Electrochemical Surface Science - American Chemical Society

Chapter 27

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

Downloaded by MONASH UNIV on November 27, 2015 | http://pubs.acs.org Publication Date: November 11, 1988 | doi: 10.1021/bk-1988-0378.ch027

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.


27.

CORIA-GARCIA & PEMBERTON

Effect of Underpotentially Deposited Lead

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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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ELECTROCHEMICAL SURFACE SCIENCE

(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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Effect of Underpotentially Deposited Lead

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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-E (Volts) vs SCE Figure 1. Intensity as a function of p o t e n t i a l f o r a) i/(Ag-Cl) band (238 cm ) and i/(Ag-Br) band (163 c m ) , and b) i/(0H) bands i n CI' (3495 cm" ) and Br" (3505 cm" ). -1

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Effect of Underpotentially Deposited Lead 4 0 3

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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Figure 2. Frequency of the i/(0H) v i b r a t i o n as a function of p o t e n t i a l i n C l " and Br" media.

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Figure 3. C y c l i c voltammetry of Pb UPD on Ag electrode i n a) 0.1 M KC1, and b) 0.1 M KBr. Sweep rate 10 mV s" , S = 47 /iA cm" . 1

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e Figure 4. Normalized SERS i n t e n s i t y as a function of Pb coverage f o r a) 3495 cm" i/(0H) band and 238 cm" y(Ag-Cl) band i n 0.1 M KC1, and b) 3505 cm" i/(0H) band and 163 cm" i/(Ag-Br) band i n 0.1 M KBr. 1

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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


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Effect of Underpotentially Deposited Lead 407

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.


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


Electrochemical Surface Science - American Chemical Society

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