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

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.