Open AccessCCS ChemistryMINI REVIEW1 Apr 2021Strategies for Scalable Gas-Phase Preparation of Free-Standing Graphene Yangyong Sun and Jin Zhang Yangyong Sun Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 Beijing Graphene Institute, Beijing 100095 Google Scholar More articles by this author and Jin Zhang *Corresponding author: E-mail Address: [email protected] Center for Nanochemistry, Beijing Science and Engineering Center for Nanocarbons, Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871 Beijing Graphene Institute, Beijing 100095 Google Scholar More articles by this author https://doi.org/10.31635/ccschem.020.202000289 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail The preparation of graphene with high quality serves as a prerequisite for its usage. Traditional methods of graphene production, represented by liquid-phase exfoliation and chemical vapor deposition, either sacrifice the quality and purity of graphene or are limited by the substrate and catalyst. Developing simple and scalable preparation methods of high-quality and high-purity graphene remains a big challenge. Herein, we have reviewed the gas-phase methods including carbonization, combustion, arc discharge, and atmospheric plasma for scalable preparation of free-standing graphene, which are catalyst-, substrate-, solvent-free, and without the need for complex post-treatment methods. The obtained graphene exhibits characteristics of high quality and high purity. Moreover, applications of free-standing graphene were also summarized. Finally, perspectives on opportunities and challenges of free-standing graphene have been discussed. Download figure Download PowerPoint Introduction As an allotrope of carbon, intrinsic graphene has unparalleled electrical,1 thermal,2 mechanical properties3 and good biocompatibility,4 enabling wide applicability of graphene in the fields of high-performance sensors,5 energy storage,6 heat dissipation,7 mechanical enhancement8, biomedical applications,9 and so on.10,11 The upstream preparation of the raw materials of graphene directly determines the downstream applications. Therefore, the first and foremost target is to realize the scalable preparation of high-quality and high-purity graphene.12,13 Since the initial preparation of graphene by mechanical exfoliation in 2004,1 numerous technologies have been developed,14–16 which can be roughly divided into two approaches. Top-down methods, such as Hummers’ method and liquid-phase exfoliation, are based on the exfoliation from graphite to graphene powder.15,17,18 This types of methods can greatly improve the yield but sacrifice graphene quality with significant contamination and defects, which puts forward higher requirements for the posttreatment of graphene. On the contrary, a bottom-up strategy represented by catalyst-assisted chemical vapor deposition realizes graphene synthesis via the assembly of carbon atoms.19,20 Such methods obtain high-quality graphene films through complex parameter adjustment, but the transfer of graphene from catalysts or substrates has always been one of the bottlenecks.12 It seems that these two types of technologies have their own pros and cons. Is there a possibility that we can achieve scalable production of high-quality and high-purity graphene but get rid of the dependence of catalysts or substrates at the same time? It has been inferred that to obtain high-quality graphene, the bottom-up growth strategy should be adopted, but the introduction of substrate or catalyst is undesirable. Therefore, graphene should be nucleated in the gas phase, where high temperature is required. Hence, if the temperature is high enough to allow graphene to nucleate in the gas phase, then it is likely to achieve graphene growth in the gas phase, which should meet the previous requirements. Herein, we have summarized the current scalable gas-phase preparation methods of graphene (Figure 1), including carbonization, combustion, arc discharge, and atmospheric plasma. In arc discharge methods, some related parameters, such as type of buffer gas, temperature, and pressure, have been elaborated. Corona discharge, radio frequency-induced thermal plasma, and surface wave-induced microwave plasma have been emphasized in the atmospheric plasma method. Based on the obtained free-standing graphene, we have also reviewed its related applications. Finally, the preparation and applications of free-standing graphene are prospected. Figure 1 | Preparation of free-standing graphene in the gas phase and related applications. Reproduced with permission from Ref. 21. Copyright 2018, Wiley-VCH. Reproduced with permission from Ref. 22. Copyright 2020, Wiley-VCH. Download