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Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots.

Qu D, Zheng M, Zhang L, Zhao H, Xie Z, Jing X, Haddad RE, Fan H, Sun Z - Sci Rep (2014)

Bottom Line: The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework.N-doping results in a great improvement of PL quantum yield (QY) of GQDs.The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China [2] University of Chinese Academy of Science, Beijing 100000, P. R. China.

ABSTRACT
Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between -NH₂ and -COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

No MeSH data available.


Related in: MedlinePlus

Representative TEM images of GQDs-U samples prepared at (A) 4 hours, (B) 6 hours, 8 hours, and (C) 24 hours.The corresponding HR-TEM images and particle size distribution are shown as left and right insets, respectively.
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f2: Representative TEM images of GQDs-U samples prepared at (A) 4 hours, (B) 6 hours, 8 hours, and (C) 24 hours.The corresponding HR-TEM images and particle size distribution are shown as left and right insets, respectively.

Mentions: To prepare high luminescent N-doped GQDs, urea was chosen because it could be used as both a base and the N source for N-doping processes. A series of samples were prepared at different reaction time (samples are denoted as GQDs-U-n, n is reaction time). Figure 2 and Supplementary Figure S1 showed the TEM images of GQDs-U-n samples at different reaction times from 2 to 24 hours. The overall size of GQDs–U increases as the reaction time increases. The average sizes of GQDs-U-n are 2.45 ± 0.26, 2.58 ± 0.34, 4.3 ± 0.7, 5.26 ± 0.78 and 7.11 ± 1.06 nm in diameter for the samples prepared at from 2, 4, 6, 8 and 24 hours, respectively. The related HR-TEM images showed that all the GQDs-U exhibited highly crystalline graphite nature. The measured lattice fringe distance is 0.24 nm, which corresponds to the (1120) crystal phase of graphite. The UV-Vis and PL spectra are showed in Supplementary Figure S1 and S2 in Supplementary Information. There are two clear absorption bands at 234 nm and 340 nm, which is close to previous reports32. These absorption bands are related to π→π* and n →π* transition of C = C and C = O bond in the GQDs. All GQDs-U samples show excitation-independent and narrow photoluminescent emission band at 450 nm and full width at half maximum is ~65 nm, which suggests both the size and the surface state of these GQDs should be uniform24. Besides, the lifetimes (τ, Supplementary Figure S1, S3) of GQDs-U samples are 8 ns, which is single exponential decay. This suggests that the origin of PL is one single species.


Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots.

Qu D, Zheng M, Zhang L, Zhao H, Xie Z, Jing X, Haddad RE, Fan H, Sun Z - Sci Rep (2014)

Representative TEM images of GQDs-U samples prepared at (A) 4 hours, (B) 6 hours, 8 hours, and (C) 24 hours.The corresponding HR-TEM images and particle size distribution are shown as left and right insets, respectively.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4061557&req=5

f2: Representative TEM images of GQDs-U samples prepared at (A) 4 hours, (B) 6 hours, 8 hours, and (C) 24 hours.The corresponding HR-TEM images and particle size distribution are shown as left and right insets, respectively.
Mentions: To prepare high luminescent N-doped GQDs, urea was chosen because it could be used as both a base and the N source for N-doping processes. A series of samples were prepared at different reaction time (samples are denoted as GQDs-U-n, n is reaction time). Figure 2 and Supplementary Figure S1 showed the TEM images of GQDs-U-n samples at different reaction times from 2 to 24 hours. The overall size of GQDs–U increases as the reaction time increases. The average sizes of GQDs-U-n are 2.45 ± 0.26, 2.58 ± 0.34, 4.3 ± 0.7, 5.26 ± 0.78 and 7.11 ± 1.06 nm in diameter for the samples prepared at from 2, 4, 6, 8 and 24 hours, respectively. The related HR-TEM images showed that all the GQDs-U exhibited highly crystalline graphite nature. The measured lattice fringe distance is 0.24 nm, which corresponds to the (1120) crystal phase of graphite. The UV-Vis and PL spectra are showed in Supplementary Figure S1 and S2 in Supplementary Information. There are two clear absorption bands at 234 nm and 340 nm, which is close to previous reports32. These absorption bands are related to π→π* and n →π* transition of C = C and C = O bond in the GQDs. All GQDs-U samples show excitation-independent and narrow photoluminescent emission band at 450 nm and full width at half maximum is ~65 nm, which suggests both the size and the surface state of these GQDs should be uniform24. Besides, the lifetimes (τ, Supplementary Figure S1, S3) of GQDs-U samples are 8 ns, which is single exponential decay. This suggests that the origin of PL is one single species.

Bottom Line: The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework.N-doping results in a great improvement of PL quantum yield (QY) of GQDs.The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

View Article: PubMed Central - PubMed

Affiliation: 1] State Key Laboratory of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Changchun 130033, Jilin, P. R. China [2] University of Chinese Academy of Science, Beijing 100000, P. R. China.

ABSTRACT
Photoluminescent graphene quantum dots (GQDs) have received enormous attention because of their unique chemical, electronic and optical properties. Here a series of GQDs were synthesized under hydrothermal processes in order to investigate the formation process and optical properties of N-doped GQDs. Citric acid (CA) was used as a carbon precursor and self-assembled into sheet structure in a basic condition and formed N-free GQD graphite framework through intermolecular dehydrolysis reaction. N-doped GQDs were prepared using a series of N-containing bases such as urea. Detailed structural and property studies demonstrated the formation mechanism of N-doped GQDs for tunable optical emissions. Hydrothermal conditions promote formation of amide between -NH₂ and -COOH with the presence of amine in the reaction. The intramoleculur dehydrolysis between neighbour amide and COOH groups led to formation of pyrrolic N in the graphene framework. Further, the pyrrolic N transformed to graphite N under hydrothermal conditions. N-doping results in a great improvement of PL quantum yield (QY) of GQDs. By optimized reaction conditions, the highest PL QY (94%) of N-doped GQDs was obtained using CA as a carbon source and ethylene diamine as a N source. The obtained N-doped GQDs exhibit an excitation-independent blue emission with single exponential lifetime decay.

No MeSH data available.


Related in: MedlinePlus