Three dimensional ZnO nanotube arrays and their optical tuning through formation of type-II heterostructures

Abstract

In this paper, we report on the first successful attempt of chemical vapor deposition (CVD) synthesis of well-aligned single-crystalline ZnO nanotube arrays on Mo wire mesh. According to detailed morphology and composition analyses, a rational growth model is proposed to illustrate the growth process of the hollow ZnO nanotubes. Metastable Zn-rich ZnO_x nanorods formed in the early stage are believed to play a vital role towards the formation of nanotube configuration. In addition, we also successfully fabricate ZnO/ZnS and ZnO/CdS core/shell nanotube arrays by simply depositing ZnS and CdS on the pre-fabricated ZnO nanotubes. Despite the existence of a large lattice mismatch, the grown ZnS and CdS layers are somewhat single-crystalline and show an epitaxial orientation relationship with the inner ZnO, that is, [0001]_{ZnS or CdS}//[0001]_ZnO and (10-10)_{ZnS or CdS}//(10-10)_ZnO. Further, room temperature cathodoluminescence (CL) characterization indicates that after surface decoration, the formed ZnO-based heterostructures show distinguished optical properties. Quenching of 67.2% and 99.7% of ZnO near-band-gap (NBE) emission is observed on ZnO/CdS and ZnO/ZnS, respectively. Efficient charge separation resulting from type-II band configuration is considered to be responsible for the variation of optical properties. Our method may represent a powerful synthesis platform for creation of ZnO nanotubes and ZnO-based hollow heterostructures with tunable properties.