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Magnetic Responsive Janus Nanosheets with Catalytic Property Dan Xue, Qingbo Meng, and Xi-Ming Song ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b21012 • Publication Date (Web): 22 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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ACS Applied Materials & Interfaces

Magnetic Responsive Janus Nanosheets with Catalytic Property Dan Xue, Qing Bo Meng, Xi-Ming Song* Liaoning Key Laboratory for Green Synthesis and Preparative Chemistry of Advanced Materials, College of Chemistry, Liaoning University, Shenyang 110036, China KEYWORDS: magnetic responsive property, single-molecular-thickness Janus nanosheet, catalytic property, surface protection method, recyclability

ABSTRACT: In this article, we describe a method to fabricate magnetic responsive Janus nanosheets with catalytic property via surface protection method. Fe3O4 nanoparticles and PW12O403- based ionic liquid are located on the two opposite sides of the Janus nanosheets, respectively. The Janus nanosheets are characterized by FTIR, SEM, TEM, AFM and Zetapotential analysis. They are used as recyclable catalyst to the esterification reaction of methanol and oleic acid for their magnetic responsive and catalytic properties. The esterification ratio is up to 80 % and it is nearly no change when Fe3O4 nanoparticles/PW12O403- based ionic liquid composite nanosheets are recycled 4 times.

1. INTRODUCTION

ACS Paragon Plus Environment

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Janus materials,1 named after ancient Roman two-faced god Janus, are composed of asymmetric compositions and constructions on the same surface of one object. In recent years, much research has been carried out on fabrication and applications of Janus materials for their intriguing properties.2-6 There are several methods to fabricate Janus materials, such as surface protection,79

self-assembly,10-13 phase separation,14-15 microfluid16-20 and so on.21-23 Among them, surface

protection method is the most simple and suitable for fabrication in large scale. The application fields of Janus materials are covered from solid surfactant,24-26 interfacial compatilizer,27-30 probe,31-33 sensor,34 catalyst35-37 to self-driven motor38-40 and so on.41-43 However, for their interface affinity property, one of the most important applications for Janus materials is to be served as solid surfactants to emulsify immiscible liquids. Theoretical studies have shown Janus materials offered advantages over those homogeneous counterparts as solid surfactants.44 Recently, the influence on the shape of Janus particles at liquid-liquid interface has been studied via molecular dynamic simulation, it shows that Janus nanosheets are greatly restricted at the interface of oil and water, and they are superior to Janus sphere on reducing interfacial intension.45 Moreover, while the Janus nanosheets are modified with some special properties, such as catalytic,46-47 responsive,48 amphipathic properties49 and so on, that would greatly broaden their application range. Herein, we introduced a method to fabricate Fe3O4 nanoparticles (NPs)/PW12O403- based ionic liquid (IL) composite nanosheets via surface protection method. The Janus nanosheets were prepared by a course of being released from the surface of template microsphere and then modified with Fe3O4 NPs and PW12O403- based IL. Compared with other methods of preparation of Janus nanosheets,49-50 the template microsphere could be easily collected by magnet and then recycled for their magnetic responsive property, and the released Janus nanosheets were uniform


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in single-molecular thickness because they were connected on the surface of template microsphere via chemical bond. This method provides a modular tool to prepare uniform Janus nanosheets in thickness and the template microsphere could be recycled. As it shown in Scheme 1, firstly, Fe3O4@SiO2 magnetic microsphere was prepared. The Fe3O4 magnetic microsphere was prepared by solvothermal method and SiO2 layer covered on it was prepared via sol-gel process of tetraethyl orthosilicate (TEOS). Secondly, the Fe3O4@SiO2 microsphere was modified with aminopropyl groups by utilizing 3-aminopropyltrimethoxysilane (APTMS), amino groups directed outwardly and thus Fe3O4@SiO2-C3H6-NH2 template microsphere was prepared. Ttriethoxsilylbutyraldehyde (TESBA) was then covalently connected on the surface of template microsphere via imide bond and the residual triethoxysilane directed outwardly. After sol-gel process, there was a single-molecular-thickness SiO2 layer covered on the surface of template microsphere. The hydroxyl groups faced outwards and butylaldehyde groups were connected with amino groups on the surface of template microsphere. In acid condition, the magnetic microsphere

was

crushed

under

ultrasonication

and

then

single-molecular-thickness

hydroxyl/butylaldehyde composite nanosheets were released from the template microsphere by breaking imide bond. Hydroxyl/butylaldehyde composite nanosheets were modified with chitosan capped Fe3O4 NPs on hydroxyl group sides via electrostatic force and PW12O403- based ionic liquid on butylaldehyde group sides via imide bond, and thus Fe3O4 NPs/PW12O403- based IL composite nanosheets were obtained.


