Large-Scale Ultrasonic Fabrication of White Fluorescent Carbon Dots

Apr 26, 2016 - dots (CDs) that can emit white fluorescence via one step ultrasound at room ... To further broaden the applications of white fluorescen...
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Large-Scale Ultrasonic Fabrication of White Fluorescent Carbon Dots Hui Dang, Li-Kai Huang, Yan Zhang, Cai-Feng Wang, and Su Chen* State Key Laboratory of Materials-Oriented Chemical Engineering and College of Chemical Engineering, Nanjing Tech University, 5 Xin Mofan Road, Nanjing 210009, People’s Republic of China ABSTRACT: We first used oligomer polyamide resin as carbon source to prepare carbon dots (CDs) that can emit white fluorescence via one step ultrasound at room temperature. We have had a further understanding in the surface morphology and chemical characteristics of the CDs by performing TEM, XPS, XRD, FT-IR, and Raman spectroscopy. It has been found that such carbon dots have good dispersion, low crystallinity, rich surface functional groups, and easy large-scale production. The quantum yield of the white fluorescent CDs was further enhanced from 3.3% to 28.3% by adding a silane coupling agent as a co-passivating agent and conducting ultrasonic treatment. We successfully prepared white-light-emitting diodes using these carbon dots as light conversion materials. To further broaden the applications of white fluorescent carbon dots, we employed the as-prepared CD solution as ink to prepare luminescent patterns, along with favorable versatile effect.

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(EDA) as a passivant (Scheme 1). The white fluorescence quantum yield (QY) of the CDs reached ∼28.3% after

INTRODUCTION There is growing interest in white-light-emitting diodes (WLEDs) because of their extensive applications in lighting field instead of traditional lighting equipment owing to the superior energy utilization, long lifetime, and lower power wastage.1 So far, most of commercial WLEDs are constructed with use of a blue InGaN chip and yellow phosphor.2,3 In recent years, much effort has been devoted to the fabrication of WLEDs by using semiconductor quantum dots (QDs) as light emitter, which show excellent performance such as wide excitation scope, low energy consumption, and rich emission spectrum.4−6 However, QDs-based WLEDs are usually easily self-quenching and have poor stability. Especially, the employed QDs are not easily large-scale-produced due to occurrence of rich heavy metal ions wastewater during the production, limiting them in the application in WLEDs industry.7,8 Photoluminescent carbon dots are promising to replace traditional nanocrystals due to the bright photoluminescence, light stability, hypotoxicity, and good biocompatibility of carbon dots.9 The emergence of CDs has presented exciting opportunities in potential applications such as bioimaging, catalysts, sensors, printing inks, and optoelectronic devices.10−16 Diverse approaches have been explored to fabricate CDs, such as plasma treatments,15 electrochemical synthesis,17 combustion/thermal oxidation,18 and hydrothermal,19 microwave,20 or ultrasonic21 preparation. Meanwhile, numerous carbon sources have been discovered such as fine carbon structures,17 chemicals,15 and waste.21 Nevertheless, there are only several examples for CDs emitting white photoluminescence or multicolor emission,22−27 which militates against their commercial applications in WLEDs. Herein, we presented a simple and inexpensive method to fabricate white fluorescent CDs (WCDs) via a facile one-step ultrasonic method, using inexpensive and easily obtained polyamide resin as the precursor, along with ethylenediamine © 2016 American Chemical Society

Scheme 1. Synthesis of WCDs from Polyamide Resin and Their Applications of WLEDs and Fluorescent Patterns

additional ultrasonic treatment with silane coupling agent as co-passivating agent. So far, only a few examples reported that the QY of the as-prepared WCDs could reach 9.0%.16 Also, contrasting with other techniques, such as plasma, hydrothermal treatment, and electrochemical synthesis, this technique is simpler and easily large-scale-produced with a high yield of 25.7%. Therefore, this finding provides an available approach to large-scale production of WCDs, might significantly improve Received: Revised: Accepted: Published: 5335

March 6, 2016 April 21, 2016 April 26, 2016 April 26, 2016 DOI: 10.1021/acs.iecr.6b00894 Ind. Eng. Chem. Res. 2016, 55, 5335−5341

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Industrial & Engineering Chemistry Research

Figure 1. (a) FT-IR spectra of the fluorescent untreated CDs and treated CDs. (b) Raman spectrum of WCDs.

