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Atomic Layer Deposition Al 2 O 3 Coatings Significantly Improve Thermal, Chemical, and MechanicalStability of Anodic TiO 2 Nanotube Layers

View Article: PubMed Central - PubMed

ABSTRACT

We report on a verysignificant enhancement of the thermal, chemical,and mechanical stability of self-organized TiO2 nanotubeslayers, provided by thin Al2O3 coatings of differentthicknesses prepared by atomic layer deposition (ALD). TiO2 nanotube layers coated with Al2O3 coatingsexhibit significantly improved thermal stability as illustrated bythe preservation of the nanotubular structure upon annealing treatmentat high temperatures (870 °C). In addition, a high anatase contentis preserved in the nanotube layers against expectation of the totalrutile conversion at such a high temperature. Hardness of the resultingnanotube layers is investigated by nanoindentation measurements andshows strongly improved values compared to uncoated counterparts.Finally, it is demonstrated that Al2O3 coatingsguarantee unprecedented chemical stability of TiO2 nanotubelayers in harsh environments of concentrated H3PO4 solutions.

No MeSH data available.


Representative STEM-HAADFimages of (a) a fragment of Al2O3 coated (10nm) TiO2 nanotube and (b) thecorresponding higher magnification of the nanotube wall. Interfacesbetween individual parts of the tubes are distinguished by solid linesand appropriate description.
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fig2: Representative STEM-HAADFimages of (a) a fragment of Al2O3 coated (10nm) TiO2 nanotube and (b) thecorresponding higher magnification of the nanotube wall. Interfacesbetween individual parts of the tubes are distinguished by solid linesand appropriate description.

Mentions: Figure 2 showsrepresentativeSTEM-HAADF images of the nanotube body (separated from the annealedAl2O3 coated (10 nm) TiO2 nanotubelayer by mechanical bending of the layers followed by sonication inmethanol) at a low (a) and at a high magnification (b). Especiallyfrom Figure 2b, theinterface between the TiO2 wall and Al2O3 coating is well distinguishable. There are actually two interfacesbetween the TiO2 wall and Al2O3 coating,as the Al2O3 coating is deposited inside (interiorcoating) and outside (exterior coating) the TiO2 tube walls.This feature is in accordance with our previous ALD work,36 where we showed very good uniformity of Al2O3 coatings on the amorphous tubes and absenceof any pinholes in the coating. As apparent from Figure 2, Al2O3 coatings remained continuous and pinhole-free even after annealing,during which thermally induced crystallization of TiO2 tubewalls occurred. Some delamination of the coating seen at the outerand inner interface between TiO2 wall and Al2O3 coating (especially at Figure 2b) stems most likely from the stress thatthese layers are exposed to during the preparation of specimens forSEM and STEM observation, which includes mechanical rupture of layers.These roughening and delamination events have no detrimental effecton coated nanotube layers that were not submitted for SEM and STEMand that completely survived soaking in H3PO4 solutions (described later in text).


Atomic Layer Deposition Al 2 O 3 Coatings Significantly Improve Thermal, Chemical, and MechanicalStability of Anodic TiO 2 Nanotube Layers
Representative STEM-HAADFimages of (a) a fragment of Al2O3 coated (10nm) TiO2 nanotube and (b) thecorresponding higher magnification of the nanotube wall. Interfacesbetween individual parts of the tubes are distinguished by solid linesand appropriate description.
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Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382572&req=5

fig2: Representative STEM-HAADFimages of (a) a fragment of Al2O3 coated (10nm) TiO2 nanotube and (b) thecorresponding higher magnification of the nanotube wall. Interfacesbetween individual parts of the tubes are distinguished by solid linesand appropriate description.
Mentions: Figure 2 showsrepresentativeSTEM-HAADF images of the nanotube body (separated from the annealedAl2O3 coated (10 nm) TiO2 nanotubelayer by mechanical bending of the layers followed by sonication inmethanol) at a low (a) and at a high magnification (b). Especiallyfrom Figure 2b, theinterface between the TiO2 wall and Al2O3 coating is well distinguishable. There are actually two interfacesbetween the TiO2 wall and Al2O3 coating,as the Al2O3 coating is deposited inside (interiorcoating) and outside (exterior coating) the TiO2 tube walls.This feature is in accordance with our previous ALD work,36 where we showed very good uniformity of Al2O3 coatings on the amorphous tubes and absenceof any pinholes in the coating. As apparent from Figure 2, Al2O3 coatings remained continuous and pinhole-free even after annealing,during which thermally induced crystallization of TiO2 tubewalls occurred. Some delamination of the coating seen at the outerand inner interface between TiO2 wall and Al2O3 coating (especially at Figure 2b) stems most likely from the stress thatthese layers are exposed to during the preparation of specimens forSEM and STEM observation, which includes mechanical rupture of layers.These roughening and delamination events have no detrimental effecton coated nanotube layers that were not submitted for SEM and STEMand that completely survived soaking in H3PO4 solutions (described later in text).

View Article: PubMed Central - PubMed

ABSTRACT

We report on a verysignificant enhancement of the thermal, chemical,and mechanical stability of self-organized TiO2 nanotubeslayers, provided by thin Al2O3 coatings of differentthicknesses prepared by atomic layer deposition (ALD). TiO2 nanotube layers coated with Al2O3 coatingsexhibit significantly improved thermal stability as illustrated bythe preservation of the nanotubular structure upon annealing treatmentat high temperatures (870 °C). In addition, a high anatase contentis preserved in the nanotube layers against expectation of the totalrutile conversion at such a high temperature. Hardness of the resultingnanotube layers is investigated by nanoindentation measurements andshows strongly improved values compared to uncoated counterparts.Finally, it is demonstrated that Al2O3 coatingsguarantee unprecedented chemical stability of TiO2 nanotubelayers in harsh environments of concentrated H3PO4 solutions.

No MeSH data available.