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Ag3VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation
Accepted Manuscript Title: Visible-light-driven ZnFe2 O4 /Ag/Ag3 VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation Authors: Liquan Jing, Yuanguo Xu, Chengcheng Qin, Jie Liu, Shuquan Huang, Minqiang He, Hui Xu, Huaming Li PII: DOI: Reference:
S0025-5408(17)30009-0 http://dx.doi.org/doi:10.1016/j.materresbull.2017.06.003 MRB 9386
To appear in:
MRB
Received date: Revised date: Accepted date:
2-1-2017 1-6-2017 3-6-2017
Please cite this article as: Liquan Jing, Yuanguo Xu, Chengcheng Qin, Jie Liu, Shuquan Huang, Minqiang He, Hui Xu, Huaming Li, Visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation
Liquan Jing,a Yuanguo Xu*,a Chengcheng Qin,a Jie Liu,a Shuquan Huang,a, Minqiang He,a Hui Xu,b Huaming Li*b a School b
of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China.
Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China.
*E-mail: [email protected]; [email protected]
Graphical Abstract
Novel ZnFe2O4/Ag/Ag3VO4 showed much higher photocatalytic activity in degrading MO and TC.
Highlights
The ZnFe2O4/Ag/Ag3VO4 composites were successfully synthesized by a hydrothermal method.
ZnFe2O4/Ag/Ag3VO4 composites show better photocatalytic activity for MO and TC degradation under visible light.
The ZnFe2O4/Ag/Ag3VO4 photocatalyst is promising for designing an environmental purification material.
Abstract A novel visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalyst were successfully fabricated through a two-step hydrothermal method. The samples were characterized by a lot of techniques including X-ray diffraction (XRD), scanning electron microscope with an energy-dispersive X-ray spectroscope (SEM-EDS), X-ray photo-electron spectroscopy (XPS) and Ultraviolet–visible absorption spectroscopy (UV-vis). The as-prepared ZnFe2O4/Ag/Ag3VO4 composites possess an excellent performance through the photocatalytic degradation of methyl orange and tetracycline under visible-light irradiation. The results of Electrochemical impedance spectroscopy (EIS) indicate that faster separation and transfer of photo-generate carriers over ZnFe2O4/Ag/Ag3VO4 composite than Ag/Ag3VO4. Among the prepared samples, the 5 % ZnFe2O4/Ag/Ag3VO4 composite shows the upmost photocatalytic activity, the degradation constant of 5 % ZnFe2O4/Ag/Ag3VO4 was as high as 1.49 times to that of Ag/Ag3VO4. It was found that that the superoxide radicals (•O2−) and hole (h+) are the predominant reactive species in the ZnFe2O4/Ag/Ag3VO4 photocatalytic system by the trapping experiments. In all, the ZnFe2O4/Ag/Ag3VO4 photocatalyst is promising for designing an environmental purification material in treatment of antibiotic pollutant. Keywords: ZnFe2O4/Ag/Ag3VO4; photocatalytic; methyl orange; tetracycline 1. Introduction Over the past several years, semiconductor photocatalytic technology has attracted considerable attention due to its gigantic potentiality in solving current the problem of environmental pollution. Among all the semiconductors, silver vanadate (Ag3VO4) reported by Konta et al. is a good candidate as visible-light-driven photocatalyst because of its narrow band gap [1, 2]. Nevertheless, owing to the high recombination of photogenerated electrone-hole pairs, the catalytic performance of pure Ag3VO4 should be further enhanced. To further facilitate the photoactivity of catalysts, coupling catalysts together are used to expedite the separation of electron−hole pairs. Recently, researchers have developed many Ag3VO4-based photocatalytic materials, such as La2O3/Ag3VO4 [3], NiO/Ag3VO4 [4], Ag3VO4/TiO2 [5], Co3O4/Ag3VO4 [6], Gd2O3/Ag3VO4 [7], Ag3VO4/ZnO [8], CoTiO3/Ag3VO4 [9], NiTiO3/Ag3VO4 [10], BiOI/Ag3VO4 [11], graphene–Ag3VO4 [12], g-C3N4/Ag3VO4 [13, 14], Ag3VO4/AgBr/Ag [15], CaFe2O4/Ag3VO4 [16], MgFe2O4/Ag3VO4 [17] and so on. Excitingly, these catalysts show different degrees of increased catalytic performance compared to pure Ag3VO4. In all, the composites exhibit higher photocatalytic activity could be ascribed to the decreased recombination probability of the photogenerated electrons and holes [18]. ZnFe2O4 is a spinel oxide with a narrow band gap. It is a visible light responsive semiconductor that has been extensively applied in the solar energy conversion and photocatalytic technology [19]. However, due to its fast recombination of charge carriers, the photocatalytic activity of ZnFe2O4 is low [20]. As a result, ZnFe2O4 could not be directly applied to the photocatalytic removal of organic pollutants [21]. According to its visible light response property, many researchers have devoted to fabricating semiconductor composites, such as ZnFe2O4/TiO2 [22, 23], ZnFe2O4@ZnO [24], ZnFe2O4/BiVO4 [25], Ag/ZnO/ZnFe2O4 [26], Ag3PO4/ZnFe2O4 [18, 27], carbon microspheres (CMSs)@ZnFe2O4/Ag3PO4 [19], ZnFe2O4–ZnO–Ag3PO4 [20], ZnFe2O4/Ag3VO4 [21] and so on. Although Zhang et al. [21] reported on the combination of
Ag3VO4 and ZnFe2O4, the preparation of ZnFe2O4 and Ag3VO4 with better control is worth being further explored. Furthermore, to our knowledge, hydrothermal preparation of these nanocomposites has not been reported. Besides, there is no report about photocatalysts based on ZnFe2O4/Ag/Ag3VO4 nanocomposites and our previous work has construct the CoFe2O4/Ag/Ag3VO4 composite photocatalyst applied to degradation of organic pollutants [28]. In this work, ZnFe2O4/Ag/Ag3VO4 nanocomposites with different ZnFe2O4 contents were synthesized by hydrothermal treatment. XRD, SEM, XPS, UV–vis DRS and EIS techniques were used to characterize the property of the samples. The photocatalyst was employed by degradation of methyl orange and tetracycline under visible-light irradiation. The trapping experiments suggest that the superoxide radicals (•O2−) and hole (h+) were the predominant reactive species on the degradation reaction for the ZnFe2O4/Ag/Ag3VO4 photocatalytic system. A possible photocatalytic mechanism based on the ZnFe2O4/Ag/Ag3VO4 composites has been put forward. 2. Experimental 2.1. Materials All chemicals (analytical reagent grade) were used without additional purification or treatment. 