AFM Characterization of Dendrimer-Stabilized Platinum

AFM provides a vertical height measurement that complements the lateral ..... out all AFM measurements using a PicoSPM AFM (Molecular Imaging, Phoenix...
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AFM Characterization of Dendrimer-Stabilized Platinum Nanoparticles Yunlong Gu, Hong Xie, Jinxin Gao, Dongxia Liu, Christopher T. Williams, Catherine J. Murphy, and Harry J. Ploehn* Departments of Chemical Engineering and Chemistry & Biochemistry, University of South Carolina, Columbia, South Carolina 29208 Received August 30, 2004. In Final Form: November 22, 2004 This work describes the use of atomic force microscopy (AFM) to measure the size of dendrimer-stabilized Pt nanoparticles (Pt DNs) deposited from aqueous solutions onto mica surfaces. Despite considerable previous work in this area, we do not fully understand the mechanisms by which PAMAM dendrimers template the formation of Pt DNs. In particular, Pt DN sizes measured by high-resolution transmission electron microscopy (HRTEM) are reported to be larger than expected if one assumes that each PAMAM molecule templates one spherical Pt nanoparticle. AFM provides a vertical height measurement that complements the lateral dimension measurement from HRTEM. We show that AFM height measurements can distinguish between “empty” PAMAM and Pt DNs. If the complexation of Pt precursor with PAMAM is prematurely terminated, AFM images and feature height distributions show evidence of arrested precipitation of Pt colloids. In contrast, sufficient Pt-PAMAM complexation time leads to AFM images and height distributions that have relatively narrow, normal distributions with mean values that increase with the nominal Pt:PAMAM ratio. The surface density of features in AFM images suggest that these Pt DNs reside on the mica surface as two-dimensional surface aggregates. These observations are consistent with an intradendrimer templating mechanism for Pt DNs. However, we cannot determine if the mechanism obeys a fixed loading law because we do not have definitive information about Pt DN shape. A second peak in the Pt DN height distribution appears when the Pt loading exceeds about 66% of PAMAM’s theoretical capacity for Pt. Excluding these secondary particles, the dependence of mean feature height on the Pt: PAMAM ratio follows a power-law relationship. Also considering the magnitudes of the measured mean height values, the data suggest that Pt DNs exist as ramified, noncompact aggregates of Pt atoms interspersed within the PAMAM framework.

Introduction Dendrimer-mediated synthesis has emerged as a viable method for producing metal1-3 and semiconductor2,4-7 nanoparticles. Several recent publications,8-13 as well as earlier reviews,1-3 provide a comprehensive overview of research in this area. This research demonstrates the utility of dendrimers for synthesizing of metal nanoparticles with sizes in the range of 1-3 nmslarger than molecular clusters containing fewer than about 20 metal atoms but smaller than precipitated colloidal particles containing thousands of metal atoms. Most research in this area employs poly(amidoamine) and poly(propylene imine) dendrimers (known as PAMAM * Author to whom correspondence should be addressed. E-mail: [email protected]. (1) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (2) Crooks, R. M.; Buford, I. L.; Sun, L.; Yeung, L. K.; Zhao, M. Top. Curr. Chem. 2001, 212, 81-135. (3) Esumi, K. Top. Curr. Chem. 2003, 227, 31-52. (4) Sooklal, K.; Hanus, L. H.; Ploehn, H. J.; Murphy, C. J. Adv. Mater. 1998, 10, 1083-1087. (5) Huang, J.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Chem. Mater. 1999, 11, 3595-3601. (6) Hanus, L. H.; Sooklal, K.; Murphy, C. J.; Ploehn, H. J. Langmuir 2000, 16, 2621-2626. (7) Lemon, B. I.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 1288612887. (8) Niu, Y.; Crooks, R. M. C. R. Chim. 2003, 6, 1049-1059. (9) Scott, R. W. J.; Ye, H.; Henriquez, R. R.; Crooks, R. M. Chem. Mater. 2003, 15, 3873-3878. (10) Oh, S. K.; Kim, Y. G.; Ye, H.; Crooks, R. M. Langmuir 2003, 19, 10420-10425. (11) Kim, Y. G.; Oh, S. K.; Crooks, R. M. Chem. Mater. 2004, 16, 167-172. (12) Esumi, K.; Isono, R.; Yoshimura T. Langmuir 2004, 20, 237243. (13) Ye, H.; Scott, R. W. J.; Crooks, R. M. Langmuir 2004, 20, 29152920.

and PPI, respectively). These dendrimers have been extensively studied, are commercially available, and have many favorable physical and chemical attributes. In particular, PAMAM features (1) interior amine and amide groups that can interact with ionic metal precursors through coordination chemistry or ligand-exchange reactions; (2) interior void space and structural flexibility that can accommodate supermolecular guest species; and (3) exterior chemical groups (amine, hydroxyl, carboxyl, etc.) that permit further functionalization, tethering to surfaces, and assembly into higher-order structures. The first two of these features make PAMAM useful as a template and stabilizer for synthesizing metal nanoparticles with controlled size, shape, and composition. The third feature provides the means to stabilize the nanoparticles in various phases and/or deliver them to desired locations on solid surfaces. These considerations immediately suggest catalysis as a promising application of dendrimer-stabilized metal nanoparticles (DNs). Most of the previous work in this area1-3,8 focused on liquid-phase homogeneous catalysis or electrocatalysis. Papers describing the use of DNs as gas-phase heterogeneous catalysts are now beginning to appear.14,15 This work attempts to show that DNs are suitable precursors that, after suitable activation, lead to active catalysts. The ultimate goal is to understand how catalyst activity, selectivity, and lifetime depend on DN size, shape, and composition, especially for bimetallic DNs. The viability of dendrimer-based catalyst design depends on our ability to use dendrimers to control the size, (14) Lang H.; May, R. A.; Iversen, B. L.; Chandler, B. D. J. Am. Chem. Soc. 2003, 125, 14832-14836. (15) Deutsch, D. S.; Lafaye, G.; Liu, D.; Chandler, B.; Williams, C. T.; Amiridis, M. A. Catal. Lett. 2004, 97, 139-143.

