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Enhanced Optical Responses of Au@Pd Core/Shell Nanobars Ke Zhang,† Yanjuan Xiang,‡ Xiaochun Wu,*,† Lili Feng,† Weiwei He,† Jianbo Liu,† Weiya Zhou,‡ and Sishen Xie*,‡ National Center for Nanoscience and Technology, Beijing 100190, P. R. China, and Institute of Physics, National Laboratory for Condensed Matter Physics, Beijing 100190, P. R. China ReceiVed September 18, 2008. ReVised Manuscript ReceiVed NoVember 18, 2008 A Pd nanoshell was epitaxially grown on a Au nanorod (NR) via simple seed-mediated growth. Compared with the cylindrical shape of the Au NR, the Au core/Pd shell (Au@Pd) nanorods change to a rectangular shape due to the disappearance of {110} facets. The Au NRs exhibit a strong longitudinal surface plasmon resonance (LSPR). As Pd is deposited, damping and broadening occur to the LSPR band. Interestingly, the LSPR band maximum first shows a small red-shift (ca. 40 nm) which then is followed by a blue-shift as the amount of Pd is increased. A thicknessdependent LSPR feature of the Pd shell is believed to contribute to the shift. At a thinner Pd thickness, the Au@Pd nanobars exhibit a well-defined LSPR band in the visible and near-infrared region, which demonstrates a higher dielectric sensitivity than that of the corresponding Au NRs. It thus opens up the potential of Pd nanostructures for SPR-based sensing. Investigations on the surface-enhanced Raman scattering (SERS) indicate that the SERS activities of the Au@Pd nanobars at thicknesses smaller than 2.5 nm mainly originate from the Au cores; thus, the SERS activities can be improved by tuning the aspect ratio of the Au core and/or the Pd shell thickness.
* To whom correspondence should be addressed. E-mail:
[email protected]. † National Center for Nanoscience and Technology. ‡ National Laboratory for Condensed Matter Physics.
can be formed through a galvanic replacement between Ag and Na2PdCl4. The alloy nanoboxes show a SPR maximum around 732 nm at the Pd/Ag ratio of 0.69.4a Recently, the same group demonstrated that by introducing Na2PdCl4 and HAuCl4 successively to the suspension of Ag nanocubes, nanocages with a high porosity are formed and show a further red-shifted SPR peak up to 850 nm.4b Although the SPR maximum of Pd nanostructues has been successfully pushed into the visible and near-infrared region by the above designs, the bandwidth is still quite broad, making exploration of SPR-based sensing difficult. Different from the SPR properties, the research activities related to the SERS of Pt and Pd have a relatively long history because SERS can provide the composition and structure information of reactive species and intermediates on the surface of the catalyst. Great efforts therefore have been made to improve the SERS response of Pd and Pt because of the low SERS activities themselves.5 Up to now, two strategies have been demonstrated to be effective. One is the deposition of an ultrathin (three to five monolayers) Pt (or Pd) film on a roughened Au electrode, thus “borrowing” the enhancement from underneath Au.6 The key point in this method is the preparation of a pinhole-free overlayer in order to avoid the interference from the Au. The other way is to electrochemically roughen a Pt or Pd electrode.7 One big drawback in the roughened electrodes is that they are, in general, highly random with ill-defined morphology, thus making the optimization of SERS activities difficult. Therefore, electrodes composed of metal nanoparticles are used to replace planar electrodes.8-10 Using a 55 nm Au nanoparticle as the core and Pt as the shell and prepared via the combination of underpotential
(1) (a) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (b) Hu, M.; Chen, J. Y.; Li, Z. Y.; Au, L.; Hartland, G. V.; Li, X. D.; Marquez, M.; Xia, Y. N. Chem. Soc. ReV. 2006, 35, 1084. (c) Lee, K. S.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 19220. (d) Jackson, J. B.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2004, 101, 17930. (2) (a) Sun, Y.; Tao, Z.; Chen, J.; Herricks, T.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940. (b) Nishihata, Y.; Mizuki, J.; Ako, T.; Tanaka, H.; Uenishi, M.; Kimura, M.; Okamoto, T.; Hamada, N. Nature 2002, 418, 164. (3) (a) Xiong, Y. J.; Chen, J. Y.; Wiley, B.; Xia, Y. N.; Yin, Y. D.; Li, Z. Y. Nano Lett. 2005, 5, 1237. (b) Xiong, Y. J.; Wiley, B.; Chen, J. Y.; Li, Z. Y.; Yin, Y. D.; Xia, Y. N. Angew. Chem., Int. Ed. 2005, 44, 7913. (4) (a) Chen, J. Y.; Wiley, B.; McLellan, J.; Xiong, Y. J.; Li, Z. Y.; Xia, Y. N. Nano Lett. 2005, 5, 2058. (b) Cobley, C. M.; Campbell, D. J.; Xia, Y. N. AdV. Mater. 2008, 20, 748.
