Pd Alloy Colloid

7980

Langmuir 1999, 15, 7980-7992

Surface-Enhanced Raman Scattering of a Cu/Pd Alloy Colloid Protected by Poly(N-vinyl-2-pyrrolidone) Ping Lu,† Jian Dong,‡ and Naoki Toshima*,§ Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, Department of Biochemistry, Case Western Reserve University, Cleveland, Ohio 44106-4935, and Department of Materials Science and Engineering, Science University of Tokyo in Yamaguchi, Onoda-shi, Yamaguchi 756-0884, Japan Received December 31, 1998. In Final Form: July 7, 1999 A novel series of poly(N-vinyl-2-pyrrolidone)-protected nanometer-sized Cu/Pd alloy colloids have been prepared by a polyol reduction method. These colloids, having high molar fractions of Cu (x ) 0.99-0.80), show remarkable stability, as good as a Pd colloid. Transmission electron microscopy and high-resolution transmission electron microscopy analyses indicate that the Cu-rich particles spanning from 2-10 nm in size display an increase in the (111) lattice spacings compared to the bulk state. X-ray absorption near-edge spectroscopy and extended X-ray absorption fine structure analysis confirm the alloy phase nature in the Cu/Pd [Cu:Pd ) 4:1 (mol/mol)] nanoparticles with Cu at the zero-oxidation state. The Raman scattering behavior of several types of molecules adsorbed on the Cu/Pd(4/1) colloids has been examined for the first time. Significant band shifts in the ring modes of p-aminobenzoic acid, thiophenol, and bis(3-carboxy-4nitrophenyl) disulfide in the colloid surface spectra reveal their particular adsorption interactions and preserve the typical traits, when compared with some of the previously reported surface-enhanced Raman scattering (SERS) spectra on pure Ag, Au, or Cu colloids or metal films. The Raman signal intensities from the molecules adsorbed on the Cu/Pd(4/1) alloy colloid are about 10-102-fold more intense than normal Raman scattering in an aqueous solution. The implied surface adsorption configurations are also discussed in terms of the surface selection rule, with the facilitation of reliable assignments of normal modes of p-aminobenzoic acid and thiophenol.

Introduction Much attention has been paid to colloidal transition metals because they possess highly efficient catalytic properties if properly controlled in size and modulated in composition.1 In recent years, they have been one of the major nanoscopic materials as well.2 Their interactions with organic materials are also of great interest from the viewpoints of the stabilization of nanoparticles and the cooperative function of the composite materials composed of colloidal metals and organic materials. They also exhibit diverse optical properties, including the surface-enhanced Raman scattering (SERS) effect for coinage metal colloids, because of the excitation of surface-plasmon resonances within a single particle or within many particles in an aggregate. The SERS effect provides one of the sensitive techniques to give information on the interaction between colloidal metals and organic materials. In fact, the SERS effect has been observed on many types of colloidal particles, in addition to noncolloidal substrates such as electrode surfaces or vapor-deposited metal island films.3 * Corresponding author. Tel.: +81-836-88-4561. Fax: +81-83688-4567. E-mail: [email protected]. † The University of Tokyo. ‡ Case Western Reserve University. § Science University of Tokyo in Yamaguchi. (1) (a) Hirai, H.; Toshima, N. In Tailored Metal Catalysts; Iwasawa, Y., Ed.; Reidel: Dordrecht, The Netherlands, 1986; Chapter 2. (b) Schmid, G. Chem. Rev. 1992, 92, 1709. (c) Hirai, H.; Toshima, N. In Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: Boca Raton, FL, 1996; Vol. 2/C, pp 1310-1316. (d) Toshima, N.; Yonezawa, T. New J. Chem. 1998, 1179. (e) Toshima, N. Shokubai (Catalysts & Catalysis) 1998, 40, 536. (2) (a) Klabunde, K. J. Free Atoms, Clusters, and Nanoscale Particles; Academic Press Inc.: San Diego, CA, 1994. (b) Schmid, G. Clusters and Colloids; VCH: Weinheim, 1994. (c) Chem. Eng. News 1996, Dec. 2, 20. (d) Chem. Ind. 1997, Sept. 1, 691.

The colloidal dispersions are often prepared by various chemical methods,1 where sodium borohydride and sodium citrate are the most frequently used reductants for preparation of SERS-active silver and gold colloids. On the other hand, copper,4 platinum,5,6 palladium,6 osmium,6 and iridium6,7 colloids have been prepared by a number of methods. One of the concerns in these procedures for metal colloids, especially for Cu, is the presence of oxidation products or the coexistence of extraneous ions when chemical methods are used to prepare the colloids. As a small change in electrostatic charges on the surface of the colloids induced by adsorption of a foreign compound, for example, can lead to aggregation and precipitation on standing, stabilizers or polymers are often added to the colloids after or during reduction in order to improve the stability. SERS of polymer-protected Ag was first reported by Lee and Meisel.8 In their study, a Ag2SO4 aqueous solution (3) (a) Chang, R. K.; Furtak, T. E. Surface Enhanced Raman Scattering; Plenum Press: New York, 1982. (b) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (c) Campion, A.; Kambhampati, P. Chem. Soc. Rev. 1998, 27, 241. (4) (a) Creighton, J. A.; Alvarez, M. S.; Weitz, D. A.; Garoff, S.; Kim, M. W. J. Phys. Chem. 1983, 87, 4793. (b) Pate, J.; Leiden, A.; Bozlee, B. J.; Garrell, R. L. J. Raman Spectrosc. 1991, 22, 477. (c) Curtis, A. C.; Duff, D. G.; Edward, P. P.; Jefferson, D. A.; Johnson, B. F. G.; Kirkland, A. I.; Wallace, A. S. J. Phys. Chem. 1988, 92, 2270. (d) Angel, S. M.; Katz, L. F.; Archibald, D. D.; Honigs, D. E. Appl. Spectrosc. 1989, 43, 367. (e) Loo, B. H.; Lee, Y. G.; Liang, E. J.; Kiefer, W. Chem. Phys. Lett. 1998, 297, 83. (f) Neddersen, J.; Chumanov, G.; Cotton, T. M. Appl. Spectrosc. 1993, 47, 1959. (5) (a) Kiwi, J.; Gra¨tzel, M. J. Am. Chem. Soc. 1979, 101, 7214. (b) Wilenzick, R. M.; Russell, D. C.; Morriss, R. H.; Marshall, S. W. J. Chem. Phys. 1967, 47, 533. (6) Hirai, H. J. Macromol. Sci., Chem. 1979, A13, 633. (7) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci., Chem. 1979, A13, 727. (8) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391.

