Preparation of porous bismuth vanadate nanorod loaded sheet-like cobalt-doped graphitic carbon nitride catalyst and application thereof

Abstract

The application provides a porous bismuth vanadate nanorod loaded sheet-shaped cobalt-doped graphite phase carbon nitride photoanode catalyst, a preparation method thereof and application thereof in the field of photoelectrocatalytic water splitting to produce hydrogen. A spin-coating and annealing process is used to tightly load Co:g-C3N4 nanosheets on the surface of porous BiVO4 nanorods, and the energy band structure of the Co:g-C3N4 nanosheet is regulated by adjusting the doping ratio of Co, and then a Co:g-C3N4 / BiVO4 heterojunction photoanode with good energy band matching is constructed. Among them, the ultrathin sheet-shaped Co:g-C3N4 has a more negative conduction band potential and a wider light absorption range than BiVO4, which is conducive to expanding the light response interval; and the energy band bending on the BiVO4 side helps to promote the directional transmission of photo-generated electrons to the photocathode. Based on the synergistic effect of the two materials, the heterojunction structure can shorten the carrier transmission path, promote the effective separation of photo-generated electron-hole pairs, and thus improve the photoelectrocatalytic water splitting to produce hydrogen performance. In addition, the preparation process is simple and clear in operation, and has potential for large-scale application.

Description

Technical Field

[0001] This invention belongs to the field of nanocomposite materials, and specifically relates to a porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphite phase carbon nitride composite photocatalyst, its preparation method, and its application in the field of photoelectric water splitting for hydrogen production. Technical Background Hydrogen energy, as an ideal energy carrier with high energy density and zero carbon emissions, is considered one of the most promising alternative energy sources. Among numerous hydrogen production technologies, photoelectrocatalytic (PEC) water splitting has attracted widespread research interest because it can directly utilize solar energy to drive the water splitting reaction, converting solar energy into chemical energy stored in the form of hydrogen. However, despite half a century of development in PEC research, this technology still faces bottlenecks such as low catalytic efficiency and insufficient long-term stability, making large-scale industrial application difficult. Therefore, developing efficient and stable PEC catalysts has become a key scientific problem that urgently needs to be solved in this field.

[0002] Bismuth vanadate (BiVO4), a representative n-type semiconductor photoanode material, has a band gap of approximately 2.4 eV, and its band edge position matches the water redox potential, thus attracting widespread attention in the field of PEC water splitting. However, BiVO4 faces several inherent challenges in practical applications, mainly manifested in its low carrier mobility, rapid electron-hole recombination rate, numerous trapped states on its surface, and sluggish water oxidation reaction kinetics, resulting in unsatisfactory photoelectrocatalytic performance. To overcome these shortcomings, researchers have proposed various modification strategies, including elemental doping, heterojunction construction, surface co-catalyst modification, and morphology control. Among these, constructing heterojunctions on the BiVO4 surface is considered an effective way to simultaneously suppress carrier recombination and fill surface trapped states. Two-dimensional sheet-like semiconductor materials, with their excellent in-plane carrier transport characteristics and good surface water molecule adsorption capacity, can effectively cover the BiVO4 surface and passivate its trapped states, and are considered ideal materials for constructing heterostructures.

[0003] Graphitic carbon nitride (g-C3N4, hereinafter referred to as CN) is a two-dimensional polymeric semiconductor material composed of carbon and nitrogen elements. With a band gap of approximately 2.7 eV, it possesses a suitable band structure, excellent chemical and thermal stability, and is widely available and inexpensive, thus attracting significant attention in the field of polycarbonate (PC). However, intrinsic CN still suffers from drawbacks such as poor conductivity, high photogenerated carrier recombination rate, and limited reactive sites, hindering further improvements in its PC performance. To address these issues, researchers typically employ elemental doping strategies to modify CN by introducing metal or non-metal heteroatoms into its framework structure to adjust its electronic structure and catalytic performance. Cobalt (Co), as a transition metal element, not only modulates the band structure and improves conductivity when incorporated into CN but also provides additional catalytic active centers, thus attracting widespread attention in CN doping modification research.

