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Xanthate Capping of Silver, Copper, and Gold Colloids O. Tzhayik,† P. Sawant,†,§ S. Efrima,*,† E. Kovalev,‡ and J. T. Klug‡ Department of Chemistry, and The Ilse Katz Center for Meso and Nanoscale Science and Technology, The Institutes for Applied Research, Ben-Gurion University, Beer Sheva, Israel 84105 Received October 29, 2001. In Final Form: January 31, 2002 We cap silver, copper, and gold nanocolloids with long-chain alkylxanthates. In comparison to thiol capping, the particles are less hydrophobic and are stable in aqueous solutions for over a month, though being less stable than the corresponding oleate-capped particles. They can be transferred into relatively polar organic media (such as dichloromethane) but not into nonpolar solvents (such as dodecane). Unlike noncapped, thiol-capped, and oleate-capped colloids, they are temperature sensitive, as a result of the thermal decomposition of the xanthate molecule itself, and can be applied as thermally decomposable colloids. They demonstrate exceptional resistivity toward cyanide-induced corrosion by oxygen, when compared to noncapped or even to oleate-capped colloids. Xanthate capping enables the production of stable copper nanocolloids in aqueous solution under ambient conditions.
Introduction Metal, metal oxide, and semiconductor colloids are inherently unstable with regard to aggregation, which eventually leads to precipitation. Capping these nanoparticles with a monolayer of judiciously selected molecules is a convenient way to stabilize them, not only with respect to aggregation but also against corrosive chemical reactions. In addition, these capping layers can modify the electronic, optical, spectroscopic, and chemical properties of the particles, providing a rich plethora of controllable nanotools. The capping layer can also affect the capability to assemble the particles in specific arrays or the ability to target desired chemical, physical, or biological environments. Unlike the coating of particles with polymers (which has its own specific advantages), capping with a single layer of “small” molecules retains the low-nanometer size of the dressed particles and enables a tight control over their chemical nature. It also allows for the precise control of the interactions between the solid cores and the ambient surroundings (which is important for catalysis or for chemical sensors, as prominent examples). The classical capping agents for metal-containing colloidal cores (metal, oxide, or chalcogenide particles) are long-chain alkanethiols or their oxidized form, disulfides.1-5 * To whom correspondence should be addressed. E-mail: efrima@ bgumail.bgu.ac.il. † Department of Chemistry. ‡ The Institutes for Applied Research. § Present affiliation: Department of Physics, The National University of Singapore, Singapore. (1) (a) Watson, K. J.; Zhu, J.; Nguyen, S. T.; Mirkin, C. A. J. Am. Chem. Soc. 1999, 121, 462. (b) Leff, D. V.; Ohara, P.; Heath, J. R.; Gelbart, W. J. Phys. Chem. 1995, 99, 7036. (c) Heath, J. R.; Knobler, C. M.; Leff, D. V. J. Phys. Chem. B 1997, 101, 198. (d) 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. (2) (a) Johnson, 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. (3) (a) Peterlinz, K. A.; Georgiadis, R. Langmuir 1996, 12, 4731. (b) Peterlinz, K. A.; Georgiadis, R. J. Phys. Chem. B 1997, 101, 8041.
