Dielectric spectroscopy of water dynamics in functionalized UiO-66 metal-organic frameworks

Abstract

We present a dielectric spectroscopy study of dipolar dynamics in the hydrated UiO-66(Zr) type metal-organic frameworks (MOFs) functionalized with −NH2 and −F groups. Experiments are performed in a broad temperature and frequency ranges allowing us to probe several dipolar relaxations. For both samples at temperature below 220 K, we observe confined supercooled water dynamics, which can be described by the Arrhenius law. At slightly higher temperature, a second less pronounced dipolar relaxation is identified, and its origin is discussed. At even higher temperature, the dielectric permittivity exhibits anomalous increase with increasing temperature due to the proton conductivity. Upon further heating, the permittivity shows a sudden decrease indicating a reversible removal of water molecules. Measurements of the dehydrated samples reveal absence of all three dipolar processes.