10.1021/la981767w CCC: $18.00 © 1999 American Chemical Society Published on Web 09/23/1999

Raman Scattering of a Cu/Pd Alloy Colloid

Langmuir, Vol. 15, No. 23, 1999 7981

Scheme 1. Schematic Illustration of the Formation Process of the Polymer-Protected Cu/Pd Colloid with an Alloy Structure

was reduced by H2 or NaBH4 in the presence of poly(vinyl alcohol) (PVA) and boiled for 1-3 h. The colloid prepared in this method showed an absorption maximum at 400 nm. Strong Raman signals could easily be observed from a dye molecule (a carbocyanine derivative) in the presence of the Ag/PVA colloid, although attempts to observe SERS from several other dyes yielded negative results. Recently, Nie et al. extended this approach to probe adsorbed molecules on selected Ag nanoparticles9 and found that the intrinsic Raman enhancement factors from certain screened Ag particles (∼200 nm in diameter) were on the order of 1014 to 1015, much larger than the ensembleaveraged values derived from conventional measurements. Siiman et al.10 investigated SERS on poly(N-vinyl-2pyrrolidone) (PVP)-protected Ag colloids prepared by the ethanol reduction procedure, which was in fact developed in this laboratory.6,7 In their study, dabsyl aspartate, a derivative of methyl orange, was used to probe the combined surface Raman and resonance Raman signals on account of its facile binding to PVP. They found that Ag colloids protected by PVP during the reduction process could show a SERS effect when excess base was present. The base requirement seemed to be linked to the promotion of growth and aggregation of the silver particles, as evidenced by absorption spectra. Other SERS studies of Ag or Au colloids involve those prepared by chemical reduction and stabilized by PVP, PVA, and so forth at the end of reduction, where good SERS was observed when aggregation of the colloidal particles occurred.11 Current knowledge on SERS of metal alloys is, however, very limited. Furtak et al. reported that SERS signals from pyridine adsorbed onto Ag1-xPdx alloy electrodes were detectable provided that the Pd concentration x e 0.04.12 The quenching of the SERS effect in x g 0.05 alloys was ascribed to introduction of the d electrons of Pd, which are involved in electron excitations that interfere with the (9) Nie, S.; Emory, S. R. Science 1997, 275, 1102. (10) (a) Siiman, O.; Lepp, A.; Kerker, M. Chem. Phys. Lett. 1983, 100, 163. (b) Lepp, A.; Siiman, O. J. Phys. Chem. 1985, 89, 3494. (11) (a) Heard, S. M.; Grieser, F.; Barraclough, C. G., Chem. Phys. Lett. 1983, 95, 154. (b) Lee, P. C.; Meisel, D. Chem. Phys. Lett. 1983, 99, 262. (12) Furtak, T. E.; Kester, J. Phys. Rev. Lett. 1980, 45, 1625.

favorable free-electron-like behavior in Ag. Takenaka et al. studied colloidal Au/Pt alloy particles with 0-25% Pt which were prepared by sodium citrate-reduction without polymer protection.13 The spectral features from such alloy particles were the same as that on pure Au, and the SERS intensity from Au/Pt alloy particles decreased markedly with increasing Pt concentration, in general agreement with the results of Ag1-xPdx alloys by Furtak et al.12 We have long been interested in the preparation, structure, and catalytic properties of bimetallic colloids.1a,d We have often used alcohol as a mild reagent to reduce noble metal ions. However, light transition metal ions such as Cu(II) and Ni(II) cannot be reduced under the same conditions as those for Pd(II), Pt(IV), Au(III), etc. Recently, we succeeded in reducing Cu(II)14 and Ni(II)15 by using glycol as a reductant at high temperature. The preparation process of Cu/Pd bimetallic colloids is illustrated in Scheme 1. Although Cu/Pd bimetallic colloids can be prepared by other methods,16 our method appears to be capable of producing stable Cu-rich Cu/Pd bimetallic colloids. This is essential in order to investigate the SERS behaviors. The previous X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) characterization work indicates that PVP-protected Pd-rich Cu/Pd bimetallic colloids have alloy structures with unusual stability and show excellent catalytic behavior in terms of activity and selectivity for several reactions.17 On the other hand, the Cu-containing alloy nanoparticles protected by PVP may retain or modulate the nonlinear optical properties of PVPprotected monometallic Cu nanoparticles, which has been pointed out very recently.18 These properties, in particular the excellent stability, provide us with a basis and an opportunity to study the SERS behavior of such colloidal dispersions of alloys, which will suggest to us the coordinating structure of organic molecules on metal (13) Takenaka, T.; Eda, K. J. Colloid Interfacial Sci. 1985, 105, 342. (14) Toshima, N.; Wang, Y. Chem. Lett. 1993, 1611. (15) Toshima, N.; Lu, P. Chem. Lett. 1996, 729. (16) (a) Esumi, K.; Tano, T.; Torigoe, K.; Meguro, K. Chem. Mater. 1990, 2, 564. (b) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993, 5, 254. (17) Toshima, N.; Wang, Y. Langmuir 1994, 10, 4574. (18) Huang, H. H.; Yan, F. Q.; Kek, Y. M.; Chew, C. H.; Xu, G. Q.; Ji, W.; Oh, P. S.; Tang, S. H. Langmuir 1997, 13, 172.

