Au Bimetallic Nanorods through Kinetics

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Controlled Growth of Ag/Au Bimetallic Nanorods through Kinetics Control Yun Yang,* Wenfang Wang, Xingliang Li, Wei Chen, Nini Fan, Chao Zou, Xian Chen, Xiangju Xu, Lijie Zhang, and Shaoming Huang* Nanomaterials and Chemistry Key Laboratory, Wenzhou University, Wenzhou, Zhejiang 325027, P. R. China S Supporting Information *

ABSTRACT: One-dimension noble nanomaterials have promising applications in many fields, and their growth pattern control is significant to property modulation. Herein, we report a facile strategy with which the growth pattern of Ag on the Au nanorod (NR) or decahedral nanoparticle (NP) surface can be precisely controlled and various structured Ag/Au NRs can be synthesized. Achievement of growth pattern control is mainly attributed to the adjustable reaction kinetics of Ag− to Ag0. Slow and moderate reaction rate favor asymmetrical growth, producing Au-tipped Ag NRs and asymmetrical Ag−Au−Ag NRs, respectively. In the case of a fast reaction rate, symmetrical growth dominates and symmetrical Ag−Au−Ag NRs form. Furthermore, the prepared bimetallic NRs can be used as starting materials to generate other novel nanostructures (nanocups, nanonails, and longer Au-tipped Ag NRs). The result presented here is vital to both exploration of growth theory and constructing nanostructures of not only the Au/Ag bimetallic system but also possibly other noble bimetallic systems. Moreover, these prepared nanostructures could provide model materials for studying the physical properties (such as structure-dependent surface plasmon) or have potential applications in the medical field. For example, hollow nanocups can serve as containers for controlled release of drug, etc. KEYWORDS: seeded growth, nanorod, selective etching shell ODNHN.9,37 However, it is a grand challenge to achieve heterogeneous growth and synthesize noncore@shell ODNHN due to their small lattice mismatch. Thus far, reports about heterogeneous growth and synthesis of noncore@shell ODNHN mainly focus on the growth of noble metal on nonnoble substances (magnetic or semiconductor) or growth of the latter on the former.34,38 Only a few groups have reported heterogeneous growth and achieved the preparation of noble noncore@shell ODNHN.39,40 Recently, several groups achieved heterogeneous growth of noble metals and prepared interesting nanostructures using different technologies.39−43 Most of all, kinetics control is more powerful. For example, Xia’s group reported heterogeneous growth of Ag on Pd nanocube and successfully synthesized diverse bimetallic nanostructures through controlling the injection rate of precursor.41 In a plasmon-mediated process, Mirkin’s group achieved heterogeneous growth of Ag on Au decahedron surface and prepared an interesting heterostructured icosahedron by adjusting the reducing ability of the agent.39 These reports inspire us that such kinetics control also might be applied to the synthesis of ODNHN and allow fabricating novel noncore@shell ODNHN.

1. INTRODUCTION Controlled synthesis of one-dimension noble heterometallic nanomaterials (ODNHN) is highly desirable because of their potential applications in biosystems,1−3 catalysis,3−12 and optics.3,13−19 According to the component, ODNHN can be divided into two categories, single metal and multimetal. Compared with the former, multimetal ones are often advantageous due to their multifunction imparted by each component.20,21 Thus far, several technologies including seeded growth22−24 and hard template25−29 have been developed to controllably synthesize multimetal ODNHN. Among them, seeded growth involving preferential growth on preformed seed surface is probably the most widely used and effective protocol for its advantages in structural control and variety.30,31 In general, two patterns are always observed in seeded growth, homogeneous growth in which the whole seed surface acts as a nucleation site and heterogeneous growth in which growth occurs only on part of the seed surface.22−24,30 It is generally accepted presently that homogeneous growth is due to a small lattice mismatch and heterogeneous growth due to a large lattice mismatch.5,32−36 The growth pattern often governs the resultant shape and structure of the product. Homogenous growth always generates core@shell ODNHN, and heterogeneous growth results in formation of tipped or island ODNHN.5,32,34 For noble metals, small lattice mismatch allows achieving homogeneous growth easily and preparing core@ © XXXX American Chemical Society

