Electro-oxidation of Au(1 1 1) in contact with aqueous electrolytes: New insight from in situ vibration spectroscopy

Abstract

We carried out a comprehensive study of Au(1 1 1) oxidation–reduction in the presence of (hydrogen-) sulfate ions on ideally smooth and stepped Au(S)[n(1 1 1)-(1 1 1)] single crystal electrodes using cyclic voltammetry, in situ scanning tunneling microscopy (STM) and vibration spectroscopy, such as surface-enhanced infrared absorption spectroscopy (SEIRAS) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS). Surface structure changes and the role of surface defects in the potential regions of double layer charging and gold oxidation/reduction are discussed based on cyclic voltammetry and in situ STM data. SEIRAS and SHINERS provide complementary information on the chemical nature of adsorbates. In particular, the potential-dependent formation and stability ranges of adsorbed sulfate, hydroxide-species and of gold surface oxide could be resolved in detail. Based on our experimental observations, we proposed new and extended mechanisms of gold surface oxidation and reduction in 1.0 M H2SO4 and 1.0 M Na2SO4.

Full text

Electrochimica
Acta
112 (2013) 853–
863
Contents
lists
available
at
ScienceDirect
Electrochimica
Acta
jo
u
r
n
al
hom
ep
age:
www.elsevier.com/locate/electacta
Electro-oxidation
of
Au(1
1
1)
in
contact
with
aqueous
electrolytes:
New
insight
from
in
situ
vibration
spectroscopy
Ulmas
Zhumaeva,1,
Alexander
V.
Rudneva,b,1,
Jian-Feng
Lia,1,
Akiyoshi
Kuzumea,1,
Thu-Hien
Vua,1,
Thomas
Wandlowskia,∗,1
aDepartment
of
Chemistry
and
Biochemistry,
University
of
Bern,
Freiestrasse
3,
3012
Bern,
Switzerland
bA.N.
Frumkin
Institute
of
Physical
Chemistry
and
Electrochemistry,
Russian
Academy
of
Sciences,
Leninskii
pr.
31,
Moscow
119991,
Russia
a
r
t
i
c
l
e
i
n
f
o
Article
history:
Received
15
November
2012
Received
in
revised
form
4
February
2013
Accepted
21
February
2013
Available online 5 March 2013
Keywords:
Electrochemical
oxidation
Stepped
gold
single
crystals
Au(1
1
1)
STM
SEIRAS
SHINERS
a
b
s
t
r
a
c
t
We
carried
out
a
comprehensive
study
of
Au(1
1
1)
oxidation–reduction
in
the
presence
of
(hydrogen-)
sulfate
ions
on
ideally
smooth
and
stepped
Au(S)[n(1
1
1)-(1
1
1)]
single
crystal
electrodes
using
cyclic
voltammetry,
in
situ
scanning
tunneling
microscopy
(STM)
and
vibration
spectroscopy,
such
as
surface-
enhanced
infrared
absorption
spectroscopy
(SEIRAS)
and
shell-isolated
nanoparticle-enhanced
Raman
spectroscopy
(SHINERS).
Surface
structure
changes
and
the
role
of
surface
defects
in
the
potential
regions
of
double
layer
charging
and
gold
oxidation/reduction
are
discussed
based
on
cyclic
voltammetry
and
in
situ
STM
data.
SEIRAS
and
SHINERS
provide
complementary
information
on
the
chemical
nature
of
adsorbates.
In
particular,
the
potential-dependent
formation
and
stability
ranges
of
adsorbed
sulfate,
hydroxide-species
and
of
gold
surface
oxide
could
be
resolved
in
detail.
Based
on
our
experimental
obser-
vations,
we
proposed
new
and
extended
mechanisms
of
gold
surface
oxidation
and
reduction
in
1.0
M
H2SO4and
1.0
M
Na2SO4.
© 2013 Elsevier Ltd. All rights reserved.
1.
Introduction
Exploring
the
molecular-scale
structure
of
solid/liquid
inter-
faces
comprises
a
key
topic
in
interfacial
electrochemistry
[1–4].
In
particular,
the
detailed
knowledge
on
structure
and
dynamics
of
water
molecules
in
direct
contact
with
electrified
electrode
sur-
faces
is
essential
for
the
further
understanding
of
electrocatalytic
and
other
interfacial
reactions
[3,5,6].
The
adsorption
and
structure
of
water
molecules
at
well-defined
solid/liquid
interfaces
has
been
studied
intensively
during
the
last
two
decades
[7–10].
Various
reports
provide
direct
evidence
for
potential-dependent
orienta-
tion
changes
of
interfacial
water
[9–13].
The
role
of
electrolyte
ions
[14–16],
neutral
(molecular)
adsorbates
as
well
as
nature
and
mor-
phology
of
the
substrate
have
been
addressed
[9].
These
factors
play
a
key
role
in
electrocatalytic
processes,
which
involve
water
and
surface
oxide
as
a
reactants
and/or
spectators.
Prominent
examples
are
the
oxidation
of
formic
acids
and/or
carbon
monoxide
[3,5,6,17].
Osawa
et
al.
pioneered
Surface-enhanced
Infrared
Reflection
Absorption
Spectroscopy
(SEIRAS)
and
its
application
in
electro-
chemical
studies
[18].
Among
others,
this
spectroscopic
approach
∗Corresponding
author
at:
Tel.:
+41
31
631
5384;
fax:
+41
31
631
3994.
E-mail
address:
Thomas.wandlowski@dcb.unibe.ch
(T.
Wandlowski).
URL:
http://wa.dcb.unibe.ch/
(T.
Wandlowski).
1ISE
member.
led
to
a
major
advancement
in
our
current
understanding
of
interfa-
cial
water
and
of
the
structure
of
the
electrochemical
double
layer.
Osawa
et
al.
proposed
three
distinct
types
of
interfacial
water,
based
on
potential-dependent
infrared
experiments
of
0.5
M
HClO4[11]
and
of
0.1
M
H2SO4in
contact
with
gold
electrodes
[19].
Weakly
hydrogen-bonded
water
with
a
hydrogen-end
“down”
orientation
was
reported
at
negative
charge
densities,
i.e.
at
E
<
Epzc,
with
Epzc
being
the
potential
of
zero
charge.
Interfacial
water
at
E
∼
Epzc is
ori-
ented
with
its
dipole
moment
parallel
to
the
surface
as
concluded
from
the
negligible
band
intensity
of
the
water-related
vibration
modes.
Strongly
hydrogen-bonded
water
in
hydrogen-end
“up”
ori-
entation
is
found
in
the
double
layer
region
at
E
>
Epzc.
The
latter
is
modified
by
the
presence
of
specifically
adsorbed
hydrophilic
and/or
hydrophobic
anions
[14,15,19].
As
an
example,
combined
infrared
and
scanning
tunneling
microscopy
experiments
revealed
the
formation
of
a
characteristic
(√3
×√7)R19.1◦adlayer
struc-
ture
of
(hydrogen-)
sulfate
ions
on
Au(1
1
1)
in
contact
with
H2SO4
[20–25].
The
latter
is
thought
to
be
stabilized
either
by
coadsorbed
hydrated
hydronium
ions
[21,25]
or
Zundel
[26]
ions,
rather
than
just
water
molecules.
Water
molecules
decompose
at
sufficiently
high
positive
poten-
tials,
which
leads
to
the
formation
of
adsorbed
oxygen-containing
species
as
the
first
step
of
surface
oxidation
[1,27].
Oxide
formation
on
noble
metals
is
a
rather
complex
process,
which
depends
on
electrode
material
[27–29],
crystallographic
orientation
[30–32],
nature
of
adsorbed
ions
[5,27,33],
pH
[5,27,32,34,35],
and
0013-4686/$
–
see
front
matter ©
2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.electacta.2013.02.105

854 U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863
temperature
[5,36].
These
factors
also
determine
the
catalytic
activ-
ity
of
substrate
in
many
electrochemical
reactions,
which
involve
oxide
species.
Conway
suggested
[27],
based
on
cyclic
voltammetry,
ellip-
sometry,
low
electron
energy
diffraction
and
Auger
electron
spectroscopy
measurements,
that
the
anodic
oxidation
of
noble
metal
surfaces
involves
the
formation
of
two-dimensional
OH/O
arrays.
Place
exchange
between
electrosorbed
OH/O
species
and
metal
surface
adatoms
leads
to
the
formation
of
a
quasi-
two-dimensional
compact
layer,
which
subsequently
grows
to
a
three-dimensional
hydrous
oxide
film
[27].
The
initial
stage
involves
the
competitive
adsorption
of
OH
with
anions,
such
as
ClO4−,
(H)SO4(2)−,
and
Cl−.
The
adsorption
strength
of
the
latter
may
lead
to
the
inhibition
of
the
onset
of
surface
oxide
formation
[30,31,37–40].
In
situ
STM
experiments
revealed
that
the
reduc-
tion
of
the
surface
oxide
on
Au(h
k
l)
[33,41–44]
and
Pt(1
1
1)
[45]
is
accompanied
by
the
formation
of
adislands
and
vacancy
islands.
In
other
words,
the
electrode
surface
is
roughened
at
an
atomic
scale.
Extended
oxidation–reduction
cycles
cause
a
major
restruc-
turing.
Kondo
et
al.
[46]
provided
direct
structure
evidence
of
the
place
exchange
between
adsorbed
OH/O
species
and
adatoms
of
the
Au(1
1
1)
and
Au(1
0
0)
electrodes
in
acidic
electrolyte
based
on
in
situ
surface
X-ray
scattering
(SXS)
measurements.
Existing
experimental
data
and
their
interpretation
suggest
that
gold
surface
oxidation
in
acidic
media
may
proceed
according
to
the
following
mechanism
[27,33]:
AuX−·
nH2O

