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Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light
Advanced Powder Technology xxx (2017) xxx–xxx
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Original Research Paper
Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light An-Cheng Sun Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 12 July 2017 Received in revised form 14 November 2017 Accepted 15 December 2017 Available online xxxx Keywords: Carbon nanodots Fe3O4 nanoparticles Hybrid material Photocatalyst Visible light
a b s t r a c t In this study, magnetic carbon nanodots (C-dots) were synthesized by connecting C-dots and magnetic Fe3O4 nanoparticles (so called magnetic C-dots) in seeking to understand the photocatalytic activity under visible light and the recyclable ability in wastewater treatment. All of the samples were synthesized by bottom-up procedure at reaction temperatures (Tr) of 140 °C and 180 °C with different reaction times (tr = 0–18 h). The results indicated that the C-dots gradually attached to Fe3O4 nanoparticles with the increase of tr at Tr = 140 °C, but suddenly approached saturation adsorption on Fe3O4 particles at Tr = 180 °C. Microstructural images confirmed that magnetic Fe3O4 nanoparticles were surrounded by C-dots 5–10 nm in size. Optical properties illuminated that magnetic C-dots presented a red-shifted emission at k = 300–450 nm attesting to their photocatalytic ability in visible light. In this study, a higher extent of degradation of the dye was noted in a larger amount of C-dots on a Fe3O4 nanoparticle surface. Methylene blue (MB) concentration can be decreased by 83% within 30-min visible light irradiation. A recyclability test evidenced that the magnetic C-dots can further photodegrade large MO concentrations by at least 10-fold or more. Therefore, magnetic C-dots exhibit good degradation ability for MB under visible light, and could be easily recycled by applying a magnetic field after photodegradation, as shown in this study. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction A life of convenience entails a great deal of pollution that can harm our natural environment, as we have witnessed from the 18th century to the present. Massive industrial wastewater has been drained into rivers, thereby infiltrating and contaminating the groundwater system. Industrial wastewater contains organic compounds and metal ions from manufacturing, such as printing, papermaking, dyeing, and the food industry. Industrial wastewater may contain organic pollutants which are colored, toxic, carcinogenic, and teratogenic; if injected into nature without proper treatment, plants, aquatic organisms, and humans will be harmed [1,2]. In addition, these colored substances are hard to remove naturally as the concentration of dye in the effluents is less than 1 ppm in water [3,4]. Adsorption and a technique to head off pollution are usually adopted to treat wastewater; however, only photocatalytic degradation can transfer organic wastewater into CO2 and H2O without any intermediate [5,6]; thus, photocatalytic degradation is a promising green material in environmental technology. It is
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well-known that TiO2 is a photocatalyst because it is clean and has low toxicity and good degradation ability [5]. The band gap of TiO2 is about 3.2 eV, implying that only high energy light, e.g. ultraviolet light, can trigger its photocatalytic response [6]. However, high energy light (k < 400 nm) is obtained in only 5% in the sunlight; the other 95% is visible light and heat [7]. A proper photocatalyst material should work in natural surroundings. Carbon nanodots (C-dots), part of the carbon family, were discovered in 2004 [8]. C-dots act as an alternative photocatalyst whose photocatalytic response can be triggered by sunlight. Cdots possess not only low toxicity and a unique optical property, but also a large range for absorption in visible light [9]. According to the quantum size effect, small size materials exhibit more effective conversion ability. The average size of C-dots is smaller than 10 nm; hence, C-dots could contribute more effective conversion ability, becoming a potential material to replace the traditional semiconductor photocatalyst. However, it is difficult to remove C-dots from the aqueous phase after putting them in wastewater and finishing the degradation due to the C-dots’ lack of polarity; hence, the result is secondary pollution [10]. In order to separate these nanoscale C-dots from wastewater, a costly recyclable procedure is needed due to the C-dots’ small particle size. Connecting
https://doi.org/10.1016/j.apt.2017.12.013 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013
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magnetic particles is one promising solution; it is an easy and inexpensive way to replace traditional separation [11]. Nanomagnetite (Fe3O4) particles show low toxicity, bio-compatibility [12], high magnetization, and superparamagnetism [13–16]. The mobility of magnetite nanoparticles could be easily controlled by applying a magnetic field. If C-dots can attach to the surface of magnetic Fe3O4 nanoparticles, the C-dots would be easily trapped, removed, and recycled by the applied magnetic field after finishing photocatalytic degradation. Hence, in this study, C-dots were fabricated on a magnetic Fe3O4 nanoparticle as a new type of photocatalytic material (so-called magnetic C-dots). Our results presented that magnetic C-dots exhibit outstanding photocatalytic activity. More than 80% degradation of dye in the solution was confirmed following 30 min of visible light irradiation. In brief, the magnetic C-dots could be easily trapped, restored, and recycled using an applied magnetic field after photodegradation. 2. Experimental methods 2.1. Materials Glucose (C6H12O6), the precursor of C-dots, was supplied by Riedel-de Haën (Morristown, NJ, USA). Sodium hydroxide (NaOH) and acetic acid (CH3COOH) were obtained from the Macron Fine Chemicals Co. (Center Valley, PA, USA) and the Sigma-Aldrich Co. (St. Louis, MO, USA), respectively. Commercially-available magnetic Fe3O4 nanoparticles, with a size of 20–40 nm, were purchased from Nanostructured and Amorphous Materials Inc. (Houston, TX, USA). For the photocatalytic degradation tests, methylene blue (MB), one of the azo dyes which is usually used in industry [17], was purchased from Acros Organics (Geel, Belgium). The chemical formula of MB is C16H18N3ClS.
h, where tr = 0 means the solution remained in a mixture state before heating. During the nucleation process (Fig. 1(b)), C-dots precipitated at the surface of Fe3O4 particles initially in an autoclave. Continuously extending the tr, the size of the C-dots grew to about 10 nm, called the growth process in Fig. 1(c). After the reaction, the solution was cooled naturally to room temperature. The produced powders were then separated from the solution using Nd-Fe-B magnets and washed with DI water and ethanol three times, respectively. The washed powders were dried at 70 °C for 12 h in a vacuum oven prior to storage.
2.3. Characteristics of magnetic C-dots In this study, magnetic hysteresis loops of magnetic C-dots were measured using a vibrating sample magnetometer, VSM (DMS Model 1660, ADE Technologies Inc., MA, USA) in the applied field range of ±10 kOe. Crystal structures of magnetic C-dots were analyzed by a D2 PHASER X-ray diffractometer, XRD (Bruker, Germany) with a scanning region of 2h from 10° to 70°. Functional groups were measured by Fourier transform infrared, FTIR (PerkinElmer Spectrum 100, PerkinElmer, Massachusetts, USA). Photoluminescence properties used photoluminescence (PL) spectroscopy (OBB Quattro II, Optical Building Blocks, New Jersey, USA). The surface morphology of magnetic C-dots prepared under various conditions was determined by a JEOL scanning electron microscope, SEM (JSM-7800F Prime, Tokyo, Japan). Microstructures and nanobeam compositions of the samples were observed by a JEOL transmission electron microscope, TEM (JSM-2010, Tokyo, Japan) with EDS analysis. The accelerating voltage during the TEM operation was 200 keV.
