Reacting Mn3O4 powders with quaternary ammonium hydroxides to form two-dimensional birnessite flakes

Abstract

There is a current interest in synthesis of two-dimensional (2D) materials that would be cost-efficient to scale to industry production. In this study, we report on a bottom-up approach to convert a manganese oxide (Mn3O4) powder into crystalline, 2D layered birnessite flakes. The Mn3O4 precursor powder, is reacted with aqueous solutions of, tetramethyl-, tetraethyl- or tetrabutylammonium hydroxide in a shaker oven at 80 °C for 4 days. X-ray diffraction confirmed that in all cases, upon filtration of colloidal suspensions, well-stacked, crystalline, 2D MnO2 birnessite layers formed, that upon characterization were found to be quite similar. The major differences were the spacings between the flakes that depended on cation size. Selected area diffraction patterns in a transmission electron microscope confirmed both the structure and the polycrystallinity of the flakes. Raman spectroscopy confirmed the structure. The zeta-potentials of the 2D flakes were measured to be ≈−35 ± 2 mV. When UV–Vis spectra were converted to Tauc plots, direct and indirect band gap energies of ≈2.15 and ≈2.60 eV were obtained, respectively. Additionally, the oxygen evolution reactivity of our birnessite flakes significantly are better than those found for Pt/C, demonstrating a high degree of catalytic reversibility. The facile conversion of a relatively inexpensive, earth-abundant oxide, Mn3O4, into a highly value-added 2D birnessite using a one-pot, simple hugely scalable, protocol at near ambient temperatures and pressures with various quats can be considered a breakthrough in the production of these nanomaterials.