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. 2026 Jul 9;16(37):39337–39349. doi: 10.1039/d6ra03849b

Efficient removal of heavy metal Cr(vi) from wastewater via synergy of defect engineering and S-scheme heterojunction

Mingxia Tian a, Dong He a, Hui Bai a, Lili Huang a, Jianhui Jiang a,✉
PMCID: PMC13347490  PMID: 42427405

Abstract

A layered indium tin sulfide (SnIn4S8, SIS)/KCl-, LiCl-, and NaCl-modified g-C3N5 (KLNCN) S-scheme heterojunction material was synthesised via thermal condensation and in situ growth. Under simulated sunlight irradiation, the degradation rate of rhodamine B (Rh B) reached 92.46%, which was 5.6 to 42.9 folds higher than that of pure SIS and KLNCN, respectively. Meanwhile, the reduction rate of Cr(vi) reached 93.60%, representing respective 5.5- and 49-fold enhancements compared with that of pure SIS and KLNCN. Moreover, systematic investigations of catalyst dosage, reusability, and catalytic performance under different pH conditions confirmed that the SIS/KLNCN heterojunction material exhibits an excellent stability and broad adaptability. The outstanding photocatalytic performance is attributed to the synergistic effect of sulfur vacancies and the S-scheme heterojunction formed by SIS and KLNCN, which reduces the recombination rate of the photogenerated electron–hole pairs and improves photocatalytic effectiveness. Free radical capture experiments confirmed that the ˙O2− radical is the primary active species, playing a key role in the degradation of Rh B. This study provides new insights into the design of efficient and stable photocatalysts and demonstrates their significant application potential.

A layered indium tin sulfide (SnIn4S8, SIS)/KCl-, LiCl-, and NaCl-modified g-C3N5 (KLNCN) S-scheme heterojunction material was synthesised via thermal condensation and in situ growth.graphic file with name d6ra03849b-ga.jpg

1. Introduction

With the acceleration of industrialisation and urbanisation, industries such as electroplating, coating, tanning, and smelting are producing increasingly large volumes of wastewater containing heavy metals (HMs). Although emission standards for HMs have been tightened, their discharge remains the primary contributor to industrial wastewater emissions in various countries.1,2 Exposure to high levels of HMs poses significant toxic risks to human health. For example, chromium (Cr) exists in two forms: trivalent (Cr(iii)) and hexavalent (Cr(vi)). Cr(iii) is very low in toxicity or even non-toxic. It is an essential element for the growth and development of plants and animals and participates in the metabolism of sugars and fats. However, Cr(vi) is highly soluble and carcinogenic and can pass through cell membranes via certain anions such as SO42− and PO43−. This facilitates its accumulation in the human body through the food chain, thereby leading to toxicity and serious risks to human health.3,4 Conventional wastewater treatment methods are often ineffective at removing some pollutants that are low in toxicity, non-biodegradable, or persistent in the environment.5 Therefore, the treatment of wastewater containing HMs is both important and urgent.

Photocatalytic technology, as a green, highly efficient, low-cost, and easy-to-operate approach, is widely used in water treatment.6 In wastewater containing dyes and HMs, the primary degradation principle involves the excitation of a photocatalyst by sunlight, which initiates a series of oxidation–reduction reactions with the pollutants in the solution.7 Under visible-light irradiation, the semiconductor photocatalyst is excited to generate electrons (e−) and holes (h+), which promote the formation of highly reactive free species in solution, including hydroxyl radicals (˙OH), superoxide radicals (˙O2−), and h+. These species then decompose organic pollutants in wastewater into non-toxic carbon dioxide and water.8–10 Therefore, photocatalysis has shown significant potential in wastewater purification and has attracted widespread research attention.11 Importantly, the photocatalytic degradation of refractory pollutants using sunlight-driven photocatalysts provides a sustainable method for environmental purification. However, the relatively poor catalytic performance of conventional photocatalysts limits their broader application, making the design of more efficient photocatalysts a central focus of photocatalytic technology.

