Mechanistic Study of the Synthesis of Au Nanotadpoles, Nanokites, and Microplates by Reducing Aqueous HAuCl₄ with Poly(vinyl pyrrolidone)

This article describes a simple approach to anisotropic Au nanostructures with various shapes by reducing HAuCl4 with poly(vinyl pyrrolidone) (PVP) in aqueous solutions without the use of any additional capping agent or reductant. In this approach, the commercially available PVP servers as a mild reducing agent thanks to its hydroxyl (−OH) end groups, enabling kinetic control over both nucleation and growth. As the volume of HAuCl4 solution added to the reaction was increased, the morphology of Au nanostructures evolved from nanotadpoles to nanokites and then triangular and hexagonal microplates. The slow reduction rate associated with the mild reducing power of PVP plays a critical role in forming nanoplates during nucleation as well as their growth into highly anisotropic nanostructures. Electron microscopy studies reveal that the nanotadpoles and nanokites are formed through the linear fusion of small Au particles (<10 nm) to the initially formed nanoplates, whereas the microplates result from the continuous addition of Au atoms to the side faces of nanoplates. Through this morphological control, the localized surface plasmon resonance peaks of these Au nanostructures can be tuned in the visible and near-IR regions.