Magnetically Separable Nanocatalyst with the Fe3O4 Core and

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Magnetically separable nanocatalyst with Fe3O4 core and polydopamine-sandwiched-Au-nanocrystals shell JIANFENG ZHANG, QUNLING FANG, Jinyu Duan, HONGMEI XU, Hua-Jian Xu, and Shouhu Xuan Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b00302 • Publication Date (Web): 16 Mar 2018 Downloaded from http://pubs.acs.org on March 16, 2018

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Magnetically separable nanocatalyst with Fe3O4 core and polydopamine-sandwiched-Au-nanocrystals shell Jianfeng Zhanga, Qunling Fanga*, Jinyu Duana, Hongmei Xua, Huajian Xua, Shouhu Xuanb*

a

School of Biological and Medical Engineering, Hefei University of Technology, Hefei, 230009, P.R. China.

b

CAS Key Laboratory of Mechanical Behavior and Design of Materials, Department of Modern Mechanics, University of Science and Technology of China, Hefei 230027, P.R. China.

*

Corresponding author:

Asso. Prof. Qunling Fang E-mail:[email protected] Tel: 86-551-62904353 Fax: 86-551-62904353 Asso. Prof. ShouhuXuan E-mail:[email protected] Tel: 86-551-63601702 Fax: 86-551-63606382

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Abstract This work reports a novel Fe3O4@polydopamine-Au-polydopamine core/shell nanocomposite

towards

magnetically

separable

nanocatalyst.

Because

the

polydopamine (PDA) layers sandwiched Au nanocrystals were prepared by a layer-by-layer method, the content of Au could be controlled by varying the Au shell number

(such

as

burger-like

Fe3O4@PDA/Au/PDA/Au/PDA).

The

Fe3O4@PDA/Au/PDA exhibited excellent catalytic activity on reducing the p-Nitrophenol since the substrate could penetrate the PDA shell. Owing to the protection of PDA shell, the Fe3O4@PDA/Au/PDA presented higher cyclability than the Fe3O4@PDA/Au. The activity of Fe3O4@PDA/Au/PDA maintained 95% after 7 cycles, while Fe3O4@PDA/Au was only 61%. The detailed cycling catalytic mechanism was investigated and it was found that the catalytic rate of Fe3O4@PDA/Au/PDA/Au/PDA was faster than Fe3O4@PDA/Au/PDA due to the higher Au content. Interestingly, this method could be extended for other magnetic nanocomposites with two different kinds of noble metal nanocrystals integrated within

one

particle,

such

as

Fe3O4@PDA/Au/PDA/Ag/PDA

Fe3O4@PDA/Au/PDA/Pd/PDA.

Keywords: Magnetic, Nanocatalyst, Fe3O4, Polydopamine, p-Nitrophenol


and

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1. Introduction Nanocatalysts, especially the precious metal nanocrystals, have attracted tremendous attention in catalysis owing to their small volumes, large specific surface areas and high exposured crystal facets.[1-3] During the past decades, various efforts have been conducted to improve the catalytic performance by tuning the size, exposured crystal facets and inner nanostructure. [4-7] However, due to the high surface energy and small size, the naked nanocrystals were very easy to be aggregated thus seriously reduced their activity.[8] To solve this problem, immobilizing the nanocatalysts onto the nanocarriers became an effective method due to their excellent catalytic properties, high temperature resistance, oxidation resistance and corrosion resistance. To date, lots of nanocarriers such as the silica, carbon, polymer, metal organic frameworks, clays, LDH, and metal oxides have been developed to support the nanocatalysts.[9-15] Besides keeping the catalytic activity, these nanocomposites exhibited well stability and recyclability during the repeated reactions. Magnetic nanocatalyst is very attractive because it provides a convenient way to remove and recycle the nanocatalyst from reaction system.[16] In order to reduce the cost, various core/shell structured magnetic nanocatalyst have been applied in nanocatalysis.[17-22] Fe3O4 was the mostly used magnetic carrier in nanocatalysis and the as-obtained Fe3O4 based core/shell particles showed both excellent catalytic activity and magnetic separation. Polymer was favorable for carrying nanocrystals because there were a large number of functional groups in the polymer chains which possessed high compatibility to the noble metal. Therefore, several kinds of polymer encapsulated

Fe3O4,

such

as

the

Fe3O4@polyaniline,

Fe3O4@polypyrrole,

Fe3O4@polydoapmine, etc. were constructed and they showed wide potential in nanocatalysis.[23-26] Since the magnetic characteristic of the Fe3O4, large aggregations were often existed in preparing magnetic core/shell structure. Therefore, many attentions have been paid on investigating the synthetic method for the well-defined Fe3O4@polymer towards carrying nanocatalyst. Usually, the nanocrystals were immobilized on the surface of the carrier thus


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they inevitably met a leaching problem. To improve the stability, a sandwiched shell structure in which the noble metal shell was further encapsulated by another porous protecting layer was developed. Typically, the reported Fe3O4@SiO2@Au@p-SiO2 and

