template simply serves as a scaffold against which other kinds of materials with
similar morphologies are synthesized. In other words, the in situ generated material is shaped into a nanostructure with its morphology complementary to that of
the template. These templates could be nanoscale channels within mesoporous
materials or porous alumina and polycarbonate membranes. They can be filled
using (i) a solution route or (ii) a sol–gel technique or (iii) an electrochemical route
to generate 1D nanowires. The produced nanowires can be released from the templates by selectively removing the host matrix [274]. Unlike the polymer membranes fabricated by track itching, porous AAO membranes containing hexagonally packed 2D array of cylindrical pores with a uniform size are prepared using
anodization of aluminum foils in an acidic medium (Figure 8.34(a)). Many materials have been fabricated into nanowires using porous anodic alumina membranes (AAM) in a templating process, including various inorganic materials such
as Au, Ag, Pt, TiO 2 , MnO 2 , ZnO, SnO 2 , electronically conducting polymers: polypyrrole, poly(3-methylthiophene), and polyaniline, and carbon nanotubules [310a].
Figure 8.34(b) shows the highly ordered In 2 O 3 nanowires uniformly assembled
into the hexagonally ordered nanochannels of the AAM by oxidizing the In nanowire arrays electrodeposited in the nanochannels of the AAM [310b]. Figure 8.34(c)
shows a TEM image of In 2 O 3 nanowires after removing the AAM from the In 2 O 3 /
AAM samples by dissolving the AAM in NaOH solution followed by washing
several times with distilled water. Besides alumina and polymer membranes,
with their high surface areas and unifirm pore sizes, mesoporous silica materials
(MCM-41 or SBA-15) have been successfully used as templates for the synthesis of
polymer and inorganic nanowires [311–315]. Ag nanowires of uniform diameters
of 5–6 nm and large aspect ratios between 100 and 1000 are synthesized by AgNO 3
solution impregnation in SBA-15 or MCM-41 template followed by thermal decomposition. Similarly Ge nanowires have been successfully synthesized within
the mesochannels of MCM-41 [315a].
Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating 1D nanostructures in
relatively large quantities. It is well known that at critical micellar concentration
(CMC) surfactant molecules spontaneously organize into rod-shaped micelles
[315c]. These anisotropic structures can be used immediately as soft templates to
promote the formation of nanorods when coupled with appropriate chemical or
electrochemical reaction. The surfactant needs to be selectively removed to collect
the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by
using surfactants such as Na-AOT or Triton X of known concentrations [238, 246].
The nanowires themselves can be used as templates to generate the nanowires
of other materials. The template may be coated onto the nanowire (physically)
forming coaxial nanocables [316a], or it may react with the nanowires forming a
new material [317a,b]. In the physical (solution or sol–gel coating) approach, the
surfaces of the nanowires could be directly coated with conformal sheaths made of
a different material to form coaxial nanocables. Subsequent dissolution of the
original nanowires could lead to nanotubes of the coated material. The sol–gel
8.4 Nanowires 267
similar morphologies are synthesized. In other words, the in situ generated material is shaped into a nanostructure with its morphology complementary to that of
the template. These templates could be nanoscale channels within mesoporous
materials or porous alumina and polycarbonate membranes. They can be filled
using (i) a solution route or (ii) a sol–gel technique or (iii) an electrochemical route
to generate 1D nanowires. The produced nanowires can be released from the templates by selectively removing the host matrix [274]. Unlike the polymer membranes fabricated by track itching, porous AAO membranes containing hexagonally packed 2D array of cylindrical pores with a uniform size are prepared using
anodization of aluminum foils in an acidic medium (Figure 8.34(a)). Many materials have been fabricated into nanowires using porous anodic alumina membranes (AAM) in a templating process, including various inorganic materials such
as Au, Ag, Pt, TiO 2 , MnO 2 , ZnO, SnO 2 , electronically conducting polymers: polypyrrole, poly(3-methylthiophene), and polyaniline, and carbon nanotubules [310a].
Figure 8.34(b) shows the highly ordered In 2 O 3 nanowires uniformly assembled
into the hexagonally ordered nanochannels of the AAM by oxidizing the In nanowire arrays electrodeposited in the nanochannels of the AAM [310b]. Figure 8.34(c)
shows a TEM image of In 2 O 3 nanowires after removing the AAM from the In 2 O 3 /
AAM samples by dissolving the AAM in NaOH solution followed by washing
several times with distilled water. Besides alumina and polymer membranes,
with their high surface areas and unifirm pore sizes, mesoporous silica materials
(MCM-41 or SBA-15) have been successfully used as templates for the synthesis of
polymer and inorganic nanowires [311–315]. Ag nanowires of uniform diameters
of 5–6 nm and large aspect ratios between 100 and 1000 are synthesized by AgNO 3
solution impregnation in SBA-15 or MCM-41 template followed by thermal decomposition. Similarly Ge nanowires have been successfully synthesized within
the mesochannels of MCM-41 [315a].
Mesophase structures self-assembled from surfactants (Figure 8.35) provide another class of useful and versatile templates for generating 1D nanostructures in
relatively large quantities. It is well known that at critical micellar concentration
(CMC) surfactant molecules spontaneously organize into rod-shaped micelles
[315c]. These anisotropic structures can be used immediately as soft templates to
promote the formation of nanorods when coupled with appropriate chemical or
electrochemical reaction. The surfactant needs to be selectively removed to collect
the nanorods/nanowires as a relatively pure sample. Based on this principle, nanowires of CuS, CuSe, CdS, CdSe, ZnS and ZnSe have been grown selectively by
using surfactants such as Na-AOT or Triton X of known concentrations [238, 246].
The nanowires themselves can be used as templates to generate the nanowires
of other materials. The template may be coated onto the nanowire (physically)
forming coaxial nanocables [316a], or it may react with the nanowires forming a
new material [317a,b]. In the physical (solution or sol–gel coating) approach, the
surfaces of the nanowires could be directly coated with conformal sheaths made of
a different material to form coaxial nanocables. Subsequent dissolution of the
original nanowires could lead to nanotubes of the coated material. The sol–gel
8.4 Nanowires 267