J. Phys. Chem. C 2007, 111, 16249-16254
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From Wormlike to Spherical Palladium Nanocrystals: Digestive Ripening K. Naoe,†,‡ C. Petit,† and M. P. Pileni*,† Laboratoire des Mate´ riaux Me´ soscopiques et Nanome´ triques, UMR CNRS 7070, UniVersite´ Pierre et Marie Curie,-Paris 6, 4 place Jussieu 75252 Paris Cedex 05, France ReceiVed: May 22, 2007; In Final Form: June 22, 2007
Similar wormlike palladium nanocrystals are produced by various synthetic methods such as reverse micelles and phase-transfer reactions. Palladium nanocrystals, stabilized by dodecylamine as a coating agent, are produced by chemical reduction of PdCl2 by sodium borohydride, NaBH4. A soft digestive ripening process, without any reflux, in which wormlike palladium particles go to sphere is observed in the presence of a rather large amount of dodecanethiol. The wormlike particle diameter is similar to that of the spheres produced.
I. Introduction In the last two decades, a very large number of groups have developed new strategies to produce nanomaterials. The use of reverse micelles was one of the first1,2 and has been followed by several others such as organometallic approaches3,4 and the phase-transfer process.5,6 A new strategy based on a general phase transfer and separation mechanism occurring at the interfaces of the liquid, solid, and solution phases is proposed.7 Palladium nanocrystals act as an excellent catalyst for hydrogenation of unsaturated hydrocarbons8,9 and many other applications like environmental catalysts.10 For this, the size of the nanocrystals has to be rather small and the coating agent has to be efficient enough to prevent coalescence and weak enough not to interact too much with the nanoparticle surface. Furthermore, the crystallinity of the nanoparticles has to be very high to be efficient. This is a real challenge and is why many groups have tried to find the best compromise. Hence, several ligands have been used in the past few years. Pd nanoparticles with a very low size distribution are extracted from dendrimer templates into toluene in the form of an alkanethiol stabilizer,11 which seems to be strongly attached to the nanocrystal interface. A recent study shows the importance of the thio-alkyl chain length in the sulfidation of the nanoparticles. For C18SH, the capping thiol occurs not only on the surface but also in the bulk. This makes the nanoparticles totally inefficient, whereas with C12SH the sulfidation rests on the surface and the core remains crystallized.12 As an example, poly(N-vinyl-2-pyrolidone) (PVP) as a coating agent produces either amorphous palladium nanocrystals13 or a crystalline phase after annealing. The opposite occurred with platinum nanocrystals.14 Furthermore, the change in size and shape with the coating agent used is still an open question.8 Digestive ripening was demonstrated with gold nanomaterials via three important steps. Several ligands such as alkylthiols, amines, and phosphines are very efficient in converting highly polydispersed particles to those that are nearly monodispersed. It was shown that it is important to reflux the colloid with only the digestive ripening agent without surfactant or others * To whom correspondence should addressed. E-mail: pileni@ sri.jussieu.fr. † Universite Paris 6. ‡ Permanent address: Department of Chemical Engineering, Nara National College of Technology, Yamato-Koriyama, Japan.
