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Surface-enhanced Raman scattering from crystal violet adsorbed on a silver electrode
CHEMICAL
Volume 89. number 6
SURFACE-ENHANCED ADSORBED Tadashi
PHYSICS LETTERS
9 July 1982
RAMAN SCATTERING FROM CRYSTAL VIOLET
ON A SILVER
ELECTRODE
WATANABE
Reccivrd 11 May
1981
SERS from ckcll~llon
crystal rlolct (CV) on a Ag clectrodc was mvcslqakd under prcrrsonsncc and resonance condltlons. The prolilc of the chcmaorbcd spccics is Me that of chsrolvcd mokculcs but mtcnsltirs ZUIL’ =I000 tnncs luger. The
Raman enhancement IS= IO8 and ckhlblts 3 spcc~lic potcntlal dcpcndcncr cvcn in ~IICabsence of processes At polentlals where rcductron CV occurs lcuco crystal wok-t wx dctcctcd.
of
1. Introduction Recent years have seen much interest in surfaceRaman (SER) scattering, both os a novel surface phenomenon to be elucidated and as a powerful spectroscopic rool for surface analysis [l-8]. However, a unified wew on the mechanism of SER scattermg has not yet been reached 19, IO]. This srtuenhanced
atron necessitates further accumulalion mental data on a variety of systems. In the present
study
we carried
employed
1. Experimental
out SER mvestlga-
in an electrochemical
cell.
The reason for the choice of this system is four-fold (i) Prehminary studies by van Duyne 131 show that a high-quality SER spectrum of CV can be obtained; however comparative investigations on the excitation pro!32 of CVadarbti and CVdlaolved were not performed. (ii) Recent resonance Raman (RR) data on CV are available, though the band assignments are not entirely consistent with each other [ 1l-141. (rii) Since this system does not require the przsence of CV molecules in the solution phase, the potentral dependence of the SER intennty, which is one of the 0 00%3614/82/0000-0000/S
crucial points for discussion oiSER mechanisms, can conveniently be ekamincd and extended to a potcntral region where protonation of the dye should occur. (IV)The occurrence of multiphwtion of SER and RR enhancements would enable an assessment of attainable sensitivity and apphcabibty of thus technique in surface analysis.
of experi-
tlons of an orgamc dye, crystal violet (CV), adsorbed at a Ag interface
adsorption-dcsorpllon
02.75 0 1982 North-Holland
As pretreatment for a massive polycrystalline silver plate (40 X 10 X 1 mm), an ouldation-reduction cycle (ORC) (-0.4 to +0.2 V and back to -0.4 V) was applied to the electrode in an elecrrolyte solution containing 5 X 10m3 m CV and 0.1 hl KCI. During this ORC. at Grst several tens of Ag layers w2r2 oxrdiz.cd and precipitated as AgCl at the surface, followed by B redeposition of these silver atoms at the interface leading to a strong roughening of the electrode. No electrochemical reactron of CV is expected in this potential range. After this procedure the Ag electrode was thoroughly rinsed with distilled water to remove ant dye not bound to silver. Then it was transferred
9 July 1981
CHEMICAL PHSSICS LCT-ERS
Volun~ 89. nun&x 6
Raman cell [ 151 which contanned3 dye-iree solution of 0.1 hl WI. Hence, elecIO an cl2ctrochcmical
trochcm~stry and Raman spectroscopy wdl b2 pcriornxd only with dye moleculeswhich wrre chemi-
!nterixe. The cornpurer-controlled Raman equipment has
sorbed at rhr sdvcr
been dcscrlbrd [ I5 1. The Imcs of an argon and a krypton laser (model 5,. CR4. and 3000 K, Coherent Radiation) were used as e\cltmg photodrcomposnion
frequcnclrs. To avold possible of CV. the mcldent power was
reduced to ~3 mW and the laser beam was line-focused usmg a hemicylindrlcal kns For borh incident and scsttcr2d s polarlzatlon (E vector perpendicular to the pl3ne of ~nculencc) w.is chosen AII potsnrials ar2 cited against the saturated calomsl elsctrods (SCE)
3. Results and discussion In the present
system, the CV molecules
exist only
the Ag electrode surface. Moreover, spontaneous dcsorptlon oi CV into the elecrrolyte IS not sigmficanr. at
because spsctra recorded around -0 1 V with an interval of30 min gave nearly the same intensities. Prebminary measurements showed that desorption of a small fracllon of surface CV givss rise to a weak broad fluorescence of dissolved CV. This in turn means that
chemisorbed CV is practically non-fluorescent. 3. i. IVmeler~gtlr deperlderrce Since the dye 1s absorbingln
thr spectral
re+n
m-
vestlg.nrd hers. selection of a sultable evcltation frequcncy is oigreat unponance, os shown by fig. I with a set of CV spectra recorded for vallous excitation frequencies a1 -0.1 V. The most intense Raman scatter-
ing was achieved with 514.5 MI excuarion, whereas rhe lowest enhancements are found wirh 457.1 and 647.1 MI.
For convenient comparison, rhe spectra have been normalized 10 the same background level by usmg the enhanced background as Internal standard. The reason to choose ttus type of normalization IS three-fold. (i) Th2 background has been reported to appsar generally in SERS. (ti) The ratio of enhanced Ramrn signal to bachground is nearly independent of the electrode history >xch as pretreatment, the degree of anodtza502
I
I
500 ENERGY
1500 [ E m-1-J
I‘lg 1 Rm;ln spectra for CV molcculcschcmlsorbedon 3 sdw clccuode at -0 I V ior KUIIOUS c\crrlv ircqucncxs norm&cd to constant enhanced bxhground IIIcclrolyrc 0.1 .\I KCI. po~cr 3 rnw.
tion. the amount of dye present at the interface. In addition. it varies only weakly with the electrode potential (iii) Independently of its ongin (though some conrroversy remams) thr enhanced background represents inslastlc scattermg occurring at specific surface
sites [ 16,171. Therefore, th2 present normalization accounts not only for the spectrometer sensirivlty changing with the frequenciesof the photonspasstng throughthe system,but alsofor the unknown density of SER active sites. By comparlon of these spectra, distinct changrs of relative band intensities are notIcSable. To explain these effects more data are required on the type of couphng between electronic and Wbronic states and on their interaction with the metal substrate.
