Templated synthesis of metal nanorods in silica nanotubes

Article abstract of DOI:10.1021/ja209647d

We report a general method for the synthesis of noble metal nanorods, including Au, Ag, Pt, and Pd, based on their seeded growth in silica nanotube templates. The controlled growth of the metals occurs exclusively on the seeds inside the silica nanotubes, which act as hard templates to confine the one-dimensional growth of the metal nanorods and define their aspect ratios. This method affords large quantities of noble metal nanorods with well-controlled aspect ratios and high yield, which may find wide use in the fields of nanophotonics, catalysis, sensing, imaging, and biomedicine.

Full text of DOI:10.1021/ja209647d

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Templated Synthesis of Metal Nanorods in Silica Nanotubes
Chuanbo Gao, Qiao Zhang, Zhenda Lu, and Yadong Yin*
Department of Chemistry, University of California, Riverside, California 92521, United States
S Supporting Information
b
Scheme 1. A General Templating Approach to the Synthesis
of Metal Nanorods
a
ABSTRACT: We report a general method for the synthesis
of noble metal nanorods, including Au, Ag, Pt, and Pd, based
on their seeded growth in silica nanotube templates. The
controlled growth of the metals occurs exclusively on the
seeds inside the silica nanotubes, which act as hard templates
to confine the one-dimensional growth of the metal nano-
rods and define their aspect ratios. This method affords large
quantities of noble metal nanorods with well-controlled
aspect ratios and high yield, which may find wide use in the
fields of nanophotonics, catalysis, sensing, imaging, and
biomedicine.
a
(a) Silica nanotube with amino groups functionalized selectively on the
inner surface; (b) Au seed@silica yolk/shell structure; (c) metal nanorod@
silica core/shell structure obtained by seeded growth; (d) metal nano-
rods after etching of silica shells.
etal nanorods have received widespread interest due to
Herein, we present a robust templating approach to the
synthesis of metal nanorods, including Au, Ag, Pt, and Pd in
composition, with well-controlled dimensions and high yield.
The success of this method should first be attributed to the
employment of silica nanotube templates with tunable dimen-
sions for which a large-scale synthesis was developed recently
M
their unique one-dimensional structure; their consequent
unusual optical, electronic, and catalytic properties; and their
potential use in chemical sensing, cellular imaging, and
1
À3
4À8
9
10,11
therapeutics.
and Pd
Specifically, noble metal Au,
Ag, Pt,
1
2À15
nanorods have garnered great interest, among which
Au nanorods have been most extensively studied due to their
plasmonic activity. Most conventional syntheses rely on solution-
phase reactions, for example, seed-mediated growth for the
22
in our laboratory. Briefly, these high-quality templates were
produced by coating uniform nickel-hydrazine rod-like nano-
crystals with a layer of silica through a solÀgel process, and then
selectively removing the nickel-hydrazine templates to yield
tubular structures.
Subsequent templated growth of metal nanorods, as outlined
in Scheme 1, involves: (1) the seeding of Au nanoparticles in the
cavity of silica nanotubes; (2) the confined growth of metal
initiated from the Au seeds, forming a metal nanorod@silica
core/shell structure; and (3) the etching of the silica shell
producing metal nanorods.
In addition to the synthesis of well-defined templates, intro-
ducing Au seeds exclusively into the cavity of these templates is
another indispensable step as it ensures that subsequent metal
growth occurs in the confined space rather than outside as a
result of self-nucleation. There are only a few reports on the
seeding of Au nanoparticles in a preexisting nanostructure, which
4
À8
synthesis of Au nanorods.
This synthesis route proves
successful in producing Au nanorods with different aspect ratios
thereby rendering them with desired properties. However, the
general applicability of the method for other metals, the scal-
ability of the reaction, the yield of nanorods relative to byprod-
ucts such as spheres, and the sensitivity of the reaction to various
parameters (and thus the reproducibility) leave a lot of room for
developing alternative approaches for this class of functional
1
6
materials.
Templating methods have also been pursued to synthesize
metal nanorods by employing porous alumina membranes and
17À21
polycarbonate filtration membranes as hard templates.
