Magnetism and plasmonic performance of mesoscopic hollow ceria spheres decorated with silver nanoparticles

Abstract

We investigate the role of interfaces and surfaces in the magnetic and surface enhanced Raman spectroscopy (SERS) properties of CeO₂ hollow spheres decorated with Ag nanoparticles (H-CeO₂@Ag). The composites, H-CeO₂@Ag, were synthesized using a newly developed two-step process. The CeO₂ hollow sphere diameter ranges from 100 nm to 2 μm and the grafted Ag nanoparticle (NP) size varies from 5 to 50 nm with a controllable coverage ratio. Spectroscopic and microscopic characterization confirms the formation of an interface between the Ag and ceria and shows different charge rearrangements occurring at both the interface and the surface. Room temperature ferro-magnetism was observed in all composites, and is associated mostly with ceria surface defects. A strong SERS effect was reported with a detection limit down to 10⁻¹⁴ M for the rhodamine 6G analyte. Scanning transmission electron microscopy and electron energy loss spectroscopy investigation reveals that hot-spots are associated with the silver NP surfaces and also with the Ag/CeO₂ interface. This interfacial hot spot occurs for metallic particles above 30 nm and is strongly red shifted with respect to the Ag surface plasmon. The strong SERS activity is then attributed to the presence of several types of hot-spots and the geometrical features (buoyant hollow sphere and size dispersion) of the composite.