Inverted core/shell

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

In summary the simultaneous reduction method usually provides alloyed bimetallic nanoparticles or mixtures of two kinds of monometallic nanoparticles. The bimetallic nanoparticles with core/shell structure also form in the simultaneous reduction when the reduction is carried out under mild conditions. In these cases, however, there is difference in redox potentials between the two kinds of metals. Usually the metal with higher redox potential is first reduced to form core part of the bimetallic nanoparticles, and then the metal with lower redox potential is reduced to form shell part on the core, as shown in Figure 2. The coordination ability may play a role in some extent to form a core/shell structure. Therefore, the simultaneous reduction method cannot provide bimetallic nanoparticles with so-called inverted core/ shell structure in which the metal of the core has lower redox potential. 

Reduction of two different precious metal ions by refluxing in ethanol/water in the presence of PVP gave a colloidal dispersion of core/shell structured bimetallic nanoparticles. In the case of Pd and Au ions, e.g., the colloidal dispersions of bimetallic nanoparticles with a Au core/Pd shell structure are produced. In contrast, it is difficult to prepare bimetallic nanoparticles with the inverted core/shell (in this case, Pd-core/Au-shell) structure. The sacrificial hydrogen strategy was used to construct the inverted core/shell structure, where the colloidal dispersions of Pd-cores are treated with hydrogen and then the solution of the second element, Au ions, is slowly added to the dispersions. This novel method, developed by us, gave the inverted core/shell structured bimetallic nanoparticles. The Pd-core/Au-shell structure was confirmed by FT-IR spectra of adsorbed CO [144]. 

In summary, we concluded that the successive reduction method easily provides the bimetallic nanoparticles with the core/shell structure according to versatile design. For example, different reducing agents may be used for the first reduction and the second one, respectively, depending on the property of the metal. In some cases of two kinds of metals with much different redox potentials, however, inverted core/shell nanoparticles are difficult to form even in the successive reduction. The inverted core/shell structure can be realized by an... 

Now let us consider the core/shell and inverted core/ shell structures. The core/shell structure and the inverted... 

Fig. 9.1.5 Preparation process of Pd-core/Pt-shell (inverted core/shell) structured bimetallic clusters by a sacrificial hydrogen reduction. (From Ref. 69.)...

Infrared Spectroscopy. Infrared (1R) spectroscopy is also used for understanding the structure of the bimetallic nanoparticles. Carbon monoxide can be adsorbed on the surface of metals, and the 1R spectra of the adsorbed CO depend on the kind of metal. These properties are used for analyzing the surface structure of metal nanoparticles. The inverted core/shell structure, constructed by sacrificial hydrogen reduction, was probed by this technique (44). 

For composite particles in water, the three extreme morphologies may be considered as core-shell, with the more hydrophilic component interfacing with water, inverted core-shell, with the more hydrophobic component interfacing with water, and separated particles. 

In the case of PMMA/PS composite particles, where the equilibrium morphology was an inverted core-shell structure, employing a chain transfer agent (CQaBr), even under conditions favoring a core-shell structure (Le. a very low rate of monomer addition), resulted in the formation of particles where the core contained a number of PS domains distributed throughout the entire FMMA [ ase [57]. This indicated that shorter PS chains have higher diffusivities in the highly viscous core particles. 

Practical methods can be used to ensure that the required particle morphology is obtained. A high Tg or molar mass for the hrst-formed polymer forces the second-formed polymer to reside on its surface due to kinetic effects. The use of an inherently more water-soluble second polymer as compared to the first will also contribute to obtaining the desired morphology. Inverted core-shell latexes are most readily obtained when a more hydrophobic monomer for the second-formed polymer is polymerized in the presence of a less hydrophobic polymer latex. An alternative is to render the second monomer coUmdaUy unstable ufien polymerized, forcing it to reside in the core of the first formed polymer. 

The above computations for the fiee energy changes for composite latex particles have shown that only core-shell, inverted core-shell and hemispherical particles are stable in a thermodynamic sense. All of the other reported morphologies (e.g. sandwich-like , raspberry or confetti-like particles or occluded domains) are non-equilibrium, kinetically controlled structures, prepared... 

Intentional structure in latexes is traditionally obtained by polymerizing a second monomer over a first-formed latex polymer to form a core-shell latex (see Chapter 9). A more recent development allows the production of a core-shell latex using an inversion process. In this so-called inverted core-shell process the second monomer is polymerized in the presence of a first-formed latex but forms the core of the core-shell latex particle. In both the core-shell and inverted core-shell procedures the final morphology is governed by the change in 6ee... 