Fe3O4@RF@Pd@-pSiO2

possessed

higher

recyclability

than

the

non-encapsulated nanocatalysts.[27,28] Recently, it was demonstrated that the polydopamine (PDA) encapsulated nanocrystals survived in many reaction cycles with stable activities since the intrinsic nano-porous shell structure.[29-38] Due to its strong adhesive property, the in situ polymerized PDA shell was formatted much easier than the porous-SiO2. Our previous study indicated the PDA layer not only prevent the encapsulated Au nanocrystals from aggregation but also enhance the recyclability[39]. Considering the separation advantage of the Fe3O4@polymer, the demand of exploring PDA shell encapsulated magnetic nanocatalyst has driven the development of our target. In this work, a simple method was developed for fabricating the Fe3O4@PDA/Au/PDA nanospheres in which the Au nanocrystals were sandwiched by two PDA layers. As depicted in Scheme 1, the Fe3O4 could be easily coated with a thin PDA shell by the simple oxidation polymerization. Then, the surface of the Fe3O4@PDA was decorated with Au nanocrystals through the electrostatic adsorption method. Following a further PDA polymerization, the target with a Fe3O4 core and a PDA/Au/PDA sandwich shell was constructed. Interesting, a thick burger-like shell could be obtained by increasing the layer numbers (Such as PDA/Au/PDA/Au/PDA). Moreover, the controllable shell contents with different or mixed metal species were achievable. Because the Au nanocrystals were protected by the porous PDA, the final magnetically separable Fe3O4@PDA/Au/PDA nanospheres exhibited cyclable catalytic activities. The detailed catalytic performance of the product was discussed and it was estimated that the composite with mixed metal nanocatalysts (Such as Fe3O4@PDA/Au/PDA/Pd/PDA, Fe3O4@PDA/Au/PDA/Ag/PDA) would possess broad application in nanocatalysis.


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Scheme 1 The synthetic procedure of Fe3O4@PDA/Au/PDA/Au/PDA nanocatalyst.

the

Fe3O4@PDA/Au/PDA

and

2. Results and Discussions

Fig. 1 SEM (a) and TEM (b,c,d,e) images of the Fe3O4@PDA/Au/PDA nanospheres.

Fig. 1a shows the typical SEM image of the Fe3O4@PDA/Au/PDA nanospheres. All particles were well dispersed on the copper grid and the average size was about 260 nm, which also agreed with the TEM analysis (Fig. 1b). Interesting, it was found that the product presented a yolk-like structure with a big core and non-continuous nanocrystals aggregated shell (Fig. 1c). To further investigate its inner nanostructure, a higher magnification TEM image was conducted (Fig. 1d). The black-pale colored structure indicated the core/shell nature of the targeting particle, in which the outer ACS Paragon Plus Environment

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pale shell was the PDA and the cluster-like core was Fe3O4. Between the pale shell and inner core, we can find many dark black nanoparticles, which must be responded for the Au nanocrystals. Although it could hardly identify the low contrast PDA interlayer between the Au shell and Fe3O4 core under high magnification, the observed yolk structure also demonstrated the succeeded product with a Fe3O4 core and two PDA layers sandwiched Au shell. Fig. 1e further proved that the Au layer was encapsulated by an outer PDA shell. Here, the Au sizes in the Fe3O4@PDA/Au/PDA nanospheres were ranged from 4 to 8 nm and the average thickness of outer PDA shell was about 5 nm.

Fig. 2 Representative HAADF-STEM images and EDX elemental mapping of Au, Fe, O, C and N (a-f), and the EDX spectrum (g) of the Fe3O4@PDA/Au/PDA nanocomposite.

High-angle annular dark field scanning TEM also confirmed the typical sandwiched nanostructure of the nanospheres (Fig. 2a). The energy dispersive X-ray


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(EDX) mapping of the Fe element revealed the Fe3O4 was mainly located within the core/shell particles (Fig. 2b). The Au element was distributed around the Fe3O4 core since the density in the center was very low. The diameters of the C and N element map were larger than the Au, which further supported that the Au nanocrystals were protected by the PDA shell (Fig. 2c,d,e,f). Based on the TEM and EDX mapping, it could be concluded that the Fe3O4@PDA/Au/PDA microparticles with four-layered structure had been successfully achieved (Fig. 2g).

Fig. 3 SEM and TEM images of the as-synthesized Fe3O4 (a, e), Fe3O4@PDA (b, f), Fe3O4@PDA/Au (c, g), and Fe3O4@PDA/Au/PDA (d, h).