Characterization. Fluorescent emission spectra of the CDs were examined with a LS-55 fluorophotometer. A Hitachi H8100 electron microscope was used to detect the transmission electron microscopy (TEM) images of the CDs. UV−vis absorption spectra were examined using a Shimadzu UV-2450 spectrophotometer. FT-IR spectra were performed via an IFS 66V/S (Bruker) IR spectrometer in the range of 400−4000 cm−1. Raman spectrum was performed using a Jobin Yvon Horiba LAB-RAM Infinity using a 325 nm laser beam. Powder X-ray diffraction (XRD) pattern was examined with a BrukerAXS X-ray diffractometer. X-ray photoelectron spectra (XPS) were recorded by a Thermo Scientific ESCALAB 250 Multitechnique surface analyzer with an Al Kα X-ray monochromator. Quantum yield (QY) was calculated by utilizing a normative reference substance of quinine sulfate in H2SO4 solution. QY was calculated using the following equation:

level of resources utilization, along with reducing production cost of WLEDs. More importantly, we successfully used the WCDs to fabricate a WLED without additional phosphors and utilized WCDs as ink that can emit white fluorescence to obtain a variety of fluorescent patterns. To a certain degree, this work might further promote commercial application of WCDs in the optoelectronic devices and fluorescent patterns.

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EXPERIMENTAL METHODS Materials. Polyamide resin (low molecular weight, 650) was purchased from Xi’an Keda Adhesive Co., Ltd., China. Ethylenediamine (EDA), ethanol, polyvinylpyrrolidone (PVP), and KH570 (CH2C(CH3)COOC3H6Si(OCH3)3, silane coupling agent) were purchased from Sinopharm Chemical Reagent Co., Ltd. Preparation of Carbon Dots (CDs). The CDs were synthesized using the following steps: a mixture of 3 g of polyamide resin and 3.5 g of EDA was put into 200 mL of deionized water, and ultrasonic treatment was performed for 3 h to obtain the WCDs. Further, to improve the quantum yield of WCDs, a silane coupling agent KH570 (2 mL) as copassivating agent was added to the above reactant system and underwent the same before-mentioned treatment to get the treated CDs (T-CDs). Preparation of White-Light-Emitting Diodes (WLEDs). To prepare a WLED, first, the UV chip (emission wavelength at 420 nm) was fixed at a LED pedestal. And then we blended the heated curable silicone with as-prepared WCDs in a weight ratio of 3:2. The mixture was added into toluene and mixed uniformly. In order to dislodge the solvent of toluene and air bubbles in the solution, we put it in a vacuous vessel for some minutes. Finally, the mixture was smeared on the chip. The LED was placed in an electric stove at 140 °C for 0.5 h. The relevant photoelectric characterization was conducted via a ZWL-600 instrument with integral sphere. Fluorescent Patterns from WCDs via Silk-Screen Printing and Inkjet Printing. We chose polyvinylpyrrolidone (PVP) as a favorable matrix. 3 mL as-prepared WCDs solutions and 20 g PVP solutions of 2% weight percent in water were mixed uniformly. Then, the WCDs/PVP ink was brushed on penetrating silk screen and attached on the paper using a scratch board. Finally, multifarious fluorescent patterns were obtained.14 Also, a photoluminescent hybrid ink that can be used to print has been fabricated via mixing the WCDs solutions and glycol at an appropriate proportion. The WCDs/ glycol ink was then injected into a printer, and finally, we obtained a series of legible patterns emitting white photoluminescence on the paper.14

Q = QR

I AR n2 IR A nR 2

(1)

In the equation, Q represents quantum yield, A expresses the measured emission intensity, I shows luminous density, n is index of refraction, and R represents normative reference substance.

