A method for preparing a carbon nitride S-type homojunction water-splitting photocatalyst and its application

Abstract

This invention discloses a method for preparing a carbon nitride S-type homojunction water-splitting photocatalyst and its application, belonging to the field of photocatalysis technology, specifically related to photocatalytic water splitting. This invention synthesizes a rod-shaped intermediate via hydrothermal supramolecular self-assembly, which is then co-calcined with urea to obtain the carbon nitride S-type homojunction water-splitting photocatalyst. The catalyst prepared by this invention exhibits a unique coral-like structure that enhances light utilization and increases the specific surface area, exposing more active sites. Due to the good interfacial matching degree of the homojunction, recombination sites at the interface are reduced. Furthermore, the built-in electric field formed between the homojunctions promotes the directional separation of charge carriers. Therefore, it essentially improves its charge carrier separation efficiency and optimizes the carbon nitride band structure, resulting in excellent photocatalytic efficiency. Moreover, the entire preparation process requires no special equipment, is simple to operate, easy to control, has good repeatability, is environmentally friendly, and is conducive to industrial production.

Description

Technical Field

[0001] This invention relates to the field of photocatalysis technology, specifically to a method for preparing a carbon nitride S-type homojunction photocatalyst for total water splitting and its application. Background Technology

[0002] With the rapid development of science and technology, environmental pollution caused by fossil fuel combustion has always been a hot issue facing the world. Therefore, how to effectively and greenly obtain clean energy has become a focus of scientists in recent years. Photocatalytic water splitting has attracted much attention due to its advantages of being green, efficient, safe, and stable. The core of photocatalytic water splitting is the photocatalyst. Semiconductor catalysts, such as graphitic carbon nitride materials, have attracted much attention in the field of photocatalytic water splitting due to their environmental friendliness, lack of pollution, and ability to generate photogenerated carriers with redox capabilities when excited by light. However, due to the easy recombination of electrons and holes, their charge separation and transfer capabilities are poor, severely limiting their overall reduction efficiency. To improve this shortcoming, different research teams have done a lot of work on modification, such as constructing heterojunctions, element doping, and surface functional group modification. Among them, the S-type heterojunction, due to the difference in the Fermi levels of the two semiconductors, forms a strong interfacial electric field as a driving force for electron transfer, which can effectively enhance carrier separation. The spatial separation of electrons and holes greatly suppresses their complexation while maintaining high redox capabilities. However, as research progresses, it has become clear that not all heterojunctions can improve photocatalytic performance. For some semiconductors with mismatched interfaces, lattice distortion at the junction leads to an increase in the number of carrier recombination sites, thereby increasing the carrier complexation rate. Therefore, pursuing heterojunction photocatalysts with matched interfaces and strong interfacial charge transfer capabilities is a feasible approach to achieving photocatalytic total water splitting. An effective method to achieve heterojunction interface matching is to construct homojunctions, which consist of two semiconductors with the same basic elements and structure. Since homojunctions are made of similar materials, they are not affected by the interface compatibility issues of ordinary heterojunctions.

[0003] Inspired by the above, this study modifies various properties of carbon nitride by adjusting its micro / nanostructure. A carbon nitride nanotube / carbon nitride nanosheet S-type homojunction was successfully synthesized using an in-situ growth method. Due to the similar chemical structures of the two carbon nitrides and the ability of the in-situ growth method to establish a large interfacial contact, a homojunction with good interfacial matching was formed, reducing recombination sites at the interface. Furthermore, the different Fermi levels between the two carbon nitrides facilitate the establishment of a strong built-in electric field. This built-in electric field plays a crucial role in enhancing interfacial electron transfer, thereby mitigating the inherent defects of carbon nitride and significantly improving carrier separation efficiency. Based on these concepts, it can be predicted that this carbon nitride S-type homojunction can greatly improve the separation efficiency of photogenerated electron-hole pairs, increase light utilization, and thus enhance photocatalytic performance, achieving photocatalytic total water splitting. Summary of the Invention

[0004] The purpose of this invention is to provide a method for preparing a carbon nitride S-type homojunction water-splitting photocatalyst. This method first involves a hydrothermal process to induce supramolecular self-assembly, partially hydrolyzing and polymerizing melamine to form a rod-shaped intermediate. This intermediate is then calcined with urea at different mass ratios to form a coral-like carbon nitride S-type homojunction water-splitting photocatalyst. The built-in electric field and unique coral-like structure effectively reduce carrier recombination rate and increase light absorption, thereby enhancing catalytic activity. This method solves the problems of low visible light utilization, easy carrier recombination, and low efficiency in existing photocatalysts.

