Alkyl Xanthates: New Capping Agents for Metal Colloids. Capping of

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Alkyl Xanthates: New Capping Agents for Metal Colloids. Capping of Platinum Nanoparticles P. Sawant,† E. Kovalev,‡ J. T. Klug,‡ and S. Efrima*,†,§ Department of Chemistry, The Institutes for Applied Research, and The Ilse Katz Center for Meso and Nanoscale Science and Technology, Ben-Gurion University of the Negev, Beer Sheva, Israel 84105 Received October 24, 2000. In Final Form: February 20, 2001 We investigate the properties of long-chain alkyl xanthate-capped platinum nanoparticles. First an uncapped platinum colloid, 4.0 ( 1 nm in diameter, is produced in an aqueous phase, and then the capping agent is added. The capped particles are hydrophobic and easily transfer into organic solvents. They can be dried and repetitively transferred to various organic liquids in a reversible manner. Xanthate-capped platinum colloids are more stable than the analogous thiol-capped particles (and certainly noncapped particles) toward chemical corrosion (by oxygen in the presence of cyanide ion) and are also stable on heating. These colloids exhibit a characteristic sharp absorption in the range 450-470 nm in addition to the extinction in the UV. This absorption might be assigned tentatively to a weak d-d transition of platinum observed in this region also for the PtCl62- salt and for its adduct with xanthate, though it is absent for uncapped Pt particles. The transition for the capped colloid, however, is much stronger compared to that observed for the salts.

Introduction There is a growing interest in nanoparticles1,2 of metals such as cobalt, copper, gold, silver, and platinum,3,4 as well as in semiconductor nanoparticles (quantum dots).5 Metal and semiconductor nanoparticles are inherently unstable with respect to aggregation and subsequent precipitation from the suspension. Capping, i.e., coating * To whom correspondence should be addressed. E-mail: [email protected]. † Department of Chemistry. ‡ The Institutes for Applied Research. § The Ilse Katz Center for Meso and Nanoscale Science and Technology. (1) (a) Brust, L. Curr. Opin. Colloid Interface Sci. 1996, 2, 197. (b) Matijevic, E. Curr. Opin. Colloid Interface Sci. 1996, 1, 176. (c) Lewis, L. N. Chem. Rev. 1993, 93, 2693. (d) Schmid, G. Chem. Rev. 1992, 92, 1709. (2) (a) Haberland, H., Ed. Clusters of atoms and molecules; SpringerVerlag: New York, 1994. (b) Schmid, G., Ed. Clusters and colloids, from Theory to applications; VCH: New York, 1994. (c) Kreibig, U., Vollmer, M., Eds. Optical properties of metal clusters; Springer-Verlag: New York, 1995. (3) (a) Zhai, X.; Efrima, S. J. Phys. Chem. 1996, 100, 1779. (b) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (c) Burshtain, D.; Zeiri, L.; Efrima, S. Langmuir 1999, 15, 3050. (d) Wang, W.; Chen, X.; Efrima, S. J. Phys. Chem B 1999, 103, 7238. (4) (a) Horswell, S. L.; Kiely, C. J.; O’Neil, I. A.; Schiffrin, D. J. Am. Chem. Soc. 1999, 121, 5573. (b) Stietz, F.; Trager, F. Philos. Mag. B 1999, 79, 1281. (c) Tinkham, M. Philos. Mag. B 1999, 79, 1267. (d) Dassenoy, F.; Philippot, K.; Ely, T. O., Amiens, C.; Lecante, P.; Snoeck, C.; Mosset, A.; Casanov, M. J.; Chaudret, B. New J. Chem. 1998, 22, 703. (e) Seshadri, R.; Sen, R.; Rao, C. N. R. Chem. Phys. Lett. 1994, 231, 308. (f) Tanori, J.; Pileni M. P. Adv. Mater. 1995, 7, 862. (g) Suryanarayan, R.; Frey, C. A.; Buhro, W. E. J. Mater. Res. 1996, 11, 449. (h) Qi, J.; Ma, J.; Shen, J. J. Colloid Interface Sci. 1997, 186, 498. (i) Huang, H. H.; Xu, G. Q.; Ji, W. Langmuir 1997, 13, 172. (j) Giersig, M.; Ung, T.; LizMarzan, l. M.; Mulvaney, P. Adv. Mater. 1997, 9, 570. (k) Majumdar, D.; Kodas, T. T.; Glicksman, H. D. Adv. Mater. 1996, 8, 1020. (l) Mulvaney, P. Langmuir 1996, 12, 788. (m) Leff, D. V.; Brandt, L.; Heath, J. R. Langmuir 1996, 12, 4723. (n) Chen S.; Kimura, K. Langmuir 1999, 15, 1075. (5) (a) Kotov, N. A.; Dekany, I.; Fendler, J. H. J. Phys. Chem. 1995, 99, 13065. (b) Cusack, L.; Rizza, R.; Fitzmaurice, D. Angew. Chem. 1997, 36, 848. (c) Trindade, T.; O’Brien, P. Adv. Mater. 1996, 8, 161. (d) Ko, M.-J.; Birnboim, M.; Plawsky, J. Adv. Mater. 1997, 9, 909. (e) Pillai, V.; Kumar, P.; Multani, M. S. Colloids Surf. 1993, 80, 69. (f) Bakuzis, A. F.; Morais, P. C.; Tourinho, F. A. J. Magn. Reson., Ser. A 1996, 122, 100. (g) Wang, W.; Chen, X.; Efrima, S. Chem. Mater. 1999, 11, 1883.

