BiPO4 / BiVO4 array structure photo-anode, preparation method and application of BiPO4 / BiVO4 array structure photo-anode

Abstract

The invention provides a BiPO4 / BiVO4 array structure photo-anode as well as a preparation method and application thereof. The BiPO4 / BiVO4 array structure photo-anode is prepared in situ through a three-step method of electro-deposition, calcination and photoelectrochemical activation. A BiPO4 / BiVO4 array structure photo-anode is used as a working electrode, a KHCO3 solution is used as an electrolyte, hydrogen peroxide is directionally generated through a double-electron water oxidation reaction under simulated sunlight irradiation, the unique photoelectrochemical self-sacrifice reconstruction in-situ growth mechanism of the BiPO4 / BiVO4 array structure photo-anode solves the problem of poor interface bonding force, and the photocatalytic performance of the BiPO4 / BiVO4 array structure photo-anode is greatly improved. The surface defect passivation and catalytic selectivity regulation and control dual functions are achieved, the preparation process is green and mild, high controllability is achieved, and the yield of photoelectrocatalytic synthesis of H2O2 is remarkably improved.

Description

Technical Field

[0001] This invention belongs to the field of photoelectric water splitting materials technology, specifically relating to a BiPO4 / BiVO4 array structure photoanode, its preparation method, and its application. Background Technology

[0002] Hydrogen peroxide (H2O2), as a green and efficient chemical oxidant, medical disinfectant, and fuel cell fuel, produces only oxygen and water after reaction, without secondary pollution. It has irreplaceable application value in environmental protection, medical and health care, fine chemical synthesis, and energy fields. Traditional H2O2 preparation is mainly based on the anthraquinone process. This process is complex, requiring organic solvents, precious metal Pd catalysts, and is carried out under high temperature and pressure conditions, resulting in huge energy consumption and the generation of large amounts of organic waste liquid, causing environmental pollution. Furthermore, the storage and long-distance transportation of the product pose significant safety hazards. To overcome these challenges, photoelectrocatalysis technology, with its unique advantage of synergistic "photo-electro-chemical" conversion, has become a research focus in the field of green H2O2 synthesis.

[0003] Among numerous semiconductor photoelectrocatalytic materials, BiVO4 is a core candidate photoanode material for the photoelectrocatalytic oxidation of water to synthesize H2O2. However, pure-phase BiVO4 has inherent performance bottlenecks: firstly, its poor electronic conductivity and slow photogenerated carrier migration rate limit the reaction kinetics; secondly, its high electron-hole recombination probability means that a large number of carriers annihilate before participating in the surface reaction, resulting in an actual photocurrent density far lower than the theoretical value, which severely restricts the synthesis efficiency and selectivity of H2O2.

[0004] Currently, various modification strategies are employed, including element doping, heterojunction construction, catalyst loading, morphology control, and surface passivation. For example, patent CN119571382B discloses a CoSnO3 / BiVO4 photoanode, its preparation method, and its application in the preparation of hydrogen peroxide. CoSn(OH)6 powder is prepared by co-precipitation and hydrothermal methods, and then the composite photoanode is prepared on an FTO substrate by electrodeposition and spin coating. This technology uses perovskite-type CoSnO3 as the composite component, but its preparation process requires the separate synthesis of CoSn(OH)6 powder, followed by multiple coating and calcination steps, making the process relatively complex and unfavorable for cost control and large-scale production.

[0005] Patent CN105749942A discloses a bitter gourd-shaped BiVO4 / BiPO4 heterojunction photocatalytic material, its preparation method, and its application. The heterojunction material is synthesized in one step using a co-precipitation hydrothermal method. This material exhibits high activity in the photocatalytic degradation of organic pollutants. However, it is prepared as a powdered catalyst, limiting its charge transport efficiency and practical application applicability. Patent CN107098429A discloses a BiVO4 / BiPO4 composite material, its preparation method, and its application. This material is prepared by in-situ acid radical ion exchange, resulting in a composite material in which BiVO4 nanoparticles are attached to the surface of BiPO4 nanorods. This method is simple, and the obtained material has broad applicability in the degradation of organic pollutants. However, the composite material is in powder form, making it difficult to directly use as a photoanode in photoelectrocatalytic systems.