Fig I. displays an additional, very interssting. feature of this system - the appearance of (enhanced) Raman scattering only within a well-defined wavelrngth region between 480 and 670 NIL Using the 457 run lime for exciralion, no Raman bands below 1000 cm-l (or in wavelength scale below 480 run) can be detected, whereas above this threshold several, usually intense, Raman lines clearly exceed the noise. Using the next two laser lines at 488 and 5 I4 run, the Raman intensity risss steeply, as indicated by the substantial noise reduction. Now, all the Raman bands of adsorbed CV appear in the spectra (see table 1). Then, the recorded count rate decreases with 568 nm excitation and de.
CHEMICAL
Volume 89, number 6
9 July 1981
PHYSICS LEITERS
Table 1 Comparison of peak wwcnumbcrs in SER and wluclon RR spectra of CV SERS
Resoruna
Rrunan
13-l
131,237
337 125 446 529 562 608 623 663
330 418 440 519 568
ref.
[ 141
-
-
‘08
ref. [ 131
ref. [ 121
ref. Ill]
6-t3
-
‘O.?
103
334 126 444 532 570 616 -
335 421 JJl 528 565 608
341 426 JJJ,970 528 562 607
124
731
743
749
710 810
168 804 -
770 811 -
912 -
918 942
91-t 941
917 944
982
975
727
13-l
746
-
165 808 838
827
919 918
97s
-
1000 1129 1178 130-t 13-G 1375 1392 1350
1176 1298 1367 -
l-483 1511
1su
IS-13 1592 1625
1588 1619
129
971
1454
990 1172,1191 1301 1311 1370 1396 1449
990 1133 1174,1192 130-t 1341 1372 1395 1452
1489
1483
1483
15J2.1550 1594 1624
1510 1589 1620
1543 1599 1617
11-12 1182, 1193,l’OO 1306 13-U 1377 1390
-
-
cays even more with the 647 run line. In the latter curve a cut-off for SERS is found around 500 cm-’
where u,, q, represent two excited kV&, citing frequency and v,, the Raman stuft.
or 670 run. Note however,
A rough estimzrtion shows that the scattcnng cross section of the chemisorbed molecules is =I000 tunes larger than that of dtssolved species tn the resonance case (owing to some uncertainties tn the data, this estimate might be low. see ref. [2]) At a silver electrode
tion/scattertng
that thus particular excna-
profile for adsorbed CV molecules re-
sembles strongly that for CV molecules dissolved tn solution;in accord with Angeloni et al. [13] we observed a similar pattern in the preresonance and reronance Raman spectra, and conclude therefore that the “thresholds” reflect the variation in the vibronic cou-
pling with distinct excited electromc states. llus behaviour is described by the preresonance Raman enhancement factor F = [(l’a Vb
+
L’f)/(+- +)(I$ - I$]
(Y, -
“,,)’ ,
Y,
the ex-
the surface enhancement combines multipkstrvely with the molecular resonance Raman enhancement processes, resultmg in a total enhancement factor in the order of 108. The chemlsorbed species responds to the excitmg frequencies as it would to an enhanced electromagnetic wave. Also of interest is that even
503
cHt3IICAL
9 July 1982
PWSlCS UTTERS
smooth clcctrodes (1000 X Ag evaporated on a glass substrdtc) show relar~rely mtensr Raman signals for CV. m contrast to the usual e~periense that interfaces have IO be rough IO be SER sctw. Obviously, the mokCUlJr resonance Raman rnh.mccmsnt alonc C~II provide h& scattermg mtcnrilies Assuming a monolayer coverage. the amount of CV molecules bemg analyzed here is as small as 0 05 nmol. w111chis suffic~rnt to yield a htgh-quahty vlbrational spectrum (IR - IO4 cps at I, < 3 mW. 5 l-l nm). Thus demonstrdtcs a high scnslttvity of SER measuremcnts in suriacc JnAysis I
X-7. Cor~~parisorr
of
SLR ad
The SER spectrum in fig 21 is compared wth the publIshed RR spectra of CV solutions [ 11 - 1-I]. Table t summar17cs rhc w~enumb2rs of observed Raman pc,tks. A fatrly good coincidence IS evtdent among rltcsc spectra. The relattre band mrcns~tics ars also comparable to etch other. From these results it may be concluded. that the vtbrational state ofCV at -0.1 V is not much influenced by the presence of the Ag electrode surface. The counterparts for the 11-l and 663 cm-l peaks tn the SER spectrum are not Lund in the RR spectra. Of these, the former b.md ISsupposed to arise from a vibration of an adsorptton bond between the Ag surface and the CV molecule (probably via the nttrogen atom). The 663 cm-t band could be asslgned IO the C-H out-ofplan2 vibration of rhe benz2ne rmg (~(1 I) in W&on’s notation [IS]). The peak splitrmg tn the 1171-l 191 cni-’ region, frequently observed m rhe RR spccrra [ I Z-14]. is not detected tn tl~r SER spectrum.
