Q Copyright 1995 American Chemical Society
DECEMBER 1995 VOLUME 11,NUMBER 12
Letters Novel Characterization of the Structure of Surfactants on Nanoscopic Metal Clusters by a Physicochemical Method Tetsu Yonezawa,tJ Toshihiro Tominaga,* and Naoki Toshima**t Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113, Japan, and Department of Applied Chemistry, Faculty of Engineering, Okayama University of Science, Ridai-cho, Okayama 700, Japan Received August 14, 1995. In Final Form: October 2, 1995@
This is the first report of the application of the Taylor dispersion method to determine the dimension of the cationic surfactants protecting the nanoscopic platinum clusters. The entire size of the surfactantprotected nanoscopic platinum cluster particles, and those of surfactant themselves, determined by the Taylor dispersion, and the size of naked particles, obtained by TEM, were compared. In the case of the surfactant with a relatively short hydrophobic alkyl chain, the Stokes' radii of clusters were shorter than those of surfactant micelles themselves,indicatingthat the surfactant layer is deformed by the hydrophobic interaction between the surface of metal clusters and the alkyl chain of surfactant molecules, which might be stronger than that between surfactant molecules themselves to form micelles. Nanoscopic metal clusters are now widely investigated because of great scientific interest and for the potential applications in the fields of physics, chemistry and biology as well as their interdisciplinary f i e l d ~ . l - Many ~ preparation methods have already been developed to have a fine nanoscopic metal clusters, e.g.,gas evaporation, and chemical preparation, involving reductive conversion of metal ions to metal atom^.^-^ Chemically prepared metal
* To whom correspondence should be addressed: FAX, +81-33812-9254; e-mail, toshimaG3tansei.cc.u-tokyo.ac.jp. + T h e University of Tokyo. t JSPS Research Fellow. Okayama University of Science. Abstract published in Aduance ACS Abstracts, November 15, 1995. (1)Clusters and Colloids; Schmid, G., Ed.; V C H Weinheim, 1994. (2) Physics and Chemistry of Finite Systems: From Clusters to Crystals; Jena, P., Khanna, S. N., Rao, B. K., Eds.; Kluwer Academic Pub.: Dordrecht, 1992; Vol. I and 11. (3) Mulvaney, P.; Grieser, F.; Meisel, D. Surf: Sci. Ser. 1991,38,303. (4) Torigoe, K.; Esumi, K. Langmuir 1992,8, 59.
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clusters are often used as the catalysts, e.g., for olefin hydrogenation,lOJ1nitrile hydration,12photoinduced electron transfer,13J4g7etc. The protective layers are used to prevent the aggregation of metal clusters to form precipitates, and furthermore, they can promote some special catalytic properties of metal clusters. For example, the ( 5 ) Touroude,R.; Girard,P.; Maire, G.; Kizling,J.; Boutonnet-Kizling, M.; Stenius, P. Colloid Surf: 1992, 67, 9. (6)Torigoe, K.; Esumi, K. Langmuir 1993,9, 1664. (7)Yonezawa, T.; Toshima, N. J. Mol. Catal. 1993,83, 167. ( 8 )Bonnemann,H.; Brijow, W.; B r i n k " , R.; Fretzen, R.; Joussen, T.; Koppler, R.; Korall, B.; Neiteler, P.; Richter, J.J.Mol. Cutal. 1994, 86, 129. (9) Bradley, J. S.; Hill, E. W.; Chaudret, B.; Duteil, A. Langmuir 1995, 11, 693. (10)Toshima,N.; Takahashi, T.Bull. Chem. Soc. Jpn. 1992,65,400. (ll)Toshima, N.; Yonezawa, T.; Kushihashi, K. J. Chem. Soc., Faraday Trans. 1993.89, 2537. (12)Toshima, N.; Wang, Y. Adu. Mater. 1994, 6, 245. (13)Toshima, N.; Takahashi, T.; Hirai, H. Chem. Lett. 1986, 1986, 35. (14)Toshima, N.; Takahashi, T.; Hirai, H. Chem. Lett. 1987,1987, 1031.
