Study On W₁₈O₄₉ Electrochromic Films

Electrochromic materials are capable of reversibly changing their optical properties induced by the applied voltage change,showing important applications in high contrast display and energy-saving smart windows.However,there are still some obstacles like short cycle life,slow coloration efficiency,low optical modulation and high cost,restricting the popularization of electrochromic devices.Non-stoichiometry tungsten trioxide(W18O49)electrochromic films were studied in this dissertation.Citric acid and oxalic acid,the common organic acids,were used as the chelate agents to optimize the structure of films through the simple solvothermal process,contributing to the good electrochromic performances with enhanced optical modulation,fast switching time and high coloration efficiency.Electrochromic films were fabricated on FTO substrate via solvothermal process.The films showed loose structure composed of nanowires with average diameter of 10-20 nm and length of several micrometers,preferably growing along the close-packed(010)planes.The influence of initial WCl6 concentrations on the structures and properties of the film was studied.As WCl6 concentrations increased around the substance,deposition of W18O49 increased on the substrate,leading to the change of films structure from nanowire tussock-like structure to dense layer at the bottom and tussock-like upper layer.Optical modulation enhanced in Vis-NIR region as WC16 concentrations increased,with the cost of worse response speed and coloration efficiency.The EIS tests indicated that the films owned slower kinects of electrochemical reaction because of the decreased contact area between films and electrolyte.With the addition of citric acid as a chelate agent,W18O49 bi-layer films were achieved at low WCl6 concentrations.The bi-layer structure with denser layer at the bottom and loose tussock-like upper layer exhibited better electrochromic performances.The results of EIS inferred that the tight contact between film and substrate,as well as the interspace at denser layer led to the maintenance of electrochemical reaction kinetics.Meanwhile,there was a proper addition for citric acid preparing electrochromic films because of its hydroscopy.The excess water molecules introduced by citric acid resulted in the hydrolysis of WCl6.When the concentration of WCl6 was set at 0.01 mol/L,W18O49 bi-layer films manifested the enhanced optical modulation of 68.6%at 633 nm with good cyclic stability,a coloration efficiency of 82.1 cm2/C,and fast switching time of 1.4 s and 2.3 s for bleaching and coloration at 633 nm in the Al3+ electrolyte.As a chelate agent,oxalic acid was introduced to solvothermal process for obtaining W18O49 bi-layer films,which had denser layer at the bottom and loose tussock-like upper layer.Various thickness and structure of W18O49 films were successfully obtained via the adjustment of oxalic acid mass.Without the property of hydroscopy compared of citric acid,there were less influence on the alcoholysis reaction of WCl6 by the excess water molecules.The bi-layer films exhibited an enhanced optical modulation of 73.2%with good cyclic performance,a coloration efficiency of 77.4 cm2/C,and fast bleaching and coloration switching time of 6.6 s and 2.8 s at 633 nm,respectively.