Size-Selective Synthesis of Cubooctahedral Palladium Particles

Synthesis and Characterization of N,N-Dimethyldodecylamine-Capped Aucore-Pdshell Nanoparticles in Toluene. Sudip Nath, Snigdhamayee Praharaj, Sudipa ...
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Langmuir 2003, 19, 4817-4824

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Size-Selective Synthesis of Cubooctahedral Palladium Particles Mediated by Metallomicelles Bernadett Veisz and Zolta´n Kira´ly* Department of Colloid Chemistry, University of Szeged, Aradi Vt. 1, H-6720 Szeged, Hungary Received January 28, 2003. In Final Form: March 16, 2003 The reduction of [PdCl4]2- by hydrazine in the postmicellar region of even-numbered alkyltrimethylammonium bromides CnTABr (with n ) 8 to 16) yielded fairly monodisperse Pd particles with mean diameters in the range 1.6-6.8 nm. Transmission electron microscopy (TEM) measurements indicated that the particle size decreases with increasing length of the alkyl chain, decreasing precursor concentration, and decreasing surfactant concentration. Kinetic measurements demonstrated that the nucleation is a fast process and well separated from the growth. TEM and high-resolution TEM micrographs of the particles suggested that the dominant morphology is cubooctahedral. Composition analysis (1H NMR, total organic carbon, and inductively coupled plasma atomic emission spectroscopy), titration microcalorimetry, and spectroscopic methods (IR, Raman, and UV-vis) revealed that [PdCl4]2- is not the real precursor species in the reduction process. In the presence of a large excess of Br-, [PdCl4]2- transforms to [PdBr4]2-. The ligand-exchange reaction is accompanied by the formation of stoichiometric complex-surfactant aggregates [CnTA]2[PdBr4], and followed by a slow, higher-order aggregation to microcrystals. The organic salt precipitates below the critical micelle concentration but undergoes solubilization above it, leading to the formation of palladate-surfactant metallomicelles. In the postmicellar region, the close proximity of the surfactant molecules to the reduction centers ensures rapid adsorption of the amphiphiles on the surface of the nascent particles, and a protective bilayer is formed, ultimately leading to long-term stability of the Pd hydrosols.

Introduction The chemical reduction of Pd(II) precursor compounds to colloidally stable Pd nanoparticles can be achieved in a variety of micellar systems, including normal micellar solutions,1-4 inverse micellar solutions,5-7 media containing swollen micelles,8,9 liquid crystals,10 inverse microemulsions,11-14 and biphasic oil/water systems.15 The particle size and size distribution are determined by the relative rates of the individual steps of nucleation, growth, and particle stabilization.16 Seed formation begins when * Corresponding author. E-mail: [email protected]. (1) Toshima, N.; Takahashi, T. Bull. Chem. Soc. Jpn. 1992, 65, 400409. (2) Nakao, Y.; Kaeriyama, K. J. Colloid Interface Sci. 1985, 110, 82-87. (3) Jana, N. R.; Wang, Z. L.; Pal, T. Langmuir 2000, 16, 2457-2463. (4) Okitsu, K.; Bandow, H.; Maeda, Y. Chem. Mater. 1996, 8, 315317. (5) Arul Dhas, N.; Gedanken, A. J. Mater. Chem. 1998, 8, 445-451. (6) Bo¨nnemann, H.; Brijoux, W.; Brinkmann, R.; Fretzen, R.; Joussen, T.; Ko¨ppler, R.; Korall, B.; Neiteler, P.; Richter, J. J. Mol. Catal. 1994, 86, 129-177. (7) Reetz, M. T.; Helbig, W.; Quaiser, S. A.; Stimming, U.; Breuer, N.; Vogel, R. Science 1995, 267, 367-369. (8) Kira´ly, Z.; Veisz, B.; Mastalir, AÄ .; Ra´zga, Zs.; De´ka´ny, I. Chem. Commun. 1999, 1925-1926. (9) Kira´ly, Z.; Veisz, B.; Mastalir, A Ä .; Ko¨farago´, Gy. Langmuir 2001, 17, 5381-5387. In this article the formula given for the apparent dispersion of quasi-spherical Pd particles is incorrect. The correct expression is Dapp ) 1.07/d, where d is the particle diameter in nm. (10) Puvvada, S.; Baral, S.; Chow, G. M.; Qadri, S. B.; Ratna, B. R. J. Am. Chem. Soc. 1994, 116, 2135-2136. (11) Boutonnet, M.; Kizling, J.; Stenius, P. Colloids Surf. 1982, 5, 209-225. (12) Chen, D.-H.; Wang, C.-C.; Huang, T.-C. J. Colloid Interface Sci. 1999, 210, 123-129. (13) Berkovich, Y.; Garti, N. Colloids Surf. 1997, 128, 91-99. (14) Arcoleo, V.; Cavallaro, G.; La Manna, G.; Turko Liveri, V. Thermochim. Acta 1995, 254, 111-119. (15) Esumi, K.; Shiratori, M.; Ishizuka, H.; Tano, T.; Torioge, K.; Meguro, K. Langmuir 1991, 7, 457-459. (16) Kizling, J. Nonionic Microemulsions as Vehicles for the Preparation of Small Metal Particles. Ph.D. Thesis, The Royal Institute of Technology, Stockholm, 1991; Chapter 3.

