Tungsten trioxide electrochromic film based on multi-tungsten oxygen cluster induced in-situ crystallization as well as preparation method and application of tungsten trioxide electrochromic film

Abstract

The invention discloses a tungsten trioxide electrochromic film for multi-tungsten oxygen cluster induced in-situ crystallization as well as a preparation method and application of the tungsten trioxide electrochromic film. The preparation method comprises the following steps: S1, adding hydrochloric acid into a sodium tungstate dihydrate aqueous solution to obtain a precipitate; washing the precipitate until the pH value of the supernate is greater than 3, and then drying the precipitate to obtain hydrated tungstic acid powder; dissolving the hydrated tungstic acid powder in a hydrogen peroxide solution to obtain a peroxy tungstic acid solution; s2, heating, evaporating and concentrating the peroxy tungstic acid solution to obtain a precursor solution containing multi-tungsten-oxygen clusters; and transferring the precursor solution containing the multi-tungsten-oxygen cluster to a substrate by adopting a film coating process, and annealing to obtain the tungsten trioxide electrochromic film with multi-tungsten-oxygen cluster induced in-situ crystallization, a matrix of the tungsten trioxide electrochromic film for multi-tungsten-oxygen cluster induced in-situ crystallization is amorphous tungsten trioxide, and hexagonal-phase tungsten trioxide nanocrystals are dispersed and distributed in the amorphous tungsten trioxide.

Description

Technical Field

[0001] The present invention relates to the technical field of electrochromic thin films, and particularly relates to a tungsten trioxide (WO3) electrochromic thin film induced by multi-tungsten oxygen clusters for in-situ crystallization, and a preparation method and application thereof. Background Art

[0002] Electrochromic devices are widely used in the fields of smart window glass, automotive anti-glare rearview mirrors, anti-counterfeiting labels, wearable devices, etc. because their optical properties (such as transmittance, reflectance) can undergo reversible and persistent changes under the stimulation of an externally applied small voltage, and have broad application prospects. As a typical inorganic electrochromic material, tungsten trioxide has a large optical modulation amplitude and a fast response rate, and thus has been widely studied.

[0003] At present, there are still many problems with tungsten trioxide materials. For example, although the crystalline tungsten trioxide thin film has good cycle stability, its dense structure leads to a low ion transport rate, so the switching response time is long. The pore structure inside the amorphous tungsten trioxide is richer for ion transport and has good optical modulation ability. However, the unstable amorphous structure is prone to collapse or expand during long-term electrochemical cycling and cannot meet the actual application requirements. The existing literature (Van T N, Anh K H, Van QL, et al. Highly stable electrochromic cells based on amorphous tungsten oxides prepared using a solution-annealing process[J]. International Journal of Energy Research, 2021, 45(5):8061-8072.) achieved the regulation of the crystallization morphology of the thin film by changing the annealing temperature. The study found that the amorphous thin film annealed at 250 °C exhibited the best electrochromic performance, but its optical contrast at 633 nm was only 59%. This is mainly because simply controlling the crystal state by temperature cannot precisely control the microscopic structural transformation, so the electrochromic performance of the thin film cannot be further optimized. In order to accurately control the crystallization state of the thin film, another literature (Yuanyang Z, Xiaoping L, Tao J, et al. Amorphous / crystalline WO3 dual phase laminated films: Fabrication, characterization and evaluation of their electrochromic performance for smart window applications[J]. Solar Energy Materials and Solar Cells, 2022, 244:111820) used the sol-gel method to prepare a crystalline tungsten trioxide substrate, and then electrochemically deposited an amorphous tungsten trioxide on the substrate to prepare a crystalline / amorphous composite thin film by a two-step method. Due to the synergistic effect of the crystalline and amorphous states, the tungsten trioxide thin film exhibits excellent electrochromic performance, such as a high optical contrast (reaching 70.6% at 633 nm) and coloring efficiency (53.6 cm at 633 nm 2 / C). However, the two-step method often has complex steps and is not conducive to large-scale popularization and application. Therefore, how to precisely induce the crystalline phase of tungsten trioxide by a one-step method, control the crystallinity and grain size, and prepare tungsten trioxide thin films with large optical modulation ability, fast color change speed and excellent cycle stability is crucial for promoting practical applications. Summary of the Invention

