Fundamentals of Sol Gel Processing

The sol-gel synthesis process [1, 2] involves the hydrolysis and the condensation of metal alkoxide (M(OR')n), where M is the metal, O the oxygen, and R' the organic group. Since the metal alkoxide and water are insoluble in each other, they are dissolved in a common alcoholic solvent in order to carry out the reaction. The alkoxide group (OR'), being highly electronegative, creates a partial positive charge on the central metal atom. On the other hand, in the water molecule, there exists a partial negative charge on the oxygen atom. As a result, metal alkoxides are highly reactive to water. The water molecule attacks the central metal atom, which results in the hydrolysis of the alkoxide. The hydrolysis and the condensation reactions are generally described as:

Hydrolysis: M(OR')4 + 4H2O ^ M(OH)4 + 4R'OH (1) Condensation: M(OH)4 + M(OH)4

Copyright © 2004 by American Scientific Publishers All rights of reproduction in any form reserved.

Encyclopedia of Nanoscience and Nanotechnology Edited by H. S. Nalwa Volume 10: Pages (27-42)

The result is the formation of the M—O—M bond within the solution. The kinetics of the hydrolysis and the condensation reactions are governed mainly by the ratio (R) of molar concentrations water to alkoxide. In general, low R-value (<3) is suitable for thin-film formation while large R-values (>3) generate powder particles.

Once the oxide particles grow in the solution, they are likely to collide with each other due to Brownian motion and this may cause excessive aggregation of the particles. Forming a stable colloidal suspension of oxide nanoparticles (sol) is hence essential. In a colloid, the dispersion forces (van der Waals force of attraction) exist between the particles, which tend to aggregate the particles. For a colloidal particle, the dispersion force is the summation of dispersion forces of all the atoms within the particle. Hence, for colloidal particles of nanometer size, the attractive dispersion force becomes significant and can cause flocculation of colloids. The attractive dispersion force also depends on the shape of the particles. For particles having plate-like shape separated by distance "h," the attractive potential is given by [1]

where A is the Hamaker constant and is the material property and h the distance away from the particle surface. It can be noted that for plate-shaped particles, the attractive force decays in proportion to 1/h2 while for atoms it decays as 1/h6. Hence, the decay of dispersion force with "h" is slower for the plate-shaped particles than for atoms. Moreover, for spherical particles, the dependence of the attractive dispersion force on "h" is logarithmic; hence, the dispersion force decays more slowly for spherical particles than for plate-shaped particles (at least for small separations comparable to the radius of sphere). The attractive dispersion force between two spherical particles (of size say in the nanometer scale) can extend over the distances of nanometers, and hence, can cause coagulation of particles. Hence, the closest approach of the particles must be avoided to prevent coagulation.

On the other hand, in a colloid, the repulsive electrostatic force is created by the electrical double layer associated with the colloidal particles. The hydrous oxides generally have OH groups on their surface. The protonation and depro-tonation of the M—OH bonds result in the creation of a charge on the particle surface:

These H+ and OH- ions are thus the charge-determining ions. Whether the particle is positively or negatively charged or is neutral, depends upon the pH of the solution. The pH at which the charge on the particle is zero is called the "point of zero charge (PZC)." At pH < PZC, the particle is positively charged, while for pH > PZC, the charge on the particle is negative.

This surface charge of the colloidal particle then attracts ions (known as "counter-ions") of opposite charge in the solution. These counter-ions bind to the particles via van der Waals force as well as electrostatic potential of charge-determining ions. The water molecules are also attracted towards the surface charge and are held by van der Waals force as well as by hydrogen bonds. Hence, in a colloid, the particle surface charge is screened by the counter-ions. The charge-determining ions and counter-ions create an electrical double layer. This electrical double layer, associated with every colloidal particle, is responsible for generating a repulsive force between two neighboring particles, thus avoiding flocculation of particles, and thus, produces a stable sol.

This stable sol is then used for producing either nano-crystalline powder by removing the solvent by heating or nanocrystalline thin films. For the latter, spin-coating and dip-coating techniques have been extensively used for producing quality thin films.

0 0

Post a comment