A method for improving the bonding ability of carbon nanotubes and repairing the electronic transmission path

Abstract

The application discloses a method for improving the bonding capacity of carbon nanotubes and repairing the electronic transmission path, which comprises the following steps: introducing carbon dots with rich surface states into the surface of oxidized carbon nanotubes to form a carbon dot-oxidized carbon nanotube composite material, wherein the amino groups on the surface of the carbon dots are bonded with the carboxyl groups on the surface of the oxidized carbon nanotubes to form amide bonds, thus repairing the phenomenon of electron loss caused by the carboxyl groups on the surface of the oxidized carbon nanotubes; and through the bridging of the amide bonds, the electronic transmission path of the entire carbon nanotube system is turned on; and other functional groups in the rich surface states of the carbon dots are also introduced into the surface of the oxidized carbon nanotubes through the bridging of the amide bonds, so as to improve the surface bonding capacity of the carbon nanotubes. By introducing the carbon dots into the surface of the oxidized carbon nanotubes, the bonding capacity of the carbon nanotubes is improved, the electronic transmission path of the carbon nanotube system is restored, the dielectric loss and the polarization loss of the composite material are improved, and the electromagnetic parameters of the system are improved, so that the system exhibits excellent wave-absorbing performance in the X wave band.

Description

Technical Field

[0001] This invention belongs to the field of carbon nanotube technology, specifically relating to a method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways. Background Technology

[0002] With the advent of the 5G era and the increasing sophistication of military equipment positioning and tracking technologies, the problems caused by electromagnetic waves (EMS) are becoming increasingly serious. Absorbing materials have become an indispensable element in both daily life and national defense. Carbon nanotubes (CNTs) possess sp... 2 The π-electron configuration of carbon nanotubes enables excellent electron transport properties. Therefore, carbon nanotube-based composite materials have enormous potential for electromagnetic absorption.

[0003] In recent years, high-performance carbon nanotube-based electromagnetic shielding materials have been developed by complexing with other electromagnetic materials to modulate dielectric properties. Research has revealed that physical blending methods can lead to uneven dispersion of the composite material, resulting in particle agglomeration and sedimentation, ultimately negatively impacting the microwave absorption performance of the composite. In contrast, covalent bonding methods can significantly improve the uniformity of the composite material and increase the electron transfer rate of the system, thereby enhancing the dielectric loss rate.

[0004] However, carbon materials typically require pretreatment to functionalize their surfaces and increase their surface binding energy before they can be covalently bonded to other materials. Carbon nanotubes are often functionalized by methods such as acid treatment to generate oxygen-containing functional groups with electron-withdrawing properties. These oxygen-containing functional groups, with their electron-withdrawing properties, induce electron cloud shifts through inductive effects, thereby disrupting electron delocalization in the π-π conjugated system and negatively impacting electron transport in carbon nanotubes, ultimately affecting their microwave absorption performance. Therefore, a modification method to improve the surface binding energy of carbon nanotubes without affecting their electron transport performance is of great significance. Summary of the Invention

[0005] To address the aforementioned problems in the prior art, this invention provides a method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, as well as a carbon dot-carbon nanotube composite material. The technical problem to be solved by this invention is achieved through the following technical solution:

[0006] A method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways involves introducing carbon dots with abundant surface states onto the surface of carbon oxide nanotubes to form a carbon dot-carbon nanotube composite material. The amino groups on the surface of the carbon dots bond with the carboxyl groups on the surface of the carbon oxide nanotubes to form amide bonds, thus repairing electron loss caused by the carboxyl groups on the surface of the carbon oxide nanotubes. The amide bonds also bridge the electron transport pathways of the entire carbon nanotube system. Other functional groups within the abundant surface states of the carbon dots are also introduced to the surface of the carbon oxide nanotubes via the amide bond bridging, thereby increasing the surface binding energy of the carbon nanotubes and thus enhancing their surface bonding ability.

[0007] In one embodiment of the present invention, the carbon nanotube composite material includes a blue fluorescent carbon dot-carbon nanotube composite material and a green fluorescent carbon dot-carbon nanotube composite material.

[0008] In one embodiment of the present invention, the preparation method of the carbon dot-carbon nanotube composite material includes the following steps:

[0009] Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes.

[0010] The carbon dots were synthesized by hydrothermal synthesis using citric acid and ethanolamine as precursors.

[0011] The carbon nanotubes and carbon dots are reacted to synthesize the carbon dot-carbon nanotube composite material.

[0012] In one embodiment of the present invention, the preparation method of the blue fluorescent carbon dot-carbon nanotube composite material includes the following steps:

[0013] Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes.

[0014] Blue fluorescent carbon dots crude samples were synthesized by hydrothermal synthesis using citric acid and ethanolamine as precursors; the reaction temperature was 145–165℃ and the reaction time was 12–18 h.

[0015] The crude sample of blue fluorescent carbon dots was purified by silica gel column chromatography, and then the solvent was removed and the sample was dispersed in water to form a blue fluorescent carbon dot solution.

