382
11 Templated Systems
Ni is the major shell component in both cases). The core–shell type nanowires thus
produced make it possible to obtain nanotubes by selectively dissolving the core
component [179].
Coverage of the template by a thin enough conductive strike layer. The big
majority of works in which the thoughtful electrosynthesis of nanotubes in template
was carried out employed a thin electrically conducting coating on one side of the
membrane. The thickness of the deposit is at least half of an order of magnitude
smaller than the pore diameter; hence, the pores remained open after the coating
process and a conducting rim occurred around the perimeter of the nanopores only.
If the deposition rate was high enough to achieve a fast depletion of the centre of the
pores with respect to the reactant, a steady-state nanotube growth could be achieved
(see Fig. 11.8a). This strategy was successful for the deposition of a great variety of
nanotubes like Au [173], Pt [171, 180], Pd [181] Co [171], Fe [182, 183], Ni [80,
184–186], Bi [187], Co–Pt [171, 188], ZnO [189], etc.
Hydrogen evolution. The strong deviation of the deposition efficiency from 1
was also found to be related to the formation of nanotubes [190–193]. In some
other cases, the current efficiency was not measured, but the potential applied fell
beyond the stability regime of water, and the hydrogen evolution was hypothesized
as the factor responsible for the nanotube formation [172]. It cannot be established
systematically in which cases the hydrogen evolved blocks the pores and when they
modify the deposition mode from dense to tubular. It seems to be a criterion that the
nanopore diameter should be at least 200 nm. It was found for various materials that
the decrease in pH and the application of a more negative deposition potential may
be enough for either inducing the growth of nanotubes instead of nanowires; if such
changes are applied in the nanotube growth regime, the pore wall of the nanotubes
becomes thinner. The latter trends are in accord with the theorem that the gas evolved
blocks the centre of the nanopores.
Nanotube-to-nanowire transition during the pore filling. It has been reported for
various metallic [194–196] and non-metallic [197, 198] materials that the initial
nanotube growth turns into nanowire growth merely by the larger time elapsed after
the start of the electrodeposition. The resulting objects in the membrane pore are
either hybrid particles having an axial cavity at the substrate side and a compact
structure at the solution side or nanowires with a protrusion at the perimeter at the
bottom side.
The transition mechanism is visualized in Fig. 11.8b. If the nucleation at the
substrate size is restricted to the perimeter but the growth is not diffusion-limited,
the sufficient reactant supply can lead to a partly sidewise deposition ranging to the
internal cavity of the nanowire. After a sufficiently long time, the cavity can close,
which is in accord with the energetic minimum due to the disappearance of a surface
element with very high curvature. The closure of the initially formed nanotube cavity
can be associated with a drop in the electrical current in the chronoamperometric
curves [196]. The initially formed cavity in compact nanowires can be visualized
by partially dissolving the template and imaging the bottom side of the nanowires
[199]. As an extreme example, the full closure of nanotubes formed all along the
11 Templated Systems
Ni is the major shell component in both cases). The core–shell type nanowires thus
produced make it possible to obtain nanotubes by selectively dissolving the core
component [179].
Coverage of the template by a thin enough conductive strike layer. The big
majority of works in which the thoughtful electrosynthesis of nanotubes in template
was carried out employed a thin electrically conducting coating on one side of the
membrane. The thickness of the deposit is at least half of an order of magnitude
smaller than the pore diameter; hence, the pores remained open after the coating
process and a conducting rim occurred around the perimeter of the nanopores only.
If the deposition rate was high enough to achieve a fast depletion of the centre of the
pores with respect to the reactant, a steady-state nanotube growth could be achieved
(see Fig. 11.8a). This strategy was successful for the deposition of a great variety of
nanotubes like Au [173], Pt [171, 180], Pd [181] Co [171], Fe [182, 183], Ni [80,
184–186], Bi [187], Co–Pt [171, 188], ZnO [189], etc.
Hydrogen evolution. The strong deviation of the deposition efficiency from 1
was also found to be related to the formation of nanotubes [190–193]. In some
other cases, the current efficiency was not measured, but the potential applied fell
beyond the stability regime of water, and the hydrogen evolution was hypothesized
as the factor responsible for the nanotube formation [172]. It cannot be established
systematically in which cases the hydrogen evolved blocks the pores and when they
modify the deposition mode from dense to tubular. It seems to be a criterion that the
nanopore diameter should be at least 200 nm. It was found for various materials that
the decrease in pH and the application of a more negative deposition potential may
be enough for either inducing the growth of nanotubes instead of nanowires; if such
changes are applied in the nanotube growth regime, the pore wall of the nanotubes
becomes thinner. The latter trends are in accord with the theorem that the gas evolved
blocks the centre of the nanopores.
Nanotube-to-nanowire transition during the pore filling. It has been reported for
various metallic [194–196] and non-metallic [197, 198] materials that the initial
nanotube growth turns into nanowire growth merely by the larger time elapsed after
the start of the electrodeposition. The resulting objects in the membrane pore are
either hybrid particles having an axial cavity at the substrate side and a compact
structure at the solution side or nanowires with a protrusion at the perimeter at the
bottom side.
The transition mechanism is visualized in Fig. 11.8b. If the nucleation at the
substrate size is restricted to the perimeter but the growth is not diffusion-limited,
the sufficient reactant supply can lead to a partly sidewise deposition ranging to the
internal cavity of the nanowire. After a sufficiently long time, the cavity can close,
which is in accord with the energetic minimum due to the disappearance of a surface
element with very high curvature. The closure of the initially formed nanotube cavity
can be associated with a drop in the electrical current in the chronoamperometric
curves [196]. The initially formed cavity in compact nanowires can be visualized
by partially dissolving the template and imaging the bottom side of the nanowires
[199]. As an extreme example, the full closure of nanotubes formed all along the