Underpotentially Deposited Layers of Bi on Au(100) in HClO4 Investigated by In Situ STM

Article abstract of DOI:10.1149/1.1646143

The underpotential deposition (UPD) of Bi on Au(100) was investigated in HClO₄ solution using in situ scanning tunneling microscopy (STM). The UPD of Bi occurred in three steps. A (3 × 3) structure with a square lattice was found for the first UPD adlayer. This structure is composed of four square clusters, each consisting of four Bi atoms. A (√2 × 3√2)R45° structure, in which Bi atoms form zigzag rows, appeared after the second UPD peaks. After the third UPD peak, a c(4 × 2) structure was observed, in which Bi atoms form a close-packed hexagonal structure.

Full text of DOI:10.1149/1.1646143

E92
Journal of The Electrochemical Society, 151 ͑3͒ E92-E96 ͑2004͒
0013-4651/2004/151͑3͒/E92/5/$7.00 © The Electrochemical Society, Inc.
Underpotentially Deposited Layers of Bi on Au„100… in HClO4
Investigated by In Situ STM
a
a,b,
a,b,
Masanori Hara,a,b, Yoshiki Nagahara, Soichiro Yoshimoto,
Junji Inukai,
*
*
*
*
,z
and Kingo Itayaa,b,
aDepartment of Applied Chemistry, Graduate School of Engineering, Tohoku University,
Sendai 980-8579, Japan
bCREST, JST, Saitama 332-0012, Japan
The underpotential deposition ͑UPD͒ of Bi on Au͑100͒ was investigated in HClO4 solution using in situ scanning tunneling
microscopy ͑STM͒. The UPD of Bi occurred in three steps. A (3 ϫ 3) structure with a square lattice was found for the first UPD
adlayer. This structure is composed of four square clusters, each consisting of four Bi atoms. A (ͱ2 ϫ 3ͱ2)R45° structure, in
which Bi atoms form zigzag rows, appeared after the second UPD peaks. After the third UPD peak, a c(4 ϫ 2) structure was
observed, in which Bi atoms form a close-packed hexagonal structure.
© 2004 The Electrochemical Society. ͓DOI: 10.1149/1.1646143͔ All rights reserved.
Manuscript submitted March 7, 2003; revised manuscript received August 18, 2003. Available electronically February 5, 2004.
The processes of the underpotential deposition ͑UPD͒ of metals
are known to be extremely sensitive to the surface structures of the
electrode.1-7 Among various UPD metals, Bi,7-10 Pb,7-9,11-15 and
Tl7,9,14-16 have been widely studied because electrochemical mea-
surements revealed considerable acceleration of the electroreduction
of H2O2 and O2 on the Au surface modified by those metals. Based
on electrochemical measurements, it was reported that Au electrodes
with fractional coverages of those metal adlayers were significantly
more active than either bare or monolayer-covered Au surfaces to-
ward the reduction of H2O2 and O2 .7-13
finer grades of Al2O3 powder, from 1 to 0.3 ␮m in diameter. The
final treatment, performed to expose a clean and ordered surface,
consisted of flame annealing, slow cooling in a stream of hydrogen
Several in situ techniques have been used to understand the re-
lationship between the electrocatalytic activity and the structure of
UPD overlayers. The adlayer structures of Pb,17-19 Bi,20-24 and
Tl25,26 on Au͑111͒, and Pb27 on Au͑100͒ were studied by using in
situ scanning tunneling microscopy ͑STM͒, in situ atomic force mi-
croscopy ͑AFM͒, and surface X-ray scattering ͑SXS͒. Among the
three admetals namely Bi, Pb, and Tl, which enhance the H2O2
reduction, Bi allows the reaction to start at the most positive poten-
tial. The adlayer structures of Bi on Au͑111͒ in HClO4 solution were
determined by Gewirth and co-workers using in situ AFM,20 in situ
STM,21 and SXS.21 It was reported that the Bi adlayer exists in two
different structures, i.e., (2 ϫ 2) and (p ϫ ͱ3), at positive and
negative potentials, respectively.20,21 It was also reported that the
H2O2 reduction proceeds at the highest rate on the (2 ϫ 2)
adlayer.20,24,30
The electrochemical measurements of the rate of H2O2 reduction
10
¨
on Bi-deposited Au͑100͒ have been reported by Juttner and Sayed.
