Ultrathin single crystal Pt nanowires grown on N-doped carbon nanotubes

Article abstract of DOI:10.1039/b916080a

Ultrathin single crystal platinum nanowires with diameters of ～2.5 nm and lengths up to 100 nm were synthesized on nitrogen-doped CNTs (N-CNTs) via a straightforward wet-chemical method in environmentally friendly water solution at room temperature, witho

Full text of DOI:10.1039/b916080a

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Ultrathin single crystal Pt nanowires grown on N-doped
carbon nanotubesw
a
a
a
a
a
b
Shuhui Sun, Gaixia Zhang, Yu Zhong, Hao Liu, Ruying Li, Xiaorong Zhou and
Xueliang Sun*a
Received (in Cambridge, UK) 5th August 2009, Accepted 22nd September 2009
First published as an Advance Article on the web 13th October 2009
DOI: 10.1039/b916080a
Ultrathin single crystal platinum nanowires with diameters
of B2.5 nm and lengths up to 100 nm were synthesized on
nitrogen-doped CNTs (N-CNTs) via a straightforward wet-
chemical method in environmentally friendly water solution at
room temperature, without using any stabilizing agent.
These ultrathin nanowires and their composites with N-CNTs
hold potential for a wide range of applications.
growth process, and the key role of nitrogen was revealed.
The synthetic procedure is very simple, and only two chemicals
(H PtCl and HCOOH) were used throughout the whole
2
6
synthetic process, without using any stabilizing agent.
Further, the reaction was conducted at room temperature in
environmentally friendly aqueous solution.
In a typical synthesis, 0.032 g hexachloroplatinic acid
(
H
2
PtCl
6
ꢀ6H
2
O, Aldrich) and 1 mL formic acid (HCOOH,
O to form a
Among various one-dimensional (1D) noble metal nano-
structures, platinum nanowires (Pt NWs) are of particular
importance and are expected to play a critical role in electronics,
catalysis, fuel cells and the automotive industries, as this metal
has outstanding catalytic and electrical properties and superior
Aldrich) were added simultaneously to 20 mL H
2
golden orange solution. The N-CNTs (nitrogen content of
10.4 at.%) grown on carbon paper, synthesized by a CVD
9
method, were immersed in the above solution, acting as the
growth substrates. Then the reaction proceeded at room
temperature without stirring for up to 3 days until the solution
color gradually changed to colorless. The final product can be
easily collected by handling the carbon paper support and
washing with deionized water.
1
,2
resistance to corrosion. To date, various methods have been
developed for the synthesis of Pt NWs. However, wet-chemical
methods often lead to the formation of relatively large diameter
(410 nm) or polycrystalline wires whereas the commonly used
template approach has problems associated with the removal
3
of the template. It is well accepted that the surface area
Fig. 1 shows the representative transmission electron
microscopy (TEM) images of N-CNTs before and after
growth of the Pt NWs. It can be seen that the pristine N-CNTs
have diameters ranging from 40 to 60 nm (Fig. 1A, and
Fig. S1, ESIw), and the bamboo-like structure can be attributed
increases with the diameter decrease of individual wire, which
4
in turn, has immediate effect on surface-related applications.
There is very limited success in producing ultrafine Pt NWs,
5
especially on the scale below 5 nm. Very recently, a flurry of
7
to the integration of N into the graphitic structure. Fig. 1B
papers has appeared in the literature regarding the synthesis
6
shows that numerous Pt NWs, with lengths up to 100 nm,
grow uniformly on N-CNTs, forming a hybrid structure of
Pt NWs/N-CNTs. The higher magnification image (Fig. 1C)
indicates that the diameter of the Pt NWs is less than 3 nm.
The aspect ratio of Pt NWs easily approaches 50. A survey of
several tens of Pt NWs under TEM investigation reveals that
the diameters of these NWs are in the range of 2 to 3 nm,
with an average of 2.5 nm. Besides the ultrathin nature, these
Pt NWs are also single crystalline. Selected area electron
diffraction (SAED) pattern recorded from a few NWs confirms
their high crystallinity (inset in Fig. 1C). The high-resolution
TEM (HRTEM) image (Fig. 1D) shows that the Pt NWs grow
along h111i direction, and the interplanar distance between
the {111} planes is 0.23 nm which matches that for bulk
Pt crystals. To the best of our knowledge, this is the first time
that single crystal Pt NWs with such thin diameters were
synthesized with such a simple method. Interestingly, our
previous work shows that under similar synthetic conditions
but without using N-incorporated supports, Pt NWs that grew
either freely in solution or on CNT and carbon black supports,
of Au ultrathin (o3 nm) NWs. However, the synthesis of
ultrathin NWs of single crystal Pt with such small diameters is
still challenging.
Recently, nitrogen-doped carbon nanotubes (N-CNTs)
have attracted much attention due to their unique properties
7
and wide applications. Specifically, they have been widely
used as diverse matrices to fabricate various nanocomposites
such as Pt/N-CNTs, which possess great potential in fuel cells,
8
electronic devices, and chemical sensors.
Here we report a straightforward solution method for the
synthesis of single crystalline ultrathin Pt NWs, with uniform
diameters of B2.5 nm and lengths up to 100 nm, on N-CNTs.
In addition, direct evidence for the formation of ultrathin Pt
NWs was provided by systematically investigating their
a
Department of Mechanical and Materials Engineering,
The University of Western Ontario, London, ON, Canada N6A 5B9.
E-mail: xsun@eng.uwo.ca; Fax: +1-519-661-2111 ext. 87759;
Tel: +1-519-661-3020
School of Materials, University of Manchester, Manchester,
