Investigation of Lattice Vibrations and Band Gaps of Cobalt Oxides by Fourier Transform Infrared, Raman Scattering and Photoluminescence Spectroscopy
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Cobalt Oxides, Co3O4 and CoO, have remarkable properties for the application of lithium ion battery, magnetic materials and the photocatalyst. Recently, people have synthesized cobalt oxide nanostructures with different morphologies. Fourier-transform infrared spectroscopy (FTIR), Raman scattering and photoluminescence spectroscopy are the generally used techniques for the characterization of materials. I have carried out a systematic and comprehensive spectroscopic study to elucidate the phonon spectrum and electronic band structures Co3O4 and CoO. First, the FTIR spectra of Co3O4 were studied by investigating angle dependent transmittance and reflectance spectra of the Co3O4 film, and diffuse reflectance FTIR (DRFTIR) of Co3O4 powders with different sizes. The transverse optical phonons (TO) and the longitudinal optical phonons (LO) were evaluated, and compared with the calculation results. Second, the FTIR spectra of CoO were studied by the reflectance configuration of CoO single crystal and calculation results from the modelling and oscillator fitting were used to identify the LO modes in CoO. DRFTIR technique was used to characterize the LO in CoO powders, as well as the evolution process from CoO to Co3O4. X-ray diffraction (XRD) was also used to characterize cobalt oxides. Third, Raman spectra of CoO were studied at various temperatures from 10 K to 295 K (room temperature). The second-order phonon scattering in CoO Raman spectrum provides evidence for the LO phonons observed in the DRFTIR spectrum. The laser-induced heating effect was also studied for the CoO powder samples. Fourth, the photoluminescence spectra (PL) of Co3O4 were studied at various temperatures. The band gap of Co3O4 was determined in broad spectral range using UV-Vis-IR spectrophotometer. The PL spectra of Co3O4 were investigated in both visible and infrared range. Last, the band gap of CoO was imparted using the UV-Vis-IR spectrophotometer. PL spectra of CoO film on Co plate and CoO single crystal were studied at various temperatures.