In this work, the formation procedure of the Fe3O4@PDA/Au/PDA nanospheres was tracked by the SEM and TEM. Firstly, the polyacrylic acid (PAA) modify Fe3O4 nanospheres were synthesized through our reported mix-solvothermal method. [40] The SEM image indicated the average size of the uniform Fe3O4 particles was about 220 nm (Fig. 3a). Similar to the previous work, the Fe3O4 particles possessed the cluster-like nanostructure (Fig. 3e). [23] Owing to its multifunctional surface, the Fe3O4 particles could be well dispersed within water and ethanol. Because the PDA exhibited a strong adhesive property to both inorganic and organic materials, the Fe3O4@PDA core/shell nanospheres could be synthesized by a simple in situ polymerization. As shown in Fig. 3b, the resultant Fe3O4@PDA possessed a near spherical morphology. In comparison to the pristine Fe3O4, although the Fe3O4@PDA showed a similar average particle size, its surface became much smoother, which suggesting the uniform PDA coating. The TEM image (Fig. 3f) further showed the typical core/shell nanostructure of Fe3O4@PDA. Obviously, the outer pale shell was ACS Paragon Plus Environment

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PDA and the thickness was about 15 nm. Then, the surface became further rough when the Au NPs were fixed on the surface of the Fe3O4@PDA (Fig. 3c and g). The TEM image with a higher magnification of Fe3O4@PDA/Au was shown in Fig. S3. The dark dots with an average size about 5 nm were Au NPs and they were tightly immobilized on the periphery of the Fe3O4@PDA through the electrostatic attraction. Finally, another PDA layer was coated to form the Fe3O4@PDA/Au/PDA with a smooth surface (Fig. 3d). Due to the presence of PDA/Au/PDA sandwich shell, the average size of the Fe3O4@PDA/Au/PDA (260 nm) was larger than Fe3O4. The contrast of the PDA was much lower than the Fe3O4 and Au, therefore, the TEM image exhibited a yolk-like structure in a low magnification (Fig. 3h).

Fig. 4 SEM and TEM images of the Fe3O4@PDA/Au/PDA/Au (a, c) and Fe3O4@PDA/Au/PDA/Au/PDA (b, d). Due to the simple layer-by-layer method, the content of the final product was easily controlled by varying the layer numbers. In this work, a thick burger-like shell could be constructed. As shown in Fig. 4a, the Fe3O4@PDA/Au/PDA/Au displayed a strawberry-like morphology because the surface was fully covered with Au nanocrystals. The TEM image also indicated the density of the Au significantly increased

(Fig.

4c).

After

the

further

coating


of

PDA,

the

smooth

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Fe3O4@PDA/Au/PDA/Au/PDA microparticles were successfully obtained (Fig. 4b). The defined multi-core/shell structure could be clearly observed in the TEM image (Fig. 4d). Obviously, the two distinctive non-continuous Au shells were constructed during the layer-by-layer method.

Fig. 5 TEM images and EDX spectra of the Fe3O4@PDA/Au/PDA/Pd/PDA (a,b,c) and Fe3O4@PDA/Au/PDA/Ag/PDA (d,e,f). The metal species were tunable in our product since the easy fabrication. Various noble metals such as Pd, Ag and Pt could be used for the magnetic nanocatalyst. Moreover, two different types of nanocatalysts could be encapsulated within one particle. To our knowledge, this kind of nanostructure has not been reported anywhere. As

shown

in

Fig.

5,

the

Fe3O4@PDA/Au/PDA/Pd/PDA

Fe3O4@PDA/Au/PDA/Ag/PDA nanocomposites

also

presented

well

and defined

core/shell nanostructure. EDX clearly demonstrated the presence of both Pd and Au component in the single Fe3O4@PDA/Au/PDA/Pd/PDA (Fig. 5c). Similarly, the big nanocrystals in the Fe3O4@PDA/Au/PDA/Ag/PDA must be responded for the Ag element since the small one was Au (Fig. 5e). The above results were also agreed well by the XRD characterization (Fig. S1, 2). Actually, the position of the Pd, Ag and Au layers could be exchanged thus they possessed high potential in step-wise nanocatalysis. Based on present results, the Fe3O4@PDA/Ag/PDA/Au/PDA,


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Fe3O4@PDA/Au/PDA/Ag/PDA,

Fe3O4@PDA/Ag/PDA/Pd/PDA,

and

Fe3O4@PDA/Pd/PDA/Ag/PDA would be also available.

(f)

Relative Intensity(a.u.)

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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(e) (d)

(c) (b)

(a)

10

20

30

40 50 2θ（ degree）

60

70

80

Fig. 6 XRD diffraction patterns of the Fe3O4 (a), Fe3O4@PDA (b), Fe3O4@PDA/Au (c), Fe3O4@PDA/Au/PDA (d), Fe3O4@PDA/Au/PDA/Au (e), and Fe3O4@PDA/Au/PDA/Au/PDA (f).

