Shape/size-controlled syntheses of metal nanoparticles for site-selective modification of carbon nanotubes

Article abstract of DOI:10.1021/ja060296u

Shape- and size-controlled syntheses of metal nanoparticles have been achieved by galvanic displacement reaction between an aqueous solution of metal salt and Cu foil substrate. In particular, cubic and spheric nanoparticles of Pt (Au) with a fairly narrow size distribution were produced by reacting K ₂PtCl₄ (HAuCl₄) with a Cu foil in an aqueous medium with and without CuCl₂ under different reaction conditions (e.g., different concentrations and reaction times). In conjunction with the substrate-enhanced electroless deposition (SEED) technique (Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806), the shape/size-controlled syntheses have been successfully exploited to site-selectively deposit these metal nanoparticles onto the outerwall, innerwall, or end-tip of carbon nanotubes (CNTs). Asymmetric sidewall modification by attaching the innerwall and outerwall of CNTs with metal nanoparticles of different shapes was also achieved. Furthermore, it was demonstrated that the nanotube-supported Pt nanoparticles could be converted into hollow Au nanoboxes by galvanic displacement of Pt with Au. These CNT-supported metal nanoparticles were shown to possess interesting optical and electrocatalytic properties.

Full text of DOI:10.1021/ja060296u

Published on Web 03/31/2006
Shape/Size-Controlled Syntheses of Metal Nanoparticles for
Site-Selective Modification of Carbon Nanotubes
Liangti Qu,† Liming Dai,*,†,‡ and Eiji Osawa§
Contribution from the Department of Chemical and Materials Engineering, School of
Engineering, UniVersity of Dayton, 300 College Park, Dayton, Ohio 45469-0240, UniVersity of
Dayton Research Institute, Dayton 45469, and NanoCarbon Research Institute, Ltd.,
c/o Toudai Kashiwa Venture Plaza, 5-4-19 Kashiwa-no-ha, Kashiwa, Chiba 277-0882, Japan
Received January 15, 2006; E-mail: ldai@udayton.edu
Abstract: Shape- and size-controlled syntheses of metal nanoparticles have been achieved by galvanic
displacement reaction between an aqueous solution of metal salt and Cu foil substrate. In particular, cubic
and spheric nanoparticles of Pt (Au) with a fairly narrow size distribution were produced by reacting K2PtCl4
(HAuCl4) with a Cu foil in an aqueous medium with and without CuCl2 under different reaction conditions
(e.g., different concentrations and reaction times). In conjunction with the substrate-enhanced electroless
deposition (SEED) technique (Qu, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806), the shape/size-controlled
syntheses have been successfully exploited to site-selectively deposit these metal nanoparticles onto the
outerwall, innerwall, or end-tip of carbon nanotubes (CNTs). Asymmetric sidewall modification by attaching
the innerwall and outerwall of CNTs with metal nanoparticles of different shapes was also achieved.
Furthermore, it was demonstrated that the nanotube-supported Pt nanoparticles could be converted into
hollow Au nanoboxes by galvanic displacement of Pt with Au. These CNT-supported metal nanoparticles
were shown to possess interesting optical and electrocatalytic properties.
Introduction
quently, much effort has been devoted to the shape and size-
controlled syntheses of metal nanoparticles.9
With the recent development in nanomaterials and nano-
technology,1-3 research on nanoparticles, in general, and metal
nanoparticles, in particular, has received renewed interest in
recent years for their potential applications in photography,
catalysis, biological labeling, photonics, optoelectronics, infor-
mation storage, surface-enhanced Raman scattering (SERS), and
many other areas.4-7 Like many other nanomaterials, metal
nanoparticles show shape/size-dependent properties.8 Conse-
On the other hand, multicomponent assemblies of nanoscale
entities are attractive for multifunctional systems, including
optoelectronic,10 sensing, and catalysis.11 In this context, a few
innovative methods have been devised to graft metal nanopar-
ticles onto carbon nanotube (CNT) structures.12 These metal
nanoparticles may act as quantum dot devices along the
nanotube molecular wires of one-dimensional electronic proper-
ties, and they may also serve as active sites for further
modification and assembling of CNTs. The detailed structure
and precise location of the metal nanoparticles along the
nanotube molecular wires plays an important role in regulating
the final assembled structure and its performance. As far as we
† University of Dayton.
‡ University of Dayton Research Institute.
§ NanoCarbon Research Institute.
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J. AM. CHEM. SOC. 2006, 128, 5523-5532
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A R T I C L E S
Qu et al.
with spherical and cubic structures can also be synthesized with aqueous
solutions of HAuCl4 under similar experimental conditions.
are aware, however, almost no attention has been paid to the
shape/size-controlled syntheses of metal nanoparticles for site-
selective modification of CNTs by attaching the nanotube
innerwall, outerwall, and/or end-tip(s) with metal nanoparticles
of different shapes in a controllable fashion.
