Structural color carbon dot superlattice and preparation method and application thereof

Abstract

The application belongs to the technical field of carbon dot superlattice preparation, and particularly relates to a structural color carbon dot superlattice and a preparation method and application thereof. The carbon dot superlattice with structural color is synthesized by using o-phenylenediamine as the only carbon source and nitrogen source through a simple and green hydrothermal method. Five kinds of structural color carbon dot superlattices are prepared by changing the oxidant, sodium sulfate and cooling speed, and the colors are powder purple, dark green, yellow green, blue purple and light green respectively. From the structure, the structural color of the carbon dot superlattice is related to the structure of the carbon dot itself and the arrangement between the carbon dots. The long-range van der Waals attractive force, long-range electrostatic repulsion and π-π stacking effect between the carbon cores are the main driving force for the formation of the carbon dot superlattice. The structural color carbon dot superlattice CD-SL has good potential in the simple and rapid preparation of large-area structural color thin films.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of carbon dot superlattice preparation, and particularly relates to a structural color carbon dot superlattice and a preparation method and application thereof. BACKGROUND

[0002] Nanocrystal superlattice is a kind of long-range ordered structure in two-dimensional or three-dimensional space formed by self-assembly of nanocrystal particles through interaction between themselves or surface-coated ligands, and is a kind of artificial "super crystal". On the one hand, this self-assembly structure has many similarities with the long-range ordered array of atomic crystal, and can be used as an intuitive model for studying the formation and evolution of crystal structure; on the other hand, due to the coupling and coordination of nanocrystals, the superlattice exhibits overall performance that individual nanocrystals do not have. When the nanocrystals are graphene quantum dots, this superlattice is called graphene quantum dot superlattice, and when the monomers are zero-dimensional carbon dots, the superlattice structure formed is called carbon dot superlattice (CDs-SL).

[0003] Structural color is a kind of pure physical optical color formed by selective emission or projection of visible light after interaction of nanostructure with visible light, and has the advantages of high stability, non-toxicity, harmlessness, environmental friendliness, long-term stability of color and the like compared with chemical color. The color production mechanism of structural color mainly includes grating diffraction, scattering coloring, metasurface, thin film interference, photonic crystal and amorphous photonic extraction, and a structural color probe can be prepared by using structure to sense the object to be detected. Liu et al. used poly(methyl methacrylate-2-hydroxyethyl methacrylate-methyl acrylate) as a functional component, and changed the refractive index of the silica inverse opal structure by forming hydrogen bonds between poly(methyl methacrylate-2-hydroxyethyl methacrylate-methyl acrylate) and alcohol, so that the reflection peak was red-shifted and the structural color was changed, thereby realizing detection of volatile alcohol. Li et al. used photonic crystal structural color to realize detection of different oils. The biggest advantage of structural color sensor is that it can sense the object to be detected and produce an intuitive and visual response signal in a complex environment without complex equipment, which makes the structural color sensor have popularization value in the sensing field.

[0004] Since the discovery of fluorescent carbon dots in 2004, carbon dots have developed rapidly in the past 20 years, especially in the fluorescent properties of carbon dots. However, in the performance research of CDs, researchers mainly focus on the luminescent properties of CDs, and the research on the construction of structural color materials with carbon dots as the unit is not enough. The research results show that the structural color of the carbon dot superlattice is related to the ordered arrangement of the carbon dots in the carbon dot superlattice. It is understood that there are 11 reports about the self-assembly of carbon dots, of which only 3 reports involve the superlattice structure (ordered structure) of carbon dots, and it is believed that hydrogen bonding, pi-pi stacking and van der Waals attraction play an important role in the self-assembly of carbon dots to form carbon dot superlattice structure (ordered structure). However, in these reports, the preparation of carbon dots and the self-assembly of carbon dots need to be completed in two different reaction systems, respectively, and it can be seen that further research is still needed for the self-assembly of carbon dots to form a highly ordered structure. SUMMARY

[0005] In view of the above problems, the purpose of the present application is to provide a structural color carbon dot superlattice and a preparation method and application thereof.

[0006] The technical content of the present application is as follows:

[0007] The present application provides a preparation method of a structural color carbon dot superlattice, comprising the following steps:

[0008] Dissolve o-phenylenediamine and sodium sulfate in deionized water, stir while heating, then add an acid solution and continue to stir, transfer to a reaction kettle, react at high temperature, after the reaction is completed, naturally cool to room temperature, centrifuge, wash and dry the reaction product, and the structural color carbon dot superlattice product is obtained;

[0009] The mass ratio of o-phenylenediamine to sodium sulfate is 1:(1.5-3.5);

[0010] The acid solution comprises nitric acid or perchloric acid;

[0011] The concentration of the acid solution is 0.15-0.45 mL / g;

[0012] The reaction temperature at high temperature is 140-220 DEG C, and the reaction time is 10-20 h.

