Ni monatomic photocatalyst with asymmetric double-coordination structure as well as preparation method and application of Ni monatomic photocatalyst

Abstract

The invention discloses a Ni monatomic photocatalyst with an asymmetric double-coordination structure and a preparation method and application thereof.The preparation method of the photocatalyst comprises the following steps that 1, melamine, urea and deionized water are mixed, ultrasonic treatment is conducted firstly, then a hydrothermal reaction is conducted, then the reaction mixture is dried and ground into powder, and the powder is obtained; and finally carrying out gradient calcination treatment to obtain g-C3N4 with surface hydroxyl and carbon defects, dispersing the g-C3N4 in a precursor salt solution of Ni, carrying out stirring treatment, then carrying out a photodeposition reaction, and carrying out centrifugal washing and drying on a final product to obtain the Ni monatomic photocatalyst with the asymmetric double-coordination structure. According to the invention, an asymmetric local coordination environment is constructed on a metal center, so that different regions of the metal center respectively have reduction and oxidation activities, and a'one-element dual-energy 'catalytic function is realized, that is, two different coordination environments are constructed around the same metal atom, so that atomic-scale multifunctional catalysis is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photocatalysts, in particular to a Ni single-atom photocatalyst with asymmetric double coordination structure and a preparation method and application thereof. BACKGROUND

[0002] With the increasingly serious global energy crisis and environmental pollution problems, developing a multifunctional photocatalytic system that can simultaneously realize the high-value conversion of carbon resources and the degradation of pollutants has become an important direction of sustainable energy technology. Traditional photocatalytic processes are usually limited to a single reaction path, such as hydrogen production, CO2 reduction or pollutant oxidation, and the solar energy utilization efficiency is low. In recent years, the construction of a dual-function catalytic system coupled with redox reactions, through the coupling of antibiotic oxidation and CO2 reduction to realize the full utilization of carriers, can significantly improve the overall quantum efficiency, and has important scientific significance and application prospects.

[0003] Single-atom catalysts (SACs) have shown great potential in the field of photocatalysis due to their 100% atom utilization rate and adjustable local coordination environment. However, most single-atom catalysts have symmetric single coordination structures (such as M–N4), and their active centers can only efficiently catalyze a single type of reaction (reduction or oxidation), making it difficult to meet the needs of dual-function catalysis. If spatially heterogeneous active sites can be constructed on the same metal center, with reduction and oxidation functions, respectively, it is expected to achieve efficient synergistic catalysis of "one dual function".

[0004] Graphitic carbon nitride (g-C3N4, abbreviated as CN) is a polymer semiconductor that responds to visible light and is rich in nitrogen coordination sites, making it an ideal carrier for anchoring transition metal single atoms. Previous studies have regulated metal coordination numbers by introducing nitrogen defects or heteroatom doping, but most of them have focused on the regulation of symmetric single coordination structures, and have not achieved spatial separation of oxidation and reduction functions on single atom sites.

[0005] Therefore, there is an urgent need to develop a new catalyst site design strategy to break the coordination symmetry of single-atom catalysts and construct multifunctional catalysts with spatially dual-functional active centers to achieve efficient and stable solar energy conversion. SUMMARY

[0006] The main purpose of the present application is to provide a Ni single-atom photocatalyst with asymmetric double coordination structure and a preparation method and application thereof. The catalyst constructs asymmetric Ni–N / Ni–O double coordination environment on a single Ni atom, realizing the spatial separation and functional synergy of reduction sites (CO2 reduction) and oxidation sites (antibiotic oxidation) at the atomic scale.

[0007] To achieve the above objectives, this invention provides a method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure, comprising the following steps: (1) Preparation of g-C3N4 with surface hydroxyl groups and carbon defects Melamine, urea and deionized water were mixed, ultrasonically treated, and then subjected to hydrothermal reaction. The reaction mixture was then dried and ground into powder, and finally subjected to gradient calcination to obtain g-C3N4 with surface hydroxyl groups and carbon defects. (2) Preparation of catalyst g-C3N4 with surface hydroxyl groups and carbon defects was dispersed in a Ni precursor salt solution, stirred first, then photodeposited, and finally the product was centrifuged, washed and dried to obtain the Ni single-atom photocatalyst with the asymmetric dual coordination structure.

[0008] Furthermore, in step (1), the temperature of the hydrothermal reaction is 160–200 °C and the time is 12–36 h.

