Bi4O5Br2 / Bi-MOF Composite Photocatalyst and Its Preparation Method and Application

Abstract

The present invention discloses a Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2, its preparation method and application. In the Bi4O5Br2 / Bi-MOF composite material, Bi4O5Br2 and Bi-MOF with a mass ratio of 1:0.5 to 2 are tightly coupled with the Bi-MOF nanorods embedded in the gaps between the nanosheets on the surface of the Bi4O5Br2 microspheres. By dropping the Bi-MOF solution into the Bi4O5Br2 solution drop by drop, after completion, the mixed solution is heated in a water bath until it is evaporated to a powder state, the heating temperature is 60 °C, and then it is placed in an oven to be dried to obtain the product. The present invention uses a simple self-assembly method to prepare the Bi4O5Br2 / Bi-MOF composite photocatalyst, and the morphology is that the nanorods are embedded in the gaps between the nanosheets on the surface of the microspheres. This catalyst has the advantages of high photocatalytic reduction performance and good stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of preparation of environmental materials and their photocatalytic reduction of CO2, and specifically relates to a Bi4O5Br2 / Bi-MOF (BOB / BM) composite photocatalyst, its preparation method and application for photocatalytic reduction of CO2. Background Art

[0002] Using solar energy to drive the conversion of greenhouse gas carbon dioxide into high-value fuels is one of the strategies to solve the "environment" and "energy" problems. For example, photocatalytic conversion of CO2 into CO, CH4, CH3OH, C2H5OH and HCOOH is an attractive sustainable energy conversion technology, which can alleviate the global warming phenomenon caused by the large consumption of fossil fuels and the increase in the concentration of CO2 in the atmosphere. However, problems such as the weak CO2 capture ability of photocatalysts, high recombination rate of photo-generated electron-hole pairs, and poor charge separation effect restrict their photocatalytic efficiency and severely limit their practical applications. Therefore, exploring the preparation of efficient and sustainable photocatalysts for reducing CO2 into high-value fuels has become a research hotspot.

[0003] Pure BiOBr materials have the disadvantages of high recombination rate and low photo-generated charge transfer efficiency, which limit their applications in photocatalytic reactions. The bismuth-rich strategy can effectively solve these problems. It can elevate the CBM position, endowing Bi4O5Br2 with stronger reduction ability. The spherical structure increases the surface area, enhancing light collection and carrier separation. Therefore, it shows great application potential in the research of photocatalytic CO2 reduction.

[0004] Porous crystalline metal-organic frameworks (MOFs) are hybrid materials composed of metal ions coordinated with organic ligands. Due to their flexible coordination environment, high specific surface area, customizable structure and strong adsorption ability for small molecules, they have attracted extensive attention in the field of photocatalysis. Bi-MOF has become a relatively new representative with a wide variety of types due to the adjustable porosity and structural stability of its pores. However, the limitations of this type of catalyst are great, and the charge separation efficiency is lower than that of traditional photocatalysts, restricting its efficiency in photocatalysis. Therefore, it is chosen to composite Bi4O5Br2 with Bi-MOF to improve the efficiency of photocatalytic reduction of CO2. Summary of the Invention

[0005] Based on the above analysis and aiming at the deficiencies in the prior art, the present invention aims to provide a Z-type heterojunction Bi4O5Br2 / Bi-MOF composite photocatalyst, its preparation method and application.

[0006] The present invention provides a Bi4O5Br2 / MOFs composite photocatalyst, namely Bi4O5Br2 / Bi-MOF. This composite photocatalyst effectively inhibits the recombination of photo-generated electron-hole pairs. In the carbon dioxide photocatalytic system, Bi4O5Br2 serves as an electron accumulation active site, which is used to extract the photo-generated electrons on the surface of Bi-MOF and effectively activate CO2 molecules, showing good visible-light photocatalytic activity.

[0007] The present invention combines microspherical Bi4O5Br2 with nanorod-shaped Bi-MOF by a self-assembly method to synthesize the photocatalytic material Bi4O5Br2 / Bi-MOF for photocatalytic reduction of CO2. The nanorod-shaped Bi-MOF is tightly coupled by embedding into the gaps between the nanosheets on the surface of the Bi4O5Br2 microspheres, forming a Z-type heterojunction. The loading amount of Bi-MOF in the Bi4O5Br2 / Bi-MOF composite material is 33% - 67%.