Received: September 11, 2012 Revised: November 30, 2012

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Figure 1. Schematic diagram of forming different bimetallic nanostructures (Etch 1, etchants are propane diamine and O2; Etch 2, etchant is HAuCI4). Red and yellow represent Au and Ag, respectively. resulting AgNO3 DEG solution was used for assisted growth of various Au nanostructures (seed) and bimetallic NRs. Preparation of Au Decahedra and NRs. Typically, 5 μL of HAuCl4 (0.48 M) aqueous solution was dried at 125 °C to remove water completely, and then 10 mL of DEG containing 0.25 mL of PDDA was added. The solution was stirred vigorously for 10 min to form a yellow solution. The preprepared AgNO3 DEG solution (2 mL for decahedra; 170 μL for NRs) was then introduced to a yellow solution, and the mixture was stirred for another 3 min. The resulting solution was heated in an oil bath without disturbance. To synthesize decahedra with different edge sizes, reaction temperature should be adjusted. For example, decahedra with a 21 nm edge (Figure S1, Supporting Information) could be fabricated at 227 °C. However, 225 °C is suited for synthesis of decahedra with a 23 nm edge. For preparation of decahedra with a 35 nm edge, 215 °C is needed. To synthesize decahedra with a 46 nm edge, temperature must be reduced to 200 °C. After 30 min reaction, the solution was cooled down to room temperature and then 12 mL of water was added. The resulting colloidal solution was used as seed for synthesis of bimetallic NRs. In order to purify products for characterization, 9 mL of water was added to 1 mL of DEG colloid and then the products were collected with a centrifuge (12 000 rpm). The collected product was dispersed in 9 mL of water and precipitated through a centrifuge again. The purifying procedure was repeated three times, and the resulting bimetallic NRs were dissolved in 0.5 mL of water. The AgNO3 introduced for synthesis of decahedra will act as precursor to prepare bimetallic Ag−Au NRs. If no other statement is given, no more AgNO3 is added again in the preparation of bimetallic Ag−Au NRs. Preparation of Bimetallic Ag−Au NRs Using Au Decahedra As Seed. Different amounts of aqueous ammonia (11.5% volume ratio) were introduced to the above 1 mL of seed, and then the mixture was stirred for 2 min. The resulting homogeneous solution was put into a 60 °C oven. After 12 h, 8 mL of water was added and the solution was subjected to centrifuge (10 000 rpm) to collect the product. The collected product was redispersed in 9 mL of water and precipitated through the centrifuge again. The purifying procedure was repeated three times, and the resulting bimetallic NRs were dissolved in 0.5 mL of water for characterization.

Here, we demonstrate that the above kinetics control strategy is also effective for th synthesis of multimetal ODNHN. As illustrated in Figure 1, controlled overgrowth of Ag on Au seed surface can be easily achieved through controlling the amounts of aqueous ammonia to tune the reaction kinetics of Ag− to Ag0, which allows producing various interesting bimetallic ODNHN (Au-tipped Ag NRs, asymmetrical Ag−Au−Ag segmental NRs, and symmetric Ag−Au−Ag NRs). Besides, temperature, which is another important factor affecting the reaction kinetics of Ag− to Ag0, also can be used to precisely control the growth pattern and product structures. Similarly, when 5-fold twinned Au NRs act as seeds, precise controlled overgrowth of Ag on their surface also can be realized. To the best of our knowledge, such precise growth pattern control and corresponding ODNHN have not been reported so far. The strategy reported here is potentially crucial to both exploring the growth mechanism of ODNHN and constructing unusual asymmetrical nanostructures. Furthermore, the prepared ODNHN could be used as starting materials to prepare other novel nanostructures (nanocup and nanonail) which might have special physical properties and applications in cancer therapy or imaging.1,2

