Atomic Layer Deposition of SnO₂-Coated Anodic One-Dimensional 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 a 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 an operating temperature of 300 °C 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.