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Synthesis and Growth Mechanism of Pentagonal Bipyramid-Shaped Gold-Rich Au/Ag Alloy Nanoparticles Xu Zhang,†,| Masaharu Tsuji,*,†,‡ Seongyop Lim,§ Nobuhiro Miyamae,‡ Michiko Nishio,‡ Sachie Hikino,† and Mitsutaka Umezu† 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 December 18, 2006. In Final Form: March 27, 2007 Pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles are synthesized in ethylene glycol (EG) in the presence of small amounts of AgNO3 and PVP without using Au seeds. The contents of Au and Ag in pentagonal nanobipyramids are determined by energy-dispersive X-ray spectroscopy (EDS). The EDS data demonstrates that this kind of nanoparticles is composed of Au/Ag alloys, not silver monolayers simply covering the surface of Au nanoparticles. Insights into the growth mechanism of pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles are discussed.
Introduction Recently, gold nanoparticles have been extensively studied as an active component not only because of their unique physical and chemical properties but also because of their important applications in catalysis, photoelectronic devices, and biomedicine.1 It is well known that some chemical and physical properties and applications of materials are dependent, to a large extent, on their shapes and textures. Thus, much effort has been made to find effective synthetic methods to prepare gold nanoparticles with different shapes. Surfactant-assisted synthesis approaches in the solution phase are currently popular in the preparation of gold nanostructures with various morphologies including nanorods,2,3 nanosheets,4 nanocubes,5 nanobelts,6 and branched nanocrystals7 for the purpose of finding new properties or applications for porous alumina templates.8 In these reports, it seems that cetyltrimethylammonium bromide (CTAB) is a unique surfactant in the preparation of rodlike gold nanoparticles. Some groups have also tried to find another effective surfactant to control the synthesis of 1D or quasi-1D gold nanostructures. Therefore, poly(vinyl pyrrolidone) (PVP), an effective morphology-directing agent in the synthesis of Ag nanowires,9 has been * 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. (1) Daniel, M. C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Kim, F.; Song, J. H.; Yang, P. D. J. Am. Chem. Soc. 2002, 124, 14316. (3) Murphy, C. J.; Tapan, K. S.; Gole, A. M.; Orendorff, C. J.; Gao, J. X.; Gou, L. F.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857 and references therein. (4) Li, C. C.; Cai, W. P.; Cao, B. Q.; Sun, F. Q.; Li, Y.; Kan, C. X.; Zhang, L. D. AdV. Funct. Mater. 2006, 16, 83. (5) (a) Sau, T. K.; Murphy, C. J. J. Am. Chem. Soc. 2004, 126, 8648. (b) Tsuji, M.; Hashimoto, M.; Nishizawa, Y.; Tsuji, T. Chem. Lett. 2003, 32, 1114. (c) Shankar, S. S.; Rai, A.; Ankamwar, B.; Singh, A.; Ahmad, A.; Sastry, M. Nat. Mater. 2004, 3, 482. (6) Zhang, J. L.; Du, J. M.; Han, B. X.; Liu, Z. M.; Jiang, T.; Zhang, Z. F. Angew. Chem., Int. Ed. 2006, 45, 1116. (7) (a) Chen, S. H.; Wang, Z. L.; Ballato, J.; Foulger, S. H.; Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (b) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp, J. T.; Li, S. Y. Nano Lett. 2004, 4, 327. (8) Van der Zande, B. M. I.; Bohmer, M. R.; Fokkink, L. G. J.; Schonenberger, C. J. Phys. Chem. B 1997, 101, 852.
