Preparation method and application of space charge separation type carbon nitride crystal

Abstract

The invention discloses a preparation method and application of a space charge separation type carbon nitride nanocrystal. According to the present invention, by using the characteristics of polymer carbon nitride deamination polymerization, the synthesis process for accelerating the carbon nitride deamination polymerization under the semi-closed condition is designed, the high-crystallinity carbon nitride nanocrystal with the space charge separation is prepared, the separation and the migration of the photon-generated carrier can be promoted, and the utilization efficiency of the photon-generated carrier can be improved; the carbon nitride nanocrystal can be used as a catalyst for photocatalytic decomposition of water to produce hydrogen. The carbon nitride nanocrystal is simple in preparation step, mild in reaction condition, high in repeatability and easy to synthesize, and has a certain industrial application prospect.

Description

Technical Field

[0001] The invention belongs to the field of multiphase catalysis and photocatalytic water decomposition, and specifically relates to a preparation method of spatial charge separation type carbon nitride crystals and application thereof. Background Art

[0002] Using solar energy to drive photocatalytic water splitting reaction to produce hydrogen is a low-cost, mild and environmentally friendly hydrogen production technology. This hydrogen production technology uses water and sunlight, which are the two most abundant natural resources on earth, and is one of the most promising ways to achieve carbon neutrality. Among many photocatalysts, carbon nitride is considered to be one of the ideal photocatalysts for water splitting because of its abundant raw material reserves and environmental friendliness.

[0003] Polytriazine-based crystalline carbon nitride (PTI) has perfect in-plane conjugation and regular interlayer structure. The electrons and holes generated after photoexcitation can be effectively separated and utilized, and its band structure can meet the requirements of full water splitting. It is expected to achieve high efficiency in solar energy to hydrogen energy conversion. However, due to the high recombination rate of photogenerated charges on the surface of PTI, the water splitting efficiency is low. In addition, the synthesis conditions of PTI are relatively harsh, and long-term deamination polymerization is required under vacuum-sealed conditions, which makes its synthesis process face a high risk of explosion. The harsh process conditions make it difficult to achieve high-efficiency water splitting and large-scale preparation. Summary of the invention

[0004] In view of the above problems, the present invention provides a method for preparing and applying spatial charge separation type carbon nitride nanocrystals. By utilizing the characteristics of carbon nitride deamination polymerization, a synthesis process for synthesizing spatial charge separation type carbon nitride nanocrystal catalysts (PTI / HCN) under semi-closed conditions is designed. The optimization of deamination polymerization conditions accelerates the deamination polymerization kinetic process, allowing heptazine-based carbon nitride to grow in situ on the surface of carbon nitride crystals, thereby forming a PTI-HCN built-in electric field. The prepared carbon nitride nanocrystals have a spatial charge separation effect, which promotes the separation and migration of photogenerated carriers, improves the utilization efficiency of photogenerated carriers, and thus improves the catalytic reaction efficiency.

[0005] In order to achieve the above object, the present invention adopts the following technical scheme: A method for preparing space charge separation type carbon nitride crystals comprises the following steps: (1) placing a carbon nitride precursor in a crucible with a lid and obtaining polymer carbon nitride after heat treatment; (2) mixing the polymer carbon nitride obtained in step (1) with lithium chloride and potassium chloride and placing the mixture in a semi-closed container, and subjecting the mixture to heat treatment under nitrogen to obtain a mixture of carbon nitride nanocrystals and molten salt; (3) The mixture of carbon nitride nanocrystals and molten salt obtained in step (2) is dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate is 0, and then dried at 80° C. overnight to obtain carbon nitride nanocrystals.

[0006] Furthermore, the carbon nitride precursor in step (1) is one of urea, melamine and melamine.

[0007] Furthermore, the heat treatment temperature in step (1) is 500-600°C, the heating rate is 60-120°C / h, and the heat treatment time is 2-4h.

