molecularly imprinted TiO2 nanocomposites and its molecular recognitive photocatalytic degradation of target contaminant

Journal of Molecular Catalysis A: Chemical 402 (2015) 10–16

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Facile synthesis of magnetically recoverable Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites and its molecular recognitive photocatalytic degradation of target contaminant Chunjing Zhang a,b , Hao Chen a , Mingjie Ma a,∗ , Zhengpeng Yang a,∗ a b

Institute of Materials Science and Engineering, Henan Polytechnic University, Jiaozuo 454000, China College of Chemistry and Chemical Engineering, Central South University, Changsha 410083, China

a r t i c l e

i n f o

Article history: Received 4 February 2015 Received in revised form 10 March 2015 Accepted 11 March 2015 Available online 13 March 2015 Keywords: Magnetic photocatalyst Molecularly imprinted TiO2 Photocatalytic activity Photocatalytic selectivity Degradation

a b s t r a c t Magnetically separable Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites with molecularly recognitive photoactivity have been prepared successfully by a multistep wet chemical process. Compared to Fe3 O4 /Al2 O3 /non-imprinted TiO2 , the Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 shows a better photocatalytic selectivity toward the template molecules. The enhanced selectivity can be attributed to the chemical interaction and size matching between template molecules and imprinted cavities. Photocatalytic activity studies for 2-nitrophenol degradation indicate an enhanced activity for the composites when the Al2 O3 interlayer and molecularly imprinted TiO2 are present in the composites. The enhanced photocatalytic activity is attributed to the decrease in the migration of photogenerated charge carriers to the inter layer by Al2 O3 interlayer and selective photodegradation of template molecules on the imprinted composites. The superparamagnetism of the imprinted composites provides a convenient route for separation of the catalyst from the reaction mixture by an external magnet. The spent catalyst could be recycled without appreciable loss of catalytic activity. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Magnetic photocatalysts are of significant interest currently due to their potential application in environmental cleaning especially contaminated water [1,2]. TiO2 -coated magnetic nanoparticles (Fe3 O4 , ␥-Fe2 O3 , NiFe2 O4 , ZnFe2 O4 etc.,) have been used for the purpose as they provide a convenient approach for photocatalyst separation and recycling from the treated water by applying external magnetic fields [3–7]. Although the nanoparticles with a magnetic core and a TiO2 shell can realize better the separation of TiO2 photocatalyst, the magnetic oxide nanoparticles show a lower photocatalytic activity and poor stability in comparison with single phase TiO2 . It has been reported that a direct contact between TiO2 photocatalyst and magnetic iron oxide usually gives rise to an unfavorable heterojunction, resulting in an increase in electron-hole recombination and photodissolution of the iron oxide [8,9]. Meanwhile, it is found that the TiO2 photocatalyst

∗ Corresponding authors. Tel.: +86 391 3986080; fax: +86 731 3986900. E-mail addresses: [email protected] (M. Ma), [email protected] (Z. Yang). http://dx.doi.org/10.1016/j.molcata.2015.03.008 1381-1169/© 2015 Elsevier B.V. All rights reserved.

degrades organic pollutants as well as nutrients indiscriminately owing to its poor selectivity [10,11]. Hence, it is necessary to design a multifunctional photocatalytic system which possesses high photocatalytic activity, cost-effective recyclability, better stability and selectivity. In order to improve the photocatalytic activity and stability of the magnetic TiO2 photocatalyst, the effect of an intermediate layer of SiO2 or Al2 O3 between the magnetic core and the TiO2 shell on the properties of the magnetic TiO2 photocatalyst has been studied and found that the presence of the intermediate layer enhances the properties of the catalyst compared to the one without the intermediate layer [12–14]. It has been proposed that the intermediate layer not only protests the magnetic core from photodissolution, but also prevents the unfavorable migration of charge carriers from TiO2 to underlying magnetic core, which will negatively affect the photocatalytic activity of the catalyst [15–17]. Magnetic TiO2 photocatalysts with the intermediate layer have been tested for the photodegradation of dyes and water pollutants [18–21]. Generally speaking, desirable results have been found in terms of photocatalytic activity, separability and catalyst stability. In the past years, several approaches have been employed to enhance the selectivity of TiO2 photocatalyst such as surface

