Photoelectrochemical H2 Generation from Suboxide TiO2 Nanotubes: Visible-Light Absorption versus Conductivity

In the present work we report on the key factors dictating the photoelectrochemical (PEC) performance of suboxide titania (TiOx) nanotubes. TiOx nanotubes were produced by a systematic variation of reduction heat treatments of TiO2 in Ar/H-2. The properties of the TiOx tubes were investigated by electron paramagnetic resonance (EPR), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), solid-state conductivity, reflectivity measurements, photocurrent spectroscopy, and photoelectrochemical hydrogen evolution. In line with earlier literature, these suboxide tubes show a drastically improved photoelectrochemical water-splitting performance compared to non-reduced anatase TiO2 tubes. In this work we show that the key improvement in water-splitting performance is due to the strongly improved conductivity of TiOx semimetalic tubes, reaching 13.5 K Omega per tube compared to 70 M Omega (for non-reduced anatase), and is not due to the enhanced visible-light absorbance.