Modified carbon nitride composite photocatalytic material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of preparation of photocatalytic materials, and particularly provides a modified carbon nitride composite photocatalytic material as well as a preparation method and application thereof.The modified carbon nitride composite photocatalytic material is prepared by introducing an aminobenzonitrile unit for site-controllable molecular modification and successfully constructing an efficient donor-pi-acceptor structure on a g-C3N4 skeleton; the donor-pi-acceptor structure forms a strong intramolecular electric field and a directional potential gradient on a molecular level, effectively drives separation and migration of photon-generated carriers, and is used for photocatalytic CO2 reduction to achieve higher catalytic efficiency. The composite material is stable in structure in reaction, simple in preparation process, low in raw material cost, short in time consumption, suitable for large-scale production and convenient to popularize and apply.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photocatalytic material preparation, in particular to a modified carbon nitride composite photocatalytic material and a preparation method and application thereof. BACKGROUND

[0002] With the acceleration of industrialization, global climate change and energy crisis caused by the massive emission of carbon dioxide have become major challenges facing human society. Photocatalytic technology can utilize solar energy to reduce CO2 into valuable carbon-based fuels, and has been widely studied. Developing efficient, stable and inexpensive photocatalysts is a key factor to realize the technology.

[0003] Graphitic carbon nitride (g-C3N4) has become a very promising metal-free photocatalytic material due to its unique visible light response, suitable energy band structure, good chemical stability, and the advantage of being prepared from inexpensive precursors such as urea and melamine. However, bulk g-C3N4 still has problems such as limited specific surface area, insufficient visible light absorption range, and rapid recombination of photo-generated electron-hole pairs, which restricts its photocatalytic CO2 reduction activity and selectivity. At present, introducing strong electron-withdrawing groups (such as cyano, -CN) into the g-C3N4 framework through molecular engineering is an effective means to regulate its electronic structure. The introduction of cyano can optimize the energy band structure of the material, extend the light absorption range, and form an internal built-in electric field by constructing a donor-acceptor structure, thereby promoting the separation and migration of photo-generated charges. However, existing researches mostly focus on the introduction of groups themselves and their electronic effects, such as simply grafting cyano-containing small molecules onto g-C3N4 through thermal condensation. There is a lack of systematic research and precise control of the spatial geometric configuration of modified molecules, and the photocatalytic CO2 reduction performance of the composite material is not significantly improved. SUMMARY

[0004] The present application aims to overcome the problem of the existing technology that the photocatalytic CO2 reduction performance of the composite material is not significantly improved by introducing strong electron-withdrawing groups (such as cyano, -CN) into the g-C3N4 framework through molecular engineering to regulate its electronic structure, and provides a modified carbon nitride composite photocatalytic material and a preparation method and application thereof.

[0005] The present application provides a modified carbon nitride composite photocatalytic material in the first aspect, the modified carbon nitride composite photocatalytic material is a composite material formed by modifying amino benzonitrile on g-C3N4 nanosheet through thermal condensation reaction;The amino benzonitrile is selected from at least one of o-amino benzonitrile, m-amino benzonitrile and p-amino benzonitrile.

[0006] The modified carbon nitride composite photocatalytic material provided by the application is modified by introducing an aminobenzonitrile unit to control the site of the molecule, successfully constructing an efficient donor-π-acceptor structure on the g-C3N4 skeleton, which forms a strong intramolecular electric field and a directional potential gradient at the molecular level, effectively driving the separation and migration of photo-generated carriers. In the photocatalytic reduction of CO2, the catalytic efficiency is high, the CO generation rate is as high as 140.0 μmol·g -1 ·h -1 , the CH4 production rate is 23.5 μmol·g -1 ·h -1 , which is 7.1 times and more than 13 times the performance of unmodified g-C3N4, respectively. The structure of the composite material is stable in the reaction, the preparation process is simple, the raw materials are low-cost, the time-consuming is short, it is suitable for large-scale production, and it is easy to popularize and apply.

[0007] The aminobenzonitrile has an electron-donating group (-NH2) and a strong electron-withdrawing group (-CN), and ortho, meta and para isomers have completely different molecular dipole moments and electron cloud distributions. When it is used to modify g-C3N4, a more delicate donor-π-acceptor (D-π-A) structure is constructed, and the dipole arrangement and charge distribution are optimized at the molecular scale to more efficiently drive the directional separation of photo-generated carriers.

