Preparation method of a dual-ligand metal organic framework-based electrochromic film

Abstract

The application discloses a preparation method of an electrochromic film based on a double-ligand metal organic framework, and comprises the following steps: performing ultrasonic cleaning, oxygen plasma activation and sodium octyl sulfate self-assembled monolayer functionalization on an ITO / FTO conductive substrate; preparing a precursor solution containing metal salt and double ligand, and depositing a uniform porous precursor film on the functionalized substrate by electrostatic spraying; and then performing solvent vapor assisted in-situ low-temperature crystallization to obtain a double-ligand MOF electrochromic film. The application adopts double-ligand collaborative design to collaboratively improve optical contrast, response speed and cycle stability; the film-substrate adhesion is significantly enhanced through substrate interface functionalization; the electrostatic spraying + vapor assisted crystallization process is adopted to realize large-area, uniform and thickness-controllable film forming, and the process is mild and suitable for large-scale preparation. The prepared film has high optical modulation range, fast response, excellent cycle stability and strong adhesion, and can be applied to the fields of smart windows, displays and anti-dazzling devices.

Description

Technical Field

[0001] This invention belongs to the field of functional materials and electrochromic devices, specifically relating to a method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework. Background Technology

[0002] Electrochromic technology refers to the phenomenon where the optical properties (such as transmittance, reflectance, and absorptivity) of a material change reversibly under the influence of an applied electric field. Electrochromic devices developed based on this effect have broad application prospects in fields such as smart windows, energy-efficient buildings, automatic anti-glare rearview mirrors, new displays, and military camouflage. Currently, mainstream electrochromic materials mainly include transition metal oxides (such as WO3 and NiO) and conductive polymers (such as polyaniline and polythiophene). However, these traditional systems still have room for improvement in terms of color control diversity, cycle stability, response speed, and fabrication cost.

[0003] In recent years, metal-organic frameworks (MOFs) have demonstrated unique advantages in the field of electrochromism due to their highly ordered pore structure, high specific surface area, tunable chemical composition, and multifunctionality. Excellent electrochromic properties can be endowed to MOFs by introducing redox-active organic ligands or metal nodes. For example, the M-MOF-74 series based on 2,5-dihydroxyterephthalic acid ligands has been shown to achieve reversible color development via quinone / hydroquinone redox pairs. Nevertheless, existing MOF electrochromic systems still face challenges in terms of synergistic performance optimization, controllable thin film preparation, and interfacial stability.

[0004] Among the valid patents retrieved, the following two technical solutions are quite similar to "a method for preparing electrochromic thin films based on dual-ligand metal-organic frameworks": Chinese patent CN106371258B discloses a method for preparing electrochromic thin films, which involves dissolving an electrochromic material in a polymer solution to form a polymeric composite electrochromic solution, and then coating it onto a substrate to form a film. While this method offers advantages such as simple processing, low cost, and applicability to large-scale fabrication, the electrochromic materials used are traditional inorganic or organic small molecules, without involving metal-organic frameworks (MOFs). In particular, it does not utilize a dual-ligand strategy to synergistically regulate the redox activity and charge transport capacity of MOFs, thus limiting its overall performance in terms of color contrast, response speed, and cycle life.

[0005] Chinese patent CN111474792B proposes a method for preparing porous electrochromic thin films. It employs electrostatic spraying technology to deposit a mixture of electrochromic materials onto a substrate surface, forming a porous structure that facilitates ion diffusion, thereby improving response speed and reducing operating voltage. While this method incorporates electrostatic spraying, a process suitable for large-area film formation, its electrochromic material system remains based on traditional polymers or inorganic oxides, without involving MOF materials, and it does not explore the film-forming behavior and performance of dual-ligand MOFs under electrostatic spraying conditions. Furthermore, this patent does not specifically optimize the interfacial adhesion between the substrate and the thin film, potentially leading to insufficient adhesion during long-term electrochemical cycling.

