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Mixed Silver/Gold Colloids: A Study of Their Formation, Morphology, and Surface-Enhanced Raman Activity L. Rivas,† S. Sanchez-Cortes,*,† J. V. Garcı´a-Ramos,† and G. Morcillo‡ Instituto de Estructura de la Materia, CSIC, Serrano 121, E-28006 Madrid, Spain, and Departamento de Quı´mica Orga´ nica y Biologı´a, Universidad Nacional de Educacio´ n a Distancia, Senda del Rey s/n, E-28040 Madrid, Spain Received April 12, 2000. In Final Form: August 21, 2000 Ag-coated Au and Au-coated Ag colloidal particles were prepared by deposition of Ag or Au through chemical reduction on Au or Ag colloids, respectively. Different amounts of the depositing metal were employed in order to obtain different compositions of the Au100-xAgx and Ag100-yAuy resulting particles. The obtained colloids were characterized by UV-vis spectroscopy and transmission electron microscopy. A different formation mechanism was deduced for the different mixed colloids obtained by this procedure. Moreover, the activity of these colloids in surface-enhanced Raman spectroscopy (SERS), that is, the SERS enhancement factor, was checked by using pyridine as an adsorbate probe. The SERS excitation profiles for each colloid were investigated in order to obtain information about the activity of these substrates at different excitation wavelengths as well as to estimate the relative exposed area of each metal in every composite colloid.

Introduction Au and Ag colloids are metallic systems largely employed in surface-enhanced Raman spectroscopy (SERS).1-5 Despite their easy preparation, these systems are rather difficult to control and characterize because of the enormous range of particle sizes and shapes. Although the Au surface shows a lower enhancement factor in the visible in comparison to that of Ag,6,7 colloids of this metal have many advantages, including an easier preparation and a higher homogeneity as concerns the dispersion of diameters.8 For this reason, many attempts to obtain composite particles by depositing Ag on preformed Au particles have been carried out in order to induce a higher homogeneity in the Ag surfaces9-12 as well as to induce a greater SERS enhancement of the Au colloid. Au particles may act as seed to induce the Ag grown by chemical reduction of Ag+ in the presence of preformed colloidal Au.4 These Au/Ag composite particles have received enormous attention in the fields of surface chemistry and electrochemistry.13,14 * To whom correspondence should be addressed. Fax: + 34 91 564 55 57. Tel: + 34 91 561 68 00. E-mail: [email protected]. † Instituto de Estructura de la Materia. ‡ Universidad Nacional de Educacio ´ n a Distancia. (1) Creighton, J. A.; Blatchford, C. G.; Albrecht, M. G. J. Chem. Soc., Faraday Trans. 2 1979, 75, 790. (2) Moskovits, M. Rev. Mod. Phys. 1985, 57, 783. (3) Sanchez-Cortes, S.; Garcia-Ramos, J. V.; Morcillo, G. J. Colloid Interface Sci. 1994, 167, 428. (4) Hayat, M. A. Colloidal Gold: Principles, Methods, and Applications; Academic Press: San Diego, 1989. (5) Blatchford, C. G.; Campbell, J. R.; Creighton, J. A. Surf. Sci. 1982, 120, 435. (6) Smith, W. E. Methods Enzymol. 1993, 226, 482. (7) Laserna, J. J. Anal. Chim. Acta 1993, 283, 607. (8) Sutherland, W. S.; Winefordner, J. D. J. Colloid Interface Sci. 1992, 148, 129. (9) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (10) Bright, R. M.; Walter, D. G.; Musick, M. D.; Jackson, M. A.; Allison, K. J.; Natan, M. J. Langmuir 1996, 12, 810. (11) Felidj, N.; Levi, G.; Pantigny, J.; Aubard, J. New J. Chem. 1998, 725. (12) Heard, S. M.; Grieser, F.; Barraclough, C. G.; Sander, J. V. J. Colloid Interface Sci. 1983, 93, 545.

