Spectrochimica Acta Part A 60 (2004) 25–29

Surface enhanced Raman scattering of benzotriazole:a molecular orientational study

S. Thomasa, S. Venkateswarana, S. Kapoorb,∗, R. D’Cunha1, T. Mukherjeeb

a Synchrotron Radiation Section, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, Indiab Radiation Chemistry and Chemical Dynamics Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India

Received 5 February 2003; accepted 3 March 2003

Abstract

Surface enhanced Raman scattering of benzotriazole in aqueous silver sol at pH∼9 has been investigated. The spectra of the moleculeat various concentrations were recorded and the maximum enhancement was observed with a 10−5 M solution. From the comparison of thesurface enhanced Raman spectra with the conventional Raman spectra and applying ‘surface selection rule’, it is inferred that the molecularion interacts with the silver sol via the nitrogen atoms of the benzotriazole ring and the molecule assumes a near ‘end-on’ orientation.© 2003 Elsevier B.V. All rights reserved.

Keywords: Surface enhanced Raman scattering (SERS); Benzotriazole (BTAH)

1. Introduction

Surface enhanced Raman scattering (SERS) is a sen-sitive spectroscopic technique for the study of ionic andmolecular species adsorbed on ‘rough’ metal surfaces ornanometer-sized metallic colloidal particles[1,2]. Theenhancement of the Raman signal in SERS has been ex-plained through two main mechanisms, the electromagneticenhancement and the chemical enhancement. Both thesemechanisms play an important role but the contributionto the total enhancement from each method is not clearlydefined. Among the many substrates, silver colloids inaqueous solutions are predominantly used because of thesimplicity of preparation, characterization and convenienceof monitoring the sols by visible absorption spectroscopy[3]. The discovery of SERS has increased the sensitivity ofRaman spectroscopic detection. In addition, the adsorptionof molecules on metal particles reduces the fluorescencebackground and hence the technique is useful in the studyof biological samples[4]. Most reported studies pertain, tothe adsorbate molecules, possessing one or more�-donoratoms such as N, O and S, many of which possess potential

∗ Corresponding author. Tel.:+91-22-559-0298;fax: +91-22-550-5151.

E-mail address: sudhirk@apsara.barc.ernet.in (S. Kapoor).1 Ex-Spectroscopy Division.

� donor systems such as an aromatic ring, which couldcompete for interaction with the surface.

The enhancement of selective vibrational modes and bandshifts observed in SERS have usually been explained interms of the charge-transfer model, and are found to be sen-sitive to the orientation of the molecules with respect to thesurface. The technique is therefore expected to provide in-teresting information on the sites through which the inter-action takes place and also the molecular orientation withrespect to the metal surfaces.

Benzotriazole (BTAH) and its derivatives have foundseveral applications in the field of corrosion of metals andhave been widely used in the surface treatment of materialsas corrosion inhibitors. Several investigations to understandand elucidate the mechanism of interaction of BTAH andother related molecules with metal surfaces have been re-ported[5,6]. Studies of BTAH adsorbed on copper, silver,iron and nickel electrodes as a function of the electrodepotential and added electrolyte have been recently reported[7–9]. SERS of BTAH on silver colloidal surface has alsobeen reported[10]. All the above studies have mainly fo-cused on the identification of the nature of the speciesformed on the surface and the assignment of the observedvibrational bands. The resistance of silver and other metalsto tarnishing in the presence of BTAH has been attributedto the formation of a stable metal-benzotriazole com-plex [7–9]. Recent studies have reported surface enhanced

1386-1425/$ – see front matter © 2003 Elsevier B.V. All rights reserved.doi:10.1016/S1386-1425(03)00213-0

26 S. Thomas et al. / Spectrochimica Acta Part A 60 (2004) 25–29

resonance Raman scattering detection of DNA labelled witha specifically designed benzotriazole azo dye[11].

In recent years there has been interest on SERS stud-ies of molecules with particular emphasis on the orienta-tion of the adsorbate on the surface. The ‘surface selectionrule’ of Moskovits and Suh[12–16] have formed the basisfor the interpretation of the SERS data leading to informa-tion on the orientation of adsorbed molecules relative to themetal surface. Moreover, significant differences in the orien-tation of molecules adsorbed on electrodes and colloidal sur-faces have been reported based on their SERS spectra[17].Changes in orientation of molecules as a function of con-centration have also been reported. In spite of many SERSstudies on BTAH, no investigation on the orientation of ben-zotriazole molecule adsorbed on silver colloidal surface, arereported.

