A CdIn2S4Sv / Bi2O3 composite material, its preparation method and application

Abstract

The application belongs to the technical field of photocatalysts, and discloses a CdIn2S4Sv / Bi2O3 composite material and a preparation method and application thereof. Bi2O3 nanofibers are synthesized by controlling calcination after electrospinning, serving as an ideal substrate for subsequent growth of CdIn2S4Sv, and then CdIn2S4Sv and CdIn2S4Sv / Bi2O3 are synthesized by a simple oil bath method. The composite material exhibits significant photocatalytic performance in the production of hydrogen peroxide without a sacrificial agent in pure water, and the production rate is much higher than that of original CdIn2S4Sv, Bi2O3 and most reported S-scheme heterojunctions. In addition, the composite material realizes efficient degradation of formaldehyde, and exhibits potential for multifunctional application. The application establishes a comprehensive design paradigm, integrating interface chemical bonding, defect engineering and heterojunction structure, to realize advanced photocatalytic application.

Description

Technical Field

[0001] This invention relates to the field of photocatalyst technology, and more specifically to a CdIn2S4Sv / Bi2O3 composite material, its preparation method, and its application. Background Technology

[0002] Hydrogen peroxide (H2O2) is an environmentally friendly green oxidant widely used in chemical synthesis, environmental remediation, and medical and health fields. Its only reaction byproduct is water, aligning with green chemistry principles. Industrially, H2O2 production mainly relies on the anthraquinone process, which is energy-intensive, cumbersome, and uses large amounts of organic solvents, posing environmental pollution risks. Photocatalytic synthesis of H2O2 utilizes water and oxygen as raw materials, generating H2O2 through the oxygen reduction reaction pathway under light irradiation. This is a highly atom-economical and environmentally friendly synthetic route. Notably, the photocatalytic hydrogen peroxide production process is particularly suitable for on-site applications because it can directly generate the low-concentration hydrogen peroxide typically required in practical applications, thus eliminating the need for dilution and transportation of high-concentration (30-70 wt%) commercially available hydrogen peroxide. Photocatalytic H2O2 synthesis mainly involves two pathways: one is an indirect pathway, where photogenerated electrons reduce O2 to generate superoxide radicals (·O2). - The first pathway involves protonation to generate H2O2; the second pathway is the direct two-electron oxygen reduction pathway (O2 + 2H+). + + 2e - → H2O2). In addition, the water oxidation pathway can also generate H2O2 through hole oxidation of water, but the contribution is relatively small. Although using hole scavengers is a common strategy to improve charge separation, its application brings additional economic costs, the risk of secondary contamination, and the complexity of subsequent purification steps. Despite the water oxidation pathway (2H2O + 2H+), H2O2 can still be generated. + →H₂O₂ + 2H⁺ + E 0 = 1.76 V vs. RHE) offers a sacrificial-free route, but it is more thermodynamically and kinetically challenging. Therefore, achieving efficient sacrificial-free hydrogen peroxide production in pure water remains a significant challenge, requiring photocatalysts with both strong redox capabilities and the ability to promote selective 2e- ... - ORR, and inhibits charge recombination and hydrogen peroxide decomposition.

[0003] To address the charge recombination problem while maintaining strong redox capabilities, heterojunction engineering is considered a promising strategy. Among numerous structural designs, the S-type heterojunction has attracted significant attention due to its unique charge transfer mechanism, which enables both spatial separation of photogenerated carriers and the preservation of the strong redox potentials of the two constituent semiconductors. In the S-type structure, the staggered band arrangement and Fermi level difference drive directional electron transfer at the interface, forming a built-in electric field that promotes low-energy charge carrier recombination while retaining high-energy electrons and holes for surface redox reactions.

[0004] Beyond charge separation, defect engineering has become a powerful strategy for customizing the electronic structure and surface reactivity of photocatalysts. Specifically, the introduction of anionic vacancies has been shown to modulate the local electronic environment, forming electron-rich sites that act as effective traps for photogenerated electrons, thereby suppressing charge recombination. These vacancy sites also serve as preferential adsorption centers for reactant molecules, lowering the activation barrier and enhancing catalytic activity. For example, Li et al. demonstrated that high concentrations of sulfur vacancies (Sv) lower the onset potential and optimize the adsorption of oxygen intermediates (…). OOH) adsorption, and inhibition of 4e - The ORR pathway enables the selective generation of hydrogen peroxide. However, charge imbalances between adjacent atoms create reverse traps by isolating adsorbates, effectively hindering the other half of the reaction. This limitation highlights the necessity of combining defect engineering with interface modulation, as interfacial bonds can alleviate vacancy-induced charge imbalances while simultaneously promoting charge transfer—a synergistic effect that has not been fully explored in hydrogen peroxide photocatalysis. Importantly, the formation of interfacial bonds has become an effective strategy for establishing tight electronic coupling between different semiconductors, promoting charge transfer, and stabilizing heterojunctions. For example, Sarkar et al. demonstrated that Bi-S bonds formed in a BiOCl / Cu2SnS3 heterostructure establish a direct Z-type charge transfer pathway, significantly enhancing photocatalytic hydrogen production. Despite significant efforts, the reported rates of sacrificial hydrogen peroxide production in pure water remain far below the threshold for industrial applications, highlighting the urgent need for advanced photocatalyst design.

[0005] Indium cadmium sulfide (CdIn2S4) is a visible-light-responsive semiconductor with a well-aligned conduction band and suitable for O2 reduction. Wu et al. recently proposed a concept for electron reversal by simultaneously introducing oxygen and sulfur vacancies into CdIn2S4. This method reverses the charge distribution around the vacancies and generates a strong internal electric field to guide charge transfer. However, pristine CdIn2S4 suffers from rapid charge recombination and 2e2- ions. -The limited selectivity of ORR (Ortho-Ortho-Resistant Photocatalytic Reaction) remains a challenge. Bismuth oxide (Bi₂O₃), with its strong oxidation valence band, provides complementary properties for constructing heterojunctions. Lee's team reported for the first time a method for preparing Bi₂O₃ nanofibers through precise controlled calcination after electrospinning, which significantly enhanced photocatalytic activity by improving charge separation. Despite these advances, the synergistic integration of interfacial Bi-S bonding, sulfur vacancy engineering, and S-scheme heterojunction design in selective hydrogen peroxide photosynthesis in pure water remains to be explored.

[0006] Therefore, researching a synthesis method for a CdIn2S4Sv / Bi2O3S heterojunction photocatalyst is a problem that urgently needs to be solved by those skilled in the art. Summary of the Invention

[0007] In view of this, the present invention provides a CdIn2S4Sv / Bi2O3 composite material, its preparation method and application.

[0008] One objective of this invention is to provide a method for preparing a CdIn2S4Sv / Bi2O3 composite material, comprising the following steps:

[0009] Preparation of S1.Bi2O3 nanofibers S11. Preparation of precursor solution: Dissolve the bismuth source in N,N-dimethylformamide (DMF) solvent and continuously stir magnetically at room temperature until a completely clear solution is formed. Add polyvinylpyrrolidone (PVP) to the clear solution and stir continuously at room temperature for 10-14 h to finally obtain a uniform, stable, and slightly white precursor solution with a certain viscosity for later use.

[0010] S12. Electrospinning The electrospinning process is carried out at room temperature, and the key process parameters are set as follows: the precursor solution propulsion speed of the injection pump is 0.02-0.06 mm / min, the distance between the needle tip and the roller collector covered with aluminum foil is fixed at 12-18 cm, and a DC high voltage of 12-18 kV is applied between the two. Under the action of the high voltage electrostatic field, the precursor solution forms a stable Taylor cone jet, the solvent evaporates rapidly, and a nanofiber membrane composed of PVP and bismuth nitrate is deposited on the collector.