Metal nanorods, typically Au in composition, have been fabri-
cated by using this method via electrochemical deposition and
subsequent dissolution of the templates. Typical limitations
include the difficulty in controlling both the width and length
of the products, low yield because only a monolayer of metal
nanorods could be prepared in a membrane template, and the
high cost associated with these templates. In general, a practically
useful templating method for nanostructure synthesis should
meet the following requirements: (1) the convenient availability
of well-defined templates in large quantity at low cost; (2) the
precise placement of seeds inside the templates; (3) a well-
controlled seeded growth process; and (4) a reliable process to
selectively remove the templates.
were achieved by infiltration of HAuCl solution into the
4
23,24
nanostructure followed by reduction by NaBH4.
In our
system, the encapsulated hydrazine inside the silica nanotubes
fails in producing Au seeds inside due to their outward diffusion
of the templates (Supporting Information (SI)). To produce Au
seeds preferentially within the inner cavity, we modified the
original silica nanotube synthesis by introducing a layer of
3-aminopropyltriethoxysilane (APS) to the surface of the
Received: October 13, 2011
Published: November 15, 2011
r 2011 American Chemical Society
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|J. Am. Chem. Soc. 2011, 133, 19706–19709

Journal of the American Chemical Society
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preventing the final products from agglomeration. The as-pre-
pared growth solution, which combines HAuCl , PVP, KI, and
4
ascorbic acid, remained stable at room temperature for longer
than 2 h without obvious self-nucleation, as evidenced by
UVÀvis spectrophotometry measurements. However, immedi-
ately after the Au seed@silica was injected into the growth
solution, growth of Au on the existing seeds occurred as
evidenced by the color change of the mixture.
The silica nanotube acts as a limiting factor to restrain the
growth of metal in one dimension, leading to the formation of
nanorods. The seeded growth stops theoretically when the
nanorods reach the end of the templates. Figure 1c shows a
TEM image of the Au nanorod@silica nanostructures obtained
by seeded growth. The final Au nanorods fill the inner space of
the silica nanotubes, taking their shape and size. Interestingly,
most silica nanotubes are not completely filled by Au nanorods,
probably because at the end of the reaction, the concentration of
the Au precursors/monomers decreases dramatically, and their
limited diffusion to the remaining small free space of the silica
nanotubes (indicated by arrows in the inset in Figure 1c) is not
able to support extensive continued growth. The low-magnifica-
tion TEM image in Figure 1c indicates the high yield of the Au
nanorods, as no obvious presence of free Au nanospheres has
been observed, which could be attributed to the exclusive seeding
of the Au nanoparticles inside the silica nanotubes and the high
stability of the growth solution against self-nucleation.
Figure 1. TEM images of the (a) silica nanotubes with inner cavity
functionalized with amino groups; (b) Au seed@silica nanotubes; (c) Au
nanorod@silica nanotubes after seeded growth; (d) Au nanorods after
removal of silica templates.
nickel-hydrazine nanorods before the deposition of a silica layer
through the hydrolysis of tetraethyl orthosilicate (TEOS). As is
À
well-known, Au species, including the anionic AuCl4 and
metallic Au, have a high affinity to amino groups due to
electrostatic and/or coordinate interactions. The selective mod-
ification of the inner surface of the silica nanotubes with amino
groups allows the retention of the Au species inside the templates
Well-defined metal nanorods could be finally obtained after
removing the silica shell by NaOH etching. The particular sample
of Au nanorods shown in Figure 1d has an average dimension of
17 nm  150 nm, which is consistent with the cavity size of the
silica nanotubes with a slight deviation. The nanorods are uni-
form in size with high yield, proving the competence of this
method in synthesizing high-quality samples.
during the chemical adsorption of HAuCl , resulting in Au seeds
4
exclusively positioned inside the silica nanotubes after reduction.
