CHEM01200604004 Shri Sanyasinaidu Boddu - Homi Bhabha ...

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charge stabilized nanoparticles, an electric double layer is formed on the surface of the
particle. This double layer acts as stabiliser due to electrostatic repulsion between layers of
different particles. In the case of sterically stabilized nanoparticles, capping agent is adsorbed
on the surface of the particles either by forming chemical bonds or by weak interactions.
These capping agents act as stabilisers due to repulsion between their tail groups. Both types
of stabilisation are schematically shown in Fig.4.
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(a)
(b)
Fig.4. Schematic representation of nanoparticles stabilization by (a) electrostatic repulsion (b)
steric repulsion/ interaction [15].
1.3.2 Sol-gel Method: Sol–gel technique is one of the most popular solution processing
methods for producing metal oxide nanoparticles. In sol–gel processing, a reactive metal
precursor, such as metal alkoxide is hydrolyzed by water, and the hydrolyzed species are
allowed to condense with each other to form precipitates of metal hydroxide/ oxide
nanoparticles. The precipitate is subsequently washed and dried and then calcined at an
elevated temperature to form crystalline metal oxide nanoparticles [16]. Hydrolysis of metal
alkoxides involves nucleophilic reaction with water (equation 3).
M(OR) y + x H 2 O → M(OR) y-x (OH) x + x ROH …………..(3)
Condensation occurs when either hydrolyzed species react with each other and release
a water molecule, or a hydrolyzed species reacts with an unhydrolyzed species and releases
an alcohol molecule. The rates at which hydrolysis and condensation reactions take place are
important parameters that affect the properties of the final product. For example, slower and
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