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Catalytic Nanoreactors of Au@FeO Yolk-Shell Nanostructures with Various Au Sizes for Efficient Nitroarenes Reduction Fang-hsin Lin, and Ruey-an Doong J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b00130 • Publication Date (Web): 13 Mar 2017 Downloaded from http://pubs.acs.org on March 13, 2017

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The Journal of Physical Chemistry

Catalytic Nanoreactors of Au@Fe3O4 Yolk-Shell Nanostructures with Various Au Sizes for Efficient Nitroarenes Reduction Fang-hsin Lin1, 2 and Ruey-an Doong1, 3* 1. Department of Biomedical Engineering and Environmental Sciences, National Tsing-Hua University, Hsinchu, 30013, Taiwan. 2. Center for Measurement Standards, Industrial Technology Research Institute, Hsinchu, 300, Taiwan 3. Institute of Environmental Engineering, National Chiao Tung University, 1001, University Road, Hsinchu, 30010, Taiwan.

ABSTRACT The Au@Fe3O4 yolk-shell nanocatalysts based on the thermal decomposition of iron

pentacarbonyl in the presence of 2.5−10 nm Au core nanoparticles were successfully fabricated for catalytic reduction of nitroarenes. The particle sizes of Au@Fe3O4 nanostructures are in the range 8−15 nm with Fe3O4 shell layer thicknesses of 2.0−2.4 nm. The Fe3O4 shell layer can not only possess magnetic shell for recovery but also enable the protection of catalytic activity of Au core nanoparticles toward the reduction of nitrobenzene derivatives

including

2-nitrophenol,

4-nitrophenol,

4-nitrotoluene

and

1-chloro-4-nitrobenzene in the presence of NaBH4. The catalytic performance of Au@Fe3O4 is highly dependent on particle size of Au core materials and substituent group of nitroarenes. 1

ACS Paragon Plus Environment


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The reduction rate of nitroarene with electron-withdrawing group is 2.3−2.6 times higher than that with electron-donating group. In addition, the reduction of nitroarenes by Au@Fe3O4 yolk-shell nanocatalysts is a surface-mediated reaction and the relationship between reduction rate and initial NaBH4 follows Langmuir-Hinshelwood kinetics. In addition, the yolk-shell nanoparticles show good separation ability and reusability which can be

repeatedly

applied

for

a

nearly

complete

reduction

of

4-nitrophenol

and

1-chloro-4-nitrobenzene for at least 5 successive cycles. These unique properties make Au@Fe3O4 nanocatalysts an ideal platform to tailor yolk-shell nanoreactors with various active materials as well as to study various heterogeneous catalytic processes.

INTRODUCTION Noble metal nanomaterials such as Au, Ag, Pd and Pt have been found to play an

important role in a wide variety of catalytic reactions including CO oxidation, epoxidation, water-gas shift reaction, selective oxidation, hydrogenation and nitroaromatic reduction.1-3 The catalytic activity of noble metal nanoparticles usually increases significantly when the particle size narrows down to less than 10 nm. However, the small size of nanoparticles often tends to agglomerate, and leads to the decrease in the catalytic activity under some reaction circumstances. Yolk-shell or rattle type nanomaterials have recently received considerable attention due to their unique configuration with interior void between core and shell.4, 5 The appealing structures and tunable physicochemical properties make yolk-shell nanomaterials suitable for numerous catalytic applications.6, 7 The yolk-shell structures not only provide a homogeneous environment around the core particle surfaces but can also prevent the aggregation of neighboring cores, resulting in the enhanced catalytic activity and high stability.8 In addition, 2


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the yolk-shell nanoparticles can be tailored by using different catalytic species as the core and shell materials.9 Several synthesis approaches have been developed for the fabrication of yolk-shell nanostructures.4 The template method is one of the most commonly used strategies to prepare the yolk-shell nanostructures by using SiO2 as the sacrificial layer.10,11 Lee et al. have prepared Au@SiO2@TiO2 nanostructures via sol-gel-based template method first and then the Au@TiO2 yolk-shell nanostructures were obtained by the removal of silica layer with NaOH.10 Although the template method is effective on preparation of yolk-shell nanomaterials, the synthesis procedure is usually multistep, complex and time-consuming. In addition, the shell materials in the yolk-shell nanostructures are usually SiO2, TiO2 and carbon, which need tedious filtration and centrifugation procedures for recycling. Therefore, the development of the simple and easy method to fabricate the yolk-shell nanostructures with magnetic property for recycling usage is thus needed. Several strategies have been developed to fabricate magnetic-based yolk-shell nanomaterials. Xuan et al. have synthesized rattle type noble metal@Fe3O4 nanocomposites through a one-step wet chemical approach by in situ reduction of noble metal nanoparticles and transformation of Fe3O4 hollow spheres from Fe-complex precursor.12 Another method for the synthesis of magnetic shell is the thermal decomposition of metal carbonyl in high boiling solvent.13,

14

The hollow Fe3O4 shell was obtained by the oxidation of metallic

nanoparticles via the nanoscale Kirkendall effect.15,

16

Shevchenko et al. have prepared

Au@Fe2O3 hollow-shell nanoparticles by oxidizing the iron shell on the surface of Au nanoparticles to form the Fe2O3 shells with thickness of 2−3 nm.17 Similar procedure has also been adapted to the synthesis of other Fe2O3-based yolk-shell nanostructures including Pt@Fe2O3, FePt@Fe2O3 and Ag@Fe2O3 nanoparticles.18,

19

It is well-known that Au

nanoparticles exhibit good catalytic activity toward nitrophenol reduction in the presence of NaBH4 and the catalytic activity is highly dependent on the morphology and size of Au-based 3



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nanomateirals.20-23 Although the fabrication and characterization of magnetic-based yolk-shell nanostructures has been investigated, the tailor of Au@Fe3O4 yolk-shell nanoparticles with various sizes of Au core nanoparticles as well as their catalytic activity toward nitroarene reduction has been rarely reported.

