Pyridine as a probe molecule for surface enhanced Raman spectroscopy of the silver-modified glassy carbon/solution interface

Article abstract of DOI:10.1016/S0039-6028(02)02271-9

Pyridine is employed as a probe molecule to study with surface enhanced Raman spectroscopy the interface between an aqueous electrolyte solution and an electrochemically activated glassy carbon surface modified with electrodeposited silver. Results demonstrate the feasibility of this approach. The surface of the silver deposits shows surface enhanced Raman scattering without further activation. Band positions are compared with those obtained with electrochemically roughened solid silver electrodes. Further simple adsorbates (halide or oxoanions) were studied; their behaviour closely resembles previous observations with solid silver electrodes.

Full text of DOI:10.1016/S0039-6028(02)02271-9

Surface Science 522 (2003) 55–63
www.elsevier.com/locate/susc
Pyridine as a probe molecule for surface enhanced
Raman spectroscopy of the silver-modified glassy
q
carbon/solution interface
Peter Mayer, Rudolf Holze *
Institut f €u r Chemie, AG Elektrochemie, Technische Universit a€ t Chemnitz, D-09107 Chemnitz, Germany
Received 7 July 2001; accepted for publication 21 August 2002
Abstract
Pyridine is employed as a probe molecule to study with surface enhanced Raman spectroscopy the interface between
an aqueous electrolyte solution and an electrochemically activated glassy carbon surface modified with electrodeposited
silver. Results demonstrate the feasibility of this approach. The surface of the silver deposits shows surface enhanced
Raman scattering without further activation. Band positions are compared with those obtained with electrochemically
roughened solid silver electrodes. Further simple adsorbates (halide or oxoanions) were studied; their behaviour closely
resembles previous observations with solid silver electrodes.
Ó 2002 Elsevier Science B.V. All rights reserved.
Keywords: Chemisorption; Electrochemical methods; Silver; Carbon; Raman scattering spectroscopy
1
. Introduction
ment effect. This enhancement could be ‘‘trans-
ferred’’ to some degree to other substrates by
electrodepositing the metal of interest onto a silver
substrate [1,2]. In case of gold a similar approach
involving deposition of the metal to be investi-
gated onto a SERS-active gold surface [3]. Laser
ablation of e.g. steel and alloys on electrochemi-
cally roughened silver electrode surfaces was also
used [4].
Pyridine has been employed as a probe mole-
cule in numerous investigations of the metal/
solution interface. In particular surface enhanced
Raman spectroscopy (SERS) has been used to
study the structure of the interface in the presence
of adsorbed pyridine molecules. SERS has been
employed frequently with d-metals (Cu, Ag and
Au) showing an especially large surface enhance-
In our ongoing research of metal-modified
carbon materials to be used as electrodes in redox
batteries [5,6] we have observed a considerable
q
Paper presented in part at the 199th Meeting of the
catalytic effect of silver deposits on electrochemi-
2þ=3þ
Electrochemical Society, Washington, DC, March 25–29, 2001.
*
Corresponding author. Tel.: +49-5311-509; fax: +49-5311-
cally activated glassy carbon for the Fe
redox
couple. In an attempt to get a deeper under-
standing of the basics of this electrocatalytic effect,
which is definitely not to be confused with a simple
8
32.
E-mail address: rudolf.holze@chemie.tu-chemnitz.de (R.
Holze).
0
039-6028/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved.
PII: S0039-6028(02)02271-9

