Anodic TiO2 nanotube arrays for photocatalytic CO2 conversion: 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 CO2 into hydrocarbon fuel precursors. TiO2 in 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 CO2 to hydrocarbon products and the defect properties of multi-walled and single-walled arrays of TiO2 nanotubes. Methanol and methane have been detected in the CO2 photoreduction process. The photocatalytic evolution rate of multi-walled TiO2 nanotubes is twice as fast for methane as for single-walled TiO2 nanotubes 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 CO2 to hydrocarbon compounds and show good prospects for applying TiO2 nanotube arrays.