One-Pot Controllable Synthesis of Au@Ag Heterogeneous Nanorods

Article pubs.acs.org/cm

One-Pot Controllable Synthesis of Au@Ag Heterogeneous Nanorods with Highly Tunable Plasmonic Absorption Cuncheng Li,*,† Lin Sun,† Yiqiang Sun,† and Toshiharu Teranishi*,‡ †

Key Laboratory of Chemical Sensing & Analysis in Universities of Shandong (University of Jinan), School of Chemistry and Chemical Engineering, University of Jinan, Jinan 250022, Shandong, People’s Republic of China ‡ Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan S Supporting Information *

ABSTRACT: It generally requires a complex workup procedure for the fabrication of Au@Ag heterogeneous nanostructures with an accurate morphology by the present multistep seed-mediated growth approaches. In this paper, we present a new and straightforward method for the controllable synthesis of uniform Au@Ag heterogeneous nanorods (NRs) by coreduction of gold and silver sources in a one-pot polyol reaction. High-quality Au@Ag heterogeneous NRs of various aspect ratios were facilely and selectively produced in high concentration by tuning the initial experimental parameters. Our synthetic approach is highlighted by its simplicity, large-scale production, and controllability of the synthesis. Our study indicates the oxidative etching by O2/Cl− pairs plays a key role for the high-yield synthesis of uniform Au@Ag heterogeneous NRs. The size-dependent optical properties of Au@Ag heterogeneous NRs were first and systematically investigated. Our experiments reveal that Au@Ag heterogeneous NRs exhibit two strong absorption peaks that, respectively, originate from the transverse and longitudinal localized surface plasmon resonances (LSPRs). Moreover, the longitudinal LSPR can be facilely tuned from the visible to the near-infrared regions by changing the aspect ratio of Au@Ag heterogeneous NRs. Importantly, Au@Ag heterogeneous NRs synthesized by our method have an excellent stability. They can maintain their optical properties over a long period of time. Au@Ag heterogeneous NRs with an interesting plasmonic property would have fascinating application in surface plasmonics, surface-enhanced Raman scattering, chemical and biological sensing, optical labeling, and information storage. KEYWORDS: gold, silver, heterogeneous nanorods, polyol synthesis, surface plasmon resonance

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appears brighter than a Ag NP of the same size in the dark field due to its heavier atomic number (ZAg = 47, ZAu = 79).27,28 Accordingly, Au@Ag bimetallic nanostructures, where Ag provides ideal optical properties and Au imparts a bright image, have been an active subject of research. To date, a series of Au@Ag core−shell nanostructures, which include polyhedral Au@Ag NPs,5,11−16 Au@Ag NRs,17−21 and Au@Ag nanoprisms,22−24 have been successfully synthesized by the seedmediated approaches. Among them, one-dimensional Au@Ag NRs are especially interesting because they have anisotropic optical properties and fascinating applications in surface plasmonics, SERS, chemical and biological sensing, optical labeling and imaging, and information storage.21 Au@Ag core− shell NRs were usually synthesized through conformal deposition of Ag on the surface of the preformed Au NRs.19−21 In these cases, the length of the Au@Ag core− shell NRs was strongly limited by that of Au NRs. Consequently, the aspect ratio of Au@Ag core−shell NRs was decreased as the thickness of the Ag shell increased, resulting in that the corresponding longitudinal LSPR gradually

INTRODUCTION Heterogeneous bimetallic nanoparticles (NPs), as an important family of inorganic materials, have attracted considerable attention due to their unique optical, electrical, and catalytic properties compared to the monometallic counterparts and alloys.1,2 In general, heterogeneous bimetallic NPs with welldefined shapes were generated through controllable deposition of a metal on the surfaces of well-faceted seeds made of another metal.3−24 A common feature of this technique is that the seed preparation and the overgrowth of secondary metal were separated into two or more distinct processes. The synthesis procedure would be simplified if the controllable synthesis of seeds and the preferential overgrowth of heterogeneous metal are achieved in a one-pot reaction. Recently, Au@Pd core− shell octahedra have been successfully prepared by a straightforward synthetic approach.25 However, up to now, it remains a great challenge for the fabrication of Au@Ag heterogeneous bimetallic NPs with an accurate morphology through a one-pot wet-chemical method. Usually, a AuAg bimetallic alloy or core−shell NP with an uncontrolled spherical shape was produced if gold and silver precursors were simultaneously reduced in the same reaction medium.26 As is known, Ag NPs have a higher extinction coefficient and SERS enhancement factor than Au NPs. The image of a Au NP © XXXX American Chemical Society

Received: January 31, 2013 Revised: May 20, 2013

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dx.doi.org/10.1021/cm400392e | Chem. Mater. XXXX, XXX, XXX−XXX

Chemistry of Materials

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Figure 1. (a) Photograph of the as-synthesized Au@Ag heterogeneous NR colloid: In this case, green color was only observed at the interface of the colloid and air due to that the concentration of the product is high. (b) Photograph of the diluted Au@Ag heterogeneous NR colloid solution (×5): green color in the whole solution was clearly observed. (c) Low- and (d) high-magnification TEM images of Au@Ag heterogeneous NRs. (e) HAADF-STEM image of a single Au@Ag heterogeneous NR. EDS line profiles of Au and Ag along (f) line 1 and (g) line 2 in (e). Scale bars for (c)−(e) are 2 um, 100 nm, and 20 nm, respectively.