[0006] In summary, existing technologies have not fully leveraged the unique advantages of MOF materials in terms of material system innovation, functional synergistic design, and interface engineering. In particular, there is a lack of systematic solutions that integrate dual-ligand strategies, electrostatic spraying processes, and substrate pretreatment techniques for the preparation of high-performance electrochromic thin films. Summary of the Invention

[0007] The purpose of this invention is to overcome the shortcomings of the prior art and provide a dual-ligand MOF electrochromic thin film that can simultaneously achieve strong interfacial bonding, large-area uniform preparation, and high performance, as well as its preparation method.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework includes the following steps: S1. Conductive substrate pretreatment and functionalization: ITO or FTO conductive substrates are ultrasonically cleaned and activated by oxygen plasma, and then immersed in sodium octyl sulfate in ethanol / water solution with a concentration of 0.5-5 mmol / L for 12-24 h at room temperature in the dark. After removal, the substrates are rinsed with ethanol and deionized water and dried to form a dense octyl sulfate self-assembled monolayer on the substrate surface. S2. Preparation of dual-ligand precursor solution: Dissolve the metal salt, the first ligand and the second ligand together in a mixed solvent composed of organic solvent and deionized water, disperse by ultrasonication for 30-60 min and stir continuously for 6-12 h to form a homogeneous and stable precursor solution. S3. Electrostatic spraying deposition of precursor film: The functionalized substrate obtained in step S1 is fixed on the grounding receiving plate of the electrostatic spraying equipment, and the precursor solution obtained in step S2 is prepared for spraying; the distance between the nozzle and the substrate is adjusted to 5-50cm, a DC high voltage of 10-30kV is applied, the flow rate of the precursor solution is controlled to 0.05-3.0mL / h, and spraying is carried out in an air environment with normal temperature and pressure to deposit a porous and uniform wet film on the surface of the functionalized substrate, and then dried at 60-80℃ for 10-30min to obtain a precursor dry film; S4. Steam-assisted in-situ crystallization: The substrate with the precursor dry film deposited is placed in a sealed container; the sealed container is then placed in an oven for heat treatment; after the heat treatment is completed, it is naturally cooled, the substrate is removed, and washed with DMF and ethanol to remove unreacted substances. After drying, a crystalline dual-ligand MOF electrochromic film is obtained on the conductive substrate.

[0009] Further, in step S1, the conditions for plasma activation are: power 50-100W, oxygen pressure 10-50Pa, and processing time 5-15min.

[0010] Further, in step S2, the metal salt is one of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, copper nitrate hexahydrate, or zinc nitrate hexahydrate; The total concentration of metal ions in the precursor solution is 0.05-0.15 mol / L.

[0011] Further, in S2, the volume ratio of organic solvent to deionized water in the mixed solvent is 4:1-10:1; The organic solvent is N,N-dimethylformamide or N,N-dimethylacetamide.

[0012] Furthermore, in S2, the molar ratio of the first ligand to the second ligand is 1:0.2-1:1; the molar ratio of the total number of metal ions to the total number of functional groups available for coordination provided by the two ligands is 1:1-1.5:1.

[0013] Furthermore, the first ligand is an aromatic polycarboxylic acid or a derivative thereof having multiple phenolic hydroxyl groups; The second ligand is a disc-shaped or rod-shaped molecule with a large planar π-conjugated structure; The second ligand is pre-connected to the first ligand via covalent bonds to form an organic linker unit.

[0014] Furthermore, in S3, the preferred parameters for electrostatic spraying are: DC high voltage 15-25kV, supply flow rate 0.2-1.0mL / h, and nozzle-to-substrate distance 15-25cm.

[0015] Furthermore, in S4, a sufficient amount of N,N-dimethylformamide solvent is placed at the bottom of the container to create a saturated solvent vapor atmosphere.

[0016] Furthermore, in S4, the preferred conditions for the heat treatment are: heat treatment at 80-120°C for 12-48 hours.

[0017] A method for preparing electrochromic films based on dual-ligand metal-organic frameworks, resulting in dual-ligand MOF electrochromic films based on interface modulation and electrostatic spraying.

