In the co-complexation synthesis, both metal precursors are introduced into the PAMAM dendrimer solution simultaneously. After the completion step is completed, the solution is typically treated with a reducing agent to form reduced metal nanoparticles (MA0MB0). Co-complexation has been used to prepare a variety of bimetallic DENs (Figure 3.2), including PdPt 164-166, PdRh 166,167, PdAu 168-170, PtAu 169, AuAg 171,172, PdCu 173,174, FePt 175, NiSn 176, AuNi 177,178, PtRu 180, AgCu 180. In preparations of G4NH2-(PdPt) and G4NH2-(PdRh) DENs, the primary amine groups of
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Figure 3.1CO oxidation catalysis by silica supported Pt32, Au32, Pt16Au16, and Pt32+Au32 NPs. Rate is reported as moles CO converted per total moles Pt per minute; Au32, the rate is in moles CO converted per total mole Au per minute.163
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dendrimer were protonated by adjusting the solution pH to 3 prior the addition of the metal precursors.167,181,182 When the complexation step was completed (76 and 24 h for G4NH2-(PdPt) and G4NH2-(PdRh), respectively), solutions were treated with NaBH4. PdPt DENs thus formed were found to be nearly uniform with spherical shape of metal particles having an average diameter of 2.5 nm, which is smaller than the diameter of the G4-OH dendrimer (4.5 nm).166 Furthermore, Pt-Ru/SiO2 catalysts prepared by the co- complexation method was found to be much more active for the liquid-phase hydrogenation of 3,4-epoxy-1-butene (EpB) than a conventional catalyst with a similar composition.179
The UV-vis spectroscopy has been used in preparations of bimetallic systems to monitor the complexation process. For example, the characteristic SPR bands at 214 and 208 nm for monometallic PtCl42- and PdCl42-, respectively, disappeared after the addition of G4OH and a new band emerged at 230 nm, indicating that both metal cations are complexed with the internal functional groups of the dendrimer.165,181 After reduction of the complexed bimetallic solution with NaBH4, the band at 230 nm completely disappeared and a new broad absorption band appears over a wide range of wavelength regardless of the Pt/Pd ratio used.181 The spectra of the resulting nanoparticles were found to be different not only from those of the monometallic Pt or Pd nanoparticles but also from those characterizing their physical mixtures, which can be attributed to the change in the dielectric function caused by mixing atoms of two different metals.183 Furthermore, the energy dispersive spectroscopy (EDS) analysis of two individual nanoparticles indicated that the atom %’s of Pd and Pt were in a good agreement with the mol %’s of PdCl42- and PtCl42- used in the original synthesis mixture. The single-particle EDS measurements suggested that bimetallic nanoparticles (rather than physical mixtures
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of monometallic nanoparticles) are present within the dendrimer.164 Finally, Crooks et al. proved the formation of bimetallic G4NH2(Pd27.5Au27.5) nanoparticles using EDS as well, showing an average composition of 48 ± 3% Pd and 52 ± 3% Au, which is in agreement with the molar percentages of PdCl42- and AuCl4- used in the original mixture (50% each).8
Unfortunately, UV-vis spectroscopy cannot always provide accurate information concerning the formation and structure of bimetallic nanoparticles. Alloying or the formation of core-shell type particles is expected to cause a shift of the surface plasmon resonance band. In the case of Au-Pt/G5.5COOH prepared by co-complexation, the shift for the gold plasmon band was not observed, although XPS data indicated that bimetallic particles are formed.169 Specifically, small shoulders due to oxidized gold (84.5 and 89 eV) were observed and these shoulders declined in intensity after the addition of platinum while bands corresponding to the oxidized Pt species appeared in spectra. This observation showed that Au atoms withdrew some electrons from platinum atoms, suggesting that gold and platinum are not separated but coexist in one metal particle. However the structure of bimetallic particles such as core-shell, random uniform alloy, reverse core-shell, and mosaic structure, etc., cannot be determined from these results.169
When metal precursors are introduced simultaneously to the dendrimer, they compete with each other for binding sites. It is feasible that the most reactive complex will occupy the majority of available sites. It is possible to control this process by adding the less reactive complex first. This method was employed for preparations of PtCu DENs. In this case, PtCl4- was allowed to interact with G5OH for 2 days before Cu(NO3)2 was added since Cu2+ binds to this dendrimer in a matter of minutes. The pH of the Pt2+ solution was adjusted to 7 before the addition of the second metal.184 The
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same procedure was followed for the preparation of PdCu DENs.174 The DEN precursor solutions exhibit strong absorptions bands at ~230 and 290 nm and an isosbestic point at 261 nm. These peaks arise from ligand-to-metal charge-transfer (LMCT) bands associated with the dendrimer/metal-ion complexes.174 The reduction of the metal- cation/dendrimer precursor complex with BH4- yields DENs with diameters of metal particles in the range of 1.2 – 1.3 nm. The coordination environments of these two metals were measured by EXAFS and the formation of an alloy structure was confirmed.174