Xiaobing
Yang
abc,
Junjie
Pan
c,
Bingcong
Xing
d,
Wenjie
Lei
b,
Yingchun
Fu
*a and
Kejun
Cheng
*cd
aCollege of Biosystems Engineering and Food Science, Zhejiang University, Hangzhou 310058, Zhejiang Province, China. E-mail: ycfu@zju.edu.cn
bFujian Provincial Key Laboratory of Eco-Industrial Green Technology, Wuyi University, Wuyishan 354300, Fujian Province, China
cChemical Biology Center, Lishui Institute of Agriculture and Forestry Sciences, Lishui 323000, Zhejiang Province, China. E-mail: chengkejun@gmail.com
dZhejiang Province Key Laboratory of Resources Protection and Innovation of Traditional Chinese Medicine, Zhejiang A&F University, Hangzhou 311300, Zhejiang Province, China
First published on 13th January 2023
Nowadays, aflatoxin B1 (AFB1) contamination is considered as one of the most common food safety issues for humans and animals. As one of the most advanced oxidation techniques, photocatalytic degradation can break down organic contaminants into nontoxic and harmless materials efficiently. In this paper, we explored the degradation efficiency of AFB1 by ZnIn2S4. To promote the photocatalytic degradation efficiency of AFB1, ZnIn2S4 was coupled with MIL-125(Ti) and then doped with La [this hybrid is denoted as La-ZnIn2S4/MIL-125(Ti)] to effectively convert it via photocatalytic generation of superoxide radicals (˙O2− and ˙OH) and achieve much enhanced photocatalytic performance, which is demonstrated by degrading 97.6% of AFB1. According to the transient photocurrent responses, the doping of La and coupling with MIL-125(Ti) can highly improve the efficient separation of the photoinduced electron–hole pairs on ZnIn2S4, leading to the effective conversion of OH− and O2 into ˙O2− and ˙OH, respectively, during the photodegradation process. This strategy of coupling with MOFs and doping with rare earth elements provides a facile and efficient method for degrading food pollutants produced by aflatoxins.
To reduce, eliminate, and prevent the risks in animals and humans, numerous methods have been utilized to remove or/and degrade aflatoxins. For example, Xu and his co-workers developed polydopamine-modified magnetic multi-wall carbon nanotubes for the adsorption and removal of aflatoxins and ochratoxins from vegetable oils.6 Xing and his team adopted roasting with a lemon juice and/or citric acid method to remove aflatoxin B1.7 Xing and his co-workers used a Spin-X centrifuge system to investigate the AFB1 degradation of atoxigenic GZ15 and JZ2.8 Although these strategies can remove or/and reduce aflatoxins to some degree, they are restricted by some factors. For example, the adsorption of aflatoxins can bring secondary contamination, the chemical method will leave some chemicals in the environment, and the gene method is restricted by the technical requirements.9,10 Moreover, with the growing demands for food safety and environmental protection, it is vital to develop newer and greener technologies that can break aflatoxins into nontoxic and harmless materials more efficiently, environment-friendly, and affordably.
As one of the most advanced oxidation techniques, photocatalysis technology can degrade organic contaminants with many benefits, such as efficiency, eco-friendliness, no secondary pollution, easy operation, and low cost.11 Photocatalysts, for example, ZnIn2S4, can break down the organic contaminants into non-toxic and harmless materials under UV or/and visible light irradiation. Hence, this technology holds great promise for detoxifying aflatoxins. However, the photocatalytic degradation efficiency of photocatalysts suffers from the rapid combination of the photoexcited electron–hole pairs.12 Some measures need to be taken to improve its photocatalytic activity. In this paper, we introduced a novel La doped ZnIn2S4 coupled with MIL-125(Ti) to form a unique composite structure of La-ZnIn2S4/MIL-125(Ti). It showed much improved photocatalytic decomposition of AFB1 with UV illumination. The influences of MIL-125(Ti) coupling and La doping on the AFB1 degradation efficiency of ZnIn2S4 were investigated, and the degradation mechanism of AFB1 by La-ZnIn2S4/MIL-125(Ti) was also systematically explored in detail.