7982

Langmuir, Vol. 15, No. 23, 1999

particles as well as the surface effect of Cu/Pd bimetallic nanoparticles. The SERS phenomenon described here also indicates a markedly modulated electronic and morphological structure of such alloy particles compared to the huge SERS enhancement well-known for copper colloids.4 Creighton and co-workers, especially, not only confirmed the earlier huge SERS effect found on electrochemically roughened Cu bulk surfaces but also extended SERS from Ag and Au colloids to Cu colloids.4a Experimental Section 2.1. Preparation of Cu/Pd Bimetallic Colloids. Cu/Pd bimetallic colloids were prepared by a cold alloying process, based on the reduction of the corresponding metal hydroxide, as reported in a previous study,17 with a minor modification outlined here. We noticed that stable Cu/Pd bimetallic nanoclusters with Cu molar fractions higher than 75% have rarely been obtained successfully by the conventional methods in the literature. It was realized that in the cold alloying process, reducing the reaction scale in one batch as far as possible, increasing reflux time from 3 h to 4-5 h, and increasing the amount of PVP to 1.5 times the conventional amount do help in obtaining Cu/Pd colloids with extremely high Cu contents. Based on our preparation experience, the smaller the preparation scale, the easier the gain of well-dispersed samples with high Cu/Pd ratios. Thus, PVP (K-30, average molecular weight 40,000, 1.0 g, 8.9 mmol in monomeric units) and cupric sulfate (CuSO4‚5H2O, 39 mg, 0.156 mmol) were dissolved in 150 mL of glycol at 80 °C in a flask, to which a 5-mL dioxane solution of palladium(II) acetate containing an appropriate molar amount was added after cooling the solution to 0-5 °C. The pH was then adjusted to about 10 by dropwise addition of an aqueous solution of sodium hydroxide (NaOH, 0.1 M). Refluxing the mixture at 198 °C for at least 3 h with a nitrogen flow passing through the reaction system to take away water and organic byproducts gave a homogeneous colloidal dispersion of the bimetallic clusters. The PVP-protected Cu/Pd bimetallic colloids were then separated from the glycol solvent by filtration under argon using an ultrafilter equipped with a membrane having a cutoff molecular weight of 10,000 and were then washed with degassed water several times. 2.2. Electron Microscopy. Samples for transmission electron microscopy (TEM) were prepared by placing a drop of the colloidal dispersions of metal nanoclusters onto a carbon-coated copper micro-grid (kindly provided by Dr. K. Adachi, the University of Tokyo), removal of most of the solvents onto a filter paper by capillary action, and vacuum-drying for 1 day. TEM photographs were taken at 125 kV acceleration voltage on a Hitachi H-7100 electron microscope. The mean diameter and standard deviation were calculated by counting 200 particles from the TEM image of 400 000 magnification using a magnifier with a magnifying power of 10 times. High-resolution transmission electron microscopy (HRTEM) measurements were carried out on a Hitachi H-9000NAR electron microscope at 300 kV. 2.3. UV-Visible Absorption Spectroscopy. UV-visible absorption spectra were taken on a Shimadzu 2100 spectrophotometer. The colloidal samples were diluted to sufficiently low concentrations before measurements were taken. 2.4. X-ray Absorption Spectroscopy (XAS). Cu K edge X-ray absorption near-edge structure (XANES) (Cu 1s binding energy 8979 eV) as well as Cu K edge and Pd K edge EXAFS measurements were carried out in a transmission mode at the BL-7B (Cu-K) and BL-10B (Pd-K) beamlines of the Photon Factory, the National Laboratory for High Energy Physics (KEKPF) at Tsukuba, Japan, using synchrotron radiation at room temperature. The channel-cut Si(311) monochromator was used. The storage ring was operated at 2.5 GeV, and the ring current was in the range of 100-300 mA. The colloidal samples for the XAS measurements were concentrated by ultrafiltration, and the concentrated dispersions were put in a glass sample cell with polyimide windows under a nitrogen atmosphere. The cells with optical path lengths of 5-10 and 50 mm were used for Cu K edge and Pd K edge measurements, respectively. The reference compounds used were monometallic Cu and Pd metal foils and bimetallic Cu/Pd(9/1, mole ratio), Cu/Pd(1/1), and Cu/Pd(1/9) alloy foils, produced by Tanaka Kikinzoku Kogyo K. K. in accordance

Lu et al. with our request, as well as CuO and Cu2O prepared as finely ground powders and fixed on adhesive Kapton tapes. For the contribution of the metal-metal bonds, Fourier transformation of k3χ(k) was carried out over the region 30-160 nm-1. The peak in the Fourier transform was filtered over 0.150.30 nm and inversely Fourier-transformed into k-space (the region 40-150 nm-1) again. The Fourier-filtered data were then analyzed with a curve-fitting technique. 2.5. Raman Spectroscopy. A wavelength-stabilized diode pumped laser (SDL, Inc., CA, model 8530) operating at 785 nm was utilized as an excitation source. The laser power at the sample cell position was kept at 200 mW. A back-illuminated chargecoupled device (CCD) detector (Princeton Instruments, Inc., NJ, model 1024EHRB/1) working at -90 °C was used to collect Raman signals. The scattered light was prefiltered by a holographic notch filter and dispersed through a single monochromator (Spex 500M) equipped with a grating with 600 grooves/mm. Typically, data accumulation was undertaken for 5 min of CCD detector exposure time. The Cu/Pd bimetallic colloids were diluted to 1 mM, unless specified in figure captions by water or a KOH solution (4 mM) to desired concentrations, before the adsorbate solution was added for SERS measurements. 2.6. Vibrational Analysis. Harmonic frequency calculations of para-aminobenzoate anion and thiophenol were carried out with the Gaussian 94 software package19 on a SGI workstation. The geometry of each molecule was completely optimized at the appropriate level of theory by analytic gradient techniques. A density functional theory (DFT) variant B3LYP, with a basis set of 6-31+G(d), was first employed to give normal modes. Then, a Hartree-Fock (HF) method with the same basis set was performed, producing Raman activities and nearly the same normal modes as those from the DFT level. The calculated frequencies at the DFT/B3LYP level were converted by a single scaling factor through a least-squares approach, according to a recent comprehensive evaluation of Scott and Radom.20 Normal modes related to ring vibrations (in Wilson’s notation 21) are described according to Colthup et al. and Dollish et al.22

Results and Discussion 3.1. Preparation of Cu/Pd(4/1) Alloy Colloid. Colloidal dispersions of Cu/Pd alloy clusters with Pd contents varying from 1 to 20 mol % were obtained by reduction of the corresponding metal hydroxide with glycol under nitrogen flow to remove water and other byproducts (cf. Scheme 1). The resulting colloids, obtained by our improved synthetic conditions, are homogeneous, yellowbrown in color, and stable for months without precipitation (apparently as stable as Pd colloids), in contrast to the conventional pure Cu colloids which last only for a few weeks. UV-vis spectroscopy is an efficient tool to study electron excitations in Cu-containing colloids. Thus, the spectral features of various metal colloids have been investigated experimentally in comparison with the theoretical reports. Recently, Creighton et al. predicted that copper colloidal particles (∼10 nm size) will give two absorption peaks at (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.; Keith, T. A.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Peng, C. Y.; Ayala, P. A.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, Revision E.2; Gaussian, Inc.: Pittsburgh, PA, 1995. (20) Scott, A. P.; Radom, L. J. Phys. Chem. 1996, 100, 16502. (21) Wilson, E. B. Phys. Rev. 1934, 45, 706. (22) (a) Dollish, F. R.; Fateley, W. G.; Bentley, F. F. Characteristic Raman Frequencies of Organic Compounds; John Wiley & Sons: New York, 1973. (b) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. Introduction to Infrared and Raman Spectroscopy; 2nd ed.; Academic Press: New York, 1975. (c) Varsanyi, G. Vibrational Spectra of Benzene Derivatives; Academic Press: New York, 1969.


Langmuir, Vol. 15, No. 23, 1999 7983

Figure 1. UV-vis absorption spectra of Cu/Pd(20/1), Cu/Pd(9/1), Cu/Pd(17/3), and Cu/Pd(4/1) colloids with Cu contents of 95, 90, 85, and 80 mol %, respectively: (a) λ ) 250-450 nm; (b) λ ) 450-600 nm. [Cu] ) 0.25 mM.