[0013] The present application also provides a structural color carbon dot superlattice obtained by the above preparation method, wherein the composition of the carbon dot superlattice is composed of carbon, oxygen and nitrogen, and the structure is a three-dimensional structure with a monomer carbon dot structure;

[0014] The structural color comprises one of powder purple, dark green, yellow green, blue purple and light green.

[0015] The present application also provides an application of a structural color carbon dot superlattice, which is used for preparing a structural color film.

[0016] The beneficial effects of the present application are as follows:

[0017] In the preparation of the structural color carbon dot superlattice of the present application, o-phenylenediamine is used as the only carbon source and nitrogen source, and a simple and green hydrothermal method is used to synthesize the carbon dot superlattice with structural color in one step. By changing the oxidant, sodium sulfate and cooling speed, five kinds of structural color carbon dot superlattices are prepared, and the colors are respectively powder purple, dark green, yellow green, blue purple and light green. The results show that the structural color of the structural color carbon dot superlattice CDs-SL of the present application is caused by the diffuse reflection optical behavior of light waves (visible light region) in CDs-SL. From the structure, the structural color of the carbon dot superlattice is related to the structure of the carbon dot itself and the arrangement between the carbon dots. The long-range van der Waals attractive force, long-range electrostatic repulsion and the π-π stacking effect between the carbon cores are the main driving force for the formation of the carbon dot superlattice. The structural color carbon dot superlattice CDs-SL of the present application has good potential in the simple and rapid preparation of large-area structural color thin films. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is the carbon dot superlattice product graph of different HNO3 amount in example 1;

[0019] Figure 2 It is the carbon dot superlattice product graph of different Na2SO4 amount in example 1;

[0020] Figure 3 It is the carbon dot superlattice product graph of different reaction temperature in example 1;

[0021] Figure 4 It is the carbon dot superlattice product graph of different HNO3 amount in example 2;

[0022] Figure 5 It is the carbon dot superlattice product graph of different Na2SO4 amount in example 2;

[0023] Figure 6 It is the carbon dot superlattice product graph of different reaction temperature in example 2;

[0024] Figure 7 It is the carbon dot superlattice product graph of different reaction time, reaction temperature and Na2SO4 amount in example 3;

[0025] Figure 8 It is the ultraviolet-visible absorption spectrum, fluorescence excitation and emission spectrum of CDs-SL1 (A), CDs-SL2 (B) and CDs-SL3 (C);

[0026] Figure 9 It is the XPS spectrum of CDs-SL1;

[0027] Figure 10 XPS spectrum of CDs-SL2;

[0028] Figure 11 XPS spectrum of CDs-SL3;

[0029] Figure 12 HRTEM images and particle size distribution plots of CDs-SL1, CDs-SL2 and CDs-SL3 (scale bars of A and B are 50 nm);

[0030] Figure 13 Optical photograph images of CDs-SL1, CDs-SL2 and CDs-SL3;

[0031] Figure 14 Color index data of CDs-SL1, CDs-SL2 and CDs-SL3 calculated from diffuse reflectance spectra and corresponding color plots of the fitting;

[0032] Figure 15 UV-Vis diffuse reflectance spectra of CDs-SL1, CDs-SL2 and CDs-SL3;

[0033] Figure 16 Optical photograph images of A: fast cooling control experiment product of CDs-SL1, B: no stirring control experiment product of CDs-SL1, C: fast cooling control experiment product of CDs-SL2;

[0034] Figure 17 SEM images of CDs-SL1, CDs-SL2 and CDs-SL3;

[0035] Figure 18 XRD images of CDs-SL1, CDs-SL2 and CDs-SL3;

[0036] Figure 19 Lattice parameter images of CDs-SL1, CDs-SL2 and CDs-SL3;

[0037] Figure 20 Image of CD2 overlapping phenomenon observed in the dispersion solution of CDs-SL2;

[0038] Figure 21 Structural color films prepared from CDs-SL1, CDs-SL2 and CDs-SL3 and PVA. DETAILED DESCRIPTION

[0039] The application will be described in further detail below with reference to specific examples and drawings, it should be understood that these examples are only used to illustrate the application and not used to limit the protection scope of the application, after reading the application, the modification of various equivalent forms of the application by those skilled in the art falls within the scope defined by the claims of the application.

[0040] Unless otherwise specified, all raw materials and reagents of the application are conventional market raw materials and reagents.

[0041] Example 1

[0042] Preparation of pink purple carbon dot superlattice (CDs-SL1)

[0043] Dissolve 1.08 g of o-phenylenediamine and 2.84 g of sodium sulfate in 19.25 mL of deionized water, heat to 80℃ while stirring, then add 0.75 mL of nitric acid, stir for 3 min, transfer to a 50 mL polytetrafluoroethylene reactor, and react at 180℃ for 10 hours. After the reaction is completed, it is naturally cooled to room temperature, the reaction product is poured out, centrifuged at 4000 rpm for 5 minutes, the precipitate is washed with 10 mL of deionized water for 2 times, and dried at 60℃. The product before and after drying is pink purple, named as CDs-SL1, the yield is 63.5%, and the corresponding carbon dots are named as CDs1.

