A bismuth oxybromide / biochar composite material for nitrogen fixation, a preparation method and application thereof in photocatalytic nitrogen fixation

Abstract

The present application relates to a kind of bismuth oxybromide / biochar composite for nitrogen fixation and preparation method and its application in photocatalytic nitrogen fixation.The composite is composed of layered porous biochar and rose-like bismuth oxybromide, bismuth oxybromide nanosheet is vertically or obliquely oriented growth in the two-dimensional surface and three-dimensional pore inside of biochar, form 0D-2D face-face contact heterostructure, rather than traditional point-face contact.The preparation method includes controllable preparation of corn straw biomass BC carrier and in-situ growth induced by chemical bond.And its application in photocatalytic nitrogen fixation.By introducing the physical support function of biochar, the problems of BiOBr nanosheet easy to agglomerate, few surface active sites, weak nitrogen adsorption and activation ability, poor cycle stability and short service life are solved.

Description

Technical Field

[0001] This invention relates to the field of photocatalytic nitrogen fixation technology, specifically to a bismuth oxybromine / biochar composite material for nitrogen fixation, its preparation method, and its application in photocatalytic nitrogen fixation. Background Technology

[0002] Ammonia (NH3), as one of the most important chemical raw materials, has an annual production exceeding 150 million tons. Its traditional Haber-Bosch synthesis process consumes approximately 2% of global energy and emits large amounts of CO2. Photocatalytic nitrogen fixation technology, using semiconductor materials to directly utilize solar energy under mild conditions to convert N2 and H2O into NH3, is considered an ideal green route for ammonia synthesis. Bismuth oxybromine (BiOBr) has attracted widespread attention in the field of photocatalysis due to its unique layered structure, suitable band structure, and good chemical stability. However, pure BiOBr suffers from the following fatal flaws: a small photoresponse range and a high electron-hole (electron-hole) ratio. - -h + The rapid recombination of these composite materials, their limited carrier concentration, surface inertness, tendency to aggregate, poor stability, and weak adsorption capacity with N2 molecules present challenges. To address these issues, new strategies for preparing composite materials are being explored.

[0003] Chinese Journal of Environmental Science, 2025, 45(01): 310-321 discloses a "biochar and oxygen vacancy co-modified bismuth oxybromine" material (BC / OV-BiOBr), but the application of this technology is limited to the photocatalytic degradation of organic pollutants such as tetracycline. Its technical effect is based on the free radical oxidation mechanism, which is fundamentally different from the electron reduction mechanism required for nitrogen fixation. This literature does not mention or imply that this technology can be used for photocatalytic nitrogen fixation reactions.

[0004] The Journal of Materials Chemistry A, 2025 reported a method for photocatalytic nitrogen fixation using iron complex-anchored BiOBr nanoflowers, but this technique uses noble metal complexes as co-catalysts, which is costly and has a complex preparation process.

[0005] Therefore, inventing a composite photocatalyst of biochar and BiOBr for nitrogen fixation has significant practical implications. Summary of the Invention

[0006] The purpose of this invention is to provide a bismuth oxybromine / biochar composite material for nitrogen fixation, its preparation method, and its application in photocatalytic nitrogen fixation. This composite material is used for nitrogen fixation for the first time, and by introducing biochar, it aims to overcome the problems of small photoresponse range, high recombination rate of photogenerated carriers, easy agglomeration, and instability of existing BiOBr-based photocatalytic nitrogen fixation materials.

[0007] The technical solution of the present invention: A bismuth oxybromine / biochar composite material for nitrogen fixation is disclosed. The bismuth oxybromine / biochar (BC) composite photocatalyst consists of layered porous biochar and rose-shaped bismuth oxybromine. Bismuth oxybromine nanosheets are vertically or obliquely oriented on the two-dimensional surface and three-dimensional pores of the biochar, forming an 0D-2D surface-to-surface contact heterostructure, rather than the traditional point-to-surface contact. This hierarchical structure of "three-dimensional porous carbon framework-two-dimensional bismuth oxybromine nanosheets" achieves interfacial electronic coupling and electron channel transport through CO-Bi chemical bonds, and simultaneously constructs a Schottky heterojunction. Due to the difference in work function between the two and the unique electron storage function of biochar, a stable electron source is provided for the subsequent nitrogen reduction reaction. A built-in electric field is formed at the interface, and dynamic electron recombination between the interfaces is completed. This electron transfer accelerates the separation of photogenerated electrons and holes in bismuth oxybromine, thereby promoting the photocatalytic nitrogen fixation rate.