of various aspect ratios by the present multistep seed-mediated or template methods. As is known, controllable synthesis of well-defined decahedral or rodlike Au seeds and the AAO template itself is also a challenging and complex task. Therefore, it is highly desirable to develop a straightforward synthetic strategy used to make high-quality Au@Ag heterogeneous NRs of various sizes for uncovering their properties and for achieving their fascinating applications. Herein, we report a straightforward and effective one-pot polyol process for the controllable synthesis of Au@Ag heterogeneous NRs by coreduction of gold and silver sources in an ethylene glycol (EG) solution, which is a frequently used polyol, using poly(diallyldimethylammonium) chloride (PDDA) as both the stabilizer and the shape-controller. The synthetic procedure requires no seed preparation procedure and further addition of any reagents until the formation of Au@ Ag heterogeneous NRs. High-yield Au@Ag heterogeneous NRs of various aspect ratios were facilely and selectively synthesized only by tuning the initial experimental parameters. The sizedependent optical properties of Au@Ag heterogeneous NRs were first and systematically investigated in this study. Our study reveals that Au@Ag heterogeneous NRs synthesized by our method have a highly tunable plasmonic absorption. Moreover, the as-synthesized Au@Ag heterogeneous NRs show an excellent stability. They can maintain their optical properties over a long period of time. Our synthetic approach is

blue shifts. Interestingly, another seed-mediated preparation pathway for the synthesis of Ag@Au@Ag heterogeneous NRs was first described by Song’s group.29a In this work, the pentatwinned Au decahedra or NRs synthesized in a diethylene glycol (DEG) solution were employed as seeds. This work provides an effective approach for the synthesis of Ag@Au@Ag heterogeneous NRs, though this method needs a purification process to remove the large aggregates. Recently, Yang et al. demonstrated a kinetic control strategy for the fabrication of heterostructured Au@Ag NRs by a modified seed-mediated growth process.29b In their study, decahedral Au or NRs colloids were first synthesized in a DEG solution in the presence of a given amount AgCl at a high temperature. Subsequently, a given amount of aqueous ammonia and water was introduced into the as-synthesized Au colloid to dissolve insoluble AgCl and form the ammoniacal silver complex, which is then reduced to a Ag atom. Besides the seed-mediated synthesis, multisegmented Au@Ag heterogeneous NRs were fabricated by sequential electrodeposition of Au and Ag ions into the pores of an anodic aluminum oxide (AAO) membrane.30 To the best of our knowledge, although a few heterostructured Au@Ag NRs were successfully synthesized, the size-dependent optical properties of Au@Ag heterogeneous NRs have never been systematically investigated until now. As aforementioned, it generally requires a complex workup procedure for the fabrication of Au@Ag heterogeneous NRs B



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Figure 2. (a) Local HRTEM image of a single Au@Ag heterogeneous NR near the Au segment. Enlarged image of (b) the frame B and (c) the frame C in (a). (d) The corresponding FFT pattern of (c). Scale bars for (a)−(c) are 5, 2, and 5 nm, respectively. NRs, were also conducted to determine their influence on the synthesis of Au@Ag heterogeneous NRs. Analysis and Instruments. The products were characterized by transmission electron microscopy (TEM, JEOL, JEM-1400), scanning TEM (STEM), EDS, and high-resolution TEM (HRTEM, JEOL, JEM-2100F). Samples for TEM examination were prepared by putting a droplet of the treated solution on a copper grid coated with a thin carbon film and were then evaporated in air at room temperature. Xray photoelectron spectra (XPS) measurement for well-washed Au@ Ag heterogeneous NRs was performed using a VG ESCA2000 X-ray photoelectron spectrometer with a Mg Kα excitation source. XRD spectra were measured on an X-ray diffractometer (Philips X’pert PRO with Cu Kα radiation, λ = 0.15418 nm) by depositing the sample on glass. For optical measurements, the products were dispersed in water and the optical extinction spectra were recorded with a spectrophotometer (Cary 5E) using an optical quartz cell with a 10 mm path length.

highlighted by its simplicity, large-scale production, and controllability of the synthesis compared to the traditional multistep seed-mediated methods.