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11

Fig. 1. Schematic illustration of the preparation process of the Fe3 O4 /Al2 O3 /2NP-TiO2 .

modification of TiO2 with specific molecules [22], control of surface electric charge by pH adjustment [23], preparation of TiO2 with {0 0 1} facets [24], fabrication of TiO2 with double-region structure [25] and molecular imprinting [26]. Among them, molecular imprinting has been recognized as the most promising and representative technique to prepare specific binding sites for given molecules in appropriate matrices. In this approach, the shape and functionality of a template can be transcribed onto microporous materials. The configuration of the functional groups in the template may be memorized within the matrix. Recently, molecularly imprinted TiO2 has been successfully prepared and used for photocatalytic degradation of wastewater, chemical sensors and removal of biomolecules [27–30], showing an excellent selectivity for target molecules. In this work, the core–shell structured Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites with photoactivity, reusability, stability and selectivity were prepared by a multistep wet chemical process. The resultant magnetic TiO2 photocatalyst was used for the degradation of the 2-nitrophenol which was selected as a model of organic pollutants. The morphology, size, structure and magnetic properties of the as-prepared Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites were characterized by transmission electron microscopy (TEM), dynamic light scattering (DLS), Fourier transformed infrared (FTIR) spectrometry, X-ray diffraction (XRD) and vibrating magnetometer. The photocatalytic activity, photocatalytic selectivity and reusability of Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites under UV irradiation were also examined in detail.

2. Experimental 2.1. Materials FeCl3 ·6H2 O, NaAc, 2-nitrophenol (2NP), 4-nitrophenol (4NP) and 2,4-dinitrophenol (2,4-DNP) were of analytical grade and obtained from Shanghai Chemical Reagent Co. Aluminum isopropoxide and Ti(O-n Bu)4 were purchased from Sigma Chem. Co., and used as received. All other chemicals were of analytical grade and used as received without further purification. Deionized (DI) water (resistivity of 18 M cm) was obtained from a Millipore MilliQ Water System (Millipore Inc.), and was used for rinsing and for makeup of all aqueous solutions.

2.2. Synthesis of photocatalysts The synthesis procedure of Fe3 O4 /Al2 O3 /molecularly imprinted TiO2 nanocomposites (Fe3 O4 /Al2 O3 /2NP-TiO2 ) is illustrated in Fig. 1. Specifically, the procedure was described in detail as follows. The Fe3 O4 nanoparticles were prepared via a solvothermal method as described previously [31]. In brief, FeCl3 ·6H2 O (2.0 g) and NaAc (5.4 g) were dissolved in 60 mL ethylene glycol under vigorous stirring. The obtained homogeneous yellow solution was transferred to a Teflon-lined stainless-steel autoclave. The autoclave was sealed and heated at 200 ◦ C for 8 h, and then allowed to cool to room temperature. The obtained black products were collected with a permanent magnet, washed for several times with ethanol and dried in vacuum at 60 ◦ C for 10 h. The Fe3 O4 /Al2 O3 core–shell nanocomposites were prepared through a sonochemical approach. Briefly, 0.1 g of as-prepared Fe3 O4 nanoparticles was dispersed into 50 mL ethanol containing a certain amount of aluminum isopropoxide with the aid of ultrasonication. Subsequently, a mixture of water and ethanol (1:5, v/v) was added into the solution with vigorous stirring. After ultrasonicated for another 2 h, the product was separated with a permanent magnet, washed with ethanol for several times, and dried in vacuum at 80 ◦ C for 12 h. A series of Fe3 O4 /Al2 O3 nanocomposites with 2–12% Al2 O3 content were prepared. The Fe3 O4 /Al2 O3 /2NP-TiO2 nanocomposites were prepared by sol–gel hydrolysis of Ti(O-n Bu)4 on Fe3 O4 /Al2 O3 nanocomposites followed by calcination treatment. Firstly, Ti(O-n Bu)4 (0.6 mmol) and 2NP (0.2 mmol) were dissolved in 8 mL of toluene-ethanol (v/v, 2:1) mixture, and then vigorously stirred at room temperature. Subsequently, the solution was diluted by 20 times with water-saturated toluene and continuously stirred for 5 h to get a sol–gel solution which was used as a dipping solution. The obtained Fe3 O4 /Al2 O3 nanocomposites were dispersed into the dipping solution for 5 min at room temperature, rinsed with toluene, hydrolyzed in water and dried in N2 gas, producing a modified Fe3 O4 /Al2 O3 microsphere whose surface was covered with a TiO2 layer containing the template molecule of 2NP. The thickness of TiO2 layer can be controlled by the number of “dipping-rinse-hydrolyzation-drying”cycles. Finally, the Fe3 O4 /Al2 O3 modified by TiO2 layer was dried at 80 ◦ C for 3 h, and then calcined in air at 400 ◦ C for 2 h to remove template molecules and crystalline the TiO2 , resulting in the formation of Fe3 O4 /Al2 O3 /2NP-TiO2 . A series of Fe3 O4 /Al2 O3 /2NP-TiO2

