Biomedical Engineering Reference
In-Depth Information
FIGURE 16.4
ALD of HfO
2
coatings on single-walled carbon nanotubes (SWNTs) (a) with and (b) without DNA func-
tionalization. (Top panel) AFM images of ≈5-nm-thick HfO
2
coatings on SWNTs suspended on SiO
2
. (Middle panel) Cross-
sectional schematics of the coating profiles. (Bottom panel) TEM images of nominally 5-nm-thick ALD-HfO
2
coating on
suspended SWNTs. Reprinted from Ref.
18
. Copyright © 2006, with permission from the American Chemical Society.
depending on the crystalline phase, TiO
2
exhibits
numerous interesting physical and chemical
properties.
Kim's group investigated the wetting proper-
ties
[39]
, electrochemical properties
[40]
, and
application potential for dye-sensitized solar
cells
[41]
of the replicated TiO
2
ribbons in a series
of works. The wetting behavior of the anatase
phase TiO
2
was switched with ultraviolet light.
Though the switchable wetting effect was known
[43]
, the novelty of this approach was primarily
the very interesting replica of the structure with
a large surface area.
The follow-up papers from the same group
showed some more interesting aspects of the pro-
duced structures from the technological point of
view. The replicated nanoribbons were proposed
as potential electrodes for Li-ion batteries
[40]
.
The highly porous structure with only 15-nm-
thick TiO
2
layers permits efficient diffusion of an
electrolyte and thus the Li
+
ions into the TiO
2
matrix. Compared to TiO
2
nanopowders, the
nanoribbons show great enhancement of the spe-
cific capacity, reaching a fivefold increase.
The last proposed application by the same
authors with the same structures relates to
dye-sensitized solar cells (DSSCs)
[41]
. In the
simplest case, a DSSC consists of crystalline TiO
2
,
an organic dye, and an electrolyte sandwiched
between two electrodes
[44]
. The nanoribbons
were used as the anode, and cis-bis(isothiocyanato)
bis(2,2'-bipyridi-4,4'-dicarboxylato)ruthenium
(II) bis-tetrabutyl-ammonium (Ru535-bisTBA) as
the dye. Depending on the calcination tempera-
ture, structures consisting of pure anatase or
mixed anatase-rutile phases were obtained.
