Preparation and characterization of Ag(Au) bimetallic core–shell nanoparticles with new seed growth method

Colloids and Surfaces A: Physicochem. Eng. Aspects 260 (2005) 79–85

Preparation and characterization of Ag(Au) bimetallic core–shell nanoparticles with new seed growth method Lei Qian a,b , Xiurong Yang a,∗ a

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Remin Street 5625, Changchun, Jilin 130022, China b Graduate School of the Chinese Academy of Sciences, Beijing 100039, China Received 30 November 2004; accepted 15 March 2005 Available online 5 May 2005

Abstract Ag(Au) bimetallic core–shell nanoparticles were prepared by a new seed growth method. Ascorbic acid was used to reduce the complex of HAuCl4 and hexadecyltrimethylammoniumbromide (CTAB). This resulted in the forming of colorless Au(I) (AuCl2 − ). It was used as the growth solution to prepare these bimetallic core–shell nanoparticles. These nanoparticles were characterized by UV–vis spectroscopy, transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). The results showed these nanoparticles exhibited core–shell shape and there was large amount of Ag in the shell. These nanoparticles could be produced in a few minutes without violent stirring and the method was easy and convenient compared with others. The effect of amount of AuCl2 − on the shape of nanoparticles was also studied. Many small gold nanoparticles were formed on the surface of bimetallic core–shell nanoparticles in the presence of excess AuCl2 − . The mechanisms were also proposed to explain the process of colloidal preparation. © 2005 Elsevier B.V. All rights reserved. Keywords: Bimetallic; Core–shell; Nanoparticles; Ascorbic acid; Seed growth method

1. Introduction The preparation of nanoparticles with various shapes attracts considerable interests in recent years. Shape of colloids has effects on their properties such as optics, electrochemistry, Raman spectroscopy and so on [1–5]. Many nanoparticles with different shapes such as nanorods, nanowires, metallic core–shell nanoparticles, nanoplates and branched nanoparticles have been reported [6–20]. These particles can be used to develop new nanomaterials because of their unique properties. Among these nanoparticles with various shapes, the preparation of nanostructures with hollow interiors has been recently reported because of their unique properties of Raman and UV–vis spectroscopy. These particles exhibit plasmon properties completely different from those of solid nanopar∗

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ticles [21,22]. Xia and co-workers [16] reported a one-step approach to synthesis of metal nanostructures with hollow interiors. This method was based on the fact that silver colloids could be oxidized by HAuCl4 because the standard reduction potential of AuCl4 − /Au (0.99 V versus SHE) was higher than that of Ag+ /Ag pair (0.80 V versus SHE) [16]. Seed growth method was a usual method to prepare nanoparticles. On the basis of surface-catalyzed reduction of Au3+ by NH2 OH, gold nanoparticles of various sizes could be prepared [23]. Ascorbic acid was also used to prepare spherical nanoparticles with different sizes [24]. Moreover, it was also widely used for preparation of gold nanorods with hexadecyltrimethylammoniumbromide (CTAB) as templates [25–29]. Ascorbic acid was a mild reducing reagent and it could not reduce HAuCl4 to Au(0) at room temperature. But it could reduce gold–CTAB complex to Au(I) which could be reduced to Au(0) at the surface of small gold nanoparticles [29]. The preparation of Ag(Au) bimetallic core–shell nanoparticles by the seed growth method was also reported [30–32]. Among

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them, NH2 OH was used as a reducing agent to deposit metal layer. But this seed growth method needed violent stirring and took much time to react completely. In this paper, an easy and convenient seed growth method to prepare these nanostructures was proposed. AuCl2 − solution was obtained by ascorbic acid reducing HAuCl4 –CTAB complex. The solution was used as a growth solution and Ag colloids protected by citrate as seeds. It could be found that nanoparticles of bimetallic core–shell shape were easy to produce. Moreover, this process would finish completely in a few minutes. It indicated that this method was easy and convenient compared with others.

