Evolution of Heterogeneous Tunnel Structures in Cryptomelane-Type Manganese Oxides and Their Geoinspired Implications

Abstract

Cryptomelane-type manganese oxides, α-MnO₂ (K_xMn₈O₁₆), play key roles in various fields such as geochemical processes, catalytic reactions, energy storage, and environmental sciences. The function of cryptomelane-type oxides can be affected by cation substitutions and the changes in tunnel structures. Research on natural cryptomelane minerals could provide geoinspiration for the design of new nanomaterials with cation substitutions, as well as a key to understanding the evolution of tunnel structures. In this study, natural cryptomelane minerals are characterized by the cosubstitution of iron and zinc. The localization of cosubstituted Fe and Zn in the tunnel framework has been revealed. Furthermore, the evolution of heterogeneous tunnel structures in cryptomelane has been demonstrated as a transition from large-size tunnels to small ones with high Mn(III) concentrations, indicating the significant role of Mn(III) in driving this transition. Lead (Pb²⁺) can be effectively trapped in the 2 × 2 tunnels. A mechanism for the attachment of cryptomelane crystals in different orientations has also been explored, showing that the migration of Mn atoms and the formation of (110) planes at specific sites contribute to lattice matching at the boundary. Our results provide geoinspired insights into controlled synthesis with Fe/Zn cosubstitution, a fundamental understanding of the evolution of tunnel structures, and functionalized applications of tunnel-based nanomaterials.