these particles by a strongly adsorbed layer of (usually organic) molecules, is often being used in order to stabilize them (“thermodynamically”).6 Functionalizing the capping agents allows one to impart to the particles a variety of additional useful properties (molecular and biological recognition, special linear and nonlinear optical behavior, specific packing and assembly capabilities). Capped colloids find potential uses in science and in technological applications such as microelectronics,7 optoelectronics, chemical sensors and biosensors,8 lubrication,9 catalysis,10 colloid modified electrodes,11 contrast agents for electron and visible microscopy,12 colloidal immunoassays,13 and hollow polymer capsules.14,15 Alkanethiols16,17 and bisulfides18 are prime examples for such capping materials. Extensive studies by many groups demonstrated that they are almost universal capping agents, having a very strong affinity to metals (and metal ions), and form compact and ordered adsorption layers.19-22 They have found a wide-spread use as capping agents of metal nanoparticles.23,24 Oleic acid (or oleate) was also found to be useful as a capping agent.25 Its weaker binding to metal, compared to thiols, is disadvantageous for stabilization; however, the electric charge it can carry compensates for this deficiency. Furthermore, oleate was found to form both hydrosols and organosol, while alkanethiol-capped particles are hydrophobic and disperse well only in organic liquids. (6) Wang, Z. L.; Yin, J. S. Mater. Sci. Eng., A 2000, 286, 39. (7) Schon, G.; Simon, U. Colloid Polym. Sci. 1995, 273, 101, 202. (8) (a) Tang, F. Q.; Meng, X. W.; Chen, D.; Ran, J. G.; Zheng, C. Q. Sci. China, Ser B: Chem. 2000, 43, 268. (b) Himmelhaus, M.; Takei, H. Sens. Actuators, B 2000, 63, 24. (c) Bauer, G.; Pittner, F.; Schalkhammer, T. Mikrochim. Acta 1999, 131, 107. (9) Rapoport, L.; Bilik, Y.; Feldman, Y.; Homyonfer, M.; Cohen, S. R.; Tene, R. Nature 1997, 387, 791. (10) Spiro, M.; de Jesus, D. M. Langmuir 2000, 16, 2464. (11) Dorn, A.; Katz, E.; Willner, I. Langmuir 1995, 11, 1313. (12) Beesley, J. E. Colloidal Gold: A new perspective for cytochemical marking; Royal microscopial Society Microscopy Handbook 17; Oxford University Press: Oxford, U.K., 1989. (13) Van Erp, R.; Gribnau, T. C. J.; Van Sommeren, A. P. G.; Bloemers, H. P. J. Immunoassay 1991, 12, 425. (14) Kuther, J.; Seshadri, R.; Nelles, G.; Assenmacher, W.; Butt, H.J.; Mader, W.; Tremel, W. Chem. Mater. 1999, 11, 1317.