Among all known photocatalytic materials, the nitrogen-rich polymer semiconductor graphitic carbon nitride (g-C3N5) has attracted considerable attention, owing to its abundant availability, narrow band gap, non-toxicity, defect-rich structure, and good chemical stability. These properties endow the material with significant potential for photocatalytic energy conversion and water treatment.11,12 However, single-component g-C3N5 exhibits relatively low conductivity, a high rate of photogenerated carrier recombination, and limited redox potential, which restrict its photocatalytic performance and application potential.13 To overcome these limitations, researchers have enhanced the application scope of g-C3N5 through strategies such as morphology engineering, defect regulation, doping, and heterostructure construction.14–18 For example, CdTe@CdS quantum dots were embedded into the g-C3N5 nanosheet structure, which significantly enhanced the specific surface area of the g-C3N5 structure and the photocatalytic activity of single g-C3N5 (ref. 19) Kong et al.20 demonstrated that a Bi2MoO6/g-C3N5 S-type heterojunction, constructed via in situ synthesis, markedly improved the migration ability and redox capacity of photogenerated carriers. This enhancement was attributed to the formation of an intrinsic electric field at the composite interface, which facilitated the directional movement of photogenerated carriers and improved photocatalytic efficiency. These findings strongly support the practical application of heterojunction-based photocatalysts in water treatment. In particular, owing to its unique charge-transfer mechanism, the S-scheme heterojunction promotes the recombination of ineffective electron–hole pairs while retaining those with strong oxidising and reducing capabilities. These carriers then participate in redox reactions, enhancing the degradation rate of pollutants.21–24

Layered indium tin sulfide, SnIn4S8, with a cubic perovskite structure, exhibits a narrow band gap, strong visible-light response, and excellent photogenerated charge-transfer efficiency, making it promising for photocatalytic applications. However, single-component SnIn4S8 is limited by rapid recombination of photogenerated charge carriers.25,26 Therefore, in this study, g-C3N5 was modified using the molten salt method, and a heterojunction was subsequently formed with SnIn4S8. The resulting intrinsic electric field promotes the effective separation of photogenerated carriers, retains electron–hole pairs with strong oxidising and reducing properties, and enhances photocatalytic performance compared to that of the individual components.

In this study, KCl-, LiCl-, and NaCl-modified g-C3N5 (KLNCN) was synthesised via thermal polymerisation. The SnIn4S8/KLNCN composite was subsequently prepared using an in situ growth method, and its performance in dye- and chromium-containing wastewater was systematically studied. The structure, morphology, and electrochemical properties of the composite were characterised using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and photoluminescence (PL) spectroscopy. The results demonstrated that the formation of heterojunctions and generation of vacancies in the SnIn4S8/KLNCN composite significantly prolonged the lifetime of the photogenerated carriers and increased the number of reactive sites, thereby enhancing its photocatalytic performance.

2. Experimental

2.1. Reagents

Potassium chloride (KCl, analytical reagent, AR), lithium chloride (LiCl, AR), sodium chloride (NaCl, AR), 3-amino-1,2,4-triazole (C2H4N4, 3-AT, AR), pentahydrate of tetrachlorosilicon (SnCl4·5H2O, AR), tetrahydrate of indium trichloride (InCl3·4H2O, 98%), l-cysteine (HSCH2CH(NH2)COOH, 99%), 1,4-benzoquinone (BQ, C6H4O2, AR), potassium iodide (KI, AR), isopropanol (IPA, C3H8O, AR), rhodamine B (Rh B, C28H31ClN2O3, AR), and potassium dichromate (K2Cr2O7, AR).

2.2. Preparation of KLNCN

Salt-modified g-C3N5 was prepared according to a previously reported method.27 Potassium chloride, lithium chloride, sodium chloride, and 3-amino-1,2,4-triazole (3-AT) were mixed at a defined mass ratio and thoroughly ground to obtain a homogenous mixture. Subsequently, the mixture was heated at a rate of 5 °C min−1 to 500 °C for 5 h. The resulting precipitate was washed with deionized water, dried, and ground into a powder denoted as KLNCN.

