SERS of Pyridine adsorbed on rhodium electrodes

8/10/2019 SERS of Pyridine adsorbed on rhodium electrodes

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Volume 171, number 1,2

CHEMICAL PHYSICS LETTERS

27 July 1990

SERS of pyridine adsorbed on rhodium electrodes *

S.A. Bilmes

Departamento de Quimica Inorghica, Anaiitica y Quimica-Fisica, Facultad de Ciencias Exactas y Nalurales,

Universidad de Buenos Aim, Ciudad UniversicariaPub II, 1428 Buenos Airef, Argentina

Received 16 January 1990; in tinal form 4 May 1990

SERS for pyridine adsorbed on a Rh electrode is presented. SERS-active Rh electrodes were electrochemically obtained in 1 M

KOH after the growing of a thick layer of rhodium oxide, Raman spectra for adsorbed pyridine are potential dependent. Results

are compared with those for pyridine adsorbed on Rh-covered SERS-active silver electrodes.

1. Introduction

Surface-enhanced Raman scattering (SERS) has

been improved as a powerful technique for in situ

analysis of the adsorbate configuration on metal sur-

faces [ 11. Ag, Au and Cu are the most common met-

als employed in SERS as they exhibit the greatest en-

hancement of the Raman-scattering cross section for

many adsorbates, ranging from lo4 to lo6 [2].

Since the discovery of SERS

[

3

1,

great effort has

been made to demonstrate its operation in metals

with high catalytic activity. Although the enhance-

ment of the Raman-scattering cross section for ad-

sorbates on d-metals is several orders of magnitude

lower than that reported for sp-metals (i.e. Ag, Au,

Cu ), it has been possible to detect Raman scattering

for pyridine adsorbed on Pt electrodes [41, sput-

tered Pt [ 51, Pt colloids [ 61 and Rh colloids [ 7 1.

Recently, a very attractive indirect method in which

layers of Pt or Rh are electrodeposited onto SERS-

active Ag electrodes has allowed SERS measure-

ments for several adsorbates on these substrates

[

8

1.

In these experiments, the Raman spectrum differs

from that measured for the same adsorbate on silver.

The origin of SERS is not yet clear; it is therefore

not possible to predict the influence of surface to-

pography in order to optimize the conditions for

maximal Raman intensity. Up to now, based on ex-

perimental evidence, all models agree that some form

*

Dedicated to the memory of Professor M. Cristina Giordano.

of roughness on an undefined scale is a necessary (but

no sufftcient) condition for SERS [ I]. Electrody-

namic calculations [ 91 predict a maximal enhance-

ment value of 134 for the local electromagnetic field

on Rh spheroids in air with 3 1 nm semi-major axes

and at 300 nm incident wavelength.

In this work, the Raman spectra of pyridine (Py

)

adsorbed on Rh electrodes are presented as a func-

tion of potential and surface conditions. Results are

compared with those reported in ref. [B] corre-

sponding to Py adsorbed on Rh-modified Ag SERS-

active electrodes.

2. Experimental

The electrochemical cell was a conventional three-

electrode system. The electrolyte solution was 1 M

KOH. The working electrode was a circular Rh foil

(Johnson Matthey, spectroscopically pure; 0.6 cm

diameter, 1 mm thick) with one of its sides in con-

tact with a copper rod for electrical contact. The

whole assembly was embedded in teflon under pres-

sure and only one side of the metal was in contact

with the electrolyte. The counter electrode was a large

Rh wire. A saturated calomel electrode (SCE) was

empolyed as reference. The reference compartment

was filled with 0.1 M Na2S04. By doing this, a po-

tential difference of a few mV builds up between the

Luggin capillary and the working solution, but the

formation of a Hg oxide at the SCE is prevented.

