Hydrothermal synthesis of highly nitrogen-doped few-layer graphene via solid–gas reaction
Graphical abstract
Introduction
Graphene, single-layer carbon atoms densely packed into a two-dimensional honeycomb lattice, has recently attracted tremendous research interest due to its fascinating properties and potential applications in many fields, including nanoelectronics, transistors, energy storage materials and catalysts [1], [2], [3], [4]. It is well known that successful applications of graphene in some areas, such as nanoelectronics and transistors, are certainly determined by its electronic properties, which will show considerable effects on the graphene-based devices [5]. Therefore, the modulation of its electronic properties (e.g., the tuning of band gap and charge carrier concentration) is of great importance to expand the potential applications for the graphene-based devices [6], [7], [8]. Both theoretical and experimental studies revealed that doping graphene with nitrogen has been considered as an effective approach to achieve this goal [9], [10], [11], [12]. The lone electron pairs of nitrogen atoms play an important role in producing a delocalized conjugated system with sp2 hybridized carbon frameworks that can significantly affect the electronic structure of graphene [13], [14]. Up to now, several methods have been reported to synthesize nitrogen-doped graphene, such as chemical vapor deposition (CVD), arc discharge of graphite in the presence of pyridine vapor or NH3, thermal annealing graphene oxide (GO) with NH3, graphene treated with nitrogen plasma [15], [16], [17], [18]. However, these methods require special equipments or rigorous conditions and suffer from difficulties in scaling up. In addition, the nitrogen precursors of pyridine and NH3 gas are environmentally hazardous, and subsequent treatments are essential. Compared with these methods, the hydrothermal method has merits of mild conditions and scale-up synthesis [19], [20]. Unfortunately, the doping of graphene in solution has also accompanied some inevitable problems. The separation of N-doped graphene from solution results in the loss of production and the separated solution can not be used again burdening the post treatments. Besides, the application of N-doped graphene is related not only to the nitrogen doping level, but also to the types of the nitrogen. Therefore, it is still highly desirable to develop a simple, cost-effective and eco-friendly approach to synthesize N-doped graphene with high nitrogen content and appropriate nitrogen species.
In this study, we present a facile and efficient synthetic protocol for producing N-doped few-layer graphene by a self-designed hydrothermal system. The GO in autoclave was kept above the ammonia solution to avoid its direct contact with aqueous solution. Bulk quantities of hydrothermally treated GO (HTGO) with high nitrogen-doping level were successfully synthesized via solid–gas reaction of GO with ammonia vapor. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) spectroscopy were employed to probe the chemical bonding and electronic structure of the resultant HTGO before and after annealing treatment. It is found that different nitrogen bonding configurations in N-doped few-layer graphene sheets can be easily achieved by thermal annealing of the resultant HTGO at various temperatures. More importantly, the remaining ammonia solution can be repeatedly used due to the solid–gas reaction. Our doping method is cost-effective and eco-friendly, and thus it will pave a viable way toward the large-scale production of N-doped graphene for wide potential applications.
Section snippets
Preparation of N-doped graphene
GO was synthesized from natural graphite flake by a modified Hummers method [21]. A self-designed glass bottle was placed in a Teflon reaction autoclave, in which a small amount of ammonia solution was added. The schematic reaction system is illustrated as Fig. 1(a). As evident in Fig. 1(a), the as-prepared GO was loaded in the bottle to completely avoid direct contact with the aqueous solution. The autoclave was heated to 180 °C and kept at this temperature for 24 h. During the hydrothermal
Results and discussion
The surface morphology and crystalline structure of graphene sheets were studied by TEM, SAED, AFM, and XRD. Fig. 1(b) and (c) shows the typical TEM images of as-prepared GO and HTGO samples along with corresponding SAED patterns in the inset. It can be observed that the materials are transparent with the crumpled sheet-like morphology. In generally, the surface morphology of these materials could be attributed to the defective structures formed during the synthesis of GO or the presence of
Conclusions
We have developed a novel method to synthesize highly N-doped few-layer graphene sheets via solid–gas reaction in a self-designed hydrothermal system. It is found that the amide and amine N can be successfully introduced into graphene sheets by the solid–gas reaction between the most commonly studied GO and ammonia vapor in hydrothermal condition, and gradually converted to pyridinic and graphitic N by the subsequent thermal annealing treatments. This newly developed doping method is simple,
Acknowledgements
We acknowledge the support from BSRF. This work is supported by grants from the National Natural Science Foundation of China (Grant No. 11105032), the Guangxi Higher Education Reform Project (2013JGZ100) and the Experimental Teaching Reform Project of Guangxi University (20120401).
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