A citric acid modified red mud-based s-type heterojunction photocatalyst, and a preparation method and application thereof

Abstract

This invention discloses a citric acid-modified red mud-based S-type heterojunction photocatalyst, its preparation method, and its application. The photocatalyst reduces the strong alkalinity of red mud through citric acid modification to obtain acid-treated red mud, and is then composited with the oxidation photocatalyst ZnIn2S4 to construct an S-type heterojunction structure with a synergistic catalytic effect. This photocatalyst, as a highly efficient and low-cost hydrogen production material, can achieve a hydrogen production rate of 2.59 mmol g / L under full-spectrum illumination. ‑1 h ‑1 The high photocatalytic hydrogen production rate is 3.28 times higher than that of pure ZnIn2S4, and it exhibits excellent catalytic stability and environmental compatibility. In addition, through the resource-based modification of red mud and the synergistic regulation of heterojunction, it achieves efficient synergy of light absorption, carrier separation and surface catalytic reaction, effectively improving hydrogen production performance and reducing preparation cost. At the same time, it provides a new path for the harmless treatment and high-value utilization of industrial solid waste red mud, showing significant application potential in the field of large-scale photocatalytic hydrogen production.

Description

Technical Field

[0001] This invention relates to a citric acid-modified red mud-based S-type heterojunction photocatalyst, and also to a method for preparing the above-mentioned photocatalyst and its application in photocatalytic hydrogen production. Background Technology

[0002] Faced with the dual challenges of global energy shortages and environmental pollution, the development of clean and renewable energy has become an international consensus. Photocatalytic hydrogen production technology is receiving widespread attention due to its low emissions and environmentally friendly characteristics. How to rationally design and develop low-cost photocatalysts that can maintain simple structure, environmental friendliness, and scalability, while also being easy to modify, is of vital importance to promoting the development of sustainable and practical photocatalytic technologies.

[0003] Red mud (RM) is a highly alkaline solid waste generated during the alkaline process of alumina production from bauxite, with an estimated generation of 0.8-2.0 tons per ton of alumina produced. Rich in alkaline oxides, red mud is inherently highly alkaline and, under long-term accumulation, is easily leached by rainwater, causing secondary environmental hazards, including soil alkalization, structural deterioration, and pollution of nearby water bodies, thus significantly exacerbating the risk of ecological pollution.

[0004] Existing technologies mostly employ inorganic acids such as hydrochloric acid, sulfuric acid, and nitric acid to acidify red mud. While these methods can neutralize alkalinity and increase specific surface area to some extent, they still have significant drawbacks: The excessive acidity of inorganic acids can lead to over-etching of the red mud lattice, causing the dissolution of active metal ions and structural collapse; sulfate ions easily form insoluble salt deposits; hydrochloric acid volatilization causes significant pollution and requires high equipment corrosion resistance; furthermore, inorganic acids lack complexation selectivity, failing to directionally remove amorphous SiO2 and thus failing to fully expose active metal oxide sites. This results in low carrier separation efficiency and insufficient catalytic activity and stability when the modified red mud is used as a photocatalytic component. While some organic acid modification processes are milder, they do not achieve selective etching of the silicon components in red mud or precise control of the band structure, making it difficult to meet the requirements for efficient photocatalytic hydrogen production. There is an urgent need to develop photocatalysts with more stable and efficient catalytic activity. Summary of the Invention

[0005] Purpose of the invention: The purpose of this invention is to provide a citric acid-modified red mud-based S-type heterojunction photocatalyst with high photocatalytic hydrogen production activity, excellent catalytic stability and environmental compatibility, and to provide a method for preparing the above photocatalyst and its application in large-scale photocatalytic hydrogen production and industrial solid waste resource utilization.

[0006] Technical Solution: The citric acid-modified red mud-based S-type heterojunction photocatalyst of the present invention is obtained by compositing acid-treated red mud ARM with a ZIS precursor to form an S-type heterojunction structure. The ZIS precursor is a ZnIn2S4 oxidation photocatalytic component, and the acid-treated red mud ARM is obtained by acidifying and modifying industrial solid waste red mud RM with citric acid. The photocatalyst of the present invention achieves a concentration of 2.59 mmol g under full-spectrum illumination. -1 h -1 The high photocatalytic hydrogen production rate is 3.28 times higher than that of pure ZnIn2S4, and it exhibits excellent catalytic stability and environmental compatibility.

[0007] The amount of acid-treated red mud ARM added is 1wt%~5wt% of the ZIS precursor mass; preferably 4wt%. The acid-treated red mud ARM is prepared by citric acid acidification and complexation modification, by mixing red mud and citric acid solution at room temperature, filtering, washing to neutrality, and then drying. The mass ratio of red mud to citric acid is 1:1~6, which removes SiO2 from the red mud, thereby exposing more active metal oxide sites inside the red mud and optimizing the electronic band structure of the material.