Pvrerlrial ilcpeirderlce
3.3.
rlectrodes eenerdly show a potenUsually one cannot distinguish to what exlent tt is an intnnsic phenomenon of rhe SER process or is caused by secondary effects. such as adsorptton. desorption and electrochemical reacttons of the surface species. In this experimental configllration with the dye present only at the interface and in a limitcd potential range (EF -0.6 V), the potential dependcncc resulrs only from the electric field variation. This is dlustrated in figs. la-2f, showing SER spectra of CV ar the Ag electrode for potentials succeswely SER
efiects
llal dependence.
504
at
I
500
RR spccrra
ENERGY
1500 [ cm-‘]
clwnnsorb~d on A_r J( xmous tb) -0.3. tc) -0.5. (d) -0.7. tr) -0.9 Luld (II - 1.0 v. A,. = 51-t 5 nm. Poacr 3 mW. Ekcrrol~w. 0 I a1KC1
I I: 1
Kdrnm
S~L!SU.I Ior CV
porcnr~~~l~ (J) -0.1.
set to -0.1. -0.3.
-0.7, -0.9, and -1.0 V. E cathodic of -0.6 V, the adsorbed CV molecules are reduced VKI a oneclectron step [ 191, and the SER Intensity, Is, decreases. At -I 0 V. the magnitude ofl, is -l/7,0 of the original tntenstty. However. as.& is returned to the initlal value of -0.1 V,I, ISrecovered fo *l/3 of the imttal inrek shy. Comparmg the mitial SER spectrum with the last one. one notes a considerable difference tn shape. Thts irreverstble alteration in the spectral characteristics is umque evidence for a change of the molecular configuration of surface species occurrtng during the reducrlon and oxidation processes when the electrode potential is set to vtiurs cathodic of -0.6 V and back to -0.1 v. The relative intensity of each Raman band is influenced remarkably by the electrode potential. For a detailed discussion, one should know the correct ortgtn of each vibrational band. Although the reported ShIftmg
-0.5.
the potentml
band assignments
[ 1 l-141
remam
controversd,
an
“averaged” view for several important bands is given in table 2. On this basis it is evident from figs. ?a-2f that, for several a-b pair bands, the a mode and b mode e.xhtbit different potential dependences espectalty IINtie range E> -0.6 V. Typical results are ,IIustrared in fig. 3. These pair bands arise from vibrations of the p-daubstituted benzene moiety in the CV molecule, and are characterized by the fact that (i) the a
CHEMICAL
Volume 89. number 6
Table 2 Tcntauve awgnmcnt of sevcr~ lmport3nr bands m the SER spectrum of CV Wavcnumbsr 134 108
AE-N? breathing of the
cnttrc rnolcculc 337 415
v(16a) v(l6b) u(6b) ~(63) v(IOa)
838
UllOb)
919 1178 1375 l.ISO l-183 1592 1615 3)
v(Ph-C-Ph)
J-16 529 561 808
9 July 1981
PHYSICS LU-l-CRS
L’(12)? v(92) N-phcnyl v(19b) vt 193) v(8b) Y(k)
The LJsymbols xc Wdson’s notation [ 181
mode 1smore symmetric than the b mode with respect to the 1,4-axis for a given pau, and (II) the former mode does not mvolve out-of-plane vtbrations of the I ,4-substituents [ 161. As mentioned above. there is no electrochemrcrrl reduction of CV in the range E >
-0.6 V, whtch means there IS no change m surface concentration of CV. Hence, the observed general mtcnstty decrease (with a shght increase near E = -0.6 V) for the a modes and the intensity increase for the b modes with cathodtc polarnation are caused by the actron of the electric field on the vrbratronal characteristics of CV. Provided that the exIent of vibrontc coupltng is different for a and b modes, the dtstmct behaviour of thcsc modes could be qualitatively explrmcd by JSsuming a change in the opttcal absorptton property. though not idcntrfied, of adsorbed CV with changing potential_ In this context we note that the N-pllenyl wbration (1375 cm- I ), which lies ‘Loutslde” the chromophore of CV, shows very httle potential dependence in the range E 2 -0.5 V (ftg. 3). However, any further drscussron IS not posstble at the present stage where the band sssrgnments are not agreed and the state of adsorbed CV molecules on the Ag SUTixe is not known. The abrupt I, decrease beyond -0.6 V can be correlated with the electrochemical reductron of CV [ 171 or IIS surface complex probably composed of Ag adaloms, hahde ions and CV molecules (ii analogy to other surface complexes formed with pyridinc, CN- etc.. see references in ref. [lo]). As a result protonahon of the surface specres followed by a color
(Bl
ICI
L 02
table 2). (A) 0’ 562. l . 1615.~ 1375 cm-’ The broben curve represents
06
08
10
electrode potentral for (A) a mode, and (B) b mode. and (C) N.phcnyl vtbrs1591.a.838.A 446.0 145Ocm-l.(C) 808.~. 425.0: 1483 cm-’ .(B)o-529.o the background RS intensity at 1800cm-‘.