2.2. Synthesis of magnetic C-dots
2.4. Photocatalytic degradation experiments
Magnetic C-dots were fabricated by connecting the C-dots and magnetic Fe3O4 nanoparticles by a bottom-up procedure. Fig. 1 describes the synthetics process. First of all, glucose (4 g) was added to acetic acid (40 ml), and the solution was stirred for 30 min. Following this, commercial magnetic Fe3O4 nanoparticles (0.4 g) were added into the solution. After stirring the solution for 30 min with a magnetic stirring apparatus (the mixture process in Fig. 1(a)), the solution was transferred into a 50 ml Teflon-lined stainless autoclave whose reaction temperature (Tr) was kept at 140 °C and 180 °C. The reaction time (tr) was varied from 0 to 18
Photocatalytic degradation of the industrial dye, methylene blue (MB), was investigated by dispersing 50 mg of photocatalyst in 25 ml of a MB dye solution (2 10 3 M) and 25 ml of NaOH solution (0.01 M). The hybrid solution was stirred for 60 min under dark conditions for physical adsorption. A xenon lamp (400 W) was the source of visible light providing a cutoff filter (k > 420 nm). The concentration of MB was detected using a UV–VIS spectrum with kmax = 663 nm. After visible light irradiation for 30 min, the photodegraded material was removed from the MB hybrid solution using Nd-Fe-B magnets.
Fig. 1. The synthetic process of magnetic C-dots.
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013
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3. Results and discussion 3.1. Characterization of C-dots/Fe3O4 nanocomposites Fig. 2(a) and (b) show hysteresis loops of magnetic C-dots synthesized with different reaction times (tr) at Tr = 140 °C and 180 °C, respectively. The amplitude of magnetization was estimated for whole samples, including C-dots and Fe3O4 nanoparticles. As seen in Fig. 2(a), the magnetization curves of all magnetic C-dots show no hysteresis; therefore, it is suggested that they have typical superparamagnetic behavior [13–16]. When Tr increased to 180 °C, all the magnetic C-dots also presented superparamagnetic behavior, but the curves achieved superposition when tr = 4–18, as shown in Fig. 2(b), implying that all samples have similar amounts of C-dots. The inset shows a profile of saturation magnetization (Ms) versus reaction time. The Ms at tr = 0 was 57 emu/g and it decreased with tr. When Tr = 140 °C, Ms dropped to 43, 25, 17, 11, and 4.7 emu/g after reacting for 4, 8, 10, 12, and 18 h, respectively. The weight fraction of C-dots in the whole samples could be calculated to be 24.6%, 56.1%, 70.2%, 80.7%, and 91.8%. A linear drop appeared from tr = 0–12, indicating that the C-dots were simply attached to Fe3O4 nanoparticles. There was less chemical reaction between C-dots and Fe3O4 nanoparticles; thus, increasing the amount of nonmagnetic C-dots in magnetic C-dots just caused a simple dilution in Ms. The nonlinear relationship of Ms at tr > 12 represented the saturated attachable C-dots on Fe3O4 nanoparticles. When Tr = 180 °C, Ms was drastically reduced to about 4 emu/g at tr = 4; it then remained constant as tr = 8–18. Apparently, entering more energy during the synthesis process is helpful in producing C-dots on Fe3O4 nanoparticles. The saturation amount of C-dots at the surface of Fe3O4 powders was about 91.8% of the total weight of magnetic C-dots. Fig. 3 shows the crystal structures of magnetic C-dots synthesized at Tr = 140 and 180 °C for tr = 0–18. The patterns identified the crystal structures of all synthesized magnetic C-dots in both Fig. 3(a) and (b). The spectrum of all samples suggested that the magnetic C-dots were cubic Fe3O4 crystal structures since peaks (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) were observed at 2h values of 18.3°, 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, and 62.5°, respectively, which agrees with the JCPDS No. 821533. Both figures presented a broad peak at 2h around 5–29°, which could be attributed to carbon reflection because it was close to graphite (0 0 2). The shift could be attributed to residual stress in the small sized C-dots [18]. However, a broad peak appeared
Fig. 3. Crystal structures of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C.
at longer reaction times (tr = 18) at Tr = 140 °C, as shown in Fig. 3 (a) and short reaction times (tr = 4) at Tr = 180 °C, as shown in Fig. 3(b), in agreement with the results in Fig. 2. Greater amounts of C-dots were produced on the Fe3O4 powders with higher
Fig. 2. Magnetic properties of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C. The inset shows a profile of saturation magnetization (Ms) versus tr.
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013
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thermal energy. A large amount of C-dots was attached to Fe3O4 powders; hence, a carbon reflection was shown at tr = 4 at 180 °C. Fig. 4 shows the FTIR spectrum results of magnetic C-dots synthesized at Tr = 140 and 180 °C for tr = 0–18. FeAO bonds were observed at a position of 560 cm 1 evidencing that all the samples had Fe3O4 powders, as shown in Fig. 4(a) and (b). The absorption positions located at 3440, 2930, 1700, 1620, 1200, and 1300 cm 1 could be indexed as OAH, CAH, C@O, C@C, and CAOAC bonds, respectively. Before reaction (tr = 0), these signals came from the solution during the mixing process. Because no C-dot was produced on the Fe3O4 nanoparticles at tr = 0, a high intensity FeAO bond was obtained. When tr increased to 4 h and Tr = 140 °C, C-dots were attached at the surface of the Fe3O4 nanoparticles. The intensity of the C@C bond was greatly enhanced, as shown in Fig. 4 (a). The clear FeAO bond was owing to insufficient C-dots on the Fe3O4 nanoparticles. Further extension of tr largely reduced the intensity of the FeAO bond, suggesting that large amounts of Cdots were adsorbed on the Fe3O4 nanoparticles, hence, resisting the signal of the FeAO bond. However, opposite results were found at Tr = 180 °C. In Fig. 4(b), a clear C@C bond appeared as tr = 4 while the intensity of the FeAO bond decayed more. The intensity of the FeAO bond eventually hidden in the background at tr = 18, indicating that too many C-dots were attached to the Fe3O4 nanoparticles. TEM bright field images of magnetic C-dots synthesized at different tr and Tr values are presented in Fig. 5. In Fig. 5(a), we see that when tr = 0 the morphology of Fe3O4 nanoparticles was spherical and the particle size ranged from 10 to 20 nm. Fig. 5(b) shows mixture of C-dots and magnetic Fe3O4 particles at Tr = 140 and tr = 4. In order to identify C-dots and Fe3O4 particles, nanobeam EDS analysis was used, confirming that the Fe3O4 nanoparticles were surrounded by carbon nanoparticles (indexed by the letter ‘‘c”) with a size of about 10 nm in nanocomposites case, evidencing that C-dots were successfully synthesized on the Fe3O4 powder surface. However, there were few C-dots due to insufficient reaction time. When tr was extended to 18 h, more C-dots were formed on Fe3O4 particles, as shown in Fig. 5(c). The size of C-dots still remained at around 10 nm. Further raising the Tr to 180 °C injected more energy during the reaction process; therefore, numerous Cdots were produced on Fe3O4 particles in a short time, as shown in Fig. 5(d). Nevertheless, the particle size of C-dots was smaller than those in the above figures. It is possible that the higher temperature synthesis process sped up the formation of C-dots on Fe3O4 particles, thus resulting in smaller C-dots. The photoluminescence (PL) properties of magnetic C-dots synthesized with tr = 0 and 4 at Tr = 140 and 180 °C are presented in
Fig. 6. Usually, the excitation wavelength (k) generates an electron-hole (e /h+) pair on the photocatalyst. The recombination of the photogenerated e /h+ pairs would contribute luminescence from localized surface states. Fig. 6(a) shows that when the excitation wavelength (k) ranged from 300 to 400 nm, Fe3O4 nanoparticles emitted a wavelength energy ranging from 400 to 500 nm, which was in agreement with previous researches [19,20]. Excitation wavelengths of k = 450 nm generated lower energy wavelengths (about 550 nm) on Fe3O4 nanoparticles, but the intensities of the emitted wavelengths declined gradually with the increase in excitation k, with low intensity emitted wavelengths eventually occurring at k = 450 nm. The spectra showed small peaks at 550 nm with k = 450 nm, which are attributed to less luminescence from localized surface states due to the few recombinations of photogenerated e /h+ pairs. Therefore, there is less interaction between Fe3O4 nanoparticles and visible light before the connection of C-dots and Fe3O4 powders. When the Cdots were attached onto the Fe3O4 nanoparticles, red-shifted emission was found at k = 300–450 nm, as shown in Fig. 6(b) and (c). The intensity of the emitted wavelength at k = 300 nm was also smaller than that with other k, indicating more generation and recombination of photogenerated e /h+ pairs happening at low energy excitation wavelengths. Thus, magnetic C-dots could conduct photocatalysis under visible light. After comparing Fig. 6 (b) and (c), we find that the intensities of emitted wavelengths at k = 300 and 350 nm in Fig. 6(c) are lower than those in Fig. 6(b), owing to high amounts of C-dots being adsorbed on Fe3O4 nanoparticles at Tr = 180. Higher intensities of emitted wavelength at k = 450, shown in Fig. 6(c), also suggest that more red-shifted emission and high amounts of recombinations of photogenerated e /h+ pairs happened on numerous C-dots. 3.2. Photocatalytic degradation experiments Fig. 7(a) shows the photodegradation activities of magnetic Cdots (tr = 0, 4, and 18 h at 140 °C) with different exposure times (te) in visible light. When tr = 0, little concentration of MB was degraded, indicating less interaction between the magnetic Cdots and visible light. These results have also been mentioned in previous studies [21,22]. The photocatalytic activity of pure Fe3O4 nanoparticles was weak under visible light [21,22] because few e /h+ pairs were generated (see Fig. 6(a)). Increasing tr triggers photocatalytic activities under visible light irradiation. The degradation abilities of magnetic C-dots with tr = 4, 12, and 18 h were 4, 30, and 83% at a 30 min exposure, respectively. High amounts
Fig. 4. FTIR spectrum of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C.
