Rational design of direct Z-scheme magnetic ZnIn2S4/ZnFe2O4 heterojunction toward enhanced photocatalytic wastewater remediation

The rationally designed heterojunction photocatalysts with magnetic semiconductors and easy recyclability have received considerable attention due to their great advantages in practical application. In our work, a series of ZnIn2S4/ZnFe2O4 Z-scheme heterojunction photocatalysts with superior magnetic properties were synthesized by a gentle chemical bath method and utilized for the effective photodegradation and Cr(VI) reduction under irradiation. Systematic evaluation experiments revealed that the derived ZnIn2S4/ZnFe2O4 photocatalysts exhibited enhanced photocatalytic efficiency for RhB degradation and Cr(VI) reduction as compared with pristine ZnIn2S4 and ZnFe2O4, which was primarily due to the close contact interface and the formation of Z − scheme charge transfer mechanism between ZnFe2O4 rods and ZnIn2S4 nanosheets. Moreover, the as-synthesized photocatalyst could be easily recycled with a remarkable photocatalytic performance because of its magnetic separation characteristic. The present work opens up a vast prospect for the design of highly efficient and magnetically separable photocatalysts for environmental remediation.


Introduction
During the last two decades, serious water pollution has become the biggest threat to mankind with the deepening of industrialization. The harmful components in wastewater generally include organic pollutants and inorganic heavy metal ions, which are characterized by stable chemical properties and difficult to biodegrade, and cause a huge threat to human health security and ecosystem sustainability (Hu et al. 2018;Xue et al. 2019;Zhou et al. 2017). Therefore, it is urgent to seek a safe, green, and economical wastewater treatment technology. Solar-driven photocatalysis has been regarded as a new technology that can directly convert solar energy into chemical energy with simple operation and rapid reaction, which has been widely used for pollutant degradation, H 2 generation, CO 2 reduction, and photocatalytic organic conversion in recent years (Li et al. 2020;Wen et al. 2017;Zhang et al. 2019;Zhao et al. 2020). However, in the process of practical application, photocatalysts still faced the problem that could not be effectively separated, which not only limited the recycling of photocatalysts, but also caused secondary pollution (Guo et al. 2019a, b, c, d;Wu et al. 2018). Numerous studies have proven that magnetic photocatalysts could be easily and efficiently separated from reaction mixture with the assistance of external magnetic field due to its unique paramagnetism and inherent insolubility in most reaction solvents, so it could effectively solve the problems of product separation and catalyst recovery existing in most photocatalysts (Cui et al. 2016;Sibi et al. 2020). Therefore, it can be predicted that the design of magnetic photocatalyst exhibits a great potential in the field of photocatalytic applications.
As a kind of magnetic semiconductor, spinel zinc ferrite (ZnFe 2 O 4 ) was considered as an emerging visible-lightresponsive photocatalyst due to its unique properties such Responsible Editor: Guilherme L. Dotto as the narrow band gap (~ 1.9 eV), excellent recyclability, and extraordinary photochemical stability (Yang et al. 2015). However, the photocatalytic activity of bare ZnFe 2 O 4 was greatly restricted by low quantum efficiency, which limited the practical application. Up to now, researchers have sought to improve the photocatalytic activity of pristine ZnFe 2 O 4 by a variety of strategies, such as morphology regulation (Zhou et al. 2015), element doping (Guo et al. 2017), and heterostructure construction (Huang et al. 2018;Wang et al. 2020). In particular, the applications of integrating ZnFe 2 O 4 and other nanostructures with suitable band gaps to construct heterojunction photocatalysts have been extensively reported. For instance, Zeng et al. (Zeng et al. 2022) synthesized TiO 2 @ZnFe 2 O 4 heterojunctions using a facile hydrothermal process by in situ growing TiO 2 nanosheets on the surface of ZnFe 2 O 4 nanorods, and the nanocomposites exhibited superior photocatalytic performances for H 2 production and tetracycline degradation than that of pristine TiO 2 and ZnFe 2 O 4 . The improvement of reaction efficiency indicated that the construction of heterojunction between TiO 2 and ZnFe 2 O 4 was beneficial to the utilization of visible light and the efficient transfer of photogenerated e − − h + pairs. Shi et al. (Shi et al. 2022) also fabricated 2D/3D yolkshell g-C 3 N 4 /ZnFe 2 O 4 composites for the decomposition of tetracycline under irradiation, and the satisfactory photocatalytic activity was attributed to the enhanced photogenerated charges separation and transfer by type-II heterojunction.
