Self-assembled lanthanum hydroxide microspheres within a reaction-diffusion framework: synthesis, characterization, control and application

We present the synthesis of spherical microparticles by self-assembly of lanthanum hydroxide nanoplatelets in a reaction-diffusion framework, which consists of diffusing an outer ammonia solution into a hydrogel matrix containing lanthanum ions (inner electrolyte). The coupling of reaction-diffusion with nucleation and growth of crystals leads to the formation of precipitation bands well separated in space. The advantage of this method lies in its simplicity to prepare crystalline lanthanum hydroxide at ambient conditions with average particle sizes with small dispersion and varying between similar to 300 nm and similar to 70 mu m, depending on the band location in space. The control over the size of the particles is furthermore studied as a function of the concentration of the inner electrolyte, the thickness of the gel matrix and temperature. The morphology of the spheres is elucidated and is shown to exhibit fascinating topology as a result of the tight packing of the nanoplatelets. Consequently, we demonstrate the remarkable potential of these spheres to swiftly adsorb Congo red (an azo dye) in aqueous solution.