Bifunctional Au−Fe3O4 Heterostructures for Magnetically Recyclable

ARTICLE pubs.acs.org/JPCC

Bifunctional Au
Fe3O4 Heterostructures for Magnetically Recyclable Catalysis of Nitrophenol Reduction Fang-hsin Lin and Ruey-an Doong* Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, 30013, Taiwan

bS Supporting Information ABSTRACT: The dumbbell- and flower-like Au
Fe3O4 heterostructures by thermal decomposition of the iron
oleate complex in the presence of Au nanoparticles (NPs) have been successfully fabricated using different sizes of Au NPs as the seeds for magnetically recyclable catalysis of p-nitrophenol and 2,4-dinitrophenol reduction. The heterostructures exhibit bifunctional properties with high magnetization and excellent catalytic activity toward nitrophenol reduction. The epitaxial linkages in dumbbell- and flower-like heterostructures are different, leading to the change in magnetic and catalytic properties of the heterostructured nanocatalysts. The pseudo-first-order rate constants for nitrophenol reduction are 0.63
0.72 min
1 and 0.38
0.46 min
1 for dumbbell- and flower-like Au
Fe3O4 heterostructures, respectively. In addition, the heterostructured nanocatalysts show good separation ability and reusability which can be repeatedly applied for nearly complete reduction of nitrophenols for at least six successive cycles. The reaction mechanism for nitrophenol reduction by Au
Fe3O4 nanocatalysts is also proposed and confirmed by XPS and FTIR analyses. These unique properties make Au
Fe3O4 heterostructures an ideal platform to study various heterogeneous catalytic processes which can be potentially applied in a wide variety of fields in purification, catalysis, sensing devices, and green chemistry.

1. INTRODUCTION Nobel metal nanostructures have recently received much attention because of their unique optical, catalytic, and electrochemical properties which make them suitable materials for potential applications in various fields.1,2 Gold nanoparticles (Au NPs) have been found to play an important role in several catalytic processes including low-temperature CO oxidation,3 reductive catalysis of chlorinated or nitrogenated hydrocarbons,4
6 and organic synthesis.7,8 Due to the high cost and limited supply, however, the improvement of the catalytic efficiency and the reduction of the used amounts are the top priorities for practical applications. The deposition of Au NPs onto porous supports such as TiO2, SiO2, and carbon is regarded as a conventional way to solve the problem by maximizing the loading of catalysts and to enhance the catalytic activity by well-tuning the surface functionality.9
12 However, the entrapment or immobilization of the nanocatalysts on solid supports normally results in a decrease in the catalytically active surface area and the reactivity of catalytic species.13 Recently, the doping of Au NPs into the interior of spherical Ag
C composites containing Ag NPs has been synthesized for reduction of 4-nitrophenol in the presence of sodium borohydride.12 The catalytic activity of bimetallic composites is highly enhanced over the monometallically doped carbon spheres. However, these catalysts are usually recycled by tedious and time-consuming centrifugation/redispersion cycles, r 2011 American Chemical Society

thus hampering the recovery and reusability of catalysts in aqueous solutions. The magnetic nanoparticles have recently emerged as viable alternatives to conventional materials for catalyst supports.14,15 Their insoluble and superparamagnetic natures enable troublefree separation of the nanocatalysts from the reaction mixture using an external magnet, which eliminates the necessity of catalyst filtration.16 Ge et al. synthesized a nanostructured composite with a high specific surface area and magnetic separation ability for 4-nitrophenol reduction.17 A complete conversion of 4-nitrophenol was obtained within 1 h, and the catalysts were recycled by an external magnet and reused eight times with almost identical reaction rate. Deng et al. also fabricated a multicomponent nanostructure composed of a magnetic-silica core, a layer of gold nanoparticles, and a mesoporous silica shell for both 4-nitrophenol reduction and styrene epoxidation.18 Although these materials show improved stability and recyclability, the synthesis procedures are complicated and the magnetic property is usually hindered by the silica shell. Dumbbell-like Au
Fe3O4 nanostructures, where one nanoparticle is linked to another, have been used as the nanocatalysts for CO oxidation as well as H2O2 Received: November 16, 2010 Revised: February 19, 2011 Published: March 17, 2011 6591

