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Synthesis and Optical Properties of Three-Dimensional Porous Core-Shell Nanoarchitectures Li-Hua Qian,† Yi Ding,‡ Takeshi Fujita,† and Ming-Wei Chen*,† Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan, and Key Laboratory of Colloid and Interface Chemistry, Ministry of Education, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, PR China ReceiVed NoVember 20, 2007. In Final Form: January 18, 2008 Three-dimensional porous core-shell nanostructures consisting of gold skeletons and silver shells were fabricated by controllable electroless plating. Optical properties of the 3D nanocomposite with a heterogeneous interface exhibit a significant shell-thickness dependence. The porous core-shell structure with an optimized shell thickness of ∼3-5 nm exhibits a considerable improvement in surface-enhanced Raman scattering. This study has important implications in the functionalization of nanoporous metals by surface modification.
Introduction Nanoporous gold (NPG) fabricated by chemical/electrochemical dealloying has recently attracted a considerable amount of attention because of its large surface area, good electrical/thermal conductivity, and open porosity in three dimensions for a wide range of applications in actuation, sensing, catalysis, and biotechnology.1-9 Moreover, the bicontinuous porous structure with a large internal surface area is also an ideal 3D nanoarchitecture for fabricating porous nanocomposites with novel properties by surface decoration. Among the potential applications, NPG has recently been developed as a promising substrate for surface-enhanced Raman scattering (SERS).5-9 By optimizing the nanoporous structures, such as tailoring nanopore sizes and changing the surface roughness of gold ligaments, improved optical properties have been observed,5-9 which are comparable to those of the assembled nanostructures.10-13 In this letter, we report that the optical properties of NPG can be modified by surface decoration. Because silver commonly exhibits much better SERS enhancements than does gold in the visible region,14,15 we developed a porous core-shell nanostructure consisting of gold skeletons and silver shells and demonstrated that the optical * Author to whom correspondence should be addressed. E-mail: mwchen@ imr.tohoku.ac.jp. † Tohoku University. ‡ Shandong University. (1) Kramer, D.; Viswanath, R. N.; Weissmuller, J. Nano Lett. 2004, 4, 793. (2) Ding, Y.; Chen, M. W.; Erlebacher, J. J. Am. Chem. Soc. 2004, 126, 6876. (3) Liu, Z.; Searson, P. C. J. Phys. Chem. B 2006, 110, 4318. (4) Yu, F.; Ahl, S.; Caminade, A.; Majoral, J.; Knoll, W.; Erlebacher, J. Anal. Chem. 2006, 78, 7346. (5) Kucheyev, S. O.; Hayes, J. R.; Biener, J.; Huser, T.; Talley, C. E.; Hamza, A. V. Appl. Phys. Lett. 2006, 89, 53102. (6) Dixon, M. C.; Daniel, T. A.; Hieda, M.; Smilgies, D. M.; Chan, M. H. W.; Allara, D. L. Langmuir 2007, 23, 2414. (7) Nyce, G. W.; Hayes, J. R.; Hamza, A. V.; Satcher, J. H. Chem. Mater. 2007, 19, 344. (8) Qian, L. H.; Yan, X. Q.; Fujita, T.; Inoue, A.; Chen, M. W. Appl. Phys. Lett. 2007, 90, 153120. (9) Qian, L. H.; Chen, M. W. Appl. Phys. Lett. 2007, 91, 83105. (10) Tao, A.; Kim, F.; Hess, C.; Goldberger, J.; He, R. R.; Sun, Y. G.; Xia, Y. N.; Yang, P. D. Nano Lett. 2003, 3, 1229. (11) Wang, H.; Levin, C. S.; Halas, N. J. J. Am. Chem. Soc. 2005, 127, 14992. (12) Zhang, X. Y.; Zhao, J.; Whitney, A. V.; Elam, J. W.; Van Duyne, R. P. J. Am. Chem. Soc. 2006, 128, 10304. (13) Wang, H. H.; Liu, C. Y.; Wu, S. B.; Liu, N. W.; Peng, C. Y.; Chan, T. H.; Hsu, C. F.; Wang, J. K.; Wang, Y. L. AdV. Mater. 2006, 18, 491. (14) Surface-Enhanced Raman Scattering: Physics and Applications; Kneipp, K., Moskovits, M., Kneipp, H., Eds.; Springer: Berlin, 2006; Vol.103. (15) Hodak, J. H.; Martini, I.; Hartland, G. V. J. Phys. Chem. B 1998, 102, 6958.