2.3. Preparation of the SnIn4S8/KLNCN heterojunction

The preparation of the SnIn4S8/KLNCN heterojunction is illustrated in Fig. 1. Varying amounts of KLNCN were added to a solvent with a water-to-alcohol ratio of 1 : 2 under ultrasonic treatment. Subsequently, SnCl4·5H2O (1 mol), InCl3·4H2O (4 mol), and l-cysteine were added, and the mixture was stirred until a homogeneous sample was obtained. The resulting mixture was then transferred to a water-cooled autoclave and heated in a water bath at 180 °C for 12 h. The added amounts of KLNCN were 100 mg, 200 mg and 400 mg, and the final products were designated as SIS/KLNCN1, SIS/KLNCN2, and SIS/KLNCN3, respectively.

Fig. 1. Schematic diagram illustrating the preparation of the SIS/KLNCN heterojunction.

Fig. 1

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2.4. Evaluation of photocatalytic performance

The prepared photocatalyst (7 mg) was added to a solution containing 10 mg L−1 of Rh B or Cr(vi) (40 mL). The mixture was stirred in the dark for 30 min to establish adsorption–desorption equilibrium, and then irradiated using a 300 W xenon lamp to simulate sunlight. For Rh B degradation, samples were collected seven times at 10 min intervals, whereas for Cr(vi) reduction, samples were collected five times at 10 min intervals. The absorbance of the supernatant was measured at 554 nm (Rh B) and 540 nm (Cr(vi)), and the degradation rate was calculated using the following formula:

η = (C0 − Ct)/C0 1

where C0 represents the initial concentration before degradation and Ct represents the concentration after a specified period of light exposure.

Details of the instruments used for photocatalyst characterisation are provided in Text S1 of the SI.

3. Results and discussion

3.1. XRD analysis

The crystal structures of the KLNCN, SIS, and SIS/KLNCN samples were determined using XRD (Fig. 2(a)). For KLNCN, characteristic peaks at 11.96° and 27.44° were assigned to the (100) and (002) crystal planes, respectively. The former reflects the ordered stacking of triazine ring units in the CN structure, while the latter corresponds to the π–π* stacking within the graphite layer of CN.28 These results are consistent with those of the standard card (JCPDS No. 87-1526) for g-C3N5. The 2θ values of the SIS samples were observed at 18.26°, 28.38°, 32.36°, 37.57°, 50.15°, 58.90°, corresponding to (202), (600), (602), (404), (006), and (007) crystal planes of the cubic SIS phase, respectively. These results are in good agreement with previous literature,29–32 confirming that the SIS samples were successfully synthesised via the hydrothermal method. Interestingly, after combining KLNCN and SIS using an in situ synthesis method, the characteristic diffraction peaks of SIS were still observed, indicating that the SIS/KLNCN composite retained the crystal structure of SIS.32 Compared to pure SIS, the diffraction angles of the SIS/KLNCN composite shifts towards smaller angles (Fig. 2(b)), suggesting the formation of a strong interface connection between KLNCN and SIS. However, no diffraction peaks corresponding to KLNCN were observed in the SIS/KLNCN composite, which may be attributed to the low content and poor crystallinity of KLNCN.33–35

Fig. 2. (a) XRD patterns of KLNCN, SIS, and SIS/KLNCN and (b) partial enlarged view highlighting the characteristic peaks.

