Method for preparing polyheptazine carbon nitride ultra-thin nanosheets by sodium chloride-calcium chloride binary molten salt and application

Abstract

The application discloses a method for preparing polyheptazine carbon nitride ultra-thin nanosheets by using sodium chloride and calcium chloride binary molten salt and application thereof. The method is characterized by simple steps, mild reaction conditions, high safety, high repeatability, cheap material and the like, and has certain industrial application prospect. The polyheptazine carbon nitride ultra-thin nanosheets obtained by the method can be used as a catalyst to decompose water to produce hydrogen under visible light. The ultra-thin structure of the polyheptazine carbon nitride ultra-thin nanosheets shortens the migration distance of photo-generated electrons and holes, improves the separation efficiency of the photo-generated electron-hole pairs, and thus improves the reaction efficiency, and has potential application prospect.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of heterogeneous catalysis and photocatalytic water splitting, and particularly relates to a preparation method of polyheptazine carbon nitride ultra-thin nanosheet catalytic material and application thereof in visible light photocatalytic water splitting for hydrogen production. BACKGROUND

[0002] Solar energy is widely considered as the most promising renewable energy to solve the environmental and global energy crisis due to its abundance, inexhaustibility and wide distribution. Photocatalytic water splitting is considered as an ideal method to convert solar energy into storable chemical fuel hydrogen (H2), and the technology of photocatalytic water splitting for hydrogen production is a process in which a catalyst decomposes water into hydrogen gas in the presence of light and a sacrificial agent. The technical focus is the absorption range of visible light (visible light accounts for about 43% in the solar spectrum) of the catalyst and whether the photo-generated electrons and holes can be efficiently separated and migrated to the catalyst surface for redox reaction. Therefore, enhancing the visible light absorption of the photocatalyst and improving the utilization efficiency of photo-generated electrons and holes are the keys to improving the efficiency of solar energy conversion into hydrogen energy. SUMMARY

[0003] In view of the above problems, the application provides a method for preparing polyheptazine carbon nitride ultra-thin nanosheet by using sodium chloride-calcium chloride binary molten salt and application thereof. The polyheptazine carbon nitride ultra-thin nanosheet has the characteristics of short photo-generated carrier migration distance, thereby improving the separation efficiency. The polyheptazine carbon nitride ultra-thin nanosheet with enhanced visible light absorption is designed and synthesized, and is applied as a catalyst in photocatalytic water splitting for hydrogen production, thereby enhancing the visible light absorption, promoting the separation and migration of photo-generated electrons and holes, and improving the catalytic reaction efficiency.

[0004] To achieve the above object, the application adopts the following technical scheme:

[0005] A polyheptazine carbon nitride ultra-thin nanosheet, the preparation thereof comprises the following steps:

[0006] 1) Melamine, sodium chloride and calcium chloride are mixed in a certain proportion, and then heat-treated under a nitrogen atmosphere to obtain a carbon-nitrogen polymer mixed with molten salt;

[0007] 2) The obtained carbon-nitrogen polymer mixed with molten salt is subjected to secondary heat treatment under vacuum to obtain polyheptazine carbon nitride ultra-thin nanosheet mixed with molten salt;

[0008] 3) The obtained polyheptazine carbon nitride ultra-thin nanosheet mixed with molten salt is dispersed in pure water, and then filtered and washed with pure water until the ion strength of the washing liquid is 0, and then dried at 60°C overnight to obtain apricot-colored powder of polyheptazine carbon nitride ultra-thin nanosheet.

[0009] Further, the mass ratio of melamine, sodium chloride and calcium chloride used in step (1) is 1:4-10:6-10.

[0010] Further, the temperature of the heat treatment under nitrogen atmosphere in step (1) is 300-400 DEG C, the time is 2-6 h, and the heating rate is 60-120 DEG C / h.

[0011] Further, the temperature of the heat treatment under vacuum in step (2) is 500-625 DEG C, and the time is 4-48 h.