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The Journal of Physical Chemistry C reduction.19,20 The combination of Au NPs with magnetic Fe3O4 NPs can not only provide catalytic activity but also be reclaimed via magnetic separation after use. Moreover, the magnetic property was enhanced via interfacial interaction.21 It is believed that electron transfer across the interface between these two NPs may lead to a dramatic change in physicochemical properties, thus offering an ideal platform to study the multifunctionality of nanomaterials.22,23 In addition, the Au
Fe3O4 heterostructures contain both magnetically, optically, and catalytically active NPs, which show high potential applications to chemical catalysis, drug delivery, and biomedical imaging.24
26 The thermal decomposition of iron precursors including iron pentacarbonyl, iron acetylacetonate, and the iron
oleate complex at high temperature is one of the most efficient techniques to synthesize monodisperse magnetic NPs.27,28 The size and morphology of Fe3O4 NPs can be tuned by simply adjusting the reaction temperature ranging from 280 to 380 °C.29
34 In addition, the synthesis of heterostructures that contain a noble metal particle in the structures has recently received considerable attention.35
37 Yu et al. fabricated Au
Fe3O4 dumbbell structures by thermal decomposition of iron pentacarbonyl in the presence of preformed Au nanoparticles in 1-octadecene followed by oxidation of iron nanocrystals in air at room temperature.38 The thermal decomposition of mixtures of metal
oleate complexes and metal
oleylamine complexes in the presence of 1,2-hexadecanediol has also been reported.39 Although these strategies produce well-crystallized nanostructures, the synthesis processes are usually expensive and may contain toxic reagents, leading to the difficulty in practical application. In addition, the fabrication of different morphologies of Au
Fe3O4 heterostructures by tuning the size of Au NPs has received less attention. Herein, we demonstrate a facile method for the synthesis of different morphologies of monodisperse Au
Fe3O4 heterostructures by thermal decomposition of iron
oleate complex (Fe(OL)3) in the presence of Au seeds at 310 °C. The designed heterostructures show excellent dual functions which can not only undergo rapid catalytic reduction of nitrophenols including p-nitrophenol and 2,4-dinitrophenol in the presence of NaBH4 but also be easily recycled using an external magnetic field. To our best knowledge, this is the first report demonstrating the use of Au
Fe3O4 heterostructures for magnetically recyclable catalysis of nitroaromatic compounds.

2. EXPERIMENTAL SECTION 2.1. Chemicals. Iron chloride (FeCl3 3 6H2O, 98%), oleylamine (>70%), oleic acid (90%), 1-octadecene (90%), and tert-butylamine
borane complex (97%) were purchased from Sigma-Aldrich. Sodium oleate (95%) was purchased from TCI. Trisodium citrate dehydrate (>99%) was purchased from Ferak. Sodium borohydride (95%) and ethanol absolute (99.8%) were purchased from Riedel-de Ha€en. p-Nitrophenol and 2,4-dinitrophenol were purchased from Fluka. Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 3 3H2O) was purchased from Alfa Aesar. Cyclohexane (99.95%) was purchased from TEDIA. nHexane was purchased from J. T. Baker. 2.2. Synthesis of Gold NPs with Different Sizes. The Au NPs with sizes of 4
5 nm were prepared by dissolving 40 mg of HAuCl4 3 3H2O in a mixture containing 4 mL of oleylamine and 4 mL of cyclohexane in air followed by magnetic stirring at 10 °C under a gentle stream of nitrogen gas. An amount of 0.2 mmol of tert-butylamine
borane complex was dissolved in 0.4 mL of

