Morphology Conserving High Efficiency Nitrogen Doping of Titanate Nanotubes by NH3 Plasma

Titanate nanotubes offer certain benefits like high specific surface area, anisotropic mesoporous structure and ease of synthesis over other nanostructured titania forms. However, their application in visible light driven photocatalysis is hindered by their wide band-gap, which can be remedied by, e.g., anionic doping. Here we report on a systematic study to insert nitrogen into lattice positions in titanate nanotubes. The efficiency of N-2 (+) bombardment, N-2 plasma and NH3 plasma treatment is compared to that of NH3 gas synthesized in situ by the thermal decomposition of urea or NH4F. N-2 (+) bombarded single crystalline rutile TiO2 was used as a doping benchmark (16 at.% N incorporated). Surface species were identified by diffuse reflectance infrared spectroscopy, structural features were characterized by scanning electron microscopy and powder X-ray diffraction measurements. The local chemical environment of nitrogen built into the nanotube samples was probed by X-ray photoelectron spectroscopy. Positively charged NH3 plasma treatment offered the best doping performance. This process succeeded in inserting 20 at.% N into nanotube lattice positions by replacing oxygen and forming Ti-N bonds. Remarkably, the nanotubular morphology and titanate crystal structure were both fully conserved during the process. Since plasma treatment is a readily scalable technology, the suggested method could be utilized in developing efficient visible light driven photocatalysts based on N-doped titanate nanotubes.