A General and Facile Method to Fabricate Yolk-like Structural

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A General and Facile Method to Fabricate Yolk-like Structural Magnetic Nanocatalysts Linfeng Bai, Wanquan Jiang, Mei Liu, Sheng Wang, Min Sang, Shouhu Xuan, and Xinglong Gong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b00291 • Publication Date (Web): 20 May 2018 Downloaded from http://pubs.acs.org on May 20, 2018

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ACS Sustainable Chemistry & Engineering

A General and Facile Method to Fabricate Yolk-like Structural Magnetic Nanocatalysts Linfeng Baia, Wanquan Jianga,*, Mei Liua, Sheng Wanga, Min Sanga, Shouhu Xuanb, Xinglong Gongb,* a

Department of Chemistry, Collaborative Innovation Center of Suzhou Nano Science and

Technology, University of Science and Technology of China (USTC), No. 96 Jinzhai Road, Baohe District, Hefei, Anhui Province, China 230026 b

CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern

Mechanics, USTC, No. 443 Huangshan Road, Shushan District, Hefei, Anhui Province, China 230027

Corresponding author: Tel: 86-551-63607605; Fax: 86-551-63600419. E-mail: [email protected] (W. Q. Jiang) [email protected] (X. L. Gong)

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KEYWORDS: yolk-like structure, magnetic nanocomposites, general method, catalysts, reusability

ABSTRACT: A general and facile method for synthesis of yolk-like nanocomposite composed of superparamagnetic core and mesoporous SiO2 (mSiO2) shell with functional nanoparticles (Pd, Au, Pt) uniformly dispersing on their inner wall is reported. Firstly, the magnetic particle (MP) was modified with resorcinol/formaldehyde resin (RF) layer. Then, the obtained MP@RF core-shell particle was decorated with functional metal nanoparticles such as Pd, Au, or Pt (defined as MP@RF-Pd, MP@RF-Au and MP@RF-Pt, respectively). To protect the metal nanoparticles, another hybrid CTAB/SiO2 (hSiO2) shell was constructed on the above particle through the surfactant-templated sol-gel process. After calcination, the yolk-like nanocomposites such as MP@Void-Pd@mSiO2, MP@Void-Au@mSiO2 and MP@Void-Pt@mSiO2 were successfully obtained. Because of the unique yolk-like structure, the as-prepared nanomaterials can be served as ideal nanoreactors for heterogeneous catalysis. The Heck reaction and the reduction of p-Nitrophenol (4-NP) were introduced as model reactions to evaluate the catalytic activities of the magnetic nanocatalysts. Benefitted from the high dispersity of noble metals, large specific surface area and the mSiO2 protective shell, all samples exhibited remarkable catalytic activity and reusability. After 7 cycles, the activities of MP@Void-Pd@mSiO2 and MP@Void-Au@mSiO2 could maintain as high as 98% and 81%, respectively.


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INTRODUCTION Noble metal nanoparticles, such as Ag, Au, Pd, Ru, and Pt etc, have received considerable attention due to their excellent electronic, photonic, and catalytic performance1-5. As an efficient catalyst, noble metal nanoparticles possess large surface area and high catalytic activity which are beneficial to promote chemical reactions, and the size of metal nanoparticles plays an important role in this process. Currently, the most common method for synthesizing metal nanoparticles is chemical reduction of noble metal ions in aqueous solution which contain different reductive and stabilizing agents6,7. However, the as-prepared noble metal nanoparticles are easily reunited and lost in catalytic reactions, resulting in the decreasing of catalytic property and poor reusability8. Taking into account the cost of metal nanoparticles, it is critically necessary to find an appropriate way to improve the stability and catalytic efficiency of the noble metal catalysts. Recently, the yolk-like structured nanocomposite with a movable core, interstitial hollow space and multifunctional shell has attached great interest because of its appealing structure and property9,10. The yolk-like structures can avoid the aggregation of the cores and prevent the cores exposing to reaction solution directly when the small molecules diffuse in or out of the shell. Moreover, because of the high surface area, large void space, and prominent loading capacity, this kind of materials possess promising application in lithium-ion batteries11, drug delivery12,13 and catalysis14-16. In the past several decades, researches paid a lot of efforts to fabricate various yolk−shell nanomaterials17-19. The defined function of the yolk-like structure could be controlled by varying the synthesis parameters such as the core and shell material, core size, void space and shell thickness. Up to now, various yolk-like structures with different properties, such as



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biocompatible and biodegradable features20,21, magnetism22,23, and super hydrophobicity24 have been synthesized. Due to superb environmental compatibility and recyclability, the magnetic materials are widely used and usually served as a core structure in sample preparation25,26. Recently, the magnetic yolk-like nanoparticles have attracted increasing interests because they can be expediently recovered and reused through the application of magnetic field. For example, the Do group27 prepared yolk-shell structured material with a Fe3O4 core and a mesoporous silica shell. The sample showed excellent catalytic reactivity and recycling efficiency in the removal of acetaminophen. Meanwhile, the shell plays an irreplaceable role in yolk-like structure. Many researchers have devoted great efforts to develop a protective shell for magnetic nanocomposites. At present, the common shell structures are made of silica or porous carbon materials28,29. For instance, Deng group have done lots of work about the combination of magnetic nanoparticles with porous silica through well-controlled interface co-assembly method which lead to yolk-shell microspheres with controlled structure and morphology30,31. Very recently, they developed a wonderful