[0008] The present invention also provides a preparation method for the above photocatalytic material Bi4O5Br2 / Bi-MOF, which includes the following steps:

[0009] (1) Disperse a set amount of microspherical Bi4O5Br2 material in a mixed solution with a volume ratio of anhydrous ethanol to deionized water of 4:1, and perform ultrasonic stirring to form a uniformly dispersed Bi4O5Br2 suspension;

[0010] (2) Disperse a set amount of nanorod-shaped Bi-MOF material in a mixed solution with a volume ratio of anhydrous ethanol to deionized water of 4:1, and perform ultrasonic stirring to form a uniformly dispersed Bi-MOF suspension;

[0011] (3) Dropwise add the Bi-MOF suspension into the Bi4O5Br2 suspension. After completion, continue stirring for 1 h until it is uniformly mixed, then heat it in a water bath with continuous stirring at a temperature of 60°C - 80°C until it evaporates to a powder state, and then dry it in an oven at 60°C to obtain the Bi4O5Br2 / Bi-MOF composite material.

[0012] Further, in the above preparation method, the mass concentration of the Bi4O5Br2 material in the Bi4O5Br2 suspension is 3 - 4 mg / mL.

[0013] The mass concentration of the Bi-MOF material in the Bi-MOF suspension is 1.5 - 7 mg / mL

[0014] The volume ratio of the Bi4O5Br2 suspension to the Bi-MOF suspension is 1:1.

[0015] In some exemplary specific embodiments of the present invention, the composite material is labeled as A-Bi4O5Br2 / Bi-MOF (A = 33%, 50% and 66%), where A represents the mass percentage of Bi-MOF in the composite material.

[0016] Optionally, the Bi4O5Br2 is obtained by the following method:

[0017] (1) Disperse an appropriate amount of Bi(NO3)3·5H2O into ethylene glycol;

[0018] (2) Disperse an appropriate amount of KBr into ethanol;

[0019] (3) Add the KBr solution to the Bi(NO3)3·5H2O solution, continue magnetic stirring and ultrasonic dispersion for 0.5 h, react the mixed solution at 180 °C for 16 h, collect the product by centrifugation, and wash it three times with deionized water and anhydrous ethanol respectively, and dry it in air at 80 °C for 6 h to obtain Bi4O5Br2.

[0020] Among them, the molar ratio of Bi:Br in the prepared Bi4O5Br2 material is 1.5:1.

[0021] Optionally, the Bi-MOF is obtained by the following method:

[0022] (1) Take an appropriate amount of Bi(NO3)3·5H2O and H3BTC and dissolve them in a DMF solution;

[0023] (2) Stir the above mixed solution for 1 h;

[0024] (3) After mixing evenly, transfer the mixture to a reaction kettle, heat it at 120 °C for 24 h, precipitate by centrifugal filtration, wash it three times with DMF and anhydrous ethanol respectively, and then dry the product at 50 °C for 10 h to obtain Bi-MOF nanorods.

[0025] Among them, the molar ratio of Bi(NO3)3·5H2O to H3BTC is 1:2.

[0026] The present invention also provides the application of the above Z-type heterojunction Bi4O5Br2 / Bi-MOF composite photocatalyst, and the Bi4O5Br2 / Bi-MOF composite photocatalyst is used for visible light catalytic reduction of CO2.

[0027] Compared with the prior art, the beneficial effects of the present invention are:

[0028] By self-assembly, nanorod-shaped Bi-MOF is tightly coupled into the gaps between the nanosheets on the surface of Bi4O5Br2 microspheres. The formed Z-scheme heterojunction can promote the separation of photo-generated electron-hole pairs, improve the charge transfer efficiency, and thus enhance the photocatalytic performance. Compared with direct solution mixing, the solution dropping method adopted in this invention makes the distribution of the two materials more uniform, avoids the agglomeration of the materials, increases the light absorption, and further improves the performance of catalyzing carbon dioxide reduction. Compared with the conventional hydrothermal reaction of the precursor solution, the heating stirring evaporation process adopted in this invention enables the materials to be fully mixed, completes the self-assembly, and promotes the uniform distribution of nanorod-shaped Bi-MOF on the surface of Bi4O5Br2 microspheres, further improving the photocatalytic performance of the composite material. This invention successfully prepares the Bi4O5Br2 / Bi-MOF composite material by a simple self-assembly method and applies it to photocatalytic reduction of CO2. The morphology is that nanorods are embedded in the gaps between the nanosheets on the surface of the microspheres. This material has the advantages of high photocatalytic reduction performance and good stability. Description of the Drawings

