A Novel Side-Selective Galvanic Reaction and Synthesis of Hollow

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J. Phys. Chem. C 2010, 114, 18073–18080

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A Novel Side-Selective Galvanic Reaction and Synthesis of Hollow Nanoparticles with an Alloy Core Xuezhong Gong, Yun Yang,* and Shaoming Huang* Nanomaterials and Chemistry Key Laboratory, Wenzhou UniVersity, Wenzhou, Zhejiang 325027, People’s Republic of China ReceiVed: June 22, 2010; ReVised Manuscript ReceiVed: August 20, 2010

Hollow nanoparticles with an alloy core (HNACs) have been prepared by a novel side-selective galvanic reaction. Seeding growth first is used to synthesize Au@Ag nanoparticles (NPs), which react with HAuCl4 to form HNAC NPs. Through adjusting the core size, the amount of HAuCl4, and the diameter of Au@Ag, fabrications of various HNACs are achieved. By using high-resolution transmission electron microscopy, energy-dispersive X-ray spectrometry, field emission scanning electron microscopy, and UV-vis spectroscopy, the galvanic etching process is observed. We find the galvanic reaction is side-selective compared to previously reported reactions. Three features are believed to play important roles: (1) interfacial alloying between the Au core and Ag shell, (2) the structure of Au@Ag (the Au core is not right center), and (3) the Au-contentdependent galvanic reaction. Prepared HNACs have a rough surface and porous structure, which indicates they can capture more molecules with affinity. Also, these hydrophobic HNACs can assemble into ordered 3-dimensional organizations. All these features are important to surface-enhanced Raman scattering (SERS). 2-Naphthalenethiol is used as a probe molecule to investigate their SERS activity, and the results demonstrate their good performance. In addition, during the galvanic reaction, interestingly, the Ag+ from oxidization can react with a Cl- of HAuCl4 to form size-controlled AgCl NPs with a uniform diameter. 1. Introduction Noble nanoparticles (NPs) mainly including Ag, Au, Pt, and Pd have attracted a lot of attention due to their promising applications in biosystems,1-4 catalysis,5-11 nanoelectronics,12 and optics.13-22 Among them, Au and Ag have aroused more interests than the others because their unique size-dependent color and tuned UV-vis spectral properties are very significant to bioapplications.1-4 Practically, their color has a strong dependence on not only size but also other parameters such as structure and composition.12-32 To tune their optical properties, numerous technologies have been developed, for example, seeding growth,33-36 the galvanic reaction,23-32 microemulsions,37 the one-spot route,38,39 and digestive repining.40-43 Of these, the galvanic reaction in which one metal corrodes preferentially when in electrical contact with another metal becomes a powerful tool to construct various NPs.44-70 Conventionally, galvanic corrosion is used to describe the electrochemical action of bulk metals. Two different bulk metals contact each other in an electrolyte to form a battery in which a less noble metal acts as the cathode and another as the anode. For the galvanic reaction in nanosystems, one NP and the surrounding solution also can combine into a nanobattery in which different parts of the NP and solution act as electrodes and an electrolyte, respectively, so some parts of the NP being cathodes corrode first, and newly formed metal atoms deposit on other parts of the NP (anodes). So far, in the galvanic reaction, the most studied NP is Ag, which can be etched by Au(III), Pd(II), and Pt(IIII) ions.44,45,47 Ag NPs have two crystalline structures (single-crystalline and polycrystalline), and they are polyhedral on the basis of TEM observations. For example, single-crystalline Ag NPs are coated by 8 {111} facets and 6 {100} facets, giving a truncated octahedral morpholgy.25-27,30,71,72 Polycrystalline Ag NPs are commonly observed to be bound by 20 * To whom correspondence should be addressed. E-mail: badbachier@ gmail.com (Y.Y.); [email protected] (S.H.).

