NANO LETTERS
Synthesis of Bimetallic Colloids with Tailored Intermetallic Separation
2002 Vol. 2, No. 1 13-16
Martin Schierhorn† and Luis M. Liz-Marza´n* Departamento de Quı´mica Fı´sica, UniVersidade de Vigo, 36200 Vigo, Spain Received August 27, 2001; Revised Manuscript Received October 29, 2001
ABSTRACT The synthesis of bimetallic colloid concentric spheres is described through an example involving Au and Ag spaced by amorphous SiO2. The procedure involves a careful combination of existing methods for synthesis of Au nanoparticles, silica coating, and deposition of silver on silica. The choice of materials is motivated by their strong plasmon absorption bands, which allow us to follow the synthetic steps spectrophotometrically. TEM images demonstrate the concentric geometry of the particles prepared.
The enormous interest currently existing on metal nanoparticles is due to their unique optical properties, which arise from the collective oscillation of free conduction electrons due to the interaction with an electromagnetic radiation.1 When two metals are combined within a single nanoparticle (bimetallic nanoparticles) the optical properties are dictated by a combination of the properties (dielectric functions) of both metals. Such a combination strongly depends on the microscopic arrangement of the metals within the particle; i.e., whether an alloy, a perfect core-shell geometry or something between is obtained, but in any of these cases there is a direct interaction between the metals. Bimetallic particles of various combinations of metals have been studied by several groups, either with a focus on the study of the optical properties,2,3 or on their catalytic applications.4,5 The case of Au and Ag is specially tricky, because of their nearly identical lattice constants (0.408 for Au and 0.409 for Ag),6 which leads to a strong tendency toward alloy formation. Colloidal AgAu alloys were studied in detail by Papavassiliou7 and later by Link et al.,8 while studies of core-shell nanoparticles of the same system were performed by Morriss and Collins,9 who prepared spheres with a constant gold core diameter of 5.9 nm, with silver shells grown epitaxially and varying in thickness from 0.5 to 23.15 nm. Nanoparticles of gold encapsulated in silver were also generated chemically by Henglein et al.10 when Ag+ reoxidized the shell of Au@Pb and Au@Tl, thereby being reduced to Ag atoms, which were subsequently deposited on the same cores in the place of the less noble metals. However, Ag+ could not completely reoxidize the Pb shell of Au@Pb, as the adatoms in the innermost shell were strongly bonded to the Au atoms. * Corresponding author. E-mail:
[email protected]. † Permanent address: Department of Chemistry, The Pennsylvania State University, PA. 10.1021/nl015616g CCC: $22.00 Published on Web 12/05/2001
© 2002 American Chemical Society
As a general rule, it is very difficult to deposit a noble metal onto a less noble metal colloid particle. For example, it was observed that a Au3+ solution oxidized 10 nm colloidal silver, forming colloidal gold.11 To avoid the problem, in the case of Ag@Au Mulvaney et al.12 employed Au(CN)2(E° ) -0.61 V), which is less noble than Ag(CN)2- (E° ) -0.31 V) for the radiolytic deposition of gold onto silver seeds, observing a dramatic red shift toward the absorption band of pure Au particles when the more noble Au was deposited on Ag cores. The same problem was more recently successfully approached by Mirkin et al.13 by simultaneous treatment of a silver colloid with HAuCl4 and NaBH4, which allowed them to use the composite nanoparticles for DNA diagnostics. In this paper we present the first example of bimetallic colloids, where the metals involved are separated by an insulating shell, so that their properties can be tuned independently. This procedure has the advantage that the interactions between the metals can be fully suppressed. Additionally, the separation between the inner and the outer metal (which can be a continuous shell) can be easily tailored through the thickness of the silica layer, so that the resulting nanostructured particles appear promising for applications as nanocapacitors where the silica shell would determine the effective capacitance of the system. We have chosen as metals gold and silver because of the mentioned tendency to form alloys and because they have very distinctive, well separated plasmon absorption bands in the visible,14 which means that they can be easily monitored through UV-visible spectroscopy. However, since the silica coating procedure used has proven successful not only for metal but also for magnetic15,16 and semiconductor17 nanoparticles, a broad range of particles with combined properties can be prepared through a similar synthetic procedure. Other examples of interesting systems that can be prepared include colored
Figure 1. Representative transmission electron micrographs of silica-coated gold nanoparticles (core diameter 15 nm), before (a) and after one (b) and two (c) electroless plating experiments. The lower row shows the same system after one (d), two (e), and three (f) seeded growth steps by reduction with formaldehyde.
