but no gas was fed on the right. During growth, we collected many crumpled GNs on the left side. However, no

Figure 1f−h show that the product consists of a few graphitic layers. The temperature (T) and methane concentration (Cp) are the two most influential parameters for the growth of GNs by conventional CVD.13,14,18,19 In our B-CVD growth process, the influence of these two parameters was investigated through nine growth recipes (recipes C1−C9 in Table S1). As shown in Figure 2a, both the production rate (RP) and the methane transfer efficiency (Xe) significantly increase with T and reach 3.2 g/h and 50%, respectively, using recipe C9 (T = 1350 °C, CP = 2%, and a total flow rate of 10 SLM). The significant increase in RP and Xe with T should be attributed to the higher decomposition rate of methane at higher temperatures (the decomposition of methane on Cu is highly endothermic).20−22 With an increase in CP, RP increases but Xe decreases slightly. This coincides with the result obtained via thermal decomposition of methane on metal catalysts, where the reaction order is ∼1.23,24 With further increases in Cp and T, RP can be as high as 9.4 g/h (recipe C10, T = 1450 °C, Cp = 5%, and a total flow rate of 10 SLM). The production of GNs in our system is continuous and stable over an 8 h growth period (Figure 2b). The quality [thickness (Tp), domain length (Dl), and defect density (ID/IG) (Section S1)] of B-CVD GNs is also affected by T and Cp, as summarized in Figure 2d−f. A higher T would lead to larger graphitic domains and fewer defects, but thicker GNs, while a higher Cp would lead to lower-quality GNs. The precursor gases go through the aerator and form bubbles in the molten copper. The surface of the bubbles containing precursor gases should act as catalytic sites for CH 4 decomposition, carbon diffusion, and GN formation. Therefore, the surface area of the bubbles may be a critical parameter. The influence of the surface area of gas bubbles on GN growth was investigated by controlling the dimension of the bubbles, which was mainly attributed to the gas flow rate at orifices, the wettability between the aerator and molten copper, and the diameter of orifices. To investigate their influence in our system, five experiments (Section S2) with five different aerators (Figure S3) were conducted and compared. The 8407

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Figure 4. Analysis of the growth mechanism. (a and b) Schematic diagram of B-CVD growth with a separated crucible. (a) Precursor gas was fed into molten copper at the left side, but no gas was fed at the right side. (b) Precursor gas was fed into molten copper at the left side, and N2 was fed at the right side. (c and d) SEM and OM images of graphitic layers grown on the copper surface at left side (c) and right side (d) in panel a. (e and f) SEM images of graphitic layers grown on the copper surface at left side (e) and right side (f) in panel b. (g) Proposed growth mechanism of GNs in the B-CVD growth system. The scale bars in panels c, e, and f are 15 μm, and that in panel d is 20 μm.

GNs were obtained on the right side, but a thick graphite film was. In addition, it was found that the weight of those GNs from the left side is much higher than that of graphite film from the right side (Table S6). Combining this result with the results of other experiments (Section S4), we conclude that (1) the methane from the aerator is the main carbon source, (2) the methane decomposes at the surface of bubbles inside the molten copper, (3) at our growth temperature, carbon atoms can diffuse through the grids, and (4) the inner mode dominates the growth. In the second design as shown in Figure 4b, we conducted an experiment similar to the first design, except a pipe was added to the right side for feeding nitrogen. Compared to the results for design 1, a similar result can be obtained except for the crumpled GNs can be obtained on the right side. With the result from the second design, we can conclude that another function of the bubbles (whether they are precursor gas or N2 bubbles) is to separate as-formed GNs on the top surface of copper and push the crumpled GNs. As shown in the SEM and TEM images (Figures 1d,f and 3c,e,f), the GNs from B-CVD are highly crumpled. The crumpled morphology can be maintained even after solution processing with vacuum filtration. It is believed that the crumpled feature can prevent GNs from restacking and provide

benefits for many applications of GNs, such as composite materials and energy storage.2,26,27 We investigated the formation of the crumpled GNs with serial growth by varying the growth time. The SEM (Figure 3) and OM (Figure S19) results for the copper surface clearly show the evolution of GNs from flat domains to a crumpled film because of the agitation of the copper surface by bursting bubbles, and then the crumpled GNs as a three-dimensional structure piles up. The corresponding Raman results (Figure S19g−j) also reflect the evolution of morphology.28−33 When the GNs evolved from flat to crumpled, the Raman D band became stronger and the 2D band became symmetrical, and a new band (L band) centered at 120 cm−1 appeared; its intensity increased with the degree of crumple. The stronger D band can be attributed to the higher curvature of folded (crumpled) GNs, which can generate double-resonance D band scattering. Through the observation of copper surfaces (Figure S5) and Raman results (Figure S12− S14) with nine growth recipes after growth for 1 h, we found both flat and crumpled features present on the copper surfaces for all recipes. This result indicates GNs locally evolve from flat to crumpled at different locations and paces. On the basis of the results and discussion given above, the growth process can be depicted as having six stages (Figure 4g). 8408

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Figure 5. Application of B-CVD GNs. (a) Pristine PU (left) and GN/PU composite (right). (b) SEM picture of the GN/PU composite. The scale bar is 2.5 μm. (c) EMI shielding property of the GN/PU composite. (d) A glove covered with the GN/PU composite. (e) Heating property of the glove in panel d under a 9 V source. (f−h) Oil absorbing properties of B-CVD GNs using recipes C5 and C9.

