Evidence for surface Ag+ complexes as the SERS-active sites on Ag electrodes

Volume 102. number 6

EVIDENCE

FOR SURFACE

CliEhlICAL

9 Dccembcr 1983

PHYSICS LETTERS

Ag+ COMPLEXES

AS THE SERSACTIVE

SITES ON Ag ELECTRODES

T. WATANABE, 0. KAWANAMI, K. HONDA Department of Synrhetic Ciicmistr-1,Faculty of Engineering, University of TokJpo.Hongo. Bunk)*o-ku. TokJpo113, Japan and B. PETTlNGER Frit2-IIabcr-blstifut der Max-Planck-Gesellschaft, Farada~rur~J-6. D-1000 Berlirl33. West Geruzan_v Reccivti 11 August 1983; in final form 1 September

1983

Evidence is given that SERS-active sites at Ag electrodes are associated with Ag+ ions, forming sparingly soluble surface complc~cs with ligands such as pyridinc molcculcs and halide ions. Such surface A$ coniplcses contribute a factor of >800 to the overall (107-fold) enhancenient, possibly via a resonance Raman effect.

1_ Introduction Despite the numerous works published to date, the mechanism of surface-enhanced Raman scattering (SERS) still remains a matter of controversy [I]. It appears widely agreed that the enormous enhancement (106-fold or more [2,3] ) comes partly from an increase in the local electric field felt by the adsorbates due to surface plasmon polariton (SPP) excitation in the rough feature of metal substrates. The requirement for roughness is obvious from the absence of SERS on a smooth Ag surface [4]. Recent results point to the importance of another mechanism (possibly working multiplicatively with the SPP excitation) wherein a resonant Raman (RR) type process at special surface sites increases the effective Raman polarizability of adsorbates. Such sites are often referred to as the “SERS-active sites”, and apparently occupy a rather small fraction of the entire surface

[2,3,5].

However, the nature of the active site remains

uncertain_ In a survey of previous SERS reports on Ag electrodes in aqueous solutions, we note that nwst of the

SERS-generating adsorbates happen to be those capable of forming A$ complexes or salts sparingly soluble in water. In effect, the solubility products (Ksp)

for simple Ag+ salts of pyridine and its derivatives, phenanthrolme, benzotriazole, mercaptobenzothiazole, citrate, thiourea, SOS-* SCN-. CN-, NT, and halides (all exhibiting SERS) are belov: lo-‘” in HZ0 at room temperature [6.7]. Further, with pyridine [8] and CN- [9], f. similarity has been noted between the SERS specrrum and the ordinary Raman spectrum of their Ag+ salts or complexes. A recent result [ 1O] suggests participation of a Ag+ complex in SERS from pyridine-2-azo-p-dimethylaniline. These findings indicate the crucial importance of Ag+ complexes adhering to the Ag surface in giving intense SERS. An adsorbate attached as a l&and to a Ag* ion can be replaced by another, stronger Bgand, resulting in the formation of a new Ag+ complex. If the latter compIex is readily soluble in water, the original Age center will be lost from the surface. One of the typical ligands having strong complexation capability to form water-soluble Age complexes is S20:-, which is

commonly used in the fixing of photographic fiis (selective dissolution of AgX as Ag(S203)zor Ag&O& while leaving the neutral Ag intact) [l I]. Similarly, at high concentrations SOS- and SCNcan serve as fixing agents, though at lower concentrations these ions form the water-insoluble salts Ag2S03 and AgSCN. 565

Volume 102. number 6

CHEMICAL

In the presenr work we st;Idy the effects of S20p aszd SO$- on the SERS from pyridine at a Ag electrode Indeed. both ions quenched the SERS irreversibly. On tlris basis we put forth the hypothesis that the RR process within surface Ag+ complexes constitulcs an integral part of the enormous enhancement

9 December 1963

PHYSICS LETTERS

Scat Li

in SERS.