2. EXPERIMENTAL SECTION Chemicals. AgNO3, propane diamine, HAuCl4, diethylene glycol (DEG), poly(diallyldimethylammonium chloride) (PDDA, Mw = 400 000−500 000, 20 wt % in H2O), poly(vinylpyrrolidone) (PVP, MW = 58 000), and cetyltrimethylammonium bromide (CTAB) were purchased from Sigma-Aldrich. Aqueous ammonia and NaOH were obtained from Aladdin reagent. All chemicals were used as received without further purification. Deionized water (18.2 MΩ·cm) from a Milli-Q Academic water purification system (Millipore Corp., Billerica, MA, USA) was used in all cases. Preparation of 11.8 mM AgNO3 DEG Solution. A 80 mg amount of AgNO3 was added to 40 mL of DEG, and then the solution was sonicated for about 10 min to dissolve AgNO3 completely. The B

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Figure 2. Typical HAADF, TEM, elemental mapping, and schematic diagrams of Ag/Au NRs prepared using different amounts of aqueous ammonia: (A1−A6) 15, (B1−B6) 30, and (C1−C6) 45 μL. Red and yellow represent Au and Ag, respectively. In all cases, the temperature is 60 °C. Arrows in the schematic diagrams represent the growth direction. To test other surfactant (PVP and CTAB), 1 mL of decahedra seeds was purified with the above procedure and then dispersed in solution (0.5 mL of DEG and 0.5 mL of water). An equimolar quantity of PVP/CTAB (based on monomer of PDDA) and AgNO3 was added. Then a similar growth and collection procedure of bimetallic products were employed. Preparation of Bimetallic Ag−Au NRs Using Au NRs As Seed. A similar preparation was used except that Au decahedra were replaced by Au NRs. It is worth noting that only 340 μL of AgNO3 was used for synthesizing Au NR seeds, and the Ag/Au is about 0.33. In order to prepare longer Ag−Au−Ag NRs, a calculated amount of AgNO3 DEG solution (2 mg/mL) should be introduced again. For example, 60 μL of AgNO3 DEG solution was needed to prepare the Ag−Au−Ag NRs (Figure S17, Supporting Information). A similar purifying procedure to that of Ag−Au bimetallic NRs was used to remove excess PDDA and DEG. Preparation of Nanobottle Using Galvanic Reaction. The purified Au-tipped Ag NRs prepared with 0.24 mM decahedron seed were dissolved in 10 mL of water containing 49.95 mg of PVP. A 10 μL amount of HAuCl4 aqueous solution (0.024 mM) was added into the above bimetallic NR colloid in a dropwise fashion, and then galvanic reaction proceeded under magnetic stirring at room temperature. After 12 h, the nanobottles were collected and purified with the same procedure as that of Ag−Au bimetallic NRs prepared using decahedra as seeds. Preparation of Nanonail Using Selective Etching. The purified Au-tipped Ag or Ag−Au−Ag segemental NRs were dissolved in 1 mL of water, and then 70 μL of propane diamine was added under stirring. Stirring was continued for 2 min, and then the etching proceeded at room temperature without disturbance. After 12 h, nanonails were collected and purified with a similar procedure to that of Ag−Au bimetallic NRs. Characterization. For transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), energydispersive spectroscopy (EDS), high-angle annular dark field (HAADF), and scanning transmission electron microscopy (STEMEDS) characterization the purified colloid was deposited on copper grids coated by a carbon membrane and dried at 80 °C. After the water was removed completely, the sample was observed on a 300 KV Tecnai G2 F30 S-Twin microscope with an attached EDS. UV−vis

spectra of NPs colloid were recorded with a Shimadzu 2450 UV−vis spectrophotometer at room temperature. For SEM characterization, a similar preparation of sample was used except that Si wafer was used as sample supporter. SEM images were obtained with a Nova Nano SEM 200.

3. RESULTS AND DISCUSSION The {111}-faceted Au decahedra seeds (Figure S1, Supporting Information) capped by PDDA were prepared via novel

Figure 3. HRTEM images of different sections in one Au-tipped NR (insets are corresponding FFT patterns): (A) close to Au tip; (B) at the other end of the NR.