310 and 560 nm in the UV-vis range.23 In most of the previous investigations, the absorption at 560 nm is frequently observed for Cu colloids, which can be explained by Mie’s scattering theory as the excitation of plasmon resonances, especially when the particle size is relatively big. Creighton et al. believe that both absorption bands at ca. 560 and ca. 310 nm have considerable interband transition character in origin. In our study, both a weak absorption peak at ca. 520 nm and a relatively strong band around 330 nm were observed (Figure 1). The two characteristic absorption bands are most conspicuous in the spectrum of PVP-protected Cu/Pd bimetallic nanoclusters with a mole ratio of Cu:Pd ) 20:1 [abbreviated hereafter as Cu/Pd(20/1)] among the spectra examined here, but they gradually decrease in intensity with increasing Pd content and at last become barely visible in the spectrum of Cu/Pd(4/1) colloid. We have also checked the possibility of Cu2+ or Pd2+ ions to account for the band at ca. 330 nm. The reference Cu2+ ions, either in water or in PVP solution, have no peaks in the region 250-400 nm. The Pd2+ ions in PVP solution show only a weak and very broad band. Cu2O and CuO have no UV peaks, either. None of the samples of PVP-protected Cu/Pd colloids showed any features of Cu2+ or Pd2+ but showed steep lines with a band at ca. 330 nm in the region of 280-400 nm. In addition, the lines are appreciably higher in absorbance than those of pure Pd colloids, which exhibit a nearly flat line in the same region. Thus, the possibility of their origin from Pd colloids can be excluded, too. The present results of UV-vis absorption experiments suggest formation of new phase(s), which are neither metallic salts, metal oxides, nor mixtures of Pd and Cu particles. Thus, nanoclusters of a certain alloy phase structure having both Cu and Pd atoms on the surface seem to be reasonable in the present case, although there is still a possibility of multiple phases within each particle (polycrystallinity) or aggregation of fine particles. 3.2. Morphological and Structural Characterization. Transmission electron micrographs (TEM) and highresolution TEM (HRTEM) were used to characterize a series of Cu/Pd bimetallic colloids. It was revealed by TEM photographs that, although being a little less uniform in size and less homogeneous in shape than those of Cu/ Pd(4/1) colloids, the bimetallic particles containing Pd from 1 to 15 mol % are rather homogeneous in size generally. For example, as shown in Figure 2, the particles are fairly (23) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881.

Figure 2. Transmission electron microscope images of Cu/Pd colloids with Cu contents of (a) 95, (b) 90, and (c) 85 mol %.

well-dispersed, ranging from 2.5 to 7.0 nm, 5.0 to 10.0 nm, and 5.0 to 12.5 nm in Figure 2a, b, and c, respectively. The TEM photographs of Cu-rich colloids with Cu contents of 99, 98, 92, and 91 mol % (data not shown here) illustrate quite similar results. There has rarely been a facile method that can give rise to such high Cu-content nanoclusters with such small sizes, as conventional methods often bring about precipitation or severe aggregation with much larger particle sizes (several hundred nanometers, actually giving properties similar to the bulky state). The HRTEM photograph (Figure 4a) of Cu/Pd(11.5/1) bimetallic colloid (Pd content, 8 mol %) exhibits two kinds of (111) lattice spacings, that is, 2.13 and 2.38 Å. This suggests the existence of polycrystalline or double-phase particles, for example, the coexistence of Pd-rich parts and Cu-rich ones, because the above lattice spacings are rather similar to those of bulk Cu (2.09 Å) and Pd (2.25 Å), respectively. Thus, the comparative nonuniformity in size and irregularity in shape, as well as the heterogeneity in lattice spacings of Cu/Pd bimetallic colloids containing less than 15 mol % of Pd, could be attributed to the formation of double-phase particles. Careful examination of the irregular shape of the individual colloidal particles in Figure 2 suggests that they seem to consist of multiple smaller particles (“microclusters”) in an aggregated state. The composition of such microclusters may vary from each other, forming multiple phases. In contrast, TEM data indicate that Cu/Pd(4/1) bimetallic colloids are rather uniform in size, with an average diameter of 5.6 nm (standard deviation σ ) 1.5 nm), and are composed of well-shaped particles, as shown in Figure 3. The HRTEM photograph (Figure 4b) reveals a single

7984


Lu et al.

Figure 3. TEM micrograph and particle size distribution of Cu/Pd(4/1) colloids: dav ) 5.57 nm, σ ) 1.47 nm.

Figure 4. HRTEM images of Cu/Pd bimetallic colloids: (a) Cu/Pd(11.5/1) (Pd content ) 8 mol %) with lattice spacings of 2.13 Å (the corresponding particles labeled as 1) and 2.38 Å (the corresponding particles labeled as 2); (b) Cu/Pd(4/1) (Pd content ) 20 mol %) with lattice spacing of 2.50 Å.

(111) lattice spacing, 2.50 Å, illustrating that the Cu/Pd(4/1) colloid consists of alloy clusters with a relatively homogeneous structure, in favor of a randomized distribution of Pd in Cu atoms. The lattice spacing of 2.50 Å is a little larger than that of the bulk Pd (2.25 Å). Although the precise reason for this elongated spacing is not yet clear, similar phenomena have been observed for Pd nanoclusters24 and Ni/Pd bimetallic colloids.25 One of the possible reasons to account for this interesting observation is the large number of surface atoms that are relaxed from their normal crystal lattice equilibrium positions by occlusion of extra elements.24 In addition, other factors, like the malleable nature of metals, may also promote the increase in the lattice spacings of the colloids. In toto, the microscopic data shows that the colloid particles are well dispersed, and because they are so wellprotected by PVP polymers, coalescence or aggregation is not evident in the photographs. This is also one of the reasons why, in the following study, the SERS intensity does not change when these super-stable colloids are disturbed in several ways in an attempt to aggregate the (24) Lamber R.; Wetjen, S.; Jaeger, N. I. Phys. Rev. B 1995, 51, 10968. (25) Lu, P.; Teranishi, T.; Asakura, K.; Miyake, M.; Toshima, N. J. Phys. Chem., in press.

particles. Noticeably, many of the known SERS studies in the past involve colloids that are not protected by stabilizing molecules or even bare colloids, where facile aggregation occurs frequently, and such colloids are unstable, giving SERS signals that are changeable with time (sometimes with uncertain reproducibility). In contrast, the method described here provides extremely stable colloids that are very resistant to aggregation. This point is of great value because stable colloids are certainly useful for quantitative analytical applications of SERSactive colloids. The fact that SERS is often easily observed on the properly aggregated colloid may be because the electromagnetic field enhancement in the space between two closely spaced spheres may be as much as an order of magnitude greater than that obtained from a single sphere, or because there is strong interparticle electromagnetic coupling.3a,26 X-ray absorption fine structure (XAFS) is a powerful technique for characterization of structures and electronic states of nanoscopic materials.27 Both extended X-ray absorption fine structure (EXAFS) and X-ray absorption near-edge structure (XANES) are crucial to the elucidation (26) Keating, C. D.; Kovaleski, K. K.; Natan, M. J. J. Phys. Chem. B 1998, 102, 9414.


