Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
E110
0013-4651/2003/150共2兲/E110/7/$7.00 © The Electrochemical Society, Inc.
Structure of Sulfur Adlayer on Cu„111… Electrode
in Alkaline Solution
Masatoshi Sugimasa,a Junji Inukai,b and Kingo Itayaa,z
a
Department of Applied Chemistry, Faculty of Engineering, Tohoku University, Sendai 980-8579, Japan
PRESTO, JST, Kawaguchi City, Saitama Pref. 332-0012, Japan
b
The structure of sulfur adlayer and the formation process of Cu2 S layer on Cu共111兲 were investigated using in situ scanning
tunneling microscopy 共STM兲 in alkaline solution. In situ STM revealed two different structures of the sulfur adlayer,
() ⫻ ))R30° and (19 ⫻ 19), in the double-layer potential region. The () ⫻ ))R30° structure observed at negative potentials was transformed into (19 ⫻ 19) upon sweeping the potential in the positive direction. The (19 ⫻ 19) structure consisted of
well-ordered triangular domains separated by boundaries running along the 具110典 direction. In each triangular domain of
(19 ⫻ 19), sulfur atoms were found to form a () ⫻ ))R30° structure. Characteristic hexagonal rings consisting of six sulfur
atoms were observed at intersections of the domain boundaries of (19 ⫻ 19). At anodic potentials, layers of Cu2 S were epitaxially
formed on Cu共111兲, on which a ( 冑7 ⫻ 冑7)R19.1° structure was observed.
© 2003 The Electrochemical Society. 关DOI: 10.1149/1.1533042兴 All rights reserved.
Manuscript received May 3, 2002. Available electronically January 6, 2003.
Sulfur atoms are known to be strongly adsorbed on metal surfaces and to play important roles in catalysis and many electrochemical reactions such as corrosion. Adlayer structures of S on
共111兲 surfaces of Pt,1 Rh,2 Ni,3 Pd,4,5 and Cu6-12 have been extensively studied in ultrahigh vacuum 共UHV兲 environment. These studies revealed that the S adlayer forms various ordered structures on
共111兲 metal surfaces depending on the coverage. Among various
metal surfaces, the adsorption of S on Cu共111兲 has been most intensively studied since S atoms act as a strong poison to Cu-based
catalysis, such as the water-gas shift reaction.9,10 Moreover, the reaction between Cu and S results in the formation of copper
sulfide,6-12 which is a compound semiconductor used in optical devices such as solar cells.13,14
Adlayer structures of S on Cu共111兲 surface have been investigated mainly by using scanning tunneling microscopy 共STM兲 and
low-energy electron diffraction 共LEED兲 under UHV. At a very low S
coverage at a low temperature, an STM image was obtained that
could be described as ( 冑43 ⫻ 冑43)R ⫾ 7.5°. 12 At a coverage of
one-third, it was found by using LEED that S formed a simple
() ⫻ ))R30° structure.6 STM studies revealed that when the S
coverage was increased, the adlayer structure was changed into
4 1 7-9
( ⫺1
At the saturation coverage, a ( 冑7 ⫻ 冑7)R19.1° structure
4 ).
4 1
was observed by LEED6,10 as well as STM.7-9 The ( ⫺1
4 ) and the
( 冑7 ⫻ 冑7)R19.1° adlayers were also studied using other techniques, such as surface X-ray diffraction11 and Auger electron
4 1
spectroscopy.10 These studies indicated that the ( ⫺1
4 ) and
冑
冑
( 7 ⫻ 7)R19.1° structures were not simply attributable to the
formation of adlayers of S on Cu共111兲, and that the formation of
copper sulfide compounds from a chemical reaction between Cu and
S atoms must also be taken into account. According to the previous
studies in UHV, the adlayer structure of S on Cu共111兲 seems to be
strongly affected by its coverage.