[0029] Figure 1 XRD and infrared spectrum images of Bi4O5Br2, Bi-MOF and 50-Bi4O5Br2 / Bi-MOF;

[0030] Figure 2 SEM images, TEM images and Mapping diagrams of Bi4O5Br2, Bi-MOF and 50-Bi4O5Br2 / Bi-MOF;

[0031] Figure 3 XPS spectra of Bi4O5Br2, Bi-MOF and 50-Bi4O5Br2 / Bi-MOF;

[0032] Figure 4 CO and CH4 yield diagrams of Bi4O5Br2, Bi-MOF and A-Bi4O5Br2 / Bi-MOF;

[0033] Figure 5 Recycling experiment of 50-Bi4O5Br2 / Bi-MOF and XRD diagram of the material after the recycling experiment;

[0034] Figure 6 UV-Vis DRS diagrams of Bi4O5Br2, Bi-MOF and 50-Bi4O5Br2 / Bi-MOF;

[0035] Figure 7 Band gaps, Mott-Schottky spectra, VB-XPS spectra of Bi4O5Br2 and Bi-MOF;

[0036] Figure 8 ESR spectra of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF;

[0037] Figure 9 PL spectra of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF;

[0038] Figure 10 Electrochemical diagrams of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF. Detailed implementation manners

[0039] The invention is not limited to the following specific implementation manners. Those of ordinary skill in the art can implement the invention in other various specific implementation manners according to the content disclosed in the present invention. Or, those that adopt the design structure and idea of the present invention and make simple changes or modifications all fall within the protection scope of the present invention. The present invention will be described in detail below in conjunction with embodiments to enable those skilled in the art to better understand the present invention. However, the present invention is not limited to the following embodiments. Example 1

[0040] (1) Preparation of Bi4O5Br2:

[0041] A. Dissolve Bi(NO3)3·5H2O (1.5 mmol) in 10 mL of ethylene glycol and sonicate for 30 minutes to form a homogeneous solution;

[0042] B. Dissolve KBr (1 mmol) in 30 mL of ethanol and sonicate for 30 minutes to form a homogeneous solution;

[0043] C. Add the KBr solution to the Bi(NO3)3·5H2O solution, continue magnetic stirring and sonication for 0.5 h, transfer the mixed solution to a 50 mL polytetrafluoroethylene stainless steel autoclave, and react at 180 °C for 16 h. After the reaction is completed, collect the product by centrifugation, wash it three times with deionized water and anhydrous ethanol respectively to remove residual ions; finally, dry the collected Bi4O5Br2 material in air at 80 °C for 6 hours.

[0044] (2) Preparation of Bi-MOF:

[0045] A. Dissolve Bi(NO3)3·5H2O (1 mmol) and H3BTC (2 mmol) in 25 mL of DMF solution and stir for 1 h to form a homogeneous solution;

[0046] B. Transfer the mixture solution to a 50 mL polytetrafluoroethylene stainless steel autoclave, heat it at 120 °C for 24 h, centrifuge and filter the precipitate, wash it three times with DMF and absolute ethanol respectively, and then dry it at 50 °C for 10 h. The obtained sample is denoted as Bi-MOF.

[0047] (3)Preparation of 50-Bi4O5Br2 / Bi-MOF (abbreviated as 50-BOB / BM) composite material:

[0048] A. Disperse 0.1 g of microspherical Bi4O5Br2 material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a homogeneous dispersion;

[0049] B. Disperse 0.1 g of nanorod-shaped Bi-MOF material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a homogeneous dispersion;

[0050] C. Dropwise add the Bi-MOF solution into the Bi4O5Br2 solution. After completion, continue to stir for 1 h until it is mixed evenly. Then, at a temperature of 60 °C, continuously stir and heat in a water bath until it evaporates to a powder state, and then place it in an oven at 60 °C for drying. Finally, obtain the 50-Bi4O5Br2 / Bi-MOF composite material. Example 2

[0051] (1)Preparation of Bi4O5Br2:

[0052] Obtain Bi4O5Br2 by using the same preparation method as in Example 1.

[0053] (2)Preparation of Bi-MOF:

[0054] Obtain Bi-MOF by using the same preparation method as in Example 1.

