The Study Of Interface Optimization On CdS/ZnO Nanorods Photoelectrode For Enhanced Photoelectrochemical Properties

Metal oxide semiconductors are widely studied because of their stability,environmental friendly and low cost. As an excellent candidate to TiO2, ZnO has a similar band gap (～3.37eV), higher electron mobility, better crystallinity and less defects. Meanwhile, one-dimensional (1-D) nanostructure is beneficial for fast and long-distance electron transport, and also owns large specific surface areas, high length-to-diameter ratios. In this paper, the 1-D oriented ZnO nanorods (NRs) are prepared by hydrothermal method, and the photoelectronchemical (PEC) properties of the photoelectrodes are enhanced by loading CdS quantum dots (QDs) to extend the absorption edge of ZnO.Firstly, to obtain the optimal deposition cycles of CdS QDs on ZnO NRs, a series of PEC performances have been studied. The results suggest that the ZnO NRs sensitized with 30 cycles CdS QDs (Z/30C NRs) have a saturated light absorption property with the edge at around 550 nm. Under standard simulated sunlight irradiation, the maximum photocurrent density of Z/30C NRs is raised up to 5.23 mA cm-2, which is 9.5 times higher than ZnO NRs. The maximum photoconversion efficiency (PCE) of 2.80 % is observed for the Z/30C NRs. And the IPCE of the Z/30C NRs reaches the highest value about 70.5 % at 350 nm. The introduction of CdS could effectively reduce the charge transfer resistance between the electrode and the electrolyte, and improves the PEC performance.Secondly, compared to one-step low temperature thermal treatment, the two-step low temperature thermal treatment is proposed to optimize the interface of ZnO/CdS electrode (Z/30C-150/250 NRs). It is found that the two-step treatment could obtain a better PEC performance than the one-step treatment. After thermal treatment, the superior photocurrent density of the Z/30C-150/250 NRs can reach 9.16 mA cm-2 and the IPCE value rises to 95 % at 350 nm. The maxrimum PCE also reaches 4.03 %.From the electrochemical impedance spectroscopy (EIS) fitting results, the charge tansfer resistance across the interface (Rct) is reduced from 8294 Ω to 5070 Ω. As a result, the two-step low temperature thermal treatment not only improves the crystallinity of the CdS layer but also optimize the interface through preferential orientation of CdS QDs on ZnO nanorods, which could facilitate the charge transfer and lead to enhanced photoelectric properties.Finally,the first-pitnciples calculations are used to analyze the electron charge distribution and the variation of the density of states around the ZnO/CdS interface.The results reveal that the distribution of electron densities surrounded the interface of ZnO/CdS is higher than that far from the interface, which could facilitate the charge transfer. Thus, interface optimization plays an important role in enhancing PEC performance of ZnO/CdS electrode.