A photocatalyst with a double heterojunction structure and a preparation method and application thereof

Abstract

This invention discloses a photocatalyst with a dual heterojunction structure, its preparation method, and its application. First, a solution containing iodide and a solution containing bismuth salt are mixed and the pH is adjusted to alkaline. A hydrothermal reaction is then carried out at 150–200°C. The resulting product is washed and dried to obtain product BiOI. Next, the prepared BiOI is calcined at 350–420°C to obtain product BiOI / Bi5O7I. Then, BiOI / Bi5O7I and Mn3O4 are dissolved in a first organic solvent, followed by a hydrothermal reaction at 150–200°C. The resulting product is washed and dried to obtain product Mn3O4 / BiOI / Bi5O7I. This invention introduces a Mn3O4 / BiOI / Bi5O7I dual heterojunction constructed using Mn3O4 as a redox electron mediator, ensuring that the photocatalyst maintains a suitable valence band position, thereby maintaining a strong redox reaction capability. Therefore, it can effectively remove NO and inhibit the formation of NO2 toxic byproducts.

Description

Technical Field

[0001] This invention belongs to the field of denitrification technology, and relates to denitrification catalyst technology, particularly to a photocatalyst with a dual heterojunction structure, its preparation method, and its application. Background Technology

[0002] One of the key aspects of photocatalytic denitrification technology is the selection of excellent photocatalysts. BiOX photocatalysts, with their typical layered structure, excellent carrier transport properties, good chemical stability, non-toxicity, and corrosion resistance, have attracted much attention in the field of denitrification. However, the rapid regeneration of photogenerated electron-hole pairs and the formation of NO2 toxic byproducts greatly limit their practical application.

[0003] To improve the catalytic efficiency of BiOI, researchers have conducted various studies, including noble metal deposition, morphology regulation, and heterostructure construction. Studies have shown that one effective method to enhance the photocatalytic activity of p-type semiconductor BiOI is coupling it with a wide-bandgap semiconductor to promote charge separation efficiency. Therefore, other photocatalysts are often grown on the surface of BiOI photocatalysts or BiOI photocatalysts are adhered to the surface of other photocatalysts to form composite structures that achieve the corresponding redox potentials and carry out corresponding redox reactions. This allows composite catalysts to be applied to areas where single catalysts are not feasible.

[0004] For example, the publicly disclosed Bi₂O₃-BiOI heterojunction has been found to exhibit higher photocatalytic activity than either BiOI or Bi₂O₃ alone in the photocatalytic reduction of Cr(VI) due to its higher photogenerated electron-hole pair separation efficiency (Chemosphere, Volume 257, 2020, 127210). However, the improved photogenerated electron-hole separation efficiency of a single heterojunction comes at the cost of reduced oxidation capacity of both semiconductor photocatalysts. Furthermore, certain redox potentials are required to drive certain reactions, which is detrimental to the occurrence of photocatalytic reactions. Summary of the Invention

[0005] The purpose of this invention is to address the aforementioned technical problems in the prior art by providing a photocatalyst with a dual heterojunction structure and its preparation method. The method combines a bismuth-rich strategy with heterojunction construction to modify BiOI, thereby improving the catalyst's removal efficiency for low-concentration (sub-ppm) NO under light irradiation, converting NO into nitrate / nitrite, and effectively suppressing the formation of the byproduct NO2.

[0006] Another object of the present invention is to provide the application of the above-mentioned photocatalyst having a dual heterojunction structure.

[0007] This invention first prepares BiOI using a one-step hydrothermal method under different pH conditions (alkaline, acidic, or neutral) to determine the optimal pH value for the preparation process. Then, a bismuth-rich strategy is employed to partially convert BiOI into bismuth-rich Bi5O7I through high-temperature calcination, constructing a BiOI / Bi5O7I heterojunction photocatalyst. Next, a secondary hydrothermal method is used to directionally assemble Mn3O4 prepared by the hydrothermal method onto the surface of BiOI / Bi5O7I, constructing a Mn3O4 / BiOI / Bi5O7I double heterojunction photocatalyst system. Here, Mn3O4 acts as a redox electron mediator, ensuring that the photocatalyst maintains a suitable valence band position, thereby maintaining strong redox reactivity. The denitrification performance of the prepared heterojunction photocatalysts was investigated under simulated sunlight conditions, providing a new approach for the removal of low-concentration gaseous pollutant NO.