impurities.15 Furthermore, the shape transformation was found to be completely reversible.9 In this paper, similar wormlike palladium nanocrystals are produced by various synthetic methods such as reverse micelles and phase-transfer reactions. Whatever the chemical route is, wormlike structures are obtained. It is shown to result from aggregation of spherical nuclei. Soft digestive ripenings allows us to recover the initial spherical particles. II. Experimental Section II.1. Materials. Palladium (II) chloride (PdCl2, 99.9%) was from Sigma-Aldrich and used as received. Tetrakisdecylammonium bromide (TDAB) and sodium bis(2-ethylhexyl)sulfosuccinate (AOT), both 99% purity, were from Fluka. The dodecylamine (C12NH2) and dodecylthiol (C12SH) (99%) used in the study were from Aldrich, as was the reducing agent, sodium borohydride, NaBH4 (99%). The organic solvents (toluene, isooctane, cyclohexane, and ethanol) were all of analytical grade. Water was purified by a Millipore water system. All glassware was cleaned carefully in aqua regia and rinsed with large quantities of water before use. II.2.Transmission Electron Microscopy Measurements. Transmission electron microscopy (TEM) images were obtained using a JEOL 1011 operated at 100 kV with magnifications between 150 000 and 300 000 times. TEM sample grids were prepared by placing a drop of a 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 the results are shown in a histogram. The standard deviation, σ, was calculated according to
σ ) {[
∑ (Di - D)2]/[n - 1]}1/2
where D is the average diameter. A polydispersity index was defined as the ratio σ/D. For high-resolution transmission electron microscopy (HRTEM) measurements of the crystallinity of the prepared palladium nanoparticles, a JEOL 2010 UHR instrument, operated at 200 kV (LaB6), was used. III. Syntheses of Palladium Nanocrystals III.1. Synthesis in Reverse Micelles (Procedure I). Two solutions of 0.25 M AOT in isooctane forming reverse micelles
10.1021/jp073957y CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007
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Figure 1. (A-C) TEM images, (D-F) HRTEM micrographs, and (G-I) histograms of the diameter of wormlike palladium particles coated by dodecylamine (A, D, and G) synthesized by the reverse micelle method (B, C, E, F, H, and I) synthesized by the phase-transfer method (B, E, and H) via procedure II.A (addition of the coating agent before reduction) (C, F, and I) via procedure II.B (addition of the coating agent after reduction).
are mixed with a fixed water content. One contains 1.125.10-3 M of PdCl2 and the other 2.10-2 M of NaBH4. The water-tosurfactant molar ratio (W) is fixed at 20. This corresponds to a water-pool diameter of 6 nm. Immediately after mixing the two reverse micelle solutions, the solution color turned from yellowbrown to black, indicating the formation of palladium nanocrystals. After 45 min, 2.17 × 10-1 M dodecylamine (C12NH2) is added as a stabilizing agent to the solution. After 1 h, isooctane is removed by evaporation and the waxy residue is redispersed in 45 mL of ethanol, followed by centrifugation at 5000 rpm for 10 min. The black precipitate is redispersed in 30 mL of fresh ethanol and again centrifuged. This washing step is repeated twice. After the washing with ethanol, the black precipitate is redispersed in 6 mL of toluene. The addition of another 10 µL of C12NH2 is required to obtain stable dispersion in the solvent. III.2. Phase-Transfer Reaction. This is based on a synthesis of Brust.5 Typically, the phase-transfer synthesis method consists of the transfer of metal ions to an organic phase with a surfactant, addition of a stabilizing agent, reaction by a reducing agent, and washing processes. An aqueous solution of 0.05 M PdCl2 is mixed with 2.17 × 10-2 M tridecylammonium bromide, TDAB, dissolved in toluene. The solution was stirred for at least 2 h upon which the water phase turned almost colorless. In this study, the dodecylamine (C12NH2) used as the stabilizing agent is added either before or after the reduction reaction: Procedure II-A. Addition of a stabilizing agent before reduction: 6.52 × 10-3 M of C12NH2 is added to the solution under continuous stirring and sodium borohydrate, NaBH4, is added to keep the ratio [NaBH4]/[PdCl2] ) 16.7. The mixture turns relatively quickly from brown to black and is stirred for
1 h. The organic phase containing the Pd nanoparticles is then separated from the aqueous one. Procedure II-B. Addition of the stabilizing agent after reduction: NaBH4, is first added with the same experimental conditions as procedure I. After 1 h of stirring, C12NH2 is added and the solution is left overnight with stirring and the organic phase is collected. In both procedures, the collected organic phase is evaporated. Ethanol (400 mL) is added to the black paste obtained at the end of the evaporation and kept at -18 °C overnight. The supernatant is removed, and the residual mixture is centrifuged at 5000 rpm for 10 min. After removing the supernatant, 30 mL of ethanol is added, inducing particle dispersion. The solution is then centrifuged again. This procedure is repeated, and ethanol is replaced by 6 mL of toluene. Addition of another 5 µL of stabilizing agent is required to obtain a stable dispersion in the toluene. IV. Results and Discussion IV.1. Wormlike Particles Made by Various Pocedures. IV.1.1. ReVerse Micelles.2 The palladium particles are synthesized as described in procedure I. At the end of the synthesis, the particles are extracted by C12NH2 and dispersed in toluene. A drop of solution is deposited on a TEM grid. Figure 1A and the inset show the formation of wormlike particles. Highresolution TEM (HRTEM) shows that the wormllike particles are well-crystallized (Figure 1D) with an fcc structure that fully agrees with other publications.13,16 Note that a change in the contrast along the particles is observed. It can thus be assumed that the wormlike particles are composed of associated elongated
Wormlike to Spherical Palladium Nanocrystals
Figure 2. (A) TEM image of particles made in reverse micelles 5 min after the reduction starts and (B) histogram of the spheres shown in A.
