Stabilized Spherical Aggregate of Palladium Nanoparticles Prepared

Feb 6, 2008 - Spherical aggregates of palladium nanoparticles were produced by stirring palladium(II) acetate with dendritic molecules (i.e., ...
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Langmuir 2008, 24, 2719-2726

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Stabilized Spherical Aggregate of Palladium Nanoparticles Prepared by Reduction of Palladium Acetate in Octa(3-aminopropyl)octasilsesquioxane as a Rigid Template Kensuke Naka,* Masahide Sato, and Yoshiki Chujo* Department of Polymer Chemistry, Graduate School of Engineering, Kyoto UniVersity, Katsura, Nishikyo-ku, Kyoto 615-8510, Japan ReceiVed September 15, 2007. In Final Form: December 8, 2007 Spherical aggregates of palladium nanoparticles were produced by stirring palladium(II) acetate with dendritic molecules (i.e., octa(3-aminopropyl)silsesquioxane octahydrochloride (POSS-NH3+) or the amine-terminated G1.0 poly(amidoamine) dendrimer (G1-NH2)) in methanol at room temperature via self-organized spherical templates of Pd(OAc)2 and the dendritic molecule. The mixing ratio of the terminal amino groups of the dendritic molecule and palladium ions (Z ) [Pd2+]/[-NH2]) affected the formation of the spherical aggregates of palladium nanoparticles. Maximum Z values with no reduction of palladium ions (the solution remained yellow) were 1.0 for POSS-NH3+ and 1.6 for G1-NH2, respectively. TEM observations suggested that the density of the palladium nanoparticles in the aggregates using POSS-NH3+ as a template was higher than that using G1-NH2. From tapping mode atomic force microscopy, shapes of the aggregates using POSS-NH3+ and G1-NH2 were a spherical form and an oval form on plates, respectively. Increasing the rigidity of the silsesquioxane core of the dendritic molecules increased the stability of the spherical form in the dry state.

Introduction Metal nanoparticles have become attractive key materials in both academic and industrial areas because of their unique chemical and physical properties, such as catalytic, optical, and magnetic properties distinct from those of bulk metals or atoms.1-4 Multiscale organization of the metal nanoparticles with a controlled manner is central to the application of a molecular system in macroscopic devices.5,6 The self-assembly of the metal nanoparticle based on a selective control of diverse interactions provides a powerful tool for the creation of structured systems at the molecular level.7-13 Several approaches have been used to obtain three-dimensional aggregates of metal nanoparticles.10-13 Previously, we reported that spherical aggregates of palladium nanoparticles were produced by stirring palladium(II) acetate with dendritic molecules such as octa(3-aminopropyl)silsesquioxane octahydrochloride (POSS-NH3+) or amine-terminated poly(amidoamine) dendrimers in methanol at room temperature via self-organized spherical templates of palladium ions and dendritic molecules.14-16 The highly ordered spherical aggregates (1) Daniel, M.-C.; Astruc, D. Chem. ReV. 2004, 104, 293. (2) Henglein, A. Chem. ReV. 1989, 89, 1861. (3) Templeton, A. C.; Wuelfing, W. P.; Murray, R. W. Acc. Chem. Res. 2000, 33, 27. (4) Schmid, G.; Corain, B. Eur. J. Inorg. Chem. 2003, 3081. (5) Schenhar, R.; Norsten, T. B.; Rotello, V. M. AdV. Mater. 2005, 17, 657. (6) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P. Chem. Soc. ReV. 2000, 29, 27. (7) Mirkin, C. A.; Letsinger, R. L.; Mucic, R. C.; Storhoff, J. J. Nature (London, U.K.) 1996, 382, 607. (8) Mann, S.; Shenton, W.; Li, M.; Connolly, S.; Fitzmaurice, D. AdV. Mater. 2000, 12, 147. (9) Teranishi, T.; Haga, M.; Shiozawa, Y.; Miyake, M. J. Am. Chem. Soc. 2000, 122, 4237. (10) Mandal, S.; Gole, A.; Lala, N.; Gonnade, R.; Ganvir, V.; Sastry, M. Langmuir 2001, 17, 6262. (11) Adachi, E. Langmuir 2000, 16, 6460. (12) Jin, J.; Iyoda, T.; Cao, C.; Song, Y.; Jiang, L.; Li, T.; Zhu, D. Angew. Chem., Int. Ed. 2001, 40, 2135. (13) Hussain, I.; Wang, Z.; Cooper, A. I.; Brust, M. Langmuir 2006, 22, 2938. (14) Naka, K.; Itoh, H.; Chujo, Y. Nano Lett. 2002, 2, 1183. (15) Naka, K.; Itoh, H.; Chujo, Y. Chem. Lett. 2004, 33, 1236. (16) Tanaka, H.; Koizumi, S.; Hashimoto, T.; Itoh, H.; Satoh, M.; Naka, K.; Chujo, Y. Macromolecules 2007, 40, 4327.

