CN111085231B - Nano-sheet assembled hierarchical porous flower-shaped carbon nitride and preparation method and application thereof - Google Patents

Abstract

A graded porous flower-like carbon nitride assembled by nanosheets and a preparation method and application thereof belong to the technical field of semiconductor photocatalytic materials. The carbon nitride is a hierarchical porous flower-like structure, which is obtained by layering and orderly assembly of porous nanosheets, wherein the flower body size of the flower-like carbon nitride is 4-10 μm, and the thickness of the nanosheets is 20-10 μm. 60nm, the specific surface area is 90～140m ² g ^‑1 , the pore volume is 0.4～1m ³ g ^‑1 , and the pore diameter is 1～6nm. The hierarchical porous flower-like carbon nitride of the present invention enhances the photocatalytic activity from three aspects: first, the hierarchical porous structure enhances the light collection efficiency, and solves the stacking problem of nanosheets; secondly, the hierarchical porous structure increases the specific surface area of the material, It provides more active sites for surface redox reactions; finally, due to the unique hierarchical structure with the porous network, a unique transport channel is formed, which promotes the diffusion of reactant molecules and carriers.

Description

Hierarchical porous flower-like carbon nitride assembled by nanosheets and its preparation method and application

technical field

The invention belongs to the technical field of semiconductor photocatalytic materials, and in particular relates to a hierarchical porous flower-shaped carbon nitride with enhanced activity of visible light photocatalytic reduction of carbon dioxide, a preparation method and application thereof.

Background technique

Today, energy consumption and environmental pollution have become two major problems in today's world. Global warming caused by excessive human activities and exhaust emissions has made environmental problems more and more serious. As the main gas that causes the greenhouse effect, the reduction of carbon dioxide is very important. Photocatalytic reduction of CO ₂ and generation of CH ₄ , CO, and other fuel gases have attracted extensive attention due to their high efficiency and environmental friendliness.

Carbon nitride (g～C ₃ N ₄ ) has been widely used in photocatalytic hydrogen production, carbon dioxide reduction and pollutant degradation due to its advantages of no metal, good chemical and thermal stability, and moderate energy band structure. The photocatalytic activity of graphitic carbon nitride synthesized by conventional methods is limited due to low specific surface area and high carrier recombination. In order to improve the photocatalytic performance and application value of carbon nitride, it is very important to design materials with ideal morphology and advanced structure. As a unique structure, hierarchical porous structure has attracted people's attention due to its ability to increase light absorption efficiency, increase specific surface area, and facilitate the transport of reactant molecules. Considering the superiority of hierarchical porous structures, hierarchical porous structures of carbon nitride with different morphologies have been designed and prepared in recent years, such as hierarchical porous carbon nitride nanotubes, hierarchical porous carbon nitride nanoribbons, etc. However, hierarchically porous flower-like carbon nitrides have rarely been reported. So far, Sun et al. (Sun, J.; Xu, J.; Grafmueller, A.; Huang, X.; Liedel, C.; Algara-Siller, G.; Willinger, M.; Yang, C.; Fu, Y.; Wang, X.; Shalom, M., Self～assembled carbon nitride for photocatalytic hydrogen evolution and degradation of p～nitrophenol.Appl.Catal.B～Environ.2017, 205, 1～10) reported that by introducing a secondary electrode After about 20 hours of post-treatment with a non-toxic solvent, a similar flower-like carbon nitride was prepared, but the sheet-like monomer was thicker (90-130) nm, and did not produce a porous network. Non-toxic solvents will pollute the environment, making it difficult to apply them on a large scale.

Contents of the invention

The object of the present invention is to propose a nanosheet-assembled hierarchical porous flower-like carbon nitride and its preparation method and application in view of the defects in the background technology.

To achieve the above object, the technical scheme adopted in the present invention is as follows:

A hierarchical porous flower-like carbon nitride assembled by nanosheets, characterized in that the carbon nitride is a hierarchical porous flower-like structure obtained by layering and orderly assembling porous nanosheets, wherein the flower-like nitrogen The size of the carbonized flower body is 4-10 μm, the thickness of the nanosheet is 20-60 nm, the specific surface area is 90-140 m ² g ^-1 , the pore volume is 0.4-1 m ³ g ^-1 , and the pore diameter is 1-6 nm.

A preparation method of hierarchical porous flower-like carbon nitride assembled by nanosheets, characterized in that, the hierarchical porous flower-like carbon nitride is formed by molecular self-assembly of cyanuric acid and melamine to form a cyanuric acid-melamine complex precursor; Then, the intercalation of ethanol molecules is introduced into the precursor by water bath heating method, and it is formed after calcining and polycondensation.