These materials were first introduced for purposes of self-assembly on metal surfaces.3,6-8 However, they have been shown to produce highly stable and robust colloidal systems as well.1-6 Other capping agents that have been studied to some degree involve alkylamines or carboxylates, which are more weakly anchored to metal cores, compared to thiols.9 An in-depth discussion of capping of nanoparticles with various capping agents and related studies has been given by Murray10 and Willner,11 where additional important references can be found. As currently capping agents are given many functions in addition to their basic role as particle stabilizers, there is a clear need to explore various alternative chemistries for their design and structure. Xanthates (dithiocarbonates) are used on a large scale as collectors in flotation processes in metal recovery, have been investigated as adsorbents on coinage metals,12 and are known to form very strong bonds with metals. Recently, we found alkylxanthates to be convenient capping agents for platinum particles.13 (4) Porter, L. A., Jr.; Ji, D.; Westcott, S. L.; Graupe, M.; Czernuszewicz, R. S.; Halas, N. J.; Lee, T. R. Langmuir 1998, 14, 7378. (5) (a) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edward, P. P. Chem. Soc. Rev. 2000, 29, 27. (b) Rao, C. N. R.; Kulkarni, G. U.; Govindaraj, A.; Satishkumar, B. C.; Thomas, P. J. Pure Appl. Chem. 2000, 72, 21. (6) (a) Heister, K.; Allara, D. L.; Bahnck, K.; Frey, S.; Zharnikov, M.; Grunze, M. Langmuir 1999, 15, 5443. (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. (7) Garg, N.; Jonathan, M.; Friedman, M.; Lee, T. R. Langmuir 2000, 16, 4266. (8) (a) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (b) Biebuyck, H. A.; Bain, C. D.; Whitesides, G. M. Langmuir 1994, 10, 1825. (c) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62. (9) (a) Wang, W.; Efrima, S.; Regev, O. Langmuir 1998, 14, 602. (b) Huang, W.; Huang, Y. Spectrosc. Spectral Anal. 2000, 20, 449. (10) Tempelton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (11) Shipway, A. N.; Willner, I. Chem. Commun. 2001, 2035. (12) (a) Woods, R.; Hope, G. A. Colloids Surf., A 1999, 146, 63. (b) Woods, R.; Hope, G. A.; Brown, G. M. Colloids Surf., A 1998, 137, 329. (c) Buckley, A. N.; Parks, T. J.; Vassallo, A. M.; Woods, R. Int. J. Miner. Process. 1997, 51, 303. (13) Sawant, P.; Kovalev, E.; Klug, J. T.; Efrima, S. Langmuir 2001, 17, 2313.
10.1021/la015653n CCC: $22.00 © 2002 American Chemical Society Published on Web 03/12/2002
Xanthate Capping of Coinage Metal Colloids
In this report, we present the behavior of alkylxanthates on silver, copper, and gold colloids. We find that xanthates strongly adsorb to noble metal nanoparticles, similarly to thiols, and can replace adsorbed oleic acid. Xanthatecapped colloids transfer to organic phases under milder conditions than those observed for oleate and can be repeatedly precipitated and redispersed. Xanthate capping protects the particles against corrosion with oxygen (induced by the presence of cyanide ions) both in aqueous and organic liquids. The resistance toward corrosion that it manifests is much stronger than that shown by oleate capping, which is also considerable. However, the stability for xanthate capping upon heating the colloidal suspension is lower than that observed when oleate or thiol is used, apparently as a result of the sensitivity of the xanthate molecule itself to temperature. Xanthates are found to be very efficient in producing and stabilizing copper colloids even in aqueous solutions. Our results show that xanthates provide an alternative choice of capping agents for metal nanoparticles, thus augmenting the selection of compounds for the emerging science and technology of nano- and mesoscale systems. Experimental Section Hexadecylxanthate (C16 xanthate) and octylxanthate (C8 xanthate) potassium salts are prepared from hexadecanol or octanol (Aldrich), respectively, as described previously.13 We use AgNO3 and Cu(NO3)2‚3H2O (Aldrich) and hexachloroauric acid (Stern Chemicals). Methanol (AR), dichloromethane (AR), and chloroform (AR) are purchased from Frutarom, dodecane (99+% pure) and NaBH4 (99% pure) from Aldrich, and isooctane (HPLC grade) from Fluka. Hexadecanethiol is from Fluka, and oleic acid is from Aldrich. 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 (TEM) is carried out using a JEOL 2010 HR-TEM, equipped with a Gatan multiscan CCD camera. Energy-dispersive X-ray spectroscopy (EDS) is performed using an Oxford linked ISIS 6498 (version 1.4). Typically, an accelerating voltage of 200 kV is used with magnifications of ×100 000 for obtaining size distributions and ×600 000 for singleparticle 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 are used. “Noncoated” colloidal sols are prepared as described elsewhere for silver.9a Briefly, a solution of 1 mM AgNO3, HAuCl4, or Cu(NO3)2 is added dropwise with stirring to an equivolume freshly prepared ice-cold 4 mM NaBH4 aqueous solution. Subsequent heating to 60-80 °C (except for the copper colloids) decomposes any residual borohydride. Capping by the xanthates is carried out by adding appropriate (methanol or water) solutions of the capping agent to aqueous suspensions of a preformed noncoated colloid. The transfer of the xanthate-capped colloids to an organic medium is obtained spontaneously, by stirring together equal volumes of the aqueous capped colloid solution and the organic liquid, usually with the addition of 0.04-0.1 M NaH2PO4. Capping by oleate is achieved by adding 2.5 × 10-3 M sodium oleate prior to the silver reduction (proc A) or after it with a Ag/oleate molar ratio of 2:1 (proc B). Thiols are applied by stirring together the respective hydrosol (for silver, as an example, 2.5 × 10-4 M in terms of silver atoms) with a tetrahydrofurane, THF, 0.002 M solution of the thiol (typical molar ratio of 2:1 Ag/thiol). The precipitate is then dispersed in an appropriate organic solvent (dichloromethane, 1,2 dibromoethane, n-hexane, and n-dodecane).
