Layered Phosphonates in Colloidal Synthesis of Anisotropic ZnO Nanocrystals

We describe the role of phosphonic acids in the synthesis of anisotropic colloidal ZnO nanocrystals (nanorods) and, specifically, the discovery of an insoluble layered Zn–phosphonate intermediate. This compound is formed by the reaction of the molecular Zn precursor Zn­(OAc)2 with phosphonic acids, and it acts as a heterogeneous Zn source during nanocrystal formation. Layered metal phosphonates have been studied extensively but have not been described in the context of nanocrystal synthesis. Layered Zn–octadecylphosphonate can be used as a sole precursor to obtain isotropic, soluble ZnO nanocrystals. However, for anisotropic rod-like shapes both the heterogeneous and the homogeneous (Zn­(OAc)2) sources of Zn are necessary. The existence of a heterogeneous metal source described here is in contrast to the mechanisms of particle nucleation and growth from homogeneous molecular precursors often used to describe nanocrystal formation. Since many metals form layered phosphonates, our findings have implications for synthesis of nanocrystals of other semiconductors.