An iridescent carbon dot thin film with photoelectric effect, its temperature-controlled preparation method, and its application.

Abstract

This invention belongs to the technical field of carbon dot thin films, specifically relating to a rainbow-colored carbon dot thin film with photoelectric effect, a temperature-controllable preparation method, and its application. The carbon dot thin film is formed by assembling nanoscale carbon dots into micrometer-scale fence structures, which then form centimeter-scale rainbow-colored carbon dot thin films. The rainbow-colored carbon dot thin film is prepared through a thermal reaction of dopamine derivatives and oxidants, employing a convenient, simple, green, and environmentally friendly method. Large-scale carbon dot thin films are synthesized using a self-assembly method. By adjusting the non-covalent bond interactions during the self-assembly process, the ordered arrangement and assembly of carbon dots in the film are achieved, providing a way to precisely control the structure and properties of carbon dots. Furthermore, its properties were investigated, revealing that it exhibits vibrant rainbow colors; natural light reflected on the film displays a bright rainbow effect, exhibiting a significant neon effect, good photoelectric conversion efficiency, and high photocurrent intensity.

Description

Technical Field

[0001] This invention belongs to the technical field of carbon dot thin films, specifically relating to an iridescent carbon dot thin film with photoelectric effect, a temperature-controllable preparation method, and its application. Background Technology

[0002] Carbon dot thin films, as an emerging material, have shown great application potential in energy storage, photoelectric conversion, and biomedicine. The nanoscale size and abundant surface functional groups of carbon dots give them unique advantages in optical, electrical, and chemical properties. Compared to traditional quantum dots containing cadmium or lead, carbon dots not only exhibit better photostability (resistance to photodecomposition, photobleaching, and photoscintillation) but also lower toxicity, lower cost, and excellent biocompatibility. However, traditional preparation methods, such as bubble blowing assembly, capillary force-assisted assembly, electric field-assisted assembly, Langmuir-Blodgett assembly, coating or deposition methods, or combining with polyvinyl alcohol to form films, are relatively cumbersome and complex, limiting the structural control of carbon dot films and often requiring complex processes and high costs. Furthermore, compared with typical thin film composites, carbon dot films have shown significant improvements in membrane water flux, desalination, chlorine resistance, antifouling properties, and antibacterial properties. Despite these advancements, the large-scale production of pure carbon dot thin films remains a major challenge. One of the most important properties of carbon materials, such as carbon quantum dots, is their hydrophobicity. While the deposition of carbon nanoparticles into thin films can degrade their electronic and optical properties, non-covalent methods can effectively preserve their excellent optical characteristics. Meanwhile, nanostructures such as carbon nanotubes and carbon quantum dots have become alternative materials for thin-film electronics, not only ensuring high electronic performance but also providing low-temperature processability. These unique properties open up new possibilities for future thin-film electronics.

[0003] The focus of nanoscience research has gradually shifted from single components to the precise manipulation of larger, ordered nanostructures according to human desires, especially in the fields of sensors, electronic devices, and novel materials with unique properties. Self-assembly is the most effective method for obtaining ordered arrangements of nanomaterials at the nanoscale. The key lies in understanding and controlling the interactions between nanoparticles and designing them rationally to form the desired structures and properties. Therefore, large-scale assembly of nanomaterials is a crucial factor in promoting the creation of high-performance devices. In this context, self-assembly has become an effective and well-controllable method for structural regulation. By controlling the interactions between carbon point molecules, molecules can spontaneously arrange themselves into hierarchical structures, constructing multidimensional structures to enhance material performance. Summary of the Invention

[0004] To address the aforementioned problems, the present invention aims to provide an iridescent carbon dot film with photoelectric effect, a temperature-controllable preparation method, and its application.

[0005] The technical content of this invention is as follows:

[0006] The present invention provides an iridescent carbon dot film with photoelectric effect, wherein the carbon dot film is formed by assembling nanoscale carbon dots into a micrometer-scale fence structure, and then forming a centimeter-scale iridescent carbon dot film.

[0007] The iridescent carbon dot film has an ordered fence structure that gives it a distinct neon effect, good photoelectric conversion efficiency, and high photocurrent intensity.

[0008] This invention also provides a method for preparing an iridescent carbon dot thin film with photoelectric effect, comprising the following steps:

[0009] Dopamine derivatives and oxidants are added to an aqueous solution containing concentrated hydrochloric acid or concentrated ammonia. After stirring and dissolving, an oil bath reaction is carried out. After the reaction is completed, the mixture is cooled to room temperature to obtain a precipitate. The precipitate is then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant is then cooled to room temperature to obtain a carbon dot film.

[0010] The dopamine derivative includes one of dopamine hydrochloride, epinephrine hydrochloride, and phenylephrine hydrochloride, preferably phenylephrine hydrochloride.

[0011] The oxidant is sodium hypochlorite;

[0012] The molar ratio of the dopamine derivative to the oxidant is (3-7.5):(40-100);

[0013] The pH value of the preparation method is 1 to 12 (the pH value is adjusted using concentrated hydrochloric acid or concentrated ammonia) (any amount of substance within this range);

[0014] The oil bath reaction is carried out at a temperature of 120–200°C for 6–12 hours.

[0015] The beneficial effects of this invention are as follows:

[0016] This invention discloses a method for preparing iridescent carbon dot films with photoelectric effects. The method involves a thermal reaction of dopamine derivatives and an oxidant, employing a convenient, simple, and environmentally friendly approach. Large-scale carbon dot films are synthesized through self-assembly. By adjusting non-covalent interactions during the self-assembly process, the ordered arrangement and assembly of carbon dots within the film are achieved, providing a precise means to control the structure and properties of carbon dots. The properties were investigated, revealing a vibrant iridescent color; natural light reflected onto the film exhibits a bright rainbow effect, resembling a neon light. The photocurrent effect of the iridescent film and its composite with TiO2 was measured. The iridescent film demonstrates excellent photocurrent performance while also enhancing the photocurrent effect of titanium dioxide. Furthermore, it exhibits a wide adhesion range, adhering to various substrates. These films possess excellent optical and electrical properties and can be repeatedly self-assembled and utilized through temperature control. This provides a new possibility for programmable optical supramolecular materials based on carbon dot self-assembly, enabling the widespread application of carbon dot films in optoelectronic devices, catalysts, and biosensors. Attached Figure Description

[0017] Figure 1 Images of rainbow-colored carbon dot films synthesized with different oxidants using water as the fixative solvent;

[0018] Figure 2 Images of rainbow-colored carbon dot films synthesized using different solvents with sodium hypochlorite as the fixed oxidant.