[0011] Preferably, the electrospinning further includes: transferring the prepared precursor solution into a 10 mL syringe and installing a 21 G stainless steel spinning needle.

[0012] S13. Calcination and crystallization treatment The composite nanofiber membrane was placed in a muffle furnace and calcined under a programmed temperature rise in air atmosphere; after the calcination process was completed, it was naturally cooled to room temperature to obtain Bi2O3 nanofibers.

[0013] Preparation of S2.CdIn2S4Sv and CdIn2S4Sv / Bi2O3 composite materials First, Bi₂O₃ nanofibers were dispersed in a mixed solvent containing deionized water and glycerol, and a homogeneous suspension was formed by ultrasonic treatment. Then, cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O), indium nitrate tetrahydrate (In(NO₃)₃·4H₂O), and thioacetamide (TAA) were added sequentially to the suspension. The mixture was magnetically stirred at room temperature for 1 h to ensure thorough mixing and dispersion of the reactants. The reactant solution was then transferred to a device equipped with a reflux condenser, and the device was immersed in a preheated solution at 80°C. o In an oil bath containing C, the mixture was heated and continuously magnetically stirred. During the reaction, the mixture gradually transformed into a uniform yellow suspension, indicating that CdIn2S4Sv was grown in situ on Bi2O3 nanofibers. After the reaction was complete, the mixture was naturally cooled to room temperature, and the precipitate was collected by centrifugation (8000 rpm, 5 min). The precipitate was thoroughly washed three times with deionized water and anhydrous ethanol, and finally dried in a vacuum oven at 60 °C for 12 h to obtain the CdIn2S4Sv / Bi2O3 composite material.

[0014] Preferably, in step S11, the bismuth source is bismuth nitrate pentahydrate (Bi(NO3)35H2O), and the concentration of bismuth nitrate pentahydrate in N,N-dimethylformamide solvent is 0.12 g / mL.

[0015] Preferably, in step S11, the molecular weight of the polyvinylpyrrolidone is approximately 1,300,000, and the mass ratio of the bismuth source to the polyvinylpyrrolidone is 12:17.

[0016] Preferably, in step S13, the programmed calcination involves heating from room temperature to 450°C at a heating rate of 2°C / min and holding for 2 hours. This heat treatment process aims to completely decompose and remove the organic polymer PVP, while simultaneously promoting the conversion of the inorganic bismuth salt precursor into a well-crystallized oxide.

[0017] Preferably, in step S2, the solid-liquid ratio of the Bi2O3 nanofibers to the mixed solvent is 1-4:6 mg / mL.

[0018] Preferably, in step S2, the concentration of the thioacetamide in the mixed solvent is 0.1 mmol / mL; the molar mass ratio of the cadmium nitrate tetrahydrate, indium nitrate tetrahydrate and thioacetamide is 1:2:6. In this step, an excess of S source is added to ensure sulfidation and vacancy formation.

[0019] Preferably, in step S2, the volume ratio of deionized water to glycerol in the mixed solvent is 29:6.

[0020] Preferably, in step S2, the oil bath heating is at 80°C. o The C reaction was carried out at this temperature for 6 hours under continuous magnetic stirring.

[0021] The second objective of this invention is to provide a CdIn2S4Sv / Bi2O3 composite material.

[0022] The third objective of this invention is to provide the application of CdIn2S4Sv / Bi2O3 composite materials in the field of photocatalytic production of hydrogen peroxide or photocatalytic degradation of formaldehyde.

[0023] As can be seen from the above technical solution, compared with the prior art, the beneficial effects achieved by the present invention are as follows: This invention successfully designed and constructed a CdIn2S4Sv / Bi2O3S-type heterojunction photocatalyst using a combination of electrospinning and hydrothermal synthesis. This photocatalyst possesses intentionally designed sulfur vacancies and interfacial Bi-S covalent bonds. Comprehensive structural and spectroscopic analyses, including synchrotron radiation-based X-ray absorption fine structure (XAFS), X-ray photoelectron spectroscopy (XPS), and electron paramagnetic resonance (ESR), ultimately confirmed the formation of interfacial Bi-S bonds and the presence of sulfur vacancies in the heterostructure. The optimized CdIn2S4Sv / Bi2O3 composite material exhibits excellent photocatalytic performance, achieving 7 mmol / h in pure water without any sacrificial agent. -1 g -1 The material exhibits a hydrogen peroxide generation rate approximately 4.01 times that of the original CdIn2S4Sv, and an apparent quantum efficiency (AQE) of 3.83% at 420 nm. Simultaneously, it demonstrates highly efficient degradation of gaseous formaldehyde, establishing its bifunctional properties. This superior photocatalytic activity stems from the synergistic effect of three key design elements: the interfacial Bi-S bond acts as an atomic-scale charge transfer bridge, promoting directional electron migration from CdIn2S4Sv to Bi2O3 and constructing a strong built-in electric field; under IEF-driven conditions, the S-scheme charge transfer mechanism achieves effective spatial separation of photogenerated carriers while maintaining the strong reducing power of CdIn2S4Sv conduction band electrons and the strong oxidizing power of Bi2O3 valence band holes; and the Sv on CdIn2S4Sv serves as a preferential O2 adsorption and activation site, as confirmed by DFT calculations. The decrease in the O2 adsorption free energy, from a thermodynamic perspective, guides the reaction toward selectivity for 2e-. - ORR development and inhibition of hydrogen peroxide decomposition. In-situ drift spectroscopy and free radical capture experiments further confirmed this. -Bismuth is the dominant reactive species in hydrogen peroxide generation and formaldehyde degradation. This study constructs an advanced photocatalytic design paradigm integrating interfacial chemical bonding, defect engineering, and S-type heterojunction structures. By revealing the synergistic mechanism of bismuth-sulfur bonds and sulfur vacancies in regulating charge transfer and surface reaction pathways, a theoretical framework is provided for the development of high-performance bifunctional photocatalysts. This technology shows broad application prospects in the fields of sustainable solar-to-chemical energy conversion and environmental remediation. Attached Figure Description

[0024] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0025] Figure 1 (a) Schematic diagram of the preparation process of CdIn2S4Sv / Bi2O3 composite material; (bd) FESEM images of original Bi2O3 nanofibers, CdIn2S4Sv and BC composite material; (e) HRTEM image of BC composite material; (f–k) STEM-EDS elemental distribution diagrams of Bi, O, S, In and Cd in BC composite material.

[0026] Figure 2 (a) Comparison of XRD patterns of synthesized CdIn2S4Sv, Bi2O3 and BC-25 with standard samples; (b) Comparison of XRD patterns of selected diffraction peaks of CdIn2S4Sv and composite materials; (c) FT-IR spectra; (d) Pore characteristics; (e) Nitrogen adsorption capacity; and (f) ESR spectra of CdIn2S4Sv, Bi2O3 and BC-25.

[0027] Figure 3 (a) UV-Vis diffuse reflectance spectra of Bi2O3, CdIn2S4Sv, and BC-25; (b) Tauc plots of Bi2O3 and CdIn2S4Sv derived from absorption spectra; Mott-Schottky curves of Bi2O3 (c) and CdIn2S4Sv (d) at different frequencies; Calculated work functions of Bi2O3 (e), ordinary CdIn2S4 (f), and CdIn2S4Sv (g); (h) Schematic diagram of band arrangement in the CdIn2S4Sv / Bi2O3 heterojunction.

[0028] Figure 4(a) XPS scan spectra of Bi₂O₃, CdIn₂S₄Sv, and BC-25. (b) High-resolution XPS spectra of Bi₄F and (c) O₁s. (d) High-resolution XPS spectra of Cd₃d, (e) In₃d, and (f) S₂p.

