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Investigation on shape variation of Au nanocrystals
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Current Applied Physics 8 (2008) 810–813 www.elsevier.com/locate/cap www.kps.or.kr
Investigation on shape variation of Au nanocrystals Sung Koo Kang a, Inhee Choi a, Jeongjin Lee a, Younghun Kim b, Jongheop Yi a
a,*
School of Chemical and Biological Engineering, Institute of Chemical Engineering, Seoul National University, Seoul 151-742, Republic of Korea b Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea Received 14 August 2006; received in revised form 20 December 2006; accepted 27 April 2007 Available online 1 October 2007
Abstract A variety of shapes, such as rod, tripod, /-shape and cube, of Au nanocrystals were synthesized by employing different reaction conditions. The nanocrystals and their shape variation were characterized by transmission electron microscopy and UV–vis spectrophotometry. The evolution of shape was accomplished by controlling the parameters used in their synthesis, the concentration of reducing agent and surface capping agent. The effect of synthetic parameters on shape was explored, to determine suitable conditions for producing each shape of nanocrystals. Nanocrystals with different shapes have different plasmon bands in the visible region of the spectrum, which is a valuable property for sensor applications. 2007 Elsevier B.V. All rights reserved. PACS: 61.46.Df; 61.46.Hk Keywords: Gold (Au); Nanoparticles; Nanorod; /-Shape; Shape control; Tripod
1. Introduction Controlling the size and shape of inorganic nanoparticles remains an important issue in the area of advanced material development, since their unique properties are size and shape dependent on a nanometer scale [1–5]. Rapid progress has been made in controlling the shape of Au nanoparticles in recent years, leading to, for example, the production of rods [6–12], triangular platelets [13] and multipods [14,15]. These new structures are of great interest in terms of their optical, electric and magnetic properties for technological applications by ‘bottom-up’ approaches in developing new nanodevices [16,17]. Metal multipods with branches are particularly desirable structures for applications in areas such as interconnections of nanocircuits because of their electrical conductivity. Although it has been reported that semiconductor materials, such as CdS
*
Corresponding author. Fax: +82 (02) 880 7438. E-mail address: [email protected] (J. Yi).
1567-1739/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.04.035
[18–20], CdSe [21,22], ZnO [23–25], and PbS [26], can be used to control multipod-shaped structures, only a few studies of multipod structures of metal have been reported. Carroll et al. synthesized Au multipod nanocrystals by the addition NaOH and silver plates to Au–CTAB mixtures [14]. Li et al. reported on the synthesis of branched Au nanocrystals in high yields [15]. In the findings reported in these papers, tiny triangular or rectangular plates, which are formed in the initial step, develop into tripod or tetrapod structures by anisotropic growth. In this paper we describe the synthesis of shape-controlled Au nanoparticles using a seed-mediated method that was originally reported by Murphy et al. [7]. This is a well known method for preparing rod type nanoparticles. However, we observed that tripod- and /-shaped nanoparticles can be produced using the same procedure, by controlling the concentrations of reducing agent and surface capping agent used. Even though the mechanism of growth is yet to be elucidated, it is likely that the evolution of the shape is controlled by the kinetics associated with the reduction of the Au ion precursors.
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
2. Experimental details The Au seeds used in this work were prepared as follows; a solution 0.6 ml of 0.1 M NaBH4 at 0 C was added to 20 mL of a solution that was 2.5 · 10 4 M in HAuCl4 and tri-sodium citrate with vigorous stirring. The color of the solution instantly turned to brown. For the growth solution, four tubes, each of which contained 10 mL of an aqueous solution of 2.5 · 10 4 M HAuCl4 and 8.23 · 10 3 M cetyltrimethylammonium bromide (CTAB) were prepared. These solutions were held at 0 C for all of the syntheses. Different amounts of a 0.1 M ascorbic acid solution (AA) were added to each tube. The amounts added were 0.1, 0.2, 0.3 and 0.4 ml, respectively. After the
811
color of the solution changed from yellow to white, 0.025 ml of the seed solution was added to each tube. The same procedure was repeated with respect to CTAB concentration, i.e., 2.74 · 10 3 and 1.37 · 10 3 M. 3. Results and discussion The evolution of the shape of Au nanoparticles was tracked by means of TEM. The results show that the shape of the Au nanoparticles is dependent on the concentration of the reducing agent, ascorbic acid (AA), and the surfactant, cetyltrimethylammonium bromide (CTAB) that are used. These two parameters were found to be interrelated; different combinations resulted in various shapes. Fig. 1
Fig. 1. TEM images of Au nanoparticles synthesized using different reaction conditions, as a function of AA and CTAB concentration. (AA) and (CTAB) were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and 8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.