[0055] (3)Preparation of 33-Bi4O5Br2 / Bi-MOF (abbreviated as 33-BOB / BM) composite material:

[0056] A. Disperse 0.1 g of microspherical Bi4O5Br2 material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a homogeneous dispersion;

[0057] B. Disperse 0.05 g of nanorod-shaped Bi-MOF material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a homogeneous dispersion;

[0058] C. Drop the Bi-MOF solution into the Bi4O5Br2 solution drop by drop. After completion, continue stirring for 1 h until it is evenly mixed. Then, at a temperature of 60 °C, continuously stir and heat in a water bath until it dries to a powder state. Then, place it in an oven at 60 °C for drying. Finally, obtain the 33-Bi4O5Br2 / Bi-MOF composite material. Example 3

[0059] (1) Preparation of Bi4O5Br2:

[0060] Obtain Bi4O5Br2 using the same preparation method as in Example 1.

[0061] (2) Preparation of Bi-MOF:

[0062] Obtain Bi-MOF using the same preparation method as in Example 1.

[0063] (3) Preparation of 66-Bi4O5Br2 / Bi-MOF (abbreviated as 66-BOB / BM) composite material:

[0064] A. Disperse 0.1 g of microspherical Bi4O5Br2 material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a uniformly dispersed solution;

[0065] B. Disperse 0.2 g of nanorod-shaped Bi-MOF material in a mixed solution of 30 ml of absolute ethanol and deionized water (volume ratio 4:1), and ultrasonically stir to form a uniformly dispersed solution;

[0066] C. Drop the Bi-MOF solution into the Bi4O5Br2 solution drop by drop. After completion, continue stirring for 1 h until it is evenly mixed. Then, at a temperature of 60 °C, continuously stir and heat in a water bath until it dries to a powder state. Then, place it in an oven at 60 °C for drying. Finally, obtain the 66-Bi4O5Br2 / Bi-MOF composite material. Comparative Example 1

[0067] The difference between this comparative example and Example 1 is that in Step C of the preparation of the 50-Bi4O5Br2 / Bi-MOF composite material, the Bi-MOF solution and the Bi4O5Br2 solution are directly mixed and stirred for 1 h until evenly mixed. Then, at a temperature of 60 °C, continuous stirring is carried out in a water bath until it is evaporated to a powder state, and then it is placed in an oven at 60 °C for drying. Finally, the 50-Bi4O5Br2 / Bi-MOF-H composite material is obtained. The 50-Bi4O5Br2 / Bi-MOF-H composite material was subjected to a photocatalytic CO2 reduction experiment at room temperature and atmospheric pressure. 20 mg of the composite material was placed in a beaker containing 80 ml of deionized water and 20 ml of triethanolamine (TEOA), and ultrasonic stirring was used to make it evenly mixed. After continuous irradiation with a 300 W xenon lamp for 5 hours, the main products detected were CO and CH4, and the CO production rate was 13.47 μmol·g -1 ·h -1 , and the CH4 production rate was 2.89 μmol·g -1 ·h -1 . Comparative Example 2

[0068] The difference between this comparative example and Example 1 is that in the preparation steps of the 50-Bi4O5Br2 / Bi-MOF composite material, first in Step A, Bi(NO3)3·5H2O (1 mmol) and H3BTC (2 mmol) are dissolved in 25 mL of DMF solution and stirred for 1 h to form a uniformly dispersed solution; then in Step B, 0.15 g of Bi4O5Br2 material is put into the above-mentioned uniformly dispersed solution, and ultrasonic stirring is continued until evenly mixed; finally in Step C, the mixture solution is transferred to a 50 mL polytetrafluoroethylene stainless steel autoclave and heated at 120 °C for 24 h. After cooling, the precipitate is centrifuged and filtered, and then washed three times with DMF and absolute ethanol respectively, and then dried at 50 °C for 10 h to obtain the 50-Bi4O5Br2 / Bi-MOF-R composite material. The 50-Bi4O5Br2 / Bi-MOF-R composite material was subjected to a photocatalytic CO2 reduction experiment at room temperature and atmospheric pressure. 20 mg of the composite material was placed in a beaker containing 80 ml of deionized water and 20 ml of triethanolamine (TEOA), and ultrasonic stirring was used to make it evenly mixed. After continuous irradiation with a 300 W xenon lamp for 5 hours, the main products detected were CO and CH4, and the CO production rate was 17.29 μmol·g -1 ·h -1 , and the CH4 production rate was 3.74 μmol·g -1 ·h -1 .

