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Enhanced photocatalytic degradation of tetracycline hydrochloride by novel porous hollow cube ZnFe2O4
Journal of Photochemistry & Photobiology A: Chemistry 364 (2018) 794–800
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Enhanced photocatalytic degradation of tetracycline hydrochloride by novel porous hollow cube ZnFe2O4 ⁎
Yang Caoa, Xianyu Leia, Qianlin Chena,b, , Chao Kanga, Wenxue Lia, Baojun Liuc,d,
T
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a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China Institute of Advanced Technology, Guizhou University, Guiyang 550025, China c College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China d Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hollow cube ZnFe2O4 Photocatalysis Tetracycline hydrochloride Degradation mechanism
As a broad-spectrum antibacterial agent, tetracycline hydrochloride (TCHC) was widely used in medical treatment and animal husbandry. However, the TCHC in various systems such as the soil, rivers and foods, had caused great harm to the environment and human health. Among the many methods for treated tetracycline hydrochloride in wastewater, the photocatalytic treatment had great advantages. In this study, we used Prussian blue as a precursor to synthesized porous hollow spinel cube ZnFe2O4 for photocatalysis. The experimental results showed that the degradation rates of the prepared porous hollow cube ZnFe2O4 (l-ZnFe2O4) significantly better than the commercially available ZnFe2O4 (g-ZnFe2O4) and the ZnFe2O4 prepared by the coprecipitation method (p-ZnFe2O4). In addition, we made a further introduction on the degradation mechanism of TCHC. The porous hollow cube ZnFe2O4 could be expected to provide a green and effective for material degradation of tetracycline hydrochloride in water.
1. Introduction As a broad-spectrum antimicrobial agent, tetracycline hydrochloride (TCHC) was widely used in the medicine and livestock husbandry, and with the discharge of the above waste water, it was widely distributed in the water environment [1–3]. TCHC in water was accumulated in plants and animals over a long period time, eventually reaching the human body by the food chain, which could cause serious harms to the human body. At present, there were some treatments for antibiotics in water, such as biological methods [4], physical methods [5] and chemical methods [6,7]. However, these methods possessed some fatal weaknesses in the degradation process, such as low efficiency, large energy consumption and giving rise to secondary pollution. The photocatalysis technology had the thorough degradation ratios, little secondary pollution, and almost all the antibiotics in the water could be degraded in the sewage treatment. Moreover, directly using sunlight in the room temperature, was a promising green technology for treatment of antibiotic in wastewater [8–10]. Since 1972, Fujishima [11] and Honda first discovered that TiO2 electrodes electrolyzed water to generate H2 and O2 under light irradiation. And in 1976, Carey et al. [12] first used TiO2 materials for
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photocatalytic degradation of highly toxic and low concentrations of PCBs (Polychlorinated biphenyls) materials. Since then, photocatalytic materials were focused by many researchers. As an important metal oxide, spinel zinc ferrite (ZnFe2O4) had been widely used in magnetic [13–15], lithium battery electrode material [16–18], gas sensing material [19] and so on. Especially for photocatalytic materials, ZnFe2O4 and its composites were widely used in the study of photocatalytic degradation of pollutants due to their good properties of narrow band gaps, good visible light response, good thermal stability and low toxicity [20–23]. For example, Xu et al. [36] reported that ZnFe2O4 was responsive to visible light, and the composite material of ZnFe2O4 and Ag/AgBr can effectively increase the visible light response range of Ag/ AgBr, and increased the photocatalytic efficiency under visible light. In addition, Xu et al. [37] using Zn-Fe mixed metal-organic framework as precursor prepared the flake ZnO/ZnFe2O4, it was obvious degradation effect on rhodamine B (RhB) and methylene blue (MB) under UV–vis light. More importantly, Zn2+ and Fe3+ elements were located at the available octahedral and tetrahedral sites of the close-packed oxygen atoms, respectively, which could enhance the separation efficiency of the photo-generated charges, and then would improve the photocatalytic degradation [24].
Corresponding author at: School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. Corresponding author at: College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China. E-mail addresses: [email protected] (Q. Chen), [email protected] (B. Liu).
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https://doi.org/10.1016/j.jphotochem.2018.07.023 Received 18 April 2018; Received in revised form 10 June 2018; Accepted 16 July 2018 Available online 17 July 2018 1010-6030/ © 2018 Published by Elsevier B.V.
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At present, various methods have been studied for the fabrication of ZnFe2O4 such as co-precipitation [33], hydrothermal [38] and sol-gel methods [39] et al. Metal-organic frameworks (MOFs) were a class of crystalline compounds that are self-assembled by metal ions and multidentate organic ligands. Due to its controllable pore size, modifiable pore surface, ultra-low density and ultra-high specific surface area, it was always as a promising new material for fabricating hollow structures [26,27]. Synthesis of ZnFe2O4 using MOFs as precursor is attracting attention of many researchers [20,28,37,40]. In addition, the hollow structure could modulate the index of refraction and enhance light scattering, which were beneficial for the degradation of organic pollutants [20]. Moreover, there were some large number of active sites for adsorption and reaction in the surface. More importantly, a large quantity of nanoparticles was accumulated in the shell, so water, oxygen and reactants could permeate through the shell through porous channels for better oxidation [25]. For example, Habibi et al. [41] studied the photocatalytic effect of different ZnFe2O4, the results showed that the porous ZnFe2O4, has better photocatalytic performance than ZnFe2O4. To our best knowledge, however, there are little reports about photocatalytic degradation of tetracycline hydrochloride by the porous hollow cube ZnFe2O4. In our work, we successfully synthesized a novel porous hollow cube ZnFe2O4 by calcination using MOF (Prussian Blue (PB)) as precursor. In addition, degradation of TCHC in aqueous were studied using the porous hollow cube ZnFe2O4. The results showed that the porous hollow cube ZnFe2O4 showed superior performance for effective degradation of TCHC in aqueous. At the same time, we also provided a reference mean in wastewater treatment and degradation of antibiotics.
