A general approach to electrochemical deposition of high quality free-standing noble metal (Pd, Pt, Au, Ag) sub-micron tubes composed of nanoparticles in polar aprotic solvent

Article abstract of DOI:10.1039/b920705h

A family of free-standing noble metal (Pd, Pt, Au, Ag) sub-micron tubes composed of nanoparticles (STNs) can be synthesized by a one-step electrochemical route in anhydrous dimethyl sulfoxide (DMSO) solution without addition of any other surfactants. The Royal Society of Chemistry 2010.

Full text of DOI:10.1039/b920705h

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A general approach to electrochemical deposition of high quality
free-standing noble metal (Pd, Pt, Au, Ag) sub-micron tubes
composed of nanoparticles in polar aprotic solventw
Chun-Hua Cui, Hui-Hui Li and Shu-Hong Yu*
Received (in Cambridge, UK) 6th October 2009, Accepted 18th November 2009
First published as an Advance Article on the web 10th December 2009
DOI: 10.1039/b920705h
A family of free-standing noble metal (Pd, Pt, Au, Ag) sub-micron
tubes composed of nanoparticles (STNs) can be synthesized by a
one-step electrochemical route in anhydrous dimethyl sulfoxide
(DMSO) solution without addition of any other surfactants.
cular adsorptions and reactions. These tubular structures may
provide more opportunities for developing new applications
with improved performance or useful properties.
In this communication, we demonstrate the first controlled
synthesis of a family of free-standing noble metal (Pd, Pt, Au, Ag)
sub-micron tubes composed of nanoparticles (STNs) by a one-
step electrochemical route in anhydrous dimethyl sulfoxide
(DMSO) solution without addition of any other surfactants.
The tubes of Pd, Pt, Au and Ag were electrodeposited into the
channels of the template in a three-electrode cell (Zahner
IM6ex, Germany) at room temperature. The scanning electron
microscopy (SEM) images on the surface morphologies of the
STNs are shown in Fig. 1. The STNs electrodeposited with
smooth orifices and surfaces were achieved indicating their
high quality and good uniformity. The outer diameters of the
tubes by SEM are about 300 nm. Herein, we choose Pd and Pt
as examples to demonstrate the effectiveness of this method
and discuss their interesting architectures.
In recent years, noble-metal Pd, Pt, Au and Ag tubular
nanostructured materials with hollow interiors have been
intensively investigated, stimulated by their potential applications
in gas sensors,1 catalysis,2 and biosensors3 because they can
provide unique nanometre dimensional paths, accessible inner
and outer surfaces, and confined reservoirs. The tubular
materials with single crystal structure or smooth surface were
realized by various routs4 and few synthetic approaches were
adopted to obtain nanoparticle nanotubes.5 The surface area
of smooth tubes is limited and the nanoparticle nanotubes can
be challenged by mechanical mechanics. For the proposed
applications, there is a need for tubes with a higher surface
area, stronger mechanical properties and more active points
for sensors and catalysis, especially for Pd and Pt nanomaterials.
However, it is difficult to break through the bottleneck and
improve the surface-to-volume ratio of tubes by all existing
methods. In addition, nanoparticles with three-dimensional
dendritic structures or nanoshells have higher surface-to-
volume ratio and catalytic activity compared to bulk noble-
metal crystals, and are also attractive building blocks for
nanomaterial architectures.6 Technically, the well dispersed
and stable nanoparticles are highly expected, but the control
assemblies of these architectures are mainly prepared by
chemical reactions in the presence of a mass of surfactants,
using templates or polymers as the host supported substrates
to improve the dispersibility or mechanical property. Bare
noble metal nanostructures that were considered as ideal
materials, which were usually prepared by a physical method,
i.e. laser evaporation, are usually quite difficult to be achieved.
It is well-known that small intestine transverse folds and villi
can enhance the surface area several hundred times compared
with the smooth intestine in physiology. Therefore, the
bio-inspired architecture assemblies of three-dimensional
villi-like structures into the tubes that can effectively improve
the surface area and provide more active channels for mole-
The resultant phase of the as-synthesized Pd sub-micron
tubes was examined by powder X-ray diffraction (XRD). All
of the reflections of the XRD patterns can be indexed to a pure
phase and face-centered cubic (fcc) structure (JPCDS No.
05-0681) (ESI, Fig. 1Sz). All the peaks are broadened, indicating
that the Pd sub-micron tubes contain tiny nanocrystals.
Fig. 1 Top-view SEM images of noble-metal STNs after removal of
AAO template with NaOH (2.0 M). (a) Palladium STNs were
obtained at ꢀ1.0 V for 1 h in anhydrous DMSO solvents mixed with
20 mM PdCl2, (b) platinum STNs at ꢀ1.3 V for 6 h with 20 mM PtCl2,
(c) gold STNs at ꢀ1.5 V for 5 h with 10 mM AuCl3ꢁHClꢁ4H2O, and (d)
silver STNs at ꢀ1.0 V for 1 h with 20 mM AgNO3. 100 mM NaNO3
was added in all of the reactions for supporting electrolyte.
