ARTICLE pubs.acs.org/JPCC
Optical Property of a Colloidal Solution of Platinum and Palladium Nanorods: Localized Surface Plasmon Resonance Sujin Jung,† Kevin L. Shuford,*,‡ and Sungho Park*,† †
Department of Chemistry & Department of Energy Science, SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon 440-746, South Korea ‡ Department of Chemistry, Drexel University, 3141 Chestnut Street, Philadelphia, Pennsylvania 19104, United States ABSTRACT: The optical properties of Pt and Pd nanorods as a function of aspect ratio were investigated and systematically compared with Au nanorods of similar physical dimensions. The overall optical properties of Pt and Pd nanorods were tailored by controlling aspect ratio and could be tuned from 400 nm up to near-IR spectral window. Their longitudinal mode red shifts as the length increases, whereas the transverse mode blue shifts very slightly. The comparison of observed LSPR modes in Pt and Pd NRs with Au NRs as a function of aspect ratio exhibited that Pd NRs are most sensitive to variations in rod length. The magnitude of longitudinal mode shift per unit change of NR length could be obtained from the slope of a linear fit of each plot. Pd NRs show the largest dependence with a value ca. 6.72 (Δλ/ΔL). For Pt and Au NRs, the slope is ca. 4.23 and 3.11, respectively.
1. INTRODUCTION A collective oscillation of loosely bound electrons on a metal surface can be induced by light waves when the incident field meets the resonance conditions. If confined to the surface of metal nanoparticles, the collective motion is referred to as a localized surface plasmon resonance (LSPR).1 The LSPR modes are sensitive to changes of the surrounding medium and can be tailored by the physical dimensions and composition of the nanoparticles.1 Among these, shape control allows one to fine-tune the LSPR modes with great versatility. From this viewpoint, commonly investigated nanoparticles are composed of Au or Ag because their LSPR excitations give rise to intense color in the visible-near-IR (vis-NIR) spectral window, which has led to various applications such as sensing and imaging of analytes.2 In comparison, the LSPR characteristics of other metals like Pt and Pd remain largely unexplored because controlling the shape of Pt and Pd nanoparticles is more difficult, and their LSPR modes are in the ultraviolet spectral range.3 As in the case of Au and Ag, altering the shape of Pt and Pd nanoparticles can push the LSPR bands to the vis-NIR region.3 Specifically, controlling the aspect ratio of Pt and Pd nanorods (NRs) allows one to fine-tune their LSPR modes in the vis-NIR region. Herein, we report the LSPR characteristics of Pt and Pd NRs as a function of aspect ratio and show how their properties differ from Au NRs. Pt (and Pd) NRs displayed transverse and longitudinal modes, which oscillate along both the short and long axes, as do Au NRs. However for Pt and Pd, the higher order longitudinal LSPR modes were not observed experimentally because of the relatively weak intensities but were predicted by the theoretical calculations. r 2011 American Chemical Society
2. EXPERIMENTAL SECTION Synthesis of AAO Templates. The synthetic procedure is based on the method established by Masuda et al.7 A highpurity (99.999%) thin plate of aluminum (from Goodfellow Cambridge) was electropolished in a mixture of ethanol (from Samchun Chemical) and perchloric acid (from Samchun Chemical) (7:3, v/v) at 20 V and 0 °C for 3 min. Then, the plate was anodized in 0.3 M oxalic acid (from Sigma-Aldrich) at 40 V and 0 °C for 12 h. The alumina layer was then dissolved using an aqueous mixture of chromic acid (from Sigma-Aldrich) (1.8 wt %) and phosphoric acid (from Samchun Chemical) (6 wt %) at 60 °C for 12 h. A second anodization step followed in 0.3 M oxalic acid at 40 V and 0 °C for 24 h, producing highly ordered porous AAO template. The residue of aluminum plate was next removed by immersing in saturated HgCl2 aqueous solution for 6 h and then immersed in 8.5 wt % phosphoric acid solution for pore widening for 30 min. The resulting pore diameter was 83 ((6) nm. Synthesis of Pt/Pd Nanorods. The synthesis of multiblock metal 1D nanostructures is based on the method developed by Martin and Moskovits.4,5 Similar approaches were adopted from our previous reports.6 In brief, a thin layer of silver (300 nm) was thermally evaporated on one side of the anodized aluminum oxide (AAO) template. It served as a working electrode in an Received: July 5, 2011 Revised: August 12, 2011 Published: August 26, 2011 19049
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Figure 1. Schematic representation of the synthesis of Pt and Pd NRs. The electrochemical deposition of Pt and Pd results in pure Pt and Pd NRs in the interior of AAO templates. The lengths of resulting NRs could be controlled by monitoring the total charge passed during electroplating step. The Ag component was dissolved with concentrated nitric acid or a mixture of methanol, hydrogen peroxide, and ammonium hydroxide (4:1:1, in volume ratio) and the AAO was dissolved with 3 M aqueous NaOH solution.
