Self-Assembly of Polyoxometalate Macroanion-Capped Pd0

When R ≥ 0.6, HPV-capped (and therefore negatively charged) 3-nm-radius Pd0 nanoparticles are formed, which can further self-assemble into stable, h...
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Langmuir 2008, 24, 5277-5283

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Self-Assembly of Polyoxometalate Macroanion-Capped Pd0 Nanoparticles in Aqueous Solution Jie Zhang,† Bineta Keita,‡ Louis Nadjo,*,‡ Israel Martyr Mbomekalle,| and Tianbo Liu*,† Department of Chemistry, Lehigh UniVersity, Bethlehem, PennsylVania 18015, Laboratoire de Chimie Physique, UMR 8000, CNRS, Electrochimie et Photoe´lectrochimie, UniVersite´ Paris-Sud, Baˆtiment 350, 91405 Orsay Cedex, France, and Department of Chemistry, Hunter College of the City UniVersity of New York, 695 Park AVenue, New York 10021 ReceiVed NoVember 23, 2007. ReVised Manuscript ReceiVed February 4, 2008 The self-assembly behavior of polyoxometalate (POM) macroanion-capped 3-nm-radius Pd0 nanoparticles in aqueous solution is reported. Pd0 nanoparticles are synthesized from reducing K2PdCl4 by using Dawson-type V-substituted POM K9[H4PVIVW17O62] (HPVIV) clusters as the reductant and stabilizer simultaneously in acidic aqueous solutions. The starting molar ratio of K2PdCl4 to HPVIV (R value) in solution is important to the formation of Pd nanoparticles. When R < 0.6, ∼20-nm-radius Pd0 colloidal nanocrystals are formed. When R g 0.6, HPV-capped (and therefore negatively charged) 3-nm-radius Pd0 nanoparticles are formed, which can further self-assemble into stable, hollow, spherical, 30-50-nm-radius supramolecular structures in solution without precipitation, as confirmed by light scattering and transmission electron microscopy studies. This structure resembles the unique supramolecular structure formed by hydrophilic POM macroanions in polar solvents, which we refer to as “blackberry” structures. It is the first evidence that the blackberry formation can occur in hydrophobic nanoparticle systems when the surface of nanoparticles is modified to be partially hydrophilic. Counterions play an important role in the self-assembly of Pd nanoparticles, possibly providing an attractive force for blackberry formation, which is the case for blackberry formation in POM macroanionic solutions. Our results suggest that the blackberry formation is not a specific property of POM macroions but most likely a general phenomenon for nanoparticles with relatively hydrophilic surfaces and suitable sizes and charges in a polar solvent.

Introduction Polyoxometalate clusters (POMs) are ideal candidates for homogeneous-phase electron-transfer reactions. Keggin or Dawson-type POM structures, with one or more metal atoms substituted by d-electron-containing first- or second-row transition-metal cations, make redox processes accessible.1–4 They undergo stepwise multielectron redox processes without structural change. These valuable properties of POMs make them suitable for the synthesis of nanosized colloidal nanoparticles of technological and fundamental interest.5–8 Furthermore, in addition to acting as photoswitchable catalysts, POMs can act as stabilizers at the same time. In such a case, a layer of POM polyanions is adsorbed onto the surface of the newly synthesized metal nanoparticles.9 The absorbed anionic POMs provide negative charges to the nanoparticles, which can prevent the agglomer* Corresponding authors. E-mail: [email protected] (L.N.), [email protected] (T.L.). † Lehigh University. ‡ Universite´ Paris-Sud. | Hunter College of the City University of New York.

