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Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer.

Kwon HJ, Ha JM, Yoo SH, Ali G, Cho SO - Nanoscale Res Lett (2014)

Bottom Line: We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene.Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared.The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ; Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea.

ABSTRACT
Graphene can be synthesized from polyacrylonitrile (PAN) polymer through pyrolysis. A metal catalyst such as nickel (Ni) is required for the conversion of the polymer to graphene. The metal catalysts can be placed either atop or underneath the polymer precursor. We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene. Formation of the flake-like graphene is attributed to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared. The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.

No MeSH data available.


FE-SEM images of the Ni layers on PAN/SiO2/Si. After the pyrolysis at 800°C/10 min (b), 750°C/10 min (c), and 700°C/10 min (d). FE-SEM image of a pristine film is also shown in (a). Their corresponding Raman spectra of the samples after Ni etching were included in the images.
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Figure 5: FE-SEM images of the Ni layers on PAN/SiO2/Si. After the pyrolysis at 800°C/10 min (b), 750°C/10 min (c), and 700°C/10 min (d). FE-SEM image of a pristine film is also shown in (a). Their corresponding Raman spectra of the samples after Ni etching were included in the images.

Mentions: In order to identify that graphene is formed from PAN even though dewetting of the catalytic Ni film occurred, Raman mapping analysis on the pyrolyzed Ni-coated PAN films was carried out (Figure 4). The first column in Figure 4 shows the FE-SEM images of the Ni-coated PAN films with different Ni coating thickness before and after Ni etching. The other four columns denote 2D, D, G, and D/G mapping images, respectively. The blue color in the Raman mapping images indicates the lowest intensity of the signal, whereas brown color indicates the highest intensity. It is clearly shown that graphene was sparsely formed on the surface of SiO2, which is caused by the dewetting of Ni catalyst film. Moreover, from the FE-SEM images (the first column of Figure 4a,c) and 2D Raman mapping images (the second column of Figure 4a,c) of the pyrolyzed Ni-coated PAN films before Ni removal, we can find that graphene was mainly formed both on and around the Ni particles. The presence of graphene both on and around the Ni particles can be explained by the solubility of carbon source on the catalytic Ni particles and the diffusion of carbon from the Ni particles to outside of the particles during the pyrolysis procedure [30,31]. In addition, the FE-SEM images (the first column of Figure 4b,d) and 2D Raman mapping images (the second column of Figure 4b,d) of the films after Ni removal show that graphene was primarily synthesized underneath the Ni particles. Therefore, very non-uniform, flake-like graphene was fabricated by the pyrolysis of the Ni-coated PAN films due to the dewetting of the catalytic Ni films during the pyrolysis process.Flake-like graphene might be useful for certain applications, but uniform and interconnected graphene is necessary for the applications to electronic devices. Since such flake-like graphene is produced because of dewetting of Ni film on PAN, suppressing of dewetting is required for the fabrication of a continuous graphene film. As shown in Figure 3, when the thickness of Ni film on PAN was increased, the dewetting phenomenon was alleviated. However, a too thick Ni film may lead to the difficulty in removing the Ni film after the pyrolysis. Alternatively, we tried to decrease the pyrolysis temperature. When the pyrolysis temperature was decreased from 1,050°C to 800°C (Figure 5b), dewetting was significantly suppressed. However, many pores were still observed on the pyrolyzed nickel layer. Both the density and the size of the pores were gradually decreased as the pyrolysis temperature was further decreased down to 700°C, and consequently, a more uniform Ni film with less defects was prepared (Figure 5d). Definitely, lower pyrolysis temperature could almost completely prevent the dewetting of Ni coating. However, the quality of the synthesized graphene was deteriorated with the decrease in the pyrolysis temperature. As can be seen in the Raman spectra of Figure 5b,c,d, the intensity of D peak was gradually increased while the intensity of 2D peak was gradually decreased as the pyrolysis temperature was decreased, indicating that graphene with lower quality is produced.After eliminating the Ni layer, Raman mapping of the graphene film prepared by the pyrolysis process at 750°C was carried out to evaluate the spatial uniformity and coverage of the synthesized graphene (Figure 6). Although a slight deviation exists in the 2D map, the intensity ratio of G to 2D exhibits a considerably identical color tone, which means that the synthesized graphene is fairly uniform.


Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer.