Lately, zinc indium sulfide (ZnIn 2 S 4 ) has been considered to be a popular photocatalytic material because of the narrow bandgap (2.06 ~ 2.85 eV), good stability, and excellent photoelectric properties (He et al. 2019;Wang et al. 2021;Yuan et al. 2014). Meanwhile, numerous studies have proved that ZnIn 2 S 4 nanosheets with unique layered structure properties could easily combine with other semiconductors to construct heterojunction system and achieve superior photocatalytic activity (Lin et al. 2018;Yang et al. 2019). For example, Chen et al. (Chen et al. 2020a, b) designed WO 2.72 /ZnIn 2 S 4 heterojunction photocatalysts by integrating 1D WO 2.72 nanorods and 2D ZnIn 2 S 4 nanosheets, and the as-synthesized samples shown significantly enhanced photocatalytic performance for tetracycline degradation. This result mainly depended on the close interfacial contact between different nanostructures, which not only provided a larger specific surface area and more reactive sites, but also greatly promoted the migration of photogenerated charges by the formation of direct Z-scheme heterojunction. As two versatile semiconductor materials, ZnFe 2 O 4 and ZnIn 2 S 4 have also been proven to be suitable for the construction of heterojunction photocatalytic system because of their appropriate bandgap structure (Yang et al. 2016). However, the photocatalytic applications of Z-scheme ZnIn 2 S 4 /ZnFe 2 O 4 heterojunctions were rarely studied up till now, especially in the field of photocatalytic Cr(VI) reduction.
Based on above investigations, we first synthesized 1D ZnFe 2 O 4 rods, and then successfully anchored 2D ZnIn 2 S 4 nanosheets to their surface to construct Z-scheme heterojunction photocatalysts for photocatalytic removal of organic pollutants and Cr(VI). The significantly enhanced photoactivity of ZnIn 2 S 4 /ZnFe 2 O 4 composites was attributed to the tight interfacial contact and the construction of Z-scheme charge transfer mode, which effectively accelerated the migration of photoexcited e − -h + , as well as enhanced the redox potential.

Synthesis of ZnIn 2 S 4 /ZnFe 2 O 4 composites
ZnFe 2 O 4 rods were synthesized by hydrothermal method according to previous report (Liang et al. 2020). ZnIn 2 S 4 / ZnFe 2 O 4 composites were synthesized via a facile chemical bath method as illustrated in Scheme 1. Typically, InCl 3 ·4H 2 O (0.6 mmol), ZnCl 2 (0.3 mmol), and thioacetamide (1.6 mmol) were dissolved in glycerol aqueous solution (40 mL, 20 v%, pH = 2), then various amounts of ZnFe 2 O 4 (0.3 mmol, 0.6 mmol, and 0.9 mmol) were dispersed into above solution and continuously stirred for 30 min. Afterward, the suspension was transferred to an oil bath at 80 °C and stirred for another 2 h. Finally, the precipitate was collected by centrifugation, washed, and dried after naturally cooling to room temperature. The obtained samples were marked as ZIS/ZFO − X (X = 1, 2, and 3), respectively, where X refers to the molar ratio of ZFO/ZIS. For comparison, pristine ZnIn 2 S 4 was synthesized under the same conditions except adding ZnFe 2 O 4 . Besides, the additional experimental details including materials characterization and photocatalytic activity assessment could also be found in Supporting Information.

Characterization
The various microscopic and spectroscopic techniques such as XRD, BET, SEM, TEM, XPS, and VSM were adopted to analyze the physicochemical properties of as-synthesized composites, and the details for materials characterization were presented in Supporting Information.

Photocatalytic experiments
The photocatalytic activity of as-synthesized samples was investigated by RhB (20 mg/L) degradation and Cr(VI) (10 mg/L) reduction. A certain amount of photocatalyst was dispersed into 50 mL RhB or Cr(VI) solution by continuous stirring for 40 min without irradiation from the Xenon lamp (300 W, λ > 420 nm). After the equilibrium adsorption, the light source was switched on to start the photocatalytic experiments. At a certain interval of irradiation, 2 mL of aliquot was taken out and centrifuged to wipe the photocatalyst particles for analysis. RhB concentration was monitored with UV − vis spectrophotometer (Shimadzu UV − 2450) at 554 nm, and the concentration of Cr(VI) was detected by using a standard diphenylcarbazide (DPC) spectrophotometric method (Zhang et al. 2020a, b).  (440), respectively. From the XRD patterns of ZIS/ZFO samples, the characteristic peaks of ZIS could be clearly observed, suggesting that ZIS nanosheets were successfully anchored to the ZFO rods. Besides, it was obvious that the peak intensity of (311), (511), and (440) facet for ZFO gradually increased with the increase of ZFO, which also implied that the two phases perfectly co − existed in the composites (Sultana et al. 2018).