[0069] Further, in combination with the attached drawings, it is described how each embodiment of the present invention embodies the object of the present invention and achieves the beneficial effects of the present invention:

[0070] The crystal structures of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF prepared in Example 1 were analyzed by XRD patterns. Please refer to Figure 1 a. The XRD pattern of the synthesized Bi4O5Br2 was in good agreement with the standard XRD pattern of the corresponding Bi4O5Br2 (JCPDS card number: 37-0699). The nine characteristic peaks at 10.948°, 24.297°, 29.697°, 31.807°, 33.295°, 37.390°, 39.716°, 43.353°, and 45.491° corresponded to the (101), (112), (411), (020), (501), (321), (322), (105), and (422) lattice planes of Bi4O5Br2, respectively. The XRD pattern of the prepared Bi-MOF was also in good agreement with the simulated XRD pattern of the reported Bi-MOF (CCDC: 1426169). At the same time, the characteristic peaks of the two monomers were reflected in the XRD pattern of the composite material 50-Bi4O5Br2 / Bi-MOF, and no other impurity phases were found, indicating that there were no impurities in the composite material, which showed that the combination of Bi4O5Br2 and Bi-MOF was very successful.

[0071] The molecular structures and functional groups of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF were studied by Fourier transform infrared (FT-IR) spectroscopy. Please refer to Figure 1 b. Clear peaks were observed in the Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF samples. In the FT-IR spectrum of Bi4O5Br2, the absorption band at 527 cm −1 belonged to the Bi-O stretching vibration, and this vibration mode only appeared in pure Bi4O5Br2 and Bi4O5Br2 / Bi-MOF. For Bi-MOF, the two obvious bands at 1681 and 1356 cm −1 belonged to the stretching vibrations of C=O and C-O, respectively. The band at 1537 cm −1 corresponded to the asymmetric vibration of the carboxylic acid functional group, and the band at 765 cm −1 belonged to the Bi-O stretching vibration mode in Bi-MOF, and all could be well corresponded one by one. It should be emphasized that 50-Bi4O5Br2 / Bi-MOF had all the characteristic bands of Bi4O5Br2 and Bi-MOF at the same time, further confirming the existence of the two expected materials in the synthesized 50-Bi4O5Br2 / Bi-MOF composite material.

[0072] The morphology, structure, and elements of the samples were analyzed using a scanning electron microscope, a transmission electron microscope, and an energy spectrometer. Please refer to Figure 2 . As can be seen from Figure 2 a, the original Bi4O5Br2 presents a microsphere composed of nanosheets, while Bi-MOF presents a nanorod structure with a cross-sectional diameter of about 100 nm, as shown in Figure 2 b. From Figure 2 c, it can be clearly seen that the Bi-MOF nanorods are embedded in the gaps between the nanosheets on the surface of the Bi4O5Br2 microspheres, indicating the successful preparation of the composite material. Figure 2 d shows that the state of the catalyst is completely consistent with the SEM results; in Figure 2 e, two lattice fringes with widths of 0.281 nm and 0.33 nm can be clearly observed, belonging to the (020) crystal plane of Bi4O5Br2 and the (013) crystal plane of Bi-MOF, respectively. At the same time, according to the elemental mapping distribution results of 50-Bi4O5Br2 / Bi-MOF in the EDS ([[]] Figure 2 f-l), it was found that only Bi, Br, C, and O elements were detected to be evenly distributed, indicating the successful construction of the heterojunction structure.