10.1021/la047843e CCC: $30.25 © 2005 American Chemical Society Published on Web 02/18/2005

Dendrimer-Stabilized Platinum Nanoparticles

shape, and composition of metal nanoparticles. This kind of control requires a fundamental understanding of the detailed mechanisms of DN synthesis. Several mechanisms have been considered, including intradendrimer templating, interdendrimer templating, and arrested precipitation. Intradendrimer templating is thought to predominate when metal ions (like Cu2+) or ionic precursors (like PtCl42-) preferentially coordinate with the dendrimers’ interior groups. In this mechanism, each dendrimer molecule coordinates with essentially the same number of metal ions. Subsequent reduction produces zero-valent metal nanoparticles, uniform in size, with one encapsulated in the interior of each dendrimer molecule. Templating follows the “fixed loading law”16 if the number of metal atoms in each nanoparticle equals the number of metal atoms initially loaded into each dendrimer molecule. This mechanism, proposed1,2,8 for Cu2+, Pd2+, Pt2+, Ni2+, Fe3+, Mn2+, Au3+, and Ru3+, offers the possibility of fine control of nanoparticle size, shape, and composition. Interdendrimer templating may occur when metal ions preferentially coordinate with the dendrimers’ exterior groups. This mechanism has been implicated,1-3,8 under certain conditions, for DNs derived from Au3+, Ag+, Pt2+, and Pt4+. Chemical reduction produces zero-valent metal nanoparticles with narrow particle size distributions, justifying this as templating, but the exact locus of particle nucleation and growth remain unclear. Observations of relatively large mean particle sizes indicate clear violations of the fixed loading law. In many cases, the nanoparticles are too large to be accommodated within the dendrimer interior. These observations suggest that interdendrimer templating may not afford good control of DN characteristics. Arrested precipitation describes a variety of interrupted nucleation and growth mechanisms used to make stable colloidal particles. In the present context, precipitation of metal salts in the presence of dendrimer leads to stable colloidal nanoparticles due to surface adsorption of dendrimer, which arrests the growth process. Nanoparticle size and polydispersity depend on the dendrimer generation, concentration, and metal-to-dendrimer (M:D) ratio. Higher generation and lower M:D generally produce smaller, more uniform nanoparticles. Insofar as the locus of particle nucleation lies outside of the dendrimer, interdendrimer templating can be viewed as a form of arrested precipitation. Discriminating among these mechanisms relies on accurate characterization of nanoparticle size. Highresolution transmission electron microscopy (HRTEM) has served admirably for this purpose. HRTEM images have conclusively demonstrated that PAMAM-mediated synthesis gives sub 2 nm nanoparticles for a variety of metals, including Au, Pt, and Pd. For example, Kim et al.11 recently reported successful intradendrimer templating of Au55 and Au140 nanoparticles in PAMAM dendrimers with primary amine and quarternized ammonium exterior groups (GnNH2 and GnQp, respectively, where n denotes the PAMAM generation number and p denotes the number of exterior quarternized groups). In all cases, the mean particle diameter from HRTEM agreed with the calculated diameter, assuming a spherical particle containing the nominal number of Au atoms per PAMAM molecule. In all cases except one, this agreement supports intradendrimer templating in accord with the fixed loading law. The exceptional case, G4Q32-Au140, also manifested (16) Gro¨hn, F.; Bauer, B. J.; Akpalu, Y. A.; Jackson, C. L.; Amis, E. J. Macromolecules 2000, 33, 6042-6050.

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agreement between the HRTEM and calculated particle diameters. However, the nominal loading, Au:D ) 140, substantially exceeds the capacity of G4Q32 under the assumption of intradendrimer templating following the fixed loading law. In contrast, HRTEM images of Pt and Pd DNs consistently show average particle diameters significantly greater than expected.8,10,13 For example, the theoretical diameter of a Pt40 sphere (assuming fcc packing) is 1.05 nm. HRTEM images of Pt40 DNs give average particle diameters of 1.4 ( 0.2 nm (G4OH,1,17,18 G4NH213) and 1.7 ( 0.3 nm (G4Q3210). Likewise, the theoretical diameter of a Pd40 sphere is 1.04 nm, but HRTEM images of Pd40 DNs give average particle diameters of 1.3 ( 0.3 nm (G4OH1,18), 1.5 ( 0.3 nm (G4NH213), and 1.7 ( 0.2 nm (G4OH,19 G4NH2,9 G4Q3210). For bimetallic G4OH-Pd30Pt10 made by co-complexation and co-reduction,20 the average HRTEM particle diameter is 1.9 ( 0.4 nm. In all cases, the HRTEM particle size is 30-70% greater than expected. The discrepancy is amplified when one considers atom numbers: a 1.4 nm diameter Pt sphere (the smallest one observed by HRTEM) ought to contain about 96 atoms, 140% more than was used1 to template G4OHPt40. As stated by Oh et al.,10 “there is something peculiar about Pt and Pd DENs that we do not understand at this time.” Discounting the unlikely possibility of systematic error in HRTEM measurements over the past six years, there are several other plausible explanations. First, Pt and Pd DNs may assume nonspherical shapes when deposited on surfaces. The theoretical diameter of a Pt40 hemisphere (for example) is 1.3 nm (assuming fcc packing) or as much as 1.4 nm (assuming random close packing). However, the theoretical and HRTEM diameters for Au DNs become inconsistent in this scenario, nor can nonspherical shape account for the much larger diameters observed in many cases. Second, Pt and Pd DNs may assume expanded, irregularly shaped, non-close-packed geometries13 not seen in Au DNs. Third, larger-than-expected spherical Pt and Pd DNs may be formed by interdendrimer templating that does not obey the fixed loading law. Measurement of DN size in all three dimensions may help resolve this question. HRTEM provides only the lateral (x-y) dimensions of nanoparticles on surfaces. Atomic force microscopy (AFM) can measure nanoparticle size in the vertical (z) dimension. Unlike HRTEM, AFM can image surfaces in gaseous or liquid environments and does not inflict e-beam damage. Sample damage caused by the probe tip can be minimized using tapping-mode AFM. In general, probe tip convolution20-23 limits the lateral resolution of surface features, but vertical resolution can be better than 0.01 nm. Since AFM images are ultimately force measurements, contributions from van der Waals, electrostatic, capillary, and specific chemical forces are expected. These depend on surface composition and may vary from place to place on one surface, complicating the problem of measuring unambiguous, absolute heights of surface features. (17) Zhao, M.; Crooks, R. M. Adv. Mater. 1999, 11, 217-220. (18) Zhao, M.; Sun, L.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364-366. (19) Niu, Y.; Yeung, L. K.; Crooks, R. M. J. Am. Chem. Soc. 2001, 123, 6840-6846. (20) Scott, R. W. J.; Datye, A. K..; Crooks, R. M. J. Am. Chem. Soc. 2003, 125, 3708-3709. (21) Vesenka, J.; Miller, R.; Henderson, E. Rev. Sci. Instrum. 1994, 65, 2249-2251. (22) Ramirez-Aguilar, K. A.; Rowlen, K. L. Langmuir 1998, 14, 25622566. (23) Mulvaney, P.; Giersig, M. J. Chem. Soc., Faraday Trans. 1996, 92, 3137-3143.

