Research On Multi-band Modulation Of Tungsten Oxide-based Electrochromic Devices

Electrochromism is the phenomenon that the optical properties（reflectance,transmittance,absorption rate,etc.）of a material change stably and reversibly under an applied electric field.Electrochromic devices made of electrochromic materials have been widely used in smart windows,military camouflage,communication technology and electrochromic displays.Tungsten oxide,as the most classical electrochromic material,has high optical contrast,coloring efficiency and cyclic stability.In the past,there have been many researches on the electrochromism of tungsten oxide materials,but generally they are focused on the modulation of transparency in the visible light band.Visible light and near-infrared（NIR）light compose the main spectrum of the sunlight.The modulation of reflection in visible range can realize the multi-color changes,while the regulation of NIR region can realize the management of solar heat.Independent control of visible and NIR irradiations can be at the core of smart window with light and heat adjustable at the same time.This thesis is mainly based on the electrochromic tungsten oxide materials,the multicolor modulation in the visible light region,the large-extent modulation of NIR transmittance,and the independent control of visible-NIR light.The preparation and characterization of related materials,the electrochromic performance of relevant films and devices are also studied.The content mainly includes the following aspects.1.In the field of color control in the visible light band,the color modulation of electrochromic materials is usually very monotonous.The ability of multicolor modulation for electrochromic technique has always been the bottleneck to expand the application to displays and imaging devices.For example,tungsten oxide can only show a bi-stage color change between colorless and blue due to its electronic structure,with the modulation of color far from the demand for colorful modulation in real life.In this thesis,a magnetron sputtered W reflective layer and a WO₃ electrochromic dielectric layer are successively deposited to construct a Fabry-Perot cavity-based electrochromic device.The W metal layer has excellent conductivity,stability,and a tight combination with WO₃ dielectric layer,which can replace the traditional FTO/ITO current collector.The desired multicolor can be obtained by adjusting the thickness of the tungsten oxide in the WO₃/W electrode.Since the refractive index of the tungsten oxide material can be reversibly adjusted by electrochemical ion insertion and extraction.The color of WO₃/W devices can be further adjusted and precisely controlled.Through the reflection peak adjustment of the WO₃/W electrode in visible light region under applied potentials,the colour of tungsten oxide film can be greatly adjusted from red to yellow and green.It can also realize the fine adjustment of tones for single color,such as blue tones ranging from steel cyan,sky blue,royal blue,azure blue,to sea blue,and rock blue.This research achievement realizes the controllable colorful display with tungsten oxide,and also provides a strategy for expanding the range of color modulation based on inorganic electrochromic materials.2.In the field of NIR electrochromism,electrochromic modulation in the 800-2000 nm wavelength range has received widespread attention in applications such as solar thermal regulation,medicine（photothermotherapy）,fiber optic communications,aerospace,and military camouflage.However,the currently reported NIR electrochromic materials are limited and the modulation rate is low.It is of great significance to explore high-performance NIR electrochromic materials with high modulation rates.Taking amorphous tungsten oxide as an example,although with good electrochromic performance in the visible region,it shows poor modulation performance in the NIR band.Its optical modulation rate at the wavelength of 1600 nm is only 56%,and the coloration efficiency is only 49 cm2 C-1.In this thesis,structured water molecules are introduced into the lattice of tungsten oxide nanoplate materials,namely,WO₃,WO₃·H2O,and WO₃·2H2O nanoplate materials with uniform surface morphology.The results show that the capacitance of the hydrated tungsten oxide material and the relevant NIR electrochromic modulation rate are greatly improved with the increase number of structured water molecules.The optimized NIR electrochromic modulation rate is 90.4%,and the coloration efficiency is 322.6 cm2 C-1,which is much higher than the reported inorganic electrochromic materials with practical application value.The strategy of using structured water molecule provides guidance for improving the NIR electrochromic performance of inorganic materials.3.The dual-band independent control of visible and NIR light endows smart windows with multimode including light mode,dark mode,hot mode and cold mode,which can improve the energy efficiency of buildings and comfort for people.Therefore,it is of great significance to find new electrochromic materials with reduced production costs,and excellent independent control capacity over dual-band light.In this thesis,W18O49 nanowires and Prussian blue are simply laminated,in which the outer PB has intrinsic internal tunnels for the transport of electrolyte ions,and the inner W18O49 has sufficient conductivity for electron transportation.Thus,electrochromic insertion ions and electrons can be transport in contact with both modulating components.Finally,the dynamic optical modulation of visible and near-infrared light is achieved in single film electrode without arduously making exterior pores.This simple and easy-to-manufacture composite system has good electrochromic modulation performance,with a modulation rate of 71.2%at 633 nm in the visible region and a modulation rate of 64.8%at 1600 nm in the NIR region,which is higher than those previous reports.The utilization of intrinsic ion tunnels in materials provides a new idea for the design of composited electrochromic materials.4.Smart windows can achieve effective modulation of light.However,for the indoor environment,air permeability is also an important factor affecting the quality of living.At present,air pollution has a huge threat to human physical and mental health,including particulate matter,toxic and harmful gases and so on.It is of great significance to develop transparent and intelligent window screens capable to prevent particulate matter and harmful gases at the same time.In this thesis,a method of fabricating porous nanofiber filter membrane was developed based on electrospinning accompanied by the phase separation technology.Using the transparent porous nanofiber film,the haze particles were effectively blocked under the condition of transparency and permeability.Moreover,when Prussian blue derivative（CuHCF）is combined with the nano-porous fiber,it can endow the filter membrane with the ability of NH₃ molecule removal and realize the visual and intelligent monitoring of NH₃ concentration.