CN115155641B - Oxygen atom in-situ self-doped high-crystallization type carbon nitride photocatalyst and preparation method thereof - Google Patents

Abstract

The invention relates to an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst and a preparation method thereof. The invention innovatively uses unimolecular carbohydrazide (CBZ) as the precursor of carbon, nitrogen and oxygen, and develops a photocatalyst with excellent NIR activity through the molten salt method. The preparation method does not require additional chemical reagents, and the structural oxygen in the precursor is directly introduced into the CN framework and replaces part of the edge N sites. At the same time, the polymerization process is adjusted by molten salt treatment to form an ordered structure to achieve efficient charge transfer rate. Compared with conventional CN, the bandgap of the synthesized sample is narrowed with the introduction of structural oxygen and the higher order degree of CN framework structure, which enhances the separation and transfer of photogenerated charge carriers. The photocatalyst has unique hydrogen production activity in the near-infrared region and has the potential for full water splitting at long wavelengths. Therefore, the present invention is of great significance for promoting the development of metal-free photocatalysts that efficiently utilize the solar spectrum.

Description

Oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst and its preparation method

technical field

The invention belongs to the field of photocatalytic materials, and relates to a highly crystalline carbon nitride photocatalyst and a preparation method thereof, in particular to a highly crystalline carbon nitride photocatalyst with near-infrared photocatalytic activity and a preparation method thereof, in-situ self-doped with oxygen atoms.

Background technique

With the intensification of the contradiction between human's growing energy demand and energy shortage, the development and utilization of new energy, especially solar energy, is also showing a more important position. At present, photocatalytic technology is an emerging technology to clean and develop renewable energy, which can directly convert solar energy into chemical energy, which is expected to solve the energy crisis and environmental problems brought about by the increasing depletion of energy. Such as the conversion of clean energy (photolysis of water to produce H ₂ , total water decomposition to produce H ₂ and O ₂ , photocatalytic conversion of CO ₂ to CH ₄ , photodegradation of pollutants in water, etc.). However, the main obstacles limiting this direct solar-to-fuel conversion process are the narrow light absorption range of photocatalysts and the slow separation/migration kinetics of photogenerated carriers. At the same time, the solar spectrum is composed of 5% ultraviolet light, 42-45% visible light and more than 50% near-infrared (NIR) light. However, most of the photocatalysts currently studied only absorb ultraviolet light or part of visible light. Therefore, it is important and urgent to design photocatalysts with NIR activity to realize the effective utilization of solar energy.

Carbon nitride (C ₃ N ₄ , CN) has unique optical and electrical properties, and is widely used in the fields of photocatalytic hydrogen production, total water splitting, and carbon dioxide reduction. However, polycarbon nitride (PCN) prepared by conventional calcination of nitrogen-containing precursors is semi-crystalline or amorphous, and incomplete deamination results in excessive hydrogen bonds and amino groups in CN, which are proved to be recombination sites for photogenerated electron-hole pairs, hindering the trans-plane conduction of electrons and leading to low electrical conductivity. To overcome the above shortcomings, crystallization engineering is regarded as an effective strategy to enhance photocatalytic activity. At present, a variety of new highly crystalline CN structures, such as polytriazinimide (PTI), polyheptazinimide (PHI), and PHI/PTI, can be formed by treating nitrogen-containing precursors or PCN with eutectic salts. At the same time, the salt treatment process will destroy the hydrogen bonds in the carbon nitride structure and amino groups that are not conducive to the separation of photogenerated electron-hole pairs, increase the specific surface area of carbon nitride and reduce the migration path of photogenerated electrons, which will further improve the photocatalytic performance of CN.

