Layered Double Hydroxide-Supported Carbon Dots as an Efficient

Article pubs.acs.org/JPCC

Layered Double Hydroxide-Supported Carbon Dots as an Efficient Heterogeneous Fenton-Like Catalyst for Generation of Hydroxyl Radicals Manlin Zhang,† Qingfeng Yao,† Weijiang Guan,† Chao Lu,*,† and Jin-Ming Lin‡ †

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People’s Republic of China ‡ Department of Chemistry, Tsinghua University, Beijing 100084, People’s Republic of China S Supporting Information *

ABSTRACT: The development of a new heterogeneous Fenton-like catalyst is highly desired. Herein, we reported a simple and efficient method for the preparation of colloidal nanocomposites consisting of carbon dots and dodecylbenzenesulfonate (DBS)-layered double hydroxides (LDHs). The resulting nanocatalyst can function as an effective heterogeneous Fenton-like catalyst for the decomposition of acidified H2O2 to generate abundant hydroxyl radicals (·OH). With the aid of chemiluminescence (CL) technique, electron spin resonance (ESR) measurements and ion chromatography (IC) separation technique, we demonstrated that the unique structural configuration of the carbon dotDBS-LDH nanocomposites was responsible for the highly efficient catalytic activities toward H2O2 decomposition. The fabricated material introduced a novel family of Fenton-like nanocatalysts with environmental friendliness, cost effectivity, and superior efficiency for the decomposition of H2O2 to ·OH radicals. Such heterogeneous Fenton-like catalyst could realize the degradation of DBS without any external energy input, showing a promising application for the oxidative degradation of organic contaminants in wastewater treatment applications.

1. INTRODUCTION

Layered double hydroxides (LDHs) are an important class of layered materials with brucite-like layers and intercalated anions in the interlayer region.14 LDHs themselves (e.g., Cu/AlLDHs) can display great potential as efficient solid catalysts for various organic reactions due to the atomic-scale uniform distribution of metal cations in the brucite-like layers and the ability to intercalate a variety of interlayer anions.15,16 In addition, LDHs as catalyst precursors/support represent an interesting opportunity for fabricating a multifunctional nanocomposite with a diverse range of nanoparticles, such as gold nanoparticles, graphene, and semiconductor nanoparticles.17−20 The catalytic sites of such nanocomposites can be preferentially orientated, highly dispersed, and firmly stabilized to afford excellent catalytic performance and recyclability.21 Carbon dots have become a rising star in the nanocarbon family in terms of high aqueous solubility, robust chemical inertness, easy functionalization, low toxicity, and good biocompatibility.22−24 Recently, carbon dots could exhibit electron-donating properties by advanced oxidation process, in which superoxide anions were generated by the electron transfer process from carbon dots to dissolved oxygen in

The introduction of novel strategies to generate abundant hydroxyl radicals (·OH) is highly desired for environmental remediation, biological sensors, and material synthesis.1−3 As an advanced oxidation process, the conventional Fenton’s reagent containing H2O2 and ferrous ion has been extremely popular for the production of ·OH radicals.4 However, some inherent drawbacks of the current Fenton process, such as the narrow pH range of reaction and the accumulation of iron sludge, limit its application on a large scale.5 Nowadays, in order to overcome these disadvantages of the homogeneous Fenton process, Fenton-like catalysts involving other transitional metals or nonmetallic materials, such as manganese, copper, cobalt, gold, and carbon could exhibit similar catalytic activities in comparison to the conventional Fenton catalyst.6−8 In addition, the use of heterogeneous solid iron-based catalysts is a highly promising alternative to prevent the accumulation and precipitation of soluble iron.9−11 However, the heterogeneous Fenton reaction itself shows weak catalytic activity, and thus most of the heterogeneous Fenton catalysts often need to rely on ultrasound or UV/visible light irradiation for the acceleration of the reaction.12,13 Accordingly, the key problem is how to design heterogeneous Fenton-like catalysts with a highly efficient activity for the production of ·OH radicals. © 2014 American Chemical Society

Received: February 4, 2014 Revised: March 26, 2014 Published: April 24, 2014 10441