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RESULTS AND DISCUSSION The synthesis of WCDs started from polyamide resin through a simple one-step ultrasonic method with ethylenediamine and silane coupling agent. The correlative properties of WCDs were investigated. The chemical structure of CDs is characterized using FT-IR spectroscopy. As shown in the FT-IR spectra of untreated CDs (Figure 1a), 3423 cm−1 arises from the −OH group and 1640 cm−1 is assigned to the −NH2 stretching, which can also be observed on the treated CDs. Besides these, there are some obvious peaks of treated CDs which are centered at about 2850, 1610, 1080, and 1390 cm−1 originating from the −CH2 vibration, CO stretching, Si−O vibration, and symmetric carboxylate stretching, respectively, demonstrating that there is carboxylic functional group.16 It is likely to come from the external deactivation of as-prepared CDs by silane coupling agent. The higher external deactivation degree may result in increasing QY of the CDs. The Raman spectrum of the as-prepared CDs (Figure 1b) displays distinct D ribbon and G ribbon centered at 1360 and 1600 cm−1, respectively, implying main sp2 carbons and sp3 blended carbons in CDs. Hence, we can basically conclude there are primarily sp2 plumbaginous and sp3 defective carbons in the as-prepared CDs. 5336

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elements. The peak of C 1s for raw material can be divided into three different peaks at 284.3, 285.5, and 287.4 eV, respectively, which are ascribed to CC, C−C/C−H/C−N, and CO bonds (Figure 3b). Similarly, as shown in Figure 3d, the peak of C 1s for WCDs can also be divided into three different peaks at 284.2, 285.3, and 287.3 eV, corresponding to above-mentioned bonds, respectively. The XPS intensity at 285.4 and 287.3 eV gradually increases from sample A (raw material) to sample B (WCDs), implying a corresponding increase in the content of carbonyl group, −CH2 and C−N in the CDs (Table 1), which

Here, in order to further confirm the formation of carbon dots via ultrasonic processing, we have performed more experiments. We characterized the raw material and treated CDs with FT-IR spectra. As shown in Figure 2, compared with

Table 1. XPS Data Analyses of the C 1s Spectra of raw material (A) and WCDs (B) sample

CC

C−C/C-H/C−N

CO

A B

68.78% 62.53%

26.23% 29.71%

4.99% 7.76%

is in accordance with the FT-IR results, implying the larger degree of passivation and carbonization of WCDs compared to raw material. These results reasonably demonstrate the formation of carbon dots from the carbon source of the oligomer polyamide resin. Figure 4a shows that the as-prepared WCDs in the transmission electron microscope (TEM) image are well-

Figure 2. FT-IR spectra of the raw material and treated CDs.

raw material, we can easily observe the strong peaks at about 2850, 1610, 1080, and 1390 cm−1 originating from the −CH2 vibration, CO stretching, Si−O vibration, and symmetric carboxylate stretching, respectively, on the treated CDs, demonstrating the existence of the carboxylate groups.16 This may derive from the passivation of WCDs by the co-passivating agent. Therefore, this implies the carbon source of oligomer polyamide resin formed brightly fluorescent carbon dots in the presence of co-passivating agent via ultrasonic processing. We have characterized the raw material and WCDs with Xray photoelectron spectra (XPS). The bond energy and elementary components in raw material and WCDs were investigated (Figure 3). Figure 3a is the XPS spectrum of raw

Figure 4. (a) TEM, (b) particle size histogram, and (c) XRD pattern of WCDs.

dispersed and well-monodispersed, along with particle size of 2−4 nm. It is in good agreement with the result of the particle size histogram with about ∼3 nm in Figure 4b. The as-prepared CDs have been characterized with XRD to confirm their crystalline degree. As shown in Figure 4c, we can see an obvious peak at about 24° from the XRD pattern, which is basically in accordance with a number of references about CDs, further demonstrating the plumbaginous structure of the WCDs. Figure 5a shows the UV−vis adsorption (black line) spectrum and photoluminescent (red line) spectrum of the CDs. An intense absorption characteristic peak at ∼375 nm can be seen in Figure 5a. We can clearly see the photoluminescence (PL) peak around at 515 nm (λex = 365 nm). As demonstrated in Figure 5b, the location of PL peak of the CDs barely changes when the excitated wavelength changes, with PL peaks red-

Figure 3. (a) XPS spectrum of the raw material. (b) High resolution XPS spectrum of C 1s of raw material. (c) XPS spectrum of WCDs. (d) High resolution XPS spectrum of C 1s of WCDs.

