Shape-Templated Growth of Au@Cu Nanoparticles - The Journal of

Article pubs.acs.org/JPCC

Shape-Templated Growth of Au@Cu Nanoparticles Alejandro F. Alvarez-Paneque,† Benito Rodríguez-González,†,‡ Isabel Pastoriza-Santos,*,† and Luis M. Liz-Marzán*,† †

Departamento Química Física, Universidade de Vigo, 36310 Vigo, Spain International Iberian Nanotechnology Laboratory, Braga 4715, Portugal

‡

S Supporting Information *

ABSTRACT: We report the formation of copper nanoparticles with various morphologies and low polydispersity, using Au nanoparticles as templates. This seeded growth strategy is based on the reduction of Cu2+ with hydrazine in water at low temperature. Additionally, the use of poly(acrylic acid) as capping agent allows synthesis under aerobic conditions. The dimensions of the resulting Au@Cu nanoparticles can be readily tuned through either the dimensions of the Au cores or the Cu/Au molar ratio. Although Au and Cu show a significant lattice mismatch, epitaxial growth of Cu onto single crystal Au nanorods was confirmed through high-resolution electron microscopy and electron diffraction analysis. The effects of core morphology on the optical properties of the core−shell nanoparticles were analyzed by vis-NIR spectroscopy and were found to agree with simulations based on the boundary element method. This work contributes to understand the strong effect of interband transitions on the optical response of Au@Cu and to confirm the importance of tuning the localized surface plasmon resonance away from the interband transitions.

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INTRODUCTION The plasmonic properties of Cu nanoparticles have received significantly less attention than those of other noble metals, mainly because of their low chemical stability, which leads to oxidation into Cu 2+ or copper oxides under ambient conditions,1 but also because copper shows a lower freeelectron character than silver or gold. Interband transitions from the valence band to the Fermi level occur at 2.1 eV for Cu, which corresponds to a wavelength of 590 nm.2,3 As a consequence, the localized surface plasmon resonance (LSPR) band often overlaps the interband transition threshold, and this electromagnetic interference produces a strong damping on plasmon resonances.4 Nevertheless, copper is much cheaper and has thus become an attractive alternative to gold and silver. In fact, a great scientific and technological interest for Cu nanoparticles synthesis has arisen during the last two decades, especially for applications in electronics and catalysis.5 Taking into account that plasmon resonances in metal nanoparticles strongly depend on particle size and even more strongly on particle morphology,6,7 control over these parameters should allow tuning the LSPR energy with respect to the onset of interband transitions and consequently obtaining Cu nanoparticles with well-defined and intense LSPR bands in the visible and the near IR. Although several wet chemistry strategies have been reported, in most cases only highly polydisperse Cu nanoparticles with a variety of morphologies were obtained, leading to poorly defined optical properties.5 Therefore, the control of particle size and shape is still a challenge for copper. Nevertheless, some reports are © 2012 American Chemical Society

worth mentioning, such as the recent synthesis of Cu nanocubes, nanowires, and bipyramids, obtained in water by reduction with glucose, using hexadecylamine (HDA) as a capping agent.8 The HDA concentration was claimed to determine the crystalline structure of the initial nuclei and therefore to control the final shape, whereas reaction time allowed a certain control on particle size. Cu nanospheres and nanowires with tunable sizes and low dispersity were also obtained by thermal decomposition in organic solvents, using a surfactant as structure-directing agent.9 Nonetheless, high quality plasmonic response is still rare, perhaps with the exception of Cu nanoplates prepared by hydrazine reduction in N,N-dimethylformamide (DMF) and poly(vinylpirrolidone) (PVP) as capping agent.10 The high yield of plate-like morphology resulted in remarkably sharp plasmon resonance bands. However, a real control over particle size and shape is missing, and the solution might be found in the use of seedmediated growth, which has proven very efficient for other noble metals. This synthetic route is based on the reduction of a metal salt onto preformed seeds, thereby avoiding the critical nucleation step. Thus, through the crystalline structure of the seed and using suitable additives, morphology control is possible.11 However, this methodology is also attractive to Special Issue: Nanostructured-Enhanced Photoenergy Conversion Received: June 26, 2012 Revised: August 2, 2012 Published: August 3, 2012 2474

