Electric field-driven one-step formation of vertical p-n junction TiO₂nanotubes exhibiting strong photocatalytic hydrogen production

In this study, vertically aligned p-n junction TiO₂nanotubes can be formed by anodization of the Ti substrate following the addition of a high electric field in a Pd precursor-containing electrolyte. The bottom region of the TiO₂nanotubes with a high concentration of Pd because of the high electric field which was induced to be p-type. In contrast, the region with a low Pd concentration (top of the TiO₂nanotubes) was determined to be n-type, similar to the pristine TiO₂. The concentration profile of the dopant in TiO₂nanotubes was investigatedviaTOF-SIMS and XPS. Defect formation energies in TiO₂nanotubes were estimated using density-functional theory calculation to understand the p-n junction formation. The p-n junction TiO₂nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm⁻²h⁻¹under solar light irradiation as a result of the enhancement of visible light photoactivity.