0009-2614/90/$03.50 0 1990 - Elsevier Science Publishers B.V. (North-Holland)

141


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Volume 71,number1,2

CHEMICALHYSICS

ETTERS

27 July 1990

The optical set-up for Raman spectroscopy was the

same as previously described [ 4 1.The Raman spec-

trometer was a Spex 1404 double monochromator

with holographic gratings (2400 grooves/mm). A

cooled RCA (C 31034-02) photomultiplier was used

as detector. The sample was irradiated by the 514.5

nm line of an Ar-ion laser operating at 100-200 mW

measured at the spectroelectrochemical cell. Light

was polarized parallel to the incident plane. To avoid

local heating, cylindrical lenses were used to focus

the laser line on the sample. The incident angle was

circa 45. Spectra were obtained with a spectral slit

width of 5 cm- and at 0.1 A s- or 0.5 8, s- de-

pending on the scanned energy range.

All reagents were analytical grade used without

further purification. Water was bidistilled in quartz.

Preparation of the electrode surface. Prior to each

experiment the electrode was polished with alumina

paper up to 1 urn and rinsed with bidistilled water.

The electrode was immersed in 1 M KOH and the

potential continuously scanned at 0.1 V/s between

- 1 O and 0.1 V for circa 15 min until a reproducible

voltammogram was obtained. The potential

range

was

then extended to 0.5 V for 2 min and the sweep

rate increased to 10 V s- for 15 min. After this &ac-

tivation procedure, the potential sweep rate was

lowered to 0.1 V

S .

Fig. 1 shows the

E/I

profiles corresponding to a

Rh electrode before (a) and after (b) the fast po-

tential perturbation. The main feature in the voltam-

mogram of the activated electrode is a reversible

couple at circa 0.3 V, indicating the formation of

Rhz03 [ 10, I 11.

In a 1 M KOH+0.036 M Py solution, the voltam-

mogram of the non-activated electrode (fig. 1,dashed

line) exhibits a decrease in the currents related to

the H-electroadsorption/electrodesorption process

( - 1 OQE -0.7 V) and to the O-monolayer for-

mation (-0.7 QEcO.~ V). These current deple-

tions indicate that pyridine is adsorbed (at least for

E~0.1

V) and inhibits both H- and O-monolayer

formation. Py adsorption does not influence the ac-

tivation process: the E/I profile after the fast PO

tential perturbation is not appreciably changed by

Py (see fig. 1b, full and dashed lines ).

Raman spectra of Py adsorbed on activated Rh

electrodes are independent of the addition of Py be-

fore or after the fast potential perturbation.

142

2-

-1.0

-8

n .4

72 0 .2 .4

Ii Icr

Fig. 1. Potentiodynamicrofiles

f a Rh electrode in 1

M KOH

(-) and

1M KOH+0.036 M Py (---) before (a) andafter

(b) the activation process. Sweep rate: 0.1 V SK;otential mea-

sured against a saturated calomel electrode.

Activated Rh electrodes are stable (both vol-

tammogram and Raman spectra are reproducible)

at least 24 h after withdrawal of the cell and being

stored in clean conditions. The smooth electrode can

be recovered by polishing or by immersion in hot

concentrated sulfuric acid.

Fast potential perturbation leads to a loss of the

metallic luster, characteristic of mirror-polished rho-

dium, and the surface becomes pale yellow. This kind

of electrode exhibits electrochromic behavior

[

111.

3. Results

Fig. 2 shows the Raman spectrum of Py adsorbed

onto a smooth Rh electrode (fig. 2a) and an acti-

vated Rh electrode (fig. 2b) both immersed in a 1

M KOH+0.036

M

Py at

E=

-0.97 V. Both spectra

cover the energy region corresponding to symmet-

rical and asymmetrical breathing modes of Py [ 121.

Smooth Rh electrodes exhibit two bands at 1004 and

1035 cm- whose location and relative intensity in-

dicate that they correspond to the solution phase close

to the electrode surface. The Raman spectrum for

activated Rh electrodes shows bands at 1004 and


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Volume

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2

CHEMICAL PHYSICS LETTERS

27 July 1990

smooth

a)

00

\

z

Ji

$

z

200

m

00

act ivattd

(b)

RAMAN SHIFT/cm-1

Fig. 2. (a) Raman spectrum of a smooth Rh electrode in I M

KOHS0.036 M Pyaqueous solutionat

E

-0.97 V.1,=514.5

nm, P= 100 mW. (b) Raman spectrum of an activated Rh

electrode under the same conditions as (a).

