Langmuir 1998, 14, 4945-4949
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Notes Preparation and Characterization of Gold and Silver Nanoparticles in Layered Laponite Suspensions Nariaki Aihara, Kanjiro Torigoe, and Kunio Esumi* Department of Applied Chemistry and Institute of Colloid and Interface Science, Science University of Tokyo, Kagurazaka, Shinjuku-ku, Tokyo 162-8601, Japan Received April 2, 1998. In Final Form: June 16, 1998
Introduction Many preparation methods of nanometallic particles have been proposed, such as photoreduction, chemical reduction in aqueous medium with or without polymers, chemical reduction in reverse micelles, or thermal decomposition in organic solvents.1 In addition, dry methods including chemical vapor deposition have also been examined.1 To obtain monodispersed nanometallic particles, protective colloids such as poly(vinylpyrrolidone)2,3 have often been required to control nucleation and prevent coagualtion of metallic particles. Although organic protective colloids have been used for many studies, there are very few reports on the preparation of metallic particles using inorganic protective colloids. For example, in optically transparent dispersions of imogolite fiber, stable and nanosized metallic nanoparticles such as platinum, gold, silver, and various bimetallic colloids have been prepared.4 Thus, this inorganic colloid may be useful for preparation of nanometallic particles. However, there are not sufficient data available for metallic particle-inorganic colloid systems. Laponite, which is a synthetic layered silicate, has been used as a model colloid, and its rheological properties have been investigated under various conditions.5,6 Generally, gelation in clay dispersions has been proposed to occur by edge-to-face as well as edge-to-edge associations in which surface charges on clay particles play an important role. It is also expected that charged sites on clay particles can provide cooperative binding with metal ions for effective protective colloids. It is therefore interesting to study the role of layered silicate for the preparation of metallic particles. There is a possibility that reduction of intercalated metallic ions provides anisotropic metallic particles in the restricted space. In this study, preparation of metallic particles in Laponite suspensions by addition of a reductant was examined. The particles obtained were characterized by using UV-vis spectrophotometry and transmission electron microscopy. * To whom correspondence may be addressed: e-mail, kuesumi@ ch.kagu.sut.ac.jp. (1) Bradley, J. S. Clusters and Colloids from Theory to Applications; Schmid, G., Ed.; VCH: Weinheim, 1994; Chapter 6 and references therein. (2) Hirai, H.; Nakao, Y.; Toshima, N. J. Macromol. Sci. Chem. 1979, A12, 1117. (3) Meguro, K.; Nakamura, Y.; Hayashi, Y.; Torizuka, M.; Esumi, K. Bull. Chem. Soc. Jpn. 1988, 61, 347. (4) Liz-Marzan, L. M.; Philipse, A. P. J. Phys. Chem. 1995, 99, 15120. (5) Ramsay, J. D. F. J . Colloid Interface Sci. 1986, 109, 44. (6) Willenbacher, N. J. Colloid Interface Sci. 1996, 182, 501.
Figure 1. (a) Change in the absorption spectra of [Au(NH3)4](NO3)3 aqueous solution in the presence of Laponite (2.11 g dm-3) as a function of elapsed time after addition of sodium borohydride. (b) Change in the absorption spectra of [Au(NH3)4](NO3)3 aqueous solution in the presence of various concentrations of Laponite, 2 h of elapsed time after addition of sodium borohydride.
Experimental Section Materials. Laponite was kindly supplied by Nippon Silica Kogyo Co., and its cationic exchange capacity and specific surface area determined by nitrogen adsorption were 0.72 mequiv/g and 224.9 m2/g, respectively. AgNO3 and HAuCl4 were kindly obtained from Tanaka Kikinzoku Kogyo K. K. [Au(NH3)4](NO3)3
S0743-7463(98)00370-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 07/28/1998
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Figure 2. TEM micrographs and size distribution of gold particles prepared in the presence of various concentrations of Laponite. was synthesized according to the literature.7 The water used was purified through a Milli-Q system. The other chemicals were of analytical grade. Methods and Measurements. A 2 cm3 aqueous solution of metal salts (1.0 mmol dm-3) was added into a 17 cm3 aqueous suspension of Laponite (0-15.0 g dm-3) with stirring, and this suspension was further stirred for 2 h. Then, to reduce metal (7) Mason, W. R.; Gray, H. B. J. Am. Chem. Soc. 1968, 90, 5723.
