Composite photocatalyst, preparation method and application thereof

Abstract

The application discloses a composite photocatalyst and a preparation method and application thereof, and belongs to the technical field of inorganic composite materials and energy storage and conversion. SrTiO3-TiO2-CaTiO3 is synthesized through a one-step hydrothermal method, and then a layer of Cu2O particles is loaded on the surface of the SrTiO3-TiO2-CaTiO3 through NaBH4 reduction, so as to prepare a composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O with double p-n heterojunctions and double II-type heterojunctions. The composite photocatalyst has the advantages of simple preparation method, obvious improvement of photoelectric conversion efficiency, and high photocatalytic water decomposition hydrogen production effect, and can be applied to hydrogen production in deionized water and natural water.

Description

Technical Field

[0001] This invention belongs to the field of inorganic composite materials and energy storage and conversion technology, specifically relating to a double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O, its preparation method, and its application in photocatalytic hydrogen production. Background Technology

[0002] In recent years, converting inexhaustible solar energy into clean energy (hydrogen energy) has been considered an effective strategy for improving the human energy structure. Therefore, semiconductor photocatalytic water splitting for hydrogen production, as a green and sustainable technology for converting solar energy into hydrogen energy, has become a research hotspot. However, this technology remains in the laboratory stage, and most studies still use deionized water as the water source. This is because, compared to natural water, deionized water does not contain impurities such as inorganic salts, organic matter, and microorganisms, thus making it easier to achieve good hydrogen production rates and maintain good stability. Clearly, replacing deionized water with natural water is a necessary condition for this technology to move beyond the laboratory, reducing hydrogen production costs and better utilizing natural water resources. It is worth noting that some researchers have already used natural water instead of deionized water and have made some progress. For example, Zhang et al. (Nano Res. 2022, 15, 2013-2022) investigated the feasibility of using Pt / brookite, Pt / anatase, and Pt / rutile TiO2 samples as photocatalysts under the full solar spectrum, using seawater as the water source. Their hydrogen production performance was 1.476, 1.045, and 0.292 mmol g, respectively. -1 h -1 Kistanov's group (Nanoscale 2022, 14, 8601-8610) discovered that (MoS2) could not be synthesized by loading Ni and Ag particles onto the surface. 84 Ag 10 Ni6 catalysts exhibit good catalytic activity in both ultrapure water and river water. However, the high cost of using precious metals as co-catalysts inevitably hinders their large-scale application. Therefore, finding inexpensive, stable, and efficient non-precious metal photocatalysts for utilizing natural water remains a significant challenge for researchers, which will provide numerous opportunities for studying the theory of hydrogen production through water splitting.

[0003] Titanates (MTiO3, M = Mg, Ca, Sr, Ba, etc.) and TiO2 have been widely studied due to their excellent resistance to photocorrosion, high physicochemical stability, non-toxicity, and low cost. Numerous studies have found that single MTiO3 or TiO2 exhibits poor catalytic activity as photocatalysts because photogenerated carrier recombination is a significant challenge in single semiconductor photocatalytic materials. Currently, researchers are attempting to design bi- or even multi-type heterojunctions to achieve rapid separation and directional migration of photogenerated carriers, ultimately leading to higher hydrogen production efficiency. Increasing research indicates that the presence of multiple interfaces in bi- or multi-type heterojunction composites promotes charge separation and directional migration, helping to delay photogenerated carrier recombination and significantly improve photocatalytic efficiency. Therefore, based on band structure, the design and preparation of multi-type heterojunction composites using MTiO3, TiO2, and Cu2O can yield highly efficient photocatalyst candidates for hydrogen production. Forming pn heterojunctions between Cu2O and CaTiO3 (or SrTiO3) and type II heterojunctions between CaTiO3 (SrTiO3) and TiO2 can effectively promote the separation and migration of photogenerated charges. Constructing composite photocatalysts with double pn heterojunctions and double type II heterojunctions using these methods and studying their hydrogen production performance in deionized water and natural water is of great significance. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide a composite photocatalyst and its application in photocatalytic hydrogen production. It is prepared by hydrothermal synthesis of SrTiO3-TiO2-CaTiO3, followed by reduction with NaBH4 to load a layer of Cu2O particles onto its surface. This method features a simple preparation process, significantly improved photoelectric conversion efficiency, and highly efficient photocatalytic water splitting for hydrogen production.