In addition, the existing technology also adopts a non-metal doping strategy to optimize the energy band structure of carbon nitride, broaden the photoresponse range, and promote and inhibit the migration and recombination of photogenerated charges, respectively. Meanwhile, the incorporation of oxygen atoms has been shown to be an effective means to enhance the n→π* electronic transition in pure carbon nitride, which can extend its visible light absorption range to 600 nm, and provides a promising strategy for developing long-wavelength responsive photocatalysts. However, when oxalic acid, O ₂ /H ₂ O ₂ , acid solution, etc. are incorporated into the carbon nitride framework as oxygen sources, the incorporation sites of oxygen atoms in the carbon nitride framework cannot be accurately predicted, and there are problems such as structural damage caused by excessive oxidation.

In summary, the construction of heterostructures has become the main strategy for NIR photocatalyst research, but these heterostructures are affected by the chemical instability and toxicity of each component, which brings great challenges to the development of NIR photocatalysts.

Contents of the invention

In order to solve the above problems, the present invention provides a crystalline carbon nitride photocatalyst and its preparation method, using the dual strategy of in-situ doping of structural oxygen in the precursor and improving the crystallinity of CN materials to improve the activity of the NIR-responsive carbon nitride photocatalyst. Compared with the introduction of additional oxygen source reagents, this method is more attractive to synthesize oxygen-doped carbon nitride with a single precursor that is economical, non-toxic, environmentally friendly and readily available, and to prepare efficient and stable NIR photocatalytic materials with a safer and faster synthesis process.

The present invention first uses unimolecular carbohydrazide (CBZ) as the precursor of carbon, nitrogen and oxygen, and develops an excellent NIR crystalline carbon nitride photocatalyst for the field of photocatalysis through a simple one-step molten salt method. This method does not require additional treatment and addition of chemical reagents. The structural oxygen in the precursor is directly introduced into the CN framework and replaces part of the edge N sites. At the same time, the polymerization process is adjusted by molten salt treatment to form a highly crystalline ordered structure to achieve efficient charge transfer rate. Compared with conventional CN, the bandgap of the synthesized sample is narrowed with the introduction of structural oxygen and the formation of local internal electric field, which can enhance the separation of photogenerated charge carriers and facilitate charge transfer. The experimental results show that the HER activity of the synthesized sample is significantly enhanced under visible light, has unique hydrogen production activity under near-infrared light range, and has the potential of full water splitting under long wavelength. This work provides a useful strategy for the design and fabrication of efficient metal-free photocatalysts that efficiently utilize the solar spectrum. The whole preparation process has simple steps, mild and environment-friendly conditions, good stability and low energy consumption, and has obvious advantages.

In order to achieve the above object, the specific technical solutions of the present invention are as follows:

A method for preparing an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst, comprising the following steps:

S1. Place the precursor under a protective atmosphere to perform a pre-condensation reaction to obtain a pre-condensation product;

S2. Mix and grind the precondensation product and the eutectic salt system evenly, then transfer it to a crucible, place the crucible in a heating furnace, roast it under a protective atmosphere and keep it warm for a period of time, and then naturally cool down to room temperature;

S3. Wash the solid mixture in the crucible with a large amount of hot water at 60-90°C, remove all the salt therein, and dry it in an oven at 60-70°C to obtain the target product.

In this method, the oxygen-containing carbon nitride precursor is pre-condensed, and then thermally polymerized with the eutectic salt. After the reaction, the hot water is washed to remove the salt and dried to obtain a high-crystalline carbon nitride photocatalyst, which is denoted as MC-CN in this paper.

Preferably, the precursor is carbohydrazide. The oxygen-doped crystalline carbon nitride of the present invention presents a polyheptazinimide (PHI) structure, and the structural oxygen in the carbohydrazide is directly introduced into the carbon nitride skeleton, and specifically replaces the double-coordination edge nitrogen site.