(1) Papaconstantinou, E. Chem. Soc. ReV. 1989, 18, 1. (2) Pope, M. T.; Muller, A. Angew. Chem., Int. Ed. 1991, 30, 34. (3) Kogan, V.; Izenshtat, Z.; Neumann, R. Angew. Chem., Int. Ed. 1999, 38, 3331. (4) Mbomekalle, I. M.; Keita, B.; Lu, Y. W.; Nadjo, L.; Contant, R.; Belai, N.; Pope, M. T. Eur. J. Inorg. Chem. 2004, 276. (5) (a) Troupis, A.; Gkika, E.; Hiskia, A.; Papaconstantinou, E. New J. Chem. 2001, 25, 361. (b) Troupis, A.; Gkika, E.; Hiskia, A.; Papaconstantinou, E. Angew. Chem., Int. Ed. 2002, 41, 1911. (6) (a) Mandal, S.; Rauntaray, D.; Sastry, M. J. Mater. Chem. 2003, 13, 3002. (b) Mandal, S.; Selvakannan, P. R.; Pasricha, R.; Sastry, M J. Am. Chem. Soc. 2003, 125, 8440. (7) Flynn, N. T.; Gewirth, A. A. Phys. Chem. Chem. Phys. 2004, 6, 1310. (8) Watzky, M. A.; Finke, R. G. Chem. Mater. 1997, 9, 3083. (9) (a) Keita, B.; Zhang, G. J.; Dolbecq, A.; Mialane, P.; Se´cheresse, F.; Miserque, F.; Nadjo, L. J. Phys. Chem. C 2007, 1, 8145. (b) Zhang, G.; Keita, B.; Dolbecq, A.; Mialane, P.; Se´cheresse, F.; Miserque, F.; Nadjo, L. Chem. Mater. 2007, 19, 5821.

ization of nanoparticles via strong electrostatic repulsion. The small countercations, which are necessary for charge balance, are usually present on the surface of POM-coated metal nanoparticles to form electric double layers. The highly charged POM macroanions have a better ability to stabilize metal nanoparticles than other types of anions in the study of iridium colloidal nanoparticles.10 Recently, different approaches to synthesizing palladium colloidal nanoparticles, an important catalyst with catalytic activity that is sensitive to particle size and morphology, have been reported.11–13 Some authors reported a new method of making Pd0 nanoparticles by reducing PdCl42- via Dawson-type vanadium-substituted POM K9[H4PVIVW17O62] (HPVIV) in water at room temperature.14 HPVIV acted simultaneously as both the reducing agent (in which it would be oxidized to HPVV) and the stabilizer, which significantly simplified the synthesis process. Pd0 nanoparticles with an average size of ∼3 nm were synthesized, as confirmed by TEM studies.14 The neutral Pd nanoparticle can absorb a layer of HPV (either HPVIV or HPVV) on its surface, making the overall nanoparticle negatively charged with a relatively hydrophilic surface. The solution behavior of such POM-coated Pd nanoparticles is still completely unknown. It could be very interesting because the POM-coated Pd nanoparticles can be treated as hydrophilic macroanions, and recently, some authors found that some hydrophilic macroions (mostly POMs) underwent unique self(10) Ozkar, S.; Finke, R. G. J. Am. Chem. Soc. 2002, 124, 5796. (11) Chen, S. W.; Huang, K.; Stearns, J. A. Chem. Mater. 2000, 12, 540. (12) Yee, C. K.; Jordan, R.; Ulman, A.; White, H.; King, A.; Rafailovich, M.; Sokolov, J. Langmuir 1999, 15, 3480. (13) Teranishi, T.; Miyake, M. Chem. Mater. 1998, 10, 594. (14) Keita, B.; Mbomekalle, I. M.; Nadjo, L.; Haut, C. Electrochem. Commun. 2004, 6, 978.

10.1021/la7036668 CCC: $40.75  2008 American Chemical Society Published on Web 04/26/2008