The crystal structures of the as-prepared samples were investigated by XRD. As shown in Fig. 6a, all the characteristic peaks were indexed to be (220), (311), (440), (422), (511) and (440) lattice planes in the face-centered-cubic Fe3O4 (JCPDS card No.19-629). The diffraction peaks of Fe3O4@PDA were similar to the Fe3O4 (Fig. 6b), indicated the crystalline phase of Fe3O4 had not be changed during the PDA coating. Because the PDA was amorphous, no new diffraction peak was observed in the Fe3O4@PDA. After further immobilizing the Au NPs onto the surface of the PDA, an obvious Au peak (111) located at 38° was found (JCPDS card no. 04-0784) (Fig. 6c). With increasing of the Au layer, the intensity of this peak sharply increased (Fig. 6e). The PDA hardly contributed the XRD diffraction intensity, thus the Fe3O4@PDA/Au and Fe3O4@PDA/Au/PDA/Au showed similar patterns to the Fe3O4@PDA/Au/PDA and Fe3O4@PDA/Au/PDA/Au/PDA, respectively.


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Transmittance(%)

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(f) (e) (d) (c) (b) (a)

500

1000

1500

2000

2500

3000

3500

4000

-1

Wavenumbers(cm )

Fig. 7 FTIR spectra of the as-prepared samples Fe3O4 (a), Fe3O4@PDA (b), Fe3O4@PDA/Au (c), Fe3O4@PDA/Au/PDA (d), Fe3O4@PDA/Au/PDA/Au (e), andFe3O4@PDA/Au/PDA/Au/PDA (f). Fig. 7a was the FTIR spectrum of pure Fe3O4, in which the broad peaks at 1413 cm-1 and 1622 cm-1 were corresponding to the vibration of COO- and O-H due to the residual PAA in the cluster-like Fe3O4 nanospheres.[8] The peak observed at 3440 cm-1 was attributed to O-H stretching vibration. As soon as the PDA was coated on the Fe3O4, the strong vibrational absorption band at 595 cm-1 (Fe-O vibration mode) gradually weaken. Here, the PDA layer was very thin and the PDA adsorption band was too weak, thus no additional crucial peak could be observed in the Fe3O4@PDA (Fig.7b, 7d, 7f). The immobilization of Au further reduced the Fe-O absorption (Fig. 7c, 7e), which must be responded for the decreased weight ratio.


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100 95 90

Weight(%)

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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(a)

85 (c)

80

(b)

75

(e)

70

(d)

65

(f)

0

100

200

300

400

500

600

700

Temperature(ºC)

Fig. 8 TG curves of the as-prepared samples Fe3O4 (a), Fe3O4@PDA (b), Fe3O4@PDA/Au (c), Fe3O4@PDA/Au/PDA (d), Fe3O4@PDA/Au/PDA/Au (e), and Fe3O4@PDA/Au/PDA/Au/PDA (f). The TG curves were tested from 25 to 700 ℃ in the air to track the synthetic procedure (Fig. 8). The weight losses of the samples were 12.1, 22.1, 18.4, 30.6, 22.9 and 33.0 wt%, respectively. For the naked Fe3O4 (Fig. 8a), the observed weight loss could be attributed to the disappearance of residual organic groups and absorbed water. After coating a PDA layer, an addition 10% weight loss was found in the Fe3O4@PDA than Fe3O4 (Fig. 8b). In comparison to the Fe3O4@PDA, the weight loss of Fe3O4@PDA/Au reduced 3.8% due to the presence of Au NPs (Fig. 8c). The second PDA shell further increased the weight loss in Fe3O4@PDA/Au/PDA (Fig. 8d). Similarly, the further coating of Au NPs increased the weight loss and the PDA shell reduced the weight loss (Fig. 8e,f). All these results indicated the successful construction of the sandwiched nanostructure.


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O1s

C1s Au4dN1s

Au4f

(f)

Relative intensity(cps)

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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(e) (d) (c) (b) Fe2p

(a)

Fe3p

0

200

400

600

800

1000

1200

Binding energy(eV)

Fig. 9 XPS spectra of the as-prepared samples Fe3O4 (a), Fe3O4@PDA (b), Fe3O4@PDA/Au (c), Fe3O4@PDA/Au/PDA (d), Fe3O4@PDA/Au/PDA/Au (e), and Fe3O4@PDA/Au/PDA/Au/PDA (f). To further detect the inner structure of the core-shell nanocomposites, X-ray photoelectron spectroscopy (XPS) was applied to analyze the surface state elemental components. The XPS spectrum of Fe3O4 was showed in Fig. 9a, which clear showed the strong signals for Fe, C and O element. For Fe3O4@PDA (Fig. 9b), the N element was found in the curve indicated the presence of PDA. Here, the Fe peak was disappeared, demonstrating the PDA shell was uniformly wrapped on the Fe3O4 core. Because the detection depth of the XPS was about 10nm, this phenomenon also indicated the thick PDA shell, which agreed with the TEM analysis. As soon as the Au NPs were fixed on the surface of Fe3O4@PDA, the Au signals were presented (Fig. 9c). When another PDA layer was formed on the surface of Fe3O4@PDA/Au, the relative intensities of Au peaks were significantly weakened (Fig. 9d). After another Au NPs layer was fixed on the surface of Fe3O4@PDA/Au/PDA, the peaks of Au NPs were further enhanced (Fig. 9e). When the third PDA layer was coated on the surface of Fe3O4@PDA/Au/PDA/Au, the relative intensities of Au peaks were weakened again (Fig. 9f).


