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Production of carbon dots during the liquid phase exfoliation of MoS2 quantum dots
Carbon 155 (2019) 243e249
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Carbon journal homepage: www.elsevier.com/locate/carbon
Production of carbon dots during the liquid phase exfoliation of MoS2 quantum dots Weimin Zhang a, b, 1, Jinhong Du a, b, 1, Zhibo Liu a, Dingdong Zhang a, Qinwei Wei a, b, Haichao Liu a, Wei Ma a, b, Wencai Ren a, b, *, Hui-Ming Cheng a, b, c, ** a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang, 110016, PR China c Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, 1001 Xueyuan Road, Shenzhen, 518055, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2019 Received in revised form 20 August 2019 Accepted 24 August 2019 Available online 26 August 2019
N-methyl-2-pyrrolidone (NMP) has been widely used as a solvent for preparing two-dimensional quantum dots (2D-QDs) by probe sonication. Here we found that carbon dots (CDs) are simultaneously produced by the carbonization of NMP when preparing MoS2 QDs in a NMP solvent by probe sonication. These two kinds of QDs are similar in color, crystal structure, excitation wavelength dependent photoluminescence (PL), and have overlaps in their absorption spectra and coincidence of PL peaks. As a result, the co-production of CDs has not previously been recognized. The CDs show strong optical interactions with the MoS2 QDs, which leads to a dramatic decrease in the fluorescence intensity of the CDs/MoS2 QDs. Moreover, we found that the carbonization also occurs for the commonly used organic solvents. These findings suggest that associated production of CDs should be considered when preparing 2D-QDs by probe sonication, and the resulting hybrid CDs/2D-QDs will provide opportunities to design their properties and uses. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction The successful isolation and applications of graphene [1] have greatly stimulated research on many two-dimensional (2D) materials including transition metal dichalcogenides (TMDs) [2], hexagonal boron nitride (h-BN) [3], black phosphorene [4], and silicene [5]. These 2D materials have provided a rich platform for exploring new physical phenomena and developing new technologies [6,7]. 2D-quantum dots (2D-QDs) are a kind of 2D material with reduced dimensions, which usually have a lateral size less than 10 nm [8,9]. The quantum confinement effect and surface/edge states give these 2D-QDs different physical and chemical properties, such as a high quantum yield, a tunable band gap, size dependent
* Corresponding author. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China. ** Corresponding author. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China. E-mail addresses: [email protected] (W. Ren), [email protected] (H.-M. Cheng). 1 These authors were equal major contributors. https://doi.org/10.1016/j.carbon.2019.08.067 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
photoluminescence, a larger number of active sites, and better dispersibility [10]. As a result, they have many promising applications in optoelectronics [11e13], catalysis [14], sensing [15], bioimaging [10,16,17], and cancer therapy [9,18,19]. Although many methods including Li/K intercalation [16,20,21], chemical etching [22,23], and solvothermal treatment [24,25], have been developed for the preparation of 2D-QDs, liquid exfoliation by probe sonication is the most commonly used method due to its easy operation under ambient conditions, environmental friendliness, and the fact that the crystal structure is usually retained. Moreover, probe sonication is usually required to assist the exfoliation of 2D-QDs by other preparation methods [17,21,26,27]. For liquid exfoliation by probe sonication, the selection of solvents is very important [17,28]. In 2011, Coleman's group tried a number of different solvents to exfoliate bulk 2D materials and found that surface tension and solubility parameters are two key factors to minimize the energy of exfoliation and make 2D-QDs that are well dispersed [28]. In this case, N-methyl-2-pyrrolidone (NMP) is considered an excellent solvent for the exfoliation of various 2DQDs from their bulk precursors. However, probe sonication can produce vacuum bubbles due to the cavitation effect [29]. The
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implosive collapse of these bubbles results in a local high temperature and high pressure [30] that can cause an increased reactivity and composition changes of the solvents. In this paper we report that probe sonication causes the carbonization of NMP to produce carbon dots (CDs). Taking MoS2 as an example, we found that CDs and MoS2 QDs were simultaneously produced during the exfoliation of MoS2 by probe sonication in NMP, forming hybrid CDs/MoS2 QDs. Comparative studies show that sonicated pure NMP (s-NMP) and NMP with MoS2 (s-NMP-MoS2) have similar features in color and excitation wavelength dependent photoluminescence (PL), have overlaps in their absorption spectra and coincidence of their PL peaks, and the CDs and MoS2 QDs have the same hexagonal structure. Therefore, the co-production of CDs has been previously overlooked. These CDs have a strong optical interaction with MoS2 QDs, leading to a great decrease in the fluorescence intensity of the hybrids. Moreover, we found that the carbonization also occurs for all commonly used organic solvents. These findings call attention to the fact that the co-production of CDs should be taken into consideration when preparing 2D-QDs by probe sonication, and that this enables the one pot preparation of CDs/2D-QDs hybrids and provides more opportunities to design the properties and uses of 2D-QDs by taking advantage of the interactions between CDs and 2D-QDs. 2. Experimental 2.1. Materials NMP (AR, 99.0%), ethanol (AR, 99.7%), acetone (AR, 99.5%), isopropanol (AR, 99.7%), N, N-dimethylformamide (DMF) (AR, 99.5%), dichloromethane (AR, 99.5%) and chlorobenzene (AR, 99.5%) were purchased from Sinopharm Chemical Reagents (Shanghai) Co. Ltd. Hexane (99%) and bulk MoS2 (99.5%, <2 mm in average size) were purchased from Aladdin Reagent Co. Ltd. They all were used without further purification. 