Morphology control of novel anodic TiO₂ nanomaterials and their energy- and environment- related applications

二氧化鈦納米材料的形貌控制及其在能源和環境領域的應用

Student thesis: Doctoral Thesis

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Author(s)

  • Hui LI

Detail(s)

Awarding Institution
Supervisors/Advisors
Award date14 Feb 2014

Abstract

Self-organized TiO2 nanostructures that are fabricated by anodizing Ti substrates in fluoride-containing electrolytes exhibit high photocatalytic efficiency and are of particularly practical importance in energy- and environment-related applications, such as dye sensitized solar cells, supercapacitors, and lithium ion batteries. One major advantage of the anodization method for generating nanostructured TiO2 lies in its capability of targeting the desired morphology by means of convenient and adjustable experimental parameters. In this dissertation, we mainly focus on the effects that can significantly vary the morphology of the TiO2 nanostructures and their impact on the energy- and environment- related applications. The first chapter introduces the background of fabricating TiO2 nanotubes by anodization and the morphological/electronic control of the resulted TiO2 nanotube arrays. A brief overview of their applications is also included. Chapters 2-5 describe different methods to fabricate or modify the anodic TiO2 nanostructures for improved performance as photocatalysts or supercapacitors. The second chapter demonstrates that the defect level of the Ti substrates has a strong impact on the morphology of the subsequently generated anodic TiO2 nanostructures. By increasing the amount of cold work applied to the Ti foil, the morphology of the subsequently generated anodic TiO2 can be converted from self-organized nanotube arrays to an exotic type of nanoporous structure. The interweaving nanoporous TiO2 structures achieved by the cold work pretreatment exhibit strong photocatalytic abilities and significantly outperform their nanotubular counterparts. In the third chapter, four different electrolytes and the asymmetric electrode configurations were adopted for growing gradient TiO2 nanotube arrays with the tube diameters and length gradually changing along the sample plane. The key factor for formation of the gradient TiO2 nanotubes lies in the voltage drop over the tube bottom at different locations along the film. Moderate electrolyte conductivities and sufficiently high anodization voltages are suitable for generating TiO2 structures with the desired gradient. In the fourth chapter, an electronic field is applied parallel to the anodic TiO2 nanotubes, resulting in the removal of the outer shells of the nanotubes. Better-separated single-walled TiO2 nanotubes were obtained and shown significantly improved photocatalytic efficiency than the non-etched counterparts. Furthermore, the novel approach introduced here offers a new route to adjust the characteristics of anodic TiO2 nanotubes, e.g., to generate exotic multilayered structures. In the fifth chapter, electrochemical doping of anatase TiO2 in an organic electrolyte of ethylene glycol through proton intercalation is proposed. The treated TiO2 nanotubes are black to the naked eye and possessed significantly lower bandgap and higher electrical conductivity. Therefore, the treated TiO2 displayed remarkably higher photoconversion efficiency (increased from 48% to 72% in the visible region, and from ~0% to 7% in the UV region), photocatalytic efficiency, and charge-storage capability (42-fold increase in specific capacitance). More importantly, the doping effects were found to persist for over one year. It should also be pointed out that the doping method reported here is highly suitable for producing novel energy-efficient electrochromatic systems that can stay at the color state for an extremely long time without supply of electrical power. Chapter 6 concludes the dissertation and suggests future work.

    Research areas

  • Titanium dioxide, Nanostructured materials, Industrial applications