Electron Tomography Analysis of Reaction Path during Formation of Nanoporous NiO by Solid State Decomposition

Using transmission electron microscopy, we have analyzed the structure of nanoporous NiO formed by thermal decomposition of Ni­(OH)2. It was found that a tabular single crystal of Ni­(OH)2 transforms pseudomorphically to an array of topotaxially aligned NiO nanocubes separated by nanoscale pores at a volume fraction of about 50%. High-resolution electron tomography on NiO produced by ex situ heating combined with direct in situ observation of the transformation under the electron beam revealed a dramatic redirection of the reaction front when pores are closed by consolidation of NiO nanocubes around the perimeter of the tabular crystals. This causes local changes in the density, structure, and connectivity of the pores. Insights from this study are of broader interest for the class of decomposition reactions that result in nanocrystalline materials with highly tailored porosity and are significant for the development of devices with impact in fields as varied as energy storage, catalysis, and sensing.