Hydrogen Kinetics on Scalable Graphene Growth by Atmospheric

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Hydrogen Kinetics on Scalable Graphene Growth by Atmospheric Pressure Chemical Vapor Deposition with Acetylene Mei Qi, Zhao-Yu Ren, Yang Jiao, Yixuan Zhou, Xinlong Xu, Weilong Li, Jiayuan Li, Xinliang Zheng, and Jintao Bai J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp403410b • Publication Date (Web): 18 Jun 2013 Downloaded from http://pubs.acs.org on June 20, 2013

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The Journal of Physical Chemistry

Hydrogen Kinetics on Scalable Graphene Growth by Atmospheric Pressure Chemical Vapor Deposition with Acetylene Mei Qi1, Zhaoyu Ren1,†,, Yang Jiao2, Yixuan Zhou1, Xinlong Xu1,*,, Weilong Li1, Jiayuan Li1, Xinliang Zheng2, and Jintao Bai1 1

Nanobiophotonic Center, State Key Lab Incubation Base of Photoelectric Technology and Functional Materials, and Institute of Photonics & Photon-Technology, Northwest University, Xi'an 710069, China 2 Physics Department, Northwest University, Xi’an 710069, China KEYWORDS graphene; hydrogen; acetylene; atmospheric pressure chemical vapor deposition; copper. ABSTRACT Acetylene (C2H2) and copper foil have been chosen as carbon precursor and catalyst respectively for the synthesis of graphene by atmospheric pressure chemical vapor deposition. Effects of hydrogen (H2) concentration on graphene growth have been studied by Raman spectroscopy and transmission electron microscope. Different to methane as carbon precursor, high-quality bilayer graphene films can be grown rapidly with the ratio of H2 and argon (Ar) flow rates (H2/Ar) range from 0.010 to 0.111. However, with the further increasing of H2 concentration (H2/Ar=0.250 and H2/Ar=0.429), multilayer graphene domains are dominant on top of the bilayer graphene films. These observations demonstrate that H2 serve as an activator of the surface bound carbon for the bilayer graphene growth, while show an etching effect that control the morphology, nucleation density, and nucleation size of the multilayer graphene domains. The results offer useful insights into the understanding of the kinetic effect of H2 on scalable synthesis of graphene with C2H2.

*

Corresponding author: [email protected].

†

Corresponding author: [email protected].

ACS Paragon Plus Environment

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1. INTRODUCTION Graphene, sp2-bonded carbon atoms in the format of honeycomb lattice, is a special twodimensional material attracted great attention 1-2. Since it was firstly mechanical exfoliated from highly-oriented pyrolytic graphite in 2004 3, various other synthetic approaches have been developed, for instant, epitaxial growth on single-crystal SiC 4, chemical reduction of exfoliated graphite oxide

5-6

, and chemical vapor deposition (CVD) on transition metals7-8. Graphene has

shown great potentials for electronic and optoelectronic applications terahertz modulator

10

and isolator

11

1-3, 9

, such as broadband

based on the intraband transition in graphene. In these

optoelectronic applications, large area, low defect density, and high uniformity graphene films are desirable. CVD on transition metals is a very effective method for such high quality graphene synthesis. Especially, graphene films grown on copper (Cu) foil

7-8, 12-13

by CVD have attracted

much interest due to the following reasons: (1) Cu has the lower solubility of carbon than nickel (Ni); Ni is also magnetic, while Cu diamagnetic; (2) Cu is cheap compared to platinum (Pt); (3) Graphene grown on Cu foil can be easily transferred to any substrates. According to the operating pressure, CVD can be classified as low-pressure CVD (LPCVD), ultrahigh vacuum CVD, and atmospheric press CVD (APCVD). The first two are popular for graphene synthesis but extra vacuum and prolonged time are needed. However, APCVD is a cost-efficient, scalable, quick, and high-throughput method for large scale graphene films synthesis. The graphene films grown on Cu foil by LPCVD or APCVD show different kinetics 14, which further control the morphology and defects of as-growth graphene

12, 15

. There are several issues

such as crystalline of Cu, thermodynamics, and kinetics, which influence the graphene growth with CVD

15-18

. Wood et al.