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Fig. 5. TEM bright field images of magnetic C-dots synthesized at different tr and Tr: (a) commercial Fe3O4 nanoparticles with purchased specifications of 20–40 nm (small Fe3O4), (b) C-dots/small Fe3O4 at Tr = 140 and tr = 4, (c) C-dots/small Fe3O4 at Tr = 140 and tr = 18, (d) C-dots/small Fe3O4 at Tr = 180 and tr = 4, (e) commercial Fe3O4 nanoparticles with purchased specifications of 100–150 nm (large Fe3O4), and (f) C-dots/large Fe3O4 at Tr = 140 and tr = 4. The inset in Fig. 5(e) shows the SEM image of large Fe3O4 particles.
of C-dots on Fe3O4 nanoparticles can massively reduce the concentration of MB. For the case of tr = 18, the concentration of MB can be decreased by 60% at te = 15 min. Finally, MB concentrations decreased to about 17% as te was extended to 30 min. Fig. 7 (b) and (c) show pictures of the degradation abilities of magnetic C-dots (tr = 18 at 140 °C) at te = 0 and 30 min, respectively. Before exposure (te = 0), the deep blue solution indicated the existence of a high concentration of MB. High degradation of MB was observed at te = 30. The light blue bottle indicated lower concentrations of MB in the solution. A Nd-Fe-B magnet was adopted to recycle and restore the magnetic C-dots. After degradation, the magnetic C-dots (indicated by the yellow arrow in Fig. 7(c)) could be collected using a Nd-Fe-B magnet which was placed outside the bottle to provide an applied magnetic field. The magnetic Fe3O4 nanoparticles could be attracted by the applied field. When the C-dots and Fe3O4 nanoparticles were conjugated together, it was clear that the magnetic C-dots were also well controlled by the Nd-Fe-B magnet, evidencing a recyclable photocatalyst application. The recyclability of magnetic C-dots (tr = 18 and Tr = 140) was tested 10 times, and the results were plotted in Fig. 7(d). Before the recyclability testing, in order to reduce the loss of the magnetic catalyst during the washing process, the as-synthesized catalyst was separated and collected from the aqueous phase using a NdFeB magnet, and then rinsed with a large amount of water. The assynthesized magnetic catalyst was then dried and weighed. We did the separation, collection, rinsing, drying, and weighing again
and again, until the mass of collected as-synthesized catalyst remained constant. The as-synthesized magnetically catalyst was then dispersed into a MB solution for photodegradation. After 30 min of photocatalysis under visible light irradiation, the catalyst was quickly collected and separated from the aqueous phase using a NdFeB magnet, and then rinsed with a large amount of water. The recovered magnetic catalyst was dried and then re-dispersed into another new MB solution to initiate a second photodecomposing cycle. The same procedure was performed in the additional cycles. From Fig. 7(d), the degradation ability of magnetic C-dots seemed not to have been significantly reduced after 10 runs of irradiation in visible light. Therefore, our study clearly points out that the Cdots/Fe3O4 nanocomposites possessed good recyclability. The photocatalytic products were collected and analyzed by FTIR. Fig. 8 presents group functions of product after photodegradation by nanocomposites with tr = 18 at 140 °C. The spectrum shows OAH peak at 3700 cm 1 for H2O, C@O vibrations at 2366 cm 1 and 1712 cm 1 for CO2 and aldehyde, respectively [23], C@N vibration at 1560 cm 1 for MB. Therefore, MB was photodecomposed into CO2 gas under visible light irradiation. 3.3. Photocatalytic degradation mechanisms According to the above results and discussion, a photocatalytic degradation process was proposed, as shown in Fig. 9. The photocatalytic ability of magnetic C-dots was mostly attributed to C-dots
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013
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Fig. 7. (a) MB concentration changes with magnetic C-dots (synthesized at tr = 0– 18 and Tr = 140 °C) versus exposure time (te) under visible light, (b) and (c) show the pictures of MB solution photodegraded by magnetic C-dots (tr = 18) at te = 0 and 30 min, respectively. (d) MB concentration changes with 10 photodegradation tests using magnetic C-dots (tr = 18 and Tr = 140 °C) within 30 min visible light irradiation.
Fig. 6. Photoluminescence properties of magnetic C-dots synthesized at (a) tr = 0 and Tr = 140 °C, (b) tr = 4 and Tr = 140 °C and (c) tr = 4 and Tr = 180 °C.
which could excite electrons to produce e /h+ pairs under visible light irradiation. The photoelectrons in the valence band (VB) of C-dots can be effectively excited to the conduction band (CB) and generate e /h+ pairs [24], becoming an excellent electron acceptor and conductor. Although the Fe3O4 nanoparticles also generated e /h+ pairs, their efficiency could be ignored because fewer amounts of e /h+ pairs were photogenerated under visible light. However, Fe3+ in Fe3O4 can promote effective separation of e /h+ pairs due to their superior electronic conductivity [25]. The photogenerated electrons can then react with the dissolved O2 on the surface of C-dots and transform to superoxide radicals (O2 ). Meanwhile, the photogenerated holes react with the adsorption H2O molecules to produce hydroxyl radicals (OH). These radicals have great oxidative ability to decompose methylene blue (MB) to CO2 and H2O. The photocatalytic degradation processes of MB could follow the reactions of (1)-(5) [26,27]. 4. Conclusion C-dots were successfully synthesized on an Fe3O4 nanoparticle surface for fabricating magnetic C-dots. The C-dots were gradually produced on an Fe3O4 nanoparticle surface with increasing
Fig. 8. FTIR spectrum of product after photodegradation by nanocomposites with tr = 18 at 140 °C.
