Shell Fe

Aug 24, 2009 - instrument. The magnetic properties (M-H curve) were evaluated ... blackberry-like morphology have been successfully obtained by using ...
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Preparation, Characterization, and Catalytic Activity of Core/Shell Fe3O4@Polyaniline@Au Nanocomposites Shouhu Xuan,†,§ Yi-Xiang J. Wang,‡ Jimmy C. Yu,† and Ken Cham-Fai Leung*,† †

The Center of Novel Functional Molecules, Department of Chemistry, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, ‡Department of Diagnostic Radiology and Organ Imaging, Prince of Wales Hospital, The Chinese University of Hong Kong, Shatin, NT, Hong Kong SAR, and §Department of Materials Science and Engineering, China Jiliang University, Hangzhou, 310018, People’s Republic of China Received April 23, 2009. Revised Manuscript Received June 28, 2009

We report a new method to synthesize magnetically responsive Fe3O4@polyaniline@Au nanocomposites. The superparamagnetic Fe3O4@polyaniline with well-defined core/shell nanostructure has been synthesized via an ultrasound-assisted in situ surface polymerization method. The negatively charged Au nanoparticles with a diameter of about 4 nm have been effectively assembled onto the positively charged surface of the as-synthesized Fe3O4@polyaniline core/shell microspheres via electrostatic attraction. The morphology, phase composition, and crystallinity of the as-prepared nanocomposites have been characterized by transmission electron microscopy (TEM) and powder X-ray diffraction (XRD). The central Fe3O4 cores are superparamagnetic at room temperature with strong magnetic response to externally applied magnetic field, thus providing a convenient means for separating the nanocomposite from solution. As-prepared inorganic/organic nanocomposite can be used as a magnetically recoverable nanocatalyst for the reduction of a selected substrate.

1. Introduction 1

In recent years, core/shell-structured composite materials, which combine with the advantageous properties of both materials of the core and the shell, have attracted increasing interest to materials scientists due to their unique physicochemical properties and great potential applications in the areas of electronics, photonics, catalysis, biotechnology, and nanotechnology.2 These specific core/shell structures which possess the ability to be modified with different charges, functions, or reactive moieties on the surface with enhanced stability and compatibility provide an avenue for the synthesis of complex composite materials. Alternatively, various types of bifunctional or trifunctional core/shell materials, such as electronic/optical, electronic/catalytic, magnetic/electronic, etc., could be technically obtained.3 Magnetite (Fe3O4), which is a common ferrite possessing a cubic inverse spinel structure, has been widely studied because of their potential applications as ferrofluids, catalysts, biological assays, chemical sensors, and electrophotographic developers.4 With respect to these properties, the design and synthesis of *Corresponding author: Tel þ852 26096342; Fax þ852 26035057; e-mail [email protected]. (1) Caruso, F. Adv. Mater. 2001, 13, 11–22. (2) (a) Li, X.; Wan, M. X.; Wei, Y.; Shen, J. Y.; Chen, Z. J. J. Phys. Chem. B 2006, 110, 14623–14626. (b) Roca, M.; Haes, A. J. J. Am. Chem. Soc. 2008, 130, 14273–14279. (c) Yi, D. K.; Lee, S. S.; Ying, J. Y. Chem. Mater. 2006, 18, 2459–2461. (d) Feng, X.; Mao, C.; Yang, G.; Hou, W.; Zhu, J. J. Langmuir 2006, 22, 4384–4389. (e) Salgueirio-Maceira, V.; Correa-Duarte, M. A.; Spasova, M.; Liz-Marzan, L. M.; Farle, M. Adv. Funct. Mater. 2006, 16, 509–514. (3) (a) Xi, Y.; Zhou, J.; Guo, H.; Cai, C.; Lin, Z. Chem. Phys. Lett. 2005, 412, 60– 64. (b) Mangeney, C.; Bousalem, S.; Connan, C.; Vaulay, M.; Bernard, S.; Chehimi, M. M. Langmuir 2006, 22, 10163–10169. (4) (a) The Iron Oxides; Cornell, R. M., Sehwertmann, U., Eds.; VCH: Weinheim, Germany, 1996.(b) Keane, M. A. J. Catal. 1997, 166, 347–355. (c) Geus., J. W. Appl. Catal. 1986, 25, 313–333. (d) Martin, B. R.; Dermody, D. J.; Reiss, B. D.; Fang, M. M.; Lyon, L. A.; Natan, M. J.; Mallouk, T. E. Adv. Mater. 1999, 11, 1021–1025. (e) Huo, L.; Li, W.; Lu, L.; Cui, H.; Xi, S.; Wang, J.; Zhao, B.; Shen, Y.; Lu, Z. Chem. Mater. 2000, 12, 790–794. (f) Matijevic, P. E.; Hench, L. L.; Ulrich, D. B. Colloid Science of Composites System Science of Ceramic Chemical Processing; Wiley: New York, 1986; p 463.