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association behavior in solution. Many types of giant POMs are hydrophilic and very soluble in polar solvents because of their negative charges and water ligands15–28 that are chemically bonded to the external surface of POMs, such as [Mo72Fe30O252(CH3COO)12{Mo2O7(H2O)}2{H2Mo2O8(H2O)}(H2O)91] ({Mo72Fe30}),15–20 [{Mo72O252(H2O)72} · {Mo2O4(CH3COOH)}30]42({Mo132}),21–24 {[Mo154O462H14(H2O)70]0.5[Mo152O457H14(H2O)68]0.5}15({Mo154}),25–27 [Cu20Cl(OH)24(H2O)12(P8W48O184)]25- ({Cu20P8W48}),28 and so forth. Unlike common small inorganic ions that exist as discrete ions in aqueous solutions, such hydrophilic macroanions tend to self-assemble further into hollow, spherical, single-layer “blackberry”-type structures even in dilute solutions.15,24,26,28 The blackberry formation differs from the aggregation of hydrophobic colloids (because of van der Waals forces), which leads to phase separation and micelle/vesicle structures formed by amphiphilic surfactants (because of hydrophobic interactions). On the contrary, the blackberry solution is thermodynamically stable. The blackberry size can be accurately tuned by solvent quality and/or the charge density of the macroions. The counterion-mediated attraction and special hydrogen bonds formed between POMs are most likely responsible for blackberry formation. A critical condition for blackberry formation is the size disparity between anions and cations, which leads to the cation association around macroanions. An important question is whether the blackberry formation is a unique phenomenon for very soluble POM macroions or a more general phenomenon for other types of hydrophilic nanoparticles with suitable sizes and charges. The POM-coated Pd0 nanoparticles in very dilute solutions can be used as a nice model system for this study. Pure Pd0 nanoparticles are typical hydrophobic colloidal particles that can form suspensions only in aqueous solutions and tend to agglomerate and precipitate from the solution eventually. However, the POM-coated Pd0 nanoparticles are very stable in solution, showing similarities to POM macroanions. A major difference is that the POM-coated nanoparticles have much higher mass, and the hydrophobic Pd surface has not been totally covered by the HPV anions. By adjusting the molar ratio between Pd(II) and POM (defined as R) before reaction, the supramolecular structure of Pd0 nanoparticles could be controlled.

Experimental Section Sample Preparation. A 2 mM K2PdCl4 (Aldrich, 99.8%) solution (pH ∼ 3.54) in water was prepared. To avoid the hydrolysis of K2PdCl4 and the formation of palladium hydroxide precipitates, which will disturb the light-scattering measurement, the pH of the K2PdCl4 solution was adjusted to 1.53 by adding HCl. HPVIV was synthesized (15) Liu, T. J. Am. Chem. Soc. 2002, 124, 10942. (16) Liu, T. J. Am. Chem. Soc. 2003, 125, 312. (17) Liu, G.; Cai, Y.; Liu, T. J. Am. Chem. Soc. 2004, 126, 16690. (18) Liu, G.; Liu, T. J. Am. Chem. Soc. 2005, 127, 6942. (19) Liu, G.; Liu, T. Langmuir 2005, 21, 2713. (20) Liu, T.; Imber, B.; Diemann, E.; Liu, G.; Cokleski, K.; Li, H.; Chen, Z.; Mu¨ller, A. J. Am. Chem. Soc. 2006, 128, 15914. (21) Zhu, Y.; Cammers-Goodwin, A.; Zhao, B.; Dozier, A.; Dickey, E. C. Chem.sEur. J. 2004, 10, 2041. (22) Chen, B.; Jiang, H.; Zhu, Y.; Cammers, A.; Selegue, J. P. J. Am. Chem. Soc. 2005, 127, 4166. (23) Mu¨ller, A.; Krickemeyer, E.; Bo¨gge, H.; Schmidtmann, M.; Peters, F. Angew. Chem., Int. Ed. 1998, 37, 3360. (24) (a) Kistler, M. L.; Bhatt, A.; Liu, G.; Casa, D.; Liu, T. J. Am. Chem. Soc. 2007, 129, 6453. (b) Liu, G.; Kistler, M. L.; Li, T.; Bhatt, A.; Liu, T. J. Cluster Sci. 2006, 17, 427. (25) Mu¨ller, A.; Diemann, E.; Kuhlmann, C.; Eimer, W.; Serain, C.; Tak, T.; Kno¨chel, A.; Pranzas, P. K. Chem. Commun. 2001, 19, 1928. (26) Liu, T.; Diemann, E.; Li, H.; Dress, A.; Mu¨ller, A. Nature 2003, 426, 59. (27) (a) Mu¨ller, A.; Das, S. K.; Fedin, V. P.; Krickemeyer, E.; Beugholt, C.; Bo¨gge, H.; Schmidtmann, M.; Hauptfleisch, B. Z. Anorg. Allg. Chem. 1999, 625, 1187. (b) Mu¨ller, A.; Das, S. K.; Krickemeyer, E.; Kuhlmann, C. Inorganic Syntheses; Shapley, J. R., Ed.; Wiley: New York, 2004; Vol. 34, p 191. (28) Liu, G.; Liu, T.; Mal, S. S.; Kortz, U. J. Am. Chem. Soc. 2006, 128, 10103.

