Surface enhanced Raman spectroscopic studies on 1H-1,2,4-triazole adsorbed on silver colloidal nanoparticles

Vibrational Spectroscopy 48 (2008) 202–205

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Surface enhanced Raman spectroscopic studies on 1H-1,2,4-triazole adsorbed on silver colloidal nanoparticles Barbara Pergolese a,*, Maurizio Muniz-Miranda b, Adriano Bigotto a a b

Department of Chemical Sciences, University of Trieste, Via L. Giorgieri 1, Trieste I-34127, Italy Department of Chemistry, University of Florence, Sesto Fiorentino, Via della Lastruccia 3, I-50019 Italy

A R T I C L E I N F O

A B S T R A C T

Article history: Received 16 July 2007 Received in revised form 6 February 2008 Accepted 18 February 2008 Available online 23 February 2008

1H-1,2,4-triazole is a very effective corrosion inhibitor for copper. The adsorption of this compound on silver colloidal nanoparticles has been studied by means of surface enhanced Raman scattering (SERS). SERS data are interpreted with the help of DFT calculations of models of the surface complex formed by 1H-1,2,4-triazole on the silver colloidal nanoparticles surface. It was found that this compound is adsorbed on metal surface in its anionic form and that it interacts with silver through the N1 and N2 atoms. The molecular plane assumes a tilted orientation with respect to the silver surface. ß 2008 Elsevier B.V. All rights reserved.

Keywords: 1H-1,2,4-triazole Corrosion inhibition SERS DFT calculations Mixed basis sets

1. Introduction The azoles are extensively used in surface treatment of materials, in particular in the field of protection of metals from corrosion. Therefore, a large number of investigations were carried out in order to clarify the interaction mechanism between these substances and metal surfaces and, for this purpose, the SERS technique has proved very informative. 1H-1,2,4-triazole (TZ4) is a very effective corrosion inhibitor for copper [1]. Even if several spectroscopic investigations were carried out on this compound [2,3], studies of the adsorption behaviour of TZ4 on metals are scarce, despite its well-known corrosion inhibition efficiency. In particular, SERS investigations on TZ4 were performed only for the molecule adsorbed on microlithographically prepared copper surfaces [4]. In this study the authors found that TZ4 adsorbed on copper in its non-dissociatively form with a flat orientation, but no conclusions about the molecular sites of interaction with the metal were drawn. In order to gain a better understanding of the adsorption behaviour of TZ4 on metal surface, we planned a SERS investigation of this compound on silver colloids. In fact, Ag, belonging to the same group of Cu, has similar chemical and physical

* Corresponding author. Tel.: +39 040 5583950; fax: +39 040 5583903. E-mail address: [email protected] (B. Pergolese). 0924-2031/$ – see front matter ß 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.vibspec.2008.02.007

properties to copper. Moreover, unlike Cu sols, Ag colloids offer the enormous advantage of stability and reproducibility. Finally, it is of note that the SERS enhancement factor of Ag is higher than that of Cu. DFT calculations of the harmonic vibrational wavenumbers of surface complexes formed by TZ4 with silver adatoms have been also carried out. In fact, these kind of calculations are able to give useful information about the interaction between adsorbates and metal substrates; in particular, DFT calculations were previously employed successfully for other ligands [5–7]. The results of this combined SERS/DFT investigation of TZ4 are reported here. 2. Experimental procedure TZ4 (98%) was obtained from Aldrich. Silver colloids were prepared following the Creighton’s procedure [8], by adding silver nitrate (Aldrich, purity 99.998%) to an aqueous solution of excess sodium borohydride (Aldrich, purity 99%), as reducing agent. MilliQ water was used in the preparation. All glassware was thoroughly cleaned with HNO3 and washed with Milli-Q water in order to avoid impurities in the colloid preparation. TZ4 was added to the sol in order to obtain a final concentration of about 103 M. The sols, before and after addition of TZ4, were characterized using UV– vis absorption spectra obtained with a UNICAM Helios spectrophotometer. Raman spectra were obtained with a SPEX Ramalog

B. Pergolese et al. / Vibrational Spectroscopy 48 (2008) 202–205

203

Table 1 Experimental, calculated data and assignment for TZ4A compared with the related SERS bands n˜ a

n˜ calc b

Symmetry species

PEDc

n˜ SERS

1487

1508

A1

1500

1384 1253

1364 1194

A1 A1

1146 1063

1197 1008

B2 A1

1025 971 – –

980 949 – –

B2 A1

21r1,2, 12r4,5, 12r3,4, 11a6,3,2,11a7,5,1, 10a7,5,4,10a6,3,4 70r1,2, 10r4,5, 10r3,4 9r3,4, 9r4,5, 7r1,2, 19a6,3,4,19a7,5,4, 9a6,3,2, 9a7,5,1 33r3,4, 33r4,5, 9a2,3,4, 9a1,5,4 22r1,5, 22r2,3, 23a3,4,5,7a7,5,1, 7a6,3,2, 5a1,5,4, 5a2,3,4 12r2,3, 12r1,5, 29a2,1,5,29a1,2,3 18r1,5, 18r2,3, 26a3,4,5,11a1,5,4, 11a2,3,4