[0019] Figure 3 Image of rainbow-colored carbon dot film samples synthesized by increasing the proportions of oxidant and substrate in equal proportions;

[0020] Figure 4 Images of rainbow-colored carbon dot films synthesized under different synthesis times, acid-base ranges, temperatures, and salt additions;

[0021] Figure 5 Figures showing the formation of iridescent carbon dot films on different substrates;

[0022] Figure 6 A schematic diagram of temperature-controlled cycling fabrication of rainbow-colored carbon dot films;

[0023] Figure 7 Lens image and particle size distribution of rainbow-colored carbon dot film (the inset image in the lower right corner of a is the lattice parameter diagram);

[0024] Figure 8To comprehensively investigate the high-resolution C1s XPS spectrum (a), O1s XPS spectrum (b), N1s XPS spectrum (c), and full XPS spectrum (d) of the iridescent carbon dot film, the Raman spectrum (e), FTIR spectrum (f), XRD pattern (g), and UV-Vis spectrum (h) of the iridescent carbon dot film were also examined.

[0025] Figure 9 A diagram of the fence-like structure of an iridescent carbon dot film;

[0026] Figure 10 The images show the color pattern (4×10x) of the rainbow-colored carbon dot film under an optical microscope, the AFM thickness image, the SEM cross-sectional image, and the film thickness diagram based on the film cross-sectional thickness image.

[0027] Figure 11 This is a magnified SEM image of a portion of the fence structure of the rainbow-colored carbon dot film.

[0028] Figure 12 C1s spectra of carbon dot films of different colors;

[0029] Figure 13 The UV-Vis diffuse reflectance spectrum of the carbon dot film (a) and the effect of simulated COOH content on the color reflectance of the film (b);

[0030] Figure 14 The image shows the green fluorescence of an aqueous solution of an iridescent carbon dot film (demonstrating the Tyndall effect).

[0031] Figure 15 The images show the regeneration of iridescent carbon dot films in aqueous solutions of different concentrations (Figure b shows a concentration of 5.6 mg / mL, and Figure c shows a concentration of 11.2 mg / mL).

[0032] Figure 16 The growth of the rainbow-colored carbon dot film was shown when the aqueous solution was reheated and cooled to different temperatures (a is when cooled to 4℃, b is when cooled to room temperature).

[0033] Figure 17 To investigate the π-cation interaction diagram of potassium chloride;

[0034] Figure 18 This image shows the result of close-range interaction between the rainbow-colored carbon dot film and the SEM electron probe (the film begins to crack and melt as the temperature rises);

[0035] Figure 19 This illustrates the film growth process after mixing aqueous solutions of carbon dots with different solvents.

[0036] Figure 20 δ corrected for different solvents HLinear relationship graph (a) and UV-Vis spectra of solutions measured before and after forming thin films by mixing different solvents (b);

[0037] Figure 21 Photographs showing iridescent carbon dot films adhered to different substrates;

[0038] Figure 22 The self-assembly growth process of rainbow-colored carbon dot films (ae);

[0039] Figure 23 The interaction between white light and the iridescent carbon dot film (exhibiting a neon-like effect);

[0040] Figure 24 The image shows the birefringence of the iridescent film under a polarizing microscope (vertical direction).

[0041] Figure 25 Photocurrent curves for rainbow-colored carbon dot films and TiO2 composite films (TiO2@CDs film-1, TiO2@CDs film-2 and TiO2@CDs film-3 represent films with different thicknesses).

[0042] Figure 26 This diagram illustrates the synthesis, related mechanisms, and applications of rainbow-colored carbon dot films. Detailed Implementation

[0043] The present invention will be further described in detail below through specific implementation examples and accompanying drawings. It should be understood that these embodiments are only used to illustrate the present invention and are not intended to limit the scope of protection of the present invention. After reading the present invention, any modifications of the present invention in various equivalent forms by those skilled in the art fall within the scope of the appended claims.

[0044] Unless otherwise specified, all raw materials and reagents used in this invention are from the conventional market.

[0045] Example 1

[0046] Preparation of an iridescent carbon dot thin film with photoelectric effect

[0047] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of aqueous solution containing 150 μL of concentrated hydrochloric acid. After stirring and dissolving, the mixture was reacted in an oil bath at 180 °C for 10 h. After the reaction was completed, the mixture was cooled to room temperature to obtain a precipitate. The precipitate was then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant was cooled to room temperature to obtain a carbon dot film.

[0048] The pH reaction environment should be between 1 and 12.

[0049] The oxidizing agents include H₂O₂, FeCl₃, CuCl₂, KClO₄, AgNO₂, K₂S₂O₈, (NH₄)₂S₂O₈, NaNO₂, and NaClO. The results are as follows: Figure 1 As shown, rainbow-colored carbon dot films only appear in the product when the oxidant is NaClO; otherwise, they are powder precipitates. This indicates that using sodium hypochlorite helps control the reaction and thus obtain a more stable product.

[0050] Example 2

[0051] Preparation of an iridescent carbon dot thin film with photoelectric effect

[0052] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of different solvents containing 150 μL of concentrated hydrochloric acid. After stirring and dissolving, the mixture was reacted in an oil bath at 180 °C for 10 h. After the reaction was completed, the mixture was cooled to room temperature to obtain a precipitate. The precipitate was then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant was cooled to room temperature to obtain a carbon dot film.

[0053] The pH reaction environment should be between 1 and 12.

[0054] Depend on Figure 2 As can be seen, the different solvents are DMF, DMSO, ethanol / water, ethanol, methanol, methanol / water 1:1, water and water, respectively. Among them, when the solvent is methanol / water, a dark gold carbon dot film appears. When the solvent is water, an iridescent carbon dot film can be synthesized (second figure from the right), and a gold carbon dot film can be separated (first figure from the right).

