Langmuir 1986,2, 259-260
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Nonaqueous Colloidal Gold. Clustering of Metal Atoms in Organic Media. 12 Shaw-Tao Lin, Matthew T. Franklin, and Kenneth J. Klabundeh Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received December 3, 1985 Gold vapor (atoms) was deposited at low temperature with excess organic solvents: acetone, ethanol, dimethylformamide,dimethyl sulfoxide, tetrahydrofuran, triethylamine,diethyl ether, toluene, and pentane. Stable colloidal solutions, usually purple in color, were formed upon warmup with the more polar solvents. Particle sizes in acetone ranged from 10 to 30 nm and these appeared spherical under the electron microscope (TEM). However, solution-phase measurements by photon correlation spectroscopy and UV-vis plasmon absorption indicated larger particles, suggesting weak particle agglomeration. Colloidal gold in aqueous solution (gold sols) is wellknown and was prepared by Michael Faraday as early at 1857.' After the classic work of Zsigmondy2and others3 a reasonably good understanding of these sols exists, and they have found uses in ceramics, medicine, and other areas.* Gold sols are usually prepared by reduction of halide salts, such as HAuCle3 The gold particles and the byproducts of the reduction remain in the aqueous solution. The colloidal particles are stabilized because of the charged double layer which is dependent on adsorbed anions, usually chloride, that remain in solution after the reduction p r o ~ e s s . ~ The preparation of colloidal gold in nonaqueous media has received less attention. Kimura and Bandow5 have successfully prepared colloidal gold by clustering gold atoms in the gas phase and exposing these particles to ethanol solvent. Fendler and co-worked and Blumencron' have reported on gold colloids in water-in-oil and waterpolymer systems, respectively. However, in general problems with organic solvents have been encountered, presumably due to lack of stabilization of the colloids in the absence of adsorbed anions and the lower dielectric constants of these solvents. We have approached this problem in a different way and considered that dispersion of single atoms of gold in excess cold solvent might lead to atom clustering moderated by solvation. Thus, we anticipated atom-atom recombination but that cluster growth would eventually be inhibited by strongly bound solvent molecules. The resulting solution would consist only of colloids and solvent; no byproducts of gold salt reduction would be present. There is some precedent for this type of metal cluster growth in cold solvents from our earlier work on the preparation of pseudoorganometallic particles of Ni and other metals, which have proven to be unique catalytic materials? We report herein our initial studies of colloidal solutions of (1)Faraday, M. Philos. Trans. R. SOC. London 1857,147,145. (2)Zsigmondy, R.;Thiessen, P. A. "Das Kolloide Gold"; Barth Leipzig, 1925;p 59. Also see: Turkevich, J.; Stevenson, P. C.; Hillier, J. J.Phys. Chem. 1953,57,670. (3)Jirgensons, B.;Straumanis, M. E. "Colloid Chemistry";Macmillan: New York, 1962;pp 119,130,258,306. (4)Frens, G. Naturephys. Sci. 1973,241,20-22. Short, 0. A.;Weaver, R. V. Gen. Pat. 1973,2, 154,093. Golla, M.; Lutz, K. Ger. Offen. 1975, 2, 329,352. Muchlpfordt, A., Experimenta 1982,38(9),1127-1128. (5)Kimura, K.; Bandow, S. J.Chem. SOC.Jpn. 1983,56,3578-3584. (6)Kurihara et al. (Kurihara, K.; Kizling, J.; Stenilus, P.;Fendler, J. H. J.Am. Chem. SOC.1983,105, 2574-2579) have reported on gold colloids in water-in-oil emulsions. (7)Water-polymer-solvent systems have also been reported Ledwith, A. Chem. Ind. (London) 1956,1310.Blumencron, W. Med..Monatsschr. 1957,11, 89. (8)Klabunde, K. J.; Tanaka, Y. J. Mol. Catal. 1983, 21, 57-79. Matauo, K.; Klabunde, K. J. J. Org. Chem. 1982,47,843-848.