[0018] The beneficial effects of this invention are: 1. Fundamental Innovation in Process Route (Decoupling of Film Formation-Crystallization): This invention pioneers a two-step method of "first forming a film through room-temperature electrostatic spraying, then assisted crystallization in a low-temperature vapor phase." Electrostatic spraying solves the challenges of large-area, rapid, and thickness-controllable film formation; the subsequent independent vapor phase crystallization process specifically optimizes the diffusion and ordered assembly of the two components, overcoming the component inhomogeneity problem caused by convection and thermal gradients in the solvothermal method, and ensuring uniform co-crystallization of the two ligands at the molecular level.

[0019] 2. Innovative Crystallization Mechanism (Steam-Assisted Diffusion and Crystallization): Unlike direct high-temperature heat treatment of the precursor film (which easily leads to decomposition of organic ligands or rapid disordered crystallization), this invention utilizes a saturated solvent vapor atmosphere. This atmosphere creates a localized high-concentration solvent microenvironment within the dry precursor film, causing partial "softening" or "dissolving" of the solid reactants and significantly increasing molecular mobility. This is similar to a mild "steam annealing" or "gas-phase transport" process, allowing metal ions sufficient time and driving force to find the lowest-energy coordination mode with two ligands (linear H4DOBDC and disk-shaped HHTP) with different spatial structures and coordination characteristics, thereby forming a thermodynamically stable, long-range ordered hybrid crystal structure, rather than a two-phase mixture.

[0020] 3. Interface Engineering Innovation (Chemical Anchoring and Hydrophobic Template): This invention introduces an octyl sulfate self-assembled monolayer. This functionalized layer utilizes -SO3... - Chemical anchoring is achieved through strong electrostatic interactions between the functional groups and metal ions on the substrate surface; the outward-facing octyl long chains form a hydrophobic interface. This hydrophobic interface not only enhances the binding force with organic components in the MOF through hydrophobic interactions, but also serves as a template, preferentially adsorbing highly hydrophobic HHTP ligands or precursor aggregates in the early stages of electrostatic spraying, inducing MOF crystals to nucleate and grow in a specific orientation, thereby further optimizing the charge transport capability and mechanical adhesion of the film.

[0021] 4. Synergistic effect of dual ligands: The HHTP-type second ligand introduced in this invention constructs an "electronic highway" within the framework, which increases the electronic conductivity of the thin film by 1-2 orders of magnitude and significantly accelerates the response speed. At the same time, its rigid framework enhances the structural stability of the MOF and improves the cycle life.

[0022] 5. Substantial differences from patents in similar fields: This invention is fundamentally different from patents that simply improve material ligands (such as various bimetallic and dual-ligand design patents). Its core contribution lies in providing a novel and universally applicable preparation process. This process is particularly suitable for preparing complex MOF thin films composed of multiple functional components, and is not limited to the field of electrochromic films, but can also be extended to the preparation of catalytic, sensing, and other functional thin films. Attached Figure Description

[0023] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0024] Figure 1 SEM images of MOFs prepared in Example 1; Figure 2 Here is a SEM image of the MOF prepared in Example 2; Figure 3 The image shows a SEM image of the MOF prepared in Comparative Example 2. Figure 4 XRD pattern of MOF prepared in Example 1; Figure 5 This is the XRD pattern of the MOF prepared in Example 2; Figure 6 The transmittance data of the MOF prepared in Example 1 is shown in the figure. Figure 7 The transmittance data of the MOF prepared in Example 2 is shown in the figure. Detailed Implementation

[0025] The present invention will be further described below with reference to the accompanying drawings and embodiments, but the scope of protection of the present invention is not limited thereto.

[0026] The technical solution of this application systematically solves the aforementioned technical problems through synergistic innovation in material systems, interface engineering, and film formation processes. Specifically, this application first pretreats the conductive substrate with a solution containing octyl sulfuric acid or its salt, constructing a functionalized self-assembled monolayer on its surface. This functionalized interface can provide strong chemical anchoring points for the subsequent growth of MOF films through electrostatic attraction and hydrogen bonding, thereby significantly enhancing the film-substrate adhesion and interfacial charge transport efficiency.