dx.doi.org/10.1021/jp3062724 | J. Phys. Chem. C 2013, 117, 2474−2479

The Journal of Physical Chemistry C

Article

obtain bimetallic core−shell nanostructures.11,12 However, very few reports can be found on the use of the seed-mediated growth to fabricate Cu nanoparticles with controlled size and shape. Xia and co-workers have recently reported the synthesis of Cu nanocubes by epitaxial growth on Pd seeds (despite a significant lattice mismatch, 7.1%). The side length of the resulting particles could be tuned from 50 to 100 nm by varying the molar ratio of Cu salt to Pd seeds.13 It is also worth mentioning the work by Tsuji et al.,14 who demonstrated the epitaxial growth of core−shell Au−Cu nanoparticles using PVP-assisted polyol reduction, however, focusing on the analysis of the epitaxial growth of Cu over Au cores with different shapes, rather than trying to control the morphology of the final nanoparticles. We propose here a strategy to synthesize Cu nanoparticles with controlled morphology and low polydispersity, using welldefined Au nanoparticles as templates. As a proof of concept, we used polycrystalline spheres and single crystal nanorods as seeds, though other sizes or morphologies can also be used. The process comprises the reduction of copper(II) acetate with hydrazine in the presence of poly(acrylic acid) (PAA), in water, and at moderate temperature (60 °C) but still with no need to use inert atmosphere. Control over the final particle size was achieved by varying either the Cu shell thickness (controlling the molar ratio between Cu salt and Au seeds) or the Au core dimensions. High-resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) analysis allowed us to determine an epitaxial growth of single crystalline Cu shells onto Au nanorods, despite of the large lattice mismatch. The optical properties of the Au@Cu core− shell nanoparticles were examined experimentally by vis-NIR spectroscopy, and compared to numerical calculations using the boundary element method (BEM).

CTAB, 0.5 mM HAuCl4, 0.019 M HCl, 0.8 mM ascorbic acid, and 0.12 mM AgNO3. The dimensions of the obtained gold nanorods were 65.7 ± 8.5 nm × 13.7 ± 2.3 nm, with an average aspect ratio of 4.2 ± 1.6. mPEG-SH Capping and Ethanol Transfer. Capping replacement of the as-prepared Au nanoparticles of different shapes was carried out following previous reports.19 Au Spheres. Twenty milliliters of the as-synthesized particles was centrifuged for 15 min at 4000 rpm to remove excess CTAB. The precipitate was collected and redispersed in 10 mL of Milli-Q water ([CTAB] ≈ 1 mM and [Au] ≈ 0.5 mM). An aqueous solution containing 4 mPEG-SH molecules/nm2 (ca. 3.4 × 10−8 moles for 57 nm particles) was then added dropwise under vigorous stirring and allowed to react for 30 min. Finally, the particles were centrifuged twice to remove excess mPEGSH and redispersed in 2 mL of ethanol. Au Nanorods. Twenty milliliters of the as-synthesized particles was centrifuged 3-fold for 15 min (first at 4000 rpm and then 2× at 8000 rpm). The resulting precipitate was redispersed in 10 mL of water ([CTAB] ≈ 1 mM and [Au] ≈ 0.7 mM). An aqueous solution containing 150 mPEG-SH molecules/nm2 (ca. 5.53 × 10−6 moles for Au nanorods) was added dropwise under vigorous stirring and allowed to react for 30 min. Finally, the particles were centrifuged twice to remove excess mPEG-SH and redispersed in 2 mL of ethanol. Preparation of Au@Cu Core Shell Particles. Au Spheres As Seeds. To 10 mL of 0.5 mM Cu(OAc)2 aqueous solution at 40 °C, 4.35 × 10−6 mol of PEG-SH capped Au spheres (57 nm) and 1 mL of PAA 10 mM were added. Then, 3.1 × 10−3 moles of N2H4 was added and allowed to equilibrate (complex formation) for 10 min. Subsequently, the temperature was increased to 60 °C at a rate of ca. 1.5 °C/min to favor the reduction of Cu(II) to Cu(0). The reaction was monitored by vis-NIR spectroscopy and was allowed to proceed until no further spectral evolution was observed. The obtained Au@Cu particles were washed by centrifugation to remove the excess of PAA and other reaction byproducts. The particles were centrifuged at least twice (first at 3500 rpm and then at 2000 rpm) for 25 min and redispersed in water. The Cu shell thickness was tuned by varying the Cu2+/Au0 molar ratio from 8 to 16, while the N2H4/Cu2+ molar ratio was kept constant at a value of 62. Au Nanorods As Seeds. The protocol was similar to that detailed for Au spheres. The only differences are the amounts of Au seeds (2.1 × 10−6 mol) and N2H4 (2.1 × 10−3 mol). Characterization. Transmission electron microscopy (TEM) was carried out with a JEOL JEM 1010 transmission electron microscope operating at an acceleration voltage of 100 kV. Size (and shell thickness) distributions were determined from TEM images using more than 100 particles. HRTEM/ SAED analysis was carried out in a JEOL JEM 2010 FEG TEM operating at an acceleration voltage of 200 kV. The specimens were prepared by depositing a droplet on pure carbon-coated grids and evaporating the solvent in air at room temperature. Vis-NIR spectra were measured with an Agilent 8453 spectrophotometer. BEM Simulations of Optical Properties.24,25 Different regions were defined with the dielectric properties of water (refractive index 1.333), Au, Cu, and Cu2O. For Au and Cu, frequency-dependent dielectric functions were taken from ref 3 and for Cu2O from ref 20. Since the BEM is particularly advantageous in axial symmetry, all core−shell particles were described by concentric spheres and rods with the average

