ABSTRACT
It is well known that morphologies of inorganic nanostructures can inϐluence their properties such as light absorption and photoluminescence. For example, CdSe nanorods of different diameters and lengths can show tunable photoluminescence peaks as a result of quantum conϐinement effects [1]. A strong absorption spectral shift has also been recorded for PbSe nanocrystals with diameters between 3 and 8 nm [2]. Gold and palladium nanorods can show a progressively red-shifted longitudinal surface-plasmonresonance (SPR) absorption band with increasing aspect ratios [3-5]. Triangular and hexagonal gold nanoplates with sizes ranging from tens of nanometers to over 1 μm can exhibit blue to brown and
then gold colors [6]. As the number of nanomaterials publications grows in the past decade, a wide variety of nanostructures has been synthesized. Various physical and chemical properties of nanomaterials have been examined. However, it still remains challenging to develop synthetic approaches for the growth of metal and semiconductor nanocrystals with systematic shape evolution such as cubic to octahedral structures for crystals with cubic unit cells [7, 8]. Control of nanocrystal shapes with well-deϐined facets is important and should be a major research goal. This is because various properties of nanocrystals are likely facet-dependent, such as their catalytic, photocatalytic, electrical, and molecular adsorption properties. The ability to synthesize nanocrystals with excellent shape control and monodisperse size distribution enables more accurate observation of their facet-dependent effects. In addition, these uniform polyhedra with sizes of tens of nanometers or less may readily form self-assembled 2-dimensional and 3-dimensional superlattice structures [9-13]. These nanocrystal superstructures may display optical properties that are different from individual nanoparticles. In this chapter, synthetic methods for the growth of metal and semiconductor polyhedral nanocrystals are ϐirst presented with emphasis on the systematic shape evolution. Growth mechanism of polyhedral nanocrystals is also discussed. The preparation of nanocrystals with systematic shape evolution provides unique opportunities for the examination of factors affecting the product morphology. The discussion will show how our general understanding of nanocrystal growth mechanism may need to be modiϐied. Using the pre-formed metal nanoparticles as cores for the overgrowth of shells of a different composition, core-shell heterostructures with well-deϐined morphologies can be fabricated. Unusual crystal morphologies may be obtained by this means. Synthesis of this type of nanocrystals is described. Another way to extend the structural diversity of polyhedral nanocrystals is by forming hollow structures such as nanocages and nanoframes. Approaches for the preparation of hollow nanostructures are also brieϐly introduced. Finally some facet-dependent properties of these nanocrystals are presented to illustrate the importance of controlling the particle shape. Ultimately, the purpose and value of such plane-selective property investigation is to provide better nanomaterials with enhanced functionality for
various applications. Thus, if we know which surface is most active for catalysis, such as the {001} facets of anatase TiO2 [14, 15], and the nanocrystals can be made by a convenient and cost-effective method, then there should be a tremendous potential for application. In this chapter, some of the nanocrystals synthesized in our laboratory are used as examples for the presentation.
