nanomaterials

Article Effects of Buffer Gases on Graphene Flakes Synthesis in Thermal Plasma Process at Atmospheric Pressure

Cheng Wang 1, Ming Song 2, Xianhui Chen 1, Dongning Li 1, Weiluo Xia 3 and Weidong Xia 1,*

1 Department of Thermal Science and Energy Engineering, University of Science and Technology of China, Hefei 230027, China; [email protected] (C.W.); [email protected] (X.C.); [email protected] (D.L.) 2 Department of Materials Science and Engineering, University of Science and Technology of China, Hefei 230027, China; [email protected] 3 Hefei Institutes of Physical Science, Chinese Academy of Sciences, Hefei 230031, China; [email protected] * Correspondence: [email protected]; Tel.: +86-0551-63602716



Received: 2 February 2020; Accepted: 9 February 2020; Published: 11 February 2020


Abstract: A thermal plasma process at atmospheric pressure is an attractive method for continuous synthesis of graphene flakes. In this paper, a magnetically rotating arc plasma system is employed to investigate the effects of buffer gases on graphene flakes synthesis in a thermal plasma process. Carbon nanomaterials are prepared in Ar, He, Ar-H2, and Ar-N2 via propane decomposition, and the product characterization is performed by transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and the Brunauer–Emmett–Teller (BET) method. Results show that spherical particles, semi-graphitic particles, and graphene flakes coexist in products under an Ar atmosphere. Under an He atmosphere, all products are graphene flakes. Graphene flakes with fewer layers, higher crystallinity, and a larger BET surface area are prepared in Ar-H2 and Ar-N2. Preliminary analysis reveals that a high-energy environment and abundant H atoms can suppress the formation of curved or closed structures, which leads to the production of graphene flakes with high crystallinity. Furthermore, nitrogen-doped graphene flakes with 1–4 layers are successfully synthesized with the addition of N2, which indicates the thermal plasma process also has great potential for the synthesis of nitrogen-doped graphene flakes due to its continuous manner, cheap raw materials, and adjustable nitrogen-doped content.

Keywords: graphene flakes; thermal plasma; magnetically rotating arc plasma; buffer gas; nitrogen-doped graphene flakes

1. Introduction Graphene is a novel nanomaterial with a single layer of carbon atoms packed in a hexagonal lattice. Since the first synthesis in 2004, graphene has emerged as highly active research field due to its fascinating physical, chemical, and mechanical properties [1–4]. In the past decade, various graphene preparation methods were developed such as mechanical cleavage [1], chemical vapor deposition [5,6], epitaxial growth [7,8], oxidation reduction [9], and arc-discharge method [10], among others. However, these methods generally operate in a batch mode. It remains challenging to obtain high-quality graphene economically, continuously, and with high throughput [11–13]. In recent years, a thermal plasma process at atmospheric pressure was developed for continuous synthesis of graphene flakes [14]. In this synthesis process, a carbon-containing precursor is delivered directly into the thermal plasma region where the precursor is decomposed to smaller reactive fragments. Then these reactive fragments recombine in the plasma environment to form graphene flakes and other products. Compared with the traditional graphene preparation methods such as chemical vapor

Nanomaterials 2020, 10, 309; doi:10.3390/nano10020309 www.mdpi.com/journal/nanomaterials Nanomaterials 2020, 10, 309 2 of 19 deposition (CVD) and the arc-discharge method, this process is a single-step, rapid, continuous method that occurs at atmospheric pressure without the use of substrates, catalysts, solvents, or acids. Thus, it is considered a promising process for graphene flakes synthesis. The effects of process parameters on the graphene flakes synthesis in a thermal plasma process have been reported in some studies. For example, Dato et al. [14–17], Tatarova et al. [18–20], and Melero et al. [21,22] established a single-step method to synthesize graphene flakes with few layers based on microwave plasma. Their research revealed that factors influencing graphene flakes synthesis include the precursor type, reactor design, flow rate of buffer gas, etc. Pristavita et al. [23–28] and Cheng et al. [29] developed a radio-frequency plasma system for graphene flakes synthesis via methane decomposition. Their study indicated that a high reaction temperature and addition of H2 may promote the transformation of products from spherical particles to graphene flakes. Kim et al. [11] and Suh et al. [12,30] proposed an atmospheric arc plasma process for graphene flake preparation. Relevant research has also uncovered the effects of reaction temperature and precursors on graphene flake formation. Recently, we developed a magnetically rotating arc plasma system for continuous synthesis of graphene flakes [31–33]. The high yield and low energy cost make this method competitive among these thermal plasma processes. Moreover, the influence of the magnetic field, gas temperature, precursor type, and precursor flow rate on the product microstructure is systematically presented. In reviewing these studies, it is found that process parameters such as precursor composition, reaction temperature, and buffer gas are essential for preparing graphene flakes in a controlled manner. However, to the authors’ knowledge, few reports have systematically addressed the dependence of different buffer gases on the graphene flakes synthesis in the thermal plasma process even though the effects of buffer gases have been widely reported in terms of the arc-discharge method [34–40] and chemical vapor deposition [41–43]. In this paper, a magnetically rotating arc plasma system is applied to study the effects of buffer gases on graphene flakes synthesis in the thermal plasma process. Carbon nanomaterials are synthesized under different buffer gases (i.e., Ar, He, Ar-H2, and Ar-N2) via propane decomposition. Transmission electron microscopy (TEM), Raman spectroscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and the Brunauer–Emmett–Teller (BET) method are employed to analyze the microstructure of the products. Based on product characterization, the influence mechanism of buffer gases on the graphene flakes formation is discussed.

2. Experimental Methods

2.1. Experimental Apparatus A schematic diagram of the experimental apparatus is exhibited in Figure1, which is mainly composed of a magnetically rotating arc plasma generator, a water-cooling deposition chamber, a gas supply system, and a direct current (DC) power system. The plasma generator is constructed with two concentric graphite electrodes (Cathode: 8 mm in diameter and 250 mm in length. Anode: 30 mm in inner diameter and 300 mm in length) and a water-cooling magnetic coil that surrounds the anode. Buffer gas and feedstock gas are introduced into the plasma generator from gas inlets around the cathode bottom. Two modulated 0–200 V DC power supplies are connected to the plasma generator and magnetic coil, respectively. The magnetic coil provides an axial magnetic field of 0.08 T by controlling the coil current. A water-cooling deposition chamber (stainless steel, cylindrical structure, 40 mm in inner diameter, and 400 mm in length) is connected to the anode. An exit is open to the atmosphere for the exhaust emission. Thus, the deposition pressure is approximately 1 atm. A more detailed description of the experimental apparatus can be found in the authors’ previous work [31,44–46]. Nanomaterials 2020, 10, 309 3 of 19 Nanomaterials 2020, 10 3 of 18

