Electrochemically shape-controlled synthesis in deep eutectic solvents of Pt nanoflowers with enhanced activity for ethanol oxidation

Article abstract of DOI:10.1016/j.electacta.2012.05.063

The electrochemically shape-controlled synthesis in deep eutectic solvents (DESs) has been applied to produce the electrocatalyst of Pt nanoflowers. The uniform Pt nanoflowers with sharp single crystal petals and high density of atomic steps were characte

Full text of DOI:10.1016/j.electacta.2012.05.063

Electrochimica Acta 76 (2012) 468–474
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Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Electrochemically shape-controlled synthesis in deep eutectic solvents
of Pt nanoflowers with enhanced activity for ethanol oxidation
Lu Weia,b, You-Jun Fana,∗, Hong-Hui Wangb, Na Tianb, Zhi-You Zhoub, Shi-Gang Sunb,∗,1
a Key Laboratory for the Chemistry and Molecular Engineering of Medicinal Resources (Ministry of Education of China), College of Chemistry and Chemical Engineering, Guangxi
Normal University, Guilin 541004, China
b State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
a r t i c l e i n f o
a b s t r a c t
Article history:
The electrochemically shape-controlled synthesis in deep eutectic solvents (DESs) has been applied to
produce the electrocatalyst of Pt nanoflowers. The uniform Pt nanoflowers with sharp single crystal petals
and high density of atomic steps were characterized by SEM, TEM, XRD, XPS and electrochemical tests. The
results illustrated that the as-prepared Pt nanoflowers exhibit higher electrocatalytic activity and stability
than commercial Pt black catalyst toward ethanol electrooxidation. The growth of Pt nanoflowers in DESs
by the simple electrochemical route is straightforward and controllable in terms of nanoflowers’ shape
and size, which can be applied in shape-controlled synthesis of other noble metal nanoparticles with
high catalytic activity.
Received 30 March 2012
Received in revised form 16 May 2012
Accepted 20 May 2012
Available online 29 May 2012
Keywords:
Deep eutectic solvents
Pt nanoflowers
© 2012 Elsevier Ltd. All rights reserved.
Electrodeposition
Shape-controlled synthesis
Ethanol electrooxidation
1. Introduction
for methanol electrooxidation by the electrodeposition method.
However, all Pt nanoflowers mentioned above were synthe-
Pt nanoparticles (NPs) are of great interest in recent years due
to their unique physicochemical properties and extensive applica-
tions in catalysis [1–3], sensors [4,5], and fuel cells [6]. It is known
that the properties of NPs strongly depend on their shape or sur-
face atomic arrangement, size and crystallinity. In particular, the
Pt NPs with high-index planes generally exhibit much higher cat-
alytic activity than those with low-index planes [7]. Therefore,
the shape-controlled synthesis of Pt NPs has attracted enormous
attention, and Pt NPs with different shapes were obtained, such
as cubes [8–10], rods [11], dendritic structures [12], nanowires
[13–15], nanoflowers [16–19], and polyhedra [20–24]. It is worth
noting that, among various Pt NPs, very recently Pt nanoflowers
have received attention because of their surface roughness, excel-
lent electrochemical active surface area and possible high catalytic
activity. For example, Mohanty et al. [16] reported a universal
approach to the synthesis of star-shaped Pt nanoflowers, which
is capable of catalyzing the Suzuki–Miyaura coupling reaction of
phenyl boronic acid and iodobenzene in water in very high yield.
In another study, Zhang et al. [17] prepared porous Pt nanoflow-
ers with higher catalytic activity and stronger poisoning-tolerance
sized in aqueous solution, less attention has been paid so far
to study the preparation of Pt nanoflowers in non-aqueous sys-
tems.
Deep eutectic solvents (DESs), a simple eutectic-based ionic liq-
uids, usually can be produced by using quaternary ammonium
salts mixed with hydrogen-bond donors such as acids, amides
and alcohols [25,26]. The properties of DESs are similar to those
of ionic liquids, namely, high conductivity, viscosity, surface ten-
sions, polarity, and thermal stability and negligible vapor pressure.
