Mussel-Inspired CatecholâFormaldehyde Resin-Coated Fe3O4 Core

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Energy, Environmental, and Catalysis Applications

Mussel-Inspired Catechol-Formaldehyde Resin Coated Fe3O4 Core-Shell Magnetic Nanospheres: An Effective Catalyst Support for Highly Active Palladium Nanoparticles Yanan Zhang, Yu Yang, Haichao Duan, and Changli Lü ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19489 • Publication Date (Web): 30 Nov 2018 Downloaded from http://pubs.acs.org on December 1, 2018

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ACS Applied Materials & Interfaces

Mussel-Inspired Catechol-Formaldehyde Resin Coated Fe3O4 Core-Shell Magnetic Nanospheres: An Effective Catalyst Support for Highly Active Palladium Nanoparticles Yanan Zhang,║ Yu Yang,║ Haichao Duan, and Changli Lü* College of Chemistry, Northeast Normal University, Changchun 130024, P. R. China

KEYWORDS. catechol-formaldehyde resin, mussel-inspired, magnetic core-shell nanospheres, Pd NPs, catalysis

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ABSTRACT Magnetic Fe3O4@catechol-formaldehyde resin (CFR) core-shell nanospheres were fabricated via a controllable hydrothermal method. The shell thickness of Fe3O4@CFR nanospheres can be effectively regulated in the range of 10~170 nm via adjusting reaction parameters. Especially, catechol groups on the surface of nanospheres also play a significant role in mussel-inspired chemistry to further combine with graphene oxide (GO) to wrap the Fe3O4@CFR spheres. The obtained Fe3O4@CFR and Fe3O4@CFR@GO nanospheres can be used as the effective catalyst supports of small Pd nanoparticles ( < 10 nm) formed via in-situ synthesis route. The asfabricated nanohybrid catalysts of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs with excellent dispersibility and stability are reusable after magnetically separation from catalytic systems. Especially, a super active performance was demonstrated for catalytic reduction of methylene blue dye with highest TOF (5260 min-1) yet reported in literatures using a very low dosage of Fe3O4@CFR@GO@PdNPs catalyst. In addition, the Fe3O4@CFR@GO@PdNPs catalyst also exhibits a highly catalytic efficiency for Suzuki coupling reaction using pure water as a green solvent at room temperature.


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INTRODUCTION The huge potential applications for metal nanoparticles (NPs) have been demonstrated in the fields of optoelectronics, catalysis, chemical sensors and biological detection, benefiting from their advantages of small size, large specific surface area, good stability, fluorescence and catalytic effect.1,2 However, metal NPs with high specific surface energy are easy to aggregate, which makes them lost a lot of prominent properties, especially catalytic activity.3 Therefore, how to prevent their aggregation is critically important in the catalysis of metal NPs. In recent years, to meet this challenge, there has been a lot of research to improve the dispersity of metal NPs by loading them onto the surface of multifarious solid supports with enhanced catalytic efficiency. Various materials such as iron oxide,4 zeolites,5 silica,6 and polymer 7 have been used as the supports of metal catalysts. Hu et al. developed a Pd NPs-loaded mesoporous wood nanomaterial which could be utilized for highly efficient wastewater treatment owing to a synergistic effect of well-distributed nanocatalyst and the channel-like microstructure inside the hardwood.8 So the separation and recycling of catalysts become more easier by immobilizing the metal NPs on support materials.9 In addition, it is very essential for nanocatalysts which not only have the outstanding catalytic property but also maintain their strong binding with support materials for long-lasting structural stability.10 It is well known that two-dimensional structural graphene or graphene oxide (GO) has a super-high specific surface area, 2D morphology and unique multi-functional surface chemical properties. Previously reports have demonstrated that graphene or GO nanosheets are the promising catalyst supports for metal NPs. In particular, GO contains abundant oxygenated functionalities which can be used as anchoring sites to form a strong interaction with other components including polymers, metals and their oxides, and so on.11,12 Graphene sheets can


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adsorb and embed metal particles to form stable nanocomposites through their surface oxygencontaining functional groups or defects as nucleation sites.13 However, it is infamous for a strong tendency to aggregate of metal particles because of their van der Waals interaction or π–π stacking, so the chemical functionalization of graphene composites are an essential method to improve or solve the above problem.14 Furthermore, the graphene-supported metal/metal oxide composite materials are a kind of catalysts with great potential and also have better catalytic performance compared with those ordinary catalysts benefiting from the formation of stable metal particles-embeded graphene hybrids. 15-18 For instance, Siril et al. synthesized CuO nanohybrids using pure graphene as the support via microwave-assisted hydrothermal route.19 Wang et al. constructed magnetic Pd/Fe3O4/PEI/ rGO nanocatalyst using the strong interaction of nanoparticles with polyethyleneimine (PEI) modified reduced graphene oxide (rGO).20 However, it is difficult to fabricate complex nanocomposites from the flat and smooth GO sheets as substrates. In addition, the synthetic conditions generally are harsh and complicated. In recent years, wrapping the GO sheets on the nonplanar interfaces of microspheres or nanoparticles has attracted great attention from researchers, and there are many widely synthetic methods for this type of nanomaterials,21 such as electrostatic assembly,22 hydrothermal method,23 emulsification method,24 aerosol-phase method,25 covalent bonding.26 Li et al. synthesized Pd nanoparticles on the rGO-wrapped polystyrene microspheres and the resulted PS/RGO@Pd hybrid could be used as the heterogeneous catalysts for organic reaction.27 Liu et al. prepared raspberrylike rGO coated silica microspheres loaded with Ag nanoparticles via a facile electrostatic assembly strategy under ultrasonic assistance. The obtained nanocatalyst with stable water dispersion displayed highly catalytic performance for the reduction of nitrophenol compound.28