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oleylamine and 0.4 mL of cyclohexane and then injected into the precursor solution. The solution color changed to deep red immediately after injecting the borane complex solution. The mixture was aged for 40 min at 10 °C followed by addition of 30 mL of ethanol to precipitate the Au NPs. The Au NPs were then harvested by centrifugation and redispersed in hexane. For preparation of 10 nm Au NPs, 40 mg of HAuCl4 3 3H2O was dissolving in a mixture containing 4 mL of 1-octadecene and 4 mL of oleylamine in air. The resulting solution was put in an oil bath at 120 °C and reaction for 30 min under N2 atmosphere. After reaction, the mixture was cooled to room temperature and followed by addition of 30 mL of ethanol to precipitate the Au NPs. The product was centrifuged and redispersed in hexane. 2.3. Synthesis of Au
Fe3O4 Heterostructures. A solution containing 0.5 mmol of oleic acid, 0.5 mmol of oleylamine, 1 mmol of Fe(OL)3, 0.1 mmol of gold colloid dispersion, and 5 mL of octadecene was heated to 110 °C for 20 min. The solution was refluxed at 310 °C for 30 min. After cooling to room temperature, the particles were separated by adding absolute ethanol, centrifugation, and redispersion into hexane. 2.4. Preparation of Water-Soluble Au
Fe3O4 NPs. The Au
Fe3O4 NPs were washed with a mixture of hexane and ethanol (1:2) several times to remove excess capping agent on the surface of NPs. The heterostructured Au
Fe3O4 NPs were then dried and added into an aqueous solution containing 50 mM sodium citrate. After reaction of 10 min, the Au
Fe3O4 heterostructures were separated by a magnet and washed with deionized water three times. The particles were then dissolved in deionized water. 2.5. Catalytic Reaction. The reduction of nitrophenol compound by water-soluble Au
Fe3O4 NPs in the presence of NaBH4 was carried out to examine the catalytic activity and recyclability of the Au
Fe3O4 nanocatalysts. Amounts of 2 mL of deionized water, 40 μL of 10 mM nitrophenol, and 0.16 mL of 0.1 M NaBH4 solutions were added into a quartz cuvette followed by addition of 2 mg of water-soluble Au
Fe3O4 NPs to the mixture. The color of the solution changed gradually from yellow to transparent as the reaction proceeded. UV
vis spectrometry was used to record the change in absorbance at a time interval of 2 min. 2.6. Surface Characterization. Transmission electron microscopy (TEM) images were obtained on a JEOL 2011 microscope operated at 120 kV. High-resolution transmission electron microscopy (HR-TEM) was carried out on a JEOL JEM-2010 microscope at 200 kV. The samples were prepared by suspension in hexane. Wide-angle XRD patterns were recorded on a Bruker D8 X-ray diffractometer with Ni-filtered Cu KR radiation (λ = 1.5406 Å) and operated at a generator voltage and an emission current of 40 kV and 40 mA, respectively. A Hitachi U-3010 UV
vis spectrometer using a 1 cm path length quartz cuvette was used to identify the change in concentration over a wavelength range from 200 to 600 nm. Magnetic measurements were carried out using a superconducting quantum interference device magnetometer (SQUID MPMS5, Quantum Design Inc.) with a maximum applied continuous field of 10 000 G at room temperature. X-ray photoelectron spectroscopy (XPS) measurements were performed by an ESCA PHI 1600 photoelectron spectrometer using an Al KR X-ray source (1486.6 eV photon energy). During data acquisition, the pressure in the sample chamber was maintained below 2.5  10
8 Torr. The binding energies of the photoelectrons were determined under the assumption that Au has a binding energy of 84.0 eV. FTIR 6592

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Figure 1. TEM images of (a) 5 nm Au NPs, (b) dumbbell-like Au
Fe3O4 heterostructures, (d) 10 nm Au NPs, and (e) flower-like Au
Fe3O4 heterostructures. Figures (c) and (f) are HRTEM images of dumbbell- and flower-like Au
Fe3O4 heterostructures, repsectively.

Figure 2. Histogram analysis of particle sizes of (a) dumbbell-like and (b) flower-like Au
Fe3O4 heterostructures.

spectra were obtained by a Horiba FT-720 spectrometer with KBr method.