[0006] Therefore, it is of great significance to develop a low-cost, simple, high-charge-transfer-efficiency, and structurally stable BiVO4 modification technology for use in photoelectrocatalytic systems. Summary of the Invention

[0007] To address the problems of the existing technologies, this invention utilizes the excellent photocatalytic performance of BiPO4, which has a good band structure matching with BiVO4. After the two are combined to form a heterojunction, the built-in electric field can promote the directional transport of photogenerated carriers and effectively suppress electron-hole recombination, thereby achieving excellent selectivity and yield in H2O2 synthesis. Simultaneously, BiPO4 also promotes the synthesis of HCO3-. - It has a stronger adsorption capacity and can enrich local HCO3 at the electrode / electrolyte interface. - The concentration provides sufficient reactants for the two-electron water oxidation pathway, and the weak adsorption of H2O2 by BiPO4 can inhibit the in-situ decomposition of the product, significantly improving the selectivity and stability of H2O2 synthesis, thus complementing the advantages of BiVO4.

[0008] The method for preparing the BiPO4 / BiVO4 array structure photoanode provided by this invention is an in-situ construction method that enables tight bonding at the heterojunction interface, producing a BiPO4 / BiVO4 composite photoanode with an array structure. This method solves the problems of existing BiPO4 / BiVO4 composite systems, which are mostly in powder form, lack array structure design, have limited specific surface area, and have poor charge transport paths, thus restricting photoelectrocatalytic performance. This application optimizes charge separation and transport efficiency and surface reaction kinetics through the synergistic effect of BiPO4 and BiVO4, thereby improving the selectivity and yield of photoelectrocatalytic synthesis of hydrogen peroxide, which is crucial for the industrial application of this technology.

[0009] Specifically, the BiPO4 / BiVO4 array structure photoanode provided by the present invention is prepared in situ by a three-step method of electrodeposition-calcination-photoelectrochemical activation; in the photoanode, BiVO4 has a monoclinic scheelite crystal structure and BiPO4 has a hexagonal phase crystal structure, which are uniformly grown on the surface of the BiVO4 array in the form of nanosheets to form a hierarchical heterojunction array structure.

[0010] Furthermore, BiVO4 is designated JCPDS 14-0688 and exhibits a porous worm-like array morphology; BiPO4 is designated JCPDS 00-015-0766 and has a plate-like structure; the mass fraction of P element is 1.0%~2.0%.

[0011] This invention also protects the method for preparing the BiPO4 / BiVO4 array structure photoanode as described above, comprising the following steps: Step S1: Conductive substrate pretreatment; Step S2: Prepare a BiVO4 array film by electrodeposition, and then convert it into a BiVO4 array by drop-coating a vanadium source, calcination, and alkali washing; Step S3: A BiPO4 layer is grown in situ on the surface of the BiVO4 array by photoelectrochemical activation to obtain a BiPO4 / BiVO4 array structure photoanode.

[0012] Furthermore, the specific steps of step S1 are as follows: S11. Use fluorine-doped tin oxide conductive glass (FTO) or indium tin oxide transparent conductive film glass (ITO) as the substrate, and cut it to a size of 1~3cm×1~3cm. S12. The cut substrate is ultrasonically cleaned in sequence with deionized water, acetone, isopropanol and ethanol, each for 5 to 20 minutes. S13. After cleaning, place the substrate in a vacuum environment at 60℃ for 30 minutes to dry. After removal, seal and store for later use. The substrate cleaning must ensure that there are no organic impurities and surface oxides, otherwise it will affect the adhesion and array regularity of the electrodeposited film.

[0013] Furthermore, before electrodepositing the BiVO4 array film in step S2, a three-electrode system is constructed. The specific steps are as follows: the pretreated conductive substrate is used as the working electrode, the Pt wire is used as the counter electrode, and the Ag / AgCl (saturated KCl) electrode is used as the reference electrode.

[0014] Furthermore, the specific steps of step S2 are as follows: S21. Prepare the mixed electrolyte: Dissolve 2-5 mmol / L bismuth nitrate pentahydrate, 10-30 mmol / L potassium iodide, and 3-6 mmol / L p-benzoquinone in deionized water, adjust the pH of the system to 1.5-2.3 with 0.5-2M nitric acid, and sonicate for 15 min. S22. Electrodeposition operation: Electrodeposition for 60~160s at a potential of -0.1~-0.2V vs. Ag / AgCl to form a BiOI array film on the substrate surface; The core reaction of the electrodeposition of BiOI in this application occurs on the surface of the working electrode (FTO / ITO) (BiOI). 3+ with I - (A BiOI array is formed by the combination and accompanying electron gain / loss). The oxidation reaction in the circuit occurs entirely on the surface of the counter electrode. Due to its strong chemical inertness, the Pt wire does not participate in its own oxidation / dissolution, but only serves as an "electron exchange station" for the reverse reaction, avoiding the consumption and contamination of the electrolyte by the counter electrode. The excellent conductivity of the Pt wire allows the current in the external circuit to pass through evenly, avoiding uneven BiOI deposition and distorted array morphology on the working electrode surface due to excessively large / small local currents. The electrodeposition time in this experiment is only 60~160 s. The inertness and conductivity of the Pt wire fully meet the requirements of short-time, low-current electrodeposition, and there is no need to replace the counter electrode.

