Atomic-Scale Imaging of Transformation of Nickel Nanocrystals to Nickel Carbides in Real Time

Transition of metal-to-metal carbide plays a key role in heterogeneous catalysis. We confined nickel nanocrystals in single-walled carbon nanotubes, stimulated delivery of carbon atoms by the 80 keV electron beam, and imaged the entire carbonization process by time-resolved aberration-corrected transmission electron microscopy at the atomic scale. Metal nanocrystal Ni₄₀ progressively capturing carbon atoms evolved from pure metal to Ni₄₀C₂₀ and then to Ni₄₀C₄₀. The carbonization is accompanied by changes in the structure of the crystal, including a two-dimensionalization process, at the Ni₄₀C₄₀ stage. This work provides valuable insights into the atomic mechanism of metal carbide formation, which may help to develop stable catalysts and provide a reliable route for synthesizing metal-based two-dimensional materials.