Jiafei Wu, Yuning Jin, Danping Wu, Xiaoying Yan, Na Ma, Wei Dai,*
1 Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, College of Chemistry and Life Science, Zhejiang Normal University, Jinhua 321004, China
2 National Center for Nanoscience and Technology, Beijing 100190, China
3 College of Geography and Environmental Sciences, Zhejiang Normal University, Jinhua 321004, China
Keywords:Degradation Catalyst Tetracycline ZIF-8 Zn2SnO4/SnO2 Adsorption
ABSTRACT Nowadays,efficient removal of antibiotic(e.g.tetracycline,TC)from water bodies has been of great global concern.Hereby, a novel Zn2SnO4/SnO2@ZIF-8 photo-catalyst was first synthesized by a facile selfassembly in-situ growth method.Zn2SnO4/SnO2@ZIF-8 exhibits a remarkable photocatalytic activity towards TC under visible-light driven with a rapid rate constant of 1.5×10-2 min-1,and a removal efficiency of 81.2%, which is superior to some catalysts in the literature.Importantly, the photocatalytic activity of Zn2SnO4/SnO2@ZIF-8 could still remain ～77% after the fifth cycle, indicating its good stability and reusability.The remarkable performances of adsorption and photocatalytic were attributed to associative effects of catalytic activity of Zn2SnO4/SnO2 and the unique porous nanostructure and stability of ZIF-8.This work could aid the future design and preparation of novel MOFs composite catalysts for efficient elimination of antibiotics from water.
As one of the most typical antibiotics,tetracycline(TC)is a kind of polycyclic and tetrastyroxylamide compound with antimicrobial properties.It has a wide utilization in animal husbandry, in agriculture, and as fungistat in medical treatment [1,2].However, it cannot be totally metabolized in the bodies of humans or animals,and most of the consumed TC being released into the environment’s water.Accumulation of trace TC in water can increase the probability of triggering antimicrobial resistance, and expand the risk of transmitting antimicrobial-resistant bacteria through the food chain [3,4].Thus, it is necessary to develop effective material and method for TC removal from wastewater.So far, semiconductor photo-catalysts have exhibited attractive prospects in energy and environmental applications [5–7].Development of semiconductor photo-catalysts with excellent performance is the research focus of researchers.Among them, thanks to the low price, environmental friendliness, simple preparation process, and high catalytic efficiency, research on n-type semiconductor photocatalyst with electrons as the main carrier for conduction is an extremely hotspot.For example, the n-type TiO2microspheres developed by Liuetal.could produce 0.2 mmol H2per gram of catalyst through photocatalytic hydrolysis [8]; Another case, the degradation rate of crystal violet by the thin films of threedimensional ordered macro-porous nano-crystalline Fe2O3catalyst could reach 97.9% within 30 min with H2O2as oxidant [9].Some other cases of the n-type semiconductor photo-catalysts such as BiVO4, ZnInS, and Zn2SnO4have also shown potential application[10–12].However most of the currently reported n-type semiconductor are attributing to single-component catalysts.The low separation efficiency and high recombination rate of the singlecomponent could restrict its further popularization and application.Typical issues also enclosed the broad band gap could only be motivated by ultraviolet or near-ultraviolet radiation[13],short distance of carrier diffusion [14], and lower electron-hole separation efficiency [10],etc.Therefore, the bottleneck issues of the ntype semiconductor photo-catalysts need to be further researched and developed urgently.
In response to the above-mentioned bottlenecks in the development of n-type single-component photo-catalysts, various strategies such as the double semiconductors heterojunctions, noblemetals modification, doping of active components, and coupled with carbon have been explored to improve the photocatalytic efficiency [15–18].Noteworthy, constructing a heterojunctions by coupled double semiconductors, and exploiting the advantages of the their associative effects is considered as an effective strategy to achieve the spatial separation of optical excitation electronhole pairs [19,20].For example, Dadigalaetal.constructed a p-n type semiconductor heterojunction material (MoS2/CoTiO3), and the photo-degradation efficiency of ciprofloxacin and biphenyl A accordingly reached 91.8% and 82.1% under sunlight [15]; A n-n type catalyst (CdS@ZnIn2S4) developed by Zhangetal.could efficiently decompose pure water into H2and H2O2under visiblelight irradiation[21];A p-n type composite (SrFe12O19/MoS2) constructed by Chenetal.can achieve a degradation rate of 97%towards methylene blue [22].Nevertheless, preparations of ntype semiconductor catalysts were carried out through two-step or multi-step methods in previous works [15,21,22].Due to the uneven distribution of elements and poor active site dispersion,it is difficult to achieve excellent photocatalytic activity by twostep or multi-step synthesis methods.Thus,it is urgent and necessary to further develop and explore novel synthetic strategy of catalysts.
