Rapid Construction of ZnO@ZIF-8 Heterostructures with Size

Mar 21, 2016 - Importantly, such ZnO@ZIF-8 core−shell heterostructures exhibit size-selective photocatalysis properties due to selective adsorption ...
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Rapid Construction of ZnO@ZIF‑8 Heterostructures with SizeSelective Photocatalysis Properties Xianbiao Wang,†,‡ Jin Liu,† Sookwan Leong,‡ Xiaocheng Lin,‡ Jing Wei,‡ Biao Kong,‡ Yongfei Xu,† Ze-Xian Low,‡ Jianfeng Yao,*,‡,§ and Huanting Wang*,‡ †

Anhui Key Laboratory of Advanced Building Materials, Key Laboratory of Functional Molecule Design and Interface Process, School of Materials and Chemical Engineering, Anhui Jianzhu University, Hefei, 230601, P. R. China ‡ Department of Chemical Engineering, Monash University, Clayton, Victoria 3800, Australia § College of Chemical Engineering, Nanjing Forestry University, Nanjing, 210037, P. R. China S Supporting Information *

ABSTRACT: To selectively remove heavy metal from dye solution, inspired by the unique pore structure of ZIF-8, we developed a synthetic strategy for rapid construction of ZnO@ZIF8 heterostructure photocatalyst for selective reduction of Cr(VI) between Cr(VI) and methylene blue (MB). In particular, ZnO@ ZIF-8 core−shell heterostructures were prepared by in situ ZIF-8 crystal growth using ZnO colloidal spheres as template and zinc source within 8−60 min. The shell of the resulting ZnO@ZIF-8 core−shell heterostructure with a uniform thickness of around 30 nm is composed of ZIF-8 crystal polyhedrons. The concentration of organic ligand 2-methylimidazole (Hmim) was found to be crucial for the formation of ZnO@ZIF-8 core−shell heterostructures. Different structures, ZnO@ZIF-8 core−shell spheres and separate ZIF-8 polyhedrons could be formed by altering Hmim concentration, which significantly influences the balance between rate of Zn2+ release from ZnO and coordinate rate. Importantly, such ZnO@ZIF-8 core−shell heterostructures exhibit size-selective photocatalysis properties due to selective adsorption and permeation effect of ZIF-8 shell. The as-synthesized ZnO@ZIF-8 heterostructures exhibited enhanced selective reduction of Cr(VI) between Cr(VI) and MB, which may find application in the dye industry. This work not only provides a general route for rapid fabrication of such core−shell heterostructures but also illustrates a strategy for selectively enhanced photocatalysis performance by utilizing adsorption and size selectivity of ZIF-8 shell. KEYWORDS: rapid construction, ZnO@ZIF-8, heterostructure, size-selective, photocatalysis potential selectivity in adsorption,10 catalytic process,11 gas separation, and capturing target materials,12 etc. Importantly, ZIF-8 has been reported for selective adsorption13 and detection.14 Nevertheless, the photocatalytic performance of ZIF-8 is not well applicated. Therefore, it is anticipated that selectively enhanced photocatalysis should be achieved by preparing ZnO and ZIF-8 heterostructures. Recently, heavy metal pollution in dye industry has been an important issue. To selective reduction of heavy metals from the dye solution, inspired by the unique pore structure and excellent photocatalytic performance of ZnO, herein, we demonstrate for the first time a ZnO@ZIF-8 core−shell heterostructures with selectively enhanced photocatalysis properties. Such core−shell heterostructures can selective photoreduction of Cr(VI) from Cr(VI) and MB mixed aqueous

1. INTRODUCTION Photocatalysis is a very important process for water treatment,1 hydrogen generation2 and CO2 conversion3 on the basis of the redox effect. Selective photocatalysis process would be an efficient route for enhanced redox reaction of target molecules without influence of other materials.4,5 It is known that molecular adsorption or permeation plays an important role in quantum efficiency of photocatalysis process.6 Selective adsorption or permeation of target molecules could lead to selectively enhanced photocatalysis. To date, there have been limited reports on developing photocatalysts for selective degradation reactions. Among a wide variety of photocatalytically active materials, ZnO is a semiconductor with a band gap of 3.3 eV and has been widely studied as a photocatalyst,7,8 but it remains challenging to selective capture the target molecules for selectively enhanced photocatalysis. Zeolitic Imidazolate Framework-8 (ZIF-8, also named zeolitic metal azolate frameworks, MAF-4) as a kind of MOFs with pore size of 11.6 Å through 3.4 Å apertures9 has © 2016 American Chemical Society