Zhang et al. as previously reported.14 An appropriate amount of the K2PdCl4 solution (0.05-0.75 mL) was mixed with 10 mL of 0.1 mM HPVIV solution in water to prepare a series of reaction solutions with different molar ratios of K2PdCl4 and POM HPVIV. The mixture was stirred vigorously for a couple of minutes before being filtered into dustfree cells through Millipore filters with a 0.10 µm pore size. After a few hours of delay, the solution color changed from blue to yellow. The reaction process was monitored by using static and dynamic light scattering techniques (SLS and DLS). Static and Dynamic Light Scattering Measurements. The BIZPM laser-light-scattering spectrometer (Brookhaven Instruments) equipped with a coherent radiation 200 mW diode-pumped solidstate (DPSS 532) laser with a wavelength of 532 nm and a BI-9000 correlator was used for both SLS and DLS measurements. DLS measured the intensity-intensity time correlation functions at scattering angles of 30-90° at 25 °C. The correlation functions from DLS were analyzed by the CONTIN method. SLS measurements were performed over a scattering-angle range of 30-150°. For aqueous solutions of Pd0, dn/dc ) -0.125 mL/g at 25 °C (wavelength 535 nm), as measured by a commercial Brookhaven Instruments differential refractometer. The data analysis of SLS is based on the Rayleigh-Gans-Debye equation.29 The principles of light scattering can be found in our previous publication.28 X-ray Photoelectron Spectrum (XPS). Black Pd nanoparticles were centrifuged from the reaction solution, and the precipitates were washed twice with deionized water to remove discrete HPVV and HPVIV polyanions or K2PdCl4. Then, the palladium powder was transferred onto the silicon substrate and dried in air. The sample was measured by Scienta ESCA-300 X-ray photoelectron spectrometer. Transmission Electron Microscopy (TEM). Transmission electron microscopy was performed on a JEOL-2000FX TEM operating at an acceleration voltage of 200 kV. A drop of aqueous solution containing Pd nanoparticles was deposited onto a Famvorcoated copper EM grid and dried in air. A high-resolution TEM photograph was taken on a JEOL 2200FS electron microscope equipped with a field-emission source. The operating voltage was 200 kV. Zeta Potential Analysis. A Brookhaven Instruments Zeta PALS analyzer was used to measure the zeta potential and the mobility of the particles in solution. The analyzer was equipped with a 35 mW solid-state laser operating at 660 nm. The samples were measured at 25 °C. According to the instrument design, particles with diameters from 10 nm to 30 µm (depending on particle density) and zeta potentials ranging from -150 to +150 mV could be measured.

Results and Discussion Monitoring the Reaction and the Self-Assembly Process by SLS and DLS. The electron exchange between K2PdCl4 and HPVIV took place at ambient temperature without any additives. The pure K2PdCl4 aqueous solution (pH ∼ 3.4) is unstable and tends to hydrolyze into insoluble palladium hydroxide. To solve this problem, the K2PdCl4 solution was acidified to pH ∼ 1.5. After mixing the K2PdCl4 solution with HPVIV, the solution experienced a gradual color change from blue to yellow within a few hours, indicating that HPVIV was continuously converted to HPVV and, consequently, that Pd(II) ions were reduced to Pd0 by the following reaction:

PdCl42-+ 2[H4PVIVW17O62]9- ) Pd0+ 2[H4PVVW17O62]8-+ 4Cl- (1) Because of the extremely low initial concentration of Pd0, the typical dark color of Pd0 cannot be observed in solution. However, dark precipitates can be separated from the yellow solution after centrifugation, implying Pd0 formation. (29) Chu, B. Laser Light Scattering, 2nd ed.; Academic Press: New York, 1991.