Figure 1a shows the TEM image of a typical sample of silica
nanotubes, synthesized by using polyoxyethylene(10) cetyl ether
(
2
Brij C10) as the surfactant. They possess an average length of
00 nm and a cavity width of 12 nm, which are determined
independently by the hydrazine/nickel ratio and the intrinsic size
This method is versatile in affording metal nanorods with
different aspect ratios, which can be conveniently achieved by
controlling the aspect ratio of the original silica nanotubes
through tuning the hydrazine/nickel ratio during the original
22
of the surfactant micelles. Modification of the inner surface
with amino groups results in no significant change to the tubular
morphology of the silica templates. After seeding, Au nanopar-
ticles ∼4 nm in size are formed inside each silica nanotube
22
template synthesis. A series of Au nanorods prepared by this
method is presented in Figure 2, showing both the TEM images
and the UVÀvisÀnear-IR (NIR) spectra. With the average
aspect ratio increasing from 3.5 to 5.7, 14.8, and 21, the long-
itudinal plasmonic band red-shifts significantly from 755 to 1132,
1885, and 2320 nm, which are close to the values predicted by
(Figure 1b). No obvious nucleation outside the silica nanotubes
has been observed. The UVÀvis spectrum of the material (SI)
shows an absorption band at ∼512 nm, characteristic of small-
sized Au nanospheres. To enhance the mass transfer through the
silica shell in the following seeded growth of metal nanorods, the
Au seed@silica sample was etched with water at 70 °C for 1 h,
increasing the pore size in the silica shell, and in the meantime,
the inner diameter of the cavity was enlarged to ∼15 nm.
The key in the seeded growth step is to maintain a low reaction
rate and minimize self-nucleation events. Taking Au for example,
we used ascorbic acid as a weaker reducing agent to replace
NaBH for the reduction of Au salt. However, on direct mixing of
the ascorbic acid with AuCl4 , the solution still quickly turned
red, suggesting self-nucleation due to the high reduction poten-
tial of AuCl4 (+0.93 V vs SHE). To further slow down the self-
3
theoretical calculations (λmax = 95R + 420 nm; R: aspect ratio).
The transverse plasmonic band, on the other hand, blue-shifts
from 512 nm (sample a) to ∼504 nm (samples bÀd) (SI), with
the intensity becoming drastically weaker compared with that of
the longitudinal bands. The absence of the plasmonic band at
530 nm suggests high yield of nanorods without significant
existence of spheres.
The hard-templating method to synthesize metal nanorods is
not restricted to Au, but can be readily extended to many other
metals, including Ag, Pt, and Pd. Typical samples of Ag, Pt, and
Pd nanorods prepared by this method are shown in the TEM
images in Figure 3.
25
4
À
À
nucleation, a coordinating ligand, KI, was introduced, which
Ag nanorods are difficult to synthesize by conventional
methods, with only a few reports documented in the literature,
but can be easily synthesized using the silica nanotube templates.
À
À
9
reacts with AuCl4 forming a stable complex AuI4 with a
decreased reduction potential (+0.56 V vs SHE) due to the
strong AuÀI affinity. An additional capping ligand, typically
polyvinylpyrrolidone (PVP), was also added to the growth
solution to stabilize the atomic monomer species and further
In preparing the growth solution for Ag nanorods, AgNO was
3
used as the metal source, ascorbic acid as the reducing agent,
acetonitrile as the coordinating ligand, and sodium citrate as the
additional capping ligand. The nitrile group on acetonitrile can
2
6
delay the self-nucleation. PVP also has the function of
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Figure 2. (aÀd) TEM images and (e) UVÀvisÀNIR spectra of Au
nanorods prepared by using silica nanotubes with different aspect ratios
as templates. Spectra were taken from (a and b) a water solution of the
samples, and (c and d) from samples deposited on glass slides.
Figure 3. TEM images of (a) Ag, (b) Pt, and (c) Pd nanorods prepared
by seeded growth in silica nanotubes.