[0008] Based on the above analysis, the method for preparing a photocatalyst with a dual heterojunction structure provided by the present invention includes the following steps:

[0009] (1) Preparation of BiOI

[0010] A solution containing iodide is mixed with a solution containing bismuth salt, and the pH is adjusted to alkaline. The mixture is then subjected to a hydrothermal reaction at 150–200 °C. The resulting product is washed and dried to obtain product BiOI. The molar ratio of the iodide salt to the bismuth salt is 1:1.

[0011] (2) Preparation of BiOI / Bi5O7I

[0012] The BiOI prepared in step (1) was calcined at 350-420℃ to obtain the product BiOI / Bi5O7I;

[0013] (3) Preparation of Mn3O4 / BiOI / Bi5O7I

[0014] BiOI / Bi5O7I and Mn3O4 are dissolved in a first organic solvent, and then subjected to a hydrothermal reaction at 150-200℃. The resulting product is washed and dried to obtain product Mn3O4 / BiOI / Bi5O7I. The mass fraction of Mn3O4 in the sum of BiOI / Bi5O7I and Mn3O4 is 1%-10%.

[0015] In step (1) above, the iodide-containing solution is obtained by dissolving the iodide in deionized water to form a colorless and transparent solution; the iodide is KI or NaI. The bismuth salt-containing solution is obtained by dissolving the bismuth salt in a mixed solution of deionized water and nitric acid to form a colorless and transparent solution; the bismuth salt is bismuth nitrate or bismuth nitrate hydrate; the volume ratio of deionized water to nitric acid in the mixed solution is 2:1. After mixing the iodide-containing solution and the bismuth salt-containing solution, the pH value is adjusted to 9-13 using KOH or NaOH solution. The hydrothermal reaction time is 16-24 hours.

[0016] In step (2) above, the temperature is raised to 350–420°C at a heating rate of 2–5°C / min, and the calcination time is 1–3 h. After calcination, the product BiOI / Bi5O7I is cooled to room temperature before being removed. The BiOI content in the prepared BiOI / Bi5O7I is 80%–90%.

[0017] In step (3) above, Mn3O4 is prepared as follows: potassium permanganate is dissolved in deionized water, and then, under stirring conditions, a second organic solvent is added dropwise to the potassium permanganate solution to obtain a black viscous mixture; the resulting viscous mixture is heated to react at 140–180°C, and the reaction product is washed and dried to obtain a brown solid, which is Mn3O4. The reaction time here is 8–16 h. The volume ratio of the second organic solvent to deionized water is 1:8–10.

[0018] In step (3) above, BiOI / Bi5O7I and Mn3O4 are mixed in an organic solvent and stirred for 1-3 hours to form a uniformly distributed suspension, followed by a hydrothermal reaction. The hydrothermal reaction time is 8-16 hours. In a preferred embodiment, the mass fraction of Mn3O4 in the sum of BiOI / Bi5O7I and Mn3O4 is 5%-10%.

[0019] The first organic solvent may be the same as or different from the second organic solvent, and may be ethylene glycol or ethanol, respectively.

[0020] The present invention also provides a photocatalyst with a dual heterojunction structure obtained by the above preparation method.

[0021] The present invention also provides the application of photocatalysts with a dual heterojunction structure in denitrification, for suppressing NO2 production and removing NO.

[0022] Compared with the prior art, the method for preparing a photocatalyst with a dual heterojunction structure provided by the present invention has the following advantages:

[0023] Beneficial effects:

[0024] (1) This invention is the first to successfully construct a photocatalyst with a Mn3O4 / BiOI / Bi5O7I double heterojunction structure using a secondary hydrothermal method:

[0025] (2) Although the Mn3O4 / BiOI / Bi5O7I dual heterojunction catalyst system constructed in this invention does not have a very high efficiency in separating photogenerated electron-hole pairs, Mn3O4, as a redox electron mediator, ensures that the photocatalyst can maintain a suitable valence conduction band position, thereby maintaining a strong redox reaction capability; therefore, it can effectively remove NO and inhibit the generation of NO2 toxic byproducts.

[0026] (3) The Mn3O4 / BiOI / Bi5O7I double heterojunction constructed in this invention can completely suppress the generation of NO2. Attached Figure Description

[0027] Figure 1 The XRD test results are for the BiOI prepared in Example 1;

[0028] Figure 2 The results of the photocatalytic activity evaluation experiment of BiOI prepared in Example 1 are shown; where (a) corresponds to NO removal rate and (b) corresponds to NO2 generation.