Figure 1. Schematic diagram of the experimental apparatus. Figure 1. Schematic diagram of the experimental apparatus. 2.2. Experimental Parameters 2.2. Experimental Parameters The input conditions used for each test are listed in Table1. The bu ffer gases include argon, helium, a mixtureThe input of argon conditions and hydrogen, used for and each a mixturetest are oflist argoned in andTable nitrogen, 1. The buffer respectively. gases includeThe feedstock argon, gashelium, is propane, a mixture and of the argon purity and of hydrogen, all gases isand more a mixture than 99.99%. of argon To and minimize nitrogen, the erespectively.ffect from a gasThe temperaturefeedstock gas di isff erence,propane, the and average the purity gas temperature of all gases isis controlledmore than as99.99%. the same To minimize for different the bu effectffer gases from bya gas adjusting temperature the input difference, power. Thethe averageaverage gasgas temperaturetemperature isis calculated controlled by as the the energy same equilibriumfor different (assumingbuffer gases the by system adjusting is in the a thermodynamicinput power. The equilibrium average gas and temperature the propane is calculated is adequately by the mixed energy up withequilibrium buffer gas. (assuming The thermal the system efficiency is in of a thethermody plasmanamic generator equilibrium ranges from and 40–60%).the propane As is suggested adequately by Pristavitamixed up etwith al. [buffer23,27] andgas. WangThe thermal et al. [31 efficiency], the essential of the gasplasma temperature generator for ranges graphene from flakes 40%–60%). synthesis As issuggested more than by 3000Pristavita K. Therefore, et al. [23,27] the averageand Wang gas et temperature al. [31], the essential in this experiment gas temperature is controlled for graphene to be 3500flakes K, synthesis and the temperature is more than error 3000 is K. roughly Therefore,300 the K. average The input gas power temperature is controlled in this by experiment changing the is ± arccontrolled current. to be 3500 K, and the temperature error is roughly ± 300 K. The input power is controlled by changing the arc current. Table 1. Experimental condition for each test. Table 1. Experimental condition for each test. Test Input Power I/U/P Feedstock Gas Buffer Gas Test Input Power I/U/P Feedstock Gas Buffer Gas Ar 85 A/61 V/~5.2 kW C3H8 1 slm Ar 35 slm HeAr 8592 AA/61/103 V/~5.2 V/~9.5 kWkW C C33HH88 11slm slm He Ar 3535 slm slm Ar-H2He 92 90 A/103 A/84 V V/~9.5/~7.6 kW kW C C33HH88 11slm slm Ar (32 Heslm), 35 H slm2 (3 slm) Ar-H2Ar-N2 90 95 A/84 A/78 VV/~7.6/~7.4 kW kW C C33HH88 11slm slm Ar (32(32 slm),slm), N H2 2(3 (3 slm) slm) Ar-N2 95 A/78 V/~7.4 kW C3H8 1 slm Ar (32 slm), N2 (3 slm) During the course of the experiment, the argon is first injected into the plasma generator, and the arcDuring is ignited the course under aof pure the experiment, Ar atmosphere. the argon Then theis first argon injected is replaced into the by plasma the buff generator,er gas. Lastly, and thethe propanearc is ignited is mixed under with a pure the buArff atmosphere.er gas in the Then mixing the cavity argon and is replaced then introduced by the buffer into thegas. plasma Lastly, generatorthe propane for is decomposition. mixed with the The buffer typical gas duration in the mixi timeng of cavity each test and is then 20 min. introduced A mass of into solid the products plasma aregenerator deposited for ondecomposition. the inner wall The of the typical deposition duration chamber. time Theof each solid test products is 20 min. are powdery A mass andof solid pile 3 togetherproducts loosely. are deposited The packing on the density inner wall of resultant of the de solidposition products chamber. is 0.1–0.12 The so glid/cm products− , and theare synthesispowdery rateand ispile about together 100–300 loosely. mg per The minute. packing The density solid of products resultant are solid continuously products is prepared 0.1–0.12 ing/cm a gas-3, and phase, the sosynthesis the formation rate is about time from100–300 feedstock mg per gas minute. to solid The products solid products is about are 20 continuously ms (i.e., residence prepared time in of a feedstockgas phase, gas so inthe the formation high temperature time from region). feedstock gas to solid products is about 20 ms (i.e., residence time of feedstock gas in the high temperature region).

2.3. Characterization

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2.3. Characterization The product morphology is characterized by transmission electron microscopy (TEM, JEM-2011, JEOL,Nanomaterials Tokyo, Japan) 2020, 10 and high-resolution transmission electron microscopy (HRTEM, JEM-2100F,4 of 18 JEOL, Tokyo, Japan). The crystalline structure of the products is analyzed using an X-ray diffractometer The product morphology is characterized by transmission electron microscopy (TEM, (TTRIII, Rigaku, Tokyo, Japan) and a Raman spectrometer (LabRam HR Evolution, Horiba Scientific, JEM-2011, JEOL, Tokyo, Japan) and high-resolution transmission electron microscopy (HRTEM, VilleneuveJEM-2100F, d’Ascq, JEOL, France). Tokyo, RamanJapan). The spectrum crystalline measurements structure of the are products carried is out analyzed by the using 532-nm an X-ray line of an 1 He-Nediffractometer laser as the (TTRIII, excitation Rigaku, source Tokyo, in the Japan) spectral and rangea Raman of 500–3000spectrometer cm −(LabRam. X-ray HR diff Evolution,raction (XRD) measurementsHoriba Scientific, are performed Villeneuve usingd'Ascq, a France). Cu-Kα Raradiationman spectrum in the measurements 2θ range of 10are to carried 70◦. Theout by elemental the analysis532-nm of theline productsof an He-Ne is laser conducted as the excitation using X-ray source photoelectron in the spectral range spectroscopy of 500–3000 (XPS, cm−1. Axis X-ray Ultra DLD,diffraction Kratos, Manchester, (XRD) measurements England). are XPSperformed spectra using are a obtained Cu-Kα radiation via a monochromatic in the 2θ range of Al 10 irradiationto 70°. sourceThe in elemental the range analysis of 200–700 of the eV. products Nitrogen is conduc adsorption–desorptionted using X-ray photoelectron isotherm spectroscopy measurements (XPS, of the productsAxis areUltra performed DLD, Kratos, using Manchester, a surface England). area analyzer XPS spectra (TristarII3020M, are obtained Micromeritics, via a monochromatic Atlanta, Al GA, irradiation source in the range of 200–700 eV. Nitrogen adsorption–desorption isotherm USA). The Brunauer–Emmett–Teller (BET) method is used to calculate the specific surface area of measurements of the products are performed using a surface area analyzer (TristarII3020M, the products. Micromeritics, Atlanta, USA). The Brunauer–Emmett–Teller (BET) method is used to calculate the specific surface area of the products. 3. Results 3. Results 3.1. TEM Images Typical3.1. TEM TEM Images images of the products obtained under an Ar atmosphere are shown in Figure2. Figure2a indicatesTypical TEM the images existence of the of threeproducts kinds obtained of products: under an semi-graphitic Ar atmosphere particles, are shown graphene in Figure flakes,2. and sphericalFigure 2a indicates particles. the The existence semi-graphitic of three kinds particles of products: have semi-graphitic a characteristic particles, of graphitized graphene flakes, forms of carbonand blacks spherical [47]. particles. These particles The semi-graphitic are stacked particles by dozens have ofa graphiticcharacteristic layers of graphitized that define forms the particle of boundary,carbon which blacks leads [47]. These to a polyhedral particles are morphology stacked by dozens and shell-like of graphitic appearance, layers that asdefine shown the inparticle Figure 2b. This isboundary, also known which as leads polyhedral to a polyhedral graphite morphology [48] or semi-graphitic and shell-like polyhedral appearance, particles as shown [49 in]. Figure The size of 2b. This is also known as polyhedral graphite [48] or semi-graphitic polyhedral particles [49]. The semi-graphitic particles is often within the range of 50–150 nm, and the number of graphitic layers size of semi-graphitic particles is often within the range of 50–150 nm, and the number of graphitic can range from 50 to several hundreds. The graphene flakes are the main products under an Ar layers can range from 50 to several hundreds. The graphene flakes are the main products under an atmosphere.Ar atmosphere. These flakesThese flakes have have the size the size (length (length and and width) width) in in the the range range ofof 50–20050–200 nm, nm, and and appear appear as irregularly-curledas irregularly-curled flakes flakes that overlap that overlap and aggregate,and aggregate, which which is, hence, is, hence, being being labeled labeled “crumpled “crumpled paper sheet”-likepaper sheet”-like carbon black carbon [50 black–53], [50–53], as revealed as revealed in Figure in Figure2c,d. 2c,d. TEM TEM image image analysis analysis indicates indicates that the graphenethe graphene flakes consist flakes consist of graphitic of graphitic layers layers with wi theth the number number of of 1–10. 1–10. The TheHRTEM HRTEM image in in Figure Figure 2d gives2d a typicalgives a edgetypical image edge ofimage the grapheneof the graphene flakes flak in whiches in which the number the number of layers of layers of graphene of graphene flakes is sevenflakes and nine, is seven respectively. and nine, respecti However,vely. these However, graphitic these layers graphitic are distortedlayers are to distorted some extent, to some which extent, reveals that somewhich disordered reveals that structuresome disordered exists in structure the graphene exists in flakes. the graphene The spherical flakes. The particles spherical have particles a diameter have a diameter range of 10–30 nm, and their content is about 30% of the products. These particles range of 10–30 nm, and their content is about 30% of the products. These particles are aggregated and are aggregated and fused together to form a branched morphology, as shown in Figure 2e. The fused together to form a branched morphology, as shown in Figure2e. The HRTEM image in Figure2f HRTEM image in Figure 2f shows the spherical particles possess many small and distorted graphitic showslayers, the spherical which are particles similar to possess the carbon many black small with and the distorted amorphous graphitic structure layers, produced which areat a similar low to the carbontemperature black [54,55]. with the amorphous structure produced at a low temperature [54,55].

