Au, Cu, Ag, Ni, and Pd particles grown in solution at different electrode potentials

Article abstract of DOI:10.1021/jp9628747

The multiply twinned particles (MTPs) of Cu, Ni, Ag, and Pd formed on an electrode in solution at low electrode potentials were systematically studied by means of transmission electron microscopy (TEM). At low electrode potentials the icosahedral and deda

Full text of DOI:10.1021/jp9628747

4030
J. Phys. Chem. B 1997, 101, 4030-4034
Au, Cu, Ag, Ni, and Pd Particles Grown in Solution at Different Electrode Potentials
Da-ling Lu and Ken-ichi Tanaka*
The Institute for Solid State Physics, The UniVersity of Tokyo, 7-22-1, Roppongi, Minato-ku, Tokyo 106, Japan
X
ReceiVed: September 19, 1996; In Final Form: February 18, 1997
The multiply twinned particles (MTPs) of Cu, Ni, Ag, and Pd formed on an electrode in solution at low
electrode potentials were systematically studied by means of transmission electron microscopy (TEM). At
low electrode potentials the icosahedral and dedahedral particles were observed for gold, silver, and palladium,
but only the decahedral particles were observed for copper and nickel. The icosahedral particles of copper
and nickel are less stable compared to the decahedral particles. The decahedral Ag particles were oxidized
2
during the observation by TEM and changed into Ag O having fcc structure. The stability of these transition
metal MTPs formed in solution is in the sequence Au > Ag > Cu and Pt > Pd > Ni. The differences of d-s
hybridization or s,p-d hybridization among 3d, 4d, and 5d transition metals will increase the surface electron
density, which results in the contraction of the lattice in the lateral direction. The decahedral and icosahedral
2
particles of Au were formed on an SnO electrode, which indicates that the MTPs of Au are grown not only
on carbon film but on SnO film depending on the electrode potential.
2
Introduction
Since decahedron and icosahedron of fine gold particles
evaporated in ultrahigh vacuum (UHV) were first observed by
1
2,3
Mihama and were analyzed by Ino who designated these
particles as “multiply twinned particles” (MTPs), several groups
6,7,10
4,5,8
4
investigated other fcc metal particles, e.g., Ag,
Ni,
Al,
4
,5,9
and Pd,
made by evaporation in UHV and found that these
particles have MTPs. MTPs were also observed while the
preparation of fine particles of these metals was carried out in
11-13
14
15
an inert gas, that is, in argon,
helium, or xenon gases.
In this paper we report the formation of MTPs of Cu, Ag, Ni,
and Pd by electrochemical deposition on amorphous carbon
electrode in a neutral electrolyte or in acid solution. The MTPs
of gold were also grown on SnO2 film. On the basis of our
previous reports on the particles of gold1
6-18
and platinum,
19
we could say that the MTPs of fcc transition metals can be
prepared not only in UHV and inert gases but also in solution
under certain electrode potentials.
Experimental Section
Figure 1. TEM images of gold particles formed in 50 mM CsClO +
4
1
mM HAuCl
b) -0.3 V for 60 s. In (a) the gray particles are SnO
are Au deposited on the SnO particles. In (b) the small black Au
particles formed on the large gray particles of SnO . Note the scales in
4
solution on SnO
2
electrode at (a) 0.3 V for 300 s and
The experimental procedure was described in detail in
(
2
, the black particles
1
6-18
previous reports.
The working electrode was a Au mesh
2
for transmission electron microscopy (TEM). The mesh was
covered with a collodion film on which an amorphous carbon
film was evaporated. SnO2 film for the electrodeposition of
Au was made by evaporating Sn on a collodion film prepared
on a Au mesh, and then the Au mesh with Sn film was heated
and kept at 500 K for 2 h in air. Before the deposition of Au,
the mesh with SnO2 film was investigated by TEM. It was
observed from the diffraction pattern that the previous Sn film
was completely oxidized to SnO2 film. The TEM observation
indicated that this SnO2 film was a polycrystalline film showing
clear lattice structures.
2
(a) and (b). A typical decahedral Au particle and an icosahedral Au
particle formed at -0.3 V are shown in (c) and (d), respectively.
water. All experiments were performed in a classic three-
electrode cell at room temperature. The particles were grown
on the carbon electrode or SnO2 electrode and observed by
means of a Hitachi 9000 high-resolution TEM.
Results
Gold. Gold particles grown on a SnO2 electrode in 0.05 M
CsClO4 + 1 mM HAuCl4 solution at different electrode
potentials are shown in Figure 1. The gold particles formed at
Electrode potentials were measured and quoted in this paper
against the saturated calomel electrode (SCE). The electrolyte
was prepared with HClO4 (Wako), HAuCl4 (Kanto), CsClO4,
AgClO4, CuClO4, NiSO4, PdSO4 (Aldrich), and triply distilled
0
.3 V are polycrystalline (Figure 1a). However, most of the
gold particles formed at -0.3 V are MTPs, that is, decahedral
and icosahedral particles (parts b-d of Figure 1). This
phenomenon is similar to the growth of gold particles on a
carbon electrode on which the octahedral fcc single-crystalline
*
To whom correspondence should be addressed. Fax: 81-3-34015169.
Telephone: 81-3-34786811.
X
Abstract published in AdVance ACS Abstracts, April 1, 1997.
S1089-5647(96)02874-X CCC: $14.00 © 1997 American Chemical Society

