Guozhong Cao. Nanostructures & Nanomaterials: Synthesis, Properties & Applications
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156
Nanostructures and Nanomaterials
Fig.
4.26.
(a) SEM micrograph
of
nanorods and (b) X-ray diffraction spectra
of
Pb(Zr,Ti)O,
nanorods
grown
by template-based
sol-gel
electrophoretic deposition. [S.J. Lirnrner,
S.
Seraji,
M.J. Forbess,
Y.
Wu,
T.P.
Chou,
C.
Nguyen, and
G.Z.
Cao,
Adv.
Muter:
13,
1269
(2001).]
Electrostatic attraction between the ZnO nanoparticles and the pore walls
then leads to tubule formation.
Miao
et
al.
I3l
prepared single crystalline TiO, nanowires by template-
based electrochemically induced sol-gel deposition. Titania electrolyte solu-
tion was prepared using a method developed by Natarajan and N~gami,
in which Ti powder was dissolved into a
H202
and NH40H aqueous solu-
tion and formed Ti02+ ionic clusters. When an external electric field was
applied, Ti02+ ionic clusters diffused to cathode and underwent hydrolysis
One-Dimensional Nanostructures: Nanowires and Nanorods
157
Fig.
4.27.
SEM
images
of
the nanorods
of
(A)
SiOz,
(B)
Ti02,
(C)
Sr2Nb207 and
(D)
BaTi03 grown by template-based sol-gel electrophoretic deposition.
[S.J.
Limmer,
S.
Seraji,
M.J.
Forbess,
Y.
Wu,
T.P.
Chou,
C.
Nguyen, and
G.Z.
Cao,
Adv.
Func.
Muter:
12,
59
(2002).]
and condensation reactions, resulting in deposition of nanorods of amor-
phous TiO, gel. Afler heat treatment at 240°C for 24 hrs in air, nanowires of
single crystal TiOz with anatase structure and with diameters of
10,
20, and
40 nm and lengths ranging from 2 to
10
pm were synthesized. However, no
axis crystal orientation was identified. The formation of single crystal TiO,
nanorods here is different from that reported by Martin's Here the
formation of single crystal Ti02 is via crystallization
of
amorphous phase at
an elevated temperature, whereas nanoscale crystalline TiO, particles are
believed to assemble epitaxially to form a single crystal nanorod. Epitaxial
agglomeration of two nanoscale crystalline particles has been reported,'34
though no large single crystals have been produced by assembling
nanocrystalline particles. Figure 4.28 shows the micrograph of single crys-
tal nanorods of Ti02 grown by template-based electrochemically induced
sol-gel deposition.
l3I
4.3.3.
Template filling
Direct template filling is the most straightforward and versatile method in
preparation of nanorods and nanotubules. Most commonly, a liquid precur-
sor or precursor mixture is used to fill the pores. There are several concerns
158
Nanostructures and Nanomaterials
Fig.
4.28.
SEM
micrograph
of
single crystal nanorods
of
TiOz grown by template-based
electrochemically induced sol-gel deposition. [Z. Miao,
D.
Xu,
J.
Ouyang, G. Guo, Z. Zhao,
and
Y.
Tang,
Nuno
Lett.
2,
71
7
(2002).]
in the template filling. First of all, the wetability of the pore wall should be
good enough to permit the penetration and complete filling of the liquid
precursor or precursor mixture. For filling at low temperatures, the surface
of pore walls can be easily modified to be either hydrophilic or hydropho-
bic by introducing a monolayer of organic molecules. Second, the template
materials should be chemically inert. Thirdly, control of shrinkage during
solidification is required. If adhesion between the pore walls and the filling
material is weak or solidification starts at the center, or from one end of the
pore, or uniformly, solid nanorods are most likely to form. However, if the
adhesion is very strong, or the solidification starts at the interfaces and pro-
ceeds inwardly, it is most likely to form hollow nanotubules.
4.3.3.1
.
Colloidal dispersion filling
Martin and his co-~orkers'~~,'~~ have studied the formation of various
oxide nanorods and nanotubules by simply filling the templates with col-
loidal dispersions. Colloidal dispersions were prepared using appropriate
sol-gel processing. The filling of the template was to place a template in
a stable sol for various periods of time. The capillary force is believed to
drive the sol into the pores, when the surface chemistry of the template
pores were appropriately modified to have a good wetability for the sol.
