Ultrathin platinum-group metal coated hierarchical flowerlike gold microstructure: Electrochemical design and characterization

Article abstract of DOI:10.1016/j.electacta.2007.10.080

The deliberate tailoring of hierarchical flowerlike gold microstructure (HFGMs) at the ultrathin level is an ongoing challenge and could introduce opportunities for new fabrication and application in many fields. In this paper, a templateless, surfactantless, electrochemical strategy for fabrication of ultrathin platinum-group metal coated HFGMs is proposed. HFGMs were prepared by simple electrodeposition on an indium tin oxide (ITO) substrate. The resulting HFGMs enables the deposition of an ultrathin overlayer through copper underpotential deposition (UPD), followed by spontaneous replacement of the copper with the platinum-group metal to yielded a uniform and ultrathin Pt-group metal layer on the HFGMs surface. Thus prepared bimetallic core-shell sample has been characterized by cyclic voltammetry, scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray maps and X-ray diffraction (XRD). This is a very general and powerful technique for the fabrication of hierarchical, ultrathin coating, bimetallic core-shell microstructure, which, in turn, opens up the possibility of building more complex multicomponent hierarchical microstructure with potential application, such as electrocatalysis, biosensors and nanodevices, etc.

Full text of DOI:10.1016/j.electacta.2007.10.080

Available online at www.sciencedirect.com
Electrochimica Acta 53 (2008) 2776–2781
Ultrathin platinum-group metal coated hierarchical flowerlike gold
microstructure: Electrochemical design and characterization
Liang Wang, Shaojun Guo, Junfeng Zhai, Xiaoge Hu, Shaojun Dong∗
State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry,
Chinese Academy of Sciences, Changchun, Jilin 130022, China
Received 11 July 2007; accepted 23 October 2007
Available online 13 November 2007
Abstract
The deliberate tailoring of hierarchical flowerlike gold microstructure (HFGMs) at the ultrathin level is an ongoing challenge and could
introduce opportunities for new fabrication and application in many fields. In this paper, a templateless, surfactantless, electrochemical strategy
for fabrication of ultrathin platinum-group metal coated HFGMs is proposed. HFGMs were prepared by simple electrodeposition on an indium
tin oxide (ITO) substrate. The resulting HFGMs enables the deposition of an ultrathin overlayer through copper underpotential deposition (UPD),
followed by spontaneous replacement of the copper with the platinum-group metal to yielded a uniform and ultrathin Pt-group metal layer on
the HFGMs surface. Thus prepared bimetallic “core-shell” sample has been characterized by cyclic voltammetry, scanning electron microscopy
(SEM), energy-dispersive X-ray spectroscopy (EDX), X-ray maps and X-ray diffraction (XRD). This is a very general and powerful technique for
the fabrication of hierarchical, ultrathin coating, bimetallic “core-shell” microstructure, which, in turn, opens up the possibility of building more
complex multicomponent hierarchical microstructure with potential application, such as electrocatalysis, biosensors and nanodevices, etc.
© 2007 Elsevier Ltd. All rights reserved.
Keywords: Gold; Platinum; Palladium; Hierarchical structure; Underpotential deposition
1. Introduction
strate, superhydrophobicity, and the nanodevice realization
[10].
Hierarchicalmicro/nanostructuresassembliesusingnanopar-
ticles, nanorods, nanobelts as building blocks and complex
nanocrystals with well-defined shape and inner structure are
of great significance for the realization of nanodevices and
have been obtained through different strategies, such as
hydrothermal strategy and electrochemical method, etc. [1–9].
Some semiconductor materials, such as ZnO, CuO, MnO2,
Bi2S3, with flowerlike nanoarchitectures (FNs), have been
fabricated by a mild hydrothermal strategy [10–15]. How-
ever, fabrication of metallic FNs is still a challenge, and
especially, the study of FNs’ morphology dependent prop-
erties is to be conducted. In addition, FNs growing on a
substrate directly, which is difficult for hydrothermal strat-
egy, is of importance for some special applications, such
as surface-enhanced Raman scattering (SERS) active sub-
Electrochemical tactic might be a good alternative approach
for meeting this matter. Several electrochemical methods
are employed to prepare complex hierarchical gold micro/
nanostructures, most of which are synthesized in the presence
of organic additives or surfactants [10], and others are based on
template [16]. Recently, we have reported a facile and low-cost
electrochemical approach to prepare diameter-controlled hier-
archical flowerlike gold microstructures (HFGMs) with “clean”
surface without introducing any template or surfactant and the
diameter of HFGMs can be easily controlled via simply control-
ling the time of electrodeposition [17].
There is currently considerable interest in the preparation
and application bimetallic “core-shell” nanostructures in part by
specific applications in catalysis, sensors and microelectronics
[18–20]. Among various alternative candidates, Pt-group metal
are of great significance in chemical catalysis [21] and also
have attracted extensive attention in the application of surface
enhanced Raman spectroscopy (SERS) [22,23]. Since surface-
catalyzed reactions are extremely sensitive to the atomic-level
∗
Corresponding author. Tel.: +86 431 85262101; fax: +86 431 85689711.
E-mail address: dongsj@ns.ciac.jl.cn (S. Dong).
0013-4686/$ – see front matter © 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2007.10.080

