Microwave-assisted green synthesis of Ni-integrated ZnCo2O4 nanospheres with enhanced photocatalytic and supercapacitive performance

Abstract

Nickel-doped zinc cobaltite nanospheres (ZnCo2-xNixO4; x = 0.0, 0.01, 0.05, 0.10, 0.15) were synthesized via a green, microwave-assisted route, a strategy that has been rarely explored for Ni-doped ZnCo2O4 spinel oxides, for dual applications in photocatalysis and electrochemical energy storage. The rapid and energy-efficient microwave process enabled uniform Ni incorporation and the formation of porous nanospheres under mild reaction conditions. Structural analyses using X-ray diffraction and Raman spectroscopy confirmed the successful substitution of Ni ions into the spinel lattice, while HR-TEM and EDX mapping revealed well-defined nanospheres with homogeneous elemental distribution. UV–Vis diffuse reflectance spectroscopy coupled with Tauc analysis indicated a reduced and optimized band gap, promoting enhanced visible-light absorption. Among the studied compositions, the ZCNO-0.15 sample demonstrated outstanding photocatalytic performance, achieving 96 % degradation of methylene blue within 60 min under visible-light irradiation, along with excellent reusability. The valence-band and conduction-band edge positions were evaluated using the Sanderson-Mulliken electronegativity approach, revealing the dominant reactive radical pathways responsible for the enhanced photocatalytic activity. The electrochemical responses indicated that ZCNO-0.15 electrode provided high specific capacitance of 331.42 F g⁻¹ in 2 M KOH electrolyte at 1 A g⁻¹ accompanied by low charge transfer and equivalent series resistance, indicative of improved charge-transport kinetics. Moreover, the electrode shows excellent durability, retaining 96 % of its initial capacitance after 3000 charge-discharge cycles. Overall, this study establishes microwave assisted synthesis as a sustainable and effective approach for producing Ni doped ZnCo2O4 spinel nanomaterials with significant potential for environmental remediation and high-performance energy storage utilization.