The Ag and Au colloids display different optical properties which have to be considered from the point of view of the electromagnetic mechanism associated with the SERS effect; whereas Ag sols display an absorption maximum at about 420 nm, the Au sols show a maximum at above 500 nm. The aggregated colloids show new absorption bands which appear toward the red region: 500-600 nm for Ag colloids and 700-900 nm for Au colloids. These new absorption bands are due to the plasmon excitation in aggregated particles that govern the SERS excitation profiles of such colloids.1 Because of this difference, the choice of an adequate excitation wavelength depending on the nature of the metal employed is important; whereas Ag colloids usually have a higher activity in the visible region, the Au ones are more active in the red but their activity highly decreases in the nearinfrared (NIR) region, being nonactive below the red. The preparation of Ag/Au mixed colloids allows for the combination of the SERS activities of both metals in a broader interval of the electromagnetic spectrum. So far, Ag/Au mixed colloids obtained by other authors for SERS have been almost exclusively prepared by depositing Ag on Au particles9-11 in an attempt to search for a more homogeneous particle size distribution of composite particles. Ag/Au alloys15,16 as well as Au-coated Ag particles have been much less investigated,17,18 although in such systems an enhancement of the optical absorption of Au in the visible region below the red excitation, where plasmons of this metal are not active, could be induced. In this work, we report the preparation of both Aucoated Ag colloids and Ag-coated Au colloids. These colloids (13) Mason, M. G.; Hansen, J. C. J. Vac. Sci. Technol., A 1994, 12, 2023. (14) Corcoran, S. G.; Chakarova, G. S.; Sieredski, K. J. Electroanal. Chem. 1994, 377, 85. (15) Aihara, N.; Torigoe, K.; Esumi, K. Langmuir 1998, 14, 4945. (16) Han, S. W.; Kim, K.; Kim, K. J. Colloid Interface Sci. 1998, 208, 272. (17) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (18) Murray, C. A. J. Electron Spectrosc. Relat. Phenom. 1983, 29, 371.

10.1021/la000557s CCC: $19.00 © 2000 American Chemical Society Published on Web 11/10/2000