In this article, we report the results of our observations ofBTAH adsorbed on Ag sols in alkaline media. The focus ofthe present work is to obtain information on the orientationof the BTAH molecules on silver sol and to study the effect ofconcentration on the surface orientation. The interpretationof the data based on the ‘surface selection rules’ suggests anear ‘end-on’ configuration for the BTAH molecule with theN-atoms of the hetero ring mainly interacting with the silversurface. The details of the studies are discussed in this article.

2. Experimental

Aqueous silver sol was prepared by the reduction of sil-ver nitrate with sodium borohydride using the method ofCreighton et al.[18]. The size of the silver particles in thesol was determined by TEM studies and was found to be15–20 nm[19]. The UV–Visible absorption spectrum wasrecorded using a Hitachi 330-A spectrophotometer. All therequired samples were prepared using millipore water. Ra-man spectra were recorded using a SPEX Ramalog doublemonochromator (Model 1401). The samples were taken in acapillary tube and were excited using the 514.5 nm line of aCoherent Ar+ laser. The power at the sample was 150 mW.The spectrometer was operated in the photon counting modeand a PC based system was used for date acquisition andmonochromator control. Each measurement was repeated atleast twice to ensure reproducibility.

3. Results

3.1. UV–Visible absorption spectra

The UV–Visible absorption spectra of the silver sol be-fore and after the addition of BTAH are shown inFig. 1.The absorption spectrum of the silver sol showed a singlesharp molar extinction maximum at 390 nm, which is dueto the resonant excitation of plasma oscillations in the con-fined electron gas of the particles[20]. After the addition ofthe sample to the sol, the absorption spectrum showed a sec-

Fig. 1. Solid line: UV–Visible spectrum of silver sol; dashed line:UV–Visible spectrum of silver sol after addition of BTAH.

ond band at about 560 nm with a decrease in the absorbanceof the 390 nm band. The appearance of the red shifted sec-ond band has been ascribed to the aggregation of the silverparticles in the presence of the adsorbed molecules.

3.2. SERS Spectra

The SERS spectra of BTAH (10−5 M) in silver sol alongwith the normal Raman spectra of a 0.5 M alkaline solu-tion of BTAH are presented inFig. 2. The SERS spectrumshows a selective enhancement of bands with relative in-tensities different from those in the solution spectrum. Themost prominent Raman active modes in the spectra are ex-pected to be in the region 700–1700 cm−1. The assignmentsof the vibrational modes, which are based on the previousstudies carried out on BTAH[7,10] and results reported for1H Indazole[21] are given inTable 1. The assignments of

Table 1Normal Raman and SERS spectral data (cm−1) of benzotriazole andtentative band assignments

Normal Raman(0.5 M BTAH)

SERS(10−5 M BTAH)

Tentative assignments

– 242 Ag–N stretch544vw 551vw CCC in plane bend631w 637vw Triazole ring torsion779s 788s Benzene ring breathing

1015s,b1017s 1033s,b In plane triagonal ring breathing1138w 1137w CH in plane bend1162w 1183b NNNsym.stretch (triazole ring

breathing)+CH in plane bend1283m 1290b CH in plane bend+CC stretch1376sh 1371b,sh Phenyl ring stretch1388s 1393vs Triazole ring stretch1574m 1576m Phenyl ring stretch

s, strong; w, weak; vw, very weak; m, medium; sh, shoulder; vs, verystrong; b, broad.

S. Thomas et al. / Spectrochimica Acta Part A 60 (2004) 25–29 27

Fig. 2. (a) SERS spectrum of BTAH (10−5 M); (b) Normal Raman spectrum of alkaline BTAH (0.5 M).

the phenyl ring modes have been made by partially keepingin mind the assignments for ortho-disubstituted benzenes(light substituents) by Varsanyi[22]. Several of the vibra-tional modes are expected to be mixed with contributionsfrom the N=N and C–N stretch of the triazole ring, the CCstretch and the CH in plane bending vibrations of the phenylring as was shown from DFT calculations for 1H Indazole[21].

4. Discussion

The BTAH molecule dissociates according to the equilib-rium

BTAH �BTA− + H+, Ka = 4.27× 10−9 M

and the pKa value of this equilibrium is 8.37[23].For the measurements carried out in the present studies

at pH ∼9, the BTAH molecule is expected to be in theionized form resulting from the dissociation of the labileH atom of the triazole ring. The loss of the proton wouldeffectively meanC2v symmetry for the molecular ion withthe delocalisation of the� electrons in the 5 and 6 memberedfused ring system, and the bond order of the two NN bondswould be equal[7]. The three nitrogen atoms of the triazolering are expected to be equivalent as demonstrated fromprevious XPS measurements of BTAH molecule adsorbedon copper surfaces[6].

