Temperature-programmed nitridation of monodispersed VOx nanoparticles into nanocrystalline superconducting oxygen-doped vanadium nitride

Abstract

A two-stage synthesis process was employed to prepare high-quality nanocrystalline vanadium nitride (VN) for superconducting applications. Firstly, monodispersed amorphous VOx nanoparticles were obtained via thermal-decomposition of the vanadium(III) acetylacetonate [V(acac)₃] precursor in phenyl ether using oleylamine as a surface stabilizing agent. In the second stage, VOx nanoparticles were nitrided using a temperature-programmed reduction reaction at 700 °C under an NH₃ atmosphere. Finally, at room temperature a nano-sized nitride sample was oxygen passivated by flowing 0.1% O₂-containing N₂ gas before removing from the furnace to avoid bulk-oxidation of nanocrystals. X-ray diffraction (XRD) peak reflection confirms the formation of phase-pure VN. The transmission electron microscopy (TEM) image displays that the particles are non-agglomerated and have a size distribution of 17.71 ± 3.59 nm. X-ray photoelectron spectroscopy (XPS) study provides evidence that the main oxidation state of vanadium lies between (III) and (0) in the sample. However, it also appears that vanadium located on the surface of VN nanocrystals is oxidized during the passivation. Hence, the present synthesis strategy leads to oxygen-doping on the surface of VN nanoparticles that results in the formation of a vanadium oxide/oxynitride thin-layer on the surface. In addition, the temperature-dependent magnetization study of the product exhibits an abrupt decrease in the magnetization susceptibility at a temperature of 7.1 K, which indicates the onset superconducting transition temperature (T_c) of the prepared VN. The exciting feature in this magnetization study is that the observed T_c value of VN nanocrystals is similar to that of the pure bulk-VN and is not affected by the existence of different atomic arrangements on the surface of nanocrystals. This property makes it a potential candidate for the future development of new superconducting materials.