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Phenolic resin as the earliest invention of polycondensate is still a large-scale commercial synthetic resin with wide applications in different fields because of the low cost and excellent performance as compared with other polymer resins.29 Especially, many efforts recently have devoted to construct phenolic-formaldehyde resin based nanostructured materials, especially, monodisperse colloidal spheres, core-shell nanoparticles and phenolic resin-derived carbon nanomaterials with different structures for various applications.30,31 Resorcinol-formaldehyde (RF) resin is one of the most studied phenolic resins in recent years.32,33 In particular, since Liu et al. innovatively proposed an extended Stöber strategy to the precisely controlled synthesis of RFbased microspheres and RF-derived carbon nanomaterials,34 the RF resins have been widely used as excellent precursors of carbon nanostructures for catalytic supports, energy-storage, electrode materials, absorption, and biomedical applications due to its attractive advantages including low cost, controllable pore structure, large specific surface area, outstanding thermal and mechanical performance.35-39 Besides phenol and resorcinol, other substituted phenols such as 2,4dihydroxybenzoic acid

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and aminophenol41 were also used to react with formaldehyde to

prepare phenolic resin nano/microspheres. So far, only a limited number of phenol derivatives including resorcinol and aminophenol have been successfully used to construct phenolic resinbased nanostructured materials. Recently the catechol and its derivatives such as dopamine have received more interesting for the researchers owing to the extensive applications for fabricating multifunctional adhesives or coatings through mussel-inspired chemistry.42,43 Catechol-derived macromolecules with odihydroxyaryl chemical function have been designed as a versatile anchor for modifying various surfaces including both organic and inorganic substrates via simple chemistry.44,55 Therefore integrating catechol groups into different functional materials is very important and a challenging


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work for many applications in energy, catalysis, biological and biomedical aspects, sensing and water treatment.46-49 Duan et al. fabricated core-shell structured Au@Ag@polydopamine (PDA) catalysts loaded rGO paper for nitrophenol reduction.48 Accordingly, Zeng et al. proposed a simple strategy to prepare Fe3O4@PDA@Ag hybrid microspheres which showed better catalytic activity and fast adsorption/removal capacity for organic dyes.50 PDA functionalized gold nanohybrids were also prepared, however, the catalytic reduction performance of the nanohybrids for nitrophenol was not better than that of some catalysts reported previously.51 Chen et al. used mercaptoundecanoic acid modified PDA particles as the support of Pd nanoparticles which could efficiently catalyze 4-nitrophenol to 4-aminophenol.52 However, it is noted that many nanocatalysts fabricated through mussel-inspired chemistry did not exhibit high catalytic activity. In this work, we report the hydrothermal preparation of catechol-formaldehyde resin (CFR) coated Fe3O4 core-shell magnetic nanospheres with controlled shell thickness (Figure 1). CFR resin is chosen as a precursor because it has several outstanding advantages: First, catechol is a low cost monomer for CFR preparation through the facile procedure. Second, the two hydroxyl groups in vicinal positions of catechol have strong chelating ability toward metal ions. Third, the catechol is similar to mussel-inspired polydopamine that can adhere to organic and inorganic substrates.61 Especially, the superior catalytic activity may be achieved if the catechol groups on the nanospheres are combined with GO and metal nanoparticle catalysts because the graphene oxide (GO) with high charge mobility characteristics has strong interaction with aromatic compound substrates via π-π stacking effect.53 Inspired by this thought, GO wrapped magnetic Fe3O4@CFR nanospheres (Fe3O4@CFR@GO) also are easily constructed based on the adhesive properties of catechol groups of CFR shell. The magnetic Fe3O4@CFR and Fe3O4@CFR@GO


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hybrid nanospheres can be used as the robust catalyst supports to combine Pd nanoparticles by using the intrinsic reactivity of GO and catechol groups on the surface of nanospheres, which revealed excellent water dispersity and superior catalytic performance in the organic reaction including Suzuki cross-coupling reaction and the reduction of methylene blue (MB).

Figure 1. Illustration of the synthesis process for Fe3O4@CFR, Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs hybrid nanospheres.

EXPERIMENTAL SECTION Materials. Tiny graphene oxide (GO) with about 200 nm lateral size was purchased from JCNANO Technology CO., Ltd. (Nanjing, China). Fe3O4 nanoparticles were prepared according to the previous reported method.54 Palladium chloride, aryl bromide, arylboronic acid, methylene blue (MB) and catechol were obtained from Shanghai Macklin Reagent Co. Ltd. Ethylene glycol (EG), anhydrous ferric chloride (FeCl3), sodium acetate anhydrous, potassium carbonate


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and trisodium citrate were supplied by Sinopharm Chemical Reagent Co., Ltd. Ammonia solution (25 wt %) and formaldehyde (37-40 wt%) were obtained from Shanghai Chemical Reagents Company. Characterization. Fourier transform infrared (FTIR) spectra were collected by a Magna 560 FTIR spectrometer. Transmission electron microscopy (TEM) images were conducted on a JEM2100F electron microscope. The test sample was obtained through dispersing the catalysts in ethanol and dropping them onto a carbon support membrane, and the solvent was evaporated at room temperature. Magnetic properties of nanospheres were tested using a vibrating sample magnetometer (Daghigh Meghnatis Kashan Co., Kashan, Iran). UV-vis spectra were measured on a SHIMADZU UV-2550 spectrometer. The X-ray photoelectron spectroscopy (XPS) was conducted on a Quantum 2000 spectrometer with non-monochromatized Al Ka radiation source. Inductively coupled plasma atomic emission spectrometry (ICP-AES) was utilized to measure the Pd content of nanocatalysts (Thermo Jarrel Ash, Franklin, MA, USA). X-ray diffraction (XRD) spectra were obtained from a Rigaku D/max-TTR-III diffractometer with Cu Ka radiation. Synthesis of Fe3O4@CFR Core-Shell Nanospheres. Easily dispersed Fe3O4@CFR nanospheres were synthesized by using different ratios of catechol and formaldehyde solution. According to Table S1, 15.0 mg of Fe3O4 was dispersed into 20 mL of deionized water containing catechol and NH4OH solution by ultrasonication. The formaldehyde was put into above system, and the solution was then transferred into Teflon-sealed stainless-steel autoclave, and heated to a certain temperature for different times. The reaction product obtained after magnetic separation was washed with water and ethanol until the solution was clarified, and finally dried in vacuum.