{111} facets and have an icosahedral shape.72 On the NP surface, the surface energies at different parts (crystal edge, different facets) are not the same, for example, E{111} < E{100} and Eedge > Efacet.30 Generally, it is widely believed that the part of the NP with a high energy acts as the anode and that with a low energy as the cathode during the galvanic reaction.71 That is to say, the galvanic reaction follows a surface-energy-selective route.25-27 For instance, the etching starts on the {100} facet (anode) when truncated octahedral Ag NPs react with HAuCl4.25-27 However, the reaction conditions can always act as modifiers of the galvanic reactive route. For example, during the galvanic reaction between icosahedral Ag NPs and HAuCl4, Xia and co-workers found that etching preferentially occurred from the center of the {111} facet rather than the highenergy crystal edge due to the existence of oxygen in the edge.30 So far, although the mechanism is not understood completely, this reaction has been a powerful tool to build hollow, porous, or other structured NPs.23,25,28-31,44-66,66-70 Several groups, specially that of Xia, have made great contributions to this field.1-3,14,17,24-27,29,51,70 In the past decade, the studies of the galvanic reaction have mostly focused on water-soluble NPs, and only a few studies have involved oil-soluble NPs.25-28,30 Oil-soluble NPs can form ordered organizations which are crucial in nanoelectronics and the study of the coupling effect. Besides, the preparation concentration of oil-soluble NPs is higher than that of water-soluble NPs due to the steric hindrance stabilizing mechanism, which is of importance to scaled preparation.38-43 Also, compared with that of pure Ag NPs, the galvanic reaction of Ag-based NPs (core-shell or alloy) in organic systems very likely follows different routes, so it is also important to pay more attention to the study of the galvanic reaction in organic solutions.30 Here, various hydrophobic hollow nanoparticles with an alloy core (HNACs) with a size of less than 20 nm are prepared in an organic solvent through the galvanic reaction between Au@Ag NPs and HAuCl4. To the best our knowledge, no

10.1021/jp105768q  2010 American Chemical Society Published on Web 10/01/2010

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SCHEME 1: Procedure of Preparing Au@Ag Core-Shell and HNAC NPs

similar results have been reported in organic systems so far. First, seeding growth is adopted to fabricate Au@Ag core-shell NPs, which then are exposed to HAuCl4 to form HNACs. By adjusting the size of the gold seed and the amount of the Ag precursor, various HNACs are synthesized. Half-hollow and other partly hollow HNACs are generated through control of the amount of HAuCl4. Here, the galvanic reaction was observed to be side-selective compared to previously reported reactions.30 The HNACs were demonstrated to be effective materials for enhancing surface Raman scattering. Interestingly, the Ag+ ion from oxidized Ag NPs can react with Cl- to form size-tuned monodispersed AgCl NPs. 2. Experimental Section 2.1. Chemicals. Oleylamine, silver acetate, tri-n-octylphospine oxide (98%), and 4-tert-butyltoluene were purchased from Tokyo Chemical Industry. Borane-tert-butylamine complex (97%) was obtained from Aldrich. Chloroauric acid tetrahydrate was provided by Sinopharm Chemical Agent Co., Ltd. Octadecylamine was obtained from Alfa. All chemicals were used as received without further treatment. 2.2. Preparation of Au@Ag. A previously reported method was used to prepare Au@Ag NPs.34 Briefly, 4.8 nm Au NPs were dissolved in 3 mL of a mixed solution which contained 2 mL of 4-tert-butyltoluene and 0.05 g of octadecylamine, giving a red solution used as the seed. Then the calculated amount of acetate silver was added to 2 mL of 4-tert-butyltoluene and 0.5 mL of oleylamine, resulting in a colorless solution after ultrasonic treatment for 5 min at 40 °C which was used as the growing solution. The growing solution and seed solution were mixed homogeneously and then heated to 170 °C under vigorous stirring. After 1 h, the heating source was removed, and the solution was cooled to room temperature. After addition of 20 mL of ethanol, the black precipitate was separated through centrifugation (3000 rpm) and then dried completely at 45 °C. 2.3. Preparation of HNAC NPs. The prepared core-shell NPs (0.06 mmol) were redispersed in 28.5 mL of chloroform and 1.5 mL of oleylamine to form transparent solution A. A 10 mg portion of HAuCl4 was dissolved in 10 mL of chloroform containing 0.25 mL of oleylamine to give solution B. The calculated amount of solution B was added to solution A under stirring, and then the system was allowed to react for 30 min. After 30 mL of ethanol was introduced, HNACs were precipitated, and the precipitates were collected through centrifugation (3000 rpm). The obtained HNACs can be easily dissolved in organic solvents such as chloroform or toluene. 2.4. HNAC NP Assembly for SERS Study. Briefly, a total of 1.0 mL of a chloroform dispersion of the HNAC NPs (0.06

