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8/10/2019 1996 J Chem Soc Ag Roughening
1/7
Roughening
of
thin silver
films
in aqueous electrolytes
Sara A. Bilmes
I N Q U I M A E , Facultad de C iencias Exactas, Universidad de Buenos Aires. Ciudad Universitaria
Pab I I . 1428)Buenos Aires, Argentina
The reactivity of silver surfaces in aqueous electrolytes, i.e. corrosion and roughening, is studied by monitoring simultaneously
changes in resistivity and transmittance of thin silver films. Atomic-scale and large-scale roughening are discriminated by the
different sensitivity of the tran sm ittan ce and resistance responses. Results for different surface conditions produced on a thin silver
film by adsorption of water, pyridine and Cl- ions, as well as the potential dependence, are interpreted by considering that
adsorb ates induce roughening
of
the surface.
The morphology and texture of silver surfaces has attracted
much attention in connection with the use of surface-
enhanced spectroscopy for the resolution of adsorbate-
sub strate structures. Surface-enhanced Ram an scattering
(SERS), second h armo nic generation and fluorescence have
allowed one to probe the electronic and geometric configu-
ration of the surface complexes formed upon chemisorp-
tion.' ** These surface-enhanced phenomena require a metallic
substrate with some degree of roughness to support optical
resonances. Owing to their unique optical and electronic
properties, thin silver films are suitable substrates for surface-
enhanced processes either in ultra-high vacuum (UHV) or in
electrolytes, and for a variety of additional applications such
as solar
cell^ ^ ^
In aqueous electrolytes, the morphology and texture of
silver surfaces can be modified by changing the potential of
the silver/electrolyte interface by electrochemical method^.
The modification of the surface occurs as a consequence of a
variety of potential-dependent processes such as adsorption-
desorption of anions, oxidation-reduction of silver. In this
way, an initially smooth silver surface can be roughened by
dissolution and redeposition of metallic ions at different
surface site^.^-''
The surface of silver electrodes which generate SERS has
been extensively studied by scanning electron microscopy
(SEM)6,7and by scanning tunneling microscopy (STM).7-'o
Recent STM work reveals that electrochemically rough silver
electrodes exhibit a nodular structure after the oxidation-
reduc t ion of the ~urface ,~nd the neighbouring nodules col-
lapse in the further relaxation of the surface. In
situ
S TM
observations of silver electrodes also indicate that C1- adso rp-
tion strongly influences surface morphology. l o ,
Sm ooth an d rough surfaces in vacuum are characterized by
their different optical properties and different specific
resistivities. Bumps on rough surfaces generate changes in the
scattering and absorption cross-sections.' 2 , 1 3 Experiments
with Ag overlayers on sm ooth silver films demonstrate large-
scale modifications of surface plasmon polariton (SPP)
reso-
nances which are partially restored to the original shape by
warming to room temperature.14-16 Bumps on the surface
also produce an increase in the electron surface scattering,
increasing the resistivity of the sample.' 7- Fo r thin Ag over-
layers on smooth silver films the change in a simple Fuchs-
type specularity parameter indicates an almost perfectly
diffuse scattering of electrons at the surface. l Adsorbates
produce changes both in the UV-VIS spectrum and in the
resistivity of thin silver Th e latter has been
recently related to th e electron-hole pair da mp ing of the frus-
trated tra nslation of adsorb ates.22
Thin silver film electrodes exhibit different behaviour than
massive silver electrodes. As soo n as the m ean free pat h of the
conduction electrons is comparable to the film thickness, elec-
tron scattering at th e surface is the dom inan t effect in thin film
electrodes, the chemical reactivity of thin films is enhanced
when compared to that of the massive metal. One of the
major problems for the use of thin silver films in aqueous
systems is their chemical instability, through dissolution and
changes in the surface roughness. These corrosion effects may
be enhanced by anions or attenuated by organic molecules.
Changes in the resistivity of thin metal films in electrochemi-
cal interface^^^ ^^ reveal that adsorp tion of anions leads to the
reconstruction of the surface with increasing roughness,
accompanied by an increase in the surface re~ is tan ce .'~ his
effect can be avoided
if
an organic molecule such as pyridine
(Py) is first adsorbed on the surface to act as a corrosion
inhibitor.23 Th e study of th e massive silver electrode/
electrolyte interface by differential reflectance, electro-
reflectance, and attenuated total reflectance (ATR),25-28
reveals that the optical properties are dependent on th e poten-
tial, the electrolyte composition and the surface roughn ess.
In this work, the reactivity of silver surfaces in aqueous
electrolytes, i.e. corrosion and roughening, is studied. By mon-
itoring simultaneously the changes in resistivity and transmit-
tance for different surface conditions produced on a thin silver
film by ad sor ptio n of water, pyridine and C1- ions, as well as
the potential dependence of these parameters, it is possible to
describe the surface chemical processes taking place at the
interface. The sensitivity of transmittance and resistivity
towards surface roughness is discussed; this allows us to
separate atomic-scale and large-scale roughness co ntribution s
which, in tur n, are related to SE RS activity.