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various core/shell architectures based on Fe3O4 are important research subjects. Studies on the modular synthesis of the magnetic core/shell composite materials with unique size- and shapedependent properties are of great interest and are actively being pursued. The most common method to synthesize magnetic core/shell composite materials was coating Fe3O4 particles with functional shells. Among these shell materials, polyaniline (PANI) has received more attention for its unique electrical property.5 Recently, Fe3O4@PANI nanocomposite has become a popular material for applications in electrical-magnetic shields, electrochemical display devices, and microwave absorbing materials.6 Several methods have been developed to synthesize Fe3O4@PANI nanocomposite.7 Wan et al.7a studied a series of PANI-containing nanomagnets which was prepared by chemical polymerization. Deng et al.7b reported the preparation of PANI/Fe3O4 nanoparticles with core/shell structure via an in situ polymerization of aniline monomer in aqueous solution, which contained Fe3O4 nanoparticles and surfactants. Besides the spherical morphology, other types of Fe3O4@PANI nanocomposite such as Y-junction structure can also be selectively prepared by using in situ self-assembly of water-soluble Fe3O4 nanoparticles.8 The as-prepared composite particles are shown to be superparamagnetic, wherein they serve as ideal candidates for biomedical applications, such as enzyme immobilization, nucleic acid extraction, cancer diagnosis, biosensors, and drug delivery.9 However, most of these nanoparticles result in fluctuations in size (5) (a) Zhang, L. J.; Wan, M. X. J. Phys. Chem. B 2003, 107, 6748–6753. (b) Liang, L.; Liu, J.; Windisch, C. F.; Exarhos, G. J.; Lin, Y. Angew. Chem., Int. Ed. 2002, 41, 3665–3668. (6) Lu, X. F.; Yu, Y. H.; Chen, L.; Mao, H. P.; Gao, H.; Wang, J.; Zhang, W. J.; Wei, Y. Nanotechnology 2005, 16, 1660–1665. (7) (a) Wan, M. X.; Li, W. C. J. Polym. Sci., Part A: Polym. Chem. 1997, 35, 2129–2136. (b) Deng, J. G.; He, C. L.; Peng, Y. X.; Wang, J. H.; Long, X. P.; Li, P.; Chan, A. S. C. Synth. Met. 2003, 139, 295–299. (c) Deng, J.; Ding, X.; Zhang, W.; Peng, Y.; Wang, J.; Long, X.; Li, P.; Chan, A. S. C. Polymer 2002, 43, 2179–2184. (8) Xia, H. B.; Cheng, D. M.; Xiao, C. Y.; Chan, H. S. O. J. Mater. Chem. 2005, 15, 4161–4166. (9) Lee, G.; Kim, J.; Lee, J. H. Enzyme Microb. Technol. 2008, 45, 466–472.

Published on Web 08/24/2009

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whereas they had a nonuniform magnetite fraction in each nanosphere due to nanoscale clustering of magnetic particles. In order to achieve steady and reproducible applications, it is therefore of great interest to prepare well-dispersed Fe3O4@PANI nanocomposite with defined core/shell nanostructure. Additionally, a rational combination of the above magnetic core/shell particles together with other functional materials (noble metals, semiconductor, etc.) would result in multicomponent nanocomposites with integrated and stable optical, catalytic, electrical, and magnetic properties.10 These species of nanocomposite exhibited promising application in many areas.11 Various complex structures, such as Fe3O4@SiO2@PABI-Tb, Fe3O4 @SiO2@Au, Fe3O4@SiO2@CdTe, Fe3O4@SiO2(FITC), etc.12 have been successfully prepared. A significant amount of work has been reported using the Fe3O4@inorganic@shell type threecomponent composite particles. However, the study of the Fe3O4@polymer@shell three-component composites is rare, which may be attributed to the synthetic difficulties of the Fe3O4@organic shell composite particles. Very recently, Wang et al.13 reported the synthesis of Fe3O4@PAH@Au multifunctional nanoparticles, which display both magnetism and nearinfrared absorption. Xu’s group14 has also successfully prepared Fe3O4@PPy@Au nanocomposite which exhibited excellent electrocatalytic properties for ascorbic acid. Additionally, Yu et al.15 prepared Fe3O4@Au/PANI nanocomposites by in situ polymerization in the presence of mercaptocarboxylic acid. However, many of these multifunctional magnetic nanocomposites do not possess monodispersity with well-defined core/shell nanostructures, which confined their properties and applications. Therefore, the construction of monodisperse Fe3O4@polymer@shell composite particles with well-defined core/shell nanostructure is a pressing need for the study of their physicochemical properties. On the basis of the above considerations, one could combine the collective advantages of Fe3O4, PANI, and Au nanoparticle to fabricate multifunctional materials with well-defined core/shell structures, sharing good stability, solvent compatibility, optical property, catalytic activity, and magnetic separability. However, the study of the Fe3O4@PANI@Au microspheres with welldefined core/shell nanostructure has not been reported, which mostly correspond to the difficulty to synthesize Fe3O4@PANI particles with defined core/shell structure, and thus further confine the synthesis of the Fe3O4@PANI@Au core/shell composite particles. In this paper, a novel method for the synthesis of magnetically responsive Fe3O4@PANI@Au nanocomposites with monodisperse core/shell nanostructure is reported. The superparamagnetic Fe3O4@PANI with well-defined core/shell nanostructure (10) (a) Kim, J.; Lee, J. E.; Lee, J.; Jang, Y.; Kim, S. W.; An, K.; Yu, J. H.; Hyeon, T. Angew. Chem., Int. Ed. 2006, 45, 4789–4793. (b) Sacanna, S.; Philipse, A. P. Langmuir 2006, 22, 10209–10216. (c) Chen, M.; Kim, Y. N.; Lee, H. M.; Li, C.; Cho, S. O. J. Phys. Chem. C 2008, 112, 8870–8874. (d) Yang, H.; Jiang, W.; Lu, Y. Mater. Lett. 2007, 61, 2789–2793. (e) Nagao, D.; Yokoyama, I.; Yamauchi, N.; Matsumoto, H.; Kobayashi, Y.; Konno, M. Langmuir 2008, 24, 9804–9808. (f) Lu, Z.; Qin, Y.; Fang, J.; Sun, J.; Li, J.; Liu, F.; Yang, W. Nanotechnology 2008, 19, 055602. (11) Stoeva, S. I.; Huo, F.; Lee, J. S.; Mirkin, C. A. J. Am. Chem. Soc. 2005, 127, 15362–15363. (12) (a) Yu, S. Y.; Zhang, H. J.; Yu, J. B.; Wang, C.; Sun, L. N.; Shi, W. D. Langmuir 2007, 23, 7836–7840. (b) Ji, X.; Shao, R.; Elliott, A. M.; Stafford, R. J.; Esparza-Coss, E.; Bankson, J. A.; Liang, G.; Luo, Z. P.; Park, K.; Markert, J. T.; Li, Chun. J. Phys. Chem. C 2007, 111, 6245–6251. (c) Lu, C. W.; Hung, Y.; Hsiao, J. K.; Yao, M.; Chung, T. H.; Lin, Y. S.; Wu, S. H.; Hsu, S. C.; Liu, H. M.; Mou, C. Y.; Yang, C. S.; Huang, D. M.; Chen, Y. C. Nano Lett. 2007, 7, 149–154. (13) Wang, L. Y.; Bai, J. W.; Li, Y. J.; Huang, Y. Angew. Chem., Int. Ed. 2008, 47, 2439–2442. (14) Zhang, H.; Zhong, X.; Xu, J. J.; Chen, H. Y. Langmuir 2008, 24, 13748– 13752. (15) Yu, Q. Z.; Shi, M. M.; Cheng, Y. N.; Wang, M.; Chen, H. Z. Nanotechnology 2008, 19, 265702.

