Role of Surface Metal Clusters in SERS Spectra of Ligands Adsorbed on Ag Colloidal Nanoparticles Maurizio Muniz-Miranda,* ,² Marco Pagliai, ² Gianni Cardini, ²,‡ and Vincenzo Schettino ²,‡ Department of Chemistry, UniVersity of Florence, Sesto Fiorentino, I-50019 Italy, and European Laboratory for Nonlinear Spectroscopy (LENS), Polo Scientifico, Sesto Fiorentino, I-50019 Italy ReceiVed: May 21, 2007; In Final Form: October 22, 2007 The role of surface metal clusters in the surface enhanced Raman scattering (SERS) enhancement mechanism was investigated by coupling Raman measurements and DFT (density functional theory) calculations for pyrazole adsorbed on Ag colloidal nanoparticles. The adsorption process of the ligand can be suitably described by a model system constituted of pyrazolide anions chemically interacting with (Ag 4 ) +2 surface clusters since the DFT approach allows a complete depiction of the SERS profile to be obtained, including both band frequencies and relative intensities. Introduction Rapid developments regarding metal clusters and nanopar- ticles clearly indicate the vital role played by these systems in many research fields. In fact, their physical and chemical properties might change dramatically with size or even geo- metrical structure, mainly concerning their optical responses and their interaction with absorbed ligands. When a molecule is adsorbed on a nanostructured surface of high-reflectivity metals such as Ag, Au, or Cu, the Raman signal of the adsorbate undergoes a huge enhancement by SERS (surface enhanced Raman scattering) effects. 1-2 For example, in single-molecule SERS experiments, 3 enhancement factors up to 10 14 - 10 15 have been observed. The SERS enhancement is generally considered to depend on two main mechanisms: (1) magnification of the local electric field near a nanostructured metal surface when the incident or scattering electromagnetic radiation wavelength matches the excitation band of the conduction electrons localized at the surface and (2) perturbation of the molecular polarizability when a ligand is chemically bound to atomic-sized defects of the metal substrate. The electromagnetic contribution plays a predominant role for the SERS enhancement, but the chemical contribution is important to determine the SERS spectral pattern, even if it improves the enhancement factor only up to 10 2 . Actually, the formation of surface ligand/metal complexes provokes signifi- cant frequency shifts of the SERS bands with respect to the normal Raman spectra of the nonadsorbed molecules. Moreover, chemisorption strongly affects the intensities of the SERS bands, as shown by the close similarity often observed between SERS spectra and Raman spectra of the corresponding coordination compounds, concerning both spectral positions and relative intensities. 4-7 This experimental evidence justified the use of DFT (density functional theory) calculations for model systems able to mimic the chemical interactions between adsorbed molecules and active sites of the metal surface. The DFT approach was very efficient in the interpretation of the SERS spectra, and it allowed us to obtain useful results, in particular the following: (1) The spectral positions of the SERS bands and the frequency shifts with respect to the normal Raman bands of the nonadsorbed molecules can be satisfactorily reproduced for different adsorbates; (2) it is possible to identify the molecular active sites involved in the chemical interactions with the metal substrate and, consequently, also the adsorption geometries; and (3) the relative intensities of the SERS bands can be predicted when ligands are adsorbed on silver colloids activated by coadsorption of chloride anions, on the basis of model systems of molecules bound to single Ag + ions, as for pyridine 8 or 4-methylpyridine. 9 Regarding the chloride activation effect of Ag colloids, which usually strongly improves the SERS enhancement of the adsorbate, it is important to remark that molecules with a large affinity to silver undergo chemisorption in Ag hydrosols, giving rise to strong SERS spectra, without any addition of halide anions, as found for several N-containing organic ligands. 6,7,10-17 In these cases, the chemical interaction with the metal surface was generally shown by the following experimental evidence: (1) In the absorption spectra of the Ag colloidal suspensions, after ligand addition, a secondary plasmon resonance band occurred in the 600-800 nm spectral region, due to silver particles aggregated by the presence of strongly adsorbed molecules, besides the usual surface plasmon resonance of nonaggregated Ag particles around 400 nm; (2) the Raman bands of the adsorbed molecules were strongly enhanced and markedly shifted with respect to those corresponding to the normal Raman spectra; and (3) in the low-frequency region (200-250 cm -1 ) of the SERS spectra, broad bands were detected, not attributable to molecular vibrations but to Ag-N stretching modes of ligand-metal chemical bonds. Very recently, 18 by using the DFT approach, we modeled the adsorption of pyridine on silver colloids: Two different positively charged metal clusters were taken into account, both consisting of four silver atoms, (Ag 4 ) + and (Ag 4 ) 2+ . In chloride- free colloids, the more efficient model of the surface complex was obtained by pyridine molecules bound to (Ag 4 ) 2+ clusters, reproducing satisfactorily SERS frequencies and intensities. By coupling SERS measurements and DFT calculations, we also studied 8,19 the adsorption of pyrazole on halide-free Ag colloids. Pyrazole is a five-membered azoaromatic molecule, which * Corresponding author. E-mail: muniz@unifi.it. ² University of Florence. ‡ LENS. 762 J. Phys. Chem. C 2008, 112, 762-767 10.1021/jp073914h CCC: $40.75 © 2008 American Chemical Society Published on Web 01/01/2008