Double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancy and preparation method thereof

Abstract

The invention discloses a double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancy and a preparation method thereof, and belongs to the technical field of photocatalytic materials, the preparation method comprises the following steps: (1) pre-pressing original g-C3N4 powder into sheets to obtain a sample, and then loading the sample into a sample cavity of a high-pressure device; (2) putting the high-pressure device filled with the sample into a pressing machine, applying pressure to 5.0-6.0 GPa, then raising the temperature to 200-500 DEG C under the pressure of 5.0-6.0 GPa, and keeping the temperature and the pressure for 10-120 minutes at the temperature of 200-500 DEG C; (3) after heat preservation is finished, heating is stopped, the sample is cooled to the room temperature along with the furnace, pressure is relieved, and the sample is taken out; and (4) washing the taken sample with water, drying and grinding to obtain the double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancy. According to the preparation method disclosed by the invention, in a single high-temperature and high-pressure process, oxygen element doping of g-C3N4 and construction of nitrogen vacancy defects are synchronously realized, two defect sites can generate a synergistic effect, an electronic structure of the photocatalyst is more effectively regulated and controlled, and separation and migration of photon-generated carriers are promoted.

Description

Technical Field

[0001] This invention belongs to the field of photocatalytic materials technology, specifically relating to a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies and its preparation method. Background Technology

[0002] Graphitic carbon nitride (g-C3N4), as a non-metallic polymer semiconductor photocatalyst, has attracted widespread attention due to its good chemical stability, suitable band structure, and ease of preparation. However, pristine g-C3N4 suffers from inherent drawbacks such as a narrow visible light absorption range and a high recombination rate of photogenerated electron-hole pairs, severely limiting its photocatalytic efficiency and application potential. To overcome these shortcomings, researchers have developed various modification strategies, including elemental doping, morphology manipulation, heterojunction construction, and defect introduction. Among these, defect introduction (such as nitrogen vacancies) and elemental doping (such as oxygen, sulfur, and phosphorus) are important means to effectively control its electronic and band structures. In particular, the synergistic introduction of oxygen (O) doping and nitrogen (N) vacancies can theoretically promote light absorption and charge separation synergistically by adjusting the band structure and constructing an in-plane built-in electric field, respectively. However, achieving efficient, controllable, and especially synergistic construction of these two defects remains a bottleneck that current technologies have struggled to overcome. Current mainstream methods generally suffer from some inherent drawbacks. For example, hydrothermal / solvothermal methods for constructing defects typically operate at relatively mild temperatures and pressures (generally <10 MPa), with the chemical potential primarily dependent on the solvent system. While this process can achieve doping, the extremely low pressure cannot fundamentally affect the bulk electronic structure of g-C3N4, making it difficult to create high concentrations of nitrogen vacancies. Furthermore, residual solvent or ionic impurities after the reaction require complex cleaning steps and are prone to secondary contamination. Another example is the post-polymerization treatment method, which is usually carried out at atmospheric pressure, using temperature to drive structural modification. At low temperatures, it is difficult to effectively break the stable heptaazine ring structure of g-C3N4. As a result, defect formation is random, the concentration is low, and it can usually only introduce a single type of defect (such as vacancies), making it difficult to achieve effective doping of heterogeneous elements such as O. Excessively high temperatures can easily lead to uncontrolled polymerization, increased structural disorder, and even dissociation.

[0003] High-temperature high-pressure (HTHP) technology, as an extreme-condition synthesis method, can provide a reaction environment far from equilibrium, promising to synergistically introduce multiple defects in a single process and endow materials with unique structures. However, the application of existing HTHP technology in the modification of g-C3N4 has a flawed approach. Most research focuses on pursuing extremely high pressures and temperatures to synthesize novel dense-phase carbon nitrides (such as c-C3N4) and obtain superhard materials. This essentially destroys the semiconductor properties of g-C3N4, often resulting in insulating or wide-bandgap materials, completely losing its original purpose as a visible-light photocatalyst. "Phase transition"-oriented research neglects to use HTHP technology as a sophisticated "defect engineering" tool to optimize and enhance the intrinsic photocatalytic activity of pristine g-C3N4. Therefore, there is an urgent need in this field for a novel technical solution that can provide a much stronger physical driving force than conventional methods to overcome the energy barrier of defect formation. This solution should achieve the synergistic and high-concentration introduction of multiple defects (such as oxygen doping and nitrogen vacancies) while maintaining the g-C3N4 semiconductor framework. At the same time, the process is simple and clean, avoiding complex chemical processes and impurity residues. Summary of the Invention

[0004] The purpose of this invention is to overcome the aforementioned shortcomings of existing g-C3N4 modification technologies and provide a relatively simple method for preparing g-C3N4 photocatalysts that requires no complex post-processing and can simultaneously introduce oxygen doping and nitrogen vacancy dual defects. Another objective of this invention is to provide a dual-defect g-C3N4 photocatalyst with excellent visible light absorption and photogenerated charge separation efficiency prepared by the above method.

