l2- nucleation and growth

8/12/2019 L2- Nucleation and Growth

1/14

Nucleation and Growth ofNanosystems


2/14

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


3/14

Nucleation and Growth


4/14

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.)


5/14

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.


6/14

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).


7/14


8/14

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)


9/14

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


10/14

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

d+

d+

d+d+

d+

d+

d+

d+d+

d+

------

-----

-

-

-

-- -

--

- - ---

- - -

-- ---

-

-

-+

+

+

+

+

+

+


11/14

Tuning of the shape of nanoparticles


12/14

Growth mechanism of gold nanorods


13/14

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

13


14/14

Gold nanoparticles. Scale bar=100nm

l2- nucleation and growth

Documents