Through review of related literature, we realized that the Zn2-SnO4had high electron mobility (10–15 cm2∙V-1∙s-1) [23], high electrical conductivity, and good stability, which might be a promising photo-catalyst.Anyway, pure Zn2SnO4is not perfect because of wide band gap (3.6 eV), and high recombination rate,etc.Literature indicated that the band gap of SnO2was consistent with that of Zn2SnO4one [24].The conduction band (CB) edge potential of Zn2SnO4(-0.22 eV) exhibits further negative than SnO2(0.05 eV) ones [24] because that the electrons could easily transfer to the CB of SnO2, resulting in effective electron-hole pair separation[25].Zn2SnO4and SnO2were composited by Lupanetal.to prepare nanowires materials, which was used as highperformance ultraviolet detectors(decay time ～100 ms)and ethanol vapor sensor devices (Igas/Iair≈3.5) [23].Zn2SnO4/SnO2was also explored for dye-sensitized solar cells and its conversion efficiency even can reach 1.18% [26].However, the reports of n-type semiconductor catalyst prepared by the bi-components materials for antibiotics capture are still scare, and deserve further study to achieve excellent catalytic materials.
Additionally, catalytic oxidation reaction would be achieved when the antibiotics molecules were adsorbed onto catalyst surface and irradiated by a light source with a certain wavelength to generate photo-generated electron-hole pairs.To our best knowledge, how to quickly enrich and restrict antibiotics molecules around the active sites of catalyst, and enhance catalytic activity is also a key scientific issue.Zeolitic imidazolate frameworks(ZIFs)are constructed from tetrahedrally coordinated transition metal(e.g.Zn,Co)ions linked by organic imidazole units,possess similar structures to zeolites [27–29].Thanks to good features of large specific surface area, uniform pore size, ZIFs exhibit extremely potential application in many fields such as adsorption,separation,gas storage,and photo-catalysis,etc[27–29].As one of typical case,the ZIF-8 owns excellent characteristics such as a higher surface area,a large pore-cage with a pore size of ～11.6 Å,good hydrothermal, and chemical stability,etc., which is applied as adsorbent for antibiotics capture from water.It has been studied and used to improve catalytic activity of semiconductor materials [30,31].In our previous works [32,33], novel CuBi2O4@ZIF-8 and ZnInS NS@ZIF-8 NFs composites were achieved.In comparison with CuBi2O4and ZnInS NS, the removal rates of CuBi2O4@ZIF-8 and ZnInS NS@ZIF-8 catalysts towards TC accordingly increased 23%and 20%.However, these previous works only focus on the single-component semiconductor materials.Thus, development of a bi-component material simultaneously with higher photocatalysis activity is great necessary.
In this work,selection reasons of the ZIF-8 was due to its better selective adsorption ability towards TC, and its porous structure can restrain TC molecules around the surface active sites of catalyst.Additionally, water resistant ability of ZIF-8 is much better than many other MOFs [30,31].Zn2SnO4and SnO2was combined to construct a bi-component catalyst Zn2SnO4/SnO2, and a novel Zn2SnO4/SnO2@ZIF-8 composite was achieved by hydrothermal synthesis method.ZIF-8 can be wrapped onto the surface of Zn2-SnO4/SnO2throughin-situgrowth technique,improving its photocatalytic activity towards TC (Fig.1).The microstructure,photocatalytic activity, and mechanism of composite were deeply investigated and discussed.This work could offer a novel strategy for advanced MOFs composite catalysts.
Samples characterizations were carried out by TGA (Thermo-Gravimetric Analysis), SEM (Scanning Electron Microscopy), EDS(Energy Dispersive X-ray Spectroscopy), FT-IR (Fourier Transform Infrared), XRD (X-ray Diffraction), XPS (X-ray photoelectron spectroscopy), N2adsorption–desorption isotherms, UV–vis and PL(Photoluminescence).Reagents and experimental process of the characterizations can be found in the Supplementary Material.
