A gradient-doped BiVO4 / WO3 photoanode material, a preparation method and application thereof

Abstract

The application discloses a gradient-doped BiVO4 / WO3 photoanode material and a preparation method and application thereof, and relates to the technical field of photoanode materials. 6+ The gradient-doped BiVO4 / WO3 photoanode material comprises a BiVO4 / WO3 heterojunction, and variable-valence transition metal ions are doped on the surface of the BiVO4 / WO3 heterojunction and the interface between BiVO4 and WO3, wherein the variable-valence transition metal ions comprise one of Mo 2+ and Co. The application adopts the above gradient-doped BiVO4 / WO3 photoanode material and the preparation method and application thereof, introduces variable-valence transition metal elements between the interface of BiVO4 and WO3, promotes the formation of an electron transport fast channel in a bulk phase, and improves the charge separation and transport efficiency of the BiVO4 / WO3 heterojunction interface; meanwhile, the variable-valence transition metal elements are constructed on the surface of the photoanode material, a CoOOH cocatalyst layer is formed on the surface, the cocatalyst layer and the interface electron transport fast channel synergistically act, the photo-generated charges of the BiVO4 / WO3 heterojunction bulk phase and surface are efficiently separated and transported, and therefore, the photo-catalytic efficiency is improved, and the service life of the photoanode material is prolonged.

Description

Technical Field

[0001] This invention relates to the field of electrode materials technology, and in particular to a gradient-doped BiVO4 / WO3 photoanode material, its preparation method, and its application. Background Technology

[0002] WO3 and BiVO4 have shown great potential as candidate materials for PEC water splitting, but when used as stand-alone electrodes, they fail to achieve ideal catalytic efficiency due to poor internal charge carrier separation efficiency. Therefore, synthesizing these two materials into a heterojunction can not only optimize the charge separation process but also significantly improve the overall performance of PEC water splitting.

[0003] However, the heterojunction formed by WO3 and BiVO4 still has the following drawbacks:

[0004] (1) Interface charge transport: Although the combination of WO3 and BiVO4 is intended to promote charge separation in the electrode bulk phase, efficient charge transport at the interface of the heterojunction remains a challenge.

[0005] (2) Stability of composite materials: The long-term stability of heterojunctions is a key issue. Under continuous light and reaction conditions, the interface may gradually degrade, affecting the catalytic efficiency and lifespan of the material. Summary of the Invention

[0006] The purpose of this invention is to provide a gradient-doped BiVO4 / WO3 photoanode material, its preparation method, and its application, in order to solve the problems of inefficient interfacial charge separation and transport, and poor stability in the above-mentioned BiVO4 / WO3 heterojunction.

[0007] To achieve the above objectives, the first aspect of the present invention provides a gradient-doped BiVO4 / WO3 photoanode material, the photoanode material comprising a BiVO4 / WO3 heterojunction, wherein the surface of the BiVO4 / WO3 heterojunction and the interface between BiVO4 and WO3 are both doped with variable-valence transition metal ions.

[0008] Preferably, the variable valence transition metal ion includes Mo. 6+ or Co 2+ One of them.

[0009] A second aspect of this invention provides a method for preparing a gradient-doped BiVO4 / WO3 photoanode material, comprising the following steps:

[0010] (1) Preparation of WO3 thin film

[0011] Sodium tungstate was dissolved in water and hydrochloric acid was added dropwise. After the tungstate precipitate was completely formed, ammonium oxalate was added until a clear precursor solution was obtained.

[0012] The pretreated FTO substrate was tilted and placed into a polytetrafluoroethylene reactor with the conductive side facing down. Then, the precursor solution was poured into the reactor for hydrothermal reaction. After the reaction was complete, the substrate loaded with WO3·H2O film was taken out, washed, dried, and heat-treated to obtain WO3 film.

[0013] (2) The WO3 film was soaked in an aqueous solution of variable valence transition metal ions, then rinsed with deionized water and air-dried to obtain the M / WO3 film.

[0014] BiOI films were prepared on M / WO3 films by electrodeposition to obtain BiOI-M / WO3 films.

[0015] (3) An organic solvent containing variable-valence transition metal ions and vanadium source is dropped onto the surface of the BiOI-M / WO3 film, and then heat-treated to obtain the M-BiVO4-M / WO3 film, which is the photoanode material.

[0016] To address the issue of poor charge carrier separation efficiency when using a single material as an independent electrode in existing technologies, heterojunctions are synthesized from two materials to optimize the charge separation process. However, catalytic water splitting is a complex process; improving catalytic efficiency is not solely achieved by separating charge carriers, but also involves interfacial charge transport between the two materials, material composite stability, and the subsequent recombination of separated charges. This invention deposits variable-valence transition metal elements on the surface of the WO3 film by immersing it in variable-valence transition metal ions before preparing BiVO4. This promotes the formation of fast electron transport channels at the heterojunction interface, improves the electron transport efficiency at the BiVO4 / WO3 heterojunction interface, and enhances the catalytic efficiency of the photoanode material.

[0017] In addition, during the preparation of BiVO4, an organic solution containing both a vanadium source and a variable-valence transition metal element is dropped onto the surface of the BiOI-M / WO3 film to form a co-catalyst layer on the surface of the photoanode material, thereby further improving the catalytic efficiency of the photoanode material.

[0018] Preferably, the substrate pretreatment process is as follows:

[0019] Soak the purchased FTO conductive glass in acetone, isopropanol, ethanol, and deionized water for 30 minutes each, then ultrasonically clean it. After drying, it is ready for use. Unused FTO can be stored in anhydrous ethanol and dried before use.