[0029] Figure 3 The XRD test results are for BiOI / Bi5O7I prepared in Example 2;

[0030] Figure 4 SEM images of BiOI-pH12 (a) and BiOI / Bi5O7I (b) prepared in Example 1;

[0031] Figure 5 The 4f spectra of BiOI-pH12 (a) and BiOI / Bi5O7I (b) prepared in Example 1;

[0032] Figure 6 The results of the photocatalytic activity evaluation of BiOI / Bi5O7I and BiOI-pH12 prepared in Example 2 are shown; where (a) corresponds to NO removal rate and (b) corresponds to NO2 generation.

[0033] Figure 7 The XRD test results are for Mn3O4 and Mn3O4 / BiOI / Bi5O7I prepared in Example 3;

[0034] Figure 8 SEM images of Mn3O4 and Mn3O4 / BiOI / Bi5O7I prepared in Example 3; where (a) corresponds to Mn3O4, and (b)-(d) correspond to BBM-1%, BBM-5%, and BBM-10%, respectively.

[0035] Figure 9 The results of the photocatalytic activity evaluation experiments of BBM-1%, BBM-5%, BBM-10%, and BiOI / Bi5O7I and Mn3O4 prepared in Example 3 are shown; where (a) corresponds to NO removal rate and (b) corresponds to NO2 generation.

[0036] Figure 10 The results of the photocatalytic cycle test of BBM-10% prepared in Example 3 are shown; where (a) corresponds to the NO removal rate and (b) corresponds to the NO2 generation. Detailed Implementation

[0037] The present invention will be specifically described below through embodiments. It should be noted that these embodiments are only used to further illustrate the present invention and should not be construed as limiting the scope of protection of the present invention. Those skilled in the art can make some non-essential improvements and adjustments based on the above content of the present invention.

[0038] The experimental steps for evaluating photocatalytic activity in the following examples are as follows:

[0039] (1) Place the petri dish containing 0.3g of photocatalyst in the reaction chamber, place a 300W xenon lamp light source directly above the petri dish, and purge the air in the reaction chamber with N2; the solvent in the reaction chamber is 0.2L;

[0040] (2) Turn on the xenon lamp light source to activate the photocatalyst. After one minute, introduce the mixed gas of N2, NO and O2 into the reaction chamber. The total flow rate of N2, NO and O2 is 100 mL / min, and the NO inlet concentration is 100 ± 5 ppm.

[0041] (3) Use an infrared flue gas analyzer to monitor and record NO and NO2 in real time;

[0042] (4) When the experiment is over, turn off the light source and introduce N2 to purge the gas from the reaction chamber.

[0043] In the following examples, the cyclic experiment was performed as follows: the photocatalyst after the reaction was completed was collected, placed in a centrifuge tube, submerged in deionized water for half an hour and washed by centrifugation 3 times, dried in a forced-air drying oven, and the photocatalytic activity evaluation experiment was performed again.

[0044] The photocatalytic activity evaluation method used in the following examples is as follows:

[0045] NO removal rate is calculated according to formula (1):

[0046]

[0047] In the formula, C0(NO) represents the initial concentration of NO (ppm); C t(NO) represents the real-time concentration of NO (ppm). The amount of NO2 generated is calculated according to formula (2):

[0048] NO2 generation (ppm) = C t (NO2)-C0(NO2)(2)

[0049] In the formula: C0(NO2) represents the initial concentration of NO2 (ppm); C t (NO2) indicates the real-time concentration (ppm) of NO2.

[0050] Example 1: Preparation of BiOI

[0051] 1.66 g (0.01 mol) of KI was dissolved in 10 mL of deionized water to form a colorless and transparent solution. 4.8507 g (0.01 mol) of Bi(NO3)3·5H2O was dissolved in a mixture of 10 mL of deionized water and 5 mL of nitric acid to form a colorless and transparent solution. Under continuous stirring, the KI solution was poured into the Bi(NO3)3·5H2O solution until fully mixed. Then, the pH of the mixed solution was adjusted to 2, 7, and 12 respectively using 2 mol / L NaOH solution. The mixed solution was transferred to a high-pressure reactor and reacted at 180 °C for 24 hours. After cooling to room temperature, the resulting solids were washed three times by centrifugation with deionized water, and then dried at 60 °C for 12 hours to obtain the products BiOI, which were named BiOI-pH2, BiOI-pH7, and BiOI-pH12, respectively.

