Photoelectrochemical Performance of Quantum dot-Sensitized TiO Nanotube Arrays: a Study of Surface Modification by Atomic Layer Deposition Coating.

Although CdS and PbS quantum dot-sensitized TiO 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 AlO, TiO, 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 TiO, and 30 cycles AlO overlayers are 5.0, 4.3, and 5.6 mA/cm at 1.0 V (vs. SCE), respectively. The photoelectrode with AlO 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 TiO and ZnO overcoatings can only ascribed to the decrease of the interfacial charge transfer impedance, besides which AlO 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.