[0016] The carbon nanotubes were added to the blue fluorescent carbon dot solution and reacted at a temperature of 180–220°C for 12–18 h to obtain the blue fluorescent carbon dot-carbon nanotube composite material.

[0017] In one embodiment of the present invention, the preparation method of the green fluorescent carbon dot-carbon nanotube composite material includes the following steps:

[0018] Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes.

[0019] Green fluorescent carbon dots were synthesized using citric acid and ethanolamine as precursors via a hydrothermal synthesis method. The reaction temperature was 170–190 °C and the reaction time was 12–18 h.

[0020] The crude green fluorescent carbon dot sample was purified by silica gel column chromatography, and then the solvent was removed and the sample was dispersed in water to form a green fluorescent carbon dot solution.

[0021] The carbon nanotubes were added to the green fluorescent carbon dot solution and reacted at a temperature of 180–220°C for 12–18 h to obtain the green fluorescent carbon dot-carbon nanotube composite material.

[0022] In one embodiment of the present invention, the molar ratio of concentrated sulfuric acid to concentrated nitric acid is 3:1.

[0023] In one embodiment of the present invention, the reaction temperature for the acidification synthesis of carbon nanotubes is 80°C and the reaction time is 2 hours.

[0024] In one embodiment of the present invention, the silica gel column chromatography method is a gradient elution method using methanol and dichloromethane in a 1:8 ratio.

[0025] A carbon dot-carbon nanotube composite material is disclosed, wherein the carbon dot-carbon nanotube composite material is formed by introducing carbon dots into the surface of carbon oxide nanotubes. Specifically, the carbon dot-carbon nanotube composite material is formed by introducing carbon dots with abundant surface states into the surface of carbon oxide nanotubes. The amino groups on the surface of the carbon dots bond with the carboxyl groups on the surface of the carbon oxide nanotubes to form amide bonds, thereby repairing the electron loss caused by the carboxyl groups on the surface of the carbon oxide nanotubes. Furthermore, the amide bonds bridge the electron transport pathways of the entire carbon nanotube system. Other functional groups within the abundant surface states of the carbon dots are also introduced to the surface of the carbon oxide nanotubes through the amide bond bridging, thereby increasing the surface binding energy of the carbon nanotubes and thus enhancing their surface bonding ability.

[0026] In one embodiment of the present invention, the carbon dot-carbon nanotube composite material includes a blue fluorescent carbon dot-carbon nanotube composite material and a green fluorescent carbon dot-carbon nanotube composite material.

[0027] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] This invention introduces carbon dots with abundant surface states onto the surface of carbon oxide nanotubes. On one hand, the amino groups on the carbon dot surface bond with the carboxyl groups on the carbon oxide nanotube surface, forming amide bonds, thus repairing the electron loss caused by the carboxyl groups on the carbon oxide nanotube surface. Furthermore, the amide bonds bridge the electron transport pathway of the entire carbon nanotube system, thereby improving the dielectric loss and polarization loss of the composite material and enhancing the electromagnetic parameters of the system, resulting in excellent X-ray absorption performance. On the other hand, other functional groups within the abundant surface states of the carbon dots are also introduced to the surface of the carbon oxide nanotubes through amide bond bridging, significantly increasing the surface binding energy of the carbon nanotubes and thus improving their surface bonding ability. In addition, this invention studies the energy transfer process of the carbon dot and carbon nanotube system using time-resolved kinetics and proves the proposed electron transport mechanism. This invention effectively restores the dielectric parameters of carbon oxide nanotubes through amide bond bridging, and the prepared carbon dot-carbon nanotube composite material exhibits excellent absorption performance in the X-ray band. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the reaction process of the present invention;

[0030] Figure 2 This is the energy-dispersive X-ray spectra (EDS) of carbon dot-carbon nanotube composite materials under a transmission electron microscope.

[0031] Figure 3 These are Fourier transform infrared (FTIR) spectra of CNTs, O-CNTs, and CDs-CNTs;

[0032] Figure 4 These are Raman spectra of CNTs, O-CNTs, and CDs-CNTs;

[0033] Figure 5 These are the X-ray diffraction energy distributions (XPS) of CNTs, O-CNTs, and CDs-CNTs; among them, Figure 5 (a) is the full XPS spectrum. Figure 5 (b) is the C1s fine spectrum. Figure 5 (c) is the O1s fine spectrum. Figure 5 (d) is the N1s fine spectrum;

[0034] Figure 6 These are electrochemical impedance (EIS) plots for CNTs, O-CNTs, and CDs-CNTs;

[0035] Figure 7 This is the photoluminescence spectrum (PL) of CDs and CDs-CNTs aqueous solutions;

[0036] Figure 8 This is a fluorescence decay curve of CDs and CDs-CNTs;

[0037] Figure 9 This is a diagram illustrating the energy transfer process and fluorescence quenching mechanism of CDs;

[0038] Figure 10 It is a two-dimensional diagram of the differential charge density after carboxyl functional groups are attached to the surface of carbon nanotubes;

[0039] Figure 11 This is a comparison of the differential charge density of carbon dots attached to the surface of carbon nanotubes via covalent bonding and van der Waals forces. Figure 11 (a) is a data graph of the covalent bond state. Figure 11 (b) is a data graph showing the adhesion state under van der Waals forces. Figure 11 (c) is a graph showing the amount of electron transfer and accumulation along the y-axis and z-axis in the CDs-CNTs system;

[0040] Figure 12 This is a graph showing the variation of ε′ and ε″ of CNTs, O-CNTs, B-CDs-CNTs, and G-CDs-CNTs with the frequency of the incident electromagnetic wave.