Characterizations of photocatalysts
The surface area and porous characteristics of the prepared ZIS, ZFO, and ZIS/ZFO were studied by N 2 adsorption/desorption isotherms, and the results were shown in Fig. 1b. The similar isotherms with the shape of type IV cures was observed for all the synthesized samples, revealing the existence of a typical mesoporous structure (Skliri et al. 2018). The data for BET surface area, pore volume, and pore sizes were presented in Table 1, it could be seen that the average pore sizes for ZFO, ZIS, and ZIS/ZFO − 2 were Scheme 1 The schematic illustration of ZIS/ZFO composites preparation Fig. 1 XRD patterns (a) and nitrogen adsorption/desorption isotherm curves (b) of prepared composites 3.11, 6.97, and 6.78 nm, respectively, which also verified the mesoporous structure of synthesized samples. Meanwhile, the BET surface area of the three samples were determined to be 80.12, 64.32, and 118.24 m 2 ·g −1 , respectively, and the enhanced specific surface area also explained the thorough exposure of ZIS nanostructures after integration (Gao et al. 2022).
The morphologies and microstructures of resultant samples were further investigated by SEM and TEM and the results were depicted in Fig. 2a ~ g. Pristine ZFO shown a regular rod structure with width size of 2 ~ 4 μm (Fig. 2a, b). It could be observed from Fig. 2c to d that pristine ZIS possessed a flower − like structure composed of a large number of nanosheets. Figure 2e and f displayed the SEM and TEM pictures of ZIS/ZFO − 2 composite, and it was easy to see that ZIS nanosheets were uniformly and tightly dispersed around the ZFO rods. The close interfacial contact between the two structures contributed to the expansion of specific surface area and the formation of heterostructures, which leaded to excellent photocatalytic performance. Finally, the EDX elemental mapping analysis was obtained and the results were presented in Fig. 2g. The uniform elemental distribution of Fe, In, S, and Zn over ZIS/ZFO-2 sample further confirmed the successful combination of ZIS and ZFO.  To analyze the surface chemical composition of ZIS, ZFO and ZIS/ZFO-2 samples, XPS analysis was conducted. As could be seen from Fig. 3a, Zn 2p, Fe 2p, In 3d, S 2p, and O 1 s appeared in XPS survey spectrum of ZIS/ZFO-2 composite, which was in line with EDX results. Figure 3b-d shown the high resolution XPS spectra of Fe 2p, In 3d, and S 2p, respectively. For ZIS/ZFO-2 composite, the characteristic peaks of Fe, In, and S elements located at 724.8 eV (Fe 2p 1/2 ), 711.0 eV (Fe 2p 3/2 ) (Guo et al. 2019a, b, c, d;Yoo et al. 2017), 453.2 eV (In 3d 3/2 ), 445.7 eV(In 3d 5/2 ) (Yang et al. 2017), 162.2 eV (S 2p 3/2 ), 163.7 eV(S 2p 1/2 ) (Zeng et al. 2019a), respectively, could be found, which were basically consistent with literature date except for slight differences. In addition, there was a weak signal peak intensity of Fe in ZIS/ZFO-2 composite since the surface of ZFO was almost completely covered by ZIS nanosheets after integration. Notably, compared with two individual units, the binding energy of ZIS/ZFO-2 composite was significantly changed with a negative shift of Fe and positive shifts of In and S, which strongly proved that the transmission of photogenerated electrons caused the change of surface electron density under the action of intimate contact interface for different nanostructures. Meanwhile, according to the values of binding energy, it could be inferred that the transfer direction of photoexcited electrons was from ZIS to the surface of ZFO (Guo et al. 2019a, b, c, d;Yu et al. 2017). Figure 4a shown the optical absorption ability of ZIS, ZFO and ZIS/ZFO-2 samples. It could be observed from the UV-vis diffuse reflectance spectra that pristine ZIS and ZFO shown an intense light absorption with the band edge around 520 nm and 650 nm, respectively, and the corresponding bandgap energies were estimated by Kubelka − Munk equation to be about 2.33 and 1.94 eV, respectively (Fig. 4b). After growing ZIS nanosheets on ZFO, the obtained ZIS/ZFO samples exhibited the enhanced light absorption in the range of 250-650 nm by comparison with pure ZIS. The spatial distribution hierarchical structure was favorable for the reflection and scattering of light, which was responsible for the improvement of light absorption capacity over ZIS/ZFO composites (Chen et al. 2021). Additionally, the VB edge potential was measured to further determined the bandgap structure of synthesized sample. As presented in Fig. 4c, the obtained values for ZFO and ZIS were 0.68 and 1.62 eV, respectively. Therefore, the corresponding CB potential were calculated to be -1.26 and -0.71 eV, respectively. Combined with CB and VB potential positions obtained above, the energy band structures were shown in Fig. 4d. It could be seen that ZIS and ZFO exhibited mutually matched energy band distribution, which was conducive to the generation of heterojunctions and the improvement of photocatalytic performance.

