Completely green synthesis of rose-shaped Au nanostructures and their catalytic applications.

The importance of and demand for eco-friendly syntheses of metal nanocrystals are increasing. In this study, a novel protocol for the one-pot, template/seed-free, and completely green synthesis of rose-shaped Au nanostructures with unique three-dimensional hierarchical structures was developed. The synthesis of the nanostructures was carried out at room temperature using water as a reaction medium and an eco-friendly biopolymer (sodium salt of alginic acid (Na-alginate)) as a reducing agent. The morphologies of the Au nanostructures were controlled by adjusting the amount of capping ligand (polyvinylpyrrolidone (PVP)) in the reaction mixture, and a limited ligand protection (LLP) strategy was used to induce the formation of rose-shaped Au nanostructures. A formation mechanism for the rose-shaped Au nanostructures was proposed on the basis of structural characterizations and the shape evolution of the nanostructures. The unique structural features of the rose-shaped nanostructures, which include a high surface roughness, a large surface area-to-volume ratio, and abundant edges and sharp tips, motivated us to use them as a high-performance catalyst. They were used as an environmentally benign catalyst in an organic reaction to remove a hazardous chemical from an aqueous medium: specifically, the hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP) by sodium borohydride. Without an additional supporting material, the rose-shaped Au nanostructures showed outstanding catalytic activity that was maintained when the catalyst was recycled and used a total of five times.