Other Modes of Formation for Complex Nanostructures

FIGURE 2.8 Schematic illustration of the formation of hollow spheres with (a) compact and (b) loose surfaces, respectively, through Ostwald ripening. Reprinted with permission from Ref. [32]. American Chemical Society. 

3 Kirkendall Effect The Kirkendall effect is a phenomenon observed frequently in solid materials [38]. It refers to a vacancy counter diffusion process through an interface of two solid materials, metals in particular, to compensate the unequal material flow formation at the interface [38a]. In metals and metallic alloys, the vacancy is atomic defect, that is, empty lattice site. Combination of excess vacancies can lead to the formation of void within the fast-diffusion side of the interface [39]. While this phenomenon has been known for a very long time, synthesis of hollow nanostructures based on Kirkendall effect was realized fairly recently [40]. Ym studied the time evolution in the formation of hollow nanospheres and found that Kirkendall diffusion followed the Tick s law [41]. This means that the diffusion of atoms and vacancies is driven by the difference in atom concentration. Wu et al. synthesized hollow nanostructures of CoCuPt alloy catalyst by using Co nanoparticles as the sacrificial templates. For this trimetallic system, Co atoms diffused faster than those of Pt or Cu to form core-shell like Co CuPt hollow nanoparticles and then the CoCuPt hollow spheres (Fig. 2.10) [42]. 

4 Defect and Twinned Plane Since many metals have fee structures, there is no strong intrinsic driving force for metals or alloys to grow into one-dimensional (ID) or two-dimensional (2D) nanocrystals. However, these two classes of highly anisotropic shapes can be obtained in cases when the cubic symmetry is broken down, which is achievable by introducing twin defects in the seed nanoparticles. For 

FIGURE 2.10 Schematic of the formation of CoCuPt hollow nanoparticles. Purple circles, Co blue circles, Cu orange circles, Pt. Reprinted with permission from Ref. [42]. Royal Society of Chemistry. (See insert for color representation of the figure.) 

Icosahedral nanoparticles were also made recently. This morphology is composed of 20 tetrahedral subunits with 30 twinned plates, and one icosahedral nanocluster is bound with 20 111 facets. Ag, An, and Pd are among the metals that can form 

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