Preparation And Properties Of Organic/Inorganic Composite Electrochromic Materials

Electrochromism refers to the phenomenon that the color of a material changes reversibly under the application of an external electric field.Electrochromic devices have attracted much attention in applications such as smart windows,rear-view mirrors for cars,and low-energy displays due to their low energy consumption,high contrast,and intelligent adjustment.At present,foreign countries are in a leading position in the research and application of electrochromic technology,and domestic electrochromic technology is still immature.Single organic electrochromic materials or inorganic electrochromic materials usually have defects in performance.The use of organic-inorganic composite electrochromic materials has gradually become the focus of researchers.In addition,there is an increasing demand for flexible,wearable electronic devices,and flexible electrochromic devices have also been extensively studied.In this paper,TiO₂ nanorods and TiO₂ nanosheets arrays were prepared by hydrothermal method,and modified with methyl viologen to prepare organic-inorganic composite electrochromic devices,the electrochromic properties such as optical modulation range,response time and cycle stability were further explored.In addition,WO₃ flexible electrochromic film was prepared by UV deposition method,and the performance of flexible electrochromic film was explored.The details are as follows:（1）The TiO₂ nanorod arrays were prepared by a simple hydrothermal method,modified with organic small molecule electrochromic material methyl viologen,and then encapsulated into composite electrochromic devices,and their electrochemical and electrochromic properties were studied.The organic electrochromic film has the advantages of low required voltage,rich color,short response time,high coloring efficiency,etc.,but the adhesion to the substrate is not strong,and the film formation is uneven,resulting in unstable electrochemical performance.In this experiment,the TiO₂ nanorod arrays not only increase the attachment area of methyl viologen,but also provide more active sites for methyl viologen,and play an"anchoring"role,making the viologen more uniform on the substrate,improving the electrochemical redox reversibility,thereby improving the stability of the electrochromic cycle.（2）The anatase phase TiO₂ nanosheets array was prepared by hydrothermal method and modified with methyl viologen to increase the contact area between methyl violet and TiO₂.After being packaged as an electrochromic device,the electrochromic properties such as cyclic voltammetry,optical modulation range,and"memory effect"were studied.It was found that the maximum optical modulation range of the methyl viologen-modified TiO₂ nanosheets electrochromic device reaches 76%,which is larger than the maximum optical modulation range of the methyl violet modified TiO₂ nanorods（55%）.In terms of cycle stability,the light transmission rate changes by 15%after removing the voltage for 1800 s,which is better than that of the methyl violet-modified TiO₂ nanorod electrochromic device（the transmittance changes by 22%after removing the voltage for 1800 s）.This is because that the TiO₂ nanosheets array has a higher specific surface area and provides more adsorption area for the methyl viologen than the nanorod array on the same FTO substrate,and the organic/inorganic composite effect is more obvious,which makes the color change more intense and the electrochemical stability is better.（3）WO₃ flexible electrochromic film was prepared on ITO/PET flexible substrate by a method of ultraviolet light deposition.The WO₃ film obtained by the method has an amorphous crystal phase,a porous surface morphology,a pore diameter of about 100 to 300 nm,and a high porosity.This not only facilitates the contact and penetration of the electrolyte,but also shortens the charge transfer path in the electrochromic process,thus the WO₃ flexible electrochromic film has a faster response speed（coloring and fading time are 15 s,5 s,respectively）.The light transmittance of the film in the colorless state near 630 nm is more than 90%,which is almost transparent.In the colored state,the transmittance of the WO₃ film is less than 40%,and the optical modulation rangeΔT of the film is above 60%,indicating the film has a large optical modulation range.In addition,the electrochromic film is flexible and bendable,which overcomes the limitation of the conventional electrochromic film using a brittle substrate such as ITO or FTO,and conforms to the current pursuit of flexible wearable devices.