Page 52 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

38 Principles and Methods

spontaneously in suspension more or less quickly to form crystalline
nanoparticles, with possibly an increase in particle size, releasing simul-
taneously the lattice energy (and decreasing the surface energy) to
decrease the free enthalpy of the system. The acidity of the suspension
during evolution is the most important parameter to control crystalline
structure and the size of the final particles. Two distinct mechanisms
are involved in the transformation.
When the suspensions are aged at a pH where the solid is partially
soluble, the concentration in solution may be enough to feed nuclei of a
more stable crystalline phase. A transfer of matter occurs via the solu-
tion from the soluble amorphous phase toward a less soluble crystalline
phase during a slow dissolution-crystallization process allowing for-
mation of well-crystallized particles. Such a process is involved in the
formation of goethite, -Fe(O)(OH), during aging of ferrihydrite in sus-
pension at pH 5 or pH 10. Because of structural anisotropy of
goethite, rod-like particles of mean dimensions 150 25 15 nm are
obtained (Figure 3.3). These particles, anisotropic in shape, form very
stable concentrated suspensions, which behave as nematic lyotropic
liquid crystals exhibiting very interesting magnetic properties [22]. The
nematic phase aligns in a very low magnetic field (20 mT for samples
20 mm thick). The particles orient along the field direction at intensi-
ties smaller than 350 mT but reorient perpendicular to the field beyond
350 mT. This behavior could have interesting applications.
In similar ranges of acidity, the aluminate gel is transformed into
, gibbsite at pH 5, and bayerite at
platelets of hydroxide Al(OH) 3
pH 8 [20]. In a rather acidic medium (pH 1), the same dissolution-
crystallization mechanism transforms the amorphous titanium oxyhy-
droxide into elongated TiO rutile nanoparticles. In these examples, the
2
final size of particles depends on the acidity of the medium: the parti-
cle size increases when the acidity is strong.

300 nm
20 nm

[002]

[110]

150 nm
(a) (b) (c)
Figure 3.3 Particles of (a) goethite -Fe(O)(OH), (b) gibbsite Al(OH) 3 , and (c) rutile TiO 2
synthesized in aqueous medium.