Crystal Structures and Growth Mechanisms of Au@Ag Core−Shell

The crystal growth of new Au@Ag core−shell nanoparticles was studied ...... Access to small size distributions of nanoparticles by microwave-assiste...
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Crystal Structures and Growth Mechanisms of Au@Ag Core-Shell Nanoparticles Prepared by the Microwave-Polyol Method Tsuji,*,†,‡

Miyamae,‡

Masaharu Nobuhiro Sachie Hikino,† and Michiko Nishio‡

Seongyop

Lim,§

Kimura,‡

Kousuke

Xu

Zhang,†,|

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 8 1801-1807

Institute for Materials Chemistry and Engineering, Graduate School of Engineering Sciences, and Art, Science and Technology Center for CooperatiVe Research, Kyushu UniVersity, Kasuga 816-8580, Japan, and Department of Chemistry, Harbin Normal UniVersity, Harbin 150080, Heilongjiang, People’s Republic of China ReceiVed February 27, 2006; ReVised Manuscript ReceiVed April 13, 2006

ABSTRACT: Au@Ag core-shell nanocrystals have been synthesized by using a microwave-polyol method, and their growth mechanisms have been studied. When HAuCl4‚4H2O was reduced in ethylene glycol in the presence of poly(vinylpyrrolidone) (PVP) as a polymer surfactant, a mixture of triangular twin platelike, octahedral, and multiple-twinned decahedral Au nanocrystals, having only {111} facets, was prepared. When Ag+ was reduced by using these Au nanocrystals as seeds, triangular-bipyramidal, cubic, and rod/wire Ag shells, having {100} facets, were overgrown, respectively. It was concluded that morphology changes between Au cores and Ag shells arise from changes in the adsorption selectivity of PVPs from {111} facets of Au to {100} facets of Ag. Total space volumes of Ag shells overgrown on Au cores were on the order of cubes > triangular-bipyramidal crystals > rods and wires. These findings provide general information on the growth mechanism of face-centered cubic (FCC) types of single crystals in the presence of a polymer surfactant, which is difficult to obtain from monometallic crystals. Introduction In recent years, bimetallic nanoparticles have received intense attention, owing to their different optical, electronic, magnetic, and catalytic properties relative to those of the individual metals. Since these properties strongly depend on the composition, shapes, and sizes of nanoparticles, extensive studies have been carried out on the composition-, shape-, and size-controlled syntheses of bimetallic core-shell and alloy nanoparticles. A number of methods have been used to synthesize bimetallic nanoparticles: for example, alcohol citrate reduction,1 alcohol reduction,2 the polyol process,3 solvent extraction reduction,4 sonochemical method,5 photolytic reduction,6 decomposition of organometallic precursors,7 dendrimer-templating method,8 electrolysis of a bulk metal,9 and laser ablation.10 Among these methods, the polyol method is a typical technique to prepare bimetallic metallic nanoparticles in solution by reducing their ionic salts. In general, a mixture of reagent and polymer surfactant in ethylene glycol (EG) was heated in an oil bath for several hours and spherical bimetallic nanoparticles were prepared. Recently, microwave (MW) heating has been coupled with the polyol method as a new rapid preparation method of metallic nanoparticles. This method, called the MW-polyol method, was applied to the synthesis of bimetallic nanoparticles.11,12 We have recently synthesized Au nanocrystals by using a MW-polyol method.13-15 When AuCl4- ions were reduced in EG in the presence of the polymer surfactant PVP, triangular, square, rhombic, and hexagonal nanoplates were preferentially produced. When these nanostructures were used as seeds, Ag+ was reduced in order to synthesize Au core-Ag shell nanostructures, denoted as [email protected] Then, we have succeeded in * To whom correspondence should be addressed. E-mail: tsuji@ cm.kyushu-u.ac.jp. † Institute for Materials Chemistry and Engineering, Kyushu University. ‡ Graduate School of Engineering Sciences, Kyushu University. § Art, Science and Technology Center for Cooperative Research, Kyushu University. | Harbin Normal University.