Figure 1. Representative TEM images and size distribution of 4.8 nm Au seed NPs (A1-A3), 12 nm polycrystalline Au@Ag NPs (B1-B3) (the inset is the schematic illustration of an icosahedron), and 16 nm single-crystalline Au@Ag NPs (C1-C3) (the inset is the schematic illustration of a truncated octahedron). SD ) standard deviation of the size distribution, and MS ) mean size.

mmol) or other NPs was dropped onto a Si wafer and dried under ambient conditions. Then the wafer was put into a beaker containing 2 mL of 1 × 10-3 M 2-naphthalenethiol-ethanol solution. After 3 h, the Si wafer was washed several times to remove absorbed molecules and then dried in the air. A Si substrate modified only with 2-naphthalenethiol was used as a reference. 2.5. Characterization. Purified NPs were dispersed in hexane. For transmission electron microscopy (TEM), the corresponding colloids were deposited on copper grids and observed on a JEOL (Japanese Electron Optics Laboratory) 2100F microscope with a 200 kV accelerating voltage. UV-vis spectra of the NP-hexane colloid were recorded with a Shimadzu 2450 UV-vis spectrophotometer at room temperature. In the case of preparations of energy-dispersive X-ray (EDX) samples, a drop of the colloid NPs was dropped onto a wafer. After the solvent evaporated completely, the wafer was analyzed using an EDX analyzer attached to a JSM-6700F scanning electron microscope. The surface-enhanced Raman spectrum was recorded on the Raman system JY-T64000 with confocal microscopy under ambient conditions by using a laser excitation of 632.8 nm (1.96 eV, 3 mW) from an air-cooled He-Ne laser. The beam size is 2 µm. 3. Results and Discussion 3.1. Preparation and Crystalline Control of Au@Ag Core-Shell NPs. In Figure 1, panels A1-A3, the Au seed NPs have a 4.8 nm mean size and a 0.8 standard deviation of the size distribution. Panels A1 and A2 indicate that the seed NPs have a narrow size distribution and most of them are spherelike. During seeding growth, two different reducing agents were used to reduce silver acetate, octadecylamine, and 1,2-tetradecanediol. When the former is employed, prepared NPs mostly have a polycrystalline structure as shown in Figure 1, panels B1 and B2. However, in the latter case, interestingly, single-crystalline NPs were generated (Figure 1, panels C1 and C2). Although the TEM images show both polycrystalline and single-crystalline

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Figure 2. (1) HAADF and STEM-EDX results of Au@Ag core-shell NPs: (2) C spectral map; (3) Ag spectral map; (4) Au spectral map.

NPs offer spherelike results similarly, in practice, polycrystalline and single-crystalline NPs have icosahedral and truncated octahedral shapes, respectively, as described above. The crystalline difference from different reducing agents is complex and needs further study to make it clear. To further confirm the core-shell structure, HAADF (Figure 1 in the Supporting Information) and STEM-EDX were used, and the results are shown in Figure 2. The Au and Ag element distributions also show the obtained NPs have a core-shell structure. 3.2. Preparation of HNACs. When the 12 nm polycrystalline Au@Ag NPs shown in Figure 3A are subjected to HAuCl4, the galvanic reaction generates differently structured NPs (Figure 3B-D). In Figure 3B, after addition of 0.5 mL of HAuCl4, only some of the NPs have a small void and most of the NPs are still holeless. When 1.5 mL of HAuCl4 is added, NPs with voids dominate, and HRTEM shows most of the NPs possess a halfhollow structure, implying that the etching prefers to start from one side (Figure 3, panels C1 and C2). When excess HAuCl4 (3 mL) is introduced, almost all the NPs exhibit a hollow structure as shown in Figure 3, panel D1. However, it was found that mostly the core is still connected to the shell and immobilized (Figure 3, panel D2 and Figure 2 in the Supporting Information). Previously, by using a similar route, other groups synthesized a nanorattle in which a movable core is coated by a hollow shell.29,65 Also, the observation of a half-hollow structure was not reported by them. In our case, a majority of the cores more or less are immobilized to a shell even if excess HAuCl4 is introduced, which, together with the presence of halfhollow NPs, indicates that our reaction mechanism is different from the previously reported ones. The UV-vis spectrum was also employed to study the galvanic reaction, and the results are shown in Figure 4. The Au@Ag template NPs have a narrow band with a peak at 425 nm as shown in Figure 4A, which is similar to that of pure Ag

Figure 4. UV-vis absorption and typical optical color of 12 nm Au@Ag (left) NPs after reaction with different amounts of HAuCl4: (A) 0 mL, (B) 0.5 mL, (C) 1.0 mL, (D) 1.5 mL, (E) 2 mL, (F) 2.5 mL, (G) 3 mL.

