Electric field-driven one-step formation of vertical p–n junction TiO2 nanotubes exhibiting strong photocatalytic hydrogen production

Abstract

In this study, vertically aligned p–n junction TiO₂ nanotubes 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 TiO₂ nanotubes 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 TiO₂ nanotubes) was determined to be n-type, similar to the pristine TiO₂. The concentration profile of the dopant in TiO₂ nanotubes was investigated via TOF-SIMS and XPS. Defect formation energies in TiO₂ nanotubes were estimated using density-functional theory calculation to understand the p–n junction formation. The p–n junction TiO₂ nanotubes showed a high photocatalytic hydrogen production rate of 25.2 μL cm⁻² h⁻¹ under solar light irradiation as a result of the enhancement of visible light photoactivity.