+
effectively coordinate to a Ag ion, lowering its reduction
nanotubes were collected at different growth stages. Prior to
quick centrifugation, at each stage additional PVP was added to
the growth solution to stabilize the monomer species and
effectively stop the growth. The Au seeds in silica nanotubes
initially grow isotropically into larger nanospheres. After reaching
the diameter of the silica nanotubes, they begin growing one-
dimensionally as a result of the confinement of the silica shell,
forming anisotropic nanorods which continue growing in the
longitudinal direction of the silica nanotube until reaching or
near to the ends of the silica nanotubes. It is worth noting that the
anisotropic growth occurs mainly from a single seed rather than
the multiple seeds contained in each nanotube, mainly because
the growth of one seed decreases the precursor concentration in
the surrounding and prohibits the extensive growth of the
adjacent seeds.
potential and thus preventing self-nucleation. The seeded growth
of Ag nanorods was initiated by injecting the Au seed@silica into
the growth solution. A significant color change was observed,
from colorless to yellow, red, blue, gray, and eventually green,
indicating a change in the aspect ratio accompanying the growth
of the Ag nanorods. The silica shell was removed by etching the
sample with NaOH, with a small amount of diethylamine added
to the solution to enhance the stability of the Ag nanorods against
base etching. Pure Ag nanorods have been obtained by this
method, as shown in Figure 3a.
Likewise, to synthesize Pt and Pd nanorods, growth solutions
were first prepared, to which the Au seed@silica was injected to
trigger seeded growth in the confined space. The growth solution
for Pt was prepared with H PtCl as the metal source, hydrazine
2
6
hydrate as the reducing agent, NaNO as the coordinating ligand,
and PVP as the additional capping ligand. Similarly, the growth
The growth of Au nanorods in the silica nanotubes is
accompanied by a change in the optical property, as shown in
Figure 4b. Shortly after the growth started, a single absorption
peak at ∼530 nm was observed, indicative of the formation of Au
nanospheres. As the growth went on, a longitudinal plasmon
band of Au at a longer wavelength appeared, suggesting the
formation of Au nanorods. The longitudinal band red-shifted
thereafter, accompanied by a blue-shift of the transverse band,
consistent with an increase in the aspect ratio of the nanorods
resulting from an elongation of the Au nanorods. The extinction
increases throughout the process, due to an increase in the
absolute amount of elemental Au. These results match very well
with the TEM observations and confirm the growth process of
Au nanorods in silica nanotubes. Similar evolution of the
plasmon resonance peaks was also observed for Ag nanorods,
as shown in Figure 4c, confirming the same seeded growth
mechanism. Apparently, harvesting the samples at different
2
solution for Pd contained Na PdCl as the metal source, ascorbic
2
4
acid as the reducing agent, and PVP as the additional capping
ligand. TEM images of the Pt and Pd nanorods after etching of
silica are displayed in Figure 3b,c, respectively, demonstrating the
high yield of the synthesis. The aqueous solutions of the
nanorods are black in color, without discernible optical features.
By comparing the high-magnification images in the insets, one
can easily notice that the Pt and Pd nanorods are grainier than the
Au and Ag nanorods, although all are polycrystalline in nature.
The cause of the different crystallinity is not clear at the current
stage and will be the subject of future investigation.
The seeded growth of metal nanorods in the silica nanotubes
has been monitored by TEM and in situ UVÀvisÀNIR spectro-
photometry studies, taking Au and Ag as typical models
(Figure 4). Growth intermediates of Au nanorods in silica
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’
AUTHOR INFORMATION
Corresponding Author
yadong.yin@ucr.edu
’
ACKNOWLEDGMENT
Financial support of this work was provided by U.S. Depart-
ment of Energy (DE-SC0002247) and the National Science
Foundation (DMR-0956081). Y.Y. thanks the Research Cor-
poration for Science Advancement for the Cottrell Scholar
Award, 3M for the Nontenured Faculty Grant, and DuPont for
the Young Professor Grant. We also thank the Central Facility for
Advanced Microscopy and Microanalysis at UCR for access to
TEM analysis.
’
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effective method to control the aspect ratio of metal nanorods.
In summary, we have presented a general method to synthe-
size noble metal nanorods in silica nanotubes, which includes Au
seeding in the cavity of the silica nanotubes, seeded growth of
metal nanorods in the confined space, and removal of the silica
templates by chemical etching. This robust templating process is
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obtained in large quantities at low costs. The unique seeding
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ASSOCIATED CONTENT
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Supporting Information. Details of experimental proce-
b
dures, additional UVÀvisÀNIR spectra, control experiments,
and multimedia on the seeded growth of metals in silica
nanotubes (avi). This material is available free of charge via the
Internet at http://pubs.acs.org.
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