[0041] Figure 13 (a) Figure 13 (b) shows the dielectric loss constant and attenuation constant curves for CNTs, O-CNTs, B-CDs-CNTs and G-CDs-CNTs, respectively.

[0042] Figure 14 (a) Figure 14 (b) Figure 14 (c) Figure 14 (d) are reflection loss diagrams of CNTs, O-CNTs, B-CDs-CNTs and G-CDs-CNTs at various thicknesses, respectively.

[0043] Figure 15 This is a schematic diagram of the specific binding sites and electron transport mechanism of CDs-CNTs. Detailed Implementation

[0044] The present invention will be further described in detail below with reference to specific embodiments, but the implementation of the present invention is not limited thereto.

[0045] The carbon dots (CDs) involved in this invention are a class of zero-dimensional carbon nanomaterials with significant fluorescence properties, composed of a core state with a π-conjugated structure and a surface state with active functional groups. The unique surface structure of carbon dots allows them to act as bridging ligands, promoting multiple bonding with CNTs through electron cloud overlap. This invention reveals that combining carbon dots with carbon oxide nanotubes introduces abundant functional groups from the carbon dot surface into the carbon nanotube surface, increasing the binding energy of the carbon nanotube surface. Furthermore, carbon dots bond with the carboxyl groups of carbon oxide nanotubes through surface amino groups, forming amide bonds to repair electron loss caused by carboxyl groups. The core structure of carbon dots is similar to the long-range conjugated π bonds of carbon nanotubes; therefore, the amide bond bridging can conduct electron transport pathways throughout the entire carbon nanotube system. This invention introduces carbon dots onto the surface of carbon nanotubes, which not only improves the bonding ability of carbon nanotubes but also restores and enhances the electron transport pathway of the carbon nanotube system. This method improves the dielectric loss and polarization loss of the composite material, thereby enhancing the electromagnetic parameters of the system and enabling it to exhibit excellent microwave absorption performance in the X-band.

[0046] This invention provides a method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways. Carbon dots with abundant surface states are introduced onto the surface of carbon oxide nanotubes to form a carbon dot-carbon nanotube composite material. The amino groups on the surface of the carbon dots bond with the carboxyl groups on the surface of the carbon oxide nanotubes to form amide bonds, thus repairing the electron loss caused by the carboxyl groups on the surface of the carbon oxide nanotubes. The amide bonds also bridge the electron transport pathways of the entire carbon nanotube system. Other functional groups within the abundant surface states of the carbon dots are also introduced to the surface of the carbon oxide nanotubes through the amide bond bridging, thereby increasing the surface binding energy of the carbon nanotubes and thus improving their surface bonding ability.

[0047] It should be noted that the carbon dots involved in this invention can be of all types. The embodiments of this invention focus on blue fluorescent carbon dots (B-CDs) and green fluorescent carbon dots (G-CDs) for illustration, and green fluorescent carbon dots (G-CDs) are preferably used to prepare carbon dot-carbon nanotube composite materials.

[0048] Accordingly, the carbon dot-carbon nanotube composite materials involved in the embodiments of the present invention include blue fluorescent carbon dot-carbon nanotube composite materials (B-CDs-CNTs) and green fluorescent carbon dot-carbon nanotube composite materials (G-CDs-CNTs).

[0049] The preparation method of carbon dot-carbon nanotube composite material provided in this embodiment of the invention includes the following steps:

[0050] S1. Carbon nanotubes are acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes.

[0051] Specifically, in this step, the molar ratio of concentrated sulfuric acid to concentrated nitric acid is 3:1. Acidification is carried out by heating at 80°C for 2 hours in a high-pressure autoclave lined with polytetrafluoroethylene. After cooling to room temperature, the acidified carbon nanotubes are repeatedly washed with ultrapure water until the pH value is neutral, thus obtaining carbon oxide nanotubes (O-CNTs).

[0052] S2. The carbon dots were synthesized using citric acid and ethanolamine as precursors via a hydrothermal synthesis method.

[0053] Specifically, the hydrothermal synthesis method involved in this step involves adding ethanolamine (EMA) to ultrapure water containing citric acid (CA), stirring until a clear solution is obtained, and then transferring the solution to a high-pressure reactor lined with polytetrafluoroethylene, heating it at 145–165°C and 180–220°C for 12–18 hours respectively, to obtain crude B-CDs and crude G-CDs samples.

[0054] After hydrothermal synthesis, the crude B-CDs and crude G-CDs solutions were purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane in a 1:8 ratio. Then, the solvent was removed from the crude B-CDs and crude G-CDs solutions by rotary evaporation. The solvent-free B-CDs and G-CDs solutions were redispersed in ultrapure water to obtain purified B-CDs and G-CDs solutions.