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EXPERIMENTAL SECTION Chemicals. Tetrachloroauric acid (HAuCl4·3H2O), copper(II) acetate (Cu(OAc)2·H2O), poly(acrylic acid) (PAA, Mw 1800), and hydrazine monohydrate (N2 H4 ·H 2O) were purchased from Aldrich. Ascorbic acid, sodium citrate (C6H5O7Na3·2H2O), silver nitrate (AgNO3), and sodium borohydride (NaBH4) were supplied by Sigma. Cetyltrimethylammonium bromide (CTAB) and O-[2-(3mercaptopropionylamino)ethyl]-O′-methyl-poly(ethylene glycol) (mPEG-SH, Mw 5000) were procured from Fluka. HCl (37%) was supplied by Panreac. All chemicals were used as received. Pure-grade ethanol and Milli-Q-grade water were used in all preparations. Gold Nanosphere Synthesis. CTAB-stabilized gold nanospheres with average size of 57.0 ± 7.6 nm were prepared by seeded growth as previously reported.15 Briefly, 2.5 mL of 0.1 M ascorbic acid was added to 250 mL of an aqueous solution at 35 °C containing HAuCl4 (0.25 mM) and CTAB (0.015 M). Subsequently, 4 mL of Au seeds (0.5 mM, 15 nm citrate stabilized Au nanoparticles synthesized through Turkevich’s method) was added under mild stirring.16 The growth process was finished in ca. 30 min. The population of nonspherical particles formed during the process was removed through CTAB-assisted shape-selective separation.17 Gold Nanorod Synthesis. Gold nanorods were prepared by Ag-assisted seeded growth.18 Au nanoparticle seeds were prepared by borohydride reduction of HAuCl4 (5 mL, 0.25 mM) in a 0.1 M aqueous CTAB solution. The seed solution (24 μL) was then added to a growth solution containing 0.1 M 2475



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Figure 1. (A,B) Representative TEM micrographs of core−shell Au@Cu nanoparticles obtained using as seeds (PEG)-capped Au polycrystalline spheres (57.0 ± 7.6 nm diameter) and single crystal nanorods (65.7 ± 8.5 nm long, 13.7 ± 2.3 nm thick). (C.1, D.1) HAADF STEM images showing mass−thickness contrast; the Au cores are brighter due to gold higher Z number. (C.2, D.2) STEM-XEDS elemental maps (Au = red; Cu = green) of the same nanoparticles shown in C.1 and D.1.

