High-yield nitrogen-doped g-C3N4 micron tube and preparation method thereof

Abstract

The invention belongs to the field of graphite phase carbon nitride preparation, and discloses a high-yield nitrogen-doped g-C3N4 micron tube and a preparation method thereof, the preparation method comprises the following steps: S1, preparing an aqueous solution dispersion of hydroxylamine, and adding melamine to obtain a hydrothermal precursor; wherein the mass percentage concentration of hydroxylamine in the aqueous solution dispersion of hydroxylamine does not exceed 3.64%; s2, carrying out hydrothermal reaction treatment, collecting a hydrothermal product, cleaning and drying to obtain a nitrogen-doped melamine intermediate powder product; and S3, calcining to obtain the nitrogen-doped graphite phase carbon nitride micron tube material. According to the preparation method, melamine is taken as a raw material, a hydroxylamine solution is taken as a unique doped nitrogen source and a morphology regulating agent, and the self-assembly behavior of a precursor intermediate is accurately regulated by utilizing the synergistic effect of introduction of hydroxylamine and a hydrothermal process, so that a micron tube with a regular structure and good dispersity is formed through induction in subsequent calcination, the yield can reach 78.5%, and the preparation method is suitable for industrial production. The problems of low doping efficiency and low g-C3N4 yield in the prior art are solved.

Description

Technical Field

[0001] This invention belongs to the field of graphitic carbon nitride preparation, and more specifically, relates to a high-yield nitrogen-doped g-C3N4 microtube and its preparation method. Furthermore, the nitrogen doping method in this invention can further control the structure, making it particularly suitable for cutting-edge fields such as photocatalysis, electrocatalysis, gas adsorption, and bioimaging. Background Technology

[0002] Graphitic carbon nitride (g-C3N4), as a material with a two-dimensional graphite-like layered structure, possesses superior properties compared to most carbon-based materials, including a relatively small band gap (2.7 eV), stable properties, low cost, and environmental friendliness, and is widely used in optical catalysis and metal ion detection. However, bulk g-C3N4 suffers from drawbacks such as severe recombination of photogenerated charge carriers, a relatively limited specific surface area, and a narrow light absorption range, which restrict its optical and electrical performance.

[0003] To overcome the aforementioned limitations, existing research proposes forming an intermediate with melamine through elemental doping and morphology control strategies. This intermediate is then used to induce the formation of a micron-sized g-C3N4 structure, effectively controlling the framework structure of g-C3N4, narrowing the band gap, and inducing more fluorescence excitation levels, thereby significantly improving the optical and electrochemical performance of g-C3N4 microtubes. Furthermore, the intermediate formation process can further enhance the fluorescence quenching reaction with metal ions by substituting surface functional groups. Among the many elements that can form intermediates with melamine, nitrogen (N) exhibits unique potential. N possesses a stable energy level structure and easily tunable optical properties; doping N into carbon nitride crystals can improve its electronic conductivity and electron mobility. Forming a nitrogen-doped g-C3N4 intermediate through morphology control of melamine by N atoms under hydrothermal conditions, and then using this intermediate for high-temperature calcination to prepare nitrogen-doped g-C3N4 microtubes, is considered a highly promising research direction for improving its overall performance. Currently, traditional methods for preparing g-C3N4 (regardless of whether the resulting g-C3N4 is undoped or elementally doped) mainly include template methods and vapor deposition methods. However, template methods require expensive templates (such as SBA-15, MCM-41, and KIT-6), resulting in high costs, low yields (typically 10-50%), and stringent process control requirements. Vapor deposition methods suffer from cumbersome processes, extremely low yields (typically <5%), and the inability to achieve large-scale production. To address these issues in the preparation of g-C3N4 microtubes, there is an urgent need to develop a simple, efficient, and high-yield synthesis method.

[0004] The research group of the inventors of this invention previously reported a method for the large-scale preparation of g-C3N4 nanotube arrays with high nitrogen ratios (see Chinese patent application 202411712772.8). This method successfully prepared g-C3N4 nanotube arrays by simultaneously using urea and hydroxylamine sulfate as multiple nitrogen sources, combined with melamine, and employing a hydrothermal + calcination process. However, the yield of this method was only 50%, indicating significant room for improvement. Summary of the Invention

[0005] To address the aforementioned deficiencies or improvement needs of existing technologies, the present invention aims to provide a high-yield nitrogen-doped g-C3N4 microtube and its preparation method. Using melamine as a raw material and hydroxylamine solution as the sole nitrogen source and morphology modifier (eliminating the need for other nitrogen sources such as urea), the self-assembly behavior of the precursor intermediate is precisely controlled through the synergistic effect of hydroxylamine introduction and hydrothermal process. This induces the formation of well-structured and well-dispersed microtubes during subsequent calcination, improving the yield and solving the problems of low doping efficiency and low g-C3N4 yield in existing technologies. The preparation method of this invention offers advantages such as simple operation, higher efficiency, lower energy consumption, environmental safety, lower raw material costs, and more precise preparation and operation times. The resulting microtubes exhibit a well-ordered and stable structure. The preparation method is simple, reproducible, and highly controllable in parameters, eliminating the need for complex templates (such as SBA-15, MCM-41, KIT-6, etc.) required by existing template methods, as well as the need for expensive vapor deposition equipment required by vapor deposition methods. The prepared nitrogen-doped g-C3N4 microtubes possess high nitrogen content, high specific surface area, ordered structure, and excellent charge transport capabilities. These properties enable the prepared nitrogen-doped g-C3N4 microtubes to demonstrate significant application potential in photocatalysis, electrocatalysis, fluorescence sensing, energy storage and conversion, and other fields.

[0006] To achieve the above objectives, according to one aspect of the present invention, a method for preparing nitrogen-doped g-C3N4 microtubes with high yield is provided, characterized by comprising the following steps: S1: Prepare an aqueous dispersion of hydroxylamine, add melamine to the dispersion, and mix thoroughly to obtain a homogeneous dispersion system as a hydrothermal precursor; wherein, the mass percentage concentration of hydroxylamine in the aqueous dispersion does not exceed 3.64%; S2: The uniform dispersion system is subjected to hydrothermal reaction treatment, and the hydrothermal products are collected to obtain a light yellow crude nitrogen-doped melamine intermediate powder; then, the crude product is washed and dried to obtain a nitrogen-doped melamine intermediate powder product after impurity removal and drying. S3: The intermediate powder product of nitrogen-doped melamine is calcined under air conditions to obtain nitrogen-doped graphitic carbon nitride microtube material.

[0007] As a further preferred embodiment of the present invention, in step S1, the mass percentage concentration of hydroxylamine in the aqueous dispersion of hydroxylamine is 2.14% to 2.65%.

[0008] As a further preferred embodiment of the present invention, in step S1, the mass ratio of the aqueous dispersion of hydroxylamine to melamine is (50.56~53.92):(0.5~3). Preferably, the mass ratio of the aqueous dispersion of hydroxylamine to melamine is (52.24~52.8):1.

[0009] As a further preferred embodiment of the present invention, in step S1, the aqueous dispersion of hydroxylamine is obtained by mixing deionized water with a hydroxylamine solution and then ultrasonically treating the mixture to obtain a homogeneous mixed dispersion. Preferably, the hydroxylamine solution contains 50% hydroxylamine by mass; the deionized water and hydroxylamine solution are mixed at a volume ratio of 50:(0.5~3.5). More preferably, the ultrasonic treatment time is 30 min to 60 min.

[0010] As a further preferred embodiment of the present invention, in step S1, the uniform mixing is achieved by magnetic stirring and / or ultrasonic treatment; Preferably, the uniform mixing is achieved by ultrasonic treatment, and the ultrasonic treatment time is 20 min to 60 min.

[0011] As a further preferred embodiment of the present invention, in step S2, the hydrothermal reaction temperature is 160 ℃~200 ℃, and the reaction time is 18 h~30 h; Preferably, the hydrothermal reaction temperature is 180 °C and the reaction time is 24 h.