(5) (a) Weaver, M. J.; Zou, S.; Chan, H. Y. H. Anal. Chem. 2000, 72, 38A. (b) Tian, Z. Q.; Ren, B. Annu. ReV. Phys. Chem. 2004, 55, 197. (6) (a) Zou, S.; Williams, C. T.; Chen, E. K. Y.; Weaver, M. J. J. Am. Chem. Soc. 1998, 120, 3811. (b) Zou, S.; Weaver, M. J. Anal. Chem. 1998, 70, 2387. (c) Mrozek, M. F.; Xie, Y.; Weaver, M. J. Anal. Chem. 2001, 73, 5953. (7) (a) Bilmes, S. A.; Rubim, J. C.; Otto, A.; Arvia, A. J. Chem. Phys. Lett. 1989, 159, 89. (b) Tian, Z. Q.; Ren, B.; Wu, D. Y. J. Phys. Chem. B 1997, 101, 1338. (c) Ren, B.; Lin, X. F.; Yang, Z. L.; Liu, G. K.; Aroca, R. F.; Mao, B. W.; Tian, Z. Q. J. Am. Chem. Soc. 2003, 125, 9598. (8) (a) Park, S.; Yang, P. T. X.; Corredor, P.; Weaver, M. J. J. Am. Chem. Soc. 2002, 124, 2428. (b) Zhang, B.; Li, J. F.; Zhong, Q. L.; Ren, B.; Tian, Z. Q.; Zou, S. Z. Langmuir 2005, 21, 7449. (c) Lu, L. H.; Sun, G. Y.; Zhang, H. J.; Wang, H. S.; Xi, S. Q.; Hu, J. Q.; Tian, Z. Q.; Chen, R. J. Mater. Chem. 2004, 14, 1005.
Introduction Noble metal nanoparticles, because of their size-, shape-, composition-, and structure-dependent surface plasmon resonance (SPR) features, have great potential in various applications such as imaging, sensing, and surface-enhanced Raman scattering (SERS).1 In comparison with Au and Ag, the SPR properties of Pd and Pt nanoparticles remain largely unexplored because of a lack of SPR features in the visible spectral region. It is wellknown that Pd is a very important catalyst in many industrial applications. For example, it plays a central role in hydrogen storage and serves as the main catalyst for the reduction of pollutant gases emitted from automobiles.2 It is therefore very intriguing to fabricate SPR-based sensors that monitor these catalytic processes. Recently, with the progress on the controlled synthesis of Pd nanostructures, their SPR bands have been pushed into the visible spectral region.3 For instance, Xia’s group have demonstrated that by forming Pd nanocubes with a size of ∼50 nm, their SPR peak could be tuned to nearly the same location (∼400 nm) as that of Ag nanoparticles.3a Moreover, a transformation from nanocubes to nanocages, via a corrosive pitting process, further red-shifted the SPR maximum to 520 nm.3b Apart from controlling the size and shape of Pd itself, doping with metals owning strong SPR features has been exhibited as another effective way to improve SPR responses.4 For example, using Ag nanocubes as a sacrificial template, Ag/Pd alloy nanoboxes
10.1021/la803060p CCC: $40.75 2009 American Chemical Society Published on Web 12/17/2008
Enhanced Optical Responses of Au@Pd Core/Shell Nanobars
deposition (upd) and spontaneous redox replacement, Park et al. observed about 10-fold higher SERS intensities than those of the corresponding Pt overlayers on planar gold electrodes.8a Considering the complexity of the above shell formation, Tian’s group employed seed-mediated growth to form the Pt (or Pd) shell.8b,c Apart from avoiding the roughening process, nanoparticle electrodes present more feasibility for tuning core size and/or shell thickness. In the case of pure Pt and Pd nanoparticles, great progress has been achieved in their shape-controlled synthesis using wet chemical methods.11 McLellan et al. investigated the SERS activities of films formed by Pd nanoparticles with different sizes (8-50 nm) and morphologies (solid cubes, hollow boxes, and porous cages).11 At optimized conditions, an enhancement factor up to 104 has been reported. In addition, Pt and Pd surfaces with a controlled structure present another way to fabricate SERSactive substrates. For example, Abdelsalam et al. fabricated a structured Pd and Pt film via a templated electrodeposition technique. By tuning the template sphere diameter and the film thickness, the SPR features can be tailored. For such films, they observed a maximum enhancement of 1800 for Pd films.12 Up to now, SERS investigations for Pd nanoparticles have been mainly carried out on the aggregates of the Pd nanoparticles, where a strong coupling of plasmon resonances is known to greatly improve SERS activities. In order to explore the relation between SPR feature and SERS enhancement from nanoparticles alone, systems containing isolated nanoparticles should be investigated. Recently, single crystalline Au NRs with aspect ratios (AR) between 2 and 4 have been synthesized with high quality.13 They show strong and tunable longitudinal SPR (LSPR) bands and enhanced SERS due to enhanced LSPR.14 It is thus desirable to form a Pd shell on the Au NR to improve the optical response of Pd. Using Au NRs as templates, Au@Pd nanorods with homogeneously coated Pd nanodots of about 2-4 nm diameter have been reported.15 The large lattice mismatch between Pd (0.389 nm) and Au (0.408 nm) is believed to induce the random nucleation of the Pd nanodots and thus a polycrystalline structure. In addition, a featureless extinction spectrum makes exploration of the SPR-related phenomena impossible. Recently, by finely tuning the deposition parameters, we have succeeded in creating an epitaxial growth condition for Pd on the Au rod, thus obtaining Au@Pd nanorods with a single crystalline structure.16 The Au NR seeds, whose side facets are composed of four {110} facets and four {100} facets, have a cylindrical shape. During the growth of the Pd shell, competition growth between the {110} and {100} side facets leads to disappearance of {110} facets and thus results in a Pd shell with four {100} side facets. In comparison with the cylindrical shape of the Au NRs, the Au@Pd nanorods have a rectangular shape. We therefore name them as nanobars. For the (9) (a) Hu, J. W.; Zhang, Y.; Li, J. F.; Liu, Z.; Ren, B.; Sun, S. G.; Tian, Z. Q.; Lian, T. Chem. Phys. Lett. 2005, 408, 354. (b) Pergolese, B.; Bigotto, A.; MunizMiranda, M.; Sbrana, G. Appl. Spectrosc. 2005, 59, 194. (c) Hu, J. W.; Zhang, Y.; Li, J. F.; Ren, B.; Sun, S. G.; Tian, Z. Q. J. Phys. Chem. C 2007, 111, 1105. (10) (a) Muniz-Miranda, M.; Pergolese, B.; Bigotto, A. Chem. Phys. Lett. 2006, 423, 35. (b) Muniz-Miranda, M.; Pergolese, B.; Bigotto, A. J. Phys. Chem. C 2008, 112, 6988. (11) McLellan, J. M.; Xiong, Y. J.; Xia, Y. N.; Hu, M. Chem. Phys. Lett. 2006, 417, 230. (12) Abdelsalam, M. E.; Mahajan, S.; Bartleet, P. N.; Baumberg, J. J.; Russell, A. E. J. Am. Chem. Soc. 2007, 129, 7399. (13) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (b) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (c) Gou, L. F.; Murphy, C. J. Chem. Mater. 2005, 17, 3668. (14) (a) Nikoobakht, B.; Wang, J. P.; El-Sayed, M. A. Chem. Phys. Lett. 2002, 366, 17. (b) Orendorff, C. J.; Gole, A.; Sau, T. K.; Murphy, C. J. Anal. Chem. 2005, 77, 3261. (c) Orendorff, C. J.; Gearheart, L.; Jana, N. R.; Murphy, C. J. Phys. Chem. Chem. Phys. 2006, 8, 165. (15) Song, J. H.; Kim, F.; Kim, D.; Yang, P. D. Chem. Eur. J. 2005, 11, 910. (16) Xiang, Y. J.; Wu, X. C.; Liu, D. F.; Jiang, X. Y.; Chu, W. G.; Li, Z. Y.; Ma, Y.; Zhou, W. Y.; Xie, S. S. Nano Lett. 2006, 6, 2290.
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Au@Pd nanobars, tunable LSPR bands can be achieved in the visible and near-infrared spectral region by varying the thickness of the shell. It thus opens a way to investigate their SPR-related properties. Previously, because of the lack of an obvious SPR band in the visible region, the correlation of the electromagnetic enhancement (EM) mechanism to the SPR features has not been explored for Pd nanostructures. In our case, a well-defined and tunable SPR band makes study of this correlation possible. In this article, we address the following three issues for the Au@Pd nanobars: (1) The tunability of the SPR band, (2) the dielectric sensitivity of the SPR band, and (3) the SERS activities of the nanobar ensembles in solution phase so that the enhancement due to the aggregation has been excluded.
Experimental Section Sodium borohydride (NaBH4), hydrogen tetrachloroaurate (III) trihydrate (HAuCl4 · 3H2O), cetyltrimethylammonium bromide (CTAB), poly(sodium p-styrensulfonate) (PSS, mw 70 000), silver nitrate (AgNO3), L-ascorbic acid (AA), and palladium chloride (PdCl2) were obtained from Alfa and used as received. Rhodamine 6G was bought from Sinopharm Chemical Reagent Co., Ltd. Milli-Q water (18MΩ.cm) was used for all solution preparations. The glassware was cleaned in a bath of a piranha solution (H2SO4/30% H2O2 ) 7: 3 v/v) and boiled for 30 min. Preparation of Au NRs. CTAB-capped Au NRs were prepared using a seed-mediated growth method.13 Briefly, CTAB-capped Au seeds were synthesized by chemical reduction of HAuCl4 with NaBH4: 7.5 mL of 0.1 M CTAB were mixed with 250 µL of 10 mM HAuCl4, the mixture was diluted to 9.4 mL, and 0.6 mL of ice-cold sodium borohydride aqueous solution (0.01 M) was added to 9.4 mL of a mixed solution of chlorauric acid and CTAB while stirring magnetically. The seeds were used within 2-5 h. Growth solution typically used for the growth of Au nanorods consisted of 100 mL of 0.1 M CTAB, 5 mL of 0.01 M HAuCl4, 1 mL of 10 mM AgNO3, 2 mL of 0.5 M H2SO4, and 800 µL of 0.1 M AA, and then 240 µL of seeds were added to the growth solution. After the preparation, excess surfactants were removed by centrifuging the solution at 8000 rpm for 10 min. The precipitation was redispersed in deionized water to keep the original volume. Synthesis of Au@Pd Nanobars with Different Shell Thickness. H2PdCl4 aqueous solution was prepared from PdCl2 (0.035 g), dissolved in HCl (2 mL, 0.2 M), and diluted