Interfacial S–O bonds specifically boost Z-scheme charge separation in a CuInS₂/In₂O₃ heterojunction for efficient photocatalytic activity

Abstract

Reducing the recombination rate of photoexcited electron–hole pairs is always a great challenging work for the photocatalytic technique. In response to this issue, herein, a novel Z-scheme CuInS₂/In₂O₃ with interfacial S–O linkages was synthesized by a hydrothermal and subsequently annealing method. The Fourier transform infrared (FT-IR) and X-ray photoelectron spectrometer (XPS) measurements confirmed the formation of covalent S–O bonds between CuInS₂ and In₂O₃. The quenching and electron spin resonance (ESR) tests revealed the Z-scheme transfer route of photogenerated carriers over the CuInS₂/In₂O₃ heterojunctions, which was further verified theoretically via density functional theory (DFT) calculations. As expected, the CuInS₂/In₂O₃ heterojunctions showed significantly boosted photocatalytic activities for lomefloxacin degradation and Cr(VI) reduction under visible light illumination compared with the bare materials. Accordingly, a synergistic photocatalytic mechanism of Z-scheme heterostructures and interfacial S–O bonding was proposed, in which the S–O linkage could act as a specific bridge to modify the Z-scheme manner for accelerating the interfacial charge transmission. Furthermore, the CuInS₂/In₂O₃ heterojunction also exhibited excellent performance perceived in the stability and reusability tests. This work provides a new approach for designing and fabricating novel Z-scheme heterostructures with a high-efficiency charge transfer route.

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1. Introduction

Along with the rapid development of modern industries like the pharmaceutical, chemical and manufacturing industries, abundant concomitant pollutants like antibiotics and heavy metals have entered into the aquatic ecosystem, posing a severe threat to the environment and human health.^1–3 In order to alleviate these environmental issues, various water purification technologies, such as adsorption,⁴ ion exchange,⁵ membrane separation,⁶ photocatalysis⁷ and electrocatalysis,⁸ have been studied and applied to remove the aqueous contaminants. Among them, semiconductor-based photocatalysis has become a research hotspot in water environment treatment due to its sustainable, eco-friendly and low-cost characteristics.^9–11 To date, multitudinous semiconductors have been developed, and typically, semiconducting metallic oxides such as TiO₂,¹² ZnO¹³ and SnO₂,¹⁴ are extensively researched because of their excellent physical and chemical properties. Nevertheless, due to their wide band gap energies, the effective light response is normally restrained to the ultraviolet region, making them less active under visible-light. Thus, seeking an effective visible-light-responsive semiconductor is of great significance in the practical application of photocatalysis.

Indium oxide (In₂O₃), as a typical n-type semiconductor, has been considered as a promising visible-light-active photocatalyst due to its relative narrow band gap (∼2.7 eV), environmental stability and low toxicity.^15,16 However, a major shortcoming on the pure In₂O₃ is its high recombination rate of photoinduced charge carries, which has seriously inhibited its photocatalytic activity.¹⁷ To address this drawback, combining In₂O₃ with another semiconductor to form a heterojunction has been regarded as an effective approach to favor the charge separation.¹⁸ In particular, the Z-scheme heterojunction, inspired by the natural photosynthesis, has sparked great interest from researchers due to its efficient carries transfer pattern and the relative higher redox capacity in comparison with the conventional type II heterostructure.^19–22 For instance, Wang et al. fabricated a Z-scheme In₂O₃/CdS photocatalyst and this heterojunction possessed outstanding degradation performance for the doxycycline hydrochloride and levofloxacin owing to the high charge separation rate.²³ Duan and coworkers constructed a ZnO/In₂O₃ Z-scheme heterojunction which showed superior photocatalytic performance for H₂ production.²⁴ Recently, copper indium disulfide (CuInS₂), a ternary chalcogenide semiconductor, has shown great potential in photocatalysis due to its narrow band energy (∼1.8 eV) and high optical absorption coefficient (α > 10⁵ cm⁻¹).^25,26 Furthermore, the CuInS₂ possesses a theoretically suitable band structures and Fermi levels with those of In₂O₃,^27,28 which makes them prospective candidates for constructing a Z-scheme heterostructure. However, to the best of our knowledge, very few reports currently focus on the assembling of such CuInS₂/In₂O₃ composite to achieve a promoted photocatalytic activity.

In general, simple combining of hybridized semiconductors may cause interfacial resistance at their contact regions, which would impede the charge transfer and severely affect their photocatalytic performance.²⁹ To overcome this issue, heterostructures with interfacial bonds were fabricated and the formed intimate linkage was considered to be efficient to accelerate the spatial carrier transportation.^30,31 For example, Shehzad et al. reported that the chemical bonded TiO₂-graphene displayed high photocatalytic activity attributing to the interfacial Ti–O–C bonds and promoted transfer of electrons.³² In Chen and Jiang's report, the covalently bonded Bi₂O₃/Bi₂WO₆ composite by cosharing of the Bi–O tetrahedron showed superior photocatalytic activities for CO₂ reduction and degradation of RhB.³³ Recently, some researchers have found that calcination treatment on sulphide semiconductors could promote the production of sulfur vacancies, which could delocalize local electrons and contribute to developing the heterogeneous interfaces with chemical bonding.^29,34 Accordingly, for the CuInS₂/In₂O₃ heterojunction, annealing of its precursor in the preparation process may be a prospective method to generate the interfacial chemical linkages between CuInS₂ and In₂O₃, which would be a key factor to boost its photocatalytic activity.