After the pores were filled with sol, the template was withdrawn from the
sol and dried prior to firing at elevated temperatures. The firing at elevated
temperatures served two purposes: removal of template
so
that free stand-
ing nanorods can be obtained and densification of the sol-gel derived
green nanorods. Figure
4.29
show
SEM
micrographs of TiOz and ZnO
nanorods made by filling the templates with s01-gel.l~~
One-Dimensional Nanostructures: Nanowires and Nanorods
159
Fig.
4.29.
SEM micrographs
of
oxide nanorods made
by
filling the templates with sol-
gels: (a) ZnO and
(b)
TiOz.
[B.B.
Lakshmi,
P.K.
Dorhout, and C.R. Martin,
Chem. Muter.
9,
857
(1
997).]
Fig.
4.30.
Hollow nanotubes formed
by
incomplete filling
of
the template.
[B.B.
Lakshmi,
P.K.
Dorhout, and C.R. Martin,
Chem. Muter:
9,
857 (1997).]
In the previous chapter, we discussed about sol-gel processing and knew
that the typical sol consists of a large volume fraction of solvent up to
90%
or higher.'** Although the capillary force may ensure the complete filling
of colloidal dispersion inside pores of the template, the amount of the solid
filled inside the pores can be very small. Upon drying and subsequent fir-
ing processes, a significant amount of shrinkage would be expected.
However, the results showed that most nanorods shrank a little only as com-
pared with the size
of
the template pores and indicated that there are some
unknown mechanisms, which enrich the concentration of solid inside
pores. One possible mechanism could be the diffusion of solvent through
the membrane, leading to the enrichment of solid along the internal surface
of template pores, a process used in ceramic slip casting.'36 The formation
of nanotubules (as shown in Fig.
4.30133)
by such a
sol
filling process may
160
Nanostructures and Nanomaterials
imply such a process is indeed present. However, considering the fact that
the templates typically were emerged into sol for just a few minutes, the
difision through membrane and enrichment of solid inside the pores had
to be a rather rapid process. This is a very versatile method and can be
applied for any material, which can be made by sol-gel processing.
However, the drawback is the difficult to ensure the complete filling of the
template pores. It is also noticed that the nanorods made by template fill-
ing are commonly polycrystalline or amorphous. The exception was found,
when the diameter of nanorods is smaller than ZOnm, single crystal Ti02
nanorods were created.
133
4.3.3.2.
Melt
and
solution
filling
Metallic nanowires can also be synthesized by filling a template with molten
metal^.'^^,^^
One example is the preparation of bismuth nanowires by pres-
sure injection of molten bismuth into the nanochannels of an anodic alumina
tem~1ate.l~~ The anodic alumina template was degassed and immersed in the
liquid bismuth at 325°C (T,
=
27 1.5"C for Bi), and then high pressure Ar
gas of
-300
bar was applied to inject liquid Bi into the nanochannels of the
template for 5 hr. Bi nanowires with diameters of
13-1
lOnm
and large
aspect ratios of several hundred have been obtained. Individual nanowires
are believed to be single crystal. When exposed to air, bismuth nanowires
are readily to be oxidized. An amorphous oxide layer of
-4
nm in thickness
was observed after
48
hr. After
4
weeks, bismuth nanowires of 65nm in
diameter were found to be totally oxidized. Nanowires of other metals, In,
Sn and Al, and semiconductors, Se, Te, GaSb and Bi2Te3 were prepared by
injection of melt liquid into anodic alumina templates.25
Polymeric fibrils have been made by filling a monomer solution, which
contain the desired monomer and a polymerization reagent, into the
template pores and then polymerizing the monomer s~lution.'~~-'~' The
polymer preferentially nucleates and grows on the pore walls, resulting
in tubules at short deposition times, as discussed previously in the growth
of conductive polymer nanowires or nanotubules by electrochemical dep-
osition and fibers at long times. Cai
et
al.
142
synthesized polymeric fibrils
using this technique.
Similarly, metal and semiconductor nanowires have been synthesized
through solution techniques. For example, Han
et
al.
143
synthesized Au,
Ag and Pt nanowires in mesoporous silica templates. The mesoporous
templates were filled with aqueous solutions of the appropriate metal
salts (such as HAuC14), and after drying and treatment with CH2C12 the
One-Dimensional Nanostructures: Nanowires and Nanorods
161
samples were reduced under H2 flow to convert the salts to pure metal.
Chen
et
filled the pores of a mesoporous silica template with an
aqueous solution of Cd and Mn salts, dried the sample, and reacted it
with H2S gas to convert to (Cd,Mn)S. Ni(OH)2 nanorods have been
grown in carbon-coated anodic alumina membranes by Matsui
et
by filling the template with ethanolic Ni(N03)2 solutions, drying, and
hydrothermally treating the sample in NaOH solution at 150°C.