[0012] The polyheptazine carbon nitride ultra-thin nanosheet can be used for visible light photocatalytic decomposition of water to produce hydrogen, and specifically, the polyheptazine carbon nitride ultra-thin nanosheet is used as a catalyst, water and triethanolamine are used as reactants, and water is decomposed into hydrogen under visible light irradiation. The application provides a green, environmentally friendly, stable and efficient use method of a metal-free carbon-nitrogen-based polymer catalyst, which has potential application prospects.

[0013] Compared with the prior art, the application has the following advantages:

[0014] (1) The polyheptazine carbon nitride ultra-thin nanosheet catalyst with enhanced visible light absorption is designed and synthesized by taking advantage of the features of polymer carbon nitride, such as adjustable morphology and structure and suitable band structure for photocatalytic water decomposition, and is introduced into the reaction of photocatalytic decomposition of water to produce hydrogen. The carbon nitride ultra-thin nanosheet has enhanced absorption of visible light, and the ultra-thin structure shortens the migration distance of photo-generated electrons and holes, thereby promoting the separation and migration of the electrons and holes, so that the catalytic reaction efficiency is improved.

[0015] (2) The preparation process of the catalyst in the application is simple, safe and reliable, low in cost and high in stability. The catalytic reaction has mild conditions, is green and environmentally friendly, and is high in efficiency, so that it is conducive to large-scale industrial production and application and alleviation of energy crisis. BRIEF DESCRIPTION OF DRAWINGS

[0016] Figure 1 The left graph is a scanning electron microscope image of the PHI ultra-thin nanosheet prepared in Example 1, and the right graph is an atomic force microscope image of the PHI ultra-thin nanosheet.

[0017] Figure 2 The left graph is a scanning electron microscope image of the PHI ultra-thin nanosheet prepared in Example 1, and the right graph is an atomic force microscope image of the PHI ultra-thin nanosheet.

[0018] Figure 3 The left graph is a scanning electron microscope image of the PHI ultra-thin nanosheet prepared in Example 1, and the right graph is an atomic force microscope image of the PHI ultra-thin nanosheet.

[0019] Figure 4 The left graph is a scanning electron microscope image of the PHI ultra-thin nanosheet prepared in Example 1, and the right graph is an atomic force microscope image of the PHI ultra-thin nanosheet.

[0020] Figure 5 Scanning electron microscope image of PHI bulk prepared for Comparative Example 1.

[0021] Figure 6 The left image is a scanning electron microscope image of PHI thicker sheet prepared for Comparative Example 2, and the right image is an atomic force microscope image of PHI thicker sheet.

[0022] Figure 7 The left image is a comparison chart of photocatalytic water splitting activity of carbon nitride synthesized at different sodium chloride-calcium chloride ratios, and the right image is a comparison chart of light absorption of carbon nitride at different ratios.

[0023] Figure 8 The left image is a comparison chart of photocatalytic water splitting activity of PHI ultrathin nanosheet prepared for Example 1-2, and the right image is a comparison chart of light absorption.

[0024] Figure 9 The left image is a comparison chart of photocatalytic water splitting activity of PHI ultrathin nanosheet prepared for Example 1, and the right image is a comparison chart of light absorption.

[0025] Figure 10 The left image is a comparison chart of photocatalytic water splitting activity of PHI ultrathin nanosheet prepared for Example 1 after two cycles of 8 h in total. DETAILED DESCRIPTION

[0026] A polyheptazine carbon nitride ultrathin nanosheet, the preparation of which comprises the following steps:

[0027] 1) Mix melamine, sodium chloride, and calcium chloride in a mass ratio of 1:4-10:6-10 in an ampoule, and heat treat at 300-400°C under nitrogen atmosphere for 2-6 h (the purpose of this step is mainly to remove part of the amino group in the precursor melamine, release ammonia gas, and prepare for the second step of vacuum sealing tube heat treatment to avoid the danger of pipe explosion), to obtain a carbon-nitrogen polymer mixed with molten salt;