or spherical nanocrystals with a few lattice planes in between. The histogram of the wormlike diameters shows a very broad distribution with an average diameter of 4.2 nm and a standard deviation of 18%. All of the presented data are obtained by mixing two reverse micelles and waiting 45 min before adding dodecylamine (C12NH2), as a stabilizing agent, to the solution. Waiting 5 min after the reaction starts instead of 45 min, very few wormlike particles (Figure 2A) and a rather large amount of spheres are observed with an average spherical diameter and distribution of 4 nm and 16.5%, respectively (Figure 2B). The close values of the average diameters of wormlike particles (Figure 1G) and spheres (Figure 2B) and the HRTEM images (Figure 1D) indicate that the wormlike particles are due to a specific association of spheres. IV.1.2. Phase-Transfer Method (Procedure II). As described above, two procedures are followed: the stabilizing agent is added either before (procedure II.A) or after (procedure II.B) the chemical reduction of palladium ions takes place. Note that the experimental conditions (PdCl2, TDAB, and C12NH2 concentrations, volume, etc.) remain the same. For either procedure, the wormlike particles are produced with a behavior similar to that observed in reverse micelles (Figure 1B and C and their insets). Note that the amount of material produced is rather large when the stabilizing agent is added before (Figure 1B and inset) than after reduction (Figure 1C and inset). Furthermore, the observed shape of the wormlike particles made by procedure II.B is less-uniform (Figure 1C) than those produced via procedure II.A. The average wormlike lengths and diameters are smaller than those obtained by reverse micelles. The histograms of the wormlike diameters (Figure 1H and I) are
J. Phys. Chem. C, Vol. 111, No. 44, 2007 16251 similar with average diameters of 3.5 and 3.2 nm and size distributions of 14.3% and 17.6% before (procedure IIA) and after (procedure II.B) reduction, respectively. The HRTEM patterns (Figure 1E and F) confirm the high fcc crystallinity of the wormlike particles, as observed from the synthesis made in reverse micelles (Figure 1D), with the appearance of spheres in the internal structures. The change in the contrast compared to reverse micelles is probably due to the difference in the wormlike size (average diameters of 4.25, 3.5, and 3.2 nm for syntheses in reverse micelles and the phase-transfer process before and after reaction, respectively). The change in the average wormlike diameter via procedures II.A (3.5 nm) and II.B (3.2 nm) is attributed to the fact that smaller particles are always observed when the stabilizing agent is added before the reduction (see below). Unfortunately, with these procedures, it is impossible to perform kinetic experiments similar to those described above in reverse micelles. From these results, it is concluded that addition of dodecylamine, C12NH2, as a stabilizing agent induces the formation of well-defined wormlike fcc crystals, probably made of associated spheres. We must note that, usually, the sizes of nanomaterials are smaller when the stabilizing agent is added before the reduction. In the present case, the opposite behavior is observed and could be attributed to the nature of the stabilizing agent (see below). Similar features were obtained by hydrogenation of the precursor.8 With a relatively low concentration of hexadecylamine, HAD, the particle growth could not be controlled and the syntheses led to spongelike particles (HAD/Pd ) 0.1). At HAD/Pd ) 10, wormlike particles with a very small amount of spheres are produced. In the present case, the C12NH2/Pd ratio is 21.7 and no spherical particles are formed. The presented data clearly show that with C12NH2 as the stabilizing agent the formation of wormlike particles is not related to the synthesis modes but mainly to the weak attachment of the coating agent. The mechanism of formation of the wormlike structure by aggregation of individual spherical nanocrystals is not clear. Similar structures have been