4.3.3.3.
Chemical vapor deposition
Some researchers have used chemical vapor deposition (CVD) as a means
to form nanowires. Ge nanowires were grown by Leon
et
al.
146
by difhs-
ing Ge2H6 gas into mesoporous silica and heating. They believed that the
precursor reacted with residual surface hydroxyl groups in the template,
forming Ge and
HZ.
Lee
et
used a platinum organometallic com-
pound to fill the pores of mesoporous silica templates, followed by pyrol-
ysis under H2/N2 flow to yield Pt nanowires.
4.3.3.4.
Deposition
by
centrifugation
Template filling of nanoclusters assisted with centrihgation force is another
inexpensive method for mass production of nanorod arrays. Figure 4.31
shows SEM images of lead zirconate titanate (PZT) nanorod arrays with
uniform sizes and unidirectional a1ig11ment.I~~ Such nanorod arrays were
Fig.
4.31.
SEM
images of the top view (left) and side view (right) of lead zirconate titanate
(PZT) nanorod arrays grown in polycarbonate membrane from PZT
sol
by centrifugation at
1500rpm for 60min. Samples were attached to silica glass and fired at 650°C in air
for
60min.
[T.L.
Wen,
J.
Zhang, T.P. Chou, and G.Z. Cao, unpublished
(2003).]
162
Nanostructures and Nanomaterials
grown in polycarbonate membrane fi-om
PZT
sol by centrihgation at
1500 rpm for
60
min. The samples were attached to silica glass and fired
at 650°C in air for
60
min. Nanorod arrays
of
other oxides including silica
and titania have also been grown in this method. The advantages of
centrifugation include its applicability to any colloidal dispersion systems
including those consisting of electrolyte-sensitive nanoclusters
or
mole-
cules. However, in order to grow nanowire arrays, the centrifugation force
must be larger than the repulsion force between
two
nanoparticles or
nanoclusters.
4.3.4.
Converting through chemical reactions
Nanorods or nanowires can also be synthesized using consumable tem-
plate~.*~~ Nanowires of compounds can be synthesized or prepared using
a template-directed reaction. First nanowires or nanorods
of
constituent
element is prepared, and then reacted with chemicals containing desired
element to form final products. Gates
et
al.
150
converted single crystalline
trigonal selenium nanowires into single crystalline nanowires of Ag2Se by
reacting with aqueous AgN03 solutions at room temperature. The trigonal
selenium nanowires were prepared first using solution synthesis method.29
Selenium nanowires were either dispersed in water or supported on TEM
grids during reaction with aqueous AgN03. The following chemical reac-
tion was suggested:
3Se(,)
+
6Ag&,)
+
3H20
+
2Ag2Se(,)
+
Ag2Se03(aq)+
6H&q)
(4.17)
The products have the right stoichiometric composition and nanowires
are single crystalline, with either tetragonal (low temperature phase) or
orthorhombic structure (high temperature phase, the bulk phase transfor-
mation temperature is 133°C). It was further noticed that nanowires with
diameters larger than 40 nm tended to have orthorhombic structure. Both
crystallinity and morphology of the template were retained with high
fidelity. Other compound nanowires can be synthesized by reacting sele-
nium nanowires with desired chemicals using a similar approach. For
example, Bi2Se3 nanowires may be produced by reacting Se nanowires
with Bi vapor.151
Nanorods can also be synthesized by reacting volatile metal halide or
oxide species with formerly obtained carbon nanotubes to form solid car-
bide nanorods with diameters between
2
and 30nm and lengths up to
20
pm as shown schematically in Fig. 4.32.1521153 Carbon nanotubes were
One-Dimensional Nanostructures: Nanowires and Nanorods
163
Fig.
4.32.
TEM images of nanorods synthesized by reacting volatile metal halide
or
oxide
species with formerly obtained carbon nanotubes to form solid titanium carbide nanorods
with diameters between
2
and
30
nm and lengths up to
20
pm: (a) an unreacted carbon
nanotube and (b) titanium carbide. The scale bars are IOnm. [E.W. Wong, B.W. Maynor,
L.D. Burns, and C.M. Lieber, Chern.
Mater.
8,
2041
(1996).]
used as removable template in the synthesis of silicon and boron nitride
nan0r0ds.l~~ Silicon nitride nanorods of NOnm in diameter were also
prepared by reacting carbon nanotubes with a mixture of silicon monox-
ide vapor and flowing nitrogen at
1500"C'55:
3SiO(,,
+
3C,,,
+
2N2(,)
-+
Si3N4,,,
+
3CO,,, (4.18)
Silicon monoxide is generated by heating a solid mixture of silicon and
silica in an alumina crucible at
1500°C.
