Preparation of Chromium Hydroxide Sub-micro- and Nanoparticles by Microwave Dielectric Heating

This paper reports the preparation of spherical amorphous chromium hydroxide particles by the forced hydrolysis method combined with microwave dielectric heating. The sols were obtained by aging both chrome alum (KCr(SO4)2·12H2O) solutions and solutions of Cr(NO3)3 - K2SO4 with different initial [Cr3+]/[SO42-] ratios, at the boiling temperature. In the first case the experiments yielded sols composed of submicroparticles. The aging times were shorter (two order of magnitude less) than those reported in the literature by using conventional heating. In the second case the mean size of particles decreased drastically from the submicrometric range for [Cr3+]/[SO42-] ratios of 0.5,0.8 and 1.0 to the nanometric one for ratios 1.6 and 2.0. This finding is the first successful demonstration of applicability of the forced hydrolysis method to obtain nanoparticles of this material. A mechanism of nanoparticles formation is proposed.