1340 1283 1158 1056d 1027d 984 669 610

a

Normal Raman spectrum of TZ4 in alkaline solution. Calculated data for TZ4A. r: stretching; a: in-plane bending; g: out-of-plane bending; t: torsion; only contributions 5 are reported. d Wavenumber values obtained by deconvolution of the SERS band at 1059 cm1. b c

instrument. An AT personal computer was used for data acquisition and monochromator control. Excitation was provided by 514.5 nm radiation from a Spectra-Physics 165 argon ion laser and the samples were contained in capillary cells. The treatment of the spectral data was performed using the PerkinElmer IRDM and the Galactic GRAMS386 software. 3. Computational procedure

Fig. 1. (a) SERS spectrum of TZ4 in silver colloid; (b) Raman spectrum in aqueous solution; (c) Raman spectrum of TZ4 in alkaline solution. Excitation: 514.5 nm.

Calculations on the 1,2,4-triazolate anion (hereafter TZ4A) and on its complexes with silver adatoms were carried out using the GAUSSIAN 03 package [9]. Optimized geometries were obtained at the density functional level of theory with the Becke 3-parameter hybrid functional combined with the Lee–Yang–Parr correlation functional (B3LYP). The 6-311++G(d,p) was used for the calculations of TZ4A, whereas, in the case of the TZ4A/Ag complexes, a mixed basis set (6-311++G(d,p) for all atoms except silver, LANL2DZ for silver) was used. By allowing that all the parameters could relax, all the calculations converged to optimized geometries, which corresponded to true energy minima, as revealed by the lack of imaginary values in the wavenumber calculation. Harmonic vibrational wavenumbers were calculated at the same level of approximation using the parameters corresponding to the structure obtained from the optimization step. Force constants in internal coordinates, which were calculated according to the procedure described elsewhere [10], were used for a standard zero-order GF-matrix treatment from which vibrational wavenumbers and potential energy distributions (PEDs) were obtained.

A satisfactory agreement between calculated and experimental vibrational wavenumbers was obtained without using scaling factors. Calculated data for TZ4A compared with normal Raman and the related SERS wavenumbers are reported in Table 1. The theoretical results of the models of the surface complexes are reported in Table 2, along with the SERS data. 4. Results and discussion The Raman spectra obtained, both normal and surfaceenhanced, are reported in Fig. 1. From the comparison between the SERS spectrum in Ag colloid and the normal Raman (hereafter RS) spectrum of TZ4 in aqueous and alkaline solution, it can be evinced that TZ4 interacts with the silver surface in the anionic form. It can be observed that some bands undergo wavenumbershifts upon going from the RS to the SERS spectrum. Moreover the SERS band at 250 cm1 can be confidently attributed to the

Table 2 SERS wavenumbers compared with the calculated data obtained adopting models A, B, C, D n˜ SERS

n˜ calc model A

n˜ calc model B

Symm. speciesa

n˜ calc model C

n˜ calc model D

Symm. speciesb

1500 1340 1283 1158 1056 1027 984 669 610 250

1498 1304 1137 1228 1030 915 983 690 652 248

1518 1305 1211 1215 1097 1038 943 676 652 252

A1 A1 A1 B2 A1 B2 A1 A2 B1 A1

1518 1305 1211 1215 1097 1038 943 676 652 252

1518 1305 1211 1215 1097 1038 943 676 652 252

A0 A0 A0 A0 A0 A0 A0 A00 A00 A0

a b

Symmetry of normal modes calculated for models A and B. Symmetry of normal modes calculated for models C and D.

204

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Fig. 2. Optimised structure for model B of TZ4/Ag surface complex.