Photocatalytic performance
The photocatalytic activities for RhB degradation of ZIS, ZFO and ZIS/ZFO composites were evaluated under visible light irradiation. As presented in Fig. 5a, there were relatively low photocatalytic RhB degradation efficiency for pristine ZFO (2.9%) and ZIS (24.4%). Oppositely, the removal efficiency of ZIS/ZFO composites were greatly improved, and 99.6% of RhB was completely degraded over ZIS/ZFO − 2 composite in just 8 min. The corresponding apparent rate constant of ZIS/ZFO − 1, ZIS/ZFO − 2, and ZIS/ZFO − 3 were 0.364, 0.695, and 0.226 min −1 , respectively (Fig. 5b), which were higher than that of ZFO and ZIS. In addition, the photocatalytic Cr(VI) reduction experiments were carried out to further verify the photocatalytic performance of as − synthesized photocatalysts and the results were presented in Fig. 5c. As expected, ZIS/ZFO − 2 composite shown a strongest Cr(VI) reduction potential with 96.6% removal efficiency during 20 min irradiation. To our knowledge, the excellent photocatalytic performance was superior to the most reported magnetic photocatalytic materials in the treatment of refractory pollutants in wastewater (Table 2). These results indicated that a close contact interface was formed between different nanostructures after growing ZIS nanosheets on ZFO, which induced the generation of abundant heterojunctions and accelerated the separation of photoexcited electrons and holes, thus improving the photocatalytic capacity. Besides, the reusability and stability of ZIS/ZFO − 2 composite was checked by recycling experiments for four times, and the result was presented in Fig. 5d. The removal efficiency of RhB over ZIS/ZFO − 2 composite did not decrease obviously after four cycles, which also confirmed that the prepared photocatalysts were basically stable in practical application process.
The magnetic behaviors of ZFO and ZIS/ZFO − 2 photocatalysts were analyzed by vibrating sample magnetometry (VSM). As shown in Fig. 6a, the magnetization curves of ZFO and ZIS/ZFO − 2 samples passed through the coordinate origin, implied that they belonged to typical soft magnetic materials superparamagnetic properties (Wu et al. 2016). Besides, although the saturation magnetization of ZIS/ZFO − 2 composite was slight lower than that of bare ZFO, it could still satisfy the effective separation of photocatalysts in the reaction system. It could be clearly seen from the inset of Fig. 6a that ZIS/ZFO − 2 photocatalyst in the reaction system was quickly attracted to the magnet and the solution returned to transparency in a short time. The results indicated that the magnetic ZIS/ZFO − 2 composite was easy to recycle from the aqueous solution.
In order to explore the lifetime and transfer resistance of photogenerated charges during the photocatalytic process, electrochemical impedance spectroscopy (EIS) of ZFO, ZIS, and ZIS/ZFO − 2 samples were employed. As depicted in Fig. 6b, the Nyquist plots of EIS suggested that Fig. 4 (a) UV-vis DRS, (b) the corresponding band energy, (c) XPS valence band, and (d) band gap structure of synthesized samples ZIS/ZFO − 2 composite has a smaller arc radius than bare ZFO and ZIS, indicating a lower charge transfer resistance of ZIS/ZFO − 2 due to the effective charge separation (Ma et al. 2019;Zeng et al. 2019b). Meanwhile, the transient photocurrent response was measured to analyze the interfacial charge separation dynamics. It could be clearly seen from Fig. 6c that ZIS/ZFO − 2 composite exhibited about 5.5 times enhanced photocurrent response intensity after growing ZIS nanosheets on ZFO, which also implied a higher separation efficiency and longer lifetime   (Guo et al. 2019a, b, c, d) Pd/ZnFe 2 O 4 / g-C 3 N 4 0.2 g/L RhB 100 mL 10 mg/L + 300 W Xe lamp 95% in 60 min (Zhang et al. 2020a, b)  of photogenerated e − -h + pairs (Meng et al. 2018). Besides, the lower peak intensity of photoluminescence curves for ZIS/ZFO − 2 photocatalyst also indicated the higher separation efficiency of photogenerated e − − h + pairs (Fig. 6d). These mutually compatible results were consistent with the remarkable photocatalytic performance of ZIS/ZFO-2 composite.