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Fig. 2. TEM images and size distribution of (a) Fe3 O4 , (b) Fe3 O4 /Al2 O3 and (c) Fe3 O4 /Al2 O3 /2NP-TiO2 .

with 30–60% TiO2 content were prepared in our experiments. For comparison, Fe3 O4 /Al2 O3 /non-imprinted TiO2 nanocomposites (Fe3 O4 /Al2 O3 /NIP-TiO2 ) were prepared using the same procedure without template molecules, and Fe3 O4 /molecularly imprinted TiO2 nanocomposites (Fe3 O4 /2NP-TiO2 ) without Al2 O3 interlayer were also synthesized using a similar procedure mentioned above.

(Malvern Instruments Ltd., Worcestershire, UK). The concentration of the organic pollutant was determined by an UV/vis spectrophotometer (UV-1601, Shimadzu, Japan), and the detection wavelengths were set at 279, 318 and 365 nm for 2NP, 4NP and 2,4-DNP, respectively. 2.4. Photocatalytic experiments

2.3. Characterization The morphology of photocatalysts was examined on a Hitachi model H-800 TEM at an accelerating voltage of 120 kV. Infrared spectra were recorded on Nicolet 200SXV FTIR spectrometer using a KBr wafer. XRD was performed on a Rigaku D/MAX-RC X-ray diffractometer using Cu K␣ radiation. The magnetization measurements were performed at room temperature using model 155 vibrating magnetometer. The size distribution of photocatalysts was investigated using DLS on a Zetasizer Nano ZS instrument

The photocatalytic procedure for the degradation of organic pollutants (2NP, 4NP and 2,4-2DNP) was described as follows. Firstly, a solution containing organic pollutant (10 mg/L) and the optimized photocatalyst (5 g/L, TiO2 amount: 2.5 g/L) was prepared and kept in the darkness for eight minutes. Subsequently, the prepared suspension was irradiated with UV light under continuous stirring, the UV lamp used was a 30 W low-pressure mercury lamp (Philips, 254 nm). After UV irradiation for some time, the photocatalyst was separated using magnet from the reaction solution to measure

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the concentration change of organic pollutants. According to the change in the concentration of organic pollutants, the photodegradation rate (X) of organic pollutants versus time was calculated as follows: C0 − C C0

where C0 is the initial concentration of organic pollutants, and C is the concentration of organic pollutants at time t. 2.5. Procedure for recycling experiment After the photocatalytic reaction, the Fe3 O4 /Al2 O3 /2NP-TiO2 was assembled on the side wall of the reactor by an external permanent magnet and the reaction solution was removed. The photocatalyst was washed several times with DI water, and then used to start the next run under the same experimental condition. 3. Results and discussion

c

Transmittance (%)

X=

13

4000

b

a

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber (cm ) Fig. 3. FTIR spectra of (a) Fe3 O4 , (b) Fe3 O4 /Al2 O3 and (c) Fe3 O4 /Al2 O3 /2NP-TiO2 .