[0028] 2) Vacuum seal the ampoule containing the carbon-nitrogen polymer mixed with molten salt, and then heat treat at 500-625°C for 4-48 h to obtain polyheptazine carbon nitride ultrathin nanosheet mixed with molten salt;

[0029] 3) Disperse the obtained polyheptazine carbon nitride ultrathin nanosheet mixed with molten salt in deionized water, and wash with deionized water until the ion strength of the washing liquid is 0, and then dry at 60°C overnight to obtain apricot-colored powder of polyheptazine carbon nitride ultrathin nanosheet.

[0030] In order to make the content of the present application more convenient to understand, the technical solutions of the present application are further described below in combination with specific embodiments, but the present application is not limited to this.

[0031] Example 1

[0032] Melamine, sodium chloride and calcium chloride were uniformly mixed in a mass ratio of 1:4:6 and then loaded into a par bottle (110 mL). The par bottle was heated at 400°C for 6 hours under a nitrogen atmosphere (the heating rate was 120°C / h), to obtain a mixture of carbon-nitrogen polymer and fused salt solid powder. The par bottle containing the mixture of solid powder was vacuumized and sealed, and then heated at 600°C for 24 hours in a furnace (the heating rate was 120°C / h), to obtain a mixture of polyheptazine carbon nitride ultrathin nanosheets and fused salt solid powder. After the par bottle was carefully opened, the solid powder was poured out and ultrasonically dispersed in deionized water, filtered and washed until the ion strength of the washing liquid was 0, and then vacuum dried at 60°C overnight. The apricot-colored powder product obtained was polyheptazine carbon nitride (PHI) ultrathin nanosheets, and the thickness thereof was about 6 nanometers.

[0033] Figure 1 The left image is a scanning electron microscope image of the PHI ultrathin nanosheets prepared in this embodiment, and the right image is an atomic force microscope image thereof. It can be proved from the images that the PHI ultrathin nanosheets have an ultrathin nanosheet morphology. This is because the calcium chloride in the binary NaCl / CaCl2 fused salt used in the present application has a strong structure-directing effect on the growth of carbon nitride. When the NaCl-CaCl2 binary fused salt is melted, Ca 2+ ions continuously move between the layers, causing the thicker layers to gradually peel off into thinner layers. In addition, during the polymerization of carbon nitride, the interaction between Ca 2+ ions and the Lewis base sites of sp 2 hybridized N on the adjacent PHI thin sheets is enhanced, causing the structure to be distorted and thus expanding the absorption of visible light.

[0034] Figure 2 The X-ray powder diffraction pattern of the PHI ultrathin nanosheets prepared in this embodiment is shown in the figure. It can be proved from the figure that the polyheptazine crystal phase structure exists.

[0035] Figure 3 The X-ray photoelectron spectroscopy of N 1s of the PHI ultrathin nanosheets prepared in this embodiment is shown in the figure. In the N 1s spectrum, the characteristic peak at 398.9 is attributed to C=N-C, the characteristic peak at 400.5 is attributed to N-(C)3, and the characteristic peak at 401.4 eV is attributed to the C-N-H bond, further proving the heptazine carbon nitride structure.

[0036] Figure 4 The Fourier infrared spectrogram of the PHI ultrathin nanosheets prepared in this embodiment is shown in the figure. As can be seen from the figure, the sample has absorption peaks at 800 cm−1 peaks in the region of 1200-1600 cm"1may be attributed to the stretching vibration of the triazine ring. -1 peaks in the region of 1200-1600 cm"1may be attributed to the stretching vibration of the triazine ring.