[0015] Ag / AgCl (saturated KCl) is a reversible reference electrode. Its electrode potential is highly stable under experimental conditions (room temperature, saturated KCl) and does not change with the composition of the electrolyte / reaction process. It can provide a fixed "potential scale" for the working electrode, allowing the potential of the working electrode to be accurately calibrated and read.

[0016] In conjunction with the constant potential mode of the electrochemical workstation, the reference electrode will detect the potential change on the surface of the working electrode in real time. Once it deviates from the set potential range, the workstation will immediately adjust the external circuit current to pull the working electrode potential back to the set value. This control is crucial for the fabrication of BiOI arrays.

[0017] In the electrodeposition preparation of BiOI, pH and potential are key parameters. Too low or too high pH can lead to uneven deposition, and potential deviations can affect the morphology of the BiOI array. The BiOI in the electrolyte... 3+ I - The reaction is extremely sensitive to potential. A potential that is too positive (too high) will result in a slow BiOI deposition rate and a sparse array; a potential that is too negative (too low) will trigger a hydrogen evolution side reaction, causing the deposited film to bubble, fall off, or even fail to form an ordered array.

[0018] The Ag / AgCl reference electrode saturated with KCl is suitable for acidic to near-neutral electrolytes (the pH of this application is 1.5~2.3, which is strongly acidic), and will not react with H+ in the electrolyte. + NO3 - I - The reaction occurs, and the potential reproducibility is good, which can ensure the consistency of electrodeposition experiments in different batches.

[0019] S23. Vanadium source drop coating and calcination: A 0.1-0.5 mol / L solution of vanadium acetylacetonate in dimethyl sulfoxide (DMSO) is uniformly drop-coated onto the surface of the BiOI array film at a rate of 50-100 μL / cm². The substrate after drop coating is heated to 450-500℃ at a heating rate of 1-5℃ / min and calcined at this temperature for 2-4 hours. The heating rate during calcination must be strictly controlled; too fast a rate can easily lead to array cracking, while too slow a rate will reduce preparation efficiency. The purpose of NaOH alkaline washing is to remove impurities generated after calcination. S24. Post-processing purification: After calcination and natural cooling, the substrate is placed in a 0.1~1 mol / L NaOH aqueous solution and stirred for 20~35 min. Then, it is repeatedly rinsed with deionized water until the washing solution is neutral. After vacuum drying, BiVO4 array is obtained.

[0020] Furthermore, the specific steps of step S3 are as follows: S31. Preparation of PBA buffer solution: Prepare 0.1~0.5M phosphate-borate-acetate PBA buffer solution; S32. Constructing a photoelectrochemical system: Using a BiVO4 array as the working electrode, a Pt electrode as the counter electrode, and Ag / AgCl as the reference electrode, the three electrodes are placed in the above-mentioned PBA buffer solution. S33. Photoelectrochemical activation treatment: Simulate sunlight with AM1.5G and apply a constant bias voltage of 0.8~1.5V vs. RHE to the working electrode, continuing the activation treatment for 30~180min. The bias voltage, illumination time, and pH of the PBA buffer solution are the core factors controlling the thickness and loading of the BiPO4 layer, directly affecting the photoelectric performance of the composite photoanode. Adjusting the bias voltage and light intensity can effectively control the concentration of a few migrants-holes on the surface, while the concentration of the PBA buffer solution determines the conductivity and the amount of surface material deposited. The pH value is a crucial factor in initiating the surface reaction.

[0021] S34. Post-treatment drying: After the reaction is completed, the working electrode is removed, the surface is rinsed three times with 50 mL of deionized water, and dried in a vacuum environment at 60 °C for 2 h to finally obtain the BiPO4 / BiVO4 array structure photoanode.

[0022] Furthermore, the PBA buffer solution in step S31 is prepared by mixing KH2PO4, H3BO3 and acetic acid solutions of equal concentration and volume, and the pH of the solution is adjusted to 3.0~8.0 by adding acetic acid or KOH dropwise.

[0023] Furthermore, the luminous intensity of sunlight in S33 is 50~150 mW / cm².

[0024] This invention also protects the application of the aforementioned BiPO4 / BiVO4 array structure photoanode, using the BiPO4 / BiVO4 array structure photoanode as the working electrode and KHCO3 solution as the electrolyte, to directionally generate hydrogen peroxide through a two-electron water oxidation reaction under simulated sunlight irradiation.

[0025] Furthermore, the pH of the KHCO3 solution is 8.3.

