Shell pressure on the core of MnO/Mn3O4 core/shell nanoparticles

A myriad of core/shell nanoparticles (NPs) have been developed and studied in the last decades aiming at the increase of functionality, stability, dispersibility, biocompatibility, and specific targeting, for instance.1 Shells can also interact synergetically with the core by changing lattice parameters, resulting in strain-engineered materials with tuned, improved, and new properties.2,3 This includes materials with strain-induced ordered structures,4 materials with tuned thermodynamic properties,5,6 and materials with tuned and new optical,7–13 electronic,14 electro-optical,15 magnetoelectric,16,17 and magnetic18–20 properties. In the case of magnetic properties, attention focused on coupling magnetic and electric properties by strain-induced multiferroic materials,16,17 on increasing anisotropy by shell-induced strain,19 and on controlling the core/shell exchange coupling by strain.18 In the context of exchange coupling, one of the systems that has received more attention in recent years is MnO/Mn3O4 core/shell NPs, an “inverted” system where the core is antiferromagnetic (AF) and the shell is ferrimagnetic, growing epitaxially on the core.21–26 This results in a rich magnetic behavior including an enhancement of the transition temperature of the shell due to the MnO core,23 a change in the nature of phase transition,27,28 and an increase of the MnO antiferromagnetic transition temperature TN in the NPs when compared to bulk MnO.23,26,28–30 Some of these phenomena are poorly understood and we anticipate that controlling surface oxidation and strain starting from nonoxidized cores can give a new insight on the rich magnetic behavior of MnO and MnO/Mn3O4 NPs. Literature devoted to the synthesis of MnO NPs shows that the NPs are always surface oxidized to a certain extent when exposed to air, resulting in a MnO/Mn3O4 core/shell structure, even when it is not explicitly mentioned. This is the case of MnO NPs formed by the decomposition of Mn-oleate in trioctylamine and oleic acid. The dispersion of NPs and nanopods is light green at high temperature (the color of bulk MnO) becoming brown when cooled (the color of Mn3O4), indicating surface oxidation.31 The formation of brownish oxidized MnO NPs and nanopods was also reported after the decomposition of H2O:Mn-ac in different ratios of trioctylamine:oleic acid.32 Another example is the decomposition of Mn-acetate in trioctylamine and oleic acid underN2, leading to the formation of MnO NPs with sizes from 7 to 20 nm.33 Again, the authors mention the formation of a black dispersion at high temperature suggesting that the MnO NPs are surface oxidized. In the case of MnO NPs formed by the decomposition of Mn–fatty acid salts in the presence of free fatty acid in octadecene, n-eicosane, and tetracosane at 300 ◦C,34 no direct evidence of oxidation is provided but the reduction of the MnO lattice parameter compared to that of the bulk (4.439 Å compared to 4.445 Å of bulk MnO35) is an indirect evidence of oxidation. Since spontaneous oxidation of MnO NPs is ubiquitous after air exposure, we have designed a set of experiments where MnO NPs are studied at different stages of air exposure. This allowed us to investigate the role of surface oxidation and strain on the magnetic properties of MnO NPs.