15

have suggested that engineering Cu foils with (111) surfaces is

more suitable for the formation of uniform monolayer graphene films. Thermodynamics such as growth temperatures 16 and thermal annealing 17 play an important role in the growth mechanism. Hydrogen (H2) flow control

18

, have a considerable kinetic contribution to the quality of as-

grown graphene, while are reported controversially in the growth process. For example, Gao et al.

12

reported high quality graphene growth on Cu by APCVD with methane (CH4) without H2.

However, Vlassiouk et al.

18

suggested that H2 is necessary for graphene synthesis with CH4 by

CVD and the size and morphology of graphene domains can be regulated by H2 concentration. Although Gao et al. 12 synthesized the graphene without H2 supply, the pyrolysis of CH4 provides active hydrogen, which also play a considerable role in the graphene synthesis 18.


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CH4 is the most used precursor for the graphene synthesis due to the low pyrolysis rate, yet ethylene (C2H4) and acetylene (C2H2) with high pyrolysis rate can dramatically decrease the defects in as-grown graphene due to the divacancy defects healing mechanism

19

. However, to

our best knowledge, there are limited investigations on the effect of H2 concentration on the growth process of the graphene by APCVD using C2H2 as carbon source. In this paper, we synthesized graphene films by APCVD on Cu foils with C2H2. Effects of H2 concentration on graphene growth were studied by Raman spectroscopy and transmission electron microscope (TEM). High-quality bilayer graphene films were rapidly synthesized with the ratio of H2 and Ar flow rates (H2/Ar) range from 0.010 to 0.111. In addition, multilayer graphene domains were observed on top of the bilayer graphene films under the relatively large H2 concentration (H2/Ar=0.250 and H2/Ar=0.429). These suggest that H2 serve as an activator of the bound carbon for the bilayer graphene growth, while show an etching effect that control the morphology, nucleation density, and nucleation size of the multilayer graphene domains. The results offer useful insights into the understanding of the kinetic effect of H2 on the quality of graphene synthesized with C2H2. 2. EXPERIMENT METHOD Graphene films were synthesized by a home-built APCVD with a quartz tube furnace. C2H2 served as the carbon precursor and Cu foil (Alfa Aesar, 25-µm thick, square area of ~6 cm2, 99.8%) served as the catalyst. The Cu foils were cleaned by an ultrasonic cleaner for ~15 min in alcohol and acetone, respectively. The synthesis process by APCVD can be described as follows: the Cu foil was heated to 1000 °C at a rate of 15 °C/min and then annealed for 20 min under the constant flowing of 300 standard-state cubic centimeters per minute (sccm) Ar and 300 sccm H2. During the growth process of graphene, the total gas flow rates of H2 and Ar were kept constantly at 1000 sccm, while the ratio of H2 and Ar are changed to understand the effects of the H2 on the graphene formation. The H2 flow rate was set to 0, 10, 50, 100, 200, and 300 sccm, respectively. The grown time was set to 10 min. Finally, the sample was cooled down to room temperature at a rate of 10 °C/min under the same H2/Ar atmosphere as the gown condition but without C2H2 supply.

For the study of the effect of growth temperature, C2H2 concentration, and H2

concentration on the graphene growth, we always keep the same cooling rate in order to minimize the effects from the change of cooling rate.


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After the synthesis, the Cu foils with as-grown graphene on top of it were etched out with the etchant (ferric nitrate (1 g) and hydrochloric acid (1 ml) in deionized water (25 ml)). Then the graphene films were transferred onto a SiO2/Si substrate for Raman spectroscopy characterization (Laboratory Ram HR800, excitation wavelength at 514 nm) and onto a copper grid for transmission electron microscope (TEM) characterization (Tecnai G2 F20 S-TWIN, point resolution: 0.24 nm, line resolution: 0.14 nm).

