MIL-Derived Hollow Tubulous-Shaped In₂O₃/ZnIn₂S₄ Z-Scheme Heterojunction for Efficient Antibacterial Performance via In Situ Composite

Abstract

In this study, a hollow tubulous-shaped In₂O₃ derived from MIL (MIL-68 (In)) exhibited an enhanced specific surface area compared to MIL. To further sensitize In₂O₃, ZnIn₂S₄ was grown in situ on the derived In₂O₃. The 40In₂O₃/ZnIn₂S₄ composite (1 mmol ZnIn₂S₄ loaded on 40 mg In₂O₃) exhibited degradation rates of methyl orange (MO) under visible light (80 mW·cm⁻², 150 min) that were 17.9 and 1.4 times higher than those of the pure In₂O₃ and ZnIn₂S₄, respectively. Moreover, the 40In₂O₃/ZnIn₂S₄ exhibited an obviously improved antibacterial performance against Pseudomonas aeruginosa, with an antibacterial rate of 99.8% after visible light irradiation of 80 mW cm⁻² for 420 min. The 40In₂O₃/ZnIn₂S₄ composite showed the highest photocurrent density, indicating an enhanced separation of photogenerated charge carriers. Electron spin resonance results indicated that the 40In₂O₃/ZnIn₂S₄ composite generated both ·O₂⁻ and ·OH radicals under visible light, whereas ·OH radicals were almost not detected in ZnIn₂S₄ alone, suggesting the presence of a Z-scheme heterojunction between In₂O₃ and ZnIn₂S₄, thereby enhancing the degradation and antibacterial capabilities of the composite. This offers fresh perspectives on designing effective photocatalytic materials for use in antibacterial and antifouling applications.

1. Introduction

Biofilm formed by marine microorganisms and their metabolites on the surfaces of marine facilities serve as carriers for the attachment of macroorganisms, leading to marine biofouling [1]. Marine biofouling is a significant factor affecting and constraining the development of marine industries. Currently, antifouling coatings containing biocides are the most commonly used method. However, the slow release of biocides may pose threats to the safety of marine ecological environments [2]. Therefore, the development of novel environmentally friendly antimicrobial and antifouling materials to replace toxic alternatives has been recognized as an inevitable choice for sustainable development [3]. Photocatalysis generates highly oxidative reactive radicals capable of effectively degrading organic compounds and disrupting microbial cell structures. Moreover, photocatalytic sterilization is environmentally friendly and does not pose issues related to biological resistance [4,5]. Since sunlight can penetrate up to 200 m in seawater, it can be fully received in the upper layers of the ocean where biofouling occurs. Therefore, photocatalytic technology holds significant potential for applications in marine antimicrobial and antifouling fields.

Indium oxide (In₂O₃) is a semiconductor material with a relatively narrow bandgap, attracting attention due to its good stability and conductivity [6,7]. However, conventional In₂O₃ photocatalysts often suffer from limitations such as a simple structure, small surface area, weak light response, and high recombination rate of photogenerated charge carriers, which restrict their further application in photocatalysis [8]. To enhance the photocatalytic performance of monolithic In₂O₃, researchers have explored various methods including elemental doping [9,10], morphology control [11], and the construction of heterojunctions [12,13], achieving significant improvements. Recently, In₂O₃ photocatalysts prepared via the pyrolysis of metal-organic frameworks (MOFs) have garnered significant attention. Liu et al. achieved the formation of rhombohedral corundum/cubic In₂O₃ by the direct annealing of NH₂-MIL-68(In) in air atmosphere. Spectroscopic and photoelectrochemical tests demonstrated that this unique structure effectively accelerates the separation and transfer of photogenerated charges within In₂O₃ [14]. Yang et al. employed a simple oil bath method to grow CdZnS on shuttle-shaped mesoporous In₂O₃ derived from NH₂-MIL-68. Under visible light irradiation, the photocatalytic hydrogen evolution rate was significantly enhanced. This improvement can be attributed to several factors: the derived In₂O₃ possessing mesopores which increase active sites, the introduction of ultrafine CdZnS nanoparticles reducing the bandgap of material, thereby enhancing the visible light response, and the construction of Type II heterojunctions which enhance the separation efficiency of photogenerated charge carriers in the composite material [15]. Research indicates that MOFs-derived In₂O₃ can retain the original framework structure of MOFs, while the resulting hollow structures and pores provide additional reactive sites for photocatalytic reactions, thereby enhancing photocatalytic activity. In situ composite semiconductor heterojunctions constructed with multi-site hollow framework MOFs derivatives can further improve photocatalytic activity [13,16].