2.2. Probe sonication of NMP and MoS2 in NMP Scheme S1 shows the probe sonication process of MoS2 in NMP. Typically, 90 mL NMP containing MoS2 (7.5 mg/mL) was placed in a glass beaker and sonicated for 1e4 h using a 13 mm diameter probe (Sonics & Materials, Inc. VCX 800 and 20 kHz) with an amplitude of 75%. To protect the sonicator from overheating, the probe sonication worked for 2 s followed by a 2 s pause for cooling. Subsequently, the suspension was centrifuged at 10,000 rpm for 30 min. Then the supernatant was taken out and centrifuged at 10,000 rpm for another 30 min. Finally, the supernatant was collected for characterization. For comparison, 90 mL of pure NMP was also treated with the same probe sonication and centrifugation processes. 2.3. Characterization UVeVis absorption spectra of s-NMP and s-NMP-MoS2 were collected using a Varian Cary 5000 spectrometer with pure NMP as a control. PL spectra were measured by a FLSP-920 Fluorescence Spectrophotometer. Transmission electron microscopy (TEM, FEI TECNAI G2 F20) and atomic force microscopy (AFM, Bruker MultiMode 8, in ScanAsyst mode) were used to characterize the structure, morphology and thickness of the QDs. The samples were prepared by dropping the dilute solutions of s-NMP and s-NMPMoS2 onto a copper grid covered with an amorphous carbon film for TEM and a mica substrate for AFM. To characterize the crystal structure and composition, s-NMP
and s-NMP-MoS2 were transferred into a dialysis bag (molecular weight cut-off: 500 Da) for 4 days to remove the NMP solvent, and then freeze-dried to get powder samples. The samples were measured by X-ray diffraction (XRD, Bruker D8 Focus diffractometer, with X-ray radiation of Cu Ka1 (l ¼ 1.5406 Å)), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka radiation sources), Raman spectroscopy (Jobin Yvon LabRam HR800, excited by a 532 nm laser), and Fourier transform infrared spectroscopy (FTIR, Bruker TENSOR 27). 3. Results and discussion 3.1. Production of CDs by the sonication of NMP We probe sonicated pure NMP for different times and measured their absorption spectra. As shown in Fig. 1a, with an increase in sonication time, the s-NMP changes from colorless to yellowish. The UVeVis absorption spectra of s-NMP with different sonication times (Fig. 1b) show sharp absorption peaks at around 300 nm, which are the characteristic absorption peak of CDs [31], giving clear evidence for the production of CDs. Moreover, with increasing sonication time, the peak intensity increases and its position shifts from ~265 nm to ~295 nm. This is because local high temperatures and high pressures caused by the cavitation effect of probe sonication [30] enable carbonization of the NMP to produce carbon cores [8,32]. With increasing sonication time, the concentration and size of these cores increase since more NMP precursor is carbonized and the cores grow along their edges [33]. Thus, the concentration of photoluminescence centers increases, which leads to the increase in peak intensity. On the other hand, due to the quantum confinement effect [10], with the decrease in diameter of the particles (usually below 10 nm) [4], their energy spacing of electronic levels increases, which results in the increase in energy gaps [34]. Thus, the size increase of carbon cores leads to decreased energy gaps and consequently slight redshift of absorption peaks. In addition, with the increase in particle concentration and size, the average distance between the photoluminescence centers decreases. The resulting aggregation of adjacent CDs is another reason for the red-shift of absorption peaks [35]. To further confirm the carbonization of NMP to produce CDs during probe sonication, TEM and AFM were used to characterize the structure and morphology of the products. Because the production of CDs is a “bottom-up” process, including bond breaking, hydrolyzation, oxidation, polymerization, and carbonization [8,32], many intermediate products, such as 3-methylaminobutyric acid [36], 5-hydroxy-Nmethyl-2-pyrrolidone and N-methyl-succinimide [37,38], can be produced during a short time sonication. These small organic precursors and the low concentration of carbon nanoparticles in s-NMP make the collection of CDs extremely difficult. We therefore used 4 h s-NMP to prepare the TEM and AFM samples. As shown in Fig. 1c, all the small particles from 4 h s-NMP are quasi-circular. High resolution TEM (HRTEM) (Fig. 1d) shows that the d-spacing of the particle is 2.11 Å, which is consistent with that of graphite (100) planes [39]. The fast Fourier transform (FFT) image (Fig. 1e) indicates that these particles have a hexagonal structure, which is consistent with the structure of reported CDs [39]. These results strongly indicate that probe sonication results in the carbonization of NMP to produce CDs. The average lateral size of the CDs obtained from TEM images of 4 h s-NMP is ~3.3 nm (Fig. S1a) and AFM images (Fig. 1f and g) with different magnifications indicate that the size of the CDs is very uniform. According to the AFM height profile (inset in Fig. 1f and g) and the statistical thickness distribution (Fig. S1b), most of them have a thickness less than 1.5 nm.
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Fig. 1. The characterization of s-NMP and CDs. (a) Optical photograph and (b) absorption spectra of 1e4 h s-NMP; (c) TEM and (d) HRTEM images of CDs from 4 h s-NMP; (e) FFT pattern of the HRTEM image of CDs in (d); (f, g) Low- and high-magnification AFM images of the CDs. Insets in f and g are the height profiles of CDs along the yellow lines. (A colour version of this figure can be viewed online.)
3.2. Simultaneous production of CDs and MoS2 QDs during liquid exfoliation Because NMP is usually used for the preparation of 2D-QDs by probe sonication, CDs could be produced at the same time by the carbonization of NMP as described above. MoS2 QDs are a representative of the 2D-QD family and have many unique physical properties such as an indirect-to-direct band gap transition and a high luminescent quantum yield [40]. To identify whether CDs are also produced, taking MoS2 as an example, we studied the products of NMP containing MoS2 after probe sonication. An optical image shows that the s-NMP-MoS2 changes from yellowish to yellowbrown with increasing sonication time (Fig. 2a). We then examined the products of s-NMP-MoS2 obtained with different sonication times by the TEM. As shown in Figs. S2a-d, all
Fig. 2. TEM characterization and the absorption spectra of hybrid CDs/MoS2 QDs. (a) Photograph of 1e4 h s-NMP-MoS2; (b) TEM image of QDs obtained from 4 h s-NMPMoS2; (c, d) HRTEM images of (c) CDs and (d) MoS2 QDs in (b), indicating the coexistence of CDs and MoS2 QDs in 4 h s-NMP-MoS2; (e, f) UVeVis absorption spectra of (e) 1e4 h s-NMP-MoS2 using pure NMP as the control and (f) 1e4 h s-NMP-MoS2 using s-NMP sonicated for the same times as the control. (A colour version of this figure can be viewed online.)