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AFM has been used extensively to measure the size of PAMAM dendrimers deposited on mica, graphite, glass, and gold surfaces.24-30 In general, individual molecules can be imaged for higher generation PAMAM (>G5) deposited on mica from very dilute solutions. All AFM studies show that PAMAM molecules on surfaces assume highly flattened configurations. The degree of flattening (relative to the hydrodynamic diameter) increases for lower-generation PAMAM due to its greater flexibility. Lower-generation PAMAM tends to form two-dimensional surface aggregates at the water-mica and water-graphite interface. AFM has also been used to study the morphology and size distribution of nanoparticles deposited onto flat surfaces. Several studies employing colloidal gold,23,31,32 polystyrene,22 silica,22 and magnetitie33 compared particle size statistics derived from AFM and TEM. In general, the particle height measured by AFM agrees closely with the particle diameter measured by TEM. However, for particle sizes below about 10 nm, long-range surface-tip forces become important and may lead to height anomalies in tapping-mode AFM measurements. Artifacts due to humidity34 and tip-surface adhesion35 can be ultimately traced to shifting of the cantilever resonance frequency due to attractive capillary forces.36 Particle deformation37 due to long-range particle-surface forces must also be considered. The objective of the present work is to measure the size of Pt DNs on mica surfaces in the vertical (z) dimension using intermittent-contact (“tapping”) mode atomic force microscopy (TMAFM). To the best of our knowledge, only Sun and Crooks38 have used AFM to image Pd DNs on mica and graphite surfaces. We aim to extend this work to achieve more quantitative results while avoiding or minimizing the possibilities for height anomalies described previously. We vary the metal-to-dendrimer ratio and PAMAM generation to prepare Pt DNs of varying size and use TMAFM to quantify the height distribution and number density of DNs deposited on mica surfaces. These data are compared with theoretical estimates and TEM results. Accurate nanoparticle size distributions derived (24) Li, J.; Piehler, L. T.; Qin, D.; Baker, J. R.; Tomalia, D. A. Langmuir 2000, 16, 5613-5616. (25) Hierlemann, A.; Campbell, J. K.; Baker, L. A.; Crooks, R. M.; Ricco, A. J. J. Am. Chem. Soc. 1998, 120, 5323-5324. (26) Tsukruk, V. V. Adv. Mater. 1998, 10, 253-257. (27) Li, J.; Qin, D.; Baker J. R., Jr.; Tomalia, D. A. Macromol. Symp. 2001, 166, 257-269. (28) Betley, T. A.; Banaszak Holl, M. M.; Orr, B. G.; Swanson, D. R.; Tomalia, D. A.; Baker, J. R., Jr. Langmuir 2001, 17, 2768-2773. (29) Betley, T. A.; Hessler, J. A.; Mecke, A.; Banaszak Holl, M. M.; Orr, B. G.; Uppuluri, S.; Tomalia, D. A.; Baker, J. R., Jr. Langmuir 2002, 18, 3127-3133. (30) Muller, T.; Yablon, D. G.; Karchner, R.; Knapp, D.; Kleinman, M. H.; Fang, H.; Durning, C. J.; Tomalia, D. A.; Turro, N. J.; Flynn, G. W. Langmuir 2002, 18, 7452-7455. (31) Vesenka, J.; Manne, S.; Giberson, R.; Marsh, T.; Henderson, E. Biophys. J. 1993, 65, 992-997. (32) Grabar, K. G.; Brown, K. R.; Keating, C. D.; Stranick, S. J.; Tang, S. L.; Natan, M. J. Anal. Chem. 1997, 69, 471-477. (33) Lacava, L. M.; Lacava, B. M.; Azevedo, R. B.; Lacava, Z. G. M.; Buske, N.; Tronconi, A. L.; Morais, P. C. J. Magn. Magn. Mater. 2001, 225, 79-83. (34) Thundat, T.; Warmack, R. J.; Allison, D. P.; Bottomley, L. A.; Lourenceo, A. J.; Ferrell, T. L. J. Vac. Sci. Technol. A 1992, 10, 630635. (35) Van Noort, S. J. T.; Van der werf, K. O.; De Grooth, Bart G.; Van Hulst, N. F.; Greve, J. Ultramicroscopy 1997, 69, 117-127. (36) Ebenstein, Y.; Nahum, E.; Banin, U. Nano Lett. 2002, 2, 945950. (37) Gro¨hn, F.; Kim, G.; Bauer, B. J.; Amis, E. J. Macromolecules 2001, 34, 2179-2185. (38) Sun, L.; Crooks, R. M. Langmuir 2002, 18, 8231-8236.

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from TMAFM may provide additional insight into the synthesis mechanisms of PAMAM-mediated metal nanoparticles. Experimental Section Chemicals and Materials. Solutions of hydroxyl-terminated poly(amidoamine) (PAMAM) dendrimers (GnOH, generation n ) 4 or 6, 10 wt% in methanol) were purchased from Aldrich. Reagents K2PtCl4 (99.99%) and NaBH4 (99.9999%) were purchased from Aldrich and used as received. For sample purification by dialysis, we used two kinds of cellulose membrane tubing (both purchased from Aldrich): “benzoylated” tubing (D2272) and “regular” tubing (D9277) having nominal cutoffs of 1200 and 12 400 Da, respectively (based on retention of cytochrome C, according to the manufacturer). Prior to use, dialysis tubing was soaked for at least 3 h and rinsed with deionized (DI) water. All water was deionized to a resistivity of 18 MΩ‚cm using a Nanopure system (Barnstead). Muscovite mica for AFM imaging was obtained as 9.5 mm disks of grade V-4 (Structure Probe, Inc., West Chester, PA). Synthesis of PAMAM-Stabilized Pt Nanoparticles. Our methods for synthesis and purification of PAMAM-stabilized Pt nanoparticles are similar to those originally developed by Crooks and co-workers.1,2 For example, synthesis of a solution of G4OHPt40 begins with weighing out as-purchased PAMAM solution (assuming 10 wt% G4OH), rotary evaporation and/or impinging flow of dry N2 gas for at least 3 h to strip out the methanol, and then dissolution of the dry G4OH in DI water to produce a 100 µM solution. We add 10 mL of this solution to 10 mL of 4.0 mM aqueous K2PtCl4 and stir for 2 h, and then store this solution in a refrigerator at least for 10 days to achieve complete equilibration. The resulting “complex” solution contains 50 µM of G4OH(Pt2+)40 (the complexed ions are actually PtClx(+2-x), x ) 0-3, as shown by 195Pt NMR39) with a nominal metal-to-dendrimer (M: D) ratio of 40. Before reduction, we first dilute the G4OH-(Pt2+)40 complex solution with DI water to a concentration of 5.0 µM and bubble through dry N2 gas for 1 h. To 10 mL of the diluted complex solution, we quickly add 0.8 mL of fresh 20 mM NaBH4 aqueous solution with vigorous stirring and bubbling with dry N2 gas for 2 h. Water evaporates during this process, so we add makeup DI water to bring the solution to 10 mL. The final concentrations are 5.0 µM in G4OH, 200 µM in Pt, and 1.6 mM in NaBH4 (an 8:1 ratio of NaBH4 to Pt). In addition to G4OH-Pt40, we synthesized G4OH-Pt20, G4OH-Pt60, G6OH-Pt100, G6OHPt150, and G6OH-Pt200 in a similar way. The product solutions are purified by dialysis against pure DI water using cellulose dialysis tubing. On the basis of careful study of the dialysis process, benzoylated tubing (1200 Da cutoff) must be used to purify G4OH-Pt solutions to prevent losses of PAMAM and Pt. Regular dialysis tubing (12 400 Da cutoff) can be safely used for G6OH-Pt solutions. A standard course of dialysis employs 1000 mL of dialysis water per mL of sample, with at least seven changes of water per day, for a minimum of 2 days. The volumes of the solution samples do not change noticeably during the course of dialysis. We monitor the mixing, equilibration, reduction, and dialysis steps by following the absorption spectra using UV-visible spectrophotometry (Shimadzu model UV-210 PC). We also verify the Pt concentrations and confirm salt removal by dialysis using flame atomic absorption spectroscopy. AFM Characterization. We prepare samples for AFM by first diluting the product solutions by 100-fold or more with DI water (to PAMAM concentrations of 50 nM or less). Typically, we place 2.5 µL of the final solution directly onto the surface of a freshly cleaved mica disk. The sample is discarded if the solution does not spread evenly across the mica. The samples are dried in air, enclosed in covered Petri dishes, at room temperature for at least 5 h. Using this method of sample preparation, AFM measurements of the number of features per unit area have been found to be highly reproducible. (39) Pellechia, P. J.; Gao, J.; Gu, Y.; Ploehn, H. J.; Murphy, C. J. Inorg. Chem. 2004, 43, 1421-1428.

