4-nitrophenol-2

Published: March 17, 2011

r 2011 American Chemical Society 6591 dx.doi.org/10.1021/jp110956k | J. Phys. Chem. C 2011, 115, 65916598

ARTICLE

pubs.acs.org/JPCC

Bifunctional AuFe3O4 Heterostructures for Magnetically RecyclableCatalysis of Nitrophenol ReductionFang-hsin Lin and Ruey-an Doong*

Department of Biomedical Engineering and Environmental Sciences, National Tsing Hua University, Hsinchu, 30013, Taiwan

bS Supporting Information

1. INTRODUCTION

Nobel metal nanostructures have recently received muchattention because of their unique optical, catalytic, and electro-chemical properties which make them suitable materials forpotential applications in various elds.1,2 Gold nanoparticles(Au NPs) have been found to play an important role in severalcatalytic processes including low-temperature CO oxidation,3

reductive catalysis of chlorinated or nitrogenated hydrocarbons,46

and organic synthesis.7,8 Due to the high cost and limited supply,however, the improvement of the catalytic eciency and thereduction of the used amounts are the top priorities for practicalapplications. The deposition of Au NPs onto porous supportssuch as TiO2, SiO2, and carbon is regarded as a conventional wayto solve the problem by maximizing the loading of catalysts andto enhance the catalytic activity by well-tuning the surface func-tionality.912 However, the entrapment or immobilization of thenanocatalysts on solid supports normally results in a decrease inthe catalytically active surface area and the reactivity of catalyticspecies.13 Recently, the doping of Au NPs into the interior ofspherical AgC composites containing Ag NPs has beensynthesized for reduction of 4-nitrophenol in the presence ofsodium borohydride.12 The catalytic activity of bimetallic com-posites is highly enhanced over the monometallically dopedcarbon spheres. However, these catalysts are usually recycled bytedious and time-consuming centrifugation/redispersion cycles,

thus hampering the recovery and reusability of catalysts inaqueous solutions.

The magnetic nanoparticles have recently emerged as viablealternatives to conventional materials for catalyst supports.14,15

Their insoluble and superparamagnetic natures enable trouble-free separation of the nanocatalysts from the reaction mixtureusing an external magnet, which eliminates the necessity ofcatalyst ltration.16 Ge et al. synthesized a nanostructuredcomposite with a high specic surface area and magnetic separa-tion ability for 4-nitrophenol reduction.17 A complete conversionof 4-nitrophenol was obtained within 1 h, and the catalysts wererecycled by an external magnet and reused eight times with almostidentical reaction rate. Deng et al. also fabricated a multicompo-nent nanostructure composed of a magnetic-silica core, a layer ofgold nanoparticles, and amesoporous silica shell for both 4-nitro-phenol reduction and styrene epoxidation.18 Although thesematerials show improved stability and recyclability, the synthesisprocedures are complicated and the magnetic property is usuallyhindered by the silica shell. Dumbbell-like AuFe3O4 nano-structures, where one nanoparticle is linked to another, havebeen used as the nanocatalysts for CO oxidation as well as H2O2

Received: November 16, 2010Revised: February 19, 2011

ABSTRACT: The dumbbell- and ower-like AuFe3O4 het-erostructures by thermal decomposition of the ironoleatecomplex in the presence of Au nanoparticles (NPs) have beensuccessfully fabricated using dierent sizes of Au NPs as theseeds for magnetically recyclable catalysis of p-nitrophenol and2,4-dinitrophenol reduction. The heterostructures exhibit bi-functional properties with high magnetization and excellent cata-lytic activity toward nitrophenol reduction. The epitaxial linkagesin dumbbell- and ower-like heterostructures are dierent, leadingto the change in magnetic and catalytic properties of the hetero-structured nanocatalysts. The pseudo-rst-order rate constants fornitrophenol reduction are 0.630.72min1 and 0.380.46min1for dumbbell- and ower-like AuFe3O4 heterostructures, respec-tively. In addition, the heterostructured nanocatalysts show goodseparation ability and reusability which can be repeatedly appliedfor nearly complete reduction of nitrophenols for at least six successive cycles. The reaction mechanism for nitrophenol reduction byAuFe3O4 nanocatalysts is also proposed and conrmed by XPS and FTIR analyses. These unique properties make AuFe3O4heterostructures an ideal platform to study various heterogeneous catalytic processes which can be potentially applied in a wide variety ofelds in purication, catalysis, sensing devices, and green chemistry.

