380
11 Templated Systems
where S is Seebeck coefficient, σ is the electrical conductivity and κ is the thermal
conductivity. (The relationship is often given in a form by multiplying both sides
with the absolute temperature.)
Although quantum confinement effects lead to a power factor enhancement if
the nanowire radius is around 10 nm, the decrease of the thermal conductivity has
a measurable contribution to the power factor enhancement with diameters below
100 nm. Also, the template synthesis can easily produce single-crystalline nanowires,
which is a great advantage due to the elimination of the grain boundary electron
scattering, hence leading to an electrical conductivity improvement as compared to
polycrystalline bulk materials. It was shown for various thermoelectric materials that
the power factor enhancement predicted on the basis if the spatial confinement can
indeed be achieved [22, 136].
11.2.8 Metallic Nanotubes Obtained by Electrodeposition
into Nanochannel Templates
It was commonly observed that nanotubes can be grown by electrodeposition into
nanochannel templates. As it was described in a comparative study [170], the
chronoamperometric transient measured during the nanotube formation is qualitatively very similar to that observed for the nanowire growth. The difference is that the
nanotube formation is a much faster process, simply because it requires the transport
of a much smaller amount of precursor material. Regarding the deposition conditions,
it must be mentioned that the elecrosynthesis of templated nanotubes often takes place
with current control instead of potential control. This is because of the necessity of
the maintenance of either a large electric field or a fully diffusion-controlled process.
Figure 11.8a shows a mass transport scheme for the cross section of a growing
nanotube within a template channel. It can be seen that the filling of the internal cavity
already formed in a nanotube is not possible since once the diffusing reactant can
reach the top zone of the nanotube, it reacts in the close vicinity of the pore ending.
Therefore, the internal cavity within the nanotube is fully depleted with respect to
the reactant. Two examples for electrodeposited nanotubes are shown in Fig. 11.9.
It can be seen in both images that the nanotubes are composed of several grains not
only as tube segments but also along the tube perimeter.
Concerning the circumstances of the nanotube formation, a large number of factors
are mentioned in the literature. The role of each factor has been plausibly evidenced;
therefore, it is possible that in some cases, the synergy of more than one single factor
prevails, resulting in the formation of nanotubes instead of nanowires. The relevant
experimental parameters are given below.
Interaction of the growing deposit with the pore walls of the template. This method
was introduced in 1991 [173] for the preparation of gold nanotubes in membranes
by using the functionalization of the internal pore walls with 2-cyanoethyl terminal
groups that serve as anchoring centres for the growing nanotube [174–176]. For
11 Templated Systems
where S is Seebeck coefficient, σ is the electrical conductivity and κ is the thermal
conductivity. (The relationship is often given in a form by multiplying both sides
with the absolute temperature.)
Although quantum confinement effects lead to a power factor enhancement if
the nanowire radius is around 10 nm, the decrease of the thermal conductivity has
a measurable contribution to the power factor enhancement with diameters below
100 nm. Also, the template synthesis can easily produce single-crystalline nanowires,
which is a great advantage due to the elimination of the grain boundary electron
scattering, hence leading to an electrical conductivity improvement as compared to
polycrystalline bulk materials. It was shown for various thermoelectric materials that
the power factor enhancement predicted on the basis if the spatial confinement can
indeed be achieved [22, 136].
11.2.8 Metallic Nanotubes Obtained by Electrodeposition
into Nanochannel Templates
It was commonly observed that nanotubes can be grown by electrodeposition into
nanochannel templates. As it was described in a comparative study [170], the
chronoamperometric transient measured during the nanotube formation is qualitatively very similar to that observed for the nanowire growth. The difference is that the
nanotube formation is a much faster process, simply because it requires the transport
of a much smaller amount of precursor material. Regarding the deposition conditions,
it must be mentioned that the elecrosynthesis of templated nanotubes often takes place
with current control instead of potential control. This is because of the necessity of
the maintenance of either a large electric field or a fully diffusion-controlled process.
Figure 11.8a shows a mass transport scheme for the cross section of a growing
nanotube within a template channel. It can be seen that the filling of the internal cavity
already formed in a nanotube is not possible since once the diffusing reactant can
reach the top zone of the nanotube, it reacts in the close vicinity of the pore ending.
Therefore, the internal cavity within the nanotube is fully depleted with respect to
the reactant. Two examples for electrodeposited nanotubes are shown in Fig. 11.9.
It can be seen in both images that the nanotubes are composed of several grains not
only as tube segments but also along the tube perimeter.
Concerning the circumstances of the nanotube formation, a large number of factors
are mentioned in the literature. The role of each factor has been plausibly evidenced;
therefore, it is possible that in some cases, the synergy of more than one single factor
prevails, resulting in the formation of nanotubes instead of nanowires. The relevant
experimental parameters are given below.
Interaction of the growing deposit with the pore walls of the template. This method
was introduced in 1991 [173] for the preparation of gold nanotubes in membranes
by using the functionalization of the internal pore walls with 2-cyanoethyl terminal
groups that serve as anchoring centres for the growing nanotube [174–176]. For