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.