the particles obtained from s-NMP-MoS2 have a similar quasicircular shape to the CDs obtained from s-NMP. The average diameter for the particles obtained with sonication times of 1, 2, 3 and 4 h (Figs. S2e-h) respectively is 3.15 nm, 3.20 nm, 3.14 nm, and 3.00 nm. We also analyzed the structures of these particles (Fig. 2b), and observed CDs with a d-spacing of 2.11 Å (Fig. 2c), corresponding to that of the graphite (100) planes [39], and MoS2 QDs with dspacing of 1.82 Å (Fig. 2d), corresponding to the (105) planes of 2HeMoS2 with a hexagonal structure (PDF#37e1492). This gives direct evidence of the associated production of CDs during the liquid exfoliation of MoS2 QDs by probe sonication in NMP. Further AFM measurements show that the thicknesses of the hybrid CDs/ MoS2 QDs obtained with different sonication times are almost the same. More than half the QDs in each sample have a thickness less than 1.5 nm (Fig. S3). Fig. 2e shows the absorption spectra of s-NMP-MoS2 obtained with different sonication times, using pure NMP as the control and they completely overlap those of s-NMP. In order to exclude the influence of CDs and obtain the absorption spectra of MoS2 QDs, sNMP was used as a control to measure the absorption spectra of sNMP-MoS2. As shown in Fig. 2f, there are five characteristic absorption peaks of MoS2 QDs located at 666, 608, 445, 394, and ~300e350 nm. The peaks at 666 and 608 nm can be ascribed to the excited-transition from the K point of the Brillouin zone, while the two peaks at 445 and 394 nm are due to the direct transition from the deep valance band to the conduction band from the M point of the Brillouin zone [41]. All these four peaks show nearly no shifts in position with the sonication time, demonstrating no obvious difference in the size and thickness of the MoS2 QDs obtained. This is consistent with the TEM and AFM characterizations discussed above. The absorption peak at ~300e350 nm originates from the convoluted long wavelength excitonic peaks caused by the quantum confinement effect when MoS2 is exfoliated into small QDs [42,43]. It shows a small red-shift with increasing sonication time, which might due to changes in both the solution environment and the surface state of MoS2. 3.3. Structure and chemical composition of CDs and hybrid CDs/ MoS2 QDs XRD, Raman, FTIR, and XPS spectra were measured to
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comprehensively investigate the structure and chemical composition of the CDs obtained from s-NMP and hybrid CDs/MoS2 QDs obtained from s-MoS2-NMP. The XRD pattern of CDs (red line in Fig. 3a) shows a weak peak centered at 21.5 , which corresponds to an interlayer spacing of 4.13 Å and can be ascribed to the (002) plane of graphite [39]. The large interlayer spacing [31] and wide diffraction peak suggest the existence of imperfect and amorphous structures of carbon with functional groups or polymer chains originating from the polymerization of the NMP precursor [35]. This is confirmed by the Raman spectrum of the CDs (red line in Fig. 3b), which shows a strong disorder-related D band at ~1343 cm1, a wide G band at ~1585 cm1 and a low G band to D band intensity ratio of ~1.0 [39]. The Raman spectrum of the QD samples obtained from s-MoS2-NMP shows the characteristic peaks of both CDs and MoS2 QDs [42,44,45], further confirming the simultaneous production of CDs and MoS2 QDs. Moreover, the CDs are defective but the MoS2 QDs have better crystallinity based on the obvious XRD diffraction peaks of MoS2 (blue lines in Fig. 3a) compared with CDs (red lines in Fig. 3a). Note that the MoS2 QDs have a weak interlayer interaction and weak coupling between electronic transitions and phonons [45] which is why, compared to bulk MoS2, the XRD diffraction peaks are negligible (Fig. S4a), and both the A1g (410.8 cm1) and E12g (383.6 cm1) peaks, respectively corresponding to the vertical vibration of S atoms and the horizontal vibration of SeMoeS are red-shifted, and the distance between the two Raman peaks is decreased (Fig. S4b) [42,45]. For the FTIR spectrum of CDs obtained from 4 h s-NMP (red line in Fig. 3c), the peaks centered at 1718 cm1, 1590 cm1, and 1288 cm1 are respectively ascribed to C]O, C]C, and CeN vibrations [39,46], and those at 3400 cm1 and 1270 cm1 are ascribed to OeH vibrations [39]. These results indicate the presence of oxygen- and nitrogen-containing functional groups on the surface and edges of the CDs, which are also revealed in their XPS spectrum (red line in Fig. 3d). As shown in Fig. S5a, the high resolution XPS C1s spectrum of the CDs has three characteristic peaks corresponding to O]CeO (287.8 eV), CeN/CeO (285.8 eV), and C] C bonds (284.6 eV) [47]. For the hybrid CDs/MoS2 QDs, the presence of similar FTIR peaks (blue line in Fig. 3c) and a XPS C1s spectrum (blue line in Fig. 3d and Fig. S5b) similar to CDs indicates the
Fig. 3. (a) XRD pattern, (b) Raman spectra, (c) FTIR spectra, and (d) XPS spectra of CDs obtained from 4 h s-NMP and hybrid CDs/MoS2 QDs obtained from 4 h s-NMP-MoS2. (A colour version of this figure can be viewed online.)
simultaneous production of CDs containing oxygen- and nitrogencontaining functional groups. However, these CD-related peaks are not obvious, indicating a small amount of CDs in hybrid QDs. In addition, the hybrid CDs/MoS2 QDs show three additional FTIR peaks (blue line in Fig. 3c), MoeOH at 1382 cm1, SeOH at 1120 cm1, and SeH at 605 cm1 [48]. Fig. S5c shows a high resolution Mo 3d XPS spectrum. The peaks located at 226.1 eV, 229.0 eV, 232.2 eV correspond to S2 2s, Mo4þ 3d5/2, and Mo4þ 3d3/2, respectively, and another weak peak located at 235.2 eV corresponds to Mo6þ (inset in Fig. S5c). Fig. S5d shows a high resolution S 2p XPS spectrum, in which the peaks located at 161.9 eV, 162.9 eV, 169.2 eV are assigned to S2 2p3/2, S2 2p1/2, and S6þ, respectively [48]. The appearance of MoeOH and SeOH vibration peaks in the FTIR spectrum as well as small Mo6þ and S6þ peaks in the XPS spectra of hybrid CDs/MoS2 QDs indicates slight oxidation of the MoS2 QDs, which is due to the local high temperature and high pressure generated during the probe sonication process under ambient condition [48]. 