BiOF-Bi2Fe4O9 S-type heterojunction material as well as preparation and application of BiOF-Bi2Fe4O9 S-type heterojunction material

Abstract

The invention discloses a BiOF-Bi2Fe4O9S type heterojunction material and preparation and application thereof.The preparation method comprises the steps that bismuth salt and fluoride are dissolved in a solvent, a bismuth salt solution and a fluoride solution are sequentially mixed and then subjected to a hydrothermal reaction, and a bismuth oxyfluoride precursor BiOF solution is obtained; and dropwise adding ferric salt into the bismuth oxyfluoride precursor BiOF solution, and adding the precipitate into an alkaline reaction solvent for hydrothermal reaction, thereby obtaining the BiOF-coated Bi2Fe4O9S type heterojunction composite material. The crystal structure is changed by adjusting the pH and hydrothermal temperature of the hydrothermal reaction solvent, so that the metal atom utilization efficiency and photocatalytic activity are improved, and the efficiency of photocatalytic reduction of CO2 is improved. The material is easy to separate and recycle, still keeps efficient catalytic performance after being recycled for multiple times, and has high environmental compatibility and low secondary pollution risk.

Description

Technical Field

[0001] The present invention relates to the field of photocatalytic materials, and in particular to a preparation method of a BiOF@Bi2Fe4O9 S-type heterojunction composite material and application thereof in photocatalytic CO2 reduction. Background Art

[0002] With the acceleration of industrialization and urbanization, the extensive use of fossil fuels has led to the emission of a large amount of carbon dioxide (CO2), the greenhouse gas effect has intensified, and the problem of climate change has become increasingly serious. How to effectively reduce CO2 emissions and alleviate the greenhouse effect has become a global focus. In recent years, photocatalytic technology has attracted widespread attention because it can use solar energy to convert CO2 into valuable fuels and chemicals.

[0003] Photocatalytic reduction of CO2 technology is a process that simulates natural photosynthesis. Photocatalysts absorb sunlight, convert light energy into chemical energy, and reduce CO2 to useful products such as hydrocarbons (such as methane and methanol). However, there are still some problems in the practical application of photocatalysts, such as narrow light absorption range, low separation efficiency of photogenerated electrons and holes, and insufficient catalytic activity and stability.

[0004] Common photocatalytic materials include titanium dioxide (TiO2), zinc oxide (ZnO), cadmium sulfide (CdS), etc., but these materials have the disadvantages of wide band gap, mainly absorbing ultraviolet light, easy recombination of photogenerated carriers, low photocatalytic efficiency, etc. In recent years, bismuth-based materials such as Bi2WO6 and BiVO4 have become a hot topic of research due to their narrow band gap, high photocatalytic activity and good stability.

[0005] Bi2Fe4O9 (BFO) is a new type of bismuth-based photocatalytic material with good photocatalytic performance, but its absorption in the visible light region and the separation efficiency of photogenerated carriers still need to be improved. Summary of the invention

[0006] The technical problem to be solved by the present invention is to overcome the deficiencies of the prior art. The purpose of the present invention is to provide a BiOF@Bi2Fe4O9 S-type heterojunction material and a preparation method thereof, and to change the crystal structure by adjusting the pH and hydrothermal temperature of the hydrothermal reaction solvent, thereby improving the metal atom utilization efficiency and photocatalytic activity thereof. At the same time, the endogenous electric field at the S-type heterojunction interface helps to separate and transport photogenerated carriers, thereby improving the efficiency of photocatalytic reduction of CO2. This catalyst is easy to separate and recycle, and maintains efficient catalytic performance after multiple cycles of use, and has high environmental compatibility and low secondary pollution risk.

[0007] The present invention is achieved through the following technical solutions.