2.2.1.SynthesisofZn2SnO4/SnO2
Zn2SnO4/SnO2was synthesized on basis of previous report with a slightly adjustment [34,35].Typically, SnCl4∙5H2O (3.5 g,0.01 mol) was added into 50 ml ethanol and stirred at 1000 rpm for 10 min to form solvent A, and then the Zn(Ac)2∙2H2O (2.2 g,0.01 mol) and sodium dodecyl sulfate (SDS, 3.5 g, 0.01 mol) were added into 100 ml distill water,and kept stirring 10 min to prepare solution B.Solutions A and B were equal volume mixed at 25°C to form solution C.Then the NaOH aqueous solution(50 ml,2 mmol)was added into the solution C,stirring at 1000 r∙min-1for 5 min at 25°C.A white product(ZnSn(OH)6cubes)can be observed and precipitated on the bottom of beaker.The solid product were obtained after centrifugation, washing (deionized water and absolute ethanol),and drying(60°C).The as-synthesized ZnSn(OH)6solid cubes were heated to 650°C for 2 h with a rising rate of 1°C∙min-1.Then,the Zn2SnO4/SnO2catalyst can be achieved after it was cooled down to 25 °C.
2.2.2.SynthesisofZn2SnO4/SnO2@ZIF-8
Firstly,2-methylimidazole(0.205 g,2.5 mmol)in 20 ml methanol with ultra-sonication treatment for 10 min to form solution D.Secondly, the as-obtained Zn2SnO4/SnO2(0.5 g) was dispersed in solution with a stirring constantly at 1000 r∙min-1for 12 h at 25 °C.Thirdly, the 0.05 g Zn(NO3)2∙6H2O was added in solution C and stirred with 1000 r∙min-1for 2 h at 50°C.Finally,resultant precipitate was washed clean by ethanol, and dried at 60 °C in an oven.The gotten sample was noted as Zn2SnO4/SnO2@ZIF-8.
Photocatalytic activities of as-synthesized photo-catalysts towards TC were investigated under visible-light driven (CELS500 Plus, B.Z.J.Technology Co., Ltd.).In detail, 20 mg Zn2SnO4/SnO2@ZIF-8 catalysts were dispersed in 80 ml TC solution including (20 mg∙L-1) in a cylindrical reactor with condensed water and stirred in the dark for 30 min to achieve the adsorption equilibrium.A 500 W Xenon lamp equipped with a 420 nm cut filter was used to simulate visible light source.At 5 min interval,a 4 ml mixture in the reaction system was extracted and centrifuged,then monitored the residual concentration of TC by dual-beam UV–vis spectrophotometer at the maximum absorption wavelength of 357 nm.In the meantime,a blank comparison test without catalyst was also carried out.
Fig.1. Schematic illustration of synthesis procedure for Zn2SnO4/SnO2@ZIF-8.
FT-IR spectra of Zn2SnO4/SnO2, ZIF-8, and Zn2SnO4/SnO2@ZIF-8 were displayed in Fig.2(a).A typical well-defined absorption bands attributed to Sn—O (583 cm-1) and Zn—O (420 cm-1) of the Zn2-SnO4/SnO2were observed clearly from the curves of Zn2SnO4/SnO2or Zn2SnO4/SnO2@ZIF-8 [36,37].It was similar with original ZIF-8 that the Zn2SnO4/SnO2@ZIF-8 displayed the same absorption of the imidazole ring stretching vibration,ring’s in-plane or out-ofplane bending vibrations peaked at 1340–1540,940–1330 or 660–810 cm-1, aromatic C—N stretching at 1146 cm-1, C—N bending vibration at 995 cm-1[38–40].Furthermore, characteristic peak of Zn2SnO4/SnO2@ZIF-8 at 420 cm-1owing to the Zn—O stretching vibration of Zn2SnO4/SnO2or Zn—N stretching vibration of ZIF-8 was accordance with the results in literature [33,36,37].Anyway,these characteristics peaks could be indirect evidences to confirm the existence of Zn2SnO4/SnO2and ZIF-8 in the composite.