In solution, adlayer structures of specifically adsorbed anions
have long been investigated, and several potential-dependent structures have been reported on electrode surfaces. For example, the
adlattices of Br and Cl on Cu共111兲 were continuously compressed as
the electrode potential was swept in the anodic direction.15 This
so-called electrocompression was also found for the I adlayers on
Au共111兲16-19 and Ag共111兲.20 It was also reported that the adlayer
structures of S on Au共111兲21,22 and Ag共111兲23 were dependent on
the electrode potential in solution. On Au共111兲, a () ⫻ ))R30°
structure of S adlayer was observed at negative potentials by in situ
STM,21,22,24 whereas at positive potentials, S clusters like S8 rings
z
E-mail: [email protected]
were found with an increase in coverage.21,22 On Ag共111兲, S formed
also a () ⫻ ))R30° structure at negative potentials. When the
electrode potential was scanned in the positive direction, triangle
clusters made of three S atoms were arranged so as to form a wellordered ( 冑7 ⫻ 冑7)R19.1° structure on Ag共111兲.23 As described
above, S atoms on Au共111兲21,22 and Ag共111兲23 in aqueous solutions
presented various structures depending on the electrode potential.
However, to our knowledge, there has been no in situ STM study
performed in solution for the adlayer structure of S on Cu共111兲.
It is well known that copper sulfide layers grow electrochemically on the Cu electrode in solution. The phase diagram of copper
sulfides at room temperature shows the existence of at least four
different phases, Cu2 S, Cu1.96S, Cu1.8S, and CuS.25 The chemical
composition of the copper sulfide layer electrochemically grown on
Cu electrode has been investigated in the interest of the application
to semiconductor devices,13,14 since electrical properties of copper
sulfides are affected by their composition. On the other hand, the
surface morphology and the process of the formation of copper sulfide layers on Cu surface have not been studied on the atomic scale.
Here, we describe the adlayer structure of S on Cu共111兲 in alkaline solution studied by using in situ STM. The STM study revealed
two different structures for the S adlayer in the double-layer potential region. One was a () ⫻ ))R30° structure observed at negative potentials, and the other was (19 ⫻ 19) at positive potentials.
The (19 ⫻ 19) structure consisted of well-ordered triangular domains, which were separated by zigzag domain boundaries running
along the 具110典 direction. The S atoms in each triangular domain
possessed a () ⫻ ))R30° structure. The characteristic hexagonal
rings of S were clearly observed with a (19 ⫻ 19) periodicity at the
intersection of domain boundaries. At anodic potentials where bulk
copper sulfide layers are formed, a hexagonal atomic structure similar to that of the 共111兲 layer of cubic Cu2 S was observed.
Experimental
A commercial Cu共111兲 single-crystal disk with a diameter of 5
mm 共MaTeck, Inc., Germany兲 was used as the substrate for both
electrochemical and in situ STM measurements. Details of the
sample preparation were described in our previous papers.15,27
Briefly, the surface was first metallographically polished and then
sonicated in acetone. To remove contaminating surface oxide layers,
the Cu共111兲 surface was electropolished in a mixed solution of phosphoric and sulfuric acids 共60 mL of 85% H3 PO4 , 10 mL of concentrated H2 SO4 , and 40 mL of pure water兲 at ca. 5 A cm⫺2 for 0.1 s.
The Cu crystal was then rinsed repeatedly with ultrapure Millipore
water. A droplet of water was left on the electrode surface to protect
it from contamination during transfer into the electrochemical cell.
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Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
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reduction of copper sulfide appeared at ⫺1.13 and ⫺1.19 V. Overall, the CV profiles shown in Fig. 1 are similar to those previously
obtained using polycrystalline Cu electrodes.13,14 When the electrode potential was scanned between ⫺1.3 and ⫺0.7 V, the separation of the reduction peaks became smaller. After ten cycles, a single
reduction peak appeared at ⫺1.17 V. The charge density for the
single reduction peak was ca. 1.4 times larger than the sum of
charge densities for the two reduction peaks observed in the first
scan. This change in the shape of reduction peak suggests that the
well-ordered Cu共111兲 surface was roughened by the oxidationreduction cycles.
Figure 1. Cyclic voltammograms recorded on Cu共111兲 in 0.1 M KOH in the
absence 共dashed line兲 and the presence 共solid line兲 of 0.5 mM Na2 S. Scan
rate ⫽ 20 mV s⫺1 .
Electrochemical measurements were carried out in a threecompartment glass cell using 0.1 M KOH containing 0.5 mM Na2 S.
Solutions were deoxygenated by bubbling purified N2 . In situ STM
measurements were carried out with a Nanoscope E 共Digital Instruments兲 in 0.1 M KOH containing 0.5 mM Na2 S. The STM tips were
prepared by electrochemically etching a tungsten wire 共0.25 mm
diam兲 in 1 M KOH. To minimize residual background currents, the
sidewall of the tips was coated with nail polish.
