2.3 Oxide nanoparticles

Introduction to oxide nanoparticles

The most widespread route to fabrication of metal oxide nanoparticles involves the “bottom-up” approach involving the precipitation from aqueous solution from metal salts. Organometallic species can also be used, but due to their cost and the difficulty in manipulating these compounds, they are used less frequently. An alternative “top-down” approach has been demonstrated for aluminum and iron oxide nanoparticles; however, it is possible that this methodology could be extended to other oxides.

From molecular species to nanoparticles

Hydroxide, oxyhydroxide or hydrated oxide solid phases obtained via precipitation are made of particles whose average size may range from a few nanometers to a few microns. Particle morphology may vary depending on synthesis conditions. Moreover, aging in aqueous solution may bring about significant dimensional, morphological and structural changes.

Use of metal salts

The dissolution of metal salts in water results in the formation of solvated coordination compounds in which the chemistry of such complexes, and especially their acid behavior, provides a framework for understanding how the solid (oxide) forms via polycondensation. The binding of water molecules to the metal cation results in the increase in the acidity such that they tend to be deprotonate spontaneously according to the hydrolysis equilibrium

[M(H ₂ O) _n ] ^z+ + h H ₂ O → [M(OH) _h (H ₂ O) _n-h ] ^(z-h)+ + h H ₃ O ⁺

or by the neutralisation with a base,

[M(H ₂ O) _n ] ^z+ + h HO- → [M(OH) _h (H ₂ O) _n-h ] ^(z-h)+ + h H ₂ O

in which h is the hydroxylation ratio of the cation. The resulting hydroxylated complexes condense via two basic mechanisms of nucleophilic substitution, depending on the nature of the coordination sphere of the cations. Condensation of aquohydroxo complexes proceeds by elimination of water and formation of hydroxo bridges (olation), while for oxohydroxo complexes, condensation proceedes via the formation of oxo bridges (oxolation).

In order to understand how small particles form and what role the experimental parameters play on their characteristics and on evolution, it is useful to review the kinetic aspects of condensation reactions. The precipitation of a solid involves four kinetic steps.

1. Formation of the zero-charge precursor. [M(OH) _z (H ₂ O) _n-z ] ⁰ , which is able to condense and form a solid phase.
2. Creation of nuclei, through condensation of zero-charge precursors.
3. Growth of the nuclei through addition of matter, until the primary particle stage is reached.
4. Nucleation and growth steps form particles under kinetic control following a reaction path of minimum activation energy under conditions imposed to the system (acidity, concentration, temperature), but the products are not necessarily thermodynamically stable.

Aging of the suspensions, which may take place over a long time scale (hours, days or months), allows the system to tend toward, or reach stability, and it is often associated with modifications of some physical or chemical characteristics of the particles.