Triggering zinc oxide nanostructure with gold and silver nanoparticles: A SERS-active hybrid Plasmonic construct

Zinc oxide nanostructures (ZnO NSs), amongst the most extensively studied materials, are often decorated with noble metals in SERS studies to synergistically combine the advantages of both components. In this work, a simple and generic route has been developed to functionalize such ZnO NSs by gold nanoparticles (Au NPs) and silver nanoparticles (Ag NPs) simultaneously. The as-developed nanostructures, called hereafter Au-ZnO-Ag NSs, were found to be highly SERS-active compared to their counterpart, ZnO NSs. To understand the growth morphology and the mechanism behind strong SERS-activity, a series of specimens such as ZnO mists coated Ag nanostructures (i.e., ZnO-Ag NSs), Ag NPs and Au NPs on glass substrates have been collected and investigated. The morphology of each individual construct has been investigated along with their elemental composition by high resolution field emission scanning electron microscope (FESEM). It was evident that the core element (i.e., Ag NPs) of the Au-ZnO-Ag NSs, was diverse in size and shape and therefore the strongest enhancement factor (EF) was achieved. For ZnO-Ag NSs, ZnO was noticed to be small mists of diameter ca. 25 nm and deposited surrounding the Ag NPs. Such arrangement was speculated to facilitate charge transfer process. The chemical state and electronic structure of the elements in the Au-ZnO-Ag NSs were investigated by X-ray photoelectron spectroscopy (XPS) measurements. SERS-activity of Ag NPs, Au NPs, ZnO-Ag NSs and Au-ZnO-Ag NSs was confirmed using Rhodamine 6G (R6G) as Raman-active dyes with two different excitations. Two lasers such as 633 nm (i.e., 1.96 eV) and 532 nm (i.e., 2.33 eV) were used as excitations in SERS measurements. The EF of SERS was estimated for each construct and was found to be in the order of 10⁶. A plausible mechanism in terms of the charge transfer process has been demonstrated. It was speculated that ZnO NSs were influenced by both Ag NPs and Au NPs and contributed to a high EF. Such a versatile fabrication approach, along with a related investigation, is essential not only for unlocking the potential of SERS applications but also for revealing the underlying plasmonic properties of complex SERS-active nanostructures.