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40 20

M (emu/g)

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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0

-20 Fe3O4 Fe3O4@PDA/Au/PDA

-40 -10000

Fe3O4@PDA/Au/PDA/Au/PDA

-5000

0

5000

10000

Magnetic Field (Oe)

Fig. 10 Room-temperature magnetization hysteresis loops of the as-prepared samples.

As shown in Fig. 10, the magnetic properties of Fe3O4, Fe3O4@PDA/Au/PDA, and Fe3O4@PDA/Au/PDA/Au/PDA were evaluated by SQUID. All the three nanospheres presented the typical superparamagnetic behavior and the remanence and coercivity were nearly zero. Obviously, the magnetic properties of the other two products were originated from the superparamagnetic Fe3O4. The saturation magnetization (Ms) of the nanocomposites decreased with the addition of nonmagnetic PDA and Au layer. Although the Ms value of Fe3O4@PDA/Au/PDA (28.5 emu/g) and Fe3O4@PDA/Au/PDA/Au/PDA (25.8 emu/g) were smaller than the Fe3O4 (47.9 emu/g), the magnetic sensitivity was enough for separating the magnetic nanocatalysts from reaction systems. Therefore, the above product possessed potential application in nanocatalysis.


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Fig. 11 Illustration of color change reaction (A), UV-Vis spectra of p-nitrophenol catalyzed by 2 mg Fe3O4@PDA/Au/PDA nanocatalyst (B), Plot of conversion of p-nitrophenol versus time at different weights of the Fe3O4@PDA/Au/PDA catalyst (0 mg a), 2 mg b), 3 mg c), 4 mg d)) (C), and Plot of conversion of p-nitrophenol versus time at the same weights of the Fe3O4@PDA/Au, Fe3O4@PDA/Au/PDA and Fe3O4@PDA/Au/PDA/Au/PDA nanocatalysts (D). Because the p-nitrophenol can be reduced to p-aminophenol by the catalysis of Au NPs (Fig. S7). Here, the as-prepared nanocatalysts were used as catalysts for the reduction of p-nitrophenol to evaluate their catalytic activities (Fig. 11A). The UV-Vis spectra of p-nitrophenol in the presence of NaBH4 and Fe3O4@PDA/Au/PDA nanocatalysts in aqueous solution were recorded (Fig. 11B). With increasing of the reaction time, the absorbance peak of intermediate at 400 nm was gradually decreased and disappeared at 10 min. At the same time, a gradual augment of a new absorbance peak at 300 nm indicated the formation of p-aminophenol, which further proved the reduction

of

p-nitrophenol

to

p-aminophenol

under

the

catalysis

of

Fe3O4@PDA/Au/PDA. Here, the catalytic performance of Fe3O4 or Fe3O4@PDA also has been investigated. As shown in Fig. S4, with increasing of the reaction time, the absorbance curves were almost the same, which indicated the p-nitrophenol has not been reduced. Similarly, no obvious change can be observed in Fig. S5. The slight decrease of the absorbance peak may be responded for the absorption of p-nitrophenol ACS Paragon Plus Environment

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by Fe3O4@PDA. Therefore, both of the Fe3O4 and Fe3O4@PDA cannot catalyze the reduction of p-nitrophenol. In this catalytic reaction (Fig.11C), the reaction speed was sharply accelerated with increasing of catalyst content. All of these results clearly demonstrated that the reaction was solely catalyzed by the Au NPs. During the reaction, the concentration of NaBH4 was regarded as a constant due to the concentration of p-nitrophenol was far lower than that of NaBH4. Therefore, the reduction rate of p-nitrophenol decreased slowly with increasing of the reaction time, which could be considered as the pseudo-first-order kinetics.[41] The

mass

percentages

of

Au

NPs

in

Fe3O4@PDA/Au/PDA

and

Fe3O4@PDA/Au/PDA/Au/PDA determined by ICP-AES were 9.3 wt% and 15.2 wt%, respectively.

As

shown

in

Fig.

11D,

the

reaction

catalyzed

by

Fe3O4@PDA/Au/PDA/Au/PDA was faster than that of Fe3O4@PDA/Au/PDA under the same reaction conditions. This result indicated that the Au loading capacity of the Fe3O4@PDA/Au/PDA/Au/PDA was better. More Au NPs can be further decorated onto the magnetic nanocarrier by using the above method to improve the catalytic performance.

Fe3O4@PDA/Au

100

Conversion (%)

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Fe3O4@PDA/Au/PDA

80 60 40 20 0 1

2

3

4

5

6

7

Cycle Times

Fig. 12 Conversion of p-nitrophenol in seven successive cycles of reduction with two kinds of nanocatalysts: Fe3O4@PDA/Au and Fe3O4@PDA/Au/PDA. The recyclability of catalysts is very important for the green catalysis. Catalysts with good separability and reusability are suitable for industrial applications.


