Polyhedral Oligomeric Silsesquioxane (POSS)-Stabilized Pd

Oct 15, 2009 - Sonia E. Létant, Amitesh Maiti, Ticora V. Jones, Julie L. Herberg, Robert S. Maxwell and Andrew P. Saab*. Lawrence Livermore National ...
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Polyhedral Oligomeric Silsesquioxane (POSS)-Stabilized Pd Nanoparticles: Factors Governing Crystallite Morphology and Secondary Aggregate Structure Sonia E. Le´tant, Amitesh Maiti, Ticora V. Jones, Julie L. Herberg, Robert S. Maxwell, and Andrew P. Saab* Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: April 27, 2009; ReVised Manuscript ReceiVed: September 29, 2009

Polyhedral oligomeric silsesquioxane (POSS)-based amine ligands are used as capping agents in the reductive growth of Pd nanocrystallites, and the effects of the specific form of the ligand on the structure of both the secondary aggregate and the primary metal nanoparticle are explored. Secondary aggregates of the ligand capped Pd particles are seen to form a uniform structure only if (a) the POSS ligand is totally functionalized with amine groups and (b) the amine groups are in the hydrochloride form. By casting these results in the context of density functional theory calculations, we show that the morphology of Pd core-shell nanomaterials is determined by the ratio of self-interaction potentials of the ligands to their interaction with solvent. The structures of the primary metal crystallites are also shown to be sensitive to the amine ligand in the hydrochloride form. Our results further show that the POSS versions of a ligand will frequently form precipitating capped metal nanoparticle structures, where the discrete molecular forms produce no isolable metal nanoparticles at all on the time scale of the experiments. A study of the diffusion rates of the reactive species in the Pd/ POSS-ligand system shows a correlation between the growth of observable metal crystallites and sufficiently slow ligand diffusion kinetics. Finally, we illustrate through a TEM time series that Pd nucleation seems to happen without a prior formation of observable self-assembled POSS ligand templates. Introduction Among metal nanoparticles, palladium attracts particular attention owing to its catalytic behavior,1-3 hydrogen storage and sensing properties,4-6 and utility as a nanostructured ohmic contact.7-9 Pd nanoparticles are frequently prepared as supported or surface capped structures so as to stabilize the desired small crystallites against excessive growth, provide structural control, or both.10-12 We reported a study on one such material: a core-shell structure of Pd nanoparticles stabilized by an adsorbed overlayer of polyhedral oligomeric silsesquioxane (POSS).13 POSS is essentially a molecular silica, with a cubic shape consisting of eight Si atoms at the corner positions bridged by oxygen along the cube edges. Each Si atom is also functionalized with a pendant group, the simplest being hydrogen. For the material in our previous report, the pendant group consisted of propylammonium chloride. In this prior study we determined that the catalytic activity of the Pd nanoparticles persisted despite the stabilizing overlayer of POSS. That the reactivity of the Pd remained intact implies that such a material can be used to make interesting reactive composites by embedding the POSS/Pd structure into an appropriate matrix. This could result in composites for applications such as separation membranes or electrochemical cell electrodes. Furthermore, POSS compounds can be chosen so as to contain functionalities that both stabilize palladium and improve compatibility or reactivity toward a desired matrix, thereby enhancing the overall material properties. However, the structure of the secondary aggregates of POSS/ Pd compounds can be highly sensitive to the initial reaction composition. This structure in turn can determine the suitability of the POSS/Pd material for creation of a composite. For * Corresponding author. E-mail: [email protected].