to 100 mL with deionized water. Au NRs were employed as seeds to initiate the deposition of Pd shell. Typically, a suspension of 2 mL of Au nanorod was mixed with 600 µL of CTAB (0.1 M). To this solution, a fixed volume of 2 mM H2PdCl4 and a 10-fold excess of AA (0.1M) was added and followed by quickly shaking. The final volume was adjusted to 6 mL with water. By changing the volume (20-400 µL) of added H2PdCl4, different shell thicknesses can be obtained. The reaction took place in several minutes accompanied by a color change from brown to gray and finished after several hours. Synthesis of Au NRs with Different Aspect Ratios and Au@Pd Nanobars with Different Au Cores. The aspect ratio of the Au NRs can be tuned simply by changing the concentration of silver ions in the growth solution. Therefore, we distinguish the different Au NRs by the concentration of silver ions used in the growth solution. For example, in the above case, when the concentration of silver ions in the growth solution is ca.100 µM, we name the obtained Au NRs as Au100. Similarly, for silver concentrations of 50, 75, and 150 µM, the corresponding Au NRs are named as Au50, Au75, and Au 150, respectively. For the formation of Au@Pd nanobars on different Au cores, 80 µL of 2 mM H2PdCl4 was added. Modification of the Au NRs and the Au@Pd Nanobars with PSS. To investigate the dielectric sensitivity of the LSPR band, the mixtures of water and DMSO in different volume ratios were used to fabricate solvents with different refractive indices. The refractive indices of water, DMSO, and water/DMSO mixtures were measured with an Abbe refractormeter. Because of aggregation of CTABcapped Au NRs in the mixtures of water and DMSO, PSS-modified
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Figure 1. UV-vis-NIR absorption spectra of the Au NR seeds (dashed line) and Au@Pd nanobars with calculated Pd/Au molar ratios from 0.04 to 0.8 (solid lines). The arrow direction indicates the gradually enhanced damping of the LSPR band of the Au core with increasing Pd/Au ratios. Inset shows the maximum of LSPR band (left) and aspect ratio (right) versus the calculated Pd/Au ratio. The aspect ratio is averaged from 100 nanorods/nanobars using TEM images. The error bar represents standard deviation.
Au NRs were employed for dielectric sensitivity measurements. The preparation of PSS-modified Au NRs is as follows: 12 mL of CTAB-coated nanorod solution (either Au NRs or Au@Pd nanobars) was centrifuged at 12 000 rpm for 10 min, and the precipitate was dispersed in 12 mL of 2 mg/mL PSS aqueous solution (containing 6 mM NaCl). Then the solution was stirred magnetically for 3 h. After that, it was centrifuged at 12 000 rpm for 10 min, and the precipitate was redispersed in water, dimethyl sulfoxide (DMSO), or the mixtures of water/DMSO with different volume ratios. Instruments. UV-visible absorption spectra were recorded from Perkin-Elmer Lamdba 950. For transmission electron microscopy (TEM, Tecnai G2 20 S-TWIN) and scanning electron microscopy (SEM, Hitachi S-4800) measurements, one more centrifugation (12 000 rpm, 10 min) was applied. Raman spectroscopic measurements were obtained in the solution phase using a Renishaw InVia Raman microscope. The excitation lines are 785 nm with a power of 5.6 mW and 632.8 nm with a power of 10.4 mW, respectively. The measurement results were averaged from four measurements.
Results and Discussion Tunable LSPR Bands of the Au@Pd Nanobars by the Variation of Pd/Au Ratios. Figure 1 presents UV-vis-NIR absorption spectra of the Au NR seeds and the Au@Pd nanobars with different shell thicknesses. In order to compare the effect of Pd amount on the LSPR feature of the Au NRs, each spectrum was normalized by its absorption at 400 nm. The Au NRs show a strong LSPR band centered at 846 nm with a full width at half-maximum (fwhm) of 0.34 eV and a weak transverse SPR band around 512 nm. As the amount of deposited Pd is increased, the intensity of the LSPR band continuously decreases, accompanying a gradual broadening in width due to the increased damping of Pd. At the Pd/Au ratio of 0.16 (Au@Pd 80), the Au@Pd nanobars exhibit a well-defined LSPR band with a medium reduction in intensity and a slight broadening in width (fwhm of 0.45 eV). When the Pd/Au ratio reaches 0.8 (Au@Pd 400), the LSPR band is strongly damped and severely broadened. Different from the variation in intensity and width, the shift in LSPR band exhibits an interesting behavior: it first shows a red-shift that is then followed by a blue-shift with increasing Pd shell thickness. This trend can be seen more clearly in Figure 1 inset (left). At Pd/Au ratios less than 0.12, the LSPR band is gradually red-shifted to longer
Zhang et al.