3.1. Characterization of Fe3 O4 /Al2 O3 /2NP-TiO2 Fig. 2 shows the TEM images and size distribution of Fe3 O4 , Fe3 O4 /Al2 O3 and Fe3 O4 /Al2 O3 /2NP-TiO2 . It was observed that the Fe3 O4 nanoparticles were nearly spherical and monodisperse, the size distribution of these spheres with rough surface was relatively uniform and their average diameter was about 85 nm (Fig. 2a). The obtained Fe3 O4 /Al2 O3 also exhibited monodispersed spherical morphology as shown in Fig. 2b, uniform Al2 O3 layer (bright, ∼4 nm in thickness) was formed on individual magnetite particle, resulting in core–shell Fe3 O4 /Al2 O3 microspheres. TEM image and DLS measurement indicated that the mean size of the Fe3 O4 /Al2 O3 was about 93 nm. The subsequent deposition of 2NP-TiO2 composites on the Fe3 O4 /Al2 O3 microspheres and calcination resulted in molecularly imprinted TiO2 shell (bright, ∼15 nm in thickness, Fig. 2c), and the average diameter of Fe3 O4 /Al2 O3 /2NP-TiO2 was about 123 nm. Compared with the Fe3 O4 and Fe3 O4 /Al2 O3 nanocomposites, the obtained Fe3 O4 /Al2 O3 /2NP-TiO2 showed more regular spherical shape with smooth surface due to the coating of TiO2 on a nano-scale in the sol–gel process. TEM and DLS analysis indicate that the obtained Fe3 O4 /Al2 O3 /2NP-TiO2 possesses a typical sandwich structure with Fe3 O4 core, Al2 O3 in middle layer and molecularly imprinted TiO2 shell in the outer layer. FTIR analysis was performed on Fe3 O4 , Fe3 O4 /Al2 O3 and Fe3 O4 /Al2 O3 /2NP-TiO2 samples to prove the sequential coating of Al2 O3 and TiO2 layers on the magnetite particles. Fig. 3 shows their FTIR spectra recorded in the range of 4000–400 cm−1 . In three curves, the bands centered at ca. 1630 and 3410 cm−1 can be assigned to the H O H stretching modes and bending vibration of the adsorbed water, respectively [32]. In the curve (a), the high-intensity band at 575 cm−1 corresponds to the Fe O stretching vibration from the magnetite phase [33]. In the curve (b), in addition to the characteristic peak of Fe3 O4 at 575 cm−1 , a new weak band around 550–890 cm−1 corresponding to the characteristic vibration of Al2 O3 was clearly observed [34], indicating that Al2 O3 was immobilized on the surfaces of Fe3 O4 nanoparticles. In the curve (c), a strong band in the range from 500 to 900 cm−1 relating to the stretching vibration of Ti O Ti bond appeared and the band at 575 cm−1 for Fe3 O4 was largely weakened. Moreover, it was noted that the weak band at 550–890 cm−1 in the curve (b) disappeared. The result strongly confirms the formation of TiO2 in the outlayer of Fe3 O4 /Al2 O3 /2NP-TiO2 . The XRD patterns for Fe3 O4 , Fe3 O4 /Al2 O3 and Fe3 O4 /Al2 O3 /2NPTiO2 were performed to further confirm the composition and phase structure of the Fe3 O4 /Al2 O3 /2NP-TiO2 . As seen in Fig. 4a, the

Fig. 4. XRD patterns of (a) Fe3 O4 , (b) Fe3 O4 /Al2 O3 and (c) Fe3 O4 /Al2 O3 /2NP-TiO2 .