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concentrated sodium hydroxide.25,26 In addition, carbon dots could exhibit intrinsic peroxidase-like activities in Fenton-like reactions.27 However, Kang and his coauthors have successfully used the electron-accepting and -transport properties of carbon dots to design efficient complex photocatalysts through the construction of nanocomposites (e.g., carbon dot/Cu2O and carbon dot/Fe2O3).28−31 We anticipate that the combination of carbon dots and LDHs seems to be an ideal strategy for promoting the exceptional catalytic activity as a result of the interaction of carbon dots and LDH support, and this novel composite is a fascinating and competitive candidate for the catalytic decomposition of H2O2. However, so far no attention has been paid to carbon dot-LDH nanocomposites. In this work, we chose dodecylbenzenesulfonate (DBS)LDHs as the catalyst support because the hydrophobic microenvironment from DBS bilayer bunches on the surface of LDHs can protect the produced active intermediates. Therefore, we report our studies on a one-step facile route to directly fabricate carbon dot-DBS-LDH nanocomposites. The as-prepared new nanocatalyst can catalyze the decomposition of acidified H2O2 to generate abundant ·OH radicals, accompanying with the significant enhancement in the chemiluminescence (CL) signals. The structure of the obtained nanocomposites and the mechanism of the CL enhancement were analyzed by X-ray diffraction (XRD) measurements, transmission electron microscopy (TEM) images, electron spin resonance (ESR) measurements, and so on. More interestingly, the catalytic performances of this new nanoscale carbon dot-LDH catalyst were used to promote the heterogeneous Fenton-like reaction by evaluating DBS degradation without any external energy input. Our findings suggest that the as-prepared nanohybrids in this work could act as an efficient catalyst with some advantageous properties, such as easy separation and excellent recyclability, which may hold great potential for the treatment of nonbiodegradable water pollutants. To the best of our knowledge, no previous example has been disclosed for heterogeneous Fenton-like reactions with LDH-supported carbon dot catalysts.

2.2. Synthesis of Mg−Al−NO3 LDHs. The coprecipitation of Mg−Al−NO3 LDHs was performed at a molar ratio of 3/1 between Mg2+ and Al3+ by reacting with NaOH. The solution A was prepared by dissolving Mg(NO3)2·6H2O (0.06 mol) and Al(NO3)3·9H2O (0.02 mol) in 80 mL of degassed/deionized water. The solution B was prepared by dissolving NaOH (0.16 mol) in 80 mL of degassed/deionized water. The two solutions were added slowly to a 250 mL four-necked flask under vigorous stirring maintaining pH 10 at room temperature. The resulting white slurry was aged for 24 h at 65 °C. The whole procedure was performed under a nitrogen atmosphere to exclude the aqueous CO2. Afterward, the precipitate was recovered by centrifugation and washed for three times with deCO2/deionized water, and then dried in vacuo at 60 °C for 24 h. The resulting solid was white and homogeneous powder. 2.3. Synthesis of Mg−Al−DBS LDHs. Mg-Al-DBS LDHs were prepared via a simple anion exchange procedure.32 A mixture of 1.0 g Mg−Al−NO3 LDHs powder and 25 mL DBS aqueous solution (0.2 M) was vigorous stirred at 80 °C for 24 h under N2 atmosphere, the Mg−Al−DBS LDHs suspension was obtained and then stored at 4 °C for further use. 2.4. Synthesis of Carbon Dots. In a typical procedure, carbon dots were prepared as follows.23 The homogeneous solution containing 15 mL glycerin and 1.0 g PEG 1500 was heated for 1 min in a microwave oven. After that, 0.5 g serine was dissolved in this homogeneous solution. Next, this mixture was further heated in a microwave oven for 6 min. The colorchanged solution was stable for several months in a refrigerator at 4 °C. 2.5. Fabrication of Carbon Dot-DBS-LDH Nanocomposites. In brief, 100 μL 1.4 M carbon dot colloidal solution was mixed with 1.9 mL as-prepared Mg−Al−DBS LDH colloidal solution. The obtained suspension was stirred for 10 min at room temperature for further use (Supporting Information, SI, Figure S1). The well-dispersed solution was stable for several months. 2.6. Apparatus. The powder XRD measurements were performed on a Bruker (Germany) D8 ADVANCE X-ray diffractometer equipped with graphite-monochromatized Cu Kα radiation (λ = 1.5406 Å). The 2θ angle of the diffractometer was stepped from 2° to 70° at a scan rate of 10°/min. The particle sizes and morphology of the samples were observed on a TEM (Tecnai G220, FEI, Hong Kong). ESR measurements were performed on a JES-FA200 spectrometer (JEOL, Tokyo, Japan). ICS-5000 ion chromatography (IC) equipment (Dionex Company, U.S.A.) consisted of an anion separation column (AS11-HC, 250 × 4.0 mm i.d.) and a conductivity detector. Thirty mmol/L KOH was used as the eluent at 1.2 mL/min. A 800 W microwave oven (G8023CSL-K3, Galanz, China) was used for the preparation of carbon dots. The CL detection was conducted on a biophysics chemiluminescence (BPCL) luminescence analyzer (Institute of Biophysics, Chinese Academy of Science, Beijing, China). UV−vis spectra were measured on a USB 4000 miniature fiber optic spectrometer in absorbance mode with a DH-2000 deuterium and tungsten halogen light source (Ocean Optics, Dunedin, FL). Fluorescence spectra were measured with a Hitachi F7000 fluorescence spectrophotometer (Tokyo, Japan). The CL spectrum of this system was measured with high-energy cutoff filters from 400 to 640 nm between the flow CL cell and the photomultiplier tube (PMT). 2.7. CL Measurements. SI Figure S2 showed the schematic diagram of the flow injection CL system. To investigate the