Fg. 3. Rcl~tive SER Intensity as a funcuon of the
tions (cl
OL
CHEMICAL PHYSICS LETTERS
Volur~~e 89. nulnbcr 6
ENERGY
[cm-‘]
I lg. 4. Kamm spccu;l ior chcmisorbcd 0
-0.9 V(C) XUI w~nance
IIICII
spectmm
numaicd
31 -0 1 V (a) mod
duiercnce (11) st 5 14 nm PIG
ior IICV in 3 hl IICI at 488 nm (d) In~cn-
Sll) SC& IclcrS IOcurve (3) OnI) hdmg could occur which would explain the drastic I, decrease of the brearhmg mode of the entire moleCUIC (208 cm-l), since rhs central carbon is situated at the midpotnt of the chromophore of the CV molecule. However. the reaction takmg place IS certamly not a simple uniform color fading, becauw the SER spectra at E G -0.6 V are remarkably &fferent from
the Raman spectrum of CV as well as of leuco-CV (111, though they etibit some vibronic fearures resemblingboth CV and WV. Tlti inillustratedin fig.4, showingspectra recordsd at -0.1 V (a), -0.9 V (c) and the numerical difierence of both (b). The former IS weighted with a factor of 0% to account for the gencnl intensity loss at -09 V. This difference spectrum yields a surprisingly clear insight on the variation of surface composition with potential: The “negalne” bands represent CV molsculrs reduced to HCV sprues which in rum contnbure to the “positlve” bands. For convenience, these spectra are shown together with a resonance Raman spectrum for HCV in acid solution (d). The necessary use of high HCI concentration IS here responuble for the shift of some 12uc0~CV bands relative to rhose observed for adsorbed CV. But there is a fairly good coincidence of bands which can be attnbuted either to HCV at the surface (b: 1175,1416,1448,1509,1591 cm-l)ortovibrations assocnted with the original CV molecules. This indeed indicates the coexistence of CV and HCV at the very cathodically polarizrd electrode. The generd irreversible decrease of the SERinten&y on applying cathodic potzntials (E G -1 .O V) is not a specific phenomenon of rhe CV-sdver system 506
9 July 198’
It resembles the partial quenching of SER scattering found for many adsorbates on Ag electrodes (e.g. pyridine, Cl-,CN-, SCN- [7,0,21]) when the applied potenual is set near, but anodic of the so-called quenching potenual. The (partial) quenching process has been shown [20,21] to occur due to the decreaseof the amount of adatom-adsorbate complsxes (iniually created m the ORC) by desorption of the adsorbate on applying sufficient cathodic potentials. Tlus sets adatoms free which ~IU be incorporated in the metal structure. After returning to more anodic potentials the number ofpossible adatom-adsorbate complexes wdl be drastically reduczd. For our particular system, however, it cannot be dlstingurshed
to what extent the electrochemical
reac-
tron acts on the complex composition or on the dye rtsclf. Although the type of change cannot be identifisd at present, the significant spectral variations represent a very impressive example of monttoring electro. chemical reactions occurring at the mterface. It is known that CV forms Ag-CV-X, (X = halide Ion) ternary complexes_ This reaction has been utllbed in quantitative deterrnmation of Ag+ ions by the extraction-photometric method [Zl. Even in aqueous solutions of CV, stable and soluble CV-Ag compleaes can be formed by addttion of ApN03 at a concentration of 10-t hl. This is indicated by a remartiable shift of the CV absorptron band by 5 nm after addilon of the
silver salt. Whether or not the stable attachment of the CV molecules on the Ag electrode surface is due to a sunilar complex has to be examined.
Acknowledgement We thank Professor H. Cerischer for his contimung support and stimulating Interest in this work.
References
[ 11 M. Flcischmann. P.J. Hendra and A J McQullhn,Chem. Phys. Lctrea 26 (1974) 163.
[Z] D L. Jeanmaire and R.P. van Duyne, J. Eleclroln31 Chcm. 84 (1977) 1.
[ 3 1 R P. van Duyne, m
Chemlcal and blochemlcal appbcanons of lasers. VoL 1. ed. C.B. Moore (Academic Press,
New York, 1979) ch 5. Ill M. Maskavits. Sohd State Commun 32 (1979) 59.
volume 89, number 6
CHEMICAL PHYSICS LETTERS
[ii] 5. Efnma and H. Meuu. 1. Chem. Phys 70 (1979) 229’1. [6] hf. Kerker. D S Wang and H. Chew, Appl. Opt. 19 (1980) 4159. 171 S S. Jha. J.R. KrrUey and J.C. Tung. Phys Rev B1, (1980) 3973. [S[ A Otto, Appl Surface SCI 6 (1980) 309. 191 T E. Turlakand J. Reyes,SurRcc SCI 93 (1980) 351.
lo] B Peltlnger and H. Wenel, Bcr. Bunscngcs Phbnk. Chcm. 85 (1981) 473. 111 1.V. Alrkundrov. Ya S. Bobowch. A T Var~anysn and A.N Sldorov, Opt Specrry. 47 (1977) 35. 121 J Glcqurl. M Carlcs and H. Bodol. J. Phys Chcm. 83 (1979) 699. 13 1 L. Angclom. G. Smulcwch and M P. MyrosL. J. Raman Spcctry 8 (1979) 305. 114) S. Sunder and H J. Bernstem, Can J CiWm. 59 (1981) 961
9 July I981
115 I B Pcttmger. U Wtnnmg and D hl. liolb, Bcr. Bunxngcs Physlk. Chcm. 82 (1978) 1326
[lb] 1. PocLnnd and A. OIIO. Sohd S~atc Commun
37 (1981) 109. [ 171 B. Pcrrmgcr and H. WCKCI. Chem. Phys Lc~~crs 78 (1981) 398 1181 C. Varunyi. VlbrationaJ spcclra ol benzcnc dcr~u~s (Academic Press, New York. 1969) [ 191 R C. Kwc and 11.1. StonchB. J. Chcm. Sac (1952)3231. [ 201 H. Wctzcl. H Ccnschcr and B. Pcrrmgcr. Chcm. Phys Lcc~ers 78 (1981) 392 [al 1 H. Wc:ctzcl.H. Ccrlschcr and B. Petnngcr, Chsm. Phys Lertcrs 80 (1981) 1.59 [??I N L Shcstldcsyatnaya. L 1. EoMyanskaya and I.A.Chuchulina, Zh. Anal. Khim. 30 (1975) 1303
507
Volume 89. number 6
SURFACE-ENHANCED ADSORBED Tadashi
PHYSICS LETTERS
9 July 1982
RAMAN SCATTERING FROM CRYSTAL VIOLET
ON A SILVER
ELECTRODE
WATANABE
Reccivrd 11 May
1981
SERS from ckcll~llon
crystal rlolct (CV) on a Ag clectrodc was mvcslqakd under prcrrsonsncc and resonance condltlons. The prolilc of the chcmaorbcd spccics is Me that of chsrolvcd mokculcs but mtcnsltirs ZUIL’ =I000 tnncs luger. The
Raman enhancement IS= IO8 and ckhlblts 3 spcc~lic potcntlal dcpcndcncr cvcn in ~IICabsence of processes At polentlals where rcductron CV occurs lcuco crystal wok-t wx dctcctcd.