Figure 2. Cont.

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FigureFigure 2. TEM 2. TEM images images of productsof products obtained obtained underunder anan Ar atmosphere. (a) ( aLow-magnification) Low-magnification TEMTEM imageimage of whole of whole products. products. (b) HRTEM (b) HRTEM image image of semi-graphitic of semi-graphitic particles. particles. (c,d) ( TEMc,d) TEM image image of graphene of flakes.graphene (e,f) TEM flakes. image (e,f) of TEM spherical image of particles. spherical particles.

WhenWhen the butheff bufferer gas gas changes, changes, the the product product morphologymorphology demonstrates demonstrates clea clearr variation, variation, as shown as shown in Figure 3. As the buffer gas changes from Ar to He, the semi-graphitic particles and spherical in Figure3. As the bu ffer gas changes from Ar to He, the semi-graphitic particles and spherical particles disappear, and all products are graphene flakes ranging in size from 50 to 200 nm, as particles disappear, and all products are graphene flakes ranging in size from 50 to 200 nm, as exhibited in Figure 3a. The number of layers of graphene flakes is always less than 10, as indicated exhibitedby the in HRTEM Figure 3imagea. The in numberFigure 3b. of Similar layers to of the graphene microstructure flakes isin alwaysFigure 2d, less the than graphene 10, as flakes indicated by theshould HRTEM exhibit image some in disordered Figure3b. structure Similar due to theto th microstructuree distorted graphitic in Figure layers.2 d,When the graphenethe buffer gas flakes shouldis Ar-H exhibit2, more some transparent disordered graphene structure flakes due with to a size the range distorted of 50–200 graphitic nm arelayers. obtained, When as shown the in bu ffer gas isFigure Ar-H 3c.2, moreThis phenomenon transparent reveals graphene that flakesthe grap withhene a flakes size range have a of fewer 50–200 number nm areof layers obtained, in as shownAr-H in2 Figure. The HRTEM3c. This image phenomenon in Figure 3d indicatesreveals thatthe gr theaphitic graphene layers are flakes straighter, have which a fewer suggests number a of well-ordered graphitic structure in the products. When the buffer gas is Ar-N2, all products are layers in Ar-H2. The HRTEM image in Figure3d indicates the graphitic layers are straighter, which graphene flakes that exhibit a smaller size (usually less than 100 nm), as shown in Figure 3e. suggests a well-ordered graphitic structure in the products. When the buffer gas is Ar-N2, all products Moreover, the graphene flakes are more transparent, which indicates fewer layers of the graphene are graphene flakes that exhibit a smaller size (usually less than 100 nm), as shown in Figure3e. flakes. The HRTEM image in Figure 3f reveals that the number of layers of graphene flakes is Moreover, the graphene flakes are more transparent, which indicates fewer layers of the graphene typically no more than four. In particular, the graphitic layers under Ar-N2 are very straight, which flakes.suggestsThe HRTEM a well-orderedimage ingraphitic Figure 3structuref reveals in that the thegraphene number flakes. of layers of graphene flakes is typically no more than four. In particular, the graphitic layers under Ar-N2 are very straight, which suggests a well-ordered graphitic structure in the graphene flakes.

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FigureFigure 3. 3. TEMTEM images images of of products products obtained under different different bubufferffer gases.gases. (a,b): He, ( c,d): Ar-H Ar-H22,, and and

((ee,,ff):): Ar-N Ar-N22. . 3.2. Raman Spectroscopy 3.2. Raman Spectroscopy Raman spectrum analysis of products under different buffer gases is displayed in Figure4. As each Raman spectrum analysis of products under different buffer gases is displayed in Figure 4. As spectrum shows, three intense peaks include the D band at about 1350 cm 1, the G band at about each spectrum shows, three intense peaks include the D band at about 1350 cm− −1, the G band at about 1580 cm 1, and the 2D band at about 2700 cm 1, respectively. The presence of the D band corresponds 1580 cm−−1, and the 2D band at about 2700 cm−−1, respectively. The presence of the D band corresponds to a disordered or defective structure in carbon materials. The G band is related to phonon vibrations to a disordered or defective structure in carbon materials. The G band is related to phonon vibrations in sp2 carbon materials, which reflects the ordered graphitic sheet [56]. The 2D band is from the in sp2 carbon materials, which reflects the ordered graphitic sheet [56]. The 2D band is from the overtone of the D band, and it is closely related to the band structure of graphene layers [57,58]. overtone of the D band, and it is closely related to the band structure of graphene layers [57,58]. The The relative intensity of the D band to the G band (ID/IG), the peak position, and the full-width at relative intensity of the D band to the G band (ID/IG), the peak position, and the full-width at half-maximum (FWHM) of the G band are widely used for characterizing the defect quantity in the half-maximum (FWHM) of the G band are widely used for characterizing the defect quantity in the samples [59,60]. The relative intensity of the 2D band to the G band (I2D/IG) is used to determine samples [59,60]. The relative intensity of the 2D band to the G band (I2D/IG) is used to determine the the thickness of graphene flakes, whereas a high value of I2D/IG indicates fewer graphene layers [5]. thickness of graphene flakes, whereas a high value of I2D/IG indicates fewer graphene layers [5]. Table 2 summarizes the peak position and FWHM of the G band, the ID/IG value, and the I2D/IG value. Table