3. RESULTS AND DISCUSSION 3.1. Characterization of Au
Fe3O4 Heterostructures. The heterostructured Au
Fe3O4 nanoparticles were prepared by thermal decomposition of the iron
oleate complex in the presence of different sizes of Au NPs. The morphology of these heterostructures is highly dependent on the size of Au seeds. Figure 1 shows the TEM and HR-TEM images of Au
Fe3O4 heterostructures synthesized by using different sizes of Au seeds ranging between 5 and 10 nm (Figure 1a, d). The nanostructured nanoparticles show a dumbbell-like structure when small-sized Au NPs are used as seeds (Figure 1b). The epitaxial relationship between Au and Fe3O4 nanoparticles was further examined by HR-TEM. The interfringe distances are measured to be 0.24 nm for Au nanoparticles and 0.24 nm for Fe3O4 nanoparticles, which correspond to the (111) plane of face-centered cubic (fcc) Au and (311) plane of inverse spinel structured magnetite, respectively (Figure 1c). In addition, the line-scan analysis was used to get information on relative locations of Au and Fe3O4 in the heterostructures. As depicted in Figure S1 (Supporting Information), different distribution patterns of Au and Fe are observed. The Fe signals mainly locate at 14
26 and 36
44 nm, while

Au signals appear at 26
33 and 44
49 nm, which confirm that the Au and Fe3O4 NPs in dumbbell-like heterostructures are in an epitaxial relationship. The histogram analysis shows that the Au and Fe3O4 NPs in dumbbell-like structures are in the range 2.8
5.8 and 11
15 nm with mean sizes of 5 and 12 nm, respectively (Figure 2a). The sizes of dumbbell-like nanostructures are also in the range of 12
16 nm. Using large Au NPs of 7
13 nm as seeds, flower-like structures with sizes of 20
28 nm are formed (Figure 2b). A previous study depicted that the crystallinity of Au seeds controlled the nucleation process, with one iron oxide leaf nucleated per monocrystalline domain of gold.40 In this study, large Au NPs provide large surface areas and multiple monocrystalline domains for Fe3O4 to nucleation, resulting in the production of flower-like heterostructures. In addition, the d-spacings of 0.24 and 0.48 nm for Au and magnetite NPs, respectively, are observed, clearly showing the growth of the Fe3O4(111) plane onto a Au(111) plane to form a flowerlike heterostructure. In addition, parallelogram-like Au
Fe3O4 heterostructures were obtained when the particle size of Au seeds increased to 20 nm (Supporting Information, Figure S2), clearly indicating that the morphology of Au
Fe3O4 heterostructures is highly dependent on the size of Au seeds. The crystallinity of Au
Fe3O4 heterostructures is characterized by XRD. Figure 3a shows the XRD patterns of different morphologies of Au
Fe3O4 NPs. Five resolved peaks at 30.10°, 6593

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Figure 3. (a) XRD patterns and (b) magnetic hysteresis loops of dumbbell- and flower-like Au
Fe3O4 heterostructures.

Figure 4. Time-dependent UV
vis spectral changes in (a) p-nitrophenol (4-NP) and (b) 2,4-dinitrophenol (2,4-DNP) catalyzed by Au
Fe3O4 heterostructures and concentration change in nitrophenol compounds (Ct/C0) in the presence of (c) dumbbell-like and (d) flower-like Au
Fe3O4 nanocrystals. Insets in Figures (c) and (d) are linear relationship of ln(Ct/C0) as a function of time for 4-NP and 2,4-DNP, respectively.

35.54°, 43.09°, 56.98°, and 62.58° 2θ, which can be assigned as the fcc Fe3O4, are observed. In addition, peaks at 38.18°, 44.39°, 64.58°, and 77.55° 2θ are common patterns for fcc-structured Au. The XRD patterns of Au
Fe3O4 NPs match well with those corresponding JCPDS standards of Au and Fe3O4 (JCPDS 040784; JCPDS 65-3107, clearly indicating the nature of heterodimer structures of Au
Fe3O4 NPs. In addition, the epitaxial linkage in heterostructures has a significant effect on the change in optical properties of Au NPs.37 The pure Au NPs show a surface plasmon resonance peak at 517 and 520 nm for 5 and 10 nm Au NPs, respectively. After conjugation with Fe3O4 NPs, the peak is broadening and red-shifts to 567 nm in dumbbell-like structure and 550 nm in flower-like structures (Supporting Information, Figure S3). The relatively weak reflectance of Au
Fe3O4 NPs is primarily attributed to the dilution effect of Fe3O4 on Au NPs in the heterostructures.9 Moreover, the magnetic measurement shows that Au
Fe3O4 NPs are superparamagnetic at room temperature (300 K) (Figure 3b). The hysteresis loops of Au
Fe3O4 NPs indicate that the saturation magnetization is 31 emu/g for dumbbell-like structures and 43 emu/g for flower-like structures at 300 K. After normalization to the unit weight of Fe3O4, the saturation magnetizations are 41