[0012] As a further preferred embodiment of the present invention, in step S2, the cleaning is performed by alternating centrifugation with deionized water and anhydrous ethanol to remove water-soluble impurities; the drying temperature is 50 ℃~80 ℃ and the drying time is 20 h~30 h. Preferably, the cleaning process involves alternating between ultrapure water and ethanol for a total of seven times, with the last cleaning performed using ultrapure water.

[0013] As a further preferred embodiment of the present invention, in step S3, the calcination temperature is 480 ℃~600 ℃, the heating rate is 1 ℃ / min~10 ℃ / min, and the holding time is 3 h~5 h.

[0014] According to another aspect of the present invention, the present invention provides a nitrogen-doped g-C3N4 microtube material obtained by the above preparation method.

[0015] As a further preferred embodiment of the present invention, the impedance of the nitrogen-doped g-C3N4 microtube material is 592.30Ω~1483.00Ω; Preferably, the impedance of the nitrogen-doped g-C3N4 microtube material is 592.30Ω~682.80Ω.

[0016] Compared with existing technologies such as template methods for preparing g-C3N4, which suffer from low yield, high cost, and difficulty in morphology control, and deposition methods that cannot be applied on a large scale, the technical solution conceived in this invention uses melamine as a raw material and hydroxylamine solution as a nitrogen dopant source. It preconstructs the morphology of a nitrogen-doped g-C3N4 intermediate using a hydrothermal method, and then calcines it to shape and stabilize the microtube structure of the nitrogen-doped g-C3N4 intermediate (wherein, by introducing hydroxylamine, melamine molecules are induced to undergo directional self-assembly under hydrothermal conditions, thereby transforming them from random particles into an ordered microtube structure, i.e., the nitrogen-doped melamine intermediate; subsequently, this intermediate is calcined in air), thus obtaining nitrogen-doped, directionally arranged one-dimensional g-C3N4 microtube material. The process exhibits good repeatability and strong parameter adjustability, providing a new path for morphology control of nitrogen-doped g-C3N4 microtube materials.

[0017] Furthermore, unlike the bulk g-C3N4 with a nitrogen content of 54.02 atom% (corresponding to Comparative Example 1 below) and the g-C3N4 microtubes without additional nitrogen doping with a nitrogen content of 65.19 atom% (corresponding to Comparative Example 2 below), the gCN MTCs (graphite phase carbon nitride microtube structures) prepared in this invention have an average nitrogen content of 68.37 atom%, with an optimal content of 70.74 atom%, exhibiting the characteristics of nitrogen doping. In addition, it also increases the specific surface area and reduces the electronic band gap.

[0018] This invention achieves the reproducible and structurally controllable fabrication of nitrogen-doped g-C3N4 microtubes. The invention utilizes hydroxylamine as the nitrogen dopant source and clarifies the effect of hydroxylamine regulation on the product structure. The fabrication method of this invention can control the electronic structure of g-C3N4, create more active sites, and broaden the light absorption range. It has advantages such as good process repeatability and strong parameter adjustability. The resulting nitrogen-doped g-C3N4 microtubes show broad application prospects in clean energy production, environmental pollution control, and novel electrochemical devices.

[0019] The method for preparing nitrogen-doped g-C3N4 microtubes in this invention, compared with traditional template methods and vapor deposition methods, has the advantages of high yield, low cost, and easier morphology control. The prepared nitrogen-doped g-C3N4 microtubes are highly efficient and stable. By constructing microtubes, more active sites are provided, and the charge transport efficiency is improved by utilizing nitrogen doping and one-dimensional tube structure. Furthermore, the photoresponse range can be controlled to a larger area, making nitrogen-doped graphitic carbon nitride microtubes show broad application prospects in cutting-edge fields such as photocatalysis, electrocatalysis, gas adsorption, biosensing, and biomedicine.

[0020] Specifically, the present invention can achieve the following beneficial effects: (1) This invention discloses a novel method for the controllable preparation of g-C3N4MTCs through the synergistic effect of hydroxylamine regulation and hydrothermal preformation precursor. This method uses melamine as the basic raw material and introduces hydroxylamine as a nitrogen dopant source. During the hydrothermal reaction, hydroxylamine guides the self-assembly of melamine molecules by regulating the microenvironment of the system, forming a gCNMTCs supramolecular intermediate with a specific morphology and structure. In this process, the hydroxylamine molecules effectively optimize the morphology and dispersibility of the intermediate. Subsequently, during the high-temperature calcination stage, based on the structural characteristics of this intermediate, the g-C3N4 micromaterial undergoes directional structural solidification and recombination, ultimately forming gCN MTCs with a one-dimensional regular microtube structure. In the experimental examples of this invention, the amount of hydroxylamine added has a significant impact on the morphology and structural integrity of the final product. Taking Example 1 below as an example, the amount of commercially available 50% hydroxylamine solution can be adjusted in the range of 0.5 mL to 4.5 mL. By precisely controlling its amount, the morphology of gCN MTCs can be controlled. Finally, it was confirmed that the amount of commercially available 50% hydroxylamine solution should not exceed 3.5 mL (the mass percentage concentration of hydroxylamine in the aqueous dispersion of hydroxylamine formed should not exceed 3.64%; Examples 2 and 3 are similar), providing diverse samples for the study of the relationship between structure and performance.

[0021] (2) This invention provides a highly reliable method for preparing nitrogen-doped graphitic carbon nitride microtubes. This method not only achieves the morphological transformation of g-C3N4 from a bulk structure to a microtube structure through hydroxylamine regulation, but also ensures the stability and uniformity of the product structure through fine optimization of various process parameters. By using a hydrothermal + calcination process (wherein, the hydrothermal reaction temperature is preferably 180 ℃, and the reaction time is preferably 24 h; the calcination temperature is preferably 520 ℃, and the calcination time is preferably 4 h), combined with 3 to 8 alternating centrifugation washing processes of deionized water and anhydrous ethanol, impurities are effectively removed and the purity of the product is ensured. The prepared gCN MTCs have a regular microtube structure, forming a distinct morphological difference from gCN bulk, providing high-quality samples for subsequent performance studies.

[0022] (3) The present invention provides a method for preparing nitrogen-doped g-C3N4 microtubes with high yield. The product has significant performance advantages: the nitrogen content of the g-C3N4 microtubes prepared by the present invention reaches 70.74 atom%, which is significantly improved compared with bulk carbon nitride materials; the product has a highly ordered structure, a yield of up to 78.5%, a tube length >30 μm, an average tube diameter of about 2.30 μm, and a prominent proportion of nitrogen element in the elemental content, which has excellent synergy between structure and performance (as exemplified in Example 1 below).

[0023] (4) To ensure that hydroxylamine molecules fully react with melamine molecules, the method of this invention first adds a hydroxylamine solution, and then adds melamine to it. This allows the melamine molecules to fully contact the nitrogen source, ensuring a thorough hydrothermal reaction. This invention controls the morphology of the melamine intermediate and the reaction characteristics during hydrothermal and calcination processes by adding a nitrogen source, further controlling the nitrogen-doped g-C3N4 microtubes produced after calcination. The hollow three-channel microtubes inside are clearly visible, indicating that nitrogen doping can effectively regulate the microstructure of the bulk g-C3N4 micromaterial, further enhancing its application prospects in cutting-edge fields such as photocatalysis, electrocatalysis, gas adsorption, and bioimaging. At the same time, the highly regular one-dimensional thin-walled tubular morphology also improves the structural stability during application, further enhancing the stability in practical testing applications.

[0024] (5) The hydrothermal pre-construction-calcination shaping preparation method adopted in this invention has significant process advantages compared with traditional g-C3N4 preparation techniques such as template method and vapor phase exfoliation method. This method does not require the use of complex templates and expensive equipment and can achieve large-scale preparation. It uses melamine as an inexpensive raw material and hydroxylamine as a green regulator. The process is simple, the reaction conditions are mild (the hydrothermal temperature is adjustable from 160 ℃ to 200 ℃, and the calcination parameters can be optimized), and the process has good repeatability and high yield. At the same time, in Comparative Examples 1 and 2 below, the regulatory mechanism of hydroxylamine was clarified by setting gCN bulk and g-C3N4 microtube control samples without additional nitrogen doping, respectively, providing reliable process support for batch preparation and suitable for large-scale industrial production.