[0008] Further, the modified carbon nitride composite photocatalytic material is a donor-π-acceptor structure, in which the amino group is a donor, the benzene ring is a π conjugated bridge, and the cyano group is an acceptor.

[0009] Further, the aminobenzonitrile is p-aminobenzonitrile.

[0010] Further, the mass ratio of the g-C3N4 nanosheet to the aminobenzonitrile is 1: (0.02-0.1). Preferably, the mass ratio of the g-C3N4 nanosheet to the aminobenzonitrile is 1: (0.02-0.05).

[0011] The second aspect of the application provides a preparation method of the modified carbon nitride composite photocatalytic material as described above, comprising the following steps: S1. Grinding g-C3N4 nanosheets and aminobenzonitrile together; S2. Calcining the mixture under an inert atmosphere, and cooling to obtain the modified carbon nitride composite photocatalytic material.

[0012] Further, in S2, the calcination temperature is 350-450°C, and / or the calcination time is 20-60 min.

[0013] Further, the g-C3N4 nanosheet is prepared by the following method: the blocky g-C3N4 obtained by pyrolysis of urea is subjected to secondary calcination at 450-550 DEG C under an inert atmosphere, the calcination time is 1-3 h, and the g-C3N4 nanosheet is obtained.

[0014] The third aspect of the present application provides application of the modified carbon nitride composite photocatalytic material or the modified carbon nitride composite photocatalytic material prepared by the preparation method in photocatalytic reduction of CO2.

[0015] The fourth aspect of the present application provides a method for photocatalytic reduction of CO2, using the modified carbon nitride composite photocatalytic material or the modified carbon nitride composite photocatalytic material prepared by the preparation method as a catalyst, reducing CO2 into CO and CH4 under the conditions of water, a sacrificial agent and light; and / or the sacrificial agent is triethanolamine, and / or the light source is a xenon lamp.

[0016] Compared with the prior art, the present application has the following beneficial effects: The present application provides a modified carbon nitride composite photocatalytic material, a preparation method and application thereof, a high-efficiency donor-π-acceptor structure is successfully constructed on the g-C3N4 skeleton by introducing an aminophenyl cyanide unit for site-controllable molecular modification, a strong intramolecular electric field and directional potential gradient are formed at the molecular level by the donor-π-acceptor structure, and the separation and migration of photo-generated carriers are effectively driven. The catalytic efficiency is high in photocatalytic reduction of CO2. The CO generation rate is as high as 140.0 μmol·g -1 ·h -1 The CH4 production rate is as high as 23.5 μmol·g -1 ·h -1 The performance is 7.1 times and more than 13 times that of unmodified g-C3N4, respectively. The composite material is stable in structure in the reaction, the preparation process is simple, the raw materials are low in cost, the time consumption is short, and the composite material is suitable for large-scale production and easy to popularize and apply. BRIEF DESCRIPTION OF DRAWINGS

[0017] Figure 1 The transmission electron microscope photo of g-C3N4 prepared in Example 1.

[0018] Figure 2 The transmission electron microscope photo of pabn@g-C3N4 prepared in Example 2.

[0019] Figure 3 The transmission electron microscope photo of mabn@g-C3N4 prepared in Example 3.

[0020] Figure 4 The transmission electron microscope photo of oabn@g-C3N4 prepared in Example 4.

[0021] Figure 5 A comparison chart of the production of CO and CH4 generated by the photocatalyst prepared for Examples 1~4 in the photocatalytic CO2 reduction reaction.

[0022] Figure 6 A CO2 reduction performance chart of the pabn@g-C3N4 photocatalyst prepared for Example 5. DETAILED DESCRIPTION

[0023] The application will be described in further detail below with specific embodiments. However, it should not be understood that the scope of the above-mentioned subject matter of the application is limited to the following examples, and any technology implemented based on the content of the application falls within the scope of the application.

[0024] In the description of the specific embodiments of the application, the orientation or positional relationship terms such as "up", "down", "left", "right", "center", "inner", "outer", etc. appearing without special indication, are based on the orientation or positional relationship expressed in the drawings, or the orientation or positional relationship of the product / equipment / device of the application when it is usually used. These orientation or positional relationship terms are only for the convenience of describing the application scheme or simplifying the description in the specific embodiments, so as to facilitate the quick understanding of the scheme by the technicians, and are not intended to indicate or imply that a specific device / component / element must have a specific orientation, or be constructed and operated in a specific positional relationship, and therefore cannot be understood as a limitation of the application.

