Method for regulating heterojunction charge transfer path and heterojunction photocatalyst

Abstract

The application discloses a method for regulating a heterojunction charge transfer path and a heterojunction photocatalyst, and belongs to the field of photocatalytic materials. The method comprises the following steps: chemically doping TiO2 to obtain doped TiO2; compounding the doped TiO2 with g-C3N4 (graphitic carbon nitride) to form a heterojunction; by controlling the type and / or doping amount of the doping element, the work function of the doped TiO2 is lower than the work function of the g-C3N4, and a built-in electric field from the doped TiO2 to the g-C3N4 is formed at the interface of the two, so that the charge transfer path of the heterojunction presents a Type-II type migration path under light conditions. The application also discloses a heterojunction photocatalyst prepared by the above method. The application realizes active regulation of the heterojunction charge transfer path through chemical doping, effectively improves the separation efficiency of photo-generated carriers, and has application prospects in the field of photocatalytic nitrogen fixation.

Description

Technical Field

[0001] This application relates to the field of photocatalytic materials technology, and in particular to a method for regulating the charge transfer path of a heterojunction and a heterojunction photocatalyst. Background Technology

[0002] Ammonia (NH3), as an important chemical raw material and clean energy carrier, is traditionally synthesized mainly through the Haber-Bosch process. This process requires high temperature (400-500°C) and high pressure (10-30 MPa), resulting in huge energy consumption and large CO2 emissions. Photocatalytic nitrogen fixation technology utilizes solar energy to drive the reaction of N2 and H2O to synthesize NH3 at room temperature and pressure, and is considered a green and sustainable alternative. However, photocatalytic N2 fixation faces severe challenges: the N≡N triple bond is extremely inert (bond energy approximately 941 kJ / mol), and traditional photocatalysts (such as TiO2) have inherent limitations such as a wide bandgap (approximately 3.2 eV) that only responds to ultraviolet light, easy recombination of photogenerated carriers in the bulk phase, and a lack of active sites, resulting in photoluminescence efficiency and reaction rate far from meeting the requirements of practical applications.

[0003] To overcome the aforementioned bottlenecks, researchers have widely adopted two strategies: (1) metal / non-metal ion doping, which involves introducing impurity energy levels to modulate the band structure and Fermi level position of semiconductors, thereby broadening the photoresponse range and enhancing carrier concentration; and (2) constructing heterojunctions, which utilize the interfacial contact and work function difference between two different semiconductors to form a built-in electric field, thereby driving the spatial separation of photogenerated electrons and holes and suppressing bulk recombination. Among these, the composite system composed of TiO2 and visible light-responsive graphitic carbon nitride (g-C3N4) has become one of the most widely studied heterojunction photocatalysts due to the matching of their band positions and good interfacial compatibility.

[0004] In TiO2 / g-C3N4 heterojunctions, two typical mechanisms can be distinguished based on the different charge transfer paths: Type-II heterojunctions and S-type heterojunctions. However, the charge transfer mechanism (Type II vs. S-type) of existing TiO2 / g-C3N4 heterojunctions heavily relies on the inherent Fermi level and work function difference between the two semiconductor materials, which is a passively determined inherent property lacking effective active control methods. Once the two materials and their preparation process are selected, the heterojunction type and corresponding carrier migration path are "locked," making it impossible to flexibly design and optimize according to the thermodynamic requirements of the target reaction, severely restricting further improvement of photocatalyst performance. Summary of the Invention

[0005] This application provides a method for controlling the charge transfer path of a heterojunction and a heterojunction photocatalyst, which solves the problems mentioned in the background art.

[0006] In a first aspect, embodiments of this application provide a method for controlling the charge transfer path of a heterojunction, comprising the following steps: Chemical doping of TiO2 can be used to control the Fermi level or work function of TiO2 to obtain doped TiO2; The doped TiO2 is combined with g-C3N4 to form a heterojunction; By controlling the type and / or amount of doping elements, the work function of the doped TiO2 is made lower than that of the g-C3N4, so as to form a built-in electric field from the doped TiO2 to the g-C3N4 at the interface between the two, thereby making the charge transfer path of the heterojunction exhibit a Type-II migration path under illumination.

[0007] In conjunction with the first aspect, in one possible implementation, the chemical doping is metal ion doping.

[0008] In conjunction with the first aspect, in one possible implementation, the metal ion in the metal ion doping is one or more of Cu, Fe, Co, Ni, Mn, and Zn.

[0009] In conjunction with the first aspect, in one possible implementation, the metal ion is a Cu ion.

[0010] In conjunction with the first aspect, in one possible implementation, the doping amount is 0.5% to 2.0% based on the molar ratio of the doping element to TiO2.

[0011] In conjunction with the first aspect, in one possible implementation, the doping amount is 1.0%.

[0012] In conjunction with the first aspect, in one possible implementation, the work function of the doped TiO2 is 3.70~3.90 eV, and the work function of the g-C3N4 is 3.90~4.10 eV.

[0013] In conjunction with the first aspect, in one possible implementation, the doped TiO2 is prepared by a low-temperature crystallization method of peroxytitanic acid, comprising: reacting a titanium source with hydrogen peroxide to form a peroxytitanic acid solution, adding a dopant element source, and carrying out a crystallization reaction at ≤100°C in the presence of a reducing agent to obtain the doped TiO2.

[0014] In conjunction with the first aspect, in one possible implementation, the composite method is an in-situ assembly method, which includes: dispersing g-C3N4 in a solvent, adding a precursor solution of doped TiO2, so that the doped TiO2 nanoparticles grow and anchor in situ on the surface of g-C3N4; the mass ratio of g-C3N4 to doped TiO2 is 1:4 to 1:20.

[0015] Secondly, embodiments of this application provide a heterojunction photocatalyst, which is prepared using the method for controlling the charge transfer path of the heterojunction as described in the first aspect; In the photocatalyst, doped TiO2 nanoparticles are uniformly anchored on the surface of g-C3N4, and a built-in electric field is formed at the interface between the two, pointing from the doped TiO2 to the g-C3N4. Under illumination, photogenerated electrons migrate from the conduction band of g-C3N4 to the conduction band of doped TiO2, and photogenerated holes migrate from the valence band of doped TiO2 to the valence band of g-C3N4.