[0004] In view of the above technical problems and deficiencies in the art, the present invention provides a tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters for in-situ crystallization, and its preparation method and application. The present invention proposes to introduce polyoxotungstate clusters into the precursor solution to effectively increase the viscosity of the solution, and then spin-coat to obtain a tungsten trioxide thin film with a suitable thickness. At the same time, the polyoxotungstate clusters act as nucleation centers for crystallization and nucleation, and will preferentially induce the precipitation of hexagonal tungsten trioxide crystals, which are thermodynamically metastable phases. Compared with other crystal phases such as monoclinic phase, hexagonal tungsten trioxide has a unique octahedral pore structure and a lower ion transport barrier, thus successfully preparing an amorphous tungsten trioxide thin film embedded with nanocrystals. This unique structure has a large number of large pore channels that facilitate ion and electron transport, improving the electrochemical activity of the material, thereby realizing rapid ion insertion and extraction. In addition, the hexagonal phase nanocrystals also play a pinning role, effectively stabilizing the amorphous structure and improving the electrochemical cycle stability of the material.

[0005] The specific technical solutions are as follows:

[0006] In the first aspect, the present invention provides a method for preparing a tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters for in-situ crystallization, including the steps of:

[0007] S1: Add hydrochloric acid to an aqueous solution of sodium tungstate dihydrate to obtain a precipitate; wash the precipitate until the pH of the supernatant is greater than 3, and then dry the precipitate to obtain hydrated tungstic acid powder; dissolve the hydrated tungstic acid powder in hydrogen peroxide solution to obtain a peroxotungstic acid solution;

[0008] S2: Heat and evaporate the peroxotungstic acid solution to concentrate it to obtain a precursor solution containing polyoxotungstate clusters; use a coating process (such as spin coating, printing, dip coating, etc.) to transfer the precursor solution containing polyoxotungstate clusters onto a substrate, and anneal to obtain a tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters for in-situ crystallization;

[0009] The matrix of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters for in-situ crystallization is amorphous tungsten trioxide, in which hexagonal tungsten trioxide nanocrystals are dispersedly distributed.

[0010] The present invention first prepares an electrochromic precursor solution, and the polyoxotungstate clusters therein can be adjusted to different polymerization degrees by controlling the temperature and / or time of heating evaporation and concentration. Further, the present invention realizes the preparation of an amorphous tungsten trioxide thin film embedded with hexagonal phase nanocrystals through the precise in-situ crystallization induced by polyoxotungstate clusters. The preparation method has a simple process, is green and environmentally friendly, and realizes the preparation of high-performance WO3 electrochromic thin films at low cost.

[0011] The present invention first introduces polyoxotungstate clusters into the precursor solution, thereby initially forming an amorphous tungsten trioxide thin film with diffusely distributed polyoxotungstate clusters. Thereafter, the polyoxotungstate clusters in the amorphous tungsten trioxide thin film matrix can induce crystallization at a relatively low temperature (lower than the conventional crystallization temperature of amorphous tungsten trioxide) to form nano tungsten trioxide grains, which have a hexagonal crystal form. At the same time, due to the relatively low crystallization temperature, the growth of grains can be effectively inhibited.

[0012] Preferably, in step S1, the mass fraction of the hydrochloric acid is 36% - 38%.

[0013] Preferably, in step S1, the temperature of the sodium tungstate dihydrate aqueous solution is 0 - 10 °C.

[0014] Preferably, in step S1, the volume ratio of the hydrochloric acid to the sodium tungstate dihydrate aqueous solution is 1 - 0.1:1.

[0015] Preferably, in step S1, the concentration of the sodium tungstate dihydrate aqueous solution is 0.5 - 2 M.

[0016] Preferably, in step S1, the mass fraction of the hydrogen peroxide solution is 10% - 50%, such as 30%, etc.

