Construction of a La-ZnIn₂S₄/MIL-125(Ti) heterojunction for highly efficient photocatalytic degradation of aflatoxin B₁

Abstract

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.

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Introduction

In recent years, consumers and producers have drawn much attention to food safety. One of the most common food safety issues comes from mycotoxin contamination, which exists in a variety of products ranging from food to feed.¹ Aflatoxins, as one of the most toxic and carcinogenic mycotoxins, are secondary fungal metabolites of Aspergillus flavus and Aspergillus parasiticus, especially aflatoxin B₁ (AFB₁), which has been listed as a type one carcinogen by the International Agency for Research on Cancer and probably causes serious effects in humans and animals, such as bleeding, chronic toxicity, carcinoma and immunosuppression even when exposed to an extremely low level.^2–4 Aflatoxins have high chemical and thermal stability. It is difficult to destroy them and still they possess toxic activity even after being treated with high temperatures.⁵ Aflatoxins not only are one of the important threats to animal and human health, but also cause great economic losses all over the world.

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.

Experimental

Materials

N,N-Dimethylformamide (DMF), ethanol, 1,4-dicarboxybenzene (C₈H₆O₄), methanol, and tetrabutyl titanate (C₁₆H₃₆O₄Ti) were bought from Sinopharm Chemical Reagent (Shanghai, China). Thioacetamide (CH₃CSNH₂), lanthanum nitrate [La(NO₃)₂], and aflatoxin B₁ (AFB₁) were obtained from Aladdin (Shanghai, China). Indium chloride (InCl₃) and zinc chloride (ZnCl₂) were supplied by Alfa Aesar (Shanghai, China). The chemicals were used directly since they were of analytical grade.

Synthesis of the La-ZnIn₂S₄/MIL-125(Ti) catalyst

The MOF material of MIL-125(Ti) was synthesized using a method adopted from the previously published solvothermal synthesis.¹³ Typically, 5 g of C₈H₆O₄ and 30 mL of ethanol were dissolved in 270 mL of DMF, by stirring at room temperature for 0.5 h. Then, 2.6 mL of C₁₆H₃₆O₄Ti was mixed with the C₈H₆O₄ solution followed by stirring for 15 min before being moved into a PTFE vessel (500 mL). The vessel was sealed and the solution was reacted for 24 h at 150 °C. White precipitates were collected by centrifugation at room temperature. The obtained MIL-125(Ti) solid sample was purified by washing with ethanol and DMF three times, and then dried at 80 °C.

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. 1 The procedure for La-ZnIn₂S₄/MIL-125(Ti) synthesis.

Characterization

The crystal structures were studied using a powder X-ray diffractometer (PXRD, D8 Advance, Bruker) using Cu Kα X-ray (λ = 1.54168 Å). The morphology of all samples was investigated by scanning electron microscopy (SEM, S4800, Hitachi, Japan). Infrared vibrational spectra were obtained using a Fourier transform infrared (FT-IR) spectrophotometer (Vector22, Bruker). Brunauer–Emmett–Teller (BET) specific surface areas were measured using the N₂ adsorption–desorption isotherms at 77 K obtained using a BET analyzer (ASAP 2000, Micromeritics). The light absorption was recorded on a UV-Vis spectrometer (UV2450) equipped with diffuse reflectance (DRS) attachments. The photoelectrochemical properties, including the electrochemical impedance spectrum (EIS), were investigated on a CHI760E workstation. A scanning transmission electron microscope (TEM, JEM-2100HR, Jeol) was used to study the structure of the samples. Surface element composition and associated oxidation states were measured using an X-ray photoelectron spectrometer (XPS, Escalab 250Xi) equipped with Al Kα emission. Radical intermediates were identified by electron spin resonance (ESR) spectroscopy (A-300, Bruker) using spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) to identify the free radicals.

Evaluation of photocatalytic activity

The photocatalytic degradation measurements were carried out in a photochemical reactor (SH-EJE-A, Shanghai EHE Instrument, China). A mercury lamp (500 W) was adopted as the UV light source. During the degradation experiment, the photocatalyst (0.05 g) was dispersed in a freshly prepared AFB₁ aqueous solution (100 mL, 5 ppm). To eliminate the adsorption, the adsorption equilibrium was reached by stirring the suspension solution for 30 min without light. During irradiation, a small amount of solution was sampled from the photochemical vessel at certain duration. The solid photocatalyst was removed by centrifugation. The concentrations of AFB₁ solution were detected using a UV-vis spectrophotometer. The stability of the photocatalyst was also evaluated by repeating the photodegradation for five recycles. At the end of each degradation reaction, the photocatalyst was collected, washed with water, and dried at 80 °C before the next degradation reaction.

