Urea-Directed Precursor Architectures and Their Inheritance in Spinel NiCo2O4 on Stainless-Steel Mesh

Abstract

Although the morphology of spinel oxides is commonly controlled through precursor formation followed by thermal conversion, the roles of these two stages in defining the final architecture remain insufficiently distinguished. To bridge this gap, the present study systematically investigates the influence of urea concentration on the morphology of spinel NiCo₂O₄ grown on stainless-steel mesh (SSM) through hydrothermal synthesis (precursor formation) followed by calcination (thermal conversion) at 350°C. Diverse microscale precursor architectures are obtained over a urea concentration range of 0 (no urea)–0.7 M, evolving from limited surface nucleation to hierarchical sea-urchin-like structures composed of radially oriented nanospikes and, at higher concentrations, nanoball-like aggregates comprising interconnected nanowires and nanosheets. This concentration-dependent morphological evolution is attributed to urea hydrolysis and the associated changes in solution alkalinity and carbonate-containing species, which influence the formation, nucleation, growth, and assembly of Ni–Co precursor phases. Structural, spectroscopic, and thermal analyses indicate the involvement of carbonate- and hydroxide-containing precursor species during hydrothermal growth. Upon calcination, these species undergo chemical and crystallographic transformations accompanied by the evolution of CO₂ and H₂O to form crystalline spinel NiCo₂O₄ with largely preserved microscale architectures. These results indicate that urea concentration predominantly defines the precursor architecture, which is subsequently inherited by the crystalline spinel oxide, whereas calcination drives precursor-to-NiCo₂O₄ conversion while largely retaining the pre-established microscale morphology. This distinction between morphology-defining hydrothermal growth and phase-forming thermal conversion provides insights into the rational synthesis of architecturally controlled spinel oxide materials.