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has been synthesized via a simple ultrasound-assisted in situ surface polymerization method. Although it has been reported that the dedoped PANI can be used as reductant for the generation of Au nanoparticles from Au metal salts,16 a method which utilizes electrostatic attractions was employed to immobilize the presynthesized Au nanoparticles onto the Fe3O4@PANI core/ shell microspheres. The negatively charged Au nanocatalysts, which possess an average diameter about 4 nm, can be effectively assembled onto the positively charged surface of the as-synthesized Fe3O4@PANI core/shell microspheres. To the best of our knowledge, this is the first report about the synthesis of monodisperse Fe3O4@PANI@Au nanocomposites with defined core/ shell nanostructure. The central magnetite cores are superparamagnetic at room temperature with strong magnetic response to externally applied magnetic fields, thus providing a convenient means for separating the nanocomposite from solution. Additionally, such nanocomposite can be used as a magnetically recoverable catalyst for the reduction of rhodamine B (RhB) with NaBH4. In a parallel comparison to the Fe3O4@SiO2@Au catalyst, the Fe3O4@PANI@Au nanoparticle is a more effective catalyst which can also be recycled for repeated catalyzes.

2. Experimental Section Materials. Ferric chloride hexahydrate, sodium acrylate, sodium acetate, hydrogen tetrachloroaurate hydrate, ethylene glycol, trisodium citrate, tetraethyl orthosilicate (TEOS), 3-aminopropyltriethoxysilane (APTES), rhodamine B (RhB), sodium borohydride, absolute ethanol (EtOH), ammonium peroxodisulfate (APS), and poly(vinylpyrrolidone) (PVP; 30 kDa) were purchased from Aldrich. All chemicals were of analytical grade and used without further purification. Aniline was also obtained from Aldrich and was distilled at a reduced pressure before use. Doubly deionized water was used through out the synthetic processes. The sonicator was operated at 35 kHz using an Elma TI-H-5 apparatus. Synthesis of Fe3O4 Microspheres. The magnetic Fe3O4 microspheres were prepared through a solvothermal reaction. Briefly, FeCl3 3 6H2O (1.08 g), sodium acrylate (3.0 g), and sodium acetate (3.0 g) were dissolved in ethylene glycol (40 mL) with magnetic stirring at room temperature for 2 h. The homogeneous yellow solution was then transferred to a Teflon-lined stainlesssteel autoclave (container volume=80 mL) and sealed to heat at 200 °C. After a reaction for 10 h, the autoclave was cooled to room temperature. The supernatant of the mixture was discarded. The obtained Fe3O4 particle suspension was washed successively with H2O and EtOH three times and then dried in vacuum for 12 h at room temperature. Synthesis of Fe3O4@PANI Nanocomposite with Core/ Shell Nanostructures. In a typical procedure, PVP (50 mg) and Fe3O4 microspheres (10 mg) were dispersed in H2O (12.5 mL) under ultrasonication at room temperature. After 30 min, aniline (1.2  10-3, 1.6  10-3, or 2.0  10-3 M) in the presence of concentrated HCl (25 μL) was added into the mixture at room temperature. The solution was shaken for 12 h at room temperature. Then, H2O (10 mL) was added to the mixture, followed by ultrasonication (1 h at room temperature). After an hour, an aqueous solution of APS (0.3 g of APS in 10 mL of H2O) was added into the above mixture instantly to start the oxidative polymerization under ultrasonic irradiation, and the reaction was allowed to proceed for 2 h at room temperature. After the reaction, the precipitate was magnetic separated and washed successively with H2O and EtOH three times. The final product

(16) (a) Wang, J.; Neoh, K. G.; Kang, E. T. J. Colloid Interface Sci. 2001, 239, 78–86. (b) Tseng, R. J.; Huang, J.; Ouyang, J.; Kaner, R. B.; Yang, Y. Nano Lett. 2005, 5, 1077–1080.

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was dried in vacuum for 12 h at room temperature to obtain the desired Fe3O4@PANI core/shell nanocomposite. Synthesis of Fe3O4@PANI@Au Nanocomposites. An electrostatic attraction method was employed for the immobilization of Au nanoparticles onto the Fe3O4@PANI core/shell microspheres to form the Fe3O4@PANI@Au microspheres. First, in order to induce positive charges at the surface of the Fe3O4 @PANI microspheres, acetic acid solution (0.1 mL) was added into the ethanolic dispersion of Fe3O4@PANI (10 mg/mL, 1 mL) at room temperature and then sonicated at room temperature for 1 h. Subsequently, an aqueous solution of Au nanoparticles with an average diameter of 4 ( 1 nm was prepared according to literature procedures.17 In brief, an aqueous solution (20 mL) containing HAuCl4 (2.5  10-4 M) and trisodium citrate (2.5  10-4 M) was prepared in a conical flask. Then, cold NaBH4 solution (0.6 mL, 0.1 M) was added with vigorous stirring. The solution turned pink immediately after the addition of NaBH4, indicating the Au nanoparticle formation. The freshly prepared Au nanoparticles solutions were used within 2-5 h after preparation. Finally, the solution of positively charged Fe3O4@PANI microspheres (1 mL) was added to the Au solution under sonication at room temperature for 30 min. The Au nanoparticles were electrostatically attracted on the surface of the Fe3O4@PANI nanocomposites; thereby, the resultant Fe3O4@PANI@Au product was removed from the solution by applying an external magnetic field, rinsed successively with H2O and EtOH three times, and then dispersed in H2O for the characterization of catalytic properties.

