NANO LETTERS
Optical Properties of Pd−Ag and Pt−Ag Nanoboxes Synthesized via Galvanic Replacement Reactions
2005 Vol. 5, No. 10 2058-2062
Jingyi Chen,† Benjamin Wiley,‡ Joseph McLellan,† Yujie Xiong,† Zhi-Yuan Li,§ and Younan Xia*,† Department of Chemistry and Department of Chemical Engineering, UniVersity of Washington, Seattle, Washington 98195, and Laboratory of Optical Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100080, PRC Received August 19, 2005; Revised Manuscript Received September 2, 2005
ABSTRACT Silver nanocubes dispersed in water were transformed into Pd−Ag or Pt−Ag nanoboxes by adding either Na2PdCl4 or Na2PtCl4. By controlling the amount of noble metal salt added, and therefore the molar ratio of Na2PdCl4 or Na2PtCl4 to Ag, we could tune the surface plasmon resonance peak of the nanostructures across the entire visible spectrum, from 440 to 730 nm. Replacement of Ag with Pd resulted in the formation of a nanobox composed of a Pd−Ag alloy single crystal, but the nanobox formed after replacement of Ag with Pt was instead composed of distinct Pt nanoparticles. DDA calculations suggest that both nanoboxes absorb light strongly, with Qabs/Qsca ≈ 5. After galvanic replacement, Pd−Ag and Pt−Ag nanostructures remain SERS active, suggesting their use as a SERS probe for studying the dependence of interfacial chemistry on composition.
The optical properties of Au and Ag nanoparticles have been under scientific scrutiny for nearly 150 years, culminating in the niche application of their surface plasmon resonance (SPR) in sensing and plasmonics.1 In contrast, the optical properties of Pd and Pt nanoparticles remain relatively unexplored. The location of the SPR peak of Pd and Pt nanoparticles in the UV region not only gives them an uninteresting black color but it also makes their SPR characteristics much more difficult to probe because of the absorption of light at this wavelength by silica and most solvents.2 However, we have demonstrated recently that the SPR peak of Pd nanocubes could be tuned to nearly the same location (∼400 nm) as that of Ag nanoparticles by increasing their sizes to ∼50 nm.3 Transformation of the Pd nanocubes into nanocages via a corrosive pitting process enabled further tuning of their SPR peak to 520 nm.4 This second approach, formation of hollow nanostructures, has been particularly effective for tuning the SPR peak of Au nanoshells across the visible spectrum because of the sensitivity of the SPR peak location to wall thickness.5 We have also shown that if Ag nanocube templates are titrated with HAuCl4 in water, galvanic replacement of Ag with Au leads to the formation of nanoboxes and nanocages in a matter of minutes.6 By controlling the molar ratio of HAuCl4 to Ag, the extinction peak of the nanostructures could be tuned from the blue (400 nm) to the near-in* Corresponding author. E-mail:
[email protected]. † Department of Chemistry, University of Washington. ‡ Department of Chemical Engineering, University of Washington. § Laboratory of Optical Physics. 10.1021/nl051652u CCC: $30.25 Published on Web 09/14/2005
© 2005 American Chemical Society
Figure 1. Formation of Ag-Pd nanoboxes through galvanic replacement between Ag nanocubes and Na2PdCl4. SEM images were taken after reacting Ag nanocubes with (A) 0.3, (B) 0.6, (C) 0.9, and (D) 1.5 mL of a 0.5 mM aqueous solution of Na2PdCl4. Insets are corresponding TEM images with a 50 nm scale bar.
frared (1200 nm).7 Here we illustrate that galvanic replacement of Ag nanocubes with either Pd or Pt salts can also produce nanoboxes with extinction peaks tunable to the nearinfrared. Experimental details are given in ref 8. To tune the spectra of the nanostructures, we added Ag nanocubes to boiling water before titrating with the desired noble metal salt (either Na2PdCl4 or Na2PtCl4). Figure 1 illustrates the change in product morphology as Na2PdCl4 was added to a suspension of Ag nanocubes. At a molar ratio (Na2PdCl4 to Ag) of 0.14,
Figure 2. (A) UV-visible extinction spectra of products from galvanic replacement of Ag nanocubes with Na2PdCl4. The amount of 0.5 mM Na2PdCl4 aqueous solution added to the initial Ag nanocubes is indicated on each curve. (B) Photographs of the aqueous Pd-Ag nanostructures corresponding to the curves in (A).
