Direct visualization and understanding of the atomic mechanisms governing the growth of nanomaterials are crucial for designing synthesis strategies of high specificity. Aside from playing a key role in numerous technological applications, palladium clusters and nanoparticles are particularly valuable due to their outstanding catalytic activity. Studies show that the properties of Pd nanomaterials depend on shape and size. Therefore, optimizing the synthesis to control the final size and shape of Pd nanoparticles is important for a large number of current and future applications. In this work, we exploit in situ liquid cell scanning transmission electron microscopy to track at the atomic scale the growth of Pd nanoparticles from the very early stage to mature, crystalline nanoparticles. We find that the formation of Pd nanoparticles consists of multiple steps. The first step in nanoparticle formation, representing a nonclassical nucleation step, can be described by the formation of agglomerates of Pd atoms. In the second step, these agglomerates grow via atomic addition to form primary nanoclusters, which coalesce to form amorphous clusters. In the third stage, these clusters continue to coalesce, leading to the formation of amorphous Pd NPs, while in parallel, growth by monomer attachment continues. Then, in the fourth step, the amorphous nanoparticles undergo a nanocrystallization process, where the continuous improvement of crystallinity and the establishment of a distinct morphology eventually give rise to the formation of facetted, crystalline nanoparticles. Similar to our earlier work with Au and Pt nanoparticles, these results confirm that even for simple systems, nonclassical nucleation and growth processes dominate and that these multi-step mechanisms are highly element-specific. Despite the fact that the synthesis conditions are identical, the element-specific interactions define the pathway of the formation of crystalline nanoparticles.

中文翻译：

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原位液体透射电子显微镜研究水溶液中 Pd 纳米晶的非经典成核和生长

直接可视化和了解控制纳米材料生长的原子机制对于设计高特异性合成策略至关重要。除了在众多技术应用中发挥关键作用外，钯簇和纳米粒子因其出色的催化活性而特别有价值。研究表明，Pd 纳米材料的特性取决于形状和尺寸。因此，优化合成以控制 Pd 纳米粒子的最终尺寸和形状对于当前和未来的大量应用非常重要。在这项工作中，我们利用原位液体细胞扫描透射电子显微镜在原子尺度上跟踪 Pd 纳米粒子从早期阶段到成熟结晶纳米粒子的生长。我们发现 Pd 纳米粒子的形成由多个步骤组成。纳米粒子形成的第一步，代表非经典的成核步骤，可以通过形成 Pd 原子团来描述。在第二步中，这些团聚体通过原子加成生长形成初级纳米团簇，这些团聚体结合形成无定形团簇。在第三阶段，这些簇继续聚结，导致形成无定形 Pd NP，同时，通过单体附着继续生长。然后，在第四步中，无定形纳米颗粒经历纳米结晶过程，其中结晶度的不断提高和独特形态的建立最终导致形成多面的结晶纳米颗粒。类似于我们之前对 Au 和 Pt 纳米粒子的研究，这些结果证实，即使对于简单系统，非经典成核和生长过程也占主导地位，并且这些多步骤机制是高度特定于元素的。尽管合成条件相同，但元素特定的相互作用定义了结晶纳米粒子的形成途径。