[0020] Preferably, the specific preparation method of the precursor solution in step (1) is as follows:

[0021] Weigh 0.231g of sodium tungstate and dissolve it in 30mL of deionized water. After it is fully dissolved, measure 6mL of 3M HCl and slowly add it dropwise to the above solution. After the tungstate precipitate is completely formed, add 0.20g of ammonium oxalate until the solution gradually becomes clear. Then add 30mL of deionized water and continue stirring for half an hour to obtain the precursor solution.

[0022] In this invention, ammonium oxalate acts as a structure-directing agent for sheet-like growth. Due to the complexation of oxalate ions, the tungstic acid precipitate gradually transforms into a clear solution. The downward orientation of the conductive surface ensures the growth of the sheet-like array and avoids disordered accumulation during the hydrothermal process.

[0023] Preferably, the temperature of the hydrothermal reaction in step (1) is 120-150°C and the time is 5-10 hours.

[0024] Preferably, the heat treatment temperature in step (1) is 400-500℃, the time is 1-3h, and the heating rate is 2℃ / min.

[0025] Preferably, the concentration of the variable-valence transition metal ion in step (2) is 0.2 mM.

[0026] Preferably, the specific preparation process of the electrodeposition solution used in step (2) is as follows:

[0027] Dissolve 3.32 g of KI in 50 mL of deionized water, stir well, adjust the pH of the solution to 1.6-1.8 using HNO3, add 0.97 g of Bi(NO3)3·5H2O, stir for 20 min, add 20 mL of 0.23 M p-benzoquinone ethanol solution, stir vigorously for 30 min, and obtain the electrodeposition solution.

[0028] The deposition temperature for the electrodeposition method was 25℃, the deposition potential was -0.1V vs. Ag / AgCl, and the deposition time was 5min.

[0029] Preferably, in step (3), the concentration of the vanadium source is 0.2 M and the concentration of the variable-valence transition metal ion is 0.2 mM.

[0030] Preferably, the volume of the organic solvent containing variable-valence transition metal ions and vanadium source added in step (3) is 100 μL.

[0031] Preferably, the heat treatment temperature in step (3) is 400-500℃, the time is 2-5h, and the heating rate is 2℃ / min.

[0032] The third aspect of this invention provides an application of a gradient-doped BiVO4 / WO3 photoanode material, specifically its application in the electrochemical splitting of water.

[0033] Therefore, the gradient-doped BiVO4 / WO3 photoanode material with the above-described structure, its preparation method, and its application have the following beneficial effects:

[0034] (1) This invention promotes the formation of fast electron transport channels at the interface between BiVO4 and WO3 by doping with variable-valence transition metal elements, thereby improving the electron transport efficiency of the BiVO4 / WO3 heterojunction interface and enhancing the catalytic efficiency of the photoanode material.

[0035] (2) In addition, the present invention also sets a variable valence transition metal element on the surface of the photoanode material to form a CoOOH cocatalyst layer on its surface. The cocatalyst layer and the formed interface transport channel work together to fully separate the photogenerated charge of the BiVO4 / WO3 heterojunction, reduce the subsequent recombination of electrons and holes, and improve the catalytic efficiency.

[0036] (3) The present invention sets a variable valence transition metal element between the BiVO4 and WO3 interface, which increases the stability of the BiVO4 / WO3 heterojunction and avoids the problem of gradual interface degradation under long-term continuous light or other reaction conditions, which affects the service life of the material.

[0037] The technical solution of the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. Attached Figure Description

[0038] Figure 1 This is the XPS spectrum of the photoanode material prepared in Example 1;

[0039] Figure 2 This is a transmission electron microscope (TEM) image of the photoanode material prepared in Example 1;

[0040] Figure 3 These are SEM images and elemental distribution diagrams of the photoanode material prepared in Example 1;

[0041] Figure 4 This is the energy spectrum of the photoanode material prepared in Example 1;

[0042] Figure 5 The LSV curves of the photoanode materials prepared in Comparative Examples 1, 2, 3 and 1, and the It curve of the photoanode material prepared in Example 1 are shown.

[0043] Figure 6 These are XPS depth profile test images prepared in Comparative Examples 4, 5 and 2;

[0044] Figure 7 This is a transmission electron microscope image of the photoanode material prepared in Comparative Example 4;

[0045] Figure 8This is the elemental distribution diagram of the photoanode material prepared in Comparative Example 4;

[0046] Figure 9 This is a transmission electron microscope image of the photoanode material prepared in Comparative Example 5;

[0047] Figure 10 This is the elemental distribution diagram of the photoanode material prepared in Comparative Example 5;

[0048] Figure 11 This is a transmission electron microscope (TEM) image of the photoanode material prepared in Example 2;

[0049] Figure 12 This is the energy spectrum of the photoanode material prepared in Example 2;

[0050] Figure 13 The energy spectrum of the photoanode materials prepared in Comparative Example 4, Comparative Example 5 and Example 2 is shown.

[0051] Figure 14 The LSV and It curves of the photoanode materials prepared in Comparative Examples 1-5 and Examples 1-2 are shown.

[0052] Figure 15 MS images of the photoanode materials prepared in Comparative Example 1, Comparative Example 3, Example 1, and Example 2;

[0053] Figure 16 The LSV curves and charge separation injection efficiency diagrams of the photoanode materials prepared in Comparative Example 1, Comparative Example 3, Example 1 and Example 2 are shown.