preparing some new Au@Ag nanocrystals having a good correlation between the Au core and Ag shell. However, their exact three-dimensional (3-D) crystal structures and the reason why a good correlation arises in shape between the Au core and Ag shell were not clarified in our previous communication. Gold and silver have the same FCC crystal structures, and the lattice constants of Au (0.0479 nm) and Ag (0.4086 nm) are very similar. Therefore, it is known that both Au/Ag alloys and various kinds of Au@Ag core-shell structures can be prepared from Au/Ag reagents using various methods.17-27 Fortunately, Au@Ag core-shell structures with thin boundary layers can easily be observed from transmission electron microscopy (TEM) images, because Au cores and Ag shells appear as clear black and blight contrasts, respectively.10,21 Therefore, we have found that there is a good correlation between Au cores and Ag shells from TEM images of various new Au@Ag core-shell structures.16 In this study, we have synthesized Au@Ag core-shell nanostructures at different [AgNO3]/[HAuCl4] molar ratios. The TEM images of Au@Ag crystals from different view angles, selected area electron diffraction (SAED) patterns, and energydispersive X-ray spectroscopy (EDS) analyses give new detailed information on 3-D crystal structures and growth mechanisms of each Au@Ag crystal in the presence of PVP. On the basis of these data, we have succeeded in the clarification of general growth mechanisms of FCC types of single crystals in the presence of a polymer surfactant, which is difficult to obtain from monometallic single crystals. Experimental Section The MW-polyol method used in this study was essentially identical with that reported previously.16 The preparation of Au@Ag core-shell nanocrystals has been carried out in two steps. The first step was the synthesis of an Au inner core, and the second step was the preparation of an Ag outer core. At first, 2.4 mM of HAuCl4‚4H2O was resolved in 20 mL of EG solution. Then, 1 M of PVP in terms of monomeric units (molecular weight 40 000) was added to the above solution. The mixture was heated by MW irradiation in a CW mode (Shikoku Keisoku, 400 W) for 2 min. After the heated solution was cooled to

10.1021/cg060103e CCC: $33.50 © 2006 American Chemical Society Published on Web 06/23/2006

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Figure 1. (a) TEM photographs of Au core nanocrystals prepared by MW heating of HAuCl4‚4H2O (2.4 mM)/PVP(1 M)/EG for 2 min. (b) TEM photographs of Au@Ag nanocrystals prepared by addition of AgNO3 (23 mM) to the solution obtained in (a) and MW heating for 2 min. room temperature, AgNO3 was added. The [AgNO3]/[HAuCl4] molar ratio was varied in the range of 3-30. The solution was heated again by MW irradiation for 2 min. Au core-Au/Ag alloy shell particles, denoted as Au@Au/Ag, were prepared by the addition of 2.4 mM of HAuCl4‚4H2O to Au@Ag nanocrystals obtained at an [AgNO3]/ [HAuCl4] molar ratio of 11. Product particles after the first and second MW irradiations were characterized by using TEM (JEOL JEM-2010 and JEM 3000F). Absorption spectra of products were measured in the 300-1400 nm region with a Shimadzu UV-3600 spectrometer.

Results and Discussion A typical TEM image of Au nanocrystals obtained by reduction of HAuCl4‚4H2O in EG under MW heating for 2 min is given in Figure 1a. A mixture of triangular, square, rhombic, and pentagonal Au nanoparticles in 2-D images was prepared. TEM images were observed at various view angles (within (25°), and SAED patterns were measured in order to determine their 3-D crystal structures. For example, the SAED pattern of a triangular plate is shown in Figure S1a (Supporting Informa-