In the above technical solution, carbohydrazide molecules are used in the reaction system to select the oxygen-containing carbon nitride precursor. The structural oxygen in the carbohydrazide can be self-doped into the carbon nitride framework and replace specific nitrogen sites, avoiding the excessive oxidative damage to the carbon nitride framework structure caused by the external oxygen source reagent. And the raw material is environmentally friendly and easy to get, economical and practical. Selecting the eutectic salt as the reaction medium can accelerate the mass transfer rate and polymerization rate between the reactants, and lead to changes in the molecular structure of the carbon nitride material. The newly generated highly ordered structure can greatly improve the separation and transfer efficiency of photogenerated charges, and in principle can broaden the light absorption range of carbon nitride and improve the catalytic activity.

Preferably, the protective atmosphere is nitrogen, oxygen, argon or air.

Preferably, the temperature of the precondensation reaction is 300-600° C., and the reaction time is 1-6 hours.

Preferably, the eutectic salt system is NaCl-KCl-LiCl system, the molar ratio is 32-36:32-36:30-34; the mass ratio of the precondensation product to the eutectic salt system is 1:5-15.

Preferably, the heating rate of the calcination is 1-5°C min ^-1 , the temperature is 300-600°C, and the holding time is 1-6h.

Another object of the present invention is to provide an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst, which is prepared by the above preparation method.

Further, the carbon nitride structure is: based on the PHI high crystal structure; the in-situ self-doping oxygen atom exists in the form of: doping into the carbon nitride framework to replace part of the N site or C site, which helps to enhance the photocatalytic activity. The specific substitution position of this self-doped oxygen is a double-coordinated edge nitrogen site, which facilitates the charge redistribution of atoms around the doped oxygen atom, thereby generating a local built-in electric field and promoting the separation of electron-hole pairs.

Further, the doping amount of the oxygen atoms is 8-24.0%, preferably 19.78%, based on the atomic percentage of oxygen atoms in the photocatalyst.

The present invention also provides the application of the above-mentioned photocatalyst in photocatalysis, which can be applied in the fields of near-infrared photocatalytic hydrogen production or total water splitting.

The beneficial effects of the present invention are:

(1) Aiming at the self-incorporation of O atoms and the preparation method of NIR-responsive crystalline CN, the present invention is the first to use single-molecule carbohydrazide to achieve a significant increase in the near-infrared activity of a single carbon nitride material, while effectively avoiding the environmental problems caused by the use of toxic reagents in the process of oxygen atom incorporation and the selection of substitution sites.

(2) Highly crystalline carbon nitride with in-situ O doping was prepared by a simple molten salt method, and the utilization of near-infrared light was realized at the same time. The incorporation of structural oxygen enhances the n→π* electronic transition in the system, effectively extending the light absorption range to about 1400nm, and the highly ordered structure greatly improves the separation and transfer efficiency of photogenerated charges. Therefore, the optimization of energy band structure and electronic conductivity endows the synthesized material with excellent visible-light photocatalytic activity, near-infrared photocatalytic activity, and potential for total water splitting at long wavelengths. At the same time, due to the orderly reorganization of the atomic structure, it has strong stability and usability.

(3) There are no expensive and toxic raw materials in the present invention, and the research concept is green and environmentally friendly. The preparation process uses unimolecular carbohydrazide as the source of C, N, and O, and no toxic solvents are used. At the same time, the eutectic salt can be recovered through recrystallization and reused, which is in line with the concept of environmental protection and sustainability of catalysts.

(4) The NIR photocatalyst MC-CN prepared by the method of the present invention has excellent photocatalytic activity and outstanding stability (the photocatalytic activity of MC-CN is 11.6 times (λ>420nm) and 300 times (λ=450nm) of CN, and the hydrogen production at λ>700nm is 57 μmol·g ^-1 ). It also showed strong hydrogen evolution stability after continuous operation for 15h.

(5) In the present invention, the MC-CN photocatalytic material is obtained after heat-treating the precondensation product by a simple molten salt method. The process is simple, the operating conditions are mild, the stability is good, the energy consumption is small, and the impact on the environment is small. Commercial production.

Description of drawings

Figure 1(a)-(e) are the morphological characterization diagrams of the product MC-CN of Example 1 at different magnifications; Figure 1(f)-(g) are the fast Fourier transform (FFT) diagrams of Figure 1(e).

