600–700
C. The ZnO nanowire templates were subsequently removed by thermal
reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the
substrates.
A wide range of examples of organic templates (organogels) and extensive coverage of the methods for the formation of morphologically interesting inorganic
materials were given in a review by Shinkai’s group [214]. The organogels present
a wider range of morphologies, such as fibrous, tubular, ribbon-like, lamellar, hollow spherical, than have been transcribed so far. These organogels are comprised
of an organic liquid and low concentrations (< 0:5 wt.%) of relatively low molecular weight molecules (namely gelators) [243], the morphology of which may be
spherical or fibrous. These organogels are usually prepared by heating a mixture
of a gelator and solvent until the solid dissolves, upon cooling the solution (sol)
thickens to form a gel. The gelation of a number of compounds such as tetraethylorthosilicate (TEOS) along with a cholesterol-based gelator, under acidic pH conditions, followed by polycondensation, exhibits a network of fibers with diameter
ranging from 50–200 nm [244]. Subsequent drying and calcination steps resulted
in silica tubes without the presence of the original organic template.
8.3.3
Structures
Some of the important inorganic nanotubes synthesized and characterized in the
last few years are the following [22]:
Chalcogenides: MoS 2 , WS 2 , MoSe 2 , WSe 2 , NbS 2 , NbSe 2 , HfS 2 , ZrS 2
Oxides: TiO 2 , ZrO 2 , VO x , SiO 2 , ZnO, Ga 2 O 3 , BaTiO 3 , PbTiO 3
Nitrides: BN, GaN
Halides: NiCl 2
Metals: Ni, Cu, Te, Co, Fe
In Figure 8.22 we show the TEM images of MoS 2 nanotubes prepared by the direct
thermal decomposition of the ammonium thiomolybdate in H 2 atmosphere (flow)
[231]. In addition to providing a direct method for the preparation of dichalcogenide nanotubes [231], the trichalcogenide or the ammonium chalcometallate route
also enables the easy synthesis of nanotubes of the other layered dichalcogenides
[232–234]. The structure of MoS 2 consists of disulfide layers stacked along the cdirection [211]. This implies that the SaS interaction between the MoS 2 slabs is
weaker than the intralayer interactions. The SaS interlayer distances are therefore susceptible to distortions during the folding of the layers. This is exemplified by the slight expansion of the c-axis (2%) in the MoS 2 nanotubes [229]. Highresolution (HREM) images of the disulfide nanotubes show stacking of the (002)
planes parallel to the tube axis. The distance between the layer fringes corresponds
to the d(002) spacing. In Figure 8.23(a) we show the SEM image of nanotubes of
HfS 2 which are quite lengthy, some being more than a micron long. Interestingly,
a large proportion of these nanostructures are nanotubes. In Figure 8.23(b) and (c),
8 Nanotubes and Nanowires
246
C. The ZnO nanowire templates were subsequently removed by thermal
reduction and evaporation, resulting in ordered arrays of GaN nanotubes on the
substrates.
A wide range of examples of organic templates (organogels) and extensive coverage of the methods for the formation of morphologically interesting inorganic
materials were given in a review by Shinkai’s group [214]. The organogels present
a wider range of morphologies, such as fibrous, tubular, ribbon-like, lamellar, hollow spherical, than have been transcribed so far. These organogels are comprised
of an organic liquid and low concentrations (< 0:5 wt.%) of relatively low molecular weight molecules (namely gelators) [243], the morphology of which may be
spherical or fibrous. These organogels are usually prepared by heating a mixture
of a gelator and solvent until the solid dissolves, upon cooling the solution (sol)
thickens to form a gel. The gelation of a number of compounds such as tetraethylorthosilicate (TEOS) along with a cholesterol-based gelator, under acidic pH conditions, followed by polycondensation, exhibits a network of fibers with diameter
ranging from 50–200 nm [244]. Subsequent drying and calcination steps resulted
in silica tubes without the presence of the original organic template.
8.3.3
Structures
Some of the important inorganic nanotubes synthesized and characterized in the
last few years are the following [22]:
Chalcogenides: MoS 2 , WS 2 , MoSe 2 , WSe 2 , NbS 2 , NbSe 2 , HfS 2 , ZrS 2
Oxides: TiO 2 , ZrO 2 , VO x , SiO 2 , ZnO, Ga 2 O 3 , BaTiO 3 , PbTiO 3
Nitrides: BN, GaN
Halides: NiCl 2
Metals: Ni, Cu, Te, Co, Fe
In Figure 8.22 we show the TEM images of MoS 2 nanotubes prepared by the direct
thermal decomposition of the ammonium thiomolybdate in H 2 atmosphere (flow)
[231]. In addition to providing a direct method for the preparation of dichalcogenide nanotubes [231], the trichalcogenide or the ammonium chalcometallate route
also enables the easy synthesis of nanotubes of the other layered dichalcogenides
[232–234]. The structure of MoS 2 consists of disulfide layers stacked along the cdirection [211]. This implies that the SaS interaction between the MoS 2 slabs is
weaker than the intralayer interactions. The SaS interlayer distances are therefore susceptible to distortions during the folding of the layers. This is exemplified by the slight expansion of the c-axis (2%) in the MoS 2 nanotubes [229]. Highresolution (HREM) images of the disulfide nanotubes show stacking of the (002)
planes parallel to the tube axis. The distance between the layer fringes corresponds
to the d(002) spacing. In Figure 8.23(a) we show the SEM image of nanotubes of
HfS 2 which are quite lengthy, some being more than a micron long. Interestingly,
a large proportion of these nanostructures are nanotubes. In Figure 8.23(b) and (c),
8 Nanotubes and Nanowires
246