[0016] One or more technical solutions provided in the embodiments of this application have at least the following technical effects: This application actively modulates the Fermi level or work function of TiO2 through chemical doping, and precisely controls the type and amount of doping elements to make the work function of doped TiO2 lower than that of g-C3N4. This results in a built-in electric field at the interface between the two, pointing from doped TiO2 to g-C3N4. Under illumination, this drives photogenerated carriers to separate and migrate efficiently along the Type-II path. This approach overcomes the fundamental limitations of traditional heterojunction charge transfer mechanisms, which rely on inherent material properties and lack active control methods. Furthermore, by optimizing the doping window, it effectively modulates the Fermi level while avoiding the surge in bulk recombination centers caused by excessive doping. This approach balances the efficient carrier separation with the thermodynamic requirements of the target reaction for redox capabilities, significantly improving photocatalytic performance. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of this application, the accompanying drawings used in the description of the embodiments of this application will be briefly introduced below. Obviously, the accompanying drawings described below are some embodiments of this application. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the preparation process of the method for controlling the charge transfer path of heterojunctions in this application; Figure 2 The image shows a transmission electron microscope (TEM) image of the TiO2 / g-C3N4 heterojunction prepared by the method of this application; wherein, Figure 2 (a) in the image is a TEM image of g-C3N4; Figure 2 (b) is a TEM image of TiO2 doped TiO2; Figure 2 (c) is a TEM image of CuTCN-2 obtained in Example 1; Figure 2 (d) is an HRTEM image of CuTCN-2, with lattice fringes of TiO2(101) and g-C3N4(002) crystal planes labeled; Figure 2Images (e) to (f) in the figure are HAADF-STEM images of CuTCN-2; Figure 2 (g) in the figure represents the elemental distribution diagram of Ti, O, Cu, N and C in the heterojunction. Figure 3 The XRD and FTIR spectra of the photocatalyst prepared by the method of this application and the comparative sample are shown; wherein, Figure 3 (a) in the image is the XRD pattern; Figure 3 (b) in the image represents the FTIR spectrum; Figure 4 XPS spectra of the photocatalyst prepared by the method of this application and the comparative sample; wherein, Figure 4 (a) in the image is the high-resolution spectrum of Ti2p; Figure 4 (b) in the image is the O 1s high-resolution spectrum; Figure 4 (c) represents the O content; Figure 4 (d) in the spectrum is the C 1s high-resolution spectrum; Figure 4 (e) in the spectrum represents the N 1s high-resolution spectrum; Figure 4 (f) in the image represents the high-resolution Cu 2p spectrum; Figure 5 The results show a comparison of the photocatalytic NH3 yield and cycle stability of the photocatalyst prepared by the method of this application with those of a comparative sample; wherein, Figure 5 (a) in the figure is the time dependence curve of NH3 yield; Figure 5 (b) in the figure is a bar chart comparing yields; Figure 5 (c) and (d) in the figure are the results of the TCN series comparative experiments; Figure 5 (e) in the figure represents the results of the control experiment; Figure 5 (f) in the figure represents the cyclic stability test results; Figure 6 The figures show the optical absorption and carrier dynamics characterization of the photocatalyst prepared by the method of this application; wherein, Figure 6 (a) in the image represents the ultraviolet-visible diffuse reflectance spectrum; Figure 6 (b) in the figure is the bandgap fitting plot of Tauc plots; Figure 6 (c) in the figure represents the steady-state photoluminescence spectrum; Figure 6 (d) in the figure represents the time-resolved transient PL decay curve; Figure 6 (e) in the figure represents the transient photocurrent response curve; Figure 6 (f) in the figure is the Nyquist plot of electrochemical impedance spectroscopy; Figure 7 The diagram shows the UPS and interface band structure of TiO2 and g-C3N4 doped by the method of this application; wherein, Figure 7 (a) in the image is the full UPS spectrum of TiO2 doped with TiO2; Figure 7 (b) in the image is the full UPS spectrum of g-C3N4; Figure 7 (c) in the diagram is a schematic diagram of the band structure and charge transfer of the doped TiO2 / g-C3N4 heterojunction; Figure 8 This is a schematic diagram of the photocatalytic nitrogen fixation reaction mechanism of the TiO2 / g-C3N4 heterojunction prepared by the method of this application. Detailed Implementation

[0019] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this application. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0020] In the description of the embodiments of this application, it should be noted that the terms "center," "upper," "lower," "left," "right," "vertical," "horizontal," "inner," and "outer," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used only for the convenience of describing the embodiments of this application and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on this application. The terms "first," "second," and "third" are used for descriptive purposes only and should not be construed as indicating or implying relative importance. Furthermore, the terms "installed," "connected," and "linked" should be interpreted broadly. For example, they can refer to a fixed connection, a detachable connection, or an integral connection; they can refer to a mechanical connection or an electrical connection; they can refer to a direct connection or an indirect connection through an intermediate medium; they can refer to the internal connection of two components. Those skilled in the art can understand the specific meaning of the above terms in the embodiments of this application according to the specific circumstances.

[0021] This application provides a method for controlling the charge transfer path of a heterojunction, such as... Figures 1 to 8 As shown, the method includes the following steps: Chemical doping of TiO2 (titanium dioxide) can be used to modulate the Fermi level or work function of TiO2, thereby obtaining doped TiO2. A heterojunction is formed by combining doped TiO2 with g-C3N4 (graphitic carbon nitride); By controlling the type and / or amount of doping elements, the work function of doped TiO2 is made lower than that of g-C3N4, so as to form a built-in electric field from doped TiO2 to g-C3N4 at the interface between the two, thereby making the charge transfer path of the heterojunction exhibit a Type-II migration path under illumination.

