Article pubs.acs.org/Langmuir
Shape-Controlled Synthesis of Magnetic Iron Oxide@SiO2−Au@C Particles with Core−Shell Nanostructures Mo Li, Xiangcun Li,* Xinhong Qi, Fan Luo, and Gaohong He* State Key Laboratory of Fine Chemicals, The R&D Center of Membrane Science and Technology, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China S Supporting Information *
ABSTRACT: The preparation of nonspherical magnetic core−shell nanostructures with uniform sizes still remains a challenge. In this study, magnetic iron oxide@SiO2−Au@C particles with different shapes, such as pseduocube, ellipsoid, and peanut, were synthesized using hematite as templates and precursors of magnetic iron oxide. The as-obtained magnetic particles demonstrated uniform sizes, shapes, and welldesigned core−shell nanostructures. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDX) analysis showed that the Au nanoparticles (AuNPs) of ∼6 nm were uniformly distributed between the silica and carbon layers. The embedding of the metal nanocrystals into the two different layers prevented the aggregation and reduced the loss of the metal nanocrystals during recycling. Catalytic performance of the peanut-like particles kept almost unchanged without a noticeable decrease in the reduction of 4-nitrophenol (4-NP) in 8 min even after 7 cycles, indicating excellent reusability of the particles. Moreover, the catalyst could be readily recycled magnetically after each reduction by an external magnetic field. prepared in the first step, and the following steps always involve encapsulation techniques, e.g., layer-by-layer self-assembly,27 aerosol process,28 Kirkendall effect,29 and Stöber method.30 In such processes, the magnetic particles were always coated by other materials or could react with coating reagents, and their magnetic properties would be hindered or changed.28 Therefore, it is interesting and important to develop a method to prepare magnetic core−shell particles using chemicals and materials without any magnetic responses, and one of the materials could be converted to magnetic in the final step so that their magnetic properties would not be influenced by further processes. Hematite colloidal particles with different shapes and uniform sizes can be easily prepared in large amounts31,32 and thus an ideal template for shape-controlled synthesis of core−shell nanostructures. In addition, hematite can be easily converted into magnetic materials such as γ-Fe2O3, Fe3O4, or metallic iron at high temperature or reductive atmosphere.25,33,34 It was reported that silica-coated hematite spheres could be reduced into magnetic core−shell nanocomposites after calcination in a H2/N2 mixture.35 Inspired by this, we suppose that in the synthesis of magnetic core−shell particles hematite can be employed as an ideal candidate for both shapecontrolling template and precursor of magnetic core.
1. INTRODUCTION Recently, considerable attention has been focused on the preparation of core−shell nanostructures with magnetic property because of their extensive potential applications in heterogeneous catalysis,1−3 photocatalysis,4−6 controlled delivery of drugs,7,8 adsorption of pollutants,9,10 DNA/protein separation,11,12 medical imaging,8,13 and liquid chromatography.14 To improve their performance in different applications, remarkable efforts have been devoted into preparing such core−shell nanostructures with different sizes and shapes.15−18 As we know, it is relatively simple to control the size of the core−shell particles by precisely manipulating the core dimensions and the shell thickness. However, most of the obtained core−shell nanostructures are spherical in shape, and it is difficult to synthesize uniform and monodispersive nonspherical core−shell nanostructures due to their highcurvature surfaces.19 If industrial and commercial factors are taken into account, nonspherical particles are necessary because different catalytic reactions and operation procedures require catalyst particles with different shapes.20 For example, pellets and extrudates are suitable for fixed bed reactors, and flakes are common in hydrogenation of fats.21 Thus, to expand the application of magnetic core−shell particles in various fields, shape-controlled synthesis of such nonspherical nanostructures with uniform sizes via a facile route is becoming an issue of importance. Moreover, in most methods to synthesize magnetic core− shell nanostructures, magnetic particles such as Fe3O4,22−24 γFe2O3,25 Co compounds,26 and Ni compounds11 have to be © XXXX American Chemical Society
Received: March 3, 2015 Revised: April 18, 2015