812
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
shows TEM images of Au nanoparticles obtained when different amounts of AA and CTAB were added to the solutions. Fig. 1a–d clearly shows a variation in shape as the addition of AA is gradually increased at a constant CTAB concentration (8.23 · 10 3 M). Particles synthesized using 1 mM AA were spherical and rod-shaped (Fig. 1a). When the concentration of AA in the solution was gradually increased, a transition from rod-shaped particles with a lower aspect ratio (Fig. 1b, 2 mM of AA), through tripods (Fig. 1c, 3.00 mM of AA), to irregular shapes (Fig. 1d, 4 mM of AA) was observed. Since the minimum concentration of AA required for the reduction of Au ions was 1 mM in these systems (minimum amount of color change from a yellow Au(III)–CTAB solution to a white Au(I)–CTAB solution), these results clearly indicate that the use of an excess of reducing agent was a major factor in the shape transitions. Interestingly, the concentration of the surface capping agent, CTAB, also led to a variation in the shape of the Au nanocrystals. When the concentration of CTAB was decreased to three times (2.74 · 10 3 M) and the other parameters of Fig. 1c were held constant, their shape was altered from tripods to /-shaped particles (Fig. 1e). In addition, particles synthesized using a 1.37 · 10 3 M CTAB solution, with the other parameters of Fig. 1c held constant, were nanocubes, as well as other shapes. The formation of various shapes is likely due to an interplay between the adsorption of surface capping agent and growth kinetics induced by the reducing agent. CTAB molecules have a stronger affinity for unstable {1 1 0} facets than other facets [27]. Because of this property of CTAB, the growth of Au nanorods is thought to involve the longitudinal surface, CTAB strongly adsorbs to the {1 1 0} facets of Au nanorods, thus conferring stability. When the AA concentration is increased with the values of other parameters held constant, tripod type nanoparticles are generated and the morphology of the tripod is likely to be related to kinetic factors. It should be noted that in the case of the Au tripod, growth occurred when the {1 1 0} and {1 1 1} facets were both favorably stabilized at the same time, assuming the growth mechanism reported by Manna et al. [21]. A higher AA concentration favors the more rapid formation of all facets and a sufficient number of CTAB molecules can simultaneously stabilize both the {1 1 0} and {1 1 1} facets, thereby producing a tripod. In the case of a slightly lower CTAB concentration with the same AA concentration as was used for a tripod, /shaped particles are produced in high yield (Fig. 1d). The preparation of /-shaped Au particles has been reported and a mechanism that explains the growth mechanism has been proposed by the El-Sayed et al. [28]. The transition in shape from rod to /-shaped particles appears to be originated from local melting by the laser pulses, followed by surface reconstruction. This surface reconstruction is driven by a decrease in the surface energy with a reduction in the {1 1 0} surface area and an increase in the {1 1 1} facets. Based on the fact that a decrease in the
concentration of CTAB led to the production of various shapes, we reasoned that an insufficient adsorption of CTAB on the facets of the Au nanocrystals occurred, thus causing the shapes to be varied via surface migration of Au atoms during the growth. Under the condition of lowest CTAB with the same AA concentration at conditions for producing a tripod induces an increase in the rate of disappearance of unstable {1 1 0} facets, because the insufficient capping of CTAB on {1 1 0} facets. As a result, the production of cubes with hexagon and triangle particles are produced, which surround the relatively stable {1 1 1} and {1 0 