plasmolysis-inspired

nanoengineering

strategy

to

synthesize

yolk−shell

Fe3O4@RF@Au-void@mSiO2 microspheres. The final product showed excellent catalysis efficiency for hydrogenation of 4-NP by NaBH432. The yolk-like structural catalysts with protective shell and magnetic property can effectively improve the stability, catalytic efficiency and recyclability of the noble metal catalysts. Due to the complex nanostructure, there is still urgent for developing an easy and general method to synthesize multifunctional yolk-like nanocomposites toward nanocatalysis. In this work, we reported a general method to synthesize yolk-like structural magnetic nanocatalyst. The MP was prepared by solvothermal reaction, followed by depositing an RF


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layer. Then the noble metal ions (take Pd2+ as an example) were reduced by NaBH4 and anchored on the surface of RF. The above nanocomposites were encapsulated with a hybrid protective shell which was constructed through the hydrolysis and condensation of TEOS and CTAB. After calcination, MP@Void-Pd@mSiO2 nanocomposite with Pd nanoparticles filling in the cavity was synthesized. The nanocomposite shows a uniform yolk-like structure with metal nanoparticles dispersed in the inner surface. The present method can effectively avoid the aggregation and loss of noble metal nanoparticles during the catalytic process and promote catalyst separating from the reaction system by magnetic separation. More importantly, this approach can be extended to synthesize other multifunctional nanocatalysts with similar structure, such as MP@Void-Au@mSiO2 and MP@Void-Pt@mSiO2.

EXPERIMENTAL SECTION Materials Iron (III) chloride hexahydrate (FeCl3·6H2O), ethylene glycol (C2H6O2), diethylene glycol (C4H10O3), sodium acetate anhydrous (CH3COONa), polyacrylic acid (PAA, 1800), resorcinol (C6H6O2), formaldehyde solution (HCHO), ammonia solution (NH3·H2O), ethanol (EtOH), gold(III) chloride (HAuCl4), palladium(II) chloride (PdCl2), (hydro)chloroplatinic acid hexahydrate (H2PtCl6·6H2O), polyvinylpyrrolidone (PVP, 30 kDa), acetic acid (CH3CHOOH), citrate

dehydrate

(C6H5Na3O7·2H2O),

sodium

borohydride

(NaBH4),

hexadecyltrimethylammonium bromide (CTAB), tetraethoxysilane (TEOS), nitrobenzene (C6H5NO3), hydrazine hydrate (H4N2·H2O), iodobenzene (C6H5I), ethyl acrylate (C5H8O2), N,N-dimethylformamide (DMF), triethylamine (Et3N) and other chemicals were bought from



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Sinopharm Chemical Reagent Co. Ltd. All chemicals are analytical grade and used without further treatment. Re-distilled water was used for all experiments. Synthesis of yolk-like MP@Void-M@mSiO2 Synthesis of MP@RF nanospheres. Firstly, the MP was synthesized via solvothermal reaction. Briefly, FeCl3·6H2O (1.08g), sodium acetate anhydrous (4.00 g) and PAA (0.10 g) were dissolved into the mixture with ethylene glycol (14 mL) and diethylene glycol (26 mL). Afterwards, the yellow mixture was transferred to the 50mL Teflon-lined stainless-steel autoclave and heated at 200°C for 12 h. Next, the obtained MP (0.14g) was dispersed into a 60 mL ethanol/water solution (V

ethanol:

V

water=1:2)

under sonication. Then, resorcinol (0.2 g),

HCHO (280 µL) and NH4OH (0.3 mL) were added into the dispersion and maintain mechanical stirring at 30°C for 24 h. The obtained slurry was transferred to the Teflon-lined stainless-steel autoclave and heated at 100°C for 24 h. The products of core-shell MP@RF nanospheres were collected by a magnet. Synthesis of MP@RF-M nanospheres. 20 mL of aqueous solution containing MP@RF (20 mg) and PdCl2 (6 mg) was placed in a flask under vigorous stirring for 6 h. Then, the cold NaBH4 (0.1 mL, 0.1M) solution was added to the mixture. The yellow solution turned to black rapidly. After 2 h, the MP@RF-Pd was separated by magnet and washed with ethanol and water. The MP@RF-Au was synthesized via an ordinary electrostatic attraction method8. For MP@RF-Pt, the extremely small Pt nanoparticles were prepared by alcohol reduction and stabilized on the surface of MP@RF by sonication33. Synthesis of MP@Void-M@mSiO2 nanospheres. Typically, the as-synthesized MP@RF-M (15 mg) was dispersed in the mixture containing ethanol (110 mL), re-distilled water (10 mL), NH3·H2O (3 mL) and CTAB (0.1 g). Subsequently, TEOS (0.1 mL) was added dropwise and the


