Core-shell materials preparation

Fleming MS, Mandal TK, Walt DR (2001) Nanosphere-microsphere assembly methods for core—shell materials preparation. Chem Mater 13(6) 2210-2216... 

This chapter will focus mainly on core-shell materials prepared by the addition of a Pt monolayer (PtML) to a preformed core, by various approaches (chemical and electrochemical), and consider the uniformity, stability, and activity of materials of... 

For the characterization of Langmuir films, Fulda and coworkers [75-77] used anionic and cationic core-shell particles prepared by emulsifier-free emulsion polymerization. These particles have several advantages over those used in early publications First, the particles do not contain any stabihzer or emulsifier, which is eventually desorbed upon spreading and disturbs the formation of a particle monolayer at the air-water interface. Second, the preparation is a one-step process leading directly to monodisperse particles 0.2-0.5 jim in diameter. Third, the nature of the shell can be easily varied by using different hydrophilic comonomers. In Table 1, the particles and their characteristic properties are hsted. Most of the studies were carried out using anionic particles with polystyrene as core material and polyacrylic acid in the shell. 

A similar strategy was used to prepare BaFe12019-Ti02 core-shell materials, with the core of BaFei20i9 used to magnetically recover the catalyst and the shell ofTi02 that promoted degradation of dyes. The activity was dependent on the shell thick-... 

Presumably these three methods for preparing bimetallic, dendrimer-encap-sulated nanoparticles can be extended to trimetallics,bi- and trimetallics having unique structures (such as core/shell materials), and interesting combinations of two (or more) zero-valent metals plus intradendrimer ions. However, analysis of such materials await more sophisticated analytical methods than are currently at OUT disposal. 

It is possible to create more complex compounds and morphologies, such as alloys or core-shell structures. The nucleation step still occurs with injection of the metal carbonyl. However, when other organometallic precursors are in the solution, you can get the alloy to grow on the nuclei through transmetallation. To prepare core-shell material, the nuclei are allowed to react, then the temperature is reduced and a second precursor is introduced. The initial injection provided the nucleation which the additional materials when decomposed grow on the initial nuclei. 

Gold lymCT nanocomposites were very stable, brush-type shells of linear macromolecules and terminal mesogen prepared by means of a quantitative termination reaction. The result was an amphiphilic core-shell material with a well-defined hydrophilic-lipophilic balance... 

The Mo-containing MFl-type core-shell HZSM-5-Silicalite-l s (HZ5 S1) materials with various core-shell ratios prepared by the epitaxial growth of Silicalite-1 on HZSM-5 demonstrated the high shape selectivity to aromatics and stability in methane dehydroaromatization [66]. The silicalite-1 layer covering the HZSM-5 core can eliminate the external acid sites, and thus prevent the formation of active Mo species associated with Brpnsted acid sites on the external surface of catalysts. However, the overgrowth of the Silicalite-1 shell may lead to a severe inhibition of the Mo species migration into zeolite pores and consequent anchoring on the Brpnsted acid sites of the HZSM-5 core. Therefore, the catalytic performance is dependent on the core-shell ratio. 

Different from simple coating, we also have to mention here the "core-shell" materials. The principle consists in preparing a thick and stable shell (micrometer order) completely encapsulating a high-energy but unstable core. S5mergetic (positive) effect of the two materials is expected. Formation process of microscale coreshell structures is tedious, and it requires, at least, three distinct steps (e.g., hydroxide co-precipitation, formation of core-shell... 

The second step is to disperse the core material being encapsulated in the solution of shell material. The core material usually is a hydrophobic or water-knmiscible oil, although soHd powders have been encapsulated. A suitable emulsifier is used to aid formation of the dispersion or emulsion. In the case of oil core materials, the oil phase is typically reduced to a drop size of 1—3 p.m. Once a suitable dispersion or emulsion has been prepared, it is sprayed into a heated chamber. The small droplets produced have a high surface area and are rapidly converted by desolvation in the chamber to a fine powder. Residence time in the spray-drying chamber is 30 s or less. Inlet and outlet air temperatures are important process parameters as is relative humidity of the inlet air stream. 

Nanoparticles of Mn and Pr-doped ZnS and CdS-ZnS were synthesized by wrt chemical method and inverse micelle method. Physical and fluorescent properties wra cbaractmzed by X-ray diffraction (XRD) and photoluminescence (PL). ZnS nanopatlicles aniKaled optically in air shows higher PL intensity than in vacuum. PL intensity of Mn and Pr-doped ZnS nanoparticles was enhanced by the photo-oxidation and the diffusion of luminescent ion. The prepared CdS nanoparticles show cubic or hexagonal phase, depending on synthesis conditions. Core-shell nanoparticles rahanced PL intensity by passivation. The interfacial state between CdS core and shell material was unchan d by different surface treatment. 

Core/shell-type nanoparticles ovm ated with higher band inorganic materials exhibit high PL quantum yield compared with uncoated dots d K to elimination of surface non-radiative recombination defects. Such core/shell structures as CdSe/CdS [6] and CdSe ZnS [7] have been prepared from organometaHic precursors. 

The preparation of both, the particles themselves and the protective surface layer, has direct influence on their cytotoxicity. It is common belief that in the case of core/shell nanoparticles, properly prepared, close shell or multiple shells such as ZnS/Si02-shells prevents the leakage of toxic elements and thus makes cytotoxicity unlikely. Naturally, a better solution is to avoid cytotoxic materials in the first place. QDs, for example, can be synthesized without utilization of any class A or B elements InP/ZnS QDs have photophysical properties comparable to those of CdSe-based systems [43, 93]. Principally, whenever a new approach for QD synthesis or coating is used or if the QDs are applied in an extreme environment that could compromise their integrity, it is recommended to assess their cytotoxicity. 

---