3. RESULTS AND DISCUSSION

Figure 1. Raman spectra of graphene films synthesized with (a) different growth temperature (850, 900, 950, and 1000 °C ) and (b) different C2H2 flow rate (1, 3, and 12 sccm), respectively. Graphene growth is determined by both thermodynamics (such as temperature) and kinetics (such as carbon source concentration and H2 concentration). To keep other thermodynamics and kinetics influence factor under the minimal impact, we have optimized the growth temperature and C2H2 flow rate firstly and then kept all the experimental parameters fixed except for the H2 concentration. Fig. 1(a) shows the Raman spectra of graphene films synthesized with different growth temperature from 850 °C to 1000 °C . During the growth process, the C2H2, H2, and Ar flow rate was set to 10, 250, and 750 sccm, respectively. The Raman spectra in Fig. 1(a) consist of D band at 1342-1355 cm-1, G band at 1581-1588 cm-1, and 2D band at 2694-2716 cm-1. The relative intensity ratio of D and G band (ID/IG) can be used to probe the defects in graphene films, while the relative intensity ratio of G and 2D band (IG/I2D) can be used to probe the number of layers 20-22. It is evident that G band becomes sharper with the increasing of growth temperature,


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while the ID/IG decreases (Fig.1 (a)). This indicates that the defect density decreases and the crystallization of graphene increases with the increasing of temperature. Since the melting point of Cu foil is 1080 °C , we keep the growth temperature under 1000 °C . Fig. 1(b) shows the Raman spectra of graphene films synthesized with different C2H2 flow rate from 12 sccm down to 1 sccm. The growth temperature was set to 1000 °C and, the H2 and Ar flow rate was set to 250 sccm and 750 sccm. It can be clearly seen that the G band also becomes sharper with the decreasing of C2H2 flow rate, while the ID/IG decreases (Fig. 1(b)). The result manifests that the lower the C2H2 flow rate is, the higher the crystallization is. In the following experiments, we set growth temperature to 1000 °C and C2H2 flow rate to 1 sccm to minimize the effects from growth temperature and C2H2 flow rate on the defect density and crystallization.

Figure 2. (a) Raman spectra of graphene films synthesized with different H2 concentration (H2 flow rate is 0, 10, 50, 100, 200 and 300 sccm, respectively, and the total H2 and Ar flow rate is set to 1000 sccm) in the growth process. (b) The ID/IG (black squares) and IG/I2G (blue squares) of the Raman spectra in Fig. 2(a). (c) The 2D FWHM (black squares) and 2D shift (blue squares) of the Raman spectra in Fig. 2(a). Fig. 2(a) shows the Raman spectra of the graphene films synthesized at different H2 flow rate of 0, 10, 50, 100, 200, and 300 sccm, respectively, but the constant H2 and Ar total flow rate of 1000 sccm. Fig. 2(b) summarizes the ID/IG (black squares) and IG/I2D (blue squares) of the Raman


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spectra in details. When the H2 and Ar flow rate is 0 sccm and 1000 sccm, the ID/IG and IG/I2D are 0.6 and 1.692, respectively. The results indicate that graphene films grown without H2 are multiple layers with much higher defect density. However, when the flow rate of H2 and Ar is set to 10 sccm and 990 sccm, the G band becomes sharper and the ID/IG is only 0.05. Although the ID/IG increases with the increasing of the H2 concentration, the maximum defect rate is only 0.208. The result suggests that the graphene films with high quality can be obtained with the presence of H2.