AuX−·
OH(1−ı)−·
(n
−
1)H2O
+
H+(aq)
+
ı¯
e
(1)
AuX−·
m
OH(1−ı)−+
(m
−
1)
Au
→
mAuOH
+
X−+
m(1
−
ı)¯
e
(2)
AuOH
→
AuO
+
H++¯
e
(3)
AuOH
→
OHAu,
AuO
→
OAu
(4)
The
process
starts
with
the
partial
discharge
of
hydrated
water,
which
originates
from
coadsorbed
anions
X−,
resulting
in
the
for-
mation
of
adsorbed
OH(1−ı)−-type
species
(reaction
(1)).
Next,
the
adsorbed
OH-species
discharge,
accompanied
by
the
desorption
of
anions
X−and
a
place
exchange
process
(reactions
(2)–(4)).
The
proposed
mechanism
is
partially
supported
by
experimen-
tal
observations,
which
are
based
on
photoelectron
spectroscopy
(XPS)
[33],
quartz
crystal
microbalance
(QCM)
[47],
ellipsomet-
ric
[48]
and
SXS
[46]
measurements.
Additional
evidence
was
provided
from
electroreflectance,
surface
plasmon
[48–50]
and
infrared
spectroscopic
experiments
on
Au(1
1
1)
in
combination
with
chronocoulometry
[51]
as
well
as
by
surface
enhanced
Raman
studies
on
rough
gold
electrodes
[52].
However,
the
nature
of
the
adsorbed
oxygen
species,
the
key
species
in
surface
oxidation,
is
not
resolved
at
all.
Possible
species
could
be
adsorbed
OH-
or
O-species
[27,46].
The
present
paper
aims
to
address
the
surface
oxidation
of
Au(1
1
1)
in
(hydrogen-)
sulfate
electrolytes
with
a
particular
focus
on
exploring
the
nature
of
the
intermediate
oxygen-
containing
adsorbed
species.
Our
experimental
strategy
is
based
on
the
combination
of
single
crystal
electrochemical
measure-
ments,
in
situ
STM
and
vibration
spectroscopies
to
characterize
the
structure
of
the
Au(1
1
1)/electrolyte
interface
under
reaction
conditions.
In
particular,
we
employed
in
situ
SEIRAS
[53]
and
shell-isolated
nanoparticle-enhanced
Raman
spectroscopy
(SHIN-
ERS)
[54]
as
an
unique
experimental
approach
to
identify,
for
the
first
time,
the
chemical
nature
of
intermediate
species
dur-
ing
surface
oxidation–reduction
on
well-defined,
ideally
smooth
and
stepped
single
crystal
Au(1
1
1)
electrodes
in
aqueous
elec-
trolytes.
We
report
experimental
data,
as
obtained
in
acidic
and
neutral
solution,
to
address
the
influence
of
pH
on
the
surface
oxidation–reduction
processes.
2.
Experimental
2.1.
Chemicals
and
glassware
Suprapur
H2SO4(96%),
Na2SO4(anhydrous)
and
hydrochloric
acid
(30%)
were
purchased
from
Merck.
D2O
(99.9
atom
%
D)
and
sodium
silicate
solution
(27%
SiO2)
were
obtained
from
Aldrich.
99.99%
HAuCl4·3H2O,
3-aminopropyl-trimethoxysilane
(97%)
and
sodium
citrate
(99%)
were
used
as
received
from
Alfa
Aesar.
All
solutions
were
prepared
with
ultrapure
water
as
produced
with
a
Milli-Q
system
(18
M
cm,
3
ppb
TOC).
The
electrolytes
were
deaerated
by
Ar
(5N,
Carbagas)
prior
to
and
during
each
experi-
ment.
All
measurements
were
carried
out
at
room
temperature.
Glassware
and
Teflon
parts
were
cleaned
either
in
concentrated
hot
HNO3(cyclic
voltammetry,
SEIRAS,
SHINERS)
or
by
soaking
in
piranha
solution
(STM),
followed
by
several
heating–rinsing
cycles
with
Milli-Q
water.
2.2.
Cyclic
voltammetry
The
voltammetric
measurements
were
conducted
in
a
three
compartment
glass
cell
using
an
Autolab
PGSTAT30.
An
ideally
smooth
Au(1
1
1)
and
four
stepped
Au(S)[n(1
1
1)-
(1
1
1)]
=
Au(n
n
n
−
2)
single
crystal
electrodes,
n
=
55,
27,
15
and
7.5,
were
custom-made
cylinders
(U.
Linke,
Research
Center
Jülich)
of
4
mm
height
and
4
mm
diameter.
n
is
the
number
of
terrace
atoms,
which
represent
the
miscut
to
the
nominal
(1
1
1)
orientation
respective
terrace
width
and
step
density.
The
gold
single
crystals
were
annealed
in
a
butane
flame
to
red
color
before
each
measurement,
and
cooled
down
in
a
high-purity
argon
atmosphere
(5N,
Carbagas).
Contact
with
the
electrolyte
was
established
under
potential
control
in
a
hanging
meniscus
configuration.
Cyclic
voltammograms
were
measured
using
a
platinum
counter
electrode
and
a
Hg/Hg2SO4reference
electrode.
All
potentials
in
this
paper
are
quoted
with
respect
to
a
Ag/AgCl
(saturated
KCl)
reference
electrode.
2.3.
STM
In
situ
STM
measurements
were
carried
out
with
a
Molecular
Imaging
Pico-SPM
system
using
disc-shaped
Au(1
1
1)
electrodes
of
10
mm
diameter
and
2
mm
height.
Tungsten
STM
tips
were
prepared
by
electrochemical
etching
in
2
M
KOH
solution
and
subsequent
coating
with
polyethylene.
Pt
wires
were
utilized
as
counter
and
quasi-reference
electrodes.
The
Au(1
1
1)
electrode
was
annealed
in
a
hydrogen
flame
to
red
color
for
5
min
and
then
slowly
cooled
down
under
an
Ar
atmosphere
before
mounting
into
a
lab-
build
Kel-F
cell.
The
electrode
was
brought
in
contact
with
the
electrolyte
under
strict
potential
control.
The
STM
images
were
recorded
in
constant
current
mode
with
typical
tunneling
currents
ranging
between
70
and
200
pA.
2.4.
SEIRAS
The
SEIRAS
experiments
were
conducted
in
an
ATR-
configuration
using
a
lab-made
Teflon
cell,
which
was
equipped
with
a
Au
wire
and
an
oxidized
Au/AuOxwire
as
counter
and
reference
electrodes,
respectively.
The
working
electrode
was
a
quasi-single
crystalline
Au(1
1
1-20
nm)
thin
film
as
prepared
by
electron-beam
evaporation
on
the
flat
side
of
a
highly
resis-
tive
(>6000

cm)
Si
hemisphere
(25
mm
diameter)
prism.
The
base
pressure
and
deposition
rate
were
<
1.0
×
10−6mbar
and
∼0.01
nm
s−1,
respectively.
The
infrared
spectra
were
acquired
with
a
Bruker
Vertex
80
v
Fourier
Transform
Spectrometer
equipped
with
a
liquid-nitrogen-
cooled
MCT
detector,
using
a
spectral
resolution
of
4
cm−1.