[0027] Secondly, this application innovatively designs and employs a dual-ligand system consisting of 2,5-dihydroxyphthalic acid (as the electrochromic active center) and 2,3,6,7,10,11-hexahydroxytriphenylene (as the charge transport enhancement and structural stability center), which is used in conjunction with transition metal salts to prepare precursor solutions. 2,5-dihydroxyphthalic acid, with its biphenyl backbone and phenolic hydroxyl groups, provides abundant redox active sites, ensuring high optical contrast. Meanwhile, 2,3,6,7,10,11-hexahydroxytriphenylene, with its large π-conjugated disk-like structure, can form an efficient charge transport pathway within the MOF framework through π-π stacking, and utilizes its hydroxyl groups to participate in coordination and hydrogen bonding networks to enhance structural stability. The synergistic effect of these two components achieves simultaneous optimization of electrochromic performance and conductivity. Finally, this application employs an electrostatic spraying process to deposit the precursor solution onto a functionalized substrate. This process, driven by a high-voltage electric field, enables uniform atomization and deposition of precursor droplets at room temperature and pressure, forming a porous precursor film with controllable thickness. Subsequent heat treatment transforms this precursor into a well-crystallized dual-ligand MOF electrochromic film in situ. Electrostatic spraying technology inherently possesses advantages such as simple equipment, low cost, and ease of large-area uniform film formation. The resulting porous structure also facilitates the rapid penetration of electrolyte ions.

[0028] By employing the aforementioned dual-ligand synergistic design, substrate interface functionalization, and electrostatic spraying deposition techniques, this application achieves the following technical effects: the prepared dual-ligand MOF electrochromic film simultaneously possesses a high optical modulation range (e.g., up to 70% at 550 nm), rapid response (coloring and fading can be completed within seconds), excellent cycling stability (e.g., minimal performance degradation after tens of thousands of cycles), and extremely strong film-substrate adhesion (e.g., passing the highest level of cross-cut adhesion testing). This preparation method is simple, mild, low-cost, and easily scalable, effectively overcoming various deficiencies mentioned in the background technology and existing patents, providing a new material system and feasible preparation path for the development of high-performance electrochromic devices.

[0029] Metal-organic frameworks (MOFs) contain at least two different organic ligands. The first ligand primarily contributes to electrochromic activity, while the second ligand primarily contributes to charge transport capabilities. The two ligands are connected by metal nodes to form a hybrid framework structure with synergistic effects. This dual-ligand design aims to overcome the difficulty of balancing optical contrast and conductivity in single-ligand MOFs, achieving an overall performance improvement through functional complementarity.

[0030] The first ligand is an aromatic polycarboxylic acid or its derivative having multiple phenolic hydroxyl groups, such as 2,5-dihydroxyterephthalic acid, 2,5-dihydroxyphthalic acid, or their alkyl or halogen-substituted analogs. The redox pairs of the phenolic hydroxyl / quinone groups on these ligands can undergo reversible electron transfer and proton coupling under an electric field, inducing significant changes in the π-π* transition energy levels of the ligand itself, thereby producing strong color changes, which is key to achieving a high optical modulation range.

[0031] The second ligand is a disc-shaped or rod-shaped molecule with a large planar π-conjugated system, such as hexahydroxytrimethylene (HHTP), hexaaminotrimethylene (HITP), tetrathiofulvalene tetracarboxylic acid (TTF-TC), or their derivatives. These ligands tend to form two-dimensional or three-dimensional electronic conduction channels throughout the entire crystal structure via face-to-face π-π stacking within the MOF framework. This significantly enhances the intrinsic electronic conductivity of the framework, thereby shortening the charge transport time during electrochromic processes and achieving a rapid response.

[0032] The metal nodes in dual-ligand MOFs are selected from transition metal ions, such as Ni. 2+ Co 2+ Cu 2+ Zn 2+ Mn 2+ or Fe 2+ / Fe 3+ These metal ions not only serve as structural centers connecting ligands, but may also possess variable oxidation states, enabling them to participate in or assist electrochemical redox processes. This provides additional electrochromic active sites or regulates the overall charge balance, further enriching the color changes and electrochemical behavior of the material.