the Pd precursor species incorporated in the micellar system in the oxidized state are reduced to zerovalent Pd atoms. For surfactant-mediated synthesis, mechanistic considerations suggested that the limited aggregation of Pd nuclei to nanoscale particles may proceed via compartmentalization10,13,14 and via the arrested growth mechanism.11,12,16,17 Compartmentalization implies that the interior of a micellar aggregate, with the inner pool either spherical in shape13,14 or a network of interconnected channels,10 may be regarded as a microreactor in which the precursor molecules are dissolved and reduced. The finite aggregation of Pd nuclei is thought to be restricted to the size of the pool insofar as the primary particles are efficiently stabilized by surfactant molecules adsorbed from the surrounding micellar microenvironment. In contrast, the arrested growth mechanism assumes that geometric constraint is not a prerequisite for nanoparticle formation, but the particle growth is limited by the rapid adsorption of surfactant molecules at a very early stage of the aggregation process, and in this way a colloidally stable hydrosol is formed.9,11,16,17 This mechanism may also be valid under compartmentalization conditions. In fact, the poor correlation between the final particle size and the initial precursor concentration in a single pool suggested that the formation of primary particles in separate compartments is followed by restricted coalescence and that compact secondary particles develop outside the micellar aggregates.11,16 Since the underlying mechanism is at present not well understood in either case, the design of novel, surfactant-mediated synthetic routes with good control of size and shape remains of great practical importance at the empirical level. Among the wide selection of organic and inorganic precursor compounds, PdCl2 is a commonly available, rather inexpensive starting material for the preparation (17) O′Sullivan, E. C.; Ward, A. J. I.; Budd, T. Langmuir 1994, 10, 2985-2992.

10.1021/la034146y CCC: $25.00 © 2003 American Chemical Society Published on Web 04/09/2003

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of Pd particles.1-4,11,15,18-21 This compound is insoluble in water and in noncomplexing organic solvents. However, it dissolves readily in aqueous solutions containing an excess of Cl-. The resulting complex anion [PdCl4]2- is charge neutralized by dissociated H+ or alkali metal cations. On the other hand, alkyltrimethylammonium bromides are prototypes of cationic surfactants whose solution properties have been thoroughly investigated.22 We report here on the size-controlled preparation of monodisperse Pd nanoparticles in K2[PdCl4]/CnTABr normal micellar solutions. The size tailoring of metal particles on the nanometer scale is of current interest in the fields of catalysis, nanoelectronics, and materials science.23-25 The high potential of the present synthesis method in heterogeneous catalysis has recently been demonstrated via surface chemical studies26 and selective hydrogenation reactions.9,27,28 Materials and Methods K2[PdCl4] (98%, Aldrich), KBr (99%, Aldrich), n-alkyltrimethylammonium bromides CnTABr with n ) 8 and 10 (98%, Fluka), 12 and 16 (99%, Sigma), and 14 (99%, Aldrich), 5 M NH2NH2‚H2O in water (p.a., Fluka), and 2-propanol (p.a., Reanal) were used as received. Reagent-grade water was produced by a Milli-Q filtration system. Phase Behavior of K2[PdCl4] with C14TABr in Aqueous Solutions. The phase diagram of the K2[PdCl4]/C14TABr system was mapped at room temperature by visual inspection of a large number of samples differing in composition. In one set of experiments, a series of vials containing C14TABr at constant concentration were prepared and the concentration of K2[PdCl4] was successively increased until precipitation occurred. Endto-end stirring was applied for 24 h, and the composition of the micellar solution at which a second phase appeared after an equilibration period of 24 h was recorded. The experiment was repeated at different C14TABr concentrations in the range 1 to 100 times the critical micelle concentration (cmc) ) 3.9 mM (for the cmc values of the CnTABr series, see, e.g., ref 22). Chemical Analysis of the Precursor/Surfactant Organic Salt. The organic salt was filtered off, thoroughly washed with water on a 0.2 µm Millipore filter, and dried. The metal-halogen bonds in the salt were investigated by IR spectroscopy (BIO-Rad Digilab Division FTS-40V, far-IR region) and Raman spectroscopy (BIO-Rad dedicated Raman spectrometer). Measurements were made on pelleted samples, prepared either directly from the salt (Raman) or from a 1:20 mixture of the salt with polyethylene (IR). The C content of the salt was determined with a Euroglas TOC 1200 total organic content analyzer, the Pd content by inductively coupled plasma atomic emission spectroscopy (ICPAES) at 229.7 and 324.3 nm (Jobin Yvon 24), and the H content by 1H NMR spectroscopy (Bruker Avance DRX 500) in CDCl3 solution with benzene as the internal standard. Precursor-Surfactant Interactions in the Premicellar and Postmicellar Regions. Interactions between K2[PdCl4] and C14TABr in the surroundings of the cmc were investigated (18) Turkevich, J.; Kim, G. Science 1970, 169, 873-879. (19) Michel, J. B.; Schwartz, J. T. In Preparation of Catalysts IV; Delmon, B., Grange, P., Jacobs, P. A., Poncelet, G., Eds.; Elsevier: Amsterdam, 1987; pp 669-687. (20) Hoogsteen, W.; Fokkink, L. G. J. J. Colloid Interface Sci. 1995, 175, 12-26. (21) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594-600. (22) van Os, N. M.; Haak, J. R.; Rupert, L. A. M. Physicochemical Properties of Selected Anionic, Cationic and Nonionic Surfactants; Elsevier: Amsterdam, 1993; pp 110-136. (23) Schmid, G. Chem. Rev. 1992, 92, 1709-1727. (24) Schmid, G. In Clusters and Colloids: From Theory to Applications; Schmid, G., Ed.; VCH: New York, 1994; pp 183-188. (25) Aiken, J. D., III.; Finke, R. G. J. Mol. Catal. A 1999, 145, 1-44. (26) Veisz, B.; Kira´ly, Z.; To´th, L.; Pe´cz, B. Chem. Mater. 2002, 14, 2882-2888. (27) Mastalir, A Ä .; Kira´ly, Z.; Szo¨llo¨si, Gy.; Barto´k, M. Appl. Catal. A. 2001, 213, 133-140. (28) Mastalir, A Ä .; Kira´ly, Z.; Szo¨llo¨si, Gy.; Barto´k, M. J. Catal. 2000, 194, 146-152.