[0055] Therefore, based on the above, a green and environmentally friendly hydrothermal method for synthesizing rainbow-colored thin film carbon dots is obtained, with water as the solvent and sodium hypochlorite as the oxidant.

[0056] Example 3

[0057] Preparation of an iridescent carbon dot thin film with photoelectric effect

[0058] Add 0.075-0.75 mmol of phenylephrine hydrochloride and 1-10 mmol of sodium hypochlorite to 20 mL of aqueous solution containing 150 μL of concentrated hydrochloric acid. After stirring to dissolve, react in an oil bath at 180 °C for 10 h. After the reaction is complete, cool to room temperature to obtain a precipitate.

[0059] The pH reaction environment should be between 1 and 12.

[0060] As the proportions of the oxidant sodium hypochlorite and the substrate phenylephrine increase proportionally, the specific material ratios are shown in Table 1:

[0061] Table 1. Investigating the effect of the material ratio of norepinephrine and sodium hypochlorite on the preparation of carbon dot films.

[0062]

[0063]

[0064] From Table 1 and Figure 3 As can be seen, a mixture of black precipitate (product 1), iridescent film (product 2), and black precipitate appears sequentially, followed by a mixture of black precipitate, iridescent film, and gold film (product 3). The order of precipitation is black precipitate to iridescent carbon dot film and then gold carbon dot film. The precipitate obtained from the reaction is reheated using water as a solvent to filter the insoluble black precipitate. During the cooling process, the iridescent and gold carbon dot films regrow, providing information that the carbon dot film has temperature-reversible growth.

[0065] The products obtained from the synthesis process show that, regardless of the adjustments made to the reaction process, a black precipitate (denoted as PDA) was consistently accompanied by the growth of an iridescent film. To further understand the synthesis process, elemental analysis was performed, as shown in Table 2.

[0066] Table 2. XPS element content and ratio of precipitates of different colors.

[0067]

[0068] *The white film appears during the regrowth of the iridescent film, specifically when the concentration of the carbon point aqueous solution is greater than 11.2 mg / mL, which will be explained further later.

[0069] As shown in Table 2, the N / C ratio of PDA is 0.09, which is within the N / C range reported in the literature on melanin, i.e., 0.08 to 0.17, consistent with other types of melanin. It is judged that PDA may be a type of melanin called polydopamine.

[0070] Example 4

[0071] Preparation of an iridescent carbon dot thin film with photoelectric effect

[0072] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of aqueous solution containing 150 μL of concentrated hydrochloric acid. After stirring and dissolving, the mixture was reacted for 10 h in an oil bath at different temperatures (100-200℃); or at 180℃ for different times (5-14 h); or at 180℃ for 10 h under different acid-base conditions (pH = 1-13). After the reaction was completed, the mixture was cooled to room temperature to obtain a precipitate. The precipitate was then reheated, filtered to remove the black precipitate, and the supernatant was obtained. The supernatant was cooled to room temperature, and the formation of a carbon dot film was observed. The effects of salts, such as potassium chloride and sodium chloride, on the synthesis of the iridescent carbon dot film were also investigated.

[0073] By adjusting the reaction time, temperature, and the acidity or alkalinity of the solution, Figure 4 (a) From left to right, the different reaction times are 5h, 6h, 8h, 10h, 12h, and 14h. It can be seen that the rainbow-colored carbon dot film was only observed after 6h of reaction, and the film could not be formed when the reaction time reached 14h. This indicates that the optimal time range for synthesizing carbon dot films is 6-12h.

[0074] Depend on Figure 4 (b) From left to right, the different acid-base ranges are pH = 13, 12, 9, 7, 5, 2, 1. It can be seen from the figure that no film was observed when pH = 13, a strong base range, but film was observed in the other acid-base ranges. Therefore, it is shown that the acid-base range that can successfully synthesize carbon dot films is 12-1.

[0075] Depend on Figure 4 (c) From left to right, the oil baths at different temperatures are 100℃, 120℃, 140℃, 160℃, 180℃, and 200℃, as well as the synthesis samples with 0.1 mmol KCl and 0.1 mmol NaCl added. It can be seen that the formation of the film was only observed when the temperature reached 120℃, and that the low concentration of salt did not inhibit the synthesis of carbon dots, and the formation of the film was still observed.

[0076] It is evident that rainbow-colored carbon dot films can only be synthesized when pH = 1–12, temperature = 120–200℃, and synthesis time = 6–12 hours. Furthermore, salt solutions do not affect the formation process, indicating that the synthesis method is universal.

[0077] To further demonstrate the feasibility of the method and facilitate large-scale production, the substrates were expanded to include other dopamine derivatives and control substrates, phenolic structural analogs.

[0078] Table 3. Studies on different substrates in the preparation of thin film carbon dots

[0079]

[0080] from Figure 5 As shown in Table 3, dopamine derivatives such as dopamine hydrochloride, phenylephrine hydrochloride, and epinephrine hydrochloride can all synthesize iridescent films. The iridescent films of dopamine hydrochloride and phenylephrine hydrochloride are more pronounced, while those of epinephrine hydrochloride are less obvious. Among other phenolic structural analogs, only o-aminophenol and p-aminophenol can produce iridescent films; resorcinol and hydroquinone cannot form iridescent carbon dot films.

[0081] Example 5

[0082] Preparation of an iridescent carbon dot thin film with photoelectric effect

[0083] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of aqueous solution (pH=1) containing 200 μL of concentrated hydrochloric acid and stirred until dissolved. The solution was then reacted in an oil bath at 180 °C for 10 h. After the reaction was completed, the solution was cooled to room temperature to obtain a precipitate. The precipitate was then reheated and filtered to remove the black precipitate, and the supernatant was obtained. The supernatant was cooled to room temperature to obtain a carbon dot film, denoted as F-1 (pH=1).

[0084] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of aqueous solution (pH=7) and stirred to dissolve. The mixture was then reacted in an oil bath at 180 °C for 10 h. After the reaction was completed, the mixture was cooled to room temperature to obtain a precipitate. The precipitate was then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant was cooled to room temperature to obtain a carbon dot film, denoted as F-2 (pH=7).

