Urea-Melt Assisted Synthesis of Ni/NiO Nanoparticles Exhibiting Structural Disorder and Exchange Bias

An easy, inexpensive urea-melt assisted route was designed for the synthesis of ∼10 nm-sized Ni/NiO nanoparticles (NPs). The method consists of the thermal decomposition of a urea-melted medium containing a Ni 2+ -salt and involves the olygomerization and self-combustion of the organic matrix and the formation of the Ni/NiO NPs within it in a two stepped process of Ni 2+ reduction yielding Ni crystallization and subsequent inward oxidation. Control over the microstructural features of the NPs within three typical regimes was accomplished by means of the initial urea quantity. Type A samples (0.2 g urea/0.2908 g nickel nitrate) consists of NiO nanostructures which might contain dispersed Ni nuclei of a few atoms; B NPs (2 g urea/0.2908 g nickel nitrate) are made of proper small Ni cores, in the limit of the coherence length of XRD, surrounded by thick NiO shells, while C (20 g urea/0.2908 g nickel nitrate) NPs correspond to relative big Ni cores and thin NiO shells. Further control over the thickness of the NiO shell was achieved within each regime by means of the calcination time of the organic matrix, painting a whole palette of Ni core −NiO shell NPs. Magnetic characterization revealed large (for A and B) and common (C) shifts of the hysteresis loops, which were ascribed to the high disorder of the (A, B and C) NPs obtained by quenching as well as to exchange bias (EB) effects (B and C). A thorough characterization of the reaction pathway is reported.