Scheme 1. Schematic illustration of the fabrication of Au@Fe3O4 yolk-shell nanoparticles and reduction of nitroarenes in the presence of NaBH4. Herein, we report the synthesis of Au@Fe3O4 yolk-shell nanoparticles by Kirkendall effect using various Au particle sizes as the core materials. The method is based on the deposition of an iron shell on the surface of various sizes of Au nanoparticles ranging from 2.5 to 10 nm by non-aqueous thermal decomposition of iron pentacarbonyl (Fe(CO)5) (Scheme 1). The hollow Fe3O4 shells are obtained by the oxidation of iron nanoparticles in air, which enables the protection of Au activity and recycling of nanocomposites. In addition, the Au core layer can actively catalyze the reduction reaction of nitrobenzene derivatives including 2-nitrophenol (2-NP),

4-nitrophenol (4-NP),

4-nitrotoluene

(4-NT) and

1-chloro-4-nitrobenzene (CNB) in the presence of NaBH4. The recyclability of Au@Fe3O4

4


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yolk-shell nanocatalysts for the reduction of 4-NP and CNB is also examined.

EXPERIMENTAL DETAILS Synthesis of Au Nanoparticles with Various Sizes. The precursor solution of Au

nanoparticles at mean particle sizes of 2.5 nm were synthesized in a three-neck bottle by dissolving 40 mg of HAuCl4⋅3H2O (Alfa Aesar) in a mixture containing 4 mL of oleylamine (Sigma-Aldrich) and 4 mL of cyclohexane (TEDIA) in air at 40 ºC under vigorous stirring conditions. In addition, the reducing agent solution was prepared by dissolving 0.2 mmol of tert-butylamine-borane complex (Sigma-Aldrich) in a solution containing 0.4 mL of oleylamine and 0.4 mL of cyclohexane, and then injected into the precursor solutions. The mixture was aged for 40 min at 40 ºC followed by the addition of 30 mL of ethanol to precipitate the Au nanoparticles. The 2.5 nm Au core nanoparticles were then harvested by centrifugation and re-dispersed in hexane. Similar procedure was used to prepare 4 nm Au nanoparticles except the change in aging temperature to 10 ºC. The 10 nm Au nanoparticles were prepared by dissolving 40 mg HAuCl4⋅3H2O into a mixture containing 4 mL of 1-octadecene (Sigma-Aldrich) and 4 mL of oleylamine in air. The resulting solution was heated at 120 °C for 30 min in N2, and then cooled down to room temperature after the reaction. 30 mL of ethanol were added to precipitate the Au nanoparticles. The product was centrifuged and re-dispersed in hexane. Synthesis of Hollow Fe3O4 Nanoparticles. The hollow Fe3O4 nanoparticles were prepared via thermal decomposition of Fe(CO)5 (Sigma-Aldrich) in the presence of oleylamine. A solution containing 0.5 mL of oleylamine and 20 mL of 1-octadecene was heated to 180 ºC in Ar to remove excess O2 in the mixture. Then, 0.1 mL of Fe(CO)5 was injected and the solution was heated to 300 ºC immediately at a rate of 20 ºC/min. The

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mixture was kept at 300 ºC for 30 min to complete the transformation of Fe(CO)5 to iron nanoparticles, and then cooled down to 220 ºC by purging with air for 60 min to oxidize the iron nanoparticles to iron oxides. The resulting particles were harvested by addition of 30 mL of ethanol, centrifugation, and then re-dispersion in hexane. Synthesis of Au-Fe3O4 Yolk-Shell Nanoparticles. The Au@Fe3O4 yolk-shell nanoparticles were synthesized according to the previous report with modification.17 For Au@Fe3O4 yolk-shell nanoparticles with 2.5 and 4 nm of Au nanoparticles as the core materials, a solution containing 0.5 mL of oleylamine, 0.1 mmol gold colloid dispersion, 0.15 mL of Fe(CO)5, and 20 mL of 1-octadecene was first heated to 120 ºC for 30 min in Ar to remove excess oxygen and hexane. 0.15 mL of Fe(CO)5 were injected into the mixture, heated to 250 ºC at a rate of 20 ºC/min , and then hold at 250 ºC for another 30 min. To form Fe3O4 hollow shell, the solution temperature was lower down to 220 ºC and purged with air for 1 h to convert iron nanoparticles into Fe3O4. After cooling down to room temperature, the nanostructures were separated by adding absolute ethanol, centrifugation, and redispersion into hexane. For 10 nm Au core layer-based Au@Fe3O4 nanoparticles, 0.2 mmol Au core particles and 0.1 mL of Fe(CO)5 solutions were used to prepared yolk-shell nanostructures using the procedure described above. Preparation of Water Soluble Au@Fe3O4 Yolk-Shell Nanoparticles. The Au@Fe3O4 yolk-shell nanoparticles were first washed with 1:2 (v/v) hexane/ethanol mixtures several times to remove capping agents on the surface of Au@Fe3O4 yolk-shell nanoparticles. The washed yolk-shell nanostructures were then added into an aqueous solution containing 2.5 wt% tetramethylammonium hydroxide (TMAH, Sigma-Aldrich) and sonicated for 10 min. The ligand exchange procedures were conducted several times to ensure the full suspension of Au@Fe3O4 nanocatalysts in bidistilled de-ionized water.