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were characterized by UV-visible absorption spectroscopy, transmission electron microscopy (TEM), and SERS. The application of these techniques was aimed toward a morphological characterization and a study of the mechanism of deposition of metals on preformed metal particles. The SERS activity of these systems was tested by using pyridine as a molecular probe at different excitation wavelengths to compare the effectiveness of each system to that of the neat metal colloids. Furthermore, an estimation of the available surface corresponding to each metal in the mixed colloids was accomplished on the basis of the different SERS profiles that pyridine displays on each metal. Experimental Section AgNO3 (99.98%), HAuCl4 (99%), trisodium citrate (99%), and NaBH4 (96%) were purchased from Merck. Pyridine (99.6%) was supplied by Carlo Erba. All the reagents employed were of analytical reagent grade. The aqueous solutions were prepared using tridistilled water. All the glass material was cleaned by using a chromic mixture and rinsing several times with triply distilled water. The initial Ag colloid was prepared according to the Lee and Meisel method:19 200 mL of a 10-3 M AgNO3 aqueous solution was heated to boiling, and then 4 mL of a 1% trisodium citrate solution was added, keeping the mixture boiling for 1 h. The initial Au sol was prepared according to the method reported by Sutherland and Winefordner:8 0.1 mL of a 4% (w/v) HAuCl4 solution was added to 40 mL of triply distilled water, then 1 mL of a 1% (w/v) trisodium citrate solution was added drop-by-drop while stirring, and the resulting mixture was boiled for 5 min. Preparation of Ag-Coated Au Colloids. Aliquots of 10-3 M AgNO3 (250, 500, 1000, and 2000 µL) were added drop-by-drop to 5 mL of a preformed citrate-reduced Au colloid with continuous stirring. Afterward, the corresponding volume of the citrate aqueous solution in relation to the added AgNO3 volume was added to induce the chemical reduction of Ag and its deposition on the Au particles. The mixture was kept boiling for 1 h. The percent composition of the resulting mixed colloids was Au85Ag15, Au73Ag27, Au54Ag46, and Au40Ag60. Preparation of Au-Coated Ag Colloids. Aliquots of 3 × 10-4 M HAuCl4 (50, 250, and 500 µL) were added to 5 mL of a preformed citrate-reduced Ag colloid drop-by-drop with continuous stirring. Afterward, the corresponding volume of the citrate aqueous solution in relation to the HAuCl4 volume was added to induce the chemical reduction of Au and its deposition on Ag particles. The mixture was kept boiling for 5 min. The percent composition of the resulting mixed colloids was Ag99.7Au0.3, Ag97Au3, and Ag93Au7. As can be seen, the relative metal fractions are apparently different in both kinds of colloids; the relative amount of Au employed to deposit this metal on Ag in the Ag100-yAuy mixed colloids was much less than that required for Ag deposition on Au colloids to form Au100-xAgx mixed colloids. This is justified by the different metal surfaces existing in the particles of initial Ag and Au colloids acting as seed. Taking into account that (1) the concentration of Ag in the initial particles is 3-fold greater than that of Au, (2) the atomic volume of Au is higher (VAu ≈ 1.28VAg),20 and (3) the radius of the Ag particles is higher (RAg ≈ 4RAu, see TEM micrographs of Figure 3), we deduced a relationship between particles (number of Ag particles/number of Au particles) of 1/27. By considering the different size of each metal particle, we concluded that the exposed metal surface ratio is SAu ≈ 2SAg in the same volume of the original colloids. From these data, a relationship between deposited atoms and available deposition area can be deduced. We have found that this relationship is equivalent in the Ag97Au3 and Au85Ag15 mixed colloids, despite their different metal fractions. Samples for UV-visible measurements were obtained by diluting the silver colloids to 10% and then placing them in 1 cm (19) Lee, P. C.; Meisel, D. J. Phys. Chem. 1982, 86, 3391. (20) CRC Handbook of Chemistry and Physics, 78th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1997.

Figure 1. UV-visible absorption spectra of mixed Ag-coated Au colloids (Au100-xAgx).

Figure 2. UV-visible absorption spectra of mixed Au-coated Ag colloids (Ag100-yAuy). optical path quartz cells. Samples for SERS measurements were prepared by adding 10 µL of a 1 M pyridine aqueous solution to 1 mL of the silver colloid. Samples for TEM were prepared on a 3.05 mm diameter copper grid, composed of 200 squares 100 µm in length. These grids were previously covered by a 0.2 M Formvar chloroform solution in order to obtain a thin transparent film. On this film, a small drop of the colloid (ca 1 µL) was deposited and evaporated at 37 °C. Ag and Au colloids were deposited on these grids without further dilution. Fourier transform (FT)-Raman spectra were recorded using a Bruker RFS 100/S model spectrophotometer connected to a liquid-air-cooled Ge detector. The 1064 nm line provided by a Nd:YAG laser was used for excitation in the NIR, with a 250 mW power output. The samples were placed in a 1 mm path length glass capillary. The resolution was set to 4 cm-1. SERS spectra in the visible were recorded by a Jobin-Yvon U-1000 spectrometer. The excitation lines at 457.9, 488.0, and

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Figure 3. Transmission electron micrographs of Ag (A) and Au (B) colloids.

Figure 4. Transmission electron micrographs of Au85Ag15 (A) and Au54Ag46 (B) colloids. 514.5 nm were provided by a Spectra-Physics model 165 Ar+ laser, and the excitation lines at 568.2, 647.1, and 676.4 nm were provided by a Spectra-Physics model 165 Kr+ laser. In all cases, the power at the sample was 60 mW. Slits were set to 5 cm-1. UV-vis absorption spectra were recorded on a Shimadzu UV2100 spectrometer. Micrographs were obtained with a Hitachi 7000 transmission electron microscope operating at 100 kV.