Received: January 2, 2016 Accepted: March 21, 2016 Published: March 21, 2016 9080

DOI: 10.1021/acsami.6b00028 ACS Appl. Mater. Interfaces 2016, 8, 9080−9087

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Zeta potential measurement was performed according to the principle of laser Doppler electrophoresis on the Malvern zetasizer nano. 2.3. Evaluation of Selective Photocatalysis Performance. All the catalysts were activated in 130 °C for 5 h before use. Selective photocatalysis performances were evaluated by photo reduction of Cr(VI) and methylene blue (MB) in a single-component solution or two-component mixed [Cr(VI) and MB] solution. Analytical pure K2Cr2O7 and MB were used to prepare solutions of Cr(VI) and MB, respectively. UVP Pen-Ray mercury lamp (USA) with light emission of 4.86 W and wavelength of 254 nm was used as a UV source. The photocatalysis experiments were carried out in open air and at room temperature. Typically, 20 mg of ZnO@ZIF-8 catalyst was added into 20 mL of Cr(VI) or MB solution with a concentration of 20 mg/L (pH = 7). Prior to irradiation, the suspension was magnetically stirred in the dark for 60 min to reach adsorption equilibrium. During the photocatalysis reaction, UV Pen-Ray mercury lamp was immersed into the stirring suspension. Sample (1.5 mL) was extracted at regular intervals using 0.45 μm syringe filter for analysis. To further evaluate selective photocatalysis performance in mixed solution, Cr(VI) and MB with the same concentration (10 mg/L) were prepared to obtain a two-component solution. For comparison, ZnO colloidal spheres were used as a catalyst as well. Every point in the photocatalytic curve was done twice for average values. The concentrations of Cr(VI), MB, and Rhodamine B (RhB) were measured by UV−vis maximum absorbance at 365, 664, and 553 nm, respectively.1,20,21

solution. To achieve this, an in situ ZIF-8 crystal growth route for rapid construction of the heterostructures was applied using ZnO colloidal spheres as a sacrifice template. In terms of core−shell heterostructures with ZIF-8 as shell, recently, Hee Jung Lee et al. reported synthesis of PS@ZIF-8 structure and subsequently obtained ZIF-8 hollow structures by removal of the PS core.15 Zhang et al. reported preparation of ZIF-8 shells around Fe3O4 cores to obtain Fe3O4@ZIF-8. Both the strategies started from modification of core with negative charge to adsorb Zn2+ and then ZIF-8 crystals grew around the cores. In addition, Zhan et al. described fabrication of ZnO@ ZIF-8 nanorods in H2O−DMF mixed solution using ZnO nanorods as a sacrifice template after long-time solvothermal treatment (24 h).14 Following that strategy, Yu et al. obtained ZnO@ZIF-8 nanospheres.16 Lu et al. developed a ZnO induced strategy to fabricate Pd/ZnO@ZIF-8 core−shell microspheres.17 However, both of their ZIF-8 shells are too thick (∼100 nm) to be applicable for selective photocatalysis.16,17 In this paper, we report an in situ crystal growth strategy for rapid fabrication of ZnO@ZIF-8 heterostructures within 8−60 min. The as-synthesized ZnO@ZIF-8 core−shell spheres are 300−400 nm in diameter with ∼30 nm shell thickness. Formation investigations indicated that the ZIF-8 crystals grew directly on the ZnO colloidal spheres. Concentration of Hmim plays an important role in fabricating such core−shell heterostructures by influencing the balance between release of Zn2+ ions and coordinate rate. More importantly, selectively enhanced photocatalysis performance was achieved by selective reduction of Cr(VI) between Cr(VI) and MB. It was found that the selective photocatalytic properties of ZnO@ZIF-8 heterostructures were based on their selective adsorption and permeation of Cr(VI) species by incorporation of ZIF-8 shell.