of
1. Introduction Recent years have seen much interest in surfaceRaman (SER) scattering, both os a novel surface phenomenon to be elucidated and as a powerful spectroscopic rool for surface analysis [l-8]. However, a unified wew on the mechanism of SER scattermg has not yet been reached 19, IO]. This srtuenhanced
atron necessitates further accumulalion mental data on a variety of systems. In the present
study
we carried
employed
1. Experimental
out SER mvestlga-
in an electrochemical
cell.
The reason for the choice of this system is four-fold (i) Prehminary studies by van Duyne 131 show that a high-quality SER spectrum of CV can be obtained; however comparative investigations on the excitation pro!32 of CVadarbti and CVdlaolved were not performed. (ii) Recent resonance Raman (RR) data on CV are available, though the band assignments are not entirely consistent with each other [ 1l-141. (rii) Since this system does not require the przsence of CV molecules in the solution phase, the potentral dependence of the SER intennty, which is one of the 0 00%3614/82/0000-0000/S
crucial points for discussion oiSER mechanisms, can conveniently be ekamincd and extended to a potcntral region where protonation of the dye should occur. (IV)The occurrence of multiphwtion of SER and RR enhancements would enable an assessment of attainable sensitivity and apphcabibty of thus technique in surface analysis.
of experi-
tlons of an orgamc dye, crystal violet (CV), adsorbed at a Ag interface
adsorption-dcsorpllon
02.75 0 1982 North-Holland
As pretreatment for a massive polycrystalline silver plate (40 X 10 X 1 mm), an ouldation-reduction cycle (ORC) (-0.4 to +0.2 V and back to -0.4 V) was applied to the electrode in an elecrrolyte solution containing 5 X 10m3 m CV and 0.1 hl KCI. During this ORC. at Grst several tens of Ag layers w2r2 oxrdiz.cd and precipitated as AgCl at the surface, followed by B redeposition of these silver atoms at the interface leading to a strong roughening of the electrode. No electrochemical reactron of CV is expected in this potential range. After this procedure the Ag electrode was thoroughly rinsed with distilled water to remove ant dye not bound to silver. Then it was transferred
9 July 1981
CHEMICAL PHSSICS LCT-ERS
Volun~ 89. nun&x 6
Raman cell [ 151 which contanned3 dye-iree solution of 0.1 hl WI. Hence, elecIO an cl2ctrochcmical
trochcm~stry and Raman spectroscopy wdl b2 pcriornxd only with dye moleculeswhich wrre chemi-
!nterixe. The cornpurer-controlled Raman equipment has
sorbed at rhr sdvcr
been dcscrlbrd [ I5 1. The Imcs of an argon and a krypton laser (model 5,. CR4. and 3000 K, Coherent Radiation) were used as e\cltmg photodrcomposnion
frequcnclrs. To avold possible of CV. the mcldent power was
reduced to ~3 mW and the laser beam was line-focused usmg a hemicylindrlcal kns For borh incident and scsttcr2d s polarlzatlon (E vector perpendicular to the pl3ne of ~nculencc) w.is chosen AII potsnrials ar2 cited against the saturated calomsl elsctrods (SCE)
3. Results and discussion In the present
system, the CV molecules
exist only
the Ag electrode surface. Moreover, spontaneous dcsorptlon oi CV into the elecrrolyte IS not sigmficanr. at
because spsctra recorded around -0 1 V with an interval of30 min gave nearly the same intensities. Prebminary measurements showed that desorption of a small fracllon of surface CV givss rise to a weak broad fluorescence of dissolved CV. This in turn means that
chemisorbed CV is practically non-fluorescent. 3. i. IVmeler~gtlr deperlderrce Since the dye 1s absorbingln
thr spectral
re+n
m-
vestlg.nrd hers. selection of a sultable evcltation frequcncy is oigreat unponance, os shown by fig. I with a set of CV spectra recorded for vallous excitation frequencies a1 -0.1 V. The most intense Raman scatter-
ing was achieved with 514.5 MI excuarion, whereas rhe lowest enhancements are found wirh 457.1 and 647.1 MI.
For convenient comparison, rhe spectra have been normalized 10 the same background level by usmg the enhanced background as Internal standard. The reason to choose ttus type of normalization IS three-fold. (i) Th2 background has been reported to appsar generally in SERS. (ti) The ratio of enhanced Ramrn signal to bachground is nearly independent of the electrode history >xch as pretreatment, the degree of anodtza502
I
I
500 ENERGY
1500 [ E m-1-J
I‘lg 1 Rm;ln spectra for CV molcculcschcmlsorbedon 3 sdw clccuode at -0 I V ior KUIIOUS c\crrlv ircqucncxs norm&cd to constant enhanced bxhground IIIcclrolyrc 0.1 .\I KCI. po~cr 3 rnw.
tion. the amount of dye present at the interface. In addition. it varies only weakly with the electrode potential (iii) Independently of its ongin (though some conrroversy remams) thr enhanced background represents inslastlc scattermg occurring at specific surface
sites [ 16,171. Therefore, th2 present normalization accounts not only for the spectrometer sensirivlty changing with the frequenciesof the photonspasstng throughthe system,but alsofor the unknown density of SER active sites. By comparlon of these spectra, distinct changrs of relative band intensities are notIcSable. To explain these effects more data are required on the type of couphng between electronic and Wbronic states and on their interaction with the metal substrate.
