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Oxidative-Etching-Assisted Synthesis of Centimeter-Sized Single-Crystalline Graphene Wei Guo, Feng Jing, Jian Xiao, Ce Zhou, Yuanwei Lin, and Shuai Wang* Chemical vapor deposition (CVD) growth of graphene has recently developed into a scalable and effective method that is superior to other strategies of graphene synthesis.[1–5] The growth of high-quality graphene is strongly desired as the candidate material for the next-generation electronics and optoelectronics,[6–8] because the scattering effect of graphene grain boundaries formed in the CVD procedure will significantly disrupt the intrinsic carrier transport performance.[9–13] Currently, the main approaches to synthesize grain boundaryfree graphene film are to: i) suppress nucleation density during the growth process in order to obtain as large single graphene domains as possible; or ii) induce the regular alignment and commensurate stitching of graphene domains in the same crystal orientation. To this end, great efforts have been made to engineer empirical CVD conditions, such as substrate roughness, temperature, chamber pressure, and reacting gas ratios,[14–16] as well as to utilize a growth–etching–regrowth strategy or O2 pretreatment prior to growth.[17–21] Among these strategies, the presence of surface O is the most critical parameter for centimeter-sized single-crystalline graphene growth. However, special substrate structures such us Cu pockets or tubes, are required for this process, and are not suitable for post-growth industrial procedures, such as roll-to-roll technology.[2] In addition, large-area grain boundary-free graphene film can be achieved by the seamless stitching of unidirectional graphene domains aligned on single-crystalline substrates.[22–24] Nevertheless, these methods primarily depend on sophisticated substrate pretreatment procedures and suffer from variability in the quality of substrate. These drawbacks have, therefore, restrained the compatibility of single-crystalline graphene growth with currently existing industrial technology. In this study, we propose a general synthetic approach that rationally controls large-sized single-crystalline graphene W. Guo, F. Jing, J. Xiao, Prof. S. Wang Key Laboratory of Material Chemistry for Energy Conversion and Storage, Minisrty of Education School of Chemistry and Chemical Engineering Huazhong University of Science and Technology Wuhan 430074, P. R. China E-mail: [email protected] C. Zhou, Y. Lin Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species College of Chemistry and Molecular Engineering Peking University Beijing 100871, P. R. China
DOI: 10.1002/adma.201503705
3152
wileyonlinelibrary.com
growth on Cu foil. It was found that trace amounts of gaseous oxidants are primarily responsible for graphene etching, which is critical for modulating graphene nucleation and subsequent growth behavior. Graphene nucleation density at the expected low level required for centimeter-scale domains can be achieved via fine-tuning the O2 concentration and balancing the relative ratios of O2/CH4 and O2/H2. Utilizing this approach, we developed a simple and reliable method of controlling the synthesis of high-quality graphene materials. First, it should be noted that an uncertain amount of H2O vapor impurities inherently exist in commercially available H2 feedstock during industrial electrolysis production. In contrast with the conventional atmospheric pressure CVD method, in which CH4 and H2 feedstock balanced in Ar directly flow into the reaction chamber, a gas purifier (a tube filled with a drying agent such as KOH) was installed into our CVD system, as illustrated in Figure 1a. This enabled the system to substantially absorb the majority of H2O vapor impurities that are mixed in the CVD atmosphere prior to the reaction. We were then able to fine-tune the concentration of O2 with an individual mass flow controller. Here, we present the synthesis of graphene achieved by three different CVD systems: O2-free (without H2O purifying), O2-free (H2O purified), and O2-assisted (H2O purified) CVD, which are defined as O2-free CVD (H2O), O2-free CVD, and O2-assisted CVD, respectively, in order to simplify the description. Additionally, the CH4 and O2 feedstock in the following discussion are both 0.9% diluted in Ar. The growth processes are shown in detail in Figure S1 (Supporting Information) and the Experimental Section. We first investigate the influence of gaseous oxidants on graphene nucleation density. Figure 