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

Abstract

Sensitization of TiO2 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 TiO2 nanotubes (N–F–TNTs) that were grown by anodization of titanium plates and sensitized with β–Bi2O3 by immersing the TNTs into a Bi2O3 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 0.1M HClO4. 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–Bi2O3 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–Bi2O3 sensitization, establishing an adequate n–n heterojunction at the N–F–TNT/Bi2O3 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–Bi2O3 and incorporation of the Bi3+ energy levels into TiO2 facilitate charge carrier separation and electron transport throughout the cell. Nevertheless, increasing the number of immersion cycles over three creates an excess of Bi3+ species at the N–F–TNT/β–Bi2O3 interface, producing an energetic barrier that hinders electron transport. The Bi–N–F–TNT/Bi2O3 photoanode was still photoactive after glycerol oxidation under visible illumination, indicating that its oxidizing power and stability remained.