used to control the morphology of gold nanoparticles. So far, gold polyhedrons10 and plates11 have been synthesized, including decahedrons, dodecahedrons, triangles, hexagons, and so forth. Pentagonal bipyramid-shaped gold nanoparticles (abbreviated as pentagonal nanobipyramids in this text), a new quasi-1D nanostructure, have also been synthesized by using a seedmediated approach in the presence of CTAB.12,13 It is found that a small amount of silver nitrate is necessary for the formation of gold pentagonal nanobipyramids in this kind of method besides the determinant role of gold seeds. However, the exact role of Ag in the growth of gold pentagonal nanobipyramids has not been clarified thoroughly. In addition, whether Ag makes coreshell particles inside or outside or alloy particles are produced in the final products has not been measured. The determination of the final location of silver is very important to the understanding of the role of silver. In this article, we have studied the growth of pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles in the presence of PVP and silver nitrate. Energy-dispersive X-ray spectroscopy (EDS) analyses are carried out to obtain detailed information on the exact composition of the pentagonal nanobipyramids. Such data lead us to conclude that the pentagonal nanobipyramids are a gold-rich Au/Ag alloy rather than a silver monolayer simply covering the surface of the gold. We found that both PVP and silver nitrate are prerequisites in the formation of pentagonal nanobipyramids. In particular, the silver not only benefits the stable existence of the steps but also dopes the Au solid to form an alloy through underpotential deposition (UPD) accompanying a great deal of gold deposition. This is the first report of pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles being prepared by using PVP as a surfactant. It is expected that pentagonal nanobipyramids with unique geometry may provide larger near-field enhancement at the plasomon resonance as a result of their sharp apex.12 (9) Sun, Y. G.; Xia, Y. N. AdV. Mater. 2002, 14, 833. (10) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. D. Angew. Chem., Int. Ed. 2004, 43, 3673. (11) Ah, C. S.; Yun, Y. J.; Park, H. J.; Kim, W. J.; Ha, D. H.; Yun, W. S. Chem. Mater. 2005, 17, 5558. (12) Liu, M. Z.; Guyot-Sionnest, P. J. Phys. Chem. B 2005, 109, 22192. (13) (a) Jana, N. R.; Gearheart, L.; Murphy, C. J. AdV. Mater. 2001, 13, 1389. (b) Jana, N. R.; Gearheart, L.; Obare, S. O.; Murphy, C. J. Langmuir 2002, 18, 922.
10.1021/la063662w CCC: $37.00 © 2007 American Chemical Society Published on Web 05/01/2007
Synthesis, Growth of Au/Ag Alloy Nanobipyramids
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Figure 2. Corresponding UV-vis spectrum of the product shown in Figure 1a.
Figure 1. (a) Typical TEM image of the product prepared using 0.5 mM HAuCl4, 0.1 M PVP, 1.65 × 10-5 M AgNO3, and 0.8 mM AA. (b) High-magnification TEM image of a pentagonal nanobipyramid.
Experimental Section In a typical procedure, a volume of 20 mL of ethylene glycol (EG) containing HAuCl4‚4H2O (5 × 10-4 mol‚L-1), PVP (0.1 mol‚L-1), AgNO3 (1.65 × 10-5 mol‚L-1), and L-ascorbic acid (AA) (8 × 10-4 mol‚L-1). The molar ratio of ascorbic acid to [AuCl4-]was kept at 1.6:1, and all of the original reactant solutions were added to a test tube in this order. Then the resulting solution was shaken vigorously for 2 min until a homogeneous solution was obtained. It was found that the yellowish solution gradually became colorless during the shaking. Finally, the solution was left undisturbed for 24 h at room temperature, and the final color was bluish purple. The precipitate was collected by centrifugal separation and dispersed in deionized water for characterization. Product particles were characterized by using transmission electron microscopy (TEM; JEOL2010) with an accelerating voltage of 200 kV. High-resolution TEM images and their EDS analyses were obtained using FE-TEM (JEOLJEM 2100F) with an accelerating voltage of 200 kV. Electronic absorption spectra were acquired with a Shimadzu UV-3600 UVvis-near-IR spectrophotometer.
Results and Discussion
Figure 3. TEM images of the as-prepared products at various AgNO3 concentrations: (a) 0, (b) 6.6 × 10-6, (c) 3.3 × 10-5, and (d) 1.65 × 10-4 mol‚L-1.
Figure 1a displays a typical TEM image of the as-obtained products. It can be seen that some pentagonal bipyramid-shaped nanoparticles are produced in addition to spherelike nanoparticles. A single pentagonal nanobipyramid is shown in Figure 1b. The yield of the pentagonal nanobipyramids is about 15% under this condition. A typical UV-vis absorption spectrum is given in Figure 2, where a typical absorption peak due to the surface plasmon resonance (SPR) band of spherical gold nanoparticles with a peak at 580 nm is observed in the 500-700 nm region.
No clear absorption peak of the pentagonal nanobipyramids is observed because of its low yield. The effects of AgNO3, L-ascorbic acid, and PVP concentrations on the formation of pentagonal nanobipyramids are examined. Figure 3 shows TEM images of products obtained in the AgNO3 concentration range of 0-1.65 × 10-4 mol‚L-1 by keeping the other conditions constant. In the absence of Ag+, besides small amounts of large, thin plates and nanorods, hexagonal and pentagonal particles are prepared as major products, and no
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Figure 4. UV-vis absorption spectra of the product solution at various AgNO3 concentrations: (1) 0, (2) 6.6 × 10-6, (3) 3.3 × 10-5, and (4) 1.65 × 10-4 mol‚L-1.