[0025] (6) Compared with gCN bulk, the gCN MTCs prepared in this invention have a unique oriented microtube structure. This structural difference provides a research model for in-depth study of the structure-property relationship of g-C3N4 materials. Its controllable morphology and stable structural characteristics enable this material to show potential application value in many cutting-edge fields such as photocatalysis, electrocatalysis, bioimaging, and electrochemical energy storage, providing important research theories for the functional expansion and practical application of g-C3N4 materials.

[0026] The introduction of heteroatoms can easily disrupt the thermal condensation pathway of precursors such as melamine, leading to a trade-off between doping and morphology control. This invention uses hydroxylamine as the sole nitrogen source (meaning that no other nitrogen source is used besides hydroxylamine and melamine). In the complex hydrothermal high-temperature and high-pressure liquid environment, hydroxylamine may undergo beneficial structure-directed reactions with melamine, or it may trigger excessive induction reactions leading to hydrogen bond breaking and polymerization. Therefore, the content of hydroxylamine in the hydrothermal precursor needs to be strictly controlled (i.e., the mass percentage concentration of hydroxylamine in the aqueous dispersion should not exceed 3.64% before adding melamine and forming the corresponding hydrothermal precursor). During the development of this invention, a "hydroxylamine-mediated hydrothermal pre-assembly" strategy was proposed. This strategy utilizes the directional bonding of hydroxylamine molecules with melamine under hydrothermal conditions to simultaneously achieve uniform introduction of nitrogen and in-situ construction of the microtube supramolecular framework. This achieves one-step synergistic control of high yield, high nitrogen content, and well-ordered microtube structure, resolving the inherent contradictions of uneven doping, morphology loss, and low yield in traditional methods. Compared to template methods and vapor deposition methods, the method of this invention has lower cost, higher yield, controllable process parameters, and good repeatability, making it suitable for large-scale production. The nitrogen-doped graphitic carbon nitride microtubes obtained have an ordered and regular structure with strong stability. They not only have the advantages of high nitrogen ratio and large specific surface area, but also optimize the electronic band structure, reduce charge transfer resistance and impedance, and significantly improve charge transport efficiency, effectively expanding the application scenarios of this type of material in photocatalysis, electrocatalysis, gas adsorption, biosensing, biomedicine and other fields. Attached Figure Description

[0027] Figure 1 This is a scanning electron microscope (SEM) image of the melamine used in Example 1.

[0028] Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 SEM images of nitrogen-doped melamine intermediates prepared in Example 1 by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution, respectively. Figure 3 The scale in the image corresponds to a size of 100 μm; Figure 5 The scale in the image corresponds to a size of 100 μm.

[0029] Figure 11XRD comparison images of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution to Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2.

[0030] Figure 12 SEM image of bulk g-C3N4 prepared by one-step pyrolysis of melamine without any nitrogen source doping, as compared to Comparative Example 1.

[0031] Figure 13 , Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 , Figure 20 , Figure 21 The images show SEM images of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution, respectively, as described in Example 1.

[0032] Figure 22 , Figure 23 , Figure 24 , Figure 25 , Figure 26 , Figure 27 , Figure 28 , Figure 29 , Figure 30 The figures show the diameter distribution of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution, respectively, as described in Example 1.

[0033] Figure 31 EDS elemental distribution diagram of the bulk phase g-C3N4 prepared in Comparative Example 1.

[0034] Figure 32 EDS elemental distribution diagram of the g-C3N4 microtube prepared in Comparative Example 2 without the introduction of additional nitrogen doping.

[0035] Figure 33 , Figure 34 , Figure 35 , Figure 36 , Figure 37 , Figure 38 , Figure 39 , Figure 40 , Figure 41The images show the EDS elemental distribution of nitrogen-doped g-C3N4 microtubes prepared in Example 1 by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution, respectively.

[0036] Figure 42 , Figure 43 Transmission electron microscope (TEM) image of nitrogen-doped g-C3N4 microtubes prepared by adding 2.5 mL of hydroxylamine solution in Example 1.

[0037] Figure 44 This is a yield graph of the nitrogen-doped g-C3N4 microtubes prepared in Example 1.

[0038] Figure 45 The Fourier transform infrared (FT-IR) spectra of the nitrogen-doped g-C3N4 microtube prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtube without additional nitrogen doping obtained in Comparative Example 2 are compared.

[0039] Figure 46 The specific surface area (BET) graphs are shown for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes obtained in Comparative Example 2 without additional nitrogen doping.

[0040] Figure 47 The pore size (BJH) diagrams are shown for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes obtained in Comparative Example 2 without additional nitrogen doping.

[0041] Figure 48 XPS images of nitrogen-doped g-C3N4 microtubes prepared in Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2.

[0042] Figure 49 Comparison of electrochemical impedance spectroscopy (EIS) of nitrogen-doped g-C3N4 microtubes prepared in Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 in a mixed solution containing 5 mM potassium ferrocyanide, potassium ferrocyanide, and 0.1 M potassium chloride.

[0043] Figure 50 Impedance quantitative analysis diagrams based on electrochemical impedance spectroscopy experiments are shown for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2.

[0044] Figure 51 Comparison of cyclic voltammetry (CV) of nitrogen-doped g-C3N4 microtubes prepared in Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 in a mixed solution containing 5 mM potassium ferrocyanide, potassium ferrocyanide, and 0.1 M potassium chloride.

[0045] Figure 52 The current quantitative analysis diagram based on cyclic voltammetry is shown for the nitrogen-doped g-C3N4 microtube prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtube without additional nitrogen doping obtained in Comparative Example 2.

[0046] Figure 53 , Figure 54 , Figure 55 , Figure 56 , Figure 57 , Figure 58 , Figure 59 , Figure 60 , Figure 61 SEM images of the nitrogen-doped melamine intermediates prepared in Example 2 by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL hydroxylamine solutions. Figure 55 The scale in the image corresponds to a size of 50 μm; Figure 56 The scale in the image corresponds to a size of 50 μm; Figure 59 The scale in the image corresponds to a size of 10 μm.

[0047] Figure 62 , Figure 63 , Figure 64 , Figure 65 , Figure 66 , Figure 67 , Figure 68 , Figure 69 , Figure 70 The image shows SEM images of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution as described in Example 2.

[0048] Figure 71 XPS images of the nitrogen-doped g-C3N4 microtubes prepared in Example 2, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes obtained in Comparative Example 2.

[0049] Figure 72 The image shows a SEM image of the nitrogen-doped melamine intermediate prepared in Example 3.

[0050] Figure 73 , Figure 74 , Figure 75 , Figure 76 , Figure 77 , Figure 78 , Figure 79 , Figure 80 , Figure 81 SEM images of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution to Example 3.

[0051] Figure 82 XPS images of the nitrogen-doped g-C3N4 microtubes prepared in Example 3, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes obtained in Comparative Example 2 without additional nitrogen doping.

[0052] Figure 83 SEM image of the melamine precursor prepared in Comparative Example 2 without the introduction of additional nitrogen doping.

[0053] Figure 84 SEM image of g-C3N4 microtubes prepared for Comparative Example 2 without the introduction of additional nitrogen doping.

[0054] Figure 85 XPS image of g-C3N4 microtubes prepared for Comparative Example 2 without the introduction of additional nitrogen doping.

[0055] Figure 86 Raman spectra of nitrogen-doped g-C3N4 microtubes prepared in Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2.

[0056] Figure 87 The diameter distribution of the g-C3N4 microtube prepared for Comparative Example 2 without the introduction of additional nitrogen doping is shown. Detailed Implementation

[0057] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0058] In general, the high-yield preparation method of nitrogen-doped g-C3N4 microtubes in this invention mainly includes the following steps: First, hydroxylamine solution and melamine are ultrasonically stirred in deionized water to form a uniformly dispersed precursor mixture; then, the mixture is subjected to a hydrothermal reaction to generate a nitrogen-doped melamine intermediate with an ordered micron columnar structure; finally, the intermediate is calcined to obtain nitrogen-doped g-C3N4 microtubes.

