One-step formation of core-shell sulfide-oxide nanorod arrays from a single precursor

Densely packed and well-aligned coaxial (core-shell) CdS-CdO and ZnS-ZnO nanorod arrays were prepared with a one-step, non-catalytic, template-free metalorganic chemical vapour deposition (MOCVD) process by using single-source molecular precursors Cd(O-EtXan)(2) and Zn(O-EtXan)(2), respectively (O-EtXan = S(2)COCH(2)CH(3)). Data from pyrolysis gas chromatography/mass spectrometry of the precursors revealed the sequence of formation, namely sulfide core first and oxide shell later, with the first formed sulfide nanorods serving as the template for subsequent in situ oxide shell coating. The coaxial CdS-CdO and ZnS-ZnO nanorod arrays were formed at collection temperatures of 180 and 200 °C, respectively. At a slightly higher collection temperature of 250 °C, coaxial ZnS-ZnO nanowires were also obtained. The coaxial nanostructure was characterized and confirmed with high-resolution transmission electron microscopy. In a photoluminescence study, near band edge emissions at around 530 nm, a composite emission of the two near band edge emissions of 524 nm of CdS and 551 nm of CdO, were observed for the coaxial CdS-CdO nanorod sample, while defect-induced emissions at around 470 nm were observed for both coaxial ZnS-ZnO nanorod and nanowire samples. A more stable oxide shell not only protects the sulfide core but also provides possible surface passivation beneficial to enhanced photoluminescence. The present work demonstrates the feasibility of growth of coaxial one-dimensional nanostructures from single-source precursors that contain all the necessary constituent elements in one compound.