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EVIDENCE FOR LOW DENSITY OF ACTIVE SITES IN SURFACE-ENHANCED RAMAN SCATTERING
B. Pettinger, L. Moerl
To cite this version:
B. Pettinger, L. Moerl. EVIDENCE FOR LOW DENSITY OF ACTIVE SITES IN SURFACE-
ENHANCED RAMAN SCATTERING. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-333-
C10-336. �10.1051/jphyscol:19831066�. �jpa-00223524�
Colloque CIO, supplément au n°12, Tome 44, décembre 1983 page C10-333
EVIDENCE FOR LOW DENSITY OF ACTIVE SITES IN SURFACE-ENHANCED RAMAN SCATTERING
B. P e t t i n g e r and L . - M o e r l
Fritz-Haber-Institut der Max-Planck-Gesellschaft, Favadayweg 4-6, D-1000 Berlin 33, F.R.G.
Résumé - La d i f f u s i o n Raman exaltée de surface (SERS), u t i l i s a n t l a p y r i d i n e comme molécule sonde, a été étudiée en m o d i f i a n t les électrodes d'Ag par des dépôts de Cu i n f é r i e u r s à l a monocouche. Dès un taux de recouvrement e = 0,003, l e s s i t e s a c t i f s pour l e SERS, associés aux atomes de Cu, deviennent plus s t a b l e s , e t de nouvelles bandes typiques de complexes de surface Cu-pyridine a p p a r a i s s e n t , t a n d i s que c e l l e s des complexes A g - p y r i d i n e s ' a f f a i b l i s s e n t . L ' a m p l i f i c a t i o n des champs é l e c t r o - magnétiques pour des é l e c t r o d e s rugueuses é t a n t estimée i n f é r i e u r e à 1000, ces
e f f e t s mettent en évidence l e r ô l e dominant des processus d ' e x a l t a t i o n l o c a l e dans SERS.
Abstract - Surface-enhanced Raman scattering (SERS) using pyridine as probe molecule was studied by modification of Ag-electrodes with depo- sition of Cu submonolayers. Already at a coverage of 0 = 0.003 (SERS) active sites, so far associated with Cu atoms, become more stable and new bands typically for Cu-pyridine surface complexes begin to rise whereas those for Ag-pyridine drop. Since electromagnetic enhancement at roughened electrodes is estimated to be less than 1000, these ef- fects evidence the dominant role of local enhancement processes in SERS.
1 - INTRODUCTION
As shown by many theoretical and experimental works /1-6/, surface-en- hanced Raman spectroscopy is based on two enhancement processes which are referred to as being of "electromagnetic" and "chemical'' (or :!lo- cal") nature. Their particular weight will depend on the current ex- perimental situation.
For a better understanding of the physical processes underlying the enhancement one has to separate the various contributions and evaluate their important parameters. An example in this direction is given in earlier papers showing that metal overlayers on Ag modify SERS for pyridine on Ag in a very significant way /4-6/. A deposit of 3 % of a ML of Tl is sufficient to destroy SERS irreversibly / 6 / . With traces of Cu equivalent to 0.003 of a monolayer the SERS quenching treat- ment is no longer as efficient as on a pure Ag electrode (the intensity drops only to 30 % instead of to 5 % of the initial value) /4,5/. This clearly speaks for the existence and low density of active sites.
In the first part of this communication we present unique spectroscopic evidence for these active sites, based upon the appearance of two well separated frequencies for the same (pyridine) vibrational mode, after modification of the surface by a small amount of Cu. In the second part we try to estimate the height of the "classical" electromagnetic en- hancement in the case of a roughened electrode, referring to our expe- rimental results and recent theoretical calculations.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831066
JOURNAL DE PHYSIQUE
2
-
EXPERIMENTALThe experimental set-up for Xaman in-situ spectroscopy has been des- cribed elsewhere /7/, and the parameters for performing the experiment are identical to those defined in Ref. /4,5/.