etch pits developed on the surfaces of the nanocubes, primarily at the corners (see Figure 1A). The atomic step edges necessarily present at the corners may present the most reactive sites for initiation of pitting at this location.9 Figure 1B and C shows the products at a molar ratio of Na2PdCl4/ Ag ) 0.28 and 0.42, respectively. By comparing the TEM and SEM images, we find that oxidation within the etch pits removed Ag from the interior rather than the surface of the nanocube. As Ag was dissolved from the interior, Pd accepted its electrons and plated onto the exterior of the nanocube and therefore may have helped to prevent etching of the nanocube surface. The Pd-Ag system is a well-known binary alloy,10 and plating of Pd will result in the formation of walls composed of a Pd-Ag alloy. Completely hollow and fully enclosed nanoboxes formed at a molar ratio of Na2PdCl4/Ag ) 0.69 as a result of further oxidation of Ag from the interior and alloying of Pd into the exterior walls of nanocubes. If more Na2PdCl4 was added, then no morphological change was observed. This suggests that the Pd atoms had diffused quickly into Ag to form a homogeneous alloy, thereby eliminating the electrochemical driving force for galvanic replacement.11 This is in contrast to the Au/Ag galvanic replacement reaction, in which addition of HAuCl4 after formation of an enclosed alloyed nanobox led to dealloying of Ag and the formation of a porous Au nanocage. Figure 2 illustrates that the UV-visible extinction peak of the nanostructures could be red-shifted in a predictable Nano Lett., Vol. 5, No. 10, 2005
Figure 3. (A) The predicted molar percentage of Pd in the alloyed hollow nanostructures assuming compete conversion of Na2PdCl4 to Pd is compared to the actual percentage of Pd as measured using AES and EDX. (B) The XRD pattern obtained from the hollow nanostructures shown in Figure 1D. The inset is the ED pattern taken from an individual nanobox by aligning the electron beam perpendicular to one of the faces. The ED lattice parameter of 4.04 ( 0.01 Å agrees with the XRD measurement.
manner by controlling the molar ratio of Na2PdCl4 to Ag. The initial peak of the pure Ag nanocubes was located at 440 nm. This peak was red-shifted to 460 nm after formation of etch pits in the corners. Even such small etch pits have also previously caused the extinction peak to red-shift for the case of pure Pd nanocubes.4 As the etch pits grew in size, and the Ag nanocubes became more hollow, the extinction peak was red-shifted further to 496 and 544 nm. The red-shift stopped at 732 nm for the final Pd-Ag alloy nanoboxes. Figure 2B displays the aqueous solutions of these nanostructures tuned to the wavelengths shown in Figure 2A, indicating that the Pd-Ag nanostructures displayed bright and distinctive colors as they were tuned across the visible spectrum. The extinction peak broadened during the transformation of Ag nanocubes into Pd-Ag alloy nanoboxes primarily as a result of the change in nanostructure composition. Figure 3A compares the calculated change in molar composition assuming complete conversion of Na2PdCl4 to Pd with the actual changes being measured with both atomic emission 2059
Figure 4. (A) UV-visible extinction spectra of products from galvanic replacement of Ag nanocubes with Na2PtCl4. The amount of 0.5 mM Na2PtCl4 aqueous solution added to the initial silver nanocubes is indicated on each curve. (B) Corresponding photographs of the aqueous Ag-Pd nanostructures analyzed in A. (CE) TEM images of the Ag-Pt nanostructures formed after addition of (C) 0.3, (D) 0.9, and (E) 1.5 mL of a 0.5 mM Na2PtCl4 aqueous solution. The scale bar is 100 nm and applicable to all three images.