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reaction was kept under sonication at 30°C for 1 h. The resultant of MP@RF-M@hSiO2 was separated using a magnet and washed with ethanol and water. Finally, the dried MP@RF-M@hSiO2 was calcinated at 600°C for 6 h in air to remove CTAB and RF. The obtained yolk-like magnetic nanospheres loaded with metal nanoparticles were expressed as MP@Void-M@mSiO2. Catalytic properties of the MP@Void-M@mSiO2 The Heck coupling reaction was chosen to study the catalytic properties of the MP@Void-Pd@mSiO2. The reaction was carried out in a 25 mL Schlenk tube with N2 protection. Iodobenzene (56 µL), ethyl acrylate (109 µL), Et3N (139 µL), DMF (2 mL) and MP@Void-Pd@mSiO2 (10 mg) were added to the reactor and the reaction system was conducted at 120°C for 3 h. After completion of reaction, the productivity was quantified by GC-MS analysis. The catalytic activity of MP@Void-Au@mSiO2 was explored by the reduction reaction of 4-NP. Typically, a certain amount of the MP@Void-Au@mSiO2 (1 mg, 1.5 mg, 2 mg) was uniformly dispersed in 4-NP solution (50 mL, 1×10-4 M). After that, the NaBH4 (0.04g) was added rapidly and the bright yellow mixture faded gradually. The reaction process was monitored by UV-vis spectroscopy. At the end of reaction, the catalyst can be separated from the solution by a magnet and reused in the next catalytic reaction. Characterization The structure and size of materials were determined by the field emission transmission electronic microscopy (FETEM, JEM-2100F) with an accelerating voltage of 200 kV and the surface features of products were observed by the field emission scanning electron microscope (FESEM, JEM-500, 20 kV). X-ray powder diffraction patterns (XRD) data were obtained by



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X-ray diffractometer (XRD, Smartlab, Rigaku, Japan). The surface area of materials was calculated by Brunauer-Emmett-Teller (BET) method. The pore volume and pore size distribution were determined using the Barrett–Joyner–Halenda (BJH) model. To investigate the surface

composition

of

samples,

an

X-ray

photoelectron

spectrophotometer

(XPS,

ESCALAB250, Thermo-VG Scientific, UK) with a monochromatized Al Kα radiation (1486.92 eV) was used. The Nicolet Model 759 Fourier transform infrared (FT-IR) spectrometer was used to measure the infrared (IR) spectra of the samples in the wavenumber range 4000-400 cm-1 with a KBr wafer. Thermogravimetric (TG) analysis was carried out on a DTG-60H thermogravimetric instrument from room temperature to 700°C at the rate of 10°C min -1 in air. The UV-vis spectra were recorded by using TU-1901 spectrophotometer. The productivity of Heck coupling reaction was analyzed by gas chromatography mass spectrometers (GC-MS), which was performed on an Agilent Technologies 5957C inert MSD with triple-axis detector. The content of Pd nanoparticles remained in the solution was analyzed using inductively coupled plasma mass spectrometer (ICP-MS). RESULTS AND DISCUSSION Synthesis and characterization of yolk-like MP@Void-M@mSiO2


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Scheme 1 The synthesis procedure for the MP@Void-Pd@mSiO2 nanocatalyst.

The overall synthesis procedure of yolk-like MP@Void-Pd@mSiO2 nanocatalyst is briefly depicted in Scheme 1. Firstly, the water dispersible MP is synthesized via a solvothermal method. Then, the well-defined core-shell structural MP@RF is obtained by an extended Stöber reaction34. The RF shell, which is formed by the polymerization of the resorcinol and formaldehyde in the alkaline solution, can directly cover the surface of MP because of the complexation between phenol groups and MP. Subsequently, the Pd nanoparticles are successfully assembled onto the surface of MP@RF by impregnating the particles in PdCl2 aqueous solution, followed by a simple NaBH4 reduction process. Next, the hSiO2 protective shell is further deposited on the MP@RF-Pd composites through the surfactant-templated sol-gel process using TEOS as the silica precursor and CTAB as the soft template. Finally, the middle RF layer and CTAB template are removed simultaneously by calcination at 600°C and the



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yolk-like MP@Void-Pd@mSiO2 is obtained. As the illustration of the scheme 1, it can be known that the frequently-used methods such as solvothermal method, Stöber method and sol-gel process, are facile and generally employed in nano-science. In addition, the cavity and mesoporous structures are achieved by one step, which simplifies the synthetic procedures. Consequently, the general and facile way is applied to prepare MP@Void-Pd@mSiO2 composites and it can be extended to manufacture yolk-like magnetic nanocatalyst loaded with different metal nanoparticles such as MP@Void-Au@mSiO2 and MP@Void-Pt@mSiO2.

Fig. 1 SEM images of as-prepared products obtained from each step. MP (a), MP@RF (b), MP@RF-Pd (c), MP@RF-Pd@hSiO2 (d), MP@Void-Pd@mSiO2 (e).

The scanning electron microscopy (SEM) is used to characterize the surface morphology of products in different stages. As shown in Fig. 1, panoramic view reveals that all samples are


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well-dispersed and exhibit uniform spherical morphology without any large aggregation. It is clearly seen from Fig. 1a that the suspended MP adopts a spherical shape with a rough surface. Fig. 1b shows SEM image of the resulting hybrid MP@RF, in which the MP is uniformly coated with RF polymer layer. The obtained MP@RF nanospheres are well dispersed and exhibit smooth surface morphology. At a higher voltage, it is obvious that MP@RF-Pd nanospheres remain the well-defined core-shell structure with the Pd nanoparticles immobilized on the exterior surface (Fig. 1c). As shown in Fig. 1d, the Pd nanoparticles are difficult to be observed and the particles possess a thicker shell compared to MP@RF-Pd, which indicates that the hSiO2 layer has been successfully decorated on the surface of MP@RF-Pd. However, some MPs are not well dispersed, thus a few MP@RF-Pd products have two cores. After calcination in the air (Fig. 1e), the final products keep their nanostructure very well. There is no broken particle in the samples, indicating that the yolk-like nanostructure is stable. Based on the above analysis, it can be found that all samples are well dispersed and their structure is very stable.



