Optimization of photoelectrochemical water splitting performance on hierarchical TiO₂ nanotube arrays

In this paper, we show that by varying the voltages during two-step anodization the morphology of the hierarchical top-layer/bottom-tube TiO ₂ (TiO₂ NTs) can be finely tuned between nanoring/nanotube, nanopore/nanotube, and nanohole-nanocave/nanotube morphologies. This allows us to optimize the photoelectrochemical (PEC) water splitting performance on the hierarchical TiO₂ NTs. The optimized photocurrent density and photoconversion efficiency in this study, occurring on the nanopore/nanotube TiO₂ NTs, were 1.59 mA cm^-2 at 1.23 V vs. RHE and 0.84% respectively, which are the highest values ever reported on pristine TiO₂ materials under illumination of AM 1.5G. Our findings contribute to further improvement of the energy conversion efficiency of TiO₂-based devices.