5.3
Routes for the Preparation of Isolated Oxide Nanoparticles
5.3.1
Hydrolysis
Traditionally, a number of oxide materials have been prepared from aqueous solution using hydrolytic methods. Many of these preparation techniques lend themselves to the preparation of nanoparticles. The term hydrolysis is used in a number
of contexts, in most of which it involves breaking up of the water molecule [34]. It
is useful to imagine that the hydrolysis of a hydrated trivalent metal ion into an
oxide takes place in the following manner:
2M
3þ (H 2 O) 6 ! M 2 O 3 # þ 6H
þ þ 9H 2 O
Such a scheme indicates that the water molecule is decomposed in the process
and, in addition, that the formation of an oxide usually requires basic conditions.
However, in the preparation of oxides of amphoteric species such as Al(III) or
Ga(III), acid hydrolysis could be employed. For example:
2[Ga(OH) 4 ]
À ! Ga 2 O 3 # þ 3H 2 O þ 2(OH)
À
An important aspect of such hydrolysis is that not all ions are amenable to it. The
alkali, alkaline earth and rare-earth ions, for example, are too stable in solution,
and too ionic for hydrolysis to take place. In fact, the hydration reaction is the one
that would be favored. For La
3þ for example, the equilibrium:
La 2 O 3 # þ 6H
þ þ 9H 2 O $ 2La
3þ (H 2 O) 6
can be shifted completely to the left only at temperatures as high as 1000 K.
Despite these limitations, hydrolytic routes have been the mainstay of many
preparations of nanoparticulate oxide materials, particularly of the first row transition metals. It is of interest to note that much of the reported literature on hydrolytic routes concerns aqueous systems and it would not be incorrect to say that
much needs to be done on the use of solvents other than water, particularly aprotic
solvents, where hydrolysis may be more effective. The use of electrochemistry to
assist in hydrolysis is a technique that has found much use in the preparation of
transition metal oxide films [35], but it’s use in the preparation of oxide nanoparticles is not widespread.
Magnetic oxide (typically ferrite spinels or g-Fe 2 O 3 ) nanoparticles, when dispersed at high concentrations in water or oil, form ferrofluids [36]. Particle sizes in
these materials are usually in the 5–15 nm range – small enough that neither
magnetic nor gravitational fields should cause their precipitation. The many uses
of ferrofluids in magnetic seals, bearings, dampers etc. [36] have resulted in an
extensive body of literature on the preparation and properties of dispersions of
5 Oxide Nanoparticles
98
Routes for the Preparation of Isolated Oxide Nanoparticles
5.3.1
Hydrolysis
Traditionally, a number of oxide materials have been prepared from aqueous solution using hydrolytic methods. Many of these preparation techniques lend themselves to the preparation of nanoparticles. The term hydrolysis is used in a number
of contexts, in most of which it involves breaking up of the water molecule [34]. It
is useful to imagine that the hydrolysis of a hydrated trivalent metal ion into an
oxide takes place in the following manner:
2M
3þ (H 2 O) 6 ! M 2 O 3 # þ 6H
þ þ 9H 2 O
Such a scheme indicates that the water molecule is decomposed in the process
and, in addition, that the formation of an oxide usually requires basic conditions.
However, in the preparation of oxides of amphoteric species such as Al(III) or
Ga(III), acid hydrolysis could be employed. For example:
2[Ga(OH) 4 ]
À ! Ga 2 O 3 # þ 3H 2 O þ 2(OH)
À
An important aspect of such hydrolysis is that not all ions are amenable to it. The
alkali, alkaline earth and rare-earth ions, for example, are too stable in solution,
and too ionic for hydrolysis to take place. In fact, the hydration reaction is the one
that would be favored. For La
3þ for example, the equilibrium:
La 2 O 3 # þ 6H
þ þ 9H 2 O $ 2La
3þ (H 2 O) 6
can be shifted completely to the left only at temperatures as high as 1000 K.
Despite these limitations, hydrolytic routes have been the mainstay of many
preparations of nanoparticulate oxide materials, particularly of the first row transition metals. It is of interest to note that much of the reported literature on hydrolytic routes concerns aqueous systems and it would not be incorrect to say that
much needs to be done on the use of solvents other than water, particularly aprotic
solvents, where hydrolysis may be more effective. The use of electrochemistry to
assist in hydrolysis is a technique that has found much use in the preparation of
transition metal oxide films [35], but it’s use in the preparation of oxide nanoparticles is not widespread.
Magnetic oxide (typically ferrite spinels or g-Fe 2 O 3 ) nanoparticles, when dispersed at high concentrations in water or oil, form ferrofluids [36]. Particle sizes in
these materials are usually in the 5–15 nm range – small enough that neither
magnetic nor gravitational fields should cause their precipitation. The many uses
of ferrofluids in magnetic seals, bearings, dampers etc. [36] have resulted in an
extensive body of literature on the preparation and properties of dispersions of
5 Oxide Nanoparticles
98