A Novel Bi2MoO6/ZIF-8 Composite for Enhanced Visible Light Photocatalytic Activity

A series of novel Bi2MoO6/zeolitic imidazolate framework-8 (ZIF-8) photocatalysts have been successfully fabricated through a facile self-assembly process. X-ray diffraction (XRD), scanning electron microscopy (SEM), UV-vis spectrophotometry, and X-ray photoelectron spectroscopy (XPS) characterized pure Bi2MoO6, pure ZIF-8, and a series of Bi2MoO6/ZIF-8 composites. The result indicated that, when compared with pure Bi2MoO6, the composite of Bi2MoO6/ZIF-8 exhibited excellent photocatalytic performance for the degradation of methylene blue (MB) under visible light. Moreover, the Bi2MoO6/ZIF-8-3 composite (the molar ratio of Bi2MoO6 to 2-MI is 3:3) has optimum photocatalytic performance because of the suitable amount of ZIF-8 decorated on the flower-like Bi2MoO6. The enhanced photocatalytic activity is probably due to the introduction of ZIF-8, which will promote the separation of electron–hole pair and the surface morphology. Benefitting from the diversity of the MOF species (ZIF-8 is one of them), this composing strategy of Bi2MoO6/MOF composite would provide new insight into the design of highly efficient visible light photocatalysts.


Introduction
Photocatalytic materials, especially semiconductor photocatalysts, have attracted a lot of attention in recent years due to the world-wide environmental problems, such as the water pollution [1][2][3]. However, as most semiconductor photocatalysts, such as TiO 2 , cannot utilize visible light and respond to other visible lights, except ultraviolet light, because of a wide band gap, the photocatalytic activity of them was reduced [4]. Therefore, for the effective use of visible lights, the semiconductor photocatalyst should be able to respond to all visible lights and its band gap should be within a suitable range.
Bismuth molybdate (Bi 2 MoO 6 ) is a typical Aurivillius oxide. When compared with the titanium dioxide, the Bi 2 MoO 6 has a smaller forbidden band width (about 2.7 eV). As a result, it has a better photocatalytic activity within the visible range [5]. As is known to all, the morphology of the semiconductor photocatalyst plays a vital role in photocatalysis process [6]. Bi 2 MoO 6 has a variety of morphologies that have been synthesized [7,8], among which the hierarchical flower-like Bi 2 MoO 6 spheres have attracted a lot of attention with their super photocatalytic performance. In addition, the catalytic efficiency of the semiconductor photocatalyst can be greatly improved by increasing the number of photogenerated electron-hole pairs, inhibiting the recombination of photogenerated electron-hole pairs and increasing the reaction sites of the catalyst with the reactants. The photocatalytic activity of the Bi 2 MoO 6 photogenerated electron-hole pairing ratio is relatively high. To further improve the photocatalytic activity of Bi 2 MoO 6 , many feasible studies have been conducted, such as ion doping [9], surface noble metal deposition [10], and compounding of semiconductor photocatalysts [11]. However, there is still much room for the exploration of the potential of bismuth molybdate-based as a photocatalyst material.
Metal-organic frameworks (MOFs) is a kind of porous crystalline material. It has a periodic multi-dimensional network structure that is formed by the coordination of metal ions or metal cluster units and the self-assemble of organic ligand [12,13]. As the metals and organic ligands can be adjusted and controlled, MOFs exhibit structural diversity and have recently attracted wide attention from the public. Researchers have found that MOFs have potential to be applied in many fields, such as gas storage and separation, catalysis, sensors, drug delivery, etc. [14][15][16][17]. In addition, zeolitic imidazolate framework-8 (ZIF-8) is one of the MOFs with outstanding photocatalytic properties, which has been researched many times in the field of photocatalysis. For example, X. Zeng et al. synthesized the TiO 2 /ZIF-8 composite, which effectively inhibits the recombination of charge carrier, resulting in an improved photocatalytic activity [18]. Y.H. Ding et al. reported that Bi 2 S 3 @ZIF-8 core-shell heterostructures exhibited photocatalytic activity to Rhodamine B (RhB) dye degradation under visible light [19]. These results indicated that ZIF-8 based materials have potential applications in the field of photocatalysis.
In this work, a novel Bi 2 MoO 6 /ZIF-8 composite that displays excellent photocatalytic activity in the degradation of methylene blue (MB) has been successfully synthesized. The nanoparticle of ZIF-8, with different content, has been decorated on the hierarchical flower-like Bi 2 MoO 6 . The result shows that the Bi 2 MoO 6 /ZIF-8 composite has excellent photocatalytic performance under visible light.