56
P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
surface area increase by the silver deposit as evi-
denced by the almost unchanged double layer ca-
pacity [5] we have tried to study the modified
electrode surface with SERS. Since the iron ions
themselves are coordinated weakly only with water
ligands spectroscopic investigations of the redox
ion as a start seemed less promising. Instead we
have tried to study the surface with a probe mol-
ecule well understood in its changes upon ad-
sorptive interaction. Pyridine is certainly a very
suitable candidate.
Silver deposit modification was done by apply-
ing a potential cycle starting and ending at the
negative potential limit. A solution of 1 M HClO
4
À3
with 10 M AgCH
or AgNO
3
COO (Riedel de Haen, p.A.)
(DEGUSSA, 99.9%) was used. The
3
À1
slow potential scan (dE=dt ¼ 5 mV s ) was stop-
ped at an electrode potential negative to the opd
peak. The charge consumed during metal deposi-
tion was obtained by integrating the cathodic
peak. A gold wire counter-electrode and a hydro-
gen reference electrode filled with the supporting
electrolyte solution (ESCE ¼ À236 mV, for ease of
comparison data are converted to the HgjHg SO
4
-
2
2
. Experimental
reference electrode scale) were used during elect-
rochemical activation and metal deposition. In
spectroelectrochemical experiments a gold wire
counter-electrode and suitable reference electrodes
Glassy carbon (Ringsdorff; GC) discs of 0.282
2
cm apparent surface area glued onto copper rods
with conductive silver cement were embedded in
epoxy (Ciba-Geigy Araldit D/HY 956). Before
electrochemical measurements the electrodes were
(HgjHg SO
4
-reference electrode filled with an
SO
2
aqueous solution of 0.1 M K
2
4
(ESCE ¼ 397
mV) or a saturated calomel electrode) were used.
Anion cross contamination was carefully avoided.
Surface Raman spectra were recorded on an
ISA 64000 spectrometer equipped with a liquid
nitrogen cooled CCD camera detector at a reso-
2 3
polished with c-Al O of 0.3 and 0.05 lm grain
size (B u€ hler Micropolish). Subsequently they were
rinsed twice and sonicated twice for 10 min with
ultrapure water (Seralpur pro 90c) in order to re-
move traces of the polishing powder. For com-
parison a solid silver electrode (99.99%) embedded
in epoxy was used. The activation procedure nec-
essary to confer SER activity has been described
previously [7]. An aqueous solution of 0.1 M
À1
lution of 2 cm . Samples were illuminated with
laser light provided by Coherent Innova 70 laser
systems. Further experimental details are provided
in the figure captions. Because of the reported
Raman band shifting of the band caused by the
K
2
SO
Electrolyte solutions were prepared from 18
MX water (Seralpur pro 90c), HClO4 (VEB
Apolda, p.A.), K SO (FERAK, p.A.) and pyri-
4
was used as indicated below.
E
used.
2g-mode [13] only laser light of k
0
¼ 514:5 nm was
Scanning electron microscope picture were ob-
tained with a Philips SEM 1515 microscope.
2
4
dine (Merck, puriss.). Further chemicals used for
additional experiments described below were of at
least ‘‘puriss.’’ quality; they were used as received.
All electrochemical experiments were performed at
room temperature using solutions purged with
nitrogen.
3. Results and discussion
A cyclic voltammogram showing the deposition
process is displayed in Fig. 1. The silver deposition
and stripping peaks are clearly visible. Integration
of the charge yielded a value equivalent to about
eight monolayers of silver assuming perfectly
smooth two-dimensional layers.
A scanning electron microscope picture of the
silver-modified surface of electrochemically acti-
vated GC is displayed in Fig. 2. It closely resem-
bles previously published micrographs [14–18].
Besides globular silver deposits finely dispersed
Glassy carbon electrodes were activated elec-
trochemically in an H-cell filled with 1 M sulphuric
acid by applying 10 electrode potential cycles be-
tween À0:36 V < ERHE < 2:1 V at a scan rate
À1
of dE=dt ¼ 50 mV s . After activation the elec-
trode was rinsed again with ultrapure water.
Changes caused by this procedure and their effects
e.g. in electrocatalysis have been discussed else-
where [8–12].