[0054] Figure 17 The graphs show the CV curves of the photoanode material prepared in Example 2 and the electrochemical active area curves of the photoanode materials prepared in Comparative Examples 1, 3, 1, and 2. Detailed Implementation

[0055] The present invention will be further described below. It should be noted that this embodiment is based on the present technical solution and provides detailed implementation methods and specific operation processes, but the present invention is not limited to this embodiment.

[0056] Example 1

[0057] A method for preparing a Co-BiVO4-Mo / WO3 photoanode material includes the following steps:

[0058] (1) Substrate pretreatment

[0059] Soak the purchased FTO conductive glass in acetone, isopropanol, ethanol, and deionized water for 30 minutes each, then ultrasonically clean it. After drying, it is ready for use. Unused FTO conductive glass can be stored in anhydrous ethanol and dried before use.

[0060] (2) Preparation of WO3 thin film

[0061] WO3 films were prepared using a hydrothermal method. First, 0.231 g of sodium tungstate was weighed and dissolved in 30 mL of deionized water. After it was fully dissolved, 6 mL of 3M HCl was slowly added dropwise to the solution. After the tungstate precipitate was completely formed, 0.20 g of ammonium oxalate was added as a structure-directing agent for sheet growth. Due to the complexation effect of oxalate ions, the solution gradually became clear. Then, 30 mL of deionized water was added and the mixture was stirred for another half hour to obtain the precursor solution.

[0062] The pre-cleaned 2×4cm 2 FTO conductive glass was tilted and placed in a 100mL polytetrafluoroethylene reactor with the conductive side facing down to ensure the growth of the sheet array and avoid disordered accumulation during the hydrothermal process. Then, the above precursor solution was poured into the reactor, sealed and placed in a forced-air drying oven, kept at 140℃ for 6 hours, and after natural cooling, the FTO conductive glass loaded with WO3·H2O film was taken out, rinsed with deionized water and ethanol, dried in an oven at 70℃ for 1 hour, and heat-treated at 450℃ for 1 hour (heating rate 2℃ / min) to obtain the desired WO3 sheet array film.

[0063] (3) The FTO conductive glass loaded with WO3 film was immersed in 0.2mM ammonium molybdate aqueous solution for 40 minutes and then removed and dried naturally to obtain FTO conductive glass loaded with Mo / WO3 film.

[0064] (4) Preparation of BiVO4-Mo / WO3 composite photoanode

[0065] First, a BiOI membrane was deposited on hydrothermally grown WO3, and then it was converted into a BiVO4 membrane.

[0066] The detailed method is as follows: Dissolve 3.32g of KI in 50mL of deionized water, stir well, adjust the pH of the solution to 1.6-1.8 using HNO3, add 0.97g of Bi(NO3)3·5H2O, stir for 20min, add 20mL of 0.23M p-benzoquinone ethanol solution, stir vigorously for 30min to obtain the electrodeposition solution.

[0067] Electrodeposition was performed using a three-electrode system at a deposition temperature of 25°C, a deposition potential of -0.1V vs. Ag / AgCl, and a deposition time of 5 min. After electrodeposition, the residual solution on the photoanode was rinsed off with deionized water and then dried in air for later use, resulting in FTO conductive glass loaded with a BiOI-Mo / WO3 film.

[0068] Subsequently, 100 μL of a DMSO solution containing 0.2 M vanadium acetylacetonate and 0.2 mM cobalt nitrate was added dropwise to the Co-BiOI-Mo / WO3 membrane, followed by heat treatment at 450 °C for 2 h at a heating rate of 2 °C·min. -1 After the reaction was completed, the sample was allowed to cool naturally to room temperature. It was then soaked in 1M NaOH solution and slowly stirred for 30 minutes to remove excess V2O5. The sample was then allowed to dry naturally for later use. The prepared photoanode was named Co-BiVO4-Mo / WO3 photoanode material.

[0069] Example 2

[0070] The difference from Example 1 is that in step (3), the FTO conductive glass loaded with WO3 film is immersed in 0.2mM cobalt nitrate aqueous solution for 40 minutes and then taken out and dried naturally to obtain FTO conductive glass loaded with Co / WO3 film. The prepared photoanode is named Co-BiVO4-Co / WO3 photoanode material.

[0071] Comparative Example 1

[0072] The difference from Example 1 is that steps (3) and (4) are omitted, and the prepared photoanode is named WO3 photoanode material.

[0073] Comparative Example 2

[0074] The difference from Example 1 is that steps (2) and (3) are omitted, and BiVO4 is directly deposited on the pretreated FTO conductive glass surface. The specific steps are as follows:

[0075] 3.32 g of KI was dissolved in 50 mL of deionized water and stirred until homogeneous. The pH of the solution was adjusted to 1.6–1.8 using HNO3. 0.97 g of Bi(NO3)3·5H2O was added and stirred for 20 min. Then, 20 mL of 0.23 M p-benzoquinone ethanol solution was added and stirred vigorously for 30 min to obtain the electrodeposition solution.

[0076] Electrodeposition was performed using a three-electrode system at a deposition temperature of 25°C, a deposition potential of -0.1V vs. Ag / AgCl, and a deposition time of 5 min. After electrodeposition, the residual solution on the photoanode was rinsed off with deionized water and then dried in air for later use, resulting in BiOI-loaded FTO conductive glass.

[0077] Subsequently, 100 μL of a DMSO solution containing 0.2 M vanadium acetylacetonate was added dropwise onto the BiOI membrane, followed by heat treatment at 450 °C for 2 h at a heating rate of 2 °C·min. -1After the reaction was completed, the mixture was allowed to cool naturally to room temperature. It was then soaked in 1M NaOH solution and slowly stirred for 30 minutes to remove excess V2O5. After natural drying, the resulting photoanode was named BiVO4 photoanode material.