Langmuir, Vol. 15, No. 23, 1999 7985

Figure 5. XANES spectra at the Cu K edge and their first derivatives for (a) Cu2O, (b) CuO, (c) metallic copper, and (d) Cu/Pd(4/1) colloid. The vertical line marks 8985 eV.

of the structural information, which is difficult if not impossible to obtain by other methods, especially for ultrafine alloy particle systems.28,29 Particularly, Bradley et al. have demonstrated the technique successfully in their composite Cu/Pd particles.28b Here, the results of XANES and EXAFS measured for Cu/Pd(4/1) bimetallic colloids are presented in order to confirm the structure. The near-edge regions of X-ray absorption spectra can give direct information on the oxidation state of copper, and these data have in fact been used in analyzing mixtures of copper compounds of different oxidation states28b,g or many Cu-containing catalyst systems.30 To (27) (a) Iwasawa, Y. X-ray Absorption Fine Structure for Catalysts and Surfaces; World Scientific Series on Synchrotron Radiation Techniques and Applications, Vol. 2; World Scientific: Singapore, 1996. (b) Koningsberger, D. C., Prins, R., Eds. X-ray Absorption: Techniques of EXAFS, SEXAFS and XANES; J. Wiley & Sons: New York, 1988. (28) (a) Sinfelt, J. H. Acc. Chem. Res. 1987, 20, 134. (b) Bradley, J. S.; Via, G. H.; Bonneviot, L.; Hill, E. W. Chem. Mater. 1996, 8, 1895. (c) Richard, D.; Couves, J. W.; Thomas, J. M. Faraday Discuss. Chem. Soc. 1991, 92, 109. (d) Duff, D. G.; Edwards, P. P.; Evans, J.; Gauntlett, J. T.; Jefferson, D. A.; Johnson, B. F. G.; Kirkland, A. I.; Smith, D. J. Angew. Chem., Int. Ed. Engl. 1989, 28, 590. (e) Kolb, U.; Quaiser, S. A.; Winter, M.; Reetz, M. T. Chem. Mater. 1996, 8, 1889. (f) Franke, R.; Rothe, J.; Pollmann, J.; Hormes, J.; Bo¨nnemann, H.; Brijoux, W.; Hindenburg, T. J. Am. Chem. Soc. 1996, 118, 12090. (g) Rothe, J.; Hormes, J.; Bo¨nnemann, H.; Brijoux W.; Siepen K. J. Am. Chem. Soc. 1998, 120, 6019. (29) (a) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448. (b) Harada, M.; Asakura, K.; Toshima, N. J. Phys. Chem. 1994, 98, 2653. (c) Harada, M.; Asakura, K.; Ueki, Y.; Toshima, N. J. Phys. Chem. 1993, 97, 10742. (30) (a) Lamberti, C.; Bordiga, S.; Salvalaggio, M.; Spoto, G.; Zecchina, A.; Geobaldo, F.; Vlaic, G.; Bellatreccia, M. J. Phys. Chem. B 1997, 101, 344. (b) Ferna´ndez-Garcı´a, M.; Anderson, J. A.; Haller, G. L. J. Phys. Chem. 1996, 100, 16247.

determine the copper oxidation state in the Cu/Pd colloid, we used Cu K edge XANES spectra. A selection of Cu K edge XANES spectra of Cu/Pd(4/1) colloid and various reference samples with their first derivatives (as insets) are presented in Figure 5. It is widely recognized that a single well-defined peak at 8980-8985 eV is the fingerprint of Cu(I) species. This peak is due to the dipole-allowed 1s f 4p electron transition of Cu(I). On the contrary, Cu(II) species exhibit: (i) a weak absorption at about 89768979 eV, attributable to the dipole-forbidden 1s f 3d electronic transition; (ii) a shoulder at about 8985-8988 eV and an intense peak at about 8995-8998 eV, both due to a 1s f 4p transition. All these features are evident in the reference spectra of Cu2O (Figure 5a) and CuO (Figure 5b). The arrow in the CuO spectrum evidences the weak absorption attributed to the dipole-forbidden 1s f 3d electronic transition of Cu(II) species. A glance at Figure 5 reveals that the spectra of the alloy colloid (Figure 5d) retain all the features of metallic copper (Figure 5c). Furthermore, the shape of the edge and the features of the XANES structure of the Cu/Pd(4/1) colloid are indicative of the absence of copper in either the Cu(I) or Cu(II) state. This result will be more pronounced by comparing their derivative spectra. Therefore, it is established that Cu(II) ions have been reduced completely to the state of zero valence in the current cold alloying process. EXAFS analysis further supports this point. The fit of the theoretical and experimental EXAFS spectra becomes worse or impossible if the Cu-O bond is considered in the calculation for the Cu/Pd(4/1 and other ratios) alloy colloid, which provides additional evidence for the absence of the Cu-O bond in the samples under study.

7986


Lu et al.

Figure 6. Fourier-transformed EXAFS spectra: (a) at Cu K edge of Cu foil, Cu/Pd(1/1) foil, and Cu/Pd(1/9) foil; (b) at Pd K edge of Pd foil, Cu/Pd(1/9) foil, Cu/Pd(1/1) foil, and Cu/Pd(9/1) foil; (c) at Cu K edge of the colloidal dispersions of PVP-protected monometallic Cu nanoclusters and Cu/Pd bimetallic nanoclusters at Cu/Pd mole ratio ) 4/1, 1/1, and 1/4; (d) at Pd K edge of the colloidal dispersions of PVP-protected monometallic Pd nanoclusters and Cu/Pd bimetallic nanoclusters at Cu/Pd mole ratio ) 1/4, 1/1, and 4/1.

To understand the detailed structure of PVP-protected Cu/Pd bimetallic colloids, we have measured the EXAFS spectra of these colloids as well as those of the corresponding metal and alloy foils at various Cu/Pd ratios. Figure 6a shows the k3-weighted Fourier-transformed EXAFS spectra at the Cu K edge of the Cu foil, the Cu/ Pd(1/1) alloy foil, and the Cu/Pd(1/9) alloy foil. The main strong peak found in the spectrum of the Cu foil should be attributed to the Cu-Cu metal bond, which was confirmed and determined to be 0.255 nm in distance by the curve-fitting analysis. In the case of the Cu/Pd(1/1) foil, the main peak splits into two. In the case of the Cu/ Pd(1/9) foil, the main peak mentioned above shifts to the rightmost side with a weak shoulder peak appearing at a shorter distance. The newly appeared peaks at longer distances are attributed mainly to the Cu-Pd bond. The phenomenon of the original main peak due to the Cu-Cu bond decreasing in intensity with increasing Pd content while the interference peak on the right increases can be clearly observed in Figure 6a. Figure 6c shows the Fourier-transformed EXAFS spectra at the Cu K edge of the colloidal dispersions of Cu nanoclusters and the Cu/Pd bimetallic nanoclusters with Cu/Pd mole ratios of 4/1, 1/1, and 1/4. The main peak for the Cu colloid should be assigned to the Cu-Cu metal bond. In a series of EXAFS spectra of Cu/Pd(4/1, 1/1, and 1/4) bimetallic nanoclusters, the main peak assigned to the Cu-Cu bond progressively decreases in height with a decreasing Cu/Pd ratio and splits into two peaks after showing a small shoulder peak at a longer distance in the Cu/Pd(4/1) colloid. This tendency is quite similar to the