0743-7463/86/2402-0259$01.50/0
Codeposition of Gold Atoms and Solvent. Gold, usually about 0.5 g, was vaporized under vacuum and deposited on the inside walls (cooled to -196 "C) of the vacuum chamberlo simultaneously with vapor of organic solvent, usually about 50 g. The frozen matrices were colored (Table I) and the color changed upon warmup and melting. During the warmup process the Au atoms are very mobile, and reactions with the solvent can complete with the clustering process. As Au-Au bonds are formed, each resultant cluster may have different reactivity with the solvent.6 In other words, atoms have different reactivities than small clusters, and small clusters have different reactivities than larger clusters. Usually the bare atoms would have the highest chemical reactivity, but this is not always the case.'l As the clusters grow and continue to move through the solvent, reacting or being solvated, eventually a stage must be reached where it is energetically costly to move two clusters together and desolvate them to allow further Au-Au bond formation. Therefore, the colloidal particles remain solvated and suspended in solution by Brownian motion. Au atoms
+
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Table I summarizes a series of experiments with different solvents. Note that polar protic or aprotic solvents yielded stable colloids but that nonpolar organics and water yielded large gold particles that precipitated. Acetone colloidal solutions have been investigated most intensively. It was noted in this system, as well as in ethanol, that the successful preparation of stable colloids is dependent upon the rate of matrix warmup. Rapid 25 "C in 30 min yielded large warming from -196 particles that precipitated. Slow warmup over a 2-h period
-
(9)Lin, S.T.;Klabunde, K. J. 58th Colloid and Surface Science Symposium, American Chemical Society, June 10-13, 1984,paper 391. (10)Klabunde, K. J.;Timms, P. L., Skell, P. S.; Ittel, S. Inorg. Synth. 1979,19, 59-61. (11)Imizu, Y.; Klabunde, K. J. Inorg. Chem. 1984,23, 3602-3605.
0 1986 American Chemical Society
260 Langmuir, Vol. 2, No. 2, 1986
solvent aeetone MesO ethanol THF triethylamine diethyl ether toluene pentane water
Letters
Table 1. Gold Colloidal Solutions Prepared by Metal Vaporization into Cold Organic Solvents gold conen: M matrix color colloidal soh color UV-vis' absorptns. nm particle size, nm 0.038 ourole 706.572. 317 0.0028 0.040 0.0078 0.0057 0.0033 0.036 0.040 0.0061
yellow purple-red red pink orange blue brown brawn
purple dark purple purple purple precipitated precipitated precipitated precipitated
530 595,559
570 (65)d 230 (70)d
.For aeetone and ethanol, about 1% gold by weight. Molarities calculated from amounts of metal evaporated and solvent added. 'The phmon absorptions were 572 (acetone),554 (DMF), 530 (ethanol), and 559 (THF). cAversgestaken from photon correlation s p e c t m p y (PCS)." dNumbers in parentheses were estimated from UV-vis plasmon absorptions.
W Figure 2. Weakly bound spherical particles.
Figure I. Transmission electron micrograph of gold colloidal particles from acetone (20 nm average particle size).
always yielded stable colloidal solutions. The implication of this finding is that finite time is needed for solvation/stabhtion to OCCUT. Whether this is due to ordering of solvent molecules or to chemical reactions with the solvent (bond breaking) is a subject for further investigation. Similar findings in the Ni atom-pentane system proved to be due to low-temperature reactions of pentane with small growing clustera of Ni.'* Figure 1 shows a transmission electron micrograph of gold particles from acetone.I3 Note the mostly spherical particle shapes. Sizes of 10-30 nm can be estimated from (12) Davis. S.C.: Sevenon. S.: Klabunde. K. J. d. Am. Chem. Sac. 1981, 103.30244029. (13) The sample was prrpared by evaporation of amtone alloaing the
pld particles to be mounted in a carbon Iilm on a copper grid.
the micrograph, and there appears to be agglomeration of some of the spheres. Table I shows particle sizes determined in solution by photon correlation spectroscopy (PCS)I4and by UV-vis plasmon absorption spectroscopy (PAS).3J5 PCS yielded values of 19&570 nm while PAS yielded 65-75 nm. If it is assumed that weak particle agglomeration occurs in solution, these differing values can be reconciled. Thus, the TEM (Figure 1) shows the true size distribution. Agglomeration in solution would lead to the larger PCS values. However, PAS would be less sensitive to agglomeration since plasmon absorption is a collective effect dependent on atoms being tightly bound together.15 Therefore plasmon absorptions in a particle made up of weakly agglomerated spheres would still yield smaller size values than PCS. Figure 2 shows a representation of such an agglomerated particle. Further work directed toward understanding the colloid solvation/stabilization mechanism, controlling particle sizes, and other metals is under way. Acknowledgment. The support of the National Science Foundation and the Office of Naval Research is acknowledged with gratitude. We also thank Professor Chris Sorenson and Dr. Thomas Taylor for assistance with photon correlation spectroscopy measurements. (14) Berne, B. J.: Pereora, R. J.; 'Dynamic Light Scattering"; Wiley: New York, 1976. (15) Mie. C. Ann phyr. (Leiprig)1508,25,377. Ferrell, T.L.:CaUcon.
T.A,; Warmsck. R. J. Am. Sci., 1985, 73,344.
