Enhancing visible-light photoelectrochemical water splitting through transition-metal doped TiO₂ nanorod arrays

Extending the photoresponse from the ultraviolet (UV) to the visible light region, while maintaining a high photocatalytic activity has been an important challenge for TiO₂. We demonstrate the use of transition-metal doping treatment as a facile and effective strategy to substantially improve the performance of TiO₂ nanorods in the visible light region for photoelectrochemical (PEC) water splitting. The effect of Fe, Mn and Co as dopants on the PEC performance of the TiO₂ nanorods was investigated, wherein the Fe doping is the most effective route to enhance the photoactivity of TiO₂. The photocurrent density of Fe-TiO₂ sample significantly increases with bias voltage and reaches 2.92 mA cm^-2 at 0.25 V vs. Ag/AgCl, which is five times higher than that of the undoped TiO₂. Even under visible light illumination (>420 nm), the photocurrent density of Fe-TiO₂ is as high as 0.96 mA cm^-2 at 0.25 V vs. Ag/AgCl. Incident-photon-to-current-conversion (IPCE) efficiency (up to ∼18%) measurements reveal that the Fe-TiO₂ nanorod sample significantly improve the photoresponse not only in the UV region, but also in the visible light region. Fe doping not only enhances the visible light absorption of TiO₂ nanorods by creating impurity states near the conduction band, but also obviously increases carrier density of TiO₂, leading to effective carrier separation and transportation and relatively long electron lifetime. Because of their relatively high photocatalytic activity, the Fe-TiO₂ nanorods can serve as a promising candidate in various areas, such as solar water splitting, dye-sensitized solar cells, and photocatalysis.