Laser 1. Experimental

Fig. I. Schermtic illustration of the method of injecting %Oispot on ZIAg or SO:- snttrtion to\\ard the laser-iIiun]i~t~

A pdycrystalline Ag platelet (IO X 10 X 1 mm) embedded in :I Teflon holder was used as a working electrode in an electrochemical cell with a capacity of== IO0 ml.The electrolyte solution contained 0.1 Rf KC1 and 0.05 hipyridine. The electrode potentials are rckrred to a saturated calomel electrode (SCE). Activation of the Ag electrode was carried out through an oxidation-reduction cycle (ORC) up to a reformation charge of about 20 mC zm-‘. While observmg a SERS signal from pyridine. an aqueous sohnion of 0.1 hi KC1 containing Na,S,03 or Na2S0, (IO-‘-10-t ht) was injected toward the vicinity of the laseriiluminated spot as shown in fig_ I. The instrumenta-

cicctrode.

I

I.-

’

1

I

I

. . .

'Z/V

Ivs. SCE

I

tion for Ramsn rneasuret xnts was the same as in pleviuus work 13 1. except that a 5 14-5 nm line {= 100 mtt’, was used here for excitation_ Unless otherwise stated. an s-beam + s-analyzer configuration was employed as the po!arization mode.

3. Results A typical result showing a significant quenching effect of S,Ogis illustrated in fig_ 3. At -O-.5 V

‘

I

Pyridine

I

I

I

50 mM + KCI 0.1 hl

Injected

1000

Raman

Shift

800 / cm-’

600

400

20

)_‘I+ 1. Quenching of IIN pyridine SCRS by injection of 1 ml of ‘J 10’ 51 $O?,-* ..qucous solution The inset (top left) shows the pnrcn11.11 prog.immin~. The ten-fold magnified spectra depict rhc absence of GO;- vibrtltiowl peaks.

Volume 102, number Ei

CHEBIICAL PHYSICS LETTERS

9 Dt?cember 1983

after an initial ORC, an intense

and stable SERS from pyridine was observed fcurve A). Inject&n of a pure KC1 (0.1 M) solution did not affect the signal inter&

selective@ suppressed whereas the depolarized SERS can be detecteq even if it is very weak, In the heavy quenching. the original SERS intensities for the v1

ty; this ineans that local dilution does not desorb SERS-active pyridine molecules. However. injection of f ml of a IO-” &I S20$solution quenched the SERS intensity by >90% in a few minutes (curve B). S,Ozis transparent in the visible region, is electrocheinicnlly inert at -0.6 V, and is inert toward neutral Ag. Further. no specific chemical interactions could be expected between S20$and pyridine or Cl- _ The present result therefore suggests trapping of surface Ag+ centers by S,@$-, followed by dissolution or Ag(SZO~)~into the bulk solution_ of Ag(S,O&Wtlen an additional ORC was applied after this quenching, tile SERS intensity became even lower fcurve C). This suggests that free Ag+ ions, generated during the ORC, are trapped by S20k more effectively than are the surface Ag+ centers. which have already been trapped by pyridine and/or Cf- to form “SERSactive sites’-, No new SERS peaks assignable to S20zW (e-g. 447 and 670 cm-l [ f 21) were detected after the queaching, as seen in the tzl.-cold magnified spectra in fig. 2. Jience the observed qrtenct&;; evidentfy does not involve competitive adsorption of 5,3{- _ on active centers. Interestingly, the quenching accompanies an increase in the 240 cnl-1 peaZc intensity reela!ive to the v1 -peak at 1006-1009 cm-r _The ratio 124’2/IPy for curves .A, B and C are, respectively, 0.15,0.47 and 0.70. Since the former peak may be assigned tz the Ag+-Clvibration, this finding can be understood by assuming that (i) on the surface, there exist a variety of SERS-active A$ complexes differing in the relative population of pyridine and Cl- as Jigands, and (ii) sites richer in pyridine are attacked by S,C$more easily than those richer in Cl-. The extent of SERS quenching depended on the SIOgcuncentration in the injected solution. Thus, injection of 2 ml of a lo-’ M solution led to an intensity decrease by *3O% within 5 min_ N%en injection of 3 ml of&l M S20T was followed by a second ORC, the pyridine SERS and even the 240 cm_1 bared

and ~~2 modes (250000 cps and 150000 cps, respectively) decreased by >70% within a few secor.ds. followed by a continuing

slow decay. After 30 min.