[0029] Figure 5 Normalized results of Bi L3 edge XANES spectrum (a), Fourier transform EXAFS (FT-EXAFS) spectrum (b), and k3-weighted EXAFS oscillations of BC-25, Bi2O3 and Bi foil in k space (c).

[0030] Figure 6 (a) Steady-state photoluminescence (PL) spectra, and (b) time-resolved PL decay curves of Bi2O3, CdIn2S4Sv and BC-25 under 350 nm excitation. (c) Photocurrent response and electrochemical impedance spectroscopy of Bi2O3, CdIn2S4Sv and BC-25.

[0031] Figure 7 Pseudo-color images of the fs-Tas signals in BC-25(a), Bi2O3(d), and CdIn2S4Sv(g). Femtosecond transient absorption spectra of BC-25(b), Bi2O3(e), and CdIn2S4Sv(h). fs-Tas decay curves of the GSB signals in BC-25(c), Bi2O3(f), and CdIn2S4Sv(i).

[0032] Figure 8 (a) Comparison of photocatalytic hydrogen peroxide yields of different samples in pure water. Recovery test (b), accumulation test (c), and wavelength-dependent AQE (d) show the photocatalytic hydrogen peroxide generation of BC-25. (e) Control experiment on photocatalytic hydrogen peroxide generation in pure water.

[0033] Figure 9 (a) Photocatalytic formaldehyde degradation performance, (b) Removal kinetic rate.

[0034] Figure 10 (a) Curves showing the change in hydrogen peroxide yield over time on BC-25 under different conditions. (b) In-situ Fourier transform infrared (FT-IR) spectra acquired under O2 light irradiation. RRDE polarization curves in O2. (c) Selectivity of hydrogen peroxide and the corresponding average number of transferred electrons. (d) RRDE polarization curves of Bi2O3, CdIn2S4Sv, and BC-25 under N2 atmosphere. (e) CV curves at different scan rates: (f) Bi2O3, (g) CdIn2S4Sv, and (h) BC-25; (i) corresponding fitted curves.

[0035] Figure 11 The composites of CdIn2S4Sv, Bi2O3, and BC-25 under light and dark conditions exhibit (a) DMPO-·OH and (b) DMPO-·O2. - (c)TEMPO-·e - (d)TEMPO-·h + The ESR spectrum.

[0036] Figure 12 Schematic diagram of the S-type heterostructure mechanism of BC composite material. Detailed Implementation

[0037] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0038] This invention presents a rationally designed and constructed heterojunction photocatalyst based on the CdIn2S4Sv / Bi2O3S scheme. A schematic diagram of the synthesis is shown below. Figure 1 As shown in Figure a, Bi₂O₃ nanofibers synthesized by electrospinning followed by controlled calcination serve as an ideal substrate for subsequent CdIn₂S₄Sv growth. The resulting heterojunction possesses intentionally designed sulfur vacancies and interfacial Bi-S covalent bonds. Through detailed structural characterization, spectroscopic analysis, and density functional theory (DFT) calculations, the synergistic mechanism of Bi-S bonds, Sv, and S-scheme charge transfer was systematically elucidated. The optimized CdIn₂S₄Sv / Bi₂O₃ composite material exhibits significant photocatalytic performance in the production of sacrificial hydrogen peroxide in pure water, with a production rate far exceeding that of the original CdIn₂S₄Sv, Bi₂O₃, and most reported S-scheme heterojunctions. Furthermore, this composite material achieves highly efficient formaldehyde degradation, demonstrating its potential for multifunctional applications. This invention establishes a comprehensive design paradigm that integrates interfacial chemical bonding, defect engineering, and heterojunction structure to achieve advanced photocatalytic applications.

[0039] Example 1: Preparation of Bi2O3 nanofibers Bi2O3 nanofibers are prepared by electrospinning technology combined with high-temperature calcination. The main process can be divided into three steps: precursor solution preparation, electrospinning and calcination treatment.

[0040] S11. First, the spinning precursor solution was prepared. 1.2 g of bismuth nitrate pentahydrate (Bi(NO3)35H2O) was used as the bismuth source and dissolved in 10 mL of N,N-dimethylformamide (DMF) solvent. The solution was continuously stirred magnetically at room temperature until a completely clear solution was formed. Subsequently, 1.7 g of polyvinylpyrrolidone (PVP, molecular weight approximately 1,300,000) was slowly added to the solution, and the mixture was continuously stirred at room temperature for 12 h, ultimately obtaining a homogeneous, stable, and slightly viscous white precursor solution.

[0041] S12. Next, electrospinning is performed. The prepared precursor solution is transferred to a 10 mL syringe, and a 21 G stainless steel spinning needle is installed. The electrospinning process is carried out at room temperature, with the following key process parameters set: the solution advance speed of the syringe pump is controlled at 0.04 mm / min, the distance between the needle tip and the roller collector covered with aluminum foil is fixed at 16 cm, and a 15 kV DC high voltage is applied between them. Under the action of the high-voltage electrostatic field, the precursor solution forms a stable Taylor cone jet, and the solvent evaporates rapidly, thereby depositing a nanofiber membrane composed of PVP and bismuth nitrate on the collector.

[0042] S13. Finally, calcination and crystallization treatment is performed. The composite fiber membrane obtained above is placed in a muffle furnace and calcined under a programmed temperature rise in air atmosphere: at 2... o Heating rate from room temperature to 450 °C / min o The sample was heated to a target temperature of C and held at that temperature for 2 hours. This heat treatment process aimed to completely decompose and remove the organic polymer PVP, while simultaneously promoting the transformation of the inorganic bismuth salt precursor into a well-crystallized oxide. After the calcination process, the sample was allowed to cool naturally to room temperature in the furnace, thus finally preparing the nanofiber material composed of Bi2O3.

[0043] Example 2 Preparation of CdIn2S4Sv and CdIn2S4Sv / Bi2O3 composite materials CdIn2S4Sv and its composite material with Bi2O3 nanofibers were synthesized by a simple oil bath method, wherein the mass ratio of Bi2O3 nanofibers in the CdIn2S4Sv / Bi2O3 composite material was 2-8%.

[0044] S2. The typical preparation steps for this composite material are as follows: First, predetermined amounts (10, 20, 25, 30, and 40 mg) of pre-synthesized Bi₂O₃ nanofibers were dispersed in a mixed solvent containing 58 mL of deionized water and 12 mL of glycerol, and a homogeneous suspension was formed by ultrasonic treatment. Subsequently, 1 mmol of cadmium nitrate tetrahydrate (Cd(NO₃)₂·4H₂O), 2 mmol of indium nitrate tetrahydrate (In(NO₃)₃·4H₂O), and 6 mmol of thioacetamide (TAA) were added sequentially to the suspension in a molar ratio of 1:2:6 (excess sulfur source was added to ensure sulfidation and vacancy formation). The mixture was magnetically stirred at room temperature for 1 h to ensure thorough mixing and dispersion of the reactants. The solution was then transferred to a 100 mL round-bottom flask equipped with a reflux condenser. The apparatus was immersed in water preheated to 80°C. o The mixture was placed in an oil bath at C and maintained at this temperature for 6 h with continuous magnetic stirring. During the reaction, the mixture gradually transformed into a uniform yellow suspension, indicating that CdIn2S4Sv was successfully grown in situ on Bi2O3 nanofibers. After the reaction was complete, the system was allowed to cool naturally to room temperature. The precipitate was collected by centrifugation (8000 rpm, 5 min), thoroughly washed three times with deionized water and anhydrous ethanol, and finally purified at 60 °C. o CdIn2S4Sv / Bi2O3 composites were obtained by drying in a vacuum oven at C for 12 h. A series of CdIn2S4Sv / Bi2O3 composites were prepared by changing the initial mass of Bi2O3 nanofibers (10, 20, 25, 30, 40 mg). These samples were labeled BC-10, BC-20, BC-25, BC-30, and BC-40, respectively.