[0085] 0.1545 g of norepinephrine hydrochloride and 0.74 g of sodium hypochlorite were added to 20 mL of aqueous solution (pH = 10) containing 100 μL of concentrated ammonia and stirred until dissolved. The solution was then reacted in an oil bath at 180 °C for 10 h. After the reaction was completed, the solution was cooled to room temperature to obtain a precipitate. The precipitate was then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant was cooled to room temperature to obtain a carbon dot film, denoted as F-3 (pH = 10).

[0086] After drying the carbon dot film at 60℃, the yields of F-1 (pH=1), F-2 (pH=7), and F-3 (pH=10) were calculated to be 136.4%, 189.6%, and 193.5%, respectively. Then, a temperature-controlled study was conducted to determine their recovery yields. First, the dried samples F-1, F-2, and F-3 were added to 30 mL of an aqueous solution, heated to 90℃ to dissolve the film, and then cooled to room temperature. Figure 6 The film was found to have regrowth. The process was repeated three times, and the film recovery rate was calculated. It was found that even after three repetitions, film regrowth was still observed, and the recovery rate was greater than 77.9% in each case.

[0087] 1. Characterization of iridescent carbon dot thin films

[0088] To further investigate the properties of the iridescent carbon dot thin film, it was characterized by lens electron microscopy. The results are as follows:

[0089] Figure 7 (a) is a lens electron microscope image of the rainbow-colored thin film carbon dots. There are two lattice spacings, 0.24 nm and 0.34 nm, which correspond to the 100 crystal plane and the 001 crystal plane of graphene, respectively. Figure 7 The inset in the lower right corner of image a is its high-resolution transmission electron microscope image, which clearly shows that the iridescent carbon dot films have distinct lattice fringes, indicating that they all have a highly crystalline structure. Figure 7 (b) shows the particle size distribution of the iridescent thin film carbon dots, which is 2.01 ± 0.67 nm. The distribution is relatively diffuse, but also spherical.

[0090] To further investigate the surface state of the carbon dot film, X-ray photoelectron spectroscopy (XPS) characterization was performed, and the results are as follows:

[0091] Depend on Figure 8 As shown in (d), the carbon dot film contains three elements: C, O, and N, and peak fitting was performed on these three elements. Figure 8 (a) is the high-resolution spectrum of C1s, which can be fitted into three peaks: the peak at 284.68 eV (C=C / CC), which is the sp2 region of carbon dots; the peak at 286.26 eV (CO / CN); and the peak at 289.77 eV (C=O). This indicates that the surface of the iridescent carbon dot film may contain -COOH or -CONH2. Figure 8 (b) is the high-resolution spectrum of O1s, which can be fitted into two parts: CO (531.84 eV) and C=O (532.45 eV). Finally, Figure 8 (c) shows the high-resolution N1s spectrum, which can also be fitted to three peaks: pyridine nitrogen at 399.26 eV, pyrrole nitrogen at 400.49 eV, and graphitic nitrogen at 401.1 eV. Therefore, the XPS results indicate that the synthesized iridescent carbon dot film contains carbon sp2 hybridized regions and surface functional groups. Raman spectroscopy can reflect the degree of graphitization of the iridescent carbon dot film, so Raman characterization was performed. Figure 8 (e) is the Raman spectrum of the iridescent carbon dot film, showing two peaks, one at 1370 cm⁻¹. -1 D-band and 1580cm -1The G-bands represent disordered and ordered graphitized carbon structures, respectively. The ID / IG ratio not only demonstrates the degree of graphitization of carbon dots but is also an important indicator related to defect density and grain size. The calculated ID / IG ratio is 0.709, which differs from the ratios of commonly synthesized carbon dots. The smaller ratio obtained in this experiment indicates a higher degree of graphitization in the synthesized iridescent carbon dot film, forming a highly ordered structure. HRTEM images also show very clear lattice parameters. To investigate the functional group composition of the carbon dot surface, Figure 8 The FTIR characterization results in (f) show that at 3298 cm⁻¹ -1 2989cm -1 The broad peak at 1676 cm⁻¹ is likely due to stretching by OH and NH. -1 and 1759.6cm -1 It is the C=O absorption peak originating from -COOH or -CONH2, at 1256 cm⁻¹. -1 Belonging to the bending vibration of C=O, 1460cm -1 The peak at 1450 cm belongs to CN. -1 and 1396cm -1 This means that C = C exists. Figure 8 The XRD pattern of (g) shows a small peak at approximately 21.5°, corresponding to the increased interlayer spacing of the graphene (0.41 nm). This increased spacing may be due to steric hindrance caused by functional groups at the graphene edges, or in-plane deformation caused by sp3 hybridization. The large peak at 26.6° corresponds to the 001 crystal plane of the graphene structure (0.34 nm). This very sharp peak indicates high conjugation and a high degree of graphitization in the iridescent carbon dot film. Furthermore, its UV-Vis spectrum was analyzed. Figure 8 The UV spectrum of (h) shows absorption peaks at approximately 265 nm and 372 nm. These two peaks originate from the π-π* transition of C=C / C=N and the n-π* transition of C=O / C=N / CN, respectively.

[0092] 2. The reason why rainbow-colored carbon dot films produce rainbow colors

[0093] The surface of the thin film was investigated using SEM and atomic force microscopy (AFM):

[0094] from Figure 9 and Figure 10 The SEM and AFM results in (d) show that the carbon dot film surface has a very neat fence structure, but the spacing between each fence is not very uniform. Figure 10 (a, b) are visible under an optical microscope. Figure 10The rainbow-colored carbon dot film in (arrow and box) has a very uniform upper layer that displays a single color, while the lower layer exhibits distinct rainbow bands due to the varying thickness of the carbon dots during self-assembly to form the rainbow film. This is reminiscent of rainbow-colored soap bubbles. Under natural light, the varying thickness of the bubbles due to gravity causes thin-film interference when light reaches the interface between the upper and lower layers. However, if the film thickness is less than 100 nm or greater than 1000 nm, thin-film interference will not occur due to the thickness, thus failing to affect the color of the reflected light. In this case, the film will appear colorless (or white, see...). Figure 10 (b) at the arrow);