[0008] Furthermore, the semi-enclosed container in step (2) is an open long-necked ampoule.

[0009] Furthermore, in step (2), the mass ratio of the polymer carbon nitride, lithium chloride and potassium chloride is 1:4.5:5.5; the heat treatment temperature is 500-600°C, the time is 4-48h, and the heating rate is 120-360°C / h. A space charge separation carbon nitride nanocrystal prepared by the above preparation method.

[0010] The application of the above-mentioned space charge separation carbon nitride nanocrystals in photocatalytic water decomposition to produce hydrogen.

[0011] Furthermore, the ratio of the spatial charge separation carbon nitride nanocrystals to water is 1 mg:1 mL, the photocatalytic temperature is 12° C., and the reaction is carried out under a 300W xenon lamp.

[0012] Compared with the prior art, the present invention has the following advantages: (1) Compared with the conventional process conditions of deamination polymerization under vacuum-sealed conditions, which are prone to explosion, the semi-enclosed process conditions of the present invention are milder and safer, and are conducive to industrial production.

[0013] (2) The present invention introduces spatial charge separation type carbon nitride nanocrystals synthesized under semi-closed conditions into the photocatalytic water decomposition reaction, and utilizes the high efficiency of spatial charge separation to improve the light absorption capacity, promote the separation and migration of photogenerated carriers, and improve the utilization efficiency of photogenerated carriers, thereby improving the efficiency of the catalytic reaction.

[0014] (3) The catalyst of the present invention has a simple preparation process, low cost and strong stability. The catalytic reaction conditions are mild and green and environmentally friendly, which is conducive to large-scale industrial application and energy security, and is of great significance to achieving "carbon peak and carbon neutrality". BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1: Scanning electron micrographs of carbon nitride nanocrystals prepared in (a) Example 1, (b) Comparative Example 1 and (c) Comparative Example 2.

[0016] Figure 2 : XRD pattern of spatial charge separation carbon nitride nanocrystals (PTI / HCN) prepared in Example 1.

[0017] Figure 3 : Kelvin probe force microscopy charge distribution image of spatial charge separation carbon nitride nanocrystals prepared in Example 1.

[0018] Figure 4 : Steady-state fluorescence intensity images of Example 1 and Comparative Example 1.

[0019] Figure 5 : Reaction activity diagram of carbon nitride nanocrystals prepared by photocatalytic decomposition of water to produce hydrogen in Example 1, Comparative Example 1 and Comparative Example 2.

[0020] Figure 6 : Catalytic effect diagram of the space charge separation carbon nitride nanocrystals prepared in Example 1 after circulation for 8 hours. DETAILED DESCRIPTION

[0021] In order to make the contents of the present invention easier to understand, the technical solution of the present invention is further described below in conjunction with specific implementation methods, but the present invention is not limited thereto.

[0022] Example 1 10g of urea was placed in a crucible with a lid, and heated in a muffle furnace at 500ºC for 2 hours at a heating rate of 120℃ / h to obtain a solid powder polymer carbon nitride. The polymer carbon nitride was mixed with lithium chloride and potassium chloride in a ratio of 1:4.5:5.5 and placed in an open long-necked ampoule, which was then placed in a nitrogen atmosphere and heated in a muffle furnace at 550ºC for 4 hours at a heating rate of 360℃ / h to obtain a mixture of space charge separation carbon nitride nanocrystals and molten salt; the mixture was taken out and ultrasonically dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate was 0, and vacuum dried overnight at 80℃ to obtain a white solid powder, which was a space charge separation carbon nitride nanocrystal catalyst (PTI / HCN).

[0023] Figure 1 a is a scanning transmission electron microscope image of carbon nitride nanocrystals. The image shows that small HCN particles are attached to PTI hexagonal nanocrystals, proving that it has a carbon nitride nanocrystal morphology with spatial charge separation.