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Fig. 2 TEM images of as-prepared products obtained from each step. MP (a), MP@RF (b), MP@RF-Pd (c), MP@RF-Pd@hSiO2 (d), MP@Void-Pd@mSiO2 (e).

The interior nanostructure and size of the samples synthesized in each step are investigated by TEM (Fig. 2). As can be seen from Fig. 2a, the as-synthesized MP is uniform and it can be well dispersed in water or ethanol solution. The average diameter of MP is about 210 nm and the size distribution is narrow. The MP is chosen as the magnetic core to prepare composite with core-shell structure. As shown in Fig. 2b, it is apparent that the uniform RF shell is well encapsulated on the surface of MP via a modified Stöber reaction and the average thickness of the RF shell is about 70 nm. Next, Pd nanoparticles can be tightly loaded on the surface of supports by using the in situ reduction method. The TEM image of Fig. 2c confirms that the Pd nanoparticles are successfully immobilized on the surface of MP@RF nanospheres. Of particular


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note is that the dispersability of Pd nanoparticles on the surface of MP@RF is uniform and no large aggregation is observed. The successful deposition of Pd nanoparticles on the RF shell can be put down to the essentials as follows. Firstly, there are a large number of functional groups in the RF shell, such as –OH and –CHO, which can adsorb and in situ reduce part of PdCl2

35

.

Secondly, after introducing NaBH4, all PdCl2 are reduced to form Pd nanoparticles. Due to high surface energy, the small Pd nucleuses generated in the solution are unstable and tend to gather with the small Pd nanoparticles on the surface of RF shell. Therefore, the metal nanoparticles would like to attach onto the surface of the RF shell. After the reaction, a few residual free Pd nanoparticles were presented in the solution. The concentration of the Pd nanoparticles remained in the solution is 3.8 µg/ml which is measured by ICP-MS. To further protect the attached Pd nanoparticles, another hSiO2 layer is deposited on the surface of MP@RF-Pd by using the surfactant-templated sol-gel process. The surfactant CTAB and silica oligomer from TEOS co-assembled in the presence of MP@RF-Pd seeds. As shown in Fig. 2d, the obtained nanospheres (MP@RF-Pd@hSiO2) exhibit well-defined dual-shelled core-shell structure and the average diameter of MP@RF-Pd@hSiO2 nanospheres is 480 nm. The Pd nanoparticles on MP@RF surface are completely encapsulated by hSiO2 shell, which will prevent the loss of metal nanoparticles in the catalytic process. After calcination at 600°C, the RF layer and CTAB are removed and final products (MP@Void-Pd@mSiO2) are shown in Fig. 2e. Obviously, the MP@Void-Pd@mSiO2 exhibits yolk-like structure and maintains the uniform spherical morphology. It is easily observed that a black dot (MP) existed in the inner hollow area and the inner cavity is distinctly revealed by the contrast between shell and hollow interior, which manifests that the as-synthesized MP@Void-Pd@mSiO2 shows a yolk-like nanostructure. The



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yolk-like structure of catalyst not only protects the functional nanoparticles primely but also possesses higher surface area and larger void space, which are favored in the future applications.

Fig. 3 TEM images of MP@Void-Pd@mSiO2 with different magnifications (a-c), size distribution of Pd nanoparticles on the inner surface of MP@Void-Pd@mSiO2 (d), HAADF STEM image of MP@Void-Pd@mSiO2 (e) and corresponding elemental mapping images (f).

The size and dispersity of Pd nanoparticles have a significant impact on the catalytic activity. To observe the distribution of Pd nanoparticles clearly, the TEM images with different magnifications are collected. As presented in Fig. 3a, the small black dots are Pd nanoparticles, which are loaded in MP@Void-Pd@mSiO2 nanospheres. It can be identified that all Pd nanoparticles are separated from each other (Fig. 3b and c). They are evenly dispersed on the cavity of MP@Void-Pd@mSiO2 and well encapsulated within the mSiO2 shell. The size distribution of Pd nanoparticles is calculated by 80 nanoparticles (Fig. 3d). On the basis of the


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statistical analysis, Pd nanoparticles show an actually narrow particle size distribution. The sizes mainly distribute at the range of 7-13 nm and the average diameter is about 8.6 nm. The high-angle annular dark-field (HAADF) STEM and element mapping are applied to acquire more precise information of structures and compositional distributions. As shown in Fig. 3e, the HAADF STEM images clearly reveal that the composite presents yolk-like nanostructure, which is determined by the element contrast in the number of atoms among core, shell and metal nanoparticles. The overlay of Si, Fe and Pd signals are consistent with corresponding HAADF STEM image (Fig. 3f).The above analysis, demonstrates that the nanoparticle consists of MP, mSiO2 and Pd as pre-designed. Moreover, high Fe signal is presented in the core area, while the Si signal is distributed throughout the overall nanoparticle and abundant in the edge, confirming that the MP core is surrounded by mSiO2 shell. Notably, there is a cavity in the nanocomposite and Pd signal is obvious in this region.