L. Wang et al. / Electrochimica Acta 53 (2008) 2776–2781
2777
details of the catalytic surface, the deliberate tailoring of the
Pt-group metal nanostructured catalysts is an ongoing challenge
and could lead to new electronic and catalytic properties. On the
other hand, Pt-group metal micro/nanostructure with sharp tips
may exhibit especially high SERS activity [24].
HFGMs with the potential of 0.5 V and the deposition time was
30 min.
2.3. Electrochemical design ultrathin Pt-group metal
coated HFGMs
Herein a promising electrochemical strategy for fabrication
of ultrathin Pt-group metal coated HFGMs is proposed. Uniform
and ultrathin Pt-group metal coating on Au or Pt nanoparti-
cles can be achieved, which involves redox replacement of an
underpotential deposition (UPD) ultrathin level layer by more
noble metal cations that could be reduced and simultaneously
deposited [25,26]. Similar tactics can now be harnessed to pre-
pare HFGMs modified with uniform Pt-group overlayers. Thus
prepared bimetallic “core-shell” HFGMs material with sharp
tips could be expected to exhibit excellent catalytic proper-
ties and especially high SERS activity [24,27]. To the best of
our knowledge, attempt to the preparation of ultrathin Pt-group
metal coated three-dimensional (3D) HFGMs has not yet been
exploited. Although 1D gold nanorod and 2D nanoporous gold
nanorod have been exploited to be coated with ultrathin Pt layer
based on the UPD technique, the strategy must use template [27].
However, the use of hard templates may complicate the syn-
thetic procedure and limit the synthesis of micro/nanostructured
materials in large quantities [28,29]. The newly electrochemical
design presented here is a very general and powerful tech-
nique for fabrication of hierarchical, ultrathin coating, bimetallic
“core-shell” microstructure, which, in turn, opens up the possi-
bility of building more complex multicomponent hierarchical
microstructure with potential application, such as electrocataly-
sis, biosensors and nanodevices, etc.
UPD-redox replacement reaction technique was adopted to
fabricate ultrathin Pt-group metal coated HFGMs. The typical
experiment for fabricating ultrathin Pd coated HFGMs is as fol-
lows: a UPD ultrathin of copper was formed on the HFGMs by
holding the potential at 0.01 V in deaerated 0.1 M H2SO4 con-
taining 1 mM CuSO4 for several minute, then transferred to a
solution of 5 mM PdCl2 in deaerated 0.1 M HClO4. The elec-
trode was left in this solution at an open circuit for 10 min to
ensure complete redox replacement of the UPD Cu, then rinsed
and dried with N2. The procedure involved spontaneous redox
replacement of the Cu layer by the Pt-group metal, similar to
that as described previously [21–23,26,27]. The electrochemical
fabrication ultrathin Pt coated HFGMs processing was similar to
that of Pd and just 5 mM K2PtCl4 replaced the 5 mM PdCl2.The
value of this method is that it provides epitaxial Pt-group lay-
ers on the HFGMs. Thus prepared ultrathin Pd and Pt coated
HFGMs were used for further characterization.
2.4. Characterization of the ultrathin Pt-group metal
coated HFGMs
Electrochemical experiments were performed with a CHI
832 electrochemical analyzer (CH Instruments, Chenhua Co.,
Shanghai, China). A conventional three-electrode cell was used,
including a Ag/AgCl (saturated KCl) electrode as reference
electrode, a platinum wire counter electrode and a HFGMs or
Pt-group metal coated HFGMs modified ITO as working elec-
trode. ThemorphologyoftheHFGMsandPt-groupmetalcoated
HFGMs was characterized with a XL30 ESEM FEG scan-
ning electron microscopy (SEM) at an accelerating voltage of
20 kV. This SEM instrument has also been used to determine the
chemical composition by energy-dispersive X-ray spectroscopy
(EDX) and characterize the elemental map of Au and Pt-group
metal of the resulting product. The X-ray diffraction (XRD)
analysis of the resulting product was carried out on a D/MAX
2500 V/PC X-ray diffractmeter using Cu (40 kV, 30 mA) radia-
tion.
2. Experimental
2.1. Reagents
HAuCl4, CuSO4 and H2SO4 were purchased from Bei-
jing Chemical Factory (Beijing, China). PdCl2 and K2PtCl4
were purchased from Chinese Chemical Reagent Companies
(Shanghai, China). Tin-doped indium oxide on glass (ITO) was
purchased from Shenzhen Hivac Vacuum Photo-electronics Co.
Ltd (Shenzhen, China). Otherchemicalswereof analytical grade
and were used without further purification. All solutions were
prepared with deionized water treated in a Millipore water purifi-
cation system (Millipore Corp.).
3. Results and discussion
2.2. Electrochemical fabrication of the HFGMs
3.1. SEM characterization of the HFGMs
The electrochemical fabrication HFGMs was prepared
according to our recent report [17]. For a typical electrochemical
fabrication of HFGMs, 24.3 mM of HAuCl4 aqueous solu-
tion was used as the source of Au and also as electrolyte.
ITO was used as working electrode. Before used, ITO was
cleaned by sonicating sequentially for 10 min each in acetone,
10% NaOH in ethanol and distilled water. The clean plat-
inum wire and Ag/AgCl (sat. KCl) electrodes were used as
counter and reference electrode, respectively. Amperometric I–t
curve technique was employed to electrochemical deposition
Fig. 1 is the typical FE-SEM images of the as-prepared
HFGMs samples growing on ITO substrate at different magni-
fications. Low-magnification image (Fig. 1A) indicates that the
as-prepared product consists of a large quantity of well dispersed
microspheres with the diameter about 5 ␮m. Higher magnifi-
cation image (Fig. 1B) demonstrates that these microspheres
exhibit flowerlike and planar-dendrite (branched) structure. The
growth processing of the HFGMs during this electrochemical
approach can be explained by the reaction (or electron transfer)