[0055] S3. React the carbon oxide nanotubes and the carbon dots to synthesize the carbon dot-carbon nanotube composite material.

[0056] The O-CNTs prepared in S1 were added to the B-CDs and G-CDs solutions prepared in S2, respectively. The mixtures were then sonicated for 120 minutes to form a homogeneous mixture. The sonicated mixtures were then transferred to a polytetrafluoroethylene-lined autoclave and heated at 180–220 °C for 12–18 h. The solutions were then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the products were dried in a vacuum drying oven at 60 °C to obtain B-CDs-CNTs and G-CDs-CNTs.

[0057] This invention also provides a carbon dot-carbon nanotube composite material, which is formed by introducing carbon dots onto the surface of carbon oxide nanotubes. Specifically, the carbon dot-carbon nanotube composite material is formed by introducing carbon dots with abundant surface states onto the surface of carbon oxide nanotubes. The amino groups on the surface of the carbon dots bond with the carboxyl groups on the surface of the carbon oxide nanotubes to form amide bonds, thus repairing the electron loss caused by the carboxyl groups on the surface of the carbon oxide nanotubes. Furthermore, the amide bonds bridge the electron transport pathways of the entire carbon nanotube system. Other functional groups within the abundant surface states of the carbon dots are also introduced to the surface of the carbon oxide nanotubes through the amide bond bridging, thereby increasing the surface binding energy of the carbon nanotubes and thus enhancing their surface bonding ability.

[0058] The carbon dot-carbon nanotube composite material includes B-CDs-CNTs and G-CDs-CNTs.

[0059] The preparation method of this carbon dot-carbon nanotube composite material is the same as the method described above, and will not be repeated here.

[0060] The following detailed description, with reference to specific embodiments, illustrates the method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, as well as the carbon dot-carbon nanotube composite material of the present invention.

[0061] Example 1

[0062] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material provided in this embodiment, include:

[0063] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a high-pressure autoclave lined with polytetrafluoroethylene. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0064] Synthesis of S2 and crude B-CDs samples: Crude B-CDs samples were synthesized using a hydrothermal synthesis method with CA and EMA as precursors. Specifically, 59.4 mmol EMA was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was stirred until a clear solution was obtained. The solution was then transferred to a polytetrafluoroethylene-lined autoclave and heated at 160 °C for 15 hours to obtain crude B-CDs samples.

[0065] Purification of S3 and crude B-CDs samples: After hydrothermal synthesis, crude B-CDs samples were purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane in a ratio of 1:8. Then, the solvent was removed from the crude B-CDs sample solution by rotary evaporation. The solvent-free B-CDs solution was redispersed in ultrapure water to obtain the purified B-CDs solution.

[0066] Synthesis of S4 and B-CDs-CNTs: 0.05 g of O-CNTs was added to 30 mL of purified B-CDs solution, and the resulting mixture was sonicated for 120 minutes to form a homogeneous solution. The solution was then transferred to a 50 mL autoclave lined with polytetrafluoroethylene and heated at 200 °C for 16 hours. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain B-CDs-CNTs.

[0067] Example 2

[0068] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material provided in this embodiment, include:

[0069] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a high-pressure autoclave lined with polytetrafluoroethylene. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0070] Synthesis of S2 and G-CDs crude samples: G-CDs crude samples were synthesized using a hydrothermal synthesis method with CA and EMA as precursors. 59.4 mmol EMA was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was stirred until a clear solution was obtained. The solution was then transferred to a 50 mL polytetrafluoroethylene-lined autoclave and heated at 180 °C for 15 hours to obtain the G-CDs crude sample.

[0071] S3. Purification of crude G-CDs sample: After hydrothermal synthesis, crude G-CDs sample was purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane (1:8). Then, the solvent was removed from the crude G-CDs sample solution by rotary evaporation, and the solvent-free G-CDs solution was redispersed in ultrapure water to obtain the purified G-CDs solution.

[0072] Synthesis of S4, G-CDs-CNTs: 0.05 g of O-CNTs was added to 30 mL of purified G-CDs solution. The mixture was then sonicated for 120 minutes to form a homogeneous solution. The solution was then transferred to a 50 mL PTFE-lined autoclave and heated at 200 °C for 16 hours. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain G-CDs-CNTs.

[0073] Example 3

[0074] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material provided in this embodiment, include:

[0075] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a high-pressure autoclave lined with polytetrafluoroethylene. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0076] Synthesis of S2 and crude B-CDs samples: Crude B-CDs samples were synthesized using a hydrothermal synthesis method with CA and EMA as precursors. 59.4 mmol EMA was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was vigorously stirred until a clear solution was obtained. The solution was then transferred to a 50 mL high-pressure reactor lined with polytetrafluoroethylene and heated at 145 °C for 18 hours to obtain the crude B-CDs samples.

[0077] Purification of S3 and crude B-CDs samples: After hydrothermal synthesis, crude B-CDs samples were purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane in a ratio of 1:8. Then, the solvent was removed from the crude B-CDs solution by rotary evaporation, and the solvent-free B-CDs solution was redispersed in ultrapure water to obtain the purified B-CDs solution.