0743-7463/95/2411-4601$09.00/00 1995 American Chemical Society
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surfactant layers protecting the metal clusters can promote the stereoselectivity in hydrogenation1°J5and the reaction rate in photoinduced hydrogen generation.14J6 To reveal the functions of protective layers surrounding metal clusters, their thickness and structure should be investigated. For this purpose, we propose here to apply a physicochemical method, the so-called Taylor dispersion method,17Js to determination of the entire structure of nanoscopic clusters with the protecting layer on their surface. Recently, the combination of STM and high-resolution TEM has been used to determine the dimensions of palladium clusters stabilized by tetraalkylammonium salt.19 The difference between the diameter determined by STM and that determined by TEM allows estimation of the thickness of the protective alkylammonium salt layer. However, the samples for STM were dried, which may change the structure, and so the STM images may not present the real structure of protected particles in a solvent. The greatest advantage of the present Taylor dispersion method to STM for analyzing the entire cluster size involving the protective layer is that the entire size can be directly measured in the solution, where the surfactant molecules on the surface of the cluster particles rapidly exchange with those (‘free”in the solution. In addition, although the surfactant molecules can form ‘yree”micelles without metal clusters, only the surfactant micelles containing metal clusters can be measured in the present method because the diffusion was detected by the UVvis absorption of the metal cluster itself. Another advantage is that the size can be measured under any conditions,for example, under catalytic reaction conditions at relatively high temperature or with reaction substrates. Furthermore, when some structural changes of surfactant micelles occur, the STM measurement cannot distinguish between “free”micelles and micelle containing platinum clusters, but the Taylor dispersion method can do it. In the Taylor dispersion method, a small amount of solution is injected into a solvent flowing through a capillary tube. As the result of a combination ofconvection and molecular diffusion, the solute is dispersed in the tube. By selection of suitable experimental conditions, a Gaussian distribution in concentration is attained, and the diffusion coefficient, D, can be calculated from the equation
D = -r2t - In 2 - 0.2310r2t 3w
W2
where r is the radius of the tube, t is the residence time of the solute, and W is the width a t half-height of the peak detected. The Stokes’ radius ( R )can be calculated from the diffusion coefficient (D)as
R = kTI6nqD
(2)
where T is the absolute temperature of a tube, k is Boltzmann’sconstant, and 7 is the viscosity ofthe solvent containing surfactants, taken from a literature.20 ~
(15)Drognat Landre, P.; Richard, D.; Draye, M.; Gallezot, P.; Lemaire, M . J. Catal. 1994,147,214. (16)Toshima, N.; Takahashi, T.; Hirai, H. J. Mucromol. Sci., Chem. 1988,A25,669. (17)Taylor, G . I. Proc. R.SOC.London, Ser. A 1953,219,186. (18)Taylor, G. I. Proc. R.SOC.London, Ser. A 1954,225,473. (19)Reetz, M.T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995,267,367. (20) Giiveli, D. E. J. Chem. SOC.,Faraday Trans. 1 1981,78,1377.
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Figure 1. Platinum cluster concentration profile at the end ofthe capillarytube (0.5mm i.d. x 50 m) in the Taylor dispersion method: Detected by the absorption at 300 nm, [Ptl = 2.0 x mol dm-3, [CI~TAB] = 2.0 x 10-1mol dm-3;open circles, experimental data; dashed line, curve-fitted Gaussian line.