Outerwall Modification of CNTs with Pt Nanocubes and Nano-
spheres. To attach Pt nanocubes onto the nanotube outerwall, we
deposited 10 droplets (∼0.5 mL) of MWNTs dispersed in ethanol (0.8
mg/10 mL) onto a Cu foil (8 cm2). Having been dried in air, the Cu-
supported CNTs were immersed into an aqueous solution of 3.8 mM
K2PtCl4 containing 5 mM CuCl2 for 1 min and then thoroughly washed
with distilled water. Following the same procedure, the Cu foil-
supported CNTs were reacted with 0.95 mM K2PtCl4 in the absence
of CuCl2 for 30 min to deposit Pt nanospheres onto the nanotube
outerwall.
Conversion of the CNT-Supported Pt Nanocubes into Au
Nanoboxes. The above-mentioned Pt nanocube-modified CNTs sup-
ported by the Cu foil were ultrasonicated (VWR model 75D ultrasoni-
cation bath ) and dispersed in distilled water (1.6 mL). To eight droplets
of this aqueous solution of the Pt nanocube-modified CNTs was added
one droplet of 1 mM HAuCl4 aqueous solution. The reaction mixture
was then kept at room temperature for 20 min, leading to the formation
of Au hollow nanoboxes along the CNT structure. The resultant Au
nanobox-modified CNTs were collected by centrifuging and repeatedly
washed with distilled water.
Modification of Aligned CNTs with Pt Nanocubes. An aligned
CNT film generated by pyrolysis of FePc was removed from the quartz
plate as a free-standing film in an aqueous HF solution (ca.10 wt
%/wt),15b followed by carefully washing with distilled water. The free-
standing nanotube film was then held on a Cu foil by stick tapes at the
film edge. The nanotube film was kept undried during the whole course
of the sample preparation so that both the top surface of the CNT film
and the nanotube outerwall were accessible by an aqueous solution of
metal salt. An alternative option to maximize the access for the metal
salt aqueous solution was to render the nanotube sidewall hydrophilic
by keeping the aligned CNT film in H2SO4/HNO3 for several hours
before the surface modification. Pt nanocubes were then deposited along
the aligned CNT length by immersing the above prewetted/preactivated
aligned CNT film supported by the Cu foil into an aqueous solution of
3.8 mM K2PtCl4 containing 5 mM CuCl2 for 1 min.
In the present study, we found that the shape- and size-
controlled syntheses of metal nanoparticles (e.g., Pt, Au) could
be achieved by simply reacting appropriate aqueous solutions
(e.g., K2PtCl4, HAuCl4) with a Cu foil via a galvanic displace-
ment reaction at different metal ion concentrations and/or
reaction times. Galvanic displacement reactions, involving
concurrent electrochemical oxidation and reduction, have been
widely used to deposit metal nanoparticles on metallic or
semiconducting substrates.13 The driving force for the galvanic
displacement reaction arises from the difference in half-cell
potentials between the metal ions to be reduced and the substrate
to be oxidized. For the deposition of metal nanoparticles, the
half-cell potential of the reduced species must be higher than
that of the oxidized substrate. Accordingly, we found that Pt or
Au nanospheres and nanocubes with a narrow size distribution
can be directly deposited onto a Cu foil without any additional
reducing or capping reagents (e.g., polyvinylpyridine, PVP, and
silver ions9a,e,13a). This, together with our previously reported
substrate-enhanced electroless deposition (SEED) technique,14
prompted us to deposit metal nanoparticles of a controllable
shape and size onto CNTs in a site-selective manner. In this
paper, we report the first site-selective modification of the
outerwall, innerwall, or end-tip(s) of either aligned or nonaligned
CNTs with the shape/size-controlled metal nanoparticles. Asym-
metric sidewall modification with the outerwall and innerwall
of individual CNTs being attached by metal nanoparticles of
different shapes will also be described.
Experimental Section
Modification of the Nanotube Innerwall and Asymmetric Side-
wall Modification with Metal Nanospheres and Nanocubes. Scheme
1 shows the procedures for modification of the nanotube innerwall and
the asymmetric sidewall modification with Pt nanospheres being
attached on the nanotube innerwall and nanocubes on the outerwall.
To facilitate the introduction of the metal salt solution into the nanotube
hollow core, we first immersed the template-synthesized aligned CNTs
within the alumina membrane (Scheme 1a) into an aqueous solution
of 1.9 mM K2PtCl4 under ultrasonication for 5 min. After removing
the remaining solution on the surface of alumina membrane by
adsorption with a filter paper, the preactivated alumina membrane-
supported aligned CNT film was perpendicularly anchored onto a
copper foil by stick tapes (Scheme 1b) and then reimmersed into the
aqueous solution of 1.9 mM K2PtCl4 for 30 min for the formation of
Pt nanospheres on the nanotube innerwall (Scheme 1, parts c and d).
