Highly ordered TiO₂ nanotubes on patterned substrates: Synthesis-in-place for ultrasensitive chemiresistors

Integrating a highly ordered TiO₂ nanotube array with patterned substrates is an effective design strategy for taking advantage of one-dimensional nanotube structures such as high surface-to-volume ratio and novel functionalities of TiO₂, but many challenges remain in synthesis. Here we report a novel synthesis-in-place method for highly ordered vertical TiO₂ nanotube films on various patterned substrates and the application of the films to chemiresistive sensors. A unique synthetic strategy, in which the Pt bottom electrodes were clamped during anodization, leads to the complete anodization of the Ti layer to TiO₂ nanotubes not only on SiO₂/Si substrates but also on glass and sapphire substrates, which can be used as chemiresistors without additional processes. TiO₂ nanotube sensors on SiO₂/Si substrates show unprecedentedly ultrahigh responses to ethanol and acetone with detection limits down to parts per billion levels. Beyond the high sensitivity, selective responses to ethanol and acetone create opportunities for application in breath analyzers to diagnose human respiratory diseases.