U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863 855
P-polarized
infrared
light
from
a
Globar
source
was
collected
on
the
detector
after
total
reflection
at
the
electrode/electrolyte
inter-
face
(incident
angle
>
60◦).
16
interferograms
were
scanned
and
averaged
during
a
simultaneous
potential
sweep
with
10
mV
s−1
for
each
spectrum.
The
acquisition
time
per
spectrum
was
about
5.4
s,
and
thus
each
spectrum
represents
the
average
IR
response
of
a
potential
interval
of
54
mV.
The
spectra
are
plotted
in
absorbance
units
defined
as
A
=
−log(I/Io),
with
I
and
Iobeing
the
intensities
of
the
reflected
light
at
the
sample
and
at
the
reference
potentials,
respectively.
2.5.
SHINERS
The
SHINERS
experiments
were
carried
out
with
Au(55
nm)@SiO2(3
nm)
core-shell
gold
nanoparticles
as
plas-
monic
antennas.
These
particles
were
prepared
by
the
standard
citrate
reduction
method
[55]
in
combination
with
a
second
step
leading
to
the
formation
of
a
pinhole-free
ca.
3
nm
thick
SiO2shell.
Details
of
the
procedure
were
reported
in
our
previous
commu-
nications
[54,56,57].
Subsequently,
a
submonolayer
of
Au@SiO2
nanoparticles
was
casted
on
a
freshly
flame-annealed
Au(1
1
1)
single-crystal
half-bead
electrode
and
dried
under
vacuum.
The
coverage
was
estimated
from
scanning
electron
microscopy
(SEM)
and
atomic
force
microscopy
(AFM)
as
ranging
between
0.2
and
0.3.
The
modified
electrodes
were
then
mounted
in
a
lab-made
Kel-F
cell
equipped
with
a
Pt
counter
electrode
and
a
Ag/AgCl
reference
electrode.
Contact
with
the
deoxygenated
electrolyte
was
established
under
potential
control.
The
spectra
were
recorded
with
a
LabRAM
HR800
confo-
cal
Raman
spectrometer
(Horiba
Jobin
Yvon).
The
excitation
wavelength
was
632.8
nm
(He–Ne
laser)
with
∼1
mW
power
on
the
sample.
A
50×
magnification
long-working-distance
(8
mm)
objective
was
used
to
focus
the
laser
onto
the
sample
and
to
collect
the
scattered
light
in
a
backscattering
geometry.
3.
Results
3.1.
Au(1
1
1)/H2SO4:
cyclic
voltammetry
and
in
situ
STM
Fig.
1
shows
typical
cyclic
voltammograms
(CVs)
of
a
freshly
flame-annealed
Au(1
1
1)
electrode
in
50
mM
H2SO4recorded
with
a
scan
rate
of
10
mV
s−1in
the
double
layer
region
(−0.30
V
≤
E
≤
1.00
V)
as
well
as
up
to
the
potential
region
of
surface
oxidation
(−0.25
V
≤
E
≤
1.40
V).
Characteristic
fea-
tures
in
the
CVs
are
labeled
as
P1/P1,
P2/P2,
P3/P3and
P4,
which
separate
distinct
potential
regions.
The
macroscopic
electrochemical
data
are
combined
with
in
situ
STM
images
to
illus-
trate
substrate
surface
structures
and
their
potential-dependent
changes.
Each
experiment
started
with
a
flame-annealed,
reconstructed
Au(1
1
1)-(p
×√3)
electrode,
which
was
brought
in
contact
with
the
electrolyte
at
E
=
−0.20
V
(Fig.
1a).
Scanning
of
the
electrode
potential
toward
more
positive
potentials
leads
to
the
lifting
of
the
thermally
induced
reconstruction,
Au(1
1
1)-(p
×√3)
→
Au(1
1
1)-
(1
×
1)
[58],
which
is
represented
in
the
voltammogram
by
the
current
peak
P1.
The
broad
feature
between
E
=
0.40
V
and
E
=
0.80
V
is
attributed
to
the
disordered
adsorption
of
(hydrogen-)
sulfate
anions
on
Au(1
1
1)-(1
×
1)
(Fig.
1b,
left
part).
A
disorder-order
phase
transition
takes
place
within
the
sulfate
adlayer
upon
reach-
ing
a
critical
coverage
of
0.20.
This
phase
transition
is
represented
Fig.
1.
Cyclic
voltammograms
of
an
Au(1
1
1)
single
crystal
electrode
in
50
mM
H2SO4in
the
double
layer
region
and
up
to
the
potential
region
of
surface
oxidation,
scan
rate
10
mV
s−1.
The
STM-images
illustrate
characteristic
surface
structures:
(a)
thermally
reconstructed
Au(1
1
1)-(p
×√3)
surface
recorded
after
flame-annealing
upon
immersion
at
−0.20
V,
(b)
atomic
resolution
image
illustrating
the
transition
of
the
disordered
(hydrogen-)
sulfate
layer
on
Au(1
1
1)-(1
×
1)
into
the
ordered
(√3
×√7)
layer
upon
scanning
the
potential
from
0.80
V
to
0.90
V,
(c)
transition
of
the
ordered
sulfate
adlayer
up
to
the
onset
of
terrace
oxidation
upon
scanning
the
potential
from
1.10
V
to
1.30
V,
(d)
surface
oxide
layer
at
1.40
V,
(e)
Au(1
1
1)-(1
×
1)
just
after
passing
the
reduction
peak
as
monitored
at
0.60
V
and
(f)
potential-induced
Au(1
1
1)-(p
×√3)
reconstruction
of
the
gold
surface
recorded
at
0.00
V.
Note
that
the
fast
scan
direction
in
the
STM
images
shown
in
panels
(b)
and
(c)
is
vertical
(indicated
by
the
white
arrows),
and
not
horizontal
as
in
all
the
other
panels.

856 U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863
Table
1
Charge
densities
obtained
by
integration
of
the
corresponding
characteristic
current
peaks
of
the
cyclic
voltammograms
as
plotted
in
Figs.
2
and
6.
See
the
text
for
further
details.
Electrode
1
M
H2SO41
M
Na2SO4
qoxP3P4 (␮C
cm−2)
qredP3(␮C
cm−2)
qoxP3P4/qredP3qoxP3P4 (␮C
cm−2)
qredP3(␮C
cm−2)
qoxP3P4/qredP3
Au(S)[7.5(1
1
1)-(1
1
1)]
572
554
1.03
530
525
1.01
Au(S)[15(1
1
1)-(1
1
1)] 603
580
1.04
554
568
0.98
Au(S)[27(1
1
1)-(1
1
1)]
620
598
1.04
573
584
0.98
Au(S)[55(1
1
1)-(1
1
1)]
639
612
1.04
605
576
1.05
Au(1
1
1),
n
=
∞
619
625
0.99
617
592
1.04
by
the
sharp
pair
of
current
peaks
P2/P2and
leads
to
a
commen-
surate
(√3
×√7)R19.1◦adlayer
at
E
>
0.85
V
(Fig.
1b,
right
part)
[20–23].
We
observed
a
small
current
increase
in
the
voltammo-
gram
around
P4
at
E
=
1.10
V
followed
by
the
main
oxidation
peak
P3
at
E
=
1.33
V.
The
corresponding
reduction
peak
P3develops
during
the
negative
potential
sweep
with
a
maximum
at
E
=
0.92
V.
The
sur-
face
oxidation
appears
to
start
at
E
=
1.10
V.
However,
the
ordered
sulfate
adlayer
remains
stable
on
wide,
(1
1
1)-oriented
terraces
until
the
main
oxidation
peak
P3
appears
(Fig.
1c).
This
result
demonstrates
that
P3
represents
the
oxidation
of
(1
1
1)
terraces,
while
P4
might
be
ascribed
to
the
onset
of
oxidation
on
defect
sites,
such
as
steps
or
(vacancy)
islands.
The
presence
of
such
defects,
although
in
small
quantities,
cannot
be
completely
avoided
on
real
single
crystal
surfaces.
Fig.
1c
illustrates
further
that
the
dissolu-
tion/desorption
of
the
(√3
×√7)R19.1◦sulfate
adlayer
is
followed
by
an
intermediate
disordered
adlayer
pattern,
and
subsequently
by
the
formation
of
2–3
nm
in
size
gold
oxide
clusters.
The
surface
oxide
phase
is
characterized
by
a
rather
even
distribution
of
equally
sized
clusters
(Fig.
1d).
However,
their
exact
morphology
is
difficult
to
determine
because
of
a
severe
superposition
of
the
tunneling
current
with
the
Faraday
current
of
the
oxidation
process
[42,43].
Passing
the
current
peak
P3marks
the
disintegration
of
the
surface
oxide,
and
is
followed
by
the
formation
of
(i)
small
gold
islands,
which
dissolve
rather
fast,
as
well
as
of
(ii)
vacancy
islands
(Fig.
1e)
[59].
The
latter
undergo
Ostwald-ripening
and
electrochemical
annealing
processes
upon
cycling
of
the
electrode
potential
in
the
double
layer
region.
The
charge
density
of
the
reduction
process
as
represented
by
P3is
estimated
to
qredP3=
625
␮C
cm−2(Table
1).
The
ratio
of
the
charge
densities
qoxP3P4/qredP3amounts
to
approx-
imately
1.
Polarization
of
the
electrode
at
more
negative
potentials,
such
as
E
<
0.20
V,
leads
finally
to
the
formation
of
an
electrochem-
ically
reconstructed
Au(1
1
1)-(p
×√3)
surface
(Fig.
1f).
The
latter
shows
small
domains
of
the
(p
×√3)
patches
with
a
higher
defect
density
as
compared
to
the
initial,
thermally
reconstructed
elec-
trode
surface.
This
pattern
reflects
the
active
role
of
surface
defects
as
nucleation
centers.
In
particular
gold
and
vacancy
islands
were
created
during
the
preceding
positive
half-cycle
upon
lifting
the
thermal
surface
reconstruction
as
well
as
upon
the
surface
oxida-
tion/reduction
process.
We
also
investigated
the
voltammetric
profiles
of
a
series
of
Au(1
1
1)
electrodes
with
well-defined
(1
1
1)-oriented
steps
to
unravel
the
role
of
terraces
and
defect
sites.
Fig.
2
shows,
as
an
example,
five
cyclic
voltammograms
of
flame-annealed
Au(S)[n(1
1
1)-(1
1
1)]
electrodes
with
equally
spaced
(1
1
1)-
oriented
monatomic
steps
[23]
in
1.0
M
H2SO4recorded
after
immersion
at
E
=
−0.10
V.
The
curves
were
acquired
for
electrodes
with
n
=
7.5
(alternating
terrace
width
of
7
and
8
atoms),
15,
27,
55
and
∞
(“ideally”
smooth
Au(1
1
1)
electrode).
Fig.
2
demon-
strates
that
the
two
peaks
P3
and
P4,
which
are
related
to
the
surface
oxidation,
decrease
and
increase
with
decreasing
terrace
width
n,
respectively.
On
the
other
hand,
the
position
of
both
fea-
tures
is
rather
independent
on
step
density.
We
conclude
that
P4
indeed
represents
the
surface
oxidation
of
step
(defect)
sites,
and
P3
is
attributed
to
the
oxidation
of
terrace
sites.
The
former
is
energetically
favored
by
approximately
0.20
eV.
We
also
found
that
the
oxidation
charges
qoxP3P4 and
the
reduction
charges
qredP3
of
the
gold
surface
oxide
are
decreasing
up
to
8%
with
increasing
step
density.
However,
the
corresponding
ratio
of
charge
densities
qoxP3P4/qredP3is
rather
constant
and
approaches
a
value
of
1
for
all
electrodes
studied
(Table
1).
3.2.
Au(1
1
1)/H2SO4:
in
situ
SEIRAS
and
SHINERS
In
an
attempt
to
explore
chemical
nature
and
structure
of
adsorbates
in
the
double
layer
region
as
well
as
during
surface
oxidation–reduction
processes,
we
performed
a
series
of
in
situ
vibration
spectroscopic
experiments
employing
well-defined
gold
surfaces.
Fig.
3a
shows
selected
SEIRA
spectra
of
quasi-single
crys-
talline
Au(1
1
1-20
nm)
thin
film
electrodes
in
contact
with
1.0
M
H2SO4.
The
spectra
were
recorded
continuously
during
a
poten-
tial
sweep
from
−0.10
to
1.40
V
and
back,
at
a
rate
of
10
mV
s−1.
A
set
of
single-beam
spectra,
acquired
at
E
=
−0.10
V,
i.e.
at
a
negatively
charged
surface
and
in
the
absence
of
adsorbed
sul-
fate
ions
[60],
was
used
as
the
reference
spectrum.
The
spectra
of
Fig.
3a
show
well-resolved
bands,
which
are
assigned
to
the
OH
stretching
(3600–3100
cm−1)
and
the
HOH
bending
modes
(1660–1600
cm−1)
of
interfacial
water,
OH and
ıHOH,
as
well
as
to
the
SO
stretching
mode
SO of
adsorbed
(hydrogen-)
sulfate
ions
[11,26,61,62].
The
potential-dependence
of
OH and
ıHOH indi-
cates
the
preference
of
weakly
hydrogen-bonded
water
at
E
<
Epzc
and
of
strongly
hydrogen-bonded
water
at
E
>
Epzc,
even
in
the
potential
region
of
surface
oxidation
at
E
>
1.20
V.
However,
since
SEIRAS
is
not
an
“absolute”
method,
but
requires
difference
spec-
tra,
the
specific
shapes
of
the
bands
attributed
to
interfacial
water
depend
strongly
on
the
chosen
reference
potential.
In
the
present
study
we
have
selected
the
spectrum
at
E
=
−0.10
V
as
reference
to
gain,
in
particular,
information
on
the
specific
adsorption
of
(hydrogen-)
sulfate
ions
as
they
are
competing
with
adsorbed
Fig.
2.
First
scan
cyclic
voltammograms
of
a
series
of
stepped
gold
single
crystal
elec-
trodes,
Au(S)[n(1
1
1)-(1
1
1)],
recorded
in
1.0
M
H2SO4with
a
scan
rate
of
10
mV
s−1,
n
represents
the
number
of
terrace
atoms.

