A method for synthesizing highly crystalline graphitic carbon nitride and its molecularly induced self-assembly.

Abstract

This invention relates to the field of semiconductor photocatalysis technology, specifically to a highly crystalline graphitic carbon nitride and its molecularly induced self-assembly synthesis method. The molecularly induced self-assembly synthesis method uses graphitic carbon nitride nanosheets as a precursor. Under ultrasonic conditions, the graphitic carbon nitride nanosheets are fully dispersed in a mixed polar solution of water and acetonitrile, with diphenylalanine added as a molecular inducer. After thorough stirring, the mixture is placed in a microwave hydrothermal apparatus for reaction under vacuum. Before the reaction, the graphitic carbon nitride nanosheets undergo at least one pretreatment. The pretreatment method involves sealing the graphitic carbon nitride nanosheets in a tube furnace, continuously introducing ammonia gas, heating to 500-550°C, holding at that temperature, and then allowing it to cool naturally. The graphitic carbon nitride obtained by this invention has a hexagonal dense structure, high crystallinity, low porosity, and visible light response. Furthermore, its surface contains abundant amino and hydroxyl functional groups, exhibiting excellent photocatalytic hydrogen production activity under visible light.

Description

Technical Field

[0001] This invention relates to the field of semiconductor photocatalysis technology, specifically to a highly crystalline graphitic carbon nitride and its molecularly induced self-assembly synthesis method. Background Technology

[0002] Photocatalytic water splitting for hydrogen production shows great potential in alleviating energy and environmental problems such as the fossil fuel crisis and environmental pollution. In photocatalytic reactions, high-performance photocatalysts exhibit two characteristics: internal crystallinity and surface defects. These two characteristics synergistically determine photocatalytic activity. The former mainly affects the scattering process of photoelectrons in the periodic lattice field of the photocatalyst, while the latter can provide abundant catalytic active sites for the photocatalytic reaction. In fact, good crystallinity within the photocatalyst facilitates the migration and transport of photoelectrons in the periodic potential field of the lattice, thereby reducing energy loss during the migration from the interior to the surface. However, many polymer photocatalysts generally have poor crystallinity, and their electrons encounter strong scattering during migration, leading to unfavorable transfer and migration of photoelectrons, and thus hindering the improvement of photocatalytic performance.

[0003] Therefore, developing novel synthesis methods for highly crystalline polymers is of research significance and innovative value for improving this type of photocatalyst. Summary of the Invention

[0004] To address the technical problem of poor photocatalytic performance caused by poor crystallinity, this invention provides a highly crystalline graphitic carbon nitride and its molecularly induced self-assembly synthesis method. The obtained graphitic carbon nitride has a hexagonal dense structure, high crystallinity, low porosity, and visible light response. Furthermore, its surface contains abundant amino and hydroxyl functional groups, exhibiting excellent photocatalytic hydrogen production activity under visible light.

[0005] In a first aspect, the present invention provides a molecularly induced self-assembly synthesis method for highly crystalline graphitic carbon nitride, using graphitic carbon nitride nanosheets as a precursor, dispersing the graphitic carbon nitride nanosheets fully in a mixed polar solution of water and acetonitrile under ultrasonic conditions, adding diphenylalanine as a molecular inducer, stirring thoroughly, and then placing the mixture in a microwave hydrothermal instrument to react under vacuum.

[0006] Before the reaction, the graphitic carbon nitride nanosheets are subjected to at least one pretreatment. The pretreatment method is to seal the graphitic carbon nitride nanosheets in a tube furnace, continuously introduce ammonia gas, raise the temperature to 500~550℃, hold the temperature, and then cool naturally.

[0007] Furthermore, the ammonia flow rate is 2~4 mL / min.

[0008] Furthermore, the heat preservation time is 3 to 5 hours.

