4. Synthesis of graphene from methane, acetonitrile, xylene and

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1 CHAPTER 4 4. Synthesis of graphene from methane, acetonitrile, xylene and ethanol 4.1 Introduction In this chapter, the synthesis of graphene from three different carbon precursors include gases (methane, ethylene), liquid (ethanol, methanol) and solid (PMMA and polystyrene) using CVD method is reported. Srivastava et al.[89] demonstrated the substrate-selective growth of centimetre size ( 3.5 cm x1.5 cm), uniform and continuous single and few-layer graphene films employing vacuum-assisted chemical vapor deposition on polycrystalline Cu foils using liquid hexane as the carbon precursor and it exhibits better FET properties as compared to exfoliated graphene. Guermoune et al. reported the synthesis of the graphene from methanol, ethanol and propanol using CVD at 850 C. Low-purity carbon sources were used to grow high purity, large area graphene films. Many researchers reported the growth of graphene from solid carbon sources included PMMA [90], silicon carbide [91],amorphous carbon [92] and highly oriented pyrolytic graphite (HOPG) [93] The solid precursors have large and complex molecular structures. Synthesis of graphene from these precursors involves complicated chemical reactions and processes. Zhancheng Li et al. reported synthesis of graphene from liquid benzene precursor at low temperature (300 C) compared to conventional high temperature. The benzene ring resembles the basic unit of graphene. In contrast to the large molecules such as PMMA or polystyrene, benzene molecules just need to dehydrogenate and connect to each other to form the graphene structure. The activation energy of benzene dehydrogenation on Cu [111] is (1.47 ev), which is lower than conventional carbon precursor [methane 1.77 ev]. Hence, growing high-quality graphene using benzene as the carbon precursor might require lower temperatures as compared using gas precursors such as methane, and acetylene [94]. Thus, there is value in further investigating the synthesis of high quality graphene (including single layer) from new liquid organic precursors. This is also significant since graphene 50

2 synthesis becomes possible without the use of expensive high purity gaseous hydrocarbons (methane, acetylene and ethylene) [95, 96]. The modulation of electronic/electrical properties of graphene is highly important for the development of electronic devices. Theoretical studies have revealed that substitutional doping can alter the Fermi level and induce the metal-to-semiconductor transition in graphene. Elements such as nitrogen and boron are ideal candidates as dopants in graphene. Nitrogen-doped graphene (NG) has been successfully developed by direct synthesis and post treatments. The promising applications of N-doped graphene in electronic devices, oxygen reduction reaction, biosensing, and energy storage devices have attracted great attention [97]. Graphene synthesis using organic liquid precursors is feasible since most organic compounds and their derivatives readily vaporize below 200 C. Doping of graphene with different elements can be conducted by using organic precursors containing those elements. E.g. Nitrogen and boron containing compounds such as pyridine and triethylborane for N-doped and B-doped graphene respectively[89]. The liquid precursors can be more easily stored and handled as compared to hydrocarbon gases. The use of inductive heating in the synthesis of carbon nanotubes and graphene by CCVD can significantly reduce the energy consumption and overall reaction time [64, 98]. This can lead to significant reduction in price, even while maintaining the quality (purity and crystallinity) of carbon nanotubes and graphene. In this chapter, we report the synthesis of large area (1 cm 2 x 4 cm 2 ), continuous graphene films using atmospheric pressure radio frequency chemical vapor deposition (RF-CVD). The catalyst used was polycrystalline copper foil, which is shown to have advantage of allowing the synthesis of single to few layer graphene[60] at a reasonable cost. The different precursors used were methane, xylene, ethanol and acetonitrile at temperatures ranging from C (Table 4-1). 4.2 Experimental method The two major steps involved in the synthesis of graphene are as follows Pre-treatment of the copper foils Growth of graphene on copper by CVD [method] 51