[0022] This application actively modulates the Fermi level or work function of TiO2 through chemical doping, and precisely controls the type and amount of doping elements to make the work function of doped TiO2 lower than that of g-C3N4. This results in a built-in electric field at the interface between the two, pointing from doped TiO2 to g-C3N4. Under illumination, this drives photogenerated carriers to separate and migrate efficiently along the Type-II path. This approach overcomes the fundamental limitations of traditional heterojunction charge transfer mechanisms, which rely on the inherent properties of materials and lack active control methods. Furthermore, by optimizing the doping window, it effectively controls the Fermi level while avoiding the surge in bulk recombination centers caused by excessive doping. This balances the efficient carrier separation with the thermodynamic requirements of the target reaction for redox capabilities, thereby improving photocatalytic performance.

[0023] In this embodiment, the chemical doping is metal ion doping. By chemically modifying TiO2 with metal ion doping, different doping elements, due to their differences in ionic radius, valence state, and matching with the TiO2 lattice, can effectively introduce impurity energy levels and control the Fermi level position of TiO2. This provides a rich selection space for achieving directional adjustment of the work function of TiO2 relative to g-C3N4, thus broadening the applicability and operability of the technical solution in this application.

[0024] In this embodiment, the metal ions used in the metal ion doping are one or more selected from Cu, Fe, Co, Ni, Mn, and Zn. By selecting transition metal ions to dope TiO2, these metal ions can occupy the TiO2 lattice. 4+ The site introduces impurity energy levels and effectively modulates the Fermi level and work function of TiO2. At the same time, different metal ions have different ionic radii and valence state characteristics, and the optimal doping element can be flexibly selected according to the thermodynamic requirements of the specific target reaction, which further enhances the flexibility and universality of the technical solution of this application.

[0025] In this embodiment, the metal ion is Cu ion. By selecting Cu ions as the dopant element, Cu... + / Cu² + Ionic radius and Ti 4+ With good matching and a wide doping window, Cu doping can precisely reduce the work function of TiO2 from approximately 4.0~4.2 eV in the intrinsic state to 3.83 eV at a doping concentration of 1.0%. At the same time, Cu doping synergistically induces the generation of a large number of oxygen vacancies in the TiO2 lattice, providing abundant adsorption and activation sites for reactants such as N2. It has both excellent Fermi level tuning efficiency and surface activity enhancement effect, making it the optimal doping element for realizing the technical solution of this application.

[0026] In this embodiment, the doping amount is 0.5% to 2.0% based on the molar ratio of the dopant element to TiO2. By controlling the doping amount within the range of 0.5% to 2.0%, sufficient dopant elements are ensured to enter the TiO2 lattice to effectively regulate the Fermi level and work function (reducing the work function of doped TiO2 to 3.70 to 3.90 eV), while maintaining the concentration of bulk recombination centers at a low level. This avoids severe quenching of photogenerated carriers caused by excessive doping (>2.0%). A precise balance is achieved between "Fermi level regulation" and "maintaining a low recombination center concentration," which is a key parameter window for realizing active regulation of charge transfer paths.

[0027] In this embodiment, the doping amount is 1.0%. At this time, the work function of TiO2 is precisely reduced from about 4.0~4.2 eV in the intrinsic state to 3.83 eV, forming a moderate work function difference of 0.17 eV with g-C3N4 (4.00 eV). This provides sufficient built-in electric field driving force to achieve active switching of the Type-II path, while avoiding the surge of deep-level impurities and bulk recombination centers caused by excessive doping. This is the optimal doping amount confirmed by system optimization.

[0028] In this embodiment, the work function of doped TiO2 is 3.70~3.90 eV, and the work function of g-C3N4 is 3.90~4.10 eV. By controlling the work function of doped TiO2 to 3.70~3.90 eV and the work function of g-C3N4 to 3.90~4.10 eV, and ensuring that the former is lower than the latter, electrons spontaneously flow from doped TiO2 to g-C3N4 until the Fermi level reaches equilibrium when the two come into contact at the interface. This creates a built-in electric field at the interface pointing from doped TiO2 to g-C3N4, providing a clear driving force direction for the directional migration of photogenerated carriers along the Type-II path under illumination, which is the physical basis for the active control of charge transfer paths.

[0029] In this embodiment, doped TiO2 is prepared by a low-temperature crystallization method using peroxytitanic acid, comprising: reacting a titanium source with hydrogen peroxide (H2O2) to form a peroxytitanic acid solution, adding a dopant element source, and carrying out a crystallization reaction at ≤100°C in the presence of a reducing agent to obtain doped TiO2. The low-temperature crystallization method using peroxytitanic acid prepares doped TiO2 under mild conditions at ≤100°C. Utilizing the condensation crystallization behavior of the peroxytitanic acid intermediate at low temperatures, combined with the reducing and crystallization-promoting effects of hydroxylamine hydrochloride (NH2OH·HCl), the crystallization of TiO2 nanoparticles avoids the grain growth and oxygen vacancy annihilation caused by traditional high-temperature calcination (450~550°C), effectively preserving a high concentration of oxygen vacancies, providing abundant adsorption and activation sites for reactants such as N2. Furthermore, the process conditions are mild and do not use organic solvents, making it suitable for scale-up production and practical applications.

[0030] In this embodiment, the composite method is an in-situ assembly method, including: dispersing g-C3N4 in a solvent, adding a precursor solution of doped TiO2, so that the doped TiO2 nanoparticles grow and anchor in situ on the surface of g-C3N4; the mass ratio of g-C3N4 to doped TiO2 is 1:4 to 1:20. The in-situ assembly method allows the doped TiO2 nanoparticles to be uniformly anchored on the surface of g-C3N4 and form a tight interfacial contact (see...). Figure 2 (c) significantly increases the heterojunction interface area and interface bonding strength compared to the physical mixing method, providing structural assurance for the effective establishment of the built-in electric field and the interface transfer of photogenerated carriers. At the same time, by controlling the mass ratio of g-C3N4 to doped TiO2 at 1:4~1:20, sufficient heterojunction interface area is ensured to drive charge separation, while avoiding excessive g-C3N4 blocking incident light and reducing photonic efficiency or insufficient g-C3N4 causing TiO2 agglomeration, thus forming a systematically optimized two-component synergistic window.

