Photocatalytic generation of multiple ROS types using low-temperature crystallized anodic TiO2 nanotube arrays

This study investigates the photocatalytic efficiency, type of reactive oxygen species (ROS) produced, and potential for structural and morphological modification of anodic TiO2 nanotubes (NTs) synthesized using a novel, energy efficient, low temperature crystallization process. These TiO2 NTs show greater photocatalytic efficiency than traditional high-temperature sintered NTs or supported Degussa P25 TiO2, as measured by degradation of methyl orange, a model organic dye pollutant. EPR analysis shows that low-temperature crystallized TiO2 NTs generate both hydroxyl radicals and singlet oxygen, while high-temperature sintered TiO2 NTs generate primarily hydroxyl radicals but no singlet oxygen. This “cocktail” of reactive oxygen species, combined with an increased surface area, contributes to the increased efficiency of this photocatalytic material. Furthermore, variation of the NT crystallization parameters enables control of structural and morphological properties so that TiO2-NTs can be optimized for scale-up and for specific treatment scenarios.