Results and Discussion UV-Vis Absorption Spectra. The optical absorption of Au and Ag colloids has been a subject of study for decades.21-24 These colloids show absorption maximums at about 420 and 520 nm for Ag and Au, respectively, because of the different plasmon excitation resonance of each metal.21 Figure 1 shows the absorption spectra of Ag-coated Au colloids (Au100-xAgx). The Au85Ag15 and Au73Ag27 colloids show only an absorption band above 500 nm, which can be exclusively attributed to plasmon resonance of Au particles. When the Ag fraction is increased, a slight downshift in the wavelength of the maximum is observed. The appearance of only one absorption band corresponding to Au indicates that homogeneous mixed colloidal particles of both metals are formed without significant formation of independent particles. When the Ag fraction is increased, that is, for Au54Ag46 and Au40Ag60, a weak, broad absorption band is observed at about 415 nm, characteristic of Ag colloidal particles. Moreover, the Au absorption band downshifts to 506 nm in the Au54Ag46 colloid and to 482 nm in the Au40Ag60 one. The presence of separate Ag and Au resonances in the latter colloid suggests the formation of separate particles in the medium.25 (21) Mie, G. Ann. Phys. 1908, 25, 377. (22) Doremus, R. H. J. Chem. Phys. 1964, 40, 2389. (23) Kreibing, U. J. Phys. F: Met. Phys. 1974, 4, 999. (24) Sanchez-Cortes, S.; Garcı´a-Ramos, J. V.; Morcillo, G. J. Colloid Interface Sci. 1995, 175, 358. (25) Smith, G. Chem. Rev. 1992, 92, 1709.

Figure 2 shows the absorption spectra of Au-coated Ag colloids (Ag100-yAuy). In the colloid having a lower amount of Au deposited on Ag (Ag99.7Au0.3), a unique, broad absorption band appears extended in the 400-600 nm region, hence, at an intermediate position in relation to the absorption of Ag and Au. A detailed analysis of the last band by deconvolution (data not shown) reveals the simultaneous contribution to this band from plasmon resonances due to both Au and Ag particles. A unique absorption band is also observed in the case of Ag97Au3 and Ag93Au7 colloids, although with maximums centered at 541 and 554 nm, respectively. The last maximums are shifted to longer wavelengths in comparison to that of the Au colloid, which appears at 520 nm, thus suggesting that these Ag/Au composite colloids are not a simple mixture of the two different monometallic particles. Transmission Electron Microscopy of Mixed Colloids. Figure 3 shows the transmission electron micrographs of Ag (Figure 3A) and Au (Figure 3B) colloids. From these micrographs, we have found that the average size of the particles existing in each colloid is completely different: 10-15 and 40-50 nm for Au and Ag colloids, respectively. On the other hand, the Au colloid displays a more uniform distribution of sizes and shapes and no rod-shaped particles are seen, in contrast to the Ag colloid. Among the TEM images obtained for the Ag-coated Au colloids, we have selected those of the Au85Ag15 and Au54Ag46 colloids (Figure 4) because they represent the two different kinds of mixed colloids that can be obtained. When the amount of Ag added to the preformed Au colloid is low, corresponding to the Au85Ag15 colloid, composite bimetallic Ag/Au particles are actually formed. These particles show a core-shell structure (Figure 4A) with a darker nucleus that may correspond to the initial Au particle and a clearer shell, corresponding to the deposited Ag. The darker contrast observed in the micrograph for the nucleus further suggests the presence of Au in the core, because this metal is more efficient in the scattering

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Figure 5. Transmission electron micrographs of Ag99.7Au0.3 (A and B), Ag97Au3 (C and D), and Ag93Au7 (E and F) colloids.

