Effect of calcination temperature on the photocatalytic reduction and oxidation processes of hydrothermally synthesized titania nanotubes

Titania nanotubes having diameters 8 to 12 nm and lengths of 50-300 nm were prepared using a hydrothermal method. Further, the titania nanotubes were calcined over the temperature range 200-800 °C in order to enhance their photocatalytic properties by altering their morphology. The calcined titania nanotubes were characterized by using X-ray diffraction and surface area analysis and their morphological features were studied by scanning and transmission electron microscopy. Nanotubes calcined at 400 °C showed the maximum extent of photocatalyitc reduction of carbon dioxide to methane, whereas samples calcined at 600 °C produced maximum photocatalytic oxidation of acetaldehyde. Electron paramagnetic resonance (EPR) spectroscopy was used to interrogate the effects of nanotube structure on the charge separation and trapping as a function of calcination temperature. EPR results indicated that undercoordinated titania sites are associated with maximum CO₂ reduction occurring in nanotubes calcined at 400 °C. Despite the collapse of the nantube structure to form nanorods and the concomitant loss of surface area, the enhanced charge separation associated with increased crystallinity promoted high rates of oxidation of acetaldehyde in titania materials calcined at 600 °C. These results illustrate that calcination temperature allows us to tune the morphological and surface features of the titania nanostructures for particular photocatalytic reactions.