[0024] Figure 2This is the XRD pattern of spatial charge separation carbon nitride nanocrystals (PTI / HCN). The diffraction peaks at 26.9 degrees and 27.9 degrees are respectively attributed to the interlayer stacking structure of triazine-based crystalline carbon nitride and heptazine-based carbon nitride, proving the existence of its polytriazine-heptazine-based crystalline phase structure.

[0025] Figure 3 This is the Kelvin probe force microscopy charge distribution image of spatial charge separation carbon nitride nanocrystals, proving that the prepared carbon nitride nanocrystals have spatial charge separation characteristics.

[0026] Example 2 10g of melamine was placed in a crucible with a lid, and heated in a muffle furnace at 500ºC for 2 hours at a heating rate of 120℃ / h to obtain a solid powder polymer carbon nitride. The polymer carbon nitride was mixed with lithium chloride and potassium chloride in a ratio of 1:4.5:5.5 and placed in an open long-necked ampoule, which was then placed in a nitrogen atmosphere and heated in a muffle furnace at 525ºC for 6 hours at a heating rate of 240℃ / h to obtain a mixture of spatial charge separation carbon nitride nanocrystals and molten salt; the mixture was taken out and ultrasonically dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate was 0, and vacuum dried overnight at 80℃ to obtain a white solid powder, namely a carbon nitride nanocrystal catalyst with spatial charge separation (PTI / HCN).

[0027] Example 3 10g of melamine was placed in a crucible with a lid, and heated in a muffle furnace at 500ºC for 4 hours at a heating rate of 120℃ / h to obtain a solid powder polymer carbon nitride. The polymer carbon nitride was mixed with lithium chloride and potassium chloride in a ratio of 1:4.5:5.5 and placed in a long-necked ampoule, which was then placed in a nitrogen atmosphere and heated in a muffle furnace at 575ºC for 12 hours at a heating rate of 120℃ / h to obtain a mixture of space charge separation carbon nitride nanocrystals and molten salt; the ampoule was opened, the mixture was taken out and ultrasonically dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate was 0, and vacuum dried overnight at 80℃ to obtain a white solid powder, namely, a space charge separation carbon nitride nanocrystal catalyst (PTI / HCN).

[0028] Comparative Example 1 Place 10g of urea in a crucible with a lid, heat it in a muffle furnace at 500ºC for 2 hours at a heating rate of 120℃ / h to obtain a solid powder polymer carbon nitride. Mix the polymer carbon nitride with lithium chloride and potassium chloride in a ratio of 1:4.5:5.5, place it in an ampoule, evacuate it and seal it, heat it in a muffle furnace at 550℃ for 4 hours at a heating rate of 360℃ / h to obtain a mixture of solid powder polytriazine-based crystalline carbon nitride and molten salt. Then open the ampoule, take out the mixture and disperse it in pure water, filter it, wash it with pure water until the ionic strength of the eluate is 0, and vacuum dry it at 80℃ overnight. The white powder product obtained is pure phase polytriazine-based crystalline carbon nitride (PTI), and its scanning transmission electron microscope image is as follows Figure 1 As shown in b.

[0029] Figure 4 The steady-state fluorescence intensity of Comparative Example 1 is compared with that of Example 1. The significantly reduced fluorescence intensity proves that the prepared spatial charge separation carbon nitride nanocrystals can promote the separation and migration of photogenerated carriers.

[0030] Comparative Example 2 10g of urea was placed in a covered crucible and heated in a furnace at 500ºC for 2 hours at a heating rate of 120℃ / h to obtain a solid powder polymer carbon nitride. The polymer carbon nitride was mixed with lithium chloride and potassium chloride in a ratio of 1:4.5:5.5 and placed in a crucible. The crucible was then placed in a nitrogen atmosphere and heated in a muffle furnace at 550ºC for 4 hours at a heating rate of 360℃ / h to obtain a mixture of solid powder heptazine-based crystalline carbon nitride and molten salt; the mixture was ultrasonically dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate was 0, and vacuum dried at 80℃ overnight to obtain a yellow solid powder heptazine-based carbon nitride, namely HCN, whose scanning transmission electron microscope image is shown below Figure 1 As shown in c.

