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Large-scale synthesis of N-doped carbon quantum dots and their phosphorescence proper es in polyurethane matrix† Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
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Jing Tan, Rui Zou, Jie Zhang, Wang Li, Liqun Zhang
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and Dongmei Yue*
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An easy, large-scale synthesis of N-doped carbon quantum dots (CQDs) was developed by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. The yield of raw N-doped CQDs was about 83 %, which is suitable for industrial-scale production. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in various organic monomers and do not need toxic organic solvents as dispersing agents. This advantage expands the range of applications of CQDs in composites. The N-doped CQDs dispersed in polyurethane (PU) matrixes emit not only fluorescence but also phosphorescence and delayed fluorescence at room temperature upon excitation by ultraviolet (UV) light. Furthermore, the phosphorescence of CQDs/PU composites is sensitive to oxygen and therefore, the obtained-CQDs could be exploited in the development of novel oxygen sensors.
1. Introduction Recently, carbon quantum dots (CQDs), as new carbon materials, have been a rapidly growing research area due to their advantages of low toxicity, environmental friendliness, low cost, and simple synthesis routes.1 Owing to their excellent properties, most studies focus on the applications of CQDs based on fluorescence, such as catalysts,2-4 chemical sensing,5-7 bioimaging,8, 9 printing inks,10-12 and light-emitting devices13, 14. However, CQDs have obvious limitations in their practical applications. First, CQDs production yields are painfully low. To the best of our knowledge, several methods have been used to improve the production of CQDs, but the yields are still too low for industrial-scale production, resulting in a waste of raw materials and an increase in product costs.15, 16 Second, most CQDs are used alone or in solutions and thus have limited applications. Some researchers have incorporated CQDs into polymers, but in order to obtain homogeneous composites, they must use organic solvents, which lead to serious environmental pollution.17 Last, most reports focus on the fluorescence of CQDs, far less attention has been paid on room temperature phosphorescence (RTP),18 and delayed fluorescence (DF) has never been reported before. RTP and DF, which have a much longer lifetime and higher electroluminescence efficiency than fluorescence, have been
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State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: [email protected] Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing 100029, China. Tel: +86-010-64436201. †Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x b.
gradually applied in sensing, bioimaging, light-emitting diodes, 19, 20 advanced security imaging, and solar cells. However, the preparation of pure organic RTP materials is still a huge challenge owing to the nonradiative deactivation of the highly 21 active excited states. Up to now, only PVA and 18 KAl(SO4)2•x(H2O) as host matrixes for RTP materials based on CQDs have been reported, and the application is typically limited in anti-counterfeiting. Therefore, more efforts have to be made to seek new host materials to obtain remarkable RTP emission and broaden the application fields. Herein, we developed for the first time a novel strategy to synthesize N-doped CQDs by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. A high yield of raw N-doped CQDs of about 83 % was achieved, exceeding that of the previously reported 11, 15, 22 CQDs. The obtained N-doped CQDs are oil soluble and can be highly dispersed in common organic solvents (e.g., methanol, ethanol, acetone, chloroform, and N,Ndimethylformamide) and monomers (e.g., IPDI, 2,4-tolylene diisocyanate (2,4-TDI), polymethyl methacrylate (PMMA), divinylbenzene (DVB), and styrene (St)). Since polyurethane (PU) is widely used in hundreds of different products, we selected IPDI to synthesis CQDs/polyurethane (CQDs/PU) 23 composites by the ‘‘monomer as solvent’’ approach, avoiding the use of toxic organic solvents as dispersing agents. More importantly, the synthesized CQDs/PU composites showed room temperature phosphorescence (RTP) and delayed fluorescence (DF). The phosphorescence of CQDs/PU composites is response to oxygen which has never been
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2. Experimental section
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2.1 Chemicals IPDI, 1,4-butanediol (1,4-BDO), dibutyltin dilaurate (DBTDL), and polytetramethylene ether glycol (PTMEG, Mn~1,000) were of analytical reagent grade and were purchased from Aladdin Chemistry Co. Ltd, China. Anhydrous alcohol of analytical grade was purchased from the Beijing Chemical Company, China. Dialysis membrane (1000 Da) was purchased from Beijing Biodee Biotechnology Co., Ltd, China. All the reagents and solvents were used as received and without further purification.
of 2.0-5.5 nm. The size of the N-doped CQDs was also measured by atomic force microscopy (AFM, Fig. S1†), and the height is approximately 0-5 nm, consistent with the TEM results. Fig. 2(c) and (d) show a high-resolution TEM (HRTEM) image and X-ray diffraction (XRD) pattern, respectively, of the N-doped CQDs. The HRTEM image shows that most of the Ndoped CQDs are amorphous in structure without any lattices. A broad peak at 2θ of 15.9° (0.54 nm) in the XRD pattern is due to the highly disordered carbon atoms. Due to the interference of the fluorescence of the N-doped CQDs, no obvious peaks 11, 24, 25 are detected in the Raman spectrum (Fig. S2†).
Fig. 1 Synthesis of N-doped CQDs. 2.2 Synthesis of N-doped CQDs IPDI (8 g, 36 mmol) was put into a polytetrafluoroethylene vessel that was sealed by an explosion-proof enclosure. Then, raw N-doped CQDs were prepared under 700 W microwave o irradiation (CEM MARS6 microwave system, USA) at 250 C for 10 min. During the irradiation, the colorless liquid became a dark brown solid, indicating the formation of raw CQDs. The obtained raw CQDs were dissolved in ethanol, centrifuged to remove the large particles, and then dialyzed with ethanol through a dialysis membrane (1000 Da) for 72 hours. The final product, over 6.66 g of CQDs, was obtained under reduced pressure.