Solutions were prepared with KOH 共Cica-Merck, Ultrapure
grade兲, Na2 S 共Aldrich, reagent plus grade兲, and ultrapure water
共Millipore-Q兲. The electrochemical and STM measurements were
preformed utilizing a reversible hydrogen electrode 共RHE兲 in 0.1 M
KOH containing no sulfur ion. A Ag/AgCl/saturated KCl electrode
was used to calibrate the RHE. All potentials reported in this paper
are referred to the Ag/AgCl/saturated KCl to facilitate comparison
with the results reported in other papers.
Results and Discussion
Voltammetric study.—We first describe the cyclic voltammetry of
Cu共111兲 and then discuss in situ STM experiments. Figure 1 shows
cyclic voltammograms 共CVs兲 of Cu共111兲 in 0.1 M KOH in the absence 共dashed line兲 and the presence 共solid line兲 of Na2 S. In pure
KOH solution, a featureless double-layer region was observed between ⫺1.4 and ⫺0.75 V. 13,28
After the CV measurement in pure 0.1 M KOH, Na2 S was introduced into the electrochemical cell at ⫺1.2 V. The concentration of
Na2 S was adjusted to 0.5 mM. The scan was started in the positive
direction after the injection of Na2 S. A featureless double-layer region was observed between ⫺1.2 and ⫺1.0 V. At ca. ⫺1.0 V, the
anodic current abruptly began to increase. This anodic current is due
to the bulk formation of copper sulfide on Cu共111兲.13,14 The anodic
current then decreased at potentials more positive than ⫺0.9 V, because the copper surface was passivated by the formation of copper
sulfide. In the subsequent negative scan starting at ⫺0.7 V, the anodic current for the sulfide formation continued until ⫺0.97 V. In
the further scan in the negative direction, two cathodic peaks for the
In situ STM on bare Cu(111).—To produce an atomically flat
Cu共111兲 surface for STM observations, a freshly polished surface
was immersed in 0.1 M KOH in the STM cell, and the electrode
potential was set at a potential slightly negative to that for the onset
of the hydrogen evolution reaction. Then, the electrode potential was
held at ⫺1.4 V for at least for 30 min to completely reduce the
natural oxide layer on the Cu surface. Figure 2a shows a large-scale
STM image of the bare Cu共111兲 electrode in 0.1 M KOH at ⫺1.0 V.
It is clear that atomically flat terraces are extended with steps of
monatomic height, and that there are many kink sites at the step
edges. Figure 2b shows a high-resolution STM image acquired on
an atomically flat terrace. The hexagonal close- packed structure can
be seen with an interatomic distance of ca. 0.26 nm, which corresponds to the lattice constant of Cu共111兲, 0.256 nm. Figure 2b demonstrates that, under the present conditions, the Cu共111兲 surface retains the unreconstructed (1 ⫻ 1) structure. The (1 ⫻ 1) structure
was consistently observed in the potential range between ⫺1.2 and
⫺1.0 V. By using in situ STM, the electrochemical oxidation of
Cu共111兲 was reported to be initiated at step edges at ca. ⫺0.6 V vs.
the standard hydrogen electrode.28 The results described above on
bare Cu共111兲 surface are consistent with those of previous studies
performed by in situ STM28 and in situ AFM29 in alkaline solutions.
Structures of S adlayer on Cu(111).—After observing the bare
Cu共111兲 surface, droplets containing Na2 S were carefully introduced
into the cell at ⫺1.2 V to avoid the formation of bulk copper sulfide
on the Cu共111兲 surface. The average concentration of Na2 S was
adjusted to 0.5 mM. The surface morphology showed little difference from that of bare Cu共111兲 surface in KOH solution. No additional pits or islands were observed on terraces. The shapes of the
steps were also unchanged. Figure 3 shows a high-resolution STM
image of Cu共111兲 acquired at ⫺1.2 V in 0.1 M KOH containing 0.5
mM Na2 S. S atoms formed a nearly perfect hexagonal adlattice. In
the middle of the image, a white arrow points to a defect, which is a
vacancy of S atom in the adlattice. The S rows are rotated by 30°
with respect to the rows of substrate Cu. The S-S distance was ca.
0.44 nm, corresponding to ) times the lattice constant of Cu共111兲.
