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Size Control and Growth Process of Alkylamine-Stabilized Platinum Nanocrystals: A Comparison between the Phase Transfer and Reverse Micelles Methods Kjell Wikander,†,‡ Christophe Petit,*,† Krister Holmberg,‡ and Marie-Paule Pileni† UniVersite´ Pierre et Marie Curie-Paris 6, UMR7070, LM2N, Paris F-75005, France and CNRS, UMR7070, Paris F-75005, France, and DiVision of Applied Surface Chemistry, Department of Chemical and Biological Engineering, Chalmers UniVersity of Technology, Go¨teborg, Sweden ReceiVed January 17, 2006. In Final Form: February 21, 2006 Alkylamine-stabilized platinum nanoparticles are synthesized either by the phase-transfer method or in reverse micelles. The phase-transfer method produces nanocrystals that are quite spherical whereas the synthesis in reverse micelles generates a large number of wormlike structures. An alkylamine is used as a stabilizing agent to prevent nanoparticle coalescence, and it is shown that there is an inverse relationship between the alkylamine chain length and the platinum nanoparticle diameter. By comparing alkylamine and alkylthiol analogues, it is found that the integrity of the different crystallites depends on the strength of the interaction between the stabilizing agent and the platinum nanocrystals. The results obtained and the comparison made between the two synthesis methods improve the understanding of the growth mechanisms of platinum nanocrystals in disperse media.
Introduction Research in the field of nanoparticles has grown tremendously during the last two decades1-9 even though the understanding of nanocrystal growth is not well defined.10 From very recent data, it seems that the growth mechanisms of nanocrystals and of the bulk phase are similar. However, selective adsorption on a given facet permits control of the shape of nanocrystals.11 Such inorganic nanocrystals are of interest for a variety of applications such as superparamagnets12 and semiconductors.13 Thanks to the small size and the large surface area, nanoparticles are of interest for use in catalysis, where both single metals and different alloys have been studied.14 In catalysis, platinum is the most frequently used metal today, but natural resources are scarce and there is a general need to minimize the use of this expensive metal. This can be done either by optimizing the size and shape of the platinum nanoparticles or by replacing pure platinum with a platinum-containing alloy. Thus, it is important to have access to methods that give particles in the nanometer range with good control over size and shape and with the potential for controlled deposition on a solid support. * Corresponding author. E-mail:
[email protected]. † Universite ´ Pierre et Marie Curie-Paris 6 and CNRS, UMR 7070. ‡ Chalmers University of Technology. (1) Petit, C.; Pileni, M. P. J. Phys. Chem. 1988, 92, 2282. (2) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961. (3) Steigewald, M. L.; Alivisatos, P. A.; Gibson, J. M.; Harris, T.D.; Korten, R.; Muller, A. J.; Thoyer, A. M.; Duncan, T. M.; Douglas, D.C.; Brus, L. J. Am. Chem. Soc. 1988, 110, 3046. (4) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801. (5) Brust, M.; Bethell, D.; Schiffrin, D. J.; Kiely, C. J. AdV. Mater. 1995, 7, 795. (6) Bradley, J. S.; Hill, E. W.; Behal, S.; Klein, C.; Duteil, A.; Chaudret, B. Chem. Mater. 1992, 4, 1234. (7) Murray. C. B.; Kagan C. R.; Bawendi M. G. Science 1995, 270, 1335. (8) Pileni, M. P. Langmuir 1997, 13, 3266. (9) Pileni, M. P. Langmuir 2001, 17, 7476. (10) Pileni, M. P. Nat. Mater. 2003, 2, 145. (11) Pileni, M. P., J. J. Exp. Nanosci., 2006, in press. (12) Aschwalom D. D.; von Molnar, S. Physical Properties of NanometerScale Magnets in Nanotechnology; Timp, G. L., Ed.; Springer: New York, 1999; Chapter 12, pp 437-470. (13) Grieve, K.; Mulvaney, P.; Grieser, F. Curr. Opin. Colloid Interface Sci. 2000, 5, 168. (14) Bo¨nnemann, H.; Richards, R. M. Eur. J. Inorg. Chem. 2001, 10, 2455.