[0073] The interfacial interaction behavior and elemental state between Bi4O5Br2 and Bi-MOF were further analyzed using X-ray photoelectron spectroscopy (XPS). Please refer to Figure 3 . As shown in Figure 3 a, there are four elements, Bi, O, Br, and C, in the composite material 50-Bi4O5Br2 / Bi-MOF, indicating the coexistence of Bi4O5Br2 and Bi-MOF in the hybrid. In the high-resolution Bi 4f XPS spectra of the original Bi4O5Br2 and 50-Bi4O5Br2 / Bi-MOF samples ( Figure 3 b), the XPS orbital scans at binding energies of 158.6 and 158.8 eV correspond to Bi4f 7 / 2 , while the XPS orbital scans at 163.9 and 164.1 eV correspond to Bi 4f 5 / 2 , both caused by Bi 3+ substances. The binding energies of the Bi 4f orbitals of 50-Bi4O5Br2 / Bi-MOF (158.8 and 164.1 eV) show a negative shift relative to Bi-MOF (159.1 and 164.4 eV). In addition, for the Figure 3 Br 3d spectrum in c, the binding energies of Br 3d in Bi4O5Br2 are 67.8 and 68.9 eV, respectively. After being combined with Bi-MOF, the signal peaks shift to 68.1 and 69.2 eV. The appearance of the doublet is due to Br 3d 5 / 2 3 / 2is caused by the binding energy, and there is a positive shift in the characteristic peaks from the monomer to the composite material. Similarly, for O 1s ( Figure 3 d), the two binding energy peaks at 529.4 and 530.4 eV correspond to the lattice oxygen and adsorbed oxygen in Bi4O5Br2, respectively. In the O 1s XPS spectrum of 50-Bi4O5Br2 / Bi-MOF, the three peaks at 529.8, 530.8, and 532.3 eV are attributed to Bi−O, surface −OH, and absorbed H2O, respectively. The binding energy peaks show a positive shift compared to Bi4O5Br2 and a negative shift compared to Bi-MOF (529.9, 531.1, and 532.5 eV). Combining these peak shifts, it can be inferred that photoelectrons tend to transfer from Bi-MOF to Bi4O5Br2 during hybridization, which leads to an increase in the electron cloud density of Bi, O, Br, and C in the composite material 50-Bi4O5Br2 / Bi-MOF and an improvement in the separation efficiency of electron-hole pairs.

[0074] The photocatalytic reduction of CO2 experiment was carried out at room temperature and atmospheric pressure. 20 mg of the composite material was placed in a beaker containing 80 ml of deionized water and 20 ml of triethanolamine (TEOA), and it was mixed evenly by ultrasonic stirring. After continuous irradiation with a 300 W xenon lamp for 5 hours, the main products detected were CO and CH4, see Figure 4 a and b. Figure 4 a compares the CO2 reduction performance of three composite materials with different mass ratios, and the optimal ratio can be obtained as 1:1, that is, 50-Bi4O5Br2 / Bi-MOF. At this time, the yield of the reduction product CO is 23.78 μmol·g -1 ·h -1 , and the yield of CH4 is 5.39 μmol·g -1 ·h -1 , reaching the highest. The main product detected for pure Bi4O5Br2 under artificial light irradiation is CO, and the average yield within 5 hours is 4.67 μmol·g -1 ·h -1 , and only trace amounts of CH4 were detected. After 5 hours of light irradiation of Bi-MOF, the CH4 yield reached 2.14 μmol·g -1 ·h -1 , and no CO was produced. Therefore, the average CO yield of the optimal ratio of 50-Bi4O5Br2 / Bi-MOF after 5 hours is about 5.09 times higher than that of Bi4O5Br2, and the average CH4 yield is about 2.52 times higher than that of Bi-MOF ( Figure 4 b), indicating that the construction of the direct Z-scheme heterojunction greatly enhances the photocatalytic CO2 reduction performance.

[0075] The stability of the photocatalyst was evaluated by performing a CO2 reduction cycle experiment using 50-Bi4O5Br2 / Bi-MOF hybrid as the photocatalyst. See Figure 5 a. It can be seen that the 50-Bi4O5Br2 / Bi-MOF composite maintained good photocatalytic activity within 25 hours of cycling, and the CO production rate was basically stable. The CO production rate in the 5th cycle was 98.2% of that in the 1st cycle. Then, the samples were collected by centrifugation, and the stability was further evaluated using XRD. See Figure 5 b. The XRD pattern of the 50-Bi4O5Br2 / Bi-MOF hybrid collected after the reaction was almost the same as the original one, indicating that its crystal structure was maintained, confirming the excellent stability of the photocatalyst. The above results show that this catalyst has superior reusability and stability.

[0076] The light absorption ability of the samples was analyzed by ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis). See Figure 6 . From Figure 6 it can be seen that the absorption edge of Bi-MOF is 357 nm, which is in the ultraviolet light absorption region. Bi4O5Br2 exhibits obvious visible light absorption, and its inherent absorption edge is at 448 nm, while the absorption edge of the 50-Bi4O5Br2 / Bi-MOF is at 435 nm, significantly greater than that of the Bi-MOF monomer, indicating that the light capture ability of the composite material has been effectively improved.