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DOI: 10.1021/acs.langmuir.5b00800 Langmuir XXXX, XXX, XXX−XXX
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Scheme 1. Illustration of Shape-Controlled Synthesis of Magnetic Iron Oxide@SiO2−Au@C Core−Shell Particles: (i) Hematite Templates with Different Shapes, (ii) Hematite@SiO2 Core−Shell Particles, (iii) Hematite@SiO2−Au Core−Shell Particles, (iv) Hematite@SiO2−Au@RF Core−Shell Particles, and (v) As-Synthesized Magnetic Iron Oxide@SiO2−Au@C Core−Shell Particles
preparation of ellipsoid-type and pseudocube-type hematite templates, the process was similar to that of peanut-type ones, except for the decreased Na2SO4 solution (10 mL) concentration of 0.20 and 0 mol/ L, respectively. For the synthesis of peanut-, ellipsoid-, and pseudocube-type hematite templates, the final Na2SO4 concentrations in the bottle were 30, 3, and 0 mmol/L, respectively. The as-synthesized hematite templates with different shapes were coated by silica with a modified Stöber method.36 0.6 g of hematite was dispersed in the mixture of 100 mL of ethanol absolute, 5 mL of DI water, and 15 mL of ammonia solution in a 250 mL Pyrex bottle, which was placed in an ultrasonic water bath at 50 °C. Then, 0.75 mL of TEOS was added dropwise, and the sealed bottle was kept in ultrasonication for 5 h. Afterward, the products were collected by centrifugation and washed with water and ethanol for 3 times before drying at 80 °C for 4 h. 2.3. Preparation of Hematite@SiO2−Au Core−Shell Particles. 0.6 g of as-synthesized hematite@SiO2 core−shell particles with different shapes was dispersed in 100 mL of ethanol absolute in a 150 mL Erlenmeyer flask, and 1 mL of APTES and 1.2 mL of ammonia solution were added; the flask was kept at 25 °C for 12 h with magnetic stirring. The amino-modified hematite@SiO2 core−shell particles were collected by centrifugation and washed with water and ethanol for 3 times before drying at 80 °C for 4 h. Then, 0.15 g of the amino-modified particles was redispersed in 70 mL of DI water. After adding 0.5 mL of 1 wt % HAuCl4 solution and stirring at 25 °C for 1 h, 10 mL of 0.05 mol/L NaBH4 solution was added dropwise under vigorous stirring. The stirring was continued for another 1 h, and the products were collected by centrifugation and washed with water for 3 times before drying at 60 °C for 8 h. 2.4. Preparation of Hematite@SiO2−Au@RF Resin Particles and Magnetic Iron Oxide@SiO2−Au@C Particles. 0.15 g of hematite@SiO2−Au core−shell particles with different shapes was dispersed in the mixture of 90 g of DI water and 30 g of ethanol absolute in a 250 mL beaker by ultrasonication, followed by the addition of 0.3 g of CTAB. Then, 0.14 g of resorcinol, 0.26 g of formaldehyde solution, and 0.55 g of ammonia solution were added to the dispersion under magnetic stirring. The final dispersion was stirred in a water bath at 25 °C for 12 h, and the product was collected by centrifugation and washed with water and ethanol for 3 times before drying at 80 °C for 4 h. Afterward, the hematite@SiO2−Au@RF resin particles were calcined under a nitrogen atmosphere at 350 °C for 2 h and 550 °C for another 4 h with the heating rate of 1 °C/min. Finally, the magnetic iron oxide@SiO2−Au@C particles were obtained. 2.5. Catalytic Reduction of 4-Nitrophenol (4-NP) with Magnetic Iron Oxide@SiO2−Au@C Particles. 20 mg of magnetic iron oxide@SiO2−Au@C particles was dispersed in 4 mL of DI water by ultrasonication. Then, 1 mL of the dispersion, 1 mL of 4-NP
Here, we report a shape-controlled synthetic method of magnetic iron oxide@SiO2−Au@C particles using hematite with different shapes as templates and precursors of magnetic iron oxide. The synthetic procedure and structural model of magnetic iron oxide@SiO2−Au@C particles with core−shell nanostructures are shown in Scheme 1. First, hematite particles with different shapes, i.e., pseudocubes, ellipsoids, and peanuts, were synthesized. Then, a uniform silica layer is coated on the surface of the hematite particles, resulting in a hematite@SiO2 core−shell structure. Next, the silica layer is modified by amino group and Au nanoparticles are introduced on the surface of amino-modified silica layer. Afterward, the hematite@SiO2−Au particles are coated by a porous RF resin polymer layer, giving rise to the hematite@SiO2−Au@RF resin particles. Finally, after a calcination process under a nitrogen atmosphere, a porous carbon layer is obtained due to the carbonation of RF resin, and the hematite core is simultaneously converted into magnetic iron oxide, leading to the magnetic iron oxide@SiO2− Au@C nanostructure.