[0044] Note: Stirring is accompanied during the reaction and cooling process.

[0045] According to the amount of reactants, reaction temperature and reaction time and other factors, the preparation process of CDs-SL1 is optimized, and the parameter design is shown in Table 1:

[0046] Table 1 Parameter optimization design for preparation of CDs-SL1

[0047]

[0048] According to different amounts of HNO3, the obtained S1 CDs-SL1 , S2 CDs-SL1 , S3 CDs-SL1 , S4 CDs-SL1 , S5 CDs-SL1 products are shown in Figure 1In the HNO3 dosage investigation, it was found that only when the dosage of HNO3 was 0.75 mL, the obtained carbon dot superlattice had bright powder purple color; under 1.20 mL of HNO3, the obtained CD-SL had dark green color, and the preparation of dark green CD-SL was further optimized; under 0.50, 1.00 and 1.50 mL of HNO3, the carbon dot superlattice product with bright color could not be obtained; therefore, 0.75 mL of HNO3 was selected as the optimal dosage of HNO3 for preparing powder purple carbon dot superlattice. HNO3 is an oxidant, which mainly plays a role in promoting the oxidation polymerization of o-phenylenediamine in the preparation of carbon dots; at the same time, HNO3 is also an acid, which can affect the dehydration and carbonization process in the formation of CDs, so HNO3 can affect the preparation process of carbon dots; that is, different dosages of HNO3 result in different carbon dots, which leads to different carbon dot superlattices with structural color formed by self-assembly of carbon dots, and carbon dots prepared under a specific dosage of HNO3 can self-assemble to form carbon dot superlattices with bright structural color.

[0049] According to different dosages of Na2SO4, the obtained S2 CDs-SL1 , S6 CDs-SL1 , S7 CDs-SL1 and S8 CDs-SL1 products are shown in Figure 2 , it was found that the carbon dot superlattices prepared under different dosages of Na2SO4 all had powder purple color, only the brightness of the color was different; that is, changing the dosage of Na2SO4 within a certain range will not completely change the structural color of the carbon dot superlattice, but the structural color of the carbon dot superlattice can be fine-tuned by changing the dosage of Na2SO4; when the dosage of Na2SO4 was 1.42 g, powder purple powder appeared; with the increase of the dosage of Na2SO4 (1.42 g, 2.13 g, 2.84 g), the powder purple color of the carbon dot superlattice became brighter and brighter; with excessive dosage of Na2SO4 (3.55 g), the powder purple color became lighter; therefore, 2.84 g of Na2SO4 was selected as the optimal dosage of Na2SO4 for preparing powder purple CDs-SL1.

[0050] According to different reaction temperatures, the obtained S9 CDs-SL1 , S10 CDs-SL1 , S7 CDs-SL1 , S11 CDs-SL1 and S12 CDs-SL1 products are shown in Figure 3, it was found that different reaction temperatures had important influence on the preparation of CDs-SL with bright structural color; when the reaction temperature was low (such as 140℃), the product after reaction had flaky structure, which was the result of incomplete reaction; with the increase of reaction temperature, i.e. the degree of dehydration and carbonization, the powder purple product was prepared by self-assembly of CDs. When the reaction temperature was too high (200℃ and 220℃), the powder would become black; therefore, 180℃ was selected as the optimal reaction temperature for the preparation of powder purple CDs-SL. In the preparation of carbon dots by hydrothermal method, the reaction temperature was a crucial factor that determined the degree of dehydration and carbonization, and affected the preparation process of CDs.

[0051] Example 2

[0052] Preparation of dark green carbon dot superlattice (CDs-SL2)

[0053] Dissolve 1.08g of o-phenylenediamine and 2.13g of sodium sulfate in 18.70mL of deionized water, heat to 80℃ while stirring, then add 1.30mL of nitric acid, stir for 3 minutes, transfer to a 50mL polytetrafluoroethylene reaction kettle, and react at 180℃ for 10 hours. After the reaction is completed, cool naturally to room temperature, pour out the reaction product, centrifuge at 4000rpm for 5 minutes, wash the precipitate with 10mL of deionized water for 2 times, and dry at 60℃. The product before and after drying is dark green, named as CDs-SL2, with a yield of 20.7%, and the corresponding carbon dots are named as CDs2.

[0054] Note: Stirring is accompanied during the reaction and cooling process.