[0008] A method for preparing a bismuth oxybromine / biochar composite material for nitrogen fixation. Step 1: Controllable preparation of corn straw biomass BC carrier: The original BC was obtained by pyrolysis of corn stalks at 600℃ under limited oxygen. Step 2: Chemical bond-induced in-situ growth: A simple one-step solvothermal method was used. Ethylene glycol was prepared in a beaker, and BC and Bi(NO3)3•5H2O were added and stirred until complete. KBr was then added and stirred thoroughly. The mixed solution was then transferred to a polytetrafluoroethylene reactor, sealed and heated to 160℃ for 16 h. BiOBr was directionally grown on the BC surface through chemical bonding induction. After cooling to room temperature, the sample was first washed by centrifugation with deionized water, then washed by centrifugation with ethanol, and finally dried in an oven at 80℃ overnight. After drying, BiOBr nanoflower powder was obtained by grinding and labeled as BC / BiOBr-X.

[0009] Application of a bismuth oxybromine / biochar composite material for nitrogen fixation in photocatalytic nitrogen fixation.

[0010] The beneficial effects of this invention are: 1. This application's composite material addresses the problems of easy aggregation, few surface active sites, weak nitrogen adsorption and activation capacity, poor cycle stability, and short service life of BiOBr nanosheets by introducing the physical support function of biochar. The stability of this composite material is optimized: after 5 cycles, the catalytic activity retention rate is >92%, mitigating the instability of BiOBr caused by photocorrosion and easy aggregation.

[0011] 2. This application's composite material addresses the problems of severe electron-hole recombination in pure-phase BiOBr, resulting in low quantum efficiency and insufficient catalytic activity in photocatalytic nitrogen fixation, by introducing biochar as an electron storage center. The composite material achieves a breakthrough in activity: in sacrificial agent-free pure water, the introduction of BC expands the catalyst's photoresponse range, enabling effective electron-hole separation and achieving photocatalytic nitrogen fixation with an ammonia formation rate reaching 174.33 μmol·g⁻¹. -1 ·h -1 It is 5 times that of the existing pure BiOBr system and close to the performance of the noble metal co-catalyst system.

[0012] 3. The composite material of this application solves the problems of high cost, insufficient environmental friendliness, and difficulty in achieving green and industrialized large-scale production of existing BiOBr modification processes by adopting high-temperature sintering and solvothermal methods.

[0013] The composite material of this application has cost advantages: the raw material is corn straw biomass, which does not require precious metals (Au, Pt, Pd) or rare earth elements, and the total cost is only 1 / 10 to 1 / 12 of the iron complex system described in WO2025 / 123456.

[0014] Environmentally friendly: The preparation process uses no toxic solvents, avoiding the environmental pollution risks caused by the large-scale use of strong reducing agents (such as sodium borohydride) in traditional methods. At the same time, the conversion of corn straw biomass into biochar also reduces carbon dioxide emissions to a certain extent. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the provided drawings without creative effort.

[0016] Figure 1 The image shows a SEM image of BC / BiOBr-0.8, illustrating the directional growth of BiOBr nanosheets on the BC surface.

[0017] Figure 2 The XRD pattern shows a comparison between the composite sample and the standard card, indicating that the sample was successfully prepared.

[0018] Figure 3 (a) Comparison of nitrogen fixation activity of composite materials; (b) Blank control experiment, confirming that the nitrogen source for ammonia production is nitrogen gas introduced into the experimental system.

[0019] Figure 4 Five photocatalytic cycle experiments showed that the composite material has high stability and potential application value.

[0020] Figure 5 (a) XRD and (b) SEM comparison of the BC / BiOBr-0.8 sample before and after reaction show that the composite material has good structural stability.

[0021] Figure 6 (a) Solid-state UV spectrum; (b) PL spectrum; (c) Photocurrent spectrum; (d) Impedance spectrum. This confirms that the BC / BiOBr-0.8 sample after BC recombination broadens the photoresponse range, accelerates the separation of photogenerated carriers, and to some extent hinders their recombination.

[0022] Figure 7 The work functions of BC and BiOBr are obtained through UPS testing.

