Temperature dependence of Raman spectra and first principles study of NiMn2O4 magnetic spinel oxide thin films. Application in efficient photocatalytic removal of RhB and MB dyes.

Magnetic spinel oxide NiMn2O4 thin films were grown on glass substrates using the spray pyrolysis deposition technique at 350 °C. First, structural optimization, electronic structure and vibrational properties were obtained for NiMn2O4 by density functional theory (DFT) with an all electron basis set and the hybrid functional B3LYP implemented in the CRYSTAL14 code. Our DFT results show good agreement with previous experimental and theoretical data. Second, Raman spectra of as-prepared thin films have been measured over the temperature range from 100 K to 620 K. We report on the temperature dependence of their dominant first-order scattering F2g and A1g phonons modes. The phonon dynamic behavior is well described by a model taking into account explicit lattice anharmonicity associated to both cubic and quartic phonon decay processes and thermal expansion related quasiharmonicity. Finally, the photocatalytic activities of the obtained thin films were evaluated through the photocatalytic degradation of Rhodamine B (RhB) and methylene blue (MB) solutions under solar light irradiation as a function of time. All the results indicate that NiMn2O4 thin film can be used as a potential solar-light-driven photocatalyst.