Nonspherical Noble Metal Nanoparticles: Colloid-Chemical Synthesis and Morphology Control By Tapan K. Sau* and Andrey L. Rogach* 1. Introduction Hardly any other material has contributed more in the human civilization than metals. Metals account for 24% of the mass of the planet. About two thirds of all elements in the periodic table are classified as metal. Metals remain equally important even in the ages of polymer and silicon. Applications of metallic materials range from household items to space ships due to their useful properties including for example strength and toughness, thermal and electrical conductivity, ductility, and high melting points. One of the key features of metallic materials is that they can be crafted into numerous useful shapes and forms. In our endeavor of exploring new materials with better performance and new functionality, we have recently invented techniques for manipulat- ing metallic materials at the nanometer scale. Currently, metal nanoparticles with a variety of morphologies and sizes can be synthesized, which have been the focus of numerous studies. Perhaps an acceptable definition of a nanoparticle can be given as follows. Nanoparticles are discrete particles with at least one characteristic dimension: the size in nanometers, typically in the range of 1–100 nm. [1] 1 nm is 6 times the atom size of Au and 4.5 times the atomic spacing in noble metals. Nanomaterials show electronic, physical, and chemical properties that are often substantially and sometimes radically different from their constituents or bulk counterparts. In addi- tion to the material composition, the size and shape are two other important factors that determine properties of a nanoparticle. The shape anisotropy in noble metals (e.g., Ag, Au, Pt and Pd) is the theme of this Review. Chemists are aware of the intimate relationships among valence, stoichiometry, molecular geometry (the way the atoms or molecules arrange themselves), and reactivity of molecules and solids. It has long been recognized that the molecular morphology exerts a direct impact on the properties of polymeric materials. Similarly, in nanomaterials where a few hundreds to thousands atoms (or molecules) are combined as a single entity, the resultant particle shape is expected to be a crucial factor in determining the properties of a nanoparticle. However, it should be noted here that nanoparticles are often larger than molecules (and small clusters). Therefore, the surface-energy factor plays a major role in determining the shapes of the nanoparticle, unlike the bond energy in the molecules (and clusters). It has been experimentally and theoretically found that deviations from spherical geometry strongly affect the properties of nanoparticles. Strict control of the particle morphology is therefore required in order to fine-tune the properties of the nanoparticles. Conversely, this allows for the generation of particles with new properties from the same materials by simply tuning the particle morphology. In addition, there are other motivations for morphology-controlled preparation of nanoparticles. For exam- ple, an ability to engineer materials on a nanometer length scale enables the investigation into the fundamental size- and shape-dependent properties of matter. Furthermore, studies of the growth mechanism leading to nanoparticle anisotropy are important in the elucidation of the crystal-growth mechanism. Anisotropic nanoparticles can also be used as templates for REVIEW www.MaterialsViews.com www.advmat.de [*] Prof. Dr. T. K. Sau International Institute of Information Technology Hyderabad 500 032 (India) E-mail: tapan.sau@iiit.ac.in Prof. A. L. Rogach Department of Physics and Materials Science City University of Hong Kong Tat Chee Avenue, Kowloon (Hong Kong) E-mail: andrey.rogach@cityu.edu.hk DOI: 10.1002/adma.200901271 Metal nanoparticles have been the subject of widespread research over the past two decades. In recent years, noble metals have been the focus of numerous studies involving synthesis, characterization, and applications. Synthesis of an impressive range of noble metal nanoparticles with varied morphologies has been reported. Researchers have made a great progress in learning how to engineer materials on a nanometer length scale that has led to the understanding of the fundamental size- and shape-dependent properties of matter and to devising of new applications. In this article, we review the recent progress in the colloid-chemical synthesis of nonspherical nanoparticles of a few important noble metals (mainly Ag, Au, Pd, and Pt), highlighting the factors that influence the particle morphology and discussing the mechanisms behind the nonspherical shape evolution. The article attempts to present a thorough discussion of the basic principles as well as state-of-the-art morphology control in noble metal nanoparticles. Adv. Mater. 2010, 22, 1781–1804 ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1781