properties of NPG can obviously be improved by surface modification. Experimental Section Commercial Ag65Au35 (atom %) films with a thickness of ∼100 nm were used to synthesize NPG by chemical etching in a 70% HNO3 aqueous solution at room temperature for 0.5 h, and the residual silver concentration was less than 3%.9,16 Silver dissolution resulted in the formation of NPG with a pore size of ∼30 nm. To plate silver onto the internal surface of NPG, we employed a modified electroless plating technique.17,18 The solution for silver plating contained 0.05 mol/L AgNO3, 2 mol/L NH3‚H2O, and 0.1 mol/L Na2EDTA‚2H2O. Hydrazine (N2H4) gas was used as a reducing agent, and silver loading was controlled by the plating time. The silver-plated nanoporous films were rinsed in pure water (18.2 MΩ‚cm) more than five times to remove the residual chemical substances. The clean samples were placed on copper grids for transmission electron microscope (TEM) characterization and onto glass slides for Raman and UV-vis spectroscopy experiments. A rhodamine 6G (R6G) aqueous solution and 1,4-benzenedithiol (1,4-BDT) dissolved in methanol were used to evaluate the SERS activities of the porous nanostructures. The porous films supported by glass slides were immersed in the solutions for 4 h to allow efficient absorption. The samples were rapidly rinsed with distilled water to remove the residual droplets on the sample surfaces and then dried in vacuum (10-5 Pa). Surface-enhanced Raman spectra were collected with a Ranishaw Raman microscope operating with a 514.5 nm Ar ion laser (beam size ∼1 µm).19
Results and Discussion Figure 1a shows the microstructure of as-prepared NPG with a pore size of ∼30 nm. Quantitative TEM tomography verified that the morphology and size of the gold ligaments and hollow channels are identical in 3D.20,21 The representative microstructure of silver-plated NPG is shown in Figure 1b,c, illustrating that the silver-plated films uniformly cover the internal surfaces of NPG to form a porous core-shell nanocomposite (Figure 1d). The core-shell structure can be identified from the mass contrast (16) Ding, Y.; Kim, Y. J.; Erlebacher, J. AdV. Mater. 2004, 16, 1897. (17) Ding, Y.; Erlebacher, J. J. Am. Chem. Soc. 2003, 125, 7772. (18) Ding, Y.; Mathur, A.; Chen, M. W.; Erlebacher, J. Angew. Chem., Int. Ed. 2005, 44, 4002. (19) Yan, X. Q.; Li, W. J.; Goto, T.; Chen, M. W. Appl. Phys. Lett. 2006, 88, 131905. (20) Fujita, T.; Qian, L. H.; Inoke, K.; Erlebacher, J.; Chen, M. W. Unpublished work. (21) Fujita, T.; Chen, M. W. Jpn. J. Appl. Phys. 2008, 47, 1161.
10.1021/la703621c CCC: $40.75 © 2008 American Chemical Society Published on Web 03/21/2008
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Figure 1. Representative TEM images of (a) as-dealloyed NPG, (b) silver-plated NPG with a plating time of 8 min, (c) a magnified micrograph of NPG silver plated for 8 min, and (d) a 3D image of the gold-silver core-shell nanostructure.
Figure 2. Amount of silver and normalized shell thickness vs deposition time.
difference between bright silver shells and dark gold cores (Figure 1c,d). The shell thickness is ∼3-5 nm for the sample that was plated for 8 min and varies slightly with pore size and gold ligament curvature. Silver content in the plated NPG samples, as determined by energy dispersive X-ray (EDX) analysis, increases gradually with plating time (Figure 2), which allows us to control the shell thickness precisely and thereby to specify the properties. The surface area of uncoated NPG determined by TEM tomography is about ∼5 m2/g. Therefore, the nominal thickness of the plated silver films (t) can be calculated by the following equation
Figure 3. (a) UV-vis absorption spectra of silver-plated NPG with different deposition times. The significant intensity reduction in the high-wavelength region around 470 nm takes place as a function of plating time. (b) Experimental spectrum of core-shell nanostructures silver plated for 16 min and deconvoluted by using multipeak fitting. (c) Plasmon resonance wavelengths of gold cores (λ1 and λ2) in the core-shell nanostructures vs shell thickness (t).
(1)
The estimated values, for example, that of the 8-minute sample (Figure 1c), are approximately consistent with TEM measurements. Heterogeneous core-shell nanostructures usually exhibit interesting optical properties that are different from their homogeneous counterparts.22-29 To investigate the effect of silver
where MAg is the weight percentage of silver in the porous composite determined by EDX analysis, FAg is the density of bulk silver, and S is the internal surface area of uncoated porous gold measured by TEM tomography. Figure 2 illustrates that the nominal thickness of the silver shell varies with the plating time.
