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Preoparation and Improved Electrochromic Properties of Metal Oxide Films

Author: CaiGuoFa
Tutor: TuJiangPing
School: Zhejiang University
Course: Materials Processing Engineering
Keywords: Electrochromism Nickel oxide Tungsten trioxide Titanium dioxide PANI Microstructure Optical modulation Coloration efficiency Cycling stability
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Type: PhD thesis
Year: 2014
Downloads: 2
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Abstract


Electrochromic materials are able to change optical properties persistently and reversibly by the application of a small electric field. As a low cost and low energy consumption technology, it is highly attractive for a wide range of potential applications, such as energy efficient smart windows, large area information displays, antiglare automotive mirrors, spacecraft thermal control and military camouflage against IR sensors. Up till the present moment, some industrial powers such as Japan, Europe and the United States take advanced in electrochromic technology application research, especially on the solid-state device, while China falls behind. Unfortunately, single color change, slow switching speed and short life time limit its commercialization application. The research topic carried out in this dissertation is about electrochromic thin films based on metal oxides (NiO, WO3and TiO2). The main objective in this work is to develop methods for improving the electrochromic properties of metal oxides, such as switching speed, optical modulation, coloration efficiency, cycling stability and color variety.Nanostructured NiO thin films were prepared by a simple electrodeposition in choline chloride-based ionic liquid followed by an oxidation in air. The NiO electrochromic films exhibit high coloration efficiency, fast switching speed and good cycling durability. This process is complicated and time-consuming, and the maximum optical modulation only has37%in transmission between the colored and bleached states. In order to overcome the disadvantages of the method above, nanostructured NiO thin films were directly prepared by a simple and efficient electrodeposition method in choline chloride-based ionic liquid with addition of oxidizing agent. The NiO film deposited at high temperature exhibits large optical modulation of67%at550nm, high coloration efficiency, good memory effect and cycling durability.A TiO2/NiO core/shell nanorod array film was prepared by the combination of hydrothermal and chemical-bath deposition. Compared to the NiO nanoflake film, the TiO2/NiO core/shell nanorod array exhibits larger optical modulation, higher coloration efficiency and better cycling performance. The enhancement of the electrochromic performances is attributed to the synergetic contribution of the single crystalline TiO2nanorod core and the ultrathin NiO nanoflake shell, as well as the ordered array geometry, which can all offer direct electrical pathways for electrons and increase the electron transport rate. The porous NiO/graphene sheet hybrid film was prepared by the combination of electrophoretic and chemical-bath deposition. The porous hybrid film exhibits higher coloration efficiency, faster switching speed and better cycling performance than the porous NiO thin film. The improved electrochromic performance is attributed to the reinforcement of the electrochemical activity of the graphene sheets and the more open space among the porous hybrid films which allows the electrolyte to penetrate through the film and shortens the proton diffusion paths within the bulk of NiO. The results indicate that synthesizing hybrid film is an efficient route for improving electrochromic performances.Vertically aligned hierarchical WO3arrays on FTO glass were produced via a hydrothermal or solvothermal method. Due to its high porosity among the nanosheets and good contact with the conductive substrate, the WO3nanosheet film exhibits advantageous electrochemical abilities. For example, fast switching speed, significant optical modulation, high coloration efficiency and excellent cycling stability are achieved for the WO3nanosheet array film. The nanostructured array (nanotree and nanowire) films produced via a solvothermal method show remarkable enhancement of the electrochromic properties in visible spectrum and infrared region. In particular, significant optical modulation (66.5%and66.0%at633nm,73.8%and53.9%at2000nm,57.7%and51.7%at8μm), fast switching speed (4.6s/3.6s and2.0s/3.4s), high coloration efficiency (126and120cm2C-1at633nm) and excellent cycling stability (transmittance discrepancies77.5%and81.7%during a4500-cycle test with cyclically applied voltage range of-0.7to1.0V for interaction/extraction of H+) are achieved for the nanotree and nanowire arrays, respectively.Ti-doped WO3thin films were prepared by a hydrothermal method. Ti doping can lead to significant change in surface morphology and low crystallization, which plays an important role on the electrochromic properties of WO3films. Their low crystallization and a star-like structure lead low charge transfer and ion diffusion resistance. The low Ti-doped WO3film exhibits large transmittance modulation, fast switching speed and high coloration efficiency. The TiO2/WO3core/shell nanorod arrays were prepared by the combination of hydrothermal and electrodeposition method. Significant optical modulation, fast switching speed, high coloration efficiency and excellent cycling performance are achieved for the core/shell nanorod arrays due to the single crystalline nanorod core and amorphous cell, which makes the ion diffusion easier, improves the structural stability and it also provides larger surface area for charge-transfer reactions.The metal oxide (TiO2, WO3)/PANI core/shell structures were prepared by the combination of hydrothermal, solvothermal and electro-polymerization methods. Significant optical modulation, high coloration efficiency, excellent cycling performance and multicolor electrochromism are achieved for the metal oxide/PANI core/shell structures.

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