Figure 2(a)-(b) are the XRD patterns of the product MC-CN of Example 1.

Figure 3(a)-(d) are the XPS diagrams of the product MC-CN of Example 1.

Fig. 4 is the ultraviolet-visible diffuse reflectance spectrum of the product MC-CN of embodiment 1.

Figure 5(a)-(c) are the photocatalytic performance diagrams, total water splitting performance diagrams and cycle stability diagrams of the product MC-CN of Example 1 at different wavelengths.

Fig. 6 is a schematic diagram of the atomic structure of MC-CN according to the present invention.

Detailed ways

The following non-limiting examples can enable those skilled in the art to understand the present invention more fully, but do not limit the present invention in any way.

The test methods described in the following examples, unless otherwise specified, are conventional methods; the reagents and materials, unless otherwise specified, can be obtained from commercial sources.

The basic idea of the present invention is: using carbohydrazide, potassium chloride, sodium chloride, and lithium chloride as raw materials, this simple preparation process produces crystalline carbon nitride containing in-situ oxygen doping, and uses the product for photocatalytic conversion into clean energy in the near-infrared region, and the material has excellent catalytic activity and good recyclability in the field of photocatalysis.

In order to further illustrate the technical effect of the present invention, the following description will be made in conjunction with specific examples.

Example 1

Using the method of the present invention to prepare an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst comprises the following steps:

20 g of carbohydrazide was placed under a nitrogen atmosphere for precondensation reaction at a reaction temperature of 450° C. and a reaction time of 2 hours to obtain a precondensation product.

The precondensation product with a mass ratio of 1:15 was mixed with NaCl-KCl-LiCl (molar ratio 34:34:32) eutectic salt system, ground evenly, and then transferred to a crucible, and the crucible was placed in a heating furnace, and roasted in an air atmosphere with a heating rate of 5°C min ^-1 , a temperature of 550°C, and a holding time of 2 hours, and then naturally cooled to room temperature.

Wash the solid mixture in the crucible with a large amount of hot water at 80°C, remove all the salt therein, and dry it in an oven at 60°C. The final yield of sample MC-CN was 20%.

Example 2

Using the method of the present invention to prepare an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst comprises the following steps:

20 g of carbohydrazide was placed under a nitrogen atmosphere for precondensation reaction at a reaction temperature of 300° C. and a reaction time of 6 hours to obtain a precondensation product.

The precondensation product with a mass ratio of 1:10 was mixed with the NaCl-KCl-LiCl (molar ratio: 36:36:34) eutectic salt system, ground evenly, and then transferred to a crucible, which was placed in a heating furnace and roasted under a nitrogen atmosphere at a heating rate of 3°C min ^-1 , a temperature of 600°C, and a holding time of 1 hour, and then naturally cooled to room temperature.

Wash the solid mixture in the crucible with a large amount of hot water at 90°C, remove all the salt therein, and dry it in an oven at 70°C. The final yield of sample MC-CN was 19.5%.

Example 3

Using the method of the present invention to prepare an oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst comprises the following steps:

20 g of carbohydrazide was placed under a nitrogen atmosphere for precondensation reaction at a reaction temperature of 600° C. and a reaction time of 1 hour to obtain a precondensation product.

The precondensation product with a mass ratio of 1:5 was mixed with NaCl-KCl-LiCl (molar ratio 32:32:30) eutectic salt system, ground evenly and then transferred to a crucible, the crucible was placed in a heating furnace, and roasted under a nitrogen atmosphere at a heating rate of 1°C min ^-1 at a temperature of 300°C for a holding time of 6 hours, and then naturally cooled to room temperature.

Wash the solid mixture in the crucible with a large amount of hot water at 60°C, remove all the salt therein, and dry it in an oven at 60°C. The final yield of sample MC-CN was 19%.