[0009] Furthermore, the pretreatment of graphitic carbon nitride nanosheets is performed 3 to 4 times. Repeated pretreatment can more fully thin and shear the graphitic carbon nitride nanosheets, thereby obtaining more small-scale graphene carbon nitride nanosheets.

[0010] Furthermore, the mass ratio of graphitic carbon nitride nanosheets to diphenylalanine is 10~15:0.21~0.38.

[0011] Furthermore, in the mixed polar solution of water and acetonitrile, the molar ratio of water to acetonitrile is 1:1 to 1.25.

[0012] Furthermore, the reaction conditions are: vacuum degree ≤ 8 Pa, microwave power 180~200 W, reaction time 2 h, and reaction temperature 180 °C.

[0013] Furthermore, after the reaction was completed, the product was purified by dialysis and then freeze-dried under vacuum to obtain highly crystalline graphitic carbon nitride.

[0014] Furthermore, the dialysis bags used in the dialysis method have a cutoff molecular weight of 100~500 Daltons, and the vacuum degree of vacuum freeze drying is ≤15Pa and the temperature is ≤-50℃.

[0015] Secondly, the present invention provides a highly crystalline graphitic carbon nitride prepared by the above-mentioned molecularly induced self-assembly synthesis method.

[0016] The technical principle of this invention is as follows:

[0017] Ammonia molecules can disrupt the NC bonds of graphitic carbon nitride nanosheets at high temperatures, thinning nanosheets smaller than 100 nm. Repeated pretreatment helps improve the yield of these extremely small nanosheets. Furthermore, the pretreated nanosheets are rich in amino and hydroxyl groups, enabling them to bind with diphenylalanine (rich in amino groups) via hydrogen bonds. Under vacuum-microwave hydrothermal activation, this induces the self-assembly of graphitic carbon nitride nanosheets into a hexagonal dense structure, increasing the crystallinity of the graphitic carbon nitride. Simultaneously, the vacuum conditions allow dissolved oxygen to precipitate from the reaction environment, reducing interfacial partial pressure and surface tension of solvent molecules, thus promoting the reaction.

[0018] The beneficial effects of this invention are as follows:

[0019] This invention synthesizes highly crystalline graphitic carbon nitride with a hexagonal dense microstructure through molecular-induced self-assembly. Graphitic carbon nitride has low porosity, exhibits visible light response, and its surface contains abundant amino and hydroxyl functional groups, resulting in stronger reducibility of its photogenerated electrons. The high crystallinity of graphitic carbon nitride leads to fewer obstacles encountered during photoelectron migration, resulting in high photoelectron-hole separation efficiency and excellent photocatalytic hydrogen production activity under visible light.

[0020] This invention provides an innovative synthesis method for developing high-performance, highly crystalline polymer photocatalysts. Attached Figure Description

[0021] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, for those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0022] Figure 1 These are scanning electron microscope (SEM) images of graphitic carbon nitride nanosheets and graphitic carbon nitride microsheets from Example 1.

[0023] Figure 2 This is an electron microscope image of the highly crystalline graphitic carbon nitride photocatalyst in Example 1.

[0024] Figure 3 This is an elemental surface distribution diagram of the highly crystalline graphitic carbon nitride photocatalyst in Example 1.

[0025] Figure 4 The image shows the X-ray diffraction and infrared spectra of the highly crystalline graphitic carbon nitride photocatalyst and graphitic carbon nitride nanosheets in Example 1.

[0026] Figure 5 The X-ray photoelectron spectra of the highly crystalline graphitic carbon nitride photocatalyst and the graphitic carbon nitride nanosheets in Example 1 are shown.

[0027] Figure 6 These are the light absorption curves and band structure diagrams of the highly crystalline graphitic carbon nitride photocatalyst and graphitic carbon nitride nanosheets in Example 1.

[0028] Figure 7 This is a comparison diagram of the pore structures of the highly crystalline graphitic carbon nitride photocatalyst and the graphitic carbon nitride nanosheets in Example 1.

