Amorphous titania
(am.-TiO2) has gained wide interest
in the field of photocatalysis, thanks to exceptional disorder-mediated
optical and electrical properties compared to crystalline TiO2. Here, we study the effects of intrinsic Ti3+ and
nitrogen defects in am.-TiO2 thin films via the atomic
layer deposition (ALD) chemistry of tetrakis(dimethylamido)titanium(IV)
(TDMAT) and H2O precursors at growth temperatures of 100–200
°C. X-ray photoelectron spectroscopy (XPS) and computational
analysis allow us to identify structural disorder-induced penta- and
heptacoordinated Ti4+ ions (Ti5/7c
4+), which are related to the formation of Ti3+ defects
in am.-TiO2. The Ti3+-rich ALD-grown am.-TiO2 has stoichiometric composition, which is explained by the
formation of interstitial peroxo species with oxygen vacancies. The
occupation of Ti3+ 3d in-gap states increases with the
ALD growth temperature, inducing both visible-light absorption and
electrical conductivity via the polaron hopping mechanism. At 200
°C, the in-gap states become fully occupied extending the lifetime
of photoexcited charge carriers from the picosecond to the nanosecond
time domain. Nitrogen traces from the TDMAT precursor had no effect
on optical properties and only little on charge transfer properties.
These results provide insights into the charge transfer properties
of ALD-grown am.-TiO2 that are essential to the performance
of protective photoelectrode coatings in photoelectrochemical solar
fuel reactors.
Oxides
are an important class of materials and are widely used,
for example, as supports in heterogeneous catalysis. In a number of
industrial catalytic processes, oxide supports actively participate
in chemical transformations by releasing lattice oxygen anions. While
this is intuitively understood for reducible oxides, the reducibility
of irreducible oxides may be modified via nanoengineering or upon
inclusion of foreign species. Our calculations predict that the ability
of irreducible monoclinic zirconia to release oxygen improves substantially
upon deposition of rhodium. Through a comprehensive screening of Rh/ZrO2 with different size of the rhodium species, we find that
a Rh adatom and a Rh4 nanocluster have the largest impact
on the reducibility of zirconia. With increasing size the effect of
rhodium decays. Our findings demonstrate that the phenomenon of enhanced
reducibility of irreducible oxides in the presence of metals should
be considered when interpreting experimental and computational results,
as reactions that involve release of oxygen from an oxide support
might be possible for irreducible oxides.
Kinetic and thermodynamic stability of single-atom and nanocluster catalysts is addressed under reaction conditions within a DFT-parametrised multi-scale thermodynamic framework combining atomistic, non-equilibrium, and nanothermodynamics.
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