Anodic TiO₂nanotube arrays for photocatalytic CO₂conversion: comparative photocatalysis and EPR study

Titania (TiO₂) is a widely used semiconductor for the photocatalytic decomposition of organic impurities in air, water and the conversion of CO₂into hydrocarbon fuel precursors. TiO₂in the form of nanotubes arrays is the most attractive for practical use because of the morphological advantages providing more favorable diffusion of photocatalytic reaction products and a low recombination rate of photogenerated electrons and holes. We have carried out a comparative study of the photocatalytic activity of gas-phase conversion of CO₂to hydrocarbon products and the defect properties of multi-walled and single-walled arrays of TiO₂nanotubes. Methanol and methane have been detected in the CO₂photoreduction process. The photocatalytic evolution rate of multi-walled TiO₂nanotubes is twice as fast for methane as for single-walled TiO₂nanotubes after four hours of irradiation and four times faster for methanol. The type and features of the structural defects have been investigated by EPR spectroscopy. For the first time, it has been shown that Ti³⁺/oxygen vacancy centers are mainly located inside the outer layer of nanotubes, while carbon dangling bonds have been observed directly on the surface of the inner layer. Carbon defects have been found to be the centers of adsorption and accumulation of photoinduced charge carriers. The results are entirely new; they clarify the role of different types of defects in the photocatalytic conversion of CO₂to hydrocarbon compounds and show good prospects for applying TiO₂nanotube arrays.