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L. Wang et al. / Electrochimica Acta 53 (2008) 2776–2781
Fig. 2. Cyclic voltammograms for HFGMs modified ITO substrate in deaerated
1 mM CuSO4 + 0.1 M H2SO4. Scan rate is 50 mV s−1
.
two anodic peaks (bꢀ and aꢀ) are obtained on the return (positive-
going) sweep, corresponding to dissolution of bulk and UPD
copper, respectively. The electrode was immersed in the cop-
per solution at a potential (∼0.01 V) just above that where bulk
deposition commences, to ensure full ultrathin coverage, and
was then transferred to a deaerated solution containing the Pt-
group cation of interest (typically 5 mM Pt and Pd salt in 0.1 M
HClO4, respectively). The electrode was left in this solution
at open circuit for 10 min. This period of time was chosen to
ensure complete redox replacement of UPD Cu and ordering of
the resulting Pt and Pd overlayer. The electrode was either trans-
ferred to the electrochemical cell for voltammetric scrutiny or
to be used for further characterization, as discussed in the next
section.
Fig. 1. Typical FE-SEM images of the HFGMs located at ITO substrate at dif-
ferent magnifications. The scale bar is 10 and 2 ␮M for (A) and (B), respectively.
and crystallization (nucleation and growth) coexist. This result
is in accordance with our recent report [17].
3.2. Electrochemical design of the ultrathin Pt-group metal
coated HFGMs
3.3. Electrochemical characterization of the ultrathin
Pt-group metal coated HFGMs
The basic electrochemical design protocol employed here
entails replacing a UPD metal adlayer on HFGMs template
with the desired Pt-group metal via a spontaneous deposition
process. The spontaneous redox replacement process, yield-
ing a late Pt-group metal overlayer, is driven by the difference
at equilibrium potentials between the Pt-group metal and the
UPD metal redox couples. For the system employed, Pd2+/Pd
and PtCl42−/Pt, the standard potentials (E0) of the correspond-
ing half-reaction are more positive than those for the Cu2+/Cu
UPD couple [21–23]. So, the spontaneous redox replace-
ment of a previously prepared UPD Cu ultrathin by Pt-group
cations can be used to form Pt-group metal deposits, consis-
tent with the anticipated balanced Cu → Cu2+/Pd(II) → Pd and
Cu → Cu2+/Pt(II) → Pt/Pt(II) electron charge flow.
Representative cyclic voltammograms obtained (at
50 mV s−1) from −0.2 to 1.5 V versus Ag/AgCl electrode
(saturated KCl) for the fabricated HFGMs modified ITO in
0.1 M H2SO4 are shown in Fig. 3 (dash traces). The presence
of a wide “double-layer” region below ca. 0.7 V and the anodic
formation and cathodic removal of oxide at higher potentials
are familiar features of gold electrochemistry [21,22]. The
solid traces in Fig. 3A and B are corresponding voltammo-
grams obtained after the deposition of ultrathin palladium and
platinum, respectively, using essentially the UPD procedure
described in Ref. [22]. Significantly, the resulting cyclic
voltammograms provide clear evidence that the HFGMs
surface is modified essentially by the Pt-group metal. Thus the
anodic–cathodic current profiles, comprised of near-reversible
peaks associated with hydrogen desorption–adsorption and
the irreversible features corresponding to the formation and
removal of surface oxide, are characteristic of conventional
Pd and Pt surfaces [22], even though only ultrathin level
films are apparently present. In particular, the presence of the
Pt-group metal oxide reduction peak at ca. 0.32 V, along with
the near-absence of the corresponding gold oxide feature at
ca. 0.87 V, shows that few exposed Au sites remain, which
After the initial preparation of the HFGMs template, a UPD
copper ultrathin was deposited. Fig. 2 shows a representative
cyclic voltammogram (20 mV s−1) for HFGMs coated ITO in
a deaerated 1 mM CuSO4 + 0.1 M H2SO4 solution. On the for-
ward (i.e., negative-going) sweep, two main cathodic features
(a and b) are observed at about +0.22 and −0.07 V, respectively.
Feature a is consistent with the formation of UPD copper, while
feature b refers to bulk copper deposition [21,23]. As expected,