Therefore, it is concluded that S atoms formed a () ⫻ ))R30°
structure. A unit cell is drawn by solid lines in Fig. 3. This
() ⫻ ))R30° structure was observed at potentials between ⫺1.3
and ⫺1.05 V. A () ⫻ ))R30° structure was reported in the previous LEED study at a low S coverage on Cu共111兲 in UHV,6 and
also on Au共111兲21,22,24 and Ag共111兲23 in solution by using in situ
STM. It has been proposed that S atoms were adsorbed at threefold
sites in the case of a S adlayer on Au共111兲 in solution.21 Somorjai
and co-workers discussed that S atoms generally prefer to be adsorbed at the fcc hollow sites on Rh共111兲 and Pt共111兲 surfaces at low
coverages in UHV.1,2 Based on those previous results, we expect
that S atoms forming a () ⫻ ))R30° structure on Cu共111兲 might
also be located at threefold sites.
After observing the () ⫻ ))R30° structure, the potential was
scanned very slowly in the anodic direction. At potentials more positive than ca. ⫺1.05 V, large triangular superstructures gradually appeared on the terrace and finally extended over the entire region of
the surface. Figure 4 shows a typical STM image of the triangular
superstructures of the S adlayer on Cu共111兲 obtained at ⫺1.0 V. It
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Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
Figure 3. Atomically resolved STM image of a () ⫻ ))R30° structure
on Cu共111兲 in 0.1 M KOH ⫹ 0.5 mM Na2 S. The arrow points at a defect
lacking S atom. Sample potential ⫽ ⫺1.2 V. Tip potential ⫽ ⫺0.85 V.
Figure 5 shows a high-resolution image of the (19 ⫻ 19) structure of the S adlayer. Although several atomic defects were found in
the domains, the S atoms in each triangular domain formed the
Figure 2. Large-scale 共a兲 and atomically resolved 共b兲 STM images of
Cu共111兲 in 0.1 M KOH. A unit cell is outlined by solid lines. Sample
potential ⫽ ⫺1.0 V. Tip potential ⫽ ⫺0.95 V.
can be seen that an atomically flat terrace is covered with large,
well-ordered triangular domains. Vacancies of S atoms are also seen
in some triangular domains. The length of the sides of the triangular
domain is ca. 3.8 nm, and each triangle contains about 50 atoms of
S on the average. These triangular domains are separated by dark
zigzag lines, which are aligned along the 具110典 direction. At the
intersection of the dark lines, bright rings are seen as marked by a
solid circle in Fig. 4. The diameter of each ring was ca. 1.0 nm, and
the corrugation amplitude of each ring was higher by ca. 0.02 nm
than that of the S atoms in the triangular domain. The distance of
5.0 ⫾ 0.3 nm between two adjacent rings was nearly 19 times
共4.864 nm兲 the lattice constant of Cu共111兲. Therefore, we assign
(19 ⫻ 19) to this structure of the S adlayer. This (19 ⫻ 19) structure was observed at potentials between ⫺1.05 and ⫺0.98 V.
Figure 4. Large-scale STM image of (19 ⫻ 19) structure on Cu共111兲 in
0.1 M KOH ⫹ 0.5 mM Na2 S. Sample potential ⫽ ⫺1.0 V. Tip potential
⫽ ⫺0.5 V.
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Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
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Figure 5. Atomically resolved STM image of (19 ⫻ 19) structure on
Cu共111兲 in 0.1 M KOH ⫹ 0.5 mM Na2 S. Sample potential ⫽ ⫺1.0 V. Tip
potential ⫽ ⫺0.45 V.
hexagonal adlattice with the same brightness. The S-S distance of
0.44 nm corresponded to ) times the lattice constant of Cu共111兲,
and the atomic rows were rotated by 30° with respect to the Cu
lattice. These results suggest that the triangular domains in the
(19 ⫻ 19) structure consist of S atoms forming a () ⫻ ))R30°
structure. The high-resolution STM image allowed us to determine
the detailed structure of bright rings located at the intersections of
the dark lines. As can be seen in Fig. 5, a bright ring is made of six
S atoms, which are marked by a set of six circles.
To describe the site arrangement of S atoms in detail, a close-up
image of a part of the ordered (19 ⫻ 19) structure is displayed in
Fig. 6a. In the STM image, lines A and B were drawn through the
centers of S atoms aligned in a row in two adjacent domains. The
STM image shows that line B is shifted by a distance of ca.