Catalytic Properties of the Fe3O4@PANI@Au Nanocomposites. The reduction of RhB by NaBH4 was chosen as a model reaction for the efficiency testing of the Au-immobilized nanoparticle. A given amount of the magnetic catalysts were added into a solution with RhB (20 mL, 5  10-5 mol/L), in which the volume of the mixture was adjusted to 40 mL with H2O. After that, an aqueous solution of NaBH4 (1 mL, 0.4 mol/L) was rapidly injected at room temperature with stirring. The color of the mixture gradually vanished, indicating the reduction of the RhB dye. Changes in the concentration of RhB were monitored by examining the variations in the maximal UV/vis absorption at 554 nm. After the catalytic reaction was completed, the nanocatalysts were separated by externally applied magnetic field and then repeated for the catalytic reaction. The recyclability of the nanoparticle catalysis was determined by measuring the maximal UV/vis absorption of RhB at the end of each catalytic degradation reaction. Characterization. X-ray powder diffraction patterns (XRD) of the products were obtained on a Japan Rigaku DMax-γA rotation anode X-ray diffractometer equipped with graphite monochromatized Cu KR radiation (λ=1.541 78 A˚). Transmission electron microscopy (TEM) photographs were taken on a FEI CM120 microscope at an accelerating voltage of 120 kV. Xray photoelectron spectra (XPS) were measured on a photoelectron spectrometer using Mg KR radiation. Infrared (IR) spectra were recorded in the wavenumbers ranging from 4000 to 500 cm-1 with a Nicolet Model 759 Fourier transform infrared (FT-IR) spectrometer using a KBr wafer. Thermogravimetric (TG) analyses were conducted with a Netzsch STA 409C thermoanalyzer instrument. The magnetic properties (M-H curve) were evaluated on a BHV-55 vibrating sample magnetometer (VSM). UV/vis spectra were obtained using a UV-365 spectrophotometer.

3. Results and Discussion 3.1. Synthesis of Monodisperse Fe3O4@PANI Microspheres with Well-Defined Core/Shell Nanostructure. To date, the effective coating of Fe3O4 particles with natural or synthetic polymers is still a challenge since the surfaces of (17) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065– 4067.

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Figure 1. XRD diffraction patterns of the as-prepared Fe3O4 (a), Fe3O4@PANI (b), Fe3O4@PANI@Au (c), and Fe3O4@SiO2@Au (d) microspheres.

magnetic particles are hydrophilic while polymers are hydrophobic. In our previous report,18 Fe3O4@PANI microspheres with blackberry-like morphology have been successfully obtained by using an in situ surface polymerization method. However, a modification of the Fe3O4 microsphere cores is inevitable before PANI coating, which renders a complex synthetic route. Moreover, a more uniform PANI coating apart from the rough blackberry-like morphology should be obtained. Therefore, a facile method must be developed to directly coat a uniform PANI layer onto Fe3O4 particles without prior surface modification. It has been reported that the coating of colloids with PANI shells is strongly influenced by the sign of charges of the materials.19 Since PANI chains are positively charged in the reaction solution, uniform PANI shells can be successfully obtained when negatively charged templates such as PSA are used.20 Therefore, in order to coat a uniform PANI layer on to the Fe3O4 core, carboxylate-functionalized Fe3O4 microspheres were used as the core template. Herein, carboxyl group functionalized Fe3O4 microspheres were synthesized through a solvothermal method in ethylene glycol using FeCl3 3 6H2O as the sole iron source. Sodium acrylate was used in the reaction system, which served both as base and surface modification agent. The XRD pattern of the as-synthesized product is shown in Figure 1a, which can be indexed to facecenter-cubic phase of Fe3O4 (JCPDS Card No. 19-629). According to the Scherrer equation,21 the average crystallite size which is calculated based on the XRD pattern (311) is ∼7.5 nm. Representative TEM images of the as-synthesized Fe3O4 particles are shown in Figure 3d. Clearly, the diameter of the spherical microsphere is about 170 nm. The microsphere is an assembly of many small Fe3O4 nanocrystals, which agrees well with the observed broad X-ray diffraction lines. Figure 2a presents the typical FTIR spectrum of the Fe3O4 microspheres. The bands appeared at 590, 1450, and 3400 cm-1 are related to the Fe-O vibration, the CH2 bending mode, and the O-H stretching vibrations, respectively.22 The two bands located at 1550 and 1405 cm-1 correspond to the COO- antisymmetrical vibration and COO- symmetric (18) Xuan, S. H.; Wang, Y. X. J.; Leung, K. C.-F.; Shu, K. Y. J. Phys. Chem. C 2008, 112, 18804–18809. (19) Lu, Y.; McLellan, J.; Xia, Y. N. Langmuir 2004, 20, 3464–3470. (20) (a) Ding, K. L.; Miao, Z. J.; Liu, Z. M.; An, G. M.; Xie, Y.; Tao, R. T.; Han, B. X. J. Mater. Chem. 2008, 18, 5406–5411. (b) Kim, B. J.; Oh, S. G.; Han, M. G.; Im, S. S. Polymer 2002, 43, 111–116. (21) Klug, H. P.; Alexander, L. E. X-ray Diffraction Procedures for Polycrystalline and Amorphous Materials; Wiley: New York, 1962; pp 491-538. (22) Xuan, S. H.; Hao, L. Y.; Jiang, W. Q.; Gong, X. L.; Hu, Y.; Chen, Z. Y. J. Magn. Magn. Mater. 2007, 308, 210–213.

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Figure 2. FTIR spectra of the as-prepared Fe3O4 (a), Fe3O4@PANI (b), Fe3O4@PANI@Au (c), and Fe3O4@SiO2 (d) microspheres.