Over the years, various methods of nanoparticle synthesis have been developed. Two main soft-chemical routes have been developed over the last two decades: in situ synthesis in the water pools of reverse micelles developed in the 1980s by Pileni et al.1,2,8-10 and the phase-transfer method developed by Brust et al. in the 1990s.4,5,15 In the first method, the inner core of the reverse micelles is considered to be a nanoreactor, and the size of the nanocrystals is controlled by the state of the water molecules inside the water pools. This method has been used for the synthesis of semiconductor materials such as CdS,1,16 of metallic nanoparticles such as Pt,17,18 Cu,2,19-21 Co,22,23 and Ag,24,25 and also of nanoalloys such as CoPt,26-28 PtPd,29 and FeCu.30 The second method involves the transfer of the metal ion from a polar phase to a nonpolar phase using a transferring agent. It has been used for the synthesis of metallic nanoparticles.4,5,15 Other chemical routes based on organometallic chemistry6,7,31 have also been used, but such synthesis procedures are more complex than those of the soft-chemistry routes presented above. Obviously, nanoparticles can also be obtained in solution by other physical techniques such as solvatation of a metal atom obtained by the vaporization of bulk materials,32 or laser processes,33 or pulse (15) Brust, M.; Bethell, D.; Kiely, C. J.; Schiffrin, D. J. Langmuir 1998, 14, 5425. (16) Petit, C., P.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1990, 94, 1598. (17) Ha¨relind Ingelsten, H.; Bagwe, R.; Palmqvist, A.; Skoglundh, M.; Svanberg, C.; Holmberg, K.; Shah, D. O. J. Colloid Interface Sci. 2001, 241, 104. (18) Ha¨relind Ingelsten, H.; Beziat, J.-C.; Bergkvist, K.; Palmqvist, A.; Skoglundh, M.; Qiuhong, H.; Falk, L. K. L.; Holmberg, K. Langmuir 2002, 18, 1811. (19) Lisiecki, I.; Pileni, M. P. J. Am. Chem. Soc. 1993, 115, 3887. (20) Tanori, J.; Pileni, M. P. AdV. Mater. 1995, 7, 862. (21) Lisiecki, I. J. Phys. Chem B 2005, 109, 12231. (22) Petit, C.; Taleb, A.; Pileni, M. P. J. AdV. Mater. 1998, 10, 259. (23) Lisiecki, I.; Albouy, P. A.; Pileni, M. P. AdV. Mater. 2003, 15, 712. (24) Petit, C.; Lixon, P.; Pileni, M. P. J. Phys. Chem. 1993, 97, 12974. (25) Taleb, A.; Petit, C.; Pileni, M. P. Chem. Mater. 1997, 9, 950. (26) Petit, C.; Rusponi, S.; Brune, H. J. Appl. Phys. 2004, 95, 4251. (27) Xiong, L.; Manthiram, A. Electrochim. Acta 2005, 50, 2323. (28) Carpenter, E. E.; Seip, C. T.; O’Connor, C. J. J. Appl. Phys. 1999, 85, 5184. (29) Yashima, M.; Falk, L. K. L.; Palmqvist, A. E. C.; Holmberg, K. J. Colloid Interface Sci. 2003, 268, 348. (30) Duxin, N.; Brun, N.; Colliex, C.; Pileni, M. P. Langmuir 1998, 14, 1984. (31) Dassenoy, F.; Philippot, K.; Ould Ely, T.; Amiens, C.; Lecante, P.; Snoeck, E.; Mosset, A.; Casanove, M.-J.; Chaudret, B. New J. Chem. 1998, 703.