Figure 5. TEM images of the as-prepared products at various AA concentrations: (a) 5 × 10-4, (b) 6.5 × 10-4, (c) 1.0 × 10-3, and (d) 1.5 × 10-3 mol‚L-1.
pentagonal nanobipyramids are produced (Figure 3a). This indicates that the presence of a trace amount of AgNO3 is essential for producing the pentagonal nanobipyramids. At a low AgNO3 concentration of 6.6 × 10-6 mol‚L-1, only a few pentagonal nanobipyramids (∼5%) are obtained, and the rest of the particles are spherelike (Figure 3b). At an AgNO3 concentration of 3.3 × 10-5 mol‚L-1, pentagonal nanobipyramids (∼10% in yield) are produced (Figure 3c). When the AgNO3 concentration is increased from 3.3 × 10-5 to 1.65 × 10-4 mol‚L-1, the size of the product decreases, and the number of pentagonal nanobi-
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Figure 6. UV-vis absorption spectra of the product solution at various AA concentrations: (1) 5 × 10-4, (2) 6.5 × 10-4, (3) 1.0 × 10-3, and (4) 1.5 × 10-3 mol‚L-1.
pyramids decreases (Figure 3d). Dumbbell-like nanoparticles appear at an AgNO3 concentration of 1.65 × 10-4 mol‚L-1. On the basis of these facts, it can be concluded that the AgNO3 concentration had a determinant effect on the morphology of the product. Figure 4 shows the absorption spectra of the product prepared at various AgNO3 concentrations. It can be seen that a narrow SPR peak appears at about 550 nm without the addition of AgNO3, which suggests that monodispersed gold nanoparticles with a mean diameter of ∼40 ( 10 nm including hexagons and pentagons are produced in this case. Then the peaks become broad and shift to the red upon increasing the AgNO3 concentration from 6.6 × 10-6 to 3.3 × 10-5 mol‚L-1, indicating that nanoparticles with a wide size distribution from ∼40 to ∼90 nm in length are prepared. At a high AgNO3 concentration of 1.65 × 10-4 mol‚L-1, only a relatively narrow peak appears at about 550 nm, indicating that the size distribution (∼35 ( 10 nm in length) becomes narrow again. The concentration of L-ascorbic acid (AA) in the reaction solution is another important parameter in the kinetically controlled process and can affect the shape of the products. Figure 5 shows the TEM images of the products obtained at various AA concentrations at room temperature while keeping the rest of the parameters the same. When the concentration of AA is reduced to 5 × 10-4 mol‚L-1 (the molar ratio of Au to AA is 1:1), the reaction is very slow, and few pentagonal nanobipyramids are produced, even after about one week (Figure 5a). When the concentration of AA is 6.5 × 10-4 mol‚L-1 (the ratio of Au to AA is 1:1.3), then the morphology of product undergoes little change except for the increase in the number of pentagonal nanobipyramids (Figure 5b). However, when the concentration of AA is increased to 1.0 × 10-3 mol‚L-1 (the ratio of Au to AA is 1:2), the morphology changes greatly. The products consist of some irregular platelike nanocrystals and asymmetric pentagonal nanobipyramids with thicker centers and longer, thinner ends (Figure 5c). These shape changes may be attributed to the appearance of different step lengths derived from different UPDs along the growth direction under the kinetic effect. The morphology does not change with further increases in AA concentration to 1.5 × 10-3 mol‚L-1 (Figure 5d), which suggests that 1.0 × 10-3 mol‚L-1 is the limit of the effect of AA concentration on the morphology. Figure 6 shows the UV-vis
Synthesis, Growth of Au/Ag Alloy Nanobipyramids
Figure 7. (a) TEM image of a pentagonal nanobipyramid. (b) TEM image of part a at an angle of rotation of -23.3°. (c) Electron diffraction pattern recorded for the entire pentagonal nanobipyramid in part b. (d) High-resolution TEM image taken in the given region of the pentagonal nanobipyramid in part b.
