NANO LETTERS
Surface Atomic Defect Structures and Growth of Gold Nanorods
2002 Vol. 2, No. 7 771-774
Pratibha L. Gai*,† and Mark A. Harmer DuPont, Central Research and DeVelopment, Experimental Station, Wilmington Delaware 19880-0356 Received April 24, 2002
ABSTRACT We present atomic and defect structural studies of technologically important gold nanorods synthesized by a wet chemical process. Our surface atomic structural studies and electron nanodiffraction from single nanorods have provided the first direct evidence of the stabilization of the highly unstable (110) surface by surfactant molecules. The insights into surface defect structures have important implications in the applications of nanorods in the molecular device technology. The origin and the development of the growth process of nanorods have been inferred from the results.
Nanorods with high aspect ratios have generated considerable interest globally due to their potential applications in the next generation of nano- and molecular electronics.1 The synthesis of controlled sizes and shapes is critical to these applications since shapes influence electrical properties. Recently, rods of metals and metallic compounds such as metal oxides, metal sulfides (CdS), and selenides have received considerable attention.2-15 Metal rods have been synthesized by various methods including electrochemical,5 inorganic and organic templating,9,11 and a range of wet chemical methods which may involve surfactant templating.2,3 However, mechanisms of the gold surface formation and the growth of nanorods, which are critical to their applications, are not well understood. In gold nanorods grown by electrochemical methods, (110) surface reconstruction has been reported5 and it is suggested that the reconstruction is due to possible defect sites where capping micelle molecules are missing and that the stabilization of the (110) is due to its possible confinement by the molecules. However, no experimental evidence was presented to support the suggestions. Elegant studies of the synthesis of gold nanorods via a bioreduction process have also been reported.16,17 The process shows surface relaxation, faceting/twinning defects, and surface reconstruction in the nanorods which have no capping molecules. The nature and dimensions of the defects are different. Combining the surface relaxation, reconstruction and faceting, a model for the nanorod structure based on a decahedral structure is proposed. It is suggested that the defects relieve stresses introduced during the nanorod synthesis. However, the role of the defect structures in the nanorod growth and in the * Corresponding author. E-mail:
[email protected] † Also at the University of Delaware, Department of Materials Science and Engineering, Newark, DE. 10.1021/nl0202556 CCC: $22.00 Published on Web 06/12/2002
© 2002 American Chemical Society
stabilization of (110) in the absence of capping molecules was not presented. Here we report atomic and defect structures and the growth mechanism of gold nanorods synthesized by wet chemical methods, using atomic resolution transmission electron microscopy and electron nanodiffraction. Based on the results, we propose a growth mechanism for the nanorods. We prepared Au nanorods by wet chemical reduction of Au salts. We used a seeded growth method to make gold nanowires with high aspect ratios, following the procedures recently described by Jana et al.2 This work is an excellent example of the high quality of rods that is attainable using a wet chemical process. A wet chemical method was preferred because it is economical and simple. The gold rods were prepared via the reduction of HAuCl4 in a surfactantbased solution (cetyltrimethylammonium bromide, CTAB). In this method a gold seed particle is made via the sodium borohydride reduction of HAuCl4, which are approximately 4 nm in size. A stepwise growth method is then used where the seed is added to a 0.1 M solution of CTAB and HAuCl4, using in this case ascorbic acid as the reducing agent. The conditions used in our experiments were the same as described in ref 2. The product was finally centrifuged at 7500 rpm and the rods washed with water to remove the bulk of the surfactant. In these preparations the surfactant is believed to enhance the rod formation, although, as has been noted in several publications, rods can form in small amounts even in the absence of CTAB. We found, for example, that the slow addition of 2.7 mM NaBH4 added to 1 mM HAuCl4 resulted in nanosized gold particles (∼ 5-10 nm) with a small population of rods present, although probably an order of magnitude lower than that found in the presence of the surfactant.
Figure 1. Low-voltage FESEM of gold nanorods by a wet chemical method.
Figure 3. (a) Mixture of nanorods and multiply twinned particles (MTP), e.g., at A. MTP were observed prior to the rod formation and all the particles were twinned. (b) Inset showing HRTEM image of a nano-MTP observed prior to rods.
Figure 2. (a) Single high aspect ratio gold nanorod. (b) Electron nanodiffraction from the single rod along the [110] zone axis. (c) EDX analysis showing the formation of high purity Au nanorod. (Cu peaks are from the support grid).
