the stable oxides, MoO 3 and WO 3 [213]. The oxides are first heated at high temperatures in a reducing atmosphere and then reacted with H 2 S. Reaction with
H 2 Se is used to obtain the selenides [230]. Recognizing that the trisulfides MoS 3
and WS 3 are likely to be the intermediates in the formation of the disulfide nanotubes, the trisulfides have been directly decomposed to obtain the disulfide nanotubes [231]. Diselenide nanotubes have been obtained from the metal triselenides
[232]. The trisulfide route is indeed found to provide a general route for the synthesis of the nanotubes of many metal disulfides such as NbS 2 [233] and HfS 2
[234]. In the case of Mo and W dichalcogenides, it is possible to use the decomposition of the precursor ammonium salt, such as (NH 4 ) 2 MX 4 (X ¼ S, Se; M ¼ Mo,
W) as a means of preparing the nanotubes [231]. Other methods employed for the
synthesis of dichalcogenide nanotubes include hydrothermal methods where the
organic amine is taken as one of the components in the reaction mixture.
The hydrothermal route has been used for synthesizing nanotubes and related
structures of a variety of other inorganic materials as well. Thus, nanotubes of
several metal oxides (e.g., SiO 2 [235], V 2 O 5 [219], ZnO [236]) have been produced
hydrothermally. Nanotubes of oxides such as V 2 O 5 are also conveniently prepared
from a suitable metal oxide precursor in the presence of an organic amine or a
surfactant [237]. Surfactant-assisted synthesis of CdSe and CdS nanotubes has
been reported. Here the metal oxide reacts with the sulfidizing/selenidizing agent
in the presence of a surfactant such as Triton X [238, 246].
Sol–gel chemistry is widely used in the synthesis of metal oxide nanotubes, a
good example being that of silica [216] and TiO 2 [239]. Oxide gels in the presence
of surfactants or suitable templates form nanotubes. For example, by coating carbon nanotubes (CNTs) with oxide gels and then burning off the carbon, one obtains nanotubes and nanowires of a variety of metal oxides including ZrO 2 , SiO 2
and MoO 3 [218, 240]. Sol–gel synthesis of oxide nanotubes is also possible in the
pores of alumina membranes. It should be noted that MoS 2 nanotubes are also
prepared by the decomposition of a precursor in the pores of an alumina membrane [241].
Boron nitride nanotubes have been obtained by striking an electric arc between
HfB 2 electrodes in a N 2 atmosphere [242]. BCN and BC nanotubes are obtained by
arcing between B/C electrodes in an appropriate atmosphere. A greater effort has
gone into the synthesis of BN nanotubes starting with different precursor molecules containing B and N. Decomposition of borazine in the presence of transition
metal nanoparticles and the decomposition of the 1:2 melamine–boric acid addition compound yield BN nanotubes [223, 224]. Reaction of boric acid or B 2 O 3 with
N 2 or NH 3 at high temperature in the presence of activated carbon, carbon nanotubes or catalytic metal particles has been employed to synthesize BN nanotubes
[225]. Goldberger et al. [226b] have synthesized single-crystal GaN nanotubes with
inner diameters of 30–200 nm and wall thicknesses of 5–50 nm by employing
‘epitaxial casting’ approach. They used hexagonal ZnO nanowires as templates for
the epitaxial overgrowth of thin GaN layers in a chemical vapor deposition system.
In a typical experiment they used trimethylgallium and ammonia as precursors
with argon or nitrogen as carrier gas and maintained the deposition temperature at
8.3 Inorganic Nanotubes 245
H 2 Se is used to obtain the selenides [230]. Recognizing that the trisulfides MoS 3
and WS 3 are likely to be the intermediates in the formation of the disulfide nanotubes, the trisulfides have been directly decomposed to obtain the disulfide nanotubes [231]. Diselenide nanotubes have been obtained from the metal triselenides
[232]. The trisulfide route is indeed found to provide a general route for the synthesis of the nanotubes of many metal disulfides such as NbS 2 [233] and HfS 2
[234]. In the case of Mo and W dichalcogenides, it is possible to use the decomposition of the precursor ammonium salt, such as (NH 4 ) 2 MX 4 (X ¼ S, Se; M ¼ Mo,
W) as a means of preparing the nanotubes [231]. Other methods employed for the
synthesis of dichalcogenide nanotubes include hydrothermal methods where the
organic amine is taken as one of the components in the reaction mixture.
The hydrothermal route has been used for synthesizing nanotubes and related
structures of a variety of other inorganic materials as well. Thus, nanotubes of
several metal oxides (e.g., SiO 2 [235], V 2 O 5 [219], ZnO [236]) have been produced
hydrothermally. Nanotubes of oxides such as V 2 O 5 are also conveniently prepared
from a suitable metal oxide precursor in the presence of an organic amine or a
surfactant [237]. Surfactant-assisted synthesis of CdSe and CdS nanotubes has
been reported. Here the metal oxide reacts with the sulfidizing/selenidizing agent
in the presence of a surfactant such as Triton X [238, 246].
Sol–gel chemistry is widely used in the synthesis of metal oxide nanotubes, a
good example being that of silica [216] and TiO 2 [239]. Oxide gels in the presence
of surfactants or suitable templates form nanotubes. For example, by coating carbon nanotubes (CNTs) with oxide gels and then burning off the carbon, one obtains nanotubes and nanowires of a variety of metal oxides including ZrO 2 , SiO 2
and MoO 3 [218, 240]. Sol–gel synthesis of oxide nanotubes is also possible in the
pores of alumina membranes. It should be noted that MoS 2 nanotubes are also
prepared by the decomposition of a precursor in the pores of an alumina membrane [241].
Boron nitride nanotubes have been obtained by striking an electric arc between
HfB 2 electrodes in a N 2 atmosphere [242]. BCN and BC nanotubes are obtained by
arcing between B/C electrodes in an appropriate atmosphere. A greater effort has
gone into the synthesis of BN nanotubes starting with different precursor molecules containing B and N. Decomposition of borazine in the presence of transition
metal nanoparticles and the decomposition of the 1:2 melamine–boric acid addition compound yield BN nanotubes [223, 224]. Reaction of boric acid or B 2 O 3 with
N 2 or NH 3 at high temperature in the presence of activated carbon, carbon nanotubes or catalytic metal particles has been employed to synthesize BN nanotubes
[225]. Goldberger et al. [226b] have synthesized single-crystal GaN nanotubes with
inner diameters of 30–200 nm and wall thicknesses of 5–50 nm by employing
‘epitaxial casting’ approach. They used hexagonal ZnO nanowires as templates for
the epitaxial overgrowth of thin GaN layers in a chemical vapor deposition system.
In a typical experiment they used trimethylgallium and ammonia as precursors
with argon or nitrogen as carrier gas and maintained the deposition temperature at
8.3 Inorganic Nanotubes 245