Fig. 2

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3.2. Morphological analysis

The morphology and microstructure of the prepared photocatalyst were systematically characterised using scanning electron microscopy (SEM) and TEM. As illustrated in Fig. 3(a), the KLNCN synthesised via the molten salt method, which includes calcination, exhibits a typical layered fragmented stacking structure. The single-component SIS sample displays a unique multilevel petal-like aggregate microstructure (Fig. 3(b)), in which the petal-shaped SIS units are formed via the layer-by-layer self-assembly of ultrathin nanosheets to form a highly distinct hierarchical porous architecture. Fig. 3(c) illustrates the unique semi-blossom-like microstructure of SIS/KLNCN, in which the two-dimensional layered KLNCN surface is interspersed with layered SIS nanosheets. This morphology significantly increases the specific surface area and availability of active sites, thereby enhancing the photocatalytic performance of the composite. Fig. 3(d–f) presents the TEM and HRTEM images of the SIS/KLNCN sample. The images further demonstrate that SIS nanosheets are tightly attached to the surface of KLNCN, significantly enhancing interfacial contact within the composite and facilitating heterojunction formation. Analysis of the crystal structure of the SIS/KLNCN composite reveals lattice spacings of 0.2644 and 0.3231 nm, corresponding to the (007) and (311) crystal planes of SIS, respectively.30,36 This confirms that the crystal structure of SIS in the SIS/KLNCN composite is preserved. Additionally, elemental mapping (Fig. S1) indicates that In, N, S, C, and Sn are present and uniformly distributed in the composite. No other impurity elements were detected, confirming the uniform growth of layered SIS nanosheets on the surface of KLNCN.26

Fig. 3. Low-magnification SEM of (a) KLNCN, (b) SIS, and (c) SIS/KLNCN2. (d and e) TEM and (f) HRTEM images of SIS/KLNCN2.

Fig. 3

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3.3. XPS analysis

The valence states of the elements in the SIS, KLNCN, and SIS/KLNCN2 samples were characterised using XPS. The formation of the heterojunction and the associated electron transfer processes were also investigated. The full XPS spectrum of the SIS/KLNCN2 sample indicates the presence of five elements: C 1s, N 1s, Sn 3d, In 3d, and S 2p (Fig. S2). The high-resolution C 1s spectrum (Fig. 4(a)) displays two peaks at 284.8 and 288.14 eV, which correspond to the C–C and N–C Created by potrace 1.16, written by Peter Selinger 2001-2019 N groups, respectively.37,38 The high-resolution N 1s spectrum (Fig. 4(b)) presents two peaks at binding energies of 398.76 and 400.32 eV, which correspond to the sp2-hybridised N and N–(C)3 groups in the N–C Created by potrace 1.16, written by Peter Selinger 2001-2019 N structure.39 Compared with the spectrum of KLNCN, the π–π* peaks at 293.10 and 404.38 eV in the SIS/KLNCN2 spectrum were absent, which may be attributed to changes in the optical absorption characteristics following the combination of SIS and KLNCN. Moreover, the high-resolution XPS spectrum of Sn 3d (Fig. 4(c)) exhibits two peaks at 486.15 and 494.62 eV, corresponding to Sn 3d5/2 and Sn 3d3/2, respectively. These results indicate that Sn in the SIS/KLNCN2 sample mainly exists in the +4 oxidation state.40 For In 3d (Fig. 4(d)), binding energies at 444.27 and 451.90 eV were observed, corresponding to In 3d5/2 and In 3d3/2, respectively, which indicates that the indium on the sample surface mainly exists in the +3 oxidation state.41 The S 2p spectrum (Fig. 4(e)) displays peaks at 160.85 and 161.85 eV, indicating that sulfur in the SIS/KLNCN2 sample is present predominantly as S2−.42

Fig. 4. High-resolution XPS spectra of (a) C 1s, (b) N 1s, (c) Sn 3d, (d) In 3d, and (e) S 2p and (f) EPR spectrum of SIS/KLNCN2.