Figure 3. (a) Four-component decomposition of the 2D band in graphene films synthesized with 100 sccm H2 and 900 sccm Ar. The inset shows the model structure of AB-stacked bilayer graphene. (b) SEM image of graphene grown with 100 sccm H2 and 900 sccm Ar, directly on Cu foil. The inset shows a photograph of the sample on a SiO2/Si substrate. (c) TEM image of the graphene grown with 100 sccm H2 and 900 sccm Ar. (d) HR-TEM image of the graphene grown with 100 sccm H2 and 900 sccm Ar. The as-grown graphene films with the H2 and Ar flow rates range from 10 and 990 sccm to 100 and 900 sccm are bilayer graphene, which are verified with the IG/I2D from 0.77 to 1.09 (Fig. 2(b)) and the 2D full width at half maximum (FWHM) from 44.04 cm-1 to 59 cm-1 (Fig. 2(c)).


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Although bilayer graphene grown on Cu surface by LPCVD has been reported, it needs 180 min to grow the second layer on the first graphene layer

23

, while our synthesis time is 10 min. Fig.

3(a) shows 2D band of the graphene growth with 100 sccm H2 and 900 sccm Ar. The spectra can be decomposed into four components of Gaussian distribution, which indicates the sample is AB-stacked as discussed by Malard 21. The inset in Fig. 3(a) shows an atomic structure model of AB-stacked bilayer graphene

24

. The carbon atoms in the first layer are indicated by the red

points, and atoms in the second layer indicated by the blue points. Fig. 3(b) shows the scanning electron microscope (SEM) image of the graphene directly grown on Cu foil, and the inset shows a photograph of the graphene films on a SiO2/Si substrate. It is evident that the large scale graphene films are uniform through the SEM and photograph. TEM image of the sample (Fig. 3(c)) indicates that no graphene nucleuses continue to form on top of the bilayer graphene, and the HR-TEM (Fig. 3(d)) suggests the bilayer graphene films have a good crystallization degree.

Figure 4. (a), (b), (c) TEM image of the graphene films synthesized with 200 sccm H2 and 800 sccm Ar. (d) The zoom-in part of the black square in (c). (e) Electron diffraction pattern from the yellow circle area (2-layer graphene films) in Fig. 4(b). (f) Electron diffraction pattern from the blue circle area (new crystalline nucleuses on the 2-layer graphene films) in Fig. 4(b).


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With the H2 and Ar flow rates increasing from 100 and 900 sccm to 300 and 700 sccm, the IG/I2D increases from 1.09 to 1.6, the 2D FWHM becomes broader from 59 cm-1 to 63 cm-1, and 2D shift is blue-shifted from 2697 cm-1 to 2705 cm-1 (as shown in Fig. 2(b), (c)). These results indicate more layers of graphene films could be deposited on top of bilayer graphene with the increasing of H2 concentration. We choose the graphene films grown with 200 sccm H2 and 800 sccm Ar as an example to identify the multilayer graphene growth with the increasing of H2 concentration. Fig. 4(a-d) shows the different magnification of TEM images of the same sample supported by a copper grid. Fig. 4(a) demonstrates that bilayer graphene films cover the whole Cu foil with some graphene domains on top of it. Zoom-in part of Fig. 4(a) shows more clearly that besides the continuous bilayer graphene (yellow circle in Fig. 4(b)), there are several graphene crystalline nucleuses on top of it (blue circle in Fig. 4(b)). Fig. 4(e) shows the selective electron diffraction from the yellow circle area in Fig. 4(b), which suggests large single crystal area of bilayer graphene films and it is consistent with the Raman spectrum. Fig. 4(f) shows the selective electron diffraction from the blue circle area in Fig. 4(b). The diffraction pattern is blurred due to the graphene nucleate on the bilayer graphene, which may cause inhomogeneity for bilayer graphene films. However, the adjacent crystalline nucleuses can join together in the red square of Fig. 4(c) and could be used for multilayer graphene synthesis. Fig. 4(d) is the zoomin part of the black square in Fig. 4(c), which show clearly multilayer graphene with the number of layers from 2-5, as shown with the yellow, red, green, and blue circle areas in Fig. 4(c) and Fig. 4(d).