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Table2 summarizes the peak position and FWHM of the G band, the ID/IG value, and the I2D/IG value. 2 indicates that the peak position and FWHM of the G band as well as the values of ID/IG and I2D/IG Table2 indicates that the peak position and FWHM of the G band as well as the values of ID/IG and are sensitive to the buffer gas. The ID/IG value under an Ar atmosphere is 0.54 and the FWHM of the I2D/IG are sensitive to the buffer gas. The ID/IG value under an Ar atmosphere is 0.54 and the FWHM −1 G band is 42.37 cm , which1 is higher than the others. This can be attributed to the existence of of the G band is 42.37 cm− , which is higher than the others. This can be attributed to the existence spherical particles in the products. These spherical particles have an amorphous structure, which of spherical particles in the products. These spherical particles have an amorphous structure, which carries a high degree of defects and disorder. This results in low product crystallinity. The ID/IG value carries a high degree of defects and disorder. This results in low product crystallinity. The ID/IG value is 0.46 for He, 0.37 for Ar-H2, and 0.25 for Ar-N2, which implies increasingly high product is 0.46 for He, 0.37 for Ar-H2, and 0.25 for Ar-N2, which implies increasingly high product crystallinity crystallinity as the buffer gas changes. Meanwhile, the FWHM of the G band decreases slightly with as the buffer gas changes. Meanwhile, the FWHM of the G band decreases slightly with the changes in the changes in buffer gases, which also confirms the increasingly high crystallinity. This buffer gases, which also confirms the increasingly high crystallinity. This phenomenon aligns with the phenomenon aligns with the variation tendency of TEM images in Figure 3 in which the graphitic variation tendency of TEM images in Figure3 in which the graphitic layers become straighter as the layers become straighter as the buffer gas changes. These straighter graphitic layers possess a more buffer gas changes. These straighter graphitic layers possess a more ordered graphitic structure, which ordered graphitic structure, which improves the crystallinity. The I2D/IG value increases as the buffer improves the crystallinity. The I2D/IG value increases as the buffer gas changes. As such, the number of gas changes. As such, the number of layers of graphene flakes diminishes gradually, in accordance layers of graphene flakes diminishes gradually, in accordance with the TEM results in Figure3. For He, with the TEM results in Figure 3. For He, Ar-H2, and Ar-N2, all products are graphene flakes, but the1 Ar-H2, and Ar-N2, all products are graphene flakes, but the FWHM of the G band is about 30 cm− , −1 −1 FWHM of the G band is about 30 cm , which is higher 1than the single-layer graphene1 (~14 cm ) or which is higher than the single-layer graphene (~14 cm− ) or graphite (~12 cm− )[61]. The relatively graphite (~12 cm−1) [61]. The relatively high FWHM indicates some defects and disorder in the high FWHM indicates some defects and disorder in the graphene flakes [60]. According to the TEM graphene flakes [60]. According to the TEM results, the graphene flakes obtained in this experiment results, the graphene flakes obtained in this experiment are small, which easily results in the formation are small, which easily results in the formation of many wrinkles and edges, so the defects and of many wrinkles and edges, so the defects and disorder are likely a consequence of the distortion of disorder are likely a consequence of the distortion of the graphitic layers and edge effects of the graphitic layers and edge effects of graphene flakes. Moreover, based on the FWHM of the G band, graphene flakes. Moreover, based on the FWHM of the G band, the crystal size (La) of graphene the crystal size (La) of graphene flakes can be estimated, which is about 20 nm. The G band for Ar-N2 flakes can be estimated, which is about 20 nm. The G band for Ar-N2 presents a small blue shift with presents a small blue shift with respect to those for other buffer gases. This phenomenon may be due respect to those for other buffer gases. This phenomenon may be due to the incorporation of to the incorporation of nitrogen into the graphene lattices, which results in the compressive/tensile nitrogen into the graphene lattices, which results in the compressive/tensile strain in the C-C bonds strain in the C-C bonds [62,63]. [62,63].

Figure 4. Raman spectra of products under different buffer gases. Figure 4. Raman spectra of products under different buffer gases. Table 2. Raman information of products obtained in different buffer gases. Table 2. Raman information of products obtained in different buffer gases. 1 1 Sample Position (cm− ) FWHM (cm− ) ID/IG Value I2D/IG Value −1 −1 SampleAr Position 1575.58 (cm ) FWHM 42.37 (cm ) ID/IG Value 0.54 I2D/IG Value 0.53 HeAr 1575.58 1573.28 42.37 31.40 0.54 0.46 0.53 0.64 Ar-H2He 1573.28 1571.17 31.40 30.81 0.46 0.37 0.64 0.69 Ar-N2Ar-H2 1571.17 1581.56 30.81 29.36 0.37 0.25 0.69 0.82 Ar-N2 1581.56 29.36 0.25 0.82

3.3. XRD Patterns XRD is a useful method for the characterization of the crystal structure of nanoparticles, which is a supplement to the Raman result. XRD spectra of products obtained under different buffer gases are shown in Figure 5a. Two diffraction peaks appear at 2θ ≈ 26° and 2θ ≈ 43°, which correspond to

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3.3. XRD Patterns NanomaterialsXRD is 2020 a useful, 10 method for the characterization of the crystal structure of nanoparticles, which8 of 18 is a supplement to the Raman result. XRD spectra of products obtained under different buffer gases arethe shown002 and in 100 Figure diffraction5a. Two peaks di ffraction of carbon, peaks respecti appearvely. at 2θ Generally,26 and the 2θ peak43 ,intensities which correspond of 002 and to ≈ ◦ ≈ ◦ the100 002are andcharacteristic 100 diffraction of a graphitic peaks of structure carbon, respectively.[64]. XRD spectra Generally, indicate the that peak all intensities products obtained of 002 and in 100our areexperiment characteristic possess of a crystalline graphitic structure structures. [64 ].Th XRDe d-spacing, spectra indicatewhich is that determined all products by obtained Bragg’s inequation, our experiment is also presented possess in crystalline Figure 5a. structures. The products The in d-spacing, Ar and He which show is002 determined peaks with by d-spacing Bragg’s equation,of 3.436 Åis and also 3.420presented Å at 25.91° in Figure and 526.03°,a. The respectively. products in Ar When and HeH2 and show N 0022 are peaksadded with to the d-spacing buffer gas, of the products have 002 peaks with d-spacing of 3.406 Å and 3.397 Å at 26.14°and 26.21°, respectively. 3.436 Å and 3.420 Å at 25.91◦ and 26.03◦, respectively. When H2 and N2 are added to the buffer gas, The shift of the 002 peak to a higher diffraction angle and declining d-spacing value further confirm the products have 002 peaks with d-spacing of 3.406 Å and 3.397 Å at 26.14◦and 26.21◦, respectively. Thea higher shift level of the of 002 crystallinity peak to a higherin the products diffraction as anglethe buffer and declininggas changes. d-spacing According value to further the TEM confirm images a higher(Figures level 2 and of crystallinity 3), the graphene in the products flakes are as thecomp buosedffer gas of changes.few graphitic According layers. to theHowever, TEM images these (Figuresgraphitic2 andlayers3), theare graphenenot perfect. flakes The are distorted composed stru ofcture few always graphitic exists layers. in th However,e graphitic these layers. graphitic Thus, layersthe d-spacing are not perfect.value is The usually distorted higher structure than that always of bulk exists graphite in the graphitic(~0.335 nm). layers. Notably, Thus, the d-spacing002 peaks valueof all isproducts usually higherare asymmetric than that of because bulk graphite of a mixed (~0.335 polycrystalline nm). Notably, thestructure 002 peaks of ofgraphitic all products and aredisordered asymmetric domains because [65]. of aIn mixed order polycrystalline to identify the structure different of graphiticcrystal regions and disordered in the products, domains XRD [65]. ® Inspectra order are to identify analyzed the using different JADE crystal software regions (version in the products, 6.5, MATERIALS XRD spectra DATA, are analyzed California, using USA) JADE to® softwaredistinguish (version different 6.5, regions MATERIALS in the DATA,002 peak. California, XRD analysis USA) toin distinguishFigure 5b shows different two regions peaks in the 002 peak.peak. XRDOne is analysis the peak in Figureat about5b 2 showsθ ≈ 26°, two which peaks reflects in the a 002 graphitic peak. One structure, is the peakand the at about other2 isθ on 26the, ≈ ◦ whichleft, which reflects corresponds a graphitic to structure, the disordered and the structure. other is on The the area left, from which the corresponds fitting peak to can the describe disordered the structure.relative content The area of graphitic from the fittingand disordered peak can describestructures the [65]. relative Figure content 5b shows of graphitic the proportions and disordered of the structuresgraphitic region [65]. Figure under5b Ar shows and theHe proportions to be 53.2% of and the graphitic63.6%, respectively. region under For Ar Ar-H and He2, the to begraphitic 53.2% structure content is 69.3%, and increases to about 75% with the addition of N2. Combined with the and 63.6%, respectively. For Ar-H2, the graphitic structure content is 69.3%, and increases to about 75% morphology in TEM images, an increase in the graphitic region is mainly due to the transformation with the addition of N2. Combined with the morphology in TEM images, an increase in the graphitic regionof products is mainly from due spherical to the transformationparticles to graphene of products flakes, from or sphericaldue to the particles straighter to graphenegraphitic flakes,layers oras duethe buffer to the straightergas changes. graphitic layers as the buffer gas changes.