Dendrimer-Stabilized Platinum Nanoparticles We carried out all AFM measurements using a PicoSPM AFM (Molecular Imaging, Phoenix, AZ) operated in the acoustically driven, intermittent contact (“tapping”) mode. We used standard silicon AFM probes (Mikromasch Ultrasharp NSC12/3) having cantilever spring constants of 2.5-8.5 N/m and resonance frequencies from 120 to 190 kHz. The manufacturer estimates that the probe tip radius of curvature is no smaller than about 5-10 nm. All AFM measurements were performed with the samples at ambient temperature. We flow dry N2 gas through the sample enclosure (PicoApex, Molecular Imaging) to maintain a relative humidity below 20% as measured by a portable humidity meter (Sper Scientific model 800016). Our measurements employed the “small” 6 µm piezoelectric z-scanner (Molecular Imaging) calibrated by the manufacturer using calibration gratings with step heights of 20, 100, and 500 nm to precisely quantify any vertical nonlinearity of the scanner. We have also calibrated this scanner using our own calibration gratings (Mikromasch) and colloidal gold particle size standards (Ted Pella, Inc.). Additional discussion of scanner calibration with AFM results for gratings and colloidal gold may be found in the Supporting Information (Figures S1 and S2). According to the manufacturer, this scanner has a minimum vertical resolution of 0.01 nm. We used a commercial software package, SPIP (version 3.0.1.1, Image Metrology A/S) to automate the analysis of AFM images. We have developed suitable protocols for processing images to discriminate surface features from background noise (flattening and thresholding) and to quantify the maximum z value (height) associated with each feature. We use these data to create a histogram of maximum feature heights, using either SPIP or exported data in MS Excel. The feature height distribution obtained from SPIP agrees closely with the value determined manually (i.e., building a histogram one particle at a time) by cross-section analysis using the PicoSPM instrument software (PicoScan 5.2, Molecular Imaging). We prepared at least three mica specimens for each PAMAMPt solution, and we imaged at least five distinct areas of each specimen using the same probe tip. Image analysis shows good consistency of the feature height distributions among specimens and selected areas on each specimen (see Supporting Information, Figure S3). All of the measurements were conducted under dry nitrogen atmosphere to minimize the possibility of artifacts due to humidity (see Supporting Information, Figure S4). We repeated many of these measurements using at least two additional, different probe tips. For each PAMAM-Pt solution, we selected several of the best quality images for image analysis from among dozens of images made as described above. We show just one typical image from the set for each solution in the Results or Supporting Information sections. Feature height data were exported from SPIP to MS Excel to compile data sets from multiple images, producing feature height histograms based on hundreds of observed features. TEM Characterization. We obtained HRTEM images using a Hitachi HF-2000 instrument (Oak Ridge National Laboratory). We prepared samples for HRTEM by depositing small drops of dilute PAMAM-Pt solutions (5.0 µM in G4OH) onto standard carbon-coated copper TEM grids resting on an absorbent filter paper. The sample grids were inserted into the instrument after allowing 5 min for removal of excess solution by the filter paper. HRTEM images were obtained using (typically) an operating voltage of 200 kV with a magnification of 300 000×. We estimated the average particle diameter by manual measurement of the size of at least 100 randomly selected particles in the HRTEM images.

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Results and Discussion

Figure 1. AFM topography images and corresponding feature height distributions for G4OH (a) and G6OH (b) on mica. The bottom panel shows the feature height distribution compiled from multiple images.

PAMAM Dendrimer. AFM results for “empty” dendrimers provide a useful baseline for characterization of PAMAM-Pt nanoparticle size. Figures 1 and S5 (Supporting Information) show typical AFM topography images of G4OH and G6OH deposited onto mica from more- and less-concentrated solutions. For deposition from more concentrated solutions (Figure S2), the PAMAM molecules appear to attract each other and form a weblike structure on the mica surface. These images, like others published

previously,24 show that PAMAM molecules adsorbed at high surface concentrations form patchy films to reduce overall surface energy. Hydrogen bonding mediated by residual water in the films may contribute to interdendrimer attraction. For deposition from much more dilute PAMAM solutions, typical AFM images (Figure 1) show many small, yet distinct features, as well as a few tall features. The

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Table 1. Feature Height Statisticsa from AFM Images of PAMAM and PAMAM-Pt on Mica counts sample

imagesb (figure no.)

G4OH

3 (1a)

G6OH

5 (1b)

G4OH-Pt20 (10 min) G4OH-Pt20

1 (2a)

G4OH-Pt40

4 (S6a)

G4OH-Pt60

4 (S6b)

G6OH-Pt100

1 (S6c)

G6OH-Pt150

1 (S6d)

G6OH-Pt200

precipitated

1 (2b)

feature heighta (nm) median std devd

featuresa N

meanc

1007 (951) 818 (808) 659 (562) 555 (540) 1967 (1852) 721 (708) 325 (318) 315 (311)

0.28 ( 0.005 (0.27 ( 0.003) 0.68 ( 0.02 (0.66 ( 0.01) 0.53 ( 0.04 (0.34 ( 0.01) 0.58 ( 0.02 (0.56 ( 0.01) 0.92 ( 0.02 (0.83 ( 0.01) 1.33 ( 0.04 (1.30 ( 0.04) 1.47 ( 0.04 (1.48 ( 0.04) 1.97 ( 0.06 (1.99 ( 0.05)

0.26 (0.26) 0.67 (0.67) 0.34 (0.32) 0.56 (0.56) 0.81 (0.79) 1.24 (1.23) 1.49 (1.49) 2.01 (2.01)

0.08 (0.05) 0.23 (0.19) 0.57 (0.08) 0.21 (0.13) 0.48 (0.31) 0.56 (0.51) 0.38 (0.35) 0.50 (0.48)

CV (%)e 29 (19) 34 (29) 106 (23) 36 (24) 52 (38) 42 (39) 26 (24) 26 (24)

a

For each sample, the second set of values (in parentheses) is based on the statistical exclusion of outliers41 from the data analysis. Numbers of distinct images used for statistical analysis (with figure number of a typical image). c Values after ( are 95% confidence limits. d Sample standard deviation. e Coefficient of variation. b

