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 MnO_x/FeO_x 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 H 2 O. Here we report a novel route to synthesize either fully dense or hollow core/shell metal-oxide nanoparticles (MnO x /FeO x ) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O 2 carried by the not properly degassed H 2 O or (CH 3 ) 3 NO) allows the formation of hollow structures by a galvanic reaction between the MnO x and FeO x phases. In particular, the use of (CH 3 ) 3 NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnO x /FeO x nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnO x /FeO x hollow nanoparticles by an intermediate step where the MnO/Mn 3 O 4 seeds are oxidized into Mn 3 O 4, thus allowing the Mn 3+ → Mn 2+ reduction by the Fe 2+ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn 3 O 4 /Fe 3 O 4 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.