10.1021/la060163m CCC: $33.50 © 2006 American Chemical Society Published on Web 03/29/2006
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radiolysis.34 However, we will focus here on two usuals methods of colloidal chemistry. To decrease the tendency of nanocrystals to agglomerate and to keep their nanoparticle size, it is necessary to use a stabilizing agent. This also allows their extraction from reactive media.35 Depending on the element, different stabilizing agents have been shown to be useful: trioctylphosphine22 and lauric acid23 are effective for cobalt, alkylthiols and alkylamines work well for noble metals4,25,36,37 and semiconductors,35,7,38 and polymers such as polyvinyl pyrrilidone (PVP) have been used for platinum.31,39,40 The interaction strength between the stabilizing agent and the nanoparticle surface varies with the type of functional group used to anchor the stabilizing agent, and the -SH group in 1-thiols usually shows the strongest interaction with platinum and other noble metals. Despite the large number of articles published on nanocrystals, there are still open questions related to their synthesis as well as to the parameters controlling their growth and shape. In the present work, we demonstrate, by using the phase-transfer method, that the length of the chains used to coat the platinum nanocrystals induces control of the nanocrystal size. Furthermore, the phasetransfer and reverse micelles methods for the preparation of fine platinum nanocrystals are compared Experimental Section Materials. Platinum(IV) chloride (PtCl4, 99%) was purchased from Agro and used as received. Tetrakisdecylammonium bromide (TDAB) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT), both of 99% purity, were from Fluka. The alkylamines used in the study (hexylamine, heptylamine, octylamine, nonylamine, decylamine, dodecylamine, hexadecylamine, and octadecylamine, all 99% pure) were from Aldrich, as was the reducing agent, sodium borohydride, NaBH4 (99%). Hexylamine, heptylamine, and octylamine are abbreviated C6NH2, C7NH2, and C8NH2, respectively, and dodecylthiol is abbreviated C12SH. The organic solvents (toluene, hexane, isooctane, and ethanol) were all of analytical grade. Water was purified with a Millipore water system. All glassware was carefully cleaned in aqua regia and rinsed with large quantities of water before use. Transmission Electron Microscopy Measurements. Transmission electron microscopy (TEM) images were obtained using a CCD camera connected to a JEOL 100 CX II operated at 100 kV with a magnification of 63 000-250 000×. TEM sample grids were prepared by placing a drop of the fine, freshly prepared nanoparticle dispersion onto a grid placed on an adsorbing paper, which removed excess solvent. To determine the mean nanoparticle size, D, and the corresponding size distribution, σ, around 500 nanoparticles were measured for each sample and presented in a histogram. The standard deviation, σ, was calculated according to σ)
{
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∑(D - D) ]
[
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i
[n - 1]
1/2
(32) (a) Zhang, F.; Barrowcliff, R.; Stecker, G.; Pan, W.; Wang, D.; Hsu, S. Jpn J. Appl. Phys. 2005, 44, L398. (b) Collier, P. J.; Iggo, J. A.; Whyman, R. J. Mol. Catal. A: Chem. 1999, 146, 149. (33) (a) Veintemillas-Verdaguer, S.; Morales, M.; Bomati-Miguel, O.; Bautista, C.; Zhao, X.; Bonville, P.; de Alejo, R.; Ruiz-Cabello, J.; Santos, M.; TendilloCortijo, F.; Ferreiros, J. J. Phys. D: Appl. Phys. 2004, 37, 2054. (b) Ayers, T. M.; Fye, J. L.; Li, Q.; Duncan, M. A. J. Cluster Sci. 2003, 14, 97. (34) Henglein, A.; Ershov, B. G.; Malow, M. J. Phys. Chem. B 1995, 14129. (35) J. Cizeron, J.; Pileni, M. P. J. Phys. Chem. 1995, 99, 17410. (36) Reetz, M. T.; Winter, M.; Tesche, B. Chem. Commun. 1997, 147. (37) Sarathy, K. V.; Raina, G.; Yadav, R. T.; Kulkarni, G. U.; Rao, C. N. R. J. Phys. Chem. B 1997, 101, 9876. (38) Motte, L.; Billoudet, F.; Pileni, M. P. J. Phys. Chem. 1995, 99, 16425 (39) Teranishi, T.; Hosoe, M.; Tanaka, T.; Miyake, M. J. Phys. Chem. B 1999, 103, 3818. (40) Yadav, O.; Palmqvist, A.; Cruise, N.; Holmberg, K. Colloids Surf., A 2003, 221, 131.
