Abstract
The present work presents a strategy to stabilize amorphous anodic self-organized TiO2 nanotube layers against morphological changes and crystallization upon extensive water soaking. The growth of needle-like nanoparticles was observed on the outer and inner walls of amorphous nanotube layers after extensive water soakings, in line with the literature on water annealing. In contrary, when TiO2 nanotube layers uniformly coated by thin TiO2 using atomic layer deposition (ALD) were soaked in water, the growth rates of needle-like nanoparticles were substantially reduced. We investigated the soaking effects of ALD TiO2 coatings with different thicknesses and deposition temperatures. Sufficiently thick TiO2 coatings (≈8.4 nm) deposited at different ALD process temperatures efficiently hamper the reactions between water and F− ions, maintain the amorphous state, and preserve the original tubular morphology. This work demonstrates the possibility of having robust amorphous 1D TiO2 nanotube layers that are very stable in water. This is very practical for diverse biomedical applications that are accompanied by extensive contact with an aqueous environment.
Highlights
Various morphologies of TiO2 with nano scale dimensions have been extensively investigated as photo catalysts for H2 evolution, dye-sensitized solar cells (DSSCs), degradation of organic compounds, methanol oxidation, CO2 reduction, self-cleaning and anti-fogging, and many other applications (Chen and Mao, 2007; Schneider et al, 2014; Wang et al, 2014)
We extend the application of atomic layer deposition (ALD) TiO2 coatings as a protective coating of amorphous TiO2 nanotube layers to prevent their morphological changes, known as water annealing effect
The blank TiO2 nanotube layers and TiO2 coated (NALD = 150 at 300◦C) TiO2 nanotube layers are imaged by SEM in two regions of the nanotube layer, i.e., the top and the bottom
Summary
Various morphologies of TiO2 with nano scale dimensions have been extensively investigated as photo catalysts for H2 evolution, dye-sensitized solar cells (DSSCs), degradation of organic compounds, methanol oxidation, CO2 reduction, self-cleaning and anti-fogging, and many other applications (Chen and Mao, 2007; Schneider et al, 2014; Wang et al, 2014). In the last 15 years, the anodic self-organized TiO2 nanotube layers have attracted scientific interests in the mentioned areas. This is mainly attributed to the controllable geometry and large specific surface area of the anodic TiO2 nanotube layers which allow higher reaction activities as well as the one-dimensional (1D) orientation which offers unidirectional charge transport from the tubes to the supporting Ti substrate (Macak et al, 2007; Lee et al, 2014; Wang et al, 2014). On top of its hemocompatibility (Huang et al, 2017), the tubular morphology is an added advantage for genes, drugs, and therapeutic carrier or reservoirs, for example, gentamicin sulfate, chitosan, bone morphogenetic protein 2, and tumor necrosis factor-related apoptosis-inducing ligand (Hu et al, 2012; Feng et al, 2016; Kaur et al, 2016)
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