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Preparation of Anisotropic Gold Particles Using a Gemini Surfactant Template
JOURNAL OF COLLOID AND INTERFACE SCIENCE ARTICLE NO.
208, 578 –581 (1998)
CS985852
NOTE Preparation of Anisotropic Gold Particles Using a Gemini Surfactant Template Anisotropic gold particles were prepared in a gemini cationic surfactant solution by reduction of HAuCl4 with UV irradiation. With increasing concentrations of both HAuCl4 and the surfactant, fibrous gold particles were obtained and their length increased. These results suggest that the gemini surfactant which provides threadlike micelles operates as a soft template for anisotropic gold particles. © 1998 Academic Press Key Words: fibrous gold particles; gemini surfactant; UV irradiation; soft template.
the addition of HAuCl4 to the surfactant solution followed by agitation at 25°C for 24 h. Then a 3 cm3 portion of the sample was placed in a rectangular quartz cell. UV irradiation was carried out with a 200 W low pressure mercury lamp (lmax 5 253.7 nm) for 4 h at 15 cm from the light source. Optical absorption spectra of the colloidal solutions were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. Since the samples were too concentrated to be observed neatly, they were diluted to 10 times with water and their absorbances were measured at 1 mm of path length. Electron micrographs of Au particles were taken with a Hitachi H-9000 NAR transmission electron microscope, operating at 200 kV.
RESULTS AND DISCUSSION INTRODUCTION Many spherical metal particles have been prepared by using many methods and their physicochemical properties have been characterized (1). On the other hand, only a few studies for the preparation of anisotropic metal particles have been reported (2–5), although they show interesting properties in optical and rheological behaviors. Anisotropic gold (2) and copper (3) particles have been prepared by using rodlike cationic micelles or microemulsions which are called “soft template.” By using hard templates like a porous aluminum oxide membrane, rodlike gold particles have also been obtained (4, 5) and their absorption spectrum is in agreement with theoretical prediction for anisotropy. Recently, we observed elongation of rodlike gold particles by the addition of a small amount of platinum by using a cationic surfactant soft template showing that platinum would enhance the growth of rodlike gold particles as a catalysis (6). In addition, micellar properties of surfactants for soft templates are considerably influenced by their chemical structure. For example, gemini or dimeric surfactants spontaneously aggregate into micelles whose shape and size are highly sensitive to the length of this hydrophobic spacer (7, 8). So, it is still interesting to study the effect of surfactant structure on the formation of anisotropic gold particles. In this study, anisotropic gold particles were prepared in aqueous solution by using a cationic gemini surfactant as a soft template through a UV irradiation. The used surfactant was 1,2-bis(dodecyldimethylammonio) ethane dichloride.
It has been reported (2) that rodlike gold particles can be obtained by using hexadecyltrimethylammonium chloride (HTAC) which provides rodlike micelles as a soft template. In this case, the concentration of HTAC was about 30 wt%. It is known (8, 10) that micellar shape is significantly influenced with surfactant structure; in particular, cationic gemini surfactants show entangled threadlike micelles at low concentration. For 2RenQCl, we confirmed by
EXPERIMENTAL
Materials HAuCl4 was kindly supplied by Tanaka Kikinzoku Kogyo. 1,2-bis(dodecyldimethylammonio)ethane dichloride (2RenQCl) was synthesized from an ion exchange of the corresponding dibromide (9). The water used in this study was purified through a Milli-Q system.
Methods and Measurements Various compositions of 2RenQCl–HAuCl4 mixed solutions containing 0.5–3.0 wt% 2RenQCl and 1– 40 mmol dm23 HAuCl4 were prepared by
0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
FIG. 1. Evolution of UV-visible spectra of (a) 20 mmol dm23 HAuCl4, (b) 2 wt% 2RenQCl, (c) 20 mmol dm23 HAuCl4 1 2 wt% 2RenQCl, before irradiation, (d) UV irradiation to the sample c for 1 h, (e) 2h, (f) 4h. The samples were diluted to 10 times and measured at 1 mm of path length.