2. EXPERIMENTAL SECTION 2.1. Chemicals and Solutions. All chemicals were of analytical grade without further purification. All solutions were prepared with deionized water (Milli Q, Millipore, Barnstead, CA, U.S.A.). Mg(NO3) 2 ·6H2 O, Al(NO 3) 3 ·9H2 O, NaCl, Na2SO4, NaNO3, NaOH, HNO3, and glycerin were purchased from Beijing Chemical Reagent Company (Beijing, China). Thiourea and NaN3 were purchased from Tianjin Fuchen Chemical Reagents Factory (Tianjin, China). Ascorbic acid was purchased from Beijing Aoboxing Biotech Co., Ltd. (Beijing, China). 2,2,6,6-Tetramethyl-4-piperidine (TEMP), 1,4diazabicyclo[2.2.2]octane (DABCO), poly(ethylene glycol) 1500 (PEG 1500), histidine, sodium dodecyl sulfate (SDS) and serine were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO, U.S.A.). Nitro blue tetrazolium chloride (NBT) was purchased from Nacalai Tesque Inc. (Tokyo, Japan). 5,5-Dimethyl-1-pyrroline N-oxide (DMPO) was obtained from Tokyo Kasei Kogyo Co. Ltd. (Tokyo, Japan). DBS was purchased from Tokyo Chemical Industry Co., Ltd. (Tokyo, Japan). A mixed working solution of 0.05 M H2O2 and 0.3 M HCl was freshly prepared by volumetric dilution of commercial 30% (v/v) H2O2 and 36% (v/v) HCl (Beijing Chemical Reagent Company, Beijing, China) with deionized water, respectively. 10442

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Figure 1. (A) TEM image, (B) fluorescence spectra, (C) UV−vis absorption spectrum, and (D) powder XRD patterns of the as-prepared carbon dots.

Figure 2. (A) TEM image of carbon dot-DBS-LDH nanocomposites and (B) fluorescence intensity of (1) 5.6 mM carbon dots and (2) supernatant obtained by centrifuging 5.6 mM carbon dots assembled DBS-LDHs.

as-prepared carbon dots were spherical and monodispersed with an average diameter of 5 nm (Figure 1A). In addition, carbon dots exhibited λex-dependent emission behavior with a weak absorption band around 380 nm (Figure 1B,C). The XRD patterns of the as-prepared carbon dots revealed a two diffraction peak attributed to turbostratic carbon phase (Figure 1D).33 However, SI Figure S3A illustrated the XRD patterns for the original DBS-LDHs, displaying a series of characteristic reflections of LDH structure with great intensity and narrow line width. A TEM image showed flat and well-defined layered platelets with fairly large crystal sizes (SI Figure S3B). Carbon dot-LDH nanocomposites can be fabricated by a simple onestep method through electrostatic attraction between negatively charged carbon dots and positive surface of LDHs.23 The effective assembly of the as-prepared carbon dot-LDHs can be determined by TEM and fluorescence spectra (Figure 2). As

effect of the carbon dot-LDH catalytic activity on the acidified H2O2 CL system, we injected the carbon dot-LDH colloidal solution into the carrier stream (H2O) through a valve injector with a 120 μL sample loop, which mixed with 0.05 M H2O2-0.3 M HCl through three-way pieces. The flow rates for the carrier stream and H2O2−HCl were 1.0 and 2.0 mL/min, separately. The CL signals were monitored by the PMT adjacent to the flow CL cell. The data integration time of the BPCL analyzer was set at 0.1 s per spectrum, and a work voltage of −1100 V was used for the CL detection. The signal was imported to the computer for data acquisition.

3. RESULTS AND DISCUSSION 3.1. Characterization of Carbon Dot-LDH Nanocomposites. Figure 1 illustrated the structural characterization of the as-prepared carbon dots. A TEM image showed that the 10443

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Figure 3. ESR spectra of (A) carbon dots (blue trace), carbon dots (red trace) or carbon dot-DBS-LDHs (black trace) in the presence of acidified H2O2, (B) ·OH radicals from carbon dot-acidified H2O2 system and carbon dot-DBS-LDH-acidified H2O2 system via their reactions with DMPO. Experimental conditions: microwave frequency, 9.055 GHz, power, 4.00 mW, modulation amplitude, 2.01 G, modulation frequency, 100 kHz. Experimental procedure: (A) 20 μL carbon dots or carbon dot-DBS-LDHs + 40 μL 0.05 M H2O2-0.3 M HCl, (B) 40 μL 0.05 M H2O2-0.3 M HCl + 20 μL 0.1 M DMPO + 20 μL carbon dots or carbon dot-DBS-LDHs.