2.3 Preparation of CQDs/PU composites N-doped CQD powder and IPDI (20 mmol, 4.45 g) were mixed in a one-neck flask under sonication to form a solution. Then, PTMEG (2 mmol, 2 g) and DBTDL (20 μl, 0.2 w/w % of total reactant weight) were dropped into the CQDs/IPDI solution. o The mixture was heated at 80 C for 1.5 h to form the prepolymer, and 1,4-BDO (18 mmol, 1.62 g) was added into the prepolymer and vacuum degassed for 5 min. Finally, the o mixture was placed in the Teflon mold and heated at 80 C in an oven for 8 h to obtain the CQDs/PU composites. The amount of N-doped CQDs was 0.2 w/w % of total reactant weight.
Fig. 2 (a) TEM image and (b) size distribution of N-doped CQDs; (c) high-resolution TEM image and (d) XRD pattern of N-doped CQDs.
3. Results and discussion Novel N-doped CQDs were synthesized by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation with an adaptable production yield of ~83 %. The synthesis route is shown in Fig. 1, no corrosive acid, alkali, or organic solvent were used. Fig. 2(a) shows a transmission electron microscopy (TEM) image of the N-doped CQDs, and Fig. 2(b) shows the particle diameter distribution of the Ndoped CQDs obtained by measuring the particle sizes of over 100 nanoparticles. The N-doped CQDs have a uniform size distribution and the particle diameters are mainly in the range
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reported before in CQDs composites. And this property makes N-doped CQDs have a potential in oxygen sensors.
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In Fig. 3(a), when excited under ultraviolet (UV) light (365 nm), the ethanol solution of N-doped CQDs displays a bright blue fluorescence. The UV-vis spectrum has two absorption peaks centered at 217 and ~300 nm in the UV region. 21, 26 According to previous reports, the absorption peaks at 217 and ~300 nm are attributed to the π−π* transition of the aromatic C=C bond and n-π* transition of the C=O bond, respectively. Photoluminescence (PL) is a significant feature of N26-29 doped CQDs. As shown in Fig. 3(b), like many CQDs, the Ndoped CQDs show excitation-dependent PL behavior in the excitation wavelength range 295 to 475 nm. This behavior may be attributed to the surface properties and size of the CQDs, .15, 30 which affect the energy levels of the electronic states In the PLE spectrum of N-doped CQDs at emission wavelength of 440 nm (Fig. 3(c)), there are two peaks at around 274 and 375 nm, which are ascribed to the allowed transition of the C=O 18 bond on the N-doped CQDs. The absolute PL quantum yield (QY) (excitation at 375 nm) of N-doped CQDs is about 11 %, which is a relatively high PL QY for CQDs without surface 31- 33 passivation. The high PL QY is mainly due to the existence of O and N atoms. These heteroatoms in the CQD lattice can 2 disrupt the sp hybridization of carbon atoms, altering the 34, electronic structures and increasing the PL intensity of CQDs. 35 The time-resolved fluorescence decay curve obtained by the time-correlated single photon counting method is illustrated in Fig. 3(d). The fluorescence decay curve can be fitted into the three lifetimes of 1.34, 3.98, and 9.75 ns (The detail results of fitting are presented in Table S1†). The average lifetime is calculated to be a short lifetime of 6.58 ns. The short lifetime of N-doped CQDs is indicative of the radiative recombination 18, 25 of the excitons, giving rise to fluorescence. Significantly, the obtained N-doped CQDs are shown to possess clear upconversion PL (UPL) properties in the excitation wavelength range 740 to 860 nm (Fig. S3†). This UPL property of CQDs may be attributed to the different emissive sites on each CQDs and 33,36 the multiphoton active processes. And the result indicates that the obtained N-doped CQDs will be of really great interest in bioscience and energy technology.
Fig. 4 FTIR spectra of N-doped CQDs, N-doped CQDs-6, and Ndoped CQDs-2. To confirm the structure and composition of the N-doped CQDs, Fourier Transform infrared spectroscopy (FTIR) and Xray photoelectron spectroscopy (XPS) were used. In the blue curve in Fig. 4, the broad absorption bands at 3400 and 3270 cm-1 are related to the stretching vibrations of O-H and N-H, -1 respectively. The strong peaks at 2951 and 2872 cm correspond to the C–H bond stretching vibrations. The peaks -1 at around 1638 and 1580 cm are assigned to the typical stretching peaks of the C=N and N-H bonds, respectively.34 The stretching vibrations peak of C=O is exhibited by the absorption band at 1695 cm-1. In Fig. 5(a), a typical XPS survey of the N-doped CQDs shows C, N, and O on the surface of the N-doped CQDs. The deconvoluted XPS C1s spectrum of Ndoped CQDs (Fig.5 (b)) displays three main components: C-C or C=C at ca. 284.88 eV, C-N at ca. 286.00 eV, and C=O or C=N at ca. 288.2 eV.34 In the XPS N1s spectrum of N-doped CQDs (Fig.5 (c)), the peaks at about 398.31 eV, 399.23 eV, 399.91 eV, and 400.69 eV confirm the presence of the nitrogen atoms of graphite-like structure, pyridinic-like N, pyrrolic-like N, and NH 10, 37 groups, respectively. The O1s XPS spectrum of N-doped CQDs (Fig.5 (d)) can be deconvoluted into two peaks at 531.9 eV and 532.9 eV, indicating the existence of C=O and C-OH, respectively.38 These XPS results indicate that N-doped CQDs are rich in oxygen and nitrogen atoms derived from the polymerization of IPDI.
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Fig. 3 (a) UV-Vis absorption spectrum of N-doped CQDs ethanol solution; insets are photographs of N-doped CQDs ethanol solution under sunlight (left) and under UV light (365 nm) (right); (b) fluorescence emission spectra of N-doped CQDs recorded from 295 to 475 nm in 20 nm increments; (c) fluorescence excitation spectrum of N-doped CQDs; (d) fluorescence decay profiles of N-doped CQDs measured with excitation and emission wavelengths of 375 and 440 nm, respectively.