of electrons. This result indicates that Au particles act as seeds for the Ag deposition, as has also been suggested by other authors.26 As a consequence of the silver deposition, a size enlargement of the initial Au particles from 15 nm to an average size of 25 nm in the composite particles is observed (Figure 4A). In contrast, the Au54Ag46 TEM image shows the formation of separate Ag and Au colloidal particles, which are clearly identified by their completely different sizes (Figure 4B). This result agrees with the UV-vis absorption spectra (Figure 1), in which the appearance of two maximums at 507 and 412 nm can be attributed to the plasmon resonances of independent Au and Ag particles. Figure 5 displays the TEM micrographs of the Au-coated Ag colloids at several scales. The formation mechanism changes depending on the amount of added gold, thus giving rise to different morphologies in the obtained particles. The TEM images of Ag99.7Au0.3 (Figure 5A,B) show the presence of large aggregates integrated by both spherical and rod-shaped particles which are coated by a large number of smaller spheres and spheroids. The larger particles have a similar size in relation to the original Ag particles, but the size of the smaller particles which cover the larger ones is of the same order as that of independent Au particles (Figure 3B). This fact suggests that the

observed aggregates are formed by Ag nuclei surrounded by small Au particles as shown in Figure 6a. The formation of such mixed particles suggests a three-step mechanism of metal deposition: (1) a previous formation of individual Au particles; (2) diffusion of these particles to the surface of the Ag ones; (3) deposition on the Ag surface, which is covered without losing its initial shape. This three-step deposition can be attributed to the low Au concentration existing in the medium. At this concentration, the diffusion step controls the deposition process of Au. Figure 5C,D shows the TEM micrographs of the Ag97Au3 colloid. This colloid is basically composed of strings with polygonal surfaces. Two main aggregate widths can be observed: (1) thick strings of about 200 nm and (2) thinner strings integrated by particles of 50-100 nm in width. The higher concentration of Au allows, in this case, a quicker diffusion of this metal toward the Ag surface in such a way that the process is now controlled by the metal deposition. In fact, the resulting mixed particles may preserve the original faceted crystalline surface existing in Ag by epitaxial growing of the Au surface, as illustrated in Figure 6b. Moreover, different final composite particles that also have a different metal composition may be formed as deduced from the two sizes observed in the appearing particle chains. The existence of actual Au-coated Ag particles is demonstrated by the UV-vis absorption

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Figure 6. Different deposition patterns in Au-coated Ag colloids.

Figure 8. Excitation profiles of Au, Ag, and Au85Ag15 colloids. UV absorption spectra of Au and Au-pyridine colloids are shown at the top of the figure.

Figure 7. Excitation profiles of Au-coated Ag colloids in comparison to those of Ag and Au colloids. The UV absorption spectra of Ag and Ag-pyridine colloids are also shown.

spectrum of this colloid (Figure 2), in which no Ag plasmon resonance is observed. In addition, the Au plasmon resonance is upshifted in connection with the enlargement of the Ag/Au mixed particle upon deposition of Au atoms on the Ag surface. In the case of Ag93Au7 (Figure 5E,F), dense particles surrounded by chainlike aggregates can be observed. The chains are formed by small particles with an average diameter of 15 nm, which corresponds to the size of the Au particles. On the other hand, the denser particles are larger (100-200 nm) than the usual Ag particles (40-50 nm) but with a size comparable to that of the dense

particles observed in the Ag97Au3 colloid (Figure 5C,D). The absence of any absorption band at 400-420 nm precludes the existence of noncoated Ag particles. We suggest that the Ag93Au7 colloid is the result of a combination of the two growing mechanisms described above because of the higher Au concentration in the medium (Figure 6c). SERS Activity of Au/Ag Mixed Colloids. The SERS activity of the above colloids was checked by using pyridine as a molecular probe. This molecule has been widely used in SERS studies since the SERS effect was discovered.1,26 One interesting aspect of this molecule, that has justified its use in this work, is the fact that its SERS spectrum pattern depends on the metallic surface used as substrate.27,28 Pyridine was employed to obtain the excitation profile of each colloid. Figures 7 and 8 display the SERS excitation profiles (represented by the SERS enhancement factor G) for some of the mixed colloids. The absorption spectra of initial and aggregated Ag and Au colloids are also shown. G was deduced from the SERS spectrum and the FT-Raman spectrum of a 0.1 M pyridine aqueous solution by the following equation:

G)

ISERSCAD IADCSERS

(1)

where ISERS and IAD are the intensities of the 1008 and 995 cm-1 ring breathing bands of pyridine (Figure 9) in the SERS and the normal Raman spectra of the solution in water, respectively, and CSERS and CAD are the concentra(26) Esumi, K.; Tano, T.; Trigol, K.; Megero, K. Chem. Mater. 1990, 2, 564. (27) Seki, H. J. Chem Phys. 1982, 76, 4412. (28) Oblonsky, L. J.; Devine, T. M.; Ager, J. W., III; Perry, S. S.; Mao, X. L.; Russo, R. E. J. Electrochem. Soc. 1994, 141, 3312.

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Figure 9. SERS spectra of pyridine on different colloids in the 900-1080 cm-1 region. Excitation at 1064 nm.

tions of pyridine in the SERS sample (10-2 M) and in the aqueous solution (1 M). The problem that we faced when using this molecule as a probe was its high aggregating activity on Au whereas on Ag a relatively high concentration (10-2 M) is required to induce the necessary colloid aggregation without adding an aggregating inductor. Because of this high pyridine concentration, only a few molecules in the vicinity of the metal surface will contribute to the Raman enhancement. Therefore, the exact value of CSERS must be much lower than the overall pyridine concentration in the mixture (10-2 M). This is the reason relatively low G values are obtained (Figures 7 and 8). Although these G values are not exact under the absolute point of view, they can be employed for comparing the relative effectiveness of each colloid at different wavelengths. We suggest that the actual G values could be about 1000-fold higher than those displayed in Figures 7 and 8. The Ag colloid shows a small G when exciting at wavelengths falling in the green-blue region (Figure 7), in contrast to the result found by Creighton et al.1 We have attributed this fact to the different Ag colloids employed by the above authors (borohydride-reduced), which show an absorption maximum at 500-600 nm when aggregated. In our case, the higher size of the Ag particles and the relatively high pyridine concentration lead to a red displacement of the aggregated particles’ plasmon resonance, which appears as a broad band in the 600800 nm region. For the Ag colloid (Figure 7), G increases toward higher excitation wavelengths, reaching a maximum in the red (647.1 nm). In the case of the Au colloid (Figure 8), the SERS spectra could be registered only by using red and infrared lines (647.1, 676.4, and 1064 nm). G excitation profiles in Ag and Au colloids follow the absorption band observed for the aggregated colloids once the pyridine is added, in accordance with the absorption of the corresponding metallic plasmons.1,5