[0009] Furthermore, in step (1), the gradient calcination treatment involves first pre-calcining at 500 °C for 0.5–2 h, and then continuing calcination at 520 °C for another 0.5–2 h.

[0010] Furthermore, in step (1), the ratio of melamine, urea and deionized water is 5 g: 10 g: 40 mL.

[0011] Furthermore, in step (2), the photodeposition reaction is carried out under full-arc irradiation of a 300W xenon lamp for a reaction time of 2.5 to 7.5 h.

[0012] Furthermore, in step (2), the drying temperature is 60 °C and the time is 12 h.

[0013] Further, in step (2), the precursor salt solution of Ni is an aqueous solution of Ni(NO3)2 with a concentration of 0.1M, and the ratio of g-C3N4 with surface hydroxyl groups and carbon defects to the aqueous solution of Ni(NO3)2 is 1 g: 1-5 mL.

[0014] The present invention also provides a Ni single-atom photocatalyst with an asymmetric dual-coordination structure, which is prepared according to the above preparation method.

[0015] This invention also provides the application of a Ni single-atom photocatalyst with an asymmetric dual-coordination structure in photocatalytic CO2 reduction coupled with pollutant oxidation.

[0016] Furthermore, the application is photocatalytic CO2 reduction coupled with tetracycline oxidation.

[0017] Design concept and principle of the invention: This invention constructs an asymmetric local coordination environment on a metal center, enabling different regions to possess both reduction and oxidation activities, thereby achieving a "uni-element dual-energy" catalytic function. In other words, it constructs two different coordination environments around the same metal atom to achieve atomic-level multifunctional catalysis.

[0018] Through research, g-C3N4 with surface hydroxyl and carbon defects was used as a support to construct Janus-type Ni single-atom photocatalysts with an asymmetric dual-coordination environment. Janus structure materials have asymmetric surface chemistry at the nanoscale. This photocatalyst structure achieves spatial integration of reduction and oxidation activities on a single metal center, effectively breaking through the bottleneck of the single function of traditional single-atom catalysts.

[0019] The beneficial effects of this invention are reflected in: (1) This invention is the first to induce the formation of Janus-type Ni single-atom active sites with Ni–N / Ni–O asymmetric dual coordination environment based on interface engineering strategy, realizing atomic-level integration of reduction and oxidation functions on the same metal center, and providing a new paradigm for breaking through the functional monotony of single-atom catalysts.

[0020] (2) In the Ni single-atom photocatalyst with an asymmetric dual-coordination structure prepared in this invention, the d-band center of the Ni–N region shifts downward, which is conducive to the adsorption of *COOH and the generation of CO; the d-band center of the Ni–O region shifts upward, which promotes the decomposition of H2O and the generation of ·OH, effectively catalyzing the oxidation of antibiotics. Through the regional regulation of the d-band center, the efficient synergy between CO2 reduction and antibiotic degradation is achieved.

[0021] (3) The Ni single-atom photocatalyst with asymmetric dual coordination structure prepared by the present invention has excellent stability and reproducibility. Thanks to the synergistic anchoring effect of carbon defects and hydroxyl groups, Ni single atoms have high thermodynamic stability on the support. After five consecutive cycles, the catalytic activity did not decrease significantly, showing good cycle durability.

[0022] (4) The preparation process of the photocatalyst of the present invention is mild, requires no special equipment, is simple to operate, and all solvents used are non-toxic and harmless.

[0023] (5) This invention breaks through the bottleneck of high carrier recombination rate in traditional photocatalysis through a dual-function synergistic catalysis mechanism. It can realize CO2 reduction coupled with antibiotic oxidation under solar energy drive, significantly improve the conversion efficiency of solar energy to chemical energy, and provide an innovative solution for building a green technology system that integrates pollution control and production capacity. Attached Figure Description

[0024] Figure 1These are aberration-corrected transmission electron microscope (TEM) images of the photocatalyst 8.7wt%Ni@DCN-OH (A is the aberration-corrected TEM image of 8.7wt%Ni@DCN-OH, B is the elemental distribution map of the corresponding sample) and synchrotron radiation X-ray absorption spectra (C is the X-ray absorption near-edge structure spectrum of 8.7wt%Ni@DCN-OH, D is the spectrum of the sample in R space after k...). 3 (Weighted Fourier transform spectrum).

[0025] Figure 2 This is the result of the photocatalytic CO2 reduction coupled with TC oxidation activity of a series of catalysts in this invention.