[0055] According to the amount of reactants, reaction temperature and reaction time and other factors, the preparation process of CDs-SL2 was optimized, and the parameter design is shown in Table 2:

[0056] Table 2 Parameter optimization design for preparation of CDs-SL2

[0057]

[0058]

[0059] According to different amounts of HNO3, the obtained S1 CDs-SL2 , S2 CDs-SL2 , S3 CDs-SL2 , S4 CDs-SL2 and S5 CDs-SL2 products are shown in Figure 4In the investigation of the amount of HNO3, when the amount of HNO3 was 1.20 mL, the dark green carbon dot superlattice appeared, and when the amount of HNO3 was increased to 1.30 mL, relatively uniform dark green carbon dot superlattice could be obtained; when the amount of HNO3 was more than 1.30 mL, the reaction product became black; when the amount of HNO3 was less than 1.20 mL, dark green carbon dot superlattice could not be obtained either; therefore, 1.30 mL of HNO3 was selected as the optimal amount of HNO3 for preparing dark green CDs-SL2; HNO3 is an oxidizing agent and also an acid, which affects the dehydration and carbonization processes in the formation of CDs during the oxidation and polymerization of o-phenylenediamine; therefore, the carbon dots prepared under different amounts of HNO3 are not completely the same, and the carbon dots prepared under a specific amount of HNO3 can self-assemble to form carbon dot superlattice with structural color; this phenomenon indicates that not all carbon dots can self-assemble to form carbon dot superlattice with structural color, only specific carbon dots can self-assemble to form carbon dot superlattice with structural color.

[0060] According to different amounts of Na2SO4, the obtained S6 CDs-SL2 , S7 CDs-SL2 , S3 CDs-SL2 , S8 CDs-SL2 , S9 CDs-SL2 , S10 CDs-SL2 , and S11 CDs-SL2 products are shown in Figure 5 ; the amount of Na2SO4 (0.71 g and 1.06 g) is too low to obtain dark green carbon dot superlattice; when the amount of Na2SO4 is 1.42-2.48 g, dark green carbon dot superlattice is obtained, among which the carbon dot superlattice prepared with 2.13 g of Na2SO4 has the most vibrant structural color, and the dark green of the carbon dot superlattice prepared with 2.48 g of Na2SO4 is darker; further increasing the amount of Na2SO4 (2.84 g), the self-assembled product of the prepared carbon dots has no dark green color. Therefore, 2.13 g of Na2SO4 was selected as the optimal amount of Na2SO4 for preparing dark green CDs-SL2. Too low or too high amount of Na2SO4 cannot obtain dark green carbon dot superlattice. Within a certain amount of Na2SO4, the carbon dot superlattices prepared all have dark green color, only the depth and vibrancy of the color are different; this is consistent with the optimization results of the amount of Na2SO4 for pink purple carbon dot superlattice, and changing the amount of Na2SO4 within a certain range does not completely change the structural color of the carbon dot superlattice, but only fine-tunes the structural color of the carbon dot superlattice.

[0061] According to different reaction temperatures, the obtained S12 CDs-SL2 , S13 CDs-SL2 , S9 CDs-SL2 , S14 CDs-SL2 , and S15CDs-SL2 See product Figure 6 In the investigation of the reaction temperature of the pinkish-purple carbon dot superlattice, it was found that the color of the carbon dot superlattice would not change completely within a certain temperature range, and only when the reaction temperature was too high would black carbon dot self-assembled products be prepared. Therefore, in the investigation of the reaction temperature of the dark green carbon dot superlattice, the temperature variation range was reduced to save energy. CDs-SL prepared at reaction temperatures of 160℃, 170℃, 180℃ and 190℃ all showed dark green to light shades. The dark green of CDs-SL was brighter at the reaction temperature of 180℃. Therefore, 180℃ was selected as the optimal reaction temperature for preparing dark green carbon dot superlattices.

[0062] Example 3

[0063] Preparation of yellow-green carbon dot superlattice (CDs-SL3)

[0064] 1.08 g of o-phenylenediamine and 3.55 g of sodium sulfate were dissolved in 19.20 mL of deionized water and heated to 80 °C while stirring. Then, 0.80 mL of perchloric acid was added, and the mixture was stirred for 3 minutes. The solution was then transferred to a 50 mL polytetrafluoroethylene (PTFE) reactor and reacted at 190 °C for 18 hours. After the reaction was complete, the mixture was allowed to cool naturally to room temperature. The reaction product was then poured off and centrifuged at 4000 rpm for 5 minutes. The precipitate was washed four times with 10 mL of deionized water and dried at 60 °C. The product was yellow-green both before and after drying and was named CDs-SL3, with a yield of 32.6%. The corresponding carbon point was named CDs3.

[0065] Note: Stirring is performed during both the reaction and cooling processes.

[0066] The preparation process of CDs-SL2 was optimized based on factors such as the amount of reactants, reaction temperature, and reaction time. The parameter design is shown in Table 3.

[0067] Table 3. Parameter optimization design for CDs-SL2 preparation

[0068]

[0069] Depending on the reaction time, the obtained S1 CDs-SL3 S2 CDs-SL3 and S3 CDs-SL3 Products such as Figure 7 As shown, the yields were 0.2%, 3.1%, and 6.2%, respectively. While extending the reaction time did increase the yield of the yellow-green CDs-SL, the final yield remained very low.