NPs due to the thick Ag coating layer. After addition of 0.5 mL of HAuCl4, the peak (Figure 4B) has a red shift to 455 nm and the absorbance band becomes broad. When 1 mL of HAuCl4 is added, a peak appears at 460 nm (Figure 4C). With 1.5 mL of HAuCl4 added (Figure 4D), the absorbance band becomes broader and a new peak is observable at 500 nm beside the peak at 460 nm. With further addition of 2 and 2.5 mL of HAuCl4 (Figure 4E,F), the intensity of the peak at 460 nm decreases and the peak at 500 nm increases, giving an almost flat absorbance band. When excess HAuCl4 (3 mL) is introduced, the peak at 500 nm is clearly visible (Figure 4G). The red shift could be attributed to the introduction of Au(0) and the electromagnetic coupling effect. The electromagnetic coupling effect also could be responsible for the broadening of the absorbance band. From HRTEM images (Figure 8 in the Supporting Information), the shell of the particle becomes porous and many surfaces with different curvatures form. It is believed that these formed surfaces are equivalent to aggregations of many small NPs with different diameters.70 It is well-known that the aggregation of small NPs leads to broadening of the absorbance band and a red shift due to their electromagnetic coupling effect.31,73-76 As well as the effect of aggregation of NPs with various diameters, the hollow interior and porous shell lead to broadening and a red shift. The optical color (inset in Figure 4) accompanies the change of the UV-vis spectral results. The color of the template Au@Ag is light yellow, and the HNACs are a little red, indicating the composition change of the NPs. EDX was used to obtain data on the composition of two metals, and the results are shown in Figure 5. For the template

Figure 3. Representative TEM images of (A1, A2) 12 nm Au@Ag NPs and of the same NPs (B1, B2) after etching using 0.5 mL of HAuCl4, (C1, C2) after etching using 1.5 mL of HAuCl4, and (D1, D2) after etching using 3 mL of HAuCl4.

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Figure 5. EDX results of (A) Au@Ag NPs and of the same NPs (B) after etching using 1.5 mL of HAuCl4 and (C) after etching using 3 mL of HAuCl4.

of Au@Ag (Figure 5A), the Ag peak possesses a higher intensity than the Au peak, which is consistent with the composition (Ag/ Au ) 10). After reaction with 1.5 mL of HAuCl4, the Ag peak decreased greatly and the intensity of the Au peak increased, indicating consumption of Ag NPs and formation of Au(0) as shown in Figure 5B. With addition of 3 mL of HAuCl4, although the Ag peak is still much weaker than the Au peak (Figure 5C), it is clearly observed, signifying that the HNACs still trap some Ag. 3.3. Possible Mechanism of Side-Selective Etching. The above observations show the etching process here should employ another route instead of those reported by another group.30 Previously, by using polycrystalline Ag icosahedral NPs, Xia demonstrated etching existed in each of the {111} facets almost simultaneously.30 In our experiment, if the process follows such a route, the void should appear around the core instead of only on one side. However, mainly the void prefers to appear only on one side, demonstrating that the etching is side-selective. Similarly, in several groups, rattle-like NPs have been prepared by using Au@Ag NPs as the template.29,65,69,77 However, such side-selective etching was not observed by them. In our system, three features are possibly responsible for the side-selectiveetching: (I) The interfacial alloying between the Au core and Ag shell changes the Au and Ag distribution. As we all know, the mobility of atoms can be highly accelerated under heat, by which Au@Ag NPs can be transformed into alloy NPs due to atom exchange between the Au core and Ag shell (Scheme 2).34,74,75 However, at room temperature, this exchange rarely happens. In these previous preparations of nanorattles, the Au@Ag NPs used by other groups are prepared at relatively low temperature.29,65,69 In our case, the Au@Ag NPs are synthesized at high temperature, and hence, the interfacial alloying should exist. The alloying can relocate Au and Ag atoms. Therefore, inside the NPs, the Au atom density decreases gradually from the Au core to the surface of the Au@Ag NPs owing to the different migration distances of the atoms; that is to say, the side near the Au core (SNC) has a higher Au atom density than the side far from the Au core (SFC). (II) The structure of Au@Ag NPs is believed to be critical to sideselective etching. The HAADF image (Figure 1 in the Supporting Information) shows Au cores are always off center rather than right center (Scheme 2), which is also confirmed by the STEM-EDX line spectra of a single NP as shown in Figure 6. In the preparation of Au@Ag NPs, if the growth occurred surrounding the seed homogenously, the richest area of Au should be right center. However, according to the STEM-EDX line spectrum, the richest area of Au is about 3 nm away from