Ternary metal sulfides, characterized by narrow bandgaps, good stability, and excellent electrical and optical properties, are considered promising photocatalytic materials with high performance and potential applications [17]. Among these, ternary metal sulfides with an AB₂X₄ structure exhibit excellent photocatalytic stability. Within the AB₂X₄ series, ZnIn₂S₄ stands out due to its narrower bandgap and absence of toxic metal ions, making it an excellent candidate for photocatalytic applications in the energy and environmental fields. ZnIn₂S₄ is commonly used to sensitize wide-bandgap semiconductor materials, enhancing the charge carrier separation efficiency of monolithic semiconductor materials through the construction of heterojunction-based multi-component photocatalytic systems. Moreover, the facile preparation method of ZnIn₂S₄ enhances its appeal as an excellent candidate for modified materials [18,19].

In this paper, the derived hollow tubulous-like In₂O₃ was synthesized through calcination. Compared to the MIL, the derived In₂O₃ exhibited a reduced bandgap, increased optical absorption threshold, and enhanced photoelectric response. The composite material based on hollow tubulous-like In₂O₃ with in situ-grown ZnIn₂S₄ exhibits significantly enhanced photosensitivity compared to individual In₂O₃ and ZnIn₂S₄. Among them, the 40In₂O₃/ZnIn₂S₄ (1 mmol ZnIn₂S₄ loaded on 40 mg In₂O₃) composite shows the highest performance. The application potential of In₂O₃/ZnIn₂S₄ composites for marine antibacterial and antifouling purposes was evaluated by measuring their capability to degrade MO dye and eradicate Pseudomonas aeruginosa (P. aeruginosa). The 40In₂O₃/ZnIn₂S₄ composite degraded 99.5% of 10 ppm MO under simulated visible light irradiation for 150 min, which is 17.9 times higher than In₂O₃ and 1.4 times higher than ZnIn₂S₄ alone. Additionally, the composite achieved a 99.8% antibacterial rate against P. aeruginosa after 420 min of light exposure, demonstrating a significant enhancement in photocatalytic performance compared to the individual photocatalytic materials. Electron Spin Resonance (ESR) studies indicate that both In₂O₃ and ZnIn₂S₄ can generate ·O₂⁻ radicals, attributed to their conduction band potentials (E_CB = −0.59 V (In₂O₃), E_CB = −0.75 V (ZnIn₂S₄)) being more negative than the reduction potential of O₂/·O₂⁻ (−0.33 V vs. NHE). Additionally, only In₂O₃ can generate ·OH radicals due to its valence band potential (E_VB = 2.75 V) being more positive than the oxidation potential of H₂O/·OH (E_VB = 2.40 V vs. NHE). However, the composite material 40In₂O₃/ZnIn₂S₄ exhibits increased production of both ·O₂⁻ and ·OH radicals, suggesting the formation of a Z-Scheme heterojunction structure between In₂O₃ and ZnIn₂S₄, thereby enhancing the separation of photogenerated charge carriers. The composite material, prepared by the in situ growth of ZnIn₂S₄ on MIL-derived hollow tubular In₂O₃, demonstrates obviously enhanced degradation and antibacterial capabilities compared to pure In₂O₃ and ZnIn₂S₄, further expanding the possibilities for practical applications. Meanwhile, this advancement provides new insights for the development and design of novel, efficient photocatalytic antibacterial and antifouling materials.

