Time dependence of water-induced phase transition in nano- and microcrystalline Eu3+-doped MOF-76(Y): different luminescence responses with memory effect

Abstract

MOF-76 compounds were solvothermally synthesized, producing micrometer-sized needle-like crystals, while bean-shaped nanoparticles were successfully obtained by modifying the synthesis conditions and using sodium acetate as a modulator. The water-induced transition of MOF-76(Y_1−xEu_x) x = 0, 0.05 in nano and microcrystalline forms was studied and followed by PXRD, FTIR, SEM, and TEM. The phase transition involves changing the 3D MOF-76 (tetragonal) structure to a 1D Ln-BTC (monoclinic) structure upon water exposure. The influence of particle size, mechanic stress (ultrasound treatment), and thermal activation on the kinetics of the structural and morphological transformation was studied. Besides, the changes in photophysical properties resulting from the phase transition are also analyzed. Photophysical properties of MOF-76(Y_0.95Eu_0.05) were studied before and after water exposure, revealing efficient UV absorption and characteristic Eu³⁺ emissions for all samples. Although the emission spectra of the samples synthesized in nanometric and micrometric forms exhibit the same peaks and almost the same intensity, the corresponding spectra of the compounds upon water immersion are different, indicating a likely memory effect in transformed MOF-76 compounds regarding the initial particle size of each one. This research provides valuable insights into the material stability and phase behavior with a MOF-76 structure under different operating conditions. These results are critical for their practical applications in sensing and other technological fields.