Oxygen vacancy modulation of titania nanotubes by cathodic polarization and chemical reduction routes for efficient detection of volatile organic compounds.

In this work, we have synthesized a highly ordered TiO2 nanotube array by an electrochemical anodization method. Then the oxygen vacancy level of the TiO2 nanotubes was tuned by two different methods: i.e. (i) cathodic polarization by the application of a reverse potential and (ii) chemical reduction using a reducing agent (e.g. hydrazine hydrate) treatment at elevated temperature. Field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) confirmed there was no morphological deformation of TiO2 nanotubes after the modulation of oxygen vacancies. X-ray diffraction spectroscopy (XRD) and TEM both confirmed the formation of highly crystalline anatase (101). The oxygen vacancy level of all the TiO2 nanotubes was tested progressively with photoluminescence (PL) spectra, Raman spectra, energy dispersive X-ray spectroscopy (EDS) and X-ray photoelectron spectra (XPS). All the spectroscopy results confirmed the successful reduction of TiO2 nanotubes with different levels of defects. All the nanotube samples with various oxygen vacancy levels were transformed to Au/TiO2 nanotube/Ti type sandwich structured sensor devices and tested under exposure to 100 ppm of ethanol in air. Highly doped cathodic reduced nanotubes offered very high response magnitudes of 99.64% and 60% under exposure to 100 ppm of ethanol at 200 °C and 50 °C, respectively. Chemically reduced TiO2 nanotubes offered moderate response magnitudes of 75.4% and 80% at 150 °C and 200 °C under exposure to 100 ppm of ethanol, which was found to be the best among all the samples due to the appreciably fast response (155 s) and recovery time (779 s). The developed sensors showed adequate stability and selectivity towards ethanol with a moderate dynamic range (20 to 200 ppm of ethanol) of detection. A general relation was drawn based on the experimental findings of this work to estimate the response magnitude of nanoscale metal oxide gas sensors with various levels of oxygen vacancies.