As mentioned before, hybrid latex particles are usually prepared by seeded emulsion polymerization. In the first stage, well-defined inorganic or organic particles are prepared, while in the second stage a monomer is polymerized in the presence of these well-defined particles. Multistage emulsion polymerization produces structures such as core-shell, inverted core-shell structures, and phase-separated structures such as sandwich structures, hemispheres, raspberry-like and void... 

The structure of bimetallic nanoclusters depends on the preparation method and the kind of elements composing the bimetallic nanoclusters. Typical examples of structures are shown in Fig. 3.18. The random alloy indicates a structure in which the atoms of both elements are located in a completely random way, resulting in the formation of a kind of solid solution. The cluster-in-cluster indicates a structure in which the atoms of one element forms clusters and these clusters aggregate to form a larger cluster by including atoms of the other element which may form small clusters as well. In the core/shell structure, atoms of one element form a core and atoms of the other element surround the core to form the shell. The inverted core/shell is similar to the core/shell structure. The difference is the kind of element that forms the core and shell. The element forming a core in the core/shell structure now forms the shell in the inverted core/shell structure and vice versa. Thermodynamically, the core/shell structure is stable, while the inverted core/shell is metastable. 

Recently, our group has invented a modified successive alcohol reduction process to prepare inverted core/shell structured bimetallic nanoclusters. In practice, PVP-capped Pd-core/Pt-shell bimetallic nanoclusters 2 nm in size were obtained. This method is based on the fact that precious metals like Pd have the ability to adsorb hydrogen and split it to form metal hydride on the surface of a precious metal. Hydride on precious metal has a very strong reducing ability, implying a quite low redox potential. As illustrated in Fig. 3.23, the Pt ions added into the dispersion of Pd nanoclusters are reduced by hydride on the surface of preformed Pd nanoclusters without oxidizing Pd atoms to Pd ions, and are deposited on the Pd nanoclusters to form Pd-core/Pt-shell bimetallic nanoclusters. Since the coreduction of Pd and Pt ions produces only Pt-core/Pd-shell bimetallic nanoclusters and this structure is controlled by nature, Pt-core/Pd-shell can be called a normal core/shell structure. The structure of Pd-core/Pt-shell is opposite to the normal structure. Thus, we call this an inverted core/shell structure. 

In the modified successive reduction process to produce the inverted core/shell structure, hydrogen is additionally used for control of the structure. We call this the sacrificial hydrogen reduction method. This method can be very useful in controlling layered bimetallic and trimetallic structures in general. 

Fig. 3.23 Sacrificial hydrogen reduction method to synthesize inverted Core/shell-structured bimetallic nanoclusters by successive reduction.

Second, the reduced free energy change for the formation of inverted core shell particles (a) with the lyophohic liquid 3 and the lyophilic liquid 1 in the shell and the core, respectively, leads to equation 47. Note that the same reference state is assumed as above. 

If the seed latex particles can barely be swollen by the second-stage monomer and a water-soluble initiator is used, then the subsequent seeded emulsion polymerization will be localized near the particle surface layer. Thus, the postformed polymer tends to form a surface layer around the seed latex particle. An example of this kind of morphological structure of latex particles is the seeded emulsion polymerization of methyl methacrylate in the presence of a polyvinylidene chloride seed latex. On the other hand, free radical polymerization can take place inside the seed latex particles. In this manner, various morphological structures of latex particles such as the perfect core/shell, inverted core/shell, dumbbell-shaped, and occluded structures can be achieved, depending on various physical parameters and polymerization conditions. 

Cho and Lee [6] used three different initiators, potassium persulfate, 2,2 -azobisisobutyronitrile, and 4,4 -azobis(4-cyanovaleric acid) (water-soluble, but less hydrophilic than potassium persulfate) to investigate their effects on the emulsion polymerization of styrene in the presence of polymethyl methacrylate seed latex particles. Inverted core/shell latex particles were observed when 2,2 -azobisisobutyronitrile or 4,4 -azobis(4-cyanovaleric acid) was used to initiate free radical polymerization. The use of potassium persulfate resulted in various morphological structures of latex particles, which were largely determined by the initiator concentration and polymerization temperature. 

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