[0078] Comparative Example 3

[0079] The difference from Example 1 is that cobalt nitrate in the drop solution is omitted in steps (3) and (4).

[0080] A method for preparing a BiVO4 / WO3 photoanode material, the specific steps of which are as follows:

[0081] (1) Substrate pretreatment

[0082] Soak the purchased FTO conductive glass in acetone, isopropanol, ethanol, and deionized water for 30 minutes each, then ultrasonically clean it. After drying, it is ready for use. Unused FTO conductive glass can be stored in anhydrous ethanol and dried before use.

[0083] (2) Preparation of WO3 thin film

[0084] WO3 films were prepared using a hydrothermal method. First, 0.231 g of sodium tungstate was weighed and dissolved in 30 mL of deionized water. After it was fully dissolved, 6 mL of 3M HCl was slowly added dropwise to the solution. After the tungstate precipitate was completely formed, 0.20 g of ammonium oxalate was added as a structure-directing agent for sheet growth. Due to the complexation effect of oxalate ions, the solution gradually became clear. Then, 30 mL of deionized water was added and the mixture was stirred for another half hour to obtain the precursor solution.

[0085] The pre-cleaned 2×4cm 2 FTO conductive glass was tilted and placed in a 100mL polytetrafluoroethylene reactor with the conductive side facing down to ensure the growth of the sheet array and avoid disordered accumulation during the hydrothermal process. Then, the above precursor solution was poured into the reactor, sealed and placed in a forced-air drying oven, kept at 140℃ for 6 hours, and after natural cooling, the FTO conductive glass loaded with WO3·H2O film was taken out, rinsed with deionized water and ethanol, dried in an oven at 70℃ for 1 hour, and heat-treated at 450℃ for 1 hour (heating rate 2℃ / min) to obtain the desired WO3 sheet array film.

[0086] (3) Preparation of BiVO4 / WO3 composite photoanode

[0087] First, a BiOI membrane was deposited on hydrothermally grown WO3, and then it was converted into a BiVO4 membrane.

[0088] The detailed method is as follows: Dissolve 3.32g of KI in 50mL of deionized water, stir well, adjust the pH of the solution to 1.6-1.8 using HNO3, add 0.97g of Bi(NO3)3·5H2O, stir for 20min, add 20mL of 0.23M p-benzoquinone ethanol solution, stir vigorously for 30min to obtain the electrodeposition solution.

[0089] Electrodeposition was performed using a three-electrode system at a deposition temperature of 25°C, a deposition potential of -0.1V vs. Ag / AgCl, and a deposition time of 5 min. After electrodeposition, the residual solution on the photoanode was rinsed off with deionized water and then dried in air for later use, resulting in FTO conductive glass loaded with a BiOI / WO3 film.

[0090] Subsequently, 100 μL of a DMSO solution containing 0.2 M vanadium acetylacetonate was added dropwise onto the BiOI / WO3 membrane, followed by heat treatment at 450 °C for 2 h at a heating rate of 2 °C·min. -1 After the reaction was completed, the mixture was allowed to cool naturally to room temperature. It was then soaked in 1M NaOH solution and slowly stirred for 30 minutes to remove excess V2O5. After natural drying, the resulting photoanode was named BiVO4 / WO3 photoanode material.

[0091] Comparative Example 4

[0092] The difference from Example 2 is that in step (4), 100 μL of DMSO solution containing 0.2 M vanadium acetylacetonate is added to the BiOI-Co / WO3 film, and the resulting photoanode is named BiVO4-Co / WO3 photoanode material.

[0093] Comparative Example 5

[0094] The difference from Example 1 is that step (3) is omitted, and the prepared photoanode is named Co-BiVO4 / WO3 photoanode material.

[0095] Experimental Example 1

[0096] Example 1 and Comparative Examples 1-3 were characterized and tested.

[0097] (1) XPS test

[0098] XPS tests were performed on the Co-BiVO4-Mo / WO3 photoanode, such as... Figure 1 As shown in (a), in addition to detecting the binding energies of W, O, Bi, and V, the binding energies of Co 2p orbitals and Mo 3d orbitals were also detected. From... Figure 1 The fine spectra of Co and Mo in (b, c) show that 795.2 eV and 796.8 eV correspond to Co, respectively. 3+ 2p1 / 2 and Co 2+ 2p 1 / 2 The orbital binding energies of 780.0 eV and 781.1 eV correspond to Co, respectively. 3+ 2p 3 / 2 and Co 2+ 2p 3 / 2 Orbital binding energy. Binding energies of 230.2 eV and 236.2 eV correspond to Mo, respectively. 6+ 3D 3 / 2 and Mo 6+ 3D 5 / 2 The orbital data proves that Mo-Co has been doped into the Co-BiVO4-Mo / WO3 photoanode. Figure 1 (d, e) are the fine spectra of W and Bi, with 164.0 eV attributed to Bi. 3+ 4f 5 / 2 158.7 eV belongs to Bi 3+ 4f 7 / 2 39.2 eV belongs to W 4+ 4p 3 / 2 This is attributed to MoO2 generated from Mo at high temperatures.

[0099] In-depth analysis and testing were conducted on the Mo and Co elements in the Co-BiVO4-Mo / WO3 photoanode, from... Figure 1 As can be seen from (f) and (g), since Mo is doped at the interface, the intensity of the binding energy of the Mo 3d orbitals increases with increasing profile depth, thus proving that Mo is graded doped inside the Co-BiVO4-Mo / WO3 photoanode. Co is doped at the surface; therefore, the strongest signal is detected at the surface, and the intensity of the binding energy of the Co 2p orbitals decreases with increasing profile depth, proving that Co is also graded doped inside the Co-BiVO4-Mo / WO3 photoanode.