2. Experimental Section

2.1. Synthesis of Rod-Shaped MIL-68 (MIL)

Using a solvothermal method, 5 mmol of indium nitrate (In(NO₃)₃) was dissolved in 35 mL of N,N-Dimethylformamide (DMF) to form solution A, while 6 mmol of phthalic acid (C₈H₆O₄) was dissolved in 35 mL of DMF to form solution B. Solution A was then added dropwise to solution B under stirring at room temperature for 30 min. The resulting mixture was transferred to a high-pressure reaction vessel and subjected to a hydrothermal reaction at 100 °C for 12 h. Afterward, the product was centrifuged, washed several times with ethanol, and dried under vacuum at 80 °C for 12 h, yielding a white powder of MIL.

2.2. Synthesis of MIL-Derived In₂O₃ Photocatalyst

A certain amount of MIL powder was spread evenly in a crucible. The temperature was then ramped up at a rate of 10 °C/min until reaching 500 °C and maintained for 2 h, resulting in off-white In₂O₃ powder.

2.3. Synthesis of ZnIn₂S₄ Photocatalyst

In total, 1 mmol of ZnCl₂, 2 mmol of InCl₃, and 4 mmol of thiourea (TAA) were dissolved in 100 mL of deionized water. The mixture was stirred at room temperature for 1 h. Subsequently, the solution was heated and stirred in a water bath at 80 °C for 2 h. After cooling, the resulting mixture was centrifuged and washed several times with deionized water and ethanol. Finally, the material was dried under vacuum at 80 °C for 12 h to obtain the ZnIn₂S₄ photocatalyst.

2.4. Synthesis of In₂O₃/ZnIn₂S₄ Composites

In total, 30, 40, 50, 60, and 300 mg of In₂O₃ were individually weighed and dispersed in 100 mL of deionized water with stirring for 30 min to ensure thorough dispersion. Subsequently, 1 mmol of ZnCl₂, 2 mmol of InCl₃, and 4 mmol of thiourea (TAA) were added sequentially to each dispersion, followed by stirring at room temperature for 1 h. The mixtures were then subjected to constant-temperature reactions at 80 °C in a water bath for 2 h. After cooling, the reaction products were collected by centrifugation, washed several times with deionized water and ethanol, and finally dried under vacuum at 80 °C for 12 h. The resulting materials were named as follows based on the amount of In₂O₃ used: 30In₂O₃/ZnIn₂S₄, 40In₂O₃/ZnIn₂S₄, 50In₂O₃/ZnIn₂S₄, 60In₂O₃/ZnIn₂S₄, and 300In₂O₃/ZnIn₂S₄. The synthesis steps of the materials are shown in the Figure 1 below.

2.5. Characterization

X-ray diffraction spectra was used to characterize the crystal structure of the prepared composites (XRD, Smart Lab, Rigaku Co., Tokyo, Japan); Scanning electron microscopy was used to observe the microstructure of the samples (SEM, JSM-7601F, NEC Co., Tokyo, Japan); and the species and distribution of elements were determined by energy dispersive X-ray spectroscopy (EDS, Oxford INCAx-sight, Oxford, UK). The microscopic bonding of the composites was further observed using transmission electron microscopy (TEM, JEOL 2100F, NEC Co., Tokyo, Japan); Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS, U-3900H, Shimadzu, Kyoto, Japan) was used to observe the optical properties of the composites and calculate their bandgap information; The valence states of reactive elements were determined by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo, Waltham, MA, USA). The production of free radicals was detected by electron paramagnetic resonance spectroscopy (EPR, EMXplus, Bruker Co., Billerica, MA, USA; Ettlingen, Germany).