On Au͑100͒, the maximum rate of H2O2 reduction was observed at
the Bi coverage of 0.65.10 However, to our knowledge, the structure
of the UPD adlayer of Bi on Au͑100͒ has not been determined.
In this paper we report the UPD structure of Bi on Au͑100͒ in
HClO4 solution. On Au͑100͒, three UPD structures of Bi (3 ϫ 3),
(ͱ2 ϫ 3ͱ2)R45°, and c(4 ϫ 2), were found in the present study.
Experimental
Single-crystal electrodes of gold were prepared by the method of
Clavilier et al.31 Briefly, a pure Au wire ͑0.7 mm in diameter͒ was
melted in a hydrogen flame to form a single-crystal bead. A well-
prepared, single-crystal bead showed eight facets of ͑111͒ in an oc-
tahedral configuration and six facets of ͑100͒ in a hexahedral con-
figuration. These facets usually exhibited a wide, atomically flat
terrace-step structure. For STM observations, one of the ͑100͒ facets
was used directly. For cyclic voltammetric measurements, the ͑100͒
surface was mechanically exposed and polished with successively
Figure 1. ͑a͒ Cyclic voltammogram of Au͑100͒ electrode in 0.1 M HClO4 ;
scan rate, 50 mV sϪ1. ͑b͒ Cyclic voltammogram of Au͑100͒ electrode in
* Electrochemical Society Active Member.
z E-mail: Itaya@atom.che.tohoku.ac.jp
0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan rate, 1 mV sϪ1
.
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Journal of The Electrochemical Society, 151 ͑3͒ E92-E96 ͑2004͒
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Figure 2. ͑a͒ STM top view recorded at a potential after the first UPD peak
of Bi on Au͑100͒ in 0.1 M HClO4 ϩ 50 ␮M Bi2O3 , scan area, 40 ϫ 40
nm2; electrode potential, 0.5 V; tip potential, 0.4 V; tunneling current, 3 nA.
͑b͒ High-resolution STM image of the above Bi adlayer on Au͑100͒ in
0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan area, 5 ϫ 5 nm2; electrode potential,
0.5 V; tip potential, 0.41 V; tunneling current, 20 nA. ͑c͒ Structure model for
the Bi adlayer deposited on Au͑100͒ with a (3 ϫ 3) unit cell.
gas, and immersion in ultrapure water. The Au electrode was then
transferred into the STM or the electrochemical cell with a drop of
water protecting the surface from contamination.
Electrochemical STM imaging was performed with Nanoscope E
͑Digital Instruments, Santa Barbara, CA͒. The STM tip was made
from a W wire ͑0.25 mm in diameter͒ electrochemically etched in 1
M KOH. The tips were then coated with transparent nail polish to
minimize the faradaic current. All STM images were taken in the
constant-current mode and unfiltered.
All electrolyte solutions were prepared with ultrapure HClO4
͑Cica-Merck͒, 99.9% Bi2O3 ͑Wako Pure Chemical Industries, Ltd.͒,
and ultrapure water ͑Millipore-Q͒. All electrode potentials are re-
ported with respect to a reversible hydrogen electrode ͑RHE͒ in 0.1
M HClO4 .
Figure 1b shows
a CV of Au͑100͒ in 50 ␮M Bi2O3
ϩ 0.1 M HClO4 . The electrode potential was initially scanned from
0.8 V in the negative direction at the scan rate of 1 mV sϪ1. The
increase in cathodic current at 0.65 V is due to the first UPD of Bi.
Peak I for the first UPD of Bi can be seen at around 0.55 V with a
sharp spike at 0.52 V. Peak II at 0.47 V and the relatively sharp peak
III at 0.28 V are associated with the second and the third UPD of Bi
on Au͑100͒ surface, respectively. The cathodic current commencing
at 0.21 V is due to the bulk deposition of Bi, which is almost the
same as the Nernst potential of Bi ͑0.20 V͒. The peaks observed in
the positive scan correspond to the dissolution of bulk Bi at 0.23 V
and the successive dissolution of the UPD layers ͑peaks III , II , and
I ͒. The shape of the CV indicates that the UPD of Bi on Au͑100͒
occurred in three steps. The small cathodic and anodic peaks near
Ј
Ј
Ј
peaks II and II at 0.4 V might be attributable to the UPD of Bi on
Ј
Results and Discussion
a small portion of the surface defects, such as steps and kinks.10 The
charges consumed during the UPD of Bi at peaks I, II, and III were
calculated to be ca. 250, 130, and 75 ␮C cmϪ2 respectively. The
theoretical total charge corresponding to the deposition of a com-
plete (1 ϫ 1) monolayer of Bi on Au͑100͒ is 579 ␮C cmϪ2, assum-
ing that Bi3ϩ in solution is transformed into Bi0 upon adsorption on
Au͑100͒. By using the above values, the surface coverages of Bi
after the first, second, and third UPD peaks were calculated to be
0.43, 0.65, and 0.78, respectively.
