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.