where D is the average diameter. The polydispersity index was defined as the ratio σ/D. For high-resolution transmission electron microscopy (HRTEM) measurements of the crystallinity of the prepared platinum nanoparticles, a JEOL 2010 UHR instrument operated at 200 kV (LaB6) was used. X-ray Diffraction Measurements. The crystal characteristics of the prepared nanoparticles were investigated using X-ray diffraction (XRD) measurements in transmittance mode using a Siemens Kristalloflex diffractometer with a STOE goniometer (Co KR, λ ) 1.7902 Å) setup. Samples were prepared by repetitive dropwise addition of a concentrated nanoparticle suspension onto an X-raytransparent tape until a sufficient sample film had been established. The intensity of the diffracted beams was detected over the interval 30-120° (2θ) with a step size of 0.05° and an integration time of 8 s. A comparison of the (111) and (220) peak broadness values after normalizing relative to the measured intensity was used as an indicator of the mean nanoparticle size for each synthesis and was treated as a complement to the TEM data Synthesis Procedures. It is known that reduction of a freshly prepared PtCl42- solution occurs more slowly than that of an aged solution because of the formation of platinum complex PtCl2(H2O)2.34 To standardize the conditions, a 0.1 M PtCl42- stock solution (MilliQ) was prepared by stirring for 48 h, followed by filtration to remove solid contaminants. Phase-Transfer Synthesis Method. We have adapted the method first presented by Brust et al.4 Fifteen milliliters of a 20 mM PtCl4 aqueous solution was mixed with 0.50 g of TDAB dissolved in 35 mL of toluene. To ensure maximum transfer of platinum ions from the water phase to the organic phase, the transferring agent (TDAB) was present in large excess. The reaction mixture was stirred for at least 2 h, after which the water phase turned almost colorless. In a typical synthesis, 1.5 mL of a stabilizing agent (e.g., C12NH2 (6.6 mmol)) dissolved in 5 mL of toluene was added to the continuously stirred reaction mixture. Reduction was then performed through the dropwise addition of a freshly prepared solution consisting of 190 mg of NaBH4 (5 mmol) dissolved in 12.5 mL of MilliQ water. The final amount of Pt is 0.3 mmol, thus both the reducing agent and the stabilizing agent were added in large stoichiometric excess relative to platinum (Pt/NaBH4 )1:17 and Pt/SA ) 1:22). The mixture changed from intense orange to dark brown relatively quickly and was allowed to react for at least 2 h. The stirring was turned off, and the organic phase, which contained the Pt-alkylamine nanoparticles, was separated and evaporated to ca. 5 mL in a rotovapor. Four hundred milliliters of ethanol was then added, and after redispersion the mixture was left at -18 °C overnight, after which a precipitate had formed. After supernatant removal, another 50 mL of fresh ethanol was added, the precipitate was redispersed in the alcohol, and the mixture was subsequently centrifuged at 5000 rpm for 10 min. This procedure was repeated twice with 30 mL of ethanol and finally with either 6 mL of toluene when C6NH2-C10NH2 was used as the stabilizing agent or 6 mL of hexane when C12NH2C18NH2 was used as the stabilizer. When C6NH2 or C7NH2 was used, the addition of another 50 µL of alkylamine was required to obtain dispersion in the toluene. Each synthesis was made in duplicate. ReVerse Micelle Synthesis Method. The preparation of platinum nanoparticles in reverse micelles was achieved by mixing two solutions of reverse micelles, one containing the platinum salt and one the reducing agent. In a typical experiment, 675 µL of water containing 15 µmol of PtCl42- was added to 15 mL of 0.25 M AOT in isooctane. The water-to-surfactant molar ratio (W) was equal to 10, which corresponds to a droplet diameter of 3 nm2. An equal volume of reverse micelles with W ) 10 but with a freshly prepared NaBH4 solution (300 µmol) in the water pools was prepared. Again, the reducing agent was present in large excess with a 1:20 Pt/NaBH4 stoichiometric ratio that is comparable to that of the previous method. Immediately after mixing the two reverse micelle solutions, the solution turned from yellow to dark brown, indicating the formation of platinum nanocrystals. Extraction of the nanocrystals was performed as follows. After 1 h of reaction, stabilizing agent dodecylamine (1.5 cm3) or dodecylthiol (60 µL) was added to the reaction mixture, which was left for another 1 h. The isooctane was
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Figure 2. XRD diffractograms illustrating the (111) and (220) peaks normalized to the detected maximum intensity of the (111) peak indicating different crystallite sizes of the platinum nanoparticles synthesized by (A) the phase-transfer method and (B) the reverse micelle method.