Veisz and Kira´ ly by UV-vis spectroscopy (UVIKON 930) at room temperature. While the concentration of K2[PdCl4] was held constant at 0.04 mM, the progressive spectral change in the K2[PdCl4]/C14TABr system was investigated in the range 0-8 mM C14TABr. Precursor-surfactant interactions were further investigated at 298.15 K by using an isotherm titration microcalorimeter (TAM 2277). Thermometric titration of 2 mL of C14TABr solution was performed with a feed solution (5 mM) of K2[PdCl4] in 20 µL aliquots, both below (2 mM) and well above (200 mM) the cmc. For elucidation of the effect of ligand exchange, if any, 2 mL of KBr solution was titrated with K2[PdCl4], under otherwise the same conditions as above. For each set of measurements, corrections were made for the enthalpies of dilution. The blank experiments included titrations of C14TABr and KBr solutions with water, and titration of water with K2[PdCl4] solution. Preparation and Characterization of the Pd Hydrosols. Nanosized Pd particles were prepared by the reduction of K2[PdCl4] (0.20-2.75 mM) in the presence of cationic surfactants (1-30 times the cmc). A series of aqueous solutions of the precursor compound and the surfactant homologues were prepared separately and then mixed to give the desired concentrations. The reaction system was subjected to vigorous stirring when the reducing agent, an aqueous solution of hydrazine hydrate, was introduced into the vessel in a 50-fold excess over the precursor metal. (In a typical experiment, 4.0 cm3 of C14TABr (100 mM) was mixed with 5.5 cm3 of K2[PdCl4] (5.0 mM), followed by the addition of 0.5 cm3 of hydrazine (2.75 M) to produce a Pd hydrosol with a mean particle diameter of 4.3 nm.) The reaction was conducted at room temperature, with the exception of C16TABr; this solution was gently heated above the Krafft temperature of 299 K,22 where the solubility of the surfactant becomes sufficient for micelle formation. The particle size distributions were determined by using a Philips CM-10 transmission electron microscope operated at 100 kV. One drop of a hydrosol was placed on a Formvar-covered copper grid, and measurements were made after solvent evaporation. The microscope was equipped with a Megaview II digital camera. Size distribution analysis was performed by counting over 200 particles and using AnalySIS 3.1 software. The poor contrast, caused by excess surfactant deposited on the sample, led to some difficulties with the identification of the morphology of the particles. However, sharp-cut projections were observed when the particles were first precipitated with 2-propanol, the surfactants were washed away, and a drop of suspension was placed on the grid. High-resolution TEM (HRTEM) images were taken with JEOL-3010 equipment operating at 300 kV and with a nominal resolution of 0.17 nm. The kinetics of formation of the Pd colloids in a micellar system (0.08 mM K2[PdCl4]; 20 times the cmc of C14TABr) was continuously monitored by using an Ocean Optics UV-vis fiberoptic spectrometer. The solution was kept under magnetic stirring in a 1 cm quartz cell. After the addition of a 20-fold excess of the reductant, the spectral change in the range from 200 to 700 nm was recorded as a function of time with a time resolution of 2.7 s per spectrum.

Results and Discussion Monophasic and Biphasic Behavior of K2[PdCl4]/ CnTABr in Aqueous Solution. Transition metal complexes in media containing surfactants often display peculiar behavior. Specific complex-surfactant interactions have been described in both organic and aqueous media.5,6,29,30,31a,b,32-37 Examples relevant to the present (29) Bouquillon, S.; du Moulinet d′Hardmare, A.; Averbuch-Pouchot, M.-T.; He´nin, F.; Muzart, J. Polyhedron 1999, 18, 3511-3516. (30) Weddle, K. S.; Aiken, J. D., III.; Finke, R. G. J. Am. Chem. Soc. 1998, 120, 5653-5666. (31) In Gmelin Handbook of Inorganic Chemistry, Palladium, Supplement; Griffith, W., Robinson, S. D., Swars, K., Eds.; Springer: Berlin, 1989; Vol. B2: (a) p 124; (b) p 127; (c) p 204; (d) p 88. (32) Torigoe, K.; Esumi, K. Langmuir 1992, 8, 59-63. (33) Kameo, A.; Suzuki, A.; Torigoe, K.; Esumi, K. J. Colloid Interface Sci. 2001, 241, 289-292. (34) Snyder, S. W.; Buell, S. L.; Demas, J. N.; DeGraff, B. A. J. Phys. Chem. 1989, 93, 5265-5271. (35) Cavasino, F. P.; Sbriziolo, C.; Cusumano, M.; Gianetto, A. J. Chem. Soc., Faraday Trans. 1 1989, 85, 4237-4236.

Size-Selective Synthesis of Palladium Particles

Figure 1. Maximum additive concentration (MAC) plotted against surfactant concentration. The straight line represents a nominal (but hypothetical) binding of 0.5 palladate complex per C14TABr micelle.