(22) Freeman, R. G.; Hommer, M. B.; Grabar, K. C.; Jackson, M. A.; Natan, M. J. J. Phys. Chem. 1996, 100, 718. (23) Jackson, J. B.; Halas, N. J. J. Phys. Chem. B 2001, 105, 2743. (24) Jana, N. R. Analyst 2003, 128, 954. (25) Cao, L. Y.; Diao, P.; Tong, L. M.; Zhu, T.; Liu, Z. F. ChemPhysChem 2005, 6, 913. (26) Liu, M. Z.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882. (27) Link, S.; El-Sayed, M. A. J. Phys. Chem. B 1999, 103, 8410.
t)
MAg FAgS
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plating on the optical properties of NPG, we systematically collected the UV-vis absorption spectra of the as-dealloyed and silver-plated NPG samples (Figure 3a). For the uncoated NPG substrate, the adsorption band, similar to that of nanorods,26,27 has a wide plateau with two intrinsic peaks from ∼350 to ∼470 nm that result from transverse and longitudinal plasmon absorptions of gold ligaments. The width spanning two peaks is much smaller than that of gold nanorods as a result of the unique features of NPG that can be rationally described as a quasiperiodic network composed of randomly oriented metallic nanorods in 3D.20 Quantitative measurements suggest that the aspect ratio of gold ligaments in the nanoporous structure is smaller than 2, leading to much more closely spaced absorption bands of the longitudinal and transverse plasmon than those of gold nanorods.26,27 Obvious changes in the shape of surface plasmon resonance (SPR) spectra can be observed with plating time. The detectable shift of the plasmon resonance band at 470 nm takes place with silver plating (Figure 3c), and the peak shifts to ∼450 nm with the thickness of the silver shell being larger than 10 nm. In contrast, the SPR band at ∼350 nm does not shift with silver plating. However, when the plating time is longer than 8 min, a new peak at ∼328 nm emerges (Figure 3b) that originates from thick silver coatings. In comparison with the other core-shell nanostructures, the nanoporous gold-silver structures appear to have some unique optical properties.30-33 For example, the opposite shifts of gold plasmon bands, including transverse and longitudinal modes, are observed in gold-silver core-shell nanorods.33 However, only one SPR band from the gold core exhibits an obvious blue shift in porous gold-silver core-shell nanostructures, which is most likely associated with the structural features of the continuous gold skeletons and the curved silver films in 3D. Figure 4a displays the representative Raman spectra of R6G absorbed onto core-shell nanostructures with different shell thicknesses. Obvious improvement in the Raman band intensities can be observed with silver plating, and the intensities monotonically increase until ∼8 min. The enhancement of the sample plated for 8 min is about 2 times higher than that of the uncoated NPG. Because microstructural features (pores, gold cores, and silver shells) are much smaller than the laser beam size, reproducible Raman spectra can be uniformly acquired from the entire core-shell nanostructures with a small (∼5%) variation in intensity. Further increasing the plating time leads to an abrupt reduction of the SERS enhancements (Figure 4b). The Raman intensities of the sample that was silver plated for 32 min are about 1/20 of that of the 8 min one, which is even lower than that of uncoated NPG. It is known that the SERS effect of R6G on gold nanostructures contains an additional resonance enhancement. To characterize the SERS performance of the nanocomposites further, 1,4-BDT dissolved in methanol is used as the probe molecule. Figure 5 represents the typical SERS spectra of 1,4-BDT on the coreshell nanostructures with different shell thicknesses. Apparently, the uncoated NPG is inactive with respect to 1,4-BDT, and no detectable Raman scattering of the molecules can be found from the spectrum. However, the silver coating dramatically changes (28) Kumar, G. V. P.; Shruthi, S.; Vibha, B.; Reddy, B. A. A.; Kundu, T. K.; Narayana, C. J. Phys. Chem. C 2007, 111, 4388. (29) Pande, S.; Ghosh, S. K.; Praharaj, S.; Panigrahi, S.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806. (30) Jain, P. K.; El-Sayed, M. A. Nano Lett. 2007, 7, 2854. (31) Prodan, E.; Radloff, C.; Halas, N. J.; Nordlander, P. Science 2003, 302, 419. (32) Pande, S.; Ghosh, S. K.; Praharaj, S.; Panigrahi, S.; Basu, S.; Jana, S.; Pal, A.; Tsukuda, T.; Pal, T. J. Phys. Chem. C 2007, 111, 10806. (33) Liu, M. Z.; Guyot-Sionnest, P. J. Phys. Chem. B 2004, 108, 5882.