[0026] Figure 3 The results are based on the photocatalytic CO2 reduction coupled with TC oxidation activity of catalysts with different supports.

[0027] Figure 4 This is the result of the catalytic activity of the catalyst of the present invention in different reaction systems. Detailed Implementation

[0028] To enable those skilled in the art to more clearly understand the technical solutions described in this invention, the following embodiments are provided for illustration. It should be noted that the following embodiments do not constitute a limitation on the scope of protection claimed by this invention.

[0029] Unless otherwise specified, the raw materials, reagents or devices used in the following embodiments can be obtained from conventional commercial sources or by existing known methods; unless otherwise specified, the methods used in the embodiments of the present invention are methods mastered by those skilled in the art.

[0030] Example 1 Preparation of Ni single-atom photocatalyst "2.9wt%Ni@DCN-OH" with asymmetric dual-coordination structure The preparation method includes the following steps: (1) Preparation of g-C3N4 with surface hydroxyl groups and carbon defects 5 g of melamine, 10 g of urea, and 40 mL of deionized water were added to a beaker and sonicated for 30 min. The resulting mixture was then transferred to a stainless steel autoclave lined with polytetrafluoroethylene and hydrothermally reacted at 180 °C for 24 h. After the reaction was completed, the reaction mixture was dried at 80 °C for 24 h, then ground into powder and placed in an air atmosphere. The powder was then pre-calcined at 500 °C for 1 h at a heating rate of 5 °C / min, and then calcined at 520 °C for another 1 h at a heating rate of 5 °C / min to obtain g-C3N4 powder with surface hydroxyl groups and carbon defects, which was named DCN-OH.

[0031] (2) Preparation of 2.9wt% Ni@DCN-OH 1 g of DCN-OH powder obtained in step (1) was added to 50 mL of water and stirred until evenly dispersed. Then, 1 mL of 0.1 M Ni(NO3)2 aqueous solution was added and stirred evenly. The mixture was then photodeposited under full-arc irradiation of a 300 W xenon lamp for 5 h. Finally, the product was washed several times by centrifugation with deionized water, and the obtained powder was collected and dried at 60 ℃ and 100 Pa vacuum for 12 h to obtain a Ni single-atom photocatalyst with an asymmetric dual-coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.029:1), named 2.9wt%Ni@DCN-OH.

[0032] Example 2 Preparation of Ni single-atom photocatalyst "2.9wt%Ni@DCN-OH-2" with asymmetric dual-coordination structure The preparation method includes the following steps: (1) Preparation of g-C3N4 with surface hydroxyl groups and carbon defects 5 g of melamine, 10 g of urea, and 40 mL of deionized water were added to a beaker and sonicated for 15 min. The resulting mixture was then transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 160 °C for 12 h. After the reaction was completed, the reaction mixture was dried at 80 °C for 24 h, then ground into powder and placed in an air atmosphere. The powder was then pre-calcined at 500 °C for 0.5 h at a heating rate of 5 °C / min, and then calcined at 520 °C for another 0.5 h at a heating rate of 2.5 °C / min to obtain g-C3N4 powder with surface hydroxyl groups and carbon defects, which was named DCN-OH-2.

[0033] (2) Preparation of 2.9wt%Ni@DCN-OH-2 1 g of DCN-OH-2 powder obtained in step (1) was added to 50 mL of water and stirred until evenly dispersed. Then, 1 mL of 0.1 M Ni(NO3)2 aqueous solution was added and stirred evenly. The photodeposition reaction was carried out for 2.5 h under full-arc irradiation of a 300 W xenon lamp. Finally, the product was washed several times by centrifugation with deionized water, and the obtained powder was collected and dried at 60 ℃ and 100 Pa vacuum for 12 h to obtain a Ni single-atom photocatalyst with an asymmetric dual-coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.029:1), named 2.9wt%Ni@DCN-OH-2.