[0008] In one aspect, the present invention provides a method for preparing a BiOF@Bi2Fe4O9 S-type heterojunction material, comprising the following steps:

[0009] a. Preparation of bismuth oxyfluoride precursor BiOF:

[0010] preparing a bismuth salt solution, a fluoride solution and an iron salt solution;

[0011] Slowly drop the bismuth salt solution into the bismuth salt solution according to the mass ratio (2-4): (6-8), stir, heat to react, let stand, and naturally cool to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution;

[0012] b. Preparation of S-type heterojunction materials:

[0013] Slowly drop the iron salt solution into the bismuth oxyfluoride precursor BiOF solution in a mass ratio of (8-20): (1-2), mix thoroughly, adjust to strong alkalinity with an alkaline solvent, and stir slowly to obtain a mixed solution;

[0014] Collect the precipitate in the mixed solution by centrifugation, add the precipitate into an alkaline reaction solvent in a mass ratio of (1-2):(5-10), and perform hydrothermal reaction after ultrasonic stirring;

[0015] The reactants were collected by centrifugation, washed repeatedly and dried at a constant temperature to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material.

[0016] Preferably, the bismuth salt is one of bismuth nitrate, bismuth chloride and bismuth sulfate; the bismuth salt solution solvent is water, ethanol, ethylene glycol or hydrochloric acid.

[0017] Preferably, the fluoride is sodium fluoride or potassium fluoride; the fluoride solution solvent is water, hydrochloric acid or hydrofluoric acid.

[0018] Preferably, the iron salt is ferric nitrate, ferric sulfate or ferric chloride; and the iron salt solution solvent is water or ethanol.

[0019] Preferably, the concentration of the fluoride solution is 1-3 g / L; the concentration of the bismuth salt solution is 5-10 g / L; and the concentration of the iron salt solution is 15-30 g / L.

[0020] Preferably, the alkaline solvent is aqueous ammonia or sodium hydroxide.

[0021] Preferably, the reaction solvent is water, ethanol or ethylene glycol.

[0022] Preferably, in step b, the pH is adjusted to 10-11 and stirred slowly for 80-100 minutes.

[0023] Preferably, in step b, after ultrasonic stirring, hydrothermal reaction is carried out at 160-180° C. for 40-60 hours; and constant temperature drying is carried out at 80-90° C. for 10-12 hours.

[0024] Another aspect of the present invention provides a BiOF@Bi2Fe4O9 S-type heterojunction composite material prepared by the method.

[0025] The BiOF@Bi2Fe4O9 S-type heterojunction composite material prepared by the method of the present invention is used in photocatalytic CO2 reduction. The composite material can convert CO2 into CO and CH4 in a photocatalytic reaction.

[0026] The present invention adopts the above technical solution, which has the following beneficial effects:

[0027] 1. The present invention adjusts the crystal structure of bismuth ferrite by adjusting the pH of the hydrothermal reaction solvent to produce bismuth ferrite (Bi2Fe4O9) with a mullite structure, further improving the metal atom utilization efficiency of bismuth ferrite and improving the catalytic activity of the catalyst.

[0028] 2. The present invention compounds Bi2Fe4O9 and BiOF through a heterojunction structure, and compounds the two bismuth-based catalysts Bi2Fe4O9 and BiOF through an S-type heterojunction, which can utilize the synergistic effect of the two to improve the photocatalytic performance; while improving the catalyst's response to visible light, a unique interface electric field is achieved through the S-type heterojunction structure, and the endogenous electric field of the S-type heterojunction interface can further improve the separation and transmission performance of photogenerated carriers, which can effectively promote the separation of photogenerated electrons and holes, thereby improving its photocatalytic reduction efficiency of CO2.

[0029] 3. The catalyst of the present invention is easy to separate and recycle, can be repeatedly recycled, and still has a high catalytic efficiency after multiple cycles.