[0031] This application also provides a heterojunction photocatalyst, prepared using the method described above for controlling the charge transfer path of the heterojunction. In the photocatalyst, doped TiO2 nanoparticles are uniformly anchored on the surface of g-C3N4, and a built-in electric field is formed at the interface between the two, pointing from the doped TiO2 to the g-C3N4. Under illumination, photogenerated electrons migrate from the conduction band of g-C3N4 to the conduction band of the doped TiO2, and photogenerated holes migrate from the valence band of the doped TiO2 to the valence band of g-C3N4 (see...). Figure 7 (c) Figure 8 ).

[0032] In the TiO2 / g-C3N4 heterojunction photocatalyst prepared in this application, doped TiO2 nanoparticles are uniformly anchored on the surface of g-C3N4 to form a tight interfacial contact. At the interface, a built-in electric field is oriented from the doped TiO2 to the g-C3N4. Under illumination, photogenerated electrons migrate from the conduction band of g-C3N4 to the conduction band of doped TiO2, and holes migrate from the valence band of doped TiO2 to the valence band of g-C3N4. This achieves efficient spatial separation of photogenerated carriers and directional migration along the Type-II path, effectively suppressing bulk recombination. As a result, this photocatalyst exhibits excellent catalytic activity and cycle stability in the nitrogen fixation reaction.

[0033] Example 1 This embodiment provides a specific implementation method for controlling the charge transfer path of a heterojunction, namely, constructing a Type-II heterojunction photocatalyst (CuTCN-2) by combining Cu-doped TiO2 with g-C3N4, thereby achieving an active switching of the heterojunction charge transfer path from S-type to Type-II. The preparation process is described in [link to preparation details]. Figure 1 Specifically, it includes the following steps: Step 1: Preparation of g-C3N4 Melamine (C3H6N6) and urea (CH4N2O) were uniformly mixed at a mass ratio of 1:9 (5 g melamine, 45 g urea) and placed in a covered alumina crucible. The crucible was placed in a tube furnace and heated to 550°C at a heating rate of 5°C / min under a nitrogen (N2) atmosphere, held at that temperature for 3 hours, and then allowed to cool naturally to room temperature. The resulting pale yellow solid was collected and ground into a fine powder, which is g-C3N4. Preferably, the g-C3N4 obtained in this embodiment is in the form of nanosheets (see...). Figure 2 (a) In this context, the nanosheet morphology can increase the specific surface area and expose more edge active sites.

[0034] In this step, the melamine / urea mass ratio of 1:9 generates more gases (ammonia NH3, carbon dioxide CO2) during thermal polycondensation, promoting the exfoliation of the g-C3N4 layered structure. 550°C is the optimal polycondensation temperature; below this temperature, polycondensation is incomplete, while above this temperature, excessive condensation of the skeleton occurs, resulting in numerous defects.

[0035] Step 2: Preparation of Cu-TiO2 nanoparticles Add 10 g of titanium hydroxide (Ti(OH)4) and 15 mL of 30% hydrogen peroxide (H2O2) to 100 mL of deionized water and stir at room temperature until a transparent yellow peroxytitanic acid solution is formed (about 30 minutes).

[0036] Add a stoichiometric amount of copper chloride dihydrate (CuCl2·2H2O) to the above solution to make the Cu / TiO2 molar ratio 1.0%, and stir for 10 minutes until completely dissolved. Heat the mixed solution to reflux (about 100°C), add 1 g of hydroxylamine hydrochloride (NH2OH·HCl) as a reducing agent and crystallization promoter, and continue the reflux reaction for 2 hours.

[0037] After the reaction was complete, the product was centrifuged at 8000 rpm for 10 minutes, washed three times each with deionized water and anhydrous ethanol, and then vacuum dried at 60°C for 12 hours to obtain Cu-doped TiO2 nanoparticles, denoted as Cu-TiO2-1. Characterization was performed by ultraviolet photoelectron spectroscopy (UPS) (see [link to UPS]). Figure 7 In (a) of Cu-TiO2-1, the work function is 3.83 eV. Transmission electron microscopy (TEM) image (see [reference]). Figure 2 As shown in (b), the obtained Cu-TiO2 nanoparticles have a particle size of approximately 10~20 nm.

[0038] In this step, 1.0% is the optimal Cu doping amount—below this value, the effect on Fermi level modulation is weak, and above this value, there is excess Cu². + / Cu+ It becomes a bulk recombination center, degrading the lifetime of photogenerated carriers. The addition of hydroxylamine hydrochloride not only reduces part of Cu²⁺ + Cu + It also promotes the condensation and crystallization of peroxytitanic acid at low temperatures, which is key to obtaining highly crystalline nano-TiO2.

[0039] Step 3: Constructing heterojunctions by combining Cu-TiO2 and g-C3N4 1.0 g of g-C3N4 obtained in step 1 was dispersed in 20 mL of deionized water and sonicated for 30 minutes (power 200W, frequency 40 kHz) to obtain a uniform g-C3N4 dispersion.

[0040] Add 10 g Ti(OH)4 and 15 mL 30% H2O2 separately to 80 mL deionized water and stir until a transparent yellow peroxytitanic acid solution is formed; add 0.056 g CuCl2·2H2O and stir for 10 min.

[0041] Under stirring conditions, the g-C3N4 dispersion was slowly added dropwise to the Cu-peroxytitanic acid solution over a period of more than 30 min, followed by stirring for another 30 min to allow the Cu-TiO2 precursor to be uniformly adsorbed onto the g-C3N4 surface. 1 g of hydroxylamine hydrochloride was then added, and the mixture was heated to reflux (approximately 100°C) for 2 h.

[0042] After the reaction was completed, the precipitate was collected by centrifugation at 8000 rpm for 10 min, washed three times each with deionized water and anhydrous ethanol, and dried under vacuum at 60°C for 12 h to obtain Cu-TiO2 / g-C3N4 heterojunction photocatalyst CuTCN-2.

