Core-to-shell ratio

An analogous synthetic approach has been used by Fox et al. [SO] for the preparation of CdSe/ZnSe core-shell particles. X-ray photoelectron and Auger and absorption spectroscopies were applied to demonstrate the assumed structure of both composites, CdSe on ZnSe and ZnSe on CdSe. It is found that the shell material dominates the optical properties of the particles, even when the narrower bandgap material was used for the core. This apparent contradiction to the above-mentioned statement of Brus et al. on the composite ZnS on CdSe is probably due to different sizes and different core-to-shell ratios in the two studies. 

Nanoshell particles (NSs) are highly functional materials with tailored properties, which are quite different from either the core or the shell material. Therefore, nanoshells are preferred over nanoparticles since their properties can be modified by changing either the constituting materials or the core-to-shell ratio. These particles are synthesized for a variety of purposes like providing chemical stability to colloids, band structures, biosensors, drug delivety, etc. 

Particles with a copolymer soft core of poly-n-butyl acrylate/PMM A and a homopolymer hard shell of PMMA were characterised by TEM and solid-state NMR spectroscopy. Two synthesis parameters were investigated, the phase ratio of the core and the shell, and the compatibility of the two phases. A series of core-to-shell ratios from 100/0 to 25/75 was synthesised and characterised. The compatibility between the phases was changed, either by using acrylic acid in either the core and the shell or in both, or by synthesising a homopolymer or a copolymer core, or by introducing crosslinking points in the core. The combination of TEM and solid-state NMR spectroscopy allowed quantitative determination of the extent of coverage of the core by the shell polymer and the... 

Based on the core-to-shell ratio, nanoshells can be designed to preferentially absorb or scatter light. Absorbed energy is mainly... 

With the newer materials, the ratio of core to shell is much better, so that the issue of loadability plays only a minor role. The first solid core materials with particle sizes between 2 and 3 pm clearly aimed at providing an improved alternative to porous sub-2 pm materials. 

CdS and CdS-ZnS core-shell nanoparticles were synthesized by inverse micelle method. Crystallinity of CdS nanoparticles was hexagonal structure under the same molar ratio of CM and S precursor. However it was changed easily to cubic structure under the condition of sonication or higher concentration of Cd than S precursor. The interfacial state betwran CdS core and shell material was unchanged by different surface treatment. 

In 1989, we developed colloidal dispersions of Pt-core/ Pd-shell bimetallic nanoparticles by simultaneous reduction of Pd and Pt ions in the presence of poly(A-vinyl-2-pyrrolidone) (PVP) [15]. These bimetallic nanoparticles display much higher catalytic activity than the corresponding monometallic nanoparticles, especially at particular molecular ratios of both elements. In the series of the Pt/Pd bimetallic nanoparticles, the particle size was almost constant despite composition and all the bimetallic nanoparticles had a core/shell structure. In other words, all the Pd atoms were located on the surface of the nanoparticles. The high catalytic activity is achieved at the position of 80% Pd and 20% Pt. At this position, the Pd/Pt bimetallic nanoparticles have a complete core/shell structure. Thus, one atomic layer of the bimetallic nanoparticles is composed of only Pd atoms and the core is completely composed of Pt atoms. In this particular particle, all Pd atoms, located on the surface, can provide catalytic sites which are directly affected by Pt core in an electronic way. The catalytic activity can be normalized by the amount of substance, i.e., to the amount of metals (Pd + Pt). If it is normalized by the number of surface Pd atoms, then the catalytic activity is constant around 50-90% of Pd, as shown in Figure 13. 

X-Ray Photoelectron Spectrometry. X-ray photoelectron spectrometry (XPS) was applied to analyses of the surface composition of polymer-stabilized metal nanoparticles, which was mentioned in the previous section. This is true in the case of bimetallic nanoparticles as well. In addition, the XPS data can support the structural analyses proposed by EXAFS, which often have considerably wide errors. Quantitative XPS data analyses can be carried out by using an intensity factor of each element. Since the photoelectron emitted by x-ray irradiation is measured in XPS, elements located near the surface can preferentially be detected. The quantitative analysis data of PVP-stabilized bimetallic nanoparticles at a 1/1 (mol/mol) ratio are collected in Table 9.1.1. For example, the composition of Pd and Pt near the surface of PVP-stabilized Pd/Pt bimetallic nanoparticles is calculated to be Pd/Pt = 2.06/1 (mol/ mol) by XPS as shown in Table 9.1.1, while the metal composition charged for the preparation is 1/1. Thus, Pd is preferentially detected, suggesting the Pd-shell structure. This result supports the Pt-core/Pd-shell structure. The similar consideration results in the Au-core/Pd-shell and Au-core/Pt-shell structure for PVP-stabilized Au/Pd and Au/Pt bimetallic nanoparticles, respectively (53). 

Synthesis of oil soluble micellar calcium thiophosphate was performed in a one-step process involving the reaction of calcium oxide, tetraphosphorus decasulfide and water in the presence of an alkylaryl sulfonic acid. This product could be defined as a calcium thiophosphate hard-core surrounded by a calcium alkylarylsulphonate shell in accordance with a reverse micelle type association in oil. Three micellar products with the same chemical nature core were prepared, each with different core/shell ratio of 0.44, 0.92 and 1.54. Better performances are expected with products of higher core/shell ratios. The antiwear performance of micellar calcium carbonates is directly linked to the size of the mineral CaC03 colloidal particles. At a concentration of 2 % micellar cores, no antiwear effect is observed whatever the micellar size. At an intermediate concentration of 4 % of micellar cores, the wear scar diameter is clearly dependent on the micellar size, slipping from 1.70 mm to 1.10 mm, then to 0.79 mm when the core diameter moves from 4.37 nm to 6.07 nm, then to 6.78 nm. Size dependence is increased at a concentration of 5 % in colloidal cores. This clearly confirms the size dependence of the micellar cores on their antiwear performance (Delfort et al.,... 

In a typical Doppler measurement only one of the two annihilation photons with on average half of the Doppler shift is observed. With second detector opposite to the first one and operated in coincidence with the first one the full Doppler shift is observed. The signal to noise ratio improves by a factor of-1000. The shell structure of tightly bound core electrons of atoms does not change much when the atoms form a solid. Doppler shifts from these electrons can be detected and permit the identification of specific elements next to the annihilation site [70]. 

The Group 12 elements differ markedly from those in Group 2 in nearly all respects except having II as their only important oxidation state. Thus, while the Zn2+ and Mg2+ ions are very similar in their 6-coordinate radii (0.88 A and 0.86 A, respectively), Zn2+ has a relatively polarizable 3d10 shell whereas the neon core of Mg2+ is very hard. This special combination of softness and a high charge-to-radius ratio appears to be responsible for the unique role played by zinc in biochemistry (see Section 15-17). 

Pd-core/Ag-shell bimetallic nanoparticles were prepared via the reduction of silver ions on the surface of Pd nanoparticles (mean radius 4.6 nm) with formaldehyde by Michaelis and Henglein [129]. The particles became larger and the deviations from the spherical shape became more pronounced with the increasing ratio of the shell to the core. As shown in Eigure 9, these Pd/Ag bimetallic nanoparticles possess a surface plasmon absorption band close to 380 nm when more than 10 monolayers of silver are deposited. When the shell thickness is less than 10 atomic layers the absorption band is located at shorter wavelengths. The band disappears when the thickness of the shell is below about three atomic layers. 

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