[0095] Secondly, to verify the above conjecture, the thickness of the rainbow-colored carbon dot film was measured. Figure 10 In this study, by analyzing the SEM cross-section and AFM to determine the thickness of the thin film, it was found that different thicknesses of the entire film correspond to different wavelength ranges of electromagnetic waves, with the thickness of the iridescent CDs film ranging from 200 to 800 nanometers. Meanwhile... Figure 10 In (f, g) (orange box), the thickness of the white CDs film is in the range of 1.0-1.2 μm, indicating that its thickness is too thick to appear white. This confirms that one possible reason for the color of the film is that different thicknesses of the film result in multiple colors due to film interference. Furthermore, when a local area of ​​the film is magnified, from... Figure 11 As can be seen, the surface of the film is composed of stacked round or elliptical particles, similar to how carbon dots of different sizes accumulate to form films of varying thicknesses. In the study of natural colors, the colors of a peacock are related to the varying thicknesses of protein particles. Therefore, it is hypothesized that the rainbow-colored carbon dot film may exhibit rainbow colors due to interference caused by differences in film thickness. From a microscopic perspective, this may be due to the uneven particle size distribution of the carbon dots. Carbon dots of different sizes, due to their varying weights during synthesis, result in films of varying thicknesses after self-assembly. Under natural light, this difference in thickness causes interference, resulting in a rainbow effect.

[0096] Secondly, XPS elemental analysis of films of different colors, as shown in Table 2, reveals that the N / C and O / C ratios gradually decrease from white, dark gold, gold, iridescent carbon dot films, to black precipitate (PDA), suggesting that the formation of iridescent films may also be related to changes in N and O elements. Furthermore, analysis of specific functional groups reveals… Figure 12As can be seen, the -COOH content decreases sequentially from white, dark gold, gold, iridescent films to black precipitate, and the -COOH content of PDA is much lower than that of iridescent films. Some reports suggest that the content of carboxyl groups is related to color, therefore it is speculated that the color of the film may be related to the content of carboxylic acids. Related DFT calculations were used to describe the emissivity changes of carbon dot structures corresponding to different carboxyl group contents. The experimental data are presented as follows: Figure 13 (a) and computational simulations such as Figure 13 Comparing the data in (b), we can see that as the content of carboxyl groups changes, the reflectance spectrum in the wavelength range of 200-800 nm increases, indicating that it reflects more light in this wavelength range, meaning that the object reflects more of the spectrum in this wavelength range and displays more color light in this wavelength range. Therefore, the content of different functional groups on the surface of carbon dots may affect the arrangement and angle of carbon dots during the self-assembly process to form a thin film, thereby affecting the interaction between light and the thin film, resulting in different reflectances and thus different colors, or more colorful ones.

[0097] 3. Temperature-reversible self-assembly preparation and self-assembly mechanism study of iridescent carbon dot thin films

[0098] To better understand and control the self-assembly of iridescent carbon dot films, the following experiments were conducted. Surprisingly, when the iridescent carbon dot film was heated to 90°C, dissolved in water, and then cooled to room temperature, the film was found to regenerate in water. The dissolved carbon dots (CDs) could reassemble into the film at low temperatures ≤90°C, making it possible to process the film at lower temperatures in water. This operation is simple, environmentally friendly, and cost-effective, greatly expanding its application areas. The above operation was repeated three times, and film regeneration was still observed, with a recovery rate remaining above 77.9% (99.9% the first time, 82.7% the second time, and 77.9% the third time). This self-healing behavior of the iridescent carbon dot film may rely on some dynamically reversible non-covalent driving forces, such as hydrogen bonding, which will be investigated in detail later.

[0099] Regarding the driving force of carbon dot self-assembly, iridescent carbon dot films were dispersed in water, from... Figure 14(Right) As can be seen, it exhibits the Tyndall effect, indicating that the carbon dot is a colloidal particle. As colloidal particles, the interaction forces between carbon dots can involve common forces between colloidal particles, such as van der Waals forces and electrostatic forces. These interaction forces determine the dispersion state, aggregation behavior, and stability of carbon dots in solution. Carbon dot spheres of different sizes gradually form a carbon dot film under their own interaction forces. At the same time, slow solvent evaporation allows the building blocks to migrate to the most suitable location to minimize energy, which is considered to be the force driving the self-assembly of the building blocks into a film. Generally speaking, self-assembly depends on the interaction between basic building blocks, or the collection / aggregation of some basic elements, so that it can achieve a stable state under a given thermodynamic environment or some external driving force (non-equilibrium state).

[0100] Looking back at the self-assembly process from a thermodynamic perspective, Figure 15 As can be seen, when the carbon dot precipitate is reheated and redissolved, the carbon dot film only regrows when the concentration reaches 0.06 mg / mL. Therefore, increasing the carbon dot concentration in the solution usually reduces the free volume between building units, thus making the originally weak attractive forces manifest. When the carbon dot concentration increases to a certain level, the interaction force between the carbon dots themselves becomes greater than the dispersing force of the solvent on the carbon dots. From the perspective of free energy, in very dilute solutions, a single building unit needs to overcome the "entropy penalty" caused by the reduction in translational and rotational degrees of freedom during self-assembly to form an ordered self-assembled structure. In this study, we assume that a single building unit: a spherical carbon dot nanoparticle with a radius of α (2-10 nm) has a model of repulsive and attractive interactions, and its interaction force and the distance from the surface of the building unit are represented by ε and λ, respectively. As the interaction force λ increases, the mutual movement distance of the single building units decreases, and they aggregate to self-assemble into an ordered structure. The aggregation process to form a self-assembled structure can be represented by an aggregation equilibrium state, where the average number of aggregates of n building units is N. n Q n It is the partition function, and the equilibrium state of these aggregates can be expressed by the formula The self-assembled aggregation system can be represented by the following formula (from George M. Whitesides, Bartosz Grzybowski. Self-Assembly at All Scales. Science, 2002, 295(5564):2418-2421; Naomi E. Chayen. Tackling the bottleneck of protein crystallization in the post-genomic era. Trends in Biotechnology, 2002, 20(3):98.):