2.6. Photoelectrochemical Test

The photoelectrochemical testing was conducted using a CHI660E electrochemical workstation (CHI660E, Shanghai Chenhua Instrument Co. Ltd., Shanghai, China). A three-electrode system was employed, with an Ag/AgCl electrode as the reference electrode, a Pt electrode as the counter electrode, and the prepared photoelectrode as the working electrode. The experiments were carried out in a 3.5 wt% NaCl solution to simulate seawater conditions. In addition, a xenon lamp (PLS-SXE300D, Beijing Perfect Light Co., Ltd., Beijing, China) was used as the simulated solar light illumination. The light was filtered through an AM 1.5 filter to obtain the simulated solar light and then adjusted to an intensity of 100 mW·cm⁻². For the Mott–Schottky measurements, the open circuit potential of the system was first determined. Then, the Mott–Schottky plot was tested over a range of ±0.6 V with a frequency of 1000 Hz and an AC voltage amplitude of 10 mV.

2.7. Photocatalytic Degradation and Photocatalytic Antibacterial Performance

For the photocatalytic degradation of methyl orange (MO), 15 mg of the sample was weighed in a 50 mL quartz tube, followed by adding 50 mL of a 10 ppm MO solution; each sample was tested three times. The quartz tube was then placed into a photocatalytic reactor (Figure 2) and stirred for 60 min to achieve adsorption–desorption equilibrium. Subsequently, photocatalytic degradation reactions were conducted under 800 W xenon lamp irradiation (with a 420 nm cut-off wavelength filter and a light intensity of 80 mW·cm⁻²). Then, 1 mL solution of each sample was taken every 30 min during the reaction, and finally, the changes in MO absorbance were measured using an enzyme-labeler (YP-96C, Youyunpu, Weifang, China) to calculate the degradation rate.

The photocatalytic antibacterial experiments used P. aeruginosa (BNCC186070), commonly found in marine fouling biofilm. In total, 50 mg of the photocatalyst was added to a 50 mL quartz tube, followed by adding 49.5 mL of sterilized 0.1 M phosphate buffer saline (PBS), and each sample requires three parallel experiments. This was inoculated with 500 μL of an appropriately diluted bacterial suspension (5.9 × 10⁸ cfu·mL⁻¹). The experiments were also conducted in a photocatalytic reactor with the same simulated light conditions. Prior to light exposure, the mixture was stirred for 60 min to achieve adsorption–desorption equilibrium in the dark condition. Then, 100 μL of the bacteria solution from the quartz tube was collected every 60 min. The antibacterial efficiency was determined by counting viable bacteria using the colony-counting method on a solid LB medium plate. The equipment used for photocatalysis is as follows:

3. Results and Discussion

3.1. Analysis of Physical Properties

The XRD patterns of MIL, derived In₂O₃, ZnIn₂S₄, and different In₂O₃/ZnIn₂S₄ composites were determined, and the results are shown in Figure 3. It is evident that the peaks of the rod-like MIL template are sharp, indicating good crystallinity, and the peak patterns correspond well with those reported in the literature [20]. The diffraction peaks of the derived In₂O₃ at 2θ = 21.5°, 30.5°, 35.4°, 45.6°, 50.9°, and 60.6° correspond to the (211), (222), (400), (400), (440), and (622) crystal planes of cubic-phase In₂O₃ (PDF#65-3170), indicating that the material derived from MIL after high-temperature calcination is In₂O₃. The characteristic peaks at 21.6°, 27.7°, and 47.2° in the synthesized ZnIn₂S₄ correspond to the (006), (102), and (110) diffraction peaks of hexagonal-phase ZnIn₂S₄ (PDF#65-2023), confirming the successful synthesis of ZnIn₂S₄. The XRD analysis of the 300In₂O₃/ZnIn₂S₄ composite shows characteristic diffraction peaks of both In₂O₃ and ZnIn₂S₄, indicating the simultaneous presence of both materials. However, in the XRD spectra of 30In₂O₃/ZnIn₂S₄, 40In₂O₃/ZnIn₂S₄, and 50In₂O₃/ZnIn₂S₄, no distinct In₂O₃ diffraction peaks are observed. This suggests that In₂O₃ is heavily loaded with ZnIn₂S₄, resulting in the characteristic peaks of In₂O₃ being less pronounced.