[0033] 2,5-Dihydroxyphthalic acid, whose biphenyl skeleton can be extended to other aromatic or heteroaromatic skeletons with conjugated systems, such as naphthalene, anthracene, phenanthrene, or thienobenzobenzene. These extended skeletons can provide a wider range of π-electron delocalization, which may result in richer redox states and a wider spectral absorption range, contributing to the achievement of multicolor or deeper color variations.

[0034] Preferably, the hydroxyl (-OH) functional group on 2,5-dihydroxyphthalic acid can be replaced or partially replaced by other redox-active functional groups, such as amino (-NH2), mercapto (-SH), or disulfide bonds (-SS-). These functional groups can also participate in electrochemical redox reactions, altering the electronic structure and optical properties of the ligands, thereby modulating the electrochromic behavior of the MOF film, such as achieving a transition from colorless to blue or green.

[0035] Particularly preferred, the two carboxylic acid groups (-COOH) of 2,5-dihydroxyphthalic acid can be replaced by other functional groups capable of coordinating with metal ions, such as phosphonic acid groups (-PO3H2), sulfonic acid groups (-SO3H), or nitrogen-containing heterocycles (such as pyridine, imidazole, and triazole). These different coordinating groups can alter the coordination mode and coordination strength with the metal nodes, thereby modulating the topology, porosity, and stability of the MOF framework and affecting the transport kinetics of ions within the framework.

[0036] 2,3,6,7,10,11-Hexahydroxytriphenylene, in which the six hydroxyl groups can be partially or completely replaced or modified by other functional groups that can participate in coordination, such as, but not limited to, amino, mercapto, carboxyl, or phosphonic acid groups. Such substitution or modification can further modulate the coordination ability of the ligand with metal ions, the coordination mode, and the topology and electronic properties of the final metal-organic framework. For example, introducing an amino group can enhance the basicity of the ligand, thereby potentially altering its coordination selectivity with specific metal ions; while introducing a carboxyl group can provide additional coordination sites, potentially leading to more complex network structures.

[0037] 2,3,6,7,10,11-Hexahydroxytriphenylene, whose triphenylene core skeleton can be replaced by other planar polycyclic aromatic hydrocarbons with extended π-conjugated systems, such as hexahydroxybenzophenanthrene, hexahydroxyanthracene, hexahydroxypyrene, or their derivatives. These extended π-conjugated systems can also construct efficient charge transport channels within the formed metal-organic framework through intermolecular π-π stacking interactions, thereby maintaining or enhancing the material's conductivity. Furthermore, the different sizes and shapes of these substituted ligands can influence the pore structure and stacking mode of the MOF, potentially modulating ion transport kinetics and electrochromic behavior.

[0038] Particularly preferred is 2,3,6,7,10,11-hexahydroxytriphenylene, which, as a second ligand, can coexist with the first ligand (2,5-dihydroxyphthalic acid) in a non-equimolar ratio, gradient distribution, or localized enrichment within the metal-organic framework. For example, in precursor films deposited by electrostatic spraying, the two ligands may form a concentration gradient along the film thickness direction or in the plane due to differences in solubility, volatility, or mobility in an electric field. This non-uniform distribution may result in the final crystallized MOF film having a layered or gradient-varying composition and structure, thereby potentially achieving differentiated electrochromic responses along the thickness direction or optimized charge / ion transport pathways.

[0039] In electrostatic spraying, the intensity of the DC high-voltage electric field applied between the nozzle and the substrate is adjustable, preferably in the range of 10kV to 30kV, and more preferably in the range of 15kV to 25kV. By adjusting the electric field intensity, the atomization degree, flight trajectory, and deposition rate of the precursor droplets can be controlled, thereby achieving precise control over the film thickness and morphology. Higher voltages help generate finer droplets, forming denser and more uniform films; while lower voltages may generate larger droplets, which helps to construct films with specific pore structures.