[0023] Figure 8 Comparison of photocatalytic ammonia yields with similar photocatalysts.

[0024] Figure 9 Carrier lifetime testing of biochar / bismuth oxybromide-0.8 and bismuth oxybromide samples. Detailed Implementation

[0025] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0026] Specifically, this invention relates to a three-dimensional close-contact photocatalyst formed by combining porous multilayer biochar (BC) derived from corn stalk biomass through high-temperature sintering with bismuth oxybromine (BiOBr) nanoflowers, its controllable preparation method, and the application of this catalyst in driving nitrogen reduction to synthesize ammonia under simulated full-spectrum sunlight.

[0027] A bismuth oxybromine / biochar composite material for nitrogen fixation is disclosed. The bismuth oxybromine / biochar composite photocatalyst consists of layered porous biochar and rose-shaped bismuth oxybromine. Bismuth oxybromine nanosheets are vertically or obliquely grown on the two-dimensional surface and within the three-dimensional pores of the biochar, forming an 0D-2D face-to-face contact heterostructure, rather than the traditional point-to-face contact. This hierarchical structure of "three-dimensional porous carbon framework-two-dimensional bismuth oxybromine nanosheets" achieves interfacial electronic coupling and electron channel transport through CO-Bi chemical bonds, and simultaneously constructs a Schottky heterojunction. Due to the difference in work function between the two materials and the unique electron storage function of biochar, a stable electron source is provided for the subsequent nitrogen reduction reaction. A built-in electric field is formed at the interface, and dynamic electron recombination between the interfaces is completed. This electron transfer accelerates the separation of photogenerated electrons and holes in bismuth oxybromine, thereby promoting the photocatalytic nitrogen fixation rate.

[0028] A method for preparing a bismuth oxybromine / biochar composite material for nitrogen fixation. Step 1: Controllable preparation of corn straw biomass BC carrier: The original BC was obtained by pyrolysis of corn stalks at 600℃ under limited oxygen. Step 2: Chemical bond-induced in-situ growth: A simple one-step solvothermal method was employed. Ethylene glycol was prepared in a beaker, and BC and Bi(NO3)3•5H2O were added and stirred until complete. KBr was then added, and the mixture was stirred thoroughly. The resulting solution was transferred to a polytetrafluoroethylene (PTFE) reactor, sealed, and heated to 160°C for 16 h. This directional growth of BiOBr on the BC surface was induced by chemical bonding. After cooling to room temperature, the sample was first washed repeatedly by centrifugation with deionized water, then repeatedly by centrifugation with ethanol. Finally, the sample was dried overnight in an oven at 80°C. The dried sample was then ground to obtain BiOBr nanoparticle powder, labeled BC / BiOBr-X. The mass ratio of BC to BiOBr was 1:4 to 1:12.

[0029] This invention utilizes corn stalk biomass-derived BC to modify BiOBr as a photocatalytic nitrogen fixation material, making it more environmentally friendly and innovative. It aims to overcome the problems of existing BiOBr-based photocatalytic nitrogen fixation materials, such as small photoresponse range, high photogenerated carrier recombination rate, easy aggregation, instability, and environmental unfriendliness, and provides a low-cost, highly active, and long-life BC / BiOBr composite photocatalyst, achieving the following innovations: 1. Cross-disciplinary application innovation: For the first time, BC / BiOBr composite materials have been extended from the field of pollutant degradation to the field of photocatalytic nitrogen fixation, realizing the resource utilization of nitrogen; 2. Structural innovation: A hierarchical structure of "three-dimensional porous carbon framework-two-dimensional BiOBr nanosheets" is constructed, and the interfacial electronic coupling and electronic channel transport are realized through CO-Bi chemical bonds; 3. This invention utilizes BC as an electron reservoir center to capture and enrich photogenerated electrons of BiOBr through interface coupling, thereby achieving the synergistic control of spatial carrier separation and local electronic state density.

[0030] This invention is the first to discover that using corn stalk BC as a three-dimensional conductive framework to construct a surface-to-surface contact heterostructure with BiOBr not only enhances N2 transfer by utilizing the microporous enrichment effect of BC, but more importantly, BC has high conductivity and a rich conjugated π electron system. As an electron reservoir center, it can first transfer electrons to BiOBr to reach a Fermi level equilibrium state, forming a built-in electric field. Driven by the direction of the built-in electric field, it can capture photogenerated electrons transferred from BiOBr, achieving effective electron-hole separation. At the same time, the stored electrons migrate directionally to the surface active sites through the interface Bi-OC bond, participating in the multi-electron reduction process of N2 molecules, enhancing the nitrogen fixation reaction kinetics, and ensuring that the composite catalyst remains inactive after five cycles of experiments. This opens up new avenues for the application of biochar-based materials in the field of nitrogen fixation.