Polyoxometalate Macroanion-Capped Pd0 Nanoparticles

Figure 1. Growth of scattering intensity with time from K2PdCl4/HPVIV solution at R ) 1.0.

The above reaction under different conditions was monitored by light-scattering techniques. Freshly mixed K2PdCl4 and HPVIV solutions showed very weak scattering intensities in SLS measurements, indicating that there was no large structure in such solutions because all of the reactants are soluble small ions. The reaction usually occurred after a short period of lag time (1 or 2 h) and was completed within a couple of days. Figure 1 shows the increase in the scattering intensity with time for a solution containing 0.1 mM HPVIV. (The molar ratio of K2PdCl4/ HPVIV, defined as the R value here, is 1.0.) The continuous increase in the scattering intensity suggests the continuous formation of larger structures based on the synthesized Pd0 particles. After several days, the scattering intensity slowly stabilized at a very high level (1600 Kcp; for comparison, pure water has a scattering intensity of ∼20 Kcp) because of the completion of the supramolecular structure formation. The solution remained clear and stable during this period and did not show any sign of precipitation. Solutions with different starting concentrations have very similar scattering intensity vs time curves. After the solutions were kept at room temperature for half a year, neither the scattering intensity nor the average Rh value of the assemblies changes. Effect of the Molar Ratio of K2PdCl4 to HPVIV It is known that changing the starting concentrations of reductants or stabilizers or their relative molar ratios can allow some control of the size of Pd nanoparticles.13,30,31 At a fixed HPVIV concentration (0.1 mM), the effect of K2PdCl4 concentration on the size of synthesized Pd nanoparticles was studied. The K2PdCl4/ HPVIV molar ratio was varied from 0.1 to 2.0. Dynamic light scattering and TEM were used to study the structure of assemblies. As shown in Figure 2, the average Rh of the large assemblies increases from 15 to 25 nm with increasing R value when R < 0.6. The TEM image (Figure 3A) reveals that regularly shaped palladium nanoparticles were formed in solution. The typical electron diffraction pattern of the cubic-shaped particle (inset of Figure 3B) shows the sharp diffraction spots corresponding to d spacings of 0.225, 0.194, 0.138, and 0.117 nm-1, respectively, which can be assigned to (111), (200), (220), and (311) planes of the Pd nanocrystals with a face-centered cubic (fcc) structure. The high-resolution TEM micrograph (Figure 3B) also clearly indicates that these nanoparticles are single crystals with a spacing of 0.191 nm along the (200) plane. It is widely accepted that for (30) Hoogsteen, W.; Fokkink, L. G. J. J. Colloid Interface Sci. 1995, 175, 12. (31) Papp, S.; Dekany, I. Colloid Polym. Sci. 2006, 284, 1049.

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Figure 2. Rh of palladium nanocrystals at different molar ratios of K2PdCl4/HPVIV (R value) for the region where R < 0.6, as measured by DLS at a 90° scattering angle.

Figure 3. (A) Typical TEM image of palladium nanocrystals synthesized at the K2PdCl4/HPVIV molar ratio of 0.5 (R ) 0.5). (B) High-resolution TEM image and electron diffraction pattern of Pd nanoparticles showing the crystal lattice of Pd atoms (inset).

the growth of nanocrystals their final size is determined by the nucleation rate and the growth rate at the very beginning of the chemical reaction. In general, an increase in the reductant concentration increases the reduction rate, whereas an increase in the stabilizer concentration decreases the growth rate. In our case, HPVIV serves simultaneously as the reductant and the stabilizer. A higher molar ratio of K2PdCl4 to HPVIV (higher R value) corresponds to a slower reduction rate and a faster growing process. As a result, the formation of larger nanocrystals can be expected. For R g 0.6, the average Rh of the large assemblies (∼40 nm) remains almost unchanged (Figure 4), and it has a totally different nature from that of single nanocrystals obtained at R < 0.6. It takes several days for the completion of the reaction. SLS and DLS measurements were performed during the reaction to monitor the change in particle size in solution. Figure 5 is typical CONTIN analysis of DLS studies on the solution with R ) 1.0 at different

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Figure 4. Rh of the palladium nanoparticles at different molar ratio of K2PdCl4/HPVIV(R) for the region where R g 0.6, as measured by DLS at a 90° scattering angle.