wavelengths. At the Pd/Au ratio of 0.08, the maximum red-shift (42 nm) is achieved. As the Pd/Au ratio is increased, the LSPR band is blue-shifted back. It shifts to 740 nm at the Pd/Au ratio of 0.8. As already known for monometallic nanorods such as Au and Ag, the shift of the LSPR bands is mainly determined by AR. Increasing AR leads to a red-shift of the LSPR band. Recently, Grzelczak et al. have synthesized Au@Pt bimetallic nanorods and observed a red-shift of LSPR band in comparison with the Au core.17 They ascribed the red-shift to the increased AR. In our case, however, the overall AR decreases with increasing thickness of the Pd shell (Figure 1 inset). The Au NRs have an average AR of 4.0 ( 0.4. For the Au@Pd nanobars with a Pd/Au ratio of 0.16, the LSPR is red-shifted to 881 nm. Its average AR is 3.9 ( 0.5. The change in AR is quite small and cannot induce such an obvious red-shift. Therefore, there exist other factors that can induce a red-shift of the LSPR band. Different from monometallic nanorods, herein we have bimetallic nanorods with a core/shell structure. Therefore, both the core and the shell contribute to the final SPR features of the nanorods. For the core/shell structure, the SPR properties with a spherical shape have been investigated.18 For metals with a well-defined SPR band, formation of a core/shell structure results in two distinct SPR features (one from the core metal and the other from shell metal) in the extinction spectrum, whose relative intensities depend on the shell thickness. For example, for Au@Ag nanoparticles, two bands at 400 and 510 nm were observed in the extinction spectra.As Ag thickness is increased, the band at 400 nm gradually increases whereas the band at 510 nm gradually blue-shifts and weakens (relative to the 400 nm band).18c In the case of spherical Au@Pd nanoparticles, as Pd nanoparticles show no observed SPR features in the visible region, the extinction spectrum mainly shows one SPR band from Au. By depositing Pd, the damping and gradual blue-shift of SPR band of the Au core were often observed.9c Different from spherical particles, because of the high sensitivity of SPR maximum on shape, a geometry-dependent SPR for Pd shell should be introduced: a thickness-dependent LSPR for the Pd shell can explain the observed spectral features. For the Pd nanocages with a diameter of 48 nm and a wall thickness of 6 nm, a broad SPR band centered at 520 nm has been found experimentally and verified by theoretical calculation.3b The SPR position shows a high sensitivity to the wall thickness. By decreasing the wall thickness to 3 nm, theoretical calculation indicates that the SPR position can be red-shifted to 870 nm.3b Herein, we have nanoboxes rather than nanocages, thus less red-shift. However, at Pd/Au ratios lower than 0.24 the calculated shell thickness is less than 0.8 nm. Thus, the LSPR position of the Pd shell can locate at wavelength longer than the LSPR maximum of the Au cores. The combination of the Pd shell and the Au core could lead to a net red-shift. The magnitude of red-shift not only depends on the shell thickness but also on the percentage of Pd in the Au@Pd nanobar. For example, at Pd/Au ratio of 0.04, although the LSPR maximum of the Pd shell itself should be red-shifted more than that with the Pd/Au ratio of 0.12, the low percentage of Pd leads to a small contribution of the Pd shell to the overall LSPR feature of the core/shell structure. Thus, it shows less red-shift compared with the nanobars with the Pd/Au ratio of 0.12. For the Au@Pd nanobars at a Pd/Au ratio of 0.8, obviously, the LSPR features of the Pd shell dominate the extinction spectrum of the core/shell (17) Grzelczak, M.; Perez-Juste, J.; De Abajo, F. J. G.; Liz-Marzan, L. M. J. Phys. Chem. Phys. 2006, 8, 165. (18) (a) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061. (b) Toshima, N.; Harada, H.; Yamazaki, Y.; Asakura, K. J. Phys. Chem. 1992, 96, 9927. (c) Kim, Y.; Johnson, R. C.; Li, J.; Hupp, J. T.; Schatz, G. C. Chem. Phys. Lett. 2002, 352, 421.
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Figure 2. UV-vis-NIR absorption spectra of the Au 100@Pd 80 nanobars in water, DMSO, and the mixtures of water and DMSO at different volume ratios. Inset: Plot depicting the linear relationship between the solvent refractive index and λLSPR.
Figure 4. (a) UV-vis-NIR absorption spectra of the Au NRs with different aspect ratios (dashed lines) and the corresponding Au@Pd 80 nanobars (solid lines). (b) Comparison of refractive index sensitivity for the Au NRs and the corresponding Au@Pd 80 nanobars. Figure 3. UV-vis-NIR absorption spectra of Au 100 NRs in water, DMSO, and the mixtures of water and DMSO at different volume ratios. Inset: Plot depicting the linear relationship between the solvent refractive index and λLSPR.