The microstructures of MIL, MIL-derived In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ composite were characterized using SEM. As shown in Figure 4a, MIL exhibits a rod-like structure with a smooth and uniform surface, and the rod width is approximately 3 μm. Figure 4b–d show the SEM images of the derived In₂O₃ products after calcination. The In₂O₃ retains the rod-like morphology of MIL but develops a hollow structure with abundant surface voids, possibly due to the removal of organic ligands during combustion [15]. Figure 4e displays the SEM image of ZnIn₂S₄, revealing a flower-ball structure composed of nanoplates. This nanoplate structure endows ZnIn₂S₄ with strong light absorption performance and provides numerous active sites [21]. Figure 4f shows the SEM image of the 40In₂O₃/ZnIn₂S₄ composite, where the surface of In₂O₃ becomes roughened following the in situ growth of ZnIn₂S₄. Combined with Figure 4g, it is evident that a multi-layered ZnIn₂S₄ nanoplate was loaded on the surface of In₂O₃. Figure 4h presents the EDS spectra of the 40In₂O₃/ZnIn₂S₄ composite, indicating the uniform distribution of In, O, Zn, and S elements on the surface, confirming the successful composite of ZnIn₂S₄ on the hollow tubulous-like In₂O₃.

XPS was employed to analyze the surface composition and chemical states of the derived In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄. Figure 5a presents the XPS survey spectra of the three samples. It can be observed that both the prepared ZnIn₂S₄ and 40In₂O₃/ZnIn₂S₄ exhibit characteristic peaks corresponding to Zn2p, In3d, and S2p. In the In3d orbital XPS spectra (Figure 5b), all three samples show two distinct peaks. For ZnIn₂S₄, the peaks at 452.7 eV and 445.2 eV are attributed to the In3d_3/2 and In3d_5/2 orbitals of ZnIn₂S₄, respectively. Similarly, for 40In₂O₃/ZnIn₂S₄, peaks corresponding to In3d_3/2 and In3d_5/2 appear at the same binding energies as ZnIn₂S₄, indicating the substantial in situ growth of ZnIn₂S₄ on the In₂O₃ carrier. In contrast, the binding energies of In3d_3/2 and In3d_5/2 in pure In₂O₃ are located at 451.9 eV and 444.3 eV, respectively, indicating the presence of In³⁺ in the samples. Figure 5c displays the orbital peaks of O1s. In In₂O₃, the O1s peak appears at a binding energy of 529.8 eV, corresponding to lattice oxygen. In contrast, the high binding energy peak at 532.0 eV for the In₂O₃/ZnIn₂S₄ composite is typically attributed to chemisorbed surface oxygen [22,23]. As shown in Figure 5d, Zn2p in ZnIn₂S₄ exhibits two characteristic peaks: Zn2p_1/2 at 1045.5 eV and Zn2p_3/2 at 1022.5 eV. For the 40In₂O₃/ZnIn₂S₄, the Zn2p orbital peaks are observed at 1045.6 eV (Zn2p_1/2) and 1022.6 eV (Zn2p_3/2), indicating a slight shift towards higher binding energies of 0.1 eV compared to ZnIn₂S₄. From the high-resolution spectra of S2p (Figure 5e), it can be observed that for ZnIn₂S₄, the S2p_1/2 and S2p_3/2 orbital peaks are located at 163.0 eV and 161.7 eV, respectively. For the 40In₂O₃/ZnIn₂S₄, the characteristic peaks of S2p_1/2 and S2p_3/2 orbitals are found at 162.9 eV and 161.8 eV, respectively. Sulfur exists in the S^2- state in both ZnIn₂S₄ and 40In₂O₃/ZnIn₂S₄ [22]. For the 40In₂O₃/ZnIn₂S₄, there is a negative shift in the binding energy of S compared to that of ZnIn₂S₄. The shift in binding energies of the Zn and S orbital peaks indicates strong interactions between In₂O₃ and ZnIn₂S₄ [24,25]. Simultaneously, it suggests that due to the interfacial coupling between In₂O₃ and ZnIn₂S₄, there is charge transfer occurring between the two phases to achieve a new equilibrium. Interfacial tight binding can promote the separation and migration of photogenerated charge carriers, reduce the electron–hole recombination rate, and enhance the charge transfer rate in photocatalytic reactions, thereby improving the activity of In₂O₃/ZnIn₂S₄ composite photocatalysts [26]. XPS analysis confirms the chemical states of In, Zn, S, and O elements in the samples as In³⁺, Zn²⁺, S²⁻, and O²⁻, respectively, verifying the presence of In₂O₃ and ZnIn₂S₄ in the composite.

