Luminescent and Transparent Nanocellulose Films Containing Europium Carboxylate Groups as Flexible Dielectric Materials

Transparent, flexible, and luminescent 2,2,6,6-tetramethylpiperidine-1-oxyl radical-oxidized cellulose nanofibril films with europium carboxylate groups (TOCN-Eu) are proposed as potential flexible dielectric materials. The TOCN-Eu films were prepared from TOCN films with sodium carboxylate groups (TOCN-Na) by immersion in Eu(NO3)3 solution and subsequent washing with water and drying of the wet TOCN-Eu hydrogel films. The nanolayered TOCN-Eu films emit red light under ultraviolet-light irradiation and exhibit greatly improved dry and wet mechanical properties, water resistance, thermal stability, oxygen-barrier properties, dielectric breakdown strength, and energy density than conventional TOCN-Na films. These characteristic properties of the TOCN-Eu films are most likely caused by the formation of strong ionic bonds between the carboxylate groups and Eu3+ ions. These high-performance cellulose nanofibril films are promising for application as multifunctional materials in light-emitting, photovoltaic, and energy storage devices.