L. Wang et al. / Electrochimica Acta 53 (2008) 2776–2781
2779
Fig. 3. Cyclic voltammograms for HFGMs modified ITO substrates in deaerated
0.1 M H2SO4 before (dash trace) and after (solid trace) coating with ultrathin
Pd (A) and Pt (B). Scan rate is 50 mV s−1
.
also indicating the essentially “pinhole-free” nature of the
ultrathin Pt-group metal coating [21–23]. The uniformity of
such Pt-group metal overlayers on a gold nanoparticle surface
formed with Pt-group metal is indeed confirmed previously by
SERS, employing carbon monoxide as a “probe” chemisorbate
[22,23].
Fig. 4. Typical FE-SEM images of ultrathin Pd coated HFGMs located at ITO
substrate at different magnifications. The scale bar is 20, 5 and 1 ␮M for (A),
(B) and (C), respectively.
3.4. SEM, EDX, X-ray map and XRD characterization
The chemical composition of the ultrathin Pd coated HFGMs
was determined by EDX and X-ray maps analysis of the prod-
ucts coated on an ITO glass slide (Fig. 5). The EDX spectrum
(Fig. 5A)testifiedtheexistingoftheAuandPdelement, inwhich
the peak of the corresponding element Au and Pd was distinct
(other peaks originated from the substrate). The corresponding
X-ray maps (Fig. 5B–D) reveal that the Pd element is relatively
uniform distributed in these microstructures. These results are
consistent with the electrochemical results.
Representative SEM images obtained for ultrathin Pt coated
HFGMs are confirmed no visible change and variation of the
morphology and diameter of the HFGMs can be observed after
being coated with ultrathin Pt films (data not shown). EDX anal-
ysis and X-ray maps of the Pt coated sample reveal Pt element is
relatively uniform distributed in HFGMs (Fig. 6). Furthermore,
Fig. 4 is the typical FE-SEM images of the as-prepared ultra-
thin Pd coated HFGMs samples growing on ITO substrate at
different magnifications. Low-magnification image (Fig. 4A)
indicates that the as-prepared product consists of a large quantity
of well dispersed microspheres with the diameter about 5 ␮m.
Higher magnification image (Fig. 4B) demonstrates that these
microspheres exhibit flowerlike structures. Local magnification
(Fig. 4C) reveals that ultrathin Pd coated HFGMs are still built
with many 2D nanoscaled flakelike or pricklike building blocks,
showing branched structure. Compared with SEM images of
bare HFGMs (Fig. 1), after coating with ultrathin Pd, there was
neither a morphology change nor a significant diameter varia-
tion. This observation is indicative of the homogeneity of the Pd
coating and ultrathin nature.

