We report low temperature synthesis of nearly monodispersed NiCo2O4 nanoparticles by a combustion method utilizing glycine as a fuel and nitrate as oxidizer. An appropriate glycine-to-metal nitrate molar ratio favors the formation of nearly monodispersed NiCo2O4 nanoparticles. We discuss the relevant synthesis chemistry and their detailed characterization using different techniques such as X-ray diffraction, high resolution transmission electron microscopy, superconducting quantum interference device magnetometry, and Fourier transform infrared spectroscopy. We also show the interesting evolution of the phase and magnetic properties of such nanoparticles upon annealing treatment. Importantly, these nanoparticles show a high (∼83%) infrared transparency that is useful for specific solar and fuel cell electrode applications as well as significant radiofrequency (RF) absorption causing substantial heating of their aqueous dispersion that should have potential applications for magnetic hyperthermia.

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In the present study, a facile one-pot synthetic route, utilizing a strong polar organic solvent, N-methyl 2-pyrrolidone (NMP), is demonstrated to obtain highly monodispersed ferrite nanocrystals. The equimolar mixture of oleic acid, C(17)H(33)COOH (R-COOH), and oleylamine, C(18)H(35)NH(2) (R'-NH(2)), was used to coat the magnetic nanocrystals. Structural and magnetic properties of the ferrite nanocrystals were studied by a multitechnique approach including X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), X-ray photoelectron spectroscopy (XPS), vibrating sample magnetometry (VSM), and Mössbauer spectroscopy. FTIR spectral analysis indicates oleylamine helps in deprotonation of oleic acid, resulting in the formation of an acid-base complex, R-COO¯:NH(3)(+)-R', which acts as binary capping agent. Structural and coordination differences of iron were studied by XPS and Mössbauer spectral analysis. XPS analysis was carried out to examine the oxidation state of iron ions in iron oxide nanocrystals. The presence of a magnetically dead layer (∼0.38 and ∼0.67 nm) and a nonmagnetic organic coating (∼2.3 and ∼1.7 nm) may substantially reduce the saturation magnetization values for CoFe(2)O(4) and Fe(3)O(4) nanocrystals, respectively. The energy barrier distribution function of magnetic anisotropy was derived from the temperature dependent decay of magnetization. A very narrow energy barrier distribution elucidates that the ferrite nanocrystals obtained in this study are highly monodispersed.

In the present investigation, we report a detailed examination of the effect of off-stoichiometry introduced in NiCo 2 O 4 by adding excess cobalt. Thus, we compare and analyze the structural and magnetic properties of the Ni 0.75 Co 2.25 O 4 and NiCo 2 O 4 cubic systems. A low temperature combustion method was utilized to synthesize stoichiometric (NiCo 2 O 4 ) and off-stoichiometric (Ni 0.75 Co 2.25 O 4 ) nanoparticles on a large scale. The X-ray diffraction pattern for the sample annealed at high temperature (773 K) shows the presence of a much less intense NiO phase (∼2−5%) in Ni 0.75 Co 2.25 O 4 as compared to that in the case of NiCo 2 O 4 sample (∼15−20%). The Ni 2p and Co 2p XPS spectra reveal the coexistence of Ni 2+ , Ni 3+ , Co 2+ , and Co 3+ species on the surface of both the NiCo 2 O 4 and Ni 0.75 Co 2.25 O 4 samples in differing proportions. In addition to the basic magnetic characterizations using PPMS, these were also analyzed by neutron diffraction. The off-stoichiometric Ni 0.75 Co 2.25 O 4 sample shows an interesting magnetic phase conversion from frustrated dipolar system to an enhanced magnetic ordering upon annealing. Local moments on the lattice sites of NiCo 2 O 4 and Ni 0.75 Co 2.25 O 4 samples are further compared by neutron diffraction confirming stronger ordered moments and enhanced structural and thermal stability for the Ni 0.75 Co 2.25 O 4 sample.