Visible Light Driven Photoelectrochemical\nWater Oxidation\non Nitrogen-Modified TiO₂ Nanowires

Abstract

We report hydrothermal synthesis of single crystalline\nTiO₂ nanowire arrays with unprecedented small feature sizes\nof\n∼5 nm and lengths up to 4.4 μm on fluorine-doped tin\noxide substrates. A substantial amount of nitrogen (up to 1.08 atomic\n%) can be incorporated into the TiO₂ lattice via nitridation\nin NH₃ flow at a relatively low temperature (500 °C)\nbecause of the small cross-section of the nanowires. The low-energy\nthreshold of the incident photon to current efficiency (IPCE) spectra\nof N-modified TiO₂ samples is at ∼520 nm, corresponding\nto 2.4 eV. We also report a simple cobalt treatment for improving\nthe photoelectrochemical (PEC) performance of our N-modified TiO₂ nanowire arrays. With the cobalt treatment, the IPCE of N-modified\nTiO₂ samples in the ultraviolet region is restored to equal\nor higher values than those of the unmodified TiO₂ samples,\nand it remains as high as ∼18% at 450 nm. We propose that the\ncobalt treatment enhances PEC performance via two mechanisms: passivating\nsurface states on the N-modified TiO₂ surface and acting\nas a water oxidation cocatalyst.