Experimental
Apparatus
The transmittance apparatus consisted of a deuterium-
halogen duplex lam p focused on the electrode with a concave
mirror in order to avoid chromatic aberrat ion. Transmitted
light was focused on a slit (also by concave mirrors) prior to
discrimination by a grating (250 lines mm -'), an d detected
with a d iode arra y (E G& G M 1412). All experiments w ere per-
formed with normal incidence. Simultaneous resistance mea-
surem ents w ere m ad e using the three-contact m e t h ~ d ~ ~ , ~
with a Wheatsto ne bridge and dc current.
The electrochemical apparatus consisted of a potentiostat
(PA R 173) and a scan g enerator (PAR 175). Th e electrochemi-
cal cell was designed with both adjustable counter electrode
J .
Chem.
SOC.,
Faraday Trans . , 1996,92(13), 2381-2387
2381
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2/7
(Pt) and reference electrode (saturated calomel electrode, SCE )
in order to optimize current-potential distribution dur ing
oxidation-reduction cycles. Fo r transmission experiments
these electrodes were kept away from the optical path. All
chemicals employed w ere p.a. or better and water w as bidistil-
led on q uar tz. Unless otherwise specified, the electrolyte was
pre-saturated with N, before contacting the silver film.
Preparation
of
thin silver film electr odes
Silver was evaporated at room temperature onto a circular
Suprasil optical substrate of 50 mm diameter at P
< 5
x
Pa. The substrate was cleaned by sonication in acetone,
immersion in hot alkaline KM n0,-satura ted solution, rinsed
in water and sonicated in acid H,O, solution. The substrate
was finally rinsed with water and exposed to water vapour.
This cleaning process ensures the total removal of organic
matter. The adhesion
of
silver onto this optical substrate is
good enough to make it unnecessary to improve adherence
with previous deposition of Cr . Before the condensation of the
working electrode film, 100 nm silver contacts were evapo-
rated. The probe was grown at 0.2 8, s - l for the first
10
nm
thickness and at 1 8, s - l up to the specified thickness, which
was typically 30-35 nm. Exp erimental results have a repro -
ducibility better than
lo ,
independent of the thickness of the
film over the above-mentioned range. Contact wires were con-
nected with silver-loaded paint and masked with Epoxigelb@.
The substrate-film assembly was then annealed at
60C
for
2
h to ensure the smoothness (on a macroscopic scale)
of
the
sample.
Results
The transmittance spectrum of a thin silver film in N, P 1
atm) exhibits an asymmetric peak a t 320 nm due t o the high
transmittance of the metal at the bulk plasma frequency,
up
as
shown in Fig.
l(a).
In this figure, the transmittance of the
cell without film T,) is taken as a reference. These kind of
films h ave a resistivity of ca. 2.5 n il m a t room temperature, in
good agreement wi th da ta in the l i t e ra t~re . ~ .~oth resist-
ance and transmittance are time-independent in N, atmo-
sphere.
Ag/water and AglO.1 mol dm-
KCI
at open circuit
The transmittance of a silver film exhibits a time-dependent
transmission spectrum when the cell is filled with
N,-
saturated water, as shown in Fig.
l(b).
The normalized trans-
mittance spectra, ATJT,
,
epresents the difference between the
transmittance at time
t
after addition of water
17;)
and that
corresponding to
1
min after the cell is filled with water
T ) .
This reference was chosen in order to monitor the changes in
the film, independent of the refractive index of the surround-
ing medium. The spectra dep icted in Fig.
l(b)
exhibit two defi-
nite regions: at il< 600 nm there is a decrease in
transmittance whereas transmitted intensity increases for
1 > 600
nm. The limit between these two regions,
ATJT
= 0,
is red-shifted with increasing time. In the low-wavelength
region a broad peak centred at
ca.
540 nm is noticed. A sta-
tionary situation is achieved after 60-90 min. Addition of 2
mol dm- KCl up to a
0.1
mol dm- final concentration pro-
duces further changes, and the system continues to evolve
with the above-described tren d [Fig. l(c)].
Dc resistance changes, AR/R, recorded simultaneously with
transmittance changes, indicate that, after contact with water,
there is an increase in the film resistivity with time (Fig.
2).
However, at longer times the resistivity is continuously
increasing, even though no significant changes in transmit-
0.6
kv 0 7
I=
0.2
I
800
700
600
500
LOO 300
r I
A g ( 3 S n m
H z o
* * l o t
0.1
0
I I
800
600
4
k
I
d
c) -
-
155 m i n ( 1 m i n )
O e 2 k
\8 9
m i n ( 3 5 m i n l
0.2
800 600 4
00
A h m
Fig. 1 a)Transmittance of a
30
nm silver film in
N,
,measured rela-
tive to the transmittance of the cell. (b) Time evolution
of
the relative
transmittance of the same film in contact with water. (c) Same
as 6) after the addition of KCl final concentration
0.1
mol dm-3).