U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863 857
Fig.
3.
(a)
Selected
SEIRA
spectra
for
a
Au(1
1
1-20
nm)
thin
film
electrode
in
contact
with
1.0
M
H2SO4as
a
function
of
the
electrode
potential.
The
reference
spec-
trum
was
acquired
at
−0.10
V.
(b)
Potential
dependence
of
the
SO band
position
as
obtained
from
SEIRA
spectra
in
1.0
M
H2SO4(䊉)
and
in
1.0
M
Na2SO4().
The
arrows
indicate
the
direction
of
potential
sweep.
oxygen-containing
species
during
the
surface
oxidation.
The
shape
and
potential
dependence
of
the
water-related
features
in
the
potential
region
of
an
ideally
polarized
gold
electrode
will
be
there-
fore
in
the
context
of
this
paper
just
at
the
above
qualitative
level.
For
more
details
we
refer
to
our
previous
experiments,
which
report
a
detailed
discussion
of
the
water
bands
in
dependence
on
the
applied
electrode
potential
[26].
At
this
stage
we
also
like
to
note,
that
the
shape
of
the
water
bands
OH and
ıHOH depends
not
only
on
the
reference
potential
but
also
on
the
morphology
of
the
gold
substrate.
Our
experiments
always
refer
to
quasi-single
crystalline
Au(1
1
1-20
nm)
thin
films
with
a
rather
low
density
of
surface
defects.
This
preparation
leads
to
distinct
differences
in
the
shape
of
the
spectra
in
the
OH and
ıHOH regions,
as
compared
to
recent
work
by
Garcia-Araez
et
al.
[61,62].
Fig.
4
illustrates
the
evolution
of
the
(hydrogen-)
sulfate
band
SO in
the
entire
potential
region
during
the
positive
(full
circles)
and
the
negative
(open
circles)
potential
sweeps.
The
integrated
intensity
of
SO is
proportional
to
the
coverage
of
the
sulfate
species
[19,26].
The
band
starts
growing
at
E
=
0.20
V,
which
is
the
onset
of
the
sulfate
adsorption
in
the
CV
[60],
and
increases
until
E
≈
0.85
V
(Fig.
4a).
Simultaneously,
the
band
position
shifts
linearly
toward
higher
wavenumbers
with
more
positive
poten-
tials
from
1145
cm−1to
1196
cm−1(Fig.
3b).
The
shift
is
proposed
to
be
due
to
electron
back-donation
or
Stark
tuning
mechanisms
[63].
The
lateral
interactions
between
adsorbates,
i.e.
(hydrogen-)
sulfate
ions
and
water
molecules,
might
also
cause
a
shift
of
the
SO mode
upon
potential
change,
because
the
(hydrogen-)
sulfate
coverage
is
potential
dependent
(Fig.
4).
However,
the
Fig.
4.
Potential
dependence
of
the
normalized
integrated
intensities
of
SO (䊉,
)
and
AuO (,
)
as
obtained
from
SEIRA
and
SHINER
spectra
during
(a)
the
positive
(filled
symbols)
and
(b)
the
subsequent
negative
potential
(open
symbols)
sweep.
The
bands
are
normalized
to
their
maximum
values.
Cyclic
voltammograms
in
the
double
layer
region
and
up
to
surface
oxidation
are
also
plotted
for
guidance.
slope
SO/E
=
86
cm−1V−1is
constant
in
the
whole
range
of
(hydrogen-)
sulfate
adsorption
(Fig.
3b).
This
result
favors
clearly
an
interpretation
based
on
Stark
tuning.
Furthermore,
this
poten-
tial
region
coincides
with
the
broad
peak
between
P1
and
P2
in
the
corresponding
cyclic
voltammogram
(Fig.
4a)
and
is
attributed
to
the
disordered
adsorption
of
negatively
charged
(hydrogen-)
sul-
fate
ions
(Fig.
1b).
The
(hydrogen-)
sulfate
coverage
levels
off
and
reaches
a
plateau
at
E
>
0.85
V,
which
coincides
with
the
formation
of
the
commensurate
(√3
×√7)R19.1◦(hydrogen-)
sulfate
adlayer.
The
integrated
intensity
of
SO (and
consequently
also
the
corre-
sponding
surface
coverage)
remains
rather
constant
until
1.10
V,
the
onset
potential
of
gold
surface
oxidation
at
step
sites,
and
sub-
sequently
decreases.
The
(hydrogen-)
sulfate
band
disappears
at
E
≥
1.40
V,
which
correlates
with
the
completion
of
surface
oxida-
tion
of
the
(1
1
1)
terrace
sites
as
indicated
by
passing
the
main
current
peak
P3.
Changing
the
direction
of
the
potential
scan
toward
negative
values
leads
to
the
reappearance
of
SO at
E
≈
1.00
V
(Fig.
4b),
which
coincides
with
the
onset
of
P3,
the
cathodic
current
peak
of
gold
surface
oxide
reduction.
A
slight
hysteresis
is
found
in
the
potential
dependence
of
the
integrated
intensities
(Fig.
4)
and
of
the
band
positions
(Fig.
3b)
of
SO during
the
negative
potential
scan
as
compared
to
the
positive
scan
in
0.0
V
<
E
≤
0.92
V.
The
strong
adsorption
of
the
Si
prism
does
not
allow
observing
the
spectral
range
below
1000
cm−1,
from
which
structure
infor-
mation
of
the
oxide
layer
could
be
extracted.
Furthermore,
shape
and
appearance
of
the
OH and
ıHOH-related
SEIRAS
bands
depend
strongly
on
the
chosen
reference
signal,
which
makes
an
unam-
biguous
structure
interpretation
rather
complicated.

