Graphene quantum dots (GQDs) have attracted tremendous attention due to their unique optical and optoelectronic properties. Doping GQDs with nitrogen atoms can effectively tune their intrinsic properties and exploit new applications in many fields. In this paper, we report a facile yet effective strategy for enhancing the photoluminescent (PL) intensity of nitrogen-doped GQDs (N-GQDs) with enriched pyrrolic-N content using urea as a dopant. The morphology, structure, components, and optical properties of the N-GQDs were characterized systematically. The average diameter of N-GQDs was about 7.5 nm, which was larger than undoped GQDs (∼3 nm). Both GQDs and N-GQDs exhibited excitation-independent PL behavior and the PL quantum yields were improved from 9% to 24% after N-doping. Moreover, the PL lifetime decay of GQDs (1.74 ns) and N-GQDs (7.40 ns) can be fitted to a single exponential function very well, indicating that GQDs/or N-GQDs have one single PL origin. Considering the results of Fourier transform infrared spectra and X-ray photoelectron spectra, –COOH groups and pyrrolic-N rings might be feature chromophores of GQDs and N-GQDs, respectively.

Nitrogen-doped graphene hydrogels (NGHs) were synthesized through a one-pot hydrothermal route with graphene oxide (GO) as raw material and urea as reducing-doping agents. The morphology, structure, and components of the NGHs were characterized by scanning electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, methylene blue adsorption, thermogravimetric analysis and electrical conductivity measurements. The results demonstrated that nitrogen was doped into the graphene plane at the same time as the GO sheets were reduced, and the nitrogen content incorporated into the graphene lattice was in the range of 3.95 to 6.61 at.% with pyrrolic N as the main component. The NGHs contained about 97.6 wt% water and have a large specific surface area (SSA) of >1300 m2 g−1 in the wet state. In addition, the electrochemical performance of the NGHs was investigated. The sample NGHs-4 with a nitrogen content of 5.86 at.% and SSA of 1521 ± 60 m2 g−1 in the wet state showed excellent capacitive behavior (308 F g−1 at 3 A g−1) and superior cycling stability (92% retention after 1200 cycles) in 6 mol L−1 KOH. The experimental results indicated that not only the N-content but also the N-type have very significant impact on the capacitive behavior. Furthermore, NGHs strongly influenced the electrochemical properties because of their high SSAs and mesoporous structure.

Nitrogen-doped graphene hydrogels (NGHs) were synthesized through a one-pot hydrothermal route with graphene oxide (GO) as raw material and urea as reducing-doping agents. The morphology, structure, and components of the NGHs were characterized by scanning electron microscopy, Raman spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, methylene blue adsorption, thermogravimetric analysis and electrical conductivity measurements. The results demonstrated that nitrogen was doped into the graphene plane at the same time as the GO sheets were reduced, and the nitrogen content incorporated into the graphene lattice was in the range of 3.95 to 6.61 at.% with pyrrolic N as the main component. The NGHs contained about 97.6 wt% water and have a large specific surface area (SSA) of >1300 m2 g−1 in the wet state. In addition, the electrochemical performance of the NGHs was investigated. The sample NGHs-4 with a nitrogen content of 5.86 at.% and SSA of 1521 ± 60 m2 g−1 in the wet state showed excellent capacitive behavior (308 F g−1 at 3 A g−1) and superior cycling stability (92% retention after 1200 cycles) in 6 mol L−1 KOH. The experimental results indicated that not only the N-content but also the N-type have very significant impact on the capacitive behavior. Furthermore, NGHs strongly influenced the electrochemical properties because of their high SSAs and mesoporous structure.