exclusively located at the center of the particles, while Cu (in green) appears at the whole particle area. The efficiency and versatility of this strategy allowed us to tune the final particle size by either varying the Cu2+/Au molar ratio or employing Au seeds with different sizes or aspect ratios. This is shown in Figure 2, where we collected TEM images of AuSP@Cu nanoparticles with different Cu shell thicknesses, ranging from ca. 15 to 38 nm, grown on 57 nm Au nanospheres by simply varying the Cu2+/Au molar ratio from 8 to 16. The thickness of the Cu shells could not be further increased since higher molar ratios would lead to the formation of isolated Cu nanoparticles. However, larger particles could indeed be obtained when larger Au cores were used as seeds (see in Figure S3, Supporting Information, TEM images of AuSP@Cu nanoparticles obtained from 100 nm Au spheres). Although both copper and gold display a face-centered cubic (fcc) crystalline structure, their lattice constants (0.4079 and 0.3615 nm for Au and Cu, respectively) present a relatively large mismatch of 11.4%. Although it is generally believed that the epitaxial growth of a metal shell onto a different metal can only take place when the lattice mismatch between the metals is below 5%, recently both Tsuji et al.14 and Xia et al.8 demonstrated that Cu can be epitaxially grown on Au and Pd nanoparticles, respectively, in spite of their large lattice mismatch (11.4% for Au@Cu and 7.1% for Pd@Cu). They also found that, as a consequence of the mismatch, Cu deposition was not completely homogeneous but strongly dependent on the core crystalline structure. Thus, the core was not always located at the particle center and the epitaxial growth occurred at a slower rate on corners and edges of the seed. The analysis of the morphology of our resulting core− shell particles revealed that the majority of the particles obtained from Au spheres maintain the morphology of the starting seeds though the Au core is not always located in the center of the particle (Figures 1A and 2). This could be not only due to the lattice mismatch but also to the polycrystalline nature of Au nanospheres. When the seeds were Au nanorods, TEM analysis also revealed several interesting features: the cores were not perfectly located at the center, not just with different shell thickness at both lateral sides but also showing a less homogeneous distribution of Cu at the tips. In fact, there

dimensions determined by TEM. Convergence was achieved by using 250 parametrization points.

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RESULTS AND DISCUSSION The new synthetic method presented here for the growth of Cu shells on preformed Au nanospheres and nanorods is based on the reduction of Cu2+ ions with hydrazine onto the Au seeds, in the presence of poly(acrylic acid) (PAA) as capping agent (see the experimental section for details). The starting Au polycrystalline spheres (57.0 ± 7.6 nm diameter, Figure S1 in Supporting Information) and single crystal nanorods (65.7 ± 8.5 nm long, 13.7 ± 2.3 nm thick; Figure S2 in Supporting Information) were first coated with thiolated polyethylene glycol (mPEG-SH), and then, Cu2+ was reduced with hydrazine in the presence of PAA, leading to the uniform growth of a Cu shell on each Au core. This route presents several advantages from the synthetic point of view since the reaction takes place in aqueous medium at moderate temperature (60 °C), and more importantly, no inert atmosphere is required. The choice of PAA as stabilizer was motivated by the work of Wang et al.,21 who recently reported the use of PAA to synthesize rather polydisperse Cu particles from 30 to 80 nm in diameter. They found that PAA protects the Cu nanoparticles from oxidation, even when the process takes place in aqueous medium under aerobic conditions. The performance of this method is exemplified in Figure 1, which shows representative TEM images of Au@Cu nanoparticles synthesized from Au spheres (AuSP@Cu, Figure 1A) and nanorods (AuNR@Cu, Figure 1B). The core−shell structure can be easily discerned due to the different electron density between Au and Cu. The Au@Cu core−shell structure was further confirmed by both high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM), which is sensitive to atomic number, and STEM X-ray energy dispersive spectroscopy (XEDS) elemental mapping. The HAADF STEM images displayed in Figure 1C.1,D.1 shows cores that are much brighter than the shells, which can be explained in terms of the higher atomic number of gold (Z = 79) as compared to that of copper (Z = 29). This analysis agrees with the STEM XEDS RGB elemental mapping displayed in Figure 1C.2,D.2, which shows that Au (in red) is 2476



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Figure 3. (A,B) HRTEM images showing a small area of Au@Cu core−shell nanoparticles grown from Au single crystal nanorods (A) and polycrystalline spheres (B). (C,D) SAED patterns from the particles shown in the insets without any in-plane rotation over the diffraction pattern. The spots for 02̅0 reflections from Cu and Au are circled in red and yellow, respectively.