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L. Wang et al. / Electrochimica Acta 53 (2008) 2776–2781
Fig. 5. (A) EDX analysis in a selected domain of ultrathin Pd coated HFGMs sample, (B) the morphological image of the as-prepared sample and X-ray maps of Au
(C) and Pd (D), respectively, corresponding to (B).
Fig. 6. (A) EDX analysis in a selected domain of ultrathin Pt coated HFGMs sample, (B) the morphological image of the as-prepared sample and X-ray maps of Au
(C) and Pt (D), respectively, corresponding to (B).

L. Wang et al. / Electrochimica Acta 53 (2008) 2776–2781
2781
conductors, etc.) that can be electrodeposited on gold [22], there
is clearly tremendous scope for generating a myriad of hierarchi-
cal microstructure materials by the present means. The further
applications of such electrochemical design technique in the
development of biosensors are currently underway.
Acknowledgements
This research was supported by National Natural Science
Foundation of China (Nos. 20575064 and 20675076).
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(Fig. 7, inset), thepeaksnumberandthelocationarenotchanges,
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of the bare HFGMs. XRD analysis provided clear evidence that
the Pt coating has not interfering with the HFGMs structure and
provided more evidence that the Pt coating is relatively uniform
and ultrathin. The similar XRD analysis also obtained for Pd
coated HFGMs (data not shown), which demonstrates the same
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4. Conclusion
Overall, the present results demonstrate a very simple, tem-
plateless, surfactantless, electrochemical strategy for fabrication
of ultrathin platinum-group metal coated HFGMs. Thus pre-
pared bimetallic “core-shell” hierarchical microstructure has
been well characterized by electrochemical method, SEM, EDX,
X-ray map and XRD, which testified the Pt-group coating hold-
ing the “pinhole-free” nature while the HFGMs still keep its
morphology. The advantages of the strategy are obvious. First,
such bimetallic “core-shell” HFGMs with sharp tips and edges
may exhibit better enhancement in SERS compared to analo-
gous nanoparticles with smooth surface [17,24]. Second, it is
grateful to design catalysts for practical application since it is
possible to save precious metal, e.g., Pt. Furthermore, given the
availability of well-developed strategies for preparing HFGMs,
together with the diversity of overlayer materials (metal, semi-