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Fig. 4 TEM images of yolk-like MP@Void-Au@mSiO2 (a, b) and MP@Void-Pt@mSiO2 (d, e) with different magnifications, elemental mapping images of MP@Void-Au@mSiO2 (c), MP@Void-Pt@mSiO2 (f).

The synthetic method for yolk-like MP@Void-Pd@mSiO2 can be extended to similar type nanocatalyst with different functional nanoparticles. Taking MP@Void-Au@mSiO2 as an example, the positively charged MP@RF is firstly obtained through sonication in the acetic acid/ethanol solution. Negatively charged, citrate-protected Au nanoparticles are prepared according to the reported approach36. Then, freshly synthesized Au nanoparticles are efficiently attached to the positively charged MP@RF surface via electrostatic attraction. Following synthetic steps are as same as that of preparing MP@Void-Pd@mSiO2 and the chemical state of samples in each stage is described concisely in Fig. S1. It can be found from Fig. S3a and d that the well-dispersed Au nanoparticles uniformly adhere to the surface of MP@RF. After coating with CTAB/SiO2 shell, the size of Au nanoparticles significantly increases, which indicates that there is an aggregation of the Au nanoparticles in the surfactant-templated sol-gel process (Fig. S3b and e). Finally, the MP@Void-Au@mSiO2 is obtained by calcination and it similarly exhibits well-defined yolk-like nanostructure with Au nanoparticles scattered in the cavity (Fig. 4a and b, Fig. S3c and f). The element mapping images are presented in Fig. 4c and Fig. S4. It is obvious that the signals for Si, Fe, and Au can all be easily observed from the sample region, which is in accordance with relevant HAADF STEM image (Fig. S4). Above results indicate that the MP@Void-Au@mSiO2 is successfully prepared as pre-designed. Compared Fig. 4b with Fig. 3b, the morphology of MP@Void-Au@mSiO2 is slightly different from MP@Void-Pd@mSiO2. In the synthesis, the hSiO2 protective shell was synthesized through the surfactant-templated


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sol-gel process using TEOS as the silica precursor and CTAB as the soft template. As we know, there are hydrolysis and condensation steps in the formation of SiO2. The TEOS precursor adsorbed on the MP@RF-M nanospheres was hydrolyzed by NH3·H2O, generating the negatively charged silica oligomers which can co-assemble with CTAB to form mesostructure nanocomposites37. The size of Au nanoparticles loaded on the surface of MP@RF is not very uniform and the size distribution calculated from above method is shown in Fig. S5. The size principally distributes at 5-15 nm and the average diameter is around 9.8 nm. Moreover, citrate-protected Au nanoparticles are negatively charged which leads to a bit rougher surface of MP@Void-Au@mSiO2.

In

addition,

the

pore

structure

is

examined

by

nitrogen

adsorption-desorption test and the BET analysis results shown in Fig. S6. The samples can be collected simply by magnet because of the strong magnetism (Fig. S7). Furthermore, Pt nanoparticles can also be fabricated to substitute Pd nanoparticles for synthesizing Pt-based magnetic yolk-like nanocomposites. The TEM images of corresponding products from each step are introduced in Fig. S3. The Pt nanoparticles with small grain sizes (about 2.6 nm) are synthesized by the modified ethanol reduction method33. Fig. S3g presents that the MP@RF-Pt nanospheres keep core-shell structure well and are uniformly dispersed on the Cu grid. From Fig. S3j, it can be seen that the Pt nanoparticles are secluded and randomly distributed on the RF shell without apparent agglomeration. In the subsequent synthesis process, all samples show pre-designed morphology without the collapse of the structure. The Pt nanoparticles maintain original shape and are encapsulated by another protective silica shell (Fig. S3h, i, k and l). As exhibited in Fig. 4d and e, the yolk-like MP@Void-Pt@mSiO2 is prepared successfully in the same way. Similarly, the signals for Si, Fe, and Pt can all be easily found from the sample region in the mapping results (Fig. 4f and Fig. S8). The XPS analysis further



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proves the successful fabrication of MP@Void-Pt@mSiO2 (Fig. S9). From the above discussion, the synthetic method is universal and facile which is suitable to prepare yolk-like magnetic nanocatalysts loaded with functional nanoparticle (Pd, Au, Pt) in their cavities.

Fig. 5 Nitrogen adsorption-desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plot (inset) of MP@RF-Pd@hSiO2 and MP@Void-Pd@mSiO2.

Nitrogen adsorption-desorption isotherms analysis is adopted to investigate the pore structural evolution of the nanocatalyst after calcination. As exhibited in Fig. 5, the nitrogen adsorption-desorption isotherms of MP@RF-Pd@hSiO2 and MP@Void-Pd@mSiO2 are significantly different. Apparently, the BET of MP@Void-Pd@mSiO2 is much higher than MP@RF-Pd@hSiO2. The results are mainly attributed to distinct cavity structure in the MP@Void-Pd@mSiO2, which is consistent with the TEM analysis, indicating that the RF layer and CTAB are removed successfully via calcination. Evidently, it is difficult to observe the pores in the silica shell from TEM images because the diameter of the pores is very small. However, it is can be deduced from the pore size distribution curves (in the inset of Fig. 5). Obviously, the MP@Void-Pd@mSiO2 has a fairly strong and narrow peak shape in mesoporous region. The mean pore size of MP@Void-Pd@mSiO2 is 2.8 nm, which is smaller than that of the metal


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nanoparticles in the cavity. Therefore, the silica shell can effectively prevent the metal nanoparticles separating from the supporter in the recycling process. Besides, the BET surface area and pore volume of MP@Void-Pd@mSiO2 are calculated to be as high as 536.6 m2/g and 0.37 m3/g, respectively. The porous nature is beneficial to enhance the catalytic property of the metal nanoparticles because of providing enough space for catalytic reaction.