[0101]

[0102]

[0103]

[0104]

[0105]

[0106] C1 is the concentration of a single spherical nanoparticle in the solution, C2 is the concentration of self-assembled aggregates, and V ε It is a phase space volume in which carbon dot particles fully interact with other carbon dot particles, where V is the volume of the solution, and when λ≤α, V ε ≈16πα 2 λ, combined with formulas (1)-(5):

[0107]

[0108] It can be seen that the magnitude of the free energy ΔF is determined by the interaction force ε (which is linearly related) and the interaction distance λ (ΔF2 ~ -ln(λ), λ << α);

[0109] Therefore, in this study, when the concentration reached 0.06 mg / mL, an iridescent film appeared. At this point, assuming C2 >> C1, combining (4) and (6), we have ε≈C2kTln(4λα). 2 Based on the above characterization, the particle size of the carbon dots is approximately 2 nm, and the interaction force ε ≈ 0.173 kT. This may explain why the assembly type varies with different temperatures. This explains why... Figure 16The state of the thin film obtained by cooling to different temperatures is different. The temperature difference between cooling at low temperature and cooling at room temperature is different. The larger the change in temperature (T), the larger the ε value, which manifests as two states: iridescent thin crystals and thin films. Secondly, for self-assembly under the same temperature change, different concentrations will result in different corresponding forces. Figure 15 It is evident that self-assembled films only occur when the carbon dot concentration exceeds a certain value, and when the concentration is increased, low concentrations ( Figure 15 b) The film state is better than that of high concentration ( Figure 15 c) appears to be thinner. This is because different temperature and concentration changes regulate the thickness and state of the film.

[0110] In summary, firstly, a single building block requires a force greater than 0.173 kT to guide the self-assembly of the solution; secondly, different cooling methods and concentrations result in different film states due to varying force magnitudes, with thinner films obtained at concentrations between 0.06 and 5.6 mg / mL, and thicker films obtained at concentrations greater than 5.6 mg / mL, exhibiting a white color. Figure 15 The red arrow (at a concentration of 11.2 mg / mL) indicates that different interaction forces lead to different assembly structures and equilibria. Furthermore, based on ε≈0.173 kT, it suggests that the reversible self-assembly of the film at temperature is essentially a change in the driving force ε. The types of driving force ε will be discussed in detail later.

[0111] According to literature reports, potassium ions can form cation-π interactions with π-electron structures, thereby occupying some of the original cation-π sites and reducing the cation-π interaction force, thus proving its existence. The magnitude of the cation-π interaction largely depends on the π-electron system and the positively charged amine functional groups in the carbon dot structure. Therefore, examining different amounts of potassium chloride, it was found that as the amount of potassium chloride added gradually increased to 10 mmol, the iridescent carbon dot film could not be formed.

[0112] Table 4 examines the effects of different potassium chloride concentrations on the synthesis of iridescent carbon dot films.

[0113]

[0114] From Table 4 and Figure 17 As can be seen, at a potassium chloride concentration of 10 mmol, no increase in the concentrations of the oxidant sodium hypochlorite and the substrate resulted in the absence of an iridescent film growth; the product was a black precipitate (as shown in the image). Figure 17 Sample 31(f)) and white crystals (e.g. Figure 17 For samples 34(d) and 35(h), only by simultaneously increasing the concentrations of both the substrate and the oxidant could the appearance of iridescent carbon dot films be observed (e.g., ...). Figure 17 Sample 36(e)). This may be because, under acidic conditions, the amine groups are protonated, and the solution contains a large number of cation sites that can form cation-π interactions with the conjugated system of carbon dots. However, when a large amount of potassium chloride is added to the system, it occupies the original sites, thus preventing the film from forming. This indicates that cation-π interactions play a crucial role in the growth of iridescent carbon dot films. Furthermore, from... Figure 4 (b) It is evident that no film grew in the strongly alkaline solution (pH=13), possibly because the cations were neutralized, and the -π cations were not manifested. Furthermore, the Zeta potentials show that the carbon point solution has potentials of -2.1 eV and -9.8 eV at pH=3 and pH=11, respectively. This is because at lower pH values, the H+ in the solution... + Higher concentrations and acidic conditions can increase the activity and positive charge of cations, making them more likely to interact with substances containing π electrons. Interestingly, to further confirm whether it is a cation-π interaction, different substrates were used as control experiments (substrates containing amine functional groups and those without). Figure 5 As shown in Table 3, p-aminophenol (d) and o-aminophenol (e) can form iridescent films, but hydroquinone (f) and resorcinol (g) cannot. This further confirms that the inability to form iridescent carbon dot films may be related to the presence or absence of amine groups in the substrate. Resorcinol and hydroquinone lack amine functional groups and cannot form conjugated structures or positively charged amine groups; that is, they cannot form cations (-π), and therefore cannot form films. Furthermore, during the reaction regulation process, films were found to form under acidic, weakly basic, and neutral conditions, indicating that the charge characteristics of the film material are relatively stable. This may be due to the presence of carboxyl, hydroxyl, and amine functional groups in the substrate, allowing it to freely adjust its charge according to environmental conditions during carbon dot synthesis. These charge characteristics may play an important driving role in the assembly and stability of the film, similar to the regulation of the isoelectric point of proteins. Furthermore, the elemental analysis of XPS shows that the carbon dot surface contains abundant functional groups such as carboxyl and amino groups. The iridescent carbon dot film is rich in oxygen. During the oxidative polymerization process to form carbon dots, the nitrogen and oxygen elements contained in the substrate (containing two phenolic hydroxyl groups and amino groups at the ends of the branches) are retained. At the same time, the FTIR spectrum shows the presence of -OH and amide bonds. The formation of hydrogen bonds depends on whether the carbon dot surface contains elements with high electronegativity, such as N, O and F. These functional groups may be the source of hydrogen bonds formed between the carbon dots in the formation of the iridescent carbon dot film. Figure 8(f) shows the FTIR spectra of the iridescent carbon dot films at 30 °C (col⁻²) and 45 °C (col⁻¹). It can be seen that the absorption peaks are identical, but their intensities change. At 3298 cm⁻¹... -1 and 2989cm -1 The OH and NH absorption peaks are at 1676 cm⁻¹. -1 and 1759.6cm -1 The absorption peaks at the carboxyl group or -CONH2 are particularly prominent, indicating that hydrogen bonding is a crucial force in the self-assembly of CDs films. Furthermore, temperature is critical to hydrogen bond formation. Observations show that the iridescent carbon dot film formed during cooling after the reaction (approximately between 70-60°C). This may be because the interaction distance between carbon dot particles differs at different temperatures. At lower temperatures, the distance between carbon dot particles decreases, which is conducive to the formation of various forces. At higher temperatures, molecular motion intensifies, tending towards a disordered state, making hydrogen bond interactions unstable and prone to breakage, leading to film dissolution. Additionally, from... Figure 18 The SEM images revealed that as the electron probe approached the film at increasing magnification, the local temperature rose due to the interaction between electron impact and the film, causing deformation and cracks. The field of view gradually blurred and the film began to melt, indicating that the film is very sensitive to temperature changes and that hydrogen bonds are a very important force in maintaining the integrity of the film.