3.2. Analysis of Photocatalytic Degradation of MO and Sterilization of P. aeruginosa

The photocatalytic activity of the composite was evaluated by assessing the degradation performance of MO solution under visible light irradiation. As shown in Figure 6a, the photocatalytic material reached adsorption–desorption equilibrium after 1 h. Under simulated visible light irradiation for 150 min, MIL, In₂O₃, and ZnIn₂S₄ exhibited degradation rates of 33.0%, 5.6%, and 69.7% respectively. The 40In₂O₃/ZnIn₂S₄ composite showed the highest degradation efficiency, reaching 99.5%, which was 3.0, 17.9, and 1.4 times higher than that of the pure MIL, In₂O₃, and ZnIn₂S₄, respectively. Other ratios of composites also significantly enhanced the degradation of MO, indicating that the combination of In₂O₃ and ZnIn₂S₄ facilitates the separation of photogenerated charge carriers, thereby improving its degradation performance. As depicted in Figure 6b, under visible light exposure, the absorbance of MO at different illumination times decreased gradually for the 40In₂O₃/ZnIn₂S₄, particularly at the peak intensity around 465 nm, indicating the effective degradation of MO.

To assess the stability of the 40In₂O₃/ZnIn₂S₄ composite in degrading MO solution through recycling tests of recovered samples, as shown in Figure 6c, its degradation efficiency remained consistently high after three cycles of testing. This indicates excellent stability during the photocatalytic process and demonstrates good repeatability. Additionally, the XRD diffraction peaks of the 40In₂O₃/ZnIn₂S₄ composite showed no significant changes before and after degradation, suggesting the robust structural stability of the composite photocatalyst before and after the reaction.

To evaluate the antibacterial and antifouling performance of In₂O₃, ZnIn₂S₄, and In₂O₃/ZnIn₂S₄ composites, their antibacterial efficiency against the typical marine fouling microorganism P. aeruginosa (5.9 × 10⁶ cfu·mL⁻¹) under visible light was assessed. In Figure 7a, it is evident that there was minimal change in the bacterial count in the control group, indicating that light exposure had a minimal effect on the activity of P. aeruginosa. Compared to the individual photocatalysts MIL, In₂O₃, and ZnIn₂S₄, the In₂O₃/ZnIn₂S₄ composite exhibited enhanced antibacterial performance, with 40In₂O₃/ZnIn₂S₄ performing the best. As shown in Figure 7b, after visible light irradiation of 80 mW cm⁻² for 420 min, 40In₂O₃/ZnIn₂S₄ achieved an antibacterial rate of 99.8% against P. aeruginosa, which is significantly higher compared to that of MIL, In₂O₃, and ZnIn₂S₄ by 45.1%, 18.9%, and 17.7%, respectively. The antibacterial rates of the 30In₂O₃/ZnIn₂S₄ and 50In₂O₃/ZnIn₂S₄ composites were 96.6% and 95.5%, respectively, also notably higher than those of the individual materials. The excellent antibacterial efficacy of the In₂O₃/ZnIn₂S₄ composite demonstrates its potential for highly effective antibacterial action in real marine environments.