[0059] Specifically, a homogeneous mixture of a quantitative hydroxylamine solution and melamine was first formed under ultrasonic treatment conditions via hydrothermal-calcination. This system was then transferred to a high-pressure reactor lined with polytetrafluoroethylene (PTFE) for hydrothermal reaction to obtain nitrogen-doped graphitic carbon nitride microtube supramolecular intermediates (gCN MTCs SI). Subsequently, the system was transferred to another high-pressure reactor lined with PTFE for hydrothermal reaction. After alternating centrifugal washing with deionized water and anhydrous ethanol, and vacuum drying, the intermediates were placed in an alumina crucible and calcined again, followed by cooling to obtain nitrogen-doped g-C3N4 microtubes.

[0060] All raw materials used in this paper were commercially available. For example, the hydroxylamine solution had a concentration of 50 wt% and a density of 1.12 g / mL and was purchased from the Aladdin Reagent website (https: / / www.aladdin-e.com / , CAS No.: 7803-49-8, item number: H164487). Melamine was purchased from the Aladdin Reagent website (https: / / www.aladdin-e.com / , CAS No.: 108-78-1, item number: M108433).

[0061] Example 1: The method for preparing high-yield nitrogen-doped g-C3N4 microtubes of the present invention specifically includes the following steps: First, take several 100 mL beakers and add 50 mL of deionized water to each. Add different volumes of commercially available hydroxylamine solution to each beaker to prepare a dispersion (in this example, the amount of hydroxylamine solution used is 0.5 mL to 4.5 mL, with a gradient of 0.5 mL). Mix thoroughly by ultrasonication to form several homogeneous dispersions. The mass percentage concentration of hydroxylamine in each dispersion is shown in the table below. [In the table, "Concentration of hydroxylamine in the obtained dispersion" = 100% × "Mass of solute hydroxylamine" / ("Mass of 50 mL of deionized water" + "Mass of solute hydroxylamine"); where "Mass of 50 mL of deionized water" = 50 g (the density of water is 1 g / cm³).] 3 The mass of the solute hydroxylamine is calculated as follows: "Volume of commercially available hydroxylamine solution used" × 1.12 g / mL × 50 wt%.

[0062] For each dispersion: The second step is to add 1 g of melamine to the dispersion and ultrasonically stir for 30 min to form a homogeneous mixture, ensuring that the hydroxylamine solution and melamine are fully mixed and dispersed. The third step involves transferring the above-mentioned mixture into a high-pressure reactor lined with 100 mL of polytetrafluoroethylene for hydrothermal reaction at a temperature of 180 °C for 24 h. After the reaction is complete, the reaction product, namely nitrogen-doped g-C3N4 microtube supramolecular intermediate (gCN MTCs SI), is collected. The fourth step involves washing the gCN MTCs SI by alternating centrifugation with deionized water and anhydrous ethanol seven times, with the last wash using ultrapure water, to remove water-soluble impurities. The purified product is then vacuum-dried at 60°C for 24 hours to obtain gCN MTCs SI dry powder (i.e., intermediate powder of nitrogen-doped melamine, with a microstructure of micron-sized columnar structures, such as...). Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 (as shown) In the fifth step, 3 g of nitrogen-doped melamine intermediate powder (gCN MTCs SI) was placed in a covered alumina crucible and tightly wrapped with aluminum foil. The crucible was then placed in a muffle furnace and calcined in air at a heating rate of 2 °C / min from room temperature to 520 °C, and held at that temperature for 4 h. After natural cooling to room temperature, a yellow, nitrogen-doped g-C3N4 microtube material powder (gCN MTCs, with a micron-like columnar structure) was obtained. The samples prepared were designated as "gCN MTCs (x mL)" samples (x mL representing the amount of hydroxylamine solution used) based on the amount of hydroxylamine solution used.

[0063] Figure 13 , Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 , Figure 20 , Figure 21 This is a SEM image of the nitrogen-doped g-C3N4 microtube prepared in this embodiment.

[0064] Example 2: The method for preparing high-yield nitrogen-doped g-C3N4 microtubes of the present invention specifically includes the following steps: First, take several 100 mL beakers and add 50 mL of deionized water to each beaker; add different volumes of commercially available hydroxylamine solution to each beaker to prepare a dispersion (in this example, the amount of hydroxylamine solution used is 0.5 mL to 4.5 mL, with a gradient of 0.5 mL), and sonicate to mix evenly to form several homogeneous mixed dispersions; For each dispersion: The second step is to add 2 g of melamine to the dispersion and ultrasonically stir for 30 min to form a homogeneous mixture, ensuring that the hydroxylamine solution and melamine are fully mixed and dispersed. The third step involves transferring the above mixture into a high-pressure reactor lined with 100 mL of polytetrafluoroethylene for hydrothermal reaction at a temperature of 190 °C for 28 h. After the reaction is complete, the reaction product, namely nitrogen-doped g-C3N4 microtube supramolecular intermediate (gCN MTCs SI), is collected. The fourth step involves washing the gCN MTCs SI by alternating centrifugation with deionized water and anhydrous ethanol seven times, with the last wash using ultrapure water, to remove water-soluble impurities. The purified product is then vacuum-dried at 80 °C for 36 h to obtain dried gCNMTCs SI powder (i.e., intermediate powder of nitrogen-doped melamine, with a microstructure of micron-sized columnar structures, such as...). Figure 53 , Figure 54 , Figure 55 , Figure 56 , Figure 57 , Figure 58 , Figure 59 , Figure 60 , Figure 61 (as shown) Fifth step: Take 4 g of nitrogen-doped melamine intermediate powder (gCN MTCs SI) and place it in a covered alumina crucible, wrap it tightly with aluminum foil, and then place it in a muffle furnace. Under air atmosphere, heat it from room temperature to 600 ℃ at a heating rate of 5 ℃ / min, and then hold it at that temperature for 4 h for calcination. After calcination, cool it naturally to room temperature to obtain yellow nitrogen-doped g-C3N4 microtube material powder (gCN MTCs, whose microstructure is a micron columnar structure).

[0065] Figure 62 , Figure 63 , Figure 64 , Figure 65 , Figure 66 , Figure 67 , Figure 68 , Figure 69 , Figure 70 This is a SEM image of the nitrogen-doped g-C3N4 microtube prepared in this embodiment.

[0066] Example 3: The method for preparing high-yield nitrogen-doped g-C3N4 microtubes of the present invention specifically includes the following steps: First, take several 100 mL beakers and add 50 mL of deionized water to each beaker; add different volumes of commercially available hydroxylamine solution to each beaker to prepare a dispersion (in this example, the amount of hydroxylamine solution used is 0.5 mL to 4.5 mL, with a gradient of 0.5 mL), and sonicate to mix evenly to form several homogeneous mixed dispersions; For each dispersion: The second step is to add 3 g of melamine to the dispersion and ultrasonically stir for 30 min to form a homogeneous mixture, ensuring that the hydroxylamine solution and melamine are fully mixed and dispersed. The third step involves transferring the above-mentioned mixture into a high-pressure reactor lined with 100 mL of polytetrafluoroethylene for hydrothermal reaction at a temperature of 210 °C for 36 h. After the reaction is completed, the reaction product, namely nitrogen-doped g-C3N4 microtube supramolecular intermediate (gCN MTCs SI), is collected. The fourth step involves washing the nitrogen-doped g-C3N4 microtube supramolecular intermediate (gCN MTCs SI) seven times by alternating centrifugation with deionized water and anhydrous ethanol, with the last wash using ultrapure water, to remove water-soluble impurities. The purified product is then vacuum-dried at 100 °C for 48 h to obtain dried gCN MTCs SI powder (i.e., the intermediate powder of nitrogen-doped melamine, whose microstructure is a micron-column structure, such as...). Figure 72 (as shown) Fifth step: Take 3 g of nitrogen-doped melamine intermediate powder (gCN MTCs SI) and place it in a covered alumina crucible. Wrap it tightly with aluminum foil and then place it in a muffle furnace. Under air atmosphere, heat it from room temperature to 540 ℃ at a heating rate of 10 ℃ / min and hold it at that temperature for 5 h for calcination. After naturally cooling to room temperature, you can obtain yellow nitrogen-doped g-C3N4 microtube material powder (gCN MTCs, whose microstructure is a micron columnar structure).