Moreover, DESs have the additional advantages of nontoxicity,
biodegradation, and easy preparation at low cost and with high
purity. Recently, DESs have fascinated the scientific community
as a promising solvent in large scale applications [27–35]. How-
ever, to the best of our knowledge, the studies on shape-controlled
synthesis of metal nanomaterials in DESs are very few. Sun and
co-workers reported the synthesis of star-shaped Au NPs enclosed
with {3 3 1} and vicinal high-index facets in DESs by chemical
reduction route, which exhibit high activity toward H2O2 elec-
trocatalytic reduction [28]. Chirea and co-workers synthesized Au
nanowire networks with high catalytic activity for the chemical
reduction of p-nitroaniline in DESs by NaBH4 reduction method
[34]. More recently, we reported the preparation of concave tetra-
hexahedral Pt nanocrystals (THH Pt NCs) bounded with {9 1 0}
and vicinal high-index facets in DESs by a programmed electrode-
position method, which exhibit high electrocatalytic activity and
stability toward ethanol oxidation [35].
∗
Corresponding authors.
E-mail addresses: youjunfan@mailbox.gxnu.edu.cn (Y.-J. Fan),
sgsun@xmu.edu.cn (S.-G. Sun).
1
ISE member.
0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2012.05.063

L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
469
10
0
1.0
0.5
-10
-20
-30
0.0
-0.5
-1.0
-2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
E vs. Pt/V
-1.6
-1.4
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
E vs. Pt/V
Fig. 1. Cyclic voltammogram (CV) of GC electrode in 19.3 mM H2PtCl6/DESs solution at 80 ◦C. The inset is the CV curve of GC electrode in DESs at 80 ◦C. Scan rate: 50 mV s−1
.
Herein we report a novel route of shape-controlled synthesis of
Pt nanoflowers. The Pt nanoflowers with sharp single crystal petals
and high density of atomic steps were successfully synthesized in
DESs by simple cyclic voltammetry (CV) method. The size and shape
of as-obtained Pt nanoflowers can be tuned easily by controlling
the deposition conditions, such as the precursor concentration, CV
number of cycle, scan rate and temperature. As compared to Pt
nanoflowers with other shapes and commercial Pt black catalyst,
Fig. 2. (a) SEM image of Pt nanoflowers electrodeposited on GC by using CV method in 19.3 mM H2PtCl6/DESs solution at 80 ◦C. Potential scan range: −1.5 to −0.2 V, scan
rate: 50 mV s−1, CV number of cycle: 80 cycles. (b) TEM image of a single Pt nanoflower, the inset is the SAED pattern of the nanoflower. (c) HRTEM image of the petal marked
in (b), showing the high density of atomic steps. (d) EDX image of Pt nanoflowers.

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L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
Fig. 3. SEM images of Pt nanostructures electrodeposited on GC in DESs at 80 ◦C under various H2PtCl6 concentrations of (a) 1.93 mM, (b) 5 mM, (c) 10 mM and (d) 19.3 mM.
Potential scan range: −1.5 to −0.2 V, scan rate: 50 mV s−1, CV number of cycle: 80 cycles.
the Pt nanoflowers with sharp single crystal petals and high density
of atomic steps show higher electrocatalytic activity and stability
for ethanol oxidation.
(H2PtCl6·6H2O, >99%) and absolute ethanol of analytical grade,
and super pure HClO4 were purchased from Shanghai Chemical
Reagent Ltd., China.
2. Experimental
2.2. Preparation of DESs
2.1. Chemicals
Choline chloride was recrystallized from absolute ethanol, fil-
tered and dried under vacuum. Urea was recrystallized from
Millipore water (18.0 Mꢀ cm) provided by a Milli-Q Lab apparatus
(Nihon Millipore Ltd.), filtered and dried under vacuum prior to use.
