Preparation method and application of porous carbon nitride microspheres

Abstract

The present invention belongs to the field of photocatalytic hydrogen production technology, discloses a preparation method of L-leucine-doped porous carbon nitride microspheres, comprising the following steps: L-leucine and thiourea are dissolved in water, stirred evenly, and an aqueous solution of L-leucine and thiourea is obtained; acetic acid is added to the obtained aqueous solution and stirred evenly, and then chitosan is added, and vigorously stirred until the chitosan dissolves to form an acetic acid hydrogel of chitosan containing L-leucine and thiourea; the obtained hydrogel is subjected to a hydrothermal reaction, and after the reaction is complete, it is cooled, centrifuged, washed with water, washed with alcohol, and dried to obtain a precursor; the obtained precursor is subjected to a heat treatment to obtain a porous carbon nitride microsphere composed of porous nanosheets interconnected. The porous carbon nitride microspheres obtained by the inventive method have a high specific surface area, a rich characteristic of active sites, and the carbon nitride microstructure and energy level structure can be regulated by changing the heat treatment time, showing a high visible light catalytic hydrogen production rate and good cycle stability.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic hydrogen production and relates to a preparation method and application of porous carbon nitride microspheres. Background Art

[0002] With the rapid development of industry, energy demand is increasing. The combustion of fossil fuels releases large amounts of nitrogen oxides, sulfur oxides, and carbon oxides into the atmosphere, causing numerous problems, including air pollution and the greenhouse effect. Using semiconductor photocatalysis to convert solar energy into hydrogen is an important approach to alleviating the energy crisis and environmental pollution. The principle is that semiconductor photocatalysts, when exposed to a certain light source, generate electron-hole pairs. When the electrons migrate to the catalyst surface, they reduce water to hydrogen, while the holes are consumed by a sacrificial agent. Therefore, the key to achieving hydrogen production through photocatalytic water splitting is to find a photocatalyst with an appropriate energy band, stable and efficient performance, and low cost.

[0003] Graphitized carbon nitride (g-C3N4) semiconductor photocatalysts are widely used in various photocatalytic research and development fields due to their advantages, such as a two-dimensional π-conjugated structure, a suitable band structure, good visible light response, and excellent thermal and chemical stability. Currently, graphitized carbon nitride is mainly prepared using nitrogen-rich compounds (such as melamine, monocyanamide, dicyandiamide, urea, and thiourea) as precursors through methods such as high-temperature thermal polycondensation, solvothermal methods, and chemical vapor deposition. However, as a photocatalytic material, g-C3N4 suffers from the difficulty of separating photogenerated electron-hole pairs, low photogenerated carrier mobility, and low electrical conductivity, which limits its application.

[0004] In recent years, structural modification, morphology control, and heterostructure construction based on carbon nitride have become research hotspots. Molecular doping is a modification method similar to elemental doping. Since g-C3N4 is synthesized by polycondensation of multiple synthetic monomers, molecular doping is to dope organic molecules that match the structure of the synthetic monomers into the synthetic monomers of graphitic carbon nitride. By introducing doping molecules into the graphitic carbon nitride skeleton through co-condensation, the photoresponse range can be broadened, the degree of electron delocalization of the catalyst can be changed, the transport capacity of photogenerated carriers can be significantly enhanced, and the electronic and optical properties of the catalyst can be improved, thereby achieving the purpose of improving the performance of carbon nitride.

[0005] For example, patent application CN113318769A discloses a method for preparing and applying an amino acid-doped carbon nitride photocatalyst. The morphology of a catalyst precursor doped with molecular amino acids (such as phenylalanine, tryptophan, tyrosine, histidine, proline, and cystine) is controlled through a hydrothermal reaction to form a rod-shaped precursor. This precursor, upon pyrolysis under nitrogen, forms tubular carbon nitride, thereby improving light utilization and the separation efficiency of photogenerated electrons and holes, thereby enhancing photocatalytic performance. However, the carbon nitride is tubular in shape, and the degradation of rhodamine B has only been tested.