3.3. Mechanism Analysis of the Promotion of Photocatalytic Performance

Figure 8a shows the UV–Vis diffuse reflectance spectra of MIL, In₂O₃, ZnIn₂S₄, and In₂O₃/ZnIn₂S₄ composite. It can be observed that pure MIL has an absorption threshold at 321 nm, while the derived In₂O₃ shows an increased absorption threshold at 447 nm, indicating enhanced light absorption performance due to high-temperature derivation from MIL. ZnIn₂S₄ exhibits a larger absorption threshold at 547 nm, which further increases the threshold of In₂O₃ to 530 nm, leading to enhanced light absorption intensity. Figure 8b presents the band gaps (E_g) of MIL, In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ calculated from the Tauc plot [27]. It can be seen that the band gaps of MIL, In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ are 4.00 eV, 3.34 eV, 2.45 eV, and 2.62 eV, respectively. Compared to MIL, In₂O₃ has a lower band gap, indicating increased sensitivity to light. After the composite with ZnIn₂S₄, the band gap of In₂O₃ further decreases, enhancing its photosensitivity, consistent with the diffuse reflectance spectroscopy (DRS) results. The Mott–Schottky test results indicate that MIL, In₂O₃, and ZnIn₂S₄ samples all exhibit positive slopes, indicating that these materials are n-type semiconductors. Therefore, the intersection of the tangent to the curve with the X-axis gives the flat band potential (E_fb) of the materials [26,27]. Figure 8c shows the flat band potentials (E_fb) for MIL, In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄, which are −0.62 V, −0.79 V, −0.95 V, and −1.05 V (vs. Ag/AgCl), respectively. For n-type semiconductors, E_fb is approximately equal to E_CB. Thus, the E_CB values for MIL, In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ are −0.62 V, −0.79 V, −0.95 V, and −1.05 V (vs. Ag/AgCl), which correspond to −0.42 V, −0.59 V, −0.75 V, and −0.85 V (vs. NHE), respectively. According to the E_VB = E_g + E_CB equation, the valence band edges (E_VB) for MIL, In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ are 3.58 V, 2.75 V, 1.70 V, and 1.77 V (vs. NHE), respectively.

Figure 8d shows the photocurrent densities of different samples in 3.5 wt% NaCl solution under simulated sunlight irradiation of 100 mW·cm⁻². It can be observed that the photocurrent density of the electrode made from pure MIL powder is relatively low, whereas the derived In₂O₃ shows a moderate increase in photocurrent density. The In₂O₃/ZnIn₂S₄ composite exhibits significantly enhanced photocurrent density compared to both In₂O₃ and ZnIn₂S₄ alone, with 40In₂O₃/ZnIn₂S₄ showing the highest photoresponse. The increased photocurrent density of the composites indicates that the In₂O₃/ZnIn₂S₄ heterojunction promotes the efficient separation of photogenerated charge carriers.

To investigate the electron transfer mechanism between In₂O₃ and ZnIn₂S₄, ESR was used to further explore the generation of radicals in In₂O₃, ZnIn₂S₄, and 40In₂O₃/ZnIn₂S₄ [28]. The results, as shown in Figure 8e,f, reveal that after 20 min of visible light irradiation, 40In₂O₃/ZnIn₂S₄ exhibits higher peaks in ·O₂⁻ and ·OH signals compared to In₂O₃ and ZnIn₂S₄ individually, indicating that the heterojunction formed by In₂O₃ and ZnIn₂S₄ promotes the separation of photogenerated electrons and holes. ESR testing confirms that ZnIn₂S₄ does not generate ·OH radicals under visible light irradiation, which correlates with its valence band being lower than the potential of H₂O/ OH. In contrast, In₂O₃ can generate both ·O₂⁻ and ·OH radicals under light exposure.

