Core-shell clusters

The possible formation of an alloyed or a core-shell cluster depends on the kinetic competition between, on one hand, the irreversible release of the metal ions displaced by the excess ions of the more noble metal after electron transfer and, on the other hand, the radiation-induced reduction of both metal ions, which depends on the dose rate (Table 5). The pulse radiolysis study of a mixed system [66] (Fig. 7) suggested that a very fast and total reduction by the means of a powerful and sudden irradiation delivered for instance by an electron beam (EB) should prevent the intermetal electron transfer and produce alloyed clusters. Indeed, such a decisive effect of the dose rate has been demonstrated [102]. However, the competition imposed by the metal displacement is more or less serious, because, depending on the couple of metals, the process may not occur [53], or, on the contrary, may last only hours, minutes, or even seconds [102]. 

Rossi, G. Rapallo, A. Mottet, C. FortuneUi, A. Baletto, D. Ferrando, R. Magic pol5acosahedral core-shell clusters. Phys. Rev. Lett. 2004, 93, 105503. 

Kim, Y.T., Lee, H., Kim, H.l. Lim, T.H. PtRu nano-dandelions on thiolated carbon nanotubes a new synthetic strategy for supported bimetallic core-shell clusters on the atomic scale. Chem. CommurL 46... 

Yue and Cohen prepared (ZnCd)S2 colloids by multiple loading experiments [164]. The carboxylic acid coordination sites are regenerated and can be re-used to make onion-type binary clusters. This technique allows for the possibility of cluster-size control and the synthesis of core-shell clusters through multiple metal-loading and reduction cycles. Recharging leads to ZnCdS colloids. In a similar fashion, bimetallic Au/Pd nanocolloids have been synthesized as catalysts for hydrogenation of dienes [244]. Eisenberg and coworkers used this method to increase the size of CdS colloids [267]. A one step precipitation approach was used to prepare the bicomponent particles Cd and Se in the presence of polyvinyl alcohol (PVA) [268]. 

The next available full-shell cluster that has been investigated by MoBbauer spectroscopy was the four-shell cluster Pt309phen 3603o+io (phen = 4,7-p-C6H4S03Na substituted 1,10-phenanthroline) [17]. Its inner core consists of 147 atoms. However, since platinum is not MoBbauer-active, the cluster sample had to be irradiated with thermal neutrons to transfer a fraction of the Pt... 

The next smaller ligand-protected nanocluster that was investigated by scanning tunneling spectroscopy (STS) was the four-shell cluster Pt309phen 36O20 [20,21]. The diameter of the Pt core is 1.8 nm, about a tenth of the former example. However, even here a Coulomb blockade could only be observed at 4.2 K, i.e. at room temperature the particle still has metallic behaviour. Since... 

Random Alloy 2) Ctuster-in-Cluster 3) Core/Shell 4) Inverted Core/Shell... 

Our first attempt of a successive reduction method was utilized to PVP-protected Au/Pd bimetallic nanoparticles [125]. An alcohol reduction of Pd ions in the presence of Au nanoparticles did not provide the bimetallic nanoparticles but the mixtures of distinct Au and Pd monometallic nanoparticles, while an alcohol reduction of Au ions in the presence of Pd nanoparticles can provide AuPd bimetallic nanoparticles. Unexpectedly, these bimetallic nanoparticles did not have a core/shell structure, which was obtained from a simultaneous reduction of the corresponding two metal ions. This difference in the structure may be derived from the redox potentials of Pd and Au ions. When Au ions are added in the solution of enough small Pd nanoparticles, some Pd atoms on the particles reduce the Au ions to Au atoms. The oxidized Pd ions are then reduced again by an alcohol to deposit on the particles. This process may form with the particles a cluster-in-cluster structure, and does not produce Pd-core/ Au-shell bimetallic nanoparticles. On the other hand, the formation of PVP-protected Pd-core/Ni-shell bimetallic nanoparticles proceeded by a successive alcohol reduction [126]. 

Figure 8. TEM and optical absorption of the sample implanted with 5 x 10 Au /cm (a) TEM cross-sectional micrograph (dashed lines represent the free surface and film-substrate interface) (b) nanoparticles size distribution (c) simulated optical spectra (1) Au cluster in a non-absorbing medium with n = 1.6 (2) Au cluster in polyimide (absorbing) (3) Au(core)-C(shell) cluster in a nonabsorbing medium with n = 1.6 (4) the experimental spectrum of Au-implanted polyimide sample, (d) X-ray diffraction patterns as a function of the implantation fiuence.

Osterloh, F. Hiramatsu, H. Porter, R. Guo, T., Alkanethiol induced structural rearrange ments in silica gold core shell type nanoparticle clusters An opportunity for chemical sensor engineering, Langmuir. 2004, 20, 5553 5558... 

Laboratory procedures are presented for two divergent approaches to covalent structure controlled dendrimer clusters or more specifically - core-shell tecto(dendrimers). The first method, namely (1) the self assembly/covalent bond formation method produces structure controlled saturated shell products (see Scheme 1). The second route, referred to as (2) direct covalent bond formation method , yields partial filled shell structures, as illustrated in Scheme 2. In each case, relatively monodispersed products are obtained. The first method yields precise shell saturated structures [31, 32] whereas the second method gives semi-controlled partially shell filled products [30, 33],... 

In general, incorporation of hydrophobic groups into PIPAAm chains decreases the LCST [29-31]. Hydrophobic groups alter the hydrophilic/ hydrophobic balance in PIPAAm, promoting a PIPAAm phase transition at the LCST, water clusters around the hydrophobic segments are excluded from the hydrophobicaUy aggregated inner core. The resulting isolated hydrophobic micellar core does not directly interfere with outer shell PIPAAm chain dynamics in aqueous media. The PIPAAm chains of the micellar outer shell therefore remain as mobile linear chains in this core-shell micellar structure. As a result, the thermoresponsive properties of PIPAAm in the outer PIPAAm chains of this structure are unaltered [23-27,32]. 

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