spectra of the products obtained at various AA concentrations. It can be seen that the SPR bands become strong and broad and that the peaks shift from 550 to 700 nm when the ratio of Au to AA is beyond 1:2, corresponding to the fact that the length of the nanoparticles changes from 30-70 nm (Figure 3a,b) to 50-250 nm (Figure 3c,d). These spectral changes are consistent with changes in the shapes and sizes of products in the TEM images. It was found that the concentration of HAuCl4 has almost no effect on the morphology of the product except for the decrease in the aspect ratio with increasing concentration of HAuCl4 (Figure S1, Supporting information) when the other conditions remain the same. The corresponding UV-vis absorption spectra are shown in Figure S2 (Supporting information). The intensity of the peak at about 550 nm becomes strong without broadening with increasing HAuCl4 concentration. The change in PVP concentration has little effect on the morphology of the product by keeping the other conditions constant in the presence of PVP and silver nitrate. Some pentagonal nanobipyramids change only their shape to be longer and thinner by changing the PVP concentration, as shown in Figure S3 (Supporting information). However, the morphology is totally different, and only some irregular aggregated particles are obtained without the addition of PVP even though silver nitrate is present, as shown in Figure S4 (Supporting information). This suggests that PVP plays an indispensable role in the formation of pentagonal nanobipyramids. The corresponding UV-vis spectra are shown in Figure S5. It can be seen that the peak position of the SPR band hardly shifts except for the intensity changing with the variation of PVP concentration when PVP is present. However, the spectrum is completely different when PVP is absent. The HRTEM images taken from different angles are shown in Figure 7a,b. There is a clear twin boundary along the growth axis of the pentagonal nanobipyramid, indicating that the structure is not single-crystalline. The structure is twinned along the long axis. Figure 7c gives the SAED pattern of the selected pentagonal
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Figure 8. TEM EDS data of (a) the Au shape, (b) the Ag shape, and (c) the Au/Ag alloy pentagonal nanobipyramid. (d) Distributions of Au and Ag components along the cross-section line shown in part c. The Au/Ag alloy pentagonal nanobipyramid was prepared at 1.65 × 10-5 mol‚L-1 AgNO3.
nanobipyramid with an angle of rotation of -23.3°. A highresolution image of the pentagonal nanobipyramid in its equatorial part is shown in Figure 7d, showing clear fringes parallel to the growth axis along both sides. The spacing is given as 0.232 nm by a direct measurement, by which the fringes are indexed as (111) planes. HRTEM research suggests that the growth is along the [110] direction. To study the exact composition of the pentagonal nanobipyramid further, TEM EDS is employed. Figure 8a-c shows TEM EDS data of Au, Ag, and the Au/Ag alloy of a pentagonal nanobipyramid, respectively. The distributions of Au and Ag components along the long axis shown in Figure 8a,b are shown in Figure 8d. The distribution curve of silver is similar to the shape of the pentagonal nanobipyramid, and the Au/Ag atomic ratios were similar in all positions, as measured for various parts. We have recently prepared Au@Ag core-shell particles through two steps by using a microwave-polyol method.14 When Au@Ag core-shell particles having thin Ag layers are produced, Au cores and Ag shells appear as black, bright contrasts, respectively, in the TEM images shown in Figure S6 (Supporting information). Such a bright Ag layer is not observed in the TEM and HRTEM images of pentagonal nanobipyramids. These results led us to conclude that pentagonal nanobipyramids are composed of Au/ Ag alloys. The average Au/Ag atomic ratios of pentagonal nanobipyramids are (97.8 ( 0.3)/(2.2 ( 0.3)%. Because the initial Au/Ag atomic ratios in reagents are 96.8/3.2%, smaller amounts of Ag are involved in the Au/Ag pentagonal nanobipyramid alloys. The above experimental results clearly suggest that silver nitrate plays a crucial role in the formation of pentagonal bipyramidshaped nanoparticles. In the seed-mediated growth approach with CTAB,12,13 a small amount of Ag+ was needed, although its exact role has not been clarified. However, it is found that silverassisted growth is significantly slower, and the slower growth rate in the presence of Ag may allow the gold atoms to be deposited at the most energy favorable place and create no defect at all.12 In contrast to the nanorods with an isometric pentafold twinned around their growth axis prepared without the addition of AgNO3, (14) Tsuji, M.; Miyamae, N.; Lim, S.; Kimura, K.; Zhang, X.; Hikino, S.; Mishio, M. Cryst. Growth Des. 2006, 6, 1801.