[0078] Synthesis of S4 and B-CDs-CNTs: 0.05 g of O-CNTs was added to 30 mL of purified B-CDs solution. The mixture was then sonicated for 120 minutes to form a homogeneous solution. The solution was then transferred to a 50 mL PTFE-lined autoclave and heated at 180 °C for 18 hours. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain B-CDs-CNTs.

[0079] Example 4

[0080] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material of this embodiment, include:

[0081] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a polytetrafluoroethylene-lined autoclave. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0082] Synthesis of S2 and crude G-CDs samples: Crude G-CDs samples were synthesized using a hydrothermal method with CA and EMA as precursors. EMA (59.4 mmol) was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was vigorously stirred until a clear solution was obtained. The solution was then transferred to a 50 mL autoclave lined with polytetrafluoroethylene and heated at 170 °C for 18 hours. The crude G-CDs sample was then obtained.

[0083] S3. Purification of crude G-CDs sample: After hydrothermal synthesis, crude G-CDs sample was purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane (1:8). Then, the solvent was removed from the crude G-CDs sample solution by rotary evaporation, and the solvent-free G-CDs solution was redispersed in ultrapure water to obtain the purified G-CDs solution.

[0084] Synthesis of G-CDs-CNTs (S4): 0.05 g of O-CNTs was added to 30 mL of purified G-CDs solution. The mixture was then sonicated for 120 min to form a homogeneous mixture. The solution was then transferred to a 50 mL PTFE-lined autoclave and heated at 180 °C for 18 h. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain G-CDs-CNTs.

[0085] Example 5

[0086] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material of this embodiment, include:

[0087] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a polytetrafluoroethylene-lined autoclave. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0088] Synthesis of S2 and crude B-CDs samples: Crude B-CDs samples were synthesized using a hydrothermal method with CA and EMA as precursors. EMA (59.4 mmol) was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was vigorously stirred until a clear solution was obtained. The solution was then transferred to a 50 mL autoclave lined with polytetrafluoroethylene and heated at 165 °C for 12 hours. The crude B-CDs samples were then obtained.

[0089] Purification of S3 and crude B-CDs samples: After hydrothermal synthesis, crude B-CDs samples were purified by silica gel column chromatography using a gradient elution method with methanol and dichloromethane in a 1:8 ratio. Then, the solvent was removed from the crude B-CDs solution using a rotary evaporator. The solvent-free B-CDs solution was then redispersed in ultrapure water to obtain the purified B-CDs solution.

[0090] Synthesis of S4, B-CDs-CNTs: 0.05 g of O-CNTs was added to 30 mL of purified B-CDs solution. The mixture was then sonicated for 120 min to form a homogeneous mixture. The solution was then transferred to a 50 mL PTFE-lined autoclave and heated at 220 °C for 12 h. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain B-CDs-CNTs.

[0091] Example 6

[0092] The method for improving the bonding ability of carbon nanotubes and repairing electron transport pathways, and the carbon dot-carbon nanotube composite material of this embodiment, include:

[0093] Synthesis of S1 and O-CNTs: A certain amount of carbon nanotubes were acidified with concentrated H2SO4 and concentrated HNO3 (molar ratio 3:1) and heated at 80°C for 2 hours in a polytetrafluoroethylene-lined autoclave. After cooling to room temperature, the acidified carbon nanotubes were repeatedly washed with ultrapure water until the pH value was neutral.

[0094] Synthesis of S2 and crude G-CDs samples: Crude G-CDs samples were synthesized using a hydrothermal method with CA and EMA as precursors. EMA (59.4 mmol) was added to 30 mL of ultrapure water containing 3.22 mmol CA, and the mixture was vigorously stirred until a clear solution was obtained. The solution was then transferred to a 50 mL autoclave lined with polytetrafluoroethylene and heated at 190 °C for 12 hours. The crude G-CDs sample was then obtained.

[0095] S3. Purification of crude G-CDs sample: After hydrothermal synthesis, crude G-CDs sample was purified by silica gel column chromatography using a gradient elution method of methanol and dichloromethane (1:8). Then, the solvent was removed from the crude G-CDs sample solution by rotary evaporation, and the solvent-free G-CDs solution was redispersed in ultrapure water to obtain the purified G-CDs solution.

[0096] Synthesis of G-CDs-CNTs (S4): 0.05 g of O-CNTs was added to 30 mL of purified G-CDs solution. The mixture was then sonicated for 120 min to form a homogeneous mixture. The solution was then transferred to a 50 mL PTFE-lined autoclave and heated at 220 °C for 12 h. The solution was then removed, centrifuged, and repeatedly washed to remove unreacted carbon dots. Finally, the sample was poured into a beaker and dried in a vacuum drying oven at 60 °C to obtain G-CDs-CNTs.

[0097] The reaction process diagram of the above embodiments of the present invention is shown below. Figure 1 As shown.

[0098] The structure and performance of the CDs-CNTs provided by this invention will be described below.

