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

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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.


SEM top-view images ofuncoated TiO2 nanotube layer(a) before and (b) after annealing; Al2O3 coated(1 nm) TiO2 nanotube layer (c) before and (d) after annealing;Al2O3 coated (10 nm) TiO2 nanotubelayer (e) before and (f) after annealing. The annealing was carriedout at 870 °C for 1 h. Insets: magnification of the correspondingSEM images. All the scale bars denote 100 nm.
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fig1: SEM top-view images ofuncoated TiO2 nanotube layer(a) before and (b) after annealing; Al2O3 coated(1 nm) TiO2 nanotube layer (c) before and (d) after annealing;Al2O3 coated (10 nm) TiO2 nanotubelayer (e) before and (f) after annealing. The annealing was carriedout at 870 °C for 1 h. Insets: magnification of the correspondingSEM images. All the scale bars denote 100 nm.

Mentions: Highly ordered TiO2 nanotube layers, witha thickness of ≈20 μm and an average diameter value of≈110 nm (aspect ratio ≈180), were prepared by anodizationof Ti foils as described in detail in the ExperimentalSection. As-prepared amorphous TiO2 nanotube layerswere coated with Al2O3 of different nominalthicknesses, namely 1, 10, and 42 nm by ALD, as verified by SEM andellipsometric measurements (1.1 ± 0.2, 10 ± 0.5, and 44± 2.1 nm). Freshly coated nanotube layers were annealed at 870°C for 1 h along with reference uncoated TiO2 nanotubelayers. Figure 1 showsSEM images of the TiO2 nanotube layers with and withoutAl2O3 coating annealed at 870 °C. UncoatedTiO2 nanotube layers (Figure 1a) collapsed during the annealing processinto a pillar nanostructure (Figure 1b). When coated, the nanotube layers were preservedafter the annealing process, regardless of the thickness of the Al2O3, as apparent for coatings of either 1 nm (Figure 1c,d), or 10 nm (Figure 1e,f). It is quitefascinating that even 1 nm thin Al2O3 coatingcan built a very thermally robust cage all over TiO2 nanotubeswith some 20–40 nm thick tube walls.


Atomic Layer Deposition Al 2 O 3 Coatings Significantly Improve Thermal, Chemical, and MechanicalStability of Anodic TiO 2 Nanotube Layers
SEM top-view images ofuncoated TiO2 nanotube layer(a) before and (b) after annealing; Al2O3 coated(1 nm) TiO2 nanotube layer (c) before and (d) after annealing;Al2O3 coated (10 nm) TiO2 nanotubelayer (e) before and (f) after annealing. The annealing was carriedout at 870 °C for 1 h. Insets: magnification of the correspondingSEM images. All the scale bars denote 100 nm.
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fig1: SEM top-view images ofuncoated TiO2 nanotube layer(a) before and (b) after annealing; Al2O3 coated(1 nm) TiO2 nanotube layer (c) before and (d) after annealing;Al2O3 coated (10 nm) TiO2 nanotubelayer (e) before and (f) after annealing. The annealing was carriedout at 870 °C for 1 h. Insets: magnification of the correspondingSEM images. All the scale bars denote 100 nm.
Mentions: Highly ordered TiO2 nanotube layers, witha thickness of ≈20 μm and an average diameter value of≈110 nm (aspect ratio ≈180), were prepared by anodizationof Ti foils as described in detail in the ExperimentalSection. As-prepared amorphous TiO2 nanotube layerswere coated with Al2O3 of different nominalthicknesses, namely 1, 10, and 42 nm by ALD, as verified by SEM andellipsometric measurements (1.1 ± 0.2, 10 ± 0.5, and 44± 2.1 nm). Freshly coated nanotube layers were annealed at 870°C for 1 h along with reference uncoated TiO2 nanotubelayers. Figure 1 showsSEM images of the TiO2 nanotube layers with and withoutAl2O3 coating annealed at 870 °C. UncoatedTiO2 nanotube layers (Figure 1a) collapsed during the annealing processinto a pillar nanostructure (Figure 1b). When coated, the nanotube layers were preservedafter the annealing process, regardless of the thickness of the Al2O3, as apparent for coatings of either 1 nm (Figure 1c,d), or 10 nm (Figure 1e,f). It is quitefascinating that even 1 nm thin Al2O3 coatingcan built a very thermally robust cage all over TiO2 nanotubeswith some 20–40 nm thick tube walls.

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.