Figure 1. Characteristic TEM pictures of platinum nanocrystals (A) synthesized by the phase-transfer method and (B) synthesized by the reverse micelle method. (Inset) Characteristic XRD patterns showing the FCC structure of platinum. then removed by evaporation, and the waxy residue was redispersed in 50 mL of ethanol, followed by centrifugation at 5000 rpm for 10 min. The black precipitate was redispersed in 30 mL of fresh ethanol and centrifugated again. This repetitive extraction procedure was continued until the supernatant was colored because of the presence of nanoparticles. The colored supernatants were collected after each washing step until the obtained supernatant was again transparent after centrifugation. After evaporation of the collected colored supernatants, the residue was dispersed in hexane. It should be noticed that in the case of dodecylthiol the extraction process is very efficient; Pt/C12SH is 5:1 when it is 1:4.4 for dodecylamine. Each synthesis was carried out in duplicate.
Results and Discussion Comparison of the Two Synthesis Routes. Figure 1 shows TEM patterns of the platinum nanocrystals obtained by the two methods. The nanocrystals synthesized by the phase-transfer method are spherical with a diameter of 1.7 nm. (Figure 1A). Synthesis in reverse micelles followed by the addition of dodecylamine gave both spherical particles (55%) and wormlike structures (45%) with a cross-sectional diameter of 2.6 nm, which is equal to the diameter of the spherical nanocrystals (Figure 1B). The XRD patterns indicate differences, as is shown in the inset of Figure 1 and in Figure 2. This confirms that the crystallite size is larger for nanocrystals synthesized in reverse micelles. The nanoparticles prepared by Brust’s phase-transfer method are well-crystallized spheres (Figure 3A) showing atomic planes with the characteristic lattices of platinum whereas nanocrystals made by the reverse micelle route are polycrystalline and consist of a mixture of spheres and wormlike structures (Figure 3B).
Figure 3. HRTEM micrographs of dodecylamine-stabilized platinum nanocrystals synthesized by (A) the phase-transfer method and (B) the reverse micelle method.
Clearly, the two methods yield different nanoparticles with respect to both size and shape. This difference is probably due to differences in the environment of the growing crystallites. No structural study made of the toluene phase in the phase-transfer method is investigated. However, this phase is likely to be very similar to a reverse micelle solution with small water droplets that contain the platinum salt in the water pools and with the
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Figure 4. TEM micrographs of platinum nanocrystals obtained by the phase-transfer method using different alkylamines as stabilizing agents: (A) C6NH2, (B) C7NH2, (C) C8NH2, (D) C9NH2, (E) C10NH2, (F) C12NH2, (G) C16NH2, and (H) C18NH2.