858 U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863
Fig.
5.
(a)
Selected
“steady-state”
SHINER
spectra
of
a
Au(1
1
1)
electrode
in
1.0
M
H2SO4plotted
as
a
function
of
the
electrode
potential.
(b)
Potential
dependence
of
the
AuO position
as
obtained
from
SHINERS
for
1.0
H2SO4(circles)
and
1.0
M
Na2SO4
(squares)
during
a
positive
(full
symbols)
and
the
subsequent
negative
(open
sym-
bols)
scans.
The
inset
demonstrates
the
band
positions
of
the
two
bending
modes,
ıAuOH and
ıAuOD,
as
a
function
of
the
applied
electrode
potential.
The
directions
of
the
potential
excursions
are
indicated
by
the
arrows.
In
an
attempt
to
resolve
this
problem
we
applied
in
situ
Raman
spectroscopy
in
the
so-called
SHINERS-mode
employing
a
well-
defined
Au(1
1
1)
single
crystal
in
contact
with
1.0
M
H2SO4.
For
details
on
the
method
we
refer
to
our
previous
publication
[57].
We
emphasize
that
the
integrity
of
the
electrochemical
response
of
the
Au(1
1
1)
electrode
in
the
presence
of
a
submonolayer
of
Au(55
nm)@SiO2(3
nm)
nanoparticles
is
guaranteed.
A
comprehen-
sive
study
will
be
reported
elsewhere
[64].
Fig.
5a
shows
typical
potential-dependent
SHINER
spectra
of
a
Au(1
1
1)
electrode
in
contact
with
1.0
M
H2SO4.
The
potential
cycle
started
from
E
=
0.70
V
in
the
double
layer
region
and
con-
tinued
to
1.40
V,
where
the
gold
surface
is
completely
oxidized.
The
subsequent
negative
scan
was
carried
out
until
E
=
0.70
V,
i.e.
past
the
reduction
peak
P3back
into
the
double
layer
region.
The
potential
was
advanced
in
increments
of
0.10
V.
The
spectra
were
acquired
after
a
stabilization
time
of
∼30
s.
The
recording
time
at
each
potential
was
∼30
s.
No
vibration
bands,
which
are
related
to
the
surface
species,
were
found
in
the
double
layer
region
in
300
cm−1<

<
900
cm−1.
The
two
bands
at
491
cm−1and
at
616
cm−1originate
from
hydrogen
sulfate
anions
of
the
bulk
elec-
trolyte
[65].
A
broad
band
with
a
center
around
590
cm−1appears
at
E
>
1.10
V.
This
potential
corresponds
to
the
onset
of
step-site
oxida-
tion
on
Au(1
1
1).
Therefore,
the
observed
Raman
signature
is
clearly
related
to
the
oxide
species
on
the
single
crystal
Au(1
1
1)
surface.
It
develops
further
during
the
positive
potential
excursion,
pass-
ing
the
terrace
oxidation
peak
P3
with
AuO/E
∼
20
cm−1V−1
(Fig.
5b)
and
disappears
during
the
subsequent
return
scan
toward
more
negative
potentials
at
E
<
EP3,
i.e.
after
the
decomposi-
tion
of
the
gold
surface
oxide
and
reestablishment
of
an
ideally
polarized
electrode
in
the
double
layer
region.
Interestingly,
the
band
position
shows
a
none-uniform
potential
dependence
dur-
ing
the
negative
potential
scan.
A
small
potential-dependent
peak
shift
of
AuO ∼
20
cm−1V−1in
1.20
V
≤
E
<
1.40
V
is
followed
by
a
larger
shift
of
∼95
cm−1V−1in
0.80
V
≤
E
<
1.20
V
(Fig.
5b).
This
non-uniform
potential
dependence
of
the
Raman
band
suggests
structure
changes
within
the
surface
oxide
layer
during
the
reduc-
tion
process.
The
comparison
with
SERS
experiments
on
rough
gold
electrodes
leads
to
the
assignment
of
the
observed
Raman
band
to
a
gold
oxide
stretching
mode,
either
AuOH and/or
AuO
[52,66].
Fig.
4
shows
the
integrated
intensity
of
this
Raman
band
in
dependence
on
the
applied
electrode
potential
for
the
positive
(full
squares)
as
well
as
for
the
negative
(open
squares)
scan
direc-
tions.
The
integrated
intensity
of
AuO increases
from
E
>
1.10
V
until
the
onset
of
terrace
oxidation
at
E
≈
1.30
V,
and
then
decreases
up
to
the
turning
point
at
E
=
1.40
V.
During
the
subsequent
negative
potential
excursion
from
1.40
V
to
1.00
V
we
observe
an
increase
in
intensity
of
AuO,
followed
by
a
maximum
in
0.90
V
<
E
<
1.00
V
and
an
abrupt
decrease
until
the
band
disappears
at
E
≤
0.70
V.
This
evolution
of
the
integrated
Raman
intensity
during
the
oxida-
tion
and
reduction
of
the
gold
surface
depends
on
several
factors.
The
integrated
intensity
increases
proportionally
to
the
generated
amount
of
the
gold
oxide.
However,
the
oxide
formation
decreases
simultaneously
the
reflectivity
of
the
Au
surface
[47,48]
and,
as
a
consequence,
quenches
the
coupling
of
the
plasmonic
antennas
Au(55
nm)@SiO2(3
nm)
with
the
gold
surface.
This
phenomenon
causes
the
decrease
of
the
integrated
Raman
intensity
of
AuO in
1.30
V
<
E
<
1.40
V
during
the
surface
oxidation
(Fig.
4a)
as
well
as
its
increase
from
E
=
1.40
V
to
E
=
1.00
V
during
the
reduction
pro-
cess
(Fig.
4b).
Interestingly,
even
a
small
amount
of
oxide
at
the
onset
potential
of
surface
reduction,
E
≈
1.10
V,
restores
the
cou-
pled
enhancement,
and
the
maximum
intensity
of
the
AuO Raman
band
is
observed
again.
3.3.
Au(1
1
1)/Na2SO4:
cyclic
voltammetry
In
an
attempt
to
generalize
our
observations
in
acidic
media
we
also
explored
the
oxidation/reduction
of
single
crystal
Au(h
k
l)
electrodes
in
1.0
M
Na2SO4,
a
neutral
electrolyte
with
pH
∼
7.5.
The
double
layer
voltammogram
and
the
corresponding
STM
images
of
the
substrate
surface
are
rather
similar
as
compared
to
the
data
in
1.0
M
H2SO4(Fig.
1),
except
that
the
onset
of
hydrogen
evolution
is
shifted
toward
more
negative
potentials
and
no
ordered
sulfate
layer
is
found
[25].
Fig.
6
displays
a
series
of
five
cyclic
voltammograms
of
Au(S)[n(1
1
1)–(1
1
1)]
electrodes,
which
were
recorded
in
1
M
Na2SO4.
Each
experiment
started
with
a
flame-annealed
electrode
at
E
=
−0.35
V
in
a
hanging
meniscus
configuration.
The
current
peaks
labeled
as
P1
and
P1represent
the
lifting
and
reforma-
tion
of
the
Au(1
1
1)-(p
×√3)
surface
reconstruction.
These
features
are
most
pronounced
for
the
ideally
smooth
gold
single
crystal
electrode.
Surface
oxidation
starts
at
step
sites
around
E
=
0.80
V
as
represented
by
P4.
The
corresponding
current
increases
with
increasing
step
density
from
Au(1
1
1)
(n
=
∞)
to
Au(15
15
11)
(n
=
7.5).
Terrace
oxidation
follows
in
0.90
V
<
E
≤
1.20
V
as
indicated
by
the
broad
main
current
peak
P3.
The
total
charge
density
of
gold
surface
oxidation,
qoxP3P4,
decreases
with
increasing
step
density
slightly
more
as
compared
to
experiments
in
1.0
M
H2SO4(Table
1).
We
also
notice
that
the
potentials
of
surface
oxidation
of
(1
1
1)
step
sites
as
well
as
of
the
(1
1
1)
terrace
sites
are
about
0.30
V
more
negative
than
in
1.0
M
H2SO4(Figs.
2
and
6).
Two
broad,
but
well-separated
reduction
peaks
P3aand
P3b
were
observed
in
the
voltammograms
during
the
negative
poten-
tial
sweep
with
maxima
at
0.70
and
0.40
V.
Reduction
of
the
gold