[0004] Current research reports on constructing type II heterojunctions by combining BiVO4 and CN to achieve PEC water splitting. However, due to the small band shift between intrinsic CN and BiVO4, the charge transfer driving force at the heterojunction interface is insufficient. Furthermore, CN struggles to effectively passivate trapped states on the BiVO4 surface, resulting in unsatisfactory PEC catalytic efficiency for the composite material. How to construct a well-matched type II heterojunction between CN and BiVO4 through band modulation to achieve efficient PEC water splitting for hydrogen production remains a critical problem to be solved in this field.

[0005] Based on the above analysis, this invention proposes a strategy to regulate the band structure of graphitic carbon nitride by cobalt doping and to load sheet-like cobalt-doped graphitic carbon nitride on the surface of porous bismuth vanadate nanorods to construct a type II heterojunction, thereby preparing a novel composite photoelectrocatalytic material, which is then applied to PEC water splitting for hydrogen production. Summary of the Invention

[0006] The first objective of this invention is to provide a porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride composite photocatalyst (Co:CN / BiVO4), wherein cobalt (Co) is doped onto the surface of ultrathin sheet-like graphitic carbon nitride (CN) through in-situ growth, and the resulting Co:CN nanosheets are loaded onto the surface of porous bismuth vanadate (BiVO4) nanorods, forming a type II heterojunction between the two.

[0007] The graphitic carbon nitride described herein is an ultrathin two-dimensional nanosheet structure obtained through multiple calcinations.

[0008] The second objective of this invention is to provide a method for preparing the above-mentioned porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphitic carbon nitride composite photocatalyst. The core aspect is to first prepare Co:CN by in-situ loading Co onto the surface of CN nanosheets using an impregnation-annealing method, and then load Co:CN onto the surface of BiVO4 nanorods using a spin-coating-annealing method to obtain the Co:CN / BiVO4 composite photocatalyst. This method includes the following steps: Step S1: Preparation of ultrathin sheet-like graphitic carbon nitride (CN): 1.1 Melamine was used as the raw material and prepared by a multiple calcination method. Specifically, 10 g of melamine was placed in a covered porcelain crucible and heated to 550 °C at a heating rate of 5 °C / min for 4 hours, followed by natural cooling. After the reaction was completed, the sample was collected.

[0009] 1.2 The sample obtained in step 1.1 was placed in a covered porcelain crucible for secondary calcination, heated to 450°C at a heating rate of 2.5°C / min, maintained for 2 hours, and then allowed to cool naturally. This process was repeated three times. After the reaction was completed, the sample was collected, which was the ultrathin sheet-like graphitic carbon nitride.

[0010] Step S2: Preparation of Co:CN catalyst by impregnation-annealing method: 2.1 Disperse 2 g of the CN sample obtained in step 1.2 in 75 mL of anhydrous ethanol, then add 25 mL of aqueous solution containing 1.48 g of Co(NO3)2·6H2O. Alternately stir and sonicate four times, each time for 30 minutes. Then, stir, impregnate, and evaporate the resulting mixture to dryness at 60°C. Anneal the resulting sample at 350°C for 2 hours under an argon atmosphere at a heating rate of 2°C / min. Collect the sample; this is Co:CN.

[0011] Step S3: Preparation of BiVO4 photoanode by electroplating: 3.1 Dissolve 3.32 g KI in 50 mL of deionized water, and precisely adjust the pH of the solution to 1.7 using HNO3. Then, add 0.9702 g Bi(NO3)3·5H2O and stir continuously until a clear solution is formed. Next, dissolve 0.52 g p-benzoquinone in 20 mL of anhydrous ethanol and quickly add it to the above clear solution, maintaining the reaction under high-speed stirring for 3 minutes. Subsequently, prepare a BiOI film (1×1 cm in size) at room temperature using a potential-controlled cathodic deposition method. 2 Using Ag / AgCl as the reference electrode, the potential was set to -0.1 V, and the deposition time was 180 seconds. The resulting film was collected and washed.

