Improvement of gas sensing performance of TiO2 towards NO2 by nano-tubular structuring

Aligned nano-tubular TiO2 layers were synthesised and their gas sensing abilities were compared to those of magnetron sputtered titania films towards NO2 and CO. TiO2 nano-tubes were grown on the titanium foils, via anodic oxidation method (hereafter anodisation) using of fluoride containing ethylene glycol (EG)-based electrolytes. Both amorphous, nano-tubular titania structures and magnetron sputtered titania layers were annealed at 450 and 700 °C. Magnetron-sputtered titania films were anatase on deposition and converted to rutile on heat-treatment in air for 3 h at 800 °C. Nano-tubular titania layers were converted to anatase at about 400 °C and above 600 °C to rutile. The sensor performance of the nano-tubular TiO2 layers annealed at 450 °C (anatase) and at 700 °C (anatase and rutile) was investigated at 300-500 °C towards NO2 concentrations of 10, 25, 50 and 100 ppm and CO concentrations of 25, 50 and 75 ppm and compared to those of the magnetron sputtered TiO2-layers annealed at 450 °C (anatase) and 800 °C (rutile and anatase). The nano-tubular TiO2-sensor layers yield very promising results for sensing NO2 with some cross-sensitivity towards CO in relatively small concentrations and in a wide concentration range up to 500 °C.