In this study, a chemically bonded CuInS₂/In₂O₃ composite was successfully synthesized using a hydrothermal method followed by annealing treatment. The obtained heterojunction between CuInS₂ and In₂O₃ was formed via interfacial S–O linkages verified by the FT-IR and XPS. Besides, we selected lomefloxacin and Cr(VI) as the target contaminants to probe the visible-light photocatalytic performance on the prepared catalysts. Furthermore, a Z-scheme charge separation route was revealed by the quenching and ESR tests, and confirmed theoretically by DFT calculations. Finally, a synergistically enhanced photocatalytic mechanism of Z-scheme heterostructure and interfacial S–O bond on CuInS₂/In₂O₃ was proposed and discussed.

2. Experimental

2.1 Chemical materials

Cuprous chloride (CuCl), indium chloride tetrahydrate (InCl₃·4H₂O), indium nitrate pentahydrate (In(NO₃)₃·5H₂O), urea (CO(NH₂)₂), N,N-dimethylformamide, thioacetamide (C₂H₅NS), anhydrous ethanol, ammonium oxalate (AO), dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA), and benzoquinone (BQ) were obtained from Sinopharm Chemical Reagent Co., Ltd. (China). The lomefloxacin (LOM) was purchased from Aladdin Chemistry Co., Ltd (China). Besides, the potassium dichromate (K₂Cr₂O₇) was supplied by Shiyi Chemical Co., Ltd. (China). All chemicals and reagents are of analytical grade and used directly in the experiment.

2.2 Preparation of CuInS₂ sample

The bare CuInS₂ catalyst was synthesized by a solvothermal method. Typically, 0.3 mmol CuCl and 0.3 mmol InCl₃·4H₂O was dissolved and stirred in 40 mL of N,N-dimethylformamide to form a mixed solution. Subsequently, 3 mmol of thioacetamide was added into the above mixture under continuous stirring for another 60 min. Then, the obtained homogeneous solution was sealed in a 100 mL teflon-lined stainless autoclave and thermally treated at 180 °C for 16 h. After cooling to the ambient temperature naturally, the precipitate was harvested via centrifugation and washed alternately with deionized water and anhydrous ethanol for six times, and then dried in a vacuum oven at 60 °C for 24 h.

2.3 Preparation of CuInS₂/In₂O₃ composites

The CuInS₂/In₂O₃ photocatalysts with different mass ratios were prepared via a solvothermal and followed calcination route. In a typical procedure, 4 mmol In(NO₃)₃·5H₂O and 30 mmol urea were introduced into 50 mL of deionized water with vortex stirring for 20 min, and then a predetermined amount of as-synthesized CuInS₂ was dispersed into the mixed solution with magnetic stirring for 90 min. Afterwards, the obtained suspension was transferred to a teflon-lined stainless autoclave (100 mL) and maintained at 140 °C for 12 h. The resultant product was separated by centrifugation and washed for several times with deionized water and anhydrous ethanol, and then dried at 80 °C for 24 h. Subsequently, the obtained power was calcined at 400 °C for 120 min in a ceramic crucible with a ramp rate of 10 °C min⁻¹ under argon atmosphere. The as-prepared CuInS₂/In₂O₃ samples with nominal mass ratio 5%, 11%, 17% and 23% CuInS₂ were denoted as 5-CuInS₂/In₂O₃, 11-CuInS₂/In₂O₃, 17-CuInS₂/In₂O₃, and 23-CuInS₂/In₂O₃, respectively. For comparison, the pure In₂O₃ was prepared via the similar procedure only without the introduction of CuInS₂.

2.4 Characterization and theoretical calculation

The details about characterization and theoretical calculation on the fabricated materials were provided in the ESI† SM1.

2.5 Photocatalytic activity evaluation

The photocatalytic performance of the synthesized catalysts were investigated through degrading lomefloxacin (LOM) and reducing Cr(VI) under visible light irradiation. The typical photocatalytic test was conducted as follows: 200 mg of each as-synthesized material was dispersed into 500 mL LOM (30 mg L⁻¹) or Cr(VI) (50 mg L⁻¹) aqueous solution under continuous stirring. To ensure the adsorption–desorption equilibrium, the suspension was vigorously stirred in dark for 30 min. Afterwards, the reaction system was irradiated by a 350 W Xe-lamp (CEL-HXUV500, CEAULIGHT, Beijing, China) equipped with a 420 nm cut-off filter, and 5 mL of the suspensions were sucked out at timed intervals and immediately centrifuged for the removal of catalyst powders. The concentrations of residual LOM were determined by an UV-vis spectrophotometer (TU-1900) at a wavelength of 286 nm, and the remaining Cr(VI) concentrations were measured colorimetrically at 540 nm with the standard diphenylcarbazide method. All the experimental data were the average of three duplicate determinations.

3. Results and discussion

3.1 XRD analysis

The crystalline phase and purity of all prepared catalysts were characterized by XRD measurements, and the results are shown in Fig. 1. In the XRD pattern of bare In₂O₃, the diffraction peaks at 21.5°, 30.6°, 35.5°, 41.8°, 45.7°, 51.0° and 60.7° could be assigned to the (211), (222), (400), (332), (431), (440) and (622) crystal planes of cubic phase In₂O₃ (JCPDS no. 06-0416), respectively.³⁵ For bare CuInS₂, the typical peaks located at 27.8°, 32.4°, 46.2° and 55.0° were observed, which corresponded well to the (112), (200), (204) and (312) crystal reflections of CuInS₂, respectively (JCPDS no. 76-1461).³⁶ In regard to the CuInS₂/In₂O₃ samples, the characteristic peaks of both In₂O₃ and CuInS₂ were identified and no other diffraction peaks of impurities could be observed. At the same time, the peak intensities of CuInS₂ became correspondingly more intense with the CuInS₂ content increasing, and not much difference was observed in the In₂O₃ diffraction peaks. The XRD characterization suggested that CuInS₂ was successfully coupled with In₂O₃, and their original crystal phases were remained well during the combining process.