Fig. 6 XRD patterns (a), XPS (b), FT-IR (c), TG (d) of as-prepared products obtained from each step.



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The crystalline structures of the as-prepared samples are identified by power X-ray diffraction (XRD) (Fig. 6a). It can be seen that the six characteristic peaks (2θ=30.4°, 35.6°, 43.3°, 53.7°, 57.3° and 62.7°) are related to (220), (311), (400), (422), (511) and (440) reflection planes of the face-centered cubic Fe3O4, respectively. The thin RF layer is non-crystalline, thus the new peak in MP@RF is attributed to a small part of Fe2O3 transformed from Fe3O4. After loading Pd nanoparticles loading, a new diffraction peak is observed in spectrum at 2θ=40°, which is corresponded to Pd (111) crystalline plane. In spectra of MP@RF-Pd@hSiO2, it is clearly seen that a broad peak (2θ=22°) is appeared, demonstrating the formation of SiO2 protective shell. Besides, the Pd peak can be recognized directly with intensity weakening. Compared with MP@RF-Pd@hSiO2, the Fe3O4 core in MP@volid-Pd@mSiO2 is partly oxidized into Fe2O3 via air calcination. Because of low content ratio in samples, no Pd peak is observed in spectrum. The chemical state of surface elements in different materials is determined by XPS (Fig. 6b). For MP sample, O1s (531 eV) and Fe (57 eV, 712 eV) peaks are obvious in spectra, and the small C1s peak at 286 eV can be ascribed to residual ethanol or reagent which was not washed cleanly. With RF coating on the surface of MP, the intensity of O1s peak become weak while C1s peak value increases. The result reveals that the MP core is covered with RF. The Pd peak at 336 eV is distinctly identified in XPS spectrum of MP@RF-Pd sample demonstrating that Pd is loaded successfully on the MP@RF, which is consistent with XRD analysis. The corresponding XPS spectrum of Pd 3d is presented in Fig. S10, in which two typical peaks at around 336.3 eV and 340.5 eV are in accordance with the Pd 3d5/2 and Pd 3d3/2 core levels of metallic Pd38. The presence of SiO2 protective shell lead to no Pd peak being detected in MP@RF-Pd@hSiO2 spectrum, but two new peaks corresponded to Si2p peaks of SiO2 are generated. After calcining


Page 21 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


in air, the sample had no significant changes in addition to the relative content of the elements, whose result implies that the structure of material does not collapse during calcination. Fig. 6c shows the FT-IR spectra of each sample, in which the peak at 589 cm-1 corresponds to the stretching mode of Fe-O, the peak at 1616 cm-1 assigns to the aromatic groups, and the peak at 2920 cm-1 represents the –CH2- groups. The above results indicate that the RF shell coats on the MP core successfully. The metal is impracticable to be analyzed by FT-IR but can be certified via TG. As shown in Fig. 6d, the final weight losses of MP@RF and MP@RF-Pd are 66.2% and 58.1%, respectively. The weight loss of MP@RF-Pd is smaller than MP@RF because the Pd nanoparticles which attached on MP@RF surface change a little between 25°C and 700°C. In spectrum of MP@RF-Pd@hSiO2, the peaks located at 795 cm−1, 1080 cm−1, 461 cm−1 can be attributed to the symmetric stretching vibration (Si-O-Si), the asymmetric stretching vibration (Si-O-Si) and the Si-O-Si bending mode. The other two peaks at 2854 cm-1 and 2900 cm-1 are assigned to the –CH2– group in CTAB. After carbonization, the characteristic peaks of CTAB are disappeared, manifesting that the CTAB is removed absolutely.

Fig. 7 M-H curves of as-prepared products obtained from each step and the photograph of the magnetic separation process (inset).



Page 22 of 38

The hysteresis loop of each product is presented in Fig. 7. The saturation magnetizations of the MP, MP@RF, MP@RF-Pd, MP@RF-Pd@hSiO2, and MP@Void-Pd@mSiO2 at an external magnetic field of 30 kOe are 67.3, 22.5, 19.5, 10.3, and 10.3 emu/g, respectively. Therefore, this composite immobilized with Pd nanoparticles can be separated and recycled from reaction systems by easy and high-efficiency method. Compared to MP, the saturation magnetization of final product is significantly reduced since the MP has a protective shell on the outside and MP core is partly oxidized into Fe2O3 during the preparation. Moreover, all products show superparamagnetic property, which is beneficial to the redispersion of the catalysts in the solution without serious aggregation. The separation schematic diagram in Fig. 7 intuitively illuminates the effective separation and redispersion process of MP@Void-Pd@mSiO2 under an external magnetic field. The catalytic properties of the as-prepared yolk-like structured magnetic nanocatalyst

Fig. 8 Schematic illustration of Heck reaction catalyzed by MP@volid-Pd@mSiO2 (a) and recycling yields of the Heck reaction by using the MP@volid-Pd@mSiO2 magnetic catalysts (b).