example, the material on which we previously reported, a compound of octa(propylamine hydrochloride)-POSS and Pd grown from methanol, consists of highly regular spherical aggregates of the primary POSS-covered Pd nanocrystallites (Figure 1a). In sharp contrast, an analogue of the OPAm-POSS material using mono(propylamine hydrochloride)-heptaethylPOSS (MPAm-POSS) results in a rapidly formed, random threedimensional network solid (Figure 1b). Understanding the forces that drive the formation of the aggregate structure during synthesis could make it possible to deliberately create a particular aggregate morphology that optimizes the mechanical properties of the composite. Likewise, knowledge of the factors controlling the primary metal nanoparticle structure is valuable in discerning possible impacts on metal reactivity or stability. In the present work, we study the formation of POSS/Pd nanoparticle aggregate compounds from solution and report characteristics of the capping ligands that determine the secondary aggregate morphology as well as the formation and structure of the primary metal nanoparticles. Reduction of Pd salt in the presence of a variety of POSS ligands of varying chemical structure indicated that the resulting Pd-POSS aggregates formed with an ordered morphology only in the single case of a POSS perfunctionalized with an amine hydrochloride. Interpreting these results in the context of density functional theory calculations, we show that the aggregate structure is determined by the ratio of self-interaction potentials of the ligands to their solvent interaction energies. The crystallinity of the primary Pd nanoparticles was likewise observed to depend upon the amine being in the hydrochloride form, implying the role of HCl as a corrosive or surfactant in this system. Further experiments that evaluated the reduction of Pd in the presence of POSS-amine ligands and their non-POSS (i.e., “free molecular”) analogues showed that only the POSS versions led to the formation of Pd

10.1021/jp903866f CCC: $40.75  2009 American Chemical Society Published on Web 10/15/2009

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Figure 1. TEM images: (a) OPAm-POSS/Pd. Pd particles capped with POSS are present as small black dots surrounded by a lighter colored material, which aggregate to form the larger spheres. (b) MPAm-POSS/Pd. In this case the POSS/Pd aggregates form a continuous network. Scale bars: 100 nm.

nanoparticles observable by TEM. A study of the diffusion rates of both ligands and Pd nucleating species demonstrates a correlation between the ligand diffusion rate and the likelihood of forming observable Pd nanoparticles. As an extension of this concept, we illustrate through a TEM time series that Pd nucleation seems to happen without prior formation of POSSligand reverse micellar templates, in contradiction to a prior study.14 Experimental Section General Synthesis. Octa(3-aminopropyl)octasilsesquioxane octahydrochloride (otherwise known as octa(propylammonium)POSS, or OPAm-POSS), monoammonium heptaethyl POSS (MPAm-POSS), octaethyl POSS (OE-POSS), and monosilane heptaisobutyl POSS (S-POSS) were purchased from Hybrid Plastics Inc., Hattiesburg, MS, and used as received. Octapropylamine POSS (OPA-POSS) and octasilane-aniline POSS (OSAn-POSS) were synthesized as described below. All POSS species used were reacted with palladium according to the method in ref 14: POSS was dissolved in 50 mL of methanol at a concentration of 8.5 × 10-3 mM, followed by addition of palladium(II) acetate (Sigma-Aldrich, St. Louis, MO) to a concentration of 9.0 × 10-2 mM. The solution was vigorously stirred at room temperature for 14 h and then allowed to react unstirred for an additional 36 h prior to sample recovery. Recovery of dry powders was performed by centrifugation at 10 000 rpm for 10 min, followed by solvent removal, two fresh methanol washes, and drying at room temperature in air. NMR and Si elemental analysis confirmed adequate solubility of both OE-POSS and S-POSS in methanol. Synthesis of Octapropylamine POSS (OPA). OPA was synthesized following a procedure from ref 32 utilizing an Amberlite IRA-400 ion-exchange resin. 1H NMR (500.2 MHz, DMSO-d6, 25 °C): δ 4.3 (s, NH2 and H2O), 2.50 (t, J ) 7.4, CH2N, 16 H), 1.42 (m, SiCH2CH2, 16 H), and 0.54 (t, J ) 7.3 Hz, SiCH2, 16 H). 13C-{1H} NMR (125.8 MHz, CD3OD, 25 °C): δ 44.87 (s, CH2N), 26.88 (s, SiCH2CH2), and 9.57 (s, SiCH2). Synthesis of Octasilane-Aniline (OSAn). A hydrosilylation reaction with vinylaniline (VA) was used to attach the aniline to the POSS cage. To a solution of octasilane POSS in THF was added octasilane and vinylaniline in a 10:1 molar ratio, and the solution was cooled to 10 °C. To the chilled solution, 20 µL of Karstedt’s catalyst Pt2{[(CH2dCH)Me2Si]2O}3 was added, triggering a color change from clear yellow to brown. The reaction proceeded overnight and was checked by TLC for