Au, Cu, Ag, Ni, and Pd
J. Phys. Chem. B, Vol. 101, No. 20, 1997 4031
V (Figure 2b). However, the decahedral copper particles were
not stable; they changed to polycrystalline particles quickly
during observations by TEM. There was no icosahedral copper
particle to be found in this experiment.
Nickel. Figure 3a shows a typical decahedral particle of
nickel formed in 0.05 M CsClO4 + 1 mM NiSO4 solution at
-
0.9 V. As in the case of decahedral copper particles, the
decahedral particles of nickel were not stable for TEM. The
icosahedral particle of nickel was not observed. Takahashi et
2 4
Figure 2. (a) Image of Cu O particles formed in 50 mM CsClO + 1
8
al. also observed only the decahedral particle of nickel
mM CuSO solution on carbon electrode at -0.1 V for 600 s. (b) Image
4
of a decahedral Cu particle grown at -0.6 V for 60 s.
evaporated in vacuum. It is supposed that the icosahedral
particles of nickel and copper cannot be formed in the present
case, which will be discussed in detail in discussion. We found
that NiO grew simultaneously with nickel particles at -0.9 V
(Figure 3b). According to the thermodynamic chart given by
2
1
Pourbaix, when the pH of the electrolyte is 7, the deposited
particle should be only nickel below -0.6 V vs SCE. Therefore,
it can be considered that the formation of NiO at -0.9 V is
controlled by kinetics, that is, the reaction of nickel cation with
H2O to form NiO is faster than the formation of nickel particle.
Silver. The phenomenon of silver particles formed at
different electrode potentials is about the same with that of gold
1
6
particles, that is, the fcc single-crystalline particles form at
higher potentials and the decahedral and icosahedral particles
grow at lower potentials. Figure 4 shows the particles of silver
formed at 0.2 V in 0.05 M CsClO4 + 1 mM AgClO4 solution.
From the diffraction patterns of the particles (parts b and e of
Figure 4) and their dark field images (parts d and g of Figure
Figure 3. (a) Image of a decahedral Ni particle formed in 50 mM
4
), there is no doubt that these silver particles are fcc single
CsClO
00 s. However, NiO and Ni particles grew simultaneously at -0.9 V.
b) Image of electron diffraction pattern of NiO and Ni particles formed
4
+ 1 mM NiSO
4
solution on carbon electrode at -0.9 V for
crystalline ones; even the shape of these particles seems to be
polycrystalline.
3
(
at -0.9 V. (c) Illustration of (b).
Parts a and b of Figure 5 show a TEM image and a
corresponding dark field image of a decahedral particle of silver
formed at 0 V, which is a little lower than the initial occurrence
potential (0.05 V) of decahedral particles of gold. It should be
pointed out that the MTPs of silver are not stable for TEM,
especially icosahedral particles, which changed very quickly
during observations with TEM. If the observation time for one
decahedral particle of silver is longer, that particle will be
oxidized to Ag2O which has a cubic structure, and the shape of
the particle will vary (see Figure 6).
gold particles are formed at 0.3 V and the decahedral and
icosahedral gold particles are formed at -0.3 V.
16,17
It is clear
that the SnO2 electrode has no effect on the growing of gold
MTPs at lower potentials; even the islands of SnO2 film are
not amorphous but a polycrystalline film because it takes lattice
structures (see the background of parts c and d of Figure 1).
Copper. The particles of copper formed at different electrode
potentials in 0.05 M CsClO4 + 1 mM Cu(ClO4)2 solution are
shown in Figure 2. At -0.1 V the grown particles are Cu2O
No silver particles were observed as the electrodeposition was
carried out at 0.3 V. At this potential the grown particles had
a hexagonal diffraction pattern (Figure 7b) and the diffraction
(
Figure 2a), which was discussed in detail in our previous
20
paper. The decahedral copper particles were observed at -0.6
Figure 4. (a) Image of Ag particles formed in 50 mM CsClO
4
4
+ 1 mM AgClO solution on carbon electrode at 0.2 V for 300 s. The particles that
are marked by 1 and 2 were investigated in detail. (b) Diffraction pattern of particle 1 taken along the [211] direction. (c) Illustration of (b). (d)
Dark field image of particle 1 using a (111) spot. (e) Diffraction pattern of particle 2 taken along the [110] direction. (f) Illustration of (e). (g) Dark
field image of particle 2 using a (111) spot. It is clear that the gathered Ag particles are fcc single-crystal particles.