and 51 emu/g-Fe3O4 for dumbbell- and flower-like Au
Fe3O4 nanostructures, respectively. It is noteworthy that saturation magnetization obtained in this study is lower than that of bulk magnetite (90 emu/g).41 However, these values are higher than those reported data prepared by the similar procedure after normalization to the unit weight of Fe3O4.28,42 3.2. Application of Au
Fe3O4 Heterostructures for Catalytic Reduction of Nitrophenols. The catalytic reduction of p-nitrophenol to their corresponding daughter derivatives, paminophenol, in the presence of NaBH4 was chosen as a model reaction to investigate the bifunctionality of Au
Fe3O4 heterostructures. Such a reaction catalyzed by Au catalysts has been reported because this reaction can be rapidly and easily characterized.43
45 In addition, 2,4-dinitrophenol was also selected as another model compound for elucidating the reaction kinetics as well as a mechanism for nitrophenol reduction. Figure 4 shows the typical UV
vis spectra and concentration change of nitrophenol compounds in the presence of different morphologies of Au
Fe3O4 heterostructures and NaBH4. The original absorption peak of p-nitrophenol is centered at 317 nm and shifts to 400 nm after addition of freshly prepared NaBH4 solution, indicating the formation of p-nitrophenolate ions (Supporting 6594

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Figure 5. Catalytically recyclable reduction of (a) p-nitrophenol and (b) 2,4-nitrophenol by dumbbell-like Au
Fe3O4 NPs in the presence of NaBH4. Conversion efficiency of (c) p-nitrophenol in six successive cycles of reduction and (d) 2,4-nitrophenol in seven successive cycles of reduction by Au
Fe3O4 and citrate-stabilized Au nanocatalysts.

Information, Figure S4).46 This peak starts to decrease when the reduction proceeds in the presence of Au
Fe3O4 nanocatalysts. Addition of NaBH4 in the absence of Au
Fe3O4 NPs has little effect on the change in absorbance at 400 nm, confirming that the reduction is mainly catalyzed by the Au
Fe3O4 NPs. In addition, the absorption peak at 400 nm decreases with the concomitant increase in peak intensity at 300 nm within 10 min after addition of Au
Fe3O4 catalysts (Figure 4a). A similar reduction pattern for 2,4-dinitrophenol is also observed in which the peak at 357 nm decreases with the increase in absorption at 450 nm (Figure 4b), clearly indicating the production of intermediate of 2-amino-4-nitrophenol.47 The concentration of 2-amino-4-nitrophenol at 450 nm decreases again with the concurrent increase in peak intensity of p-aminophenol at 300 nm. This means that 2,4-dinitrophenol undergoes the consecutive reaction to form 2-amino-4-nitrophenol and then to p-aminophenol in the presence of Au
Fe3O4 nanocatalysts and NaBH4. The pseudofirst-order kinetics can be applied to evaluate the rate constants for nitrophenol reduction because the concentration of NaBH4 is higher than those of nitrophenols and can be considered as a constant during the reaction period. The concentration of p-nitrophenol and 2,4-dinitrophenol at time t is denoted as Ct, and the initial concentration of nitrophenols at t = 0 is regarded as C0. The Ct/C0 is measured from the relative intensity of absorbance (At/A0). The linear relationship of ln(Ct/C0) versus time (t) indicates that the reduction of nitrophenols by Au
Fe3O4 heterostructures follows the pseudofirstorder kinetics. The rate constants for nitrophenol reduction are 0.63 min
1 for p-nitrophenol and 0.72 min
1 for 2,4-dinitrophenol by using dumbbell-like Au
Fe3O4 nanocatalysts (Figure 4c). In addition, the flower-like Au
Fe3O4 NPs are also used as nanocatalysts for reduction of nitrophenols. The rate constants for p-nitrophenol and 2,4-dinitrophenol reduction catalyzed by flower-like Au
Fe3O4 nanocatalysts are 0.38 and 0.46 min
1, respectively (Figure 4d). The catalytic efficiency as well as the rate

