Preparation of Tungsten Trioxide and Nickel Oxide Films and Their Electrochromic Performance

Author:Qu Hui Ying

Supervisor:zhao jiu peng


Degree Year:2019





Electrochromism refers to the phenomenon that the optical properties of a material can be changed reversibly by applying an electrical signal.Since the electrochromic materials can adjust their optical properties with the application of a low voltage,they can be used as the electrochromic layers in smart windows in buildings to adjust the indoor temperature and light intensity,achieving the purpose of saving energy and protecting the environment.The smart windows are consisted by an ion conductor,a cathodically electrochromic film,an anodically electrochromic film and transparent electrical conductors.The entire arrangement is backed by a transparent substrate or located between two such substrates.Apparently,the electrochromic films are the core part of the entire device.Tungsten trioxide(WO3) and nickel oxide(NiO) are cathodically and anodically electrochromic materials,respectively;they are often used as the electrochromic films for smart windows due to their excellent electrochromic performance.Therefore,improving the electrochromic performance as well as enhancing the durability of WO3 and NiO are significant to the further improvement and optimization of the smart windows,and also to save energy in a more effective way.In this dissertation,we studied the electrochromic performance of WO3 and NiO by designing nanostructures,composite materials and using electrochemical treatments,in order to improve their electrochromic performance.The electrochromic performance of the cathodic WO3 film was studied.Face-centered cubic structured polystyrene colloid crystal templates were prepared on the surface of indium-tin oxide(ITO)/glass by vertical deposition.Three-dimensional ordered macroporous(3DOM) structured crystalline WO3 thin films were obtained by galvanostatically cathodic electrodeposition.Since the wall of the 3DOM structured WO3 film is very thin,the ion diffusion distance can be remarkably reduced and ion transport can be facilitated,so that the film has the advantage of fast response with short coloring time of 5.1 s and bleaching time of 0.9 s.In order to enlarge the transmittance modulation as well as develop the application of the WO3 film in flexible electronic devices,three kinds of WO3·2H2O/PEDOT flexible composite films were prepared using silver/ethylene vinyl acetate/polyethylene terephthalate(Ag/EVA/PET)as the flexible conductive substrates.Moreover,by comparing the electrochromic performance of three kinds of WO3·2H2O/PEDOT composite films,the effect of the PEDOT content on the electrochromic performance of the composite films was studied.The electrochromic performance of the composite film is greatly improved due to the outstanding electrical conductivity of PEDOT.Together with short coloring time of 4.4 s and bleaching time of 2.6 s,large optical modulation(63.1%) and high coloration efficiency(180.2 cm2 C-1) were achevied.Electrochemical pretreatment was carried out in order to extend the cycle life of the WO3 film by applying a voltage of 6 V vs.Li/Li+.Results show that the electrochemical pretreatment can yield unprecedented durability for charge exchange and optical modulation and effectively evades harmful trapping of Li.The pretreat film was quite stable when cycled in the potential range of 1.5-4 V vs.Li/Li+.In order to study the changes of the WO3 film during electrochemical pretreatment,associated compositional and structural modifications were probed by several techniques.It is indicated that changes of the film were related to compositional differences at the WO3/ITO and ITO/glass interfaces,and the formation of carbon-containing solid-electrolyte interfacial layers on top of the WO3film.The electrochromic performance of the anodic NiO film was also studied.During the electrochemical cycling,the electrochromic performance of the NiO thin film degraded,which was mainly reflected in the decrease of the optical modulation and the charge capacity in 100 cycles.Li+and Cl-based anions were found to participate in the electrochromic process of NiO in the LiClO4/PC electrolyte.At high voltages,desorption of Li+and adsorption of Cl-based anions occurred;at low voltages,adsorption of Li+and desorption of Cl-based anions occurred.In addition,the amount of Li+and Cl-based anions involved in the reaction were very small,indicating that the electrochromic process of NiO is a surface process.The electrochromic performance of the degraded NiO film was rejuvenated by electrochemical treatments,including the recovery of optical modulation,charge capacity and coloring efficiency.During the rejuvenation process,the Li+ content was decreased and the Cl-based anion content was increased,and the difference between the two(-0.32) was recovered to the initial value(-0.34).