Electrochemistry.—A cyclic voltammogram ͑CV͒ of a well-
defined Au͑100͒ electrode in 0.1 M HClO4 ͑pH 1͒ is shown in Fig.
1a. The electrode potential was initially scanned from 0.75 V in the
negative direction. It can be seen that a double-layer region extends
from 0 to 0.75 V. The increase in cathodic current at potentials more
negative than 0 V is due to hydrogen evolution. In the positive scan,
an anodic peak was observed at 0.85 V due to the phase transition of
the Au͑100͒ surface from a (5 ϫ 20) reconstruction structure to
(1 ϫ 1). The overall shape of the CV of Fig. 1a is the same as that
described in a previous paper.32 This result shows that the well-
defined surface of Au͑100͒ was exposed to the electrolyte solution.
It is noteworthy that the shape of the CV in Fig. 1b is very
sensitive to the rate of the potential scan. When the scan rate was
higher than 5 mV sϪ1, the potential of peak II moved in the cathodic
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Journal of The Electrochemical Society, 151 ͑3͒ E92-E96 ͑2004͒
Figure 3. ͑a͒ STM top view recorded at a potential after the second UPD
peak of Bi on Au͑100͒ in 0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan area, 40
ϫ 40 nm2; electrode potential, 0.35 V; tip potential, 0.3 V; tunneling cur-
rent, 5 nA. ͑b͒ High-resolution STM image of the above Bi adlayer on
Au͑100͒ in 0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan area, 5 ϫ 5 nm2; elec-
trode potential, 0.35 V; tip potential, 0.33 V; tunneling current,
25 nA. ͑c͒ Structure model for the Bi adlayer deposited on Au͑100͒ with a
(ͱ2 ϫ 3ͱ2)R45° unit cell.
direction in the negative scan. Peak III could not be seen because it
was shifted into the potential region where the bulk deposition of Bi
occurred. These results indicate that the UPD of Bi on Au͑100͒ is a
slow process. A similar dependence of UPD processes on the rate of
the potential scan at various metal surfaces was previously
reported.28,29,33,34 The CV for UPD of Bi on Au͑100͒ reported by
a square lattice on the Au surface, and the rows are running along
the atomic rows of the underlying Au͑100͒ (1 ϫ 1) lattice. The
phase boundary of two domains is indicated by a white dashed line
in Fig. 2a. The STM image shows that the domain size is as large as
50 nm. The high-resolution STM image shown in Fig. 2b reveals
details of the Bi adlayer structure. Surprisingly, each spot in Fig. 2a
is seen to consist of four bright spots forming a square cluster. The
average atomic distance between the spots that form the square clus-
ter is about 0.3 nm. This length is the same as the Bi-Bi distance in
a Bi single crystal, suggesting that each bright spot corresponds to a
deposited Bi atom. Sides of a Bi cluster and the rows of clusters
were both aligned in the ͗110͘ directions to form the square adlat-
tice. The average distance between the square clusters is ca. 0.86
nm, which is three times the Au-Au distance. The result described
above shows that the Bi adlayer on Au͑100͒ formed at 0.5 V is of a
(3 ϫ 3) structure, whose model is depicted in Fig. 2c. In order to
determine the adsorption sites of Bi atoms, we carried out a cross-
sectional analysis. It was found that the height of the Bi atoms was
0.18 Ϯ 0.03 nm compared to the Au substrate. From this result, we
assigned Bi atoms to 4-fold sites, because theoretically the height
between substrate Au atoms and Bi atoms at 4-fold sites is calcu-
lated to be 0.22 nm. In addition, UPD metals such as Pb,27 Cu,35 and
Se36 are reported to be adsorbed at 4-fold sites on Au͑100͒. The
coverage of Bi calculated from this structure is 0.44, which agrees
well with the value obtained by the charge density measurement
͑0.43͒, as described previously.