6592 dx.doi.org/10.1021/jp110956k |J. Phys. Chem. C 2011, 115, 65916598

The Journal of Physical Chemistry C ARTICLE

reduction.19,20 The combination of AuNPs with magnetic Fe3O4NPs can not only provide catalytic activity but also be reclaimedvia magnetic separation after use. Moreover, the magnetic propertywas enhanced via interfacial interaction.21 It is believed thatelectron transfer across the interface between these twoNPs maylead to a dramatic change in physicochemical properties, thusoering an ideal platform to study the multifunctionality ofnanomaterials.22,23 In addition, the AuFe3O4 heterostructurescontain both magnetically, optically, and catalytically active NPs,which show high potential applications to chemical catalysis,drug delivery, and biomedical imaging.2426

The thermal decomposition of iron precursors including ironpentacarbonyl, iron acetylacetonate, and the ironoleate com-plex at high temperature is one of themost ecient techniques tosynthesize monodisperse magnetic NPs.27,28 The size and mor-phology of Fe3O4 NPs can be tuned by simply adjusting thereaction temperature ranging from 280 to 380 C.2934 Inaddition, the synthesis of heterostructures that contain a noblemetal particle in the structures has recently received considerableattention.3537 Yu et al. fabricated AuFe3O4 dumbbell struc-tures by thermal decomposition of iron pentacarbonyl in thepresence of preformed Au nanoparticles in 1-octadecene fol-lowed by oxidation of iron nanocrystals in air at room temper-ature.38 The thermal decomposition of mixtures of metaloleatecomplexes and metaloleylamine complexes in the presence of1,2-hexadecanediol has also been reported.39 Although thesestrategies produce well-crystallized nanostructures, the synthesisprocesses are usually expensive and may contain toxic reagents,leading to the diculty in practical application. In addition, thefabrication of dierent morphologies of AuFe3O4 heterostruc-tures by tuning the size of Au NPs has received less attention.

Herein, we demonstrate a facile method for the synthesis ofdierent morphologies of monodisperse AuFe3O4 hetero-structures by thermal decomposition of ironoleate complex(Fe(OL)3) in the presence of Au seeds at 310 C. The designedheterostructures show excellent dual functions which can notonly undergo rapid catalytic reduction of nitrophenols includingp-nitrophenol and 2,4-dinitrophenol in the presence of NaBH4but also be easily recycled using an external magnetic eld. Toour best knowledge, this is the rst report demonstrating the useof AuFe3O4 heterostructures for magnetically recyclable cata-lysis of nitroaromatic compounds.

2. EXPERIMENTAL SECTION

2.1. Chemicals. Iron chloride (FeCl3 3 6H2O, 98%), oleyl-amine (>70%), oleic acid (90%), 1-octadecene (90%), andtert-butylamineborane complex (97%) were purchased fromSigma-Aldrich. Sodium oleate (95%) was purchased from TCI.Trisodium citrate dehydrate (>99%) was purchased from Ferak.Sodium borohydride (95%) and ethanol absolute (99.8%) werepurchased from Riedel-de Haen. p-Nitrophenol and 2,4-dinitro-phenol were purchased from Fluka. Hydrogen tetrachloroaurate-(III) trihydrate (HAuCl4 3 3H2O) was purchased from AlfaAesar. Cyclohexane (99.95%) was purchased from TEDIA. n-Hexane was purchased from J. T. Baker.2.2. Synthesis of Gold NPs with Different Sizes. The Au

