Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 189

reinsertion of Al in regenerated catalysts that are de-aluminated due to re-
moval of the deposited coke via calcination.

d n 9 r 4 n g | 7

7.3.2 Case 2: Introducing Photo-Active Nanoparticles in
Mesoporous Films
TiO 2 nanomaterials play an important role in photocatalytic air and water
remediation and in solar energy conversion. 86,107,108 Nanosized powders are
among the best photocatalysts but their separation after a water purification
process is time consuming. Nanopowders also present a risk to human
health making their immobilization of vital importance in gas phase ap-
plications. Another issue is the loss in exposed surface area due to ag-
glomeration of powder particles. These problems can be circumvented by
direct synthesis of surface-immobilized TiO 2 nanostructures. Because ana-
tase TiO 2 nanoparticles exhibit enhanced photocatalytic activity for very
small particle sizes ( o 10 nm), 107,109 the TiO 2 nanostructures should ideally
consist of defect-free nanometer-sized anatase particles stacked in a porous
arrangement or well dispersed on a large surface area support.
Mesoporous silica films are often used for the preparation of photoactive
surfaces. Incorporation of TiO 2 in mesoporous silica films is mostly realized
during synthesis of the film, 110-116 e.g., by spin coating a suspension of silica
precursor, polymeric templates and colloidal TiO 2 nanoparticles on a sub-
strate, followed by thermal treatment. 114-116 These direct approaches typi-
cally result in TiO 2 -SiO 2 composite films with anatase nanocrystals
contained within the walls of the porous structure. Only limited papers have
used a post-synthesis method for introducing TiO 2 nanoparticles in the pore
channels of mesoporous silica thin films; in the studies by Hua et al. 117 and
Kohno et al. 118 nanometer-sized TiO 2 crystals were synthesized in the
mesopores of an ordered silica network through impregnation of a Ti pre-
cursor and subsequent calcination.
This case study presents an alternative post-synthesis approach com-
prising ALD followed by a thermal treatment to introduce anatase particles
into mesopores. 51 Mesoporous silica films with an exceptional high porosity
(75%) and an average pore size of 12 nm were selected as support ma-
terial. 119 An amorphous TiO 2 layer was conformally deposited using 50 ALD
cycles of the TDMAT/H 2 O process at 200 1C. According to ellipsometric
porosimetry measurements the average pore size decreased to 6 nm, indi-
cating a TiO 2 film thickness of ca. 3 nm. After ALD, the amorphous as-
deposited TiO 2 layer was converted into crystalline anatase by calcination of
the sample in air for 5 h at 550 1C starting at room temperature and heating
at a rate of 1 1Cmin 1 .
Quantitative electron tomography was used to investigate the morphology
of the crystallized TiO 2 coating. Figure 7.18a shows an XZ-orthoslice through
the 3D reconstruction of the porous film obtained using conventional
reconstruction algorithms. 120

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The white dots correspond to TiO 2