their intensities were SO00 and 2500 cps. Both hands showed a potential dependence, and their peak frequency and halfgvidth were different from those in the initial spectrum as well as from those of pyridine in solution; this indicates the presence of at least two surts of SERS-active sites which, due most probably to different effective charges on the Ag+ center, show different rates of attack by S20k. _4t 30 min. a potential scanning from -0.6 V to -0-f V and back IO --0.6 V promoted rhe intensity decay, although the reason for this is not tbar, At this stage the SERS signal was indistinguishable from the normal Raman signal of pyridine in solution (300 cps for s-p configuration). We therefore estimate the quenching factor to be X300_ With an s-s configuration, quenching by a factor of >200 made it impossible to separate the SERS from the solution signal. A quenching effect was observed also with SO:ion, but to a much lesser extent. At -0.6 V, injection of 4 tnf of a 0. I M Na-,SO~ solution resulted in a decrease of the pyridine>ERS by <30%. We suppose that the effects of both ions are of common origin: complexation with Ag+ followed by dissolution. The difference in the quenching efficiency may be related to the difference in the formation constant (Kf) of walerwiuble corn lexes, namely ITf = 1013-3 for Ag(S20.$P and K,= 10 *7 for Ag(SO,);[6]_ in view of a difference in the KsP between AgCi (1 O-g-8) and Agi (1 U- 16*1 ) f6] f a preliminary measurement of the quenching effect of SrOgwas made in 0.05 M pyridine f 0-l M KI. Injection of 2 ml of solution quenched the pyridine a IO-’ M S,C$ SERS at - 12 V by =50’% within a few minutes compared to X0% in O-1 M KQ (fig_ 2)_ ~uaJJtativeIy, this is consistent with the expected higher stability of surface Ag+ centers trapped by I- than those trapped by Cl-.

almost disappeared.

This heavy quenching was studied in more detail by using an s-beam -f p-analyzer configuration, by which the Raman signal from polarized (breathing) vibrations from dissalved pyridine is

567

Volume 101, number 6

CHEMICXL PHYSICS LETTERS

9 December 1983

4. Discussion 4. I. Surface Ag+ compleres

as the SERS-active

sites

The idea that surface _4gi complexes constitute tfte SERS-active sites is consistent with experimental tkdings reported w far: (1) The irreversible SERS quenching by suffkiently negative polarization [13] is now understood by invoking a direct electron transfer from the electrode to the Ag* centers. leading to disintegration of the Ag* complexes_ A similar picture may hold for SERS quenching by underpotentialiy deposited foreign metal (Me) ]2.~.5.14.15]. but in this case electron transfer should be from a Me atoni to a AS+ center (an electrocatalytic process via surface diffusion of Me). Fig. 3 schematically ilhrstrates the three types of SERS qntnching. (2) Ag*-pyridine salts. practically insolttble in water. are soluble in several organic solvents [7]. Presumably. this renders the pyridine SERS virturlly undetectable in ormnic solvents ]lC;]. even though sdsorption is not ~~rol~ibited. (3) Aquation of metal ions is 3 well known process. In vie\\ of this. the SERS from kg20 on Ag electrodes [ 171 is probably associated with the trapping of Ii20 molecules at surface i5g+ centers. (3) The SER spectrum observed by introduction of g~scous X0 onto Ag powder was identical w&h that from NOT f I 81. The cationic species. required for the fuhilhnent of electroneutrality. must be Ag+ serving as a center for surface complex (or salt) formation.