figure Download PowerPoint Scalable gas-phase preparation of free-standing graphene Gas-phase preparation of free-standing graphene is a catalyst- and substrate-free way to synthesize graphene without the introduction of solvent. The four associated methods based on carbonization, combustion, arc discharge, and atmospheric plasma are reviewed here. Carbonization Carbon-containing precursors can be transformed into graphene when dehydration and carbonization are conducted at high temperature in the absence of air. Both graphene films and graphene powder have been synthesized in this way. It should be mentioned that the mechanisms vary depending on different carbonization processes. As for graphene films, in addition to being synthesized on the substrate surface,19,20 they can also be fabricated in a substrate-free gas phase, which was reported by Liu et al.23 By heating the volatile camphor at 180 °C, followed by carbonization at a relatively high temperature (800–850 °C) in a furnace, graphene films were deposited on the quartz tube wall, which were easily peeled off (Figure 2a). The graphene films exhibit a regular shape with inner angles of adjacent edges approaching 120° or 105° (Figure 2b). It was speculated that hexagonal or pentagonal carbon rings were formed during pyrolysis of the camphor molecule, and then the carbon rings were cross-linked with each other and stacked in the [0001] direction. As a new method for growing graphene films, the layers and quality of the graphene need to be improved further. Figure 2 | Production of graphene by carbonization and combustion. (a and b) Schematic diagram of gas-phase preparation of graphene films and the corresponding SEM image. Reproduced with permission from Ref. 23. Copyright 2010, Elsevier Ltd. (c) Growth process for strutted graphene by sugar-blowing. Reproduced with permission from Ref. 24. Copyright 2013, Macmillan Publishers Ltd. (d and e) Schematic diagram of flash Joule heating process and Raman spectra of graphene. Reproduced with permission from Ref. 27. Copyright 2020, Springer Nature Ltd. (f–h) Basic characterizations of graphene by combustion of the mixture of C2H2 and O2. Reproduced with permission from Ref. 32. Copyright 2013, IOP Publishing Ltd. Download figure Download PowerPoint Different from the growth of thin films, carbonization is often used to prepare graphene powder. A method of sugar-blowing was developed to prepare three-dimensionally strutted graphene, which was reported by Wang et al.24 They introduced volatile salts to make pores via pyrolysis. Similar to fermentation, the decomposition of NH4Cl at a relatively low temperature of 250 °C creates bubbles. Then glucose is pyrolyzed at a very high temperature (up to 1350 °C) to obtain 1–6 layers of strutted graphene (Figure 2c). Raman spectra indicates that this graphene exhibits ID/IG>1 and I2D/IG∼0.9. In addition to glucose, polymers,25 coal,26 and so on can also be used to produce graphene. Zhang et al. reported a catalytic pyrolysis to obtain high-quality graphene powder. They used FeCl3 as a catalyst in the process of pyrolyzing glucose and found that the quality of the graphene was improved with increasing carbonization temperature. The addition of FeCl3 also significantly improved the quality of graphene as can be seen that ID/IG was reduced from ∼1.1 to ∼0.4, while I2D/IG was increased from ∼0.1 to ∼0.7, but it is inevitable that the introduction of heterogeneous elements will reduce graphene purity, and, therefore, post-treatments are often necessary. High temperature makes it easier to regularize the arrangement of carbon atoms, that is, to improve crystallinity. Very recently, Luong et al.27 proposed the use of Joule heating of inexpensive carbon sources to obtain graphene in less than 1 s. This graphene is called flash graphene (FG). They have calculated that the energy consumption per gram of FG is only 7.2 kJ. As shown in Figure 2d, an extremely high temperature of greater than 3000 K was realized within 100 ms, causing an I2D/IG as high as 17 to appear in some regions (Figure 2e). Additionally, the temperature generated by this Joule heat can be adjusted by changing the applied voltage, directly affecting the quality of FG. Simulations reveal that temperature has a great impact on the formation process of graphene, which is greatly accelerated at higher temperature (even 5000 K). This method provides further possibilities for waste recycling, but the obtained FG is often small in size, ranging from 5 to 30 nm. Carbonization of carbon sources can easily achieve scalable production of graphene powder, and gram-level graphene can be obtained in one batch from