[0005] The specific technical solution for achieving the objective of this invention is as follows:

[0006] A method for preparing a carbon nitride S-type homojunction water-splitting photocatalyst, comprising the following steps:

[0007] (1) Add a certain amount of melamine, hydroxylamine sulfate and water to the polytetrafluoroethylene liner and stir to disperse;

[0008] (2) Place the above-mentioned inner liner into a stainless steel autoclave, then place it in an oven and keep it at a certain temperature for a certain period of time. Take it out after it cools to room temperature.

[0009] (3) Remove the inner liner, and the resulting solid can be separated by centrifugation, washed and dried to obtain the intermediate;

[0010] (4) Grind a certain proportion of intermediate with urea, place it in a crucible, heat it to a certain temperature at a certain rate in a muffle furnace and keep it for a certain time. After cooling to room temperature, the resulting solid is ground evenly and is carbon nitride S-type homojunction water-dissolving photocatalyst.

[0011] Furthermore, in step (1), the mass of melamine is 1-2g, the mass of hydroxylamine sulfate is 2-3g, and the volume of water is 30-45mL.

[0012] Furthermore, the oven temperature in step (2) is 100-150°C, and the holding time is 8-13 hours.

[0013] Furthermore, the intermediate in step (4) has a mass of 0.6 to 1.2 g, a mass ratio of intermediate to urea of ​​0.5:1.5 to 2:0, a heating rate of 1 to 4 °C / min, a reaction temperature of 450 to 550 °C, and a reaction time of 3 to 6 h.

[0014] Compared with the prior art, the beneficial effects achieved by the present invention are: the present invention,

[0015] (1) By adjusting the micro-nano structure, carbon nitride with different morphologies and photochemical properties is synthesized, thereby constructing an S-type homojunction;

[0016] (2) Different Fermi levels in the S-type heterojunction lead to the formation of a strong built-in electric field, which promotes the directional migration and separation of charge carriers and maintains a relatively high redox capability.

[0017] (3) The unique coral-like structure improves the utilization rate of light. In addition, the good interfacial contact of the homogeneous structure reduces the formation of recombination sites.

[0018] (4) The carbon nitride S-type homojunction water-splitting photocatalyst exhibits excellent hydrogen evolution activity and water-splitting activity. The hydrogen evolution activity is 3.4 times and 4.3 times that of carbon nitride nanotubes and carbon nitride nanosheets monomers, respectively, and the overall water separation activity is achieved from zero.

[0019] (5) The preparation method of the material of this invention has no special requirements for equipment, has extremely high output, is simple to operate, easy to control, has good repeatability, is green and environmentally friendly, and is conducive to large-scale industrial production. Attached Figure Description

[0020] The accompanying drawings are provided to further illustrate the invention and form part of the specification. They are used in conjunction with embodiments of the invention to explain the invention and do not constitute a limitation thereof. In the drawings:

[0021] Figure 1 The synthesis method, scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in this invention are shown.

[0022] Figure 2 The electron paramagnetic resonance image is shown for the carbon nitride S-type homojunction photocatalyst for total water splitting prepared in this invention.

[0023] Figure 3 The image shows the ultraviolet photoelectron spectrum of the carbon nitride S-type homojunction photocatalyst for total water splitting prepared in this invention.

[0024] Figure 4 This is a schematic diagram of the S-type homojunction of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in this invention.

[0025] Figure 5 The electron spin resonance diagram is shown for the carbon nitride S-type homojunction photocatalyst for total water splitting prepared in this invention.

[0026] Figure 6 This is a diagram showing the water-splitting activity of the carbon nitride S-type homojunction photocatalyst prepared in this invention.

[0027] Figure 7 The structure and morphology of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in this invention after a 25-hour stability test are shown.

[0028] Figure 8The electrostatic potential diagram is shown for the carbon nitride S-type homojunction photocatalyst for total water splitting prepared in this invention.

[0029] Figure 9 The hydrogen adsorption Gibbs free energy diagram of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in this invention.

[0030] Figure 10 The differential charge density diagram and Bader charge diagram are shown for the carbon nitride S-type homojunction total water splitting photocatalyst prepared in this invention.

[0031] Figure 11 The diagram shows the energy barrier diagram of the oxygen production pathway of the carbon nitride S-type homojunction photocatalyst for total water splitting prepared in this invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] Example 1

[0034] A method for preparing a bulk carbon nitride photocatalyst, specifically including the following steps:

[0035] Place 2g of melamine in a crucible, heat it to 550℃ in a muffle furnace at 2℃ / min and maintain it for 4h. After cooling, grind the resulting solid evenly to obtain bulk carbon nitride.

