A tungsten oxide / titanium oxide heterogeneous composite electrochromic film and a one-step hydrothermal preparation method thereof

Abstract

This invention discloses a tungsten oxide / titanium oxide (WO₃ / TiO₂) heterogeneous composite electrochromic thin film and its one-step hydrothermal preparation method, belonging to the fields of functional materials and optoelectronic technology. The method first involves heating tungstic acid and hydrogen peroxide to form a peroxytungstic acid solution, which is then mixed with acetonitrile and hydrochloric acid to prepare a stable precursor solution. Tetrabutyl titanate is pre-reacted with hydrochloric acid to obtain a titanium source solution. The two are then mixed in a specific ratio to prepare a composite precursor solution. A thin film is directly deposited on a conductive substrate using this precursor solution via a one-step hydrothermal reaction, followed by annealing to obtain the WO₃ / TiO₂ heterogeneous composite thin film. This invention achieves the synergistic growth and uniform composite of WO₃ and TiO₂ during the hydrothermal process through precise design of the precursor solution system. The process is simple, low-cost, and has good reproducibility. The resulting thin film exhibits uniform two-phase distribution and a distinct heterojunction structure, combining high optical modulation amplitude, fast response, and excellent cycling stability. The optical modulation amplitude can reach over 45% at a wavelength of 633 nm, and the retention rate exceeds 70% after 1000 cycles, making it suitable for high-performance electrochromic devices such as smart windows and displays.

Description

Technical Field

[0001] This invention belongs to the field of functional materials and optoelectronic technology, specifically relating to an electrochromic thin film and its preparation method, and more particularly to a WO3 / TiO2 thin film with a heterogeneous composite structure and its controllable synthesis method via a one-step hydrothermal method. Background Technology

[0002] Electrochromic materials can undergo reversible changes in optical properties (such as transmittance, reflectance, and absorptivity) under the influence of an electric field, and have broad application prospects in fields such as smart windows, anti-glare rearview mirrors, and energy-saving displays. WO3, as a typical cathodic electrochromic material, has the advantages of high coloring efficiency and relatively low cost, but its cycle stability and ion diffusion kinetics still have room for improvement. TiO2 has good chemical stability and excellent ion transport characteristics, and is often used as a supporting framework or composite component of electrochromic materials to improve overall performance.

[0003] Currently, the main methods for preparing WO3 / TiO2 composite films include layer-by-layer deposition, stepwise deposition (such as depositing TiO2 nanostructures first, then loading WO3), or physical mixing. These methods are typically cumbersome, may have insufficient interfacial bonding, and struggle to achieve uniform and tight composites of the two phases at the nanoscale. While the one-step hydrothermal method is simple, directly mixing the two metal precursors can easily lead to phase separation due to their different hydrolysis and polymerization rates, making it difficult to obtain heterogeneous composite films with uniform structure and controllable properties, especially in terms of precisely controlling the molar ratio and composite morphology of the two phases.

[0004] Therefore, developing a simple, low-cost preparation method that can achieve uniform and stable composite of WO3 and TiO2 at the molecular / nanoscale and precisely control their composition and structure is of great significance for obtaining high-performance WO3 / TiO2 heterogeneous composite electrochromic films. Summary of the Invention

[0005] The technical problem to be solved by this invention: To address the problems of complex processes, uneven two-phase composite, weak interfacial bonding, and difficulty in precisely controlling composition in existing WO3 / TiO2 composite film preparation methods, this paper provides a simple and highly controllable one-step hydrothermal preparation method, as well as a WO3 / TiO2 heterogeneous composite film with excellent electrochromic properties prepared by this method.

