Environmental function heterojunction material and preparation and application thereof

Abstract

The invention belongs to the technical field of environment function materials, and particularly relates to an environment function heterojunction material and preparation and application thereof. According to the heterojunction material, a Mn2V2O7 phase and a BiVO4 phase are subjected to gas phase ion exchange in-situ construction to form a tight epitaxial interface, BiVO4 nanoparticles are uniformly anchored to the surface of a Mn2V2O7 nanosheet, and the surface electronic structure and key intermediate adsorption capacity of the heterojunction material can be systematically regulated and controlled by adjusting the oxygen vacancy concentration, so that the performance of the heterojunction material is improved. Therefore, selective switching of two electron paths and four electron paths of water oxidation reaction is realized in the same material system. The material shows excellent overall water decomposition performance, a high-oxygen-vacancy sample realizes product distribution of H2 / O2 = 2: 1 close to a theoretical value in a pure water system, and a non-oxygen-vacancy sample shows high-selectivity H2O2 generation capacity. Through oxygen vacancy mediated reaction path regulation and control and an in-situ constructed heteroepitaxial interface, selective generation of photocatalytic water decomposition products is realized, and a new design strategy is provided for development of efficient multifunctional photocatalytic materials.

Description

Technical Field

[0001] This invention relates to the field of environmental functional materials technology, and in particular to an environmental functional heterojunction material and its preparation and application. Background Technology

[0002] With the escalating global energy crisis and the continued advancement of carbon neutrality strategies, the development of highly efficient environmental functional materials that combine environmental governance and energy conversion has become an important research direction in the fields of materials science and environmental technology. Environmental functional composite materials can play a key role in processes such as pollutant conversion, energy acquisition, and resource utilization, and are considered an important material foundation for supporting green and low-carbon technology systems.

[0003] Among numerous environmental functional materials, semiconductor-based photocatalytic materials exhibit unique advantages in clean hydrogen production, pollutant conversion, and environmental energy conversion because they can directly utilize solar energy to drive redox reactions. Photocatalytic water splitting, in particular, is considered a crucial technological pathway for converting solar energy into chemical energy because it can directly convert water into hydrogen.

[0004] However, the oxygen evolution reaction (OER) involved in the overall photocatalytic water splitting process is a multi-proton coupled electron transfer process, characterized by slow kinetics and high energy barriers, which remains a key bottleneck limiting the overall efficiency and stability of the photocatalytic system. To improve the OER rate, researchers have proposed strategies such as introducing hole sacrificial agents, loading co-catalysts, and establishing polarized electric fields. However, due to insufficient understanding of the reaction mechanism and key intermediates, the precise structural design and performance regulation of catalysts still face significant challenges. Recent studies have shown that the surface atomic structure of photocatalytic materials can overcome traditional kinetic limitations by altering intermediate adsorption methods and reaction pathways. By controlling the surface atomic coordination environment, valence state distribution, and electronic structure, key elementary steps in the water oxidation process can be effectively modified, thereby lowering the reaction energy barrier and increasing the reaction rate. Among numerous structural regulation strategies, oxygen vacancy engineering has attracted widespread attention due to its outstanding advantages in regulating the electronic structure of metal oxides, enhancing charge transfer capabilities, and optimizing intermediate adsorption. High-energy surface oxygen vacancies can selectively anchor reaction intermediates and act as high-speed electron channels, promoting the injection of electrons from the active sites of metal oxides into reactants, thereby achieving an order-of-magnitude improvement in reaction kinetics.

[0005] To further optimize the spatial separation and interfacial transport efficiency of photogenerated carriers, heterojunction photocatalysts have gradually become a core strategy for achieving efficient photocatalysis. By constructing heterojunctions with complementary energy bands, a stable built-in electric field can be formed between different semiconductors, enabling the directional migration of electrons and holes, effectively suppressing carrier recombination, and synergistically promoting the reaction kinetics of OER and Hydrogen Evolution Reaction (HER).

[0006] Although existing studies have explored the impact of oxygen vacancies on photocatalytic reactions, current technologies mainly focus on the generation and regulation of oxygen vacancies in single material systems. Furthermore, the formation and distribution of oxygen vacancies are often difficult to control precisely. Excessively high oxygen vacancy concentrations may induce lattice distortion or decrease interfacial stability, making it difficult to simultaneously achieve high stability and high reaction controllability. On the other hand, the synergistic regulation mechanism of oxygen vacancies on interfacial charge migration, reaction pathways, and product selectivity in heterojunction systems still lacks systematic research, making it difficult for existing systems to achieve selective switching between oxidation products such as H2O2 and O2 through material structure adjustments.