Results and Discussion Xanthate-Capped Silver Hydrosols. Figure 1 shows that adding C16 xanthate to a silver hydrosol red-shifts
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Figure 1. UV-visible spectra of C16 xanthate capped silver hydrosols at various xanthate/Ag molar ratios. Inset: UVvisible spectra of a C16 xanthate capped silver hydrosol with a xanthate/Ag molar ratio of 0.5, as a function of time.
the UV-visible absorption band from 398 nm by 15-20 nm and broadens it, with a concomitant change of the color from yellow to orange. The peak at 300 nm is the residual absorption of xanthate itself ( ) 11945 cm-1 M-1 in water/methanol solution), incompletely compensated by the aqueous/methanol xanthate solution used as a blank. Unlike the case of platinum,13 we do not observe a separate new transition. Changing the amount of the xanthate has only a slight effect on the spectra, mostly attributed to the dilution as additional aliquots of the xanthate-bearing solution are added. Note also that a hint of absorption at ∼350 nm becomes apparent, which can be seen more clearly by deconvoluting the spectra. At xanthate/Ag molar ratios exceeding 0.5, the spectra tend in time to shift slightly further to the red and broaden, indicating that some aggregation is taking place (inset in Figure 1). We also find that the xanthate/Ag molar ratio of 0.5 is optimal in terms of stability of the colloid and the ability to transfer it to organic phases (see below). A typical electron transmission micrograph of the xanthate-coated colloid is shown in Figure 2. In all of the micrographs, mostly round particles are observed with a radius of 5 ( 1 nm, with the larger particles being a bit oval. Energy-dispersive analysis of X-rays (EDAX) gives a S/Ag molar ratio of 0.47/0.79, from which a xanthate/Ag molar ratio of 0.24/0.79 ) 0.3 is obtained, consistent with the optimal ratio of 0.5. Assuming a monodisperse suspension of spherical particles completely coated with xanthate molecules, attached to the surface of the particles via their acid functionality, the xanthate/ silver molar ratio is given by
3MWAg nxan ) nAg NarAgFAgaxan where MWAg is the atomic mass of silver, FAg is its density, rAg is the average radius of the particles, axan is the area occupied by the xanthate molecules adsorbed on the particles, and Na is Avogadro’s number. Taking axan ) 0.2 nm2 (as for a carboxylic acid), we find this ratio to be 0.05, which is about 6-fold smaller than the actual optimal value we reported above. This discrepancy probably results, in part, from the simple model
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Figure 2. Transmission electron micrograph of a C16 xanthate capped silver hydrosol.
Figure 3. UV-visible spectra of a C16 xanthate capped silver colloid transferred from water to chloroform (xanthate/Ag molar ratio ) 0.5).
we used (for a polydisperse sample, as judged from Figure 3, the smaller particles would increase the ratio by a factor of up to ∼2) and also may indicate that there is some loss of the xanthate. Also, the semiquantitative nature of EDAX can contribute to the discrepancy. Nevertheless, we view this at least as a near agreement and as indicative that the general picture of xanthate-capped silver particles is basically correct. Formation of Organosols. Silver xanthate passes into chloroform, dichloromethane, and 1,2-dibromoethane (for the latter there is no need to add the phosphate), typically with 50% efficiency. The remainder of the colloid coats the walls of the vessel. Figure 3 shows the spectra measured in the aqueous phase and that of the organosol, prior to and after the particle transfer to chloroform. Xanthate is not sufficiently hydrophobic to enable transfer into dodecane, even when using high concentrations of the phosphate salt. Colloid Stability on Storage. Xanthate-capped silver hydrosols are stable for a few weeks; however, typically within a month or two they partially precipitate out of the solution. The fraction that still remains suspended seems to consist of small clusters of silver particles, as can be judged from the broadening of the spectrum and the
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Figure 4. UV-visible spectra of a C16 thiol capped silver colloid in n-hexane (xanthate/Ag molar ratio ) 0.5).