[0005] To achieve the above objectives, the present invention provides the following technical solution: A composite photocatalyst is a double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O.

[0006] A method for preparing a composite photocatalyst, characterized in that: SrTiO3-TiO2-CaTiO3 is synthesized by a one-step hydrothermal method, and then a layer of Cu2O particles is loaded on its surface by NaBH4 reduction, so as to prepare a double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O.

[0007] Furthermore, the preparation method of the composite photocatalyst specifically includes the following steps: I. Preparation of SrTiO3-TiO2-CaTiO3 photocatalyst: First, 2.5–5 mmol SrCl2•6H2O and 2.5–5 mmol CaCl2 were mixed into 30–60 mL of deionized water and stirred magnetically for 5–10 min. Then, 1.7–3.4 mL Ti(C4H9O)4 and 0.4–0.8 g NaOH were added. Optimal mixing was achieved after 1–2 h of magnetic stirring. The sample was then transferred to a 30–100 mL high-pressure reactor and reacted continuously at 180–200 °C for 24–36 h. After the reaction is complete, the sample is washed with 1 to 2 mL of glacial acetic acid to neutralize the pH of the solution. Then, it is repeatedly centrifuged and washed with alcohol solvent and deionized water, and dried at 60 to 80 °C for 10 to 16 h to obtain the SrTiO3-TiO2-CaTiO3 sample, abbreviated as Sr-Ti-Ca.

[0008] II. Preparation of SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst: 30-100 mg SrTiO3-TiO2-CaTiO3, 1-70 mg water-soluble Cu salt, and 10-30 mg PVP were sequentially dissolved in 30-70 mL of aqueous solution containing 10-30 mL of alcohol solvent. The solution was stirred for 3-7 min and then sonicated for 5-15 min. Subsequently, 0.05 g-0.15 g NaBH4 was added to the mixed solution under magnetic stirring in a fume hood, and the mixture was stirred for 3-6 h. Then, it was sonicated for 30-60 min and allowed to stand for 3-6 h. Finally, the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, abbreviated as Sr-Ti-Ca / Cu, was obtained by multiple centrifugations using deionized water and alcohol solvents and drying at 60-80 ℃ for 8-12 h.

[0009] Furthermore, in step two above, the Cu salt can be any one of Cu(COOH)2, CuSO4, CuCl2, or Cu(NO3)2•3H2O.

[0010] Furthermore, in step two above, PVP can be any one of K15, K30, K60, or K90.

[0011] Furthermore, the alcohol solvent can be one of the water-soluble alcohol compounds such as anhydrous ethanol, propanol, butanol, and ethylene glycol.

[0012] Furthermore, the magnetic stirrer speed during the preparation process is 600-800 rpm; the centrifugation speed is 6000-8000 rpm.

[0013] Furthermore, the magnetic stirrer rotates at 700-800 rpm during the preparation process.

[0014] The double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O prepared by this invention can be used to decompose deionized water and natural water to produce hydrogen, and has good hydrogen production performance.

[0015] After loading Cu₂O particles onto the Sr-Ti-Ca surface, the Sr-Ti-Ca / Cu sample showed a response in the UV-Vis region, and expanding the light absorption range is beneficial to improving the photocatalytic hydrogen evolution activity. The photoluminescence intensity of Sr-Ti-Ca / Cu is lower than that of Sr-Ti-Ca, indicating that the recombination rate of photogenerated carriers in Sr-Ti-Ca / Cu is lower than that in Sr-Ti-Ca. This is because a heterojunction is formed between Cu₂O and Sr-Ti-Ca, which inhibits the recombination of photogenerated carrier pairs and is conducive to improving the hydrogen evolution activity.