[0017] Preferably, in step S1, the concentration of the peroxotungstic acid solution is 0.1 - 2 M, such as 0.6 M, etc.

[0018] In step S2, the viscosity of the precursor solution containing polyoxotungstate clusters can be adjusted by changing the temperature and time of heating evaporation and concentration, so as to be suitable for conventional coating film processes such as spin coating, printing, dip coating, etc.

[0019] Preferably, in step S2, the heating temperature is 50 - 100 °C, such as 80 °C, etc.

[0020] Preferably, in step S2, the heating is water bath heating.

[0021] Preferably, in step S2, the heating time is 3 - 24 h, such as 6 h, 7 h, 8 h, etc.

[0022] Preferably, in step S2, the end point of concentration is 30% - 100% of the initial volume of the peroxotungstic acid solution, excluding 100%.

[0023] Preferably, in step S2, the substrate is a conductive substrate. More preferably, the conductive substrate is a fluorine-doped tin oxide substrate or an indium tin oxide substrate or other transparent conductive substrates.

[0024] The temperature and time for heating and evaporating and concentrating the peroxotungstic acid solution may affect the subsequent crystallization temperature. For example, when the heating and evaporation concentration time of the peroxotungstic acid solution is prolonged, the radius and quantity of the polytungsten oxygen clusters will also increase, and the crystallization temperature will decrease, resulting in a decrease in the temperature required for annealing. Preferably, in step S2, the annealing temperature is 150 - 450 °C, such as 200 °C, 250 °C, 300 °C, 350 °C, 400 °C, etc., and the time is 0.5 - 6 h, such as 1.5 h, etc.

[0025] In a second aspect, the present invention provides a tungsten trioxide electrochromic thin film with in-situ crystallization induced by polytungsten oxygen clusters prepared by the preparation method described in the first aspect.

[0026] The present invention uses polytungsten oxygen clusters as nucleation centers to preferentially induce the precipitation of hexagonal tungsten trioxide nanocrystals. The amorphous structure inlaid with hexagonal nanocrystals has rich ion and electron transport paths and good electrochemical cycling stability.

[0027] The tungsten trioxide electrochromic thin film with in-situ crystallization induced by polytungsten oxygen clusters of the present invention has a large optical modulation ability, a fast color change speed, and excellent cycling stability in the visible light and near-infrared regions.

[0028] Furthermore, the thickness of the tungsten trioxide electrochromic thin film with in-situ crystallization induced by polytungsten oxygen clusters can be 100 - 600 nm, and can be obtained by spin coating once or more than twice.

[0029] In a third aspect, the present invention provides the application of the tungsten trioxide electrochromic thin film with in-situ crystallization induced by polytungsten oxygen clusters described in the second aspect in the preparation of electrochromic materials and devices. Furthermore, the devices include but are not limited to smart glass windows, automotive anti-glare rearview mirrors, anti-counterfeiting labels, wearable devices, etc.

[0030] The present invention solves the problems such as insufficient viscosity of the electrochromic coating solution, difficult control of the coating thickness, difficult control of the grain size and crystal form during the crystallization process of the amorphous tungsten trioxide thin film, and weak bonding between the amorphous tungsten oxide thin film and the conductive substrate. Compared with the prior art, the beneficial effects of the present invention are:

[0031] (1) The present invention provides a novel electrochromic coating liquid, which is a precursor liquid containing polyoxotungstate clusters. By heating the peroxotungstic acid solution, in this process, the octahedral network of peroxotungstic acid undergoes processes such as losing crystal water, depolymerization, and concentration into polynuclear tungstates, i.e., polyoxotungstate clusters. According to different heating evaporation concentration temperatures and times, the viscosity of the precursor liquid can be adjusted (without adding organic additives, etc., to increase the viscosity of the coating liquid), making it suitable for conventional coating processes such as spin coating, printing, and dip coating.

[0032] (2) The novel electrochromic coating liquid has the advantages of simple coating and easy control of film thickness.