0.12 ⫾ 0.02 nm with respect to line A. If every S atom were adsorbed at an identical adsorption site, lines A and B would form a
single straight line. The shift of atomic rows is attributed to the
difference in adsorption site of S atoms in the two domains.
Figure 6b shows the structural model. In this model, we postulated that the adsorption sites of S in the neighboring domains are
different, namely, face-centered cubic 共fcc兲 and hexagonal closepacked 共hcp兲 sites. In the model, three Cu atoms underneath an S
atom are marked by triangles. The triangles a 共pointing left兲 and b
共pointing right兲 indicate the fcc and hcp sites of Cu共111兲, respectively. One can see in the model that all S atoms in the upper right
domain are located at the fcc site while those in the lower left
domain at the hcp site. Lines A and B in Fig. 6b mark the rows of S
atoms on fcc and hcp sites, respectively. According to our model,
line B is dislocated from line A by a distance of 0.128 nm, which is
equal to the radius of a substrate Cu atom. This is in good agreement
with the in situ STM measurement 共Fig. 6a兲. Therefore, we conclude
that S atoms in the neighboring domains are adsorbed at two different threefold sites, fcc and hcp, respectively.
An ideal model for the (19 ⫻ 19) structure is proposed in Fig. 7.
In the model, S atoms in each triangular domain form a
() ⫻ ))R30° structure, located at threefold sites. The ()
Figure 6. 共a兲 High-resolution STM image of a part of the (19 ⫻ 19) structure. 共b兲 Structural model.
⫻ ))R30° structure is separated by domain boundaries, which appear as dark zigzag lines in the STM images 共Fig. 4 and 5兲. These
domain boundaries are aligned along the 具110典 direction. S atoms
are adsorbed at fcc sites in one domain, and at hcp sites in the
neighboring domains 共Fig. 7兲. The triangular domain structure of S
atoms on Cu共111兲 shown in Fig. 4-6 is similar to that for the first
UPD layer of Te on Au共111兲,30-32 recently proposed as (13 ⫻ 13) by
Stickney and co-workers based on in situ STM and ex situ LEED
measurements.32 In their model, the UPD layer consists of
() ⫻ ))R30° triangular domains with a network of dark boundaries forming a (13 ⫻ 13) unit cell, which is similar to our model.
They suggested that Te atoms on each side of the domain boundary
are at different threefold sites, namely, fcc and hcp sites, because
slight shifts of atomic rows of Te were also observed in their STM
image around domain boundaries.32
At the intersection of the domain boundaries, six S atoms make a
hexagonal ring, and the set of the rings forms a (19 ⫻ 19) periodicity. These S atoms can be located at bridge sites of Cu共111兲 as
shown in Fig. 7, because they appeared brighter than the other S
atoms in the triangular domains at threefold sites 共Fig. 4 and 5兲. In
the case of the first UPD layer of Te, the intersection of domain
boundaries appeared simply dark.30-32 On the contrary, we clearly
observed bright hexagonal rings at the intersections of domain
boundaries in our STM images in Fig. 4 and 5. Generally, S atoms
have a pronounced tendency for catenation and prefer to form mol-
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Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
Figure 8. Atomically resolved STM image of ( 冑7 ⫻ 冑7)R19.1° structure
of Cu2 S layers on Cu共111兲 in 0.1 M KOH ⫹ 0.5 mM Na2 S. A unit cell is
drawn by solid lines. Sample potential ⫽ ⫺0.95 V. Tip potential
⫽ ⫺0.45 V.
Figure 7. Structural model for the (19 ⫻ 19) adlayer of S on Cu共111兲 surface. Solid lines indicate a unit cell.
ecules in various phases such as rings and chains.33 It is assumed
that S atoms situated at the apices of six triangular domains are
unstable, resulting in the formation of a more stable configuration.
To realize a more stable structure, the six atoms at the apices should
form a hexagonal ring as observed at the intersection of domain
boundaries in our STM images. S6 rings, as well as S8 , are reported
to be very stable based on symmetrical considerations.33
After observing the (19 ⫻ 19) structure as shown in Fig. 4, the
electrode potential was scanned back in the negative direction. At a
potential more negative than ca. ⫺1.05 V, the triangular domains
and hexagonal rings of the S adlayer disappeared, and a simple
() ⫻ ))R30° structure reappeared. The phase transition of the S
adlayer is thus reversible.