various solid or liquid carbon sources, which is an obvious advantage of this method. The quality of graphene is much related to the pyrolysis temperature. Generally speaking, long-term high-temperature carbonization will polish graphene well, but usually requires great energy consumption by traditional heating methods. Developing new types of carbonization methods, such as Joule heat, will be critical for scalable production of high-quality graphene. Combustion Combustion is a self-propagating, high-temperature synthesis technology.28 Generally, solid graphene is obtained by either incomplete combustion of carbon sources with oxygen or reactions of active metals with carbon-containing oxides where the enthalpy change is less than 0, therefore the system can release heat to provide high temperature. Such high temperature may contribute to the formation of free radicals during the combustion of carbon-containing species, which helps the formation of aromatic molecules and ultimately promotes graphene preparation.29 The liberated heat can be the driving force for spontaneous propagation in the form of a combustion wave,30,31 enabling a quick reaction, so that a large amount of graphene can be synthesized in a short time. Nepal et al.32 reported a way to prepare graphene by combustion of the mixture of C2H2 and O2. It was found that combustion of different molar ratios of O2/C2H2 generated high temperatures, bringing about diverse effects on the obtained graphene (Figures 2f–2h). In particular, a molar ratio of O2/C2H2 of 0.6 reached 4200 ± 200 K in the system. Based on this method, a production rate of 300 g/h of graphene was achieved. Another simpler method for preparing graphene by combustion was studied by Chakrabarti et al.,33 who reacted Mg with CO2 [2Mg(s) + CO2(g) → 2MgO(s) + C(s)]. Characterizations, such as Raman spectroscopy and transmission electron microscopy (TEM), also indicated that the materials were graphene, but the by-products, such as MgO, must be removed with acid at the same time and, typically, oxygen impurities remain in the graphene. Li et al.31 also prepared graphene with the same strategy, but used electricity in the reaction chamber to provide the initial thermal stimulus. Low oxygen content of ∼1.2% and high specific surface area (SSA) of ∼709 m2/g of graphene were realized in a few seconds but was accompanied with a low I2D/IG of 0.22. Other metals, such Ca and Li, can also react with CO2 to produce graphene with similar parameters.34,35 Combustion is an economical and eco-friendly way to produce graphene. Compared with carbonization, the temperature of combustion is higher, where the latter is more conducive to topological defect removal for the regularization of the graphene structure. However, depending on the type of combustion, oxidants (such as O2) or active metals (such as Mg) will participate in the graphene preparation, resulting in a certain amount of oxygen contained in the product or the necessary posttreatment process. Arc discharge Arc discharge has played an important role in the discovery of carbon nanomaterials. Fullerenes,36 carbon nanotubes,37 carbon nanohorns,38 and so on were originally prepared using this method. In 2009, Subrahmanyam et al.39 reported the idea for preparing graphene by arc discharge for the first time. In this section, we will discuss some important parameters and analyze their respective effects during the synthesis of graphene by arc discharge. Equipment and growth mechanism A general setup of arc discharge for fabrication of graphene is shown in Figure 3a, where the system is maintained at low pressure and filled with buffer gases.40 Two carbon rods, serving as electrodes and carbon sources, are used to generate arcs when direct current (DC) or alternating current (AC) are applied. To accelerate the heat dissipation, the steel chamber is always cooled by water. Graphene powder is deposited on the surface of the electrodes or chamber. Figure 3 | Schematic diagram of equipment and growth mechanism of arc discharge. (a) Schematic diagram of conventional equipment of arc discharge. Reproduced with permission from Ref. 40. Copyright 2010, Elsevier Ltd. (b) Schematic diagram of equipment of arc discharge using other feedstock gases, such as CH4, C2H4, and C2H2, as carbon sources. Reproduced with permission from Ref. 41. Copyright 2019, Elsevier Ltd. (c–e) Schematic diagram of the growth mechanism of arc discharge. Reproduced with permission from Ref. 47. Copyright 2016, Royal Society of Chemistry. Download figure Download PowerPoint Other feedstock gases, such as methane (CH4), ethylene (C2H4), and acetylene (C2H2), can also be used to prepare graphene