Table 2. Average Feature Density in AFM Images of PAMAM and PAMAM-Pt on Mica feature density (1/µm2)

sample parameters sample G4OH G6OH G4OH-Pt20 (10 min) G4OH-Pt20 G4OH-Pt40 G4OH-Pt60 G6OH-Pt100 G6OH-Pt150 G6OH-Pt200 a

image count (figure no.)

concn (nM)

drop vol (µL)

scan area (µm2)

observeda

predicted

3 (1a) 5 (1b) 1 (2a)

50 10 50

2.5 2.5 2.5

2.25 2.25-4 6.25

149 ( 10 50 ( 11 105

1060 212 1060

14 23 10

1 (2b) 4 (S6a) 4 (S6b) 1 (S6c) 1 (S6d) precipitated

50 50 50 25 25

2.5 2.5 2.5 2.5 2.5

4 2.25-4 4 4 2.25

139 139 ( 13 45 ( 8 81 140

1060 1060 1060 530 530

13 13 4.2 15 26

ratio (%)

Values after ( are 95% confidence limits based on data from multiple images from different scan areas.

feature height histograms (Figure 1c) manifest a single peak with a relatively narrow width, indicating a uniform distribution (Tables 1 and 2 provide detailed statistical information). The median feature heights (0.26 nm for G4OH, 0.67 nm for G6OH) are less than 10% of the corresponding hydrodynamic sizes in solution2,40 (4.5 nm for G4OH, 6.7 nm for G6OH). As seen in many previous studies,24-30,38 PAMAM molecules assume highly flattened configurations on mica surfaces. The measured height for G6OH agrees with a value reported previously38 (0.6 nm) but is smaller than published values28 for G6NH2. We do not know of any conclusive measurements of the heights of individual G4OH molecules, but values of 0.5-0.8 nm have been reported for G4NH2 on gold. Are the features in Figure 1 individual dendrimers? Preparation of specimens by drop evaporation permits us to compare the observed feature density in the images (number per area) with theoretical values (calculated from the known concentration, drop volume, and mica disk area), assuming PAMAM deposition as individual molecules. For G4OH, the observed feature density in Figure 1a is only 14% of the theoretical value (Table 2). Placeto-place variability cannot account for the difference between the observed and predicted feature density. Thus, the features in Figure 1a must be surface aggregates averaging about seven G4OH molecules each. Nevertheless, the remarkable uniformity of feature heights in the G4OH images suggests that the aggregates are two(40) Tomalia, D. A.; Naylor, A. M.; Goddard, W. A., III. Angew. Chem. Int. Ed. Engl. 1990, 29, 138-175.

dimensional on the surface, consisting of close-packed, perhaps interpenetrating G4OH molecules, instead of three-dimensional stacks. This interpretation is supported by a previous study27 which presents AFM images of individual G9NH2 molecules arranged in ordered and disordered surface aggregates on mica. The “flatness” of the G4OH features can be attributed to the considerable flexibility of the G4OH molecule, molecular collapse on the surface due to water removal, and sufficient G4OH-mica attraction. Sample preparation and AFM imaging issues may explain why such features have not been observed before, despite previous efforts.24 Previous AFM measurements published by others did not attempt to control the humidity of the sample environment. We have observed surface diffusion of G4OH on mica under ambient humidity conditions over the course of several days. Freshly prepared specimens of G4OH on mica, imaged within a few hours of preparation, yield images such as Figure 1a. Over the course of a few days, the images evolve into ones that look like Figure 2 of ref 24. We are currently working to quantify these observations. Consistent trends are observed for the larger G6OH molecules on mica. The median feature height is greater than that of G4OH, as expected. The height distribution for G6OH is broader, perhaps due to a distribution of inherent molecular sizes and/or variation of surface packing and thus the degree of deformation. The observed G6OH feature density is 23% of the predicted value, so the features in Figure 1b are also surface aggregates averaging about four G6OH molecules. This suggests that,

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compared to G4OH, the G6OH molecules are more dispersed on mica, perhaps due to stronger G6OH-mica adhesion. Still, considering the narrow distribution of feature heights, the dominant features in Figure 1b are probably two-dimensional surface aggregates having heights corresponding to individual G6OH molecules. This conclusion is in accord with previous observations of twodimensional aggregates of G9NH2 PAMAM dendrimers on mica surfaces.27-29 PAMAM-Pt Nanoparticles. Varying Complexation Time. The interaction between platinum precursor (PtCl42-) and PAMAM functional groups is a slow ligandexchange reaction1-3,8 involving successive substitution of interior amine and amide groups for chloride anions.39 The elapsed time for PAMAM-PtCl42- complexation is therefore a relevant parameter. Sufficient complexation time is a prerequisite for effective templating. To investigate this hypothesis, we prepared two G4OHPt20 samples: one allowing 10 days for G4OH-PtCl42complexation, and one allowing only 10 min for complexation. All other synthesis conditions were identical. Figure 2 shows AFM topography images and feature height distributions for these samples. A typical image for the “10 min” sample (Figure 2a) shows many features with heights greater than 2 nm (the bright ones in the image), and even more short, uniform features in the background (appearing dimly in the image). We observed no gross precipitation in this solution. Figure 2c quantifies these observations: the feature height distribution has a narrow peak centered at about 0.3 nm and a broad, flat tail extending beyond 3.0 nm. The feature density (10%) appears to be slightly lower than that observed for empty G4OH (13%), although the statistical significance of this difference is not established. These observations are consistent with a superposition of distributions for empty G4OH and PAMAM-stabilized Pt colloids formed by arrested precipitation. Excluding outliers41 from the feature statistics (Table 1), the dominant peak centered at 0.3 nm corresponds to that observed for empty G4OH dendrimer (Figure 1a). Due to the short equilibration time, almost none of the PtCl42would have been complexed with the G4OH. Thus, the broad tail of taller features in the image probably corresponds to polydisperse, G4OH-stabilized Pt colloids formed by arrested precipitation of uncomplexed PtCl42-. In contrast, the AFM image for the “10 day” G4OHPt20 sample (Figure 2b) shows features with a much narrower distribution of feature heights (Figure 2, bottom). The distribution is normal (i.e., Gaussian) with equal mean and median feature heights (0.56 nm, after excluding outliers). The corresponding coefficient of variation (24%) reflects a narrow distribution that, although not monodisperse (CV < 10%), is comparable to that of empty G4OH (19%). Compared to the median feature height for empty G4OH (0.26 nm), the greater median height for G4OHPt20 can be attributed to the presence of Pt nanoparticles stabilized by G4OH. The observed feature density, 13% of the predicted value (Table 2), is low but consistent with that of empty G4OH. These observations lead us to conclude that the features seen in Figure 2b are two-dimensional “rafts” of G4OHPt20 nanoparticles. The narrow, normal distribution and lack of significant tail at taller heights suggest that the median feature height in Figure 2b is that of individual G4OH-Pt20 nanoparticles, rather than random stacks of (41) Montgomery, D. C.; Runger, G. C. Applied Statistics and Probability for Engineers; John Wiley and Sons: New York, 1994; Chapter 1.