Fig. 1 XRD patterns recorded on bare In₂O₃, CuInS₂ and CuInS₂/In₂O₃ composites.

3.2 Micromorphology and element analysis

The surface morphologies of as-prepared In₂O₃, CuInS₂ and 17-CuInS₂/In₂O₃ composite were depicted by SEM and TEM. As displayed in Fig. 2(a), the bare In₂O₃ presented stacked small-size plate structure. In Fig. 2(b), it was observed that the pure CuInS₂ was constructed by dense nanoparticles with concomitant aggregation. For 17-CuInS₂/In₂O₃ composite, it was found that the lamellar In₂O₃ and CuInS₂ particles were both in its morphology (Fig. 2(c)). Besides, to further investigate the detailed information of the morphology and crystal structure, TEM and HRTEM analysis on 17-CuInS₂/In₂O₃ were carried out. As illustrated in Fig. 2(d and e), there was intimate interface contact between In₂O₃ and CuInS₂. In addition, the HRTEM image clearly showed the lattice fringes of the two substances. Among, the interplanar spacings of 0.272 and 0.319 nm corresponded to the (200) and (112) crystal planes of CuInS₂,^36,37 and the spacings of the 0.261 and 294 nm were attributed to the (200) and (222) planes of In₂O₃,^16,38 respectively, which was in good agreement with the results from XRD analysis. Moreover, the high-angle annular dark field (HAADF) image and corresponding element mapping of 17-CuInS₂/In₂O₃ (Fig. 2(f)) demonstrated the presence of Cu, In, S and O elements, further testifying the co-existing of both CuInS₂ and In₂O₃ in this composite.

Fig. 2 SEM images of (a) In₂O₃, (b) CuInS₂ and (c) 17-CuInS₂/In₂O₃ sample; TEM images (d, e) and HRTEM image (f) of 17-CuInS₂/In₂O₃; (g) HAADF-STEM image and corresponding EDS elemental mappings of 17-CuInS₂/In₂O₃.

3.3 FT-IR and XPS analysis

The FT-IR spectra were performed to explore the structure and bond characteristics of the as-prepared samples. As observed in Fig. 3, the FT-IR spectrum recorded on bare In₂O₃ revealed the In–O stretching vibrations at 426 and 560 cm⁻¹ and the In–O bending vibration at 604 cm⁻¹.^27,35 In addition, the bands centered at 3456 and 1592 cm⁻¹ could be ascribed to the stretching and bending vibration of –OH groups from physical adsorbed water, respectively.³⁹ In the FT-IR spectrum of bare CuInS₂, the bands at 1700 and 1449 cm⁻¹ belonged to the S–In and S–Cu vibrations, respectively.^40,41 For the composite of 17-CuInS₂/In₂O₃, all the typical bands of surface bonds on In₂O₃ and CuInS₂ were detected, suggesting their coexistence in this composite. Of note, a new band located at 1061 cm⁻¹ was observed, which could be assigned to the vibration of S–O bond.²⁹ This S–O bond might be formed via the reaction of the unsaturated S vacancies in CuInS₂ and O atoms in In₂O₃, suggesting the construction of interfacial linkages in CuInS₂/In₂O₃ heterostructure.

Fig. 3 FT-IR spectra of bare In₂O₃, CuInS₂ and 17-CuInS₂/In₂O₃ composite.

The XPS determination was used to probe more details about the surface element composition and chemical states on the as-synthesized catalysts. As depicted in Fig. 4(a), the In 3d spectrum for bare In₂O₃ was divided into two symmetrical peaks at 452.2 and 444.6 eV, corresponding to In 3d_3/2 and 3d_5/2, respectively.²⁹ The energy peaks of Cu 2p_1/2 and 2p_3/2 on bare CuInS₂ were located at 950.2 and 930.8 eV,⁴² respectively (Fig. 4(b)). While for the 17-CuInS₂/In₂O₃ composite, the In 3d energy positions were observed to shift to lower states (Fig. 4(a)), and meanwhile the Cu 2p states showed a positive shift (Fig. 4(b)) in comparison with the individual materials. Fig. 4(c) presents the high resolution XPS spectra of S 2p on bare CuInS₂ and 17-CuInS₂/In₂O₃ sample. The S 2p spectrum on bare CuInS₂ could be decomposed into two peaks at 163.1 eV (S–In) and 161.9 eV (S–Cu), respectively.⁴³ In the case of 17-CuInS₂/In₂O₃ composite, the fitted S 2p peaks of S–In (163.6 eV) and S–Cu (162.4 eV) exhibited a positive shift, and notably, a new peak was observed at 164.6 eV associated with the S–O bonds.^29,34 In Fig. 4(d), the fitting curve of O 1s on bare In₂O₃ involved two peaks at 532.6 and 529.8 eV, which were ascribed to the –OH from surface adsorbed water and In–O–In bond in the structure of In₂O₃, respectively.¹⁶ Furthermore, an additional peak was developed at 531.6 eV in the O 1s spectrum of 17-CuInS₂/In₂O₃ composite, which could corresponded to the S–O bonds.^29,44 According to the above fitting results, it was found that each element in 17-CuInS₂/In₂O₃ showed a negative or positive shift in the banding energy compared with that of bare materials as indicated in Fig. 4(a–d), and moreover, the formation of interfacial S–O bonds was testified by XPS measurement in consistence with the results form FT-IR, indicating the intimate interfacial contact between CuInS₂ and In₂O₃.

