Electrochemical Surface Science - American Chemical Society

Furtak, T. E.; Roy, D. Phys. Rev. Lett. 1983, 50, 1301. 7. Roy, D.; Furtak, T. E. J. Chem. Phys. 1984, 81, 4168. 8. Guy, A. L.; Bergami, B.; Pemberton...
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Chapter 27

Effect of Underpotentially Deposited Lead on the Surface-Enhanced Raman Scattering of Interfacial Water at Silver Electrode Surfaces 1

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Jose C. Coria-Garcia and Jeanne E. Pemberton

Department of Chemistry, University of Arizona, Tucson, AZ 85721 The influence of underpotentially deposited Pb monolayers and submonolayers on the surface enhanced Raman s c a t t e r i n g (SERS) of i n t e r f a c i a l H 0 at roughened Ag electrodes i n aqueous chloride and bromide solutions i s presented. With laser e x c i t a t i o n at 5145 Å, the SERS i n t e n s i t y of thev(OH)v i b r a t i o n of i n t e r f a c i a l H 0 a t 3495 cm and 3505 cm in aqueous chloride and bromide, respectively, decreases as the first monolayer of Pb is deposited. The rate of the decrease is e s s e n t i a l l y independent of the nature of the supporting e l e c t r o l y t e anion and is s i g n i f i c a n t l y d i f f e r e n t than the rate of decrease of the v(Ag-X) (X=C1 , B r ) v i b r a t i o n s i n these media. The presence of i n t e r f a c i a l H 0 at all coverages of UPD Pb precludes i n t e r p r e t a t i o n of these r e s u l t s strictly in terms of a decrease in adsorbate surface coverage. The r e s u l t s are interpreted as a combination of changes in electromagnetic enhancement r e s u l t i n g from a l t e r a t i o n of the surface o p t i c a l properties of the electrode in the presence of UPD Pb and decreased e f f i c i e n c y of photoassisted charge transfer r e s u l t i n g from a l t e r a t i o n of the Fermi l e v e l at microscopic surface active s i t e s i n the presence of UPD Pb. 2

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Several recent experimental investigations have been directed towards i n v e s t i g a t i n g the influence of underpotentially deposited foreign metal monolayers on the surface enhanced Raman s c a t t e r i n g (SERS) a b i l i t y of Ag and Au electrodes.(1-16) These studies have been undertaken with the intent of further e l u c i d a t i n g the mechanism of the SERS phenomenon. SERS i s generally thought to r e s u l t from a combination of two mechanisms. The f i r s t i s c l a s s i c a l electromagnetic enhancement of the e l e c t r i c f i e l d at the interface between an appropriately roughened metal substrate and another medium. An a d d i t i o n a l mechanism i s a resonance-like enhancement postulated to involve photoassisted charge transfer between the 1

Address correspondence to this author. c

0097-6156/88/0378-0398$06.00/0 1988 American Chemical Society

In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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metal and the adsorbate at unique s i t e s on the metal surface known as a c t i v e s i t e s . These a c t i v e s i t e s have been documented to involve c l u s t e r s of adatoms at the surface.(17) In general, the deposition of monolayer and submonolayer amounts of nonenhancing foreign metals onto an enhancing metal electrode through the underpotential deposition (UPD) process r e s u l t s i n a decrease i n SERS i n t e n s i t y f o r a l l of the adsorbate probes studied to date. This decrease has been recently interpreted i n terms of two e f f e c t s , a change i n substrate o p t i c a l properties causing a decrease i n the c l a s s i c a l electromagnetic enhancement e f f e c t , and a l t e r a t i o n of the energy l e v e l s of the a c t i v e s i t e s rendering the charge transfer process s i g n i f i c a n t l y l e s s e f f i c i e n t . ( 1 4 ) One aspect of these studies which has yet to be adequately addressed, however, i s the influence of the f o r e i g n metal layer on the surface coverage of the adsorbate during the deposition process. I t i s very l i k e l y that, i n a l l of the studies performed thus f a r , the observed response represents a convolution of the e f f e c t s noted above and a change i n the adsorbate surface coverage. In f a c t , i n several studies, a decrease i n adsorbate surface coverage was proposed as a major cause of the SERS i n t e n s i t y decrease.(5-7,10) Although previous studies i n t h i s laboratory have been undertaken with the intent of addressing t h i s issue using the e x - s i t u probe of x-ray photoelectron spectroscopy as a measure of adsorbate surface coverage (11), i t i s c l e a r l y more desirable to probe such changes i n - s i t u to avoid complications r e s u l t i n g from removing the i n t e r f a c e from the electrochemical environment. The work reported here was designed to address the issue of adsorbate surface coverage i n the e f f e c t on SERS of UPD Pb on Ag electrodes i n aqueous chloride and bromide media using i n t e r f a c i a l H 0 species as the probe molecule. No studies have been reported on the e f f e c t of UPD layers on the SERS of i n t e r f a c i a l solvent molecules previously. However, the solvent i s an i d e a l choice f o r such studies, because i t w i l l always remain i n intimate contact with the electrode surface. Moreover, the SERS of i n t e r f a c i a l H 0 has been characterized quite extensively i n aqueous h a l i d e media (18-29) and allows the possible influence of anion on the response of the system to be assessed. 2