spectroscopy (AES) and energy-dispersive X-ray (EDX) analysis. As Na2PdCl4 was added to the suspension of Ag nanocubes, the molar fraction of Pd in alloyed nanostructures increased until it reached between 33 (AES) and 49 (EDX) percent. If more Na2PdCl4 was added, the percentage of Pd in the nanobox remained constant. This may be the result of a decreased electrochemical driving force for oxidation of Ag in the alloy relative to bulk Ag. An X-ray diffraction (XRD) pattern (Figure 3B) obtained from a sample of Pd-Ag nanoboxes further confirms that they are composed of a Pd-Ag alloy. The inset of Figure 3B displays an electron diffraction pattern obtained from a single nanobox, indicating that even though the nanobox walls alloyed, they retained their single crystallinity. Addition of Na2PtCl4 to the Ag nanocubes also resulted in predictable extinction peak red-shifts. Figure 4A shows that through controlled addition of Na2PtCl4, the extinction 2060
peaks of Pt-Ag alloy nanoboxes could be tuned continuously to 670 nm. Aqueous suspensions of these alloy nanoboxes displayed bright colors similar to those of the Pd-Ag nanoboxes (Figure 4B). However, the resulting morphology changes were distinctly different from those observed in the Pd-Ag system. This is because, unlike Pd, Pt does not readily undergo solid-solid diffusion with Ag at temperatures below 900 K.12 Even when melted together in a crucible, it is impossible to form an alloy richer than 5% in Pt.13 Figure 4C shows that, as pitting of the Ag nanocube was initiated, small Pt particles nucleate on its surface. When Ag was further oxidized from the interior (Figure 4D and E), these nuclei grew in size, resulting in a nanobox with bumpy outer walls composed mainly of Pt nanoparticles. EDX analysis performed on the nanoboxes with a SPR peak at 670 nm indicated that they were 50% Pt. Further addition of Na2PtCl4 past this point resulted in collapse of the nanoboxes into nanoparticle aggregates. Evidently, because their outer walls were not a single-crystal alloy, Pt-Ag nanoboxes could not evolve into the porous nanocages observed for the Au-Ag system and tuning of the spectra was limited to ∼700 nm for the Ag nanocubes used in the present study (∼50 nm in edge length). To enhance our understating of the UV-visible extinction spectra exhibited by Pd-Ag and Pt-Ag alloyed nanoboxes, we performed DDA calculations on nanoboxes with dimensions and compositions approximating those of the experimental samples.14 Figure 5A shows the calculated extinction, absorption, and scattering coefficients from a Pd0.4Ag0.6 nanobox with an outer edge length of 63 nm and an inner edge length of 51 nm (wall thickness ) 6 nm). The peak location of 755 nm shows a good match with that obtained from the synthesized Ag-Pd nanoboxes (732 nm). The calculated peak has a full width half-maximum (fwhm) of ∼300 nm, which is significantly broader than the ∼80 nm fwhm of the DDA calculated extinction peaks for Au nanoboxes of similar sizes. This result suggests that the broadening of the experimental peak as the ratio of Pd increased is due primarily to the change in composition. The calculation also suggests that Pd-Ag nanoboxes absorb much more light than they scatter; Qabs/Qsca ≈ 5 at the peak maximum, compared with Qabs/Qsca ≈ 1 for Au nanoboxes of similar sizes. Figure 5B shows the DDA results for a Pt0.5Ag0.5 nanobox with an outer edge length of 63 nm and an inner edge length of 49 nm (wall thickness ) 7 nm). Although the wall here was assumed to be an alloy rather than separate Pt and Ag layers, the calculated peak location (680 nm) is very close to that of the synthesized Pt nanoboxes (670 nm). The fwhm of ∼280 nm and Qabs/Qsca ≈ 5 are similar to that predicted for the Pd-Ag nanobox. The experimental extinction peaks for Au-Ag, Pd-Ag, and PtAg alloyed nanoboxes consistently exhibit a fwhm broader than that predicted by DDA calculations. This is likely the result of small variations in wall thickness among the particles; even a 2 nm difference in wall thickness causes a 110 nm extinction peak shift.7 The exceptionally broad range of wavelengths across which Pd-Ag and Pt-Ag nanoboxes absorb light strongly could make them particularly useful as nanoscale photothermal heating elements.15 Nano Lett., Vol. 5, No. 10, 2005
Figure 6. Raman spectra of 4-mercaptopyridine on (A) Pd0.15Ag0.85, (B) Pd0.30Ag0.70, and (C) Pt0.15Ag0.85 nanostructures. The insets give TEM images of the nanostructures used as the substrates, with the scale bar being 50 nm.
Figure 5. The extinction, absorption, and scattering coefficients as predicted by DDA calculation from (A) a Pd0.4Ag0.6 nanobox 63 nm in edge length and 6 nm in wall thickness and (B) a Pt0.5Ag0.5 nanobox 63 nm in edge length and 7 nm in wall thickness.