[0067] Figure 73 , Figure 74 , Figure 75 , Figure 76 , Figure 77 , Figure 78 , Figure 79 , Figure 80 , Figure 81 This is a SEM image of the nitrogen-doped g-C3N4 microtube prepared in this embodiment.

[0068] Comparative Example 1: Comparative Example 1 yielded a bulk g-C3N4 (gCN bulk), and the preparation method specifically included the following steps (hydroxylamine was not used when preparing the bulk g-C3N4, and the amount of melamine and the calcination process parameters were matched with those in the example): First, take 3 g of melamine powder and place it in a covered alumina crucible, then wrap it tightly with aluminum foil. The second step involves placing the sample in a muffle furnace and heating it from room temperature to 520 °C at a rate of 10 °C / min under air atmosphere. This temperature is then maintained for 4 hours, followed by natural cooling to room temperature to obtain a yellow, bulk g-C3N4 material. This sample is designated as "gCN bulk".

[0069] Figure 12 SEM image of the bulk g-C3N4 prepared by one-step pyrolysis of melamine without additional nitrogen source doping, which is used as a comparative example. Figure 31 EDS elemental distribution diagram of the bulk phase g-C3N4 prepared in this comparative example. Figure 11 , Figure 45 , Figure 46 , Figure 47 The gCN bulk curves in the figure are the XRD, FT-IR, BET, and BJH spectra of the bulk phase g-C3N4 prepared in this comparative example. Figure 48 , Figure 71 , Figure 82 , Figure 85 The gCN bulk curve in the figure is the XPS plot of the bulk g-C3N4 prepared in this comparative example. Figure 49 , Figure 50 , Figure 51 , Figure 52 The gCN bulk curves in the figure are the EIS diagram, impedance quantitative analysis diagram, CV comparison diagram, and current quantitative analysis diagram of the bulk phase g-C3N4 prepared in this comparative example. Figure 86 The gCN bulk curve in the figure is the Raman plot of the bulk phase g-C3N4 prepared in this comparative example.

[0070] Comparative Example 2: This comparative example does not add hydroxylamine solution and prepares g-C3N4 microtubes without introducing additional nitrogen doping. The preparation method specifically includes the following steps: First, take a 100 mL beaker, add 50 mL of deionized water, add 1 g of melamine separately, and sonicate for 30 min to form a milky white suspension of a uniformly dispersed melamine mixture. The second step involves transferring the above mixture into a high-pressure reactor lined with 100 mL of polytetrafluoroethylene for hydrothermal reaction at a temperature of 180 °C for 24 h. After the reaction is complete, the reaction product, namely g-C3N4 microtube supramolecular intermediate (gCN MTCs SI), is collected. The third step involves washing the gCN MTCs SI by alternating centrifugation with deionized water and anhydrous ethanol seven times, with the last wash using ultrapure water, to remove water-soluble impurities. The purified product is then vacuum-dried at 60 °C for 24 h to obtain gCNMTCs SI dried powder (i.e., intermediate powder of nitrogen-doped melamine, with a microstructure of micron-sized columnar structures, such as...). Figure 84 (as shown) Fourth step: Take 3 g of nitrogen-doped melamine intermediate powder (gCN MTCs SI) and place it in a covered alumina crucible, then tightly wrap it with aluminum foil. Place it in a muffle furnace and calcine it in air at a heating rate of 2 °C / min from room temperature to 520 °C, holding it at that temperature for 4 h. After cooling to room temperature, a yellow g-C3N4 microtube material powder (gCN MTCs) is obtained, with a micron-column structure, such as... Figure 85 (As shown). This sample is labeled "gCN MTCs(0mL)" (corresponding to 0 mL of hydroxylamine solution).

[0071] Figure 32 EDS elemental distribution diagram of the g-C3N4 microtube prepared in this comparative example without the introduction of additional nitrogen doping. Figure 87 The diameter distribution of the g-C3N4 microtube prepared for this comparative example without the introduction of additional nitrogen doping is shown in the diagram. Figure 11 , Figure 45 , Figure 46 , Figure 47 The 0 mL hydroxylamine solution-doped g-C3N4 microtube curves are XRD, FT-IR, BET, and BJH plots of the g-C3N4 microtubes prepared in this comparative example without additional nitrogen doping. Figure 48 , Figure 71 , Figure 82 , Figure 85 The 0 mL hydroxylamine solution-doped g-C3N4 microtube curve is the XPS plot of the g-C3N4 microtube prepared in this comparative example without additional nitrogen doping. Figure 50 , Figure 51 , Figure 52 , Figure 53 The curves of the 0 mL hydroxylamine solution-doped g-C3N4 microtubes in the comparative example are the EIS plot, impedance quantitative analysis plot, CV comparison plot, and current quantitative analysis plot of the g-C3N4 microtubes prepared without additional nitrogen doping. Figure 86The 0 mL hydroxylamine solution doped g-C3N4 microtube curve is the Raman plot of the g-C3N4 microtube prepared in this comparative example without the introduction of additional nitrogen doping.

[0072] Figure 1 This is a scanning electron microscope (SEM) image of the melamine used in Example 1. The image shows that the melamine exhibits an irregularly sized and arranged granular distribution under electron microscopy.

[0073] Figure 2 , Figure 3 , Figure 4 , Figure 5 , Figure 6 , Figure 7 , Figure 8 , Figure 9 , Figure 10 The images show SEM images of nitrogen-doped melamine intermediates prepared in Example 1 with the addition of 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution. The images show that after hydrothermal treatment, hydroxylamine molecules undergo a supramolecular self-assembly reaction with melamine molecules. When the amount of hydroxylamine solution is ≤3.5 mL, the resulting nitrogen-doped melamine intermediate exhibits a directionally arranged microtubular structure. When the amount of hydroxylamine solution exceeds 3.5 mL (e.g., reaching 4.0 mL and 4.5 mL), a short columnar structure is observed. It is evident that the morphology of the nitrogen-doped melamine intermediates after sintering differs depending on the volume of hydroxylamine solution added.

[0074] Figure 11 XRD patterns of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution to Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 are compared. The figures show the 12.7° and 27.4° diffraction peaks of the nitrogen-doped g-C3N4 microtubes on the (100) and (002) crystal planes, respectively, corresponding to the bulk g-C3N4 structure. These peaks can be attributed to the interlayer stacking of tris-triazine units and the interlayer stacking of conjugated aromatic compounds, respectively. However, the formation of hollow microtube structures in the nitrogen-doped g-C3N4 microtube structure and the reduction of interlayer size in the thin-layer structure lead to the disappearance of the (100) crystal plane and the weakening of the (002) crystal plane intensity. This indicates that the nitrogen-doped melamine intermediate undergoes a fundamental chemical transformation from bulk g-C3N4 to nitrogen-doped g-C3N4 microtubes through a supramolecular self-thermal polycondensation reaction.

[0075] Figure 12The image shows a SEM image of the bulk g-C3N4 phase prepared by one-step pyrolysis of melamine without any nitrogen source doping, as compared to Comparative Example 1. After calcination, the irregular melamine particles were transformed into a poorly dispersed bulk structure.