Fig. 4

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Notably, compared with those of the single-component KLNCN and SIS samples, the C 1s and N 1s peaks in the SIS/KLNCN2 composite shift towards higher binding energies, whereas the Sn 3d, In 3d, and S 2p peaks shift towards lower binding energies. These results demonstrate that charge transfer occurs between KLNCN and SIS, with electrons flowing from KLNCN, which has a higher electron cloud density, to SIS, which has a lower electron cloud density. Following heterojunction formation in the SIS/KLNCN2 composite, charge redistribution occurs throughout the system. In addition, this charge transfer may be associated with sulfur vacancies (Sv), which generate holes in the SIS sample. To further verify the generation of Sv defects, electron paramagnetic resonance (EPR) spectroscopic analysis was conducted on the SIS/KLNCN2 samples (Fig. 4(f)). The characteristic peak at a g value of 2.004 in the SIS sample was attributed to Sv generation.43,44 After coupling SIS and KLNCN, the Sv peak intensity increased significantly, indicating a higher Sv concentration in the composite. These vacancies act as electron and hole capture centres, efficiently promoting charge separation, thereby enhancing the photocatalytic reactivity of the SIS/KLNCN2 sample.45,46

3.4. UV-vis diffuse reflectance spectroscopy (DRS) analysis

The optical absorption properties of the prepared KLNCN, SIS, and SIS/KLNCN photocatalysts were investigated using UV-vis DRS. Pure SIS exhibited strong absorption in the visible-light region (Fig. 5(a)), demonstrating its potential for effective light harvesting and utilisation of visible-light energy.32 Compared with KLNCN alone, the SIS/KLNCN composite exhibited significantly enhanced visible-light absorption, accompanied by a clear redshift at the absorption edge. This may be due to the formation of dense SIS nanosheets on the surface of KLNCN, which respond strongly to visible light. Using the Tauc method,32,47 the band gap energies (Eg) of the KLNCN, SIS, and SIS/KLNCN photocatalysts were determined as 1.71, 2.12, and 2.27 eV, respectively (Fig. 5(b–d)). Furthermore, the Mott–Schottky plots of SIS and KLNCN exhibit positive slopes (Fig. 5(e and f)), indicating that both materials are n-type semiconductors.48 The flat-band potentials of SIS and KLNCN are −1.3 and −0.66 eV, respectively, suggesting that KLNCN possesses a stronger reduction capability and can promote the single-electron reduction of O2 to ˙O2−. Based on the flat-band potentials, the conduction band positions of SIS and KLNCN were estimated to be −1.1 and −0.84 eV, respectively. Accordingly, the valence band positions of SIS and KLNCN were calculated to be 0.61 and 1.66 eV, respectively.

Fig. 5. (a) UV-vis DRS spectra and band gap width diagrams of (b) KLNCN, (c) SIS, and (d) SIS/KLNCN2. Mott–Schottky plots of (e) SIS and (f) KLNCN.

Fig. 5

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3.5. Photoelectric properties

In general, the separation and transfer efficiency of photogenerated electrons in a photocatalyst play a crucial role in determining its performance. Therefore, PL spectroscopy, transient photocurrent response measurements, and electrochemical impedance spectroscopy (EIS) were employed to characterise the KLNCN, SIS, and SIS/KLNCN materials. The PL spectra indicate that a higher emission peak intensity corresponds to a higher recombination rate of photogenerated charge carriers, which is detrimental to photocatalytic performance.49 As shown in Fig. 6(a), the PL intensity of the SIS/KLNCN2 photocatalyst is significantly reduced compared with that of SIS and KLNCN2. This indicates that the formation of the heterojunction in the composite material generates a built-in electric field that effectively suppresses the recombination of photogenerated charge carriers. Fig. 6(b) shows that the transient photocurrent response of SIS/KLNCN is higher than the response of the single KLNCN and SIS materials, indicating enhanced charge separation and optimal solar-driven redox capacity. Fig. 6(c) presents the EIS Nyquist plots, where KLNCN displays the largest arc radius, followed by SIS. Notably, the SIS/KLNCN composite presents the smallest arc radius, indicating the lowest interfacial charge-transfer resistance.50 Conclusively, the excellent photoelectric performance of the developed SIS/KLNCN composite can be mainly attributed to the presence of Sv defects, which reorganise charge distribution during the synthesis process and act as electron traps, thereby playing an important role in regulating charge-carrier transport.51 Meanwhile, the construction of an S-scheme heterostructure in SIS/KLNCN, together with the synergistic interaction between the heterojunction and defect sites, enables efficient separation and transport of photogenerated charge carriers, thereby significantly enhancing the photocatalytic efficiency.