3. RESULTS AND DISCUSSION 3.1. Structure and Morphology. The structure of the products were identified by XRD measurements, as shown in Figure 1a, which revealed that the white powder obtained in the

2. EXPERIMENTAL SECTION 2.1. Synthetic Procedure. 2.1.1. Preparation of ZnO Colloidal Spheres. ZnO colloidal spheres were prepared and used as template and zinc source for preparation of ZnO@ZIF-8 core−shell heterostructures. The ZnO colloidal spheres with sizes of 200−300 nm were prepared with a modified strategy according to previously reported works.18,19 Typically, 5.49 g of zinc acetate dihydrate was added into 250 mL of diethylene glycol (DEG) and refluxing at 160 °C for 1 h. After the mixture was cooled down to room temperature, the white powder was collected by centrifugation and washed with ethanol for several times. 2.1.2. Preparation of ZnO@ZIF-8 Core−shell Heterostructures. Typically, 0.05 g of as-synthesized ZnO colloidal spheres were mixed with 13.5 mL of 2-methylimidazole (Hmim) methanol solution with concentration of 3.66 M. Then the white powders were obtained by centrifugation after aged for 8−60 min. The ZnO@ZIF-8 core−shell heterostructures were dried at 60 °C overnight after washing with ethanol for three times. By altering the concentrations of Hmim solution, a large amount of ZIF-8 separate polyhedron crystals could be produced. 2.2. Characterization. X-ray diffraction (XRD) measurements were carried out on a Philips PW1140/90 diffractometer with CuKα radiation (25 mA and 40 kV) at a scan rate of 2° per minute with a step size of 0.02°. Fourier transform infrared spectroscopic (FTIR) spectra were obtained on a Nexus spectrometer. Scanning electron microscopy (SEM) images were taken by using a FEI-NOVA Nano SEM 450 microscope. Transmission electron microscopy (TEM) analysis was carried out on a JEOL-2010 microscope. Nitrogen adsorption−desorption isotherms were performed at 77 K with a volumetric adsorption analyzer (Micromeritics ASAP 2010). Thermal gravimetric (TG) measurement was taken on a thermal instrument (Shimadzu Corp. Japan) with a heating rate of 10 °C/min. An UV2600 spectrometer was chosen to obtain UV−vis absorption spectra.

Figure 1. (a) XRD patterns and (b) FTIR spectra of as-prepared ZnO colloidal spheres and ZnO@ZIF-8 heterostructures. The inset in part a is the whole XRD pattern of ZnO colloidal spheres from 10° to 70°.

DEG solution has a wurtzite-type ZnO phase (JCPDS No. 361451). SEM images (Figure 2a) indicate that the morphology

Figure 2. SEM images of (a) ZnO colloidal spheres, (b) ZnO@ZIF-8 heterostructures, and (c) TEM image of ZnO@ZIF-8 heterostructures.

of ZnO is spherical with a uniform size ∼200−300 nm. The high magnification image (Figure 2a, inset) shows that the surface of ZnO colloids is rough owing to aggregation of nanoparticles.18,19 After treatment with Hmim solution, the product has two phases as shown in Figure 1a. Except for ZnO diffraction peaks, the residual peaks are fit well with ZIF-8 phase with a cubic space group (I4̅3m)9 by comparing with the simulated ZIF-8 pattern. FTIR spectra further demonstrate the 9081

DOI: 10.1021/acsami.6b00028 ACS Appl. Mater. Interfaces 2016, 8, 9080−9087

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ACS Applied Materials & Interfaces existence of ZIF-8, as shown in Figure 1b. In addition to the peaks of ZnO, the peaks of 1306, 1145, and 759 cm−1 corresponds to the bending signals of the imidazole ring.17 The peak located at 1444 cm−1 ascribes to the stretching vibration of the imidazole ring.17 Peaks at 2927 and 3133 cm−1 are aromatic and aliphatic C−H stretches, respectively.22 Further observation from Figure 2b shows that many polyhedrons (∼50 nm) appeared on the surface of ZnO colloids. In order to investigate their inner structure, TEM analysis was performed (Figure 2c), which demonstrated that ZnO@ZIF-8 core−shell heterostructures were produced. The ZIF-8 crystal polyhedrons are covered around ZnO colloidal spheres with a thickness of ∼30 nm. Figure 3 shows the nitrogen adsorption−desorption isotherms of ZnO colloids, ZnO@ZIF-8 heterostructures, and

Figure 4. TG curve of the ZnO@ZIF-8 heterostructures.