[0031] Example 1: Preparation method of bismuth oxybromide / biochar composite material for nitrogen fixation: Step 1: Controllable preparation of corn straw biomass BC carrier: The original BC was obtained by pyrolysis of corn stalks at 600℃ under limited oxygen. Step 2: Chemical bond-induced in-situ growth: A simple one-step solvothermal method was employed. Ethylene glycol was prepared in a beaker, and BC and Bi(NO3)3•5H2O were added and stirred until complete. KBr was then added, and the mixture was stirred thoroughly. The resulting solution was transferred to a polytetrafluoroethylene (PTFE) reactor, sealed, and heated to 160°C for 16 h. This directional growth of BiOBr on the BC surface was achieved through chemical bonding induction, with a BC to BiOBr mass ratio of 1:4. After cooling to room temperature, the sample was first washed repeatedly by centrifugation with deionized water, then repeatedly by centrifugation with ethanol. Finally, the sample was dried overnight in an oven at 80°C. The dried sample was then ground to obtain BiOBr nanoparticle powder, labeled BC / BiOBr-X.

[0032] The key features of the bismuth oxybromine / biochar composite material for nitrogen fixation in this application are: Hierarchical structure: BiOBr nanosheets grow vertically or obliquely on the two-dimensional surface and three-dimensional pores of BC, forming an 0D-2D surface-to-surface contact heterostructure, rather than the traditional point-to-surface contact.

[0033] Unique electron storage effect: The high conductivity of the BC graphitized carbon framework, the electron buffering capacity of the delocalized π bonds, and the abundant surface defect sites can serve as efficient electron storage centers, receiving and enriching photogenerated electrons transferred from BiOBr, providing a stable electron source for subsequent nitrogen reduction reactions.

[0034] Optimize the mass ratio: The mass ratio of BC to BiOBr is 1:4 to 1:12, preferably 1:6 to 1:10, to balance conductivity and exposure of active sites.

[0035] Example 2: The mass ratio of BC to BiOBr is 1:6, and the other preparation methods are the same as in Example 1.

[0036] Example 3: The mass ratio of BC to BiOBr is 1:7, and the other preparation methods are the same as in Example 1.

[0037] Example 4: The mass ratio of BC to BiOBr is 1:8, and the other preparation methods are the same as in Example 1.

[0038] Example 5: The mass ratio of BC to BiOBr is 1:9, and the other preparation methods are the same as in Example 1.

[0039] Example 6: The mass ratio of BC to BiOBr is 1:10, and the other preparation methods are the same as in Example 1.

[0040] Application of a bismuth oxybromine / biochar composite material for nitrogen fixation in photocatalytic nitrogen fixation.

[0041] The aforementioned application, specifically the process steps for photocatalytic nitrogen fixation using the bismuth oxybromide / biochar composite material, includes: a. At 3℃, 0.05g of bismuth oxybromine / biochar composite material was added to a reaction system containing 250mL of deionized water to construct a photocatalytic nitrogen fixation reaction system; b. Carry out the photocatalytic reaction under xenon lamp illumination, and take 5 mL of reaction solution every 1 hour during the reaction process; c. Add a small amount of potassium sodium tartrate and 50 μL Nessler's reagent to the 5 mL reaction solution and let stand for 15 min to develop color. d. After 15 minutes of color development, the absorbance of the reaction solution at a wavelength of 420 nm was measured using a UV-Vis spectrophotometer. The measured value was then substituted into a pre-established standard curve to obtain the amount of ammonia generated.

[0042] The dosage of the bismuth oxybromide / biochar composite material in step a is 0.05 g, the total volume of the reaction solution is 250 mL, and the reaction temperature is 3 °C.

[0043] The potassium sodium tartrate and Nessler's reagent mentioned in step c are used for colorimetric detection of ammonia in the reaction solution, and the standard curve mentioned in step d is a pre-established curve showing the relationship between ammonia concentration and absorbance.