powder were obtained in the 2θ range from 10°–80° by the D8 ADVANCE X-ray diffraction (BRUKER-AXS, Germany). The surface properties and composition are conducted by the ESCALAB 250XI multi-functional imaging X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, America). The test conditions are as follows: monochromatic Al Ka (hv = 1486.6 eV), power 150 W, 500 μm beam spot and the binding energy is calibrated with C 1s 284.8 eV. The morphology and particle size of the ZnFe2O4 were analyzed by the ΣIGMA + X-Max20 field emission scanning electron microscope (FESEM) (Carl Zeiss, Germany) with an accelerating voltage of 5 kV. The UV–vis absorption spectrum was measured using U4100 UV Spectrometer (Hitachi, Japan), and the wavelength range was from 200 nm to 800 nm. The TEM images, HRTEM images and SAED images of the samples were obtained by the Tecnai G2 F20 field emission transmission electron microscope (FEI, America) with an acceleration voltage of 200 kV. Adsorption-desorption curve of nitrogen in the sample was analyzed by the ASAP2020 (M) automatic surface area micro-gap analyzer (Micromeritics Instrument, America). 2.4. Photocatalytic performance evaluation The photocatalytic activity of the porous hollow cube ZnFe2O4 for the degradation of TCHC in aqueous was evaluated using a 300 W Xe lamp equipped with 350 nm–780 nm reflection filter and 420 nm cutoff filter (irradiation wavelength of 420 nm–780 nm) as the light source at ambient temperature. First, 500 mg/L of the catalyst was added into 100 mL solution of 40 mg/L of TCHC, and magnetic stirred 30 min to achieve sufficient adsorption/desorption equilibrium in dark. The liquid level of solution and the cut-off filter distance was 10 cm. Then, the sample was collected at given time intervals under light irradiation and throughout the reaction under constant magnetic stirring. The removed sample was centrifuged at 12,000 r/min for 5 min, and the supernatant was aspirated with a syringe and the concentration of TCHC was measured at a wavelength of 356 nm using the Alpha 1860 A UV–vis spectrophotometer (Puyuan, China). Degradation rate of TCHC can be calculated by the following formula:
2. Experimental sections 2.1. The experimental materials Polyvinylpyrrolidone (PVP, K30, MW∼40,000) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). K4Fe (CN)63H2O (≧99.5%) was purchased from Aladdin (Shanghai, China). Zinc acetate (≧98%) was purchased from Sinopharm Chemical ReagentCo., Ltd (Shanghai, China). Tetracycline hydrochloride (TCHC) (≧96%) was purchased from Aladdin (Shanghai, China). Eehanol (≧99.7%) was purchased from Fuyu Fine Chemical Co., Ltd (Tianjing, China).
Degradation Rate (DR) % = Ct/CO(%)
(1)
where CO was the absorbance of the sample before the dark reaction and Ct was the absorbance of the sample after degradation.
2.2. Synthesis of the porous hollow cube ZnFe2O4
3. Results and discussion
The porous hollow cube ZnFe2O4 material was prepared by using PB as the MOF precursor [28]. First, 20 g of PVP was dissolved into 250 ml of 0.1 M HCl solution under magnetic stirring 30 min to obtain a clear solution. Second, 0.6 g of K4Fe(CN)63H2O was added into the solution under magnetic stirring 30 min to obtain a bright yellow solution. The solution was sealed into an airtight container, and incubated at 80 °C for 24 h to get the blue insolubles. Third, the blue insolubles (PB) was centrifuged at 11,000 r/min and washed three times with distilled water and ethanol. The obtained precipitants were dried at 70 °C for 12 h in vacuum. The cubic PB was successfully synthesized. Fourth, 0.2 g of homemade PB was weighed accurately and dissolved into 40 ml of ethanol under magnetic stirring for 10 min. Fifth, 0.14 g of anhydrous zinc acetate was added into the solution under magnetic stirring for 10 min. Sixth, the solution was heated at 70 °C to evaporate the ethanol. The blue solid was collected and ground for 5 min in agate. At last, it was heated at 700 °C at a heating rate of 2 °C/min for 6 h. The porous hollow spinel cube ZnFe2O4 was prepared. The above steps were mainly divided into two major steps as shown in Fig. S1.
3.1. The formation mechanism of the material The fabrication process of the porous hollow cube ZnFe2O4 involved two steps. The First step was shape-controlled synthesis of PB precursor. In this step, Fe2+ was oxidized to Fe3+ by HCl, and then formed PB. During nucleation and growth processes of PB, polyvinylpyrrolidone (PVP) was employed as an effective capping gent could drive the Fe2+ and [Fe(CN)6]3− to self-assemble with a cubic shape [29]. In the second step, The Zn2+ was evenly dispersed in the PB surface, during calcination treatment in air and 2 °C/min, the ZnFe2O4 are formed on the surface of cubic. Simultaneously, the organic of MOF (PB) precursor including PVP and cyanide ligand could be slowly decomposed [30], and the small voids were generated when the gas diffused outward passing through ZnFe2O4 oxide layer in the process. The porous hollow cube ZnFe2O4 was synthesized when PB precursor completely consumed. The formation mechanism of the porous hollow cube ZnFe2O4 could be illustrated by Fig. 1. 3.2. Structural analysis
2.3. Characterization of the porous hollow cube ZnFe2O4 Fig. 2 showed the XRD patterns of the porous hollow cube ZnFe2O4. Diffraction peaks of the sample at 2θ were 18.20, 30.10, 35.00, 42.80,
X-ray diffraction (XRD) data for crystalline structure of ZnFe2O4 795
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Fig. 1. Schematic illustration of the formation mechanism of the porous hollow ZnFe2O4.
oxidized into Fe3+ [43]. Through Gaussian–Lorentzian fitting of the O 1s characteristic peak (Fig. 3d), the band energies at 531.35 eV was attributed to terminal of the OeH bond on the catalyst surface, while the band at 529.68 eV was ascribed to the surface lattice defect O2− [31]. 3.3. Morphology analysis The morphology of the porous hollow cube ZnFe2O4 and PB was investigated by SEM analysis. The samples in the SEM were measured by dispersing the sample on the silicon wafer with ethanol as the dispersant. As shown in Fig. 4, PB has a good cubic structure and an average particle size of 600 nm. The prepared ZnFe2O4 had a substantially monodisperse cube shape and an average particle size of the cube was 600 nm (Fig. 5a). And Fig. 5d shows clearer appearance of cube, and the cubic surface was formed by the accumulation of nanoparticles. This result revealed the ZnFe2O4 still maintains the cubic structure of PB. The more detailed morphology of porous hollow cube ZnFe2O4 was exposed using TEM, HRTEM and SAED. As shown in Fig. 5b, the sample had a cubic shape and an average particle size of the cube was about 600 nm. The surface of the cube was composed of many dense nanoparticles. This was consistent with the results of SEM. In Fig. 5e, the cube has part of the lighter in center, indicating the sample was hollow cubic shape. The lattice spacing of around 0.259 nm in the HRTEM image corresponds to the distance of < 311 > plane of spinel ZnFe2O4 (Fig. 5c). Finally, the ring-shaped pattern of SAED showed that the cubic spinel phase ZnFe2O4 had a polycrystalline structure (Fig. 5f), and four clear concentric rings to the center distance of 0.257, 0.214, 0.171 and 0.11 were assigned to < 311 > , < 400 > , < 422 > , and < 533 > planes of spinel ZnFe2O4 respectively, which were in agreement with the results of XRD.