structure. Therefore, for the bimetallic nanorods with a core/ shell structure, tuning shell thickness presents another effective way to tailor the SPR properties. Increased Dielectric Sensitivity of the Au@Pd Nanobars. To investigate the dielectric sensitivity, the absorption spectra of the Au@Pd nanobars were collected in water, DMSO, and the mixtures of water/DMSO at different volume ratios (Figure 2). The spectra in different solvents show no obvious change in the width of the LSPR band, indicating that individual nanobars remain. As expected, the LSPR peak red-shifts with an increase in refractive index of the solvent. A good linear relationship is achieved, with a slope of 439 nm RIU-1. For comparison, the results of the corresponding Au NR cores are also demonstrated in Figure 3. For the Au cores, the dielectric sensitivity is 360 nm RIU-1 and is lower than that of the Au@Pd nanobars. In order to obtain more information concerning the increased dielectric sensitivity, we investigated another three Au@Pd nanobars (Au 50@Pd 80, Au 75@Pd 80, Au 150@Pd 80). In all, we have four Au@Pd nanobars with different Au cores but fixed Pd/Au ratio of 0.16. The four Au cores have ARs from 2.5 to 4.9. The absorption spectra of Au NRs and Au@Pd nanobars are shown in Figure 4a. The number after the Au indicates the concentration of Ag+ ions in the growth solution (see Experimental Section). Ag+ ions are used to control the aspect ratio of the Au NRs. As illustrated in Figure 4a, by increasing Ag+ concentration, the LSPR band of the Au NRs can be continuously
tuned to the longer wavelength region because of an increase in AR. Upon the deposition of a thin Pd shell, the LSPR bands of the Au@Pd nanobars all red-shift compared with those of the Au cores and further verified the red-shift of the LSPR maximum at thinner shell thicknesses. Although the LSPR band is damped and broadened slightly, a well-defined band still exists. The dielectric sensitivities of the Au NRs and Au@Pd nanobars are illustrated in Figure 4b. For the Au NRs, by tuning the LSPR maximum from 662 to 897 nm, the sensitivity increases from 160 to 430 nm RIU-1. In the case of Au@Pd nanobars, by increasing the AR of the inside Au cores, the LSPR maximum red-shifts from 676 to 926 nm and the sensitivity increases from 222 to 487 nm RIU-1. All Au@Pd nanobars show increased sensitivity relative to the corresponding Au cores, verifying their enhanced dielectric sensitivity. For monometallic nanorods, the dielectric sensitivity increases linearly with increasing AR.21 From Figure 5, both Au NRs and Au@Pd nanobars obey this linear relationship. The slope of the straight line for the Au NRs is 117 with a correlation coefficient of 0.995. For the Au@Pd nanobars, the slope increases to 126 with a correlation coefficient of 0.975. This means that at the same AR, the Au@Pd nanobars have a higher dielectric sensitivity. Although the ARs of the Au@Pd nanobars are lower (20) (a) McFarland, A. D.; Van Duyne, R. P. Nano Lett. 2003, 3, 1057. (b) Mock, J. J.; Smith, D. R.; Schultz, S. Nano Lett. 2003, 3, 485. (c) Raschke, G.; Brogl, S.; Susha, A. S.; Rogach, A. L.; Klar, T. A.; Feldmann, J.; Fieres, B.; Petkov, N.; Bein, T.; Nichtl, A.; Kuezinger, K. Nano Lett. 2004, 4, 1853. (d) Yang, J.; Wu, J. C.; Wu, Y. C.; Wang, J. K.; Chen, C. C. Chem. Phys. Lett. 2005, 416, 215. (21) Lee, K.; El-Sayed, M. A. J. Phys. Chem. B 2006, 110, 19220.
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Figure 5. The dielectric sensitivity versus aspect ratio for Au NRs and Au@Pd nanobars. The straight lines are linear regressions of corresponding data points.
Figure 7. Surface enhanced Raman spectra of Rh6G (1.7 × 10-4 M) in the suspensions of the AuNRs (a) and the Au@Pd nanobars with different shell thicknesses (b-d). Parts b, c, and d correspond to Au@Pd 80, Au@Pd 120, and Au@Pd 310, respectively. For comparison, the normal Raman spectrum of Rh6G in water (10-3 M) is also shown in part e. Excitation wavelength is 633 nm with an acquisition time of 10 s. Insets are TEM images of the Au NRs and three Au@Pd nanobars (a to d is from left to right). All scale bars correspond to 50 nm.
Figure 6. The relationship between the refractive index sensitivity of the Au NRs and the Au@Pd nanobars and the maximum of the LSPR band in water. The data points from the Au NRs and the Au@Pd nanorods are represented by solid squares and solid triangles, respectively. The straight line is a linear regression to all data points.
than those of corresponding Au cores, they have LSPR band maximum in the longer wavelength. In order to get more insights into the enhanced sensitivity of the Au@Pd nanobars, the dielectric sensitivity is plotted against the LSPR maximum as shown in Figure 6. Very interestingly, a linear relationship is found between the dielectric sensitivity and the LSPR maximum. It seems that the longer the wavelength of the LSPR maximum, the larger the dielectric sensitivity. Similar observations have been demonstrated by other metal nanostructures. For example, for spherical Au nanoparticles, as the diameter is increased, the SPR band redshifts and the dielectric sensitivity also increases. In the case of Au2S@Au core/shell nanoparticles, they have a higher dielectric sensitivity than that of pure Au nanoparticles with the same size.19 Compared with the Au nanoparticles, Au2S@Au nanoparticles have a red-shifted SPR maximum. We therefore believe that the enhanced sensitivity of the Au@Pd nanobars comes from the red-shifted LSPR band. In addition, it seems that the Au NRs and the Au@Pd nanobars adhere to the same straight line. For the Au NRs, this linear relationship has been verified. Herein, it seems that this linear relationship can be extended to the Au@Pd nanobars. We believe, because of the very thin thickness of the Pd layer (an average thickness of 0.4 nm), the (19) Averitt, R. D.; Sarkar, D.; Halas, N. J. Phys. ReV. Lett. 1997, 78, 4217.