Subsequently, the innerwall-modified CNTs (Scheme 1e) were released
from the alumina membrane using an aqueous HF solution (Scheme
1f). To demonstrate the asymmetric sidewall modification, we deposited
the innerwall-modified nanotubes onto a Cu foil with a sidewall-on
configuration (Scheme 1g) before being exposed to an aqueous solution
of 3.8 mM K2PtCl4 containing 5 mM CuCl2 for 1 min. The resultant
asymmetrically modified CNTs (Scheme 1h-j) were then thoroughly
washed with distilled water and characterized on a scanning electron
microscopy (SEM) and transmission electron microscopy (TEM).
Tip Modification of CNTs with Pt Nanocubes and Nanospheres.
To demonstrate the tip modification, we used the perpendicularly
aligned CNTs produced by pyrolysis of FePc.15 In particular, we
removed the FePc-generated aligned CNT film from the quartz substrate
as a free-standing film in an aqueous HF solution (ca.10 wt %/wt),15b
Materials. Potassium tetrachloroplatinate(II) (K2PtCl4) and hydrogen
tetrachloroaurate (III) (HAuCl4) were purchased from Aldrich and Alfa
Aesar, respectively, and were used as received. Copper foils were
obtained from EMS US. CNTs for the outerwall and tip modification
were prepared by pyrolysis of iron (II) phthalocyanine (FePc),15 while
those for the innerwall and asymmetric sidewall modification were
produced by the template-synthesis method using commercially avail-
able alumina membrane (Whatman) templates with pore size of about
200-500 nm according to the reported procedure.16
Controlled Syntheses of Metal Nanoparticles. In a typical experi-
ment, Pt (Au) nanoparticles were produced by immersing Cu foils into
an aqueous solution of K2PtCl4 (HAuCl4) at room temperature for a
certain period of time, followed by thoroughly rinsing with distilled
water. We used a low metal salt concentration (e1.9 mM) for the
formation of spheric nanoparticles. However, a relatively high metal
salt concentration (g3.8 mM), with or without the addition of CuCl2,
was used for the nanocube formation (vide infra). Au nanoparticles
(13) See, for example: (a) Song, H.; Kim, F.; Connor, S.; Somorjai, G. A.;
Yang, P. D. J. Phys. Chem. B 2005, 109, 188. (b) Dryfe, R. A. W.; Walter,
E. C.; Penner, R. M. ChemPhysChem 2004, 5, 1879. (c) Magagnin, L.;
Maboudian, R.; Carraro, C. J. Phys. Chem. B 2002, 106, 401. (d) Porter,
L. A.; Choi, H. C.; Schmeltzer, J. M.; Ribbe, A. E.; Elliott, L. C. C.; Buriak,
J. M. Nano Lett. 2002, 2, 1369. (e) Aizawa, M.; Cooper, A. M.; Malac,
M.; Buriak, J. M. Nano Lett. 2005, 5, 815.
(14) Qu, L. T.; Dai, L. J. Am. Chem. Soc. 2005, 127, 10806.
(15) (a) Li, D. C.; Dai, L.; Huang, S.; Mau, A. W. H.; Wang, Z. L. Chem.
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Chem. B 1999, 103, 4223.
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A R T I C L E S
Scheme 1. Procedures for the Nanotube Innerwall Modification and the Asymmetric Modification of the Nanotube Innerwall with Pt
Nanospheres and the Outerwall with Nanocubesa
aFor the sake of clarity, only a few of the many CNTs on the Cu foil are shown.
which was then perpendicularly anchored onto a Cu foil by stick tapes
(Scheme 1b). Having been thoroughly washed with distilled water, the
Cu foil-supported aligned CNT film was then floated on aqueous
K2PtCl4 solutions of different concentrations with each of the constituent
CNTs within the aligned nanotube film perpendicularly aligned at the
air/liquid interface17 for depositing Pt nanoparticles on the nanotube
end-tips only. An aqueous solution of 3.8 mM K2PtCl4 containing 5
mM CuCl2 and a deposition time of 1 min were used for the deposition
of Pt nanocubes, while Pt nanospheres were deposited onto the nanotube
tips from an aqueous solution of 0.95 mM K2PtCl4 with a deposition
time of 30 min.
Characterization and Optoelectronic Properties. SEM imaging
was performed on a Hitachi S-4800 high-resolution scanning electron
microscope. An energy-dispersive X-ray spectroscopic (EDS) detecting
unit was used for the element analysis. TEM images and electron
diffraction (ED) patterns were taken on a Hitachi H-7600 transmission
electron microscope. UV-vis spectra were recorded on a Lambda 900
UV/VIS/NIR spectrometer (Perkin-Elmer).
containing 2 M CH3OH + 0.1 M H2SO4 was used as the electrolyte.
Cyclic voltammetric measurements were performed with a scan rate
of 50 mV s-1 on an eDAQ potentiostat electrochemical analyzer
(Australia).