[0070] In the preparation condition optimization of powder purple carbon dots superlattice and ink green carbon dots superlattice, it is found that the increase of reaction temperature and the change of sodium sulfate dosage have little effect on the structural color of carbon dots superlattice in a certain range, so it is guessed that the reaction temperature and the sodium sulfate dosage can be changed in a certain range in the preparation of yellow-green carbon dots superlattice without greatly changing the color of carbon dots superlattice.

[0071] According to different reaction temperatures, when the reaction temperature is 190℃ (product S4 CDs-SL3 ), the reaction time is still 18 hours, and other reaction conditions remain unchanged, the product is yellow-green carbon dots superlattice, and the yield of yellow-green carbon dots superlattice increases to 10.6%. When the reaction temperature is 200℃ (product S5 CDs-SL3 ), the reaction time is still 18 hours, and other reaction conditions remain unchanged, the product is black. Therefore, 190℃ is selected as the optimal reaction temperature for preparing yellow-green carbon dots superlattice.

[0072] According to different Na2SO4 dosages, when the reaction temperature is set to 190℃ and the reaction time is set to 18 hours, when the Na2SO4 dosage is 2.13g, 2.84g, 3.55g, 4.26g and 4.97g (products S4 CDs-SL3 , S6 CDs-SL3 , S7 CDs-SL3 , S8 CDs-SL3 and S9 CDs-SL3 ), the carbon dots superlattice is yellow-green, and the yield is 10.6%, 27.4%, 32.6%, 28.5% and 21.6% respectively. Under low sodium sulfate dosage, with the increase of Na2SO4 dosage, the yield of ink green carbon dots superlattice also increases; when the Na2SO4 dosage exceeds a certain range, with the increase of Na2SO4 dosage, the yield of yellow-green carbon dots superlattice decreases instead. Therefore, the optimal conditions for preparing yellow-green CDs-SL3 are selected as Na2SO4 dosage of 3.55g, reaction temperature of 190℃ and reaction time of 18 hours.

[0073] The above prepared carbon dots superlattice is formed by oxidation, polymerization, dehydration and carbonization of o-phenylenediamine under high temperature and high pressure to form carbon dots, and then assembled to form CDs-SL. CDs-SL1, CDs-SL2 and CDs-SL3 are dispersed in dimethyl sulfoxide (DMSO) respectively, and each DMSO solution has red fluorescence. Their excitation spectrum, emission spectrum and ultraviolet-visible absorption spectrum are shown in Figure 8 , and their monomers are red fluorescent carbon dots.

[0074] As shown in Figure 9 A-D: XPS spectrum of CDs-SL1 (A: full spectrum, B: C1s spectrum, C: N1s spectrum, D: O1s spectrum); Figure 10XPS spectra of CDs-SL2 (E: survey spectrum, F: C1s spectrum, G: N1s spectrum, H: O1s spectrum); Figure 11 XPS spectra of CDs-SL3 (I: survey spectrum, J: C1s spectrum, K: N1s spectrum, L: O1s spectrum) It can be seen from the XPS spectra that CDs-SL1, CDs-SL2 and CDs-SL3 all have three strong peaks at 284.9 eV, 400.3 eV and 531.8 eV, which are C1s, N1s and O1s, respectively, indicating that they are mainly composed of carbon, nitrogen and oxygen elements. Among them, the relative content of carbon element of CDs-SL3 is 78.2%, the relative content of nitrogen element is 16.9%, the relative content of oxygen element is 3.2%, and a small amount of chlorine element (relative content is 1.6%) is also contained. The original source of chlorine element is perchloric acid. The carbon content of CDs-SL2 is also very high, the relative content is 74.4%, the relative content of nitrogen element and the relative content of oxygen element are 11.3% and 12.9% respectively, and a small amount of sulfur element (relative content is 1.5%) is also contained. The source of sulfur element is sodium sulfate. But the relative content of carbon element of CDs-SL1 is only 53.3%, which is lower than that of CDs-SL2 and CDs-SL3, the relative content of nitrogen element and oxygen element is 17.0% and 24.0% respectively, the relative content of sulfur element is 5.7%, and the original source of sulfur element is sulfuric acid.

[0075] As Figure 12 shown in the HRTEM, when CDs-SL1, CDs-SL2 and CDs-SL3 are dispersed in DMSO respectively, carbon dots with very good monodispersity can be obtained. The particle sizes of CDs1, CDs2 and CDs3 are 3.24 ± 0.59 nm (sample number: n = 60), 1.88 ± 0.30 nm (sample number: n = 56) and 2.30 ± 0.28 nm (sample number: n = 35) respectively. It is also shown that CDs1, CDs2 and CDs3 have lattice fringes with a lattice spacing of 0.21 nm, which is the characteristic lattice spacing of the graphene crystal plane (1, 0, 0), indicating that the carbon core of CDs1, CDs2 and CDs3 is mainly composed of graphene fragments, and the carbon core has a large conjugated structure.