[0099] (I) Structural characterization of CDs-CNTs

[0100] The morphology of CDs-CNTs was characterized by transmission electron microscopy (TEM). Energy dispersive X-ray spectroscopy (EDS) was also used to characterize the morphology. Figure 2 Elemental analysis of CDs-CNTs revealed that nitrogen exhibited a regional distribution, unlike the uniform distribution of C and O. This, combined with the premise that nitrogen was added during the synthesis of only carbon points, suggests that nitrogen was present in the process. Figure 1 This indicates that CDs successfully complexed with O-CNTs.

[0101] The surface functional groups of CNTs, O-CNTs, and CDs-CNTs were analyzed by Fourier transform infrared (FT-IR) spectroscopy. Figure 3 As shown, C=C(1620cm⁻¹) appeared in all modified carbon nanotube samples. -1 ) and CC key (1201cm -1 The text describes a series of typical carbon nanotube vibrational peaks exhibiting tensile vibrations, including those associated with O-CNTs. O-CNTs show a peak at 1656 cm⁻¹. -1The prominent absorption band at 1656 cm⁻¹ is attributed to the C=O vibration, indicating successful functionalization of the carboxyl group on the carbon nanotube. The introduction of carbon dots brings numerous functionalized groups (NH₂, OH, etc.) to CDs-CNTs, demonstrating that the introduction of carbon dots significantly increases the surface energy of the carbon nanotubes, thereby enhancing their bonding ability. In addition to the absorption bands corresponding to common functional groups, a band at 1656 cm⁻¹ was observed in the FT-IR spectrum of CDs-CNTs. -1 The C=O vibration peak at this location is significantly enhanced and reaches 1607 cm⁻¹. -1 The observation of new NH bond vibrations, attributed to amide group vibrations, confirms the successful amide bond between CNTs and CDs. The graphitic carbon content of CNTs, O-CNTs, and CDs-CNTs was confirmed by the ratio of the D peak intensity to the G peak intensity (ID / IG) in the Raman spectra of CNTs, O-CNTs, and CDs-CNTs. Figure 4 As shown, CNTs exhibit good crystallinity; oxidation of carbon nanotubes partially disrupts their conjugated structure, leading to a decrease in crystallinity (ID / IG decreases from 0.625 to 0.701); CDs, due to their unique growth process, generate numerous active functional groups on their surface and generate many defects within their core, resulting in poor crystallinity. When carbon dots are attached to carbon nanotubes, the crystallinity further decreases (ID / IG decreases from 0.701 to 0.701). The changes in the overall Raman data are consistent with our proposed composite process, confirming the successful composite of carbon dots and carbon nanotubes. Furthermore, XPS analysis was used to investigate the relative elemental content and surface structure (…). Figure 5 (a)- Figure 5 (d)). The binding energy peaks of the changes in the proportions of the main elements (C, N, O) in the three groups of samples are consistent with the structural characteristics described above. The C1s spectrum shows four peaks at 284.1, 284.8, 286.1, and 290.1 ​​eV, which are attributed to C=C, CC, C=O, and -COOH, respectively. Simultaneously, due to the oxidation of CNTs and the attachment of CDs, the concentration of surface functional groups also changed accordingly. The O1s spectrum shows two peaks at 531.1 eV and 532.5 eV, which are attributed to CO and C=O, respectively. The oxidation of CNTs led to an increase in the C=O content from 0.54% to 2.85%, and subsequently, the introduction of oxygen-rich CDs into the CNT surface through the formation of amide bonds resulted in a further increase in the C=O content (from 2.85% to 3.85%). Notably, among all samples, only CDs-CNTs showed a binding energy peak in the N1s spectrum. In the N1s spectrum of CDs-CNTs, the peaks observed at 399.1, 400.1, and 401.1 eV belong to CN bonds, graphite N bonds, and NH bonds, respectively. Considering the synthesis process of CDs and CNTs, only the precursors of carbon dots contain nitrogen, thus fully demonstrating the bonding between CDs and CNTs.

[0102] Based on the structural and chemical characterization analysis of CDs-CNTs, carbon dots improve the binding energy of the carbon nanotube surface without changing the structural properties of the carbon nanotube itself, and are stably linked to CNTs through amide bonds.

[0103] (II) Dynamic Investigation of CDs-CNTs

[0104] The electron transport properties of CNTs, O-CNTs, and CDs-CNTs were analyzed by electrochemical impedance spectroscopy. Figure 6 The electron transport properties of CNTs were observed to be the best in the spectrum; the electron transport properties of carbon nanotubes deteriorated significantly after oxidation; and the electron transport properties of the system were restored when carbon dots were attached with oxidized carbon nanotubes.

[0105] To verify the electron transfer process between carbon dots and carbon nanotubes, this invention tested the photoluminescence (PL) spectra of carbon dots and carbon nanotubes before and after compositing. Figure 7 ) and fluorescence decay curve spectrum ( Figure 8 ).from Figure 8 It can be observed that the fluorescence is significantly weakened after the carbon dots are combined with carbon nanotubes, indicating that the excited-state charge carriers are not emitted as photons, but dissipate energy in other ways. The fluorescence decay curve was fitted with the exponential function shown in eq1.