Comparative example 1

In order to further explore the effect of self-doping and crystallinity of oxygen on the photocatalysis of materials, polymeric carbon nitride was prepared as a control group by using melamine as a precursor and adopting a common thermal condensation polymerization method, including the following steps:

Put 20g of melamine in a muffle furnace for thermal polycondensation reaction at a reaction temperature of 550°C, a heating rate of 5°C min ^-1 , a holding time of 2h, and then cool down to room temperature naturally. Grind to obtain a yellow powder, and the finally obtained sample is denoted as CN.

test result

Using detection means such as SEM, TEM, EDS, XPS and DRS, the product MC-CN of Example 1 and the product CN of Comparative Example 1 have been studied on the aspects of morphology, structure, light absorption properties and photocatalytic activity. The specific contents are as follows:

MC-CN morphology characterization

Referring to Fig. 1, it is the SEM and TEM images of the product MC-CN material of Example 1. Under different magnifications, MC-CN exhibits a hierarchical nanoflower morphology composed of small nanorods with a diameter of about 1.5 μm (Fig. EDS analysis (Fig. 1c) showed that C, N, and O elements were uniformly distributed in the sample, confirming that O was successfully integrated into the structure of MC-CN. The TEM image (Fig. 1d) shows that MC-CN contains layered stacked nanosheets (transparent regions) with nanorods (opaque regions). The high-resolution TEM image (Fig. 1e) confirmed the highly crystalline form of MC-CN, and the lattice spacing measured from the fast Fourier transform (FFT) pattern of HRTEM (Fig. 1f and Fig. 1g) was 0.32 and 1.05 nm, corresponding to the (002) and (100) crystal planes of PHI.

Referring to Fig. 2, it is the XRD pattern of the sample in the present invention, MC-CN has two main peaks at 8.1 ° and 27.54 °, they are with the extension distance of the repeating unit between the planes in the (100) diffraction plane and the stacking of π-π conjugated aromatic rings on the (002) diffraction plane. Compared with the product CN (27.38°) of Comparative Example 1, the (002) peak of MC-CN shifted to 27.54° to the right, indicating that the strong π-π interaction leads to a closer distance between adjacent heptazine layers. The calculated lattice spacing based on the XRD pattern is 0.323 nm, which is consistent with the lattice data derived from the lattice fringe results in Fig. 1f. The extended in-plane periodicity and reduced interlayer distance can significantly promote the transfer of photogenerated carriers within the stacked structure, reduce the surface barrier, and benefit the improvement of photocatalytic performance.

MC-CN structure characterization

Referring to Fig. 3, the chemical state and composition of MC-CN were carefully studied by XPS. A surge of O signal was found in the full XPS spectrum of MC-CN (19.78% O content) compared to CN (8.58% O content), indicating successful incorporation of O species (Fig. 3a). In the C1s spectrum (Fig. 3b), there are three peaks at 288.2eV, 286.6eV and 284.8eV, corresponding to NC=N, C≡N/C-NHx and CC in the oxygen-containing CN framework. Furthermore, the new peak at 288.9 eV is associated with the CO bond, which suggests that the oxygen atom is directly bonded to the ^sp2 hybridized carbon by substituting the nitrogen atom. The four peaks at 398.2, 399.3, 400.5, and 403.7 eV of the N1s spectra of CN and MC-CN were assigned to CN=C(N2), N-(C) ₃ (N3), CNH groups and charge effects (Fig. 3d). In which the N2/N3 ratio decreased from 2.69 (CN) to 2.39 (MC-CN), indicating that some sp ^2- bonded N in the tri-s-triazine ring (N2) in MC-CN were replaced by O atoms. The O1s spectrum analysis of MC-CN (Fig. 3c) revealed the presence of CO bonds at 532 eV in addition to the adsorbed water and adsorbed oxygen peaks at 533 and 534 eV. This result further confirms the existence of structural oxygen in MC-CN and replaces some of the dicoordinated edge N sites.