[0034] Example 3 Preparation of Ni single-atom photocatalyst "2.9wt%Ni@DCN-OH-3" with asymmetric dual-coordination structure The preparation method includes the following steps: (1) Preparation of g-C3N4 with surface hydroxyl groups and carbon defects 5 g of melamine, 10 g of urea, and 40 mL of deionized water were added to a beaker and sonicated for 45 min. The resulting mixture was then transferred to a stainless steel autoclave lined with polytetrafluoroethylene and subjected to a hydrothermal reaction at 200 °C for 36 h. After the reaction was completed, the reaction mixture was dried at 80 °C for 24 h, then ground into powder and placed in an air atmosphere. The powder was then pre-calcined at 500 °C for 2 h at a heating rate of 5 °C / min, and then calcined at 520 °C for 2 h at a heating rate of 7.5 °C / min to obtain g-C3N4 powder with surface hydroxyl groups and carbon defects, which was named DCN-OH-3.

[0035] (2) Preparation of 2.9wt%Ni@DCN-OH-3 1 g of DCN-OH-3 powder obtained in step (1) was added to 50 mL of water and stirred until evenly dispersed. Then, 1 mL of 0.1 M Ni(NO3)2 aqueous solution was added and stirred evenly. The photodeposition reaction was carried out for 7.5 h under full-arc irradiation of a 300 W xenon lamp. Finally, the product was washed several times by centrifugation with deionized water, and the obtained powder was collected and dried at 60 ℃ and 100 Pa vacuum for 12 h to obtain a Ni single-atom photocatalyst with an asymmetric dual coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.029:1), named 2.9wt%Ni@DCN-OH-3.

[0036] Example 4 Preparation of Ni single-atom photocatalyst "5.8wt%Ni@DCN-OH" with asymmetric dual-coordination structure The preparation method of this embodiment is basically the same as that of Example 1, except that in step (2), the amount of Ni(NO3)2 aqueous solution added is adjusted to 2 mL, and finally a Ni single-atom photocatalyst with an asymmetric dual coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.058:1) is obtained, named 5.8wt%Ni@DCN-OH.

[0037] Example 5 Preparation of Ni single-atom photocatalyst "8.7wt%Ni@DCN-OH" with asymmetric dual-coordination structure The preparation method of this embodiment is basically the same as that of Example 1, except that in step (2), the amount of Ni(NO3)2 aqueous solution added is adjusted to 3 mL, and finally a Ni single-atom photocatalyst with an asymmetric dual coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.087:1) is obtained, named 8.7wt%Ni@DCN-OH.

[0038] Example 6 Preparation of Ni single-atom photocatalyst "12wt%Ni@DCN-OH" with asymmetric dual-coordination structure The preparation method of this embodiment is basically the same as that of Example 1, except that in step (2), the amount of Ni(NO3)2 aqueous solution added is adjusted to 4 mL, and a Ni single-atom photocatalyst with an asymmetric dual coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.12:1) is obtained, named 12wt%Ni@DCN-OH.

[0039] Example 7 Preparation of Ni single-atom photocatalyst "15wt%Ni@DCN-OH" with asymmetric dual-coordination structure The preparation method of this embodiment is basically the same as that of Example 1, except that in step (2), the amount of Ni(NO3)2 aqueous solution added is adjusted to 5 mL, and finally a Ni single-atom photocatalyst with an asymmetric dual coordination structure (Ni(NO3)2:DCN-OH mass ratio = 0.15:1) is obtained, named 15wt%Ni@DCN-OH.

[0040] Comparative Example 1 Comparison of the preparation of Ni single-atom photocatalyst "8.7wt%Ni@CN" (1) Preparation of CN Five g of melamine was placed in an air atmosphere and pre-calcined at 500 °C for 1 h at a heating rate of 5 °C / min. Then, it was calcined at 520 °C for another 1 h at a heating rate of 5 °C / min to obtain graphitic carbon nitride, which was named CN.

[0041] (2) Preparation of 8.7wt%Ni@CN The preparation method is the same as in Example 5, except that DCN-OH is replaced with CN, and the final Ni single-atom photocatalyst is named 8.7wt%Ni@CN.

[0042] Comparative Example 2 Comparison of the preparation of Ni single-atom photocatalyst "8.7wt%Ni@CN-OH" (1) Preparation of CN-OH 5 g of melamine and 40 mL of deionized water were added to a beaker and sonicated for 30 min. The resulting mixed solution was then dried at 80 °C for 24 h, ground into powder, placed in an air atmosphere, and pre-calcined at 500 °C for 1 h at a heating rate of 5 °C / min. Then, it was further calcined at 520 °C for 1 h at a heating rate of 5 °C / min to obtain graphitic carbon nitride with surface hydroxyl groups, named CN-OH.