0} facets. Fig. 2 shows UV–vis absorption spectra of Au nanoparticles that correspond to the TEM images shown in Fig. 1. In the case of rod type Au nanoparticles (Fig. 1a–b), two peaks appear for the Au surface plasmon band. One appears at around at 540 nm which is a conventional plasmon band for spherical particles (transverse) and the other appears in the longer wavelength region, at 1100 nm (aspect ratio = 7, Fig. 1a) and 900 nm (aspect ratio = 5, Fig. 1b), corresponding to the longitudinal plasmon band for rod-shaped particles. In general, the position of the longitudinal absorption band is dependent on the aspect ratio of the rod-shaped particles [13]. The aspect ratio was decreased with increasing AA concentration, and the longitudinal absorption band was gradually blue-shifted in this system. Interestingly, these two split peaks were merged in the range of 630–650 nm for tripod particles (Fig. 2c and d). This result is consistent with data reported in a previous study of the optical properties Au tripods. Hao et al. showed that the color of a branched Au tripod was blue and the peak position of the absorption spectrum varied in the range from 650 to 700 nm [15]. In addition, the characteristics of /-shaped particles were intermediate in
Fig. 2. Absorption spectra of Au nanocrystals synthesized using different reaction conditions, as a function of (AA) and (CTAB). (AA) and (CTAB) were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and 8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
behavior between nanorods and tripods with regard to peak splitting and the position of peaks (Fig. 2e). That is, the spectrum of /-shaped particles showed two split peaks, transverse (600 nm) and longitudinal (1000 nm). The position of transverse peak, however, is shifted to 600 nm, intermediate between peak position of nanorods and tripods. The position of the longitudinal peak implies that /-shaped particles have optical properties that are similar to rod-typed particles. Furthermore, colloidal solutions of tripod and /-shaped particles are blue. Finally, the peak for nanocubes at 530 nm is similar to that for spherical particles (Fig. 2f). Based on these results, absorption spectra of the solutions containing Au nanoparticles of various shapes show clear differences in optical properties. 4. Conclusion In summary, a variety of Au nanoparticle shapes were produced in aqueous media, using a seed-mediated method. Their shape could be controlled by the appropriate adjustment of the concentrations of reducing agent and surface capping agent used. From the results, we conclude that the morphological control of Au nanoparticles is related to an interplay between the adsorption of the surface capping agent and the kinetics of reduction of Au ions. In addition, their unique optical properties permitted us to classify their morphology. Acknowledgment This work was supported by Grant No. (R01-2006-00010239-0) from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] J.A. Creighton, D.G.J. Eadon, Faraday Trans. 87 (1991) 3881– 3892.
813
[2] Y.W. Cao, R. Jin, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7961– 7962. [3] S. Nie, S.R. Emory, Science 275 (1997) 1102–1106. [4] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. [5] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153–1175. [6] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Commun. (2001) 617– 618. [7] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065–4067. [8] B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) 6368–6374. [9] N.R. Jana, Chem. Commun. (2003) 1950–1951. [10] J. Gao, C.M. Bender, C.J. Murphy, Langmuir 19 (2003) 9065–9070. [11] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, J. Phys. Chem. B 101 (1997) 6661–6664. [12] C.J. Johnson, E. Dujardin, S.A. Davis, C.J. Murphy, S. Mann, J. Mater. Chem. 12 (2002) 1765–1770. [13] J.E. Millstone, S. Park, K.L. Shuford, L. Qin, G.C. Schatz, C.A. Mirkin, J. Am. Chem. Soc. 127 (2005) 5312–5313. [14] S. Chen, Z.L. Wang, J. Ballato, S.H. Foulger, D.L. Carroll, J. Am. Chem. Soc. 125 (2003) 16186–16187. [15] E. Hao, R.C. Bailey, G.C. Schatz, J.T. Hupp, S. Li, Nano Lett 4 (2004) 327–330. [16] S.K. Kang, Y. Kim, I. Choi, J. Lee, J. Yi, Microelectron. Eng. 81 (2005) 389–393. [17] J. Zheng, Z. Zhu, H. Chen, Z. Liu, Langmuir 16 (2001) 4409–4412. [18] Y.W. Jun, S.M. Lee, N.J. Kang, J. Cheon, J. Am. Chem. Soc. 123 (2001) 5150–5151. [19] M. Chen, Y. Xie, J. Lu, Y. Xiong, S. Zhang, Y. Qiang, X. Liu, J. Mater. Chem. 12 (2002) 748–753. [20] J. Joo, H.B. Na, T. Yu, J.H. Yu, Y.W. Kim, F. Wu, J.Z. Zhang, T. Hyeon, J. Am. Chem. Soc. 125 (2003) 11100–11105. [21] L. Manna, D.J. Milliron, A. Meisel, E.C. Scher, A.P. Alivisatom, J. Am. Chem. Soc. 122 (2000) 12700–12706. [22] X. Peng, Adv. Mater. 15 (2003) 459–463. [23] C.C. Tang, S.S. Fan, M.L. de la Chapelle, P. Li, Chem. Phys. Lett. 333 (2001) 12–15. [24] Y. Dai, Y. Zhang, Q.K. Li, C.W. Nan, Chem. Phys. Lett. 358 (2002) 83–86. [25] V.A.L. Roy, A.B. Djurisic, W.K. Chan, J. Gao, H.F. Lui, C. Surya, Appl. Phys. Lett. 83 (2003) 141–143. [26] S.M. Lee, Y.W. Jun, S.N. Cho, J. Cheon, J. Am. Chem. Soc. 124 (2002) 11244–11245. [27] T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648–8649. [28] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 7867–7870.
Current Applied Physics 8 (2008) 810–813 www.elsevier.com/locate/cap www.kps.or.kr
Investigation on shape variation of Au nanocrystals Sung Koo Kang a, Inhee Choi a, Jeongjin Lee a, Younghun Kim b, Jongheop Yi a
a,*
School of Chemical and Biological Engineering, Institute of Chemical Engineering, Seoul National University, Seoul 151-742, Republic of Korea b Department of Chemical Engineering, Kwangwoon University, Seoul 139-701, Republic of Korea Received 14 August 2006; received in revised form 20 December 2006; accepted 27 April 2007 Available online 1 October 2007
Abstract A variety of shapes, such as rod, tripod, /-shape and cube, of Au nanocrystals were synthesized by employing different reaction conditions. The nanocrystals and their shape variation were characterized by transmission electron microscopy and UV–vis spectrophotometry. The evolution of shape was accomplished by controlling the parameters used in their synthesis, the concentration of reducing agent and surface capping agent. The effect of synthetic parameters on shape was explored, to determine suitable conditions for producing each shape of nanocrystals. Nanocrystals with different shapes have different plasmon bands in the visible region of the spectrum, which is a valuable property for sensor applications. 2007 Elsevier B.V. All rights reserved. PACS: 61.46.Df; 61.46.Hk Keywords: Gold (Au); Nanoparticles; Nanorod; /-Shape; Shape control; Tripod
1. Introduction Controlling the size and shape of inorganic nanoparticles remains an important issue in the area of advanced material development, since their unique properties are size and shape dependent on a nanometer scale [1–5]. Rapid progress has been made in controlling the shape of Au nanoparticles in recent years, leading to, for example, the production of rods [6–12], triangular platelets [13] and multipods [14,15]. These new structures are of great interest in terms of their optical, electric and magnetic properties for technological applications by ‘bottom-up’ approaches in developing new nanodevices [16,17]. Metal multipods with branches are particularly desirable structures for applications in areas such as interconnections of nanocircuits because of their electrical conductivity. Although it has been reported that semiconductor materials, such as CdS
*
Corresponding author. Fax: +82 (02) 880 7438. E-mail address: [email protected] (J. Yi).