Based on the present results, we propose the following mechanism for SERS. On a smooth Ag surface before an ORC, no SERS occurs 143 even though adsorption of pyridine and Cl- is taking place. An ORC produces rouglt features of the kg surface. which can significantly augment the incident electric field via SPP excitation 4 small fraction of _4g+ ions. generated in the positive scan of ORC. is trapped by pyridine and Clto form sparingly soluble surface Ag+ complexes (active sites). Their surface coverage at the end of ORC is rather low, typically below 10% [2.3_5]. This corresponds to a surface charge of <20 PC cmM2. which is barely detectable in the total ORC charge (20 mC ~nr--~) by ceulometry. We now assume, although this is not experimentally verified yet. that the surface Ag+ complexes absorb at the laser wavelength. or at least their absorption tail extends into the visible region. This could result in a ~‘moleculai RR type process, leading to an increased Raman polar&ability. As another RR process. a chargetrausfrr excitation (CTE) between an adsorbed Ag atom and an adsorbate [5.19] may be envisaged_ We could thus formulate. in a manner similar to that proposed by Otto [20]. tltt total Raman intensity gGn G as a product of three terms: G(et_. E- 0

= GSPP(eL- e)G,,,,#&L.

x (;CTL$EL. E. EL

e- 0 0)

where EL denotes the incident laser photon energy. E the scattered photon energy, and E the electrode potential. The term G,rnple, (>800), introduced explicitly for the first time, represents an RR-type contribution from surface Ag+ complexes. in what follows. consequences of the present model are brrefly discussed. 4.3. SERS exciratiort profile

Drssolut~on

of Ag+ Complex as Ag[S2031~3Ag*

Complex

If one of the three terms on t?re right-hand side of eq. (1) ltas a much smaller half\wdth Aer,, than tlte other two terms. the spectral dependence-of G is governed by that particular term. In general, the &si,-) for Gspp is rather small (=OS eV) [2 1] _This is prob-ably the main cause for the observed quasi-equivalence of the SERS excitation profiles for different adsorbates [22]_

Volume 102, number 6

4-C Potent&d dependence

of SERS

In electrochemical systems, the SERS signal is considerably influenced by the electrode potential E. In previous work f23] we demonstrated that the intensity and shape of SER spectra show a marked E depen-

dence even in the absence of adsorption/desorption processes, According to the present result. the “SERS-active sites” must be Ag* complexes adhering to the surface- Since free Ag+ ions are thermodynamicalfy unstabfe at potentials giving intense SERS (from 0.0 to -0.7 V for the Ag/pyridine/Cl- system). one cannot expect new production of surface Ag+ complexes in the course of a potential scan after an ORC. The number of A$ centers must, due ro the limited sol&iIity, remain ’ 'frozetz -*rimi cI “~~~e~lc~~~~~~ potential” is reached (cf. fig. 3). Thus, the E dependence of SERS has to be sought in the E dependence of the enhancement mechanism(s). Referring to eq. (l), the problem is to determine which of the Go,nu,lex and G,, terms is more reasonable to account for the observed E dependence of SERS. The E dependence of G,, results from a change, with a shift ofE, in the relative position of the Ag Fermi level against the pyridine vacant level. On the other hand. for the E dependence of Gcompl~~we could make a reasonable assumption that the nature (structure and composition of ligands) of the Ag* complexes changes with E. through adsorptionjdesorption of Cl- and by partial c~larg~g~disc~l~ging of Ag+ centers. and such a change in turn entails a shift of their absorption maxinnm~. Among the various aspects of the E dependence of SERS, of particular interest is a recent finding by Furtak and co-workers

[5.24]

9 December 1983

CHEhlICAL PHYSICS LETTERS

and Billmann and Otto [ I9 J

that a change in eL causes a shift of the intensity versus

E profile along the E axis. These authors interpret this result in terms of a CTE process. But it is hard to reconcile the observed ratio &JAE = I-5-7 eV/V [5,f9,24] with the CTE concept alone, which should predict &J&C < 1 as a first approximation. The E dependence of G,mplni would provide a clue to rationalize these results without recourse to an improbable “overshooting” of the potential at the interface [19,24]. Another important but as yet unsolved problem is the difference in the intensity versus E profile among different vibrational modes at a fixed eL [I 6,191. A