[0005] A dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies and its preparation method, comprising the following steps: (1) Initial sample preparation: The original g-C3N4 powder is pre-pressed into sheets to obtain the sample, and then loaded into the sample chamber of the high-pressure device; (2) High temperature and high pressure treatment: Place the high pressure device containing the sample in the press, apply pressure to 5.0-6.0 Gpa, then raise the temperature to 200-500 °C under 5.0-6.0 GPa pressure, and keep it at 200-500 °C for 10-120 minutes; (3) Depressurization and cooling: After the heat preservation is completed, stop heating, cool the sample with the furnace to room temperature, release the pressure, and take out the sample; (4) Post-processing: The extracted sample is washed with water, dried and ground to obtain a double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies.

[0006] Further, the original g-C3N4 powder in step (1) is obtained by thermal polymerization of one or more of melamine, urea, thiourea or cyanamide.

[0007] Furthermore, in step (1), the pre-compression pressure of the original g-C3N4 powder is 2-5 MPa, and the diameter of the sample is 15 mm.

[0008] Furthermore, the pressure applied in step (2) is 5.5 GPa.

[0009] Furthermore, in step (2), the heating rate for raising the temperature to 200-500 °C is 50-300 °C / minute.

[0010] Furthermore, the heat preservation and pressure preservation time in step (2) is 30-60 minutes.

[0011] Furthermore, the high-pressure device is a hinged six-sided top press or a diamond anvil cell, and the sealing material of the sample chamber is pyrophyllite and sodium chloride.

[0012] A dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies was prepared according to the above-described preparation method.

[0013] In summary, due to the adoption of the above technical solution, the beneficial effects of the present invention are: 1. Synergistic Effect: This invention simultaneously achieves oxygen doping and nitrogen vacancy defect construction in g-C3N4 using a single high-temperature, high-pressure process. Under ultra-high pressure, the g-C3N4 framework is distorted by strong hydrostatic pressure, leading to the breakage of some unstable CN bonds and the escape of nitrogen atoms in gaseous form, thus forming nitrogen defects. Simultaneously, the electron cloud distribution of the entire π-conjugated system changes, activating and weakening some chemical bonds, making them unstable and reacting with trace amounts of oxygen (such as H2O or O2) in the reaction environment to achieve oxygen doping. The entire process is a dynamic synergistic mechanism: nitrogen defects provide anchoring sites for oxygen doping, while the intervention of oxygen further promotes the breaking of surrounding CN bonds, more effectively regulating the electronic structure of the photocatalyst and promoting the separation and migration of photogenerated carriers.

[0014] 2. Unique conditions and remarkable results: The 5.5 GPa high pressure combined with medium-temperature heat treatment is a non-equilibrium process that can achieve defect structure control that cannot be achieved by atmospheric pressure methods, thereby significantly enhancing the light absorption capacity and photocatalytic activity of the material.

[0015] 3. Green and efficient: This method does not require the introduction of additional chemical dopants or template agents, avoids complicated post-processing cleaning steps, and is simple, efficient and environmentally friendly.

[0016] 4. Significant performance improvement: The prepared dual-defect g-C3N4 photocatalyst has an oxygen atomic doping concentration of 6.7% (measured by XPS) and a significant concentration of nitrogen vacancy defects (proven by the characteristic signal peak at g≈2.003 in the ESR spectrum). Its absorption in the visible light region is significantly enhanced, the photogenerated electron-hole separation efficiency is high, and the fluorescence quenching effect is significant. It exhibits far superior activity to the original g-C3N4 in photocatalytic water splitting. Attached Figure Description

[0017] Figure 1 The image shows a comparison of the XRD patterns of the double-defect g-C3N4 prepared in Example 1 of this invention and the original g-C3N4. Figure 2 The XPS O 1s spectrum of the double-defect g-C3N4 prepared in Example 1 of this invention is shown to demonstrate the presence and chemical state of the O element. Figure 3 The electron paramagnetic resonance (ESR) spectrum of the double-defect g-C3N4 prepared in Example 1 of this invention is shown to prove the existence of N vacancies. Figure 4 This is a comparison of the UV-Vis diffuse reflectance spectra (UV-Vis DRS) of the double-defect g-C3N4 prepared in Example 1 of the present invention and the original g-C3N4 in the comparative example. Figure 5 This is a comparison of the fluorescence spectra of the double-defect g-C3N4 prepared in Example 1 of the present invention under 380nm excitation light. Figure 6 This is a comparison chart of the photocatalytic hydrogen production rates of the double-defect g-C3N4 prepared in Example 1 of the present invention under simulated sunlight irradiation. Detailed Implementation

[0018] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. The components of the embodiments of the present invention described and shown in the accompanying drawings can generally be arranged and designed in various different configurations.