3.2. Effect of Hmim Concentration. Concentration of Hmim in methanol is crucial to the formation of ZnO@ZIF-8 core−shell heterostructures. With higher concentration of Hmim (4.51 M), ZnO@ZIF-8 core−shell heterostructures and separate ZIF-8 polyhedrons are produced (Figure 5a).

Figure 3. Nitrogen adsorption−desorption isotherm of (a) ZnO colloidal spheres, (b) ZnO@ZIF-8 heterostructures, and (c) ZIF-8.

ZIF-8. The curves of ZIF-8 (Figure 3c) are a typical type I nitrogen adsorption−desorption isotherm,23 which fit well with the microporous frameworks of ZIF-8. The initial stage of the isotherm could be attributed to adsorption by micropores. The curves for ZnO colloids (Figure 3a) are a type IV isotherm,23 which shows low adsorption at low pressure and multilayer adsorption at high pressures owing to slits caused by assembly of ZnO colloids. The specific BET surface area is 44.4 m2/g. For comparison, the BET surface area of ZnO@ZIF-8 is 334.0 m2/g, higher than that of ZnO colloidal spheres. The initial adsorption of ZnO@ZIF-8 is higher than that of ZnO (Figure 3b,a), indicating that micropores exist in ZnO@ZIF-8 heterostructures,23 which can be attributed to the frameworks of ZIF-8. The multilayer adsorption at high pressure corresponds to spherical morphology of the sample, similar to that of ZnO colloidal spheres. To determine the content of ZIF-8 in the ZnO@ZIF-8 composites, TG analysis was carried out in air atmosphere, as shown in Figure 4. The weight loss at the beginning stage (∼0.5%) is the vaporization of adsorbed water in the sample. Then oxidation of ZIF-8 was initialized to produce ZnO and release gases. The obvious weight loss occurred in the range of 248−535 °C owing to rapid decomposition of ZIF-8 molecules. When the temperature reaches 600 °C, ZIF-8 molecules are transformed to ZnO completely. The weight loss remains constant (25.87%) with further increasing the temperature. Therefore, the content of ZIF-8 in the ZnO@ZIF-8 sample is calculated to be 40.27 wt % on the basis of the weight loss of pure ZIF-8 in air (64.24%).

Figure 5. SEM and TEM (inset) images of products obtained with different concentration of Hmim (a) 4.51 M, (b) 1.83 M, [inset, selected area electron diffraction (SAED) pattern of the colloidal sphere in TEM image], (c) 0.915 M, and (d) 0.366 M.

Further observation shows that the shell thickness of core−shell sphere is ∼22 nm thinner than that of product obtained with a concentration of 3.66 M (∼30 nm) and the separate crystal polyhedrons are dodecahedrons and hexahedrons with a size of around 140 nm. Lower the Hmim concentration to 3.66 M (Figure 2), pure ZnO@ZIF-8 core−shell heterostructures are obtained. The relative contents of ZnO to ZIF-8 in both samples are similar based on relative intensity of XRD patterns (Figure 6a,e and Table 1), which agrees well with results of SEM and TEM. Further decrease the concentration of Hmim solution to 1.83 M results in ZIF-8 polyhedrons and a few colloidal spheres as shown in Figure 5b. The selected area electron diffraction (SAED) pattern (inset of Figure 5b) indicates that the colloidal spheres is the remaining ZnO colloidal spheres21 which also demonstrated by weak ZnO characteristic peaks in Figure 6b. As the concentration of Hmim decreased to 0.915 M and further to 0.366 M, core−shell structures with a shell thickness of ∼30 nm (Figure 5c) and ∼15 nm (Figure 5d) and separate ZIF-8 polyhedron crystals are 9082

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Figure 6. XRD patterns of the product obtained with different Hmim concentration: (a) 4.51 M, (b) 1.83 M, (c) 0.915 M, (d) 0.366 M, and (e) 3.66 M Note: ZIF-8 peaks were labeled as “*” and ZnO peaks were labeled as “#”.