[0012] 3.2 The BiOI film obtained in step 3.1 was converted into a BiVO4 film. A 0.4 mol / L vanadium acetylacetonate solution was drop-coated onto the BiOI electrode (50 μL / cm²). 2 The electrode was heated to 450°C in air at a heating rate of 2°C / min and annealed for 2 hours. Subsequently, the resulting BiVO4 electrode was immersed in a 1 mol / L sodium hydroxide solution for 10 minutes while gently stirring to remove excess V2O5, and finally rinsed with deionized water.

[0013] Step S4: Preparation of Co:CN / BiVO4 catalyst by spin coating-annealing method: 4.1 100 mg of Co:CN sample was dispersed in 20 mL of anhydrous ethanol to prepare a homogeneous suspension. Then, the suspension was spin-coated onto a BiVO4 surface at 2500 rpm for 1 minute. This spin-coating operation was repeated three times. The resulting sample was then placed in a N2 atmosphere and annealed at 350°C with a heating rate of 10°C / min for 1 hour. After natural cooling, the sample was collected, which is the Co:CN / BVO4 sample.

[0014] The third objective of this invention is to provide the application of the aforementioned porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphitic carbon nitride composite photocatalyst in the field of PEC water splitting for hydrogen production. Specifically, under visible light irradiation (300 W xenon lamp), the H2 generation rate of the Co:CN / BiVO4 composite photoanode reaches as high as 15.236 μmol cm⁻¹. -2 h -1 .

[0015] Compared with existing composite functional photoelectrocatalytic materials, the beneficial effects of this invention are as follows: (1) This invention uses a simple and efficient impregnation-annealing method to in-situ dope cobalt atoms into ultrathin graphitic carbon nitride nanosheets. While not destroying the intrinsic structure of carbon nitride, it effectively regulates the band structure of carbon nitride, significantly enhances the visible light absorption capacity, and makes it well matched with the band structure of bismuth vanadate, laying the foundation for the construction of type II heterojunction.

[0016] (2) In this invention, cobalt-doped graphitic carbon nitride nanosheets are loaded onto the surface of porous bismuth vanadate nanorods, and a type II heterojunction is successfully constructed without changing the basic structure and properties of both. This heterojunction maintains the high reduction driving force of BiVO4 conduction band electrons, while significantly improving the light absorption capacity of BiVO4 in the visible light region; thanks to the charge transport mechanism of the type II heterojunction, the recombination efficiency of photogenerated carriers is greatly reduced, thereby significantly improving the PEC catalytic activity of the photoanode material.

[0017] (3) The preparation process of the present invention is simple, the conditions are mild and the steps are clear, which facilitates large-scale production and provides the possibility for future commercial applications. It can also provide technical reference for the design and preparation of other heterojunction structure photoanode materials.

[0018] (4) The porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride photocatalyst of the present invention does not require the addition of any sacrificial agent or co-catalyst in the process of PEC water cracking to produce hydrogen, which greatly saves economic costs and does not cause environmental pollution. Attached image description: Figure 1 This is a scanning electron microscope image of the porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention.

[0019] Figure 2 The X-ray diffraction pattern of the porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention is shown.

[0020] Figure 3 The UV-Vis diffuse reflectance spectrum and corresponding Tauc diagram of the porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention are shown.

[0021] Figure 4 This is a comparison diagram of the electrochemical performance of the porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphite carbon nitride catalyst of the present invention.

