Fig. 2

X-ray diffraction (XRD) and Fourier-transform infrared (FTIR) spectra of Co₃O₄ NPs. a XRD spectrum of Co₃O₄ NPs before heating in the furnace (AMP), after heating at 500 °C for 2 h (Co₃O₄ NPs-h 500 °C), and after heating at 600 °C for 2 h (Co₃O₄ NPs-h 600 °C). b FTIR spectrum of Co₃O₄ NPs before annealing, and c after annealing at 600 °C for 2 h