In order to elucidate the proposed mechanism of photocatalytic RhB degradation process over ZIS/ZFO-2 composite, radical trapping experiments were performed. AO (0.01 M), IPA (0.01 M), and BQ (0.02 M) were added to capture h + , • OH, and • O 2 − , respectively (Chen et al. 2019;Chen et al. 2020a, b). As shown in Fig. 6e, there was a distinct inhibitory effect on the removal efficiency of RhB in the presence of different scavengers, indicating the existence of h + , • OH, and • O 2 − in the photocatalytic process. Concretely, when AO, IPA, and BQ were added into solution, the removal efficiency of RhB decreased from 99.6% to 75.9%, 90.8%, and 29.6%, respectively. This phenomenon suggested that • OH and h + radicals gave a minor contribution, while • O 2 − played a crucial role in the oxidation of RhB. Additionally, the existence of • O 2 − radical in the degradation process was further confirmed by EPR measurement. As shown in Fig. 6f, the signal peaks of DMPO − • O 2 − adduct could be clearly observed after 2 min illumination.
Based on the above analysis, a direct Z − scheme charge transfer mode of ZIS/ZFO − 2 composite was proposed and elucidated schematically in Fig. 7. Under visible light irradiation, ZIS and ZFO were simultaneously excited to produce e − and h + on the CB and VB. Then, driven by the inner electric field at the ZIS/ZFO heterojunction interface, the photogenerated e − located on CB of ZIS transferred rapidly to VB of ZFO and consumed h + . As a result, the retained photogenerated e − and h + in ZIS/  Proposed photocatalytic mechanism over ZIS/ZFO − 2 composite ZFO − 2 photocatalyst could exhibit a strong redox ability. Especially, the photogenerated e − accumulated on the CB of ZFO with more negative edge potential (-1.26 eV) shown a strong reduction effect on the conversion of O 2 / • O 2 − (-0.33 eV). Part of • O 2 − could be taken up by H + to produce • OH radicals ( • O 2 − + H + → • OH) and most of them contributed to the degradation of RhB, which was consistent with the results of trapping experiment. The retained photogenerated h + in the VB of ZIS with strong oxidation potential could also directly oxidized RhB. Besides, Cr(VI) was simultaneously converted to Cr(III) by the accumulated e − on the CB of ZFO. Ultimately, the photogenerated e − − h + pairs in ZIS/ZFO − 2 photocatalyst were effectively separated and redox potential was enhanced, which was beneficial to the improvement of photocatalytic performance.

Conclusions
In summary, the magnetically separable ZIS/ZFO composites were controllably constructed by a facile chemical bath method, and exhibited excellent photocatalytic activity for RhB degradation and Cr(VI) reduction. The optimized ZIS/ ZFO − 2 photocatalyst showed the highest photocatalytic removal efficiency with 99.6% of RhB and 96.6% of Cr(VI) in a short reaction time, and could easily achieve magnetic separation, which was superior to the most reported magnetic photocatalysts in the treatment of refractory contaminants in wastewater. The Z − scheme heterojunction formed between different nanostructures contributed to the rapid separation of photoexcited charges and the significant improvement of redox capacity, thus greatly improving the photocatalytic capability. The magnetic ZIS/ZFO heterojunction will become a promising visible − light − driven photocatalyst for the efficient removal of refractory organic and inorganic contaminants in wastewater. Zhao F, Liu Y, Hammouda S et al (2020) MIL-101(Fe)/g-C 3 N 4 for enhanced visible-light-driven photocatalysis toward simultaneous reduction of Cr(VI) and oxidation of bisphenol A in aqueous media. Appl Catal B 272:119033 Zhou X, Li X, Sun H et al (2015) Nanosheet-Assembled ZnFe 2 O 4 Hollow Microspheres for High-Sensitive Acetone Sensor. ACS Appl Mater Inter 7:15414-15421 Zhou Y, Liu X, Xiang Y et al (2017) Modification of biochar derived from sawdust and its application in removal of tetracycline and copper from aqueous solution: adsorption mechanism and modelling. Bioresource Technol 245:266-273 Publisher's note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
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