In general, two processes are possible in anodic dissolution of a single phase (both solid solutions and intermetallic phases): In order to understand the shell formation, the electrochemical behaviour of the matrix and the precipitate phase must be considered. The two-phase alloys used for ESPD include a solid solution matrix and an intermetallic precipitate phase. The particle shape and size is completely preserved, indicating that the matrix was only dissolved and the shell formed on top of the particle during the dissolution process. Further, it shows that the shell is of uniform thickness on the completely exposed particle surfaces, but becomes thinner near the region where the particle is still connected to the matrix. Figure 3 clearly shows that the shell has formed even before the particles are completely detached from the bulk alloy. Cross-sectional (CS) specimens were prepared from this partially etched sample for microstructural observation by TEM directly at the prior sampleelectrolyte interface. they remained partly embedded in the alloy surface. In order to study how the shell is forming, the ESPD process was arrested in between, when some of the nanoparticles were only partially extracted from the bulk alloy, i.e. The core and the shell compositions, which were measured by energy dispersive X-ray spectroscopy (EDS), showed that the shell is made of Si and O atoms, while the core contains Ni and Si atoms.įor the first time, the core-shell structure is observed in Ni3Si-type nanoparticles extracted from binary Ni-Si and ternary Ni-Si-Al alloys. 2b) is obtained from a digitised IP image by azimuthally averaging the intensities. IP, having a higher definition image recording on a large usable area, recorded intensity pattern over a wide Q range, where the very low intensities from the superlattice reflections and the diffused amorphous peaks can be well resolved in the presence of other high intensity Bragg peaks. Typical Debye-Scherrer ring pattern from powder samples was recorded using Imaging Plate (IP) in TEM. The crystalline core and the amorphous shell structure are also confirmed by electron diffraction measurements on powder samples (fig. In contrast, the shell shows contrast typical of an amorphous phase. The atomically resolved high resolution electron microscopy image (HREM) of the particle oriented in zone clearly reveals that the particle-core has a cubic lattice structure. The shell is homogeneous with uniform thickness (~10–15 nm). The particle surface shows a different contrast than its core, indicating a shell is formed around the particle. 1), a Ni3(Si,Al) nanonanoparticles of ~100 nm size is seen (top right). In the transmission electron microscopy (TEM) image (fig. Structure and Composition of the Core and Shell ESPD can also be used to produce coreshell nanoparticles of other compositions. Such coreshell nanoparticles are unique and may find attractive functional applications. A typical core-shell nanoparticle produced by this method is seen in figure 1. In the present paper, we report an extension of this method and show that nanoparticles covered with amorphous shell can also be produced by ESPD. The selection is achieved through the suitable choice of electrolyte and the ESPD processing parameters. In this process, nano sized precipitates are extracted from a two phase alloy by selectively dissolving the matrix phase (by electrochemical selective phase dissolution – ESPD). Recently, a new method for producing nanoparticles of intermetallic phases was reported by us. A few examples of core/shell type nanoparticles are Ag/ Au, CdSe/ZnS or MgO/Fe2O3.
Amorphous Si and SiOx are also suitable shell material in some applications. Further, magnetic nanoparticles are covered by biocompatible organic shell material. for enhanced specific chemical functions, nanoparticles may be covered by a catalytically active shell material. Naturally, occurring inert oxide shells on metal particles are not uncommon. This distinction is important because for many applications, a shell covering the nanoparticles is deliberately formed either to protect the core or enhance a specific property on the surface of the particle. Here, we distinguish shells obtained naturally and artificially. Most research, however, focuses on bare nanoparticles in many applications, however, nanoparticles covered with a shell having a specific function is actually needed. Nanoparticles are finding innovative applications as functional material. The core-shell nanoparticles are extracted from a two phase metallic alloys by selective phase separation using electrochemical dissolution technique. A method to produce nanoparticles with core-shell structure, having crystalline Ni3Si core and amorphous Si(O) shell is reported. Core-Shell Nanoparticles: Imaging and Structure Determination of the Core and the Shell Regions.
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