under arc discharge, in which another setup shown in Figure 3b is employed.41 Gases are inputted from the outside, and the graphene is synthesized through the arc generated by the hollow cylindrical anode and rod cathode, which are made of graphite as well. There are two possible reasons why graphene can be obtained by arc discharge. Generally speaking, the gasification temperature of graphite is above 4000 K, much lower than the temperature in the core region of the arc, which usually reaches above 5000 K42 or even 10,000 K.43 Such a high temperature will cause the graphite on the electrode to be gasified to volatile carbon atoms. After the temperature in the near region is slightly lowered, carbon atoms will combine with each other to form carbon fragments, such as C2, C3, and so on, followed by nucleation.44 Pristavita et al.45 reported that the nucleation temperature of graphene is 3000–5000 K, while 2500 K is obtained by Chen et al.46 through dynamic simulation. When the temperature further away from the central area of the arc is lower, graphene will continue to grow under the attack of carbon fragments and will form graphene flakes in the end. Besides, high temperature is an important factor affecting the growth of graphene. Li et al.47 believed that arc force is another important factor, which acts as the driving force for sending buffer gas molecules into the arc area and promoting the movement of ionized species to ambient space (Figures 3c–3e). Apart from this mechanism, it is widely accepted that the exfoliation of graphite electrodes during the arc discharge process is another possible mechanism that contributes to graphene deposition on the wall of the chamber.48 Type of buffer gas Buffer gases in the arc discharge have a certain effect on the quality, number of layers of graphene, or whether graphene can be synthesized. In the process of fabricating fullerenes by arc discharge, helium (He) is often used as a buffer gas,36 but is not suitable for graphene alone. Subrahmanyam et al.39 used the mixture of H2 and He during DC arc discharge and found that graphene flakes with 2–4 layers on the inner wall of the chamber were formed, which were ascribed to the effects of terminating the dangling bonds and preventing the formation of closed structures of graphene by H2. Shen et al.49 conducted further research on the effect of H2 in DC arc discharge. They explored different buffer gases, such as H2, N2, air, He, H2–He, H2–N2, and H2–N2–He when maintaining the same pressure. It seemed that the effect on graphene was found to be the same in the cases of pure He, N2, or air, where only carbon nanospheres and caky graphite flakes were found. However, graphene sheets could only be obtained in the presence of H2. Further, they found that graphene exhibited smaller ID/IG (∼0.31) in a H2–N2 atmosphere than H2–He and H2–N2–He, indicating that graphene had the best crystallinity in this case. Similar results were also reported by Chen et al.,50 who found that graphene in the H2–N2 atmosphere showed higher thermal stability than in H2–Ar and H2–He. Zhang et al.51 determined the effects of H2 by comparing it with Ar and N2 through experimentation (Figures 4a–4e). They believed that C–X clusters (X is a carbon atom or heteroatom) were important during the formation of graphene. Figure 4a shows three possible mechanisms where carbon atoms cannot bond with Ar, thus random bonds between carbon clusters are formed into amorphous carbon in Ar (Figure 4b), while carbon nanohorns appear in N2 because of the formation of C–N bonds causing the graphite sheet to bend (Figure 4c), and only graphene sheets were obtained by eliminating the dangling bonds in H2 (Figure 4d and 4e). Figure 4 | Influencing factors in arc discharge. (a–e) Effects of buffer gases and the corresponding TEM images of obtained carbon nanomaterials in Ar (b), N2 (c), and H2 (d–e) during arc discharge. Reproduced with permission from Ref. 51. Copyright 2018, Elsevier Ltd. (f and g) Effects of cooling rate on graphene during arc discharge. Reproduced with permission from Ref. 54. Copyright 2013, Elsevier Ltd. (h) Effects of pressure on graphene in arc discharge. Reproduced with permission from Ref. 51. Copyright 2018, Elsevier Ltd. (i) Effects of magnetic field on graphene in arc discharge. Reproduced with permission from Ref. 41. Copyright 2019, Elsevier Ltd. Download figure Download PowerPoint Chen et al.46 explored the role of H2 in preparing graphene using other gases instead of graphite rods as carbon sources during arc discharge. The setup is shown in Figure 3b, where they chose C2H2 as the carbon source with the assistance of Ar and H2. By controlling