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Experimental The l a s e r Raman system used f o r these studies has been described i n d e t a i l i n previous reports from t h i s laboratory.(8.9) A l l spectra reported here were obtained with 5145 A e x c i t a t i o n from an A r l a s e r . Laser power at the sample was t y p i c a l l y 200 mW. Spectra of CI" and Br" were acquired at 0.5 cm" increments over a 0.5 s i n t e g r a t i o n period. Spectra of i n t e r f a c i a l H 0 were acquired at 1.0 cm" increments over a 0.5 s i n t e g r a t i o n period. A l l spectra were acquired as s i n g l e scans with a 6 cm" bandpass. Peak areas were d i g i t a l l y determined assuming a s t r a i g h t - l i n e background i n a l l frequency regions. In the low frequency region where the i/(Ag-Cl) and i/(Ag-Br) v i b r a t i o n s are observed, s i g n i f i c a n t background i n t e n s i t y can obscure these v i b r a t i o n a l features i f they are weak i n i n t e n s i t y . In order to improve the s e n s i t i v i t y of these measurements, the spectra acquired i n t h i s region before an oxidation-reduction cycle +

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(ORC) pretreatment were d i g i t a l l y subtracted from the spectra acquired a f t e r the ORC. The r e s u l t i n g spectra i n t h i s frequency region demonstrate a smaller influence from the background and allow accurate q u a n t i t a t i o n of peak areas even at low i n t e n s i t y . In the frequency region where the i/(0H) v i b r a t i o n s of i n t e r f a c i a l H 0 are observed, the normal Raman s c a t t e r i n g from the bulk s o l u t i o n can obscure the SERS of i n t e r f a c i a l H 0 i f appropriate precautions are not taken. In the studies reported here, the SERS of i n t e r f a c i a l H 0 was acquired with the electrode surface positioned as close to the electrochemical c e l l window as possible to minimize contributions from the bulk s o l u t i o n . When a l t e r i n g the electrode p o t e n t i a l to deposit Pb onto the Ag electrode surface, the electrode was p u l l e d away from the window several mm, the surface allowed to e q u i l i b r a t e at the new conditions, and the electrode repositioned near the c e l l window f o r s p e c t r a l a c q u i s i t i o n . Electrochemical equipment and c e l l s used f o r these investigations have also been described previously.(8.9) P o l y c r y s t a l l i n e Ag (Johnson Matthey, 99.9%) was mechanically polished with alumina (Buehler) to a mirror f i n i s h and sonicated i n t r i p l y d i s t i l l e d H 0 before each run. A l l p o t e n t i a l s were measured and are reported versus a saturated calomel reference electrode (SCE). The solutions consisted of 5 x 10" M Pb(N0 ) i n e i t h e r 0.1 M KC1 or 0.1 M KBr made s l i g h t l y a c i d i c to pH 5.5 by the a d d i t i o n of HC1 or HBr as appropriate, i n order to maintain the s o l u b i l i t y of the P b . A l l chemicals were reagent grade and were used as received. A l l solutions were prepared from t r i p l y d i s t i l l e d , deionized H 0, the l a s t d i s t i l l a t i o n being from basic permanganate. A l l solutions were deaerated by bubbling with N p r i o r to use. The Ag electrodes were subjected to p o t e n t i a l sweep ORCs at 10 mV s" i n e i t h e r 0.1 M KC1 or 0.1 M KBr from an i n i t i a l p o t e n t i a l of -0.30 V to more p o s i t i v e p o t e n t i a l s . A f t e r ca. 30 mC cm" of anodic charge was passed, the d i r e c t i o n of p o t e n t i a l sweep was reversed to reduce the Ag halide surface to Ag metal. The roughened electrode was then removed under p o t e n t i a l c o n t r o l at -0.30 V and immersed i n the P b - c o n t a i n i n g t e s t s o l u t i o n f o r SERS studies. The f r a c t i o n a l Pb monolayer coverages were c a l c u l a t e d by comparing the charge under the s t r i p p i n g wave obtained at d i f f e r e n t p o t e n t i a l s with that obtained for a f u l l monolayer on a given electrode surface. A value of 300 /iC cm" was used f o r a complete Pb monolayer as reported by Dickertmann, Koppitz, and Schultze.(30) 2