[0007] Therefore, how to selectively adjust the photo-oxidation reaction pathway and product type in environmental functional heterojunction materials by precisely controlling the surface defect structure without destroying the heterojunction structure and interface stability is a technical problem that urgently needs to be solved in this field.

[0008] The information disclosed in this background section is only intended to enhance the understanding of the background technology of this application and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0009] To address the shortcomings of existing technologies, this invention provides an environmentally functional heterojunction material and its preparation and application. The environmentally functional heterojunction composite material of this invention refers to a composite material that, through heterojunction structural design and surface atomic configuration control, achieves functions such as material transformation, energy conversion, or pollutant disposal in environmentally related reaction systems. The environmental functions include at least the control of oxidation reaction pathways, selective generation of oxidation products, and efficient energy conversion in photocatalytic water splitting reactions. By controlling the oxygen vacancy concentration on the surface of the Mn2V2O7 / BiVO4 heterojunction, selective switching between two-electron and four-electron pathways for oxidation reactions is achieved within the same material system, allowing for the preferential generation of H2O2 or O2 as needed. Furthermore, thanks to the tightly constructed heterojunction interface and the high oxidation activity of Bi sites, the catalyst exhibits efficient hole transport and rapid water oxidation kinetics, thereby achieving an overall water splitting product distribution close to the theoretical value of H2 / O2 = 2:1 in a pure water system.

[0010] This invention proposes an environmentally functional heterojunction material, wherein the heterojunction material is a heterojunction structure formed in situ by Mn2V2O7 phase and BiVO4 phase, and the two phases form a tightly contacted heteroepitaxial interface at the interface. The surface of the heterojunction material contains an adjustable concentration of oxygen vacancies, wherein the molar fraction of oxygen vacancies is 0%~30%, preferably 0%~28.03%, which is used to control the water oxidation reaction pathway and product selectivity.

[0011] Preferably, the Mn2V2O7 has a nanosheet structure, and the BiVO4 is uniformly anchored on the surface of the Mn2V2O7 in the form of nanoparticles.

[0012] Preferably, the introduction of oxygen vacancies enhances the electron density of Bi sites, promotes the formation of *OOH intermediates, and thereby drives the water oxidation reaction to shift from a two-electron pathway to a four-electron pathway.

[0013] Preferably, when there are no oxygen vacancies or the oxygen vacancy concentration is low, the heterojunction material preferentially stabilizes the *HOOH intermediate, thereby promoting the generation of H2O2.

[0014] Preferably, the oxygen vacancies are distributed in the surface region of the heterojunction structure, and the stability of the Mn2V2O7 / BiVO4 crystal structure is maintained after the introduction of oxygen vacancies.

[0015] A method for preparing an environmentally functional heterojunction material as described above includes the following steps: (1) Preparation of sacrificial template: An array of oxide nanosheets is grown on a conductive substrate by hydrothermal method to form a sacrificial template; (2) First gas-phase ion exchange: Heated under a protective atmosphere with a vacuum degree of 2×10 -2 tor causes the sacrificial template to interact with the Mn source and V source in the gas phase, resulting in a topological ion exchange reaction to form Mn2V2O7 nanosheets. (3) Second gas-phase ion exchange in-situ construction of heterojunction: heating under a protective atmosphere with a vacuum degree of 2×10 - 2 tor, so that the Mn2V2O7 nanosheets interact with the Bi source gas phase to carry out a partial ion exchange reaction and form a Mn2V2O7 / BiVO4 heterojunction; (4) Oxygen vacancy control: The Mn2V2O7 / BiVO4 heterojunction is annealed in a hydrogen atmosphere to control the oxygen vacancy concentration.

[0016] Preferably, the conductive substrate in step (1) is a fluorine-doped tin oxide substrate, and the oxide is selected from ZnO, CoO or WO3.

[0017] Preferably, the Mn source in step (2) is MnCl2, MnAc2 or Mn(NO3)2, and the V source is V2O3, VCl3 or VOC2O4·nH2O.

[0018] Preferably, the Bi source in step (3) is selected from BiCl3, Bi2O3, (NH4)3Bi(C6H5O7)2 or Bi(NO3)3·5H2O.

[0019] Preferably, the protective atmosphere in steps (2) and (3) is nitrogen atmosphere with a flow rate of 50 sccm, and the heating temperature is 425~550 ℃, and the holding time is 5~35 min.

[0020] Preferably, the annealing temperature in step (4) is 200~400 ℃.