interface stabilized by TDAB and the alkylamine used as the stabilizing agent. This is supported by the very low conductivity (20-100 nS) of the solution. Thus, as in reverse micelles the reactions occur in confined media. The major difference between these two methods is related to the moment that the stabilizing agent is added. In reverse micelles, it is added 1 h after the synthesis starts whereas in the phase-transfer method it is part of the reactant providing sufficient protection to block the coalescence after the nanocrystals have been formed. In reverse micelles, this permits the formation of wormlike structures induced by coalescence of the spherical nanoparticles (i.e., there is no simultaneous growth of worms and spheres). As expected, the efficient stabilization of the nanoparticles obtained with the alkylamine in the phase-transfer method triggered an experiment to use an alkylamine in the reverse micelle method. However, an attempt to synthesize platinum in AOT reverse micelles with preadded dodecylamine was unsuccessful. No platinum nanoparticles were formed even after several hours of reaction time. This could be attributed to complex formation between the amino group and the fcc nuclei formed immediately after reduction of the platinum ions. However, we know that the interface rigidity41 increases by adding long-chain amines. This reduces the intermicellar exchange process, which is one of the key parameters in nanocrystal size control.10,24,42 Chaudret et al.31 have shown (41) Shiao, S. Y.; Chabra, V.; Patist, A.; Free, M. L.; Huibers, P. D. T.; Gregory, A.; Patel, S.; Shah, D. O. AdV. Colloid Interface Sci. 1998, 74, 1.
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Figure 5. Size histograms of platinum nanocrystals obtained by the phase-transfer method using different alkylamines as stabilizing agents: (A) C6NH2, (B) C7NH2, (C) C8NH2, (D) C9NH2, (E) C10NH2, (F) C12NH2, (G) C16NH2, and (H) C18NH2.
that alkylamines are not very efficient as stabilizing agents for such metallic particles in pure toluene. They are only weakly coordinated to the nanocrystals and can easily be displaced from the surface. However, in this study it is shown that it is possible to block the growth process and to obtain spherical nanocrystals by using a long-chain alkylamine as a stabilizer in toluene. The high efficiency of the alkylamine in this case can probably be attributed to the reaction taking place in a micro-heterogeneous medium with water droplets stabilized by a combination of TDAB and the alkylamine rather than a homogeneous organic medium. The compartmentalization of the reaction medium is likely to slow the reaction, which allows better control of the growth process. Effect of the Stabilizing Ligand on the Platinum Nanoparticle Diameter. The phase-transfer method does not allow for efficient control of the size of the nanocrystals. Post-treatment in the form of a germination process43 or heat treatment44 is often used to increase the size of the nanocrystals. It is relevant in the case of platinum, where the size is almost always smaller than 2 nm. Allowing the solution to age prior to addition of the stabilizing agent could be an alternative method, and the procedure works well for gold45 but not for platinum.46 Thus, it is not trivial to increase the size while maintaining the spherical shape of the (42) Lisiecki, I.; Pileni. M. P. J. Phys. Chem. 1995, 99, 5077. (43) Murphy, C. J.; Sau, T. K.; Gole, A. M.; Orendorff, C. J.; Gao, J.; Gou, L.; Hunyadi, S. E.; Li, T. J. Phys. Chem. B 2005, 109, 13857. (44) Shimizu, T.; Teranishi, T.; Hasegawa, S.; Mayake, M. J. Phys. Chem. B 2003, 107, 2719.
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Table 1. Data on the Size of the Platinum Nanoparticles Obtained from the Histograms in Figure 4a synthesis method
SA
moles of SA added (× 103)
molar ratio SA/Pt4+
nanoparticle diameter, D (nm)
SD σ (nm)
polydispersity index (σ/D)
PTM PTM PTM. PTM PTM PTM PTM PTM RMM RMM
C6NH2 C7NH2 C8NH2 C9NH2 C10NH2 C12NH2 C16NH2 C18NH2 C12NH2 C12SH
11.36 10.12 9.08 8.19 7.55 6.58 5.05 4.52 19.73 2.5 × 10-4
37.8 33.7 30.3 27.3 25.2 21.9 16.8 15.1 3496 0.05
2.60 ( 0.17 2.53 ( 0.43 2.09 ( 0.13 2.0 2.00 ( 0.03 1.64 ( 0.03 1.57 ( 0.02 1.61 ( 0.10 2.61b 2.69b
0.48 ( 0.01 0.25 ( 0.06 0.26 ( 0.02 0.46 0.37 ( 0.02 0.25 ( 0.02 0.24 ( 0.10 0.24 ( 0.00 0.59b 0.73b
0.19 ( 0.01 0.35 ( 0.20 0.12 ( 0.01 0.23 0.19 ( 0.01 0.15 ( 0.02 0.15 ( 0.01 0.15 ( 0.01 0.23b 0.27b
a The polydispersity index, obtained from two identical preparations, is also given. Abbreviations: PTM, phase-transfer method; RMM, reverse micelle method; SA, stabilizing agent; SD, standard deviation. b Data based on the measurement of isolated spherical nanoparticles (i.e., no wormlike structures).