Time in parenthesis are referred to the
KCI
addition.
A T / T =
W
- ~(O)I/T(O).
tance are noticed. The same results are obtained, with respect
to both transmittance and resistance, when the same experi-
ment is carried out in air-saturated water instead of N,-
saturated w ater.
2382
J .
Chem.
SOC.,
Faraday Trans., 1996, Vol . 92
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3/7
2.5
I
0.4
2.0
-
1.5
-
U
50 1 0 0 1 5 0 200 250
timelmin
Fig.
2 Relative resistance ARIR)
(-)
and relative transmittance
AT/T)
---), at
L
= 800 A) nd 400 nm
a),
f a 35 nm silver film
in
water. KCl is added at t
= 155
min.
a1
, , , I ,
Ag
30
myo.1mol dm-3 KCI
-2
-
0
- 1
2
- 3
- c
am 600 LOO
800
600 LOO
2nrn
Aglpyridine
A
thin silver film exposed t o the vapour of 0.1 cm3 of pure
liquid pyridine add ed to the N,-saturated cell exhibits a
change both in transmittance and resistance with a time
0.6
I
I
- 1 . 3 -1 .1 0.9 -0.7 -0.5 0.3 -0.1
EN
i=
a
800 600
L O O
AJnm
3.5 I I I 3.0
k u
- - - ,
1 6
32 48 64 80
tirne/min
Fig. 3 (a) Time evolution of the relative transmittance of a
35
nm
silver film exposed to pyridine vapour. (b) Relative resistance
ARIR)
(-)
and relative transmittance AT/T) ---), at L = 800 A) nd
400 nm
a),
f a
35
nm silver film exposed to pyridine vapour.
Fig. 4 (a)
Differential transmittance spectra
of
a 30 nm Ag thin film
in 0.1 mol dm-3 KCl, and in 0.1 mol dm- 3 KC1-0.04 mol dm-3
Pyridine. Spectra are recorded at the indicated potentials during a
potential sweep at
u
= 0.01 V s - l from - .3 to -0.2 V.
(b)
Relative
resistance change during a potential sweep,
u
=
0.01
V
s - ,
from
-
.3
to
-0.2
V and back to 1.3 V.
ATJT
= [ T E )- T -0.2
V)]/
T -
.2
V).
dependence similar to that described for liquid water. This
situation is shown in Fig. 3. The same trend has been found
for the resistance variation of annealed silver films during
exposure to Py a t 40 K in UHV.30
Although adsorption of pyridine on the bare surface modi-
fies both the resistivity and the transmittance of the sample, as
depicted in Fig. 3, the addition of pyridine to the Ag/H,O
stabilized interface did not produce significant changes. On
the other hand, the addition of pyridine inhibits the effect of
C l- ions show n in Fig. l(c). This effect has already been dis-
cussed by Korwer
et
aLZ3
AglO.1 mol dm-3 KCl under potential control
Tra nsm ittanc e spectra are influenced by the potential applied
to the interface, as can be seen in Fig. qa) . The difference
between the transmittance at potential E and that a t
E
=
-0.2
V,
ATE,
normalized to that at E
=
-0.2 V is
recorded during a potential sweep at 10mV s-' , starting from
-0.2
V. An absorption band at
380
nm is the main feature of
the relative transmission spectra with a small structure at 340
nm. As the interface is polarized to more negative potentials
the intensity of the main feature increases and the structure at
higher energy is more clearly defined. There are no significant
differences between the spectra taken with and without dis-
solved p yridine.
The relative resistance change of the same film is shown in
Fig. 4 b) in the potential range where the silver surface is not
oxidized ( - 0.2 =- E/V > - .3). For potentials more positive
J . Chem. SOC.,
Faraday Trans.,
1996, V o l .
92 2383
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4/7
0
- 2
4
- 6
=
- a
= o
e :
G?
? - 2
N
4
- 6
- 8
0.02
1
I
I
I
(4
Ag
35 nm)/O.l mot dmJ
KCI
+
0 02
mot dm Py
- -
Ag
35
m)/O.l mol
dm4
KCI
00
700 600
500
400 300
&nrn
Fig.
5 Transmittance spectra of a 35 nm silver film in
0.1
mol dm-3
KCI at - .2 V after ORC between - .2 and +0.2 V (TAR) relat ive to
that of the same film at the same potential before ORC (T R).
Q A
z
m C cm -2 .
than the potential of zero charge E p z cx -0.9 V) the adsorp-
tion of C1- induces an increase in t he resistivity of the film of
ca.