[0021] The application of the above-mentioned environmental functional heterojunction material in photocatalytic water splitting achieves selective regulation of water oxidation products by controlling the oxygen vacancy concentration. Reducing the surface oxygen vacancy concentration of the heterojunction material makes the water oxidation reaction tend to the two-electron pathway, mainly generating H2O2, while increasing the surface oxygen vacancy concentration of the heterojunction material makes the water oxidation reaction tend to the four-electron pathway, mainly generating O2.

[0022] Preferably, the heterojunction material with high oxygen vacancy concentration can achieve an overall water splitting product distribution close to the theoretical value of H2 / O2 = 2:1 in a pure water system.

[0023] Preferably, the heterojunction material is used for visible light-driven sacrificial-free monolithic water splitting.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention achieves selective control of the photo-oxidation reaction pathway by regulating the concentration of oxygen vacancies on the surface of heterojunction materials. Under oxygen-vacancy-free conditions, the heterojunction materials tend to use a two-electron pathway and selectively generate H2O2, while a high oxygen vacancy concentration promotes the key conversion step OH→OOH, thereby driving the water oxidation reaction to selectively shift from a two-electron pathway to a four-electron pathway and preferentially generate O2. This pathway regulation is achieved entirely within the same heterojunction material system, changing its surface electronic structure only by the difference in surface oxygen vacancy concentration, without changing the material's nature, and without relying on co-catalysts or sacrificial agents; (2) The in-situ heteroepitaxial interface constructed in this invention ensures rapid carrier separation and efficient transport of photogenerated holes from the interior to the surface. At the same time, the Bi sites have high surface oxidation activity, which significantly improves the kinetics of water oxidation reaction. The synergy of these two factors enables the catalyst to achieve a stable product ratio of H2 / O2=2:1 close to the theoretical value in a pure water system, proving that it has efficient surface oxidation kinetics and excellent hole migration ability, which significantly improves the overall photocatalytic water splitting performance. (3) This invention breaks through the technical limitations of existing technologies where oxygen vacancies are mainly used to regulate reaction rates and it is difficult to achieve intrinsic regulation of reaction pathways. In the same heterojunction photocatalytic system, it realizes intrinsic regulation of water oxidation reaction pathways and product types through surface oxygen vacancy engineering, providing a new technical approach for the functional design of the overall photocatalytic water splitting system and the controllable generation of products. Attached Figure Description

[0025] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the following description is only one embodiment of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of heterogeneous MBO.

[0027] Figure 2 SEM images of ZnO nanosheets and Mn2V2O7: (a) ZnO; (b) Mn2V2O 7。

[0028] Figure 3 XRD patterns of heterojunction material samples: (a) MBO, MBOv-1, MBOv-2 and MBOv-3; (b) Magnified partial spectra.

[0029] Figure 4 SEM images of heterojunction material samples: (a)~(b) MBO; (c) MBOv-1; (d) MBOv-2.

[0030] Figure 5 Microstructure and surface chemical state analysis of MBOv-3: (a) SEM image of MBOv-3; (b) TEM image of MBOv-3; (c) HRTEM image of MBOv-3; High-resolution XPS spectrum of MBOv-3; (d) Bi 4 f (e) V 2 p(f) Mn 2 p And (g) O 1s; (h) the ratio of lattice oxygen, defect oxygen and adsorbed oxygen in the sample.

[0031] Figure 6 UV-Vis absorption spectrum of H2O2 and standard curve for quantification: (a) Absorption spectrum of H2O2; (b) Standard curve for quantitative determination of H2O2.

[0032] Figure 7 Comparison of parameters in the photocatalytic water splitting reaction: (a) Standard curve of hydrogen peroxide concentration versus absorbance; (b) Photocatalytic performance of water oxidation products of different heterojunction materials; (c) Comparison of the performance of different heterojunction materials in water oxidation to H2O2; (d) Total water splitting performance and H2 / O2 molar ratio of MBO and MBOv heterojunction materials.

[0033] Figure 8 To assess the photocatalytic hydrogen production stability of the MBOv-3 sample.

[0034] Figure 9 To characterize the oxygen vacancy structure and oxygen species in the oxidation reaction of the sample: (a) EPR spectra of MBO and MBOv samples; (b) Changes in the selectivity of water oxidation products after adding different free radical scavengers; (c) DMPO-•O2 - EPR spectra of the adduct and (d) DMPO-•OH adduct under dark and light conditions for capturing active groups; (e) in-situ Raman spectra of the original MBOv-3 and (f) MBO samples under different applied bias voltages.

