Evolution\nof Heterogeneous Tunnel Structures in Cryptomelane-Type\nManganese Oxides and Their Geoinspired Implications

Abstract

Cryptomelane-type manganese oxides, α-MnO₂ (K_<i>x</i>Mn₈O₁₆), play\nkey roles\nin various fields such as geochemical processes, catalytic reactions,\nenergy storage, and environmental sciences. The function of cryptomelane-type\noxides can be affected by cation substitutions and the changes in\ntunnel structures. Research on natural cryptomelane minerals could\nprovide geoinspiration for the design of new nanomaterials with cation\nsubstitutions, as well as a key to understanding the evolution of\ntunnel structures. In this study, natural cryptomelane minerals are\ncharacterized by the cosubstitution of iron and zinc. The localization\nof cosubstituted Fe and Zn in the tunnel framework has been revealed.\nFurthermore, the evolution of heterogeneous tunnel structures in cryptomelane\nhas been demonstrated as a transition from large-size tunnels to small\nones with high Mn(III) concentrations, indicating the significant\nrole of Mn(III) in driving this transition. Lead (Pb²⁺)\ncan be effectively trapped in the 2 × 2 tunnels. A mechanism\nfor the attachment of cryptomelane crystals in different orientations\nhas also been explored, showing that the migration of Mn atoms and\nthe formation of (110) planes at specific sites contribute to lattice\nmatching at the boundary. Our results provide geoinspired insights\ninto controlled synthesis with Fe/Zn cosubstitution, a fundamental\nunderstanding of the evolution of tunnel structures, and functionalized\napplications of tunnel-based nanomaterials.