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EXPERIMENTAL SECTION

Materials. Chloroauric acid (HAuCl4, 99.9%), silver nitrate (AgNO3, ≥99.8%), PDDA (Mw = 400 000−500 000, 20 wt % in water), and PDDA (Mw = 100 000−200 000, 20 wt % in water) were purchased from Sigma-Aldrich. Ethylene glycol (ACS reagent) was purchased from Xilong Chemical Industry Incorporated Co., Ltd. Deionized water was prepared with a Milli-Q water purification system. All chemicals were used without further purification. Representative Synthesis of Au@Ag Heterogeneous NRs. For a typical synthesis of Au@Ag heterogeneous NRs, 0.025 mL of 0.5 M HAuCl4 (0.0125 mmol) aqueous solution, 1 mL of PDDA (1.25 mmol), and 0.25 mL of 0.5 M AgNO3 (0.125 mmol) aqueous solution were introduced into 49 mL of EG solution in a brown glass vial. The mixture solution was vigorously stirred for about 30 s at room temperature and ambient conditions. In this process, AgNO3 was converted to AgCl due to a mass of Cl− ions (from PDDA) that existed in the EG solution. The final concentrations of [AuCl4]− ions, AgCl, and PDDA were 0.25, 2.5, and 25 mM, respectively, giving a molar ratio of AgCl to [AuCl4]− ions of 10. The as-prepared mixture solution was subsequently reacted at 200 °C for 60 h in air in an oil bath. The final product was collected by centrifugation at 14 500 rpm and washed repeatedly with pure water and ultrasound to remove any residual EG or PDDA for characterization and optical property study. After centrifugation, the green products were deposited on the bottom of the tubes and the supernatant solution became clear, indicating that no residual product, such as Au, Au@Ag, or Ag NPs, existed in the solution. Further experiments, in which the amount of the reagents was controlled to be identical to that of the Au@Ag heterogeneous

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RESULTS AND DISCUSSION Characterization of Au@Ag Heterogeneous NRs. The final color of the colloid solution synthesized in an EG solution containing 0.25 mM HAuCl4, 2.5 mM AgNO3, and 25 mM PDDA appears green, as displayed in Figure 1a,b, which is definitely diverse from the color of spherical Au, Ag, and AuAg bimetallic NPs. Transmission electron microscopy (TEM) images demonstrate that the product (without purification, yield > 98%) was dominated by uniform NRs with lengths of 80 ± 5 nm and widths of 30 ± 2 nm (Figure 1c,d). The average aspect ratio of these NRs is 2.7. Interestingly, a single dark object embedded in the body of each particle was clearly observed in the high-magnification TEM image (Figure 1d). High-angle annular dark-field scanning TEM (HAADF-STEM) C



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and energy-dispersive X-ray spectroscopy (EDS) analysis were further applied to confirm the morphology and the composition of these NRs. High contrast between the center and two ends of the NRs was distinctly observed in the HAADF-STEM images (see Figure 1e and Figure S1, Supporting Information). The compositional analysis using EDS on a single Au@Ag heterogeneous NR along its long and short axes shows clear boundaries of Ag, Au, and Ag segments (Figure 1f,g). Moreover, our results confirmed that Au NP was completely embedded into Ag. They are thus better described as Au@Ag heterogeneous NRs rather than alloy or core−shell NRs because the distribution of Ag on the surfaces of Au decahedra is not conformal. No pure Ag or alloy AuAg NRs were observed, though our experiments were performed at a high temperature. Moreover, no aggregates were observed in the as-obtained Au@Ag heterogeneous NRs colloid solution in our experiments. Twin boundaries were distinctly observed on the surfaces of Au@Ag heterogeneous NRs, as illustrated in the HRTEM image of a single Au@Ag heterogeneous NR (Figure 2a). Three types of lattice fringes with interplanar spacings of 0.14, 0.20, and 0.23 nm, which, respectively, are attributed to {220}, {002}, and {111} planes of face-centered cubic (fcc) Ag, are clearly observed in the enlarged HRTEM image (Figure 2b). This suggests that Au@Ag heterogeneous NRs grew along the ⟨220⟩ direction, bound by {002} planes as the lateral surfaces, and terminated by {111} planes. The local enlarged HRTEM image of Figure 2a near the Au segment is shown in Figure 2c. The continuous lattice fringe at the junction of Au and Ag along the long axis suggests that the Ag segments were epitaxially grown on the surface of Au NP. The corresponding fast Fourier transform (FFT) pattern of Figure 2c further confirmed that the growth direction of Ag was along the ⟨220⟩ direction (Figure 2d). Optical Absorption of Au@Ag Heterogeneous NRs. The UV−vis-NIR optical absorption spectrum of the assynthesized Au@Ag heterogeneous NRs dispersed in water is shown in Figure 3. Interestingly, Au@Ag heterogeneous NRs with an aspect ratio of 2.7 exhibit two strong absorption peaks that are located at 393 and 675 nm. It is well-known that metal NRs exhibit transverse and longitudinal LSPRs that, respec-