2.2. Catalysts preparation ZnFe2O4 was synthesized by a sol–gel method. Firstly, 1.4875 g Zn(NO3)2·6H2O and 4.0402g Fe(NO3)3·9H2O were dissolved in 100 mL distilled water to form a transparent solution (A solution). Then, 3.1521 g citric acid was dissolved in 100 mL deionized water to form a clear solution (B solution). Then, B solution was slowly added into solution A and the pH of the mixed solution was adjusted to 7 with ammonia solution. The mixture was stirred for 1 h at 80oC. The resulting solution was dried at 90°C overnight. Finally, the dried solid sample was calcined at 600°C for 2 h. After cooling down to room temperature, the red products were obtained. The ZnFe2O4/Ag/Ag3VO4 composites were synthesized by hydrothermal treatment. A certain amount of ZnFe2O4 was augmented to aqueous solution by ultrasonic treatment for 30 min. Subsequently, 0.24 g of AgNO3 was augmented into the above solution and then the mixture was stirred magnetically for 30 min. Afterwards, 0.1884 g Na3VO4·12H2O was dissolved in 40 mL H2O and the mixture solution was slowly dropped into the above solution. Then the mixture was magnetically stirred for 60 min. Moreover, the mixture was transferred into a 25 mL Teflon-lined stainless steel autoclave, and heated at 140oC for 6 h. Finally, the resulting product was washed by deionized water and ethanol, dried at 60oC to obtain ZnFe2O4/Ag/Ag3VO4 composites. Ag/Ag3VO4 was fabricated in the absence of ZnFe2O4 by the same procedure. 2.3. Characterization XRD measurements was monitored by a Bruker D8 advance diffractometer with Cu Kα radiation in order to demonstrate the crystal phase composition and fineness. The micrographs of prepared samples and sample surfaces were observed by using (JEOL-JEM-2010, accelerating voltage 200 kV) scanning electron microscope and the energy-dispersive spectrometry (EDS) analysis was employed to an energy-dispersive X-ray spectrometer attached to the FE-SEM apparatus. The X-ray photo-electron spectra (XPS) were employed to detect the chemical compositions and valence state of the composites (VG MultiLab 2000, Mg Kα, 20 kV). The UV– vis diffuse reflectance spectroscopy (DRS) spectra of the catalysts were recorded by a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) with the range of 200–800 nm, using BaSO4 as
the reflectance standard material. Electrochemistry analysis was employed on an electrochemical workstation (CHI 660B, CH Instruments, Inc., Shanghai). 2.4. Photocatalytic tests The activity of ZnFe2O4/Ag/Ag3VO4 was evaluated via the degradation of methyl orange (MO) and tetracycline (TC) at room temperature under visible-light irradiation. In each experiment, 70 mg of composites and 70 mL of MO (10 mg/L) aqueous solution of (40 mg of composites and 80 mL of TC (10 mg/L) aqueous solution) were added into the light reaction bottle. The suspension was magnetically stirred in the dark reaction for 30 min to obtain adsorption equilibrium between samples and MO (or TC). During the illumination, an aliquot (4 mL) was taken out and centrifuged at 13,000 rpm for 5 min, then the absorbance of the solution was monitored by UV-vis spectrometer (UV-2450, Shimadzu). 3. Results and discussion 3.1 XRD analysis The crystalline phases of the synthetical catalysts were investigated by XRD analysis. Fig. 1 displays the XRD patterns of ZnFe2O4, Ag/Ag3VO4 and ZnFe2O4/Ag/Ag3VO4 photocatalysts with different ZnFe2O4 concentrations. It can be seen that the distinctive peaks at 30.14°, 35.10°, 42.80°, 53.40 °, 56.80°, 62.30°can be ascribed to (220), (311), (400), (422), (511) and (440) crystal planes of ZnFe2O4 (JCPDS no. 82-1042) [25], respectively. In the case of Ag3VO4, the diffraction peaks at 19.40°, 31.08°, 32.48°, 35.26°, 36.16°, 39.08°, 41.44°, 48.44°, 51.46°, 54.30° were indexed to (011), (−121), (121), (301), (202), (022), (400), (−322), (132) and (331) planes of the monoclinic crystalline phase Ag3VO4 (JCPDS 43-0542) [21]. The weak peaks at 38.14° and 44.38°can be matched well with metal Ag0 (JCPDS No. 04-0783) [29]. Moreover, the peaks of ZnFe2O4 in the composites are not obviously observed, which may be due to the fact that the diffraction peaks of Ag3VO4 are too strong to mask the diffraction peaks of ZnFe2O4 or ZnFe2O4 to be highly dispersed in the composite. The peaks of Ag3VO4 have no obvious changes, indicating ZnFe2O4 is not incorporated into the lattice of Ag3VO4. 3.2 SEM-EDS analysis The morphologies of the obtained Ag/Ag3VO4, ZnFe2O4, 5 % ZnFe2O4/Ag/Ag3VO4 are shown in Fig. 2. As shown in Fig. 2A, the Ag/Ag3VO4 shows irregular particles with sizes of about 200-300 nanometers. The ZnFe2O4 phase is constituted by spherical structures with a diameter of ∼50 nm (in Fig. 2B). Fig. 2C and 2D show the low and high SEM image of 5 % ZnFe2O4/Ag/Ag3VO4 composites, respectively. Fig. 2D clearly shows that the ZnFe2O4 nanoparticles are tightly adhered on the surface of Ag/Ag3VO4. Besides, the Ag/Ag3VO4 particles show the larger irregular particles with sizes of about 1-2 micrometer with the introduction of ZnFe2O4. The EDS of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 were shown in Fig. 2E and 2F, respectively. These results showed that the Fe and Zn element were detected in addition to the initial Ag, V and O elements in Ag/Ag3VO4. It is indicated that ZnFe2O4 exists in the 5 % ZnFe2O4/Ag/Ag3VO4 composites. 3.3 XPS analysis The surface chemical composition of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composites, and the interaction between ZnFe2O4 and Ag/Ag3VO4 was further examined by XPS spectra. In Fig. 3A, the survey XPS spectrum show that Ag 3s, Ag 3d, V 2p and O 1s peaks for Ag/Ag3VO4
and 5 % ZnFe2O4/Ag/Ag3VO4 composites, as well as Zn 2p and Fe 2p peaks for ZnFe2O4. For the Ag/Ag3VO4 sample (in Fig. 2B), the Ag 3d peaks located at 368.4 eV and 374.4 eV, which corresponded to the Ag 3d5/2 and Ag 3d3/2 binding energies and revealed the presence of Ag+ [13]. However, the binding energies of Ag 3d5/2 and Ag 3d3/2 of 5 % ZnFe2O4/Ag/Ag3VO4 composites were 368.3 eV and 374.3 eV, which were lower than that of Ag/Ag3VO4 sample. A similar phenomenon was also observed in the XPS spectra of V 2p (Fig. 2C). The peaks at 516.9 eV and 524.4 eV in Ag/Ag3VO4 were corresponding to V 2p3/2 and V 2p1/2 binding energies, which showed the characteristic of V5+ [13]. The V 2p3/2 binding energy of 516.7 eV for 5 % ZnFe2O4/Ag/Ag3VO4 was slightly lower than that of 516.9 eV in Ag/Ag3VO4. On the basis of the high resolution XPS results of Ag 3d and V 2p, it can be attributed to the interaction between ZnFe2O4 and Ag/Ag3VO4 in ZnFe2O4/Ag/Ag3VO4 composite. The two peaks at 1021.8 eV and 1045.1 eV are assigned to Zn 2p3/2 and Zn 2p1/2, respectively, implying that Zn element exists in the form of Zn2+ in the composites [19, 25, 32]. The main peaks of Fe 2p3/2 and 2p1/2 are observed at 712.4 eV and 725.1 eV, respectively. Furthermore, a peak of the Ag 3s also can be observed at the position of 719.2 eV, which is in agreement with the reported paper [28, 33]. 3.4 UV-vis analysis The light absorption property of ZnFe2O4, Ag/Ag3VO4 and the ZnFe2O4/Ag/Ag3VO4 composites with different ZnFe2O4 contents is analyzed via UV-Vis absorption spectroscopy analysis, and the result is depicted in Fig. 4A. All the samples exhibit strong absorption from ultraviolet to visible light. The corresponding band gap energy for pure ZnFe2O4 and Ag/Ag3VO4 can be calculated from the following equation: αhν = A (hν-Eg)n/2. Where α, h, ν, A, and Eg are the absorption coefficient, Planck's constant, light frequency, a constant and band gap energy, respectively. For ZnFe2O4 and Ag/Ag3VO4, the value of n is 1 for the direct transition semiconductor. Fig. 4B showed the estimated band gaps for ZnFe2O4 and Ag/Ag3VO4 were about 1.97 eV and 2.05 eV, respectively. 