Fig. 4 High-resolution XPS spectra of (a) In 3d, (b) Cu 2p, (c) S 2p, and (d) O 1s on bare In₂O₃, CuInS₂ and 17-CuInS₂/In₂O₃ composite.

3.4 UV-vis DRS analysis

UV-vis diffuse reflectance spectra (DRS) were recorded for investigating the photo-response range on the prepared photocatalysts. As displayed in Fig. 5(a), the optical absorption edge of bare In₂O₃ was found at approximate 500 nm. After combining with CuInS₂, the CuInS₂/In₂O₃ composites revealed gradually promoted visible light response in the range of 500–800 nm with the CuInS₂ loading content increasing, which was attributed to the excellent light absorption on CuInS₂. In addition, the band gap energies of the bulk CuInS₂ and In₂O₃ was analyzed via the following equation:⁴⁵ αħν = A (ħν − E_g)^n/2, in which α and A represent the absorption coefficients, ħ, ν and E_g stand for the Planck constant, light frequency and band gap energy, respectively, and n depends on the transition feature of electrons in the semiconductor with 1 and 4 for a direct transition and indirect transition gap one, respectively. In this case, both CuInS₂ and In₂O₃ were disclosed to reveal direct transitions.^27,41 Accordingly, the band gap values of CuInS₂, In₂O₃, 5-CuInS₂/In₂O₃, 11-CuInS₂/In₂O₃, 17-CuInS₂/In₂O₃ and 23-CuInS₂/In₂O₃ were estimated to be 1.81, 2.75, 2.73, 2.70, 2.67 and 2.62 eV, respectively (Fig. 5(b)).

Fig. 5 (a) UV-vis DRS spectra of different samples and (b) Plots of (aħv)² versus ħv on pure CuInS₂ and In₂O₃.

3.5 Photoluminescence and photocurrent analysis

The photoluminescence (PL) emission spectra were used to explore the recombination rate of photoinduced electrons and holes on the prepared photocatalysts. As illustrated in Fig. 6(a), the CuInS₂/In₂O₃ samples exhibited obviously declining PL emission intensities in comparison with the pure In₂O₃ and CuInS₂, indicated that the recombination of charge carries was effectively restrained in the composites.^46,47 In addition, it was found that the loading content of CuInS₂ in the composites had a great influence on the PL intensity. Notably, the 17-CuInS₂/In₂O₃ possessed the lowest PL emission intensity, implying its highest separation efficiency of electron–hole pairs in the photocatalytic process. Furthermore, to confirm the charge separation efficiency on the obtained catalysts, photocurrent response tests were carried out and the results are shown in Fig. 6(b). Normally, the higher photocurrent intensity indicates the larger charge transfer and separation rate on the semiconductor.^48,49 It was observed that the CuInS₂/In₂O₃ composites displayed enhanced photocurrent response densities compared to the bare materials, and the 17-CuInS₂/In₂O₃ showed the strongest response density, suggesting the outstanding charge separation efficiency inside 17-CuInS₂/In₂O₃. This result was kept in line with the analysis of PL spectra, which might be due to the tight interfacial contact with covalent S–O bonds between CuInS₂ and In₂O₃.

Fig. 6 Photoluminescence spectra (a) and photocurrent density curves (b) on bare In₂O₃, CuInS₂ and CuInS₂/In₂O₃ composites.

3.6 Photocatalytic activity

Photocatalytic experiments of degrading LOM and reducing Cr(VI) conducted under visible light irradiation were used to evaluate the photocatalytic performance of the as-fabricated photocatalysts. In addition, the blank tests were also carried out to estimate the photostability of the selected pollutants under light irradiation. Fig. 7(a) presents the photo-degradation activities of different samples towards LOM solution. It could be found that there was very little degradation rate of LOM with direct photolysis (without solids), suggesting the tenacious stability of aqueous LOM. Besides, after the addition of bare In₂O₃, only 29.9% of LOM was degraded in 90 min, and meanwhile the pure CuInS₂ also displayed unsatisfactory photocatalytic performance with 37.7% LOM degradation rate. By contrast, all the CuInS₂/In₂O₃ composites showed boosted photocatalytic activities. Among them, the 17-CuInS₂/In₂O₃ possessed the highest photo-degradation rate of LOM (98.8%) after 90 min visible light irradiation, which might mainly due to its excellent separation efficiency of electron–hole pairs demonstrated by the PL and photocurrent measurements. Additionally, the photo-reaction kinetics of LOM over different catalysts were analyzed via the pseudo-first order kinetic formula (ln(C₀/C_t) = kt).⁵⁰ As illustrated in Fig. 7(b), the fitted curves were in good accordance with the kinetic model, and the 17-CuInS₂/In₂O₃ revealed the fastest rate constant (0.0474 min⁻¹), which was 10.1 and 13.9 times higher than that of bare CuInS₂ (0.0047 min⁻¹) and In₂O₃ (0.0034 min⁻¹), respectively.

Fig. 7 Photocatalytic degradation of LOM (a) and reduction of Cr(VI) (c), with (b) and (d) the corresponding pseudo-first-order kinetics on the as-prepared photocatalysts under visible light irradiation.