Figure 3. TEM images of the as-prepared Fe3O4@PANI (a-c) and Fe3O4 (d) microspheres.

vibration, indicating the bidentate bonding of the carbonyl groups to the surface Fe atoms.23 Additionally, a weak band appeared at 1710 cm-1 is attributed to the CdO stretching mode of the residue carboxylic acid groups, which form dimer pairs and sideway chains with hydrogen bonds.24 Although large amounts of carboxylate groups are coordinated strongly to the iron cations, there are still many polymer chains located on the particles’ surface with uncoordinated residue carboxylic acid groups. All these results indicate that carboxyl functionalized Fe3O4 microspheres have been successfully obtained. Therefore, it is noteworthy that uniform PANI layer can be directly coated onto the as-prepared carboxylic acid-functionalized Fe3O4 microspheres. The preparation of the Fe3O4@PANI microspheres with defined core/shell nanostructure using the ultrasonic-assisted in situ polymerization method can be divided into several steps. (23) Lin, C. L.; Lee, C. F.; Chiu, W. Y. J. Colloid Interface Sci. 2005, 291, 411– 420. (24) Ge, J.; Hu, Y.; Biasini, M.; Dong, C.; Guo, J.; Beyermann, W. P.; Yin, Y. Chem.;Eur. J. 2007, 13, 7153–7161.

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First, the water-soluble carboxylic acid-functionalized Fe3O4 microspheres were dispersed in an aqueous solution. Then, the aniline monomers were added and partly reacted with the surface carboxylic acid groups to give the -COO-H3Nþ- ion pairs. The adsorption of aniline onto the particle surface results in a close proximity. It is believed that the initial polymerization of aniline under this circumstance is favorable.25 Once the aniline nucleation is generated, the polymerization takes place preferentially and continuously on the particle’s surface rather than in solution.18 Eventually, the monodisperse, well-defined Fe3O4@PANI core/shell structure is formed. TEM images permit the comparison of the particle morphology between the Fe3O4 particles (Figure 3d) and the Fe3O4@PANI microspheres (Figure 3a-c). As shown in Figure 3a, the as-prepared Fe3O4@PANI nanocomposites were well dispersed on the copper grid without large aggregations. Figure 3b shows the TEM image of Fe3O4@PANI nanocomposites with a higher resolution; it is clear that the composite particles are constituted of two distinct components: a black core of Fe3O4 and a gray shell of PANI. No obvious changes in size or shape of the Fe3O4 cores are observed; thereby a well-defined Fe3O4@PANI core/shell structure is successfully achieved. Figure 3c shows the TEM image of a single Fe3O4@PANI particle with a typical core/shell nanostructure. The Fe3O4 core is well wrapped by the coating layer, and the average thickness of the PANI coating shell is about 25 nm. Moreover, there exists a clear interface between the PANI shell and the Fe3O4 core (Figure 3c), indicating a tight encapsulation. For the XRD spectrum of Fe3O4@PANI nanocomposites (Figure 1b), the major peaks are similar to the pristine Fe3O4 particles (Figure 1a), revealing that the as-prepared core/shell composites consist of the Fe3O4 component. Because of the relatively thin layer and amorphous crystallinity of the PANI prepared under this in situ polymerization method, no obvious diffraction peak for the PANI is observed. Figure 2b shows the typical FTIR spectrum of the as-prepared Fe3O4@PANI composites. The characteristic peaks at approximately 1591, 1315, 1177, and 820 cm-1 are similar to that of the pure PANI sample, which further support that the PANI shell is successfully coated on the Fe3O4 microspheres’ surface.18 In our system, both the ultrasonic irradiation treatment and usage of PVP are particularly important for the synthesis of the monodisperse Fe3O4@PANI core/shell nanoparticles. The negatively charged Fe3O4 microspheres are in favor of coating by positively charged PANI; however, the major problem regarding the coating of PANI onto the Fe3O4 microspheres is that the polymerization is required to perform in acidic condition. As the carboxyl-functionalized Fe3O4 microspheres are easily to form aggregates in acidic solution, it has resulted in large conglomeration which responds to the strong hydrogen-bonding interaction between PANI and Fe3O4 microspheres during the polymerization process. Once the coating process was performed under mechanical stirring without the usage of PVP surfactant, only PANI encapsulated, large Fe3O4 microspheres aggregates were observed (Figure 4a). Therefore, PVP macromolecules were added into the reaction solution in order to stabilize the Fe3O4 microspheres. Figure 4b represents the typical TEM image of the Fe3O4@PANI nanocomposites which were synthesized with the use of PVP. It is clear that all the Fe3O4 microspheres are well wrapped in the PANI coating layer. However, the Fe3O4@PANI core/shell particles are still terribly linked with the PANI layers which indicate that the addition of PVP surfactant was (25) Zhu, C. L.; Chou, S. W.; He, S. F.; Liao, W. N.; Chen, C. C. Nanotechnology 2007, 18, 275604.

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Figure 4. TEM images of the as-prepared Fe3O4@PANI without PVP surfactant by mechanical stirring (a), with PVP surfactant by mechanical stirring (b), without PVP surfactant by ultrasonic irradiation (c), and with PVP surfactant by ultrasonic irradiation (d). All other synthetic parameters were the same.

insufficient to prevent the Fe3O4 aggregation. It has been reported that ultrasound has been widely used in chemical reactions, dispersion, emulsifying, crushing, and polymerization because the ultrasonic cavitation can generate high local temperatures and high local pressure.26 Hence, in the present study, the ultrasonic irradiation technique was employed to prepare the Fe3O4@PANI core/shell microspheres. The TEM image of the Fe3O4@PANI nanocomposites which were synthesized under ultrasonication without PVP surfactant (Figure 4c) shows that all the Fe3O4 particles are well encapsulated in the PANI latex without large aggregates. This is because the ultrasonic irradiation can effectively break down the aggregation of the Fe3O4 microspheres. However, it is also noticed that some particles are lightly agglomerated by the coalescence of the PANI shell. To solve this problem, the PVP surfactant was added into the reaction system so that the monodisperse Fe3O4@PANI microspheres with core/ shell nanostructure were successfully obtained (Figure 4d). Under the intense dispersion and stirring effects offered by the ultrasonic irradiation, Fe3O4 microspheres were well dispersed in the reaction system. After the addition of an oxidant (APS), the polymerization of aniline begins. The hydrogen bond and coordinative interactions between the carboxylic acid-functionalized Fe3O4 microspheres and the PANI chains favor the absorption of PANI on the surface of Fe3O4, thereby leading to the formation of the PANI-encapsulated Fe3O4 composite particles. Because of the fast polymerization reaction, large amounts of PANI nucleates appeared and favored to aggregate together to form large particles, which further lead to large Fe3O4@PANI agglomerates by coalescence of the PANI shell. With the presence of PVP surfactant, once PANI nucleation was generated, the grains might be stabilized by the PVP molecules. Therefore, large and irregular PANI particles often caused by the linkage of nearby Fe3O4@PANI microspheres would be avoided. Finally, a homogeneous, continuous, and uniform PANI shell was formed on the surface of the Fe3O4 core. In this process, various thicknesses of PANI shell were obtained by varying the aniline monomer concentrations (Figure 5). When the amount of Fe3O4 (26) Suslick, K. S. Science 1990, 247, 1439–1445.