[0033] (3) The process method for preparing a polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic film of the present invention is simple, and high-efficiency production can be achieved under normal pressure conditions. Polyoxotungstate clusters are introduced into the precursor liquid, and tungsten trioxide crystallization is induced by the polyoxotungstate clusters, realizing the preparation of an amorphous tungsten trioxide film embedded with hexagonal phase nanocrystals. The main amorphous structure and the large pores of the hexagonal phase provide a transmission path for the transmission of ions / electrons, improving the electrochemical activity of the material. The nanocrystals distributed in the amorphous structure also play a pinning role, improving the electrochemical cycling stability. The high interface contact area and the low interface diffusion barrier in the crystalline / amorphous composite structure reduce the hindrance of ion transport and accelerate the color change speed. The polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic film prepared by the present invention has a strong binding force with the conductive substrate and will not easily fall off during the production and device preparation processes, which is beneficial for practical commercial applications. Description of the Drawings

[0034] Figure 1 It is a digital photo of the color change of the solution during the process of preparing a polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic film in Example 1 with different water bath heating evaporation concentration times.

[0035] Figure 2 It is a transmission electron microscope (TEM) photo of the polyoxotungstate cluster-induced tungsten trioxide electrochromic film obtained at different annealing temperatures, where: (a) is the TEM photo of the polyoxotungstate cluster-induced tungsten trioxide electrochromic film in Example 2, (b) is the TEM photo of the tungsten trioxide electrochromic film prepared in Example 1, (c) is the TEM photo of the tungsten trioxide electrochromic film prepared in Example 3, and (d) is the TEM photo of the tungsten trioxide electrochromic film prepared in Example 4.

[0036] Figure 3 It is an X-ray diffraction (XRD) pattern of the polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic film prepared in Example 1.

[0037] Figure 4It is the X-ray photoelectron spectroscopy (XPS) diagram of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared in Example 1 for in-situ crystallization, where: (a) is the fine spectrum of tungsten element, and (b) is the fine spectrum of oxygen element.

[0038] Figure 5 It is the SEM photograph of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared in Example 1 for in-situ crystallization, where: (a) is the surface SEM photograph, and (b) is the cross-section SEM photograph.

[0039] Figure 6 It is the transmittance curve spectrum of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared in Example 1 for in-situ crystallization at different voltages.

[0040] Figure 7 It is the in-situ transmittance spectrum of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared in Example 1 for in-situ crystallization at 633 nm and the relationship diagram between optical density and charge density, where: (a) is the in-situ transmittance spectrum at 633 nm, and (b) is the curve of optical density changing with charge density.

[0041] Figure 8 It is the in-situ optical transmittance change spectrum at 633 nm of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared in Example 1 for in-situ crystallization under the cyclic voltage of -0.2 v and +0.7 v for 1000 cycles. Detailed implementation manners

[0042] The present invention will be further described below in conjunction with the accompanying drawings and specific embodiments. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention. For the operation methods without specific conditions noted in the following embodiments, they are usually carried out under conventional conditions or according to the conditions recommended by the manufacturer. In the following embodiments and test examples, the raw materials and preparation methods used are conventional materials and technologies in the art unless otherwise specified.

[0043] Example 1:

[0044] Example 1 provides a preparation method of a tungsten trioxide electrochromic film induced by polyoxotungstate clusters and prepared by in-situ crystallization. The preparation steps are as follows:

[0045] In this example, fluorine-doped tin oxide (FTO) with a size of 30 mm × 30 mm is used as the substrate. Before coating, the following treatments are carried out first: ultrasonic cleaning with deionized water and isopropanol for 30 min each, then drying at 60 °C, and surface plasma treatment for 120 s before use.

[0046] S1: Under ice bath conditions, add 10 mL of hydrochloric acid (12 M) to 30 mL of an aqueous sodium tungstate dihydrate solution (0.67 M) to form a pale yellow precipitate. Add 500 mL of deionized water to the pale yellow precipitate at room temperature, stir vigorously for 2 min, then let it stand for 30 min, and take the lower layer precipitate. Repeat the operation until the pH of the supernatant after precipitation is greater than 3. Dry the precipitate at 60 °C for 12 h to obtain white hydrated tungstic acid powder.

