CHAPTER 9: Nucleation and Crystal Shape Engineering

Given the dominance of surface energy, small crystallites are disfavoured relative to larger crystallites, and nucleation has to overcome an energy barrier before the volume energy starts to drive crystal growth. Small particles are formed under conditions of high oversaturation because this triggers instant formation of many nuclei and homogeneous growth, until after a short time span growth stops simultaneously for all of them, owing to starvation. Equilibrium crystal shapes are determined by Wulff's law, but inhibition due to selective adsorption or liquid layer formation at the different crystal facets leads to different growth rates and to very different morphologies at the same crystal structures. Thus, crystal shape engineering is the art of handling non-equilibrium conditions and kinetic rather than thermodynamic control, with temperature and degree of oversaturation being the major parameters. Liquid droplets can act as collectors of materials from the gas phase, catalysing unidirectional growth of needle-type structures or carbon nanotubes. Growth rate and shape can also be influenced electrochemically, and densely packed layers of oriented metal oxide nanotubes can be grown by anodisation of the metal foils.