3 4.2.1 Pretreatment of copper foil The catalyst used for graphene synthesis is copper. Figure 4-1: Copper foils (Alfa Aesar 25μ thick) substrates for the synthesis of graphene. The copper foil is cut with a flange and a hole in the centre to the dimension [1 1cm 2 ] shown in Figure 4 1 and Figure 4 2 shows the flange helps in holding the foil without causing damage to the surface of copper while transferring and it is maintained as a reference for positioning the foil. Also, for viewing the sample in optical microscopy the punched hole acts as a reference point. Figure 4-2: Schematic of the copper foil with dimension in mm The foil is placed between two glass slides to make it wrinkle free. The foil is taken from the glass slide and it is rinsed in iso propyl alcohol (IPA) and acetone for 10 seconds respectively to decrease the surface of organic impurities. Then the foil is dipped in acetic acid at 35 C for 10 min to remove if any oxide layer was formed and it is weighed. Before weighing, the traces of acetic acid are cleaned using lint-free tissue paper. The cleaned copper foil is placed in the graphite boat immediately, to avoid further formation of oxide layer, according to the 52

4 arrangement shown in Figure 4-3 to study the deposition gradient. The graphite susceptor is then carefully placed in the quartz tube. A summary of the procedure is shown in Figure 4-4 Figure 4-3: Arrangements of copper foils Figure 4-4: Pretreatment steps in synthesis of graphene Graphene growth in CVD A pressure gauge is placed in between the flow control rotameter and quartz tube as shown in Figure 4-5. Argon gas is introduced into the one end of quartz tube and the other end of the quartz tube is closed. So the pressure in the tube builds up and it will be indicated in pressure gauge. After this process, the argon flow is stopped and a leak test is performed using soap solution at every joint to identify any leakage and this process is repeated till the system is leak proof. 53

5 Figure 4-5: Pressure gauge for leak test If the pressure remains stable then the CVD system is considered to be leak proof and the quartz tube is purged using argon gas with a flow rate of 600 ml/min for 20 minutes. Figure 4-6 shows the removal of oxygen and the increase in the concentration of Ar as a function of time. These curves were obtained from a simple model considering the flow of argon and air in the quartz tube as a plug flow (with no axial or radial counter-diffusion). It is clear from the figure that after about 10 min, the level of oxygen becomes low enough to be undetected. Yet the purging time for all experiments is kept at 20 min as a factor of safety before introducing the hydrocarbon precursor. This is to prevent possible ignition or explosion of the hydrocarbon gas in the presence of oxygen, as well as possible defects that might arise in the graphene during its synthesis. Table 4-1: Reaction condition of graphene synthesis Precursor Flow rate Temperature ( C) Reaction Time (min) Methane 15 ml/min Acetonitrile 1 ml/hr Ethanol 2 ml/ min Xylene 1 ml/hr The temperature is then increased to 1000 C and the argon flow rate is reduced to 200 ml/min. Once the temperature reaches 1000 C, H 2 at 100mL/min is introduced for 30min to reduce the copper oxide layer from the copper foil. Then carbon precursors are introduced 54

6 according to Table 4-1. The samples are then cooled down to room temperature in Ar atmosphere. Figure 4-6: Purging time chart Transfer of Graphene The fabrication of samples and devices using other types of substrates is relevant for fundamental studies or the optimization of graphene s performance. The identification of graphene sheets, down to one layer in thickness, with optical microscopy is possible via colour contrast caused by the light interference effect on the SiO 2 which is modulated by the graphene layer. This makes the preparation of graphene samples and devices not only possible but also efficient. However, the observation of a clear colour contrast requires Si substrates with a specific SiO 2 thickness ( nm), thus limiting the fabrication of graphene devices to these substrates. Therefore, it is necessary to develop transfer methods which can integrate graphene sheets with a wider variety of substrate materials [PMMA] while also allowing an efficient identification of the graphene. The graphene films synthesized from the Cu foils were removed by etching in an aqueous solution of ferrous nitrate. The etching time was found to be a function of the etchant concentration, the area, and thickness of the Cu foils. Typically, a 1 cm 2 by 25-μm thick Cu foil can be dissolved by a 0.05 g/ml ferrous nitrate solution overnight. For a 2 cm 2 and 25-μm thick Cu foil, the optimized concentration of iron nitrate used was 0.083g/ml. The blank 55