metrical arrangement of Moiré fringes are determined by the differences in the lattice parameters of core and shell and the relative orientation between both lattices. Further confirmation of the single crystalline epitaxial growth was provided by selected area electron diffraction. The SAED pattern shown in Figure 3C was obtained from the AuNR@Cu nanoparticle shown in the inset, with the in plane rotation compensated, and resembles a single crystal pattern in the [001] zone axis, which is common to Au and Cu. This pattern, however, presents double-diffraction spots, the brighter spots corresponding to metallic Au fcc reflections, while the weakest are due to copper double diffraction. In Figure 3C, Cu 02̅0 and Au 02̅0 spots were marked with a red and a yellow circle, respectively, showing that these two spots are surrounded by double-diffraction spots, originated from the path of the electron beam through copper, gold core, and again through the bottom copper shell. In this figure, spots from both Au and Cu lie on the same line, passing by the center of the diffraction pattern in the same reciprocal space direction, meaning that there is a common crystalline direction of the Au and Cu lattices and again suggesting epitaxial growth of Cu over Au. Further, in some of the analyzed Au@Cu nanoparticles, the SAED pattern presents four faint double-diffraction spots centered on the main fcc reflections, which correspond to the {110} planes of Cu2O (see Figure S4 in Supporting Information, green arrows). In these SAED patterns, there are also some broad spots (white arrows) in the same reciprocal direction as Cu{020} and Au{020}, which might result from the overlap of Cu2O{020} planes and the double diffraction of Au{020} planes. The fact that the faint spots were not observed in all the particles analyzed by SAED suggests that the amount of Cu2O onto the core−shell particles is rather small (a thin passivating shell). In the case of AuSP@Cu spheres, although Moiré patterns can also be seen on the core, they are not well-defined and do

Figure 2. TEM micrographs of core−shell AuSP@Cu nanoparticles with different Cu shell thicknesses: 18.1 ± 4.1 nm (A), 26.0 ± 3.9 nm (B), and 37.7 ± 5.5 nm (C). The core diameters are ∼57 nm in all cases. The Cu2+/Au molar ratios were 8 (A), 12 (B), and 16 (C).

are particles where the tip coating seems to be rather thin, which could be explained in terms of both lattice mismatch and capping agent effect. It has been reported that Ag preferentially deposits at the lateral sides of single crystalline Au nanorods when thiolated PEG (capping agent used here) was used as stabilizer.22 Figure 3A,B shows HRTEM images of the Au@Cu core− shell nanoparticles, synthesized from single crystalline rods (Figure 3A) and polycrystalline spheres (Figure 3B). Interestingly, the HRTEM image of an AuNR@Cu nanorod shows fringes, known as Moiré patterns, which originate from the overlap of two or more crystalline lattices (Au and Cu lattices in this case) in the electron beam path. It is important to note that the interference fringes are almost parallel and cover the whole nanorod core area, which suggests a single crystalline epitaxial growth of Cu onto the single crystalline Au nanorods. Generally, the separation distance and the geo2477



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Figure 4. (A,B) Time evolution of the LSPR bands during the growth of a Cu shell via reduction of Cu2+ ions with hydrazine onto Au spheres and nanorods, respectively. The reaction time displayed in the labels is the time after reaching 60 °C. Dashed lines correspond to the absorption spectra of the resulting core−shell nanoparticles after washing (normalized to the maximum before washing to facilitate comparison). (C,D) Calculated extinction cross-section of an Au@Cu core−shell nanoparticle comprising a 57 nm Au sphere (C) or a 67 × 14 nm Au nanorod (D) and different Cu shell thickness, from 0 through 33 nm. For the calculations, water was used as the surrounding medium. The geometrical models used in the simulations are depicted in the plots.

