TY  - THES
T1  - Advanced Electrochemical Approaches for the Self-organized Formation of One-Dimensional Oxide Nanoarchitectures:From Transition Metals to Superlattices
A1  - Wei,Wei
Y1  - 2012/09/07
N2  - Over the past decades, synthesis of One-Dimensional (1D) nanostructured transition metal oxide materials has been attracting significant research interests. Among the numerous synthesis routes, the electrochemical self-organized approach offers a low-cost, straightforward and parallel process leading to grow vertically aligned, self-organized oxide nanoarchitectures (such as nanotubular or nanoporous arrays). 
By a simple but optimized electrochemical anodization of a titanium metal sheet in fluoride containing electrolytes, high aspect ratio, self-ordered TiO2 nanotube arrays could be formed. However, similar self-ordered metal oxide nanoarchitectures (nanotubular or nanoporous structures) with high aspect ratio have not been achieved for some transition metals, such as tantalum. In this thesis advanced anodization approaches in organic fluoride containing electrolytes are employed to fabricate various high aspect ratio, self-ordered metal oxide nanoarchitectures on the pristine transition metals (such as tantalum and titanium), on binary Ti-Ta alloys, on Ti-Ta alloy thin films and Ti/Ta multilayers. It is also within the scope of this work to achieve the controllable formation of anodic oxide 1D nanoarchitectures with tunable geometries (e.g., nanopores or nanotubes, length, and tube/pore diameter) and crystallinity (e.g., amorphous or completely crystallized). 
In the first part of this thesis, by using an optimized glycerol / ethylene glycol mixture electrolyte with addition of NH4F and the background salt (NH4)2SO4, long-range self-ordered, high aspect ratio Ta2O5 nanoporous layers with thicknesses of more than 11 ?m are grown. The nanopore diameter of the formed nanoporous layers can be altered in the range of 7-18 nm by controlling the applied voltage. For Ti, by anodization of Ti in the NH4F containing ethylene glycol electrolyte with precisely controlled water content (0 to 0.7 vol.%), the transition from self-ordered anodic nanotubular to nanoporous TiO2 layer is achieved. 
In the second part, we show that the tunable formation of long-range hexagonally ordered anodic nanoporous / nanotubular TiO2-Ta2O5 layers with thicknesses of more than 10 ?m can be achieved on the Ti-19Ta, Ti-10Ta and Ti-40Ta alloys, by carefully adjusting the small amount of water (5vol.%~15 vol.%) in the NH4F containing ethylene glycol electrolyte. The pore diameter of the formed anodic TiO2-Ta2O5 1D nanostructures can be tailored from 27 nm to 50 nm with variation of the applied voltage from 20 V to 60 V.  
The third part of this thesis is devoted to form self-ordered anodic TiO2-Ta2O5 1D nanostructured thin films on Al foil and Si wafer substrates. In the case of the sputtered    Ti-19Ta alloy/Si substrates, an entire large area of self-ordered anodic nanoporous TiO2-Ta2O5 thin film with a thickness of 3.16 ?m can be formed on the Si wafer. For the sputtered Ti-19Ta alloy/Al composite thin film, self-ordered TiO2-Ta2O5 nanotubular arrays with open bottom are obtained.
In the fourth part, by the optimization of the anodization process of the Ti/Ta multilayers, 3D TiO2/Ta2O5 superlattice nanotube arrays with regular 20 nm-spaced modulations in the tube wall composition are fabricated in a simple electrochemical process for the first time, without expensive UHV equipment. The TiO2/Ta2O5 confined heterojunction nanotubes, where electronic properties alternate with 20 nm precision, have shown enhanced photocatalytic efficiencies and phonon confinement effects, and are expected to drastically enhance efficiencies in charge transfer devices.
In the final part, some key factors towards the formation of high aspect ratio, self-ordered anodic oxide nanoarchitectures are discussed, such as the organic electrolyte, the water content of the electrolyte as well as the involved metallic substrates.

KW  - Elektrochemie
KW  - Nanoröhre
KW  - Titandioxid
KW  - Tantal
KW  - Titanblech
CY  - Erlangen
PB  - Universitätsbibliothek der Universität Erlangen-Nürnberg
AD  - Universitätsstraße. 4, 91054 Erlangen
L2  - http://www.opus.ub.uni-erlangen.de/opus/volltexte/2012/3446
ER  -