[0026] Compared with the prior art, the present invention has the following outstanding features and advantages: (1) The unique “photoelectrochemical self-sacrifice reconstruction” in-situ growth mechanism solves the problem of poor interfacial bonding.

[0027] Existing technologies for constructing heterostructures often employ physical deposition or chemical bath deposition, which frequently results in lattice mismatch or loose contact at the interface, hindering charge transport. This invention utilizes a photoelectrochemical activation strategy in PBA buffer solution, leveraging the strong oxidizing properties of photogenerated holes in BiVO4 to selectively etch vanadium (V) sites at high-energy states or defects on the surface. This induces residual bismuth (Bi) sites to react in situ with phosphate ions in the solution to generate BiPO4. This "self-sacrificing" in-situ growth mode allows the BiPO4 layer to grow directly from the BiVO4 lattice, achieving atomically close contact and minimizing interfacial electron transport impedance. The needle-like BiPO4 structure and the worm-like BiVO4 structure are tightly bonded together, achieving atomically close contact and reducing electron transport distance.

[0028] (2) It achieves the dual functions of surface defect passivation and catalytic selectivity regulation.

[0029] The surface of pure BiVO4 contains numerous defects such as vanadium vacancies, which not only serve as electron-hole recombination centers but also lead to photocorrosion. The BiPO4 layer constructed in this invention effectively fills and passivates the defect states on the BiVO4 surface, significantly improving the separation efficiency of photogenerated charges and the stability of the photoanode. Furthermore, the unique acidic sites and band structure of BiPO4 effectively alter the kinetic pathway of the water oxidation reaction, suppressing the thermodynamically more favorable four-electron oxygen production reaction and promoting the two-electron water oxidation reaction, thus significantly improving the selectivity and Faraday efficiency of hydrogen peroxide (H2O2) formation.

[0030] (3) The preparation process is green and mild, and has a high degree of controllability.

[0031] Compared to high-temperature annealing or complex vacuum deposition techniques, the photoelectrochemical activation process of this invention is carried out at room temperature in a neutral / weakly acidic aqueous solution, resulting in low energy consumption and environmental friendliness. More importantly, this method has a "self-limiting" characteristic: as the BiPO4 deposition time increases, the hole etching effect on the surface gradually weakens, thereby automatically controlling the film thickness within the optimal range (typically at the nanometer scale), avoiding the problem of excessively thick capping layers hindering charge transport, and ensuring good process repeatability.

[0032] (4) It significantly improved the yield of photoelectrocatalytic synthesis of H2O2.

[0033] The BiPO4 / BiVO4 array structure photoanode prepared by the method of this invention not only exhibits a higher photocurrent density than pure BiVO4 under simulated sunlight, but also demonstrates excellent hydrogen peroxide accumulation capacity in a bicarbonate electrolyte system. Experiments show that this photoanode can effectively redirect hole flow, originally intended for oxygen production, towards hydrogen peroxide synthesis, overcoming the bottleneck problem of low H2O2 selectivity in traditional BiVO4-based photoanodes, and possesses promising prospects for industrial applications. Attached Figure Description

[0034] Figure 1 SEM images of the BiVO4 array (left) and the BiPO4 / BiVO4 array structured photoanode (right) in Example 1; Figure 2 TEM image of the photoanode of the BiPO4 / BiVO4 array structure in Example 1; Figure 3 EDX spectrum and elemental mapping of the BiPO4 / BiVO4 array structure photoanode in Example 1; Figure 4 XRD patterns (left) and UV-Vis spectra (right) of BiVO4 and BiPO4 / BiVO4 array structure photoanodes. Figure 5 XPS full spectrum and high-resolution spectrum of BiVO4 and BiPO4 / BiVO4 array structure photoanodes in Example 1; Figure 6 LSV curves and EIS spectra of BiPO4 / BiVO4 array structure photoanodes prepared with BiVO4 and different activation times in Example 1, under dark and light conditions; Figure 7 Graphs showing the changes in photoelectrochemical hydrogen peroxide yield and Faraday efficiency of the BiPO4 / BiVO4 array structure photoanodes over time in Examples 1-3. Detailed Implementation

[0035] 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 some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. The technical solutions of the present invention will be further described below with reference to implementation examples. Unless otherwise stated, the raw materials, reagents and instruments used in the embodiments of the present invention are all commercially available conventional products.

[0036] Example 1 This embodiment provides an application of a BiPO4 / BiVO4 array structure photoanode.