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Figure 4. (a) SERS spectra of silver-plated NPG with distinct deposition times. The probed solution is 10-7 mol/L R6G. (b) Deposition time dependence of the integrated intensities of the 1650 cm-1 Raman band.
the properties of the NPG, and Raman scattering corresponding to the vibrational modes of 1,4-BDT can be readily acquired from the porous core-shell nanostructure. Similar to the observations of R6G, the SERS enhancement of 1,4-BDT also exhibits a noticeable dependence of the silver shell thickness, and the strongest Raman enhancement is attained from the samples plated for 8-10 min. It is known that 1,4-BDT has different absorption behaviors on silver and gold.34 Two sulfur atoms of the molecule can be coordinated to the silver surface, whereas the benzene ring tends to maintain a flat geometry with respect to the surface. As for gold, the S-H bond is parallel to the gold surface, leading to the inactive SERS effect. Thus, the surface plating presented in this study paves the way to modify the chemical performance of nanoporous metals. On the basis of our experimental observations, the SERS improvement with silver plating can be explained by the fact that silver itself has a stronger SERS enhancement than gold in the visible region14,15 and thicker silver layers endow the porous nanostructure with more silver characteristics. However, the noticeable phenomenon of SERS improvement arising from the core-shell nanoarchitecture has a strong thickness dependence of the silver shells, which appears to be independent of the probe molecule species and result solely from the core-shell nanostructure.22-27 It has been reported that the core-shell (34) Joo, S. W.; Han, S. W.; Kim, K. J. Colloid Interface Sci. 2001, 240, 391.
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substrate.25,35 As an example, the SERS enhancement in goldcopper core-shell nanoparticles has been successfully evaluated with an assumption that the efficiencies of charge transfer between the different substrates and the adsorbates are comparable.25 In this work, we adopt the model developed in refs 25 and 35 to estimate the enhancement factor of the 3D gold-silver coreshell nanostructures following the equation
( )( )
IAg FAg ) IAu FAu
Figure 5. (a) SERS spectra of silver-plated NPG with distinct deposition times. (b) Deposition time dependence of the integrated intensities of the 1563 cm-1 Raman band. The probe is 10-4 mol/L 1,4-BDT dissolved in methanol.
nanostructure can change the frequency of the surface plasmon resonance (SPR) bands considerably by core-shell interaction, leading to an improved SERS enhancement.22,23 However, the absorption intensity around 514.5 nm (the wavelength of the laser applied in this work) in the UV-vis spectra of the silverplated NPG (Figure 3) does not exhibit a remarkable improvement with silver plating. Therefore, improved SERS enhancements and the strong thickness dependence cannot be well explained by the variation of SPR with the silver coating. According to the Raman enhancement mechanisms, chemical enhancement due to charge transfer between the adsorbate and SERS substrate may be an alternative explanation for the observed SERS improvement of the porous core-shell nanostructure. The chemical enhancement from distinct metallic substrates can be estimated according to the density of states in the metals and the overlap of energy levels between absorbed molecules and the
2
ωMK(Au)
ωMK(Ag)
2
(2)
where IAg and IAu are the intensities of Raman bands acquired from silver and gold substrates, FAg and FAu are the densities of states for silver and gold, and and ωMK is the frequency of the molecular transition between states M and K. The parameters used in eq 2 can be found in the literature,36,37 and the Raman enhancement factor of the gold-silver core-shell nanostructure with respect to the uncoated NPG is ∼1.8, which is fairly close to the experimental value shown in Figure 4. Therefore, the improved SERS enhancements of the porous nanocomposite most likely result from the increased chemical enhancement by the 3D core-shell nanostructure. On the basis of the calculations and the SPR measurements (Figure 3a), the strong dependence of the SERS enhancements on the thickness of the silver shells appears to result from the competition between the decay of the SPR absorption strength at 514.5 nm and the improvement of chemical enhancements with silver plating. When the plating time is shorter than 8 min, the absorption strength does not change very much (Figure 3a). The improved SERS enhancements mainly result from the increased chemical effects from silver coatings. The dramatic decrease in SERS intensities with plating time longer than 8 min can be explained by the abrupt reduction of the absorption strength around 514.5 nm in the UV-vis spectra (Figure 3a), which results in the partial loss of electromagnetic enhancements from gold ligaments and therefore a dramatic decrease in SERS intensities.
Conclusions We have successfully developed a porous core-shell nanostructure with open, bicontinuous porosity by plating silver onto the internal surfaces of NPG. The novel core-shell nanostructure exhibits improved SERS enhancements for both R6G and 1,4BDT molecules. This study has important implications in the functionalization of nanoporous metals by surface decoration. Acknowledgment. This research was sponsored by the Grantin-Aid for Exploratory Research, a Global COE for Materials Science, and a project of the World Premier International Research Center (WPI) for Atoms, Molecules and Materials, the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. Y.D. is grateful for support from the JSPS visiting scholar program, the National Science Foundation of China, the National 863 Project, and the 973 Program of China. LA703621C (35) Wang, X. Q.; Wen, H.; He, T. J.; Zuo, J.; Xu, C. Y.; Liu, F. C. Spectrochim. Acta, Part A 1997, 53, 2495. (36) Ashcroft, N. W.; Mermin, N. D. Solid State Phys. 1976, 38. (37) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001; p 808.