[0100] (2) TEM test

[0101] Figure 2 (a) is a TEM image of the Co-BiVO4-Mo / WO3 photoanode. The Mo-Co doped photoanode retains the basic structure of BiVO4 / WO3. From Figure 2 (b) shows an amorphous CoOOH layer with a thickness of 0.4 nm on the surface of the Co-BiVO4-Mo / WO3 photoanode. Figure 2(ce) is the HRTEM image of the Co-BiVO4-Mo / WO3 photoanode. The 0.363 nm lattice spacing corresponds to the (200) crystal plane of WO3, the 0.302 nm lattice spacing corresponds to the (121) crystal plane of BiVO4, and the 0.378 nm and 0.358 nm lattice spacings both correspond to the (-130) crystal plane of BiVO4. Figure 2 (f) is the SEAD pattern of the Co-BiVO4-Mo / WO3 photoanode. The polycrystalline structure of Co-BiVO4-Mo / WO3 exhibits unique ring diffraction.

[0102] from Figure 3 The distribution of elements in the Co-BiVO4-Mo / WO3 photoanode can be observed. W, O, Bi, and V are uniformly distributed in the photoanode, while Mo is present in lower concentrations on the surface due to interfacial doping. Surface-doped Co, on the other hand, exhibits a higher concentration. This is also evident from... Figure 4 The energy spectrum shown indicates that the atomic percentage and mass percentage of Mo are 0.25 and 0.19, respectively, while the atomic percentage and mass percentage of Co are 0.60 and 0.28, respectively, proving that Mo-Co elements are doped into the photoanode.

[0103] (3) Photoelectrochemical performance testing

[0104] Using 0.5M Na2SO4 solution as the electrolyte at AM 1.5G (100mW cm⁻¹) -2 The photoelectrochemical performance of the prepared Co-BiVO4-Mo / WO3 photoanode was tested under simulated sunlight. The photocurrent curve is shown below. Figure 5 As shown in (a). At 1.23V RHE At that time, the Co-BiVO4-Mo / WO3 photoanode reached 1.61 mA cm⁻¹. -2 The photocurrent density was increased by approximately 50% compared to BiVO4 / WO3, and the onset potential shifted negatively by about 0.5 eV. The LSV curves show that the Mo-doped photoanode at the interface lowers the onset potential. This is likely due to the CoOOH coating layer formed by surface Co doping acting as a co-catalyst, promoting carrier separation efficiency and thus reducing the OER overpotential of the Co-BiVO4-Mo / WO3 photoanode. In summary, the Co-BiVO4-Mo / WO3 photoanode, achieved by introducing anion ions to achieve co-doping of cations and anions, effectively promotes the migration of photogenerated carriers, accelerates the water oxidation kinetics, and reduces recombination of photogenerated charges. These improvements significantly enhance the photoelectrochemical performance of the photoanode.

[0105] To further investigate the photoresponse performance of the photoanode, transient photoresponse tests were conducted. The Co-BiVO4-Mo / WO3 photoanode was tested at 1.23V.RHE The photocurrent-time (IT) curve under the applied bias condition is shown in Figure 5 (b) shows that the photocurrent of the photoanode increases and decreases rapidly with the switching on and off of the light source, reflecting the photoanode's excellent photoresponse capability. Figure 5 As shown in (b), the Co-BiVO4-Mo / WO3 photoanode, like other photoanodes, exhibits a transient current spike at the moment the light source is turned on. This is due to the large number of charge carriers accumulated at the semiconductor / electrolyte interface before the light is turned on. Furthermore, after the 3600s transient photoresponse test, the photocurrent density of the Co-BiVO4-Mo / WO3 photoanode showed almost no decay. This may be because the Mo and W elements doped at the interface belong to the same group and have very similar chemical properties, thus being less affected by photocorrosion. Therefore, the Co-BiVO4-Mo / WO3 photoanode exhibits high stability.

[0106] Experimental Example 2

[0107] Example 2 and Comparative Examples 4-5 were characterized and tested.

[0108] (1) XPS test

[0109] To investigate the doping mechanism of transition metal ions in BiVO4 / WO3, XPS was used to perform in-depth profiling analysis on BiVO4-Co / WO3, Co-BiVO4 / WO3, and Co-BiVO4-Co / WO3 photoanodes. The XPS results after profiling showed that the content of the doped metal in the samples also changed.

[0110] like Figure 6 As shown in (a), BiVO4-Co / WO3 contains less Co than the other two, and no obvious signal peak was found. However, it showed a weak signal peak at a resolution of 30 nm, and was not detected at other resolutions. Figure 6 As shown in (b), the Co content in Co-BiVO4 / WO3 gradually decreases from a depth of 0 nm to 30 nm. Figure 6As shown in (c), the Co content in Co-BiVO4-Co / WO3 changes relatively uniformly with increasing analysis depth, except for strong signal peaks at analysis depths of 0 nm and 20 nm. This indicates that the doping of transition metals within BiVO4 / WO3 is gradient-dependent. The Co content in Co-BiVO4-Co / WO3 first decreases, then increases, and then decreases again from an analysis depth of 0 nm to 30 nm, showing a gradually decreasing trend. No signal peak was detected for Co on the surface of BiVO4-Co / WO3, but a weak signal peak was detected at a depth of 30 nm, thus proving that Co exists in BiVO4 / WO3 in a gradient-doped form.