Figure 7. SEM images of the obtained products at different reaction times: (a) 3 min, (b) 8 min, (c) 20 min, and (d) 60 min.

Table 1. Dependence of pH, IZIF‑8, IZnO, and Structure on Concentration of Hmima concn of Hmim (M)

pH

IZIF‑8/(IZIF‑8 + IZnO)

4.51

10.69

0.6

3.66 1.83 0.915

10.54 10.22 9.81

0.53 0.88 0.35

0.366

9.42

0.21

Table 1. Then, the released Zn2+ would coordinate with Hmim to form ZIF-8 crystals. Generally, formation of ZIF-8 is controlled by the nucleation process during the whole nucleation and crystal growth stages.27 In addition, a high mole ratio of Hmim/Zn2+ leads to a high nucleation rate.25,28 Thus, the coordination rate of Hmim with Zn2+ is significantly controlled by the mole ratio of Hmim/Zn2+. In this case, the key factor of core−shell structure formation is the balance between the coordination rate and the release rate of Zn2+ (dissolution of ZnO).14 Generally, the rate of dissolution and coordination are both increased along with increased concentration of Hmim while keeping the concentration of ZnO unchanged. In the cases of Hmim solutions with concentrations of 4.51 or 0.915 M or 0.366M, the rate of dissolution and coordination are competitive. Hmim molecules could react with released Zn2+ ions rapidly, and then most of ZIF-8 nucleation occurred on the surface of ZnO colloidal spheres, leading to formation of ZnO@ZIF-8 core−shell heterostructures (Figure 5a,c,d). However, there are still some Zn2+ ions that diffuse into solution, resulting in separate ZIF-8 polyhedrons (Figure 5a,c,d). Higher Hmim concentrations lead to a higher release rate of Zn2+ and coordination rate, finally resulting in higher content of ZIF-8 in the product which is demonstrated by relative intensity of XRD patterns (Figure 6d,c,a) and Table 1. The formation procedure is illustrated in Scheme 1a. When the concentration of Hmim is fixed to 1.83 mol/L (Scheme 1c), the rate of coordination is greatly decreased and could not react with Zn2+ ions on the surface of ZnO rapidly. Therefore, all the ZIF-8 crystals are produced in the form of polyhedrons. There are no obvious peaks of ZnO observed in Figure 6b, indicating that only a little amount of ZnO colloidal spheres remained in the solution, as shown in Figure 5b. The complete ZnO@ZIF-8 core−shell heterostructures can only be obtained when the concentration of Hmim is 3.66 M (Scheme 1b). ZnO@ZIF-8 core−shell heterostructures are formed and no other structure is observed through SEM and TEM images (Figure 2b,c). In this circumstance, all the released Zn2+ ions are captured by Hmim ligands rapidly and nucleation on the surface of ZnO colloidal spheres, owing to a much higher coordination rate compared to the rate of dissolution. Therefore, ZnO@ZIF-8 core−shell heterostructures can be obtained by controlling the concentration of Hmim solution.

structure core−shell and polyhedrons core−shell polyhedrons and ZnO core−shell and polyhedrons core−shell and polyhedrons

a

IZIF‑8 and IZnO refer to intensity of XRD peak located at 7.42° [(001) plane of ZIF-8] and 36.16° [(101) plane of ZnO] (Figure 6), respectively.

obtained. Except that higher ZnO content (Figure 6c,d and Table 1), the obtained structures are similar to the product prepared with 4.51 M Hmim solution. The dependence of structure on concentration of Hmim is illustrated in Table 1. 3.3. Effect of Reaction Time. To investigate the rapid formation process of such core−shell heterostructures, timedependent experiments were carried out. Morphologies of the resulting products were observed by SEM, as shown in Figure 7. As the reaction time was limited to 3 min, there are some ZIF-8 polyhedron crystals covered on the surface of ZnO colloidal spheres (Figure 7a). However, some ZnO colloidal spheres are still exposed (inset of Figure 7a) and not completely covered (marked with black line in Figure 7a) within the short time. Further, there are still some ZnO colloidal spheres that remain totally uncovered, indicating that 3 min is not enough for the whole reaction. Prolong the reaction time to 8 min, all the ZnO spheres are covered with ZIF-8 polyhedrons (Figure 7b), implying fast fabrication of ZnO@ZIF-8 core−shell heterostructures. Further prolonging of the reaction time to 20 and 60 min, no obvious morphological change occurs except that more coarse surfaces are observed from Figure 7c,d. 3.4. Formation Mechanism. As we know, Zn2+ ions are released from the surface of ZnO colloids when mixed with methanol solution of Hmim (pKa,14.2)24,25 as ZnO is an amphoteric metal oxide which can be dissolved in basic or acidic solutions.26 The release rate of Zn2+ ions is relevant to concentrations of Hmim with different pH values, as shown in 9083