appearance of a split with maxima at 350 and 445 nm (not shown for brevity). As remarked above, a hint of the 350 nm band was observed already in fresh preparations. The xanthate molecule itself decomposes to a significant degree under the same conditions and might be the major reason for the instability of the colloid. In comparison, an noncapped silver colloid or an oleate-capped hydrosol is stable over at least a few months. In organic phases, we find xanthate-capped colloids to be stable only for a few weeks. However, in these solvents, when the colloid deteriorates, the 350 nm peak does not appear, and a more substantial aggregation is evident in the spectrum (with a larger broadening and shift of the band compared to water). In comparison, oleate-capped silver organosols (prepared by proc B) are stable for many months, practically indefinitely. Silver colloids protected with a hexadecane thiol (C16SH) layer in organic phases are also more stable than the xanthate-capped particles, though a 30% decrease of the extinction is observed in 1 month (Figure 4). Stability on Heating. Our samples of noncapped silver colloid are completely stable to heating for 3 h (at 96 °C), while the xanthate-capped hydrosols and organosols show significant precipitation within 1 h, which is practically completed in 3 h (Figure 5). A control experiment reveals that xanthate itself is unstable under these conditions and totally decomposes in 1 h. Thus, the stability of the xanthate-capped colloid is determined essentially by the time and temperature instability of the capping agent itself. An instability is, of course, undesirable when one seeks long-range stability or heat resistance; however, it can be useful when one wants a built-in, quick way to destabilize a colloid. Also, the reactivity of the xanthate provides a convenient means to modify chemically the nature of the particle surface. Oleate-capped silver hydrosols also deteriorate by heating at 96 °C, at a somewhat slower rate compared to that of the xanthate-capped particles (after 3 h about 50% of the colloid is still present in suspension). In contrast, thiol (C16)-stabilized silver colloids in dodecane are quite immune to the heat treatment. After 3 h of treatment at 96 °C, the spectrum loses merely ∼10% of its initial intensity, even though free thiol in solution is sensitive to heating. Resistance to Oxidation of the Hydrosols. A different measure for the protective power of the xanthate
Xanthate Capping of Coinage Metal Colloids
Figure 5. UV-visible spectra of a heat-treated C16 xanthate capped silver hydrosol.
Figure 6. UV-visible spectra of a xanthate-capped silver hydrosol treated with cyanide in aerated solutions.
capping is obtained when testing for the chemical inertness of the particles. We investigated the stability with respect to oxidation by oxygen (and eventual dissolution) induced in the presence of cyanide ions. Adding a solution of potassium cyanide to a noncapped silver hydrosol (1.6 × 10-3 M cyanide and 2.5 × 10-4 M in terms of silver atoms) results in a total dissolution of the colloid within 5 s. In comparison, under the same conditions a xanthate-capped silver hydrosol is highly stable. Even after 4 days, most of it is retained (Figure 8). The appearance of the 350 nm band, which we assign to aggregates (see above), indicates that most of the decrease of the colloid extinction at ∼400 nm stems from aggregation and eventual precipitation rather than from oxidation. This conclusion is supported by the disappearance of the xanthate signature at 300 nm seen in Figure 6. A control experiment shows that in a deaerated solution the noncapped colloid is stable for at least 3 h even at a 2-fold higher cyanide concentration. Capping by oleate too increases the resistance of the colloids toward oxidation in the presence of cyanide, but not to the degree exhibited
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by the xanthate. Oleate-capped silver hydrosols totally disappear after 5 h of cyanide treatment. Alkanethiolcapped silver colloids do not disperse in water, so a comparison to them cannot be carried out. Exchange of Oleate by Xanthate or Thiol. Previously, ligand exchange has been demonstrated by several groups.14 We carry out the exchange in the following manner. First, oleate-capped silver colloids are prepared (using proc B) and transferred into chloroform. Next, a methanolic solution of C16 xanthate or a THF solution of the C16 alkanethiol is added, to affect a replacement of the oleate adsorbed on the particles by the presumably more strongly adsorbing xanthate or thiol molecules. Note that the amount of xanthate or the thiol we use in this experiment is adjusted so it can replace only part of the oleate and can coat only about 1/6 of the total (estimated) available surface area of the particles. We want to demonstrate here the molecular exchange on the basis of the relative affinity to the silver surface, rather than that which is simply due to concentration