Optimization Of Preparation Process And Electrochromic Property Of Titanium-doped Nanoporous WO₃ Films

As a new energy-saving technology,electrochromic smart windows can realize the dynamic adjustment of house lighting and shading energy according to human will.Compared with other electrochromic materials,WO₃ has become the most widely used electrochromic material due to its large discoloration amplitude and low driving voltage.The WO₃ film prepared by traditional magnetron sputtering method has a dense structure,which makes it difficult to improve the response rate and cycle stability.Therefore,it is of great significance to explore new ways to control the physical morphology and chemical structure of WO₃ films,which is to improve the electrochromic performance.In this thesis,tungsten metal and"mosaic"tungsten-titanium metal were used as the target materials,and metal W and Ti:W films were deposited on ITO substrates by DC magnetron sputtering technology,and finally the WO₃ and Ti:WO₃ films with porous structures were prepared by anodizing and annealing.The effects of annealing temperature and Ti-doping on the on the structure and electrochromic properties of thin films were investigated by SEM,XRD,Raman,electrochemical workstation and UV-Vis spectrophotometer.The electronic and optical properties of pure WO₃ and Ti-doped systems are analyzed by first-principles calculation.The main research results are as follows:The orthogonal experimental design determined that the factors affecting the charge storage capacity of WO₃ nanoporous films are NH₄F concentration,oxidation time and oxidation voltage from largest to smallest,among which NH₄F concentration of 10g/L,oxidation time of 8min and oxidation voltage of 10V are the best anodizing processes,based on which WO₃ nanoporous films with good charge storage capacity can be obtained,and the maximum charge storage capacity is about 24 m C/cm².Low-temperature annealing can effectively improve the electrochromic properties of WO₃nanoporous films.The WO₃ film annealed at 300°C remains amorphous,with the best optical modulation amplitude（59.5%）and the highest coloring efficiency（72.72cm²/C）.At 350°C,the film begins to crystallize and is in a state of coexistence between crystalline and amorphous states.After subsequent electrochemical tests,the internal mechanism explains the reason for the improvement of electrochromic performance.The WO₃ film annealed at 350°C shortens the ion diffusion path due to its loose microstructure and ordered porous morphology,thereby showing a greater diffusion coefficient of 1.62×10^-10cm²/s,faster coloring,fading response time（t_c=2.20s,t_b=1.34s）,and good cycle stability after 200 cycles.Compared with pure WO₃ films,Ti:WO₃ films change from monoclinic to cubic system,which reduces the lattice stress and energy barrier caused by the discoloration process of the film.At the same time,Ti doping distorts the octahedral structure of WO₃,inhibits the accumulation of Li⁺in the film and improves the cycling stability.In addition,the length of the W-O-W bond is longer,which facilitates the binding with Li⁺and improves the charge storage capacity.Therefore,while ensuring the discoloration amplitude,the charge storage of Ti:WO₃film increased（38m C/cm²）,the response rate was significantly accelerated（t_c=1.20s,t_b=0.8s）,and the cycle reversibility of more than 92%remained after 200 cycles;the cycle life of the film is obviously improved after WO₃ doped with Ti.First-principle calculations show that the lattice volume of WO₃ increases and the stability of the system increases after doping with Ti.The optical absorption edge is redshifted,the band gap is reduced,and the conductivity is enhanced.