Formation of copper sulfide on Cu(111).—After observing the S
adlayer with the (19 ⫻ 19) structure, the electrode potential was
scanned in the anodic direction to form a layer of copper sulfide. At
ca. ⫺0.97 V at the foot of the anodic current for the formation of
copper sulfide 共Fig. 1兲, the step lines started to move, and the atomic
resolution on terraces was lost in the STM imaging. This structural
change continued for nearly a minute at ⫺0.97 V, when most of the
step lines became atomically straight. The total charge density of ca.
0.3 mC cm⫺2 was required for the formation of sulfide on Cu共111兲.
As we described in the introductory section, at least four phases
of copper sulfides, Cu2 S, Cu1.96S, Cu1.8S, and CuS, exist in the
phase diagram at room temperature.25 Based on the investigations
involving electrochemical measurements, electrochemical quartz
crystal microgravimetry, X-ray photoelectron spectroscopy, and Raman spectroscopy, it was reported that only Cu2 S was electrochemically grown on Cu electrode in the early stages of the formation of
copper sulfide film.13,14,26 If the reaction for the formation of sulfide
proceeds according to Eq. 1,13,14 one layer of Cu2 S should form on
Cu共111兲 at ⫺0.97 V with the charge density of ca. 0.3 mC cm⫺2
2Cu ⫹ HS⫺ → Cu2 S ⫹ H⫹ ⫹ 2e⫺
关1兴
Figure 8 shows a typical STM image of the surface structure of
Cu2 S on Cu共111兲. The Cu2 S surface consists of atomically flat terraces and monatomic steps. The step-height was ca. 0.22 nm, which
is equal to that of bare Cu共111兲. On terraces, a hexagonal structure
of bright spots was observed. The distance between the nearestneighbor spots was ca. 0.64 nm, corresponding to 冑7 times 共0.67
nm兲 the lattice constant of Cu共111兲. The rows of the bright spots
were in parallel with the step lines and rotated by ca. 20° with
respect to the 具110典 direction of Cu共111兲. The structure shown in
Fig. 8, therefore, can be described as ( 冑7 ⫻ 冑7)R19.1°.
The STM image in Fig. 8 is identical to that of the monolayer
structure of sulfide previously obtained on Cu共111兲 in UHV by dosing H2 S at room temperature.7-9 In the case of the monolayer structure of ( 冑7 ⫻ 冑7)R19.1° for sulfide in UHV, a structure similar to
a monolayer of cubic Cu2 S crystals34 has been believed to form on
Cu共111兲.6,7 Figure 9 shows a structural model of a monolayer of
sulfide in UHV formed on Cu共111兲 with the ( 冑7 ⫻ 冑7)R19.1°
structure. By using techniques such as ion scattering spectroscopy10
and surface X-ray diffraction,11 it is now established that the copper
sulfide surface with the ( 冑7 ⫻ 冑7)R19.1° structure in UHV is not
terminated by Cu ions but by S ions. In Fig. 9, each S ion in the
topmost lattice coordinates to three underlying Cu ions. In the
model, two S ions in the ( 冑7 ⫻ 冑7)R19.1° unit cell 共drawn by
thick solid lines in Fig. 9兲 are located above threefold sites of the
Cu共111兲 substrate, whereas S ions at the corners of the unit cell are
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Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
E115
cally moved, and the atomic resolution was completely lost on terraces for 1 min. When the current ceased 共the charge density was
0.6 mC cm⫺2 ), the STM images similar to that in Fig. 9 reappeared.
According to Eq. 1, ca. four layers of Cu2 S should now have been
formed on Cu共111兲. The STM images showed that the direction of
the monoatomic steps was again rotated by ca. 20° with respect to
that for the Cu共111兲 substrate. The S ions were also arranged in the
( 冑7 ⫻ 冑7)R19.1° symmetry. Therefore, it can be concluded that
the Cu2 S(111) layers were epitaxially grown on Cu共111兲 in solution.