[0031] Compared with the closed system of Comparative Example 1, in which the product is PTI, and the open system of Comparative Example 2, in which the product is HCN, Examples 1-3 prepared in a semi-closed system can allow HCN to grow in situ on the surface of PTI crystals, thereby forming crystals with spatial charge separation, which greatly promotes the separation and utilization of photogenerated carriers, thereby greatly improving its photocatalytic water decomposition performance.

[0032] Application Example 1 Fully decomposed water activity experiment The hydrogen content of the prepared polymer after the catalytic experiment was analyzed by gas chromatography. Specific operation steps: weigh 100 mg of space charge separation carbon nitride nanocrystal catalyst and add it to 100 mL of aqueous solution, mix it evenly by ultrasonic mixing, and pour it into the reactor. After completely removing the air in the system using the vacuum system, keep the reaction temperature at 12 ° C and turn on the 300W xenon lamp for reaction. After the reaction is completed for 1 hour, the amount of hydrogen and oxygen generated is analyzed by gas chromatography (Shimadzu GC-8A).

[0033] Figure 5 The reaction activity diagram of the photocatalytic decomposition of water to produce hydrogen by preparing carbon nitride nanocrystals (PTI / HCN) in Example 1, PTI-type carbon nitride in Comparative Example 1, and HCN-type carbon nitride in Comparative Example 2. As can be seen from the figure, compared with the PTI-type and HCN-type carbon nitrides prepared under traditional conditions, the activity of the space charge separation carbon nitride nanocrystals prepared in Example 1 is significantly improved.

[0034] Figure 6 This is a diagram of the catalytic effect of the space charge separation carbon nitride nanocrystals prepared in Example 1 after circulation for 8 hours. The carbon nitride nanocrystals show high catalytic stability.

[0035] The above description is only a preferred embodiment of the present invention. All equivalent changes and modifications made according to the scope of the patent application of the present invention should fall within the scope of the present invention.

Claims

1. A method for preparing space charge separation carbon nitride crystals, characterized in that: The following steps are involved: (1) placing a carbon nitride precursor in a crucible with a lid and obtaining polymer carbon nitride after heat treatment; (2) mixing the polymer carbon nitride obtained in step (1) with lithium chloride and potassium chloride and placing the mixture in a semi-closed container, and subjecting the mixture to heat treatment under nitrogen to obtain a mixture of carbon nitride nanocrystals and molten salt; (3) The mixture of carbon nitride nanocrystals and molten salt obtained in step (2) is dispersed in pure water, filtered, washed with pure water until the ionic strength of the eluate is 0, and then dried at 80° C. overnight to obtain carbon nitride nanocrystals.

2. The preparation method according to claim 1, characterized in that: The carbon nitride precursor in step (1) is one of urea, melamine and melamine.

3. The preparation method according to claim 1, characterized in that: The heat treatment temperature in step (1) is 500-600°C, the heating rate is 60-120°C / h, and the heat treatment time is 2-4h.

4. The preparation method according to claim 1, characterized in that: The semi-enclosed container in step (2) is an open long-necked ampoule.

5. The preparation method according to claim 1, characterized in that: The mass ratio of the polymer carbon nitride, lithium chloride and potassium chloride in step (2) is 1:4.5:5.5; the temperature of the heat treatment is 500-600°C, the time is 4-48h, and the heating rate is 120-360°C / h.

6. A spatial charge separation type carbon nitride nanocrystal prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the spatial charge separation carbon nitride nanocrystals as claimed in claim 6 in photocatalytic water decomposition to produce hydrogen.

8. The use according to claim 7, characterized in that: The usage ratio of the space charge separation carbon nitride nanocrystals to water is 1 mg:1 mL, the photocatalytic temperature is 12° C., and the reaction is carried out under a 300W xenon lamp.

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