U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863 859
Fig.
6.
First
scan
cyclic
voltammograms
of
a
series
of
stepped
gold
single
crys-
tal
electrodes,
Au(S)[n(1
1
1)-(1
1
1)],
recorded
in
1.0
M
Na2SO4with
a
scan
rate
of
10
mV
s−1,
n
represents
the
width
of
the
terraces.
surface
oxide
appears
to
be
completed
at
E
<
0.20
V.
The
total
charge
density
qP3,
obtained
by
integration
of
the
current
density
dur-
ing
the
negative
half
cycle
in
0.20
V
≤
E
≤
0.90
V,
decreases
slightly
with
increasing
step
density.
The
ratio
qoxP3P4/qredP3is
rather
con-
stant.
However,
the
total
charge
densities
qoxP3P4 and
qredP3are
slightly
smaller
for
experiments
in
1.0
M
Na2SO4as
compared
to
1.0
M
H2SO4(c.f.
Table
1).
The
current
peaks
P3
and
P3a/P3bfor
the
neutral
solution
are
much
broader.
This
may
reflect
either
a
slow
interfacial
kinetics
and/or
a
less
uniform
surface
oxide
structure.
Hamelin
suggested
that
the
reduction
peak
P3acould
be
attributed
to
local
changes
of
the
solution
pH
[37].
She
demonstrated
that
P3a
decreases
significantly
upon
stirring
the
solution.
This
observation
might
reflect
a
more
uniform
surface
pH.
A
detailed
investigation
of
this
effect
is
in
progress
in
our
group
[67].
The
current
study
is
con-
fined
to
a
quiescent
electrolyte
to
guarantee
comparable
conditions
between
the
voltammetric
and
spectroscopic
investigations.
3.4.
Au(1
1
1)/Na2SO4:
in
situ
SEIRAS
and
SHINERS
Fig.
7
displays
selected
SEIRA
spectra
of
a
Au(1
1
1-20
nm)
thin
film
electrode
in
1.0
M
Na2SO4recorded
during
a
positive
potential
sweep
from
E
=
−0.35
V
to
E
=
1.25
V
with
a
scan
rate
of
10
mV
s−1.
The
reference
spectrum
was
acquired
at
−0.35
V.
All
other
acqui-
sition
parameters
are
identical
to
the
measurements
in
1.0
M
H2SO4(Section
3.2).
The
bands
related
to
the
water
stretching
and
bending
modes,
OH and
ıHOH,
are
observed
in
3640–3000
and
1660–1600
cm−1,
respectively.
Similar
to
findings
in
our
pre-
vious
paper
[26],
we
observed
slightly
less
pronounced
spectral
Fig.
7.
Selected
SEIRA
spectra
for
a
Au(1
1
1-20
nm)
thin
film
electrode
in
contact
with
1.0
M
Na2SO4as
a
function
of
the
electrode
potential
during
a
positive
potential
scan.
The
reference
spectrum
was
acquired
at
−0.35
V.
Fig.
8.
Potential
dependence
of
the
normalized
integrated
intensities
of
SO (䊉,
),
AuO (,
)
and
ıAuOH (,
)
as
obtained
from
SEIRA
and
SHINER
spectra
during
(a)
a
positive
(full
symbols)
and
(b)
the
subsequent
negative
(open
symbols)
potential
sweeps.
The
bands
are
normalized
to
their
maximum
values.
The
cyclic
voltam-
mograms
in
the
double
layer
region
and
up
to
surface
oxidation
are
also
plotted
for
guidance.
signatures
of
strongly
hydrogen-bonded
interfacial
water
in
neutral
electrolyte
at
positive
charge
densities
(E

0.20
V)
in
the
dou-
ble
layer
region
and
in
the
region
of
surface
oxidation/reduction.
The
band
in
1140
cm−1<

<
1220
cm−1is
assigned
to
the
sulfate
stretching
mode
SO [19,21,26,68].
The
position
of
this
band
shifts
linearly
toward
higher
wavenumbers
with
SO/E
=
88
cm−1V−1
in
0.30
V
<
E
≤
1.25
V
(Fig.
3b).
Fig.
8
displays
the
integrated
inten-
sity
of
SO in
function
of
the
applied
electrode
potential.
The
adsorption
of
sulfate
ions
starts
during
a
positive
potential
scan
at
E
=
0.20
V
(Fig.
8a).
The
corresponding
integrated
intensity
increases
monotonously
until
a
plateau
is
reached
at
E
=
0.70
V,
and
subse-
quently
decreases
gradually
from
the
onset
of
step-site
oxidation
at
E
=
0.80
V
(current
feature
P4
in
the
voltammograms
of
Fig.
6)
up
to
E
=
1.20
V,
where
the
surface
oxide
displaces
adsorbed
sul-
fate
completely
from
the
surface.
The
subsequent
reduction
of
the
oxide
layer
leads
to
a
severe
corrosion
of
the
Au(1
1
1-20
nm)
film
electrode,
as
represented
by
a
tilted
baseline
of
the
IR
spectrum
at
1.20
V
(Fig.
7).
The
reappearance
of
adsorbed
sulfate
could
be
detected
qualitatively
by
the
SO band
evolving
again
at
E
≥
0.2
V
in
the
second
positive
scan.
Next,
we
investigated
the
surface
oxide
formation
and
reduc-
tion
processes
by
in
situ
SHINERS
in
1.0
M
Na2SO4.
Fig.
9
displays
typical
spectra
recorded
during
a
potential
excursion
from
0.00
V,
i.e.
in
the
double
layer
region,
to
1.20
V
and
subsequently
back
to
0.00
V.
The
experimental
protocol
followed
a
similar
strategy
as
described
before
in
1.0
M
H2SO4.
No
Raman
signatures,
which
could
be
related
to
the
surface
species
is
found
up
to
0.40
V.
How-
ever,
at
slightly
higher
potentials,
E
≥
0.50
V,
two
bands
with
their
center
positions
at
590
and
807
cm−1appear.
The
latter
shifts
in

