Anions and cations distribution in M 5+ /N 3- co-alloyed TiO 2 nanotubular structures for photo-electrochemical water splitting

Co-alloyed (M,N) titanium dioxide nanotubes were synthesized by a simple anodization process with M-Ti alloys (M = Ta or Nb), followed by a thermal treatment in ammonia to introduce nitrogen in those nanostructure. The photo-electrochemical performances of these co-alloyed samples was compared with the ones of N doped and undoped TiO 2 NTs. Different conditions of thermal treatment under ammonia were studied in order to control the amount of nitrogen introduced in the TiO 2 structure and tend to achieve a balance of charges between Ta 5+ /Nb 5+ cations and N 3- anions. The structure and composition of these materials were characterized by X-Ray Diffraction (XRD), X-ray Photoelectron Spectroscopy (XPS) and Transmission Electron Microscopy (TEM) EDX mapping. This combination of methods confirm the successful introduction of doping atoms, and allowed us to estimate the amount of M 5+ cations and N 3- anions in the TiO 2 NTs. TEM EDX mapping indicates a heterogeneous distribution of Nb 5+ and Ta 5+ cations in the nanotubes that can originate from the use of highly concentrated alloys. External quantum efficiency measurements were used to determine the photo-electrochemical activity of our samples in different spectral domains. A significant improvement of activity in the visible region of the solar spectra was observed for co-alloyed TiO 2 samples when compared to undoped nanotubes but also when compared to N doped TiO 2 . The obtained results indicate that the parameters of the NH 3 thermal treatments should be finely controlled to improve the conversion efficiency in the visible domain for the co-alloyed samples by introducing a precise amount of N 3- in substitution of oxygen in the TiO 2 lattice and to avoid the damaging of the nanotubular structure.