Page 53 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials Fabrication 39

If the suspensions are aged at an acidity where the solubility of the
solid is very low or at a minimum, the concentration of soluble species
in equilibrium with the solid phase does not allow an efficient transport
of matter, and crystallization of the early amorphous material will occur
more easily by a transformation in situ, in the solid state. The trans-
formation involves the diffusion of ions within the solid with partial
dehydration, and the formation of crystalline domains of very small
size. Nanoparticles of hematite, -Fe O , are so obtained from ferrihy-
3
2
drite at 6 pH 8 [23]. Very small nanoparticles of boehmite,
2 1
-Al(O)(OH), (around 300 m g ) are similarly obtained by aging of
aluminate gels at the same pH range (6 to 8) [20]. Although boehmite
is not the most thermodynamically stable phase at room temperature,
it is probably kinetically stabilized because the system is constrained
to evolve without heating and transforms on the lowest activation energy
path. Between pH 2 and pH 7, where the solubility of titania is very low,
the amorphous solid is transformed into TiO anatase nanoparticles
2
[21]. Over this acidity range, the particle size of anatase depends on the
pH of precipitation and aging. This effect of acidity on particle size will
be discussed later.
Precipitation by addition of a base at room temperature may also lead
to stable crystalline nanoparticles without involving any transformation by
the above mechanisms. For instance, magnetite Fe O is easily obtained by
3
4
3 2
coprecipitating aqueous Fe and Fe ions with x 0.66 [24]. Iron ions are
distributed into the octahedral (Oh) and tetrahedral (Td) sites of the face
3 3 2
centered cubic (fcc) stacking of oxygen according to [(Fe ) (Fe Fe ) O ].
Oh
Td
4
Magnetite is characterized by a fast electron hopping between the iron
cations on the octahedral sublattice. Crystallization of spinel is quasi-
2
immediate at room temperature, and electron transfer between Fe and
3
Fe ions plays a fundamental role in the process [25, 26]. In effect,
3 3
maghemite, –Fe O , [(Fe ) (Fe 5/3 V ) O ] (where V stands for a
Td
3
1/3 Oh
4
2
cationic vacancy) does not form directly in solution by precipitation of
2
ferric ions, but a small proportion of Fe ( 10 mol %) induces the
crystallization of all the iron into spinel. Studies of the early precipitate
2 2
revealed that all Fe ions were incorporated into a Fe -ferrihydrite,
forming a short-range ordered, mixed valence material exhibiting fast
electron hopping, as evidenced by Mössbauer spectroscopy [26]. Electron
mobility brings about local structural rearrangements and drives spinel
ordering. Besides this topotactic process, crystallization of spinel can also
proceed by dissolution crystallization, resulting in two families of non-
III III II
stoichiometric spinel particles [(Fe ) (Fe 1+2z/3 Fe 1 z V ) O ] with very
4
z/3 Oh
Td
different mean size [25]. The relative importance of these two path-
2
ways depends on the Fe level in the system, and the end products of
the coprecipitation are single phase only for 0.60 x 0.66. The com-
2 2
parison with the cases where M is different from Fe emphasizes the