As the calcination temperature of Co₃O₄ nanosheets decreased from 600 °C to 300 °C, systematic changes in structure, electronic state, and catalytic activity were observed. X-ray diffraction (XRD) confirmed that all samples retained the spinel crystal structure, while transmission electron microscopy (TEM) revealed consistent nanosheet morphology with a trend of decreasing particle size at lower temperatures.

Linear sweep voltammetry (LSV) measurements demonstrated that the 300 °C-calcined sample achieved the best oxygen evolution reaction (OER) performance, exhibiting the lowest overpotential of 1.61 V.

X-ray photoelectron spectroscopy (XPS) indicated an increasing Co²⁺/Co³⁺ surface ratio with decreasing calcination temperature, reflecting a more reduced and catalytically active surface.

Soft X-ray absorption spectroscopy (sXAS) further revealed higher populations of tetrahedral high-spin Co²⁺ and greater Co²⁺/Co³⁺ (HS) ratios in low-temperature samples, associated with more reactive spin states.

Extended X-ray absorption fine structure (EXAFS) analysis showed that as the temperature decreased, Co–O coordination numbers increased while Debye–Waller factors rose, suggesting a transition from ordered to moderately disordered local structures.

These temperature-dependent transformations culminated in the 300 °C sample, which featured the highest Co²⁺ content, diverse Co³⁺ spin states, moderately disordered coordination, and the best electrochemical performance.

This work demonstrates that lowering calcination temperature effectively tunes the valence, spin configuration, and local structure of Co₃O₄ nanosheets, offering a rational strategy for designing high-efficiency cobalt-based OER electrocatalysts.