We report hydrothermal synthesis of single crystalline TiO(2) nanowire arrays with unprecedented small feature sizes of ~5 nm and lengths up to 4.4 μm on fluorine-doped tin oxide substrates. A substantial amount of nitrogen (up to 1.08 atomic %) can be incorporated into the TiO(2) lattice via nitridation in NH(3) flow at a relatively low temperature (500 °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 TiO(2) samples is at ~520 nm, corresponding to 2.4 eV. We also report a simple cobalt treatment for improving the photoelectrochemical (PEC) performance of our N-modified TiO(2) nanowire arrays. With the cobalt treatment, the IPCE of N-modified TiO(2) samples in the ultraviolet region is restored to equal or higher values than those of the unmodified TiO(2) samples, and it remains as high as ~18% at 450 nm. We propose that the cobalt treatment enhances PEC performance via two mechanisms: passivating surface states on the N-modified TiO(2) surface and acting as a water oxidation cocatalyst.

We report a synthesis of N- and Ta-coincorporated TiO2 (N,Ta:TiO2) and Ta-incorporated TiO2 (Ta:TiO2) nanowire (NW) arrays and their application as photoanodes for water photooxidation. Tantalum is incorporated into TiO2 NWs with concentrations ranging from 0.11 to 3.47 atomic % by a simple solvothermal synthesis. N,Ta:TiO2 nanowires are prepared via nitridation of Ta:TiO2 nanowires in NH3 flow at a relatively low temperature (500 °C). N,Ta:TiO2 NWs with the optimum Ta concentration of 0.29 atomic % also demonstrate significant enhancement in photoelectrochemical performance with the photocurrent reaching 0.52 and 0.18 mA/cm2 under AM 1.5 G and visible light (>420 nm) illumination, compared with 0.26 and 0.13 mA/cm2 for that of N:TiO2 NWs, although the active spectrum of the N,Ta:TiO2 NW sample only extends to ∼520 nm (2.38 eV), compared to ∼540 nm (2.30 eV) for N:TiO2 NWs. We believe that the enhancement shown by the N,Ta-coincorporated sample is due to fewer recombination centers from charge compensation effects and suppression of the formation of an amorphous layer on the nanowires during the nitridation process.

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Heavily doped oxide nanocrystals exhibit a tunable localized surface plasmon resonance (LSPR) in the infrared, a property that is promising for applications in photonics, spectroscopy, and photochemistry. Nanocrystal carrier density and, thus, spectral response are adjustable via chemical reaction; however, the fundamental processes that govern this behavior are poorly understood. Here, we study the time dependence of the LSPR supported by indium tin oxide (ITO) nanocrystals during O2 and N2 annealing with in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). We show that the LSPR red-shifts upon oxidation in O2 and blue-shifts to its original position upon reduction in N2. A reaction–diffusion model allows us to rationalize the underlying physicochemical processes and quantitatively connect nanocrystal redox chemistry, solid-state diffusion, carrier density, and the LSPR.