Bifunctional Effects of CdSe Quantum Dots and Nb₂O₅ Interlayer for ZnO Nanorods-based Photoelectrochemical Water-Splitting Cells

Single crystalline ZnO nanorods (ZONRs) with a length of 3.6 μm and a diameter of 100 nm were prepared using a hydrothermal method. Based on photoactive ZONR films, the thin Nb₂O₅ layers were introduced using spin-coating, followed by the deposition of CdSe quantum dots (QDs) as an absorber. The Nb₂O₅ layers, denoted as Nb₂O₅(1) and Nb₂O₅(2), were spin-coated one and two times, respectively, to demonstrate the thickness effect of Nb₂O₅. The Nb₂O₅(1)-coated ZONR (ZONR/Nb₂O₅(1)) exhibited a higher photoelectrochemical (PEC) response and better stability than the bare ZONR, due to the surface-related defect states passivated by the Nb₂O₅. In contrast, the PEC response of the ZONR/Nb₂O₅(2) was reduced about 40% due to the insulating nature of the thicker Nb₂O₅. CdSe QD-sensitized ZONR and ZONR/Nb₂O₅ films were compared, revealing that the ZONR/Nb₂O₅(1)/CdSe film exhibited significantly enhanced PEC performance when compared with the other films. Herein, the CdSe QDs acted as the light absorber and, in addition, the QDs on the exposed ZONR passivated the surface-related defect states of the ZONR. The Nb₂O₅ layers also passivated the surface-related defect states of the ZONR; furthermore, the Nb₂O₅ located between the ZONR and the QDs led to the stepwise energy band edge alignment, resulting in the faster charge separation. These binfunctional effects contributed to the significantly enhanced PEC response of the ZONR/Nb₂O₅(1)/CdSe