3.4. The optical interactions between CDs and MoS2 QDs We compared the PL properties of s-NMP (CDs) and s-NMPMoS2 (hybrid CDs/MoS2 QDs). The 1e4 h s-NMP shows only one PL peak under different excitation wavelengths (Fig. 4a, c, e and g), and the peak position showed a red-shift with an increase in excitation wavelength from 280 to 440 nm. However, the 1e4 h s-NMP-MoS2 showed two distinct PL peaks under different excitation wavelengths (Fig. 4b, d, f and h). One peak is consistent with that of CDs, and the other is assigned to MoS2 QDs. The peak intensity for the CDs is lower than that for the MoS2 QDs due to its small amount in the hybrids, and it becomes lower and lower with an increase in sonication time. Moreover, these two peaks show a red-shift with an increase in excitation wavelength from 280 to 440 nm and finally merge, which might be due to the larger red-shift of the PL peak for CDs than that for MoS2 QDs. It is worth noting that although the PL peak of CDs in s-NMPMoS2 can be detected (Fig. 4b, d, f and h), its intensity is drastically decreased compared with that in s-NMP (Fig. 4a, c, e and g). Moreover, the decrease becomes more obvious with increasing sonication time. These results suggest the existence of an optical interaction between associated CDs and MoS2 QDs. To understand this optical interaction, we used 365 nm UV light to excite the 1e4 h s-NMP and s-NMP-MoS2 and measured the corresponding PL spectra. As shown in Fig. 5a, there is no evident fluorescence for pure NMP, but strong fluorescence appeared after it was sonicated for 1 h due to the production of CDs. With increasing sonication time, the fluorescence color changed from blue-white to light yellow and the corresponding emission peaks (Fig. 5b) were redshifted. This is in good agreement with the red-shift of the ~300 nm absorption peaks of s-NMP (Fig. 1b) caused by the size increase of the carbon cores [49] and the aggregation of adjacent photoluminescence centers with increased CD concentration [41]. Additionally, because of the continuous increase in the concentration of CDs, the peak intensity of s-NMP also increases. Different from s-NMP, with an increase in sonication time the fluorescence color of s-NMP-MoS2 changes from blue-green to weak yellowgreen (Fig. 5c). The PL peak intensity of CDs in s-NMP-MoS2 (Fig. 5d) is ~10, 30, 75 and 300 times lower than that in s-NMP (Fig. 5b) for 1, 2, 3, 4 h sonication, respectively. Moreover, the fluorescence intensity of s-NMP-MoS2 is much lower than that of sNMP and gradually decreases with an increase in sonication time. For example, the fluorescence intensity of 1 h s-NMP-MoS2 is ~7.4 times lower than that of s-NMP, which is due to the increased concentration of MoS2 QDs in the solvent, which results in selfabsorption, consequently quenching of PL intensity [50,51]. In
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Fig. 4. PL spectra of (a, c, e, g) s-NMP and (b, d, f, h) s-NMP-MoS2 with sonication times of (a, b) 1 h, (c, d) 2 h, (e, f) 3 h and (g, h) 4 h, under different excitation wavelengths. (A colour version of this figure can be viewed online.)
Fig. 5. Photographs and PL spectra of (a, b) 1e4 h s-NMP and (c, d) 1e4 h s-NMP-MoS2 under 365 nm UV light. (A colour version of this figure can be viewed online.)
addition, there are great overlaps between the absorption spectra of MoS2 QDs (Fig. 2b) and the emission spectra of CDs (Fig. 4a, c, e, g). Such an overlap provides the possibility of resonance energy transfer [15] between CDs and MoS2 QDs, which can result in a
decrease in fluorescence intensity. The above results show that the CDs associated with the MoS2 QDs have an optical interaction with them, which decreases their PL strength and may influence their use in bioimaging and fluorescence. However, CDs and MoS2 QDs have different physical and chemical properties [52,53]. These hybrid QDs may have uses in different fields by taking advantage of their combined effect. For example, the CDs with oxygen- and nitrogen-containing functional groups have a high absorption ability to reactants [54], and the MoS2 QDs have a good catalytic activity [17,42], which might be beneficial to catalysis. In addition, both CDs and MoS2 QDs have been widely used in photothermal and photodynamic cancer therapy [9,19], and as the functional layers of photoelectronic devices such as organic photovoltaic solar cells and organic lightemitting diodes [6]. The functional groups on CDs may improve the dispersibility of MoS2 QDs and make these uses easier. Therefore, the associated production of CDs provides more opportunity to design the properties and uses of MoS2 QDs, which is worth further study. Finally, we investigated the optical properties of the commonly used organic solvents, including ethanol, acetone, isopropanol, hexane, DMF, dichloromethane, chlorobenzene, before and after 2 h probe sonication. As shown in Fig. 6a, similar to s-NMP, all the sonicated organic solvents show an absorption peak at around 300 nm, indicating that these organic solvents were carbonized.
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Fig. 6. Optical properties of the commonly used solvents before and after probe sonication. (a) Absorption spectra of the different solvents after sonication. (b) Ratio of PL intensity after sonication to before sonication for different solvents under 365 nm UV light. (c) Photographs of the solvents before (left) and after (right) sonication under 365 nm UV light. (A colour version of this figure can be viewed online.)
Note that there is no obvious absorption peak for sonicated deionized water (DI water), which excludes the influence of other factors, such as a residue from the probe. Due to the different carbonization degrees, they show different PL intensity ratios after to before sonication (Fig. 6b) and different fluorescence (Fig. 6c) under the 365 nm UV light. The results show that the carbonization of organic solvents is universal, therefore, the associated production of CDs should be considered when preparing 2D-QDs by probe sonication. However, the technique provides for the one pot fabrication of hybrid CDs/2D-QDs which might have wide uses in catalysis, optoelectronic devices, therapy, and so on.
Technology of China (No. 2016YFA0200101), National Science Foundation of China (Nos. 51325205, 51290273, 51572265 and 51861135201), Postdoctoral Science Foundation of China (No. 2018M630309), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000), Liaoning Revitalization Talents Program (No. XLYC1808013), and the Economic, Trade and Information Commission of Shenzhen Municipality for the “2017 Graphene Manufacturing Innovation Center Project” (No. 201901171523).
4. Conclusion
Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.08.067.