578
NOTE
579
FIG. 2. Effect of HAuCl4 concentration on the formation of anisotropic Au particles. [2RenQCl] 5 2.0 wt%, [HAuCl4] 5 (a) 1, (b) 5, (c) 10, (d) 40 mmol dm23. UV irradiation for 4 h.
dynamic light scattering that anisotropic micelles were formed above 2 wt% in salt-free solution. The 2RenQCl dissociates in aqueous solution to a double-positive cation, 2RenQ21, due to two quaternary ammonium groups in the molecule, and 2Cl2. On the addition of HAuCl4, the 2RenQ21 immediately reacts with AuCl2 4 to give yellow precipitate. This precipitate is found to be an adduct of a 2RenQ21 and two AuCl2 4 ions. The adduct is hardly soluble in water, since it does not have formal charge and has two long alkyl chains associated to the surfactant. In fact, when the AuCl2 4 was mixed with 2RenQCl in a 2:1 molar ratio, e.g., 20 mmol dm23 AuCl2 4 and 0.5 wt% (10 mmol dm23) 2RenQ21, the adduct completely settled down and the supernatant became transparent. However, when the cationic surfactant was in excess, the adduct was dispersed homogeneously, making turbid suspension. It was found that the high dispersity of the adduct was crucial for the reduction of Au(III) by UV irradiation. Figures 1a–1c show UV-visible extinction spectra of aqueous 2RenQCl (2 wt%), HAuCl4 (20 mmol dm23), and corresponding mixed suspension. In the surfactant-free solution, HAuCl4 shows two intense charge transfer (CT) bands, peaking at 212 and 234 nm, and a moderate d– d transition band at 312 nm. On the other hand, aqueous 2RenQCl solution shows no absorption except an intense peak at 190 nm due to chloride ions. On mixing the two solutions, the two CT bands of AuCl2 4 merge into a broad band at 242 nm. At the same time, the
d– d transition band undergoes a significant red-shift to 334 nm. These band shifts are indicative of the formation of the adduct by a strong interaction between the quaternary amine groups of 2RenQ21 and the negative charge on the AuCl2 4 . Another change by mixing the two components is a remarkable increase in the turbidity, which is evident from the high absorbance at longer wavelengths. Figures 1d–1f show the evolution of the optical extinction spectra in the course of UV irradiation. The CT and d– d bands of AuCl2 4 decreased to one sixth after 1 h UV irradiation and completely disappeared at 2 h. Instead, another absorption band appeared at 550 nm at 1 h, followed by a gradual red-shift. Since the latter band is known to be a surface plasmon band of metallic gold, these spectral changes can be attributed to the decomposition of the 2RenQ21 2 AuCl2 4 adduct and formation of colloidal gold, respectively. In the following, morphology of the gold particles was investigated by TEM. Figure 2 shows the effect of HAuCl4 concentration. In this series, the surfactant concentration was held constant at 2 wt%. At 1 mmol dm23 HAuCl4 (a), many polyhedral Au particles of 20 –25 nm in size were observed, but no fibrous particles were found. The fibrous Au particles were observed only for more than 5 mmol dm23 HAuCl4 (b– d). An increment of the HAuCl4 concentration leads to elongation of the rodlike particles as well as an increase in the number. For instance, an average-
580
NOTE
FIG. 3. Effect of 2RenQCl concentration on the formation of anisotropic Au particles. [HAuCl4] 5 20 mmol dm23, [2RenQCl] 5 (a) 0.5, (b) 1, (c) 2, (d) 3 wt%. UV irradiation for 4 h.
