Insights into Nucleation, Growth and Shape Control for Designing Anisotropic Nanostructures and Heterostructures

Abstract

The properties of nanomaterials significantly depend on the size and shape of the nanocrystals. Thus the size and shape control becomes an important and interesting aspect of nanocrystal synthesis. Wet chemical method of synthesis of nanomaterials is an efficient bottom-up approach. It starts at the molecular level and the rate of the reaction can be monitored and regulated at various stages of reaction, thus enabling one to have a better control over the final morphology of the nanocrystal. In order to carry out a morphology controlled synthesis strong understanding of nucleation and growth of a nanocrystal is essential. According to the classical theory of nucleation, the nucleus at its critical radius of nucleation has a shape that is geometrically identical to the equilibrium morphology of the crystal. Into the growth regime the growth of the nucleus happens by attachment of atoms at the growing interfaces of the nucleus. If the rate of attachment is equal at all the growing interfaces the final morphology is called an “equilibrium morphology”. On the other hand, if the attachment of atoms is not equal at all the growing interfaces it results in the generation of a host of different kinetic shapes or “growth morphologies”. The variation in the rate of growth in different facets can be brought about by selectively attaching capping agents to the different facets, for example. Equilibrium shape is the thermodynamic shape of the crystal and is obtained on minimization of the total surface free energy of the system. Growth morphologies on the other hand can be subdivided into – (a) symmetry conserving growth morphologies or (b) symmetry breaking / anisotropic growth morphologies depending on whether the point group symmetry of the nucleus has been retained in the final growth morphology or not. The symmetry will be retained if all the equivalent planes in the nucleus are affected in the same way during the course of the reaction. For example, if the growth of all the 8 {111} planes of a cuboctahedron is stopped and all the 6 {100} planes outgrow the {111} planes, the resulting shape is that of an octahedron. Under some special circumstances, if the growth of one set of {111} planes is different from the rest of the {111} planes, the resulting shape becomes anisotropic or symmetry breaking. Through this dissertation, a fundamental insight into the heterogeneous nucleation and equilibrium shapes is obtained and with the help of this understanding, few interesting 1 dimensional anisotropic nanostructures and heterostructures have been designed by wet chemical synthesis method. Some of these nanostructures have shown excellent activity as electrocatalysts and substrates for surface enhanced Raman spectroscopy. In conclusion, a discussion of the future prospects involving these vast variety of nanostructures has been presented. The knowledge and insights gained from the study in this dissertation can be utilised in predicting and designing growth of similar or related nanostructures and nanoheterostructures