2. EXPERIMENTAL SECTION 2.1. Chemicals and Materials. FeCl3·6H2O, NaOH, Na2SO4, ethanol absolute, ammonia solution (∼28%), chloroauric acid (HAuCl4·4H2O), sodium borohydride (NaBH4), resorcinol, and formaldehyde solution (35−40%) were obtained from Sinopharm Chemical Reagent Co. (Shanghai, China). Tetraethyl orthosilicate (TEOS), cetyltrimethylammonium bromide (CTAB), and (3aminopropyl)triethoxysilane (APTES) were purchased from SigmaAldrich. All the chemicals and materials were analytical grade and used as received without further purification. Deionized water was used in all experiments. 2.2. Preparation of Hematite@SiO2 Core−Shell Particles with Different Shapes. Hematite templates with different shapes were synthesized based on a method described in the literature.31,32 In a typical synthesis of peanut-type hematite template, 90 mL of 6.0 mol/L NaOH solution was slowly added into 100 mL of 2.0 mol/L FeCl3 solution in a 250 mL Pyrex bottle under vigorously stirring in about 5 min, followed by adding 10 mL of 0.60 mol/L Na2SO4 solution, and the stirring was continued for another 10 min. The tightly sealed Pyrex bottle was placed in a 100 °C oven for 8 days. Afterward, the bottle was withdrawn from the oven and quenched to room temperature in running water. The supernatant was removed by decanting, and the precipitate was washed by DI water for three times and collected by centrifugation before drying at 60 °C for 12 h. For the B
DOI: 10.1021/acs.langmuir.5b00800 Langmuir XXXX, XXX, XXX−XXX
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Figure 1. SEM images of hematite templates with different shapes: (a, d) pseudocubes, (b, e) ellipsoids, and (c, f) peanuts. FESEM images of magnetic iron oxide@SiO2−Au@C core−shell particles with different shapes: (g) pseudocubes, (h) ellipsoids, and (i) peanuts. solution (0.1 mol/L), and 12 mL of DI water were mixed under stirring, followed by the addition of 1 mL of NaBH4 solution (0.5 mol/ L). The stirring was continued at 25 °C until the yellow solution became colorless. During the reaction, the solution was sampled at a 2 min interval, and the suspension was removed by an external magnetic field. The concentration of 4-NP in the clarified solution was analyzed by measuring UV−vis absorption spectra. In order to test the catalytic reusability of the magnetic iron oxide@ SiO2−Au@C particles, after the 4-NP reduction reaction, the particles were washed by DI water for three times and cycled for further 4-NP reduction experiments. The cycling tests were performed for seven times. 2.6. Characterizations. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) were performed on the Hitachi TM3000 and FEI NovaSEM 450 electron microscopy, respectively. Transmission electron microscopy (TEM) images were taken by a JEOL JEM-2000EX electron microscopy operating at 120 kV. The chemical compositions of the synthesized particles were analyzed by an energy dispersive X-ray spectroscopy (EDX) analyzer attached to the TEM. The powder X-ray diffraction (XRD) patterns were recorded by a Rigaku D/MAX-2400 diffractometer with Cu Kα radiation (λ = 0.1541 nm). Magnetization curves of the particles were obtained from a Quantum Design MPMS XL-7 superconducting quantum interference magnetometer. Nitrogen sorption isotherms were measured at 77 K with a Quantachrome Quardasorb SI automated surface area and pore size analyzer. Specific surface areas were calculated with the Brunauer−Emmett−Teller (BET) method. The mass fraction of Au was analyzed by PerkinElmer Optima 2000DV inductively coupled plasma atomic emission spectrometer (ICP-AES). The UV−vis spectra was recorded by a
Persee TU1900 UV−vis spectrometer in the wavelength range of 200−500 nm.