We found that probe sonication causes some carbonization of NMP solvents to form CDs when preparing MoS2 QDs in NMP. These two kinds of QDs show similar features in color, crystal structure, excitation wavelength dependent photoluminescence (PL), and have an overlap of their absorption spectra and coincidence of the PL peaks, which is why the associated production of CDs has not previously been recognized. The CDs associated with the MoS2 QDs have an optical interaction with them, which leads to a dramatic decrease in the fluorescence intensity. We also found that the production of CDs by probe sonication is universal for the commonly used organic solvents. These observations not only require that we take CDs into consideration when preparing 2DQDs by probe sonication in organic solvents, but also facilitate the fabrication of hybrid CDs/2D-QDs, which may promote the wide use of 2D-QDs in catalysis, optoelectronic devices, therapy, and so on. Acknowledgements This work is supported by the Ministry of Science and
Appendix A. Supplementary data
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Carbon journal homepage: www.elsevier.com/locate/carbon
Production of carbon dots during the liquid phase exfoliation of MoS2 quantum dots Weimin Zhang a, b, 1, Jinhong Du a, b, 1, Zhibo Liu a, Dingdong Zhang a, Qinwei Wei a, b, Haichao Liu a, Wei Ma a, b, Wencai Ren a, b, *, Hui-Ming Cheng a, b, c, ** a
Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China School of Materials Science and Engineering, University of Science and Technology of China, 72 Wenhua Road, Shenyang, 110016, PR China c Shenzhen Geim Graphene Center, Tsinghua-Berkeley Shenzhen Institute, Tsinghua University, 1001 Xueyuan Road, Shenzhen, 518055, PR China b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 June 2019 Received in revised form 20 August 2019 Accepted 24 August 2019 Available online 26 August 2019
N-methyl-2-pyrrolidone (NMP) has been widely used as a solvent for preparing two-dimensional quantum dots (2D-QDs) by probe sonication. Here we found that carbon dots (CDs) are simultaneously produced by the carbonization of NMP when preparing MoS2 QDs in a NMP solvent by probe sonication. These two kinds of QDs are similar in color, crystal structure, excitation wavelength dependent photoluminescence (PL), and have overlaps in their absorption spectra and coincidence of PL peaks. As a result, the co-production of CDs has not previously been recognized. The CDs show strong optical interactions with the MoS2 QDs, which leads to a dramatic decrease in the fluorescence intensity of the CDs/MoS2 QDs. Moreover, we found that the carbonization also occurs for the commonly used organic solvents. These findings suggest that associated production of CDs should be considered when preparing 2D-QDs by probe sonication, and the resulting hybrid CDs/2D-QDs will provide opportunities to design their properties and uses. © 2019 Elsevier Ltd. All rights reserved.
1. Introduction The successful isolation and applications of graphene [1] have greatly stimulated research on many two-dimensional (2D) materials including transition metal dichalcogenides (TMDs) [2], hexagonal boron nitride (h-BN) [3], black phosphorene [4], and silicene [5]. These 2D materials have provided a rich platform for exploring new physical phenomena and developing new technologies [6,7]. 2D-quantum dots (2D-QDs) are a kind of 2D material with reduced dimensions, which usually have a lateral size less than 10 nm [8,9]. The quantum confinement effect and surface/edge states give these 2D-QDs different physical and chemical properties, such as a high quantum yield, a tunable band gap, size dependent
* Corresponding author. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China. ** Corresponding author. Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang, 110016, PR China. E-mail addresses: [email protected] (W. Ren), [email protected] (H.-M. Cheng). 1 These authors were equal major contributors. https://doi.org/10.1016/j.carbon.2019.08.067 0008-6223/© 2019 Elsevier Ltd. All rights reserved.
photoluminescence, a larger number of active sites, and better dispersibility [10]. As a result, they have many promising applications in optoelectronics [11e13], catalysis [14], sensing [15], bioimaging [10,16,17], and cancer therapy [9,18,19]. Although many methods including Li/K intercalation [16,20,21], chemical etching [22,23], and solvothermal treatment [24,25], have been developed for the preparation of 2D-QDs, liquid exfoliation by probe sonication is the most commonly used method due to its easy operation under ambient conditions, environmental friendliness, and the fact that the crystal structure is usually retained. Moreover, probe sonication is usually required to assist the exfoliation of 2D-QDs by other preparation methods [17,21,26,27]. For liquid exfoliation by probe sonication, the selection of solvents is very important [17,28]. In 2011, Coleman's group tried a number of different solvents to exfoliate bulk 2D materials and found that surface tension and solubility parameters are two key factors to minimize the energy of exfoliation and make 2D-QDs that are well dispersed [28]. In this case, N-methyl-2-pyrrolidone (NMP) is considered an excellent solvent for the exfoliation of various 2DQDs from their bulk precursors. However, probe sonication can produce vacuum bubbles due to the cavitation effect [29]. The
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implosive collapse of these bubbles results in a local high temperature and high pressure [30] that can cause an increased reactivity and composition changes of the solvents. In this paper we report that probe sonication causes the carbonization of NMP to produce carbon dots (CDs). Taking MoS2 as an example, we found that CDs and MoS2 QDs were simultaneously produced during the exfoliation of MoS2 by probe sonication in NMP, forming hybrid CDs/MoS2 QDs. Comparative studies show that sonicated pure NMP (s-NMP) and NMP with MoS2 (s-NMP-MoS2) have similar features in color and excitation wavelength dependent photoluminescence (PL), have overlaps in their absorption spectra and coincidence of their PL peaks, and the CDs and MoS2 QDs have the same hexagonal structure. Therefore, the co-production of CDs has been previously overlooked. These CDs have a strong optical interaction with MoS2 QDs, leading to a great decrease in the fluorescence intensity of the hybrids. Moreover, we found that the carbonization also occurs for all commonly used organic solvents. These findings call attention to the fact that the co-production of CDs should be taken into consideration when preparing 2D-QDs by probe sonication, and that this enables the one pot preparation of CDs/2D-QDs hybrids and provides more opportunities to design the properties and uses of 2D-QDs by taking advantage of the interactions between CDs and 2D-QDs. 2. Experimental 2.1. Materials NMP (AR, 99.0%), ethanol (AR, 99.7%), acetone (AR, 99.5%), isopropanol (AR, 99.7%), N, N-dimethylformamide (DMF) (AR, 99.5%), dichloromethane (AR, 99.5%) and chlorobenzene (AR, 99.5%) were purchased from Sinopharm Chemical Reagents (Shanghai) Co. Ltd. Hexane (99%) and bulk MoS2 (99.5%, <2 mm in average size) were purchased from Aladdin Reagent Co. Ltd. They all were used without further purification. 