Surface conditions of nano-sized inorganic particles and pore structure control of their agglomerates

The aggregation phenomena or several metal oxide nanoparticles were analyzed and discussed based on the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory of colloidal stability. The attractive van der Waals (VDW) and the repulsive electrostatic double-layer (EDL) forces acting upon a pair of approaching particles are two kinds of the fundamental interaction forces that control their aggregation. The surface potential and the Debye length of the particles are also important factors. The particles that disperse in an aqueous solution were forced to aggregate under unstable dispersion conditions wherein the EDL interaction becomes much weaker than the VDW interaction between two particle surfaces. Under this condition, the agglomerated nanoparticles having a disorderly arranged structure, i.e. a lot of small pores, are created. In order to control the surface potential and the Debye length on the nanoparticles of the Ti-, Zn-, Al-, Sn- and Sb-oxides, the electrolyte (KBr) and acid/alkali were added into the colloidal solutions. Agglomerated their nanoparticles with pore structure were prepared after evaporating the solutions and removing KBr. In the case of the Ti- and Zn-oxides, a strong aggregate force was observed among the nanoparticles. The pore size and the pore volume of the agglomerated nanoparticles increased with decreasing energy barrier of the interaction energy among these particles. The other oxide nanoparticles also formed an agglomerate body in each pH-controlled (metal oxide/KBr) solution. However, the aggregate force was so weak that the particles could not keep the aggregate state in the removal stage of KBr, which resulted in the re-dispersion of the agglomerated particles into their primary particles. From the DLVO calculations of the experimental systems, it was confirmed that the dispersion /aggregation phenomena of the nanoparticles observed in each stage of the experiments are determined by the balance of the attractive and the repulsive interactions among them and their thermal kinetic energy.