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Catalytic Reaction. The reduction of nitroaromatic compounds by water soluble Au@Fe3O4 yolk-shell nanoparticles in the presence of NaBH4 was carried out to investigate the catalytic activity of the Au@Fe3O4 yolk-shell nanoparticles. After addition of 40 µL of 10 mM nitroaromatic compounds and 0.2 mL of 0.1 M NaBH4 solutions into a quartz cuvette containing 2.74 mL of bidistilled deionized water, 20 µL of 1 g/L water soluble Au@Fe3O4 were added into the mixture. The color of solution changed gradually as the reaction proceeded and the reduction efficiency of nitroarenes by Au@Fe3O4 was determined by recording the change in absorbance using UV-Vis spectrophotometer. Characterization. Transmission electron microscopy (TEM) images were obtained by using a JEOL 2011 microscope operated at 120 kV. The crystallinity of yolk-shell nanoparticles were identified by using a Bruker D8 X-ray diffractometer with Ni-filtered Cu Kα radiation (λ = 1.5406 Å) and operated at a generator voltage and an emission current of 40 kV and 40 mA, respectively. The optical property as well as catalytic activity of Au@Fe3O4 was examined using the Hitachi U-3010 UV–Vis spectrometer using a 1-cm path length quartz cuvette. In addition, the magnetic hysteresis loops of Au@Fe3O4 were carried out using a superconducting quantum interference device magnetometer (SQUID, Quantum Design, MPMS5). X-ray photoelectron spectroscopy (XPS) measurements were performed by an ESCA Ulvac PHI 1600 photoelectron spectrometer from Physical Electronics using Al Kα radiation photon energy at 1486.6 eV.

RESULTS AND DISCUSSION Morphology and Structural Properties of Au@Fe3O4 Yolk-Shell Nanocatalysts. The

various sizes of Au core nanoparticles were prepared by chemical reduction of HAuCl4 using tert-butylamine borane complex (2.5 and 4 nm Au) or oleylamine (10 nm Au) as the reducing agent. Figure 1 shows the TEM images of Au core nanoparticles and corresponding

7



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Au@Fe3O4 yolk-shell nanocatalysts. The Au nanoparticles are monodisperse and the size of Au nanoparticles increases from 2.5 to 4 nm when reaction temperature decreases from 40 to 10 ºC, presumably attributed to the slow nucleation process at low reaction temperature (Figure 1(a) and (b)). Since the total amount of nuclei generated at low temperature is less than that at high temperature, more Au precursors are available for nuclei to grow into larger nanoparticles. Therefore, the size of Au nanoparticles increases from 2.5 nm at 40 ºC to 4 nm at 10 ºC. Similar to the growth conditions at low temperature, the mild reducing agent of oleylamine produces few nuclei and more Au precursors are available to increase the Au nanoparticles up to 10 nm (Figure 1(c)).

(a)

(c)

(b)

5nm

(d)

(f)

(e)

20 nm

10 nm

(g)

(i)

(h)

Au (111)

Au (111)

Au (111)

Fe3O4 (311)

Fe3O4 (311) Fe3O4 (311) 2 nm

2 nm

2 nm

Figure 1. The TEM images of Au nanoparticles at (a) 2.5, (b) 4 and (c) 10 nm and the corresponding Au@Fe3O4 yolk-shell nanocatalysts using (d) 2.5, (e) 4 and (f) 10 nm Au 8


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nanoparticles as the core materials. The HRTEM images of Au@Fe3O4 yolk-shell nanocatalysts at various Au core sizes of (g) 2.5, (h) 4 and (i) 10 nm.

The Au@Fe3O4 yolk-shell nanoparticles were prepared by using various sizes of Au nanoparticles ranging from 2.5 to 10 nm as the core materials. It is clear that all the Au nanoparticles are encapsulated by a thin shell of iron oxide (Figure 1 (d)-(f)). The particle sizes of Au@Fe3O4 are in the range of 8−15 nm with shell thickness of 2.0−2.4 nm. The HRTEM images of the corresponding Au@Fe3O4 yolk-shell nanoparticles are shown in Figure 1 (g)-(i). The inter-fringe distances of all the yolk-shell nanoparticles are measured to be 0.24 nm for Au seeds and 0.26 nm for Fe3O4 nanoparticles, which correspond to the (111) plane of face-centered cubic (fcc) Au and (311) plane of fcc Fe3O4, respectively. Wei et al. have synthesized Ag@Fe2O3 yolk-shell nanoparticles at 180 ºC in the presence of 0.2 mL of Fe(CO)5 and oleylamine and found that the resulting thickness of Fe2O3 shell was 3 nm.19 Shevchenko et al. have synthesized Au@Fe2O3 core-hollow shell nanoparticles at 180 ºC in the presence of 0.1 mL of Fe(CO)5. 17 The thickness of Fe2O3 shell was around 3 nm when oleylamine was used as the sole capping agent. However, the shell thickness of Fe2O3 decreased with the increase in oleic acid/oleylamine ratio when oleic acid was added. In this study, the shell thickness of Fe3O4 is in the range of 2.0−2.4 nm, which is thinner than those reported values.17, 19 This may be attributed to the different added amounts of Fe(CO)5 and temperatures used for synthesis. When changing the added amount of Fe(CO)5 to 0.1 mL in the absence of Au seeds, the thickness of iron oxide shell was 3 nm (Figure S1, Supporting Information), showing that the added amount of Fe(CO)5 plays an important role in determination of shell thickness. It is noteworthy that the formation of yolk-shell nanoparticles needs to deposit an iron shell around the Au core first, and then oxidize the iron shell to form the hollow iron oxide shell.17 In this study, the Au@Fe3O4 yolk-shell nanoparticles was firstly synthesized by thermal decomposition of Fe(CO)5 under Ar 9



atmosphere to form an iron shell on the surface of Au seeds. Figure S2 (Supporting Information) shows the formation of Au@Fe core-shell structures by using various sizes of Au nanoparticles as the seeds and the nanoparticles tend to aggregation due to the ferromagnetic property of iron shell. This result indicates that the formation of Fe3O4 shells is mainly from the oxidation of Au@Fe core-shell nanoparticles. In addition, the thin shell thickness of Fe3O4 can accelerate the diffusion of chemicals into the yolk-shell nanoparticles, and results in the enhancement of catalytic efficiency and rate of chemicals by Au@Fe3O4 yolk-shell nanoparticles.