constants for nitrophenol reduction by both dumbbell- and flowerlike Au
Fe3O4 are higher than those previously reported values obtained from the catalysis of p-nitrophenol by Au-based materials (Supporting Information, Table S1).48,49 This result clearly indicates that Au
Fe3O4 heterostructures are superior nanocatalysts which can enhance the catalytic efficiency and minimize the used amounts of catalysts for reaction, especially only when trace amounts of Au catalysts are used (0.38
0.96 mg Au) for reduction. It is noteworthy that the catalytic efficiency of flower-like Au
Fe3O4 is lower than that of dumbbell-like structures (Supporting Information, Figure S5), presumably due to that the Au surfaces in flower-like structures are mainly occupied by the Fe3O4 leaves and thus suppress the reaction rate of nitrophenols. Therefore, the dumbbell-like Au
Fe3O4 heterostructures are selected as the model nanocatalysts for further experiments. The as-prepared Au
Fe3O4 heterostructures show both catalytic and magnetic properties which can be easily recycled by an external magnet after the catalytic reduction. Figure 5 shows the magnetically recyclable reduction of nitrophenols in the presence of dumbbell-like Au
Fe3O4 nanocatalysts. The catalysts can be successfully recycled and reused for at least six successive cycles of reaction with stable conversion efficiency of around 100%. The hydrodynamic size of Au
Fe3O4 nanoparticles remains unchanged after several cycles of catalytic reactions (Supporting Information, Figure S6), indicating the stability of the dumbbell-like nanoparticles in aqueous solutions. In addition, the citrate-stabilized Au NPs were used as the catalysts to reduce nitrophenol compounds for comparison. The conversion efficiency of p-nitrophenol and 2,4-dinitrophenol by citratestabilized Au NPs drops dramatically after the second cycle, which is primarily attributed to the loss of Au NPs after periodic centrifugation/redispersion cycles. The average particle sizes of citrate-stabilized Au NPs before and after the centrifugation, determined by TEM images, are 11.2 ( 0.8 and 11.7 ( 1.8 nm, respectively, clearly indicating that the decrease in conversion 6595

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Scheme 1. Possible Mechanism for Magnetically Recyclable Catalysis of Nitrophenols by Au
Fe3O4 Heterostructures

efficiency of nitrophenols by Au NP is primarily attributed to the loss of Au NPs after periodic centrifugation/redispersion cycles. In addition, the same Au seeds used for synthesis of Au
Fe3O4 heterostructures were also employed for reduction of p-nitrophenol. The single-component Au NP catalysts were obtained by etching Fe3O4 away from the Au
Fe3O4 NPs in 0.5 M H2SO4 solutions.10 The catalytic efficiency of p-nitrophenol by Au seeds is low when compared with that by dumbbell-like Au
Fe3O4 heterostructures, presumably due to the aggregation of Au seeds during the etching step (Supporting Information, Figure S7). When adding sodium citrate to the solution containing p-nitrophenol and NaBH4, little p-nitrophenol was reduced, suggesting that citrate has no effect on nitrophenol reduction. These results demonstrate that the Au
Fe3O4 NPs are superior catalysts than Au itself and other supported Au catalysts, presumably attributed to the electronic junction effect of Au and Fe3O4 NPs.20
22 This electronic junction effect can also be observed in reduction catalysis of H2O2 by Au
Fe3O4 dumbbell-like structure.20 In addition, the Au NPs in dumbbell-like structure were stable against aggregation during harvest procedure, resulting in the enhanced catalysis of nitrophenol compounds. It is obvious that the presence of Fe3O4 NPs makes the dumbbell-like heterostructures a promising bifunctional probe for magnetically recyclable catalytic reduction. When the reduction is complete, the Au
Fe3O4 nanocatalysts can be separated easily and rapidly from the solution within 10 s by a magnet and then be redispersed into deionized water for the next cycle of catalysis (Supporting Information, Figure S8). Although the Au
Fe3O4 heterostructures show superior catalytic and recycling efficiencies, the rate constant for nitrophenol reduction decreased when Au
Fe3O4 NPs were reused (Supporting Information, Figure S9). The decrease in rate constants for nitrophenol reduction may probably be attributed to the generation of aminophenols after catalytic reduction and then bound to the surface of Au NPs. Scheme 1 shows the catalytic mechanisms for nitrophenol reduction by Au
Fe3O4