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Currently, noble metal Pd has been widely studied as an effective catalyst39,40. However, its application is limited because of agglomeration and poor recycling performance. Here, the catalytic properties of MP@volid-Pd@mSiO2 are determined by the Heck reaction (Fig. 8a). The specific process of the reaction is described in the experimental section. As shown in Fig. 8b, the reactants can convert completely in 2 h without any by-product. To investigate the recycling performance of MP@volid-Pd@mSiO2, the sample is recovered through magnetic action and washed repeatedly with DMF solvent. In cycling test, the catalytic activity has no evident decrease after 7 cycles and the yields of product in Heck reaction still reach 98% (Fig. 8b), signifying excellent cycle performance of MP@void-Pd@mSiO2.

Fig. 9 Color changes and reaction scheme associated with the reduction of 4-NP catalyzed by MP@Void-Au@mSiO2 (a), UV-vis spectra of the 4-NP reduction which catalyzed by 1.5 mg MP@Void-Au@mSiO2 catalyst (b), plot of conversion (%) of 4-NP versus time (t, min) at different concentrations of MP@Void-Au@mSiO2 (c), conversion of 4-NP in 7 successive cycles of reduction with MP@Void-Au@mSiO2 (d).



Analogously,

Au

nanoparticles

biotechnology43, catalysis44,

45

present

excellent

and other fields46,

47

Page 24 of 38

performance

in

photonics41,

42

,

. In this work, the reduction of 4-NP to

4-aminophenol (4-AP) in the presence of NaBH4 is chosen as a model reaction to assess the catalytic activity of MP@Void-Au@mSiO2. The concentration changes of 4-NP can be easily monitored by UV-vis absorbance spectroscopy and no byproducts are formed. More importantly, the 4-NP has the severe toxicity to humans and environment. As shown in Fig. 9a, the color of the solution changed from light yellow to luminous yellow. Above phenomena are ascribed to the

formation

of

the

4-nitrophenolate

ion

intermediate.

After

introducing

the

MP@Void-Au@mSiO2 into the solution, the color of the reaction system changes quickly from luminous yellow to colorless and the characteristic adsorption peak of intermediate at 400 nm is gradually decreased, while the characteristic absorption peak of 4-AP at 300 nm is arosen (Fig. 9b). Therefore, the reduction of 4-NP to 4-AP in the presence of NaBH4 is commendably catalyzed by MP@Void-Au@mSiO2 and the reaction is completed within 4.5 min. As exhibited in Fig. 9c, the reaction time is sharply shortened with increasing of catalyst content. Furthermore, the reaction rate decreased slowly over time because of the pseudo-first-order kinetics, similar to the previous report48. The linear relationship of ln (Ct/C0) versus reaction time (t) is shown in Fig.S11. Where Ct and C0 represent the concentration of 4-NP at reaction time t and 0 min, respectively. The rate constant (k) is calculated to be 1.04 min-1 for the reaction catalyzed by MP@Void-Au@mSiO2.

The

comparison

of

catalytic

performance

between

MP@Void-Au@mSiO2 and previous similar catalysts is collected in Table S1. Compared with other catalysts, the as-prepared catalyst shows good catalytic performance for the 4-NP reduction. Importantly, some reported catalysts exhibit higher catalytic activity such as catalysts


Page 25 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesized by Deng’s group. Therefore, more work will be done to further improve the catalytic performance and investigate the influence factors. The recyclability, an important aspect for catalyst is investigated to evaluate catalytic performance. As shown in Fig. 9d, no significant change in catalytic activity is discovered even after circulating for 7 times at the same reaction time (4.5 min), in which the 4-NP’s conversion rate only decreased from 100% to 81%, suggesting an excellent stability and a long life of the composite catalyst.

Fig. 10 UV-vis spectra of the 4-NP reduction which catalyzed by MP@Void-Pt@mSiO2 catalyst (a) the linear relationship of ln (Ct/C0) versus reaction time (t) (b) (4.5 mg MP@Void-Pt@mSiO2) Similarly, the catalytic activity of MP@Void-Pt@mSiO2 was evaluated by the reduction of 4-NP with NaBH4. As shown in Fig. 10a, with increasing of reaction time, the characteristic absorption peak of intermediate at 400 nm was successively decreased and disappeared in 10 min. Meanwhile, a gradually increscent new absorption peak at 300 nm manifested the formation of 4-AP, which sufficiently demonstrated the reduction of 4-NP to 4-AP was catalyted by MP@Void-Pt@mSiO2 catalyst. Considering the concentration of NaBH4 is far higher than that



