Enhanced Photoelectrochemical Performance Of Quantum Dot-Sensitized TiO₂ Nanotube Arrays With Oxide Modification By Atomic Layer Deposition

Among numerous semiconductor materials, TiO2 has been greatly researched due to its advantages of non-toxic, good stability and so forth. One dimensional TiO2 nanotube arrays (TNTAs) prepared by anodization has highly regular tubular structure, which is beneficial for charge transport, thus achieving high photocatalytic and photoelectric activity. However, limited by its wide band gap (anatase:3.2 eV), TiO2 cannot fully absorb sunlight, so it is rarely used alone. In this study, TNTAs were co-sensitized by CdS and PbS quantum dots (QDs) through sonication-assisted successive ionic layer adsorption and reaction method to expand the light absorption range to visible light. Although this compound semiconductor materials have been widely researched, serious carrier recombination, self-oxidation and low loads of QDs still exsit, which limits its applications.Based on the above problems, ALD used as a surface treatment technology to deposite various thickness of Al2O3 coating layer on the surface of TNTAs/QDs, and each samples’s photoelectrochemical (PEC) performance was studied. Under simulated solar illumination, the photoelectrode deposited with 30 ALD cycles of Al2O3 overlayer exhibited the maximum photocurrent density of 5.19 mA/cm2, which was 52 times and 1.6 times higher than that of pure TNTAs and TNTAs/QDs, respectively. Furthermore, the incident photon-to-current conversion efficiency of the TNTAs/QDs with 30 ALD cycles of Al2O3 overlayer photoelectrode can reach the record value of 83% at 350 nm and remain above 30% up to 450 nm. Electrochemical impedance spectra (EIS) and Mott-Schotty analysis indicate that small ALD cycles of Al2O3 overlayer could markedly decrease the interfacial charge transfer resistance and increase the carrier density, thus making the photoelectrode more stable.The loading amounts of QDs on TNTAs could be controlled by adjusting the TNTAs length. PEC measurements indicate that the 7 h TNTAs (2.4 μm) displayed the highest photocurrent. In addition, the ZnO interlayer through ALD technique deposited on the surface of TNTAs could further increase the loading amounts of QDs. Through morphology and performance characterizations, we found that the loading amounts of QDs was increased with an increase on the thickness of ZnO interlayer. While the photoelectrode deposited with ten cycles of the ZnO interlayer showed the optimal PEC performance, the photoconversion efficiency of which was as high as 4.9%.