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Preparation of Fe3O4@CFR@GO Hybrid Nanospheres. 15.0 mg tiny GO sheets and 15.0 mg of the as-prepared Fe3O4@CFR nanospheres (CFR-18 in Table S1) in 75 mL tris solution (pH=8.5) were dispersed under ultrasound for 45 min, respectively. Then, the aqueous suspension of Fe3O4@CFR nanospheres was dropwise added into above GO suspension within 20 min via ultrasound, and then allowed to stir for 24 h at 25 oC. The final sample was obtained with the help of a magnet, followed by washing several times with deionized water and ethanol to remove unadhered GO sheets, dried for 12 h in vacuum. Synthesis of Fe3O4@CFR@PdNPs Hybrid Nanospheres. Fe3O4@CFR@PdNPs hybrid nanospheres were fabricated by a modified method. 50 mg of Fe3O4@CFR spheres (CFR-18 in Table S1) were dispersed into 50 mL absolute ethanol under ultrasound for 30 min, and the formed black suspension was mixed with 3.7 mL PdCl2 solution (0.01 M) by ultrasonication for 1.0 h. The obtained mixture was continuously stirred at room temperature for 24 h without any reducing agent. The products were collected through a magnet, washed several times with deionized water and ethanol to remove any unloaded palladium nanoparticles and PdCl2, followed by drying for 12 h in vacuum. About 1.9 wt % of Pd content in the Fe3O4@CFR catalyst was determined via ICP-AES. Preparation of Fe3O4@CFR@GO@PdNPs Nanospheres. 50 mg of Fe3O4@CFR@GO nanospheres were dispersed into 50 mL absolute ethanol using ultrasonication for 30 min. 3.7 mL PdCl2 (0.01 M) solution was mixed with above suspension under ultrasonication. And the reaction solution was continuously stirred at room temperature for 24 h without any reducing agent. The products were collected using a magnet, followed by washing several times with deionized water and ethanol to remove any unloaded palladium nanoparticles and PdCl2, dried


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for 12 h in vacuum. About 2.2 wt % of Pd content in Fe3O4@CFR catalyst was determined via ICP-AES. Catalytic Reduction Reaction of Methylene Blue (MB). The catalytic performance of different catalysts (Fe3O4@CFR@Pd NPs and Fe3O4@CFR@GO@PdNPs) for the reduction of MB to leuco methylene blue was evaluated. In detail, fresh NaBH4 solution (1.0 mL, 0.5 M) and MB aqueous solution (2.0 mL, 5 mg L-1) were added to a quartz cell. After the mixture solution was immediately agitated for 10 s, twenty microliters of catalysts Fe3O4@CFR@PdNPs (0.0167 mg mL-1) or Fe3O4@CFR@GO@PdNPs (0.0144 mg mL-1) was added to the aforementioned mixed solution. And then the solution was agitated again for 5 s. The catalytic reduction process was monitored using a UV-Vis spectrophotometer. To study the reusability of the catalysts, the used magnetic nanocatalysts were separated with a magnet from reaction solution when the reduction process was completed. The collected Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@ PdNPs catalysts were washed with water and ethanol three times respectively for the following cycle. Evaluation of Catalytic Performance for Suzuki Cross-Coupling Reactions.55,56 The reactants of aryl halide (0.5 mmol), phenylboronic acid (0.6 mmol), K2CO3 (1.5 mmol), and catalysts Fe3O4@CFR@PdNPs or Fe3O4@CFR@GO@PdNPs (0.14 mol % Pd) in deionized water (5.0 mL) were reacted for 24 h at 25oC under stirring. And the catalyst was separated using magnetic decantation, and the organic product was extracted with ethyl acetate (10 mL, three times). The reaction products were quantitatively analyzed by GC. For recycling catalysis experimental, the catalyst was separated from system by magnet. The collected sample was washed with lots of ethyl acetate and deionized water to remove adsorbed organics and salts for the next cycle.


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Catalytic Reduction Reaction of 4-Nitrophenol (4-NP). The catalytic activity measurements for Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs catalysts were also performed using model reduction reaction of 4-NP to 4-aminophenol (4-AP). Fresh NaBH4 solution (1.0 mL, 0.5 M) and 4-NP solution (1.5 mL, 10-4 M) were added to a quartz cell. After the mixture solution was agitated for 1.0 min, thirty microliters of Fe3O4@CFR@PdNPs (0.1 mg mL-1) or Fe3O4@CFR@GO@PdNPs (0.086 mg mL-1) catalyst was added into the above solution. The study on the reusability of Fe3O4@CFR@Pd NPs and Fe3O4@CFR@GO@PdNPs catalysts was the same as the catalytic reduction of MB.