work include the binding of [PdCl4]2-,31a,b [AuCl4]-,32,33 anionic and cationic Ru(II) complexes,34 and cationic Pd(II) complexes35-37 to a variety of oppositely charged surfactants in aqueous solution, in both the premicellar and postmicellar regions. Our visual observations during the preparation of Pd hydrosols likewise indicated that [PdCl4]2- interacts with cationic surfactants before the reduction process is induced. To clarify this point, C14TABr was selected for use in a systematic study on the nature of the specific complex-surfactant interactions. Upon addition of an excess of C14TABr to K2[PdCl4], both dissolved in water, the pale-yellow solution turned orangered in the premicellar region, with a further color intensification in the postmicellar region. We attributed these observations to the formation of complex-surfactant premicellar and complex-surfactant postmicellar aggregates below and above the cmc, respectively. Upon storage, microphase separation occurred below the cmc and an orange-red organic salt precipitated slowly. Above the cmc, the apparent solubility of the organic salt increased with increasing surfactant concentration. This observation can be explained in terms of solubilization in the classical sense, i.e., the incorporation of the alkyl chains of the preformed organic salt in the hydrophobic core of the micelles. Alternatively, binding of the complex anions to the palisade layer of the micelles may occur. The structure and composition of these metallomicelles are practically the same in either case, provided that the number of individual palladate-surfactant species incorporated in a micelle, or the number of palladate anions bound to one micelle, is small in comparison with the aggregation number of the micelle. The saturation concentration of a solubilizate for a given concentration of surfactant is termed the maximum additive concentration (MAC),38 conveniently expressed here in terms of (equivalent) K2[PdCl4] concentration. The phase diagram of the K2[PdCl4]/C14TABr system determined by visual observations is given in Figure 1. Each bar in the figure represents saturation, i.e., the MAC at the given surfactant concentration. In the composition range which covers the area below the bars, the solubilized moieties are incorporated into the system with maintenance of a single isotropic solution. Outside this region, i.e., when the concentration of K2[PdCl4] is increased to the area above the bars, (36) Calvaruso, G.; Cavasino, F. P.; Sbriziolo, C. J. Chem. Soc., Faraday Trans. 1996, 92, 2263-2268. (37) Cavasino, F. P.; Sbriziolo, C.; Turco Liveri, M. L. J. Phys. Chem. B 1998, 102, 3143-3146. (38) Attwood, D.; Florence, A. T. Surfactant Systems: Their Chemistry, Pharmacy and Biology; Chapman and Hall: London and New York, 1983; Chapter 5.

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supracolloidal aggregates are formed with concominant sedimentation. The straight line in Figure 1 conforms with the surfactant aggregation number of 70 reported for spherical C14TAB micelles at 298 K22 and a complex-tomicelle ratio of 1:2. The experimental data are not too far from this theoretical line. However, we do not claim that one complex anion binds to two micelles. Since solubilization is more dynamic, the picture of a palladate ion sandwiched between two micelles appears rather illusory: we suggest that metallomicelles are in dynamic equilibrium with free micelles. Composition of the K2PdCl4/CnTABr ComplexSurfactant Aggregates. It seemed plausible to assume that the microcrystallization in the premicellar region, and also in the postmicellar region above the MAC, originates from electrostatic attractions between the complex anions and the organic cations, and the composition of the organic salt therefore simply follows the 1:2 stoichiometry required for charge neutralization. In fact, the existence of stoichiometric laurylammonium31a and cetylammonium salts31b of [PdCl4]2- has already been reported in the literature. Further, charged complexes of Au(III),32,33 Ru(II),34 and Pd(II)35-37 are known to undergo binding with oppositely charged surfactants, and both monophasic and biphasic behavior has been observed, depending on the concentration conditions. The composition of the organic salt is not clear-cut, for in aqueous solutions containing a mixture of Cl- and Br- a series of mixed ligand complexes [PdClmBr4-m]2-, with m ) 0 to 4, may coexist to extents depending on the relative concentrations of the two halide ions.31c,39,40 Measurements of the stability constants of the five complexes (in the absence of organic cations) indicated that Pd2+ binds Br- more strongly than Cl-. The overall stability constant of [PdBr4]2- is nearly 104 times higher than that of [PdCl4]2-,39,40 and at [Br-]/[Cl-] > 10 the proportion of [PdBr4]2- is close to 100%.31c,40 For the present preparation of Pd particles, the surfactant concentration, and hence the Br- concentration, was in a large excess over [PdCl4]2-. We presumed, therefore, that the ligand-exchange proceeded to completion and that the composition of the organic salt is [C14TA]2[PdBr4]. This hypothesis was justified by the results of IR, Raman, and UV-vis spectroscopy and elemental analysis. [PdCl4]2- and [PdBr4]2- ions are square-planar and exhibit D4h symmetry with seven fundamental modes of vibration.31d,41-45 Three of them are IR active, Eu (stretching, ν6), Eu (in-plane deformation, ν7), and A2u (out-of-plane deformation, ν2), and three of them are Raman active, A1g (stretching, ν1), B1g (stretching, ν3), and B2g (in-plane deformation, ν4). The seventh mode of vibration, B2u (out-of-plane deformation, ν5) is inactive. The IR and Raman spectra of the organic salt, and those of K2[PdCl4], were recorded in the ranges from 400 to 240 cm-1 and from 350 to 100 cm-1, respectively. The solid-phase spectra are displayed in Figures 2 and 3. For [PdCl4]2-, one IR-active vibration (ν6Eu) is positioned at 335 cm-1; the other two bands, (39) Srivastava, S. C.; Newman, L. Inorg. Chem. 1966, 5, 15061510. (40) Feldberg, S.; Klotz, P.; Newman, L. Inorg. Chem. 1972, 11, 28602865. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1986; pp 141-145. (42) Ferraro, J. R. Low-Frequency Vibrations of Inorganic and Coordination Compounds; Plenum: New York, 1971; Chapter 6. (43) Hendra, P. J. J. Chem. Soc. A 1967, 1298-2301. (44) Perry, C. H.; Athans, D. P.; Young, E. F.; Durig, J. R.; Mitchell, B. R. Spectrochim. Acta 1967, 23A, 1137-1147. (45) Goggin, P. L.; Mink, J. J. Chem. Soc., Dalton Trans. 1974, 14791483.

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Figure 2. IR spectra of K2[PdCl4] and the organic salt precipitated in a micellar system of K2[PdCl4]/C14TABr above the maximum additive concentration.