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it is observed that the {100} facets of the pentagonal nanobipyramids prepared with silver nitrate have to be stepped periodically along the growth direction to fit the geometry. This suggests that silver nitrate has an effect on the deposition behavior of gold atoms under the selected reaction condition. In fact, as the Au nanostructure grows along the twinning axis, steps will also appear on the Au {100} side walls along the growth direction.12 However, these steps are not thermodynamically stable and finally disappear from the nanorods in the absence of silver nitrate so that isometric pentafold twinned nanorods are formed. On the contrary, if silver nitrate is added to the reaction solution, then the silver species generated from AgNO3 may be selectively bound to {100} and will restrict the surface growth.15 Thus, the steps are stabilized, leading to the formation of pentagonal bipyramid-shaped nanoparticles. With regard to the detailed mechanism in which silver nitrate affects the morphology of the product, the recent report on silver UPD on growing gold nanoparticles does provide some useful insights into the role of silver.12 Silver UPD on gold surfaces is the reduction of Ag+ to Ag0 at a gold substrate with a surface potential that is less than the standard reduction potential of Ag+. In this article, we propose that the different morphology of the product in the presence of silver(I) can be ascribed to the effect of silver UPD (i.e., silver deposition occurs concomitantly with gold deposition) from the beginning of nucleation to the end of the growth of the pentagonal nanobipyramids, forming the Au/Ag alloy. Lorenz et al. have reported that silver is preferentially grown on Au(100) but not on Au(111) at the intermediate deposition rates in the UPD.16 Moreover, the abovementioned steps provide open sites with probably a larger UPD shift for silver deposition, making the reduction of Ag+ to Ag0 much easier. After silver deposition, the gold atoms have to change their method of packing because of the incomplete match of the two metals’ lattices so that the stepped side walls are retained. With the formation of silver and gold depositions, the side {100} facets tilt gradually toward the {111} facets along the growth axis, and finally the pentagonal nanobipyramids are obtained. Liu et al. thought that when the tip angle is between 26 and 30° the average step length should be ∼3.5 atoms.12 It is obvious that the change in the step can lead to the variation of the tip angle. Therefore, it is easy to understand the reason that the tip angles are different under various reaction conditions in the presence of silver(I). Because the silver underpotential will change with changing detailed reaction conditions so that the amounts of silver deposited are different in various reactions, the lengths of the steps are different as well. Analogously, (15) Seo, D.; Park, J. C.; Song, H. J. Am. Chem. Soc. 2006, 128, 14863. (16) Garcia, S.; Salinas, D.; Mayer, C.; Schmidt, E.; Staikov, G.; Lorenz, W. J. Electrochim. Acta 1998, 43, 3007.
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nanoparticles with other morphologies are also transformed as a result of the silver UPD. Therefore, it is reasonable to think that the as-obtained products are gold-rich Au/Ag alloys. In addition, PVP and EG also play essential roles in the formation of pentagonal nanobipyramids. Pentafold twinned nuclei will be formed in the presence of PVP and will grow into pentagonal nanobipyramids assisted by silver(I). No twinned nuclei are formed without PVP, and nanopyramids do not appear; that is, the final nanostructures are strongly dependent on the nuclei structure, which has also been demonstrated by our experimental results. In this study, EG acts as a solvent rather than as a reducing agent because the reaction is carried out at room temperature. Generally speaking, EG will lose its reducing power at room temperature and will show its reducing property at relatively high temperature.17 By comparison with the case in which distilled water is a solvent, it is reasonable to think that the viscidity of EG plays a key role in the formation of nanopyramids because the morphology is completely different under these two different solvents.18
Conclusions Gold-rich Au/Ag alloy pentagonal nanobipyramids have been prepared by reducing HAuCl4‚4H2O with L-ascorbic acid in EG in the presence of small amounts of AgNO3 and PVP. The crystal structures and growth mechanism of pentagonal nanobipyramids are examined by using HRTEM and EDS analysis. TEM EDS data demonstrate that pentagonal nanobipyramids consist of Au/ Ag alloy pentagonal nanobipyramids because similar distributions of Au and Ag are observed throughout the pentagonal nanobipyramids. It is found that the addition of a trace amount of silver nitrate can trigger different arrangements of gold atoms so that pentagonal bipyramid-shaped gold-rich Au/Ag alloy nanoparticles are formed. TEM images show that the structure is twinned along the long axis. HRTEM research suggests that growth is along the [110] direction. Acknowledgment. This work was partially supported by JSTCREST, a Grant-in-aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology (nos. 19033003 and 19310064), and the Joint Project of Chemical Synthesis Core Research Institutions. Supporting Information Available: Additional views of the crystals obtained in this study. This material is available free of charge via the Internet at http://pubs.acs.org. LA063662W (17) (a) Wiley, B.; Sun, Y.; Mayers, B.; Xia, Y. Chem.sEur. J. 2005, 11, 454. (b) Sun, Y.; Xia, Y. Science 2002, 298, 2176. (c) Jiang, P.; Zhou, J. J.; Li, R.; Wang, Z. L.; Xie, S. S. Nanotechnology 2006, 17, 3533. (18) Zhang, X.; Tsuji, M.; Lim, S.; Miyamae, N.; Nishio, M.; Hikino, S. To be submitted for publication.