effects. Finally, all the samples are treated with an aqueous 0.2 M KCN solution with vigorous stirring. We use the kinetics of oxidation as a diagnostic of the substitution. The oleate-capped organosol (and several controls with appropriate amounts of methanol or THF added to the oleate-capped chloroform suspension) fully reacts within 3 min. However, the organosols treated with the xanthate or the thiol manifest a much better resistance. With the xanthate, some colloid (∼10%) remains even after an hour. With the thiol, some remains even after 3 days. It is yet to be explored whether the difference between the stabilities of the thiol-exchanged colloid and the xanthatesubstituted one results from a more substantial replacement of the oleate by the thiol than by the xanthate or whether the thiol intrinsically provides better protection (perhaps by establishing a more hydrophobic environment around the particle or lowering the energy of the surface atoms more efficiently). Understanding the stability of organosols with respect to oxidation is even more indirect than it is for hydrosols, as it involves also transport of cyanide ions from the aqueous phase to the organic phase and that of the silver cyanide complex ions (and perhaps some of the released capping agents) in the reverse direction. The overall process is also complicated by the issue of solubility of oxygen and the capping agents in water and in the organic liquid. All of these effects are medium dependent. It is, however, quite clear that even a partial involvement of thiols or xanthates in the capping layer very significantly improves the resistance to corrosion of the organosols, in line with what has been observed for the xanthate-treated hydrosols. UV-Visible Spectra of Silver Nanocolloids Stabilized by C16 Alkanethiol. Figure 4 shows an interesting extinction spectrum of a silver organosol in n-hexane capped by C16SH. Five peaks are obtained at 446, 418, 396, 371, and 325 nm by deconvolution. The exact positions and intensities vary with the solvent (we studied also dichloromethane, 1,2-dibromoethane, and dodecane). The spectra in hexane and dodecane are practically identical except for a red shift of 2-4 nm in dodecane. A similar (14) (a) Hostetler, M. J.; Green, S. J.; Stokes, J. J.; Murray, R. W. J. Am. Chem. Soc. 1996, 118, 4212. (b) Green, S. J.; Stokes, J. J.; Hostetler, M. J.; Pietron, J.; Murray, R. W. J. Phys. Chem. 1997, 101, 2663. (c) Ingram, R. S.; Hostetler, M. J.; Murray, R. W. J. Am. Chem. Soc. 1997, 119, 9175. (d) Ingram, R. S.; Murray, R. W. Langmuir 1998, 14, 4115. (e) Green, S. J.; Pietron, J. J.; Stokes, J. J.; Hostetler, M. J.; Vu, H.; Wuelfing, W. P.; Murray, R. W. Langmuir 1998, 14, 5612. (f) Hostetler, M. J.; Templeton, A. C.; Murray, R. W. Langmuir 1999, 15, 3782. (g) Zamborini, F. P.; Gross, S. M.; Murray, R. W. Langmuir 2001, 17, 481.
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Figure 8. UV-visible spectra of a C8 xanthate capped gold hydrosol at a Au/xanthate molar ratio of 25 as a function of time. Figure 7. UV-visible spectra of a xanthate-capped copper hydrosol.
observation is made for the two more polar liquids, except that the main peaks are shifted to the red by ∼6 nm in 1,2-dibromoethane compared to dichloromethane. There are larger differences between the spectra observed in the two groups of solvents, the polar solvents on one hand and the nonpolar on the other, though the overall fivepeak shape is retained. The main changes are that in the nonpolar liquids the two longest wavelength spectral features carry most of the intensity (over 85%), whereas in the polar liquids the third peak, at 396 nm, also carries a substantial intensity. Also, the widths of the bands are larger in the polar liquids. The shape of the spectra suggests the presence of small multiplets of the silver particles, composed of a few particles (perhaps 1-4) with definite aggregation geometries. This is an interesting feature which warrants a complete, separate study. At present, suffice it to say that it seems that the behvior in the polar liquids indicates enhanced aggregation, in line with the hydrophobic nature of the thiol-capped colloids. As noted above, the thiol-capped colloids are very stable and also resist heat treatment, retaining the general fivepeak shape of the spectrum. Production and Capping of Copper Colloids with Xanthates. Copper colloids are produced here by reduction of 2.5 × 10-4 M Cu(NO3)2 with borohydride. In general, these colloids are unstable in air, as they precipitate out of solution or oxidize within a few minutes. However, addition of a methanolic solution of C16 xanthate immediately after the reduction, reaching a low concentration of 1.1 × 10-4 M, yields a stable copper colloid in water. Its spectrum (Figure 7) exhibits a peak at 400 nm (appearing as a shoulder on the broad UV extinction), characteristic of small copper particles (with a radius smaller than 4 nm6a). TEM (not shown) indicates that the particle diameter is smaller than 5 nm. The capped copper colloid seems to be stable for more than 2 weeks. It is also quite resistant to oxidation in the presence of 2 mM cyanide ions, maintaining most of the spectrum intensity even after 8 days. In comparison, a noncapped metallic copper precipitate obtained a few minutes after the reduction of the copper salt totally oxidizes in the presence of cyanide within 2 s. The xanthate-capped copper colloid transfers into chloroform or dichloromethane with 50-70% efficiency by adding 0.36 M sodium phosphate. Similar to silver