(b) 15

Magnetization (emu/g)

2.5 nm Au@Fe3O4

4 nm Au@Fe3O4

10 nm Au@Fe3O4

30

40

50

60

2θ (degree)

70

80

5

0.02 0.01 0.00 -0.01 -0.02 -0.03 -10

-8

-6

-4

-2

0

2

4

6

8

10

Applied magetic field (Oe)

0

Hollow Fe3O4 -5

2.5 nm Au@Fe3O4 4 nm Au@Fe3O4

-10

Fe O JCPDS 65-3107 3 4 Au JCPDS 04-0784

20

10

0.03

Magnetization (emu/g)

(a)

Intensity (a.u.)

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90

-15

10 nm Au @Fe3O4 -10000

-5000

0

5000

10000

Applied magnetic field (Oe)

Figure 2. The (a) XRD patterns and (b) magnetic hysteresis loops of Au@Fe3O4 yolk-shell nanoparticles synthesized with various sizes of Au core nanoparticles. The inset of Figure (b) is the enlarged magnetic hysteresis loops in the range of -10 to 10 Oe. The crystallinity of Au@Fe3O4 yolk-shell nanoparticles was further identified by XRD. Figure 2(a) shows the XRD patterns of Au@Fe3O4 nanocatalysts using various sizes of Au nanoparticles as the core materials. Five resolved peaks centered at 38.37°, 44.34°, 64.79°, 77.85º and 82.01º 2θ are clearly observed, which can be assigned as the face-centered cubic (fcc) Au nanoparticles (JCPDS 04-0784). In addition, several small peaks at 30.40°, 35.53°, 57.23°, and 74.14° 2θ appear, which are the characteristic peaks of fcc Fe3O4 (JCPDS

10


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65-3107). The weak XRD patterns of Fe3O4 are mainly attributed to the thin thickness of shell layer. In addition, the XRD patterns of Au@Fe3O4 nanoparticles match well with the corresponding standards of Au and Fe3O4, clearly indicating the formation of Au@Fe3O4 yolk-shell structures. Figure 2(b) show the magnetic hysteresis loop of Au@Fe3O4 yolk-shell nanoparticles. The magnetic property of pure hollow Fe3O4 nanoparticles was also measured and compared. All the four samples exhibit superparamagnetic behaviors at 300 K. As shown in the inset of Figure 2(b), the magnetic remanences are 0.0267, 0.078, 0.002 and 0.001 emu/g for hollow Fe3O4, 2.5 nm, 4 nm and 10 nm Au@Fe3O4 yolk-shell nanoparticles, respectively. In addition, the coercivities of all the samples are lower than 4 Oe, clearly indicating the superparamagnetic property of Au@Fe3O4 yolk-shell nanoparticles. The saturation magnetization of pure hollow Fe3O4 nanoparticles is 13.1 emu/g. The magnetization of Au@Fe3O4 decreases as the size of Au nanoparticles in yolk-shell structures increases, and the saturation magnetization decreases from 10.64 to 4.27 emu/g when the particle size of Au core layer increases from 2.5 to 10 nm. The high saturation magnetization of 2.5 nm Au@Fe3O4 is mainly attributed to high Fe/Au weight ratio. In addition, all the Fe3O4-based nanomaterials cannot be saturated magnetically in the high magnetic field of 10,000 Oe, presumably attributed to the spin canting effect resulting from the lack of full alignment of the spins in surface atoms of nano-sized magnetic nanoparticles.14,

24

Kim et al. have

synthesized various sizes of iron oxide nanoparticles and found that the magnetic moments at room temperature decreased from 49 to 3.5 emu/g as the particle sizes decreased from 12 to 1.5 nm.24 In this study, the thin Fe3O4 shells (20−2.4 nm) in yolk-shell structures result in the decrease in the magnetization of Au@Fe3O4 yolk-shell nanoparticles.

11



(a)

0.6

(b) 2.5 nm Au 4 nm Au 10 nm Au

0.5

0.7

2.5 nm Au@Fe3O4 4 nm Au@Fe3O4

0.6

10 nm Au@Fe3O4

0.4

0.5

Absorbance

Absorbance

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0.3 0.2

0.4 0.3 0.2

0.1 0.1

0.0 400

0.0 450

500

550

600

650

700

750

800

400

450

500

Wavelength (nm)

550

600

650

700

750

800

Wavelength (nm)

