Catalytic Au Wool-Ball-Shaped Nanostructures

Gold nanostructures, particularly Au nanoparticles (NPs) have attracted significant scientific and technological interest due to their physical properties and applications in electronics, optics, catalysis, biology, and sensing. The sizes, shapes, and surface features of Au NPs constitute the fundamental parameters affecting their properties and applications. Accordingly, broad research efforts have focused on the development of synthetic routes aimed at tailoring Au NP structure and surface properties. While most Au NPs reported thus far exhibit spherical shapes and smooth surface architectures, gold particles exhibiting rough or corrugated surfaces have been sought for important applications such as sensing and catalysis, which benefit from extended surface areas. Varied wrinkled/branched Au NP morphologies have been reported, including polyhedral NPs, dendritic nanostructures, star-shaped, flowery NPs, urchin-like, and others. The challenges, however, for fabricating irregularly shaped and non-smooth Au NPs in a controlled and practical manner are significant. Most reported strategies are relatively complex, consisting of multistep processes, and requiring the addition of reducing agents, seeds, and/or templates. Furthermore, uniformity and structural reproducibility have been generally limited. Au NPs have been employed as catalytic substrates in varied reactions, including aerobic oxidation of alcohols, carbon monoxide oxidation, degradation of nitro-aromatic compounds, and others. Importantly, the catalytic efficiency of Au NPs has been linked to their sizes, shapes, and surface properties. In particular, wrinkled/branched Au NP morphologies are of interest for catalysis applications, as such particles exhibit high specific surface areas and abundant edges and crevices presumed to contribute to catalytic activity. Porosity is another important determinant for the catalytic activity of Au NPs; recent studies have demonstrated, for example, 50% enhancement of electrocatalytic activity attained in nanoporous Au NPs compared to porous gold sheets. Au “nanoflowers”, which combine nanoporosity and wrinkled surface morphology, have similarly displayed good catalytic properties for the degradation of nitroaromatic compounds. Herein, we describe the generation of unique Au “nanowool balls” and their application as broad-based catalytic substrates. The Au nanowool balls were prepared through a simple process comprising self-assembly and reduction of Au(SCN)4 in water without co-addition of nucleation seeds, templates, or reducing agents. The nanowool-balls were highly uniform in size and shape, which could be tuned by modulating the reaction conditions. We demonstrate that the Au nanowool balls exhibited excellent catalytic activity for methanol oxidation and 4-nitrophenol degradation, ascribed to the intricate surface nanoribbon morphology.