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Scheme 1. Mechanism for formation of N-doped CQDs.
Fig. 5 (a) XPS spectrum of N-doped CQDs; (b-d) high resolution C 1s, O 1s, and N 1s spectra of N-doped CQDs. Due to the harsh and/or complicated synthesis conditions, few reports provided a detailed mechanism for the formation of CQDs.10, 22, 39 Herein, a possible mechanism for the formation of N-doped CQDs from IPDI, as shown in Scheme 1, is proposed. At high temperatures, the isocyanate groups react with themselves. The self-polymerization of isocyanate groups follows one of the three reactions: (1) dimerization cyclization reaction, while the dimer is not stable and will dissociate above 175 oC, (2) trimerization or multimerization 40,41 o reaction, and (3) carbodiimide reaction. At 250 C, IPDI first reacts with itself to form trimers, multimers, and carbodiimide structures. And carbodiimide self-polymerizes or reacts with isocyanate. Then the polymers are formed through reactions among trimers, multimers, and carbodiimide isocyanurates. Finally N-doped CQDs are formed by probable unclear burst. To confirm the mechanism, we performed FTIR analyses of Ndoped CQD samples formed under different irradiation times. IPDI was microwave irradiated for 2 and 6 min to form Ndoped CQDs-2 and N-doped CQDs-6, respectively. As shown in -1 Fig. 4, the sharp peaks at 2263 cm indicate large amounts of unreacted isocyanate groups. With increasing irradiation time, -1 the peaks at 2120 and 1714 cm become more and more pronounced, indicating the formation of carbodiimide and trimerisation/multimerisation structures. At an irradiation time of 10 min, the disappearance of the absorption peaks at -1 -1 -1 2120 cm and the shift of the peak at 1714 cm to 1695 cm indicate that the carbodiimide reacted with itself or isocyanate.
To date, CQDs have been mainly used alone or in solution, and only a few researchers have focused their attention on the incorporation of CQDs with polymers.14, 17, 39, 42 The incorporation of CQDs with polymers is one of the most significant means to expand the range of applications of CQDs because they combine the advantages of CQDs and organic materials. However, the inherently high specific surface energies and hydrophilic character of nanoparticles often lead to aggregation and phase separation in the prepared 43, 44 composites. In order to disperse the CQDs into monomers or polymers homogeneously at the nanometer scale, 17, 39, 45 dispersing agents are usually used. Due to the lack of hydrophilic groups, applicability of the as-synthesized N-doped CQDs will be hindered by the poor stability in water media, but they can be dispersed homogeneously in organic monomers (e.g., IPDI, TDI, PMMA, DVB, and St) (Fig. S4†), and these monomers often can be used to produce transparent materials for potential optical applications. We can use the ‘‘monomer as solvent’’ approach to synthesis CQDs composites, avoiding 23 the use of toxic organic solvents. FTIR results (Fig. S5†) show that CQDs/PU composites were successful prepared. In the scanning electron microscopy (SEM) images (Fig. S6†), the fractured section of the CQDs/PU composite presents a homogeneous dispersion of N-doped CQDs in PUs. The digital photograph (Fig. 6(a), left) under daylight further confirms the transparency and good optical quality of CQDs/PU composites with a large 3D macrostructure. Surprisingly, the N-doped CQDs in PUs emit not only fluorescence under UV light (365 nm) excitation (middle), but also an obvious afterglow when the UV light is turned off (right), which is visible to the naked eye for seconds (a corresponding video is provided in the ESI†). The PL emission spectra of the CQDs/PU composites in Fig. 6(b) show the strongest emission peak at 436 nm under excitation at 375 nm. Compared with those of the N-doped CQDs dispersed in ethanol, the emission peaks of the CQDs/PU composites have a blue shift at excitation wavelengths shorter than 375 nm. In addition, the corresponding PL spectra and phosphorescence spectrum of CQDs/PU composites by 375 nm excitation were shown in Fig. 6(c). The phosphorescence spectrum of the CQDs/PU composites shows a broad band at 450-500 nm and a large Stokes shift between fluorescence. Moreover, there is a partly overlap peak between florescence and phosphorescence of the CQDs/PU composites, corresponding to type DF. The phosphorescence excitation
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spectrum of the CQDs/PU composites (Fig. 6(d)) with emission at 500 nm show broad peaks at 285 and 377 nm, indicating that the phosphorescence behaviour come from the C=O bonds on N-doped CQDs. According to literature reports, the aromatic carbonyl group exhibits some degree of spin-orbit coupling that usually leads to intrinsic triplet generation 46, 47 through intersystem crossing. In addition, like O atom, N atom also favour n–π ∗ transition and hence facilitate the spinforbidden transfer of singlet-to-triplet excited states through intersystem crossing to populate triplet excitons.48 However, the lowest triplet excited state at room temperature in air can easily go through nonradiative and quenching processes. The rigidity of PU matrixes and the hydrogen bonding between the urethane backbone and N-doped CQDs both play a key role in rigidifying the C=O bonds on the surface of N-doped CQDs, minimizing the non-radiation transitions of triplet excitons, because similar RTP was not observed from N-doped CQDs dispersed in ethanol or a polystyrene matrix, probably because compared with those of the N-doped CQDs dispersed in ethanol, the emission peaks of the CQDs/PU composites have a blue shift at excitation wavelengths shorter than 375 nm.