Preparation of Bi 2 MoO 6
All of the chemicals were of analytical grade and they can be used without further purification. Bi 2 MoO 6 powder was prepared by the hydrothermal method [20], Na 2 MoO 4 ·2H 2 O (1 mmol, 0.4210 g) and Bi(NO 3 ) 3 ·5H 2 O (2 mmol, 1.6866 g) was dissolved in 5 mL ethylene glycol (EG), respectively. After being stirred for 10 min, the above two solutions were mixed together. Subsequently, 20 mL ethanol was added into the above mixture and then stirred for 10 min. Afterwards, the mixture was transferred into a Teflon lined steel autoclave and heated at 160 • C for 20 h. After the autoclave was cooled to room temperature, the yellow products was recovered by filter, washed with deionized water and ethanol, and collected under vacuum at 80 • C for 12 h. Figure 1 illustrates the preparation procedure for the Bi 2 MoO 6 /ZIF-8 composites. In general, the as-synthesized flower-like Bi 2 MoO 6 spheres were dispersed into a methanol solution containing 2-methylimidazole (2.4 mmol, 0.66 g), and then stirred for 10 min. Zn(NO 3 ) 2 ·6H 2 O (0.3 mmol 0.03 g) was dispersed into 10 mL methanol. After being stirred for 10 min, the above two solutions were mixed together and stirred for 10 min. The samples were collected by centrifugation (7000 rpm, 5 min), washed by methanol for three times, and finally the Bi 2 MoO 6 /ZIF-8 composites were dried at 80 • C for 12 h. All of the Bi 2 MoO 6 /ZIF-8 composites with different molar ratios of Bi 2 MoO 6 to 2-methylimidazole (2-MI), i.e., 5:3, 4:3, 3:3, and 2:3, respectively, were also prepared under the similar procedure. The samples were named Bi 2 MoO 6 /ZIF-8-1, Bi 2 MoO 6 /ZIF-8-2, Bi 2 MoO 6 /ZIF-8-3, and Bi 2 MoO 6 /ZIF-8-4, respectively according to the different molar ratio of Bi 2 MoO 6 to 2-MI mentioned above. The syntheses of pure ZIF-8 without Bi 2 MoO 6 were prepared in the same way.

Evaluation of Photocatalytic Performance
In this experiment, 20 mg photocatalysts were added to 80 mL aqueous solution of Methylene blue (20 mg/L). To ensure an adsorption/desorption equilibrium, the solution was continuously stirred for 30 min in the dark before light irradiation. Afterwards, turn on the light source, which is made up of 300 W Xe lamps with 420 nm cut-off filters. Take about 4 mL of suspension every 20 min and centrifuge it at the same time. The degradation of methylene blue solution and the UV-visible absorption/diffuse reflectance spectroscopy were characterized by a UV-2450 spectrometer (Shimadzu, Kyoto, Japan) Figure 2a shows the X-ray diffraction pattern of simulated ZIF-8, synthesized pure ZIF-8, pure Bi2MoO6, and a series of the composite Bi2MoO6/ZIF-8-1, Bi2MoO6/ZIF-8-2, Bi2MoO6/ZIF-8-3, and Bi2MoO6/ZIF-8-4. The diffraction peak of the synthesized ZIF-8 conforms to that of the simulated ZIF-8. It indicates that the synthesized samples are pure. The diffraction peak of synthesized Bi2MoO6 can be indexed as Bi2MoO6 (JCPDS No. 21-0102). Moreover, in Figure 2b, with the increased content of ZIF-8 in the series of composites of Bi2MoO6/ZIF-8-(1 to 4), the intensity of the ZIF-8 peak will also be enhanced, but it is not obvious. It is probably because of the low content and crystallinity of ZIF-8 [21]. Subsequent characterization results further support the successful combination of ZIF-8 and Bi2MoO6.