[0044] The amount of ammonia generated in the reaction system was calculated based on the absorbance value measured at 420 nm using the standard curve, in order to characterize the photocatalytic nitrogen fixation performance of the bismuth oxybromine / biochar composite material.

[0045] The unique application conditions of the composite photocatalyst of this invention in photocatalytic nitrogen fixation are as follows: the difference in work function between biochar and BiOBr in this catalyst leads to the spontaneous redistribution of electrons from BC to BiOBr, thereby achieving Fermi level equilibrium and forming a built-in electric field with interfacial polarization. Under illumination, the built-in electric field drives photogenerated electrons toward BC while retaining holes in BiOBr, thus enhancing charge separation and suppressing recombination. The conductive BC acts as an electron storage center, enabling surface electron accumulation to sustain the multi-electron nitrogen reduction reaction. The synergistic effect of interfacial electric field modulation, efficient carrier separation, and surface electron enrichment creates the unique application conditions of this composite catalyst in photocatalytic nitrogen fixation.

[0046] Sacrificial agent-free system: Nitrogen fixation is carried out in pure water to avoid the oxidation of photogenerated holes by sacrificial agents. In this system, the ammonia formation rate is 61.95-174.33 μmol·g. -1 ·h -1 .

[0047] This application has achieved unexpected technical effects: In the prior art, bismuth oxybromine / biochar composite photomaterials were originally used for catalytic degradation of pollutants. This application is the first to use them for photocatalytic nitrogen fixation. Pure BiOBr has low photocatalytic nitrogen fixation performance, while this invention utilizes the micropore enrichment effect of BC, which not only transfers electrons in BiOBr to the BC surface and accelerates the separation of photogenerated carriers, but also increases the N2 adsorption capacity due to the porous multilayer structure, thereby improving the photocatalytic nitrogen fixation performance. This is a parameter that does not need to be considered in the pollutant degradation system.

[0048] The differences between this application and the closest prior art are: 1. Comparative literature (China Environmental Science, 2025, 45(1): 310-318): Although the BC / BiOBr composite material is disclosed, its technical problem is the degradation of organic pollutants. The technical effect is based on the oxidation of ·OH free radicals, utilizing the oxidation end of the catalyst, and the activity evaluation index is the tetracycline removal rate. The technical problem of this invention is N2 reduction, utilizing the reduction end of the catalyst. The reaction mechanism is electron transfer reduction, and the evaluation index is the ammonia generation rate. The two are fundamentally different in terms of reaction thermodynamics, kinetics, and active site requirements, and belong to technical problems in different technical fields. At the same time, the composite material synthesized in the literature has a plate-like morphology, which is significantly different from the rose-like morphology of BiOBr grown in situ on biochar in this invention. This morphology increases the specific surface area and increases the active sites, thereby improving the photocatalytic nitrogen fixation performance. This is an essential difference in morphology caused by different synthesis methods.

[0049] 2. Patent WO2025 / 123456 adopts an iron-based complex strategy, while this invention adopts a biochar and BiOBr composite strategy. The raw material sources, cost structure, and preparation process are completely different, and it has independent intellectual property space.

[0050] 3. For example Figure 8 The table below shows a detailed comparison of existing photocatalytic nitrogen fixation photocatalysts.

[0051] 4. For example Figure 9 As shown in the table below, the carrier lifetime tests of the biochar / bismuth oxybromine-0.8 and bismuth oxybromine-0.8 samples of this application showed that the carrier lifetime of pure bismuth oxybromine-0.8 was 0.384 ns, while that of the biochar / bismuth oxybromine-0.8 composite sample increased to 0.403 ns. This extended lifetime directly reflects the effective suppression of photogenerated carrier recombination and optimization of charge dynamics by biochar, playing a crucial role in photocatalytic nitrogen fixation. The extended carrier lifetime of biochar / bismuth oxybromine-0.8 demonstrates the role of biochar as an electron trap and transport medium, effectively separating photogenerated carriers, mitigating their recombination to a certain extent, and improving charge utilization efficiency. This provides more sufficient and longer-lasting electrons for N2 reduction, which is the core basis for the improved nitrogen fixation performance of this composite material.

[0052]

[0053] The present invention and its embodiments have been described above illustratively. This description is not restrictive, and the figures shown are only one embodiment of the present invention; the actual structure is not limited thereto. Therefore, if those skilled in the art, inspired by this description, design similar structures and implementations to the above embodiments without departing from the technical essence of the present invention, such designs should fall within the protection scope of the present invention.