Fig I. displays an additional, very interssting. feature of this system - the appearance of (enhanced) Raman scattering only within a well-defined wavelrngth region between 480 and 670 NIL Using the 457 run lime for exciralion, no Raman bands below 1000 cm-l (or in wavelength scale below 480 run) can be detected, whereas above this threshold several, usually intense, Raman lines clearly exceed the noise. Using the next two laser lines at 488 and 5 I4 run, the Raman intensity risss steeply, as indicated by the substantial noise reduction. Now, all the Raman bands of adsorbed CV appear in the spectra (see table 1). Then, the recorded count rate decreases with 568 nm excitation and de.
CHEMICAL
Volume 89, number 6
9 July 1981
PHYSICS LEITERS
Table 1 Comparison of peak wwcnumbcrs in SER and wluclon RR spectra of CV SERS
Resoruna
Rrunan
13-l
131,237
337 125 446 529 562 608 623 663
330 418 440 519 568
ref.
[ 141
-
-
‘08
ref. [ 131
ref. [ 121
ref. Ill]
6-t3
-
‘O.?
103
334 126 444 532 570 616 -
335 421 JJl 528 565 608
341 426 JJJ,970 528 562 607
124
731
743
749
710 810
168 804 -
770 811 -
912 -
918 942
91-t 941
917 944
982
975
727
13-l
746
-
165 808 838
827
919 918
97s
-
1000 1129 1178 130-t 13-G 1375 1392 1350
1176 1298 1367 -
l-483 1511
1su
IS-13 1592 1625
1588 1619
129
971
1454
990 1172,1191 1301 1311 1370 1396 1449
990 1133 1174,1192 130-t 1341 1372 1395 1452
1489
1483
1483
15J2.1550 1594 1624
1510 1589 1620
1543 1599 1617
11-12 1182, 1193,l’OO 1306 13-U 1377 1390
-
-
cays even more with the 647 run line. In the latter curve a cut-off for SERS is found around 500 cm-’
where u,, q, represent two excited kV&, citing frequency and v,, the Raman stuft.
or 670 run. Note however,
A rough estimzrtion shows that the scattcnng cross section of the chemisorbed molecules is =I000 tunes larger than that of dtssolved species tn the resonance case (owing to some uncertainties tn the data, this estimate might be low. see ref. [2]) At a silver electrode
tion/scattertng
that thus particular excna-
profile for adsorbed CV molecules re-
sembles strongly that for CV molecules dissolved tn solution;in accord with Angeloni et al. [13] we observed a similar pattern in the preresonance and reronance Raman spectra, and conclude therefore that the “thresholds” reflect the variation in the vibronic cou-
pling with distinct excited electromc states. llus behaviour is described by the preresonance Raman enhancement factor F = [(l’a Vb
+
L’f)/(+- +)(I$ - I$]
(Y, -
“,,)’ ,
Y,
the ex-
the surface enhancement combines multipkstrvely with the molecular resonance Raman enhancement processes, resultmg in a total enhancement factor in the order of 108. The chemlsorbed species responds to the excitmg frequencies as it would to an enhanced electromagnetic wave. Also of interest is that even
503
cHt3IICAL
9 July 1982
PWSlCS UTTERS
smooth clcctrodes (1000 X Ag evaporated on a glass substrdtc) show relar~rely mtensr Raman signals for CV. m contrast to the usual e~periense that interfaces have IO be rough IO be SER sctw. Obviously, the mokCUlJr resonance Raman rnh.mccmsnt alonc C~II provide h& scattermg mtcnrilies Assuming a monolayer coverage. the amount of CV molecules bemg analyzed here is as small as 0 05 nmol. w111chis suffic~rnt to yield a htgh-quahty vlbrational spectrum (IR - IO4 cps at I, < 3 mW. 5 l-l nm). Thus demonstrdtcs a high scnslttvity of SER measuremcnts in suriacc JnAysis I
X-7. Cor~~parisorr
of
SLR ad
The SER spectrum in fig 21 is compared wth the publIshed RR spectra of CV solutions [ 11 - 1-I]. Table t summar17cs rhc w~enumb2rs of observed Raman pc,tks. A fatrly good coincidence IS evtdent among rltcsc spectra. The relattre band mrcns~tics ars also comparable to etch other. From these results it may be concluded. that the vtbrational state ofCV at -0.1 V is not much influenced by the presence of the Ag electrode surface. The counterparts for the 11-l and 663 cm-l peaks tn the SER spectrum are not Lund in the RR spectra. Of these, the former b.md ISsupposed to arise from a vibration of an adsorptton bond between the Ag surface and the CV molecule (probably via the nttrogen atom). The 663 cm-t band could be asslgned IO the C-H out-ofplan2 vibration of rhe benz2ne rmg (~(1 I) in W&on’s notation [IS]). The peak splitrmg tn the 1171-l 191 cni-’ region, frequently observed m rhe RR spccrra [ I Z-14]. is not detected tn tl~r SER spectrum.
