Developing Synthetic Strategies for Ligand-Free Noble Metal Nanostructures

Abstract

As an important class of nanomaterials, noble metal nanostructures (NMNs) have attracted significant attention because of their wide variety of applications in catalysis, photonics, sensing and medicine. The catalytic and optical properties of NMNs can be effectively tailored by tuning their size and shape. Till date all the reported methods mainly used capping ligands to control the size and shape of NMNs. But the presence of surfactants on the surface of NMNs may drastically affect their catalytic activity, stability in harsh conditions, and usage in biological applications. Therefore, our research objective is to design novel synthetic methods for the synthesis of ligand-free noble metal nanoparticles with controllable size, shape, composition and exposed facets. In this regard, we mainly employed mesoporous hollow silica shells (mHSS) as nanoreactors for tuning the size and shape of noble metal nanostructures. mHSS mainly act as a nano-container for entrapping noble metal precursor molecules. Firstly, we developed a new volume-confined method to tune the size of naked gold nanoparticles inside mHSS by soaking mHSS in aqueous chloroauric acid solution followed by a simple heating process. The resulting ligand-free Au nanoparticles exhibit enhanced catalytic activity compared to sodium citrate-capped gold nanoparticles. A new photochemical reduction method is also introduced for the first time to tune the shapes of gold nanoparticles inside the nanocavity of mHSS without using any capping ligands. The results confirm that the presence of Ag+ ions promotes the growth of Au triangular nanoplates inside mHSS. Finally, we introduce a novel facile method which can be used as a universal procedure for the synthesis of noble metal M@SiO2 (M=Ag, Au, Pd, Pt) yolk-shell nanoparticles. In this study, we demonstrate that nanocavity of mHSS plays a critical role in enhanced nucleation rate inside the cavity which eventually leads to the formation of yolk-shell nanoparticles. The growth of nanoparticles can be controlled by tuning deprotonated species (=Si-O-) present on the mHSS with different pHs.