Evaluating the mechanism of visible light activity for N,F-TiO₂ using photoelectrochemistry

The improvement of the solar efficiency of photocatalytic materials is important for solar driven environmental remediation and solar energy harvesting applications. Photoelectrochemical characterization of nitrogen and fluorine codoped titanium dioxide (N,F-TiO₂) was used to probe the mechanism of visible light activity. The spectral photocurrent response under visible irradiation did not correlate with the optical absorption spectrum of the N,F-TiO₂; however, open-circuit photopotential measurements provided better correlation to the optical absorption spectra. These observations suggest that electrons excited to the conduction band from the N-induced midgap state are rapidly trapped by defect levels below the conduction band. Reactive oxygen species (ROS) can be produced via the reduction of molecular oxygen by conduction band electrons leading to the oxidative degradation of organic pollutants, and singlet oxygen may play a role. If there is no loss in the band gap activity, as compared to undoped titania, then any additional visible light activity may give an overall improvement in the solar efficiency. The photocurrent response should not be used as a direct measure of photocatalytic activity for doped titania as the oxygen reduction pathway is vitally important for the generation of ROS, whereas hole transfer from dopant midgap states may not be so critical.