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cores was fixed, the average thickness of the PANI was from 7 to 15 and 25 nm (Figure S1) by changing the concentration of the aniline monomer from 1.2  10-3 to 1.6  10-3 and 2.0  10-3 M, respectively. 3.2. Synthesis of Well-Dispersed Fe3O4@PANI@Au Core/Shell Nanocomposite. It has been reported that PANI and its derivatives can act as reactive templates for polymersupported gold nanoparticles, which will facilitate the synthetic procedures for polymer-supported gold nanoparticles within one step.27 Very recently, Guo’s group28 has reported that welldefined gold nanoparticles with tunable size and morphology can be supported by the shells of hollow microspheres of poly(o-phenylenediamine) (PoPD), a derivative of PANI. The synthesis relies on a method from which the hollow microspheres of PoPD act as both reductant and template/stabilizer. In our experiment, the as-prepared Fe3O4@PANI microspheres can also be used as the reactive template to synthesize the Fe3O4@PANI@Au nanocomposites from Au(III) salt. However, the sizes of the as-prepared Au nanoparticles which are supported on Fe3O4@PANI microspheres are not uniform. Therefore, direct immobilization of presynthesized Au nanoparticles via electrostatic interactions could be employed to synthesize the monodisperse Fe3O4@PANI@Au nanocomposites. For the synthesis of the Fe3O4@PANI@Au nanocomposites, amine groups on the Fe3O4@PANI microspheres’ surface were protonated such that the citrate-coated, negatively charged Au seeds could be immobilized by electrostatic interactions.29 When the Fe3O4@PANI nanocomposites with a shell thickness of 25 nm were introduced into the Au colloid solution, Fe3O4@PANI@Au composite microspheres were formed by virtue of multiple electrostatic interactions. As shown in Figure 6a, Fe3O4@PANI@Au nanocomposites with well-defined core/shell nanostructure are well-dispersed on the copper grid. From the TEM image with a higher resolution (Figure 6b), clearly, all the Fe3O4 cores (dark) are wrapped by the composite PANI (gray) coating layer. Furthermore, there is another dark line-like coating on the PANI surface, which can be indexed to be Au nanoparticles. Figure 6c shows a representative TEM image of a single Fe3O4@PANI@Au core/shell microspheres. It is clear that all the Au nanoparticles are homogeneously located on the Fe3O4@PANI particles’ surface. No individual nanoparticles are found in the solution, indicating that the adsorption of the noble metal nanoparticles takes place completely. The TEM image with higher resolution (Figure 6d) shows that all the Au nanoparticles (4 ( 1 nm) are uniform in size and well-dispersed on the surface without forming large aggregates. Compare to the TEM image of the preprepared Au nanoparticles and the immobilized Au nanoparticles, it is obvious that the immobilization process does not affect the size of the Au nanoparticles (Figures S2 and S3). Moreover, these hybrid composite structures are stable and not disintegrated even after a sonication for 2 h. Figure 7 shows the UV-vis absorption spectra of the asprepared colloidal gold solution (a) and the gold solution after immobilization on the Fe3O4@PANI microspheres after magnetic separation (b). The characteristic UV-vis absorption peak of the citrate-stabilized colloidal Au nanoparticles appears at 518 nm. A dramatic decrease in the intensity at 518 nm is observed after the Au nanoparticle deposition, indicating that all of the Au (27) (a) Feng, X. M.; Mao, C. J.; Yang, G.; Hou, W. H.; Zhu, J. J. Langmuir 2006, 22, 4384–4389. (b) Gao, Y.; Chen, C. A.; Gau, H. M.; Bailey, J. A.; Akhadov, E.; Williams, D.; Wang, H. L. Chem. Mater. 2008, 20, 2839–2844. (28) Han, J.; Liu, Y.; Guo, R. Adv. Funct. Mater. 2009, 19, 1112–1117. (29) Fang, C. L.; Qian, K.; Zhu, J.; Wang, S.; Lv, X.; Yu, S. H. Nanotechnology 2008, 19, 125601.

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Figure 5. TEM images of the as-prepared Fe3O4@PANI core/shell nanocomposite with the PANI shell thickness of 0 (a-c), 7 (d-f), 15

(h-j), and 25 nm (k-m), which were prepared at a concentration of aniline in reaction system with 0, 1.2  10-3, 1.6  10-3, and 2.0  10-3 M, respectively.