[0076] Figure 13 , Figure 14 , Figure 15 , Figure 16 , Figure 17 , Figure 18 , Figure 19 , Figure 20 , Figure 21 The images show SEM images of nitrogen-doped g-C3N4 microtubes prepared in Example 1 with 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution added. The images show that after high-temperature pyrolysis of the nitrogen-doped melamine intermediate, the hydrogen bonds in the original micropillar structure are broken. After exfoliation, the micropillars further form slightly fluffy nitrogen-doped g-C3N4 microtubes with an average tube length >30 μm. In Example 1, as the amount of hydroxylamine solution added increased from 0.5 mL to 4.5 mL, the microtubular structure of the nitrogen-doped g-C3N4 microtubes exhibited a pattern of disorder to order and then back to disorder. The most ordered structures were those formed by the directional assembly of the nitrogen-doped melamine intermediate particles into regularly arranged tubular structures at 2 mL and 2.5 mL of hydroxylamine solution addition. In this example, when the amount of hydroxylamine solution added was higher than 2.5 mL, the nitrogen-doped g-C3N4 microtubes exhibited a random appearance. Nitrogen-doped g-C3N4 microtubes exhibited a random aggregated structure when hydroxylamine solution was added at concentrations of 4 mL and 4.5 mL. This is because the addition of excessive hydroxylamine interfered with the self-assembly process of the originally directional tubular structure, causing excessive cross-linking of the precursor. The microtubes changed from directional extension to random stacking, thus forming an aggregated structure.

[0077] Figure 22 , Figure 23 , Figure 24 , Figure 25 , Figure 26 , Figure 27 , Figure 28 , Figure 29 , Figure 30The figures show the diameter distribution of nitrogen-doped g-C3N4 microtubes prepared in Example 1 with 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution added. The figures show that the average diameter of the fitted nitrogen-doped g-C3N4 microtubes is 2.30 μm. During the thermal polycondensation process, hydroxylamine and melamine molecules form supramolecular intermediates that induce the supramolecular assembly of gCN MTCs microparticles, controlling the pore growth. The nitrogen-doped g-C3N4 microtubes prepared in Example 1 (with 2.5 mL of hydroxylamine solution added) have the smallest average pore size (1.44 μm).

[0078] Figure 31 The figure shows the EDS elemental distribution of the bulk phase g-C3N4 prepared in Comparative Example 1. The figure shows that the elemental distribution of the bulk phase g-C3N4 includes three categories: carbon (60.88 wt%; all percentages measured by EDS in this invention are mass percentages, the same below), nitrogen (27.2 wt%), and oxygen (11.92 wt%).

[0079] Figure 32 The image shows the EDS elemental distribution of the g-C3N4 microtube without additional nitrogen doping prepared in Comparative Example 2. The figure shows that the elemental distribution of the g-C3N4 microtube without additional nitrogen doping includes three categories: carbon (48.76 wt%), nitrogen (39.86 wt%), and oxygen (11.88 wt%).

[0080] Figure 33 , Figure 34 , Figure 35 , Figure 36 , Figure 37 , Figure 38 , Figure 39 , Figure 40 , Figure 41The images show the EDS elemental distribution of nitrogen-doped g-C3N4 microtubes prepared in Example 1 by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution, respectively. The figure shows the elemental distribution of nitrogen-doped g-C3N4 microtubes prepared using hydroxylamine solution as a nitrogen source, including carbon (42.37 wt%~55.94 wt%, with an average of 51.13 wt%), nitrogen (29.64 wt%~51.10 wt%, with an average of 39.53 wt%), and oxygen (3.14 wt%~14.42 wt%, with an average of 9.34 wt%). The nitrogen content of the nitrogen-doped g-C3N4 microtubes with the addition of 2.5 mL of hydroxylamine solution is the highest, at 51.1 wt%. Comparing bulk g-C3N4 and g-C3N4 microtubes without additional nitrogen doping, it can be found that the nitrogen content of the nitrogen-doped g-C3N4 microtubes prepared in Example 1 is increased by 1.88 times (51.1% / 27.2% = 1.88) and 1.28 times (51.1% / 39.86% = 1.28), respectively.

[0081] Figure 42 , Figure 43 The image shows a transmission electron microscope (TEM) image of the nitrogen-doped g-C3N4 microtubes prepared by adding 2.5 mL of hydroxylamine solution to Example 1. The image shows a highly regular one-dimensional thin-walled tubular morphology, indicating that the nitrogen-doped g-C3N4 microtubes prepared by using hydroxylamine solution as the nitrogen source are composed of hollow three-channel microtubes (the cross-section of the microtube is ∞-shaped, with two '0's corresponding to two channels, and the area between them corresponding to the third channel).

[0082] Figure 44 The figure shows the yield of nitrogen-doped g-C3N4 microtubes prepared using the 2.5 mL hydroxylamine solution addition method in Example 1. The figure shows that, following the preparation method of Example 1, after repeated experiments with an average of 3 g of nitrogen-doped melamine precursor per 5 experiments, the average mass of the nitrogen-doped g-C3N4 microtubes formed after high-temperature pyrolysis in air atmosphere was 2.356 g, resulting in a high yield of 78.5%.

[0083] Figure 45 The Fourier transform infrared (FT-IR) spectra of the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 are compared. The figures show that the nitrogen-doped g-C3N4 microtubes maintain the same chemical structure as the bulk g-C3N4 and the g-C3N4 microtubes without additional nitrogen doping, indicating that the preparation strategy in Example 1 did not disrupt the core conjugated framework of g-C3N4. (810 cm⁻¹)-1 The absorption peak at 1200-1650 cm⁻¹ corresponds to the bending vibration mode of the tris-s-triazine unit. -1 The absorption peaks within this range correspond to the characteristic stretching vibration modes of the carbon-nitrogen bonds in the heptaazine heterocyclic unit. (3000-3500 cm⁻¹) -1 Multiple absorption peaks within the range are attributed to vibrational modes of amino, imino, and adsorbed water that are not fully condensed on the surface.

[0084] Figure 46 The specific surface area (BET) plots are shown for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. The plots show that the specific surface area of ​​the bulk g-C3N4 is approximately 6.27 m². 2 / g. The figure shows that the specific surface area of ​​g-C3N4 microtubes without additional nitrogen doping is approximately 39.22 m². 2 The average specific surface area of ​​nitrogen-doped g-C3N4 microtubes prepared by nitrogen doping is approximately 68.52 m² / g. 2 / g (16.65 m 2 / g~137.07 m 2 The specific surface area of ​​nitrogen-doped g-C3N4 microtubes was increased by 10.92 times (68.52 / 6.27 = 10.92) and 1.75 times (68.52 / 39.22 = 1.75) compared to bulk g-C3N4 and g-C3N4 microtubes, respectively. The specific surface area reached a maximum of 137.07 m² when 2.5 mL of hydroxylamine solution was added. 2 / g. This indicates that a higher specific surface area can provide more active sites for a wide range of applications such as catalysis, sensing, and adsorption.

[0085] Figure 47 The figures show the pore size measurements (BJH) of the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. The figures show that the pore size of the bulk g-C3N4 structure is 32.26 nm, and the pore size of the g-C3N4 microtubes without additional nitrogen doping is approximately 29.47 nm. The average pore size of the nitrogen-doped g-C3N4 microtubes prepared in Example 1 is 35.234 nm (25.20 nm ~ 62.20 nm). The pore size of the nitrogen-doped g-C3N4 microtubes reaches a minimum of 25.20 nm with the addition of 2.5 mL of hydroxylamine solution. This indicates that during the thermal polycondensation process, hydroxylamine molecules undergo a supramolecular self-assembly reaction with melamine molecules, acting as an intermediate to induce supramolecular assembly of g-C3N4 microparticles, directionally controlling pore growth, reducing pore size, and improving their electronic band structure.

[0086] Figure 48 XPS images of nitrogen-doped g-C3N4 microtubes prepared in Example 1, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. The figure shows the relative contents of C, N, and O in nitrogen-doped g-C3N4 microtubes and bulk g-C3N4. The nitrogen content increased by an average of 26.57% (68.37% / 54.02% = 1.2657) from bulk g-C3N4 (54.02 atom%; all percentages measured by XPS in this invention are atomic percentages) to the nitrogen-doped g-C3N4 microtubes prepared in Example 1 (65.99 atom%~70.74 atom%, average nitrogen content 68.37 atom%). Compared to the g-C3N4 microtubes prepared in Comparative Example 2 without additional nitrogen doping (65.19 atom%), the average increase was approximately 4.88% (68.37% / 65.19% = 1.0488). The nitrogen-doped g-C3N4 microtubes prepared with 2.5 mL of hydroxylamine solution added had the highest nitrogen content (70.74 atom%). (atom%) indicates that the nitrogen-doped g-C3N4 microtubes prepared in Example 1 contain a high nitrogen content.