colloids, it does not transfer into dodecane, even when higher concentrations of phosphate are used. Instead, it forms an interfacial colloid at the dodecane/water interface. This suggests that the xanthate-capped copper colloid is not as hydrophobic as the thiol-capped silver particles. Capping of Gold Colloids with Xanthates. Gold colloids can also be capped with C8 and C16 xanthates and show the characteristic plasmon peak of gold shifting (and broadening) from 522 to 528-530 nm, with the color changing to red-purple. C8 xanthate is more soluble than C16 xanthate and can be introduced in aqueous as well as methanolic solutions, with similar results. Xanthate-capped gold particles transfer into chloroform or dichloromethane by adding sodium phosphate. The corresponding spectra show a maximum at 532 nm for both liquids, very close to the 528-530 nm observed in water. The colloids can be dried and then redispersed in the organic liquids. As with silver and copper colloids, xanthate-capped gold particles do not transfer into dodecane but rather concentrate in the water/organic interface. At low concentrations of the C8 xanthate relative to the colloid (at a xanthate/Au molar ratio of ∼0.04), a broad feature at ∼700 nm appears and grows in time (Figure 8), with a concomitant change of the color from red to purple and then to blue. A similar but much weaker behavior is observed for C16 xanthate. Once this transition sets in, the colloids do not transfer into organic phases anymore. We attribute this transition to a partial aggregation of the colloid, in line with results reported for thianicotinamide on a gold sol.15 When higher xanthate concentrations are used (with a xanthate/Au molar ratio of ∼10 and higher), this aggregation is absent. Au-xanthate colloids, similar to the silver colloids, become unstable when heated. Boiling the noncapped gold hydrosol for hours has no effect on it, while the xanthatecapped hydrosols exhibit definite aggregation within 1 h, as evidenced by the change of color (from red to blue with C8 xanthate capped colloids). C16 gold colloids are a little more stable, developing a purple color in the same time span. In contrast, and as was found for silver and copper, etching the colloids in aerated cyanide solutions is vastly different for the noncapped and the xanthate-capped particles. The noncapped colloids completely react within 30 s, with the suspension becoming colorless. In com(15) Murakoshi, K.; Nakato, Y. Adv. Mater. 2000, 12, 791.
Xanthate Capping of Coinage Metal Colloids
parison, the xanthate-protected gold colloids are stable, with significant deterioration appearing only after about 1 month. C8 and C16 xanthate cappings behave in a similar fashion. Conclusions We investigated the capping of coinage metal nanocolloids with long alkyl chain (C16 and, for gold, also C8) xanthates. In comparison to thiol capping, the particles are much less hydrophobic, but compared to oleate capping, they are less stable in aqueous solutions over long periods (months). They can be transferred into relatively polar organic media but not into nonpolar solvents (such as dodecane). Unlike noncapped, thiolcapped, or oleate-capped colloids, they are temperature sensitive, probably as a result of the thermal decomposition of the xanthate molecule itself. They demonstrate exceptional resistivity toward corrosion by oxygen (in the presence of cyanide), when compared to noncapped or even oleate-capped colloids (that react typically within a few seconds).
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Xanthate capping seems to be of interest when high resistance to chemical attack is desired or when there is a need at some stage to dispose of the colloid, which can be easily attained by heating moderately. It also enables us to produce stable copper nanocolloids in aqueous solution under ambient conditions. Xanthates join the ever increasing arsenal of chemical nanotools, providing a variety of capabilities for the design and the fabrication of nanosystems. Also, just as alkylthiols which were developed for adsorption on “macroscopic” metal substrates were found to be very useful for capping of nanoparticles, thus, but in the reverse, xanthates shown here as efficient capping agents for particles might be also interesting adorbates for modifying metal surfaces, in general. Acknowledgment. We acknowledge the support of the Infrastructure program of the Israeli Ministry of Science. LA015653N