Figure 5. Cont.

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Figure 5. (a) XRD spectra. (b) Fitted details of d002 peaks of products under different buffer gases. Figure 5. (a) XRD spectra. (b) Fitted details of d002 peaks of products under different buffer gases. 3.4. XPS Spectra 3.4. XPS Spectra The XPS spectrum can effectively reflect the elemental components of the sample surface. In our experiment,The XPS the spectrum products can are effectively prepared by reflect an in the situ elemen synthetictal components method. Thus, of the elementalsample surface. components In our ofexperiment, the internal the region products and surfaces are prepared are basically by an the same,in situ in synthetic principle. method. In this section, Thus, XPSthe spectraelemental are employedcomponents to analyzeof the internal the element region composition and surfaces of are products basically under the disame,fferent in buprinciple.ffer gases, In asthis shown section, in FigureXPS spectra6. Figure are6 aemployed indicates to that analyze there arethe two element characteristic composition peaks: of aproducts C1s (carbon) under peak different at 284.9 buffer eV andgases, an as O1s shown (oxygen) in Figure peak at6. 532.6Figure eV 6a when indicates the bu thatffer there gas is are Ar. two The characteristic high-resolution peaks: C1s a XPS C1s spectrum (carbon) inpeak Figure at 284.96a can eV be and deconvolved an O1s (oxygen) into two peak peaks at 532.6 centered eV when at 284.7 the andbuffer 286.4 gas eV, is Ar. respectively. The high-resolutionThe main peakC1s XPS at 284.7 spectrum eV is assigned in Figure to 6a the can C-C be bonds, deconvolved while the into minor two peak peaks at highercentered binding at 284.7 energy and (286.4286.4 eV)eV, isrespectively. very close to The C-O main bonds peak [66 at]. 284.7 The low-intensity eV is assigned O1s to the peak C-C and bonds, C-O bonds while showthe minor a very peak high at carbonhigher binding energy (286.4 eV) is very close to C-O bonds [66]. The low-intensity O1s peak and C-O content in the products. The XPS spectra for He and Ar-H2 are basically the same, and are not displayed bonds show a very high carbon content in the products. The XPS spectra for He and Ar-H2 are in this case. When N2 is added, a small N1s (nitrogen) peak at 399.9 eV is also presented besides the C1s peakbasically and O1sthe same, peak, and as shown are not in displayed Figure6b. in The this high-resolution case. When N C1s2 is added, XPS spectrum a small inN1s Figure (nitrogen)6b reveals peak threeat 399.9 peaks eV locatedis also atpresented 284.7, 285.8, besides and 288.6 the C1s eV, whichpeak consistsand O1s of peak, C-C, C-N,as shown and O-C-O in Figure/C=O 6b. groups, The respectivelyhigh-resolution [19 ,C1s67]. XPS Thus, spectrum the XPS in spectrum Figure 6b indicates reveals three the formation peaks located of nitrogen-doped at 284.7, 285.8, grapheneand 288.6 flakes.eV, which The elementalconsists of components C-C, C-N, ofand products O-C-O/C=O obtained groups, in diff erentrespectively buffer gases [19,67]. is shown Thus, in the Table XPS3. spectrum indicates the formation of nitrogen-doped graphene flakes. The elemental components of For Ar, He, and Ar-H2, the carbon content is more than 98.7%, and the oxygen content is less than 1.3%. Inproducts our opinion, obtained the oxygenin different is from buffer the ambientgases is airshown because in Table the products 3. For Ar, are He, prepared and Ar-H in the2, the absence carbon of content is more than 98.7%, and the oxygen content is less than 1.3%. In our opinion, the oxygen is oxygen. When the products are exposed to the ambient air, the products can be oxidized by H2O/O2 from the ambient air because the products are prepared in the absence of oxygen. When the species due to the existence of active sites, so as to form C-O, C-O-C, or C=O groups. For Ar-N2, the totalproducts amount are ofexposed nitrogen to incorporatedthe ambient intoair, the the products graphene can flakes be isoxidized about 1.9%. by H The2O/O oxygen2 species content due to is the up toexistence 2.3%, which of active is higher sites, than so as those to fo inrm other C-O, bu ffC-O-C,er gases. or Usually,C=O groups. nitrogen-doped For Ar-N2, graphene the total flakesamount have of nitrogen incorporated into the graphene flakes is about 1.9%. The oxygen content is up to 2.3%, more active sites [68]. This is why the products in Ar-N2 own a high level of oxygen content. However, thewhich oxygen is higher content than measured those in other by the buffer XPS gases. method Usually, is likely nitrogen-doped a little higher gr thanaphene true flakes values. have This more is becauseactive sites the [68]. oxidation This is mainly why the occurs products at the in surface Ar-N2 and own edge a high ofthe level products of oxygen instead content. of the However, internal region.the oxygen In order content to distinguish measured theby nitrogen-dopedthe XPS method types, is likely high a resolutionlittle higher analysis than true of the values. N1s peakThis isis depictedbecause the in Figureoxidation6c. Themainly deconvolution occurs at the of surface this peak and shows edge fourof the types products of N-bonding: instead of pyridinic the internalN (peakregion. at In 398.4 order eV), to pyrrolic distinguishN (peak the nitrogen-doped at 399.8 eV), graphitic types,N high(peak resolution at 400.9 eanalysis V), and of oxidized the N1s N peak (peak is atdepicted 402.2 eV). in Figure Their content 6c. The is deconvolution 0.43%, 0.85%, 0.36%,of this andpeak 0.26%, shows respectively. four types of N-bonding: pyridinic N (peak at 398.4 eV), pyrrolic N (peak at 399.8 eV), graphitic N (peak at 400.9 e V), and oxidized N (peak at 402.2 eV). Their content is 0.43%, 0.85%, 0.36%, and 0.26%, respectively.

