Nano spinel cobaltites and their catalytic and electrochemical properties: facile synthesis of metal (Co, Ni, and Zn) and mixed metal (Co–Ni and Co–Zn) complexes of Schiff bases prepared from α-ketoglutaric acid and ethyl carbazate

Abstract

Novel divalent Co(II), Ni(II) and Zn(II) Schiff base complexes have been synthesized by reacting a prepared Schiff base (H₂L, 2-(ethoxycarbonyl-hydrazono)-pentanedioic acid) with metal salts. Ligand H₂L was prepared through condensation of α-ketoglutaric acid with ethyl carbazate. Solid solutions [Ni_1/3Co_2/3(HL)₂] and [Zn_1/3Co_2/3(HL)₂] were prepared by performing the reaction with stoichiometric proportions of metal ions. The crystal structures of metal (Co and Zn) and mixed metal (Ni–Co and Zn–Co) complexes were isomorphic and crystallized in a monoclinic system of space group C2/c. Interestingly, prepared solid solutions were employed as precursors for producing nanospinels such as nickel cobaltite (NiCo₂O₄) and zinc cobaltite (ZnCo₂O₄). As-synthesized spinel cobaltite nanoparticles (NPs) were characterized by Fourier-transform infrared (FT-IR) spectroscopy, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HR-TEM) techniques. PXRD revealed a predominant spinel-type structure with a crystalline nature and high purity. Lattice parameters were calculated to be 8.1120 Å for NiCo₂O₄ and 8.1178 Å for ZnCo₂O₄. The morphology studies (by FESEM and HRTEM) revealed that the spinels were built by self-assembled aggregation of NPs with irregular shapes. Based on the TEM and HRTEM images, the mean sizes of the NPs synthesized by as-prepared solid solutions were 22–24 nm for NiCo₂O₄ and 16–18 nm for ZnCo₂O₄. In addition, photocatalytic degradation activity was evaluated using rhodamine dyes in visible light. Surprisingly, nanospinels employed as electrode materials showed a good specific capacitance C_sp of 115 F g⁻¹ (NiCo₂O₄), 177 F g⁻¹ (ZnCo₂O₄) at a 0.25 A g⁻¹ current density. Furthermore, the as-prepared nanospinels showed desirable stability in alkaline electrolyte with excellent cycling performance.