7 experiment of copper etching in ferrous nitrate is shown in Figure 4-7 and the graphene grown on copper is etched in ferrous nitrate as shown in Figure 4-8. We used two methods to transfer the graphene from the Cu foils: (1) The copper film is dissolved, a substrate is brought into contact with the graphene film and it is pulled from the solution, (2) the surface of the graphene-on-cu is coated with poly-methyl methacrylate (PMMA) and the Cu is dissolved in the PMMA-graphene which is lifted from the solution. The first method is simple, but the graphene films experiences more tear damage. The graphene films are easily transferred with the second method to other desired substrates such as SiO 2 /Si, with significantly fewer holes or cracks (<5% of the film area). Figure 4-7: Etching of copper (without graphene) by ferrous nitrate Figure 4-8: Etching of graphene grown copper by ferrous nitrate Figure: 4-9: Steps involved in the transfer of graphene 56

8 The overall steps involved in the transfer of graphene are listed below and shown in Figure 4-8 and depicted in Figure: 4-9 Remove the graphite boat out of the quartz tube slowly. Use tweezers to gently remove the copper foil with graphene on a glass slide as shown in Figure: 4-9a Use a micropipette to drop coat a known amount PMMA solution in acetone (0.0467g/mL) so that it just covers the surface of graphene as shown in Figure: 4-9 b Carefully place the glass slide containing PMMA-coated graphene in the oven at 180 C for 1 min to dry the PMMA coating. Immediately remove the glass slide from the oven and place the PMMA/ graphene/copper foil onto the ferrous nitrate solution using tweezers so that it floats as shown in Figure: 4-9c After the completion of etching the PMMA/graphene film will remain floating on nitrate solution where as the copper foil may or may not completely dissolve but it will detach from the PMMA/graphene film as shown in Figure: 4-9d. Gently scoop the PMMA/graphene from the iron nitrate solution using the required substrate like silicon wafer or Cu grid coated with Formvar or glass slide Depending on the convenience for further characterization as shown in Figure: 4-9d e Figure 4-10: Steps in the transfer of graphene. 57

9 4.2.3 Characterization High-resolution scanning electron microscopy (HRSEM) observations of the nanoparticles were performed with a HRSEM Hitachi S4800 with EDX at an acceleration voltage of 5.0 kv (This SEM instrument was used in Chapter 6, 7 & 8). Raman spectra were obtained with a WITec Alpha 300 Confocal Raman system equipped with a Nd:YAG laser (532 nm) as the excitation source (for used methane, acetonitrile and xlyene). X-ray photoelectron spectroscopy (XPS) utilizing a PHOIBOS 100 analyser with an Al Ka radiation ( ev) as an excitation source. 4.3 Results and Discussions Synthesis of graphene from methane and acetonitrile The second-order Raman spectrum (2D band) of graphene is quite sensitive in determining the number of layers of graphene [99]. The graphene layers were estimated from the FWHM of the 2D band and the intensity ratio of the 2D and G band (I 2D /I G ). According to literature, FWHM values of the 2D band < 32 cm -1 indicate single layer graphene, cm -1 indicate bilayer graphene and >70 cm -1 indicate few layer-graphene. Also, when the ratio of intensity of the G and 2D bands (I G /I 2D ratio) is less 0.5, then it is an indication of single-layer graphene. Similarly, an I G /I 2D ratio ~ 1 indicates bilayer and I G /I 2D ratio >1.5 indicates fewlayer graphene [100, 101, 102]. The I G /I D ratio indicates the crystalline nature of graphene and a higher value indicates a better quality material with low defects [103, 104]. The presence of disorder in the crystalline lattice is indicated by the D band, which arises from the A 1g mode breathing vibrations of six-membered sp 2 carbon rings [105]. 58