3
-
RESULTS AND DISCUSSIONsERS-and-cu-Ge~ositL~~i
As is well known, an oxidation reduction cycle (ORC) is necessary to generate intense SERS. It leads in our ezperiments to a fivefola Ln- crease of the current surface area. With this enlargement and with a deposition rate of 2.7 VC cm /min for a solution containing 2 M cu2+
the Cu coverage rises at a rate of 0.0015 monolayer/min (ML/min), which is low enough to monitor the development of Raman bands with growing coverage. For details in determining the deposition rate the reader is referred to Ref. /4/.
Table 1
Strong modes for pyridine on Ag and Cu electrodes
assignment Ag A = 514 647 Cu A = 647 nm
6 a 623 m m 635 m
1 1007 vs vs 1 0 1 4 vs
12 1035 s m 1041 w
9 a 1215 s w 1219 w
8 a 1593 s m 1602 m
Among the pyridine modes listed in Table 1 the 6a vibration responds most significantly to a change of the substrate, and is also quite sen-
sitive to a small amount of copper deposition. This is shown in Fig-la with a three-dimensional plot of the SERS intensity vs. energy [l/cml and coverage
[@I.
Since it takes only two minutes to record each spec- trum, the coverage rises by about 0.003 ML between subsequent spectra.L
Q Q Q
-
Ln c-3
-
F I- -
G 8
Z Q -
yR-
zH 8 8 .
Ln 8
ENERGY [ c rn- '1 ENERGY [ c rn- '1
Fig, 1: Influence of Cu deposition on SERS 6a vibrations of pyridine on Ag-electrodes. USCE = -0.7 V; Electrolyte: 0.1 M C1-, 0.05 M pyridine and CuC12. Laser power: 100
mE,
647 nm. a) Sequence of spectra during continuous Cu deposition (10 M CuC12). b) Continuous Cu deposition, left part of a-y: spectra at O = 0.016, OCU = 0.023, O = 0.16;right part a-y: series of spectra, after quenching pulse F%.7 Cu V +
-1.7 V for 5 s + 19.7 V) applied at above OCU values, during continuous Cu deposition (10 M CuC12)
.
~g/pyridine/chloride system drops, and a new band rises at 635 [l/cml, identical to that of the 6a vibration of the pure Cu/pyridine/chloride system. Most of the drop of the former band and most of the rise of the latter one takes place in a coverage range of 0 < O < 0.1; with con- tinuing copper deposition, further spectroscopic changes are minlmal.
Obv~ously, a replacement of pyridine-Ag bonding by a pyridine-Cu one occurs, leading to the growth of a new band with the deposited amount of Cu.
Very interesting is the effect of an SERS quenching treatment of the electrode. It consists of applying a cathodic potential pulse of -1.7 V vs. SCE for 5 sec to an SERS-displaying electrode. Returning to the initial potential at -0.7 V, those species such as chloride and pyri- dine which have been desorbed temporarily due to the very cathodic po- potential, readsorb at a surface which is still rough. However, we found a drop of the initial SERS intensity, defined as 100 % before the quenching pulse, down to 10 % afterwards in the case of pure Ag- electrodes. This behaviour changes significantly with the deposition of Cu on the substrate as shown in Fig. Ib. In these experiments, copper deposition takes place as in Fig. la. However, at O = 0.013, 0.023 and 0.16 (Fig. Ib, a-y) a quenching pulse is applied to the electrodes;
afterwards Cu deposition goes on as usual. The left parts of Fig. Ib, a-y show the spectrum recorded just before the quenching pulse, where- as the right parts give an impression of the sudden spectroscopic change due to the pulse and the following (slow) developments in the spectra. With growing copper coverage the quenching treatment becomes more and more inefficient and the percentage of remaining intensity
rises, although it is very different for the 6a(Ag) and 6a(Ag/Cu) bands
(a: 9 %, 43 %; B: 16 %, 51 %; y: 50 %, 77 % ) . Obviously, the more Cu
deposited, the more stable are the SERS sites and, hence, the less the reduction in SERS intensity. This is essentially the same finding as earlier reported for the v(l) and v(12) modes of pyridine /4,5/; how- ever, the individual developments of the v(6a,~g) and v(6a,Ag/Cu) vi- bration can be followed from the beginning of the Cu deposition. ~t evidences that only a fraction Of a ML of adsorbed pyridine molecules contributes to SERS.
