Environmental Engineering Reference
In-Depth Information
confirms the coating of silica on g-Fe
2
O
3
. By the analysis of XPS narrow-scan
spectra, semi-quantitative analysis showed an area ratio of silica to iron band
(Si/Fe) of 0.7, 4.1, and 4.1 for particles coated using the dense-liquid, sol-gel, and
two-step processes, respectively. A higher Si/Fe ratio indicates a high degree of
silica coating. It is evident that the silica-coating efficiency is comparable for the
sol-gel and two-step processes, higher than that of the dense-liquid process.
The formation of silaxone bonds was confirmed for all the coatings by
DRIFTS [48]. In contrast to the dense-liquid or two-step-coating process,
unhydrolyzed ethoxy was detected for particles coated using only the sol-gel
process. The presence of Si-OEt terminal groups is partially responsible for the
porous nature of the coated films by the sol-gel process.
To further study the surface properties of the magnetic nanocomposite
particles, the electrokinetics of the particles was measured, and the obtained
IEP for maghemite particles coated with different methods are summarized in
Table 6.1. As shown, the IEP of pH 3 for maghemite coated using the dense-
liquid process is significantly lower than that for uncoated maghemite particles
(pH
IEP
4.5). However, this value is higher than the IEP of fused silica (pH
IEP
2.1), suggesting that silica was coated on maghemite, but only partially (prob-
ably in the form of islands) by the dense-liquid process, as illustrated schema-
tically in Fig. 6.6b. When the sol-gel process was used, an IEP of pH 2.4 was
obtained, showing an improved coating compared with the dense-liquid pro-
cess. This value, however, remains slightly higher than the IEP of fused silica,
indicating that the surface coating is either incomplete or porous, as confirmed
later by leaching tests (Table 6.1). When the two-step process was used, an
identical electrokinetic behavior between the coated particles and fused silica
was observed over the pH range studied, indicating the same surface properties
between the two and confirming a full coverage of particles with silica. It is
important to note that silica-like electrokinetics of silica-coated maghemite with
the two-step process ensures its dispersion, as required in many of its techno-
logical applications.
Leaching tests were conducted to further examine the state of silica film on
g-Fe
2
O
3
. The results also are summarized in Table 6.1 along with the saturation
magnetization of coated particles. It is evident that the amount of iron leached
out was below the detection limits for the particles coated with the two-step
process in contrast to the single-step, either sol-gel or dense-liquid process
where 1.1 or 2.8 mg of Fe per gram of particles was detected, respectively.
Table 6.1 Characteristics and stability of silica-coated g-Fe
2
O
3
by dense liquid, sol-gel, and
two-step coating processes
Sample and methods
IEP (pH)
Fe leached in 0.01 M HCl (mg/g)
M
s
(emu/g)
g-Fe
2
O
3
4.5
60.3
52.0
DL coating
3.0
2.8
48.5
Sol-gel
2.4
1.1
43.2
Two-step
2.2
42.5