[0040] During electrostatic spraying, the supply flow rate of the precursor solution is controllable, preferably within the range of 0.05 mL / h to 3.0 mL / h, and more preferably within the range of 0.2 mL / h to 1.0 mL / h. Matching the flow rate with the applied voltage maintains stable Taylor cone formation and droplet atomization, preventing excessively large droplets or jet formation, and ensuring the continuity and uniformity of film deposition. Lower flow rates promote sufficient solvent evaporation, resulting in a drier deposit and reducing film flow; while higher flow rates improve deposition efficiency.

[0041] The deposition environment for electrostatic spraying is atmospheric pressure air, and the distance between the nozzle and the conductive substrate is adjustable, preferably in the range of 5 cm to 50 cm, and more preferably in the range of 15 cm to 25 cm. This distance affects the droplet flight time in the electric field and the degree of solvent evaporation. A longer flight distance allows for more complete solvent evaporation, which helps to form a precursor deposition layer with higher dryness and reduces structural collapse during subsequent processing; while a shorter flight distance is suitable for systems with low requirements for solvent evaporation rate or for applications requiring a higher deposition rate.

[0042] Example 1: This embodiment describes a method for preparing a dual-ligand metal-organic framework electrochromic thin film, specifically following these steps: S1. Conductive Substrate Pretreatment and Functionalization: The FTO glass was ultrasonically cleaned sequentially in detergent, acetone, ethanol, and deionized water for 20 min each, and then dried with nitrogen. It was then treated in an oxygen plasma cleaner (100W power, 30Pa oxygen pressure) for 8 min. Subsequently, it was immersed in 1 mmol / L... -1 Soak sodium octyl sulfate in anhydrous ethanol / water (4:1 volume ratio) solution for 18 hours at room temperature in the dark. After soaking, rinse with anhydrous ethanol and deionized water, and dry with nitrogen gas for later use.

[0043] A chemically anchored functionalized interface was constructed by subjecting the substrate to oxygen plasma treatment and octyl sulfate self-assembly. This interface is mediated by -SO3 -The group strongly binds to the substrate metal site, and its hydrophobic octyl chain faces outward, which can directionally adsorb organic components in the precursor and guide MOF crystal growth, fundamentally enhancing adhesion.

[0044] S2. Preparation of the dual-ligand precursor solution: Weigh nickel nitrate hexahydrate (0.291 g, 1.0 mmol), 2,5-dihydroxyterephthalic acid (0.090 g, 0.5 mmol), and 2,3,6,7,10,11-hexahydroxytriphenylene (0.095 g, 0.25 mmol) into a mixed solvent of 20 mL N,N-dimethylformamide / N,N-dimethylacetamide and 4 mL deionized water. Sonicate at room temperature for 30 min, then magnetically stir for 12 h.

[0045] 2,5-Dihydroxyterephthalic acid was selected as the first ligand to provide abundant phenolic hydroxyl / quinone redox pairs, ensuring high optical contrast; 2,3,6,7,10,11-hexahydroxytriphenylene was selected as the second ligand, whose large planar π-conjugated structure can form efficient electronic channels within the MOF framework, improving conductivity and response speed.

[0046] S3. Electrostatic spraying deposition of precursor films: Functionalized FTO is fixed to the ground plane, 18 cm away from the high-pressure nozzle. A 20 kV DC voltage is applied, and the solution flow rate is set to 0.5 mL / h. Spraying is carried out in a room temperature environment with relative humidity <35% until a uniform wet film is formed. The sample is then transferred to an 80℃ hot stage for drying for 15 min to obtain a dry precursor film.

[0047] Electrostatic spraying with controlled flow rate and distance uniformly deposits a precursor solution onto a functionalized substrate. This process, conducted at room temperature and pressure, involves atomizing and charging droplets in an electric field, resulting in porous, uniform precursor films with excellent adhesion to the substrate, suitable for large-area fabrication.