860 U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863
Fig.
9.
Selected
“steady-state”
SHINER
spectra
of
a
Au(1
1
1)
electrode
in
1.0
M
Na2SO4plotted
as
a
function
of
the
electrode
potential.
The
direction
of
the
potential
excursion
is
indicated
by
the
arrow.
deuterated
water
(D2O)
toward
lower
wavenumbers
(red
shift)
with
H2O/nD2O=
1.15
±
0.01
(inset
in
Fig.
5b),
while
the
former
is
not
influenced
by
the
exchange
of
the
solvent
(not
shown).
Based
on
these
observations
and
on
a
comparison
with
literature
data
of
gold
oxo-
and
hydroxo-complexes
[69–71],
we
attribute
the
bands
at
590
and
807
cm−1to
the
gold
oxide
stretching
mode
AuO and
to
the
gold-hydroxide
bending
mode
ıAuOH,
respectively.
We
notice
that
the
band
attributed
to
the
AuO mode
might
originate
from
both
AuO
and/or
AuOH
surface
species,
since
their
vibration
fre-
quencies
are
rather
close
to
each
other
[69–71].
This
situation
is
different
in
acidic
electrolyte
(Fig.
5a),
where
AuO originates
only
from
AuO
species.
The
potential
dependence
of
the
experimentally
observed
band
positions
of
AuO is
rather
complex
in
1
M
Na2SO4(Fig.
5b).
It
decreases
from
E
=
0.60
V
up
to
E
=
1.00
V
during
the
positive
poten-
tial
scan
and
subsequently
increases
up
to
1.20
V.
The
position
of
the
ıAuOH band
is
almost
constant
at
∼808
cm−1.
Reversing
the
direc-
tion
of
the
potential
scan
leads
to
a
linear
decrease
in
the
position
of
AuO with
a
slope
of
∼20
cm−1V−1in
1.20–0.80
V.
ıAuOH is
not
detectable
any
more
at
E
<
0.60
V,
i.e.
after
the
first
surface
oxide
reduction
peak
P3a,
while
AuO reaches
its
highest
band
position
AuO(max)
=
609
cm−1.
The
latter
decreases
subsequently
with
a
slope
of
∼
105
cm−1V−1in
0.2
V
≤
E
≤
0.5
V,
i.e.
under
the
second
reduction
peak
P3b.
The
inflection
point
in
the
potential
depend-
ence
of
the
position
of
AuO demonstrates
the
distinctly
different
nature
of
the
two
gold
surface
oxide
reduction
peaks
P3aand
P3b.
This
aspect
will
be
addressed
in
Section
4
in
detail.
Fig.
8
displays
the
potential
dependence
of
the
integrated
intensities
of
the
two
Raman
modes
AuO and
ıAuOH.
The
plot
of
ıAuOH shows
that
the
adsorption
of
OH
species
increases
in
0.50
V
≤
E
≤
0.90
V
monotonically.
The
maximum
of
the
integrated
intensity
is
reached
around
0.90
V,
i.e.
at
the
onset
of
surface
oxida-
tion
on
(1
1
1)
terrace
sites.
Subsequently,
the
intensity
decreases
monotonously
and
finally
ceases
at
1.20
V
(Fig.
8a).
The
reverse
potential
scan
leads
to
the
reappearance
of
ıAuOH in
the
region
of
the
first
reduction
peak
P3a,
e.g.
0.50
≤
E
≤
0.80
V
(Fig.
8b).
On
the
other
hand,
the
integrated
intensity
of
AuO increases
slightly
from
E
=
0.50
V
to
E
=
0.90
V
followed
by
a
steep
rise
between
E
=
1.00
V
and
E
=
1.20
V
(Fig.
8a).
Changing
the
direction
of
the
potential
scan
reveals
an
abrupt
decrease
in
the
integrated
inten-
sity
of
AuO.
Then
a
rather
shallow
region
evolves
between
1.10
V
and
0.70
V.
The
integrated
intensity
increases
again
upon
passing
the
first
reduction
peak
P3a,
reaches
a
maximum
at
E
=
∼0.55
V,
and
finally
approaches
zero
at
E
≤
0.10
V,
just
after
the
second
surface
reduction
peak
P3b(Fig.
8b).
The
finite
integrated
intensity
of
the
AuO band
during
the
negative
potential
scan
at
E
≥
0.90
V
shows
that
the
disappearance
of
the
ıAuOH band
in
this
potential
region
is
directly
related
to
the
desorption
and/or
conversion
of
the
formerly
adsorbed
OH
species,
rather
than
to
a
decrease
in
the
enhancement
of
the
SHINERS
signal.
4.
Discussion
4.1.
Adsorption–desorption
of
anions
The
potential
dependence
of
the
integrated
intensities
of
SO may
be
considered
as
an
adsorption
isotherm.
Our
results
demonstrate
that
the
experimentally
obtained
isotherms
of
sul-
fate
adsorption
in
1.0
M
H2SO4and
in
1.0
M
Na2SO4are
similar
(Figs.
4a
and
8a).
An
initial
S-shaped
increase
of
the
integrated
SO-intensity
is
followed
by
a
plateau
region,
and
subsequently
a
gradual
decrease
due
to
the
onset
of
substrate
surface
oxidation.
Likewise,
the
position
of
SO changes
linearly
with
the
slopes
of
86
or
88
cm−1V−1in
0.3
V
≤
E
≤
1.25
V
(Fig.
3b).
Based
on
the
dissociation
constants
of
H2SO4,
pKa1 =
−3
and
pKa2 =
2
[72],
we
obtained
pH
=
0
in
deoxygenated
and
CO2-free
1.0
M
H2SO4and
pH
=
7.5
in
1.0
M
Na2SO4,
respectively
The
cor-
responding
bulk
concentration
ratio
of
SO42−and
HSO4−ions,
cSO2−
4
/cHSO−
4,
amounts
to
0.01
and
106in
1.0
M
H2SO4and
1.0
M
Na2SO4.
This
difference
appears
to
have
no
significant
influence
on
the
adsorption
isotherms
and
on
the
position
of
SO.
Therefore,
we
propose
that
the
(hydrogen-)
sulfate
ions
adsorb
on
Au(1
1
1)
electrode
surfaces
in
a
single
form,
either
SO42−or
HSO4−.
We
note
that
the
ion
concentrations
of
SO42−and
HSO4−change
by
a
factor
of
100
and
10−6when
comparing
the
two
electrolytes.
These
concentration
changes
should
lead
to
shifts
of
+0.12
V
or
−0.36
V
of
the
(hydrogen-)
sulfate
adsorption
isotherm
in
neutral
solution
as
compared
to
the
acidic
one
if
the
adsorbate
is
SO42−
or
HSO4−,
respectively.
The
shifts
are
estimated
from
the
Nernst
equation
assuming
z
=
1
for
the
number
of
transferred
electrons
per
adsorbed
(hydrogen-)
sulfate
[73].
The
experimentally
observed
shift
in
the
position
of
the
adsorption
isotherms
is
approximately
+0.11
V
(Figs.
4a
and
8a),
which
provides
strong
support
for
SO42−
being
the
preferentially
adsorbed
species
on
Au(1
1
1).
A
similar
conclusion
was
reached
by
Weaver
et
al.
based
on
a
combined
in
situ
STM
and
IR
study
[21].
No
spectroscopic
indication
of
OH
adsorption
was
observed
in
1.0
M
H2SO4(Fig.
5a).
The
decrease
in
intensity
of
SO is
accompa-
nied
with
the
growth
of
the
AuO feature
(Fig.
4a).
The
AuO band
appears
at
∼1.10
V,
simultaneously
with
the
onset
of
oxidation
on
defect
sites,
and
continues
to
grow
upon
oxidation
of
the
(1
1
1)
terrace
sites
(Figs.
2
and
4a).
Thus,
the
desorption
of
SO42−ions
in
acidic
media
is
only
triggered
by
the
formation
of
gold
surface
oxide
of
AuO-type,
and
proceeds
sequentially
from
nearby
defect
sites,
such
as
(vacancy)
islands
and/or
step
sites,
to
the
(1
1
1)
terrace
sites.
On
the
other
hand,
OH
adsorption
is
observed
clearly
in
1
M
Na2SO4(Fig.
9).
The
integrated
intensity
of
SO is
rather
constant
in
0.60
V
≤
E
≤
0.80
V,
while
the
integrated
intensity
of
the
ıAuOH
band
increases
gradually
(Fig.
8a).
This
finding
indicates
the
co-
adsorption
of
OH
species
with
SO42−anions
(reaction
(1)
with
X−=
SO42−,
Section
1).
Changing
the
potential
from
E
=
0.80
V
to
E
=
1.20
V
leads
to
sulfate
desorption
(reaction
(2))
accompanied
with
a
decrease
and
an
abrupt
increase
of
the
integrated
band
inten-
sities
of
ıAuOH and
AuO,
respectively
(Fig.
8b).
The
changes
in
the
integrated
intensities
of
ıAuOH and
AuO in
0.80
V
≤
E
≤
1.20
V
are
assigned
to
the
oxidation–deprotonation
of
adsorbed
OH
due
to
the
formation
of
gold
oxide
AuO
(reaction
(3)).
The
latter
starts
at
defect
sites
and
proceeds
subsequently
toward
the
atomically
smooth
(1
1
1)
terraces,
as
nicely
supported
by
cyclic
voltammetry
(Fig.
6)
and
STM
data
(not
shown)
with
the
stepped
single
crystal
gold
electrodes
Au(S)[n(1
1
1)-(1
1
1)].

