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Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light
Journal Pre-proof Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light
BoYin Zhai, Ying Chen, YuNing Liang, YanHua Gao, JianLing Shi, HuiLi Zhang, Yuan Li PII:
S0254-0584(19)30874-0
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122078
Article Number:
122078
Reference:
MAC 122078
To appear in:
Materials Chemistry and Physics
Received Date:
15 April 2019
Accepted Date:
27 August 2019
Please cite this article as: BoYin Zhai, Ying Chen, YuNing Liang, YanHua Gao, JianLing Shi, HuiLi Zhang, Yuan Li, Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j. matchemphys.2019.122078
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Graphical Abstract
Journal Pre-proof Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light BoYin Zhaia, Ying Chena,*, YuNing Lianga, YanHua Gaoa, JianLing Shia, HuiLi Zhanga, Yuan Lia a Key Laboratory of Oil and Natural Gas Processing, School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China *Corresponding
author at: School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China (Y. Chen) Tel: 0459-6503786
E-mail addresses: [email protected] (Y. Chen)
Abstract: A novel composite photocatalyst Ag3VO4/ZIF-8 was prepared by a facile two-step method. It was characterized by XRD, SEM/EDS, TEM, EDX mapping, XPS, FT-IR, BET, UV-Vis DRS and PL, and its photocatalytic degradation activity of RhB was studied. The possible mechanism of photocatalytic degradation of RhB was put forward. The results illustrate that the specific surface area of a series of hybrid catalysts is larger than that of the single Ag3VO4 material, and the photocatalytic efficiency of the composite catalyst is much higher than that of pure Ag3VO4 and ZIF-8. Among them, the photocatalytic activity of 60%AZ composites is the highest, about 4.2 times of pure Ag3VO4 and 22.8 times of bare ZIF-8 respectively. At the same time, the Ag3VO4/ZIF-8 composite photocatalyst can maintain a stable photocatalytic activity and structure after five cycles. KEYWORD: Ag3VO4; Metal-organic framework; Photocatalytic activity; RhB degration
1. Introduction In recent years, the high discharge of dye wastewater and water pollution caused by toxic and refractory organic pollutants has become a serious global environmental problem[1]. Therefore, it is of great significance to control the pollution of organic dyes in water for the protection of environment and the maintenance of human health[2]. Many kinds of wastewater treatment methods, such as coagulation/flocculation, adsorption, photocatalysis, oxidation and other wastewater treatment methods have been reported[3]. However, to some extent, these have some defects in operation more or less. Fortunately, the conventional semiconductor has been widely concerned by the public and research groups owing to its advantages of high efficiency, easy operation, low energy consumption and non-secondary pollution[4-6]. However, it still has many disadvantages, such as the easy recombination of charges and the low utilization of solar energy, which hinders its practical application[7, 8]. Therefore, from the point of view of making full use of sunlight, to develop a novel catalytic agent with superior efficiency, long-lasting stability and visible light response is one of the current research hotspots[9, 10]. Metal-organic frameworks (MOFs) are novel and multifunctional crystal composed of *Corresponding
author at: School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China (Y. Chen) Tel: 0459-6503786 E-mail address: [email protected]
Journal Pre-proof organic linkers and metal ions that have garnered extensive attention from the public and researcher’s group[11]. It has several advantages of high porosity, large specific surface area, rich structure, adjustable composition and high crystallinity. With its rapid development, it has important applications in the fields of gas storage/separation[12], sensor[13], catalysis[14] and drug delivery[15]. Among the numerous sub-classes of MOFs, ZIF-8 is composed of Zn(II) ions and 2-methylimidazole, and has zeolite-like topologies structure with micropore and thermal/chemical stability[16-18]. Nevertheless, ZIF-8 with a large band gap (5.1 eV) could merely response in UV region, which curbs limit its application in photocatalysis[19]. In order to tackle this problem, many composites that semiconductor coupled with ZIF-8 have been developed, of which post-synthetic modification is the most widely used. Yuan etal used ZIF-8/g-C3N4 composites to enhance the photocatalytic performance to degrade organic pollutants owing to the special hybrid structures and effective abruption of the photon-generated carriers[20]. Zhang et al have synthesized MoO3@ZIF-8 composites for efficiently reducing Cr (VI) to Cr (Ⅲ), because this composite not merely possessed big specific surface area, but availably separated the photo-generated carriers[21]. For some common semiconductor with photoactivity, Ag3VO4 is ascribed to n-type and attracted focus due to its narrow band gap[22]. Ag3VO4 nanoparticles could be to degrade organic contaminant under the visible-light, while its disadvantages contain fast photogenerated carriers recombination rate and unstability[23]. In order to overcome these shortcomings, researchers have carried on much more modified experiments for pristine Ag3VO4. For instances, Wang et al is successfully prepared BiOCl/Ag3VO4 nanocomposite, which showed higher efficiency to remove the organic dye under visible light[24]. A novel Z-scheme g-C3N4/Ag/Ag3VO4 composites is developed by Wu et al to decompose Rhodamine B under visible light[25]. Herein, we were to utilize the advantage of MOFs and Semiconductor to report a novel Ag3VO4/ZIF-8 composite via facile two-step method, and the reaction route displayed in Scheme 1. The results showed that the hybrid material has expanded optical absorption and photoactivity, which were attributed to Ag3VO4 and ZIF-8 form the closely contacted interfaces and produce a synergistic effect. The recombination rate of photo-generated charges and catalytic efficiency of Ag3VO4 was reduced and enhance respectively. What’s more, the adsorption equilibrium of dye molecules around the catalyst can be broken when the introduction of ZIF-8, which can accelerate the degradation rate. The ZIF-8 with large specific surface area can supply a big touch area for the valid hybrid of the two materials, which were beneficial to the formation of the composites. In addition, the photodegradation experiments showed that the photocatalytic activity of a series of Ag3VO4/ZIF-8 materials was higher than that of ZIF-8 and Ag3VO4. In particular, 60% AZ have the best photocatalytic activity. At the same time, the possible mechanism of RhB degradation by Ag3VO4/ZIF-8 composites was proposed.
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Scheme 1. Ideal composite route of Ag3VO4/ZIF-8 composite
2. Experimental 2.1.
Chemicals
All chemicals in this work were commercially available. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 2-methylimidazole (C4H6N2), methanol (MeOH), Silver nitrate (AgNO3), Sodium vanadium (Na3VO4·12H2O), ethanol (C2H5O).
2.2.