material, and Figure 3b is the high-resolution XPS spectra of C 1s of raw material. Figure 3c and Figure 3d are the corresponding spectra of WCDs. From Figure 3a, we can see that XPS spectrum of raw material displays C 1s, N 1s, O 1s, and Si 2p peaks at 285.0, 400.0, 531.0, and 100.1 eV, respectively, which is similar to WCDs in Figure 3c. This indicates that the raw material and WCDs include the same 5337

DOI: 10.1021/acs.iecr.6b00894 Ind. Eng. Chem. Res. 2016, 55, 5335−5341

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Figure 5. (a) Photoluminescent emission spectrum (λex = 365 nm) and UV−vis absorption spectrum of the as-prepared CDs. (b) PL emission spectra at incremental excitation wavelengths from 340 to 450 nm.

Figure 6. Time-resolved fluorescence decay curves for untreated CDs (a) and treated CDs (b) (λex = 405 nm).

fluorescent perssad. This might result from the external deactivation of silane coupling agent and the higher carbonization of T-CDs. Meanwhile, to investigate the role of EDA during the preparation of WCDs, we characterized the WCDs with PL spectra in the presence of EDA and not, respectively. As shown in Figure 7, we can see the fluorescence intensity of WCDs with

shifted from 500 to 530 nm via increasing excitation wavelength from 340 to 450 nm. With excitation at 420 nm, a PL peak centering at 510 nm with maximum intensity and a full width at half-maximum (fwhm) of about 110 nm can be observed. The as-prepared CDs exhibit excellent photoluminescent properties. Although the fluorescent mechanism is not clear at present, it may reasonably result from the nonuniformity of grain size of the as-prepared CDs and the diverse emission positions upon every nanoparticle16 and surface energy traps that are created by surface groups28,29 according to previous reports. In addition, we inferred that the formation process of WCDs is as follows. Decomposition between polyamide resin with the presence of two kinds of passivants may come into being with ultrasonic conditions. Under the specific physical environment formed by ultrasonic conditions, there could be more profound carbonization and passivation on the medium.30,31 In this case, the presence of silane coupling agent might further promote the increase of QY of the WCDs. We further investigated the typical fluorescence decay lifetime of CDs (seen in Figure 6), which was obtained by using multidimension TCSPC technique. The attenuation trace of CDs was fitted utilizing the following equation: Y (t ) = α1 exp( −t /τ1) + α2 exp(−t /τ2)

Figure 7. PL spectra of the as-prepared CDs with EDA and without EDA (λex = 365 nm).

EDA is obviously higher than these without EDA and has a smaller fwhm. Therefore, we can conclude that EDA contributes to the synthesis of WCDs as a passivant and is necessary during the fabrication of bright WCDs. And on the basis of the EDA, we can obtain WCDs with more intense fluorescence assisted with silane coupling agent. The fluorescence stability of as-prepared CDs was thoroughly investigated. Figure 8a is the PL spectra (λex = 420 nm) of the CDs under diverse pH surroundings, and Figure 8b shows the graph of fluorescence intensity under diverse pH surroundings (λex = 420 nm). From Figure 8a, we can observe that fluorescence intensities of the CDs only have a slight decrease along with a slight blue shift of PL peak with the decrease of pH on the whole. Accordingly, we can easily

(2)

where α1 and α2 are the sectional effect on the attenuation lifetime with τ1 and τ2.14 The mean lifetime is calculated via the following equation: τ̅ =

α1τ12 + α2τ2 2 α1τ1 + α2τ2

(3)

The calculated mean lifetimes for untreated CDs and treated CDs (T-CDs) are 1.92 and 3.23 ns. Compared with CDs, this profound increase of fluorescent lifetime of T-CDs indicates there are distinct decrease of radiationless pitfall and increase of 5338

DOI: 10.1021/acs.iecr.6b00894 Ind. Eng. Chem. Res. 2016, 55, 5335−5341

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Figure 8. (a) PL spectra (λex = 420 nm) of the CDs under diverse pH surroundings. (b) Graph of fluorescence intensity under diverse pH surroundings (λex = 420 nm).