NPs with sizes of 45 nm were prepared by dissolving 40 mg ofHAuCl4 3 3H2O in a mixture containing 4 mL of oleylamine and4 mL of cyclohexane in air followed by magnetic stirring at 10 Cunder a gentle stream of nitrogen gas. An amount of 0.2 mmolof tert-butylamineborane complex was dissolved in 0.4 mL of

oleylamine and 0.4 mL of cyclohexane and then injected into theprecursor solution. The solution color changed to deep redimmediately after injecting the borane complex solution. Themixture was aged for 40 min at 10 C followed by addition of30 mL of ethanol to precipitate the Au NPs. The Au NPs werethen harvested by centrifugation and redispersed in hexane. Forpreparation of 10 nm Au NPs, 40 mg of HAuCl4 3 3H2O wasdissolving in a mixture containing 4 mL of 1-octadecene and4 mL of oleylamine in air. The resulting solution was put in an oilbath at 120 C and reaction for 30 min under N2 atmosphere.After reaction, the mixture was cooled to room temperature andfollowed by addition of 30 mL of ethanol to precipitate the AuNPs. The product was centrifuged and redispersed in hexane.2.3. Synthesis of AuFe3O4 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, and5 mL of octadecene was heated to 110 C for 20 min. Thesolution was refluxed at 310 C for 30 min. After cooling to roomtemperature, the particles were separated by adding absoluteethanol, centrifugation, and redispersion into hexane.2.4. Preparation of Water-Soluble AuFe3O4 NPs. The

AuFe3O4 NPs were washed with a mixture of hexane andethanol (1:2) several times to remove excess capping agent onthe surface of NPs. The heterostructured AuFe3O4 NPs werethen dried and added into an aqueous solution containing50 mM sodium citrate. After reaction of 10 min, the AuFe3O4heterostructures were separated by a magnet and washed withdeionized water three times. The particles were then dissolved indeionized water.2.5. Catalytic Reaction. The reduction of nitrophenol com-

pound by water-soluble AuFe3O4 NPs in the presence ofNaBH4 was carried out to examine the catalytic activity andrecyclability of the AuFe3O4 nanocatalysts. Amounts of 2 mLof deionized water, 40 L of 10 mM nitrophenol, and 0.16 mLof 0.1 M NaBH4 solutions were added into a quartz cuvettefollowed by addition of 2 mg of water-soluble AuFe3O4 NPs tothe mixture. The color of the solution changed gradually fromyellow to transparent as the reaction proceeded. UVvis spec-trometry was used to record the change in absorbance at a timeinterval of 2 min.2.6. Surface Characterization. Transmission electron micro-

scopy (TEM) images were obtained on a JEOL 2011microscopeoperated at 120 kV. High-resolution transmission electronmicroscopy (HR-TEM) was carried out on a JEOL JEM-2010microscope at 200 kV. The samples were prepared by suspensionin hexane. Wide-angle XRD patterns were recorded on a BrukerD8 X-ray diffractometer with Ni-filtered Cu KR radiation ( =1.5406 ) and operated at a generator voltage and an emissioncurrent of 40 kV and 40 mA, respectively. A Hitachi U-3010UVvis spectrometer using a 1 cm path length quartz cuvettewas used to identify the change in concentration over a wave-length range from 200 to 600 nm. Magnetic measurements werecarried out using a superconducting quantum interference devicemagnetometer (SQUID MPMS5, Quantum Design Inc.) with amaximum applied continuous field of 10 000 G at room tem-perature. X-ray photoelectron spectroscopy (XPS) measure-ments were performed by an ESCA PHI 1600 photoelectronspectrometer using an Al KR X-ray source (1486.6 eV photonenergy). During data acquisition, the pressure in the samplechamber was maintained below 2.5 108 Torr. The bindingenergies of the photoelectrons were determined under theassumption that Au has a binding energy of 84.0 eV. FTIR



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

3. RESULTS AND DISCUSSION

3.1. Characterization of AuFe3O4 Heterostructures. Theheterostructured AuFe3O4 nanoparticles were prepared bythermal decomposition of the ironoleate complex in thepresence of different sizes of Au NPs. The morphology of theseheterostructures is highly dependent on the size of Au seeds.Figure 1 shows the TEM and HR-TEM images of AuFe3O4heterostructures synthesized by using different sizes of Au seedsranging between 5 and 10 nm (Figure 1a, d). The nanostructurednanoparticles show a dumbbell-like structure when small-sizedAu NPs are used as seeds (Figure 1b). The epitaxial relationshipbetween Au and Fe3O4 nanoparticles was further examined byHR-TEM. The interfringe distances are measured to be 0.24 nmfor Au nanoparticles and 0.24 nm for Fe3O4 nanoparticles, whichcorrespond to the (111) plane of face-centered cubic (fcc) Auand (311) plane of inverse spinel structured magnetite, respec-tively (Figure 1c). In addition, the line-scan analysis was used toget information on relative locations of Au and Fe3O4 in theheterostructures. As depicted in Figure S1 (Supporting Infor-mation), different distribution patterns of Au and Fe are obser-ved. The Fe signals mainly locate at 1426 and 3644 nm, while