Results and Discussion
1. Shape/Size-Controlled Synthesis of Metal Nanoparticles.
1.1. Shape-Controlled Syntheses of Metal Nanoparticles.
Figure 1 shows SEM images of Pt nanostructures obtained by
a typical galvanic displacement reaction13,18 between Cu foils
and aqueous K2PtCl4 solutions of different concentrations for a
constant deposition time of 30 min. Interestingly, both spherical
and cubic nanostructures were produced, and a concentration-
dependent shape evolution was clearly evident. Figure 1, parts
a and b, shows the formation of smooth Pt nanoparticles between
150 and 400 nm in diameter when Cu foil was immersed into
aqueous solutions of K2PtCl4 at low concentrations (e1.9 mM).
Upon increasing the metal salt concentration up to about 3.8
mM, Pt cubic nanoparticles of a fairly uniform size (150-300
nm in the side length) were formed (Figure 1c). Further
increasing the metal salt concentration caused the nanocubes
Electrochemical measurements were carried out in a single-compart-
ment, three-electrode glass cell with a KCl-saturated Ag/AgCl reference
electrode and a platinum counter electrode. A Pt nanocube-grafted
aligned CNT film of 4 mm2 supported by a thin layer of sputter-coated
Au was used as the working electrode, while a similar pristine aligned
CNT film was also measured for comparison. N2-saturated solution
(18) (a) Porter, L. A.; Choi, H. C.; Ribbe, A. E.; Buriak, J. M. Nano Lett. 2002,
2, 1067.
(17) Lee, K.; Li, L.; Dai, L. J. Am. Chem. Soc. 2005, 127, 4122.
9
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Qu et al.
Figure 1. SEM images of Pt nanostructures formed by immersing Cu foils into aqueous solutions of K2PtCl4 with different concentrations for 30 min: (a)
0.38 mM; (b) 1.9 mM; (c) 3.8 mM; (d) 38 mM.
The shape-controlled syntheses of metal nanoparticles have
been previously achieved by using capping reagents, such as
surfactants and polymers, which selectively bound to certain
crystalline face(s) of the growing nanoparticles to retard or
enhance the growth from these face(s).9a,c,d Recently, Yang and
co-workers9e,13a reported that shape-controlled syntheses of metal
nanoparticles could also be realized by introducing foreign metal
ions. Elegant work carried out by these authors showed the
formation of gold nanocubes through the addition of a small
amount of silver ions. By analogy, Cu2+ ions produced in the
galvanic displacement reaction (i.e., reaction 1) between the Cu
foil and K2PtCl4 salt solution in our present study could also
act as an in situ capping reagent to play the role in regulating
the shape of the resulting Pt nanoparticles.
Cu(s) + PtCl42-(aq) f Pt(s) + Cu2+(aq) + 4Cl-(aq)
(reaction 1)
The above scenario was consistent with our observation that
the reaction mixture became pale blue, characteristic of Cu2+
ions in an aqueous medium, by increasing the K2PtCl4 concen-
tration up to 3.8 mM, at which Pt nanocubes were produced
(Figure 1c). This observation indicates the importance of Cu2+
Figure 2. TEM images of (a) spheric and (b) cubic Pt nanoparticles. (c)
EDS spectrum and (d) ED pattern of these particles.
ions in the formation of Pt nanocubes. The important role of
Cu2+ ions was further checked by carrying out a control
experiment, in which CuCl2 was deliberately added into an
aqueous solution with a low concentration of K2PtCl4 (e.g., e1.9
mM). As expected, the addition of CuCl2 led to the formation
of Pt nanocubes even at the low K2PtCl4 concentration (Figure
3, parts a and b), which otherwise should produce Pt nano-
spheres only (see Figure 1, parts a and b). By prolonging the
immersion time of a Cu foil in an aqueous solution of pure
K2PtCl4 at a low concentration (e.g., e1.9 mM K2PtCl4), we
further noticed the formation of hybrid structures with Pt
nanocubes growing out from the spheric nanoparticles formed
at the low K2PtCl4 concentration (Figure 3, parts c and d),
indicating a sufficient amount of Cu2+ ions have been ac-
cumulated during the late stage of the galvanic displacement
reaction (i.e., reaction 1). These results provide an effective route
toward the formation of hierarchical metal nanostructuressa
challenging research topic yet to be explored.
to aggregate into clusters (Figure 1d). Similar Au nanospheres
and nanocubes have also been produced by immersing the Cu
foils into aqueous solutions of HAuCl4 under similar conditions.
As revealed by the TEM images in Figure 2, parts a and b,
the nanospheres and nanocubes are not hollow. The chemical
nature of these nanoparticles can be detected by EDS measure-
ments (Figure 2c) to be dominated by Pt signals with several
Cu peaks arising from the copper foil substrate. ED patterns
for the Pt nanospheres and nanocubes show similar features with
several bright concentric rings (Figure 2d), attributable to [111],
[200], [220], and [311] crystal planes of a face-centered cubic
(fcc) polycrystalline metal Pt (International Centre for Diffrac-
tion Data, no. 04-0802).19
(19) Zoval, J. V.; Lee, J.; Gorer, S.; Penner, R. M. J. Phys. Chem. B 1998, 102,
1166.