completeness. The THF was evaporated, and the compound was purified by column chromatography in a 1:1 dichloromethane: hexane solution to yield a yellow-brown solid. 1H NMR results show >90% substitution at the Si-H vertices. 1H NMR (500.2 MHz, DMSO-d6, 25 °C): δ 4.3 (s, NH2 and H2O), 2.50 (t, J ) 7.4, CH2N, 16 H), 1.42 (m, SiCH2CH2, 16 H), and 0.54 (t, J ) 7.3 Hz, SiCH2, 16 H). 13C-{1H} NMR (125.8 MHz, CD3OD, 25 °C): δ 44.87 (s, CH2N), 26.88 (s, SiCH2CH2), and 9.57 (s, SiCH2). 1H NMR (500.2 MHz, DMSO-d6, 25 °C): δ 4.3 (s, NH2 and H2O), 2.50 (t, J ) 7.4, CH2N, 16 H), 1.42 (m, SiCH2CH2, 16 H), and 0.54 (t, J ) 7.3 Hz, SiCH2, 16 H). 13C-{1H} NMR (125.8 MHz, CD3OD, 25 °C): δ 44.87 (s, CH2N), 26.88 (s, SiCH2CH2), and 9.57 (s, SiCH2). Microscopy. High-resolution TEM characterization was performed using a Philips CM300FEG microscope. Typically, a few drops of sample were let to dry on a lacey carbon grid and mounted in the microscope. TEM images were recorded with a typical acceleration voltage of 300 keV. Nuclear Magnetic Resonance. 1H NMR measurements were performed on a Bruker Avance 500 MHz spectrometer with a magnetic field of 11.75 T, giving a resonance frequency of 500 MHz for 1H (spin ) 1/2). The 1H diffusion-ordered NMR spectroscopy (DOSY) measurements were carried out in a Bruker 5 mm tunable multinuclear triple (TBI) Z-gradient resonance probe. The gradient amplifier was used with a 5.35 G/cmA for the maximum gradient capability. The 1H chemical shifts were referenced to 99.95% perdeuterated D2O at a chemical shift of 4.8 ppm. A 2D sequence for diffusion measurement using stimulated echo using bipolar pulses for diffusion23 with a diffusion time of 100 ms, diffusion gradient length of 1 ms, 16 squared increments for a gradient levels, and 16 transients was used. The Bruker DOSY package was used to process all of the data. In addition, the 1H DOSY NMR data were recorded at 25 °C for each sample. For each sample, 2 mg of OPAm-POSS, diaminooctane, and palladium acetate were dissolved in 1 mL of deuterated methanol. Results and Discussion As described above, POSS functionalized with a single ammonium hydrochloride group was able to form an aggregate material of Pd nanoparticles with multiple structural differences from another Pd material formed from POSS perfunctionalized with the same ligand. To explain these differences, we examined the separate contributions of the POSS molecule and its side groups on the formation of both secondary aggregation and primary metal crystallite formation by performing two types of

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TABLE 1: POSS-Based Ligands and TEM Images of Their Resulting Pd Nanostructures and Aggregates As Grown from Ethanol

a OPAm ) octa(propylamine hydrochloride). octa(silylaniline).

b

MPAm ) mono(propylamine hydrochloride).

experiments. In the first, a series of Pd reductions in methanol were carried out in the presence of (1) no additional chemical species (i.e., direct reduction of Pd-acetate in methanol); (2) metal coordinating ligands not pendant to a POSS structure, including 1,8-diaminooctane (DAO), DAO-hydrochloride, and 4-vinylaniline (VA); and (3) POSS compounds functionalized with unreactive groups, namely octaethyl POSS (OE-POSS) and silane heptaisobutyl POSS (S-POSS). In the second type of experiment, Pd was reduced in the presence of POSS molecules bearing coordinating groups similar to those used as free molecules in the first experiment type. In the first series of experiments, large Pd crystals formed when the reduction of Pd was carried out without additional chemical species or in the presence of the unreactive POSS (OEPOSS or S-POSS). In the second set of experiments, summarized in Table 2, only in the presence of POSS ligands were isolable Pd nanoparticles formed on the time scale of the experiments. Also, within the set of POSS/Pd nanomaterials that did form, both the secondary aggregate structure and the structures and sizes of the primary Pd particles varied in accordance with the number of metal coordinating functionalities per POSS and whether or not the amines were present as neutral or HCl forms. In the following subsections, we analyze the

c

OPA ) octapropylamine.

d

OSAn )