tively, originate from light-induced electron oscillations oriented parallel to the short and long axes of the NRs. Generally, the transverse LSPR appears in the shorter wavelength region of the electromagnetic spectrum, whereas longitudinal LSPR always appears in the longer wavelength region.19−21,31−34 We thus attribute the absorption at 393 and 675 nm to transverse and longitudinal LSPRs of Au@Ag heterogeneous NRs, respectively. Note that, compared to the pure Au NRs, the transverse and longitudinal LSPRs of Au@Ag heterogeneous NRs have a comparable intensity, which is in accordance with the theoretical calculation of Ag NRs and Au@ Ag core−shell NRs.19,21,32 Moreover, the transverse LSPR absorption peak of Au@Ag heterogeneous NRs obviously blue shifts compared to that of Ag NPs with random shapes, which is located at ∼420 nm (vide infra). Formation Mechanism of Au@Ag Heterogeneous NRs. A series of experiments were performed by only changing the reaction time to investigate the formation mechanism of the Au@Ag heterogeneous NRs. A spectacular color evolution ranging from red (0.5 h) to green (60 h) through magenta (1 h), orange (2.5 h), yellow (10 h), orange (20 h), rose (30 h), deep blue (40 h), and sky blue (50 h) was clearly observed, as shown in Figure 4a. The corresponding UV−vis-NIR spectra of these samples measured in an EG solution is presented in Figure 4b,c. An absorption band at 567 nm (curve 1 in Figure 4b), a well-known LSPR peak of Au NPs, appears when the asprepared precursor was heated for 0.5 h (reddish color, sample 1 in Figure 4a), reflecting the formation of Au NPs. TEM observation reveals that decahedral NPs with a little broad size distribution (their edge lengths range from 14 to 20 nm) were exclusively produced (Figure 5a). Further EDS analysis confirmed that these decahedral NPs only consist of gold (Figure S2a,b, Supporting Information). As the reaction time increased, the average size of Au decahedra gradually decreased. For example, the mean edge length of decahedra harvested at 1 h decreased from 18.5 to 16.5 nm with a narrow size distribution (Figure 5b). No silver was detected for the decahedra obtained at 1 h (Figure S2c,d, Supporting Information), indicating that they are also decahedral Au NPs (reddish color, sample 2 in Figure 4a). This is well consistent with the results of the corresponding optical absorption spectra shown in Figure 4b. The LSPR absorption peak of Au decahedra blue shifts from 567 to 558 nm due to a decrease of particle size. Interestingly, the intensity of LSPR absorption increased, reflecting that new Au decahedra were formed during this process. These results suggest that a digestive ripening: smaller decahedral Au NPs grew, whereas the bigger Au decahedra partially dissolved with the increase of reaction time to reach a uniform size distribution, rather than Ostwald ripening occurred in our experiments.35 As is known, although the dissolution concentration of oxygen in a solution will gradually decrease as the temperature is increased, it is very difficult to completely remove oxygen from the reaction system only by a high-temperature heating when the experiments were performed in air. In our reaction system, besides oxygen, a mass of Cl− ions (from PDDA) together with a few H+ and NO3− ions (from HAuCl4 and AgNO3, respectively) existed; the strong oxidative etching from O2/Cl− pairs is inevitable. We thus believe that the dissolution of the bigger Au decahedra resulted from the oxidative etching of O2/Cl− pairs, which provides the Au sources for the further growth of the smaller Au decahedra and the formation of new Au decahedra. This suggests that the oxidative etching of O2/Cl− pairs plays a

Figure 3. Optical absorption spectrum of Au@Ag heterogeneous NRs with an aspect ratio of 2.7 dispersed in water. Two strong absorption peaks located at 393 and 675 nm were observed, which, respectively, correspond to the transverse and longitudinal LSPR absorptions of Au@Ag heterogeneous NRs. D



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Figure 4. (a) Photographs of the reaction products obtained at (1) 0.5, (2) 1, (3) 2.5, (4) 10, (5) 20, (6) 30, (7) 40, (8) 50, and (9) 60 h; photographs of samples (6)−(9) were recorded when the as-synthesized colloid solution was diluted 10 times (×10) with EG solution to clearly observe its color. (b, c) The corresponding UV−vis-NIR spectra of samples (1)−(9) in (a). Note that, for UV−vis-NIR spectrum measurement, the as-obtained reaction colloid solutions were diluted 10 times (×10) in (b) and 25 times (×25) in (c) with an EG solution of 25 mM PDDA.

Figure 5. TEM images of an EG solution containing 0.25 mM HAuCl4, 2.5 mM AgNO3, and 25 mM PDDA, which was reacted at 200 °C for different times: (a) 0.5, (b) 1, (c) 2.5, (d) 10, (e) 20, (f) 30, (g) 40, (h) 50, and (i) 60 h; the inset of (a) shows a schematic illustration of the decahedral structure. (j) Schematic illustration for the growth of Au@Ag heterogeneous NRs. All scale bars are 100 nm.

positive role for yielding uniform decahedral Au NPs, which is a key factor for the formation of Au@Ag heterogeneous NRs with uniform diameters. Further experiment was conducted to reveal that the oxidative etching by O2/Cl− pairs plays a key

role for the high-yield synthesis of uniform Au@Ag heterogeneous NRs. If the as-prepared precursor containing HAuCl4, AgNO3, and PDDA was heated under nitrogen (oxygen in the solution was removed by freeze−pump−thaw), E



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Figure 6. XPS spectra of the repeatedly washed Au@Ag heterogeneous NRs: (a) a survey scan and (b) Ag 3d, (c) Au 4f, and (d) N 1s energy regions.