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Scheme 1. Preparation of magnetic responsive Janus nanosheets with catalytic property. Single-molecular-thickness hydroxyl/butylaldehyde composite nanosheets were released from the surface of template microsphere by breaking imide bond. Hydroxyl group side and butylaldehyde group side were respectively modified with Fe3O4 NPs and PW12O403based IL, and thus Fe3O4 NPs/PW12O403- based IL composite nanosheets were prepared. 2. EXPERIMENTAL SECTION Materials Ethylene glycol (EG), Iron chloride hexahydrate (FeCl3·6H2O), Iron dichloride tetrahydrate (FeCl2·4H2O), Sodium acetate trihydrate (NaAC), Sodium citrate, Ammonium hydroxide (NH3·H2O, 25 wt. %), TEOS, Hydrochloric acid (HCl, 36-38 wt. %), Chitosan (Mw = 1.526 kDa), Acetic acid (HAc), Paraffin (52-54 ºC), Tetrahydrofuran (THF), n-Heptane, Cyclohexane, 1-methylimidazole, oleic acid, methanol, phosphotungstic acid, acetonitrile, sodium hydroxide (NaOH) and sodium borohydride (NaBH4) were purchased from Sinopharm


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Chemical Reagent. APTMS, Coumarine-6 and 2-bromoethylamine hydrobromide were purchased from J&K. TESBA was purchased from Gelgest. All the reagents were used as received. Preparation of Fe3O4 microspheres 3.76 g of FeCl3·6H2O and 10.03 g of NaAC were dispersed in 139 ml of EG under ultrasonication for 0.5 h. The dispersion was shifted to hydrothermal reactor and sealed. After heating at 200 ºC for 8 h, the reactor was cooled down to room temperature and black precipitates were obtained. The crude product was separated with a magnet and washed with deionized water and ethanol, and then dried in vacuum oven at 35 ºC for 12 h to achieve Fe3O4 microspheres. Preparation of Fe3O4@SiO2 microspheres 0.1 g of Fe3O4 microspheres was etched by 20 mL of HCl (2 M) for 2 minutes under ultrasonication, collected with a magnet and then dispersed it in 20 mL of sodium citrate solution (10 wt. %) under stirring for 0.5 h. The as-treated Fe3O4 microspheres were collected with a magnet and dispersed in the solution of 54 mL of ethanol, 9 mL of water and 0.9 mL of NH3·H2O under ultrasonication for 1 h. 0.3 mL of TEOS was added to it and stirred at room temperature for 10 h. The product was collected with a magnet and washed with deionized water and ethanol, then dried in vacuum oven at 35 ºC for 12 h to achieve Fe3O4@SiO2 microspheres. Preparation of template microspheres 150 mg of Fe3O4@SiO2 microspheres and 3 μL of APTMS were dispersed in 100 mL of ethanol under ultrasonication, and stirred at room temperature for 10 h. The crude product were collected by a magnet, washed with deionized water and ethanol for three times, and then dried in vacuum oven at 35 ºC for 12 h to get Fe3O4@SiO2-C3H6-NH2 microspheres, denoted as template microspheres.


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Preparation of hydroxyl/butylaldehyde composite nanosheets

150 mg of template

microspheres were dispersed in the solution of 100 mL of toluene and 1 mL of NH3·H2O under ultrasonication. 3 μL of TESBA was then added to the dispersion and stirred for 10 h at room temperature. The resulting microspheres were collected with a magnet, and washed with deionized water and ethanol for three times. The as-prepared magnetic microsphere was dispersed in the solution of 95 mL of ethanol and 5 mL of water, and then stirred at room temperature for 12 h to get black precipitates. They were collected by a magnet and crushed in the solution of 20 μL of HCl and 20 mL of ethanol under ultrasonication at room temperature. The hydroxyl/butylaldehyde composite nanosheets were released from the surface of template microsphere and freeze dried. Preparation of chitosan capped Fe3O4 nanoparticles (NPs)51 0.7 g of FeCl3·6H2O and 0.3 g of FeCl2·4H2O were dispersed in 43 mL of deionized water under ultrasonication, accompanied by purging with nitrogen to remove dissolved oxygen in deionized water. 200 mL of HAc aqueous solution (0.25 %, V/V) and 0.6 g of chitosan were added to the dispersion, and then vigorously stirred for 0.5 h at 40 ºC. 20 mL of NH3·H2O was then added dropwise within 0.5 h until pH value was 13. The stirring was continued for 1 h at 40 ºC and got black precipitates. They were washed with deionized water for several times until the solution was neutral, and then freeze dried to achieve chitosan capped Fe3O4 NPs. Preparation