Fig. 6. (a) PL spectra, (b) transient photocurrent responses, and (c) electrochemical impedance spectra of KLNCN, SIS, and SIS/KLNCN2.

Fig. 6

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3.6. Photocatalytic activity

The photophysical performance of the materials was evaluated using Rh B and Cr(vi). As shown in Fig. 7(a), compared to single-component KLNCN and SIS, the SIS/KLNCN composites exhibited outstanding photocatalytic activity. With increasing KLNCN content, the degradation efficiency of Rh B by SIS/KLNCN first increased and then decreased, which might be due to the excessive presence of the photocatalyst, which blocked the light transmission rate. Among the composite samples, SIS/KLNCN2 exhibited the most pronounced performance, achieving a degradation rate of 92.46%. Notably, a similar trend was observed for the photocatalytic reduction of Cr(vi), with SIS/KLNCN2 reaching a reduction efficiency of 93.60% (Fig. 7(d)). This enhanced performance was attributed to the formation of a heterojunction between SIS and KLNCN, which facilitates efficient charge transfer within the composite. This, in turn, markedly improves the separation of photogenerated electron–hole pairs, enhancing the effectiveness of the composite in wastewater photodegradation. The first-order kinetic plots for Rh B degradation and Cr(vi) reduction Fig. 7(b–e) further confirm that SIS/KLNCN2 exhibits the highest photocatalytic activity. The degradation process of SIS/KLNCN2 is consistent with the full-spectrum scanning Fig. 7(c–f). Fig. S3 also demonstrates that the lower-quality SIS/KLNCN2 photocatalyst achieves substantial dye removal from dye-containing wastewater. Furthermore, the pH of the wastewater solution has a significant impact on the degradation efficiency (Fig. S4). Under acidic conditions, the adsorption of dye molecules by the SIS/KLNCN2 photocatalyst is significantly enhanced, leading to improved photocatalytic activity. In contrast, under alkaline conditions, changes in the surface charge of SIS/KLNCN2 decrease its adsorption capacity for dye molecules, ultimately reducing the degradation efficiency of Rh B. Notably, SIS/KLNCN2 exhibits good adaptability under different acidic and alkaline conditions, indicating its wide applicability in real-world environments. The stability and reproducibility of a photocatalyst are also key factors in evaluating its performance. After conducting five repeated experiments, the removal rate of Rh B by SIS/KLNCN2 still reached 74.5% (Fig. 7(g)). The slight decrease in photocatalytic activity after five cycles may be attributed to the formation of by-products or impurities during the cycling process, which occupy some active sites and thus reduce the number of effective sites available for degradation. Overall, this SIS/KLNCN material with a slit-like porous structure was both stable and highly efficient, demonstrating good application potential, especially in wastewater treatment. Table 1 compares the removal rate of SIS/KLNCN2 with those reported for other photocatalytic materials, highlighting the excellent photocatalytic performance of SIS/KLNCN2.

Fig. 7. (a) Photocatalytic degradation, (b) first-order kinetic plot and (c) full wavelength spectra of Rh B degradation; (d) reduction, (e) first-order kinetic plot and (f) full wavelength spectra of Cr(vi) reduction; (g) cyclic stability test of SIS/KLNCN2.

Fig. 7

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Table 1. Photocatalytic effectiveness to removal of pollutants in recent research.