Based on the types and intensities of the generated radicals, the ·O₂⁻ and ·OH signals of the 40In₂O₃/ZnIn₂S₄ composite are stronger compared to those of In₂O₃ and ZnIn₂S₄ alone. This indicates that the heterojunction formed between In₂O₃ and ZnIn₂S₄ enhances the separation of photogenerated charge carriers. In ESR results, In₂O₃ can simultaneously produce ·O₂⁻ and ·OH under light exposure because its valence band (E_VB = 2.75 V vs. NHE) is higher than the potential of H₂O/·OH (E_VB = 2.40 V vs. NHE), and its conduction band (E_CB = −0.59 V vs. NHE) is lower than the potential of O₂/·O₂⁻ (E_CB = −0.33 V vs. NHE). Conversely, the valence band of ZnIn₂S₄ (E_VB = 1.70 V vs. NHE) is lower than the potential of H₂O/·OH; thus, it cannot generate ·OH radicals but can produce ·O₂⁻ due to its lower conduction band (E_CB = −0.75 V), capable of reducing O₂ to generate radicals. Therefore, based on these deductions, it is more likely that a Z-scheme heterojunction forms between In₂O₃ and ZnIn₂S₄. This Z-scheme heterojunction preserves the highly reductive photogenerated electrons of ZnIn₂S₄ and highly oxidative photogenerated holes of In₂O₃. Then, this retains the high reducibility of ZnIn₂S₄ while leveraging the strong oxidizing capability of In₂O₃, enhancing the degradation and sterilization performance. Below is a schematic diagram illustrating the potential charge transfer pathways in the In₂O₃/ZnIn₂S₄ composite, as shown in Figure 9:

According to the above conclusion (Figure 9), a possible mechanism [29] of the bactericidal action of In₂O₃/ZnIn₂S₄ is as follows:

In₂O₃ → In₂O₃ (e⁻ + h⁺)     ZnIn₂S₄ → ZnIn₂S₄ (e⁻ + h⁺)

(1)

In₂O₃ (e⁻ + h⁺) + ZnIn₂S₄ (e⁻ + h⁺) → In₂O₃ (h⁺) + ZnIn₂S₄ (e⁻)

(2)

ZnIn₂S₄ (e⁻) + O₂ → ·O₂⁻

(3)

In₂O₃ (h⁺) + H₂O → ·OH

(4)

Live bacteria +·O₂⁻ + ·OH → Dead bacteria

(5)

4. Conclusions

The hollow tubulous-shaped In₂O₃ derived from metal-organic framework (MOF) structure MIL was obtained via calcination, which exhibits an increased specific surface area, reactive sites, light absorption performance, and photoelectric response performance. Adopting this derived In₂O₃ as a template, nanoflower-shaped ZnIn₂S₄ was in situ-composited on it and obtained the In₂O₃/ZnIn₂S₄ composite, resulting in enhancing performance in MO degradation and P. aeruginosa sterilization. The optimal 40In₂O₃/ZnIn₂S₄ composite exhibited degradation rates of MO under visible light (80 mW·cm⁻², 150 min) that were 17.9 and 1.4 times higher than those of the pure In₂O₃ and ZnIn₂S₄, respectively. And the 40In₂O₃/ZnIn₂S₄ shows the highest antibacterial performance against P. aeruginosa, with an antibacterial rate of 99.8% after visible light irradiation of 80 mW cm⁻² for 420 min. Even after three cycles of reuse, the 40In₂O₃/ZnIn₂S₄ maintained high degradation activity without significant structural changes.

SEM, TEM, and XPS characterizations confirmed the tight contact between In₂O₃ and ZnIn₂S₄. The enhanced photocurrent density further indicates the presence of heterojunction. The ESR analysis of free radicals indicated that the 40In₂O₃/ZnIn₂S₄ generated both ·O₂⁻ and ·OH radicals under visible light, whereas ·OH radicals were not detected in ZnIn₂S₄ alone, suggesting the presence of a Z-scheme heterojunction between In₂O₃ and ZnIn₂S₄. The Z-scheme heterojunction preserves the highly reductive photogenerated electrons of ZnIn₂S₄ and highly oxidative photogenerated holes of In₂O₃, simultaneously enhancing the separation of photogenerated charge carriers.

This in situ growth of ZnIn₂S₄ on MIL-derived In₂O₃ produces an efficient composite for photocatalytic degradation and sterilization, offering guidance for applications in photocatalytic antimicrobial and antifouling treatments. Additionally, the synthesis conditions of the materials in this article are relatively mild, the raw materials are widely sourced, and only a small amount is needed to achieve significant degradation and bactericidal effects. At the same time, the physicochemical properties are stable, allowing for multiple cycles of use, which better meets the cost control requirements in practical applications and demonstrates considerable potential for application.