were composed of the palladium nanoparticles, in which the dendritic molecules acted as cross-linkers and stabilizers for the palladium nanoparticles. Palladium nanoparticles are attractive materials for catalysis17,18 and hydrogen storage.18 Building the nanoparticles into stable hierarchical structures is required for these applications. POSS-NH3+ is regarded as a structure equivalent of the G1.0 PAMAM dendrimer (G1-NH2).19-21 Contrary to the PAMAM dendrimer, POSS-NH3+ has an inner cubic ∼0.5 nm rigid inorganic core containing silicon and oxygen, which offers uniform interparticle spacing and good thermal, mechanical, and solvent-resistant properties. Since the cubic silica core is rigid, the eight organic functional groups of POSS-NH3+ are appended to the vertexes of the cubic silica core via the spacer linkage. Contrary to the synthesis of the PAMAM dendrimer, POSSNH3+ is simply prepared and isolated as precipitates by hydrolysis of aminopropyltriethoxysilane in an aqueous acidic methanol.22 The present study elucidated the difference between G1-NH2 and POSS-NH3+ for the formation processes and structures of the spherical aggregates of palladium nanoparticles. We found that increasing the rigidity of the core of the dendritic molecules increased the stability of the spherical form. Further stable spherical aggregates of palladium nanoparticles using a coating of gold nanoparticles were also reported. Experimental Procedures Materials. Palladium(II) acetate, hydrogen tetrachloroaurate(III) trihydrate, potassium carbonate, formaldehyde, and sodium hydroxide were purchased from Wako Pure Chemical Industry Co. Ltd. Tetraethyl orthosilicate, the G1.0 PAMAM dendrimer denoted as (17) Bo¨nnemann, H.; Braun, G.; Brijoux, W.; Brinkmann, R.; Tilling, A. S.; Seevogel, K.; Siepen, K. J. Organomet. Chem. 1996, 520, 142. (18) Mayer, A. B. R.; Mark, J. E.; Morris, R. E. Polym. J. 1998, 30, 197. (19) Laine, R. M.; Zhang, C.; Sellinger, A.; Viculis, L. Appl. Organomet. Chem. 1998, 12, 715. (20) Feher, F. J.; Wyndham, K. D. Chem. Commun. 1998, 323. (21) Feher, F. J.; Wyndham, K. D.; Soulivong, D.; Nguyen, F. J. Chem. Soc., Dalton Trans. 1999, 1491. (22) Naka, K.; Fujita, M.; Tanaka, K.; Chujo, Y. Langmuir 2007, 23, 9057.

10.1021/la7027109 CCC: $40.75 © 2008 American Chemical Society Published on Web 02/06/2008

