Transparent Sn-Decorated W‑Doped TiO₂ Multiphase Nanotube Arrays as Efficient Photocatalysts for Solar-Driven Water Splitting

Photoelectrochemical water splitting is a crucial step in realizing a green hydrogen economy. Herein, we demonstrate, for the first time, the rational design and optimized fabrication of earth-abundant W-doped TiO2 thin nanotubular structures decorated with Sn in mixed polymorphs (anatase/brookite/rutile) on F-doped tin oxide substrates via a direct-current sputtering technique and subsequent electrochemical anodization followed by thermal treatment. Sn decoration and W doping along with optimized annealing conditions were elucidated as determinant factors controlling the structural and optical features of the fabricated nanotubes. The resulting highly crystalline polymorphic nanotubes showed an enhanced photocurrent at 0.4 VSCE (1.75 mA/cm2) with a noticeable narrowing in the optical band gap and 25 times compared to pristine TiO2 nanotubes. The photoluminescence spectra revealed enriched oxygen (O) vacancies upon doping with W and decoration with Sn. The Mott–Schottky plot unveils the variations in the density of charge carriers upon annealing at different temperatures and their role in annihilating these O vacancies. Importantly, the electrochemical impedance spectroscopy elucidation helps to reveal the charge-transfer mechanism in the mixed polymorph junctions of the fabricated nanotubes under dark and illumination conditions, revealing reduced charge-transfer resistance under light-on conditions, which is consistent with the produced photocurrent, lifetime of the electrons, and density of the impurity levels. Moreover, the density functional theory investigation unraveled the development of a homojunction between the brookite and rutile phases that improved the photocatalytic activity toward the oxygen evolution reaction, in agreement with the experimental findings. The findings of this work represent a crucial milestone toward the development of visible-light, earth-abundant, light-absorbing photoanodes for solar-assisted water splitting