Fig. 6 (a) Digital photographs of CQDs/PU composites dispersed in PU matrix under daylight (left), UV light (middle), and right after UV light has been turned off (right); (b) fluorescence emission spectra of CQDs/PU composites; (c) fluorescence and phosphorescence emission spectra of CQDs/PU composites exposed to UV light for 2 min; (d) phosphorescence excitation spectrum of CQDs/PU composites with emission at 500 nm; (e) effect of exposure time under UV light on intensity of phosphorescence. The time-resolved phosphorescence spectra with an excitation wavelength of 375 nm and emission wavelengths of 436 and 500 nm are shown in Fig. 7. The results of fitting the two spectra with multi-exponential functions are presented in Table S2†. The average RTP and DF lifetimes are 8.7 and 5.0 ms, respectively. The three discrete exponential components
imply different electronic transition processes, which may be attributed to a wide range of chemical environments for 21 aromatic carbonyls on the surface of N-doped CQDs.
Fig. 7 Time-resolved (a) delayed fluorescence and (b) phosphorescence spectra of CQDs/PU composites. In addition, if the CQDs/PU composites are put in air for a period of time, no obvious afterglow can be observed after a very short UV exposure time. With increasing exposure time under UV light, the intensity of phosphorescence increases and eventually reaches a steady-state value (Fig. 6(e)). It is wellknown that oxygen can quench the triplet states.46, 49 Although the PU matrixes have some barrier to oxygen, oxygen will gradually penetrate the matrix if exposed in air for a long time. With increasing time of UV irradiation, the triplet state of the material and the ground state of molecular oxygen (3O2) 48 3 undergo triplet-triplet quenching. The concentration of O2 in the PU matrixes gradually decreases, and a significantly low 3 concentration of O2 is preserved for some time due to the slow diffusion of oxygen into the PU matrixes. To validate the above mechanism, the CQDs/PU composites exposed to UV light for 5 min were put in a N2 and an air atmosphere for 48 h each. As shown in Fig. S7†, the intensity of phosphorescence hardly changes in the N2 atmosphere, but decreases significantly in the air atmosphere and after purging the oxygen media with nitrogen for 12 h, the intensity of phosphorescence is recovered. Moreover, pure organic room temperature phosphorescent (RTP) materials based on Ndoped CQDs are attractive alternatives to inorganics or metal complexes RTP materials because of their low cost, chemical 19 inertness and nontoxic. We suggested that this research will promote the practical application of the CQDs in oxygen sensors.
Conclusions We synthesized novel oil-soluble N-doped CQDs by using IPDI as a single carbon source under microwave irradiation. Notably, the yield of raw N-doped CQDs was about 83 % and suitable for industrial-scale production. A detail mechanism for the formation of N-doped CQDs from IPDI was proposed. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in organic monomers and can use the ‘‘monomer as solvent’’ approach to synthesis CQDs composites, avoiding the use of toxic organic solvents as dispersing agents. Surprisingly, obvious phosphorescence and delayed fluorescence were discovered at room temperature when the CQDs/PU composites were excited by UV light, in addition to
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fluorescence. The PU matrixes play a key role in suppressing the nonradiation transitions of triplet excitons. In addition, we are the first to find that the phosphorescence of CQDs/PU composites is sensitive to oxygen and makes the obtained Ndoped CQDs have potential applications in oxygen sensors.
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Acknowledgements The financial support of the National Natural Science Foundation of China under Grant No. 20774009 is gratefully acknowledged.
Notes and references 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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23. H. D. Josh H. Golden, Francis J. DiSalvoa, Jean M. J. Fréchet, Patrick M. Thompson, Science, 1995, 268, 14631466. 24. M. Xu, G. He, Z. Li, F. He, F. Gao, Y. Su, L. Zhang, Z. Yang and Y. Zhang, Nanoscale, 2014, 6, 10307-10305. 25. Z. Yang, M. Xu, Y. Liu, F. He, F. Gao, Y. Su, H. Wei and Y. Zhang, Nanoscale, 2014, 6, 1890-1895. 26. Y. Wang, S. Kalytchuk, Y. Zhang, H. Shi, S. V. Kershaw and A. L. Rogach, J. Phys. Chem. Lett., 2014, 5, 1412-1420. 27. F. Wang, Z. Xie, H. Zhang, C.-y. Liu and Y.-g. Zhang, Adv. Funct. Mater., 2011, 21, 1027-1031. 28. P. Zhang, W. Li, X. Zhai, C. Liu, L. Dai and W. Liu, Chem. Commun., 2012, 48, 10431-10433. 29. Y. P. Sun, B. Zhou, Y. Lin, W. Wang, K. A. S. Fernando, P. Pathak, M. J. Meziani, B. A. Harruff, X. Wang, H. Wang, P. G. Luo, H. Yang, M. E. Kose, B. Chen, L. M. Veca and S.-Y. Xie, J. Am. Chem. Soc., 2006, 128, 7756-7757. 30. L. Bao, C. Liu, Z.-L. Zhang and D.-W. Pang, Adv. Mater., 2015, 27, 1663-1667. 31. Y. Yang, D. Wu, S. Han, P. Hu and R. Liu, Chem. Commun., 2013, 49, 4920-4922. 32. D. Pan, L. Guo, J. Zhang, C. Xi, Q. Xue, H. Huang, J. Li, Z. Zhang, W. Yu, Z. Chen, Z. Li and M. Wu, J. Mater. Chem., 2012, 22, 3314-3318. 33. H. Li, X. He, Z. Kang, H. Huang, Y. Liu, J. Liu, S. Lian, C. H. Tsang, X. Yang and S. T. Lee, Angew. Chem. Int. Ed., 2010, 49, 4430-4434. 34. Y. Li, Y. Zhao, H. Cheng, Y. Hu, G. Shi, L. Dai and L. Qu, J. Am. Chem. Soc., 2012, 134, 15-18. 35. H. Ding, J.-S. Wei and H.-M. Xiong, Nanoscale, 2014, 6, 13817-13823. 36. B. Yin, J. Deng, X. Peng, Q. Long, J. Zhao, Q. Lu, Q. Chen, H. Li, H. Tang, Y. Zhang and S. Yao, The Analyst, 2013, 138, 6551-6557. 37. Y. Xu, M. Wu, Y. Liu, X.-Z. Feng, X.-B. Yin, X.-W. He and Y.K. Zhang, Chem. - Eur. J., 2013, 19, 2276-2283. 38. L. Lin, M. Rong, S. Lu, X. Song, Y. Zhong, J. Yan, Y. Wang and X. Chen, Nanoscale, 2015, 7, 1872-1878. 39. W. F. Zhang, L. M. Jin, S. F. Yu, H. Zhu, S. S. Pan, Y. H. Zhao and H. Y. Yang, J. Mater. Chem. C, 2014, 2, 1525-1531. 40. P. Krol, Prog. Mater. Sci., 2007, 52, 915-1015. 41. A. A. Caraculacu and S. Coseri, Prog. Polym. Sci., 2001, 26, 799-851. 42. Z. Xie, F. Wang and C.-y. Liu, Adv. Mater., 2012, 24, 17161721. 43. C. Lü, Y. Cheng, Y. Liu, F. Liu and B. Yang, Adv. Mater., 2006, 18, 1188-1192. 44. S. Chen, J. Zhu, Y. Shen, C. Hu and L. Chen, Langmuir, 2007, 23, 850-854. 45. M. Sun, S. Qu, Z. Hao, W. Ji, P. Jing, H. Zhang, L. Zhang, J. Zhao and D. Shen, Nanoscale, 2014, 6, 13076-13081. 46. X. Xiong, F. Song, J. Wang, Y. Zhang, Y. Xue, L. Sun, N. Jiang, P. Gao, L. Tian and X. Peng, J. Am. Chem. Soc., 2014, 136, 9590-9597. 47. O. Bolton, K. Lee, H.-J. Kim, K. Y. Lin and J. Kim, Nat. Chem., 2011, 3, 207-212. 48. Z. An, C. Zheng, Y. Tao, R. Chen, H. Shi, T. Chen, Z. Wang, H. Li, R. Deng, X. Liu and W. Huang, Nat. Mater., 2015, 14, 685-690. 49. S. Mukherjee and P. Thilagar, Chem. Commun., 2015, 51, 10988-11003.