[0036] Example 2

[0037] A method for preparing a carbon nitride nanotube photocatalyst, specifically including the following steps:

[0038] (1) Add 1g of melamine, 2g of hydroxylamine sulfate and 40mL of water to the polytetrafluoroethylene liner and stir to disperse;

[0039] (2) Place the above-mentioned inner liner into a stainless steel autoclave, then place it in an oven and keep it at 120°C for 12 hours. Take it out after it cools to room temperature.

[0040] (3) Remove the inner liner, and the resulting solid can be separated by centrifugation, washed and dried to obtain the intermediate;

[0041] (4) The intermediate obtained in step (3) is heated to 550°C at 2°C / min in a muffle furnace and held for 4 hours. After cooling, the resulting solid is ground evenly to obtain carbon nitride nanotubes, which are named TCN.

[0042] Example 3

[0043] A method for preparing a carbon nitride nanosheet photocatalyst, specifically including the following steps:

[0044] 2g of urea was placed in a crucible and heated to 550℃ in a muffle furnace at 2℃ / min and held for 4h. After cooling, the resulting solid was ground evenly to obtain carbon nitride nanosheets, which were named SCN.

[0045] Example 4

[0046] A method for preparing a carbon nitride S-type homojunction water-splitting photocatalyst specifically includes the following steps:

[0047] (1) Add 1g of melamine, 2g of hydroxylamine sulfate and 40mL of water to the polytetrafluoroethylene liner and stir to disperse;

[0048] (2) Place the above-mentioned inner liner into a stainless steel autoclave, then place it in an oven and keep it at 120°C for 12 hours. Take it out after it cools to room temperature.

[0049] (3) Remove the inner liner, and the resulting solid can be separated by centrifugation, washed and dried to obtain the intermediate;

[0050] (4) Grind 1.2g of intermediate and 0.8g of urea together, then place them in a crucible, heat them in a muffle furnace to 550℃ at 2℃ / min and keep them for 4h. After cooling, grind the resulting solid evenly to obtain carbon nitride S-type homojunction water-splitting photocatalyst and name it TSCN.

[0051] Figure 1 a is a synthesis route diagram of the carbon nitride S-type homojunction total water-splitting photocatalyst prepared in Example 4. Figure 1 bi are scanning electron microscope (SEM) and transmission electron microscope (TEM) images of the photocatalysts prepared in Examples 1, 2, 3, and 4. As shown in the figures, the bulk carbon nitride exhibits a blocky structure, the carbon nitride nanotubes exhibit a tubular structure, the carbon nitride nanosheets exhibit a sheet-like structure, and the carbon nitride S-type homojunction water-splitting photocatalyst exhibits a unique coral-like structure.

[0052] Figure 2 The electron sequence diagrams are shown for the carbon nitride nanotube / nanosheet physical mixtures prepared in Examples 2 and 3 and the carbon nitride S-type homojunction water-splitting photocatalyst prepared in Example 4. The lower signal structure of the carbon nitride S-type homojunction demonstrates that the efficient interface contact effectively reduces the formation of interface defects.

[0053] Figure 3The images show the ultraviolet photoelectron spectra of the carbon nitride nanotubes and carbon nitride nanosheets photocatalysts prepared in Examples 2 and 3. The alternating band positions and different Fermi levels of the two catalysts provide the necessary conditions for the formation of an S-type homojunction.

[0054] Figure 4 This is a schematic diagram of the S-type homojunction of the carbon nitride S-type homojunction photocatalyst prepared in Example 4. Before contact, the two carbon nitrides exhibit an interleaved band structure. After contact, electrons transfer from high to low energy levels until the Fermi level is flat, which leads to band bending and thus creates a built-in electric field.

[0055] Figure 5 The electron spin resonance diagrams are for the carbon nitride nanotubes, carbon nitride nanosheets, and carbon nitride S-type homojunction water-splitting photocatalysts prepared in Examples 2, 3, and 4. None of the three showed a signal in the dark. Due to differences in band structure leading to different redox capabilities, carbon nitride nanotubes and carbon nitride nanosheets exhibited higher modulo values ​​in superoxide radical and hydroxyl radical tests, respectively. However, because the S-type homojunction retained the valence band and inverted band positions where the redox capabilities were strongest in both, the carbon nitride S-type homojunction showed the highest response signal in both tests.

[0056] Figure 6 The diagram shows the total water-splitting activity of bulk carbon nitride, carbon nitride nanotubes, carbon nitride nanosheets, and carbon nitride S-type homojunctions prepared in Examples 1, 2, 3, and 4. Due to suitable interface matching and the formation of a built-in electric field, the carbon nitride S-type homojunction exhibits better redox activity and carrier splitting capacity. Therefore, compared to bulk carbon nitride, carbon nitride nanotubes, and carbon nitride nanosheets, the carbon nitride S-type homojunction demonstrates significantly improved total water-splitting activity and exhibits better stability in a 25-hour stability test.