[0037] First, a BiPO4 / BiVO4 array structured photoanode is prepared, including the following steps: Step S1: Select fluorine-doped tin dioxide conductive glass (FTO) as the substrate and cut it to a size of 2cm×3cm; Step S2: The cut substrate is ultrasonically cleaned sequentially with deionized water, acetone, isopropanol and ethanol, for 10 minutes each time. Step S3: After cleaning, place the substrate in a vacuum environment at 60°C for 30 minutes to dry, then remove and seal for later use; Step S4: Using the pretreated conductive substrate as the working electrode, the Pt wire as the counter electrode, and the Ag / AgCl (saturated KCl) electrode as the reference electrode; Step S5: Dissolve 3 mmol / L bismuth nitrate pentahydrate, 20 mmol / L potassium iodide, and 4 mmol / L p-benzoquinone in deionized water, adjust the pH of the system to 1.7 with 1M nitric acid, and sonicate for 15 min. Step S6: Electrodeposit at a potential of -0.14V vs. Ag / AgCl for 100s to form a BiOI array film on the substrate surface; Step S7: A 0.2 mol / L solution of vanadium acetylacetonate in dimethyl sulfoxide (DMSO) was uniformly drop-coated onto the surface of the BiOI array film at a rate of 80 μL / cm²; the substrate after drop-coating was heated to 450 °C at a heating rate of 2 °C / min and calcined at the same temperature for 2 h. Step S8: After calcination and natural cooling, the substrate was placed in a 0.1 mol / L NaOH aqueous solution and stirred for 25 min. Then, it was repeatedly rinsed with deionized water until the washing solution was neutral. After vacuum drying, the BiVO4 array (denoted as BVO) was obtained. Step S9: Prepare a 0.1M phosphate-borate-acetate (PBA) buffer solution; the PBA buffer solution is prepared by mixing equal concentrations and volumes of KH₂PO₄, H₃BO₃ and acetic acid solutions, and adjusting the pH of the solution to 4.0 by adding acetic acid dropwise; Step S10: Using the BiVO4 array as the working electrode, the Pt electrode as the counter electrode, and Ag / AgCl as the reference electrode, place the three electrodes in the above-mentioned PBA buffer solution. Step S11: Turn on AM1.5G simulated sunlight (light intensity 100mW / cm²), illuminate the effective area of ​​the light electrode to 0.283cm², apply a constant bias voltage of 1.23V vs. RHE to the working electrode, and continue the activation treatment for 60min. Step S12: After the reaction is complete, remove the working electrode, rinse the surface with deionized water to remove residual electrolyte, and dry it in a vacuum environment at 60°C for 2 hours. After drying, a BiPO4 / BiVO4 array structure photoanode (denoted as BVO-P-60) is obtained.

[0038] Secondly, the prepared BiVO4 and BiPO4 / BiVO4 array structure photoanodes were applied to the photoelectrocatalytic synthesis of hydrogen peroxide. Using BVO-P-60 as the working electrode and 2 M KHCO3 solution (pH=8.3) as the electrolyte, under AM 1.5G simulated sunlight (100 mW / cm²) irradiation and at a potential of 1.23 V vs. RHE, the photocurrent density reached 2 mA / cm², and the hydrogen peroxide yield was approximately 8.8 μmol·h⁻¹. -1 ·cm -2 The Faraday efficiency was approximately 73%. Photoelectrocatalytic performance was tested in a standard three-electrode H-type photoelectrolysis cell at 25 °C. The anode and cathode chambers were separated by a Nafion 117 proton exchange membrane (DuPont) to prevent the products generated at the cathode from diffusing to the anode and causing a reverse reaction. A 2.0 M KHCO3 aqueous solution was prepared as the electrolyte. The prepared BiPO4 / BiVO4 array structure photoanode was fixed in the anode chamber, with the effective illumination area controlled at 0.283 cm². 2 Connect the electrochemical workstation and turn on the simulated sunlight to illuminate the back of the working electrode (FTO glass side). Apply a constant bias voltage of 1.6 V vs. RHE (reversible hydrogen electrode) for chronoamperometry (it) testing. Let the reaction continue for 1 hour, then extract 1 mL of electrolyte from the anode chamber for product concentration analysis, and immediately replenish the anode chamber with an equal volume of fresh electrolyte to maintain a constant volume.