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Large-scale synthesis of N-doped carbon quantum dots and their phosphorescence proper es in polyurethane matrix† Received 00th January 20xx, Accepted 00th January 20xx DOI: 10.1039/x0xx00000x www.rsc.org/
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Jing Tan, Rui Zou, Jie Zhang, Wang Li, Liqun Zhang
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and Dongmei Yue*
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An easy, large-scale synthesis of N-doped carbon quantum dots (CQDs) was developed by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. The yield of raw N-doped CQDs was about 83 %, which is suitable for industrial-scale production. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in various organic monomers and do not need toxic organic solvents as dispersing agents. This advantage expands the range of applications of CQDs in composites. The N-doped CQDs dispersed in polyurethane (PU) matrixes emit not only fluorescence but also phosphorescence and delayed fluorescence at room temperature upon excitation by ultraviolet (UV) light. Furthermore, the phosphorescence of CQDs/PU composites is sensitive to oxygen and therefore, the obtained-CQDs could be exploited in the development of novel oxygen sensors.
1. Introduction Recently, carbon quantum dots (CQDs), as new carbon materials, have been a rapidly growing research area due to their advantages of low toxicity, environmental friendliness, low cost, and simple synthesis routes.1 Owing to their excellent properties, most studies focus on the applications of CQDs based on fluorescence, such as catalysts,2-4 chemical sensing,5-7 bioimaging,8, 9 printing inks,10-12 and light-emitting devices13, 14. However, CQDs have obvious limitations in their practical applications. First, CQDs production yields are painfully low. To the best of our knowledge, several methods have been used to improve the production of CQDs, but the yields are still too low for industrial-scale production, resulting in a waste of raw materials and an increase in product costs.15, 16 Second, most CQDs are used alone or in solutions and thus have limited applications. Some researchers have incorporated CQDs into polymers, but in order to obtain homogeneous composites, they must use organic solvents, which lead to serious environmental pollution.17 Last, most reports focus on the fluorescence of CQDs, far less attention has been paid on room temperature phosphorescence (RTP),18 and delayed fluorescence (DF) has never been reported before. RTP and DF, which have a much longer lifetime and higher electroluminescence efficiency than fluorescence, have been
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State Key Laboratory of Organic-Inorganic Composites, Beijing University of Chemical Technology, Beijing 100029, China. E-mail: [email protected] Key Laboratory of Beijing City on Preparation and Processing of Novel Polymer Materials, Beijing 100029, China. Tel: +86-010-64436201. †Electronic Supplementary Information (ESI) available: [details of any supplementary information available should be included here]. See DOI: 10.1039/x0xx00000x b.
gradually applied in sensing, bioimaging, light-emitting diodes, 19, 20 advanced security imaging, and solar cells. However, the preparation of pure organic RTP materials is still a huge challenge owing to the nonradiative deactivation of the highly 21 active excited states. Up to now, only PVA and 18 KAl(SO4)2•x(H2O) as host matrixes for RTP materials based on CQDs have been reported, and the application is typically limited in anti-counterfeiting. Therefore, more efforts have to be made to seek new host materials to obtain remarkable RTP emission and broaden the application fields. Herein, we developed for the first time a novel strategy to synthesize N-doped CQDs by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation. A detailed formation mechanism for N-doped CQDs involving self-polymerization and condensation of IPDI was demonstrated. A high yield of raw N-doped CQDs of about 83 % was achieved, exceeding that of the previously reported 11, 15, 22 CQDs. The obtained N-doped CQDs are oil soluble and can be highly dispersed in common organic solvents (e.g., methanol, ethanol, acetone, chloroform, and N,Ndimethylformamide) and monomers (e.g., IPDI, 2,4-tolylene diisocyanate (2,4-TDI), polymethyl methacrylate (PMMA), divinylbenzene (DVB), and styrene (St)). Since polyurethane (PU) is widely used in hundreds of different products, we selected IPDI to synthesis CQDs/polyurethane (CQDs/PU) 23 composites by the ‘‘monomer as solvent’’ approach, avoiding the use of toxic organic solvents as dispersing agents. More importantly, the synthesized CQDs/PU composites showed room temperature phosphorescence (RTP) and delayed fluorescence (DF). The phosphorescence of CQDs/PU composites is response to oxygen which has never been
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2.1 Chemicals IPDI, 1,4-butanediol (1,4-BDO), dibutyltin dilaurate (DBTDL), and polytetramethylene ether glycol (PTMEG, Mn~1,000) were of analytical reagent grade and were purchased from Aladdin Chemistry Co. Ltd, China. Anhydrous alcohol of analytical grade was purchased from the Beijing Chemical Company, China. Dialysis membrane (1000 Da) was purchased from Beijing Biodee Biotechnology Co., Ltd, China. All the reagents and solvents were used as received and without further purification.