[0006] The technical solution of the present invention: This invention provides a one-step hydrothermal preparation method for a WO3 / TiO2 heterogeneous composite electrochromic thin film, characterized by comprising the following steps: S1. Preparation of precursor solution: First, tungstic acid, hydrogen peroxide, and deionized water are mixed and heated and stirred until clear to form a peroxytungstic acid solution; then, deionized water, acetonitrile, and concentrated hydrochloric acid are added to the solution in sequence and stirred evenly to obtain a clear and stable peroxytungstic acid precursor solution A with a pH of approximately 1.5-2.5; then, tetrabutyl titanate is mixed with an acidic medium composed of concentrated hydrochloric acid and deionized water and stirred thoroughly until the reaction is complete to obtain a titanium source solution B; finally, the peroxytungstic acid precursor solution A and the titanium source solution B are mixed at a certain volume ratio and stirred thoroughly to obtain a WO3 / TiO2 heterogeneous composite film precursor solution C; wherein, by adjusting the amount of tetrabutyl titanate added to the titanium source solution B, the molar ratio of W to Ti in the precursor solution C is adjustable within the range of 1:(0.1-2.0); S2. Thin film deposition: Place the cleaned conductive substrate (preferably FTO conductive glass) with the conductive side facing down in a hydrothermal reactor at an angle; transfer an appropriate amount of the WO3 / TiO2 heterogeneous composite thin film precursor solution C prepared in step S1 into the reactor, so that the liquid level submerges most of the conductive surface of the substrate; seal the reactor and place it in an oven at 160-200℃ for 1-4 hours. S3. Post-processing: After the reaction is complete, the conductive substrate is removed, cleaned and dried, and then annealed in air at 250-400℃ for 1-3 hours to obtain the WO3 / TiO2 heterogeneous composite electrochromic film.

[0007] Further, in step S1, the preparation of the peroxytungstic acid precursor solution A is as follows: 0.4-0.6 g of tungstic acid is mixed with 10-14 mL of deionized water, and 3-5 mL of hydrogen peroxide is added dropwise while stirring. The mixture is heated and stirred at 90-100°C until clear. Then, 20-28 mL of deionized water, 50-60 mL of acetonitrile, and 3-5 mL of concentrated hydrochloric acid are added sequentially.

[0008] Furthermore, in step S1, when preparing titanium source solution B, the volume ratio of concentrated hydrochloric acid to deionized water is (0.5-2):1.

[0009] Furthermore, in step S2, the conductive substrate is FTO or ITO conductive glass.

[0010] Furthermore, in step S2, the hydrothermal reaction temperature is 170-190℃, and the reaction time is 1.5-2.5 hours.

[0011] Furthermore, in step S3, the annealing temperature is 280-320℃ and the annealing time is 1.5-2.5 hours.

[0012] The present invention also provides a WO3 / TiO2 heterogeneous composite electrochromic thin film prepared according to any of the above methods.

[0013] Furthermore, the film is a nanostructured film directly grown on a conductive substrate, in which WO3 and TiO2 exist in a heterogeneous composite form, the two phases are uniformly distributed, and the molar ratio of W to Ti is 1:(0.1-2.0).

[0014] Furthermore, the optical modulation amplitude of the film at a wavelength of 633 nm is not less than 45%, and after 1000 coloring / fading cycles, the optical modulation amplitude retention rate is above 70%.

[0015] Compared with the prior art, the present invention has the following advantages: 1. Innovative Process, One-Step Synthesis: This invention utilizes a carefully designed precursor solution system, mixing a stable peroxytungstic acid solution with a pre-hydrolyzed tetrabutyl titanate acidic solution to obtain a homogeneous and stable composite precursor solution. This successfully enables the direct synthesis of WO3 / TiO2 heterogeneous composite films via a one-step hydrothermal method. This method simplifies the process, reduces preparation costs, and avoids interface contamination and adhesion problems that may arise from stepwise deposition.

[0016] 2. Uniform Composite and Controllable Structure: Due to the uniformity and stability of the precursor solution C, W and Ti species can synergistically nucleate and grow during the hydrothermal process, thereby achieving a uniform and compact composite of WO3 and TiO2 phases at the nanoscale, forming an effective heterojunction structure. By simply adjusting the amount of titanium source added, the W / Ti molar ratio in the final film can be precisely controlled, achieving controllable synthesis of composition and structure.