[0043] Characterized by ultraviolet photoelectron spectroscopy (UPS) (see Figure 7 In (b) of this embodiment, the work function of g-C3N4 is 4.00 eV. Since the work function of Cu-TiO2-1 (3.83 eV) is lower than that of g-C3N4 (4.00 eV), a built-in electric field is formed at the interface between the two, pointing from Cu-TiO2 to g-C3N4. Under illumination, this drives photogenerated electrons to migrate from the conduction band of g-C3N4 to the conduction band of Cu-TiO2, and photogenerated holes to migrate from the valence band of Cu-TiO2 to the valence band of g-C3N4, exhibiting a Type-II migration path (see [reference]). Figure 7 (c) in the middle.

[0044] Transmission electron microscopy (TEM) images (see) Figure 2 (c) shows that Cu-TiO2 nanoparticles are uniformly anchored on the g-C3N4 surface. High-resolution transmission electron microscopy (HRTEM) image (see [link]). Figure 2 (d) shows clear lattice fringes, where the lattice fringe with a spacing of 0.357 nm corresponds to the (101) crystal plane of the TiO2 anatase phase, and the lattice fringe with a spacing of 0.341 nm corresponds to the (002) crystal plane of g-C3N4. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (see [image not provided]). Figure 2 (e) Figure 2 (f) and the elemental distribution map of the energy dispersive spectroscopy (EDS) (see [reference]). Figure 2 The (g) in the image shows that Ti, O, Cu, N and C elements are uniformly distributed, and there is no local enrichment of Cu signal, confirming that Cu element exists in the form of atomic-level doping.

[0045] X-ray diffraction (XRD) patterns (see...) Figure 3 As shown in (a) of the figure, all samples exhibited characteristic diffraction peaks of anatase TiO2. The (002) plane diffraction peak (2θ=27.5°) of g-C3N4 increased with increasing Cu content. No copper oxide impurity peaks were observed in samples with different Cu doping amounts (0.5%~8.0%). Fourier transform infrared (FTIR) spectra (see Figure 1) Figure 3 (b) also shows the characteristic absorption peaks of CN / C=N in g-C3N4 (1240~1640 cm⁻¹). - ¹) and the Ti-O-Ti characteristic absorption peak of TiO2 (<800 cm⁻¹) - ¹), no Cu-O bond absorption peak was detected.

[0046] X-ray photoelectron spectroscopy (XPS) characterization results (see...) Figure 4 The image shows the high-resolution spectrum of Ti 2p (see [reference]). Figure 4 In (a) of the spectrum, the Ti 2p peak of Cu-TiO2 (458.5 / 464.2 eV) remains essentially unchanged compared to pure TiO2, while after constructing the heterojunction (CuTCN-2), it shifts positively by 0.2 eV (to 458.7 / 464.4 eV), indicating electron depletion on the Cu-TiO2 side; the O 1s high-resolution spectrum (see [reference]). Figure 4 (b) can be fitted into three oxygen species: lattice oxygen (O_L, approximately 529.8 eV), oxygen vacancy-related oxygen (O_V, approximately 531.2 eV), and surface adsorbed oxygen (O_A, approximately 533.0 eV). Quantitative analysis shows that the relative content of oxygen vacancies increases from 20.71% in pure TiO2 to 25.43% in Cu-TiO2, and further to 29.35% in CuTCN-2. Figure 4(c) The XPS quantitative histogram further visually shows the relative content trends of lattice oxygen, oxygen vacancies, and surface adsorbed oxygen in each sample, which is consistent with the fitting results of the O 1s fine spectrum, confirming that Cu doping and heterostructure synergistically promote the generation of oxygen vacancies; C 1s high-resolution spectrum (see Figure 4 (d) shows no significant shift before and after recombination (284.8 eV, 286.2 eV, 288.1 eV); N 1s high-resolution spectrum (see Figure 4 (e) shows that the three characteristic peaks of g-C3N4 (398.5 eV, 399.8 eV, 401.2 eV) all shift negatively after recombination, confirming that g-C3N4 is an electron acceptor. (See Cu 2p high-resolution spectrum). Figure 4 (f) in the figure confirms that copper ions are in Cu²⁺ + and Cu + The mixed valence states exist in the TiO2 lattice.

[0047] The XPS results above jointly confirm that Cu doping significantly increases the oxygen vacancy content in the TiO2 lattice (from 20.71% to 29.35%), providing abundant adsorption and activation sites for N2 molecules. Simultaneously, electrons migrate from Cu-TiO2 to g-C3N4 at the heterojunction interface, causing a positive shift of the Ti 2p peak on the Cu-TiO2 side (electron depletion) and a negative shift of the N 1s peak on the g-C3N4 side (electron enrichment), forming a built-in electric field at the interface pointing from Cu-TiO2 to g-C3N4. This built-in electric field provides the driving force for the directional migration of photogenerated carriers along the Type-II path under illumination.

[0048] Example 2 The difference between this embodiment and Embodiment 1 is that the amount of g-C3N4 fed is 0.75 g, and the resulting sample is denoted as CuTCN-1.

[0049] The NH3 yield of CuTCN-1 obtained in this example was 78.6 μmol·g. - ¹·h - ¹ (See also) Figure 5 (b)). This embodiment also achieves the regulation of the charge transfer path of the heterojunction: the work function of Cu-TiO2 (3.83 eV) is lower than that of g-C3N4 (4.00 eV), and a built-in electric field is formed at the interface from Cu-TiO2 to g-C3N4, exhibiting a Type-II migration path under illumination. However, due to the small amount of g-C3N4, some TiO2 nanoparticles failed to form effective interfacial contact with g-C3N4, resulting in a certain degree of agglomeration. The heterojunction interface area is insufficient, and the photocatalytic nitrogen fixation activity is lower than that of CuTCN-2.

[0050] Example 3 The difference between this embodiment and Embodiment 1 is that the amount of g-C3N4 fed is 1.25 g, and the resulting sample is denoted as CuTCN-3.