TABLE 2: DFT Values of the Binding Energies of Species in the POSS/Pd/Methanol System interaction (kJ/mol) with species

Pd {100}

Pd {111}

POSS

methanol

Pd octaethyl POSS MPAm POSS MPAe POSS OPAm POSS OPA POSS

8.37 0.79 1.42 1.59 2.76 4.52

7.62 0.63 1.30 1.38 2.59 4.14

0.23 0.23 0.23 0.54 0.26

0.054 0.054 0.054 0.32 0.11

impact of the ligand on structure, first with respect to the secondary aggregate and then in terms of the primary metal nanocrystallites. Secondary Aggregate Structure. The agglomerated structure of the POSS/Pd materials was generally a random 3D network, with the exception of the case of the OPAm-POSS/Pd, which formed relatively monodisperse spheres. This compound is distinct from the others tested in that the POSS species is perfunctionalized with amine ligands, and the amines are in the hydrochloride form. A comparison across the POSS ligands used clearly shows that neither the presence of a maximum number of coordinating species on the POSS nor the hydrochloride form

POSS-Stabilized Pd Nanoparticles

Figure 2. Computational supercell with an OE-POSS molecule (shown in the “stick” representation) bound to the Pd(100) surface, which consists of three layers of Pd atoms (shown in the “ball” representation). The supercell is periodic in three dimensions with dimensions 16.5 Å × 16.5 Å in the (100) surface plane and 33.9 Å normal to the surface. The total number of atoms is 184.

of the amine was individually sufficient to drive the formation of the spherical structure. In order to rationalize these observations, all-electron first-principles density-functional theory (DFT) computations were performed to estimate the binding energies of the various species in solution. Binding energies were computed between (1) Pd atoms, (2) POSS particles, (3) POSS particles and the (111) and (100) surfaces of a Pd crystallite, and (4) POSS particles and methanol. The functional groups attached to the POSS corners were varied to reflect the experimental conditions. As in our previous work,15 the Pd surfaces were represented by periodic slabs cleaved appropriately so as to expose the (100) and the (111) planes. For all calculations three Pd layers were used, with the bottom layer fixed in order to mimic the semi-infinite bulk Pd. However, additional functionalization of the POSS molecules in this work necessitated more extended supercells as compared to previous work. For instance, Figure 2 displays a 6 × 6 extended periodic Pd(100) surface with an all-ethyl (OE) POSS on top. This structure consists of a total of 184 atoms. A vacuum of 30 Å is placed normal to the plane in order to limit interaction of periodic images in this direction. For consistency, all calculations were performed on the supercell of this same size, with the OEPOSS replaced by other species whose binding energy was of interest. For each species, the binding energy was computed simply by the difference in energy between structures in which the species was relaxed on the surface and those in which the species was placed in the middle of the vacuum. The solvent

