Designed Synthesis Of Tungsten Trioxide-based Composite Nanowire Arrays And Their Electrochromic Properties

Electrochromic materials can achieve reversible color change and dynamic adjustment of light transmission,absorption and reflectivity at a low voltage(usually<5V),so that the solar energy can be effectively utilized,which are a kind of environmentally-friendly and energy-saving material and have broad application prospects in smart windows,static displays,automotive anti-glare rearview mirrors,electronic paper and other energy-saving fields.As a typical inorganic electrochromic material,WO3 has the advantages of high color contrast,good stability and facile synthesis.Among different polymorphs of WO3,hexagonal phase WO3 has unique advantages in the field of electrochromism due to its large quadrilateral and hexagonal channels for ion transport,however,disadvantages of single color change,low coloring efficiency and poor cycle stability.In the research work of this dissertation,a series of WO3-based nanostructure electrochromic materials and devices were obtained by means of nanoscale control,heterogeneous element doping,interface control and structural recombination strategies,including WO3 nanorods/V2O5 dots hybrid array structure,which realized multi-color display with high color-changing efficiency;crystalline/amorphous tungsten trioxide core/shell nanowire array structure with fast switching response;WO3/TiO2 core-shell nanowire heterostructures with excellent cycle stability;crystalline WO3/Ti doped amorphous WO3 core-shell nanowire structures,and their electrochromic-energy storage dual-function characteristics were studied.The specific research contents and results are as follows:1)As is known,there have few studies on WO3-based multicolor inorganic electrochromic nanomaterials,in the third chapter,WO3 nanorods/V2O5 dots hybrid arrays were prepared by the combination of electrodeposition and solvothermal technique.Structural characterization results show that the diameter of the WO3 nanorods tapers from the base(80 nm)to the tip(30 nm),and the average diameter of the obtained V2O5 dots(2.7 nm)is smaller than the exciton Bohr radius of V2O5(4.5 nm).Performance tests have shown that the color of the hybrid nanorod arrays can be switched between orange-yellow,green-yellow and black,enriching the color changing of WO3-based composites.And compared with the individual WO3 and V2O5 component,the transmittance contrast,switching time,coloring efficiency and cycle stability of the hybrid nanorod array are significantly improved.The authors propose that the enhancement of performance and reaction kinetics can be attributed to the synergistic effect between WO3 nanorods and V2O5 nanodots as well as vertically oriented alignment of the nanoarray structures.2)Effect of crystallographic characteristics and lattic matching on electrochromic properties was investigated.In the fourth chapter,crystalline/amorphous WO3 core/shell nanowire arrays were rationally constructed based on the strategy of constructing tungsten oxide composites with different crystallinity by combining magnetron sputtering and solvothermal methods.Microstructural characterization demonstrated that the single crystalline hexagonal phase WO3 nanocores were encapsulated by amorphous WO3 nanoshells.The thickness of the nanoshell can be precisely adjusted by sputtering techniques,which significantly improves the performance of the core-shell nanowires.After the recombination,the coloring time of the nanowires was reduced from 5.6 s to 3.6 s,the coloring efficiency was increased from 52.1 to 83.6 cm2·C-1,and the optical contrast and cycle stability were improved as well.The improved properties are mainly due to the complementary effect between the amorphous nanoshells and crystalline nanocores as well as the interfacial interaction.3)Performance enhancement mechanism was investigated based on transition metal oxide heterostructures with band-matching relationships.In the fifth chapter,by combining magnetron sputtering and solvothermal methods,novel WO3/TiO2 core-shell nanowire heterostructures have been rationally constructed.HRTEM characterization reveals a semi-coherent interface between the anatase TiO2 shell and the hexagonal WO3 core.Optical absorption spectra shows a red shift in the absorption edge of the heterostructures and X-ray photoelectron spectroscopy(XPS)detects shifts in peaks before and after the compositing,confirming an interfacial interaction between the shell and the core.The effect of the thickness of TiO2 shell on the properties of core-shell nanowires was investigated.The coloring efficiency of the optimized WO3/TiO2 core-shell nanowires is twice that of the pure WO3 nanowires and the heterostructures exhibit good optical contrast in both visible and near-infrared regions,and the contrast retention rate of 95.6%can be maintained and after 3000 cycles.4)In the sixth chapter,the crystalline tungsten oxide/titanium doped amorphous tungsten oxide core-shell nanowire array structures with dual functional electrochromic-energy storage characteristics were synthesised by solvothermal and magnetron sputtering techniques.It can be observed by high-resolution transmission electron microscopy that the surface of the crystalline tungsten oxide nanowires is uniformly coated with an amorphous titanium-doped tungsten oxide nanoshell.Raman spectroscopy analysis shows that Ti-doping increases the crystallization temperature of the tungsten oxide film and enhances the degree of amorphization of the film,thereby promoting ion transport and enhancing cyclic reversibility.The coloring core-shell nanowires have transmittances as low as 5%and 1%in visible(633 nm)and near-infrared(1500 nm)regions,and the transmission range are 88.8%and 90.0%,respectively.The coloring speed(coloring time:2.5 s)and coloring efficiency(101.3 cm2·C-1)are improved by about 2.5 times compared to the sample before compositing.In terms of energy storage,the specific capacity(32.2 mF/cm2)of the sample after compositing is 1.4 times that before compositing,and its capacity shows no obvious attenuation after galvanostatic charging/discharging for 2000 cycles.Moreover,the color of the material changes during the charging and discharging process,thereby realizing the visualization of the energy storage state,which provides a possibility for the development of intelligent energy-saving energy storage materials and devices.