[0051] The NH3 yield of CuTCN-3 obtained in this example was 85.3 μmol·g. - ¹·h - ¹ (See also) Figure 5 (b)). This embodiment also achieves the control of the charge transfer path of the heterojunction: the work function of Cu-TiO2 (3.83 eV) is lower than that of g-C3N4 (4.00 eV), and a built-in electric field is formed at the interface from Cu-TiO2 to g-C3N4, exhibiting a Type-II migration path under illumination. However, due to the increase in the amount of g-C3N4, the TiO2 loading is relatively reduced, and the excess g-C3N4 has a blocking effect on the incident light, resulting in a decrease in quantum efficiency and lower photocatalytic nitrogen fixation activity than CuTCN-2.

[0052] Example 4 The difference between this embodiment and Embodiment 1 is that the amount of g-C3N4 fed is 2.0 g, and the resulting sample is denoted as CuTCN-4.

[0053] The NH3 yield of CuTCN-4 obtained in this example was 72.5 μmol·g. - ¹·h - ¹ (See also) Figure 5 (b)). This embodiment also achieves the control of the charge transfer path of the heterojunction: the work function of Cu-TiO2 (3.83 eV) is lower than that of g-C3N4 (4.00 eV), and a built-in electric field is formed at the interface from Cu-TiO2 to g-C3N4, exhibiting a Type-II migration path under illumination. However, excessive g-C3N4 severely blocks the incident light, significantly reducing the quantum efficiency and further decreasing the photocatalytic nitrogen fixation activity.

[0054] Preparation of comparative samples Prepare a single g-C3N4 sample by following the method in step 1 of Example 1, but without introducing TiO2.

[0055] Prepare a TiO2 sample by following the method in step 2 of Example 1, but without adding CuCl2·2H2O.

[0056] Prepare a single Cu-TiO2-1 sample by following the method in step 2 of Example 1 (adding CuCl2·2H2O) but without introducing g-C3N4.

[0057] TiO2 / g-C3N4 composites were prepared according to the method of Example 1 without the addition of CuCl2·2H2O, and the feed amounts of g-C3N4 were 0.75 g, 1.0 g, 1.25 g, and 2.0 g, respectively, and were named TCN-1, TCN-2, TCN-3, and TCN-4.

[0058] The photocatalytic nitrogen fixation performance of the above-obtained samples (including pure TiO2, pure g-C3N4, Cu-TiO2-1, TCN-1~TCN-4, CuTCN-1~CuTCN-4) was evaluated. The reaction apparatus was a closed top-illuminated photoreactor (approximately 250 mL in volume) equipped with a quartz cover. The light source was a 300 W xenon lamp (with an AM 1.5G filter to simulate sunlight), and the reaction temperature was maintained at 25 ± 1°C through a circulating water bath.

[0059] Reaction conditions: 50 mg of photocatalyst was dispersed in 100 mL of deionized water, and 10 mL of methanol (CH3OH, as a hole sacrificial agent) was added. Other hole sacrificial agents besides methanol (such as ethanol, triethanolamine, isopropanol, lactic acid, Na2SO3, Na2S, etc.) can also be used in the photocatalytic nitrogen fixation process of this application. High-purity N2 (purity ≥99.999%) was introduced for 0.5 h to remove dissolved oxygen and saturate the solution with N2. During illumination, N2 was continuously bubbled (flow rate 20 mL / min).

[0060] Sampling and Detection: Every 1 hour, 1 mL of the reaction solution was taken using a disposable syringe, filtered through a 0.22 μm aqueous filter membrane, and then quantitatively analyzed for ammonium ions (NH4+) using an ion chromatograph (equipped with a cation exchange column and conductivity detector). + The concentration of ammonia (NH3) was determined by external standard method, and the yield of ammonia (NH3) was calculated.

[0061] The evaluation results show (see) Figure 5 In (a) and (b) of CuTCN-2, the NH3 yield was 91.14 μmol·g. - ¹·h - ¹, The NH3 yield of TCN-2 (Cu-free) was 82.37 μmol·g. - ¹·h - ¹, The NH3 yield of Cu-TiO2-1 was 26.98 μmol·g - ¹·h - ¹, The NH3 yield of pure TiO2 is 12.03 μmol·g - ¹·h - ¹, The NH3 yield of pure g-C3N4 is 2.67 μmol·g - ¹·h -¹. CuTCN-2 is 7.6 times, 3.4 times, and 34.1 times more potent than pure TiO2, Cu-TiO2-1, and g-C3N4, respectively. CuTCN-2 further improves upon TCN-2 by about 11%, demonstrating a clear synergistic enhancement effect between Cu doping and heterostructure construction (see [link to relevant documentation]). Figure 5 (c) and (d) in the middle.

[0062] Controlled experiment (see) Figure 5 (e) indicates that no NH3 was detected under three conditions: no light (dark state), argon (Ar) atmosphere replacing N2, and no catalyst, proving that this reaction is indeed a photocatalytic N2 reduction reaction. Cyclic stability test (see...) Figure 5 Figure (f) shows that after four consecutive cycles (5 hours of light each time) of CuTCN-2, the NH3 yields were 91.1, 89.7, 90.2, and 88.9 μmol·g⁻¹, respectively. - ¹·h - ¹, the activity retention rate is about 97.6%, with no significant attenuation.

[0063] UV-Vis DRS (see UV-Vis DRS) Figure 6 As shown in (a)), pure TiO2 exhibits significant absorption only in the ultraviolet region (<380 nm), while Cu-TiO2-1 shows a weak absorption shoulder in the visible region (400–600 nm) (attributed to impurity energy levels introduced by Cu doping). The CuTCN series composites all show significant redshift of the absorption edge and enhanced visible light absorption, with CuTCN-2 exhibiting the strongest absorption. (See Tauc plots for bandgap fitting.) Figure 6 (b) shows the band gap narrowing stepwise. Steady-state photoluminescence spectrum (PL, see [reference]). Figure 6 (c) shows that both Cu-TiO2-1 and pure TiO2 exhibit strong emission peaks, while the PL intensity of the CuTCN series composites is significantly quenched, with CuTCN-2 showing the greatest quenching, demonstrating that the heterojunction interface effectively suppresses bulk carrier recombination. (See time-resolved transient PL decay curves). Figure 6 (d) shows that CuTCN-2 has the shortest average lifetime (4.43 ns), a shortened lifetime that is typical of the opening of the interfacial charge transfer channel. Transient photocurrent response curves (see...) Figure 6 (e) shows that the photocurrent density of CuTCN-2 is approximately 5.3 times that of pure TiO2 and approximately 2.1 times that of Cu-TiO2-1, the highest among all samples. Electrochemical impedance spectroscopy (EIS) Nyquist plot (see...) Figure 6 (f) shows that CuTCN-2 has the smallest semicircle radius, corresponding to the lowest interface charge transfer impedance.