The La-ZnIn2S4/MIL-125(Ti) composite was synthesized using a modified hydrothermal process. 0.05 g of La(NO3)2, 0.677 g of InCl3, 0.455 g of CH3CSNH2, and 0.206 g of ZnCl2 were added to DMF (100 mL) followed by stirring at room temperature for 0.5 h until all the added chemical reagents were fully dissolved before adding 0.9 g of the synthesized MIL-125(Ti) crystallized particles. The mixture was stirred for 10 min before sealing in a 200 mL PTFE container. The sample was heated to 180 °C for 10 h. After reaction, a yellow powder was obtained by centrifugation, washed with ethanol three times, and dried at 80 °C. For comparison, ZnIn2S4 and ZnIn2S4/MIL-125(Ti) were fabricated using the same process without adding La(NO3)2. The synthesis procedure is illustrated in Fig. 1.
Fig. 2 (a) XRD patterns and (b) FT-IR spectra of ZnIn2S4, MIL-125(Ti), ZnIn2S4/MIL-125(Ti) and La-ZnIn2S4/MIL-125(Ti). |
FT-IR spectra were recorded to study the functional groups and chemical bonding in different samples. As shown in Fig. 2b, all samples have a broad peak centered at 3470 cm−1 and two distinct absorption bands at 1610 and 1397 cm−1 from the absorbed water molecules.19 ZnIn2S4 exhibits similar characteristic peaks to the previous reports.20 The broad absorption band between 400 and 800 cm−1 for MIL-125(Ti) is assigned to the O–Ti–O vibration. Two clear absorptions at 1406 and 1653 cm−1 are linked to the O–C–O vibrational stretching.21 The CC stretching in the aromatic ring peak is at 1510 cm−1.22 The feature at 1705 cm−1 is typical for a pristine BDC.23 ZnIn2S4/MIL-125(Ti) shows almost identical absorption bands to the MOF. However, a slight peak shift for the characteristic O–Ti–O peak was observed for ZnIn2S4/MIL-125(Ti), suggesting the chemical coupling between the MOF and ZnIn2S4. La-ZnIn2S4/MIL-125(Ti) has an almost identical IR spectrum to ZnIn2S4/MIL-125(Ti), inferring that the doping of La does not affect the molecular structure and the chemical bonding of ZnIn2S4/MIL-125(Ti).
The SEM images were used to analyze the morphologies of the precursors and composites. As shown in Fig. 3a, the MIL-125(Ti) MOF exhibited a plate-like crystallized morphology with a smooth surface, and its particle size is 350–600 nm. Fig. 3b shows the SEM image of ZnIn2S4, which shows a flower-like microsphere morphology with an average diameter of about 2 μm and the flower-like microsphere is assembled by numerous cross-linked nanosheets. As shown in the SEM image of ZnIn2S4/MIL-125(Ti) in Fig. 3c, many ZnIn2S4 nanosheets were attached on the surface facets of the MOF. According to Yuan's report, coating ZnIn2S4 nanosheets on MOFs can weaken the characteristic peaks,24 which is in agreement with the XRD patterns. No significant change of the morphology was observed in La-ZnIn2S4/MIL-125(Ti) (Fig. 3d) with respect to ZnIn2S4/MIL-125(Ti) (Fig. 3c). Hence, the structure of the ZnIn2S4/MIL-125(Ti) composite did not change after the La doping. TEM and HRTEM images were used to investigate further the morphology and internal structure of La-ZnIn2S4/MIL-125(Ti). As shown in Fig. 3b, ZnIn2S4 exhibits a flower-like spherical morphology with numerous aggregated nanosheets. However, when ZnIn2S4 is coupled with MIL-125(Ti) (Fig. 3e), its cross-linked nanosheets are disconnected and evenly distributed on the crystal planes of MIL-125(Ti). The high-magnification image in Fig. 3f shows the surface unfolded nanosheets. The high-magnification image of these unfolded nanosheets displays a clear lattice fringe of 0.32 nm, assigned to the ZnIn2S4 (102) crystal plane,25 indicating the successful coupling of ZnIn2S4 on the MIL-125(Ti) crystal surfaces. There are no signals for the La element in the unfolded nanosheets of ZnIn2S4, possibly due to low content.