LSPR properties are still dominated by the Au core. Thus, a linear relationship for the pure Au NRs can be extended to the Au@Pd nanobars. SERS Activities of the Au@Pd Nanobars. As described above, the ensembles of Au@Pd nanobars in solution show an obvious LSPR band in the visible and near-infrared spectral region. This makes a study of the EM enhancement of SERS from isolated nanoparticles possible. For such investigations, Au 100 NRs were used as cores. Different amounts of Pd were deposited on the Au core, thus forming Au@Pd nanobars with different shell thicknesses. The SERS spectra of the Au cores and three Au@Pd nanobars are shown in Figure 7. TEM images indicate that Au NRs have a cylindrical shape whereas the Au@Pd nanobars show a rectangular shape. The Au NRs have an average length of 63 ( 5 nm and width of 16 ( 2 nm. For three Au@Pd nanobars, the average lengths and widths are 66 ( 8 nm and 18 ( 2 nm (Au@Pd 80), 67 ( 7 nm and 19 ( 2 nm (Au@Pd 120), and 68 ( 8 nm and 21 ( 2 nm (Au@Pd 310), respectively. Obviously, the Au NRs have the strongest SERS signal. By depositing a Pd shell, the SERS signal is significantly reduced. In order to obtain the effects of the shell thickness on the SERS activity, surface enhancement factors (EF) are determined. Estimated EFs of Au NRs and Au@Pd nanobars are shown in Table 1, which are calculated using the following function:
EF ) [ISERS]/[IRaman] × [NBulk]/[Nads] where ISERS is the intensity of a vibrational mode in the surfaceenhanced spectrum, IRaman refers to the intensity of the same mode in the Raman spectrum, Nbulk is the number of R6G molecules in the bulk, and Nads is the number of molecules adsorbed on the SERS-active nanorods/nanobars. Nads is calculated using the number of nanorods/nanobars in the illuminated volume, the surface area of each nanorod/nanobar, and the binding density of Rh6G molecules. The SERS signal of Rh 6G molecules in the
Enhanced Optical Responses of Au@Pd Core/Shell Nanobars
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Table 1. Pd/Au Molar Ratio, Shell Thickness (d), and EFs of Rh6G Molecules on Au NRs and Au@Pd Nanobars at 633 and 785 nm sample Au Au Au Au Au Au Au
100 100@Pd 100@Pd 100@Pd 100@Pd 100@Pd 100@Pd
Pd/Au d (nm) 80 120 160 200 260 310
0 0.4 0.5 0.6 0.7 0.9 1.0
0 1.1 1.3 1.6 1.8 2.2 2.5
EF633nm
EF785nm
5.7 ( 1.1 × 102 4.6 ( 0.5 × 101 2.8 ( 0.5 × 101 3.2 ( 0.3 × 101 2.2 ( 0.3 × 101 2.1 ( 0.3 × 101 1.6 ( 0.2 × 101
1.5 ( 0.1 × 103 2.3 ( 0.4 × 102 7.6 ( 0.8 × 101 5.8 ( 1.8 × 101 4.6 ( 1.7 × 101 3.7 ( 0.8 × 101 2.7 ( 1.1 × 101
Au nanorod suspension increases linearly with the concentration of Rh6G molecules until saturation is approached at values above 5 × 10-6 M. We therefore assumed monolayer coverage of Rh6G molecules on the nanorods/nanobars. The employed concentration (1.7 × 10-4 M) is much higher in order to guarantee a saturation adsorption on the nanorods/nanobars. For the Au NRs, the EF at 785 nm is 1.5 × 103 and is 2.6 times larger than that at 633 nm (5.7 × 102). For the Au@Pd 80 nanobars, it has a Pd thickness of ca. 1 nm. The EFs at 785 and 633 nm decrease to 15% and 8% EFs of the Au cores, respectively. For the Au@Pd 310 nanobars with a Pd thickness of ca. 2.5 nm, the EFs at 785 and 633 nm reduce to 2% and 3% EFs of the Au cores. In order to see the relationship between the EF and shell thickness more clearly, EF values versus shell thicknesses (d) are plotted and shown in Figure 8. The EF value decreases dramatically in the first 1 nm shell thickness and reaches a constant at ca. 2 nm. Both the EF values at 633 nm and at 785 nm show the same trend. This can be seen more clearly from the Figure 8 inset where normalized EFs are plotted with shell thickness. The normalized EFs can be fitted well with a single exponential decay (EF633 ) e-d/0.46 with a R2 of 0.996 and EF785 ) e-d/0.52 with a R2 of 0.998). This means that with a shell thickness of ca. 0.5 nm, the EF is reduced to 37% of the original EF value. As shown in Table 1, for the Au 100 NRs, the EF at 785 nm is 2.5 times higher than that at 633 nm. For a strong EM enhancement, a coupling of the SPR band with both incident light and scattered light is essential.22 The Au 100 NRs have a LSPR band centered at 791 nm. For excitation at 785 nm, the overall coupling with the incident light and the scattered light (890 nm) is stronger in comparison with 633 nm excitation (Figure 9). Thus, for the Au 100 NRs, the larger enhancement at 785 nm is attributed to a stronger coupling with both the incident light and the scattered light and is consistent with the theoretical prediction of wavelength-dependent EM contributions. For Au@Pd nanobars, a similar trend is observed. Generally, the EF values at 785 nm are higher than those at 633 nm. This is also because of the stronger coupling of the LSPR resonance with both the incident light and the scattered light at 785 nm