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Figure 2. AFM topography images and corresponding feature height distributions for G4OH-Pt20 on mica, allowing (a) 10 min and (b) 10 days for G4OH-PtCl42- complexation. The inset shows the tail of the distribution on an expanded ordinate scale.

nanoparticles or aggregates. The low feature density implies surface aggregates rather than individual G4OHPt20 nanoparticles, similar to what is seen in the images for empty G4OH and G6OH. If this interpretation is correct, these results provide guidance for interpreting other images of PAMAMstabilized Pt nanoparticles and feature height statistics derived from them. A single, relatively narrow peak with

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a normal distribution quantifies the dominant feature height, which we presume characterizes height of PAMAMstabilized Pt nanoparticles. A broad tail of taller features suggests a significant level of arrested precipitation or other forms of aggregation in solution. Viewing these taller features as “defects” in the putative templating of Pt nanoparticles, we shall use the median feature height as our primary measure of Pt nanoparticle size. Varying Metal:Dendrimer Ratio. Next, we examine feature height statistics as a function of the metal-todendrimer (Pt:D) ratio and PAMAM generation. Following the standard procedures outlined earlier, we prepared and characterized PAMAM-stabilized Pt nanoparticles with Pt:D ) 20, 40, and 60 using G4OH, and Pt:D ) 100 and 150 using G6OH. We also attempted to synthesize G6OH-Pt200. No visible precipitation was seen in any of the samples except G6OH-Pt200. In the latter case, we observed a white precipitate at the complexation stage, consistent with precursor-induced PAMAM precipitation, rather than brown precipitate indicative of colloidal Pt. PAMAM precipitation due to excessive precursor loading has been observed previously16 for aqueous solutions of HAuCl4 and amine-terminated PAMAM, but no similar observations have been reported for K2PtCl4 and hydroxylterminated PAMAM. No further work was done on the G6OH-Pt200 sample. Typical AFM topography images for all of the samples are shown in Figure 2b (G4OH-Pt20) and in the Supporting Information (Figure S6). Figures 3 and 4 show feature height distributions, and Tables 1 and 2 provide complete statistical analysis. The most obvious trend in Figure 3 is the shift of the mean feature height to larger values with increasing Pt:D ratio. The mean heights are all significantly greater than those of the corresponding “empty” dendrimers. In general, the peaks are described adequately by normal distributions. None of the distributions have broad, flat tails indicative of arrested precipitation of colloidal Pt. These observations are all consistent with PAMAM-mediated templating as the dominant synthesis mechanism for these Pt nanoparticles. The feature densities (Table 2) are all less than theoretical values based on deposition of individual PAMAM-Pt nanoparticles. Thus, like “empty” PAMAM on mica, PAMAM-stabilized Pt nanoparticles must be associated on the surface to form two-dimensional aggregates, or “rafts”. The breadths of the distributions (see StDev or CV, Table 1) increase with Pt:D for a given PAMAM generation. Additional insight can be gained by looking more closely at the results for each sample. As discussed earlier, G4OH-Pt20 has a narrow feature height distribution with CV (Table 1) and feature density (Table 2) comparable to “empty” G4OH. The distribution shows very few features taller than 2 nm (3 out of 555). In this case, templating by G4OH is predominant, with little evidence of larger colloidal Pt particles formed by arrested precipitation. At first glance, the distribution for G4OH-Pt40 appears to be similar to that of G4OH-Pt20. G4OH-Pt40 has a dominant primary peak with normal distribution, and a feature density (Table 2) consistent with that seen for empty G4OH. This again suggests templating as the main synthesis mechanism. However, breadth of the distribution (StDev or CV, Table 1) exceeds that of empty G4OH. The distribution shows increasing numbers of taller features: every bin between 1.5 and 3.0 nm has some features, and almost 2% of the total features are taller than 2.5 nm. The incidence of the tallest features implies that (1) some templated nanoparticles may have aggregated in solution, or (2) a few Pt nanoparticles are

Gu et al.

Figure 3. Feature height distributions for PAMAM-Pt solutions deposited on mica. From top to bottom: G4OH-Pt20, G4OH-Pt40, G4OH-Pt60, G6OH-Pt100, and G6OH-Pt150. The normal distributions (solid curves) are based on the outliercorrected means and standard deviations given in Table 1.

formed via arrested precipitation. Our previous NMR results39 indicate that, even after 10 days of equilibration, about 15% of the PtCl42- precursor (for Pt:D ) 40) does not undergo ligand exchange to bind with G4OH. Arrested precipitation of nonbound PtCl42- could account for the tallest features (>2.5 nm).

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Figure 4. Feature height distributions for G4OH-Pt40 (top) and G4OH-Pt60 (bottom) solutions deposited on mica. The normal distributions (solid curves) are based on the fitting procedure described in the text.

The G4OH-Pt40 features with heights between 1.5 and 2.5 nm seem to be grouped into a secondary peak (Figure 4, top). Distributions derived from individual images (not shown) all suggest two populations separated by a natural break at about 1.5 nm. Using this break, we divide the combined feature height data into two sub-populations (1.5 nm) and exclude large aggregates (>2.5 nm). We fit the subpopulations with normal distributions, locating the peaks at the calculated subpopulation means. The subpopulation standard deviations have been adjusted upward in order to minimize the root-mean-square error between the experimental and theoretical distributions (effectively compensating for the error in splitting the population in two at 1.5 nm). Both of the G4OH-Pt40 subpopulations (Figure 4a) do appear to be normally distributed. Based on this procedure, about 91% of the G4OH-Pt40 features are in the primary peak in Figure 4a (mean height 0.80 nm), about 8% are in the secondary peak (mean 1.89 nm), and about 1% are large aggregates. Likewise, the distribution for G4OH-Pt60 (Figure 4, bottom) also shows two distinct peaks, both normally distributed, and some larger aggregates. Distributions from individual images suggest dividing the population at about 1.6 nm, excluding features taller than 2.6 nm, and fitting the two peaks with normal distributions. The primary peak (mean height 1.05 nm) contains about 72% of all features and likely corresponds to templated Pt nanoparticles. The tallest features (>2.6 nm) include almost 3% of the total and probably represent aggregates or particles formed by arrested precipitation. The secondary peak (mean height 1.98 nm) contains the remaining 25% of the features. This sample also has a remarkably low feature densitysonly 4.2% of the theoretical value based on deposition of individual templated nanoparticles (Table 2). If the primary peaks represent intradendrimer-templated Pt nanoparticles, what are the features in the secondary peaks? The secondary features could be randomly deposited “stacks” consisting of two or three layers

