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In this paper, we show that by varying the voltages during two-step anodization the morphology of the hierarchical top-layer/bottom-tube TiO2 (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.
Zhang et al. (Sun,) studied this question.