Au signals appear at 2633 and 4449 nm, which confirm thatthe Au and Fe3O4 NPs in dumbbell-like heterostructures are inan epitaxial relationship. The histogram analysis shows that theAu and Fe3O4 NPs in dumbbell-like structures are in the range2.85.8 and 1115 nm with mean sizes of 5 and 12 nm, res-pectively (Figure 2a). The sizes of dumbbell-like nanostructuresare also in the range of 1216 nm. Using large Au NPs of713 nm as seeds, flower-like structures with sizes of 2028 nmare formed (Figure 2b). A previous study depicted that thecrystallinity of Au seeds controlled the nucleation process, withone iron oxide leaf nucleated per monocrystalline domain ofgold.40 In this study, large Au NPs provide large surface areasand multiple monocrystalline domains for Fe3O4 to nucleation,resulting in the production of flower-like heterostructures. Inaddition, the d-spacings of 0.24 and 0.48 nm for Au and mag-netite NPs, respectively, are observed, clearly showing the growthof the Fe3O4(111) plane onto a Au(111) plane to form a flower-like heterostructure. In addition, parallelogram-like AuFe3O4heterostructures were obtained when the particle size of Au seedsincreased to 20 nm (Supporting Information, Figure S2), clearlyindicating that the morphology of AuFe3O4 heterostructures ishighly dependent on the size of Au seeds.The crystallinity of AuFe3O4 heterostructures is character-

ized by XRD. Figure 3a shows the XRD patterns of dierentmorphologies of AuFe3O4 NPs. Five resolved peaks at 30.10,

Figure 2. Histogram analysis of particle sizes of (a) dumbbell-like and (b) ower-like AuFe3O4 heterostructures.

Figure 1. TEM images of (a) 5 nm Au NPs, (b) dumbbell-like AuFe3O4 heterostructures, (d) 10 nm Au NPs, and (e) ower-like AuFe3O4heterostructures. Figures (c) and (f) are HRTEM images of dumbbell- and ower-like AuFe3O4 heterostructures, repsectively.



35.54, 43.09, 56.98, and 62.58 2, which can be assigned asthe fcc Fe3O4, are observed. In addition, peaks at 38.18, 44.39,64.58, and 77.55 2 are common patterns for fcc-structuredAu. The XRD patterns of AuFe3O4 NPs match well with thosecorresponding JCPDS standards of Au and Fe3O4 (JCPDS 04-0784; JCPDS 65-3107, clearly indicating the nature of hetero-dimer structures of AuFe3O4 NPs. In addition, the epitaxiallinkage in heterostructures has a signicant eect on the changein optical properties of Au NPs.37 The pure Au NPs show asurface plasmon resonance peak at 517 and 520 nm for 5 and10 nm Au NPs, respectively. After conjugation with Fe3O4 NPs,the peak is broadening and red-shifts to 567 nm in dumbbell-likestructure and 550 nm in ower-like structures (SupportingInformation, Figure S3). The relatively weak reectance ofAuFe3O4 NPs is primarily attributed to the dilution eect ofFe3O4 on Au NPs in the heterostructures.

9 Moreover, the mag-netic measurement shows that AuFe3O4 NPs are superpara-magnetic at room temperature (300 K) (Figure 3b). Thehysteresis loops of AuFe3O4 NPs indicate that the saturationmagnetization is 31 emu/g for dumbbell-like structures and 43emu/g for ower-like structures at 300 K. After normalization tothe unit weight of Fe3O4, the saturation magnetizations are 41

and 51 emu/g-Fe3O4 for dumbbell- and ower-like AuFe3O4nanostructures, respectively. It is noteworthy that saturationmagnetization obtained in this study is lower than that of bulkmagnetite (90 emu/g).41 However, these values are higher thanthose reported data prepared by the similar procedure afternormalization to the unit weight of Fe3O4.