Figure 3. Raman spectra of K2[PdCl4] and the organic salt precipitated in a micellar system of K2[PdCl4]/C14TABr above the maximum additive concentration.

assigned to (ν7Eu) and (ν2A2u), lie in the far-IR region and were not measured in the present work. The three Raman-active vibrations are at 307 (ν1A1g), 273 (ν3B1g), and 197 cm-1 (ν4B2g). For the organic salt, one IR-active vibration at 252 cm-1 (ν6Eu) and two Ramanactive vibrations at 190 (ν1A1g) and 175 cm-1 (ν3B1g) were detected in the present wavenumber ranges. In general, the vibrations of the metal-halide bond in square-planar tetrahalo Pd(II) complexes are affected only slightly by the nature and/or the size of the counterion.42 The pronounced shifts of the vibrations to lower wavenumbers observed for the organic salt with respect to the inorganic salt result from the mass effect of halides: the Pd-Br vibrations lie well below the corresponding Pd-Cl vibrations, in both the stretching and bending modes.31d,42-45 Previously reported IR and Raman data for K2[PdCl4] were reproduced here to within (3 cm-1. The IR absorption band frequencies and the Raman shifts for the present organic salt were also very close ((8 cm-1) to the values reported earlier for various inorganic and organic derivatives of [PdBr4]2-, both in solid forms (films and pastilles) and in solution.42-45 Partial ligand-exchange or halidebridged complex formation may be safely excluded since this would lower the symmetry of the complex to C2v or D2h, and the number and the position of the various vibrations would change accordingly.41,42,44,46 Thus, analysis of the IR and Raman spectra of the metal-halogen bonds in the surfactant-free Pd complex and the surfactant-treated Pd complex provided evidence of a completed ligand-exchange reaction in which [PdCl4]2- is transformed to [PdBr4]2- in the presence of a large excess of C14TABr. (46) Goggin, P. L. J. Chem. Soc., Dalton Trans. 1974, 1483-1486.

Veisz and Kira´ ly

Figure 4. The progressive spectral change in aqueous solutions of K2[PdCl4] (0.04 mM) upon the transformation, in the close proximity of the cmc (3.9 mM), of complex-surfactant premicellar aggregates to complex-surfactant postmicellar aggregates. The concentration of C14TABr was increased stepwise in increments of 0.1 mM from (A) 3.7 mM to (H) 4.4 mM.

This finding implies that the stoichiometric composition of the precipitated organic salt is [C14TA]2[PdBr4]. In fact, the molar mass of the organic salt calculated from the results of elemental analysis (1H NMR, total organic carbon (TOC), and ICP-AES) complied with this composition to within 5% (H, 8.7%; C, 36.7%; Pd, 11.5%). Further, the organic salt was readily soluble in common organic solvents such as chloroform, toluene, THF, etc. We propose, therefore, that the composition of the complex-surfactant aggregate formed at saturation (i.e., below the cmc and above the MAC) is [C14TA]2[PdBr4] and that the initial 2:1 electrostatic binding is followed by a slow, higherorder aggregation to attain an equilibrium state. Higherorder aggregates undergo precipitation or spontaneous solubilization, depending on the concentration of the surfactant relative to that of the organic salt. Binding in the Premicellar and the Postmicellar Regions. UV-vis spectroscopy is a versatile tool with which to elucidate the binding of charged probe molecules to oppositely charged, single surfactant molecules in the premicellar region and to oppositely charged micelles in the postmicellar region.47-49 We applied this method to investigate the progressive spectral change in the K2[PdCl4]/C14TABr system involving the cmc region. In a series of blank experiments, when KBr was used instead of C14TABr, we obtained two absorption maxima in the UV region, at 222 and 279 nm for [PdCl4]2- (when [Br-] ) 0), and at 247 and 332 nm for [PdBr4]2- (when [Br-] . [Cl-]). These values compare well with those for the parent species in the deconvolution spectra of the mixed-ligand complexes [PdClmBr4-m]2-.39,40 The absorption spectrum of the primary aggregates [C14TA]2[PdBr4] formed in situ in aqueous solution below the cmc was markedly similar to that of the organic salt dissolved in THF. [C14TA]2[PdBr4] displayed two predominant peaks, at 291 and 423 nm, which were 1 order of magnitude higher in intensity than those of K2[PdBr4] at 247 and 332 nm, otherwise at the same concentration. The pronounced red shift and the higher absorptivity are attributed to the association of C14TA+ with [PdBr4]2- to form 2:1 ion pairs, in a manner similar to the 1:1 coupling of CnTA+ with [AuCl4]- via Coulombic attraction.32,33 Figure 4 depicts the spectral change in the K2[PdCl4]/C14TABr system in the close (47) Karukstis, K. K.; Savin, D. A.; Loftus, C. T.; D’Angelo, N. D. J. Colloid Interface Sci. 1998, 203, 157-163. (48) Buwalda, R. T.; Jonker, J. M.; Engberts, J. B. F. N. Langmuir 1999, 15, 1083-1089. (49) Kuiper, J. M.; Buwalda, R. T.; Hulst, R.; Engberts, J. B. F. N. Langmuir 2001, 17, 5216-5224.

Size-Selective Synthesis of Palladium Particles

Figure 5. Thermometric titrations of KBr with K2[PdCl4] for measurement of the enthalpies of ligand exchange and C14TABr with K2[PdCl4] for measurement of the enthalpies of formation of complex-surfactant premicellar aggregates and complex-surfactant postmicellar aggregates.