[0045] Comparative Example 1 In contrast, pure-phase CdIn2S4Sv was prepared using the same method as in Example 2, but without the addition of Bi2O3 nanofibers.

[0046] The following describes the characterization, photocatalytic activity, and photocatalytic mechanism of the composite materials obtained in the examples and comparative examples.

[0047] (I) Characterization and analysis of materials: 1. Structural and morphological characteristics The morphology and microstructure of the prepared samples were studied using scanning electron microscopy and transmission electron microscopy systems. For example... Figure 1As shown in b, the original Bi2O3 exhibits a continuous fibrous structure with a smooth surface and a diameter of approximately 200 nm, which is the result of electrospinning followed by calcination. In contrast, pure CdIn2S4Sv exhibits aggregated nanosheets with a lateral dimension of 3 μm (Fig. 1c). After hybridization, the BC composite material retains the one-dimensional morphology of Bi2O3, while the uniform CdIn2S4Sv is anchored on the surface of the nanofibers, forming an integrated heterostructure (Fig. 1d). The rod-like morphology of CdIn2S4Sv in the composite material indicates that the presence of the Bi2O3 substrate guides the anisotropic growth of CdIn2S4Sv during the oil bath process. High-resolution TEM (HRTEM, Fig. 1e) further resolved the tight contact of the interface. Lattice fringes with spacings of 0.327 nm and 0.315 nm can be clearly observed, corresponding to the (311) plane of cubic CdIn2S4Sv and the (120) plane of monoclinic Bi2O3, respectively. The clear interface connection between the two phases indicates coherent interfacial coupling, which is beneficial for charge transfer. The spatial elemental distribution was analyzed using STEM-EDS mapping. Figure 1 The bismuth signal was concentrated on the nanofiber framework, while indium, cadmium, and sulfur were distributed in a shell-like structure around the fiber core, confirming the uniform growth of CdIn2S4Sv on the Bi2O3 support without obvious aggregation or phase separation. These structural features provide an ideal foundation for efficient interfacial charge separation and transport, which is crucial for the enhanced photocatalytic activity observed in hydrogen peroxide production and formaldehyde degradation.

[0048] X-ray diffraction (XRD) is used to analyze the crystal structure of prepared samples. For example... Figure 2As shown in Figure a, all diffraction peaks of the original Bi₂O₃ can be attributed to the monoclinic α-Bi₂O₃ phase, with characteristic signals appearing at 2θ = 25.7, 27.4, 28.0, 33.2, 45.1, and 46.3, corresponding to the (002), (120), (012), (200), (023), and (041) crystal planes, respectively. Meanwhile, the XRD pattern of CdIn₂S₄Sv is highly consistent with the cubic CdIn₂S₄Sv standard. The main diffraction peaks at 2θ = 14.1, 23.1, 27.2, 33.0, 43.3, and 47.4 can be attributed to the (111), (220), (311), (400), (511), and (440) crystal planes, respectively, indicating good crystallinity. More importantly, for the CdIn2S4Sv / Bi2O3 composite material (BC-25), the characteristic diffraction peaks of both Bi2O3 and CdIn2S4Sv were clearly visible, confirming the successful construction of the CdIn2S4Sv / Bi2O3 heterostructure. Furthermore, the diffraction intensity of the Bi2O3 phase (especially the reflection peaks at 45.1 and 46.3) showed a monotonically increasing trend as its nominal content increased from BC-10 to BC-40. Figure 2 b). The relatively low and broad peak intensity of CdIn2S4Sv in the composite material may be due to the glycerol-induced surface reduction during the oil bath synthesis process. This effect leads to the formation of defect structures and locally disrupts the long-range crystal order, thereby weakening coherent X-ray scattering.

[0049] according to Figure 2 The Fourier transform infrared (FT-IR) spectrum in c was used to analyze the surface functional groups of the sample. At 530 cm⁻¹ -1 A prominent absorption band in the vicinity is attributed to the Bi-O stretching vibration in αBi₂O₃. (3100-3600 cm⁻¹) -1 and 1630cm -1 The broad features in the vicinity correspond to the stretching and bending modes of OH generated by adsorbed water. Furthermore, at 1049 cm⁻¹... -1 The weak peak at this location belongs to the Bi-S bond, further confirming the successful construction of the CdIn2S4Sv / Bi2O3 composite material. Figure 2 As shown in Figures d and e, Brunauer-Emmett-Teller (BET) measurements were performed to evaluate the specific surface area and pore structure of the synthesized materials. All samples exhibited a type IV isotherm and an H3 hysteresis loop in the relative pressure range of 0.4–1.0, indicating a mesoporous structure. The pore size distribution (Figure 2d) shows that BC-25 has a pore volume of 0.412 cm³. 3 ·g -1 The pore volumes of Bi₂O₃ and CdIn₂S₄Sv are 0.016 cm⁻¹, respectively. 3 ·g-1 and 0.636 cm 3 ·g -1 It is worth noting that the BC-25... Figure 2 The highest specific surface area (132.908 cm²) was observed in e. 3 ·g -1 This is attributed to its well-developed mesoporous framework, which provides abundant active sites for adsorption and catalytic reactions, thereby enhancing photocatalytic performance.

[0050] To investigate the defect characteristics of the synthesized materials, electron spin resonance (ESR) measurements were performed, as shown in Figure 2f. The ESR spectra show a significant difference in signal intensity between BC-25 and CdIn2S4Sv within a magnetic field range of 3450–3560 Gauss. Specifically, the characteristic signal with a g-value of 2.003 in both BC-25 and CdIn2S4Sv confirms the presence of sulfur vacancies (Sv). This significant signal enhancement in BC-25 is mainly attributed to the Bi-S bonds formed during synthesis (based on DFT calculations / XPS analysis). Notably, these introduced sulfur vacancies disrupt the electron localization around the metal sites, thereby favoring Pauling-type O2 adsorption and enhancing the adsorption of 2e-. - Selectivity of the oxygen reduction reaction (ORR).

[0051] 2. Band Structure and Charge Transfer Figure 3a shows the UV-Vis diffuse reflectance spectra of the original Bi₂O₃, CdIn₂S₄Sv, and the composite material represented by BC-25. Bi₂O₃ exhibits a steep absorption edge at approximately 450 nm, corresponding to its intrinsic bandgap. In contrast, CdIn₂S₄Sv shows a significant absorption tail extending into the visible light region to approximately 650 nm, indicating its narrower bandgap. Notably, the spectrum of the BC-25 composite material clearly combines the absorption characteristics of both components, demonstrating enhanced visible light capture capability compared to Bi₂O₃ alone. This extended optical response is attributed to the efficient integration of the two phases in the heterostructure, which facilitates broader utilization of the solar spectrum during photocatalysis. The bandgap energies (Eg) of Bi₂O₃ and CdIn₂S₄Sv were determined using Tauc plots of the UV-Vis absorption spectra. Figure 3 As shown in b, the direct band gap of Bi2O3 is estimated to be ≈2.88 eV, while CdIn2S4Sv shows a narrow band gap of ≈2.18 eV.

