J. Phys. Chem. B 2001, 105, 2743-2746
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Silver Nanoshells: Variations in Morphologies and Optical Properties J. B. Jackson and N. J. Halas* Rice Quantum Institute, Rice UniVersity, Houston, Texas ReceiVed: October 20, 2000; In Final Form: January 18, 2001
The production of silica core-silver shell nanoparticles (silver nanoshells) by three different chemical methods is described. Each method produces a silver outer layer with a unique morphology, ranging from smooth and uniform, to rough on the 2-5 nm length scale, to spiky with extremely sharp, needlelike protrusions extending outward from the surface of the nanoparticle. The nanoshells produced by each growth method were studied using transmission electron microscopy and UV-vis spectroscopy. In particular, one reduction method produced silver nanoshells with smooth and highly uniform morphologies whose optical extinction corresponded quantitatively to Mie scattering theory for a silica core-silver shell nanostructure.
Introduction Metal nanoshells are layered nanoparticles consisting of a dielectric (silica) core (40-250 nm radius) surrounded by a thin (10-30 nm) metallic shell. Metal nanoshells with shell layers consisting of metals with strong plasmon resonances exhibit a strong, plasmon-derived optical resonance, typically shifted to much longer wavelengths than the plasmon resonance of the corresponding solid metal nanosphere. The resonance frequency for a metal nanoshell is a sensitive function of the relative thicknesses of the core and shell layers.1 Nanoshell structures can currently be fabricated which exhibit this structural tunability of optical resonances from the visible into the infrared.2,3 The resonant frequencies and line widths of metal nanoshells have been shown to agree with Mie scattering theory to quantitative accuracy, provided changes in the dielectric function of the ultrathin metallic layer due to enhanced electron scattering are accounted for.1 Gold-silica nanoshells have recently been successfully fabricated and have been shown to be useful as band-pass optical filters, Raman enhancers,4 polymer oxidation quenchers,5 and drug delivery substrates.6 Because its plasmon resonance occurs at energies distinct from any bulk interband transition, silver colloid has a stronger and sharper plasmon resonance than that of gold.7 Also, the plasmon resonance of a solid silver nanosphere occurs at shorter wavelengths than the corresponding gold plasmon resonance, so the nanoshell geometry will allow for the shifting of the silver plasmon resonance across more of the visible spectral range. A direct comparison between the theoretical plasmon response of a gold and a silver nanoshell in water ( ) 1.77) of equal dimensions is shown in Figure 1. The dielectric constant for gold was taken from Johnson and Christy,8 and the dielectric for silver was taken from Hagemann et al.9 It is apparent that the plasmon-derived resonance of silver nanoshells is only slightly stronger (∼10%) than the corresponding gold nanoshell resonance, and that the silver nanoshell resonance appears at a shorter wavelength (∼100 nm in this example) than that of the analogous gold nanostructure. We also observe an enhanced contribution of the higher order multipole resonances in the silver nanoshell relative to the same size gold nanoshell structure. Several routes have been reported for the possibility of silver nanoshell fabrication onto latex substrates, including using silver
Figure 1. Comparison of the theoretically predicted extinction spectrum in water of a silver nanoshell with core radius of 50 nm and shell thickness of 10 nm (solid line) with that of a gold nanoshell of the same dimensions (dashed line).