1b illustrates the results of the as-grown graphene from different CVD processes. In the O2-free CVD (H2O) process, the fluctuation in nucleation density is due to uncertain amounts of H2O vapor impurities, which dramatically disrupt the steady growth conditions. Following the purification of H2O vapor, graphene growth is steady in both O2-free CVD and O2-assisted CVD, with the nucleation density gradually decreasing as the O2 concentration increases. Figure 2 shows the typical experimental observations from different CVD processes under the same growth conditions of 0.8 sccm CH4, 4.5 sccm H2, and 320 sccm Ar, in which H2 is from different cylinders. It is clearly shown in Figure 2a–d that graphene nucleation density varies in significance because of different contents of H2O vapor impurities. However, following the purification of H2O vapor, the reaction atmosphere is free of oxidants and the graphene nucleation density increases (Figure 2e). In addition, as a growing amount of excess O2 is introduced into the system, graphene nucleation density gradually decreases (Figure 2f–h). The nucleation density of
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 3152–3158
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a) Drawing of the improved CVD system. A tube filled with KOH as the gas purifier is installed between the reacting chamber and mass flow controllers. The majority of the H2O vapor impurities can be absorbed through the purifier. b) Illustration of grown graphene from different CVD processes. The C(H2O) and C(O2) represent the concentration of H2O vapor impurities and introduced O2, respectively.
repeatedly grown graphene from different CVD processes with H2 feedstock from six different cylinders is summarized in Figure S2 (Supporting Information). We found that the value for both the O2-free CVD and O2-assisted CVD, atmosphere remains fairly constant, but is highly variable in the O2-free CVD (H2O) process, with a maximum difference of approximately two orders of magnitude. Therefore, this demonstrates that graphene growth can be well-controlled by precisely tuning the concentration of O2 oxidants.
Since the H2 concentration (>1%) is much greater than the introduced O2 (
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Oxidative-Etching-Assisted Synthesis of Centimeter-Sized Single-Crystalline Graphene Wei Guo, Feng Jing, Jian Xiao, Ce Zhou, Yuanwei Lin, and Shuai Wang* Chemical vapor deposition (CVD) growth of graphene has recently developed into a scalable and effective method that is superior to other strategies of graphene synthesis.[1–5] The growth of high-quality graphene is strongly desired as the candidate material for the next-generation electronics and optoelectronics,[6–8] because the scattering effect of graphene grain boundaries formed in the CVD procedure will significantly disrupt the intrinsic carrier transport performance.[9–13] Currently, the main approaches to synthesize grain boundaryfree graphene film are to: i) suppress nucleation density during the growth process in order to obtain as large single graphene domains as possible; or ii) induce the regular alignment and commensurate stitching of graphene domains in the same crystal orientation. To this end, great efforts have been made to engineer empirical CVD conditions, such as substrate roughness, temperature, chamber pressure, and reacting gas ratios,[14–16] as well as to utilize a growth–etching–regrowth strategy or O2 pretreatment prior to growth.[17–21] Among these strategies, the presence of surface O is the most critical parameter for centimeter-sized single-crystalline graphene growth. However, special substrate structures such us Cu pockets or tubes, are required for this process, and are not suitable for post-growth industrial procedures, such as roll-to-roll technology.[2] In addition, large-area grain boundary-free graphene film can be achieved by the seamless stitching of unidirectional graphene domains aligned on single-crystalline substrates.[22–24] Nevertheless, these methods primarily depend on sophisticated substrate pretreatment procedures and suffer from variability in the quality of substrate. These drawbacks have, therefore, restrained the compatibility of single-crystalline graphene growth with currently existing industrial technology. In this study, we propose a general synthetic approach that rationally controls large-sized single-crystalline graphene W. Guo, F. Jing, J. Xiao, Prof. S. Wang Key Laboratory of Material Chemistry for Energy Conversion and Storage, Minisrty of Education School of Chemistry and Chemical Engineering Huazhong University of Science and Technology Wuhan 430074, P. R. China E-mail: [email protected] C. Zhou, Y. Lin Beijing National Laboratory for Molecular Sciences State Key Laboratory for Structural Chemistry of Unstable and Stable Species College of Chemistry and Molecular Engineering Peking University Beijing 100871, P. R. China