of

Fe3O4

NPs/butylaldehyde

composite

nanosheets

1

mg

of

hydroxyl/butylaldehyde composite nanosheets was dispersed in the solution of 20 μL of HCl and 200 mL of deionized water. 1 mg of chitosan capped Fe3O4 NPs was added to the dispersion and stirred for 12 h at room temperature. The chitosan capped Fe3O4 NPs covered on the hydroxyl group side and thus Fe3O4 NPs/butylaldehyde composite nanosheets were prepared. They were


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separated from the redundant chitosan capped Fe3O4 NPs by centrifugation, washed with deionized water for three times and then freeze dried to obtain Fe3O4 NPs/butylaldehyde composite nanosheets. Emulsification with Janus nanosheets (1) 1 mg of the hydroxyl/butylaldehyde composite nanosheets was added to the mixture of 2 mL of deionized water and 500 μL of n-heptane, and nheptane in water emulsion was formed after stirring under ultrasonication for 3 minutes at room temperature. In order to better observation, n-heptane was dyed with coumarine-6. (2) At 70 ºC, 5 mg of hydroxyl/butylaldehyde composite nanosheets and Fe3O4 NPs/butylaldehyde composite nanosheets were added to the solution of 1 g of paraffin and 8 g of deionized water, respectively. The paraffin in water emulsions were obtained after stirring them at 2000 r/min for 10 minutes. Preparation of (2-aemin)+Br- 0.5 g of 1-methylimidazole and 5.125 g of 2-bromoethylamine hydrobromide were dispersed in 30 mL of acetonitrile and stirred at 80 ºC for 4 h, and then yellow solution was obtained. The solution was cooled down to room temperature, followed by adding 1 g of NaOH and stirring for 1 h to neutralize excess hydrobromic acid in the solution. After a short while, a mixture of white solid powder and yellow thick liquid was sedimented. The upper colorless and transparent liquid was decanted and the sediment was dispersed in ethanol. The white solid powder was precipitate and then removed by filtration. Ethanol in the solution was eliminated by rotary evaporator and the obtained yellow thick liquid was dried in a vacuum oven at 35 ºC for 12 h to achieve 1-(2-amino-ethyl)-3-methylimidazole bromate, which was denoted as (2-aemin)+Br-. Preparation of IL based Janus nanosheets 10 mg of (2-aemin)+Br-, 10 mg of NaBH4 and 10 mg of Fe3O4 NPs/butylaldehyde composite nanosheets were dispersed in 10 mL of deionized


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water under ultrasonication. The dispersion was stirred at room temperature for 6 h and then freeze dried to achieve Fe3O4 NPs/Br- based IL composite nanosheets. 10 mg of Fe3O4 NPs/Brbased IL composite nanosheets and 10 mg of phosphotungstic acid were dispersed in 10 mL of deionized water under ultrasonication, followed by stirring at room temperature for 1 h. The crude product was washed with deionized water, and then freeze dried. The resulting product was Fe3O4 NPs/PW12O403- based IL composite nanosheet. Catalysis with Fe3O4 NPs/PW12O403- based IL composite nanosheet A trifle of Fe3O4 NPs/PW12O403- based IL composite nanosheets was dispersed in the solution of 1 mL of methanol (0.79 g, 0.02 mol) and 10 mL of oleic acid (8.91 g, 0.03 mol) under ultrasonication. The dispersion was stirred at 70 ºC for 12 h and standing for a while, grey precipitate was formed and the solution was turning into yellow color. The grey precipitate was collected with a magnet and washed with water and ethanol, and then freeze dried for re-using. The yellow solution was the resulting product, methyl oleate. General characterization Morphology of the samples was characterized by scanning electron microscopy (Hitachi S-4800 at 15KV) and transmission electron microscopy (JEOL1011 at 100 kV). The samples for SEM observation were ambient dried and vacuum sputtered with Pt. The samples for TEM observation were prepared by spreading very dilute emulsions in ethanol onto carbon-coated copper grids. The X-ray diffraction (XRD) patterns from 3 to 80 degree were collected on Rigaku D/max-2500 with an incident wavelength of 0.154 nm (Cu K radiation) and a Lynx-Eye detector. The magnetic properties were measured with vibrating sample magnetometer (VSM) (LakeShore, 7404). FTIR spectroscopy was performed on the sample/KBr pressed pellets after scanning samples for 32 times using a BRUKER EQUINOX55 spectrometer. Thickness and shape of the sample were measured by AFM of Bruker Multimode


