Red crystalline carbon nitride as well as preparation method and application thereof

Abstract

The invention belongs to the field of photocatalytic materials, and particularly relates to red crystalline carbon nitride as well as a preparation method and application thereof. A bottom-to-top strategy is adopted, a molten salt auxiliary calcination method is utilized, a nitrogen-rich precursor melamine and molten salt potassium thiocyanate and ammonium chloride are uniformly mixed and then subjected to a thermal polycondensation reaction, and then the red crystalline carbon nitride is obtained through cleaning and drying treatment. According to the invention, the defects of low visible light utilization rate and poor crystallinity of carbon nitride are overcome, and sunlight can be utilized to the greatest extent, so that the reaction of photocatalytic decomposition of water to produce hydrogen under the irradiation of visible light is realized.

Description

Technical Field

[0001] The invention belongs to the field of photocatalytic materials, and specifically relates to red crystalline carbon nitride and a preparation method and application thereof. Background Art

[0002] Graphitic carbon nitride (CN) has become one of the most promising photocatalysts in the field of photocatalysis due to its suitable energy band structure, good optical properties and excellent chemical stability. However, due to the kinetic problems of traditional cyanamide carbon-nitrogen small molecules during high-temperature polymerization, the synthesized carbon nitride has a low crystallinity and short-range disordered heptazine melon structure, which contains a large number of incompletely polymerized terminal amino groups. The hydrogen bonds formed between the amino groups greatly reduce the transmission efficiency of carriers between the heptazine conjugated rings, limiting the photocatalytic activity of carbon nitride.

[0003] In addition, in the solar spectrum, near-infrared light accounts for about 50% of the solar flux, and has stronger tissue penetration and lower phototoxicity than ultraviolet-visible light. However, due to the compatibility problem between the high separation efficiency of photoexcited carriers and the narrow band gap (0.5-1.8eV), near-infrared light has rarely been effectively used in photocatalytic reactions so far. It is estimated that in order to achieve industrial application, the conversion efficiency of solar energy to hydrogen energy must reach more than 10%, corresponding to the light response range of semiconductor photocatalysts to be above 520nm, and the apparent quantum efficiency must reach 100%. Ordinary carbon nitride is light yellow, with a band gap value of 2.7eV, and can only absorb visible light below 460nm. Therefore, the existing technology needs to be improved and developed. Summary of the invention

[0004] In order to overcome the drawbacks of narrow light absorption range and low crystallinity of carbon nitride, the present invention provides a red crystalline carbon nitride and a preparation method and application thereof.

[0005] To achieve the above object, the technical solution of the present invention is as follows:

[0006] The first aspect of the present invention is a method for preparing red crystalline carbon nitride, comprising the following steps: uniformly mixing nitrogen-rich precursor melamine with molten salt potassium thiocyanate and ammonium chloride, and then subjecting the mixture to a thermal polycondensation reaction, followed by washing and drying to obtain red crystalline carbon nitride.

[0007] Furthermore, the conditions of the thermal polycondensation reaction are: in an argon atmosphere, the temperature is raised to 400° C. and kept for 1 hour, and then the temperature is continued to be raised to 500-610° C. and kept for 1-2 hours.

[0008] Furthermore, the molar ratio of potassium thiocyanate to ammonium chloride is (4-2):1.

[0009] Furthermore, the mass ratio of melamine to potassium thiocyanate is (2-3):2.

[0010] Furthermore, the drying temperature is 60°C.

[0011] The second aspect of the present invention is red crystalline carbon nitride obtained by the preparation method described in the first aspect.

[0012] The third aspect of the present invention is the use of the red crystalline carbon nitride described in the second aspect as a photocatalyst.

[0013] Furthermore, it is applied to the photocatalytic decomposition of water to produce hydrogen.