[0076] According to XPS, HRTEM and fluorescence spectrum, it is proved that zero-dimensional carbon dots are the smallest and unique unit cells that constitute these superlattices. Therefore, these structural color superlattices are called carbon dot superlattices.

[0077] For the reason why carbon dot superlattices produce structural color, the diffuse reflectance spectrum in the ultraviolet-visible region is used to study the structural color of CDs-SL. As Figure 13 shown in the diffuse reflectance spectrum of pink CDs-SL1 in the visible region, it can be seen that CDs-SL1 has a high light reflectivity in the purple and red regions, which is consistent with the result that the structural color of CDs-SL1 is pink.

[0078] The corresponding color was further calculated and fitted using the diffuse reflectance spectrum of CDs-SL1 in the visible light region. Figure 14 (From left to right: pinkish-purple, dark green, and yellowish-green). The fitted colors matched those of CDs-SL1, indicating that CDs-SL1's color originates from its diffuse reflection in the visible light region. Based on the diffuse reflectance spectrum of the dark green CDs-SL2, there is a high reflectance peak near 567nm in the visible light region, exhibiting a good peak shape. Figure 15 The reflection peak is located in the green light region, therefore the color of CDs-SL2 is in the green range. Because CDs-SL2 has low diffuse reflectance in the visible light region, its color is darker than that of CDs-SL1; therefore, the structural color of CDs-SL2 is dark green. The fitted color is calculated based on the diffuse reflectance spectrum of CDs-SL2 in the visible light region. Figure 14 The color of CDs-SL2 is consistent with that of CDs-SL2, indicating that the color of CDs-SL2 comes from the diffuse reflection of CDs-SL2 in the visible light region. In the diffuse reflectance spectrum of the yellow-green CDs-SL3 ( Figure 15 The maximum reflectance wavelength of the diffuse reflectance peak is 574 nm, which belongs to the yellow light region, but it also has a large diffuse reflectance in the green light region. Therefore, the color of CDs-SL3 is yellow-green. The fitted color calculated based on the diffuse reflectance spectrum of CDs-SL3 in the visible light region is yellow-green. Figure 14 This color is consistent with that of CDs-SL3. Therefore, the color of CDs-SL3 mainly comes from its diffuse reflection in the yellow-green light band. In summary, the structural colors of CDs-SL1, CDs-SL2, and CDs-SL3 all originate from the diffuse reflection of visible light waves by the carbon dot superlattice. CDs-SL1 and CDs-SL3 both have higher diffuse reflectance, which is why CDs-SL1 and CDs-SL3 have more vibrant structural colors than CDs-SL2.

[0079] As can be seen from the above, the structural color of the carbon dot superlattice is determined by the structure of the carbon dots themselves and the arrangement between the carbon dots. In the optimization of the preparation conditions of CDs-SL1 and CDs-SL2, it was found that the structural color of the prepared CDs-SL was different when the amount of nitric acid was different. When the amount of nitric acid was 0.75 mL, the carbon dot superlattice was powder purple; and when the amount of nitric acid was 1.20-1.30 mL, the carbon dot superlattice was dark green (other conditions were o-phenylenediamine 1.08 g, sodium sulfate 1.42 g, reaction temperature 180°C and reaction time 10 h). Therefore, the amount of oxidizing agent is very important for the preparation of CDs-SL with specific structural color, and it is necessary to obtain carbon dot superlattice with specific structural color under the appropriate amount of oxidizing agent. Nitric acid is an oxidant, which oxidizes and polymerizes o-phenylenediamine during the preparation of carbon dots. At the same time, nitric acid is also an acid, which can affect the dehydration and carbonization process during the formation of CDs. When other preparation conditions remain unchanged, the amount of oxidizing agent (nitric acid) is different, the structure of the prepared carbon dots is different, and finally leads to the structural color of the carbon dot superlattice being different. That is, in the same self-assembly environment, different carbon dots can self-assemble into carbon dot superlattices with different structural colors. At the same time, the following experimental phenomena also support this conclusion: in S6 CDs-SL1 , S9 CDs-SL2 and S1 CDs-SL3 experimental conditions, only the oxidizing agent is different (0.75 mL of nitric acid, 1.30 mL of nitric acid and 0.80 mL of perchloric acid, respectively), and other experimental conditions are the same, and the obtained carbon dot superlattices present different colors (powder purple, dark green and yellow green, respectively). That is, the structure of the carbon dots themselves is one of the factors affecting or determining the structural color of the carbon dot superlattice. Different carbon dot structures lead to different carbon dot superlattice structures, different interactions between light waves (visible light region) and carbon dot superlattices, and finally different structural colors of carbon dot superlattices.