Claims

1. A bismuth oxybromine / biochar composite material for nitrogen fixation, characterized in that, The bismuth oxybromine / biochar composite material consists of layered porous biochar and rose-shaped bismuth oxybromine. Bismuth oxybromine nanosheets grow vertically or obliquely on the two-dimensional surface and inside the three-dimensional pores of the biochar, forming an 0D-2D surface-to-surface contact heterostructure, rather than the traditional point-to-surface contact. This hierarchical structure of "three-dimensional porous carbon framework-two-dimensional bismuth oxybromine nanosheets" achieves interfacial electronic coupling and electron channel transport through CO-Bi chemical bonds, and simultaneously constructs a Schottky heterojunction. Due to the difference in work function between the two and the unique electron storage function of biochar, a stable electron source is provided for the subsequent nitrogen reduction reaction. A built-in electric field is formed at the interface, and dynamic electron recombination between the interfaces is completed. This electron transfer accelerates the separation of photogenerated electrons and holes in bismuth oxybromine, thereby promoting the rate of photocatalytic nitrogen fixation.

2. A method for preparing a bismuth oxybromine / biochar composite material for nitrogen fixation, characterized in that: Step 1: Controllable preparation of corn straw biomass BC carrier: The original BC was obtained by pyrolysis of corn stalks at 600℃ under limited oxygen. Step 2: Chemical bond-induced in-situ growth: A simple one-step solvothermal method was used. Ethylene glycol was prepared in a beaker, and BC and Bi(NO3)3•5H2O were added and stirred until complete. KBr was then added and stirred thoroughly. The mixed solution was then transferred to a polytetrafluoroethylene reactor, sealed and heated to 160℃ for 16 h. BiOBr was directionally grown on the BC surface through chemical bonding induction. After cooling to room temperature, the sample was first washed by centrifugation with deionized water, then washed by centrifugation with ethanol, and finally dried in an oven at 80℃ overnight. After drying, BiOBr nanoflower powder was obtained by grinding and labeled as BC / BiOBr-X.

3. The method for preparing a bismuth oxybromine / biochar composite material for nitrogen fixation according to claim 2, characterized in that: The mass ratio of BC to BiOBr is 1:4 to 1:

12.

4. The method for preparing a bismuth oxybromine / biochar composite material for nitrogen fixation according to claim 3, characterized in that: The mass ratio of BC to BiOBr is 1:6, 1:7, 1:8, 1:9, or 1:

10.

5. The application of a bismuth oxybromine / biochar composite material for nitrogen fixation according to any one of claims 1-4 in photocatalytic nitrogen fixation.

6. The application according to claim 5, characterized in that: The process steps for photocatalytic nitrogen fixation using the aforementioned bismuth oxybromide / biochar composite material include: a. At 3℃, 0.05g of bismuth oxybromine / biochar composite material was added to a reaction system containing 250mL of deionized water to construct a photocatalytic nitrogen fixation reaction system; b. Carry out the photocatalytic reaction under xenon lamp illumination, and take 5 mL of reaction solution every 1 hour during the reaction process; c. Add a small amount of potassium sodium tartrate and 50 μL Nessler's reagent to the 5 mL reaction solution and let stand for 15 min to develop color. d. After 15 minutes of color development, the absorbance of the reaction solution at a wavelength of 420 nm was measured using a UV-Vis spectrophotometer. The measured value was then substituted into a pre-established standard curve to obtain the amount of ammonia generated.

7. The application according to claim 6, characterized in that: The dosage of the bismuth oxybromide / biochar composite material in step a is 0.05 g, the total volume of the reaction solution is 250 mL, and the reaction temperature is 3 °C.

8. The application according to claim 6, characterized in that: The potassium sodium tartrate and Nessler's reagent mentioned in step c are used for colorimetric detection of ammonia in the reaction solution, and the standard curve mentioned in step d is a pre-established curve showing the relationship between ammonia concentration and absorbance.

9. The application according to claim 6, characterized in that: The amount of ammonia generated in the reaction system was calculated based on the absorbance value measured at 420 nm using the standard curve, in order to characterize the photocatalytic nitrogen fixation performance of the bismuth oxybromine / biochar composite material.

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