[0014] Compared with the prior art, the present invention has the following beneficial effects:

[0015] (1) The present invention proposes a method for synthesizing crystalline carbon nitride by a simple bottom-up strategy, which uses melamine as a nitrogen-rich precursor source and ammonium thiocyanate synthesized by the reaction of potassium thiocyanate and ammonium chloride in a high-temperature thermal polycondensation process as another precursor source, and finally prepares red crystalline carbon nitride. The preparation method uses low-cost materials, is simple and easy to operate, has low requirements on production equipment, and is easy to industrialize.

[0016] (2) The preparation method of red crystalline carbon nitride provided by the present invention can improve the crystallinity of crystalline carbon nitride, promote the separation and transmission of photogenerated carriers, and significantly improve the photocatalytic hydrogen production activity.

[0017] (3) The present invention solves the drawbacks of the narrow visible light response range and low crystallinity of carbon nitride, and can maximize the use of sunlight, achieve high visible light photochemical water decomposition hydrogen production activity, and realize efficient use of sunlight. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The X-ray diffraction patterns of the red crystalline carbon nitride prepared in Comparative Example 1 and Examples 1 to 4 are shown, wherein the abscissa 2θ is the diffraction angle (degree) and the ordinate Intensity is the intensity (au).

[0019] Figure 2 The X-ray diffraction patterns of the red crystalline carbon nitride prepared in Comparative Example 2 and Examples 5-6, wherein the abscissa 2θ is the diffraction angle (degree), and the ordinate Intensity is the intensity (au).

[0020] Figure 3 It is the ultraviolet-visible diffuse reflectance spectra of the red crystalline carbon nitride prepared in Comparative Example 1 and Examples 1 to 4, wherein the abscissa Wavelength is the wavelength (nm) and the ordinate Absorbance is the intensity (au).

[0021] Figure 4 It is the ultraviolet-visible diffuse reflectance spectra of the red crystalline carbon nitride prepared in Comparative Example 1, Comparative Example 2 and Examples 5-6, wherein the abscissa Wavelength is the wavelength (nm) and the ordinate Absorbance is the intensity (au).

[0022] Figure 5 The photocatalytic hydrogen production performance of the red crystalline carbon nitride prepared in Comparative Example 1 and Examples 1 to 4 under light irradiation with a wavelength greater than 510 nm is shown in Figure 1, where the horizontal axis H2 Production is the hydrogen production rate (mmol·g -1 ·h -1 ), the vertical axis represents each sample.

[0023] Figure 6 The photocatalytic hydrogen production performance of the red crystalline carbon nitride prepared in Comparative Examples 1 to 2 and Examples 1 to 6 under light irradiation with a wavelength greater than 420 nm is shown in Figure 1, where the horizontal axis H2 Production is the hydrogen production rate (mmol·g -1 ·h -1 ), the vertical axis represents each sample. DETAILED DESCRIPTION

[0024] In order to facilitate the understanding of the present invention, the present invention will be described more fully below. However, the present invention can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present invention more thorough and comprehensive.

[0025] Embodiment 1:

[0026] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0027] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0028] (2) Weigh 3 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.546 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0029] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept at that temperature for 1 h. Then, it was continued to be heated to 550°C at a rate of 10°C / min and kept at that temperature for 2 h. Then, it was cooled to room temperature in the furnace.

[0030] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0031] Embodiment 2:

[0032] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0033] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0034] (2) Weigh 3 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.546 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0035] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept at that temperature for 1 h. Then, it was continued to be heated to 575°C at a rate of 10°C / min and kept at that temperature for 2 h. Then, it was cooled to room temperature in the furnace.

[0036] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0037] Embodiment 3:

[0038] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0039] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0040] (2) Weigh 3 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.546 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0041] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept at that temperature for 1 h. Then, it was continued to be heated to 600°C at a rate of 10°C / min and kept at that temperature for 2 h. Then, it was cooled to room temperature in the furnace.

[0042] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0043] Embodiment 4:

[0044] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0045] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0046] (2) Weigh 3 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.546 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0047] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept at that temperature for 1 h. Then, it was continued to be heated to 610°C at a rate of 10°C / min and kept at that temperature for 2 h. Then, it was cooled to room temperature in the furnace.

