The approach for preparing dendrimer-encapsulated Pt metal particles is similar to that used for preparation of the Cu composites: chemical reduction of an aqueous solution of G4-OH(Pt2)n yields dendrimer-encapsulated Pt nanoparticles [G4-OH(Ptn)]. A spectrum of G4-OH(Pt60) is shown in Fig. 6a; it displays a much higher absorbance than G4-OH(Pt2)60 throughout the wavelength range displayed. This change results from the interband transition of the encapsulated zero-valent Pt metal particles. Spectra of G4-OH(Pt)n, n 12, 40, 60, obtained between 280 and 700 nm and normalized to A 1 at 450 nm, are shown in Fig. 6b; all of these spectra display the interband transition of Pt nanoparticles. Control experiments clearly demonstrate that the Pt clusters are sequestered within the G4-OH dendrimer. For example, BH4 reduction of G4-NH2(Pt2)n, which exist as cross-linked emulsions, results in immediate precipitation of large Pt clusters. In contrast, Gn-OHencapsulated particles do not agglomerate for up to 150 days, and they redissolve in solvent after repeated solvation/drying cycles. The absorbance intensity of the encapsulated Pt nanoparticles is related to the particle size. A plot of log A versus log provides qualitative information about particle size: the negative slopes are known to decrease with increasing particle size. For aqueous solutions of G4-OH(Pt12), G4-OH(Pt 40), and G4-OH(Pt 60), the slopes are 2.7, 2.2, and 1.9, respectively (Fig. 6b, inset). These results confirm that the size of the intradendrimer particles increases with increasing Pt2 loading. High-resolution transmission electron microscopy (HRTEM) images (Fig. 7) clearly show that dendrimer-encapsulated particles are nearly monodisperse and that their shape is roughly spherical. For G4-OH(Pt 40) and G4-OH(Pt 60) particles, the metal-particle diameters are 1.4 0.2 and 1.6 0.2 nm, which are slightly larger than the theoretical values of 1.1 and 1.2 nm, respectively, calculated by assuming that particles are contained within the smallest sphere circumscribing a fcc Pt crystal. When prepared in aqueous solution, Pt nanoparticles usually have irregular shapes and a large size distribution. The observation of very small, predominantly spherical particles in this study is a consequence of the dendrimer cavity, that is, the template in which they are prepared. Note that when the dendrimers are loaded with metal ions at maximum capacity (middle frame of Fig. 7), the resulting nanoparticles are more monodisperse than when a lower Pt2+ loading is used (top frame). This is an expected statistical consequence of the substoichiometric loading. X-ray energy dispersive spectroscopy (EDS) and X-ray photoelectron spectroscopy (XPS) analyses were also carried out, and they unambiguously identify the particle composition as zero-valent Pt (77,90). Interestingly, the XPS data also indicate the presence of Cl prior to reduction, but it is not detectable after reduction. This finding is consistent with the change in valency from the chloridecontaining G4-OH(Pt2)n complex to the zero-valent metal. Results similar to those discussed for dendrimer-encapsulated Cu and Pt also obtain for Pd, Ru, and Ni nanoclusters. An example of 40-atom Pd nanoclusters confined within G4-OH is shown in the bottom micrograph of Fig. 7.
Chemistry of metal nanoparticles