nanoparticles as a precursor,10,11 silver ions as precursors,10,12 palladium as a precursor,10 and thermal evaporative techniques.13,14 The most promising (and successful) report11 describes several reduction techniques onto 450 nm diameter latex spheres. In this paper, we report three different chemical methods for producing core-shell nanoparticles by reducing a silver layer onto silica nanoparticle “substrates” to which a layer of gold nanoparticles have been bound. Since monodisperse silica nanoparticles can be synthesized over an extremely broad size range,18 this core provides a broad range of tunability for the resultant optical properties of the completed nanoshells.2-4 By varying the reductant and reaction conditions, we have found that the nanoscale morphologies of the resultant silver layers varied dramatically between methods. Spiky, rough, and smooth film morphologies were all observed and grown reproducibly. Each of these types of particles gave rise to unique optical signatures consistent with their observed variation in structure. In particular, one reduction method yielded uniform, smooth growth of an Ag layer, resulting in a nanoparticle whose optical response was quantitatively consistent with Mie scattering theory for a silica core-silver shell nanostructure. The observed variations in nanoscale morphology resulting from these various
10.1021/jp003868k CCC: $20.00 © 2001 American Chemical Society Published on Web 03/16/2001
2744 J. Phys. Chem. B, Vol. 105, No. 14, 2001
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growth methods may provide insight into the role of reducing agents, chemical stabilizers, and overall reaction kinetics in the deposition of ultrathin metal layers onto various substrates. Experimental Section Autometallography is the deposition, or “staining”, of silver on to immunogold for the purpose of locating tagged intracellular structures microscopically. Silver is reduced onto nanoscale gold-antibody conjugates too small to be optically resolved in order to view the location of the probes in a cell using optical microscopy. Various techniques from this field were explored for the fabrication of silver nanoshells. The methods of Danscher,15 Burry,16 and Zsigmondy17 were investigated for their applicability to growing silver nanoshells. The deposition substrates for all studies were silica nanoparticles which had been functionalized with small (1-3 nm) gold colloid. Monodisperse silica cores were grown using the Sto¨ber method.18 This method is known to yield solutions of monodisperse silica particles in the size range of 80-500 nm, where the particle size is dependent on relative reactant concentrations. The silica nanoparticle surfaces were then functionalized with 3-aminopropyltrimethoxysilane (APTMS). This reaction provides an amine-moiety coating for the exterior of the silica nanoparticles. The amine-functionalized silica particles are then added to a solution of ultrasmall gold colloid (1-3 nm), coating the silica nanoparticles with a surface coverage of nominally 25%.19 Small gold colloid was chosen instead of silver because of its simplicity and reliability of synthesis in this size regime. The gold colloid bonds stably to the amine-terminated surface and provides nucleation sites for the chemical deposition of silver. All concentrations were low to avoid any supersaturation effects. All bright field images were acquired using a JEOL JEM-2010 transmission electron microscope (TEM) operating at 200 kV. The UV-visible extinction spectra were obtained with a Hitachi U-2001 UV-visible scanning spectrophotometer within the range 190-1050 nm. Results and Discussion Method 1. This method was performed utilizing a deposition protocol previously reported by Dansher.15 Varying amounts of gold-decorated silica are added to 1.2 mL of Acacia (500 mg/L) with a 0.2 mL buffer solution (1.5 M citric acid, 0.5 M sodium citrate, pH ) 3.5) and 0.3 mL of silver lactate (37 mM in water). Then, 0.3 mL of hydroquinone (0.52 M in water) is added while being stirred vigorously. Hydroquinone is the reducing agent, while the citrate solution and Acacia stabilize the silver ions and slow the kinetics of the silver reduction. This results in the growth of needlelike silver “spikes” on to the silica nanoparticle surface, in addition to a deposition of Ag that coats the surface in a nonuniform manner. The reaction took approximately 30-45 s with no aggregation observed. A TEM image of a representative particle produced by this process is shown in Figure 2a. The optical extinction spectra of these nanoparticles for varying degrees of silver deposition are also shown in Figure 2b. In these broad and relatively featureless spectra, a very weak plasmon like feature appears to shift toward longer, then shorter wavelengths as the amount of silver deposited on the nanoparticle surface is increased. Although this spectral behavior is likely to be related to an increasing thickness of metal on the nanoparticle surface,2 the overall irregular morphology of the silver deposited on the nanoparticle surfaces by this method makes a comparison with Mie scattering theory intractable for these nanoparticles.
Figure 2. (a) TEM image of hydroquinone deposited silver onto 120 nm diameter silica particle (Method 1). (b) UV/Vis spectra of increasing deposited silver, where spectra 1-4 represent increasing amounts of silver deposition.