[0025] In addition, if the terms "horizontal", "vertical", "overhanging", "parallel", etc. appear, it does not mean that the corresponding device / component / element must be absolutely horizontal or vertical or overhanging or parallel, but can be slightly inclined or have a deviation. For example, "horizontal" only means that its direction is relatively more horizontal than "vertical", and does not mean that the structure must be completely horizontal, but can be slightly inclined. Alternatively, it can be simplified to mean that the corresponding device / component / element is set in the "horizontal", "vertical", "overhanging", "parallel" direction, and can have an error / deviation of ±10% with respect to the corresponding direction setting, more preferably an error / deviation of ±8% or less, more preferably an error / deviation of ±6% or less, more preferably an error / deviation of ±5% or less, and more preferably an error / deviation of ±4% or less. As long as the corresponding device / component / element is within the error / deviation range, it can still achieve its role in the application scheme.

[0026] In addition, the terms "first", "second", "third", etc. appearing in the terms are only used to distinguish the description of the same or similar parts, and should not be understood as emphasizing or implying the relative importance of the specific parts.

[0027] In addition, in the description of the embodiments of the present application, "several" "a plurality of" "several" represents at least 2. It can be 2, 3, 4, 5, 6, 7, 8, 9, etc. Any case, it can even be more than 9 cases.

[0028] In addition, in the description of the technical solutions of the present application, unless otherwise specified / limited / limited, the terms "set", "install", "connect", "connect", "set", "lay", "arrange" should be understood broadly, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrally connected, which can be welding, riveting, bolting, screwing, etc. The connection means commonly used in the art. Such connection can be mechanical connection, electrical connection or communication connection; it can be directly connected or indirectly connected through an intermediate medium; it can be the communication between two elements.

[0029] At present, introducing strong electron-withdrawing groups (such as cyano, -CN) into the g-C3N4 skeleton through molecular engineering to regulate its electronic structure is an effective means. The introduction of cyano can optimize the energy band structure of the material, expand the light absorption range, and form a built-in electric field inside the material by constructing a donor-acceptor structure, thereby promoting the separation and migration of photo-generated charges. However, existing researches mostly focus on the introduction of groups themselves and their electronic effects, such as simply grafting cyano-containing small molecules onto g-C3N4 through thermal condensation, etc. There is a lack of systematic research and precise control of the spatial geometric configuration of the modified molecules, and the photocatalytic CO2 reduction performance of the composite material is not obviously improved.

[0030] Aminobenzonitrile has both electron-donating groups (-NH2) and strong electron-withdrawing groups (-CN), and ortho, meta and para isomers have completely different molecular dipole moments and electron cloud distributions. It is used to modify g-C3N4 to construct a more refined donor-π-acceptor (D-π-A) structure, which optimizes the dipole arrangement and charge distribution at the molecular scale, thereby more efficiently driving the directional separation of photo-generated carriers.

[0031] The first aspect of the embodiment provides a modified carbon nitride composite photocatalytic material. The modified carbon nitride composite photocatalytic material is a composite material formed by aminobenzonitrile modified on g-C3N4 nanosheets through thermal condensation reaction; the aminobenzonitrile is selected from at least one of o-aminobenzonitrile, m-aminobenzonitrile and p-aminobenzonitrile.

[0032] In some embodiments, the modified carbon nitride composite photocatalytic material is a donor-π-acceptor structure, wherein the amino group is a donor, the benzene ring is a π conjugated bridge, and the cyano group is an acceptor.

[0033] In some embodiments, the aminobenzonitrile is p-aminobenzonitrile.

[0034] In some embodiments, the mass ratio of the g-C3N4 nanosheets to aminobenzonitrile is 1:(0.02~0.1). Preferably, the mass ratio of the g-C3N4 nanosheets to aminobenzonitrile is 1:(0.02~0.05).

[0035] The second aspect of this embodiment provides a method for preparing the modified carbon nitride composite photocatalyst material as described above, comprising the following steps: S1. Grind g-C3N4 nanosheets with aminobenzonitrile; S2. The mixture is calcined and cooled under an inert atmosphere to obtain the modified carbon nitride composite photocatalytic material.