sized fibrous Au particle prepared at 5 mmol dm23 HAuCl4 (b) is 100 nm wide and 2.6 mm long. With increasing HAuCl4 concentration to 40 mmol dm23 (d), the length of the fibroid increased to 15–25 mm whereas the width remained in the range of 130 –150 nm. Next the effect of the 2RenQCl concentration was investigated at a constant HAuCl4 concentration (20 mmol dm23). As shown in Fig. 3c, many fibrous Au particles were present at above 2.0 wt% 2RenQCl. However, it should be noted that the fibrous Au particles are formed at lower concentrations, e.g., 0.5 or 1.0 wt% 2RenQCl, as shown in Figs. 3a and 3b. Apparently the latter result is not consistent with our speculation that formation of the fibrous Au particles requires threadlike micelles of the surfactant. However, the 2RenQCl concentration to form threadlike micelles may significantly decrease in the presence of HAuCl4, as is the case for the other salts such as NaCl. Finally, the effect of the irradiation time was studied. Figure 4 shows TEM micrographs of Au particles after 1 to 4 h of UV irradiation. Although there is a wide distribution in the lengths of fibrous particles, it is obvious that longer irradiation time provides longer particles. From these results, we propose the formation mechanism of the fibrous Au particles as follows. At first, 2RenQ21 and AuCl2 4 form the adduct at a molar ratio of 1:2. Although the adduct is hardly soluble in water, it can be homogeneously dispersed by the surfactant in excess. Upon UV irradiation,
AuCl2 4 in the adduct undergoes photolysis and consequently the surfactant is liberated. hy
~2RenQ! 21 2 ~AuCl2 3 ~2RenQ!Cl2 · · · ~AuIIICl3! 2 7 4 ! 2 ™™ ~2RenQ!2Cl8 · · · ~AuIICl3! 2 2 As shown above, in the transition state, a negative charge would be shared between AuCl3 ion and Cl atoms. Once the AuCl2 3 ion is liberated, the Au (II) can be reduced to Au metal in the same way as in the surfactant-free solution (11). 2 2 2 AuCl2 3 3 AuCl4 1 AuCl2 0 2 AuCl2 2 3 Au 1 Cl° 1 Cl
It is difficult to say that in which stage the formation of fibrous Au particles is induced. However, it is clear at least that the adduct or its aggregates have nothing to do with the fibrous particles, since they must decompose in the course of UV irradiation. Probably, one or some small particles adsorb on the surface of threadlike micelles of the free 2RenQCl and then growth to fibrous particles proceeds by diffusion of Au ions onto the micellar surface. In this
581
NOTE
FIG. 4. TEM micrographs of Au particles at various UV irradiation times. [2RenQCl] 5 2.0 wt%, [HAuCl4] 5 20 mmol dm23. Irradiation time 5 (a) 1h, (b) 2h, (c) 3h, (d) 4h.
model, direction of the crystal growth is determined by the elongative direction of the threadlike micelle. At the same time, it can explain the coexistence of isotropic (spherical or polygonal) particles by supposing adsorption of the surfactant monomer on the Au particles. In summary, we found that gemini surfactants such as 2RenQCl operates as a soft template to provide fibrous gold particles through UV photolysis of HAuCl4 where the gemini concentration used is considerably smaller than that of HTAC.
8. Rosen, M. J., CHEMTECH, 30 (1993). 9. Esumi, K., Taguma, K., and Koide, Y., Langmuir 12, 4039 (1996). 10. Israelachivili, J. N., and Ninham, B. W., J. Chem. Soc., Faraday Trans. 1, 72, 1525 (1976). 11. Kurihara, K., Kizling, J., Stenius, P., and Fendler, J. H., J. Am. Chem. Soc. 105, 2574 (1983). Kunio Esumi1 Junko Hara Nariaki Aihara Kiyoshi Usui Kanjiro Torigoe
REFERENCES 1. Bradley, J. S., “Clusters and Colloids. From Theory to Applications” (G. Schmid, Ed.), Chap. 6, VCH, Weinheim, 1994. 2. Esumi, K., Matsuhisa, K., and Torigoe, K., Langmuir 11, 3285 (1995). 3. Tanori, J., and Pileni, M. P., Adv. Mater. 7, 862 (1995). 4. Foss, C. A., Hornyak, G. L., Stockert, J. A., and Martin, C. R., J. Phys. Chem. 96, 7497 (1992). 5. van der Zande, B. M. I., Bohmer, M. R., Fokkink, L. G. J., and Schonenberger, C., J. Phys. Chem. B 101, 852 (1997). 6. Esumi, K., Nawa, M., Aihara, N., and Usui, K., New J. Chem. 22, 719 (1998). 7. Zana, R., Curr. Opin. Colloid Interface Sci. 1, 566 (1996).
Department of Applied Chemistry and Institute of Colloid and Interface Science Science University of Tokyo Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received May 11, 1998; accepted August 31, 1998
1
To whom correspondence should be addressed.