[0106]

[0107]

[0108] Where t represents the time variable, and A1 and A2 represent the relative amplitude of each attenuation channel. The results are shown in Table 1.

[0109] Table 1

[0110]

[0111] Fitting the fluorescence decay curve using the above formula, the decay curve of the carbon dots before recombination follows a single exponential decay, and the decay is relatively slow. After recombination, the fluorescence decay channels of the carbon dots change significantly, from the original single exponential decay to double exponential decay. Furthermore, it was found that the new decay channels not only have a higher proportion but also a significantly faster decay rate. This indicates that most electrons in the excited state return to the ground state through a rapid nonradiative transition process rather than as photons, which is consistent with the weak fluorescence emission spectrum (…). Figure 7 They match perfectly.

[0112] This indicates that carbon dots achieve electron transfer with CNTs through non-radiative transition channels. The energy transfer process and fluorescence quenching mechanism of carbon dots are as follows: Figure 9As shown, before CDs bond with CNTs, excitons and holes in the excited state recombine and release energy in the form of photons. After recombination, CDs transfer the energy of the excited excitons to the CNTs system through nonradiative transitions via amide bonds, thereby greatly reducing the fluorescence emission of the original CDs.

[0113] This invention further investigates the electron transfer between carbon dots and carbon nanotubes based on first-principles density functional theory (DFT). First, a carbon nanotube model with carboxyl groups attached was constructed and optimized, and its differential charge density was calculated. Figure 10 In the two-dimensional cross-sectional diagram, red represents electron gain and blue represents electron loss. From Figure 10 It can be observed that after carboxyl groups are attached to the surface of CNTs, a large number of electrons on the surface are captured by the carboxyl groups, thus hindering electron transport in the original CNT system. The previous section demonstrated that energy transfer can occur after carbon dot bonding, thereby repairing the electron transport pathway blocked by the carboxyl groups. To further demonstrate the electron transfer characteristics of amide bond bridging, a carbon dot layer composed of 24 carbon atoms and 2 amino groups was constructed and optimized to compare the differential charge density distribution of CDs and CNTs under covalent bonds (amide bonds) and van der Waals interactions. Figure 11 (a) and Figure 11 (b) Calculate the electron transfer and accumulation along the y-axis and z-axis in the CDs-CNTs system. Figure 11 (c) Clearly, the amide bond significantly enhances the electron transfer between CDs and CNTs. This demonstrates that the amino groups on the carbon dot surface and the carboxyl groups on the carbon nanotubes conduct the electron pathway of the entire system through the conjugation effect and the electron transfer effect of the covalent bond, thereby effectively enhancing the conductivity of the system and thus increasing the conductivity loss of CDs-CNTs.

[0114] (III) Electromagnetic wave absorption test of various CNT composite materials

[0115] The electromagnetic wave absorption performance of dielectric materials is highly dependent on their complex permittivity (ε = ε' + iε″), where ε′ represents the ability of the absorbing material to store electrical energy, and ε″ represents the ability to dissipate electrical energy. Figure 12 The curves showing the variation of ε′ and ε″ as a function of the incident electromagnetic wave frequency are displayed for various carbon nanotube composites, including pure carbon nanotubes (CNTs), carbon oxide nanotubes (O-CNTs), and carbon dot (blue and green light carbon dots)-carbon nanotube composites (B-CDs-CNTs; G-CDs-CNTs). Figure 12It can be observed that the O-CNTs have the smallest values ​​for both ε′ and ε″, indicating that the electromagnetic wave absorption performance of carbon nanotubes is greatly affected after oxidation. After the two types of carbon dots are combined with oxidized carbon nanotubes, ε′ is significantly restored and the dissipation capacity is significantly enhanced, indicating that the CDs-CNTs composite material has excellent dielectric loss performance, with high dielectric loss constant (ε″ / ε′) and low attenuation constant. This phenomenon was confirmed as supplementary evidence. Figure 13 (a) and Figure 13 (b) The electromagnetic wave absorption performance of the material is calculated using reflection loss (RL), as shown in eq 3.

[0116]

[0117]

[0118] Z in is the input impedance, c is the speed of light in a vacuum, d is the thickness of the absorbing material, and h is Planck's constant. Figure 14 (a)- Figure 14 (d) The microwave absorption performance results of the materials are consistent with the trend predicted by our simulation. The attachment of carbon dots greatly improves the microwave absorption performance of carbon nanotubes (this phenomenon is observed in both blue and green carbon dots). Among them, the green carbon dot-carbon nanotube composite material has the widest frequency range of RL < -10dB, reaching 5.02GHz, and the strongest microwave absorption can reach -38.6dB.

[0119] The above text demonstrates that CDs form amide bonds with carbon nanotubes, repairing the electron transport pathway blocked by carboxyl groups, bridging two materials with π-conjugated structures, accelerating electron flow between adjacent carbon nanotubes, and enhancing electron transport in CDs-CNTs. Based on the theoretical analysis above, the following section uses G-CDs-CNTs as an example to illustrate the specific binding sites of the amide bonds in G-CDs-CNTs and the microwave absorption mechanism of the composite system. Figure 15 As shown, CDs first form amide bonds with CNTs. The abundant surface states significantly improve the surface binding energy of CNTs, thereby enhancing their surface bonding ability. In addition, the amide bonds enable CDs to adhere stably to the surface of CNTs, repair the electron transport pathway of CNTs blocked by carboxyl groups, accelerate the electron flow between adjacent carbon nanotubes, and enhance the electron transport of the entire system.