Fig. 7(c) shows the photocatalytic performance for Cr(VI) reduction on the prepared catalysts. Similar to the LOM degradation, the removal rate of Cr(VI) was almost negligible in the blank experiment. Besides, both bare CuInS₂ and In₂O₃ presented low catalytic performance with only 37.1% and 25.2% reduction rate of Cr(VI), respectively. However, markedly improved activities of CuInS₂/In₂O₃ were acquired for the removal of Cr(VI), and in line with the LOM degradation tests, the 17-CuInS₂/In₂O₃ also displayed the best reduction rate of Cr(VI) (98.1%) in the 90 min reaction time. Furthermore, the obtained rate constants resulted from the pseudo-first order kinetic curves disclosed that the k value on the 17-CuInS₂/In₂O₃ (0.0439 min⁻¹) was 9.14 and 14.6 times higher than that of pure CuInS₂ (0.0048 min⁻¹) and In₂O₃ (0.0030 min⁻¹), respectively (Fig. 7(d)). In addition, the photocatalytic activity of the 17-CuInS₂/In₂O₃ composite was also compared with those of other reported photocatalysts. As displayed in Table S1,† the 17-CuInS₂/In₂O₃ showed a quite competitive performance in the photocatalytic removal of lomefloxacin and Cr(VI).

The cyclic stability of the 17-CuInS₂/In₂O₃ photocatalyst was evaluated via the consecutive LOM degradation and Cr(VI) reduction for five times, respectively. As can be obtained from Fig. 8(a), the photocatalytic performance of 17-CuInS₂/In₂O₃ had no obvious deactivation with only 2.8% and 3.6% decline in LOM degradation and Cr(VI) reduction after five runs, respectively, disclosing its remarkable reusability. Besides, it could be concluded from the XRD patters (Fig. 8(b)) and S 2p and O 1s XPS spectra (Fig. S1†) of 17-CuInS₂/In₂O₃ before and after five cycles that its crystal structure basically remained unchanged during the reactions, which further verifying its chemical stability.

Fig. 8 (a) Recycling experiments of 17-CuInS₂/In₂O₃ for LOM degradation and Cr(VI) reduction, (b) XRD patterns of the fresh and used 17-CuInS₂/In₂O₃.

3.7 Mechanism analysis

In order to elucidate the photocatalytic mechanism on the as-synthesized materials, density functional theory (DFT) calculation was used to explore the electronic band structures of bulk CuInS₂ and In₂O₃. As presented in Fig. 9(a and b), the direct band gaps of the CuInS₂ and In₂O₃ were calculated to be 1.77 and 2.71 eV, respectively, which were very close to the Tauc plot results of 1.81 eV (CuInS₂) and 2.75 eV (In₂O₃) from UV-vis DRS spectra (Fig. 5(b)). In addition, the Mott–Schottky (M–S) tests were performed to determine the band potentials of the CuInS₂ and In₂O₃. As can be seen from the M–S plots (Fig. 9(c and d)), these two materials both belonged to the n-type semiconductors, and the flat band potentials on the CuInS₂ and In₂O₃ were obtained as −0.67 and −0.19 V vs. Ag/AgCl (pH = 6.8), respectively, and meanwhile their flat band potentials could be converted to be −0.47 and 0.01 V vs. NHE (E_NHE = E_Ag/AgCl + 0.2 V),⁵¹ respectively. For the n-type semiconductor, the conduction band (CB) potential is normally more negative by approximately 0.1 V than the flat band potential,¹⁰ thus the CB potentials of the CuInS₂ and In₂O₃ were determined to be −0.57 and −0.09 V vs. NHE, respectively. In addition, according to the formula E_VB = E_CB + E_g, the corresponding VB potentials were calculated to be 1.24 eV for CuInS₂ and 2.66 eV for In₂O₃, which agreed well with the results from VB-XPS spectra presented in Fig. S2.†

Fig. 9 DFT calculated electronic band structures (a and b) and Mott–Schottky curves (c and d) of CuInS₂ and In₂O₃.

To clearly clarify the effect of reactive species involving the photocatalytic activity over 17-CuInS₂/In₂O₃, quenching tests were carried out by adding ammonium oxalate (AO), dimethyl sulfoxide (DMSO), isopropyl alcohol (IPA) and benzoquinone (BQ) as sacrificial agents for holes (h⁺), electrons (e⁻), hydroxyl radicals (˙OH) and superoxide radicals (˙O₂⁻), respectively.⁵² As displayed in Fig. 10(a), there was a sharp decline (from 98.8% to 18.6%) in the photo-degradation rate of LOM when the IPA was added as the scavenger, suggesting that ˙OH played the key role in degrading LOM. Besides, the introductions of AO and BQ had apparently negative effects on LOM degradation with the degradation rates decreasing to 48.2% and 35.1%, respectively, indicating that the h⁺ and ˙O₂⁻ species both played important roles in LOM degradation. For the trapping experiments of Cr(VI) reduction, it was observed that adding DMSO could significantly restrain the reduction of Cr(VI) (17.4%), revealing that the photoinduced e⁻ directly participated in the Cr(VI) reduction and acted as the dominant role in the reaction. In addition, the Cr(VI) reduction rate was greatly suppressed in the presence of BQ (30.2%), implying that ˙O₂⁻ radicals also played a crucial role in Cr(VI) reduction.

Fig. 10 Photocatalytic removal of LOM and Cr(VI) over 17-CuInS₂/In₂O₃ with different sacrificial agents (a), ESR spectra of ˙O₂⁻ (b) and ˙OH (c) radicals trapped by DMPO on 17-CuInS₂/In₂O₃.