3.5. Photocatalytic performance The photocatalytic activities of the ZnFe2O4/Ag/Ag3VO4 composites is employed by measuring the decomposition of methyl orange and tetracycline under visible-light irradiation (Fig. 5). In Fig. 5A, MO solution showed little decrease without photocatalyst after 32 min visible-light irradiation, suggesting the photolysis can be ignored. The absorbance of MO solution is hardly changed in the presence of pure ZnFe2O4, which indicates that ZnFe2O4 has low photocatalytic activity in this experiment condition. Additionally, the degradation of MO for Ag/Ag3VO4, 3 % ZnFe2O4/Ag/Ag3VO4, 5 % ZnFe2O4/Ag/Ag3VO4, 10 % ZnFe2O4/Ag/Ag3VO4, 20 % ZnFe2O4/Ag/Ag3VO4 was about 66%, 68%, 81%, 60%, and 50%, respectively. The 5 % ZnFe2O4/Ag/Ag3VO4 composite shows the highest activity and nearly 81% of MO was decomposed within 32 min, which is even better than that of Ag/Ag3VO4, as well as the composites with different ZnFe2O4 contents. Meanwhile, the photocatalytic degradation kinetics of MO by using ZnFe2O4/Ag/Ag3VO4 composites were employed, and the results were shown in Fig. 5B. A pseudo-first-order model was employed to investigate the kinetics of MO degradation. In Fig. 5C, it can be seen that the reaction rate constants of Ag/Ag3VO4, 3 % ZnFe2O4/Ag/Ag3VO4, 5 % ZnFe2O4/Ag/Ag3VO4, 10 % ZnFe2O4/Ag/Ag3VO4, 20 % ZnFe2O4/Ag/Ag3VO4 and ZnFe2O4 for MO photocatalytic degradation were 0.02778, 0.03054, 0.04132, 0.02494, 0.01695 and
0.0003439 min-1, respectively. The 5 % ZnFe2O4/Ag/Ag3VO4 composite exhibits the upmost degradation rate, which is about 1.49 times higher than that of Ag/Ag3VO4. Tetracycline (TC) was chosen as a typical antibiotic agents for estimating the photocatalytic activity of ZnFe2O4/Ag/Ag3VO4 composites under visible light irradiation. Fig. 5D showed that ZnFe2O4/Ag/Ag3VO4 composite exhibited higher photocatalytic activity for TC degradation as compared to Ag/Ag3VO4. The results of photocatalytic degradation suggest the ZnFe2O4/Ag/Ag3VO4 composite was an effective photocatalyst for antibiotic agent removal. 3.6 EIS analysis The electrochemical impedance spectroscopy (EIS) of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composite is carried out to investigate the approximation of the charge transport (in Fig. 6). The smaller arc radius represents the higher charge-transfer resistance [34]. Fig. 6 shows that the Nyquist plots of 5 % ZnFe2O4/Ag/Ag3VO4 composite is smaller than that of Ag/Ag3VO4 sample, suggesting the faster separation and transfer of photo-generate carriers over ZnFe2O4/Ag/Ag3VO4 composite, which may because of the synergetic effect between ZnFe2O4 and Ag/Ag3VO4 [21]. The characterization results of EIS measurements confirm that the introduction of ZnFe2O4 is an impressive way to improve electron-hole separation efficiency. 3.7 Possible photocatalytic mechanisms To investigate the possible reaction mechanism of degredation of MO dye under visible-light irradiation over ZnFe2O4/Ag/Ag3VO4 and explore the predominant active species generated, individual scavengers were added in the photocatalytic system. Isopropanol (IPA) was employed as the hydroxyl radical (•OH) scavenger, triethanolamine (TEOA) was employed as the hole (h+) scavenger, N2 was employed as the superoxide radical (•O2−) scavenger. The detailed results of the experiment are shown in Fig. 7. The significant effect in the degradation of MO is observed after the addition of IPA, TEOA and N2, which implies that h+ plays a principal role in the oxidation of MO and •O2− produces a marginal impact in this photocatalytic experiments. According to the above results, a possible photoelectron and hole transfer mechanism for the photodegradation of MO dye over ZnFe2O4/Ag/Ag3VO4 composites can be shown in Fig. 9. The estimated band gaps by UV-vis spectra for ZnFe2O4 and Ag/Ag3VO4 were about 1.97 eV and 2.05 eV, respectively. The valence-band (VB) potentials and conduction band (CB) potentials of ZnFe2O4 are obtained by the formula calculation, so the value of VB potentials and CB potentials of ZnFe2O4 were predicted to be 2.38 eV and 0.41 eV, respectively. The valence band (VB) potentials of Ag/Ag3VO4 is obtained by the valence-band XPS spectra of Ag/Ag3VO4 (Fig. 8), ECB of Ag/Ag3VO4 could be achieved by the equation of ECB = EVB-Eg. The value of VB and CB for Ag/Ag3VO4 were calculated to be 1.62 eV and -0.43 eV, respectively. As observed in Fig. 9, Ag nanoparticles displayed the surface plasmon resonant effect and accelerates the separation process of photo-generated electron–hole pair in the catalysts. The photoexcited electron on the surface of Ag nanoparticles migrated into the CB of Ag3VO4 and transformed the dissolved O2 to •O2-. Afterwards, the CB of Ag3VO4 was more negative than that of ZnFe2O4, the abundant electrons migrated into the CB of ZnFe2O4. Holes in the valence-band of ZnFe2O4 transfer into the VB of Ag3VO4, which is consistent with the trapping experiment. Obviously, the valence-band potential of ZnFe2O4 is more positive than the redox potential of OH-/•OH (1.99 V vs NHE), which indicates that the OH− can be oxidized to yield •OH by hole (h+) in the ZnFe2O4, but the trapping
experiment indicates that •OH can’t be generated in this process [28]. Meanwhile, the VB of Ag/Ag3VO4 (1.62 eV) are more negative than the redox potential of OH-/•OH(1.99 V vs NHE), which indicates that the •OH can’t be generated in the VB of Ag/Ag3VO4. Accordingly, the holes in the valence-band of ZnFe2O4 can not oxidize MO directly and will transfer to the valence-band of Ag3VO4. Thereby the photogenerated electrons accumulated on ZnFe2O4 and holes collected on Ag3VO4 are beneficial for the even higher separation efficiency of photogenerated carriers, which can enhance the photocatalytic activity. 4. Conclusion In summary, a new ZnFe2O4/Ag/Ag3VO4 composite was resoundingly synthesized through an unfancy hydrothermal method. The ZnFe2O4/Ag/Ag3VO4 composite displayed the synergistically enhanced photocatalytic activity for MO and TC degradation. The experiment results displayed that the degradation rate of the 5 % ZnFe2O4/Ag/Ag3VO4 composite was about 1.49 times than that of Ag/Ag3VO4. The EIS spectra illustrated that it can be attributed to the synergistic effect between Ag/Ag3VO4 and ZnFe2O4 promote the enhanced photoactivity efficiency of MO and TC. The experimental results of trapping experiment was suggested that h+ was the main reactive species to play a critical role in the degradation progress. Acknowledgements This work is financially supported by the National Natural Science Foundation of China for Youths (No. 21407065), Natural Science Foundation of Jiangsu Province for Youths (BK20140533), China Postdoctoral Science Foundation (2015T80514).