different ratios of H/C, they found that when increasing H content, the proportion of carbon nanospheres continued to decrease while graphene nanoflakes increased, which shows a similar principle to the graphene growth described earlier. In addition, other gases, such as CO2 and O2, have the same effects.52 Wu et al. reported the usage of a mixture of CO2 and He for graphene production during DC arc discharge.52,53 Graphene with a size of 100–300 nm and 4–5 layers were obtained in such cases. X-Ray photoelectron spectroscopy (XPS) revealed that functional groups such as C–O and O–C=O existed, which were attributed to the weak oxidation of CO2. Research conducted by Qin et al.53 confirmed the role of active molecules, such as O2, in the formation of graphene. They believed that molecules such as H2, O2, and CO2 affected the graphene edges by either oxidation by O2 and CO2 or reduction by H2, thus terminating carbon-centered radicals and preventing graphene curling. This research further confirmed that carbon volatilization and then nucleation was a possible mechanism for graphene formation. Temperature Temperature plays an important role in graphene quality, which is usually very high in arc discharge and contributes to the good crystallinity of graphene. Wang et al. changed the temperature by alternating the current.41 The higher the current is, the higher the temperature of arc will be, making it possible for the pyrolysis products to be transformed from spherical carbon nanoparticles to graphene sheets. Chen et al.46 used C2H2 as the carbon source to grow graphene during arc discharge and found that graphene quality was related to temperature in the range of 3200-3600 K, that is, the higher the temperature, the higher the quality of graphene. The temperature gradient also affects the arc discharge-derived graphene. Li et al. explored the effect of cooling rate on graphene in He/H2 arc discharge. By building a fan in the chamber, the cooling rate was adjusted by regulating the fan speed.54 Based on the design, they found that the quality of the graphene increased as the cooling rate increased (Figure 4g). A low ID/IG of ∼0.206 of graphene was realized when the fan speed reached 11500 rpm. It was interesting that the layers of graphene were also continuously reduced in this case. This was ascribed to the high cooling rate, which was conducive to accelerating the migration of carbon clusters out of the high-temperature region, and the excess carbon atoms were reduced (Figure 4f). Pressure During arc discharge, buffer gas pressure is another essential factor. Low pressure is typically used in arc discharge because it facilitates gas ionization for arc generation.51,55,56 Wang et al.56 explored the effects of pressure during air arc discharge and found a dependence between product and pressure. They found that higher air pressure (from 400 to 1000 torr) yielded more components of the graphene sheets. Conversely, carbon nanohorns or carbon nanospheres were obtained at low air pressure. They speculated that the reasons may be associated with the mechanism of active molecules acting on the graphene edges. Kumar et al.55 performed the experiment in Ar. It was found that the (002) peak of graphene in X-ray diffraction (XRD) weakened with increasing Ar pressure within 300–500 torr, indicating the graphene layers had decreased. The same trend of decreasing ID/IG in the Raman spectra confirmed the increased graphene crystallinity. However, further increases in Ar pressure increased the layers and reduced the graphene quality. Zhang et al.51 conducted the experiment in different buffer gases at different pressures. It was found that worse crystallinity of aggregated amorphous carbon or carbon nanohorns was obtained in pure Ar or N2 with increasing pressure, respectively, while better crystallinity of graphene sheets was realized in H2 at higher pressures from 40 to 70 kPa (Figure 4h). Magnetically enhanced arc discharge Magnetically enhanced arc discharge was first invented by Levchenko et al.40 It is possible to obtain a nonuniform magnetic field by installing a permanent magnet in a nonmagnetic vacuum chamber (Figure 3a). The introduction of a magnetic field not only condenses the arc but also enables electron magnetization to promote ionization and increase the plasma density, resulting in an increase in the temperature of the arc and enhancing the mixture of the buffer gas and the thermal plasma.41,57 Wang et al.41 designed magnetic field-assisted arc discharge for continuous graphene preparation. The magnetic coil provides an axial magnetic field within 0–0.15 T. Assisted by the magnetic field, the arc rotates counterclockwise