Additionally, ESR measurements were conducted to further examine the existence of ˙O₂⁻ and ˙OH active species generated in the photocatalytic system. As shown in Fig. 10(b), there was no distinct peak in the absence of light, and the bare In₂O₃ also displayed no characteristic peak in its ESR spectrum for DMPO-˙O₂⁻, which could be due to that the induced e⁻ in the CB of In₂O₃ (−0.09 eV vs. NHE) was not strong enough to reactive with O₂ to generate ˙O₂⁻ (O₂/˙O₂⁻, −0.33 eV vs. NHE).⁵³ Meanwhile, four characteristic peaks of DMPO-˙O₂⁻ were detected on the bare CuInS₂ after 8 min visible light irradiation. Noticeably, the typical signals of DMPO-˙O₂⁻ were extremely enhanced on the 17-CuInS₂/In₂O₃, indicating that abundant O₂⁻ radicals had been produced in this catalytic system. Fig. 10(c) depicts the ESR spectra of DMPO-˙OH performed on the as-prepared samples. It was found that no characteristic signal could be observed in dark, while four typical peaks of DMPO-˙OH appeared in the ESR spectrum on the bare In₂O₃, suggesting the existence of ˙OH species in the photoreaction. As for the bare CuInS₂, no obvious peak could be perceived in its ESR curve for DMPO-˙OH, on account of the lower oxidation ability of h⁺ in the VB of CuInS₂ (1.24 eV vs. NHE) relative to the standard potential of H₂O/˙OH (2.4 eV vs. NHE).⁵⁴ However, the 17-CuInS₂/In₂O₃ exhibited a significant enhancement of DMPO-˙OH peaks compared with bare In₂O₃, revealing that the combination of In₂O₃ and CuInS₂ could greatly promote the production of ˙OH radicals.

Based on the above band structure analysis of CuInS₂ and In₂O₃, it was assumed that the 17-CuInS₂/In₂O₃ was constructed within a traditional type II band alignment, the photoexcited e⁻ in the CB of CuInS₂ (−0.57 eV vs. NHE) should move down to that of In₂O₃ (−0.09 eV vs. NHE), and simultaneously, the generated h⁺ in the VB of In₂O₃ (2.66 eV vs. NHE) would transfer to that of CuInS₂ (1.24 eV vs. NHE). Nevertheless, in this case, neither ˙O₂⁻ nor ˙OH could be produced in the photocatalytic reactions with 17-CuInS₂/In₂O₃ composite, which be in contradiction to the results from the quenching and ESR tests. Thereby, the charge transfer mechanism of this CuInS₂/In₂O₃ heterojunction may belong to a Z-scheme pattern. To further testify this supposition and make certain the charge transfer model at interface, work functions of CuInS₂ and In₂O₃ were calculated by the DFT method. As indicated in Fig. 11, the surface work functions (Φ) of CuInS₂ and In₂O₃ were simulated to be 3.28 and 5.76 eV, respectively. According to the formula of E_f = E_va − Φ,⁵⁵ in which E_f and E_va stand for the Fermi level and electrostatic potential of vacuum level, respectively, the Fermi energy of CuInS₂ was higher than that of In₂O₃. Thus, when CuInS₂ and In₂O₃ form a intimate heterostructure, the e⁻ at the CuInS₂/In₂O₃ interface will be rearranged with a migration from CuInS₂ to In₂O₃ until their Fermi energies attain an equilibrium, and as a result, the internal electric field (IEF) will be formed with the orientation from CuInS₂ to In₂O₃. Thereby, due to the IEF, the photoexcited e⁻ in In₂O₃ should transfer to CuInS₂ in the 17-CuInS₂/In₂O₃ reaction system, forming the Z-scheme transfer path.

Fig. 11 Calculated work functions of (a) CuInS₂ and (b) In₂O₃.

As a whole, the Z-scheme photocatalytic mechanism over CuInS₂/In₂O₃ heterostructure is proposed and illustrated in Fig. 12. Firstly, once the CuInS₂/In₂O₃ was irradiated by the visible light, both CuInS₂ and In₂O₃ were excited to yield electrons and holes in their CB and VB, respectively. Secondly, owing to the formation of IEF, it would drive the photoexcited electrons to move from the CB of In₂O₃ to the VB of CuInS₂ via interfacial S–O bonds, and then the recombination of electrons and holes occurred there. In the meantime, the electrons congregated in the CB of CuInS₂ could directly reduce Cr(VI) to Cr(III) or produce ˙O₂⁻ via reacting with O₂, and subsequently the generated ˙O₂⁻ could not only convert Cr(VI) to Cr(III) but also oxidize LOM to smaller molecules. Besides, in the VB of In₂O₃, the accumulated holes would degrade LOM or react with H₂O to produce ˙OH radicals, further contributing to the degradation of LOM. Based on the aforementioned reaction process, the Z-scheme heterostructure and the formed interfacial S–O bonds between CuInS₂ and In₂O₃ were regarded as the two synergistic factors for the enhanced photocatalytic performance on CuInS₂/In₂O₃ heterojunctions. On the one hand, the built IEF in the Z-scheme heterostructure provided an efficient separate route for e⁻–h⁺ pairs. On the other hand, the covalent S–O linkage served as a specific bridge to further accelerate the charge separation. By virtue of this synergistic effect, the recombination of e⁻–h⁺ pairs was markedly suppressed on the CuInS₂/In₂O₃ composite, thus greatly enhancing its photocatalytic performance for aqueous LOM and Cr(VI).

Fig. 12 Schematic representation of proposed photocatalytic mechanism on CuInS₂/In₂O₃ for LOM degradation and Cr(VI) reduction under visible light irradiation.