Nanomaterials 2020, 10, 309 10 of 19 Nanomaterials 2020, 10 10 of 18

Figure 6. (a) XPS spectra of products in Ar. (b) XPS spectra of products in Ar-N .(c) Fitted details of Figure 6. (a) XPS spectra of products in Ar. (b) XPS spectra of products in Ar-N22. (c) Fitted details of N1s peak with nitrogennitrogen addition.

Table 3. Elemental components of products obtained in different buffer gases.

Sample C O N Ar 98.9% 1.1% - He 98.7% 1.3% - Ar-H2 98.9% 1.1% -

Nanomaterials 2020, 10, 309 11 of 19

Table 3. Elemental components of products obtained in different buffer gases.

Sample C O N Ar 98.9% 1.1% - He 98.7% 1.3% - Ar-H2 98.9% 1.1% - Ar-N2 95.8% 2.3% 1.9%

Nitrogen-doped contents for different N2 flow rates are summarized in Table4 in which the buffer gas is Ar-N2, and the total buffer gas flow is controlled to be 35 slm. Table4 shows that the nitrogen-doped content rises with an increase in the N2 flow rate. The nitrogen-doped content is only 0.7% for 1 slm of N2, and rises to 1.9% for 3 slm of N2 and to 2.8% for 5 slm of N2. Noticeably, the pyrrolic N dominates the N-bonding types, which is more than 40% of total nitrogen-doped contents. The pyrrolic N is considered to be easily formed in a rich H environment [69], so the presence of large quantities of pyrrolic N is due to the abundant H atoms, which come from propane decomposition. Moreover, the pyrrolic N and pyridinic N are mainly located at the edges of graphene [70,71]. The presence of N-bonding may restrain the lateral growth of graphene by inhibiting the formation of carbon-carbon bonds at the edge. Thereby, the graphene flakes have a smaller size when the nitrogen is incorporated into the products, as shown in Figure3.

Table 4. Nitrogen-doped content for different N2 flow rates.

N2 Flow Rate N-Doped Content Pyridinic N Pyrrolic N Graphitic N Oxidized N 1 slm 0.7% 0.16% 0.31% 0.12% 0.11% 3 slm 1.9% 0.43% 0.85% 0.36% 0.26% 5 slm 2.8% 0.76% 1.31% 0.41% 0.32%

3.5. BET Surface Area

The N2 adsorption–desorption isotherms of the products prepared under different buffer gases are revealed in Figure7, which indicates all the products exhibit a type IV adsorption isotherm based on the classification of International Union of Pure and Applied Chemistry (IUPAC). Each adsorption isotherm has a steep adsorption in the high relative pressure region (>0.9 P/P0), and presents a typical hysteresis loop in the desorption branch. The characteristic of N2 adsorption–desorption isotherms indicates the formation of mesopores in the products [34,72]. Considering the planar structure of graphene flakes, the predominant nitrogen adsorption at P/P0 > 0.9 may occur on its external surface and the internal surface of pores is formed by the re-stacking of graphene flakes. Textural data of products obtained in different buffer gases is shown in Table5. It can be seen that the BET surface area of products obtained 2 2 2 2 in Ar, He, Ar-H2, and Ar-N2 is 138.26 m /g, 172.63 m /g, 281.94 m /g, and 353.77 m /g, respectively. All products have an average pore size in the range of 13–20 nm. The products in Ar have the smallest BET surface area, which is mainly due to the presence of spherical particles and semi-graphitic particles. Usually, the specific surface area of graphene increases as the layer number decreases. As suggested by TEM images and Raman results, the layer number of graphene flakes in Ar-N2 is fewer than those in He and Ar-H2. Thus, the BET surface area of graphene flakes in Ar-N2 is the largest. In addition, the graphene flakes in Ar-N2 have the smallest average pore size. The possible reason is that the smaller graphene flakes in Ar-N2 are easier to form smaller mesopores compared with those produced in He and Ar-H2. Nanomaterials 2020, 10 11 of 18

Ar-N2 95.8% 2.3% 1.9%

Nitrogen-doped contents for different N2 flow rates are summarized in Table 4 in which the buffer gas is Ar-N2, and the total buffer gas flow is controlled to be 35 slm. Table 4 shows that the nitrogen-doped content rises with an increase in the N2 flow rate. The nitrogen-doped content is only 0.7% for 1 slm of N2, and rises to 1.9% for 3 slm of N2 and to 2.8% for 5 slm of N2. Noticeably, the pyrrolic N dominates the N-bonding types, which is more than 40% of total nitrogen-doped contents. The pyrrolic N is considered to be easily formed in a rich H environment [69], so the presence of large quantities of pyrrolic N is due to the abundant H atoms, which come from propane decomposition. Moreover, the pyrrolic N and pyridinic N are mainly located at the edges of graphene [70,71]. The presence of N-bonding may restrain the lateral growth of graphene by inhibiting the formation of carbon-carbon bonds at the edge. Thereby, the graphene flakes have a smaller size when the nitrogen is incorporated into the products, as shown in Figure 3.

Table 4. Nitrogen-doped content for different N2 flow rates.

N2 Flow Rate N-Doped Content Pyridinic N Pyrrolic N Graphitic N Oxidized N 1 slm 0.7% 0.16% 0.31% 0.12% 0.11% 3 slm 1.9% 0.43% 0.85% 0.36% 0.26% 5 slm 2.8% 0.76% 1.31% 0.41% 0.32%

3.5. BET Surface Area

The N2 adsorption–desorption isotherms of the products prepared under different buffer gases are revealed in Figure 7, which indicates all the products exhibit a type IV adsorption isotherm based on the classification of International Union of Pure and Applied Chemistry (IUPAC). Each adsorption isotherm has a steep adsorption in the high relative pressure region (>0.9 P/P0), and presents a typical hysteresis loop in the desorption branch. The characteristic of N2 adsorption–desorption isotherms indicates the formation of mesopores in the products [34,72]. Considering the planar structure of graphene flakes, the predominant nitrogen adsorption at P/P0 > 0.9 may occur on its external surface and the internal surface of pores is formed by the re-stacking of graphene flakes. Textural data of products obtained in different buffer gases is shown in Table 5. It can be seen that the BET surface area of products obtained in Ar, He, Ar-H2, and Ar-N2 is 138.26 m2/g, 172.63 m2/g, 281.94 m2/g, and 353.77 m2/g, respectively. All products have an average pore size in the range of 13–20 nm. The products in Ar have the smallest BET surface area, which is mainly due to the presence of spherical particles and semi-graphitic particles. Usually, the specific surface area of graphene increases as the layer number decreases. As suggested by TEM images and Raman results, the layer number of graphene flakes in Ar-N2 is fewer than those in He and Ar-H2. Thus, the BET surface area of graphene flakes in Ar-N2 is the largest. In addition, the graphene flakes in Ar-N2 have the smallest average pore size. The possible reason is that the smaller graphene flakes in Ar-N2 Nanomaterials 2020, 10, 309 12 of 19 are easier to form smaller mesopores compared with those produced in He and Ar-H2.