spectral change in the Cu/Pd foil, as Figure 6a shows. The peak observed at the bimetallic colloid with a high Pd content can be attributed mainly to a Cu-Pd bond, although there is some contribution from the phase shift arising from the interference between Cu and Pd atoms. The successive intensification of the peak on the right side with the increase of Pd content just shows the gradual increment in Cu-Pd bonds in these nanoclusters. Similar changes in the EXAFS spectra have been found in our previous studies of Pt/Rh and Pd/Pt bimetallic clusters.29a,b These results demonstrate that both Cu-Cu and Cu-Pd bonds exist at the same time within each particle of Cu/ Pd bimetallic nanoclusters, i.e., formation of the alloy structure. Figure 6b shows the comparison of the Fouriertransformed EXAFS spectra at the Pd K edge of the Pd foil and the Cu/Pd alloy foils with different compositions, with mole ratios of 1/9, 1/1, and 9/1. The main peak for the Pd foil can be assigned to the Pd-Pd bond, which was proven and determined to be 0.274 nm in distance by curvefitting analysis. Although the peak-splitting phenomenon for the Cu/Pd alloy foil at the intermediate Cu/Pd ratio is not as severe as that at the Cu K edge shown in Figure 6a, the main peak tends to shift to the left side gradually in an analogous manner that can be expected for this case. Fourier-transformed EXAFS spectra at the Pd K edge of the colloidal dispersions of the Pd nanoclusters and the Cu/Pd bimetallic nanoclusters with Cu/Pd ratios at 1/4, 1/1, and 4/1 are shown in Figure 6d. In the case of the Pd nanoclusters, the main peak that should be attributable to the Pd-Pd bond is found at the distance that was


determined to be 0.274 nm by curve-fitting analysis. Just like the corresponding foils’ case (Figure 6b), as the Cu/ Pd ratio becomes larger, the peak gradually shifts to a shorter distance. In the last case, the Cu/Pd(4/1) colloid, the peak features already bear a strong resemblance to that of the Cu/Pd(9/1) foil in Figure 6b. These results clearly indicate that these bimetallic nanoparticles contain both metal elements in a particle and have an alloy structure. A reasonable model obtained from these spectra is that the bimetallic clusters shown in Figure 6 have a random alloy structure. In other words, enough Cu atoms are located on the surface of Cu/Pd bimetallic nanoclusters, especially for Cu-rich particles. The presence of Cu atoms on the surface has previously been demonstrated in a few catalysis reactions, as well.17 The surface area of the particles shown in Figures 2 and 3 are very large. Specifically, average particles in Figure 3 correspond to a 10-shell structural model, composed of 3871 atoms, in which 1002 atoms are located on the surface and 2869 atoms in the inner core, that is, a high population (26%) of surface atoms. Considering the high copper molar fraction (a content of 80%), the number of surface Cu atoms will be 802 when the colloid has a random structure, or there will be 228 Cu atoms even if, in an extreme case, all Pd atoms are located on the surface. One may conclude that the composition of the particle surface contains a large number of Cu atoms. On the base of the present results and the previous catalytic activity investigations, it is worthwhile to examine Raman scattering on the Cu/ Pd alloy nanoclusters. Because the Cu/Pd(4/1) colloid was found to be the most homogeneous one by TEM and HRTEM observations, with strong alloy character among all the Cu-rich Cu/Pd colloids studied here, we have employed this type of colloid for surface Raman scattering experiments. 3.3. SERS Spectra of p-Aminobenzoic Acid. pAminobenzoic acid (PABA) was chosen as a compound for the present study of surface-enhanced Raman scattering (SERS) spectra, using a Cu/Pd(4/1) colloid, because this compound is easy to adsorb onto metals, and considerable information is available in the literature concerning its interaction with various SERS substrates.31 A Raman spectrum of PABA adsorbed onto the Cu/Pd(4/1) colloidal dispersion is shown in Figure 7b. For comparison, a Raman spectrum of PABA in water without the Cu/Pd colloids is displayed in Figure 7a. Table 1 lists the main Raman bands and assignments of the p-aminobenzoate anion (NH2-C6H4-COO-) based on our quantum mechanical calculations,19 because no affirmative assignments have been made so far. In an aqueous solution, in which PABA exists mainly as NH3+-C6H4-COO-, the benzene ring CdC-like stretching vibrations (modes 8a and 8b, in Wilson’s notation for ring modes,21,22 hereinafter, cf. Table 1 and the footnote) appear as a strong band at 1609 cm-1 (Figure 7a). A ring C-H in-plane bending mode (mode 9a) appears at 1181 cm-1, whereas a ring breathing-like mode (mode 1) is observed near 851 cm-1. Other major ring modes are observed at 1524 cm-1 (mode 19a, ring CdC-like stretching vibration), 1278 cm-1 (mode 7a′, ring X-sensitive band involving N-C stretching coupled with C-H in-plane bending), 1137 cm-1 (mode 7a, another ring (31) (a) Bornhaus, R.; Benner, R. E.; Chang, R. K.; Chakay, I. Surf. Sci., 1980, 101, 367. (b) Tsang, J. C.; Avouris, P.; Kirtley, J. R. J. Chem. Phys. 1983, 79, 493. (c) Tsang, J. C.; Avouris, P.; Kirtley, J. R. Chem. Phys. Lett. 1983, 94, 172. (d) Suh, J. S.; DiLella, D. P.; Moskovits, M. J. Phys. Chem. 1983, 87, 1540. (e) Venkatachalam, R. S.; Boerio, F. J.; Roth, P. G. J. Raman Spectrosc. 1988, 19, 281. (f) Bello, J. M.; Narayanan, V. A.; Vo-Dinh, T. Spectrochim. Acta 1992, 48A, 563.

Langmuir, Vol. 15, No. 23, 1999 7987

Figure 7. Raman spectra of (a) 0.833 mM p-aminobenzoic acid in water, (b) 0.417 mM p-aminobenzoic acid in a CuPd(4/1) colloidal dispersion (0.833 mM), (c) 0.833 mM p-aminobenzoic acid in pH 11 water, and (d) 0.833 mM p-aminobenzoic acid in a CuPd(4/1) colloidal dispersion (0.833 mM) at pH 11.

X-sensitive mode, involving ring C-carboxyl C stretching coupled with C-H in-plane bending), and 643 cm-1 (mode 6b, ring in-plane bending). Since the carboxyl group mainly exists as an ionized form (COO-), the band at 1385 cm-1 is due to a COO- symmetric stretching vibration in the carboxylate group. The band at 851 cm-1 is actually coupled with an NH2 wagging and COO- group scissoring vibration. A weak band at 1690 cm-1 is due to COO asymmetric stretching in the COOH form coexisting as a small amount in the solution (i.e., NH2-C6H4-COOH). When the pH is raised to 11 so that PABA exists solely as an anion form (NH2-C6H4-COO-), the intensity of the band at 1387 cm-1 is increased, whereas the ring modes of 8a and 8b at 1609 cm-1 and 9a at 1181 cm-1 remain almost at the same positions (Figure 7c), being insensitive to the ionization state of the NH2- or -COO- group. The band at 1145 cm-1 reflects the increase of the COO- group concentration, which is attributed to the ring mode (7a) containing C-COO- stretching vibration. When PABA is adsorbed to the Cu/Pd(4/1) colloid at neutral pH (∼6.5), significant changes in the Raman bands can be observed in several ring modes. The band at 1609 cm-1 in Figure 7a is shifted downward to about 1588 cm-1 in Figure 7b, whereas the band at 1181 cm-1 in Figure 7a also undergoes a downward shift to 1171 cm-1 in Figure 7b. An even larger band shift occurs for the broad band at 1214 cm-1 in Figure 7b; the band at 1141 cm-1 in Figure 7b increases its intensity if compared with its counterpart band at 1137 cm-1 in Figure 7a. These shifts in benzene ring vibrational modes indicate that the PABA molecule is adsorbed on the Cu/Pd metal surface rather than simply bound to the protective polymer, PVP. A certain degree

7988


Lu et al.