[0030] 4. The catalyst of the present invention has high environmental compatibility and low metal leaching rate during use, thereby reducing the risk of secondary pollution. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings described herein are used to provide a further understanding of the present invention, constitute a part of the present application, and do not constitute an improper limitation of the present invention. In the drawings:

[0032] Figure 1 The scanning electron microscope (SEM) images of (a) BOF, (b) BFO and (c) BB in Example 2 of the present invention are shown;

[0033] Figure 2Elemental energy spectrum scanning (EDS-mapping) of (a) BOF, (b) BFO and (c) BB in Example 2 of the present invention;

[0034] Figure 3 (a) X-ray diffraction pattern (XRD) and (b) Fourier transform infrared spectrum (FT-IR) of BOF, BFO and BB in Example 2 of the present invention;

[0035] Figure 4 (a) XPS broad spectrum, (b) Bi 4f, (c) F 1s and (d) Fe2p fine spectrum of BOF, BFO and BB in Example 2 of the present invention;

[0036] Figure 5 is the yield of CO and CH4 during the photocatalytic reduction of CO2 under visible light by BOF, BFO and different proportions of BB in Example 3 of the present invention;

[0037] Figure 6 This is a transient photocurrent curve of 1:8BB under visible light in Example 3 of the present invention. DETAILED DESCRIPTION

[0038] The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments. The exemplary embodiments and descriptions of the present invention are used to explain the present invention but are not intended to limit the present invention.

[0039] The method for preparing the BiOF@Bi2Fe4O9 S-type heterojunction composite material provided in an embodiment of the present invention comprises the following steps:

[0040] Step A. Preparation of bismuth oxyfluoride precursor BiOF:

[0041] a. Dissolve bismuth salts of bismuth nitrate, bismuth chloride and bismuth sulfate in a solvent to prepare a bismuth salt solution having a mass concentration of 5-10 g / L, wherein the solvent is water, ethanol and ethylene glycol or hydrochloric acid, and stir after ultrasonication to fully dissolve the bismuth salt solution to obtain a bismuth salt solution A;

[0042] b. adding a fluoride source of sodium fluoride or calcium fluoride to a solvent to prepare a fluoride solution with a mass concentration of 1-3 g / L, wherein the solvent is water, hydrochloric acid or hydrofluoric acid, and stirring until dissolved after ultrasonic treatment to obtain a fluoride solution B;

[0043] c. dissolving an iron salt of ferric nitrate, ferric sulfate or ferric chloride in a solvent to prepare an iron salt solution with a mass concentration of 15-30 g / L, wherein the solvent is water or ethanol, and stirring by ultrasonication until dissolved to obtain an iron salt solution C;

[0044] d. Slowly add the fluoride solution B to the uniformly stirred bismuth salt solution A in a mass ratio of (2-4): (6-8), stir, heat to react, let stand, and naturally cool to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution.

[0045] Step B, preparing S-type heterojunction material:

[0046] e. Slowly add the iron salt solution dropwise to the bismuth oxyfluoride precursor BiOF solution in a mass ratio of (8-20): (1-2), and after thorough mixing, adjust the mixed solution to a pH of 10-11 with an alkaline solvent, wherein the alkaline solvent is aqueous ammonia or NaOH solution, and stir slowly for 80-100 min to obtain a mixed solution;

[0047] f. Collect the precipitate in the mixture by centrifugation, add the precipitate to a 10-20 mol / L NaOH or KOH alkaline reaction solvent according to a mass ratio of (1-2): (5-10), and perform a hydrothermal reaction at 160-180°C for 40-60 hours after ultrasonic stirring;

[0048] g. Collect the reactants by centrifugation, rinse them repeatedly with ethanol and deionized water, and dry them at 80-90°C for 10-12 hours to obtain the BiOF@Bi2Fe4O9 S-type heterojunction composite material.