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8. Emulsion was characterized by inverted fluorescence microscopy (Olympus IX83) and Olympus optical microscope. Size distribution and zeta potential were measured using a ZetaSizer (Nano Series, Malvern Instruments) at 25 ºC. Thermogravimetric analysis (TGA) was measured by PerkinElmer Pyris 1 TGA in air at a scan rate of 5 ºC /min. 3-5 mg sample was used for each TGA measurement.

3. RESULTS AND DISCUSSION The Fe3O4 microspheres were prepared by solvothermal method and SiO2 layer covered on it via sol-gel process of TEOS. The surface of Fe3O4 microsphere was rough (Figure S1a) and it became smooth after being covered with a SiO2 layer (Figure S1b). Meanwhile, the diameters of Fe3O4 and Fe3O4@SiO2 microspheres were increased from 200 nm to 230 nm (Figure S1c), it indicated that the diameter of Fe3O4 microspheres was 200 nm and the thickness of SiO2 layer was 30 nm. The values of saturation magnetization for Fe3O4 and Fe3O4@SiO2 microspheres were respectively 71.24 and 57.28 emu/g (Figure S1d) indicating both of them were magnetic responsive. The lower value of Fe3O4@SiO2 microspheres compared to that of Fe3O4 microspheres, it should be attributed to the layer of amorphous SiO2 covered on the surface of Fe3O4 microsphere. The excellent magnetic properties indicated the microspheres could be easily collected with a magnet. All the peaks in XRD patters were indexed to standard pattern of Fe3O452 (PDF card NO.19-0629) and amorphous SiO253-54 (PDF card NO.29-0085) (Figure S1e). The results confirmed that Fe3O4 and Fe3O4@SiO2 microspheres were achieved. The template microspheres, Fe3O4@SiO2-C3H6-NH2 microspheres, were achieved via sol-gel process of silica hydroxyl on the surface of Fe3O4@SiO2 microspheres and APTMS. Their surface was smooth and diameters were nearly no change (Figure 1a). The zeta potential of the Fe3O4@SiO2 was


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shifted from -28.70 to -7.78 mV after the surface modification (Figure S2a). The zeta potential value was increased to some degree but still negative, because the polarity of hydroxyl group was stronger than that of amino group. The results proved that APTMS had covered on the surface of Fe3O4@SiO2 microspheres and amino group directed outwardly. A monolayer of TESBA covered on the surface of template microsphere via formation of imide bond between aldehyde group in TESBA and amino group on the surface of template microsphere (Figure S2b). After sol-gel process, a single-molecular-thickness SiO2 layer was formed on the template microsphere, and the surface of resulting microsphere was still smooth after modification (Figure S2c). The magnetic microsphere was crushed in acid condition to break the imine bond, and hydroxyl/butylaldehyde composite nanosheets were released from the surface of template microsphere. They were separated by a magnet due to magnetic responsiveness of the template microspheres. The surface of template microsphere was still smooth after releasing hydroxyl/butylaldehyde composite nanosheets (Figure S2d). The hydroxyl/butylaldehyde composite nanosheets were ultrathin and flexible with two smooth faces. They could be distinguished by their wrinkles in TEM (Figure 1b) and SEM image (Figure 1c). Their thickness measured by AFM was uniform in 0.93 nm (Figure 1d) and it was rather close to the calculated value.55 The compositions of above mentioned magnetic microspheres were measured by FTIR spectrum (Figure S3a). The characteristic peaks at 506 cm-1 and 1090 cm-1 indicated presence of Fe-O and Si-O-Si bonds, respectively. It indicated that Fe3O4@SiO2 microsphere was achieved. Other two characteristic peaks between 3500-3400cm-1 were assigned to amino group, and it proved the template microsphere had been achieved. The peak at 1690 cm-1 assigned to imine bond and the broad peak between 3300-3100cm-1 indicated associating hydroxyl group. The FTIR curves showed that TESBA and monolayer SiO2 had successfully covered on the surface