[0048] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0049] Embodiment 5:

[0050] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0051] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0052] (2) Weigh 2 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.27 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0053] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept warm for 1 h. Then, it was continued to be heated to 500°C at a rate of 10°C / min and kept warm for 1 h, and then cooled to room temperature in the furnace.

[0054] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0055] Embodiment 6:

[0056] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0057] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0058] (2) Weigh 2 g of melamine, 2 g of potassium thiocyanate dried in step (1), and 0.55 g of ammonium chloride in an agate mortar and grind and mix them evenly.

[0059] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept warm for 1 h. Then, it was continued to be heated to 500°C at a rate of 10°C / min and kept warm for 1 h, and then cooled to room temperature in the furnace.

[0060] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0061] Comparative Example 1:

[0062] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0063] (1) Weigh 3 g of melamine, place it in a porcelain boat, and wrap it with aluminum foil.

[0064] (2) Place the porcelain boat in step (1) into a tubular furnace, and heat to 400°C at a rate of 10°C / min and keep warm for 1 h under the protection of an argon atmosphere with a flow rate of 20 mL / min. Then continue to heat to 600°C at a rate of 10°C / min and keep warm for 1 h, and then cool to room temperature with the furnace.

[0065] (3) Grind the sample in step (2) into powder in an agate mortar.

[0066] Comparative Example 2:

[0067] A method for preparing red crystalline carbon nitride, the specific steps are as follows:

[0068] (1) First, weigh an excess amount of potassium thiocyanate and pre-dry it in a vacuum drying oven at 140°C for 2 h.

[0069] (2) Weigh 2 g of melamine and 2 g of potassium thiocyanate dried in step (1) in an agate mortar and grind and mix them evenly.

[0070] (3) The sample in step (2) was placed in a porcelain boat, wrapped with aluminum foil and placed in a tubular furnace. Under the protection of an argon atmosphere with a flow rate of 20 mL / min, it was heated to 400°C at a rate of 10°C / min and kept warm for 1 h. Then, it was continued to be heated to 500°C at a rate of 10°C / min and kept warm for 1 h, and then cooled to room temperature in the furnace.

[0071] (4) The sample in step (3) was thoroughly washed with deionized water to remove the remaining metal salt and then dried in an oven at 60°C.

[0072] Structural characterization:

[0073] Figure 1The X-ray diffraction patterns of comparative example 1 and embodiments 1 to 4 are shown. Comparative example 1 has two typical diffraction peaks at 12.7° and 27.7°, which respectively represent the (100) crystal plane of the repeated heptazine unit in the carbon nitride plane and the (002) crystal plane of the interlayer stacking of the conjugated aromatic system; while the diffraction peak of the (100) crystal plane of embodiments 1 to 4 shifts to 8° on the left, which means that the heptazine skeleton in the plane is more complete, and the diffraction peak of the (002) crystal plane shifts to 28.2° on the right, corresponding to a decrease in the interlayer spacing to 3.16 nm, indicating that the method has successfully synthesized crystalline carbon nitride.

[0074] Figure 2 The X-ray diffraction patterns of Comparative Example 2 and Examples 5-6 are similar in diffraction patterns of the three samples, all of which are crystalline carbon nitride structures. It can be seen from the figure that compared with Comparative Example 2, the intensities of the diffraction peaks at (100) and (002) crystal planes of Examples 5 and 6 are significantly increased, indicating that the crystalline carbon nitride synthesized by this method has a higher degree of crystallinity.

[0075] Figure 3 The UV-visible diffuse reflectance spectra of Comparative Example 1 and Examples 1-4 are shown in Figure 1. The absorption edge of Comparative Example 1 is 464 nm, while the absorption edges of Examples 1 to 4 are all significantly red-shifted, corresponding to sp 2 The intrinsic π→π* electronic transition of hybridized C and N, and the emergence of a prominent new absorption band between 449 and 800 nm, originate from the n→π* electronic transition of the lone pair electrons in the edge N atoms in the heptazine skeleton, which significantly increases the light response range of Examples 1 to 4.