4032 J. Phys. Chem. B, Vol. 101, No. 20, 1997
Lu and Tanaka
was observed at -0.3 V (Figure 8c). It indicates that at lower
potentials not only the MTPs are formed but also the growth of
platelike particles with (111) orientation are preferred because
of the morphology with lower surface free energy.
Discussion
The decahedron and icosahedron of particles are surrounded
by 10 and 20 close-packed faces, respectively. In our previous
Figure 5. Decahedral Ag particle (a) formed in 50 mM CsClO
4
+ 1
mM AgClO solution at 0 V for 60 s and its dark field image (b).
4
16
paper it was pointed out that Au atoms on the low-index clean
Au surfaces are compressed in the lateral direction on the
surfaces at lower potentials, which is responsible for the
potential-induced reconstruction. The gold particles grown in
this potential range prefer to take a hexagonal stacking at the
outermost layer, which results in the growth of decahedral and
icosahedral particles. Platinum was known to have the recon-
struction only on the (100) and (110) surfaces, but recently, it
has been shown that the reconstruction can occur on Pt(111)
2
3,24
surface at 1330 K
or at 400 K in the presence of
2
5,26
supersaturated platinum vapor.
We also observed the
decahedron and icosahedron of platinum particles grown in salt
solution at lower electrode potentials, and fcc single-crystal
platinum particles formed at higher electrode potentials.19 It
seems that if the close-packed stacking could occur on all the
low-index planes, the decahedral and icosahedral particles of
metals can be formed in solution as well as in UHV. To confirm
this postulate, we studied the electrochemical deposition of Cu-
2
7
Au alloy particles. We found formation of decahedral and
icosahedral Cu-Au alloy at the electrode potentials at which
Au particles give no decahedron or icosahedron but only fcc
single-crystal ones. Taking account of the reconstruction of
Au single-crystal surfaces induced by alkali metal adsorption,2
it was inferred that the underpotential deposited copper takes
8,29
+
Cu on the Au layer of the alloy particles, which may increase
the electron density of the gold layer at the surface. As a result,
the interatomic distance of the Au outermost layer is compressed
as in the case at lower electrode potentials, and the decahedral
and icosahedral alloy particles are formed. From the above
results if the surface of a metal can be compressed by some
means, the decahedral and/or icosahedral particles of the metal
will be produced in solution.
(
f)
Figure 6. (a) Decahedral Ag particle formed in 50 mM CsClO
mM AgClO solution at 0 V for 60 s. (b) After 35 min observation by
TEM the twins of this particle disappeared. (c) After 50 min observation
the particle was oxidized to Ag O and its shape changed. (d) Dark
field image of (c). (e) Diffraction patter of this oxidized particle. (f)
Illustration of (e). The lattice structure of this Ag O particle is fcc.
4
+ 1
4
2
As a reminder, no reconstruction occurs on the clean low-
index surfaces of copper, silver, nickel, and palladium in UHV.
2
patterns of these particles were identical, that is, all the particles
had the same crystalline orientations[001]. The element
analysis of these particles by SEM (Figure 7e) suggests that
these particles contain silver. By the calculation of the
diffraction pattern and the comparison with the calculation
results and the powder diffraction data, these particles are
assigned as Ag2O. It is interest that the structure of these Ag2O
particles is hexagonal, which is different from the cubic structure
of Ag2O particles oxidized from the decahedral particles of silver
during the TEM observation as mentioned above. From the
thermodynamic chart given by Pourbaix2 Ag2O should exist
above 0.5 V vs SCE under pH 7. It is supposed that this case
is the same as that of Cu2O, that is, the formation of Ag2O is
controlled by the kinetics.
30-32
Theoretical calculations
suggest that the reconstructions of
Au and Pt are favored and that Pd and Ag have about the same
energy for the reconstruction and the unreconstruction. In
contrast, the unreconstruction is energetically preferred for Ni
and Cu. However, when alkali metal is deposited on the
surfaces of these metals in UHV, the reconstruction is induced
33,34
35,36
37
38,39
(Cu,
Ag,
Ni, Pd
). This phenomenon is interpreted
by the charge transfer from alkali adatom to substrate surface,
and then the s-p electron density is increased at the surface
and reconstruction is induced.
From our experimental results it can be concluded that the
stability of MTPs of these metals formed in solution is as
follows:
2
Palladium. Palladium particles grown in 1 M HClO4 + 1
mM PdSO4 solution at higher electrode potentials are different
with the particles of copper, nickel, or silver, that is, there was
neither single-crystal particle nor oxide to be formed; only the
polycrystalline palladium particles were formed (Figure 8a).
However, the MTPs were formed at lower electrode potentials
Au > Ag > Cu and Pt > Pd > Ni
These sequences are in quite good agreement with the theoretical
prediction for reconstruction. Therefore, it is possible that the
sequence of stability of these metal MTPs relates to the electron
density in s-p and/or s,p-d orbitals. For a certain metal, e.g.,
Au, at low electrode potentials a negative charge induced at
the surface leads to a strengthened in-plane s-p bonding.40
Increase of the electron density at the surface favors a more
(Figure 8b). Parts c and d of Figure 8 show typical decahedral
and icosahedral particles of palladium, respectively. It should
be noticed that the MTPs of palladium are more stable than
those of silver. The platelike palladium particle with (111) face
41
densely packed surface to lower the surface energy. Therefore,