Fig. 2. XRD patterns of porous hollow ZnFe2O4.
52.50, 57.20, 62.25 and 74.50 corresponding to < 111 > , < 220 > , < 311 > , < 400 > , < 422 > , < 511 > , < 420 > and < 533 > crystal planes of the spinel phase ZnFe2O4, respectively. The results were consistent with the standard card JCPDS No. 22-1012, indicating that the ZnFe2O4 was spinel structure. To further study the surface components and chemical states of porous hollow ZnFe2O4, the samples were characterized by XPS. From the full spectrum of porous hollow ZnFe2O4 in Fig. 3a, it could be seen that the sample is composed of Zn, Fe, O and C elements with no other impurities. The peak for C 1s at 284.8 eV was introduced during the reaction process or exogenously substances during the test. In Fig. 3b, the binding energies of Zn 2p1/2 and Zn 2p3/2 are 1044.5 eV and 1021.4 eV respectively. Compared with the standard spectra, it can be determined that Zn was mainly formed by Zn2+ [42]. The XPS spectrum of the Fe 2p was provided in Fig. 3c, in which the peaks at 725.3 eV and 711.25 eV could be assigned to the binding energies of Fe 2p1/2 and Fe 2p3/2, respectively, corresponding to the characteristic spectrum of Fe3+. In addition, the peak of 710.7 eV was due to the Fe2+ species, which may come from some amorphous phase in the final product [26]. However, the satellite peak appearing at 719.2 eV provided evidence for the existence of Fe3+. Thus, Fe2+ in PB had been
3.4. The optical property, BET analysis and magnetic property The optical property of the synthesized porous hollow cube ZnFe2O4 was investigated by UV–Vis diffuse reflection spectra (DRS). Exhibited in Fig. 6a, it could be seen that the porous hollow ZnFe2O4 had strong 796
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Fig. 3. The XPS spectra of the porous hollow ZnFe2O4 sample, (a) survey of the sample, (b) Zn 2p spectrum, (c) Fe 2p spectrum, (d) O 1s spectrum.
sample was ascribed to the microporous material. The magnetization-magnetic field ((M−H)) hysteresis loops of the porous hollow ZnFe2O4 were shown in Fig. S2. The results indicate that the composites were magnetic, the saturation magnetization (Ms) of the porous hollow ZnFe2O4 was about 15 emu/g, and the sample has soft and weak superparamagnetic nature, which was favorable for the magnetic recycling of the photocatalyst.
absorption intensity in the region of UV and visible light. The results suggested that the catalyst could show catalytic performance under visible light. In addition, the band-gap energy of the sample could be calculated using Tauc's plots: (αE)n = A·(E − Eg)
(2)
where α, Eg, E and A were indicative of the absorption coefficient, band gap energy of the semiconductor, hv (Planck's constant multiplied by light frequency) and constant value, respectively. As ZnFe2O4 was an indirect band-gap [32], so the value of n was 1/2, and (AE) 1/2 makes a straight line to the relation between E and the Eg of porous hollow ZnFe2O4, which could be estimated from the intercept between the straight line and the X axis (Fig. 6a inset). It could be estimated from the figure that the Eg of porous hollow ZnFe2O4 was about 1.5 eV. The N2 adsorption/desorption at 77 K was performed to characterize the pore structure and specific surface area of the porous hollow cube ZnFe2O4. As illustrated in Fig. 6b, the specific surface area of the sample was calculated to be 13.44 m2/g by the Brunauer–Emmett–Teller (BET) method. The pore size distribution of porous hollow ZnFe2O4 could be estimated by the Barrett–Joyner–Halenda (BJH) as showed in the inset of Fig. 6b. The result suggested that the
3.5. The catalytic performance evaluation In order to investigated the catalytic performance of the porous hollow cube ZnFe2O4 on the treatment of tetracycline hydrochloride, this study was investigated by the degradation of TCHC under visible light (780 nm > λ > 420 nm). l-ZnFe2O4, g-ZnFe2O4 and p-ZnFe2O4 represented the prepared porous hollow cube ZnFe2O4, commercially available ZnFe2O4 (Aladdin, China) and the ZnFe2O4 prepared by the coprecipitation method [33], respectively. And the 30 min before reaction without light. The results were shown in Fig. 7, and the TCHC in the absence of visible light catalyst without degradation, from the dark reaction stage of l-ZnFe2O4, g-ZnFe2O4, p-ZnFe2O4, TCHC adsorption capacity decreased in turn. From the catalytic results of these three
Fig. 4. SEM images of the PB. 797
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Fig. 5. (a, b) SEM images of the sample, (c, d) TEM, (e) HRTEM and (f) SAED pattern of the porous hollow ZnFe2O4 sample, respectively.
Fig. 7. The curves of the Photocatalytic degradation of TCHC by different photocatalysts.
Fig. 8. Kinetic studies for the photodegradation of TCHC over the different photocatalysts.
Fig. 6. (a) UV–vis DRS of the porous hollow ZnFe2O4, and (inset) Tauc’s plots to determine the band gaps for porous hollow ZnFe2O4, (b) N2 adsorption–desorption isotherms and corresponding size distribution curves (inset) of the porous hollow ZnFe2O4.
70 min illumination, while that of l-ZnFe2O4 was 85%, which was almost the same with 20 min. From the above results, the porous hollow ZnFe2O4 photocatalytic degradation of TCHC was shorter degradation time and higher degradation efficiency than the other two methods prepared ZnFe2O4 under visible light. The main reasons are that porous
samples, the degradation rates of l-ZnFe2O4, g-ZnFe2O4 and p-ZnFe2O4 were 80%, 42% and 22% after 20 min illumination, respectively. Each the degradation rate of g-ZnFe2O4 and p-ZnFe2O4 was about 55% after 798
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Table 1 Different catalysts in visible decline rate of the degradation of TCHC, kinetic constants and correlation coefficient. Catalysts
After 60 min, the degradation of TCHC
The kinetic equations
Kinetic constants of 60 min(k1, min−1, k2, mol−1 dm3 s−1)
Correlation coefficient (R2)
l-ZnFe2O4 g-ZnFe2O4 p-ZnFe2O4
84.08% 59.50% 49.97%
y = 0.5932x + 5.2881 y = 0.0118x y = 0.0096x
k2 = 0.0672 k1 = 0.0118 k1 = 0.0096
0.9999 0.9897 0.9952
Fig. 9. Schematic mechanism for TCHC degradation over the porous hollow ZnFe2O4.
