Phase evolution, texture behavior, and surface chemistry of hydrothermally derived scandium (hydrous) oxide nanostructures

Nanostructured scandium hydrous oxides were hydrothermally synthesized at 180 degrees C for 18 h, using NaOH, NH(4)OH, and KOH as the bases. They were characterized by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption, thermogravimetry and differential thermal analysis (TG-DTA), infrared and Raman spectroscopy, and pyridine adsorption. XRD and TEM measurements showed that the nature and concentration of the bases played key roles in determining the phasic composition, texture behavior (shape and size), and surface chemistry of the hydrothermal products. In addition, the shape evolution of the crystalline products seemed to be closely connected with their crystal structures. As the basicity value was raised from pH 10 to 5 mol L(-1) NaOH (or KOH), alpha-ScOOH nanorods, alpha-ScOOH nanosized hexagonal-like plates, and cubic Sc(OH)3 cubes/cuboids in micrometer size were produced in turn; while within pH 10-12 using NH4OH, gamma-ScOOH nanosized lozenge-like plates were mainly obtained. According to XRD, TEM, and TG-DTA results, all the as-prepared nanostructured ScOOH and micrometric Sc(OH)3 could be converted to cubic Sc2O3 with sustained crystalline shape via calcination at 500 degrees C. Pyridine adsorption revealed the existence of Lewis acid sites on the surfaces of the nanostructured alpha-ScOOH samples and some of their Sc2O3 counterparts calcined at 700 degrees C. The alpha-ScOOH nanorod sample displayed the strongest Lewis acidity among all the samples tested, due to its highest surface area as determined by N2 adsorption. Finally, an olation-oxolation process based on a dissolution/recrystallization mechanism accounts for the formation of various ScOOH polymorphs and Sc(OH)3 with different shapes.