Wang Yi, Goudeli Eirini
Particle Technology Laboratory, Department of Mechanical and Process Engineering, ETH Zürich, Zürich, CH-8092, Switzerland.
Center for Combustion Energy, Key Laboratory of Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China.
Nanoscale. 2024 Oct 3;16(38):17942-17953. doi: 10.1039/d4nr02359e.
The crystallization of gold nanoparticles is investigated in the gas-phase by molecular dynamics (MD) that is most relevant to their synthesis by aerosol processes (flame, plasma, or cluster beam deposition). A particle size-dependent metastable region, 200-300 °C wide, is revealed between the melting and freezing points of Au. This region decreases as the MD heating or cooling rates decrease. Two separate stages, subcritical and supercritical cluster formation, are distinguished during isothermal crystallization of 2.5-11 nm Au nanoparticles at 500-1000 K. The degree of Au crystallization (face-centered cubic or hexagonal close-packing) is quantified based on the Au atom local crystalline disorder. The onset of crystallization is identified by the steep rise of the fraction of atoms that retain their crystallinity in the largest subcritical cluster, accompanied by a sharp drop of the amorphous fraction of the Au nanoparticle. Crystallization starts from, at least, one atom layer below the surface of the nanoparticle and then quickly expands to its surface and bulk. Two crystallization nucleation pathways are identified: (a) explosive nucleation well below the Au freezing point resulting in many small and broadly distributed crystals; and (b) accretion nucleation near the freezing point where narrowly distributed and larger crystals are formed that grow by accretion and coalescence. X-ray diffraction (XRD) patterns are generated by MD, from which the dynamics of crystal growth are elucidated, consistent with the literature and in excellent agreement with direct tracing of crystal sizes.
通过分子动力学（MD）在气相中研究金纳米颗粒的结晶过程，该过程与通过气溶胶工艺（火焰、等离子体或团簇束沉积）合成金纳米颗粒最为相关。在金的熔点和凝固点之间发现了一个与粒径相关的亚稳区，宽度为200 - 300°C。随着MD加热或冷却速率的降低，该区域减小。在500 - 1000 K等温结晶2.5 - 11 nm金纳米颗粒的过程中，区分出两个不同阶段，即亚临界团簇形成和超临界团簇形成。基于金原子的局部晶体无序度对金的结晶程度（面心立方或六方密堆积）进行了量化。通过在最大亚临界团簇中保持结晶性的原子分数的急剧上升以及金纳米颗粒非晶部分的急剧下降来确定结晶的起始点。结晶至少从纳米颗粒表面以下的一个原子层开始，然后迅速扩展到其表面和整体。确定了两种结晶成核途径：（a）在远低于金凝固点的温度下发生爆发性成核，产生许多小且分布广泛的晶体；（b）在凝固点附近发生凝聚成核，形成分布狭窄且较大的晶体，这些晶体通过凝聚和合并生长。通过MD生成X射线衍射（XRD）图谱，并据此阐明晶体生长的动力学，这与文献一致，并且与直接追踪晶体尺寸的结果非常吻合。