ions in the suspension, a 1 cm3 freshly prepared aqueous solution of sodium borohydride (10 mmol dm-3) was added to the suspension with stirring at room temperature. The final concentrations of Laponite, metal salts, and sodium borohydride were 0-12.7 g dm-3, 0.1 mmol dm-3, and 0.5 mmol dm-3, respectively. For preparation of bimetallic particles, the same procedure was employed as mentioned above except that the total concentration of two metal salts of various mixed ratios was 0.1 mmol dm-3 and that of Laponite was 2.11 g dm-3.
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UV-vis spectra of the metallic particle solutions were measured with a Hewlett-Packard 8452A diode array spectrophotometer at 1 cm of path length. Observation of metal colloids was performed with a Hitachi H-9000 NAR transmission electron microscope, operating at 100 or 200 kV. Particle size distributions of metal particles were obtained by counting sizes of about 200 particles.
Results and Discussion When the reduction of metal salts in the presence of Laponite was made by UV irradiation or by addition of reductants such as hydrazine and sodium borohydride, relatively small metallic particles were obtained by addition of sodium borohydride compared with that by UV irradiation and by addition of hydrazine. These results can be correlated with the reduction strength; sodium borohydride shows stronger reduction power than the others. Since the objectives of this study are to prepare ultrafine metallic particles, sodium borohydride was used as a reductant. At first, the reduction of [Au(NH3)4](NO3)3 in the presence of Laponite was traced as a function of elapsed time after addition of sodium borohydride. In Figure 1a, the aqueous suspension had a red color immediately after the addition of sodium borohydride in which a narrow absorption band was observed at around 520 nm. It should be noted that before the addition of sodium borohydride, the absorbance of the aqueous suspension is very low, indicating transparent Laponite suspensions. With increasing elapsed time, the absorbance at 520 nm increased and became constant after 30 min. On the other hand, in the absence of Laponite, gold particles were formed by addition of sodium borohydride, but they flocculated suddenly and then sedimented so that the supernatant became transparent. When the amount of Laponite increased in the aqueous suspension under a constant [Au(NH3)4](NO3)3 concentration, the absorption band shifted from 520 to 550 nm with increasing Laponite concentration (Figure 1b). A similar shift of the surface plasmon band has been observed for the inorganic fibers, imogolite-gold system.4 Thus, Laponite plays an important role in obtaining stable metallic particles as an inorganic protective colloid. Since Laponite is a cationic exchanger, [Au(NH3)4]3+ can adsorb on the basal plane of Laponite platelets by exchanging with Na+ ions. By addition of sodium borohydride, the nucleation would start at two sites: one is [Au(NH3)4]3+ adsorbed on the basal plane and the other is free [Au(NH3)4]3+ in aqueous suspension. If the concentration of free metal ions is quite large, the action of Laponite is less efficient so that large gold particles are formed. Also, there is a great possibility that gold particles formed in the bulk solution adsorb on Laponite particles, resulting in a high dispersion stability. It is interesting to note that when HAuCl4 is reduced by the addition of sodium borohydride in the presence of Laponite, only larger and unstable gold particles are obtained. Furthermore, since stable gold particles are only obtained when the concentration of [Au(NH3)4]3+ ions is below the cation exchange capacity of the Laponite, it is suggested that negatively charged sites on the surface of basal plane of Laponite operate predominatly as adsorption sites for [Au(NH3)4]3+ ions as well as protective ones. Figure 2 shows transmission electron microscopy (TEM) micrographs and size distribution of gold particles prepared in the presence of Laponite. It is clearly seen that the average size of gold particles decreases from 2.4 to 1.4 nm with an accompanying narrower size distribution with increasing concentration of Laponite. In addition, the relative standard deviation, which is obtained by
Figure 3. Change in the absorption spectra of AgNO3 aqueous solution in the presence of Laponite (2.11 g dm-3) as a function of elapsed time after addition of sodium borohydride.