decomposition of H2O2 under various conditions.39 Herein, under the optimum experimental conditions (SI Figure S4), the CL technique was carried out to verify the involving reactive oxygen species during carbon dot-DBS-LDH nanocompositecatalyzed decomposition of acidified H2O2. In a flow-injection CL setup (SI Figure S2), Figure 4 showed that the as-prepared

illustrated in Figure 2A, a TEM image of carbon dot-DBS-LDH nanocomposites indicated that carbon dots were well dispersed on the surface of DBS-LDHs. In addition, Figure 2B showed that the fluorescence intensity of supernatant obtained by centrifuging carbon dots assembled DBS-LDHs was weaker than that from the aqueous carbon dots. These phenomena indicated that a large number of carbon dots could be assembled on the surface of LDHs. 3.2. Confirmation of Abundant Hydroxyl Radicals Using ESR Measurement. Carbon dots can act as electron donors or electron acceptors during the electron transfer process.34 In this work, the ground-state properties of luminescent species in the carbon dots were investigated by ESR measurements. The ESR signal of carbon dots was shown at g = 2.0099 (Figure 3A), indicating a singly occupied orbital in ground-state carbon dots.35,36 However, the g-value of the carbon dots was reduced from 2.0099 to 2.0031 after they reacted with acidified H2O2. Moreover, the g-value of the carbon dots immobilized on the surface of DBS-LDHs caused an obvious reduction in comparison to the carbon dots, indicating the occurrence of greater change of singly occupied orbital in the carbon dots in this case. These findings demonstrated that electron transfer happened between carbon dots/carbon dot-LDH nanocomposites and acidified H2O2. The room-temperature ESR spectroscopy can further confirm the generated reactive oxygen species in the present system. Acting as a specific target molecule of ·OH radicals, DMPO can be used to identify the amount of ·OH radicals available during the reactions.37 Figure 3B presented the production of the DMPO−OH adducts during the reaction of carbon dots with acidified H2O2. Furthermore, a stronger signal of ESR was obtained from carbon dot-LDH-acidified H2O2 system. Therefore, carbon dot-DBS-LDH nanocomposites are able to catalyze the decomposition of acidified H2O2 to generate abundant ·OH radicals. 3.3. Generated Hydroxyl Radicals Triggered Carbon Dot-DBS-LDH Nanocomposite CL. CL is defined as the production of electromagnetic radiation observed when a chemical reaction yields an electronically excited intermediate, which either luminesces or transfers its energy to another molecule.38 CL technique is often employed as a sensitive and selective detection mode for reactive oxygen species from the

Figure 4. CL intensity of acidified H2O2 mixed with different solutions including carbon dots, carbon dot-DBS, DBS-LDHs, and the assembly of carbon dots on the surface of DBS-LDHs (inset: CL spectrum of the carbon dot-DBS-LDHs reacted with acidified H2O2).

carbon dot-DBS-LDH nanocomposites could catalyze the decomposition of acidified H2O2 to generate the intense CL signals. Moreover, a similar phenomenon was also observed with the addition of carbon dot-DS-LDHs (SI Figure S5A). Note that a same CL intensity was obtained when HNO3 was used to replace HCl (SI Figure S5B). The DBS-LDHs had two major advantages in the CL amplification. First, the hydrophobic microenvironment of the DBS bilayer bunches on the surface of LDHs can protect the produced active intermediates.19 However, carbon dots were adsorbed on the surface of LDHs to produce a compact nanostructure, which could greatly reduce the distance between carbon dots and DBS, facilitating the occurrence of the reaction between carbon dots and DBS. Blank experiments with the same experimental conditions were 10444

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Figure 5. (A) ESR spectra of nitroxide radicals generated via the reaction of TEMP probe for carbon dot-DBS-LDH-acidified H2O2 system. Experimental conditions: 40 μL 0.05 M H2O2-0.3 M HCl + 20 μL carbon dot-DBS-LDHs + 20 μL 0.03 M TEMP, (B) UV−vis absorption spectra for NBT before and after reaction.

Figure S6, the fluorescence intensity of carbon dots was partly reduced after the reaction, which was ascribed to the redox reaction between carbon dots and acidified H2O2. However, most of carbon dots remained their original properties after the reaction, enabling carbon dots to act as CL acceptors. Finally, the production of SO2* in the proposed CL system was confirmed by IC separation technique using KOH as eluent. As shown in Figure 6A,B, three strong Cl−, SO42−, and NO3− peaks appeared after the carbon dots-DBS-LDH colloidal solution was reacted with HCl-H2O2. Note that NO3− appeared in this system was attributed to the incomplete ion exchanges in the preparation process of DBS-LDHs using NO3-LDHs as precursors. In addition, the formation of SO42− was due to the fact that carbon dot-DBS-LDHs could catalyze acidified H2O2