Usually, XRD was explored to detect the phases of as-prepared samples.Herein, the XRD spectra were further compared to confirm successful growth of ZIF-8 components onto the surface of Zn2SnO4/SnO2cubes.Typical XRD pattern (Fig.2(b)) depicted the Bragg reflections of pristine Zn2SnO4cubes, matching well with the standard rutile phase of Zn2SnO4standard cards (JCPDS Card no.24-1470).Diffraction peaks at 26.5° could be indexed to the(1 1 0) plane of SnO2(JCPDS Card no.46-1088).In the meantime,all the peaks observed from original ZIF-8 were in the same positions compared with the simulated XRD spectra of ZIF-8(obtained by Material Studio 7.0), indicating as-prepared sample possessed the same crystalline structure of ZIF-8.No other peaks appearances indicated there was no other impurities in the composite.Characteristic peaks of the Zn2SnO4/SnO2cubes can be obviously detected in the diffraction peaks of Fig.2(b), and the diffraction peaks at 7.3°, 10.3°, and 12.7° could be indexed to ZIF-8s (0 1 1), (0 0 2),and(1 1 2)planes(Fig.2(c))[33,41].These interesting results were another indirect evidences for ZIF-8 to be coated onto the surface of Zn2SnO4/SnO2.
In order to provide a deep sight into the surface topography of as-synthesized samples, the morphology and composition of Zn2-SnO4/SnO2@ZIF-8 were accordingly detected by SEM and EDS(Fig.3).The pristine Zn2SnO4/SnO2exhibited a uniform cubes with rounded edges and corners(Fig.3(a)–(c)).After surface coated with ZIF-8 crystals (Fig.3(d)–(f)), the overall cube morphology of Zn2-SnO4/SnO2@ZIF-8 hybrids was retained.However, after ZIF-8 nanoparticles were coated and accumulated onto Zn2SnO4/SnO2surface, the Zn2SnO4/SnO2@ZIF-8 composite turned into rough originally and larger particle size (50–100 nm) compared with the Zn2SnO4/SnO2, indicating that the ZIF-8 was successfully coated onto the surface of Zn2SnO4/SnO2.Moreover,EDS elemental mapping (Fig.3(g)) revealed that C, O, N, Zn, and Sn were all uniformly distributed across the whole particle.
N2adsorption–desorption isotherms of as-prepared Zn2SnO4/SnO2, Zn2SnO4/SnO2@ZIF-8, and ZIF-8 samples were accordingly plotted in Fig.4(a).ZIF-8 curve was identified as a typical type-I isotherm on basis of the IUPAC definition [42], fitting perfect with the microporous feature of ZIF-8 crystal.The beginning of N2adsorbed stage with a steep increase for N2uptakes at a low relative pressure could be attributed to microporous nature.The curves of Zn2SnO4/SnO2cubes displayed an obvious hysteresis loop(type-IV shape) atP/P0of 0.8–1.0 [42], indicating the existence of mesoporous structure.Specific BET surface area of the Zn2SnO4/SnO2sample was only 18.48 m2∙g-1.But the BET surface area of Zn2SnO4/SnO2@ZIF-8 reached 64.06 m2∙g-1,attributing to a contribution of porous ZIF-8 in the composite.It should be noted that the initial stage of N2uptake onto Zn2SnO4/SnO2@ZIF-8 was higher than that of Zn2SnO4/SnO2(Fig.4(a)), demonstrating that more microporous and higher surface area of the Zn2SnO4/SnO2@ZIF-8[39], which was also confirmed by the pore size distribution(Fig.4(b)).
In order to determine and compare the thermal stabilities of asprepared samples, TG tests were explored under air atmosphere(Fig.4(c)).Initial mass decrease of Zn2SnO4/SnO2@ZIF-8 composite(～4%) was attributed to the evaporation of water molecules from the catalyst particles.Obviously, the mass decrease appeared within 567–673 °C region, attributing to a quickly decomposition of ZIF-8 framework.While the heating temperature reached 700°C,the ZIF-8 structure completely generated to ZnO.The mass decrease remained steady trend (～12%) with further increasing of the heating temperature.Importantly,we could calculate and conclude the content of ZIF-8 in the Zn2SnO4/SnO2@ZIF-8 sample was～19%(mass)according to the mass loss of pure ZIF-8 in air(～64%).
Fig.2. (a) FTIR spectra of Zn2SnO4/SnO2, Zn2SnO4/SnO2@ZIF-8, and ZIF-8.(b) XRD patterns of Zn2SnO4/SnO2, Zn2SnO4/SnO2@ZIF-8, and ZIF-8 and (c) magnified image.