Figure 2. FESEM images of Pt NRs. With a fixed diameter of D = 83 ((6) nm, the lengths (L) of NRs are (A) 146 ((13), (B) 230 ((38), (C) 248 ((33), (D) 295 ((30), (E) 337 ((33), and (F) 456 ((66) nm. The inset in panel A is an EDS spectrum showing pure Pt component.
electrochemical setup after making an electrical contact with aluminum foil in the Teflon cell. An Ag/AgCl reference electrode and a Pt wire counter electrode were utilized to form a threeelectrode configuration. For Pt NR synthesis, the interior of the nanopores was filled with Ni (nickel sulfamate RTU solution from Technic) at a constant potential, 0.95 V, by passing 0.5 C/cm2. Then, Pt was electroplated from a buffer (pH ∼1.8) solution containing 0.1 M HCl, 0.1 M glycine, and 0.01 M H2PtCl6 at 0.30 V. The length versus charge profiles are represented in Figure 1. The backing Ag layer and predeposited Ni layer were dissolved by concentrated nitric acid. AAO could be etched by 3 M sodium hydroxide solution. The resulting samples were rinsed with distilled water and dispersed in D2O for UV vis-NIR extinction measurements. For Pd NR synthesis,
the interior of the nanopores was filled with Ag instead of Ni (Technic ACR silver RTU solution from Technic) at a constant potential, 0.95 V, by passing 0.5 C/cm2. Then, Pd was electroplated from a solution containing 0.05 M PdCl2 and 5 wt % HCl at 0.05 V. The Ag backing and predeposited Ag layer were dissolved by a solution containing methanol, hydrogen peroxide, and ammonium hydroxide (= 4:1:1, in volume ratio). The other steps are the same as those in the case with Pt NRs. Discrete Dipole Approximation Calculations. The optical properties of Pt NRs have been calculated here using the discrete dipole approximation (DDA).8,9 The NR is represented by a square array of point dipoles, each of which obtains an oscillating polarization from the local field at that site. The local field contains contributions from the incident plane wave and the fields radiated 19050
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Figure 3. FESEM images of Pd NRs. In a fixed diameter of D = 83 ((6) nm, lengths (L) of NRs are (A) 188 ((25), (B) 205 ((20), (C) 260 ((24), (D) 282 ((48), (E) 353 ((36), and (F) 497 ((51) nm. The inset in panel A is an EDS spectrum showing pure Pd component.
from the other dipoles composing the array. The dipole polarizability incorporates the optical constants of the metal, and here we have utilized experimentally determined values for the refractive index of Pt.10 The set of coupled dipole equations composes a large, dense matrix equation that is solved iteratively for the induced polarizations, which are then used to calculate the NR extinction. The NRs were modeled as perfect cylinders. The number of dipoles used for the calculations was varied to yield an interdipole spacing of ∼3 nm for all NRs, which ensured adequate convergence.
3. RESULTS AND DISCUSSION Homogeneous and pure NRs composed of Pt and Pd were synthesized by potentiostatic electrochemical deposition in AAO templates. The dimensions were 83 ((6) nm in diameter and up to ∼500 nm in length. The diameter was determined by the template pore size, whereas the length could be tailored by adjusting the total charge passed through the electrochemical cell. The resulting length of NRs displayed a linear relationship with the total charge, as illustrated in Figure 1. After dissolving the templates and subsequently releasing the NRs into solution, the optical properties of a colloidal suspension of such nanostructures were characterized. Typical field-emission scanning electron microscopy (FESEM) images of Pt and Pd NRs exhibit a fairly homogeneous size distribution, and energy dispersive spectroscopy (EDS) shows their corresponding pure composition (Figure 2). With a fixed diameter of D = 83 ((6) nm, the length of Pt NRs was systematically varied to be L = 146 ((13), 230 ((38), 248 ((33), 295 ((30), 337 ((33), and 456 ((66) nm, as shown in Figure 1. The corresponding FESEM images and EDS spectrum of Pd NRs were obtained, and the lengths were determined to be L = 188 ((25), 205 ((20), 260 ((24), 282 ((48), 353 ((36), and 497 ((51) nm (Figure 3). The surface of NRs was smooth, but the tip showed a corrugated morphology, which originates from the irregular contact between the thermally evaporated Ag layer and AAO templates.
