Stabilized Brookite Nanotube Array Films Grown on Transparent Substrates for Photoelectrochemical Water Splitting

Abstract

Photoelectrochemical water splitting is a promising route to realizing a clean energy economy. Herein, we report on the ability to selectively fabricate optimized Ni-doped brookite nanotube array films on fluorine-doped tin oxide (FTO) substrate using ultrarapid radio frequency sputtering followed by electrochemical anodization and thermal annealing under ambient air. Ni doping promotes photocatalytic performance by introducing impurity bands within the TiO2 bandgap. Further, annealing at various temperatures helps to tune the photoactivity of the fabricated photoelectrodes, which is in agreement with the nature of the trap states. The nanotubes enjoy high crystallinity and enhanced photoresponse with a noticeable bandgap of 2.7 eV. The Mott–Schottky analysis reveals the variation in the charge carriers density of the fabricated nanotubes. Most importantly, the electrochemical impedance spectroscopy (EIS) analysis unravels the involved kinetics and helps to determine the charge transfer properties of the annealed nanotubular films under open-circuit voltage (OCV) and with applied potential under dark and illumination conditions. EIS analysis reveals decreased charge transfer resistance under illumination, in agreement with the obtained photocurrent, electron lifetime, and density of trap states.