vicinity of the cmc. As the surfactant concentration is increased, and the cmc is gradually approached and then exceeded, the absorbances at 291 and 423 nm, attributed to complex-surfactant premicellar aggregates, gradually decrease, and new absorbances develop at 251 and 342 nm, attributed to the formation of complex-surfactant postmicellar aggregates. The existence (and coexistence) of the two different absorbing species is further evidenced by the appearance of the isobestic points; i.e., the wavelengths at which the two substances, which are interconvertible, have equal absorptivity.50 As regards the formation of the metallomicelles, a further point of interest is how the cmc of C14TABr is affected by the complex as a nucleating site.34 Since the induced cmc (icmc) is close to the unperturbed cmc, the palladate complex seems to be a suitable probe (spectral indicator) for rapid measurement of the cmc’s of cationic surfactants. Calorimetric measurements provided further insight into the thermodynamic aspects of the complex-surfactant interactions. The cumulative enthalpies of titration of C14TABr and KBr solutions with K2[PdCl4] are plotted against the concentration of the titrant in Figure 5. The linearity of the titration curves is noteworthy. The molar enthalpies for the formation of complex-surfactant (premicellar) and complex-micelle (postmicellar) aggregates and for the ligand exchange (which transforms [PdCl4]2- to [PdBr4]2-) were calculated from the slopes of the straight lines. ∆Hl ) -37 kJ‚mol-1 was obtained for the ligand exchange, independently of the concentration of Br-, provided that [Br-] . [Cl-]. This value can be compared with -32 kJ‚mol-1 51 and with ca. -37 kJ‚mol-1,52 measured in aqueous solution in the presence of background electrolytes (at constant ionic strength). The enthalpy of the formation of complex-surfactant aggregates at below the cmc is ∆H2 ) -70 kJ‚mol-1, from which -37 kJ‚mol-1 is attributed to the ligand exchange and -33 kJ‚mol-1 to the electrostatic coupling of one [PdBr4]2- with two C14TA+. For the formation of metallomicelles at above the cmc, ∆H3 ) -32 kJ‚mol-1 is obtained; this process involves the ligand-exchange reaction and the binding of the resulting [PdBr4]2- to the micelles. Alternatively, the process may be subdivided into three consecutive steps instead of two steps: ligand-exchange, binding of the complex anion to surfactant cations, and embedding of (50) Jaffe´, H. H.; Orchin, M. Theory and Applications of Ultraviolet Spectroscopy; John Wiley & Sons: New York, 1965; p 589. (51) Izatt, R. M.; Watt, G. D.; Eatough, D.; Christensen, J. J. J. Chem. Soc. A 1967, 1304-1308. (52) Ryhl, T. Acta Chem. Scand. 1972, 26, 2961-2962.

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the complex-surfactant aggregates into the micelles. The effect of competitive binding of free Cl- and Br- to charged C14TABr micelles can be safely neglected, because [Br-] . [Cl-] and the selectivity coefficient has a value of about 6 in favor of Br-.53 Since ∆H1 is included in both ∆H2 and ∆H3, the enthalpy of solubilization can be estimated from the above set of measurements as ∆Hsolub ) ∆H3 - ∆H2 ) +38 kJ‚mol-1. The positive sign implies that it is the hydrophobic effect (the incorporation of complex-surfactant aggregates into the micelles) rather than electrostatic interactions (binding of the complex anion to the positively charged micelles) which dominates in the net solubilization process. Accordingly, the solubilization of the organic salt by the micelles is endothermic, and the process is therefore controlled by a favorable (positive) entropy term. Size Control of Pd Nanoparticles in K2[PdCl4]/ CnTABr Micellar Solutions. The formation of complexsurfactant premicellar and postmicellar aggregates was investigated in detail for an alkyl chain length with n ) 14. Visual observations suggested that such aggregates exist for the other homologue members from n ) 8 to 16. The addition of hydrazine to a micellar solution of K2[PdCl4]/CnTABr below the MAC led to the formation of nanosized Pd particles, mediated by the metallomicelles. In general, the reduction was complete in a matter of seconds, and the final size distribution was achieved in a matter of tens of minutes. Hydrosol samples that were stable for months were obtained in this way. We propose that the high local concentration of the stabilizing agent at the reduction center provided a suitable microenvironment for the generation of ultrafine Pd particles via the arrested growth mechanism. The limited aggregation of zerovalent metal atoms is likely to be associated with the strong and rapid adsorption of the surfactant molecules on the nascent particles with which they are in close contact from a very early stage of the reaction. In one set of experiments, we investigated the effects of the alkyl chain length on the size and the size distribution of the Pd particles generated in K2[PdCl4]/CnTABr micellar solutions. For each sample, the same reduced concentration of c/cmc ) 10 was applied at a precursor concentration of 0.2 mM. Transmission electron micrographs and the size distributions of the particles are displayed in Figure 6. It appears that, as the alkyl length of the stabilizing agent increases, the size distribution becomes narrower and the mean diameter decreases. Figure 7 depicts the correlation between the average particle size and the carbon number of the alkyl skeleton. For n ) 8 to 16, the size can be controlled in the range from 5.3 to 1.6 nm. The increase in size with decreasing chain length is further accompanied by an increase in polydispersity. These observations are in line with the general view that the higher members of a surfactant homologue series are more efficient stabilizing agents in colloid synthesis. For a given chain length, further variation in size can be achieved by variation of the precursor concentration and the surfactant concentration. The results of a systematic study for n ) 14 are displayed in Figure 8. Pd particles in the size range from 1.6 to 6.8 nm were synthesized. As expected, the size increases with increasing K2[PdCl4] concentration. In contrast, an increase in size with increasing C14TABr concentration is surprising. Nevertheless, a similar observation was made for the Stokes radii of nanoscopic Pt clusters stabilized by CnTABr surfactants in aqueous media.54 The reason for this trend is at present not understood. A possible reason is that the rate of adsorption (53) Abuin, E.; Lissi, E. J. Colloid Interface Sci. 1991, 143, 97-102. (54) Yonezawa, T.; Tominaga, T.; Toshima, N. Langmuir 1995, 11, 4601-4604.

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Figure 8. 3D representation of the size control of Pd nanoparticles as functions of K2[PdCl4] concentration and C14TABr concentration.

Figure 6. TEM micrographs and particle size distributions of Pd nanoparticles stabilized by CnTABr with n ) 8, 10, 12, 14, and 16 (see text for synthesis conditions).