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Ag
Intensity (a.u.)
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Ag/Ag3VO4 3% 5% 10% 20%
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ZnFe2O4 70
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2Theta( degree)
Fig. 1 (A) XRD pattern of Ag/Ag3VO4, 3 % ZnFe2O4/Ag/Ag3VO4, 5 % ZnFe2O4/Ag/Ag3VO4, 10 % ZnFe2O4/Ag/Ag3VO4, 20 % ZnFe2O4/Ag/Ag3VO4 and ZnFe2O4.
F
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Fig. 2 SEM images of as-prepared samples: (A) Ag/Ag3VO4, (B) ZnFe2O4, (C) and (D) 5 % ZnFe2O4/Ag/Ag3VO4; EDS of (E) Ag/Ag3VO4 and (F) 5 % ZnFe2O4/Ag/Ag3VO4
A
B
Ag/Ag3VO4
Ag 3d
368.3 eV 374.3 eV
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Zn 2p
Ag 3s Fe 2p Ag 3s
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V 2p
O 1s
Ag 3d Ag 3d
Intensity(a.u.)
368.4 eV
5 % ZnFe2O4/Ag/Ag3VO4
Ag 3d5/2
374.4 eV
Ag 3d3/2 Ag/Ag3VO4 5 % ZnFe2O4/Ag/Ag3VO4
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516.7 eV
Intensity(a.u.)
Intensity (a.u.)
1021.8 eV
524.4 eV
V 2p5/2
V 2p3/2
1045.1 eV
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Zn 2p1/2
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E 712.4 eV
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725.1 eV Ag 3s
Fe 2p3/2
Fe 2p1/2
710
715
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Band Energy (eV)
Fig. 3 XPS spectra of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composites: (A) the survey scans; (B) Ag 3d; (C) V 2p; (D) Zn 2p and (E) Fe 2p of 5 % ZnFe2O4/Ag/Ag3VO4 composites.
A
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0 1
ZnFe2O4
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2 2
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hv (eV)
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hv (eV) Fig. 4 (A) UV-vis absorption spectra of (a) Ag/Ag3VO4, (b) 3 % ZnFe2O4/Ag/Ag3VO4, (c) 5 % ZnFe2O4/Ag/Ag3VO4, (d) 10 % ZnFe2O4/Ag/Ag3VO4, (e) 20 % ZnFe2O4/Ag/Ag3VO4 and (f) ZnFe2O4. (B) Plots of (αhν)2 versus hv for Ag/Ag3VO4 and ZnFe2O4.
A
B
1.0
5 % ZnFe2O4/Ag/Ag3VO4
0.8
3 % ZnFe2O4/Ag/Ag3VO4
0.8
Ag/Ag3VO4
0.6
MO ZnFe2O4
0.4
20 % ZnFe2O4/Ag/Ag3VO4
-In(C/C0)
C/C0
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10 % ZnFe2O4/Ag/Ag3VO4
0.0
20 % ZnFe2O4/Ag/Ag3VO4 ZnFe2O4
0.4 0.2
Ag/Ag3VO4
0.2
10 % ZnFe2O4/Ag/Ag3VO4
3 % ZnFe2O4/Ag/Ag3VO4 0.0
5 % ZnFe2O4/Ag/Ag3VO4 0
4
8
12
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28
32
0
4
Time (min)
12
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D
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Ag/Ag3VO4
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5 % ZnFe2O4/Ag/Ag3VO4 0.8
0.030 0.025
C/C0
-1
-1
k(molL min )
C
8
Time (min)
0.020
0.6
0.015 0.010 0.4
0.005
0
0.000 Ag/Ag3VO4
3%
5%
10 %
20 %
ZnFe2O4
2
4
6
8
10
Time (min)
Fig. 5 (A) Photocatalytic performance of as-prepared samples for degradation of MO under visible-light illumination; (B) The kinetics of photocatalytic degradation of MO by the samples; (C) Pseudo-first-order rate constant for MO photocatalytic degradation under different photocatalysts; (D) Photocatalytic degradation of TC in the presence of pure Ag/Ag3VO4, 5 wt% ZnFe2O4/Ag/Ag3VO4 under visible light irradiation
2500
-Z" (Ohm)
2000 1500 Ag/Ag3VO4 5 % ZnFe2O4/Ag/Ag3VO4
1000 500 0
0
1000
2000
3000
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Z' (Ohm) Fig. 6 Electrochemical impedance spectra of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 photocatalysts.
80
(%)
60
40
20
0
Blank
TEOA
IPA
In N2
Fig. 7 Trapping experiment of active species during the photocatalytic degradation of MO over 5 % ZnFe2O4/Ag/Ag3VO4 sample under visible light irradiation.
Intensity (a.u.)
VB
1.62 eV
-10
0
10
20
Binding Energy (eV) Fig. 8 Valence-band XPS spectra of Ag/Ag3VO4.
30
Fig.9 Schematic diagram of the photocatalytic mechanism of the composite.
S0025-5408(17)30009-0 http://dx.doi.org/doi:10.1016/j.materresbull.2017.06.003 MRB 9386
To appear in:
MRB
Received date: Revised date: Accepted date:
2-1-2017 1-6-2017 3-6-2017
Please cite this article as: Liquan Jing, Yuanguo Xu, Chengcheng Qin, Jie Liu, Shuquan Huang, Minqiang He, Hui Xu, Huaming Li, Visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation, Materials Research Bulletinhttp://dx.doi.org/10.1016/j.materresbull.2017.06.003 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalysts with enhanced photocatalytic activity under visible light irradiation
Liquan Jing,a Yuanguo Xu*,a Chengcheng Qin,a Jie Liu,a Shuquan Huang,a, Minqiang He,a Hui Xu,b Huaming Li*b a School b
of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang 212013, PR China.
Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China.
*E-mail: [email protected]; [email protected]
Graphical Abstract
Novel ZnFe2O4/Ag/Ag3VO4 showed much higher photocatalytic activity in degrading MO and TC.
Highlights
The ZnFe2O4/Ag/Ag3VO4 composites were successfully synthesized by a hydrothermal method.
ZnFe2O4/Ag/Ag3VO4 composites show better photocatalytic activity for MO and TC degradation under visible light.
The ZnFe2O4/Ag/Ag3VO4 photocatalyst is promising for designing an environmental purification material.
Abstract A novel visible-light-driven ZnFe2O4/Ag/Ag3VO4 photocatalyst were successfully fabricated through a two-step hydrothermal method. The samples were characterized by a lot of techniques including X-ray diffraction (XRD), scanning electron microscope with an energy-dispersive X-ray spectroscope (SEM-EDS), X-ray photo-electron spectroscopy (XPS) and Ultraviolet–visible absorption spectroscopy (UV-vis). The as-prepared ZnFe2O4/Ag/Ag3VO4 composites possess an excellent performance through the photocatalytic degradation of methyl orange and tetracycline under visible-light irradiation. The results of Electrochemical impedance spectroscopy (EIS) indicate that faster separation and transfer of photo-generate carriers over ZnFe2O4/Ag/Ag3VO4 composite than Ag/Ag3VO4. Among the prepared samples, the 5 % ZnFe2O4/Ag/Ag3VO4 composite shows the upmost photocatalytic activity, the degradation constant of 5 % ZnFe2O4/Ag/Ag3VO4 was as high as 1.49 times to that of Ag/Ag3VO4. It was found that that the superoxide radicals (•O2−) and hole (h+) are the predominant reactive species in the ZnFe2O4/Ag/Ag3VO4 photocatalytic system by the trapping experiments. In all, the ZnFe2O4/Ag/Ag3VO4 photocatalyst is promising for designing an environmental purification material in treatment of antibiotic pollutant. Keywords: ZnFe2O4/Ag/Ag3VO4; photocatalytic; methyl orange; tetracycline 1. Introduction Over the past several years, semiconductor photocatalytic technology has attracted considerable attention due to its gigantic potentiality in solving current the problem of environmental pollution. Among all the semiconductors, silver vanadate (Ag3VO4) reported by Konta et al. is a good candidate as visible-light-driven photocatalyst because of its narrow band gap [1, 2]. Nevertheless, owing to the high recombination of photogenerated electrone-hole pairs, the catalytic performance of pure Ag3VO4 should be further enhanced. To further facilitate the photoactivity of catalysts, coupling catalysts together are used to expedite the separation of electron−hole pairs. Recently, researchers have developed many Ag3VO4-based photocatalytic materials, such as La2O3/Ag3VO4 [3], NiO/Ag3VO4 [4], Ag3VO4/TiO2 [5], Co3O4/Ag3VO4 [6], Gd2O3/Ag3VO4 [7], Ag3VO4/ZnO [8], CoTiO3/Ag3VO4 [9], NiTiO3/Ag3VO4 [10], BiOI/Ag3VO4 [11], graphene–Ag3VO4 [12], g-C3N4/Ag3VO4 [13, 14], Ag3VO4/AgBr/Ag [15], CaFe2O4/Ag3VO4 [16], MgFe2O4/Ag3VO4 [17] and so on. Excitingly, these catalysts show different degrees of increased catalytic performance compared to pure Ag3VO4. In all, the composites exhibit higher photocatalytic activity could be ascribed to the decreased recombination probability of the photogenerated electrons and holes [18]. ZnFe2O4 is a spinel oxide with a narrow band gap. It is a visible light responsive semiconductor that has been extensively applied in the solar energy conversion and photocatalytic technology [19]. However, due to its fast recombination of charge carriers, the photocatalytic activity of ZnFe2O4 is low [20]. As a result, ZnFe2O4 could not be directly applied to the photocatalytic removal of organic pollutants [21]. According to its visible light response property, many researchers have devoted to fabricating semiconductor composites, such as ZnFe2O4/TiO2 [22, 23], ZnFe2O4@ZnO [24], ZnFe2O4/BiVO4 [25], Ag/ZnO/ZnFe2O4 [26], Ag3PO4/ZnFe2O4 [18, 27], carbon microspheres (CMSs)@ZnFe2O4/Ag3PO4 [19], ZnFe2O4–ZnO–Ag3PO4 [20], ZnFe2O4/Ag3VO4 [21] and so on. Although Zhang et al. [21] reported on the combination of
Ag3VO4 and ZnFe2O4, the preparation of ZnFe2O4 and Ag3VO4 with better control is worth being further explored. Furthermore, to our knowledge, hydrothermal preparation of these nanocomposites has not been reported. Besides, there is no report about photocatalysts based on ZnFe2O4/Ag/Ag3VO4 nanocomposites and our previous work has construct the CoFe2O4/Ag/Ag3VO4 composite photocatalyst applied to degradation of organic pollutants [28]. In this work, ZnFe2O4/Ag/Ag3VO4 nanocomposites with different ZnFe2O4 contents were synthesized by hydrothermal treatment. XRD, SEM, XPS, UV–vis DRS and EIS techniques were used to characterize the property of the samples. The photocatalyst was employed by degradation of methyl orange and tetracycline under visible-light irradiation. The trapping experiments suggest that the superoxide radicals (•O2−) and hole (h+) were the predominant reactive species on the degradation reaction for the ZnFe2O4/Ag/Ag3VO4 photocatalytic system. A possible photocatalytic mechanism based on the ZnFe2O4/Ag/Ag3VO4 composites has been put forward. 2. Experimental 2.1. Materials All chemicals (analytical reagent grade) were used without additional purification or treatment. 2.2. Catalysts preparation ZnFe2O4 was synthesized by a sol–gel method. Firstly, 1.4875 g Zn(NO3)2·6H2O and 4.0402g Fe(NO3)3·9H2O were dissolved in 100 mL distilled water to form a transparent solution (A solution). Then, 3.1521 g citric acid was dissolved in 100 mL deionized water to form a clear solution (B solution). Then, B solution was slowly added into solution A and the pH of the mixed solution was adjusted to 7 with ammonia solution. The mixture was stirred for 1 h at 80oC. The resulting solution was dried at 90°C overnight. Finally, the dried solid sample was calcined at 600°C for 2 h. After cooling down to room temperature, the red products were obtained. The ZnFe2O4/Ag/Ag3VO4 composites were synthesized by hydrothermal treatment. A certain amount of ZnFe2O4 was augmented to aqueous solution by ultrasonic treatment for 30 min. Subsequently, 0.24 g of AgNO3 was augmented into the above solution and then the mixture was stirred magnetically for 30 min. Afterwards, 0.1884 g Na3VO4·12H2O was dissolved in 40 mL H2O and the mixture solution was slowly dropped into the above solution. Then the mixture was magnetically stirred for 60 min. Moreover, the mixture was transferred into a 25 mL Teflon-lined stainless steel autoclave, and heated at 140oC for 6 h. Finally, the resulting product was washed by deionized water and ethanol, dried at 60oC to obtain ZnFe2O4/Ag/Ag3VO4 composites. Ag/Ag3VO4 was fabricated in the absence of ZnFe2O4 by the same procedure. 2.3. Characterization XRD measurements was monitored by a Bruker D8 advance diffractometer with Cu Kα radiation in order to demonstrate the crystal phase composition and fineness. The micrographs of prepared samples and sample surfaces were observed by using (JEOL-JEM-2010, accelerating voltage 200 kV) scanning electron microscope and the energy-dispersive spectrometry (EDS) analysis was employed to an energy-dispersive X-ray spectrometer attached to the FE-SEM apparatus. The X-ray photo-electron spectra (XPS) were employed to detect the chemical compositions and valence state of the composites (VG MultiLab 2000, Mg Kα, 20 kV). The UV– vis diffuse reflectance spectroscopy (DRS) spectra of the catalysts were recorded by a UV–vis spectrophotometer (UV-2450, Shimadzu, Japan) with the range of 200–800 nm, using BaSO4 as