Figure 7. UV-vis absorption spectra of the as-prepared colloidal Au solution (a) and the Au solution after immobilization on the Fe3O4@PANI microspheres after magnetic separation (b). Inset shows the photographs of the colloidal Au solution (1), reaction solution (2), and after magnetic separation (3). Figure 6. TEM images of the as-prepared Fe3O4@PANI@Au nanocomposite with core/shell nanostructure with different magnifications.

nanoparticles are adsorbed effectively on the PANI surface. The absorption intensity of the Au colloid is not affected by the magnetic separation. A fraction of the Au nanoparticles in solution that can be deposited on PANI is up to 98% (calculated by UV adsorption), which is further supported by the TG analysis (Figure S4). Moreover, the color transformation from red to colorless after the immobilization reaction also indicates the success of effective immobilization (inset of Figure 7). The compositions of the Fe3O4@PANI@Au composites were investigated by XRD. In comparison to the XRD diffraction of the Fe3O4@PANI (Figure 1b), three additional peaks at 38°, 43°, and 65°, which represent the Bragg reflections from (111), (200), and (200) planes of Au, are observed, showing clearly the existence of Au nanoparticles in the Fe3O4@PANI@Au composites (Figure 1c). The EDX result in Figure 8 11840 DOI: 10.1021/la901462t

Figure 8. EDX spectra of the as-prepared Fe3O4@PANI@Au composite with core/shell structure.

shows that the elemental compositions of the nanocomposites are Fe, C, O, and Au, which agrees with the XRD analysis. All the Langmuir 2009, 25(19), 11835–11843

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Figure 9. XPS spectroscopy of the as-prepared Fe3O4@PANI@Au core/shell composites. Inset shows the enlarged area corresponded to the Fe element.

above results indicate that Au nanoparticles were well-immobilized onto the Fe3O4@PANI microspheres to form the monodisperse Fe3O4@PANI@Au composites. XPS has often been used for the surface characterization of various materials, and unambiguous results are readily obtained when various surface components contain unique elemental markers. To further analyze the PANI/Au composite shell in the Fe3O4@PANI@Au system, XPS (Figure 9) was employed to examine the composition of the Fe3O4@PANI@Au particles’ surface. It is clear that the elemental contents of the surface are C, O, N, and Au. The binding energy at 710.20 eV for Fe 2p3 cannot be detected, which further supports that all the Fe3O4 cores in the composite are confined within a shell of PANI/Au, in agreement with the above TEM and XRD analyses. In this case, the PANI has served as a better support for noble metal nanoparticle dispersion with strong affinity and more effectively in suppressing agglomeration and aggregation of Au nanoparticles during the immobilization process. In a parallel experiment, amine-functionalized Fe3O4@SiO2 core/shell microspheres with a shell thickness of 25 nm (Figure S5) were employed to investigate the immobilization efficiency. Figure 10 shows the TEM images of the Fe3O4@SiO2@Au microspheres which were synthesized using the previously described method with aminefunctionalized Fe3O4@SiO2 microspheres as the catalyst support. In comparison to the Fe3O4@PANI@Au nanocomposite (Figure 6), although most of the Au nanoparticles were well located on the core/shell particles’ surface (Figure 10a, Figure S6), some large aggregates of Au nanoparticles are also clearly observed (Figure 10b). Further investigations demonstrate that the Au nanoparticles which immobilized on the Fe3O4@SiO2 microspheres are less stable (Figure 10c) than those immobilized on the Fe3O4@PANI microspheres (Figure 10d) under ultrasonic treatment. Thus, these results suggest that the Fe3O4@PANI microsphere serves as a better catalyst support than the Fe3O4@SiO2. 3.3. Catalytic Properties of the As-Prepared Fe3O4@PANI@Au Core/Shell Nanocomposites. Metallic gold catalysts usually possess high activity and selectivity.28 The immobilization of Au nanoparticles with magnetic support may render them a practical recyclable nanocatalyst. By way of an example, we employ the Fe3O4@PANI@Au nanocomposites for the catalytic reduction of rhodamine B dye in the presence of NaBH4 as a model system as well as to demonstrate the recyclability and repeated catalytic activities of our Fe3O4@PANI@Au nanocomposites. One of the advantages for the reduction of RhB with NaBH4 is that the reaction is easily monitored by Langmuir 2009, 25(19), 11835–11843

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Figure 10. TEM images of the as-prepared Fe3O4@SiO2@Au with core/shell nanocomposite with different magnification (a-c) and the high-magnification TEM image of the as-prepared Fe3O4@PANI@Au (d).

UV/vis absorption spectroscopy with no appreciable of byproduct formation. In agreement with previous results,30 the reduction of RhB, if it occurs at all, is insignificant in the absence of Au nanoparticles. Without the Au catalyst, the reduction of RhB does not proceed even with large excess of NaBH4, as evidenced by an absorption spectrum with the major peak located at 554 nm;a characteristic absorption band of RhB. Additionally, when RhB solutions were tested for NaBH4 reduction in the presence of Fe3O4@PANI composite spheres, the color of the RhB dyes remained unchanged. That is, the peak at 554 nm was still observed, indicating that the RhB was not reduced. However, when catalytic amounts of our Fe3O4@PANI@Au composite catalyst are introduced into the RhB solutions, the absorption at 554 nm decreases gradually, indicating that the reduction of RhB occurs spontaneously. This evidence confirms that the Fe3O4@PANI@Au composite has a good catalytic performance. Since the concentration of the Fe3O4@PANI@Au catalyst is relatively low, light scattering that is caused by the carrier colloids does not interfere with the absorption measurements. Figure 11 shows the UV/vis absorption spectra of the RhB dyes during their catalytic reductions with Fe3O4@PANI@Au. Evidently, the maximal absorption (λmax = 554 nm) of the dyes gradually decreases in time, indicating the reduction of the RhB dyes. The catalytic reduction of the dyes proceeds successfully, wherein no deactivation or poisoning of the catalyst is observed. The rate of the catalyzed reduction of the dyes increases with increased concentrations of the magnetic composite catalyst (Figure 12). Clearly, the as-prepared Fe3O4@PANI@Au composite catalyst can be efficiently recovered and recycled in the reaction mixtures by magnetic separation. As an example, when the composite spheres are dispersed in water to give a black suspension, upon applying an external magnetic field, the black powder is readily harvested within 5 min; thereby the solution becomes transparent. As a recyclable catalyst, another important factor; the renewable catalytic activity;was also investigated by performing the RhB reductions five times using the recycled composite nanocatalysts. The catalytic activity of the recycled composite nanocatalysts was tested five times with magnetic separations. (30) (a) Jiang, Z. J.; Liu, C. Y.; Sun, L. W. J. Phys. Chem. B 2005, 109, 1730– 1735. (b) Jana, N. R.; Sau, T. K.; Pal, T. J. Phys. Chem. B 1999, 103, 115–121.

