Environmental Engineering Reference
In-Depth Information
different chemical compositions; e.g., SiO
2
on g-Fe
2
O
3
); (ii) homogeneous coat-
ings (on a substrate of the same chemical compositions; e.g., SiO
2
on SiO
2
); and
(iii) homogeneous nucleation (e.g., formation of SiO
2
nuclei) followed by
homogeneous coatings. In general, the lowest supersaturation level is required
for homogeneous coatings, followed by heterogeneous coatings, and finally
homogeneous nucleation, which requires an excess energy, as predicted by the
Kelvin equation, to account for extremely high curvatures of nuclei [87, 88].
Although in general, homogeneous nucleation can be avoided by careful con-
trol of (silica) supersaturation level just above the critical concentration of
heterogeneous coatings (on maghemite), the homogeneous surface coating
often presents a challenge to uniform surface coatings. It is evident that, as
soon as the substrate is coated with silica even at the submonolayer level, the
growth of the coated area (a process similar to homogeneous coating) prevails
because it requires a lower supersaturation level. As a result, a non-uniform and
patchwise (island) coating, as schematically shown in Fig. 6.6b, is often
obtained. With the dense-liquid-coating process, it is therefore inevitable to
expose substrate cores to the environment and poison the system by the released
species, unless a thick coating layer is applied.
6.4.3.3 Two-Step Coating
It is clear that neither the sol-gel nor the dense-liquid process could meet the
requirement of making magnetic nanocomposites of certain technological
applications. To coat magnetic particles with a thin protective silica layer and
minimize reduction of saturation magnetization, a novel two-step coating
process (the sol-gel followed by the dense-liquid coating) has been developed
in our laboratory [89]. This approach is based on the idea that the sol-gel
process can coat a surface uniformly, although the film is often porous, as
shown in Fig. 6.6a. In the second step, using the dense-liquid process, the
residual ethoxy groups in nano or microsize pores of the silica film prepared
using the sol-gel process are further hydrolyzed, and the pores are anticipated
to be closed by and filled with silica under low supersaturation conditions. It is
clear that the two-step silica-coating process integrates the advantages of uni-
form coatings by the sol-gel process and a low supersaturation level required
for homogeneous coating by the dense-liquid process. As a result, a uniform
thin silica layer, as shown in Fig. 6.6c, can be coated on maghemite or any other
magnetic nanoparticles to protect the particles with minimal reduction in
saturation magnetization (a key feature of magnetic nanocomposite sorbent)
and to provide a surface for further functionalization. It is important to note
that the objective of the two-step coating is not to coat more silica on the
particles but rather to protect the substrate particles with the thinnest silica
coatings possible to maximize the magnetic property of the coated particles.
For comparison, the silica was coated on g-Fe
2
O
3
at the 11 wt% silica level
using these three methods. The presence of a silicon band at 103.4 eV and an
additional oxygen band at 532.8 eV on the XPS spectra of coated particles