[0049] The present invention dissociates bismuth nitrate in solution to generate bismuth ions (Bi 3+ ), and at the same time, fluoride dissociates to produce fluoride ions (F - ); secondly, bismuth ions and fluoride ions react synergistically with oxygen molecules in the solution to form a precursor of bismuth oxyfluoride. As the reaction proceeds, bismuth oxyfluoride gradually crystallizes to form a precursor structure with a regular structure. 3+ After the source is added, it can react with excess Bi ions, and generate Bi2Fe4O9 under the action of oxygen molecules in the solution; by adjusting the pH conditions of the reaction solvent, it can generate Bi2Fe4O9 with a mullite structure near the interface of the BiOF precursor. Further, by adjusting the hydrothermal temperature of the solution, the effect of the solvent molecules can be overcome to allow the crystal to grow slowly in the homogeneous medium, so that the interface of BiOF and Bi2Fe4O9 is fully in contact and an effective charge separation zone is formed, thereby improving the separation of photogenerated electrons and holes in the two materials. Specifically, the p-type material BiOF absorbs photons to generate electron-hole pairs, and the electrons are transmitted to the n-type material Bi2Fe4O9 through the conduction band, retaining the holes in the p-type material. Due to the asymmetry of the two materials, an S-type heterojunction structure is formed on their surfaces, which reduces the recombination efficiency of photogenerated carriers and enables the carriers to effectively participate in the photocatalytic reaction.

[0050] The preparation of BiOF@Bi2Fe4O9 S-type heterojunction material according to the present invention is further described below through specific examples.

[0051] Example 1

[0052] a. Dissolve 5 g of Bi(NO3)3·5H2O in 1 L of ethylene glycol solution, ultrasonicate for 30 min, and stir on a constant temperature magnetic stirrer for 30 min until fully dissolved to obtain a Bi(NO3)3 solution A with a mass concentration of 5 g / L;

[0053] b. Dissolve 2 mg of NaF in 1 L of deionized water, sonicate for 30 min, and stir until dissolved to obtain a NaF solution B with a mass concentration of 2 g / L;

[0054] c. Dissolve 25 mg of Fe(NO3)3·9H2O in 1 L of deionized water, ultrasonicate for 30 min, and stir thoroughly until dissolved to obtain Fe(NO3)3 solution C with a mass concentration of 25 g / L.

[0055] d. Under constant stirring, slowly add 2 parts of NaF solution B to 7 parts of Bi(NO3)3 solution A according to the mass ratio. After thorough mixing, heat the reaction at 180°C for 24 hours, let it stand, and naturally cool to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution.

[0056] e. Slowly add 10 parts of Fe(NO3)3 solution C to 5 parts of the precursor BiOF solution according to the mass ratio. After fully mixing, adjust the mixed solution to pH = 10 with ammonia water and stir slowly for 80 minutes to obtain a mixed solution.

[0057] f. The mixture was centrifuged at 5000 rpm for 10 min to collect the precipitate, and the precipitate collected by centrifugation was added into a 15 mol / L NaOH solution at a mass ratio of 2:7, stirred to make it evenly dispersed, and then transferred to a high-pressure reactor with a polytetrafluoroethylene liner and subjected to hydrothermal reaction at 180°C for 48 hours.

[0058] g. After the reaction is completed, the high-pressure reactor is allowed to stand, and after naturally cooling to room temperature, the obtained solid is collected by centrifugation. The solid is repeatedly rinsed with ethanol and deionized water to remove the unreacted precursor substances. After rinsing, it is transferred to a constant temperature drying oven at 80°C and dried for 12 hours to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material (1:2BB).

[0059] Example 2

[0060] a. Dissolve 6 g of BiCl3 in 1 L of ethanol solution, ultrasonicate for 30 min, and stir on a constant temperature magnetic stirrer for 30 min until fully dissolved to obtain a BiCl3 solution A with a mass concentration of 6 g / L;

[0061] b. Dissolve 1g of CaF2 in 1L of hydrofluoric acid, sonicate for 30min, and stir until dissolved to obtain a CaF2 solution B with a mass concentration of 1g / L;

[0062] c. Dissolve 20g of Fe2(SO4)3 in 1L of deionized water, ultrasonicate for 30min, and stir thoroughly until dissolved to obtain Fe2(SO4)3 solution C with a mass concentration of 20g / L.