Evaluation of Photocatalytic Performance
In this experiment, 20 mg photocatalysts were added to 80 mL aqueous solution of Methylene blue (20 mg/L). To ensure an adsorption/desorption equilibrium, the solution was continuously stirred for 30 min in the dark before light irradiation. Afterwards, turn on the light source, which is made up of 300 W Xe lamps with 420 nm cut-off filters. Take about 4 mL of suspension every 20 min and centrifuge it at the same time. The degradation of methylene blue solution and the UV-visible absorption/diffuse reflectance spectroscopy were characterized by a UV-2450 spectrometer (Shimadzu, Kyoto, Japan). Figure 2a shows the X-ray diffraction pattern of simulated ZIF-8, synthesized pure ZIF-8, pure Bi 2 MoO 6 , and a series of the composite Bi 2 MoO 6 /ZIF-8-1, Bi 2 MoO 6 /ZIF-8-2, Bi 2 MoO 6 /ZIF-8-3, and Bi 2 MoO 6 /ZIF-8-4. The diffraction peak of the synthesized ZIF-8 conforms to that of the simulated ZIF-8. It indicates that the synthesized samples are pure. The diffraction peak of synthesized Bi 2 MoO 6 can be indexed as Bi 2 MoO 6 (JCPDS No. 21-0102). Moreover, in Figure 2b, with the increased content of ZIF-8 in the series of composites of Bi 2 MoO 6 /ZIF-8-(1 to 4), the intensity of the ZIF-8 peak will also be enhanced, but it is not obvious. It is probably because of the low content and crystallinity of ZIF-8 [21]. Subsequent characterization results further support the successful combination of ZIF-8 and Bi 2 MoO 6 .

Scanning Electron Microscopy (SEM) Analysis
As can be clearly observed in Figure 3a, the SEM image of pure ZIF-8 exhibits regular nanoparticle morphology, with average length about 50 nm. As shown in Figure 3b, the hierarchical flower-like Bi2MoO6 spheres with diameter of about 2 μm are composed of many smooth and clean nanosheets. Figure 3c-f shows the morphologies of Bi2MoO6/ZIF-8-1, Bi2MoO6/ZIF-8-2, Bi2MoO6/ZIF-8-3 and Bi2MoO6/ZIF-8-4. The amount of ZIF-8 is too small to be observed in Figure 3c. It is obvious that a combination of different content of ZIF-8 is formed on the nanosheets of Bi2MoO6 in Figure 3df. As the content of ZIF-8 increases, the amount of ZIF-8 on the sheet of Bi2MoO6 increases. When the molar ratio of Bi2MoO6 to 2-MI is 2:3, Bi2MoO6 is wrapped by ZIF-8 and the photocatalytic performance of Bi2MoO6/ZIF-8 will also be affected. Figure 4 shows more SEM images of Bi2MoO6/ZIF-8-3, which will further demonstrate that the proper amount of ZIF-8 were successfully combined on the Bi2MoO6 with an optimal morphology.

Scanning Electron Microscopy (SEM) Analysis
As can be clearly observed in Figure 3a, the SEM image of pure ZIF-8 exhibits regular nanoparticle morphology, with average length about 50 nm. As shown in Figure 3b, the hierarchical flower-like Bi 2 MoO 6 spheres with diameter of about 2 µm are composed of many smooth and clean nanosheets.