[0006] Therefore, it is necessary to find a preparation method to improve the photocatalytic hydrogen production performance of carbon nitride. Summary of the Invention

[0007] The present invention addresses the technical problem of low visible light hydrogen production performance of existing carbon nitride and provides a method for preparing L-leucine-doped porous carbon nitride microspheres. The porous carbon nitride microspheres can be obtained, which are composed of interconnected porous nanosheets. The microspheres have high specific surface area and abundant active sites, and can significantly improve the photogenerated charge separation efficiency of carbon nitride. In addition, the microstructure and energy level structure of carbon nitride can be controlled by changing the heat treatment time, showing a high visible light catalytic hydrogen production rate and good cycle stability.

[0008] To achieve the above object, the present invention adopts the following technical solutions:

[0009] In a first aspect, the present invention provides a method for preparing porous carbon nitride microspheres, comprising the following steps:

[0010] Step 1): Dissolve L-leucine and thiourea in water and stir evenly to obtain an aqueous solution of L-leucine and thiourea;

[0011] Step 2): Add acetic acid to the aqueous solution obtained in step 1 and stir thoroughly, then add chitosan and stir vigorously until the chitosan is dissolved to form an acetate hydrogel of chitosan containing L-leucine and thiourea;

[0012] Step 3): The hydrogel obtained in step 2 is subjected to a hydrothermal reaction. After the reaction is complete, the hydrogel is cooled, centrifuged, washed with water, washed with alcohol, and dried to obtain a precursor;

[0013] Step 4): The precursor obtained in step 3 is heat-treated to obtain porous carbon nitride microspheres composed of interconnected porous nanosheets.

[0014] In one technical solution, the mass ratio of L-leucine to thiourea in step 1) is 1:0.7-1.5.

[0015] In one technical solution, the mass ratio of chitosan to thiourea in step 2) is 1-2:1.

[0016] In one technical solution, the ratio of acetic acid to chitosan in step 2) is 1-3 ml: 1 g.

[0017] In one technical solution, the temperature of the hydrothermal reaction in step 3) is 160-180° C., and the reaction time is 8-14 h.

[0018] In one technical solution, the specific operation of the heat treatment in step 4) is: heating to 400-640°C at a rate of 1-10°C / min, keeping the temperature for 1-24 hours, and naturally cooling to room temperature.

[0019] In a second aspect, the present invention provides the use of porous carbon nitride microspheres prepared by the above-mentioned method for preparing porous carbon nitride microspheres in visible light photocatalytic hydrogen production.

[0020] Compared with the prior art, the present invention has the following beneficial effects:

[0021] (1) The design of the present invention is ingenious. L-leucine is used as a molecular polymer to dope thiourea, and chitosan is added to form a gel under acidic conditions. L-leucine is grafted onto the long-chain chitosan molecules through hydrogen bonding. Under hydrothermal conditions, chitosan is condensed into spheres in the form of dehydration condensation between hydroxyl groups to obtain a micron-sized spherical precursor. Then, porous carbon nitride microspheres are obtained after a simple heat treatment.

[0022] (2) The porous carbon nitride microspheres prepared by the present invention are composed of porous two-dimensional sheets dominated by mesopores connected to each other. They have the characteristics of high specific surface area and rich active sites, which can significantly improve the efficiency of photogenerated charge separation of carbon nitride. In addition, the microstructure and energy level structure of carbon nitride are related to the heat treatment time. The microstructure and energy level structure of carbon nitride can be regulated by changing the heat treatment time.

[0023] (3) The porous carbon nitride microspheres prepared by the present invention are used for visible light photocatalytic hydrogen production, showing a high visible light photocatalytic hydrogen production rate and good cycle stability. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 These are SEM images of the precursor and porous carbon nitride microspheres prepared in Examples 1 and 2 of the present invention.

[0025] Figure 2 These are SEM images of the precursors and porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0026] Figure 3 XRD patterns of porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0027] Figure 4 The UV-visible absorption spectra and corresponding Tauc curves of the porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0028] Figure 5 This is the XPS valence band spectrum of the porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0029] Figure 6 This is a model Schottky diagram of the porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0030] Figure 7 Schematic diagram of the energy band structure of porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0031] Figure 8 Steady-state fluorescence spectra of porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0032] Figure 9 Transient fluorescence spectra of porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0033] Figure 10 These are the photocurrent diagrams of the porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention.