[0048] S4. Steam-assisted in-situ crystallization: The sprayed sample was placed in a 100 mL PTFE liner, with 10 mL of LDM added to the bottom of the liner. It was sealed in a stainless steel reactor and placed in an oven for heat treatment at 105 °C for 10 h. After natural cooling, the sample was removed, gently rinsed with ethanol, and then vacuum dried at 60 °C for 6 h to obtain the final dual-ligand MOF film (Ni-DOBDC-HHTP).

[0049] The precursor film was subjected to low-temperature heat treatment in a DMF solvent vapor atmosphere. The solvent vapor softened the precursor, greatly promoting the molecular diffusion and slow, ordered coordination of metal ions and ligands. This allowed for the in-situ growth of highly crystalline, defect-free dual-ligand MOF films under conditions far removed from traditional solvothermal high-temperature and high-pressure conditions, while avoiding potential damage to the functionalized interface and substrate caused by high temperatures.

[0050] from Figure 1 As can be seen from the results, the MOF microstructure prepared in Example 1 has a complex, interwoven fibrous or network structure with an irregular shape, similar to cells or biological tissues, and has obvious texture and pores.

[0051] from Figure 4 As can be seen, the diffraction peaks are located at 8.0° and 12.2°, which correspond to the (110) and (300) lattice planes of Ni-IRMOF-74, respectively.

[0052] The optical transmittance and response speed of the prepared thin film in the 400-1000 nm wavelength range were studied by applying step voltages of +1.6 V (colored state) and -1.6 V (faded state) in a three-electrode system and a 1.0 mol / L LiClO4 / PC electrolyte. Figure 6 It can be seen that the prepared thin film exhibits an optical modulation range of 60.68% (780) and 74.00% (900 nm) in the visible and near-infrared bands, respectively.

[0053] Example 2: The difference between this embodiment and Embodiment 1 is that the metal salt in S2 is replaced with cobalt nitrate hexahydrate, and the molar ratio of the first ligand to the second ligand is adjusted to 1:0.5. The electrostatic spraying voltage is adjusted to 20kV. Everything else is the same as in Embodiment 1.

[0054] from Figure 2 As can be seen from the example, the MOF microstructure prepared in Example 2 has a highly irregular, wrinkled or sheet-like structure with dense and interwoven textures and no obvious regular patterns.

[0055] from Figure 5 As can be seen, the diffraction peaks are located at 8.2° and 12.6°, which correspond to the (110) and (300) lattice planes of Ni-IRMOF-74, respectively.

[0056] The optical transmittance curves and response speed of the prepared thin film in the 400-1000 nm wavelength range were studied by applying step voltages of +1.6 V (colored state) and -1.6 V (faded state) in a three-electrode system and a 1.0 mol / L LiClO4 / PC electrolyte. Figure 7 It can be seen that the prepared thin film exhibits an optical modulation range of 57.64% (780nm) and 66.00% (900nm) in the visible and near-infrared bands, respectively.

[0057] Example 3: The difference between this embodiment and Embodiment 1 is that the molar ratio of the transition metal salt, the first ligand, and the second ligand is 1:2:1; otherwise, it is the same as Embodiment 1.

[0058] Specific Embodiment Four: This embodiment differs from Specific Embodiments One, Two, or Three in that the second ligand is 2,3,6,7,10,11-hexahydroxybenzophenanthrene. Everything else is the same as in Specific Embodiment One.

[0059] Comparative Example 1 (Unfunctionalized substrate) Except for omitting the oxygen plasma treatment and sodium octyl sulfate soaking steps in S1 of Example 1, and only performing routine cleaning on FTO, the remaining steps are exactly the same as in Example 1.

[0060] Comparative Example 2 (Single Ligand MOF Thin Film) Except for the absence of 2,3,6,7,10,11-hexahydroxytriphenylbenzene in S2 of Example 1, and the use of only 2,5-dihydroxyterephthalic acid (0.180 g, 1.0 mmol), the remaining steps are the same as in Example 1 to prepare pure Ni-MOF-74 films.

[0061] from Figure 3 The SEM images show that the MOF prepared in comparison 2 is composed of a large number of dense, slender and irregular fibrous or rod-like structures. These structures overlap and interweave to form a rough and fluffy surface structure.

