Phase‐pure spinel‐type magnetic nickel ferrite (NiFe2O4) nanocrystals in the size range of 4 to 11 nm were successfully synthesized by a fast and energy‐saving microwave‐assisted approach. Size and accessible surface areas can be tuned precisely by the reaction parameters. Our results highlight the correlation between size, degree of inversion, and magnetic characteristics of NiFe2O4 nanoparticles, which enables fine‐tuning of these parameters for a particular application without changing the elemental composition. Moreover, the application potential of the synthesized powders for the electrocatalytic oxygen evolution reaction in alkaline media was demonstrated, showing that a low degree of inversion is beneficial for the overall performance. The most active sample reaches an overpotential of 380 mV for water oxidation at 10 mA cm−2 and 38.8 mA cm−2 at 1.7 V vs. RHE, combined with a low Tafel slope of 63 mV dec−1.
Dedicated to Prof. Dr. Josef Breu on the occasion of his 60th birthdayPhase-pure 6 nm spinel MnFe 2 O 4 nanoparticles with high specific surface area of 145 m 2 g À 1 were successfully prepared via microwave-assisted non-aqueous sol-gel synthesis. The phase evolution during postsynthetic thermal treatment was investigated systematically by various methods, including powder X-ray diffraction (PXRD), pair distribution function (PDF) analysis, and Raman spectroscopy. Our results show that the material decomposes to non-spinel binary compounds α-Mn 2 O 3 and α-Fe 2 O 3 at temperatures between 400 and 600 °C. The application potential of MnFe 2 O 4 nanoparticles with 3d 5 Mn(II) and Fe(III) ions with respect to the magnetic properties was demonstrated by superconducting quantum interference device (SQUID) magnetometry, with the as-synthesized nanoparticles reaching a high saturation magnetization of 2.62 μ B per formula unit (63.5 Am 2 kg À 1 ) at 10 K. We further highlight the visiblelight response of synthesized powders, making the materials promising for light-related applications, e. g. photocatalytic hydrogen evolution. An important additional feature of MnFe 2 O 4 nanoparticles is their good dispersibilty in polar or non-polar media, as a result of postsynthetic colloidal stabilization with betaine hydrochloride, oleic acid combined with oleylamine, or citric acid.
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