Chemical states of as-prepared samples were also examined by XPS.The Fig.5 displayed the typical XPS spectra of Zn2SnO4/SnO2@ZIF-8 nanocomposite before and after adsorption of TC.The Fig.5(a) confirmed the existences of C (C 1s, 284.8 eV), N (N 1s, 398.7 eV), O (O 1s, 531.4 eV), Zn (Zn 2p, 1022.2 eV), and Sn(Sn 3d, 486.5 eV) elements both in the XPS spectra of Zn2SnO4/SnO2@ZIF-8 and TC adsorbed onto Zn2SnO4/SnO2@ZIF-8 samples.Fig.5(b) displayed the C 1s XPS spectra with deconvolution of peaks (Table S1 in Supplementary Material).The peak centered at 284.8 eV was accordance with sp2C—C bonding, while sp2-hybridized carbon of N—C=N bonding in dimethylimidazole ligand addressed a peak centered at 286.03 eV [43].In comparison with Zn2SnO4/SnO2@ZIF-8, the C 1s spectra for TC adsorbed onto Zn2-SnO4/SnO2@ZIF-8 sample can be divided into three peaks, and an additional group of C—OH bonds at 285.7 eV appeared, which was related to the existence of phenolic hydroxyl group generated from TC molecules.Fig.5(c)exhibited the O 1s spectra,and deconvoluted constituent peaks were collected in Table S2.By fitting,there were two peaks located at binding energy of 530.8 eV and 531.9 eV for the high-resolution O 1s spectrum,which were attributed to the lattice oxygen within host Zn2SnO4/SnO2cubes,and the surface oxygen species (e.g.hydroxyls) [35].In comparison with Zn2SnO4/SnO2@ZIF-8, TC adsorbed onto the material further decreased in binding energy and improvement of Zn-O(530.5 eV) indicated the existence of =C—OH groups and their coordination with Zn.High-resolution doublet peaks of Zn 2p(Fig.5(d)) were ascribed to the binding energy of 1021.9 eV (Zn 2p3/2)and 1044.8 eV(Zn 2p1/2),respectively[37].With the advance process of TC adsorbed, the Zn 2p spectra exhibited coincident movement towards strong energy of Zn 2p1/2(1021.9 eV),demonstrating the combination of Zn with electronegative O atom.The N 1s peak of Zn2SnO4/SnO2@ZIF-8 (Fig.5(e) and Table S3) can be divided into two constituent peaks.The peak position at 398.7 eV was attributed to sp2-hybridized aromatic nitrogen atoms bonded to carbon atoms (C=N—C), and the peak position at 399.6 eV can be ascribed to the N bonded with H atoms (N—H)[44].After TC adsorbed, the N 1s spectra can be divided into three peaks, adding a group of tertiary amine group (N—(C)3) peaks at 399.2 eV, which was related to the groups of the tertiary amine or terminal amino.There were two strong binding energies at 486.5 eV and 495.0 eV of Sn 3d in Fig.5(f),attributing to the bonding energies of Sn 3d3/2 and Sn 3d5/2 [37].
Fig.3. (a) Overall and (b, c) magnified SEM images of Zn2SnO4/SnO2.(d) Overall and (e, f) magnified SEM images of Zn2SnO4/SnO2@ZIF-8.(g) EDS mapping of Zn2SnO4/SnO2@ZIF-8.