U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863 861
4.2.
Surface
oxidation–reduction
In
situ
surface
X-ray
scattering
(SXS)
experiments
of
Au(1
1
1)
in
0.05
M
H2SO4by
Kondo
et
al.
revealed
the
existence
of
a
place
exchange
between
oxygen
species
and
gold
surface
atoms
upon
oxidation
[46].
The
place
exchange
leads
to
a
surface
roughening
at
an
atomic
scale.
This
observation
is
supported
by
our
own
data
(Fig.
1d)
as
well
as
by
published
STM
results
from
other
groups
[33,41–43,59].
The
rough
surface
morphology
remains
rather
unal-
tered
until
the
onset
of
the
main
reduction
peak
P3,
which
indicates
the
stability
of
the
place-exchanged
gold
oxide
AuO
during
the
cathodic
scan
in
1.10
V
<
E
≤
1.40
V.
SHINERS
experiments
in
1.0
M
H2SO4demonstrate
that
the
shift
of
the
AuO mode
during
the
positive
as
well
as
during
the
negative
potential
sweep
amounts
to
∼20
cm−1V−1in
1.20
V
≤
E
≤
1.40
V.
This
slope
is
significantly
lower
than
AuO/E
=
95
cm−1V−1,
which
is
observed
for
E
<
1.20
V
until
the
complete
reduction
of
the
surface
oxide
(Fig.
5b).
Referring
to
previous
SXS
and
STM
data
[42,43,45]
and
our
in
situ
STM
observations,
we
interpret
these
spectroscopic
findings
as
evidence
of
a
place
exchange
pro-
cess
within
the
gold
surface
oxide
at
E
≥
1.20
V.
The
lower
slope
may
be
considered
as
a
Raman-spectroscopic
signature
of
the
place-exchanged
oxide
on
Au(1
1
1)
(Fig.
5b).
The
small
slope
of
∼20
cm−1V−1can
be
explained
in
terms
of
a
Stark
tuning
mech-
anism.
The
place
exchange
process
orients
AuO
dipoles
in
an
antiparallel
configuration
[27],
which
decreases
the
effective
local
electric
field
and
hence
the
Stark
tuning
rate.
The
current
response
of
surface
oxide
reduction
on
Au(1
1
1)
in
1.0
M
H2SO4is
characterized
by
a
single
and
rather
sharp
symmetric
peak
P3(Fig.
2).
The
integrated
intensity
of
AuO starts
decreasing
simultaneously
at
potentials
close
to
the
position
of
P3,
which
indi-
cates
a
loss
of
gold
surface
oxide
(Fig.
4b).
STM
images
recorded
at
E
=
0.60
V,
i.e.
after
completion
of
surface
oxide
reduction,
display
an
atomically
smooth
surface
with
monoatomic
vacancy
islands
(Fig.
1e).
These
vacancy
islands
result
from
the
reduction
of
place
exchanged
oxide.
The
oxide
reduction
is
coupled
with
the
read-
sorption
of
SO42−as
can
be
seen
in
Fig.
4b.
The
abrupt
decrease
in
the
integrated
intensities
of
AuO is
paralleled
by
an
intensity
increase
of
SO in
1.00
V
≥
E
≥
0.80
V
indicating
the
replacement
of
AuO-species
by
adsorbed
SO42−ions.
Sulfate
ions
desorb
at
lower
potentials,
due
to
electrostatic
reasons.
The
potential
dependence
of
the
AuO-band
position
is
rather
complex
during
the
anodic
scan
from
E
=
0.80
V
to
E
=
1.20
V
in
1.0
M
Na2SO4due
to
overlapping
contributions
of
adsorbed
AuO
and
AuOH
species
(Fig.
5b,
see
also
Section
3.4).
The
subsequent
cathodic
scan
leads
initially
to
a
linear
slope
AuO/E
=
∼20
cm−1V−1in
1.20
V
≥
E
>
0.80
V
(Fig.
5b).
This
result
is
similar
to
the
characteristics
of
place
exchange
within
the
oxide
phase
in
1.0
M
H2SO4.
We
therefore
conclude
that
the
oxide
layer,
as
formed
in
neutral
Na2SO4electrolyte,
exists
dominantly
in
the
place-exchanged
form
until
the
onset
of
its
reduction
at
E
≤
0.80
V.
We
notice
that
the
place
exchange
of
adsorbed
species
would
result
in
a
considerable
shift
in
the
position
of
the
correspond-
ing
band
with
potential.
However,
we
found
a
weak
potential
dependence
of
the
bending
mode
ıAuOH in
the
same
potential
range,
both
in
H2O
and
in
D2O
(inset
in
Fig.
5b).
These
finding
suggest
that
adsorbed
OH
is
not
involved
in
the
place
exchange
process,
neither
on
defect
sites
nor
on
terraces.
We
conclude
that
the
formation
of
AuO
from
AuOH
(reaction
(3))
on
both,
defect
and
terrace
sites
takes
place
prior
to
the
place
exchange
process
in
1.0
M
Na2SO4.
The
oxide
reduction
in
1.0
M
Na2SO4is
characterized
by
two
current
peaks
in
the
voltammograms,
P3aand
P3b,
as
recorded
under
stationary
conditions
(Figs.
6
and
8).
The
ıAuOH band
is
not
detected
during
the
negative
potential
scan
in
the
oxide
region
from
E
=
1.20
V
to
E
=
0.80
V.
However,
it
reappears
in
the
potential
range
of
the
first
reduction
peak
P3aat
E
≤
0.80
V
(Fig.
8b).
This
observa-
tion
suggests
that
the
place-exchanged
surface
oxide
is
partially
reduced
through
the
formation
of
adsorbed
OH,
which
appears
to
be
stable
on
the
surface
during
the
negative
potential
excur-
sion
in
0.80
V
≥
E
>
0.50
V.
The
ıAuOH band
disappears
at
E
≤
0.50
V
(Fig.
8b),
while
the
integrated
intensity
of
the
gold
surface
oxide
stretching
mode
AuO decreases
gradually.
The
integrated
intensity
of
the
AuO mode
approaches
zero
upon
passing
the
second
reduc-
tion
peak
P3b,
which
is
centered
at
E
∼
0.40
V.
Thus,
we
attribute
the
second
peak
to
the
reduction
of
residual
surface
oxide
without
the
formation
of
adsorbed
OH
as
an
intermediate.
The
position
of
AuO shifts
in
0.20
V
≤
E
≤
0.50
V
toward
lower
wavenumbers
with
AuO/E
∼
l05
cm−1V−1.
This
slope
is
rather
close
to
the
poten-
tial
dependence
of
AuO during
the
reduction
of
place-exchanged
oxide
in
1.0
M
H2SO4,
which
amounts
to
∼95
cm−1V−1.
This
agree-
ment
suggests
a
similar
mechanism
of
surface
oxide
reduction
for
1.0
M
H2SO4in
1.10
V
≥E
≥
0.80
V
and
for
1.0
M
Na2SO4in
0.50
V
≥E
≥
0.20
V.
4.3.
Mechanisms
of
Au(1
1
1)
oxidation–reduction
in
H2SO4and
Na2SO4electrolyte
solutions
Based
on
the
results
obtained
by
cyclic
voltammetry,
in
situ
STM,
SEIRAS
and
SHINERS
of
Au(1
1
1)
oxidation
in
1.0
M
H2SO4and
1.0
M
Na2SO4we
propose
the
following
new
and
extended
reaction
mechanisms.
(i)
Oxidation–reduction
mechanism
in
1
M
H2SO4(Fig.
10a):
Sul-
fate
ion
adsorption
dominates
in
the
potential
range
between
E
=
0.20
V
and
1.05
V.
Oxide
formation
and
AuO
place-exchange
start
at
defect
sites
(steps)
at
E
=
1.10
V
and
proceed
until
E
=
1.30
V
(peak
P4).
The
process
is
coupled
with
the
desorp-
tion
of
sulfate
ions.
Oxide
formation
on
terrace
sites
and
AuO
place-exchange
take
place
in
E
=
1.30–1.45
V
(peak
P3).
The
remaining
sulfate
ions
desorb
from
terraces
sites
simul-
taneously.
The
place-exchanged
surface
oxide
appears
to
be
rather
stable
during
the
subsequent
negative
going
poten-
tial
scan
until
E
=
1.10
V.
Reduction
of
the
gold
surface
oxide
occurs
in
1.10
V
>
E
>
0.70
V
(peak
P3),
which
leads
to
the
readsorption
of
sulfate
ions
as
well
as
to
the
formation
of
gold
islands
and
vacancy
islands.
The
latter
undergo
Ostwald
ripening
processes.
In
particular
the
gold
islands
are
rather
quickly
incorporated
into
larger
islands
and/or
nearby
step
sites
[59].
(ii)
Oxidation–reduction
mechanism
in
1
M
Na2SO4(Fig.
10b):
Scanning
the
potential
from
E
=
−0.20
V
toward
more
positive
values
leads
to
the
onset
of
detectable
sulfate
ion
adsorption
at
E
=
0.20
V.
Coadsorption
of
sulfate-
and
hydroxide-ions
takes
place
in
E
>
0.50
V.
At
E
≥
0.80
V
(peak
P4)
this
process
is
cou-
pled
with
the
place
exchange
of
the
surface
oxide
species
at
defects
and
the
desorption
of
sulfate
ions.
The
oxidation
of
OH
on
terrace
sites
starts
at
E
=
0.90
V
and
continues
up
to
E
=
1.20
V.
Complete
desorption
of
sulfate
ions
takes
place
simultaneously.
In
consequence,
the
entire
surface
is
covered
by
the
place-
exchanged
oxide.
Reduction
of
the
place-exchanged
surface
oxide
starts
during
the
subsequent
cathodic
potential
scan
at
E
≤
0.80
V.
The
partial
reduction
of
this
place-exchanged
surface
oxide
leads
first
to
the
formation
of
adsorbed
OH
in
0.80
>
E
>
0.50
V
(peak
P3a).
The
complete
desorption
of
this
OH-species
and
the
reduction
of
residual
place-exchanged
oxide
(without
the
formation
of
a
stable
intermediate
OH-
species)
proceed
from
E
=
0.50
V
to
0.00
V
(peak
P3b).
These
processes
are
coupled
with
the
formation
of
vacancy
islands
and
the
readsorption
of
sulfate
ions.

862 U.
Zhumaev
et
al.
/
Electrochimica
Acta
112 (2013) 853–
863
Fig.
10.
Proposed
models
representing
the
structure
changes
of
Au(1
1
1)
in
(a)
1.0
M
H2SO4and
(b)
1.0
M
Na2SO4during
surface
oxidation
and
reduction.
The
corresponding
voltammograms
are
plotted
for
guidance
in
a
current
vs.
time
scale,
scan
rate
10
mV
s−1.
The
characteristic
current
peaks
are
also
labeled.
5.
Conclusions
We
have
carried
out
a
comprehensive
study
of
processes
involved
in
surface
oxidation
and
reduction
on
well-defined
smooth
and
stepped
Au(S)[n(1
1
1)-(1
1
1)]
single
crystal
electrodes
in
sulfate
ion
containing
electrolytes
employing
cyclic
voltam-
metry,
in
situ
STM,
SEIRAS
and
SHINERS.
Based
on
complementary
results
of
these
experimental
approaches,
we
propose
a
new
and
an
extended
mechanism
of
gold
surface
oxidation
and
reduc-
tion
in
1.0
M
H2SO4and
1.0
M
Na2SO4.
We
demonstrate
that
the
Au(1
1
1)
surface
in
1
M
H2SO4is
oxidized
directly
to
gold
oxide.
Surface
oxidation
and
reduction
in
1
M
Na2SO4proceeds
through
the
formation
of
adsorbed
OH
intermediates.
SHINERS
experiment
allowed
mapping
out
their
stability
range.
We
also
demonstrate
that
only
AuO
is
actively
involved
in
the
surface
place-exchange
process
in
both
acidic
and
neutral
electrolytes.
Furthermore,
the
quantitative
analysis
of
our
SEIRAS
data
provides
clear
evidence
that
the
adsorbed
anion
in
the
double
layer
region
is
sulfate,
rather
than
hydrogen
sulfate,
in
both
electrolytes.
Acknowledgements
This
work
was
supported
by
the
European
Union
through
the
FP7
BacWire
Project
(Contract
No:
MNP4-SL-2009-2293337),
the
Swiss
National
Science
Foundation
(Grant
nos.
200020-144471
and
200021-124643)
and
the
University
of
Bern.
A.K.
acknowledges
support
through
a
MC
fellowship
within
the
project
ELCAMI.
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