J. Phys. Chem. C, Vol. 113, No. 45, 2009 19427 (methanol) environment above the surface was mimicked by an implicit solvent model, as described in the following paragraph. Prior to structural relaxation with DFT, each structure was initially optimized with the classical force field COMPASS.16 For the DFT simulations, we used the commercially available code DMol3,17-20 employing the double numeric polarized (DNP) basis set on a Fine integration grid and the gradient corrected PBE exchange correlation function.21 All calculations employed periodic supercells, for which accurate Brillouin zone sampling was ensured by summing over a finite set of K-points chosen according to the Monkhorst-Pack scheme.22 Interactions with methanol were estimated using the COSMO implicit solvent model, which was recently implemented in DMol3 under periodic boundary conditions.23 Table 2 summarizes DFT results for the binding energies normalized per unit area of a Pd crystallite. Generally, the binding energies follow the trend Pd-Pd > POSS-Pd > POSS-POSS>POSS-methanol.TheexceptionisOPAm-POSS, for which the POSS self-interaction energy exceeds the POSS-Pd binding energy. This relative order of binding energies reflects the observed structural hierarchy in our experiments, i.e., (1) a small Pd particle (2-5 nm), (2) which is coated with a single layer of POSS species, and (3) which coalesce together to form a superstructure. Beyond these trends, the specific energy values provide insight into the structures of the various POSS/Pd precipitates. In all cases, the POSS cointeraction energies largely exceed the POSS-solvent interactions, consistent with the tendency of all the POSS/Pd variants to aggregate. While the OPAm-POSS species has the largest cointeraction energy among the POSS ligands, it also has the highest solvent interaction energy by a sizable margin, with a ratio of POSS-POSS to POSS-solvent interaction of 1.7. These strong interactions are attributable to the large dipolar component arising from the hydrochloride form of the amine. Conversely, the cointeraction to solvent-interaction ratio of the other ligands (which are neutral amines) ranges from 2.3 to greater than 4. The stronger solvent interaction for the OPAm-POSS case may permit the formation equilibrium to shift more toward dissolution, thereby increasing the probability of forming a low-energy but kinetically unfavorable structure such as a sphere. This could also underlie the observations that although its POSS/Pd structures readily aggregate (owing to the high degree of POSS cointeraction), the aggregates will remain dispersed in methanol for months after synthesis. In contrast, the weak relative interaction strength of the other species with methanol allows their aggregates to precipitate out to a considerable extent, while the relatively low POSS-POSS binding permits the formation of a more random structure than that observed for the strongly self-interacting OPAm-POSS/Pd. It is necessary to point out that if self-interaction were the main factor controlling the aggregation of the OPAm-POSS/ Pd, it would be reasonable to expect that the process would continue beyond what is observed. The observation of mostly discrete particles of relatively low dispersity implies a sizedependent balance between further aggregation and solvation. Therefore, additional experiments were performed to reveal some indication of the nature of the forces controlling the aggregate formation. Initially, concentrations of the reactants were increased and decreased over those used in the original procedure by a factor of 5. For the 5× case, the mean sizes gauged by TEM images still appeared to be in the 50-70 nm range, though the distribution apparently broadened. For the 0.2× case, the mean size became somewhat smaller, on the order

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Figure 3. TEM micrographs of OPAm-POSS/Pd material formed following the procedure published by Naka et al.14 followed by the addition of POSS and Pd starting reagents in concentrations 5 times higher than the original synthesis starting concentrations. The second addition of reagents was performed after 14 h, and the second reaction was stirred for an additional 2 days before the sample was aliquotted for TEM analysis. (a) is an image recorded with a larger field of view, and (b) is a close-up of the OPAm-POSS/Pd material. Scale bars: (a) 100 nm; (b) 20 nm.

Figure 4. UV-vis absorption spectrum of (a) 9 × 10-5 M Pd(II) acetate in methanol and (b) the same solution 4 h after the addition of 8.5 × 10-6 M diaminooctane.

of 20 nm. Another experiment was performed in which a solution of OPAm-POSS/Pd that had been made according to the original procedure was charged with additional POSS and Pd reactants in concentrations equal to 5× those for the original solution. The results, presented in Figure 3, show that the parent particles did not increase their spherical diameter through the uniform addition of POSS-stabilized Pd nanoparticles. Instead, the solution was predisposed to have new, though much smaller, POSS-Pd clusters form and grow at points on the surface of the existing spheres. These results taken together suggest that, at least at the concentrations at which these syntheses are usually performed, the aggregate size represents an energetically favorable limit. Given the large number of atoms comprising these particles, addressing their stability is beyond the scope of either DFT or even classical Monte Carlo simulations. While it is arguable that charges on the surface NH4+ groups and Cl- ions might play a role in determining size through the formation of a repulsive Stern layer, we presently confine ourselves to the more general speculation that 50 nm appears to correspond to a critical size beyond which the barrier to rearrangement of constituent POSS/Pd particles within the superparticle becomes prohibitive at experimental temperature and time scales. Primary Metal Nanoparticle Structure. Only the POSS variants of the ligands tested resulted in the formation of isolable Pd nanoparticles. UV-vis spectra of the DAO and VA solutions after several hours displayed a time-invariant absorption band at 300 nm, indicating that terminal complexes of either Pd nucleation clusters or individual Pd atoms had formed (see Figure 4). Given that the specific amine bonding characteristics would likely be the same irrespective of whether it was in the form of the free molecular species tested, or present as a POSS

