Oxidation-assisted alkaline precipitation of nanoparticles using gas-diffusion electrodes

Abstract

Metal oxide nanoparticles become increasingly important as functional materials because their diversity in composition and structure allow the control of their physical properties. This work investigates gas-diffusion electrocrystallization (GDEx) as a method to synthesize metal (oxy)(hydr)oxide nanoparticles (NPs) via oxidation-assisted alkaline precipitation (Ox-AP) by using gas-diffusion electrodes. GDEx was benchmarked against alkaline titration (AT). NPs were synthesized from ZnCl₂, MnCl₂, or FeCl₂ precursor solutions at room temperature. Using AT, Zn(OH)₂, Mn₃O₄, and FeO NPs were synthesized, respectively. Using GDEx, Zn(OH)₂, Mn₃O₄, and Fe₃O₄ NPs were synthesized, respectively. The AT and GDEx process of the ZnCl₂ and MnCl₂ solutions demonstrated very similar pH behavior during precipitation and the Zn(OH)₂ and Mn₃O₄ NPs synthesized with either technique were similar in size, morphology, and composition. For these cases, AT and GDEx both elicited alkaline precipitation and were considered equivalent processes for NP synthesis. In contrast, the AT and GDEx process of the FeCl₂ solution demonstrated very different pH behavior during precipitation. Moreover, the FeO NPs, synthesized with AT, were much larger and of different shape and composition than the Fe₃O₄ NPs, synthesized with GDEx. The smaller sizes obtained with GDEx are suggested to result from an Ox-AP mechanism caused by the oxidation of Fe(II) to Fe(III) by H₂O₂ or HO₂⁻ during precipitation which presumably improves condensation kinetics and increases the supersaturation, both well-known size-determining factors.