Scanning Electron Microscopy (SEM) Analysis
As can be clearly observed in Figure 3a, the SEM image of pure ZIF-8 exhibits regular nanoparticle morphology, with average length about 50 nm. As shown in Figure 3b, the hierarchical flower-like Bi2MoO6 spheres with diameter of about 2 μm are composed of many smooth and clean nanosheets. Figure 3c-f shows the morphologies of Bi2MoO6/ZIF-8-1, Bi2MoO6/ZIF-8-2, Bi2MoO6/ZIF-8-3 and Bi2MoO6/ZIF-8-4. The amount of ZIF-8 is too small to be observed in Figure 3c. It is obvious that a combination of different content of ZIF-8 is formed on the nanosheets of Bi2MoO6 in Figure 3df. As the content of ZIF-8 increases, the amount of ZIF-8 on the sheet of Bi2MoO6 increases. When the molar ratio of Bi2MoO6 to 2-MI is 2:3, Bi2MoO6 is wrapped by ZIF-8 and the photocatalytic performance of Bi2MoO6/ZIF-8 will also be affected. Figure 4 shows more SEM images of Bi2MoO6/ZIF-8-3, which will further demonstrate that the proper amount of ZIF-8 were successfully combined on the Bi2MoO6 with an optimal morphology.

X-ray Photoelectron Spectroscopy (XPS) Analysis
The X-ray photoelectron spectroscopy spectrum demonstrates the chemical compositions and the oxidation state of pure Bi2MoO6 and Bi2MoO6/ZIF-8-3. From the full survey spectrum in Figure 5, it is obvious that a Bi, Mo, O, and C element exits in the sample of pure Bi2MoO6. In addition, when compared with pure Bi2MoO6, the composite of Bi2MoO6/ZIF-8 has two more elements, i.e., Zn and N. In Figure 6a, for the pure Bi2MoO6, the two peaks centered at 159.2 eV and 164.6 eV can be ascribed to the Bi 4f7/2 and Bi 4f5/2 of Bi 3+ [22]. In addition, the peaks of Bi2MoO6/ZIF-8 composite showed a tiny shift when compared with pure Bi2MoO6. In Figure 6b, the two peaks that are centered at 232.6 eV and 235.6 eV correspond to the Mo 3d5/2 and Mo 3d3/2 binding energies of Mo 6+ [23]. For the composites of Bi2MoO6/ZIF-8, the binding energy of Mo 3d5/2 and Mo 3d3/2 increase 0.4 eV. In Figure 6c, the two peaks that were centered at 1023.3 eV and 1046.2 eV were attributed to the Zn 2p3/2 and Zn 2p1/2 of Zn 2+ in the composites of Bi2MoO6/ZIF-8 [24]. These results showed that ZIF-8 has been successfully decorated in Bi2MoO6 spheres.

X-ray Photoelectron Spectroscopy (XPS) Analysis
The X-ray photoelectron spectroscopy spectrum demonstrates the chemical compositions and the oxidation state of pure Bi 2 MoO 6 and Bi 2 MoO 6 /ZIF-8-3. From the full survey spectrum in Figure 5, it is obvious that a Bi, Mo, O, and C element exits in the sample of pure Bi 2 MoO 6 . In addition, when compared with pure Bi 2 MoO 6 , the composite of Bi 2 MoO 6 /ZIF-8 has two more elements, i.e., Zn and N. In Figure 6a, for the pure Bi 2 MoO 6 , the two peaks centered at 159.2 eV and 164.6 eV can be ascribed to the Bi 4f 7/2 and Bi 4f 5/2 of Bi 3+ [22]. In addition, the peaks of Bi 2 MoO 6 /ZIF-8 composite showed a tiny shift when compared with pure Bi 2 MoO 6 . In Figure 6b, the two peaks that are centered at 232.6 eV and 235.6 eV correspond to the Mo 3d 5/2 and Mo 3d 3/2 binding energies of Mo 6+ [23]. For the composites of Bi 2 MoO 6 /ZIF-8, the binding energy of Mo 3d 5/2 and Mo 3d 3/2 increase 0.4 eV. In Figure 6c, the two peaks that were centered at 1023.3 eV and 1046.2 eV were attributed to the Zn 2p 3/2 and Zn 2p 1/2 of Zn 2+ in the composites of Bi 2 MoO 6 /ZIF-8 [24]. These results showed that ZIF-8 has been successfully decorated in Bi 2 MoO 6 spheres.