[0111] (2) TEM test

[0112] The structures of three photoanodes, namely BiVO4-Co / WO3, Co-BiVO4 / WO3, and Co-BiVO4-Co / WO3, were further characterized by transmission electron microscopy. First, Figure 7 (ab) shows the TEM image of BiVO4-Co / WO3. The image shows that it still retains the sheet-like structure of WO3 with a length of 3μm and a width of 5μm and the microstructure of BiVO4 nanoparticles with a diameter of 10nm. Figure 7 (cd) is the HRTEM image of BiVO4-Co / WO3. 0.361 nm corresponds to the (200) crystal plane of WO3, and 0.306 nm and 0.319 nm correspond to the (121) and (-130) crystal planes of BiVO4, respectively. Figure 7 (b) The SEAD plot in the inset also shows the diffraction rings of each crystal plane of BiVO4-Co / WO3. This was achieved through surface scan analysis of the elemental distribution, such as... Figure 8 As shown, the W element in the bottom layer is distributed in a sheet-like pattern, the Bi and V elements are uniformly distributed on the photoanode surface, and the Co element is uniformly distributed in BiVO4, proving the successful doping of Co element at the interface.

[0113] Figure 9 (ab) are TEM images of Co-BiVO4 / WO3. The basic structure remains unchanged. The WO3 sheet structure is 3 μm long and 3 μm wide, and the BiVO4 nanoparticles are 10 nm in size. Figure 9 In (ce), the 0.317 nm lattice spacing corresponds to the (-112) crystal plane of WO3, the 0.308 nm lattice spacing corresponds to the (121) crystal plane of BiVO4, and the 0.321 nm and 0.323 nm lattice spacings correspond to the (-130) crystal plane of BiVO4. Figure 9 The diffraction rings in (f) also correspond to the crystal planes of the Co-BiVO4 / WO3 photoanode. From Figure 10The surface distribution diagram of each element shows that each element is evenly distributed, but due to the large amount of surface doping, the surface distribution of Co is more obvious.

[0114] Figure 11 TEM and HRTEM images of the Co-BiVO4-Co / WO3 photoanode, as well as SEAD images, from Figure 11 As shown in (a), the basic structure of 10nm-sized BiVO4 particles loaded on a sheet-like WO3 with a length of 6μm and a width of 2μm can be seen, and... Figure 11 An amorphous layer loaded on the surface was observed in (b). In HRTEM Figure 11 In (cf), it was observed that 0.358 nm and 0.363 nm correspond to the (-130) crystal plane of BiVO4, and the lattice spacings of 0.359 nm, 0.365 nm, and 0.367 nm correspond to the (200) crystal plane of WO3. Figure 11 The diffraction spots and rings in (gi) correspond one-to-one with the crystal planes of WO3, BiVO4, and Co-BiVO4-Co / WO3 photoanodes. This indicates that BiVO4 is loaded onto the WO3 surface, forming a heterostructure.

[0115] Figure 12 The image shows the TEM elemental distribution of the Co-BiVO4-Co / WO3 photoanode, with all elements uniformly distributed. Due to the co-doping mode of Co, the surface distribution of Co elements is most pronounced compared to the other two doping modes.

[0116] Figure 13 (ac) shows the energy dispersive spectroscopy (EDS) spectra of three photoanodes: BiVO4-Co / WO3, Co-BiVO4 / WO3, and Co-BiVO4-Co / WO3. The figures show that Co is present in all three photoanodes, confirming successful Co doping. Due to the lower amount of doping at the interface, the BiVO4-Co / WO3 photoanode has a lower Co content, followed by the surface doping. Figure 13 As can be seen in (c), the co-doped photoanode has the highest Co content, thus proving that the Co element of the three doping methods was successfully gradient doped inside the photoanode.

[0117] (3) Photoelectrochemical performance testing

[0118] At AM 1.5G (100mW cm -2 Under simulated sunlight conditions, using 0.5M Na₂SO₄ solution as the electrolyte, the photoelectrochemical performance of different photoanodes was tested. Figure 14 (a) Although the photoanode with Co doping at the interface successfully shifts the overpotential of BiVO4 / WO3 negatively, it does not show a significant improvement in the first half of the load bias, only at 1.1V.RHE A more noticeable improvement only occurs at 1.23V. RHE The value obtained was 1.48 mA cm. -2 The photocurrent density was increased by approximately 36% compared to the BiVO4 / WO3 photoanode. The surface-doped Co photoanode successfully shifted the overpotential of BiVO4 / WO3 negatively, and the increase in photocurrent density was greater than that of the interface-doped photoanode, reaching 1.23V. RHE The value obtained was 1.42 mAcm -2 The photocurrent density is approximately 35% higher than that of the BiVO4 / WO3 photoanode.

[0119] In addition, it was found that the photocurrent density of the surface-doped photoanode was significantly improved at low potentials, but the improvement at higher potentials was not particularly significant. The reason for this is likely that the Co ions doped in the BiVO4 / WO3 bulk phase act as fast charge transport channels, promoting the separation and transport of charge carriers within the bulk phase. Therefore, the improvement at low potentials is not significant, but as the load voltage increases, the charge transport channels within the bulk phase begin to play a greater role, resulting in a significant improvement in photoelectrochemical performance at higher potentials. The reason for the improved current density of the surface-doped photoanode is likely due to the Co doping on the surface acting as a co-catalyst, which corresponds to the amorphous layer encapsulating the surface of the BiVO4 nanoparticles detected by SEM and TEM. This layer forms a CoOOH layer on the outside, acting as a co-catalyst for the BiVO4 / WO3 heterojunction photoanode. When this layer acts on the photoanode surface, photogenerated holes can be captured and stored, greatly suppressing the recombination of holes and electrons, and enabling the photoanode to oxidize water at a lower potential, thus reducing the onset potential. Therefore, the overpotential of the surface-doped photoanode is significantly increased, resulting in a substantial increase in photocurrent density under low load bias. However, the effect of the co-catalyst is not particularly pronounced under higher load bias.