The total transformation of carbon
nanotubes into silicon nitride nanorods was observed.
ZnO nanowires were prepared by oxidizing metallic zinc nan0~ires.
In the first step, polycrystalline zinc nanowires without preferential crys-
tal orientation were prepared by electrodeposition using anodic alumina
membrane as a template, and in the second step, grown zinc nanowires
were oxidized at
300°C
for up to
35
hr in air, yielding polycrystalline ZnO
nanowires with diameters ranging from
15
to 90nm and length of
-50
pm.
Although ZnO nanowires are embedded in the anodic alumina
membranes, free standing nanowires may be obtained by selectively
dissolving alumina templates.
Hollow nanotubules of MoS2 of
-30
pm long and 50nm in external
diameter with wall thickness of
10
nm were prepared by filling a solution
mixture
of
molecular precursors, (NH4)2MoS4 and (NH&Mo3S
13
into the
pores of alumina membrane templates. Then template filled with the
164
Nanostructures and Nanomaterials
molecular precursors was heated to an elevated temperature and the
molecular precursors thermally decomposed into MoS~.~~~
Certain polymers and proteins were also reported to have been used
to direct the growth of nanowires of metals or semiconductors. For
example, Braun
et
~11.l~~
reported a two-step procedure to use
DNA
as a
template for the vectorial growth of a silver nanorods of 12 bm in length
and lOOnm in diameter. CdS nanowires were prepared by polymer-
controlled growth.159 For the synthesis of CdS nanowires, cadmium ions
were well distributed in a polyacrylamide matrix. The Cd2+ containing
polymer was treated with thiourea (NH2CSNH2) solvothermally in ethyl-
enediamine at
1
70°C, resulting in degradation of polyacrylamide. Single
crystal CdS nanowires
of
40
nm in diameter and up to
100
pm in length
with a preferential orientation of
[OOl]
were then simply filtered from the
solvent
.
4.4.
Electrospinning
Electrospinning, also known as electrostatic fiber processing, technique
has been originally developed for generating ultrathin polymer
fibers.
160,161
Electrospinning uses electrical forces to produce polymer
fibers with nanometer-scale diameters. Electrospinning occurs when the
electrical forces at the surface of a polymer solution or melt overcome the
surface tension and cause an electrically charged jet to be ejected. When
the jet dries or solidifies, an electrically charged fiber remains. This
charged fiber can be directed or accelerated by electrical forces and then
collected in sheets or other useful geometrical forms. More than
30
poly-
mer fibers with diameters ranging from
40
nm to
500
nm have been suc-
cessfully produced by electrospinning.162,'63 The morphology of the fibers
depends on the process parameters, including solution concentration,
applied electric field strength, and the feeding rate of the precursor solu-
tion. Recently, electrospinning has also been explored for the synthesis of
ultrathin organic-inorganic hybrid For example, porous
anatase titania nanofibers was made by ejecting an ethanol solution con-
taining both poly(viny1 pyrrolidone) (PVP) and titanium tetraisopropoxide
through a needle under a strong electric field, resulting in the formation of
amorphous Ti02/PVP composite nanofibers as shown in Fig.
4.33.'66
Upon pyrolysis
of
PVP at 500°C in air, porous Ti02 fibers with diameter
ranging from
20
to 200nm, depending on the processing parameters are
obtained.
One-Dimensional Nanostructures: Nanowires and Nanorods
165
Fig.
4.33.
(A)
TEM image of Ti02/PVP composite nanofibers fabricated by electrospin-
ning an ethanol solution that contained 0.03 g/mL PVP and
0.1
g/mL Ti(OP:),.
(B)
TEM
image of the same sample after it had been calcined in air at
500°C
for
3
hr. (C,
D)
TEM
images of nanofibers made
of
anatase that were prepared under the same conditions except
that the precursor solution contained (C)
0.025
g/mL and
(D)
0.15
g/mL Ti(OP:)4, respec-
tively.
(E,
F)
High-magnification SEM images taken from the samples shown in
C
and
D,
respectively.
No
gold coatings were applied to the samples
for
all SEM studies.
[D.
Li
and
Y.
Xia,
Nuno
Lett.
3,
555 (2003).]
4.5.
Lithography
Lithography represents another route to the synthesis of nanowires.
Various techniques have been explored in the fabrication of nanowires,
such as electron beam lith~graphy,l~~,l~~ ion beam lithography,
STM
lithography, X-ray lithography, proxial-probe lithography and near-field