[0035] Figure 10 Theoretical calculations for heterojunction material samples: (a) Gibbs free energy diagrams of H2O2 generation on MBO and MBOv samples; (b) Gibbs free energy diagrams of oxygen evolution reaction (OER) on MBO and MBOv samples; (c) Calculated d-band center results for MBO and MBOv samples; (d) PDOS (partial density of states) diagrams for MBO and MBOv; (e) and (f) three-dimensional differential charge density diagrams for MBO and MBOv. Detailed Implementation

[0036] This invention proposes an environmentally functional heterojunction material, its preparation method, and its application. To facilitate understanding of this invention by those skilled in the art, specific embodiments are described below with reference to the accompanying drawings. In the following examples and comparative examples, unless otherwise specified, the equipment and reagents used are commercially available or commonly used in the field.

[0037] like Figure 1 As shown, the Mn2V2O7 / BiVO4 heterojunction materials were prepared in situ using a gas-phase ion exchange method based on a ZnO sacrificial template in both the examples and comparative examples. The in-situ synthesis ensured the formation of a tight heteroepitaxial interface between BiVO4 and Mn2V2O7, which promoted electron transport at the interface and suppressed the recombination of photogenerated carriers at the interface.

[0038] Example 1: Preparation of MBOv-1 heterojunction material The specific steps are as follows: (1) Growth of ZnO nanosheet array ZnO nanosheet arrays were grown on fluorine-doped tin oxide (FTO) conductive glass using a hydrothermal method. Figure 2 a) Obtain a uniformly distributed sheet-like structure, which can be used as a sacrificial template for subsequent ion exchange; (2) Preparation of Mn2V2O7 nanosheets Mn2V2O7 nanosheets were prepared by gas-phase ion exchange in a vacuum tube furnace using ZnO nanosheets as sacrificial templates. Figure 2 b) Specifically, an FTO substrate loaded with ZnO nanosheets was fixed in the central isothermal zone of a tube furnace; MnCl2 and dried VCl3 were placed upstream of the center at distances of 4.5 cm and 9.5 cm, respectively. Under a nitrogen flow of 50 sccm, the temperature was raised to 525 °C and held for 30 min to achieve Zn²⁺ nanosheet loading. + With Mn² + / V³ + Topological ion exchange is used to generate Mn2V2O7 nanosheets; (3) Construction of Mn2V2O7 / BiVO4 heterojunction MBO heterojunctions were constructed in a vacuum tube furnace using Mn2V2O7 nanosheets as precursors via partial ion exchange. Mn2V2O7 nanosheets were placed in the center of the furnace tube, and BiCl3 was placed 10 cm upstream. Partial ion exchange was carried out under the same thermal regime as step (2) (N250 sccm, 525 ℃, 30 min) to form a tightly coupled Mn2V2O7 / BiVO4 heterojunction. (4) Oxygen vacancy regulation To prepare oxygen-vacancy-rich Mn2V2O7 / BiVO4 heterojunction samples, the synthesized Mn2V2O7 / BiVO4 heterojunctions were placed in a vacuum tube furnace and annealed under a hydrogen atmosphere (flow rate 50 mL / min). The obtained Mn2V2O7 / BiVO4 heterojunction material was placed in a tube furnace and heated to 200 °C under a hydrogen atmosphere at a rate of 50 mL / min. After holding at this temperature for 20 min, it was naturally cooled to room temperature to obtain the heterojunction material with the lowest oxygen vacancy concentration, which was named MBOv-1.

[0039] Example 2: Preparation of MBOv-2 heterojunction material Example 2 is similar to Example 1, except that 200 °C in step (4) is replaced with 300 °C, and a heterojunction material with moderate oxygen vacancy concentration is finally obtained, named MBOv-2.

[0040] Example 3: Preparation of MBOv-3 heterojunction material Example 3 is similar to Example 1, except that 200 °C in step (4) is replaced with 400 °C, and the heterojunction material with the highest oxygen vacancy concentration is finally obtained, named MBOv-3.

[0041] Comparative Example 1: Preparation of MBO heterojunction materials Comparative Example 1 is similar to Example 1, except that step (4) is not performed. The Mn2V2O7 / BiVO4 heterojunction material obtained in step (3) is named MBO, which does not contain oxygen vacancies.

[0042] Material characterization The crystal structure of the material was characterized by X-ray diffraction (XRD); the microstructure was observed by field emission scanning electron microscopy (SEM); the elemental distribution and lattice structure were analyzed by transmission electron microscopy (TEM); and the surface chemical state was determined by X-ray photoelectron spectroscopy (XPS).