RESULTS AND DISCUSSION Synthesis and Characterization of Fe3O4@CFR Nanospheres. As shown in Figure 1, Fe3O4@CFR core-shell magnetic nanospheres were prepared through in situ polycondensation of catechol and formaldehyde catalyzed by ammonium hydroxide using magnetic Fe3O4 NPs as the seeds under the hydrothermal condition without any surfactants. The Fe3O4 NPs (150 nm) are prepared in ethylene glycol via a solvothermal method using sodium citrate as electrostatic stabilizers (Figure S1a). The surface of magnetic Fe3O4 NPs made up of many small primary nanocrystals is not smooth, but is rough. It can be seen that the CFR shells are uniformly coated onto the Fe3O4 NPs, which should be attributed to following two reasons: (1) there is a strong coordination interaction between catechol groups and Fe-O moieties; (2) the spherical structure can reduce surface energy when the CFR shell grow on the surface of Fe3O4 NPs.41,44,45 We detailed studied the effect of different synthetic conditions on the structure and morphology of Fe3O4@CFR magnetic core-shell nanospheres. The shell thickness of Fe3O4@CFR core-shell nanospheres is strongly depending on the amount of reactants of formaldehyde and catechol.


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Table S1 lists the statistical analysis of shell thickness based on TEM images. As shown in Figure 2, when the ratio of catechol and formaldehyde is fixed, with the increase of their concentration in the reaction systems, the average shell size of Fe3O4@CFR core-shell nanospheres (Sample 1-6 in Table S1) increases from 0 to about 180 nm. In addition, the morphology of the nanospheres becomes more spherical and tough gradually with the increase of catechol and formaldehyde. Therefore, the feeding amounts of catechol and formaldehyde play a critical role in the preparation of well-defined core-shell structure.

Figure 2. TEM images of Fe3O4@CFR nanospheres synthesized from different amounts of formaldehyde and catechol: (a) 0.014 mL and 0.01g, (b) 0.035 mL and 0.025 g, (c) 0.07 mL and 0.05 g, (d) 0.14 mL and 0.1 g, (e) 0.179 mL and 0.125 g, (f) 0.21 mL and 0.15 g. Reaction conditions: Fe3O4 15.0 mg, water 20 mL, ammonia solution 0.075 mL, temperature: 160 oC and reaction time: 100 min.


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We also studied the effect of only one parameter of catechol or formaldehyde on the structure of Fe3O4@CFR core-shell nanospheres. As seen from Figure S2, the change of the morphology and the average shell thickness is similar to the above discussion. The average shell thickness of Fe3O4@CFR nanospheres (Sample 7-11 in Table S1) increases from 0 to about 80 nm depending on the change of catechol from 0.01 to 0.16 g when the formaldehyde amount is fixed. The formation of this structure is because the CFR molecules could reduce the surface energy of nanostructure. From Figure S3, the average shell thickness of Fe3O4@CFR nanospheres increases from about 10 to 100 nm depending on the increasing content of formaldehyde solution from 0.01 to 0.14 mL (Sample 12-16 in Table S1). However, when the added quantity of formaldehyde is excessive (above 0.14 mL), there are no more catechols that can be used to generate CFR shell anymore. So the shell thickness of Fe3O4@CFR core-shell nanospheres is kept about 100 nm. In the same way, the morphology of the products also gradually was transformed into perfect spherical structure with the increase of catechol or formaldehyde. From the above results, the catechol and formaldehyde play a vital role in the generation of CFR polymer shell. The reaction temperature and time of hydrothermal synthesis of Fe3O4@CFR nanospheres are also the important parameters to control the shell thickness of nanostructures. As exhibited in Table S1 (Sample 17-22) and Figure S4, the CFR shell thickness increases from 0 to about 70 nm as reaction temperature increases from 80 to 180 oC. The coated CFR shells are comparatively loose and thin at a relatively low temperature of 100 oC. However, when the temperature exceeds 160 oC, the thickness of CFR shell remains almost unchanged. The reason should be that the conversion of reactants to CFR polymers reaches the maximum at a relatively high temperature, and therefore the thickness of CFR shell remains nearly unchanged. Compared


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with the reaction temperature, the reaction time also exhibited a similar effect on the coating process of Fe3O4@CFR core-shell structure (Sample 23-28 in Table S1). The CFR shell thickness increases from 0 to about 65 nm with the reaction time ranging from 20 to 120 min, and it remains no change until the reaction time extends to a certain degree (Figure S5). The thin CFR shell at its early formation stage with high roughness may be attributed to the early formed oligomers or polymers scatteredly assembled onto the magnetic Fe3O4 NPs. As the reaction time was extended, the shell kept growing and became smooth gradually. According to the above results, it can be seen that the reaction time and temperature has no obvious effect on size distribution of nanosphere as compared with the dosage of catechol and formaldehyde. But it also has an influence on the formation of Fe3O4@CFR nanospheres. Thus, we could control the size and morphology of Fe3O4@CFR nanospheres easily by changing the dosage of formaldehyde and catechol, reaction time and temperature. Fabrication and Characterization of Fe3O4@CFR@PdNPs, and Fe3O4@CFR@GO@ PdNPs Hybrid Nanospheres. The as-prepared Fe3O4@CFR core-shell nanospheres are also allowed to be encapsulated by GO using mussel-inspired chemistry based on the CFR shell with catechol groups like polydopamine in alkaline solution (Figure 1). The Pd nanoparticles (NPs) can also be generated via in situ reduction on the surface of nanospheres to efficiently construct Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs hybrid nanocatalysts by utilizing the surface coordination of vicinal hydroxyl groups for Fe3O4@CFR and adsorption capacity of GO on Fe3O4@CFR@GO nanospheres.20,28,57 The lateral size of GO used to fabricate Fe3O4@CFR@GO nanospheres is about 200 nm (Figure S1b). The morphologies of the asprepared Fe3O4@CFR, Fe3O4@CFR@GO, Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@ PdNPs samples were fully detected by TEM (Figure 3). The CFR shell thickness of Fe3O4@CFR


