Nanoscale Kirkendall effect has been widely used for rationally fabricating high-quality hollow nanocrystals, but often requires the intrinsic diffusion coefficient of out-diffusion materials higher than that of in-diffusion components. Here we demonstrate an unexpected Kirkendall effect that occurs in diffusing intrinsically faster Cu atoms into Pd icosahedra, leading to the formation of PdCu alloyed hollow nanocrystals. The control experiment with Pd octahedra replacing icosahedra indicates the critical role of twin boundaries in facilitating such unexpected Kirkendall effect. In addition, geometric phase analysis and density functional theory calculation show that out-diffusion of Pd atoms in the icosahedra is faster than in-diffusion of Cu atoms, particularly through the twin boundaries, upon the strain gradient with an inward distribution from tensile to compressive strains. The unexpected Kirkendall effect is also found in the interdiffusion of Ag and Pd atoms in Pd icosahedra. Our finds break the limitation of the intrinsic diffusion coefficient for the synthesis of hollow nanocrystals through Kirkendall effect and are expected to enormously enrich the family of hollow nanocrystals which have shown great potential in broad areas, such as fine chemical production, energy storage and conversion, and environmental protection. This work also provides a deep understanding in the diffusion behavior of atoms upon the strain gradient.

纳米级的肯肯德尔效应已被广泛用于合理地制造高质量的中空纳米晶体，但是通常要求外扩散材料的本征扩散系数高于内扩散组分的本征扩散系数。在这里，我们证明了在将本质上较快的Cu原子扩散到Pd二十面体中时发生的出乎意料的Kirkendall效应，导致形成了PdCu合金化的中空纳米晶体。用钯八面体代替二十面体的对照实验表明，孪晶边界在促进这种出乎意料的柯肯德尔效应中起着至关重要的作用。此外，几何相位分析和密度泛函理论计算表明，二十面体中Pd原子的向外扩散要快于Cu原子的内部扩散，特别是通过孪晶边界时，在应变梯度上具有从拉伸应变到压缩应变的向内分布。在Pd二十面体中Ag和Pd原子的相互扩散中也发现了出乎意料的Kirkendall效应。我们的发现突破了通过Kirkendall效应合成空心纳米晶体的本征扩散系数的局限性，并有望极大地丰富空心纳米晶体的家族，这些家族在广泛的领域中显示出巨大的潜力，例如精细化工生产，能量存储和转化，和环保。这项工作还对应变梯度下原子的扩散行为提供了深入的了解。我们的发现突破了通过Kirkendall效应合成空心纳米晶体的本征扩散系数的局限性，并有望极大地丰富空心纳米晶体的家族，这些家族在广泛的领域中显示出巨大的潜力，例如精细化工生产，能量存储和转化，和环保。这项工作还对应变梯度下原子的扩散行为提供了深刻的理解。我们的发现突破了通过Kirkendall效应合成空心纳米晶体的本征扩散系数的局限性，并有望极大地丰富空心纳米晶体的家族，这些家族在广泛的领域中显示出巨大的潜力，例如精细化工生产，能量存储和转化，和环保。这项工作还对应变梯度下原子的扩散行为提供了深刻的理解。