2. Experimental 2.1. Chemicals and reagents Hydrogen tetrachloroaurate(III) trihydrate (HAuCl4 · 3H2 O) and AgNO3 were obtained from Acros. Hexadecyltrimethylammoniumbromide (99%) and ascorbic acid was purchased from Sigma. Sodium citrate was from Beijing Chemical Factory. Milli-Q grade water (18.2 M) was used for all solution preparation and experiments. All commercial chemicals were used as received without further purifying. 2.2. Preparation of silver colloids All glassware used in the following procedures were cleaned in a bath of fresh prepared 3:1 HCl:HNO3 and rinsed thoroughly in Milli-Q grade water before experiments. The silver nanoparticles were prepared by sodium citrate reduction of AgNO3 [33]. One hundred milliliters of aqueous solution containing 18 mg AgNO3 was heated to boiling, then 2 ml of 1% sodium citrate solution was added, and at last, the mixture was kept boiling for 1 h. The color of Ag nanoparticles prepared was greenish yellow. 2.3. Preparation of core–shell Ag(Au) bimetallic colloids 2.3.1. Preparation procedure I (PI) Ten, five and twenty microliters of 6 mM HAuCl4 was added to 200 ␮l of water, and the three solutions were called A, B and C for abbreviation. Then, 5 ␮l of 0.1 M CTAB was added to each solution. After that, 3, 1.5 and 6 ␮l of 0.1 M ascorbic acid were added to solutions A, B and C, respectively. At last, 500 ␮l of Ag colloids was added to the three solutions. The color of solutions rapidly changed from yellow to orange, red or blue in a few minutes depending on HAuCl4 concentration. This indicated that nanoparticles of new shape were appeared. 2.3.2. Preparation procedure II (PII) The procedure II was based on the same amount of HAuCl4 and ascorbic acid. Ten microliters of 6 mM HAuCl4

and 5 ␮l of 0.1 M CTAB were added to 200 ␮l of water, and the three solutions called D, E and F. Three microliters of 0.1 M ascorbic acid was added to each solution. Eight hundred, five hundred and two hundred microliters of Ag colloids were added to solutions D, E and F. The color of these solutions rapidly changed within a few minutes. 2.3.3. Characterization UV–vis absorption spectra were acquired using a Cary 50 UV–vis NTR spectrometer (Varian, USA) with 1 cm light path quartz curette. The transmission electron microscopy (TEM) images were obtained on a JEOL-2000 EX transmission electron microscope operating at 160 kV. The samples for TEM were prepared by drop-casting one drop of the asprepared colloids onto carbon-coated Formvar films on copper grids. X-ray photoelectron spectroscopy (XPS) analysis was preformed on an ESCLABMK II using Mg as the exciting source. For XPS experiments, the samples were prepared by placing one drop of the prepared nanoparticles onto a clean glass slide and then drying in air.

3. Results and discussion 3.1. UV–vis absorption spectra of Ag seeds UV–vis absorption spectrum was usually to characterize Au and Ag nanoparticles because of their different optical properties. The two colloids showed different maximum absorbance for the different plasmon excitation resonance of each metal [34]. Fig. 1A showed the UV–vis spectrum of Ag colloids prepared by sodium citrate reduction of AgNO3 . The peak of maximum absorbance was about 418 nm which was the typical surface plasmon resonance of silver nanoparticles [33], indicating spherical large silver nanoparticles appeared. TEM images (Fig. 1B) showed a relatively homogeneous distribution of particles. Most of them were spheroidal particles, but some nanorods could be found. 3.2. UV–vis absorption spectra and morphology of bimetallic core–shell nanoparticles Metal nanoshell exhibited a strong plasmon resonance, which shifted to much longer wavelengths compared with the solid metal nanoparticles [32,35,36]. By changing the relative size of core and the thickness of the metallic overlayer, the plasmon resonance of the nanoshells could be tuned [35]. Fig. 2 showed the absorption spectra of core–shell bimetallic colloids prepared by procedure I. When Ag colloids were added, the color of solutions changed in a few minutes, which indicated the change of size and shape of particles. With the concentration of AuCl2 − increasing, the color of solutions after interaction was from orange, to red or blue. The peak of maximum absorbance all shifted to longer wavelengths compared with that of Ag nanoparticles. Moreover, there were not separate Ag and Au plasmon resonances of these col-

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Fig. 3. The UV–vis spectra of Ag(Au) bimetallic core–shell nanoparticles prepared by PII. The spectra a–c represented the solutions D–F.

Fig. 1. (A) The UV–vis absorbance of Ag colloids prepared by sodium citrate reduction of AgNO3 . (B) TEM images of Ag colloids as seeds.

