Photoelectrochemical Performance of Quantum dot-Sensitized TiO₂ Nanotube Arrays: a Study of Surface Modification by...

Although CdS and PbS quantum dot-sensitized TiO₂ nanotube arrays (TNTAs/QDs) show photocatalytic activity in the visible-light region, the low internal quantum efficiency and the slow interfacial hole transfer rate limit their applications. This work modified the surface of the TNTAs/QDs photoelectrodes with metal-oxide overlayers by atomic layer deposition (ALD), such as coating Al₂O₃, TiO₂, and ZnO. The ALD deposition of all these overlayers can apparently enhance the photoelectrochemical performance of the TNTAs/QDs. Under simulated solar illumination, the maximum photocurrent densities of the TNTAs/QDs with 10 cycles ZnO, 25 cycles TiO₂, and 30 cycles Al₂O₃ overlayers are 5.0, 4.3, and 5.6 mA/cm² at 1.0 V (vs. SCE), respectively. The photoelectrode with Al₂O₃ overlayer coating presents the superior performance, whose photocurrent density is 37 times and 1.6 times higher than those of the TNTAs and TNTAs/QDs, respectively. Systematic examination of the effects of various metal-oxide overlayers on the photoelectrochemical performance indicates that the enhancement by TiO₂ and ZnO overcoatings can only ascribed to the decrease of the interfacial charge transfer impedance, besides which Al₂O₃ coating can passivate the surface states and facilitate the charge transfer kinetics. These results could be helpful to develop high-performance photoelectrodes in the photoelectrochemical applications.