the reflectance standard material. Electrochemistry analysis was employed on an electrochemical workstation (CHI 660B, CH Instruments, Inc., Shanghai). 2.4. Photocatalytic tests The activity of ZnFe2O4/Ag/Ag3VO4 was evaluated via the degradation of methyl orange (MO) and tetracycline (TC) at room temperature under visible-light irradiation. In each experiment, 70 mg of composites and 70 mL of MO (10 mg/L) aqueous solution of (40 mg of composites and 80 mL of TC (10 mg/L) aqueous solution) were added into the light reaction bottle. The suspension was magnetically stirred in the dark reaction for 30 min to obtain adsorption equilibrium between samples and MO (or TC). During the illumination, an aliquot (4 mL) was taken out and centrifuged at 13,000 rpm for 5 min, then the absorbance of the solution was monitored by UV-vis spectrometer (UV-2450, Shimadzu). 3. Results and discussion 3.1 XRD analysis The crystalline phases of the synthetical catalysts were investigated by XRD analysis. Fig. 1 displays the XRD patterns of ZnFe2O4, Ag/Ag3VO4 and ZnFe2O4/Ag/Ag3VO4 photocatalysts with different ZnFe2O4 concentrations. It can be seen that the distinctive peaks at 30.14°, 35.10°, 42.80°, 53.40 °, 56.80°, 62.30°can be ascribed to (220), (311), (400), (422), (511) and (440) crystal planes of ZnFe2O4 (JCPDS no. 82-1042) [25], respectively. In the case of Ag3VO4, the diffraction peaks at 19.40°, 31.08°, 32.48°, 35.26°, 36.16°, 39.08°, 41.44°, 48.44°, 51.46°, 54.30° were indexed to (011), (−121), (121), (301), (202), (022), (400), (−322), (132) and (331) planes of the monoclinic crystalline phase Ag3VO4 (JCPDS 43-0542) [21]. The weak peaks at 38.14° and 44.38°can be matched well with metal Ag0 (JCPDS No. 04-0783) [29]. Moreover, the peaks of ZnFe2O4 in the composites are not obviously observed, which may be due to the fact that the diffraction peaks of Ag3VO4 are too strong to mask the diffraction peaks of ZnFe2O4 or ZnFe2O4 to be highly dispersed in the composite. The peaks of Ag3VO4 have no obvious changes, indicating ZnFe2O4 is not incorporated into the lattice of Ag3VO4. 3.2 SEM-EDS analysis The morphologies of the obtained Ag/Ag3VO4, ZnFe2O4, 5 % ZnFe2O4/Ag/Ag3VO4 are shown in Fig. 2. As shown in Fig. 2A, the Ag/Ag3VO4 shows irregular particles with sizes of about 200-300 nanometers. The ZnFe2O4 phase is constituted by spherical structures with a diameter of ∼50 nm (in Fig. 2B). Fig. 2C and 2D show the low and high SEM image of 5 % ZnFe2O4/Ag/Ag3VO4 composites, respectively. Fig. 2D clearly shows that the ZnFe2O4 nanoparticles are tightly adhered on the surface of Ag/Ag3VO4. Besides, the Ag/Ag3VO4 particles show the larger irregular particles with sizes of about 1-2 micrometer with the introduction of ZnFe2O4. The EDS of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 were shown in Fig. 2E and 2F, respectively. These results showed that the Fe and Zn element were detected in addition to the initial Ag, V and O elements in Ag/Ag3VO4. It is indicated that ZnFe2O4 exists in the 5 % ZnFe2O4/Ag/Ag3VO4 composites. 3.3 XPS analysis The surface chemical composition of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composites, and the interaction between ZnFe2O4 and Ag/Ag3VO4 was further examined by XPS spectra. In Fig. 3A, the survey XPS spectrum show that Ag 3s, Ag 3d, V 2p and O 1s peaks for Ag/Ag3VO4
and 5 % ZnFe2O4/Ag/Ag3VO4 composites, as well as Zn 2p and Fe 2p peaks for ZnFe2O4. For the Ag/Ag3VO4 sample (in Fig. 2B), the Ag 3d peaks located at 368.4 eV and 374.4 eV, which corresponded to the Ag 3d5/2 and Ag 3d3/2 binding energies and revealed the presence of Ag+ [13]. However, the binding energies of Ag 3d5/2 and Ag 3d3/2 of 5 % ZnFe2O4/Ag/Ag3VO4 composites were 368.3 eV and 374.3 eV, which were lower than that of Ag/Ag3VO4 sample. A similar phenomenon was also observed in the XPS spectra of V 2p (Fig. 2C). The peaks at 516.9 eV and 524.4 eV in Ag/Ag3VO4 were corresponding to V 2p3/2 and V 2p1/2 binding energies, which showed the characteristic of V5+ [13]. The V 2p3/2 binding energy of 516.7 eV for 5 % ZnFe2O4/Ag/Ag3VO4 was slightly lower than that of 516.9 eV in Ag/Ag3VO4. On the basis of the high resolution XPS results of Ag 3d and V 2p, it can be attributed to the interaction between ZnFe2O4 and Ag/Ag3VO4 in ZnFe2O4/Ag/Ag3VO4 composite. The two peaks at 1021.8 eV and 1045.1 eV are assigned to Zn 2p3/2 and Zn 2p1/2, respectively, implying that Zn element exists in the form of Zn2+ in the composites [19, 25, 32]. The main peaks of Fe 2p3/2 and 2p1/2 are observed at 712.4 eV and 725.1 eV, respectively. Furthermore, a peak of the Ag 3s also can be observed at the position of 719.2 eV, which is in agreement with the reported paper [28, 33]. 3.4 UV-vis analysis The light absorption property of ZnFe2O4, Ag/Ag3VO4 and the ZnFe2O4/Ag/Ag3VO4 composites with different ZnFe2O4 contents is analyzed via UV-Vis absorption spectroscopy analysis, and the result is depicted in Fig. 4A. All the samples exhibit strong absorption from ultraviolet to visible light. The corresponding band gap energy for pure ZnFe2O4 and Ag/Ag3VO4 can be calculated from the following equation: αhν = A (hν-Eg)n/2. Where α, h, ν, A, and Eg are the absorption coefficient, Planck's constant, light frequency, a constant and band gap energy, respectively. For ZnFe2O4 and Ag/Ag3VO4, the value of n is 1 for the direct transition semiconductor. Fig. 4B showed the estimated band gaps for ZnFe2O4 and Ag/Ag3VO4 were about 1.97 eV and 2.05 eV, respectively. 