Nanomaterials 2020, 10 12 of 18

Figure 7. N2 adsorption–desorption isotherms and BET surface areas of the products obtained in Figure 7. N2 adsorption–desorption isotherms and BET surface areas of the products obtained in different buffer gases. (a) Ar, (b) He, (c) Ar-H2, and (d) Ar-N2. different buffer gases. (a) Ar, (b) He, (c) Ar-H2, and (d) Ar-N2. Table 5. Textural data of products obtained in different buffer gases. Table 5. Textural data of products obtained in different buffer gases. Sample BET Surface Area (m2/g) Pore Volume (cm3/g) Average Pore Size (nm) Sample BET Surface Area (m2/g) Pore Volume (cm3/g) Average Pore Size (nm) Ar 138.26 0.57 19.18 ArHe 138.26 172.63 0.57 0.68 19.18 16.26 Ar-HHe 2 172.63281.94 0.68 0.94 16.26 14.89 Ar-HAr-N2 2 281.94353.77 0.94 1.31 14.89 13.21 Ar-N2 353.77 1.31 13.21 The BET surface area of graphene flakes prepared in this paper is lower than the theoretical value. The BET surface area of graphene flakes prepared in this paper is lower2 than the theoretical For example, the BET surface area of graphene flakes in Ar-N2 is 353.77 m /g, which is lower than 2 thevalue. theoretical For example, surface the area BET for surface the four-layer area of graphene graphene flakes flakes in (about Ar-N2 660 is 353.77 m2/g). m The/g, significantwhich is lower loss 2 ofthan accessible the theoretical surface surface area is area likely for a consequencethe four-layer of graphene the inhomogeneity flakes (about as 660 well m as/g). overlap The significant or severe aggregationloss of accessible of graphene surface flakesarea is [72 likely]. However, a consequence the BET of surface the inhomogeneity area of graphene as well flakes as obtained overlap or in thissevere experiment aggregation is much of graphene larger than flakes that [72]. in the However, arc method the inBET which surface the BETarea surface of graphene area is aboutflakes 20–90obtained m2 /ing [this34]. experiment This phenomenon is much larger indicates than the that graphene in the arc flakes method prepared in which in the the BET thermal surface plasma area 2 process,is about which20–90 m may/g possess[34]. This fewer phenomenon layers, less indicates overlap, the or weakergraphene aggregation flakes prepared than the in arcthe method.thermal Therefore,plasma process, the characteristic which may ofpossess a large fewer specific layers, surface less area overlap, makes or these weaker graphene aggregation flakes havethan the a good arc applicationmethod. Therefore, prospect, the such characteristic as catalyst carriersof a large and sp superecific capacitors. surface area makes these graphene flakes have a good application prospect, such as catalyst carriers and super capacitors. 3.6. Synthesis Rate/Yield 3.6. Synthesis Rate/Yield The synthesis rate/yield of the target product is an important indicator to evaluate the thermal plasmaThe process. synthesis Figure rate/yield8 displays of the that target the synthesisproduct is rate an/ yieldimportant of carbon indicator nanomaterials to evaluate is the remarkably thermal distinctplasma process. in different Figure buff 8er displays gases. The that synthesis the synthesi rates israte/yield about 250–300 of carbon mg nanomaterials per minute in pureis remarkably Ar or He atmosphere.distinct in different When buffer H2 is added,gases. The the synthesis synthesis rate rate is is about less than 250–300 200 mg per minute minute, in and pure only Ar aboutor He atmosphere. When H2 is added, the synthesis rate is less than 200 mg per minute, and only about 100 mg per minute with the addition of N2. Accordingly, the yield of carbon nanomaterials decreases from 18% to 7%. This result indicates H2 and N2 may be involved in forming more gaseous products.

Nanomaterials 2020, 10, 309 13 of 19

100 mg per minute with the addition of N2. Accordingly, the yield of carbon nanomaterials decreases Nanomaterialsfrom 18% to 2020 7%., 10 This result indicates H2 and N2 may be involved in forming more gaseous products.13 of 18

Figure 8. Synthesis rate and yield of carbon nanomaterials under different buffer gases. Figure 8. Synthesis rate and yield of carbon nanomaterials under different buffer gases.

Recent research suggests that the C2 radicals are the main precursor species of graphene Recent research suggests that the C2 radicals are the main precursor species of graphene flakes flakes [21,25,28]. In the Ar-H2 atmosphere, the formed C2 radicals can be consumed due to the [21,25,28]. In the Ar-H2 atmosphere, the formed C2 radicals can be consumed due to the formation of formation of hydrocarbons by collision with the hydrogen molecules through the reaction [73,74]: hydrocarbons by collision with the hydrogen molecules through the reaction [73,74]: C2 + H2 → C2H C2 + H2 C2H + H, so the synthesis rate of graphene flakes decreases. Similarly, the presence of + H, so the→ synthesis rate of graphene flakes decreases. Similarly, the presence of N2 in plasma favors N2 in plasma favors the formation of cyanides due to the interaction of nitrogen molecules with C2 the formation of cyanides due to the interaction of nitrogen molecules with C2 radicals through the radicals through the reaction [75,76]: C2 + N2 CN + CN. The formation of CN species suppresses reaction [75,76]: C2 + N2 → CN + CN. The formation→ of CN species suppresses the production of C2 the production of C radicals. Thus, the synthesis rate of graphene flakes is also reduced. Even so, radicals. Thus, the synthesis2 rate of graphene flakes is also reduced. Even so, the synthesis rate/yield the synthesis rate/yield of graphene flakes in this paper can still compete with that in the microwave of graphene flakes in this paper can still compete with that in the microwave plasma process [17,21] plasma process [17,21] in which the synthesis rate is about 1.33–2 mg per minute and the yield is less in which the synthesis rate is about 1.33–2 mg per minute and the yield is less than 5%. than 5%.

4. Discussion Discussion Formation ofof carbon carbon nanomaterials nanomaterials in thein the thermal thermal plasma plasma process process is a complicated is a complicated activity becauseactivity becausethe plasma the environmentplasma environment is a complex, is a complex, multi-component multi-component system system including including electrons, electrons, ions, atoms, ions, atoms,molecules, molecules, and more. and Themore. formation The formation mechanism mechan remainsism remains unclear unclear although although some meaningfulsome meaningful work workhas been has made been in recentmade yearsin recent [15,20 ,years32,52, 54[15,20,32,52,54].]. However, on theHowever, basis of producton the characterizationbasis of product by TEM,characterization the product by formation TEM, the mechanism product formation can be analyzed mechanism briefly. can The be sphericalanalyzed particlesbriefly. The include spherical many particlessmall and include distorted many graphitic small and layers, distorted which graphitic implies alayers, formation which mechanism implies a formation at a low temperature, mechanism atsimilarly a low totemperature, the typical carbonsimilarly black to processthe typical [54]. carbon Semi-graphitic black process particles [54]. possess Semi-graphitic graphitized particles forms of possesscarbon blacksgraphitized [47]. Thus, forms such of carbon particles blacks are suggested [47]. Thus, to such undergo particles a two-step are suggested process into the undergo thermal a plasmatwo-step [54 process]: (i) primary in the thermal growth ofplasma the particles [54]: (i) at primary low temperature growth of to the form particles spherical at low particles, temperature and (ii) particleto form graphitizationspherical particles, in high and temperature (ii) particle regions graphitization to cause in apolyhedral high temperature morphology regions and to shell-like cause a polyhedralappearance. morphology The graphene and flakes shell-like are mainly appearance. composed The of orderedgraphene graphitic flakes layers,are mainly corresponding composed to of a orderedhigh formation graphitic temperature layers, corresponding [52,54]. The distorted to a high structure formation in the temperature graphitic layers [52,54]. emerges The distorted through structurethe formation in the of graphitic five-member layers rings. emerges The through extent of th distortione formation in of this five-member case is thought rings. to The be governed extent of distortionby competition in this between case is the thought formation to andbe governed destruction by of competition five-member between rings, which the isformation controlled and by destructionthe plasma energyof five-member [15,77]. In rings, thermal which plasmas, is contro the unevenlled by distribution the plasma ofenergy temperature [15,77]. unavoidably In thermal plasmas,exists due the to uneven the plasma distribution fluctuation. of temperature Under an Arunavoidably atmosphere, exists the due input to powerthe plasma is relatively fluctuation. low. TheUnder uneven an Ar distributionatmosphere, of the temperature input power easily is relative causesly thelow. occurrence The uneven of distribution a low temperature of temperature region, whicheasily causes leads to the the occurrence formation of sphericala low temperature particles. Itregion, should which be noted leads that to thethe semi-graphiticformation of spherical particles particles.have a characteristic It should be of noted near-spherical that the semi-graphitic morphology, and, particles essentially, have theya characteristic belong to spherical of near-spherical particles withmorphology, many curved and, oressentially, closed structures. they belong Hence, to the spherical co-existence particles of three with kinds many of productscurved (sphericalor closed structures. Hence, the co-existence of three kinds of products (spherical particles, semi-graphitic particles, and graphene flakes) under an Ar atmosphere is potentially due to the uneven distribution of temperature. As listed in Table 1, the input power for He, Ar-H2, and Ar-N2 is clearly higher than that for Ar because of the higher enthalpy for He, H2, and N2, even though the average gas temperature is always controlled to be 3500 K. Hence, the addition of high-enthalpy gas indicates an environment

