independent of their structure and morphology. Thus, BN tubes can be used as
nano-insulating devices for encapsulating conducting materials like metallic wires.
Filled BN nanotubes are expected to be useful in nanoscale electronic devices and
for the preparation of nano-structured ceramics.
Electrochemical studies have been performed with the alkylammonium intercalated VO x nanotubes [256] as well as Mn intercalated VO x nanotubes [257]. Cyclic
voltammetry studies of alkylammoniumaVO x nanotubes showed a single reduction peak, which broadened, on replacing the amine with Na, with an additional
peak. Li ion reactivity has also been tested with MnaVO x nanotubes by reacting
with n-butyllithium, and it was found that @2 lithiums per V ion are consumed.
Electrochemical Li intercalation of MnaVO x nanotubes shows that 0.5 Li ions per
V atom were intercalated above 2 V [257]. This observation may be relevant to battery applications.
CNTs have been used as AFM tips and there appears to be every likelihood that
extremely narrow structures can be probed [179]. WS 2 could be mounted on the
ultrasharp Si tip following a similar methodology. These tips were tested in an
AFM microscope by imaging a replica of high aspect ratio, and it was observed that
these WS 2 nanotube tips provide a considerable improvement in the image quality
compared to the conventional ultrasharp Si tips [258].
The most likely application of the chalcogenide nanotubes is as solid lubricants.
Mo and W chalcogenides are widely used as solid lubricants. It has been observed
that the hollow nanoparticles of WS 2 show better tribological properties and act
as a better lubricant compared to the bulk phase in every respect (friction, wear
and life-time of the lubricant) [259]. Tribological properties of 2HaMoS 2 and WS 2
powder can be attributed to the weak van der Waals forces between the layers,
which allow easy shear of the films with respect to each other. The mechanism in
the WS 2 nanostructures is somewhat different and the better tribological properties may arise from the rolling friction allowed by the round shape of the nanostructures.
Recently, open-tipped MoS 2 nanotubes were prepared by the decomposition of
ball-milled ammonium thiomolybdate powder under a H 2 athiophene atmosphere,
and used as catalyst for the methanation of CO with H 2 [260]. The conversion of
CO to CH 4 was achieved at a much lower temperature compared to polycrystalline
MoS 2 particles, and there was no deterioration even after 50 h of consecutive catalysis cycles. This observation is of importance in the context of energy conversion
of global CO 2 .
Inorganic nanotubes have emerged as a group of interesting materials. Although
this area of research started with the layered metal chalcogenides, recent results
suggest that other inorganic materials can also be prepared in the form of nanotubes, as typified by the metal oxides. It is likely that many new types of inorganic
nanotubes will be made in the near future. These would include metal nanotubes
as well as nanotubes of inorganic compounds such as MgB 2 , GeO 2 and GaSe.
Theoretical calculations indeed predict a stable nanotubular structure for GaSe
[261]. Various layered materials could be explored for this purpose.
8 Nanotubes and Nanowires
254
nano-insulating devices for encapsulating conducting materials like metallic wires.
Filled BN nanotubes are expected to be useful in nanoscale electronic devices and
for the preparation of nano-structured ceramics.
Electrochemical studies have been performed with the alkylammonium intercalated VO x nanotubes [256] as well as Mn intercalated VO x nanotubes [257]. Cyclic
voltammetry studies of alkylammoniumaVO x nanotubes showed a single reduction peak, which broadened, on replacing the amine with Na, with an additional
peak. Li ion reactivity has also been tested with MnaVO x nanotubes by reacting
with n-butyllithium, and it was found that @2 lithiums per V ion are consumed.
Electrochemical Li intercalation of MnaVO x nanotubes shows that 0.5 Li ions per
V atom were intercalated above 2 V [257]. This observation may be relevant to battery applications.
CNTs have been used as AFM tips and there appears to be every likelihood that
extremely narrow structures can be probed [179]. WS 2 could be mounted on the
ultrasharp Si tip following a similar methodology. These tips were tested in an
AFM microscope by imaging a replica of high aspect ratio, and it was observed that
these WS 2 nanotube tips provide a considerable improvement in the image quality
compared to the conventional ultrasharp Si tips [258].
The most likely application of the chalcogenide nanotubes is as solid lubricants.
Mo and W chalcogenides are widely used as solid lubricants. It has been observed
that the hollow nanoparticles of WS 2 show better tribological properties and act
as a better lubricant compared to the bulk phase in every respect (friction, wear
and life-time of the lubricant) [259]. Tribological properties of 2HaMoS 2 and WS 2
powder can be attributed to the weak van der Waals forces between the layers,
which allow easy shear of the films with respect to each other. The mechanism in
the WS 2 nanostructures is somewhat different and the better tribological properties may arise from the rolling friction allowed by the round shape of the nanostructures.
Recently, open-tipped MoS 2 nanotubes were prepared by the decomposition of
ball-milled ammonium thiomolybdate powder under a H 2 athiophene atmosphere,
and used as catalyst for the methanation of CO with H 2 [260]. The conversion of
CO to CH 4 was achieved at a much lower temperature compared to polycrystalline
MoS 2 particles, and there was no deterioration even after 50 h of consecutive catalysis cycles. This observation is of importance in the context of energy conversion
of global CO 2 .
Inorganic nanotubes have emerged as a group of interesting materials. Although
this area of research started with the layered metal chalcogenides, recent results
suggest that other inorganic materials can also be prepared in the form of nanotubes, as typified by the metal oxides. It is likely that many new types of inorganic
nanotubes will be made in the near future. These would include metal nanotubes
as well as nanotubes of inorganic compounds such as MgB 2 , GeO 2 and GaSe.
Theoretical calculations indeed predict a stable nanotubular structure for GaSe
[261]. Various layered materials could be explored for this purpose.
8 Nanotubes and Nanowires
254