reaction time at 140 °C, causing the Ms of magnetic C-dots to decrease. The applied magnetic field could easily control the movement of these magnetic C-dots. A higher extent of photodegradation of the dye always accompanies larger amounts of C-dots on Fe3O4 nanoparticle surfaces. Such magnetic C-dots with a reaction time of 18 h showed a higher degradation ability of MB. The MB concentration decreased by 83% within 30 min of visible light irradiation. The recyclability test clearly pointed out that the magnetic C-dots had the potential of more than 10-fold the photocatalytic degradation. In this study, a new type of photocatalytic material,
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Fig. 9. The sketch of the photodegradation mechanisms of MB on magnetic C-dots’ surfaces through interactions in visible light irradiation.
magnetic C-dots, was synthesized, exhibiting good degradation abilities and recyclability. Acknowledgement Financial support for this work through grants from the Ministry of Science and Technology, Taiwan (No. MOST 106-2221-E155-015) is gratefully appreciated. References [1] X. Gao, Y. Zhang, Y. Dai, F. Fu, High-performance magnetic carbon materials in dye removal from aqueous solutions, J. Solid State Chem. 239 (2016) 265–273. [2] Z. Zhang, J. Kong, Novel magnetic Fe3O4@C nanoparticles as adsorbents for removal of organic dyes from aqueous solution, J. Hazard. Mater. 193 (2011) 325–329. [3] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [4] Ibrahim M. Banat, P. Nigam, D. Singh, R. Marchant, Microbial decolorization of textile-dye-containing effluents: a review, Bioresour. Technol. 58 (1996) 217– 227. [5] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 116 (2014) 515–582.
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[6] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997–3027. [7] P.S. Saud, B. Pant, A.M. Alam, Z.K. Ghouri, M. Park, H.Y. Kim, Carbon quantum dots anchored TiO2 nanofibers: effective photocatalyst for waste water treatment, Ceram. Int. 41 (2015) 11953–11959. [8] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments, J. Am. Chem. Soc. 126 (2004) 12736–12737. [9] P. Roy, P.C. Chen, A.P. Periasamy, Y.N. Chen, H.T. Chang, Photoluminescent carbon nanodots: synthesis, physicochemical properties and analytical applications, Mater. Today 18 (2015) 447–458. [10] L. Ai, Y. Zhou, J. Jiang, Removal of methylene blue from aqueous solution by montmorillonite/CoFe2O4 composite with magnetic separation performance, Desalination 266 (2011) 72–77. [11] A.H. Lu, E.L. Salabas, F. Schuth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222– 1244. [12] S.M. Hussain, K.L. Hess, J.M. Gearhart, K.T. Geiss, J.J. Schlager, In vitro toxicity of nanoparticles in BRL 3A rat liver cells, Toxicol In Vitro 19 (2005) 975–983. [13] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, Y. Li, Monodisperse magnetic singlecrystal ferrite microspheres, Angew. Chem. Int. Ed. 44 (2005) 2782–2785. [14] D.P. Yang, F. Gao, D.X. Cui, M. Yang, Microwave rapid synthesis of nanoporous Fe3O4 magnetic microspheres, Curr. Nanosci. 5 (2009) 485–488. [15] Cuiyan Li, Yajun Wei, Anti Liivat, Yihua Zhu, Jiefang Zhu, Microwavesolvothermal synthesis of Fe3O4 magnetic nanoparticles, Mater. Lett. 107 (2013) 23–26. [16] Wanquan Jiang, H.C. Yang, S.Y. Yang, H.E. Horng, J.C. Hung, Y.C. Chen, Chin-Yih Hong, Preparation and properties of superparamagnetic nanoparticles with narrow size distribution and biocompatible, J. Magn. Magn. Mater. 283 (2004) 210–214. [17] M. Chivukula, J. Spadaro, V. Renganathan, Lignin peroxidase-catalyzed oxidation of sulfonated azo dyes generates novel sulfophenyl hydroperoxides, Biochemistry 34 (1995) 7765–7772. [18] H. Dong, M. Roming, C. Feldmann, Unexpected fluorescence of polyols and PEGylated nanoparticles derived from carbon dot formation, Part. Part. Syst. Charact. 32 (2015) 467–475. [19] S.K. Jana, S. Majumder, B. Satpati, S.K. Mishra, R.K. Srivastava, S. Banerjee, Enhancement of photoluminescence emission andanomalous photoconductivity properties of Fe3O4@SiO2 core–shell microspheres, RSC Adv. 5 (2015) 37729–37736. [20] J. Jayabharathi, A. Prabhakaran, C. Karunakaran, V. Thanikachalam, M. Sundharesan, Structural, optical and photoconductivity characteristics of pristine FeO-Fe2O3 and NTPI–FeO-Fe2O3 nanocomposite: aggregation induced emission enhancement of fluorescent organic nanoprobe of thiophene appended phenanthrimidazole derivative, RSC Adv. 6 (2016) 18718–18736. [21] Guangcheng Xi, Bing Yue, Junyu Cao, Jinhua Ye, Fe3O4/WO3 Hierarchical core– shell structure: high-performance and recyclable visible-light photocatalysis, Chem. Eur. J. 17 (2011) 5145–5154. [22] Yuanjie Zhu, Jinjuan Xue, Xu. Tingting, Guangyu He, Haiqun Chen, Enhanced photocatalytic activity of magnetic core–shell Fe3O4 @ Bi2O3-RGO heterojunctions for quinolone antibiotics degradation under visible light, J. Mater. Sci: Mater. Electron. 28 (2017) 8519–8528. [23] Yu. Zhiqiang, Steven S.C. Chuang, Probing methylene blue photocatalytic degradation by adsorbed ethanol with in situ IR, J. Phys. Chem. C. 111 (2007) 13813–13820. [24] X.F. Chen, J.F. Dai, G.F. Shi, L. Li, G.Y. Wang, H. Yang, Visible light photocatalytic degradation of dyes by b-Bi2O3/graphene nanocomposites, J. Alloy. Compd. 649 (2015) 872–877. [25] Y. Li, J. Yang, S.H. Zheng, W.W. Zeng, N. Zhao, M. Shen, One-pot synthesis of 3D TiO2-reduced graphene oxide aerogels with superior adsorption capacity and enhanced visible-light photocatalytic performance, Ceram. Int. 42 (2016) 19091–19096. [26] F. Li, G. Wang, H. Li, Y. Li, S. Zhou, One-pot solvothermal synthesis of highly photoluminescent carbon nanoparticles and their photocatalytic application, Mater. Lett. 122 (2014) 352–354. [27] H.Q. Wang, Z.F. Ye, C. Liu, J.Z. Li, M.J. Zhou, Q.F. Guan, P. Lv, P.W. Huo, Y.S. Yan, Visible light driven Ag/Ag3PO4/AC photocatalyst with highly enhanced photodegradation of tetracycline antibiotics, Appl. Surf. Sci. 353 (2015) 391– 399.