P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
57
hexagonal Brillouin zone [25,26]. In the present
case glassy carbon in its polished state shows a
high degree of disordering as expected. Because
the scattered intensity decreases slightly and a
quantitative comparison of band intensities is in-
herently unsafe with this surface Raman spectro-
scopic technique the visible decrease of the band
intensity of the A -mode cannot be taken as a
1g
quantitative measure of a changed degree of or-
dering. Since a decrease would be taken as an in-
dication of an increased degree of ordering such an
interpretation would be unlikely anyway. Al-
though electrochemical activation of graphite has
been employed frequently before the effect of this
treatment on the vibrational properties has not yet
been studied. Based on the suggestions of Com-
pagnini et al. [24] it seems to be likely, that acti-
vation causes additional spectral features at higher
wavenumbers, which are merged with the band
already present before activation causing the visi-
ble band shift. Essentially very similar spectra of
silver coated glassy carbon electrodes before and
after various treatments have been reported by
Alsmeyer and McCreery [27,28] and other authors
[29,30]. The latter author also reported bands of
Fig. 1. Cyclic voltammogram of an electrochemically activated
glassy carbon electrode, aqueous solution of 1 M HClO w/o 1
mM AgCH
COO, dE=dt ¼ 5 mV/s.
4
3
particles of nanometre size can be seen. The size of
the small particle corresponds exactly to the opti-
mum surface roughness established with nano-
particles of approx. 100 nm size for silver surfaces
reported by Mo et al. [19,20].
Because GC itself causes two distinct Raman
bands spectra were recorded before and after
electrochemical activation as a reference. They are
displayed in Fig. 3; they closely resemble those
reported by e.g. Ray and McCreery [21,22]. The
À
À
adsorbed anions (NO , SCN ) on silver layers of
3
unspecified thickness. According to the experi-
mental data about 10 or more monolayers were
deposited although the displayed micrographs
seem to imply a localised deposit similar to the one
observed here.
À1
positions of the E -mode around 1607 Æ 4 cm
2g
À1
and of the A -mode around 1360 Æ 7 cm (this
1g
mode is not present with highly ordered graphite,
consequently it has also been called D-band (dis-
order band)) differ only slightly from the values of
SER spectra of a silver-modified GC electrode
À1
1360 and 1582–1590 cm observed by the latter
authors (values for the former band have been
reported down to 1316 cm [23]). The electro-
2 4
in contact with a solution of 0.1 M K SO are
displayed in Fig. 4, reproducibility and details are
visible in the inset. A direct comparison with the
spectrum obtained with the unmodified surface
and the mere fact, that significant bands can be
discerned easily implies, that the silver deposits
being present on the GC substrate are SER active
materials without any further activation procedure
(oxidation–reduction cycling) commonly em-
ployed with solid silver electrodes in order to
confer SER activity to this material. Besides the
bands assigned to the modes of the carbon mate-
À1
chemical roughening causes a decrease of scattered
intensity; in addition it induces a small shift of the
E -mode. Compagnini et al. have studied Raman
2g
spectra of virgin and damaged edge graphite
planes extensively [24]. The A1g-mode found with
highly oriented pyrolytic graphite only when the
edge plane is illuminated was taken as an indicator
of crystallographic orientation with this material,
its intensity with respect to the E -mode is also
2g
frequently used as a measure of the degree of
disordering of a carbon material. Knight and
White have assigned this band to the first order
phonon at the M point (the 1000 zone point) of the
rial only a single band of considerable intensity
À1
around 230 cm
band around 985 cm is caused by the symmetric
is found. A further weak
À1
m
mode of dissolved sulphate ions; the small
1

58
P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
Fig. 2. Scanning electron microscope pictures of a silver-modified GC surface after electrochemical activation, magnifications as in-
dicated.
À1
difference with respect to the value of 981 cm
reported for this mode found with a solution of
sulphate ions is too small to warrant assignment of
this mode to an adsorbed species [31–33]. The
band at low wavenumbers is caused by the silver–
oxygen stretching mode of adsorbed sulphate ions
on the silver deposits as suggested by Chen et al.
electrodes in chloride containing solutions can def-
initely be excluded as all solutions were free of
chloride. In addition the mAg–Cl is located signifi-
À1
cantly higher at about 247 cm [7,35].
The previously reported [36] assignment of a
À1
band at 237 cm as being caused by the silver–
nitrogen mode of adsorbed pyridine (an electrolyte
[
34]. Confusion with the silver–chloride stretching
2 4
solution containing 0.1 M K SO and 0.1 M
mode caused by traces of chloride being possibly
present on the electrode after activation of silver
pyridine (not explicitly stated) was used) is highly
unlikely here. Most likely this assignment was