Division of Nanomaterials & Chemistry, Hefei National Laboratory
for Physical Sciences at Microscale, Department of Chemistry,
University of Science and Technology of China, Hefei 230026,
P. R. China. E-mail: shyu@ustc.edu.cn; Fax: +86 5513603040
w Electronic supplementary information (ESI) available: Supplementary
figures S1–S5. See DOI: 10.1039/b920705h
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Fig. 3 (a) TEM image of single Pd STNs with smaller tube diameter
of 180 nm. (b) Schematic diagram of a flocky-like hemisphere at
different visual angles.
investigated by high-resolution TEM (HRTEM). The image
obtained at a lower magnification image in Fig. 2e revealed that
the flocky-like hemispheres are formed by plenty of tiny
nanocrystals, about 3–6 nm, which are close together to lower
the surface energy and explained by the XRD data above with
broadening peaks. This geometry is of particular interest
because it can improve the sensitivity in sensors or activity in
catalytic properties. The HRTEM image taken from edge of a
flocky-like hemisphere demonstrates that the nanocrystals with
a lattice spacing of ca. 0.23 nm, correspond to the lattice spacing
of the (111) planes in the fcc palladium.
Fig. 2 (a,b) Side-view SEM images at different magnifications of the Pd
STNs after one hour of electrodeposition in anhydrous DMSO solvent.
(c) TEM image of the Pd STNs under high intensity ultrasound treatment.
(d) Energy-disperse X-ray spectrum (EDS) of the Pd STNs. (e) TEM and
HRTEM images of the Pd STNs under stepwise magnifications.
The surface area of the Pd STNs with the diameters of about
300 nm was investigated (ESI, Fig. 3Sw). As we know, most of the
enhanced BET results were obtained usually by measuring
the mesoporous substrate supported Pd compound, such as
mesoporous silica templates. However, the surface area of one-
dimensional pure Pd nanostructure materials is difficult to be
improved due to the limited exposure and the smooth inner and
outer surfaces for sensor and catalytic applications. Here, we
demonstrated the enhanced surface area of the Pd STNs with a
single component. The BET surface area of the Pd STNs is
11.71 m2 gꢀ1, which is very close to those reported in the recent
literatures for the pure Pd nanoporous morphology with rando-
mized small particles (11.58 m2 gꢀ1) and nanowires with a small
As shown in the side-view SEM image of Fig. 2a, the average
diameter of the Pd tubes of about 300 nm corresponds closely to
the inner diameter of the template channels and the tube walls
are porous. The length of the Pd tubes can be tuned by the
reaction time and the maximum length of the tubes is decided
by the length of the template channels. Small holes appeared on
the surfaces of the whole tubes, indicating that the thickness of
the tube walls is even and that increasing the reaction time only
lengthens the tubes but not the wall thickness (Fig. 2a,b)
(see also the dispersed Pd tubes in ESI, Fig. 2Sw). Fig. 2c shows
a typical transmission electro-microscope (TEM) image of
palladium tubes under high intensity ultrasonic dispersion
treatment. Although the tube walls are porous and very thin,
the tubes can also retain integrality and only few collapses at the
open ends of the tubes, revealing their unique mechanical
properties. The energy-dispersive X-ray spectroscopy (EDS)
was performed on the surfaces of the tubes, showing that the
tubes are composed only of single palladium (Fig. 2d). The
copper, carbon and chromium peaks are from the TEM
support grid. As shown in Fig. 2e, it is obvious that the tubule
wall was built by a great deal of three-dimensional flocky-like
hemispheres of about 40 nm diameter (when the tube diameter
was decreased by using smaller channel AAO template, the size
of flocky-like hemispheres is also about 40 nm in Fig. 3), and
the flocky-like hemispheres aggregate together can leave a gap
that is the reason why the holes formed on the surface of the
tubes. These flocky-like hemispheres tube walls with a larger
size are somewhat difficult to be observed by TEM due to
the poor black-white contrast. In order to further study the
geometry, the dispersed hemispheres at the open ends were
diameter (12 m2 ꢀ1).7 The contribution of this BET value is
g
mainly due to the enhanced surface area of the flocky-like hemi-
spheres of the inner tube walls, not the outer surface because the
outer surface is smooth and the tube diameter is very large.
The similar deposition procedure was extended for synthesis
of Pt STNs by using PtCl2 in place of PdCl2, resulting in the
formation of similar uniform tubes. The XRD pattern of
the Pt STNs can be indexed as a fcc phase of Pt (JCPDS
no.04-0802) (ESI, Fig. 4Sw). As shown in the ESI Fig. 5S,w
both the lower position close to the bottom marked ‘‘I’’ and
the point at the tip marked ‘‘II’’ are tubular, indicating the
uniform wall thickness of the tubes. However, if in water or
acetonitrile solution by direct electrochemical deposition, the
thickness of the wall at the bottom are previously reported to
be usually quite thicker than that at the open tip due to the
over-faster mass transfer of deposition ions to the bottom.8 In
the present experiment, the Pt STNs also show the holes on the
surfaces and the obvious tubular structure. The image of the
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area marked by a square revealing the flocky-like hemisphere
aggregates like Pd (ESI, Fig. 5Sw). The EDS was used to verify
the presence of Pt. The HRTEM images also confirmed the
hemisphere-like aggregates with small nanocrystals and a
lattice spacing of ca. 0.227 nm (ESI, Fig. 5Sw).