[0052] XRD analysis was performed on the prepared BiOI-pH2, BiOI-pH7, and BiOI-pH12, and the results are as follows: Figure 1 As shown in the figure, the diffraction peaks of the three samples are consistent with the standard card of BiOI. A small number of other diffraction peaks were detected in BiOI-pH2 and BiOI-pH7, while no impurity peaks were detected in BiOI-pH12 and the peak shape was sharp, indicating that its phase is pure and the target sample was successfully prepared.

[0053] Using the prepared BiOI-pH2, BiOI-pH7, and BiOI-pH12 as photocatalysts, the photocatalytic activity evaluation experiments were conducted on each sample according to the previously given experimental procedures. The test results are as follows: Figure 2 As shown.

[0054] Figure 2(a) shows the NO removal rate of the three samples. pH has a significant impact on the photocatalytic activity of BiOI. BiOI-pH12 exhibits the best photocatalytic performance, with a NO removal efficiency of approximately 15.9% after 30 min of reaction. BiOI-pH2 is the second best, with a NO removal efficiency of 11.8% after 30 min. BiOI-pH7, after reaching adsorption-desorption equilibrium, shows a NO concentration that rises to approximately 118 ppm, gradually decreasing to the initial value of 100 ppm after 60 min of reaction, indicating almost no photocatalytic activity. Since the three samples have the same band gap, the lack of photocatalytic activity in BiOI-pH7 may be due to the excessively high recombination rate of photogenerated electron-hole pairs.

[0055] Figure 2 (b) shows the NO2 production of the three samples. At the beginning of the photocatalytic activity evaluation, the NO2 production becomes negative due to adsorption, and then the increase tends to level off. BiOI-pH2 had the highest NO2 production, close to 5 ppm. Considering both NO removal rate and NO2 production, BiOI-pH12 performed best. However, its NO removal rate was only 15.9%.

[0056] Example 2: Preparation of BiOI / Bi5O7I

[0057] The BiOI-pH12 prepared in Example 1 was placed in a crucible, covered, and placed in a muffle furnace. The temperature was increased to 390°C at a rate of 5°C / min and calcined for 2 hours. After the muffle furnace cooled to room temperature, the sample was taken out and named BiOI / Bi5O7I.

[0058] XRD analysis was performed on the prepared BiOI / Bi5O7I, and the results are as follows: Figure 3 As shown. From Figure 3 It can be seen that BiOI remains the main crystalline phase of the catalyst, with the BiOI content in the prepared BiOI / Bi5O7I being approximately 80%–90%. The peak position shifts to a smaller angle relative to the standard card, and its characteristic diffraction peak at 45.26° corresponds to the (405) plane of Bi5O7I, indicating that BiOI is partially converted to Bi5O7I, forming a BiOI / Bi5O7I composite photocatalyst. The shift of the spectral diffraction peak to a smaller angle caused by high-temperature calcination may be due to the expansion and deformation of the BiOI crystal structure caused by iodine defects and bismuth enrichment resulting from the phase transformation.

[0059] SEM images of BiOI-pH12 and BiOI / Bi5O7I are shown below. Figure 4As shown, both BiOI-pH12 and BiOI / Bi5O7I exhibit nanoflower structures assembled from nanosheets. The BiOI / Bi5O7I nanosheets obtained by high-temperature calcination are more densely stacked, the microsphere diameter is slightly smaller, and the spherical morphology is more obvious, which may be more conducive to light absorption.

[0060] Figure 5 The Bi 4f spectrum of the catalyst is shown. For BiOI-pH12, the two binding peaks at 159.2 eV and 164.5 eV correspond to Bi 4f, respectively. 7 / 2 and Bi 4f 5 / 2 This indicates that Bi is in Bi 3+ It exists in the form of BiOI; for BiOI / Bi5O7I, two binding peaks at 159.0 eV and 164.3 eV can be observed, shifting to a lower binding energy by 0.2 eV compared to BiOI-pH12. This may be due to the formation of Bi5O7I caused by high-temperature calcination, and the interaction between the two interfaces of BiOI and Bi5O7I altering the Bi... 3+ This is due to the change in electron cloud density caused by the electronic properties of the electron.

[0061] Using the prepared BiOI-pH2 and BiOI / Bi5O7I as photocatalysts, the photocatalytic activity evaluation experiments were conducted on each sample according to the previously given experimental procedures. The test results are as follows: Figure 6 As shown.

