Hierarchical porosity via layer-tunnel conversion of macroporous delta-MnO2 nanosheet assemblies

摘要

This work reports the layer-tunnel conversion of porous dehydrated synthetic alkali-free delta-MnO2 analogs prepared by exfoliation, flocculation, and heat treatment of nanosheets derived from highly crystalline potassium birnessite. High surface area porous solids result, with specific surface areas of 90-130 m(2) g(-1) and isotherms characteristic of both micro and macropores. The microstructures of the re-assembled floccules are reminiscent of crumpled paper where single and re-stacked nanosheets form the walls of interconnected macropores. The atomic and local structures of the floccules heat treated from 60-400 degrees C are tracked by Raman spectroscopy and synchrotron X-ray total scattering measurements. During heating, the nanosheets comprising the pore walls condense to form tunnel-structured fragments beginning at temperatures below 100 degrees C, while the microstructure with high surface area remains intact. The flocc microstructure remains largely unchanged in samples heated up to 400 degrees C while an increasing fraction of the sample is transformed, at least locally, to possess 1D tunnels characteristic of alpha-MnO2. Cyclic voltammetry in Na2SO4 aqueous electrolyte reflects the nanoscale structural evolution, where intercalative pseudocapacitance diminishes with the degree of transformation. Collectively, these results demonstrate that it is feasible to tailor the materials for applications incorporating nanoporous solids and nanofluidics, and specifically imply strategies to maintain a kinetically accessible interlayer contribute to Na intercalative pseudocapacitance.