28,42

3.2. Application of AuFe3O4 Heterostructures for Cata-lytic Reduction of Nitrophenols. The catalytic reduction ofp-nitrophenol to their corresponding daughter derivatives, p-aminophenol, in the presence of NaBH4 was chosen as a modelreaction to investigate the bifunctionality of AuFe3O4 hetero-structures. Such a reaction catalyzed by Au catalysts has beenreported because this reaction can be rapidly and easily charac-terized.4345 In addition, 2,4-dinitrophenol was also selected asanother model compound for elucidating the reaction kinetics aswell as a mechanism for nitrophenol reduction. Figure 4 showsthe typical UVvis spectra and concentration change of nitro-phenol compounds in the presence of different morphologies ofAuFe3O4 heterostructures and NaBH4. The original absorp-tion peak of p-nitrophenol is centered at 317 nm and shifts to400 nm after addition of freshly prepared NaBH4 solution,indicating the formation of p-nitrophenolate ions (Supporting

Figure 3. (a) XRD patterns and (b) magnetic hysteresis loops of dumbbell- and ower-like AuFe3O4 heterostructures.

Figure 4. Time-dependent UVvis spectral changes in (a) p-nitrophenol (4-NP) and (b) 2,4-dinitrophenol (2,4-DNP) catalyzed by AuFe3O4heterostructures and concentration change in nitrophenol compounds (Ct/C0) in the presence of (c) dumbbell-like and (d) ower-like AuFe3O4nanocrystals. 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.



Information, Figure S4).46 This peak starts to decrease when thereduction proceeds in the presence of AuFe3O4 nanocatalysts.Addition of NaBH4 in the absence of AuFe3O4 NPs has littleeffect on the change in absorbance at 400 nm, confirming that thereduction is mainly catalyzed by the AuFe3O4NPs. In addition,the absorption peak at 400 nm decreases with the concomitantincrease in peak intensity at 300 nm within 10 min after additionof AuFe3O4 catalysts (Figure 4a).A similar reduction pattern for 2,4-dinitrophenol is also

observed in which the peak at 357 nm decreases with the increasein absorption at 450 nm (Figure 4b), clearly indicating the pro-duction of intermediate of 2-amino-4-nitrophenol.47 The con-centration of 2-amino-4-nitrophenol at 450 nm decreases againwith the concurrent increase in peak intensity of p-aminophenolat 300 nm. This means that 2,4-dinitrophenol undergoes theconsecutive reaction to form 2-amino-4-nitrophenol and then top-aminophenol in the presence of AuFe3O4 nanocatalysts andNaBH4.The pseudorst-order kinetics can be applied to evaluate the

rate constants for nitrophenol reduction because the concentra-tion of NaBH4 is higher than those of nitrophenols and can beconsidered as a constant during the reaction period. The con-centration of p-nitrophenol and 2,4-dinitrophenol at time t isdenoted as Ct, and the initial concentration of nitrophenols at t =0 is regarded as C0. The Ct/C0 is measured from the relativeintensity of absorbance (At/A0). The linear relationship ofln(Ct/C0) versus time (t) indicates that the reduction of nitro-phenols by AuFe3O4 heterostructures follows the pseudorst-order kinetics. The rate constants for nitrophenol reduction are0.63 min1 for p-nitrophenol and 0.72 min1 for 2,4-dinitrophe-nol by using dumbbell-like AuFe3O4 nanocatalysts (Figure 4c).In addition, the ower-like AuFe3O4 NPs are also used asnanocatalysts for reduction of nitrophenols. The rate constantsfor p-nitrophenol and 2,4-dinitrophenol reduction catalyzed byower-like AuFe3O4 nanocatalysts are 0.38 and 0.46 min1,respectively (Figure 4d). The catalytic eciency as well as the rate

constants for nitrophenol reduction by both dumbbell- and ower-like AuFe3O4 are higher than those previously reported valuesobtained from the catalysis of p-nitrophenol by Au-based materials(Supporting Information, Table S1).48,49 This result clearly indicatesthat AuFe3O4 heterostructures are superior nanocatalystswhich can enhance the catalytic eciency and minimize the usedamounts of catalysts for reaction, especially only when traceamounts of Au catalysts are used (0.380.96 mg Au) for reduc-tion. It is noteworthy that the catalytic eciency of ower-likeAuFe3O4 is lower than that of dumbbell-like structures(Supporting Information, Figure S5), presumably due to thatthe Au surfaces in ower-like structures are mainly occupiedby the Fe3O4 leaves and thus suppress the reaction rate ofnitrophenols. Therefore, the dumbbell-like AuFe3O4 hetero-structures are selected as the model nanocatalysts for furtherexperiments.The as-prepared AuFe3O4 heterostructures show both