Figure 5. (a), (b) TEM images of the graphene films grown with 300 sccm H2 and 700 sccm Ar and (c) HR-TEM image of the hexagon graphene grains in (b).


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Besides the effect of H2 on the layer of graphene domains, H2 concentration also affects graphene nucleation size and nucleation density. Fig. 5 shows the TEM images of the graphene films grown with 300 sccm H2 and 700 sccm Ar. Compared to the graphene films grown with 200 sccm H2 and 800 sccm Ar (Fig. 4(a)), lower nucleation density on top of the continuous graphene films is observed in Fig. 5(a), while the nucleation shape is hexagon and the size is about 400 nm as shown in Fig. 5(b). The result indicates that the nucleation size becomes bigger with a lower density of nucleation when the H2 flow rates increase from 200 sccm to 300 sccm. Fig. 5(c) shows the HR-TEM image from the hexagon graphene grains, it reveals that graphene grains are highly ordered and crystalline over the whole 400 nm hexagon. Recently, Vlassiouk et

al.

18

observed hexagonal domains when they used CH4 as carbon source, and they attributed

them to the etching effect from the H2, which could control the size and morphology of the graphene domains. Similarly, our results also suggest that the multilayer graphene nucleation is dominant by the etching effect of the H2. Usually, ID/IG ratios can be used to probe the defects in graphene films. As shown in Figure 2(b), the ID/IG ratios follow two stages. In Stage 1, ID/IG decrease with the increasing of H2 concentration, which suggest the decreasing of defects in graphene films due to better crystallization in the existence of H2. In Stage 2, with increasing of H2 concentration, the nucleation grains as shown in Figure 5 increase, which reduce the nucleation density. Since the increasing of nucleation grains, the defects of edges and grain boundaries will decrease, resulting in decreasing of ID/IG ratios, which is against the observation in Figure 2(b). These defects are mainly one-dimensional defects, which frequently separate domains of different crystal orientation25. However, besides the defects from edges and grain boundaries, the defects of graphene can also come from point defects, such as Stone-wales defects, single vacancies, multiple vacancies, carbon adatoms, foreign adatoms, substitutional impurities, topology of defects, and so on.25-26. All the defects may lay within grains and also can cause the increasing of ID/IG ratio. With a small amount of H2, H2 can act as catalyst for the synthesis of graphene. However, with the increasing of H2, etching effect will be promoted 18. Redundant H2 can also produce enough active hydrogen, which in turn will hydrogenate graphene 27. This will produce sp3-style defects with the high H2 concentration increasing from 100 sccm to 300 sccm, resulting in the ID/IG ratios increasing 28.


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Scheme 1. The reaction process of graphene formation with C2 H 2 Cu + H2 → 2Hs Cu + C2H2 → (C2H)s + Hs Cu + C2H2 → (C2H2)s (C2H2)s + Hs → (C2H)s + H2 (C2H)s + graphene → (graphene + C) + H2 Hs + graphene → (graphene - C) + (C2H)s

(1) (2) (3) (4) (5) (6)

Zhang et al. 29 have studied the thermodynamics of graphene growth on Cu surfaces with firstprinciples, which suggested that the active carbon species for graphene synthesis mainly be CHx instead of atomic carbon. Similarly to the CH4 process

18

, the reaction process of graphene

formation with C2H2 is suggested in Scheme 1 including adsorption of H2 to form active Hs (1), pyrolysis of C2H2 (2), adsorption of C2H2 (3), hydrogen assisted pyrolysis of C2H2 (4), graphene growth from active species (5), and etching reagent for graphene (6). Graphene can grow without H2, however, with H2, it will promote the process through reactions 1 and 4, which suggest the catalytic effect with H2. On the other hand, for the multilayer graphene synthesis, only graphene domains shown, which also illustrate that the H2 can etch the graphene and limit the continuous growth of graphene domains through reaction 6. The graphene growth process is actually the competition between reaction 5 and reaction 6. Reactions for graphene growth are related to the chemical potential of carbon ( µ C ). The carbon chemical potential for CH4 has been discussed by Zhang et al., who have proposed