subsequent electrodeposition of the desired metal. Pt, Pd, Au and Ag
STNs were synthesized potentiostatically at ꢀ0.3–ꢀ2.0 V in anhydrous
DMSO solvents mixed with 20 mM PdCl2 for Pd, 20 mM PtCl2 for Pt,
20 mM AgNO3 for Ag, and 10 mM AuCl3ꢁHClꢁ4H2O for Au,
respectively. 100 mM NaNO3 was added in all of the reactions as
the supporting electrolyte. All electrode potentials were measured
relative to an Ag/AgCl reference electrode using a Pt foil as a counter
electrode. Charaterization: Prior to XRD, EDS, TEM, and HRTEM
analyses, the AAO template can be removed in 2.0 M NaOH for 1 h
and washed with deionized water several times. The resultant phase of
the as-synthesized STNs was examined by XRD (Cu-Ka radiation,
l = 0.154056 nm). In order to investigate the mechanical properties,
the samples should be treated with high intensity ultrasound treatment
before SEM and HRTEM analyses. SEM images were obtained with a
Zeis SupraTM 40 high resolution field emission scanning electron
microscope operating at 5 kV. Energy-dispersive X-ray spectra (EDS)
were taken with a JEOJ-2010 transmission electron microscope with
an acceleration voltage of 200 kV. TEM and HRTEM images were
obtained using JEOJ-2010 TEM. The wide views of TEM images were
achieved by an H-7650 transmission electron microscope. The
Brunauer-Emmett-Teller (BET) surface area of the STNs was deter-
mined by N2 adsorption using an ASAP-2020 surface area analyzer.
The key factor for successful synthesis of high-quality free-
standing STNs with high mechanical properties is the use of
the DMSO as solvent medium. Firstly, DMSO is a superior
polar aprotic solvent in electrochemistry and has direct inter-
action with surfaces of the noble-metals,9 and this interaction
can induce the formation of nanocrystals, acting in the role of
a capping surfactant absorbing colloidal particles in solution,
to control the size and shape. Secondly, the solvent viscosity of
DMSO is two more times higher than water.10 This physical
parameter can effectively slow the mass transfer of metal ions
to the bottom tubes, and then the metal ions discharge to
deposit on the inner bottom tube walls, and finally favor the
formation of uniform tubes. What’s more, the PdCl2 or PtCl2
can dissolve into anhydrous DMSO to form M((CH3)2SO)2Cl2
(M = Pd, Pt) complexes,11 which will strengthen steric
hindrance, and increase the number of crystal nuclei and
reduce the rate of crystal growth. Meanwhile, the mass
transfer rate of the complexes M((CH3)2SO)2Cl2 can be slowed
down again due to metal ions surrounded with DMSO and Cl
ions compared with the bare Pd ions. The M((CH3)2SO)2Cl2
complexes are reduced and released in DMSO under applied
power and the crystal nucleus formed at the cathode, but the
DMSO can absorb weakly on the surfaces of nanocrystals.9 So
under the multi-effect of DMSO, tiny particles can be obtained
and the growth of tubes can be parallel to the AAO channel.
In summary, we have successfully developed a simple
electrochemical method to synthesize a family of unique
high-quality free-standing noble metal (Pt, Pd, Au, and Ag)
STNs with a high surface-to-volume ratio and high mecha-
nical properties against ultrasonication. The Pd and Pt tubes
are built by many flocky-like nanohemispheres which are
assembled by plenty of tiny nanocrystals of 3–6 nm. These
tubular structures, just like the transverse folds and villi in the
small intestine inner wall, have a large surface area and highly
active points for sensors and catalysis application. This
synthesis strategy can open a new avenue towards the
syntheses of other metals, alloys and their related tublar
materials with unique shape and structural features, which
may have promising applications in sensors and catalysts.
This work is supported by the National Basic Research
Program of China (2010CB934700), the National Science Foun-
dation of China (NSFC) (Grants nos. 50732006, 20671085), and
the Partner Group of the CAS and the Max Planck Society.
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Notes and references
z Experimental section: Chemicals: NaNO3, PdCl2, AuCl3ꢁHClꢁ4H2O,
AgNO3, dimethyl sulfoxide (99% w/w), C2H5OH ( Z 99.7% w/w),
and NaOH were commercially available from Shanghai Chemical
Reagent Co. Ltd and PtCl2 from Alfa Aesar. All the chemical reagents
were analytical grade, and used as received without further purifica-
tion. Electrochemical synthesis of Pt, Pd, Au and Ag STNs: The
commercial anodic aluminium oxide template (Anodisc 47, Whatman
Co., UK) with two different channel diameters (about 300 nm and
180 nm by SEM) sputtered with thin Au layer on one side about 40 nm
thickness to form an annular base electrode at the bottom of the
nanopores. The thin Au layer was used as the working electrodes in the
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942 | Chem. Commun., 2010, 46, 940–942