One of our research work is dealing with the synthesis of particles by electrochemical method. One of them is synthesis of magnetite (Fe3O4) nanoparticles. How can electrochemical process induce particle formation? This article will give some discussion about electrochemical-induced particle formation with an example about the formation of magnetite nanoparticles.
One important thing that must be noted is that particles may be formed from solution if the solubility of a dissolved solid (solute) in solvent is exceeded, i.e., the solutiion must be supersaturated. Supersaturation refers to a state in which liquid (solvent) contains more solute than can ordinarily be accommodated at that temperature. Such process often referred to as crystallization, or precipitation if the particle formation is induced by chemical reaction. Crystallization is widely used in industries to produce some important products such as sugar, salt, monosodium glutamate, etc.
On an industrial scale, a large supersaturation driving force is necessary to initiate primary nucleation (the first step of nucleation). The supersaturation may be driven by a combination of high solute concentration and rapid cooling. Once the condition for nucleation is met, nucleation begins. Then, particles may be grown through surface growth and coagulation, and particle size distribution begins to take shape.
Regarding with the formation of magnetite nanoparticles from aqueous system in an electrochemical system, what is the driving force to initiate primary nucleation, i.e., to reach supersaturation? It has been shown that, as illustrated by the Pourbaix diagram, iron oxides and/or hydroxides (Fe2O3, Fe3O4, Fe(OH)2, Fe(OH)3) can be in electrochemical equilibrium with water at high potentials in neutral and basic solutions. Under basic conditions, Fe3O4 may be formed by the corrosion process in an aqueous medium. Therefore, if a layer of iron film adheres weakly to an anode surface, Fe2+ ions would be released from the surface during the electrooxidation of the iron.
Hence, the mechanisms of formation of Fe3O4 can be described as follows. Fe2+ ions produced by iron oxidation at anode react with OH- ions coming from water reduction at cathode. Fe(OH)2 is then partially oxidized by O2 produced by water oxidation at anode to form FeOOH. Fe3O4 is formed when an appropriate proportion of Fe(OH)2 and FeOOH is created. The overall mechanism may be illustrated schematically below (Fajaroh et al, Adv. Powder Technol., 23, 328-333, 2012).