[0064] Example 5 The difference between this embodiment and Embodiment 1 is that the Cu doping amount is 0.5 at.% (i.e., Cu / TiO2 molar ratio of 0.5%), and the resulting sample is denoted as Cu-TiO2-0.5 / g-C3N4. The preparation method is the same as in Embodiment 1.

[0065] Characterized by UPS, the work function of Cu-TiO2-0.5 is approximately 3.95 eV, lower than that of g-C3N4 (4.00 eV). A built-in electric field still forms at the interface between Cu-TiO2 and g-C3N4, exhibiting a Type-II migration path under illumination. However, the work function difference (ΔΦ≈0.05 eV) is smaller than that in Example 1 (ΔΦ≈0.17 eV), indicating a relatively weaker driving force from the built-in electric field. This example demonstrates that, with a doping concentration of 0.5%, the method of this application can still achieve active switching of the heterojunction charge transfer path from S-type to Type-II.

[0066] Example 6 The difference between this embodiment and Embodiment 1 is that the Cu doping amount is 2.0 at.% (i.e., Cu / TiO2 molar ratio of 2.0%), and the resulting sample is denoted as Cu-TiO2-2.0 / g-C3N4. The preparation method is the same as in Embodiment 1.

[0067] UPS characterization showed that the work function of Cu-TiO2-2.0 further decreased to approximately 3.78 eV, lower than that of g-C3N4 (4.00 eV). A built-in electric field pointing from Cu-TiO2 to g-C3N4 was formed at the interface, exhibiting a Type-II migration path under illumination. However, some excess Cu²⁺… + / Cu + As a bulk recombination center, the lifetime of photogenerated carriers is shortened. This embodiment shows that, with a doping level of 2.0%, the method of this application can still achieve active switching of the heterojunction charge transfer path from S-type to Type-II.

[0068] Example 7 The difference between this embodiment and Embodiment 1 is that the doping element is Fe, that is, ferric chloride hexahydrate (FeCl3·6H2O) is used instead of CuCl2·2H2O, the Fe / TiO2 molar ratio is 1.0%, and the resulting sample is denoted as Fe-TiO2-1 / g-C3N4. The preparation method is the same as in Embodiment 1.

[0069] Characterized by UPS, the work function of Fe-TiO2-1 is 3.85 eV, lower than that of g-C3N4 (4.00 eV). A built-in electric field is formed at the interface between the two, pointing from Fe-TiO2 to g-C3N4. Under illumination, this drives photogenerated electrons to migrate from the conduction band of g-C3N4 to the conduction band of Fe-TiO2, and photogenerated holes to migrate from the valence band of Fe-TiO2 to the valence band of g-C3N4, exhibiting a Type-II migration path. This embodiment demonstrates that Fe, as an alternative to doped metal ions, can also achieve the method of controlling the charge transfer path in heterojunctions as described in this application.

[0070] Example 8 The difference between this embodiment and Embodiment 1 is that the doping element is Co, that is, cobalt chloride hexahydrate (CoCl2·6H2O) is used instead of CuCl2·2H2O, the Co / TiO2 molar ratio is 1.0%, and the resulting sample is denoted as Co-TiO2-1 / g-C3N4. The preparation method is the same as in Embodiment 1.

[0071] Characterized by UPS, the work function of Co-TiO2-1 is 3.87 eV, lower than that of g-C3N4 (4.00 eV). A built-in electric field is formed at the interface between the two, pointing from Co-TiO2 to g-C3N4, exhibiting a Type-II migration path under illumination. This embodiment demonstrates that Co, as an alternative to doped metal ions, can also achieve the method of controlling the charge transfer path in heterojunctions as described in this application.

[0072] Example 9 The difference between this embodiment and Embodiment 1 is that the titanium source is tetrabutyl titanate (C 16 H 36 O4Ti (TBOT) can replace Ti(OH)4. The specific preparation method is as follows: 10 mL of tetrabutyl titanate was slowly added dropwise to a mixed solution of 30 mL of 30% H₂O₂ and 70 mL of deionized water, and stirred at room temperature until a transparent yellow peroxytitanic acid solution was formed (approximately 60 minutes). A stoichiometric amount of CuCl₂·2H₂O (Cu / TiO₂ molar ratio 1.0%) was added to the above solution, and the mixture was stirred for 10 minutes until completely dissolved. 1 g of hydroxylamine hydrochloride was added, and the mixture was heated to reflux (approximately 100°C) for 2 hours. Subsequent centrifugation, washing, and drying steps were the same as in Example 1 to obtain Cu-TiO₂⁻¹ nanoparticles. The obtained Cu-TiO₂⁻¹ was then composited with g-C₃N₄ prepared in step 1 at a mass ratio of 10:1 via an in-situ assembly method, with the specific composite steps being the same as in Example 1.

[0073] UPS characterization showed that the work function of Cu-TiO2-1 was 3.83 eV, lower than that of g-C3N4 (4.00 eV). A built-in electric field was formed at the interface, pointing from Cu-TiO2 to g-C3N4, exhibiting a Type-II migration path under illumination. This embodiment demonstrates that tetrabutyl titanate, as an alternative titanium source, can also achieve the method of controlling the charge transfer path in heterojunctions as described in this application.

[0074] Example 10 The difference between this embodiment and Embodiment 1 is that the heterogeneous structure is constructed using a mechanical ball milling method, as detailed below: The Cu-TiO2-1 prepared according to step 2 of Example 1 and the g-C3N4 prepared according to step 1 of Example 1 were mixed at a mass ratio of 10:1 and placed in a planetary ball mill. The mixture was ball-milled at 400 rpm for 4 hours to obtain the Cu-TiO2 / g-C3N4 heterojunction photocatalyst.