observed due to dipolar interaction with CdTe nanocrystals17 or gold nanorods.18 However, in the absence of external excitation, the apparition of dipoles can be excluded in our case. The branched or wormlike structure observed (Figures 1 and 3) are in fact more related to a diffusionlimited aggregation, as observed in the case of weakly interacting nanocrystals yielding to branched or wormlike structures.19 IV.2. Soft Digestive Ripening at Room Temperature. Dodecylthiol (0.84 M C12SH) was added to the wormlike particles, produced by different synthesis modes, and dispersed in toluene. These solutions were kept overnight at room temperature with continuous stirring for more than 12 h, though digestive ripening takes only a few hours. Some differences are observed on ripening for the various syntheses: IV.2.1. ReVerse Micelles. Wormlike particles produced from reverse micelles show a small decrease in their number with the appearance of very few spherical nanocrystals (Figure 3A). Both particles are highly crystalline (Figures 3D1 and D2). The histograms corresponding to spheres are based on 50 particles because of the small number of spherical nanocrystals produced compared to wormlike ones. Nevertheless, it shows that the average diameters of spheres and wormlike particles are close, 3.8 and 4.4 nm with size distributions of 9.5% and 13.5%, respectively. This demonstrates that the addition of C12SH breaks down the wormlike structures slightly. Furthermore, these values are in the same experimental range as those obtained when C12NH2 is added 5 min after the reduction starts (4 nm, 16.5%). However, this process is not very efficient and a relatively small
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Figure 3. Effect of additional dodecylthiol on the shape and size of dodecylamine-stabilized Pd nanocrystals by each procedure. (A-C) TEM and (D-F) HRTEM micrographs and (G-I) histograms of spheres and diameter of wormlike palladium nanocrystals (A, D, and G) by the reverse micelle method and by the phase-transfer method with the stabilizing agent addition (B, E, and H) before reduction or (C, F, and I) after reduction.
amount of spheres is produced. To favor the digestive ripening of C12SH to wormlike particles, the amount of C12SH is increased (0.84 M < [C12SH] < 3.36 M) without any differences in the experimental conditions. No more spheres are produced, and wormlike particles are still formed. This means that, compared to what is observed below, the spheres forming the wormlike particles are highly aggregated. This is probably due to the fact that some AOT molecules remaining from the synthesis are highly bound to the wormlike surface. IV.2.2. Phase-Transfer Method. After C12SH addition of the colloidal solution containing the wormlike particles made via procedure II, either isolated spherical particles or aggregates composed of spheres (Figure 3B and C) appear. Again, in all cases, the HRTEM patterns show a high crystallinity (fcc) of the particles (Figure 3E and F). The average diameter and size distribution of the spherical nanocrystals produced before reduction (procedure II.A) are 3.6 nm and 8.5%, respectively. These values are in the same experimental error range as the average width particle diameter and size distribution (3.5 nm 14.3%) of wormlike particles. This confirms that, as claimed above, the wormlike particles are made of spherical particles. Hence, homogeneous wormlike particles obtained via procedure II.A produce spherical particles with the same average width and size distribution (3.6 nm, 8.5%) and (3.5 nm, 14.3%) respectively. Figure 4 shows that the average diameter of spheres remains unchanged whatever the concentration of the added C12SH (3.7 nm). However, the size distribution of the spherical nanocrystals (Figure 4D-F) decreases on increasing the C12SH from 12% to 10.3% to 8.5% at 0.42 M, 0.56 M, and