Materials synthesis
The pure ZIF-8 was prepared accorded to the previous work[26]. Typically, 1mmol Zn(NO3)2·6H2O was dissolved in methanol (15 mL). Next, 15 mL methanol (containing 8 mmol 2-methylimidazole) was added slowly into the above solution. Following 120 min of stirring at room temperature, the white precipitate was appeared in the bottom of beaker, and collected it via centrifugation and clean with methanol for several times. Finally, white products were dried at 75 °C for 12 hours. The Ag3VO4/ZIF-8 composite was prepared with facile synthesis method. In the first place, several amount ZIF-8 was dispersed in H2O (20 mL) used the ultrasonication and agitation. Next, introduce 0.2548 g AgNO3 into the above solution and stir for 30 min, and this mix solution denoted as A. 20 mL H2O (containing 0.0919 g Na3VO4) was slowly dropped into A and stirred for 7h in the dark. In the end, the product were collected and cleaned with H2O and ethanol and dried at 60 °C. Ag3VO4/ZIF-8 hybrid with diverse weight ratios were developed respectively, which expressed as 0, 20, 40, 60 and 80% AZ.
2.3.
Material Characterization
Journal Pre-proof The microstructure of samples was observed at 20 kV, and the grain shape was analyzed and recorded with the help of a Field Emission Scanning Electron Microscope (Japan Hitachi S-3400) equipped with an energy dispersive X-ray spectrometer (EDS) (SEM/EDS) and transmission electronic microscope (TEM, JEOL JEM-2100). The X-ray diffraction (XRD) patterns was operated by Bruker D8 advance, and the radiation source is Cu Kα line (λ = 1.5406 Å) with the 5 (°)/min scan rate over the range of 5°-80° (2θ). The surface electronic state was measured by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, UK). Fourier transform infrared spectroscope (FT-IR) was used to test functional groups in the composites and adopted potassium bromide tablet method. N2 adsorption-desorption curve and pore size analysis of (BET) was measured by American NOVA2000e type specific surface area and porosity analyzer. The diffuse reflectance spectra (UV-Vis DRS) were characterized by UV-4100 spectrometer. A fluorospectrophotometer (Perkin Elmer LS-55), with an excitation wavelength of 370nm, was used to test the photoluminescence (PL) spectra.
2.4.
Photocatalytic experiments
In visible light activity test, 50 mg photocatalyst was dispersed in 50 mL 10 mg/L RhB aqueous. First of all, the suspension needs to undergo dark adsorption for 0.5 h to ensure the dark adsorption equilibrium. With 300 W xenon lamp and 420 nm cutting filter, the radiation below 420 nm is completely shielded. After irradiation, 1mL solution was collected periodically and centrifuged before the analysis. The residual concentration was surveyed by UV-Vis spectrophotometer at 553 nm with the different irradiation time.
3. 3.1.
RESULTS AND DISCUSSION XRD analyses
The XRD patterns of ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 composite are shown in Fig. 1. From Fig. 1a, we observed that the characteristic peaks of ZIF-8 were resembled dramatically to the simulated ZIF-8 phase structure, which indicated the sample comprised a simplex ZIF-8 phase. Fig. 1b displays the XRD patterns of Ag3VO4/ZIF-8 with various ZIF-8 amounts. It clearly showed that the Ag3VO4 particle was corresponding to the monoclinic structure (JCPDS No. 43-0542). The peaks at 2θ = 19.20º, 30.86º, 32.33º, 34.15º, 35.06º, 35.94º, 38.92º, 41.38º, 48.27º, 51.13ºand 54.06º can be assigned to (011), (-121), (121), (220), (301), (202), (022), (400), (-322), (132), and (331) crystal planes of monoclinic Ag3VO4, respectively[27]. For pure ZIF-8, the diffraction peaks fitted very well with the literature reported previously[16]. The XRD peak at 7.352º, 10.40º,12.75º,14.73º,16.48º,18.07º, 22.17º, 24.55º and 26.73º can be indexed as the (011), (002), (112), (022), (013), (222), (114), (233) and ((134) diffraction planes. Meantime, the XRD pattern of Ag3VO4/ZIF-8 hybrid materials exhibited peaks corresponding to the bare Ag3VO4 and ZIF-8, indicating the Ag3VO4/ZIF-8 was successful formation. Additionally, for Ag3VO4/ZIF-8, the intensity of the ZIF-8 peaks is gradually growing stronger, which is attributed to increase the amount of ZIF-8 in Ag3VO4/ZIF-8 composites.
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Fig. 1. XRD patterns of ZIF-8(a), Ag3VO4 and Ag3VO4/ZIF-8(b) composite material
3.2.
Morphology analysis
According to the SEM images, Fig. 2 shows the picture of ZIF-8, Ag3VO4 and 60%AZ samples. In Fig.2a-b, the average size of ZIF-8 was mainly centered on the scope of 80-90 nm in diameter. Meantime, they have an irregular polyhedron morphological feature, whereas Ag3VO4 shows the pebble-like shape morphology. When Ag3VO4 is in situ-grown on the surface and around of the ZIF-8, Fig. 2c-d were obviously displayed the ZIF-8 and Ag3VO4 nicely anchor together. Energy Dispersive Spectroscopic (EDS) analysis was performed for elemental composition of nanocomposites. Fig. 2e shows the EDS spectrum of Ag3VO4/ZIF-8 composite, which clearly makes known the existence of Ag, V, Zn, C, O and N elements in the composite, and it is identical with the component elements in the composites. Combined with the XRD pattern, these could be concluded that Ag3VO4/ZIF-8 composites were successfully prepared. The TEM and elemental mapping images of 60%AZ composite are displayed in Fig. 3. Fig. 3a-b indicated that irregular pebble-like particles (Ag3VO4) and ZIF-8 nanoparticles mixed with each other. This result is corresponds with that of SEM observations. From elemental mapping images (Fig. 3c-h), it is obviously seen that the Ag3VO4 and ZIF-8 firmly anchored and ZIF-8 evenly distributed in Ag3VO4 particles surface and surroundings.
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Fig. 2. SEM images of ZIF-8 (a), Ag3VO4 (b), 60%AZ (c-d) and EDS spectra of 60%AZ (e)
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Fig. 3. TEM image of 60%AZ (a-b) and elemental mapping patterns of C (c), O (d), Ag (e), V (f), Zn (g), N (h)
3.3.
FT-IR analysis
From the FT-IR spectra in Fig. 4, for pure ZIF-8(Fig. 4f), the characteristic Zn-N stretching and C=N stretching vibration of imidazole mode in ZIF-8 appears at 421 cm-1 and 1528 cm-1. The peaks of C-N stretching frequency were displayed at 1150 and 1350 cm-1[28]. Pristine Ag3VO4 particle display peaks at 712 cm-1 and 850 cm-1 in Fig. 3a, which could liken to Ag-V and V-O bond vibrations[29]. Comparing the spectrogram of pure ZIF-8 and Ag3VO4/ZIF-8 composites (Fig. 4b-e), we can observe that Ag-V and V-O absorption bands emerged in the composites. Moreover, the intensity of these peaks will reduce as the rise of ZIF-8 content. Particularly, the locations of the characteristic peaks of Ag3VO4 were slight red shift because of the interaction between the ZIF-8 and Ag3VO4[30].
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Fig. 4. FT-IR spectra of Ag3VO4(a), 20%AZ(b), 40%AZ(c), 60%AZ(d), 80%AZ(e) and ZIF-8 (f) materials
3.4.