2720 Langmuir, Vol. 24, No. 6, 2008 G1-NH2, and tetrakis(hydroxymethyl)phosphonium chloride (THPC, 80% solution in water) were purchased from Aldrich. These chemicals were used as received. POSS-NH3+ was prepared from (3aminopropyl)triethoxysilane following the detailed experimental procedures described in the literature.19,20 Gold colloids 2-3 nm in diameter were prepared according to ref 23.23 Measurements. Scanning electron microscopy (SEM) images were obtained using a JEOL JSM-5600 operated at an accelerating voltage of 15 kV. A JEOL JED-2300 was used as an energy dispersive X-ray spectroscopy (EDX) detector. SEM and EDX samples were prepared by putting dried samples on a conducting tape attached to a SEM grid. Transmission electron microscopy (TEM) images were observed with a JEOL JEM-1025 operating at an accelerating voltage of 100 kV. A TEM sample was prepared by depositing two drops of a desired solution on a 200 mesh copper grid covered with a carbon film and dried to remove the solvent at room temperature. UV-vis-NIR absorption spectra were obtained using a SHIMADZU UV-3600 UV-vis-NIR spectrophotometer. Dynamic light scattering (DLS) analysis was performed using an Otsuka FPAR-1000. Surface images were measured using a tapping mode atomic force microscopy (TM-AFM) (SPA-400, SEIKO Instruments) instrument operated at room temperature. Nanoprobe cantilevers (SI-DF20, SEIKO Instruments) were utilized. X-ray powder diffraction (XRD) patterns were recorded on a Shimadzu X-ray diffractometer-6000 with highintensity Cu KR radiation at a scanning rate of 0.02° S-1 in 2θ ranging from 2 to 90°. FT-IR spectra were recorded on a PerkinElmer 2000 spectrophotometer. Synthesis. Preparation of Aggregates of Palladium Nanoparticles. Typical experimental conditions are as follows. The two dilute solutions of Pd(OAc)2 (3.6 × 10-2 M) in DMF and POSSNH3+ (1.7 × 10-2 M) in methanol were independently prepared. The Pd(OAc)2 solution (1.45 mL) and the dendrimer solution (0.25 mL) were added to 30 mL of methanol and heated at 50 °C for 18 h. After being stirred for 18 h, aggregates of palladium nanoparticles were obtained. A methanol solution of G1-NH2 (1.5 × 10-2 M) was used for aggregation of palladium nanoparticles in G1-NH2 as a template.24 Coating of Gold Nanoparticles on Aggregates of Palladium Nanoparticles. The process of coating the aggregates of palladium nanoparticles with gold nanoparticles referred to the method of preparation of gold/silica nanospheres.25,26 For the following experiment, aggregates of palladium nanoparticles were prepared from the Pd(OAc)2 solution (1.91 mL) and the POSS-NH3+ solution (0.5 mL) with 13 mL of methanol (Z ) 1.0 and [POSS-NH3+] ) 0.57 mM). A methanol solution (1 mL) of the aggregates of the palladium nanoparticles (0.5 mg) was placed in a centrifuge tube along with an aqueous solution (5 mL) of the gold colloids (2.1 mg). The centrifuge tube was shaken gently for a few minutes and then was allowed to stand for 2 h. The mixture was then centrifuged, and a brown pellet was observed to settle to the bottom of the tube. The supernatant was decanted, leaving the slightly brown pellet, which was redispersed and sonicated in water. The purified precursors were then redispersed in 5 mL of water. In a reaction flask, 10 mg (7.2 × 10-5 mol) of K2CO3 was dissolved in 40 mL of water. After 10 min of stirring, 0.6 mL (8.0 × 10-3 mmol) of a solution of 1% HAuCl4 in water was added. The transparent yellow solution initially appeared and slowly became colorless over the course of 30 min, indicating the formation of gold hydroxide. To a vigorously stirred 4 mL aliquot of the colorless solution, 0.5 mL of the solution containing the precursor was injected, and then a 20 µL (0.72 mmol) aliquot of formaldehyde was added. (23) Duff, D. G.; Baiker, A.; Gameson, I.; Edward, P. P. Langmuir 1993, 9, 2310. (24) Addition of 1.4 molar equiv of HCl to a methanol solution of G1-NH2 showed no obvious difference from that without the addition of HCl by TEM analysis 15. (25) Oldenburg, S. J.; Averitt, R. D.; Westcott, S. L.; Halas, N. J. Chem. Phys. Lett. 1998, 288, 243. (26) Loo, C.; Lin, A.; Hirsch, L.; Lee, M.; Barton, J.; Halas, N.; West, J.; Drezek, R. Technol. Cancer Res. Treat. 2004, 3, 33.