10 Table 4-2: Raman features of graphene and N-doped graphene Sample Reaction Temperature C G- Band cm -1 D- Band cm -1 2D band cm -1 2D- FWHM cm -1* I G /I D cm -1* I 2D /I G cm -1* Methane ± ± ±0.014 Acetonitrile ± ± ±0.057 * Standard deviation value Figure 4-11 indicates the Raman spectra of the graphene obtained from methane at 1000 C. We observed peak positions at 1354, 1575 and 2713 cm -1 for D, G and 2D bands respectively. The 2D band is the second order of the D band and its shape as well as its position is used to identify a single layer sheet of graphene. The ratio of I 2D /I G value was ± and 2D band FWHM value was 90±2.12 cm -1. These results indicate the presence of good quality few-layer graphene (I G /I D was 2.42±0.166) [94, 96]. Figure 4-12 (ab) show SEM images of graphene in which different layers of graphene with wrinkles and folds can be seen clearly. Figure 4-12(c) shows graphene sheets and multilayer graphitic flakes along with nanoparticulate impurities, presumably from the catalyst (Cu) and etchant (Fe (NO 3 ) 2 [106]. Figure 4-12(d) shows an HRTEM image of graphene, in which few layer to multilayer graphene is clearly visible (5 10 layers). Figure 4-13 indicates the Raman spectrum of N-doped graphene, with I G /I D value of 0.82± This value is indicative of higher defect concentration, which is expected because of the doping of nitrogen in graphene leading to a more pronounced D band. In comparison, the I G /I D of graphene from methane was 2.42±0.166 indicating fewer defects. Further, the Raman spectrum of N-doped graphene also shows a pronounced D peak, which is absent in the graphene synthesized from methane. Since nitrogen atoms on graphene lattice positions act as defects, the appearance of a D peak in the Raman spectrum can be related to an incorporation of nitrogen [107]. The Figure 4-14 (a-b) indicates X-ray photoelectron spectroscopy of the N- doped graphene, which is synthesized from acetonitrile at 1000 C.Figure 4-14 (a) indicates the main peak at 59

11 284.6eV corresponds to the graphite-like sp 2 C, indicating that most of the carbon atoms are arranged in honeycomb lattice. The small peaks (Figure 4-14a) at and eV can be attributed to C-N bonding. (Figure 4-14b) N1s line scan of NG sample, which confirms the presence of substitutional (400.6eV) and pyridine-like (399.41eV) nitrogen dopants[97]. The morphology analysis of N-doped graphene is shown in Figure 4-15 (a-b). Figure 4-15(a) shows a large area graphene layer and its one edge was folded. Figure 4-15 (b) shows graphene sheets with a darker region possibly corresponding to multi or few layers of graphene. Figure 4-11: Raman Spectrum of graphene from methane carbon precursor 60

12 Figure 4-12: (a-b) SEM, (c-d) TEM image of graphene from methane Figure 4-13: Raman Spectrum of graphene from acetonitrile carbon precursor 61

13 Figure 4-14: XPS spectra of N-doped graphene from acetonitrile at 1000 C Figure 4-15: TEM images of N-doped graphene from Acetonitrile at 1000 C (a-b) Synthesis of graphene from xylene at different temperature ( C) Graphene was synthesized using xylene precursor in the temperature range C. From the Raman analysis, the formation of single to double layer graphene at lower temperature, and multilayer graphene at higher temperature was observed [108]. Figure 4-16 (a-d) shows the Raman features of graphene synthesized from xylene at C. For graphene obtained from xylene, with an increase in synthesis temperature from 700 to 1000 C, there was an increase in the FWHM of the 2D band from 53±2.5 to 100±5.1 cm -1. These results reveal the presence of double-layer graphene at 700 C. As the reaction temperature increases, few-layer graphene is formed, and the 2D peak position shifts to higher wave number. Among the samples the highest I G /I D ratio of 2.14±0.25 was obtained at 62

14 800 C, while for other samples, the ratio was in the range of This indicates the presence of graphene with the best sp 2 -hybridized graphitic nature at 800 C. a b c d Figure 4-16: Raman Analysis of graphene from xylene at C Table 4 3: Raman features of graphene synthesized from xylene at C Sample Reaction Temperature C G- Band cm -1 D- Band cm -1 2D band cm -1 2D- FWHM cm -1 I G /I D cm -1 I 2D /I G cm ± ± ± ± ± ± ± ± ± ± ± ±