[0037] Example 2

[0038] To compare the effect of reaction time on the activity of carbon nitride, experiments with different reaction times were set up. Melamine, sodium chloride, and calcium chloride were uniformly mixed in a mass ratio of 1:4:6 and then loaded into an ampoule (110 mL) and heated at 400°C for 6 hours (the heating rate was 120°C / h) under a nitrogen atmosphere to obtain a mixture of carbon-nitrogen polymer and fused salt solid powder. The ampoule containing the solid powder was evacuated and sealed, and then heated at 600°C for 12 hours (the heating rate was 120°C / h) in a furnace to obtain a mixture of solid powder polyheptazine-based carbon nitride ultrathin nanosheets and fused salt. The ampoule was opened, the solid powder was removed, and ultrasonic dispersion was performed in deionized water, followed by filtration and washing until the ion strength of the washing liquid was 0, and then vacuum drying at 60°C overnight to obtain a light apricot-colored powder product, which was polyheptazine-based carbon nitride (PHI) ultrathin nanosheets, which had a thickness comparable to that of the nanosheets in Example 1.

[0039] Comparative Example 1

[0040] To highlight the role of sodium chloride-calcium chloride binary fused salt in the synthesis of carbon nitride nanosheets, an experiment with only sodium chloride was set up as a comparative example. Melamine and sodium chloride were uniformly mixed in a mass ratio of 1:10 and then loaded into an ampoule (110 mL) and heated at 400°C for 6 hours (the heating rate was 60°C / h) under a nitrogen atmosphere to obtain a mixture of carbon-nitrogen polymer and fused salt solid powder. The ampoule containing the mixture was evacuated and sealed, and then heated at 600°C for 24 hours (the heating rate was 120°C / h) in a furnace to obtain a mixture of solid powder polyheptazine-based crystalline carbon nitride and fused salt. The ampoule was opened, the solid powder was removed, and ultrasonic dispersion was performed in deionized water, followed by filtration and washing until the ion strength of the washing liquid was 0, and then vacuum drying at 60°C overnight to obtain a white powder product, which was bulk polyheptazine-based carbon nitride (PHI), which had an irregular massive morphology with a size greater than 10 microns.

[0041] Figure 5 A scanning electron microscope image of the PHI bulk phase prepared in Comparative Example 1, which had an irregular massive morphology, was significantly different from the ultrathin nanosheets in Example 1. Figure 1

[0042] Comparative Example 2

[0043] ​To prove that calcium chloride has a strong guiding effect on the formation of sheet-like carbon nitride structure, an experiment of adding only calcium chloride is set as a comparative example. Melamine, calcium chloride are uniformly mixed in a mass ratio of 1:10, then loaded into a vial (110 mL), heated at 400°C for 6 hours under nitrogen atmosphere (the heating rate is 120°C / h), and a mixture of carbon nitride polymer and molten salt solid powder is obtained. The vial containing the solid powder is vacuumed and sealed, and heated at 600°C for 24 hours in a furnace (the heating rate is 120°C / h), and a mixture of polyheptazine carbon nitride nanosheet and molten salt solid powder is obtained. Open the vial, take out the solid powder and disperse it in deionized water by ultrasonic, filter and wash until the ion strength of the washing liquid is 0, then vacuum dry at 60°C overnight, and the obtained white powder product is polyheptazine carbon nitride (PHI) thicker nanosheet, with a thickness of about 30 nanometers.

[0044] Figure 6 The scanning electron microscope image and atomic force microscope image of the PHI thicker nanosheet prepared for Comparative Example 2 have a morphology of thicker sheets, which is similar to the ultrathin nanosheet in Figure 1 .

[0045] Full decomposition of water activity experiment:

[0046] Specific operation steps: 50 mg of polyheptazine carbon nitride ultrathin nanosheet is weighed and added to 100 mL of 10wt% triethanolamine aqueous solution, then ultrasonically mixed and poured into a reactor, and the reaction temperature is kept at 12°C. After adding 3% platinum as a hydrogen production catalyst, the system is completely evacuated by a vacuum system, and then a xenon lamp is turned on for photocatalytic reaction. After the reaction is completed, the amount of hydrogen generated is analyzed by gas chromatography (Shimadzu GC-8A). At the same time, the bulk polyheptazine carbon nitride prepared in the comparative example is used as a comparison.