Figure 3. UV-Vis spectra of (a) pure Au and (b) Au@Fe3O4 yolk-shell nanoparticles with various Au nanoparticle sizes ranging from 2.5 to 10 nm. The optical properties of Au@Fe3O4 yolk-shell nanoparticles were further characterized by UV-Vis. Generally, the Au nanoparticles in the range of 3-20 nm show a clear surface plasmon resonance (SPR) peak at around 520 nm.17, 25 As shown in Figure 3(a), the pure Au nanoparticles show SPR peaks at 513, 515, and 521 nm for 2.5, 4 and 10 nm Au nanoparticles, respectively. The peak becomes narrow as the Au particle size increases, which is mainly attributed to the increased electron density in large Au nanoparticles.26 After the formation of yolk-shell nanoparticles, the SPR peaks of 4 and 10 nm Au-based Au@Fe3O4 nanocatalysts become broad and red-shift to 580 and 567 nm, respectively (Figure 3 (b)). However, the SPR peak of Au@Fe3O4 nanocatalysts using 2.5 nm Au as the core nanoparticles diminishes. A previous study has indicated that the coating of dielectric material onto Au nanoparticles leaded to a shift in SPR wavelength.27 It is noteworthy that the refractive index of iron oxide (2.3–3.1) is much higher than that of Au (0.47).17 Therefore, the presence of iron oxide shell strongly influences the position and intensity of plasmon band and results in the red-shift in SPR peak. XPS was further used to understand the interaction between Au core and Fe3O4 shell. Figure S3 shows the XPS spectra of Au 4f and Fe 2p core level regions of the 2.5 and 10 nm 12


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Au@Fe3O4 yolk-shell nanoparticles. The Au 4f core level of 10 nm Au@Fe3O4 yolk-shell nanoparticles is characterized by two peaks centered at 83.4 and 87.1 eV, which can be assigned as Au 4f7/2 and Au 4f5/2 of the elemental gold (Au0), respectively. It is noteworthy that the binding energy of Au 4f7/2 for 2.5 nm Au@Fe3O4 shifts slightly to 83.5 eV. The slight increase in binding energy for small Au core nanoparticles is attributed to the depletion of d charge, which is in good agreement with the previous study.28 The XPS spectra of Fe 2p in both 2.5 and 10 nm Au@Fe3O4 yolk-shell nanoparticles show the pair of peaks at 710.5 and 724.3 eV, which are attributed to the Fe 2p3/2 and Fe 2p1/2 spin-orbital coupling, respectively. In addition, no peak shift is observed for 2.5 and 10 nm Au@Fe3O4 yolk-shell nanoparticles. Our previous study has indicated the clear shift in Au 4f and Fe 2p peaks when a charge transfer occurs between Au and Fe3O4 in dumbbell-like nanoparticles fabricated by the epitaxial growth.29 In this study, the Au@Fe3O4 yolk-shell nanoparticles were prepared by Au@Fe first and then oxidized to Au@Fe3O4 in the presence of air. The interaction between Au and Fe3O4 would thus be less than that of dumbbell-like nanoparticles, which means the little interaction between Au core and iron oxide shell. Concentration Effect of NaBH4 on Catalytic Activity of Au@Fe3O4. After the successful fabrication of Au@Fe3O4 yolk-shell nanoparticles, these materials were used as the nanocatalysts for nitroarene reduction. Since the yolk-shell nanoparticles were prepared in non-aqueous solutions, the capping agents on the surface of Au@Fe3O4 were removed by ligand exchange before catalytic reactions to make the yolk-shell nanoparticles more hydrophilic. Figure S4 (Supporting Information) shows the TEM images of Au@Fe3O4 yolk-shell nanoparticles in aqueous solutions after ligand exchange. It is clear that the yolk-shell Au@Fe3O4 nanoparticles are dispersed in aqueous solutions with some extent of aggregation. The particle sizes of water soluble Au@Fe3O4 yolk-shell nanoparticles are still in the range of 8-15 nm. However, the average hydrodynamic particles sizes, determined by 13



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dynamic light scattering, are 468, 453 and 256 nm for 2.5, 4 and 10 nm-based Au@Fe3O4 yolk-shell nanoparticles, respectively (Figure S5, Supporting Information). The large hydrodynamic diameter as well as the wide particle size distribution pattern of water soluble Au@Fe3O4 yolk-shell nanoparticles is mainly attributed to the decrease in capping agents on the surface after ligand exchange, resulting in the aggregation of Au@Fe3O4 yolk shell nanoparticles in aqueous solutions. The effect of NaBH4 concentration on the reduction activity of Au@Fe3O4 yolk-shell nanocatalysts was further examined. As shown in Figure 4(a), the removal efficiency of CNB increases upon increasing NaBH4 concentration, and a nearly complete reaction is observed within 70 s at 5−10 mM NaBH4. The kobs for CNB reduction increases positively from 0.681

± 0.106 min-1 at 0.5 mM NaBH4 to 2.013 ± 0.136 min-1 at 10 mM NaBH4 (Figure 4(b)). It is noteworthy that the relationship between kobs for CNB reduction and NaBH4 concentration follows the Langmuir-Hinshelwood kinetic model:

kobs =

KC dC = kmax F NaBH4 dt 1 + KF CNaBH

(1)

4

where CNaBH4 is the aqueous concentration of NaBH4, kmax is the intrinsic maximum rate constant for CNB reduction, and KF is the Langmuir adsorption coefficient of CNB. A good linear relationship between the initial NaBH4 concentration and the kobs for CNB reduction with KF and kmax of 0.934 mM and 2.128 min-1, respectively, is obtained (r2 = 0.965), clearly showing that the reduction of CNB is the surface-mediated reaction and BH4- can be readily diffused into the magnetite shell to reach the surface of Au core nanoparticles.

