Effect of surface and defect chemistry on the photo-catalytic properties of intentionally defect-rich ZnO nanorod arrays

Abstract

Due to the abundance of intrinsic defects in zinc oxide (ZnO) the material properties are often governed by same. Knowledge of the defect chemistry has proven to be highly important, especially in terms of the photo-catalytic degradation of pollutants. Given the fact that defect-free materials or structures exhibiting only one type of defect are extremely difficult to produce, it is necessary to evaluate what influence various defects may have when present together in the material. In this study, intentionally defect-rich ZnO nanorod (NR) arrays are grown using a simple low-temperature solution-based growth technique. Upon changing the defect chemistry using rapid thermal annealing (RTA) the material properties are carefully assessed and correlated to the resulting photo-catalytic properties. Special focus is put on the investigation of these properties for samples showing strong orange photoluminescence (PL). It is shown that intense orange emitting NR arrays exhibit improved dye-degradation rates under UV-light irradiation. Furthermore strong dye-adsorption has been observed for some samples. This behavior is found to stem from a graphitic surface structure (e.g. shell) formed during RTA in vacuum. Since orange-luminescent samples also exhibit an enhancement of the dye-adsorption a possible interplay and synergy of these two defects is elucidated. Additionally, evidence is presented suggesting that in annealed ZnO NRs structural defects may be responsible for the often observed PL emission at 3.31 eV. However, a clear correlation with the photo-catalytic properties could not be established for these defects. Building on the specific findings presented here, this study also presents some more general guidelines which it is suggested, should be employed when assessing the photo-catalytic properties of defect-rich ZnO.