[0039] The concentration of hydrogen peroxide was quantitatively analyzed using a modified cobalt ion spectrophotometric method. This method is based on the reaction of H₂O₂ and Co. 2+A soluble pink complex (Co(CO3)2OOH·H2O, etc.) is formed in bicarbonate solution, exhibiting a characteristic absorption peak at a specific wavelength. Standard curve preparation: A series of H2O2 standard solutions of known concentrations (0, 0.1, 0.2, 0.5, 1.0, 2.0 mM) are prepared. 1 mL of each concentration standard solution is taken, and 8.75 mL of 2 M KHCO3 solution and 0.25 mL of colorimetric reagent (0.07 M CoSO4) are added respectively, and mixed thoroughly. After standing at room temperature in the dark for 1 hour, the absorbance is measured at 257 nm using a UV-Vis spectrophotometer. A standard curve is plotted with H2O2 concentration on the x-axis and absorbance on the y-axis, and a linear regression equation is obtained by fitting the curve. 1 mL of the unknown sample solution extracted from the photoelectrocatalytic reaction is subjected to the same steps as above for color development and absorbance measurement. The concentration of H2O2 in the sample solution is calculated by substituting the values ​​into the standard curve equation.

[0040] Faraday efficiency refers to the percentage of the total charge passing through a circuit that is used to generate the target product (H₂O₂). For the two-electron water oxidation reaction, 2 mol of electrons need to be transferred to generate 1 mol of H₂O₂, as calculated by the following formula: FE(%) = 2 × n(H₂O₂) × F / Q total ×100%; In the formula: n(H2O2) is the amount of H2O2 produced (mol), Q total Let C be the amount of charge passing through, and F be the Faraday constant (96485 C / mol).

[0041] Scanning electron microscopy (SEM) was performed using a Hitachi SU8010 field emission scanning electron microscope (SEM) with an operating voltage of 5 kV, used to observe the surface morphology and microstructure of the samples. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HR-TEM) were performed using a Tecnai G2 F20 TEM from FEI (USA) with an accelerating voltage of 200 kV, used to analyze lattice fringes and heterojunction interface structures. X-ray diffraction (XRD) was performed using a Bruker D8 Advance X-ray diffractometer (Germany) with a Cu Kα radiation source (λ=0.15406 nm) and a scanning range of 10°–80°, used to analyze crystal structure and phase composition. X-ray photoelectron spectroscopy (XPS) was performed using a Thermo Fisher Scientific ESCALAB 250Xi XPS with an Al Kα radiation source, used to analyze surface elemental composition and valence states. The UV-Vis diffuse reflectance spectrometer (UV-Vis DRS) was a Shimadzu UV-2600 spectrophotometer, using BaSO4 as the standard reference, with a test wavelength range of 200-800 nm. The electrochemical workstation was a CHI 660E from Shanghai Chenhua Instruments Co., Ltd., with a xenon lamp source (Beijing Zhongjiao Jinyuan, CEL-HXF300) for photoelectrochemical testing. High-performance liquid chromatography (HPLC) was performed using an Agilent 1260 Infinity III Prime HPLC system.

[0042] Scanning electron microscope (SEM) images show, for example Figure 1 As shown, pure BiVO4 prepared by electrodeposition-calcination exhibits a typical porous worm-like nanoarray structure. This structure is beneficial for increasing the contact area between the photoanode and the electrolyte, providing abundant reaction sites. Figure 2 As shown, the atomic-level close-contact interface displayed by TEM after photoelectrochemical activation treatment is one of the core advantages of this invention. Figure 3 Elemental mapping revealed that vanadium (V) was primarily distributed at the center of the nanorod framework, while phosphorus (P) was uniformly and continuously distributed across the entire contoured surface of the nanoarray. This indicated a reduction in surface vanadium sites, with needle-like BiPO4 structures tightly bonded to worm-like BiVO4 structures, achieving atomic-level close contact and reducing electron transport distance. The spectra showed that the sample contained Bi, V, P, O, and Sn (from an FTO substrate), and the elemental mapping confirmed the uniform distribution of each element.

[0043] like Figure 4The XRD pattern shows characteristic diffraction peaks of BiVO4 and BiPO4, and the UV-Vis spectrum shows that the composite photoanode maintains good absorption in the visible light region (400~550 nm). This interface, "grown" from within the crystal lattice (rather than physically attached), greatly reduces the interfacial contact resistance, promotes the rapid transfer of photogenerated holes from the interior of BiVO4 to the surface BiPO4 layer, and effectively suppresses electron-hole recombination at the interface. This is the key structural basis for improving photocurrent density.

[0044] like Figure 5 As shown, XPS analysis of the full spectrum confirmed the presence of P, and the shifts in the Bi 4f and V 2p peaks indicated electron transfer between heterojunctions. In the O 1s spectrum of the composite sample, the peak area ratio corresponding to oxygen vacancies (Ov) decreased significantly, while the lattice oxygen ratio increased. This result directly confirms the "defect passivation" mechanism of this invention. The in-situ grown BiPO4 layer preferentially filled the high-energy defect sites (such as vanadium vacancies and oxygen vacancies) on the BiVO4 surface, eliminating the electron-hole recombination centers on the surface.

