Page 44 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

30 Principles and Methods

Specificity and Requirements
in the Fabrication Methods of Nanoparticles
Ultra-dispersed systems, such as dispersions of nanoparticles, are intrin-
sically thermodynamically metastable, in large part due to the very
high interfacial areas. Nanoparticle surface area represents a positive
contribution to the free enthalpy of the system. If the activation ener-
gies are not too high, spontaneous evolution of a nanoparticle dispersion
can occur causing an increase in nanoparticle size or the formation of
nanostructured domains and leading to the decrease of the surface area.
Consequently, it follows that:

■ An ultra-dispersed system with a high surface energy can be only
kinetically stabilized.
■ Ultrafine powders cannot be synthesized by methods involving ener-
gies that exceed a threshold, but rather through methods of “soft
chemistry” that maintain the forming particles in a metastable state.
■ Additives and/or synthesis conditions that reduce the surface energy
are needed to form nanoparticles stabilized against sintering, recrys-
tallization, and aggregation.
Under these conditions, any solid matter such as metal oxides, chalco-
genides, metals, or carbon can be obtained at the nanometric scale.
Synthesis methods for nanoparticles are typically grouped into two
categories:
■ The first involves division of a massive solid into smaller portions. This
“top-down” approach may involve milling or attrition (mecano-
synthesis), chemical methods for breaking specific bonds (e.g., hydro-
gen bonds) that hold together larger repeating elements of the bulk
solid, and volatilization of a solid by laser ablation, solar furnace, or
some other method, followed by condensation of the volatilized
components.
■ The second category of nanoparticle fabrication methods involves con-
densation of atoms or molecular entities in a gas phase or in solution.
This is the “bottom-up” approach in which the chemistry of metal com-
plexes in solution holds an important place. This approach is far more
popular in the synthesis of nanoparticles, and many methods have
been developed to obtain oxides, chalcogenides, and metals.

The liquid-phase colloidal synthetic approach is an especially powerful
tool for convenient and reproducible shape-controlled synthesis of
nanocrystals—not only because this method allows for the resulting
nanocrystals to be precisely tuned in terms of their size, shape, crys-
talline structure, and composition on the nanometer scale, but also