[0052] Mott-Schottky measurements can elucidate semiconductor type and flat-band potential; these parameters are crucial for understanding the band structure of catalysts. (See Mott-Schottky diagram). Figure 3As shown in (c and d), the positive slope of pure CdIn2S4Sv clearly indicates that it is an n-type semiconductor, while the negative slope of pure Bi2O3 confirms its p-type characteristics. The flat-band potentials (E0) of Bi2O3 and CdIn2S4Sv are also shown. fb The values ​​were measured to be 2.76 V and -0.75 V (relative to Ag / AgCl at pH=7). Using the Nernst equation, these values ​​were converted to 2.54 V and -0.97 V relative to the standard hydrogen electrode (NHE) at pH=0 for subsequent band structure analysis. Based on n-type semiconductors, E... fb Considering that the conduction band (CB) potential is typically about 0.2 V higher than its lowest point, the CB potential of CdIn₂S₄Sv is estimated to be -1.17 V (relative to NHE at pH=0). Combining this with the optical band gap derived from the Tauc plot, the maximum valence band (VB) potential of CdIn₂S₄ is calculated to be 1.01 V (relative to NHE at pH=0). Similarly, since the Efb potential of p-type semiconductors is typically about 0.2 V higher than its maximum valence band (VB), the CB and VB potentials of Bi₂O₃ are determined to be -0.54 V and +2.34 V vs. NHE, respectively.

[0053] The work function (Φ) is a key parameter for evaluating the Fermi level position and elucidating the charge transfer kinetics between CdIn2S4Sv and Bi2O3. The theoretical work functions of Bi2O3 (120), CdIn2S4Sv (311), and sulfur-rich vacancy CdIn2S4Sv (311) are 5.08, 4.94, and 4.69 eV, respectively. Figure 3 (e.g.) The significantly lower work function of CdIn2S4Sv indicates that its Fermi level is higher than that of the original CdIn2S4Sv and Bi2O3. This observation is consistent with recent reports that sulfur vacancies can effectively enhance the Fermi level of sulfide semiconductors by increasing the donor carrier concentration. The widened Fermi level difference between CdIn2S4Sv and Bi2O3 provides a stronger thermodynamic driving force for directional electron transfer during heterojunction formation. Therefore, this Fermi level gradient not only confirms the S-scheme charge transfer path (electron migration from CdIn2S4Sv to Bi2O3) but also establishes a stable internal electric field at the interface. These findings collectively support the proposed S-scheme mechanism and highlight the dual role of sulfur vacancies in regulating electronic structure and interface charge transfer dynamics.

[0054] Based on a combination of experimental and theoretical analysis, the band structure of each component was established. Figure 3h). This staggered band arrangement helps to create an internal electric field driven by the Fermi level difference, thereby enhancing solar energy collection and carrier separation, especially in systems combining n-type and p-type semiconductors. Furthermore, the conduction band of CdIn2S4Sv (-1.17 V vs. NHE, pH=0) has a sufficiently negative potential to thermodynamically drive 2e - Oxygen is reduced to hydrogen peroxide (O2 / hydrogen peroxide, 0.68 V vs. NHE; O2 / O2) - The valence band of Bi₂O₃ is at +2.34 V vs. NHE, providing a significant overpotential for the direct hole-mediated oxidation of formaldehyde (CH₂O / CO₂, +0.23 V vs. NHE). Furthermore, the generated superoxide radicals (O₂)... - ) and hydroxyl radicals (OH, 2.73 V vs. NHE) can initiate and propagate the oxidative degradation chain, thereby achieving effective mineralization of formaldehyde.

[0055] This well-defined band structure arrangement, verified through work function calculations and experimental measurements, provides a comprehensive framework for understanding the synergistic effect of interfacial Bi-S bonds and sulfur vacancies in driving S-scheme charge transfer and achieving excellent bifunctional photocatalytic performance.

[0056] In-situ X-ray photoelectron spectroscopy (XPS) was used to probe interfacial chemical probes and charge transfer kinetics in the synthesized heterojunction. The scanning results (Figure 4a) confirmed the coexistence of Bi, O, Cd, In, and S elements in the composite material, which is highly consistent with the target chemical composition. In the high-resolution Bi 4f spectrum (Figure 4b), the original Bi 2O 3 reference sample showed a high concentration of 164.51 eV (Bi 4f... 5 / 2 ) and 159.19 eV (Bi 4f 7 / 2 The characteristic peak is shown at (), which is similar to Bi. 3+ The oxidation states are consistent within the Bi₂O₃ lattice. After heterojunction formation, the Bi₄f peaks shift by 0.32 eV and 0.16 eV towards lower binding energies, respectively, stabilizing at 164.19 eV and 159.03 eV. Simultaneously, the new characteristic peaks are attributed to S₂p. 1 / 2 and S 2p 3 / 2 This indicates the formation of the BC composite material. Notably, the overlap of the Bi 4f and S 2p signals directly reflects the close alignment of their respective electronic energy levels. This energy compatibility is a fundamental prerequisite for effective orbital hybridization, thereby driving the formation of covalent Bi-S bonds at the heterojunction interface. As for the high-resolution O 1s spectrum (Figure 4c), it is characterized by three distinct components. The binding energy peak at 530.41 eV belongs to [Bi₂O₃]. 2+The binding energy peaks at 531.73 eV and 532.77 eV originate from oxygen-containing species adsorbed on the material surface. Interestingly, the formation of the heterojunction simultaneously leads to a negative shift in the binding energy of the Bi 4f and O 1s core levels. For CdIn2S4Sv, Figure 4 Figure 4d shows the high-resolution XPS spectra of Cd 3d, In 3d, and S 2p. The Cd 3d spectrum (Figure 4d) is shown at 411.82 eV (3d... 3 / 2 ) and 405.08 eV (3d 5 / 2 Cd is displayed at ) 2+ The characteristic double peak. Similarly, the In 3d peak is at 451.58 eV (3d). 3 / 2 ) and 444.03 eV (3d 5 / 2 ) corresponds to In 3+ (Figure 4e). Compared to CdIn2S4Sv, the XPS spectrum of the composite material shows an additional peak at 442.21 eV. This feature is attributed to the formation of Bi-S bonds at the heterojunction interface. The S 2p spectrum was decomposed into two main components: S 2p at approximately 163.67 eV. 1 / 2 And S 2p of approximately 162.57 eV 3 / 2 The S lattice elements belonging to CdIn2S4Sv are respectively... 2- Notably, the simultaneous negative shifts of the Bi 4f and O 1s peaks and the positive shifts of the Cd 3d, In 3d, and S 2p peaks after heterojunction formation confirm the directional transfer of electrons from CdIn2S4Sv to Bi2O3. This charge redistribution creates a strong internal electric field (IEF) pointing from CdIn2S4(+) to Bi2O3(-) at the interface, which is the fundamental driving factor of the S-scheme charge separation mechanism. To directly probe the photoinduced charge dynamics in the BC-25 heterostructure, in-situ X-ray photoelectron spectroscopy (XPS) was performed under illumination (λ=365 nm). After illumination, the nuclear level peaks of Bi 4f and O 1s in Bi2O3 shifted towards higher binding energies, while the nuclear level peaks of Cd 3d, In 3d, and S 2p in the CdIn2S4Sv component shifted in the negative direction. This opposite shift in the photoelectron signal provides direct in-situ evidence for the transfer of electrons from Bi₂O₃ to CdIn₂S₄Sv under photoexcitation, which is consistent with the characteristics of the S-scheme charge transfer path. In this invention, the strong reducing power of electrons in CdIn₂S₄Sv and the strong oxidizing power of holes in Bi₂O₃ are preserved.