DOI: 10.1002/adma.201503705
3152
wileyonlinelibrary.com
growth on Cu foil. It was found that trace amounts of gaseous oxidants are primarily responsible for graphene etching, which is critical for modulating graphene nucleation and subsequent growth behavior. Graphene nucleation density at the expected low level required for centimeter-scale domains can be achieved via fine-tuning the O2 concentration and balancing the relative ratios of O2/CH4 and O2/H2. Utilizing this approach, we developed a simple and reliable method of controlling the synthesis of high-quality graphene materials. First, it should be noted that an uncertain amount of H2O vapor impurities inherently exist in commercially available H2 feedstock during industrial electrolysis production. In contrast with the conventional atmospheric pressure CVD method, in which CH4 and H2 feedstock balanced in Ar directly flow into the reaction chamber, a gas purifier (a tube filled with a drying agent such as KOH) was installed into our CVD system, as illustrated in Figure 1a. This enabled the system to substantially absorb the majority of H2O vapor impurities that are mixed in the CVD atmosphere prior to the reaction. We were then able to fine-tune the concentration of O2 with an individual mass flow controller. Here, we present the synthesis of graphene achieved by three different CVD systems: O2-free (without H2O purifying), O2-free (H2O purified), and O2-assisted (H2O purified) CVD, which are defined as O2-free CVD (H2O), O2-free CVD, and O2-assisted CVD, respectively, in order to simplify the description. Additionally, the CH4 and O2 feedstock in the following discussion are both 0.9% diluted in Ar. The growth processes are shown in detail in Figure S1 (Supporting Information) and the Experimental Section. We first investigate the influence of gaseous oxidants on graphene nucleation density. Figure 1b illustrates the results of the as-grown graphene from different CVD processes. In the O2-free CVD (H2O) process, the fluctuation in nucleation density is due to uncertain amounts of H2O vapor impurities, which dramatically disrupt the steady growth conditions. Following the purification of H2O vapor, graphene growth is steady in both O2-free CVD and O2-assisted CVD, with the nucleation density gradually decreasing as the O2 concentration increases. Figure 2 shows the typical experimental observations from different CVD processes under the same growth conditions of 0.8 sccm CH4, 4.5 sccm H2, and 320 sccm Ar, in which H2 is from different cylinders. It is clearly shown in Figure 2a–d that graphene nucleation density varies in significance because of different contents of H2O vapor impurities. However, following the purification of H2O vapor, the reaction atmosphere is free of oxidants and the graphene nucleation density increases (Figure 2e). In addition, as a growing amount of excess O2 is introduced into the system, graphene nucleation density gradually decreases (Figure 2f–h). The nucleation density of
© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Mater. 2016, 28, 3152–3158
www.advmat.de www.MaterialsViews.com
COMMUNICATION Figure 1. a) Drawing of the improved CVD system. A tube filled with KOH as the gas purifier is installed between the reacting chamber and mass flow controllers. The majority of the H2O vapor impurities can be absorbed through the purifier. b) Illustration of grown graphene from different CVD processes. The C(H2O) and C(O2) represent the concentration of H2O vapor impurities and introduced O2, respectively.
repeatedly grown graphene from different CVD processes with H2 feedstock from six different cylinders is summarized in Figure S2 (Supporting Information). We found that the value for both the O2-free CVD and O2-assisted CVD, atmosphere remains fairly constant, but is highly variable in the O2-free CVD (H2O) process, with a maximum difference of approximately two orders of magnitude. Therefore, this demonstrates that graphene growth can be well-controlled by precisely tuning the concentration of O2 oxidants.
Since the H2 concentration (>1%) is much greater than the introduced O2 (