Fig. 3 SEM images of (a) MIL-125(Ti), (b) ZnIn2S4, (c) ZnIn2S4/MIL-125(Ti), and (d) La-ZnIn2S4/MIL-125(Ti), and (e) TEM and (f) HRTEM images of La-ZnIn2S4/MIL-125(Ti). |
Fig. 4 shows the BET measurement results, summarized in Table 1, with the specific surface area (SBET, m2 g−1), total volume of the pore (Vtotal, cm3 g−1), total volume of the micropore (Vmicro, cm3 g−1), and pore diameter (D, nm). As displayed in Fig. 4a, ZnIn2S4 shows the type-IV isotherm with a distinct H3 hysteresis loop in the range from 0.47 to 1.00 (P/P0), confirming its mesoporous structure.26 The corresponding BET surface area is 45.14 m2 g−1. MIL-125(Ti) showed initial high adsorption of N2 at a relative pressure of 0–0.1 (P/P0), which belongs to the type-I isotherm and indicates its microporous structure.27Fig. 4c shows the adsorption–desorption curves of ZnIn2S4/MIL-125(Ti). It shows a high uptake at a relative pressure of 0–0.1 (P/P0) and the H3 hysteresis loop in the range from 0.47 to 1.00 (P/P0), illustrating that ZnIn2S4/MIL-125(Ti) has both the microporous and mesoporous structures. La-ZnIn2S4/MIL-125(Ti) (Fig. 4d) has a similar adsorption–desorption behavior to ZnIn2S4/MIL-125(Ti). The SBET of La-ZnIn2S4/MIL-125(Ti) is 650.65 m2 g−1 while the corresponding D is 2.91 nm, as shown in Table 1, which are almost the same as those of ZnIn2S4/MIL-125(Ti), confirming that the doping of La does not change the textural properties of ZnIn2S4/MIL-125(Ti).
Fig. 4 N2 adsorption–desorption isotherms of (a) ZnIn2S4, (b) MIL-125(Ti), (c) ZnIn2S4/MIL-125(Ti) and (d) La-ZnIn2S4/MIL-125(Ti). |
Sample | S BET | V total | V micro | D |
---|---|---|---|---|
a Specific surface area (m2 g−1). b Total pore volume (cm3 g−1). c T-plot micropore volume (cm3 g−1). d Average pore diameter (nm). | ||||
ZnIn2S4 | 45.14 | 0.144 | 0.004 | 6.49 |
MIL-125(Ti) | 959.93 | 0.512 | 0.430 | 2.13 |
ZnIn2S4/MIL-125(Ti) | 653.05 | 0.469 | 0.258 | 2.87 |
La-ZnIn2S4/MIL-125(Ti) | 650.65 | 0.473 | 0.253 | 2.91 |
The optical absorption of the powder was analyzed using a UV-Vis DRS spectrophotometer. The following equation can be used to calculate the band gap energy (Eg): Eg = 1240/λ (λ is the absorption edge).28 MIL-125(Ti) displays a strong absorption between 300 and 350 nm, as shown in Fig. 5, which is assigned to the intrinsic band gap. ZnIn2S4 shows a visible light absorption edge near 494 nm with the corresponding band gap energy of 2.51 eV. When ZnIn2S4 is combined with MIL-125(Ti), its absorption edge is slightly broadened to 498 nm (Eg = 2.49 eV). More interestingly, the doping of La on ZnIn2S4/MIL-125(Ti) can further broaden its absorption edge from 498 to 504 nm (Eg = 2.46 eV). According to Table 1, MIL-125(Ti) recombining on ZnIn2S4 can improve its BET surface area. The doping of La and MIL-125(Ti) recombining on ZnIn2S4 can raise its adsorption capacity of contaminants and promote it to excite more photoexcited electrons and holes during the photocatalysis.