simple CTE concept cannot account for such behavior_ The G mmplex term, representing a molecular RR process, could give a qualitative answer also to this problem: we have otiy to assume, as in ordinary RR processes, that the absorption spectrum of a Ag+ complex is composed of muitipIe electronic transitions, each showing a distinct E dependence, and that differ-

ent vibrational modes are tuned to different transitions. Much work is of course needed to quantify these interpretations. 4.5. SERS-inactive

adsorbates

As a consequence of the present model, it appears difficult to observe intense SERS from adsorbates

which do not form Ag+ salts or complexes sparingly soluble in the solvent. In particular, a strong reducing agent could not be SERS-active. since it would reduce Ag+ to Ago during an ORC. In a preliminary experiment we xere indeed unable to detect intense SERS from hydroquinone electrodes_

or pmethylaminophenol

on Ag

The authors are grateful to Dr. T.E. Furtak. Rensselaer Polytechnic Institute, USA, for Vahdbk

discussions.

References 11j TX. Fur%& and R.K. Chang:,eds., Surface-enhanced Ram&n sptctroscop&’(Pienum Press, New York, 19Sl). f2] L. Moerl and B. Pettinser. Solid State Commun. 43 (1982) 215. [3] T. Watanabe. N. Yanagihara. K. Honda, 13. Pettingx and L. Moerl, Chem. Phys Letters 96 (1983) 639_ [4] A. Campion and D-R. Mullins, Chem. Phys. Letrcrs 94 (1983) 576. is] T.E. Furtak and D. Roy. Phys. Rev. Letters50 (1~83) 1301. [6] J. Pourddier. A. Pailliotet and CR. Berry, in: The theory of the photographic process, 4th Ed.. ed. T.H. James (Macmillan, New York, 1977) pp_ l-12. [7] W-i. Peard and R-T_ P&mm, J- Am. Chem. Sot. 80 (I 958) 1593. [S] J.A. Creighton. Surface Sci. 124 (1983) 209. [9] R_ Dornhaus, MB. Long, RX. Benner and R.K. Chang. Surface Sci. 93 (1980) 240.

569

Vohune 102. number 6

CHEMICAL PHYSICS LElXERS

f 10 J J.R. Cwnpbelt and J.A. Crtighton. J. Electroanal. Chcm. 143 (1983) 353. j 1%1 G.I.P. Levcnson,in: The theory of photogaphic procesCF.ed. TM. James (Macmillan, New York. 1977) pp. 437-461. f f :! J A. T.~nka. 0. Takahashr and I;. Shimizu. Denki Ka_gaku -17 (1979) 311. 113 1 Ii. Wctzel. ff. Ccrischer and B Pe?tinger. Cbem. Pbys. Letws 7s (1961) 392. 1I-l] T. N’&mabe. 0. Kaxxan~mi, h;. Honda and B. Pettinser. to bc publislled. { 15) J 1. Ke\tcr. J. Cbcm. Plr>s. 75 f 1983) 7466. [ 161 R I’. un DIQ ne, m: Chemical and biological sppficrtians of t.wrs, ed. C.B. Moore {Academic Press. NW York, 1979) pp. 101-185. 117 1 S.ll. M.wxnbcr. TX. Fur:& and T.M. Devmc. Surface sci. 122 ( 1982) 556.

9 December 1983

[IS] K.li. ton Raben. P.B. Dorain, T.T. Chen and RX. Chang. Cbem. Ph>s. Letters 95 (1983) 269. 1191 3. Billmann and A. Otto, Solid State Commun. 44 (1982) IOX 120 J A. Otra. in: Lisht seatterinz in solids, VoL4, cds. hl. Cardo= and G. Gucntherodt (Springer, Berlin). to be published). [ZI j A. Wokaun, J.P_ Gordon and P-F_ Liao, f’hys. Rev. Letters18 (1982)957_ [22] C.G. Blatchford, J-R. Campbell and J-A. Crcighton. Surface Sci. 108 (1YSl) 411. [23 ] T. \%‘&arube and B. Pettinger, Cbem. P&s. Letters 89 (1982) 501. I24 1 TX. Furtak and S.X %~dcombrr. Chem. Phyc. Letters 95 (1983) 323.