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Therefore, the recyclability of the as-prepared samples was investigated. After the reaction, both the Fe3O4@PDA/Au and Fe3O4@PDA/Au/PDA nanocatalysts were magnetically separated from the reaction solution and reused for the next reduction reaction. As shown in Fig. 12, after 7 recycles, the catalytic effect of Fe3O4@PDA/Au/PDA catalyst did not show a clear change, and the conversion rates of p-nitrophenol were larger than 95%. However, the catalytic activity of Fe3O4@PDA/Au critically decreased and the conversion rate of p-nitrophenol was reduced to 60.7%, which could be attributed to the Au NPs leached during the repeated catalytic processes. Although the reaction rate of Fe3O4@PDA/Au was a little quicker than Fe3O4@PDA/Au/PDA and Fe3O4@PDA/Au/PDA/Au/PDA (Fig. 11D), its cyclability was bad. Obviously, the better reusability of the Fe3O4@PDA/Au/PDA nanocatalyst could be attributed to the external PDA layer. Based

on

the

above

analysis,

the

Fe3O4@PDA/Au/PDA

and

Fe3O4@PDA/Au/PDA/Au/PDA were estimated to be widely applied in nanocatalysis because of the good catalytic effect, excellent reusability, and green synthesis process. Here, the reaction rate of Au NPs was quicker than Fe3O4@PDA/Au/PDA because the p-nitrophenol could reach to the Au NPs easier (Fig. S6 and S7). As shown in Fig. S6, when 3 mg Fe3O4@PDA/Au/PDA nanocomposite was used as the nanocatalyst, the absorbance at 400 nm decreased and nearly disappeared at 5 min. As a comparison, after adding the similar weights of Au NPs (0.279 mg), the absorbance at 400 nm disappears within 30s. Clearly, the Au NPs exhibited an excellent catalytic activity on the reduction reaction of p-nitrophenol. Therefore, although the PDA layer increased the stability and cycle performance of the nanocatalysts, it also slightly reduced the reaction rate. The catalytic activity of Fe3O4@PDA/Au/PDA/Pd/PDA was also

investigated

in

our

work.

As

shown

in

Fig.

S8,

when

3

mg

Fe3O4@PDA/Au/PDA/Pd/PDA was used to catalyze the p-nitrophenol, the reaction was completed within 2 min. In comparison to Fe3O4@PDA/Au/PDA/Au/PDA (3.5 min),

the

reaction

was

much

quicker.

This

result

indicated

that

Fe3O4@PDA/Au/PDA/Pd/PDA possessed a better catalytic performance in the reduction of p-nitrophenol. However, due to the different metal species, the detail ACS Paragon Plus Environment

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catalytic dynamics of the hybrid nanocatalysts was very difficult to be clarified. More works will be done to analyze the catalytic mechanism.

3. Conclusion In summary, a facile method was developed to synthesize the core/shell Fe3O4@PDA/Au/PDA nanospheres as magnetic catalyst. In order to increase the content of Au NPs in the nanocarrier, the Fe3O4@PDA/Au/PDA/Au /PDA multi-layer nanocomposite was successfully constructed. This method also could be extended for the magnetic nanocomposite with two different nanocatalysts (such as Pd, Ag, Au) integrated with in one nanosphere. The Fe3O4@PDA/Au/PDA exhibited excellent catalytic activity on reducing the p-nitrophenol since the substrate could penetrate the PDA shell. The Fe3O4@PDA/Au/PDA showed better cyclability than the Fe3O4@PDA/Au because the PDA shell prevented the Au from leaching during the reaction. Meanwhile, the reaction rate of Fe3O4@PDA/Au/PDA was a little slower than Fe3O4@PDA/Au because of the existence of PDA shell. Due to the higher Au content, the catalytic rate of Fe3O4@PDA/Au/PDA/Au/PDA was faster than Fe3O4@PDA/Au/PDA. The catalytic mechanism for the as-prepared nanocomposites was discussed and more works will be done for studying their practical catalytic performance.

4. Experimental section 4.1. Materials Iron (III) chloride hexahydrate (FeCl3 · 6H2O), diethylene glycol (DEG), ethylene glycol (EG), sodium acetate (NaAc), polyacrylic acid (PAA), ethanol (EtOH), trihydroxymethyl aminomethane (Tris), hydrochloric acid(HCl), trisodium citrate dehydrate (C6H5Na3O7 · 2H2O), acetic acid (CH3COOH), gold (III) chloride (HAuCl4), palladium chloride (PdCl2), (3-Aminopropyl)triethoxysilane (APTES), silver nitrate (AgNO3), sodium tetrahydridoborate (NaBH4), and p-nitrophenol (C6H5NO3) are analytical grade and obtained from Sinopharm Chemical Reagent Co.


