Langmuir 1995,11, 3285-3287
3285
Preparation of Rodlike Gold Particles by UV Irradiation Using Cationic Micelles as a Template Kunio Esumi," Keiichi Matsuhisa, and Kanjiro Torigoe Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan Received April 27, 1995. In Final Form: July 20, 1995@ Colloidal gold was prepared by irradiation of HAuC14 solutions with 253.7nm light in the presence of rodlike micelles of hexadecyltrimethylammonium chloride. Rodlike colloidal gold was obtained at above some concentrations of HAuC14 with increasing irradiation time. This result suggests that the rodlike micelle operates as a template for the preparation of anisotropic gold particles.
Introduction Many ultrafine metal particles have been preparedl by various methods, such as photochemical reduction, chemical reduction, chemical liquid deposition, and thermal decomposition. For silver, gold, and copper, in particular, it is very interesting to elucidate their optical properties since they strongly absorb light in the visible region due to surface plasma resonance. Until now, many monodispersed metal colloids have been obtained, but most of their shapes have been limited to spheres. Recently, self-assembled lipid tubules have been used as templates for preparation of anisotropic inorganic compounds which can be processed into higher order assemblies, such as for the crystallization of inorganic oxides2 Some unique anisotropic compound^^-^ have also been prepared using organic self-assembly systems. We have prepared6spherical gold colloids using cationic surfactants in which the surfactant concentrations used
was chosen to induce spherical micelle formation. Since micellar shape of surfactant is transformed from spherical to rodlike with increasing surfactant concentration, there is a possibility that anisotropic metal colloids can be obtained in such an anisotropic environment. The objective of this work was to prepare anisotropic gold colloids by W irradiation in rodlike micellar solution of a cationic surfactant.
Experimental Section Hexadecyltrimethylammonium chloride (HTAC) was obtained from Kokusan Kagaku Kogyo and recrystallized twice from acetone. HAuC14 was kindly supplied by Tanaka Kikinzoku Kogyo. The transition from sphericalto rodlike micelle of HTAC in aqueous solution was estimated by a static light scattering. A 3 cm3 portion of aqueous solution of HTAC and HAuc1.1was transferred to a rectangular quartz cell. UV irradiation was carried out with a 200 W low-pressure mercury lamp (Amax = 253.7 nm). W-vis spectra of the colloidal solutions were
i
1 500nm
Figure 1. TEM micrographs of gold particles obtained at various irradiation times: (a) 3;(b) 6; (c) 12;(d) 18; (e) 24 h. The concentrations of HTAC and HAuC14 were 30 w t % and 5 mmol dm-3, respectively.
0743-7463/95/24 11-3285$09.00/0 0 1995 American Chemical Society
3286 Langmuir, Vol. 11,No. 9, 1995
Letters 4.0 n
1
I
I
I
1
I
3.5
-
3.0
8
2.0
0
-II 2.5
W
c
-E 0
1.5
v)
1.0
a
0.5
4
0.0 L 200 300 400 500 600 700 800 I
I
Wavelength / nm
I
Figure 3. Change in W-vis spectra of HAuC14 aqueous solution (5 mmol dm-3) in the presence of HTAC (30 wt %) as a function of irradiation time: (a) 0; (b) 6; (c) 12; (d) 18; (e) 24 h. aqueous solutions (30wt %) containing various amounts of HAuC14(0.5-40 mmol dm-3) were prepared, followed by UV irradiation. Only spherical gold particles were obtained below concentrations of 1 mmol dm-3 HAuc14 with irradiation time up to 48 h. However, a t concentration of 5 mmol dm-3 HAuC4 or above, formation of rodlike gold particles was observed together with some spherical particles, and the length of rodlike gold particles was increased with increasing W irradiation time (Figure 1). In addition, at 48 h of irradiation the width of rodlike gold particles was increased with an increase of AuC14 concentration (Figure2). The rodlike gold particles were also I obtained by a 40 W low-pressure mercury lamp.8 In this case, longer W irradiation time was required to obtain rodlike particles. Figure 3 shows the change in UV-vis spectra of HTAC aqueous solutions containing 5 mmol dm-3 Hkiuc14 by W irradiation. It can be seen that an absorption band a t 320 nm, which is assigned to AuC14-, disappears, and a plasmon band at around 526-530 nm appears with UV irradiation time. Also, the absorbance of the plasmon band increased and reached a maximum, then decreased with a further increase of UV irradiation time. Furthermore, it is noteworthy that at more than 12 h of UV irradiation, the plasmon band shifts to a longer wavelength. Calculated absorption spectragfor spherical gold particles show that in a longitudinal resonance the band moves quickly to longer wavelengths as the aspect ratios of the spheroids increase, while the band remains near the wavelength of the spherical particle absorption band 1 . 5m ~ (1)(a) Henglein, A. J . Phys. Chem. 1993,97,5457. (b) Yonezawa,
Figure 2. TEM micrographs of gold particles obtained using various concentrationsof HAuC14 at 48 h of W irradiation: (a) 16; (b) 20; (c)40 mmol dm-3. The concentration of HTAC was 30 wt %.
recorded on a Hewlett-Packard 8452A diode array spectrophotometer. Electron micrographs of the metal colloids were taken with a Hitachi H-800 transmission electron microscope,operating at 200 kV.
