Electric field driven manipulation of nanostructured metal oxide thin films: applications in chromic devices

摘要

Chromism by definition is the process that induces coloration change in a material and is generally favourable for many applications when it is reversible. Many modern applications such as optical modulators, smart windows and optical displays are based on the chromic effect. This chromic effect is always aided with a compatible stimulus. In semiconductors, particularly those made of thin films, an induced coloration is often initiated by the intercalation of positive ions such as Li + or H + into the exposed active sites of the material’s structure. Nanostructure synthesis of semiconducting crystals continues to expand and evolve. Each synthesis method offers unique prospects that affect morphology, stoichiometry, crystallinity, dopant behaviour and eventually performance of the semiconducting crystals. Electric field driven methods such as anodization and electrodeposition are especially applicable since they are often carried out under ambient conditions with non-toxic electrolytes. In addition, manipulation of anodization and electrodeposition parameters is simple, depending upon facile changes of parameters such as voltage, current and electrolytes while delivering astounding results. In this PhD thesis, the author focuses on the electric field deposited and manipulated transition metal oxides (TMOs). The target TMOs are molybdenum trioxide (MoO 3 ), titanium dioxide (TiO 2 ) and niobium pentoxide (Nb 2 O 5 ) which have suitable band energy diagrams and crystal structure for chromic devices. The author shows that these TMOs can be synthesised into high surface area, highly crystalline and homogenous nanostructures using the two aforementioned techniques. However, these potential candidates also experience inherent limitations that restrict their chromic performances. Therefore the author of this thesis intended to seek out solutions in overcoming these limitations.  In order to achieve the goals of this PhD research program, the author comprehensively investigated chromic properties of nanostructured MoO 3 , TiO 2 and Nb 2 O 5 and assessed their chromic performance and potential strategies to enhance them. Based on the strategies and investigations by the author, the PhD project was conducted in four distinct stages that each resulted in novel outcomes. In the first stage, the author demonstrated gasochromic devices based on MoO 3 . MoO 3 was implemented due to its well-known chromic capabilities. The author developed a novel method to fabricate chromic devices based on selective electrodeposition of α- and β-MoO 3 . In the second stage, the author carried out a novel combination of anodised TiO 2 ordered nanotubular template and electrodeposited α-MoO 3 chromic interface as complimentary binary EC semiconducting materials in order to overcome the chromic limitations of each of these individual TMO. In the third stage, the author demonstrated EC devices based on ordered anodized Nb 2 O 5 , where a coloration efficiency (CE) value of 47.0 cm 2 C −1 was calculated. The calculated CE value was the highest in comparison to all other Nb 2 O 5 based EC devices at the time. In the final stage, to overcome the limitations Nb 2 O 5 , the author applied the concept of binary complimentary TMO system incorporating MoO 3 as the chromic layer while the ordered Nb 2 O 5 nanostructure functions as the template. In summary, the author believes that the outcomes of this PhD research provide an in-depth analysis of chromic devices based on TMOs including MoO 3 , TiO 2 , Nb 2 O 5 and their selected binary systems synthesised using electric field driven techniques. The author also believes that this study has contributed significantly towards improving TMO capabilities for chromic applications.