XPS analysis
The surface chemical composition of Ag3VO4/ZIF-8 hybrid material was measured by XPS. XPS survey spectra and high-resolution XPS spectra of Zn 2p, Ag 3d, O 1s, N 1s, V 2p and C 1s regions for 60%AZ have been displayed in Fig. 5. Fig. 5a exhibits the existence of Zn, Ag, O, N, V and C elements, and two small signals near 500.0 eV of 60%AZ are attributed to the Zn LMM Auger line[31-33]. The Zn 2p spectrum in Fig. 5b shows two well-defined peaks at 1044.5 eV and 1021.6 eV, which ascribed to Zn 2p1/2 and Zn 2p3/2 and belonged to Zn2+[34]. In Fig. 5c, Ag 3d spectrum emerges two peaks at 374.7 eV and 367.7 eV are attributed to Ag 3d3/2 and Ag 3d5/2 of Ag+ in 60% AZ, respectively[32]. The O 1s spectrum in Fig. 5d appears three peaks. Among them, peak at 529.4 eV can be matched with the Zn-O bond, indicating the interaction between ZIF-8 and Ag3VO4[35]. Peaks at 530.5 eV and 531.2 eV can be assigned to lattice oxygen and V-O in the crystalline Ag3VO4[32]. The high-resolution XPS spectra of N 1s located at 398.6 eV, corresponding to N-C bond or N=C bond from 2-methylimidazole in ZIF-8[35]. In V 2p spectrum (Fig. 5f), two peaks at 523.9 eV and 516.5 eV could be corresponding to V2p5/2 and V2p3/2 characteristic of V5+ in Ag3VO4. Additionally, the C 1s spectrum is split into three characteristic peaks, which attributed to C-C (284.2 eV), C-N (285.3 eV) from imidazole ring in ZIF-8[35]. Combined the FT-IR spectra, the XPS analysis indicates that Ag3VO4/ZF-8 composites have been synthesized successfully.
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Journal Pre-proof Fig. 5. XPS spectra of 60%AZ: (a) survey, (b) Zn 2p, (c) Ag 3d, (d) O 1s, (e) N 1s, (f) V 2p and (g) C 1s
3.5.
N2 absorption-desorption analysis
Fig. 6 illustrates the N2 adsorption-desorption curve together with the homologous pore-size distribution. The pure ZIF-8 exhibited the type I isotherm and the type H4 shape hysteresis loop, which indicating the presence of microporous structure. Meanwhile, ZIF-8 has a higher specific surface area (1175.462 m2/g), which is a good adsorbent and can provide more active sites during the reaction[36]. Additionally, the isotherms and the hysteresis loop of Ag3VO4 and 60%AZ present the type IV and H3 shape, implying these samples are mesoporous structure. The specific surface areas of Ag3VO4 and 60%AZ were 8.193 and 25.705 m2/g, separately (see detailed in Table 1). The specific surface area of 60%AZ is higher than pure Ag3VO4, which results from the introduction of ZIF-8 particle in the hybrids. Meantime, the big specific surface area will supply numerous active sites for reactant to promote photocatalytic reaction process[37].
Fig. 6. N2 adsorption-desorption isotherms curves(a) and pore-size distribution curves of ZIF-8, Ag3VO4 and 60%AZ composite (b)
Table 1 Specific surface area (SBET), pore size and pore volume of the pure Ag3VO4, ZIF-8 and 60%AZ sample
3.6.
Sample
Surface area /m2·g-1
Pore volume/cm3·g-1
pore size /nm
ZIF-8
1175.462
0.946
1.63
Ag3VO4
8.193
0.065
2.75
60%AZ
25.705
0.468
2.23
Analysis of Optical Properties Based on UV-Vis DRS
Fig. 7 shows the UV-vis DRS of catalyst, and in the light of αℎν = k(ℎ𝜈 ― 𝐸𝑔)1/𝑛 to calculate the band gap, Where α, h, ν, Eg and k refer to absorption coefficients, Plank constant, light frequency, band gap and a constant[38]. In this system, n value is equal to 2. The band gap of Ag3VO4, 20%AZ, 40%AZ, 60%AZ, 80%AZ and ZIF-8 was equivalent to 2.0, 2.11, 2.21, 2.25, 2.83 and 5.06 eV, respectively. Additionally, it is worth noting that the Eg value abated and the absorption edge of the AZ composite is gradually red-shifted when the Ag3VO4 content increased. This showed that the addition of Ag3VO4 particles not merely expands the light capturing range of
Journal Pre-proof ZIF-8, but also conduced to extend the electron transfer distance according to the metal-to-ligand charge transfer mechanism[39]. Hence, the range of light response of Ag3VO4/ZIF-8 nanocomposites was expanded, which can availably heighten the photocatalytic activity of the single Ag3VO4.
Fig. 7. UV-Vis diffuse reflectance spectra of ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 composite material (a) and relationships between (αhν)2 and photo energy (hν) (b)
3.7.
PL analysis
PL spectrum can be used to analyze the recombination situation of photo-generated carriers in the fluorescence intensity. The weak or strong emission intensity of the PL spectrum indicates that the degree of photo-generated carrier recombination is low or high. Fig. 8 shows the PL spectrum analysis of different samples. It displayed that the fluorescence intensity order at the same excitation wavelength are as follows: ZIF-8>Ag3VO4>60%AZ, which indicates that the 60%AZ composite can effectively promote the electron-holes transfer, prolong the lifetime of photon-generated carrier and improve the photocatalytic activity.
Fig. 8. PL spectra of ZIF-8, Ag3VO4, Ag3VO4/ZIF-8 samples
3.8.
Evaluation of photocatalytic activity based on RhB degradation
The photoactivity of pure ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 samples was measured by degrading dye solutions under visible light. As depicted in Fig. 9a, this solution needs to undergo dark adsorption for 0.5 h to ensure the dark adsorption equilibrium. Owing to the nanopores
Journal Pre-proof existed in ZIF-8 that similar to zeolite sodalite structure, we can be obviously seen that Ag3VO4/ZIF-8 have adsorption phenomenon, which caused by the nanopores connected to through a 0.34 nm windows[40]. Additionally, we can see that the amount of ZIF-8 increased from 0% to 60%, 60%AZ exhibited the highest photocatalytic activity. However, comparing with the 60%AZ, a slight drop of the photocatalytic activity occurred when the ZIF-8 content was increased to 80%, which might owning to the agglomeration of ZIF-8 curbed the visible-light penetration to Ag3VO4 and cutting down the charge transfer rate[20]. Hence, we could be inferred that the introduction of the proper dosage of ZIF-8 availably improved the degradation ability of Ag3VO4/ZIF-8 composites. The possible cause is a synergistic effect of these two substances, further enhancing separation of the photogenerated carriers. The photocatalytic activity of 60%AZ photocatalyst is measured by monitoring the successive change of Rh B residue concentration at 554 nm with the change of reaction time. Fig. 9b shows gradual reduction in the characteristic absorbance (λ max=554 nm), which corresponds to the dye concentration in solution. Moreover, 60%AZ sample shows higher removal rate of Rh B in 100 min under visible light irradiation. As shown in Fig. 9c, k value of 60%AZ was calculated to 0.01707 min-1, which is the maximal value among all catalysts, where k is the pseudo-first-order rate constant by calculating the kinetic rate constant from ln(C0/Ct) vs. t[41]. The k value of pristine ZIF-8 (0.000749 min-1), Ag3VO4 (0.0040 min-1), 20%AZ (0.00462min-1), 40%AZ (0.00677min-1) and 80%AZ (0.01254 min-1) are less than 60%AZ composite. The reason is that 60%AZ composites may have the best content ratio of Ag3VO4 and ZIF-8, and the specific surface area is relatively larger compared with Ag3VO4. At the same time, the existence of the compact interface of the composite greatly reduces the electron-hole recombination rate and thus improves the photodegradation rate.