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nanospheres used to fabricate Fe3O4@CFR@GO is about 15 nm (Figure 3a). When the surface of Fe3O4@CFR spheres is wrapped by GO, a distinct thin layer (2-5 nm) of material (GO) is observed in Figure 3b and Figure S6. which indicates that the catechol groups on the CFR shell of nanospheres can effectively adhere to the GO by two hydroxyl groups in its vicinal positions. Actually, the overall morphologies of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs samples have no considerable change after the palladium particles adhere to the hybrid nanosphere surface (Figure 3c and d). But it can be observed from Fe3O4@CFR@PdNPs that many palladium particles are homogeneously anchored to the inside and surface of CFR shell with a small size of 2-4 nm in diameter (Figure 3c). However, the relative large palladium particles (3-6 nm) are immobilized onto GO surface of Fe3O4@CFR@GO@PdNPs nanospheres as compared with that of the CFR shells (Figure 3d). The formed palladium particles on the shell of Fe3O4@CFR spheres are smaller and more uniform resulting from the catechol coordination to metal particles, but the palladium particles grow bigger and especially their size distribution is wider onto the Fe3O4@CFR@GO because the GO layer has a relatively weak interaction with metal particles as compared with that CFR shell of Fe3O4@CFR. The above results illustrate Pd nanoparticles have been successfully in situ formed onto the Fe3O4@CFR or Fe3O4@CFR@GO magnetic nanospheres. The Pd contents were determined to be 1.9 and 2.2 wt% in the two samples of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs by ICP, respectively. The surface chemical information of the samples was further identified by XPS analysis. The signals of Fe2p, C1s, O1s and Pd3d can be clearly observed in the XPS survey scans of different samples (Figure S7 and S8). The binding energies for Fe 2p3/2 and Fe 2p1/2 in Fe3O4 are found at 711.3 and 724.8 eV respectively (Figure S7b), indicating that the oxidation state of Fe3O4 exists in the samples. The C1s spectrum (Figure 3e) of Fe3O4@CFR displays three peaks


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at 287.9 (C=O), 286.4 (C-O) and 284.7 eV (C-C), respectively.58 However, four peak components for O-C=O, C=O, C-O and C-C bands can be clearly observed at 288.8, 288.0, 286.5 and 284.6 eV in the C1s spectrum (Figure 3f) of Fe3O4@CFR@GO, which is very similar to that of GO (Figure S7d), and the peak of C-C becomes dominant as compared with that of Fe3O4@CFR, indicating the immobilization of GO on Fe3O4@CFR nanospheres.59 From Figure S8b and d, the splitting patterns of Pd 3d3/2 and Pd 3d5/2 XPS spectra of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs display two series of doublet peaks (337~342.3 eV and 334.8~340 eV ) corresponding to Pd(II) and Pd(0) species, respectively. Especially, the existence of Pd(0) demonstrates that the deposited Pd NPs belong to zero oxidation state, further indicating that the Pd NPs are loaded at the surface of Fe3O4@CFR and Fe3O4@CFR@GO spheres.60


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Figure 3. TEM images of (a) Fe3O4@CFR, (b) Fe3O4@CFR@GO, (c) Fe3O4@CFR@PdNPs and (d) Fe3O4@CFR@GO@PdNPs nanospheres. XPS C1s core-level spectra of (e) Fe3O4@CFR and (f) Fe3O4@CFR@GO nanospheres. The chemical structures of the as-prepared nanohybrids were ascertained via FTIR and XRD spectra (Figure S9). The characteristic absorption bands at 592 cm-1 assigned to Fe-O bonds of Fe3O4 NPs are observed in different samples.54,59 The bands located at 1460-1600 and 3420 cm-1 belong to the characteristic vibrations of benzene rings and O-H of the CFR shells, respectively.58 The band at 1270 cm-1 is from aromatic C-O stretching vibration for catechol


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groups in [email protected],62 The above result also suggests that CFR polymers have been grafted onto the Fe3O4 surface successfully. Compared with Fe3O4@CFR, a broad absorption band at about 3330 cm-1 belongs to O-H stretching vibration

on the surface of GO for

Fe3O4@CFR@GO, confirming the existence of GO around CFR shell.20,58 After the Pd nanoparticles are immobilized onto Fe3O4@CFR or Fe3O4@CFR@GO nanospheres, there is almost no change in the FTIR curves of Fe3O4@CFR@PdNPs and Fe3O4@CFR@ GO@PdNPs probably because of the low Pd loading content. For XRD patterns of different samples, a series of characteristic diffraction peaks indexed to different planes of cubic Fe3O4 phase (JCPDS 190629) are observed at 62.8, 57.1, 53.8, 43.3, 35.5 and 30.2 o.61,62 After coating the Fe3O4 NPs with CFR polymers, the XRD spectrum of Fe3O4@CFR provides a broad peak at 2θ=20-30° corresponding to CFR amorphous polymers. When Pd particles are further formed on Fe3O4@CFR and Fe3O4@CFR@GO, the XRD patterns reveal three new prominent Bragg reflections at 2θ =67.8, 46.1 and 39.8° assigned to (220), (200) and (111) planes of cubic Pd particles, which demonstrates that the Pd NPs are loaded at the surface of Fe3O4@CFR and Fe3O4@CFR@GO spheres.49,63 The above results also indicate that Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs have been successfully constructed by our facile method.