AgNO3-assisted growth (detailed procedure can be seen in the Supporting Information). More precisely, the seeds are alloy NPs because a small amount of Ag is incorporated (Figure S2, Supporting Information), and herein they are named as Au decahedra simply. Initially, the Au seeds formed in polyolcontaining PDDA under the effect of AgNO3. For subsequent synthesis of ODNHN, the prepared seeds need no any purification to remove AgNO3, excess PDDA, and polyol. They will play appropriate roles in the synthesis of ODNHN (AgNO3 as Ag precursor; polyol as reducing agent; PDDA as surfactant), which is beneficial to cost reduction and friendly to the environment. An equal volume of water was added into polyol Au colloid to prepare seed solution. Then aqueous C

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Figure 6. Typical HAADF, SEM, TEM, and schematic images of nanostructures prepared by modifying the reaction: (A−D) Au-tipped Ag NRs prepared using NRs shown in Figure 3A as seeds; (E and F) Au-tipped Ag NRs prepared using decahedron Au with 35 nm edge as seeds; (G and H) Ag−Au NRs prepared using Au NRs as seeds (15 μL aqueous ammonia; no AgNO3 was added again); (I and J) Ag−Au−Ag segmental NRs prepared using Au NRs as seeds (45 μL aqueous ammonia; Ag/Au = 1); (K and L) Ag−Au−Ag segmental NRs prepared using Au NRs as seeds (45 μL aqueous ammonia; Ag/Au = 3). In all cases, the reaction temperature is 60 °C.

Figure 4. Typical HAADF images and schematic diagrams of bimetallic NRs at different temperatures: (A) 60, (B) 40, and (C) 20 °C. Amount of aqueous ammonia is the same in three cases (30 μL). Arrows in the schematic diagrams represent the growth direction. Insets in A1 and B1 are the mean longitudinal length.

15 μL amount of aqueous ammonia resulted in formation of Au-tipped Ag NRs (Figure 2A), indicating that the overgrowth is along the [110] direction from only one end of the Au decahedra. The HAADF image (Figure 2A1) shows a clear tipped structure: the bright part is attributed to the Au element and the gray one to the Ag element. The HRTEM image in Figure 2A2 also gives the same result. Such structure is further confirmed by the elemental distribution mapping analyses with STEM-EDS (Figure 2A3−A6). The HRTEM images and fast Fourier transform (FFT) patterns (Figure 3) show that they are a typical 5-fold twinned structure.44,45 When 30 μL of aqueous ammonia was added, the overgrowth of Ag occurred on both ends of seed along [110] unequally and asymmetric Ag−Au− Ag segmental NRs formed (Figure 2B1−B6). The asymmetric structure indicates that the growth rate along the two ends was different and the rate along the longer Ag part was faster (Figure 2B2). When the amount of aqueous ammonia increased to 45 μL, symmetrical Ag−Au−Ag segmental NRs dominated (Figure 2C), revealing that the overgrowth of Ag occurred equally on both ends of Au decahedra seeds. Meanwhile, the structure change is accompanied by the length change: 64, 70, and 87 nm NRs were created with the amount of ammonia increasing, revealing that the forming rate of Ag is not the same in the three cases. In comparison with the short NRs, formation of long NRs indicates that more Ag− ions are reduced to Ag atoms in fixed time (the same seeds were used), that is, the forming rate of Ag is fast during formation of long NRs. It can be easily understood that aqueous ammonia accelerates the forming rate of Ag because ammonia can increase the alkalinity of the solution and thus render polyol a stronger reducing ability.46 We also carried out the overgrowth of Ag on the same decahedra seeds without adding aqueous ammonia and found no any change even after 120 h, demonstrating that aqueous ammonia indeed can accelerate the reduction of Ag− to Ag0.

Figure 5. Typical HAADF images and schematic diagrams of bimetallic NRs at different temperatures: (A) 50, (B) 70, and (C) 90 °C. Amount of aqueous ammonia is the same in three cases (15 μL). Arrows in the schematic diagrams represent the growth direction. Insets in A1, B1, and C1 are the mean longitudinal length.

ammonia (11.5% volume ratio) was introduced to promote reduction of Ag− to Ag0. Interestingly, it was found that the overgrowth pattern of Ag on seed was governed by the amount of aqueous ammonia. A D

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Figure 7. Typical, HAADF, STEM-EDS, HRTEM, TEM, and schematic images of nanostructures prepared by etching the NRs (Figure 5C): (A−C) nanonails (etchant, propane diamine and O2); (D−F) nanocups prepared with galvanic reaction.