Journal Pre-proof Fig. 9. Photocatalytic degradation of Rh B over different photocatalysts under visible light irradiation (a), time-dependent degradation spectral pattern of Rh B solution over 60% AZ (b) and the corresponding apparent rate constants for the degradation of Rh B over different photocatalysts (c)
3.9.
Regeneration and reusability
The stability and recyclability of photocatalysts are important parameters in the process of application. In order to explore the recyclability of photocatalyst, under the same conditions, the Ag3VO4/ZIF-8 composite was measured to degrade dye solution up to five cycles (Fig.10). Before each cycle, the catalyst was gathered via centrifugation, and washed and dried for the next run. It is obviously displayed that the Ag3VO4/ZIF-8 composite exhibits decreased trend after the each cycle in photodegradation efficiency, as compared with that of the first cycle. A small decrease in the photocatalytic activity was probably caused by the mass loss during the separation and washing process or the active site of catalyst was occupied [42, 43]. Nevertheless, it still maintains the photocatalytic efficiency of Ag3VO4/ZIF-8 composite at a high level after five cycles. Generally speaking, the reusability results illustrated that the Ag3VO4/ZIF-8 composite has excellent stability and recycling performance.
Fig. 10. The cycling runs of the degradation of RhB over 60%AZ
3.10.
Mechanism of enhanced photocatalysis
TEOA, IPA, BQ were served as scavengers of active species (such as h+, ·OH and ·O2-) to explore the photocatalytic mechanism of Ag3VO4/ZIF-8 composite, due to these scavengers can react with the produced active species and curb the photo-degradation efficiency. Fig.11 illustrates a series of active species trapping experiments by introducing these scavengers. When TEOA and BQ were added, the photodegradation rate was remarkably decreased, while the influence of adding IPA was slight. To sum up, h+ and ·O2- ions play the critical role in the photocatalytic degradation process.
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Fig. 11. The effects of various scavengers on the visible light photocatalytic acitivty of 60%AZ
According to the above PL spectra, UV–Vis DRS spectra, active species trapping experiment and related literature, the possible photodegradation mechanism of RhB by Ag3VO4/ZIF-8 composites is shown in Scheme 2. Unlike other semiconductors, for metal-organic frameworks, the transition after photo-excitations primarily relates to the charge transfer transition of metal to-ligand (MLCT). Combined UV-Vis DRS and PL analysis, when Ag3VO4 is coupled with ZIF-8, it can be suppress the rapid electron-hole recombination rate of pure photocatalyst, which is beneficial to photocatalysis[44].Therefore, visible light cannot be excite ZIF-8 owing to the wide band gap but that is likely to excite the electrons of Ag3VO4[45, 46]. The electron at the valence band level of Ag3VO4 can be excite to a higher level orbit under visible light excitation of E < 2.95 eV (λ > 420nm), which leads to form h+ and e−. When Ag3VO4 nanoparticles and ZIF-8 form the dense and firm interface area, the electrons located at the CB of Ag3VO4 particles were forced to migrate to the methylimidazole ring by the metal to-ligand charge transfer mechanism (MLCT)[20]. The lowered electron density from methylimidazole ring and the electrons migration from Ag3VO4 restricted the recombination of photo-induced carrier. RhB molecule also was triggered to produce electrons, which will deliver to Ag3VO4 and ZIF-8[47, 48]. Photo-generated electrons in ZIF-8 can reduce O2 absorbed on the surface of photocatalyst to ·O2−[20]. Meantime, ZIF-8 also served as adsorbent due to the large specific surface area, which can capture the dye molecule, and ·O2- produced by photocatalyst furtherly react with RhB/RhB* to breakdown of the dye molecular structures. What’s more, h+ of Ag3VO4 could directly oxidize the dye molecules. Therefore, the following equations can illustrate this degradation process: RhB + hv→RhB∗ → RhB+• + e− Ag3VO4+hv →e− (Ag3VO4+RhB) + h+ (Ag3VO4) ZIF-8 + e− (Ag3VO4+RhB) +O2→ ZIF-8(e-) +·O2·O2−/ h+ + RhB+•/RhB→less organic matter→CO2+H2O
(1) (2) (3) (4)
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Scheme 2. The possible routes of photoelectron generation and transport in the 60%AZ composite
4. CONCLUSIONS A series of novel Ag3VO4/ZIF-8 composites with different ZIF-8 contents were prepared by a simple hydrothermal method. Among them, the photodegradation rate of RhB in 60%AZ composites was 0.01707min-1, which is about 4.2 and 22.7 times higher than pristine Ag3VO4 and ZIF-8. That can be attributed to the following reasons: first: the indirect dye photosensitization and the broke dye adsorption equilibrium could to promote the separation of the charges between Ag3VO4 and ZIF-8, second: the formation of a close interface between Ag3VO4 and ZIF-8 and produce a synergistic effect. Therefore, it is reasonable that this composite has good optical properties and photocatalytic activity. The free radicals trapping experiments showed h+ and •O2have the critical role in the photocatalytic degradation process. The cyclic experiments are exhibited that the Ag3VO4/ZIF-8 has excellent stability and recycling performance. We hope that this work lays a foundation for the development of high efficiency semiconductor/MOFs composites, in order to treat the real-time wastewater and protect the environment.
Acknowledgement We wish to express our gratitude to the National Natural Science Foundation of China (grant number 51146008).
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Journal Pre-proof Research Highlights （1） Ag3VO4/ ZIF-8 nanoparticles were acted as an effective visible-light-driven photocatalysts. （2） A novel photocatalyst (Ag3VO4/ ZIF-8) exhibits enhanced visible photocatalytic activity compared with pure Ag3VO4 and ZIF-8. （3） The mechanism of the enhanced photocatalytic performance was discussed in detail.