[0034] Figure 11 This is a graph showing the visible light photocatalytic hydrogen production rate of carbon nitride prepared in Examples 3 to 5 of the present invention. DETAILED DESCRIPTION

[0035] The following examples are used to illustrate the present invention but are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the technical means used in the examples are conventional means well known to those skilled in the art. The test methods in the following examples are all conventional methods unless otherwise specified.

[0036] Example 1

[0037] (1) Add 1 g of L-leucine and 1 g of thiourea to 60 mL of water and stir at room temperature for 30 min to obtain an aqueous solution of L-leucine and thiourea.

[0038] (2) Add 2 mL of acetic acid and 0.7 g of chitosan to the aqueous solution of L-leucine and thiourea, and stir vigorously until the chitosan is dissolved to form an acetic acid hydrogel of chitosan containing L-leucine and thiourea.

[0039] (3) The hydrogel is subjected to a hydrothermal reaction at 180°C for 12 h, and then centrifuged, washed with water, washed with alcohol, and dried to obtain the precursor.

[0040] (4) Place the precursor in a muffle furnace at 5°C min -1 The temperature was raised to 550 °C and kept for 16 h, and then naturally cooled to room temperature to obtain porous carbon nitride microspheres.

[0041] Example 2

[0042] This embodiment is basically the same as embodiment 1, except that the amount of chitosan used in step 2) is 1.7 g.

[0043] Example 3

[0044] This example is basically the same as Example 1, except that the amount of chitosan used in step 2) is 1 g, the holding time in step 4) is 8 h, and the temperature is naturally lowered to room temperature to obtain porous carbon nitride microspheres, which are recorded as g-CN-8.

[0045] Example 4

[0046] This embodiment is basically the same as embodiment 3, except that the holding time in step 4) is 16 h, and the obtained porous carbon nitride is denoted as g-CN-16.

[0047] Example 5

[0048] This embodiment is basically the same as embodiment 3, except that the holding time in step 4) is 24 h, and the obtained porous carbon nitride is denoted as g-CN-24.

[0049] The SEM images of the precursor and porous carbon nitride microspheres prepared in Examples 1 and 2 of the present invention are as follows: Figure 1 As shown, Figure 1 Figures a and b are SEM images of the precursor and porous carbon nitride microspheres prepared in Example 1, respectively. Figure 1 Figures c and d are SEM images of the precursor and porous carbon nitride microspheres prepared in Example 2, respectively. As can be seen from the figure, the diameter of the precursor microspheres is about 5 μm; the porous carbon nitride microspheres are composed of interconnected porous nanosheets and have a diameter of about 4 μm.

[0050] The SEM images of the precursors and porous carbon nitride microspheres prepared in Examples 3 to 5 of the present invention are as follows: Figure 2 As shown, Figure 2 Where a is the precursor prepared in Example 3, Figure 2 Figures b through d represent the porous carbon nitride microspheres prepared in Examples 3 through 5, respectively. As can be seen from the figures, the precursor microspheres have a diameter of approximately 5 μm; the porous carbon nitride microspheres are composed of interconnected porous nanosheets and have a diameter of approximately 4 μm. After heat treatment for 24 h, the porous carbon nitride microspheres decompose into nanosheets and no longer retain the microsphere structure.

[0051] Figure 3 XRD patterns of porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figure, the diffraction peaks at 2θ = 13° and 26.5° correspond to the (100) and (002) crystal planes of carbon nitride, indicating the successful preparation of carbon nitride.

[0052] Figure 4 UV-visible absorption spectra and corresponding Tauc plots for the porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figure, with increasing heat treatment time, the absorption band edge of carbon nitride red-shifts from 461 nm to 540 nm, with corresponding band gaps of 2.72, 2.58, and 2.4 eV, respectively.

[0053] Figure 5 The XPS valence band spectra of the porous carbon nitride microspheres prepared in Examples 3 to 5 show that the valence band position of the carbon nitride obtained with different heat treatment times is 1.76 eV.

[0054] Figure 6 Model Schottky plots of porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figures, with increasing heat treatment time, the flat band potentials of carbon nitride decrease to 0.063, -0.14, and -0.38 eV (V vs. RHE), respectively.