Figure 5. CONTIN analysis of the DLS results (Rh values and their distributions) in the K2PdCl4/HPVIV solution at R ) 1.0 at a scattering angle of 90° at different times.

times. Two different modes with average Rh values being around 3 and 35 nm at a scattering angle of 90° can be identified and correspond to two different types of structures in solution. Both have relatively narrow distributions and coexist in solution during the whole process. These two modes can be identified as 3-nmradius single Pd nanoparticles (capped with POMs) and their self-associated supramolecular structures. The scattering intensity kept increasing within several days as we mentioned above, but the average Rh value of the large supramolecular structures basically does not change with time. At low solute concentrations, the scattering intensity I follows the relation I ∝ CM, with C and M being the concentration and molar mass of the solutes, respectively. In the present case, because the size of the large assembles (which are responsible for most of the scattering intensity from the solution) does not change with time, we can assume that the total mass of the assemblies also remains nearly constant with time. Therefore, the continuous increase in scattering intensity should be attributed to the increase in the number of supramolecular structures. Characterization of the Supramolecular Structures Formed by HPV-Capped Pd Nanoparticles. After the scattering intensity became stable, that is, no further supramolecular structure formation occurred in solution, detailed SLS, DLS, and TEM characterizations were carried out to obtain structural information

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Figure 6. Zimm plot based on the SLS experiments on the stable supramolecular structures formed by HPV-capped Pd nanoparticles in an aqueous solution that originally contained K2PdCl4 and HPVIV with a molar ratio of 1.0 (R ) 1.0).

on the assembled Pd nanoparticles. As we mentioned above, two types of Pd species, Rh ∼3 and ∼35 nm, coexist in solution when R g 0.6. For the large assemblies, DLS measurements show a weak angular dependence of the average Rh value. Extrapolating the apparent diffusion coefficients to zero scattering angle and zero concentration results in absolute average Rh value (Rh,0). For the solution with R ) 1.24, Rh,0 ) 48.5 ( 1.5 nm. The same solution (and also those after dilution) was measured by SLS. From the Zimm plots shown in Figure 6, the average radius of gyration (Rg,0) of the large assemblies is 50.2 ( 2.0 nm. Because the 3 nm single Pd0 nanoparticles make a negligible contribution to the scattering intensity, the measured average Rg,0 represents the radius of gyration of the large assemblies. A Rg,0/Rh,0 ratio of 1.03 ( 0.05 is obtained from the study. For a solid, homogeneous sphere, Rg,0/Rh,0) 0.778. The Rg,0/Rh,0 ratio will increase if more mass is distributed closer to the surface of the sphere. If the spherical objects have all of their mass distributed on the surface, then Rg,0/Rh,0 ) 1, which is the current case. (The spherical morphology can be confirmed by TEM studies, as shown in Figure 7.) This is a typical model for a hollow, vesiclelike structure. In addition, the weight-average molecular weight Mw of the vesiclelike assemblies can be calculated from the SLS results. Considering that large particles scatter the incident beam more strongly than do small particles (proportional to Rh6), a considerable number of 3 nm single Pd0 nanoparticles exist in solution. To calculate the molecular weight of the large assemblies, we need to exclude the concentration of single Pd nanoparticles from the total concentration. This can be achieved by comparing the relative area ratio of two types of particles from their Rh distribution plot in Figure 4. Therefore, Mw ≈ (8.2 ( 0.8) × 108 g/mol was obtained after the concentration correction, which is equivalent to ∼(9 ( 1) × 102 Pd nanoparticles. TEM studies, as shown in Figure 7, indicate that the diameters of the Pd assemblies are about 50-100 nm, consistent with DLS results. Interestingly, from the TEM images we can see clearly that the large Pd0 assemblies are composed of 3-nm-radius small particles, which should be attributed to the POM-capped single Pd nanoparticles. There is still a certain amount of space between the single Pd nanoparticles in the assemblies, indicating that the nanoparticles interact weakly with each other, very possibly because of the electrostatic repulsion introduced by the absorbed anionic HPV. The relatively low electron densities at the center of the assemblies support our previous argument that the spherical