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A R T I C L E S
Figure 3. SEM images of Pt nanostructures formed on Cu foils immersed in aqueous solutions of (a) 0.38 mM K2PtCl4 and 15 mM CuCl2 for 30 min, (b)
1.9 mM K2PtCl4 and 11 mM CuCl2 for 5 min, (c) 1.9 mM K2PtCl4 for 80 min, and (d) 0.38 mM K2PtCl4 for 140 min.
1.2. Size-Controlled Syntheses of Metal Nanoparticles. As
can be seen in Figure 4, there is a strong time dependence of
the particle size for the Pt nanoparticles formed via the galvanic
displacement reaction (i.e., reaction 1). Upon immersion of the
Cu foil into an aqueous solution of K2PtCl4 (3.8 mM), cubes of
∼60 nm in size (i.e., the side length) formed on the copper
surface within about 30 s (Figure 4a). After about 2 min, the
cubic size increased from ∼60 nm to ∼110 nm (Figure 4b).
Further increasing the reaction time up to 17 h caused a gradual
increase in the cubic size to ∼700 nm (Figure 4c-i). Also
included in Figure 4j is a plot of the cubic size as a function of
the reaction time, which clearly indicates a rapid increase in
the cubic size at the initial stage of the reaction (less than ca.
30 min), followed by a relatively slow growth of the nano-
particles.
Similar growth kinetics was also observed for the formation
of Pt nanospheres on a Cu foil in aqueous solutions of low
K2PtCl4 concentrations (e.g., e1.9 mM) at an early stage of
the reaction before the appearance of the sphere-cube hybrid
structure (cf., Figure 3, parts c and d).
2. Site-Selective Modification of CNTs with the Shape/
Size-Controlled Metal Nanoparticles. 2.1. Modification of
the Nanotube Outerwall with Metal Nanospheres and
Nanocubes. As we can see from the above discussion, the
shape- and size-controlled synthetic approaches developed in
this study are rather versatile and effective for preparing various
metal nanostructures from metal ions of a higher redox potential
than that of the Cu foil. This, in conjunction with our previously
reported SEED technique,14 enabled us to site-selectively modify
CNTs with metal nanoparticles of a controlled shape and size,
as described below.
nanotube outerwall in both cases. The corresponding SEM
(Figure 5, parts c and d) and TEM (Figure 5, parts e and f)
images under higher magnifications clearly show a class of
“necklace-like” hybrid structures with individual nanotubes even
threading through the nanoparticles. Figure 5g reproduces a
typical EDS spectrum for the Pt nanoparticle-modified CNTs
supported by a Cu foil, which, as expected, shows a series of
Cu, C, and Pt peaks, attributable to the Cu foil substrate, CNTs,
and Pt nanoparticles, respectively.
In view of the previous work on gold nanobox formation by
a galvanic displacement reaction,9a we further used the nanotube-
supported metal nanoparticles prepared above as sacrificial
templates for the formation of hollow nanoboxes with a well-
defined shape and size (see the Experimental Section). As can
be seen in Figure 6, Pt nanocubes (Figure 6, parts a and b; see
also Figure 5, parts a and c) can be effectively converted into
gold hollow nanoboxes (Figure 6c-f) by depositing Au (nano-
particles) onto the Pt nanocubic faces and dissolving Pt via the
following galvanic displacement reaction (i.e., reaction 2).
3Pt(s) + 2AuCl4 (aq) f 2Au(s) + 3Pt2+(aq) + 8Cl-(aq)
-
(reaction 2)
As expected, the newly formed hollow nanoboxes having side
lengths between 200 and 400 nm (Figure 6c-f) are slightly
larger than their corresponding nanocube templates (Figure 6,
parts a and b) with a fairly rough surface composed of small
Au nanoparticles (Figure 6, parts c and d). To image the inside
structure of these nanoboxes, we have deliberately removed
some cubic face(s) of the nanoboxes by prolonged ultrasoni-
cation. As can be clearly seen in Figure 6e, the broken
nanoboxes have a hollow chamber with an individual nanotube
threading through their cavities. TEM images, as the one shown
in Figure 6f, also revealed a hollow structure for these nanoboxes
(cf., Figure 2b). The EDS spectrum given in Figure 6g further
confirmed the chemical nature of these Au nanoboxes with no
detectable Pt. The observed C and Cu peaks are attributable to
CNTs and the copper foil substrate, respectively.