[0047] S2: Dissolve 3 g of the hydrated tungstic acid powder obtained in step S1 in 20 mL of a 30% hydrogen peroxide solution by mass fraction, stir at a speed of 450 rpm at room temperature for 1 h to obtain a clear and transparent peroxotungstic acid solution with a concentration of 0.6 M. Heat the peroxotungstic acid solution in a water bath at 80 °C for 6 h to obtain a pale yellow precursor solution containing polyoxotungstate clusters. Concentrate the precursor solution to 30% of its initial volume at 80 °C to obtain a precursor solution with a higher concentration of polyoxotungstate clusters.

[0048] Spin-coat the precursor solution with a higher concentration of polyoxotungstate clusters obtained in step S2 on a pretreated FTO glass substrate by a two-step method (the spin-coating speeds and times for the first and second steps are 600 rpm, 60 s and 1500 rpm, 10 s respectively) to obtain a tungsten trioxide thin film. Then anneal it at 300 °C for 1.5 h to obtain a polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic thin film.

[0049] The polyoxotungstate cluster-induced in-situ crystallization tungsten trioxide electrochromic thin film of this example is prepared by the above method.

[0050] Example 2:

[0051] The difference from Example 1 is only that the annealing temperature is changed from 300 °C to 250 °C, and the rest are the same.

[0052] Example 3:

[0053] The difference from Example 1 is only that the annealing temperature is changed from 300 °C to 350 °C, and the rest are the same.

[0054] Example 4:

[0055] The difference from Example 1 is only that the annealing temperature is changed from 300 °C to 400 °C, and the rest are the same.

[0056] Test Example 1:

[0057] Analysis of the formation of polyoxotungstate clusters and the crystallization process of the induced tungsten trioxide electrochromic thin film:

[0058] 1) Analysis of the formation process of polyoxotungstate clusters.

[0059] Figure 1Digital photos of the color changes of solutions with different water bath heating and concentration times during the preparation of tungsten trioxide electrochromic films induced by in-situ crystallization of polyoxotungstate clusters in Example 1. Hydrated tungstic acid reacts with hydrogen peroxide to form peroxotungstic acid soluble in aqueous solution, so the initial state of the solution is clear and transparent. As the evaporation concentration time of the water bath heating increases, it can be found that the color of the solution gradually changes from transparent to dark yellow, indicating that peroxotungstic acid decomposes (opening of W-W and W-O2) and recombines into polynuclear polyoxotungstate clusters with larger radii. The reaction equations occurring in this process are as follows:

[0060] Equation 1: H2WO4·nH2O + H2O2 → [WO2(O2)(H2O)]·nH2O + H2O

[0061] Equation 2: xWO2(O2)H2O → [W x O (4x-2y) m- + xH2O + yO2↑

[0062] Among them, [W x O (4x-2y) m- in Equation 2 is a polynuclear tungstate ion (or polynuclear polyoxotungstate cluster, or polyoxotungstate cluster). Among them, x is the degree of aggregation of tungsten ions. The larger the x value, the greater the degree of aggregation, and the larger the radius of the polynuclear polyoxotungstate cluster, making it easier to become a nucleation center. According to the length of the heating evaporation concentration time, a precursor solution containing polyoxotungstate clusters with different polymerization degrees and distributions can be obtained, thereby adjusting the viscosity of the precursor solution; after the precursor solution containing polyoxotungstate clusters with various polymerization degrees is spin-coated into a film, an initially formed amorphous tungsten trioxide film is obtained, in which polyoxotungstate clusters with a higher polymerization degree are diffusely distributed and can serve as nucleation centers for inducing crystallization.

[0063] 2) Analysis of the crystallization process of tungsten trioxide electrochromic films induced by polyoxotungstate clusters.

[0064] The crystallization process of tungsten trioxide electrochromic films induced by polyoxotungstate clusters was observed using TEM, as Figure 2 shown.