X-ray Photoelectron Spectroscopy (XPS) Analysis
The X-ray photoelectron spectroscopy spectrum demonstrates the chemical compositions and the oxidation state of pure Bi2MoO6 and Bi2MoO6/ZIF-8-3. From the full survey spectrum in Figure 5, it is obvious that a Bi, Mo, O, and C element exits in the sample of pure Bi2MoO6. In addition, when compared with pure Bi2MoO6, the composite of Bi2MoO6/ZIF-8 has two more elements, i.e., Zn and N. In Figure 6a, for the pure Bi2MoO6, the two peaks centered at 159.2 eV and 164.6 eV can be ascribed to the Bi 4f7/2 and Bi 4f5/2 of Bi 3+ [22]. In addition, the peaks of Bi2MoO6/ZIF-8 composite showed a tiny shift when compared with pure Bi2MoO6. In Figure 6b, the two peaks that are centered at 232.6 eV and 235.6 eV correspond to the Mo 3d5/2 and Mo 3d3/2 binding energies of Mo 6+ [23]. For the composites of Bi2MoO6/ZIF-8, the binding energy of Mo 3d5/2 and Mo 3d3/2 increase 0.4 eV. In Figure 6c, the two peaks that were centered at 1023.3 eV and 1046.2 eV were attributed to the Zn 2p3/2 and Zn 2p1/2 of Zn 2+ in the composites of Bi2MoO6/ZIF-8 [24]. These results showed that ZIF-8 has been successfully decorated in Bi2MoO6 spheres.

Photocatalytic Activities
As shown in Figure 7a, the degradation of MB under the visible light irradiation determined the photocatalytic degradation activities of different photocatalysts. The MB photodegradation rate of pure Bi2MoO6 is 47.89% and that of pure ZIF-8 is 34.05%. When the composite is Bi2MoO6/ZIF-8-3, the degradation of MB increased to 66.88%. From the time dependent UV-vis spectrum changes of MB catalyzed by Bi2MoO6/ZIF-8-3, as shown in Figure 7b, it is obvious that it has excellent adsorption performance and photocatalytic performance. In addition, the photocatalytic performance of the composite of Bi2MoO6/ZIF-8 is improved when compared with that of the pure Bi2MoO6. Therefore, ZIF-8 plays an important role in the degradation process. In the first three groups of composites (Bi2MoO6/ZIF-8- (1 to 3)), the photocatalytic performance of the composite gradually increases with the increase of photocatalysts content. Continuing to increase the content of ZIF-8 will reduce its performance, which is possibly due to the fact that the generation of exceeding ZIF-8 on the sheet of Bi2MoO6 inhibited its absorption of light. This phenomenon also corresponds to the results of the previous SEM. Accordingly, in order to understand the photocatalytic mechanism of these composites, a possible mechanism for the photocatalytic degradation of MB over Bi2MoO6/ZIF-8 composites is proposed. In the Bi2MoO6/ZIF-8 composites, the conduction band (CB) and valence band (VB) of Bi2MoO6 are −0.32 eV and 2.34 eV [25] and the CB and VB of ZIF-8 are −3.41 eV and 1.68 eV [24]. Under the irradiation of the light source, Bi2MoO6 is more prone to generating the photogenerated electronhole pair and the VB of Bi2MoO6 is more positive than that of ZIF-8, the photogenerated hole that is generated by Bi2MoO6 would transfer the VB of ZIF-8, which would promote the separation of electron-hole pairs and improve the photocatalytic performance of the composite.

Photocatalytic Activities
As shown in Figure 7a, the degradation of MB under the visible light irradiation determined the photocatalytic degradation activities of different photocatalysts. The MB photodegradation rate of pure Bi 2 MoO 6 is 47.89% and that of pure ZIF-8 is 34.05%. When the composite is Bi 2 MoO 6 /ZIF-8-3, the degradation of MB increased to 66.88%. From the time dependent UV-vis spectrum changes of MB catalyzed by Bi 2 MoO 6 /ZIF-8-3, as shown in Figure 7b, it is obvious that it has excellent adsorption performance and photocatalytic performance. In addition, the photocatalytic performance of the composite of Bi 2 MoO 6 /ZIF-8 is improved when compared with that of the pure Bi 2 MoO 6 . Therefore, ZIF-8 plays an important role in the degradation process. In the first three groups of composites (Bi 2 MoO 6 /ZIF-8-(1 to 3)), the photocatalytic performance of the composite gradually increases with the increase of photocatalysts content. Continuing to increase the content of ZIF-8 will reduce its performance, which is possibly due to the fact that the generation of exceeding ZIF-8 on the sheet of Bi 2 MoO 6 inhibited its absorption of light. This phenomenon also corresponds to the results of the previous SEM.