Figure 7. The mean diameter of Pd nanoparticles plotted against the alkyl chain length of the stabilizing agent (see text for synthesis conditions).

of the surfactant molecules on the growing metal particles decreases with increasing surfactant concentration in the postmicellar region. This explanation assumes that only free surfactant molecules are adsorbed and that the

micelles are not directly involved in the stabilization. Since the number of free surfactant molecules is constant above the cmc, the number of these molecules, relative to the number of micelles, decreases with increase of the overall surfactant concentration. Therefore, the rate of adsorption may be modulated by the concentration of the micelles in dynamic equilibrium with the free surfactant molecules. Although this explanation seems plausible, the process is certainly more complex and deserves further investigations. Stability of Pd Nanoparticles Generated in K2[PdCl4]/CnTABr Micellar Solutions. Long-term stability of the Pd hydrosols was readily achieved under preparation conditions above the cmc, but not far below the cmc. Further, we found that Pd hydrosols prepared in the postmicellar region can be diluted down to the premicellar region without any appreciable agglomeration, up to 0.3 times the cmc of C14TABr. Further dilution to 0.2 times the cmc, however, led to irreversible aggregation, and precipitation occurred in several hours. The conditions for stability must be dictated by the structure of the surfactant adsorption layer, which varies along the adsorption isotherm. We propose a model for the progressive development of the adsorption layer around the particles, which provides a qualitative explanation for the observed stability behavior. The specific adsorption of Bron the surface of the Pd particles occurs spontaneously from aqueous solutions, thereby creating a negatively charged layer. Surfactant adsorption at low concentrations occurs with cationic headgroups toward the surface. As the adsorption proceeds with increasing concentration, the negative surface charge is gradually diminished and a vertical monolayer is formed, which renders the Pd surface hydrophobic. The conditions for stability are not satisfied at concentrations far below the cmc. However, as the surfactant concentration is further increased, a second layer is adsorbed in a tail-to-tail arrangement, either on top of the monolayer or with an overlap region due to interdigitation. A positive surface charge is formed in this way, and the adsorption levels off at the cmc. The conditions for stability via charge and steric stabilizations are achieved when the bilayer is sufficiently developed. Although the adsorption of surfactants on Pd surfaces has not been yet quantified by means of adsorption isotherms, support for the above model can be provided by previous results for closely related systems. The adsorption isotherms of alkylpyridinium bromides (n ) 12, 14, and 16) in a suspension of gold correlated well with the electrokinetic potential of the gold fines; both

Size-Selective Synthesis of Palladium Particles

measured up to the cmc.55 The shapes of the isotherms were of the double-plateau type, and the electrophoretic data exhibited a surface charge reversal from negative to positive. These observations are consistent with the progressive building up of a bilayer, both the first and the second layers being in a vertical orientation with facing or interdigitated tail groups. Ellipsometric measurements at the metal-electrolyte solution interface revealed that Br- is strongly adsorbed on Ag, Au, and Pt surfaces.56 Surface-enhanced Raman spectroscopy studies on the adsorption of C16TABr from aqueous solutions onto Cu57 and Ag58,59 surfaces indicated that at least one layer of surfactant is adsorbed with the ionic trimethylammonium headgroups pointing toward the surface of the metal, with Br- between the surface and the heads. For Ag particles, the formation of a secondary monolayer of cationic surfactants60 and fatty acids61 on top of the close-packed primary monolayer was described recently. Various experimental techniques have been used for the characterization of this bilayer, including the LangmuirBlodgett technique, contact angle measurements, quartz crystal microgravimetry, and thermoanalytical methods. X-ray photoelectron spectroscopy and electrochemical measurements likewise indicated that C16TABr forms a bilayer on a Ag core, with interdigitated segments between the two adjacent layers.62 A recent atomic force microscopy study in the dilute micellar regime provided supporting evidence that the vertical monolayer of C14TABr adsorbed on a macroscopic, planar Au surface induces the formation of cylindrical surface micelles.63 On the highly curved surfaces afforded by nanoscale particles, however, surfactants cannot self-assemble into full cylindrical micelles; a bilayer is a more adequate assumption for the morphology of the surface aggregates. To summarize, supporting evidence has accumulated in favor of the proposed model: the stability of Pd particles in CnTABr solution is implemented by a fully or partially developed adsorbed bilayer, which can provide an efficient protective shell around the particles via electrostatic and steric stabilization. Unfortunately, the perspectives for quantification of the stability behavior of Pd particles by measurement of the adsorption isotherm are not straightforward. In contrast with Au, Pd and other noble metal particles are very sensitive to atmospheric oxygen,26 and the formation of a thin oxide layer on the Pd surface is therefore difficult to eliminate during such studies. Kinetics of Formation of Pd Colloids in K2[PdCl4]/ C14TABr Micellar Solutions. As mentioned above, the [C14TA]2[PdBr4] complex-surfactant aggregates embedded in the micelles are regarded as the real precursor species for the reduction of Pd2+ to Pd0 by hydrazine. Timeresolved UV-vis spectroscopy was applied to gain an insight into the kinetics of particle formation. Figure 9 demonstrates that seed formation is a very fast process: (55) Benton, D. P.; Sparks, B. D. Trans. Faraday Soc. 1966, 62, 32443252. (56) Paik, W.; Genshaw, M. A.; Bockris, J. O’M. J. Phys. Chem. 1970, 74, 4266-4274. (57) Dendramis, A. L.; Schwinn, E. W.; Sperline, R. P. Surf. Sci. 1983, 134, 675-688. (58) Sun, S.; Birke, R. L.; Lombardi, J. R. J. Phys. Chem. 1990, 94, 2005-2010. (59) Wiesner, J.; Wokaun, A.; Hoffmann, H. Prog. Colloid Polym. Sci. 1988, 76, 271-277. (60) Suga, K.; Bradley, M.; Rusling, J. F. Langmuir 1993, 9, 30633066. (61) Patil, V.; Mayya, K. S.; Pradhan, S. D.; Sastry, M. J. Am. Chem. Soc. 1997, 119, 9281-9282. (62) Cheng, W.; Doung, S.; Wang, E. Electrochem. Commun. 2002, 4, 412-416. (63) Jaschke, M.; Butt, H. J.; Gaub, H. E.; Manne, S. Langmuir 1997, 13, 1381-1384.