The orientation of the adsorbate on the metal surface willdepend on the active sites through which the interaction takesplace. Two chemically plausible orientations of the benzotri-azole molecular ion with respect to the silver surface can beenvisaged: ‘flat’ or lying down on the metal surface through

�-bonding with the 5 and 6 membered benzotriazole ringsystem or ‘standing up’ (end-on) with� bonding throughthe lone pair of the nitrogen atom with silver.

4.1. Vibrational modes in the region200–1700 cm−1 region

A comparison of the spectrum of the alkaline BTAH solu-tion with the SERS spectrum inFig. 2shows no major shiftsin the positions of the bands relative to each other except forthe triazole ring breathing mode. Greater enhancement wasobserved for the bands at 1393 and 1033 cm−1. The bandat 1393 cm−1 which shows the maximum enhancement inSERS is assigned to the triazole ring stretching mode withsome contributions from CC stretch and CH bending. It maybe noted that the two bands observed, one at 1388 cm−1 andthe other a shoulder at 1376 cm−1 in the solution merge togive the very strong enhanced band located at 1393 cm−1

with a distinct broad asymmetry on the lower frequencyside. This could be due to the broadening of the CC stretchband at 1376 cm−1, similar to the broadening observedfor the other CC stretch modes. The substantial increase inintensity of the peak at 1393 and the 1033 cm−1 band in-dicates that the nitrogen atoms of the triazolate ring play amajor role in interacting with the silver surface. This is alsoconfirmed by the earlier SERS studies[10]. The SER spec-trum shows two overlapping bands at 1015 and 1033 cm−1

in the place of the 1015 cm−1 band seen in the solution,which is assigned to the inplane triagonal ring breathingmode. The above observation is also reported in the SERSstudies of BTAH on silver electrodes which is attributed tothe possibility of co-adsorption of BTA− and Ag(1)BTA[8]. The symmetric N=N=N ring stretching vibration seen

28 S. Thomas et al. / Spectrochimica Acta Part A 60 (2004) 25–29

at 1162 cm−1 in solution is shifted to 1183 cm−1 in theSERS spectra. This vibration is expected to be sensitive tocoordination of the triazole ring with silver and the shift isattributed to the formation of Ag(1)BTA. A similar situationhas been reported in the case of benzotriazole adsorbed ona silver electrode[8]. This is also confirmed by the appear-ance of the band at 242 cm−1 in the SERS spectrum, whichcorresponds to the Ag–N stretch (inset inFig. 2).

The intensity enhancement in SERS has been explainedon the basis of ‘surface selection rules’ based on imagedipole theory as predicted by Allen and Van Duyne[24]and Moskovits and Suh[12–16]. According to the ‘surfaceselection rule’, the normal modes of adsorbed moleculesinvolving changes in molecular polarizability with a com-ponent perpendicular to the surface are subject to the great-est enhancement. If we assume the BTAH molecule to be‘standing up’ in the end-on configuration with the nitrogenatom of the triazole ring interacting with the Ag surface,apart from the vibrations involving the N-atoms, the modesof the phenyl ring deriving their intensity fromαzz wherez is the local normal to the surface, would be expected tobe candidates for maximum enhancement. We find the CCstretch in plane ring modes at 1576, 1371 and 788 cm−1

all of which related to the A1g modes of benzene withmaximum contribution toαzz showing relatively higher en-hancements compared to the other modes. The non-totallysymmetric in plane motions, which involve theαyz andαxz

polarisability components are also expected to show surfaceenhancement. Accordingly, the CH in-plane bending modesat 1290, 1183, 1137 cm−1 are found to be enhanced. For vi-brations involvingαxy no surface enhancement is expected.All out of plane vibrations are weak for this type of orien-tation. The out-of-plane bending modes of the phenyl ring(contributing toαxy), which are usually weak in Raman arenot seen in the SERS spectrum, while the 631 cm−1 bandin the spectrum of the solution attributed to the out-of-planetorsional mode of BTAH is completely suppressed. If themolecule were lying ‘flat’ on the surface one would expectthe out of the plane modes to show enhancement in intensity.