4. Conclusions

In summary, a covalently bonded CuInS₂/In₂O₃ composite photocatalyst was successfully prepared via a hydrothermal and followed annealing route. The synthesized CuInS₂/In₂O₃ heterostructure possessed an intimate interface with covalent S–O bonds confirmed by the FT-IR and XPS characterizations. In addition, the trapping and ESR tests disclosed a Z-scheme charge transfer pattern associated with the CuInS₂/In₂O₃ heterojunction, and this transfer model was further demonstrated theoretically via the DFT calculations. As a result, in comparison with the individual materials, the optimal 17-CuInS₂/In₂O₃ composite exhibited extremely promoted photocatalytic performance in degrading LOM and reducing Cr(VI) under visible light irradiation. In this CuInS₂/In₂O₃ system, the built IEF provided an efficient Z-scheme separation path for charge carriers, and meanwhile the formed interfacial S–O bond acted as a bridge to further boost this separation rate. Thus, the Z-scheme heterostructure and S–O linkages showed a synergistic effect for the enhanced photocatalytic activities over CuInS₂/In₂O₃ composites. In addition, the fabricated heterojunctions also presented good stability and reusability in five test cycles. This work provides an innovative route to constructing high-performance Z-scheme photocatalysts for environmental pollution.

Author contributions

Xiaofei Fu: conceptualization, validation, formal analysis, writing-original draft, writing-review & editing. Junwu Tao: formal analysis, visualization, data curation. Zizhou Zhao: formal analysis, methodology, data curation. Siwen Sun: formal analysis, visualization, data curation. Lin Zhao: formal analysis, data curation. Zuming He: investigation, supervision, writing-review & editing. Yong Gao: supervision, formal analysis, visualization, supervision. Yongmei Xia: formal analysis, writing-review & editing. All authors have approved the final version of the manuscript.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Specialized Research Fund for the Doctoral Program of Jiangsu University of Technology (KYY16018) and the Innovation and Entrepreneurship Training Program for Jiangsu college students (202211463073Y).

