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In-Depth Information
7.2.3 ALD for Photocatalysis
ALD has been used to synthesize photoactive nanostructures by conformal
deposition of TiO
2
on nanoporous anodic aluminium oxide (AAO) mem-
branes and arrays of Ni nanorods.
82,83
In other studies, AAO membranes
were used as template for the fabrication of TiO
2
nanotube arrays by
ALD.
84,85
Formation of the active anatase phase was achieved during the
deposition process
82
or via post-deposition annealing.
84,85
Recent work,
detailed in the next section (Case 2), used ALD of TiO
2
followed by a thermal
treatment for the synthesis of anatase nanoparticles supported on a meso-
porous silica film.
51
All nanostructured TiO
2
materials exhibited higher
photocatalytic activity than planar TiO
2
reference samples, due their larger
surface area.
51,82-85
Adding noble metal particles to the surface of a TiO
2
photocatalyst can
improve the photocatalytic performance. This effect is often attributed to the
noble metal acting as an electron trap, thus suppressing electron-hole re-
combination.
86
Kemell et al. prepared a photocatalyst by first applying TiO
2
ALD and then Ir ALD on cellulose fibers.
87
The Ir process using Ir(acac)
3
resulted in Ir islands on the TiO
2
surface, which caused an enhancement of
the photocatalytic activity of the TiO
2
/cellulose composite, as evaluated in
the photo-degradation of methylene blue.
More recently, Zhou et al. showed that the photoactivity of TiO
2
nano-
powder
888
could be enhanced by loading it with highly dispersed Pt nano-
particles using ALD.
88
The authors systematically studied the effect of the
number of ALD cycles and the deposition temperature on the Pt deposition
and the photocatalytic activity, and concluded that one single MeCpPtMe
3
/
O
2
cycle deposited at 350 C1 was optimal for the photodegradation of
methylene blue. In a second study of the same authors, a magnetic photo-
catalyst was created by ALD of TiO
2
thin films on iron-based magnetic
nanoparticles.
89
d
n
9
r
4
n
g
|
7
.
7.2.4 Synthesis of Catalytic Membranes by ALD
Using AAO membranes as supports, Stair and co-workers demonstrated the
suitability of ALD for synthesizing catalytic membranes with precise control
of pore wall composition and pore size (10 to 100 nm).
52,90-92
AAO mem-
branes supported by an aluminium ring were infiltrated with 1 nm Al
2
O
3
and
subsequently loaded with VO
x
by either wet impregnation or ALD.
92
The
prepared catalysts were tested in the oxidative dehydrogenation of cyclo-
hexane. In order to force the reactants to flow through the nanopores of the
membrane, the coated AAO structures were placed in a standard Swagelok
VCR fitting. This sealed structure was called a nanolith. It was found that the
effective catalytic activity of the ALD-loaded catalysts was substantially
888
Aeroxide P25 TiO
2
(Evonik): surface area 50 m
2
g
1
.
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