[0080] It is found that the arrangement structure of carbon dots is another factor affecting or determining the structural color of carbon dot superlattice: in the rapid cooling control experiment, the reaction kettle is immediately moved to the ventilation place after the reaction, and cooled for 30 minutes, and then placed in a flowing water bath to accelerate the cooling speed until it is cooled to room temperature; the no stirring control experiment is to stop stirring immediately after the reaction, that is, there is no stirring effect in the cooling stage. The carbon dots obtained in the rapid cooling control experiment group, the no stirring control experiment and the corresponding experimental group are the same, except that the self-assembly environment of the carbon dots is different (different cooling rate of the reaction system and with or without stirring effect). In the rapid cooling control experiment of CDs-SL1, CDs-SL with light green color are obtained; in the no stirring control experiment of CDs-SL1, CDs-SL large particles with light green color and near spherical shape are obtained; but the structural color of CDs-SL1 in the experimental group is powder purple. That is, the self-assembly environment is different, and the structural color of the carbon dot superlattice obtained is different. Similarly, the product of the rapid cooling control experiment of CDs-SL2 is black carbon dot powder, but the structural color of CDs-SL2 in the experimental group is dark green. The products of the cooling control experiment and the no stirring control experiment are shown in Figure 16 These experimental phenomena show that the carbon dot self-assembly process of powder purple CDs-SL1 and dark green CDs-SL2 occurs in the slow cooling stage. At the same time, it also shows that the same carbon dots can self-assemble into carbon dot superlattices with different structural colors under different self-assembly environments. That is, the same carbon dots can self-assemble into carbon dot superlattices with different structural colors through different arrangement structures, which shows that the arrangement structure of monomer carbon dots is another factor affecting or determining the structural color of carbon dot superlattice. Different arrangement of monomer carbon dots, different carbon dot superlattice structures, different interaction of light wave (visible light region) and carbon dot superlattice, and finally different structural colors of carbon dot superlattice.

[0081] In the SEM images of Figure 17 , it can be seen that the CDs are not randomly stacked, but form a three-dimensional structure with certain appearance and morphology. The arrangement of carbon dots is analyzed by XRD. The XRD graphs of CDs-SL1, CDs-SL2 and CDs-SL3 are shown in Figure 18The XRD pattern revealed that CDs-SL1, CDs-SL2, and CDs-SL3 all exhibited strong and sharp diffraction peaks, with the strongest and sharpest FWHMs at 0.14° (peak position: 2θ = 5.98°), 0.22° (peak position: 2θ = 6.40°), and 0.22° (peak position: 2θ = 6.46°), respectively. This indicates that the carbon dots within CDs-SL1, CDs-SL2, and CDs-SL3 are highly ordered. XRD characterization analysis showed that the maximum interplanar spacing of CDs-SL1 was 1.48 nm, while the maximum interplanar spacings of CDs-SL2 and CDs-SL3 were 1.38 nm and 1.37 nm, respectively. Based on the XRD patterns, the lattice parameters of CDs-SL1, CDs-SL2, and CDs-SL3 were calculated using FullProf Suite Toolbar software, indicating that all three belong to the triclinic crystal system. The relevant lattice parameters are shown below. Figure 19 The crystal planes corresponding to the diffraction peaks are shown in Table 4.

[0082] Table 4 shows the crystal planes corresponding to the CDs-SL diffraction peaks.

[0083]

[0084]

[0085] Taking CDs-SL3 as an example, the arrangement structure between carbon dots in the carbon dot superlattice was studied based on lattice parameters and carbon dot particle size. For CDs-SL3, the only monomer is CDs3. According to the calculated lattice parameters, the maximum edge length of the CDs-SL3 unit cell is 1.51 nm, that is, the spatial distance between adjacent CDs3 is less than or equal to 1.51 nm, but the particle size of CDs3 is 2.30 ± 0.28 nm. Figure 10 F), greater than 1.51 nm. This indicates that there is partial overlap between adjacent CDs3 monomers in CDs-SL3. The carbon core of CDs3 contains sp 2 The conjugated structure of hybrid carbon suggests that the overlapping arrangement of CDs3 is related to the π-π stacking interaction between carbon nuclei. This arrangement is similar to a partially facet-to-face π-π stacking structure, unlike the fixed arrangement between planes of inorganic nanocrystals, which limits the tunability of interactions between nanocrystals. The partially overlapping arrangement differs in the overlapping portion and area, leading to different interactions between carbon dots, resulting in different carbon dot superlattice structures and ultimately different superlattice structural colors. The experimental phenomenon described earlier, where different structural colors of carbon dot superlattices can be prepared by adjusting the self-assembly environment, and the analysis of how different amounts of sodium sulfate lead to slight differences in the structural color of the carbon dot superlattice, both confirm this point.