1567-1739/$ - see front matter 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.cap.2007.04.035
[18–20], CdSe [21,22], ZnO [23–25], and PbS [26], can be used to control multipod-shaped structures, only a few studies of multipod structures of metal have been reported. Carroll et al. synthesized Au multipod nanocrystals by the addition NaOH and silver plates to Au–CTAB mixtures [14]. Li et al. reported on the synthesis of branched Au nanocrystals in high yields [15]. In the findings reported in these papers, tiny triangular or rectangular plates, which are formed in the initial step, develop into tripod or tetrapod structures by anisotropic growth. In this paper we describe the synthesis of shape-controlled Au nanoparticles using a seed-mediated method that was originally reported by Murphy et al. [7]. This is a well known method for preparing rod type nanoparticles. However, we observed that tripod- and /-shaped nanoparticles can be produced using the same procedure, by controlling the concentrations of reducing agent and surface capping agent used. Even though the mechanism of growth is yet to be elucidated, it is likely that the evolution of the shape is controlled by the kinetics associated with the reduction of the Au ion precursors.
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
2. Experimental details The Au seeds used in this work were prepared as follows; a solution 0.6 ml of 0.1 M NaBH4 at 0 C was added to 20 mL of a solution that was 2.5 · 10 4 M in HAuCl4 and tri-sodium citrate with vigorous stirring. The color of the solution instantly turned to brown. For the growth solution, four tubes, each of which contained 10 mL of an aqueous solution of 2.5 · 10 4 M HAuCl4 and 8.23 · 10 3 M cetyltrimethylammonium bromide (CTAB) were prepared. These solutions were held at 0 C for all of the syntheses. Different amounts of a 0.1 M ascorbic acid solution (AA) were added to each tube. The amounts added were 0.1, 0.2, 0.3 and 0.4 ml, respectively. After the
811
color of the solution changed from yellow to white, 0.025 ml of the seed solution was added to each tube. The same procedure was repeated with respect to CTAB concentration, i.e., 2.74 · 10 3 and 1.37 · 10 3 M. 3. Results and discussion The evolution of the shape of Au nanoparticles was tracked by means of TEM. The results show that the shape of the Au nanoparticles is dependent on the concentration of the reducing agent, ascorbic acid (AA), and the surfactant, cetyltrimethylammonium bromide (CTAB) that are used. These two parameters were found to be interrelated; different combinations resulted in various shapes. Fig. 1
Fig. 1. TEM images of Au nanoparticles synthesized using different reaction conditions, as a function of AA and CTAB concentration. (AA) and (CTAB) were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and 8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.
812
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
shows TEM images of Au nanoparticles obtained when different amounts of AA and CTAB were added to the solutions. Fig. 1a–d clearly shows a variation in shape as the addition of AA is gradually increased at a constant CTAB concentration (8.23 · 10 3 M). Particles synthesized using 1 mM AA were spherical and rod-shaped (Fig. 1a). When the concentration of AA in the solution was gradually increased, a transition from rod-shaped particles with a lower aspect ratio (Fig. 1b, 2 mM of AA), through tripods (Fig. 1c, 3.00 mM of AA), to irregular shapes (Fig. 1d, 4 mM of AA) was observed. Since the minimum concentration of AA required for the reduction of Au ions was 1 mM in these systems (minimum amount of color change from a yellow Au(III)–CTAB solution to a white Au(I)–CTAB solution), these results clearly indicate that the use of an excess of reducing agent was a major factor in the shape transitions. Interestingly, the concentration of the surface capping agent, CTAB, also led to a variation in the shape of the Au nanocrystals. When the concentration of CTAB was decreased to three times (2.74 · 10 3 M) and the other parameters of Fig. 1c were held constant, their shape was altered from tripods to /-shaped particles (Fig. 1e). In addition, particles synthesized using a 1.37 · 10 3 M CTAB solution, with the other parameters of Fig. 1c held constant, were nanocubes, as well as other shapes. The formation of various shapes is likely due to an interplay between the adsorption of surface capping agent and growth kinetics induced by the reducing agent. CTAB molecules have a stronger affinity for unstable {1 1 0} facets than other facets [27]. Because of this property of CTAB, the growth of Au nanorods is thought to involve the longitudinal surface, CTAB strongly adsorbs to the {1 1 0} facets of Au nanorods, thus conferring stability. When the AA concentration is increased with the values of other parameters held constant, tripod type nanoparticles are generated and the morphology