Visible Light Driven Photoelectrochemical Water Oxidation on Nitrogen-Modified TiO2 Nanowires

We report hydrothermal synthesis of single crystalline TiO2 nanowire arrays with unprecedented small feature sizes of similar to 5 nm and lengths up to 4.4 mu m on fluorine-doped tin oxide substrates. A substantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into the TiO2 lattice via nitridation in NH3 flow at a relatively low temperature (500 degrees C) because of the small cross-section of the nanowires. The low-energy threshold of the incident photon to current efficiency (IPCE) spectra of N-modified TiO2 samples is at similar to 520 nm, corresponding to 2.4 eV. We also report a simple cobalt treatment for improving the photoelectrochemical (PEC) performance of our N-modified TiO2 nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO2 samples in the ultraviolet region is restored to equal or higher values than those of the unmodified TiO2 samples, and it remains as high as similar to 18% at 450 nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-modified TiO2 surface and acting as a water oxidation cocatalyst.