Synthesis of flake-like graphene from nickel-coated polyacrylonitrile polymer
© Kwon et al.; licensee Springer. 2014
Received: 5 September 2014
Accepted: 11 October 2014
Published: 18 November 2014
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.
Ever since the discovery of graphene , research on graphene, a flat monolayer of carbon atoms arranged in a two-dimensional (2D) honeycomb lattice , has progressed rapidly. Due to the relatively simple and cheap procedures to obtain high-quality graphene  and its outstanding properties such as high electron mobility at room temperature , high intrinsic mechanical strength , high thermal conductivity , and complete impermeability to gas , graphene can be exploited in a variety of fields like electronics, photonics, energy generation and storage, sensor, and bio applications . So far, many methods to obtain graphene have been developed, including mechanical cleavage of graphite , chemical exfoliation [8–10], epitaxial growth [11, 12], chemical vapor deposition (CVD) [13–16], and solid-phase method [17–23].
The solid-phase method employs transition metals such as Ni and Cu as a catalyst to form graphene from solid-state carbon sources such as polymer, SiC, small molecule, and self-assembled monolayer. Particularly, in the method using polymer as a precursor of graphene, various polymers like polyacrylonitrile (PAN), polystyrene, and polymethylmethacrylate were used, and the polymer placed either atop or underneath metal catalysts were pyrolyzed in a reductive gas to form graphene [20–23]. When graphene is synthesized from a polymer precursor on a metal catalyst, an additional process to transfer the synthesized graphene on an insulator such as SiO2 is required for the application to electronic device [14, 24]. This transfer procedure can result in the degradation of the synthesized graphene. The opposite case where polymer precursor is underneath a metal catalyst can solve this problem; however, few results have been reported on this case [22, 23].
Here, we present systematic experimental results to synthesize graphene on a SiO2/Si substrate from PAN coated with a Ni film through pyrolysis. The Ni coating layer tends to be aggregated to form particulates due to dewetting  at a high pyrolysis temperature, and hence, the synthesized graphene was not generally continuous. The effects of the Ni film and pyrolysis temperature on the quality of graphene were investigated. As a consequence, continuously connected graphene could be prepared by reducing the pyrolysis temperature.
Polyacrylonitrile (PAN, Sigma-Aldrich, St. Louis, MO, USA, Mw =150,000) (0.5 wt.%) dissolved in N,N-Dimethylformamide (DMF, Showa Chemical, Tokyo, Japan) was spin-coated on 1 × 1 cm2 SiO2 (300 nm thickness)/Si wafers. Subsequently, a Ni layer was coated on the spin-coated PAN/SiO2/Si substrates with a magnetron sputtering system. The sputtering rate was approximately 10 nm/min, and the thickness of the Ni layer was changed by the sputtering time. The Ni-coated PAN/SiO2/Si samples were pyrolyzed in a high-vacuum furnace; the vacuum level in the furnace was roughly 10-5 Torr.
During the pyrolysis process, the samples were gradually annealed with a heating rate of 8°C/min to a maximum temperature and then were quickly cooled down by moving the heating zone of the furnace to the opposite side. The maximum temperature was changed from 1,050°C to 700°C (Since we exploited a high-vacuum furnace made of quartz for pyrolysis, the maximum temperature has to be limited up to 1,100°C. Besides, although the melting point of nickel is approximately 1,450°C at 1 atm., a very thin nickel thickness (up to 200 nm) is easily agglomerated in the vacuum atmosphere. So the temperature of 1,050°C was selected as the maximum temperature. In the case of the minimum temperature range, the temperature where graphene is formed and the agglomeration of the nickel layer is suppressed was selected as the minimum temperature. Therefore, the temperature ranged from 1,050°C to 700°C was selected for the pyrolysis).
Results and discussion
Evaluation of FWHM 2D , I (D) / I (G) , and I (G) / I (2D) for the Ni-coated PAN/SiO 2 /Si films after pyrolysis and subsequent nickel removal
We have demonstrated that spatially non-uniform and flake-like graphene is synthesized when Ni-coated PAN film is pyrolyzed at a high temperature. Such non-uniform graphene is produced due to the dewetting of the Ni layer coated on the PAN film. Dewetting phenomenon can be reduced by increasing the Ni thickness and/or by decreasing the pyrolysis temperature. However, as the pyrolysis temperature is decreased, graphene with lower quality is synthesized. Therefore, it is important to optimize both the Ni thickness and the pyrolysis temperature considering the necessary quality of the synthesized graphene and required spatial uniformity for certain applications. In addition, non-uniform and flake-like graphene is not so good for the application to electronic devices; however, such flake-like graphene might be useful for certain applications of graphene (e.g., gas sensor and energy storage [32, 33]) if the flake size can be controlled through future studies.
This work was supported by a National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST) (no. 2012-0009523).
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