Influence of immersion cycles during n–β–Bi2O3 sensitization on the photoelectrochemical behaviour of N–F–codoped TiO2 nanotubes

Abstract Sensitization of TiO 2 nanotube (TNT)-based photoanodes with narrow-band gap semiconductors is an important alternative to improving the photoelectrochemical properties of the material. However, the interaction between the sensitizer and TNT is not understood deeply enough to relate charge carrier transport into the composite photoanode with its photoactivity. In this contribution, we studied the photoelectrochemical behaviour of N–F–self codoped TiO 2 nanotubes (N–F–TNTs) that were grown by anodization of titanium plates and sensitized with β–Bi 2 O 3 by immersing the TNTs into a Bi 2 O 3 sol solution by dip–coating. The number of immersion cycles was varied. The as–fabricated photoanodes were characterized by FESEM, GIXRD, DRS and XPS, while their photoelectrochemical and semiconducting properties were investigated by photovoltammetry, electrochemical impedance spectroscopy and Mott–Schottky analysis in 0.1 M HClO 4 . The photoelectrocatalytic activity of the composite photoanodes was evaluated for glycerol oxidation under acidic and alkaline conditions. The N–F–TNTs exhibit a well–oriented structure after β–Bi 2 O 3 deposition. The presence of substitutions of both N and F, identified by XPS, indicates the self–doping of the TNTs during anodization. The visible–light harvesting of the N–F–TNT photoanode was enhanced after three –immersion cycles during β–Bi 2 O 3 sensitization, establishing an adequate n–n heterojunction at the N–F–TNT/Bi 2 O 3 interface. In addition, bismuth migration from the sensitizer to the TNT lattice was promoted during thermal treatment, forming Bi–N–F–tridoping of TNT (Bi–N–F–TNT). The suitable band alignment between TNT and β–Bi 2 O 3 and incorporation of the Bi 3+ energy levels into TiO 2 facilitate charge carrier separation and electron transport throughout the cell. Nevertheless, increasing the number of immersion cycles over three creates an excess of Bi 3+ species at the N–F–TNT/β–Bi 2 O 3 interface, producing an energetic barrier that hinders electron transport. The Bi–N–F–TNT/Bi 2 O 3 photoanode was still photoactive after glycerol oxidation under visible illumination, indicating that its oxidizing power and stability remained.