[0076] Figure 4 The UV-visible diffuse reflectance spectra of Comparative Examples 1-2 and Examples 5-6 are as follows. Compared with Comparative Example 1, the absorption edges of Examples 5-6 and the comparative example are obviously red-shifted, and a prominent new absorption band also appears near 500nm. This is due to the n→π* electron transition of the lone pair of electrons in the edge N atoms in the heptazine skeleton, which significantly increases the light response range of Examples 5-6.

[0077] Photocatalytic test:

[0078] The photocatalytic performance of the photocatalyst was evaluated by taking photolysis of water to produce hydrogen as an example: First, 100 mg of the catalyst was dispersed in a quartz reactor filled with 90 mL of deionized water and ultrasonicated for 10 minutes, followed by adding 10 mL of triethanolamine as a sacrificial agent, followed by adding chloroplatinic acid aqueous solution and irradiating with xenon light to load 3.0 wt% of Pt nanoparticles as a co-catalyst. An offline hydrogen production system was used, and 1 mL of gas was extracted every 1 hour under the conditions of irradiation with a 300W xenon lamp equipped with a UVCUT-420nm or UVCUT-510nm filter, and then injected into a gas chromatograph equipped with a thermal conductivity detector to determine the hydrogen content, and the hydrogen production rate per hour was calculated based on the standard curve of hydrogen. Figure 5 The photocatalytic hydrogen production performance diagram of Comparative Example 1 and Examples 1 to 4 under visible light with a wavelength greater than 510 nm shows that the photocatalytic hydrogen production rates of Examples 1 to 4 are improved to varying degrees, among which the photocatalytic hydrogen production rate of Example 3 is 7.1 times that of Comparative Example 1. Figure 6 The photocatalytic activities of Comparative Examples 1-2 and Examples 1-6 under visible light irradiation with a wavelength greater than 420 nm, the photocatalytic hydrogen production activities of Examples 1-6 are higher than those of Comparative Examples 1 and 2, among which Example 6 has the highest photocatalytic activity, which is 12.2 times that of Comparative Example 1.

[0079] The results of the embodiment show that the present invention utilizes the molten salt method to synthesize red crystalline carbon nitride, and the synthesized crystalline carbon nitride has higher crystallinity and a wider visible light absorption range, which solves the disadvantage of low visible light utilization rate of carbon nitride, promotes the separation and transmission of photogenerated carriers, and has better photocatalytic hydrogen production activity.

[0080] The above description is only for better explaining the embodiments of the present invention, and is not intended to limit the present invention. Any modification or equivalent substitution that does not depart from the spirit and scope of the present invention shall fall within the scope of the present invention.

Claims

1. A method for preparing red crystalline carbon nitride, characterized in that: The method comprises the following steps: uniformly mixing nitrogen-rich precursor melamine with molten salt potassium thiocyanate and ammonium chloride, performing a thermal polycondensation reaction, and then washing and drying to obtain red crystalline carbon nitride.

2. The method for preparing red crystalline carbon nitride according to claim 1, characterized in that: The conditions of the thermal polycondensation reaction are: in an argon atmosphere, the temperature is raised to 400° C. and kept at this temperature for 1 hour, and then the temperature is further raised to 500-610° C. and kept at this temperature for 1-2 hours.

3. The method for preparing red crystalline carbon nitride according to claim 1, characterized in that: The molar ratio of potassium thiocyanate to ammonium chloride is (4-2):

1.

4. The method for preparing red crystalline carbon nitride according to claim 1, characterized in that: The mass ratio of melamine to potassium thiocyanate is (2-3):

2.

5. The method for preparing red crystalline carbon nitride according to claim 1, characterized in that: The drying temperature is 60°C.

6. Red crystalline carbon nitride obtained by the preparation method according to any one of claims 1 to 5.

7. Use of the red crystalline carbon nitride according to claim 5 as a photocatalyst.

8. The use of red crystalline carbon nitride according to claim 7, characterized in that: Applied in photocatalytic water decomposition to produce hydrogen.

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