Page 68 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

54 Principles and Methods

controlling the molar ratio Fe(CO) to oleic acid. Thermal decomposition
5
of Fe(CO) in solution containing dodecylamine as a capping ligand and
5
under aerobic conditions forms also -Fe O nanoparticles with diamond,
2
3
sphere, and triangle shapes with similar 12-nm size [69]. Uniform-sized
MnO nanospheres and nanorods are obtained by heating at 300 C the
mixture of Mn (CO) with oleylamine in trioctylphosphine (TOP) [70].
10
2
The size of nanospheres can be varied from 5 to 40 nm depending on the
duration of heating, using phosphines both as solvent and stabilizing
agent (Figure 3.14). With TOP, 10 nm MnO particles can be obtained.
If the surfactant complex is rapidly injected into a solution of TOP at
330 C, nanorods 8 140 nm of MnO are produced. In fact, these rods
are polycrystalline. They are formed by an aggregation of spheres with
oriented attachment and having a core shell structure with a thin Mn O 4
3
shell. Heating of W(CO) at 270 C for 2 hours in trimethylamine oxide
6
in the presence of oleylamine forms uniform nanorods of tungsten oxide
with an X-ray diffraction pattern matching the W O reflections [71].
49
18
The lengths of the nanorods are controlled by the temperature and the
amount of oleylamine.
From minerals to materials
As discussed earlier, precursor sol-gels are traditionally prepared via the
hydrolysis of metal compounds. This “bottom-up” approach of reacting
small inorganic molecules to form oligomeric and polymeric materials is
a common approach for a wide range of metal and nonmetal oxides.
However, in the case of aluminum oxide nanoparticles, the relative rate
of the hydrolysis and condensation reactions often makes particle size
control difficult. The aluminum-based sol-gels formed during the hydrol-
ysis of aluminum compounds belong to a general class of compounds: alu-
moxanes. The term alumoxane is often given to aluminum oxide
macromolecules formed by the hydrolysis of aluminum compounds or
where X R, OR, OSiR , or O CR; however, it may also be used
salts, AlX 3 3 2 2
for any species containing an oxo (O ) bridge binding (at least) two alu-
minum atoms—that is, Al-O-Al. Alumoxanes were first reported in 1958
by Andrianov and Zhadanov [72], however, they have since been prepared
with a wide variety of substituents on aluminum. The structure of alu-
moxanes was proposed to consist of linear or cyclic chains (Figure 3.15)
analogous to that of poly-siloxanes [73]. Strictly speaking, the classifica-
tion of alumoxanes as polymers is slightly misleading since they are not
polymeric per se, but exist as three-dimensional cage structures [74–76].
For example, siloxy-alumoxanes, [Al(O)(OH) x (OSiR 3 ) 1 x ] n , consist of an
aluminum-oxygen nanoparticle ore structure (Figure 3.15c) analogous to
that found in the mineral boehmite, [Al(O)(OH)] n , with a siloxide substi-
tuted periphery [77–79]. Based on the knowledge of the boehmite-like
nanoparticle core structure of hydrolytically stable alumoxanes, it was