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In this report we introduce another type of potential capping agent, alklylxanthates (alkyldithiocarbonates). Xanthates are heavily used in the mining and ore extraction industry as collectors in froth flotation.26,27 The sulfur-containing headgroup has a strong affinity to metals and metal ions, similar to thiols. Here we investigate the effect of capping by potassium salts of hexadecyl- and octylxanthates on the properties and on the stability of platinum colloids. The hydrosols and the organosols are characterized by UV-vis absorption spectroscopy, by transmission electron microscopy (TEM), and by various stability tests (storage, heat, and chemical corrosion). We find that xanthates can serve as efficient capping agents producing transferable colloids with considerable resistance to heat and chemical oxidation. Experimental Section Hexdecylxanthate (C16 xanthate) and octylxanthate (C8 xanthate) potassium salts are prepared by heating 0.04 mol of hexadecanol (or octanol) to 150 °C with an equimolar amount of KOH. The melt is suspended with mechanical stirring in 25 mL of toluene at 100 °C, to which 0.058 mol of carbon disulfide is added dropwise at room temperature with cooling and vigorous stirring. The thick suspension formed is stirred for an hour and then diluted with 100 mL of petroleum ether and stirred for additional 2 h. The product is filtered on a sintered glass funnel, washed with petroleum ether, and dried, yielding 95% of crude xanthate. The product is purified by washing with 20 mL of cold water, followed by vacuum-drying and then washing again with petroleum ether. H2PtCl6‚6H2O (5% w/v) is obtained from BDH, methanol (AR), dichloromethane (AR), and chloroform (AR), are purchased from Frutarom, dodecane (99+% pure) and NaBH4 (99% pure) is obtained from Aldrich, and isooctane (HPLC grade) is obtained from Fluka. All reagents are used without further purification. Water is of 18 MΩ cm resistivity, from a Barnsted E-pure water purifier. UV-visible spectra are recorded with a 8452A HP diode array spectrophotometer in the range 190-820 nm, with a resolution of 2 nm. Transmission electron microscopy is carried out using a JEOL 2010 HR-TEM, equipped with a Gatan multiscan CCD camera. (15) (a) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (b) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; Blain-House, A.; Edeki, E. M.; Feldause, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518. (c) Wu, M.; O’Neill, S. A.; Brousseau L. C.; McConnell, W. P.; Shultz, D. A.; Linderman, R. J.; Feldheim, D. L. J. Chem. Soc., Chem. Commun. 2000, 1001. (16) (a) Jonson, S. R.; Evans, S. D.; Mahon, S. W.; Ulman, A. Langmuir 1997, 13, 51. (b) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (c) Brust, M.; Fink, J.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1995, 1655. (d) Sarathi, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (17) Peterlinz, K. A.; Georgiadis, R. J. Phys. Chem. B 1997, 101, 8041; Langmuir 1996, 12, 4731. (18) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (19) (a) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5440. (b) Bumm, L. A.; Arnold, J. J.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Phys. Chem B 1999, 103, 8122. (c) Bumm, L. A.; Arnold, J. J.; Charles, L. F.; Dunbar, T. D.; Allara, D. L.; Weiss, P. S. J. Am. Chem. Soc. 1999, 121, 8017. (d) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (20) Garg, N.; Jonathan, M.; Friedman, M.; Lee, T. R. Langmuir 2000, 16, 4266. (21) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Che. Soc. 1989, 111, 321. (b) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (22) Bain, C. D.; Whitesides G. M. Science 1988, 240, 62. (23) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edward, P. P. Chem. Soc. Rev. 2000, 29, 27. (24) Rao, C. N. R.; Kulkarni, G. U.; Govindaraj, A.; Satishkumar, B. C.; Thomas, P. J. Pure Appl. Chem. 2000, 72, 21. (25) Huang, W.; Huang, Y. Spectrosc. Spectral Anal. 2000, 20, 449. (26) Woods, R.; Hope, G. A. Colloids Surf., A 1999, 146, 63. (27) (a) Woods, R.; Hope, G. A.; Brown, G. M. Colloids Surf., A 1998, 137, 329. (b) Buckley, A. N.; Park, T. J. Vassallo, A. M., Woods, R. Int. J. Miner. Process. 1997, 51, 303.

Sawant et al. Energy-dispersive X-ray spectroscopy (EDS) is performed using an Oxford linked ISIS 6498 (version 1.4). A typical accelerating voltage of 200 kV is used with magnifications of 100000× for obtaining size distributions, and 600000× for single-particle analysis. A drop of a suspension of the colloid in dichloromethane is placed on a Lacey carbon-Formvar, 300 mesh copper grid (Ted Pella 01883-F) and allowed to evaporate. C16 xanthate is introduced using 0.02 M xanthate in methanol solutions. Fresh aqueous solutions of C8 xanthate were used. “Native” colloidal platinum sols are prepared as described elsewhere for silver.3b Briefly, a solution of 1 mM H2PtCl6‚6H2O is added dropwise with stirring to an equivolume of freshly prepared ice-cold 4 mM NaBH4 aqueous solution. The black Pt sol forms immediately. The Pt particles are found by electrophoresis to be negatively charged with 14.7 ( 0.8 electrons/ particle. Capping is carried out by adding methanol or water solutions of the capping agent to aqueous suspensions of a preformed native Pt colloid. The transfer of a xanthate-capped Pt colloids to an organic media is obtained spontaneously, by stirring together equal volumes of the aqueous capped colloid solution and the organic liquid. It can be aided by the addition of 1.16 M NaH2PO4.