excitation. However, with increasing Pd thickness, the difference in the coupling magnitude between 633 and 785 nm decreases because of the blue-shift and broadening of the LSPR band. Accordingly, the difference in EF value between these two excitations also decreases. For example, for the Au@Pd 80, the EFs are 2.3 × 102 at 785 nm and 4.6 × 101 at 633 nm, respectively (5 times higher). For the Au@Pd 310, the EFs are 2.7 × 101 at 785 nm and 1.6 × 101 at 633 nm, respectively (1.7 times higher). For each Au@Pd nanobar, the difference in EF at 785 and 633 nm can be explained by a wavelength-dependent EM mechanism. However, the difference in EF among different Au@Pd nanobars (22) (a) Otto, A.; Mrozek, I.; Grabhorn, H.; Akemann, W. J. Phys.: Condens. Matter 1992, 4, 1143. (b) Knoll, W.; Philpott, M. R.; Swalen, J. D.; Girlando, A. J. Chem. Phys. 1982, 77, 2254. (c) Baumberg, J. J.; Kelf, T. A.; Sugawara, Y.; Cintra, S.; Abdelsalam, M. E.; Bartlett, P. N.; Russell, A. E. Nano Lett. 2005, 5, 2262.
Figure 8. The relationship between the enhancement factors (solid squares for 633 nm and solid circles for 785 nm) and the thickness of the Pd shell. Solid lines are used to guide the eyes. Inset: Normalized EFs versus shell thickness. Solid lines are the exponential fitting of the data.
Figure 9. UV-vis-NIR absorption spectra of the Au NRs and Au@Pd nanobars with different Pd/Au ratios (from 0.4 to 1.0). Two dashed lines (633 nm, 785 nm) and two dotted lines (705 nm, 890 nm) indicate the excitation wavelengths and scattering wavelengths, respectively, used in the SERS measurement and calculation.
is strongly affected by the shell thickness. For example, at 633 nm excitation, the Au@Pd 300 experiences a coupling with both incident light and scattered light stronger than that of the Au@Pd 80. However, the EF value of the former is only 20% of the latter. This means that for the Au@Pd nanobars, the enhancement mainly comes from the underneath Au core and a “borrowing” mechanism dominates. In order to obtain a high SERS activity for the Au@Pd nanostructure, a thinner Pd shell is preferred. Recently, Ordendorff et al. investigated the SERS activities of a series of Au NRs and Ag NRs ensemble solutions.13b For the Au NRs with a SPR resonance with an excitation line (633 nm), they observed a 1.4 × 105 enhancement factor for 4-mercaptopyridine molecules. The difference in EF value for the resonant and the nonresonant excitation is 2.2, similar to our result (2.5) for the Au 100 NRs. Notes that the EF value they obtained under resonant conditions is two magnitudes higher than our EF value. We speculate that this difference might be related to interaction strength between analytes and particles. Compared with Rh 6G molecules, thiols have a stronger affinity to the Au surface because of the formation of the S-Au bond. The stronger interaction induces higher enhancement. Interestingly, they also noticed the variations in EF values among the different analytes where thiol molecules give surface enhancements larger than that of 2,2′-
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bipyridine on all nanorods investigated. For instance, for the Ag NRs, the EF value under resonant conditions is 2.3 × 107 for 4-mercaptopyridine whereas it decreases to 8.5 × 105 for 2,2′bipyridine. They guess that adsorption of negatively charged thiols should be facilitated on CTAB-capped nanorods because of an electrostatic interaction, leading to a greater overall enhancement for thiols over 2,2′-bipyridine. For the aggregates of the Au NRs, the same group reported a 108 enhancement factor for 4-mercaptobenzoic acid.14a We believe that by borrowing only 1% enhancement from the Au NRs, in the aggregates of the Au@Pd nanobars, an EF value of 106 should be expected under optimized conditions. This endows a great potential for the Au@Pd nanobars as SERS-active candidates.
Conclusion We present a simple way to prepare Pd nanostructures by a seed-mediated growth, with Au NRs as seeds. By using this
Zhang et al.
method, the growth of the Pd shell can be finely controlled. The optical responses of Au@Pd nanobars are determined both by the inner core and the outer shell. At a suitable shell thickness, the Au@Pd nanobars exhibit a well-defined LSPR band in the visible and near-infrared region. They show a dielectric sensitivity higher than that of the corresponding Au cores, thus making exploration of SPR-based sensing for Pd nanostructures possible. Investigations on the SERS activity for the ensembles of the Au@Pd nanobars indicate that the SERS activities mainly come from the Au core underneath, and the enhancement factors decrease exponentially with the shell thickness. Acknowledgment. The work was supported by National Natural Science Foundation of China (Grant No. 20773032) and the “973” National Key Basic Research Program of China (2006CB705600 and 2006CB932602). LA803060P