Figure 6. Diameter of the platinum nanocrystals obtained by the phase-transfer method as a function of the chain length of the alkylamine used as the stabilizing agent.
nanoparticles, and spherical particles are needed to obtain platinum nanocrystal self-organization. The effect of the length of the surface-active molecule on the final size of the nanocrystals has been shown from digestive ripening experiments on gold nanocrystals.47 Figure 4 shows the TEM micrographs, and Figure 5 shows the corresponding histograms of the platinum nanocrystal size obtained by systematically varying the number of carbon atoms in the alkyl chain of the stabilizer between 6 and 18. The calculated mean diameter, the polydispersity, and the polydispersity index are presented in Table 1. The data, plotted in Figure 6, indicate that there is an inverse relationship between the size of the nanoparticles and the length of the alkylamine. Inspection of the TEM micrographs reveals that the platinum nanoparticles are more spherical in shape when short-chain alkylamines are used rather than their longer-chain homologues. For instance, the effect can clearly be seen by comparing the nanoparticles obtained with octylamine (Figure 4C) and octadecylamine (Figure 4H). XRD characterization of the different samples confirms that the nanoparticles are all crystalline; see Figure 7. There is a clear relationship between the length of the alkyl chain of the stabilizer and the peak widths in the diffractogram: the longer the chain, the broader the peaks. For the longest alkylamine, octadecylamine, the (220) peak is observed only as a shoulder. Use of this alkylamine as a stabilizing agent (45) Saunders, A. E.; Sigman, M. B.; Korgel, B. A. J. Phys. Chem. B 2004, 108, 193. (46) Kumar, A.; Joshi, H.; Mandale, A.; Srivastava, R.; Adyanthaya S.; Oasricha, R.; Sastry, M. J. Chem. Sci. 2004, 116, 293. (47) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Langmuir 2002, 18, 7515.
Figure 7. XRD diffractogram comparing the normalized (111) and (220) peaks of platinum nanocrystals synthesized by the phasetransfer method using (A) C6NH2, (B) C8NH2, and (C) C18NH2 as stabilizing agent.
results in some wormlike structures, whereas the shorter alkylamines seem to give only spheres. The difference in stabilization efficiency seen with alkylamines of varying chain length may seem surprising considering that the strength of the interaction with the metal surface, which is mediated by the amino group, should be the same for all alkylamines. This effect is opposite to what is observed in digestive ripening, where the final equilibrium size increases with the chain length.47 It was attributed to a curvature effect because the longer ligands prefer a less-curved surface favoring larger particles.47,48 Clearly this is not what happened here probably because the passivating agent is added prior to the reduction of the platinum salt and not in a postsynthetic step. However, we propose that the difference in effect is due to differences in the partitioning of the alkylamines, which in turn is due to differences in their solubility in toluene. As a matter of fact, Table 1 clearly shows the strong decrease in the number of moles of alkylamines needed to stabilize the nanocrystals by increasing the alkyl chains (the ratio SA/Pt decrease from 37.8 to 15.1). An alkylamine with a short hydrocarbon tail is much more soluble in toluene, which is slightly more polar (as a matter of fact, the solubility of water in toluene is 0.53 g/L but is lower than 0.01 g/L in isooctane) than a long-chain alkylamine, thus the former will partition more into the bulk phase than the latter. The longer alkylamines will, to a higher degree than their shorter homologues, be located at the interface between the small water (48) Marin, J. E.; Wilcoxon, J. P.; Odinek, J.; Provencio, P. J. Phys. Chem. B 2000, 104, 9475.
