Atomic scale reversible opto-structural switching of few atom luminescent silver clusters confined in LTA zeolites

Abstract

Luminescent silver clusters (AgCLs) stabilized inside partially Ag exchanged Na LTA zeolites show a remarkable reversible on–off switching of their green-yellowish luminescence that is easily tuned by a hydration and dehydration cycle, making them very promising materials for sensing applications. We have used a unique combination of photoluminescence (PL), UV-visible-NIR Diffuse Reflectance (DRS), X-ray absorption fine structure (XAFS), Fourier Transform-Infrared (FTIR) and electron spin resonance (ESR) spectroscopies to unravel the atomic-scale structural changes responsible for the reversible optical behavior of the confined AgCLs in LTA zeolites. Water coordinated, diamagnetic, tetrahedral AgCLs [Ag₄(H₂O)₄]²⁺ with Ag atoms positioned along the axis of the sodalite six-membered rings are at the origin of the broad and intense green-yellowish luminescence in the hydrated sample. Upon dehydration, luminescent [Ag₄(H₂O)₄]²⁺ clusters are transformed into non-luminescent (dark), diamagnetic, octahedral AgCLs [Ag₆(O_F)₁₄]²⁺ with Ag atoms interacting strongly with zeolite framework oxygen (O_F) of the sodalite four-membered rings. This highly responsive on–off switching reveals that besides quantum confinement and molecular-size, coordinated water and framework oxygen ligands strongly affect the organization of AgCLs valence electrons and play a crucial role in the opto-structural properties of AgCLs.