15 Figure 4-17: SEM (a) and TEM (b-c) images of the graphene from xylene at 700 C Figure 4-18: SEM (a, c) and TEM (b, d) images of the graphene from xylene at 800 C and 900 C 64

16 Figure 4-19: SEM (a) and TEM (b) images of the graphene from xylene at 1000 C The morphology analysis of the graphene samples synthesized at 700 C, 800 C, 900 C and 1000 C are obtained by SEM and TEM analysis shows in Figure 4-17 to Figure These results indicate a very transparent single to few layer graphene obtained from growth temperature at 700 and 800 C. Figure 4-17(c) shows presence of 1 3 layers of graphene sheet (700 C). Figure 4-18 (a-b) SEM (a) and TEM (b) images of graphene from 800 C, Figure 4-18(b) indicates more wrinkles and folded graphene sheets with less transparent as compared to graphene synthesized at 700 C. However, Figure 4-18 (c) and Figure 4-19 (a) shows the less transparent and more wrinkles with light dark colour graphene sheet [synthesized at 900 and 1000 C]. The same result also observed from TEM [Figure 4-18 (d) and Figure 4-19 (b)] analysis, higher temperatures leads to the formation of fast decomposition of carbon precursor and carbon solubility which leads to the formation of multilayer graphene Mechanism of graphene formation using xylene Here reported, graphite boat with length 90mm and it is connected to K-type thermocouple, RF-induction coil is used as a heating element. Induction heating leads to the heating of the graphite boat surface alone. As a result, the xylene decomposes, leaving carbon deposits on a small area since the boat length is 90mm. The decomposition of the xylene molecules is uniform due to this induction heating process. 65

17 Shevel'kova et al has investigated the pyrolysis of p-xylene at temperatures of C with different hydrocarbon partial pressures (argon/p-xylene=10:1, 20:1, 30:1). The pyrolysed product contained following free radicals namely (aromatic hydrocarbon), naphthalene, methylnaphthalene, biphenyl, diphenylmethane, diphenylethane, phenanthrene, methyl- and dimethylphenanthrene, anthracene, methyl- and dimethylanthracene, di-pxylene, and dimethylstilbene. Methane and ethylene molecules were obtained from pyrolysis of xylene at higher temperature. The formations of methane and ethylene molecule increased at higher temperatures ( C) from 22 to 25.0 mol % and 0.1 to 1.7 mol% respectively. At higher temperatures, ring opening of xylene molecules is favored, which leads to the formation of non-aromatic compounds [109]. Further, the pyrolysis of the xylene also shows 5.3 mole % of toluene as a byproduct at lower temperature (830 C, andargon: xylene = 10:1). Based on the above discussion, it could be concluded that at lower temperatures, aromatic free radicals are present in significant concentrations, which deposit on copper surface. They further undergo surface diffusion on copper until they are stabilized by London dispersive forces and then cyclization of aromatic radical occurs by dehydrogenation. Finally, graphene is formed on copper foil. The temperature, for the decomposition of xylene is at 1000 C, results produced more number of free radicals with aromatic and non-aromatic compounds. So most of the carbon radicals are deposited on copper foil, which leads to the formation of multilayer graphene sheet Synthesis of graphene from ethanol Raman characterization was carried out on a JobinYvon Horiba LABRAM-HR 800 instrument with a laser source of 488nm (E= 2.53ev) and optical lens with optical magnification is 50x, a spot size of 0.59μm, a single monochromator and a Peltier cooled charge-coupled device. Raman mapping images show the typical Raman spectra of graphene films at selected locations, the graphene films grown from ethanol as carbon precursor. All the CVD graphene samples are etched from copper foil using ferric nitrate solution and then transfer to silicon wafer. Raman mapping was performed over an area of 20 x 16 μm on each sample, with each map consisting of 30 points of measurement Figure 4-20 shows the Raman mapping of 2D band features (a) FWHM (b) intensity (c) peak position of the 2D band. The 66