cage pendant, the inability of the free forms to produce nanocrystallites suggested that ligand diffusion kinetics might play a role in determining the likelihood of nanoparticle formation. Thus, diffusion coefficients of both the POSS-based and discrete molecular ligands in methanol were measured by diffusion-ordered 2D NMR spectroscopy (DOSY), a pulsed field gradient NMR method,24,25 and compared to both the NMR diffusion coefficients of Pd(II) acetate and calculations of the relative diffusion rates of Pd atoms in methanol. It is known that Pd(II) acetate exists in solution as a complex, the parent structure of which is an acetate-bridged trinuclear Pd cluster with slightly distorted D3h symmetry that may undergo varying degrees of either solvolysis or aggregation depending on solvent.26-28 Consequently, we take the diffusion coefficient as we determined it by the 1H acetate DOSY NMR signal to represent that of the likely Pd prenucleating species. It is necessary to point out that despite what is known of the solution nature of Pd(II) acetate, there is some uncertainty as to the actual stepwise speciation of the Pd as it is reduced from prenucleating species to a crystallizing nucleus. Although energetically unfavorable, the formation from fast reduction of Pd(II) of “naked” Pd(0) atoms, which then go on to form a nucleation particle, has not been entirely dismissed.29 To address this contingency, we estimated the diffusion coefficient of spherical Pd(0) in methanol using the Stokes-Einstein formula D ∼ kBT/ (6πηR), where D is the diffusivity, kB the Boltzmann constant, T the temperature, η the viscosity of methanol (solvent), and R the radius of the Pd particle. Assuming R ∼ 1 nm, and using an experimental room-temperature viscosity of 0.54 Pa · s for methanol, the above formula yields a diffusion constant of D ∼ 10-4 cm2/s at room temperature. The diffusion coefficients of OPAm-POSS, diaminooctane, and palladium acetate were determined by NMR to be (2.58 ( 0.97) × 10-6 cm2/s, (7.64 ( 1.94) × 10-6 cm2/s, and (1.25 ( 0.50) × 10-5 cm2/s, respectively, at room temperature. Alongside the synthesis observations, these results suggest two limiting cases for a presumed mechanism of nucleating Pd clusters (subscript “cluster”) accreting to form a Pd nanocrystal (subscript “particle”): (1) DPd-cluster . Dligand: Pd particle growth occurs until DPd-particle is on the order of Dligand, leading to greater ligand-metal binding. Thus, the relatively fast Pd prenucleating/nucleating species slow as they grow into larger particles, until their rates of displacement are comparable to those of the relatively slow POSS-ligand, thereby increasing the probability for binding. (2) DPd-cluster g Dligand: DPd-particle becomes comparable to Dligand at

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Figure 5. TEM images of OPAm-POSS/Pd nanocrystals formed after 5, 15, 30, and 90 min of reaction time between the palladium acetate and OPAm-POSS starting materials. Scale bars: (a-c) 5 nm; (d) 10 nm.

Figure 6. HRTEM micrographs of (a) OPAm-POSS/Pd, with inset showing (200) lattice spacing, and (b) OSAn-POSS/Pd. Scale bars: (a) 5 nm, except inset; (b) 2 nm.

smaller Pd particle sizes, leading to ligand binding and inhibition of growth before precipitation; hence, relatively fast moving molecular ligands can bind with Pd at very small sizes, effectively capping growth before an observable nanocrystallite forms. One implication of the mechanism just described is that it may exclude the formation of stable reverse micellar structures of POSS ligands in which Pd particles nucleate and grow, considering that the slow diffusion of such large structures could permit the growth of a considerable fraction of Pd crystallites exterior to the templates. The formation of templating structures was previously observed by Naka,14 who reported that TEM images showed the relatively rapid formation of voids in a methanol solution of Pd acetate and OPAm-POSS into which Pd particles began to grow many hours later. These voids were assumed to be self-assemblies of POSS ligands. Figure 5 shows a TEM time series of the growth of OPAm-POSS/Pd in methanol using the procedure described by Naka. Pd particles become evident in minutes and quickly begin to accrete to form the secondary aggregate sphere. This suggests that the growth process does not occur by the template mechanism reported by Naka, but rather by a process that is consistent with the diffusion-controlled mechanism above. The absence of nanoparticle formation for the free molecular species may also be attributable to more efficient packing of the smaller species around Pd precrystallite clusters difficult to observe by TEM, as compared to the larger POSS variants. In

