Anodic TiO2 nanotube arrays for photocatalytic CO2 conversion: comparative photocatalysis and EPR study

Titania (TiO2) is a widely used semiconductor for photocatalytic decomposition of organic impurities in air, water and conversion of CO2 into hydrocarbon fuel precursors. TiO2 in form of nanotube arrays is the most attractive for practical use because of morphology advantages providing more favorable diffusion of photocatalytic reaction products and low recombination rates of photogenerated electrons and holes. Herein we have performed a comparative study of photocatalytic activity gas-phase CO2 conversion to hydrocarbon products and defect properties of multi-walled and single-walled TiO2 nanotube arrays. The methanol and methane was detected during CO2 photoreduction process. The photocatalytic evolution rate of multi-walled TiO2 nanotubes more by two times for methane than single-walled TiO2 nanotubes after four hour irradiation time and for methanol by four times more. The type and features of structural defects was investigated by EPR spectroscopy. It was revealed that the Ti3+/oxygen vacancies centers are mostly located in outer layer of the nanotubes while carbon dangling bonds were observed in inner layer. It is established that carbon defects are the centers of adsorption and accumulation of photoinduced charge carriers. The obtained results are completely new, elucidate the role of defects of different type in the photocatalytic conversion of CO2 into hydrocarbon compounds and show good application prospects of the TiO2 nanotube arrays.