Study On Modulated Growth Of Tungsten Trioxide-Based Nanostructures And Their Electrochromic Properties

Electrochromic materials feature modulated solar transmittance,reflection or absorption in both visible and infrared regions while realizing color change driven by a low voltage(<5 V),allowing for the efficient use of solar energy,which promise great potential in smart windows,electronic paper and other related energy-saving devices.Owing to their non-emissive characteristics,electrochromic materials also promise distinct advantages in developing next-generation eye-friendly information displays.Simultaneously,the ion intercalation/deintercalation in the reversible redox reactions during an electrochromic process can also generate pseudo-capacitive behaviors which make the material a candidate for producing smart electrochromic energy storage devices,which is expected to promote the development of smart electrochromic supercapacitors and realize the visualization of energy storage state for smart energy-efficient devices.WO₃ is a typical electrochromic material with good environmental stability,whose performance is dominated greatly on its crystallinity,micro-morphology and composition.However,response speed,coloring efficiency and memory effect of WO₃ are still not satisfactory.The specific research contents and results are as follows:1)Tungsten trioxide electrochromic materials with excellent memory effect and high dual-band optical modulation were obtained by controlling oxygen deficiencies.In the third chapter,we design a mixed crystalline WO_x thin film implanted with massive oxygen deficiencies on a conventional direct current reactive magnetron sputtering method by dynamically controlling the oxygen partial pressure in the system.In this way,massive and stable internal oxygen deficiencies have been implanted inside the films,which not only provides more binding sites with Li⁺-ions for higher optical contrast but also creates sufficient deep trapping sites towards Li⁺-ions for stronger memory performance.The obtained WO_x film exhibits high dual-band optical modulation in both visible(VIS,99.0%in 633 nm)and near infrared(NIR,94.2%in 1300 nm)regions as well as exceptional memory effect(the colored transmittance increases only by 0.04%at 633 nm after 50 days).The strategy in this work bestows the WO_x thin film a promising candidate for developing electrochromic information displays and other energy efficient devices as well.2)The performance of tungsten trioxide electrochromic materials is dominated greatly on its crystallinity.In the fourth chapter,porous amorphous WO₃nanostructures are obtained by a pulsed radio frequency(PRF)magnetron sputtering technique.Compared to conventional direct current(DC)and radio frequency(RF)magnetron sputtering,the PRF magnetron sputtering circumvents the obstacle of a self-annealing temperature effect during the deposition,which may induce irregular crystallization or uneven chemical composition.The porous homogeneous amorphous WO₃ film prepared by the PRF magnetron sputtering shows large dual-band optical modulation(both visible and near infrared regions,93.6%in 633 nm and 90.6%in1500 nm)and quick response speed(3.2 s for coloring and 5.6 s for bleaching).We have also assembled the single-chip all-solid-state electrochromic device as Glass/FTO/PRF WO₃/Li Ta O₃/Ni O/FTO sandwich structure.3)The crystallization matching and the microstructure design of WO₃ can effectively improve the electrochromic properties.In the fifth chapter,porous bilayer hybrid WO₃nanoarray devices are constructed in which the active materials are composed of a crystalline WO₃ nanobowl arrays layer modified by a nanoholey amorphous WO₃ layer.The hybrid nanoarrays exhibit outstanding electrochromic performance with high color contrast in both visible and near infrared regions(93.9%at 633 nm,89.6%at 1500 nm),fast response speed(3.0 s for coloring and 3.6 s for bleaching)and exceptional cycling stability(95.4%contrast retention after 10000cycles).In addition,the hybrid nanoarrays display high areal capacitance(47.4m F/cm²),superior rate capability and cyclic stability(areal capacitance remains 84.0%after 2000 cycles).An electrochromic supercapacitor nanodevice is constructed based on the excellent electrochromic and capacitive performance of the hybrid nanoarrays.The enhanced electrochemical properties can be ascribed to the synergistic effect between the unique top amorphous layer structure and the underlying crystalline WO₃layer.In addition,oxygen partial pressure plays a crucial role in modulating the microstructure of the amorphous layer as well as electrochemical performance of the nanodevice.4)In the sixth chapter,high-quality organic/inorganic hollow nanostructure is constructed to improve the electrochromic performance.The hollow WO₃/PEDOT/WO₃ multilayer hybrid nanosphere arrays film is obtained by template-assisted magnetron sputtering combined with on-pot sequential electrochemical deposition.The hollow nanosphere arrays can provide large contact area with electrolyte to benefit ion exchange and the constructed symmetry sandwich structure is designed to overcome the instability of the PEDOT layer as well as to bestow good synergistic effect to improve the electrochromic and capacitive performance.The prepared WO₃ layer is responsible for offering a lot of ion binding sites due to the large capacity of amorphous WO₃ for small ions,and the PEDOT layer is used to serve as unique conductive network,which can effectively facilitate electron transportation and connect separated color centers.The obtained hollow hybrid nanosphere film exhibits outstanding electrochromic performance with decent coloring efficiency(115.6 cm²C^-1)at low colored/bleached potentials(-1.0/1.0 V)and good capacitive performance.In addition,due to the good environmental stability of WO₃,the sandwich system also shows better UV resistance.The obtained hybrid nanosphere arrays as well as the organic/inorganic hybridization promise great potential in developing high quality smart energy-efficient devices.