4.2. CH Stretch

The CH stretching vibration of the benzene ring, in theregion 3040–3080 cm−1 has been shown to be an unam-biguous probe in the determination of surface orientation ofsubstituted aromatics[25]. Since CH stretching modes donot mix significantly with other vibrational modes of thearomatic ring, their SERS intensities are known to providethe most specific evidence of the orientation of the adsor-bate with respect to the surface[12]. The contribution to theSERS intensity associated with the stretching of a specificCH bond will depend upon the angle it makes in relationto the surface and the distance (location) of the CH bondfrom the metal surface. Vibrations of atoms interacting in-timately with the surface are expected to contribute moresignificantly to the enhancement. In the ‘end-on’ geometry

the CH stretch vibrations of the phenyl ring, in the planeperpendicular to the metal surface would be expected tocontribute to significant enhancement of these bands. How-ever, this test is known to fail in the case of phthalazine[12,13], where all other evidence points to an end-on orien-tation of the molecule. The CH stretch modes in the solutionof BTAH, which are barely seen in the Raman spectrum, donot show up with any significant intensity in the SERS spec-trum. The absence of enhancement of CH stretching modescould be explained with reference to the structure of BTAions as envisaged on the surface (inset inFig. 1). The twoCH bonds closest to the surface vibrate parallel to the sur-face. Hence, according to the surface selection rules, theydo not contribute to the enhancement, while the two otherCH bonds, which are tilted, are located further away fromthe surface. Identical observations have been reported forphthalazine wherein a similar situation prevails[13].

Finally, a comment on the width of the phenyl ring vi-brational modes is in order. Most of the ring modes arefound to be broader in SERS as compared to those in thesolution. This could indicate the possible interaction of thedelocalised� electrons of the fused ring systems with thesurface—due to the slight tilting of the molecular ion fromits end-on position. The small shifts observed for the ringmodes could also be attributed to somewhat limited inter-action of the� electrons with the surface. Changes in thewidth of the benzene ring modes have been attributed to thepresence of surface-ring� interaction[26,27].

4.3. Concentration effects

The effect of concentration on SERS in relation to theorientation of the molecule, with respect to metal surfaceshave been reported[21,12,28]. In a recent paper on SERSof 1H Indazole on silver sols[21], the authors have no-ticed changes of relative intensities of bands, particularlythe out-of-plane modes as a function of concentration. Anincrease in the relative intensities of the out-of-plane modeshas been interpreted to indicate that the molecule assumesa more tilted orientation upon lowering the concentrationof the adsorbate. A complete change in orientation from‘end-on’ to ‘flat’ has also been reported on dilution[12]. Toinvestigate the effect of concentration on the orientation inthe present studies, the SER spectra of BTAH at various di-lution levels ranging from 10−2 to 10−5 M were recorded.The spectra at various concentrations are shown inFig. 3.No significant changes in the nature of the spectra or linepositions were observed (the maximum enhancement wasobtained with the 10−5 M solution). This suggests that themolecular orientation is more rigid in our case and is notaffected by changes in concentration, unlike what has beenreported for 1H Indazole. The possible reason for the dif-ference in the two cases could be that in 1H Indazole themolecule is non dissociatively adsorbed while BTAH inter-acts in the ionized form with the silver surface causing thestructure to be more rigid.

S. Thomas et al. / Spectrochimica Acta Part A 60 (2004) 25–29 29

Fig. 3. Concentration dependence of BTAH in silver sol: (a) 10−2 M (b)10−4 M (c) 10−5 M.

5. Summary

On the basis of SERS studies of BTAH in Ag sol (pH∼9)we propose that the molecular ion, which is chemisorbed onthe silver surface through the nitrogen atoms of the triazolering assumes a near ‘end-on’ orientation. These conclusionsare based on the following observations:

(1) The marked enhancement of the band at 1393 cm−1

assigned to the triazolate ring stretching vibration, indicatingthe involvement of the N-atoms in the interaction with thesurface.

(2) This is substantiated by the appearance of the bandat 242 cm−1 in the SER spectrum which is attributed to theAg–N stretch. This indicates the direct interaction of the Natoms through�-bonding with the Ag atoms.

(3) The enhancement of the CC stretching phenyl skeletalring modes at 1576 cm−1, 1371 cm−1 (overlapping with the1393 cm−1 band) and 788 cm−1. All these modes relate tothe totally symmetric modes of ortho di-substituted benzene(C2v symmetry) and are observed with very small shiftsof <5 cm−1 from the corresponding bands of the normalRaman spectra of the solution.