dividing the standard deviation by the average size, decreased from 0.38 to 0.16 with increasing concentration of Laponite. These results imply that a highly monodispersity of gold particles obtained is due to control of nucleation and prevention of gold cluster-cluster mutual contact by Laponite. Figure 3 shows UV-visible spectra of AgNO3 aqueous solution in the presence of Laponite with elapsed time after addition of sodium borohydride. A distinct peak of silver particles near 400 nm developed with the elapsed time until 30 min. From 30 to 60 min of the elapsed time the band shifted to longer wavelength from 390 to 408 nm accompanying a band broadening. This change seems to be due to the adsorption of excess reducing agent or secondary products of the reducing agent on silver particles. The intensity of absorption band of silver particles near 400 nm increased with increasing concentration of Laponite. Figure 4 shows TEM micrographs and size distribution of silver particles; the average size decreased from 3.6 to 3.0 nm and the relative standard deviation from 0.25 to 0.16 with increasing concentration of Laponite. This result also demonstrates an effective protective action by Laponite. The mixtures of [Au(NH3)4](NO3)3 and AgNO3 aqueous solutions with various mixed ratios in the presence of Laponite (2.11 g dm-3) were also reduced by addition of sodium borohydride. Figure 5 shows the observed absorption spectra of gold/silver bimetallic particles after 2 h of elapsed time. With increasing molar ratio of [Au(NH3)4](NO3), the plasmon band remained as a single peak and shifted continuously from 408 to 534 nm. These spectra were completely different from those of mixtures of gold and silver particles in which the intensity of respective plasmon bands is proportional to the mixed ratios. There are two possibilities for formation of gold/ silver bimetallic particles: (i) core-shell particles, gold core with silver shell or vice versa; (ii) alloy particles. In the case of (i), the theoretical spectra of two types of coreshell particles obtained using the equations given by Bohren and Huffmann8 were different9,10 from the ob(8) Bohren, C. F.; Huffmann, D. F. Absorption and Scattering of Light by Small Particles; Wiley: New York, 1983; p 183. (9) Itakura, T.; Torigoe, K.; Esumi, K. Langmuir 1995, 11, 4129.
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Figure 4. TEM micrographs and size distribution of silver particles prepared in the presence of various concentrations of Laponite.
served spectra shown in Figure 5. Mulvaney et al.11 have prepared gold-coated silver particles and they compared the observed spectra with the calculated ones using the equations in the form given by Bohren and Huffmann.8 Baba et al.12 have derived a theoretical equation for alloy (10) Mulvaney, P. Langmuir 1996, 12, 788. (11) Mulvaney, P.; Giersig, M.; Henglein, A. J. Phys. Chem. 1993, 97, 7061.
particles without using dielectric constants in the following form;
1/λbimetallic ) [(xAu /λ2Au) + (1 - xAu)/ (λ2Ag)]1/2 where λ is the maximum wavelength of particles and x is the molar fraction. Maximum wavelengths of observed and calculated absorption spectra are plotted against the
Notes
Figure 5. Observed absorption spectra of various gold/silver bimetallic particles prepared in the presence of Laponite, 2 h of elapsed time after addition of sodium borohydride.
molar fraction of gold in Figure 6. Although the observed peak curve exhibited a longer wavelength deviation from the calculation peak one over the whole molar fraction of gold, the tendency of the former was very similar to that of the latter. This small deviation seems to be due to adsorption of excess reducing agent on metal particles. It is known that gold and silver are miscible in all proportions due to the almost identical lattice constants.13 Accord(12) Baba, K.; Okuno, T.; Miyagi, M. J. Opt. Soc. Am. B 1995, 12, 2372. (13) LeBlanc, M.; Erier, W. Ann. Phys. 1933, 16, 321.
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Figure 6. Change in the maximum wavelength of observed and calculated spectra plotted against molar fraction of gold.
ingly, the above results lead to a conclusion that bimetallic gold/silver particles prepared in this study are alloy particles rather than core-shell particles. The average sizes were almost constant at 1.7 nm despite various molar fractions of gold, and the relative standard deviation ranged between 0.23 and 0.34, which was larger than that of monometallic particles. It was also found that Laponite does not operate as a hard template for the preparation of anisotropic metallic particles. Thus, the above results indicate that Laponite acts as an effective protective colloid to obtain nanometallic particles. LA980370P