Method 2. This method is based on a previously reported recipe given by Burry.16 Gold-decorated silica is mixed with 2 mL of silver nitrate (0.17 mM) under vigorous stirring. This is followed by the addition of 100 µL of an n-propyl gallate (NPG) solution and 10 µL of NH4OH (4.7 mM). The NPG solution is prepared by the addition of 15 mg of NPG dissolved in 250 µL of ethanol and then diluted to a total volume of 5 mL with distilled water. Variations in the amount of silver nitrate available for deposition leads to particles with differing optical extinction spectra (Figure 3a). There was no aggregation observed, and the reaction took 30-60 s. The morphology of the deposition method in this case is quite “bumpy”, more characteristic of aggregated silver colloid attached to the nanoparticle surface than that of a continuous silver layer (Figure 3b). Indeed, the extinction spectra do not show even qualitative agreement with what would be anticipated for a nanoshell optical response. A competing reaction with this deposition is the formation of silver colloid in solution: preparation of these types of nanoparticles therefore requires the separation of the larger nanoparticles from the silver colloid by centrifugation. As more metal is deposited on the surface of the silica particle, the magnitude of the extinction spectrum increases and the plasmon resonance begins to shift to longer wavelengths (Figure 3a, spectra 3 and 4). Figure 3 also shows spectral evidence of the formation of larger silver colloid present in solution in the form of a shoulder located around 380 nm, also evident in the TEM images of the products of this reaction (not shown). The density of the larger silver colloid formed in this reaction is similar enough to the silver/silica nanoparticles formed that separation by centrifugation proved exceedingly difficult. Method 3. Smooth silver nanoshells can be made using a variation of the method reported by Zsigmondy.17 The gold-
Silver Nanoshells
J. Phys. Chem. B, Vol. 105, No. 14, 2001 2745
Figure 4. TEM image of decorated silica particle before (left) and after (right) the rapid pH change in the Method 3 silver deposition process.
Figure 5. UV/vis (dashed) and Mie scattering theory (solid) of spectra for various core and shell sizes grown by Method 3. The theoretical and experimental dimensions of the nanoshell samples to which these spectra correspond are displayed in Table 1.
Figure 3. (a) UV/vis spectra of silica particle as more silver is deposited using NPG as the reducing agent (Method 2). (b) TEM images corresponding to the silver silica particles in solution.
decorated silica particles are mixed with a 0.15 mM solution of fresh silver nitrate (AgNO3) and stirred vigorously. A small amount (typically 25-75 µL) of 37% formaldehyde is added to begin the reduction of the silver on to the gold particles on the surface of the silica particle. This step is followed by the addition of 50 µL of doubly distilled concentrated ammonium hydroxide (NH4OH). The NH4OH causes a rapid increase in the pH of the solution, resulting in the reduction of Ag+ and their deposition onto the nanoparticle surface forming a silver shell. This rapid pH change appears to result in preferential deposition onto the existing silver-coated nanoparticle surfaces rather than the nucleation of additional silver colloid. This also results in a minimal amount of silver colloid accompanying the silver nanoshells in solution. If small nanoparticles are formed in the solution, deposition of the nanoshell silver layer will continue at the expense of these particles. No aggregation was observed, and the reaction occurred almost instantaneously. It is likely that the particles are charge-stabilized in solution due to the high pH. A TEM image of a nanoparticle before and
after this rapid pH change is shown in Figure 4. This method produces smooth, complete silver nanoshells and allows for tunability of the plasmon resonance through the visible and into the infrared wavelengths. UV/visible extinction spectra for a few representative core/shell ratios are shown as broken lines in Figure 5. Theory The plasmon-derived extinction spectra of silver nanoshells can be compared to far-field extinction spectra calculated using Mie scattering theory,20 represented by the solid lines in Figure 5. Mie solved the problem of light scattering from a solid sphere, and Aden and Kerker21 expanded this solution for the case of a core-shell particle. The calculated spectra shown here follow a series solution developed by Sarkar.22 Modifications in the dielectric function of the metal due to increased electron scattering in the confined shell geometry are included in this calculation. These modifications are relevant for any metallic nanostructure with at least one spatial dimension smaller than the bulk electron mean free path in the metal: for silver, this is 55 nm. The modification depends on the bulk plasma frequency, bulk collisional frequency, and the Fermi velocity, which are calculated using Drude theory.23 The core and shell dimensions for silver nanoshells corresponding to four different sizes obtained experimentally from the TEM images of those nanostructures are compared to the dimensions used in the theoretical
2746 J. Phys. Chem. B, Vol. 105, No. 14, 2001
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TABLE 1: Mie Theory Calculated and Experimentally Measured (TEM) Core Shell Dimensions for the Silver Nanoshells Fabricated Using Method 3a
spectrum
calculated core (nm)
shell (nm)
total radius (nm)
experimental core (nm)
total radius (nm)
1 2 3 4
92 ( 4 49 ( 6 65 ( 5 42 ( 5
15 ( 2 16 ( 1 11 ( 1 18 ( 1
107 ( 6 65 ( 7 76 ( 6 60 ( 6
89 ( 7 49 ( 7 66 ( 7 42 ( 7
107 ( 8 65 ( 8 76 ( 8 60 ( 8
a There were approximately 200-300 particles measured of the silica substrates, all with less than 10% polydispersity, and 20-30 particles measured for the statistics of total size (core plus shell). All of the core size and shell thickness variations are assumed to be Gaussian.