[0063] d. Under constant stirring, 3 parts of CaF2 solution B were slowly added to 7 parts of BiCl3 solution A according to the mass ratio. After thorough mixing, the mixture was heated at 180°C for 40 hours, allowed to stand, and naturally cooled to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution.

[0064] e. Slowly add 8 parts of Fe2(SO4)3 solution C to 2 parts of precursor BiOF solution according to the mass ratio. After fully mixing, adjust the mixed solution to pH = 11 with NaOH and stir slowly for 90 minutes to obtain a mixed solution.

[0065] f. The mixture was centrifuged at 8000 rpm for 10 min to collect the precipitate, and the precipitate collected by centrifugation was added to a 10 mol / L KOH solution at a mass ratio of 1:6, stirred to make it evenly dispersed, and then transferred to a high-pressure reactor with a polytetrafluoroethylene liner and heated at 180°C for 40 hours.

[0066] g. After the reaction is completed, the high-pressure reactor is allowed to stand, and after naturally cooling to room temperature, the obtained solid is collected by centrifugation. The solid is repeatedly rinsed with ethanol and deionized water to remove the unreacted precursor substances. After rinsing, it is transferred to a constant temperature drying oven at 90°C and dried for 10 hours to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material (1:4BB).

[0067] The scanning electron microscope (SEM) images of (a) BOF, (b) BFO and (c) BB in this example are shown in Figure 1 As shown; the element energy spectrum scanning (EDS-mapping) of this embodiment (a) BOF, (b) BFO and (c) BB is shown in Figure 2 In this embodiment, (a) X-ray diffraction pattern (XRD) and (b) Fourier transform infrared spectrum (FT-IR) of BOF, BFO and BB are shown in Figure 3In this embodiment, the detailed spectra of (a) XPS broad spectra, (b) Bi 4f, (c) F 1s and (d) Fe 2p of BOF, BFO and BB are shown in FIG. Figure 4 shown.

[0068] Example 3

[0069] a. Dissolve 8 g of Bi2(SO4)3 in 1 L of dilute hydrochloric acid solution, ultrasonicate for 30 min, and stir on a constant temperature magnetic stirrer for 30 min until fully dissolved to obtain a Bi2(SO4)3 solution A with a mass concentration of 8 g / L;

[0070] b. Dissolve 3 g of NaF in 1 L of deionized water, sonicate for 30 min, and stir until dissolved to obtain a NaF solution B with a mass concentration of 3 g / L;

[0071] c. Dissolve 15 g of FeCl3 in 1 L of deionized water, ultrasonicate for 30 min, and stir thoroughly until dissolved to obtain a FeCl3 solution C with a mass concentration of 15 g / L.

[0072] d. Under constant stirring, 4 parts of NaF solution B were slowly added to 6 parts of Bi2(SO4)3 solution A according to the mass ratio. After thorough mixing, the mixture was heated at 175°C for 60 hours, allowed to stand, and naturally cooled to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution.

[0073] e. Slowly add 8 parts of FeCl3 solution C to 1 part of the precursor BiOF solution according to the mass ratio, mix thoroughly, adjust the mixed solution to pH = 10 with ammonia water, stir slowly for 85 minutes, and obtain a mixed solution.

[0074] f. The mixed solution was centrifuged at 8000 rpm for 10 min to collect the precipitate, and the precipitate collected by centrifugation was added to a 20 mol / L NaOH solution at a mass ratio of 1.5:5, stirred to make it evenly dispersed, and then transferred to a high-pressure reactor with a polytetrafluoroethylene liner and heated at 175°C for 60 hours.

[0075] g. After the reaction is completed, the high-pressure reactor is allowed to stand, and after naturally cooling to room temperature, the obtained solid is collected by centrifugation. The solid is repeatedly rinsed with ethanol and deionized water to remove the unreacted precursor substances. After rinsing, it is transferred to a constant temperature drying oven at 85°C and dried for 10 hours to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material (1:8BB).