growth active sites where Ag atoms were orientedly deposited, whereas the deposition of Ag atoms on the lateral facets was restrained. In the corresponding spectroscopy, both the transverse and the longitudinal LSPR absorptions were enhanced with the increase of the reaction time owing to the growth of Au@Ag heterogeneous NRs (Figure 4c). Moreover, the longitudinal LSPR absorption peak red shifts from 500 to 700 nm because the aspect ratio of the Au@Ag heterogeneous NRs increased. Generally, selective adsorption of surfactant on a given plane can significantly inhibit the crystal growth along one specific direction, leading to the corresponding crystalline plane becoming the basal surface of final products.36,37 Polyelectrolyte PDDA can serve as both stabilizer and shape-controller for the synthesis of metal NPs with well-defined shapes.37 In the present work, when the as-prepared EG solution containing HAuCl4 and AgNO3, but without PDDA, was heated at 200 °C, large agglomerates were formed in a few minutes due to lack of protection (Figure S4, Supporting Information). This suggests that PDDA was doubly significant for the synthesis of Au@Ag heterogeneous NRs. X-ray photoelectron spectra (XPS) measured for the repeatedly washed sample directly verifies that PDDA molecules strongly adsorbed on the surface of Au@ Ag heterogeneous NRs (Figure 6). In the XPS pattern, the signal of Ag was much stronger than that of Au, indicating that Au NPs were embedded into Ag. The peaks of both Ag 3d5/2 (368.4 eV) and Ag 3d3/2 (374.4 eV) shifted by ca. 0.2 eV toward higher binding energies relative to pure Ag. The peaks of both Au 4f7/2 (84.0 eV) and Au 4f5/2 (87.7 eV) did not shift

besides Au@Ag heterogeneous NRs, a large number of NPs with random shapes were obtained in the final product. Moreover, compared with the results shown in Figure 1, both the diameter and the length of Au@Ag heterogeneous NRs obtained under nitrogen have a broad size distribution (about 50−110 nm in diameter, about 200−700 nm in length, Figure S3, Supporting Information). With a reaction time of 2.5 h, the majority of the product was also uniform decahedral NPs with an edge length of 15 nm (Figure 5c). However, the color of the solution (sample 3 in Figure 4a) appears orange, which deviates from the color of pure Au NPs. EDS analysis indicates that the harvested product was Au@Ag core−shell bimetallic NPs (Figure S2e,f, Supporting Information), reflecting that Ag grew on the surfaces of Au NPs rather than homogeneous nucleation to form pure Ag NPs. As the precursor was heated for 10 h, the product was mainly composed of uniform Au@Ag heterogeneous NRs with an aspect ratio of 1.3 (Figure 5d; Figure S2g,h, Supporting Information). In this case, the LSPR splits into two absorption bands located at 403 and 500 nm (a shoulder) due to the rodlike morphology (curve 4 in Figure 4b,c).19−21,31−34 Our results indicate that the length of the NRs gradually increased with increasing the reaction time until the silver precursors were depleted, but their diameter was almost unchanged, resulting in that the aspect ratio of the harvested Au@Ag heterogeneous NRs gradually increased (Figure 5d−i). This reflects that the Au@Ag heterogeneous NR grew along its long axis, as illustrated in Figure 5j. It is rational to conclude that the two end facets of Au@Ag heterogeneous NRs were the F



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typical fingerprint for Ag NPs (Figure S6b, Supporting Information). This reflects that in situ-formed Au decahedra play a key role for the formation of Au@Ag heterogeneous NRs. On the contrary, if only HAuCl4 were introduced into the EG solution of PDDA, the final product was dominated by Au octahedra rather than decahedra (Figure 7a), which is in

compared to that of the pure Au. The binding energy peak at 400.3 eV originated from the charged nitrogen (N+) of the PDDA, which shifted by ∼1.7 eV toward lower binding energy compared to that of N+ in the pure PDDA.38 This verifies that PDDA molecules strongly interact with Ag atoms on the surface of Au@Ag heterogeneous NRs. Such a type of interaction between Ag and PDDA was attributed to the donation of an electron pair from Ag to PDDA, which is responsible for the shift in the binding energy. According to our results, we infer that PDDA molecules adsorb onto the {002} planes of Ag more strongly than others, and thus, the growth rate along the ⟨002⟩ direction is inhibited. Our results clearly demonstrated that, in the present polyol synthesis, the reduction of gold salt is prior to that of silver salt, although they coexist in the as-prepared precursor (Figures 4 and 5). It is because a mass of Cl− ions existed in the solution, resulting in that AgNO3 was transformed into AgCl NPs. The formation of AgCl NPs was confirmed by the powder X-ray diffraction (PXRD) measurement and TEM observation, as illustrated in Figure S5 (Supporting Information). Compared to AgCl, [AuCl4]− ions with a higher reduction potential can be readily reduced, thus generating Au NPs. It has been revealed that AgCl can release free Ag+ ions with a considerably slow rate at a high temperature,39 and this, in turn, results in a low generation rate of Ag atoms. In general, the Gibbs free energy needed for heterogeneous nucleation is lower than that for homogeneous nucleation.40 Thus, heterogeneous nucleation is preferable when sufficient nucleation sites for secondary metals are available and the supersaturation of solute monomers is small. Therefore, under a slow reaction like in our case, heterogeneous nucleation of Ag atoms on the surfaces of Au NPs occurs more easily than their homogeneous nucleation, leading to the formation of Au@Ag heterogeneous NPs. Previous results indicated that a slow reaction favors the anisotropic growth of a metal NP with the assistance of shapecontrolled polymers.36,37 It has been revealed that twin defects on an NP are more reactive sites than other regions for heterogeneous nucleation.29,34 Meanwhile, as aforementioned, PDDA molecules selectively adsorb onto the {002} planes of Ag, which inhibits the growth of the {002} planes. The reduced Ag atoms are thus preferentially deposited on the {111} twin planes of Au decahedra, resulting in the growth of Au decahedra along their ⟨220⟩ directions. Consequently, Au@Ag heterogeneous NRs with {002} planes as lateral facets were rationally produced in high yield. In our present case, the generation rate of Ag atoms is not so rapid due to the low solubility of AgCl. Therefore, Au@Ag heterogeneous NRs of various aspect ratios can be facilely and selectively harvested by tuning the reaction time (Figure 5). Influence Factors and Size Tailoring of Au@Ag Heterogeneous NRs. The influence factors for the formation of Au@Ag heterogeneous NRs were systematically investigated. We first found that the introduction of HAuCl4 in company with an appropriate amount of AgNO3 into the EG solution of PDDA is important for high-yield synthesis of Au@Ag heterogeneous NRs. For instance, if only AgNO3 was introduced into the EG solution containing PDDA, the final color of the solution appears yellow, a typical color of Ag NPs. TEM observation indicates that the as-obtained product was composed of 95% Ag NPs with random shapes and a mean size of 30 nm, 2% Ag nanocubes, and 3% Ag NRs (Figure S6a, Supporting Information). The corresponding optical absorption spectrum has only one LSPR peak located at 420 nm, a