[0047] Figure 7 The left graph is a comparison of the photocatalytic water decomposition activity of carbon nitride prepared under different sodium chloride-calcium chloride ratios under visible light. Only the salt ratio is changed, and the other synthesis conditions are the same as Example 2. From the graph, it can be seen that when the ratio of sodium chloride to calcium chloride is 4:6, the activity is the highest. The reason may be that the melting point of the molten salt at this ratio (575°C) is relatively close to the reaction temperature (600°C), and the reaction environment is relatively mild, which is more conducive to the synthesis of carbon nitride material. In addition, from the right graph, it can be seen that the light absorption of the carbon nitride obtained at this ratio is the strongest, which may be one of the reasons for the high activity.

[0048] Figure 8The left graph is a comparison chart of the photocatalytic activity of PHI nanosheets prepared in Example 1-2 for hydrogen production by photocatalytic decomposition of water under visible light. As can be seen from the graph, the photocatalytic activity of PHI ultra-thin nanosheets prepared in Example 1 under visible light is significantly better than that of Example 2. The reason may be that the longer reaction time is conducive to better polymerization and growth of the carbon nitride material. In addition, as can be seen from the right graph, the carbon nitride obtained under this reaction time has the strongest light absorption, which may be one of the reasons for the high activity.

[0049] Figure 9 The left graph is a comparison chart of the photocatalytic activity of PHI ultra-thin nanosheets prepared in Example 1, bulk PHI prepared in Comparative Example 1, and PHI thicker sheets prepared in Comparative Example 2 for hydrogen production by photocatalytic decomposition of water under visible light. As can be seen from the graph, the activity of PHI ultra-thin nanosheets is significantly improved compared with carbon nitride in bulk PHI and thicker PHI. The reason may be that under the same mass, the specific surface area of PHI ultra-thin sheets is larger than that of bulk PHI and PHI thicker sheets, and the number of active sites is more. In addition, as can be seen from the right graph, the light absorption of PHI ultra-thin sheets is the strongest, which may be one of the reasons for the high activity. Figure 10 The graph is the catalytic effect of PHI ultra-thin nanosheets prepared in Example 1 after two cycles of a total of 8h. As can be seen from the graph, PHI ultra-thin nanosheets show high cycle stability.

[0050] The above is only the preferred embodiment of the present application, and any equivalent changes and modifications made within the scope of the patent application of the present application shall be included in the scope of the present application.

Claims

1. A method for preparing polyheptazine-based carbon nitride ultrathin nanosheets, characterized in that, The method comprises the following steps: 1) mixing melamine, sodium chloride and calcium chloride in a certain proportion, and then performing heat treatment under a nitrogen atmosphere to obtain carbon-nitrogen polymer precursors mixed with molten salt; 2) performing second heat treatment on the obtained carbon-nitrogen polymer precursors mixed with molten salt under a vacuum state to obtain polyheptazine-based carbon nitride ultra-thin nanosheets mixed with molten salt; 3) ultrasonic dispersion of the obtained polyheptazine-based carbon nitride ultra-thin nanosheets mixed with molten salt in deionized water, and filtration and washing with deionized water until the ion strength of the washing liquid is 0, and then vacuum drying at 60℃ overnight to obtain apricot-colored powder, which is the polyheptazine-based carbon nitride ultra-thin nanosheets; The mass ratio of melamine, sodium chloride and calcium chloride used in step (1) is 1:4-10:6-10; The temperature of heat treatment under a nitrogen atmosphere in step (1) is 300-400℃, and the time is 2-6 h; The temperature of second heat treatment in step (2) is 500-625℃, and the time is 4-48 h.

2. Polyheptazine-based carbon nitride ultra-thin nanosheets prepared by the method of claim 1.

3. Application of the polyheptazine-based carbon nitride ultra-thin nanosheets of claim 2 in visible light photocatalytic decomposition of water to produce hydrogen.

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