[0087] Figure 49 The electrochemical impedance spectroscopy (EIS) spectra of the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 are compared in a mixed solution containing 5 mM potassium ferrocyanide, potassium ferrocyanide, and 0.1 M potassium chloride. It can be seen that the charge transfer resistance (Rct) of the nitrogen-doped g-C3N4 microtubes is significantly lower than that of the bulk g-C3N4. Furthermore, the nitrogen-doped g-C3N4 microtubes prepared in this example with 0.5 mL to 3.5 mL of hydroxylamine solution added are significantly lower than those without additional nitrogen doping, indicating that the synergistic effect of nitrogen doping and the microtube structure greatly promotes electron transfer. In this embodiment, the Rct of nitrogen-doped g-C3N4 microtubes prepared with 4 mL and 4.5 mL hydroxylamine solution additions is higher than that of g-C3N4 microtubes without additional nitrogen doping. This is because excess hydroxylamine disrupts the long-range order of the self-assembly process, transforming the low-dimensional continuous conductive network that should have been formed into disordered agglomerates, resulting in a decrease in charge mobility and an increase in contact resistance, ultimately manifesting as an increase in electrical impedance.

[0088] Figure 50Impedance quantitative analysis diagrams based on electrochemical impedance spectroscopy (EIS) were obtained for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. In Example 1, the average impedance of the nitrogen-doped g-C3N4 microtubes obtained with different amounts of hydroxylamine solution was 1234.62 Ω. The nitrogen-doped g-C3N4 microtube with the lowest impedance (592.30 Ω) was obtained with 2.5 mL of hydroxylamine solution, significantly lower than the g-C3N4 microtube without additional nitrogen doping (1661 Ω) and the bulk g-C3N4 (2116 Ω), demonstrating the impedance advantage of the nitrogen-doped g-C3N4 microtubes.

[0089] Figure 51 The table below shows the cyclic voltammetry (CV) comparisons of the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping in Comparative Example 2, in a mixed solution containing 5 mM potassium ferrocyanide, potassium ferrocyanide, and 0.1 M potassium chloride. The specific oxidation peak potential, oxidation peak current, reduction peak potential, and reduction peak current are shown in the table below. Analysis reveals that when the amount of hydroxylamine solution is ≤3.5 mL, the oxidation peak current of the nitrogen-doped g-C3N4 microtubes (68.24 μA~96.5 μA) is significantly higher than that of the g-C3N4 microtubes without additional nitrogen doping (60.7 μA) and the bulk g-C3N4 (47.7 μA).

[0090]

[0091] Figure 52 The current-quantitative analysis diagrams based on cyclic voltammetry are shown for the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. Analysis reveals that the reduction and oxidation peak currents of the nitrogen-doped g-C3N4 microtubes (-91 μA to -47.1 μA, 50.28 μA to 96.5 μA) are significantly higher than those of the g-C3N4 microtubes (-116 μA, 47.6 μA) and the bulk g-C3N4 (-47.9 μA, 47.6 μA), indicating that nitrogen doping introduces more active sites and improves the conductivity of the material.

[0092] Depend on Figures 49 to 52 It can be seen that the nitrogen-doped g-C3N4 microtubes prepared by the method of the present invention exhibit lower resistance and increased charge transport current, demonstrating superior charge transport capability.

[0093] Figure 53 , Figure 54 , Figure 55 , Figure 56 , Figure 57 , Figure 58 , Figure 59 , Figure 60 , Figure 61 The images show SEM images of nitrogen-doped melamine intermediates prepared in Example 2 with the addition of 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution. The images show that after hydrothermal treatment, hydroxylamine molecules undergo a supramolecular self-assembly reaction with melamine molecules. When the amount of hydroxylamine solution is ≤3.5 mL, the resulting nitrogen-doped melamine intermediate exhibits a directionally arranged microtubular structure. When the amount of hydroxylamine solution exceeds 3.5 mL (e.g., reaching 4.0 mL and 4.5 mL), a short columnar structure is observed. It is evident that the morphology of the nitrogen-doped melamine intermediates after sintering differs depending on the volume of hydroxylamine solution added.

[0094] Figure 62 , Figure 63 , Figure 64 , Figure 65 , Figure 66 , Figure 67 , Figure 68 , Figure 69 , Figure 70 The images show SEM images of nitrogen-doped g-C3N4 microtubes prepared in Example 2 with 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution. The images show that after high-temperature pyrolysis of the nitrogen-doped melamine intermediate, the hydrogen bonds in the original micropillar structure are broken. After exfoliation, the micropillars further form slightly fluffy nitrogen-doped g-C3N4 microtubes with an average tube length >30 μm. In Example 2, as the amount of hydroxylamine solution added increased from 0.5 mL to 4.5 mL, the microtubular structure of the nitrogen-doped g-C3N4 microtubes exhibited a pattern of disorder to order and then back to disorder. The most ordered structures were those formed by the directional assembly of the nitrogen-doped melamine intermediate particles into regularly arranged tubular structures at 2 mL and 2.5 mL of hydroxylamine solution addition. In this example, when the amount of hydroxylamine solution added was higher than 2.5 mL, the nitrogen-doped g-C3N4 microtubes exhibited a random appearance. Nitrogen-doped g-C3N4 microtubes exhibited a random aggregated structure when hydroxylamine solution was added at concentrations of 4 mL and 4.5 mL. This is because the addition of excessive hydroxylamine interfered with the self-assembly process of the originally directional tubular structure, causing excessive cross-linking of the precursor. The microtubes changed from directional extension to random stacking, thus forming an aggregated structure.

[0095] Figure 71XPS images (C1s plots) of C-containing chemical bonds in nitrogen-doped g-C3N4 microtubes prepared in Example 2, bulk g-C3N4 obtained in Comparative Example 1, and g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2. The figure shows the relative content of C, N, and O chemical bonds in nitrogen-doped g-C3N4 microtubes and bulk g-C3N4. The nitrogen content increased by an average of 10.38% from bulk g-C3N4 (60.41 atom%) to the nitrogen-doped g-C3N4 microtubes prepared in Example 2 (59.73 atom%~70.38 atom%, with an average nitrogen content of 66.68 atom%). The nitrogen-doped g-C3N4 microtubes prepared with 2 mL and 2.5 mL of hydroxylamine solution added had higher nitrogen content in the C chemical bonds (70.37 atom%, 70.38 atom%), indicating that even with reference to the C1s diagram of the nitrogen-doped g-C3N4 microtubes prepared in Example 2, the nitrogen-doped g-C3N4 microtubes prepared in Example 2 also contained a high nitrogen content.

[0096] Figure 72 The image shows a SEM image of the nitrogen-doped melamine intermediate prepared by adding 2.5 mL of hydroxylamine solution in Example 3. The image shows that after hydrothermal treatment, hydroxylamine molecules undergo a supramolecular self-assembly reaction with melamine molecules, resulting in a nitrogen-doped melamine intermediate exhibiting a directionally arranged micron-tube structure. However, the morphology of the nitrogen-doped melamine intermediate after sintering differs depending on the volume of hydroxylamine solution added.