From the fact that only a minority of adsorbed pyridine molecules is involved in SERS, it is obvious that the local part of the enhancement for roughened electrodes is by about two orders of magnitude larger than earlier estimated / 4 / , indicating an overestimation of the role of the electromagnetic contribution to SERS. This conclusion is confirmed by theoretical calculations. Here the arbitrary roughness is replaced by a sinusoidal grating with grating constants much below the wave- length of light (D = 100 9 ) . Applying the Fourier Series Method (FSM) /8/, the EM fields can be calculated with sufficient accuracy, provided the Rayleigh expansion is taken to high enough orders (N > 30). Fig. 2 shows a nearly linear dependence of the square of the surface field with the ratio of grating amplitude to grating constant (A/D). Due to the weak increase of the EM fields with the grating amplitude, the EM enhancement factor, which is roughly proportional to the fourth power of the EM surface field / 9 / , reaches only a factor of % 125 at A/D = 50 %. Calculations for higher A/D ratios may not be sufficiently accu- rate due to the limited memory of the computer used (for an n x n ma- trix: n = 2*(2-N+I), N >> 301, but preliminary results indicate a weaker rise of < ES >2. Since this linear behaviour for A/D is found for a wide range of D (10 < D < 2000
8
was investigated), one must con- clude that EM enhancement is weak, as long as one deals with sinusoidalJOURNAL DE PHYSIQUE
Fig. 2: Square of surface fields < E >2 at the top of sinusoidal grating amplitude: D = 100 2, 0 5 A 50
9.
Calculations based on the FSM method /8/.gratings with grating constants D < 5000 3 . For D > 5000
2
the calcu- lations show that such gratings exhibit typical reflection anomalies due to surface plasmon polariton excitation, at well defined angles of incidence. The normalized fourth power of the surface EM field and, hence, the enhancement factor can reach a maximum of about 10000 at a ratio of A/D % 4 %. With larger ratios the possible enhancement factor drops.4
-
CONCLUSIONOnly for very special surface geometries such as spheres, ellipsoids, gratings, is the EM enhancement large. A rough surface, represented by a superposition of a set of sinusoidal gratings with corrugation length much below the wavelength of light, displays only a weak EM enhancement factor (F % 100). This theoretical estimation based on "exact" field calculations according to the grating theory is in agreement with our experimental results. Since the total enhancement (for pyridine on Ag) ranges between
lo6
and lo7, our results speak for the important role of local, short-range enhancement processes in SERS.ACKNOWLEDGEMENTS: We thank Professor H. Gerischer for his continuing support and stimulating interest, and Professor F. Forstmann, Profes- sor R. R. Gerhardts and Dr. K. Kempa for valuable discussions.
REFERENCES
I . "Surface Enhanced Raman Scattering", ed. by R. K. Chang, T. E. Fur- tak (Plenum Publ. Co., 1982)
.
2. P. K. Aravind, E. Hood and H. Netiu, Surf. Sci. 109 (1981) 95.
3. B. Billmann, A. Otto, Solid State Comrnun. 44 (1982) 105.
4.
F.
Moerl and B. Pettinger, Solid State ~ o K n .43
(1982) 315.5. B. Pettinger and L. Moerl, J. Electron. Spectr. 29 (1983) 383.
6. T. Watanabe, N. Yanagihara, K. Honda, B. petting= and L. Moerl, Chem. Phys. Lett.
96
(1983) 649.7. B. Pettinger, U. Wenning, D. M. Kolb, Ber. Bunsenges. Phys. Chem. 82 (1978) 1326.
"Electromagnetic Theory of Gratings", ed. by R. petit, springer-Ver- lag Berlin-New York 1980.
L. kccall, P. M. ~latzmann, P. A. Wolf, Phys. Lett.