Figure 7. TEM images of the products synthesized at 200 °C for 60 h in an EG solution containing 0.25 mM HAuCl4, 25 mM PDDA, and different concentrations of AgNO3: (a) 0, (b) 0.025, (c) 0.0625, (d) 0.0833, (e) 0.625, (f) 1.25, (g) 1.875, (h) 2.5, (i) 3.125, and (j) 3.75 mM. The corresponding molar ratios of Ag to Au are (a) 0, (b) 1:10, (c) 1:4, (d) 1:3, (e) 2.5:1, (f) 5:1, (g) 7.5:1, (h) 10:1, (i) 12.5:1, and (j) 15:1. The aspect ratios of Au@Ag heterogeneous NRs in Figure 5f−j are (f) 1.8, (g) 2.2, (h) 2.7, (i) 3.6, and (j) 4.3, respectively. All scale bars are 100 nm.

accordance with our previous report.37 We thus infer that the presence of AgCl is important for the generation of decahedral Au seeds in the present polyol synthesis. Experiments were thus performed by changing the introduction amount of AgNO3 in the initial precursors and keeping all other experimental parameters identical to the case of Figure 1, giving various molar ratios of Ag to Au in the initial precursors. The dramatic G



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tration. Interestingly, our results indicated that high-quality Au@Ag heterogeneous NRs with various aspect ratios were dominantly produced over a wide PDDA concentration range from 5 to 35 mM (Figure 8). Moreover, the aspect ratio of

morphology evolution of Au@Ag bimetallic NPs was observed with an increase of the molar ratio of Ag to Au, as shown in Figure 7b−j. Uniform Au@Ag octahedra with round corners were produced when the molar ratio of Ag to Au was lower than 1:10 (Figure 7b). With a molar ratio of Ag/Au ranging from 1:4 to 1:3, the product was dominated by truncated Au@ Ag bimetallic nanocubes (Figure 7c,d). Interestingly, decahedral Au@Ag bimetallic NPs were the major product when the Ag/Au molar ratio reaches 2.5 (Figure 7e). High-yield Au@Ag heterogeneous NRs of various aspect ratios were selectively and exclusively harvested when the Ag/Au molar ratio was above 5. Moreover, the aspect ratio of Au@Ag heterogeneous NRs was increased from 1.8 to 4.3 when the Ag/Au molar ratio was increased from 5 to 15 (Figure 7f−j). These results indicated that, besides the reaction time, the aspect ratio of Au@Ag heterogeneous NRs can be rationally manipulated over a broad range by simply tuning the Ag/Au molar ratio in the initial precursor. Consequently, the PDDA-mediated polyol process presented here is a straightforward and effective synthetic strategy for high-yield preparation of Au@Ag heterogeneous NRs of various aspect ratios. It is well-known that the morphology of the initial nucleation plays a key role in determining the shape of a final NP. For gold with an fcc structure, its nuclei can take a single-crystalline, single-twinned, or multiply twinned structure, which was greatly affected by the reduction rate of gold ions.39 In our experiments, the reddish color was observed when the EG solution containing 0.25 mM HAuCl4, 2.5 mM AgCl, and 25 mM PDDA was heated at 200 °C for 4 min, whereas the reddish color appeared at 8 min in the absence of AgCl. Apparently, the reduction rate of [AuCl4]− ions was kinetically enhanced by AgCl, and this, in turn, affected the initial gold nucleation process, resulting in the final product with different morphologies and shapes. On the basis of our results, we infer that, in the present polyol synthesis, (i) AgCl in the initial precursor affects the reduction rate of [AuCl4]− ions; (ii) the morphology of Au nuclei strongly depends on the reduction rate of gold ions; (iii) the reaction is slow when the concentration of AgCl is lower than 0.08 mM, leading to the formation of single-crystalline Au seeds; and (iv) correspondingly, a relatively high reduction rate induced by a high concentration AgCl (>0.625 mM) favors the generation of multiply twinned decahedral Au NPs. That the nucleation behavior of Au atoms depends on the reduction rate of gold ions presented here is consistent with the recent experimental observations in the synthesis of Au and Pd NPs.41 As described above, the PDDA-mediated polyol process is an effective route for the preparation of Au@Ag heterogeneous NRs. In the experiments, when PDDA was dissolved into an EG solution, besides [PDDA] + ions, Cl − ions were synchronously released. Oxidative etching by O2/Cl− pairs inevitably occurred. It has been revealed that the oxidative etching ability of O2/Cl− pairs will be enhanced if the Cl− ion concentration was increased.42 As is known, metal NPs, especially for multiply twinned metal NPs, are highly susceptible to oxidative etching in the reaction conditions.35,42 Therefore, the population of Au decahedra should be rationally decreased if oxidative etching was kinetically enhanced, and this, in turn, results in the formation of Au@Ag NRs with a high aspect ratio. That is, the PDDA concentration may greatly affect the final aspect ratio of Au@Ag heterogeneous NRs. Further experiments were thus performed without changing other experimental parameters except for the PDDA concen-

