Laser-Induced Surface Modification at Anatase TiO₂ Nanotube Array Photoanodes for Photoelectrochemical Water Oxidation

Nanostructured titanium dioxide (TiO2) presents considerable potential as a photoanode in low cost, sustainable photoelectrochemical systems for solar water splitting. The wide band gap combined with the presence of trap states and reduced water oxidation kinetics limit, however, the photocurrent performance to only ∼1 mA cm–2. Increasing the disorder of the crystal structure at the surface, on the other hand, has been proven to increase light absorption via band gap narrowing, and conversion efficiency. In this work, anodized TiO2 nanotubes have been irradiated with a pulsed UV laser in deionized water environment to introduce lattice disorder. As a result, the photocurrent improved by 1.6-fold under simulated sunlight compared with pristine TiO2 nanotube arrays at 1.23 VRHE. For all samples the water oxidation reaction kinetics is determined to be the limiting step for the solar-to-current conversion at low bias (0.5–0.7 VRHE), while modified nanotube arrays display a 78% water oxidation selectivity at 1.23 VRHE, compared to 65% for the pristine TiO2 nanotubes. The electronic density of states of the modified nanotubes is evaluated using electrochemical impedance spectroscopy, revealing that selective laser irradiation improved the number density of shallow donors while reducing the density of deep trap states.