2.3 Oxide nanoparticles  (Page 2/3)

Use of metallo-oragnic compounds

Metallo-organic compounds, and especially metal alkoxides, are used in so-called sol-gel chemistry of oxide nanomaterials. Metal alkoxides are also precursors of hybrid organic-inorganic materials, because such compounds can be used to introduce an organic part inside the mineral component. Sol-gel chemistry mainly involves hydrolysis and condensation reactions of alkoxides M(OR) _n in solution in an alcohol ROH, schematically represented as follows.

M(OR) _n + n H ₂ O → M(OH) _n + n ROH → MO _n/2 + ⁿ / ₂ H ₂ O

These reactions, hydroxylation and condensation, proceed by nucleophilic substitution of alkoxy or hydroxy ligands by hydroxylated species according to.

M(OR) _n + x HOX → [M(OR) _n-x (OX) _x ] + n ROH

If X = H, the reaction is a hydroxylation. For X = M, it is a condensation and if X represents an organic or inorganic ligand, the reaction is a complexation. The reactivity of metal alkoxides towards hydrolysis and condensation is governed by three main parameters: the electrophilic character of the metal (its polarizing power), the steric effect of the alkoxy ligands and the molecular structure of the metal alkoxide. Generally, the reactivity of alkoxides towards substitutions increases when the electronegativity of the metal is low and its size is high. The reactivity of metal alkoxides is also very sensitive to the steric hindrance of the alkoxy groups. It strongly decreases when the size of the OR group increases. The acidity of the medium also influences the rate of hydrolysis and condensation reaction to a great extent as well as the morphology of the products.

Non-hydrolytic routes

Non hydrolytic sol-gel chemistry has proved to be a promising route to metal oxides, and it has become a widely explored approach to synthesize metal oxide nanoparticles under various conditions. These methods involve either the self-condensation of various metal compounds or the thermolysis of metal coordination compounds. However, since water may be produced by the thermolysis of the organic derivatives, a hydrolytic pathway cannot be excluded. One of the most studied approaches involves the thermolytic decomposition of an inorganic complex at high temperatures. Two approaches include: the decomposition of Fe(acac) ₃ or FeCl ₃ and M(acac) ₂ salts, and the decomposition of Fe(CO) ₅ and M(acac) ₂ salts. For simple oxides (e.g., Fe ₃ O ₄ ) the precursor, e.g., Fe(acac) ₃ , is added to a suitable solvent heated to a temperature that allows for the rapid decomposition of the precursor. The choice of temperature and the temperature control (i.e., variation of the temperature during the reaction) are important in defining the resulting nanoparticle size and size distribution. By this method highly uniform nanoparticles can be obtained ( [link] ).

In addition to simple metal oxides (M _x O _y ) a range of mixed metal oxides can also be prepared. For example, nanospheres and nanocubes of cobalt ferrite can be obtained from cobalt and iron acetylacetonates, Co(acac) ₂ and Fe(acac) ₃ in solution in phenyl ether and hexadecanediol in the presence of oleic acid and oleylamine. Heating at 260 °C forms CoFe ₂ O ₄ spherical nanocrystals with a diameter of 5 nm. These nanocrystals serve as seeds for a new growth as the second step of the synthesis, giving perfect nanocubes from 8 to 12 nm, depending on the conditions. Nanocubes in the 8 nm range can also be used as seeds to obtain spheres. The tuning of the shape of ferrite nanocrystals is managed by the parameters of growth such as heating rate, temperature, reaction time, ratio of seed to precursors, and ratio of oleic acid, acting as surfactant stabilizing the nanocrystal, to oleylamine providing basic conditions needed for the formation of spinel oxide.