Fig. 5 The UV-vis absorbance spectra of ZnIn2S4, MIL-125(Ti), ZnIn2S4/MIL-125(Ti) and La-ZnIn2S4/MIL-125(Ti). |
Generally, the excellent photoactivity of a photocatalyst is associated with the good charge separation efficiency. The photoelectrochemical technique was used to analyze the interfacial charge separation of photocatalysts. Fig. 6a shows the photocurrent response with the light on and off from different photoanodes under UV illumination for seven on–off cycles. With UV light, the photocurrents from the samples without La doping decreased rapidly to zero, attributed to the fast combination of photoinduced electron–hole pairs. The photocurrent density of ZnIn2S4/MIL-125(Ti) (1.48 μA cm−2) is higher than that of ZnIn2S4 (1.02 μA cm−2), indicating that the coupling of MIL-125(Ti) enhanced the photoactivity of ZnIn2S4. The photocurrent of La-ZnIn2S4/MIL-125(Ti) is 1.87 μA cm−2 and La-ZnIn2S4/MIL-125(Ti) exhibits the highest photocurrent density. Noticeably, when the UV light is switched on, the photocurrent of La-ZnIn2S4/MIL-125(Ti) decreases to a certain value and then slows down to zero, confirming that La-ZnIn2S4/MIL-125(Ti) has the best charge separation efficiency. The results verify that the doping of La and the coupling with MIL-125(Ti) can effectively enhance the efficiency of charge separation due to the transfer of photoinduced electrons from ZnIn2S4 to La and MIL-125(Ti). To explore the enhancement mechanism, the EIS measurements were carried out for different photoanodes. The semicircle size in the Nyquist plot for La-ZnIn2S4/MIL-125(Ti) is the smallest among all the samples, representing the lowest interface resistance of La-ZnIn2S4/MIL-125(Ti). Thus, La doping and MIL-125(Ti) coupling have improved the charge separation in ZnIn2S4 in the photoexcitation process.
Fig. 6 (a) Transient photocurrent responses and electrochemical impedance spectra of (b) ZnIn2S4, (c) ZnIn2S4/MIL-125(Ti) and (d) La-ZnIn2S4/MIL-125(Ti). |
The elemental composition and the chemical state of La-ZnIn2S4/MIL-125(Ti) were established by XPS. Fig. 7a shows the XPS survey spectrum of La-ZnIn2S4/MIL-125(Ti), in which the elements La, Zn, In, S, Ti, Ti, N, and O with sharp photoelectron peaks can be identified. However, the N intensity is relatively low, indicating low content of N, which may have come from the adsorbed N2. Fig. 7b shows the high-resolution La 3d XPS spectrum, which depicts four peaks with their binding energies centered at 834.7, 838.1, 852.5, and 854.9 eV, revealing the existence of La3+ ions.29 Compared with the standard XPS peaks of La 3d in pure La2O3, the La 3d spectrum of La-ZnIn2S4/MIL-125(Ti) shows a slight shift to a lower binding energy direction, indicating that La3+ ions are dissolved into the ZnIn2S4 lattice.30 The Zn 2p high resolution XPS spectrum in Fig. 7c shows two distinctive signals at 1021.8 and 1044.9 eV, associated with the spin orbit coupling. The high resolution In 3d XPS spectrum in Fig. 7d shows the In spin orbit coupling signals at 444.7 and 451.5 eV. As for the S 2p spectrum (Fig. 7e), its 2p3/2 and 2p1/2 signals were observed at 161.4 and 162.7 eV. As shown in Fig. 7f the C 1s spectrum has four components. The two peaks at lower energies of 284.4 and 285.1 eV are assigned to the CC and C–C bonds, while the other two peaks at higher energies of 286.2 and 288.9 eV are associated with the carbon atoms bonded with oxygen in the C–O, and CO structures in BDC. Our XPS results verify the successful doping of La3+ in the La-ZnIn2S4/MIL-125(Ti) composite.