P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
59
Fig. 3. Surface Raman spectrum of a polished GC surface
measured ex situ before and after electrochemical activation,
Fig. 5. SER spectrum of a solid silver electrode (electrochem-
ically roughened) in contact with an electrolyte solution of 0.1
P
laser ¼ 100 mW, k
0
¼ 514:5 nm.
M K SO , electrode potentials as indicated, P
laser
¼ 200 mW,
2
4
k
0
¼ 514:5 nm.
À1
have assigned the shoulder at 240 cm on the
major band assigned to the mAg–Cl-mode to the
Ag–N-stretching mode.
Further proof is provided by SER spectra of an
electrochemically roughened silver electrode in
2 4
contact with a solution of 0.1 M K SO as shown
in Fig. 5. Only a single band is found around 230
À1
cm , its position depends on the electrode po-
tential as expected for a physisorbed species. The
slight upshift as compared to the position the band
with a silver-modified GC-electrode may be caused
by differences in surface properties of the respec-
tive silver adsorption sites. This assignment is
corroborated by data reported by Pagannone et al.
Fig. 4. Top: SER spectrum of a silver-modified GC electrode
electrochemically activated) in contact with an electrolyte so-
lution of 0.1 M K SO
, E ¼ open circuit, Plaser ¼ 100 mW,
(
2
4
k
0
¼ 514:5 nm; insert: detail of surface-sulphate mode; same
[
chemically etched silver surface at 233 cm and
39], who found the silver-sulphate mode with a
À1
conditions, new spectrum. Bottom: surface Raman spectrum of
a polished GC surface (electrochemically activated) in contact
À1
À1
with an electrolyte solution of 0.1 M K
¼ 514:5 nm.
2
SO
4
, E ¼ open circuit,
the internal sulphate mode at 968 cm (as com-
pared to the band position of 981 cm for the
dissolved species (see above)).
P
laser ¼ 100 mW, k
0
A SER spectrum of a silver-modified GC-elec-
trode recorded with an electrolyte solution con-
taining in addition 50 mM of pyridine as shown in
Figs. 6 and 7 closely resembles previously reported
spectra obtained with silver colloids by Creighton
et al. [40,41] and with solid silver electrodes after
ORC activation procedures [42,43]. The similari-
ties to spectra obtained with an electrochemically
roughened silver electrode are also close although
made without recording blank spectra with an
electrolyte solution containing no pyridine (The
report by the same authors quoted in support of
this assignment [37] does not contain any infor-
mation pertaining to adsorbed pyridine.). This
interpretation is straightforwardly supported by
the observation, that in a preceding paper by these
authors [2] in a corresponding spectrum recorded
with a supporting aqueous electrolyte solution of
À1
especially the bands around 600–700 cm are less
intense here. For direct comparison a Raman
spectrum of liquid pyridine is included in Fig. 7. A
0.1 M KCl no such band was found. It should
nevertheless be mentioned, that Rogers et al. [38]

60
P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
Fig. 8. SER spectrum of a solid silver electrode (electrochem-
ically roughened) in contact with an electrolyte solution of
Fig. 6. SER spectrum of a silver-modified GC electrode (elec-
trochemically activated) in contact with an electrolyte solution
50 mM pyridine þ 0:1 M K SO , E
¼ À400 mV circuit,
2
4
Hg2SO4
P
laser
¼ 100 mW, k ¼ 514:5 nm.
0
of 50 mM pyridine þ 0:1 M K
00 mW, k
¼ 514:5 nm.
2
SO
4
, EHg SO4 ¼ À600 mV, Plaser
¼
2
2
0
tetra-n-butylammonium bromide) Pemberton re-
À1
corded a value of 221 cm [48]. With a silver sol a
À1
value of 239 cm was found [49]. It is noteworthy,
that the Ag–N-stretching mode has been found
shifted to other values with related nitrogen-con-
taining compounds. Compagnini et al. [50] have
À1
reported a value of 240 cm for 1-methyl-2-pyr-
rolidinone. Yu et al. [51] have assigned a mode
À1
found at 228 cm to the Ag–N-stretching mode of
0
adsorbed 2,2 -bi-pyridine adsorbed on a silver sol.
On the contrary Kim and Itoh could not identify
this band [52]. Under UHV-conditions the absence
of this band has been stated repeatedly [53–55].
Assignment of the observed vibrational bands
as listed in Table 1 is based on literature sources
Fig. 7. SER spectrum of a silver-modified GC electrode (elec-
trochemically activated) in contact with an electrolyte solution
[
56–58].
The position of the ring breathing mode is ob-
of 50 mM pyridine þ 0:1 M K
00 mW, k
2
SO
¼ 514:5 nm; top spectrum: liquid pyridine.
4
, EHg SO4 ¼ þ200 mV, Plaser
¼
2
2
0
À1
served around 1010 cm . In a study of pyridine
adsorbed on electrochemical roughened platinum
this was taken as indication of pyridine–metal in-
teraction via the lone electron pair of the nitrogen
atom [59] in turn implying a more or less perpen-
dicular adsorbate orientation. The dominance of
SER spectrum obtained with a silver electrode
electrochemically roughened in a sulphate elec-
trolyte solution in order to avoid chloride con-
tamination obtained under similar experimental
conditions is displayed in Fig. 8.
further a -modes (in-plane molecular motions)
1
The mAg–N-mode of adsorbed pyridine on
roughened silver electrodes has been reported in-
frequently only (it is also missing here), a value
also implies a preferably perpendicular orientation
of the molecule based on the arguments (selection
rules [60,61]) previously provided.
A flat adsorbate orientation at low pyridinium
concentration has been derived in detail by Weitz
et al. [62] followed by a transition to a tilted or
perpendicular one at high pyridine concentration
À1
around 222 cm has been mentioned [44] whereas
À1
Hwang and Chang found a value of 230–240 cm
À1
[45] and Loo at 237–239 cm [46,47]. Using a
nonaqueous electrolyte solution (pyridine and