A preparation method of hierarchical porous flower-like carbon nitride assembled by nanosheets, characterized in that it specifically comprises the following steps:

Step 1, preparation of cyanuric acid-melamine complex precursor:

1.1 Dissolve melamine in dimethyl sulfoxide, stir until the solution is clear and transparent, and obtain a dimethyl sulfoxide solution of melamine with a mass concentration of 1×10 ⁴ ~4×10 ⁴ mg/L; dissolve cyanuric acid in di In methyl sulfoxide, stir until the solution is clear and transparent to obtain a dimethyl sulfoxide solution of cyanuric acid with a mass concentration of 1×10 ⁴ ~4×10 ⁴ mg/L;

1.2 Mix the dimethyl sulfoxide solution of melamine and the dimethyl sulfoxide solution of cyanuric acid prepared in step 1.1, and stir for 5 to 30 minutes until a white precipitate is formed to obtain a suspension; wherein, when mixing, the moles of melamine and cyanuric acid The ratio is 1: (0.1～2);

1.3 Centrifuge and wash the suspension obtained in step 1.2, and dry it at 50-70°C, and the white product obtained is the precursor of the cyanuric acid-melamine complex;

Step 2, preparation of hierarchical porous flower-like carbon nitride:

2.1 Disperse the cyanuric acid-melamine complex precursor obtained in step 1 in ethanol and stir evenly to obtain a dispersion with a mass concentration of 2×10 ⁴ to 5×10 ⁴ mg/L;

2.2 Stir and react the dispersion liquid obtained in step 2.1 in a water bath at 20-80°C for 1-5 hours, and centrifuge, wash and dry the obtained white suspension to obtain a sample;

2.3 Place the sample obtained in step 2.2 in a muffle furnace, raise the temperature from room temperature to 550 °C at a rate of 2-5 °C/min, and keep it at 550 °C for 2-8 hours. After completion, naturally cool to room temperature. Take it out to get hierarchical porous flower-like carbon nitride.

The present invention also provides the application of the above-mentioned hierarchical porous flower-like carbon nitride as a photocatalyst.

Further, the present invention also provides the application of the above-mentioned hierarchical porous flower-shaped carbon nitride in photocatalytic reduction of carbon dioxide.

Compared with prior art, the beneficial effect of the present invention is:

(1) The hierarchical porous flower-like carbon nitride prepared by the present invention enhances the photocatalytic activity from three aspects: first, the hierarchical porous structure enhances the light collection efficiency, and solves the accumulation problem of nanosheets; secondly, the hierarchical porous structure increases the The specific surface area of the material provides more active sites for the redox reaction on the surface; finally, due to the unique hierarchical structure combined with the porous network, a unique transport channel is formed, which promotes the diffusion of reactant molecules and carriers.

(2) The method for preparing hierarchical porous flower-like carbon nitride of the present invention has the advantages of being simple, time-saving and efficient, and easy to control the shape.

(3) The present invention uses the method of molecular self-assembly combined with porogen molecular intercalation to prepare a unique hierarchical porous structure so as to improve the activity of photocatalytic reduction of carbon dioxide, which can be promoted in the field of photocatalysis.

Description of drawings

Fig. 1 is the XRD diffraction pattern of the material prepared in embodiment 1, comparative examples 1～3;

Fig. 2 (a), (b), (c), (d) are respectively the scanning electron microscope picture (SEM) of the material prepared in embodiment 1, comparative example 1, comparative example 2, comparative example 3;

Figure 3 is a photocatalytic carbon dioxide reduction performance diagram of the materials prepared in Example 1 and Comparative Examples 1 to 3 under visible light radiation; wherein, (a) is prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3 The carbon monoxide performance diagram produced by the photoreduction of carbon dioxide of the material, (b) is the methane performance diagram produced by the photoreduction of carbon dioxide of the material prepared in Example 1, Comparative Example 1, Comparative Example 2, and Comparative Example 3;

Fig. 4 is the synthesizing mechanism diagram of embodiment 1.

Detailed ways

The technical solutions of the present invention will be further described in detail below in conjunction with specific embodiments. It should be understood that the purpose of the examples given is to further illustrate the content of the present invention, and should not be construed as limiting the protection scope of the present invention in any sense.