intermediate. Finally, %OH could oxidize TCHC to small molecules. Photocatalytic degradation of TCHC using the porous hollow cube ZnFe2O4 could be expressed as follows:
hollow structure can increase the times of light scattering, so that the photocatalytic material has a larger light irradiation area, more light absorption and better photocatalytic activity in photocatalytic degradation of tetracycline hydrochloride. Because l-ZnFe2O4 was close to the degradation equilibrium at 60 min, 60 min was chosen as the research object to investigate the reaction kinetics. According to the fitting, g-ZnFe2O4 and p-ZnFe2O4 followed the first-order kinetic equation, while l-ZnFe2O4 followed the second-order kinetic equation (Fig. 8). The fitting equations were as follows: First-order kinetic equation: ln(CO/Ct) = k1 t
(5)
ZnFe2O4(e−) + O2 → %O2−
(6)
−
%O2 + H → %OOH +
−
%OOH + H + 2e → %OH + 2OH +
(7) −
−
ZnFe2O4(h ) + OH → %OH +
−
h , %O2 , %OOH, %OH + TCHC → small molecules +
(3)
(8) (9) (10)
The generation of active species (%OH) under visible-light irradiation was investigated using the electron paramagnetic resonance (EPR) technique to capture active free radicals. As shown in Fig. S3, there are no distinct EPR peaks in the dark, however, after illumination with visible light, quartet characteristic peaks of the DMPO-OH% are observed. The result indicating that %OH was generated after light irradiation.
Second-order kinetic equation: t/ln(CO/Ct) = t/a+1/(k2a2)
ZnFe2O4 + hv → ZnFe2O4(e− + h+)
(4)
Where CO and Ct was the mass concentration of TCHC before and after photoreaction, k1 and k2 was reaction rate constant, a was the value of ln (CO/Ct) when reaction equilibrium. The fitting results were as shown in Table 1, the constant of p-ZnFe2O4 was less when compared to gZnFe2O4 and the reaction rate of l-ZnFe2O4 was proportional to the quadratic of the reactant concentration. The higher rate constant values for l-ZnFe2O4 confirmed their superior photocatalytic activity.
4. Conclusions In summary, the novel porous hollow cube ZnFe2O4 was successfully synthesized by using MOF (PB) as the precursor. The porous hollow cube ZnFe2O4 degrade 80% of TCHC (40 ml/L, 100 ml) in 20 min under visible light, and its degradation effect is obviously higher than ZnFe2O4 materials prepared by other methods. This shows that controlled microstructure of the material has a significant impact effect on the photocatalysis, and the porous hollow structure can increase the times of light scattering, so that the photocatalytic material has a larger light irradiation area, more light absorption and better photocatalytic activity in catalytic performances, indicating the relationships between the microstructure of the material and the photocatalytic activity.
3.6. Mechanism of photocatalytic activity According to the previous literatures [1,8,10,34], the key factors for the degradation of the TCHC were generated oxidation species that the photo-induced holes, super-oxide radical (%O2−) and hydroxyl radical (%OH) in the photocatalytic process. The possible mechanism for the TCHC over the porous hollow ZnFe2O4 could be proposed, as illustrated by Fig. 9. Firstly, the porous hollow ZnFe2O4 had intricate pore structures that could possess more effective light absorption for multiple scattering. Then, the porous hollow ZnFe2O4 were irradiated by visible light, electrons in the valence band (VB) could be excited to the conduction band (CB) and simultaneously the same amounts of h+ were generated in the VB. The electrons formed can react with the adsorbed molecular oxygen to yield %O2− in the CB [35]. The generated %O2− then further combine with H2O and H+ to produce %OH via series of radical reactions. This process could be occurred %OOH that was the
Acknowledgments This work was supported financially by Natural Science Foundation of China (No. 21461005), the Major applied basic research project (Guizhou branch in JZ word [2013]6004) and science and technology planned project in Guizhou Province (Qian kehe foundation [2018] 799
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1052).
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Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.07. 023. References [1] Q.S. Yan, M.M. Xu, C.P. Lin, J.F. Hui, Y.G. Lin, R.Q. Zhang, Efficient photocatalytic degradation of tetracycline hydrochloride by Ag3PO4, under visible-light irradiation, Environ. Sci. Pollut. Res. 23 (2016) 14422–14430. [2] R. Hao, X. Xiao, X.X. Zuo, J.M. Nana, W.D. Zhang, Efficient adsorption and visiblelight photocatalytic degradation of tetracycline hydrochloride using mesoporous BiOI microspheres, J. Hazard. Mater. 209–210 (2012) 137–145. [3] R.A. Palominos, M.A. Mondaca, A. Giraldo, G. Penuela, M. Perez-Moya, H.D. Mansilla, Photocatalytic oxidation of the antibiotic tetracycline on TiO2, and ZnO suspensions, Catal. Today 144 (2009) 100–105. [4] S. Chelliapan, T. Wilby, P. Sallis, Performance of an up-flow anaerobic stage reactor (UASR) in the treatment of pharmaceutical wastewater containing macrolide antibiotics, Water Res. 40 (2006) 507–516. [5] X.C. Song, D.F. Liu, Gw. Zhang, M. Frigon, X.R. Meng, K.X. Li, Adsorption mechanisms and the effect of oxytetracycline on activated sludge, Bioresource Technol. 151 (2014) 428–431. [6] J.L. De Morais, P.P. Zamora, Use of advanced oxidation processes to improve the biodegradability of mature landfill leachates, J. Hazard. Mater. 23 (2005) 181–186. [7] E.U. Cokgor, I. Arslan-Alaton, D. Orhon, Biodegradability improvement of penicillin formulation effluent by ozonation, Fresen. Environ. Bull. 13 (2004) 1053–1056. [8] C.Y. Wang, X. Zhang, H.B. Qiu, G.X. Huang, H.Q. Yu, Bi24O31Br10 nanosheets with controllable thickness for visible–light–driven catalytic degradation of tetracycline hydrochloride, Appl. Catal. B: Environ. 205 (2017) 615–623. [9] Z.M. Shaykhi, A.A.L. Zinatizadeh, Statistical modeling of photocatalytic degradation of synthetic amoxicillin wastewater (SAW) in an immobilized TiO2, photocatalytic reactor using response surface methodology (RSM), J. Taiwan Inst. Chem. E 45 (2014) 1717–1726. [10] X.D. Zhu, Y.J. Wang, R.J. Sun, D.M. Zhou, Photocatalytic degradation of tetracycline in aqueous solution by nanosized TiO2, Chemosphere 92 (2013) 925–932. [11] A. Fujishima, K. Honda, Photolysis-decomposition of water at surface of an irradiated semiconductor, Nature 238 (1972) 37–38. [12] J.H. Carey, J. Lawrence, H.M. Tosine, Photodechlorination of PCB’s in the presence of titanium dioxide in aqueous suspensions, Bull. Environ. Contam. Toxicol. 