also carried out including carbon dots, DBS-LDHs, and carbon dot-DBS. The results indicated that a weak CL emission was observed in the presence of carbon dots and carbon dot-DBS. However, DBS-LDHs could catalyze the decomposition of acidified H2O2 to induce a stronger CL emission. These phenomena indicated that the unique structural configuration of the carbon dot-DBS-LDH assembly played a predominant role in the catalytic decomposition of acidified H2O2. 3.4. Emitting Species. The scavengers of a variety of reactive oxygen species were used to confirm the exact emitting species of the present system. The results were shown in SI Table S1. As two effective ·OH radical scavengers, 10 mM ascorbic acid and 50 mM thiourea could induce a significant decrease in the CL intensity, indicating that ·OH radicals were released in the reaction.37 Moreover, 30 mM histidine, DABCO or NaN3 (scavengers of singlet oxygen, 1O2) did not quench the CL intensity, providing strong evidence that 1O2 did not contribute to the observed CL.23 In addition, it was not observed that a specific signal of 2,2,6,6-tetramethyl-4piperidine-N-oxide (TEMPO) adduct by the reaction of TEMP with the carbon dot-LDH-acidified H2O2 system (Figure 5A), meaning that it cannot produce 1O2 in the present system.36 When 30 mM NBT (an indicator of superoxide radicals,26 ·O2−) was added to the present CL system, there was not any remarkable change in CL intensity. In addition, no any new UV−vis absorption peak was observed (Figure 5B). To further confirm the emitting species in carbon dot-DBSLDH-acidified H2O2 system, the CL spectrum was measured with high-energy cutoff filters from 400 to 640 nm between the flow CL cell and the PMT. It could see clearly from the inset of Figure 4 that there were two peaks observed in the present CL system. The first peak at 430 nm belonged to the emission of excite sulfur dioxide molecules (SO2*).40 The other peak located at 490 nm, corresponding to the excited carbon dots formed by a CL resonance energy transfer between SO2* (donors) and carbon dots (acceptors). The emission band in the range of 490−550 nm may arise from the excited singlet oxygen dimol species ((O2)2*).32 However, Figure 5A and SI Table S1 showed that 1O2 and ·O2− could not be generated in the present CL reaction, and thus (O2)2* was not easily formed by recombination of 1O2 or ·O2−. Therefore, we could come to the conclusion that the CL peak located at 490 nm was attributed to the emission from carbon dots rather than the (O2)2*. Moreover, we compared the fluorescence intensity of carbon dots before and after addition of acidified H2O2 into carbon dot-DBS-LDH colloidal solution. As illustrated in SI

Figure 6. IC of (A) standard mixed solution containing 170 μM Cl−, 70 μM SO42−, and 120 μM NO3−, (B) carbon dot-DBS-LDH-acidified H2O2 (blue trace) and carbon dot-DBS-LDH-acidified H2O2 spiked with 700 μM Na2SO4 (red trace). Experimental conditions: the mixed solution of 1.0 mL carbon dot-DBS-LDHs and 2.0 mL 0.05 M H2O20.3 M HCl was filtered through a 0.22 μm filter and diluted before IC analysis. The standard addition method was carried out by spiking 700 μM Na2SO4 standard solutions to carbon dot-DBS-LDH-acidified H2O2. 10445

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decomposition to produce abundant ·OH radicals, which could degrade DBS to generate sulfite radical (·SO3−), and then the recombination of ·SO3− anionic radicals could generate SO42− with a weak CL emission.37,41 3.5. Mechanism Discussion. In alkaline medium, the major decomposition product of H2O2 is OOH− anions, which could easily react with ·OH radicals, resulting in a decrease in the amount of ·OH radicals.42 On the basis of the discussion above, the mechanism of the reaction can be illustrated in Figure 7. First, carbon dots assembled on the surface of DBS-

Article

ASSOCIATED CONTENT

S Supporting Information *

Effect of stirring time for the CL intensity of the carbon dotDBS-LDH-acidified H2O2 system; schematic diagram of the flow injection CL analysis system; powder XRD patterns and TEM image of DBS-LDHs; effects of various conditions on the CL intensity; CL intensity of acidified H2O2 mixed with carbon dot-DBS-LDHs or carbon dot-DS-LDHs, CL intensity of carbon dot-DBS-LDH-H2O2 mixed with HCl or HNO3; and fluorescence spectra of carbon dot-DBS-LDHs before and after reaction. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax/Tel.: +86 10 64411957; e-mail: [email protected]. Author Contributions

M.L.Z. and Q.F.Y. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by National Basic Research Program of China (973 Program, 2014CB932103), the National Natural Foundation of China (21375006), the 973 Program (2011CBA00503), and the Fundamental Research Funds for the Central Universities (JD1311). We also thank Prof. Xue Duan, Beijing University of Chemical Technology, for his valuable discussions.

Figure 7. Possible mechanism for carbon dot-DBS-LDH nanocatalyst for the decomposition of acidified H2O2 to produce abundant ·OH radicals.

