Fine structure and facet analyses of tunnel-structured FeOOH nanocrystals

The development of energy storage systems in the future relies on identifying suitable target materials and synthesizing target materials with controllable physicochemical properties to unlock their electrochemical contributions. Based on the structure-property-performance relationship established in materials science, it is crucial to determine the fine structure of key functional materials. Among these materials, tunnel-structured β-FeOOH is widely studied due to its low-cost, decent performance and environmental friendliness; yet, its fine structure in terms of the atomic framework and surface configuration has not been well understood. In this work, we successfully synthesized β-FeOOH nanorods and analyzed their crystallographic features via transmission electron microscopy (TEM). By combining ultramicrotome and aberration-corrected scanning TEM with atomic resolution, we first experimentally revealed the structural details of β-FeOOH featuring 1 ​× ​1 tunnels and 2 ​× ​2 tunnels along the c-axis ([001]), the same as the nanorod growth direction. The dominant exposed facets of β-FeOOH nanorods were further revealed to be {010}. Such fine structural elucidation unveils the long-existing mystery in this material system, which is expected to contribute to structural/compositional engineering toward functionality optimization in the near future.