[0043] (2) Preparation of 8.7wt% Ni@CN-OH The preparation method is the same as in Example 5, except that DCN-OH is replaced with CN-OH, and the final Ni single-atom photocatalyst is named 8.7wt%Ni@CN-OH.

[0044] Comparative Example 3 Comparison of the preparation of Ni single-atom photocatalyst "8.7wt%Ni@DCN" (1) Preparation of DCN 5 g of melamine, 10 g of urea and 40 mL of deionized water were added to a beaker and sonicated for 30 min. The resulting mixture was then dried at 80 °C for 24 h and ground into powder. The powder was placed in an air atmosphere and pre-calcined at 500 °C for 1 h at a heating rate of 5 °C / min. Then, it was calcined at 520 °C for another 1 h at a heating rate of 5 °C / min to obtain graphitic carbon nitride with carbon defects, which was named DCN.

[0045] (2) Preparation of 8.7wt%Ni@DCN The preparation method is the same as in Example 5, except that DCN-OH is replaced with DCN, and the final Ni single-atom photocatalyst is obtained and named 8.7wt%Ni@DCN.

[0046] Experimental Example 1 Structural Feature Analysis of Ni Single-Atom Photocatalysts with Asymmetric Dual-Coordination Structure Taking the 8.7wt%Ni@DCN-OH photocatalyst prepared in Example 5 as an example, the microstructure of its Ni single atom was observed.

[0047] Figure 1 The image shows aberration-corrected transmission electron microscopy (TEM) image of the photocatalyst 8.7 wt% Ni@DCN-OH (A is the aberration-corrected TEM image of 8.7 wt% Ni@DCN-OH, B is the elemental distribution map of the corresponding sample) and synchrotron radiation X-ray absorption spectra (C is the X-ray absorption near-edge structure spectrum of 8.7 wt% Ni@DCN-OH, D is the k³-weighted Fourier transform spectrum of the sample in R-space). Figure 1As shown in A and B, Ni atoms are uniformly distributed on the surface of the DCN-OH support in an atomically dispersed manner, indicating that the Ni single-atom structure has been successfully constructed. Furthermore, based on the synchrotron X-ray absorption spectrum results ( Figure 1 (C and D), Ni atoms coordinate not only with nitrogen atoms but also with oxygen atoms, further confirming the successful preparation of Janus-type Ni single-atom photocatalysts with asymmetric coordination structures.

[0048] Experiment Example 2 Activity experiment of photocatalytic CO2 reduction coupled with TC (tetracycline) oxidation of Ni single-atom photocatalyst with asymmetric dual-coordination structure. Experimental samples: 2.9wt% Ni@DCN-OH (Example 1), 5.8wt% Ni@DCN-OH (Example 4), 8.7wt% Ni@DCN-OH (Example 5), 12wt% Ni@DCN-OH (Example 6), 15wt% Ni@DCN-OH (Example 7).

[0049] Experimental Method: 50 mg of photocatalyst was added to the photocatalytic reactor, followed by 50 mL of TC aqueous solution (20 mg / L), and the mixture was ultrasonically dispersed for 5 min. The reactor was then vacuum-evacuated. 50 mL of CO2 (99.999% purity) was then introduced into the reactor, and the reaction was carried out under 300W xenon lamp irradiation. The gaseous products of the photocatalytic reaction were determined using gas chromatography, and the absorbance of the TC supernatant after the catalytic reaction was measured using UV-Vis spectrophotometry.

[0050] Figure 2 The activity results of a series of catalysts prepared in this invention in the photocatalytic CO2 reduction coupled with TC oxidation are presented, with DCN-OH as a control. As shown in the figure, DCN-OH exhibits low photocatalytic redox performance; however, the catalytic activity of Ni@DCN-OH is significantly improved after loading Ni single atoms. With increasing Ni atom loading, the photocatalytic activity of Ni@DCN-OH increases accordingly, reaching a peak performance when the Ni content reaches 8.7 wt%; however, further increasing the Ni loading leads to a gradual decrease in catalytic performance.

[0051] Experimental Example 3 Experiment on the effect of different supports on the photocatalytic activity of photocatalysts Experimental samples: 8.7wt% Ni@DCN-OH (Example 5), 8.7wt% Ni@CN (Comparative Example 1), 8.7wt% Ni@CN-OH (Comparative Example 2), 8.7wt% Ni@DCN (Comparative Example 3).

[0052] Experimental method: Same as in Experiment Example 2.