16 (1976) 697–701. [13] M. Mozaffari, J.T. Sims, Phosphorus availability and sorption in an atlantic coastal plain watershed dominated by animal-based agriculture, Soil Sci. 157 (1994) 97–107. [14] C. Luadthong, V. Itthibenchapong, N. Viriya-Empikul, P. Faungnawakij, W. Pavasant, Tanthapanichakoon, Synthesis, structural characterization, and magnetic property ofnanostructured ferrite spinel oxides (AFe2O4, A = Co, Ni and Zn), Mater. Chem. Phys. 143 (2013) 203–208. [15] C.W. Yao, Q.S. Zeng, G.F. Goya, J.Q. Jiang, ZnFe2O4 nanocrystals: synthesis and magnetic properties, Phys. Chem. C 111 (2007) 12274–12278. [16] Y.F. Deng, Q.M. Zhang, S.T. Tang, L.T. zhang, S.G. Deng, Z.C. Shi, G.H. Chen, Onepot synthesis of ZnFe2O4/C hollow spheres as superior anode materials for lithium ion batteries, Chem. Commun. 47 (2011) 6828–6830. [17] F. Zou, X.L. Hu, Z. Li, L. Qie, C.C. Hu, R. Zeng, MOF-derived porous ZnO/ZnFe2O4/C octahedra with hollow interiors for high-rate lithium-ion batteries, Adv. Mater. 26 (2014) 6622–6628. [18] H.Y. Guo, Y.M. Zhang, A.C. Marschilok, K.J. Takeuchi, E.S. Takeuchi, P. Liu, A first principles study of spinel ZnFe2O4 for electrode materials in lithium-ion batteries, Phys. Chem. Chem. Phys. 19 (2017) 26322–26329. [19] G.Y. Zhang, C.S. Li, F.Y. Cheng, J. Chen, ZnFe2O4 tubes: synthesis and application to gas sensors with high sensitivity and low-energy consumption, Sens. Actuators B: Chem. 120 (2017) 403–410. [20] B.J. Liu, X.Y. Li, Q.D. Zhao, Y. Hou, G.H. Chen, Self-templated formation of ZnFe2O4 double-shelled hollow microspheres for photocatalytic degradation of gaseous odichlorobenzene, J. Mater. Chem. A 5 (2017) 8909–8915.
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Journal of Photochemistry & Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem
Enhanced photocatalytic degradation of tetracycline hydrochloride by novel porous hollow cube ZnFe2O4 ⁎
Yang Caoa, Xianyu Leia, Qianlin Chena,b, , Chao Kanga, Wenxue Lia, Baojun Liuc,d,
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a
School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China Institute of Advanced Technology, Guizhou University, Guiyang 550025, China c College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China d Department of Electrical Engineering, The Hong Kong Polytechnic University, Hung Hom, Kowloon, Hong Kong b
A R T I C LE I N FO
A B S T R A C T
Keywords: Porous hollow cube ZnFe2O4 Photocatalysis Tetracycline hydrochloride Degradation mechanism
As a broad-spectrum antibacterial agent, tetracycline hydrochloride (TCHC) was widely used in medical treatment and animal husbandry. However, the TCHC in various systems such as the soil, rivers and foods, had caused great harm to the environment and human health. Among the many methods for treated tetracycline hydrochloride in wastewater, the photocatalytic treatment had great advantages. In this study, we used Prussian blue as a precursor to synthesized porous hollow spinel cube ZnFe2O4 for photocatalysis. The experimental results showed that the degradation rates of the prepared porous hollow cube ZnFe2O4 (l-ZnFe2O4) significantly better than the commercially available ZnFe2O4 (g-ZnFe2O4) and the ZnFe2O4 prepared by the coprecipitation method (p-ZnFe2O4). In addition, we made a further introduction on the degradation mechanism of TCHC. The porous hollow cube ZnFe2O4 could be expected to provide a green and effective for material degradation of tetracycline hydrochloride in water.
1. Introduction As a broad-spectrum antimicrobial agent, tetracycline hydrochloride (TCHC) was widely used in the medicine and livestock husbandry, and with the discharge of the above waste water, it was widely distributed in the water environment [1–3]. TCHC in water was accumulated in plants and animals over a long period time, eventually reaching the human body by the food chain, which could cause serious harms to the human body. At present, there were some treatments for antibiotics in water, such as biological methods [4], physical methods [5] and chemical methods [6,7]. However, these methods possessed some fatal weaknesses in the degradation process, such as low efficiency, large energy consumption and giving rise to secondary pollution. The photocatalysis technology had the thorough degradation ratios, little secondary pollution, and almost all the antibiotics in the water could be degraded in the sewage treatment. Moreover, directly using sunlight in the room temperature, was a promising green technology for treatment of antibiotic in wastewater [8–10]. Since 1972, Fujishima [11] and Honda first discovered that TiO2 electrodes electrolyzed water to generate H2 and O2 under light irradiation. And in 1976, Carey et al. [12] first used TiO2 materials for
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photocatalytic degradation of highly toxic and low concentrations of PCBs (Polychlorinated biphenyls) materials. Since then, photocatalytic materials were focused by many researchers. As an important metal oxide, spinel zinc ferrite (ZnFe2O4) had been widely used in magnetic [13–15], lithium battery electrode material [16–18], gas sensing material [19] and so on. Especially for photocatalytic materials, ZnFe2O4 and its composites were widely used in the study of photocatalytic degradation of pollutants due to their good properties of narrow band gaps, good visible light response, good thermal stability and low toxicity [20–23]. For example, Xu et al. [36] reported that ZnFe2O4 was responsive to visible light, and the composite material of ZnFe2O4 and Ag/AgBr can effectively increase the visible light response range of Ag/ AgBr, and increased the photocatalytic efficiency under visible light. In addition, Xu et al. [37] using Zn-Fe mixed metal-organic framework as precursor prepared the flake ZnO/ZnFe2O4, it was obvious degradation effect on rhodamine B (RhB) and methylene blue (MB) under UV–vis light. More importantly, Zn2+ and Fe3+ elements were located at the available octahedral and tetrahedral sites of the close-packed oxygen atoms, respectively, which could enhance the separation efficiency of the photo-generated charges, and then would improve the photocatalytic degradation [24].
Corresponding author at: School of Chemistry and Chemical Engineering, Guizhou University, Guiyang 550025, China. Corresponding author at: College of Resource and Environmental Engineering, Guizhou University, Guiyang 550025, China. E-mail addresses: [email protected] (Q. Chen), [email protected] (B. Liu).