Choline chloride (HOC2H4N(CH3)3Cl, 99%), urea (NH2CONH2,
>99%),
hydrogen
hexachloroplatinate(IV)
hexahydrate
Fig. 4. SEM images of Pt nanostructures electrodeposited on GC in 19.3 mM H2PtCl6/DESs solution at 80 ◦C by using different CV number of cycle: (a) 5, (b) 10, (c) 40 and (d)
100 cycles. Potential scan range: −1.5 to −0.2 V, scan rate: 50 mV s−1
.

L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
471
method. The electrodeposition conditions were as follows: poten-
tial scan range of −1.5 to −0.2 V, scan rate of 50 mV s−1 and CV
number of 80 cycles. Moreover, the effects of preparation condi-
tions (such as the precursor concentration, CV number of cycle,
scan rate and temperature) on the size and shape of Pt NPs were
systematically investigated.
2.4. Physical characterization of Pt nanoflowers
The size and morphology of Pt nanoflowers were analyzed by
scanning electron microscopy (SEM, LEO-1530) and transmission
electron microscopy (TEM, FEI Tecnai-F30). The X-ray diffraction
(XRD) patterns of Pt samples were recorded with an X-ray powder
diffraction apparatus (Panalytical X’pert PRO) using a Cu K␣ source
and 2ꢁ scan rate of 6◦ min−1. The X-ray photoelectron spectroscopy
(XPS) measurements were carried out using a Physical Electronics
PHI Quantum 2000 Scanning ESCA Microprobe with the Al K␣ X-
ray source (1486.6 eV), while the chamber pressure was held below
5 × 10−9 Torr. All reported values of electron binding energy were
calibrated with respect to the principal peak of C1s at 284.6 eV as
an internal standard.
2.5. Electrocatalytic performance measurements
The electrocatalytic performances of Pt samples were mea-
sured in 0.1 M ethanol + 0.1 M HClO4 solution at room temperature
(25 ◦C). The solutions were deaerated by purging with pure N2 gas
before experiment, and a flux of N2 was kept over the solution
during measurements to prevent the interference of atmospheric
oxygen. Pt black sheet and a saturated calomel electrode (SCE)
were used as the counter and reference electrodes, respectively.
All potentials in the electrochemical performance tests are quoted
vs. the SCE scale.
3. Results and discussion
Cyclic voltammogram (CV) of Pt electrodeposition on GC elec-
trode in 19.3 mM H2PtCl6/DESs solution at 80 ◦C is shown in
Fig. 1. From this figure, in the negative-going potential scan,
it can be clearly observed that two current peaks of reduction
occurred at near −0.93 V and −1.29 V (vs. Pt), in comparison
with the voltammogram (inset of Fig. 1) recorded on the same
GC substrate in DESs. X-ray photoelectron spectra (XPS) results
demonstrated that these two reduction peaks are corresponding
to the electrochemical reduction of Pt(IV) to Pt(II) and Pt(II) to
Pt(0), respectively (Fig. S1 and Table S1 in supplementary infor-
mation). This is consistent with the electrodeposition behaviors
of Pt in hydrophilic 1-n-butyl-3-methylimidazolium tetrafluorobo-
rate (BMIMBF4) and hydrophobic 1-n-butyl-3-methylimidazolium
hexafluorophosphate (BMIMPF6) room-temperature ionic liquids
[36].
Fig. 5. SEM images of Pt nanostructures electrodeposited on GC in 19.3 mM
H2PtCl6/DESs at 80 ◦C by potentiostatic method at the potential of (a) −0.95, (b)
−0.90 and (c) −0.80 V. The insets are the corresponding high-magnification SEM
images. Scale bar of the inset is 200 nm.
DESs were formed by stirring the above two components together,
in the stated proportion (molar ratio of choline chloride/urea = 1:2),
at 80 ◦C until a homogeneous, colorless liquid formed [25,28]. The
prepared DESs, once formulated, were kept in a vacuum at 80 ◦C
prior to use.