2.2. Probe sonication of NMP and MoS2 in NMP Scheme S1 shows the probe sonication process of MoS2 in NMP. Typically, 90 mL NMP containing MoS2 (7.5 mg/mL) was placed in a glass beaker and sonicated for 1e4 h using a 13 mm diameter probe (Sonics & Materials, Inc. VCX 800 and 20 kHz) with an amplitude of 75%. To protect the sonicator from overheating, the probe sonication worked for 2 s followed by a 2 s pause for cooling. Subsequently, the suspension was centrifuged at 10,000 rpm for 30 min. Then the supernatant was taken out and centrifuged at 10,000 rpm for another 30 min. Finally, the supernatant was collected for characterization. For comparison, 90 mL of pure NMP was also treated with the same probe sonication and centrifugation processes. 2.3. Characterization UVeVis absorption spectra of s-NMP and s-NMP-MoS2 were collected using a Varian Cary 5000 spectrometer with pure NMP as a control. PL spectra were measured by a FLSP-920 Fluorescence Spectrophotometer. Transmission electron microscopy (TEM, FEI TECNAI G2 F20) and atomic force microscopy (AFM, Bruker MultiMode 8, in ScanAsyst mode) were used to characterize the structure, morphology and thickness of the QDs. The samples were prepared by dropping the dilute solutions of s-NMP and s-NMPMoS2 onto a copper grid covered with an amorphous carbon film for TEM and a mica substrate for AFM. To characterize the crystal structure and composition, s-NMP
and s-NMP-MoS2 were transferred into a dialysis bag (molecular weight cut-off: 500 Da) for 4 days to remove the NMP solvent, and then freeze-dried to get powder samples. The samples were measured by X-ray diffraction (XRD, Bruker D8 Focus diffractometer, with X-ray radiation of Cu Ka1 (l ¼ 1.5406 Å)), X-ray photoelectron spectroscopy (XPS, ESCALAB 250, Al Ka radiation sources), Raman spectroscopy (Jobin Yvon LabRam HR800, excited by a 532 nm laser), and Fourier transform infrared spectroscopy (FTIR, Bruker TENSOR 27). 3. Results and discussion 3.1. Production of CDs by the sonication of NMP We probe sonicated pure NMP for different times and measured their absorption spectra. As shown in Fig. 1a, with an increase in sonication time, the s-NMP changes from colorless to yellowish. The UVeVis absorption spectra of s-NMP with different sonication times (Fig. 1b) show sharp absorption peaks at around 300 nm, which are the characteristic absorption peak of CDs [31], giving clear evidence for the production of CDs. Moreover, with increasing sonication time, the peak intensity increases and its position shifts from ~265 nm to ~295 nm. This is because local high temperatures and high pressures caused by the cavitation effect of probe sonication [30] enable carbonization of the NMP to produce carbon cores [8,32]. With increasing sonication time, the concentration and size of these cores increase since more NMP precursor is carbonized and the cores grow along their edges [33]. Thus, the concentration of photoluminescence centers increases, which leads to the increase in peak intensity. On the other hand, due to the quantum confinement effect [10], with the decrease in diameter of the particles (usually below 10 nm) [4], their energy spacing of electronic levels increases, which results in the increase in energy gaps [34]. Thus, the size increase of carbon cores leads to decreased energy gaps and consequently slight redshift of absorption peaks. In addition, with the increase in particle concentration and size, the average distance between the photoluminescence centers decreases. The resulting aggregation of adjacent CDs is another reason for the red-shift of absorption peaks [35]. To further confirm the carbonization of NMP to produce CDs during probe sonication, TEM and AFM were used to characterize the structure and morphology of the products. Because the production of CDs is a “bottom-up” process, including bond breaking, hydrolyzation, oxidation, polymerization, and carbonization [8,32], many intermediate products, such as 3-methylaminobutyric acid [36], 5-hydroxy-Nmethyl-2-pyrrolidone and N-methyl-succinimide [37,38], can be produced during a short time sonication. These small organic precursors and the low concentration of carbon nanoparticles in s-NMP make the collection of CDs extremely difficult. We therefore used 4 h s-NMP to prepare the TEM and AFM samples. As shown in Fig. 1c, all the small particles from 4 h s-NMP are quasi-circular. High resolution TEM (HRTEM) (Fig. 1d) shows that the d-spacing of the particle is 2.11 Å, which is consistent with that of graphite (100) planes [39]. The fast Fourier transform (FFT) image (Fig. 1e) indicates that these particles have a hexagonal structure, which is consistent with the structure of reported CDs [39]. These results strongly indicate that probe sonication results in the carbonization of NMP to produce CDs. The average lateral size of the CDs obtained from TEM images of 4 h s-NMP is ~3.3 nm (Fig. S1a) and AFM images (Fig. 1f and g) with different magnifications indicate that the size of the CDs is very uniform. According to the AFM height profile (inset in Fig. 1f and g) and the statistical thickness distribution (Fig. S1b), most of them have a thickness less than 1.5 nm.
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Fig. 1. The characterization of s-NMP and CDs. (a) Optical photograph and (b) absorption spectra of 1e4 h s-NMP; (c) TEM and (d) HRTEM images of CDs from 4 h s-NMP; (e) FFT pattern of the HRTEM image of CDs in (d); (f, g) Low- and high-magnification AFM images of the CDs. Insets in f and g are the height profiles of CDs along the yellow lines. (A colour version of this figure can be viewed online.)
3.2. Simultaneous production of CDs and MoS2 QDs during liquid exfoliation Because NMP is usually used for the preparation of 2D-QDs by probe sonication, CDs could be produced at the same time by the carbonization of NMP as described above. MoS2 QDs are a representative of the 2D-QD family and have many unique physical properties such as an indirect-to-direct band gap transition and a high luminescent quantum yield [40]. To identify whether CDs are also produced, taking MoS2 as an example, we studied the products of NMP containing MoS2 after probe sonication. An optical image shows that the s-NMP-MoS2 changes from yellowish to yellowbrown with increasing sonication time (Fig. 2a). We then examined the products of s-NMP-MoS2 obtained with different sonication times by the TEM. As shown in Figs. S2a-d, all
Fig. 2. TEM characterization and the absorption spectra of hybrid CDs/MoS2 QDs. (a) Photograph of 1e4 h s-NMP-MoS2; (b) TEM image of QDs obtained from 4 h s-NMPMoS2; (c, d) HRTEM images of (c) CDs and (d) MoS2 QDs in (b), indicating the coexistence of CDs and MoS2 QDs in 4 h s-NMP-MoS2; (e, f) UVeVis absorption spectra of (e) 1e4 h s-NMP-MoS2 using pure NMP as the control and (f) 1e4 h s-NMP-MoS2 using s-NMP sonicated for the same times as the control. (A colour version of this figure can be viewed online.)