Results and Dicussion C16 Xanthate-Capped Pt Colloids. Figure 1 shows UV-visible spectra of 5 mM H2PtCl6 in water, of H2PtCl6 and C16 xanthate in methanol at a xanthate/Pt molar ratio of 11.2, and of a xanthate-capped colloid in a water/ methanol (0.79:0.21 v:v ratio) solution. With the addition of alcoholic xanthate to the “native” Pt colloid, a peak around 470 nm develops, which saturates at a xanthate/ Pt molar ratio of 11.2 as shown in the inset to Figure 1. The color of the suspension changes from black to green. This peak is absent in the spectrum of the uncapped Pt sol or that of xanthate. We assign the spectra from a comparison between these spectra (and other spectra we measure for the capped-colloid in various organic liquids). The absorption appearing in the region 450-476 nm of a Pt xanthate-capped colloid has an apparent molar extinction coefficient (in terms of total platinum concentration) in the range of 300 (as observed in methanol) and 3000 (in water). The difference in the apparent extinction coefficient probably results from a loss of material upon the transfer to methanol. This peak is also present in the spectra of the C8-capped colloids (in organic liquids). It is not associated with xanthate by itself, having maxima at ∼300 and 350 nm. H2PtCl6 in water has a spectral feature at 463 nm. However, it is much weaker (Pt ) 24 calculated in terms of platinum concentration) and probably originates from a d-d transition in the octahedral complex. In the colloid we do not expect that any significant amount of H2PtCl6 remains after the reduction. Even if present, H2PtCl6 would not transfer into the organic liquids, all of which exhibit the strong 450-476 nm absorption with the xanthate-capped colloid. Thus the H2PtCl6 solution species cannot be the direct origin of the 450-470 nm transition observed for the colloid. A solution of C16 xanthate and H2PtCl6 (with the same molar ratio as in the colloidal suspension) also shows a band at 471 nm, but it too is too weak ( ) 108), and we again associate it with a d-d transition. The intensity is higher than that for the hexachloro complex probably because of the lower symmetry of the complex with xanthate. The ∼470 nm feature cannot be a platinum plasmon peak. Such a peak has not been observed for any other Pt colloids, including thiol-capped colloids.28 Its λmax does not shift with a change of solvents in the direction expected (28) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 4314.

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Figure 1. UV-visible extinction spectra of 5 mM H2PtCl6 in water, H2PtCl6-C16 xanthate in 53% methanol (0.4 mM Pt and 3.9 mM C16 xanthate), and a C16 xanthate-capped Pt colloid (0.4 mM Pt and 3.9 mM C16 xanthate), right-hand side ordinate. Inset: the 425-525 nm UV-vis spectrum of a C16 xanthate-capped platinum colloid with varying concentrations of C16 xanthate in methanol. The xanthate/Pt molar ratios are (0) 0, (1) 1.6, (2) 3.08, (3) 4.46, (4) 6.4, (5) 8.12, (6) 9.68, and (7) 11.23.

Figure 2. UV-visible extinction spectrum of an oleate-capped Pt colloid after treatment with C16 xanthate.

of a plasmon excitation (see below), and native platinum colloids only exhibit a threshold absorption in the UV. It is implausible that a well-defined and sharp plasmon band can be pulled out of the UV broad plasmon excitation of platinum, a shift of over 150 nm. Thus, we conclude, that this feature is associated with a d-d transition of a Ptxanthate adduct, adsorbed on the Pt colloids, and enhanced (∼30×) by the particle. The C16 xanthate-capped Pt colloid tends to gradually precipitate out of solution and cannot be redispersed in water by either shaking or sonication. This is probably a result of the particles becoming highly hydrophobic, apparently because of the long alkyl chain of the capping agent. This conclusion is confirmed by the ease of transferring the colloid into organic solvents, described in a later part of the paper, as well as by changing other system parameters, as described below. We attempted the replacement of an oleate capping agent (well-known for silver3b) by xanthate. An oleatecapped Pt colloid is prepared similarly to the “native” sol, except that sodium oleate (0.125 mM) is added to the NaBH4 solution. Then methanolic C16 xanthate is added (at a molar ratio xanthate/Pt ) 6.4). Figure 2 shows that the 470-nm peak appears, indicating a larger affinity of xanthate to platinum compared to oleate, as expected. Unlike C16 xanthate-capped Pt, C16 xanthate/oleatecapped Pt does not precipitate out and forms a stable sol. This suggests that C16 xanthate only partially replaces