[0057] Figure 7 The images show the structure and morphology of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in Example 4 after a 25-hour stability test. The carbon nitride S-type homojunction water-splitting photocatalyst retains the classic carbon nitride chemical structure and its morphology still maintains the distinctive coral-like structure.

[0058] Figure 8 The image shows the electrostatic potential diagram of the carbon nitride S-type homojunction water-splitting photocatalyst prepared in Example 4. The electrostatic potential strengths of carbon nitride nanotubes and carbon nitride nanosheets differ, theoretically demonstrating that the formation of the S-type heterojunction generates a strong built-in electric field that promotes the directional separation of photogenerated carriers.

[0059] Figure 9The hydrogen adsorption Gibbs free energy diagrams for the carbon nitride nanotubes, carbon nitride nanosheets, and carbon nitride S-type homojunction water splitting photocatalysts prepared in Examples 2, 3, and 4 are shown. The hydrogen adsorption Gibbs free energy is approximately zero, indicating that the adsorption and desorption of hydrogen ions on the material surface are in equilibrium, which is conducive to the continued photocatalytic hydrogen production reaction. The carbon nitride S-type homojunction has the closest Gibbs free energy to zero, indicating that the hydrogen evolution reaction is more easily carried out on the carbon nitride S-type homojunction.

[0060] Figure 10 The differential charge density diagram and Bader charge diagram are shown for the carbon nitride S-type homojunction water-splitting photocatalyst prepared in Example 4. At the initial contact with the semiconductor, electrons transfer from the carbon nitride nanosheets to the carbon nitride nanotubes. After simulating the built-in electric field with an external electric field, the electrons exhibit the opposite transfer direction. This theoretically confirms the successful preparation of the S-type homojunction and demonstrates that the built-in electric field can effectively promote the directional transfer of photogenerated charges.

[0061] Figure 11 This is an energy barrier diagram of the oxygen production pathway for the carbon nitride nanotubes, carbon nitride nanosheets, and carbon nitride S-type homojunction water splitting photocatalysts prepared in Examples 2, 3, and 4. The energy difference between each state in the oxygen production pathway represents the size of the energy barrier that needs to be overcome for that step of the reaction. Comparing the maximum energy barriers that need to be overcome in each photocatalyst pathway, it can be seen from the figure that the carbon nitride nanosheets require the largest energy barrier, followed by the carbon nitride nanotubes. The carbon nitride S-type homojunction requires the smallest energy barrier, indicating that its oxygen production reaction is easier to occur compared to other photocatalysts.

[0062] Finally, it should be noted that the above descriptions are merely preferred embodiments of the present invention and are not intended to limit the present invention. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art can still modify the technical solutions described in the foregoing embodiments or make equivalent substitutions for some of the technical features. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a carbon nitride S-type homojunction overall water splitting photocatalyst, characterized in that, The catalyst is prepared by supramolecular self-assembly and thermal condensation construction, and is applied to photocatalytic overall water splitting, and the specific steps are as follows: (1) a certain amount of melamine, hydroxylamine sulfate and water are added into a polytetrafluoroethylene lining bladder, and stirring and dispersion are carried out; (2) the lining bladder is placed into a stainless steel autoclave, and then is placed into an oven to keep for a certain time at a certain temperature, and after being cooled to room temperature, the lining bladder is taken out; (3) the lining bladder is taken out, and the obtained solid is centrifuged, washed and dried to obtain an intermediate; (4) 1.2g of the intermediate and 0.8g of urea are ground, are placed into a crucible, and are heated to a certain temperature at a certain rate in a muffle furnace and kept for a certain time, and after being cooled to room temperature, the obtained solid is uniformly ground to obtain a carbon nitride S-type homojunction overall water splitting photocatalyst, which presents a special coral-like structure.

2. The preparation method of the carbon nitride S-type homojunction overall water splitting photocatalyst according to claim 1, characterized in that, In the step (1), the mass of melamine is 1-2g, the mass of hydroxylamine sulfate is 2-3g, and the volume of water is 30-45mL.

3. The preparation method of the carbon nitride S-type homojunction overall water splitting photocatalyst according to claim 1, characterized in that, The oven temperature of the step (2) is 100-150 DEG C, and the keeping time is 8-13h.

4. The preparation method of the carbon nitride S-type homojunction overall water splitting photocatalyst according to claim 1, characterized in that, The heating rate of the step (4) is 1-4 DEG C / min, the reaction temperature is 450-550 DEG C, and the reaction time is 3-6h.

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