[0029] Figure 8 This is a comparison diagram of the photocatalytic hydrogen production performance and carrier dynamics of the highly crystalline graphitic carbon nitride photocatalyst and the graphitic carbon nitride nanosheets in Example 1.

[0030] Figure 9 This is a scanning electron microscope image of the highly crystalline graphitic carbon nitride photocatalyst in Example 2.

[0031] Figure 10 This is a scanning electron microscope image of the graphitic carbon nitride photocatalyst in Comparative Example 1.

[0032] Figure 11 This is a scanning electron microscope image of the graphitic carbon nitride photocatalyst in Comparative Example 2.

[0033] Figure 12 This is a scanning electron microscope image of the graphitic carbon nitride photocatalyst in Comparative Example 3. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of the embodiments of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this invention, and not all embodiments. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of this invention.

[0035] In the specific embodiments of the present invention, the graphitic carbon nitride used is prepared by urea pyrolysis.

[0036] In a specific embodiment of the present invention, the microwave hydrothermal instrument is connected to a vacuum pump. The vacuum pump is used to evacuate the reaction chamber of the microwave hydrothermal instrument. During the reaction, the hydrothermal reactor is placed inside the reaction chamber of the microwave hydrothermal instrument, the vacuum pump is turned on, and after the vacuum degree inside the reaction chamber reaches the experimental requirements, the microwave source and heating element are turned on to carry out the microwave hydrothermal reaction.

[0037] Example 1

[0038] (1) The precursor graphitic carbon nitride nanosheets (UCN) were sealed in a tube furnace and ammonia gas was continuously introduced at a rate of 2.5 mL / min. The tube furnace was then heated to 500 °C and held for 4 h. After natural cooling, the above operation was repeated 3 times to obtain a yield of graphitic carbon nitride nanosheets of more than 90%.

[0039] (2) Take 10 mg of graphitic carbon nitride nanosheets and disperse them thoroughly in a mixed polar solution of water and acetonitrile at a molar ratio of 1:1.1 under ultrasonic conditions. Add 0.3 mg of diphenylalanine as a molecular inducer. After thorough stirring, place the solution in a microwave hydrothermal apparatus for self-assembly under vacuum. The reaction conditions are set as follows: vacuum degree 8 Pa, microwave power 180 W, reaction time 2 h, and reaction temperature 180 °C. After the reaction, the sample is purified by dialysis. The molecular weight cutoff of the dialysis bag is 300 Daltons. The vacuum degree of vacuum freeze-drying is 15 Pa, and the temperature is -50 °C to obtain highly crystalline graphitic carbon nitride photocatalyst (HPCN).

[0040] Scanning electron microscopy was used to observe the precursor graphitic carbon nitride nanosheets and the pretreated graphitic carbon nitride nanosheets, respectively. The results are as follows: Figure 1 As shown in (a) and (b), graphitic carbon nitride nanosheets are formed into extremely small graphitic carbon nitride nanosheets with a size of a few nanometers after shearing and thinning under the high temperature environment of ammonia gas flow.

[0041] The highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) was observed using scanning electron microscopy and transmission electron microscopy. The results are as follows: Figure 2 As shown, the highly crystalline graphitic carbon nitride photocatalyst exhibits a hexagonal dense structure with a hexagonal prism shape. High-resolution transmission electron microscopy results reveal its clear lattice fringes, indicating high crystallinity.

[0042] The elemental distribution of the highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) was analyzed, and the results are as follows: Figure 3 As shown, highly crystalline graphitic carbon nitride is mainly composed of carbon and nitrogen elements.