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The mixed Ag/Au colloids obtained by depositing a lower amount of the second metal onto the preformed initial particles (Ag97.3Au0.3 and Au85Ag15) show a behavior that can be considered as an intermediate between those of the two metals: excitation maximums in the red region and a SERS signal that is extended from the blue to the near-infrared. In the case of the Ag97.3Au0.3, this behavior is due to the incomplete coverage of Ag by Au, as shown by the TEM images (Figure 5A,B). In consequence, this colloid can be excited with a blue or green light. However, the Ag97Au3 and Ag93Au7 colloids show optical properties that are closer to those of the Au surface, as they do not show any SERS signal below the red region. This is again in agreement with the TEM micrographs (Figure 5C-F), which show an almost complete surface coverage of Ag. For these two colloids, an increase of G is seen toward the near-infrared, in contrast to the Au colloid whose SERS activity markedly decreases at 1064 nm. In the case of the Ag93Au7 colloid, G is increased remarkably (1000-fold) in comparison to the value for Au. These results point out that the effectiveness of the mixed colloids depends on the excitation wavelength used. In general, it is observed that SERS activity decreases for Ag when this metal is covered by Au and, in contrast, an improvement of this activity is observed in Au particles when covered with Ag. This is not new, as the SERS effectiveness of Ag is usually higher than that of Au.2,29,30 But in all cases, the mixed colloids represent an improvement in relation to Au alone, whose SERS activity is markedly enhanced when this metal is deposited on Ag. This effect is more evident in the NIR region and can be attributed to (1) the high absorbance of Ag particles at this wavelength, which also represents an advantage for the Au that covers the Ag particle, and (2) the morphology change of Au when deposited on the Ag particles, which increases its plasmon resonance toward the NIR region. Relative Exposed Areas of Each Metal in Composite Colloids. Another interesting aspect that can be considered in the preparation of composite colloids is the extent to which the initial particle has been covered by deposition of the second metal. To accomplish that, the use of an adsorbate showing different Raman spectra when adsorbed on the two different metal surfaces could be of great interest. Pyridine fulfills this condition, because the position and relative intensity of the ring breathing motions appearing at 990 (ν1) and 1030 cm-1 (ν2) in aqueous solution vary depending on the surface where this molecule is adsorbed.27,28 In particular, pyridine shows a very different spectral profile when adsorbed on Ag or Au colloids (Figure 9), as has also been demonstrated by several authors.1,31 On Ag, the above bands are shifted to 1008 and 1036 cm-1, their intensity being approximately the same when exciting at 647.1 nm. On Au colloids, the first band is further shifted to 1011 cm-1 and the band at 1036 cm-1 markedly decreases to half of the ν1 band intensity at 647.1 nm. These changes can be attributed to a different interaction of pyridine with the surface, leading to a molecular electronic density redistribution. This fact can be used to estimate the relative exposed area of each metal existing on the mixed particle. The above bands also undergo a change at different excitation wavelengths. Figure 10 shows the Iν1/Iν2 relationship as a function of the excitation line expressed in eVs. The changes observed with the excitation line are similar to those observed for pyridine adsorbed on metal (29) Garrell, R. L. Anal. Chem. 1989, 61, 401. (30) Otto, A.; Mrozek, I.; Crabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (31) Wetzel, H.; Gerischer, H. Chem. Phys. Lett. 1980, 76, 460.

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Figure 10. Intensity ratio of pyridine ring breathing bands on the different mixed colloids and at different excitation wavelengths.

electrodes on increasing the applied potential.32,33 For this reason, we have attributed the Iν1/Iν2 variation to a different contribution of the charge-transfer mechanism at different excitation wavelengths or surface potentials, as in the case of other adsorbates.34 The pyridine Iν1/Iν2 ratio varies from 1.2 to 0.5 on Ag by changing the excitation line from 1064 to 457.9 nm. A similar behavior is also observed on Au, where a decrease of this ratio is observed from 2.35 to 2.0 when changing the excitation wavelength from 1064 to 647.1 nm. Excitations with lower wavelengths give rise to no SERS signal for this metal. Each composite colloid shows a different behavior as concerns the Iν1/Iν2 ratio. The Ag97Au3 colloid is closer to the Au colloid; the ratio is very high, and no SERS signal can be detected out of the NIR and red excitations. This result agrees with TEM and the absorption data shown above. The Ag99.7Au0.3 colloid shows a high Iν1/Iν2 ratio (above 2.0) in the NIR region, as in the case of Au, but this ratio remarkably decreases toward the blue region, being only 0.83 at 457.9 nm. This is again in accordance with the morphology observed in this colloid; the existence of a high Au surface because of the small Au particles which cover the Ag surface leads to a Au-pyridine profile predominance in the NIR region (Figure 10). The higher Iν1/Iν2 ratio measured for this colloid in relation to the Ag97Au3 one even if the amount of Au is lower can also be explained by their different morphologies. However, the still-available Ag surface exposed to the medium induces a SERS signal in the blue region from the pyridine adsorbed on this Ag surface. This colloid can be considered as a real mixture of metal surfaces where the enhancement and spectral profiles are governed by both metals. In the case of the Ag93Au7 colloid, no information about the above ratio could be obtained because only one band (32) Wetzel, H.; Gerischer, H.; Pettinger, B. Chem. Phys. Lett. 1982, 85, 187. (33) Creighton, J. A. Selection Rules for Surface-Enhanced Raman Spectroscopy. In Spectroscopy of Surfaces; Clark, R. J. H., Hester, R. E., Eds.; John Wiley & Sons: Chichester, 1988. (34) Sanchez-Cortes, S.; Garcı´a-Ramos, J. V. Langmuir 2000, 16, 764.