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Scheme 1. Schematic Illustration of Structures Obtained at Different Concentrations of Hmim (a) 4.51 M or 0.915 M, or 0.366 M, (b) 3.66 M, and (c) 1.83M

3.5. Structure Dependence on Hmim Concentration. To give deep insight into the structure dependence on concentration of Hmim (CHmim), relationship between the obtained structure and balance between Zn2+ release rate (Rr) and coordination rate (Rc) was further investigated, as shown in Figure 8 (details are illustrated in the Supporting Information).

to 1.83 M, the ratio of Rc/Rr decreased (zone III), resulting in formation of separate ZIF-8 polyhedrons. Further increase or decrease the concentration to 4.51 M, 0.915 M, and 0.366 M, the experimental points are in the range of zone II, resulting in both core−shell and polyhedron structures. Additional experiments with Hmim concentrations of 3 M (between 1.83 and 3.66 M) and 4 M (between 3.66 and 4.51 M) were also carried out. We got core−shell structures and polyhedrons morphology in both experiments, as shown in Figure.S1, indicating that ZnO@ZIF-8 core−shell heterostructures can only be obtained with Hmim concentration of 3.66 M. 3.6. Selectively Enhanced Photocatalysis Performance. 3.6.1. UV−Visible Absorption Analysis. UV absorptions of the obtained ZnO spheres and ZnO@ZIF-8 heterostructures are shown in Figure 9a. Both the absorptions occurred in the wavelength range of 200−400 nm. Further observation indicates that the absorption edge of ZnO@ZIF-8 heterostructures is slightly red shifted as compared with that of ZnO colloidal spheres. Correspondingly, the band gaps of ZnO spheres and ZnO@ZIF-8 heterostructures (Figure 9b) indicate that the band gap of ZnO@ZIF-8 heterostructures (3.24 eV) is a little narrower than that of ZnO spheres (3.27 eV), owing to the heterostructural effect caused by incorporation of the ZIF-8 shell.20 3.6.2. Selectively Enhanced Photoreduction of Cr(VI). To selectively remove heavy metal from dye solution, methylene blue (MB) and Cr(VI) were chosen to evaluate selective

Figure 8. Structure dependence of the obtained product on Hmim concentration.

In the case of 3.66 M, the experimental point located in the I zone, indicating high ratio of Rc/Rr, leading to formation of core−shell heterostructure. Decrease the Hmim concentration

Figure 9. (a) UV−vis absorption spectra and (b) band gap measurement of ZnO colloidal spheres and ZnO@ZIF-8 heterostructures. 9084

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Figure 10. Photocatalytic removal curves of Cr(VI) and MB in the presence of ZnO@ZIF-8 heterostructures or ZnO spheres in (a) monocomponent aqueous solutions and (b) two-component mixed Cr(VI) and MB aqueous solutions under UV irradiation.