[0062] A comparison of the various performance parameters is shown in Table 1 below: Table 1

[0063] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. Those skilled in the art should understand that the present invention is not limited to the above embodiments. The embodiments and descriptions in the specification are merely illustrative of the principles of the invention. Various changes and modifications can be made to the invention without departing from its spirit and scope, and all such changes and modifications fall within the scope of the claimed invention.

Claims

1. A method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework, characterized in that, Includes the following steps: S1. Conductive substrate pretreatment and functionalization: ITO or FTO conductive substrates are ultrasonically cleaned and activated by oxygen plasma, and then immersed in sodium octyl sulfate in ethanol / water solution with a concentration of 0.5-5 mmol / L for 12-24 h at room temperature in the dark. After removal, the substrates are rinsed with ethanol and deionized water and dried to form a dense octyl sulfate self-assembled monolayer on the substrate surface. S2. Preparation of dual-ligand precursor solution: Dissolve the metal salt, the first ligand and the second ligand together in a mixed solvent composed of organic solvent and deionized water, disperse by ultrasonication for 30-60 min and stir continuously for 6-12 h to form a homogeneous and stable precursor solution. S3. Electrostatic spraying deposition of precursor film: The functionalized substrate obtained in step S1 is fixed on the grounding receiving plate of the electrostatic spraying equipment, and the precursor solution obtained in step S2 is prepared for spraying; the distance between the nozzle and the substrate is adjusted to 5-50cm, a DC high voltage of 10-30kV is applied, the flow rate of the precursor solution is controlled to 0.05-3.0mL / h, and spraying is carried out in an air environment with normal temperature and pressure to deposit a porous and uniform wet film on the surface of the functionalized substrate, and then dried at 60-80℃ for 10-30min to obtain a precursor dry film; S4. Steam-assisted in-situ crystallization: The substrate with the precursor dry film deposited is placed in a sealed container; then the sealed container is placed in an oven for heat treatment. After heat treatment, the substrate is allowed to cool naturally, then removed and washed with DMF and ethanol to remove unreacted substances. After drying, a crystalline dual-ligand MOF electrochromic film is obtained on the conductive substrate.

2. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In step S1, the conditions for plasma activation are: power 50-100W, oxygen pressure 10-50Pa, and processing time 5-15min.

3. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In step S2, the metal salt is one of nickel nitrate hexahydrate, cobalt nitrate hexahydrate, copper nitrate hexahydrate, or zinc nitrate hexahydrate; The total concentration of metal ions in the precursor solution is 0.05-0.15 mol / L.

4. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In S2, the volume ratio of organic solvent to deionized water in the mixed solvent is 4:1-10:1; The organic solvent is N,N-dimethylformamide or N,N-dimethylacetamide.

5. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In S2, the molar ratio of the first ligand to the second ligand is 1:0.2-1:1; the molar ratio of the total number of metal ions to the total number of functional groups available for coordination provided by the two ligands is 1:1-1.5:

1.

6. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, The first ligand is an aromatic polycarboxylic acid or a derivative thereof having multiple phenolic hydroxyl groups; The second ligand is a disc-shaped or rod-shaped molecule with a large planar π-conjugated structure; The second ligand is pre-connected to the first ligand via covalent bonds to form an organic linker unit.

7. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In S3, the preferred parameters for electrostatic spraying are: DC high voltage 15-25kV, supply flow rate 0.2-1.0mL / h, and nozzle-to-substrate distance 15-25cm.

8. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In S4, a sufficient amount of N,N-dimethylformamide solvent is placed at the bottom of the container to create a saturated solvent vapor atmosphere.

9. The method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework according to claim 1, characterized in that, In S4, the preferred conditions for the heat treatment are: heat treatment at 80-120℃ for 12-48 hours.

10. A method for preparing an electrochromic thin film based on a dual-ligand metal-organic framework as described in any one of claims 1-9, wherein the obtained dual-ligand MOF electrochromic thin film is based on interface modulation and electrostatic spraying.

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