[0065] As Figure 2 shown in a, no nanocrystals were found in the tungsten trioxide electrochromic film at 250 °C. As Figure 2 shown in b, at 300 °C, polyoxotungstate clusters act as nucleation centers to induce the precipitation of metastable hexagonal phase nanocrystalline particles in the tungsten trioxide electrochromic film, forming a nanocrystal-embedded amorphous film. As Figure 2 shown in c, at 350 °C, the grains further grow and the area of the amorphous region decreases. As Figure 2 ​​As shown in d, when the temperature reaches 400 °C, the crystal phase induced by the polyoxotungstate cluster transforms into a more stable monoclinic phase, and crystallization occurs in most regions.

[0066] Experimental Example 2:

[0067] Composition and surface morphology analysis of tungsten trioxide electrochromic thin films induced by polyoxotungstate clusters through in-situ crystallization.

[0068] The crystal phase of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 was analyzed by X-ray diffraction (XRD), as Figure 3 shown. The valence states of the elements in the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 were analyzed by X-ray photoelectron spectroscopy (XPS), as Figure 4 shown. The surface morphology of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 was analyzed by scanning electron microscopy (SEM), as Figure 5 shown.

[0069] Figure 3 is the XRD pattern of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1. It can be observed that hexagonal tungsten trioxide is precipitated at this time. There are also broad diffraction peaks of amorphous in the pattern, which is consistent with the TEM results in Experimental Example 1.

[0070] Figure 4 is the XPS pattern of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1, Figure 4 a is the fine spectrum of tungsten element, Figure 4 b is the fine spectrum of oxygen element. Observing Figure 4 a, it can be seen that W in the thin film exists in the highest valence state, that is, W 6+ form. From Figure 4 b, it can be seen that there are oxygen vacancies O 2- in the thin film.

[0071] Figure 5 is the SEM image of the tungsten trioxide electrochromic thin film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1, Figure 5 a is the surface SEM image, Figure 5 b is the cross-section SEM image. From Figure 5 a, it can be obtained that the surface of the thin film is smooth, and there are many cracks caused by annealing thermal stress. From Figure 5 b, it can be obtained that the combination of the thin film and the substrate is good, and the thin film is very dense without obvious grain precipitation.

[0072] Experimental Example 3

[0073] Performance characterization of tungsten trioxide electrochromic thin films induced by polyoxotungstate clusters through in-situ crystallization.

[0074] The electrochemical and electrochromic properties of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization were tested using an electrochemical workstation with a three-electrode system and a UV-Vis spectrophotometer.

[0075] Using 0.5 M sulfuric acid as the electrolyte, the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 was used as the working electrode, Ag / AgCl as the reference electrode, and a platinum foil as the counter electrode. Voltages of -0.2 V and +0.7 V were applied to test the transmittance change of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization in the wavelength range of 250 - 1100 nm. See Figure 6 。

[0076] A periodic square wave voltage with an interval of 120 s between -0.2 V and +0.7 V was applied to the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 to test the in-situ transmittance spectrum of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization at 633 nm and the relationship between the optical density and the charge density. See Figure 7 。

[0077] A periodic square wave voltage with an interval of 30 s between -0.2 V and +0.7 V was applied to the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 to test the optical cycling stability of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization at 633 nm. See Figure 8 。

[0078] Figure 6 It is the transmittance curve map of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 at different voltages. It can be observed that the transmittance change of the film at the colored state and the bleached state at 633 nm reached 83%.

[0079] Figure 7 a is the in-situ transmittance spectrum of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters through in-situ crystallization prepared in Example 1 at 633 nm under voltages of -0.2 V and 0.7 V, Figure 7 b is the curve of the optical density changing with the charge density. From Figure 7 a, it can be seen that the coloring time and the bleaching time of the film are 11.5 s and 7 s respectively. From Figure 7 b, the coloring efficiency reaches 68.3 cm 2 / C.

[0080] Figure 8In-situ optical transmittance change spectra at 633 nm of the tungsten trioxide electrochromic film induced by polyoxotungstate clusters prepared in Example 1 under the application of a periodic square wave voltage of -0.2 V and 0.7 V for 1000 cycles. From Figure 8 It can be seen that after 1000 cycles of electrochemical cycling, only 5.8% of the optical modulation range is lost. Thus, the tungsten trioxide electrochromic film induced by polyoxotungstate clusters prepared in Example 1 has a large optical modulation ability, a fast color change speed, and excellent cycle stability.