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(b)

0.5 mM NaBH4

1.0 0.8

1.8

3 mM NaBH4

1.6

5 mM NaBH4

1.4

7.5 mM NaBH4

-1

0.6

2.0

1 mM NaBH4

kobs (min )

(a) Remaining ratio (C/C0)

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


10 mM NaBH4 0.4

1.2 1.0 0.8

kobs =

0.6 0.2

( 2 .1 2 8) × ( 0 .9 3 4) C N a B H4 1 + 0 .9 3 4C N a B H

4

2

r = 0.965

0.4 0.2

0.0

0.0 0

50

100

150

Reaction time (sec)

200

250

0

2

4

6

8

10

NaBH4 concentration (mM)

Figure 4. (a) Effect of NaBH4 concentration on the reduction of CNB by Au@Fe3O4 yolk-shell nanocatalysts and (b) the kobs for CNB reduction as a function of NaBH4 concentration. Catalytic Activity of Au@Fe3O4 Yolk-Shell Nanocatalysts. The catalytic activity of Au@Fe3O4 yolk-shell nanoparticles toward nitroarene reduction was further examined in the presence of NaBH4. 4-NP was first selected as the model compound due to the fact that the catalytic activity of Au@Fe3O4 yolk-shell nanoparticles is easy to compare with the reported data. As shown in Figure 5, the Au@Fe3O4 nanoparticles show excellent catalytic activity toward 4-NP reduction and a nearly complete removal of 4-NP is observed within 5 min. The addition of pure hollow Fe3O4 nanoparticles has little effect on the reduction of 4-NP within 5 min, indicating that the Au core nanoparticles play a crucial role in reduction of 4-NP. It is noteworthy that the removal efficiency of 4-NP is enhanced as the Au nanoparticle size decreases. Wang et al. also indicated that the reduction rate of 4-NP by Au@hollow mesoporous silica microsphere catalysts increased as the size of Au nanoparticles decreased15, which is in good agreement with the results obtained in this study.

15



1.0

Remaining ratio (C/Co)

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

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2.5 nm Au@Fe3O4

0.8

4 nm Au@Fe3O4 10 nm Au@Fe3O4

0.6

Hollow Fe3O4 0.4

0.2

0.0 0

50

100

150

200

250

300

Reaction time (sec)

Figure 5. The catalytic activity of Au@Fe3O4 yolk-shell nanoparticles toward 4-nitrophenol reduction. The catalytic performance of Au@Fe3O4 yolk-shell nanoparticles is also compared with those of Au seeds with various sizes. As shown in Figure S6 (Supporting Information), the reduction efficiencies of 4-NP by various sizes of Au nanoparticles are lower than those of Au@Fe3O4 yolk-shell nanoparticles, which are in good agreement with the result obtained from previous study.20 It is noteworthy that the reduction efficiency of 4-NP increases upon increasing the size of Au nanoparticles, presumably attributed to the fact that aggregation of Au seeds after the phase transfer decreases the active sites on Au surface for 4-NP reduction. Our previous study has used acid solution to etch Fe3O4 in Au-Fe3O4 heterostructures and found that only 58% of 4-NP was reduced by 5 nm Au seeds after 500 s of reaction,20 which is in good agreement with the results obtained in this study by using 4 nm Au seeds as the catalysts. This result also indicates the excellent catalytic performance of Au@Fe3O4 yolk-shell nanoparticles on nitroarene reduction. The reduction efficiency and rate of 4-NP by Au@Fe3O4 yolk-shell nanoparticles can be determined by using the pseudo-first-order reaction rate equation.20, 30 The pseudo-first-order 16


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rate constants (kobs) for 4-NP reduction are 0.97, 0.71, and 0.45 min-1 for yolk-shell Au@Fe3O4 nanocatalysts using 2.5, 4, and 10 nm Au nanoparticles as the core materials, respectively, indicating that Au@Fe3O4 nanoreactors with small Au core nanoparticles possess high catalytic activity toward 4-NP reduction. Several studies have used yolk-shell catalysts containing noble metal nanoparticles for the reduction of 4-NP and found that the reduction rate is size-dependent.31-33 Xiao et al. have immobilized various sizes of Ag nanoparticles within the PAA/PVA nanofibers and found the catalytic reduction rate of 4-NP followed the order of 5.8 > 6.3 > 10.8 ≥ 20.4 nm Ag.31 Sen et al. have synthesized 5.3−15 nm Au nanoparticles in glucan for catalytic reduction of 4-NP and the rate constant for 4-NP reduction decreased with the increase in Au particle size.32 It is noteworthy that the kobs for 4-NP reduction by 2.5 nm Au-based yolk-shell nanocatalysts is higher than that by using dumbbell-like Au-Fe3O4 heterostructures and Au-based nanomaterials.20, 30, 34, 35 Therefore, Au@Fe3O4 yolk-shell nanocatalysts with 2.5 nm Au core nanoparticles were chosen for further experiments. To understand the effect of substituent composition as well as position of nitro group on catalytic activity of Au@Fe3O4 yolk-shell nanocatalysts, another three nitroarenes including 2-NP, 4-NT, and CNB were chosen. Figure S7 (Supporting Information) show the optical property and concentration change of various nitroarenes in the presence of Au@Fe3O4 yolk-shell nanocatalysts and NaBH4. The main absorption peaks of 4-NP, 2-NP, 4-NT, and CNB are centered at 400, 415, 284, and 280 nm, respectively, and the intensities decrease when NaBH4 is added into the solutions, clearly indicating the occurrence of reduction reactions. In addition, isosbestic points of 313 nm for 4-NP, 310 nm for 2-NP, 249 nm for 4-NT and 252 nm for CNB are observed. As shown in Figure 6, a nearly complete reduction of all nitroarenes is observed. However, the reduction rate of nitrophenols (2-NP and 4-NP) by Au@Fe3O4 yolk-shell nanocatalysts is lower than those of 4-NT and CNB. It is 17