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LDHs could donate an electron to acidified H2O2 to produce lots of ·OH radicals. As a strong oxidant, ·OH radicals can easily attack the DBS molecules on the surface of LDHs to generate SO42− ions,41 accompanying with the CL emissions from the excited SO2*.37 Meanwhile, the excited SO2* could transfer its energy to the adjacent carbon dots to generate the excited carbon dots. Finally, the excited carbon dots returned to their ground states with an intense CL emission. Note that the compact nanostructure of carbon dot-DBS-LDHs could greatly reduce the distance between carbon dots and DBS,43 facilitating the occurrence of the reaction between carbon dots and DBS. Additionally, the hydrophobic microenvironment of the DBS bilayer bunches on the surface of LDHs can protect the produced radical intermediates or emitting species.19

REFERENCES

(1) Yang, X.-J.; Xu, X.-M.; Xu, J.; Han, Y.-F. Iron Oxychloride (FeOCl): An Efficient Fenton-Like Catalyst for Producing Hydroxyl Radicals in Degradation of Organic Contaminants. J. Am. Chem. Soc. 2013, 135, 16058−16061. (2) Radich, J. G.; Kamat, P. V. Making Graphene Holey. GoldNanoparticle-Mediated Hydroxyl Radical Attack on Reduced Graphene Oxide. ACS Nano 2013, 7, 5546−5557. (3) Granados-Oliveros, G.; Gómez-Vidales, V.; Nieto-Camacho, A.; Morales-Serna, J. A.; Cárdenas, J.; Salmón, M. Photoproduction of H2O2 and Hydroxyl Radicals Catalysed by Natural and Super AcidModified Montmorillonite and Its Oxidative Role in the Peroxidation of Lipids. RSC Adv. 2013, 3, 937−944. (4) Hartmann, M.; Kullmann, S.; Keller, H. Wastewater Treatment with Heterogeneous Fenton-Type Catalysts Based on Porous Materials. J. Mater. Chem. 2010, 20, 9002−9017. (5) Tušar, N. N.; Maučec, D.; Rangus, M.; Arčon, I.; Mazaj, M.; Cotman, M.; Pintar, A.; Kaučič, V. Manganese Functionalized Silicate Nanoparticles as a Fenton-Type Catalyst for Water Purification by Advanced Oxidation Processes (AOP). Adv. Funct. Mater. 2012, 22, 820−826. (6) Yecheskel, Y.; Dror, I.; Berkowitz, B. Catalytic Degradation of Brominated Flame Retardants by Copper Oxide Nanoparticles. Chemosphere 2013, 93, 172−177. (7) Yang, X. J.; Tian, P.-F.; Zhang, C. X.; Deng, Y.-Q.; Xu, J.; Gong, J. L.; Han, Y.-F. Au/Carbon as Fenton-Like Catalysts for the Oxidative Degradation of Bisphenol A. Appl. Catal., B 2013, 134−135, 145−152. (8) Saputra, E.; Muhammad, S.; Sun, H. Q.; Wang, S. B. Activated Carbons as Green and Effective Catalysts for Generation of Reactive Radicals in Degradation of Aqueous Phenol. RSC Adv. 2013, 3, 21905−21910. (9) Zhang, G. K.; Gao, Y. Y.; Zhang, Y. L.; Guo, Y. D. Fe2O3-Pillared Rectorite as an Efficient and Stable Fenton-Like Heterogeneous Catalyst for Photodegradation of Organic Contaminants. Environ. Sci. Technol. 2010, 44, 6384−6389.

4. CONCLUSIONS In summary, we demonstrated that carbon dot-DBS-LDH nanocomposites are an effective heterogeneous Fenton-like catalyst for the decomposition of acidified H2O2 to produce abundant ·OH radicals. Such capabilities of the as-prepared heterogeneous Fenton-like nanocatalyst are attributable to unique structural configuration of the carbon dot-DBS-LDH nanocomposites, facilitating the formation of radical intermediates. The as-prepared heterogeneous Fenton-like nanocatalyst could degrade DBS without any external energy input. Our findings will inspire further development in heterogeneous Fenton-like nanocatalysts, making them highly promising nanocatalysts in advanced oxidation processes for the degradation of environmental pollutants, such as wastewater treatment, soil remediation, and other emerging environmental problems. 10446

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The Journal of Physical Chemistry C