[0115] To further investigate the underlying reasons for the self-assembly process, equal volumes of aqueous solutions containing carbon dots capable of self-assembling into thin films were mixed with various solvents, and the results were studied. It was found that even after heating, the carbon dots could dissolve in the solvents, but the self-assembly behavior varied significantly depending on the carbon dot concentration. Some carbon dots failed to regrow into films even after a long period because the solvent effect was far greater than the interaction forces within the carbon dots themselves, preventing film formation. As the concentration increased, the solvent's effect on the carbon dots themselves became far greater than its effect on the carbon dots. Only when the driving force of the solvent on the self-assembly of carbon dots was negligible did it become meaningful to further explore the influence of external factors on self-assembly. Therefore, a carbon dot concentration of 5.6 mg / mL was used. The Hansen solubility constant (HSP) was introduced, with all constant information obtained from the Hansen user manual. HSP quantitatively determines the cohesive energy density δ of a species through three weak interactions: van der Waals or dispersive interactions (δ0). D ), dipole-dipole or polar interaction (δ P ) and hydrogen bond interactions (δ H The Hansen solubility parameter is based on thermodynamic theory, and the modified Hansen solubility constant is obtained using formulas (1) and (2) when the solvents are mixed.

[0116]

[0117]

[0118] δ i s Represents the Hansen constant before the solvents are mixed, and re-δi represents the Hansen constant after mixing. Represents volume fraction.

[0119] When an aqueous solution containing carbon dots is mixed with various solvents, the film growth varies depending on the solvent's properties. Maintaining a constant volume of carbon dot solution, and ensuring that this solution can grow an iridescent film, changes in the HSP (Highly Saturated Polymer) of the mixed solvent result in different regrowth products from the carbon dot solution.

[0120] Table 5. Film-forming states (solution, solid, and film) of carbon dot solutions mixed with various solvents and the corresponding corrected Hansen constants.

[0121]

[0122]

[0123] from Figure 19 As shown in Table 5, the film self-assembly process involved three phase separation states: first, the solution state; second, the solid state; and finally, the film state. This is likely because the interaction forces between carbon dots change with the properties of the solvent, leading to different self-assembly structures. Since the molecules are amphiphilic, the self-assembly outcome may depend on phase separation or aggregation at the microscopic level, and the competition and cooperation of non-covalent forces such as hydrogen bonds and van der Waals forces are the essential reasons for phase separation or aggregation. Analysis of the specific Hansen constant revealed that hydrogen bond interactions (δ0.05) do not affect the formation of the film during the self-assembly process of carbon dots in aqueous solution. H The value of ) is concentrated in three intervals, namely 42.30≤re-δ H ≤38.01, 33.73≤re-δ H ≤30.85 and 24.75≤re-δ H ≤22.15. Hydrogen bonding interactions were discovered (δ) H The size of the re-δ value shows a linear relationship with the state of carbon dot solution self-assembly (re-δ). H= -0.8934x + 42.551 (R = 0.99), where x is an arbitrary value. When x is in the range of 1-6, 10-13, and 19-24, it is a thin film state; when x is in the range of 7-9, it is a solution state; and when x is in the range of 14-18, it is a solid state. This indicates that the hydrogen bonding force plays an important role in the self-assembly of carbon dots. Mixing the Hansen constant with different solvents, by externally perturbing the self-assembly of carbon dots, disrupts the equilibrium barrier of their self-assembly forces. It was found that when the hydrogen bonds and van der Waals forces change, the state of self-assembly also changes, indicating that hydrogen bonds and van der Waals forces also regulate the self-assembly process. Furthermore, through... Figure 20 (b) The UV-Vis spectrophotometer shows that π-π interactions are also a significant driving force in the formation of the iridescent carbon dot film. Separating the solution and the film after film formation and measuring the UV-Vis spectrum of the solution, compared with the H₂O₂ curve (carbon dot film solution, before film formation), reveals a decrease in the absorption peaks around 250 nm and 340 nm in the solution curve after film formation. This indicates a change in the intensity of the π-π absorption peaks before and after film formation, suggesting that π-π interactions also play a role in the film's self-assembly process. Furthermore, the film's ability to grow from water or float on the surface suggests that hydrophobic and hydrophilic interactions may also influence the self-assembly process of the carbon dot film.

[0124] In summary, if the concentration is high enough, aqueous solutions of carbon dots can spontaneously self-assemble to form carbon dot films. Secondly, the driving force behind the self-assembly of carbon dot films is essentially a combination of multiple forces, including cation-π (essentially electrostatic forces), hydrogen bonds, and van der Waals forces. However, among non-covalent interactions, the cation-π interaction is the strongest, several times stronger than other forces, and its strength can be adjusted depending on the type of cation and the properties of the π system. Therefore, it is hypothesized that the dominant force is the cation-π interaction, resulting from the competition and cooperation of non-covalent forces such as hydrophilic / hydrophobic interactions, van der Waals forces, and hydrogen bonds. Thus, the study of self-assembly ultimately boils down to the study of the interactions among these various forces within the system. These forces can be precisely and rationally adjusted using kinetic parameters such as pH, temperature, concentration, and solvent. In fact, in many cases, there is no clear boundary between thermodynamic control and kinetic control. Kinetic control achieved through molecular design or alteration of solvent properties is essentially a form of thermodynamic control, as these kinetic controls disrupt the balance of the original driving forces or change the energy barrier.