[0016] This invention, based on band structure, constructs two similar charge (electron and hole) transfer pathways (Cu2O⇄CaTiO3 (SrTiO3)⇄TiO2) in a Sr-Ti-Ca / Cu photocatalyst. These two pathways are relatively complex; for simplicity, we only introduce the charge transfer pathway in CaTiO3-TiO2 / Cu2O (Cu2O⇄CaTiO3⇄TiO2). Under simulated sunlight irradiation, the Sr-Ti-Ca / Cu heterojunction composite photocatalyst initially generates photogenerated electrons and holes. Subsequently, under the combined action of the pn heterojunction (Cu2O and CaTiO3) and the type II heterojunction (CaTiO3 and TiO2), photoelectrons migrate from the conduction band (CB) of Cu2O to the CB of CaTiO3, and finally to the CB of TiO2 (Cu2O→CaTiO3→TiO2), ultimately used to reduce H2O. +H2 is produced. Simultaneously, photogenerated holes migrate from the valence band (VB) of TiO2 to the VB of CaTiO3, and finally to the VB of Cu2O (TiO2→CaTiO3→Cu2O), and are ultimately consumed by the sacrificial reagent (methanol). Similarly, the same charge transfer pathway exists between Cu2O, SrTiO3, and TiO2, except that SrTiO3 replaces CaTiO3. Therefore, double pn heterojunctions and double type II heterojunctions were successfully designed and constructed in the Sr-Ti-Ca / Cu quaternary composite catalyst. The spatial separation of charge carrier pairs effectively suppressed their recombination, thereby enhancing the photocatalytic activity for hydrogen production from water splitting. Since the composite photocatalyst Sr-Ti-Ca / Cu has strong light absorption in the ultraviolet-visible region, it can enhance photoelectric conversion performance, making it promising for applications in the field of solar cells.

[0017] This invention introduces a novel Sr-Ti-Ca / Cu composite photocatalyst with dual pn heterojunctions and dual type II heterojunctions, which improves photocatalytic efficiency in two ways: 1. After uniformly loading Cu2O particles onto the Sr-Ti-Ca surface, the light absorption range of the Sr-Ti-Ca / Cu2O composite photocatalyst extends to the entire ultraviolet-visible region, thus enhancing its photocatalytic hydrogen production activity. 2. The dual pn heterojunctions and type II heterojunctions of the Sr-Ti-Ca / Cu composite photocatalyst effectively promote the separation of photogenerated carriers and delay their recombination, thereby improving its photocatalytic hydrogen production activity. Furthermore, we tested the hydrogen production rate of this catalyst in deionized water and natural water (tap water, Tianyi Lake water (Nanchang Aviation University), and Ganjiang River water (Nanchang City, Jiangxi Province)), both showing good hydrogen production performance, indicating that it can be used as a candidate photocatalyst for water splitting to produce hydrogen. In addition, the photoelectric material of the present invention has the advantages of simple preparation method, significantly improved photoelectric conversion efficiency and high efficiency of photocatalytic water splitting to produce hydrogen. Attached Figure Description

[0018] Figures 1a-1c The images show actual photographs of the Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts used in Example 1, respectively. Figure 2 The X-ray diffraction patterns of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1 are shown below. Figures 3a-3c The images are scanning electron microscope (SEM) images of the Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1, respectively. Figure 4 The bar charts show the hydrogen production rates of the photocatalysts Cu2O, Sr-Ti-Ca, Sr-Ti-Ca / Cu-S, Sr-Ti-Ca / Cu-C, Sr-Ti-Ca / Cu-N, and Sr-Ti-Ca / Cu-Cl in Examples 1, 2, 3, and 4. Figure 5 The bar chart shows the hydrogen production rates of the Sr-Ti-Ca / Cu-Cl-1, Sr-Ti-Ca / Cu-Cl-2, Sr-Ti-Ca / Cu-Cl-6, Sr-Ti-Ca / Cu-Cl-7.4, Sr-Ti-Ca / Cu-Cl-10, Sr-Ti-Ca / Cu-Cl-20, and Sr-Ti-Ca / Cu-Cl-50 photocatalysts in Examples 1, 5, 6, 7, 8, 9, and 10. Figure 6 The graphs show the hydrogen production rates of the composite photocatalyst Sr-Ti-Ca / Cu-Cl-6 in Example 11, which decomposes natural water (tap water, Tianyi Lake water, and Ganjiang River water). Figure 7 The images show the UV-Vis diffuse reflectance spectra of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1, respectively. Figure 8 The photoelectric response curves of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts under UV-Vis irradiation are shown in Example 1. Detailed Implementation

[0019] The technical solution of the present invention will be described in more detail below with reference to specific embodiments, but the embodiments do not constitute a limitation on the present invention.