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Figure 9. (A and B) TEM and (C and D) HRTEM micrographs of platinum nanocrystals synthesized in reverse micelles and subsequently stabilized either by (A and C) C12NH2 or by (B and D) C12SH. Figure 8. Schematic model of the palisade layer surrounding the growing platinum nanoparticle. (A) Short-chained alkylamines have a higher solubility in toluene than (B) long-chained alkylamines. The palisade layer obtained with the long-chained alkylamines will be more static, and access to new platinum-TDA complexes will be restricted as compared to the situation with short-chained alkylamines.
droplets, in which the platinum nanoparticles are being generated, and the bulk toluene phase. That location is, of course, a prerequisite for a good stabilizing effect. Only when the alkylamine is situated in a surfactant-like way at the interface will the amino group be able to coordinate to the platinum surface. Figure 8 gives a schematic illustration of the difference in character of the interface for the cases of a short-chain and a long-chain alkylamine. The situation at the interface bears a close resemblance to that of a water-in-oil microemulsion, formulated with one very hydrophobic cationic surfactant and one cosurfactant, in this case represented by TDAB and the alkylamine, respectively. The tendency of the long-chain alkylamine to stay at the interface is likely to make this palisade layer static as compared to the palisade layer formed with the short-chain alkylamine. This effect is analogous to that seen for other surfactant systems, where the lifetime of the aggregate has been found to depend on the length of an added cosurfactant. For instance, in a microemulsion system containing sodium stearate as the surfactant, higher droplet stability was found when hexadecanol was used as a cosurfactant compared to when octanol was used.41 It is reasonable to assume that a more dynamic interface will favor the transport of new platinum-TDA complexes from the bulk toluene phase, where they exist in the form of ion pairs, into the water droplets, where they release the platinum to the growing nanoparticles. This difference in permeability of the palisade layer will then explain why short-chain alkylamines give larger particles than long-chain alkylamines. Consequently, it is possible to control the size of the nanoparticles formed by the phasetransfer method simply by adjusting the solubility of the stabilizing agent in the bulk organic medium.
The effect of the solvency of the stabilizing agent on the size of the nanoparticles formed in the phase-transfer method probably apply only to stabilizing agents that have moderate affinity for the metal surface. A stabilizing agent that forms strong bonds to the metal, such as an alkylthiol to platinum, will remain at the interface even if it has good solubility in the bulk organic solvent. Such a rigid palisade layer will be difficult for the platinumTDA complex to penetrate. As an example, the yield of platinum nanoparticles is low with dodecylthiol as the stabilizing agent as compared to the yield with dodecylamine as the stabilizer. Again, the comparison with the reverse micelle process helps us to understand the effect. Figure 9 shows TEM (A and B) and HRTEM (C and D) pictures of nanocrystals synthesized by the reverse micelle method and subsequently stabilized with either dodecylamine or dodecylthiol. As can be seen, the particles are less agglomerated when dodecylthiol, which forms a covalent bond to the metal, is used than when the more loosely bound dodecylamine is employed as the stabilizing agent.
Conclusions Platinum nanocrystals with controlled size and shape were synthesized either by a phase-transfer method or by reaction in AOT reverse micelles. The comparison between the two synthesis routes allows a better understanding of the mechanisms of growth and size control. The phase transfer method requires a stabilizing agent, and a range of alkylamines were tested for the purpose. It was found that there is a reverse relationship between the length of the alkyl chain of the stabilizer and the size of the nanoparticles formed. Acknowledgment. K.W. and K.H. thank Mistra (The Swedish Foundation for Strategic Environmental Research)/Jungner Centre for financial support via the program “Fuel Cells in a Sustainable Society”. K.W. also thanks H.M. King Carl XVI Gustaf of Sweden for a grant to spend time at Universite´ Pierre et Marie Curie, Paris, where this work was performed. C.P. and M.-P.P. thank the European Union for funding via the project GSOMEN (Growth and Supra Organization of Transition and Noble Metal Nanoclusters, contract no. NMP4-CT-2004-001594). LA060163M