[0077] Relationship diagrams of (αhμ) of Bi4O5Br2 and Bi-MOF 2 versus photon energy (hμ) (a, b), Mott-Schottky spectra of Bi4O5Br2 (c) and Bi-MOF (d), VB XPS spectra of Bi4O5Br2 (e) and Bi-MOF (f). See Figure 7 . Based on the calculation results of UV-vis DRS and the Tauc equation, the band gap energies (E g ) of Bi4O5Br2 and Bi-MOF were calculated to be 2.53 and 3.20 eV respectively ( Figure 7 a, b). The energy level positions of the catalysts were studied by Mott-Schottky curves. A positive slope indicates that both Bi4O5Br2 and Bi-MOF are n-type semiconductors. As shown in Figure 7 c, d, according to the x-axis intercept of the linear part of the Mott–Schottky plot, the flat band potential (E FB ) values of Bi4O5Br2 and Bi-MOF were observed to be approximately -0.53 and -0.25 V (vs. Ag / AgCl, pH = 7) respectively. Generally, the E CB of n-type semiconductors is more negative than EFB It is about -0.2 V negative. Therefore, the E of Bi4O5Br2 and Bi-MOF CB are about -0.73 and -0.45 V (vs. Ag / AgCl, pH = 7) respectively. From the formula E NHE = E Ag / AgCl + 0.2 eV, the E of Bi4O5Br2 and Bi-MOF are calculated to be about -0.53 and -0.25 eV (vs. NHE) respectively. In addition, according to the VB-XPS spectra of Bi4O5Br2 and Bi-MOF shown in ( CB e, f), the valence band values can be seen to be 2.0 and 2.95 eV respectively, which are consistent with the results calculated by the formula E Figure 7 e, f). It is consistent with the calculation results. VB =E CB +E g

[0078] DMPO was used as the scavenger for the selected active radical species (•OH and •O2 - ) of the material, and the electron spin resonance spectrum (ESR) of 50-Bi4O5Br2 / Bi-MOF was measured. Please refer to Figure 8 . Figure 8 a is the ESR spectrum of DMPO / •O2 - . It can be seen that there is almost no signal under dark conditions. However, after 5 minutes of light irradiation, the signal of 50-Bi4O5Br2 / Bi-MOF is significantly stronger than that of Bi4O5Br2 and Bi-MOF, proving that more •O2 - is generated in 50-Bi4O5Br2 / Bi-MOF. As shown in Figure 8 b, there is no signal in the dark. After 5 minutes of light irradiation, the characteristic signal with a relative peak intensity ratio of 1:2:2:1 detected belongs to DMPO / •OH, and it can be clearly seen that the characteristic signal intensity on 50-Bi4O5Br2 / Bi-MOF is higher than that of pure Bi4O5Br2 and Bi-MOF, proving that more •OH is generated in the composite material. Therefore, the ESR results support the inference of the Z-type heterojunction.

[0079] The charge separation efficiency of the prepared photocatalyst was studied by steady-state photoluminescence (PL) spectroscopy. Please refer to Figure 9 . Compared with Bi4O5Br2 and Bi-MOF, the emission peak intensity of 50-Bi4O5Br2 / Bi-MOF is the lowest, and the recombination rate of electron-hole pairs is the lowest, indicating that the successful construction of the heterostructure inhibits the recombination of photo-generated carriers, accelerates the migration speed of interfacial electrons, and effectively improves the separation efficiency of photo-generated charges.

[0080] ​For the electrochemical diagrams of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF, please refer to Figure 10 . To explore the mechanism of enhanced photocatalytic CO2 reduction, the charge transfer resistance at the photocatalyst reaction interface was measured using electrochemical impedance spectroscopy (EIS) Figure 10 a). From Bi4O5Br2, Bi-MOF to 50-Bi4O5Br2 / Bi-MOF, the semicircle radius of the composite material is the smallest, indicating that it has the lowest charge transfer resistance and the fastest charge transfer speed at the electrolyte solution interface. Fast electron transfer is beneficial to improving the separation efficiency of photo-generated electron-hole pairs and enhancing the photocatalytic performance. In addition, in order to further determine the important influence of the transfer efficiency of photo-generated carriers on the photocatalytic reaction, transient photocurrent-time (I-t) curve characterization was carried out. As can be seen from Figure 10 b, under the irradiation of a xenon lamp, the photocurrent response intensity of the composite material is higher than that of the original Bi4O5Br2 and Bi-MOF. Therefore, the 50-Bi4O5Br2 / Bi-MOF hybrid has the fastest separation efficiency of photo-generated carriers, and thus its photocatalytic activity is more excellent, which corresponds to the results of the electrochemical impedance spectroscopy. The LSV curves of Bi4O5Br2, Bi-MOF, and 50-Bi4O5Br2 / Bi-MOF are shown in Figure 10 c. Different initial potentials represent different CO2 reduction capabilities. A lower initial potential indicates that the interfacial charge transfer speed between the sample and the electrode is significantly accelerated, and the CO2 reduction ability of the sample is stronger. At the same current density, the initial potential of the 50-Bi4O5Br2 / Bi-MOF composite photocatalyst is the lowest, indicating that 50-Bi4O5Br2 / Bi-MOF has a stronger CO2 reduction ability than Bi4O5Br2 and Bi-MOF.