[0097] Figure 73 , Figure 74 , Figure 75 , Figure 76 , Figure 77 , Figure 78 , Figure 79 , Figure 80 , Figure 81SEM images of nitrogen-doped g-C3N4 microtubes prepared by adding 0.5 mL, 1 mL, 1.5 mL, 2 mL, 2.5 mL, 3 mL, 3.5 mL, 4 mL, and 4.5 mL of hydroxylamine solution to Example 3 are shown. The images show that after high-temperature pyrolysis of the nitrogen-doped melamine intermediate, the hydrogen bonds in the original micropillar structure are broken. After exfoliation, the micropillars further form slightly fluffy nitrogen-doped g-C3N4 microtubes with an average tube length >30 μm. In Example 3, as the amount of hydroxylamine solution added increased from 0.5 mL to 4.5 mL, the microtubular structure of the nitrogen-doped g-C3N4 microtubes exhibited a pattern of disorder to order and then back to disorder. The most ordered structures were those formed by the directional assembly of the nitrogen-doped melamine intermediate particles into regularly arranged tubular structures at 2 mL and 2.5 mL hydroxylamine solution additions. In this example, when the amount of hydroxylamine solution added was higher than 2.5 mL, the nitrogen-doped g-C3N4 microtubes exhibited a random appearance. Nitrogen-doped g-C3N4 microtubes exhibited a random aggregated structure when hydroxylamine solution was added at concentrations of 4 mL and 4.5 mL. This is because the excessive addition of hydroxylamine interfered with the self-assembly process of the originally directional tubular structure, causing excessive cross-linking of the precursor. The microtubes changed from directional extension to random stacking, thus forming an aggregated structure.

[0098] Figure 82 XPS images of the nitrogen-doped g-C3N4 microtubes prepared in Example 3, the bulk g-C3N4 obtained in Comparative Example 1, and the g-C3N4 microtubes obtained in Comparative Example 3. The figure shows the relative content of C, N, and O chemical bonds in nitrogen-doped g-C3N4 microtubes and bulk g-C3N4. The nitrogen content increased by an average of 26.93% from bulk g-C3N4 (54.02 atom%) to the nitrogen-doped g-C3N4 microtubes prepared in Example 3 (66.11 atom%~71.37 atom%, with an average nitrogen content of 68.56 atom%). Compared with the g-C3N4 microtubes prepared in Comparative Example 2 without additional nitrogen doping (65.19 atom%), the nitrogen content increased by an average of about 5.18%. The nitrogen-doped g-C3N4 microtubes prepared with 2.5 mL of hydroxylamine solution added had the highest nitrogen content (71.37 atom%), indicating that the nitrogen-doped g-C3N4 microtubes prepared in Example 3 contained a high nitrogen content.

[0099] Figure 83 The image shows a SEM image of the nitrogen-free melamine precursor prepared in Comparative Example 2. The image shows that after hydrothermal treatment, melamine molecules undergo supramolecular self-assembly to promote the in-situ formation of micron-sized columnar structures from randomly arranged melamine particles, resulting in a relatively disordered arrangement.

[0100] Figure 84The image shows a SEM image of the nitrogen-free doped g-C3N4 microtubes prepared in Comparative Example 2. The image shows that after high-temperature pyrolysis of the nitrogen-free doped melamine precursor, the original micro-pillar structure was peeled off, and a micro-tubular structure was further formed in situ.

[0101] Figure 85 XPS plot of nitrogen-free g-C3N4 microtube prepared in Comparative Example 2. The plot shows the relative contents of C, N, and O chemical bonds in the nitrogen-free g-C3N4 microtube, with nitrogen content of 65.19 atom, oxygen content of 8.97 atom, and carbon content of 25.84 atom.

[0102] Figure 86 Raman spectra of the nitrogen-doped g-C3N4 microtubes prepared in Example 1, the bulk g-C3N4 microtubes obtained in Comparative Example 1, and the g-C3N4 microtubes without additional nitrogen doping obtained in Comparative Example 2 are shown. The bulk g-C3N4, g-C3N4 microtubes, and nitrogen-doped g-C3N4 microtubes in the figures maintain the same chemical structure, containing an s-triazine ring (483 cm⁻¹). -1 733 cm -1 ), CN and C=N (988 cm) -1 1178 cm -1 ), D band (1350 cm) -1 ), G-band (1580 cm) -1 The characteristic peaks correspond to the bending vibrations of the triazine ring, the stretching vibrations of the CN and C=N bonds, the vibrations of structural defects or disordered regions, and the in-plane stretching vibrations of the conjugated system, respectively. Under different hydroxylamine solution addition conditions, the characteristic Raman peaks corresponding to the stretching vibrations of the s-triazine ring and CN in nitrogen-doped g-C3N4 microtubes show an initial enhancement followed by a steady-state response. This indicates that the bulk g-C3N4 prepared in Comparative Example 1, the g-C3N4 microtubes prepared in Comparative Example 2 without additional nitrogen doping, and the nitrogen-doped g-C3N4 microtubes prepared in Example 1 maintain the same crystal structure.

[0103] Figure 87 The figure shows the diameter distribution of the g-C3N4 microtubes prepared for Comparative Example 2 without additional nitrogen doping. The figure shows that the average diameter of the fitted g-C3N4 microtubes is 3.61 μm. During the thermal polycondensation process, melamine molecules undergo supramolecular self-assembly, leading to the directional growth of disordered g-C3N4 microparticles.

[0104] Those skilled in the art will readily understand that the above description is merely 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 scope of protection of the present invention.

Claims

1. A method for preparing nitrogen-doped g-C3N4 microtubes with high yield, characterized in that, Includes the following steps: S1: Prepare an aqueous dispersion of hydroxylamine, add melamine to the dispersion, and mix thoroughly to obtain a homogeneous dispersion system as a hydrothermal precursor; wherein, the mass percentage concentration of hydroxylamine in the aqueous dispersion does not exceed 3.64%; S2: The uniformly dispersed system is subjected to hydrothermal reaction treatment, and the hydrothermal products are collected to obtain a light yellow crude nitrogen-doped melamine intermediate powder; then, the crude product is washed and dried to obtain a nitrogen-doped melamine intermediate powder product after impurity removal and drying. S3: The intermediate powder product of nitrogen-doped melamine is calcined under air conditions to obtain nitrogen-doped graphitic carbon nitride microtube material.

2. The preparation method according to claim 1, characterized in that, In step S1, the mass percentage concentration of hydroxylamine in the aqueous dispersion of hydroxylamine is 2.14% to 2.65%.

3. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of the aqueous dispersion of hydroxylamine to melamine is (50.56~53.92):(0.5~3). Preferably, the mass ratio of the aqueous dispersion of hydroxylamine to melamine is (52.24~52.8):

1.

4. The preparation method according to claim 1, characterized in that, In step S1, the aqueous dispersion of hydroxylamine is obtained by mixing deionized water with a hydroxylamine solution and then ultrasonically treating the mixture to obtain a homogeneous dispersion. Preferably, the hydroxylamine solution contains 50% hydroxylamine by mass; the deionized water and hydroxylamine solution are mixed at a volume ratio of 50:(0.5~3.5). More preferably, the ultrasonic treatment time is 30 min to 60 min.

5. The preparation method according to claim 1, characterized in that, In step S1, the uniform mixing is achieved by magnetic stirring and / or ultrasonic treatment; Preferably, the uniform mixing is achieved by ultrasonic treatment, and the ultrasonic treatment time is 20 min to 60 min.

6. The preparation method according to claim 1, characterized in that, In step S2, the hydrothermal reaction temperature is 160 ℃~200 ℃, and the reaction time is 18 h~30 h; Preferably, the hydrothermal reaction temperature is 180 °C and the reaction time is 24 h.

7. The preparation method according to claim 1, characterized in that, In step S2, the cleaning is performed by alternating centrifugation with deionized water and anhydrous ethanol to remove water-soluble impurities; the drying temperature is 50 ℃~80 ℃ and the drying time is 20 h~30 h. Preferably, the cleaning process involves alternating between ultrapure water and ethanol for a total of seven times, with the last cleaning performed using ultrapure water.

8. The preparation method according to claim 1, characterized in that, In step S3, the calcination temperature is 480 ℃~600 ℃, the heating rate is 1 ℃ / min~10 ℃ / min, and the holding time is 3 h~5 h.

9. A nitrogen-doped g-C3N4 microtube material obtained by the preparation method according to any one of claims 1-8.

10. The nitrogen-doped g-C3N4 microtube material as described in claim 9, characterized in that, The impedance of the nitrogen-doped g-C3N4 microtube material is 592.30Ω~1483.00Ω; Preferably, the impedance of the nitrogen-doped g-C3N4 microtube material is 592.30Ω~682.80Ω.

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