Photoelectrochemical System for Unassisted High-Efficiency Water-Splitting Reactions Using N-Doped TiO2 Nanotubes

In this study, we report a facile one-step synthesis of porous and defect-rich nitrogen (N)-doped TiO2 nanotubes by the electrochemical anodization process with in situ N-doping on TiO2 nanotubes using nitrogen gas as a nitrogen source. N-doped TiO2 exhibits enhanced visible-light photocatalytic activity toward the photoelectrochemical water-splitting reaction and high stability due to nitrogen doping and band-gap narrowing. Nitrogen-doped nanotubes exhibit unique and efficient photoelectrochemical cell (PEC) water-splitting activity for H-2 generation in aqueous electrolytes with a pH gradient. The nitrogen doping and the concurrent defects increase the charge density, which can effectively tune the band gap toward the visible light region, hindering the electron-hole recombination process compared to that in bare TiO2 nanotubes. The PEC has been assembled with chemical (pH) bias (unassisted) by the imposition of electrolytes of different pH's, resulting in a pH gradient between the photoanode and the metal cathode. The pH-biased photoelectrochemical cell has been constructed with 1 M NaOH on the photoanode side and 1 M H2SO4 on the cathode side. The PEC of N-doped TiO2 nanotubes exhibits an open circuit potential of 1.3 V with a photocurrent density of 0.76 mA/cm(2) (at 0.5 V) and solar-to-hydrogen (STH) efficiency of 0.5% in an outdoor sunlight measurement. The photoelectrode exhibits a long-term stability of 120 h in an outdoor measurement.