diffraction peaks at 2 = 30.1◦ , 35.4 ◦ , 43.1◦ , 56.9◦ and 62.5◦ can be perfectly indexed to those XRD patterns of Fe3 O4 nanoparticles [33], indicating a cubic spinel structure of magnetite. The XRD pattern of Fe3 O4 /Al2 O3 in Fig. 4b was similar with that of Fe3 O4 but showed weak and broad peaks at 2 = 37.5◦ , 46.2◦ and 67◦ , assigned to amorphous Al2 O3 [35]. For the sample of Fe3 O4 /Al2 O3 /2NP-TiO2 as shown in Fig. 4c, relatively strong and sharp peaks at 2 = 25◦ , 38.1◦ , 48◦ , 54◦ and 54.8◦ revealed the formation of anatase TiO2 [36]. No characteristic peaks of Al2 O3 could be detected in the spectra of Fe3 O4 /Al2 O3 /2NP-TiO2 , which suggested that amorphous Al2 O3 has formed a thin layer over Fe3 O4 , and retained a small presence in the multi-component samples. Moreover, the weak peaks of Fe3 O4 in Fe3 O4 /Al2 O3 /2NP-TiO2 were observed, suggesting the unchanged magnetite phase of the composite microspheres and the existence of TiO2 in the outlayer of microspheres. The magnetic properties of samples were characterized at room temperature as shown in Fig. 5. The magnetic saturation (Ms) values were 80.7, 68.3 and 43.5 emu/g for Fe3 O4, Fe3 O4 /Al2 O3 and Fe3 O4 /Al2 O3 /2NP-TiO2 , respectively. The decrease of Ms in the order of Fe3 O4, Fe3 O4 /Al2 O3 and Fe3 O4 /Al2 O3 /2NP-TiO2 could be attributed to the contribution of the volume of the non-magnetic coating layer to the total sample volume. As seen in Fig. 5, the magnetization and demagnetization curves were coincidence, no hysteresis phenomenon existed, and remanent magnetization and coercivity were equal to zero. The results indicate that all samples

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100

0.75

60

0.70

c

40 20 0

0.65

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Magnetization (emu/g)

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80

-20 -40

0.60 -60 -80 -100 -6000

0.55 -4000

-2000

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2000

4000

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6000

4

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H (Oe) Fig. 5. The magnetic hysteresis loop of (a) Fe3 O4 , (b) Fe3 O4 /Al2 O3 and (c) Fe3 O4 /Al2 O3 /2NP-TiO2 .

0.75

10

12

B

0.70

0.65

X

are superparamagnetic. The strong magnetization and superparamagnetism of Fe3 O4 /Al2 O3 /2NP-TiO2 suggest its suitability for magnetic separation and targeting. The Fe3 O4 /Al2 O3 /2NP-TiO2 photocatalyst can be easily separated by an applied magnetic field, and it can be well redispersed in reaction solution after removing the external magnetic field.

8

Al2O3 amount (wt%)

0.60

3.2. Photocatalytic activity of Fe3 O4 /Al2 O3 /2NP-TiO2

0.55

The photocatalytic activity of Fe3 O4 /Al2 O3 /2NP-TiO2 depends on the amount of Al2 O3 and TiO2 in photocatalyst samples. Fig. 6A shows the effect of Al2 O3 amount on the photodegradation rate of 2NP at 50% TiO2 , the X value increased with the increase in Al2 O3 amount from 2% to 9%, and after that a further increase of Al2 O3 in Fe3 O4 /Al2 O3 /2NP-TiO2 caused a slight increase in the X value. Such results can be closely related to the presence of an intermediate layer of Al2 O3 between the Fe3 O4 core and the TiO2 shell, which avoids photodissolution of iron and prevents the Fe3 O4 core from acting as an electron-hole recombination center. Therefore, the high Al2 O3 amount in Fe3 O4 /Al2 O3 /2NP-TiO2 will enhance the photacatalytic activity of Fe3 O4 /Al2 O3 /2NP-TiO2 . However, the exorbitant Al2 O3 will not effectively increase the photodegradation rate of 2NP. On the contrary, it will negatively affect the magnetization of Fe3 O4 /Al2 O3 /2NP-TiO2 . Fig. 6B shows the effect of TiO2 amount on the photodegradation rate of 2NP at 9% Al2 O3 , the X value increased with the increase of TiO2 amount up to 50% and then kept almost unchanged. The observation can be understood from two aspects. On one hand, the increase of TiO2 amount will increase the number of photogenerated electrons and also the number of the 2NP molecules absorbed on Fe3 O4 /Al2 O3 /2NPTiO2 , causing an enhanced photodegradation rate of 2NP. On the other hand, at high TiO2 amount, the hindrance of TiO2 matrix in Fe3 O4 /Al2 O3 /2NP-TiO2 will be enhanced, causing that it is difficult for UV light and 2NP to enter into the inner layer of more thick TiO2 shell, so an almost constant X value was observed at TiO2 amount above 50%. According to the analysis from Fig. 6A and B, the Fe3 O4 /Al2 O3 /2NP-TiO2 with 9% Al2 O3 and 50% TiO2 possesses high photocatalytic activity, and were used in our experiments. The photocatalytic activities of Fe3 O4 , Fe3 O4 /Al2 O3 , Fe3 O4 /Al2 O3 /NIP-TiO2 , Fe3 O4 /2NP-TiO2 and Fe3 O4 /Al2 O3 /2NPTiO2 were evaluated by the degradation of 2NP under UV irradiation. Fig. 7 shows the related photodegradation rate of 2NP with time over various catalysts. A slight increase in the X value was observed in the absence of catalyst or in the presence of Fe3 O4 and Fe3 O4 /Al2 O3 , indicating a weak photocatalytic activity of Fe3 O4