[0053] Experimental results are as follows Figure 3 As shown in the figure, 8.7wt%Ni@DCN-OH exhibits superior photocatalytic redox performance compared to 8.7wt%Ni@DCN, 8.7wt%Ni@CN-OH, and 8.7wt%Ni@CN. This is mainly attributed to the Janus-type Ni atom active sites with asymmetric coordination structures formed by the co-anchoring of C defects and hydroxyl groups. This structure is a key factor in improving photocatalytic performance. In contrast, traditional supports such as CN, CN-OH, and DCN can only anchor single-coordinated Ni atoms, failing to achieve multi-coordination regulation or the construction of bifunctional active sites, thus limiting their performance in complex catalytic reactions (such as CO2 reduction coupled with antibiotic oxidation).

[0054] Experiment Example 4 Experiment on the effect of different reaction systems on the catalytic activity of photocatalysts Experimental sample: 8.7wt% Ni@DCN-OH (Example 5).

[0055] Experimental Methods: Following the method in Example 2, different reaction systems were constructed (a single TC oxidation system, a coupled reaction system of CO2 reduction coupled with TC oxidation, and a single CO2 reduction system). The activity of the catalyst 8.7wt% Ni@DCN-OH was tested in each of these systems. The coupled reaction system corresponds to the photocatalytic CO2 reduction coupled with TC oxidation system in Example 2; the single TC oxidation system is the system in Example 2 without the addition of CO2; and the single CO2 reduction system is the aqueous solution in Example 2 without the addition of TC.

[0056] Test results as follows Figure 4 As shown. By Figure 4 It can be seen that the 8.7wt% Ni@DCN-OH photocatalyst exhibits the best photocatalytic redox performance in the "coupled reaction system of photocatalytic CO2 reduction coupled with TC oxidation," which is significantly superior to other "single reaction systems." This confirms that the coupled reaction system achieves full utilization of charge carriers, thus exhibiting significant photocatalytic oxidation performance. Therefore, the Ni single-atom photocatalyst with an asymmetric dual-coordination structure prepared in this invention has potential application prospects for synergistically alleviating the energy crisis and treating industrial wastewater.

[0057] 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, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure, characterized in that, Includes the following steps: (1) Preparation of g-C3N4 with surface hydroxyl groups and carbon defects Melamine, urea and deionized water were mixed, ultrasonically treated, and then subjected to hydrothermal reaction. The reaction mixture was then dried and ground into powder, and finally subjected to gradient calcination to obtain g-C3N4 with surface hydroxyl groups and carbon defects. (2) Preparation of catalyst g-C3N4 with surface hydroxyl groups and carbon defects was dispersed in a Ni precursor salt solution, stirred first, then photodeposited, and finally the product was centrifuged, washed and dried to obtain the Ni single-atom photocatalyst with the asymmetric dual coordination structure.

2. The method for preparing the Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1, characterized in that, In step (1), the temperature of the hydrothermal reaction is 160–200 °C and the time is 12–36 h.

3. The method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1 or 2, characterized in that, In step (1), the gradient calcination treatment involves first pre-calcining at 500 ℃ for 0.5 to 2 h, and then continuing calcination at 520 ℃ for another 0.5 to 2 h.

4. The method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1 or 2, characterized in that, In step (1), the ratio of melamine, urea and deionized water is 5 g: 10 g: 40 mL.

5. The method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1 or 2, characterized in that, In step (2), the photodeposition reaction is carried out under full-arc irradiation of a 300W xenon lamp for 2.5 to 7.5 h.

6. The method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1 or 2, characterized in that, In step (2), the drying temperature is 60 °C and the time is 12 h.

7. The method for preparing a Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 1 or 2, characterized in that, In step (2), the precursor salt solution of Ni is an aqueous solution of Ni(NO3)2 with a concentration of 0.1 M. The ratio of g-C3N4 with surface hydroxyl groups and carbon defects to the aqueous solution of Ni(NO3)2 is 1 g: 1-5 mL.

8. A Ni single-atom photocatalyst with an asymmetric dual-coordination structure, characterized in that, Prepared according to the preparation method described in any one of claims 1 to 7.

9. The application of the Ni single-atom photocatalyst with an asymmetric dual-coordination structure as described in claim 8 in photocatalytic CO2 reduction coupled with pollutant oxidation.

10. The application as described in claim 9, characterized in that, The application is photocatalytic CO2 reduction coupled with tetracycline oxidation.

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