10

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of template microsphere in turn and all the desired magnetic microspheres were achieved. In TGA curves, the value of weight loss was increased according with the sequence of resulting products (Figure S3b). It proved that more organic composition covered on the surface of Fe3O4@SiO2 microsphere and all the desired magnetic microspheres were achieved. The composition of hydroxyl/butylaldehyde composite nanosheets had been characterized by FTIR spectrum (Figure 1e). The characteristic peaks at 1090, 1735 and 3192 cm-1 respectively indicated presence of Si-O-Si, -CHO and Si-OH groups, while the peaks at 2848 and 2924 cm-1 indicated presence of -CH2- groups. The results showed that hydroxyl/butylaldehyde composite nanosheets were composed of silica hydroxyl and butylaldehyde groups.

a)

b)

c)

d)


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e) Figure 1. Morphologies of template microspheres and hydroxyl/butylaldehyde composite nanosheets: (a) SEM and insert TEM images of template microspheres; (b), (c), (d) and (e) were TEM, SEM, AFM and FTIR images of hydroxyl/butylaldehyde composite nanosheets. The hydroxyl group was negatively charged and butylaldehyde group was neutral in aqueous solution, thus the hydroxyl group side of hydroxyl/butylaldehyde composite nanosheets was easily covered with positively charged particles via static force. The chitosan capped Fe3O4 NPs were about 5 nm in diameter (Figure 2a) and their zeta potential was 31.9 mV, indicating that the Fe3O4 NPs were positively charged. It was confirmed by XRD to verify the elemental composition of the Fe3O4 NPs and chitosan capped Fe3O4 NPs (Figure S4). The Fe3O4 NPs were stirred with hydroxyl/butylaldehyde composite nanosheets in HCl aqueous solution for 12 h, and thus Fe3O4 NPs/butylaldehyde composite nanosheets were obtained. The hydroxyl group side was covered with Fe3O4 NPs and its surface became rough (Figure 2b), meanwhile the other side was still smooth (Figure 2c). Comparison of zeta potential between hydroxyl/butylaldehyde and Fe3O4 NPs/butylaldehyde composite nanosheets, it was shifted from -21.5 mV to 7.6 mV (Figure 2d). That indicated that the Fe3O4 NPs had covered on hydroxyl group side and Fe3O4 NPs/butylaldehyde composite nanosheets were prepared.


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a)

b)

d) c) Figure 2. Morphologies and zeta potential of chitosan capped Fe3O4 NPs and Fe3O4 NPs/butylaldehyde composite nanosheets: (a) TEM image of chitosan capped Fe3O4 NPs; (b) and (c) were TEM and SEM images of Fe3O4 NPs/butylaldehyde composite nanosheets; (d) zeta potential curves of hydroxyl/butylaldehyde (curve a), Fe3O4 NPs/butylaldehyde (curve b) composite nanosheets and chitosan capped Fe3O4 NPs (curve c). The hydroxyl/butylaldehyde composite nanosheets were amphipathic since their two sides were respectively covered with hydrophobic aldehyde group and hydrophilic hydroxyl group. There were different morphologies for hydroxyl/butylaldehyde composite nanosheets when they were respectively dispersed in solvent, non-solvent, and co-solvent for one side of them. Concretely, when dispersing the composite nanosheets in a solvent for hydroxyl groups, such as deionized water, the nanosheets were stacked layer by layer and looked like shells, as shown in the SEM images (Figure S5a, S5b). It was resulted from the self-assembly sol-gel process between hydroxyl groups covered on different hydroxyl/butylaldehyde composite nanosheets. However, if dispersing the nanosheets in a non-solvent for hydroxyl groups but a solvent for aldehyde


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groups, such as cyclohexane, the nanosheets stacked into a micellar superstructure with aldehyde groups facing to cyclohexane (Figure S5c, S5d). In solvent for both sides of the nanosheets, such as THF, the composite nanosheets were homogeneously dispersed (Figure S5e, S5f). According to the above results, it could be found the hydroxyl/butylaldehyde composite nanosheets were wrinkled in non-solvent and solvent for one side of them, but homogeneously dispersed in cosolvent for the two sides. It confirmed that the hydroxyl/butylaldehyde composite nanosheets were amphipathic. The amphipathic and ultrathin hydroxyl/butylaldehyde composite nanosheets could be severed as solid

surfactants

to

stabilize

immiscible

liquids.