Fig. 2a displays the typical SEM image of Pt nanoflowers elec-
trodeposited on GC by using CV method in 19.3 mM H2PtCl6/DESs
solution at 80 ◦C. It can be seen that the Pt nanoflowers with
sharp petals were homogeneously formed, and their size was
about 200 nm. The crystal structure of Pt nanoflowers was further
investigated by high resolution transmission electron microscopy
(HRTEM). Fig. 2b shows the TEM image of a single Pt nanoflower,
and the inset is the corresponding selected area electron diffrac-
tion (SAED) pattern, which indicates that the petals of as-prepared
nanoflowers possess the single-crystalline structure. The HRTEM
image of a petal marked in Fig. 2b is displayed in Fig. 2c. The contin-
uous fringe pattern further verifies the single crystalline property
of the petal. The lattice spacing of 0.23 nm agrees with the distance
between two {1 1 1} planes of Pt. As compared to other Pt nanoflow-
ers and nanothorn assemblies reported previously [16–19,37], the
2.3. Synthesis of Pt nanoflowers in DESs
The electrochemical synthesis of Pt nanoflowers in DESs was
carried out under an ambient environment, using a standard three-
electrode cell connected to a CHI 631 electrochemical workstation
(Shanghai Chenhua Instrumental Co., Ltd., China), with a Pt wire
counter electrode and a Pt quasi-reference electrode. The work-
ing electrode was a glassy carbon disk (GC, Ø 3 mm), which was
polished with 5.0 ␮m, 1.0 ␮m, 0.3 ␮m Al2O3 powder and washed
ultrasonically in ultrapure water before each experiment. In a typ-
ical procedure, Pt nanoflowers with sharp single crystal petals and
high density of atomic steps were electrodeposited directly on a GC
substrate in 19.3 mM H2PtCl6/DESs solution at 80 ◦C using the CV

472
L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
Fig. 6. (a) TEM image of a single Pt nanosheet in Fig. 5b, (b and c) HRTEM images of the Pt nanosheet, (d) SAED pattern of the Pt nanosheet.
unique characteristic of as-prepared Pt nanoflowers is the for-
mation of high density of atomic steps at the edge of the petals
(Fig. 2c), which are crucial for the enhanced activity of Pt nanoflow-
ers toward ethanol electrooxidation. The energy dispersive X-ray
spectroscopy (EDX) analysis of Pt nanoflowers confirms the pres-
ence of only Pt, C and O elements (Fig. 2d), indicating no DESs
residue on the surface of Pt nanoflowers.
were obtained, as shown in Fig. 4. Some irregular quasi-spherical
nanoparticles with low surface coverage were produced in the
lower CV number of cycle (Fig. 4a and b), which acted as the
nuclei for subsequently producing Pt nanoflowers [17,19,38]. Since
the nucleation process is relatively slow and irreversible, newly
deposited Pt favors growing on the small Pt cores instead of gen-
erating more new nuclei [39]. Increasing the CV number of cycle
would result in complex monodisperse nanoflowers with more
sharp petals and larger size (Fig. 4c). Further increasing the CV
number of cycle to 80 cycles, the perfect Pt nanoflowers with sharp
petals were formed on the GC substrate (Fig. 3d). When the CV
number of cycle was increased to 100 cycles, the aggregation phe-
nomenon of Pt nanoflowers was observed (Fig. 4d). In addition, the
scan rate plays important roles in the electrodeposition of Pt owing
to its possible influence on the anisotropic growth of nanoparticles.
At the lower scan rate of 1 mV s−1, the rough nanosheets formed
on the top of Pt nanostructures (Fig. S2 in supplementary informa-
tion). Further increasing the scan rate would result in the growth
of Pt nanoflowers with sharp petals. It is worthwhile noting that
all electrodeposition processes discussed above were carried out
at 80 ◦C, when the lower or higher deposition temperature was
used, the sharp petals in the resultant Pt NPs were poorly devel-
oped (Fig. S3 in supplementary information). The reason for such
an obvious difference in size and shape of Pt NPs prepared at dif-
ferent temperatures could be the variation of DESs viscosity with
the temperature [25], which influences the diffusion of reactive
species in DESs and thus the nucleation/growth process of Pt on GC
substrate.