1035 m-, and a shoulder at circa 1010 cm-. For

these surfaces, the intensity of the 1004 cm- band

is greatly increased in respect to the 1036 cm- band.

Fig. 3 shows the Raman spectrum of an acti-

vated Rh electrode in the same experimental con-

ditions as fig. 2 in the energy region covering the most

intense Raman bands for the pyridine molecule

[

121.

Raman bands are observed at energy values that dif-

fer from those of free pyridine or Py dissolved in

water (table 1). This is an indication that Raman

spectra of figs. 2b and 3 correspond to Py molecules

interacting with the metal surface. The relative in-

tensity of the Raman bands in fig, 3 is qualitatively

different from those found for pure or aqueous

pyridine.

As the potential is varied from - 0.97 to -0.

I

V,

there is a change in the Raman spectra. Fig. 4 shows

the Raman spectra in the 950 to 1050 cm- region

for different potential values. The full half-width at

half-maximum (fhwhm) increases as the potential

goes from -0.97 to -0.4 V. The shoulder at 1010

cm- becomes more resolved and the intensity of the

1004 cm- band decreases. At

E? -0.12

V, where

the first stage of rhodium surface oxidation occurs,

the Raman signal is totally quenched and the spec-

trum is that corresponding to the solution phase close

to the electrode surface. The band at 1004 cm- is

totally recovered at -0.97 V after oxide reduction.

4. Discussion

For many adsorbates, the Stokes shifts differ from

those of free or solvated molecules because of the

metal-molecule interaction. The electric field in the

electrode-electrolyte double layer also modifies the

molecular dipole moment, farce constants and atom-

atom radii. These criteria are usually employed for

vibrational assignment of adsorbates. In electro-

chemical systems, band intensity and location are

expected to be potential dependent. In this frame-

work, Raman spectra depicted in figs. 2-4 corre-

spond to the Stokes shifts for Py adsorbed on Rh

electrodes. Table 1 summarizes the most important

vibrational modes for some Py/Rh systems in com-

parison with free Py and 0.36 M Py in water.

The relative intensities of the Raman bands (fig.

3) for Py adsorbed on activated Rh electrodes is

different from that found for liquid Py and Py dis-

solved in water. It should be noted that the sym-

metrical breathing ( 1004 cm-) to ring stretching

( 1590 cm-l

)

intensity ratio is nearly unity in fig. 3

whereas this ratio is circa 30 for aqueous Py, circa

5 for SERS of Py adsorbed on Ag electrodes at - 0.7

V and circa 3 for Py adsorbed on electrodispersed Pt

electrodes at -0.2 V [4]. Hence, the Raman-scat-

tering cross section is relatively enhanced for the dif-

ferent vibrational modes as usually found in SERS

ill*

The location for the breathing mode of pyridine

indicates that Py is bounded to the Rh surface

through the N lone-pair electrons, i.e. o interaction.

For a flat position, i.e. IC nteraction, greater shifts

are expected. The same configuration was proposed

for Py adsorbed on electrodispersed Pt electrodes

[4], and for Rh-covered Ag SERS active surfaces

[

81. However, there is a smaller shift for Py ad-

sorbed on Rh electrodes probably due to a weaker

Py-metal interaction.

From the dependence of Raman spectra on po-

tential (fig. 4), it is apparent that the maximal band

143


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Volume 171, number I,2

CHEMICALPHYSICSLETTERS 27 July 1990

--I)

Rh/ l MKOH+O. O36M CSHIN

h ,,=51&.5nm

B-0.97V

P*l SOmW

1590

-1

200

600 1550

RAMAN SHI FT/ cm

Fig. 3. Raman spectrum of pyridine adsorbed on an activated rhodium electrode in the 600-l 700 cm- range. In the energy regions

not plotted, no bands were detected.

Table 1

Vibrational frequencies of pyridine in systems nvolving Rh-Py interaction )

Symmetry

Mode Description

Pure Py b, Py/H,O b Py/HzO =I

Py/Rh(lll)d

Py/Rh )

-0.97 V)

chemis. multil.