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https://doi.org/10.1016/j.jphotochem.2018.07.023 Received 18 April 2018; Received in revised form 10 June 2018; Accepted 16 July 2018 Available online 17 July 2018 1010-6030/ © 2018 Published by Elsevier B.V.
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At present, various methods have been studied for the fabrication of ZnFe2O4 such as co-precipitation [33], hydrothermal [38] and sol-gel methods [39] et al. Metal-organic frameworks (MOFs) were a class of crystalline compounds that are self-assembled by metal ions and multidentate organic ligands. Due to its controllable pore size, modifiable pore surface, ultra-low density and ultra-high specific surface area, it was always as a promising new material for fabricating hollow structures [26,27]. Synthesis of ZnFe2O4 using MOFs as precursor is attracting attention of many researchers [20,28,37,40]. In addition, the hollow structure could modulate the index of refraction and enhance light scattering, which were beneficial for the degradation of organic pollutants [20]. Moreover, there were some large number of active sites for adsorption and reaction in the surface. More importantly, a large quantity of nanoparticles was accumulated in the shell, so water, oxygen and reactants could permeate through the shell through porous channels for better oxidation [25]. For example, Habibi et al. [41] studied the photocatalytic effect of different ZnFe2O4, the results showed that the porous ZnFe2O4, has better photocatalytic performance than ZnFe2O4. To our best knowledge, however, there are little reports about photocatalytic degradation of tetracycline hydrochloride by the porous hollow cube ZnFe2O4. In our work, we successfully synthesized a novel porous hollow cube ZnFe2O4 by calcination using MOF (Prussian Blue (PB)) as precursor. In addition, degradation of TCHC in aqueous were studied using the porous hollow cube ZnFe2O4. The results showed that the porous hollow cube ZnFe2O4 showed superior performance for effective degradation of TCHC in aqueous. At the same time, we also provided a reference mean in wastewater treatment and degradation of antibiotics.
powder were obtained in the 2θ range from 10°–80° by the D8 ADVANCE X-ray diffraction (BRUKER-AXS, Germany). The surface properties and composition are conducted by the ESCALAB 250XI multi-functional imaging X-ray photoelectron spectroscopy (XPS) (Thermo Fisher Scientific, America). The test conditions are as follows: monochromatic Al Ka (hv = 1486.6 eV), power 150 W, 500 μm beam spot and the binding energy is calibrated with C 1s 284.8 eV. The morphology and particle size of the ZnFe2O4 were analyzed by the ΣIGMA + X-Max20 field emission scanning electron microscope (FESEM) (Carl Zeiss, Germany) with an accelerating voltage of 5 kV. The UV–vis absorption spectrum was measured using U4100 UV Spectrometer (Hitachi, Japan), and the wavelength range was from 200 nm to 800 nm. The TEM images, HRTEM images and SAED images of the samples were obtained by the Tecnai G2 F20 field emission transmission electron microscope (FEI, America) with an acceleration voltage of 200 kV. Adsorption-desorption curve of nitrogen in the sample was analyzed by the ASAP2020 (M) automatic surface area micro-gap analyzer (Micromeritics Instrument, America). 2.4. Photocatalytic performance evaluation The photocatalytic activity of the porous hollow cube ZnFe2O4 for the degradation of TCHC in aqueous was evaluated using a 300 W Xe lamp equipped with 350 nm–780 nm reflection filter and 420 nm cutoff filter (irradiation wavelength of 420 nm–780 nm) as the light source at ambient temperature. First, 500 mg/L of the catalyst was added into 100 mL solution of 40 mg/L of TCHC, and magnetic stirred 30 min to achieve sufficient adsorption/desorption equilibrium in dark. The liquid level of solution and the cut-off filter distance was 10 cm. Then, the sample was collected at given time intervals under light irradiation and throughout the reaction under constant magnetic stirring. The removed sample was centrifuged at 12,000 r/min for 5 min, and the supernatant was aspirated with a syringe and the concentration of TCHC was measured at a wavelength of 356 nm using the Alpha 1860 A UV–vis spectrophotometer (Puyuan, China). Degradation rate of TCHC can be calculated by the following formula:
2. Experimental sections 2.1. The experimental materials Polyvinylpyrrolidone (PVP, K30, MW∼40,000) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). K4Fe (CN)63H2O (≧99.5%) was purchased from Aladdin (Shanghai, China). Zinc acetate (≧98%) was purchased from Sinopharm Chemical ReagentCo., Ltd (Shanghai, China). Tetracycline hydrochloride (TCHC) (≧96%) was purchased from Aladdin (Shanghai, China). Eehanol (≧99.7%) was purchased from Fuyu Fine Chemical Co., Ltd (Tianjing, China).
Degradation Rate (DR) % = Ct/CO(%)
(1)
where CO was the absorbance of the sample before the dark reaction and Ct was the absorbance of the sample after degradation.
2.2. Synthesis of the porous hollow cube ZnFe2O4
3. Results and discussion
The porous hollow cube ZnFe2O4 material was prepared by using PB as the MOF precursor [28]. First, 20 g of PVP was dissolved into 250 ml of 0.1 M HCl solution under magnetic stirring 30 min to obtain a clear solution. Second, 0.6 g of K4Fe(CN)63H2O was added into the solution under magnetic stirring 30 min to obtain a bright yellow solution. The solution was sealed into an airtight container, and incubated at 80 °C for 24 h to get the blue insolubles. Third, the blue insolubles (PB) was centrifuged at 11,000 r/min and washed three times with distilled water and ethanol. The obtained precipitants were dried at 70 °C for 12 h in vacuum. The cubic PB was successfully synthesized. Fourth, 0.2 g of homemade PB was weighed accurately and dissolved into 40 ml of ethanol under magnetic stirring for 10 min. Fifth, 0.14 g of anhydrous zinc acetate was added into the solution under magnetic stirring for 10 min. Sixth, the solution was heated at 70 °C to evaporate the ethanol. The blue solid was collected and ground for 5 min in agate. At last, it was heated at 700 °C at a heating rate of 2 °C/min for 6 h. The porous hollow spinel cube ZnFe2O4 was prepared. The above steps were mainly divided into two major steps as shown in Fig. S1.
