Titanium dioxide has a band-gap in the ultra violet region and there
have been many efforts to shift light absorption to the visible region. In this
regard, surface modification with metal oxide clusters has been used to promote
band-gap reduction. CeO<sub>x</sub>-modified<sub> </sub>TiO<sub>2</sub> materials
have exhibited enhanced catalytic activity in water gas shift, but the
deposition process used is not well-understood or suitable for powder materials.
Atomic layer deposition (ALD) has been used for deposition of cerium oxide on
TiO<sub>2</sub>. The experimentally reported growth rates using typical Ce
metal precursors such as β-diketonates and cyclopentadienyls are low, with reported growth rates
of <i>ca. </i>0.2-0.4 Å/cycle. In this
paper, we have performed density functional theory calculations to reveal the reaction
mechanism of the metal precursor pulse together with experimental studies of ALD
of CeO<sub>x</sub> using two Ce precursors, Ce(TMHD)<sub>4</sub> and Ce(MeCp)<sub>3</sub>.
The nature and stability of hydroxyl groups on anatase and rutile TiO<sub>2</sub>
surfaces are determined and used as starting substrates. Adsorption of the
cerium precursors on the hydroxylated TiO<sub>2</sub> surfaces reduces the coverage
of surface hydroxyls. Computed activation barriers for ligand elimination in
Ce(MeCp)<sub>3</sub> indicate that ligand elimination is not possible on
anatase (101) and rutile (100) surface, but it is possible on anatase (001) and
rutile (110). The ligand elimination in Ce(TMHD)<sub>4</sub> is via breaking the
Ce-O bond and hydrogen transfer from hydroxyl groups. For this precursor, the ligand elimination on the majority surface
facets of anatase and rutile TiO<sub>2</sub> are endothermic and not favourable.
It is difficult to deposit Ce atom onto hydroxylated TiO<sub>2</sub> surface
using Ce(TMHD)<sub>4</sub> as precursor. Attempts for deposit cerium oxide on TiO<sub>2 </sub>nanoparticles
that expose the anatase (101) surface show at best a low deposition rate and
this can be explained by the non-favorable ligand elimination reactions at this
surface.
Air Annealing Effect on Oxygen Vacancy Defects in Al-doped ZnO Films Grown by High-Speed Atmospheric Atomic Layer Deposition