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Original Research Paper
Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light An-Cheng Sun Department of Chemical Engineering and Materials Science, Yuan Ze University, Chung-Li, Taoyuan 32003, Taiwan
a r t i c l e
i n f o
Article history: Received 12 July 2017 Received in revised form 14 November 2017 Accepted 15 December 2017 Available online xxxx Keywords: Carbon nanodots Fe3O4 nanoparticles Hybrid material Photocatalyst Visible light
a b s t r a c t In this study, magnetic carbon nanodots (C-dots) were synthesized by connecting C-dots and magnetic Fe3O4 nanoparticles (so called magnetic C-dots) in seeking to understand the photocatalytic activity under visible light and the recyclable ability in wastewater treatment. All of the samples were synthesized by bottom-up procedure at reaction temperatures (Tr) of 140 °C and 180 °C with different reaction times (tr = 0–18 h). The results indicated that the C-dots gradually attached to Fe3O4 nanoparticles with the increase of tr at Tr = 140 °C, but suddenly approached saturation adsorption on Fe3O4 particles at Tr = 180 °C. Microstructural images confirmed that magnetic Fe3O4 nanoparticles were surrounded by C-dots 5–10 nm in size. Optical properties illuminated that magnetic C-dots presented a red-shifted emission at k = 300–450 nm attesting to their photocatalytic ability in visible light. In this study, a higher extent of degradation of the dye was noted in a larger amount of C-dots on a Fe3O4 nanoparticle surface. Methylene blue (MB) concentration can be decreased by 83% within 30-min visible light irradiation. A recyclability test evidenced that the magnetic C-dots can further photodegrade large MO concentrations by at least 10-fold or more. Therefore, magnetic C-dots exhibit good degradation ability for MB under visible light, and could be easily recycled by applying a magnetic field after photodegradation, as shown in this study. Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
1. Introduction A life of convenience entails a great deal of pollution that can harm our natural environment, as we have witnessed from the 18th century to the present. Massive industrial wastewater has been drained into rivers, thereby infiltrating and contaminating the groundwater system. Industrial wastewater contains organic compounds and metal ions from manufacturing, such as printing, papermaking, dyeing, and the food industry. Industrial wastewater may contain organic pollutants which are colored, toxic, carcinogenic, and teratogenic; if injected into nature without proper treatment, plants, aquatic organisms, and humans will be harmed [1,2]. In addition, these colored substances are hard to remove naturally as the concentration of dye in the effluents is less than 1 ppm in water [3,4]. Adsorption and a technique to head off pollution are usually adopted to treat wastewater; however, only photocatalytic degradation can transfer organic wastewater into CO2 and H2O without any intermediate [5,6]; thus, photocatalytic degradation is a promising green material in environmental technology. It is
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well-known that TiO2 is a photocatalyst because it is clean and has low toxicity and good degradation ability [5]. The band gap of TiO2 is about 3.2 eV, implying that only high energy light, e.g. ultraviolet light, can trigger its photocatalytic response [6]. However, high energy light (k < 400 nm) is obtained in only 5% in the sunlight; the other 95% is visible light and heat [7]. A proper photocatalyst material should work in natural surroundings. Carbon nanodots (C-dots), part of the carbon family, were discovered in 2004 [8]. C-dots act as an alternative photocatalyst whose photocatalytic response can be triggered by sunlight. Cdots possess not only low toxicity and a unique optical property, but also a large range for absorption in visible light [9]. According to the quantum size effect, small size materials exhibit more effective conversion ability. The average size of C-dots is smaller than 10 nm; hence, C-dots could contribute more effective conversion ability, becoming a potential material to replace the traditional semiconductor photocatalyst. However, it is difficult to remove C-dots from the aqueous phase after putting them in wastewater and finishing the degradation due to the C-dots’ lack of polarity; hence, the result is secondary pollution [10]. In order to separate these nanoscale C-dots from wastewater, a costly recyclable procedure is needed due to the C-dots’ small particle size. Connecting
https://doi.org/10.1016/j.apt.2017.12.013 0921-8831/Ó 2017 The Society of Powder Technology Japan. Published by Elsevier B.V. and The Society of Powder Technology Japan. All rights reserved.
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magnetic particles is one promising solution; it is an easy and inexpensive way to replace traditional separation [11]. Nanomagnetite (Fe3O4) particles show low toxicity, bio-compatibility [12], high magnetization, and superparamagnetism [13–16]. The mobility of magnetite nanoparticles could be easily controlled by applying a magnetic field. If C-dots can attach to the surface of magnetic Fe3O4 nanoparticles, the C-dots would be easily trapped, removed, and recycled by the applied magnetic field after finishing photocatalytic degradation. Hence, in this study, C-dots were fabricated on a magnetic Fe3O4 nanoparticle as a new type of photocatalytic material (so-called magnetic C-dots). Our results presented that magnetic C-dots exhibit outstanding photocatalytic activity. More than 80% degradation of dye in the solution was confirmed following 30 min of visible light irradiation. In brief, the magnetic C-dots could be easily trapped, restored, and recycled using an applied magnetic field after photodegradation. 2. Experimental methods 2.1. Materials Glucose (C6H12O6), the precursor of C-dots, was supplied by Riedel-de Haën (Morristown, NJ, USA). Sodium hydroxide (NaOH) and acetic acid (CH3COOH) were obtained from the Macron Fine Chemicals Co. (Center Valley, PA, USA) and the Sigma-Aldrich Co. (St. Louis, MO, USA), respectively. Commercially-available magnetic Fe3O4 nanoparticles, with a size of 20–40 nm, were purchased from Nanostructured and Amorphous Materials Inc. (Houston, TX, USA). For the photocatalytic degradation tests, methylene blue (MB), one of the azo dyes which is usually used in industry [17], was purchased from Acros Organics (Geel, Belgium). The chemical formula of MB is C16H18N3ClS.
h, where tr = 0 means the solution remained in a mixture state before heating. During the nucleation process (Fig. 1(b)), C-dots precipitated at the surface of Fe3O4 particles initially in an autoclave. Continuously extending the tr, the size of the C-dots grew to about 10 nm, called the growth process in Fig. 1(c). After the reaction, the solution was cooled naturally to room temperature. The produced powders were then separated from the solution using Nd-Fe-B magnets and washed with DI water and ethanol three times, respectively. The washed powders were dried at 70 °C for 12 h in a vacuum oven prior to storage.
2.3. Characteristics of magnetic C-dots In this study, magnetic hysteresis loops of magnetic C-dots were measured using a vibrating sample magnetometer, VSM (DMS Model 1660, ADE Technologies Inc., MA, USA) in the applied field range of ±10 kOe. Crystal structures of magnetic C-dots were analyzed by a D2 PHASER X-ray diffractometer, XRD (Bruker, Germany) with a scanning region of 2h from 10° to 70°. Functional groups were measured by Fourier transform infrared, FTIR (PerkinElmer Spectrum 100, PerkinElmer, Massachusetts, USA). Photoluminescence properties used photoluminescence (PL) spectroscopy (OBB Quattro II, Optical Building Blocks, New Jersey, USA). The surface morphology of magnetic C-dots prepared under various conditions was determined by a JEOL scanning electron microscope, SEM (JSM-7800F Prime, Tokyo, Japan). Microstructures and nanobeam compositions of the samples were observed by a JEOL transmission electron microscope, TEM (JSM-2010, Tokyo, Japan) with EDS analysis. The accelerating voltage during the TEM operation was 200 keV.
2.2. Synthesis of magnetic C-dots
2.4. Photocatalytic degradation experiments
Magnetic C-dots were fabricated by connecting the C-dots and magnetic Fe3O4 nanoparticles by a bottom-up procedure. Fig. 1 describes the synthetics process. First of all, glucose (4 g) was added to acetic acid (40 ml), and the solution was stirred for 30 min. Following this, commercial magnetic Fe3O4 nanoparticles (0.4 g) were added into the solution. After stirring the solution for 30 min with a magnetic stirring apparatus (the mixture process in Fig. 1(a)), the solution was transferred into a 50 ml Teflon-lined stainless autoclave whose reaction temperature (Tr) was kept at 140 °C and 180 °C. The reaction time (tr) was varied from 0 to 18
Photocatalytic degradation of the industrial dye, methylene blue (MB), was investigated by dispersing 50 mg of photocatalyst in 25 ml of a MB dye solution (2 10 3 M) and 25 ml of NaOH solution (0.01 M). The hybrid solution was stirred for 60 min under dark conditions for physical adsorption. A xenon lamp (400 W) was the source of visible light providing a cutoff filter (k > 420 nm). The concentration of MB was detected using a UV–VIS spectrum with kmax = 663 nm. After visible light irradiation for 30 min, the photodegraded material was removed from the MB hybrid solution using Nd-Fe-B magnets.