[0020] Example 1 1) Preparation of Cu2O photocatalyst 50 mg of anhydrous CuCl2 and 20 mg of PVP (K30) were dissolved in 50 mL of an aqueous solution containing 15 mL of ethylene glycol. The mixture was stirred for 5 min and then sonicated for 10 min. Subsequently, 0.1 g of NaBH4 was added to the above solution in a fume hood with magnetic stirring. The mixture was stirred for 4 h, then sonicated for 30 min and allowed to stand for 4 h. The mixture was then washed multiple times by centrifugation with deionized water and anhydrous ethanol, and dried at 60 °C for 12 h to obtain Cu2O particles.

[0021] 2) Preparation of SrTiO3-TiO2-CaTiO3 photocatalyst First, 2.5 mmol SrCl2•6H2O and 2.5 mmol CaCl2 were dissolved in 30 mL of deionized water and magnetically stirred for 10 min. Then, 1.7 mL of [Ti(C4H9O)4] and 0.4 g of NaOH were added to the solution. After magnetic stirring for another 1 h, the white suspension was transferred to a 50 mL high-pressure reactor and hydrothermally treated at 180 ℃ for 24 h. After the reaction was completed, the reactor was allowed to cool naturally to room temperature. The white precipitate was then washed with 1 mL of glacial acetic acid until neutral, and finally washed multiple times by centrifugation with deionized water and anhydrous ethanol. The precipitate was then dried at 60 ℃ for 12 h to obtain SrTiO3-TiO2-CaTiO3, abbreviated as Sr-Ti-Ca.

[0022] 3) Preparation of SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst 50 mg Sr-Ti-Ca, 6 mg anhydrous CuCl2, and 20 mg PVP were dissolved sequentially in 50 mL of an aqueous solution containing 15 mL of ethylene glycol. The mixture was stirred for 5 min and then sonicated for 10 min. Subsequently, 0.1 g NaBH4 was added to the above solution in a fume hood with magnetic stirring. The mixture was stirred for 4 h, sonicated for 30 min, and allowed to stand for 4 h. The solution was then washed multiple times by centrifugation with deionized water and anhydrous ethanol, and dried at 60 °C for 12 h to obtain SrTiO3-TiO2-CaTiO3 / Cu2O, abbreviated as Sr-Ti-Ca / Cu-Cl.

[0023] Figure 1 shows photographs of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1, respectively. After Cu2O is loaded onto the surface of the Sr-Ti-Ca sample, the Sr-Ti-Ca / Cu-Cl composite photocatalyst turns green and can absorb ultraviolet-visible light.

[0024] Figure 2 shows the X-ray diffraction patterns of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1, indicating that SrTiO3-TiO2-CaTiO3 was successfully synthesized by a one-step hydrothermal method, and then the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst was successfully designed by the NaBH4 reduction method.

[0025] Figure 3a, b, and c are scanning electron microscope (SEM) images of the Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1. Figure 3c shows that Cu2O is uniformly distributed on the Sr-Ti-Ca surface, which is beneficial for increasing the photocatalytic hydrogen production rate from water splitting.

[0026] Figure 4 shows the hydrogen production rates of the Cu₂O, Sr-Ti-Ca, Sr-Ti-Ca / Cu-S, Sr-Ti-Ca / Cu-C, Sr-Ti-Ca / Cu-N, and Sr-Ti-Ca / Cu-Cl photocatalysts in Examples 1, 2, 3, and 4. Under the same copper content, Sr-Ti-Ca / Cu-Cl exhibited the best photocatalytic water splitting performance, with a hydrogen production rate of 4.564 mmol g. -1 h -1 Therefore, we selected Sr-Ti-Ca / Cu-Cl as a candidate photocatalyst for further research.