Figure 4 presents UV vis-NIR extinction spectra of the Pt and Pd NRs. Typically, Pt NRs (L ≈ 146 nm) show a very broad feature over 200 1800 nm (panel A, spectrum a). When the NR length becomes longer, the broad band resolves into two distinct peaks with one centered at ca. 400 nm and the other peak shifts to longer wavelengths. The band at shorter wavelengths is assigned to a transverse dipole mode and the other at longer wavelengths to a longitudinal dipole mode. Similar trends have been observed previously for a colloidal suspension of Au NRs.11 Spherical Pt nanoparticles with a diameter of ca. 73 nm show a characteristic broad extinction feature over 200 800 nm with a peak maximum at 400 nm.12 This peak is expected to red shift as the size of spherical nanoparticles increases. In the current system, the large diameter (D = 83 ((6) nm) of Pt NRs reveals their transverse dipole modes at ca. 400 nm. As expected, the transverse dipole mode is less sensitive to changes in the aspect ratio showing only very slight blue shifts, but the longitudinal dipole mode red shifts to a large extent upon increasing the length. We have calculated the corresponding spectra for Pt using the DDA, and the calculations show good agreement with the experiment (Figure 4B). Experimentally, when the NR length becomes L ≈ 295 nm (panel A, spectrum d), two peaks are well-resolved at 425 and 1350 nm. Calculations indicate a similar result with an additional feature at 600 nm (panel B, spectrum d). This shoulder red shifts as the length of NRs increases, and it eventually becomes a distinctive peak when L ≈ 456 nm (in panel B). In our previous study with Au NRs (with the same diameter), we could observe the higher-order LSP modes when the length exceeded L ≈ 250 nm, resulting from phase retardation of the applied field inside the material.11 Although DDA calculations predict the presence of a higher-order longitudinal peak in Pt NRs, we were unable to resolve the peak in the experiment. The UV vis-NIR spectra obtained from Pd NRs revealed a similar trend as in the case of Pt NRs (panel C). When the length is L ≈ 188, there are two peaks at 450 and 1110 nm. The former is assigned to a transverse dipole mode and the latter is assigned to a longitudinal dipole mode. As in the case of Pt NRs, the longitudinal mode red 19051
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tunable spectral window for the longitudinal mode shift in Pt and Pd might allow one to design more versatile plasmonic sensors that are sensitive and specific for monitoring refractive index changes caused by molecular interactions in the vicinity of NRs.
4. CONCLUSIONS In conclusion, we have observed transverse and longitudinal LSPRs from Pt and Pd NRs. We have shown that one can systematically control the length of NRs and their corresponding optical properties. The longitudinal dipole LSPR modes systematically red-shifted as the length increased, which is similar to the characteristics observed in Au NRs. However, the expected higher-order longitudinal modes of Pt and Pd NRs could not be observed experimentally even though the modes could be resolved with Au NRs that were prepared under similar experimental conditions. Controlling and understanding the optical properties of Pt and Pd NRs will be relevant to the design of more sophisticated plasmon waveguides and chemical sensors with these materials that are capable of operating over a larger frequency range than traditional plasmonic materials. This application is under progress and will be published elsewhere in the near future. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected] (K.L.S.);
[email protected] (S.P.). Figure 4. (A) UV vis-NIR spectra of Pt NRs as a function of length. Each spectrum a f corresponds to samples shown in Figure 1A F, respectively. (B) Corresponding calculated spectra to panel A obtained using DDA. (C) UV vis-NIR spectra of Pd NRs as a function of length. Each spectrum a f was obtained when L = 188 ((25), 205 ((20), 260 ((24), 282 ((48), 353 ((36), and 497 ((51) nm, respectively. (D) Plot for dipole transverse (solid symbols) and longitudinal (open symbols) modes versus length of NRs from experimental measurements. Black, red, and blue lines correspond to Au, Pd, and Pt NRs.