Light Absorption Properties of MC-CN

Referring to Fig. 4, the DRS diffuse reflectance spectrum shows that the absorption edge of MC-CN extends significantly from the visible region to the near-infrared region and also absorbs at 1400 nm, which is rare in previous photocatalytic studies. At the same time the sample itself also showed a dark brown color. In contrast, CN exhibits only an absorption edge at ∼462 nm (with a bandgap of 2.68 eV). In addition, two absorption states for π-π* and n-π* electronic transitions can be observed in the spectrum of MC-CN with absorption shoulders between 400–450 nm and 450–900 nm. The intensity of the shoulder between 400–450 nm is higher than that of CN, indicating the enhanced ordering of the MC–CN structure, while the newly formed shoulder between 450–900 nm is attributed to the incorporation of O into the carbon nitride. The bandgap of MC-CN is also narrowed to 1.42eV (inset).

Photocatalytic activity of MC-CN materials

Referring to Figure 5, in order to evaluate the photocatalytic activity of MC-CN, the hydrogen production activity (Figure 5a), cycle stability (Figure 5b) and total water splitting performance (Figure 5c) of the material were measured at different wavelengths. The results show that the photocatalytic reactivity of MC-CN is 11.6 times higher than that of conventional CN materials (λ>420nm), and the hydrogen production rate in the near-infrared region (λ>700nm) is 57μmol·g ^-1 . It is worth mentioning that MC-CN has an overall water-separating ability even under long-wavelength irradiation of λ = 600 nm. This study proposes a unique strategy to enhance the visible light photocatalytic activity of the material and stimulate near-infrared photocatalytic activity through the introduction of O atoms and the generation of highly ordered structures, which can provide effective applications in the field of near-infrared photocatalysis. It also provides a method for the design and preparation of efficient metal-free photocatalysts that efficiently utilize the solar spectrum.

Claims (9)

1. a kind of preparation method of oxygen atom in-situ self-doping high crystalline carbon nitride photocatalyst, it is characterized in that: comprise the following steps: S1. Place the precursor under a protective atmosphere to perform a pre-condensation reaction to obtain a pre-condensation product; S2. Mix and grind the precondensation product and the eutectic salt system evenly, then transfer to a crucible, put the crucible into a heating furnace, roast it under a protective atmosphere and keep it warm for a period of time, and then naturally cool down to room temperature; S3. Wash the solid mixture in the crucible with a large amount of hot water at 60-90°C, remove all the salt therein, and dry it in an oven at 60-70°C to obtain the target product; The precursor is carbohydrazide; The eutectic salt system is NaCl-KCl-LiCl system. 2. The method according to claim 1, characterized in that: the protective atmospheres in steps S1 and S2 are nitrogen, oxygen, argon or air. 3. The method according to claim 1, characterized in that: the temperature of the precondensation reaction is 300-600 °C, and the reaction time is 1-6 h. 4. The method according to claim 1, characterized in that: the molar ratio of the NaCl-KCl-LiCl system is 32-36:32-36:30-34; the mass ratio of the precondensation product to the eutectic salt system is 1:5-15. 5. The method according to claim 1, characterized in that: the heating rate of the roasting is 1-5 °C/min, the temperature is 300-600 °C, and the holding time is 1-6 h. 6. An oxygen atom in-situ self-doping highly crystalline carbon nitride photocatalyst, characterized in that it is prepared according to the preparation method described in any one of claims 1-5. 7. The photocatalyst according to claim 6, characterized in that: the carbon nitride structure is: a crystal structure based on PHI high crystallinity; the existence form of the in-situ self-doping oxygen atom is: incorporated into the carbon nitride framework to replace part of the N site or C site. 8. The photocatalyst according to claim 6, characterized in that: the doping amount of the oxygen atoms is 8-24.0% based on the atomic percentage of oxygen atoms in the photocatalyst. 9. The application of the photocatalyst according to claim 6 in the field of near-infrared photocatalytic hydrogen production or total water splitting.