in the presence of NaBH4. When Au
Fe3O4 NPs are used for catalytic reduction, BH4
and nitrophenols (p-nitrophenol and 2,4-dinitrophnol) are first diffused from aqueous solution to the Au surface, and then the bare Au NPs on heterostructures serve as catalysts to transfer electrons from BH4
to nitrophenols, leading to the production of amino derivatives, 2-amino-4-nitrophenol and p-aminophenol.50 It is noteworthy that the amine (
NH2) group in aminophenols has a strong binding ability with Au NPs and, therefore, adsorbs onto the surface of Au NPs, resulting in the block of reactive sites on Au NPs. To verify the hypothesis of surface blocking by NH2 groups, aniline is used to pretreat the Au
Fe3O4 nanocatalysts prior to the reduction of p-nitrophenol. The rate constants for p-nitrophenol reduction by aniline pretreated Au
Fe3O4 nanocatalysts decrease dramatically after the second cycle (Supporting Information, Figure S10), which is similar to the results shown in Figure S9 (Supporting Information). In addition, XPS and FTIR are used to characterize the change in surface species on Au
Fe3O4 heterostructures before and after the reduction (Figure 6). The XPS of N1s spectra show no peak before the reaction, while one predominant peak appears at 400 eV after the catalytic reduction, which is consistent with the result of p-aminophenol, and can be assigned as the amine (
NH2) group after peak deconvolution (Figure 6a).51 This result clearly indicates the chemisorption of aminophenol onto the surface of Au NPs. In addition, all the FTIR spectra exhibit symmetric and asymmetric stretching vibrations of the
CH3 bond at 2840 and 2950 cm
1, respectively (Figure 6b). The Fe
O bonding in the range 570
600 cm
1 is also clearly observed. The Au
Fe3O4 heterostructures in the organic phase show weak CdO and C
N stretching peaks at 1704 and 1439 cm
1, respectively, indicating the presence of oleylamine and oleic acid on the surface of nanoparticles. After ligand exchange with sodium citrate in aqueous solution, the peak of C
N stretching disappears, while the CdO peak at 1613 cm
1 is 6596

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Figure 6. (a) XPS N1s spectra of of dumbbell-like Au
Fe3O4 NPs before and after catalytic reaction. (b) FT-IR spectra of dumbbell-like Au
Fe3O4 NPs capped with different ligands.

observed. After the catalytic reduction of nitrophenols, the C
N stretch in Au
Fe3O4 reappears at 1421 cm
1, confirming the attachment of aminophenols onto the catalyst surfaces.

4. CONCLUSIONS In this study, we have first demonstrated that the heterostructured Au
Fe3O4 nanocatalysts synthesized via thermal decomposition of the iron
oleate complex in the presence of Au seeds have excellent bifunctional characteristics for reusability and catalytic reduction. The size and morphology of Au
Fe3O4 nanocatalysts are highly dependent on the size of Au seeds. The catalytic performance of both dumbbell- and flower-like Au
Fe3O4 NPs is excellent for nitrophenol reduction in the presence of NaBH4. In addition, the catalytic efficiency of dumbbell-like Au
Fe3O4 NPs is higher than that of flower-like heterostructures because of the high surface coverage of the Au surface by Fe3O4 nanocrystals in flower-like heterostructures. The dumbbell-like nanoparticles also show good separability and reusability in successive cycles of reduction. The reaction mechanism of successive reduction of nitrophenols by Au
Fe3O4 heterostructures has been proposed and confirmed. Results obtained in this study open an avenue to the fabrication of highly efficient heterodimer nanocatalysts for serving as an ideal platform to study the various heterogeneous catalytic processes. ’ ASSOCIATED CONTENT