µ = −2 µ − 10.152 + 0.112 ln C

H

1300 K

29

PCH 4 PH 2 , and µ H = −0.975 + 0.056 ln , for growth temperature at P0 P0

. Where P 0 is a reference pressure, and PCH 4 , PH 2 are real pressures of CH4 and H2,

respectively. Therefore, at a fixed the

same

method,

we

µ = − µ − 8.352 + 0.056 ln C

H

PCH 4 , µ C is related to the H2 partial pressures. According to P0

obtain

the

carbon

chemical

potential

for

C 2 H2

as,

PC 2 H 2 . The carbon chemical potential difference between C2H2 and P0

CH4 can be written as ∆µ C = µ H + 1.8 + 0.056 ln

PC 2 H 2 PCH 4 − 0.112 ln . The calculation indicates P0 P0


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that C2H2, different with CH4, is sensitive to the hydrogen chemical potential. However, at the same partial pressure of H2, C2H2 is less sensitive to the pressure ratio of H2 to precursor.

Figure 6. Illustration of the graphene films formation with different H2 concentration in the growth process. Fig. 6(a-c) illustrates vividly the H2 concentration on the graphene films formation with C2H2. Without additional H2 in the growth process, more graphene nucleation and some C-C bonds form irregularly on the Cu surface, resulting in multilayer graphene with lower quality (Fig. 6(a)). With the addition of H2 such as H2/Ar range from 0.010 to 0.111, high quality bilayer graphene films are synthesized (Fig. 6(b)). With the H2 concentration continue increasing (H2/Ar from 0.250 to 0.429), new graphene nucleation deposits on the top of bilayer graphene (Fig. 6(c)).

4. CONCLUSION


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In summary, we have studied the effects of H2 concentration on graphene growth by APCVD with C2H2. High-quality bilayer graphene films can be grown rapidly with the H2/Ar flow ratio range from 0.010 to 0.111. Then, we have analyzed the multilayer graphene domains on top of the bilayer graphene in the growth process under the relatively large H2 concentration (H2/Ar=0.25 and H2/Ar=0.429). These suggest that H2 serve as an activator of the surface bound carbon for bilayer graphene, while show an etching effect that control the morphology, nucleation density, and nucleation size of the multilayer graphene domains. The results offer useful insights into the understanding of the kinetic effect of H2 on the quality of graphene synthesis with C2H2.

AUTHOR INFORMATION Corresponding Author *Telephone: +86-29-88303336. Fax: +86-29-88303336. E-mail: [email protected]. †Telephone: +86-29-88303336. Fax: +86-29-88303336. E-mail: [email protected].

ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (No. 61275105, 10974152, 21006079), Natural Science Basic Research Plan in Shaanxi Province of China (No. 2012KJXX-27),

Ph.D.

Programs

Foundation

of

Ministry

of

Education

of

China

(No.20106101110017), and Northwest University Cross-discipline Fund for Postgraduate Students (YZZ12027). Dr. Xu acknowledges support from the open foundation of State Key Lab Incubation Base of Photoelectric Technology and Functional Materials (No. ZS12018).

ABBREVIATIONS CVD, chemical vapor deposition; APCVD, atmospheric press CVD; LPCVD, low-pressure CVD; sccm, standard-state cubic centimeters per minute.

REFERENCES


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(29) Zhang, W.; Wu, P.; Li, Z.; Yang, J. First-principles Thermodynamics of Graphene Growth on Cu Surfaces. J. Phys. Chem. C 2011, 115, 17782-17787.

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Hydrogen Kinetics on Scalable Graphene Growth by Atmospheric

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