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Figure 1. Plot of [Pd2+]/[-NH2] vs diameters of the aggregates of palladium nanoparticles in (a) G1-NH2 and (b) POSS-NH3+ as templates. The diameters were determined by DLS analysis. Arrows indicate maximum Z values with no reduction of palladium ions. Over the course of 2-4 min, the solution changed from colorless to blue. The products were centrifuged and redispersed in 2 mL of water.

Results and Discussion Formation of Spherical Aggregates of Palladium Nanoparticles. The two solutions of Pd(OAc)2 in DMF and the dendritic molecules in methanol were mixed together and heated at 50 °C for 18 h. Immediately after the two solutions were mixed, the solution became slightly yellow and slightly turbid, which indicated aggregate formation before the reduction of palladium ions. With increasing the reaction time, the solution turned gradually brownish, indicating the formation of palladium nanoparticles. The mixing ratio of the terminal amino groups of the dendritic molecule and palladium ions (Z ) [Pd2+]/[-NH2]) affected the formation of the spherical aggregates of the palladium nanoparticles. Figure 1a,b shows plots of the Z value versus the diameters of the aggregates of palladium nanoparticles using G1-NH2 and POSS-NH3+, respectively. The average diameters were measured by a DLS analysis. Maximum Z values with no reduction of palladium ions, in which the solution remained yellow, were 1.6 for G1-NH2 and 1.0 for POSS-NH3+, respectively. The TEM images of the aggregates below the Z values of the critical points showed no palladium nanoparticles (Figures 2a and 3a). Beyond these values, the reaction mixtures were turned black, which indicated the formation of palladium

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Figure 2. TEM images of the aggregates of palladium nanoparticles with G1-NH2 obtained at Z ) 0.16 (a), 1.59 (b), 2.40 (c), and 4.39 (d).

nanoparticles. According to the previous report, we assume that the aggregation of the dendrimers was derived from the decrease of solubility of the dendrimers with the loading of Pd(OAc)2 in methanol. Since the rigid cubic core of POSS-NH3+ might decrease the capacity for binding Pd(OAc)2, the Z value of the critical point for POSS-NH3+ is lower than that for G1-NH2. The spherical aggregates of POSS-NH3+ obtained at Z ) 1.0 were isolated by centrifugation and washed with methanol several times. A thermogravimetric analysis (TGA) of the isolated aggregates gave a larger weight loss at 200-340 °C and a weight loss of 55.0 wt % at 350 °C. According to the previous report,16 we proposed that Pd(OAc)2 acts as a binder for cross-linking between the different dendrimer molecules to form spherical aggregates. Since the maximum Z value with no reduction of palladium ions, in which the solution remained yellow, was 1.0 for POSS-NH3+, the spherical aggregates of POSS-NH3+ obtained at Z ) 1.0 seemed to be consisted with POSS-NH3+ and Pd(OAc)2. On the basis of this assumption, a theoretical weight loss of a mixture of POSS-NH3+ and Pd(OAc)2 at Z ) 1.0 was 55.1 wt %, in which the value agreed with the observed weight loss. In the case of the spherical aggregates of POSS-NH3+ obtained in Z ) 1.3, TGA of the isolated aggregates showed a weight loss of 52.4 wt %.14 A theoretical weight loss of a mixture of POSS-NH3+ and Pd(OAc)2 at Z ) 1.3 was 76.6 wt %, in which the value did not agree with the observed weight loss. Beyond a Z value of 1.0, the formation of the palladium nanoparticles proceeded in the spherical aggregates with POSSNH3+. Excess Pd(OAc)2 beyond a Z value of 1.0 might reduce the palladium nanoparticles, and the acetate anion in excess Pd(OAc)2 might be removed from the aggregate. On the basis of this assumption, a theoretical weight loss of the aggregates of POSS-NH3+ obtained at Z ) 1.3 is 51 wt %, which agrees with the observed data. In the case of spherical aggregates of G1-NH2 obtained at Z ) 3.0, TGA of the isolated aggregates showed a weight loss of