[0008] The ZIS precursor was obtained by low-temperature hydrothermal synthesis of zinc chloride, InCl3·4H2O, and thioacetamide.

[0009] The preparation method of the above-mentioned citric acid modified red mud-based S-type heterojunction photocatalyst includes the following steps:

[0010] (1) Dissolve the red mud in a saturated citric acid solution in a beaker, stir to obtain a uniform suspension, separate and purify to obtain acid-treated red mud RM, wash with water until neutral and dry to obtain acid-treated red mud ARM;

[0011] (2) Dissolve the acid-treated red mud ARM, zinc chloride, InCl3·4H2O and thioacetamide in water, respectively, and then mix them after ultrasonic treatment and ultrasonic treatment again to obtain a mixture. After hydrothermal reaction, the mixture is cooled, washed and dried to obtain citric acid modified red mud-based S-type heterojunction photocatalyst.

[0012] In step (1), the mass percentage of the saturated citric acid solution is 37.78%~38.7%; the stirring is carried out at room temperature of 25±2℃ for 4~6 hours; the stirring time is preferably 4 hours.

[0013] In step (1), the drying process involves drying in a convection oven for more than 24 hours at a temperature of 60-70°C. The drying time is preferably 24 hours and the temperature is preferably 60°C.

[0014] In step (2), the molar ratio of zinc chloride, InCl3·4H2O and thioacetamide is 1:2:4.

[0015] In step (2), the hydrothermal reaction temperature is 170~190℃ and the heating time is 2~3h; preferably, the hydrothermal reaction temperature is 180℃ and the heating time is 2h.

[0016] Specifically, step (1) involves dissolving the RM sample in a saturated CA solution in a beaker; the reaction system is continuously stirred with a magnetic stirrer at room temperature (25±2℃) for 4 hours to obtain a uniform suspension. After the stirring process, the resulting red slurry is separated from the reaction mixture by vacuum filtration. The subsequent purification process includes centrifugation to obtain acid-treated RM, followed by repeated rinsing with deionized water until a neutral pH is reached. Finally, the acid-treated RM is dried in a 60℃ convection oven for 24 hours and labeled as ARM.

[0017] Specifically, step (2) involves dissolving acid-treated red mud ARM, zinc chloride, InCl3·4H2O, and thioacetamide in water, sonicating each solution for 10 minutes, mixing them, and sonicating again for 10 minutes to obtain a mixture, ensuring that as much ARM as possible dissolves. The mixture is then transferred to a polytetrafluoroethylene-lined autoclave and heated at 180°C for 2 hours. After cooling to room temperature, the final product is washed three times with deionized water and ethanol, and then vacuum dried at 60°C to obtain the citric acid-modified red mud-based S-type heterojunction photocatalyst of this invention.

[0018] The aforementioned citric acid-modified red mud-based S-type heterojunction photocatalyst is applied in photocatalytic hydrogen production.

[0019] Invention Principle: This invention relates to a citric acid-modified red mud-based S-type heterojunction photocatalyst. Raw industrial solid waste red mud (RM) is modified with citric acid (CA) and then combined with ZIS via a one-step hydrothermal method to obtain a novel S-type heterojunction photocatalyst. The photocatalyst of this invention exhibits distinct characteristics in its microstructure and composite morphology: ARM is added during ZIS synthesis, allowing it to be tightly embedded between the vertical and horizontal layers of the ZIS nanostructure. High-resolution transmission electron microscopy images confirm that pure ZIS exhibits a flower-like morphology, while the composite material shows obvious black ARM blocks within its flower-like structure, visually verifying the successful composite and tight interfacial coupling of ARM and ZIS. After CA etching, silica in the RM is effectively removed, significantly improving the separation and utilization efficiency of photogenerated carriers, while simultaneously greatly reducing alkalinity and optimizing the photocatalytic reaction. Under illumination, the ARM / ZIS composite material exhibits significantly superior photocatalytic performance compared to the original ARM and ZIS materials.