[0081] The tungsten trioxide electrochromic film induced by polyoxotungstate clusters prepared in Example 1 of the present invention has excellent electrochromic properties compared with other examples (Example 2 has almost no crystallization, Example 3 has grain growth, and Example 4 has a monoclinic tungsten trioxide crystal phase). For example, in a 0.5 M H2SO4 solution electrolyte, the transmittance change at 633 nm reaches 83%, the coloring time and the fading time are 11.5 s and 7 s, the coloring efficiency is 68.32 cm 2 / C, and it still maintains 94.2% of the initial optical modulation range after 1000 cycles of electrochemical cycling.

[0082] In addition, it should be understood that after reading the above description of the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

Claims

1. A preparation method of a tungsten trioxide electrochromic film induced by multi-tungsten oxygen clusters for in-situ crystallization, characterized in that, It includes the steps: S1: Add hydrochloric acid to an aqueous solution of sodium tungstate dihydrate to obtain a precipitate; wash the precipitate until the pH of the supernatant is greater than 3, and then dry the precipitate to obtain hydrated tungstic acid powder; dissolve the hydrated tungstic acid powder in hydrogen peroxide solution to obtain a peroxotungstic acid solution; S2: Heat and evaporate the peroxotungstic acid solution to concentrate it to obtain a precursor solution containing polyoxotungstate clusters; Adopt a coating process to transfer the precursor solution containing polyoxotungstate clusters onto a substrate and anneal it to obtain a tungsten trioxide electrochromic film with polyoxotungstate cluster-induced in-situ crystallization; The matrix of the tungsten trioxide electrochromic film with polyoxotungstate cluster-induced in-situ crystallization is amorphous tungsten trioxide, in which hexagonal tungsten trioxide nanocrystals are diffusely distributed.

2. The preparation method according to claim 1, characterized in that, In step S1: The mass fraction of the hydrochloric acid is 36% - 38%; The temperature of the aqueous solution of sodium tungstate dihydrate is 0 - 10 °C; The volume ratio of the hydrochloric acid to the aqueous solution of sodium tungstate dihydrate is 1 - 0.1:1; The concentration of the aqueous solution of sodium tungstate dihydrate is 0.5 - 2 M.

3. The preparation method according to claim 1, characterized in that, In step S1: The mass fraction of the hydrogen peroxide solution is 10% - 50%; The concentration of the peroxotungstic acid solution is 0.1 - 2 M.

4. The preparation method according to claim 1, characterized in that, In step S2: The temperature of the heating is 50 - 100 °C; The heating is water bath heating; The time of the heating is 3 - 24 h; The end point of concentration is 30% - 100% of the initial volume of the peroxotungstic acid solution, excluding 100%.

5. The preparation method according to claim 1, wherein In step S2, the substrate is a conductive substrate, and the conductive substrate is a fluorine-doped tin oxide substrate or an indium tin oxide substrate or other transparent conductive substrates.

6. The preparation method according to claim 1, characterized in that In step S2, the temperature of the annealing is 150 - 450 °C, and the time is 0.5 - 6 h.

7. A tungsten trioxide electrochromic film with polyoxotungstate cluster-induced in-situ crystallization prepared by the preparation method according to any one of claims 1 - 6.

8. The tungsten trioxide electrochromic film induced by polyoxotungstate clusters for in-situ crystallization according to claim 7, wherein The thickness of the tungsten trioxide electrochromic film with polyoxotungstate cluster-induced in-situ crystallization is 100 - 600 nm.

9. Application of the tungsten trioxide electrochromic film with polyoxotungstate cluster-induced in-situ crystallization according to claim 7 or 8 in the preparation of electrochromic materials and devices.

10. The application according to claim 9, wherein The devices include smart glass windows, automotive anti-glare rearview mirrors, anti-counterfeiting labels, and wearable devices.

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