3.1. The formation mechanism of the material The fabrication process of the porous hollow cube ZnFe2O4 involved two steps. The First step was shape-controlled synthesis of PB precursor. In this step, Fe2+ was oxidized to Fe3+ by HCl, and then formed PB. During nucleation and growth processes of PB, polyvinylpyrrolidone (PVP) was employed as an effective capping gent could drive the Fe2+ and [Fe(CN)6]3− to self-assemble with a cubic shape [29]. In the second step, The Zn2+ was evenly dispersed in the PB surface, during calcination treatment in air and 2 °C/min, the ZnFe2O4 are formed on the surface of cubic. Simultaneously, the organic of MOF (PB) precursor including PVP and cyanide ligand could be slowly decomposed [30], and the small voids were generated when the gas diffused outward passing through ZnFe2O4 oxide layer in the process. The porous hollow cube ZnFe2O4 was synthesized when PB precursor completely consumed. The formation mechanism of the porous hollow cube ZnFe2O4 could be illustrated by Fig. 1. 3.2. Structural analysis
2.3. Characterization of the porous hollow cube ZnFe2O4 Fig. 2 showed the XRD patterns of the porous hollow cube ZnFe2O4. Diffraction peaks of the sample at 2θ were 18.20, 30.10, 35.00, 42.80,
X-ray diffraction (XRD) data for crystalline structure of ZnFe2O4 795
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Fig. 1. Schematic illustration of the formation mechanism of the porous hollow ZnFe2O4.
oxidized into Fe3+ [43]. Through Gaussian–Lorentzian fitting of the O 1s characteristic peak (Fig. 3d), the band energies at 531.35 eV was attributed to terminal of the OeH bond on the catalyst surface, while the band at 529.68 eV was ascribed to the surface lattice defect O2− [31]. 3.3. Morphology analysis The morphology of the porous hollow cube ZnFe2O4 and PB was investigated by SEM analysis. The samples in the SEM were measured by dispersing the sample on the silicon wafer with ethanol as the dispersant. As shown in Fig. 4, PB has a good cubic structure and an average particle size of 600 nm. The prepared ZnFe2O4 had a substantially monodisperse cube shape and an average particle size of the cube was 600 nm (Fig. 5a). And Fig. 5d shows clearer appearance of cube, and the cubic surface was formed by the accumulation of nanoparticles. This result revealed the ZnFe2O4 still maintains the cubic structure of PB. The more detailed morphology of porous hollow cube ZnFe2O4 was exposed using TEM, HRTEM and SAED. As shown in Fig. 5b, the sample had a cubic shape and an average particle size of the cube was about 600 nm. The surface of the cube was composed of many dense nanoparticles. This was consistent with the results of SEM. In Fig. 5e, the cube has part of the lighter in center, indicating the sample was hollow cubic shape. The lattice spacing of around 0.259 nm in the HRTEM image corresponds to the distance of < 311 > plane of spinel ZnFe2O4 (Fig. 5c). Finally, the ring-shaped pattern of SAED showed that the cubic spinel phase ZnFe2O4 had a polycrystalline structure (Fig. 5f), and four clear concentric rings to the center distance of 0.257, 0.214, 0.171 and 0.11 were assigned to < 311 > , < 400 > , < 422 > , and < 533 > planes of spinel ZnFe2O4 respectively, which were in agreement with the results of XRD.
Fig. 2. XRD patterns of porous hollow ZnFe2O4.
52.50, 57.20, 62.25 and 74.50 corresponding to < 111 > , < 220 > , < 311 > , < 400 > , < 422 > , < 511 > , < 420 > and < 533 > crystal planes of the spinel phase ZnFe2O4, respectively. The results were consistent with the standard card JCPDS No. 22-1012, indicating that the ZnFe2O4 was spinel structure. To further study the surface components and chemical states of porous hollow ZnFe2O4, the samples were characterized by XPS. From the full spectrum of porous hollow ZnFe2O4 in Fig. 3a, it could be seen that the sample is composed of Zn, Fe, O and C elements with no other impurities. The peak for C 1s at 284.8 eV was introduced during the reaction process or exogenously substances during the test. In Fig. 3b, the binding energies of Zn 2p1/2 and Zn 2p3/2 are 1044.5 eV and 1021.4 eV respectively. Compared with the standard spectra, it can be determined that Zn was mainly formed by Zn2+ [42]. The XPS spectrum of the Fe 2p was provided in Fig. 3c, in which the peaks at 725.3 eV and 711.25 eV could be assigned to the binding energies of Fe 2p1/2 and Fe 2p3/2, respectively, corresponding to the characteristic spectrum of Fe3+. In addition, the peak of 710.7 eV was due to the Fe2+ species, which may come from some amorphous phase in the final product [26]. However, the satellite peak appearing at 719.2 eV provided evidence for the existence of Fe3+. Thus, Fe2+ in PB had been
3.4. The optical property, BET analysis and magnetic property The optical property of the synthesized porous hollow cube ZnFe2O4 was investigated by UV–Vis diffuse reflection spectra (DRS). Exhibited in Fig. 6a, it could be seen that the porous hollow ZnFe2O4 had strong 796
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Fig. 3. The XPS spectra of the porous hollow ZnFe2O4 sample, (a) survey of the sample, (b) Zn 2p spectrum, (c) Fe 2p spectrum, (d) O 1s spectrum.
sample was ascribed to the microporous material. The magnetization-magnetic field ((M−H)) hysteresis loops of the porous hollow ZnFe2O4 were shown in Fig. S2. The results indicate that the composites were magnetic, the saturation magnetization (Ms) of the porous hollow ZnFe2O4 was about 15 emu/g, and the sample has soft and weak superparamagnetic nature, which was favorable for the magnetic recycling of the photocatalyst.
absorption intensity in the region of UV and visible light. The results suggested that the catalyst could show catalytic performance under visible light. In addition, the band-gap energy of the sample could be calculated using Tauc's plots: (αE)n = A·(E − Eg)
(2)
where α, Eg, E and A were indicative of the absorption coefficient, band gap energy of the semiconductor, hv (Planck's constant multiplied by light frequency) and constant value, respectively. As ZnFe2O4 was an indirect band-gap [32], so the value of n was 1/2, and (AE) 1/2 makes a straight line to the relation between E and the Eg of porous hollow ZnFe2O4, which could be estimated from the intercept between the straight line and the X axis (Fig. 6a inset). It could be estimated from the figure that the Eg of porous hollow ZnFe2O4 was about 1.5 eV. The N2 adsorption/desorption at 77 K was performed to characterize the pore structure and specific surface area of the porous hollow cube ZnFe2O4. As illustrated in Fig. 6b, the specific surface area of the sample was calculated to be 13.44 m2/g by the Brunauer–Emmett–Teller (BET) method. The pore size distribution of porous hollow ZnFe2O4 could be estimated by the Barrett–Joyner–Halenda (BJH) as showed in the inset of Fig. 6b. The result suggested that the
3.5. The catalytic performance evaluation In order to investigated the catalytic performance of the porous hollow cube ZnFe2O4 on the treatment of tetracycline hydrochloride, this study was investigated by the degradation of TCHC under visible light (780 nm > λ > 420 nm). l-ZnFe2O4, g-ZnFe2O4 and p-ZnFe2O4 represented the prepared porous hollow cube ZnFe2O4, commercially available ZnFe2O4 (Aladdin, China) and the ZnFe2O4 prepared by the coprecipitation method [33], respectively. And the 30 min before reaction without light. The results were shown in Fig. 7, and the TCHC in the absence of visible light catalyst without degradation, from the dark reaction stage of l-ZnFe2O4, g-ZnFe2O4, p-ZnFe2O4, TCHC adsorption capacity decreased in turn. From the catalytic results of these three
Fig. 4. SEM images of the PB. 797
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Fig. 5. (a, b) SEM images of the sample, (c, d) TEM, (e) HRTEM and (f) SAED pattern of the porous hollow ZnFe2O4 sample, respectively.