noteworthy that the reduction efficiency of nitroaromatic compounds by Au@Fe3O4 yolk-shell nanoparticles is relatively low at the first 10 s, presumably due to the delayed effect of diffusion on the reduction of reactants. Since the reduction of nitroaromatic compounds by Au@Fe3O4 yolk-shell nanoparticles follows the Langmuir-Hinshelwood kinetics, nitroarenes need to diffuse from the thin layer of Fe3O4 to the surface of Au nanoparticles, resulting in the decrease in reduction efficiency at the first reaction stage. The reduction of nitroarenes by Au@Fe3O4 yolk-shell nanoparticles also obeys the pseudo-first-order kinetics. The kobs values for 4-NP, 2-NP, 4-NT, and CNB are 0.94, 0.83, 1.75, and 2.12 min-1, respectively, showing that the reduction efficiency of substituents on nitro compounds followed the order -Cl > -CH3 > -OH. Table S1 (see Supplementary data) summaries the catalytic activity of Au-based nanomaterials for the reduction of nitroarenes. The kobs of Au@Fe3O4 yolk-shell nanoparticles for the catalytic reduction of 4-NP are in the range of 0.45-094 min-1. In addition, the kobs values for nitroarene reduction by 2.5 nm Au-based Au@Fe3O4 study is higher than or comparable to those reported data in most previous studies,36-45 showing that Au@Fe3O4 yolk-shell nanoparticles with small-sized Au seeds is a promising nanocatalyst for nitroarene reduction.

1.0

Remaining ratio (C/C0)

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

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4-NP 2-NP 4-NT CNB

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18


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


Figure 6. The catalytic reduction of various nitroarenes by Au@Fe3O4 yolk-shell nanoparticles in the presence of NaBH4. The particle size of Au core is 2.5 nm. The Au@Fe3O4 yolk-shell nanocatalysts show different chemoselective activities on nitroarenes. In this study, the catalytic reduction rate of 4-NP is higher than that of 2-NP because of the resonance stability and steric effect.46-49 The negative charge on oxygen of 4-nitrophenolate ion, the intermediate of nitrophenol reduction, would be delocalized throughout the benzene ring and becomes more resonance-stabilized than that of 2-nitrophenolate ions.46 In addition, the steric hindrance effect lowers the inductive effect of nitro group on 2-nitrophenolate ions when compares with that of 4-nitrophenolate ions. Therefore, the catalytic reduction efficiency and rate of 4-NP is higher than that of 2-NP. Shin et al. have synthesized the silver-deposited silanized magnetite beads for catalytic reduction of nitrophenols and found that the reduction rate of nitrophenols followed the order 4-NP > 2-NP > 3-NP.47 The catalytic reduction rate of 4-NP is slightly higher than that of 2-NP, which is in good agreement with our catalytic results by Au@Fe3O4 yolk-shell nanocatalysts. The highest reduction rate of CNB reduction is mainly attributed to the electron-withdrawing nature of chlorine substituent. When Au-based nanoparticles are used for catalytic reduction of nitroaromatics in the presence of NaBH4, BH4- and nitro compounds are first diffused through the Fe3O4 shells, enter inside the hollow cavity of yolk-shell structures, and then adsorb onto the surface of Au nanoparticles. The Au nanoparticles serve as nanocatalysts to transfer electrons from BH4- to nitroarenes, resulting in the production of amino derivatives.20 During this reduction process, the electron-withdrawing nature of chlorine substituent makes the nitro group on benzene ring more positively charged, and leads to a rapid electron transfer from Au surface to nitro group. The hydroxyl substituents on nitroarenes, on the contrary, decrease the reduction rate of nitro group by Au@Fe3O4 yolk-shell nanocatalysts due to the electro-donating nature. Dozauer et al. have synthesized 19



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Au nanoparticle/polyelectrolyte ﬁlms for catalytic reduction of nitroaromatic compounds and found that the overall reduction rate of nitrophenol is lower than that of nitrobenzene because of the inhibition of produced nitrosophenol during the reduction of nitrophenol.50 Fountoulaki et al. investigated the mechanism for X-substituted nitroarenes (X= OMe, H, Cl, COOMe, and CN) reduction in the presence of NaBH4 by Au nanoparticles supported onto mesoporous TiO2 and found that the kinetic activity of various nitroarenes was remarkably influenced by the nature of X-substituent groups, in which the reduction proceeded faster as the electro-withdrawing ability of the substituent group improved.21 In this study, the electron-withdrawing ability also follows the order -Cl > -CH3 > -OH, which is in good agreement with the kobs for various nitroarenes obtained in this study. Therefore, 4-CNB has been selected as the target chemical because of the highest reduction rate. Recyclability of Au@Fe3O4 Yolk-Shell Nanoparticles. Figure 7 shows the recyclable reduction of 4-NP and CNB by Au@Fe3O4 yolk-shell nanocatalysts. The target nitroarenes were re-injected into the solution after the complete reduction by Au@Fe3O4 yolk-shell nanoparticles. It is clear that 4-NP can be completely reduced to 4-aminophenol for at least 5 successive cycles. The kobs for 4-NP reduction is 0.92 min-1 for the first reduction, and then decreases to 0.62−0.76 min-1 for the 2nd−5th cycle, clearly indicating the excellent reusability of Au@Fe3O4 yolk-shell nanoparticles for reduction of nitroaromatic compounds. Several studies have reported that the catalytic activity of Au-based nanoparticles would decrease during the recycling processes.20, 23, 51 An et al. fabricated Pd-Fe3O4@SiO2 nanocatalysts for repeatedly reduction of 4-NP and found that the reduction rate of 4-NP decreased from 0.82 to 0.53 min−1 after 10 successive cycles.51 Our previous study has used dumbbell-like Au-Fe3O4 heterostructures to reductively degrade 4-NP and 2,4-dinitrophenol. The kobs of nitroaromatic reduction decreased from 0.63−0.78 to 0.16−0.27 min-1 after 5 cycling times because of the interference of 4-aminophenol during the recovery processes.20 It is 20