[0043] 1.1 Crystal Structure The crystal structure of the obtained catalyst was characterized by XRD, such as... Figure 3 As shown in Figure a, the XRD patterns of MBO, MBOv-1, MBOv-2, and MBOv-3 all exhibit a series of characteristic diffraction peaks: the diffraction peaks at 2θ = 27.5°, 29.2°, and 33.8° correspond to the (021), (-201), and (130) crystal planes of the monoclinic Mn2V2O7 (JCPDS No. 38-0034); while the strong peaks at 28.82° and 30.55° are attributed to the (-121) and (040) crystal planes of the monoclinic BiVO4 (PDF#14-0688), confirming the successful construction of the MBO-based heterojunction material and that the crystal phase conforms to expectations. With increasing oxygen vacancy concentration, all diffraction peaks systematically shift towards higher angles ( Figure 3(b) This peak shift originates from lattice shrinkage caused by the introduction of oxygen vacancies, which can be explained by the Bragg equation "d sinθ = nλ". Although different concentrations of surface oxygen vacancies were introduced at different annealing temperatures, all samples maintained the main crystal phase structure of Mn2V2O7 / BiVO4, and no impurity phases were observed to form. This indicates that the oxygen vacancies are mainly concentrated on the surface layer, without disrupting the main crystal structure, and only causing slight lattice shrinkage.

[0044] 1.2 Microstructure The microstructures of MBO and MBOv-3 were systematically characterized by SEM and TEM. The results are shown in [Figure number missing]. Figure 4 and Figure 5 a~c. Mn2V2O7 prepared using ZnO nanosheets as a sacrificial template completely inherits the three-dimensional nanosheet array configuration of the sacrificial template, macroscopically exhibiting an ordered framework composed of stacked nanoparticles. After secondary ion exchange, BiVO4 nanoparticles are uniformly anchored on the surface of Mn2V2O7, and their particle size (≈ 30 nm) is much smaller than that of Mn2V2O7 particles (≈ 300 nm). Figure 5 a~b). High-resolution TEM (HRTEM) further revealed a tight heteroepitaxial interface between BiVO4 and Mn2V2O7: the 0.23 nm lattice fringes correspond to the (141) plane of BiVO4, while the 0.33 nm interplanar spacing belongs to the (021) plane of Mn2V2O7. Figure 5 c). Multi-scale microstructure characterization confirmed that the composite catalyst is a heterojunction system composed of Mn2V2O7 and BiVO4 nanoparticles.

[0045] 1.3 Surface Chemical State XPS analysis was used to resolve the oxygen vacancy concentration and its chemical environment evolution in MBO and MBOv series samples. For example... Figure 5 As shown in d, with increasing oxygen vacancy concentration, Bi₄⁴ f The peak shifts towards lower binding energies, indicating a significant increase in electron density around the Bi sites; while V2 p With Mn 2 p The minimal peak shift indicates that the electronic structure of the V and Mn sites is minimally disturbed by oxygen vacancies. Figure 5 e~f). The O 1s fine spectrum can be decomposed into three characteristic peaks: lattice oxygen (O L 528.5 eV), oxygen vacancies or defects (O V 530.0 eV) and chemically adsorbed or dissociated oxygen species (O C 531.3 eV Figure 5 g). It is worth noting that O V The vacancy component increases monotonically with increasing hydrogen annealing temperature. Figure 5h), directly related to the continuous generation of oxygen vacancies in BiVO4. XPS and XRD together elucidated the dynamic formation mechanism of oxygen vacancies during hydrogen annealing.

[0046] Performance Characterization 2.1 Photocatalytic performance Photocatalytic activity was evaluated using an online gas chromatography system equipped with a Shimadzu GC2014 thermal conductivity detector (TCD) and argon as the carrier gas. All water splitting experiments were conducted under continuous irradiation with a 300 W xenon lamp at an intensity of 240 mW·cm². -2 Typical procedure: 100 mg of catalyst powder is ultrasonically dispersed in 100 mL of deionized water to obtain a stable suspension; before light irradiation, high-purity argon gas is introduced for 30 min to remove dissolved oxygen. After magnetic stirring reaches equilibrium, timed sampling begins: gas samples are extracted from the closed reactor every 30 min and injected into the GC to quantitatively analyze the H2 / O2 yield.

[0047] 100 mg of catalyst powder was ultrasonically dispersed in 100 mL of deionized water to form a homogeneous and stable suspension. Before photocatalysis, high-purity argon gas was passed through for 30 min to remove dissolved air. Subsequently, the suspension was placed under a 300 W xenon lamp to initiate the photocatalytic H2O2 generation reaction. During the reaction, 2 mL of sample solution was taken every 15 min, filtered through a 0.22 µm nylon membrane needle filter, and then 2 mL of 0.1 mol / L precipitate was added. -1 KI solution and 50 μL 0.01 mol L -1 (NH4)6Mo7O 24 Mix the solution thoroughly for 5 min. Measure the absorbance of the mixture at 352 nm using a UV-Vis spectrophotometer, using the generated triiodine ion complex (I3). - The concentration is used as the basis for quantitative analysis of H2O2, and interpolation calculation is performed using a pre-established H2O2 concentration-absorbance standard curve.

