© Copyright 2001 by the American Chemical Society
VOLUME 105, NUMBER 33, AUGUST 23, 2001
LETTERS Preparation of AucoreAgshell Nanorods and Characterization of Their Surface Plasmon Resonances Chil Seong Ah, Seung Do Hong, and Du-Jeon Jang* School of Chemistry and Molecular Engineering, Seoul National UniVersity, Seoul 151-742, Korea ReceiVed: April 11, 2001; In Final Form: June 27, 2001
The surface of gold nanorods is coated with thickness-controlled silver by reducing AgCl43- exclusively on the metallic surface to form AucoreAgshell nanorods and then restored by selectively removing the silver coating. AucoreAgshell nanorods show much sharper, stronger, and shorter-wavelengthed surface plasmon absorption than gold nanorods. However, the absorption wavelength is much longer than the one calculated for pure Ag nanorods. This means that the core-shell structure of AucoreAgshell nanorods must be explicitly taken into account in these calculations.
Introduction Nanostructured metals are of considerable interest not only scientifically but also industrially,1-7 for they can be used in such diverse applications as catalysts, single electron transistors, and photonic crystals. A variety of inventive techniques involving chemical, electrical, and optical processes have been employed to prepare monodispersive materials of various polyhedral nanoparticles, nanorods (NRs), and nanowires with controlling their sizes and shapes.8-10 Nanostructured metals of core-shells as well as alloys exhibit characteristic electronic, optical, and catalytic properties that are absent in the nanostructured materials of individual constituent metals, attracting a great deal of interest.5,11-13 Neither the synthesis of metallic core-shell NRs nor the surface plasmon absorption of pure silver NRs has been reported yet. Here we report for the first time that AucoreAgshell NRs can be prepared and that the surface plasmon absorption of silver NRs has been experimentally characterized. Experimental Section Initial gold NRs were prepared electrochemically using a simple three-electrode cell containing hexadecyltrimethyl* To whom correspondence should be addressed. Telephone: +82-2875-6624. Fax: +82-2-889-1568. E-mail:
[email protected].
ammonium bromide (hydrophilic cationic surfactant) and tetraoctylammonium bromide (hydrophobic cationic cosurfactant).10 AucoreAgshell NRs were prepared by reducing AgCl43-(aq)14 with NH2OH selectively on the surface of gold NRs. We could control the thickness of the silver shell by adjusting the concentrations of AgCl43- and NH2OH. Gold NRs were restored by removing the silver shell with HCl. All the colloids were under ultrasonification at ∼28 °C during preparation. Samples were contained in 2 mm quartz cells to take absorption spectra using a spectrophotometer (Sinco, UVS2040). A drop of colloidal solution was applied to a carbon-coated copper grid for transmission electron micrographic (TEM) examination using a microscope (JEOL, JEM2000). Results and Discussion The surface plasmon absorption spectrum of gold NR colloids increasingly shifts to the blue with AgCl43- reduction, finally becoming the spectrum of AucoreAgshell NR colloids (Figure 1a). Then it returns gradually to the spectrum of gold NR colloids with adding HCl, although the absorbance of the restored gold NR colloids, r, is significantly smaller than that of the initial gold NRs colloids, i (Figure 1b). The bleached absorbance of r, compared with the absorbance of i, suggests that HCl slowly
10.1021/jp0113578 CCC: $20.00 © 2001 American Chemical Society Published on Web 08/01/2001
7872 J. Phys. Chem. B, Vol. 105, No. 33, 2001
Letters TABLE 1: Average Length (l), Average Diameter (d), Average Aspect Ratio (R), Wavelength of the Maximum Transverse Absorption (λT), and Wavelength of the Maximum Longitudinal Absorption (λL) for the Colloids of Figures 2 and 3 figure
metal
2a, 3a 2b, 3b 2c, 3c 2d, 3d
Au AucoreAgshell AucoreAgshell Au
l d (nm) (nm) 45.0 56.5 58.0 50.3
10.3 22.5 25.0 11.2
R 4.45 2.53 2.33 4.59
λT λL λL,cala (nm) (nm) (nm) λL/λL,cal 518 404 405 517
814 681 572 826
814 494 464 829
1.00 1.38 1.23 1.00
a Calculated by fixing all the medium dielectric constants (m) as that (3.35) of Figure 2a and assuming that the AucoreAgshell NRs are pure Ag NRs.
Figure 1. Changes in the surface plasmon absorption of a solution with time (in min) after adding electrochemically prepared gold NR colloids (Cechem, 0.1 mL) into the mixture of AgCl-sat. 2.6 M NaCl (aq, 1 mL), H2O (1.4 mL), and 50% NH2OH (aq, 50 µL) (a) and after adding 35% HCl (aq, 0.3 mL) to the above final mixture at the moment giving the spectrum t (b).
Figure 3. TEM images of the samples described in Figure 2.