Photocatalytic Activities
As shown in Figure 7a, the degradation of MB under the visible light irradiation determined the photocatalytic degradation activities of different photocatalysts. The MB photodegradation rate of pure Bi2MoO6 is 47.89% and that of pure ZIF-8 is 34.05%. When the composite is Bi2MoO6/ZIF-8-3, the degradation of MB increased to 66.88%. From the time dependent UV-vis spectrum changes of MB catalyzed by Bi2MoO6/ZIF-8-3, as shown in Figure 7b, it is obvious that it has excellent adsorption performance and photocatalytic performance. In addition, the photocatalytic performance of the composite of Bi2MoO6/ZIF-8 is improved when compared with that of the pure Bi2MoO6. Therefore, ZIF-8 plays an important role in the degradation process. In the first three groups of composites (Bi2MoO6/ZIF-8-(1 to 3)), the photocatalytic performance of the composite gradually increases with the increase of photocatalysts content. Continuing to increase the content of ZIF-8 will reduce its performance, which is possibly due to the fact that the generation of exceeding ZIF-8 on the sheet of Bi2MoO6 inhibited its absorption of light. This phenomenon also corresponds to the results of the previous SEM. Accordingly, in order to understand the photocatalytic mechanism of these composites, a possible mechanism for the photocatalytic degradation of MB over Bi2MoO6/ZIF-8 composites is proposed. In the Bi2MoO6/ZIF-8 composites, the conduction band (CB) and valence band (VB) of Bi2MoO6 are −0.32 eV and 2.34 eV [25] and the CB and VB of ZIF-8 are −3.41 eV and 1.68 eV [24]. Under the irradiation of the light source, Bi2MoO6 is more prone to generating the photogenerated electronhole pair and the VB of Bi2MoO6 is more positive than that of ZIF-8, the photogenerated hole that is generated by Bi2MoO6 would transfer the VB of ZIF-8, which would promote the separation of electron-hole pairs and improve the photocatalytic performance of the composite. Accordingly, in order to understand the photocatalytic mechanism of these composites, a possible mechanism for the photocatalytic degradation of MB over Bi 2 MoO 6 /ZIF-8 composites is proposed. In the Bi 2 MoO 6 /ZIF-8 composites, the conduction band (CB) and valence band (VB) of Bi 2 MoO 6 are −0.32 eV and 2.34 eV [25] and the CB and VB of ZIF-8 are −3.41 eV and 1.68 eV [24]. Under the irradiation of the light source, Bi 2 MoO 6 is more prone to generating the photogenerated electron-hole pair and the VB of Bi 2 MoO 6 is more positive than that of ZIF-8, the photogenerated hole that is generated by Bi 2 MoO 6 would transfer the VB of ZIF-8, which would promote the separation of electron-hole pairs and improve the photocatalytic performance of the composite.

Conclusions
In summary, a series of Bi 2 MoO 6 /ZIF-8 composites with a different content of ZIF-8 have been successfully fabricated by a facile self-assembly method. The photocatalysts of Bi 2 MoO 6 /ZIF-8 have better performance than pure Bi 2 MoO 6 in the degradation of MB under the visible light. The successful composition of ZIF-8 and Bi 2 MoO 6 inhibited the recombination of a photogenerated electron-hole pair. When the molar ratio of Bi 2 MoO 6 to 2-MI is 3:3, the amount of ZIF-8 that is decorated on flower-like Bi 2 MoO 6 is suitable and the ability of photocatalysis would reach the maximum. This composing strategy of Bi 2 MoO 6 /ZIF-8 would provide a new route in the designing of highly efficient visible light photocatalysts.