BoYin Zhai, Ying Chen, YuNing Liang, YanHua Gao, JianLing Shi, HuiLi Zhang, Yuan Li PII:
S0254-0584(19)30874-0
DOI:
https://doi.org/10.1016/j.matchemphys.2019.122078
Article Number:
122078
Reference:
MAC 122078
To appear in:
Materials Chemistry and Physics
Received Date:
15 April 2019
Accepted Date:
27 August 2019
Please cite this article as: BoYin Zhai, Ying Chen, YuNing Liang, YanHua Gao, JianLing Shi, HuiLi Zhang, Yuan Li, Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light, Materials Chemistry and Physics (2019), https://doi.org/10.1016/j. matchemphys.2019.122078
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Graphical Abstract
Journal Pre-proof Modifying Ag3VO4 with metal-organic frameworks for enhanced photocatalytic activity under visible light BoYin Zhaia, Ying Chena,*, YuNing Lianga, YanHua Gaoa, JianLing Shia, HuiLi Zhanga, Yuan Lia a Key Laboratory of Oil and Natural Gas Processing, School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China *Corresponding
author at: School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China (Y. Chen) Tel: 0459-6503786
E-mail addresses: [email protected] (Y. Chen)
Abstract: A novel composite photocatalyst Ag3VO4/ZIF-8 was prepared by a facile two-step method. It was characterized by XRD, SEM/EDS, TEM, EDX mapping, XPS, FT-IR, BET, UV-Vis DRS and PL, and its photocatalytic degradation activity of RhB was studied. The possible mechanism of photocatalytic degradation of RhB was put forward. The results illustrate that the specific surface area of a series of hybrid catalysts is larger than that of the single Ag3VO4 material, and the photocatalytic efficiency of the composite catalyst is much higher than that of pure Ag3VO4 and ZIF-8. Among them, the photocatalytic activity of 60%AZ composites is the highest, about 4.2 times of pure Ag3VO4 and 22.8 times of bare ZIF-8 respectively. At the same time, the Ag3VO4/ZIF-8 composite photocatalyst can maintain a stable photocatalytic activity and structure after five cycles. KEYWORD: Ag3VO4; Metal-organic framework; Photocatalytic activity; RhB degration
1. Introduction In recent years, the high discharge of dye wastewater and water pollution caused by toxic and refractory organic pollutants has become a serious global environmental problem[1]. Therefore, it is of great significance to control the pollution of organic dyes in water for the protection of environment and the maintenance of human health[2]. Many kinds of wastewater treatment methods, such as coagulation/flocculation, adsorption, photocatalysis, oxidation and other wastewater treatment methods have been reported[3]. However, to some extent, these have some defects in operation more or less. Fortunately, the conventional semiconductor has been widely concerned by the public and research groups owing to its advantages of high efficiency, easy operation, low energy consumption and non-secondary pollution[4-6]. However, it still has many disadvantages, such as the easy recombination of charges and the low utilization of solar energy, which hinders its practical application[7, 8]. Therefore, from the point of view of making full use of sunlight, to develop a novel catalytic agent with superior efficiency, long-lasting stability and visible light response is one of the current research hotspots[9, 10]. Metal-organic frameworks (MOFs) are novel and multifunctional crystal composed of *Corresponding
author at: School of Chemistry and Chemical Engineering, Northeast Petroleum University, DaQing, HeiLongJiang 163000, PR China (Y. Chen) Tel: 0459-6503786 E-mail address: [email protected]
Journal Pre-proof organic linkers and metal ions that have garnered extensive attention from the public and researcher’s group[11]. It has several advantages of high porosity, large specific surface area, rich structure, adjustable composition and high crystallinity. With its rapid development, it has important applications in the fields of gas storage/separation[12], sensor[13], catalysis[14] and drug delivery[15]. Among the numerous sub-classes of MOFs, ZIF-8 is composed of Zn(II) ions and 2-methylimidazole, and has zeolite-like topologies structure with micropore and thermal/chemical stability[16-18]. Nevertheless, ZIF-8 with a large band gap (5.1 eV) could merely response in UV region, which curbs limit its application in photocatalysis[19]. In order to tackle this problem, many composites that semiconductor coupled with ZIF-8 have been developed, of which post-synthetic modification is the most widely used. Yuan etal used ZIF-8/g-C3N4 composites to enhance the photocatalytic performance to degrade organic pollutants owing to the special hybrid structures and effective abruption of the photon-generated carriers[20]. Zhang et al have synthesized MoO3@ZIF-8 composites for efficiently reducing Cr (VI) to Cr (Ⅲ), because this composite not merely possessed big specific surface area, but availably separated the photo-generated carriers[21]. For some common semiconductor with photoactivity, Ag3VO4 is ascribed to n-type and attracted focus due to its narrow band gap[22]. Ag3VO4 nanoparticles could be to degrade organic contaminant under the visible-light, while its disadvantages contain fast photogenerated carriers recombination rate and unstability[23]. In order to overcome these shortcomings, researchers have carried on much more modified experiments for pristine Ag3VO4. For instances, Wang et al is successfully prepared BiOCl/Ag3VO4 nanocomposite, which showed higher efficiency to remove the organic dye under visible light[24]. A novel Z-scheme g-C3N4/Ag/Ag3VO4 composites is developed by Wu et al to decompose Rhodamine B under visible light[25]. Herein, we were to utilize the advantage of MOFs and Semiconductor to report a novel Ag3VO4/ZIF-8 composite via facile two-step method, and the reaction route displayed in Scheme 1. The results showed that the hybrid material has expanded optical absorption and photoactivity, which were attributed to Ag3VO4 and ZIF-8 form the closely contacted interfaces and produce a synergistic effect. The recombination rate of photo-generated charges and catalytic efficiency of Ag3VO4 was reduced and enhance respectively. What’s more, the adsorption equilibrium of dye molecules around the catalyst can be broken when the introduction of ZIF-8, which can accelerate the degradation rate. The ZIF-8 with large specific surface area can supply a big touch area for the valid hybrid of the two materials, which were beneficial to the formation of the composites. In addition, the photodegradation experiments showed that the photocatalytic activity of a series of Ag3VO4/ZIF-8 materials was higher than that of ZIF-8 and Ag3VO4. In particular, 60% AZ have the best photocatalytic activity. At the same time, the possible mechanism of RhB degradation by Ag3VO4/ZIF-8 composites was proposed.
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Scheme 1. Ideal composite route of Ag3VO4/ZIF-8 composite
2. Experimental 2.1.
Chemicals
All chemicals in this work were commercially available. Zinc nitrate hexahydrate (Zn(NO3)2·6H2O), 2-methylimidazole (C4H6N2), methanol (MeOH), Silver nitrate (AgNO3), Sodium vanadium (Na3VO4·12H2O), ethanol (C2H5O).
2.2.