[0020] Specifically, this invention uses industrial solid waste red mud (RM) and ZnIn2S4 (ZIS) as core raw materials, and constructs an ARM / ZIS S-type heterojunction structure through citric acid acidification modification and in-situ composite technology. Citric acid, as an environmentally friendly etchant, can effectively remove amorphous SiO2 components from the red mud, exposing more active metal oxide sites and optimizing the electronic band structure, making the ARM a highly efficient reduction photocatalytic electrode (RP) in the S-type heterojunction. The heterojunction structure achieves synergistic regulation of light absorption, charge separation, and catalytic reaction: the band matching of ARM and ZIS forms a directional charge migration path. Under full-spectrum irradiation, photogenerated electrons are positioned in the ARM band gap through the S-type transfer path, catalyzing the generation of hydrogen from H⁺ with strong reducing ability; the active holes in the ZIS valence band are rapidly consumed by the sacrificial agent TEOA, avoiding photocorrosion; the tight interfacial coupling and internal electric field further suppress the non-productive recombination of photogenerated carriers. This structure can also ensure catalytic stability through dynamic reaction equilibrium, with the optimal ratio of 4wt% ARM / ZIS composite material achieving 2.59 mmol g -1 h -1 The high photocatalytic hydrogen production rate is 3.28 times higher than that of pure ZIS, and it maintains excellent stability in multiple cycles. At the same time, it realizes the high-value utilization of industrial solid waste red mud, ensuring that the catalyst has both high efficiency and economy in large-scale applications.

[0021] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages:

[0022] (1) This invention uses specially selected citric acid to acidify and complex red mud, directionally remove amorphous SiO2 and optimize the band structure, and then combines it with conventional ZIS precursor to construct an S-type heterojunction. This specifically solves the common bottleneck of existing acidification modification technologies. Its photocatalytic hydrogen production performance is excellent. Under full-spectrum illumination, the hydrogen production rate of the optimal 4wt%-ARM / ZIS composite material is as high as 2.59 mmol g. -1 h -1 Compared to pure ZnIn2S4, it is 3.28 times more efficient, and its charge separation and catalytic efficiency are significantly better than those of traditional photocatalytic materials;

[0023] (2) It has excellent catalytic stability and environmental compatibility. After multiple cycle tests, it can still maintain high hydrogen production activity, effectively avoid photocorrosion, and is suitable for complex light and reaction environments.

[0024] (3) It has excellent solid waste resource utilization value and environmental benefits, and successfully transforms highly polluting industrial solid waste red mud into high-value photocatalytic materials. At the same time, the citric acid modification process is green and environmentally friendly, effectively removing the strong alkalinity and amorphous SiO2 of red mud and reducing the environmental burden.

[0025] (4) It is low in cost, simple in process and easy to scale up. The raw materials are mainly industrial solid waste. The preparation process does not require complicated equipment. The in-situ composite and acidification modification process can be scaled up in batches, which not only reduces the cost of catalyst preparation, but also provides an economical and feasible solution for the large-scale application of solar hydrogen production technology, showing great potential for practical application. Attached Figure Description

[0026] Figure 1 This describes the preparation process and performance characterization of citric acid-modified red mud, among which... Figure 1 a is a schematic diagram of the ARM synthesis process. Figure 1 b shows the X-ray diffraction patterns of ARM in citric acid solutions of different concentrations. Figure 1 c and 1d are the Fourier transform infrared spectra of ARM in citric acid solutions of different concentrations. Figure 1 e represents the dissolution curves of silica in RM in citric acid solutions of different concentrations. Figure 1 f is a schematic diagram of the corresponding dissolution mechanism;

[0027] Figure 2 This refers to the changes in the surface morphology and internal elemental distribution of RM after citric acid acidification, among which... Figure 2 a is a scanning electron microscope image of the ARM after acidification with different concentrations of citric acid. Figure 2 b represents the energy dispersive spectroscopy (EDS) results of ARM after acidification with different concentrations of citric acid. Figure 2 c. Elemental mapping images of ARM after acidification with different concentrations of citric acid;

[0028] Figure 3 The effect of different concentrations of citric acid acidification treatment on the light absorption capacity of RM, among which Figure 3 a represents the ultraviolet-visible diffuse reflectance absorption spectrum. Figure 3 b is (αhv) 2 The curves showing the change in photon energy (hv) demonstrate the band gap energy of the ARM under different concentrations of citric acid solution. Figure 3 Figure c shows the Mott-Schottky diagram of the RM and ARM, and Figure d shows the band structure diagram of the RM and ARM. Figure 3 e represents the transient photocurrent response curves of RM and ARM. Figure 3 f represents the electrochemical impedance curves of RM and ARM under illumination;

[0029] Figure 4 It is the preparation process and performance characterization of composite materials, among which Figure 4 a is a schematic diagram of the synthesis process of ARM / ZIS composite photocatalyst. Figure 4 b is a scanning electron microscope image of ARM / ZIS. Figure 4 c is a high-resolution transmission electron microscope image from ZIS. Figure 4d is a high-resolution transmission electron microscope image from ARM / ZIS. Figure 4 e represents the X-ray diffraction patterns of ARM, ZIS, and ARM / ZIS. Figure 4 f represents the Fourier transform infrared spectra of ARM, ZIS, and ARM / ZIS;