(4) The in-plane CH bending (wagging) modes at 1290,1183, 1137 cm−1 also show enhancement.

(5) The out-of-plane bending modes, do not show any en-hancement is the SER spectra. We believe that the above ob-servations support the ‘end-on’ orientation of the molecularion on the Ag metal surface.

(6) The lack of enhancement of the aromatic CH stretchvibrations that is expected for such a configuration is ex-plained: it is the orientation and location of the C–H bonds

relative to the surface that leads to the negligible enhance-ment of these modes in the ‘end-on’ position.

(7) The slight broadening of most of the skeletal ringstretching and CH bending modes suggests a possibleslight tilt of the molecule leading to the interaction of the�-electrons of the ring system with the surface.

All the above observations are in conformity with the sur-face selection rules and with studies of several other relatedmolecular systems reported in the literature.

References

[1] R.K. Chang, T.E. Furtak (Eds.), Surface Enhanced Raman Scattering,Plenum, New York, 1982,

[2] M. Moskovits, Rev. Mod. Phys. 57 (1985) 783.[3] M. Fleischman, P.J. Hendra, J.A. McQuilan, Chem. Phys. Lett. 26

(1974) 163.[4] K. Kneipp, H. Kneipp, I. Itzkan, R.R. Dasari, M.S. Field, Curr. Sci.

77 (1999) 915.[5] C. Tornkvist, D. Thierry, J. Bergman, B. Leidberg, C. Leygraf, J.

Electrochem. Soc. 136 (1989) 58.[6] G.A. Hope, D.P. Schweinsberg, P.M. Fredericks, Spectrochim. Acta

50A (1994) 2019.[7] J.C. Rubim, I.G.R. Gutz, O. Sala, W.J. Oriville Thomas, J. Mol.

Struct. 100 (1983) 571.[8] J.C. Rubim, I.G.R. Gutz, O. Sala, J. Mol. Struct. 101 (1983) 1.[9] P.G. Cao, J.L. Yao, J.W. Zheng, R.A. Gu, Z.Q. Tian, Langmuir 18

(2002) 100.[10] Z. Pengxiang, L. Erjun, L. Xiuying, Chin. Phys. Lett. 5 (1988) 329.[11] D. Graham, R. Brown, W.E. Smith, Chem. Commun. 7 (2001) 1002.[12] M. Moskovits, J.S. Suh, J. Phys. Chem. 92 (1988) 6327.[13] M. Moskovits, J.S. Suh, J. Phys. Chem. 88 (1984) 5526.[14] M. Moskovits, J.S. Suh, J. Phys. Chem. 88 (1984) 1293.[15] M. Moskovits, J.S. Suh, J. Chem. Phys. 77 (1982) 4408.[16] M. Moskovits, J.S. Suh, J. Am. Chem. Soc. 107 (1985) 6826.[17] Y.J. Kwon, D.H. Swon, S.J. Ahn, M.S. Kim, K. Kim, J. Phys. Chem.

98 (1994) 8481.[18] J.A. Creighton, C.G. Blatchford, M.G. Albrecht, J. Chem. Soc.,

Faraday Trans. II 75 (1979) 790.[19] S. Kapoor, Langmuir 14 (1998) 1021.[20] J.A. Creighton, Surface enhanced Raman scattering in: R.K. Chang,

T.E. Furtak (Eds.), Metal Colloids, Plenum Press, New York, 1982,pp. 315–337.

[21] B. Pergolese, A. Bigotto, Spectrochim. Acta 57A (2001) 1191.[22] G. Varsanyi, Vibrational Spectra of Benzene Derivatives, Academic

Press, New York, 1969.[23] L.D. Hansen, B.D. West, E.J. Baca, C.L. Blank, J. Am. Chem. Soc.

90 (1986) 6588.[24] C.S. Allen, R.P. Van Duyne, Chem. Phys. Lett. 63 (1979) 455.[25] Y.J. Kwon, D.H. Son, S.J. Ahn, M.S. Kim, K. Kim, J. Phys. Chem.

98 (1994) 8481.[26] S.H. Cho, H.S. Han, D.J. Jang, K. Kim, M.S. Kim, J. Phys. Chem.

99 (1995) 10594.[27] X. Gao, J.P. Davies, M.V. Weaver, J. Phys. Chem. 94 (1990)

6858.[28] W. Chen, Q. Xu, H. Jiang, Z. Xu, Spectrosc. Lett. 33 (2000) 69.