around the cathode at a high speed (Figure 4i). The introduction of the magnetic field significantly reduced the contents of spherical carbon nanoparticles and increased graphene crystallinity, exhibiting a decrease in ID/IG from ∼1.05 (0 T) to ∼0.45 (0.09 T), but the effect on the layers of graphene was small. However, in follow-up studies, impurities, such as carbon tubes, were still found in the product, which is a main problem of arc discharge when preparing graphene.58 During the process of arc discharge, a suitable buffer gas must be selected to facilitate the success of graphene production based on the principle of avoiding graphene edge curling. Appropriate temperature and pressure windows are also of great necessity. By adjusting the relevant parameters, quantitative preparation of graphene can be achieved. The temperature of the arc discharge is indeed very high, which helps to create high-quality graphene with very good crystallinity, but makes it easy to obtain impurities such as carbon nanotubes due to interactions with built-in electrodes or the stainless chamber, which is difficult to avoid. Besides, the high nucleation density and sufficient cracking of graphene usually limit the size of graphene to several nanometers. Atmospheric plasma In contrast with arc discharge, plasma discharge is a method that depends on the gas discharge behavior. Drawbacks of the interference of the built-in electrodes and by-products are greatly avoided, leading to high purity of the prepared graphene. In this section, we will review the current research progress in the preparation of graphene by atmospheric plasma. Mechanism Plasma is an ionized gas composed of free electrons, atoms, and freely moving positive or negative ions. There are two kinds of temperatures expressed in plasma, which is because the energy transfer of electrons to atoms or ions is not very effective, enabling a deviation from the thermodynamic equilibrium.59,60 One is the electron temperature (Te) estimated based on the kinetic energy of the electrons. During plasma discharge, the electrons affect the excited kinetic energy by either direct collision or the intermediate states.59 The other is the gas temperature (Tgas) derived from the kinetic energy of heavy particles, such as atoms or ions. A higher gas temperature reflects an enhanced ability of plasma to atomize molecules, which can be calculated based on nonintrusive emission spectra techniques, such as in situ optical emission spectroscopy (OES).61,62 For thermal plasma, the energy loss due to radiation is far less than the energy exchange caused by collision, and the equilibrium state is close to the thermodynamic equilibrium, therefore Te≈Tgas in this case,63 which is known as local thermodynamic equilibrium.59 In surface wave-driven plasma, electrons absorb surface waves and then transfer energy to atoms or ions by collision, generating a high Te greater than 10,000 K and a high Tgas above 3000 K,59,61,64 therefore, radicals and excited species can be adequately generated.65 The gas temperature changes with input power, location, and radial distribution.66 Graphene produced by plasma discharge in the gas phase is usually performed at atmospheric or low pressure, and torch-like plasma is obtained (Figure 5a).67 Figure 5b is a schematic diagram of the main process for preparing graphene using ethanol (EtOH) as the carbon source by atmospheric plasma, where multiple reactions occur in the hot plasma and assembly zones.66 The stages of cracking of carbon sources, nucleation, and subsequent growth also appear during graphene synthesis. Tsyganov et al.66 explored the mechanism of growing free-standing graphene in an Ar/EtOH/H2 system by atmospheric plasma. It is seen from the simplified equilibrium diagram (Figure 5c) that the temperature can be divided into four regions. The main species in the regions of above 3500, 2000–3500, 1000–2000, and below 1000 K are atomic H and C, H2 molecules, solid carbon or graphene, and CH4 molecules, respectively. The cracking of EtOH starts in the hot plasma zone at a distance of ∼0.5 cm (Figure 5d). The boundary temperature of graphene nucleation is ∼1800 K, which is lower than that of graphene using methane as carbon source, where the latter is approximately 1.4 times of the former, ranging from 2200 to 2500 K.68 Figure 5 | Mechanism of growing graphene in atmospheric plasma. (a) The generated plasma torch. Reproduced with permission from Ref. 67. Copyright 2010, American Institute of Physics. (b–d) Scheme of the main processes and the variation of species with temperature or location. Reproduced wit
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