[0043] X-ray diffraction and infrared spectroscopy were performed on the highly crystalline graphitic carbon nitride photocatalyst and the precursor graphitic carbon nitride nanosheets prepared in step (2). Figure 4 As shown in (a), the XRD diffraction pattern of the highly crystalline graphitic carbon nitride photocatalyst is sharp and the peak half-width at half-maximum is narrow, indicating its high crystallinity. Figure 4 As shown in (b), the infrared spectrum shows that the surface of the highly crystalline graphitic carbon nitride photocatalyst contains abundant hydroxyl and amino groups. These polar groups can form strong bonds with the inducing molecules during the reaction.

[0044] X-ray photoelectron spectroscopy analysis was performed on the highly crystalline graphitic carbon nitride photocatalyst and the precursor graphitic carbon nitride nanosheets prepared in step (2). The results are as follows: Figure 5 As shown, the highly crystalline graphitic carbon nitride synthesized using diphenylalanine as a molecular inducing agent has the same molecular structure as ordinary carbon nitride.

[0045] Comparing the light absorption curves and band structures of the highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) and the precursor graphitic carbon nitride nanosheets, the results are as follows: Figure 6 As shown, compared with the precursor graphitic carbon nitride nanosheets, the absorption sideband of highly crystalline graphitic carbon nitride is red-shifted to 450 nm. Simultaneously, the band structure reveals that the conduction band of highly crystalline graphitic carbon nitride is more negative, indicating stronger photoreduction of its photogenerated electrons, which helps improve photocatalytic performance.

[0046] Comparing the pore structures of the highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) and the precursor graphitic carbon nitride nanosheets, the results are as follows: Figure 7 As shown, the highly crystalline graphitic carbon nitride phase has a low porosity and a relatively dense structure, which is consistent with the SEM results.

[0047] The photocatalytic hydrogen production performance of the highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) and the precursor graphitic carbon nitride nanosheets was compared, and the carrier kinetics of the two materials were studied. Figure 8 As shown in (a), under visible light irradiation, the photocatalytic hydrogen production performance of highly crystalline graphitic carbon nitride reaches 1856 μmol·h⁻¹. -1 ·g -1 The performance is significantly higher than that of the precursor graphitic carbon nitride nanosheets. For example... Figure 8 As shown in (b), after five cycles of photocatalytic performance testing, the photocatalytic performance of highly crystalline graphitic carbon nitride showed almost no degradation, indicating its high stability. Figure 8 As shown in (c) and (d), photoelectron dynamics tests indicate that highly crystalline carbon nitride has higher electron-hole separation efficiency and photoelectron transmission efficiency.

[0048] Example 2

[0049] (1) The graphite phase carbon nitride nanosheets were sealed in a tube furnace and ammonia gas was continuously introduced at a rate of 2.5 mL / min. Then the tube furnace was heated to 500 °C and kept at that temperature for 4 h. After natural cooling, the above operation was repeated 3 times to obtain a graphite phase carbon nitride nanosheet yield of more than 90%.

[0050] (2) Take 10 mg of graphitic carbon nitride nanosheets and disperse them thoroughly in a mixed polar solution of water and acetonitrile at a molar ratio of 1:1.1 under ultrasonic conditions. Add 0.3 mg of diphenylalanine as a molecular inducer. After thorough stirring, place the solution in a microwave hydrothermal apparatus for self-assembly under vacuum. The reaction conditions are set as follows: vacuum degree 8 Pa, microwave power 200 W, reaction time 2 h, and reaction temperature 180 °C. After the reaction, the sample is purified by dialysis. The molecular weight cutoff of the dialysis bag is 300 Daltons. The vacuum degree of vacuum freeze-drying is 15 Pa, and the temperature is -50 °C to obtain highly crystalline graphitic carbon nitride photocatalyst (HPCN).

[0051] The highly crystalline graphitic carbon nitride photocatalyst prepared in step (2) was observed using scanning electron microscopy, and the results are as follows: Figure 9 As shown, when the microwave power is increased to 200W, the prepared highly crystalline graphitic carbon nitride still has a hexagonal dense junction, but its diameter is larger, indicating that increasing the microwave power can cause the graphitic carbon nitride to grow excessively.