Materials synthesis
The pure ZIF-8 was prepared accorded to the previous work[26]. Typically, 1mmol Zn(NO3)2·6H2O was dissolved in methanol (15 mL). Next, 15 mL methanol (containing 8 mmol 2-methylimidazole) was added slowly into the above solution. Following 120 min of stirring at room temperature, the white precipitate was appeared in the bottom of beaker, and collected it via centrifugation and clean with methanol for several times. Finally, white products were dried at 75 °C for 12 hours. The Ag3VO4/ZIF-8 composite was prepared with facile synthesis method. In the first place, several amount ZIF-8 was dispersed in H2O (20 mL) used the ultrasonication and agitation. Next, introduce 0.2548 g AgNO3 into the above solution and stir for 30 min, and this mix solution denoted as A. 20 mL H2O (containing 0.0919 g Na3VO4) was slowly dropped into A and stirred for 7h in the dark. In the end, the product were collected and cleaned with H2O and ethanol and dried at 60 °C. Ag3VO4/ZIF-8 hybrid with diverse weight ratios were developed respectively, which expressed as 0, 20, 40, 60 and 80% AZ.
2.3.
Material Characterization
Journal Pre-proof The microstructure of samples was observed at 20 kV, and the grain shape was analyzed and recorded with the help of a Field Emission Scanning Electron Microscope (Japan Hitachi S-3400) equipped with an energy dispersive X-ray spectrometer (EDS) (SEM/EDS) and transmission electronic microscope (TEM, JEOL JEM-2100). The X-ray diffraction (XRD) patterns was operated by Bruker D8 advance, and the radiation source is Cu Kα line (λ = 1.5406 Å) with the 5 (°)/min scan rate over the range of 5°-80° (2θ). The surface electronic state was measured by X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, UK). Fourier transform infrared spectroscope (FT-IR) was used to test functional groups in the composites and adopted potassium bromide tablet method. N2 adsorption-desorption curve and pore size analysis of (BET) was measured by American NOVA2000e type specific surface area and porosity analyzer. The diffuse reflectance spectra (UV-Vis DRS) were characterized by UV-4100 spectrometer. A fluorospectrophotometer (Perkin Elmer LS-55), with an excitation wavelength of 370nm, was used to test the photoluminescence (PL) spectra.
2.4.
Photocatalytic experiments
In visible light activity test, 50 mg photocatalyst was dispersed in 50 mL 10 mg/L RhB aqueous. First of all, the suspension needs to undergo dark adsorption for 0.5 h to ensure the dark adsorption equilibrium. With 300 W xenon lamp and 420 nm cutting filter, the radiation below 420 nm is completely shielded. After irradiation, 1mL solution was collected periodically and centrifuged before the analysis. The residual concentration was surveyed by UV-Vis spectrophotometer at 553 nm with the different irradiation time.
3. 3.1.
RESULTS AND DISCUSSION XRD analyses
The XRD patterns of ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 composite are shown in Fig. 1. From Fig. 1a, we observed that the characteristic peaks of ZIF-8 were resembled dramatically to the simulated ZIF-8 phase structure, which indicated the sample comprised a simplex ZIF-8 phase. Fig. 1b displays the XRD patterns of Ag3VO4/ZIF-8 with various ZIF-8 amounts. It clearly showed that the Ag3VO4 particle was corresponding to the monoclinic structure (JCPDS No. 43-0542). The peaks at 2θ = 19.20º, 30.86º, 32.33º, 34.15º, 35.06º, 35.94º, 38.92º, 41.38º, 48.27º, 51.13ºand 54.06º can be assigned to (011), (-121), (121), (220), (301), (202), (022), (400), (-322), (132), and (331) crystal planes of monoclinic Ag3VO4, respectively[27]. For pure ZIF-8, the diffraction peaks fitted very well with the literature reported previously[16]. The XRD peak at 7.352º, 10.40º,12.75º,14.73º,16.48º,18.07º, 22.17º, 24.55º and 26.73º can be indexed as the (011), (002), (112), (022), (013), (222), (114), (233) and ((134) diffraction planes. Meantime, the XRD pattern of Ag3VO4/ZIF-8 hybrid materials exhibited peaks corresponding to the bare Ag3VO4 and ZIF-8, indicating the Ag3VO4/ZIF-8 was successful formation. Additionally, for Ag3VO4/ZIF-8, the intensity of the ZIF-8 peaks is gradually growing stronger, which is attributed to increase the amount of ZIF-8 in Ag3VO4/ZIF-8 composites.
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Fig. 1. XRD patterns of ZIF-8(a), Ag3VO4 and Ag3VO4/ZIF-8(b) composite material
3.2.
Morphology analysis
According to the SEM images, Fig. 2 shows the picture of ZIF-8, Ag3VO4 and 60%AZ samples. In Fig.2a-b, the average size of ZIF-8 was mainly centered on the scope of 80-90 nm in diameter. Meantime, they have an irregular polyhedron morphological feature, whereas Ag3VO4 shows the pebble-like shape morphology. When Ag3VO4 is in situ-grown on the surface and around of the ZIF-8, Fig. 2c-d were obviously displayed the ZIF-8 and Ag3VO4 nicely anchor together. Energy Dispersive Spectroscopic (EDS) analysis was performed for elemental composition of nanocomposites. Fig. 2e shows the EDS spectrum of Ag3VO4/ZIF-8 composite, which clearly makes known the existence of Ag, V, Zn, C, O and N elements in the composite, and it is identical with the component elements in the composites. Combined with the XRD pattern, these could be concluded that Ag3VO4/ZIF-8 composites were successfully prepared. The TEM and elemental mapping images of 60%AZ composite are displayed in Fig. 3. Fig. 3a-b indicated that irregular pebble-like particles (Ag3VO4) and ZIF-8 nanoparticles mixed with each other. This result is corresponds with that of SEM observations. From elemental mapping images (Fig. 3c-h), it is obviously seen that the Ag3VO4 and ZIF-8 firmly anchored and ZIF-8 evenly distributed in Ag3VO4 particles surface and surroundings.
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Fig. 2. SEM images of ZIF-8 (a), Ag3VO4 (b), 60%AZ (c-d) and EDS spectra of 60%AZ (e)
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Fig. 3. TEM image of 60%AZ (a-b) and elemental mapping patterns of C (c), O (d), Ag (e), V (f), Zn (g), N (h)
3.3.
FT-IR analysis
From the FT-IR spectra in Fig. 4, for pure ZIF-8(Fig. 4f), the characteristic Zn-N stretching and C=N stretching vibration of imidazole mode in ZIF-8 appears at 421 cm-1 and 1528 cm-1. The peaks of C-N stretching frequency were displayed at 1150 and 1350 cm-1[28]. Pristine Ag3VO4 particle display peaks at 712 cm-1 and 850 cm-1 in Fig. 3a, which could liken to Ag-V and V-O bond vibrations[29]. Comparing the spectrogram of pure ZIF-8 and Ag3VO4/ZIF-8 composites (Fig. 4b-e), we can observe that Ag-V and V-O absorption bands emerged in the composites. Moreover, the intensity of these peaks will reduce as the rise of ZIF-8 content. Particularly, the locations of the characteristic peaks of Ag3VO4 were slight red shift because of the interaction between the ZIF-8 and Ag3VO4[30].
