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Nucleation and Growth ofNanosystems
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The growth of nanocrystals in solution involves two importantprocesses, the nucleation followed by the growth of the nanocrystals(Rao et al., 2007).
Nucleation is the phenomenon of initiation of formation of the first
nanocrystals in solution. It involves the appearance of very small particles or nuclei of the
new phase which are capable of growing.
There are two types of nucleation
1) Homogenous-nucleiform simultaneously and uniformly throughout
the solution.2) Heterogeneous-nuclei form preferentially at structural in-
homogeneities like container surface, insoluble impurities etc.
During growth the nuclei increase in size
Nucleation and growth
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Nucleation and Growth
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Nucleation is the creation of nuclei upon which growth canoccur.
Nucleation plays an important role in controlling theproperties of the final product, size distribution and nature of
the phase.
Nano particles need strong nucleation and slow growth.
Highly monodisperse nanoparticles are formed if the
processes of nucleation and growth can be successfully
separated.
The distance among nucleation sites has to be bigger than
diffusion length.
Nucleation (Cont.)
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Nucleation (Cont.)
Nucleationis the process by which atoms (or ions) that are freein solution come together to produce a thermodynamically
stable cluster.
The cluster must exceed a specific size (the critical size). Oncethe critical size is exceeded, the cluster is capable of further
growth. If the nucleus is smaller than the critical size,
spontaneous dissolution can occur(Tojo et al., 2006).
LaMer et al. (1950) have studied extensively nucleation and
growth in sulfur solutions, from which they developed an
understanding of the mechanism for the formation of colloids or
nanocrystals from a homogeneous medium.
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LaMers Diagram (Concentration vs Time)
In the first step, the concentration increases continuously with increasing time. As the concentration reaches
the critical super saturation value, nucleation occurs. This leads to a decrease of the concentration. Between
the concentrations C max and C min the nucleation occurs.
Later, the decrease of the concentration is due to the growth of the particles by diffusion. This growthoccurs until the concentration reaches the solubility value. Nucleation occurs in the first part of the reaction
and later only growth of the particles occurs.
If this model is followed, the size of the particles will increase continuously with the concentration of the
precursor or a minimum in the variation of the size with the concentration can also be expected. This
stems from the fact that the number of nuclei is constant and the increase of concentration leads to an
increase of the size of the particles(Destree et al., 2006).
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Tuning of the size of nanoparticles
Tuning of the size of nanoparticles can be achieved by control over
nucleation and growth rates, as illustrated in Scheme.
Fast nucleation provides a high concentration of nuclei, ultimately yielding
smaller nanocrystals, whereas slow nucleation provides a low concentration
of seeds consuming the same amount of precursors, thus resulting in larger
particles.
Schematic representation of the synthesis of nanocrystals.
Nanoparticles: From Theory to Application. Edited by Gunter Schmid (2004)
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Aspects of Nanoparticle Growth in
Solution
Arrested Precipitation
Precipitation under starving conditions: a large number of nucleation
centers are formed by vigorous mixing of the reactant solutions.
If concentration growth is kept small, nuclei growth is stopped due
to lack of material.
Oswald Ripening
The growth mechanism where small particles dissolve, and are consumedby larger particles. As a result the average nanoparticle size increases with
time and the particle concentration decreases. As particles increase in size,
solubility decreases.
Particles had to be protected from Ostwald ripening by stabilizers
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Stabilization of Nanoclusters Against
Aggregation
1. Electrostatic stabilizationAdsorption of ions to the surface.
Creates an electrical double layer
which results in a Coulombic
repulsion force between individualparticles
2. Steric Stabilization
Surrounding the metal center by
layers of material that are sterically
bulky, called capping agents
Examples: polymers, surfactants,
etc
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Tuning of the shape of nanoparticles
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Growth mechanism of gold nanorods
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Synthesis of gold nanoparticles of different
shapes
General idea is the same as the growth of gold
nanorods (seed-mediated method)
Slightly change the conditions when growingnanorods (concentration of different reactants)
Cubes, hexagon, triangle, tetropods, branched
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Gold nanoparticles. Scale bar=100nm