0.84 M C12SH concentrations, respectively. It is thus concluded that C12SH breaks down the wormlike particles to produce mainly spherical particles keeping the same diameter. In the latter case, the size distribution of the spherical palladium nanocrystals is low enough to form highly compact selforganizations in 2D superlattices. Unfortunately, small wormlike particles remain and this is sufficient to favor such an assembly on a large scale. Hence, by using procedure II.A, for any C12SH concentration employed (0.42 M < [C12SH] < 0.84 M), the TEM patterns show spheres and residues of wormlike particles (Figure 3). The histograms of spheres remain similar with the same average size (3.7 nm), indicating that a rather low amount of C12SH is needed to produce spheres. However, it is not enough to eliminate the wormlike particles. With respect to the heterogeneous wormlike particles (Figure 1C) obtained via procedure II.B, addition of 0.84 M C12SH induces formation of spheres but a number of wormlike particles remain (Figure 3C). The histogram of the spherical particles (Figure 3I) shows a bimodal distribution of spherical nanocrystals with average diameters of 2.1 and 3.7 nm. As mentioned above, the wormlike particles produced via procedure II.B are not very uniform. This is confirmed by the presence of a bimodal distribution of spheres when C12NH2 is added after the reduction. In fact, we would expect, as is observed (Figure 5), a decrease in size by replacing C12NH2 by C12SH as the coating agent. The average nanocrystal diameters are smaller (Figure 5C) when the coating agent is added before (2.3 nm) rather than after (3 nm) the reduction as shown in Figure 5B. However, the size distributions (20.8% and 22.8%, respectively) are rather large.
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Figure 4. Effect of amount of additional dodecylthiol on (A-C) shape and (D-F) size of Pd nanocrystals synthesized by the phase-transfer method with dodecylamine before reduction. The amount of additional dodecylthiol: (A and D) 0.208 mmol, (B and E) 0.313 mmol, and (C and F) 0.417 mmol.
Figure 5. (A-C) TEM micrographs and (D-F) size histograms of Pd nanocrystals (A and D) synthesized by the reverse micelle method and by the phase-transfer method with C12SH as the stabilizing agent. (B and E) addition before reduction and (C and F) after reduction.
In reverse micelles, most of the particles produced are spherical nanocrystals (Figure 5A) with an average diameter of 4.6 nm and a size distribution of 14.8% (Figure 4D) in the same range of the average diameter of wormlike particles (4.2 nm) with a size distribution of 18% (Figure 1G). High-resolution TEM imaging shows that highly crystalline fcc nanoparticles are produced (Inset Figure 5A). This agrees with several groups.20 From these data, it seems that C12NH2 plays a key role in producing well-defined wormlike particles with a mechanism differing markedly from those described over the past decade. This has to be related to the nature of C12NH2 molecules, which favor a weak interaction between PdCl2 and a homogeneous crystal growth when they are present before the reaction. Conversely, when the reduction occurs before adding C12NH2,
it just plays the weak role of stabilizing nanoparticles already produced. It does not play any role in procedure I because the reduction occurs in a confined medium that favors homogeneous growth. V. Conclusions From the above data, it seems clear that dodecylamine, C12NH2, is a weak coating agent and is not able to prevent coalescence of palladium spherical nanocrystals. However, it is a good agent for homogeneous nucleation. The coalescence process is far from being complete and well-defined wormlike particles are produced. This was observed previously by hydrogenation of the precursor in the presence of a weakly coordinated ligand such as hexadecylamine.8 The main results