[0086] Similarly, the maximum edge length of the CDs-SL1 unit cell is 2.82 nm, meaning the spatial distance between adjacent CDs1 monomers is less than or equal to 2.82 nm, but the particle size of CDs1 is 3.24 ± 0.59 nm. Figure 12 D), greater than 2.82 nm. This indicates that there is partial overlap between adjacent CDs1 in CDs-SL1. The maximum edge length of the CDs-SL2 unit cell is 1.47 nm, meaning that the spatial distance between adjacent CDs2 is less than or equal to 1.47 nm, but the particle size of CDs1 is 1.88 ± 0.30 nm. Figure 12 E), greater than 1.47 nm. This indicates that there is partial overlap between adjacent CDs2 in CDs-SL2. Most importantly, HRTEM characterization of CDs-SL2 dispersed in DMSO directly observed the phenomenon of partial overlap between CDs2, see [reference needed]. Figure 20 (Note: The observed partial overlap between CDs2 does not necessarily mean that this is the true arrangement of CDs2 in true CDs-SL2).

[0087] For the preparation of structural color thin films using CDs-SL1, CDs-SL2, and CDs-SL3, see [link to documentation]. Figure 21 A dark green film was prepared by adding dark green CDs-SL2 using PVA as a crosslinking agent and water as a solvent. (See...) Figure 21 B. The color of the film comes from the dark green of CDs-SL2. The process for preparing the dark green film is as follows: PVA is dissolved in 95℃ hot water, a dark green carbon dot superlattice is added, and the mixture is stirred in a 95℃ water bath for 2 hours. The mixture is then poured out and spread evenly on a clean glass surface, and allowed to cool and solidify. With other conditions unchanged, a yellow-green film is prepared using yellow-green CDs-SL3 instead of dark green CDs-SL2 (e.g., ...). Figure 21 C), indicating that the prepared carbon dot superlattices CDs-SL2 and CDs-SL3 can withstand strong interference and maintain stable structural colors under strong external forces. This is unlike other structural color materials, such as photonic crystals, which struggle to maintain stable structural colors under solvent and strong stirring conditions. For the pinkish-purple CDs-SL1, the color of the film prepared under stirring and high temperature differs from the structural color of CDs-SL1 itself (e.g., ...). Figure 21 A) This indicates that the pinkish-purple color of CDs-SL1 is not very stable.

[0088] According to the SEM and XRD of CDs-SL, the growth of CDs-SL is anisotropic, that is, the total interaction between CDs is directional, and according to the analysis above, there is partial overlap between adjacent carbon dots in CDs-SL. However, according to the emission spectra of CDs1, CDs2 and CDs3, the emission peak FWHMs of monodisperse CDs1, CDs2 and CDs3 are very narrow, indicating that the uniformity of CDs1, CDs2 and CDs3 is very good. Therefore, it is believed that the long-range van der Waals force and long-range electrostatic repulsion between near-spherical CDs1, CDs2 and CDs3 are non-directional forces. In carbon dots, in addition to long-range van der Waals attraction and long-range electrostatic force between carbon dots, there should also be a large π-π stacking effect. In HRTEM, the carbon core of CDs1, CDs2 and CDs3 has lattice fringes with a lattice spacing of 0.21 nm Figures 3-13 ), indicating that the carbon core contains graphene fragments. The carbon core contains a conjugated structure composed of sp 2 hybrid carbon, and π-π stacking occurs between carbon cores. Therefore, the π-π stacking between carbon cores should also play an important role in the anisotropic growth of CDs-SL and the partial overlap arrangement between carbon dots. In summary, the main driving force for the self-assembly of CDs to form superlattices is the long-range van der Waals force between carbon dot particles, the long-range electrostatic interaction between carbon dots in an acidic water system, and the π-π stacking between carbon cores. As for the growth mode of CDs-SL, it can be divided into stable growth (OR) or directional attachment growth (OA) process, and according to the XRD analysis, there is partial overlap between adjacent carbon dots in the carbon dot superlattice, that is, CDs grow into CDs-SL through a partial overlap OA process.

Claims

1. A method for preparing a structural color carbon dot superlattice, characterized in that, The method comprises the following steps: Dissolve o-phenylenediamine and sodium sulfate in deionized water, heat while stirring, then add an acid solution and continue stirring, transfer to a reaction kettle, react at high temperature, after the reaction is completed, naturally cool to room temperature, centrifuge, wash and dry the reaction product to obtain a structural color carbon dot superlattice product; The mass ratio of the o-phenylenediamine to the sodium sulfate is 1:(1.5-3.5); The acid solution comprises nitric acid or perchloric acid; The reaction temperature at high temperature is 140-220 DEG C, and the reaction time is 10-20 h; The concentration of the acid solution is 0.15-0.45 mL / g.

2. The structural color carbon dots superlattice obtained by the preparation method of claim 1, characterized in that, The carbon dot superlattice is composed of carbon, oxygen and nitrogen, and has a three-dimensional structure with a monomer carbon dot structure.

3. The structural color carbon dot superlattice of claim 2, wherein, The structural color comprises one of powder purple, dark green, yellow green, blue purple and light green.

4. Use of the structural color carbon dot superlattice of claim 2 or 3, characterized in that, The method is used for preparing a structural color thin film.

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