Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 15

example, an attrition method can break the bulk material into nanomaterials ranging
from tens to several hundreds of nanometers in diameter. However, broad size
distribution, varied shape or geometry, and impurities from the milling medium are
factors of concern for products made using this method. For this reason, bottom-up
approaches for nanomaterial synthesis are becoming more popular, as this rapidly
growing research area has great potential for use in the creation of technologically
advanced and useful materials. A bottom-up approach directly forms nanomaterials from
different kinds of precursors via mainly a chemical reaction. For example, homogeneous
nucleation from a liquid or vapor, or heterogeneous nucleation on a substrate, are
concepts for synthesizing nanomaterials using a bottom-up approach. There are several
techniques related to bottom-up approaches. These techniques can be divided into
thermodynamic approaches, equilibrium approaches, and kinetic approaches. For
thermodynamic approaches, generation of supersaturation, nucleation, and subsequent
growth are the primary procedures for nanomaterial synthesis. In kinetic approaches, the
amount of precursors required for efficient nanomaterial growth or the space required
for the reaction are the main limiting factors. Hence, synthesis of nanoparticles
introduced in this section will follow bottom-up approaches.

2.2.1 Sol-Gel Process

The sol-gel process is a wet chemical process that results in the formation of
either inorganic or organic-inorganic nanomaterials from a liquid phase (Livage et al.,
1988). The sol-gel process is especially applicable for the synthesis of oxide
nanomaterials (Cousin and Ross, 1990).

The sol-gel process uses inorganic or metal-organic precursors. Molecular
precursors are induced to undergo hydrolysis and condensation in solution to form
bridging hydroxyl [M-p(OH)-M] or oxo (M-O-M) bonds. The most commonly used
precursors are metal alkoxides [M(OR) n ], where R is an alkyl group, inorganic salt, or
organic salt. Otherwise, organic or aqueous solvents may be used to dissolve precursors,
and catalysts are then added to promote hydrolysis and condensation reactions (Gesser
and Goswami, 1989; Chandler et al., 1993; Brinker and Hurd, 1994). The overall
reaction can be represented by following steps:

Hydrolysis:

MOR + H 2 O MOH + ROH (Eq. 2.1)

Condensation:

MOR+ HOM MOM + ROH (alcoxolation) (Eq. 2.2)

MOH+ HOM MOM + H 2 O (oxolation) (Eq. 2.3)