Large-scale in-situ synthesis of nitrogen-doped graphene using magnetically rotating arc plasma
Graphical abstract
Introduction
Graphene is a two-dimensional sheet of sp2-hybridized carbon that has ignited a research boom in the scientific community since it was prepared by Novoselov and Geim using tape-exfoliated graphite in 2004 [[1], [2], [3]]. Due to its excellent physical, chemical, electrical, and optical properties, significant efforts have been devoted to the development of graphene applications in the past decade. Chemical doping is a simple and effective modification method to broaden graphene's application prospects. Doping heteroatoms (e.g., N, B, S, and P) into graphene can modify its electronic properties, magnetic properties, and band structure [[4], [5], [6]]. Nitrogen is the most widely-used element to dope graphene due to its similar size to the carbon atom, which causes less lattice distortion of graphene. When nitrogen atoms are incorporated into graphene, they usually create specific configurations such as pyridinic-N, pyrrolic-N, and graphitic-N, which play a key role in modulating various properties [7,8]. However, the lower yields and doping concentrations restrict practical applications; therefore, the development of methods to controllably prepare high-quality nitrogen-doped graphene (NG) has become a popular research trend.
NG is mainly obtained through two routes: in-situ synthesis and post-treatment methods [2,3,9]. Post-treatment methods include heat treatment, plasma treatment, etc. The post-treated NG usually has a low level of surface doping and contains a large number of defects. For example, annealing graphene oxide with urea generates NG doped with 10% [10]; however, the formation of CN bonds requires oxygen-containing functional groups as active sites, so a high amount of oxygen is also required. Shao et al. [11] used nitrogen plasma treatment of graphene to prepare NG, and the oxygen-containing functional groups increased from 3.5% to 8.6%. The treatment of high-quality graphene with nitrogen-containing plasma increases the defects in graphene and activates carbon atoms at the edges, resulting in the formation of active sites for oxygen adsorption. Compared with post-treatment, in-situ synthesis produces NG with more uniform N atom distributions and fewer defects. The CVD method is a common technique for growing high-quality few-layered NG [12,13]. It is possible to prepare NG with different doping concentrations and N-functional groups by adjusting the conditions such as feedstock types, temperature, and flow rate; however, expensive equipment, extremely low yields, and metal substrate residues limit the further application of CVD methods.
Plasmas have unique chemical activity and can be used to grow nanostructures in gas phase without catalysts by providing favorable conditions for nucleation and growth processes [[14], [15], [16]]. N Bundaleska et al. [[17], [18], [19], [20]] used a microwave plasmato synthesize NG with a doping concentration of 0.4–8% and applied the product for electrochemistry and secondary electron emission. Pristavita et al. [21,22] used an inductively coupled plasma to synthesize NG with a doping level of 2% and complexed it with Fe atoms for catalytic reactions. However, these gas-phase plasma processes, although promising, produce NG with uncontrollable doping at a very low rate (~3 mg/h). The arc plasma is competitive in the plasma process due to its high yield and low energy cost. Recently, our group has a relatively deep accumulation in the preparation of graphene by arc plasma [[23], [24], [25], [26]]. Wang et al. [25] used a magnetically rotating arc plasma for the large-scale continuous synthesis of high-quality graphene and systematically investigated the effects of factors such as buffer gas on graphene synthesis. The results showed that when Ar and N2 were used as the buffer gases, the products were doped with 1.9% N atoms with a yield of about 100 mg/min. It exhibited great potential in the large-scale preparation of NG.
In this paper, we further investigated the synthesis of free-standing NG by magnetically rotating arc plasma using pure nitrogen as the buffer gas and nitrogen source. NG dominated by pyrrole functional groups was prepared in one step by cracking hydrocarbon gases under a N2 atmosphere, and the highest doping concentration reached 14.05%. Combining theoretical calculations and experiments, the effects of gas injection position, reaction temperature, and carbon precursors on the product microstructure were studied and discussed. The formation mechanism of NG in arc plasma was also briefly discussed.
Section snippets
Experimental arrangements
The experimental setup is shown in Fig. 1 which mainly contains a magnetically rotating arc plasma generator and a water-cooled deposition chamber. The plasma generator mainly consists of a hollow cylindrical anode (length 300 mm, inner diameter 25 mm), a rod-shaped cathode (diameter 8 mm) and a high strength magnet (providing 0.08 T axial magnetic field) surrounding the anode. The anode and cathode were made of graphite (99.99% purity) to avoid impurities evaporating from the electrode. The
Effect of gas injection position
Two different gas injection positions were adopted to investigate the effects on product properties because different gas injection positions can allow the gas to flow through different plasma areas. The experimental conditions are listed in Table 2. In the experimental R1, N2 and C2H4 entered the reactor through inlet 1 and inlet 2 respectively. In the experimental R2, N2 changed to enter from inlet 2. The input power for both experiments were about 12 kW. Fig. 2 shows the TEM and HRTEM images
Conclusions
Under atmospheric pressure, NG was synthesized by magnetically rotating arc plasma in one step, and the bonding configuration of N atoms was mainly pyrrolic-N. The effects of gas injection position, reaction temperature, and the carbon precursors on the product morphology, doping level, and doping configuration were investigated. The results showed that a high reaction temperature favored the high N doping level of the NG, but it decreased the NG yield and led to the nanoflakes curl
Funding
This work is supported by National Natural Science Foundation of China (No. 12075242, 11475174, 11675177 and 11705202), Anhui Provincial Natural Science Foundation (No. 1808085MA12), Anhui Province Scientific and Technological Project (No. 1604a0902145).
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
The authors acknowledge the Instruments Center for Physical Science of USTC for use of their equipment.
Credit author statement
Ming Song: Conceptualization, Investigation, Data Curation, Writing - Original Draft. Cheng Wang: Methodology, Writing - Review & Editing, Funding acquisition. Xianhui Chen: Software. Jing Ma: Software. Weidong Xia: Resources, Funding acquisition.
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