Pvrerlrial ilcpeirderlce
3.3.
rlectrodes eenerdly show a potenUsually one cannot distinguish to what exlent tt is an intnnsic phenomenon of rhe SER process or is caused by secondary effects. such as adsorptton. desorption and electrochemical reacttons of the surface species. In this experimental configllration with the dye present only at the interface and in a limitcd potential range (EF -0.6 V), the potential dependcncc resulrs only from the electric field variation. This is dlustrated in figs. la-2f, showing SER spectra of CV ar the Ag electrode for potentials succeswely SER
efiects
llal dependence.
504
at
I
500
RR spccrra
ENERGY
1500 [ cm-‘]
clwnnsorb~d on A_r J( xmous tb) -0.3. tc) -0.5. (d) -0.7. tr) -0.9 Luld (II - 1.0 v. A,. = 51-t 5 nm. Poacr 3 mW. Ekcrrol~w. 0 I a1KC1
I I: 1
Kdrnm
S~L!SU.I Ior CV
porcnr~~~l~ (J) -0.1.
set to -0.1. -0.3.
-0.7, -0.9, and -1.0 V. E cathodic of -0.6 V, the adsorbed CV molecules are reduced VKI a oneclectron step [ 191, and the SER Intensity, Is, decreases. At -I 0 V. the magnitude ofl, is -l/7,0 of the original tntenstty. However. as.& is returned to the initlal value of -0.1 V,I, ISrecovered fo *l/3 of the imttal inrek shy. Comparmg the mitial SER spectrum with the last one. one notes a considerable difference tn shape. Thts irreverstble alteration in the spectral characteristics is umque evidence for a change of the molecular configuration of surface species occurrtng during the reducrlon and oxidation processes when the electrode potential is set to vtiurs cathodic of -0.6 V and back to -0.1 v. The relative intensity of each Raman band is influenced remarkably by the electrode potential. For a detailed discussion, one should know the correct ortgtn of each vibrational band. Although the reported ShIftmg
-0.5.
the potentml
band assignments
[ 1 l-141
remam
controversd,
an
“averaged” view for several important bands is given in table 2. On this basis it is evident from figs. ?a-2f that, for several a-b pair bands, the a mode and b mode e.xhtbit different potential dependences espectalty IINtie range E> -0.6 V. Typical results are ,IIustrared in fig. 3. These pair bands arise from vibrations of the p-daubstituted benzene moiety in the CV molecule, and are characterized by the fact that (i) the a
CHEMICAL
Volume 89. number 6
Table 2 Tcntauve awgnmcnt of sevcr~ lmport3nr bands m the SER spectrum of CV Wavcnumbsr 134 108
AE-N? breathing of the
cnttrc rnolcculc 337 415
v(16a) v(l6b) u(6b) ~(63) v(IOa)
838
UllOb)
919 1178 1375 l.ISO l-183 1592 1615 3)
v(Ph-C-Ph)
J-16 529 561 808
9 July 1981
PHYSICS LU-l-CRS
L’(12)? v(92) N-phcnyl v(19b) vt 193) v(8b) Y(k)
The LJsymbols xc Wdson’s notation [ 181
mode 1smore symmetric than the b mode with respect to the 1,4-axis for a given pau, and (II) the former mode does not mvolve out-of-plane vtbrations of the I ,4-substituents [ 161. As mentioned above. there is no electrochemrcrrl reduction of CV in the range E >
-0.6 V, whtch means there IS no change m surface concentration of CV. Hence, the observed general mtcnstty decrease (with a shght increase near E = -0.6 V) for the a modes and the intensity increase for the b modes with cathodtc polarnation are caused by the actron of the electric field on the vrbratronal characteristics of CV. Provided that the exIent of vibrontc coupltng is different for a and b modes, the dtstmct behaviour of thcsc modes could be qualitatively explrmcd by JSsuming a change in the opttcal absorptton property. though not idcntrfied, of adsorbed CV with changing potential_ In this context we note that the N-pllenyl wbration (1375 cm- I ), which lies ‘Loutslde” the chromophore of CV, shows very httle potential dependence in the range E 2 -0.5 V (ftg. 3). However, any further drscussron IS not posstble at the present stage where the band sssrgnments are not agreed and the state of adsorbed CV molecules on the Ag SUTixe is not known. The abrupt I, decrease beyond -0.6 V can be correlated with the electrochemical reductron of CV [ 171 or IIS surface complex probably composed of Ag adaloms, hahde ions and CV molecules (ii analogy to other surface complexes formed with pyridinc, CN- etc.. see references in ref. [lo]). As a result protonahon of the surface specres followed by a color
(Bl
ICI
L 02
table 2). (A) 0’ 562. l . 1615.~ 1375 cm-’ The broben curve represents
06
08
10
electrode potentral for (A) a mode, and (B) b mode. and (C) N.phcnyl vtbrs1591.a.838.A 446.0 145Ocm-l.(C) 808.~. 425.0: 1483 cm-’ .(B)o-529.o the background RS intensity at 1800cm-‘.