P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
61
Table 1
Assignment of vibrational modes observed with Raman spectroscopy, all values in cm
À1
Wilson # Symmetry Mode de-
scription
Neat liquid Ag=C þ pyridine
Solid Ag, EHg2SO4
¼ À400 mV (Fig. 8)
(Fig. 7)
EHg2SO4 ¼ À600
E
Hg SO
2
E
Hg SO ¼ 200
4
2
4
(
Fig. 6)
¼ À200
(Fig. 7)
–
–
–
–
6
6
1
–
–
–
–
a
b
b
–
–
290
402
495
544
621
–
–
–
–
–
321
–
–
(429)
–
–
–
–
(448)
–
–
–
–
–
a
b
1
1
2
1
i.p. def.
i.p. def.
604
653
621
–
629
–
–
666
–
o.o.p. C–H 706
def.
–
–
–
m
1
1
SO4
–
a
a
–
Ring breath 993
–
–
–
987
–
1
1
1005
1035
1006
1036
1009
1037
1023
1053
2
a
Trig. ring
breath
1032
–
9
–
–
i.p. C–H
–
1220
1157
1208/1217
–
1216
–
1215
–
1229
a
1
def.
–
3
b
2
–
–
1275
1357
–
–
–
–
–
–
A
GC
1g-mode,
–
a
1
1
8
4
b
2
a
1
a
1
–
–
–
–
–
1353
–
9a
a
Ring def.
Ring def.
–
1485
1580
–
–
–
–
1594
1596
1597
–
1598
–
1613
–
E
2g-mode,
GC
–
–
–
–
3057
1651
–
–
–
–
–
–
–
2
a
1
m
C–H
a
This mode is sometimes also called D-band (disorder band).
comparable to those employed here. The per-
pendicular orientation concluded from the data
reported above has also been observed before
[
63,64].
As further proof of the surface enhancement
capability of the silver-modified GC surface vari-
ous other simple adsorbates (halide ions, various
oxo anions) were studied. As an example SER
spectra of adsorbed bromide anions on silver-
modified GC electrodes are displayed in Fig. 9.
Because of the high concentration of bromide a
maximum coverage is always ensured. The posi-
tion of the band assigned to the silver–bromide
stretching mode shifts downwards with decreasing
electrode potential. This is in agreement with
previous calculations by Nichols and Hexter [65]
and experimental observations with silver colloids
reported by Garrell et al. [66] as well as with silver
electrodes [67–71].
Fig. 9. SER spectrum of a silver-modified GC electrode (elec-
trochemically activated) in contact with an electrolyte solution
of 0.1 M KBr, electrode potentials as indicated, Plaser ¼ 200
mW, k
0
¼ 514:5 nm.

6
2
P. Mayer, R. Holze / Surface Science 522 (2003) 55–63
[25] D.S. Knight, W.B. White, J. Mater. Res. 4 (1989) 385.
Acknowledgements
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26] See also F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53 (1970)
126;
R.J. Nemanich, S.A. Solin, Phys. Rev. B 20 (1979) 392.
1
Support of this work by the Deutsche Fors-
chungsgemeinschaft, the Fonds der Chemischen
Industrie and the Freistaat Sachsen is gratefully
appreciated.
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1
[29] H. Ishida, H. Fukuda, G. Katagiri, A. Ishitani, Appl.
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