As shown in FE-SEM and TEM (Fig. 1(a) and 1(b)), these images demonstrate thin and wrinkled sheets transparent to electrons. Unlike ammonia-assisted hydrothermal method^{[ 19]} and hydrothermal reaction of graphene oxide and urea^{[ 20]}, this method prepared N-GNSs has many tiny graphene sheets, which can be seen clearly from Fig. 1(b) (the red dotted circle). These tiny graphene sheets are about 10 nm, also known as graphene quantum dots, which is generated by solvothermal cutting process^{[ 17]}. Indeed, the typical AFM image and thickness analysis of the N-GNSs sample reveal that the thickness of N-GNSs is about 0.95 nm (Fig. 1(c) and 1(d)). This thickness is thicker than pristine graphene with 0.34 nm, which may be caused by the resultant defects during the preparation of GO^{[ 21]}.

Fig. 1

	Figure Option ViewDownloadNew Window
	Fig. 1 Morphological characterization of as-synthesized N-GNSs(a) FE-SEM image; (b) TEM image, the inset of red dotted circle represents the typical graphene quantum dot (GQDs); (c) AFM; (d) Corresponding height analysis taken around the line in (c), the thickness of sheets is 0.947 nm

The typical XPS survey spectra of the as-synthesized GO and N-GNSs are displayed in Fig. 2(a). The N1s peak can be observed in the N-GNSs but not in GO, which indicates that N from DMF doped into GO during the solvothermal process. As shown in Fig. 2(b), the N1s peak in the N-GNSs sample could be well fitted into two components, pyrrolic-N (N1, 399.9 eV, 52.1at%) and graphitic N (N2, 400.8 eV, 47.9at%)^{[ 19]}. The existence of pyrrolic-N suggests that there are many defects in initial GO and nitrogen atoms preferring to form a pyrrole structure, while graphitic N is due to substitute carbon atoms to form a hexagonal graphite structure. Figure 2(c) shows the well-fitted C1s spectra of GO (up) and N-GNSs (down). The C1s spectra of GO can be divided into four different peaks, which corresponds to the signal of C-C (284.6 eV), C-O (285.5 eV), C=O (286.5 eV), and O-C=O (288.6 eV). Comparing with the C1s spectra of GO, N-GNSs appears a new peak at 285.2 eV attributing to C-N bonds^{[ 14]}. Moreover, as shown in Fig. 2(d) (Table 1), the oxygen content in GO reduce sharply from 23.65at% to 16.8at%, while the nitrogen content increase from 0.45at% to 1.98at%, further revealing that the solvothermal process can make GO reduced and doped nitrogen atoms simultaneously.

Fig. 2

	Figure Option ViewDownloadNew Window
	Fig. 2 (a) XPS survey spectra of as-synthesized GO (black) and N-GNSs (blue); (b) Well-fitted XPS N1s spectra of N-GNSs; (c) Well-fitted XPS C1s spectra of GO (up) and N-GNSs (down); (d) XPS analysis of GO and N-GNSs samples

The electrochemical properties of the N-GNSs electrode were investigated under a three-electrode system in 2 mol/L KOH electrolyte. Figure 3(a) shows the CV curves of the N-GNSs electrode at different scan rates from 5 to 200 mV/s. It can be seen that CV curves of the N-GNSs electrode are close to the rectangular shape, which indicates that the electrochemical behavior based on N-GNSs is more like double layer capacitance (EDLC). However, a pair of reversible peaks appears in the CV profile which attributed to the redox reactions of N-GNSs, behaving as pseudocapacitance. Thus, the high specific capacitance of the N-GNSs electrode results from the simultaneous contributions of the EDLC and the pseudocapacitance. Figure 3(b) shows the charge/discharge curves of the N-GNSs electrode at different current densities. It can be observed that the shape of all the charge-discharge curves of the electrode is closely linear and show a typical triangle symmetrical distribution, displaying a good capacitive property. The specific capacitance ( C_g) of the electrode calculated from galvanostatic discharge is 181.3 F/g at a current density of 0.5 A/g, which is higher than some reported graphene^{[ 22, 23]} and N-doped graphene^{[ 19]}. Figure 3(c) presents the relationship between specific capacitance and charge/discharge current densities. It is important to note that the C_g value of the N-GNSs is still as high as 127 F/g even at a high current density of 10 A/g. The superior performance of the N-GNSs can be attributed to dope graphene structure with nitrogen heteroatom and appropriate nitrogen species^{[ 19]}, which can provide pseudocapacitance and lead to excellent electric conductivity. The facilitated ion-transport kinetics and electrode conductivity were further confirmed by EIS (Fig. 3(d)). To better understand the processes taking place at the electrode/electrolyte interface, the Nyquist plot is divided into two regions, the high-frequency region and the low-frequency region. In the high-frequency region, the intercept of the semicircle with the real axis represents the equivalent series resistance (ESR), where the resistance of the electrolyte solution is usually predominant. As the plots shown, in the high-frequency region, the N-GNSs electrodes have small ESR and charge transfer resistance (Fig. 3(d) inset). Furthermore, in the low-frequency region, the slope of the plot for the N-GNSs shows a straight line, indicating that the N-GNSs shows characteristic of better capacitive behavior^{[ 20]}.

Fig. 3

	Figure Option ViewDownloadNew Window
	Fig. 3 Electrochemical performance of the N-GNSs electrode in 2 mol/L KOH electrolyte(a) CV curves at different scan rates; (b) Charge/discharge curves at different current densities; (c) Specific capacitance as a function of discharge current density; (d) Nyquist plots of the N-GNSs. The inset shows the expanded high-frequency region of the plots

The cycle life of the N-GNSs electrode was examined by galvanostatic charge-discharge measurement at a current density of 5 A/g. As shown in Fig. 4, the special capacitance of as-prepared N-GNSs still remains 92.5% of the initial specific capacitance after 2000 cycles. This indicates that, within the voltage window of -1~0 V, the re peating charge-discharge process does not seem to induce a significant structural breakage for the N-GNSs electrode. The long-term stability implies that our N-GNSs can be used as an excellent potential electrode material for supercapacitors.

Fig. 4

	Figure Option ViewDownloadNew Window
	Fig. 4 Cycle life of the N-GNSs electrode at the current density of 5 A/g in KOH electrolyte. The inset shows the charge-discharge curves for N-GNSs electrode at the end of different cycles

In summary, a simple and effective solvethermal route has been employed to prepare N-GNSs with nitrogen level about 2%. The N-GNSs exhibit excellent electrochemical properties and the specific capacitance reaches 181.3 F/g in 2 mol/L KOH electrolyte. In addition, the N-GNSs sample shows excellent cycle stability after 2000 cycles (maintained at 92.5%). Thus, the N-GNSs could be used as a kind of promising electrode material for high-performance supercapacitors.