Fluorescent Ag⁺-exchanged zeolite nanoparticles with improved photoluminescence properties via X-ray irradiation

Fluorescent Faujasite (FAU)-Y Ag⁺-exchanged zeolite is a lanthanide-free phosphor with a high photoluminescence (PL) quantum yield to convert near-ultraviolet (near-UV) to visible light. Here, we focused on nanometer-sized fluorescent FAU Ag⁺-exchanged zeolite as a wavelength converter for optoelectronic devices. The prepared Ag⁺-exchanged zeolite nanoparticles were irradiated with X-rays to improve their PL properties because X-ray irradiation does not possess the disadvantages of other treatments, such as aggregation and sintering of nanoparticles exposed to thermal treatment, excessive growth of silver particles with visible light absorption following chemical reduction, and amorphization of zeolite by high-energy electron beam irradiation. FAU-Y zeolite nanoparticles were prepared by template-free hydrothermal synthesis and then fluorescent Ag⁺-exchanged zeolite nanoparticles were obtained through the ion exchange of Na⁺ with Ag⁺. The prepared Ag⁺-exchanged zeolite nanoparticles were irradiated by X-rays from a commercial tabletop device. The average size of the as-prepared zeolite nanoparticles was 35.7 ± 12.6 nm according to transmission electron microscopy (TEM) images. TEM also confirmed that Ag-related particles such as silver clusters and/or silver nanoparticles were generated and grew from Ag⁺, possibly through reduction accompanied with oxidation of oxide ions of the zeolite framework during X-ray irradiation. PL quantum efficiency was enhanced by X-ray irradiation by up to ~3 times. Analysis of PL properties and electron spin resonance spectra implied that fluorescent Ag₄²⁺ clusters were formed under X-ray irradiation, resulting in the enhancement of PL quantum efficiencies. Similar behavior was also observed for micrometer-sized Ag⁺-exchanged zeolites. X-ray irradiation of fluorescent Ag⁺-exchanged zeolites is therefore a useful method to improve their PL properties.