[0048] By constructing a photocatalytic system without additives and using pure water, selective and precise control of the distribution of vanadate heterojunction products was achieved.

[0049] First, a standard curve of H2O2 concentration-absorbance was established based on ultraviolet-visible absorption spectroscopy. Figure 6 Photocatalytic water splitting experiments showed that both MBO and MBOv series catalysts could simultaneously release H2, O2, and H2O2. Figure 7 ac). With increasing oxygen vacancy concentration, the H2O2 yield of MBOv decreased significantly (from 1569 µmol h). - ¹g -1 Reduced to 1180 µmol h - ¹g -1O2 yield showed an increasing trend (from 103.8 µmol h⁻¹). - ¹Increased to 362.2 µmol h - ¹)( Figure 7 b). The original MBO catalyst performed best in H2O2 production, achieving a yield of 1569 µmol / h. - ¹g -1 It outperforms most reported metal oxide photocatalysts. Figure 7 c). Among them, the optimized MBOv-3 composite system achieved the highest hydrogen evolution rate (734.4 µmol h⁻¹). - ¹) and oxygen evolution rate (362.2 µmol h⁻¹) - ¹)( Figure 7 d), and exhibits excellent stability in the total water splitting reaction ( Figure 8 Furthermore, the quantitative analysis of the gaseous products of the MBOv series provides the key H2 / O2 molar ratio. For example... Figure 7 As shown in d, this ratio decreases monotonically with increasing oxygen vacancy concentration, and the H2 / O2 ratio of MBOv-3 is close to the theoretical water splitting ratio of 2:1. This indicates that oxygen vacancy concentration can effectively regulate the selectivity of photocatalytic water splitting products: defect-free conditions favor H2O2 generation, while the introduction of oxygen vacancies significantly enhances O2 precipitation.

[0050] 2.2 Mechanism Investigation Oxygen vacancies, hydroxyl radicals and superoxide radicals were monitored and characterized by electron paramagnetic resonance (EPR) technology. In-situ Raman spectroscopy was performed on a LabRAM HR800 system with 532 nm laser excitation and a 100× objective lens.

[0051] To directly verify the changes in oxygen vacancy concentration in the MBOv composite catalyst, EPR spectroscopy was used to characterize the defect structure of the sample. Figure 9 As shown in a, the MBO sample only exhibits background noise signal, while MBOv-1, MBOv-2, and MBOv-3 all show significant signals at g = 2.003, corresponding to the characteristic peak of oxygen vacancy capturing single electrons; the signal intensity order is MBOv-3>MBOv-2>MBOv-1, indicating that the higher annealing temperature in the hydrogen atmosphere promotes the formation of oxygen vacancies.

[0052] To accurately identify key reactive species and elucidate their roles in the reaction system, a systematic free radical capture experiment was conducted. p-Benzoquinone (BQ) was used as the superoxide radical (•O2). -Isopropanol (IPA) is a specific quencher for hydroxyl radicals (•OH), and is also a highly efficient scavenger of hydroxyl radicals. Results showed that after introducing BQ into the MBOv-3 catalytic system, the O2 selectivity plummeted from 78.4% to 46.9%; while after adding IPA to the MBO system, the H2O2 selectivity significantly decreased from 93.8% to 55.4%. Figure 9 b). The above quantitative analysis clearly indicates that: •OH is the main active species driving the formation of H2O2, •O2 - It plays a decisive role in O2 evolution. Further EPR testing using 5,5-dimethyl-1-pyrrolidone-N-oxide (DMPO) as a spin trapping agent showed almost no signal in the dark; under illumination, MBO exhibited a strong •OH characteristic signal, while MBOv-3 showed a high concentration of •O2. - The response is in perfect agreement with the results of the free radical capture experiment. Figure 9 c~d).

