Mn and Fe-doped Co3O4 nanoparticles were prepared by a simple precipitation method. The synthesized particles were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), UV-Vis absorption spectroscopy, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, and vibrating sample magnetometer (VSM) techniques. XRD analysis showed the cubic structure of Co3O4. SEM and TEM images confirmed the formation of interconnected nanoparticles. Mn and Fe-doped Co3O4 showed broad absorption in the visible region compared to undoped sample and the band gap values are red shifted. Five Raman active modes were observed from the Raman spectra. FTIR spectra confirmed the spinel structure of Co3O4 and the doping of Mn and Fe shifts the vibrational modes to lower wave number region. The magnetic measurements confirmed that Fe-doped Co3O4 shows a little ferromagnetic behavior compared to undoped and Mn-doped Co3O4, which could be related to the uncompensated surface spins and the finite size effects.

Undoped and Mn-doped (1 at.%) Co 3 O 4 were synthesized by simple precipitation method. The morphology of the material was investigated by scanning electron microscopy which confirmed the formation of nanorods (NRs). The NRs were formed from interconnected nanoparticles. X-ray diffraction analysis confirmed the presence and crystalline nature of the Co 3 O 4 NRs. FTIR studies revealed that the defects are present in undoped Co 3 O 4 . Raman spectra confirmed the presence of five Raman active modes and the doping of Mn shifts the Raman peaks to higher wave number. Spectral responses of clad modified fiber optic gas sensor are studied for various concentrations (0-500 ppm) of ethanol at room temperature, where the clad portion is replaced with undoped and Mn-doped Co 3 O 4 and used as gas sensing material. Since the refractive index of the samples calculated from UV-Vis absorption spectra is greater than that of core, the fiber optic sensor works under leaky mode condition. The sensor showed linear variation in the spectral peak intensity with ethanol concentration. Undoped Co 3 O 4 exhibits higher sensitivity compared to Mn-doped Co 3 O 4 due to defects present in the undoped sample which offers more reactive sites for ethanol adsorption.

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