Pd-Ag and Pt-Ag nanostructures might be particularly useful for examining the compositional dependence of oxidation, adsorption, and ligand formation with surfaceenhanced Raman scattering (SERS).16 As an initial demonstration, we have obtained the SERS spectra of 4-mercaptopyridine on Pd0.15Ag0.85, Pd0.30Ag0.70 and Pt0.15Ag0.85 nanostructures. As can be seen from Figure 6, well-defined SERS spectra can still be observed using these composite nanoboxes, even for those containing as much as 30% Pd. The surface enhancement seen from these composite nanostructures is likely due to the fact they still contain a large proportion of Ag because the coupling between conduction electrons and interband electronic transitions in Pt and Pd gives these metals comparatively little SERS activity.17 In future work, we expect that these nanostructures will extend the capability of SERS to supply valuable information as to what surface species form in a particular reaction, and how metal composition affects this surface chemistry. Greater understanding often comes from comparison and contrast. By extending the compositional range of nanostructures with SPR peaks in the visible region of light, we Nano Lett., Vol. 5, No. 10, 2005
hope to illuminate the dependence of SPR characteristics on the material. We have shown that boiling Ag nanocubes in water with Na2PdCl4 or Na2PtCl4 results in the galvanic replacement of Ag with Pd or Pt and formation of welldefined nanoboxes. The extinction peaks of resultant nanostructures can be tuned continuously across the visible spectrum by titrating in controlled amounts of these noble metal salts. In the case of Pd, galvanic replacement stopped once a nanobox composed of a Pd-Ag alloy (Pd/Ag ≈ 0.40) was formed, suggesting that rapid formation of a homogeneous alloy eliminated the electrochemical driving force for further oxidation of silver. In contrast, Pt and Ag do not alloy at 100 °C, and the nucleation and growth of Pt particles on the Ag nanocube resulted in the formation of rough, noncrystalline nanobox walls that collapsed upon further addition of Pt salt. DDA calculations suggest that the broadness of Pd-Ag and Pt-Ag is due to their composition, and although each nanobox had a distinctive morphology, the Qabs/Qsca ratio was ∼5 for both. These composite nanoboxes represent two new systems in which SERS can be contrasted with that from either Au or Ag. In particular, the tunable surface chemistries of these nanoboxes will allow additional studies of chemical enhancement, as well as the affect of surface composition on interfacial chemistry. The specific chemical sensitivity of Pd toward hydrogen makes the Pd-Ag nanoboxes potentially useful for hydrogen storage and SPR-based sensors for hydrogen gas.18 The Pd-Ag and Pt-Ag nanoboxes might also find use as catalysts, photothermal heating elements, absorption contrast agents, and chemically specific optical sensors. Acknowledgment. This work was supported in part by a DARPA-DURINT subcontract from Harvard University and a fellowship from the David and Lucile Packard Foundation. Y.X. is a Camille Dreyfus Teacher Scholar 2061
(2002). B.W. thanks the Center for Nanotechnology at the UW for a Nanotech fellowship. Z.-Y.L. was supported by the National Key Basic Research Special Foundation of China (no. 2004CB719804). This work used the Nanotech User Facility (NTUF), a member of the National Nanotechnology Infrastructure Network (NNIN) funded by the NSF. References (1) See, for example, Xia, Y.; Halas, N. J. MRS Bull. 2005, 30, 338. (2) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881. (3) Xiong, Y.; Chen, J.; Wiley, B.; Xia, Y.; Yin, Y.; Li, Z.-Y. Nano Lett. 2005, 5, 1237. (4) Xiong, Y.; Wiley, B.; Chen, J.; Li, Z.-Y.; Yin, Y.; Xia, Y. Angew. Chem., Int. Ed., submitted for publication, 2005. (5) (a) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (b) Selvakannan, P. R.; Sastry, M. Chem. Commun. 2005, 1684. (6) (a) Sun, Y.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 3892. (b) Chen, J.; Saeki, F.; Wiley, B.; Cang, H.; Cobb, M. J.; Li, Z.-Y.; Au, L.; Zhang, H.; Kimmey, M. B.; Li, X.; Xia, Y. Nano Lett. 2005, 5, 473. (7) Chen, J.; Wiley, B.; Li, Z.