were observed in monocomponent or two-component aqueous solutions under UV irradiation (Figure 10). A few instances in the literature reported preparation of ZnO@ZIF-8 heterostructures and evaluation of their selective photocatalysis properties. Zhan et al. reported preparation of ZnO@ZIF-8 nanorod arrays with selective photoelectrochemical response to H2O2.14 Lin et al. reported fabrication of the Pd/ZnO@ZIF-8 structure with selective catalytic hydrogenation performances.17 To the best of our knowledge, it is the first time to report the construction of ZnO@ZIF-8 heterostructures and their application in selective photoreduction of Cr(VI) from the dye solution. It is worthy to note that ZIF-8 shell is stable during the whole photocatalysis process. The XRD patterns before and after photocatalysis are almost the same, as shown in Figure.S2. In addition, no obvious morphology change was observed after the photocatalysis process (inset of Figure.S2), indicating good cycle life performance. To further evaluate the performance of cycle life of the ZnO@ZIF-8 photocatalysts, photocatalytic reduction of Cr(VI) by irradiation for 4 h using the recycled photocatalysts was conducted. The cycle life performance was shown in Figure.S3. After three cycles, the removal (or reduction) efficiency of Cr(VI) was still maintained compared with the performance of fresh photocatalysts, which also demonstrated the good cycle life and stability of the ZnO@ ZIF-8 heterostructures. 3.7. Adsorption and Size Selectivity Induced Selectively Enhanced Photocatalysis Performance. During the photocatalysis process, ZnO produces electrons and holes under UV irradiation.30 Thus, Cr(VI) was reduced by electrons and organic molecules were mainly degraded by holes on the surface of photocatalysts.1 In terms of ZnO@ZIF-8 heterostructured photocatalysts, target molecules have to penetrate through ZIF-8 frameworks to get effective photoreduction or photodegradation. The selectively enhanced photocatalysis performance of ZnO@ZIF-8 heterostructures toward Cr(VI) could be attributed to adsorption and size selectivity of the ZIF8 shell toward Cr(VI). 3.7.1. Adsorption Selectivity. ZIF-8 crystals exhibit positive charge in acidic or neutral conditions.31 In addition, the main species of Cr(VI) in aqueous solution is CrO42− anions at pH value of 7.21 Therefore, Cr(VI) anions could be selectively adsorbed around the photocatalyst by electrostatic interaction32 as compared with MB molecules (a cationic dye), leading to enhanced photocatalysis performance.5 To demonstrate this,

photocatalysis properties of the ZnO@ZIF-8 core−shell heterostructures. During the photocatalytic reduction process, Cr(VI) could be reduced to Cr(III) which easily precipitates in neutral or alkaline conditions20 and the photocatalytic redox also degrade MB molecules,29 both leading to removal from aqueous solution. The photocatalysis performance of ZnO@ ZIF-8 core−shell heterostructures in Cr(VI) or MB monocomposition aqueous solutions are shown in Figure 10a, as compared with that of ZnO colloidal spheres. For the ZnO colloidal spheres photocatalyst, both Cr(VI) and MB could be effectively removed with a faster degradation rate of MB. After 80 min irradiation, MB molecules are degraded completely, whereas Cr(VI) still remains ∼50%. After incorporation of ZIF8 shell, the photocatalytic selectivity of ZnO@ZIF-8 core−shell heterostructures toward Cr(VI) is obviously enhanced. For the photocatalytic reduction of Cr(VI), around 88% of Cr(VI) is removed after 240 min irradiation and still under reduction. The removal efficiency is better than that of ZnO, exhibiting enhanced photocatalysis properties. In addition, the ZIF-8 shell also slows down the degradation of MB, which remains higher than 50% after irradiation for 80 min to reach equilibrium. Thus, the ZnO@ZIF-8 heterostructures have selectively enhanced photocatalysis performance toward Cr(VI) as compared with MB. To further investigate the selectivity of ZnO@ZIF-8 heterostructures in mixed aqueous solution, a two-component aqueous solution [Cr(VI) and MB] was prepared with the same initial concentrations. The relationship between removal capacity and irradiation time is shown in Figure 9b. Similarly, ZnO@ZIF-8 heterostructures exhibit the same selectivity toward Cr(VI) in mixed solution. Almost all Cr(VI) species are degraded after 50 min irradiation and then reach complete removal when the irradiation time prolong to 70 min. The MB degradation process reaches equilibrium within 30 min, with more than 60% MB molecules left. For ZnO colloidal spheres photocatalyst, both Cr(VI) and MB could be effectively reduced or degraded with complete removal after 180 min irradiation, exhibiting low selectivity. Similarly, the core−shell ZnO@ZIF-8 heterostructures reduce more Cr(VI) species than that of ZnO colloidal spheres in twocomponent mixed aqueous solution, also exhibiting enhanced photocatalysis reduction for Cr(VI). Importantly, the selectively enhanced photocatalytic reduction toward Cr(VI) in mixed solution may find application in the dye industry in the case of removing heavy metals from dye solutions. Without catalyst, no obvious concentration decreases of Cr(VI) and MB 9085

