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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.

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Possible formation process of N-doped GQDs.
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f4: Possible formation process of N-doped GQDs.

Mentions: Figure 3 and Supplementary Figure S1B showed the XPS results of GQDs-U-n samples. N signal can be barely seen in the GQDs-U-2 sample. In high-resolution N1s XPS of GQDs-U-2, there is a weak N 1s signal at 400.0 eV, which is close to the pyrrolic N. These results indicate that the graphene framework has formed at this stage, but only trace amount of N can enter the graphene framework. In full scan XPS spectra of GQDs-U-4~24 (column A in Figure 3), the N peak intensity turned stronger and stronger with extended reaction time. The peak intensity ratios of N1s/C1s (RN/C) were calculated from full survey XPS spectra. The RN/C increased from 0.22 to 0.48 (Supplementary Figure S4). This indicates that N doping degree of GQDs increases with reaction time. The high-resolution C1s XPS spectra were shown in column B of Figure 3. The spectra can be fitted into 3 Gaussian peaks at 284.5, 286.1, 288.6 eV, which correspond to the sp2 carbon (C-C/C = C) in graphene, the sp3 carbon (C-O and C-N), and the C = O in carboxyl group. The relative intensity ratio of sp3 and sp2 increases with the increasing of reaction time because more N atoms enter the graphene structure. High-resolution N1s XPS spectra were shown in column C of Figure 3. The high-resolution N1s XPS peak of GQDs-U-2 appeared at 400 eV, which contributed from the pyrrolic N. The spectrum can be fitted with 2 Gaussian peaks at 399.8 and 401.5 eV that correspond to the pyrrolic N and graphite N, respectively2533. Existence of Pyrrolic N indicates that N bonds with C in 5 member-ring structures that are formed from dehydrolysis between the neighbor carboxyl and amide groups (Figure 4). Graphite N originates from the N atoms that are bonded with 3 neighbor C atoms. With the increase of reaction time, more and more N atoms enter the graphene layer, which leads to the increase of the sp3 C. This is consistent with the fact that the relative amount of graphite N in the GQDs-U increases with the reaction time. Element analysis results also show the same trend that the amount of N increases with the reaction time (Supplementary Table S1), indicating that the extent of N-doping increases with the reaction time. Supplementary Figure S5 showed the FTIR spectra of N-doped GQDs-U prepared at different reaction time. There is a shoulder peak at 1712 cm−1 that can be assigned to the carboxyl acid groups (-COOH). Along with the reaction, this peak disappears, suggesting the consumption of –COOH groups due to the dehydrolysis. In addition, two new shoulder peaks at 1700 and 1642 cm−1 gradually show up with the increase of reaction time. These new peaks contribute to formation of amide in the N-doped GQDs. The absolute PL QY increases from 58% to 81% with increasing reaction time from 2 to 24 hours (Table 1).


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)

Possible formation process of N-doped GQDs.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

f4: Possible formation process of N-doped GQDs.
Mentions: Figure 3 and Supplementary Figure S1B showed the XPS results of GQDs-U-n samples. N signal can be barely seen in the GQDs-U-2 sample. In high-resolution N1s XPS of GQDs-U-2, there is a weak N 1s signal at 400.0 eV, which is close to the pyrrolic N. These results indicate that the graphene framework has formed at this stage, but only trace amount of N can enter the graphene framework. In full scan XPS spectra of GQDs-U-4~24 (column A in Figure 3), the N peak intensity turned stronger and stronger with extended reaction time. The peak intensity ratios of N1s/C1s (RN/C) were calculated from full survey XPS spectra. The RN/C increased from 0.22 to 0.48 (Supplementary Figure S4). This indicates that N doping degree of GQDs increases with reaction time. The high-resolution C1s XPS spectra were shown in column B of Figure 3. The spectra can be fitted into 3 Gaussian peaks at 284.5, 286.1, 288.6 eV, which correspond to the sp2 carbon (C-C/C = C) in graphene, the sp3 carbon (C-O and C-N), and the C = O in carboxyl group. The relative intensity ratio of sp3 and sp2 increases with the increasing of reaction time because more N atoms enter the graphene structure. High-resolution N1s XPS spectra were shown in column C of Figure 3. The high-resolution N1s XPS peak of GQDs-U-2 appeared at 400 eV, which contributed from the pyrrolic N. The spectrum can be fitted with 2 Gaussian peaks at 399.8 and 401.5 eV that correspond to the pyrrolic N and graphite N, respectively2533. Existence of Pyrrolic N indicates that N bonds with C in 5 member-ring structures that are formed from dehydrolysis between the neighbor carboxyl and amide groups (Figure 4). Graphite N originates from the N atoms that are bonded with 3 neighbor C atoms. With the increase of reaction time, more and more N atoms enter the graphene layer, which leads to the increase of the sp3 C. This is consistent with the fact that the relative amount of graphite N in the GQDs-U increases with the reaction time. Element analysis results also show the same trend that the amount of N increases with the reaction time (Supplementary Table S1), indicating that the extent of N-doping increases with the reaction time. Supplementary Figure S5 showed the FTIR spectra of N-doped GQDs-U prepared at different reaction time. There is a shoulder peak at 1712 cm−1 that can be assigned to the carboxyl acid groups (-COOH). Along with the reaction, this peak disappears, suggesting the consumption of –COOH groups due to the dehydrolysis. In addition, two new shoulder peaks at 1700 and 1642 cm−1 gradually show up with the increase of reaction time. These new peaks contribute to formation of amide in the N-doped GQDs. The absolute PL QY increases from 58% to 81% with increasing reaction time from 2 to 24 hours (Table 1).

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