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of templated nanoparticles. Alternately, a limited amount of postsynthesis aggregation in solution could account for the features in the secondary peaks. PAMAM dendrimers have a significant incidence of defects,42 such as missing arms, that would render them less effective as stabilizers. Under either of these scenarios, we would expect to see evidence for stacks or aggregates in the images for other samples. However, the feature height distributions for the Pt20, Pt100, and Pt150 samples show no evidence of secondary peaks. These “secondary” particles are probably produced through a form of arrested precipitation. Because the distributions appear to be normal and relatively narrow, rather than broad and flat like that of “10 min” G4OHPt20 (Figure 2a), another mechanism must play a role. We hypothesize that sharing of weakly complexed Pt precursor among several PAMAM molecules could be responsible for the features in the secondary peaks. As mentioned earlier, as much as 15% of the Pt precursor (for Pt:D ) 40) does not bind to G4OH via the ligand-exchange reaction, even after 10 days of equilibration.39 Although not chemically bound, the remaining PtCl42- precursor may still complex with G4OH via weak electrostatic interactions. Moreover, previous dynamic light scattering measurements6 show that amine-terminated PAMAM associates into large aggregates in solution (hydrodynamic radii exceeding 300 nm). We expect similar association of hydroxyl-terminated PAMAM in water via hydrogen bonding. The association must be weak and reversible since aqueous PAMAM solutions are stable and clear. These large PAMAM aggregates would carry a significant number of weakly associated, shared PtCl42- anions. Upon reduction, these Pt atoms could be “pooled” to nucleate and grow Pt nanoparticles stabilized by the surrounding PAMAM aggregate. One might call this mechanism “secondary templating”, since a secondary PAMAM structure (the weak solution aggregate) serves as the template. Two observations support this hypothesis. First, we expect the fraction of weakly complexed PtCl42- to increase with Pt:D ratio, up to the nominal dendrimer capacity (62 for G4OH, 254 for G6OH). For G4OH-Pt20, G6OH-Pt100, and G6OH-Pt150, the Pt:D ratio is less than 60% of the nominal capacity. We see no evidence for secondary templating in these samples because virtually all of the Pt is chemically bound to the PAMAM by ligand exchange. For G4OH-Pt40 and G4OH-Pt60, the Pt:D ratios are 65% and 97% of the nominal capacity of G4OH. These samples would have significant fractions of weakly complexed PtCl42- available for secondary templating. The larger secondary peak for G4OH-Pt60 (Figure 4b), compared to G4OH-Pt40 (Figure 4a), would be consistent with increasing amounts of weakly complexed PtCl42-. Second, the secondary peaks for G4OH-Pt40 and G4OH-Pt60 (Figures 4a and b) are centered at about the same feature height (about 1.9 nm), consistent with the expectation that weak G4OH aggregates would have the same structure in these two precursor solutions. We are currently carrying out additional experiments involving dialysis, AA analysis of Pt content, and AFM size analysis to further test this hypothesis. Nanoparticle Shape. HRTEM Results. Figure 5 shows a typical HRTEM image for G4OH-Pt40 and the corresponding particle diameter distribution. The mean and median particle diameters from HRTEM are 1.4 ( 0.1 and 1.4 nm with a standard deviation of 0.3 nm (22% COV) based on N ) 142 particles. These values agree (42) Zhou, L.; Russell, D. H.; Zhao, M.; Crooks, R. M. Macromolecules 2001, 34, 3567-3573.

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Figure 6. log-log plot of the mean PAMAM-Pt feature height as a function of nominal Pt:D ratio. Data points are based on overall mean heights (open squares) and the mean heights of the dominant peak (solid circles); the solid line is the linear regression of the latter. The dashed and dotted lines are the theoretical feature heights of spheres and hemispheres containing the corresponding number of Pt atoms in random closepacked arrangements.

Figure 5. HRTEM image of G4OH-Pt40 and the corresponding distributions of HRTEM diameter and AFM feature height. Note that the “binning” of the AFM data in this figure differs compared to that in Figure 4.

precisely with those measured previously1,17,18 for G4OHPt40 (mean ) 1.4 nm, standard deviation ) 0.2 nm). Comparing the AFM and HRTEM particle size distributions, we see that the mean diameter (HRTEM) is significantly greater than the mean feature height (AFM). Thus, the data from AFM and HRTEM images provide direct evidence indicating that the Pt nanoparticles on the surface are not spherical. Are these Pt nanoparticles hemispheres? A hemisphere encompassing 40 Pt atoms in an fcc lattice has a radius of 0.66 nm and a diameter of 1.32 nm. The mean AFM height (0.83 nm, Table 1) is somewhat larger than the hemisphere radius, perhaps due to the contribution of the PAMAM dendrimer to the feature height as measured by AFM. The mean diameter from HRTEM (1.4 nm) is slightly greater than the predicted hemisphere diameter. Nonetheless, the hemispherical shape is plausible. However, close examination of the HRTEM images in Figure 5 and Figures S7-S9 (Supporting Information) shows that only a few particle images manifest lattice fringes consistent with crystalline order of the constituent atoms. Most of the particles show no lattice fringes and, in fact, have a speckled appearance suggesting a disordered arrangement of Pt atoms. A hemisphere encompassing 40 Pt atoms in a random-close packed (rcp) arrangement has a radius of 0.7 nm and a diameter of 1.40 nm, both consistent with the corresponding dimensions measured by AFM and HRTEM. Thus we believe that the HRTEM images provide evidence for noncompact, disordered arrangement of Pt atoms within the dendrimer, with the overall G4OH-Pt40 composite particle having a quasi-hemispherical shape.

AFM Results. The AFM distributions in Figure 3 clearly show that the mean feature height increases with the Pt:D ratio. Analysis of this trend may tell us something about the shape of the PAMAM-Pt nanoparticles on the mica surface and the internal arrangement of Pt atoms. Figure 6 shows the mean feature height plotted as a function of the nominal Pt:D ratio (open squares). If we only consider features in the primary peaks for G4OHPt40 and G4OH-Pt60 (filled circles), the data lie on a line (slope 0.63), indicating a power-law dependence of mean feature height on Pt:D. However, the power-law exponent (0.63) is not consistent with that expected for a hemisphere or a sphere (0.33). Although the mean height of G4OHPt20 features is consistent with that of a hemisphere containing 20 Pt atoms (rcp), the height of G6OH-Pt100 is comparable to that of a 100 atom sphere. The mean height of G6OH-Pt150 is significantly greater than that expected for a 150 atom Pt sphere. These comparisons assume solid Pt nanoparticles with rcp order but do not account for any contribution of the PAMAM to the measured height. To a first approximation, adding a constant PAMAM contribution to the height estimate shifts the dotted and dashed lines upward in Figure 6 but does not rationalize the slope. Another possibility is that the shape of the Pt nanoparticles varies with Pt:D due to the interaction with the mica. However, we do not see evidence of any curvature in the data trend in Figure 6, nor can this explanation account for the shape and size of G6OH-Pt150. Finally, it is possible that the actual Pt:D ratio differs from the nominal value. Lessthan-complete complexation would make the actual Pt:D ratio less than the nominal value and shift the experimental data points to the left in Figure 6. This makes it even more difficult to rationalize Pt nanoparticle shape (especially Pt100 and Pt150) in terms of solid spherical sections. Actual Pt:D values greater than the nominal values are difficult to rationalize physically and would not be consistent with the concept of intradendrimer templating following the fixed loading law. We believe that the AFM results summarized in Figure 6 support the hypothesis that these nanoparticles are noncompact, random aggregates of Pt atoms interspersed within the dendrimer. Transposing the x and y axes in Figure 6 indicates how the nominal mass of Pt in the nanoparticle (proportional to the nominal Pt:D ratio) varies with a characteristic nanoparticle length scale. The slope, 1.6 ()1/0.63), represents a mass/length (fractal)