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Figure 4. Successive reduction reaction of MB using catalysts (a) Fe3O4@CFR@GO@PdNPs (20 μL), (b) Fe3O4@CFR@GO@PdNPs (10 μL), (c) Fe3O4@CFR@PdNPs (20 μL) and (d) Fe3O4@CFR@PdNPs (10 μL). (e) ln (ct/c0) as a function of reaction time (t) for different catalysts. (f) The reusability of the catalysts for MB reduction reaction.

Catalytic

Activity

of

Fe3O4@CFR@GO@PdNPs

and

Fe3O4@CFR@PdNPs

Nanospheres. The catalytic activity of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs nanohybrids as the newly developed catalysts were checked by choosing the reduction reaction of a common MB dye which can be transformed into leuco methylene blue (LMB) as a model


19


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reaction.60,64,65 When 20 μL of Fe3O4@CFR@PdNPs (0.0167 mg mL-1) or Fe3O4@CFR@GO@ PdNPs (0.0144 mg mL-1) were injected into the blue mixed solution containing MB and NaBH4, the blue solution instantly became colorless and transparent through macroscopic observation. Figure 4a-d show the UV-vis spectra for MB reduction reaction catalyzed by using Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs nanospheres as catalysts, respectively. It is apparent from the UV-vis spectra that the characteristic absorption band of MB at 662.7 nm gradually drops with catalytic reaction time after adding the catalysts, accompanied by the emergence of another adsorption peak at 256 nm, indicating the reduction of MB to LMB based on the color change of reaction solution from blue to colorless (see inset in Figure 4a). The catalytic reduction of MB using different dosage of two catalysts was also studied. As expected, the catalytic reaction of MB dye is successful, and the reaction rate for Fe3O4@CFR@GO@ PdNPs catalyst (Figure 4a and c) is much faster than that of Fe3O4@CFR@PdNPs catalyst (Figure 4b and d). As a control experiment, the systems catalyzed with GO@Pd, Fe3O4@Pd, GO, Fe3O4@CFR and Fe3O4 were conducted with 0.05 mg mL-1 catalyst in the catalytic reaction of MB with a large excess of NaBH4 under the other same conditions (Figure S10). The Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs nanocatalysts show better catalytic activity as compared with these control samples (Figure S10a-e). The following kinetic equation is usually used to determine catalytic reduction rate with respect to MB concentration, denoted as ln(ct / c0) = -kt (Figure 4e).59,60 In addition, another parameter for turnover frequency (TOF) is usually utilized to evaluate the catalytic activity of catalyst. TOF is defined as the number of substrate molecules per mole of the metal in catalyst per minute or hour and the unit is min-1 or h -1

(for the calculated method of TOF, see Eq(1) in supporting information). Rate constants for


20

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Fe3O4@CFR@GO@PdNPs (20μL), Fe3O4@CFR@PdNPs (20 μL), Fe3O4@ CFR@GO@PdNPs (10 μL), and Fe3O4@CFR@PdNPs (10 μL) catalyzed systems are also determined to be about 23.58, 14.07, 5.82 and 2.47 min-1, respectively. Compared with Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs, as shown in Figure S10f, GO@Pd, Fe3O4@Pd, GO, Fe3O4@CFR and Fe3O4 as catalysts display relative lower reaction rates with the values of 0.540, 0.011, 0.007, 0.007 and 0.004 min-1, respectively. The result demonstrates that the nanocatalysts have excellent catalytic performance under the joint action of CFR, GO and Pd, and the CFR polymers exhibit a positive influence on the catalytic reaction. Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@ PdNPs catalysts display super high catalytic activity for MB reduction with TOF values of 5260 and 3156 min-1, respectively. To our knowledge, this is the highest TOF value as compared with that of previously reported systems in the reduction of MB (Table S2).60,66 The

high

catalytic

activity

for

MB

reduction

with

the

as-prepared

Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs catalysts should be ascribed to the favorable interaction of nanocatalysts with the substrate MB.60 First of all, the MB molecules have a positive charge, while the CFR shell or GO layers on the surface of catalysts possesses a negative charge at a broad range of pH values. Thus, the electrostatic attraction between MB and CFR or GO carriers results in rapid enrichment of the substrate toward the surfaces of the two kinds of catalysts, creating a locally concentrated layer of MB. And the reduction reaction occurs through the electron-transfer or electron relay systems that generate an intermediate redox potential between the donor and the acceptor on the surface of PdNPs.60 Secondly, the π-π interactions and van der Waals forces can also contribute to the interactions between MB and CFR or GO. Especially, for Fe3O4@CFR@GO@PdNPs catalyst, the GO attached to the CFR shell of Fe3O4@CFR nanospheres gives the catalyst a greater specific surface area, and this


21


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characteristic is conducive to enriching reactant molecules MB on the surface of catalyst via the intense π-π interactions.60,64,65 As a result, the reaction rate catalyzed by Fe3O4@CFR@GO@ PdNPs

is

faster

than

that

of Fe3O4@CFR@PdNPs. In addition, the as-prepared

Fe3O4@CFR@PdNPs or Fe3O4@CFR@GO@PdNPs catalysts with small size of PdNPs possess corner metal atoms and highly active edges, excellent water-dispersion stability and active interaction between PdNPs and CFR or GO supports. And these favorable features make the catalysts well-optimized in the heterogeneous catalysis. To

evaluate

the

reusability

of

catalysts,

the

Fe3O4@CFR@PdNPs

or

Fe3O4@CFR@GO@PdNPs nanospheres were separated using a magnet from the mixed systems for reusing in following cycle catalysis. The magnetization curves of Fe3O4, Fe3O4@CFR@Pd NPs and Fe3O4@CFR@GO@PdNPs are given in Figure 5a. It can be seen that almost no magnetic hysteresis loops were observed, confirming that all hybrid nanospheres exhibited a superparamagnetic behavior. The saturation magnetization values (Ms) of Fe3O4@CFR@ PdNPs or Fe3O4@CFR@GO@PdNPs nanospheres are measured to be 48.5 and 47.3 emu g-1. Compared with Fe3O4 (Ms: 75.5 emu g-1), despite the relative low Ms, the two nanocatalysts still possess sufficient magnetic separation characteristics (see inset in Figure 5a). Thus, the Fe3O4@CFR@PdNPs or Fe3O4@CFR@GO@PdNPs catalysts can be reused repeatedly with an efficient catalytic activity after recycling seven times with a super high conversion ( > 98%) (Figure 4f).