of 2.0-5.5 nm. The size of the N-doped CQDs was also measured by atomic force microscopy (AFM, Fig. S1†), and the height is approximately 0-5 nm, consistent with the TEM results. Fig. 2(c) and (d) show a high-resolution TEM (HRTEM) image and X-ray diffraction (XRD) pattern, respectively, of the N-doped CQDs. The HRTEM image shows that most of the Ndoped CQDs are amorphous in structure without any lattices. A broad peak at 2θ of 15.9° (0.54 nm) in the XRD pattern is due to the highly disordered carbon atoms. Due to the interference of the fluorescence of the N-doped CQDs, no obvious peaks 11, 24, 25 are detected in the Raman spectrum (Fig. S2†).
Fig. 1 Synthesis of N-doped CQDs. 2.2 Synthesis of N-doped CQDs IPDI (8 g, 36 mmol) was put into a polytetrafluoroethylene vessel that was sealed by an explosion-proof enclosure. Then, raw N-doped CQDs were prepared under 700 W microwave o irradiation (CEM MARS6 microwave system, USA) at 250 C for 10 min. During the irradiation, the colorless liquid became a dark brown solid, indicating the formation of raw CQDs. The obtained raw CQDs were dissolved in ethanol, centrifuged to remove the large particles, and then dialyzed with ethanol through a dialysis membrane (1000 Da) for 72 hours. The final product, over 6.66 g of CQDs, was obtained under reduced pressure.
2.3 Preparation of CQDs/PU composites N-doped CQD powder and IPDI (20 mmol, 4.45 g) were mixed in a one-neck flask under sonication to form a solution. Then, PTMEG (2 mmol, 2 g) and DBTDL (20 μl, 0.2 w/w % of total reactant weight) were dropped into the CQDs/IPDI solution. o The mixture was heated at 80 C for 1.5 h to form the prepolymer, and 1,4-BDO (18 mmol, 1.62 g) was added into the prepolymer and vacuum degassed for 5 min. Finally, the o mixture was placed in the Teflon mold and heated at 80 C in an oven for 8 h to obtain the CQDs/PU composites. The amount of N-doped CQDs was 0.2 w/w % of total reactant weight.
Fig. 2 (a) TEM image and (b) size distribution of N-doped CQDs; (c) high-resolution TEM image and (d) XRD pattern of N-doped CQDs.
3. Results and discussion Novel N-doped CQDs were synthesized by using isophorone diisocyanate (IPDI) as a single carbon source under microwave irradiation with an adaptable production yield of ~83 %. The synthesis route is shown in Fig. 1, no corrosive acid, alkali, or organic solvent were used. Fig. 2(a) shows a transmission electron microscopy (TEM) image of the N-doped CQDs, and Fig. 2(b) shows the particle diameter distribution of the Ndoped CQDs obtained by measuring the particle sizes of over 100 nanoparticles. The N-doped CQDs have a uniform size distribution and the particle diameters are mainly in the range
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reported before in CQDs composites. And this property makes N-doped CQDs have a potential in oxygen sensors.
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In Fig. 3(a), when excited under ultraviolet (UV) light (365 nm), the ethanol solution of N-doped CQDs displays a bright blue fluorescence. The UV-vis spectrum has two absorption peaks centered at 217 and ~300 nm in the UV region. 21, 26 According to previous reports, the absorption peaks at 217 and ~300 nm are attributed to the π−π* transition of the aromatic C=C bond and n-π* transition of the C=O bond, respectively. Photoluminescence (PL) is a significant feature of N26-29 doped CQDs. As shown in Fig. 3(b), like many CQDs, the Ndoped CQDs show excitation-dependent PL behavior in the excitation wavelength range 295 to 475 nm. This behavior may be attributed to the surface properties and size of the CQDs, .15, 30 which affect the energy levels of the electronic states In the PLE spectrum of N-doped CQDs at emission wavelength of 440 nm (Fig. 3(c)), there are two peaks at around 274 and 375 nm, which are ascribed to the allowed transition of the C=O 18 bond on the N-doped CQDs. The absolute PL quantum yield (QY) (excitation at 375 nm) of N-doped CQDs is about 11 %, which is a relatively high PL QY for CQDs without surface 31- 33 passivation. The high PL QY is mainly due to the existence of O and N atoms. These heteroatoms in the CQD lattice can 2 disrupt the sp hybridization of carbon atoms, altering the 34, electronic structures and increasing the PL intensity of CQDs. 35 The time-resolved fluorescence decay curve obtained by the time-correlated single photon counting method is illustrated in Fig. 3(d). The fluorescence decay curve can be fitted into the three lifetimes of 1.34, 3.98, and 9.75 ns (The detail results of fitting are presented in Table S1†). The average lifetime is calculated to be a short lifetime of 6.58 ns. The short lifetime of N-doped CQDs is indicative of the radiative recombination 18, 25 of the excitons, giving rise to fluorescence. Significantly, the obtained N-doped CQDs are shown to possess clear upconversion PL (UPL) properties in the excitation wavelength range 740 to 860 nm (Fig. S3†). This UPL property of CQDs may be attributed to the different emissive sites on each CQDs and 33,36 the multiphoton active processes. And the result indicates that the obtained N-doped CQDs will be of really great interest in bioscience and energy technology.