panels d and e of Figure 5. Such heating elements can greatly enhance the delicate operation capability in a cold environment (e.g., for military staff performing missions during the winter). Because of the naturally crumpled three-dimensional structure, GNs are ideal candidates for the absorption of oils and other organic pollutants.36 As depicted in Figure 5f, when a small amount of GNs was placed on lubricating oil (stained with Sudan 3) floating on artificial ocean water, the lubricating oil was immediately absorbed by the GNs and completely disappeared within 20 s, suggesting an effective way to clean spilled oil. The B-CVD GNs exhibited excellent absorption efficiency for various oils and organic solvents, resulting in the capacity ranging from 85 to 165 times its weight (Figure 5g), which is better than the best reported results for GNs and graphene powder and comparable to the absorption capability of most graphene/CNT foam, sponge, and aerogel (Table S8). In addition, absorbed oils and organic solvents could simply be removed by heating absorbed samples above the boiling point. Using this simple sorption heating method, the GNs could be reused multiple times without significant degradation in absorption capacity within the times we tested (Figure 5h).

In stage 1, the bubbles containing precursor gas form through the aerator in molten Cu. In stage 2, the precursor gas dissociates into carbon atoms and hydrogen atoms and carbon atoms will diffuse on the bubble surface and dissolve into molten Cu. In stage 3, flat GN domains initially take form on the bubble surface and then are carried to the top Cu surface by bubbles. In stage 4, flat GN domains connect with each other on the top Cu surface to form large films, which will be pushed wrinkled and folded by the agitation of the molten Cu surface driven by bubbles. In stage 5, the crumpled GNs finally pile up and are blown away from the Cu surface by the gas flow (for stages 3−5, if the top surface of copper is static, GNs grown on the copper surface will not be pushed crumpled and will continue to grow thick to form graphite). In stage 6, a new growth process consisting of stages 1−5 continues for the next production cycle once a carbon source is available. Consequently, we can continuously obtain GNs collected by a filter. The quality of GNs produced by B-CVD is expected to be significantly higher than that produced by the modified Hummers method or the liquid-phase exfoliation method. To test the conductivity, an indicator of the quality of GNs, GN/ PU composites (Figure 5a,b) were fabricated. The loading level of GNs in PU can reach 20 wt % (7 vol %) with well-dispersed GNs. At this loading level, the volume electrical resistivity of this composite can be as low as 0.04 Ω cm, which is more than 1 order of magnitude lower than that from previous research using carbon as a conductive filler and comparable to that of the commercial conductive polymer using metal particles as fillers (Table S7). Because of its high electrical conductivity, this composite has potential application in EMI shielding.34,35 For a film of this composite with a thickness of 1 mm, the EMI shielding effectiveness (EMI SE) was >70 dB at X band [8−12 GHz (Figure 5c)], far surpassing that of any other conductive polymer with carbon fillers, and comparable to that of the pure metallic shielding material and metal-filled polymers (Table S7) but much lighter. We also demonstrated the application of the GN/PU composite in wearable devices by using electrically heated gloves. A 9 V battery can drive the glove with a temperature that is 30 °C higher than ambient, as shown in

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CONCLUSIONS In summary, we report a bubbling chemical vapor deposition (B-CVD) method for mass production of high-quality graphite nanosheets (GNs) using molten copper as the catalyst for a continuous growth. The production rate can be as high as 9.4 g/h. Moreover, the extremely low level of chemicals in the process makes it an environmentally friendly approach. Natural gas can also be used as the precursor, making it a low-cost production. A conductive polymer filled with the B-CVD GNs shows an electrical conductivity of 20 S/cm and an EMI SE as high as 70 dB at X band. In addition, the naturally crumpled feature of the GNs should allow it to be used in multiple applications, because restacking can be prevented. This approach may also be applied to fabricate other twodimensional materials in powders by selecting appropriate precursors and catalysts. 8409

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b02958. Expanded discussions and additional data (PDF) GNs floating in the collector during growth (Movie S1) and bubbles produced by orifices with radii of 0.17, 0.17, 0.34, and 0.17 mm at gas flow rates of 400, 600, 600, and 800 SCCM, respectively, in the N2−water−Teflon system (Movies S2−S5, respectively) (ZIP)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Telephone: +86 28 83201896. Fax: +86 28 83201896. *E-mail: [email protected]. Telephone: +1 512 2451826. Fax: +1 512 2457771. ORCID

Qingkai Yu: 0000-0002-1859-1895 Author Contributions

Q.Y. designed the project. Z.L. set up the apparatus for growth. Y.T. grew the GNs, characterized their microstructures, simulated bubbles in water, and tested the EM shielding and absorption of organic substances. P.P. made the GN/PU composite. Q.Y. and Y.T. wrote the manuscript. All authors participated in the discussion of the results. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Q.Y. is thankful for the start-up fund at Texas State University and discussion with Dr. Qinghong Yuan on the thickness control of graphene. X.Z. thanks the National Natural Science Foundation of China (61178018) and the NSAF Joint Foundation of China (U1630126 and U1230124).

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