catalytic and magnetic properties which can be easily recycledby an external magnet after the catalytic reduction. Figure 5 showsthe magnetically recyclable reduction of nitrophenols in thepresence of dumbbell-like AuFe3O4 nanocatalysts. The cata-lysts can be successfully recycled and reused for at least sixsuccessive cycles of reaction with stable conversion eciency ofaround 100%. The hydrodynamic size of AuFe3O4 nanoparti-cles remains unchanged after several cycles of catalytic reactions(Supporting Information, Figure S6), indicating the stability ofthe dumbbell-like nanoparticles in aqueous solutions. In addi-tion, the citrate-stabilized Au NPs were used as the catalysts toreduce nitrophenol compounds for comparison. The conversioneciency of p-nitrophenol and 2,4-dinitrophenol by citrate-stabilized Au NPs drops dramatically after the second cycle,which is primarily attributed to the loss of Au NPs after periodiccentrifugation/redispersion cycles. The average particle sizes ofcitrate-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

Figure 5. Catalytically recyclable reduction of (a) p-nitrophenol and (b) 2,4-nitrophenol by dumbbell-like AuFe3O4 NPs in the presence of NaBH4.Conversion eciency of (c) p-nitrophenol in six successive cycles of reduction and (d) 2,4-nitrophenol in seven successive cycles of reduction byAuFe3O4 and citrate-stabilized Au nanocatalysts.



eciency of nitrophenols by Au NP is primarily attributed to theloss of Au NPs after periodic centrifugation/redispersion cycles.In addition, the same Au seeds used for synthesis of AuFe3O4heterostructures were also employed for reduction of p-nitro-phenol. The single-component AuNP catalysts were obtained byetching Fe3O4 away from the AuFe3O4 NPs in 0.5 M H2SO4solutions.10 The catalytic eciency of p-nitrophenol by Au seedsis low when compared with that by dumbbell-like AuFe3O4heterostructures, presumably due to the aggregation of Au seedsduring the etching step (Supporting Information, Figure S7).When adding sodium citrate to the solution containing p-nitro-phenol and NaBH4, little p-nitrophenol was reduced, suggestingthat citrate has no eect on nitrophenol reduction. These resultsdemonstrate that the AuFe3O4 NPs are superior catalysts thanAu itself and other supported Au catalysts, presumably attributedto the electronic junction eect of Au and Fe3O4 NPs.

2022 Thiselectronic junction eect can also be observed in reductioncatalysis of H2O2 by AuFe3O4 dumbbell-like structure.20 Inaddition, the Au NPs in dumbbell-like structure were stableagainst aggregation during harvest procedure, resulting in theenhanced catalysis of nitrophenol compounds. It is obvious thatthe presence of Fe3O4 NPs makes the dumbbell-like hetero-structures a promising bifunctional probe for magnetically re-cyclable catalytic reduction. When the reduction is complete, theAuFe3O4 nanocatalysts can be separated easily and rapidlyfrom the solution within 10 s by a magnet and then be redis-persed into deionized water for the next cycle of catalysis (Sup-porting Information, Figure S8).Although the AuFe3O4 heterostructures show superior cataly-

tic and recycling eciencies, the rate constant for nitrophenolreduction decreased when AuFe3O4 NPs were reused (Sup-porting Information, Figure S9). The decrease in rate con-stants for nitrophenol reduction may probably be attributed tothe generation of aminophenols after catalytic reduction andthen bound to the surface of Au NPs. Scheme 1 shows thecatalytic mechanisms for nitrophenol reduction by AuFe3O4

in the presence of NaBH4. When AuFe3O4 NPs are used forcatalytic reduction, BH4

and nitrophenols (p-nitrophenoland 2,4-dinitrophnol) are rst diused from aqueous solutionto the Au surface, and then the bare Au NPs on heterostruc-tures serve as catalysts to transfer electrons from BH4