0.50

0.45 30

40

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TiO2 amount (wt%) Fig. 6. (A) Effect of Al2 O3 amount on the photocatalytic degradation of 2NP at 50% TiO2 . (B) Effect of TiO2 amount on the photocatalytic degradation of 2NP at 9% Al2 O3 . 10 mg/L 2NP, 5 g/L Fe3 O4 /Al2 O3 /2NP-TiO2 , irradiation time of 60 min, average of three measurements (mean ± S.D.).

0.9

UV+Fe3O4/Al2O3/2NP-TiO2

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UV+Fe3O4/2NP-TiO2

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UV+Fe3O4/Al2O3/NIP-TiO2

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UV+Fe3O4/Al2O3 UV+Fe3O4

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0.4 0.3 0.2 0.1 0.0 0

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Time (min) Fig. 7. Comparison of photocatalytic activities of the various samples for the photocatalytic degradation of 2NP aqueous solution at ambient temperature. 10 mg/L 2NP, 5 g/L sample, average of three measurements (mean ± S.D.).

C. Zhang et al. / Journal of Molecular Catalysis A: Chemical 402 (2015) 10–16

0.7

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Fe3O4/Al2O3/NIP-TiO2 0.6

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Fe3O4/Al2O3/2NP-TiO2

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Number of cycles Fig. 8. Comparison of photocatalytic selectivity of Fe3 O4 /Al2 O3 /NIP-TiO2 and Fe3 O4 /Al2 O3 /2NP-TiO2 . Concentration of 2NP, 4NP and 2,4-DNP: 10 mg/L, photocatalyst: 5 g/L, irradiation time 60 min, average of three measurements (mean + S.D.).

and Fe3 O4 /Al2 O3 due to no photoactive component sensitive to UV light in these cases. Illumination in the presence of TiO2 resulted in an enhanced photodegradation of 2NP. After UV irradiation for 60 min, the X value over Fe3 O4 /Al2 O3 /2NP-TiO2 was 0.73, being about 1.74 times of that over Fe3 O4 /2NP-TiO2 (0.42) and approximately 3.0 times of that over Fe3 O4 /Al2 O3 /NIP-TiO2 (0.25). It is clear that molecularly imprinted TiO2 and Al2 O3 interlayer play important roles in improving the photocatalytic activity of Fe3 O4 /Al2 O3 /2NP-TiO2 . This enhanced photocatalytic activity can be attributed to the inhibition of charge injection from the outlayer photocatalyst by Al2 O3 interlayer and selective photodegradation of target molecules on the Fe3 O4 /Al2 O3 /2NP-TiO2 , a result of steric match between imprinted cavities and target molecules. The photodegradation mechanism of 2NP on Fe3 O4 /Al2 O3 /2NP-TiO2 can be described as follows. The 2NP molecules are firstly adsorbed onto the Fe3 O4 /Al2 O3 /2NP-TiO2 surface. When Fe3 O4 /Al2 O3 /2NP-TiO2 is illuminated with the UV light, the electron-hole pairs will be produced. The valence band (hvb + ) potential is positive enough to generate the hydroxyl radicals and the conduction band (ecb − ) potential is negative enough to reduce the molecular oxygen. The resulting hydroxyl radicals are a very strong oxidizing agent which can oxidize the 2NP and cause the photodegradation of 2NP according to the following reactions: Fe3 O4 /Al2 O3 /2NP-TiO2 + hv → Fe3 O4 /Al2 O3 /2NP-TiO2 (ecb − + hvb + )

Fig. 9. Reusability of Fe3 O4 /Al2 O3 /2NP-TiO2 . 10 mg/L 2NP, 5 g/L Fe3 O4 /Al2 O3 /2NPTiO2 , irradiation time of 60 min.