3. RESULTS AND DISCUSSION The hematite templates with different shapes, i.e., pseudocubes, ellipsoids, and peanuts, were synthesized by changing the Na2SO4 amount in the reaction system. Figures 1a−f show the SEM images of the hematite templates with different shapes, indicating that the particles are monodispersive and uniform in size ranging from 0.5 to 1.5 μm. After SiO2, Au, and C are deposited on the surface of hematite templates, magnetic iron oxide@SiO2−Au@C particles are obtained, and FESEM images (Figures 1g−i) indicate that their shapes are maintained the same as the templates. The average sizes of coated pseudocube, ellipsoid, and peanut particles are 1.29 ± 0.14, 0.73 ± 0.07, and 1.38 ± 0.10 μm, respectively (Figure S1, Supporting Information). Thus, thickness of SiO2−Au@C coating layer is about 80−150 nm. TEM images in Figure 2a−c clearly reveal the core−shell structure of the obtained magnetic iron oxide@SiO2−Au@C particles. The shell has a sandwich-like structure with a SiO2 layer of ∼60 nm in the middle and a porous carbon layer of ∼20 nm outside. Au nanoparticles are uniformly scattered between SiO 2 and carbon layers. Besides, shapes of pseudocubes, ellipsoids, and peanuts of hematite templates are well preserved in the magnetic iron oxide cores. The TEM image of Figure 2d shows that Au nanoparticles are highly C
DOI: 10.1021/acs.langmuir.5b00800 Langmuir XXXX, XXX, XXX−XXX
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Figure 2. TEM images of magnetic iron oxide@SiO2−Au@C core−shell particles with different shapes: (a) pseudocubes, (b) ellipsoids, (c) peanuts, (d) sandwich-like nanostructure with Au nanoparticles (∼6 nm) between SiO2 inner shell and C outer shell, (e) EDX element mapping of a pseudocubic magnetic iron oxide@SiO2−Au@C particle, and (f) EDX line-scanning patterns across a selected particle with demonstration of atomic ratio of C, Si, Fe, and Au.
Figure 3. (a) XRD patterns of hematite@SiO2−Au@RF core−shell particles before calcination and as synthesized magnetic iron oxide@SiO2−Au@ C core−shell particles after calcination (■: peaks of α-Fe2O3; ★: peaks of Fe3O4). (b) Hysteresis curves of magnetic iron oxide@SiO2−Au@C core−shell particles with different shapes (inset: attraction of particles dispersed in water to a magnet in 1 min).
monodispersive and have a uniform size of ∼6 nm. For instance, EDX element mapping of a pseudocubic magnetic iron oxide@SiO2−Au@C particle (Figure 2e) also proves the sandwich-like core−shell structure and the well-distributed Au nanoparticles. It can be observed that Fe is concentrated in the core region, while Si and C form a hollow shell outside the Ferich core with much higher atomic density in the edge than that in the center. Moreover, Au nanoparticles are distributed uniformly outside the SiO2 shell. From the EDX line-scanning patterns across the selected particle, it is further illustrated the existence of Fe, Si, C, and Au elements (O is excluded) as well as their location, which accords with the core−shell structure. Figure S2 displays the nitrogen adsorption−desorption isotherm of peanut-like magnetic iron oxide@SiO2−Au@C particles, and the specific area of the particles is calculated to be
57.3 m2/g. In addition, the total pore volume of the particles is 0.032 cm3/g for pores smaller than 40.2 nm. On the basis of specific area and pore volume, we can conclude that micro- and mesopores exist on the surface of the particles. The XRD pattern of hematite@SiO2−Au@RF resin particles (Figure 3a) is in agreement with the pure hematite (α-Fe2O3, JCPDS #33-0664) phase, possibly due to the large mass fraction of the hematite template. However, after calcination, apart from the well-retained hematite diffraction peaks, peaks at the 2θ values of 30.10° (220), 37.09° (222), 43.09° (400), and 56.98° (511) can also be observed on the XRD pattern of the obtained magnetic iron oxide@SiO2−Au@C particles (Figure 3a), which can be ascribed to the Fe3O4 phase (JCPDS #653107). Meanwhile, sharp peaks at 2θ values of 35.45° (311) and 62.57° (440) overlap with peaks of hematite, but their D
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Figure 4. (a) Time-dependent UV−vis spectra of 4-NP reduction catalyzed by as synthesized magnetic iron oxide@SiO2−Au@C core−shell particles, (b) plots of 4-NP reduction catalyzed by magnetic iron oxide@SiO2−Au@C core−shell particles with different shapes, (c) kinetic analysis of 4-NP reduction catalyzed by magnetic iron oxide@SiO2−Au@C core−shell particles with different shapes, and (d) cycling performance of peanut-like magnetic iron oxide@SiO2−Au@C particles in catalytic reduction of 4-NP.