SCHEME 2: Description of the Alloying Effect on the Galvanic Reactiona

a

The circular shaded area is the section with a high Au content.

Figure 6. Representative STEM-EDX line spectra of a single Au@Ag NP: (A) Ag-M, (B) Au-M. (C) HAADF image. The red arrow is the scanning direction.

the center (Figure 6A,B), which is in agreement with the HAADF result (Figure 6C). (III) Whether the galvanic reaction can occur depends on the molar ratio of Au to Ag. In our other study, Au/Ag alloy NPs were prepared and exposed to the galvanic reaction with HAuCl4 (Figures 3-5 in the Supporting Information). When the molar ratio of Au to Ag is more than 0.17, the galvanic reaction has difficulty occurring, indicating a higher Au content can protect Au/Ag alloy NPs against galvanic etching. Xia’s report in which alloy Au/Ag NPs can protect themselves from galvanic etching also demonstrated this result.29 Also, Aherne and co-workers found that the deposition of a Au thin layer on the Ag nanoprism can render them able to resist galvanic etching. Very possibly, the addition of Au changes the Ag reducing potential and damages the nanobattery.

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Figure 9. SERS spectra of (1) different sizes of the HNACs ((A) without NPs, (B) 12 nm, (C) 16 nm, and (D) 20 nm) and (2) hollow NPs (B) and solid Au NPs (A). FE-SEM images of (3) 12 nm and (4) 16 nm hollow NPs.

Figure 7. HRTEM images of various intermediate NPs during the galvanic reaction (A1, B1, C1, D1, F1-F3) and schematic description of etching (A2, B2, C2, D2, E2) (the circular shaded area is the section with a high Au content).

When one Au@Ag NP is exposed to HAuCl4, the high Au content of its SNC offers this side the strong ability to resist galvanic etching compared with the SFC; that is to say, the Ag

of the SFC is more vulnerable to oxidization than that of the SNC. Hence, galvanic etching prefers to start at the SFC. With more HAuCl4 added, the etching gradually extends from the SFC to the SNC, generating a variously hollow structure. For a more distinct observation of the etching process, some NPs which are intermediates during the evolution from Au@Ag to HNACs were characterized using HRTEM (Figure 7). At the beginning of the galvanic reaction, in a nanobattary (Ag NPs, Au3+), the {111} facets of the SFC first act as cathodes of the nanobattery and the twin boundary of the SFC as the anode (Figure 7B), generating a small void.29 With further etching, compared with the SNC, the twin boundary far from the core is changed to a cathode and etched (Figure 7, panels C1, C2, D1, and D2), which makes the core come out. In the end, almost all boundaries and {111} facets disappear (Figure 7, panels E1 and E2) and only part of the Ag with a low reducing potential is left as an alloy of Au/Ag. The distances between the SFC/ SNC and the core are carefully measured and shown in Figure 7, panels B1, C1, D1, and E1 (oa is the distance between the

Figure 8. Representative STEM-EDX line spectra of a single Au@Ag NP: (A) Ag-M, (B) Au-M. (C) HAADF image. The red arrow is the scanning direction.

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Figure 10. SEM image and size distribution of AgCl NPs with different sizes: (A1-A3) 210 nm, (B1-B3) 230 nm, (C1-C3) 310 nm, (D1-D3) 350 nm, (E1-E3) 390 nm. SD ) standard deviation of the size distribution.