10
as the surface is positively charged, whereas for
potentials more negative than the pzc no resistivity changes
are observed. This result is coincident with that of Korwer et
01.23
0.01
I
1
0.011
0
-
0.0
0.02
800 700
600 500 400 300
Unm
r
1
I I
P
Ag 35 nm)/O.l mot
d m 3 KCI
+
0.02 mot dm3
Py
E= -O .8V
OL
I
I
0 10 20
3 0
40 5 0 60
Urnin
Fig. 6
(a)
Relative transmittance spectra of a 35 nm silver film in 0.1
rnol dm-3 KCI-0.04 mol dm-3
Py,
at -0 .8 V after a cathodic pulse
of 2 s to E
=
- .5 V TAg ) .
TBQ
s the transmittance at the same
potential before the cathodic pulse. b ) Relative resistance change in
the same conditions as a).
SERS active silver surfaces
Electrochemical roughening of the silver surface was per-
formed by oxidation-reduction cycles (OR C), sweeping the
potential between -0.2 V and +0.2
V
at 10 mV s-'. The
anodic charge involved in this process
(ca.
9 mC ern-,) allows
one to estimate that ca. 3 of the total Ag atom s are oxidized.
Com parison of anodic and cathodic charges during OR C
demo nstrates that Ag' ions are reduced back on to the film
during the cathodic sweep. This procedure is the usual one t o
obtain SERS-active silver surfaces in electrochem-
The relative change in the transmittance spectrum for this
process, TAR/ TBR is shown in Fig. 5. TAR is the transmittance of
the rougher surface generated by the ORC; T B R , the corre-
sponding value before the ORC. Immediately after the ORC
the ratio TAR/TBR exhibits a broad structure at
400
nm, and
2 decrease in transmittance for 2 > 600 nm. The band at
400 nm is still present even after 20-30 min O RC , whereas the
transmittance at lower energy is comparable to that of the
smooth surface throughout this time interval.
In SERS experiments a potential pulse at E
600
nm immediately after the ORC has also been found in
reflectance studies of massive silver electrode^,^^.^' and arises
from the change in optical constants of the surface by the for-
matio n of layers of [Ag(Py),]Cl. Th e brea thing m ode of Py in
this planar coo rdination com poun d is located at 1020 cm - ,
and is detected in SERS experiments at -0.2 V after the acti-
vation of the surface by ORC. Owing to the applied poten-
tial, a reduction process occurs:
[Ag(Py),]Cl
+
e
-+
Ago + C1-
+
2Py(ad)
(4)
increasing the transmitted intensity. In SERS experiments, the
1020
cm - band, characteristic of the [Ag(Py),]Cl complex
also decreases with time at -0.2
V . 3 5
The differences found for the transmission spectra between
adsorbate-induced and electrochemically roughened silver sur-
faces can be related to the m acroscopic roughn ess necessary
to p roduce SER S active surfaces. Roug h silver surfaces gener-
ated by a dsorb ed polar molecules can be thoug ht of as being
formed by randomly distributed hills and valleys with a broad
distribution of size and shape. These surfaces are not SERS
active. On the other hand, Ag films roughed by ORC are
SERS active, and exhibit nodular deposits of ca.
20-50
nm
diameter.6,7
SERS activity is mainly related to the existence of atomic-
scale roughness which is clearly monitored by resistivity
changes but not by optical methods. The different sensitivity
of resistance and transmittance is noticed in the relaxation of
the interface after the SE RS quenching pulse (Fig. 6). This per-
turbation of the surface produces a decrease in the resistance
due to digestion of small silver nuclei or clusters and/or
desorption of Py and C1-. This process involves short-scale
roughness chang es which are no t detected as differences in the
transmitted intensity. Moreover, the increase in resistance
after the quenching pulse is not accompanied by any change
in the transmittance spectrum, indicating that small clusters
are growing at the surface. These small nuclei of silver being
formed at sites onto a macroscopic rough surface are
undoubtedly associated with the recovering of SERS signal
after quenching.
Conclusion
Annealed thin silver films undergo roughening upon contact
with adsorbates. This effect is further enhanced by the pres-
ence of aggressive anions such as Cl-. This roughening is a
consequence of nucleation and growth of silver atoms at sites
on the surface with high reduction potential. Roughening of
the surface producing spheres with radius higher than some
critical value can be followed either by resistivity or transm it-
tance changes in the sample. Atomic-scale roughening can be
detected by the increase in the resistivity, the transmittance
being insensitive to these changes in the surface. The simulta-
neous monitoring of resistivity and transmittance allows one,
therefore, to demonstrate the operation of atomic-scale and
large-scale roughening as separate phenom ena.
This research project was financially supported by the Uni-
versity of Buenos Aires and the Fu nda cion An torchas.
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