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Ltd. 3-Hydroxytyramine hydrochloride (DA-HCl) was purchased from Aladdin Reagent Co. All chemicals were used as received without any further purification. Ultrapure water used during the entire experiment was prepared by using Ulupure SP reagent water system (Xi'an, China). 4.2.

Preparation of

Fe3O4@PDA/Au/PDA,

Fe3O4@PDA/Au/PDA/Au/PDA,

Fe3O4@PDA/Au/PDA/Pd/PDA and Fe3O4@PDA/Au/PDA/Ag/PDA core/shell nanocomposites Firstly, the magnetic Fe3O4 nanospheres were synthesized through a one-pot solvothermal reaction.[33] Typically, FeCl3 · 6H2O (1.08 g), NaAc (4.0 g), and Polyacrylic acid (0.1 g) were dissolved in the mixture of ethylene glycol (14 mL) and diethylene glycol (26 mL) by magnetic stirring for 20 min. Then, the resulting homogeneous mixture was transferred and sealed in the Teflon-lined stainless-steel autoclave, and heated at 200 ℃ for 12 h. After being cooled to room temperature, the obtained black Fe3O4 nanospheres were washed with deionized water and ethanol for five times, respectively. Finally, the Fe3O4 nanospheres were dried under vacuum at 45 ℃ for 12 h. In order to prepare monodispersed Fe3O4@PDA core-shell nanospheres, firstly, 20 mg of the Fe3O4 was dispersed in 60 mL C2H5OH by sonication. Then, 60 mL DA (6.6 mmol/L) in Tris (pH=8.5) was added into the solution under sonication. After 3 h, the product was magnetically separated and collected from the solution. The product was washed with deionized water and ethanol, and then dried in vacuum for 12 h to obtain a black powder. The Fe3O4@PDA/Au nanocomposite was prepared by using a typical electrostatic attraction method. Firstly, 20 mg of the Fe3O4@PDA nanocomposite and acetic acid (0.8 mL) were added into ethanol (4 mL), and the resulting mixture was disposed under sonication at room temperature for 20 min to ensure thoroughly dispersed. Then, an aqueous solution of Au NPs was prepared according to the previously reported reduction method.[39] Typically, 8 mL of a NaBH4 solution (0.01mol/L) was added into 80 mL of mixed aqueous solution of HAuCl4 (2.5×


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10-4mol/L) and sodium citrate (3.0×10-4mol/L) rapidly under vigorous stirring. With the addition of NaBH4, the solution turned cardinal red, indicating the citrate modified Au NPs was successfully prepared. Subsequently, the positively charged Fe3O4@PDA nanocomposite was added into the freshly Au NPs solution under sonication at room temperature for 30 min. The Au NPs were immobilized on the surface of the Fe3O4@PDA nanocomposite by electrostatic attraction. Finally, the product was separated by a magnet and washed with water and ethanol. The obtained Fe3O4@PDA/Au nanocomposite was dried under vacuum for 12 h. The synthesis of Fe3O4@PDA/Au/PDA nanocomposite is similar to the Fe3O4@PDA. Firstly, 20 mg of the as-prepared Fe3O4@PDA/Au was dispersed in 60 mL C2H5OH by sonication for 1 h. Then, 60 mL DA (6.6 mmol/L) in Tris (pH=8.5) was added into the solution under sonication for 3 h. Finally, the resultant product was washed and dried in vacuum for 12 h. The synthesis of Fe3O4@PDA/Au/PDA/Au nanospheres is similar to the Fe3O4@PDA/Au.

20

mg

of

the

positively

charged

Fe3O4@PDA/Au/PDA

nanocomposite was added into the freshly Au NPs solution under sonication at room temperature for 30 min. The obtained Fe3O4@PDA/Au/PDA/Au nanocomposite was dried under vacuum for 12 h. At last, the Fe3O4@PDA/Au/PDA/Au/PDA was fabricated by the above described in situ polymerization method. 20 mg of the Fe3O4@PDA/Au/PDA/Au was dispersed in 60 mL C2H5OH by sonication. Then, 60 mL DA (6.6mmol/L) in Tris (pH=8.5) was added into the solution and the reaction was conducted under sonication for

3

h.

After

being

washed

and

dried

in

vacuum,

the

final

Fe3O4@PDA/Au/PDA/Au/PDA was obtained. In order to prepare Fe3O4@PDA/Au/PDA/Pd core/shell nanocomposites. Firstly, 30 mg Fe3O4@PDA/Au/PDA was dispersed in 30 mL deionized water by sonication. Then, 9 mg PdCl2 was added into the solution under sonication. After 6 h, 1 mL of a NaBH4 solution (0.01 mol/L) was added into the mixed aqueous solution and the reaction was kept under sonication for 2 h. The product was magnetically separated from the solution and washed with deionized water and ethanol, and then dried in ACS Paragon Plus Environment