Fig. 2. The UV–vis spectra of Ag(Au) bimetallic core–shell nanoparticles prepared by PI. The spectra a–c corresponded to solutions A–C.

loids. This indicated that these nanoparticles were not mixture of two different metal particles [34]. From solution A to C, the peak of maximum absorbance was 459, 509 and 605 nm (curves a–c). The peak red-shifted with amount of HAuCl4 increasing, and the full width at half maximum of the absorption band increased from solution A to C. This result suggested that the nanoparticle size distribution became wide and particles of various shape were produced. Fig. 3 showed the same property of UV–vis spectra. The peak redshifted with decreasing amount of Ag colloids (curves a–c) and the maximum absorbance of solution F located at about 620 nm, which indicated the size of colloids was very big. The intensity of peak decreased from solution D to F, and this could be attributed to the decrease of the amount of Ag nanoparticles added as seeds. TEM could directly characterize the size and shape of nanoparticles. The images of bimetallic core–shell nanoparticles prepared by PI were shown in Fig. 4. Fig. 4A corresponded to the images of nanoparticles obtained from solution A. Some core–shell nanoparticles were produced, but shape of most particles was similar to Ag nanoparticles as seeds. There was only small light portion for most nanoparticles in solution A (Fig. 4B). This was consistent with the results of UV–vis spectroscopy (Fig. 2). The maximum absorbance of solution A only red-shifted 40 nm compared with that of Ag seeds. With amount of AuCl2 − increasing, the amount of nanoparticles of core–shell shape increased. Fig. 4C showed the images of solution B, and there was obvious light portion for the nanoparticles (Fig. 4D). This showed bimetallic core–shell nanoparticles were successfully prepared by this method. The images of solution C were shown in Fig. 4E and F. Fig. 4G gave an enlarged image of some core–shell nanoparticles and the shape of core–shell could be obviously observed. We could found most nanoparticles exhibited shape of core–shell, but there was small nanoparti-

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Fig. 4. TEM images of bimetallic core–shell nanoparticles prepared by PI. (A and B) TEM images of solution A; (C and D) TEM images of solution B; (E and F) TEM images of solution C; (G) the enlarged TEM image of core–shell nanoparticles.

cles on the surface of these nanoparticles (Fig. 4F). This was attributed to small gold nanoparticles produced by reduction of AuCl2 − on the surface. This indicated amount of AuCl2 − had an effect on the shape of nanoparticles. To test the effect of AuCl2 − , bimetallic nanoparticles were prepared by further increasing amount of HAuCl4 . Because yellow precipitation was easy to produce with 40 ␮l of 6 mM HAuCl4 added, the amount of 0.1 M CTAB was increased to 10 ␮l. UV–vis spectrum showed the maximum absorbance located at about 630 nm (not shown here), suggesting that the size of particles became larger. TEM images showed many small

nanoparticles appeared on surface of these colloids (Fig. 5). This indicated that in the presence of excess AuCl2 − , small gold nanoparticles were produced on the colloidal surface. So the size of particles increased and the shape of them changed. 3.3. Analysis composition of bimetallic core–shell nanoparticles Using XPS to determine composition of shell and core of nanoparticles was reported [30,32]. XPS could provide direct

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Fig. 5. TEM images of bimetallic core–shell nanoparticles prepared in the presence of 40 ␮l of 6 mM HAuCl4 and 10 ␮l of 0.1 M CTAB.

evidence of the presence of Au shell enrich with Ag because the binding energies of Au and Ag shifted for Ag–Au alloy [32]. In our XPS experiments, the sampling depth of photoelectron was about 4–5 nm. So the measurement intensity of signal almost came from the shell parts of particles. Fig. 6 showed the results of XPS analysis. Ag colloids prepared by citrate reduction of AgNO3 showed two obvious peaks of Ag 3d (Fig. 6A). Fig. 6B–G corresponded to the results of core–shell bimetallic nanoparticles prepared by PI. Only the signal of Ag could be obtained, but there was less signal of Au. This could be explained by the amount of Au in the shell was too small to be measured. When Ag seeds were added, Ag could be oxidized by AuCl2 − absorbed on the surface of Ag seeds and Ag+ was again reduced to Ag in the presence of ascorbic acid. Moreover, the process of silver nanoparticles reproduced by reduction of ascorbic acid on the seeds was slow. So, most gold nanoparticles were produced inside the shell. We could not find the binding energies of Ag 3d negatively shifted. It could be attributed to there were small amount of Au in the shell. This made the alloying between Ag and Au become unobvious. Silver colloids formed in surfactant media were found to be very easily oxidized by O2 in the presence of excess sodium borohydride. Sodium borohydride absorbed on the surface of Ag nanoparticles, which could decrease the oxidation potential of Ag+ /Ag. Surfactant removed the oxidized surface silver atoms, and this caused Ag colloids to be oxidized by O2 [37]. To further test the composition of shell of bimetallic core–shell nanoparticles, we studied the interactions between O2 and these nanoshells in the presence of excess of sodium borohydride. Excess of 0.1 M NaBH4 solutions were added to