[0036] In some embodiments, in S2, the calcination temperature is 350°C-450°C, and / or the calcination time is 20 min-60 min.

[0037] In some embodiments, g-C3N4 nanosheets are prepared by the following method: blocky g-C3N4 obtained by pyrolysis of urea is calcined twice at 450-550 °C under an inert atmosphere for 1-3 h to obtain g-C3N4 nanosheets.

[0038] The third aspect of this embodiment provides the application of the modified carbon nitride composite photocatalyst material described above or the modified carbon nitride composite photocatalyst material prepared by the above preparation method in photocatalytic CO2 reduction.

[0039] The fourth aspect of this embodiment provides a method for photocatalytic reduction of CO2, using the modified carbon nitride composite photocatalytic material described above or the modified carbon nitride composite photocatalytic material prepared by the above preparation method as a catalyst, reducing CO2 to CO and CH4 under water, sacrificial agent and light irradiation conditions; and / or, the sacrificial agent is triethanolamine, and / or, the light source is a xenon lamp.

[0040] To better understand the above technical solution, the following more specific implementation methods are provided for further explanation: Example 1

[0041] This embodiment provides a method for preparing g-C3N4 nanosheets, including the following steps: (1) Weigh 1 g of urea powder, grind it evenly, place it in an alumina crucible and seal it, and put the crucible into a tube furnace.

[0042] (2) Under a high-purity nitrogen atmosphere (>99.999%), the mixture was heated at 550 °C for 4 h and then naturally cooled to obtain blocky g-C3N4.

[0043] (3) The block g-C3N4 was placed in an alumina crucible and sealed. It was then placed in a tube furnace and heated at 500 °C for 2 h in a high-purity nitrogen atmosphere. After natural cooling, g-C3N4 nanosheets were obtained.

[0044] The g-C3N4 nanosheets prepared in step (3) were tested by transmission electron microscopy (TEM), and the results are as follows: Figure 1 As shown, Figure 1 Figure 1a shows that the prepared g-C3N4 exhibits a two-dimensional nanosheet structure. In Figure 1b, the measured lattice spacing is 0.33 nm, corresponding to the (002) crystal plane of g-C3N4. Elemental mapping ( Figure 1 c) indicates that C and N elements are uniformly distributed in the g-C3N4 nanosheets.

[0045] The following examples and comparative examples are based on the preparation method of g-C3N4 nanosheets prepared by Example 1. Example 2

[0046] This embodiment provides a method for preparing pabn@g-C3N4, including the following steps: (1) Weigh 100 mg g-C3N4 powder and 5 mg p-aminobenzonitrile powder, grind them evenly to mix thoroughly.

[0047] (2) The mixed powder was placed in an alumina crucible and heated to 400 °C in a tube furnace at a heating rate of 5 °C / min. The temperature was held for 40 min and then naturally cooled to obtain pabn@g-C3N4.

[0048] The pabn@g-C3N4 obtained in step (2) was subjected to TEM testing, and the results are as follows: Figure 2 As shown: Figure 2 a shows that the D-π-A conjugated system constructed by functionalizing p-aminobenzonitrile retains the original layered morphology of g-C3N4. Figure 2 The measured lattice spacing in b is 0.33 nm, corresponding to the (002) crystal plane of g-C3N4. Element mapping ( Figure 2 c) indicates that C and N elements are uniformly distributed in the pabn@g-C3N4. Example 3

[0049] This embodiment provides a method for preparing mabn@g-C3N4, including the following steps: (1) In the preparation of mabn@g-C3N4 material, p-aminobenzonitrile was replaced with an equal mass of m-aminobenzonitrile, and the amount of other reagents and operation steps were the same as in Example 2.

[0050] The mabn@g-C3N4 obtained in step (1) was subjected to TEM testing, and the results are as follows: Figure 3 As shown, Figure 3 a and Figure 3 b shows that the D-π-A conjugated system constructed by functionalizing m-aminobenzonitrile retains the layered morphology of the original g-C3N4. Furthermore, the measured lattice spacing is 0.33 nm, corresponding to the (002) crystal plane of g-C3N4. Example 4

[0051] This embodiment provides a method for preparing oabn@g-C3N4, including the following steps: (1) In the preparation of oabn@g-C3N4 material, p-aminobenzonitrile was replaced with an equal mass of o-aminobenzonitrile, and the amount of other reagents and operation steps were the same as in Example 2.

