Synthesis and Characterization of Anisotropic Nanostructures of Noble Metals (Au, Ag) for Enhanced Optical and Catalytic Applications

Abstract

The most fascinating aspect of nanotechnology is the controlling physical and chemical properties of particles in nanoscale via changing the size, shape and, materials. Nanoparticles (NPs) and noble metal (Au, Ag, Pt, Pd etc.) NPs in particular, have become one of the most expanding fields in nanoscience because of their unique quantum confinement, larger surface energies and surface plasmon resonance. The rates of scattering and absorption, as well as the position of the plasmon band (SPR peak) highly depends upon the shape, size, composition of materials, dielectric environment, and particle–particle separation distance of nanoparticles and surface area of the nanometals.
Hence, giving shape anisotropy (non-spherical) to the noble metal nanomaterials by tuning the particle geometry can produce great changes in the surface plasmon peak position of a metallic nanoparticle, which show better plasmonic properties over isotropic nanomaterials (spherical). Apart from this the spatial confinement of electrons, phonons, asymmetric axes, electric fields around the particles and, large surface to volume ratios make anisotropic noble metal nanoparticles a good candidate in varieties of optical, magnetic, sensing and catalytic applications. Hollow anisotropic nanoparticles have higher surface to volume ratios than other anisotropic nanoparticles and consequently, extra corner atoms with higher energy per atom, exposure to different crystallographic facets with lower surface energies, which can offer different types of catalytic sites. In addition, the optical properties of anisotropic plasmon materials are tuneable throughout the visible, near infrared (NIR), and infrared regions of the spectrum, as a function of their aspect ratios.
Importing metal core to the dielectric composite nanostructures such as SiO2 can protect them from degradation; catalytic poisoning, enhance their biocompatibility, thermal stability and dispersion, which makes a revolutionary change mainly in biomedical applications. Moreover, the combination of different plasmonic nanoparticles with SiO2 can make an advancement in the synthesis of bimetallic (alloys and core-shell) nanostructures with enhanced thermal and chemical stability in comparison with the individual metal nanoparticles, making them prospective candidates for different cost-effective practical applications.
Among the various routes of synthesis, galvanic replacement reactions provide an effective and simple route for generating metal nanostructures with accessible hollow interior and porous walls. Reported herein is simple “galvanic replacement method” to prepare triangular nanoframe structures of gold-silver alloy with a high morphological yield. Ag exhibits the sharpest and strongest bands among all noble metals, and due to this fact Ag NPs are highly used in different catalytic applications. Apart from that Au being inert in nature and biocompatible is preferred for biological applications in human cells. Therefore, out of all plasmon metals; we have taken Ag as sacrificial core to give it a Au-Ag triangular frame like structure by galvanic replacement reaction. A comparative study of the optical as well as catalytic applications such as fluorescence enhancement, 4-nitrophenol reduction, singlet oxygen (1O2) evolution for four different synthesized NPs; Spherical Au nanoparticles, Ag nanotriangles, triangular Au nanoplates an Au-Ag triangular nanoframes has given. Apart from this, a trial on the synthesis of triangular Au-Ag@SiO2 core-shell structure has also been done for the further study of it in different optical, catalytic and sensing applications.