SNC and the core; ob is the distance between the SFC and the core). The results show that oa always is shorter than ob, confirming the core is off center. For example, in Figure 7, panel E1, oa is 5 nm and ob is 4 nm longer than oa. During the whole reaction, the alloying process in which the formed Au first deposits on the anode (twin boundaries or SNC) and second migrates to the generated void of the {111} facet must accompany the galvanic reaction and rebuild the NP morphology. As stated by Xia and Lee, the alloying process should be very fast so that the intermediate is not observed.25,30 We carefully checked the TEM results and found some NPs are starfish-like, which uniformly have five protrusions and probably are intermediate NPs (Figure 7, panels F1-F3). The dents and the protrusions of starfish-like NPs are caused by selective etching and selective deposition, respectively. Besides, the SFC always has more distinct deformations than the SNC. It is wellknown that selective etching and selective deposition can lead

to deformation of NPs, and therefore, the strong ability to resist galvanic reaction in the SNC avoids deformations compared to that in the SFC. Further evidence is obtained by the STEMEDX line spectrum technology as shown in Figure 8. The line spectrum result (in Figure 8A) clearly shows the connecting area (Figure 8, panel C1) has more Ag than the other side (Figure 8, panel C3), demonstrating that Ag in the connecting area has a stronger ability to survive than that in other side in galvanic etching. In a recent study, an asymmetric hollow nanorod in which an alloy ellipsoidal core was covered by a hollow nanorod has been prepared by Song through a partial galvanic reaction.67 They thought that the formation of a single pit at the beginning of the galvanic reaction was the key factor for etching, and the mechanism reported by them could also be the cause of our results. However, a full understanding of these processes needs further research, which is under way.

Galvanic Reaction and Synthesis of HNACs To further confirm the process, single-crystalline Au@Ag NPs with various sizes were fabricated and used in the galvanic reaction, such as 12, 16, and 20 nm (Figures 9-11 in the Supporting Information). All results confirm the side-selective etching route. FE-SEM was also used to observe HNACs, and structures with a core in a shell were visible (Figure 10G,H in the Supporting Information). It is worth noting that the size of the core is much larger than that of the seed for hollow NPs from 16 nm Au@Ag (Figures 10 and 11 in the Supporting Information). During the preparation of Au@Ag NPs, the synthesis of larger sized NPs requires a longer heating time, which increases the alloying level and causes swelling of the core. 3.4. Applications of HNACs in SERS. Rough surfaces and aggregation of NPs have practical applications in SERS, and they can generate much stronger Raman enhancement compared with smooth surfaces and single NPs. In our experiments, the HRTEM results provide evidence that HNACs have a larger specific surface area and roughness, offering promising materials for SERS. Here, through use of 2-naphthalenethiol as the probe molecule and organization of hollow NPs as the substrate, their SERS is studied, and the results show such structured NPs can be employed as highly active SERS substrates (Figure 9). In Figure 9, panels 3 and 4, after the solvent evaporated, colloid HNACs form a closely packed assembly. For the 12 nm HNACs, an ordered organization was clearly visible on the wafer. When the wafers with organizations were immersed in ethanol containing 2-naphthalenethiol, the alkylamine was replaced by 2-naphthalenethiol due to the strong bonding ability of the probe molecule. Under 633 nm laser excitation, the substrate which has no coating layer of HNACs possesses no Raman signals except for a peak of Si (Figure 9, panel 1, spectrum A). On the substrate covered by assemblies of NPs, the Raman signals are enhanced greatly (Figure 9, panel 1, spectra B-D). The Raman signal intensity has a distinct size independence, larger HNACs giving a stronger signal than small ones as shown in Figure 9, panel 1, spectra B-D. The wafer was treated in the same way except that 13.4 nm solid gold NPs were used instead of HNACs. The Raman signal generated by HNACs is 2 times that generated by solid NPs, although the size of the HNACs is 12 nm and less than that of the solid NPs, proving a hollow structure is more effective for Raman enhancement than its solid counterpart. It is thought that the hollow HNAC can capture more probe molecule due to its larger surface area and in turn the Raman signal is improved and that the roughness caused by etching is also responsible for enhancement. 3.5. Formation of Size-Controlled Monodispersed AgCl NPs. In the galvanic reaction, insoluble AgCl precipitations are always observed because of the introduction of Cl- and formation of Ag+.30 Here, under our experimental conditions, formed AgCl precipitated accompanying HNACs after ethanol was added. Precipitations of HNACs can dissolve in an organic solvent easily. However, AgCl cannot be dispersed again, so it can be separated readily through centrifugation (3000 rpm) or by being left still for 1 h. Interestingly, when the AgCl precipitations are observed through SEM, it is found that they have a regular shape and high monodispersity as shown in Figure 10, which has not been reported so far. Moreover, through tuning the amount of HAuCl4, the size control is achieved from 210 to 390 nm. For example, when 0.5 mL of HAuCl4 is added, 210 nm AgCl NPs are obtained as shown in Figure 10A. With the amount of HAuCl4 increasing to 0.75 mL, 230 nm NPs are fabricated as shown in Figure 10B. AgCl NPs