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Degradation under Visible Light at Room Temperature. New J. Chem. 2012, 36, 1031−1035. (30) Zhang, H. C.; Ming, H.; Lian, S. Y.; Huang, H.; Li, H. T.; Zhang, L. L.; Liu, Y.; Kang, Z. H.; Lee, S.-T. Fe2O3/Carbon Quantum Dots Complex Photocatalysts and Their Enhanced Photocatalytic Activity under Visible Light. Dalton Trans. 2011, 40, 10822−10825. (31) Zhang, H. C.; Huang, H.; Ming, H.; Li, H. T.; Zhang, L. L.; Liu, Y.; Kang, Z. H. Carbon Quantum Dots/Ag3PO4 Complex Photocatalysts with Enhanced Photocatalytic Activity and Stability under Visible Light. J. Mater. Chem. 2012, 22, 10501−10506. (32) Zhang, M. C.; Han, D. M.; Lu, C.; Lin, J.-M. Organo-Modified Layered Double Hydroxides Switch-On Chemiluminescence. J. Phys. Chem. C 2012, 116, 6371−6375. (33) Liu, Y.; Xiao, N.; Gong, N. P.; Wang, H.; Shi, X.; Gu, W.; Ye, L. One-Step Microwave-Assisted Polyol Synthesis of Green Luminescent Carbon Dots as Optical Nanoprobes. Carbon 2014, 68, 258−264. (34) Hu, S. L.; Tian, R. X.; Wu, L. L.; Zhao, Q.; Yang, J. L.; Liu, J.; Cao, S. R. Chemical Regulation of Carbon Quantum Dots from Synthesis to Photocatalytic Activity. Chem.Asian J. 2013, 8, 1035− 1041. (35) Lin, Z.; Xue, W.; Chen, H.; Lin, J.-M. Classical Oxidant Induced Chemiluminescence of Fluorescent Carbon Dots. Chem. Commun. 2012, 48, 1051−1053. (36) Lin, Z.; Xue, W.; Chen, H.; Lin, J.-M. Peroxynitrous-AcidInduced Chemiluminescence of Fluorescent Carbon Dots for Nitrite Sensing. Anal. Chem. 2011, 83, 8245−8251. (37) Zhang, L. J.; Zhang, Z. M.; Lu, C.; Lin, J.-M. Improved Chemiluminescence in Fenton-Like Reaction via DodecylbenzeneSulfonate-Intercalated Layered Double Hydroxides. J. Phys. Chem. C 2012, 116, 14711−14716. (38) Wang, Z. H.; Teng, X.; Lu, C. Universal Chemiluminescence Flow-Through Device Based on Directed Self-Assembly of Solid-State Organic Chromophores on Layered Double Hydroxide Matrix. Anal. Chem. 2013, 85, 2436−2442. (39) Lis, S.; Kaczmarek, M. Chemiluminescent Systems Generating Reactive Oxygen Species from the Decomposition of Hydrogen Peroxide and Their Analytical Applications. Trends Anal. Chem. 2013, 44, 1−11. (40) Xue, W.; Lin, Z.; Chen, H.; Lu, C.; Lin, J.-M. Enhancement of Ultraweak Chemiluminescence from Reaction of Hydrogen Peroxide and Bisulfite by Water-Soluble Carbon Nanodots. J. Phys. Chem. C 2011, 115, 21707−21714. (41) Zhang, Z. H.; Deng, Y. Q.; Shen, M. L.; Han, W. M.; Chen, Z. L.; Xu, D. P.; Ji, X. T. Investigation on Rapid Degradation of Sodium Dodecyl Benzene Sulfonate (SDBS) under Microwave Irradiation in the Presence of Modified Activated Carbon Powder with Ferreous Sulfate. Desalination 2009, 249, 1022−1029. (42) Georgi, A.; Kopinke, F.-D. Interaction of Adsorption and Catalytic Reactions in Water Decontamination Processes Part I. Oxidation of Organic Contaminants with Hydrogen Peroxide Catalyzed by Activated Carbon. Appl. Catal., B 2005, 58, 9−18. (43) Wang, Z. P.; Li, J.; Liu, B.; Hu, J. Q.; Yao, X.; Li, J. H. Chemiluminescence of CdTe Nanocrystals Induced by Direct Chemical Oxidation and Its Size-Dependent and Surfactant-Sensitized Effect. J. Phys. Chem. B 2005, 109, 23304−23311.