[0030] Figure 5 This is an evaluation of the hydrogen production performance of ARM / ZIS composite materials, among which... Figure 5 a represents the photocatalytic H2 precipitation curve. Figure 5 b represents the H2 yield of ARM, ZIS, and composite materials under 300W xenon lamp irradiation. Figure 5 c represents the AQE values ​​of the ARM / ZIS composite material at wavelengths of 420, 550, and 660 nm. Figure 5 d represents the stability cycling test of the ARM / ZIS composite material. Figure 5 e represents the H2 yield compared to other ZIS-based photocatalysts;

[0031] Figure 6 It refers to the light absorption properties of the composite material compared to the original material, among which... Figure 6 a represents the ultraviolet-visible absorption spectrum. Figure 6 b is (αhv) 2 The relationship between photon energy (hv) and photon energy. Figure 6 c is the Mott-Schottky diagram of ARM and ZIS. Figure 6 d and Figure 6 e represents the Kelvin probe force microscopy potential images of the ARM / ZIS composite material under dark and light conditions, respectively. Figure 6 f represents the corresponding surface potential curve. Figure 6 g is a schematic diagram of the charge transfer process in the ARM / ZIS S-type heterojunction;

[0032] Figure 7 This is a schematic diagram of the hydrogen production mechanism of the ARM / ZIS S-type heterojunction photocatalyst;

[0033] Figure 8 This is the XRD pattern of the ARM crystal structure. Detailed Implementation

[0034] The present invention will now be described in detail with reference to specific embodiments.

[0035] Example 1

[0036] The citric acid-modified red mud-based S-type heterojunction photocatalyst of the present invention is prepared by the following steps:

[0037] (1) 10 g of RM sample was dissolved in a saturated CA solution in a beaker, wherein the mass percentage of citric acid in the saturated CA solution was 38.6%; the reaction system was continuously stirred with a magnetic stirrer at room temperature for 4 hours to obtain a homogeneous suspension. After the stirring process, the resulting red slurry was separated from the reaction mixture by vacuum filtration. The subsequent purification process included centrifugation to obtain acid-treated RM, followed by repeated rinsing with deionized water until a neutral pH was reached. Finally, the acid-treated RM was dried in a convection oven at 60 °C for 24 hours and labeled as ARM;

[0038] (2) Dissolve 8 mg of ARM obtained in step (1) in 20 mL of water to form a 4 wt% ARM aqueous solution. Simultaneously, dissolve 0.068 mg of zinc chloride, 0.293 mg of InCl3·4H2O, and 0.15 mg of TAA in 30 mL of water. After sonicating each solution for 10 minutes, mix them and sonicate again for 10 minutes to ensure that as much ARM as possible is dissolved. Transfer the mixture to a polytetrafluoroethylene-lined autoclave and heat at 180°C for 2 hours. After cooling to room temperature, wash the final product three times with deionized water and ethanol, and dry it under vacuum at 60°C to obtain the photocatalyst of the present invention, denoted as 4 wt%-ARM / ZIS.

[0039] Example 2

[0040] Compared with Example 1, the mass percentages of citric acid in the CA solution were changed in step (1) to 7.72 wt%, 15.44 wt%, 23.16 wt%, and 30.88 wt%, respectively, to obtain acid-treated RMs, which were denoted as 20-ARM, 40-ARM, 60-ARM, and 80-ARM.

[0041] Example 3

[0042] Compared with Example 1, the amount of ARM added in step (2) was changed to 2 mg, 4 mg, 6 mg, and 10 mg to obtain the photocatalyst of the present invention, which is denoted as 1 wt%-ARM / ZIS, 2 wt%-ARM / ZIS, 3 wt%-ARM / ZIS and 5 wt%-ARM / ZIS.

[0043] Comparative Example 1

[0044] Synthesis of ZnIn2S4 via a low-temperature hydrothermal method:

[0045] 0.5 mmol ZnCl3, 1.0 mmol InCl3·4H2O, and 1.5 mmol TAA were dissolved in 50 mL of deionized water and sonicated for 15 minutes. The mixture was then transferred to a polytetrafluoroethylene-lined autoclave and heated at 180 °C for 2 hours. After cooling to room temperature, the final product was washed three times with deionized water and ethanol, and then dried under vacuum at 60 °C to obtain pure ZnIn2S4, denoted as ZIS.