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tion). In addition to strong {220} reflections of FCC crystals, weak forbidden 1/3{422} reflections are observed, indicating the presence of twin Au single crystals within the {111} plane,28 as shown in Figure S1b. On the basis of SAED patterns and TEM images of each Au nanocrystal observed from different view angles, it was concluded that all square and some rhombic 2-D images arise from octahedral nanocrystals and all pentagonal and some rhombic 2-D images arise from multiple twin decahedron (MTD) nanocrystals. The 3-D crystal structures of Au cores will also be shown in the later figures used to demonstrate the growth mechanisms of Au@Ag core-shell crystals. It is known that FCC crystals give various shapes due to different growth rates between the 〈111〉 and 〈100〉 directions.29 Facets having faster growth rates disappear, and those having slower growth rates finally remain. It is known that surface energies of FCC crystals are generally on the order of γ{111} < γ{100} < γ{110} facets.29 According to Wulff’s rule, crystals covered by such low-energy facets as {111} and/or {100} facets were expected to be grown as equilibrium shapes.30 It should be noted that triangular twin plate (TTP) like, octahedral, and MTD Au nanocrystals obtained in this study are surrounded solely by the most stable {111} facets (see Figures S1b, 4d, and 8a). This indicates that only Au crystals surrounded by {111} facets were grown under the present experimental conditions. In the presence of a sufficient amount of PVP, the selective adsorption of PVPs to {111} facets of Au nanoparticles probably plays a significant role in the preferential growth of {111} facets in each Au crystal. Figure 1b shows typical TEM images obtained after Ag addition with an [AgNO3]/[HAuCl4] ratio of 10. It should be noted that various kinds of Au@Ag core-shell nanocrystals are prepared. It was found that inverted triangular Ag shells are prepared via a triangular Au core, square or rectangular Ag shells are prepared via square or rhombic Au cores, respectively, and Ag rods are produced via rhombic Au cores. To clarify 3-D crystal structures of these Au@Ag nanocrystals and their

Figure 2. TEM photographs of (a) the triangular twin Au core and (b-h) Au@Ag nanocrystals prepared by addition of various amounts of AgNO3 to Au cores with MW heating for 2 min. Dotted lines are triangular twin Au core plates, which can be observed using photographs with better contrast. (i-l) Growth mechanism of triangular-bipyramidal Au@Ag crystals from triangular twin Au cores.

Crystal Growth of Au@Ag Core-Shell Nanoparticles

Figure 3. TEM EDS data of (a) the Au core, (b) the Ag shell, and (c) the triangular-bipyramidal Au@Ag core-shell crystal. (d) Distributions of Au and Ag components along the cross section line shown in (c). The Au@Ag core-shell was prepared at an [AgNO3]/[HAuCl4] molar ratio of 11.

growth mechanisms, Au@Ag nanocrystals were prepared in the wide [AgNO3]/[HAuCl4] ratio range of 3-30. When mixtures of HAuCl4‚4H2O and AgNO3 were reduced in one pot with MW heating, only spherical and highly crystalline Au/Ag alloys were prepared and no core-shell crystals were obtained.31 This indicates that the two-step process is required for the preparation of Au@Ag core-shell nanocrystals. Triangular-Bipyramidal Au@Ag Core-Shell Nanocrystals. Figure 2 shows TEM photographs of Au core and Au@Ag nanocrystals overgrown over TTP-like Au cores in a [AgNO3]/ [HAuCl4] molar ratio range of 5-30. At low [AgNO3]/[HAuCl4] molar ratios of 5-9, truncated-triangular-bipyramidal crystals are overgrown until the Ag edge lengths observed from the top view become twice of those of Au core plates, as shown in Figure 2b-e. At further higher [AgNO3]/[HAuCl4] molar ratios of 11-30, the same triangular-bipyramidal shells were over-