208, 578 –581 (1998)
CS985852
NOTE Preparation of Anisotropic Gold Particles Using a Gemini Surfactant Template Anisotropic gold particles were prepared in a gemini cationic surfactant solution by reduction of HAuCl4 with UV irradiation. With increasing concentrations of both HAuCl4 and the surfactant, fibrous gold particles were obtained and their length increased. These results suggest that the gemini surfactant which provides threadlike micelles operates as a soft template for anisotropic gold particles. © 1998 Academic Press Key Words: fibrous gold particles; gemini surfactant; UV irradiation; soft template.
the addition of HAuCl4 to the surfactant solution followed by agitation at 25°C for 24 h. Then a 3 cm3 portion of the sample was placed in a rectangular quartz cell. UV irradiation was carried out with a 200 W low pressure mercury lamp (lmax 5 253.7 nm) for 4 h at 15 cm from the light source. Optical absorption spectra of the colloidal solutions were recorded on a Hewlett-Packard 8452A diode array spectrophotometer. Since the samples were too concentrated to be observed neatly, they were diluted to 10 times with water and their absorbances were measured at 1 mm of path length. Electron micrographs of Au particles were taken with a Hitachi H-9000 NAR transmission electron microscope, operating at 200 kV.
RESULTS AND DISCUSSION INTRODUCTION Many spherical metal particles have been prepared by using many methods and their physicochemical properties have been characterized (1). On the other hand, only a few studies for the preparation of anisotropic metal particles have been reported (2–5), although they show interesting properties in optical and rheological behaviors. Anisotropic gold (2) and copper (3) particles have been prepared by using rodlike cationic micelles or microemulsions which are called “soft template.” By using hard templates like a porous aluminum oxide membrane, rodlike gold particles have also been obtained (4, 5) and their absorption spectrum is in agreement with theoretical prediction for anisotropy. Recently, we observed elongation of rodlike gold particles by the addition of a small amount of platinum by using a cationic surfactant soft template showing that platinum would enhance the growth of rodlike gold particles as a catalysis (6). In addition, micellar properties of surfactants for soft templates are considerably influenced by their chemical structure. For example, gemini or dimeric surfactants spontaneously aggregate into micelles whose shape and size are highly sensitive to the length of this hydrophobic spacer (7, 8). So, it is still interesting to study the effect of surfactant structure on the formation of anisotropic gold particles. In this study, anisotropic gold particles were prepared in aqueous solution by using a cationic gemini surfactant as a soft template through a UV irradiation. The used surfactant was 1,2-bis(dodecyldimethylammonio) ethane dichloride.
It has been reported (2) that rodlike gold particles can be obtained by using hexadecyltrimethylammonium chloride (HTAC) which provides rodlike micelles as a soft template. In this case, the concentration of HTAC was about 30 wt%. It is known (8, 10) that micellar shape is significantly influenced with surfactant structure; in particular, cationic gemini surfactants show entangled threadlike micelles at low concentration. For 2RenQCl, we confirmed by
EXPERIMENTAL
Materials HAuCl4 was kindly supplied by Tanaka Kikinzoku Kogyo. 1,2-bis(dodecyldimethylammonio)ethane dichloride (2RenQCl) was synthesized from an ion exchange of the corresponding dibromide (9). The water used in this study was purified through a Milli-Q system.
Methods and Measurements Various compositions of 2RenQCl–HAuCl4 mixed solutions containing 0.5–3.0 wt% 2RenQCl and 1– 40 mmol dm23 HAuCl4 were prepared by
0021-9797/98 $25.00 Copyright © 1998 by Academic Press All rights of reproduction in any form reserved.
FIG. 1. Evolution of UV-visible spectra of (a) 20 mmol dm23 HAuCl4, (b) 2 wt% 2RenQCl, (c) 20 mmol dm23 HAuCl4 1 2 wt% 2RenQCl, before irradiation, (d) UV irradiation to the sample c for 1 h, (e) 2h, (f) 4h. The samples were diluted to 10 times and measured at 1 mm of path length.