[0057] To investigate the local electronic structure and coordination environment of bismuth atoms in the BC-25 heterojunction, X-ray absorption spectroscopy (XAS) analysis was performed in this invention. Figure 5Figure 5a shows the normalized Bi L3 edge XANES spectrum of BC-25, with Bi2O3 and bismuth foil as references. The energy absorption edge position of BC-25 is slightly negatively shifted compared to the original Bi2O3, indicating that bismuth underwent partial reduction after coupling with CdIn2S4Sv. As shown in Figure 5b, the Fourier transform EXAFS (FT-EXAFS) spectrum reveals significant differences between the reference sample and the BC-25 sample. The bismuth foil exhibits a dominant peak at 3 Å, which is a characteristic peak of Bi-Bi metallic bonding. For Bi2O3, two distinct peaks are observed: a weak peak at approximately 1.64 Å is attributed to the first shell Bi-O coordination, and a strong peak at approximately 3.6 Å corresponds to the Bi-O-Bi interaction in the oxide lattice. Notably, BC-25 displays a new characteristic peak at 2.2 Å, which is not present in either of the reference samples. This feature is clearly attributed to the Bi-S coordination shell, providing direct spectroscopic evidence for the formation of covalent Bi-S bonds at the heterojunction interface. Figure 5c illustrates the k3-weighted EXAFS oscillations in k-space. Compared to the Bi₂O₃ and bismuth foil reference samples, BC-25 exhibits unique oscillation characteristics in the high-k region of 4–10 Å⁻¹, where an additional frequency component appears. This modulation originates from the superposition of scattering paths caused by the newly formed Bi-S coordination shell and the existing Bi-O contribution. Notably, the evolution observed in the k-space oscillations is perfectly consistent with the appearance of the Bi-S peak in the Fourier transform EXAFS spectrum (Figure 5b) and the electronic state evidence provided by XPS analysis (Figure 4e), thus providing complementary and mutually reinforcing evidence for the formation of Bi-S bonds at the heterojunction interface. The appearance of Bi-S bonds confirms the successful chemical bridging between Bi₂O₃ and CdIn₂S₄Sv, which constitutes the atomic-scale basis for interfacial charge transfer under the S-scheme mechanism.

[0058] The charge transfer and utilization kinetics within the CdIn2S4Sv / Bi2O3 heterostructure were investigated using photophysical characterization. Figure 6Figure 6a shows the steady-state photoluminescence (PL) spectra of the original Bi₂O₃, CdIn₂S₄Sv, and BC-25 composite material under 350 nm excitation. A strong emission band was observed at approximately 525 nm for pure CdIn₂S₄Sv, characteristic of band-edge radiative recombination. Notably, the PL intensity of the BC-25 composite material was significantly quenched with the addition of Bi₂O₃. This indicates that the formation of the heterojunction effectively suppressed the recombination of photogenerated carriers, thereby promoting their separation and subsequent utilization in photocatalytic reactions. Similarly, time-resolved PL decay measurements (Figure 6b) show that the average carrier lifetime of BC-25 (6.48 ns) is longer than that of Bi₂O₃ (4.15 ns) and CdIn₂S₄Sv (4.75 ns). This extended lifetime suggests that the formation of the BC-25 composite material (possibly through Bi-S interfacial bonding) effectively suppressed the non-radiative recombination pathway and promoted carrier separation. The extended lifetime, combined with the quenching of steady-state charge transfer (PL) after heterojunction formation, clearly indicates an accelerated interfacial charge transfer process. This efficient separation and migration of photogenerated carriers promotes photocatalytic processes (such as 2e⁻). - ORR and formaldehyde degradation are extremely beneficial.

[0059] To systematically analyze the charge transfer kinetics and interfacial behavior of the photocatalyst, this invention conducted photocurrent response and electrochemical impedance spectroscopy (EIS) tests. As shown in Figure 6c, the transient photocurrent response of the composite material exhibits significantly enhanced characteristics, rapidly and stably generating photocurrent upon illumination. The consistently high photocurrent density and the characteristic of almost no decay during multiple switching cycles confirm that the composite material possesses excellent photoinduced charge separation performance and suppressed recombination kinetics. Furthermore, the Nyquist plot (Figure 6d) shows that compared to the original material, the semicircular diameter of the heterostructure is significantly reduced, indicating a decrease in charge transfer resistance and an improvement in interfacial charge separation efficiency.

[0060] 3. Femtosecond time-resolved fluorescence spectroscopy (fs-TA) test Figure 7 shows the transient absorption (TA) spectroscopic analysis of the CdIn2S4Sv / Bi2O3 heterojunction under 400 nm excitation, a powerful technique for revealing ultrafast charge carrier dynamics in semiconductor heterostructures. Figures 7b, e, and h show representative TA spectra of BC-25, Bi2O3, and CdIn2S4Sv over selected delay time spans, respectively. For CdIn2S4Sv, a broad photoinduced absorption (PIA) band is clearly resolved in the visible region (450–700 nm), while a significant ground-state bleaching (GSB) signal appears at approximately 510 nm. This GSB feature is a clear spectral characteristic of band-edge electronic transitions in CdIn2S4Sv, indicating photogenerated carrier depletion at the valence band maxima. The temporal evolution of the normalized ΔA signal monitored at 510 nm (Fig. 7i) fits well with the double exponential decay function, yielding two characteristic time constants: a fast component (τ1≈3 ps) attributed to rapid charge carrier trapping caused by surface defects or exciton recombination within the original CdIn2S4Sv lattice, and a slow component (τ2≈396.8 ps) corresponding to the prolonged charge separation and interfacial charge transfer in the CdIn2S4Sv / Bi2O3 heterojunction. Two-dimensional TA contour plots (Fig. 7c) further visualize the spectral-temporal evolution of the photoinduced signal. The persistent PIA signal remains stable beyond 1 ns without significant decay, directly confirming the suppression of charge recombination and the formation of long-lived charge-separated states in the heterojunction. By directly comparing the dynamic trajectories detected at 500 nm (GSB) and 600 nm (PIA) (Fig. 7d), the complementary dynamics of charge carrier evolution can be clearly observed: the GSB signal at 500 nm reflects the rapid depletion of photogenerated holes in CdIn2S4Sv, while the PIA signal at 600 nm corresponds to the accumulation of photogenerated electrons at the lowest point of the Bi2O3 conduction band. This confirms the directional charge transfer driven by the internal electric field in the S-type heterojunction.

[0061] The long lifetime phenomenon of photogenerated carriers revealed by TA spectroscopy analysis is in high agreement with the results observed in electrochemical impedance spectroscopy (EIS) measurements, which showed a significant enhancement in photocurrent response and a substantial reduction in charge transfer resistance. In summary, these TA experimental data provide direct, time-resolved spectroscopic evidence for the efficient interfacial charge separation, suppressed recombination effect, and long-lived charge-separated states in the CdIn2S4Sv / Bi2O3 heterojunction, which is key to its excellent photocatalytic performance in formaldehyde degradation.

[0062] (II) Photocatalytic activity testing and analysis: 1. Photocatalytic hydrogen peroxide production testing and analysis The hydrogen peroxide generation rate of the prepared photocatalyst is shown in Figure 8a.

[0063] Under illumination, the CdIn2S4Sv / Bi2O3 composite material exhibited significantly enhanced photocatalytic activity in saturated oxygen-rich aqueous solution compared to the original CdIn2S4Sv and Bi2O3. Notably, among the CdIn2S4Sv / Bi2O3 composite material series, the optimized sample demonstrated the highest performance, reaching 7 mmol·g⁻¹. -1 ·h -1 This value is approximately three times that of the original CdIn2S4Sv, clearly demonstrating the synergistic effect of sulfur vacancies and Bi2O3 co-modification in promoting photocatalytic hydrogen peroxide generation. The apparent quantum efficiency (AQE) of BC-25 exhibits a significant wavelength dependence, closely correlated with its optical absorption spectrum (Figure 8d).