[0052] The oabn@g-C3N4 obtained in step (1) was subjected to TEM testing, and the results are as follows: Figure 4 As shown, Figure 4 a and Figure 4 b shows that the D-π-A conjugated system constructed by functionalizing o-aminobenzonitrile retains the layered morphology of the original g-C3N4. Furthermore, the measured lattice spacing is 0.33 nm, corresponding to the (002) crystal plane of g-C3N4.

[0053] The test results show that the g-C3N4 framework structure is well preserved in the D-π-A conjugated photocatalyst constructed by aminobenzonitrile functionalization. Example 5

[0054] This embodiment provides a method for preparing pabn@g-C3N4 photocatalysts with different proportions, including the following steps: Different masses of p-aminobenzonitrile (2 mg, 5 mg, and 10 mg, respectively) were mixed and ground evenly with 100 mg g-C3N4 nanosheets; The mixture was placed in an alumina crucible and heated to 400 °C at a rate of 5 °C / min in a tube furnace under a nitrogen atmosphere (>99.999%), and held for 40 min. After natural cooling, pabn@g-C3N4 samples with different proportions were obtained. The amounts of other reagents and the operating procedures were the same as in Examples 2-4.

[0055] The products obtained in Examples 1-5 were used as photocatalysts to test the photocatalytic CO2 reduction performance. Specifically, the photocatalytic CO2 reduction reaction was conducted using a closed-loop, fully automated online trace gas analysis system (Labsolar-6A, Perfectlight Co., Beijing) equipped with a top-irradiated reactor and a 300 WXe full-spectrum lamp as the light source. 10 mg of photocatalyst was dispersed in 6 mL of a sacrificial agent mixture and continuously stirred. The samples were collected and quantitatively analyzed using an online gas chromatography system (GC 7920, Techcomp, China) with a thermal conductivity detector (TCD). The test results are shown in [Figure number missing]. Figure 5 and Figure 6 .

[0056] (1) The g-C3N4 photocatalyst prepared in Example 1 had photocatalytic reduction yields of CO to CO and CH4 of 19.7 and 1.8 μmol g, respectively. -1 h -1 .

[0057] (2) The pabn@g-C3N4 sample prepared in Example 2 was constructed using a D-π-A conjugated system with p-aminobenzonitrile functionalization, which effectively promoted the separation of photogenerated charges and could most effectively promote the separation and migration of photogenerated carriers, thereby achieving a much higher-than-expected photocatalytic CO2 reduction performance, with a CO generation rate as high as 140.0 μmol g. -1 h -1 It is about 7 times that of g-C3N4.

[0058] (3) The CO yield of mabn@g-C3N4 prepared in Example 3 was 48.9 μmol g. -1 h -1 The CO yield of the oabn@g-C3N4 sample prepared in Example 4 was 35.4 μmol g. -1 h -1 All were lower than pabn@g-C3N4.

[0059] (4) Among the different ratios of pabn@g-C3N4 photocatalysts prepared in Example 5, the CO yields of the pabn@g-C3N4 (0.02:1) and pabn@g-C3N4 (0.1:1) samples were 6 and 3.9 times that of the g-C3N4 catalyst, respectively.

[0060] Comparative Example 1 Take 100 mg g-C3N4 nanosheets and 5 mg p-aminobenzonitrile, grind and mix them, without performing the thermal calcination treatment in step (2). Use this physical mixture directly as a catalyst for testing.

[0061] The test results showed that the sample prepared in Comparative Example 1 had a photocatalytic CO2 reduction rate of 20.55 μmol g⁻¹ to CO. -1 h -1 .

[0062] Comparative Example 2 Benzonitrile@g-C3N4 and Aniline@g-C3N4 were prepared and tested by replacing p-aminobenzonitrile with benzonitrile (only -CN, no -NH2) and aniline (only -NH2, no -CN), respectively, in exactly the same manner as in Example 2 (including thermal calcination).

[0063] Test results showed that Benzonitrile@g-C3N4 and Aniline@g-C3N4 achieved photocatalytic CO2 reduction to CO rates of 24.87 μmol g⁻¹. -1 h -1 and 30.42 μmol g -1 h -1 .

