Unveiling core-shell structure formation in a Ni3Fe nanocatalyst with in situ multi-Bragg coherent diffraction imaging.

Solid state reactions play a key role in materials science. The evolution of the structure of a single 350 nm Ni3Fe nanoparticle, i.e. its morphology (facets) as well as its deformation field, have been followed by applying multi-reflection Bragg coherent diffraction imaging. Through this approach, we unveiled a demixing process that occurs at high temperatures (600°C). This process leads to the gradual emergence of a highly strained core-shell structure, distinguished by two distinct lattice parameters with a di erence of 0.4%. Concurrently, this transformation causes the facets to vanish, ultimately yielding a rounded core-shell nanoparticle. This final structure comprises a Ni3Fe core surrounded by a 40 nm Ni-rich outer shell. Providing an in situ 3D imaging of the lattice parameters at the nanometer scale while varying the temperature, this study – with the support of atomistic simulations – not only showcases the power of in situ multi-reflection BCDI but also provide valuable insights into the mechanisms at work during a solid state reaction characterized by a core-shell transition.

固态反应在材料科学领域占据核心地位。本研究采用多反射布拉格相干衍射成像（multi-reflection Bragg Coherent Diffraction Imaging, BCDI）技术，追踪了单个350 nm Ni₃Fe纳米颗粒的结构演化过程，包括其形貌（晶面）与形变场。通过该实验手段，我们揭示了600℃高温下发生的相分离过程。该过程逐步催生了高应变核壳结构，该结构以存在两种晶格参数为特征，二者差值达0.4%。与此同时，该结构转变会导致晶面消失，最终形成圆润的核壳纳米颗粒。该最终结构由Ni₃Fe内核与包裹其外的40 nm富镍外壳构成。本研究在变温条件下实现了纳米尺度晶格参数的原位三维成像，并结合原子级模拟手段，不仅展示了原位多反射BCDI技术的强大能力，也为以核壳结构转变为特征的固态反应机理提供了极具价值的研究见解。