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noteworthy that the rate constants for 4-NP reduction by Au@Fe3O4 yolk-shell nanocatalysts after recycling are much higher than those reported Au-based nanocatalysts, indicating that the Fe3O4 shell layer in yolk-shell structure can serve as the shielding of Au nanoparticles to significantly decrease the adsorbed amount of aminophenol onto the surface, and results in the enhanced reduction rate and efficiency of 4-NP by Au@Fe3O4 nanocatalysts. To prove the shielding effect of Fe3O4 shell, a reference material by direct deposition of 2.5 nm Au nanoparticles onto hollow Fe3O4 nanoparticles was prepared for 4-NP reduction. Figure S7 (Supporting Information) shows the normalized rate constants for 4-NP reduction by Au@Fe3O4 yolk-shell nanoparticles and by Au/hollow Fe3O4 reference materials as a function of cycling time. Results show that 2.5 nm Au-based Au@Fe3O4 yolk-shell nanoparticles can retain the catalytic efficiency at around 80 % after 4 cycles of reaction. However, the catalytic reduction rate constant for 4-NP reduction by Au/hollow Fe3O4 mixture decreases dramatically and only 25 % of catalytic activity is retained after 4 cycles of reaction, presumably attributed to the loss of surface activity of Au nanoparticles in the presence of aminophenol. This result clearly proves that the Fe3O4 shell plays an important role in protection of Au nanoparticles from aggregation and poisoning by aminophenol.

(a) 1.0

(b)

Remaining ratio of CNB (C/Co)

4-NP

Remaining ratio of 4-NP (C/Co)

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


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Figure 7. The recyclability of (a) 4-nitrophenol (4-NP) and (b) 1-chloro-4-nitrobenzene (CNB) by Au@Fe3O4 yolk-shell nanocatalysts using 2.5 nm Au nanoparticles as the core 21



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Page 22 of 30

materials. The Au@Fe3O4 yolk-shell nanocatalysts also show superior recyclable activity toward CNB reduction. As shown in Figure 7(b), a nearly complete reduction of CNB is observed within 100 s and the yolk-shell nanocatalysts can be repeatedly used for at least 6 cycling times. The kobs for CNB reduction is initially 1.98 min-1 and then increases to 4.86 and 6.02 min-1 at 2nd and 3rd cycles, respectively. However, the rate constant decreases to 5.37−2.04 min-1 when the recycling process continues. The increase in kobs for CNB reduction may be attributed to the effect of remaining NaBH4 in the hollow sphere of yolk-shell structures. During the reduction processes, NaBH4 is in excess amount and residual NaBH4 remains inside the yolk-shell structures of Au@Fe3O4 during recovery process. Since CNB is a readily reducible compound, the added CNB can be reacted rapidly with NaBH4 inside the yolk-shell structures after addition of fresh solutions. However, the loss of nanocatalysts during the recycling processes would result in the decrease in rate constants for the 4th−6th cycles.23 These results clearly show that the developed Au@Fe3O4 yolk-shell nanostructures are ideal nanoreactors to rapidly and repeatedly catalyze the reduction of nitroarenes in the presence of NaBH4.

CONCLUSIONS In this study, we have demonstrated the successful fabrication of Au@Fe3O4 yolk-shell

nanocatalysts using various sizes of Au nanoparticles as the core materials by Kirkendall effect. The as-prepared Au@Fe3O4 nanocatalysts show distinctive structures with 2.5−10 nm Au core nanoparticles and 2.0−2.4 nm Fe3O4 shell layer. The Fe3O4 shell serves as the protective layer to maintain the excellent catalytic activity of Au core nanoparticles toward nitroarene reduction. The catalytic activity of Au@Fe3O4 yolk-shell nanocatalysts is highly size-dependent and small size of Au core nanoparticles in yolk-shell nanostructures exhibits

22


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superior

catalytic

performance

on

nitroarene

reduction.

The

nitroarenes

with

electron-withdrawing substituents also exhibit high reduction efficiency and rate by Au@Fe3O4 nanocatalysts in the presence of NaBH4. The yolk-shell nanoparticles show good reusability of 4-NP and CNB for at least 5 cycles. To the best of our knowledge, it is the first report that Au@Fe3O4 yolk shell nanocatalysts with various Au core nanoparticles are prepared for systematically catalytic reduction of nitroarenes with various substituents. Results obtained in this study clearly indicate the Au-based yolk-shell nanostructures are excellent catalysts and the interior hollow cavity can sever as the ideal platform for nitroarene reduction, which can pave the way for tailoring Au-containing nanoreactors for various heterogeneous catalytic processes.

ASSOCIATED CONTENT

Supporting Information. TEM images of hollow Fe3O4, Au/Fe core-shell and water soluble Au@Fe3O4, DLS and XPS spectra of Au@Fe3O4 yolk shell nanoparticles, reduction of 4-NP by various sizes of Au nanoparticles, UV-Vis spectra of nitroarenes, normalized rate constants for 4-NP reduction by Au@Fe3O4 and Au/hollow Fe3O4 and summary of catalytic activity of Au-based nanomaterial for nitroarene reduction. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION

Corresponding Author * Telephone: +886-3-5726785, fax: +886-3-5725958, e-mail address: [email protected]; [email protected]. Notes

23



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The authors declare no competing financial interest.

ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology (MOST), Taiwan for financial

support under grant Nos 102-2113-M-007-002-MY3 and 104-2221-E-009-020-MY3.

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