Facilitating light utilization and electron transfer of TiO₂ nanotube arrays for catalyzing photoelectrochemical water reduction via acid etching and copper doping

Photoelectrochemical catalytic capability mainly relies on light absorption and charge transfer efficiency. TiO₂ nanotube array (TNA) having suitable band edges and one-dimensional hollow structure is regarded as one of efficient photoelectrocatalysts toward water reduction. In this study, two strategies are firstly applied to modify TNA as the photocatalyst of water reduction. Ti foils are etched by acid to produce multiple light reflections planes for improving light absorption, and Cu is doped in TNA to enhance charge transfer efficiency. The optimal Cu-doped TNA electrode on the acid-etched Ti foil presents a much smaller overpotential of 91 mV at 10 mA/cm² compared with that of the undoped TNA (524 mV). A smaller Tafel slope of 134 mV/dec is obtained for the optimal Cu-doped TNA electrode than that of the undoped TNA (167 mV/dec). After measuring the photoelectrochemical performance for 1000 cycles, overpotential and charge-transfer resistance of modified TNA are largely reduced owing to activation. Current retention of 96.7% is achieved after continuously illuminating for 10 h. Highly improved photoelectrochemical catalytic ability of TNA modified by acid etching and Cu doping provides new pages for material modifications on substrate and active material.