Atomic Layer Deposition in Nanoporous Catalyst Materials - Atomically-Precise Methods for Synthesis of Solid Catalysts - page 172

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Figure 7.4 Electron tomography reconstructions of a thin HfO 2 ALD layer deposited
in a mesoporous titania film (initial porosity: 30%, thickness: 120 nm).
(a) 3D visualization of the reconstruction. (b) XZ-orthoslice through the
3D reconstruction with HfO 2 coating in light gray, titania matrix in dark
gray and voids in black. The detail shows conformally coated ink-bottle
shaped mesopores.
Adapted with permission from ref. 25. Copyright 2012 American
Chemical Society.

minimization (TVM) reconstruction algorithm. 35 To examine the inner
structure in more detail, slices were made through the 3D reconstruction
(Figure 7.4b). Dark areas represent voids, light gray zones correspond to the
HfO 2 coating and dark gray zones to the titania pore walls. The conformal
deposition of an ultra-thin HfO 2 layer (2.2 0.5 nm) was confirmed
throughout the whole mesoporous film.
More insights on the conformal coating of the mesoporous titania film
with HfO 2 were obtained from in situ synchrotron-based XRF measure-
ments. 36,37 Figure 7.5 shows the Hf XRF intensity against the number of ALD
cycles for deposition on the porous film as well as on a planar SiO 2 reference
substrate. During the first ca. 19 ALD cycles, the XRF intensity increased
faster in the mesoporous film than on the planar substrate, proving that
HfO 2 was deposited on the internal surface of the porous thin film. y With
progressing growth, the slope of the Hf XRF intensity curve decreased and
became more similar to the slope obtained for ALD on the planar reference
sample. This suggests that the pore necks became too narrow and were no
longer accessible for the TEMAH precursor, and that ALD continued on top
of the coated mesoporous film.
An X-ray reflectivity measurement after 70 ALD cycles on the planar ref-
erence substrate yielded a growth rate of 0.12 nm per cycle for the TEMAH/
H 2 O process. Assuming a similar growth rate in the titania ink-bottle pores,
19 ALD cycles would result in a 2.3 nm thick coating, which is in agreement

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y While a gradually decreasing slope was expected during the first ca. 19 ALD cycles, due to a
decreasing surface area with progressing growth, the slope of the Hf XRF intensity curve in-
creased during the first ca. 13 cycles and only then started to decrease. This could possibly be
explained by substrate-inhibited growth of HfO 2 on the mesoporous titania film, implying an
increasing growth rate during the first ALD cycles (ref. 17).