Further epitaxial growth of Cu2 S layers was possible at more positive potentials. It is interesting to note that at ca. 10-20 layers, every
S ion on Cu2 S(111) became visible by in situ STM, contrary to the
case of the monolayer of Cu2 S on Cu共111兲.7 The influence of the Cu
substrate, therefore, should become negligible when the Cu2 S layers
grow thick in solution. We successfully produced epitaxial layers of
Cu2 S as thick as 100 layers on Cu共111兲. On the other hand, the
formation of sulfides on Cu共100兲 and 共110兲 resulted in hilly structures, probably because of the lattice mismatches between Cu and
Cu2 S on the two surfaces. On Cu共111兲, high-resolution STM images
were steadily obtained at a tip bias between ⫺0.2 and 0.6 V.
During the observation of semiconductor electrodes in solution
by STM, one must give attention to the potentials of both the sample
and the tip.35-42 Böer reported that Cu2 S is a p-type semiconductor
whose bandgap is equal to 1.2 eV.43 In the case of p-type semiconductors, electrons tunnel from the valence band in the sample to the
vacant level in the metal tip when the sample potential is more
negative than the flatband potential and the tip potential is more
positive than the edge of the valence band which is located at a
potential slightly positive than the flat band potential.38 According to
the energetics of the semiconductor/liquid interface, the tunneling of
electrons is prohibited when the tip potential is more negative than
the sample potential. However, as described above, the highresolution STM imaging was possible on Cu2 S(111) even at a negative tip bias. A possibility is the existence of surface states or defects
in the Cu2 S layers, as we previously reported in the case of the AgBr
layers electrochemically formed on Ag共100兲 in solution.42 A further
detailed investigation is needed to elucidate the electron tunneling
mechanism in the electrochemically formed Cu2 S layers.
Conclusions
Figure 9. Structural model of a sulfide monolayer with ( 冑7 ⫻
structure on Cu共111兲.
冑7)R19.1°
located above on-top sites. The distances between the nearest S-S
ions on the S-terminated Cu2 S(111) surfaces on Cu共111兲 and on a
Cu2 S crystal34 are 0.391 and 0.393 nm, respectively. The mismatch
is less than 0.6 %. As we described above, only Cu2 S has been
reported to be electrochemically grown on Cu electrode in the early
stages of the formation of copper sulfide.13,14,26 Therefore, it can be
concluded that the S-terminated Cu2 S(111) monolayer 共Fig. 9兲 was
also formed on Cu共111兲 in our case in solution. In the previous STM
imaging in UHV, it was reported that not all S ions on the
S-terminated Cu2 S(111) monolayer on Cu共111兲 were observable.7-9
It was then assumed that only S ions located above on-top sites of
Cu共111兲 might be observable by STM 共Fig. 9兲,7 because of an influence of the Cu(111)-(1 ⫻ 1) substrate lattice. In the case of the
monolayer of Cu2 S in solution, we also observed S ions only at the
corners of the unit cell 共Fig. 9兲 as previously reported in UHV.7
In the next experiment, the electrode potential was stepped in the
positive direction from ⫺0.97 to ⫺0.95 V. When the current for the
formation of Cu2 S layers started to flow, the step lines again drasti-
High-resolution STM imaging revealed the adlayer structures of
S atoms on Cu共111兲 electrode in alkaline solution. Two structures,
() ⫻ ))R30° and (19 ⫻ 19), were found within the doublelayer charging region. These structures were dependent on the electrode potential. At positive potentials, a () ⫻ ))R30° structure
was transformed into (19 ⫻ 19), in which well-ordered triangular
domains separated by dark boundaries appeared on atomically flat
terraces. Characteristic hexagonal rings were found at the intersections of domain boundaries. The atomic-resolution STM image
shows that S atoms in the triangular domains formed a
() ⫻ ))R30° structure and that the hexagonal rings consisted of
six S atoms. It was proposed that S atoms in the triangular domains
were located at two different threefold sites, fcc and hcp sites, on
each side of a domain boundary. At the foot of the anodic peak for
the formation of copper sulfide, a ( 冑7 ⫻ 冑7)R19.1° structure was
observed.
Acknowledgments
This work was supported by Grants-in-Aid for Science Research
共A兲 共12305055兲 and 共C兲 共13650875兲 from the Ministry of Education,
Culture, Sports, Science and Technology, Japan, and partially by
Precursory Research for Embryonic Science and Technology
共PRESTO兲 organized by the Japan Science and Technology Corporation 共JST兲. The authors thank Dr. Y. Okinaka for his help in writing this manuscript.
Tohoku University assisted in meeting the publication costs of this article.
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E116
Journal of The Electrochemical Society, 150 共2兲 E110-E116 共2003兲
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