[0125] 4. Potential applications of carbon dot thin films

[0126] Figure 21The rainbow-colored carbon dot film demonstrates broad adhesion, adhering to both metal substrates and skin; fingerprints were even observed on thin sections of the film. Furthermore, it peels off very easily when submerged in water. In addition, the film regrowths upon redissolving the carbon dot precipitate in hot water and cooling, indicating reproducibility. This film is also highly temperature-sensitive, with reversible growth influenced by temperature. Even after drying the rainbow-colored carbon dot film at 60°C, heating it to 90°C with water resulted in self-assembly growth. Figure 22 As shown in (abcde), the iridescent carbon dot film can still recover its growth from the solution, and the film gradually covers the entire beaker interface. This temperature-induced supramolecular depolymerization has potential applications in the biomedical field. Furthermore, since the substrate is a dopamine derivative, it is clear that as a carbon dot, it not only has photothermal effects but is also a component of the biological system, and its degradation process is well-defined. This avoids the possibility of non-degradability and toxic reactions caused by using precious metal ions and some polymer particles. Therefore, it is believed that it can be used as a carrier in clinical applications, playing a unique role when loaded onto carriers for therapeutic tumor drugs. Figure 14 (Right) As can be seen, although the formed film does not fluoresce in its physical state, it exhibits a pale green fluorescence when dispersed in a solution such as hot water. Therefore, if applied to photothermal therapy, the degree of fluorescence remaining on the cell surface can be used to determine whether the coverage is complete, thus increasing the means of visual assessment. Secondly, such as Figure 23 When an iridescent film is grown on glass, it is observed that when a flashlight interacts with the film, it reflects iridescent bands. Such films typically possess good color vibrancy and optical properties. Furthermore... Figure 24 Under a polarizing microscope, the thin film also exhibits birefringence, indicating that the iridescent carbon dot film possesses excellent neon-like properties. Furthermore, the photocurrent effect of the iridescent film and TiO2 composite was measured. Figure 25 The photocurrent effect indicates that the iridescent film not only has a good photocurrent effect, but can also enhance the photocurrent effect of titanium dioxide.

[0127] Table 6 Comparison of carbon dot thin films and their composite photocurrents

[0128]

[0129] Furthermore, Table 6 shows that the photocurrent obtained from the iridescent thin film is higher than that from ordinary carbon dots dissolved and dispersed in conductive glass. The reason for this is likely that the thin film forms a highly regular fence structure through self-assembly of the carbon dots. Figure 26It is evident that this increases the speed of movement between light and charge, significantly improving photoelectric conversion efficiency. In the synthesis of the carbon dot film of this invention, the main role is played by cation-π, with hydrogen bonding, electrostatic forces, and hydrophobic-hydrophilic interactions assisting in competition, C( K+ Increasing the concentration of ions weakens the strength of the -π cations in the carbon dot film, preventing its formation. Furthermore, the carbon dot film prepared in this invention can be used to fabricate various iridescent photoelectric conversion devices, such as LEDs, photodiodes, and photoelectric agar. Carbon dot films have broad application prospects in multiple fields, and their desired objectives can be achieved through the regulation of their structure and properties.

[0130] References [1]: Prasanta K. Raul, Angshuman Thakuria, Bodhaditya Das, etal. Carbon Nanostructures As Antibacterials and Active Food-PackagingMaterials: AReview.ACS Omega, 2022, 7(14): 1555-11559.

[0131] [2] Shujuan Yu, Shiyan Lu, Guangjian Zheng. Reusable flexible poly(vinylalcohol) / chitosan-based polymer carbon dots composite film for acidblue 93 dyeadsorption. Luminescence: the journal of biological and chemical luminescence, 2023, 38(9): 1552-1561.

[0132] [3] Yin Jifan, Huang Yuxin, Hameed Saima, et al. Large scale assembly of nanomaterials: mechanisms and applications. Nanoscale, 2020, 12(34): 17571-17589.

[0133] [4]Y Cui,C M Lieber.Functional nanoscale electronic devices assembledusingsilicon nanowire building blocks.Science,2001,291(5505):851-853.

Claims

1. A method for preparing an iridescent carbon dot thin film with photoelectric effect, characterized in that, Includes the following steps: Dopamine derivatives and oxidants are added to an aqueous solution containing concentrated hydrochloric acid or concentrated ammonia. After stirring and dissolving, an oil bath reaction is carried out. After the reaction is completed, the mixture is cooled to room temperature to obtain a precipitate. The precipitate is then reheated and filtered to remove the black precipitate, resulting in a supernatant. The supernatant is then cooled to room temperature to obtain a carbon dot film. The oxidant is sodium hypochlorite; The molar ratio of the dopamine derivative to the oxidant is (3~7.5):(40~100); The oil bath reaction is carried out at a temperature of 120-200℃ for 6-12 hours.

2. The method for preparing an iridescent carbon dot thin film with photoelectric effect according to claim 1, characterized in that, The dopamine derivatives include one of dopamine hydrochloride, epinephrine hydrochloride, and phenylephrine hydrochloride.

3. The method for preparing an iridescent carbon dot thin film with photoelectric effect according to claim 1, characterized in that, The dopamine derivatives include phenylephrine hydrochloride.

4. The method for preparing an iridescent carbon dot thin film with photoelectric effect according to claim 1, characterized in that, The reaction pH value of the preparation method is 1~12.

5. A rainbow-colored carbon dot film with photoelectric effect obtained by the preparation method according to any one of claims 1-4, characterized in that, The carbon dot film is formed by assembling nanoscale carbon dots into a micrometer-scale fence structure, and then forming a centimeter-scale iridescent carbon dot film.

6. An application of the iridescent carbon dot thin film with photoelectric effect as described in claim 5, characterized in that, This includes applying iridescent carbon dot films to the fabrication of various iridescent photoelectric conversion devices.

Images