Chemistry Reference
In-Depth Information
Figure 2.1
A crystal-growth diagram illustrates the generation of atoms,
nucleation, and subsequent growth [1].
For the reduction route, the precursor compound is in a higher
oxidation state than the atomic species. Either the precursor
compounds are reduced to zero-valent atoms and agglomerate
further into nanocrystals, or the unreduced metal species form nuclei
prior to reduction [1]. Structural fluctuations become energetically
unfavorable beyond a critical size of the atom cluster and then it
locks into a well-defined structure, giving rise to a seed crystal [44].
In general, the seeds may be in the form of a single-crystal, singly
twinned, or multiply twinned structure as shown in Fig. 2.2. In a
typical synthesis, one or more type of seeds may coexist. The key to
obtaining the desired type of nanocrystals and exclusion of others is
generally dependent on precise control of the synthesis parameters,
especially those of thermodynamics and kinetics. Figure 2.2 shows
general reaction pathways that lead to seeds formation, and finally
to fully formed nanostructures of various morphologies. These
pathways are illustrative for homogeneous nucleation. However, for
heterogeneous nucleation, shape control is less stringent. Core-shell
