The Study Of Composite Film Based On Saturated Polyoxometallate And TiO₂:Fabrication And Electrochromic Properties

Polyoxometalates?POMs?,a well-known class of cathode electrochromic materials with intriguling structures and controllable properties,have a great application prospect in the field of electrochromic?EC?.In the past,the research of EC materials based on polyoxometalates mostly been focused on the design and property control of single materials,there have been few studies on the composite film.In this paper,we have designed three kinds of POMs-TiO₂ nanocomposite EC films containing saturated-type POMs and another excellent cathode material titanium dioxide.The influences of the structure of POMs,the preparation method and the surface morphology on the EC performance of nanocomposite films were systematically studied,and EC film with high optical contrast,fast response time,good stability and suitable for commercialization is obtained.The research details and results are as follows:The NW-PW12 composite film was synthesized by growing single-crystalline nanowires on the FTO substrates using a hydrothermal method,followed by depositing negatively charged PW12and positively charged PEI onto the TiO₂ nanowires by Layer-by-Layer?LbL?method.The scanning electron microscopy?SEM?images exhibited that the FTO substrate was covered by randomly oriented TiO₂ nanowires.The nanowires in the composite films became stronger and tighter,due to the deposition of PW12 and PEI,and the thickness of the NW-PW12 film was estimated to be approximately 600 nm.The fabrication of multilayer films was monitored by UV-vis absorption spectroscopy?UV-vis?and cyclic voltammetry?CV?,results indicated the growth of each deposition cycle is regular and uniform during the LbL process.Electrochemical and UV-vis techniques were synergistically used to investigate the electrochromic properties of the PW₁₂,TiO₂ nanowires and NW-PW₁₂ films.The color of composite film could change from transparent to blue at different potentials.And compared to individual PW12 film,the nanocomposite film exhibited improved electrochromic performances.The composite film shows higher optical contrast up to 5.3%,faster switching times?2.87 s for coloration and 1.13 s for bleaching?,and the coloring efficiency at 650 nm was 39.79 cm²/C.Although the response time is very fast,the light contrast and coloring efficiency of Keggin-type POMs PW12composite film are low,which limits its application in electrochromic field.Dawson-type POMs P₂W₁₈8 were constructed on TiO₂ nanowires by LbL,sol-gel and electrodeposition methods subsequently.The EC performances of the nanocomposite films by LbL method are better.The composite film shows higher optical contrast up to 45.1%at 650 nm,faster switching times?1.9 s for coloration and 6.73 s for bleaching?and the coloring efficiency at 650 nm was 69.0 cm²/C.SEM shows that the surface of the composite film is covered with stone-shaped nanoparticles,and that of the composite film is covered with lawn-shaped nanowires,on which the nanoparticles are attached.Thus,there are more reaction sites for the insertion and extraction of the electrolyte ions,thereby enhancing the electrochromic performance of the material.Compared with prepared by the other two methods,the nanocomposite film prepared by LbL method exhibits higher EC performance.However,sol-gel and electrodeposition methods are more suitable for large-scale industrial production.The composite EC material was synthesized by constructing Preyssler type POMs P₅W₃₀ on TiO₂ nanowires with LbL method and electrodeposition methods.Consistent with the results in chapter 3,the EC properties of the composite films prepared by LbL method are better.The nanocomposite film shows better EC properties such as higher transmittance modulation?46.03%at 650 nm?,faster switching times?7.03 s for coloration and 2.55 s for bleaching?and outstanding coloration efficiency?163.4 cm²/C at 650 nm?.The composite film indicated good cycling durability,and the response time and optical contrast changed slightly after 1000 cycles.The enhancement of EC performance could due to the effective combination of two kinds of cathode EC materials.