We report on a very
significant enhancement of the thermal, chemical,
and mechanical stability of self-organized TiO2 nanotubes
layers, provided by thin Al2O3 coatings of different
thicknesses prepared by atomic layer deposition (ALD). TiO2 nanotube layers coated with Al2O3 coatings
exhibit significantly improved thermal stability as illustrated by
the preservation of the nanotubular structure upon annealing treatment
at high temperatures (870 °C). In addition, a high anatase content
is preserved in the nanotube layers against expectation of the total
rutile conversion at such a high temperature. Hardness of the resulting
nanotube layers is investigated by nanoindentation measurements and
shows strongly improved values compared to uncoated counterparts.
Finally, it is demonstrated that Al2O3 coatings
guarantee unprecedented chemical stability of TiO2 nanotube
layers in harsh environments of concentrated H3PO4 solutions.
The formation of strong metal support interactions (SMSI) is known for many metal/metal oxide systems and its consequences are well established in the field of heterogeneous catalysis, but this knowledge has only been recently transferred to the field of electrocatalysis. In this study, Pt was deposited via atomic layer deposition (ALD) onto TiO2−Y, which allowed a good control of the particle size through the number of ALD cycles. During the ALD process, a thin-film of reduced titania is formed on the Pt surface, which leads to SMSI effects. With increasing Pt particle size, the fraction of the titania-covered Pt surface decreases. As a result, the extent of platinum oxide formation in cyclic voltammetry (CV) measurements scales with the size of the Pt particles. The influence of these thin titanium oxide films, which cover the Pt surface, on the catalytic behavior with respect to oxygen reduction reaction (ORR), hydrogen oxidation reaction (HOR), CO oxidation and oxygen evolution reaction (OER) is investigated by using an RDE setup. The covering TiOX thin-films reduce the ability to catalyze ORR, OER and CO oxidation, while it does not influence the HOR and Pt H-UPD formation. These findings indicate that proton and hydrogen transport are possible through the thin TiOX film, while oxygenated species suffer from transport limitations through the thin-film. Due to this selective permeability, these materials are able to oxidize hydrogen well beyond 1.2 VRHE.
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