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Fig. 4. FT-IR spectra of Ag3VO4(a), 20%AZ(b), 40%AZ(c), 60%AZ(d), 80%AZ(e) and ZIF-8 (f) materials
3.4.
XPS analysis
The surface chemical composition of Ag3VO4/ZIF-8 hybrid material was measured by XPS. XPS survey spectra and high-resolution XPS spectra of Zn 2p, Ag 3d, O 1s, N 1s, V 2p and C 1s regions for 60%AZ have been displayed in Fig. 5. Fig. 5a exhibits the existence of Zn, Ag, O, N, V and C elements, and two small signals near 500.0 eV of 60%AZ are attributed to the Zn LMM Auger line[31-33]. The Zn 2p spectrum in Fig. 5b shows two well-defined peaks at 1044.5 eV and 1021.6 eV, which ascribed to Zn 2p1/2 and Zn 2p3/2 and belonged to Zn2+[34]. In Fig. 5c, Ag 3d spectrum emerges two peaks at 374.7 eV and 367.7 eV are attributed to Ag 3d3/2 and Ag 3d5/2 of Ag+ in 60% AZ, respectively[32]. The O 1s spectrum in Fig. 5d appears three peaks. Among them, peak at 529.4 eV can be matched with the Zn-O bond, indicating the interaction between ZIF-8 and Ag3VO4[35]. Peaks at 530.5 eV and 531.2 eV can be assigned to lattice oxygen and V-O in the crystalline Ag3VO4[32]. The high-resolution XPS spectra of N 1s located at 398.6 eV, corresponding to N-C bond or N=C bond from 2-methylimidazole in ZIF-8[35]. In V 2p spectrum (Fig. 5f), two peaks at 523.9 eV and 516.5 eV could be corresponding to V2p5/2 and V2p3/2 characteristic of V5+ in Ag3VO4. Additionally, the C 1s spectrum is split into three characteristic peaks, which attributed to C-C (284.2 eV), C-N (285.3 eV) from imidazole ring in ZIF-8[35]. Combined the FT-IR spectra, the XPS analysis indicates that Ag3VO4/ZF-8 composites have been synthesized successfully.
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Journal Pre-proof Fig. 5. XPS spectra of 60%AZ: (a) survey, (b) Zn 2p, (c) Ag 3d, (d) O 1s, (e) N 1s, (f) V 2p and (g) C 1s
3.5.
N2 absorption-desorption analysis
Fig. 6 illustrates the N2 adsorption-desorption curve together with the homologous pore-size distribution. The pure ZIF-8 exhibited the type I isotherm and the type H4 shape hysteresis loop, which indicating the presence of microporous structure. Meanwhile, ZIF-8 has a higher specific surface area (1175.462 m2/g), which is a good adsorbent and can provide more active sites during the reaction[36]. Additionally, the isotherms and the hysteresis loop of Ag3VO4 and 60%AZ present the type IV and H3 shape, implying these samples are mesoporous structure. The specific surface areas of Ag3VO4 and 60%AZ were 8.193 and 25.705 m2/g, separately (see detailed in Table 1). The specific surface area of 60%AZ is higher than pure Ag3VO4, which results from the introduction of ZIF-8 particle in the hybrids. Meantime, the big specific surface area will supply numerous active sites for reactant to promote photocatalytic reaction process[37].
Fig. 6. N2 adsorption-desorption isotherms curves(a) and pore-size distribution curves of ZIF-8, Ag3VO4 and 60%AZ composite (b)
Table 1 Specific surface area (SBET), pore size and pore volume of the pure Ag3VO4, ZIF-8 and 60%AZ sample
3.6.
Sample
Surface area /m2·g-1
Pore volume/cm3·g-1
pore size /nm
ZIF-8
1175.462
0.946
1.63
Ag3VO4
8.193
0.065
2.75
60%AZ
25.705
0.468
2.23
Analysis of Optical Properties Based on UV-Vis DRS
Fig. 7 shows the UV-vis DRS of catalyst, and in the light of αℎν = k(ℎ𝜈 ― 𝐸𝑔)1/𝑛 to calculate the band gap, Where α, h, ν, Eg and k refer to absorption coefficients, Plank constant, light frequency, band gap and a constant[38]. In this system, n value is equal to 2. The band gap of Ag3VO4, 20%AZ, 40%AZ, 60%AZ, 80%AZ and ZIF-8 was equivalent to 2.0, 2.11, 2.21, 2.25, 2.83 and 5.06 eV, respectively. Additionally, it is worth noting that the Eg value abated and the absorption edge of the AZ composite is gradually red-shifted when the Ag3VO4 content increased. This showed that the addition of Ag3VO4 particles not merely expands the light capturing range of
Journal Pre-proof ZIF-8, but also conduced to extend the electron transfer distance according to the metal-to-ligand charge transfer mechanism[39]. Hence, the range of light response of Ag3VO4/ZIF-8 nanocomposites was expanded, which can availably heighten the photocatalytic activity of the single Ag3VO4.
Fig. 7. UV-Vis diffuse reflectance spectra of ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 composite material (a) and relationships between (αhν)2 and photo energy (hν) (b)
3.7.
PL analysis
PL spectrum can be used to analyze the recombination situation of photo-generated carriers in the fluorescence intensity. The weak or strong emission intensity of the PL spectrum indicates that the degree of photo-generated carrier recombination is low or high. Fig. 8 shows the PL spectrum analysis of different samples. It displayed that the fluorescence intensity order at the same excitation wavelength are as follows: ZIF-8>Ag3VO4>60%AZ, which indicates that the 60%AZ composite can effectively promote the electron-holes transfer, prolong the lifetime of photon-generated carrier and improve the photocatalytic activity.
Fig. 8. PL spectra of ZIF-8, Ag3VO4, Ag3VO4/ZIF-8 samples
3.8.
