Synthesis and Delamination of Layered Manganese Oxide Nanobelts

This paper describes systematic studies on the synthesis and delamination of manganese oxide nanobelts with the birnessite-type layered structure. K-birnessite nanobelts of K_0.33MnO₂·0.5H₂O, which typically had a length of several tens of micrometers, a width of hundreds of nanometers, and a thickness of ∼15 nm, were synthesized by hydrothermally treating a KMnO₄–MnCl₂ mixture in a highly concentrated KOH aqueous solution. The nanobelt growth was found to be controlled by the KOH concentration and the molar ratio of Mn²⁺/MnO₄⁻ in the starting reaction mixture. The K-birnessite nanobelts were converted to H-birnessite, H_0.08MnO₂·0.7H₂O, by treatment with a (NH₄)₂S₂O₈ aqueous solution, retaining their high crystallinity and beltlike morphology. Swelling and delamination behaviors of the H-birnessite nanobelts in aqueous solutions of quaternary ammonium hydroxides were studied in detail. In tetrabutylammonium hydroxide (TBAOH) solutions, the H-birnessite showed a limited swelling and delamination behavior, whereas the compound underwent osmotic swelling in tetramethylammonium hydroxide (TMAOH) solutions. Water-washing the TMA⁺-treated samples greatly enhanced the degree of swelling, while maintaining the three-dimensional crystalline order. Upon further contact with a TBAOH solution, the highly swollen phase was dominantly delaminated into unilamellar nanosheets. The nanosheets thus obtained inherited the morphology of the parent nanobelts in their long-axis direction and had lateral sizes of micrometer order. The colloidal suspension of nanosheets showed an optical absorption band around 380 nm, which was drastically changed from the rather constant and featureless absorption of UV to visible light for the birnessite before delamination.