Dendrimer-Stabilized Platinum Nanoparticles

scaling consistent with a noncompact, ramified aggregate of Pt atoms. This kind of scaling explains why the G6OHPt150 nanoparticles can have mean heights greater than what we would expect for a solid 150 atom Pt sphere. Ye et al. suggested13 that Pt and Pd DNs have irregular shapes with non-close-packed geometries in order to rationalize larger-than-expected Pt nanoparticle size from HRTEM images. This concept is supported by the HRTEM images of Gro¨hn et al.37 which show multiple small Pt nanoparticles within each dendrimer after reduction of H2PtCl6 in the presence of G9 and G10 PAMAM (similar to their earlier results for Au DNs16). Our own HRTEM images, as discussed earlier, are also dominated by particles lacking lattice fringes. None of the previously published HRTEM images of Pt DNs show any evidence of lattice fringes characteristic of crystalline order. In view of this information, as well as our AFM results, we believe that the Pt-PAMAM nanoparticles in this work are ramified, noncompact aggregates of Pt atoms interspersed within the PAMAM dendrimer frameworksa true nanocomposite. This explanation also resolves the apparent overestimate of DN size by HRTEM in the many previous studies of Pt and Pd DNs1,8-10,13,17-20 as discussed in the Introduction. We are currently conducting a variety of complementary spectroscopy-based experiments to further test this hypothesis. Summary and Conclusions In this work, we have shown that AFM measurements provide useful information on the size and shape of PAMAM-stabilized platinum nanoparticles. This information helps us rationalize and control the mechanisms of PAMAM-mediated templating of metal nanoparticles in solution. Although we were not able to image individual PAMAM dendrimer molecules using tapping mode AFM, our images are in accord with previous efforts in this area. The feature height distributions and surface density of features are consistent with PAMAM molecules residing on mica as two-dimensional surface aggregates. The mean height values show that PAMAM molecules assume flat surface configurations, as is well known. This work also shows that AFM can distinguish between “empty” PAMAM molecules and PAMAM-Pt nanoparticles. In all cases, the AFM mean heights for PAMAMPt samples deposited on mica are greater than the corresponding values for “empty” PAMAM. When we reduced the Pt precursor in the presence of G4OH without allowing sufficient time for PAMAM-Pt complexation, the AFM images and feature height distributions show two populations: one has narrow peak with a mean feature height consistent with empty G4OH, and the other has a broad distribution suggesting colloidal Pt nanoparticles formed by arrested precipitation. In contrast, allowing sufficient time for the PAMAMPt complexation via ligand exchange results in qualitatively different AFM images and feature height distribution. The narrow distribution of feature heights and the absence of significant large features suggest successful PAMAM-mediated templating of Pt nanoparticles. Thus, the AFM-based size measurement can distinguish between mechanisms, namely arrested precipitation and dendrimer-mediated templating. In general, AFM feature height and surface density data suggest that these PAMAM-Pt nanoparticles reside on the mica surface as two-dimensional surface aggregates. The feature heights have normal distributions and are relatively narrow. The mean feature height increases with Pt loading (nominal Pt:D ratio) in a systematic way,

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obeying a power-law relationship. These observations are consistent with intradendrimer templating. However, we cannot draw a firm conclusion as to whether the synthesis mechanism obeys the fixed loading law because we do not have definitive information about the shape of the PAMAM-Pt nanoparticles on mica or carbon surfaces. The feature height distributions are monomodal for Pt loading (Pt:D) below about 66% of the theoretical capacity of PAMAM1,2 but show evidence of a secondary particle population for Pt loading above 66% of capacity. The secondary population also has a normal distribution, implying that these particles are formed through a templating process rather than simple arrested precipitation. We speculate that these particles may be formed through an interdendrimer templating process involving PAMAM aggregates in aqueous solution. A comparison of AFM height data with HRTEM diameter data for G4OH-Pt40 suggests that Pt DNs assume nonspherical shapes when deposited onto carbon and mica surfaces. The likelihood of significant deformation of nanoparticle shape due to particle-surface van der Waals attraction has been recognized for many years.43-47 If the difference between mean TEM diameter and mean AFM height is indeed due to the flattened, nonspherical shape of DNs on carbon and mica surfaces, this would explain the apparent overestimate of nanoparticle size by HRTEM in the many previous studies of Pt and Pd DNs.1,8-10,13,17-20 Full three-dimensional characterization by HRTEM and AFM is needed to establish the shape of surface-supported DNs. The variation of AFM mean height on Pt loading (Pt:D) cannot be rationalized by modeling the nanoparticle as a close-packed cluster of Pt atoms with fixed shape. Although we expect the shape to change with Pt loading and hence nanoparticle size, the largest Pt DNs have mean heights significantly greater than equivalent spheres with the same nominal number of Pt atoms. This observation, the power-law dependence AFM mean height on Pt loading, and our HRTEM images lead us to the conclusion that these Pt DNs are ramified, noncompact aggregates of Pt atoms interspersed within the dendrimer framework. A definitive answer awaits further HRTEM imaging, spectroscopic experiments, molecular modeling of Pt clusters on surfaces, and consideration of existing analytical models of surface-induced nanoparticle deformation, all of which are currently in progress in our group. Acknowledgment. The authors wish to acknowledge the National Science Foundation (Award No. CTS0103135) for financial support of this work. We also wish to thank Dr. Larry Allard and staff at Oak Ridge National Laboratory for access to HRTEM and instruction on its operation. Supporting Information Available: Additional discussion and images pertaining to (1) AFM scanner calibration, (2) accuracy and reproducibility of feature height measurements, (3) effects of humidity, (4) images of PAMAM films deposited from higher concentration solutions, (5) typical AFM images of Pt DNs not shown in the main text, and (6) additional HRTEM images. This material is available free of charge via the Internet at http://pubs.acs.org.

LA047843E (43) Maugis, D.; Pollock, H. M. Acta Metall. 1984, 32, 1323-1334. (44) Israelachvili, J. Intermolecular and Surface Forces, 2nd ed.; Academic Press: London, 1992; Chapter 15. (45) Mahoney, W.; Schaefer, D. M.; Patil, A.; Andres, R. P.; Reifenberger, R. Surf. Sci. 1994, 316, 383-390. (46) Rimai, D. S.; Quesnel, D. J.; Busnaina, A. A. Colloids Surf. A 2000, 165, 3-10. (47) Kogut, L.; Etsion, I. J. Colloid Interface Sci. 2003, 261, 372378.