Photoelectrochemical Water Splitting Using Dense and Aligned TiO2 Nanorod Arrays

Abstract

Dense and aligned TiO₂ nanorod arrays are fabricated using oblique-angle deposition on indium tin oxide (ITO) conducting substrates. The TiO₂ nanorods are measured to be 800–1100 nm in length and 45–400 nm in width with an anatase crystal phase. Coverage of the ITO is extremely high with 25 × 10⁶ mm⁻² of the TiO₂ nanorods. The first use of these dense TiO₂ nanorod arrays as working electrodes in photoelectrochemical (PEC) cells used for the generation of hydrogen by water splitting is demonstrated. A number of experimental techniques including UV/Vis absorption spectroscopy, X-ray diffraction, high-resolution scanning electron microscopy, energy-dispersive X-ray spectroscopy, and photoelectrochemistry are used to characterize their structural, optical, and electronic properties. Both UV/Vis and incident-photon-to-current-efficiency measurements show their photoresponse in the visible is limited but with a marked increase around ≈400 nm. Mott–Schottky measurements give a flat-band potential (V_FB) of +0.20 V, a carrier density of 4.5 × 10¹⁷ cm⁻³, and a space-charge layer of 99 nm. Overall water splitting is observed with an applied overpotential at 1.0 V (versus Ag/AgCl) with a photo-to-hydrogen efficiency of 0.1%. The results suggest that these dense and aligned one-dimensional TiO₂ nanostructures are promising for hydrogen generation from water splitting based on PEC cells.