Figure 5 shows SEM and TEM images for CNTs modified
with Pt nanospheres and/or nanocubes along their outerwall by
the SEED technique. The Pt nanocube-modified CNTs were
prepared by exposing the Cu foil-supported CNTs to an aqueous
solution of 3.8 mM K2PtCl4 containing 5 mM CuCl2 for 1 min.
For the formation of Pt spheric nanoparticles on CNTs, however,
the Cu foil-supported CNTs were reacted with an aqueous
solution of 0.95 mM K2PtCl4 without the addition of CuCl2.
Figure 5, parts a and b, shows multisite modification of the
Using aligned CNTs as the starting materials, we found that
the SEED technique could even be used to deposit metal
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Figure 4. SEM images of Pt nanocubes formed on Cu foils in an aqueous solution of K2PtCl4 (3.8 mM) at different reaction times: (a) 30 s; (b) 2 min;
(c) 5 min; (d) 15 min; (e) 30 min; (f) 60 min; (g) 2 h; (h) 10 h; (i) 17 h. (j) A typical plot of the cube size vs the reaction time.
nanoparticles along the aligned nanotube structure. As shown
in Figure 7a, for instance, Pt nanocubes have been attached onto
the aligned CNT outerwall while largely retaining their align-
ment structure (Figure 7b). Owing to unique physicochemical
characteristics of the metal nanoparticles and large surface/
interface areas of the aligned nanotube supports, the aligned
CNTs decorated with Pt nanoparticles thus prepared offer
additional advantages for their use as efficient electrodes, for
example in fuel cell, electrochemical sensing, and electrocata-
lytic systems (vide infra).
aligned CNTs can also be modified with metal nanoparticles
by the SEED technique. In conjunction with the shape-controlled
syntheses of metal nanospheres and nanocubes, we have
successfully demonstrated in this study that both the innerwall
and outerwall of the template-synthesized aligned CNTs can
be asymmetrically modified with metal nanoparticles of different
shapes. Figure 8 shows SEM and TEM images for both the
innerwall-modified CNTs (Figure 8, parts a and b) and the
asymmetrically modified CNTs with their innerwall being
attached with nanospheres and outerwall with nanocubes (Figure
8, parts c and d). To demonstrate the asymmetric modification
of the nanotube sidewalls, we used the template-synthesis
technique to directly deposit aligned CNTs into a commercially
available alumina membrane according to the reported proce-
2.2. Modification of the Nanotube Innerwall and Asym-
metric Sidewall Modification with Metal Nanospheres and
Nanocubes. In our previous publication14 we have briefly
mentioned that the inner walls of the template-synthesized
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Controlled Syntheses of Metal Nanoparticles
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Figure 5. SEM (a-d) and TEM (e and f) images of Pt nanoparticles grafted on the CNTs prepared by immersing Cu-supported CNTs into the aqueous
solutions of K2PtCl4 under different conditions: (a, c, and e) 3.8 mM K2PtCl4 and 5 mM CuCl2 for 1 min; (b, d, and f) 0.95 mM K2PtCl4 for 30 min; (g)
a typical EDS spectrum of the Pt nanoparticle-CNT hybrid “necklaces”.
dures.13,18 We then carried out the deposition of Pt nanospheres
onto the innerwall of the template-synthesized, perpendicularly
aligned CNTs supported by a Cu foil (Scheme 1b), in which
the template acts as a protective layer for the nanotube outerwall.
Upon the completion of the innerwall modification, the inner-
wall-modified nanotubes were released by dissolving the
alumina template in aqueous HF. As expected, Figure 8, parts
a and b, shows a very smooth outerwall for the innerwall-
modified CNTs. The presence of nanospheres within the
nanotube innerwall was clearly seen in Figure 8b. To deposit
Pt nanocubes onto the outerwall, the innerwall-modified CNTs
were supported in a sidewall-on configuration (Scheme 1g) on
a Cu foil and exposed to an aqueous K2PtCl4 solution (3.8 mM)
containing 5 mM CuCl2 for 1 min. While Figure 8c clearly
shows the presence of nanocubes on the nanotube outerwall,
Figure 8d reveals the asymmetrically modified CNTs with
nanocubes and nanospheres deposited onto the nanotube out-
erwall and innerwall, respectively.
Our TEM examination showed that the outerwall modification
did not cause any obvious change of the predeposited Pt
nanospheres or any further deposition of Pt nanoparticles on
the nanotube innerwall. This is presumably because the sidewall-
on configuration (Scheme 1g), instead of the perpendicular
configuration (Scheme 1b) used for the nanotube innerwall
modification, of CNTs on the Cu foil allowed electrons required
for reducing Pt2+ ions to transport from the Cu foil substrate
preferentially through the directly touched nanotube outerwalls
with a minimized resistance,20 and hence no further Pt deposition
on the nanotube innerwall occurred.