The measurements were performed with the apparatus below. In a coiled narrow and long capillary tube (0.5mm i.d. x 50 m) placed in a temperature-controlled bath at 25 f 0.02 “C, the solvent containing the same concentration of protective reagents is allowed to flow quite slowly but constantly with a HPLC pump. At the front end, the solute (colloidal solutions of metal clusters) is injected 8-functionally into the capillary tube with a injector for HPLC. By measurement of the concentration profile at the end of the stream in UV-vis absorption, the diffusion coefficient can be calculated. Some samples were measured twice for confirming the data reproducibility. The error of the obtained diffusioncoefficients of micelles themselves is ca. 2% (see ref 21) and that of the surfactant-protected Pt clusters can be considered as ca. 5% (see Figure 3). A typical representative time-concentration profile of the absorbance of colloidal dispersion of surfactantprotected platinum clusters and the curve-fitted Gaussian curve are collected in Figure 1. The observed curve can well fit to a Gaussian curve. More than five injections were repeated for each sample and the mean value was used for discussion. In the present study, surfactant-protected platinum clusters were prepared by photoreduction,1°a simple and mild reduction system, without any extra contamination with the reduction. Thus, no structural change of the surfactant micelles may occur by the products except platinum clusters. A typical TEM photograph of the resulting platinum clusters, taken by a Hitachi H-7000 at an acceleration voltage of 100 kV at a magnitude of 100 000, is shown in Figure 2. The diameter of each particle was determined from 3 times enlarged TEM photographs ( x 300000) using a magnifier. A histogram of the particle size and the average diameter were obtained on the basis of the measurements of a t least 300 particles in an arbitrarily chosen area. The error was ‘0.1 nm. This photograph and the diameter histogram indicate that platinum clusters have a spherical structure with a quite narrow size distribution. This observation allows us to determine the average Stokes’radii obtained by the Taylor dispersion method. Figure 3 collects some diffusion coefficientdata of “free” micelles of C,TAB (n-alkyl(C,)trimethylammoniumbromide) themselves (pyrene was used as a tracer) and the C,TAB micelles containing platinum clusters. Micelles of the surfactants with relatively shorter alkyl chains @e., CloTAB and C12TAB) containing platinum clusters have larger than or almost the same diffusion coefficients as
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Langrnuir, Vol. 11,No. 12, 1995 4603
0
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Diameter / nm
50 nm
Figure 2. TEM photograph and particle size distributionhistogram of dodecyltrimethylammonium bromide protected platinum clusters. [Pt] = 2.0 x mol dm-3, [CI~TABI= 1.0 x 10-1 mol dm-3. Table 1. Size of Cationic Surfactant-ProtectedPlatinum Cluster Measured by the Taylor Dimersion Method radius of Pt clustela sample CUTAB, 30 mM C14TAB,lOO mM CuTAB, 100 mM CloTAB, 100 mM a
(nm,Rmet) 0.8 0.9 0.9 1.0
v/Cp 0.980 1.117 1.044 0.960
“free”surfactant micelle D/10-lo Stokes’radiusb m2s-l (nm,Rmic) 0.59 0.64 0.85 1.20
3.8 3.1 2.5 1.9
surfactant-protected Pt cluster D/lO-1° Stokes’radius* calcd radiusC m2 s-1 (nm, Robs) (nm,Rcalc) 0.512 0.534 0.895 1.31
Measured from TEM photographs. Calculated from diffusioncoefficients(0). Calculated radius (&IC) + Pt cluster radius measured from TEM photographs (Rmet).
4.4 3.7 2.3
1.7
4.6 4.0 3.4 2.9
= Stokes’radius of surfactant
micelle (Rmic)
E
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u)
!E
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0.0 0.0
0.1
0.2
Surfactant Concentration/ mol dm
Figure 3. Diffusion coefficient of the cationic surfactant micelles and the micelle-protected platinum clusters as a function of surfactant concentration. Surfactant micelles: A, CloTAB; 0, C12TAB; 0, c14TAB; 0,cl6TAB. Surfactantprotected metal clusters: A, CloTAB; 0, C12TAB; +, &TAB; a, cl6TA.B.