We investigated atomic structures of the surfaces of the nanorods using a novel atomic resolution environmental transmission electron microscope (ETEM) capable of operating in gas/vacuum/liquid environments and at different temperatures18-21 attached with a scanning-TEM (STEM) system, and a novel field emission gun Tecnai (FE-(S)TEM),22 using electron accelerating voltage of 200 keV. Powerful FE STEM/TEM, the so-called 2-2-2 200 keV field emission Tecnai STEM/TEM, combines atomic resolution imaging with atomic level chemical and crystallographic analyses with 2 Å (0.2 nm) resolution in each of the TEM, 772
STEM, and chemical analyses modes (hence 2-2-2).22 Both the atomic resolution ETEM in controlled gas and liquid environments and the 2-2-2 FE(S)TEM are providing new opportunities in the atomic structural and chemical studies of nanomaterials.22-24 Samples of Au nanorods were supported on carbon filmed Cu grids for electron microscopy. In addition to atomic structures, we carried out complex nanodiffraction studies from single nanorods using nanoelectron probes in the FE(S)TEM. The (S)TEMs were fitted with X-ray spectrometers. For nanochemical composition, nanoprobe analyses from individual nanorods were recorded using energy-dispersive X-ray spectroscopy (EDX) in the electron microscopes. ETEM studies in liquid environments (wet-ETEM)24 and HRTEM were carried out to understand a range of growth effects such as the ionic concentration, CTAB concentration, and time and temperature on the surface structural formation of the nanorods. The details will be reported separately. In our preparations a large concentration of nanorods was observed. Figure 1 shows a large concentration of rods imaged in a low-voltage SEM (LVSEM). Figure 2a shows a high aspect ratio single rod. Electron nanodiffraction (inset in b) from the single rod shows it to be along the [110] crystallographic zone axis. Chemical composition of the single rod by EDX shown in Figure 2c illustrates the formation of high purity nanorods (Cu peak is from the support grid). Our studies have shown that concentrations in the starting solutions and of the reducing agent influence the shapes of the particles. Our detailed investigations have shown that multiply twinned particles (MTP) are observed prior to the rod formation. MTP are considered as 5 or 20 fcc tetrahedra joined by, for example, twin boundaries to yield decahedral or icodahedral particles.25-27 These tetrahedra do not form completely space-filling structures and require twin boundaries or elastic strains to do so. MTP have weaker bonding in twin boundary areas and can provide sites Nano Lett., Vol. 2, No. 7, 2002
Figure 5. Atomic structure image of the single nanorod showing (110) and (111) surfaces and growth along [100]. Defected area due to twins is visible.
Figure 4. (a) TEM low resolution image of an individual gold nanorod revealing twin lamellar structure on the (110) surface of the rod. The lamellar extent is arrowed. (b) HRTEM of part of the gold nanorod revealing the dimension of the twin defect lamellae on the nanorod (110) surface. The dimension of the twin lamellae is about 4 nm. The twins give rise to an uneven or ridged surface. (c) Power spectrum nanodiffraction from the region of twin lamellae, showing additional reflections due to the twins along the [100] direction.
for agglomeration.28 Figure 3a shows a mixture of MTPs and rods and the inset (b) shows HRTEM image of a nanoMTP. All the particles observed in our studies were MTP (e.g., at A). They were dominated by decahedral and icosahedral particle shapes. Particles were generally in [110] crystallographic zone axis orientations with (111) lattice planes parallel to the twin boundaries. Nano Lett., Vol. 2, No. 7, 2002
TEM images of a single Au nanorod at low and high resolution are shown in Figure 4a and b, respectively. The rod is along the [110] crystallographic zone axis. The images provide the direct evidence of twin defect structures on the (110) crystal surface (or face) (arrowed in a and b). The atomic structural detail in Figure 4b reveals that the defects are twin lamellae with the dimension of the lamellae of about 4 nm. The image also shows that the defects are along the [100] direction. Power spectrum electron nanodiffraction (which is equivalent to an optical nanodiffractogram) was obtained with Fourier transforming of the twinned area of the image in Figure 4b and is shown in Figure 4c. The electron nanodiffraction shows extra reflections along the [100] direction, confirming the presence and the orientation of the twin defects shown in Figure 4b. The dimension of the twin lamellae region of ∼4 nm on (110) surfaces of the nanorods is consistent with the diameter of the micelle of ∼4 nm. Our atomic scale results have thus provided the first direct evidence of the (110) surface stabilization of gold nanorods. Our observations also reveal that the (110) faces are not atomically flat but are uneven or ridged due to the presence of twin defect structures. This has important implications in the applications of nanorods in the molecular device technology. In the wet synthesis described above, all nanorods were observed to contain the twin defects. Atomic resolution image of a part of a single nanorod revealing the structural relationships of the various crystal faces is shown in Figure 5. The image demonstrates that the rods grow along [100] directions and the (110) faces are bounded by (111) surfaces. We believe that surface energy minimization is responsible for the nucleation and growth of the nanorods. The energy minimization with the reduction to a stable form of Au is the driving force for the formation of nanorods. Initially MTPs are formed that have weaker bonding in twinned areas 773
the first direct evidence of the stabilization of the highly unstable (110) surface of gold nanorods by surfactant molecules. The studies have also shown that the (110) faces are not atomically flat due to the presence of twin defects. This has important implications in the applications of nanorods in nanoelectronic and molecular electronics. We have proposed the growth mechanism of nanorods via the diffusion of Au at the weakly bonded defect sites of multiply twinned particles (MTP). Acknowledgment. We thank M. Barker and L. Hanna for technical assistance. References
Figure 6. Model for nanorod growth: (a) MTP. (b) Growth of nanorod along [100] via Au diffusion at twin sites. (c) Projection of the nanorods with (110), (111) faces and [100] direction, some of which are observed in Figure 5.
