Facile synthesis of noble metal nanotubes by using ZnO nanowires as sacrificial scaffolds and their electrocatalytic properties

Article abstract of DOI:10.1039/c0cc05701k

Pt and Pd nanotubes could be prepared rapidly with high yields and good uniformity through the reduction of metal precursors in the presence of ZnO nanowires as inexpensive sacrificial templates. The prepared nanotubes showed efficient electrocatalytic pr

Full text of DOI:10.1039/c0cc05701k

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COMMUNICATION
Facile synthesis of noble metal nanotubes by using ZnO nanowires
as sacrificial scaffolds and their electrocatalytic propertiesw
a
b
a
b
a
Young Wook Lee,z Mi Ae Lim,z Shin Wook Kang, Inkyu Park* and Sang Woo Han*
Received 21st December 2010, Accepted 30th March 2011
DOI: 10.1039/c0cc05701k
Pt and Pd nanotubes could be prepared rapidly with high yields
and good uniformity through the reduction of metal precursors
in the presence of ZnO nanowires as inexpensive sacrificial
templates. The prepared nanotubes showed efficient electro-
catalytic properties toward alcohol oxidation.
of simple, fast, low-cost, and versatile techniques for the
production of 1D nanostructures is still highly required and
challenging for their broader applications.
In this work, we demonstrate a facile chemical process for
the controlled synthesis of uniform 1D noble metal nanotubes
by using ZnO nanowires as inexpensive sacrificial templates.
Pt and Pd nanotubes could be prepared rapidly with high
yields and good uniformity through the reduction of metal
precursors with sodium citrate in the presence of ZnO nano-
wires. The formation of tubular structures could be attributed
to the in situ dissolution of ZnO during the reaction.
Since such nanotube structures have commonly exhibited
relatively large surface areas, it can be expected that the
prepared nanotubes may show enhanced catalytic efficiencies.
In this regard, we have also investigated the electrocatalytic
activities of the synthesized nanotubes for methanol or ethanol
oxidation.
Pt- and Pd-based nanostructures have attracted a great deal of
attention as catalytic materials due to their prominent activities
in a wide range of heterogeneous chemical reactions and electro-
catalytic oxidation/reduction reactions in polymer electrolyte
1
membrane fuel cells (PEMFCs). In this regard, large efforts
have been focused on the controlled synthesis of Pt- and
Pd-based nanostructures with specific morphologies because
their catalytic properties can be tuned by tailoring their shapes
2
and sizes. In particular, the synthesis of one-dimensional (1D)
nanostructures including nanotubes and nanowires is currently
attracting considerable interest because of their unique structural
and catalytic properties. Accordingly, various synthetic routes
In a typical synthesis of metal nanotubes, an aqueous
solution of the metal precursor was added to an aqueous
solution of sodium citrate, and then the ZnO nanowire array
grown on a Si wafer was immersed into this growth solution.
The average length and diameter of ZnO nanowires are about
1.5 mm and 55 nm, respectively (Fig. S1, ESIw). The reaction
mixture was then heated at B90 1C for about 1 h in a drying oven.
3
have been developed to prepare these types of nanomaterials.
The controlled synthesis of 1D nanostructures has been
usually achieved by template-based methods, wherein the
inorganic templates such as anodized aluminium oxide (AAO)
4
serve as structure-directing scaffolds. Most of the previous
template-assisted strategies have required extra template-
removal steps. Recently, the galvanic replacement reaction
of metal nanowires has been used as an alternative method to
prepare the 1D nanostructures. For instance, Pt, Pd, and
Pt–Pd nanotubes were synthesized by using Ag, Se, and Te
5
–9
nanowires as sacrificial templates, and their electrocatalytic
7
activities toward methanol oxidation and oxygen reduction
,9
6
reaction were investigated. However, a long reaction time was
required for the complete replacement of nanowire templates
and additional etching steps for the remaining template were
necessary in some cases. Moreover, these methods are not
cost-effective because rather expensive materials are used as
sacrificial templates. On the basis of this fact, the development
a
Department of Chemistry and KI for the NanoCentury, KAIST,
Daejeon 305-701, South Korea. E-mail: sangwoohan@kaist.ac.kr
Department of Mechanical Engineering and KI for the NanoCentury,
b
KAIST, Daejeon 305-701, South Korea. E-mail: inkyu@kaist.ac.kr
w Electronic supplementary information (ESI) available: Details for
synthesis and characterization, and additional experimental data. See
DOI: 10.1039/c0cc05701k
Fig. 1 (a) SEM and (b) TEM images of the Pt nanotubes. (c) HAADF-
STEM image and the corresponding EDS mapping images of a Pt
nanotube. (d) Changes in the pH of the reaction mixture and the mole%
of Zn of the nanotubes during the reaction.
z These authors contributed equally to this work.
This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6299–6301 6299