shifts as the length increases, whereas the transverse mode blue shifts very slightly. No features attributable to higher-order LSPR modes were detected in the Pd samples investigated. Of central interest here is the comparison of observed LSPR modes in Pt and Pd NRs with Au NRs as a function of aspect ratio. In panel D, we have plotted the Pt, Pd, and Au LSPR modes together. Regardless of composition, the longitudinal modes red shift as the length increases for a given diameter. Pd NRs are most sensitive to variations in rod length. The magnitude of longitudinal mode shift per unit change of NR length could be obtained from the slope of a linear fit of each plot. Pd NRs show the largest dependence with a value ca. 6.72 (Δλ/ΔL). For Pt and Au NRs, the slope is ca. 4.23 and 3.11, respectively. In contrast, transverse mode peak positions show little dependence on the length variation. The transverse modes of Pt and Pd are blueshifted in comparison with Au NRs. Au NRs of this diameter produce a transverse mode at ca. 550 nm, whereas Pt and Pd NRs show transverse modes at shorter wavelengths, ca. 400 nm. The short wavelength region is difficult to tune with Au NRs. Another noticeable feature is the energy separation between the transverse and longitudinal peaks. As clearly shown in panel D, Pd NRs show the largest energy separation. Typically, when the length is L ≈ 250 nm, the energy difference (in wavelength) is ca. 1060, 890, and 550 nm for Pd, Pt, and Au NRs, respectively. The large
’ ACKNOWLEDGMENT This work was supported by the National Research Foundation of Korea (World Class University (WCU): R31-2008-10029, Nano R&D program: 2010-0019149, 2010-0015457, and Priority Research Centers Program: NRF-20100029699). K.L.S. thanks Drexel University for start-up funding. ’ REFERENCES (1) (a) Barnes, W. L.; Dereux, A.; Ebbesen, T. W. Nature 2003, 424, 824–830. (b) Oldenburg, S. J.; Jackson, J. B.; Westcott, S. L.; Halas, N. J. Appl. Phys. Lett. 1999, 75, 2897–2899. (c) Kumbhar, A. S.; Kinnan, M. K.; Chumanov, G. J. Am. Chem. Soc. 2005, 127, 12444–12445. (2) (a) Thaxton, C. S.; Mirkin, C. A. Nat. Biotechnol. 2005, 23, 681–2. (b) Ko, H.; Singamaneni, S.; Tsukruk, V. V. Small 2008, 4, 1576–1599. (c) Hutter, E.; Fendler, J. H. Adv. Mater. 2004, 16, 1685–1706. (d) Jana, N. R.; Gearheart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065–4067. (e) Kim, F.; Song, J. H.; Yang, P. J. Am. Chem. Soc. 2002, 124, 14316–14317. (f) Wang, H.; Huff, T. B.; Zweifel, D. A.; He, W.; Low, P. S.; Wei, A.; Cheng, J.-X. Proc. Natl. Acad. Sci. U.S.A. 2005, 102, 15752–15756. (3) (a) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. 1995, 99, 14129–14136. (b) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z.-Y.; Xia, Y. J. Am. Chem. Soc. 2005, 127, 17118–17127. (c) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia, Y. Angew. Chem., Int. Ed. 2005, 44, 7913–7917. (4) Martin, C. R. Science 1994, 266, 1961–1966. (5) Routkevitch, D.; Haslett, T. L.; Ryan, L.; Bigioni, T.; Douketis, C.; Moskovits, M. Chem. Phys. 1996, 210, 343–352. (6) Park, S.; Lim, J. H.; Chung, S.-W.; Mirkin, C. A. Science 2004, 303, 348–351. (7) Masuda, H.; Fukuda, K. Science 1995, 268, 1466–1468. (8) Draine, B. T. Astrophys. J. 1988, 333, 848–871. (9) Draine, B. T.; Flatau, P. J. J. Opt. Soc. Am. A 1994, 11, 1491–1499. (10) Lynch, D. W.; Hunter, W. R. Comments on the Optical Constants of Metals and an Introduction to the Data of Several Metals. In Handbook of Optical Constants of Solids; Palik, E., Ed.; Academic Press: Orlando, FL, 1985. 19052
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