Results and Discussion It is found from static light scattering that rodlike micelles of HTAC are formed a t concentrations exceeding 25 wt % of HTAC in aqueous solution, which is in good agreement with the literature.' In this study HTAC @
Abstract published in Advance ACS Abstracts, September 1,
1995.
Y.;Sato, T.; Ohno, M.; Hada, H. J. Chem. Soc., Faraday Trans. 1 1987, 83,1559. (c) Torigoe, IC;Esumi, K. Langmuir 1992,8,59.(d) Hirai, H.;Nakamura,Y.; Toshima,N. J. Macromol. Sei., PartA 1979,13,727. (e)Torigoe, K.;Nakajima, Y.; Esumi, K. J . Phys. Chem. 1993,97,8304. (0Cardenas-Trivino, G.;Klabunde, K. J.; Dale, B. E.Langmuir 1987, 3,986. (g) Kimura, IC; Bandow, S. Bull. Chem. SOC.J p n . 1983,56, 3578. (h) Esumi, IC;Tano, K.; Torigoe, IC; Meguro, K. Chem. Muter. 1990,2, 564. (i) Bradley, J. S.; Hill, E. W.; Klein, C.; Chaudret, B.; Duteil, A. Chem. Mater. 1993,5,254. (2) Archibald, D. D.; Mann, S. Nature 1993,364,430. (3)Sataka, IC;Kunitake, T. Chem. Lett. 1989,2159. (4) Baral, S.;Schoen, P. Chem. Mater. 1993,5,145. (5)Bunker, B.C.; Rieke, P. C.; Tarasevich, B. J.; Champbell, A. A.; Fryxell, G. E.; Graff, G. L.; Song, L.; Liu, J.; Virden, J. W.; McVay, G. L. Science 1994,264,48. (6)Ishizuka, H.;Tano, T.;Torigoe, K.; Esumi, K.; Meguro, K. Colloids Surf. 1992,63,337. (7) Rubingh, D. N.; Holland, P. M. Cationic Surfactant-Physical Chemistry; Marcel Dekker: New York, 1991. (8)Esumi, IC;Matsuhisa, K.; Torigoe, K. J. Jpn. Soc. ColourMater., in press. (9)Creighton, J. A.; Eadon, D. G. J. Chem. SOC.,Faraday Trans. 1991,87,3881.
Letters
Langmuir, Vol. 11, No. 9, 1995 3287
in the transverse resonance. This result suggests that the absorption over 600 nm is significantly reflected by rodlike gold particles having large aspect ratios. When an aqueous solution of HAuC14 is added to an aqueous solution of a cationic surfactant, AuC14- forms a 1:l complex with the cationic surfactant, resulting in a precipitate.l0 With a further increase of the surfactant concentration, the precipitation is redispersed and then the solution becomes clear. This situation can be understood by a view that AuC14- binds preferentially to the surface of the rodlike micelle because of the hydrophilic property of AuC14- and the neutralization of micellar charge. It seems likely that these binding AuC14- to the surface of the rodlike micelle is reduced by U V irradiation, resulting in formation of rodlike gold particles. According
to Kurihara et uZ.,ll photolysis of AuC14- provides gold
(10)Kaneko, S.; Torigoe, K.; Esumi, K. J . Jpn. SOC.Colour Mater. 1993,66, 14.
(11) Kurihara, K.;Kizling, J.;Stenius, P.; Fendler,J. H. J.Am.Chem. SOC.1983,105, 2574.
atoms, which coagulate to form colloidal gold in water and microemulsions. The particles in microemulsion are seen to be smaller, more uniform, and less agglomerated than those in water. In this study, at low concentrations ofHAuC14, a sufficient quantity ofAuCl4- ions is not bound properly to surface of the rodlike micelle so that only spherical gold particles are obtained by nucleation of partially bound AuC14- ions. Thus, the above results demonstrate that anisotropic gold particles are prepared using rodlike micelles of cationic surfactant as template. This method can also be applied to other metals. LA9503360