Sayed and Juttner10 is different from the CV described above, peaks
¨
I and II were broader and peak III was absent. They measured the
CV of UPD of Bi at a scan rate of 10 mV sϪ1. The CV we obtained
at the scan rate of 10 mV sϪ1 was identical to that reported by Sayed
10
¨
and Juttner.
In situ STM.—After a well-defined terrace-step structure was ob-
served on the well-prepared Au͑100͒ surface in pure HClO4 , the
solution was replaced by 50 ␮M Bi2O3 ϩ 0.1 M HClO4 . The STM
image was first obtained at 0.7 V, a potential more anodic than that
for the UPD ͑Fig. 1b͒. The well-ordered (1 ϫ 1) structure was
observed on the Au͑100͒ surface in HClO4 solution containing Bi3ϩ
ions. Bi3ϩ ions, therefore, are not adsorbed on a well-prepared
Au͑100͒ surface in the potential region more anodic than that for the
UPD.
After the well-ordered (1 ϫ 1) structure was observed in the
anodic potential range, the potential was stepped to a value more
cathodic than that for the first UPD. Figure 2a shows an STM image
acquired at 0.5 V, a potential between peak I and peak II ͑Fig. 1b͒. It
is clear that atomically flat terraces are extended with a winding step
of a monatomic height. On the terraces, bright spots are seen to form
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Journal of The Electrochemical Society, 151 ͑3͒ E92-E96 ͑2004͒
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Figure 4. ͑a͒ STM top view recorded at a potential after the third UPD peak
of Bi on Au͑100͒ in 0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan area, 20 ϫ 20
nm2; electrode potential, 0.23 V; tip potential, 0.2 V; tunneling current, 12
nA. ͑b͒ High-resolution STM image of the above Bi adlayer on Au͑100͒ in
0.1 M HClO4 ϩ 50 ␮M Bi2O3 ; scan area, 5 ϫ 5 nm2; electrode potential,
0.23 V; tip potential, 0.2 V; tunneling current, 10 nA. ͑c͒ Structure model for
a Bi adlayer deposited on Au͑100͒ with c(4 ϫ 2) unit cells and domain
boundaries.
The electrode potential was then stepped to 0.35 V, which is
between peaks II and III ͑Fig. 1b͒. In the first minute after stepping
the potential, it was observed that the steps, which had been wind-
ing, became straight and grew longer. Figure 3a shows an STM
image acquired at 0.35 V 10 min after the potential step. It is notable
that the monoatomic steps now run parallel to the ͗001͘ direction,
rotated by 45° with respect to the atomic rows of the substrate.37-41
A close examination of this image reveals the presence of stripes on
the terraces Au͑100͒. The distance between two stripes is less than 1
nm. These stripes are aligned in the & direction, rotated by 45° with
respect to the direction of the atomic rows of the underlying
Au͑100͒ lattice. In Fig. 3a, two domains are observed to rotate by
90° with respect to each other on the two different terraces separated
by a step. These stripes run parallel with the step lines. The high-
resolution STM image in Fig. 3b reveals the structure of the Bi
adlayer at 0.35 V. It can be seen that the bright spots form a zigzag
feature running unidirectionally. Each bright line in Fig. 3a corre-
sponds to this zigzag row in Fig. 3b. The average atomic distance
between the bright spots in a zigzag row is ca. 0.31 nm, indicating
that each bright spot corresponds to a Bi atom. On the basis of the
symmetry of the atomic arrays, a unit cell can be drawn as outlined
in Fig. 3b. The average lengths of the sides of unit cells are ca. 1.22
nm ͑3& times Au-Au distance͒ and ca. 0.41 nm ͑& times Au-Au
distance͒, respectively. The unit cell of the Bi adlayer is rotated by
45° with respect to the direction of a unit cell of the underlying
Au͑100͒ lattice. The result described above shows that the Bi ad-
the (3 ϫ 3) structure described in the previous section. The cover-
age of Bi calculated from this structure is 0.66, which corresponds
to that obtained from the charge density of the sum of peaks I and II
from the CV measurement ͑0.65͒. The feature observed in Fig. 3b is
very similar to the structure of Pb electrochemically deposited on
Au͑100͒.27
The potential was then stepped to a value more cathodic than that
of peak III. Here the shape of the step line changed again. Soon after
applying the potential step, the characteristic linear steps seen in Fig.