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Results and Discussion 2

SERS of HgO i n the Absence of Pb *. The SERS of H 0 at Ag electrodes has been investigated thoroughly. (18-29) The i/(0H) v i b r a t i o n can be e a s i l y seen i n electrochemical SERS studies i f the electrode surface i s separated from the c e l l window by only a t h i n f i l m of s o l u t i o n . Using t h i s approach, the SERS spectra of H 0 at Ag electrodes i n 0.1 M KC1 and 0.1 M KBr were obtained. The i/(0H) v i b r a t i o n i s observed at 3495 cm" and 3505 cm' i n CI" and Br", r e s p e c t i v e l y . The r e l a t i v e l y high p o s i t i o n of t h i s symmetric 0-H v i b r a t i o n has been observed previously and has been interpreted as evidence f o r d i s r u p t i o n of hydrogen bonding between water molecules by these anions, and weak hydrogen bonding between the H 0 and the 2

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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anion.(18-20.26) Such interactions between the anion and the H 0 species give r i s e to a considerably narrower linewidth of t h i s v i b r a t i o n than observed i n bulk solution. Moreover, the a c q u i s i t i o n of these spectra i n K solutions, a low hydration energy cation, implies that the H 0 molecules are p r e f e r e n t i a l l y aligned with t h e i r 0 ends facing the p o s i t i v e l y charged Ag electrode, as noted i n e a r l i e r reports.(27) I t has been previously observed that the nature of the anion has a marked e f f e c t on the frequency of t h i s band. In 1.0 M and 4.0 M KC1, bands are observed at 3498 cm" and 3433 cm" , respectively.(29) In 1.0 M KBr and 1.0 M KI, bands are observed at 3523 cm" and 3553 cm" , respectively.(18) In 0.5 M KCN, t h i s band i s observed at 3521 cm" .(18) These r e s u l t s show that at a s i m i l a r h a l i d e concentration of 1.0 M, a decrease i n frequency of the band i s observed from 3553 cm' to 3523 cm" to 3498 cm" f o r I " , Br", and CI", respectively. The r e s u l t s obtained here show a s i m i l a r decrease i n p o s i t i o n from 3505 cm' i n Br" to 3495 cm' i n CI". These bands are also s h i f t e d with respect to those observed i n 1.0 M solutions of these anions, consistent with the anion concentration r e s u l t s noted above. Evidence that H 0 species also i n t e r a c t with the Ag electrode independent of adsorbed anions comes from the p o t e n t i a l dependence of the i/(0H) i n t e n s i t y as compared with the i/(Ag-X) (X=C1", Br") i n t e n s i t i e s . The normalized i n t e n s i t i e s of the i/(Ag-X) (X=C1", Br") v i b r a t i o n s i n 0.1 M KC1 and 0.1 M KBr are shown i n Figure l a , and the corresponding i n t e n s i t i e s of the i/(0H) v i b r a t i o n shown i n Figure l b . The observation that the i n t e n s i t y of the i/(0H) v i b r a t i o n reaches a maximum at more negative potentials than the i/(Ag-X) (X=C1", Br") v i b r a t i o n s has been interpreted as i n d i c a t i o n that the H 0 molecules can become maximally adsorbed on the surface when the p o s i t i v e charge has decreased to allow p a r t i a l desorption of the anions.(23) Obviously, the p o t e n t i a l at which t h i s occurs depends on the strength of i n t e r a c t i o n of the anion with the electrode. The frequency of the y(0H) band i s also observed to decrease with p o t e n t i a l i n both KC1 and KBr environments, as shown i n Figure 2. This e f f e c t can be interpreted i n terms of a weakening of the i n t e r a c t i o n strength of the H 0 with the Ag electrode surface as the p o t e n t i a l and the charge on the electrode are made more negative. As the H 0 interacts less with the electrode, i t can hydrogen bond more with other H 0 molecules, giving r i s e to the observed decrease i n frequency of the i/(0H) v i b r a t i o n . This i n t e r p r e t a t i o n i s consistent with the o r i e n t a t i o n of the H 0 molecule i n K solutions discussed above. 2

