Hydrothermal synthesis of MnCO3 thin film and its conversion to Mn-oxides by annealing in different atmospheres

Transition metal carbonates are of great interest, not only as alternative electrode materials in Li/Zn/Na/K-ion batteries or supercapacitors, but also as solid precursors for the synthesis of transition metal oxides via thermal decomposition. In this work, thin films of MnCO3 were deposited on quartz substrates via a mild one-pot hydrothermal process without using any catalysts or templates and the conversion of these films to manganese oxides through post-deposition thermal annealing for 1 h in air and nitrogen atmospheres was investigated. The films were analyzed by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectroscopy, Raman spectroscopy, Fourier transform infrared spectroscopy, and optical absorption measurements before and after annealing. The obtained results showed that as deposited films composed of MnCO3 with an impurity phase of Mn3O4 and annealing in air atmosphere caused the conversion of the films to Mn2O3, whereas nitrogen atmosphere led to the conversion to MnO with a secondary phase of Mn3O4. After annealing in both atmospheres, the size and shape of the microcubic structure were mainly retained, except for porosity caused by the release of CO2 and O2 gases. From the analysis of the room temperature optical absorption measurements, direct optical band gaps of the as deposited and air annealed films were determined as 3.27 eV and 2.30 eV, respectively, whereas the nitrogen annealed film was found to have two band gaps of 2.50 eV and 2.71 eV. © 2023, The Author(s), under exclusive licence to Springer Science+Business Media, LLC, part of Springer Nature.