this event, the formation of terminal Pd complexes would be favored by both faster diffusional kinetics and greater stabilization of subnanometer high surface energy clusters owing to higher net coordinative saturation. Within the set of POSS ligand experiments, the structure of Pd nanoparticles varied from highly regular crystal structures (OPAm-POSS/Pd, MPAm-POSS/Pd) to more disordered particles displaying polycrystallinity (OPA-POSS/Pd, OSAn-POSS/ Pd) (TEM photos of Table 1 and Figure 6). Evidently, the choice of Pd structure depended on whether or not the amine was in the HCl form. Similar results have been observed for various metals in the presence of free Cl anion30,31 and for Au precipitated in solutions of differing solvent polarity.32 The results obtained for Cl- in the presence of dissolved oxygen strongly suggested that these species acted to oxidatively dissolve the high-energy interfaces formed by nanocrystallites that grew together in an unoriented, mismatched fashion, thus permitting more stable single crystal equilibrium structures to form. For the particular case of silver, Xiong30,31 reported the presence of chloride and oxygen determined structures that were generally monocrystalline and highly symmetric. Noting that none of our solutions were sparged, Xiong’s results parallel our own for Pd grown in the presence of POSS-amine-HCl ligands (OPAm-POSS and MPAm-POSS of Table 1), which, by TEM, show mostly uniform spherical shapes that appear to be highly crystalline cuboctahedra presenting primarily {100} family spacings, as shown in Figure 6. Alternatively, the results

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obtained by Xie32 for the growth of gold particles could indicate that Pd growth in the presence of HCl is mediated by surface repulsion effects. Xie demonstrated the formation of highly fused gold crystallites in aqueous solution following the addition of ethanol. The ethanol was assumed to lower the polarity of the solvent shell surrounding the nascent nanocrystallites, thereby reducing electrostatic repulsion between the particles and allowing them to approach closely enough to fuse. Conversely, the presence of HCl on the Pd surface may form an electrostatic barrier to Pd-Pd particle fusion, thus aiding in the formation of discrete metal structures as such as those for the OPAmPOSS and MPAm-POSS cases. It is arguable that for the OPA-POSS case the relatively nonuniform Pd structure is the result of selective binding of the OPA-POSS to differing crystal faces, resulting in asymmetric rodlike growth. However, the structures appear polycrystalline with roughly uniform grain sizes of approximately normal aspect, instead of having either single crystal rods or rod-shaped grains that are usually presented under conditions of preferential facet growth. Conclusions We have revealed some of the important factors that control the formation of Pd nanocrystallite core-shell materials by using POSS-derived ligands. The second-order structure of the aggregated POSS/metal particles was seen to depend on the number of coordinating amine ligands per POSS and whether or not the amine ligand was in the hydrochloride form. In the absence of both total, symmetrical functionalization of POSS with a binding ligand and the presence of HCl, the secondary structure invariably formed as a random network. When both of these conditions were met, as in the OPAm-POSS variant, the resulting material formed well-ordered spheres as a secondary aggregate, offering new possibilities for the synthesis of controlled palladium nanoparticle aggregates. DFT simulations suggested that a relatively low ratio of POSS-POSS interaction energy to POSS-solvent interaction energy for the OPAmPOSS case drives the formation of the resulting regular spherical morphology of the aggregates, possibly by shifting the particle condensation equilibrium toward dissolution. Conversely, higher values of this ratio for other POSS ligands were consistent with random structures and low solubilities after synthesis. With respect to the primary metal particle formation, ligand condensation onto a metal nanoparticle surface appears to be closely related to the species diffusion coefficients, which in turn is related to molecular structure, composition, solubility, and charge. Our results indicate that in solution the relatively slow-moving POSS ligand allows time for fast-moving Pd prenucleating clusters to undergo reduction and growth into a nanocrystal, which itself becomes slow enough with continued growth to allow for efficient binding by the ligand. In contrast, fast-moving amines that are free molecule versions of the binding functionalities of the POSS compounds used do not form nanoparticles, consistent with the shutting off of the metal growth process at a very early stage. This implies that the size of nanoparticles could be controlled by tailoring the ligand diffusion behavior, which itself can be controlled via the molecular structure and composition of the ligand. Another effect, consistent with prior reports, is that the presence of chloride and oxygen supports the formation of low-energy single crystal structures, either by dissolving high-energy lattice mismatches between particles growing together or by altering the surface charge to form an electrostatic barrier that prohibits particle fusion.

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