References

1. H. Zhang, L. chao Nengzi, Z. Wang, X. Zhang, B. Li and X. Cheng, J. Hazard. Mater., 2020, 383, 121236.
2. Q. Wu, H. Yang, L. Kang, Z. Gao and F. Ren, Appl. Catal., B, 2020, 263, 118282.
3. J. W. Wang, F. G. Qiu, P. Wang, C. Ge and C. C. Wang, J. Cleaner Prod., 2021, 279, 123408.
4. A. M. Ismail, A. A. Menazea and H. Ali, J. Mater. Sci.: Mater. Electron., 2021, 32, 19352.
5. P. Czupryński, M. Płotka, P. Glamowski, W. Żukowski and T. Bajda, RSC Adv., 2022, 12, 5145–5156.
6. C. A. Kozlowski and W. Walkowiak, Water Res., 2002, 36, 4870–4876.
7. L. Kang, Z. Han, H. Yu, Q. Wu and H. Yang, Sep. Purif. Technol., 2022, 278, 119482.
8. J. Han, S. Zhang, Q. Song, H. Yan, J. Kang, Y. Guo and Z. Liu, Sustainable Energy Fuels, 2021, 5, 509–517.
9. L. Li, H. Gao, G. Liu, S. Wang, Z. Yi, X. Wu and H. Yang, Adv. Powder Technol., 2022, 33, 103481.
10. S. V. P. Vattikuti, A. K. R. Police, J. Shim and C. Byon, Appl. Surf. Sci., 2018, 447, 740–756.
11. X. Dong, H. Hao, N. Wang, H. Yuan and X. Lang, J. Colloid Interface Sci., 2021, 590, 387–395.
12. J. Juay, J. C. E. Yang, H. Bai and D. D. Sun, RSC Adv., 2022, 12, 25449.
13. Z. He, Y. Xia, B. Tang, X. Jiang and J. Su, Mater. Lett., 2016, 184, 148–151.
14. Y. Lv, Z. Yu, S. Huang, F. Deng, K. Zheng, G. Yang, Y. Liu, C. Lin, X. Ye and M. Liu, Chemosphere, 2021, 271, 129452.
15. A. Saroni, M. Alizadeh, S. A. Rahman, W. Meevasana and B. T. Goh, J. Power Sources, 2020, 480, 228829.
16. J. Shen, L. Qian, J. Huang, Y. Guo and Z. Zhang, Sep. Purif. Technol., 2021, 275, 119239.
17. S. Wang, B. Y. Guan and X. W. D. Lou, J. Am. Chem. Soc., 2018, 140, 5037–5040.
18. H. Xu, H. Chen, S. Chen, K. Wang and X. Wang, Int. J. Hydrogen Energy, 2021, 46, 32445–32454.
19. Y. Wang, F. Xu, L. Sun, Y. Li, L. Liao, Y. Guan, J. Lao, Y. Yang, T. Zhou, Y. Wang, B. Li, K. Zhang and Y. Zou, RSC Adv., 2022, 1, 541004.
20. Z. He, M. S. Siddique, H. Yang, Y. Xia, J. Su, B. Tang, L. Wang, L. Kang and Z. Huang, J. Cleaner Prod., 2022, 339, 130634.
21. Z. He, H. Yang, J. Su, Y. Xia, X. Fu, L. Wang and L. Kang, Fuel, 2021, 294, 120399.
22. H. Hao and X. Lang, ChemCatChem, 2019, 11, 1378–1393.
23. T. Wang, Z. Zeng, J. Yang, Z. Pan, X. Zheng, Y. Guo and T. Huang, Colloids Interface Sci. Commun., 2022, 51, 100674.
24. Y. Duan, J. Xue, J. Dai, Y. Wei, C. Wu, S. H. Chang and J. Ma, Appl. Surf. Sci., 2022, 592, 153306.
25. X. Fu, J. Tao, Z. He, Y. Gao, Y. Xia and Z. Zhao, J. Mater. Sci.: Mater. Electron., 2022, 33, 24663–24676.
26. H. Tang, Y. Deng, H. Zou, Y. Tan, Y. Xiang, Y. Xu, W. Wu and Y. Zhou, ChemistrySelect, 2022, 5, 8258–8264.
27. Z. Pan, L. Qian, J. Shen, J. Huang, Y. Guo and Z. Zhang, Chem. Eng. J., 2021, 426, 130385.
28. H. Hou, Y. Yuan, S. Cao, Y. Yang, X. Ye and W. Yang, J. Mater. Chem. C, 2020, 8, 11001–11007.
29. Y. Zhang, Y. Huang, S. S. Zhu, Y. Y. Liu, X. Zhang, J. J. Wang and A. Braun, Small, 2021, 17, 2100320.
30. C. Xu, C. Jin, W. Chang, X. Hu, H. Deng, E. Liu and J. Fan, Catal. Sci. Technol., 2019, 9, 4990–5000.
31. X. Fu, H. Yang, H. Sun, G. Lu and J. Wu, J. Alloys Compd., 2016, 662, 165–172.
32. N. Shehzad, M. Tahir, K. Johari, T. Murugesan and M. Hussain, J. Environ. Chem. Eng., 2018, 6, 6947–6957.
33. Z. Xie, Y. Xu, D. Li, S. Meng, M. Chen and D. Jiang, ACS Appl. Energy Mater., 2020, 3, 12194–12203.
34. H. Chai, L. Gao, P. Wang, F. Li, G. Hu and J. Jin, Appl. Catal., B, 2022, 305, 121011.
35. X. Zhou, J. Wu, Q. Li, T. Zeng, Z. Ji, P. He, W. Pan, X. Qi, C. Wang and P. Liang, J. Catal., 2017, 355, 26–39.
36. F. Deng, X. Lu, Y. Luo, J. Wang, W. Che, R. Yang, X. Luo, S. Luo and D. D. Dionysiou, Chem. Eng. J., 2019, 361, 1451–1461.
37. X. Lu, W. Che, X. Hu, Y. Wang, A. Zhang, F. Deng, S. Luo and D. D. Dionysiou, Chem. Eng. J., 2019, 356, 819–829.
38. A. Das, M. Patra, M. Kumar P, M. Bhagavathiachari and R. G. Nair, Mater. Chem. Phys., 2021, 263, 124431.
39. M. Lu, Z. Sun, Y. Zhang, Q. Liang, M. Zhou, S. Xu and Z. Li, Synth. Met., 2020, 268, 116480.
40. T. Yang, K. Xue, J. Wang, R. He, R. Sun, U. Omeoga, T. Yang, W. Wang, J. Wang and Y. Hu, J. Alloys Compd., 2020, 818, 152873.
41. F. Guo, W. Shi, M. Li, Y. Shi and H. Wen, Sep. Purif. Technol., 2019, 210, 608–615.
42. W. Ye, J. Hu, X. Hu, W. Zhang, X. Ma and H. Wang, ChemCatChem, 2019, 11, 6372–6383.
43. W. Yue, S. Han, R. Peng, W. Shen, H. Geng, F. Wu, S. Tao and M. Wang, J. Mater. Chem., 2010, 20, 7570–7578.
44. J. Park, T. H. Lee, C. Kim, S. A. Lee, M. J. Choi, H. Kim, J. W. Yang, J. Lim and H. W. Jang, Appl. Catal., B, 2021, 295, 120276.
45. L. Li, H. Gao, Z. Yi, S. Wang, X. Wu, R. Li and H. Yang, Colloids Surf., A, 2022, 644, 128758.
46. Z. Gao, H. Yang, J. Li, L. Kang, L. Wang, J. Wu and S. Guo, Appl. Catal., B, 2020, 267, 118695.
47. L. Kang, L. Xu, Z. Han, H. Yu, Q. Wu, M. Wu, Z. He, L. Wang and H. Yang, Chem. Eng. J., 2022, 429, 132355.
48. T. Cheng, H. Gao, G. Liu, Z. Pu, S. Wang, Z. Yi, X. Wu and H. Yang, Colloids Surf., A, 2022, 633, 127918.
49. Q. Zhao, Z. Liu, J. Li, W. Yan, J. Ya and X. Wu, Int. J. Hydrogen Energy, 2021, 46, 36113–36123.
50. Q. Qiu, P. Zhu, Y. Liu, T. Liang, T. Xie and Y. Lin, RSC Adv., 2021, 11, 3333–3341.
51. Z. He, H. Yang, J. Sunarso, N. H. Wong, Z. Huang, Y. Xia, Y. Wang, J. Su, L. Wang and L. Kang, Chemosphere, 2022, 303, 134973.
52. Z. He, Y. Xia, J. Su and B. Tang, Opt. Mater., 2019, 88, 195–203.
53. Z. He, Y. Xia, B. Tang and J. Su, Mater. Res. Express, 2017, 4, 095501.
54. X. Wei, C. C. Wang, Y. Li, P. Wang and Q. Wei, Chemosphere, 2021, 280, 130734.
55. X. Wang, X. Wang, H. Yang, A. Meng, Z. Li, L. Yang, L. Wang, S. Li, G. Li and J. Huang, Chem. Eng. J., 2022, 431, 134000.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3ra00043e

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