[0053] In-situ Raman spectroscopy was used to analyze the evolution of oxygen species in MBO and MBOv-3 during the oxidation reaction. Characteristic vibrational modes were observed in both materials within a potential window of 0.02–1.6 V (vs. RHE): 578 cm⁻¹. - ¹Peak corresponds to Mn³ + Characteristic vibration of ions, 834 cm -1 The signal corresponds to the adsorbed perhydroxyl (OOH) intermediate ( Figure 9 e). For the original MBO, when the applied potential rises to 1.2 V, at 10¹⁹ cm⁻¹ -1 A new response peak appears at 834 cm⁻¹, originating from hydroxyl (OH) species adsorbed on the surface; as the potential continues to increase, the intensity of the OH adsorption peak continues to increase, while at 834 cm⁻¹... - The characteristic peak of OOH at ¹ gradually weakens ( Figure 9 f) provides direct spectroscopic evidence for the two-electron pathway that preferentially generates H2O2 in MBO catalysis. Conversely, the OOH adsorption peak intensity of MBOv-3 consistently increases across the entire test potential range, and no OH adsorption signal is detected. The results indicate that oxygen vacancy engineering can precisely control the water oxidation reaction pathway: the presence of oxygen vacancies significantly accelerates the key OH→OOH conversion step, optimizes the formation kinetics of active oxygen species on the catalyst surface, and thus drives the water oxidation reaction from a two-electron pathway to a four-electron pathway.

[0054] 2.3 Theoretical Calculation Density functional theory (DFT) calculations were performed using the Vienna ab initio Simulation Package (VASP) software. Ion-electron interactions were described using the projector augmented wave (PAW) method. The exchange-correlation energy was treated with a Perdew-Burke-Ernzerhof (PBE) functional under the generalized gradient approximation (GGA), with a plane wave cutoff energy set to 500 eV. Grimme's DFT-D3 dispersion correction was introduced to account for long-range van der Waals forces. For structural optimization, a 2×2×1 Monkhorst-Pack k-point grid was used for the Brillouin zone integral, and the convergence criteria for interatomic forces and total energy were set to 0.02 eV Å, respectively. - ¹ and 10 -5 eV. To address the strong correlation effect between transition metals V and Mn, this work employs a spin-polarized GGA+U method, where the Hubbard U parameters for V and Mn are set to 3.1 eV and 4 eV, respectively.

[0055] To elucidate the modulation mechanism of electronic structure by oxygen vacancies, partial density of states (PDOS) analysis was performed on the MBO and MBOv systems. Figure 10 As shown in figure a, in the initial water dissociation steps of lattice oxygen sites and defect oxygen sites on the surfaces of MBO and MBOv, the transition state Gibbs free energies of the *OH species are 0.17 eV and 0.41 eV, respectively. Significant differences emerge in the second step: lattice oxygen sites tend to generate H2O2 via the HOOH intermediate (energy barrier 1.83 eV), while forming *O requires overcoming an energy barrier as high as 3.64 eV. In contrast, defect oxygen sites are more conducive to forming the *O intermediate (energy barrier 1.62 eV), which subsequently combines with *OH to form *OOH, ultimately releasing O2 (…). Figure 10 b). These results indicate that oxygen sites directly influence the selection of the water oxidation reaction pathway by regulating the energy barrier formed by intermediates. Furthermore, the Fermi level at Bi sites is mainly contributed by the Bi-6p orbital, and the p-band center shifts significantly upward after the introduction of oxygen vacancies. Figure 10 c) enhances electronic delocalization and adsorption capacity. Orbital hybridization analysis shows that the oxygen-vacancy system exhibits strong Bi-6p and O-2p hybridization above the Fermi level, stemming from the increased antibonding energy caused by the upward shift of the p-band center. Figure 10d). This electronic reconstruction significantly promotes oxygen adsorption by strengthening the Bi-O interaction. Crystal Orbital Hamiltonian Population (COHP) bond-level analysis shows that the |ICOHP| value of the Bi-O bond is larger in the MBOv system, confirming that the bonding strength and adsorption capacity are enhanced simultaneously. Three-dimensional differential charge density reveals that oxygen vacancy-induced charge transfer changes from localization of a single Bi site to synergistic action of two Bi sites, forming spatially distributed active sites to stabilize the adsorption of *O species. Figure 10 This electronic reconstruction not only enhances bonding through Bi-6p and O-2p hybridization, but also optimizes the adsorption configuration of intermediate *O, significantly improving the adsorption stability and catalytic activity of oxygen species.