Figure 3. UV-visible extinction spectra of C16 xanthatecapped Pt colloid in various solvents: dodecane (boxes); chloroform (triangles); methanol (dashed lines); dichloromethane (circles); water (thin solid line).

oleate. It is possible that the oleate remains only in a second layer with its hydrocarbon chain “dissolved” in the hydrophobic, alkyl-chain region of the xanthates, which are anchored directly to the metal particle via their headgroup. This would explain the higher solubility of the xanthate/oleate colloid in water. A C8 xanthate-capped colloid is more stable in suspension than the C16 xanthate capped Pt but also eventually separates out, with the solution becoming progressively turbid (within 30 min). Phase Transfer of Xanthate-Capped Pt Colloids to Organic Solvents. Equal volumes of xanthate-capped Pt hydrosols are stirred or shaken vigorously with either dodecane or dichloromethane or chloroform for 10-15 min. The colloid spontaneously transfers into the organic phase and forms stable organosols. Adding Na2PO4 helps. A blue shift of ∼20-30 nm in the xanthate-Pt colloid long wavelength peak is observed in methanol, chloroform, and dichloromethane, compared to water, while for dodecane it is a little smaller (Figure 3). These shifts cannot result from a simple refractive index effect on plasmon excitations.29 A plasmon in the organic liquids is expected to be red-shifted compared to water, and methanol should show a smaller shift, while dodecane should show a larger

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Figure 5. UV-visible 400-500 nm region extinction spectra of a C16 xanthate-capped Pt colloid transferred from dichloromethane (triangles) to chloroform (boxes) and finally to isooctane (circles). The spectra are baseline adjusted and arbitrarily shifted to avoid overlapping views, 0.5 mM Pt.

Figure 4. A TEM micrograph of a C16 xanthate-capped Pt colloid.

shift than those observed. Anyway, as we already discussed, a plasmon excitation for Pt at ∼470 nm is not known. Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. Figure 4 shows a typical electron micrograph of a C16 xanthate-capped Pt colloid (deposited on the grid from dichloromethane). The mean particle diameter is 4.0 ( 2.2 nm. EDS analysis gives Pt/S weight percentages of 93/7 (corresponding to a Pt/S molar ratio of ∼2.3). The Pt/S molar ratio for completely coated 4 nm (diameter) particles, with an area of 0.2 nm2 per adsorbed xanthate, should be ∼6. This is larger than the ratio we actually observe, though it is within the same order of magnitude. This discrepancy in the molar ratio cannot be explained by the particle polydispersity, as this ratio is linearly dependent on the particle size, and would average out at the mean size. The extra sulfur might come from deposition of some xanthate together with xanthatecapped colloids, despite our washing of the deposit. In addition, as some of the platinum and sulfur peaks overlap, this estimate can be in considerable error, especially regarding the sulfur content. X-ray photoelectron spectroscopy (XPS) of deposits of the colloids (on indium) shows an even lower Pt/S molar ratio, 0.6-0.7, as expected from the higher surface sensitivity of XPS. Interestingly, XPS shows that about 20% of the paltinum is in a higher oxidation state (as expected for surface atoms subjected to electrostatic forces from the nearby xanthate anions). This value is plausible in light of control XPS measurements of xanthate-coated platinum substrates that show that ∼10% platinum atoms are in a higher oxidation state. The difference is accounted for by the semi-infinite nature of the Pt substrate compared to the spherical, nanosized particles. Some of the particle micrographs exhibit clear fringes within the particles. The fast Fourier transform (FFT) analysis is not compatible with a bulk face-center cubic (fcc) crystal structure. This finding requires a more detailed study and will be discussed elsewhere. (29) Barber, P. W., Chang, R. K., Eds. Optical Effects Associated With Small Particles; Advanced Series in Applied Physics; Ramseshan, S., Ed.; 1990.