[0046] like Figure 1 As shown, according to Figure 1 The schematic diagram of the synthesis process in Figure a shows that RM is dissolved in citric acid solution by continuous stirring to obtain a homogeneous suspension, which is then filtered and dried to obtain the target product ARM. To investigate the effect of different concentrations of citric acid solution on the microstructure of ARM, X-ray diffraction (XRD) was used to analyze the crystal structure of the ARM obtained in the examples. Figure 1 b). For example Figure 8 XRD results showed that the main components of ARM were ferric oxide, titanium dioxide, and silicon dioxide, while the diffraction peak observed at 21.2° was attributed to the silicon dioxide component. Due to differences in surface atomic configurations, the solubility of citric acid solution for different crystal planes varied. After etching, the diffraction peak intensity of the ARM sample decreased significantly with increasing citric acid solution concentration, indicating that higher concentrations of citric acid solution enhanced the etching ability on silicon dioxide. Simultaneously, diffraction peaks at other positions showed a certain degree of shift, indicating that the etching of ARM by citric acid solution led to a reduction in silicon dioxide. This also affected the crystal structure of ARM. Fourier transform infrared spectroscopy (FT-IR) was used to analyze the changes in chemical composition and functional groups of ARM after acidification with different concentrations of citric acid solution. Figure 1 c). 3700-3200 cm -1 The broad peak at this point corresponds to the OH stretching vibration, with the original peak being stronger, indicating an increase in the number of hydroxyl groups and water adsorption. After acidification, this peak gradually weakens, indicating a decrease in the number of hydroxyl groups and disruption of the surface hydration layer. The most significant changes occur in the range of 500–1500 cm⁻¹. Figure 1 d) Vibrational absorption peaks were mainly observed for Si-O, Al-O, and Fe-O. Acidification treatment enhanced or caused slight shifts in the positions of these peaks, indicating structural rearrangement and reconstruction of the silicate, alumina, and iron oxide frameworks in the red mud. Notably, at 1100 cm⁻¹... -1 The absorption peak at [location] is attributed to the Si-O stretching vibration. With increasing citric acid solution concentration, the vibration amplitude gradually decreases and its position shifts, indicating that the ability of red mud to dissolve silica gradually increases. The dissolution curves of silica in citric acid solutions with different weight percentages are shown below. Figure 1As shown in Figure e, the highest solubility of silica was observed in the saturated citric acid solution, confirming its excellent etching performance on silica in red mud. Based on the above calculation results, the acidic modification mechanism of RM by citric acid solution is as follows: Figure 1 As shown in f, the dissolution process of silica in RM in the presence of citric acid follows a protonation-hydrolysis-complexation sequence mechanism, which can be understood as the following four steps: (i) Under acidic conditions, protons preferentially attack the bridging oxygen atoms on the Si–O–Si groups on the silica surface, leading to protonation and significantly weakening the originally strong Si–O bonds; (ii) After activation, these protonated Si–O–Si bonds are susceptible to nucleophilic attacks by water molecules, leading to bond breakage and the formation of soluble monosilicic acid (H4SiO4), which marks the transformation of solid-phase SiO2 into molecularly dispersed silicic acid; (iii) Subsequently, the carboxylic acid group of citric acid approaches the silicon center of monosilicic acid, and its oxygen atom is positioned to provide lone pairs of electrons and establish a geometric configuration conducive to chelation; (iv) Finally, the carboxylic acid oxygen atom of citric acid forms a coordinate bond with the silicon atom, generating a stable CA-Si complex, which effectively inhibits the repolymerization of silicic acid into silica, thereby stabilizing the dissolved silicon species and continuously driving the dissolution process. The results above show that citric acid solution has a significant etching effect on RM and can greatly reduce the silica content in RM.

[0047] like Figure 2 As shown, to investigate whether the surface morphology and internal elemental distribution of ARM changed after acidification with citric acid, scanning electron microscopy (SEM) was used to observe the morphological changes of ARM prepared in Example 1 after acidification with different concentrations of citric acid. Figure 2 a). The distribution of RM particles at the microscopic level becomes more uniform, with larger particles becoming smaller, resulting in a denser overall structure. Furthermore, the elemental distribution map ( Figure 2 (b) shows that as the concentration of citric acid solution increases, the distribution of Si element changes significantly from dense to sparse, indicating that high-concentration citric acid solution has a better etching effect on silicon dioxide in RM. Meanwhile, energy dispersive spectroscopy (EDS) results confirm that the Si content gradually decreases, which is consistent with the above findings. Figure 2 c). The above test results indicate that the acidification effect of citric acid on RM alters its microporous structure to some extent, reduces the silica content, achieves a more uniform distribution at the microscopic level, and may provide more active sites for the active metal oxide substrate.