Table 1. Density Functional Theory-Predicted Normal Modes of p-Aminobenzoate Aniona frequencies

Raman activities

normal modesb,c

1677.4 1668.7 1639.0 1614.2 1525.8 1449.3 1352.4 1341.6 1312.0 1259.6 1181.9 1138.6 1124.4 1080.0 1025.2 979.8 966.2 852.0 837.7 811.9 790.1 774.7 731.8 680.2 648.8 610.2 508.2 493.7 425.1

12.7 3.0 56.7 4.3 0.1 0.2 2.4 14.1 0.0 21.1 1.4 0.4 18.0 0.1 0.0 0.1 0.1 2.3 38.0 2.4 5.1 3.0 1.8 0.8 6.4 0.3 0.4 0.8 0.1

NH2 scissoring COO- asym. stretching + ring (ν8b) ring CdC stretching (ν8a) + NH2 scissoring ring CdC stretching (ν8b) ring CdC stretching (ν19a) ring CdC stretching (ν19b) ring C-H in-plane bending (ν14) + NH2 rocking COO- sym. stretching + C-COO- stretching ring C-H in-plane bending (ν3) ring X-sensitive mode (ν7a′): N-ring carbon stretching + C-H in-plane bending ring C-H in-plane bending (ν9a) ring C-H in-plane bending (ν15) + NH2 rocking ring X-sensitive mode (ν7a): C-COO- stretching + C-H in-plane bending NH2 rocking + ring C-H in-plane bending (ν15) ring in-plane bending + C-H in-plane bending (ν18a) ring C-H out-of-plane bending (ν17a) ring C-H out-of-plane bending (ν5) ring C-H out-of-plane bending (ν11) + NH2 wagging ring breathing (ν1) + ring C-H out-of-plane bending (ν11) + NH2 wagging + COO- scissoring ring C-H out-of-plane bending (ν10a) ring C-H out-of-plane bending (ν5) + COO- wagging + NH2 wagging ring out-of-plane bending (ν16b) + ring in-plane bending (ν13) ring out-of-plane bending (ν16b) + NH2 wagging C-H out-of-plane bending + C-COO- out-of-plane bending + C-N out-of-plane bending (ν4) ring in-plane bending (ν6b) ring in-plane bending (ν13) ring in-plane bending (ν9b) ring out-of-plane bending (ν16b′) ring out-of-plane bending (ν16a)

observedc

1609 s 1609 s 1522 w 1387 s 1281 br 1181s 1145 s

857 s

643 m

a A density functional theory (DFT) variant with Becke’s three-parameter exchange functional and the gradient-corrected functional of Lee, Yang, and Parr (B3-LYP) was employed in conjunction with a basis set of 6-31+G(d) to give normal modes. [ Lee, C; Yang, W.; Parr, R. G. Phys. Rev. 1988, B 37, 785; Becke, A. D. J. Chem. Phys. 1993, 98, 5648]. Hartree-Fock (HF) method with the same basis set was also performed, producing Raman activities and nearly the same normal modes as those from the DFT method. b Description of the normal modes follows the numerical-alphabet notation of Wilson, see ref 22. c Abbreviations: s, strong; m, medium; w, weak; br, broad; asym, asymmetric; sym, symmetric.

of charge transfer between PABA molecules and metal atoms may cause such shifts. The intensities of the bands at 1588 and 1171 cm-1 in Figure 7b are about 3-5-fold stronger than those in the normal Raman spectrum (Figure 7a) for the same PABA concentration under identical instrumental conditions. This is consistent with a previous report that SERS signals can be observed for Au/Pt alloy colloidal particles with a Pt % as high as 25%.13 The observed band shifts in the SERS study of PABA on Ag colloid particles were discussed by Suh et al.,31d although no explicit explanation was given for the shifts of several bands (e.g., the 1214 cm-1 band) in the SERS spectrum of PABA on the Ag colloid in their work. Bello et al. also observed changes in the SERS spectra of PABA on Ag substrates and attributed them to different adsorption orientations on the Ag surface.31f A large perturbation also takes place in the PABA molecules at pH 11 on going from a solvated state in an aqueous solution to an adsorbed state on the Cu/Pd(4/1) colloid, as shown in Figure 7c and d. Here, the ring modes at 1609, 1522, and 1181 cm-1 in Figure 7c are shifted downward to 1590, 1508, and 1171 cm-1 in Figure 7d, respectively. The intensities of the bands at 1590 and 1171 cm-1 in Figure 7d are 3 times stronger than those in the normal Raman spectrum (Figure 7c) under identical experimental conditions. Remarkably, the band at 1281 cm-1 also undergoes a large shift to 1222 cm-1. Although the band at 1145 cm-1 increases its intensity, the band at 857 cm-1 reduces its intensity. These changes imply strong interaction between the colloidal metal particle and the adsorbed PABA molecule. Besides, the spectra of PABA in the Cu/Pd colloids (Figure 7b and 7d) have several other common features. For example, the band due to the COO- symmetric vibration near 1380 cm-1 is not signifi-

cantly enhanced; the band involving several coupled vibrations near 855 cm-1 is weak. It is concluded that at both pH values, the PABA molecule adsorbs onto the metal surface as the same species, namely, the deprotonated aminobenzoate anion form, NH2-C6H4-COO-, and with the same orientation, although PABA molecules exist as different forms in the bulk solutions at pH 6.5 and 11. In general, the PABA molecule could bind to the metal through the lone pair on either the carboxylate or the amine group. Changes in relative Raman band intensities caused by the adsorption on metal surfaces have been discussed with an electromagnetic (EM) and/or a chargetransfer (CT) mechanism.3a According to the “surface selection rule” based on electromagnetic theory, vibrations deriving their intensities from a large value of the polarizability tensor Rzz (z being the direction of the local surface normal) would be the most intense in the SERS spectrum, whereas vibrations derived from Rxx, Ryy, and Rxy are the least intense (x and y being parallel to the surface).32 If we consider for simplicity that one of the principal molecule-fixed symmetry axes of the adsorbate coincides with the surface normal, the above rule indicates that for a planar ring molecule A-C6H4-B, such as PABA, adsorbed with its A-B long axis (C2 axis) along the surface normal, the vibrations of certain in-plane normal modes will be most intense since Rxx, Ryy, and Rzz contribute to the Raman intensity of these symmetric modes, provided that that of the derived polarizability component Rzz is large. Of the three components Rxx, Ryy, and Rzz, the two whose subscripts define the plane in which the vibrational motion (32) (a) Moskovits, M. J. Chem. Phys. 1982, 77, 4408. (b) Creighton, J. A. Surf. Sci. 1983, 124, 209. (c) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 1293. (d) Moskovits, M.; Suh, J. S. J. Phys. Chem. 1984, 88, 5526.