shell nanoparticles must also be taken into account to explain the optical effects. In fact, the experimental observations are in good agreement with calculations of the extinction crosssection for the same morphologies based on the boundary element method (BEM).24,25 As shown in Figure 4C, the LSPR band of a 57 nm Au sphere shows a gradual red-shift and intensity increase when the Cu shell thickness is increased. As the LSPR shifts away from the region of the interband transitions (below 590 nm), the plasmon band becomes more intense and better defined. In the case of Au nanorods, the simulated spectra (Figure 4D) resemble the experimental spectral evolution in terms of band position but not quite in terms of band intensities. Thus, the LSPR damping produced by electromagnetic interference between interband and LSPR is not evidenced in the experimental data (see Figure 4B). In general, the discrepancies between experimental and calculated spectra may arise from the presence of byproducts that present a strong scattering contribution (easily removed by centrifugation, dashed lines in Figures 4A,B). Minor changes may additionally arise from the presence of a copper oxide outer shell, as confirmed by HRTEM. This agrees with BEM calculations for a 57 nm Au sphere coated with 14 nm Cu shell plus 3 nm Cu2O, which shows a red-shift of 16 nm as compared to the same sphere coated with a 17 nm Cu shell (see Figure S7, Supporting Information). Finally, additional simulations performed for pure Cu spheres and nanorods of the same size as the Au@Cu nanoparticles confirm that core−shell particles with thin Cu shells already present similar optical properties to those of the pure Cu particles (see Figure S8, Supporting Information).

not cover the whole core area (Figure 3B). This is likely due to the polycrystalline structure of the core, which may induce the growth of a polycrystalline Cu shell. The SAED pattern for these particles is almost a ring pattern containing diffraction spots from both the Au core and Cu shell. The presence of numerous spots that are not arranged in a regular manner clearly points toward a polycrystalline Cu shell. The presence of less defined Moiré patterns in the shell area for both samples (see Figure S5 in Supporting Information) can be explained in terms of a polycrystalline structure of the Cu shell for the spheres, but this should not hold for the nanorods. The presence of faint diffraction spots in the SAED pattern and the tendency of Cu to oxidize in air23 suggest that the Moiré arise from the presence of a thin copper oxide outer layer. The growth of the Cu shells could be monitored by vis-NIR spectroscopy through changes in the plasmonic response when Cu covered the Au nanoparticle seeds. Figures 4A,B shows the evolution of the LSPR bands of Au spheres (centered at 547 nm) and rods (centered at 850 nm) during coating with Cu. The deposition of Cu, and thus the gradual shift of the LSPR band, starts once the reaction medium reaches 60 °C.10 Below this temperature, the Cu[(N2H4)2]2+·2AcO− complex does not decompose, and therefore, the Cu reduction does not take place (see Figure S6, Supporting Information). However, the evolution of the LSPR bands is different for Au spheres and rods. Whereas a gradual red-shift is observed for spheres (from 547 to 578 nm), the longitudinal LSPR band for Au nanorods blue-shifts from 853 to 578 nm (Figure 4B). An increase of the LSPR intensity is observed in both cases. This different behavior partly stems from the geometrical changes due to Cu deposition, i.e., an increase of the particle volume in spheres and a decrease of the aspect ratio for nanorods, as observed in the TEM images (Figure 1). Notwithstanding, the influence of Cu on the effective dielectric function of the core−

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CONCLUSIONS The seed mediated method can be used to grow Cu nanoparticles with controlled size and morphology, determined 2478



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(13) Xia, Y.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (14) Tsuji, M.; Yamaguchi, D.; Matsunaga, M.; Alam, M. J. Cryst. Growth Des. 2010, 10, 5129. (15) Rodríguez-Fernández, J.; Pérez-Juste, J.; García de Abajo, F. J.; Liz-Marzán, L. M. Langmuir 2006, 22, 7007. (16) Enüstün, B. V.; Turkevich, J. J. Am. Chem. Soc. 1963, 85, 3317. (17) Jana, N. R. Chem. Commun. 2003, 1950. (18) Nikoobakht, B.; El-Sayed, M. A. Chem. Mater. 2003, 15, 1957. (19) Fernández-López, C.; Mateo-Mateo, C.; Á lvarez-Puebla, R. A.; Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M. Langmuir 2009, 25, 13894. (20) Palik, E. D., Ed. Handbook of Optical Constants of Solids; Academic Press: New York, 1985; Vols. 1 and 2. (21) Wang, Y.; Biradar, A. V.; Wang, G.; Sharma, K. K.; Duncan, C. T.; Rangan, S.; Asefa, T. Chem.Eur. J. 2010, 16, 10735. (22) Sánchez-Iglesias, A.; Carbó-Argibay, E.; Glaria, A.; RodríguezGonzález, B.; Pérez-Juste, J.; Pastoriza-Santos, I.; Liz-Marzán, L. M. Chem.Eur. J. 2010, 16, 5558. (23) Kim, J. H.; Ehrman, S. H.; Germer, T. A. Appl. Phys. Lett. 2004, 84, 1278. (24) García de Abajo, F. J.; Howie, A. Phys. Rev. Lett. 1998, 80, 5180. (25) García de Abajo, F. J.; Howie, A. Phys. Rev. B 2002, 65, 115418.