[0017] 3. Excellent performance: The resulting heterogeneous composite film combines the advantages of WO3's high coloring ability and TiO2's excellent stability and ion transport characteristics. The introduction of TiO2 optimizes the film's microstructure and electron / ion transport path, thereby significantly improving the film's overall electrochromic performance, manifested as high optical modulation amplitude, improved response speed, and significantly enhanced cycling stability.

[0018] 4. Good reproducibility and easy scale-up: The method has clear process parameters, simple operation, and good reproducibility, providing a reliable way for the large-scale preparation of high-performance WO3 / TiO2 heterogeneous composite electrochromic films. Figure 1 The images show the XRD patterns of WO3 / TiO2 heterogeneous composite films with different W / Ti molar ratios prepared in Examples 1, 2, and 3 of this invention. Figure 2 The images show the SEM surface morphology and cross-sectional view of the WO3 / TiO2 heterogeneous composite film prepared in Example 1 of this invention. Figure 3 The static optical modulation amplitude at wavelengths of 300-1100 nm is given by the thin films prepared in the colored / faded states of Examples 1, 2, 3 and the comparative examples of the present invention. Figure 4 The curves showing the in-situ transmittance of the films prepared in Examples 1, 2, 3 and the comparative examples of the present invention as a function of time (coloring / fading kinetic curves) at a wavelength of 633 nm. Figure 5 The graph shows a comparison of the optical modulation amplitude retention rates of the thin films prepared in Examples 1, 2, 3 and the comparative examples of the present invention after 1000 cycles of testing.

Claims

1. A one-step hydrothermal preparation method of WO3 / TiO2 heterogeneous composite electrochromic thin film, characterized in that, Includes the following steps: S1. Preparation of precursor solution: Prepare peroxytungstate precursor solution A; prepare an acidic solution of tetrabutyl titanate as titanium source solution B; mix precursor solution A and titanium source solution B and stir evenly to obtain WO3 / TiO2 heterogeneous composite film precursor solution C. S2. Thin film deposition: Using a conductive substrate as the substrate, the precursor solution C is subjected to a hydrothermal reaction to deposit a thin film on the substrate; S3. Post-processing: The substrate after hydrothermal reaction is cleaned and annealed to obtain the WO3 / TiO2 heterogeneous composite electrochromic film.

2. The method of claim 1, wherein, In step S1, the preparation method of the peroxytungstic acid precursor solution A includes: mixing tungstic acid, deionized water and hydrogen peroxide, heating and stirring until clear, and then adding deionized water, acetonitrile and concentrated hydrochloric acid.

3. The method of claim 2, wherein, In the peroxytungstic acid precursor solution A, the ratio of tungstic acid, hydrogen peroxide, acetonitrile, and concentrated hydrochloric acid is (0.4-0.6)g:(3-5)mL:(50-60)mL:(3-5)mL.

4. The method of claim 1, wherein, In step S1, the titanium source solution B is prepared by mixing and stirring tetrabutyl titanate with an acidic medium composed of concentrated hydrochloric acid and deionized water, wherein the volume ratio of concentrated hydrochloric acid to deionized water is (0.5-2):

1.

5. The method of claim 1, wherein, In step S1, the amount of tetrabutyl titanate added to the titanium source solution B is adjusted so that the molar ratio of W to Ti in the precursor solution C is in the range of 1:(0.1-2.0).

6. The method according to claim 1, characterized in that, In step S2, the hydrothermal reaction is carried out at a temperature of 160-200℃ for 1-4 hours. In step S3, the annealing is carried out in an air atmosphere at a temperature of 250-400℃ for 1-3 hours.

7. A WO3 / TiO2 heterogeneous composite electrochromic thin film prepared by the method according to any one of claims 1 to 6.

8. The thin film according to claim 7, characterized in that, The molar ratio of W to Ti in the thin film is 1:(0.1-2.0).

9. An electrochromic device comprising the WO3 / TiO2 heterogeneous composite electrochromic thin film as described in claim 7 or 8.

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