[0052] Comparative Example 1

[0053] (1) The graphite phase carbon nitride nanosheets were sealed in a tube furnace and ammonia gas was continuously introduced at a rate of 2.5 mL / min. Then the tube furnace was heated to 500 °C and kept at that temperature for 4 h. After natural cooling, the above operation was repeated 3 times to obtain a graphite phase carbon nitride nanosheet yield of more than 90%.

[0054] (2) Take 10 mg of graphitic carbon nitride nanosheets and disperse them thoroughly in a mixed polar solution of water and acetonitrile with a molar ratio of 1:0.8 under ultrasonic conditions. Add 0.3 mg of diphenylalanine as a molecular inducer. After thorough stirring, place the solution in a microwave hydrothermal apparatus for self-assembly under vacuum. The reaction conditions are set as follows: vacuum degree 8 Pa, microwave power 180 W, reaction time 2 h, and reaction temperature 180 °C. After the reaction, the sample is purified by dialysis. The molecular weight cutoff of the dialysis bag is 300 Daltons. The vacuum degree of vacuum freeze-drying is 15 Pa, and the temperature is -50 °C to obtain the graphitic carbon nitride photocatalyst.

[0055] The graphitic carbon nitride photocatalyst prepared in step (2) was observed using scanning electron microscopy, and the results are as follows: Figure 10 As shown in the figure. In Comparative Example 1, the molar ratio of water to acetonitrile was adjusted to 1:0.8. The resulting graphite-like carbon nitride exhibited an inhomogeneous and irregular structure, failing to form a regular hexagonal dense packing structure. This indicates that the appropriate polarity of the solvent plays a crucial role in the formation of a hexagonal dense structure of highly crystalline graphitic carbon nitride.

[0056] Comparative Example 2

[0057] (1) The graphite phase carbon nitride nanosheets were sealed in a tube furnace and ammonia gas was continuously introduced at a rate of 2.5 mL / min. Then the tube furnace was heated to 500 °C and kept at that temperature for 4 h. After natural cooling, the above operation was repeated 3 times to obtain a graphite phase carbon nitride nanosheet yield of more than 90%.

[0058] (2) Take 10 mg of graphitic carbon nitride nanosheets and disperse them thoroughly in a mixed polar solution of water and acetonitrile at a molar ratio of 1:1.1 under ultrasonic conditions. After thorough stirring, place the solution in a microwave hydrothermal apparatus for self-assembly under vacuum. The reaction conditions are set as follows: vacuum degree 8 Pa, microwave power 180 W, reaction time 2 h, and reaction temperature 180 °C. After the reaction, the sample is purified by dialysis. The molecular weight cutoff of the dialysis bag is 300 Daltons. The vacuum degree of the freeze-drying is 15 Pa, and the temperature is -50 °C to obtain the graphitic carbon nitride photocatalyst.

[0059] The graphitic carbon nitride photocatalyst prepared in step (2) was observed using scanning electron microscopy, and the results are as follows: Figure 11 As shown in the figure. In Comparative Example 2, the molecular inducer diphenylalanine was not added, and the results showed that a hexagonal prism-shaped highly crystalline graphitic carbon nitride photocatalyst could not be formed without the addition of diphenylalanine.

[0060] Comparative Example 3

[0061] (1) The graphite phase carbon nitride nanosheets were sealed in a tube furnace and ammonia gas was continuously introduced at a rate of 2.5 mL / min. Then the tube furnace was heated to 500 °C and kept at that temperature for 4 h. After natural cooling, the above operation was repeated 3 times to obtain a graphite phase carbon nitride nanosheet yield of more than 90%.