Figure 8. TEM images of the products synthesized at 200 °C for 60 h in an EG solution containing 0.25 mM HAuCl4, 2.5 mM AgNO3, and different PDDA concentrations: (a) 5, (b) 10, (c) 15, (d) 20, (e) 25, (f) 30, (g) 35, and (h) 40 mM. The aspect ratios of Au@Ag heterogeneous NRs are (a) 1.2, (b) 1.7, (c) 2.0, (d) 2.4, (e) 2.7, (f) 3.0, and (g) 3.1, respectively. All scale bars are 100 nm.

Au@Ag heterogeneous NRs increased with the increase of PDDA concentration (Figure 8a−g). This suggests that the aspect ratio of Au@Ag heterogeneous NRs can also be manipulated by the concentration of surfactant PDDA, which provides another simple and effective method to yield Au@Ag NRs of various aspect ratios. When the PDDA concentration reaches 40 mM, the major product is Au@Ag heterogeneous NRs, but with a broad size distribution (Figure 8h). In addition, the average aspect ratio of Au@Ag heterogeneous NRs was decreased. Experiments using PDDA with a low molecular weight were also conducted. The surfactant PDDA with a molecular weight of Mw = 100 000−200 000 still favors the formation of uniform Au@Ag heterogeneous NRs in high yield (Figure S7, Supporting Information). This reflects that PDDA with an appropriate concentration is crucial for the formation of high-quality Au@Ag heterogeneous NRs. Optical Properties and Stability of Au@Ag Heterogeneous NRs. The selective synthesis of high-yield Au@Ag heterogeneous NRs of different sizes allows us to investigate their size-dependent optical properties. The UV−vis-NIR H



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they could have promising applications for multicolor labeling and high density of optical storage devices.44 Beside that, the strong absorption of Au@Ag heterogeneous NRs in the NIR region, a region where optical absorption in tissue is minimal and radiation penetration is optimal,45 would make it an ideal candidate for photothermally triggered drug release in tissues as well as for the hyperthermia treatment of tumors. We finally investigated the stability of Au@Ag heterogeneous NRs harvested by the present PDDA-mediated polyol process. Interestingly, in contrast to Ag, Au, and Au@Ag NRs synthesized by the seed-mediated approach in the cetyltrimethylammonium bromide (CTAB) micelles at a low temperature (the NRs-to-NPs transition usually occurred upon storage, exhibiting a gradual blue shift in longitudinal LSPR over a period of hours to days),20,46,47 Au@Ag heterogeneous NRs presented here are very stable, although they have multiply twinned structures on their surfaces. No change in shape and optical absorption spectrum was observed when our Au@Ag heterogeneous NRs were stored in an aqueous solution over a long period of time (Figure 10). These results suggest that the

optical absorption spectra of Au@Ag heterogeneous NRs of various aspect ratios dispersed in water are presented in Figure 9. Our results indicate that a Au@Ag heterogeneous NR with

Figure 9. UV−vis-NIR absorption spectra for Au@Ag heterogeneous NRs of various aspect ratios dispersed in water. The aspect ratios of Au@Ag heterogeneous NRs from curve a to curve i were (a) 1.2, (b) 1.5, (c) 1.8, (d) 2.0, (e) 2.3, (f) 2.7, (g) 3.1, (h) 3.6, and (i) 4.3.