The effect of deposition conditions, namely, the precursor con-
centration, CV number of cycle, scan rate and temperature on the
size and morphology of Pt nanostructures electrodeposited in DESs
was examined. Fig. 3 shows the SEM images of Pt nanostructures
prepared by using the different concentrations of H2PtCl6. We
can see that, at the H2PtCl6 concentration of 1.93 mM, the quasi-
spherical Pt NPs were formed (Fig. 3a), and their size ranged from 45
to 95 nm. When the H2PtCl6 concentration was 5 mM, the flower-
like Pt NPs without sharp petals and several cubic Pt NPs appeared
(Fig. 3b). Further increasing the H2PtCl6 concentration to 10 mM,
the sharp petals started to appear at the edge of the Pt nanoflowers
(Fig. 3c). Finally, the perfect Pt nanoflowers with sharp petals were
homogeneously formed at the H2PtCl6 concentration of 19.3 mM
(Fig. 3d). The concentration dependence of the above Pt nanostruc-
tures maybe results from the high viscosity of DESs [25,26], which
decreased the mass transportation of reactive species in DESs, lead-
ing to the difficult formation of Pt nanoflowers with sharp petals at
the lower H2PtCl6 concentration.
Among all deposition conditions, the CV number of cycle exerts
a leading influence on the process of particle growth. Differ-
ent Pt nanostructures generated by various CV number of cycle

L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
473
Table 1
Pt nanoflowers in Fig. 2a
Pt nanoflowers in Fig. 5a
Pt nanoflowers in Fig. 5b
Pt nanoflowers in Fig. 5c
Pt black
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
Comparison of peak current densities (jp) and steady current densities (j1800 s) for
different Pt samples toward ethanol oxidation in Fig. 7.
(a)
II
I
Sample
jp(I)
jp(II)
j1800 s
(mA cm−2
)
(mA cm−2
)
(mA cm−2
)
Pt nanoflowers in Fig. 2a
Pt nanoflowers in Fig. 5a
Pt nanoflowers in Fig. 5b
Pt nanoflowers in Fig. 5c
Pt black
1.484
0.481
0.567
0.780
0.582
1.414
0.598
0.716
0.983
0.849
0.184
0.047
0.081
0.128
0.094
hydrogen adsorption/desorption on Pt samples (Fig. S5 in supple-
mentary information). The corresponding CV parameters are listed
in Table 1. We can see that, in the positive-going potential scan, the
ethanol electrooxidation on the perfect Pt nanoflowers with sharp
petals gives two current peaks at near 0.56 and 0.95 V (vs. SCE), their
peak current densities are 1.484 and 1.414 mA cm−2, respectively,
which are much higher than those obtained on the Pt nanoflowers
with other shapes and Pt black catalyst. These results demonstrate
that the electrocatalytic activity of the perfect Pt nanoflowers with
sharp petals for ethanol oxidation is much higher than that of the Pt
nanoflowers with other shapes and Pt black catalyst. Fig. 7b shows
the current–time curves of ethanol oxidation on various Pt samples
at 0.45 V (vs. SCE). It is found that the current density (t = 1800 s) of
ethanol oxidation on the perfect Pt nanoflowers with sharp petals is
significantly higher than that of other Pt nanoflowers and Pt black
catalyst (Table 1). This further demonstrates that the electrocat-
alytic activity and stability of the Pt nanoflowers with sharp petals
for ethanol oxidation are superior to those of other Pt nanoflowers
and Pt black catalyst. The enhancement in electrocatalytic perfor-
mance of the perfect Pt nanoflowers with sharp petals could be
evidently attributed to their high density of atomic steps, which are
beneficial for the C C bond breaking and the oxidation of adsorbed
CO in ethanol oxidation [40,41].