Stability of Xanthate-Capped Platinum Colloids. The stability on storage or on heating was evaluated on the basis of the precipitation of the colloid or changes in the intensity of its spectrum. At times “native” Pt hydrosols aggregate and settle out of solution within a few hours. However, they are reversibly redispersable. C16 xanthate-capped Pt precipitates out of an aqueous solution (though boiling redisperses it) but is stable when dispersed in organic liquids. Aqueous solutions of C8 xanthate-capped Pt do not settle for more than 2 months, though the suspension becomes turbid in ∼30 min. Xanthate/oleate-capped Pt in water is stable, the peak intensity at 474 nm decreases to 50% only after 3 months. C16 xanthate itself decomposes in water in a few months. This can be the reason for the eventual instability of the xanthate or xanthate/oleate colloidal suspensions. In fact, in the latter case, no precipitate forms, even when the spectrum shows a decrease of the 470 nm peak. Probably oleate replaces the decomposed xanthate, reverting to a Pt/oleate colloid. In dodecane and chloroform C16 xanthate platinum colloids are stable for months, with the 450 nm peak intensity decreasing by 22-24% after a month. C8 xanthate-capped Pt colloids behave similarly. At 50 °C, “native” Pt colloids irreversibly precipitate after 1 h. C16 and C8 thiol-capped platinum sols require 1 h at 100 °C to aggregate and precipitate. In contrast, aqueous dispersions of C16 xanthate-capped Pt actually become stable after boiling (for 1 h), and the peak at 470 nm is pronounced. The colloid can still be transferred to other solvents and seems not to have lost any of its properties. In dodecane the colloids start to break down and precipitate only at 150 °C. C8 xanthate-capped colloids are stable at 100 °C for 1 h, when a white precipitate starts to form, indicating a destruction of the xanthate capping. Like C16 xanthate, aqueous solutions of C8 xanthate-capped Pt at 50 °C exhibit the 450-60 nm spectral feature. The stability of the colloids toward chemical corrosion is studied by introducing cyanide into suspensions open to air. Addition of 0.1 mL of 0.1 M KCN aqueous solution to 2 mL of colloidal hydrosols (0.5 mM Pt) results in a half-life of 10 min for the uncapped colloid, 15 min for C8 xanthate-capped Pt ([xanthate]/[Pt] ) 4.8), and 30 min for C16 xanthate-capped colloids ([xanthate]/[Pt] ) 4.8). Thus xanthate capping provides some measure of chemical protection, at least against the cyanide-induced corrosion of the metal. At a lower xanthate/Pt ratio of 2.4 and a higher cyanide concentration (15 mM compared to 5 mM) the colloid corrodes much faster. Uncapped colloids react within a few seconds, C8 thiol-capped Pt reacts in 30 s, while C8

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isooctane, in turn. Similarly C8-capped colloids can be repetitively transferred from one solvent to another, as demonstrated in Figure 6. Conclusions

Figure 6. UV-visible 400-500 nm region extinction spectra of a C8 xanthate-capped Pt colloid transferred from dichloromethane (triangles) to chloroform (boxes) and to isooctane (circles). The spectra are baseline adjusted and arbitrarily shifted to avoid overlapping views, 1.0 mM Pt.

xanthate-capped Pt requires 50 s. C16 thiol-capped colloids cannot be disperesed in water, so we did not measure their corrosion resistance. Xanthate-capped platinum colloids are stable also with respect to the evaporation of the liquid. After transfer from water to dichloromethane the solvent can be evaporated completely (even at 50 °C), leaving a dry colloidal powder. This powder can be redispersed in dichloromethane or in other solvents without any deterioration. Figure 5 shows the spectra obtained when a dichloromethane C16-capped colloid is subjected to cycles of evaporation and transfer to chloroform, and then to

We produced 4 nm platinum particles and capped them with C16 and C8 alkyl xanthates. The colloids easily transfer to organic liquids and can be dried and then redispersed repetitively and reversibly. The xanthatecapped platinum colloids exhibit good resistance toward chemical attack by oxygen (induced in the presence of cyanide), as well as considerable thermal stability. A platinum-xanthate optical transition is observed and found to be significantly enhanced compared to transitions in the same wavelength range observed for the corresponding platinum salts. Xanthates are promising materilas for serving as efficient capping agents of metal (and perhaps semiconductor) colloids. Further studies of other metals, as well as studies of the effect of xanthates on the nucleation and production of metal nanoparticles, and investigations of the packing of these molecules on the surface are under way. Acknowledgment. We acknowledge the support of the Infrastructure program of the Israel Ministry of Science. LA0014961