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SERS of H 2 O i n the Presence of Pb *. The presence of 5 x 10" M Pb i n 0.1 M KC1 and 0.1 M KBr i n the p o t e n t i a l region p o s i t i v e of UPD has no e f f e c t on the frequency of the i/(0H) band i n these media. The only e f f e c t s observed are that the 3495 cm" band i n CI" has measurable i n t e n s i t y to more negative potentials than i n the absence of P b , and the absolute i n t e n s i t i e s of t h i s band are lower i n both h a l i d e solutions than i n the absence of P b . The f a c t that the 3495 cm' band i n CI" can be observed at p o t e n t i a l s where i t i s not observed i n the absence of P b can be explained i n terms of the Pb /X" (X=C1", Br") species present i n the interface. Previous work i n t h i s laboratory has demonstrated the existence of P b 2

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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halide complexes i n the i n t e r f a c e at p o t e n t i a l s more p o s i t i v e than Pb UPD.(12) These complexes s t a b i l i z e the halides i n the i n t e r f a c e such that they remain at more negative p o t e n t i a l s . The decrease i n absolute i n t e n s i t y of the H 0 band i n the presence of P b most l i k e l y r e s u l t s from the f a c t that there are fewer free h a l i d e ions i n the i n t e r f a c e with which the H 0 can i n t e r a c t . The p o t e n t i a l regions w i t h i n which the UPD of Pb occur are shown i n the c y c l i c voltammograms i n Figure 3. In CI" media, Pb UPD occurs between -0.40 and -0.48 V. In Br" media, Pb UPD i s observed at s l i g h t l y more negative p o t e n t i a l s between -0.43 and -0.52 V. Considering a l l of the data discussed above, i t i s u n l i k e l y that d r a s t i c decreases i n surface coverage by H 0 species occur i n these r e l a t i v e l y narrow p o t e n t i a l regions. In f a c t , i t i s known that during the UPD of Pb on Ag, the surface coverage of CI" and Br" decreases. Therefore, the surface coverage of H 0 should a c t u a l l y increase during Pb UPD as h a l i d e ions are p a r t i a l l y desorbed. These considerations are c r i t i c a l i n understanding the SERS response of the i/(0H) band during UPD of Pb. Figure 4 shows the normalized SERS i n t e n s i t i e s of the 3495 cm" i/(0H) band i n CI" media and the 3505 cm" band i n Br" media as a function of Pb coverage from zero to one monolayer. Also shown are the corresponding normalized i n t e n s i t i e s of the i/(Ag-Cl) and i/(AgBr) v i b r a t i o n s as a function of Pb coverage f o r comparison. As these data demonstrate, the i n t e n s i t y of the i/(0H) v i b r a t i o n decreases as the Pb coverage increases i n both h a l i d e environments. The i n t e n s i t i e s of the i/(Ag-X) (X-Cl", Br") v i b r a t i o n s also decrease with increasing Pb coverage. These decreases are consistent with the r e s u l t s of other studies i n v o l v i n g the e f f e c t of UPD t h i n f i l m s on the SERS of other adsorbate probes i n t h i s (8-14) and other (17.15.16) l a b o r a t o r i e s . The data i n Figure 4 show d i f f e r e n t behavior i n three d i f f e r e n t coverage regions. The f i r s t region occurs between zero coverage and ca. 30% of a Pb monolayer. In t h i s region, the i n t e n s i t i e s of a l l v i b r a t i o n s decrease. The rate of increase of the y(Ag-Br) i n t e n s i t y i s greater than that f o r the i/(Ag-Cl) band. This r e s u l t has been observed and explained previously.(13) The rate of i n t e n s i t y decrease i n the i/(0H) band i n t h i s region i n both h a l i d e environments i s s i g n i f i c a n t l y less than that of e i t h e r i/(Ag-X) (X-Cl", Br") v i b r a t i o n . Moreover, the rate of decrease i s independent of the anion. This l a t t e r observation i s f u r t h e r evidence that the H 0 species being monitored are i n t e r a c t i n g with the Ag electrode independent of the s p e c i f i c a l l y adsorbed h a l i d e ions. The second region of d i s t i n c t behavior occurs between ca. 40% and 80% of a Pb monolayer. In t h i s region, the i n t e n s i t i e s of the i/(Ag-X) (X=C1", Br") bands decrease to unmeasurable values. I n contrast, the i n t e n s i t i e s of the i/(0H) v i b r a t i o n s remain e s s e n t i a l l y constant i n both media. The t h i r d region of behavior occurs f o r Pb coverages greater than 80% of a monolayer. The i n t e n s i t i e s of the i/(Ag-X) (X*C1", Br") bands remain at unmeasurable l e v e l s . The i n t e n s i t i e s of the y(0H) bands decrease r a p i d l y to unmeasurable l e v e l s between 80% of a monolayer and one complete monolayer. The behavior of the i/(0H) bands i n the second and t h i r d regions are observed to depend only s l i g h t l y on the nature of the anion. 2+

