Electrochemical fabrication of nanostructured TiO₂ materials and their applications
Student thesis: Doctoral Thesis
Titanium oxide (TiO2), due to its nontoxicity, chemical and physical stability, and photocatalytic capability, is of great practical importance particularly in health-, energy- and environment- related applications, such as dye-sensitized solar cells, drug carriers, photocatalysts, self-cleaning surfaces, smart windows and lithium ion batteries. This dissertation elaborates a variety of different morphologies of TiO2 nanostructures achieved by electrochemical techniques, proposes the formation mechanisms, and investigates their applications for electrochromic devices and photocatalysis. The first chapter provides an overview of the fabrication approaches and applications of TiO2 nanostructures. In particular, anodization, a convenient bottom-up solution-based electrochemical method, is introduced, with an emphasis on the effects of the defect level in the starting Ti metals. The second chapter studies the impact of periodically pulsed voltage waveforms on the morphologies of the subsequently generated anodic TiO2 nanostructures. It was found that the morphology of the obtained TiO2 films can be readily tuned between the two extremes of self-ordered nanotubular arrays and 3-D nanoporous structures featured interwoven tiny tubular channels by varying the durations of voltage limits. Besides, the resultant anodic TiO2 nanopores/nanotubes exhibited longer film thickness than their counterparts fabricated with constant voltages. The formation mechanism and chemical reactions were proposed. The third chapter focusses on the effect of the negative voltage limit values and the durations of the pulsed voltage waveforms on the morphologies of the generated anodic TiO2 nanostructures. It was found that -4 V and its durations provided the best results of nanotubular morphologies of anodic TiO2 structures by exerting two main effects in the anodizing processes. On one hand, the dramatic current spikes that are observed when the voltage was switched from -4 V to a positive voltage accelerate ion diffusion from the electrolyte/oxide interface towards the oxide/metal interface and thus promote tube formation. On the other hand, the steps at -4 V lead to adsorption of NH4+ on the surface of the anodic oxide and effectively protect it from chemical attack by the fluoride ions. The fourth chapter reports an exotic type of multilayered nanoporous TiO2 films by anodizing Ti foils of high imperfection levels using a novel multipulsed voltage waveform. The fabricated TiO2 films featured tens of well- defined layers whose long-range structural periodicity leaded to photonic band gaps in the visible wavelengths and vivid film colors. Moreover, the optical responses (or the film color) of the fabricated TiO2 multilayers not only were sensitive to environmental chemicals but also could be electrically switched on and off repeatedly, demonstrating their novel potential applications for, e.g., colored smart windows or electric display boards. The fifth chapter further demonstrates that multilayered nanotubular TiO2 structures can be achieved through the novel multipulse technique invented in Chapter 4. The fabricated nanotubes feature tens of well-defined layers. The structural periodicity was embodied in the periodically spaced "bamboo nodes" in the well-aligned "bamboo"-like nanotubes. The formation mechanisms and their application in photocatalysis were investigated.
- Titanium dioxide, Nanostructured materials