Fig. 7 The XPS spectra of La-ZnIn2S4/MIL-125(Ti) with (a) survey spectrum, (b) La 3d, (c) Zn 2p, (d) In 3d, (e) S 2p, and (f) C 1s. |
The experiment for the degradation of AFB1 solution by photocatalysts was performed under UV light irradiation. Before the photocatalytic degradation, the photocatalysts were dispersed in the AFB1 solution by stirring for 30 min without light to achieve the adsorption equilibrium. Then, the solution was exposed to UV light. As shown in Fig. 8a, the concentration of AFB1 exhibits a slight decrease (the black line) without adding the catalyst under UV light irradiation, indicating the negligible self-degradation of AFB1. After 28 min of illumination, the photodegradation efficiencies of MIL-125(Ti) (51.4%), ZnIn2S4 (85.5%), ZnIn2S4/MIL-125(Ti) (89.5%), and La-ZnIn2S4/MIL-125(Ti) (97.6%) were achieved. The best photocatalytic performance was achieved by La-ZnIn2S4/MIL-125(Ti). Besides that, stability is also very important for the photocatalyst.
The experimental cycling runs of La-ZnIn2S4/MIL-125(Ti) are used to evaluate its stability. As displayed in Fig. 8b, after five times repetition, the degradation efficiency of La-ZnIn2S4/MIL-125(Ti) reduces by 3.1%, without changes in XRD patterns (Fig. 8c). Hence La-ZnIn2S4/MIL-125(Ti) offers excellent photocatalytic performance stability, in addition to its good structural stability.
The photoinduced reactive species (RS) are essential as the active intermediates produced during photocatalysis. ESR spectroscopy was used to examine the generated RS by La-ZnIn2S4/MIL-125(Ti) under UV light irradiation with DMPO as the radical trapper. As shown in Fig. 9, no ESR peak was visible without UV illumination. However, the four-line characteristic peak of DMPO-˙O2− and the 1:1:1 triplet characteristic peak of DMPO-˙OH are detected under UV light irradiation for 5 min, indicating that both ˙O2− and ˙OH are generated by La-ZnIn2S4/MIL-125(Ti) during the photocatalysis.
The photocatalysis performance of ZnIn2S4 was improved by the doping of La and the coupling of MIL-125(Ti), with improved charge separation. Herein, the enhanced photocatalytic performance of AFB1 degradation on the La-ZnIn2S4/MIL-125(Ti) photocatalyst with UV excitation is explored, and the possible schematic diagram is shown in Fig. 10. With UV illumination, valence electrons in ZnIn2S4 are excited to the conduction band. The excited electrons are easily moved to La and MIL-125(Ti), resulting in an effective charge separation. Then, the photoinduced electron can combine with O2 to form ˙O2− and the photoinduced hole can capture the electron from OH− to generate ˙OH. These generated ˙O2− and ˙OH have high oxidation. AFB1 can be oxidized by superoxide radicals (˙O2− and ˙OH). Furthermore, MIL-125(Ti) and ZnIn2S4 exhibit excellent adsorption properties for AFB1. They can adsorb AFB1 molecules on their surface or/and in their pores to form a layer of high concentration AFB1 molecules. It is also beneficial for superoxide radicals (˙O2− and ˙OH) to degrade AFB1 molecules. The elementary reaction equations are shown as follows:
La-ZnIn2S4/MIL-125(Ti) + AFB1 MIL-125(Ti2S4/MIL-125(Ti) + AFB1)adsorption | (1) |
La-ZnIn2S4/MIL-125(Ti) + hv → La-ZnIn2S4/MIL-125(Ti)(e−/h+) → (e− + h+) | (2) |
e− + O2 + ˙O2− | (3) |
e− + OH− + ˙OH | (4) |
˙O2− + (La-ZnIn2S4/MIL-125(Ti) + AFB1)adsorption dsoxidant products | (5) |
˙OH + (La-ZnIn2S4/MIL-125(Ti) + AFB1)adsorption dsoxidant products | (6) |
Fig. 10 Illustration of the photocatalytic mechanism of AFB1 degradation on the La-ZnIn2S4/MIL-125(Ti) photocatalyst under UV light irradiation. |
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