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and in the ordered 3D mesoporous network of Zeotile-4 material,
respectively.
7.1.2.2.1 Isotropic Infiltration of Al
2
O
3
ALD in Silica Gel. Elam et al.in-
vestigated the infiltration of Al
2
O
3
ALD in mesoporous silica gel spheres
with a particle size of 75-200 mm.
38
The silica gel exhibited an internal
surface area of 99.6 m
2
g
1
and an average pore size of 30 nm. The con-
ditions for reaching saturation in 1 g of this mesoporous material were
determined via weight-gain experiments. At a TMA partial pressure of
ca. 20 Pa, 90 s of exposure were sucient to fully cover the interior surface
of the spherical silica particles. Using cross-sectional scanning electron
microscopy (SEM) in combination with energy dispersive X-ray analysis
(EDX), the spreading of the Al
2
O
3
material in a silica gel sphere was investi-
gated for unsaturated exposure times of 5, 15 and 30 s (Figure 7.6). The
EDX elemental maps of Al revealed the diffusion-limited nature of the TMA/
H
2
O ALD process in mesoporous silica gel. Coating of the interior surface
occurred progressively from the outside of the particles to the core. The
authors further reported that the rate of Al
2
O
3
uptake in the silica gel was
not only governed by diffusional limitations but also by the rate of pre-
cursor delivery in their experiments.
d
n
9
r
4
n
g
|
7
7.1.2.2.2 Anisotropic Infiltration of TiO
2
ALD in Zeotile-4 Material. Sree
et al. studied the infiltration of TiO
2
ALD in Zeotile-4 powder with an
average particle size of 0.5 mm and a total surface area of 1224 m
2
g
1
.
39
Zeotile-4 material is characterized by a 3D mesoporosity that is achieved
via stacking and annealing of silicalite nanoslabs.
40,41
The specific organ-
ization of the nanoslabs measuring ca. 8
4
2nm
3
creates about 8-10 nm
wide hexagonal parallel channels connected to each other via 2-4 nm wide
slits (Figure 7.7). The nanoslabs themselves have a zeolitic character and
present internal microporosity.
0.1 g of Zeotile-4 powder was highly dispersed onto the ca. 25 cm
2
bottom
of an aluminium tray and exposed to 30 cycles of the TDMAT/H
2
O process
.
Figure 7.6 Cross-sectional EDX elemental maps revealing the spreading of Al atoms
deposited in a spherical silica gel particle using five TMA/H
2
O ALD cycles
and TMA exposure times of 5, 15, 30 and 90 s. With progressing
deposition, an isotropic deposition front is penetrating to the core of
the unordered silica gel particle.
Adapted with permission from ref. 38. Copyright 2010 American
Chemical Society.
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