Fig. 1. The synthetic process of magnetic C-dots.
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3. Results and discussion 3.1. Characterization of C-dots/Fe3O4 nanocomposites Fig. 2(a) and (b) show hysteresis loops of magnetic C-dots synthesized with different reaction times (tr) at Tr = 140 °C and 180 °C, respectively. The amplitude of magnetization was estimated for whole samples, including C-dots and Fe3O4 nanoparticles. As seen in Fig. 2(a), the magnetization curves of all magnetic C-dots show no hysteresis; therefore, it is suggested that they have typical superparamagnetic behavior [13–16]. When Tr increased to 180 °C, all the magnetic C-dots also presented superparamagnetic behavior, but the curves achieved superposition when tr = 4–18, as shown in Fig. 2(b), implying that all samples have similar amounts of C-dots. The inset shows a profile of saturation magnetization (Ms) versus reaction time. The Ms at tr = 0 was 57 emu/g and it decreased with tr. When Tr = 140 °C, Ms dropped to 43, 25, 17, 11, and 4.7 emu/g after reacting for 4, 8, 10, 12, and 18 h, respectively. The weight fraction of C-dots in the whole samples could be calculated to be 24.6%, 56.1%, 70.2%, 80.7%, and 91.8%. A linear drop appeared from tr = 0–12, indicating that the C-dots were simply attached to Fe3O4 nanoparticles. There was less chemical reaction between C-dots and Fe3O4 nanoparticles; thus, increasing the amount of nonmagnetic C-dots in magnetic C-dots just caused a simple dilution in Ms. The nonlinear relationship of Ms at tr > 12 represented the saturated attachable C-dots on Fe3O4 nanoparticles. When Tr = 180 °C, Ms was drastically reduced to about 4 emu/g at tr = 4; it then remained constant as tr = 8–18. Apparently, entering more energy during the synthesis process is helpful in producing C-dots on Fe3O4 nanoparticles. The saturation amount of C-dots at the surface of Fe3O4 powders was about 91.8% of the total weight of magnetic C-dots. Fig. 3 shows the crystal structures of magnetic C-dots synthesized at Tr = 140 and 180 °C for tr = 0–18. The patterns identified the crystal structures of all synthesized magnetic C-dots in both Fig. 3(a) and (b). The spectrum of all samples suggested that the magnetic C-dots were cubic Fe3O4 crystal structures since peaks (1 1 1), (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1), and (4 4 0) were observed at 2h values of 18.3°, 30.1°, 35.4°, 43.1°, 53.4°, 56.9°, and 62.5°, respectively, which agrees with the JCPDS No. 821533. Both figures presented a broad peak at 2h around 5–29°, which could be attributed to carbon reflection because it was close to graphite (0 0 2). The shift could be attributed to residual stress in the small sized C-dots [18]. However, a broad peak appeared
Fig. 3. Crystal structures of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C.
at longer reaction times (tr = 18) at Tr = 140 °C, as shown in Fig. 3 (a) and short reaction times (tr = 4) at Tr = 180 °C, as shown in Fig. 3(b), in agreement with the results in Fig. 2. Greater amounts of C-dots were produced on the Fe3O4 powders with higher
Fig. 2. Magnetic properties of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C. The inset shows a profile of saturation magnetization (Ms) versus tr.
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thermal energy. A large amount of C-dots was attached to Fe3O4 powders; hence, a carbon reflection was shown at tr = 4 at 180 °C. Fig. 4 shows the FTIR spectrum results of magnetic C-dots synthesized at Tr = 140 and 180 °C for tr = 0–18. FeAO bonds were observed at a position of 560 cm 1 evidencing that all the samples had Fe3O4 powders, as shown in Fig. 4(a) and (b). The absorption positions located at 3440, 2930, 1700, 1620, 1200, and 1300 cm 1 could be indexed as OAH, CAH, C@O, C@C, and CAOAC bonds, respectively. Before reaction (tr = 0), these signals came from the solution during the mixing process. Because no C-dot was produced on the Fe3O4 nanoparticles at tr = 0, a high intensity FeAO bond was obtained. When tr increased to 4 h and Tr = 140 °C, C-dots were attached at the surface of the Fe3O4 nanoparticles. The intensity of the C@C bond was greatly enhanced, as shown in Fig. 4 (a). The clear FeAO bond was owing to insufficient C-dots on the Fe3O4 nanoparticles. Further extension of tr largely reduced the intensity of the FeAO bond, suggesting that large amounts of Cdots were adsorbed on the Fe3O4 nanoparticles, hence, resisting the signal of the FeAO bond. However, opposite results were found at Tr = 180 °C. In Fig. 4(b), a clear C@C bond appeared as tr = 4 while the intensity of the FeAO bond decayed more. The intensity of the FeAO bond eventually hidden in the background at tr = 18, indicating that too many C-dots were attached to the Fe3O4 nanoparticles. TEM bright field images of magnetic C-dots synthesized at different tr and Tr values are presented in Fig. 5. In Fig. 5(a), we see that when tr = 0 the morphology of Fe3O4 nanoparticles was spherical and the particle size ranged from 10 to 20 nm. Fig. 5(b) shows mixture of C-dots and magnetic Fe3O4 particles at Tr = 140 and tr = 4. In order to identify C-dots and Fe3O4 particles, nanobeam EDS analysis was used, confirming that the Fe3O4 nanoparticles were surrounded by carbon nanoparticles (indexed by the letter ‘‘c”) with a size of about 10 nm in nanocomposites case, evidencing that C-dots were successfully synthesized on the Fe3O4 powder surface. However, there were few C-dots due to insufficient reaction time. When tr was extended to 18 h, more C-dots were formed on Fe3O4 particles, as shown in Fig. 5(c). The size of C-dots still remained at around 10 nm. Further raising the Tr to 180 °C injected more energy during the reaction process; therefore, numerous Cdots were produced on Fe3O4 particles in a short time, as shown in Fig. 5(d). Nevertheless, the particle size of C-dots was smaller than those in the above figures. It is possible that the higher temperature synthesis process sped up the formation of C-dots on Fe3O4 particles, thus resulting in smaller C-dots. The photoluminescence (PL) properties of magnetic C-dots synthesized with tr = 0 and 4 at Tr = 140 and 180 °C are presented in
Fig. 6. Usually, the excitation wavelength (k) generates an electron-hole (e /h+) pair on the photocatalyst. The recombination of the photogenerated e /h+ pairs would contribute luminescence from localized surface states. Fig. 6(a) shows that when the excitation wavelength (k) ranged from 300 to 400 nm, Fe3O4 nanoparticles emitted a wavelength energy ranging from 400 to 500 nm, which was in agreement with previous researches [19,20]. Excitation wavelengths of k = 450 nm generated lower energy wavelengths (about 550 nm) on Fe3O4 nanoparticles, but the intensities of the emitted wavelengths declined gradually with the increase in excitation k, with low intensity emitted wavelengths eventually occurring at k = 450 nm. The spectra showed small peaks at 550 nm with k = 450 nm, which are attributed to less luminescence from localized surface states due to the few recombinations of photogenerated e /h+ pairs. Therefore, there is less interaction between Fe3O4 nanoparticles and visible light before the connection of C-dots and Fe3O4 powders. When the Cdots were attached onto the Fe3O4 nanoparticles, red-shifted emission was found at k = 300–450 nm, as shown in Fig. 6(b) and (c). The intensity of the emitted wavelength at k = 300 nm was also smaller than that with other k, indicating more generation and recombination of photogenerated e /h+ pairs happening at low energy excitation wavelengths. Thus, magnetic C-dots could conduct photocatalysis under visible light. After comparing Fig. 6 (b) and (c), we find that the intensities of emitted wavelengths at k = 300 and 350 nm in Fig. 6(c) are lower than those in Fig. 6(b), owing to high amounts of C-dots being adsorbed on Fe3O4 nanoparticles at Tr = 180. Higher intensities of emitted wavelength at k = 450, shown in Fig. 6(c), also suggest that more red-shifted emission and high amounts of recombinations of photogenerated e /h+ pairs happened on numerous C-dots. 3.2. Photocatalytic degradation experiments Fig. 7(a) shows the photodegradation activities of magnetic Cdots (tr = 0, 4, and 18 h at 140 °C) with different exposure times (te) in visible light. When tr = 0, little concentration of MB was degraded, indicating less interaction between the magnetic Cdots and visible light. These results have also been mentioned in previous studies [21,22]. The photocatalytic activity of pure Fe3O4 nanoparticles was weak under visible light [21,22] because few e /h+ pairs were generated (see Fig. 6(a)). Increasing tr triggers photocatalytic activities under visible light irradiation. The degradation abilities of magnetic C-dots with tr = 4, 12, and 18 h were 4, 30, and 83% at a 30 min exposure, respectively. High amounts
Fig. 4. FTIR spectrum of magnetic C-dots synthesized with tr = 0–18 at Tr = (a) 140 °C and (b) 180 °C.