by Au nanoparticle seeds, thus allowing surface plasmon resonance tuning. Uniform deposition of Cu onto Au nanospheres and nanorods was achieved in water, at low temperature, and under aerobic conditions. HRTEM and SAED analysis showed that epitaxial growth of Cu occurred on single crystalline Au nanorods but could not be confirmed for Au spheres due to their polycrystalline nature. The optical response of the Au@Cu nanoparticles was studied by vis-NIR spectroscopy, and the experimental data agreed with simulations using the boundary element method. Both experiments and calculations confirmed the importance of tuning the LSPR energy with respect to the onset of interband transitions for obtaining Cu nanoparticles with well-defined and intense optical response.

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ASSOCIATED CONTENT

S Supporting Information *

Vis-NIR spectra and TEM images of starting Au spheres and rods, TEM of AuSP@Cu from 100 nm Au spheres, SAED pattern showing the presence of oxide layer, HRTEM showing Moiré patterns outside the core area, optical spectra during the reduction process, calculated spectra including the oxide outer shell and comparing Au@Cu with pure Cu particles. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (I.P.-S.); [email protected] (L.M.L.-M.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.F.A.-P. acknowledges a scholarship from the Spanish Ministerio de Asuntos Exteriores y de Cooperación - Agencia Española de Cooperación Intenacional para el Desarrollo (MAEC-AECID Grant 0000584999 2011). This work has been supported by the Spanish MINECO (grant MAT2010-15374)) and by the Xunta de Galicia/FEDER (grant 10PXIB314218PR). L.M.L.-M. acknowledges funding from the European Research Council (ERC Advanced Grant 267867-PLASMAQUO).

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REFERENCES

(1) Xia, Y.; Xiong, Y. J.; Lim, B.; Skrabalak, S. E. Angew. Chem., Int. Ed. 2009, 48, 60. (2) Ehrenreich, H.; Philipp, H. R. Phys. Rev. 1962, 128, 1622. (3) Johnson, P. B.; Christy, R. W. Phys. Rev. B 1972, 6, 4370. (4) Pakizeh, T. J. Phys. Chem. C 2011, 115, 21826. (5) Lignier, P.; Bellabarba, R.; Tooze, R. P. Chem. Soc. Rev. 2012, 41, 1708. (6) Kelly, K. L.; Coronado, E.; Zhao, L. L.; Schatz, G. C. J. Phys. Chem. B 2003, 107, 668. (7) Liz-Marzán, L. M. Langmuir 2006, 22, 32. (8) Jin, M.; He, G.; Zhang, H.; Zeng, J.; Xie, Z.; Xia, Y. Angew. Chem., Int. Ed. 2011, 50, 10560. (9) Wei, Y.; Chen, S.; Kowalczyk, B.; Huda, S.; Gray, T. P.; Grzybowski, B. A. J. Phys. Chem. C 2010, 114, 15612. (10) Pastoriza-Santos, I.; Sánchez-Iglesias, A.; Rodríguez-González, B.; Liz-Marzán, L. M. Small 2009, 5, 440. (11) Grzelczak, M.; Pérez-Juste, J.; Mulvaney, P.; Liz-Marzán, L. M. Chem. Soc. Rev. 2008, 37, 1783. (12) Chaudhuri, R. G.; Paria, S. Chem. Rev. 2012, 112, 2373. 2479


Shape-Templated Growth of Au@Cu Nanoparticles - The Journal of

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