[0048] like Figure 3 As shown, to comprehensively investigate the effect of different citric acid acidification treatments on the RM light absorption capacity of the samples prepared in Example 1, the optical absorption characteristics of the samples were detected by ultraviolet-visible diffuse reflectance spectroscopy. Figure 3As shown in Figure a, the absorption peak position of ARM did not shift significantly with increasing citric acid concentration, and it still maintained strong light absorption near 670 nm, indicating that acidification treatment did not substantially change the inherent light absorption range of ARM. Further analysis using the Kubelka-Munk formula to process diffuse reflectance data showed that the optical band gap (Eg) of ARM was approximately 2.02 eV, and its variation was minimal under different acidification concentrations. Figure 3 b). Despite fluctuations in citric acid concentration during acidification, the absorption characteristics and band gap of the metal oxide frameworks (such as Fe2O3 and TiO2) in the ARM showed negligible changes, confirming that the structural integrity of the metal oxides controlling electron transport was maintained throughout the treatment. Mott-Schottky analysis based on the material band structure showed that the Efb values ​​for RM and ARM were -0.72 eV and -0.82 eV (vs. SCE, pH=7), respectively, corresponding to -0.06 and -0.16 eV relative to the standard hydrogen electrode (NHE, E0). NHE = E SCE +0.0591 pH + 0.24 eV Figure 3 c). It should be noted that E fb The conduction band (CB) potential is 0.1 V higher than that in n-type semiconductors, therefore the CB potentials of RM and ARM are estimated to be -0.16 eV and -0.26 eV (relative to NHE), respectively. Figure 3 As shown in the d-band structure diagram, experiments demonstrate that acid etching removes silica, leading to a negative band shift in the material's band structure, resulting in an increased band gap and significantly enhanced photoelectron reduction capability. By optimizing the band structure design, the applicability of RM in the hydrogen reduction reaction potential is significantly improved. To investigate the acidification-induced carrier separation efficiency, electrochemical impedance spectroscopy (EIS) and transient photocurrent response under illumination were performed. Figure 3 As shown in Figure e, the RM exhibits a significantly lower photocurrent response, while the ARM demonstrates excellent photocurrent density. Furthermore, as... Figure 3 As shown in f, the impedance of ARM is significantly reduced after acidification, indicating that ARM has better photogenerated charge separation capability and faster interfacial electron transfer rate, which is more conducive to the directional migration and utilization of electrons.

[0049] like Figure 4As shown, based on the above experimental analysis, it can be confirmed that acid treatment plays a key role in improving the transfer and separation efficiency of the support. Its core mechanism lies in effectively removing amorphous or low-crystallinity silica components from the ARM, thereby optimizing the material's surface properties, enhancing active sites, and improving its electron transport capability. Simultaneously, this modification makes it more suitable as a reduction photocatalyst (RP) in heterojunctions to participate in photocatalytic reactions. To further study the photocatalytic performance of ARM as a support, ZIS was selected as a typical oxidation photocatalyst (OP) to construct an S-type heterojunction composite material, and the ARM / ZIS composite material was successfully synthesized via a one-step hydrothermal method. Figure 4 a). The morphology and microstructure of the samples were examined in detail using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). For example... Figure 4 As shown in b, ARM is added during the synthesis of ZIS to ensure its tight embedding between the vertical and lateral layers of the nanostructure. High-resolution transmission electron microscopy (HRTEM) images of ZIS are shown below. Figure 4 As shown in c, the previously reported flower-like morphology of pure ZIS is confirmed, and the presence of obvious black ARM blocks within the flower-like structure confirms the successful composite of ARM and ZIS. Figure 4 d). Subsequently, the diffraction peak changes of the ARM / ZIS complex were determined by X-ray diffraction (XRD), such as... Figure 4 As shown in Figure e, its diffraction peaks appear at 2θ degrees of 21.6°, 27.8°, and 47.2°, corresponding to the (006), (102), and (112) crystal planes of ZIS, respectively. In the ARM / ZIS composite, the peaks at the corresponding positions remain clearly visible, and characteristic diffraction patterns of mineral phases such as anatase in ARM can be clearly detected. These findings collectively confirm the good integration and thorough coupling between ARM and ZIS. Furthermore, Fourier transform infrared spectroscopy (FT-IR) was used to analyze the chemical composition and functional group variations of ARM, ZIS, and ARM / ZIS. Figure 4 f). At approximately 3400 cm -1 and 1600 cm -1 The broad absorption band at this location is attributed to the stretching and bending vibrations of the OH groups, indicating the presence of hydroxyl groups and adsorbed water on the material surface []. In the ARM / ZIS composite material, the band at approximately 1100 cm⁻¹ is [missing information]. -1 The characteristic vibrational peaks of Si-O at the location are no longer clearly observable. This is because the Si-O framework of the RM has been severely damaged during the citric acid etching process, and the low proportion of ARM in the composite material further weakens the intensity of the detectable Si-O signal. Figure 4 g in 800-2000 cm -1 When the area is magnified, it can be 1100 cm. -1Characteristic absorption peaks of In-S bonds were observed nearby, and these peaks remained unchanged in the composite material, further confirming the successful integration of ARM and ZIS. In summary, the above characterization analysis confirms the successful integration of ARM and ZIS at the microscopic level.