[0064] 2. Photocatalytic degradation of formaldehyde: testing and analysis Changes in formaldehyde concentration over time in Bi2O3, CdIn2S4Sv and BC-X composite materials under visible light irradiation ( Figure 9 a) Before illumination, the system was kept in darkness for 20 min to establish adsorption-desorption equilibrium. During this period, all samples showed negligible formaldehyde adsorption, indicating that subsequent removal was mainly attributed to photocatalytic degradation rather than physical adsorption. After visible light irradiation, the formaldehyde removal efficiency of the BC-X composite material was significantly higher than that of the original Bi2O3 and CdIn2S4Sv. Among all samples, BC-25 exhibited the highest photocatalytic activity, achieving approximately 86% formaldehyde degradation within 60 min. The degradation efficiency order was BC-25>BC-30>BC-20>BC-10>BC-40>CdIn2S4Sv>Bi2O3, indicating that the optimal Bi2O3 content was crucial for maximizing the heterojunction synergistic effect. The rate constant of BC-25 was approximately 3.14 times that of Bi2O3 and 2.75 times that of CdIn2S4Sv, confirming the superior photocatalytic activity of the optimized heterojunction. This performance improvement is attributed to the synergistic effect of interfacial Bi-S bonds and sulfur vacancies, which promote efficient charge separation and surface reaction kinetics.

[0065] (III) Photocatalytic mechanism analysis and testing: 1. Free radical active quenching test and analysis To elucidate the origin of photocatalytic hydrogen peroxide formation, this invention conducted control experiments using BC-25 under different atmospheres. As shown in Figure 10a, the hydrogen peroxide yield in the O2-saturated solution was significantly higher than that in the air-saturated environment, where only a small amount of hydrogen peroxide was detected. This result clearly indicates that hydrogen peroxide formation is mainly due to the two-electron oxygen reduction reaction (2e... -The photocatalytic generation of hydrogen peroxide (ORR) is dominated by photogenerated electrons (ORR), while the contribution of water oxidation is negligible. To further investigate the active species in the photocatalytic generation of hydrogen peroxide, this invention conducted a series of quenching experiments using silver nitrate (electron scavenger), EDTA-2Na (hole scavenger), TEMPOL (superoxide radical scavenger), and tert-butanol (TBA) (hydroxyl radical scavenger). As shown in Figure 10a, the addition of silver nitrate, EDTA-2Na, or TEMPOL leads to a sharp decrease in hydrogen peroxide production, indicating that photogenerated electrons (ORR) are dominant, while the contribution of water oxidation is negligible. - ), hole (h + ) and superoxide radicals (O2) - Hydroxyl radicals play a crucial role in hydrogen peroxide formation. In contrast, the introduction of the selective OH scavenger TBA significantly enhanced hydrogen peroxide formation. This result, along with the significant increase observed after the addition of ethanol (a hole scavenger that indirectly inhibits OH formation), indicates that hydroxyl radicals are not essential for hydrogen peroxide formation; rather, they are the main species leading to hydrogen peroxide decomposition.

[0066] 2. In-situ Infrared Testing and Analysis The evolution of key intermediates in the oxygen reduction reaction (ORR) was monitored using in-situ diffuse reflectance infrared Fourier transform spectra (drift) acquired over time under visible light illumination. Under prolonged illumination, the intensity of characteristic vibrational bands gradually increased (Fig. 10b). These bands were located at 1172, 1205, and 1385 cm⁻¹. -1 The feature bands at each location are respectively assigned to O2. - , OOH and HOOH intermediate. The gradual increase in signal intensity with prolonged illumination provides strong evidence for the accumulation of reactive oxygen species on the catalyst surface, confirming the proposed two-step single-electron ORR mechanism. Meanwhile, approximately 870 cm⁻¹ -1 The peak at 1043 cm⁻¹ corresponds to the OO stretching vibration of surface-adsorbed O₂. It is noteworthy that... -1 Belonging to The banding of OH species indicates the involvement of intermediates related to the direct two-electron water oxidation pathway. In summary, these results demonstrate that photocatalytic hydrogen peroxide generation on the BC heterojunction occurs simultaneously via both the ORR and WOR pathways.

[0067] 3. Rotating Ring Disk Electrode (RRDE) and Electrochemical Active Site Testing and Analysis The reaction pathway selectivity and electron transfer number were systematically investigated using a rotating ring-disk electrode (RRDE) with a three-electrode configuration under controlled atmosphere and applied potential. In O2-saturated 0.1 M sodium sulfate electrolyte, the disk current generated by the oxygen reduction reaction (ORR) gradually increased with the applied potential scanned from 0.2 V toward the cathode to -0.8 V (vs. Ag / AgCl) (Fig. 10c). Simultaneously, the ring current, originating from hydrogen peroxide diffusion from the disk to the ring, confirmed its generation. Among the tested samples, the BC-25 heterojunction exhibited the highest current, demonstrating its excellent hydrogen peroxide generation capability. Furthermore, based on RRDE data collected between -0.80 V and -0.60 V, the hydrogen peroxide selectivity and average electron transfer number (n) were calculated. Figure 10 d). BC-25 exhibited an electron transfer number close to 2 (n≈2.3), confirming its preference for the two-electron ORR pathway and achieving the highest hydrogen peroxide selectivity of approximately 81% in the test samples. The water oxidation reaction was detected using an RRDE device under N2 atmosphere. Figure 10 e). When the Pt ring electrode detects O2 at -0.23 V (vs. Ag / AgCl), both the ring current and disk current show significant changes as the potential scans from 1.4 V to 2.2 V, indicating that 4e2O2 has occurred. - Water oxidation. However, when the platinum ring was set to +0.6 V to detect hydrogen peroxide, no significant change in the ring current was observed (Figure 10e inset). This implies that under the current conditions, 2e - WOR (2H2O → H2O2 + 2H) + + 2e - This was not feasible. The electrochemically active surface area (ECSA) of the samples was evaluated by cyclic voltammetry (CV) in the non-Radida region. The double-layer capacitance (C...) dl It is proportional to ECSA, and its value is derived from the linear relationship between current density and scan rate. For example... Figure 10 As shown in fh, the CV curves of Bi2O3, CdIn2S4Sv, and BC-25 were recorded at different scan rates. The corresponding linear fits ( Figure 10 i) Give the C2 of Bi2O3, CdIn2S4Sv, and BC-25. dl The values ​​were 29.2, 52.7, and 68.2 μF·cm, respectively. -2 The significantly higher Cdl of BC-25 confirms that the heterojunction interface exposes significantly more electrochemically active sites, which synergistically promotes its photocatalytic performance.

[0068] 4. In-situ ESR testing and analysis As shown in Figures 11a and 11b, using DMPO as a trapping agent, no DMPO-·OH or DMPO-·O2 was detected in any of the samples (BC-25, CdIn2S4Sv, Bi2O3) under dark conditions. - Characteristic signals; under illumination, all three samples showed obvious DMPO-·OH and DMPO-·O2. - The characteristic peaks, in order of signal intensity, are: BC-25 > CdIn2S4S v >Bi2O3. This order is completely consistent with the activity order of photocatalytic H2O2 production, indicating that ·O2 - (Generated by the reduction of O2 by photogenerated electrons) is a key intermediate in the generation of H2O2, and its generation efficiency directly determines the final yield.