[0055] Figure 7 Schematic diagram of the energy band structure of porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figure, with increasing heat treatment time, the position of the carbon nitride conduction band gradually decreases, and the flat band potential gradually increases, closer to the conduction band, indicating that the catalyst energy level structure can be controlled by adjusting the heat treatment time.

[0056] Figure 8 Steady-state fluorescence spectra of porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figure, g-CN-16 prepared with a heat treatment time of 16 hours exhibits the best photogenerated charge separation efficiency. This is mainly due to the small band gap and the microsphere structure composed of interconnected two-dimensional nanosheets, which provides a convenient transmission channel for the generation and rapid transfer of photogenerated charges.

[0057] Figure 9 Transient fluorescence spectra of porous carbon nitride microspheres prepared in Examples 3 to 5. As can be seen from the figure, g-CN-16 prepared with a heat treatment time of 16 h exhibits the longest photogenerated charge lifetime, which again demonstrates that the microsphere structure composed of interconnected two-dimensional nanosheets helps reduce the recombination rate of photogenerated charges.

[0058] Figure 10 The photocurrent graphs of the porous carbon nitride microspheres prepared in Examples 3 to 5 show that the g-CN-16 prepared with a heat treatment time of 16 h has the best photocurrent response compared to carbon nitride prepared with other heat treatment times.

[0059] Photocatalytic testing system: Beijing Perfect Photocatalytic 6A system. Light source: Beijing Perfect Photocatalytic 300w Xe lamp with a 420 nm filter. Gas chromatograph (GC): Shanghai Tianmei GC-7900. Triethanolamine was used as the sacrificial reagent in the visible light photocatalytic hydrogen evolution solution. Pt was used as the co-catalyst for the visible light photocatalytic hydrogen evolution test. Figure 11 The graph shows the visible light photocatalytic hydrogen production rate of carbon nitride prepared in Examples 1 to 3. As can be seen from the figure, the visible light photocatalytic hydrogen production rate of carbon nitride prepared when the heat treatment time is 16 h is the best, reaching 11.21 mmol g -1 h -1 .

[0060] The embodiments described above are only preferred embodiments of the present invention and are only used to explain the present invention, not to limit the scope of implementation of the present invention. For those skilled in the art, it is of course possible to easily make other implementation methods by replacing or changing the technical content disclosed in this specification. Therefore, all changes and improvements made on the principles of the present invention should be included in the scope of the patent application of the present invention.

Claims

1. A method for preparing porous carbon nitride microspheres, characterized in that: The following steps are involved: Step 1): Dissolve L-leucine and thiourea in water and stir evenly to obtain an aqueous solution of L-leucine and thiourea; Step 2): Add acetic acid to the aqueous solution obtained in step 1) and stir thoroughly, then add chitosan and stir vigorously until the chitosan is dissolved to form an acetate hydrogel of chitosan containing L-leucine and thiourea; Step 3): The hydrogel obtained in step 2) is subjected to a hydrothermal reaction. After the reaction is complete, the hydrogel is cooled, centrifuged, washed with water, washed with alcohol, and dried to obtain a precursor; Step 4): The precursor obtained in step 3) is subjected to heat treatment to obtain porous carbon nitride microspheres composed of interconnected porous nanosheets.

2. The preparation method according to claim 1, characterized in that The mass ratio of L-leucine to thiourea in step 1) is 1:0.7-1.

5.

3. The preparation method according to claim 1, characterized in that The mass ratio of chitosan to thiourea in step 2) is 1-2:

1.

4. The preparation method according to claim 1, characterized in that The ratio of acetic acid to chitosan in step 2) is 1-3 ml:1 g.

5. The preparation method according to claim 1, characterized in that In step 3), the temperature of the hydrothermal reaction is 160-180° C., and the reaction time is 8-14 h.

6. The preparation method according to claim 1, characterized in that The specific operation of the heat treatment in step 4) is: heating to 400-640°C at a rate of 1-10°C / min, keeping the temperature for 1-24 hours, and then cooling naturally to room temperature.

7. Use of the porous carbon nitride microspheres prepared by the preparation method according to any one of claims 1 to 6 in visible light photocatalytic hydrogen production.

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