[0022] Figure 5 This is a comparison chart of the photoelectrochemical catalytic hydrogen production from water splitting using porous bismuth vanadate nanorods supported on sheet-like cobalt-doped graphite carbon nitride catalysts of the present invention. Detailed implementation method: The porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention will be described in detail below with reference to the accompanying drawings. Embodiments of the present invention will also be described in detail below with reference to the accompanying drawings, but the scope of protection of the present invention is not limited to these embodiments.

[0023] Example 1: Step S1: Preparation of ultrathin sheet-like graphitic carbon nitride (CN): 1.1 Melamine was used as the raw material and prepared by a multiple calcination method. Specifically, 10 g of melamine was placed in a covered porcelain crucible and heated to 550 °C at a heating rate of 5 °C / min for 4 hours, followed by natural cooling. After the reaction was completed, the sample was collected.

[0024] 1.2 The sample obtained in step 1.1 was placed in a covered porcelain crucible for secondary calcination, heated to 450°C at a heating rate of 2.5°C / min, maintained for 2 hours, and then allowed to cool naturally. This process was repeated three times. After the reaction was completed, the sample was collected, which was the ultrathin sheet-like graphitic carbon nitride.

[0025] Step S2: Preparation of Co:CN catalyst by impregnation-annealing method: 2.1 Disperse 2 g of the CN sample obtained in step 1.2 in 75 mL of anhydrous ethanol, then add 25 mL of aqueous solution containing 1.48 g of Co(NO3)2·6H2O. Alternately stir and sonicate four times, each time for 30 minutes. Then, stir, impregnate, and evaporate the resulting mixture to dryness at 60°C. Anneal the resulting sample at 350°C for 2 hours under an argon atmosphere at a heating rate of 2°C / min. Collect the sample; this is Co:CN.

[0026] Step S3: Preparation of BiVO4 photoanode by electroplating: 3.1 Dissolve 3.32 g KI in 50 mL of deionized water, and precisely adjust the pH of the solution to 1.7 using HNO3. Then, add 0.9702 g Bi(NO3)3·5H2O and stir continuously until a clear solution is formed. Next, dissolve 0.52 g p-benzoquinone in 20 mL of anhydrous ethanol and quickly add it to the above clear solution, maintaining the reaction under high-speed stirring for 3 minutes. Subsequently, prepare a BiOI film (1×1 cm in size) at room temperature using a potential-controlled cathodic deposition method. 2 Using Ag / AgCl as the reference electrode, the potential was set to -0.1 V, and the deposition time was 180 seconds. The resulting film was collected and washed.

[0027] 3.2 The BiOI film obtained in step 3.1 was converted into a BiVO4 film. A 0.4 mol / L vanadium acetylacetonate solution was drop-coated onto the BiOI electrode (50 μL / cm²). 2 The electrode was heated to 450°C in air at a heating rate of 2°C / min and annealed for 2 hours. Subsequently, the resulting BiVO4 electrode was immersed in a 1 mol / L sodium hydroxide solution for 10 minutes while gently stirring to remove excess V2O5, and finally rinsed with deionized water.

[0028] Step S4: Preparation of Co:CN / BiVO4 catalyst by spin coating-annealing method: 4.1 100 mg of Co:CN sample was dispersed in 20 mL of anhydrous ethanol to prepare a homogeneous suspension. Then, the suspension was spin-coated onto the BiVO4 surface at 2500 rpm for 1 minute. This spin-coating operation was repeated three times. The resulting sample was then placed in a N2 atmosphere and annealed at 350°C with a heating rate of 10°C / min for 1 hour. After natural cooling, the sample was collected, which is the Co:CN / BiVO4.

[0029] Figure 1 These are scanning electron microscope (SEM) images of the porous bismuth vanadate nanorods supported on sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention. (a) and (c) are SEM images of the BiVO4 photoanode, showing that BiVO4 exhibits a porous nanorod structure; (b) and (d) are SEM images of Co:CN / BiVO4, showing that sheet-like Co:CN is tightly attached to the surface of the BiVO4 nanorods, confirming successful Co:CN loading.