[0045] like Figure 6 As shown, the LSV curve reveals a significant increase in the photocurrent density of BiPO4 / BiVO4, while the EIS spectrum indicates a decrease in charge transfer resistance after illumination. The increase in the LSV curve and the decrease in the EIS arc together demonstrate that the composite photoanode has a lower charge transfer resistance (Ro). ct This demonstrates that the BiPO4 layer not only does not hinder charge transport, but also accelerates the surface reaction kinetics of holes by passivating defects and building-in electric field. The H2O2 yield versus Faraday efficiency curve shows that the cumulative H2O2 concentration of the BiPO4 / BiVO4 photoanode (BVO-P-60) increases linearly and rapidly over time, with a yield as high as 11.7 μmol·h⁻¹. -1 ·cm -2 Meanwhile, its Faraday efficiency (FE) is stable between 70% and 77%, while the FE of pure BiVO4 is usually below 10%.

[0046] Example 2 This embodiment refers to Embodiment 1. The difference between this embodiment and Embodiment 1 is that in this embodiment, step S11 involves continuous activation treatment for 30 min. The prepared BiPO4 / BiVO4 array structure photoanode is designated as BVO-P-30. The photocurrent density of BVO-P-30 was tested to be 1.4 mA / cm², and the hydrogen peroxide yield was approximately 6.7 μmol·h⁻¹. -1 ·cm -2 Faraday efficiency is approximately 61%.

[0047] Example 3 This embodiment refers to Embodiment 1. The difference between this embodiment and Embodiment 1 is that in this embodiment, step S11 involves continuous activation treatment for 150 min. The prepared BiPO4 / BiVO4 array structure photoanode is designated as BVO-P-150. Testing showed that the photocurrent density of BVO-P-150 was 1.2 mA / cm², and the hydrogen peroxide yield was approximately 11.7 μmol·h⁻¹. -1 ·cm -2 Faraday efficiency is approximately 77%.

[0048] Figure 7 This is a graph showing the changes in photoelectrochemical hydrogen peroxide yield and Faraday efficiency over time for the BiPO4 / BiVO4 array structure photoanodes in Examples 1-3. It reveals the acidic sites on the BiPO4 surface and their effect on HCO3-. - The specific adsorption capacity of the array structure photoanode successfully altered the kinetic pathway of the water oxidation reaction, suppressing the thermodynamically more favorable four-electron oxygen production reaction and highly selectively guiding the two-electron water oxidation reaction. This demonstrates the enormous application potential of this array structure photoanode in the green synthesis of hydrogen peroxide.

[0049] Example 4 The method for preparing the BiPO4 / BiVO4 array structure photoanode in this embodiment is the same as steps S1-S12 in Example 1, except that the pH of the PBA buffer solution is 4.0.

[0050] A standard three-electrode system was used, with a 2M KHCO3 solution (pH=8.3) as the electrolyte, and the test temperature was 25 ℃. The illumination conditions were AM 1.5G simulated sunlight (100mW / cm²). 2 ), Illuminated area 0.283 cm² 2 .

[0051] This invention constructs a BiPO4 layer in one step via in-situ photoactivation, eliminating the need for high temperature, high pressure, or complex template agents. The process is simple and controllable, avoiding the cumbersome steps of separately preparing BiPO4 powder and then recombining it, thus reducing production costs and making it suitable for large-scale production. The BiPO4 array provides ordered charge transport channels, and the in-situ growth of BiPO4 nanosheets forms a tight heterojunction, increasing the specific surface area by 2-3 times compared to traditional powder composites. This enhances light absorption efficiency, shortens carrier transport distance, and reduces recombination probability. The visible light response characteristics of BiPO4 are related to its charge separation ability and HCO3-. -The synergistic effect of enrichment significantly optimizes the two-electron water oxidation reaction pathway, inhibits the in-situ decomposition of H2O2, and achieves better hydrogen peroxide yield and Faraday efficiency than pure BiPO4 and existing composite photoanodes, while exhibiting good stability. This photoanode can be directly used in the photoelectrocatalytic synthesis of hydrogen peroxide, realizing the on-site preparation of H2O2 and avoiding the environmental and safety issues of traditional processes. It has significant application value in wastewater treatment, medical disinfection, fuel cells, and other fields.

[0052] The embodiments described above are merely examples of several implementations of the present invention, and while the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the present invention, and these modifications and improvements all fall within the scope of protection of the present invention.

Claims

1. A BiPO4 / BiVO4 array structure photoanode, characterized in that, The BiPO4 / BiVO4 array structure photoanode was prepared in situ using a three-step method of electrodeposition, calcination, and photoelectrochemical activation. The BiVO4 in the photoanode has a monoclinic scheelite crystal structure, and the BiPO4 has a hexagonal crystal structure. The photoanode is uniformly grown on the surface of the BiVO4 array in the form of nanosheets, forming a hierarchical heterojunction array structure.