Figure 2. Surface plasmon absorption spectra of initial gold NRs (a), AucoreAgshell(thin) NRs (b), AucoreAgshell(thick) NRs (c), and restored gold NRs (d): (a) with Cechem diluted by 24 times 3 days after the preparation; (b) a day after adding Cechem (0.1 mL) into the mixture of AgCl-sat. 2.6 M NaCl (aq, 0.6 mL), H2O (1.8 mL), and 50% NH2OH (aq, 30 µL); (c) a few hours after adding Cechem (0.1 mL) into the mixture of AgCl-sat. 2.6 M NaCl (aq, 2 mL), H2O (0.4 mL), and 50% NH2OH (aq, 0.1 mL); (d) 1 h after adding 35% HCl (aq, 0.3 mL) into the sample (1.2 mL) used to take the spectrum c. Absorbances are normalized by the 24-times diluted Cechem concentration.
deteriorates gold NRs also after dissolving the silver coating of AucoreAgshell NRs. We can compare one another for the surface plasmon absorption spectra of initial gold NRs, thinly coated AucoreAgshell NRs, thickly coated AucoreAgshell NRs, and reconstructed gold NRs (Figure 2 and Table 1). Both transverse and longitudinal plasmon absorption bands shift extremely to the blue with silver coating, and then return to near their original positions when the silver coating is removed. It is interesting that the thickly coated core-shell NRs show the longitudinal surface plasmon absorption at a much shorter wavelength than the thinly coated NRs although both types show the transverse plasmon absorption at similar wavelengths. This can be understood with Figure 3. The R, ratio of length to diameter, of a thicker NR is smaller than that of a thinner one (Figure 3). As R decreases, λL is known to shift to the blue significantly, whereas λT shifts slightly to the red.10 As a gold NR is coated with silver, both the length and diameter of the NR become longer by the same amount so that R gets smaller. The high- and low-density regions of a NR in the TEM image of Figure 3b correspond to the gold core
and silver coat regions, respectively. Furthermore, Figure 2b shows neither gold plasmon absorption nor intermediate AuAg alloy absorption. This excludes the possibility of Au-Ag alloy NR formation. The dumbbell shapes in Figure 3c suggest that the edge {111} facet of a NR is more easily accessible than the side {110} facet.15 This may be useful for understanding the formation mechanism and capping-surfactant structure of metallic NRs. Figures 2d and 3d indicate that the silver coating can be successfully removed to restore gold NRs. Using an extension of the Mie theory,10,16-18 we have attempted to explore theoretically the relationship between aspect ratios and surface plasmon absorption maximum positions for gold and silver NRs. The results show that the strength of the longitudinal plasmon absorption decreases sharply for both types of gold and silver NRs as R decreases (Figure 4). However, the longitudinal absorption of the AucoreAgshell NRs with R ) 2.53 has almost the same strength as that of the gold NRs having the R of 4.45 (Figure 2). This suggests that the longitudinal, as well as transverse, plasmon absorption of a silver NR or a silvercoated gold NR is much stronger than the absorption of a gold NR having the same R. Our calculation shows that the sharper longitudinal absorption of a silver NR has stronger (by ∼5 times) peak absorption, compared with that of a gold NR having the same R. This signifies that silver and silver-coated NRs have good optical properties for applications. While the observed λL is the same as the calculated one for gold NRs, the observed λL for AucoreAgshell NRs is much longer than the one calculated for silver NRs. This suggests that the surface plasmon absorption of a nanothickly coated metal shell is affected by the core metal as well as by the surrounding media such as capping species and solvent molecules. This is not found in a thickly coated
Letters
J. Phys. Chem. B, Vol. 105, No. 33, 2001 7873 restored by removing the silver coating only. We have also shown that the thickness of the silver coating is controllable. While AucoreAgshell NRs are more stable, they have sharper, stronger, and shorter-wavelengthed longitudinal plasmon absorption than gold NRs. Their observed λL is much longer than the one calculated considering that the NRs are pure silver. Acknowledgment. The Korea Research Foundation (KRF2000-015-DP0193) supported this work. D.J.J. and C.S.A. also acknowledge the Center for Molecular Catalysis and the Brain Korea 21 Program, respectively.
Figure 4. Surface plasmon absorption spectra of silver and gold NRs having indicated aspect ratios, calculated using the m of 3.35 and the known complex dielectric constants of gold and silver.10,18
bulk object where the surface plasmon absorption is not affected by the core metal. It is reported that metastable gold NRs are able to form at ambient conditions as surfactant molecules surround the unstable {110} surface of the NRs.15 Regardless of reports on coreshell nanospheres,5,11,12 no report on core-shell metallic NRs is probable owing to difficulties in coating the surface with metals without damaging the structure of the capping micelle. We have overcome the problem by using a mild reductant, with which AgCl43- can be reduced only at the metallic surface. We have found that neither silver NRs nor silver nanodots form from AgCl43- in the absence of gold NRs. Because the unstable {110} facet is coated with silver, the core-shell NRs may be more stable than gold NRs. Our preliminary chemical examinations also show that AucoreAgshell NRs are actually more stable than gold NRs. We assert that the stability increase by coating unstable facets with another metal enhances the applicability of metallic NRs greatly. Conclusions The surface of gold NRs has been coated with silver by reducing AgCl43- exclusively on the metallic surface and then
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