Figure 11. (a) Photoluminescence spectra of initial CDs and CDs stored for 3 months (λex = 365 nm). (b) CIE color coordinate graph of the WLED based on WCDs. Illustration is the diagram of a WLED.

observe the variational tendency of fluorescence intensity in Figure 8b. These results indicate the good fluorescence stability to pH. Figure 9a and Figure 9b show PL spectra (λex = 420 nm)

quenching in this case after storage for 3 months at room temperature, presenting favorable photostability for as-prepared CDs. The photostability of phosphors is of key importance to fabricate LED with good performance. Herein, we tried to fabricate a WLED by using the as-prepared WCDs. As seen in Figure 11b, the device provides a bright white illumination. The as-prepared WLED displays the CIE color coordinate of (0.30, 0.28) (Figure 11b), which is very adjacent to the color coordinate of (0.33, 0.33) of the pure white fluorescence. This result demonstrates success in white LEDs’ fabrication based on as-prepared white emission of CDs. The first example for fabrication WLED with use of CDs is reported by Chen et al.,32 which employed the CDs from monodispersed polystyrene colloid to produce WLED. The precursor is expensive, making that kind of CD difficult to scale up. On the contrary, in our case, the as-prepared WCDs possess advantages of easy largescale production, lower cost, and lower toxicity over conventional YAG:Ce and semiconductor QDs. We further applied as-prepared WCDs as the “ink” to inkjet printing and silk-screen printing (seen in Figure 12). Figure 12a and Figure 12b are the fluorescent patterns made by silk-screen printing, and the fluorescent patterns in Figure 12c and Figure 12d are made from inkjet printing. Compared with silk-screen printing, fluorescent inkjet printing patterns are well-defined and more legible. This indicates the as-prepared WCDs might

Figure 9. (a) PL spectra (λex = 420 nm) of the CDs under diverse temperature surroundings. (b) Graph of fluorescence intensity under diverse temperature surroundings (λex = 420 nm).

of the CDs under diverse temperature surroundings and the graph of fluorescence intensity under diverse temperature surroundings (λex = 420 nm). As shown in Figure 9a and Figure 9b, we can see that there is a little of variation of fluorescence intensity when temperature increases from 30 to 90 °C with a slight blue shift of PL peak, which implies the favorable fluorescence stability to temperature. Figure 10a and Figure 10b

Figure 10. (a) PL spectra (λex = 420 nm) of the CDs under diverse storage times. (b) Graph of fluorescence intensity under diverse storage times (λex = 420 nm).

show PL spectra (λex = 420 nm) of the CDs under different storage times and the graph of fluorescence intensity under different storage times (λex = 420 nm). Similar to pH and temperature, with the increasing storage times, there is nearly no decrease of fluorescence intensity and the shift of PL peak. From above-mentioned discussion, we can conclude that the obtained CDs have favorable fluorescence stability to pH, temperature, and storage times. We also have seriously investigated fluorescence stability of the CDs. As shown in Figure 11a, there exists no fluorescence

Figure 12. A series of fluorescent patterns prepared by silk-screen printing (a, b) and inkjet printing (c, d). 5339

DOI: 10.1021/acs.iecr.6b00894 Ind. Eng. Chem. Res. 2016, 55, 5335−5341

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Industrial & Engineering Chemistry Research be good candidates as great potential ink for fluorescent patterns and printing industry via inkjet printing.

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CONCLUSIONS To sum up, we reported a facile and inexpensive route for largescale fabrication of fluorescent WCDs using polyamide resin by one-step ultrasonic method for the first time. The CDs showed bright white fluorescence with QY of ∼28.3% after additional ultrasonic treatment with KH570 as co-passivating agent. Moreover, the as-prepared WCDs exhibited excellent water solubility and highly stable fluorescence. In consideration of the excellent performance, we successfully utilized white-luminescent CDs as a white phosphor to construct WLEDs and photoluminescent ink to obtain diverse patterns by silk-screen printing and inkjet printing. These results allow us to conclude that the polyamide can be exploited as a new kind of carbon source to synthesize CDs with extensive applications.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 86-25-83172258. Fax: 86-25-83172258. E-mail: chensu@ njtech.edu.cn. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Natural Science Foundation of China (Grant 21474052) and Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).

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REFERENCES

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