[0064] Comparative Example 3 Comparative Example 3 prepared pabn@g-C3N4 composite material using the same method as Example 2. The difference was that the mass ratio of g-C3N4 nanosheets to aminobenzonitrile was modified to 1:0.01 during the preparation process, while the rest of the preparation process was the same as in Example 2.

[0065] The test results showed that the sample prepared in Comparative Example 3 had a photocatalytic CO2 reduction rate of 31.56 μmol g⁻¹ to CO. -1 h -1 .

[0066] Comparative Example 4 Comparative Example 4 prepared pabn@g-C3N4 composite material using the same method as in Example 2. The difference was that the mass ratio of g-C3N4 nanosheets to aminobenzonitrile was modified to 1:0.15 during the preparation process, while the rest of the preparation process was the same as in Example 2.

[0067] The test results showed that the sample prepared in Comparative Example 4 had a photocatalytic CO2 reduction rate of 37.81 μmol g⁻¹ to CO. -1 h -1 .

[0068] The modified carbon nitride composite photocatalytic material provided by this invention achieves site-controllable molecular modification by introducing aminobenzonitrile units, successfully constructing a highly efficient donor-π-acceptor structure on the g-C3N4 framework. This donor-π-acceptor structure generates a strong intramolecular electric field and directional potential gradient at the molecular level, effectively driving the separation and migration of photogenerated carriers. It achieves high catalytic efficiency in photocatalytic CO2 reduction, with a CO generation rate as high as 140.0 μmol·g⁻¹. -1 ·h -1 The CH4 yield reached 23.5 μmol·g. -1 ·h -1 The performance of the composite material is 7.1 times and more than 13 times that of the unmodified g-C3N4, respectively. Furthermore, the composite material exhibits structural stability during the reaction, and its preparation process is simple, uses inexpensive raw materials, and is quick, making it suitable for large-scale production and easy to promote and apply.

[0069] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A modified carbon nitride composite photocatalytic material, characterized in that, The modified carbon nitride composite photocatalytic material is a composite formed by modifying amino benzonitrile on g-C3N4 nanosheets through thermal condensation reaction; the amino benzonitrile is at least one selected from o-amino benzonitrile, m-amino benzonitrile and p-amino benzonitrile.

2. The modified C3N4 composite photocatalytic material according to claim 1, characterized in that, The modified carbon nitride composite photocatalytic material is a donor-π-acceptor structure, in which the amino group is a donor, the benzene ring is a π conjugated bridge, and the cyano group is an acceptor.

3. The modified C3N4 composite photocatalytic material according to claim 1, characterized in that, The amino benzonitrile is p-amino benzonitrile.

4. The modified carbon nitride composite photocatalytic material according to any one of claims 1-3, characterized in that, The mass ratio of the g-C3N4 nanosheet to the amino benzonitrile is 1:(0.02-0.1).

5. The modified carbon nitride composite photocatalytic material according to claim 4, characterized in that, The mass ratio of the g-C3N4 nanosheet to the amino benzonitrile is 1:(0.02-0.05).

6. A method for producing the modified carbon nitride composite photocatalytic material according to any one of claims 1 to 5, characterized by, The method comprises the following steps: S1. Mixing and grinding g-C3N4 nanosheets with amino benzonitrile; S2. Calcining the mixture under an inert atmosphere, and cooling to obtain the modified carbon nitride composite photocatalytic material.

7. The production method according to claim 6, wherein In S2, the calcination temperature is 350-450°C, and / or the calcination time is 20-60 min.

8. The preparation method according to claim 6, characterized in that, The g-C3N4 nanosheet is prepared by the following method: performing secondary calcination on blocky g-C3N4 obtained by pyrolysis of urea under an inert atmosphere at 450-550°C for 1-3 h to obtain g-C3N4 nanosheets.

9. Use of the modified carbon nitride composite photocatalytic material according to any one of claims 1-5 or the modified carbon nitride composite photocatalytic material prepared by the preparation method according to any one of claims 6-8 in photocatalytic reduction of CO2.

10. A method of photocatalytic reduction of CO2, characterized by, The modified carbon nitride composite photocatalytic material according to any one of claims 1-5 or the modified carbon nitride composite photocatalytic material prepared by the preparation method according to any one of claims 6-8 is used as a catalyst to reduce CO2 into CO and CH4 under the conditions of water, a sacrificial agent and light; and / or the sacrificial agent is triethanolamine, and / or the light source is a xenon lamp.

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