[0069] Furthermore, Figures 11c and 11d illustrate the effect of using TEMPO as a trapping agent on photogenerated electrons (e.g., electrons). - ) and holes (h + The results of the tracking were as follows. Under dark conditions, TEMPO-e was not detected in any of the samples. - Signal, while TEMPO-h + The signal is clearly visible. After illumination, TEMPO-e - The signal was significantly enhanced, with the intensity order being: BC-25 > CdIn2S4Sv > Bi2O3; simultaneously, TEMPO-h + The signal attenuates, and the order of signal strength after attenuation is: BC-25 <CdIn2S4Sv<Bi2O3。

[0070] The above phenomena exhibit a clear pattern: BC-25 shows the strongest ·OH / ·O2 ratio under light. - Free radical signals and e - Signal, but its h + The signal was the weakest in the CdIn2S4Sv composite, while pure Bi2O3 showed the opposite trend. This seemingly contradictory result can be uniformly explained by the S-type heterojunction charge transfer mechanism: in the BC-25 composite material, the built-in electric field at the interface drives the selective recombination of holes in CdIn2S4Sv and electrons in Bi2O3 at the contact interface, thereby effectively suppressing the recombination of bulk carriers and achieving efficient spatial separation. This process retains the photogenerated electrons with strong reducing ability on the Bi2O3 side (used for efficient reduction of O2 to generate H2O2) and maintains the photogenerated holes with strong oxidizing ability on the CdIn2S4Sv side. It is this S-type charge transfer path that enables BC-25 to maximize the suppression of carrier recombination while maintaining strong redox ability, ultimately exhibiting photocatalytic H2O2 production performance far superior to that of single components.

[0071] (iv) Mechanism analysis of S-type heterojunction To reveal the origin of the excellent photocatalytic performance of the (Bi2O3 / CdIn2S4Sv containing sulfur vacancies) composite material, this invention combines band structure analysis and defect chemistry to propose a charge transfer mechanism based on the synergistic effect of the S-type heterojunction and sulfur vacancies, such as... Figure 12 As shown. UV-vis DRS and Mott-Schottky measurements determined that the conduction band bottom (CB) and valence band top (VB) positions of the sulfur-vacant CdIn2S4Sv were approximately -1.17 eV and +1.01 eV (vs. NHE), respectively, showing a slight negative conduction band shift compared to the defect-free sample. This is attributed to the increased electron reduction capability of the donor levels introduced by the sulfur vacancies. The band positions of Bi2O3 were approximately CB = -0.54 eV and VB = +2.34 eV. The interleaved band structure of both provides a thermodynamic basis for constructing an S-type heterojunction. Under visible light irradiation, both CdIn2S4Sv and Bi2O3 were excited to generate photogenerated carriers. Due to the Fermi level difference, a built-in electric field was formed at the interface pointing from CdIn2S4Sv to Bi2O3. Driven by the synergistic effect of the built-in electric field, band bending, and Coulomb attraction, useless photogenerated electrons in the Bi₂O₃ conduction band and useless photogenerated holes in the CdIn₂S₄Sv valence band rapidly recombine at the interface, effectively preserving photogenerated electrons with strong reducing power (CB≈-1.17 eV) in the CdIn₂S₄Sv conduction band and photogenerated holes with strong oxidizing power (VB≈+2.34 eV) in the Bi₂O₃ valence band. Notably, the sulfur vacancies introduced in CdIn₂S₄Sv play a crucial role in charge separation and surface reactions: on the one hand, as shallow donor levels, sulfur vacancies can act as trapping centers for photogenerated electrons, further inhibiting bulk electron-hole recombination and extending the lifetime of conduction band electrons; on the other hand, sulfur vacancies are enriched with localized electrons, becoming preferred sites for oxygen molecule adsorption and activation, significantly promoting the two-step single-electron oxygen reduction reaction (O₂→·O₂). - →H2O2).

[0072] The synergistic effect of this S-shaped heterojunction and sulfur vacancies is directly reflected in its photocatalytic performance. In pure water, the composite material achieved a high efficiency of 7 mmol·g⁻¹. -1 ·h -1 The high H2O2 yield is mainly attributed to the following: the S-type heterojunction retains the extremely negative reduction potential of the CdIn2S4Sv conduction band, providing a thermodynamic driving force for oxygen reduction; sulfur vacancies enhance electron-hole separation efficiency and provide abundant oxygen reduction active sites. Meanwhile, in the formaldehyde degradation experiment, the holes retained in the Bi2O3 valence band can directly oxidize water to generate ·OH radicals, while the electrons captured by sulfur vacancies can also activate dissolved oxygen to generate reactive oxygen species (such as ·O2). -Together with H2O2, it participates in the oxidative decomposition of formaldehyde, ultimately achieving a degradation rate of 86%. The introduction of sulfur vacancies not only optimizes the electronic structure of CdIn2S4, but also maximizes the spatial separation and surface utilization of highly reducing electrons and highly oxidizing holes through synergistic interfacial charge transfer with the S-type heterojunction, providing a new approach for simultaneously achieving efficient solar energy conversion and environmental purification.

[0073] The above description of the disclosed embodiments enables those skilled in the art to make or use the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A method for preparing a CdIn2S4Sv / Bi2O3 composite material, characterized in that, Includes the following steps: Preparation of S1.Bi2O3 nanofibers S11. Preparation of precursor solution: Dissolve bismuth source in N,N-dimethylformamide solvent and stir until a clear solution is formed. Add polyvinylpyrrolidone to the clear solution and continue stirring to obtain a homogeneous and stable precursor solution for later use. S12. Electrospinning The electrospinning process was carried out at room temperature, and the key process parameters were set as follows: the precursor solution propulsion speed of the injection pump was 0.02-0.06 mm / min, the distance between the needle tip and the roller collector covered with aluminum foil was fixed at 12-18 cm, and a DC high voltage of 12-18 kV was applied between the two to deposit the composite nanofiber membrane. S13. Calcination and crystallization treatment The composite nanofiber membrane was placed in a muffle furnace and calcined under a programmed temperature rise in air atmosphere; after the calcination process was completed, it was naturally cooled to room temperature to obtain Bi2O3 nanofibers. Preparation of S2.CdIn2S4Sv and CdIn2S4Sv / Bi2O3 composite materials First, Bi2O3 nanofibers were dispersed in a mixed solvent containing deionized water and glycerol, and a uniform suspension was formed by ultrasonic treatment. Then, cadmium nitrate tetrahydrate, indium nitrate tetrahydrate, and thioacetamide were added to the suspension in sequence, and the mixture was stirred to ensure that the reactants were fully mixed and dispersed. The reactant solution was then transferred to a device equipped with a reflux condenser, heated in an oil bath, and continuously stirred magnetically. After the reaction was completed, the mixture was naturally cooled to room temperature. Finally, the mixture was centrifuged, washed, and vacuum dried to obtain the CdIn2S4Sv / Bi2O3 composite material.

2. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S11, the bismuth source is bismuth nitrate pentahydrate, and the concentration of bismuth nitrate pentahydrate in N,N-dimethylformamide solvent is 0.1~0.15 g / mL.

3. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S11, the molecular weight of the polyvinylpyrrolidone is approximately 1,300,000, and the mass ratio of the bismuth source to the polyvinylpyrrolidone is 12:16-12:

18.

4. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S13, the programmed calcination is performed at a temperature of 1-5°C. o Heating rate from room temperature to 400-500 °C / min o C, and keep it for 1-4 hours.

5. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S2, the solid-liquid ratio of the Bi2O3 nanofibers to the mixed solvent is 1-4:6 mg / mL.

6. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S2, the concentration of the thioacetamide in the mixed solvent is 0.05-0.15 mmol / mL; the molar mass ratio of the cadmium nitrate tetrahydrate, indium nitrate tetrahydrate and thioacetamide is 1:2:

6.

7. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S2, the volume ratio of deionized water to glycerol in the mixed solvent is 29:5-29:

7.

8. The method for preparing a CdIn2S4Sv / Bi2O3 composite material according to claim 1, characterized in that, In step S2, the oil bath heating is at 60-100°C. o C reaction takes 4-12 hours.

9. The CdIn2S4Sv / Bi2O3 composite material obtained by any of the preparation methods described in claims 1-8.

10. The application of the CdIn2S4Sv / Bi2O3 composite material according to claim 9 in the field of photocatalytic production of hydrogen peroxide or photocatalytic degradation of formaldehyde.

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