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Figure 5. (a) Magnetization curves of Fe3O4, Fe3O4@ CFR, Fe3O4@CFR@GO nanospheres. (b) Recycling results of the Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs catalysts for Suzuki cross-coupling reaction. Reaction conditions: phenylboronic (0.6 mmol), iodobenzene (0.5 mmol), water (5.0 mL), K2CO3 (1.5 mmol), Fe3O4@CFR@GO@PdNPs or Fe3O4@CFR@ PdNPs catalysts (0.14 mol % Pd), 24 h, 25 oC.

We

further

tested

the

catalytic

activity

of

Fe3O4@CFR@

PdNPs

and

Fe3O4@CFR@GO@PdNPs as environmentally benign nanocatalysts for Suzuki coupling reaction of phenylboronic acid and bromobenzene under green reaction conditions in pure water at aerobic conditions (Table 1). To our excitement, the Fe3O4@CFR@GO@PdNPs catalyst is extraordinary high active and the conversion in water as a green solvent reaches > 99 % both in room temperature and 80 oC. Our designed catalysts present a higher catalytic efficiency in Suzuki cross-coupling reactions over other reported catalysts, especially at room temperature in pure water (Table S3). In addition, it can be found that Fe3O4@CFR@GO@PdNPs catalyst displays higher catalytic efficiency than Fe3O4@CFR@PdNPs catalyst in water, while the


23


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catalytic efficiency of Fe3O4@CFR@PdNPs catalyst is close to that of Fe3O4@CFR@GO@ PdNPs catalyst in the catalytic system with water and ethanol as mixed solvent at 80 oC. This catalytic characteristic should be ascribed to the following two features of nanocatalysts: (1) Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs catalysts are relatively homogeneous in the mixed solvent, providing enhanced accessibility of reaction substrates to metal catalytic center at molecular level. However, the thin layer of GO on the surface of catalyst Fe3O4@CFR@GO@PdNPs is closer to the water molecules and has better dispersion than that of Fe3O4@CFR@PdNPs catalyst. (2) The π-π interactions between reaction substrates and CFR shell or GO layer of nanosphere catalysts can encourage them to better contact with each other. But the π-π interaction between aromatic reactant molecules and GO with larger surface area is more strong. Thus, the above reasons resulted in a higher conversion rate in Suzuki crosscoupling reaction for the Fe3O4@CFR@GO@PdNPs catalyst with GO layers as a support of PdNPs. The recyclability of Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs catalysts was also studied for the Suzuki coupling reaction. The recovered nanocatalysts can be reused for five times without a noticeable loss in catalytic efficiency, especially for Fe3O4@CFR@GO@ PdNPs catalyst with 90% conversion under each cycle (Figure 5b). The result also suggests that the catalytic efficiency of Fe3O4@CFR@GO@PdNPs nanocatalyst is far superior to that of Fe3O4@CFR@PdNPs catalyst.


24

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Table 1. Influence of Nanocatalysts, Solvent and Reaction Condition on Suzuki CrossCoupling Reactions.a Br

a

+

B(OH)2

Catalyst, K2CO3 Reaction Conditions conversion (%) c

entry

solvent

catalyst

reaction conditions

1

H2O

Fe3O4@CFR@GO@Pd (0.14 mol% Pd)

RT, 24h

99.2

2

H2O

Fe3O4@CFR@Pd (0.14 mol% Pd)

RT, 24h

91.6

3

H2O


80 oC, 2h

99.2

4

H2O


80 oC, 2h

94.2

5

EtOH/H2O b


RT, 24h

96.7

6

EtOH/H2O


RT, 24h

84.9

7

EtOH/H2O


80 oC, 2h

99.9

8

EtOH/H2O


80 oC, 2h

98.3

Reaction conditiond: bromobenzene (0.5 mmol), phenylboronic (0.6 mmol), K2CO3 (1.5 mmol), water (5.0 mL). b The volume

ratio of EtOH/H2O is 1:1. c Conversion was measured by GC analysis.

To evaluate the applicability of the as-fabricated nanocatalysts, the catalytic performance of Fe3O4@CFR@GO@PdNPs was investigated by Suzuki cross-coupling reaction of various substituted aryl halides with phenylboronic acid (Table S4). The Fe3O4@CFR@GO@PdNPs displays high catalytic efficiency for the reaction of phenylboronic acid and aryl iodides to produce desired cross-coupled products with excellent reaction conversions (Table S4, entries 912). The highest reaction conversion (99.7%) is obtained for the reaction of phenylboronic acid and 4-iodobenzonitrile to afford cyano-biphenyls (Table S4, entry 12). Interestingly, the aryl