As

a

proof

of

this

concept,

hydroxyl/butylaldehyde composite nanosheets were served as surfactants to emulsify n-heptane and deionized water at room temperature when n-heptane was the minor phase. For better observation of the morphology of emulsion droplet, n-heptane was dyed with coumarine-6. The emulsion was stable as no serious coalescence was happened after 6 months, and the emulsion droplet diameter was about 5 µm (Figure 3a). Using hydroxyl/butylaldehyde composite nanosheets as solid surfactants to emulsify deionized water and melted paraffin at 70 ºC, the orientation of the nanosheets could be observed in the emulsion as paraffin was the minor phase. The stability of the paraffin-in-water emulsion was similar to the n-heptane-in-water one, and the average emulsion droplets diameter was about 3 µm (Figure 3b). The paraffin-in-water emulsion was cooled down to room temperature, and the paraffin was solidified. It could be observed that the surface of the paraffin droplet was smooth although the emulsion droplet diameter decreased and a slight deformation was also observed (Figure 3c). Meanwhile, using Fe3O4 NPs/butylaldehyde composite nanosheets as solid surfactants to emulsify the same mixture of deionized water and melted paraffin at 70 ºC, the surface of paraffin droplet was rough after


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cooling down the emulsion to room temperature (Figure 3d). That was because of the chitosan capped Fe3O4 NPs uniformly covered on the surface of paraffin, although the Fe3O4 NPs were too small to be observed in the SEM image. It proved that Fe3O4 NPs side was hydrophilic and faced to water phase, meanwhile, the butylaldehyde side was hydrophobic and faced to paraffin phase. That was to say, the Fe3O4 NPs/butylaldehyde composite nanosheets were amphipathic and they could be dispersed well in solution.

a)

b)

c) d) Figure 3. Emulsion performance of the Janus nanosheets: hydroxyl/butylaldehyde composite nanosheets served as solid surfactant (a) inverted fluorescence image of n-heptane in water emulsion; (b) polarizing optical image of melted paraffin in water emulsion; (c) and (d) were SEM images of the paraffin droplets in emulsion when hydroxyl/butylaldehyde (c) and Fe3O4 NPs/butylaldehyde (d) were respectively served as solid surfactants. The compositions of (2-aemin)+Br- were characterized by FTIR spectrum (Figure S6a). The two characteristic peaks between 3600 cm-1 and 3400 cm-1 indicated presence of -NH2 group. Other


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two characteristic peaks at 1628 cm-1 and 1471 cm-1 indicated presence of imidazole group, meanwhile, the 2920 cm-1 and 2845 cm-1 were respectively assigned to -CH2- and -CH3 groups. That proved (2-aemin)+Br- was achieved. Fe3O4 NPs/Br- based IL composite nanosheets were prepared via formation of imide bonds between amino groups in (2-aemin)+Br- and aldehyde groups on the surface of Fe3O4 NPs/butylaldehyde composite nanosheets, and then Fe3O4 NPs/PW12O403- based IL composite nanosheets were prepared by anion exchange of Br- and PW12O403-. The chemical composition of the IL composite nanosheets was confirmed by FTIR spectrum. In the spectrum, the characteristic peaks at 1080 cm-1, 506 cm-1 and 1650 cm-1 in curve a and b indicated Si-O-Si, Fe-O and C-N bonds, while the peaks at 820 cm-1, 890 cm-1 and 989 cm-1 in curve b indicated O-b1-W, O-b2-W and W=O bonds, respectively (Figure S6b). All the above peaks confirmed Fe3O4 NPs/Br- and Fe3O4 NPs/PW12O403- based IL composite nanosheets were prepared. Owing to the high specific surface area of Fe3O4 NPs/PW12O403- based IL composite nanosheets resulted from their ultrathin thickness, large amount of PW12O403- groups could be loading on the surface of them. Utilizing the catalytic property of PW12O403- groups, the Fe3O4 NPs/PW12O403based IL composite nanosheets were used as catalyst in the esterification reaction of oleic acid and methanol. 0 mg, 1 mg, 3 mg, 5 mg and 7 mg of Fe3O4 NPs/PW12O403- based IL composite nanosheets were added to the solution of 10 mL of oleic acid and 1 mL of methanol, respectively, and the nanosheets were well dispersed under ultrasonication for 10 minutes, resulting milky and homogeneous dispersion (Figure 4a). The dispersion was stirred at 70 ºC for 12 h and then cooled down to room temperature, grey precipitate was observed at the bottom and the upper part was yellow solution (Figure 4b). The grey precipitate was the IL composite nanosheet catalysts and could be collected with a magnet for reusing, while the yellow solution was composed of