AI 8a ring stretching 1582

1593 1595

19a ring stretching 1483 1488 1489

9a in-plane CH bend 1217

1217 1217

18a in-plane CH bend 1069

1069 1070

12

assym.

ring breathing 1030

1033 1035

1

symm.

ring breathing 991

1001 1003

6a in-plane ring deform. 604

616 617

RI 8b ring stretching 1574

1576 1576

19b ring stretching 1437

1441

3 in-plane CC bend 1227

1231 1234

15 in-plane CH bend 1146

1150 1152

6b in-plane ring deform. 654

653 652

) Some bands are assigned to more than one mode due to uncertainity in the assignment.

b, Wilson numbers. c, Ref. [ 121. d,This work. c, HREELSdata, ref. [ 131.

1550 1590 1590

1420 1450

1240 1220 1209

1068

1025 1000 1004

635 626

1550 1590 1590

1420 1450

1240 1220 1209

1130

635 626

intensity is obtained at the most:negative potentials.

quenching of the signal is noticed when the inter-

face potential is greater than -0.12 V. This fact can

be interpreted either by a decrease in Py surface con-

centration as the surface is covered by O-containing

species or by an increase in the radiation absorption

by the surface. The splitting of the symmetrical

breathing modes in two bands at 1Cl04 nd 10 10 cm-

is more evident at more positive potentials. It may

be possible that two forms of pyridine are coad-

sorbed on different sites of the Rh surface, the rel-

ative surface concentration of each one being poten-

tial dependent. These two species can also be assigned

to physisorbed Py and chemisorbed Py, the latter

predominating at more positive potentials.

The location of Raman bands for Py adsorbed on

activated Rh electrodes presented in this work are

different from that reported by Feilchenfeld et al.

[

81

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Volume 17 1, number 1,2 CHEMICAL PHYSICS LETTERS

27 July 1990

------141Loo

1050

1025 1000 975

R M N SHIFT/et@

Fig. 4, Potential dependence for breathing mode of adsorbed Py.

Spectra were run in the same experimental conditions as in tig. 2

holding the potential at the indicated values.

with Rh-covered SERS-active silver electrodes. These

authors found that at

E=

-0.1 V, the symmetrical

breathing mode for adsorbed Py lies between 1015

and 1020 cm- and the ring deformation mode be-

tween 638 and 643 cm-. The

location

of these bands

depends on the amount of electrodeposited rho-

dium. This difference can be attributed to the dif-

ferent experimental conditions, namely, pH and ap-

plied potential. It is possible that under the pH

conditions of ref. [ 81, some amount of pyridinium

is also present at the interface, shifting the vibra-

tional modes

[

141.

On

the other hand, the sym-

metrical/asymmetrical breathing-mode intensity ra-

tio in this work is similar to that reported in ref. [ 81

for high Rh coverages (> 20 ML) on silver.

The real surface area of both smooth and acti-

vated Rh electrodes cannot be measured by the H-

monolayer charge due to the overlap between oxide

reduction and H electroadsorption. However, the

current in the - 1.0 to -0.5 V range is lower than

that of H electroadsorption on electrodispersed Pt

electrodes with roughness factor,

R

greater than

10. On the other hand, in the present system the

symmetrical breathing-mode intensity is of the same

order of magnitude as that measured on electrodis-

persed Pt electrodes with

R > 50

(using thelsame op-

tical set-up and laser power). Taking into account

that the estimated Raman-scattering cross section of

Py adsorbed on electrodispersed Pt electrodes is two

orders of magnitude larger than that of Py dissolved

in water, and assuming that the real surface area of

activated Rh electrodes is lower than that of elec-

trodispersed Pt, one can roughly estimate an en-

hancement factor of about 100 for the symmetrical

breathing mode of Py adsorbed on Rh electrodes.

The method employed for the preparation of a

SERS-active Rh electrode

is

conceptually equivalent

to that employed for Pt: the oxide thick layer formed

by potential perturbation provides, upon electrore-

duction, a topography able to produce enhancement

of the Raman cross section of the adsorbate. There-

fore, it can be postulated that a general procedure to

generate suitable surfaces for SERS is the electro-

reduction of thick layers of insoluble metal surface

compounds

(oxides, halides, sulfates) previously

built-up by oxidation of the metal surface.