Samples Solution concentration Catalytic conditions Removal rate Literature
TiO2/Vo-Bi4Ti3O12 5 mg L−1 Cr(vi), pH = 2 300 W xenon lamp 99% 52
90 min
C3N5/Bi2WO6 10 mg L−1 Cr(vi) pH = 2.5 Visible light (300 W, λ > 420 nm) 97% 53
50 min
γ-Ga2O3/g-C3N4 20 mg L−1 Cr(vi) pH = 2 300 W xenon lamp 30% 54
60 min
Bi12GeO20/Nb2O5 15 mg L−1 Cr(vi) pH = 7 400 W metal halide lamp 92.72% 55
120 min
SnS2/Ag2S 10 mg L−1 Cr(vi) pH = 6.2 Visible light (300 W xenon lamp, λ > 400 nm) 99.4% 56
180 min
SnIn4S8/KLNCN 10 mg L−1 Cr(vi) pH = 7 300 W xenon lamp 93.6% This work
50 min
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3.7. Photocatalytic mechanism

The formation and mechanism of the active species in the photocatalytic reaction were investigated through free-radical trapping experiments and EPR spectroscopy. In this study, BQ, KI, and IPA were employed as scavengers for ˙O2− radicals, h+, and ˙OH radicals, respectively, to investigate the contribution of each species in the system. When BQ was used, the degradation of Rh B decreased significantly (Fig. 8(a)), indicating that ˙O2− is the primary active species in the photocatalytic degradation process. The addition of KI resulted in pronounced enhance in degradation compared with that observed with BQ, suggesting that h+ acts as the second most active species. When IPA was employed, the degradation effect was intermediate between that of KI and the blank experiment, indicating that ˙OH radicals are produced under light irradiation and play a promoting role in the photodegradation reaction. The EPR results (Fig. 8(b–d)) further demonstrate that in the absence of light, neither ˙O2− nor ˙OH radicals are detected. As the duration of light exposure increases, the signals of ˙O2− and ˙OH gradually increase, while the signal of h+ decreases. This indicates that the SIS/KLNCN structure itself does not contain ˙O2− and ˙OH; however, under light irradiation, these species are generated and significantly promote the photocatalytic process. In summary, the synergistic effect of ˙O2−, ˙OH, h+, and e− effectively removes the target pollutant, Rh B.57

Fig. 8. (a) Active species trapping experiment and EPR spectra of the (b) ˙O2− radicals, (c) h+, and (d) ˙OH radicals.

Fig. 8

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The degradation mechanism of the SIS/KLNCN photocatalyst, based on the above experimental results, is illustrated in Fig. 9. Under visible-light irradiation, the conduction (CB) and valence (VB) bands of SIS and KLNCN in the SIS/KLNCN composite are excited to generate photogenerated e− and h+. The recombination rate of these carriers is significantly lower than that of the individual SIS and KLNCN materials. This is attributed to the formation of a heterojunction between SIS and KLNCN, which promotes the flow of photogenerated electrons from KLNCN to SIS. This, in turn, results in an upward bending of the KLNCN energy band and a downward bending of the SIS energy band. This electron flow generates an internal electric field from KLNCN to SIS, which is consistent with the XPS results. The intrinsic electric field at the SIS/KLNCN interface promotes recombination of h+ in the VB of KLNCN with e− in the CB of SIS, while retaining the high redox potentials of the remaining photogenerated e− and h+ in KLNCN and SIS. This photogenerated charge separation mechanism corresponds to the S-type heterojunction model,58–60 which significantly improves the separation efficiency of photogenerated carriers in the SIS/KLNCN composite. Meanwhile, S vacancies in the SIS/KLNCN composite act as electron capture centres, effectively improving the electronic structure and band characteristics, significantly enhancing the mobility of photogenerated charges, and promoting the formation of an S-scheme heterojunction with strong redox capability. According to the S-scheme heterojunction mechanism, the CB position of KLNCN is more negative than the O2/˙O2− potential (−0.33 eV), which enables the e− in the CB of KLNCN to react with O2 to generate ˙O2− radicals. These radicals subsequently oxidise Rh B into non-toxic CO2 and H2O. Meanwhile, the VB of SIS accumulates h+; however, its potential is insufficient to oxidise H2O to ˙OH (H2O/˙OH = 1.99 eV) under thermodynamic reactions. Notably, the VB has a finite width, allowing holes to be excited to lower energy levels within it. Thus, the generation of ˙OH radicals in the SIS VB is still possible,60 as confirmed by the radical trapping experiments and EPR measurements. Therefore, under the combined action of ˙OH and h+, Rh B can be oxidised to non-toxic CO2 and H2O, while Cr(vi) can be reduced to Cr(iii). The possible mechanism for the degradation of Rh B and the reduction of Cr(vi) by the SIS/KLNCN composite is as follows:

SIS/KLNCN + hv → SIS (e− + h+) + KLNCN (e− + h+) 2
KLNCN (h+) + SIS (e−) → SIS/KLNCN (e− − h+) 3
H2O/OH− + h+ → ˙OH 4
˙OH + Rh B → CO2 + H2O 5
˙OH + Cr(vi) → Cr(iii) 6
h+ + Rh B → CO2 + H2O 7
h+ + Cr(vi) → Cr(iii) 8

Fig. 9. Schematic illustration of the degradation mechanism over the SIS/KLNCN S-scheme heterojunction photocatalyst.

Fig. 9

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4. Conclusion

In this study, an SIS/KLNCN S-scheme heterojunction with high photocatalytic performance was prepared using the molten salt and hydrothermal methods. The composite maintained high catalytic activity after multiple cycles, demonstrating its good stability. EPR analysis revealed that the surface of SIS/KLNCN is rich in Sv, which serve as effective sites for photogenerated electron trapping and interfacial charge transfer. Compared with the single-component KLNCN and SIS, the SIS/KLNCN2 sample exhibited excellent photocatalytic activity for Rh B degradation and Cr(vi) reduction, as confirmed by various characterisation techniques including XRD, XPS, SEM, TEM, UV-vis DRS, PL, EIS, and Mott–Schottky analysis. This improvement can be attributed to the growth of SIS nanosheets on the surface of KLNCN, which established a strong interfacial connection, formed an S-scheme heterojunction structure, and promoted the migration of the photogenerated carriers. The increased Sv in the composite also provided a platform for the transfer of photogenerated charges. The synergistic effect of the heterojunction and these defects enhanced the photocatalytic activity of SIS/KLNCN. This study proposes a sustainable, environmentally friendly, and cost-effective strategy for water treatment.

Author contributions

Mingxia Tian: conceptualization; formal analysis; investigation; writing – review and editing. Dong He: conceptualization; data curation; methodology; project administration; writing – original draft. Hui Bai: formal analysis; writing – review and editing. Lili Huang: formal analysis; writing – review and editing. Jianhui Jiang: conceptualization; funding acquisition; investigation; methodology; supervision; validation.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Supplementary Material

RA-016-D6RA03849B-s001

Acknowledgments

This work is sponsored by Science and Technology Program of XPCC (2025BC009) and Natural Science Support Program of XPCC (2024DA036).

Data availability

Data will be made available on request.

Supplementary information (SI): characterization information, XPS survey spectra, EDS of the SIS/KLNCN2 samples, the catalyst loading, and the effect of the RhB solution on photodegradation. See DOI: https://doi.org/10.1039/d6ra03849b.

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Associated Data

This section collects any data citations, data availability statements, or supplementary materials included in this article.

Supplementary Materials

RA-016-D6RA03849B-s001
RA-016-D6RA03849B-s001.pdf (601.4KB, pdf)

Data Availability Statement

Data will be made available on request.

Supplementary information (SI): characterization information, XPS survey spectra, EDS of the SIS/KLNCN2 samples, the catalyst loading, and the effect of the RhB solution on photodegradation. See DOI: https://doi.org/10.1039/d6ra03849b.

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