Example 1:

The preparation method of hierarchical porous flower-shaped carbon nitride (marked as CMN), the steps are as follows:

Step 1, preparation of cyanuric acid-melamine complex precursor:

1.1 Dissolve 0.5g melamine in 20mL dimethyl sulfoxide, stir for 3h until the solution is clear and transparent to obtain a dimethyl sulfoxide solution of melamine; dissolve 0.51g cyanuric acid in 20mL dimethyl sulfoxide, stir for 3h until Solution is clear and transparent, obtains the dimethyl sulfoxide solution of cyanuric acid;

1.2 Mix the dimethyl sulfoxide solution of melamine and the dimethyl sulfoxide solution of cyanuric acid prepared in step 1.1, stir for 15 minutes to form a white precipitate, and obtain a suspension; wherein, when mixing, the molar ratio of melamine and cyanuric acid is 1 :1;

1.3 Centrifuge the suspension obtained in step 1.2, wash it several times with deionized water and ethanol, and then dry it in an oven at 50°C. The white product obtained is the precursor of cyanuric acid-melamine complex (cyanuric acid～melamine for short) for CM);

Step 2, preparation of hierarchical porous flower-like carbon nitride:

2.1 Disperse 0.6g of the cyanuric acid-melamine complex precursor (CM) obtained in step 1 in 20mL of ethanol, stir evenly to obtain a dispersion;

2.2 The dispersion liquid obtained in step 2.1 was stirred and reacted in a water bath at 60°C for 3 hours, the obtained white suspension was centrifuged, washed several times with deionized water and ethanol, and then dried in an oven at 50°C to obtain a white powder sample;

2.3 Place the white powder sample obtained in step 2.2 in a muffle furnace, raise the temperature from room temperature to 550 °C at a rate of 2.5 °C/min, and keep it at 550 °C for 4 hours. After completion, naturally cool to room temperature and take it out. The obtained brown fluffy powder is hierarchical porous flower-like carbon nitride CMN.

Comparative Example 1: In order to verify the effect of the hierarchical porous structure in the enhancement of photocatalytic activity, the hierarchical porous flower-like carbon nitride CMN obtained in Example 1 was ultrasonically treated in deionized water for 2h (marked as CMN-U). It is to destroy the hierarchical porous flower-like morphology. As shown in Figure 2b, Comparative Example 1 exhibited a bulk-packed morphology, confirming the collapse of the hierarchical porous flower-like structure. Combining Figure 3(a) and (b), the photocatalytic reduction of carbon dioxide to produce carbon monoxide and methane formation rate diagrams, the carbon monoxide generation rate and methane generation rate of CMN-U are lower than those of CMN, which is due to the destruction of the hierarchical porous structure , the active sites for the reaction are reduced, resulting in a decrease in photocatalytic activity. The important role of the hierarchical porous structure in the enhancement of photocatalytic activity is confirmed.

Comparative Example 2: In order to verify the importance of ethanol molecular intercalation for the formation of hierarchical porous flower-like carbon nitride, compared with Example 1, the steps of introducing ethanol molecular intercalation in steps 2.1 and 2.2 were omitted, and the cyanide obtained in step 1 was directly The uric acid-melamine complex precursor is calcined, and the obtained product is marked as CMN-D. As shown in Figure 2(c), Comparative Example 2 shows a hierarchical porous flower-like structure similar to Example 1, the thickness of the nanosheets in Example 1 is about 40nm, but the thickness of the nanosheets is about 70nm thicker, and There is no porosity in the sheet. Combining Figure 3(a) and (b), the photocatalytic reduction of carbon dioxide to produce carbon monoxide and methane formation rate diagrams shows that the carbon monoxide formation rate and methane formation rate of CMN-D are lower than those of CMN, which is due to the absence of ethanol molecules. Intercalation, making the nanosheets thicker and void-free, impedes the diffusion of carriers and reactant molecules. The importance of ethanol molecular intercalation for hierarchical porous flower-like structures is confirmed.

Comparative example 3

The preparation of carbon nitride (marked as CN) is as follows: 1.2g of urea and 2.5g of melamine are uniformly ground, then placed in a muffle furnace, and the temperature is raised from room temperature to 550°C at a heating rate of 5°C/min, and Keep it warm at 550°C for 4 hours. After completion, cool it down to room temperature naturally, take it out, and the product obtained is CN.

Comparative Example 3 is the carbon nitride prepared by the traditional method. It can be seen from the scanning electron microscope image in Figure 2d that the carbon nitride prepared by the traditional method has serious problems such as interlayer stacking, and in Figure 3 (a) and (b) the photocatalytic The reduction of carbon dioxide to carbon monoxide and methane formation rate plots showed that CN showed the lowest photocatalytic activity, which also confirmed the superiority of hierarchically porous flower-like carbon nitride.