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grown, as shown in Figure 2f-h. The edge length of Ag shells increases on increasing the [AgNO3]/[HAuCl4] molar ratio and becomes about 7 times larger than that of the TTP-like Au core (about 50 nm) at the highest [AgNO3]/[HAuCl4] molar ratio of 30. When the total sizes of Au@Ag crystals become large, it becomes difficult to see the exact position of the Au cores. However, better contrast TEM images definitely demonstrated that all cores located in the middle are inverted to outer larger triangular-bipyramidal Ag shells. The position of the Au core was also confirmed by etching Ag shells using AuCl4- ions.23,25 Due to a replacement reaction between Ag and Au, 3Ag(s) + AuCl4-(aq) f Au(s) + 3Ag+(aq) + 4Cl-(aq), Ag shells could partly be broken and an Au core can be observed in the center of Au/Ag alloys shells (see Figure S2a (Supporting Information)). It was known that Ag+ produced by etching reaction is reduced again and provides Au/Ag alloy shells.23,25 The 3-D crystal structures were examined by measuring TEM images at incident electron beam angles within (25°: TEM images changed from triangular forms to pyramidal ones, indicating that the products are not 2-D platelike structures but 3-D pyramidal structures (see Figure S1c,d,f (Supporting Information)). The SAED patterns at incident angles of 0 and 16.1° indicated that the incident electron beam was perpendicular to the {100} and {111} planes of product crystals, respectively (see Figure S1e,g (Supporting Information)). Figure 3 shows TEM EDS data of a triangular Au@Ag core-shell nanocrystal. It is clear from these data that the triangular Au@Ag core-shell nanocrystal consists of an Au core and Ag shell and no Au/Ag alloys exist in this Au@Ag core-shell structure. Distributions of Au and Ag components along a diagonal line in Figure 3c suggest that an inverted triangular Ag shell is covered not only on side {111} facets of the Au core but also on its wide top and bottom {111} facets. This finding is consistent with TEM images observed from various view angles. These data led us to conclude that products are not 2-D platelike structures but 3-D triangular-bipyramidal ones. On the basis of the above findings, we propose the growth mechanism of these triangular-bipyramidal crystals (Figure 2i-l). Twin Au triangular core plates consist of two wide top and bottom {111} facets and six narrow side {111} facets (Fig-

Figure 4. TEM photographs of (a) the octahedral Au core and (b, c) Au@Ag nanocrystals prepared by addition of different amounts of AgNO3 to Au cores with MW heating for 2 min. (d-g) Growth mechanism of cubic Au@Ag crystals from the octahedral Au core.

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Figure 5. TEM EDS data of (a) the Au core, (b) the Ag shell, and (c) the cubic Au@Ag core-shell crystal. (d) Distributions of Au and Ag components along the cross section line shown in (c). The Au@Ag core-shell was prepared at an [AgNO3]/[HAuCl4] molar ratio of 11.

ure 2i and Figure S1b (Supporting Information)). Ag shells having 2 {111} facets on the top and bottom and a twin pair of total of 12 alternative {100} and {111} side facets are overgrown on these TTP-like Au cores, as shown in Figure 2j. PVPs are selectively adsorbed on the six side twin {100} facets and enhance the crystal growth of {100} facets. Therefore, triangular-bipyramidal structures, surrounded solely by the six equivalent {100} facets, are finally grown (Figure 2k). It should be noted that after the formation of triangular-bipyramidal structures, crystal growth continues to occur from Figure 2k to Figure 2l, leading to the same larger structures. This implies that the reduction of Ag+ over the {100} facets can take place after formation of triangular-bipyramidal structures under our conditions.

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Cubic Au@Ag Core-Shell Nanocrystals. Figure 4 shows TEM photographs of Au core and Au@Ag nanocrystals overgrown on octahedral Au cores at [AgNO3]/[HAuCl4] molar ratios of 9 and 18. At low [AgNO3]/[HAuCl4] molar ratios of 5-9, two types of square Au@Ag crystals are observed from the top side view, as shown in Figure 4b. When the incident angles of the electron beam are changed within (25°, one TEM image changed to another one (see Figure S3 (Supporting Information)). This shows that the two TEM images originate from the same crystal structures with different view angles. We found that square Au core and square Ag shell cubic crystals are observed from the view angle in Figure S3a, while rhombic Au core and rectangular Ag shell crystals are observed from the inclined view angle in Figure S3c. SAED patterns of cubic Au@Ag crystals gave typical {100} patterns of FCC crystals (see Figure S3e). TEM EDS data are shown in Figure 5, where a cubic Au core and Ag shell are clearly observed. Distributions of Au and Ag components are consistent with octahedral Au core and cubic Ag shell structures, shown in Figure 4f. At higher [AgNO3]/[HAuCl4] molar ratios of 11-30, further larger cubic crystals involving octahedral Au cores in the center were overgrown, as shown in Figure 4c. The edge lengths of cubic crystals increased with increasing [AgNO3]/[HAuCl4] molar ratio, and they were about 5 times larger than those of Au cores (about 40 nm) at the highest [AgNO3]/[HAuCl4] molar ratio of 30. Parts d-g of Figure 4 show the growth mechanism of cubic Au@Ag core-shell nanocrystals. The octahedral FCC crystals consist of eight stable {111} facets. Crystal growth occurs in the eight 〈111〉 direction. Since PVPs are adsorbed selectively on {100} facets, the area of the {111} facets decreases, while that of {100} facets increases, and finally cubic crystals surrounded by six {100} facets are overgrown. After a full cover of cubic Ag shells on Au cores, the crystal growth takes place continuously, resulting in the same cubic structures having larger edge lengths (Figure 4f f Figure 4g). This implies that the reduction of Ag+ over the {100} facets can occur after formation of cubic Au@Ag nanocrystals. Although it became difficult to observe the Au core when the size of cubic Au@Ag crystals was increased, it can be seen by