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vacuum for 12 h to obtain a black powder. In order to prepare Fe3O4@PDA/Au/PDA/Pd/PDA core/shell nanocomposites. 20 mg Fe3O4@PDA/Au/PDA/Pd was dispersed in 60 mL C2H5OH by sonication. Then, 60 mL DA (6.6 mmol/L) in Tris (pH=8.5) was added into the solution and the reaction was conducted under sonication for 3 h. After being washed and dried in vacuum, the final Fe3O4@PDA/Au/PDA/Pd/PDA was obtained. In order to prepare Fe3O4@PDA/Au/PDA/Ag core/shell nanocomposites. Firstly, 30 mg Fe3O4@PDA/Au/PDA was dispersed in the mixed solution of deionized water (6 mL) and ethanol (24 mL). Then, 0.3 mL APTES, 3 mL AgNO3 (3 mg/mL) and 3 mL sodium citrate (4 mg/mL) were added into the solution under sonication. After 30 min, the mixed solution was poured into the three-necked flask in the oil bath (100 ℃), stirred and reflux condensation for 30 min. After natural cooling of the solution, the product was magnetically separated from the solution and washed with ethanol, and then dried in vacuum for 12 h. In order to prepare Fe3O4@PDA/Au/PDA/Ag/PDA core/shell nanocomposites. 20 mg Fe3O4@PDA/Au/PDA/Ag was dispersed in 60 mL C2H5OH by sonication. Then, 60 mL DA (6.6 mmol/L) in Tris (pH=8.5) was added into the solution and the reaction was conducted under sonication for 3 h. After being washed and dried in vacuum, the final Fe3O4@PDA/Au/PDA/Ag /PDA was obtained. 4.3. Catalytic reduction of p-nitrophenol The reduction of p-nitrophenol with NaBH4 was taken as the model reaction to investigate the catalytic activity of the nanocatalysts. 50 mL p-nitrophenol aqueous solution (1×10-4 mol/L) and 37.8 mg NaBH4 were added into a beaker, then a certain amount of nanocatalyst was introduced into the above solution. As the reaction proceeded, the color of the mixed solution changed from yellow to colorless due to the reduction of p-nitrophenol. The transformation of p-nitrophenol was traced by ultraviolet-visible (UV-Vis) spectrometry, and the characteristic absorption at 400nm was monitored. After the reaction was completed, the nanocatalysts were magnetically separated and collected from the mixed solution. Then, it was reused in the subsequent catalysis. ACS Paragon Plus Environment

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4.4. Characterization The structure of the nanocomposites was investigated by JEM-2100F field emission transmission electronic microscopy (FE-TEM) operating at 200 kV accelerated voltage. The morphology of the nanocomposites was observed by using a JEOL JSM-6700F SEM. X-ray powder diffraction patterns (XRD) of the nanocomposites were recorded on a Bruker D8 Advance diffractometer equipped with graphite monochromatized Cu Kα radiation (λ = 1.5406 Å). Fourier transform infrared (FTIR) spectra of all samples in the wavenumber range 4000-400 cm−1 was obtained in KBr pressed pellets on a TENSOR Model 27 Fourier transform infrared (FT-IR) spectrometer. Thermogravimetric (TG) analysis was recorded with a DTG-60H thermogravimetric instrument under the atmosphere of air flowing from room temperature to 700℃ at a heating rate of 10℃/min. X-ray photoelectron spectra (XPS) measurements were performed using an ESCALAB 250. UV-Vis spectra of the reactions were recorded on a TU-1901 spectrophotometer. The Au contents of the samples were analyzed using an Optima 7300DV inductive coupled plasma atomic emission spectrometer (ICP-AES). Their magnetic properties (M-H curve) were measured at room temperature on a MPMS XL magnetometer made by Quantum Design Corp.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI:

.

XRD patterns of different products (Figure S1, S2), TEM images for Fe3O4@PDA/Au (Figure S3), UV-Vis spectra for the catalyzation under different conditions (Figure S4-9)

Acknowledgement This work is supported by the National Natural Science Foundation of China (11572310, 21205026) and the Fundamental Research Funds for the Central Universities. ACS Paragon Plus Environment

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Graphic abstract:

Scheme The synthetic procedure of Fe3O4@PDA/Au/PDA/Au/PDA nanocatalyst.

the

Fe3O4@PDA/Au/PDA

and

A novel magnetically separable nanocatalyst with a Fe3O4 core and a polydopamine-sandwiched-Au-nanocrystals shell was designed via a layer-by-layer method. It exhibited excellent catalytic activity since the substrate could penetrate the


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Page 28 of 28

outer PDA shell, while the PDA shell protected the inner nanocatalysts. This method could be extended to integrate two different noble metal nanocrystals within one particle,

such

as

thick

burger-like

Fe3O4@PDA/Au/PDA/Ag/PDA

and

Fe3O4@PDA/Au/PDA/Pd/PDA, which would be favorable for multi-step catalysis.


Magnetically Separable Nanocatalyst with the Fe3O4 Core and

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