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ꢁ
This might be due to the consumption of OH ions during the
11
2
ꢁ
reduction of PtCl
4
by citrate (Fig. S5, ESIw). It has been
known that ZnO can be dissolved in acidic solution by the reac-
+
+
2+
12
tion with H ions (ZnO(s) + 2H (aq) - Zn (aq) + H O(l)).
2
Therefore, the hollow interiors of the nanotubes are formed by
+
this in situ dissolution of ZnO nanowires by H ions that is
enhanced by the decrease of pH in the solution via the
reduction of metal precursors. In fact, dissolved Zn has been
identified by the ICP-AES measurements on the growth
solution after the reaction. On the other hand, the pH of
solution decreased more rapidly in the absence of the ZnO
template under otherwise identical synthetic conditions (Fig. 1d).
This is another experimental evidence for the consumption of
+
H
by the ZnO nanowires.
Fig. 2 XRD patterns of the Pt and Pd nanotubes and the pristine
By using the same synthesis protocol, the Pd nanotubes
ZnO nanowires.
could also be prepared with K PdCl as a metal precursor.
2
4
Fig. 1a shows a representative scanning electron microscopy
Representative SEM and TEM images of the Pd nanotubes
are shown in Fig. 3a and b, respectively. The average outer
diameter and wall thickness of the Pd nanotubes are 130 ꢀ 30
and 55 ꢀ 12 nm, respectively. The compositional line profiles
on a single nanotube (Fig. 3c) and elemental mapping of Pd
and Zn (Fig. 3d) obtained by the HAADF-STEM-EDS clearly
indicate that the synthesized nanostructure is a Pd nanotube.
The XRD pattern of the prepared Pd nanotubes exhibits
characteristic diffraction peaks which can be indexed to the
(SEM) image of the samples prepared by using K PtCl as a metal
2 4
precursor, which clearly demonstrates the formation of nanotubes
with open tips. The mean length of nanotubes is B1.5 mm and
their outer diameter is ranging from 40 to 60 nm. The transmission
electron microscopy (TEM) image shown in Fig. 1b also confirms
the successful preparation of the nanotubes; each nanotube has a
well-defined hollow interior and a uniform wall with an average
thickness of 13 ꢀ 4 nm. Elemental mapping of Pt and Zn (Fig. 1c)
and the compositional line profiles on a single nanotube
˚
fcc Pd (Fig. 2). A d-spacing of 2.24 A for adjacent lattice
(
Fig. S2, ESIw) obtained by the high-angle annular dark-field
planes shown in the HRTEM image of the Pd nanotubes
reveals the predominance of {111} planes on the surface of the
scanning TEM energy-dispersive X-ray spectroscopy (HAADF-
STEM-EDS) reveal that the prepared nanostructure is definitely a
Pt nanotube, though there remains a trace amount of Zn. The
Pt/Zn ratio was estimated to be 97 : 3 by using an inductively-
coupled plasma-atomic emission spectrometer (ICP-AES).
2
d
Pd nanotubes (Fig. S3b, ESIw).
To investigate the catalytic performance of the prepared
nanotubes, their electrocatalytic activities toward alcohol
oxidation were tested. For electrochemical experiments, Pt
and Pd nanotubes were prepared by using ZnO nanowire
arrays grown on indium tin oxide (ITO) glass as templates
(Fig. S6, ESIw). Fig. 4a shows the cyclic voltammogram (CV)
of methanol oxidation at the Pt nanotube in 0.1 M KOH
solution containing 0.1 M methanol. Pt has been known to
have a good electrocatalytic activity for methanol oxidation in
Fig. 2 shows X-ray diffraction (XRD) patterns of the prepared
Pt nanotubes as well as the pristine ZnO nanowire templates. Pt
nanotubes exhibit four diffraction peaks which can be indexed to
the (111), (200), (220), and (311) reflections of the face-centered
cubic (fcc) structure of metallic Pt while the ZnO peaks
completely disappeared, showing that the Pt nanotubes are
well-crystallized along with the consumption of the ZnO
template. High-resolution TEM (HRTEM) measurements of
the Pt nanotubes identified the lattice planes forming the nano-
7
,9,13
alkaline media.
For comparison, the CV of methanol
oxidation at the Pt nanoparticles (Experimental details and
Fig. S7a, ESIw) is also shown in Fig. 4a. The current values
were normalized with respect to the electrochemically active
˚
tubes (Fig. S3a, ESIw). A d-spacing of 2.30 A for adjacent lattice
10
planes corresponds to the (111) planes of fcc Pt, indicating that
most of the exposed facets of the Pt nanotubes are {111}.
The structural evolution of the Pt nanotubes could be
monitored by TEM measurements on different reaction stages
(
Fig. S4, ESIw). Initially, Pt deposited onto the surface of ZnO
nanowires which served as a seeding template. Interestingly,
we found that the void spaces in the nanotubes appeared by
increasing the reaction time to 20 min. As the reaction
proceeded, the Pt deposited more densely and the hollow
feature in each nanotube became more distinct. Beyond 50 min,
no significant change in the structure was observed. This was
also reflected in the change of mole% of Zn during the reac-
tion measured from EDS and ICP-AES analyses (Fig. 1d). The
observed growth of nanotubes could be attributed to the
selective dissolution of ZnO templates during the reduction of
Pt precursors. As shown in Fig. 1d, the pH of the reaction
mixture was decreased rapidly with increasing reaction time.
Fig. 3 (a) SEM and (b) TEM images of the Pd nanotubes. (c) HAADF-
STEM image and cross-sectional compositional line profiles of a Pd
nanotube. (d) HAADF-STEM-EDS mapping images of the Pd nanotube.
6
300 Chem. Commun., 2011, 47, 6299–6301
This journal is c The Royal Society of Chemistry 2011