32 wt %. On the basis of this assumption in the case of the aggregates of POSS-NH3+ described previously, a theoretical weight loss of the aggregates of G1-NH2 obtained at Z ) 3.0 is 53 wt %. Even if all Pd(OAc)2 in the aggregates reduced to form palladium nanoparticles, the weight loss was calculated to be 37 wt %. This value is still larger than the observed value. We assume that some part of G1-NH2 in the aggregate might be desorbed during the isolation process. This may suggest that the aggregates of G1-NH2 are not stable as compared to those of POSS-NH3+. Increasing the rigidity of the core of the dendritic molecule might increase the stability of the spherical form. Figure 1a indicates that the diameters of the aggregates of palladium nanoparticles using G1-NH2 were almost constant with increasing Z values beyond a maximum Z value of 1.6. On the other hand, the diameters of the aggregates of palladium nanoparticles using POSS-NH3+ increased with an increase of Z values. Figure 2 shows TEM images of the aggregates of palladium nanoparticles obtained at various Z values. At Z ) 2.4 and 4.4, the diameters of the aggregates of palladium nanoparticles using G1-NH2 were 61 ( 16 and 50 ( 10 nm, respectively. The diameters of the aggregates of palladium nanoparticles with POSS-NH3+ obtained at Z ) 2.0 and 4.0 were 68 ( 11 and 146 ( 31 nm, respectively. The diameters of the aggregates of palladium nanoparticles were also dependent on the concentration of the reaction mixture. Figure 4 shows a plot of the diameters of the aggregates of palladium nanoparticles using POSS-NH3+ as a template determined by a DLS analysis against the concentration of the reactants with the same molar ratio (Z ) 1.5). The diameters of the resulting aggregates increased with increasing the concentration. A further increase in the concentration to 3 mM [POSSNH3+] gave an average diameter of 680 ( 80 nm by DLS analysis, and some network structures of the aggregates were observed by TEM measurements.

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Figure 4. Plot of the diameters of the aggregates of palladium nanoparticles in POSS-NH3+ determined by DLS analysis against the concentration of POSS-NH3+. The Pd(OAc)2 solution and the methanol solution of POSS-NH3+ (4.26 × 10-3 mmol) (Z ) 1.5) were added to different volumes of methanol and heated at 50 °C for 18 h.

Figure 3. TEM images of the aggregates of palladium nanoparticles with POSS-NH3+ obtained at Z ) 0.6 (a) and 2.0 (b and c).

The TEM observation suggested that the density of the palladium nanoparticles in the aggregates using POSS-NH3+ as a template was higher than that using G1-NH2, even in the case of higher Z values. The TEM image shows that the palladium nanoparticles in the spherical aggregates were well-separated and that their average diameter was 6.0 ( 1.3 nm. The XRD pattern of the aggregates of palladium nanoparticles using G1NH2 as a template indicated the formation of palladium nanoparticles by peaks at 40.1, 46.6, and 68.1°, which can be assigned to the (111), (200), and (220) crystalline plane diffraction peaks, respectively (Figure 5a).27 The average size of the palladium nanoparticles was 7.1 nm as calculated by the Scherrer formula, in good agreement with the result of the TEM image. The TEM image of the aggregates using POSS-NH3+ showed that the palladium nanoparticle size is 5.8 ( 0.49 nm observed at the edge of the aggregates (Figure 3b). The size of the palladium nanoparticles was also estimated to be 6.8 nm by XRD line (27) Joint Committee on Powder Diffraction Standards. Diffraction Data File; JCPDS International Center for Diffraction Data: Newtown Square, PA, 1991.