25


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iodides with electron-donating groups (para -CH3 and -OCH3) still have good reaction conversions which dropped slightly as expected. In addition, the activity of the nanocatalyst in the presence of aromatic chlorides was studied, and the biaryl conversions are relatively lower than that of aryl bromides and iodides. On the whole, the above results reveal that the designed nanocatalyst Fe3O4@CFR@GO@PdNPs has good catalytic performance for Suzuki coupling reactions. To clarify the impact of each component on catalytic reaction, a series of control experiments were conducted with different catalysts including GO@Pd, Fe3O4@Pd, Fe3O4@CFR, Fe3O4 and GO. As exhibited in Table S5, the reaction did not take place in the presence of Fe3O4@CFR, Fe3O4 and GO as catalysts without Pd (Table S5, entries 5-7). The result shows that these nanomaterials can not catalyze the reaction without a metallic catalyst. In addition, when using GO@Pd and Fe3O4@Pd as catalysts, the catalytic reaction can proceed and has relative low conversions of 62.0 and 55.9 %, respectively (Table S5, entries 3 and 4). Hence, this result indicates that the catalysts containing metal palladium can catalyze Suzuki coupling reaction. However, GO@Pd has a better conversion than Fe3O4@Pd, illustrating that the combination of the GO and Pd can synergize the catalytic reaction. Furthermore, it can be seen that the catalytic conversions of GO@Pd and Fe3O4@Pd are relatively lower than that of Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs. Thus, the high catalytic performance of Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs nanocatalysts in Suzuki cross-coupling reaction still should contributed to their excellent water-dispersion stability and synergistic interaction between PdNPs and CFR or GO supports. Moreover, the ICP analysis was used to evaluate the leaching of metal Pd in reaction system after four cycles. The result shows the


26

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leakage of 0.16 ppm Pd (0.1% Pd loss in total amount of catalyst) in reaction system, suggesting good stability of the nanocatalyst with very small leached amount of Pd after four catalysis cycle.

Figure 6. UV-vis spectra of 4-NP reduction using catalysts (a) Fe3O4@CFR@GO@PdNPs ((30 μL)) and (b) Fe3O4@CFR@PdNPs

(30

μL).

(c)

ln (ct/c0) as a function of reaction time (t) for

Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs catalysts. (d) The reusability of Fe3O4@CFR@GO@ PdNPs and Fe3O4@CFR@PdNPs catalysts in 4-NP reduction reaction.

Another model catalytic reaction for the reduction of 4-nitrophenol (4-NP) was also studied using the as-prepared Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs as catalysts and NaBH4 as reducing agent. Uv-vis spectra in Figure 6 a and b display the reduction process of 4NP with reaction time using the two nanocatalysts. It can be seen that upon adding NaBH4, the


27


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maximum absorbance band at 400 nm attributed to 4-nitrophenolate ions formed by the action of alkaline NaBH4 gradually weakens and completely disappears after about 150 s and 300 s for Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs catalysts. As shown in Figure 6c, the rate constants for 4-NP reduction are determined to be 2.458 and 0.956 min-1 for Fe3O4@CFR@GO@ PdNPs and Fe3O4@CFR@PdNPs catalysts, respectively. This result indicates that the Fe3O4@ CFR@GO@ PdNPs has higher catalytic activity than Fe3O4@CFR@PdNPs for the reduction of nitrophenol. For comparison, the reduction ability for 4-NP was also evaluated by using different control catalysts GO@Pd, Fe3O4@Pd, GO, Fe3O4@CFR and Fe3O4 (Figure S11). However, for the control catalysts GO, Fe3O4@CFR and Fe3O4, the characteristic absorption intensity of 4-NP at 400 nm almost unchanged within 10 min, which suggests the three catalysts in the absence of metallic catalyst have very low catalytic activity. The TOF values are also calculated to be 6720 h-1 for Fe3O4@CFR@GO@PdNPs and 2800 h-1 for Fe3O4@CFR@PdNPs based on metal Pd contents in two nanocatalysts which provided higher TOF values than GO@PdNPs (TOF: 605.4 h-1) and Fe3O4@PdNPs (TOF: 7.9 h-1) catalysts. Thus, our as-designed nanocatalysts of Fe3O4@CFR@GO@PdNPs and Fe3O4@CFR@PdNPs exhibit very high catalytic performance for 4-NP reduction and presents an evident advantage over other reported catalysts (Table S6). In addition, the excellent reusability of the two nanocatalysts was obtained in 4-NP reduction due to their superparamagnetic behavior. From Figure 6d, almost identical catalytic performance of 4-NP for the two catalysts was observed from at least seven catalytic cycles.


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CONCLUSIONS In summary, the stable CFR coated magnetic core-shell nanospheres with controllable shell thickness have been successfully fabricated by a convenient and efficient route for the first time. The as-prepared magnetic Fe3O4@CFR@PdNPs and Fe3O4@CFR@GO@PdNPs recyclable nanocatalysts were demonstrated to possess an exceptionally rapid catalytic property for different model reactions including Suzuki cross-coupling reaction in pure water and the reduction of MB and 4-NP. Thus, the novel magnetic CFR based hybrid nanospheres as the stable support of high active metal nanoparticles can find more potential commercial applications for heterogeneous catalysis. Moreover, our strategy also provides a new opportunity to develop mussel-inspired catechol-based multifunctional nanohybrids with special adhesion properties for various applications across the chemical, materials, and biological sciences, as well as interdisciplinary fields.

ASSOCIATED CONTENT Supporting Information. Additional TEM image and XPS spectra of different materials; Properties and synthesis parameters of Fe3O4@CFR core-shell nanospheres; Substrate study for the Fe3O4@CFR@GO@PdNPs-catalyzed Suzuki cross-coupling reaction; and a comparison for the catalytic performance of different materials in Suzuki cross-coupling reactions and the reduction of MB and 4-NP. This material is available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * E-mail:

[email protected] (L.C.)

Author Contributions ║Y.N.

Zhang and Y. Yang contributed equally to this work.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (21574017).

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Graphical Abstract


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Mussel-Inspired CatecholâFormaldehyde Resin-Coated Fe3O4 Core

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