Fe3 O4 /Al2 O3 /2NP-TiO2 possessed higher photocatalytic selectivity for template molecules. In comparison, Fe3 O4 /Al2 O3 /NIP-TiO2 exhibited an almost equal photodegradation rate for 2NP, 4NP and 2,4-DNP, indicating that the photocatalytic selectivity of Fe3 O4 /Al2 O3 /NIP-TiO2 was poor. The above results reveal that the Fe3 O4 /Al2 O3 /2NP-TiO2 photocatalyst is highly selective to template molecules, which is important for improving its photocatalytic avtivity.

3.4. Reusability of Fe3 O4 /Al2 O3 /2NP-TiO2 The reusability of the photocatalyst is of great important in the photodegradation of organic pollutants. To evaluate the reusability of Fe3 O4 /Al2 O3 /2NP-TiO2 , the photodegradation experiment of 2NP was carried out repeatedly for ten times under the same experimental condition. As seen in Fig. 9, the photodegradation rate of 2NP exhibited a slight change, and remained as high as 95.9% after 10 cycles. This indicates that Fe3 O4 /Al2 O3 /2NP-TiO2 can be recoverable by using an external permanent magnet and reusable with meager loss in activity during the photocatalytic oxidation of pollutant molecules.

hvb + + OH− (ads) → • OH(ads)

4. Conclusions

hvb + + H2 O(ads) → H+ + • OH(ads)

We fabricate a new Fe3 O4 /Al2 O3 /2NP-TiO2 composite photocatalyst, consisting of superparamagnetic Fe3 O4 nanosphere as inner core, inactive Al2 O3 as interlayer and photoactive 2NP-TiO2 as shell. Compared with Fe3 O4 /2NP-TiO2 and Fe3 O4 /Al2 O3 /NIP-TiO2 , the Fe3 O4 /Al2 O3 /2NP-TiO2 not only shows higher photocatalytic selectivity for target pollutant, but also exhibits enhanced photocatalytic activity in degrading the target contaminant. The enhanced molecular recognitive photocatalytic activity can be attributed to a combined effect of selective photodegradation of target molecules on Fe3 O4 /Al2 O3 /2NP-TiO2 and the minimization of the transfer of photogenerated charge carriers from TiO2 to Fe3 O4 . Such multifunctional photocatalyst can effectively degrade organic pollutants and can be easily recovered by an external magnetic field, which was reused without any appreciable reduction in photocatalytic activity. The as-prepared Fe3 O4 /Al2 O3 /2NP-TiO2 photocatalysts with superior selectivity, magnetism and photocatalytic activity have a promising perspective in the wastewater treatment.

ecb − + O2(ads) → • O2 − (ads) • OH (ads)

+ 2NP → degradationof2NP

hvb + + 2NP → oxidationof2NP

3.3. Photocatalytic selectivity of Fe3 O4 /Al2 O3 /2NP-TiO2 The photocatalytic selectivity of Fe3 O4 /Al2 O3 /2NP-TiO2 and Fe3 O4 /Al2 O3 /NIP-TiO2 toward 2NP, 4NP and 2,4-DNP was tested and the obtained results are shown in Fig. 8. It could be seen that the Fe3 O4 /Al2 O3 /2NP-TiO2 showed much better photodegradation rate for 2NP in comparison with 4NP and 2,4-DNP, indicating that

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Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 51202059), the Research Foundation for Youth Scholars of Higher Education of Henan Province (No. 2013GGJS047), the Key Foundation of He’nan Educational Committee (No. 14A430014), Foundation for Distinguished Young Scientists of Henan Polytechnic University (J2014-05) and the Opening Project of Henan Key Discipline Open Laboratory of Mining Engineering Materials (No. MEM13-4). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14]

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