Results and discussion

Fig. 2 shows the XRD patterns and the FT-IR spectrum of La-ZnIn₂S₄/MIL-125(Ti) is compared with that of the ZnIn₂S₄/MIL-125(Ti) composite, together with those of the ZnIn₂S₄ and MIL-125(Ti) precursors. As shown in Fig. 2a, ZnIn₂S₄ exhibits three main peaks at 21.2°, 27.5°, and 47.7°, which are ascribed to the (006), (102), and (110) crystal planes of ZnIn₂S₄ (JCPDS No. 65-2023), respectively.¹⁴ MIL-125(Ti) exhibits six characteristic peaks at 7.10°, 9.86°, 11.99°, 15.35°, 16.89°, and 18.22°, indicating its formation.^15,16 With the coupling of MIL-125(Ti) with ZnIn₂S₄, it shows the representative peaks of both MIL-125(Ti) and ZnIn₂S₄. Interestingly, the intensity corresponding to MIL-125(Ti) is not as strong as that without the coupling of ZnIn₂S₄. Thus, the coupling with ZnIn₂S₄ caused the structure and morphology changes of MIL-125(Ti). Other researchers have reported similar changes in the XRD patterns of the ZnIn₂S₄/MOF composites in the literature.^17,18Fig. 2d shows the XRD spectrum of the La-ZnIn₂S₄/MIL-125(Ti) composite, which is almost the same as that of the composite without La coupling, indicating that the doping of La has no effects on the crystal structure.

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 C=C 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 (S_BET, m² g⁻¹), total volume of the pore (V_total, cm³ g⁻¹), total volume of the micropore (V_micro, 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).

Table 1 Textural properties of all samples

Sample	S BET	V total	V micro	D
a Specific surface area (m^2 g^−1). b Total pore volume (cm^3 g^−1). c T-plot micropore volume (cm^3 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

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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 (E_g): E_g = 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 (E_g = 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 (E_g = 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 2p_3/2 and 2p_1/2 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 C=C 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 C=O 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.

Fig. 8 (a) Photocatalytic degradation of AFB₁ by different catalysts; (b) five repetitive cycles for the photocatalytic degradation of AFB₁ by La-ZnIn₂S₄/MIL-125(Ti); and (c) XRD patterns of La-ZnIn₂S₄/MIL-125(Ti) before and after the reaction.

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.

Fig. 9 ESR spectra of (a) DMPO-˙OH and (b) DMPO-˙O₂⁻ for La-ZnIn₂S₄/MIL-125(Ti).

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)

where (La-ZnIn₂S₄/MIL-125(Ti) + AFB₁)_adsorption represents the AFB₁ molecules adsorbed on the surface or/and in the pores of La-ZnIn₂S₄/MIL-125(Ti).

Fig. 10 Illustration of the photocatalytic mechanism of AFB₁ degradation on the La-ZnIn₂S₄/MIL-125(Ti) photocatalyst under UV light irradiation.

Conclusions

In conclusion, we successfully fabricated the La doped ZnIn₂S₄/MIL-125(Ti) composite via a solvothermal method, which exhibits the best photocatalytic performance in the AFB₁ degradation with UV excitation. After 28 min of UV light irradiation, La-ZnIn₂S₄/MIL-125(Ti) can degrade 97.6% of AFB₁, the highest among all samples. It was demonstrated that the doping of La and the coupling with MIL-125(Ti) could narrow the band gap energy, improve the BET surface area and enhance the charge separation and electron transfer of ZnIn₂S₄, resulting in the drastic enhancement of photocatalysis. The electron spin resonance (ESR) technique also verified that ˙OH and ˙O₂⁻ are the reactive radicals for the decomposition of AFB₁ during the photodegradation process. This study provided a novel strategy to fabricate semiconductor-based photocatalysts with enhanced photocatalytic performance to eliminate and prevent the AFB₁ contamination in the food security areas.

Author contributions

Xiaobing Yang: formal analysis, preparation and characterization of the samples, and writing – original draft preparation. Junjie Pan, Bingcong Xing and Wenjie Lei: preparation and characterization of the samples and carrying out the photocatalytic degradation testing. Yingchun Fu and Kejun Cheng: providing the idea of this work and designing relevant experiments, writing – review and editing, and funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors wish to acknowledge financial support from the Postdoctoral Science Fund of Lishui City (20201030-Y), the Natural Science Foundation of Fujian Province (2022J011201), the Program for Outstanding Young Scientific Research Talents in Fujian Province University (MinKeJiao, 2018, No 47) and the Shiyanjia Lab (http://www.shiyanjia.com) for the XPS analysis.

Notes and references

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