3a became curved. Figure 4a shows an STM image acquired at 0.23
V in the potential region between the third UPD and the bulk depo-
sition. A mesh-like structure consisting of hexagons is seen. In the
high-resolution STM image shown in Fig. 4b, it can be clearly seen
that the bright spots form a hexagonal structure. Those spots corre-
spond to deposited Bi atoms, and they exhibit different corrugation
heights at different sites. Four dark spots are clearly seen on the
corners of the unit cell outlined by solid lines in Fig. 4b. The aver-
age atomic distances between the dark spots in directions A and B in
Fig. 4b are ca. 0.58 ͑two times the Au-Au distance͒ and ca. 0.65
(ͱ5 times the Au-Au distance͒, respectively. The rows in direction A
are along the atomic rows of the underlying Au͑100͒ lattice, and the
angle between arrows A and B is 63° Ϯ 2°. The result described
above shows that the structure of Bi is (21 20). This structure can be
designated also as c(4 ϫ 2), whose unit cell is outlined by dashed
lines in Fig. 4b. This symmetry is the same as that observed for Bi
on Au͑100͒ in ultrahigh vacuum ͑UHV͒ studied by using low energy
electron diffraction ͑LEED͒.42 The coverage of Bi calculated from
the c(4 ϫ 2) structure is 0.75. This value corresponds to the cov-
layer on Au͑100͒ at 0.35 V has the (ͱ2 ϫ 3 2)R45° symmetry. A
ͱ
structure model is shown in Fig. 3c. It is considered that each Bi
atom is adsorbed at a 4-fold site, which is the same as in the case of
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Journal of The Electrochemical Society, 151 ͑3͒ E92-E96 ͑2004͒
Tohoku University assisted in meeting the publication costs of this
article.
erage calculated from the CV measurement ͑0.78͒. A structure
model with unit cells is shown in Fig. 4c. Dark spots and bright
spots in Fig. 4b are assigned to Bi atoms at 4-fold hollow sites and
bridge sites, respectively, in the model.
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The detailed investigation of Fig. 4b revealed that there are many
domain boundaries in the ͗011͘ direction indicated by the arrows C,
D, and E. The distance between the neighboring two domain bound-
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Au-Au distance, respectively. The domain boundaries consisted of
atoms forming a zigzag line with an atom-atom distance of 0.35 nm.
The structural model of this domain boundary is shown in Fig. 4c by
thick open circles for Bi atoms on domain boundaries separating
c(4 ϫ 2) domains. The Bi atoms on domain boundaries must be
assigned to locations near bridge sites to explain the STM image in
Fig. 4b.
It is known that the structure of the UPD layer is very sensitive
to anions in supporting electrolytes.1,2 For comparison, we also in-
vestigated the UPD process of Bi in H2SO4 solution, but found that
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observed in HClO4 solution. Gewirth reported that, by using chro-
nocoulometry, OHϪ was co-adsorbed on the Au͑111͒ surface after
the UPD of Bi and affected the formation of the (2 ϫ 2)
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that most significantly affects the UPD process of Bi on Au͑100͒.
However in our STM images, the adsorbed OHϪ was not seen.
Further investigation such as chronocoulomety is needed to under-
stand the effect of OHϪ on the UPD processes of Bi.
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(ͱ2 ϫ 3 2)R45° structure, in which Bi atoms formed zigzag rows.
ͱ
After the third UPD peak, a c(4 ϫ 2) structure was observed with
Bi atoms forming a close-packed hexagonal structure at different
adsorption sites. The UPD process observed in this study was com-
pletely reversible.
Acknowledgments
This work was supported in part by Core Research for Evolu-
tional Science and Technology ͑CREST͒ organized by the Japan
Science and Technology Corporation ͑JST͒ and a Grant-in-Aid for
the COE project, Giant Molecules and Complex Systems from the
Ministry of Education, Culture, Sports, Science and Technology
͑MEXT͒. The authors acknowledge Dr. Okinaka for his assistance in
writing this manuscript.
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