18 Figure 4-20 (a) 2D FWHM mapping (a) where size of the mapping with 30 points of the Raman measurement on 20 x16 μm area of graphene film. The 2D FWHM maps we see that most of the sample area had values cm -1 (red color region), corresponding to few layer graphene. The yellow colour region has cm -1 and green color region is above 76 cm -1 which indicates the presence of multilayer graphene. Figure 4-20 (c) mapping of the 2D peak position shows that the peak position at various points on the sample lie in the range of 2716 to 2720 cm -1. The peak position of the 2D band shifting towards a higher wave number is indicative of the presence of few layers to multilayer graphene. Figure 4-20 (b) shows the intensity of the 2D band, which lies between a.u (red to green colour) and a higher intensity is generally indicative of a better quality of graphene. Figure 4-21(a) shows the I 2D /I G ratio of 2D and G bands, which is related to number of the layer of graphene, where the ratio ranges between indicating few to multilayer graphene as also inferred from the FWHM of 2D band. Figure 4-22(a, b &c) shows the mapping of the D band features, FWHM of the D band 44±0.8 cm -1 due to presence of the structural defects and Figure 4-22 (b) shows the intensity of the D band, red colored area indicates the less defect compared to green colored (more defect due the graphene transferred from copper foil to silicon wafer). The quality of the graphene sheet measured by FWHM and the intensity of G band. Figure 4-23 (a) indicates the FWHM of the G-band with cm -1. A sharp G band indicates good quality of graphene sheet (lower values of FWHM of G band). The G band intensity map revealed that most of the sample area has higher intensity shown in yellow to green color region in Figure 4-23 (b). The G-peak position shifted cm -1, which indicates the change of the electronic properties of graphene due to the presence of oxygen atoms. The graphene layer quality and crystallinity are determined by intensity ratio of the G band and D band, Figure 4-21 (b) shows the mapping of I G /I D ratio, the ratio is with 20 16µM (Raman mapping area). The results show most of the area having good quality graphene sheet (light yellow to dark green colour) and few areas D band intensity higher compared to G band intensity. 67

19 Figure 4-20: Raman mapping of 2D-band features of graphene synthesized from ethanol at 1000 C. (a) FWHM, (b) Intensity and (c) Peak position of the 2D-band Figure 4-21: Raman mapping of features of graphene synthesized from ethanol at 1000 C (a) Intensity ratio of 2D/G and (b) intensity ratio of G/D 68

20 Figure 4-22: Raman mapping of D-band features of graphene synthesized from ethanol at 1000 C. (a) FWHM, (b) Intensity and (c) Peak position of the D-band. 69

21 Figure 4-23: Raman mapping of G-band features of graphene synthesized from ethanol at 1000 C. (a) FWHM, (b) Intensity and (c) Peak position of the G-band s Figure 4-24: TEM images of graphene from ethanol precursor Figure 4-24 (a-b) shows the TEM image of graphene samples, which is obtained from ethanol carbon precursor at 1000 C. Figure 4-24 (b) shows high resolution TEM image, where observed 5-10 layer of graphene and Figure 4-24 (a) shows lager area graphene folded with wrinkles. These results are well supported for micro Raman mapping (No. of the graphene layers). 70

22 4.4 Conclusions We have synthesized graphene and N-doped graphene from different carbon precursors at different temperatures ( C) on copper foil using atmospheric RF-CVD method. Raman analysis indicates that few layer graphenes are obtained from methane and acetonitrile precursor at 1000 C. The quality of the graphene as measursed by the intensity ratio of I G /I D, for the few-layer graphene sheets obtained from methane [I G /I D is 2.42] was better as compared to N-doped graphene [I G /I D is 0.82] due to presence of nitrogen atoms in the latter. This was also confirmed with XPS results. Graphene was also synthesized from aromatic carbon precursor (xylene) for the growth of graphene in the temperature range C using CVD. The results indicate that an increase in the synthesis temperature favors the formation few layer graphene sheets, while lower temperatures are more favorable for the synthesis of single layer graphene. Graphene was also synthesized from ethanol precursor on copper foil at 1000 C and the Raman features (FWHM, peak intensity, and peak position of the G, D, and 2D bands as well as the I G /I D and I 2D /I G ratios) were mapped. 71