When a small amount of aqueous ammonia (15 μL) was used, the formation rate of the Ag atom was slow, producing a low concentration of Ag atom at the nucleation stage. As a result, the collision frequency between the Ag atom and the Au seed surface is low, creating a small amount of adatoms. In this case, NNFA is equal to NMA and growth of Ag happens only on one end of the seed. When 30 μL was added, fast reduction of Ag− to Ag0 causes a high concentration of Ag atom at the initial nucleation stage. Consequently, the collision frequency between Ag atoms and Au seed surface increased. Under the circumstances, NNFA is more than NMA and an asymmetrical double-ended growth pattern happens. A 45 μL amount of aqueous ammonia produces a very high concentration of Ag atom and increases the collision frequency significantly. A large amount of active adatoms form, and NNFA is much more than NMA. For this reason, a symmetrical growth pattern dominates. A clear mechanism needs more studies, which are underway in our group. Besides the growth pattern, another important remaining issue is why Ag preferentially grows along [110]. Previously, Tsuji’s and Song’s group also studied Ag growth on Au decahedron along [110].39,48−50 Their cases are quite similar to ours, and our mechanism probably follows theirs. It is believed that aqueous ammonia is the key factor of our strategy. Except for tuning reaction kinetics, aqueous ammonia has another two important impacts, solubilizing AgCl and facilitating a homogeneous reaction. AgNO3 introduced in the synthesis of seeds converted to insoluble AgCl precipitation owing to the presence of Cl− (HAuCl4), which does not make much contribution to the growth of NRs. However, in the presence of aqueous ammonia, AgCl changes to soluble Ag(NH3)2+, which can act as a utilizable precursor for growth of NRs (Figure S5, Supporting Information).52 Meanwhile, the distribution of Ag precursor in the reaction system becomes homogeneous (Figure S5, Supporting Information), facilitating formation of the monodispersed product. If NaOH was used to replace aqueous ammonia (pH was the same), only dimer-like NPs formed even after 24 h (Figure S6, Supporting Information), indicating most of Ag+ has no contribution to

It is a big challenge to explain how the amount of ammonia affects the growth pattern due to the complexity of the influencing factor and lack of in situ observation tools.47−51 Here, based on the above result and the mechanism reported by Xia’s groups, we give a possible mechanism.41 Generally, overgrowth always happens on active sites, and the distribution of active sites on the seed surface is the key factor in determining the growth pattern.41b In our system, once Ag atoms form adatoms through heterogeneous nucleation on the Au seed surface, those positions serve as active sites to induce growth. Subsequent growth of Ag selectively occurs on these active sties rather than other sites due to small lattice mismatch and self-catalytic behavior.34,41 The active site distribution is mainly affected by two possible factors:41 (I) Collision frequency and formation rate of Ag atom. A fast formation rate of Ag produces a high concentration of Ag atoms at the nucleation stage which increases the collision frequency between the Ag atom and the Au seed surface. A high collision frequency promotes heterogeneous nucleation of the Ag atom on the Au seed surface and produces more self-catalytic active sites (adatoms). Conversely, less active sites are created.41b (II) Ag adatom migration from one end to the other end on the Au seed decahedron surface. Due to lattice mismatch and surfaceenergy minimization, the Ag adatoms rapidly diffuse from one end to the other end on the Au decahedron seed surface, changing the active site distribution and growth pattern.41 In the initial nucleation stage, if the number of newly formed atdatoms (NNFA) is equal to that of migrated adatoms (NMA) on one given end of the Au decahedron seed, that is, all adatoms (self-catalytic active site) migrate to one end, oneended growth along [110] is favored and Au-tipped NRs form. If NNFA is a little more than NMA, the active site number of one end is more than that of the other end, resulting in the asymmetrical double-ended growth pattern and formation of asymmetrical Ag−Au−Ag NRs. When NNFA is much more than NMA, adatom migration cannot produce a distinct difference of active site number on two ends. As a result, a symmetrical growth pattern dominates and corresponding products are symmetrical Ag−Au−Ag NRs. E