magnetic particles for magnetooptic devices or inks, or conducting magnetic particles. The approach used in this work to prepare onion-like bimetallic particles involves three consecutive steps: synthesis of Au nanoparticles, deposition of a silica shell of the desired thickness, and second coating with a metallic shell using electroless plating and subsequent seeded growth. The experimental details on the synthesis of monodisperse (15 ( 2 nm) Au nanoparticles in water and their homogeneous coating with silica have been extensively discussed elsewhere.18 The basic idea involves the modification of citratestabilized Au particles (15 nm av diameter, 0.5 mM) with a silane coupling agent ((aminopropyl)trimethoxysilane, APS, 0.05 mM), followed by slow silica deposition from a sodium silicate solution (0.02 wt %) at pH ∼ 8.0. After centrifugation (3500 rpm, 3 h) to remove excess silicate and redispersion in ethanol (50 mL), extensive growth of the silica shell is performed by addition of concentrated ammonia (29% NH4OH, 15 mL) and tetraethoxysilane (TES, daily additions of 0.5 mL), which hydrolyzes and condenses as silica on the existing particles in solution.19 After the total diameter of interest has been reached (12.5 mL for a total diameter of 120 nm), the core-shell particles can be centrifuged and redispersed in water. An example of 15 nm Au particles coated with 50 nm thick silica shells is shown in Figure 1a. The initial deposition of silver particles on the silica surface was performed following a recently developed procedure20 that makes use of the concept of electroless plating. This procedure comprises the adsorption of Sn2+ ions on the surface by mixing 1 mL of Au@SiO2 dispersion (0.7 wt %) in 50% (w/w) methanol/water with 9 mL of a solution containing SnCl2 (0.029 M) and CF3COOH (0.072 M) in 50% w/w methanol/water. After 45 min, the dispersion is centrifuged and the particles redispersed in 1 mL of water, followed by addition of 9 mL of ammonical AgNO3 solution, 14
so that the surface Sn2+ ions get oxidized into Sn4+, while Ag+ ions get reduced into metallic Ag, in the form of tiny silver nanoparticles that remain attached on the silica surface. The process can be repeated to increase the silver nanoparticle density on the surface, as shown in Figure 1. It is clear in Figure 1b,c that the Ag nanoparticles deposited initially are smaller than the Au cores, while in the second step the deposited particles are larger, so that it becomes more difficult to distinguish the cores. Once the silica surface has been covered with a high density of Ag nanoparticles, reduction of additional AgNO3 with formaldehyde in the presence of NH4OH leads to the formation of full, continuous shells, in agreement with ref 21. We have found that it is more advantageous to perform this procedure in steps, to prevent the formation of larger aggregates. More specifically, 0.5 mL of the colloid were added to 9.5 mL of a solution of AgNO3 (0.15 µM), and then 25 µL of formaldehyde was added, immediately followed by 25 µL of concentrated ammonia. A first addition of AgNO3, formaldehyde, and NH4OH leads to partial coverage of the surface (see Figure 1d), while after three successive additions basically all the particles are fully coated with a metallic silver shell (Figure 1f). This stepwise procedure permits us to analyze the change in the properties of the system along the successive steps, as well as to select the stage at which the procedure should be stopped to achieve the desired properties. The evolution of the UV-visible spectra during the several synthetic steps is shown in Figure 2. The upper plot contains the spectra during the successive deposition of silver nanoparticles on the silica surface, while the gold plasmon resonance is still visible. Curve 0 shows the plasmon absorption band of the core gold nanoparticles, which is slightly red-shifted with respect to the starting, aqueous colloid because of the refractive index increase due to silica deposition around the particles.18 Additionally, there is a clear Nano Lett., Vol. 2, No. 1, 2002
Figure 2. Spectral evolution during the deposition of silver on silica-coated gold spheres by electroless plating (top) and seeded growth (bottom). The labels indicate the number of deposition cycles for each procedure.