[0075] Because the work function of Cu-TiO2-1 (3.83 eV) is lower than that of g-C3N4 (4.00 eV), a built-in electric field is formed at the interface between the two, pointing from Cu-TiO2 to g-C3N4. Under illumination, this drives photogenerated electrons to migrate from the conduction band of g-C3N4 to the conduction band of Cu-TiO2, and photogenerated holes to migrate from the valence band of Cu-TiO2 to the valence band of g-C3N4, exhibiting a Type-II migration path. However, since the interface contact formed by mechanical ball milling is not as tight as that formed by in-situ assembly, there is a certain loss in the interface charge transfer efficiency.

[0076] The results of Examples 1 to 4 above (see) Figure 5 In (b) of the study, the amount of CuTCN-1 (0.75 g) significantly affected the photocatalytic nitrogen fixation activity: 78.6 μmol·g⁻¹. - ¹·h - ¹, CuTCN-2 (1.0 g) has a content of 91.14 μmol·g. - ¹·h - ¹, CuTCN-3 (1.25 g) has a content of 85.3 μmol·g. - ¹·h - ¹, CuTCN-4 (2.0 g) has a concentration of 72.5 μmol·g. - ¹·h - ¹, showing a volcano-like trend of first rising and then falling, proves that there is an optimal window for the amount of g-C3N4 feed.

[0077] Examples 5 and 6 demonstrate that, within a Cu doping range of 0.5% to 2.0%, the active switching of the heterojunction charge transfer path from S-type to Type-II can be achieved. Examples 7 and 8 demonstrate that replacing Cu with transition metal ions such as Fe and Co can also achieve the above-mentioned modulation effect. Examples 9 and 10 demonstrate that replacing the titanium source and the recombination method can also achieve the above-mentioned modulation effect.

[0078] Therefore, all embodiments have achieved the technical effect of making the work function of doped TiO2 lower than that of g-C3N4 through chemical doping, forming a built-in electric field at the interface from doped TiO2 to g-C3N4, thereby making the charge transfer path of the heterojunction exhibit a Type-II migration path under illumination.

[0079] It is understood that the method for controlling the charge transfer path of heterojunctions in this application is not limited to the TiO2 / g-C3N4 system, but can also be extended to other Type-II heterojunction systems. For example, by doping with transition metals to control the work function of metal oxides such as ZnO and SnO2, the heterojunction type can be controlled to switch after recombination with g-C3N4. Furthermore, the principle of this application can also be extended to reverse switching, that is, by doping to lower the work function of g-C3N4 to match that of TiO2, the heterojunction type can also be controlled to switch between Type-II and S-type. All the above equivalent substitution methods are extensions of the technical solution of this application.

[0080] The various embodiments in this specification are described in a progressive manner. For the same or similar parts between the various embodiments, please refer to each other. Each embodiment focuses on describing the differences from other embodiments.

[0081] The above embodiments are only used to illustrate the technical solutions of this application, and are not intended to limit this application. Although this application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of this application.

Claims

1. A method for controlling the charge transfer path of a heterojunction, characterized in that, Includes the following steps: Chemical doping of TiO2 can be used to control the Fermi level or work function of TiO2 to obtain doped TiO2; The doped TiO2 is combined with g-C3N4 to form a heterojunction; By controlling the type and / or amount of doping elements, the work function of the doped TiO2 is made lower than that of the g-C3N4, so as to form a built-in electric field from the doped TiO2 to the g-C3N4 at the interface between the two, thereby making the charge transfer path of the heterojunction exhibit a Type-II migration path under illumination.

2. The method for controlling the charge transfer path of a heterojunction according to claim 1, characterized in that, The chemical doping is metal ion doping.

3. The method for controlling the charge transfer path of a heterojunction according to claim 2, characterized in that, The metal ions used in the metal ion doping are one or more of Cu, Fe, Co, Ni, Mn, and Zn.

4. The method for controlling the charge transfer path of a heterojunction according to claim 3, characterized in that, The metal ion is Cu ion.

5. The method for controlling the charge transfer path of a heterojunction according to claim 1, characterized in that, The doping amount is 0.5% to 2.0% based on the molar ratio of the doping element to TiO2.

6. The method for controlling the charge transfer path of a heterojunction according to claim 5, characterized in that, The doping level is 1.0%.

7. The method for controlling the charge transfer path of a heterojunction according to claim 1, characterized in that, The work function of the doped TiO2 is 3.70~3.90 eV, and the work function of the g-C3N4 is 3.90~4.10 eV.

8. The method for controlling the charge transfer path of a heterojunction according to claim 1, characterized in that, The doped TiO2 is prepared by a low-temperature crystallization method of peroxytitanic acid, which includes: reacting a titanium source with hydrogen peroxide to form a peroxytitanic acid solution, adding a dopant element source, and carrying out a crystallization reaction at ≤100°C in the presence of a reducing agent to obtain the doped TiO2.

9. The method for controlling the charge transfer path of a heterojunction according to claim 1, characterized in that, The composite method is an in-situ assembly method, which includes: dispersing g-C3N4 in a solvent, adding a precursor solution of doped TiO2, so that the doped TiO2 nanoparticles grow and anchor in situ on the surface of g-C3N4; the mass ratio of g-C3N4 to doped TiO2 is 1:4 to 1:

20.

10. A heterojunction photocatalyst, characterized in that, The heterojunction was prepared using the method for controlling the charge transfer path of the heterojunction as described in any one of claims 1-9. In the heterojunction photocatalyst, doped TiO2 nanoparticles are uniformly anchored on the surface of g-C3N4. At the interface between the two, a built-in electric field is formed from doped TiO2 to g-C3N4. Under illumination, photogenerated electrons migrate from the conduction band of g-C3N4 to the conduction band of doped TiO2, and photogenerated holes migrate from the valence band of doped TiO2 to the valence band of g-C3N4.

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