578
NOTE
579
FIG. 2. Effect of HAuCl4 concentration on the formation of anisotropic Au particles. [2RenQCl] 5 2.0 wt%, [HAuCl4] 5 (a) 1, (b) 5, (c) 10, (d) 40 mmol dm23. UV irradiation for 4 h.
dynamic light scattering that anisotropic micelles were formed above 2 wt% in salt-free solution. The 2RenQCl dissociates in aqueous solution to a double-positive cation, 2RenQ21, due to two quaternary ammonium groups in the molecule, and 2Cl2. On the addition of HAuCl4, the 2RenQ21 immediately reacts with AuCl2 4 to give yellow precipitate. This precipitate is found to be an adduct of a 2RenQ21 and two AuCl2 4 ions. The adduct is hardly soluble in water, since it does not have formal charge and has two long alkyl chains associated to the surfactant. In fact, when the AuCl2 4 was mixed with 2RenQCl in a 2:1 molar ratio, e.g., 20 mmol dm23 AuCl2 4 and 0.5 wt% (10 mmol dm23) 2RenQ21, the adduct completely settled down and the supernatant became transparent. However, when the cationic surfactant was in excess, the adduct was dispersed homogeneously, making turbid suspension. It was found that the high dispersity of the adduct was crucial for the reduction of Au(III) by UV irradiation. Figures 1a–1c show UV-visible extinction spectra of aqueous 2RenQCl (2 wt%), HAuCl4 (20 mmol dm23), and corresponding mixed suspension. In the surfactant-free solution, HAuCl4 shows two intense charge transfer (CT) bands, peaking at 212 and 234 nm, and a moderate d– d transition band at 312 nm. On the other hand, aqueous 2RenQCl solution shows no absorption except an intense peak at 190 nm due to chloride ions. On mixing the two solutions, the two CT bands of AuCl2 4 merge into a broad band at 242 nm. At the same time, the
d– d transition band undergoes a significant red-shift to 334 nm. These band shifts are indicative of the formation of the adduct by a strong interaction between the quaternary amine groups of 2RenQ21 and the negative charge on the AuCl2 4 . Another change by mixing the two components is a remarkable increase in the turbidity, which is evident from the high absorbance at longer wavelengths. Figures 1d–1f show the evolution of the optical extinction spectra in the course of UV irradiation. The CT and d– d bands of AuCl2 4 decreased to one sixth after 1 h UV irradiation and completely disappeared at 2 h. Instead, another absorption band appeared at 550 nm at 1 h, followed by a gradual red-shift. Since the latter band is known to be a surface plasmon band of metallic gold, these spectral changes can be attributed to the decomposition of the 2RenQ21 2 AuCl2 4 adduct and formation of colloidal gold, respectively. In the following, morphology of the gold particles was investigated by TEM. Figure 2 shows the effect of HAuCl4 concentration. In this series, the surfactant concentration was held constant at 2 wt%. At 1 mmol dm23 HAuCl4 (a), many polyhedral Au particles of 20 –25 nm in size were observed, but no fibrous particles were found. The fibrous Au particles were observed only for more than 5 mmol dm23 HAuCl4 (b– d). An increment of the HAuCl4 concentration leads to elongation of the rodlike particles as well as an increase in the number. For instance, an average-
580
NOTE
FIG. 3. Effect of 2RenQCl concentration on the formation of anisotropic Au particles. [HAuCl4] 5 20 mmol dm23, [2RenQCl] 5 (a) 0.5, (b) 1, (c) 2, (d) 3 wt%. UV irradiation for 4 h.