Figure 6. TEM photographs of (a) the decahedral Au core, (b) shapes of the decahedral Au core, and (c-f) Au@Ag nanorods and nanowires prepared by addition of various amounts of AgNO3 to Au cores with MW heating for 2 min.

Crystal Growth of Au@Ag Core-Shell Nanoparticles

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Figure 8. Growth mechanism of Au@Ag nanorods and nanowires from the decahedral Au core. Figure 7. TEM EDS data of (a) the Au core, (b) the Ag shell, and (c) the Au@Ag rod. (d) Distributions of Au and Ag components along the cross section line shown in (c). The Au@Ag rod was prepared at an [AgNO3]/[HAuCl4] molar ratio of 9.

etching the Ag shell using AuCl4- ions (see Figure S2b (Supporting Information)). Au@Ag Nanorods and Nanowires. Figure 6 shows TEM photographs of Au core and Au@Ag nanocrystals overgrown on an MTD Au core in a [AgNO3]/[HAuCl4] molar ratio range of 5-30. When the [AgNO3]/[HAuCl4] molar ratios are increased from 5 to 30, the aspect ratios of rods and wires increase from 1.5 to 62. It should be noted that a very small MTD core can clearly be seen just in the center of a long wire (Figure 6f). TEM SAED patterns of rods and wires were similar to those of Ag rods and wires,32,33 indicating that 1-D Au@Ag nanorods and nanowires have pentagonal cross sections. TEM EDS data are given in Figure 7. It is clear from distributions of Au and Ag components that Ag shells are overgrown over {111} facets of Au MTD crystals and that no Au/Ag alloys are formed in the nanorod. These data led us to two important findings: that the crystal growth occurs selectively in two 〈110〉 directions at the same growth rates and little crystal growth occurs in the short axis direction. Figure 8 shows the growth mechanism of Au@Ag nanorods and nanowires. The MTD nanocrystals consist of 10 {111} facets. The crystal growth occurs in two opposite 〈110〉 directions with the same growth rates. Since PVPs adsorb selectively on the five rectangular side {100} facets, crystal growth occurs on the less covered active {111} facets. Thus, Ag shells are continuously overgrown on MTD particles because active {111} facets with a constant total area remain during the crystal growth. Xia et al.32 and Tsuji et al.33 have already studied crystal structures and the growth mechanism of Ag nanorods and wires in the presence of PVP on the basis of TEM SAED pattern analysis. Both groups predicted that they are grown from MTD seeds due to selective adsorption of PVP on side {100} facets. We have found here that Ag rods and wires are overgrown on the MTD Au crystals located in the center of the 1-D particles. This observation provides the first definite experimental evidence that the previously proposed growth mechanism of 1-D Ag nanorods and naowires is valid. Growth Rate of Each Au@Ag Core-Shell Structure. Parts a and b of Figure 9 show the dependence of the edge length of