[0027] Figure 5 shows the hydrogen production rate histograms of the Sr-Ti-Ca / Cu-Cl-1, Sr-Ti-Ca / Cu-Cl-2, Sr-Ti-Ca / Cu-Cl-6, Sr-Ti-Ca / Cu-Cl-7.4, Sr-Ti-Ca / Cu-Cl-10, Sr-Ti-Ca / Cu-Cl-20, and Sr-Ti-Ca / Cu-Cl-50 photocatalysts in Examples 1, 5, 6, 7, 8, 9, and 10. With increasing Cu content, the hydrogen production rate of the Sr-Ti-Ca / Cu-Cl composite photocatalyst first increases and then decreases. When the mass of CuCl2 is 6 mg, the Sr-Ti-Ca / Cu-Cl-6 sample exhibits the highest hydrogen production activity at 5.088 mmol g. -1 h -1 Approximately Sr-Ti-Ca (0.026 mmol g) -1 h -1 The sample was 195.7 times larger, indicating that the double pn heterojunction and double type II heterojunction composite photocatalyst composed of SrTiO3, TiO2, CaTiO3 and Cu2O can greatly improve the photocatalytic hydrogen production rate.

[0028] Figure 6 is a line graph showing the hydrogen production rate of the composite photocatalyst Sr-Ti-Ca / Cu-Cl-6 in Example 11, which decomposes natural water (tap water, Tianyi Lake water (Nanchang Aviation University), and Ganjiang River water (Nanchang City, Jiangxi Province)). The hydrogen production rates of tap water, lake water, and river water reached 0.692, 0.530, and 0.647 mmol g, respectively.-1 h -1 This indicates that the composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O has high practicality in natural water sources.

[0029] Figure 7 shows the UV-Vis diffuse reflectance spectra of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts in Example 1. The light absorption range of Sr-Ti-Ca / Cu-Cl extends into the UV-Vis region compared to Sr-Ti-Ca. This expanded light absorption range is beneficial for improving the photocatalytic hydrogen production rate.

[0030] Figure 8 shows the photoelectric response curves of Cu2O, Sr-Ti-Ca, and Sr-Ti-Ca / Cu-Cl photocatalysts under UV-Vis irradiation in Example 1. Among the three samples, Sr-Ti-Ca / Cu-Cl had the highest photocurrent intensity, indicating that the tight phase interface between Cu2O and Sr-Ti-Ca can enhance the separation and migration rate of photoinduced charges, thereby improving catalytic performance.

[0031] Example 2 Similar to the steps in Example 1, except that in Example 2, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we replaced CuCl2 with CuSO4 of the same copper content to obtain the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-S.

[0032] Example 3 Similar to the steps in Example 1, except that in Example 3, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we replaced CuCl2 with Cu(CH3COO)2 of the same copper content to obtain the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-C.

[0033] Example 4 Similar to the steps in Example 1, except that in Example 4, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we replaced CuCl2 with Cu(NO3)2•3H2O of the same copper content to obtain the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-N.

[0034] Example 5 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 1 mg, resulting in the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-1.

[0035] Example 6 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 2 mg, resulting in the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-2.

[0036] Example 7 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 7.4 mg, resulting in the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-7.4.

[0037] Example 8 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 10 mg, resulting in the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-10.

[0038] Example 9 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 20 mg, and obtained the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-20.

[0039] Example 10 Similar to the steps in Example 1, except that in Example 5, when preparing the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, we changed the mass of CuCl2 from 6 mg to 50 mg, resulting in the SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst Sr-Ti-Ca / Cu-Cl-50.