[0050] like Figure 5 As shown, to further evaluate the photocatalytic activity of ARM / ZIS, the hydrogen production performance in the presence of a sacrificial agent and under full-spectrum irradiation was investigated. Figure 5 As shown in Figure a, the ARM / ZIS composite material exhibits excellent photocatalytic hydrogen production performance, outperforming both single-phase ARM and ZIS. Furthermore, the hydrogen production rate increases with increasing ARM doping concentration. The hydrogen production rate reaches its maximum of 2.59 mmol g when the ARM doping concentration reaches 4 wt%. -1 h -1 It is 3.28 times that of single-phase ZIS. Figure 5 b). However, further addition of ARM leads to a decrease in hydrogen production, possibly because excess ARM hinders electron transport and active sites. Apparent quantum efficiency (AQE) results show that the AQE values ​​of ARM / ZIS at wavelengths of 420, 550, and 660 nm are 2.81%, 0.216%, and 0.0026%, respectively, consistent with the light absorption characteristics. Figure 5 c). Furthermore, the stability and recyclability of the photocatalyst are crucial for practical applications. Cyclic testing was conducted to evaluate the recyclability and stability of the ARM / ZIS composite. After five cycles, no significant deactivation was observed in the sample, indicating that the material exhibits excellent stability. Figure 5 d). In comparison with other ZIS-based photocatalysts, the photocatalytic efficiency of the ARM / ZIS composite material is significantly higher than that of most materials reported in recent years, demonstrating high feasibility for practical application. Figure 5 e).

[0051] like Figure 6 As shown, to evaluate the light absorption performance of the composite material compared to the original material, we recorded the UV-Vis and diffuse reflectance (DRS) spectra of the prepared samples. Figure 6 As shown in figure a, pure ZIS exhibits a maximum absorption edge at approximately 500 nm, while the ARM / ZIS composite material shows a redshift, shortening the electron energy transfer distance, with its maximum absorption edge shifting to approximately 550 nm, significantly enhancing the light absorption intensity. Furthermore, the band gaps (Eg) of ZIS and ARM were calculated using the Kubelka-Monk formula. g The values ​​are 2.37V and 1.94V respectively. Figure 6 b). Based on the MS test results, after converting the SCE flat-band potential to the NHE flat-band potential, the E values ​​of ZIS and ARM were calculated.fb The values ​​are 0.17 volts and -0.16 volts respectively. Figure 6 c). A significant difference in the Fermi levels between the two semiconductors was observed, leading to a built-in electric field at the interface from ARM to ZIS, providing a thermodynamic basis for carrier separation. To further investigate the carrier transport path, we used Kelvin probe force microscopy (KPFM) to analyze the surface potential variations in the composite material in detail. Figure 6 In the dark, the surface potential difference of the ARM / ZIS composite material remains relatively low, indicating a high Fermi level, a weak internal electric field, and easy recombination of photogenerated electrons and holes. Under illumination, the surface potential difference increases significantly, directly confirming the efficient photoinduced electron transport and separation within the material. Figure 6 e). Furthermore... Figure 6 The surface potential variation curve in f further verifies that under illumination, the potential increases significantly with scanning distance, strongly supporting the conclusion of photoinduced charge separation. Photoexcited electrons transition from the valence band to the conduction band and migrate to the surface under the influence of the built-in electric field, leading to the formation of surface charges. This effective charge separation mechanism significantly reduces the probability of electron-hole recombination, allowing more photogenerated electrons to participate in the surface proton reduction reaction. This is the key reason why the ARM / ZIS composite material exhibits excellent photocatalytic hydrogen production performance. Figure 6 The charge transfer behavior of the ARM / ZIS S-type heterojunction in g shows that before contact, ZIS and ARM have independent band structures, with the conduction and valence bands of ZIS being higher than the corresponding energy levels in ARM. After contact, the difference in Fermi levels drives electrons to spontaneously migrate from ZIS to ARM, while holes migrate from ARM to ZIS, thus forming an internal electric field at the interface and causing band bending, ultimately achieving Fermi level equilibrium. Based on the above mechanism analysis, this system demonstrates a clear division of labor between ARM and ZIS in photocatalysis, acting as reactive (RP) and reactive (OP) processes, respectively, forming a highly efficient synergistic OP / RP S-type heterojunction photocatalytic system. Under illumination, the two materials generate electron-hole pairs through photon absorption, at which point the internal electric field dominates the charge transfer process of the S-type heterojunction. Low-activity electrons from the ZIS band gap migrate to the valence band of ARM and recombine with low-activity holes in the ARM valence band, ultimately allocating highly active electrons from the ARM band gap for hydrogen production and highly active holes from the ZIS valence band for oxidation. This process effectively suppresses charge matching while maintaining the strong redox capability of charge carriers, becoming the core mechanism for enhancing the photocatalytic performance of ARM / ZIS heterojunctions.