UK M60 1QD
b
2b,5b,c
always have relative larger diameters of 4 nm
(also see
w Electronic supplementary information (ESI) available: SEM images
of pristine N-CNTs; SEM and TEM images of Pt NWs grew freely and
on CNT as well as CB supports; XPS peak separation of Pt4f. See
DOI: 10.1039/b916080a
Fig. S2–S4, ESIw). These reveal that N-doping into CNTs is
crucial to the formation of ultrathin Pt NWs with diameter less
than 3 nm.
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048 | Chem. Commun., 2009, 7048–7050
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Fig. 1 TEM images of N-CNTs before (A) and after (B) the growth
of Pt NWs; TEM (C) and HRTEM (D) images of ultrathin Pt NWs.
In order to investigate the possible reasons for the formation
of ultrathin Pt NWs on N-CNTs, X-ray photoelectron
spectroscopy (XPS) was employed to study the chemical
composition and status of the product. The asymmetric C1s
high-resolution spectrum (Fig. 2A, centered at B285 eV) is
deconvoluted into five peaks, which can be assigned, respectively,
to the C–C bonds in graphite (PC1), alternate defect carbon
structures associated with C–N and C–O bonds overlapping
(286–289 eV, PC2–4), and PC1 p* ’ p shake-up satellite
(PC5). Comparing the carbon spectra before and after Pt
growth (inset of Fig. 2A), the latter one shows a narrower
band with significant decrease of PC2–4 peaks corresponding
11
to the localized (defect) carbon atoms, which can be attributed
to the block effect of Pt NWs to the X-ray. This indicates that
Pt NWs are more easy to deposit on the defects of the N-CNTs
than on the perfect graphite structures. The deconvolution of
the Pt4f spectrum (Fig. S5, ESIw) indicates pure metallic Pt,
without obvious carbide and nitride formation. Fig. 2B shows
the N1s spectra of the N-CNTs before and after Pt deposition.
Fig. 2 Typical XPS spectra of N-CNTs before and after Pt deposition.
(
A) C1s, and (B) N1s.
1
0
The pristine N1s spectrum was deconvoluted into 5 peaks.
It is believed that the main peak (398.7 eV) corresponding to
the pyridine-like N within the predominantly graphitic framework
is responsible for both the wall roughness and interlinked
N-CNTs, they are more prevalent on the wall ruffles and the
interlinked parts (nodes) of N-CNTs which may be assigned
1
1
mainly to the pyridine-like N. This is consistent with our
XPS results since much higher concentration pyridine-like
than other types N was detected in pristine N-CNTs. According
to our XPS results, these defects induced by N incorporation
would provide preferred anchoring sites for Pt deposition.
To reveal the underlying growth mechanism of ultrathin Pt
NWs on N-CNTs, products were collected as a function of
growth time, and their morphologies were evaluated by TEM
and HRTEM. The HRTEM image of a pristine N-CNT
(Fig. 4A) shows that the intersects are usually consist of 4–6
graphitic layers. Between these intersects, there are many
irregular web-like structures (2–3 atomic layers) across the
surface. Careful inspection reveals the existence of many
broken graphitic layers (defects) along the entire surface of
1
1
morphologies observed in the N-doped structure. After
Pt deposition, in addition to the significant decrease of the
whole N1s signal and the almost complete disappearance of
two peaks corresponding to the oxide N atoms (403–405 eV),
the area ratio of pyridine-like (398.7 eV) to graphite-like
(
401.0 eV) N decreased from 1.81 to 1.21, further implying
that Pt NWs grow preferentially on the defect sites that more
associated with pyridine N, of the N-CNTs.
The distribution of nitrogen and carbon in the N-CNTs was
studied by elemental mapping using electron energy loss
spectroscopy (EELS) (Fig. 3). The brighter regions represent
a higher concentration of the element. Fig. 3C shows that
while the N atoms seem to be present throughout of the
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Fig. 4 TEM images show the growth process of ultrathin Pt NWs on
N-CNTs. (A) pristine N-CNT; (B) initial (nuclei), (C) intermediate
and (D) final growth stages.
Fig. 3 EELS mapping of a pristine N-doped CNT: (A) bright field
image; (B) carbon map; (C) nitrogen map.
This work was supported by NSERC, CRC, CFI, ORF,
ERA and UWO. S.S. is grateful to the NSERC scholarship.
G.Z. thanks Ontario PDF program. We are in debt to David
Tweddell for his kind help and fruitful discussion.
the nanotube, suggesting that N incorporation into CNTs
promotes not only the bamboo-structured morphology but
also strongly affects the tube defect structure. At the initial
growth stage (Fig. 4B and inset), Pt nuclei readily attach to the
sites with higher density defects along the N-CNTs, since
nitrogen can enhance the Pt adsorption on the CNT surface
1
2
due to its large electron affinity. Interestingly, the size of the
Pt nanoparticles (nuclei) ranges from 2 to 3 nm with an
average of 2.5 nm, which is smaller than those on carbon
nanotubes and nanospheres. We believed that the broken
graphitic layers, which are more associated with the pyridinic N,
confine the Pt atoms and provide the main initial nucleation
sites for the formation of Pt nanoparticles with small
diameters. This may be the key for the formation of ultrathin
Pt NWs on N-CNTs. As the growth time increased, a large
amount of nanoparticles progressively cover the whole surface
of the N-CNTs. These small Pt nuclei act as the seeds to direct
the anisotropic growth of Pt NWs, in that the growth rate
along the closed-packed h111i direction is enhanced at very
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050 | Chem. Commun., 2009, 7048–7050
This journal is ꢁc The Royal Society of Chemistry 2009