of the tripod is likely to be related to kinetic factors. It should be noted that in the case of the Au tripod, growth occurred when the {1 1 0} and {1 1 1} facets were both favorably stabilized at the same time, assuming the growth mechanism reported by Manna et al. [21]. A higher AA concentration favors the more rapid formation of all facets and a sufficient number of CTAB molecules can simultaneously stabilize both the {1 1 0} and {1 1 1} facets, thereby producing a tripod. In the case of a slightly lower CTAB concentration with the same AA concentration as was used for a tripod, /shaped particles are produced in high yield (Fig. 1d). The preparation of /-shaped Au particles has been reported and a mechanism that explains the growth mechanism has been proposed by the El-Sayed et al. [28]. The transition in shape from rod to /-shaped particles appears to be originated from local melting by the laser pulses, followed by surface reconstruction. This surface reconstruction is driven by a decrease in the surface energy with a reduction in the {1 1 0} surface area and an increase in the {1 1 1} facets. Based on the fact that a decrease in the
concentration of CTAB led to the production of various shapes, we reasoned that an insufficient adsorption of CTAB on the facets of the Au nanocrystals occurred, thus causing the shapes to be varied via surface migration of Au atoms during the growth. Under the condition of lowest CTAB with the same AA concentration at conditions for producing a tripod induces an increase in the rate of disappearance of unstable {1 1 0} facets, because the insufficient capping of CTAB on {1 1 0} facets. As a result, the production of cubes with hexagon and triangle particles are produced, which surround the relatively stable {1 1 1} and {1 0 0} facets. Fig. 2 shows UV–vis absorption spectra of Au nanoparticles that correspond to the TEM images shown in Fig. 1. In the case of rod type Au nanoparticles (Fig. 1a–b), two peaks appear for the Au surface plasmon band. One appears at around at 540 nm which is a conventional plasmon band for spherical particles (transverse) and the other appears in the longer wavelength region, at 1100 nm (aspect ratio = 7, Fig. 1a) and 900 nm (aspect ratio = 5, Fig. 1b), corresponding to the longitudinal plasmon band for rod-shaped particles. In general, the position of the longitudinal absorption band is dependent on the aspect ratio of the rod-shaped particles [13]. The aspect ratio was decreased with increasing AA concentration, and the longitudinal absorption band was gradually blue-shifted in this system. Interestingly, these two split peaks were merged in the range of 630–650 nm for tripod particles (Fig. 2c and d). This result is consistent with data reported in a previous study of the optical properties Au tripods. Hao et al. showed that the color of a branched Au tripod was blue and the peak position of the absorption spectrum varied in the range from 650 to 700 nm [15]. In addition, the characteristics of /-shaped particles were intermediate in
Fig. 2. Absorption spectra of Au nanocrystals synthesized using different reaction conditions, as a function of (AA) and (CTAB). (AA) and (CTAB) were (a) 1 and 8.31 mM, (b) 2 and 8.31 mM, (c) 3 and 8.31 mM, (d) 4 and 8.31 mM, (e) 3 and 2.74 mM, and (f) 3 and 1.37 mM, respectively.
S.K. Kang et al. / Current Applied Physics 8 (2008) 810–813
behavior between nanorods and tripods with regard to peak splitting and the position of peaks (Fig. 2e). That is, the spectrum of /-shaped particles showed two split peaks, transverse (600 nm) and longitudinal (1000 nm). The position of transverse peak, however, is shifted to 600 nm, intermediate between peak position of nanorods and tripods. The position of the longitudinal peak implies that /-shaped particles have optical properties that are similar to rod-typed particles. Furthermore, colloidal solutions of tripod and /-shaped particles are blue. Finally, the peak for nanocubes at 530 nm is similar to that for spherical particles (Fig. 2f). Based on these results, absorption spectra of the solutions containing Au nanoparticles of various shapes show clear differences in optical properties. 4. Conclusion In summary, a variety of Au nanoparticle shapes were produced in aqueous media, using a seed-mediated method. Their shape could be controlled by the appropriate adjustment of the concentrations of reducing agent and surface capping agent used. From the results, we conclude that the morphological control of Au nanoparticles is related to an interplay between the adsorption of the surface capping agent and the kinetics of reduction of Au ions. In addition, their unique optical properties permitted us to classify their morphology. Acknowledgment This work was supported by Grant No. (R01-2006-00010239-0) from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] J.A. Creighton, D.G.J. Eadon, Faraday Trans. 87 (1991) 3881– 3892.