[0062] from Figure 6 (a) It can be seen that the NO removal rate of BiOI / Bi5O7I is 30.6%, which is nearly double that of BiOI-pH12 (15.9%). This may be attributed to the stronger visible light response and higher photogenerated electron-hole pair separation efficiency of BiOI / Bi5O7I formed after high-temperature calcination. BiOI and Bi5O7I are typical p-type and n-type semiconductors, respectively. In the pn heterojunction, electrons flow on the surface of the two types of semiconductors to form an internal electric field. When exposed to light, photogenerated electrons on the CB of BiOI are transferred to the CB of Bi5O7I, while holes remain in VB, thus effectively separating the photogenerated electron-hole pairs.

[0063] from Figure 6 (b) It can be seen that the NO2 production decreases due to adsorption, but as time goes on, the NO2 production increases and then levels off at around 8.9 ppm. This indicates that BiOI / Bi5O7I is not very effective at suppressing NO2 byproducts.

[0064] Example 3: Preparation of Mn3O4 / BiOI / Bi5O7I

[0065] Preparation of Mn3O4: 1.308 g of KMnO4 was dissolved in 20 mL of deionized water. Under stirring, 2 mL of ethylene glycol was slowly added to the solution, resulting in a brownish-black viscous mixture. The mixture was then transferred to a high-pressure reactor and reacted at 160 °C for 12 hours. After cooling to room temperature, the solid obtained after the reaction was washed three times with deionized water by centrifugation and then dried at 60 °C for 12 hours. The resulting brown solid was Mn3O4.

[0066] Preparation of Mn3O4 / BiOI / Bi5O7I: 0.88 g (0.0025 mol) of BiOI / Bi5O7I and three different amounts of Mn3O4 were dissolved in 15 mL of ethylene glycol. After stirring for 2 hours, the solution was transferred to a high-pressure reactor and reacted at 180 °C for 12 hours. After cooling to room temperature, the resulting solid was washed three times with deionized water by centrifugation and then dried at 60 °C for 12 hours to obtain Mn3O4 / BiOI / Bi5O7I. The amounts of the three types of Mn3O4 used accounted for 1%, 5%, and 10% of the total mass fraction of BiOI / Bi5O7I and Mn3O4, respectively. The three samples were named BBM-1%, BBM-5%, and BBM-10%, respectively.

[0067] The XRD patterns of the synthesized Mn3O4 and the BBM-1%, BBM-5%, and BBM-10% samples synthesized by combining it with BiOI / Bi5O7I are shown below. Figure 7 As shown, the characteristic peaks of Mn3O4 are weak and broad, which may be due to the amorphous structure of the synthesized Mn3O4. For the BBM sample, its diffraction peaks are basically consistent with the BiOI standard card (JCPDS No. 10-0445) and slightly shifted to a lower angle, while the peaks belonging to Mn3O4 are almost unobservable. This is mainly because Mn3O4 is highly dispersed on the highly crystalline BiOI / Bi5O7I. The above results indicate that the Mn3O4 / BiOI / Bi5O7I composite catalyst has been successfully constructed.

[0068] SEM images of the synthesized Mn3O4 and the BBM-1%, BBM-5%, and BBM-10% samples synthesized by combining it with BiOI / Bi5O7I are shown below. Figure 8 As shown. From Figure 8 (a) It can be seen that the prepared Mn3O4 exhibits a regular octahedral morphology. From Figure 8 As can be seen from (b)-(d), Mn3O4 particles are attached to BiOI / Bi5O7I nanosheets, and the number of particles deposited on BiOI / Bi5O7I nanosheets increases significantly with the increase of the Mn3O4 composite ratio; these results indicate that the Mn3O4 / BiOI / Bi5O7I photocatalyst has been successfully constructed.

[0069] Using the prepared Mn3O4, BiOI / Bi5O7I, and BBM-1%, BBM-5%, and BBM-10% as photocatalysts, the photocatalytic activity evaluation experiments were conducted on each sample according to the previously given experimental procedures. The test results are as follows: Figure 9 As shown. From Figure 9 (a) It can be seen that Mn3O4 has almost no photocatalytic activity; the NO removal rate of BBM-1% is lower than that of pure BiOI / Bi5O7I, at only 12.4%; the composite ratios of 5% and 10% BBM-5% and BBM-10% show significant improvements compared to pure BiOI / Bi5O7I, at 64.3% and 83.8%, respectively. The enhanced light absorption capacity and reduced band gap may be the reasons for the high efficiency of BBM-5% and BBM-10% in photocatalytic NO removal. Figure 9 (b) It can be seen that the introduction of Mn3O4 inhibited the generation of NO2 by the photocatalyst. After 30 min of reaction, the NO2 generation of BBM-1% was 4.5 ppm, while BBM-10% completely inhibited the generation of NO2. Although the photogenerated electron-hole pair separation efficiency of BBM-10% is not very high, it has strong light absorption capacity and low band gap, and can efficiently remove NO by photocatalysis while inhibiting the generation of NO2 poisonous byproducts. It is a high-performing photocatalyst.