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Figure 7. (a) Typical TEM image of the supramolecular structures formed by HPV-capped 3-nm-radius palladium nanoparticles at R ) 1.0 and (b) an enlarged image at high magnification.

assemblies are not solid. The hollow structure is a more stable state than the single 3-nm-radius palladium nanoparticles in solution. According to the molecular weight obtained by SLS, each hollow aggregate is made up of ∼900 small, 3-nm-radius Pd0 nanoparticles. If the aggregates were solid, then each spherical aggregate would contain 6100 single, 3-nm-radius Pd0 nanoparticles. This large discrepancy strongly suggests that the aggregates cannot be solid. Mechanism of Hollow Structure Formation. Some previous reports show that self-organized spherical metal aggregates consisting of small particles were casually observed in the TEM studies, but the nature of their suprastructures in solution has never been discovered.32–34 Those results showed that a weak reducing agent was a prerequisite for the formation of spherical aggregates of the metal nanoparticles. However, a protective agent absorbed onto the surface of nanoparticles provided the appropriate affinity between individual particles because of their chemical cross-linking or crystalline properties, which played an important role in the self-organization of metal nanoparticles. In the present case, HPVIV or HPVV is very different from other types of stabilizers such as polymers and surfactants, as reported in the literature. The HPVs are completely hydrophilic and negatively charged. The adsorption of POM onto the palladium nanoparticles (the absorption of anions to the coordinatively unsaturated electron-deficient, initially neutral metal surface) creates an electrical layer that results in an electrostatic repulsive force between individual nanoparticles. The surface of Pd nanoparticles is modified to be (at least partially) hydrophilic because of the absorbed HPVs. However, the parts of Pd0 nanoparticles not covered by HPVs are still hydrophobic. Considering that the hydrophobic interaction is a short-range interaction, we find that there is distance (that might be as large as 1 to 2 nm but cannot be accurately determined on the basis of the current experiments) between adjacent Pd nanoparticles in their assemblies (Figure 7B). We can conclude that the hydrophobic interaction is unlikely to be responsible for the self-organization of Pd nanoparticles. The blackberry-type structure formation differs from the aggregation of hydrophobic colloids (because of van der Waals forces), which leads to phase separation. The Pd0 solutions are transparent and thermodynamically stable, without any phase separation. (32) Chen, M; Feng, Y. G.; Wang, L. Y.; Zhang, L.; Zhang, J. Y. Colloids Surf., A 2006, 281, 119. (33) Adachi, E. Langmuir 2000, 16, 6460. (34) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183.

Scheme 1. Schematic Representation of Hollow Suprastructures of Palladium Aggregatesa

a The 3-nm-radius Pd nanoparticles with a layer of POM HPVIV or HPVV on the surface self-organize into large, hollow 60-100 nm aggregates. Small counter-ion cations might be incorporated into the large hollow structures.

We demonstrated in our previous papers that 2-6 nm hydrophilic POM macroanions can self-assemble into a unique hollow, spherical, single-layer blackberry structure because of the counterion-mediated attraction and hydrogen bonds.18,20,24,28 Meanwhile, major contributions from hydrophobic interaction and van der Waals forces have been excluded. The presence of a moderate number of charges on POM clusters is critical to blackberry structure formation. Small Pd0 nanoparticles have comparable sizes and charge densities to the POM clusters, and it is interesting to see that they can also form similar hollow, spherical supramolecular structures in solution. Even though the partially hydrophobic surface of the Pd0 nanoparticles makes it slightly different from the POM clusters, the hydrophobic interaction is not the dominant driving force as we mentioned above. The self-organization mechanism of palladium nanoparticles is probably similar to that of POM clusters, that is, counterion-mediated attraction, which is typical for POM macroanions. The presence of these associated counterions may arise prior to hollow structure formation, and they build a bridge between negatively charged Pd0 nanoparticles, as represented in Scheme 1. Status of Counterions in the Supramolecular Structures of HPV-Capped Pd0 Nanoparticles. When realizing that the counterions are important, it is critical to estimate the charge density on Pd nanoparticles in order to understand the attractive force among them. Consequently, we need to determine how many HPV anions are absorbed on average on each 3 nm Pd nanoparticle. XPS analysis was performed for this purpose. In XPS spectra, as shown in Figure 8, the binding energies due to Pd 5d3/2 and W 4f7/2 are located at 335.6 and 35.7 eV, respectively.