Visible-driven photocatalytic activities of Zn2SnO4/SnO2, Zn2-SnO4/SnO2@ZIF-8, and ZIF-8 towards TC were evaluated.Fig.6(a)–(c) exhibited the temporal UV–vis absorption spectra derived from the dark test and the photocatalytic reaction of Zn2SnO4/SnO2@ZIF-8, Zn2SnO4/SnO2, and ZIF-8, respectively.The Zn2SnO4/SnO2@ZIF-8 behaved a quick degradation rate than that of the pristine Zn2SnO4/SnO2(Fig.6(d)).In comparison with Zn2SnO4/SnO2,the absorption peaks of Zn2SnO4/SnO2@ZIF-8 appeared continuous red-shifts,attributing to the different intermediates(Fig.6(a),(b)).The red-shifts phenomenon were usually considered to the formations of expanded conjugate structure of TC-metal complex compounds.In addition, the UV–vis spectra positions of Zn2SnO4/SnO2@ZIF-8, Zn2SnO4/SnO2, and ZIF-8 demonstrated that redshifts occurred under darkness condition, indicating the TC was adsorbed through coordination with Zn2+of ZIF-8 structure(Fig.6(a)–(c)) [32,40].At the same time, photo-degradation rate of the Zn2SnO4/SnO2@ZIF-8 (Fig.6(d)) also increased form 1.08 × 10-3min-1(ZIF-8) and 6.6 × 10-3min-1(Zn2SnO4/SnO2)to 1.5 × 10-2min-1.The photo-degradation rate of Zn2SnO4/SnO2@ZIF-8 was about 13.9 and 2.3 times than those of ZIF-8 and Zn2SnO4/SnO2.Through comparisons, the photocatalytic degradation activity of optimized Zn2SnO4/SnO2@ZIF-8 towards TC was superior to most of the previously reported catalysts related to SnO2and ZIF-8 (Table S5).To evaluate the chemical or photocatalytic stability, photocatalytic evaluations of Zn2SnO4/SnO2@ZIF-8 for five times recycles were executed (Fig.6(f)).After fifth round reaction,the removal rate of TC decreased slightly from 81.2% to 77.1% within 60 min, indicating a very good stability of the composite.TC reaction kinetic rate and degradation performance were investigated and presented in Fig.S1.In Fig.S1, theXaxis was accordingly denoted as ‘‘2”, ‘‘1.5”, ‘‘1”, ‘‘0.5”, and‘‘0.25”when the dosages of 2-methylimidazole and Zn(NO3)2∙6H2-O were 0.41 and 0.1 g,0.3075 and 0.075 g,0.205 and 0.05 g,0.1025 and 0.025 g,0.0513 and 0.0125 g,respectively.It was clear that the kinetic rate and degradation performance reached the maximum value when the dosage was ‘‘1”.The photocatalytic activity towards TC by Zn2SnO4/SnO2@ZIF-8 was significantly increased with an increase of ZIF-8 coating amount.While if more ZIF-8 nanoparticles were introduced into the support,the photocatalytic activity had a slightly decrease,possibly due to the‘‘shading effect”[45,46].
Fig.4. (a)Nitrogen adsorption–desorption isotherms of Zn2SnO4/SnO2,Zn2SnO4/SnO2@ZIF-8,and ZIF-8.(b)Pore size distribution of Zn2SnO4/SnO2@ZIF-8.(c)TGA of Zn2SnO4/SnO2, Zn2SnO4/SnO2@ZIF-8, and ZIF-8.
Fig.5. XPS spectra of Zn2SnO4/SnO2@ZIF-8 and TC adsorbed Zn2SnO4/SnO2@ZIF-8 samples: (a) Survey, (b) C 1s, (c) O 1s, (d) Zn 2p, (e) N 1s, and (f) Sn 3d.
Fig.6. The UV–vis absorption of(a)Zn2SnO4/SnO2@ZIF-8,(b)Zn2SnO4/SnO2,and(c)ZIF-8,respectively.(d,e)Photocatalytic degradation curves and first-order kinetic fitting of curves of reactions catalyzed by series samples (λ ≥420 nm).(f) Cycle experiments of TC degradation.
The associative effects of Zn2SnO4/SnO2and ZIF-8 can definitely exhibit extraordinary photocatalytic activity towards TC.Electrochemical Impedance Spectra(EIS)was further explored to analysis the interfacial carriers’kinetics and their roles(Fig.7(a)).Zn2SnO4/SnO2@ZIF-8 appeared distinctly decrease of EIS than either single sample, demonstrating an increase of carriers’ transportation.Fluorescence spectra of TC, ZIF-8, and TC adsorbed over ZIF-8 were accordingly presented in Fig.7(b).The fluorescence intensity increased significantly after TC adsorbed onto ZIF-8, accompanied by evidently red-shifts than that of ZIF-8.This was consistent with the reported fluorescence of tetracycline metal complex, which confirmed the zinc binding of tetracycline to ZIF-8[47–50].Further photoluminescence(PL)lifetime research indicated that PL lifetime of TC adsorbed over ZIF-8 was more than these of TC and ZIF-8,which was in accordance with the conjecture to form a new conjugate structure (Fig.7(c) and Table S4).Compared with Zn2SnO4/SnO2@ZIF-8, the PL intensity of TC obviously decreased and the PL lifetime reduced at the meantime (Fig.7(d)–(e) and Table S4),and the emission intensity at 505 nm enhanced, suggesting a increase of the carriers’ transfer ability through Zn2SnO4/SnO2.