2. The BiPO4 / BiVO4 array structure photoanode according to claim 1, characterized in that, BiVO4, designated JCPDS 14-0688, exhibits a porous worm-like array morphology; BiPO4, designated JCPDS 00-015-0766, has a plate-like structure; the mass fraction of P element is 1.0%~2.0%.

3. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to any one of claims 1-2, characterized in that, Includes the following steps: Step S1: Conductive substrate pretreatment; Step S2: Prepare a BiVO4 array film by electrodeposition, and then convert it into a BiVO4 array by drop-coating a vanadium source, calcination, and alkali washing; Step S3: A BiPO4 layer is grown in situ on the surface of the BiVO4 array by photoelectrochemical activation to obtain a BiPO4 / BiVO4 array structure photoanode.

4. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 3, characterized in that, The specific steps of step S1 are as follows: S11. Use fluorine-doped tin dioxide conductive glass or indium tin oxide transparent conductive film glass as the substrate, and cut it to a size of 1~3cm×1~3cm. S12. The cut substrate is ultrasonically cleaned in sequence with deionized water, acetone, isopropanol and ethanol, each for 5 to 20 minutes. S13. After cleaning, place the substrate in a vacuum environment at 60°C for 30 minutes to dry, then remove and seal for later use.

5. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 3, characterized in that, Before electrodepositing the BiVO4 array film in step S2, a three-electrode system is constructed. The specific steps are as follows: the pretreated conductive substrate is used as the working electrode, the Pt wire is used as the counter electrode, and the Ag / AgCl electrode is used as the reference electrode.

6. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 3, characterized in that, The specific steps of step S2 are as follows: S21. Prepare the mixed electrolyte: Dissolve 2-5 mmol / L bismuth nitrate pentahydrate, 10-30 mmol / L potassium iodide, and 3-6 mmol / L p-benzoquinone in deionized water, adjust the pH of the system to 1.5-2.3 with 0.5-2M nitric acid, and sonicate for 15 min. S22. Electrodeposition operation: Electrodeposition for 60~160s at a potential of -0.1~-0.2V vs. Ag / AgCl to form a BiOI array film on the substrate surface; S23. Vanadium source drop coating and calcination: A 0.1~0.5 mol / L solution of vanadium acetylacetonate in dimethyl sulfoxide is uniformly drop-coated onto the surface of the BiOI array film at a rate of 50~100 μL / cm². The substrate after drop coating is heated to 450-500℃ at a heating rate of 1-5℃ / min and calcined at a constant temperature for 2-4 hours. S24. Post-processing purification: After calcination and natural cooling, the substrate is placed in a 0.1~1 mol / L NaOH aqueous solution and stirred for 20~35 min. Then, it is repeatedly rinsed with deionized water until the washing solution is neutral. After vacuum drying, BiVO4 array is obtained.

7. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 3, characterized in that, The specific steps of step S3 are as follows: S31. Preparation of PBA buffer solution: Prepare 0.1~0.5M phosphate-borate-acetate buffer solution; S32. Constructing a photoelectrochemical system: Using a BiVO4 array as the working electrode, a Pt electrode as the counter electrode, and Ag / AgCl as the reference electrode, the three electrodes are placed in the above-mentioned PBA buffer solution. S33, Photoelectrochemical activation treatment: Turn on AM1.5G simulated sunlight, apply a constant bias voltage of 0.8~1.5V vs. RHE to the working electrode, and continue the activation treatment for 30~180min; S34. Post-treatment drying: After the reaction is completed, the working electrode is removed, the surface is rinsed three times with 50 mL of deionized water, and dried in a vacuum environment at 60 °C for 2 h to finally obtain the BiPO4 / BiVO4 array structure photoanode.

8. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 7, characterized in that, The buffer solution in step S31 is prepared by mixing KH2PO4, H3BO3 and acetic acid solutions of equal concentration and volume, and the pH of the solution is adjusted to 3.0~8.0 by adding acetic acid or KOH dropwise.

9. The method for preparing the BiPO4 / BiVO4 array structure photoanode according to claim 7, characterized in that, In S33, the intensity of sunlight is 50~150 mW / cm².

10. The application of the BiPO4 / BiVO4 array structure photoanode according to any one of claims 1-9, characterized in that: Using a BiPO4 / BiVO4 array structured photoanode as the working electrode and KHCO3 solution as the electrolyte, hydrogen peroxide is directionally generated through a two-electron water oxidation reaction under simulated sunlight irradiation.

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