Biomedical Engineering Reference
In-Depth Information
expected because Al
2
O
3
and ZnO have a great
tendency for a nonequilibrated interfacial diffu-
sion: The Zn ions diffuse much more quickly into
Al
2
O
3
than the Al ions into ZnO, which is com-
pensated by a diffusion of voids from the Al
2
O
3
into ZnO, eventually resulting in a void forma-
tion, the Kirkendall voids
[91]
. The resulting
large surface area per unit mass improves the
photocatalytic activity, which was confirmed
with investigations of the degradation of two
dyes, poly{1-4[4-(3-carboxy-4-hydroxyphenylazo)
benzenesulfonamido]-1,2-ethanediyl sodium salt
(PAZO) and sulforhodamine B, upon ultraviolet
irradiation.
Besides the metal-oxide compounds, ALD
offers the possibility to synthesize noble-metal
nanoparticles. Although ALD is designed to pro-
duce thin films, there is a possibility to grow
nanoparticles with good control to a certain level
because, during the nucleation and initial
growth, the deposited material forms islands on
the substrate. Dependent on the substrate and
the deposited material, the compact and pinhole-
free film may be formed within few cycles, as
expected from the ALD process, but also the film
formation can be hindered and the initial clus-
ters or islands grow to form particles. Examples
of the latter case were shown with the deposition
of Pt nanoparticles on carbon aerogels and stron-
tium titanate nanocubes
[92, 93]
. In a similar
fashion, catalytic Pd and bimetallic PtRu nano-
particles with a narrow size distribution were
also synthesized (
Figure 16.9
)
[94, 95]
.
Nanosized structures from those metals are
highly desired for their catalytic properties
[96, 97]
. A significant application of such cata-
lysts is in biomedicine, where there is now a
focus on nanoparticles as enzyme mimetics
[98]
.
For example, Pt nanoparticles show catalytic
activities that resemble those of enzymes super-
oxide dismutase (SOD) and catalase, which help
cells survive under oxidative stress
[99, 100]
.
Nanoalloys of AgM (M
=
Au, Pd, and Pt) mimic
the enzymes peroxidase and oxidase, which
FIGURE 16.9
(a) TEM images of ALD-grown metal
nanoparticles decorating an alumina sphere. The histo-
gram gives the nanoparticle size distribution measured
from TEM where the mean particle diameter is 1.2 nm
with a distribution width of 0.3 nm. (b) High-resolution
TEM image showing lattice fringes for the Al
2
O
3
and the
PtRu nanoparticles. Reprinted from Ref.
95
. Copyright ©
2010, with permission from the American Chemical
Society.