Fig.7. (a)The electrochemical impedance spectra of Zn2SnO4/SnO2@ZIF-8,Zn2SnO4/SnO2,and ZIF-8.(b)The PL and(c)the transient PL of ZIF-8,TC adsorbed ZIF-8 and TC.(d)The PL and (e) the transient PL of Zn2SnO4/SnO2@ZIF-8, TC adsorbed Zn2SnO4/SnO2@ZIF-8 and TC.(f) The FTIR spectra of ZIF-8, TC, and TC adsorbed ZIF-8.
The interaction and coordination existed between TC and ZIF-8 were further analyzed by FT-IR.The FT-IR spectra curves of ZIF-8,TC, and TC adsorbed over ZIF-8 were accordingly presented in Fig.7(f).There were obvious changes for the groups of the hydroxyl and carbon after TC adsorbed for 30 min.Accompanied by significant width broaden, the characteristic peak position of TC shifted from 3357 cm-1to high frequency.This was in accordance with previous results about FT-IR variation induced by associated hydrogen binding, demonstrating the hydrogen bonds through TC adsorbed can be formed [47–50].At the meantime, C=O stretching vibration of TC at 1670 cm-1shifted toward lower energy position after TC adsorbed, demonstrating carbonyl groups participated in the formation of conjugated regions.The weakened C=O stretching vibration and enhanced Zn-N vibration further confirmed the formation of relevant hydrogen bonds and the complexation of these hydroxyls with the opened Zn sites on the surface of ZIF-8, which could also demonstrate the UV red-shift to 374 nm after the darkness reaction process.The changed mechanism of photocatalytic degradation would also change the TC degradation rates, attributing to the effects of TC-(ZIF-8)associations.
Additionally, the light absorption performances of materials were also tested by UV–vis spectra (Fig.8(a)), and optical band gaps of Zn2SnO4/SnO2@ZIF-8,Zn2SnO4/SnO2,and ZIF-8 were calculated through the Tauc formula.Plot of(αhv)2vs.hvwas displayed in Fig.8(b) and (c), where α was absorbed coefficient,hvwas the photon energy [51].ZIF-8 exhibits a broad absorbance intensity at ～300 nm, which are mainly ascribed to the ligand-to-metal charge transfer.Owing to the existence of ZIF-8 nanoparticles,there is obvious red-shifts for the absorption edges of Zn2SnO4/SnO2@ZIF-8,demonstrating the ZnN4structure of Zn2SnO4/SnO2@-ZIF-8 and ZIF-8 matrix could induce a metal-to-ligand charge transfer through an energy transfer by the organic imidazole ligand.In addition, the synergistic interactions between ZIF-8 and Zn2SnO4/SnO2lead to a change of electronic structure.These results significantly improved the light absorption intensity of Zn2-SnO4/SnO2@ZIF-8 under visible light, indicating its great potential application for photocatalysis [52–55].Fig.8(d) displayed valence band spectra of the Zn2SnO4/SnO2, ZIF-8, and Zn2SnO4/SnO2@ZIF-8, whose valence bands were 2.98 eV, 2.63 eV, and 2.75 eV,respectively.
Fig.8. (a)UV–vis absorption spectra of Zn2SnO4/SnO2,ZIF-8,and Zn2SnO4/SnO2@ZIF-8.Optical band gap energy plots of(b)Zn2SnO4/SnO2,Zn2SnO4/SnO2@ZIF-8 and(c)ZIF-8.(d) The valence band spectra of Zn2SnO4/SnO2, ZIF-8, and Zn2SnO4/SnO2@ZIF-8.