Au, Cu, Ag, Ni, and Pd
J. Phys. Chem. B, Vol. 101, No. 20, 1997 4033
Figure 7. (a) Ag
2
O particles formed in 50 mM CsClO
4
+ 1 mM AgClO
4
solution at 0.3 V for 300 s. (b) Diffraction pattern of (a) taken along the
[001] direction. It should be noted that the diffraction patterns of these particles are identical. (c) Illustration of (b). The lattice structure of these
Ag O particles is hexagonal. (d) Dark field image of (a) using a (100) spot. (e) Element analysis of these particles by SEM. The peaks of Au came
2
from the Au mesh.
transition metals, there is a stronger tendency to decrease the
surface charge density in the sequence 3d > 4d > 5d.
Therefore, the surface structure of MTPs of 3d transition metals
formed in solution at lower electrode potentials is less com-
pressed than that of 4d and 5d transition metals. The icosahe-
dron, which has a more compressed structure than that of
decahedron, of Cu and Ni are not formed, and the MTPs of
these metals are easier to change into a fcc polycrystalline.
Conclusion
We presented the results of MTPs of Cu, Ni, Ag, and Pd
grown in solution at low electrode potentials. This phenomenon
is similar to that of Au and Pt. However, at higher electrode
potentials the formed particles are Pd polycrystalline ones for
Pd but Cu2O for Cu and Ag2O for Ag. NiO particles and Ni
decahedral particles grow simultaneously at -0.9 V. The
icosahedron and decahedron of Au formed on the SnO2 electrode
below -0.3 V, which indicates that the surface structure of SnO2
film has no effect on the growing of the MTPs of Au.
Figure 8. Images of Pd particles formed in 1 M HClO
4
+ 1 mM PdSO
4
solution on carbon electrode at 0 V for 180 s (a) and -0.2 V for 120
s (b). (c) Decahedral Pd particle formed at -0.2 V. (d) and (e) are an
icosahedral Pd particle and a plate of Pd particle formed at -0.3 V,
respectively.
Acknowledgment. We gratefully acknowledge Mr. K.
Suzuki and Mr. M. Ichihara of ISSP for their help in the TEM
experiment. This work was supported by a Grant-in-Aid for
Science Research (05403011) of the Ministry of Education,
Science and Culture of Japan.
the MTPs, which have a more dense structure than fcc, of IB
and IIB transition metals can be formed at lower electrode
potentials. However, one should consider the contribution of
the d electrons when the stability of MTPs of IB or IIB metals
31,32
References and Notes
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