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methyl oleate, water, as well as unreacted methanol and oleic acid. The esterification ratio was calculated according to the value of acidity in the solution56. It was shifted from 5 % to 78 % as increasing of added Fe3O4 NPs/PW12O403- based IL composite nanosheet from 0 mg to 5 mg in the solution. It proved that Fe3O4 NPs/PW12O403- based IL composite nanosheets were dispersed well in the solution and the effective area to load PW12O403- became larger with increase of their addition amount in the solution. As the addition amount of Fe3O4 NPs/PW12O403- based IL composite nanosheet was up to 7 mg in the solution, the esterification ratio was slowly declining, because abundant Fe3O4 NPs/PW12O403- based IL composite nanosheet aggregated in the solution and the effective area to load PW12O403- group became smaller. Moreover, after stirring 10 mL of oleic acid and 1 mL of methanol at 70 ºC for 12 h without the catalysts, the esterification ratio was calculated to be only 5 %, which was much lower than the above mentioned one (Figure 4c). Fe3O4 NPs/PW12O403- based IL composite nanosheets could be collected with a magnet due to their magnetic responsive property. Based on these characteristics, the IL based composite nanosheets could be served as recyclable catalyst for the esterification reaction of oleic acid and method. Fixing 5 mg of Fe3O4 NPs/PW12O403- based IL composite nanosheet in the solution of 10 mL of oleic acid and 1 mL of methanol, the esterification ratios were still about 78 % even after 4 times of recycling (Figure 4d). It is proved that the Fe3O4 NPs/PW12O403- based IL composite nanosheet had excellent catalytic performances and good recyclability.

a)

b)


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c) d) Figure 4. Catalytic performance of Fe3O4 NPs/PW12O403- based IL composite nanosheet: (a) and (b) were the solution photos before and after esterification reaction of oleic acid and methanol; (c) and (d) were the esterification ratios with the increase of addition amount and reuse times of Fe3O4 NPs/PW12O403- based IL composite nanosheet.

4. CONCLUSIONS In summary, we introduced an effective method to fabricate magnetic responsive Janus nanosheets with catalytic property via surface protection method. Combination of hydrothermal method and sol-gel process, single-molecular-thickness hydroxyl/butylaldehyde composite nanosheets was achieved by releasing from the surface of template microsphere via breaking imide bond. Their thickness was uniform in 0.93 nm. The hydroxyl/butylaldehyde composite nanosheets could be served as solid surfactants to emulsify immiscible liquids for their amphipathic property and ultrathin thickness. The composite nanosheets were modified with chitosan capped Fe3O4 NPs on hydroxyl group side via static force and PW12O403- based IL on butylaldehyde group side with imide bond, respectively, to obtain Fe3O4 NPs/PW12O403- based IL composite nanosheets. The IL based composite nanosheets were evaluated as recyclable catalyst in the esterification reaction of methanol and oleic acid in view of their catalytic and magnetic responsive properties. The esterification ratio was up to 80 % and showed almost no change after 4 times recycling.


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ASSOCIATED CONTENT Supporting Information. Morphologies and properties of Fe3O4 and Fe3O4@SiO2 microspheres; Properties and morphologies of representative magnetic microspheres; FTIR and TGA spectrums of different magnetic microspheres; The XRD spectrum indicated Fe3O4 NPs and chitosan capped Fe3O4 NPs; Amphipathic performance of hydroxyl/butylaldehyde composite nanosheets; FTIR spectrum of (2-aemin)+ Br- and IL based Janus nanosheets. Corresponding Author * E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors greatly appreciate Prof. Zhenzhong Yang and Prof. Fuxin Liang (Institute of Chemistry, Chinese Academy of Sciences) for their technical supports and the helpful discussions. This work was financially supported by the National Science Foundation of China (NO. 51233007 and 51622308).

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