obtain nanotube structures of metal oxides [217, 218]. Oxides such as V 2 O 5 have
good catalytic activity in the bulk phase. Redox catalytic activity is also retained in
the nanotubular structure. There have been efforts to prepare V 2 O 5 nanotubes by
chemical methods as well [219].
Boron nitride (BN) crystallizes in a graphite-like structure and can be simply
viewed as replacing a CaC pair in the graphene sheet with the iso-electronic BaN
pair. It can, therefore, be considered as an ideal precursor for the formation of
BN nanotubes. Replacement of the CaC pairs partly or entirely by the BaN pairs in
the hexagonal network of graphite leads to the formation of a wide array of twodimensional phases that can form hollow cage structures and nanotubes. The
possibility of replacing CaC pairs by BaN pairs in the hollow cage structure of C 60
was predicted [221] and verified experimentally [222]. BN-doped carbon nanotubes
have been prepared [112, 115]. Pure BN nanotubes have been generated by employing several procedures, yielding nanotubes with varying wall thickness and
morphology [223–225]. It is therefore quite possible that nanotube structures of
other layered materials can be prepared as well. For example, many metal halides
(e.g., NiCl 2 ), oxides (e.g., ZnO) and nitrides (e.g., GaN) crystallize in layered structures and nanotubes of such materials have indeed been characterized [226].
Nanotubes of elemental materials such as Te [227a] and Ni [227b] have also been
prepared. There is considerable interest at present in preparing exotic nanotubes
and studying their properties. In this section, we discuss the synthesis and characterization of nanotubes of chalcogenides of Mo, W and other metals, metal oxides, BN and other materials and present the current status of the subject. We
briefly examine some of the important properties of the inorganic nanotubes and
indicate possible future directions.
8.3.2
General Synthetic Strategies
Several strategies have been employed for the synthesis of carbon nanotubes [20a].
In addition to arc evaporation and pyrolysis methods, carbon nanotubes have been
prepared by laser ablation of graphite, electrochemical and templating (using porous alumina membrane) techniques [228]. The above methods broadly fall into
two categories. Methods such as the arc evaporation of graphite employ processes
which are far from equilibrium. The chemical routes are generally closer to equilibrium conditions. Nanotubes of metal chalcogenides and boron nitride are also
prepared by employing techniques similar to those of carbon nanotubes, although
there is an inherent difference in that the nanotubes of inorganic materials such as
MoS 2 or BN would require reactions involving the component elements or compounds containing the elements. Decomposition of precursor compounds containing the elements is another possible route.
Nanotubes of dichalcogenides such as MoS 2 , MoSe 2 and WS 2 are also obtained
by employing processes far from equilibrium, such as arc discharge and laser ablation [229]. By far the most successful routes employ appropriate chemical reactions. Thus, MoS 2 and WS 2 nanotubes are conveniently prepared by starting with
8 Nanotubes and Nanowires
244
good catalytic activity in the bulk phase. Redox catalytic activity is also retained in
the nanotubular structure. There have been efforts to prepare V 2 O 5 nanotubes by
chemical methods as well [219].
Boron nitride (BN) crystallizes in a graphite-like structure and can be simply
viewed as replacing a CaC pair in the graphene sheet with the iso-electronic BaN
pair. It can, therefore, be considered as an ideal precursor for the formation of
BN nanotubes. Replacement of the CaC pairs partly or entirely by the BaN pairs in
the hexagonal network of graphite leads to the formation of a wide array of twodimensional phases that can form hollow cage structures and nanotubes. The
possibility of replacing CaC pairs by BaN pairs in the hollow cage structure of C 60
was predicted [221] and verified experimentally [222]. BN-doped carbon nanotubes
have been prepared [112, 115]. Pure BN nanotubes have been generated by employing several procedures, yielding nanotubes with varying wall thickness and
morphology [223–225]. It is therefore quite possible that nanotube structures of
other layered materials can be prepared as well. For example, many metal halides
(e.g., NiCl 2 ), oxides (e.g., ZnO) and nitrides (e.g., GaN) crystallize in layered structures and nanotubes of such materials have indeed been characterized [226].
Nanotubes of elemental materials such as Te [227a] and Ni [227b] have also been
prepared. There is considerable interest at present in preparing exotic nanotubes
and studying their properties. In this section, we discuss the synthesis and characterization of nanotubes of chalcogenides of Mo, W and other metals, metal oxides, BN and other materials and present the current status of the subject. We
briefly examine some of the important properties of the inorganic nanotubes and
indicate possible future directions.
8.3.2
General Synthetic Strategies
Several strategies have been employed for the synthesis of carbon nanotubes [20a].
In addition to arc evaporation and pyrolysis methods, carbon nanotubes have been
prepared by laser ablation of graphite, electrochemical and templating (using porous alumina membrane) techniques [228]. The above methods broadly fall into
two categories. Methods such as the arc evaporation of graphite employ processes
which are far from equilibrium. The chemical routes are generally closer to equilibrium conditions. Nanotubes of metal chalcogenides and boron nitride are also
prepared by employing techniques similar to those of carbon nanotubes, although
there is an inherent difference in that the nanotubes of inorganic materials such as
MoS 2 or BN would require reactions involving the component elements or compounds containing the elements. Decomposition of precursor compounds containing the elements is another possible route.
Nanotubes of dichalcogenides such as MoS 2 , MoSe 2 and WS 2 are also obtained
by employing processes far from equilibrium, such as arc discharge and laser ablation [229]. By far the most successful routes employ appropriate chemical reactions. Thus, MoS 2 and WS 2 nanotubes are conveniently prepared by starting with
8 Nanotubes and Nanowires
244