2.3. Modification of the Nanotube End-Tips with Metal
Nanospheres and Nanocubes. Apart from the nanotube side-
wall modification discussed above, we have also demonstrated
site-selective deposition of metal nanoparticles on the end-tips
of aligned CNTs. Figure 9 shows SEM and TEM images of
(20) Bourlon, B.; Miko, C.; Forro, L.; Glattli, D. C.; Bachtold, A. Phys. ReV.
Lett. 2004, 93, 176806.
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Figure 6. (a and b) SEM images of CNTs grafted with Pt nanocubes at different magnifications prepared by immersing the Cu foil-supported CNTs into
an aqueous solution of 3.8 mM K2PtCl4 and 5 mM CuCl2 for 1 min. (c-e) SEM images of the nanotube-supported Au nanoboxes generated by exposing
the nanotube-supported Pt nanocubes to an aqueous solution of 1 mM HAuCl4 via reaction 2. (f and g) TEM image and EDS pattern of the Au nanobox-
decorated CNTs, respectively.
the aligned CNTs with their free end-tips being modified by Pt
nanoparticles.
optoelectronic properties of the metal nanoparticle-modified
CNTs, UV-vis absorption was measured for aqueous solutions
(by ultrasonication) of the resultant Pt nanoparticles supported
with and without CNTs. Figure 10 shows intensive absorption
peaks around 460 nm for the Pt nanocubes and nanospheres.
These newly observed optical absorption peaks arise, probably,
from the scattering/surface plasmon resonances of the Pt
nanoparticles, though further evidence is needed for a more
definitive assignment. The appearance of the weak absorption
peaks over 270-290 nm are attributable to the presence of some
fragmented small Pt nanoparticles21 generated through the
sonication-induced fragmentation. The corresponding UV-vis
absorption spectra for Pt nanocubes and nanospheres attached
onto the CNT structure also show similar intensive optical
absorption bands as their nanotube-free counterparts but with a
The top-view SEM image given in Figure 9a shows the
deposition of Pt nanocubes on the top surface of the aligned
CNT film, while the tilt, cross-sectional view SEM image in
Figure 9b clearly reveals that these nanocubes were deposited
onto the aligned nanotube end-tips with many of the nanotube
tips even protruding through the cubes (cf., Figure 5). This
peculiar structure was further confirmed by SEM (Figure 9c)
and TEM (Figure 9d) images of individual nanotubes dispersed
from the tip-modified aligned CNT arrays by ultrasonication.
Similar tip-modified nanotube structures were also produced
with Pt nanospheres, as exemplified by Figure 9, parts e and f.
3. Optoelectronic Properties of the Nanoparticle-
Decorated CNTs. 3.1. Optical Absorption. To study the
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the visible region for the Pt nanoparticle-modified CNTs arise
mainly from the nanotube-supported Pt nanoparticles.
3.2. Electrochemical Activities. To study the electrochemical
activity of the CNTs modified with metal nanoparticles, we used
the FePc-generated aligned CNTs modified with Pt nanocubes
as the working electrode (cf., Figure 7a) for electrocatalytic
oxidation of methanol. In a control experiment, the pristine
aligned CNT electrode (cf., Figure 7b) was measured under the
same condition for comparison. As can be seen in Figure 11,
the pristine aligned CNT electrode showed a capacitive current
only due to the double layer capacitance (Figure 11a), whereas
the Pt nanoparticle-grafted aligned CNT electrode showed two
strong redox peaks at 0.45 and 0.7 V characteristic of methanol
oxidation.23 These results indicate the usefulness of the metal
nanoparticle-modified aligned CNT arrays as efficient electrodes
in methanol direct fuel cells and perhaps many other electro-
chemical systems.
Conclusions
In summary, we have demonstrated a simple and versatile
route for shape/size-controlled syntheses of Pt and Au nano-
particles by the galvanic displacement reaction of a Cu foil with
K2PtCl4 and HAuCl4 in an aqueous medium under different
conditions. Both the shape and size of the resultant metal
nanoparticles were found to strongly depend on the metal salt
concentration and reaction time, providing considerable room
for regulating morphologic features of the resultant nanopar-
ticles. Of particular interest, Pt nanospheres, nanocubes, and
hybrid nanoparticles with the nanocube(s) growing out from
the nanospheres were produced.
Figure 7. SEM images of (a) aligned CNTs decorated with Pt nanocubes
prepared by immersing the Cu foil-supported aligned CNTs into an aqueous
solution of 3.8 mM K2PtCl4 and 5 mM CuCl2 for 1 min and (b) the pristine
aligned CNTs generated by pyrolysis of FePc.
In conjunction with our previously reported SEED technique,
the shape/size-controlled syntheses have been successfully
exploited to deposit the metal nanoparticles onto CNTs in a
site-selective fashion. In particular, Pt nanocube(s) or nano-
sphere(s) have been site-selectively grafted onto the nanotube
outerwall, innerwall, or end-tip. Furthermore, we have also
demonstrated the asymmetric sidewall modification by attaching
the innerwall and outerwall of individual CNTs with metal
nanoparticles of different shapes. The nanotube-supported Pt
nanoparticles were further converted into Au hollow nanoboxes
by galvanic displacement reaction of Pt with Au.
