Micro- to Macroscopic Observations of MnAlPO-5 Nanocrystal Growth in Ionic-Liquid Media
Résumé
Micro-and macroscopic studies of nucleation and growth processes of MnAlPO-5 nanosized crystals under ionothermal synthesis conditions are reported herein. The samples treated at 150 degrees C were extracted from the reaction mixture at various stages of crystallization, and characterized by XRD; SEM; thermogravimetric analysis (TGA); (31)P and (27)Al solid-state magic angle spinning (MAS) NMR, Raman, UV/Vis, and X-ray fluorescence spectroscopy (XRF). The starting raw materials (alumina, manganese, and phosphorous) were dissolved completely in the ionic liquid and transformed into an amorphous solid after 5 h of ionothermal treatment. This amorphous solid then undergoes structural changes over the following 525 h, which result in an intermediate phase that consists of octahedral Al species linked to the manganese and phosphate species. The first MnAlPO-5 nuclei on the surface of the intermediate can be observed after 50 h ionoheating. These nuclei further grow, as the surface of the intermediate is in full contact with the ionic liquid, to give crystalline MnAlPO-5 nanoparticles with a mean diameter of 80 nm. The crystals become fully detached from the intermediate and are then liberated as discrete particles after 90 h heating. The transformation process from amorphous to intermediate and then to the crystalline MnAlPO-5 nanoparticles shows that nucleation starts at the solid-liquid interface and continues through surface-to-core reversed-growth until the entire amorphous solid is transformed into discrete nanocrystals.