[0056] In summary, this invention uses the Mn2V2O7 / BiVO4 (MBO) heterostructure as a model system and elucidates the intrinsic correlation mechanism between intermediate adsorption behavior and the water oxidation reaction pathway by precisely controlling the electronic structure of oxygen sites, resulting in a photocatalyst capable of producing excellent hydrogen evolution rates and selectively controlling products. The mutual verification between experimental evidence and DFT calculations shows that oxygen vacancy-induced local electron rearrangement can effectively stabilize the key *OOH intermediate, thereby significantly improving the kinetic efficiency of the four-electron oxygen evolution reaction (OER). The optimized MBOv-3 catalyst achieves a near-theoretical H2 / O2 stoichiometric ratio (2:1) in pure water, reaching 734.4 μmol·h⁻¹. - ¹ Excellent hydrogen evolution rate. Conversely, the pristine lattice oxygen sites tend to promote the formation of the *HOOH intermediate, dominating the two-electron transfer pathway, thus enabling the defect-free MBO catalyst to exhibit superior H₂O₂ selectivity (1569 μmol·h⁻¹). - ¹g -1 By employing a strategy of controlling the reaction pathway through oxygen coordination environment engineering, this study not only overcomes the limitation of single-product output in traditional photocatalysts but also achieves, for the first time, selective regulation of the distribution of photocatalytic water splitting products in a vanadate heterostructure system. This work establishes the structure-activity relationship between surface atomic configuration, reaction pathway, and catalytic performance, providing a new paradigm and theoretical guidance for the rational design of high-performance, multifunctional photocatalysts.

[0057] The embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. The environmentally functional heterojunction composite material of the present invention can be applied to photocatalytic water splitting for hydrogen production, environmental oxidation reactions, in-situ generation of hydrogen peroxide, and related environmental reaction systems, and is particularly suitable for environmental and energy coupling scenarios where the oxidation reaction pathway and oxidation product type require controllability. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. An environmentally functional heterojunction material, characterized in that: The heterojunction material is a heterojunction structure formed in situ by Mn2V2O7 phase and BiVO4 phase. The two phases form a tightly contacted heteroepitaxial interface at the interface. The surface of the heterojunction material contains oxygen vacancies with an adjustable concentration, and the molar fraction of the oxygen vacancies is 0%~30%.

2. The environmentally functional heterojunction material according to claim 1, characterized in that: The Mn2V2O7 has a nanosheet structure, and the BiVO4 is uniformly anchored on the surface of the Mn2V2O7 in the form of nanoparticles.

3. A method for preparing an environmentally functional heterojunction material as described in claim 1, characterized in that, Includes the following steps: (1) Preparation of sacrificial template: An array of oxide nanosheets is grown on a conductive substrate by hydrothermal method to form a sacrificial template; (2) First gas-phase ion exchange: Heated under a protective atmosphere with a vacuum degree of 2×10 -2 tor causes the sacrificial template to interact with the Mn source and V source in the gas phase, resulting in a topological ion exchange reaction to form Mn2V2O7 nanosheets. (3) Second gas-phase ion exchange in-situ construction of heterojunction: heating under a protective atmosphere with a vacuum degree of 2×10 -2 tor, so that the Mn2V2O7 nanosheets interact with the Bi source gas phase to carry out a partial ion exchange reaction and form a Mn2V2O7 / BiVO4 heterojunction; (4) Oxygen vacancy control: The Mn2V2O7 / BiVO4 heterojunction is annealed in a hydrogen atmosphere to control the oxygen vacancy concentration.

4. The preparation method according to claim 3, characterized in that: The conductive substrate in step (1) is a fluorine-doped tin oxide substrate, and the oxide is selected from ZnO, CoO or WO3.

5. The preparation method according to claim 3, characterized in that: In step (2), the Mn source is MnCl2, MnAc2 or Mn(NO3)2, and the V source is V2O3, VCl3 or VOC2O4·nH2O.

6. The preparation method according to claim 3, characterized in that: The Bi source in step (3) is selected from BiCl3, Bi2O3, (NH4)3Bi(C6H5O7)2 or Bi(NO3)3·5H2O.

7. The preparation method according to claim 3, characterized in that: The protective atmosphere in steps (2) and (3) is nitrogen atmosphere with a flow rate of 50 sccm. The heating temperature is 425~550 ℃ and the holding time is 5~35 min.

8. The preparation method according to claim 3, characterized in that: The annealing temperature in step (4) is 200~400℃.

9. The application of an environmentally functional heterojunction material as described in any one of claims 1 to 8 in photocatalytic water splitting, characterized in that: The selective regulation of water oxidation products is achieved by controlling the surface oxygen vacancy concentration of the heterojunction material. Decreasing the surface oxygen vacancy concentration of the heterojunction material causes the water oxidation reaction to tend towards a two-electron pathway, mainly generating H2O2. Increasing the surface oxygen vacancy concentration of the heterojunction material causes the water oxidation reaction to tend towards a four-electron pathway, mainly generating O2.

10. The application according to claim 9, characterized in that: High oxygen vacancy concentration Mn2V2O7 / BiVO4 heterojunction materials can achieve an overall water splitting product distribution close to the theoretical value of H2 / O2 = 2:1 in a pure water system.

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