3.5. Photocatalytic performance The photocatalytic activities of the ZnFe2O4/Ag/Ag3VO4 composites is employed by measuring the decomposition of methyl orange and tetracycline under visible-light irradiation (Fig. 5). In Fig. 5A, MO solution showed little decrease without photocatalyst after 32 min visible-light irradiation, suggesting the photolysis can be ignored. The absorbance of MO solution is hardly changed in the presence of pure ZnFe2O4, which indicates that ZnFe2O4 has low photocatalytic activity in this experiment condition. Additionally, the degradation of MO for Ag/Ag3VO4, 3 % ZnFe2O4/Ag/Ag3VO4, 5 % ZnFe2O4/Ag/Ag3VO4, 10 % ZnFe2O4/Ag/Ag3VO4, 20 % ZnFe2O4/Ag/Ag3VO4 was about 66%, 68%, 81%, 60%, and 50%, respectively. The 5 % ZnFe2O4/Ag/Ag3VO4 composite shows the highest activity and nearly 81% of MO was decomposed within 32 min, which is even better than that of Ag/Ag3VO4, as well as the composites with different ZnFe2O4 contents. Meanwhile, the photocatalytic degradation kinetics of MO by using ZnFe2O4/Ag/Ag3VO4 composites were employed, and the results were shown in Fig. 5B. A pseudo-first-order model was employed to investigate the kinetics of MO degradation. In Fig. 5C, it can be seen that the reaction rate constants of Ag/Ag3VO4, 3 % ZnFe2O4/Ag/Ag3VO4, 5 % ZnFe2O4/Ag/Ag3VO4, 10 % ZnFe2O4/Ag/Ag3VO4, 20 % ZnFe2O4/Ag/Ag3VO4 and ZnFe2O4 for MO photocatalytic degradation were 0.02778, 0.03054, 0.04132, 0.02494, 0.01695 and
0.0003439 min-1, respectively. The 5 % ZnFe2O4/Ag/Ag3VO4 composite exhibits the upmost degradation rate, which is about 1.49 times higher than that of Ag/Ag3VO4. Tetracycline (TC) was chosen as a typical antibiotic agents for estimating the photocatalytic activity of ZnFe2O4/Ag/Ag3VO4 composites under visible light irradiation. Fig. 5D showed that ZnFe2O4/Ag/Ag3VO4 composite exhibited higher photocatalytic activity for TC degradation as compared to Ag/Ag3VO4. The results of photocatalytic degradation suggest the ZnFe2O4/Ag/Ag3VO4 composite was an effective photocatalyst for antibiotic agent removal. 3.6 EIS analysis The electrochemical impedance spectroscopy (EIS) of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composite is carried out to investigate the approximation of the charge transport (in Fig. 6). The smaller arc radius represents the higher charge-transfer resistance [34]. Fig. 6 shows that the Nyquist plots of 5 % ZnFe2O4/Ag/Ag3VO4 composite is smaller than that of Ag/Ag3VO4 sample, suggesting the faster separation and transfer of photo-generate carriers over ZnFe2O4/Ag/Ag3VO4 composite, which may because of the synergetic effect between ZnFe2O4 and Ag/Ag3VO4 [21]. The characterization results of EIS measurements confirm that the introduction of ZnFe2O4 is an impressive way to improve electron-hole separation efficiency. 3.7 Possible photocatalytic mechanisms To investigate the possible reaction mechanism of degredation of MO dye under visible-light irradiation over ZnFe2O4/Ag/Ag3VO4 and explore the predominant active species generated, individual scavengers were added in the photocatalytic system. Isopropanol (IPA) was employed as the hydroxyl radical (•OH) scavenger, triethanolamine (TEOA) was employed as the hole (h+) scavenger, N2 was employed as the superoxide radical (•O2−) scavenger. The detailed results of the experiment are shown in Fig. 7. The significant effect in the degradation of MO is observed after the addition of IPA, TEOA and N2, which implies that h+ plays a principal role in the oxidation of MO and •O2− produces a marginal impact in this photocatalytic experiments. According to the above results, a possible photoelectron and hole transfer mechanism for the photodegradation of MO dye over ZnFe2O4/Ag/Ag3VO4 composites can be shown in Fig. 9. The estimated band gaps by UV-vis spectra for ZnFe2O4 and Ag/Ag3VO4 were about 1.97 eV and 2.05 eV, respectively. The valence-band (VB) potentials and conduction band (CB) potentials of ZnFe2O4 are obtained by the formula calculation, so the value of VB potentials and CB potentials of ZnFe2O4 were predicted to be 2.38 eV and 0.41 eV, respectively. The valence band (VB) potentials of Ag/Ag3VO4 is obtained by the valence-band XPS spectra of Ag/Ag3VO4 (Fig. 8), ECB of Ag/Ag3VO4 could be achieved by the equation of ECB = EVB-Eg. The value of VB and CB for Ag/Ag3VO4 were calculated to be 1.62 eV and -0.43 eV, respectively. As observed in Fig. 9, Ag nanoparticles displayed the surface plasmon resonant effect and accelerates the separation process of photo-generated electron–hole pair in the catalysts. The photoexcited electron on the surface of Ag nanoparticles migrated into the CB of Ag3VO4 and transformed the dissolved O2 to •O2-. Afterwards, the CB of Ag3VO4 was more negative than that of ZnFe2O4, the abundant electrons migrated into the CB of ZnFe2O4. Holes in the valence-band of ZnFe2O4 transfer into the VB of Ag3VO4, which is consistent with the trapping experiment. Obviously, the valence-band potential of ZnFe2O4 is more positive than the redox potential of OH-/•OH (1.99 V vs NHE), which indicates that the OH− can be oxidized to yield •OH by hole (h+) in the ZnFe2O4, but the trapping
experiment indicates that •OH can’t be generated in this process [28]. Meanwhile, the VB of Ag/Ag3VO4 (1.62 eV) are more negative than the redox potential of OH-/•OH(1.99 V vs NHE), which indicates that the •OH can’t be generated in the VB of Ag/Ag3VO4. Accordingly, the holes in the valence-band of ZnFe2O4 can not oxidize MO directly and will transfer to the valence-band of Ag3VO4. Thereby the photogenerated electrons accumulated on ZnFe2O4 and holes collected on Ag3VO4 are beneficial for the even higher separation efficiency of photogenerated carriers, which can enhance the photocatalytic activity. 4. Conclusion In summary, a new ZnFe2O4/Ag/Ag3VO4 composite was resoundingly synthesized through an unfancy hydrothermal method. The ZnFe2O4/Ag/Ag3VO4 composite displayed the synergistically enhanced photocatalytic activity for MO and TC degradation. The experiment results displayed that the degradation rate of the 5 % ZnFe2O4/Ag/Ag3VO4 composite was about 1.49 times than that of Ag/Ag3VO4. The EIS spectra illustrated that it can be attributed to the synergistic effect between Ag/Ag3VO4 and ZnFe2O4 promote the enhanced photoactivity efficiency of MO and TC. The experimental results of trapping experiment was suggested that h+ was the main reactive species to play a critical role in the degradation progress. Acknowledgements This work is financially supported by the National Natural Science Foundation of China for Youths (No. 21407065), Natural Science Foundation of Jiangsu Province for Youths (BK20140533), China Postdoctoral Science Foundation (2015T80514).