tonitrophenols, leading to the production of amino derivatives,2-amino-4-nitrophenol and p-aminophenol.50 It is note-worthy that the amine (NH2) group in aminophenols hasa strong binding ability with Au NPs and, therefore, adsorbsonto the surface of Au NPs, resulting in the block of reactivesites on Au NPs. To verify the hypothesis of surface blockingby NH2 groups, aniline is used to pretreat the AuFe3O4nanocatalysts prior to the reduction of p-nitrophenol. The rateconstants for p-nitrophenol reduction by aniline pretreatedAuFe3O4 nanocatalysts decrease dramatically after the sec-ond cycle (Supporting Information, Figure S10), which issimilar to the results shown in Figure S9 (Supporting Infor-mation). In addition, XPS and FTIR are used to characterizethe change in surface species on AuFe3O4 heterostructuresbefore and after the reduction (Figure 6). The XPS of N1sspectra show no peak before the reaction, while one predo-minant peak appears at 400 eV after the catalytic reduction,which is consistent with the result of p-aminophenol, and canbe assigned as the amine (NH2) group after peak deconvo-lution (Figure 6a).51 This result clearly indicates the chemi-sorption of aminophenol onto the surface of Au NPs. Inaddition, all the FTIR spectra exhibit symmetric and asym-metric stretching vibrations of the CH3 bond at 2840 and2950 cm1, respectively (Figure 6b). The FeO bonding inthe range 570600 cm1 is also clearly observed. TheAuFe3O4 heterostructures in the organic phase show weakCdO and CN stretching peaks at 1704 and 1439 cm1,respectively, indicating the presence of oleylamine and oleicacid on the surface of nanoparticles. After ligand exchangewith sodium citrate in aqueous solution, the peak of CNstretching disappears, while the CdO peak at 1613 cm1 is

Scheme 1. Possible Mechanism for Magnetically Recyclable Catalysis of Nitrophenols by AuFe3O4 Heterostructures



observed. After the catalytic reduction of nitrophenols, theCN stretch in AuFe3O4 reappears at 1421 cm1, conrm-ing the attachment of aminophenols onto the catalyst surfaces.

4. CONCLUSIONS

In this study, we have rst demonstrated that the hetero-structured AuFe3O4 nanocatalysts synthesized via thermaldecomposition of the ironoleate complex in the presence ofAu seeds have excellent bifunctional characteristics for reusabilityand catalytic reduction. The size and morphology of AuFe3O4nanocatalysts are highly dependent on the size of Au seeds. Thecatalytic performance of both dumbbell- and ower-like AuFe3O4 NPs is excellent for nitrophenol reduction in the presenceof NaBH4. In addition, the catalytic eciency of dumbbell-likeAuFe3O4 NPs is higher than that of ower-like heterostruc-tures because of the high surface coverage of the Au surface byFe3O4 nanocrystals in ower-like heterostructures. The dumb-bell-like nanoparticles also show good separability and reusabilityin successive cycles of reduction. The reaction mechanism ofsuccessive reduction of nitrophenols by AuFe3O4 heterostruc-tures has been proposed and conrmed. Results obtained in thisstudy open an avenue to the fabrication of highly ecient hetero-dimer nanocatalysts for serving as an ideal platform to study thevarious heterogeneous catalytic processes.

ASSOCIATED CONTENT

bS Supporting Information. Line-scan analysis ofAuFe3O4;TEM image of parallelogram-like heterostructures; UVvis spectraof AuFe3O4 and p-nitrophenol; concentration change of nitrophe-nol compounds with time; hydrodynamic size of AuFe3O4 NPs;pictures of magnetic separation of nanocatalysts; pseudorst-orderrate constants for nitrophenol reduction as a function of recyclingtimes; and pseudorst-order rate constants for nitrophenol reductionby various Au catalysts. This material is available free of charge via theInternet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author*E-mail: [email protected]. Phone number: 886-3-5726785. Fax number: 886-3-5718649.

ACKNOWLEDGMENT

The authors thank the National Science Council, Taiwan, fornancial support under Contract No. NSC 99-2113-M-007-007-MY3. The authors thank Prof. Hong-Ping Lin at National Cheng-Kung University, Tainan, for help with the HR-TEM analysis.

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