Page 26 of 38

of 4-NP, the rate constant for the reaction is calculated by the pseudo-first-order kinetics. The linear relationship of ln (Ct/C0) versus reaction time (t) is shown in Fig. 10b. Herein, the Ct and C0 represent the concentration of 4-NP at reaction time t and 0 min, respectively. The ratios of Ct to C0 are obtained from the ratios of the respective absorbance at 400 nm. The rate constant (k) is calculated to be 0.348 min-1 for the 4-NP reaction catalyzed by MP@Void-Pt@mSiO2. The ratio between rate constant k and the weight of catalyst is 1.28 s-1•g-1. Previous reports indicated that the ratio between rate constant k and the weight of catalyst are 1.58 s-1•g-1, 0.09 s-1•g-1 and 0.014 s-1•g-1 for hollow multiple-Ag-nanoclusters-C-shell nanostructures, pure Ag NPs and Ag-NP-doped hollow poly (N-isopropylacrylamide) spheres, respectively49, 50. And for bimetallic nanostructures, the ratio of k over the weight of the TiO2 NW@hollow Ag/Pt catalyst is 2.56 s-1•g-1 51. Compared with previous works, the as-prepared MP@Void-Pt@mSiO2 catalyst shows good catalytic activity for the reduction of 4-NP, but even that will require huge improvements in next work. In previous report52, Pt composites could be served as an effective catalyst in hydrogen generation. Similarly, the cycle catalytic performance of MP@Void-Pt@mSiO2 is also attractive because our product possessed a higher Ms value which is favorable for magnetic separation. Furthermore, the catalytic performance comparison of as-prepared three yolk-like structured magnetic nanocatalysts for the reduction of 4-NP is presented in Fig. S12 and S13. CONCLUSIONS In summary, we developed a general and facile method to fabricate a new type of yolk-like nanocatalyst with a movable magnetic core, mesoporous silica shell and a hollow space between the core and the shell. The well-confined functional metal nanoparticles are uniformly immobilized in the interior cavity. In this method, by changing the metal nanoparticles such as Pd,

Au

and

Pt,

the

yolk-like

MP@Void-Pd@mSiO2,

MP@Void-Au@mSiO2

and


Page 27 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


MP@Void-Pt@mSiO2 nanocatalysts are obtained. The unique structure of the catalyst protects the functional metal nanoparticles and provides the hollow interior for catalytic reaction, allowing the efficient diffusion of reactants and products. Additionally, the nanocatalysts show superparamagnetic property, which will benefit for the recycling catalysis. These multifunctional nanospheres exhibit remarkable catalytic performance and wonderful reusability for the Heck reaction and the reduction of 4-NP. More importantly, this method can be extended to synthesize other multifunctional nanocatalysts with similar structure.

ASSOCIATED CONTENT Supporting Information Thirteen figures and brief statement, showing the XRD patterns, XPS, FT-IR, TG and M-H curves of as-prepared products obtained from each step, XPS of MP@RF-Pt and MP@Void-Pt@mSiO2, XPS spectrum of Au 4f, Pd 3d and Pt 4f, the TEM images of MP@RF-Au, MP@RF-Au@hSiO2, MP@Void-Au@mSiO2, MP@RF-Pt, MP@RF-Pt@hSiO2, MP@Void-Pt@mSiO2, the HAADF STEM images and the corresponding elemental mapping images for Fe3O4@Void-Au@mSiO2 and Fe3O4@Void-Pt@mSiO2, the size distribution of Au nanoparticles on the inner surface of MP@Void-Au@mSiO2, the Nitrogen adsorption-desorption isotherms and Barrett–Joyner–Halenda (BJH) pore size distribution plot of MP@RF-Au@hSiO2 and MP@Void-Au@mSiO2, the linear relationship of ln (Ct/C0) versus reaction time (t) (2 mg MP@Void-Au@mSiO2), UV-vis spectra of the 4-NP reduction which catalyzed by 1.5 mg MP@Void-Pd@mSiO2 or MP@Void-Pt@mSiO2 catalyst. One table showing the catalytic performance of different catalysts for 4-NP reduction.



Page 28 of 38

AUTHOR INFORMATION Corresponding Author Tel: 86-551-63607605; Fax: 86-551-63600419. E-mail: [email protected] (W. Q. Jiang) [email protected] (X. L. Gong)

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT Financial supports from the National Natural Science Foundation of China (Grant No.11572310), and the Strategic Priority Research Program of the Chinese Academy of Sciences, Grant No. XDB22040502 are gratefully acknowledged. This study was also supported by the Collaborative Innovation Center of Suzhou Nano Science and Technology. ABBREVIATIONS mSiO2, mesoporous SiO2; MP, magnetic particles; RF, resorcinol/formaldehyde resin; hSiO2, hybrid CTAB/SiO2; 4-NP, p-Nitrophenol; 4-AP, 4-aminophenol; PAA, polyacrylic acid; PVP, polyvinylpyrrolidone; CTAB, hexadecyltrimethylammonium bromide; TEOS, tetraethoxysilane; DMF, N,N-dimethylformamide ; Et3N, trimethylamine; FETEM, field emission transmission


Page 29 of 38 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electronic microscopy; FESEM, field emission scanning electron microscope; XRD, X-ray powder diffraction patterns; BET, Brunauer-Emmett-Teller; BJH, Barrett–Joyner–Halenda; XPS, X-ray

photoelectron

spectrophotometer;

FT-IR,

Fourier

transform

infrared;

TG,

Thermogravimetric; GC-MS, gas chromatography mass spectrometers.

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The prepared nanocatalyst shows well-defined yolk-like structure and exhibits remarkable catalytic performance for Heck reaction and the reduction of 4-NP



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A General and Facile Method to Fabricate Yolk-like Structural

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