[0019] Therefore, the following detailed description of the embodiments of the invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to illustrate selected embodiments of the invention. All other embodiments obtained by those skilled in the art based on the embodiments of the invention without inventive effort are within the scope of protection of the invention.

[0020] Example 1 Refer to the instruction manual. Figure 1-6; A dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies and its preparation method, comprising the following steps: (1) Using melamine as a precursor, heat treatment was carried out in a muffle furnace at 550°C for 4 hours to obtain the original g-C3N4 powder.

[0021] (2) Take 2 g of the above original g-C3N4 powder, pre-press it into a 15 mm sample sheet under a pressure of 3 MPa, and fill it into the sample cavity of the hinged six-sided top press device sealed with pyrophyllite and sodium chloride.

[0022] (3) Place the hinged six-sided top press device into the press and apply pressure to 5.5 GPa.

[0023] (4) After the pressure stabilizes, raise the temperature to 300 °C at a rate of 100 °C / min and keep it at this temperature and pressure for 30 minutes.

[0024] (5) After the heat preservation is completed, stop heating and let the sample cool naturally to room temperature under pressure, and then release the pressure.

[0025] (6) Take out the sample, wash it with water, dry and grind it to obtain the double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancy.

[0026] Characterization revealed that the catalyst sample had an O atom doping concentration of 6.7%, a strong ESR signal, and an optical band gap of 2.76 eV (compared to 2.81 eV for the original sample). Compared to the original g-C3N4, the visible light absorption edge showed a significant red shift, and the optical band gap decreased by 0.05 eV. Under irradiation with a 300W xenon lamp (equipped with an AM1.5 filter), using methanol as a sacrificial agent, its hydrogen production rate reached 63.4 μmol / h. -1 g -1 It is twice that of the original g-C3N4.

[0027] Example 2 The other conditions were the same as in Example 1, except that the applied pressure in steps (3) and (4) was changed to 6 GPa and the heat treatment temperature was changed to 200 °C, so as to obtain a double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies.

[0028] Example 3 The other conditions are the same as in Example 1, except that the applied pressure in steps (3) and (4) is changed to 3 GPa and the heat treatment temperature is changed to 500 °C, so as to obtain a double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies.

[0029] The above description constitutes an embodiment of the present invention. The foregoing descriptions are preferred embodiments of the present invention. Unless there is a clear contradiction or a prerequisite for a particular preferred embodiment, the preferred embodiments can be arbitrarily combined and used. The embodiments and specific parameters described are merely for clearly illustrating the verification process of the invention and are not intended to limit the scope of patent protection of the present invention. The scope of patent protection of the present invention is still determined by its claims. Similarly, any equivalent structural changes made based on the description and drawings of the present invention should also be included within the scope of protection of the present invention.

Claims

1. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies, characterized in that, Includes the following steps: (1) Initial sample preparation: The original g-C3N4 powder is pre-pressed into sheets to obtain the sample, and then loaded into the sample chamber of the high-pressure device; (2) High temperature and high pressure treatment: Place the high pressure device containing the sample in the press, apply pressure to 5.0-6.0 GPa, then raise the temperature to 200-500 °C under 5.0-6.0 GPa pressure, and keep it at 200-500 °C for 10-120 minutes; (3) Depressurization and cooling: After the heat preservation is completed, stop heating, cool the sample with the furnace to room temperature, release the pressure, and take out the sample; (4) Post-processing: The extracted sample is washed with water, dried and ground to obtain a double-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies.

2. The method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, The original g-C3N4 powder in step (1) is obtained by thermal polymerization of one or more of melamine, urea, thiourea or cyanamide.

3. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, In step (1), the pre-compression pressure of the original g-C3N4 powder is 2-5 MPa, and the diameter of the sample is 15 mm.

4. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, The pressure applied in step (2) is 5.5 GPa.

5. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, In step (2), the heating rate to raise the temperature to 200-500 °C is 50-300 °C / minute.

6. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, The heat preservation and pressure preservation time in step (2) is 30-60 minutes.

7. A method for preparing a dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies according to claim 1, characterized in that, The high-pressure device is a hinged six-sided top press or a diamond anvil cell, and the sealing material of the sample chamber is pyrophyllite and sodium chloride.

8. A dual-defect g-C3N4 photocatalyst with oxygen doping and nitrogen vacancies, characterized in that, Prepared according to the preparation method according to any one of claims 1-7.

Images