Photoelectrochemical System for Unassisted High-Efficiency Water-Splitting Reactions Using N-Doped TiO2Nanotubes

Abstract

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 H2 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/cm2 (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.