those of “free”micelles of the corresponding surfactants themselves, while micelles of surfactants with relatively long alkyl chain (i.e., C14TAB and CISTAB) containing platinum clusters have smaller values than the corresponding “free” micelles. Then, Stokes’ radii were determined by eq 2 and collected in Table 1. The calculated radii of the surfactant micelles containing platinum clusters were obtained from the following equation
(3) where Rmic is the Stokes’ radius of a “pee” surfactant
micelle itself and Rmet is the average naked radius of the platinum clusters measured by TEM observation. Rcalc is an ideal radius of a surfactant micellecontaining platinum cluster or an entire particle in which the platinum cluster particle is simply located in a hydrophobic hole of surfactant micelles without any special structural change of the surfactant molecules from “free” micelles. The radii of platinum clusters, measured by a TEM technique, resemble each other, i.e., 0.8-1.0 nm. The Stokes’ radius of the micelles of the cationic surfactants increases with increasing the length of the alkyl chain of the surfactant. For this measurement, it is assumed that no deformation of the surfactant micelle is caused by the existence of pyrene in solution.2f In the case of CloTAB and CI~TAB,the Stokes’ radii of the entire particle of micelle containing platinum clusters (Robs), measured by the UV-vis spectral change of platinum clusters at 300 nm, are a little smaller than not only those of the calculated radii (Rclc) but also those of “free”micelles themselves (Rmic). In the case of C14TAB and CISTAB,&s)S are smaller than RcaJs. These results clearly indicate that the platinum particles are not located simply in the hydrophobic hole produced by the cationic surfactant micelles and that, therefore, the structure of surfactant molecules is changed by introducing platinum clusters. To stabilize a relatively large hydrophobic material, for example the platinum clusters, in water, surfactant molecules can be adsorbed on the surface of materials not only at the terminal carbon but also on the whole chain of the hydrophobic part. However, the surfactant molecules have a tendency to aggregate themselves by their hydrophobic interaction between the alkyl chains and to expand their hydrophilic parts into an aqueous phase. In the presence of a platinum cluster, the hydrophobic alkyl chain of the surfactant can interact both with alkyl chains of other surfactants and with the surface of the (21)Tominaga, T.; Nishinaka, M.J. Chem. Soc., Faraday Trans. 1993,89,3459.
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4604 Langmuir, Vol. 11, No. 12, 1995
Figure 4. Possible models of (a) CloTAB-or C12TAB-protected and (b) C14TAB- or ClsTAEbprotected platinum clusters.
platinum cluster. When the hydrophobic alkyl chain is relatively short, the interaction among alkyl chains should be relatively weak in comparison with the interactionwith the surface of the platinum particle. Then, the surfactant molecules may allow adsorption on the surface of the platinum cluster. In fact, the hydrophobic part of the nonionic surfactant molecules is adsorbed on the surface of the platinum particles.22 In contrast, as the length of the hydrophobic alkyl chain increases, the hydrophobic interaction among alkyl chains becomes strong. Accordingly, the surfactant molecules have a tendency to keep their original micellar structure even in the presence of the platinum cluster particle inside the micelle. On the basis of these considerations, the possible structure of cationic surfactant-stabilizing platinum clusters has been proposed. Figure 4 illustrates the models of the cluster stabilized by cationic surfactants with (22) Yonezawa, T.; Gotoh, Y.; Toshima, N. React. Polym. 1994,23, 43.
relatively short alkyl chains, Le., by CloTAB and C12TAB, and with relatively long alkyl chains, ie., by C14TAB and ClsTAB, respectively. In the case of CloTAB and &TAB, the surfactant molecules have a tendency to be adsorbed on the surface of the platinum cluster particles at their hydrophobic alkyl chains, which transforms from the normal micelle structure resulting in the model shown in Figure 4a. This result may require a n adequate modification of the concept that nanoscopic hydrophobic (metal) materials should be located simply in the hydrophobic core of surfactant micelles and, in addition, may provide an important structural concept for the specific properties of enzymes as well as metal cluster catalysts.
Acknowledgment. The authors express their sincere acknowledgment to Professors Masaru Nakahara (Kyoto University) and Hiroyasu Nomura (Nagoya University) for their advice on the physical chemistry of surfactants. The authors also express their thanks to Mr. Masayuki Nishinaka (Okayama University of Science) for his aid with the Taylor dispersion experiments and Drs. Koichi Adachi and Satoru Fukuda (The University of Tokyo) for their assistance with the TEM experiments. This work was supportedby Grant-in-Aid for ScientificResearch on Priority-Area-Research of "Nonequilibrium Process in Solutions" and other Grant-in-Aids for ScientificResearch from the Ministry of Education, Science, Sports and Culture, Japan. T.Y. acknowledges a Fellowship for Young Researchers, provided by the Japan Society for the Promotion of Science (JSPS) and a Grant-in-Aid for the Encouragement of Young Scientists from the Ministry of Education, Science, Sports and Culture, Japan. LA950682H
