| Summary: | Herein, we report the synthesis of spinel cobalt oxide nanorods (Co<sub>3</sub>O<sub>4</sub> NRs) by a modified co-precipitation approach and examine their larvicidal activity against <i>Culex pipiens.</i> The structure and morphology of the as-prepared Co<sub>3</sub>O<sub>4</sub> NRs were emphasized using X-ray diffraction (XRD), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDAX), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It was found that Co<sub>3</sub>O<sub>4</sub> nanostructures have a face-centered spinel cubic crystal structure with a mean crystallite size of 38 nm. These nanostructures have a rod like shape with a mean diameter of 30 nm and an average length of 60 nm. The TGA measurements revealed the high stability of the formed spinel cubic structure at 400 °C. The optical behavior indicates the direct transition of electrons through an optical band gap in the range of 2.92–3.08 eV. These unique chemical and physical properties of Co<sub>3</sub>O<sub>4</sub> NRs enabled them to be employed as a strong agent for killing the <i>C. pipiens</i>. A comparison study was employed between the as-prepared Co<sub>3</sub>O<sub>4</sub> and the entomopathogenic fungus <i>Metarhizium brunneum</i> as a control agent of <i>C. pipiens</i> larvae. The results revealed that the as-prepared nanorods have higher mortality against <i>C. pipiens</i> larvae compared with the well-known <i>M. brunneum</i>.
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