[0040] Example 11 Similar to the steps in Example 1, the difference was that in the photocatalytic water splitting for hydrogen production experiment, the same volume of tap water, Tianyi Lake water, and Ganjiang River water were used instead of deionized water. To investigate the effects of inorganic salt ions and organic pollutants on the hydrogen production rate, based on the results of 42 routine water quality monitoring tests conducted by Nanchang Water Industry Group Co., Ltd. in February 2023, it was found that chloride ions and calcium in tap water... 2+ SO4 2- NO3 - The content of inorganic salt ions was relatively high. Therefore, we selected the following inorganic salt ions and organic pollutants for experiments. The study found (Table 1) that, except for CaCl2 (which enhanced the rate by 5.7%), almost all inorganic salts and organic pollutants reduced the hydrogen production rate. Cu(NO3)2•3H2O had the greatest impact on the hydrogen production rate, reducing it by approximately 86.3%. The results indicate that NO3... - and SO4 2- These two ions have a significant negative impact on hydrogen production rates. Therefore, if fresh water is chosen as the water source, these two ions should be removed to the greatest extent possible.

[0041] .

Claims

1. A composite photocatalyst, characterized in that: The composite photocatalyst is a double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O; SrTiO3-TiO2-CaTiO3 was synthesized via a one-step hydrothermal method, and then a layer of Cu2O particles was loaded onto its surface by NaBH4 reduction, thus preparing a double pn heterojunction and double type II heterojunction composite photocatalyst SrTiO3-TiO2-CaTiO3 / Cu2O.

2. The method for preparing a composite photocatalyst according to claim 1, characterized in that, Specifically, the steps include: I. Preparation of SrTiO3-TiO2-CaTiO3 photocatalyst: First, 2.5–5 mmol SrCl2•6H2O and 2.5–5 mmol CaCl2 were mixed into 30–60 mL of deionized water and magnetically stirred for 5–10 min. Then, 1.7–3.4 mL Ti(C4H9O)4 and 0.4–0.8 g NaOH were added. After 1–2 h of magnetic stirring to achieve optimal mixing, the sample was transferred to a 30–100 mL high-pressure reactor and reacted continuously at 180–200 °C for 24–36 h. After the reaction was complete, the sample was washed with 1–2 mL glacial acetic acid to neutralize the pH. Then, the sample was repeatedly washed by centrifugation with alcohol solvent and deionized water, and subjected to a reaction at 60–80 °C for 10–16 minutes. After drying for h, a SrTiO3-TiO2-CaTiO3 sample, abbreviated as Sr-Ti-Ca, can be obtained; II. Preparation of SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst: 30-100 mg of SrTiO3-TiO2-CaTiO3, 1-70 mg of water-soluble Cu salt, and 10-30 mg of PVP were sequentially dissolved in 30-70 mL of aqueous solution containing 10-30 mL of alcohol solvent. The solution was stirred for 3-7 min and then sonicated for 5-15 min. Subsequently, 0.05 g-0.15 g of NaBH4 was added to the mixed solution under magnetic stirring in a fume hood, and the mixture was stirred for 3-6 h. Following this, it was sonicated for 30-60 min and then allowed to stand for 3-6 h. Finally, the mixture was centrifuged multiple times using deionized water and an alcohol solvent, and then subjected to a reaction at 60-80 °C for 8-12 h. The SrTiO3-TiO2-CaTiO3 / Cu2O composite photocatalyst, abbreviated as Sr-Ti-Ca / Cu, was obtained by drying.

3. The method for preparing a composite photocatalyst according to claim 2, characterized in that, In step two above, the Cu salt can be any one of Cu(COOH)2, CuSO4, CuCl2, or Cu(NO3)2•3H2O.

4. The method for preparing a composite photocatalyst according to claim 2, characterized in that: In step two above, PVP can be any one of K15, K30, K60, or K90.

5. The method for preparing a composite photocatalyst according to claim 2, characterized in that: The alcohol solvent is one of the following: anhydrous ethanol, propanol, butanol, and ethylene glycol.

6. The method for preparing a composite photocatalyst according to claim 2, characterized in that: During the preparation process, the magnetic stirrer speed is 600 ~ 800 rpm; the centrifugation speed is 6000 ~ 8000 rpm.

7. The application of the composite photocatalyst according to claim 1 or the composite photocatalyst prepared by any one of claims 2-6 in the decomposition of deionized water or natural water to produce hydrogen.

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