Fig. 4 FTIR spectra of N-doped CQDs, N-doped CQDs-6, and Ndoped CQDs-2. To confirm the structure and composition of the N-doped CQDs, Fourier Transform infrared spectroscopy (FTIR) and Xray photoelectron spectroscopy (XPS) were used. In the blue curve in Fig. 4, the broad absorption bands at 3400 and 3270 cm-1 are related to the stretching vibrations of O-H and N-H, -1 respectively. The strong peaks at 2951 and 2872 cm correspond to the C–H bond stretching vibrations. The peaks -1 at around 1638 and 1580 cm are assigned to the typical stretching peaks of the C=N and N-H bonds, respectively.34 The stretching vibrations peak of C=O is exhibited by the absorption band at 1695 cm-1. In Fig. 5(a), a typical XPS survey of the N-doped CQDs shows C, N, and O on the surface of the N-doped CQDs. The deconvoluted XPS C1s spectrum of Ndoped CQDs (Fig.5 (b)) displays three main components: C-C or C=C at ca. 284.88 eV, C-N at ca. 286.00 eV, and C=O or C=N at ca. 288.2 eV.34 In the XPS N1s spectrum of N-doped CQDs (Fig.5 (c)), the peaks at about 398.31 eV, 399.23 eV, 399.91 eV, and 400.69 eV confirm the presence of the nitrogen atoms of graphite-like structure, pyridinic-like N, pyrrolic-like N, and NH 10, 37 groups, respectively. The O1s XPS spectrum of N-doped CQDs (Fig.5 (d)) can be deconvoluted into two peaks at 531.9 eV and 532.9 eV, indicating the existence of C=O and C-OH, respectively.38 These XPS results indicate that N-doped CQDs are rich in oxygen and nitrogen atoms derived from the polymerization of IPDI.
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Fig. 3 (a) UV-Vis absorption spectrum of N-doped CQDs ethanol solution; insets are photographs of N-doped CQDs ethanol solution under sunlight (left) and under UV light (365 nm) (right); (b) fluorescence emission spectra of N-doped CQDs recorded from 295 to 475 nm in 20 nm increments; (c) fluorescence excitation spectrum of N-doped CQDs; (d) fluorescence decay profiles of N-doped CQDs measured with excitation and emission wavelengths of 375 and 440 nm, respectively.
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Scheme 1. Mechanism for formation of N-doped CQDs.
Fig. 5 (a) XPS spectrum of N-doped CQDs; (b-d) high resolution C 1s, O 1s, and N 1s spectra of N-doped CQDs. Due to the harsh and/or complicated synthesis conditions, few reports provided a detailed mechanism for the formation of CQDs.10, 22, 39 Herein, a possible mechanism for the formation of N-doped CQDs from IPDI, as shown in Scheme 1, is proposed. At high temperatures, the isocyanate groups react with themselves. The self-polymerization of isocyanate groups follows one of the three reactions: (1) dimerization cyclization reaction, while the dimer is not stable and will dissociate above 175 oC, (2) trimerization or multimerization 40,41 o reaction, and (3) carbodiimide reaction. At 250 C, IPDI first reacts with itself to form trimers, multimers, and carbodiimide structures. And carbodiimide self-polymerizes or reacts with isocyanate. Then the polymers are formed through reactions among trimers, multimers, and carbodiimide isocyanurates. Finally N-doped CQDs are formed by probable unclear burst. To confirm the mechanism, we performed FTIR analyses of Ndoped CQD samples formed under different irradiation times. IPDI was microwave irradiated for 2 and 6 min to form Ndoped CQDs-2 and N-doped CQDs-6, respectively. As shown in -1 Fig. 4, the sharp peaks at 2263 cm indicate large amounts of unreacted isocyanate groups. With increasing irradiation time, -1 the peaks at 2120 and 1714 cm become more and more pronounced, indicating the formation of carbodiimide and trimerisation/multimerisation structures. At an irradiation time of 10 min, the disappearance of the absorption peaks at -1 -1 -1 2120 cm and the shift of the peak at 1714 cm to 1695 cm indicate that the carbodiimide reacted with itself or isocyanate.