813
[2] Y.W. Cao, R. Jin, C.A. Mirkin, J. Am. Chem. Soc. 123 (2001) 7961– 7962. [3] S. Nie, S.R. Emory, Science 275 (1997) 1102–1106. [4] S. Link, M.A. El-Sayed, J. Phys. Chem. B 103 (1999) 8410–8426. [5] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153–1175. [6] N.R. Jana, L. Gearheart, C.J. Murphy, Chem. Commun. (2001) 617– 618. [7] N.R. Jana, L. Gearheart, C.J. Murphy, J. Phys. Chem. B 105 (2001) 4065–4067. [8] B. Nikoobakht, M.A. El-Sayed, Langmuir 17 (2001) 6368–6374. [9] N.R. Jana, Chem. Commun. (2003) 1950–1951. [10] J. Gao, C.M. Bender, C.J. Murphy, Langmuir 19 (2003) 9065–9070. [11] Y.Y. Yu, S.S. Chang, C.L. Lee, C.R.C. Wang, J. Phys. Chem. B 101 (1997) 6661–6664. [12] C.J. Johnson, E. Dujardin, S.A. Davis, C.J. Murphy, S. Mann, J. Mater. Chem. 12 (2002) 1765–1770. [13] J.E. Millstone, S. Park, K.L. Shuford, L. Qin, G.C. Schatz, C.A. Mirkin, J. Am. Chem. Soc. 127 (2005) 5312–5313. [14] S. Chen, Z.L. Wang, J. Ballato, S.H. Foulger, D.L. Carroll, J. Am. Chem. Soc. 125 (2003) 16186–16187. [15] E. Hao, R.C. Bailey, G.C. Schatz, J.T. Hupp, S. Li, Nano Lett 4 (2004) 327–330. [16] S.K. Kang, Y. Kim, I. Choi, J. Lee, J. Yi, Microelectron. Eng. 81 (2005) 389–393. [17] J. Zheng, Z. Zhu, H. Chen, Z. Liu, Langmuir 16 (2001) 4409–4412. [18] Y.W. Jun, S.M. Lee, N.J. Kang, J. Cheon, J. Am. Chem. Soc. 123 (2001) 5150–5151. [19] M. Chen, Y. Xie, J. Lu, Y. Xiong, S. Zhang, Y. Qiang, X. Liu, J. Mater. Chem. 12 (2002) 748–753. [20] J. Joo, H.B. Na, T. Yu, J.H. Yu, Y.W. Kim, F. Wu, J.Z. Zhang, T. Hyeon, J. Am. Chem. Soc. 125 (2003) 11100–11105. [21] L. Manna, D.J. Milliron, A. Meisel, E.C. Scher, A.P. Alivisatom, J. Am. Chem. Soc. 122 (2000) 12700–12706. [22] X. Peng, Adv. Mater. 15 (2003) 459–463. [23] C.C. Tang, S.S. Fan, M.L. de la Chapelle, P. Li, Chem. Phys. Lett. 333 (2001) 12–15. [24] Y. Dai, Y. Zhang, Q.K. Li, C.W. Nan, Chem. Phys. Lett. 358 (2002) 83–86. [25] V.A.L. Roy, A.B. Djurisic, W.K. Chan, J. Gao, H.F. Lui, C. Surya, Appl. Phys. Lett. 83 (2003) 141–143. [26] S.M. Lee, Y.W. Jun, S.N. Cho, J. Cheon, J. Am. Chem. Soc. 124 (2002) 11244–11245. [27] T.K. Sau, C.J. Murphy, J. Am. Chem. Soc. 126 (2004) 8648–8649. [28] S. Link, Z.L. Wang, M.A. El-Sayed, J. Phys. Chem. B 104 (2000) 7867–7870.