[0070] Taking BBM-10% as an example, a cyclic experiment was conducted to test its stability, and the results are as follows: Figure 10 As shown in the figure, the samples after the denitrification experiment were washed and dried, and then their photocatalytic activity was evaluated again. In four consecutive cycles, BBM-10% exhibited NO removal efficiencies of 83.8%, 75.4%, 80.5%, and 76.8%, respectively. The NO removal efficiency decreased slightly after cycling, but remained at a high level, indicating that simple deionized water washing can effectively restore the photocatalyst's activity. This may be because water washes away the products generated in the photocatalytic reaction, exposing the active sites and continuously generating oxygen vacancies in new reactions. Simultaneously, monitoring of NO2 in the cycling experiment revealed that BBM-10% still inhibited NO2 during cycling, although the inhibition ability was reduced. This may also be because the active sites occupied by the reaction products could not be completely recovered. Therefore, simple deionized water washing can remove most of the reaction products, release most of the active sites, essentially restore the NO removal capacity of BBM-10% and partially restore its NO2 inhibition ability, thus extending the photocatalyst's lifespan.

[0071] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, simplifications, or combinations made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A method for preparing a photocatalyst with a dual heterojunction structure, characterized in that, Includes the following steps: (1) Preparation of BiOI A solution containing iodide is mixed with a solution containing bismuth salt, and the pH is adjusted to alkaline. The mixture is then subjected to a hydrothermal reaction at 150-200°C. The resulting product is washed and dried to obtain product BiOI. The molar ratio of iodide to bismuth salt is 1:

1. (2) Preparation of BiOI / Bi5O7I The BiOI prepared in step (1) was calcined at 350~420℃ to obtain the product BiOI / Bi5O7I; (3) Preparation of Mn3O4 / BiOI / Bi5O7I BiOI / Bi5O7I and Mn3O4 were dissolved in a first organic solvent, and then subjected to a hydrothermal reaction at 150~200℃. The resulting product was washed and dried to obtain the product Mn3O4 / BiOI / Bi5O7I; the mass fraction of Mn3O4 in the sum of BiOI / Bi5O7I and Mn3O4 was 5%~10%. Mn3O4 is prepared by the following method: potassium permanganate is dissolved in deionized water, and then a second organic solvent is added dropwise to the potassium permanganate solution under stirring to obtain a black viscous mixture; the obtained viscous mixture is heated to react at 140~180℃, and the reaction product is washed and dried to obtain a brown solid, which is Mn3O4.

2. The method for preparing the photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, In step (1), the iodide-containing solution is obtained by dissolving the iodide in deionized water to form a colorless and transparent solution; the iodide is KI or NaI; the bismuth salt-containing solution is obtained by dissolving the bismuth salt in a mixed solution formed by deionized water and nitric acid to form a colorless and transparent solution; the bismuth salt is bismuth nitrate or bismuth nitrate hydrate; the volume ratio of deionized water to nitric acid in the mixed solution is 2:1; after mixing the iodide-containing solution and the bismuth salt-containing solution, the pH value is adjusted to 9-13 using KOH or NaOH solution.

3. The method for preparing the photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, In step (1), the hydrothermal reaction time is 16~24h.

4. The method for preparing the photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, In step (2), the temperature is raised to 350-420℃ at a heating rate of 2-5℃ / min, and the calcination time is 1-3h; the content of BiOI in the prepared BiOI / Bi5O7I is 80%-90%.

5. The method for preparing a photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, The resulting viscous mixture is heated for 8-16 hours; the volume ratio of the second organic solvent to deionized water is 1:8-10.

6. The method for preparing a photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, The first organic solvent may be the same as or different from the second organic solvent, namely ethylene glycol or ethanol.

7. The method for preparing a photocatalyst with a dual heterojunction structure according to claim 1, characterized in that, In step (3), BiOI / Bi5O7I and Mn3O4 are mixed in the first organic solvent and stirred for 1 to 3 hours, and then a hydrothermal reaction is carried out; the hydrothermal reaction time is 8 to 16 hours.

8. A photocatalyst with a dual heterojunction structure prepared by any one of claims 1 to 7.

9. The application of the photocatalyst with a dual heterojunction structure as described in claim 8 in denitrification.

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