Unraveling the Origin of Visible Light Capture by Core-Shell TiO2 Nanotubes

A black TiO2 nanotube (NT) heterostructure with an anatase-core and an amorphous-shell has been synthesized by NH3 annealing of amorphous NT grown by the anodization of a Ti substrate. Remarkable photoabsorption behavior of these black TiO2 NTs is observed: strong absorption throughout the entire optical wavelength region from ultraviolet to near-infrared. X-ray absorption near-edge structure (XANES), X-ray photoelectron spectroscopy (XPS) and resonant inelastic X-ray scattering (RIXS) have been used to elucidate the origin of this spectacular light capture phenomenon. Surface-sensitive XANES recorded in total electron yield and XPS show that the surface layer is amorphous with a chemical composition approaching that of Ti4O7. Bulk-sensitive XANES using X-ray partial fluorescence yield and Ti 2p RIXS confirm the presence of a rich amount of Ti3+ in the crystalline bulk (core of the NT with anatase structure) of black TiO2 NTs, which exhibits a dispersive d-d energy loss at similar to 2 eV corresponding to the broad visible light absorption at similar to 600 nm. Our results suggest that the extraordinary photoabsorption behavior of these black TiO2 NTs is due to the stabilization of Ti3+ in this special N-doped core-shell assembly having structure varying between TiO2 (bulk anatase) and Ti4O7 (surface, amorphous).