an aspect ratio lower than 1.2 displays a broad absorption peak at 410 nm (curve a in Figure 9). The transverse and longitudinal absorption peaks, respectively, located at 393 and 507 nm were definitely distinguished for Au@Ag heterogeneous NRs with an aspect ratio of 1.5 (curve b in Figure 9). The longitudinal LSPR absorption peak red shifts from 507 to 950 nm when the aspect ratio of Au@Ag heterogeneous NRs was increased from 1.5 to 4.3 (curve b to curve i in Figure 9). It is because the moment of the longitudinal plasmon was increased. Correspondingly, the transverse LSPR absorption at 393 nm did not show remarkable peak shifts, suggesting that the diameter of the as-synthesized Au@Ag heterogeneous NRs is almost same. It has been revealed that the LSPR features of Au@Ag nanostructures mainly result from the Ag component and the fingerprint of the Au core disappeared if the shell of Ag is thicker than five monolayers (∼1.2 nm).16a,43 In our experiments, when the aspect ratio of the Au@Ag heterogeneous NR reaches 1.2, the thickness of the Ag shell is above 1.2 nm; that is, Au seeds were completely covered by Ag shell. Thus, we believe that the LSPR features of the Au@Ag heterogeneous NR mainly originated from the Ag component. As aforementioned, both the transverse and the longitudinal LSPR absorptions of Au@Ag heterogeneous NRs were enhanced as their aspect ratio was increased during the growth process (Figure 4c). Our experiments revealed that the intensity of the transverse LSPR absorption of Au@Ag heterogeneous NRs is stronger than that of the corresponding longitudinal LSPR absorption when the aspect ratio is lower than 2.0 (curve b to curve d in Figure 9), whereas, as the aspect ratio reaches above 2.3, the longitudinal LSPR absorption of Au@Ag heterogeneous NRs depicts a distinct peak maximum (curve e to curve i in Figure 9). These results indicate that the intensity enhancement of the LSPR longitudinal mode of Au@ Ag heterogeneous NRs is higher than that of the LSPR transverse mode as the aspect ratio increased, which is consistent with the theoretical calculation of Ag NRs and Au@Ag core−shell NRs.19,21,32 Au@Ag heterogeneous NRs synthesized by our method first show fascinating colors and a highly tunable longitudinal plasmonic absorption. Therefore,

Figure 10. (a) UV−vis-NIR absorption spectra of an aqueous suspension of Au@Ag heterogeneous NRs with an aspect ratio of 2.7: as-synthesized (black curve) and after being stored for 90 days (red curve). TEM images of (b) as-synthesized Au@Ag heterogeneous NRs and (c) after being stored for 90 days. Scale bars for both (b) and (c) are 100 nm.

configuration of our Au@Ag heterogeneous NRs really reached an intrinsic equilibrium under a slow growth condition at a high temperature. It has been revealed that the driving force for the fragmentation of metal NRs generally originates from the thermal perturbation or/and oxidative etching.46,47 In our experiments, Au@Ag heterogeneous NRs have stood the test of oxidative etching of O2/Cl− pairs during the high-temperature synthesis process. Their stability is thus greatly enhanced compared to those metal NRs obtained by fast growth at a low temperature. I



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CONCLUSIONS We develop a straightforward and powerful method for the selective and controllable synthesis of high-quality Au@Ag heterogeneous NRs. High-yield Au@Ag heterogeneous NRs of various aspect ratios were successfully synthesized in large scale using PDDA as both the stabilizer and the shape-controller. Structural characterization by TEM, HAADF-STEM, EDS, and HRTEM indicated that Au@Ag heterogeneous NRs grew along the ⟨220⟩ direction, bound by {002} planes as the lateral surfaces, and terminated by {111} planes. It has been revealed that the oxidative etching by O2/Cl− pairs plays a key role for the high-yield synthesis of uniform Au@Ag heterogeneous NRs. Formation of such Au@Ag heterogeneous NRs is attributed to the preferential adsorption of PDDA on the {002} planes of Ag that inhibited the growth rate along the ⟨002⟩ direction. The size-dependent optical property of Au@ Ag heterogeneous NRs was first and experimentally investigated. Our study reveals that Au@Ag heterogeneous NRs exhibit two strong absorption peaks that, respectively, are attributed to the transverse and longitudinal LSPRs. Moreover, the longitudinal LSPR can be facilely tuned from the visible to the NIR regions by changing the aspect ratio of Au@Ag heterogeneous NRs. Importantly, Au@Ag heterogeneous NRs synthesized by our method have an excellent stability. They can maintain their optical properties even after several months, which is very important for their various plasmonic applications. Au@Ag heterogeneous NRs with a highly tunable plasmonic property and excellent stability are of interest for application in surface plasmonics, SERS, chemical or biological sensing, optical labeling, photothermally triggered drug release in the tissue, hyperthermia treatment of tumors, and information storage. In addition, our synthetic strategy is a one-pot reaction, which is quite different from the prevalent multistep seed-mediated synthesis approaches for the fabrication of heterogeneous bimetallic nanostructures. This will bring some insights on the controllable synthesis of heterogeneous bimetallic nanostructures by using a straightforward one-pot reaction.

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Professor acknowledges the support from Shandong province and UJN.

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ASSOCIATED CONTENT

S Supporting Information *

Additional characterization including low-magnification HAADF-STEM images of Au@Ag heterogeneous NRs, composition analysis of the product harvested at different reaction times, XRD pattern and TEM image of AgCl particles, and TEM images of products synthesized at other reaction conditions (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (C.L.), [email protected]. jp (T.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant No. 21103068) and KAKENHI Grant-Aid for Scientific Research A (Grant No. 23245028) (T.T.). C.L. as a Taishan Scholar Endowed J



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One-Pot Controllable Synthesis of Au@Ag Heterogeneous Nanorods

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