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at 1025 cm-1 was obtained (Figure 9). The origin of this band is still uncertain. It has been attributed to protonated pyridine35,36 or to a chemisorbed pyridine linked to the surface.37 The presence of such a band only in this composite colloid is an interesting result that deserves a deeper investigation. For Au100-xAgx colloids, the Iν1/Iν2 ratio shows values which are closer to Ag at the different excitation lines. This indicates that Ag may be uniformly deposited on the initial Au particles. However, the still-high Iν1/Iν2 ratio in comparison to the Ag colloid reveals the existence of a large Au surface that was not covered by Ag deposition or the formation of independent metal particles, as suggested by the TEM and the absorption spectra of the colloids having a high Ag fraction. For these colloids, an increase of the Iν1/Iν2 ratio is seen on increasing the Ag fraction when exciting at 1064 nm. This fact is related to the formation of separated Ag particles at these higher concentrations of this metal. In summary, the analyzed mixed colloids show excitation and interfacial properties that can be considered as intermediate between those of the two metals, although the characteristics of each substrate tend toward one or the other metal in each case. As a first conclusion, we can deduce from this intermediate behavior that a total coverage of the initial metal particles employed as nucleation seeds is never accomplished. Conclusions The composite Ag/Au colloidal particles’ formation mechanism and morphology depends on the relative amount of the depositing metal. In the deposition process, factors such as the depositing metal atoms/surface ratio must be taken into account to control the deposition process by one of the steps involved: diffusion of metal ions or epitaxial growth on the preformed metal particle. For Ag-coated Au mixed colloids, low Ag fractions lead to core-shell particles, in agreement with the data found in the literature.9 In contrast, higher Ag fractions lead to the formation of independent particles of each metal. In the preparation of Au-coated Ag colloids, a low Au fraction leads to the previous formation of individual particles of each metal, because of the small metal atoms/ surface ratio. Then, these particles migrate to the Ag surface and are immobilized on it. Only when the Au fraction is increased to 3% are actual Au-coated Ag particles obtained. At higher Au fractions (7%), the deposition mechanism seems to be a combination of the above processes. In general, the SERS activity of the mixed colloids is placed at a intermediate position between the activities of Ag and Au. The coverage of Au with Ag induces an increase of the enhancement factor corresponding to Au, although it does not reach the value corresponding to Ag. The Ag coverage by Au represents an improvement of the SERS activity of the deposited metal. In the last case, a high portion of the Ag is covered by Au, as indicated by the SERS spectrum profile of pyridine. Acknowledgment. This work has been supported by the Direccio´n General de Ensen˜anza Superior e Investigacio´n Cientı´fica (DGESIC) Project No. PB97-1221. We also acknowledge the Consejo Superior de Investigaciones Cientı´ficas for a contract to S. S.-C. LA000557S (35) Notholt, J.; Ludwig, P. K. Chem. Phys. Lett. 1989, 154, 101. (36) Regis, A.; Dumas, P.; Corset, J. Chem. Phys. Lett. 1984, 107, 502. (37) Fleischmann, M.; Hill, I. R. J. Electroanal. Chem. 1983, 146, 353.