sized fibrous Au particle prepared at 5 mmol dm23 HAuCl4 (b) is 100 nm wide and 2.6 mm long. With increasing HAuCl4 concentration to 40 mmol dm23 (d), the length of the fibroid increased to 15–25 mm whereas the width remained in the range of 130 –150 nm. Next the effect of the 2RenQCl concentration was investigated at a constant HAuCl4 concentration (20 mmol dm23). As shown in Fig. 3c, many fibrous Au particles were present at above 2.0 wt% 2RenQCl. However, it should be noted that the fibrous Au particles are formed at lower concentrations, e.g., 0.5 or 1.0 wt% 2RenQCl, as shown in Figs. 3a and 3b. Apparently the latter result is not consistent with our speculation that formation of the fibrous Au particles requires threadlike micelles of the surfactant. However, the 2RenQCl concentration to form threadlike micelles may significantly decrease in the presence of HAuCl4, as is the case for the other salts such as NaCl. Finally, the effect of the irradiation time was studied. Figure 4 shows TEM micrographs of Au particles after 1 to 4 h of UV irradiation. Although there is a wide distribution in the lengths of fibrous particles, it is obvious that longer irradiation time provides longer particles. From these results, we propose the formation mechanism of the fibrous Au particles as follows. At first, 2RenQ21 and AuCl2 4 form the adduct at a molar ratio of 1:2. Although the adduct is hardly soluble in water, it can be homogeneously dispersed by the surfactant in excess. Upon UV irradiation,
AuCl2 4 in the adduct undergoes photolysis and consequently the surfactant is liberated. hy
~2RenQ! 21 2 ~AuCl2 3 ~2RenQ!Cl2 · · · ~AuIIICl3! 2 7 4 ! 2 ™™ ~2RenQ!2Cl8 · · · ~AuIICl3! 2 2 As shown above, in the transition state, a negative charge would be shared between AuCl3 ion and Cl atoms. Once the AuCl2 3 ion is liberated, the Au (II) can be reduced to Au metal in the same way as in the surfactant-free solution (11). 2 2 2 AuCl2 3 3 AuCl4 1 AuCl2 0 2 AuCl2 2 3 Au 1 Cl° 1 Cl
It is difficult to say that in which stage the formation of fibrous Au particles is induced. However, it is clear at least that the adduct or its aggregates have nothing to do with the fibrous particles, since they must decompose in the course of UV irradiation. Probably, one or some small particles adsorb on the surface of threadlike micelles of the free 2RenQCl and then growth to fibrous particles proceeds by diffusion of Au ions onto the micellar surface. In this
581
NOTE
FIG. 4. TEM micrographs of Au particles at various UV irradiation times. [2RenQCl] 5 2.0 wt%, [HAuCl4] 5 20 mmol dm23. Irradiation time 5 (a) 1h, (b) 2h, (c) 3h, (d) 4h.
model, direction of the crystal growth is determined by the elongative direction of the threadlike micelle. At the same time, it can explain the coexistence of isotropic (spherical or polygonal) particles by supposing adsorption of the surfactant monomer on the Au particles. In summary, we found that gemini surfactants such as 2RenQCl operates as a soft template to provide fibrous gold particles through UV photolysis of HAuCl4 where the gemini concentration used is considerably smaller than that of HTAC.
8. Rosen, M. J., CHEMTECH, 30 (1993). 9. Esumi, K., Taguma, K., and Koide, Y., Langmuir 12, 4039 (1996). 10. Israelachivili, J. N., and Ninham, B. W., J. Chem. Soc., Faraday Trans. 1, 72, 1525 (1976). 11. Kurihara, K., Kizling, J., Stenius, P., and Fendler, J. H., J. Am. Chem. Soc. 105, 2574 (1983). Kunio Esumi1 Junko Hara Nariaki Aihara Kiyoshi Usui Kanjiro Torigoe
REFERENCES 1. Bradley, J. S., “Clusters and Colloids. From Theory to Applications” (G. Schmid, Ed.), Chap. 6, VCH, Weinheim, 1994. 2. Esumi, K., Matsuhisa, K., and Torigoe, K., Langmuir 11, 3285 (1995). 3. Tanori, J., and Pileni, M. P., Adv. Mater. 7, 862 (1995). 4. Foss, C. A., Hornyak, G. L., Stockert, J. A., and Martin, C. R., J. Phys. Chem. 96, 7497 (1992). 5. van der Zande, B. M. I., Bohmer, M. R., Fokkink, L. G. J., and Schonenberger, C., J. Phys. Chem. B 101, 852 (1997). 6. Esumi, K., Nawa, M., Aihara, N., and Usui, K., New J. Chem. 22, 719 (1998). 7. Zana, R., Curr. Opin. Colloid Interface Sci. 1, 566 (1996).
Department of Applied Chemistry and Institute of Colloid and Interface Science Science University of Tokyo Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received May 11, 1998; accepted August 31, 1998
1
To whom correspondence should be addressed.