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asymmetrical Ag−Au−Ag NRs marked by the number in Figure 5B and Au-tipped NRs were observed, indicating that increasing temperature can change the growth pattern from one ended to double ended. When reaction proceeds at 90 °C, most of the products are symmetrical Ag−Au−Ag NRs and only a small amount are other structured (Au tipped and asymmetrical Ag−Au−Ag NRs), implying that 90 °C favors the symmetrical growth pattern. This observation is consistent with the results presented in Figures 2 and 4, confirming that the tunable reaction kinetics is the key factor of growth pattern. The length change trend with reaction temperature in Figure 5 is also in agreement with the results presented in Figures 2 and 4 that the fast growth rate of Ag facilitates formation of long NRs. Figure 5C also reveals that increasing reaction temperature is not very effective to prepare monodispersed product. When reaction temperature was further increased to 100 or 110 °C, lots of pure Ag NPs were observed (Figures S11 and S12, Supporting Information), indicating that the high temperature caused heavy additional nucleation of Ag. Besides, when the amount of aqueous ammonia and temperature are unchanged, the resultant structure is also governed by the seed size and small sized seeds favor asymmetrical growth (Figure S15, Supporting Information). If the prepared Au-tipped Ag NRs (Figure 2A) was subjected to seeded growth again, their length increased to 120 nm (Figure 6A and 6B) and third seeded growth produced 150 nm NRs (Figure 6C and 6D). When decahedra with a 35 nm edge (Figure S15B1, Supporting Information) were used as seeds, thick NRs formed (Figure 6E and 6F). The above kinetics control strategy is also suited to the case that Au NRs (Figure S16, Supporting Information) with the same 5-fold twinned structure as decahedra served as seeds. Rod(Ag)−rod(Au) (Figure 6G and 6H) and rod(Ag)−rod(Au)−rod(Ag) structures (Figure 6I, 6J, 6K, 6L, and Figure S17, Supporting Information) formed, respectively, with the amount of aqueous ammonia increasing. The length of NRs can be controlled by introducing different amounts of AgNO3 (Figure 6I−L). The low-magnification HAADF and SEM images (Figure S17A, S17G, S17H, and S17I, Supporting Information) show that high-quality synthesis can be achieved with our method. Therefore, it could be used as an alternative route for synthesizing unit materials for self-assembly. Presently, reshaping nanostructures is another significant route to construct novel functional nanomaterials.57−63 For example, surface-energy-dependent selective etching of Ag NPs can offer hollow nanostructures as drug carrier and multipod nanostructures for SERS.58,61 It is well known that nanoscaled Ag can be slowly oxidized to Ag2O in air, which transforms to soluble complex in the presence of ammonia.58 As a result, the Ag can be etched stepwise and this route is often utilized to reshape Ag NPs.58 In our experiment, propane diamine and O2 in air were used as etchant to selectively etch the Au-tipped Ag (Figure 6E) and Ag−Au−Ag segmental NRs (Figure 6K), producing single-end (Figure 7A−C) and double-end (Figure S19, Supporting Information) bimetallic nail-like nanostructures, respectively. Galvanic reaction was always employed as a tool to fabricate hollow nanostructures.50,61−66 In the present work, the Au-tipped Ag NRs (Figure 6E) were used as templates to construct hollow nanostructures with galvanic reaction. The galvanic etching from Au3+ created cup-like nanostructures (Figure 7D−F). From Figure 7F1−F3, their cup structure can be seen clearly. The lattice distance (0.2 nm) in the wall of the nanocup is related with the (200) facet of Au