-Y.; Campbell, D.; Saeki, F.; Cang, H.; Au, L.; Lee, J.; Li, X.; Xia, Y. AdV. Mater., in press, 2005. (8) In a typical procedure, 100 µL of a dispersion of Ag nanocubes (∼50 nm in edge length) was added to 5 mL of deionized water. This diluted dispersion was then refluxed for 8 min before dropwise addition of 0.5 mM Na2PdCl4 (or Na2PtCl4). After addition of the noble metal salt, the solution was refluxed until its color became stable (about 10 min). Vigorous magnetic stirring was maintained during the entire process. After the solution had been cooled to room temperature, white AgCl precipitated and settled at the bottom of the container. The AgCl was dissolved by adding a saturated solution of NaCl. The solution was then transferred into a tube and centrifuged at 10 000 rpm for 20 min. The supernatant containing the dissolved AgCl could be removed easily using a pipet. The solid was rinsed with water and centrifuged six more times before final dispersion in water for further analysis. SEM and TEM samples were prepared by drying a drop of final product on a small piece of Si wafer or a carbon-coated Cu grid under vacuum. SEM images were taken using a Sirion XL field-emission microscope (FEI, Hillsboro, OR) operated at an acceleration voltage of 15 kV. Energy-dispersive X-ray (EDX) measurements were conducted with EDAX system attached to the same microscope. TEM imaging and electron diffraction patterning was done using a Philips 420 microscope at 120 kV. The percentage of Pd and Ag in the nanostructures was analyzed using an atomic emission spectrophotometer (Thermo Jarrel Ash Corp., Franklin, MA) equipped with an inductively coupled plasma system. The emission lines at 328.0 and 340.5 nm were used to measure the contents of Ag and Pd, respectively. The UV-visible spectra were obtained using a Hewlett-Packard 8452A diode array spectrophotometer. XRD
2062
(9) (10) (11) (12) (13) (14)
(15)
(16)
(17)
(18)
patterns were recorded with a Philips 1820 diffractometer equipped with a Cu KR radiation source (E ) 8.04 keV, λ ) 1.544 Å). SERS substrates were prepared by drying 3 µL of the final product onto a 25 nm thick Al film, which was itself supported on a Si wafer. The substrate was then incubated in a 4 mM aqueous 4-mercaptopyridine solution for 1 h, rinsed with deionized water, and dried in air. SERS spectra were obtained at 785 nm laser excitation (Renishaw HPNIR785) with 0.03 mW and a spot size of 1.6 µm at the surface using a Leica DM IRBE optical microscope equipped with a Renishaw inVia Raman system, a 1200 l/mm grating, and a thermoelectrically cooled CCD detector. Kottmann, J. P.; Martin, O. J. F.; Smith, D. R.; Schultz, S. Phys. ReV. B 2001, 64, 235402. Moffatt, W. G. The Handbook of Binary Phase Diagrams; Genium Pub. Corp.: Schenectady, NY, 1976-; p 11/88. Berzins, D. W.; Kawashima, I.; Graves, R.; Sarkar, N. K. Dent. Mater. 2000, 16, 266. Batzill, M.; Koel, B. E. Surf. Sci. 2004, 553, 50. Koifmann, I. Arch. Sci. Phys. Nat. 1915, 40, 12509. The extinction (Qext), absorption (Qabs), and scattering (Qsca) coefficients (Qext ) Qabs + Qsca) of the nanoboxes are computed using the discrete dipole approximation (DDA) method. For the calculation, the nanobox was divided into an array of N consecutive cells, with each cell being approximated as a polarizable point electric dipole with a moment Pi. Under the excitation of a monochromatic optical field, these dipoles oscillate, couple with each other, and reradiate light. The complex refractive index of the alloy was obtained by a linear superposition of those of the composite metals: nalloy ) nAg + (1 - XAg) nPd, where XAg is the volume fraction of Ag. In addition, the nanobox was assumed to be surrounded and filled by water. Both absorption and scattering cross-sections could be obtained directly from Pi. In plotting the spectra, the cross-sections were divided by πaeff2 (aeff is related to the effective volume by 4πaeff3/3 ) d3, where d is the outer edge length of the nanobox) to obtain the dimensionless coefficients (Q). Hirsch, L. R.; Stafford, R. J.; Bankson, J. A.; Sershen, S. R.; Rivera, B.; Price, R. E.; Hazle, J. D.; Halas, N. J. Proc. Natl. Acad. Sci. U.S.A. 2003, 100, 13549. (a) Tian, Z.-Q.; Ren, B.; Wu, D.-Y. J. Phys. Chem. B 2002, 106, 9463. (b) Haes, A. J.; Haynes, C. L.; McFarland, A. D.; Schatz, G. C.; Van Duyne, R. P.; Zhou, S. MRS Bull. 2005, 30, 368. (c) Tessier, P. M.; Velev, O. D.; Kalambur, A. T.; Rabolt, J. F.; Lenhoff, A. M.; Kaler, E. W. J. Am. Chem. Soc. 2000, 122, 9554. (a) Zou, S.; Chan, H. Y.; Williams, C. T.; Weaver, M. J. Langmuir 2000, 16, 754. (b) Tian, Z.-Q.; Ren, B. Annu. ReV. Phys. Chem. 2004, 55, 197. (a) Sun, Y.; Tao, Z.; Chen, J.; Herricks, T.; Xia, Y. J. Am. Chem. Soc. 2004, 126, 5940. (b) Tobisˇka, P.; Hugon, O.; Trouillet, A.; Gagnaire, H. Sens. Actuators, B 2001, 74, 168.
NL051652U
Nano Lett., Vol. 5, No. 10, 2005
