10.3 Electrodeposition on Surfaces with Step Edges
335
not be evidenced since l was typically unknown. In contrast, the current decayed in
time for MnO 2 deposition, which was accompanied with a lower exponent in the r(t)
relationship. This is probably due to the activation limitation of the MnO 2 deposition
process via the MnO 4
−
+ 2H 2 O + 3e MnO 2 + 4OH
− reaction leading to a pH
change in the unbuffered solution.
Not only may the kinetics of the deposition but that of the dissolution play an
important role in the regulation of the diameter of nanowires obtained by ESED
[42]. When a dissolution pulse with moderate anodic current was applied after the
deposition of the nanowires deposited onto HOPG step edges (see step D in Fig. 10.1),
the experience was that the grains along the step edge forming the nanowire remain
interconnected down to a thickness at which only segregated grains can be seen
during the deposition step. This opens a way for tuning the nanowire diameter in
a range that is inaccessible without a dissolution pulse. However, if the dissolution
conditions were not mild enough, the dissolution took place in the intergranular
zone, hence leading to segregated grains. Concerning the kinetics of the slow-rate
dissolution (i.e., in the nanowire regime), a linear relationship was obtained between
the diameter and the anodization time. This can be also understood from a quantitative
approach. It was assumed that at a moderate dissolution rate a uniform current density
predominates at the surface of the hemicylindrical nanowire. Then, the relationship
between the charge and wire diameter is
Q = −π r l jt
(10.2)
with the same notations as for Eq. 10.1, with a negative sign indicating that cathodic
charge corresponds to growth. By expressing the volume of the object from the charge
and from geometric data of the hemicylindrical wire, one obtains that
−π r l jt V M
z F
=
π r
2 l
2
(10.3)
which leads to
dr
dt
= −
2 j V M
z F
.
(10.4)
It is also remarkable that, if the current distribution along the nanowire is indeed
uniform, the wire corrugation does not change during the anodic dissolution. This is
what was shown in the SEM study of thinned nanowires regardless of the composition
[40, 42].
The practical application of the ESED process stems form the opportunity that the
nanowire bundle deposited on the HOPG surface can be removed with a gentle liftoff procedure that does not damage the wires. The steps of this process are presented
in Fig. 10.7 where the electrical connection mode of the wires on a non-conducting
new substrate is also shown. The sensoric application mode depends on the material
of the nanowires. Palladium nanowires can be used as hydrogen sensor [28, 30].
335
not be evidenced since l was typically unknown. In contrast, the current decayed in
time for MnO 2 deposition, which was accompanied with a lower exponent in the r(t)
relationship. This is probably due to the activation limitation of the MnO 2 deposition
process via the MnO 4
−
+ 2H 2 O + 3e MnO 2 + 4OH
− reaction leading to a pH
change in the unbuffered solution.
Not only may the kinetics of the deposition but that of the dissolution play an
important role in the regulation of the diameter of nanowires obtained by ESED
[42]. When a dissolution pulse with moderate anodic current was applied after the
deposition of the nanowires deposited onto HOPG step edges (see step D in Fig. 10.1),
the experience was that the grains along the step edge forming the nanowire remain
interconnected down to a thickness at which only segregated grains can be seen
during the deposition step. This opens a way for tuning the nanowire diameter in
a range that is inaccessible without a dissolution pulse. However, if the dissolution
conditions were not mild enough, the dissolution took place in the intergranular
zone, hence leading to segregated grains. Concerning the kinetics of the slow-rate
dissolution (i.e., in the nanowire regime), a linear relationship was obtained between
the diameter and the anodization time. This can be also understood from a quantitative
approach. It was assumed that at a moderate dissolution rate a uniform current density
predominates at the surface of the hemicylindrical nanowire. Then, the relationship
between the charge and wire diameter is
Q = −π r l jt
(10.2)
with the same notations as for Eq. 10.1, with a negative sign indicating that cathodic
charge corresponds to growth. By expressing the volume of the object from the charge
and from geometric data of the hemicylindrical wire, one obtains that
−π r l jt V M
z F
=
π r
2 l
2
(10.3)
which leads to
dr
dt
= −
2 j V M
z F
.
(10.4)
It is also remarkable that, if the current distribution along the nanowire is indeed
uniform, the wire corrugation does not change during the anodic dissolution. This is
what was shown in the SEM study of thinned nanowires regardless of the composition
[40, 42].
The practical application of the ESED process stems form the opportunity that the
nanowire bundle deposited on the HOPG surface can be removed with a gentle liftoff procedure that does not damage the wires. The steps of this process are presented
in Fig. 10.7 where the electrical connection mode of the wires on a non-conducting
new substrate is also shown. The sensoric application mode depends on the material
of the nanowires. Palladium nanowires can be used as hydrogen sensor [28, 30].