Evaluation of photocatalytic activity based on RhB degradation
The photoactivity of pure ZIF-8, Ag3VO4 and Ag3VO4/ZIF-8 samples was measured by degrading dye solutions under visible light. As depicted in Fig. 9a, this solution needs to undergo dark adsorption for 0.5 h to ensure the dark adsorption equilibrium. Owing to the nanopores
Journal Pre-proof existed in ZIF-8 that similar to zeolite sodalite structure, we can be obviously seen that Ag3VO4/ZIF-8 have adsorption phenomenon, which caused by the nanopores connected to through a 0.34 nm windows[40]. Additionally, we can see that the amount of ZIF-8 increased from 0% to 60%, 60%AZ exhibited the highest photocatalytic activity. However, comparing with the 60%AZ, a slight drop of the photocatalytic activity occurred when the ZIF-8 content was increased to 80%, which might owning to the agglomeration of ZIF-8 curbed the visible-light penetration to Ag3VO4 and cutting down the charge transfer rate[20]. Hence, we could be inferred that the introduction of the proper dosage of ZIF-8 availably improved the degradation ability of Ag3VO4/ZIF-8 composites. The possible cause is a synergistic effect of these two substances, further enhancing separation of the photogenerated carriers. The photocatalytic activity of 60%AZ photocatalyst is measured by monitoring the successive change of Rh B residue concentration at 554 nm with the change of reaction time. Fig. 9b shows gradual reduction in the characteristic absorbance (λ max=554 nm), which corresponds to the dye concentration in solution. Moreover, 60%AZ sample shows higher removal rate of Rh B in 100 min under visible light irradiation. As shown in Fig. 9c, k value of 60%AZ was calculated to 0.01707 min-1, which is the maximal value among all catalysts, where k is the pseudo-first-order rate constant by calculating the kinetic rate constant from ln(C0/Ct) vs. t[41]. The k value of pristine ZIF-8 (0.000749 min-1), Ag3VO4 (0.0040 min-1), 20%AZ (0.00462min-1), 40%AZ (0.00677min-1) and 80%AZ (0.01254 min-1) are less than 60%AZ composite. The reason is that 60%AZ composites may have the best content ratio of Ag3VO4 and ZIF-8, and the specific surface area is relatively larger compared with Ag3VO4. At the same time, the existence of the compact interface of the composite greatly reduces the electron-hole recombination rate and thus improves the photodegradation rate.
Journal Pre-proof Fig. 9. Photocatalytic degradation of Rh B over different photocatalysts under visible light irradiation (a), time-dependent degradation spectral pattern of Rh B solution over 60% AZ (b) and the corresponding apparent rate constants for the degradation of Rh B over different photocatalysts (c)
3.9.
Regeneration and reusability
The stability and recyclability of photocatalysts are important parameters in the process of application. In order to explore the recyclability of photocatalyst, under the same conditions, the Ag3VO4/ZIF-8 composite was measured to degrade dye solution up to five cycles (Fig.10). Before each cycle, the catalyst was gathered via centrifugation, and washed and dried for the next run. It is obviously displayed that the Ag3VO4/ZIF-8 composite exhibits decreased trend after the each cycle in photodegradation efficiency, as compared with that of the first cycle. A small decrease in the photocatalytic activity was probably caused by the mass loss during the separation and washing process or the active site of catalyst was occupied [42, 43]. Nevertheless, it still maintains the photocatalytic efficiency of Ag3VO4/ZIF-8 composite at a high level after five cycles. Generally speaking, the reusability results illustrated that the Ag3VO4/ZIF-8 composite has excellent stability and recycling performance.
Fig. 10. The cycling runs of the degradation of RhB over 60%AZ
3.10.
Mechanism of enhanced photocatalysis
TEOA, IPA, BQ were served as scavengers of active species (such as h+, ·OH and ·O2-) to explore the photocatalytic mechanism of Ag3VO4/ZIF-8 composite, due to these scavengers can react with the produced active species and curb the photo-degradation efficiency. Fig.11 illustrates a series of active species trapping experiments by introducing these scavengers. When TEOA and BQ were added, the photodegradation rate was remarkably decreased, while the influence of adding IPA was slight. To sum up, h+ and ·O2- ions play the critical role in the photocatalytic degradation process.
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Fig. 11. The effects of various scavengers on the visible light photocatalytic acitivty of 60%AZ
According to the above PL spectra, UV–Vis DRS spectra, active species trapping experiment and related literature, the possible photodegradation mechanism of RhB by Ag3VO4/ZIF-8 composites is shown in Scheme 2. Unlike other semiconductors, for metal-organic frameworks, the transition after photo-excitations primarily relates to the charge transfer transition of metal to-ligand (MLCT). Combined UV-Vis DRS and PL analysis, when Ag3VO4 is coupled with ZIF-8, it can be suppress the rapid electron-hole recombination rate of pure photocatalyst, which is beneficial to photocatalysis[44].Therefore, visible light cannot be excite ZIF-8 owing to the wide band gap but that is likely to excite the electrons of Ag3VO4[45, 46]. The electron at the valence band level of Ag3VO4 can be excite to a higher level orbit under visible light excitation of E < 2.95 eV (λ > 420nm), which leads to form h+ and e−. When Ag3VO4 nanoparticles and ZIF-8 form the dense and firm interface area, the electrons located at the CB of Ag3VO4 particles were forced to migrate to the methylimidazole ring by the metal to-ligand charge transfer mechanism (MLCT)[20]. The lowered electron density from methylimidazole ring and the electrons migration from Ag3VO4 restricted the recombination of photo-induced carrier. RhB molecule also was triggered to produce electrons, which will deliver to Ag3VO4 and ZIF-8[47, 48]. Photo-generated electrons in ZIF-8 can reduce O2 absorbed on the surface of photocatalyst to ·O2−[20]. Meantime, ZIF-8 also served as adsorbent due to the large specific surface area, which can capture the dye molecule, and ·O2- produced by photocatalyst furtherly react with RhB/RhB* to breakdown of the dye molecular structures. What’s more, h+ of Ag3VO4 could directly oxidize the dye molecules. Therefore, the following equations can illustrate this degradation process: RhB + hv→RhB∗ → RhB+• + e− Ag3VO4+hv →e− (Ag3VO4+RhB) + h+ (Ag3VO4) ZIF-8 + e− (Ag3VO4+RhB) +O2→ ZIF-8(e-) +·O2·O2−/ h+ + RhB+•/RhB→less organic matter→CO2+H2O
(1) (2) (3) (4)
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Scheme 2. The possible routes of photoelectron generation and transport in the 60%AZ composite
4. CONCLUSIONS A series of novel Ag3VO4/ZIF-8 composites with different ZIF-8 contents were prepared by a simple hydrothermal method. Among them, the photodegradation rate of RhB in 60%AZ composites was 0.01707min-1, which is about 4.2 and 22.7 times higher than pristine Ag3VO4 and ZIF-8. That can be attributed to the following reasons: first: the indirect dye photosensitization and the broke dye adsorption equilibrium could to promote the separation of the charges between Ag3VO4 and ZIF-8, second: the formation of a close interface between Ag3VO4 and ZIF-8 and produce a synergistic effect. Therefore, it is reasonable that this composite has good optical properties and photocatalytic activity. The free radicals trapping experiments showed h+ and •O2have the critical role in the photocatalytic degradation process. The cyclic experiments are exhibited that the Ag3VO4/ZIF-8 has excellent stability and recycling performance. We hope that this work lays a foundation for the development of high efficiency semiconductor/MOFs composites, in order to treat the real-time wastewater and protect the environment.
Acknowledgement We wish to express our gratitude to the National Natural Science Foundation of China (grant number 51146008).
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Journal Pre-proof Research Highlights （1） Ag3VO4/ ZIF-8 nanoparticles were acted as an effective visible-light-driven photocatalysts. （2） A novel photocatalyst (Ag3VO4/ ZIF-8) exhibits enhanced visible photocatalytic activity compared with pure Ag3VO4 and ZIF-8. （3） The mechanism of the enhanced photocatalytic performance was discussed in detail.