-0.4 -0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
E vs. SCE/V
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Pt nanoflowers in Fig. 2a
Pt nanoflowers in Fig. 5a
Pt nanoflowers in Fig. 5b
Pt nanoflowers in Fig. 5c
Pt black
(b)
0
300
600
900
t / s
1200
1500
1800
Fig. 7. (a) Stable cyclic voltammograms (50 mV s−1) and (b) chronoamperometric j–t
curves recorded at 0.45 V (vs. SCE), for ethanol oxidation on various Pt nanoflowers
and commercial Pt black catalyst in 0.1 M ethanol + 0.1 M HClO4 solution at 25 ◦C.
The Pt nanoflowers of various shapes can be also obtained by
adjusting the deposition potential of the potentiostatic reduction.
Typical SEM images of Pt nanoflowers prepared at different poten-
tials are shown in Fig. 5. At the deposition potential of −0.95 V,
many pricks appeared and extruded from the surfaces of the stud-
ded particles (Fig. 5a). When the deposition potential was −0.90 V,
the nanosheet assemblies were formed on the top of Pt nanostruc-
tures (Fig. 5b). As the deposition potential became more positive,
e.g. −0.80 V, only flower-like Pt NPs with rough surface structure
were observed (Fig. 5c). To further characterize the structure of
Pt nanosheets in Fig. 5b, TEM measurements were carried out. A
single Pt nanosheet as indicated in Fig. 6a was carefully observed
by the HRTEM imaging technique. As shown in Fig. 6b, the Pt
nanosheet is in fact an agglomerate of Pt nanoparticles. The lat-
tice spacing between adjacent {1 1 1} planes is determined to be
0.23 nm, which is in agreement with the {1 1 1} d-spacing value of
face centered cubic (fcc) Pt (Fig. 6c). Interestingly, unlike the sharp
petals described in Fig. 2, the high density of atomic steps cannot
be observed in the Pt nanosheet. Furthermore, Fig. 6d shows the
corresponding SAED pattern, where the five main diffraction rings
can be indexed as the characteristic rings of Pt fcc crystal, which is
fully consistent with the result of the XRD pattern (Fig. S4 in sup-
plementary information).
4. Conclusions
In summary, we have reported the electrochemically shape-
controlled synthesis in DESs for producing Pt nanoflowers. The Pt
nanoflowers with sharp single crystal petals and high density of
atomic steps were obtained in DESs by simple cyclic voltammetry.
Thanks to their high density of atomic steps, the Pt nanoflowers
with sharp petals exhibit higher electrocatalytic activity and sta-
bility than commercial Pt black catalyst toward ethanol oxidation.
Furthermore, the size and shape of Pt nanoflowers could be eas-
ily controlled by adjusting the electrodeposition conditions. This
study is of significance for shape-controlled synthesis of other noble
metal NPs with high catalytic activity in DESs, as well as in potential
applications of electrocatalysis, electrochemical sensors and fuel
cells.
Acknowledgments
This work was supported by the National Natural Science Foun-
dation of China (20833005, 21021002), the Opening Foundation
Project of the State Key Laboratory of Physical Chemistry of Solid
Surfaces (Xiamen University), Guangxi Natural Science Foundation
of China (0991093, 2010GXNSFF013001) and the S&T Project of
Guangxi Education Department of China (201012MS024).
The electrocatalytic performance of the perfect Pt nanoflowers
with sharp petals in Fig. 2a was evaluated by CV and chronoam-
perometry methods in acidic solution at room temperature. Fig. 7a
compares the stable cyclic voltammograms of the Pt nanostruc-
tures with different shapes and commercial Pt black catalyst
(Johnson Matthey) in 0.1 M ethanol + 0.1 M HClO4 solution. The
oxidation current has been normalized to the electroactive sur-
face area, which was calculated from the electric charge of
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/
j.electacta.2012.05.063.

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L. Wei et al. / Electrochimica Acta 76 (2012) 468–474
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