16254 J. Phys. Chem. C, Vol. 111, No. 44, 2007 in this paper show that the procedure to make wormlike particles made of spheres does not depend on the experimental method used. From this, we could expect to produce the same wormlike particles with various methods, which cannot be made with a strong coating agent. However, the reactants involved in the formation of such wormlike nanocrystals (tridecylammonium bromide or AOT) obviously play an important role in the stability of the wormlike particles. From the above data, dodecanethiol (C12SH) is an efficient agent for producing digestive ripening, as described already by Klabunde et al.15 for gold nanoparticles. However, Klabunde et al. claimed that three important steps are involved in the digestive ripening. In the first step, ligand is added to break down large and polydispersed colloids into smaller ones, whereas the second one is devoted to isolating the smaller particles from reactive products. The last step is refluxing the solution containing the smaller colloids at the solvent boiling temperature in presence of a surface-active ligand to decrease the size distribution markedly. In the present case, the one step is needed to break down wormlike particles into spheres with about the same diameters. Unfortunately, some elongated particles remain and prevent the formation of a very regular self-organization of palladium nanocrystals in a compact hexagonal network. Other ligands are probably needed to eliminate such a small number of wormlike particles. Note that the method described here is very soft because the digestive ripening takes place in a few hours at room temperature. In most cases, annealing is needed to improve the particle crystallinity, whereas here both wormlike particles and spheres are highly crystallized with an fcc structure. An additional effect is that dodecylamine does not behave as a normal coating agent, favoring more homogeneous growth with larger particles when it is added before the reduction. This shows how important it is to have a detailed knowledge of the synthesis mode to understand the crystal growth because of the influence of the reactive and residual entities on this growth.21
Naoe et al. Acknowledgment. K.N. thanks Institute of National Colleges of Technology, Japan for a grant to spend time at Universite´ Pierre et Marie Curie, Paris, where this work was performed. References and Notes (1) Pileni, M.-P. J. Phys. Chem. 1993, 97, 6961. (2) Petit, C.; Pileni, M. P. J. Phys. Chem. 1989, 92, 2282. (3) Murray, C. B.; Norris, D. J.; Bawendi J. Am. Chem. Soc. 1993, 15, 8706. (4) Peng, X. G.; Manna, L.; Yang, W.; Wickham, J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (5) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 4, 1234. (6) Brust, M.; Fink, J.; Schiffrin, D. J.; Kiely, C. J. Chem. Soc., Chem. Commun. 1994, 4, 1234. (7) Wang, H.; Zhuang, J.; Peng, Q.; Li, Y. Nature 2005, 437, 121. (8) Ramized, E.; Jansat, S.; Philippot, K.; Lecante, P.; Gomez, M.; Masdeu-Bulto, A.; Chaudret, B. J. Organomet. Chem. 2004 689, 4601. (9) Stoeva, S. I.; Zaikovski, V.; Prasad, B. L. V.; Stoimenov, P. K.; Sorensen, C.; Klabunde, K. J. Langmuir 2005, 21, 10280. (10) Nutt, M. O.; Hughes, J. B.; Wong, M. S. EnViron. Sci. Technol. 2005, 39, 1346. (11) Garcia-Martinez, J. C.; Scott, R. W. J.; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 11190. (12) Amallo-Lopez, J. M.; Giovanetti, L.; Craievich, A. F.; Vicentin, F. C.; Marin-Almazo, M.; Jose´-Yacaman, M.; Requejo, F. C. Physica B 2006. (13) Wang, W. L. B.; Wang, K.; Wand, Y.; Hou, I. G. Langmuir 2003, 19, 5887. (14) Dassenoy, F.; Phillippot, K.; Ould-Ely, T.; Amiens, C.; Lecante, P.; Snoeck, E.; Mosset, A.; Casanove, M. J.; Chaudret, B. New J. Chem. 1998, 703. (15) Prasad, B. L. V.; Stoeva, S. I.; Sorensen, C. M.; Klabunde, K. J. Chem. Mater. 2003, 15, 935. (16) Sun, Y.; Frenkel, L.; Isseroff, R.; Shondrun, C.; Forman, M.; Shin, K.; Koga, T.; White, H.; Zhang, L.; Zhu, Y.; Rafailovish, M. H.; Sokolov, J. C. Langmuir 2006, 22, 807. (17) Tang, Z.; Ozturk, B.; Wang, Y.; Kotov, N. A. J. Phys. Chem. B 2004, 108, 6927. (18) George Thomas, K.; Barazzouk, S.; Ipe, B. I.; Shibu Joseph, S. T.; Kamat, P. V. J. Phys. Chem. B 2004, 108, 13066. (19) Witten, T. A.; Sander, L. M. Phys. ReV. B 1983, 27, 5686. (20) Quiros, I.; Yamada, M.; Kubo, K.; Mizutani, J.; Kurihaha, M.; Nashihara, H. Langmuir 2002, 18, 1413. (21) Pileni, M. P. J. Phys. Chem. C, in press.