Several trapping experiments were performed to illustrate the major active species of Zn2SnO4/SnO2@ZIF-8 catalyst towards TC photo-degradation.We selected the isopropyl alcohol (IPA,1 mmol), ethylenediamineteraacetic acid disodium salt dihydrate(EDTA, 1 mmol), and p-benzoquinone (BQ, 1 mmol) as scavengers for,·OH, and h+[56].As exhibited in Fig.9(a), because of the doping of EDTA, the TC degradation rate dropped from 81% to 0%,and after the addition of BQ, the degradation rate of TC also decreased to 38%.This result demonstrated thatand h+played a decisive status during the photocatalytic degradation process under visible light irradiation.As IPA scavenger was putted to the photo-degradation system, the TC elimination rate decreased slightly, indicating that·OH played little role in the reaction process.According to the effect of the scavenger on the TC elimination rate,an order as follows:No scavenger>IPA>BQ>EDTA.Thus, it can be conclude that theand h+were the main active species for the TC degradation process.
Fig.9. (a) Radicals quenching tests of Zn2SnO4/SnO2@ZIF-8.(b) Mott-Schottky plots of Zn2SnO4/SnO2 and ZIF-8.
Probable photo-catalysis mechanism of Zn2SnO4/SnO2@ZIF-8 towards TC was illustrated in Fig.10.Adsorption testing for 30 min in darkness, the TC and oxygen molecules were adsorbed onto the surface of the composite, and captured by the active capacities.According to Mott-Schottky plots (Fig.9(b)), both Zn2-SnO4/SnO2and ZIF-8 exhibited n-type semiconductor feature with positive slopes for the curves of 1/C2versuspotential,and their flat band potentials at Ag/AgCl electrodes were -0.46 V.Based on the formula ofENHE=EAg/AgCl+ 0.1976 (25 °C) [57], it was found that the flat band potential of them and ordinary hydrogen electrode(vs.NHE) were -0.26 V.XPS spectra of valence-band indicated in-depth that the energy gap between VB and Fermi level were accordingly 2.98 and 2.63 eV for Zn2SnO4/SnO2and ZIF-8 (Fig.8(d)).Assuming that the flat band potential of n-type semiconductor was Fermi level for n-type semiconductor,the VB sites of Zn2SnO4/SnO2and ZIF-8 accordingly reached 2.36 and 2.01 V (vs.NHE).While the CB of Zn2SnO4/SnO2and ZIF-8 were accordingly -0.38 and-2.98 V(vs.NHE)according to the band gap.Under dark reaction conditions, ZIF-8 could enrich TC on Zn2SnO4/SnO2@ZIF-8 composite surface.Then, by visible light irradiation, the Zn2SnO4/SnO2can generate and separate electron hole pairs, which can react with dissolved O2to form·O2–, promoting multi-step oxidation of TC.The construction of Zn2SnO4/SnO2heterojunction material and the coating of ZIF-8 to construct the composite not only promotes the separation of electron holes, but also restricts the active species and intermediates in a smaller spatial, which was advantage to the coupling of continuous reaction steps.Especially,through shortening diffusion distance between reactants, this‘‘nano-reactors”structure would be favor with the transformations towards active species with an exceedingly short lifetime [40].Thus, the Zn2SnO4/SnO2@ZIF-8 not only owned the favors of the individual component, but also exhibited highly catalytic activity because of the associate effects of Zn2SnO4/SnO2and ZIF-8.
Fig.10. Schematic diagram of TC photo-degradation mechanism with Zn2SnO4/SnO2@ZIF-8.
In conclusion, the facile and efficient fabrication of a new-type perfect Zn2SnO4/SnO2@ZIF-8 catalyst was successfully developedviaan‘‘in-situgrowth”strategy.The FT-IR,XRD,SEM,EDS,nitrogen adsorption–desorption isotherm and TG confirmed the microstructure, further XPS, EIS, PL, FT-IR and UV–vis illustrated the surface states,chemical/composition states,and interaction between catalyst and TC.The Zn2SnO4/SnO2@ZIF-8 emerged superior photocatalytic activity towards TC with a constant rate of 1.5×10-2min-1and an elimination rate of 81.2%,which was much higher than that of Zn2SnO4/SnO2.Steady-transient PL and EIS indicated that the combination of Zn2SnO4/SnO2and ZIF-8 favored the carriers’separation and transfer.Active substances and intermediates were restricted in space to enhance the coordination between multistep degradation reactions.This ZIF-8 coated onto the surface of the catalyst strategy enriched the synthesis idea of other kinds of catalyst@MOFs structures and provides opportunities for potential application for catalysis and selective adsorption.
Data Availability
Data will be made available on request.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by the National Natural Science Foundation of China (No.22178325).
Supplementary Material
Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2022.04.016.
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