Nanomaterials 2020, 10, 309 14 of 19 particles, semi-graphitic particles, and graphene flakes) under an Ar atmosphere is potentially due to the uneven distribution of temperature. As listed in Table1, the input power for He, Ar-H 2, and Ar-N2 is clearly higher than that for Ar because of the higher enthalpy for He, H2, and N2, even though the average gas temperature is always controlled to be 3500 K. Hence, the addition of high-enthalpy gas indicates an environment of high energy. In a recent study, Whitesides et al. [77] investigated the growth of graphene-edges using kinetic Monte Carlo simulations and indicated that a high-energy environment can suppress the formation of a five-member-ring, which corresponds to the curved or closed structures, and, consequently, lead to the formation of planar graphene flakes. Recent experimental studies also confirm that a high-energy environment can facilitate the formation of planar graphene flakes rather than spherical particles [11,23,27,31]. Thus, variations in gas enthalpy may represent an important factor behind changes in product morphology. The thinner and straighter graphene flakes with the addition of H2 and N2 indicate that H2 and N2 also play key roles in graphene flakes synthesis. Several experimental studies have confirmed that H atoms are beneficial to graphene formation [38,39,78]. It is believed that H atoms can effectively terminate the dangling carbon bonds by forming carbon-hydrogen bonds, and, thus, prevent the rolling and closing of graphitic layers. In addition, H atoms have other functions of etching the amorphous carbon [79]. Thus, the formation of a spherical structure and distorted graphitic layers can be suppressed effectively in a rich H environment. In thermal plasmas, H2 can be easily cracked to H atoms, and the existence of H atoms has been reported in relevant research [25,29]. As a result, the addition of H2 facilitates the formation of straighter graphene flakes. Moreover, the thickness of the graphene flakes depends on the cooling rate in the formation process, and, normally, the faster cooling rate leads to thinner graphene flakes [37]. H2 has a more efficient quenching capability because its thermal conductivity is larger than that of Ar and He [80]. This may explain why the graphene flakes have fewer layers with the addition of H2. The formation of nitrogen-doped graphene flakes shows an interesting result. In the arc-discharge method, the addition of N2 usually results in the formation of products with low crystallinity, such as spherical particles [34] and carbon nano-horns [39,81], because the carbon-nitrogen bond easily leads to the bending of graphitic layers [82,83]. On the contrary, TEM images, Raman, and XRD spectra in this experiment indicate that the graphene flakes possess greater crystallinity when applying N2. The variation of the synthesis rate/yield in Figure8o ffers a clue to understand this phenomenon. The addition of N2 may increase the relative content of H atoms since many C2 radicals are removed via CN species formation. The existence of large quantities of pyrrolic N in graphene flakes also indicates a rich H environment. Given this, formation of thin and straight graphene flakes in the Ar-N2 atmosphere is likely attributed to three factors. First, high enthalpy N2 can maintain a high-energy environment. Second, abundant H atoms are produced in the plasma region through propane decomposition. Third, the addition of N2 may improve the relative content of H atoms due to the CN species formation. Thus, the product morphology with N2 addition is very similar to nitrogen-doped graphene flakes produced by the arc-discharge method under an atmosphere that contains NH3 [35]. Nitrogen-doped graphene flakes have been found to have broad applications [70] such as an electrocatalyst for the fuel cell, a field-effect transistor, lithium ion batteries, and devices in other fields. This paper indicates that the thermal plasma process has great potential for the synthesis of nitrogen-doped graphene flakes due to its continuous manner, cheap raw materials, and adjustable nitrogen-doped content.

5. Conclusions In this paper, a magnetically rotating arc plasma system is used to prepare carbon nanomaterials by propane decomposition. The products obtained in different buffer gases (i.e., Ar, He, Ar-H2, and Ar-N2) are characterized by TEM, HRTEM, Raman spectra, XRD, XPS, and the BET method. Experimental results indicate that the product microstructure depends on buffer gases. Under an Ar atmosphere, three kinds of products (spherical particles, semi-graphitic particles, and graphene Nanomaterials 2020, 10, 309 15 of 19

flakes) coexist. Under a He atmosphere, all products transform into graphene flakes. For Ar-H2 and Ar-N2, graphene flakes with fewer layers, higher crystallinity, and larger BET surface area are obtained. As such, the buffer gas with high enthalpy, and the addition of some reactive molecules (i.e., H2 and N2), can promote the graphene flakes formation. Initial analysis indicates that a high-energy environment and abundant H atoms can prevent the formation of curved or closed structures, which produces graphene flakes with high crystallinity. In particular, nitrogen-doped graphene flakes with 1-4 layers and adjustable nitrogen-doped contents are successfully synthesized with the addition of N2. In summary, this study reveals that the thermal plasma process has great potential for graphene flakes synthesis because the morphology and composition of products can be effectively regulated via changes in buffer gases.

Author Contributions: Conceptualization, C.W. and W.X. (Weidong Xia) Methodology, X.C. and D.L. Investigation, C.W. and M.S. Data curation, W.X. (Weiluo Xia) Writing—original draft preparation, C.W. Writing—review and editing, C.W. and W.X. (Weidong Xia) Funding acquisition, C.W. and W.X. (Weidong Xia). All authors have read and agreed to the published version of the manuscript. Funding: The National Natural Science Foundation of China, grant number: 11705202, 11675177 and 11475174; Anhui Provincial Natural Science Foundation, grant number: 1808085MA12, Anhui Province Scientific and Technological Project, grant number: 1604a0902145 funded this research. Acknowledgments: Characterizations (TEM, HRTEM, Raman spectra, XRD, XPS, and the BET method) were performed using the facilities of the Physical and Chemical Science Experiment Center at University of Science and Technology of China. Conflicts of Interest: The authors declare no conflict of interest.

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