Biomedical Engineering Reference
In-Depth Information
1.9.3 Titanium Dioxide (TiO
2
)
Another semiconductor that has similar bandgap with ZnO,
titanium dioxide (TiO
2
) thin films have been widely exploited in
many applications such as microelectronics [152], highly efficient
catalysts [153], microorganism photolysis [154], antifogging and
self-cleaning coatings [155], biosensors [156], gate oxides in metal-
oxide-semiconductor field effect transistors (MOSFET) [157], and
more recently in dye-sensitized solar cells (DSSCs) [158].
The geometry of anodic TiO
nanotubes can be controlled
over a wide range by the applied potential in H
2
/HF aqueous
electrolytes in contrast to other electrolytes. It was found that for
potentials between 1 and 25 V, tubes could be grown with any
desired diameter ranging from 15 to 120 nm combined with tube
length from 20 nm to 1 μm. The diameter and the length depend
linearly on the voltage [159]. Aqueous HCl electrolyte can be used
as an alternative to fluoride containing electrolytes to obtain the
vertically oriented TiO
PO
3
4
nanotube arrays by anodization of titanium
thin films [160]. Nanotube arrays up to 300 nm in length, 15 nm
inner pore diameter, and 10 nm wall thickness can be made using
3 M HCl aqueous electrolyte for anodization potentials between 10
and 13 V.
A highly ordered TiO
2
nanotube array with a unique surface
morphology can be fabricated by electrochemical anodization of
titanium in an organic electrolyte (e.g., 1:1 mixture of DMSO and
ethanol) containing 4% HF. The TiO
2
2
nanotube arrays with improved
photochemical response can be obtained using electrochemical
anodization of titanium in fluorinated organic electrolytes by
optimizing etching time, applied potential, solvents and the HF
concentration [161]. Combining the electrochemical parameters in
an optimal way, ordered TiO
2
nanotube layers with a length of over
250 μm have been obtained at 120 V with 0.2 M HF [162]. The tubes
can grow as a hexagonal close-packed pore array. Although the TiO
2
nanotubes fabricated in organic solution have the longest length
and the largest surface area, its conductivity may be lower than the
one synthesized in aqueous solutions [156]. Crucial parameters
that decide on the dimensions are the fluoride ion concentration,
the voltage and the anodization time. The different length of TiO
2
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