[0120] This invention introduces carbon dot-carbon nanotube composites onto the surface of O-CNTs by incorporating abundant surface-state carbon ionomers (CDs). The CDs introduce rich surface functional groups into the CNTs surface, significantly improving the surface binding energy and thus enhancing their surface bonding ability. Furthermore, the amino groups on the carbon dot surface bond with the carboxyl groups on the O-CNTs surface, forming amide bonds. This repairs the electron loss caused by the carboxyl groups on the O-CNTs surface and, through the bridging of the amide bonds, opens up the electron transport pathways throughout the carbon nanotube system. This demonstrates that the carbon dot-carbon nanotube composite material prepared by this invention exhibits excellent absorption performance in the X-ray band. This research can provide new thinking and future development directions for CNT-based microwave absorbing materials.

[0121] The above description, in conjunction with specific preferred embodiments, provides a further detailed explanation of the present invention. It should not be construed that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, various simple deductions or substitutions can be made without departing from the concept of the present invention, and all such modifications and substitutions should be considered within the scope of protection of the present invention.

Claims

1. An application of a carbon dot-carbon nanotube composite material as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, Carbon dots with abundant surface states are introduced into the surface of carbon oxide nanotubes to form the carbon dot-carbon nanotube composite material. The amino groups on the surface of the carbon dots bond with the carboxyl groups on the surface of the carbon oxide nanotubes to form amide bonds, which repairs the electron loss caused by the carboxyl groups on the surface of the carbon oxide nanotubes. The electron transport pathway of the entire carbon nanotube system is also opened through the bridging of the amide bonds. Other functional groups in the abundant surface states of the carbon dots are also introduced into the surface of the carbon oxide nanotubes through the bridging of the amide bonds to improve the surface binding energy of the carbon nanotubes and thus improve their surface bonding ability.

2. The application of the carbon dot-carbon nanotube composite material according to claim 1 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The carbon dot-carbon nanotube composite materials include blue fluorescent carbon dot-carbon nanotube composite materials and green fluorescent carbon dot-carbon nanotube composite materials.

3. The application of the carbon dot-carbon nanotube composite material according to claim 1 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The preparation method of the carbon dot-carbon nanotube composite material includes the following steps: Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes. The carbon dots were synthesized by hydrothermal synthesis using citric acid and ethanolamine as precursors. The carbon nanotubes and carbon dots are reacted to synthesize the carbon dot-carbon nanotube composite material.

4. The application of the carbon dot-carbon nanotube composite material according to claim 2 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The preparation method of the blue fluorescent carbon dot-carbon nanotube composite material includes the following steps: Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes. Blue fluorescent carbon dots crude samples were synthesized by hydrothermal synthesis using citric acid and ethanolamine as precursors; the reaction temperature was 145–165℃ and the reaction time was 12–18 h. The crude sample of blue fluorescent carbon dots was purified by silica gel column chromatography, and then the solvent was removed and the sample was dispersed in water to form a blue fluorescent carbon dot solution. The carbon nanotubes were added to the blue fluorescent carbon dot solution and reacted at a temperature of 180–220°C for 12–18 h to obtain the blue fluorescent carbon dot-carbon nanotube composite material.

5. The application of the carbon dot-carbon nanotube composite material according to claim 2 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The preparation method of the green fluorescent carbon dot-carbon nanotube composite material includes the following steps: Carbon nanotubes were acidified with concentrated sulfuric acid and concentrated nitric acid to synthesize carbon oxide nanotubes. Green fluorescent carbon dots were synthesized using citric acid and ethanolamine as precursors via a hydrothermal synthesis method. The reaction temperature was 170–190 °C and the reaction time was 12–18 h. The crude green fluorescent carbon dot sample was purified by silica gel column chromatography, and then the solvent was removed and the sample was dispersed in water to form a green fluorescent carbon dot solution. The carbon nanotubes were added to the green fluorescent carbon dot solution and reacted at a temperature of 180–220°C for 12–18 h to obtain the green fluorescent carbon dot-carbon nanotube composite material.

6. The application of the carbon dot-carbon nanotube composite material according to claim 3, 4, or 5 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The molar ratio of concentrated sulfuric acid to concentrated nitric acid is 3:

1.

7. The application of the carbon dot-carbon nanotube composite material according to claim 3, 4, or 5 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The reaction temperature for the acidification synthesis of carbon nanotubes is 80℃, and the reaction time is 2h.

8. The application of the carbon dot-carbon nanotube composite material according to claim 4 or 5 as a microwave absorbing material in the field of electromagnetic wave absorption, characterized in that, The silica gel column chromatography method used is a gradient elution method using methanol and dichloromethane in a 1:8 ratio.

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