[0081] The above-described embodiments are the preferred specific embodiments of the present invention, but the protection scope of the present invention is not limited thereto. Any improvement, substitution, or modification made by those skilled in the art within any technical scope covered by the present invention to the technical solutions and their concepts shall fall within the protection scope of the present invention.

Claims

1. Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2, characterized in that: The Bi4O5Br2 / Bi-MOF composite material includes microspherical Bi4O5Br2 and nanorod-shaped Bi-MOF, wherein the nanorod-shaped Bi-MOF is embedded between nanosheets on the surface of the Bi4O5Br2 microspheres to form a Z-type heterojunction; the loading amount of Bi-MOF in the Bi4O5Br2 / Bi-MOF composite material is 33% to 67%; the Bi4O5Br2 / Bi-MOF composite material is prepared by adding a Bi-MOF suspension dropwise into a Bi4O5Br2 suspension, stirring continuously until the mixture is evenly mixed, and then heating in a water bath at a temperature of 60°C to 80°C with continuous stirring to evaporate to powder, and drying.

2. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 1, characterized in that: The following steps are involved: (1) Dispersing the microspherical Bi4O5Br2 material in a mixed solution of anhydrous ethanol and deionized water, and stirring with ultrasound to form a uniformly dispersed Bi4O5Br2 suspension; (2) Dispersing the nanorod-shaped Bi-MOF material in a mixed solution of anhydrous ethanol and deionized water, and ultrasonically stirring to form a uniformly dispersed Bi-MOF suspension; (3) The Bi-MOF suspension was added dropwise into the Bi4O5Br2 suspension. After completion, the mixture was stirred until uniformly mixed. Then, the mixture was heated in a water bath at 60°C-80°C with continuous stirring and evaporated to powder. The mixture was dried to obtain a Bi4O5Br2 / Bi-MOF composite material.

3. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 2, characterized in that: The mass ratio of Bi4O5Br2 to Bi-MOF is 1:0.5~2.

4. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 2, characterized in that: The mass concentration of Bi4O5Br2 material in the Bi4O5Br2 suspension is 3~4 mg / mL; the mass concentration of Bi-MOF material in the Bi-MOF suspension is 1.5~7 mg / mL; the volume ratio of Bi4O5Br2 suspension to Bi-MOF suspension is 1:

1.

5. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 2, characterized in that: The mixed solution of anhydrous ethanol and deionized water is prepared by mixing anhydrous ethanol and deionized water in a volume ratio of 4:

1.

6. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 2, characterized in that: Microspherical Bi4O5Br2 material was prepared by the following method: Bi(NO3)3·5H2O was dispersed in ethylene glycol to obtain a Bi(NO3)3·5H2O solution; KBr was dispersed in ethanol to obtain a KBr solution; the KBr solution was added to the Bi(NO3)3·5H2O solution and mixed evenly to obtain a mixed solution, the mixed solution was reacted at 180°C for 16 hours, the product was collected by centrifugation, washed, and dried to obtain Bi4O5Br2, wherein the molar ratio of Bi to Br was 1.5:

1.

7. The method for preparing the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 according to claim 2, characterized in that: The nanorod-shaped Bi-MOF material was prepared by the following method: Bi(NO3)3·5H2O and H3BTC were dissolved in DMF solution and stirred to mix evenly; the mixture was heated at 120°C for 24 hours, and the product was collected by centrifugation, washed, and dried to obtain Bi-MOF; wherein the molar ratio of Bi(NO3)3·5H2O to H3BTC was 1:

2.

8. The use of the Bi4O5Br2 / Bi-MOF composite material for photocatalytic reduction of CO2 as claimed in claim 1, characterized in that: Application method: Deionized water, sacrificial agent and Bi4O5Br2 / Bi-MOF composite material are added to a photoreactor, CO2 gas is introduced, a xenon lamp is used to simulate sunlight, and a photocatalytic reaction is carried out under normal pressure.

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