[0030] Figure 2 This is the X-ray diffraction pattern of the porous bismuth vanadate nanorods supported on sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention. The diffraction peak positions of Co:CN / BVO and BVO are in perfect agreement with the standard card (PDF#75-1866) of monoclinic BiVO4, and diffraction peaks from the FTO substrate (PDF#46-1088) can also be observed. After loading Co:CN, no additional diffraction peaks appeared in the XRD pattern of the composite sample, and the characteristic peak positions and intensities of BiVO4 did not change significantly, indicating that the introduction of Co:CN did not destroy the crystal structure of BiVO4.

[0031] Figure 3 The UV-Vis diffuse reflectance spectra and corresponding Tauc diagrams of the porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphitic carbon nitride catalyst of this invention are shown. (a) shows the UV-Vis diffuse reflectance absorption spectra of BVO and Co:CN / BVO, and (b) shows the corresponding Tauc diagrams. As can be seen from (b), the direct band gap of Co:CN / BVO (~2.7 eV) is narrower than that of BVO (~2.8 eV), indicating that the loading of Co:CN is beneficial for broadening the light absorption range.

[0032] Figure 4 This is a comparison of the electrochemical performance of the porous bismuth vanadate nanorods supported on sheet-like cobalt-doped graphitic carbon nitride catalyst of the present invention. (a) The linear sweep voltammetry (LSV) curve shows that the photocurrent density of Co:CN / BVO at 1.23 V (vs. RHE) is approximately 2.1 mA / cm². 2 It is approximately BVO (~1.1 mA / cm). 2 (a) The photocurrent of Co:CN / BVO is twice that of BVO; (b) The transient photocurrent response curve shows that the photocurrent of Co:CN / BVO is higher and the response is more stable; (c) The photovoltage of Co:CN / BVO (~0.48 V) in the open circuit potential test is significantly higher than that of BVO (~0.27 V); (d) The arc radius of Co:CN / BVO in the electrochemical impedance spectroscopy (EIS) is significantly smaller than that of BVO, indicating that the charge transfer resistance is reduced.

[0033] Figure 5This is a comparison of the hydrogen production from photoelectrochemical water splitting using porous bismuth vanadate nanorods supported on sheet-like cobalt-doped graphitic carbon nitride catalysts of the present invention. (a) shows the H2 and O2 production curves of the BVO photoanode over time; (b) shows the corresponding curves for Co:CN / BVO. After loading with Co:CN, the production of both H2 and O2 is significantly increased, and the hydrogen production rate of this composite photoanode can reach 15.236 μmol·cm⁻¹. -2 ·h -1 .

[0034] Example 2: The difference between Example 2 and Example 1 is that the amount of Co(NO3)2·6H2O in step S2 is adjusted to 0.74g, while the rest of the steps are the same as in Example 1.

[0035] Example 3: The difference between Example 3 and Example 1 is that the amount of Co(NO3)2·6H2O in step S2 is adjusted to 2.96g, while the rest of the steps are the same as in Example 1.

[0036] Example 4: The difference between Example 4 and Example 1 is that the concentration of the Co:CN suspension in step S3 is adjusted to 50 mg / 20 mL, the number of spin coatings is adjusted to 5, and the remaining steps are the same as in Example 1.

Claims

1. A porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphitic carbon nitride composite photocatalyst, characterized in that: Using porous bismuth vanadate (BiVO4) nanorods as a substrate, sheet-like cobalt-doped graphitic carbon nitride (Co:CN) nanosheets are loaded on its surface, forming a type II heterojunction between the two.

2. The porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphite-phase carbon nitride composite photocatalyst as described in claim 1, characterized in that: The graphitic carbon nitride described herein is an ultrathin two-dimensional nanosheet structure obtained through multiple calcinations.