Ag–N stretching mode. These evidences are diagnostic of a noticeable chemisorption of TZ4A. When a compound is chemisorbed on the metal surface, DFT calculations are a powerful tool to get a better insight into the molecule/metal interaction. In particular, useful information can be obtained from the calculations of the wavenumbers and the related normal modes of vibrations of reliable models of the molecular adsorption on the metal surface. To evaluate the reliability of models, calculated wavenumbers have to be obtained for several models of the molecule/metal complex and then compared to the experimental data. A molecule/metal interaction model can be considered reliable when provides a good agreement with the experimental wavenumbers. Being TZ4A chemisorbed on the silver surface, its adsorption can be mimicked by models of the TZ4A/Ag(I) surface complex. The compound can interact with silver through three possible sites: N1, N2 or N4 atoms. Four different models were adopted in our calculations. In model A, TZ4A is bound to one Ag(I) through N4; in model B, TZ4A interacts with 2 Ag(I) through N1 and N2. Finally TZ4A interacts with one Ag(I) through N1 in model C and N2 in model D. It is of note that TZ4A has a C2v symmetry and, upon adsorption on the metal surface through both the N1 and N2 molecular sites or via N4 (models A and B), the symmetry group of the surface complex can be assumed to remain the same. On the other hand, when the interaction with the silver ions takes place through the N1 or N2 atoms (models C and D), the symmetry of the complex lowers to Cs. Therefore, in order to make the correct assignment of the SERS bands, in the case of the models C and D, the correlation schemes between the Cs and C2v symmetries should be taken into account. The best agreement between experimental and calculated wavenumbers is provided by the model B, being the standard deviation between calculated and experimental data of 49.4, 22.7, 23.2 and 23.2 cm1 for models A, B, C and D respectively. Therefore, the interaction of TZ4A with the silver colloidal nanoparticles takes place through both the N1 and N2 sites and the results obtained from calculations of model B have been taken into account for the assignment of SERS spectra. In Fig. 2 the optimized structure for model B is reported. The most-enhanced bands in the SERS spectrum are located at 1158, 984, 669 and 610 cm1. The latter two have no counterparts in the normal Raman spectrum of TZ4 in alkaline solution, whereas the ones at 1158 and 984 cm1 are matched with the RS bands at 1146 and 971 cm1. The peaks at 1158, 984, 669, 610 cm1 are related to normal modes of B2, A1, A2, B1 symmetry respectively

(see Table 2). The molecule is considered to lie in the yz plane, where z corresponds to the molecular C2 axis. On the basis of the surface selection rules related to the electromagnetic SERS effect [11], the occurrence of enhanced bands corresponding to A2 and B2 vibrational modes is diagnostic of a molecular orientation that is not normal nor parallel with respect to the metal surface, but inclined on it. In fact, in an edge-on adsorption, the SERS bands corresponding to A2 normal modes, which span in a plane (xy) parallel to the metal surface, should be the least-enhanced ones; in a flat-on adsorption, the B2 normal modes, spanning in a plane (yz) parallel to the metal surface, should give rise to the least-enhanced SERS bands. Finally, it is to be stressed that, even if the electromagnetic effect is generally the main mechanism in the SERS enhancement, also the ‘chemical effect’ should be taken into account, as pointed out in other cases [12,13]. The chemical effect, however, is quite important in the SERS spectra of compounds adsorbed on metal colloids activated by coadsorption of halide anions, because these latter are able to promote an efficient charge-transfer effect between the adsorbate and the metal [14]. In our case, instead, these anions are not present in the colloids and, consequently, the electromagnetic enhancement effect should be considered as predominant. 5. Conclusion The adsorption behavior of 1H-1,2,4-triazole on silver colloidal nanoparticles has been studied by means of SERS spectroscopy. From the comparison of the SERS spectrum with the normal Raman spectra of TZ4 in neutral and alkaline aqueous solutions, it can be deduced that the molecule interacts in the ionized form with silver colloidal nanoparticle surface. From DFT calculations of different models of the surface complex formed by TZ4 on the silver surface, it can be evinced that the molecular interaction with silver takes place through the lone pairs of N1 and N2 atoms. By considering the surface selection rules applied to the mostenhanced SERS bands, it is deduced that the molecule interacts with silver with a tilted orientation with respect to the metal surface. Acknowledgment The authors gratefully thank the Italian Ministero dell’Universita` e Ricerca for the financial support. References [1] F. Zucchi, M. Fonsati, G. Trabanelli, Proceedings of the 13th International Corrosion Congress, Paper 322/1–Paper 322/9, Australasian Corrosion Association, Clayton, Australia, 1996. [2] D. Bougeard, N. Le Calve`, B. Saint Roch, A. Novak, J. Chem. Phys. 64 (1976) 5152. [3] F. Billes, H. Endre´di, G. Keresztury, J. Mol. Struct. (Theochem.) 530 (2000) 183. [4] D. Thierry, C. Leygraf, J. Electrochem. Soc. 133 (1986) 2236. [5] B. Pergolese, M. Muniz-Miranda, A. Bigotto, J. Phys. Chem. B 108 (2004) 5698. [6] B. Pergolese, M. Muniz-Miranda, A. Bigotto, J. Phys. Chem. B 109 (2005) 9665. [7] B. Pergolese, M. Muniz-Miranda, G. Sbrana, A. Bigotto, Faraday Discuss. 132 (2006) 111. [8] J.A. Creighton, C.G. Blatchford, M.G. Albrecht, J. Chem. Soc., Faraday Trans. II 75 (1979) 790. [9] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I.

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