[0076] Example 4

[0077] a. Dissolve 10 g of BiCl3 in 1 L of ethanol solution, sonicate for 30 min, and stir on a constant temperature magnetic stirrer for 30 min until fully dissolved to obtain a BiCl3 solution A with a mass concentration of 10 g / L;

[0078] b. Dissolve 1.5 g of NaF in 1 L of deionized water, sonicate for 30 min, and stir until dissolved to obtain a NaF solution B with a mass concentration of 1.5 g / L;

[0079] c. Dissolve 30g of Fe2(SO4)3 in 1L of deionized water, ultrasonicate for 30min, and stir thoroughly until dissolved to obtain Fe2(SO4)3 solution C with a mass concentration of 30g / L.

[0080] d. Under constant stirring, 3 parts of NaF solution B were slowly added to 8 parts of BiCl3 solution A according to the mass ratio. After thorough mixing, the mixture was heated at 170°C for 40 hours, allowed to stand, and naturally cooled to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution.

[0081] e. According to the mass ratio, 20 parts of Fe2(SO4)3 solution C were slowly added to 2 parts of the precursor BiOF solution. After thorough mixing, the mixed solution was adjusted to pH = 11 with NaOH and stirred slowly for 100 minutes to obtain a mixed solution.

[0082] f. The mixed solution was centrifuged at 8000 rpm for 10 min to collect the precipitate, and the precipitate collected by centrifugation was added to 18 mol / L KOH solution at a mass ratio of 2:9, stirred to make it evenly dispersed, and then transferred to a high-pressure reactor with a polytetrafluoroethylene liner and heated at 160°C for 40 hours.

[0083] g. After the reaction is completed, the high-pressure reactor is allowed to stand, and after naturally cooling to room temperature, the obtained solid is collected by centrifugation. The solid is repeatedly rinsed with ethanol and deionized water to remove the unreacted precursor substances. After rinsing, it is transferred to a constant temperature drying oven at 90°C and dried for 11 hours to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material (1:10BB).

[0084] The performance of the composite material prepared by the present invention is further illustrated by experiments below.

[0085] Test Example 1

[0086] In the photocatalytic reaction device CEL-SPH2N-D9 (CeAulight, China), the photothermal effect was prevented by a condensed water circulating water filter (5°C) to verify the photocatalytic reduction of CO2. Each sample (5 mg) of Examples 1-5 was dispersed in triethanolamine (2 mL) as a sacrificial agent, ultrasonically treated to produce a suspension, and transferred to a reaction cell with a diameter of 4 cm. Ultrapure CO2 gas (80 kPa, 99.99999%) was injected into the vacuum reaction system. Photocatalytic experiments were carried out under a 300W xenon lamp using an AM 1.5G filter. The gas products were detected and quantified by an online gas chromatograph (GC-9720Plus) equipped with an FID detector. Figure 5 The yields of CO and CH4 produced by catalytic reduction of CO2 by different photocatalysts obtained in Example 3 under visible light are as follows: Figure 5 It can be seen that after BOF and BFO are compounded, the ability of the composite material to convert CO2 into CO and CH4 is rapidly improved. Among them, 1:8BB has the highest CO generation yield, reaching 39.69μmol / g.

[0087] Test Example 2

[0088] Figure 6 Under the experimental conditions of Experimental Example 1, a transient photocurrent test was performed on the 1:8BB of Example 3. Figure 6 It can be seen that under illumination conditions, 1:8BB can generate a relatively high photogenerated current. In addition, its photocurrent response shows a relatively stable periodicity, which further verifies that the 1:8BB composite material prepared in Example 3 has a strong photocatalytic activity.

[0089] The following is a comparison of the performance tests of the embodiments of the present invention and the comparative examples.