[0120] This invention designs a novel photoanode, Co-BiVO4-Co / WO3, with co-doped surfaces and interfaces, and the results are as follows: Figure 14 As shown in (b), the co-doped photoanode exhibits excellent performance enhancements at both low and high potentials, particularly at 1.23V. RHE The value obtained was 2.09 mAcm -2The photocurrent density of the photoanode was increased by approximately 2 times and 30% compared to that of the BiVO4 / WO3 and Co-BiVO4-Mo / WO3 photoanodes, respectively. This is attributed to the fact that Co in the bulk phase acts as a bulk charge transport channel for the reaction, and Co on the surface acts as a co-catalyst, which improves the light absorption range of the photoanode and modulates its conduction band energy level. This not only promotes the effective separation and transport of photogenerated charges, but also enhances the photoelectrochemical efficiency of the photoanode.

[0121] By continuously switching between light and dark states on the sample for 3600 s, the transient photocurrent density of the Co-BiVO4-Co / WO3 photoanode under a constant external bias voltage (1.23V vs. RHE) as a function of time was obtained, as shown in the figure. Figure 14 As shown in (c), the results indicate that the co-doped photoanode exhibits excellent transient photoresponse characteristics. The large accumulation of charge carriers during lamp switching causes a significant peak in the instantaneous transient photocurrent density curve. After a 3600s transient photoresponse test, the co-doped photoanode did not show particularly large attenuation; the relatively small attenuation may be due to the susceptibility of Co to photocorrosion. At the end of the test, the photoanode still maintained approximately 1.6 mA cm⁻¹. -2 vs. 1.23V RHE The photocurrent density indicates that the co-doped Co-BiVO4-Co / WO3 composite photoanode possesses superior photostability.

[0122] Experimental Example 3

[0123] The performance of Examples 1-2 and Comparative Examples 1 and 3 was tested.

[0124] (1) Electrochemical impedance spectroscopy

[0125] The charge transfer properties of the Co-BiVO4-Co / WO3 photoanode were investigated by electrochemical impedance spectroscopy (EIC). The size of the semicircular arc represents the magnitude of the charge transfer resistance during the PEC process. Figure 15 The EIS results in (a) show that the co-doped Co-BiVO4-Co / WO3 photoanode has a smaller semi-circular arc. The equivalent circuit diagram of the Co-BiVO4-Co / WO3 photoanode was obtained using ZView software (inset in (a)), and its charge transfer resistance was found to be 21.15 Ω and 178.9 Ω (Table 1). This is smaller than the charge transfer resistance of WO3, BiVO4 / WO3, and Co-BiVO4-Mo / WO3, indicating that the co-doped Co-BiVO4-Co / WO3 photoanode has faster water oxidation kinetics. The reason for this is likely that the Co doped in the bulk phase and on the surface act as a charge transport channel in the bulk phase and a surface co-catalyst, respectively, thereby promoting the separation and transfer of photogenerated carriers at the interface.

[0126] Table 1. Parameter values ​​of each component in the equivalent circuit diagram fitting.

[0127] Photoanode <![CDATA[R1(Ω)]]> <![CDATA[R2(Ω)]]> <![CDATA[WO3]]> 32.33 486 <![CDATA[BiVO4 / WO3]]> 26.85 306.5 <![CDATA[Co-BiVO4-Mo / WO3]]> 23.49 216.1 <![CDATA[Co-BiVO4-Co / WO3]]> 21.15 178.9

[0128] The photoelectric conversion efficiency (ABPE) of the photoanode was calculated using linear sweep voltammetry (LSV) data. Figure 15 (b) As demonstrated, the surface- and interface co-doped Co-BiVO4-Co / WO3 photoanode exhibits the highest ABPE value. Specifically, at 0.84V... RHE At that time, the ABPE of the Co-BiVO4-Co / WO3 photoanode reached 0.455%, which is higher than that of Co-BiVO4-Mo / WO3, and is nearly 10 times and 4.5 times the maximum ABPE of WO3 and BiVO4 / WO3 photoanodes, respectively.

[0129] The reasons for the high PEC performance of the Co-BiVO4-Co / WO3 photoanode were further analyzed using the Mott-Schottky test. Figure 15 (c) The slopes of all samples are positive, indicating that all samples are n-type semiconductors with predominantly electronic conductivity. The slope of the linear portion of the sample curve is inversely proportional to the carrier concentration; the larger the slope, the smaller the carrier concentration. A smaller slope indicates a larger carrier concentration and more efficient separation and transport. Extending the tangent to its intersection with the X-axis yields the flat-band potential. For n-type semiconductors, this flat-band potential approximates their Fermi level. The data in the figure show that the slope of the linear segment of the Co-BiVO4-Co / WO3 photoanode is lower than that of the WO3, BiVO4 / WO3, and Co-BiVO4-Mo / WO3 photoanodes, while the carrier concentration is the highest, at 1.3 × 10⁻⁶. 24 cm -3 This indicates that the co-doping strategy significantly reduces the photogenerated charge recombination rate, thereby enhancing photoelectrochemical performance. This finding is consistent with the linear sweep voltammetry (LSV) results, further validating the role of co-doping in improving photoelectrochemical efficiency.