the particles obtained from s-NMP-MoS2 have a similar quasicircular shape to the CDs obtained from s-NMP. The average diameter for the particles obtained with sonication times of 1, 2, 3 and 4 h (Figs. S2e-h) respectively is 3.15 nm, 3.20 nm, 3.14 nm, and 3.00 nm. We also analyzed the structures of these particles (Fig. 2b), and observed CDs with a d-spacing of 2.11 Å (Fig. 2c), corresponding to that of the graphite (100) planes [39], and MoS2 QDs with dspacing of 1.82 Å (Fig. 2d), corresponding to the (105) planes of 2HeMoS2 with a hexagonal structure (PDF#37e1492). This gives direct evidence of the associated production of CDs during the liquid exfoliation of MoS2 QDs by probe sonication in NMP. Further AFM measurements show that the thicknesses of the hybrid CDs/ MoS2 QDs obtained with different sonication times are almost the same. More than half the QDs in each sample have a thickness less than 1.5 nm (Fig. S3). Fig. 2e shows the absorption spectra of s-NMP-MoS2 obtained with different sonication times, using pure NMP as the control and they completely overlap those of s-NMP. In order to exclude the influence of CDs and obtain the absorption spectra of MoS2 QDs, sNMP was used as a control to measure the absorption spectra of sNMP-MoS2. As shown in Fig. 2f, there are five characteristic absorption peaks of MoS2 QDs located at 666, 608, 445, 394, and ~300e350 nm. The peaks at 666 and 608 nm can be ascribed to the excited-transition from the K point of the Brillouin zone, while the two peaks at 445 and 394 nm are due to the direct transition from the deep valance band to the conduction band from the M point of the Brillouin zone [41]. All these four peaks show nearly no shifts in position with the sonication time, demonstrating no obvious difference in the size and thickness of the MoS2 QDs obtained. This is consistent with the TEM and AFM characterizations discussed above. The absorption peak at ~300e350 nm originates from the convoluted long wavelength excitonic peaks caused by the quantum confinement effect when MoS2 is exfoliated into small QDs [42,43]. It shows a small red-shift with increasing sonication time, which might due to changes in both the solution environment and the surface state of MoS2. 3.3. Structure and chemical composition of CDs and hybrid CDs/ MoS2 QDs XRD, Raman, FTIR, and XPS spectra were measured to
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comprehensively investigate the structure and chemical composition of the CDs obtained from s-NMP and hybrid CDs/MoS2 QDs obtained from s-MoS2-NMP. The XRD pattern of CDs (red line in Fig. 3a) shows a weak peak centered at 21.5 , which corresponds to an interlayer spacing of 4.13 Å and can be ascribed to the (002) plane of graphite [39]. The large interlayer spacing [31] and wide diffraction peak suggest the existence of imperfect and amorphous structures of carbon with functional groups or polymer chains originating from the polymerization of the NMP precursor [35]. This is confirmed by the Raman spectrum of the CDs (red line in Fig. 3b), which shows a strong disorder-related D band at ~1343 cm1, a wide G band at ~1585 cm1 and a low G band to D band intensity ratio of ~1.0 [39]. The Raman spectrum of the QD samples obtained from s-MoS2-NMP shows the characteristic peaks of both CDs and MoS2 QDs [42,44,45], further confirming the simultaneous production of CDs and MoS2 QDs. Moreover, the CDs are defective but the MoS2 QDs have better crystallinity based on the obvious XRD diffraction peaks of MoS2 (blue lines in Fig. 3a) compared with CDs (red lines in Fig. 3a). Note that the MoS2 QDs have a weak interlayer interaction and weak coupling between electronic transitions and phonons [45] which is why, compared to bulk MoS2, the XRD diffraction peaks are negligible (Fig. S4a), and both the A1g (410.8 cm1) and E12g (383.6 cm1) peaks, respectively corresponding to the vertical vibration of S atoms and the horizontal vibration of SeMoeS are red-shifted, and the distance between the two Raman peaks is decreased (Fig. S4b) [42,45]. For the FTIR spectrum of CDs obtained from 4 h s-NMP (red line in Fig. 3c), the peaks centered at 1718 cm1, 1590 cm1, and 1288 cm1 are respectively ascribed to C]O, C]C, and CeN vibrations [39,46], and those at 3400 cm1 and 1270 cm1 are ascribed to OeH vibrations [39]. These results indicate the presence of oxygen- and nitrogen-containing functional groups on the surface and edges of the CDs, which are also revealed in their XPS spectrum (red line in Fig. 3d). As shown in Fig. S5a, the high resolution XPS C1s spectrum of the CDs has three characteristic peaks corresponding to O]CeO (287.8 eV), CeN/CeO (285.8 eV), and C] C bonds (284.6 eV) [47]. For the hybrid CDs/MoS2 QDs, the presence of similar FTIR peaks (blue line in Fig. 3c) and a XPS C1s spectrum (blue line in Fig. 3d and Fig. S5b) similar to CDs indicates the
Fig. 3. (a) XRD pattern, (b) Raman spectra, (c) FTIR spectra, and (d) XPS spectra of CDs obtained from 4 h s-NMP and hybrid CDs/MoS2 QDs obtained from 4 h s-NMP-MoS2. (A colour version of this figure can be viewed online.)
simultaneous production of CDs containing oxygen- and nitrogencontaining functional groups. However, these CD-related peaks are not obvious, indicating a small amount of CDs in hybrid QDs. In addition, the hybrid CDs/MoS2 QDs show three additional FTIR peaks (blue line in Fig. 3c), MoeOH at 1382 cm1, SeOH at 1120 cm1, and SeH at 605 cm1 [48]. Fig. S5c shows a high resolution Mo 3d XPS spectrum. The peaks located at 226.1 eV, 229.0 eV, 232.2 eV correspond to S2 2s, Mo4þ 3d5/2, and Mo4þ 3d3/2, respectively, and another weak peak located at 235.2 eV corresponds to Mo6þ (inset in Fig. S5c). Fig. S5d shows a high resolution S 2p XPS spectrum, in which the peaks located at 161.9 eV, 162.9 eV, 169.2 eV are assigned to S2 2p3/2, S2 2p1/2, and S6þ, respectively [48]. The appearance of MoeOH and SeOH vibration peaks in the FTIR spectrum as well as small Mo6þ and S6þ peaks in the XPS spectra of hybrid CDs/MoS2 QDs indicates slight oxidation of the MoS2 QDs, which is due to the local high temperature and high pressure generated during the probe sonication process under ambient condition [48]. 