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Figure 9. The kinetics of nucleation of Pd nanoparticles, obtained from time-resolved UV-vis spectra. Insert: diminution of the spectrum of the metallomicelles from 1 to 20 s (see text for synthesis conditions).

Figure 10. The kinetics of growth of Pd nanoparticles, obtained from time-resolved UV-vis spectra: (9) 229 nm; (0) 600 nm. Insert: evolution of the spectrum of the growing Pd clusters in the range from 5 to 120 min (see text for synthesis conditions).

the disappearance of the characteristic peaks at about 251 and 342 nm ascribed to the precursor species was complete in 20 s. The rapid burst of nucleation is followed by the limited aggregation of zerovalent Pd atoms through metal clusters to metal colloids. For small particles of most of the d-block metals, the absorption in the UV-vis range is continuous across the range, whereas in the case of colloidal Pd the absorption spectrum depicts a definite structure. The evolution of the UV-vis spectrum for the Pd clusters is presented in Figure 10; the spectra agree fairly well with that calculated from the Mie theory for small spherical Pd particles.64 Although the distinct absorption band in the near-UV range cannot be assigned to plasma resonance, it is nevertheless a characteristic property of the metallic state. The appearance of a new absorption band at about 229 nm may be taken as a demonstration of the onset of free conduction electron behavior in the clusters. When the diameter of the clusters is less than the mean-free path of the conduction electrons of the metal, the height of the absorption peak is related to the size of the clusters.64 The normalized absorbance maxima are plotted against time in Figure 10 to mimic the particle growth kinetics. Alternatively, as the rather broad absorption continuum extends through the visible range, the kinetic curve was constructed by taking normalized absorbances at an arbitrarily chosen wavelength of 600 nm; the resulting curve is close to that plotted at 229 nm. Figures 9 and 10 further depict that nucleation is nearly 2 orders of magnitude faster than growth on the time scale of the experiment. This marked difference in (64) Creighton, J. A.; Eadon, D. G. J. Chem. Soc., Faraday Trans. 1991, 87, 3881-3891.

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Figure 12. HRTEM micrograph of an eight-shell Pd cuboctahedron in a hexagonal projection.

Figure 11. TEM micrograph of precipitated Pd nanoparticles, showing the abundance of cubooctahedral morphology in hexagonal projections.

rate favored the formation of small, nearly monodisperse particles in the presence of the stabilizing agent. For the kinetic experiment, the final particle size was less than 2 nm. However, the hydrosol was too diluted for a sufficient number of particles to be counted in the TEM micrographs, and thus a statistically reliable size distribution could not be obtained in this case. It may be of interest to note that the mechanism of particle formation in the present micellar system appears to be quite different from a new mechanism proposed for the nonmicellar synthesis of metal nanoclusters, which involves slow, continuous nucleation followed by fast, autocatalytic surface growth.65 Morphology of the Pd Particles Formed in K2[PdCl4]/C14TABr Micellar Solutions. A close inspection of the TEM micrograph in Figure 11, taken of Pd particles precipitated from the parent hydrosol by using 2-propanol,26 suggests that the dominant morphology is cubooctahedral rather than spherical. In fact, the characteristic hexagonal periphery can also be observed in the HRTEM image in Figure 12. In this favorable projection, one can identify the atomic lattice structure of an eightshell cubooctahedral particle in the [110] crystallographic orientation.26 Pd particles with this shape have been synthesized earlier in other laboratories21,24,66,67 but under experimental conditions different from those in the present study. Whether the abundance of the cubooctahedral morphology generally dominates for all of the systems (65) Watzky, M. A.; Finke, R. G. J. Am. Chem. Soc. 1997, 119, 1038210400. (66) Schmid, G. Polyhedron 1988, 7, 2321-2329. (67) Schmid, G.; Harms, M.; Malm, J.-O.; Bovin, J.-O.; van Ruitenbeck, J.; Zandbergen, H. W.; Fu, W. T. J. Am. Chem. Soc. 1993, 115, 2046-2048.

involved in Figures 7 and 8, independently of the solution composition, remains to be seen. Nevertheless, we observed a high population of hexagons, with sharp contours as in Figure 11, whenever the surfactant was carefully washed away before the preparation of the sample for the TEM measurements. It is very likely, therefore, that the complex-micelle aggregates play a crucial role in the mechanism of particle formation: the final morphology is mediated by the postmicellar aggregates and the structure of these metalloaggregates possesses some specificity for the resulting cubooctahedral shape. This is further supported by our preliminary results for related systems; Pt particles generated in K2[PtCl4]/C14TABr micellar solutions displayed a high abundance of cubooctahedra. Conclusions The net interaction between CnTABr and K2[PdCl4] in aqueous solution results in the formation of [CnTA]2[PdBr4]. This organic salt can be solubilized by the surfactant micelles to produce metallomicelles which are the real precursor entities for the hydrazine reduction of Pd2+ to Pd0. Rapid nucleation is followed by limited aggregation to ultrafine particles via the arrested growth mechanism. A vertical surfactant bilayer is likely to be formed on the surface of the Pd particles, which ensures the long-term stability of the Pd hydrosols. In general, the particle size decreases with decreasing precursor concentration, decreasing surfactant concentration, and increasing alkyl length. The dominant morphology is cubooctahedral. Acknowledgment. The IR and Raman spectra were obtained with the expert assistance of Dr. O. Berkesi at the Laboratory for IR and Raman Spectroscopy, University of Szeged. Financial support was provided by the Hungarian Scientific Research Fund, Grant OTKA T042521. LA034146Y