and provide sites for Au diffusion to those sites. This leads to the growth of the rods. Figure 6 shows our proposed model for the growth of nanorods. Figure 6a shows a MTP. Figure 6b shows the growth of the nanorod along the [100] direction via Au diffusion to twin sites. Projection of the nanorod exhibiting (110), (111) faces and [100] growth direction is shown in Figure 6c and is similar to some of those revealed in Figure 5. The formation of twin lamellae on the (110) surface and the surface stabilization of the elongated nanorods can be understood as follows. It is well known that Au (110) surfaces have high energy and therefore weaker atomic bonding. This presumably leads to stronger bonding with the micelle. Our observed dimension of twin lamellae structure of ∼4 nm is consistent with the micelle size and provides the direct evidence for the surface reaction with the micelle. The twin defects are generated to accommodate the small strains due to the small size difference between the micelle and the (001) gold atom layers on (110) surfaces, leading to the stabilization of the (110) surface. In summary, we have shown the formation of surface atomic and defect structures of high aspect ratio gold nanorods synthesized by a wet chemical method. The atomic scale studies of the dimensions of the twin defect structures on (110) surfaces and electron nanodiffraction have provided
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(1) Service, R. F. Science 2001, 294, 2442 and references therein. (2) Jana, N.; Gearhart, L.; Murphy, C. J. J. Phys. Chem. B 2001, 105, 4065. (3) Chen, M.; Lu, J.; Xiong, Y.; Zhang, S.; Quian Y.; Liu, X. J. Mater. Chem. 2002, 12, 748. (4) Huang, Y.; Duan, X.; Cui, Y.; Lieber, C. Nano Lett. 2002, 2, 101. (5) Wang, Z.; Gao, R.; El Sayed, M.; Nikoobakt, B. J. Phys. Chem. B 2000, 104, 5417. (6) Li, L.; Hu, J.; Yang, W.; Alivisatos, A. P. Nano Lett. 2001, 1, 349. (7) Chen, C.; Chao, C.; Lang, Z. Chem. Mater. 2000, 12, 1516. (8) Zhang, Z.; Dai, S.; Blom, D.; Shen, J. Chem. Mater. 2002, 14, 965. (9) Hernandez, B.; Chang, K.; Fisher E.; Dorhout, P. Chem. Mater. 2002, 14, 480. (10) Zhang, X.; Zhang, L. D.; Chen, W.; Meng, G.; Zheng, M.; Zhao, X. Chem. Mater. 2001, 13, 2511. (11) Zhang, X.; Zhang, L.; Lei, Y.; Zhao, X.; Mao, Y. J. Mater. Chem. 2001, 11, 1732. (12) Sun, Y.; Gates B.; Mayers, B.; Xia, Y. Nano Lett. 2002, 2, 165. (13) Tanori, J.; Pileni, M. P. Langmuir 1997, 13, 639. (14) Cordente, N.; Respaud, M.; Senocq, F.; Casanove, M; Amiens, C.; Chaudret, B. Nano Lett. 2001, 1, 565. (15) Abdelouas, A.; Gong, W.; Lutze, W.; Shelnutt J.; Franco, R.; Moura, I. Chem. Mater. 2000, 12, 1510. (16) Yacaman, M. J.; Ascencio, J. A.; Canizal, G. Surf. Sci. Lett. 2001, 486, L449. (17) Canizal, G.; Ascencio, J. A.; Gardea-Torresday, J.; Yacaman, M. J. J. Nanoparticle Res. 2001, 3, 475. (18) Gai, P. L.; Kourtakis, K. Science 1995, 267, 661. (19) Boyes, E. D.; Gai P. L. Ultramicroscopy 1997, 67, 219. (20) Gai, P. L. AdV. Mater. 1998, 10, 1259. (21) Haggin, J. C&E News 1995, 73(30), 39. (22) Boyes, E. D.; Ringnalda, J.; van der Stam, M. A. J.; Flievoet, T. F.; van Cappellen, E. Microscop. Microanal. 2001, 7, 232. (23) Cronin, S. B.; LinY.; Rabin, O.; Black, M; Dresselhaus, G.; Dresselhaus, M. S.; Gai, P. L. Microscop. Microanal. 2002, 8, 58. (24) Gai, P. L. Microscop. Microanal. 2002, 8, 21. (25) Ino, S. J. Phys. Soc. Jap. 1966, 21, 346. (26) Gillet, M. Surf. Sci. 1977, 67, 139. (27) Marks, L. D.; Smith, D. J. J. Cryst. Growth 1981, 5, 12. (28) Gai-Boyes, P. L. (Gai, P. L.) Catal. ReV. Sci. Eng. 1992, 34, 1.
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