the NR growth in this case because NaOH is not able to solubilize AgCl precipitation (Figure S5, Supporting Information). Other factors (type of surfactants, reactive time, and absence of seed) also have a strong impact on the product structure (Figures S7−S15, Supporting Information). For example, CTAB and PVP used as surfactants result in formation of core@shell and Au−Ag dimer structures, respectively, which might be attributed to their different affinity with specific facets. Previously, Mirkin’s group studied pH-dependent overgrowth of Ag on Au decahedron and found that the product changed from icosahedra NPs to symmetrical Ag−Au−Ag NRs with pH increasing.39 There is a distinct difference between their growth pattern and ours possibly due to the difference of synthetic medium and reactive conditions. For example, they used an irradiation source which can affect the growth pattern of Ag nanostructure.39,53−57 However, in our system, no any irradiation source was involved. Song’s group also studied the overgrowth of Ag on Au decahedra. They observed symmetrical growth along [110] and prepared high-quality symmetrical Ag− Au−Ag NRs.49,50 However, asymmetric growth was not observed by them, probably because the high reaction temperature (260 °C) caused a very fast nucleation and growth rate. It is well known that many factors can affect the formation rate of the Ag atom and collision frequency, such as temperature, reducing ability of agent, etc.41b In particular, the reaction temperature largely determines particle motion (molecular, atom and ion) and consequently governs reaction kinetics.41b Therefore, it is reasonable that temperature adjustment should cause a similar structure change as well as the amount of aqueous ammonia. With this consideration, we carried out another set of experiments in which we kept the amount of aqueous ammonia (30 μL) constant and investigated the product structure change with temperature. We demonstrated before that the products prepared at 60 °C are asymmetrical Ag−Au−Ag NRs (Figure 2B and Figure 4A; these two figures were taken from the same sample). Here, we further explored the overgrowth of Ag on decahedral seeds at 40 and 20 °C. In theory, decreasing reaction temperature slows down the reaction rate of Ag− to Ag0 and the collision frequency of Ag atoms with Au seed surface. On the basis of the above reaction kinetics discussion, very possibly NNFA is equal to NMA in this case. Therefore, the structure should evolve from asymmetrical Ag−Au−Ag to Au tipped. As expected, overgrowth of Ag on seeds at 40 °C indeed yielded Au-tipped NRs (Figure 4B). Further decreasing the temperature to 20 °C produced a dimer-like nanostructure (Figure 4 C). Only small amount are short Au-tipped NRs (marked by the arrows in Figure 4C1). Both Au-tipped NRs and dimer-like NPs indicate that one-ended growth of Ag on seeds happens at a low temperature. This is in agreement with the observation presented in Figure 2. With reaction temperature decreasing, the length reduces from 70, 66, to 27 nm because the forming rate of Ag becomes slow. Besides, we also carried out 50, 70, and 90 °C syntheses in which the amount of aqueous ammonia was set to 15 μL. In Figure 2A, it has been demonstrated that 15 μL of aqueous ammonia prefers formation of Au-tipped NRs at 60 °C. Therefore, it can be predicted that a lower reaction temperature (50 °C) also should facilitate formation of the Au-tipped structure because the collision and formation rate of Ag further decrease. The HAADF image shown in Figure 5A demonstrates this. When the temperature increased to 70 °C, both F

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(Figure 7F4), implying Ag is oxidized to Ag+ mostly and the main composition of the cup is Au.

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4. CONCLUSION In summary, we have shown that kinetics control allows tuning the Ag growth pattern on the Au seed surface (decahedron or NR) and fabricating various Ag/Au bimetallic NRs (Au-tipped, asymmetrical Ag−Au−Ag, and symmetrical Ag−Au−Ag NRs). This may provide a strategy to grow other unusual asymmetrical bimetallic nanostructures. Furthermore, through changing the reaction conditions (the size and shape of the seeds) or using the prepared Au-tipped Ag NRs as starting materials, other bimetallic nanostructures (different sized Autipped Ag NRs, Ag−Au NRs, Ag−Au−Ag segmental NRs, nanonails, and nanocups) also could be synthesized via selective etching and secondary seeded growth. These nanostructures might have potential applications in cancer therapy (nearinfrared absorption of NRs at 815 nm) (Figure S18, Supporting Information),67−69 drug delivery (hollow cup),65 chemical sensing,70 biological sensing, or enhancing upconversion emission.70,71

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ASSOCIATED CONTENT

S Supporting Information *

Other TEM, EDS, HRTEM results. 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]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the NSFC (21101120, 21173159, 51025207), Zhejiang Science and Technology Project (2010C31039), Zhejiang Natural Science Program (Y4110391), and Lucheng Science and Technology Project (T100106).

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