bS

Supporting Information. Line-scan analysis of Au
Fe3O4; TEM image of parallelogram-like heterostructures; UV
vis spectra of Au
Fe3O4 and p-nitrophenol; concentration change of nitrophenol compounds with time; hydrodynamic size of Au
Fe3O4 NPs; pictures of magnetic separation of nanocatalysts; pseudofirst-order rate constants for nitrophenol reduction as a function of recycling times; and pseudofirst-order rate constants for nitrophenol reduction by various Au catalysts. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Phone number: þ886-35726785. Fax number: þ886-3-5718649.

’ ACKNOWLEDGMENT The authors thank the National Science Council, Taiwan, for financial support under Contract No. NSC 99-2113-M-007-007MY3. The authors thank Prof. Hong-Ping Lin at National ChengKung University, Tainan, for help with the HR-TEM analysis. ’ REFERENCES (1) Sau, T. K.; Rogach, A. L.; Jackel, F.; Klar, T. A.; Feldmann, J. Adv. Mater. 2010, 22, 1805. (2) Jain, P. K.; Huang, X. H.; El-Sayed, I. H.; El-Sayed, M. A. Acc. Chem. Res. 2008, 41, 1578. (3) Haruta, M.; Yamada, N.; Kobayashi, T.; Iijima, S. J. Catal. 1989, 115, 301. (4) Orlov, A.; Jefferson, D. A.; Macleod, N.; Lambert, R. M. Catal. Lett. 2004, 92, 41. (5) S. Praharaj, S.; Nath, S.; Ghosh, S. K.; Kundu, S.; Pal, T. Langmuir 2004, 20, 9889. (6) Zeng, J.; Zhang, Q.; Chen, J.; Xia, Y. Nano Lett. 2010, 10, 30. (7) Gong, J. L.; Mullins, C. B. Acc. Chem. Res. 2009, 42, 1063. (8) Hutchings, G. J. Top. Catal. 2008, 48, 55. (9) Xu, C.; Xie, J.; Ho, D.; Wang, C.; Kohler, N.; Walsh, E. G.; Morgan, J. R.; Chin, Y. E.; Sun, S. Angew. Chem., Int. Ed. 2008, 47, 173. (10) Chen, M. S.; Goodman, D. W. Science 2004, 306, 252. (11) Comotti, M.; Li, W. C.; Spliethoff, B.; Schuth, F. J. Am. Chem. Soc. 2006, 128, 917. (12) Tang, S. C.; Vongehr, S.; Meng, X. K. J. Mater. Chem. 2010, 20, 5436. (13) Lim, C. W.; Lee, I. S. Nano Today 2010, 5, 412. (14) Shylesh, S.; Sch€unemann, V.; Thiel, W. R. Angew. Chem., Int. Ed. 2010, 49, 3428. (15) Zhu, Y.; Stubbs, L. P.; Ho, F.; Liu, R.; Ship, C. P.; Maguire, J. A.; Hosmane, N. S. ChemCatChem 2010, 2, 365. (16) Polshettiwar, V.; Varma, R. S. Green Chem. 2010, 12, 743. (17) Ge, J. P.; Huynh, T.; Hu, Y. P.; Yin, Y. D. Nano Lett. 2008, 8, 931. (18) Deng, Y. H.; Cai, Y.; Sun, Z. K.; Zhao, D. Y. J. Am. Chem. Soc. 2010, 132, 8466. (19) Wang, C.; Yin, H. F.; Dai, S.; Sun, S. H. Chem. Mater. 2010, 22, 3277. (20) Lee, Y. M.; Garcia, M. A.; Huls, N. A. F.; Sun, S. H. Angew. Chem., Int. Ed. 2010, 49, 1271. (21) Lopes, G.; Vargas, J. M.; Sharma, S. K.; Be’ron, F.; Pirota, K. R.; Knobel, M.; Rettori, C.; Zysler, R. D. J. Phys. Chem. C 2010, 114, 10148. (22) Costi, R.; Saunders, A. E.; Banin, U. Angew. Chem., Int. Ed. 2010, 49, 4878. 6597

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