Figure 5. XRD patterns of the aggregates of palladium nanoparticles with (a) G1-NH2 and (b) POSS-NH3+. The aggregates were prepared under the conditions described in the Experimental Procedures.

broadening of the (111), (200), and (220) peaks using the Scherrer equation (Figure 5b). This indicates that the spherical aggregates are consistent with the palladium nanoparticles. From the TM-AFM image (Figure 6a), the height of the aggregate using G1-NH2 as a template was about 30 nm. However, the TEM image of the aggregates shows a 80 nm diameter. Therefore, the shape of the aggregates using G1-NH2 might be an oval on the plate. The well-separated individual palladium nanoparticles in the TEM images also support this observation. From the TM-AFM image (Figure 6b), the height of the aggregates of palladium nanoparticles using POSS-NH3+ as a template was 100 nm, and the diameter of the aggregates was about 100 nm from the TEM image. Therefore, the aggregates of the palladium nanoparticles seem to maintain their spherical form on the plate. We assume that increasing the rigidity of the core of the dendritic molecules increased the stability of the spherical form in the dry state. Surface Coating of the Aggregate of Palladium Nanoparticles with Gold Nanoparticles. The procedure for the synthesis of surface-coated spherical aggregates of palladium nanoparticles with gold nanoparticles to improve stability of the aggregates in

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Figure 6. Tapping mode AFM images of the aggregates of palladium nanoparticles with (a) G1-NH2 and (b) POSS-NH3+. The aggregates were prepared under the conditions described in the Experimental Procedures.

Figure 7. Scheme of surface coating of the spherical aggregates of palladium nanoparticles with gold nanoparticles.

Figure 9. UV-vis absorption spectra of (a) gold colloids, (b) precursor, (c) PdNPs@AuNPs-1, and (d) PdNPs@AuNPs-2. Figure 8. Appearance of the solutions of (A) gold colloid, (B) precursor, (C) PdNPs@AuNPs-1, and (D) PdNPs@AuNPs-2.

a dry state is schematically shown in Figure 7. The colloidal solution of gold clusters with a diameter of 2-3 nm was added

to a methanol solution of the spherical aggregates of palladium nanoparticles prepared using POSS-NH3+. The aggregate used here was prepared under a Z value of 1.0 because minimized density of the palladium nanoparticles in the aggregates was

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Figure 10. TEM images of aggregates of palladium nanoparticles (A), precursor (B), PdNPs@AuNPs-1 (C), and PdNPs@AuNPs-2 (D).

Figure 11. SEM images of aggregates of palladium nanoparticles (A and B) and PdNPs@AuNPs-2 (C and D) after removing the solvents under reduced pressure.

appropriate to estimate the surface coating of the aggregates with gold nanoparticles by TEM analysis. After 2 h of incubation at room temperature, the resulting aggregates were isolated as a brown pellet by centrifugation and were redispersed in water. The obtained precursor solution and formaldehyde as a reducing

agent were added to an aqueous solution containing HAuCl4. Over the course of 2-4 min, the solution changed from colorless to blue. Afterward, two kinds of surface-coating aggregates of palladium nanoparticles with gold nanoparticles (i.e., PdNPs@ AuNPs-1 and -2) were obtained by using 1.0 and 0.5 mL of the

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Figure 12. Distribution of Au, Pd, and Si in the SEM image of Figure 11D (from POSS-NH3+).

methanol solution of the spherical aggregates of palladium nanoparticles with constant amounts of other reagents, respectively. Both the samples were centrifuged and redispersed in water. Appearances and UV-vis absorption spectra of the colloidal gold clusters, the precursor solution, PdNPs@AuNPs-1, and