is contained are usually the largest. In NH2-C6H4-COO-, the polarizability tensors are calculated by quantum mechanical (ab initio and density functional theory at HF/ 6-31+G(d) and B3LYP/6-31+G(d) levels, respectively) methods19 and found to be Rxx ) 113.5, Rxy ) 0.0, Ryy ) 62.4, Rxz ) 0.0, Ryz ) 0.0, and Rzz ) 146.9, where x is along the molecular plane but perpendicular to the molecular C2 axis, y is perpendicular to the molecular plane, and z is the direction of the C2 axis. Thus, the component perpendicular to the molecular plane is smaller than that lying in the molecular plane. Hence, the in-plane modes, such as those in Figure 7, whose motions are directed normal to the surface will, by and large, be the most intense. A standing-up geometry of PABA (or at least tilted off the surface with the ring pendant) is a good candidate for the appearance of the strong bands such as the 7a′ mode containing N-ring carbon stretching and the 7a mode containing C-COO- stretching, both involving vibrations in the direction along the molecular long axis. Likewise, the bands of 8a,b and 9a modes would have large components normal to the metal surface in the standingup configuration. An intrinsically large value of Rzz produces a SERS spectrum dominated by these vibrations. This is consistent with the essential absence of enhancement of out-of-plane bending modes, which would be expected to appear in a flat orientation. The coupled band near 857 cm-1 contains an appreciable contribution from a C-H out-of-plane bending vibration (Table 1). If the PABA molecule is adsorbed flat on the surface, this band is also expected to show a remarkable enhancement, deriving its intensity from the polarizability tensor perpendicular to the molecular plane. In fact, the opposite situation is seen in Figure 7, in which it is the least intense band compared with the counterparts in solutions. Similar arguments were used by Weaver et al. and van Duyne et al.33 Our bonding scheme differs from that of PABA adsorbed on Ag. In that report,31d the bands at 797 and 754 cm-1 (out-of-plane modes) were prominent and the bands at 1139, 1172, and 1214 cm-1 were not significantly enhanced, although the band at 1515 cm-1 was strongly enhanced. All these features, which were interpreted as a flat orientation of PABA on Ag, are quite opposite to what we see for the PABA anion in our data. Thus, comparison with the SERS data of PABA on Ag also lends support to the standing-up orientation in our case. It is very likely that the adsorption is dominated by the standing-up orientation, giving rise to ring bands that derived their intensities from the large components (Rxx and Rzz) of the polarizability tensors. 3.4. SERS Spectra of Thiophenol and Bis(3-carboxy-4-nitrophenyl) Disulfide. Recently, much attention has been shown to the adsorption of sulfur-containing compounds onto metal nanoparticles because they can easily coordinate to metal nanoparticles and stabilize the small particles.34 From this point of view, we measured the SERS spectra of thiophenol and bis(3-carboxy-4nitrophenyl) disulfide adsorbed on Cu/Pd(4/1) bimetallic colloids. Figure 8 compares Raman data of thiophenol adsorbed on a Cu/Pd(4/1) alloy colloid with that free in water. The spectrum of thiophenol in water shows several major bands which are listed in Table 2. Because of the (33) (a) Gao, X.; Davies, J. P.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6858. (b) Allen, C. S.; Van Duyne, R. P. Chem. Phys. Lett. 1979, 63, 455. (34) (a) Yonezawa, T.; Sutoh, M.; Kunitake, T. Chem. Lett. 1997, 619. (b) Andres, R. P.; Bielefeld, J. D.; Henserson, J. I.; Janes, D. B.; Kolagunta, V. R.; Kubiak, C. P. Science 1996, 273, 1690. (c) Yau, S. T.; Mulvaney, P.; Xu, W.; Spinks, G. M. Phys. Rev. B; Condensed Matter 1998, 57, R15124. (d) Xu, H.; Tseng, C. H.; Vickers, T. J.; Mann, C. K.; Schlenoff, J. B. Surf. Sci. 1994, 311, L707.

Langmuir, Vol. 15, No. 23, 1999 7989

Figure 8. Raman spectra of (a) 50 mM thiophenol in water, and (b) 0.75 mM thiophenol in a CuPd(4/1) colloidal dispersion.

importance of self-assembled monolayers of thiophenol and related compounds, we present a new normal-mode analysis, derived from a density functional theory, in Table 2 instead of the empirical assignments reported in the previous work.35 The present assignments in Table 2 not only clarify the controversial assignments of some bands, but also keep us in a good position to understand the SERS spectrum. In the Raman spectrum of thiophenol in the Cu/Pd(4/1) colloid (Figure 8b), the intensity of the 1000 cm-1 band was found to be 8 times stronger than that in the normal Raman spectrum, whereas the band at 1573 cm-1 is 32 times more intense than the band at 1584 cm-1 in the normal Raman spectrum (Figure 8a) for the same concentration and under the same instrumental conditions. The Raman spectrum of thiophenol in the Cu/Pd colloid resembles previous SERS reports in a Cu colloid or on Ag, Au, and Cu film surfaces.4a,35 In particular, in the SERS spectrum, the band at 1573 cm-1 significantly enhances its intensity and shifts downward compared with the band at 1584 cm-1 in the normal Raman spectrum. In the SERS spectrum, an intense band is seen at 1076 cm-1, which has been related to the 1095 cm-1 counterpart in the normal Raman spectrum. The latter is actually due to a ring C-H in-plane bending mode strongly coupled with C-S stretching. The band at 1023 cm-1, due to the ring 18a mode coupled with C-S stretching, also increases its relative intensity in the SERS spectrum, whereas the band at 1000 cm-1, which is attributed to a ring trigonal bending mode coupled with an S-H bending mode, is shifted slightly downward. The band at 922 cm-1 in the normal Raman spectrum, which is attributed to the S-H bending mode, disappears in the SERS spectrum. The (35) (a) Carron, K. T.; Hurley, L. G. J. Phys. Chem. 1991, 95, 9979. (b) Joo, T. H.; Kim, M. S.; Kim, K. J. Raman Spectrosc. 1987, 18, 57.

7990


Lu et al.

Table 2. Density Functional Theory-predicted Normal Modes of Thiophenola frequencies

Raman activity

normal modes

observed

1607.8 1595.4 1493.8 1456.5 1337.4 1308.8 1192.9 1168.1 1096.4 1089.3 1028.4 993.0 975.5 951.6 921.7 883.0 824.2 727.9 692.0 674.9 618.0 465.7 402.5 402.2

23 9 2

Pd Alloy Colloid

Recommend Documents