spectra and are shown in Table 1. There were approximately 200-300 particles measured of the silica substrates, all with less than 10% polydispersity, and 20-30 particles measured for the statistics of total size (core plus shell). In the calculations, the core size and shell thickness variations are assumed to be Gaussian. Agreement between the theoretically and experimentally obtained dimensions for these nanostructures is excellent, which allows us to conclude that nanostructures grown by this method are indeed uniform, layered concentric sphere structures. The small discrepancies that occur between the calculated and the measured spectra could be due to the presence of small gold colloid in the nanoshell, nonuniform size distributions of the nanoshell, and nonspherical shape of the Sto¨ber particles. Conclusions The reduction of silver on the nanometer scale is strongly dependent on the chemical stabilizers and pH of the reaction involved. Chemical stabilizers added to prevent the selfnucleation of silver ions and slow the kinetics of silver reduction along with the overall pH of the solution dictate the morphology of the nanoscale surface. The Danscher method of silver reduction results in large spiked structures with broad plasmon resonances. The Burry method results in the appearance of large silver aggregates on the surface of the silica nanoparticle along with large silver colloid in solution that cannot be removed by conventional means. By performing a variation on the Zsigmondy method, smooth nanoshells can be produced. The key to this deposition is the rapid change in pH of the solution during the reduction process. The uniform silver layer grown by this
method results in a silica core-silver shell nanoparticle whose optical properties agree quantitatively with Mie scattering theory for this geometry. It is likely that these particles will prove to be useful in applications ranging from optical filters to surfaceenhanced Raman substrates. Acknowledgment. The authors gratefully acknowledge the National Science Foundation, the Office of Naval Research, the Robert A. Welch Foundation, and the National Aeronautics and Space Administration. References and Notes (1) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217. (2) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243-247. (3) Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Appl. Phys. Lett. 1999, 111, 2897. (4) Oldenburg, S. J.; Westcott, S. L.; Averitt, R. D.; Halas, N. J. J. Chem. Phys. 1999, 111, 4729. (5) Hale, G.; Halas, N. J. Appl. Phys. Lett., submitted for publication. (6) Sershen, S. R.; Westcott, S. L.; Halas, N. J.; West, J. L. J. Biomed. ical Mater. Res. 2000, 51, 293. (7) Kreibig, U.; Vollmer, M. Optical Properties of Metal Clusters; Springer: New York, 1995. (8) Johnson P. B.; Christy R. W. Phys. ReV. B 1972, 6, 4370. (9) Hagemann, H. J.; Gudat, W.; Kunz, C. J. Opt. Soc. Am. 1975, 65, 742. (10) Warshawsky, R.; Upson, D. A. J. Polym. Sci. A: Polym. Chem. 1989, 27, 2963. (11) Mayer, A. B. R.; Gregner, W.; Wannemacher, R. J. Phys. Chem. B. 2000, 104, 7278. (12) Tamai, H.; Sakurai, H.; Hirota, Y.; Nishiyama, F.; Yasuda, H. J. Appl. Polym. Sci. 1995, 56, 441. (13) Schueler, P. A.; Ives, J. T.; De La Croix, F.; Lacy, W. B.; Becker, P. A.; Li, J.; Caldwell, K. D.; Drake, B.; Harris, J. M. Anal. Chem. 1993, 65, 3177. (14) Moody, R. L.; Vo-Dinh, T.; Fletcher, W. H. Appl. Spectrosc. 1987, 41, 966. (15) Danscher, G. Histochem. 1981, 71, 177. (16) Burry, R. W.; Vandre, D. D.; Hayes, D. M. J. Histochem. Cytochem. 1992, 40, 1849. (17) Zsigmondy, R. Kolloidchemie I and II; Spamer: Leipzig, 1927. (18) Sto¨ber, W.; Fink, A.; Bohn, E. J. Colloid Interface Sci. 1968, 26, 62. (19) Westcott, S. L.; Oldenburg, S. J.; Lee, T. R.; Halas, N. J. Langmuir 1998, 14, 5396. (20) Mie, G. Ann. Phys. 1908, 24, 377. (21) Aden, A. L.; Kerker, M. J. App. Phys. 1951, 22, 1242. (22) Sarkar, D.; Halas, N. J. Phys. ReV. 1997, 56, 1102. (23) Ashcroft, N. W.; Mermin, N. D. Solid State Physics; Harcourt: New York, 1976.