5. Conclusion

Rhodium electrodes activated by the formation

and reduction of a thick oxide layer in

I

M KOH al-

low the detection of the Raman spectrum of ad-

sorbed pyridine. The potential dependence of the

Raman spectra indicates that two forms of Py are

coadsorbed on the Rh surface, both coordinated to

the metal through the N atom. The possibility of eas-

ily detecting a monolayer of adsorbed Py indicates

that some enhancement of the Raman-scattering

cross section of the adsorbate is involved.

xl

Polycrystalline Rh and polycrystalline Pt electrodes have nearly

the same number of surface sites per unit area, see ref. [ 15

1.

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CHEMICAL PHYSICS LETTERS 27 July 1990

Acknowledgement

These experiments were made in the Physikal-

isches Institut III at the Heinrich-Heine-Universittit

Diisseldorfas a Fellow of Consejo de Investigaciones

Cientificas y TCcnicas (CONICET) from Argentina.

I am indebted to Professor A. Otto and his research

group for the kind hospitality, experimental help and

stimulating discussions. I greatly acknowledge the

assistance of Professor A.J. Arvia in giving me the

rhodium electrodes and for many fruitful discussions.

References

[ 11 R.K. Chang, Ber. Bunsenges. Physik. Chem. 91 ( 1987) 296;

M. Moskovits, Rev. Mod. Phys. 57 (1985) 783;

A. Otto, in: Light scattering in solids, Vol. 4, eds. M. Cardona

and G. Guntherodt (Springer, Berlin, 1983).

[21 M.J. Weaver, S. Farquarson and M.A. Tadayyoni, J. Chem.

Pbys. 82 1985) 4867;

M.G. Albrecht and J.A. Crieghton, J. Am. Chem. Sot. 99

(1977) 5215.

[ 31 M. Fleischmann, P.J. Hendra and A.J. McQuillan, Chem.

Phys. Letters 26 (1974) 163;

D.L. Jeanmaire and R.P. van Duyne, J. Electroanal. Chem.

84 (1977) 1.

[4] S.A. Bilmes, J.C. Rubim, A. Otto and A.J. Arvia, Chem.

Phys. letters 159 1989) 89.

[

51 H. Yamada and Y. Yamamoto, Chem. Phys. Letters 77

(1981) 520.

[6] R.E. Benner, K.U. von Raben, KC. Lee, J.F. Owen, R.K.

Chang and B.L. Laube, Chem. Phys. Letters 96 (1983) 65.

[7] W.L. Parker, R.M. Hexter and A.R. Siedle, Chem. Phys.

Letters 107 (1984) 96.

[8] H. Feilchenfeld, X. Gao and M.J. Weaver, Chem. Phys.

Letters 161 (1989) 321.

[9] M.P. Cline, P.W. Barber and R.K. Chang, J. Opt. Sot. Am.

B3(1986) 15.

[ IO] L.D. Burke and E.J.M. OSullivan, J. Electroanal. Chem. 93

(1978) 11.

[ 1I] S. Gottesfeld, J. Electrochem. Sot. 127 I 980) 272.

[ 121H.D. Stidhamm and D.P. DiLella, J. Raman Spectry. 8

(1979) 180.

[

131C.M. Mate, G.A. Somorjai, H.W.K. Tom, X.D. Zhu and

Y.R. Shen, J. Cbem. Pbys. 88 ( 1988) 441.

[ 141 S.C. Sun, I. Bernard, R.L. Birke and J.R. Lombardi, J.

Electroanal. Chem. 195 1985) 359;

D.J. Rogers, SD. Luck, D.E. Irish, D.A. Guzonas and G.F.

Atkinson, J. Electroanal. Chem. 167 1984) 237.

[

151R. Woods, Electroanal. Chem. 9 ( 1977)

1,

146

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SERS of Pyridine adsorbed on rhodium electrodes