(10) Xu, L. J.; Wang, J. L. Magnetic Nanoscaled Fe3O4/CeO2 Composite as an Efficient Fenton-Like Heterogeneous Catalyst for Degradation of 4-Chlorophenol. Environ. Sci. Technol. 2012, 46, 10145−10153. (11) Lim, H.; Lee, J.; Jin, S. M.; Kim, J.; Yoon, J.; Hyeon, T. Highly Active Heterogeneous Fenton Catalyst Using Iron Oxide Nanoparticles Immobilized in Alumina Coated Mesoporous Silica. Chem. Commun. 2006, 463−465. (12) Zhang, D. G.; Wu, J. B.; Zhou, B. P.; Hong, Y. Y.; Li, S. B.; Wen, W. J. Efficient Photocatalytic Activity with Carbon-Doped SiO2 Nanoparticles. Nanoscale 2013, 5, 6167−6172. (13) Bagal, M. V.; Gogate, P. R. Wastewater Treatment Using Hybrid Treatment Schemes Based on Cavitation and Fenton Chemistry: A Review. Ultrason. Sonochem. 2014, 21, 1−14. (14) Wang, Q.; O’Hare, D. Recent Advances in the Synthesis and Application of Layered Double Hydroxide (LDH) Nanosheets. Chem. Rev. 2012, 112, 4124−4155. (15) Liotta, L. F.; Gruttadauria, M.; Di Carlo, G.; Perrini, G.; Librando, V. Heterogeneous Catalytic Degradation of Phenolic Substrates: Catalysts Activity. J. Hazard. Mater. 2009, 162, 588−606. (16) Navalon, S.; Alvaro, M.; Garcia, H. Heterogeneous Fenton Catalysts Based on Clays, Silicas and Zeolites. Appl. Catal., B 2010, 99, 1−26. (17) Varade, D.; Haraguchi, K. Efficient Approach for Preparing Gold Nanoparticles in Layered Double Hydroxide: Synthesis, Structure, and Properties. J. Mater. Chem. 2012, 22, 17649−17655. (18) Mitsudome, T.; Mikami, Y.; Funai, H.; Mizugaki, T.; Jitsukawa, K.; Kaneda, K. Oxidant-Free Alcohol Dehydrogenation Using a Reusable Hydrotalcite-Supported Silver Nanoparticle Catalyst. Angew. Chem., Int. Ed. 2008, 47, 138−141. (19) Dong, S. C.; Liu, F.; Lu, C. Organo-Modified HydrotalciteQuantum Dot Nanocomposites as a Novel Chemiluminescence Resonance Energy Transfer Probe. Anal. Chem. 2013, 85, 3363−3368. (20) Huo, R. J.; Jiang, W.-J.; Xu, S. L.; Zhang, F. Z.; Hu, J.-S. Co/ CoO/CoFe2O4/G Nanocomposites Derived from Layered Double Hydroxides towards Mass Production of Efficient Pt-Free Electrocatalysts for Oxygen Reduction Reaction. Nanoscale 2014, 6, 203−206. (21) He, S.; An, Z.; Wei, M.; Evans, D. G.; Duan, X. Layered Double Hydroxide-Based Catalysts: Nanostructure Design and Catalytic Performance. Chem. Commun. 2013, 49, 5912−5920. (22) Baker, S. N.; Baker, G. A. Luminescent Carbon Nanodots: Emergent Nanolights. Angew. Chem., Int. Ed. 2010, 49, 6726−6744. (23) Shi, J. X.; Lu, C.; Yan, D.; Ma, L. N. High Selectivity Sensing of Cobalt in HepG2 Cells Based on Necklace Model MicroenvironmentModulated Carbon Dot-Improved Chemiluminescence in Fenton-Like System. Biosens. Bioelectron. 2013, 45, 58−64. (24) Bourlinos, A. B.; Zbořil, R.; Petr, J.; Bakandritsos, A.; Krysmann, M.; Giannelis, E. P. Luminescent Surface Quaternized Carbon Dots. Chem. Mater. 2012, 24, 6−8. (25) Zhao, L. X.; Di, F.; Wang, D. B.; Guo, L.-H.; Yang, Y.; Wan, B.; Zhang, H. Chemiluminescence of Carbon Dots under Strong Alkaline Solutions: A Novel Insight into Carbon Dot Optical Properties. Nanoscale 2013, 5, 2655−2658. (26) Dou, X. N.; Lin, Z.; Chen, H.; Zheng, Y. Z.; Lu, C.; Lin, J.-M. Production of Superoxide Anion Radicals as Evidence for Carbon Nanodots Acting as Electron Donors by the Chemiluminescence Method. Chem. Commun. 2013, 49, 5871−5873. (27) Shi, W. B.; Wang, Q. L.; Long, Y. J.; Cheng, Z. L.; Chen, S. H.; Zheng, H. Z.; Huang, Y. M. Carbon Nanodots as Peroxidase Mimetics and Their Applications to Glucose Detection. Chem. Commun. 2011, 47, 6695−6697. (28) Li, H. T.; Liu, R. H.; Liu, Y.; Huang, H.; Yu, H.; Ming, H.; Lian, S. Y.; Lee, S.-T.; Kang, Z. H. Carbon Quantum Dots/Cu2O Composites with Protruding Nanostructures and Their Highly Efficient (Near) Infrared Photocatalytic Behavior. J. Mater. Chem. 2012, 22, 17470−17475. (29) Yu, H.; Zhang, H. C.; Huang, H.; Liu, Y.; Li, H. T.; Ming, H.; Kang, Z. H. ZnO/Carbon Quantum Dots Nanocomposites: One-Step Fabrication and Superior Photocatalytic Ability for Toxic Gas 10447

dx.doi.org/10.1021/jp5012268 | J. Phys. Chem. C 2014, 118, 10441−10447