[0130] (2) Linear scan voltammetry

[0131] To further investigate the charge separation injection efficiency of the Co-BiVO4-Co / WO3 photoanode, we used Na2SO3 as a hole scavenger and performed linear sweep voltammetry (LSV) tests on the photoanode, calculating η. bulk and η surface .like Figure 16 (a) Even after the addition of the sacrificial agent, the Co-BiVO4-Co / WO3 photoanode still has the highest photocurrent density. Figure 16(b, c) show that the charge injection efficiency and charge separation efficiency are also higher than those of WO3, BiVO4 / WO3, and Co-BiVO4-Mo / WO3 photoanodes. The significant improvement in both efficiencies fully demonstrates that the co-doped sample does indeed possess excellent charge separation and transport capabilities.

[0132] Figure 17 (a, b) show the WO3 and BiVO4 / WO3 photoanodes at voltages ranging from 0.47 to 0.57 V, respectively. RHE Cyclic voltammetry curves at different scan rates within the electrochemical window and the straight line obtained by fitting with the capacitor current are shown in the figure. It can be seen from the figure that the Co-BiVO4-Co / WO3 photoanode has the largest ECSA value of 0.6 mF cm⁻¹. -2 This indicates that the co-doped Co-BiVO4-Co / WO3 photoanode has more bulk charge transport channels and active sites on its bulk and surface during the PEC reaction, resulting in improved PEC performance.

[0133] Therefore, this invention provides a gradient-doped BiVO4 / WO3 photoanode material with the above-mentioned structure, its preparation method, and its application. By setting a variable-valence transition metal element between the BiVO4 and WO3 surfaces, the electron transport efficiency at the interface of the BiVO4 / WO3 heterojunction is improved, promoting the formation of fast electron transport channels in the bulk phase. Simultaneously, by setting the variable-valence transition metal element on the surface of the photoanode material, a co-catalyst layer is formed on its surface. The co-catalyst layer and the formed fast electron transport channels at the interface work synergistically to fully separate the charges formed in the BiVO4 / WO3 heterojunction, reducing subsequent electron-hole recombination, improving catalytic efficiency, and extending the service life of the photoanode material.

[0134] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit them. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the technical solutions of the present invention, and these modifications or equivalent substitutions cannot cause the modified technical solutions to deviate from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a gradient-doped BiVO4 / WO3 photoanode material, characterized in that: The photoanode material includes a BiVO4 / WO3 heterojunction. The surface of the BiVO4 / WO3 heterojunction and the interface between BiVO4 and WO3 are doped with variable valence transition metal ions, which are distributed in a gradient inside the photoanode. Variable valence transition metal ions include Mo 6+ or Co 2+ One of them; The preparation method of gradient-doped BiVO4 / WO3 photoanode material includes the following steps: (1) Preparation of WO3 thin film Sodium tungstate was dissolved in water and hydrochloric acid was added dropwise. After the tungstate precipitate was completely formed, ammonium oxalate was added until a clear precursor solution was obtained. The pretreated FTO substrate was tilted and placed into a polytetrafluoroethylene reactor with the conductive side facing down. Then, the precursor solution was poured into the reactor for hydrothermal reaction. After the reaction was complete, the substrate loaded with WO3·H2O film was taken out, washed, dried, and heat-treated to obtain WO3 film. (2) The WO3 film was soaked in an aqueous solution of variable valence transition metal ions, then rinsed with deionized water and air-dried to obtain the M / WO3 film. BiOI films were prepared on M / WO3 films by electrodeposition to obtain BiOI-M / WO3 films. (3) An organic solvent containing variable-valence transition metal ions and vanadium source is dropped onto the surface of the BiOI-M / WO3 film, and then heat-treated to obtain the M-BiVO4-M / WO3 film, which is the photoanode material.

2. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: The specific preparation method of the precursor solution in step (1) is as follows: Weigh 0.231g of sodium tungstate and dissolve it in 30mL of deionized water. After it is fully dissolved, measure 6mL of 3M HCl and slowly add it dropwise to the above solution. After the tungstate precipitate is completely formed, add 0.20g of ammonium oxalate until the solution gradually becomes clear. Then add 30mL of deionized water and continue stirring for half an hour to obtain the precursor solution.

3. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step (1) is 120-150℃ and the time is 5-10h.

4. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: In step (1), the heat treatment temperature is 400-500℃, the time is 1-3h, and the heating rate is 2℃ / min.

5. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: In step (2), the concentration of variable-valence transition metal ions is 0.2 mM.

6. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: The specific preparation process of the electrodeposition solution used in step (2) is as follows: Dissolve 3.32 g of KI in 50 mL of deionized water, stir well, adjust the pH of the solution to 1.6-1.8 using HNO3, add 0.97 g of Bi(NO3)3·5H2O, stir for 20 min, add 20 mL of 0.23 M p-benzoquinone ethanol solution, stir vigorously for 30 min, and obtain the electrodeposition solution. The deposition temperature for the electrodeposition method was 25℃, the deposition potential was -0.1V vs. Ag / AgCl, and the deposition time was 5min.

7. The method for preparing a gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: In step (3), the concentration of the vanadium source is 0.2 M and the concentration of the variable-valence transition metal ion is 0.2 mM.

8. The application of the gradient-doped BiVO4 / WO3 photoanode material obtained by the preparation method of the gradient-doped BiVO4 / WO3 photoanode material according to claim 1, characterized in that: Application of photoanode materials in photoelectrochemical water splitting.

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