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Au
10
KeV
Fig. 2 SEM images of as-prepared samples: (A) Ag/Ag3VO4, (B) ZnFe2O4, (C) and (D) 5 % ZnFe2O4/Ag/Ag3VO4; EDS of (E) Ag/Ag3VO4 and (F) 5 % ZnFe2O4/Ag/Ag3VO4
A
B
Ag/Ag3VO4
Ag 3d
368.3 eV 374.3 eV
Intensity (a.u.)
Zn 2p
Ag 3s Fe 2p Ag 3s
V 2p
O 1s
V 2p
O 1s
Ag 3d Ag 3d
Intensity(a.u.)
368.4 eV
5 % ZnFe2O4/Ag/Ag3VO4
Ag 3d5/2
374.4 eV
Ag 3d3/2 Ag/Ag3VO4 5 % ZnFe2O4/Ag/Ag3VO4
600
400
200
0
800
1000
1200
364
366
368
Band Energy (eV)
C
370
372
374
376
378
380
Band Energy (eV)
516.9 eV
V 2p
D
Zn 2p
516.7 eV
Intensity(a.u.)
Intensity (a.u.)
1021.8 eV
524.4 eV
V 2p5/2
V 2p3/2
1045.1 eV
Zn 2p3/2
Ag/Ag3VO4
Zn 2p1/2
5 % ZnFe2O4/Ag/Ag3VO4 512
516
520
524
528
Band Energy (eV)
1025
1030
1035
1040
1045
1050
Fe 2p Ag 3s
719.2 eV
Intensity (a.u.)
1020
Band Energy (eV)
E 712.4 eV
1015
725.1 eV Ag 3s
Fe 2p3/2
Fe 2p1/2
710
715
720
725
730
Band Energy (eV)
Fig. 3 XPS spectra of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 composites: (A) the survey scans; (B) Ag 3d; (C) V 2p; (D) Zn 2p and (E) Fe 2p of 5 % ZnFe2O4/Ag/Ag3VO4 composites.
A
1.6 1.4
Absorbance
1.2 1.0 ZnFe2O4 20 % ZnFe2O4/Ag/Ag3VO4
0.8 0.6
10 % ZnFe2O4/Ag/Ag3VO4
0.4
5 % ZnFe2O4/Ag/Ag3VO4 3 % ZnFe2O4/Ag/Ag3VO4
0.2
Ag/Ag3VO4
0.0 200
300
400
500
600
700
800
Wavelength (nm)
B
10 10 8 (ahv) 2 (eV)
6
2
(ahv) (eV)
8
4
6
Ag/Ag3VO4
4
0 1
ZnFe2O4
2.07 eV
2 2
3
4
hv (eV)
2 0 1.0
1.97 eV
1.5
2.0
2.5
3.0
hv (eV) Fig. 4 (A) UV-vis absorption spectra of (a) Ag/Ag3VO4, (b) 3 % ZnFe2O4/Ag/Ag3VO4, (c) 5 % ZnFe2O4/Ag/Ag3VO4, (d) 10 % ZnFe2O4/Ag/Ag3VO4, (e) 20 % ZnFe2O4/Ag/Ag3VO4 and (f) ZnFe2O4. (B) Plots of (αhν)2 versus hv for Ag/Ag3VO4 and ZnFe2O4.
A
B
1.0
5 % ZnFe2O4/Ag/Ag3VO4
0.8
3 % ZnFe2O4/Ag/Ag3VO4
0.8
Ag/Ag3VO4
0.6
MO ZnFe2O4
0.4
20 % ZnFe2O4/Ag/Ag3VO4
-In(C/C0)
C/C0
0.6
10 % ZnFe2O4/Ag/Ag3VO4
0.0
20 % ZnFe2O4/Ag/Ag3VO4 ZnFe2O4
0.4 0.2
Ag/Ag3VO4
0.2
10 % ZnFe2O4/Ag/Ag3VO4
3 % ZnFe2O4/Ag/Ag3VO4 0.0
5 % ZnFe2O4/Ag/Ag3VO4 0
4
8
12
16
20
24
28
32
0
4
Time (min)
12
16
D
20
1.0
0.040
Ag/Ag3VO4
0.035
5 % ZnFe2O4/Ag/Ag3VO4 0.8
0.030 0.025
C/C0
-1
-1
k(molL min )
C
8
Time (min)
0.020
0.6
0.015 0.010 0.4
0.005
0
0.000 Ag/Ag3VO4
3%
5%
10 %
20 %
ZnFe2O4
2
4
6
8
10
Time (min)
Fig. 5 (A) Photocatalytic performance of as-prepared samples for degradation of MO under visible-light illumination; (B) The kinetics of photocatalytic degradation of MO by the samples; (C) Pseudo-first-order rate constant for MO photocatalytic degradation under different photocatalysts; (D) Photocatalytic degradation of TC in the presence of pure Ag/Ag3VO4, 5 wt% ZnFe2O4/Ag/Ag3VO4 under visible light irradiation
2500
-Z" (Ohm)
2000 1500 Ag/Ag3VO4 5 % ZnFe2O4/Ag/Ag3VO4
1000 500 0
0
1000
2000
3000
4000
Z' (Ohm) Fig. 6 Electrochemical impedance spectra of Ag/Ag3VO4 and 5 % ZnFe2O4/Ag/Ag3VO4 photocatalysts.
80
(%)
60
40
20
0
Blank
TEOA
IPA
In N2
Fig. 7 Trapping experiment of active species during the photocatalytic degradation of MO over 5 % ZnFe2O4/Ag/Ag3VO4 sample under visible light irradiation.
Intensity (a.u.)
VB
1.62 eV
-10
0
10
20
Binding Energy (eV) Fig. 8 Valence-band XPS spectra of Ag/Ag3VO4.
30
Fig.9 Schematic diagram of the photocatalytic mechanism of the composite.