[0062] (2) Take 10 mg of graphitic carbon nitride nanosheets and disperse them thoroughly in a mixed polar solution of water and acetonitrile at a molar ratio of 1:1.1 under ultrasonic conditions. Add 0.3 mg of diphenylalanine as a molecular inducer. After thorough stirring, place the solution in a microwave hydrothermal apparatus for self-assembly under vacuum. The reaction conditions are set as follows: vacuum degree 8 Pa, microwave power 150 W, reaction time 2 h, and reaction temperature 180 °C. After the reaction, the sample is purified by dialysis. The molecular weight cutoff of the dialysis bag is 300 Daltons. The vacuum degree of vacuum freeze-drying is 15 Pa, and the temperature is -50 °C to obtain the graphitic carbon nitride photocatalyst.

[0063] The graphitic carbon nitride photocatalyst prepared in step (2) was observed using scanning electron microscopy, and the results are as follows: Figure 12 As shown in the figure, in Comparative Example 3, when the microwave power was reduced to 150 W, the prepared graphitic carbon nitride photocatalyst exhibited neither a distinct hexagonal dense structure nor a clear columnar structure, resulting in an uneven structure. This indicates that low-energy microwave power is detrimental to the activation of the precursor's self-assembly into highly crystalline graphitic carbon nitride.

[0064] Although the present invention has been described in detail with reference to the accompanying drawings and preferred embodiments, the present invention is not limited thereto. Various equivalent modifications or substitutions can be made to the embodiments of the present invention by those skilled in the art without departing from the spirit and essence of the invention, and such modifications or substitutions should all be within the scope of the present invention. Any variations or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention should also be covered within the protection scope of the present invention.

Claims

1. A method for the molecularly induced self-assembly synthesis of highly crystalline graphitic carbon nitride, characterized in that, Using graphitic carbon nitride nanosheets as a precursor, the graphitic carbon nitride nanosheets were fully dispersed in a mixed polar solution of water and acetonitrile under ultrasonic conditions, and diphenylalanine was added as a molecular inducer. After thorough stirring, the mixture was placed in a microwave hydrothermal apparatus and reacted under vacuum with a microwave power of 180~200W. Before the reaction, the graphitic carbon nitride nanosheets are subjected to at least one pretreatment. The pretreatment method is to seal the graphitic carbon nitride nanosheets in a tube furnace, continuously introduce ammonia gas, raise the temperature to 500~550℃ and hold it at that temperature, and then cool it naturally. In a mixed polar solution of water and acetonitrile, the molar ratio of water to acetonitrile is 1:1 to 1.

25.

2. The molecularly induced self-assembly synthesis method as described in claim 1, characterized in that, The ammonia flow rate is 2~4 mL / min.

3. The molecularly induced self-assembly synthesis method as described in claim 1, characterized in that, The heat preservation time is 3 to 5 hours.

4. The molecularly induced self-assembly synthesis method as described in claim 1, characterized in that, The pretreatment of graphitic carbon nitride nanosheets is performed 3 to 4 times.

5. The molecularly induced self-assembly synthesis method as described in claim 1, characterized in that, The mass ratio of graphitic carbon nitride nanosheets to diphenylalanine is 10~15:0.21~0.

38.

6. The molecularly induced self-assembly synthesis method according to claim 1, characterized in that, The reaction conditions were: vacuum degree ≤ 8 Pa, reaction time 2 h, and reaction temperature 180 °C.

7. The molecularly induced self-assembly synthesis method according to claim 1, characterized in that, After the reaction was completed, the product was purified by dialysis and then freeze-dried under vacuum to obtain highly crystalline graphitic carbon nitride.

8. The molecularly induced self-assembly synthesis method as described in claim 7, characterized in that, The dialysis bags used in the dialysis method have a cutoff molecular weight of 100~500 Daltons, and the vacuum degree of vacuum freeze drying is ≤15Pa and the temperature is ≤-50℃.

9. A highly crystalline graphitic carbon nitride prepared by the molecularly induced self-assembly synthesis method as described in any one of claims 1 to 8.

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