the three solutions of Ag(Au) core–shell nanoparticles. After shaking, the color of solutions slowly became colorless. At last, black precipitants appeared in the solutions. This could be attributed to the presence of Ag2 O or AgO which produced by interaction of Ag+ and OH− . This indicated that Ag(Au) bimetallic nanoparticles could be oxidized by O2 , and showed there were large amount of Ag in the shell of these nanoparticles. 3.4. Mechanisms of preparation of bimetallic core–shell nanoparticles Preparation mechanisms of bimetallic core–shell nanoparticles were reported [30,31]. Ag colloids could be oxidized by AuCl2 − because of the different reduction potential between AuCl4 − /Au and Ag+ /Ag. Reduction of HAuCl4 by NH2 OH did not proceed unless Ag seeds were added. These seeds had electroactivity for the reduction, which resulted in electronless deposition of gold nanoparticles on the surface of Ag colloids [30,31]. At the same time, Ag colloids could be reproduced because Ag+ was again reduced in the presence of NH2 OH [32]. So Ag(Au) bimetallic nanoshells could be prepared. XPS showed these silver and gold nanoparticles alloyed in the shell and heating could prompt further surface alloying [32]. In our experiments, when CTAB and HAuCl4 solutions were mixed, the color of solutions turned from yellow to brown-yellow, which attributed to the presence of ligandsubstituted anions or complexes. Adding of the ascorbic acid resulted in colorless solutions and it showed that the Au(III) complexes were initially reduced to aqueous Au(I) [29]. Because the standard potential of AuCl2 − /Au (1.11 V versus

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SHE) was higher than Ag+ /Ag (0.80 V versus SHE), Ag colloids could be fast oxidized by AuCl2 − at the surface:

Ag+ could be reduced back to Ag by ascorbic acid at the surface of nanoparticles:

Ag + AuCl2 − → Au + Ag+ + Cl−

Ag+ → Ag

(1)

(2)

Fig. 6. XPS measurements for silver colloids and bimetallic core–shell nanoparticles. (A) Ag 3d of silver colloids produced by sodium citrate; (B, D and F) Ag 3d of bimetallic nanoshells of solutions A–C; (C, E and G) Au 4f of bimetallic nanoshells of solutions A–C.

L. Qian, X. Yang / Colloids and Surfaces A: Physicochem. Eng. Aspects 260 (2005) 79–85

Otherwise, excess of AuCl2 − could be reduced on the surface of nanoparticles. These nanoparticles could be used as seeds to reduce AuCl2 − : AuCl2 − → Au

(3)

From solution A to B, small nanoparticles could not be found on the colloidal surface. But small nanoparticles were produced in the presence of large amount of AuCl2 − (Fig. 5). Because the standard potential of Ag+ /Ag (0.8 V versus SHE) was small than that of AuCl2 − /Au (0.99 V versus SHE), the process (3) was slow compared with process (2). This showed that the processes (1) and (2) could process more easily than reaction (3). Moreover, process (1) was the fastest among them. This resulted in bimetallic core–shell nanoparticles appearing with large amount of Ag in the shell. When there was excess AuCl2 − , large number of small nanoparticles appeared on the surface of core–shell nanoparticles by process (3).

4. Conclusions Ag(Au) bimetallic core–shell nanoparticles were prepared by a new growth method. UV–vis spectroscopy, XPS and TEM were used to characterize the shape and size of nanoparticles. Nanoparticles with core–shell shape could be obtained in a few minutes and this method was easy to process compared with the other methods. The amount of AuCl2 − had an effect on the shape of nanoparticles. Excess of them resulted in the appearance of small gold nanoparticles on the surface of colloids. These nanoparticles produced by this way will be used to develop new type of nanomaterials.

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Acknowledgements This work was supported by the National Nature Science Foundation of China with the Grant No. 20475052 and the National Key Basic Research Development Project Research on Human Major Disease Proteomics (No. 2001CB5102).

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