Galvanic replacement onto complex metal-oxide nanoparticles: impact of water or other oxidizers in the formation of either fully dense onion-like or multicomponent hollow MnOx/FeOx structures

Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metal-oxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of HO. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnO/FeO) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O carried by the not properly degassed HO or (CH)NO) allows the formation of hollow structures by a galvanic reaction between the MnO and FeO phases. In particular, the use of (CH)NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnO/FeO nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnO/FeO hollow nanoparticles by an intermediate step where the MnO/MnO seeds are oxidized into MnO, thus allowing the Mn → Mn reduction by the Fe ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/MnO/FeO particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored.Financial support from the 2014-SGR-1015, MAT2010-20616-C02, MAT2013-48628-R, MAT2013-41506, and CSD2009-00013 projects is acknowledged. ALO acknowledges the Juan de la Cierva Program (MINECO IJCI-2014-21530). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).Peer Reviewe