synthesis, and carbon is a common reduction agent that can convert hematite into magnetic compound in some specific conditions. Finally, it is known that hematite can be reduced into Fe3O4 or Fe by carbon under inert gas atmosphere at high temperature.39 Thus, in the calcination process, it is possible that the hematite templates react with carbon, which is a product of RF resin carbonization, and be partially “in situ” reduced into Fe3O4. Both the XRD pattern with hematite and Fe3O4 peaks and magnetic hysteresis loops of magnetic iron oxide@SiO2−Au@C particles support the assumption above. Moreover, the partial transformation from hematite to magnetic iron oxide takes place at particle level. It is presented in TEM images and EDX mapping in Figure 2 that each of the hematite cores is confined in the silica/carbon shell, and contact of hematite with each other is prevented. Therefore, all hematite@ SiO2−Au@RF resin particles transformed individually into magnetic iron oxide@SiO2−Au@C particles. To evaluate the catalytic activity of magnetic iron oxide@ SiO2−Au@C particles, reduction of 4-nitrophenol (4-NP) to 4aminophenol (4-AP) in the presence of NaBH4 was selected as the model reaction. The reaction process with reaction time was monitored by a UV−vis spectrometer. When the aqueous solution of NaBH4 was added, 4-NP solution exhibited a light yellow color and a strong adsorption peak at 400 nm because of the formation of 4-nitrophenolate ions.40 In addition, the absorbance at 400 nm did not change with time even excess
relative intensities correspond with that of Fe3O4. Using the Debye−Scherrer equation, the collective mean crystallite size of Fe3O4 is estimated to be about 33 nm from mean value of the (220), (400), and (511) peaks. The Fe3O4 fraction is determined to be 35.9 wt % using the relative intensity ratio (RIR) method.37,38 Figure 3b shows the magnetic hysteresis loops of magnetic iron oxide@SiO2−Au@C particles with different shapes. No detectable remanence or coercivity is observed so it is indicated that all samples reflect a superparamagnetic property. The inset of Figure 3b illustrates the attraction of the peanut-like particles to a commercial permanent magnet in 1 min. Saturation magnetization values of pseudocubic, ellipsoid, and peanut magnetic iron oxide@SiO2− Au@C particles are 24.2, 28.1, and 16.4 emu/gFe, respectively. Because of their superparamagnetic property, after homogeneously dispersed in liquid, the magnetic iron oxide@SiO2− Au@C particles show fast motion when external magnetic field is applied, indicating that the particles can be easily collected and recycled in liquid catalysis. The superparamagnetic property of the magnetic iron oxide@SiO2−Au@C particles can be attributed to following essentials. First, hematite, which is slightly ferromagnetic, rather than other components (SiO2, Au, and C, which show no apparent magnetic properties in any phases), is the only possible reactant to be converted into magnetic iron oxide. Second, the RF resin coating layer acts as carbon source in the E
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that the particles preserved their catalytic activity during the reaction period of 10 min, with negligible loss (
