TiO2 ALD Coating of Amorphous TiO2 Nanotube Layers: Inhibition of the Structural and Morphological Changes Due to Water Annealing

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

ALD SnO2 coated anodic 1D TiO2 nanotube layers for low concentration NO2 sensing

The continuous emission of nitrous oxides contributes to the overall air pollution and deterioration of air quality. In particular, an effective NO2 sensor capable of low concentration detection for continuous monitoring is demanded for safety, health, and wellbeing. The sensing performance of a metal oxide based sensor is predominantly influenced by the availability of surface area for O2 adsorption and desorption, efficient charge transport and size or thickness of the sensing layer. In this study, we utilized anodic one-dimensional (1D) TiO2 nanotube layers of 5 µm thick which offer large surface area and unidirectional electron transport pathway as a platform to accommodate thin SnO2 coatings as a sensing layer. Conformal and homogeneous SnO2 coatings across the entire inner and outer TiO2 nanotubes were achieved by atomic layer deposition with controlled thickness of 4, 8 and 16 nm. The SnO2 coated TiO2 nanotube layers attained a higher sensing response than a reference Figaro SnO2 sensor. Specifically, the 8 nm SnO2 coated TiO2 nanotube layer has recorded up to ten-fold enhancement in response as compared to the blank nanotubes for the detection of 1 ppm NO2 at the operating temperature of 300 oC with 0.5 V applied bias. This is attributed to the SnO2/TiO2 heterojunction effect and controlled SnO2 thickness within the range of the Debye length. We demonstrated in this work, a tailored large surface area platform based on 1D nanotubes with thin active coatings as an efficient approach for sensing applications and beyond