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PdNPs@AuNPs-2 are shown in Figures 8 and 9, respectively. The solutions of PdNPs@AuNPs-1 and -2 show blue colors derived from surface plasmon absorption. The surface plasmon bands were shifted from 526 to 570 nm with increasing the feed ratio of the amount of HAuCl4 to the aggregate of palladium nanoparticles. This was a consequence of the overlap of the dipole resonances between neighboring gold nanoparticles on the surface of aggregates of palladium nanoparticles using POSSNH3+.23 The TEM analyses (Figure 10) show that the diameters of pristine aggregates of palladium nanoparticles used in this experiment, precursors, PdNPs@AuNPs-1, and PdNPs@AuNPs-2 were 161 ( 20, 173 ( 18, 172 ( 12, and and 199 ( 14 nm, respectively. The TEM images of PdNPs@AuNPs-1 and -2 (Figure 10C,D) were darker than that of the precursor (Figure 10B). It seemed that the colloidal gold nanoparticles on the aggregates of the palladium nanoparticles grew and that the gold nanoparticles covered most of the surface of the aggregates of the palladium nanoparticles. After removing the solvents from the methanol solution of the spherical aggregates of the palladium nanoparticles prepared using POSS-NH3+ under reduced pressure, a remaining solid sample was analyzed by SEM. Obvious spherical structures around 100 nm in diameter were not observed (Figure 11A,B). This indicates that the spherical aggregates of palladium nanoparticles were fused during the drying process. Increasing the concentration increased the diameter of the aggregates as described previously. On the other hand, the SEM image of PdNPs@AuNPs-1 shows spherical particles 174 ( 27 nm in diameter (Figure 11D). This value was in agreement with that of the TEM image. This suggests that the stability of the particles increased with surface coating. The composition of PdNPs@AuNPs was further probed by EDX analysis. From the distribution of Au, Pd, and Si in the individual particles measured by EDX analysis (Figure 12(Au), (Pd), and (Si)) in the same area as shown in Figure 11D, each element was uniformly distributed on the nanocomposites. These data gave a clue that the particles were nanocomposites consisting of Pd, Au, and POSS-NH3+. An EDX spectrum shows three Au peaks as well as three Pd peaks and one Si peak (Figure 13). The

Figure 13. EDX spectrum of Au, Pd, and Si for the sample in Figure 11D.

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surface weight ratio of Si/Pd/Au varied around 1.0:1.3:3.3. Although the atomic ratio of Si/Pd is comparable to the feed ratio of the aggregates of palladium nanoparticles using POSSNH3+, the weight ratio of Au against Si and Pd was twice as much as a theoretical weight ratio if gold metal is uniformly distributed on the surface of the aggregates of the palladium nanoparticles. This may be explained because the gold nanoparticles are mainly distributed on the surface of the samples.

Conclusion Spherical aggregates of palladium nanoparticles were produced by stirring palladium(II) acetate with POSS-NH3+ or G1-NH2 in methanol at room temperature via self-organized spherical templates of palladium ions and the dendritic molecules. The mixing ratio of the terminal amino groups of the dendritic molecules and palladium ions (Z ) [Pd2+]/[-NH2]) affected the formation of the spherical aggregates of palladium nanoparticles. Maximum Z values with no reduction of palladium ions, in which the solution remained yellow, were 1.0 for POSS-NH3+ and 1.6 for G1-NH2, respectively. TEM observations suggested that the density of palladium nanoparticles in the aggregates using POSSNH3+ as a template was higher than those using G1-NH2. From the TM-AFM image, the shapes of the aggregates using POSSNH3+ and G1-NH2 were an oval and a spherical form on the plate, respectively. Increasing the rigidity of the core of the

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dendritic molecule increased the stability of the spherical form in the dry state. Self-organized nanocomposites comprised of gold nanoparticles coated with carboxyl groups (Au-COOH) and POSS-NH3+ recently have been reported by us,28 in which POSS-NH3+ was employed as a rigid cross-linker of Au-COOH via electrostatic interactions between the deprotonated carboxyl groups on AuCOOH or Au-COO- and the ammonium groups on the vortexes of POSS-NH3+. Such nanostructures are expected to exhibit excellent catalytic properties because of their large surface and the synergistic effects of the palladium and gold nanoparticles. However, this process provided precipitates and did not resuspend in solution. In the present study, we prepared a colloidal form of PdNPs@AuNPs by stirring the aggregates of palladium nanoparticles protected by POSS-NH3+ with the colloidal gold nanoparticles at room temperature. Acknowledgment. This work was supported by a Grantin-Aid for Scientific Research (B) (16310086) and the 21st Century COE Program for a United Approach to New Material Science. LA7027109 (28) Wang, X.; Naka, K.; Zhu, M.; Itoh, H.; Chujo, Y. Langmuir 2005, 21, 12395.