To date, CQDs have been mainly used alone or in solution, and only a few researchers have focused their attention on the incorporation of CQDs with polymers.14, 17, 39, 42 The incorporation of CQDs with polymers is one of the most significant means to expand the range of applications of CQDs because they combine the advantages of CQDs and organic materials. However, the inherently high specific surface energies and hydrophilic character of nanoparticles often lead to aggregation and phase separation in the prepared 43, 44 composites. In order to disperse the CQDs into monomers or polymers homogeneously at the nanometer scale, 17, 39, 45 dispersing agents are usually used. Due to the lack of hydrophilic groups, applicability of the as-synthesized N-doped CQDs will be hindered by the poor stability in water media, but they can be dispersed homogeneously in organic monomers (e.g., IPDI, TDI, PMMA, DVB, and St) (Fig. S4†), and these monomers often can be used to produce transparent materials for potential optical applications. We can use the ‘‘monomer as solvent’’ approach to synthesis CQDs composites, avoiding 23 the use of toxic organic solvents. FTIR results (Fig. S5†) show that CQDs/PU composites were successful prepared. In the scanning electron microscopy (SEM) images (Fig. S6†), the fractured section of the CQDs/PU composite presents a homogeneous dispersion of N-doped CQDs in PUs. The digital photograph (Fig. 6(a), left) under daylight further confirms the transparency and good optical quality of CQDs/PU composites with a large 3D macrostructure. Surprisingly, the N-doped CQDs in PUs emit not only fluorescence under UV light (365 nm) excitation (middle), but also an obvious afterglow when the UV light is turned off (right), which is visible to the naked eye for seconds (a corresponding video is provided in the ESI†). The PL emission spectra of the CQDs/PU composites in Fig. 6(b) show the strongest emission peak at 436 nm under excitation at 375 nm. Compared with those of the N-doped CQDs dispersed in ethanol, the emission peaks of the CQDs/PU composites have a blue shift at excitation wavelengths shorter than 375 nm. In addition, the corresponding PL spectra and phosphorescence spectrum of CQDs/PU composites by 375 nm excitation were shown in Fig. 6(c). The phosphorescence spectrum of the CQDs/PU composites shows a broad band at 450-500 nm and a large Stokes shift between fluorescence. Moreover, there is a partly overlap peak between florescence and phosphorescence of the CQDs/PU composites, corresponding to type DF. The phosphorescence excitation
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spectrum of the CQDs/PU composites (Fig. 6(d)) with emission at 500 nm show broad peaks at 285 and 377 nm, indicating that the phosphorescence behaviour come from the C=O bonds on N-doped CQDs. According to literature reports, the aromatic carbonyl group exhibits some degree of spin-orbit coupling that usually leads to intrinsic triplet generation 46, 47 through intersystem crossing. In addition, like O atom, N atom also favour n–π ∗ transition and hence facilitate the spinforbidden transfer of singlet-to-triplet excited states through intersystem crossing to populate triplet excitons.48 However, the lowest triplet excited state at room temperature in air can easily go through nonradiative and quenching processes. The rigidity of PU matrixes and the hydrogen bonding between the urethane backbone and N-doped CQDs both play a key role in rigidifying the C=O bonds on the surface of N-doped CQDs, minimizing the non-radiation transitions of triplet excitons, because similar RTP was not observed from N-doped CQDs dispersed in ethanol or a polystyrene matrix, probably because compared with those of the N-doped CQDs dispersed in ethanol, the emission peaks of the CQDs/PU composites have a blue shift at excitation wavelengths shorter than 375 nm.
Fig. 6 (a) Digital photographs of CQDs/PU composites dispersed in PU matrix under daylight (left), UV light (middle), and right after UV light has been turned off (right); (b) fluorescence emission spectra of CQDs/PU composites; (c) fluorescence and phosphorescence emission spectra of CQDs/PU composites exposed to UV light for 2 min; (d) phosphorescence excitation spectrum of CQDs/PU composites with emission at 500 nm; (e) effect of exposure time under UV light on intensity of phosphorescence. The time-resolved phosphorescence spectra with an excitation wavelength of 375 nm and emission wavelengths of 436 and 500 nm are shown in Fig. 7. The results of fitting the two spectra with multi-exponential functions are presented in Table S2†. The average RTP and DF lifetimes are 8.7 and 5.0 ms, respectively. The three discrete exponential components
imply different electronic transition processes, which may be attributed to a wide range of chemical environments for 21 aromatic carbonyls on the surface of N-doped CQDs.
Fig. 7 Time-resolved (a) delayed fluorescence and (b) phosphorescence spectra of CQDs/PU composites. In addition, if the CQDs/PU composites are put in air for a period of time, no obvious afterglow can be observed after a very short UV exposure time. With increasing exposure time under UV light, the intensity of phosphorescence increases and eventually reaches a steady-state value (Fig. 6(e)). It is wellknown that oxygen can quench the triplet states.46, 49 Although the PU matrixes have some barrier to oxygen, oxygen will gradually penetrate the matrix if exposed in air for a long time. With increasing time of UV irradiation, the triplet state of the material and the ground state of molecular oxygen (3O2) 48 3 undergo triplet-triplet quenching. The concentration of O2 in the PU matrixes gradually decreases, and a significantly low 3 concentration of O2 is preserved for some time due to the slow diffusion of oxygen into the PU matrixes. To validate the above mechanism, the CQDs/PU composites exposed to UV light for 5 min were put in a N2 and an air atmosphere for 48 h each. As shown in Fig. S7†, the intensity of phosphorescence hardly changes in the N2 atmosphere, but decreases significantly in the air atmosphere and after purging the oxygen media with nitrogen for 12 h, the intensity of phosphorescence is recovered. Moreover, pure organic room temperature phosphorescent (RTP) materials based on Ndoped CQDs are attractive alternatives to inorganics or metal complexes RTP materials because of their low cost, chemical 19 inertness and nontoxic. We suggested that this research will promote the practical application of the CQDs in oxygen sensors.
Conclusions We synthesized novel oil-soluble N-doped CQDs by using IPDI as a single carbon source under microwave irradiation. Notably, the yield of raw N-doped CQDs was about 83 % and suitable for industrial-scale production. A detail mechanism for the formation of N-doped CQDs from IPDI was proposed. Moreover, the obtained N-doped CQDs can be homogeneously dispersed in organic monomers and can use the ‘‘monomer as solvent’’ approach to synthesis CQDs composites, avoiding the use of toxic organic solvents as dispersing agents. Surprisingly, obvious phosphorescence and delayed fluorescence were discovered at room temperature when the CQDs/PU composites were excited by UV light, in addition to
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fluorescence. The PU matrixes play a key role in suppressing the nonradiation transitions of triplet excitons. In addition, we are the first to find that the phosphorescence of CQDs/PU composites is sensitive to oxygen and makes the obtained Ndoped CQDs have potential applications in oxygen sensors.
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Acknowledgements The financial support of the National Natural Science Foundation of China under Grant No. 20774009 is gratefully acknowledged.
Notes and references 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
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