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Fig. 5. TEM bright field images of magnetic C-dots synthesized at different tr and Tr: (a) commercial Fe3O4 nanoparticles with purchased specifications of 20–40 nm (small Fe3O4), (b) C-dots/small Fe3O4 at Tr = 140 and tr = 4, (c) C-dots/small Fe3O4 at Tr = 140 and tr = 18, (d) C-dots/small Fe3O4 at Tr = 180 and tr = 4, (e) commercial Fe3O4 nanoparticles with purchased specifications of 100–150 nm (large Fe3O4), and (f) C-dots/large Fe3O4 at Tr = 140 and tr = 4. The inset in Fig. 5(e) shows the SEM image of large Fe3O4 particles.
of C-dots on Fe3O4 nanoparticles can massively reduce the concentration of MB. For the case of tr = 18, the concentration of MB can be decreased by 60% at te = 15 min. Finally, MB concentrations decreased to about 17% as te was extended to 30 min. Fig. 7 (b) and (c) show pictures of the degradation abilities of magnetic C-dots (tr = 18 at 140 °C) at te = 0 and 30 min, respectively. Before exposure (te = 0), the deep blue solution indicated the existence of a high concentration of MB. High degradation of MB was observed at te = 30. The light blue bottle indicated lower concentrations of MB in the solution. A Nd-Fe-B magnet was adopted to recycle and restore the magnetic C-dots. After degradation, the magnetic C-dots (indicated by the yellow arrow in Fig. 7(c)) could be collected using a Nd-Fe-B magnet which was placed outside the bottle to provide an applied magnetic field. The magnetic Fe3O4 nanoparticles could be attracted by the applied field. When the C-dots and Fe3O4 nanoparticles were conjugated together, it was clear that the magnetic C-dots were also well controlled by the Nd-Fe-B magnet, evidencing a recyclable photocatalyst application. The recyclability of magnetic C-dots (tr = 18 and Tr = 140) was tested 10 times, and the results were plotted in Fig. 7(d). Before the recyclability testing, in order to reduce the loss of the magnetic catalyst during the washing process, the as-synthesized catalyst was separated and collected from the aqueous phase using a NdFeB magnet, and then rinsed with a large amount of water. The assynthesized magnetic catalyst was then dried and weighed. We did the separation, collection, rinsing, drying, and weighing again
and again, until the mass of collected as-synthesized catalyst remained constant. The as-synthesized magnetically catalyst was then dispersed into a MB solution for photodegradation. After 30 min of photocatalysis under visible light irradiation, the catalyst was quickly collected and separated from the aqueous phase using a NdFeB magnet, and then rinsed with a large amount of water. The recovered magnetic catalyst was dried and then re-dispersed into another new MB solution to initiate a second photodecomposing cycle. The same procedure was performed in the additional cycles. From Fig. 7(d), the degradation ability of magnetic C-dots seemed not to have been significantly reduced after 10 runs of irradiation in visible light. Therefore, our study clearly points out that the Cdots/Fe3O4 nanocomposites possessed good recyclability. The photocatalytic products were collected and analyzed by FTIR. Fig. 8 presents group functions of product after photodegradation by nanocomposites with tr = 18 at 140 °C. The spectrum shows OAH peak at 3700 cm 1 for H2O, C@O vibrations at 2366 cm 1 and 1712 cm 1 for CO2 and aldehyde, respectively [23], C@N vibration at 1560 cm 1 for MB. Therefore, MB was photodecomposed into CO2 gas under visible light irradiation. 3.3. Photocatalytic degradation mechanisms According to the above results and discussion, a photocatalytic degradation process was proposed, as shown in Fig. 9. The photocatalytic ability of magnetic C-dots was mostly attributed to C-dots
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Fig. 7. (a) MB concentration changes with magnetic C-dots (synthesized at tr = 0– 18 and Tr = 140 °C) versus exposure time (te) under visible light, (b) and (c) show the pictures of MB solution photodegraded by magnetic C-dots (tr = 18) at te = 0 and 30 min, respectively. (d) MB concentration changes with 10 photodegradation tests using magnetic C-dots (tr = 18 and Tr = 140 °C) within 30 min visible light irradiation.
Fig. 6. Photoluminescence properties of magnetic C-dots synthesized at (a) tr = 0 and Tr = 140 °C, (b) tr = 4 and Tr = 140 °C and (c) tr = 4 and Tr = 180 °C.
which could excite electrons to produce e /h+ pairs under visible light irradiation. The photoelectrons in the valence band (VB) of C-dots can be effectively excited to the conduction band (CB) and generate e /h+ pairs [24], becoming an excellent electron acceptor and conductor. Although the Fe3O4 nanoparticles also generated e /h+ pairs, their efficiency could be ignored because fewer amounts of e /h+ pairs were photogenerated under visible light. However, Fe3+ in Fe3O4 can promote effective separation of e /h+ pairs due to their superior electronic conductivity [25]. The photogenerated electrons can then react with the dissolved O2 on the surface of C-dots and transform to superoxide radicals (O2 ). Meanwhile, the photogenerated holes react with the adsorption H2O molecules to produce hydroxyl radicals (OH). These radicals have great oxidative ability to decompose methylene blue (MB) to CO2 and H2O. The photocatalytic degradation processes of MB could follow the reactions of (1)-(5) [26,27]. 4. Conclusion C-dots were successfully synthesized on an Fe3O4 nanoparticle surface for fabricating magnetic C-dots. The C-dots were gradually produced on an Fe3O4 nanoparticle surface with increasing
Fig. 8. FTIR spectrum of product after photodegradation by nanocomposites with tr = 18 at 140 °C.