[0090] Table 1 Performance comparison

[0091] project Example 1 Example 2 Example 3 Example 4 CO yield, umol / g 34.67 36.42 39.69 37.62 <![CDATA[CH4 production rate, umol / g]]> 18.65 18.01 19.23 16.23

[0092] As can be seen from Table 1 above, the composite material prepared by the present invention has an S-type heterojunction formed at its interface, which can promote the separation and transmission of photogenerated carriers compared to a single catalyst, so that it has a higher photocatalytic CO2 reduction activity and CO and CH4 production efficiency. The composite material prepared by the method of the present invention has a CO yield of not less than 34.67umol / g and a CH4 yield of not less than 16.23umol / g. Therefore, the present invention is a method for preparing a BiOF@Bi2Fe4O9 S-type heterojunction composite material with excellent photocatalytic CO2 reduction CO performance.

[0093] The above content is a further detailed description of the present invention in combination with a specific preferred embodiment. It cannot be determined that the specific embodiments of the present invention are limited to this. For ordinary technicians in the technical field to which the present invention belongs, several simple deductions or substitutions can be made without departing from the concept of the present invention, which should be regarded as belonging to the present invention and the scope of patent protection determined by the submitted claims.

Claims

1. A method for preparing a BiOF@Bi2Fe4O9 S-type heterojunction material, characterized in that: The following steps are involved: a. Preparation of bismuth oxyfluoride precursor BiOF: preparing a bismuth salt solution, a fluoride solution and an iron salt solution; Slowly drop the fluoride solution into the bismuth salt solution according to the mass ratio (2-4): (6-8), stir, heat to react, let stand, and naturally cool to room temperature to obtain a bismuth oxyfluoride precursor BiOF solution; b. Preparation of S-type heterojunction materials: Slowly drop the iron salt solution into the bismuth oxyfluoride precursor BiOF solution in a mass ratio of (8-20): (1-2), mix thoroughly, adjust to strong alkalinity with an alkaline solvent, and stir slowly to obtain a mixed solution; Collect the precipitate in the mixed solution by centrifugation, add the precipitate into an alkaline reaction solvent in a mass ratio of (1-2):(5-10), and perform hydrothermal reaction after ultrasonic stirring; The reactants were collected by centrifugation, washed repeatedly and dried at a constant temperature to obtain a BiOF@Bi2Fe4O9 S-type heterojunction composite material.

2. The preparation method according to claim 1, characterized in that The bismuth salt is one of bismuth nitrate, bismuth chloride and bismuth sulfate; the bismuth salt solution solvent is water, ethanol, ethylene glycol or hydrochloric acid.

3. The preparation method according to claim 1, characterized in that: The fluoride is sodium fluoride or potassium fluoride; the fluoride solution solvent is water, hydrochloric acid or hydrofluoric acid.

4. The preparation method according to claim 1, characterized in that: The iron salt is iron nitrate, iron sulfate or iron chloride; the iron salt solution solvent is water or ethanol.

5. The preparation method according to claim 1, characterized in that: The concentration of the fluoride solution is 1-3 g / L; the concentration of the bismuth salt solution is 5-10 g / L; and the concentration of the iron salt solution is 15-30 g / L.

6. The preparation method according to claim 1, characterized in that: The alkaline solvent is aqueous ammonia or sodium hydroxide; The alkaline reaction solvent is NaOH or KOH with a concentration of 0.2-1 mol / L.

7. The preparation method according to claim 1, characterized in that: In the step b, the pH is adjusted to 10-11 and stirred slowly for 80-100 minutes.

8. The preparation method according to claim 1, characterized in that: In the step b, after ultrasonic stirring, hydrothermal reaction is carried out at 160-180° C. for 40-60 hours; and constant temperature drying is carried out at 80-90° C. for 10-12 hours.

9. A BiOF@Bi2Fe4O9 S-type heterojunction material prepared by the method according to any one of claims 1 to 8.

10. Use of the BiOF@Bi2Fe4O9 S-type heterojunction material according to claim 9 in photocatalytic CO2 reduction.

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