DOI: 10.1021/acsami.6b00028 ACS Appl. Mater. Interfaces 2016, 8, 9080−9087

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4. CONCLUSION In this paper, in order to selective removal of heavy metal from dye solution, we reported an effective strategy for rapid construction of ZnO@ZIF-8 heterostructures with sizeselective photocatalysis properties. The concentration of Hmim methanol solution was investigated to be crucial for the preparation of core−shell heterostructures by significantly influencing the balance between the rate of ZnO dissolution and Zn2+ coordination rate. As a result, the obtained heterostructures exhibited selectively enhanced photocatalysis performance by selective reduction of Cr(VI) between Cr(VI) and MB, which was induced by selective adsorption and the permeation effect of ZIF-8 shell.

zeta potential of the ZnO@ZIF-8 heterostructures were analyzed and shown in Figure.S4, exhibiting a positively charged surface in the experimental condition (pH = 7). 3.7.2. Size Selectivity. In addition to adsorption selectivity, size selectivity of ZIF-8 shell might be a key role in the selectively enhanced photocatalysis process. To permeate into the ZIF-8 shell, target molecules should go through the aperture of the ZIF-8 frameworks with a size of 3.4 Å. Thus, CrO42− ions with an ionic diameter of 4 Å33 could penetrate into the ZIF-8 shell based on their smaller molecular size and deformation of ZIF-8 frameworks.17,34 However, for MB molecules with the minimum cross-section size of 8 Å could not enter into the pores with a size less than 13 Å.35 As a result, they could not permeate into the apertures of ZIF-8 (3.4 Å). Therefore, selective adsorption and permeation of Cr(VI) leads to selectively enhanced photocatalysis. On the contrary, no permeation of MB molecules results in decrease of photocatalysis performance, as shown in Scheme 2.

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ASSOCIATED CONTENT

S Supporting Information *

. The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b00028. TEM images of the products obtained with Hmim concentrations of 3 M and 4 M, XRD patterns of ZnO@ ZIF-8 photocatalysts before and after photocatalysis process (inset, SEM image), cycle life performance of photocatalysts, zeta-potential of the ZnO@ZIF-8 core− shell heterostructures, degradation curves of RhB in the presence of ZnO@ZIF-8 heterostructures or ZnO colloidal spheres under UV irradiation, and notes for Figure 8 (PDF)

Scheme 2. Schematic Illustration of Selectively Enhanced Photocatalysis Properties of the ZnO@ZIF-8 Core−Shell Heterostructures

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AUTHOR INFORMATION

Corresponding Authors

*E-mail:[email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. 21371005 and 21001002), the Australian Research Council through a Future Fellowship (Huanting Wang, Grant FT100100192), the Excellent Talent Foundation for Young Scholars in Universities of Anhui Province (Grant No. gxyqZD2016152), and the China Scholarship Council (Grant 201308340032). X. Wang would like to thank Anhui Jianzhu University and the Department of Chemical Engineering at Monash University for supporting his visiting research.

To further demonstrate the size selectivity of ZnO@ZIF-8 heterostructures, Rhodamine B (RhB) with a molecular size of 15.9 Å × 11.8 Å × 5.6 Å36 was also chosen as a target, as shown in Figure.S5. Similarly, the size-selective properties play an important role in the photocatalysis process for RhB degradation. Photocatalysis degradation reaches equilibrium within 80 min by using ZnO@ZIF-8 heterostructures as a photocatalyst whereas ZnO colloidal spheres degrade RhB completely within 180 min. It should be noted that the concentration decreases of MB or RhB in the presence of ZnO@ZIF-8 heterostructures (Figure 10 and Figure.S5) could be ascribed to the weak photocatalytic effect of the ZIF-8 shell with a band gap of 5.16 eV.29 For practical application, more works are needed to gain a better understanding of the ZnO@ ZIF-8 heterostructure photocatalysts, such as the chemical stability of the core−shell photocatalysts in different pH solutions, photocatalytic selectivity in solutions containing more components, etc.

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