reaction time at 140 °C, causing the Ms of magnetic C-dots to decrease. The applied magnetic field could easily control the movement of these magnetic C-dots. A higher extent of photodegradation of the dye always accompanies larger amounts of C-dots on Fe3O4 nanoparticle surfaces. Such magnetic C-dots with a reaction time of 18 h showed a higher degradation ability of MB. The MB concentration decreased by 83% within 30 min of visible light irradiation. The recyclability test clearly pointed out that the magnetic C-dots had the potential of more than 10-fold the photocatalytic degradation. In this study, a new type of photocatalytic material,
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013
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Fig. 9. The sketch of the photodegradation mechanisms of MB on magnetic C-dots’ surfaces through interactions in visible light irradiation.
magnetic C-dots, was synthesized, exhibiting good degradation abilities and recyclability. Acknowledgement Financial support for this work through grants from the Ministry of Science and Technology, Taiwan (No. MOST 106-2221-E155-015) is gratefully appreciated. References [1] X. Gao, Y. Zhang, Y. Dai, F. Fu, High-performance magnetic carbon materials in dye removal from aqueous solutions, J. Solid State Chem. 239 (2016) 265–273. [2] Z. Zhang, J. Kong, Novel magnetic Fe3O4@C nanoparticles as adsorbents for removal of organic dyes from aqueous solution, J. Hazard. Mater. 193 (2011) 325–329. [3] T. Robinson, G. McMullan, R. Marchant, P. Nigam, Remediation of dyes in textile effluent: a critical review on current treatment technologies with a proposed alternative, Bioresour. Technol. 77 (2001) 247–255. [4] Ibrahim M. Banat, P. Nigam, D. Singh, R. Marchant, Microbial decolorization of textile-dye-containing effluents: a review, Bioresour. Technol. 58 (1996) 217– 227. [5] A. Fujishima, X. Zhang, D.A. Tryk, TiO2 photocatalysis and related surface phenomena, Surf. Sci. Rep. 116 (2014) 515–582.
7
[6] M.N. Chong, B. Jin, C.W.K. Chow, C. Saint, Recent developments in photocatalytic water treatment technology: a review, Water Res. 44 (2010) 2997–3027. [7] P.S. Saud, B. Pant, A.M. Alam, Z.K. Ghouri, M. Park, H.Y. Kim, Carbon quantum dots anchored TiO2 nanofibers: effective photocatalyst for waste water treatment, Ceram. Int. 41 (2015) 11953–11959. [8] X. Xu, R. Ray, Y. Gu, H.J. Ploehn, L. Gearheart, K. Raker, W.A. Scrivens, Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments, J. Am. Chem. Soc. 126 (2004) 12736–12737. [9] P. Roy, P.C. Chen, A.P. Periasamy, Y.N. Chen, H.T. Chang, Photoluminescent carbon nanodots: synthesis, physicochemical properties and analytical applications, Mater. Today 18 (2015) 447–458. [10] L. Ai, Y. Zhou, J. Jiang, Removal of methylene blue from aqueous solution by montmorillonite/CoFe2O4 composite with magnetic separation performance, Desalination 266 (2011) 72–77. [11] A.H. Lu, E.L. Salabas, F. Schuth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222– 1244. [12] S.M. Hussain, K.L. Hess, J.M. Gearhart, K.T. Geiss, J.J. Schlager, In vitro toxicity of nanoparticles in BRL 3A rat liver cells, Toxicol In Vitro 19 (2005) 975–983. [13] H. Deng, X. Li, Q. Peng, X. Wang, J. Chen, Y. Li, Monodisperse magnetic singlecrystal ferrite microspheres, Angew. Chem. Int. Ed. 44 (2005) 2782–2785. [14] D.P. Yang, F. Gao, D.X. Cui, M. Yang, Microwave rapid synthesis of nanoporous Fe3O4 magnetic microspheres, Curr. Nanosci. 5 (2009) 485–488. [15] Cuiyan Li, Yajun Wei, Anti Liivat, Yihua Zhu, Jiefang Zhu, Microwavesolvothermal synthesis of Fe3O4 magnetic nanoparticles, Mater. Lett. 107 (2013) 23–26. [16] Wanquan Jiang, H.C. Yang, S.Y. Yang, H.E. Horng, J.C. Hung, Y.C. Chen, Chin-Yih Hong, Preparation and properties of superparamagnetic nanoparticles with narrow size distribution and biocompatible, J. Magn. Magn. Mater. 283 (2004) 210–214. [17] M. Chivukula, J. Spadaro, V. Renganathan, Lignin peroxidase-catalyzed oxidation of sulfonated azo dyes generates novel sulfophenyl hydroperoxides, Biochemistry 34 (1995) 7765–7772. [18] H. Dong, M. Roming, C. Feldmann, Unexpected fluorescence of polyols and PEGylated nanoparticles derived from carbon dot formation, Part. Part. Syst. Charact. 32 (2015) 467–475. [19] S.K. Jana, S. Majumder, B. Satpati, S.K. Mishra, R.K. Srivastava, S. Banerjee, Enhancement of photoluminescence emission andanomalous photoconductivity properties of Fe3O4@SiO2 core–shell microspheres, RSC Adv. 5 (2015) 37729–37736. [20] J. Jayabharathi, A. Prabhakaran, C. Karunakaran, V. Thanikachalam, M. Sundharesan, Structural, optical and photoconductivity characteristics of pristine FeO-Fe2O3 and NTPI–FeO-Fe2O3 nanocomposite: aggregation induced emission enhancement of fluorescent organic nanoprobe of thiophene appended phenanthrimidazole derivative, RSC Adv. 6 (2016) 18718–18736. [21] Guangcheng Xi, Bing Yue, Junyu Cao, Jinhua Ye, Fe3O4/WO3 Hierarchical core– shell structure: high-performance and recyclable visible-light photocatalysis, Chem. Eur. J. 17 (2011) 5145–5154. [22] Yuanjie Zhu, Jinjuan Xue, Xu. Tingting, Guangyu He, Haiqun Chen, Enhanced photocatalytic activity of magnetic core–shell Fe3O4 @ Bi2O3-RGO heterojunctions for quinolone antibiotics degradation under visible light, J. Mater. Sci: Mater. Electron. 28 (2017) 8519–8528. [23] Yu. Zhiqiang, Steven S.C. Chuang, Probing methylene blue photocatalytic degradation by adsorbed ethanol with in situ IR, J. Phys. Chem. C. 111 (2007) 13813–13820. [24] X.F. Chen, J.F. Dai, G.F. Shi, L. Li, G.Y. Wang, H. Yang, Visible light photocatalytic degradation of dyes by b-Bi2O3/graphene nanocomposites, J. Alloy. Compd. 649 (2015) 872–877. [25] Y. Li, J. Yang, S.H. Zheng, W.W. Zeng, N. Zhao, M. Shen, One-pot synthesis of 3D TiO2-reduced graphene oxide aerogels with superior adsorption capacity and enhanced visible-light photocatalytic performance, Ceram. Int. 42 (2016) 19091–19096. [26] F. Li, G. Wang, H. Li, Y. Li, S. Zhou, One-pot solvothermal synthesis of highly photoluminescent carbon nanoparticles and their photocatalytic application, Mater. Lett. 122 (2014) 352–354. [27] H.Q. Wang, Z.F. Ye, C. Liu, J.Z. Li, M.J. Zhou, Q.F. Guan, P. Lv, P.W. Huo, Y.S. Yan, Visible light driven Ag/Ag3PO4/AC photocatalyst with highly enhanced photodegradation of tetracycline antibiotics, Appl. Surf. Sci. 353 (2015) 391– 399.
Please cite this article in press as: A.-C. Sun, Synthesis of magnetic carbon nanodots for recyclable photocatalytic degradation of organic compounds in visible light, Advanced Powder Technology (2017), https://doi.org/10.1016/j.apt.2017.12.013