Fg. 3. Rcl~tive SER Intensity as a funcuon of the
tions (cl
OL
CHEMICAL PHYSICS LETTERS
Volur~~e 89. nulnbcr 6
ENERGY
[cm-‘]
I lg. 4. Kamm spccu;l ior chcmisorbcd 0
-0.9 V(C) XUI w~nance
IIICII
spectmm
numaicd
31 -0 1 V (a) mod
duiercnce (11) st 5 14 nm PIG
ior IICV in 3 hl IICI at 488 nm (d) In~cn-
Sll) SC& IclcrS IOcurve (3) OnI) hdmg could occur which would explain the drastic I, decrease of the brearhmg mode of the entire moleCUIC (208 cm-l), since rhs central carbon is situated at the midpotnt of the chromophore of the CV molecule. However. the reaction takmg place IS certamly not a simple uniform color fading, becauw the SER spectra at E G -0.6 V are remarkably &fferent from
the Raman spectrum of CV as well as of leuco-CV (111, though they etibit some vibronic fearures resemblingboth CV and WV. Tlti inillustratedin fig.4, showingspectra recordsd at -0.1 V (a), -0.9 V (c) and the numerical difierence of both (b). The former IS weighted with a factor of 0% to account for the gencnl intensity loss at -09 V. This difference spectrum yields a surprisingly clear insight on the variation of surface composition with potential: The “negalne” bands represent CV molsculrs reduced to HCV sprues which in rum contnbure to the “positlve” bands. For convenience, these spectra are shown together with a resonance Raman spectrum for HCV in acid solution (d). The necessary use of high HCI concentration IS here responuble for the shift of some 12uc0~CV bands relative to rhose observed for adsorbed CV. But there is a fairly good coincidence of bands which can be attnbuted either to HCV at the surface (b: 1175,1416,1448,1509,1591 cm-l)ortovibrations assocnted with the original CV molecules. This indeed indicates the coexistence of CV and HCV at the very cathodically polarizrd electrode. The generd irreversible decrease of the SERinten&y on applying cathodic potzntials (E G -1 .O V) is not a specific phenomenon of rhe CV-sdver system 506
9 July 198’
It resembles the partial quenching of SER scattering found for many adsorbates on Ag electrodes (e.g. pyridine, Cl-,CN-, SCN- [7,0,21]) when the applied potenual is set near, but anodic of the so-called quenching potenual. The (partial) quenching process has been shown [20,21] to occur due to the decreaseof the amount of adatom-adsorbate complsxes (iniually created m the ORC) by desorption of the adsorbate on applying sufficient cathodic potentials. Tlus sets adatoms free which ~IU be incorporated in the metal structure. After returning to more anodic potentials the number ofpossible adatom-adsorbate complexes wdl be drastically reduczd. For our particular system, however, it cannot be dlstingurshed
to what extent the electrochemical
reac-
tron acts on the complex composition or on the dye rtsclf. Although the type of change cannot be identifisd at present, the significant spectral variations represent a very impressive example of monttoring electro. chemical reactions occurring at the mterface. It is known that CV forms Ag-CV-X, (X = halide Ion) ternary complexes_ This reaction has been utllbed in quantitative deterrnmation of Ag+ ions by the extraction-photometric method [Zl. Even in aqueous solutions of CV, stable and soluble CV-Ag compleaes can be formed by addttion of ApN03 at a concentration of 10-t hl. This is indicated by a remartiable shift of the CV absorptron band by 5 nm after addilon of the
silver salt. Whether or not the stable attachment of the CV molecules on the Ag electrode surface is due to a sunilar complex has to be examined.
Acknowledgement We thank Professor H. Cerischer for his contimung support and stimulating Interest in this work.
References
[ 11 M. Flcischmann. P.J. Hendra and A J McQullhn,Chem. Phys. Lctrea 26 (1974) 163.
[Z] D L. Jeanmaire and R.P. van Duyne, J. Eleclroln31 Chcm. 84 (1977) 1.
[ 3 1 R P. van Duyne, m
Chemlcal and blochemlcal appbcanons of lasers. VoL 1. ed. C.B. Moore (Academic Press,
New York, 1979) ch 5. Ill M. Maskavits. Sohd State Commun 32 (1979) 59.
volume 89, number 6
CHEMICAL PHYSICS LETTERS
[ii] 5. Efnma and H. Meuu. 1. Chem. Phys 70 (1979) 229’1. [6] hf. Kerker. D S Wang and H. Chew, Appl. Opt. 19 (1980) 4159. 171 S S. Jha. J.R. KrrUey and J.C. Tung. Phys Rev B1, (1980) 3973. [S[ A Otto, Appl Surface SCI 6 (1980) 309. 191 T E. Turlakand J. Reyes,SurRcc SCI 93 (1980) 351.
lo] B Peltlnger and H. Wenel, Bcr. Bunscngcs Phbnk. Chcm. 85 (1981) 473. 111 1.V. Alrkundrov. Ya S. Bobowch. A T Var~anysn and A.N Sldorov, Opt Specrry. 47 (1977) 35. 121 J Glcqurl. M Carlcs and H. Bodol. J. Phys Chcm. 83 (1979) 699. 13 1 L. Angclom. G. Smulcwch and M P. MyrosL. J. Raman Spcctry 8 (1979) 305. 114) S. Sunder and H J. Bernstem, Can J CiWm. 59 (1981) 961
9 July I981
115 I B Pcttmger. U Wtnnmg and D hl. liolb, Bcr. Bunxngcs Physlk. Chcm. 82 (1978) 1326
[lb] 1. PocLnnd and A. OIIO. Sohd S~atc Commun
37 (1981) 109. [ 171 B. Pcrrmgcr and H. WCKCI. Chem. Phys Lc~~crs 78 (1981) 398 1181 C. Varunyi. VlbrationaJ spcclra ol benzcnc dcr~u~s (Academic Press, New York. 1969) [ 191 R C. Kwc and 11.1. StonchB. J. Chcm. Sac (1952)3231. [ 201 H. Wctzcl. H Ccnschcr and B. Pcrrmgcr. Chcm. Phys Lcc~ers 78 (1981) 392 [al 1 H. Wc:ctzcl.H. Ccrlschcr and B. Petnngcr, Chsm. Phys Lertcrs 80 (1981) 1.59 [??I N L Shcstldcsyatnaya. L 1. EoMyanskaya and I.A.Chuchulina, Zh. Anal. Khim. 30 (1975) 1303
507