Tungsten oxide nanostructures and their electrochromic performance
Date: 3 April 2018
University of Exeter
PhD in Engineering
The electrochromic behaviour of tungsten oxide (WOx) bulk forms has attracted huge research interest for decades owing to advantages of fast response time, good reversibility and high colouration efficiency compared with other electrochromic materials. Nanomaterials have certainly brought in new opportunities and opened the door for ...
The electrochromic behaviour of tungsten oxide (WOx) bulk forms has attracted huge research interest for decades owing to advantages of fast response time, good reversibility and high colouration efficiency compared with other electrochromic materials. Nanomaterials have certainly brought in new opportunities and opened the door for better, higher and smarter devices fabrication. This thesis will first investigate, explore, and understand the electrochromic performance of WOx in the nanoscale, and identify ways to enhance its performance via effective doping electrolyte selection and heat treatment. Moreover, the thesis will evaluate the prototype device performance based on our new understandings obtained in this project. The main findings are as follows: • Successfully synthesised crystalline WOx-based nanomaterials using a simple solvothermal technique, and achieved a series of La-, Ce- and Na-doped nanomaterials. The results show that the dopants caused distortion of the parental WOx¬ frameworks and increased the oxygen vacancy inside the structure, which is beneficial for the chromic properties. • The best electrochromic performance was obtained from Ce/W = 1 : 15 samples which presented 44.3% for optical contrast colouration efficiency of 67.3 cm2 C−1. • Conducted in-situ phase transition investigations using both WO3 nanoparticles and W18O49 nanowires, and found out that temperature was affecting the relaxation of W-O framework and phase transition. Based on the investigation, the 187.6 cm-1 band has identified as a fingerprint band for the phase transition from γ- to β- of the WO3 nanoparticle at 275 °C. W18O49 nanowires exhibit better thermal stability than the WO3 nanoparticles. • Intensive electrochemical investigations of La- and Ce-doped WOx structures were exhibit better diffusion kinetics, stability and colouration efficiency compare with plain WOx. These improvements are contributed to the improved oxygen vacancy (Vo). DLi+ of the Ce-doped samples were much higher than that of the plain W18¬O49 nanowires, by 177%, 102% and 84% for the 1:15, 1:10 and 1:5 samples respectively. DLi+ values of all La-doped samples were over 100% higher than those of the plain W18O49. The La-doped thin films increased the stability by 9%, 4% for intercalation, and 25% and 23% for de-intercalation, for La/W = 1:15 and 1:10 samples respectively, against the plain W18O49. • Provided experimental evidence to explain the degradation of chromic thin films, which is related to the Li+ trapping and loss of Vo in the WOx the structures. • The 350 °C annealed W18O49 thin film sample showed better diffusion kinetics by 25% for intercalation and 30% for de-intercalation compared with the un-annealed W18O49 samples. Stabilities also showed 31% improvement for the de-intercalation, against the un-annealed W18O49 sample. • Fabricated electrochromic device prototypes, and investigated the influence of various electrolytes, an optimal combination of LiClO4/PPC/PC polymer electrolyte has been developed, to improve the performance in ion kinetics and switching time of W18O49. These results have shown that WOx nanomaterials via further effective modification including doping with rare-earth elements or proper heat treatment are promising and practical candidate for the creation of fast, reliable and highly efficient electrochromic devices/smart windows for various applications.
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