the Ag shell and the total space volume of Ag shells on the [AgNO3]/[HAuCl4] molar ratio, respectively. The edge length and space volume were estimated by averaging measured values of several particles. The edge length of the Ag shell increases in the following order: rods and wires . triangular-bipyramidal crystals > cubes. The lengths of 1-D rods and wires increase more rapidly than those of other two 3-D structures. On the other hand, the total space volume of the Ag shell increases in the opposite order: cubes > triangular-bipyramidal crystals > rods and wires. This indicates that larger amounts of Ag+ ions are reduced on TTP-like and octahedral Au cores. During the crystal growth, the active surface area of 10 {111} facets is constant for 1-D rods and wires, while those of TTP-like and octahedral crystals increase due to the formation of 3-D Ag shell structures. Therefore, total space volumes of triangular-bipyramidal and cubic nanocrystals are larger than those of rods and wires. UV, Visible, and Near-Infrared Spectra. Figure 10 shows UV, visible, and near-infrared absorption spectra obtained in the [AgNO3]/[HAuCl4] molar ratio range of 0-30. The absorption spectra of Au@Ag crystals are broader than those of monometallic Au and Ag particles.15,16 There are some structures in broad bands. Among them, peaks around 400 nm correspond to the surface plasmon-band component of Ag shells and those around 550 nm fit that of Au cores. It should be noted that when the [AgNO3]/[HAuCl4] ratio is increased, not only does the weaker Ag plasmon-band component become strong but also the stronger Au plasmon-band component is enhanced and gives a strong tail band above 1000 nm, even though the Au core volume is constant. This implies that the strong effects of Au cores remain, even though their concentration relative to that of Ag is 30 times smaller. UV and visible absorption spectra of spherical Au@Ag core-shell nanoparticles and spherical Au/ Ag nanoparticles have been measured by Xu et al.26 and Hodak et al.,10 respectively. For both particles, strong peaks due to the Au component appear in the 510-520 nm region and plasmon bands decrease rapidly above them and disappear above about the 700 nm region. In this study, we found strong absorption above the 700 nm region. Thus, absorption spectra of mixtures of higher crystalline Au@Ag nanocrystals are significantly different from those of spherical Au@Ag core-shell and Au/ Ag alloy nanoparticles. This indicates that the optical properties of Au@Ag core-shell particles strongly depend on their crystal structures.

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Figure 9. Dependence of (a) the edge length of Ag shells and (b) the total space volume of Ag shells on the [AgNO3]/[HAuCl4] molar ratio at a constant HAuCl4 concentration of 2.4 mM.

discussions. This work was partially supported by JST-CREST, a Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (No. 15651046), and the Joint Project of Chemical Synthesis Core Research Institutions. Supporting Information Available: Figures giving additional views of the crystals obtained in this study. This material is available free of charge via the Internet at http://pubs.acs.org.

References

Figure 10. UV, visible, and near-infrared absorption spectra of reagent, pure Au nanocrystals, and Au@Ag core-shell nanocrystals obtained at various [AgNO3]/[HAuCl4] ratios. Product solutions were diluted in EG by a factor of 100 before measurements.

Conclusion Au@Ag core-shell single crystals have been prepared by the MW-polyol method. The crystal structures and their growth mechanisms have been clarified from detailed crystal analyses using TEM. In the syntheses of FCC metallic nanocrystals such as Au and Ag in solution in the presence of polymer capping agents, triangular, truncated triangular, and cubic single nanocrystals and 1-D nanorods and nanowires have often been obtained.13-15,23-25 Some of them probably grow via mechanisms similar to those shown in this paper. We think that studies on core-shell nanocrystals give fundamental information not only on the preparation of component-, shape-, and sizecontrolled synthesis of composite nanocrystals but also on the clarification of growth mechanisms of single crystals in the presence of a polymer capping agent. The morphology changes between Au cores and Ag shells were attributed to the different adsorption sites of PVP on Au and Ag single crystals. Au@Ag nanocrystals gave absorption spectra in the 300-1400 nm region, which were broader than those of spherical Au@Ag core-shell and Au/Ag alloy nanoparticles. To clarify why such dramatic changes occur, further detailed experimental and theoretical studies will be required. Acknowledgment. We acknowledge Profs. I. Mochida, S. Yoon, and Y. Tomokiyo of Kyushu University for their valuable

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