Electrochromic And Photoluminescent Properties Of Tungsten Oxide Film Doped With Rare Earth Elements

Electrochromism is a phenomenon in which a continuous,and reversible color change occurs under the stimulation of an external electric potential or current.In recent years,electrochromic materials have received widespread attentions in many potential applications,such as smart windows for energy-saving buildings,anti-glare rearview mirrors,aircraft portholes,displays,electronic skins,and energy storage devices.A typical electrochromic device is a sandwich structure,composed of a transparent conductor layer,an electrochromic film,an electrolyte layer,and an ion storage film.Among them,the most critical electrochromic layer can change its own optical properties(absorption,transmittance,and reflectivity)under the action of a voltage pulse,and can be restored to its original state after fading when a reverse voltage is applied.The reported electrochromic materials mainly include transition metal oxides(tungsten oxide and nickel oxide),small organic molecules(viologen and triphenylamine),and conjugated conductive polymers(polythiophene and polypyrrole).Because of large optical modulation window,excellent cycle stability and good memory effect,tungsten oxide(WO3)has gradually become a research hotspot.In recent years,some significant breakthroughs have been made in WO3-based electrochromic materials,involving the development of new materials and structures,element doping,design of composite materials,dual-band(visible and near-infrared light)control,and self-powered batteries.We are also committed to developing multi-functional electrochromic materials and devices.In this work,the introduction of rare earth elements gives WO3 materials a novel photoluminescence function and further expands its application range.The influence of key reaction conditions on the structure and performance of WO3 films was systematically explored.In addition,through the doping of different rare earth elements,WO3 film exhibits different luminescence characteristics.First,rare earth-europium(Eu)doped WO3 material was prepared by hydrothermal method.Taking the pH value of precursor solution as an example,we systematically investigated the impact of key conditions on the morphology,structure,electrochemistry,electrochromic and photoluminescent properties of WO3 film.As the pH value increases,some crystalline peaks in the high 2? region disappear and are replaced by a broad hump,which indicates the existence of amorphous structure,besides,WO3 film gradually tends to construct an open porous morphology.A new phase of monoclinic W17O47 appears at a higher pH,causing an increase in crystallinity of the film.More crystal grains form at the crossing point of network structure and reduce the pore size.Electrochromic and photoluminescent properties of Eu-WO3 film could also be modulated by tuning pH value.On one hand,Eu-WO3 film achieves the optimal transmission modulation of 70.1%and high coloration efficiency of about 55.95 cm2 C-1 at pH 2.2.On the other hand,due to the introduction of Eu3+ ions,WO3 film exhibits red emission,and its intensity can be enhanced by increasing pH value.The second part of this work is mainly based on different luminescence characteristics of rare earth elements,offering WO3 film various photoluminescent functions.By introducing terbium(Tb)ions,WO3 film added a new green fluorescent property.Tb ions exist in WO3 lattice in the form of+3 valence,and its larger ion radius inhibits the orientation growth of WO3 crystal,resulting in the amorphous structure.With the optimal 13%Tb ion doping,WO3 film exhibits a larger-sized porous network structure,providing more ion transport channels and reaction sites,in favor of higher electrochromic performance:optical contrast of 66.71%,response time of less than 10 seconds,coloring efficiency of 48.33 cm2 C-1,and switching stability of more than 600 cycles.In addition,under 260 nm ultraviolet irradiation,WO3 film shows visible green fluorescence.Under the stimulus of external potential,the green light is quenched when the film is colored,but can be restored after fading,which creates possibility for the adjustable fluorescence display of WO3 materials.This work not only opens up a new direction for the functional application of WO3 materials,but also demonstrates the influence of reaction conditions on the microstructure of film,providing a guiding strategy for further improving performances.The research is beneficial for achieving a smart display which can apply in the day and night scenarios.