[0052] Based on experimental results and analysis, Figure 7The photocatalytic mechanism of the ARM / ZIS composite system in hydrogen production was investigated. Results showed that the S-shaped heterojunction formed by embedding band structures in ARM and ZIS effectively suppressed the non-productive recombination of photogenerated electrons and holes, thus achieving efficient charge separation and transport. Under sunlight irradiation, photogenerated electrons generated by photoexcitation in the material are ultimately located in the band gap of the ARM via the S-shaped charge transfer path, exhibiting stronger reduction ability and catalyzing the hydrogen production process. + The reduction generates hydrogen gas. At the same time, the highly active holes retained in the valence band of ZIS are rapidly consumed by the sacrificial agent TEOA, preventing photocatalyst photocorrosion caused by hole accumulation, significantly reducing the recombination probability of photogenerated carriers, and greatly improving the photocatalytic performance of the material.

[0053] Therefore, this invention uses CA as an environmentally friendly etchant to alter the surface composition and microstructure of chemically reduced metals, effectively removing amorphous SiO2 components, exposing more active metal oxide sites, and optimizing the electronic band structure, enabling ARM to function as a reduction electrode in an S-type heterojunction for catalytic reactions. Experimental results show that the optimized 4 wt% -ARM / ZIS composite material exhibits a significant hydrogen release rate of 2.59 mmol g. -1 h -1 The efficiency was 3.28 times higher than that of pure ZIS, and it exhibited excellent stability over multiple cycles. Comprehensive structural and electrochemical analyses confirmed the tight interfacial coupling between ARM and ZIS in the S-type heterojunction, which facilitated the formation of the internal electric field and directional charge migration pathways. This invention not only provides a sustainable and value-added utilization pathway for the industrial chemical reduction of metal waste, but also verifies the feasibility of constructing a highly efficient, stable, and low-cost solar-powered hydrogen photocatalyst.

Claims

1. A citric acid-modified red mud-based S-type heterojunction photocatalyst, characterized in that, The photocatalyst is obtained by combining acid-treated red mud ARM and ZIS precursor to form an S-shaped heterojunction structure. The ZIS precursor is a ZnIn2S4 oxidation photocatalytic component. The acid-treated red mud ARM is obtained by acidifying and modifying industrial solid waste red mud RM with citric acid.

2. The citric acid-modified red mud-based S-type heterojunction photocatalyst according to claim 1, characterized in that, The amount of ARM added to acid-treated red mud is 1wt%~5wt% of the ZIS precursor mass.

3. The citric acid-modified red mud-based S-type heterojunction photocatalyst according to claim 1, characterized in that, The acid-treated red mud ARM is prepared by citric acid acidification and complexation modification. It is made by stirring red mud and citric acid solution at room temperature, filtering, washing until neutral, and then drying. The mass ratio of red mud to citric acid is 1:1~6.

4. The citric acid-modified red mud-based S-type heterojunction photocatalyst according to claim 1, characterized in that, The ZIS precursor was obtained by low-temperature hydrothermal synthesis of zinc chloride, InCl3·4H2O, and thioacetamide.

5. A method for preparing the citric acid-modified red mud-based S-type heterojunction photocatalyst according to claim 1, characterized in that, Includes the following steps: (1) Dissolve the red mud in a saturated citric acid solution in a beaker, stir to obtain a uniform suspension, separate and purify to obtain acid-treated red mud RM, wash with water until neutral and dry to obtain acid-treated red mud ARM; (2) Dissolve the acid-treated red mud ARM, zinc chloride, InCl3·4H2O and thioacetamide in water, respectively, and then mix them after ultrasonic treatment and ultrasonic treatment again to obtain a mixture. After hydrothermal reaction, the mixture is cooled, washed and dried to obtain citric acid modified red mud-based S-type heterojunction photocatalyst.

6. The preparation method according to claim 5, characterized in that, In step (1), the mass percentage of the saturated citric acid solution is 37.78%~38.7%; the stirring is carried out at room temperature of 25±2℃ for 3~4 hours.

7. The preparation method according to claim 5, characterized in that, In step (1), the drying process involves drying in a convection oven for more than 24 hours at a temperature of 60-70°C.

8. The preparation method according to claim 5, characterized in that, In step (2), the molar ratio of zinc chloride, InCl3·4H2O and thioacetamide is 1:2:

4.

9. The preparation method according to claim 5, characterized in that, In step (2), the hydrothermal reaction temperature is 170~190℃ and the heating time is 2~3h.

10. The application of the citric acid-modified red mud-based S-type heterojunction photocatalyst of claim 1 in photocatalytic hydrogen production.

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