J. Phys. Chem. C, Vol. 114, No. 42, 2010 18079 of 310 and 350 nm size are obtained when the amounts of HAuCl4 are 1.5 and 2 mL, respectively. When excess HAuCl4 (3 mL) is introduced, the final size of AgCl is 390 nm as shown in Figure 10E. Seeding growth could result in size enlargement. At the beginning of the galvanic reaction, small-sized AgCl NPs appear and act as seeds. With further addition of HAuCl4, newly formed AgCl deposits centering seed NPs to produce larger sized NPs. X-ray photoelectron spectroscopy (XPS) and EDX technology were used to obtain the composition of AgCl. Both results show that the molar ratio of Ag to Cl is close to the theoretical value of 1, which confirms the obtained NPs are AgCl (Figures 12-14 and Tables 1 and 2 in the Supporting Information). 4. Conclusion In summary, we reported the synthesis of HNACs through use of Au@Ag core-shell NPs as starting materials and the galvanic reaction. By adjusting the reactive conditions, various HNACs were prepared, with different diameters and core sizes. On the basis of TEM observations, we found a novel etching route in which the oxidization of Ag starts from one side of the NPs rather than every part. We figured out a possible mechanism in which the side-selective etching is attributed to interfacial alloying between the Au core and Ag shell, the Au-contentdependent galvanic reaction, and the structure of Au@Ag. The SNC has a strong ability against galvanic etching because it has more Au than the SFC. However, the SFC is exposed to etching easily due to the low composition of Au. These prepared HNACs are used as active substrates for SERS, and the results demonstrate they have a good performance in enhancing Raman signals of probe molecules (2-naphthalenethiol). Moreover, it was found that AgCl, which is formed from Ag+ in etching and added Cl- of HAuCl4, appears as monodispersed NPs. Also, the size of the AgCl NPs ranging from 210 to 390 nm is controllable. So far, the galvanic reaction between Ag and another noble ion is still not understood completely, and hopefully the etching process reported here can provide some data to uncover this reaction. In addition, the prepared hollow NPs could have some other applications due to their larger surface area and porous structure, such as drug delivery and cancer therapy.1-3 Acknowledgment. This work was supported in part by the NSFC (Grant 50772076), NSFZJ (Grant R4090137), ZJED Innovative Team, and Research Fund of College Student Innovation of Zhe Jiang Province (Grant 3150601107090420). Supporting Information Available: Other XPS, EDX, and TEM images and HAADF and UV-vis spectral results. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Au, L.; Zheng, D.; Zhou, F.; Li, Z.; Li, X.; Xia, Y. ACS Nano 2008, 2, 1645–1652. (2) Song, K. H.; Kim, C.; Cobley, C. M.; Xia, Y.; Wang, L. Nano Lett. 2009, 9, 183–188. (3) Chen, J.; Wang, D.; Xi, J.; Au, L.; Siekkinen, A.; Warsen, A.; Li, Z. Y.; Zhang, H.; Xia, Y.; Li, X. Nano Lett. 2007, 7, 1318–1322. (4) Giljohann, D. A.; Seferos, D. S.; Prigodich, A. E.; Patel, P. C.; Mirkin, C. J. Am. Chem. Soc. 2009, 131, 2072–2073. (5) Pasricha, R.; Bala, T.; Biradar, A. V.; Umbarkar, S.; Sastry, M. Small 2009, 5, 1467–1473. (6) Lee, H.; Habas, S. E.; Somorjai, G. A.; Yang, P. J. Am. Chem. Soc. 2008, 130, 5406–5407. (7) Narayanan, R.; El-Sayed, M. A. J. Am. Chem. Soc. 2003, 125, 8340– 8347.

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