3. The porous bismuth vanadate nanorod-supported sheet-like cobalt-doped graphite-phase carbon nitride composite photocatalyst as described in claim 1, characterized in that: The bismuth vanadate described is a porous nanorod structure prepared by electroplating.

4. The preparation method of the porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphitic carbon nitride composite photocatalyst according to claims 1-3, the core point of which is to first prepare Co:CN by in-situ loading Co onto the surface of CN nanosheets through an impregnation-annealing method, and then load Co:CN onto the surface of BiVO4 nanorods through a spin-coating-annealing method, including the following steps: Step S1: Preparation of ultrathin sheet-like graphitic carbon nitride (CN); 1.1 Melamine was used as the raw material and prepared by a multiple calcination method. Specifically, 10 g of melamine was placed in a covered porcelain crucible and heated to 550 °C at a heating rate of 5 °C / min, maintained for 4 hours, and then allowed to cool naturally. After the reaction was completed, the sample was collected. 1.2 The sample obtained in step 1.1 was placed in a covered porcelain crucible for secondary calcination, heated to 450°C at a heating rate of 2.5°C / min, maintained for 2 hours, and then allowed to cool naturally. This process was repeated three times. After the reaction was completed, the sample was collected, which was the ultrathin sheet-like graphitic carbon nitride. Step S2: Preparation of Co:CN catalyst by impregnation-annealing method; 2.1 Disperse 2 g of the CN sample obtained in step 1.2 in 75 mL of anhydrous ethanol, then add 25 mL of aqueous solution containing 1.48 g of Co(NO3)2·6H2O. Alternately stir and sonicate four times, each time for 30 minutes. Then, stir, impregnate, and evaporate the resulting mixture to dryness at 60°C. Anneal the resulting sample at 350°C for 2 hours under an argon atmosphere at a heating rate of 2°C / min. Collect the sample; this is Co:CN. Step S3: Prepare BiVO4 photoanode by electroplating; 3.1 Dissolve 3.32 g KI in 50 mL of deionized water, and precisely adjust the pH of the solution to 1.7 using HNO3. Then, add 0.9702 g Bi(NO3)3·5H2O and stir continuously until a clear solution is formed. Next, dissolve 0.52 g p-benzoquinone in 20 mL of anhydrous ethanol and quickly add it to the above clear solution, maintaining the reaction under high-speed stirring for 3 minutes. Subsequently, prepare a BiOI thin film (1 × 1 cm in size) at room temperature using a potential-controlled cathodic deposition method. 2 Using Ag / AgCl as the reference electrode, the deposition potential was set to -0.1 V, and the deposition time was 180 seconds. The resulting film was collected and washed. 3.2 The BiOI film obtained in step 3.1 was converted into a BiVO4 film. A 0.4 mol / L vanadium acetylacetonate solution was drop-coated onto the BiOI electrode (50 μL / cm²). 2 The electrode was heated to 450°C in air at a heating rate of 2°C / min and annealed for 2 hours. Subsequently, the resulting BiVO4 electrode was immersed in a 1 mol / L sodium hydroxide solution for 10 minutes while gently stirring to remove excess V2O5, and finally rinsed with deionized water. Step S4: Preparation of Co:CN / BiVO4 catalyst by spin coating-annealing method; 4.1 100 mg of Co:CN sample was dispersed in 20 mL of anhydrous ethanol to prepare a homogeneous suspension. Then, the suspension was spin-coated onto a BiVO4 surface at 2500 rpm for 1 minute. This spin-coating operation was repeated three times. The resulting sample was then placed in a N2 atmosphere and annealed at 350°C with a heating rate of 10°C / min for 1 hour. After natural cooling, the sample was collected, which is the Co:CN / BiVO4 sample.

5. The application of the porous bismuth vanadate nanorod supported sheet-like cobalt-doped graphite phase carbon nitride composite photoelectrocatalyst of claim 1 in the field of photoelectrochemical (PEC) water splitting for hydrogen production.

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