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Figure 8. XPS spectra of Pd0 nanoparticles with (A) palladium in the core and (B) tungsten on the surface.

This demonstrates that the HPV anions have absorbed onto the surface of Pd0 nanoparticles. There are still a small number of Pd(II) atoms present, as evidenced by a corresponding peak at 337.0 eV that was identified, probably as a result of the oxidation of the surface of Pd0 during the drying period in air. The relative contents of W and Pd presented in the large aggregates are 3.8 and 96.2%, respectively. This indicates that on average 10 HPV anions are absorbed on each Pd0 nanoparticle. The vanadium

content is too small to be measured by XPS; therefore, we cannot determine the valence state of V in the absorbed HPV anions. Considering this uncertainty (HPVIV or HPVV), we can roughly estimate that each 3-nm-radius Pd0 nanoparticle carries ∼80-90 negative charges. Zeta potential analysis was employed to study the state of counterions around the surface of hollow palladium aggregates. Zeta potential analysis is widely used to determine the zeta

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potentials of colloidal systems by measuring the electrophoretic mobility of colloid particles in an external electric field. The average charges are calculated from the Hu¨ckel equation

µ0 ) q/6πηr

(2)

with µ0, q, η, and r being the absolute mobility of particles at zero buffer ionic strength, the charge on particles, the solvent viscosity, and the particle radius, respectively. The mean mobility of the hollow spherical structures with Rh ) 39 nm is -1.28 ( 0.16 (µ/s)/(V/cm) with the negative sign denoting negative charges. The average negative charge on each hollow assembly is -52 ( 0.6. Considering that each assembly contains ∼900 Pd nanoparticles, every 3 nm Pd particle carries only about -0.05 net charge, suggesting that those particles are almost chargeless. According to the XPS results, each 3 nm Pd0 nanoparticle carries 80-90 negative charges on the surface. This difference indicates a strong counterion association around those large assemblies. This is consistent with our previous observations on the POM blackberry system.24a Before the hollow assemblies formed, some counterions might have already been associated with HPV-capped Pd nanoparticles, which shield the negative charges on the Pd nanoparticles partially and allow for counterion-mediated attraction between them.

Conclusions In summary, Dawson-type V-substituted polyoxometalate HPVIV exhibits good reducing and stabilizing ability during the

facile synthesis of 3-nm-radius Pd0 nanoparticles. The variation of the metal precursor/POM ratio allows some control of the supramolecular structure formation of the Pd nanoparticles. When the molar ratio of the K2PdCl4 and HPVIV reactants is below 0.6, the POM-capped Pd0 nanoparticles are single nanocrystals. When the ratio is greater than 0.6, they are found to self-organize into 30-50-nm-radius, stable, hollow, spherical, supramolecular structures showing certain similarities to the unique self-assembly of hydrophilic polyoxometalate macroanions (blackberry formation). Our results suggest that the blackberry-type structure formation is most likely a general phenomenon for hydrophilic macroions with a suitable size and moderate amount of charge in polar solvents and is not a specific feature of giant polyoxometalate clusters. From a practical point of view, work in progress will check the ability of this Pd0-based blackberry-type structure to function as a better hydrogen reservoir than isolated Pd0 nanoparticles. Acknowledgment. T.L. is grateful for support from the NSF (CHE-0545983), ACS-PRF (46294-G3), and Lehigh University. This work was also supported by University Paris Sud 11 and the CNRS (UMR 8000). We thank Wu Zhou in the Department of Materials Sciences and Engineering at Lehigh University for performing high-resolution TEM studies. LA7036668