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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I t i s c l e a r from these H 0 r e s u l t s that the decreases i n SERS i n t e n s i t i e s as UPD layers grow on electrodes cannot be s o l e l y a t t r i b u t e d to decreases i n adsorbate coverage. This report represents the f i r s t concrete evidence f o r t h i s f a c t i n the electrochemical SERS environment. I t i s l i k e l y that the decreases observed can be r a t i o n a l i z e d i n terms of two contributions. Changes i n surface o p t i c a l properties r e s u l t i n g from modification by the foreign metal have been shown to decrease the electromagnetic enhancement c o n t r i b u t i o n to SERS. However, f o r the case of Pb UPD on Ag, t h i s e f f e c t has been shown to account f o r only ca. 40% of the decrease i n going from zero coverage to one monolayer.(14) Moreover, t h i s model does not account f o r the r e l a t i v e l y rapid decrease i n i n t e n s i t y observed with the deposition of small ( i . e . , less than 20% of a monolayer) amounts of Pb on the Ag surface. The second contribution to these observed decreases that i s proposed to be p a r t i c u l a r l y important at small Pb coverages i s a l t e r a t i o n of the microscopic Fermi energy at Ag active s i t e s , presumably adatom c l u s t e r s , rendering photoassisted charge transfer less e f f i c i e n t . ( 1 4 ) In t h i s model, the Fermi energy i s proposed to increase as the Pb coverage increases so that, at a constant e x c i t a t i o n energy, the charge transfer from the metal at s i t e s of atomic scale roughness to acceptor l e v e l s of the adsorbate gradually goes out of resonance. I t i s proposed that t h i s type of mechanism i s responsible f o r the quenching of SERS of H 0 i n the f i r s t coverage region reported here. Due to the f a c t that the H 0 species remain at the surface, the quenching of the charge transfer process i s not complete, and some atomic scale roughness remains, the SERS i n t e n s i t y of the i/(OH) v i b r a t i o n i s retained at ca. 50% throughout the second coverage region. In the t h i r d region of coverage, most of the atomic scale roughness has been proposed to be i r r e v e r s i b l y destroyed as the Pb layer rearranges to assume the f i n a l hexagonal close packed configuration of the monolayer.(9) This loss of atomic scale roughness r e s u l t s i n the i r r e v e r s i b l e decrease i n i/(0H) i n t e n s i t y to e s s e n t i a l l y unmeasurable l e v e l s . This observation further emphasizes the importance of the chemical enhancement mechanism c o n t r i b u t i o n to SERS i n electrochemical systems.

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Conclusions The e f f e c t of UPD Pb on the SERS of H 0 at Ag electrodes i s presented i n t h i s report. The work reported here demonstrates that a s i g n i f i c a n t decrease i n adsorbate coverage i s not responsible f o r the quenching of SERS at electrodes i n the presence of UPD layers. These r e s u l t s lend credence to a previously postulated model i n which the quenching e f f e c t i s interpreted i n terms of both a decrease i n the electromagnetic enhancement at these surfaces r e s u l t i n g from a change i n the macroscopic o p t i c a l properties of the surface as the UPD layer i s formed, and a decrease i n the e f f i c i e n c y of photoassisted charge transfer r e s u l t i n g from a l t e r a t i o n of the Fermi energy at microscopic active s i t e s on the surface at which charge transfer occurs. 2

In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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Acknowledgments The authors g r a t e f u l l y acknowledge the f i n a n c i a l support of t h i s work by the National Science Foundation (CHE-8614955). L i t e r a t u r e Cited 1. 2. 3.

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4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

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RECEIVED M a y 19, 1988

In Electrochemical Surface Science; Soriaga, M.; ACS Symposium Series; American Chemical Society: Washington, DC, 1988.