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Figure 11. UV-vis absorption spectra of RhB during the reduction catalyzed by Fe3O4@PANI@Au composite microspheres. [RhB] = 2.5  10-5 mol/L, [NaBH4] = 1  10-2 mol/L, [Au]0 = 2.5  10-5 mol/L. The arrows mark the increase of reaction time, showing the gradual reduction of RhB with Fe3O4@PANI@Au catalyst.

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Figure 13. Conversion of RhB in five successive cycles of reduction and magnetic separation with two types of catalysts: Fe3O4@PANI@Au and Fe3O4@SiO2@Au. [RhB] = 2.5  10-5 mol/L, [NaBH4] = 1  10-2 mol/L, [Au]0 = 6.25  10-5 mol/L.

Figure 14. M-H curve of the Fe3O4@PANI@Au composite at room temperature. Figure 12. Plot of conversion (C, %) of RhB versus time (t, min) at different concentrations of Au nanocatalysts: (a) [Au]0 = 2.5  10-5 mol/L, (b) [Au]0 = 3.13  10-5 mol/L, (c) [Au]0 = 3.75  10-5 mol/L, (d) [Au]0 = 6.25  10-5 mol/L. [RhB] = 2.5  10-5 mol/L and [NaBH4] = 1  10-2 mol/L keep constant.

UV/vis absorption spectroscopy was employed to monitor the changes in absorbance at 554 nm. Figure 13 shows the conversion for each run which was measured by UV/vis spectroscopy. For Fe3O4@PANI@Au, the reduction of RhB drops slightly after each cycle (Figure 13a), and it decreased gradually in subsequent runs to 89% at run 5. TEM micrographs (Figure S7) revealed many large Au nanoparticles exist in the Fe3O4@PANI@Au catalysts after 5 runs. This could have led to the reduced catalytic activity after multiple recycling processes. However, the reduction ratio drops rapidly in the following reaction cycles when the Fe3O4@SiO2@Au nanocomposite (Figure 13b) was employed as the catalyst (only keep 40% conversion). The catalytic results reveal that the as-synthesized Fe3O4@PANI@Au nanocatalyst show higher catalytic performance than the Fe3O4@SiO2@Au, which may be partly caused by the effective contact between PANI and Au nanocatalysts. Moreover, in this catalytic reaction, the relatively high concentration of NaBH4 slowly etches the silica surfaces, which lead to the gradual detachment of Au nanoparticles from the support surface and dramatically reduced their catalytic activity.31 Therefore, compared to the Fe3O4@SiO2@Au catalyst that has been studied, Fe3O4@PANI@Au

nanocatalyst exhibited much higher stability in various chemical and physical environments. It has been experimentally demonstrated that metal nanoparticles have high catalytic activities for hydrogenation, hydroformylation, carbonylation, and so forth. In our system, with the Au nanocatalyst, the RhB was reduced to give its leuco form (Figure S8).32 The Au nanoparticles which are supported on Fe3O4@PANI microspheres serve as an electron relay in the system for an oxidant and a reductant, and electron transfer occurs via the supported Au nanoparticles.30 In the reaction, the nucleophile NaBH4 can donate electrons to Au nanoparticles, and the electrophile RhB dyes would capture electrons from Au nanoparticles. Therefore, in such reaction, the rate of catalytic reaction should be determined by the concentration of the catalyst. The magnetic properties of the composite nanocatalysts have been investigated using a VSM. Figure 14 shows a hysteresis loop of typical Fe3O4@PANI@Au nanocomposites measured by sweeping the external field between -1 and 1 T at room temperature. The magnetization curve shows no remanence or coercivity at room temperature, suggesting the superparamagnetic character. Superparamagnetism is the responsiveness to an applied magnetic field without retaining any magnetism after removal of the applied magnetic field. With the superparamagnetic property, capillary blockage by aggregations formed by

(31) Ge, J. P.; Zhang, Q.; Zhang, T. R.; Yin, Y. D. Angew. Chem., Int. Ed. 2008, 47, 8924–8928.

(32) (a) Kr€uger, U.; Memming, R. Ber. Bunsen-Ges. Phys. Chem. 1974, 78, 670– 678. (b) Li, Y. X.; Lu, G. X.; Li, S. B. J. Photochem. Photobiol. A: Chem. 2002, 152, 215–228.

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residual magnetism after removal of the applied field will be avoided. The saturated mass magnetization is estimated to be 29.7 emu/g. Therefore, the as-prepared Fe3O4@PANI@Au nanocatalyst can be easily separated from the reaction solution by applying a relatively low magnetic field gradient, which renders the catalyst cost-effective and promising for various applications.

4. Conclusions In summary, we have demonstrated a simple, reproducible, and facile method of preparing core/shell Fe3O4@PANI@Au nanocomposites. On the basis of the carboxyl-functionalized Fe3O4 microspheres, PANI shell with various thicknesses from 7 to 15 and 25 nm can be directly coated on the cores to form the monodisperse Fe3O4@PANI microspheres with well-defined core/shell nanostructure. By means of the electrostatic interactions between positively charged PANI coating and citrate-stabilized Au nanoparticles, plentiful Au nanoparticles with ∼4 nm in size are well assembled onto the surface of Fe3O4@PANI carriers

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to form the Fe3O4@PANI@Au composite microspheres. These Au-immobilized nanocomposites exhibit excellent catalytic properties as the demonstration on the reduction of RhB with NaBH4 in comparison to the Fe3O4@SiO2@Au nanocatalyst. Most importantly, these results offer a powerful platform to construct other multicomponent composite spheres, which are likely found many potential catalytic and biomedical applications derived from their rational combination of magnetic properties with surface plasmon resonance, luminescence, or catalysis. Acknowledgment. The authors acknowledge the support from a Strategic Investments Scheme and Direct Grant of Research (2060336 and 2041339) administrated by The Chinese University of Hong Kong as well as The Research Grants Council of Hong Kong (CUHK401709). Supporting Information Available: Experimental details, TEM, TGA and EDX data. This material is available free of charge via the Internet at http://pubs.acs.org.

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