The resultant CNTs modified with metal nanoparticles of
different shape and size showed interesting optoelectronic
properties. The Pt nanocubes (100-200 nm) and nanospheres
(200-500 nm) showed intensive optical absorption bands
centered at about 460 nm, while their counterparts supported
by CNTs showed similar optical absorption bands with a red-
shift (ca. 50 nm) due to the alignment of these metal nanopar-
ticles along the nanotube structure. On the other hand, the Pt
nanoparticle-decorated aligned CNT electrodes were demon-
strated to be highly electroactive with respect to the correspond-
Figure 8. SEM (a) and TEM (b) images of CNTs having their innerwall
modified with Pt nanospheres by immersing the template-synthesized,
perpendicularly aligned CNTs into an aqueous solution of 1.9 mM K2PtCl4
for 30 min. SEM (c) and TEM (d) images of the innerwall-modified CNTs
followed by outerwall modification in an aqueous solution of 3.8 mM
K2PtCl4 containing 5 mM CuCl2 for 1 min with the innerwall-modified
CNTs being supported by a Cu foil in a sidewall-on configuration (cf.
Scheme 1g).
(21) (a) Chen, C. W.; Akashi, M. Langmuir 1997, 13, 6465. (b) Fu, X. Y.;
Wang, Y.; Wu, N. Z.; Gui, L. L.; Tang, Y. Q. Langmuir 2002, 18, 4619.
(c) Huang, J. C.; Liu, Z. L.; Liu, X. M.; He, C. B.; Chow, S. Y.; Pan, J. S.
Langmuir 2005, 21, 699. (d) Zhou, M.; Chen, S. H.; Ren, H. P.; Wua, L.;
Zhao, S. Y. Physica E 2005, 27, 341. (e) Henglein, A.; Ershov, B. G.;
Malow, M. J. Phys. Chem. 1995, 99, 14129.
(22) Correa-Duarte, M. A.; Pe´rez-Juste, J.; Sa´nchez-Iglesias, A.; Giersig, M.;
Liz-Marza´n, L. M. Angew. Chem. Int. Ed. 2005, 44, 4375.
(23) (a) Yahikozawa, K.; Fujii, Y.; Mitlsuda, Y.; Nishimura, K.; Takasu, Y.
Electrochim. Acta 1991, 36, 973. (b) Guo, D. J.; Li, H. L. J. Electroanal.
Chem. 2004, 573, 197. (c) Tang, H.; Chen, J. H.; Wang, M. Y.; Nie, L. H.;
Kuang, Y. F.; Yao, S. Z. Appl. Catal., A 2004, 275, 43.
slight red-shift (ca. 50 nm). The red-shift may be attributed to
the alignment of metal nanoparticles along the CNT structure.22
No absorption peak was observed above 400 nm for the pristine
CNTs, indicating that the intensive absorption peaks seen in
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Figure 9. SEM (a-c) and TEM (d) images of Pt nanocubes deposited onto the nanotube tips by floating the Cu foil-supported aligned CNT films on an
aqueous solution of 3.8 mM K2PtCl4 containing 5 mM CuCl2 for 1 min. SEM (e) and TEM (f) images of Pt nanospheres deposited onto the nanotube tips
by floating the Cu foil-supported aligned CNT films on an aqueous solution of 0.95 mM K2PtCl4 for 30 min.
Figure 11. Cyclic voltammograms of (a) the pristine aligned CNT and
(b) Pt nanoparticle-modified aligned CNT electrodes supported by a sputter-
coated Au layer in a N2-saturated aqueous solution of 2 M CH3OH + 0.1
Figure 10. UV-vis spectra of the pristine CNTs, Pt nanocubes (9, ∼100-
M H2SO4 solution (scan rate, 50 mV/s).
200 nm), Pt nanospheres (b, ∼200-500 nm), and the corresponding metal
nanoparticle-modified CNTs dispersed in water by ultrasonication.
Acknowledgment. We thank NSF (CCF-0403130), NEDO
(04IT4), ACS (PRF 39060-AC5M), WCI/CMPND, AFRL/ML,
ing pristine aligned CNT electrodes. Therefore, the newly
the Wright Brothers Institute, the Dayton Development Colla-
developed technique for the shape/size-controlled syntheses of
tion, and the University of Dayton for financial support. Thanks
metal nanoparticles for the site-selective modification of CNTs
also to the NEST Lab at UD for the access of the SEM and
are very attractive for producing various multicomponent
TEM facilities. We are grateful to Dr. Dave Anderson and Dr.
nanoparticle-nanotube hybrid structures useful in a wide range
Tia Benson Tolle for their kind help and strong support.
of potential applications, including fuel cell, catalytic, sensing,
and optoelectronic systems.
JA060296U
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