3.4. The optical interactions between CDs and MoS2 QDs We compared the PL properties of s-NMP (CDs) and s-NMPMoS2 (hybrid CDs/MoS2 QDs). The 1e4 h s-NMP shows only one PL peak under different excitation wavelengths (Fig. 4a, c, e and g), and the peak position showed a red-shift with an increase in excitation wavelength from 280 to 440 nm. However, the 1e4 h s-NMP-MoS2 showed two distinct PL peaks under different excitation wavelengths (Fig. 4b, d, f and h). One peak is consistent with that of CDs, and the other is assigned to MoS2 QDs. The peak intensity for the CDs is lower than that for the MoS2 QDs due to its small amount in the hybrids, and it becomes lower and lower with an increase in sonication time. Moreover, these two peaks show a red-shift with an increase in excitation wavelength from 280 to 440 nm and finally merge, which might be due to the larger red-shift of the PL peak for CDs than that for MoS2 QDs. It is worth noting that although the PL peak of CDs in s-NMPMoS2 can be detected (Fig. 4b, d, f and h), its intensity is drastically decreased compared with that in s-NMP (Fig. 4a, c, e and g). Moreover, the decrease becomes more obvious with increasing sonication time. These results suggest the existence of an optical interaction between associated CDs and MoS2 QDs. To understand this optical interaction, we used 365 nm UV light to excite the 1e4 h s-NMP and s-NMP-MoS2 and measured the corresponding PL spectra. As shown in Fig. 5a, there is no evident fluorescence for pure NMP, but strong fluorescence appeared after it was sonicated for 1 h due to the production of CDs. With increasing sonication time, the fluorescence color changed from blue-white to light yellow and the corresponding emission peaks (Fig. 5b) were redshifted. This is in good agreement with the red-shift of the ~300 nm absorption peaks of s-NMP (Fig. 1b) caused by the size increase of the carbon cores [49] and the aggregation of adjacent photoluminescence centers with increased CD concentration [41]. Additionally, because of the continuous increase in the concentration of CDs, the peak intensity of s-NMP also increases. Different from s-NMP, with an increase in sonication time the fluorescence color of s-NMP-MoS2 changes from blue-green to weak yellowgreen (Fig. 5c). The PL peak intensity of CDs in s-NMP-MoS2 (Fig. 5d) is ~10, 30, 75 and 300 times lower than that in s-NMP (Fig. 5b) for 1, 2, 3, 4 h sonication, respectively. Moreover, the fluorescence intensity of s-NMP-MoS2 is much lower than that of sNMP and gradually decreases with an increase in sonication time. For example, the fluorescence intensity of 1 h s-NMP-MoS2 is ~7.4 times lower than that of s-NMP, which is due to the increased concentration of MoS2 QDs in the solvent, which results in selfabsorption, consequently quenching of PL intensity [50,51]. In
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Fig. 4. PL spectra of (a, c, e, g) s-NMP and (b, d, f, h) s-NMP-MoS2 with sonication times of (a, b) 1 h, (c, d) 2 h, (e, f) 3 h and (g, h) 4 h, under different excitation wavelengths. (A colour version of this figure can be viewed online.)
Fig. 5. Photographs and PL spectra of (a, b) 1e4 h s-NMP and (c, d) 1e4 h s-NMP-MoS2 under 365 nm UV light. (A colour version of this figure can be viewed online.)
addition, there are great overlaps between the absorption spectra of MoS2 QDs (Fig. 2b) and the emission spectra of CDs (Fig. 4a, c, e, g). Such an overlap provides the possibility of resonance energy transfer [15] between CDs and MoS2 QDs, which can result in a
decrease in fluorescence intensity. The above results show that the CDs associated with the MoS2 QDs have an optical interaction with them, which decreases their PL strength and may influence their use in bioimaging and fluorescence. However, CDs and MoS2 QDs have different physical and chemical properties [52,53]. These hybrid QDs may have uses in different fields by taking advantage of their combined effect. For example, the CDs with oxygen- and nitrogen-containing functional groups have a high absorption ability to reactants [54], and the MoS2 QDs have a good catalytic activity [17,42], which might be beneficial to catalysis. In addition, both CDs and MoS2 QDs have been widely used in photothermal and photodynamic cancer therapy [9,19], and as the functional layers of photoelectronic devices such as organic photovoltaic solar cells and organic lightemitting diodes [6]. The functional groups on CDs may improve the dispersibility of MoS2 QDs and make these uses easier. Therefore, the associated production of CDs provides more opportunity to design the properties and uses of MoS2 QDs, which is worth further study. Finally, we investigated the optical properties of the commonly used organic solvents, including ethanol, acetone, isopropanol, hexane, DMF, dichloromethane, chlorobenzene, before and after 2 h probe sonication. As shown in Fig. 6a, similar to s-NMP, all the sonicated organic solvents show an absorption peak at around 300 nm, indicating that these organic solvents were carbonized.
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Fig. 6. Optical properties of the commonly used solvents before and after probe sonication. (a) Absorption spectra of the different solvents after sonication. (b) Ratio of PL intensity after sonication to before sonication for different solvents under 365 nm UV light. (c) Photographs of the solvents before (left) and after (right) sonication under 365 nm UV light. (A colour version of this figure can be viewed online.)
Note that there is no obvious absorption peak for sonicated deionized water (DI water), which excludes the influence of other factors, such as a residue from the probe. Due to the different carbonization degrees, they show different PL intensity ratios after to before sonication (Fig. 6b) and different fluorescence (Fig. 6c) under the 365 nm UV light. The results show that the carbonization of organic solvents is universal, therefore, the associated production of CDs should be considered when preparing 2D-QDs by probe sonication. However, the technique provides for the one pot fabrication of hybrid CDs/2D-QDs which might have wide uses in catalysis, optoelectronic devices, therapy, and so on.
Technology of China (No. 2016YFA0200101), National Science Foundation of China (Nos. 51325205, 51290273, 51572265 and 51861135201), Postdoctoral Science Foundation of China (No. 2018M630309), the Strategic Priority Research Program of Chinese Academy of Sciences (XDB30000000), Liaoning Revitalization Talents Program (No. XLYC1808013), and the Economic, Trade and Information Commission of Shenzhen Municipality for the “2017 Graphene Manufacturing Innovation Center Project” (No. 201901171523).
4. Conclusion
Supplementary data related to this article can be found at https://doi.org/10.1016/j.carbon.2019.08.067.
We found that probe sonication causes some carbonization of NMP solvents to form CDs when preparing MoS2 QDs in NMP. These two kinds of QDs show similar features in color, crystal structure, excitation wavelength dependent photoluminescence (PL), and have an overlap of their absorption spectra and coincidence of the PL peaks, which is why the associated production of CDs has not previously been recognized. The CDs associated with the MoS2 QDs have an optical interaction with them, which leads to a dramatic decrease in the fluorescence intensity. We also found that the production of CDs by probe sonication is universal for the commonly used organic solvents. These observations not only require that we take CDs into consideration when preparing 2DQDs by probe sonication in organic solvents, but also facilitate the fabrication of hybrid CDs/2D-QDs, which may promote the wide use of 2D-QDs in catalysis, optoelectronic devices, therapy, and so on. Acknowledgements This work is supported by the Ministry of Science and
Appendix A. Supplementary data
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