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attributed to their high surface area and the presence of active
6,15
sites on their surfaces. Moreover, the enhanced mass transport
and utilization of catalyst could be expected for the nanotube
7,16
catalysts because of their anisotropic tubular morphologies.
Taken together, the present work might provide a promising
strategy for developing efficient fuel cell catalysts.
In summary, we have presented a facile and cost-effective
aqueous synthesis method for the preparation of Pt and Pd
nanotubes by the reaction between ZnO nanowires and aqueous
solution of a metal precursor under moderate temperature
and mild chemical conditions. The successful synthesis of
nanotubes could be achieved via the selective dissolution of
ZnO templates during the reduction of metal precursors. The
prepared nanotubes showed enhanced electrocatalytic properties
toward alcohol oxidation. It is expected that our strategy
can be extended to the fabrication of other single- or multi-
component metal nanotubes for the improvement of catalytic
performance in diverse chemical reactions.
Fig. 4 (a) CVs in 0.1 M KOH + 0.1 M methanol of the Pt nanotubes
ꢁ1
and Pt nanoparticles on ITO electrodes. Scan rate: 50 mV s . (b) CA
curves for the Pt nanotubes and Pt nanoparticles at 0.0 V vs. Ag/AgCl.
(
c) CVs in 0.1 M KOH + 0.1 M ethanol of the Pd nanotubes and Pd
This work was supported by Basic Science Research
Programs (2008-0062042, 2010-0029149, 2010-0015290), Future-
based Technology Development Program (Nano Fields) (2009-
ꢁ1
nanoparticles on ITO electrodes. Scan rate: 50 mV s . (d) CA curves
for the Pd nanotubes and Pd nanoparticles at ꢁ0.2 V vs. Ag/AgCl.
0
082640), and Pioneer Research Center Program (2009-0082813)
surface area (ECSA), which were calculated by measuring the
coulombic charge for oxygen desorption (Fig. S8, ESIw). As
shown in Fig. 4a, characteristic well-separated anodic peaks in
the forward and reverse sweeps associated with methanol
oxidation were observed. It is noticeable that the current
density of the Pt nanotubes is much higher than that of
the Pt nanoparticles; the peak current densities are 5.01 and
through the National Research Foundation (NRF) funded
by the Korean government (MEST), and Open Innovation
Research Program of Hewlett Packard (HP) Company, USA.
Notes and references
–2
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This journal is c The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 6299–6301 6301