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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.


SEM top-view images of annealed Al2O3-coated(1 nm) TiO2 nanotube layers before (a) and after soakingin H3PO4 solutions with different concentrations:(b) 50 wt %, (c) 70 wt %, and (d) 85 wt % in total for 48 h (last8 h at 60 °C). SEM top-view images of reference uncoated amorphous(e) and anatase (f) TiO2 nanotube layers after soakingin H3PO4 solutions of 10 and 40 wt %, respectively,for 24 h. All the scale bars denote 100 nm.
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fig5: SEM top-view images of annealed Al2O3-coated(1 nm) TiO2 nanotube layers before (a) and after soakingin H3PO4 solutions with different concentrations:(b) 50 wt %, (c) 70 wt %, and (d) 85 wt % in total for 48 h (last8 h at 60 °C). SEM top-view images of reference uncoated amorphous(e) and anatase (f) TiO2 nanotube layers after soakingin H3PO4 solutions of 10 and 40 wt %, respectively,for 24 h. All the scale bars denote 100 nm.

Mentions: Finally, the chemical stabilityof Al2O3-coated TiO2 nanotube layerswas investigated in strongly acidic environment, namely in H3PO4 solutions with different concentration. Such stabilityis significant for the nanotube layers to sustain in environments,where up to now they could not preserve their morphological integrity.For example, in various biological environments with low pH, the knowledgeabout the stability threshold is important. As it can be seen in Figure 5, even the thinnestcoating (1 nm Al2O3) completely preserved theTiO2 nanotube layers from degradation in concentrated H3PO4. In line with that no degradation was observedfor any of the thicker Al2O3 coatings: 10 and42 nm (data not shown here). The soaking tests were performed on the40 h time scale for all nanotube layers. The Al2O3 coated ones survived without any change in H3PO4 of all used concentrations. This expands the already wide rangeof environments, where ALD Al2O3 coatings arestable, in addition to published stability results of these coatingsin various acidic (H2SO4, HNO3, HCl)and alkaline (KOH) environments60 and water.61 To make the H3PO4 environmenteven more harsh, the H3PO4 solutions were heatedup to 60 °C, and soaking was carried out for an additional 8h (in total 48 h). Since again no visible changes were observed, soakingexperiments were terminated afterward. Reference uncoated layers (namelyas-anodized amorphous and annealed (400 °C for 1 h) nanotubelayers) did not survive these conditions. In order to determine thechemical threshold conditions for these reference uncoated layers,lower H3PO4 concentrations had to be used. Thestability threshold was revealed to be 10 wt % (Figure 5e) and 40 wt % (Figure 5f) on the scale of 24 h for the amorphousand annealed case, respectively, without any heating. All in all,the results presented in Figure 5 for Al2O3-coated nanotube layersconfirm the outstanding enhancement of the chemical stability of TiO2 nanotube layers provided by uniform Al2O3 coatings.


Atomic Layer Deposition Al 2 O 3 Coatings Significantly Improve Thermal, Chemical, and MechanicalStability of Anodic TiO 2 Nanotube Layers
SEM top-view images of annealed Al2O3-coated(1 nm) TiO2 nanotube layers before (a) and after soakingin H3PO4 solutions with different concentrations:(b) 50 wt %, (c) 70 wt %, and (d) 85 wt % in total for 48 h (last8 h at 60 °C). SEM top-view images of reference uncoated amorphous(e) and anatase (f) TiO2 nanotube layers after soakingin H3PO4 solutions of 10 and 40 wt %, respectively,for 24 h. All the scale bars denote 100 nm.
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fig5: SEM top-view images of annealed Al2O3-coated(1 nm) TiO2 nanotube layers before (a) and after soakingin H3PO4 solutions with different concentrations:(b) 50 wt %, (c) 70 wt %, and (d) 85 wt % in total for 48 h (last8 h at 60 °C). SEM top-view images of reference uncoated amorphous(e) and anatase (f) TiO2 nanotube layers after soakingin H3PO4 solutions of 10 and 40 wt %, respectively,for 24 h. All the scale bars denote 100 nm.
Mentions: Finally, the chemical stabilityof Al2O3-coated TiO2 nanotube layerswas investigated in strongly acidic environment, namely in H3PO4 solutions with different concentration. Such stabilityis significant for the nanotube layers to sustain in environments,where up to now they could not preserve their morphological integrity.For example, in various biological environments with low pH, the knowledgeabout the stability threshold is important. As it can be seen in Figure 5, even the thinnestcoating (1 nm Al2O3) completely preserved theTiO2 nanotube layers from degradation in concentrated H3PO4. In line with that no degradation was observedfor any of the thicker Al2O3 coatings: 10 and42 nm (data not shown here). The soaking tests were performed on the40 h time scale for all nanotube layers. The Al2O3 coated ones survived without any change in H3PO4 of all used concentrations. This expands the already wide rangeof environments, where ALD Al2O3 coatings arestable, in addition to published stability results of these coatingsin various acidic (H2SO4, HNO3, HCl)and alkaline (KOH) environments60 and water.61 To make the H3PO4 environmenteven more harsh, the H3PO4 solutions were heatedup to 60 °C, and soaking was carried out for an additional 8h (in total 48 h). Since again no visible changes were observed, soakingexperiments were terminated afterward. Reference uncoated layers (namelyas-anodized amorphous and annealed (400 °C for 1 h) nanotubelayers) did not survive these conditions. In order to determine thechemical threshold conditions for these reference uncoated layers,lower H3PO4 concentrations had to be used. Thestability threshold was revealed to be 10 wt % (Figure 5e) and 40 wt % (Figure 5f) on the scale of 24 h for the amorphousand annealed case, respectively, without any heating. All in all,the results presented in Figure 5 for Al2O3-coated nanotube layersconfirm the outstanding enhancement of the chemical stability of TiO2 nanotube layers provided by uniform Al2O3 coatings.

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.