Kwon HJ, Ha JM, Yoo SH, Ali G, Cho SO - Nanoscale Res Lett (2014)

FE-SEM images of the Ni layers on PAN/SiO2/Si. After the pyrolysis at 800°C/10 min (b), 750°C/10 min (c), and 700°C/10 min (d). FE-SEM image of a pristine film is also shown in (a). Their corresponding Raman spectra of the samples after Ni etching were included in the images.
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Figure 5: FE-SEM images of the Ni layers on PAN/SiO2/Si. After the pyrolysis at 800°C/10 min (b), 750°C/10 min (c), and 700°C/10 min (d). FE-SEM image of a pristine film is also shown in (a). Their corresponding Raman spectra of the samples after Ni etching were included in the images.
Mentions: In order to identify that graphene is formed from PAN even though dewetting of the catalytic Ni film occurred, Raman mapping analysis on the pyrolyzed Ni-coated PAN films was carried out (Figure 4). The first column in Figure 4 shows the FE-SEM images of the Ni-coated PAN films with different Ni coating thickness before and after Ni etching. The other four columns denote 2D, D, G, and D/G mapping images, respectively. The blue color in the Raman mapping images indicates the lowest intensity of the signal, whereas brown color indicates the highest intensity. It is clearly shown that graphene was sparsely formed on the surface of SiO2, which is caused by the dewetting of Ni catalyst film. Moreover, from the FE-SEM images (the first column of Figure 4a,c) and 2D Raman mapping images (the second column of Figure 4a,c) of the pyrolyzed Ni-coated PAN films before Ni removal, we can find that graphene was mainly formed both on and around the Ni particles. The presence of graphene both on and around the Ni particles can be explained by the solubility of carbon source on the catalytic Ni particles and the diffusion of carbon from the Ni particles to outside of the particles during the pyrolysis procedure [30,31]. In addition, the FE-SEM images (the first column of Figure 4b,d) and 2D Raman mapping images (the second column of Figure 4b,d) of the films after Ni removal show that graphene was primarily synthesized underneath the Ni particles. Therefore, very non-uniform, flake-like graphene was fabricated by the pyrolysis of the Ni-coated PAN films due to the dewetting of the catalytic Ni films during the pyrolysis process.Flake-like graphene might be useful for certain applications, but uniform and interconnected graphene is necessary for the applications to electronic devices. Since such flake-like graphene is produced because of dewetting of Ni film on PAN, suppressing of dewetting is required for the fabrication of a continuous graphene film. As shown in Figure 3, when the thickness of Ni film on PAN was increased, the dewetting phenomenon was alleviated. However, a too thick Ni film may lead to the difficulty in removing the Ni film after the pyrolysis. Alternatively, we tried to decrease the pyrolysis temperature. When the pyrolysis temperature was decreased from 1,050°C to 800°C (Figure 5b), dewetting was significantly suppressed. However, many pores were still observed on the pyrolyzed nickel layer. Both the density and the size of the pores were gradually decreased as the pyrolysis temperature was further decreased down to 700°C, and consequently, a more uniform Ni film with less defects was prepared (Figure 5d). Definitely, lower pyrolysis temperature could almost completely prevent the dewetting of Ni coating. However, the quality of the synthesized graphene was deteriorated with the decrease in the pyrolysis temperature. As can be seen in the Raman spectra of Figure 5b,c,d, the intensity of D peak was gradually increased while the intensity of 2D peak was gradually decreased as the pyrolysis temperature was decreased, indicating that graphene with lower quality is produced.After eliminating the Ni layer, Raman mapping of the graphene film prepared by the pyrolysis process at 750°C was carried out to evaluate the spatial uniformity and coverage of the synthesized graphene (Figure 6). Although a slight deviation exists in the 2D map, the intensity ratio of G to 2D exhibits a considerably identical color tone, which means that the synthesized graphene is fairly uniform.

Bottom Line: We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene.Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared.The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea ; Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, 29 Geumgu-gil, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea.

ABSTRACT
Graphene can be synthesized from polyacrylonitrile (PAN) polymer through pyrolysis. A metal catalyst such as nickel (Ni) is required for the conversion of the polymer to graphene. The metal catalysts can be placed either atop or underneath the polymer precursor. We observed that spatially non-uniform and disconnected graphene was fabricated when PAN film coated with a Ni layer was pyrolyzed, resulting in flake-like graphene. Formation of the flake-like graphene is attributed to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by decreasing the pyrolysis temperature, and hence, more uniform graphene could be prepared. The effects of Ni coating thickness and the pyrolysis temperature on the fabricated graphene have been experimentally analyzed.

No MeSH data available.