Figure 3. UV-visible spectra of Au-SiO2-Ag nanoparticle dispersions after the first electroless plating step. The thickness of the silica shells are 50 (solid line) and 10 nm (dashed line).
increase in absorbance at lower wavelengths, which arises from scattering due to the larger size after silica coating. Once the first deposition of silver particles has been performed, a second band shows up between 400 and 500 nm, due to the coupled silver plasmon band of an array of silver particles on the surface,22 while the position of the gold core plasmon band remains unaltered; i.e., there is no interaction at all between the inner and the outer metals due to the presence of the insulating silica layer. Further deposition promotes the growth of individual silver particles, as well as contact between them, so that new optical features develop, characteristic of arrays of strongly interacting particles such as a valley below 400 nm, with a minimum at ca. 325 nm, while there is a progressive absorbance increase at higher wavelengths.21 Nano Lett., Vol. 2, No. 1, 2002
The lower plot of Figure 2 contains the spectra after a full silver shell has formed, so that the whole optical behavior of the sample is dominated by the plasmon resonance of the silver shell, mainly because the relative amount of Ag per particle is much larger than that of Au. The optical properties of metallic shells have been studied in detail by Halas and co-workers,21,23 and interpreted on the basis of Mie theory. If we compare our spectra with those reported in ref 21 for silica spheres coated with silver, we find a good agreement with those corresponding to complete shells. This is supported by the TEM micrographs shown in Figure 1. A similar synthetic procedure was used with Au@SiO2 nanoparticles with thinner (10 nm) silica shells. Although it is experimentally very difficult to deposit full shells on such small nuclei with no aggregation (several nuclei were usually incorporated within single silver spheres), it is interesting to compare the spectra after the first electroless plating step. The spectra for thick and thin shells are shown in Figure 3, showing that in both cases the Au plasmon band remains basically centered at the same wavelength, the main difference being a lower intensity for the Ag plasmon band of the colloid with thinner silica shells, due to a lower silver concentration per gold core (less surface area is available for the plating process). This constitutes evidence that the proposed method for the preparation of bimetallic particles with no contact between metals can be tailored to the desired particle size and intermetallic separation. Thus, we have demonstrated here that it is possible to synthesize bimetallic colloid nanostructures by using silica as a spacer in order to prevent interparticle interactions. The stepwise deposition of silver on silica-coated gold spheres allows a fine-tuning of the structure by adjusting the core size and silica shell thickness, which promises a full range of interesting properties to be achieved. Further work is in progress to implement different functionalities by use of nanoparticles of various nature. Acknowledgment. We are indebted to J. Me´ndez and J. B. Rodrı´guez of the CACTI from Vigo University for assistance with TEM measurements. This work has been supported by the Spanish Ministerio de Educacio´n y Cultura (Project no. PB98-1088) and Xunta de Galicia (Project no. PGIDT00PXI30108PN). M.S. acknowledges financial support from The Pennsylvania State University for a stay at the University of Vigo. References (1) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: Berlin, 1995. (2) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (3) Henglein, A.; Holzwarth, A.; Mulvaney, P. J. Phys. Chem. 1992, 96, 8700. (4) Toshima, N.; Harada, M.; Yamazaki, K.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (5) Toshima, N.; Harada, M.; Yonezawa, T.; Kushihashi, K.; Asakura, K. J. Phys. Chem. 1991, 95, 7448. (6) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 1996. (7) Papavassiliou, G. C. Prog. Solid State Chem. 1980, 12, 185. 15
(8) Link, S.; Wang, Z. L.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 3529. (9) Morriss, R. H.; Collins, L. F. J. Chem. Phys. 1964, 41, 3357. (10) Henglein, F.; Henglein, A.; Mulvaney, P. Ber. Bunsen-Ges. Phys. Chem. 1994, 98, 180. (11) Ung, T.; Liz-Marza´n, L. M.; Mulvaney, P. Langmuir 1998, 14, 3740. (12) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (13) Cao, Y. W.; Jin, R.; Mirkin, C. A. J. Am. Chem. Soc. 2001, 123, 7961. (14) Mulvaney, P. Langmuir 1996, 12, 788. (15) Philipse, A. P.; van Bruggen, M. P. B.; Pathmamanoharan, C. Langmuir 1994, 10, 92. (16) Correa-Duarte, M. A.; Giersig, M.; Kotov, N. A.; Liz-Marza´n, L. M. Langmuir 1998, 14, 6430.
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(17) Correa-Duarte, M. A.; Giersig, M.; Liz-Marza´n, L. M. Chem. Phys. Lett. 1998, 286, 497. (18) Liz-Marza´n, L. M.; Giersig, M.; Mulvaney, P. Langmuir 1996, 12, 4329. (19) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (20) Kobayashi, Y.; Salgueirin˜o-Maceira, V.; Liz-Marza´n, L. M. Chem. Mater. 2001, 13, 1630. (21) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743. (22) Bright, R. M.; Musick, M. D.; Natan, M. J. Langmuir 1998, 14, 5695. (23) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243.
NL015616G
Nano Lett., Vol. 2, No. 1, 2002