Fig. 7. The curves of the Photocatalytic degradation of TCHC by different photocatalysts.
Fig. 8. Kinetic studies for the photodegradation of TCHC over the different photocatalysts.
Fig. 6. (a) UV–vis DRS of the porous hollow ZnFe2O4, and (inset) Tauc’s plots to determine the band gaps for porous hollow ZnFe2O4, (b) N2 adsorption–desorption isotherms and corresponding size distribution curves (inset) of the porous hollow ZnFe2O4.
70 min illumination, while that of l-ZnFe2O4 was 85%, which was almost the same with 20 min. From the above results, the porous hollow ZnFe2O4 photocatalytic degradation of TCHC was shorter degradation time and higher degradation efficiency than the other two methods prepared ZnFe2O4 under visible light. The main reasons are that porous
samples, the degradation rates of l-ZnFe2O4, g-ZnFe2O4 and p-ZnFe2O4 were 80%, 42% and 22% after 20 min illumination, respectively. Each the degradation rate of g-ZnFe2O4 and p-ZnFe2O4 was about 55% after 798
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Table 1 Different catalysts in visible decline rate of the degradation of TCHC, kinetic constants and correlation coefficient. Catalysts
After 60 min, the degradation of TCHC
The kinetic equations
Kinetic constants of 60 min(k1, min−1, k2, mol−1 dm3 s−1)
Correlation coefficient (R2)
l-ZnFe2O4 g-ZnFe2O4 p-ZnFe2O4
84.08% 59.50% 49.97%
y = 0.5932x + 5.2881 y = 0.0118x y = 0.0096x
k2 = 0.0672 k1 = 0.0118 k1 = 0.0096
0.9999 0.9897 0.9952
Fig. 9. Schematic mechanism for TCHC degradation over the porous hollow ZnFe2O4.
intermediate. Finally, %OH could oxidize TCHC to small molecules. Photocatalytic degradation of TCHC using the porous hollow cube ZnFe2O4 could be expressed as follows:
hollow structure can increase the times of light scattering, so that the photocatalytic material has a larger light irradiation area, more light absorption and better photocatalytic activity in photocatalytic degradation of tetracycline hydrochloride. Because l-ZnFe2O4 was close to the degradation equilibrium at 60 min, 60 min was chosen as the research object to investigate the reaction kinetics. According to the fitting, g-ZnFe2O4 and p-ZnFe2O4 followed the first-order kinetic equation, while l-ZnFe2O4 followed the second-order kinetic equation (Fig. 8). The fitting equations were as follows: First-order kinetic equation: ln(CO/Ct) = k1 t
(5)
ZnFe2O4(e−) + O2 → %O2−
(6)
−
%O2 + H → %OOH +
−
%OOH + H + 2e → %OH + 2OH +
(7) −
−
ZnFe2O4(h ) + OH → %OH +
−
h , %O2 , %OOH, %OH + TCHC → small molecules +
(3)
(8) (9) (10)
The generation of active species (%OH) under visible-light irradiation was investigated using the electron paramagnetic resonance (EPR) technique to capture active free radicals. As shown in Fig. S3, there are no distinct EPR peaks in the dark, however, after illumination with visible light, quartet characteristic peaks of the DMPO-OH% are observed. The result indicating that %OH was generated after light irradiation.
Second-order kinetic equation: t/ln(CO/Ct) = t/a+1/(k2a2)
ZnFe2O4 + hv → ZnFe2O4(e− + h+)
(4)
Where CO and Ct was the mass concentration of TCHC before and after photoreaction, k1 and k2 was reaction rate constant, a was the value of ln (CO/Ct) when reaction equilibrium. The fitting results were as shown in Table 1, the constant of p-ZnFe2O4 was less when compared to gZnFe2O4 and the reaction rate of l-ZnFe2O4 was proportional to the quadratic of the reactant concentration. The higher rate constant values for l-ZnFe2O4 confirmed their superior photocatalytic activity.
4. Conclusions In summary, the novel porous hollow cube ZnFe2O4 was successfully synthesized by using MOF (PB) as the precursor. The porous hollow cube ZnFe2O4 degrade 80% of TCHC (40 ml/L, 100 ml) in 20 min under visible light, and its degradation effect is obviously higher than ZnFe2O4 materials prepared by other methods. This shows that controlled microstructure of the material has a significant impact effect on the photocatalysis, and the porous hollow structure can increase the times of light scattering, so that the photocatalytic material has a larger light irradiation area, more light absorption and better photocatalytic activity in catalytic performances, indicating the relationships between the microstructure of the material and the photocatalytic activity.
3.6. Mechanism of photocatalytic activity According to the previous literatures [1,8,10,34], the key factors for the degradation of the TCHC were generated oxidation species that the photo-induced holes, super-oxide radical (%O2−) and hydroxyl radical (%OH) in the photocatalytic process. The possible mechanism for the TCHC over the porous hollow ZnFe2O4 could be proposed, as illustrated by Fig. 9. Firstly, the porous hollow ZnFe2O4 had intricate pore structures that could possess more effective light absorption for multiple scattering. Then, the porous hollow ZnFe2O4 were irradiated by visible light, electrons in the valence band (VB) could be excited to the conduction band (CB) and simultaneously the same amounts of h+ were generated in the VB. The electrons formed can react with the adsorbed molecular oxygen to yield %O2− in the CB [35]. The generated %O2− then further combine with H2O and H+ to produce %OH via series of radical reactions. This process could be occurred %OOH that was the
Acknowledgments This work was supported financially by Natural Science Foundation of China (No. 21461005), the Major applied basic research project (Guizhou branch in JZ word [2013]6004) and science and technology planned project in Guizhou Province (Qian kehe foundation [2018] 799
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1052).
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