For the above-mentioned Example 1, the materials obtained in Comparative Examples 1-3 were subjected to photocatalytic carbon dioxide reduction activity experiments, and the specific steps were as follows:

(1) Take 30mg of sample and put it in a shallow dish, add 3ml of ethanol and ultrasonically disperse for 5min;

(2) The ultrasonic sample is dried in an oven until it forms a film, and then 0.5ml of deionized water is added dropwise;

(3) Vacuum the reactor, and then introduce carbon dioxide; set the photocatalytic online analysis system (Perfect LightLabsolar 6A) for 30min each cycle; keep the system pressure at about 70-80Kpa, and turn on the 300W xenon lamp to start the photocatalytic reduction of carbon dioxide reduction experiment. Products of carbon dioxide photoreduction were detected by gas chromatography.

3 is a photocatalytic carbon dioxide reduction performance diagram of the carbon nitride materials prepared in Example 1 and Comparative Examples 1-3 under simulated sunlight. Methane and carbon monoxide are the main products in the carbon dioxide reduction process. Figure 3(a) and (b) shows that Example 1 has excellent photocatalytic carbon dioxide reduction activity, and the production rate of carbon monoxide and methane is the highest. The enhanced photocatalytic activity of Example 1 is mainly attributed to the hierarchical porous structure, which enhances light collection and improves the aggregation of nanosheets, exposing a large specific surface area and facilitating the diffusion of reactants and supports.

It should be pointed out that the specific embodiments described above can enable those skilled in the art to understand the present invention more comprehensively, but do not limit the present invention in any way. Therefore, those skilled in the art should understand that the present invention can still be modified or equivalently replaced; and all technical solutions and improvements that do not depart from the spirit and technical essence of the present invention should be covered by the protection scope of the patent of the present invention.

Claims (4)

1. A preparation method of hierarchical porous flower-like carbon nitride assembled by nanosheets, characterized in that, specifically comprising the following steps: Step 1, preparation of cyanuric acid-melamine complex precursor: 1.1 Dissolve melamine in dimethyl sulfoxide, stir until the solution is clear and transparent, and obtain a dimethyl sulfoxide solution of melamine with a mass concentration of 1×10 ⁴ ~4×10 ⁴ mg/L; dissolve cyanuric acid in di In methyl sulfoxide, stir until the solution is clear and transparent to obtain a dimethyl sulfoxide solution of cyanuric acid with a mass concentration of 1×10 ⁴ ~4×10 ⁴ mg/L; 1.2 Mix the dimethyl sulfoxide solution of melamine and the dimethyl sulfoxide solution of cyanuric acid prepared in step 1.1, and stir for 5 to 30 minutes until a white precipitate is formed to obtain a suspension; wherein, when mixing, the moles of melamine and cyanuric acid The ratio is 1: (0.1～2); 1.3 Centrifuge the suspension obtained in step 1.2, wash and dry to obtain the cyanuric acid-melamine complex precursor; Step 2, preparation of hierarchical porous flower-like carbon nitride: 2.1 Disperse the cyanuric acid-melamine complex precursor obtained in step 1 in ethanol and stir evenly to obtain a dispersion with a mass concentration of 2×10 ⁴ to 5×10 ⁴ mg/L; 2.2 Stir and react the dispersion liquid obtained in step 2.1 in a water bath at 20-80°C for 1-5 hours, and centrifuge, wash and dry the obtained suspension to obtain a sample; 2.3 Place the sample obtained in step 2.2 in a muffle furnace, raise the temperature from room temperature to 550 °C at a rate of 2-5 °C/min, and keep it at 550 °C for 2-8 hours. After completion, naturally cool to room temperature. Take it out to get hierarchical porous flower-like carbon nitride. 2. the preparation method of the hierarchical porous flower-like carbon nitride of nanosheet assembly according to claim 1, is characterized in that, described carbon nitride is the flower-like structure of hierarchical porous, is layered by porous nanosheet, orderly Assembled, wherein the flower body size of the flower-like carbon nitride is 4-10 μm, the thickness of the nanosheet is 20-60 nm, the specific surface area is 90-140 m ² g ^-1 , and the pore volume is 0.4-1 cm ³ g ^{- 1} , the pore size is 1-6nm. 3. The application of the hierarchical porous flower-like carbon nitride prepared by the method according to any one of claims 1-2 as a photocatalyst. 4. The application of the hierarchical porous flower-like carbon nitride prepared by the method according to any one of claims 1-2 in the photocatalytic reduction of carbon dioxide.

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