Sytwu Katherine, Hayee Fariah, Narayan Tarun C, Koh Ai Leen, Sinclair Robert, Dionne Jennifer A
Department of Applied Physics , Stanford University , 348 Via Pueblo , Stanford , California 94305 , United States.
Department of Electrical Engineering , Stanford University , 350 Serra Mall , Stanford , California 94305 , United States.
Nano Lett. 2018 Sep 12;18(9):5357-5363. doi: 10.1021/acs.nanolett.8b00736. Epub 2018 Aug 30.
Surface faceting in nanoparticles can profoundly impact the rate and selectivity of chemical transformations. However, the precise role of surface termination can be challenging to elucidate because many measurements are performed on ensembles of particles and do not have sufficient spatial resolution to observe reactions at the single and subparticle level. Here, we investigate solute intercalation in individual palladium hydride nanoparticles with distinct surface terminations. Using a combination of diffraction, electron energy loss spectroscopy, and dark-field contrast in an environmental transmission electron microscope (TEM), we compare the thermodynamics and directly visualize the kinetics of 40-70 nm {100}-terminated cubes and {111}-terminated octahedra with approximately 2 nm spatial resolution. Despite their distinct surface terminations, both particle morphologies nucleate the new phase at the tips of the particle. However, whereas the hydrogenated phase-front must rotate from [111] to [100] to propagate in cubes, the phase-front can propagate along the [100], [11̅0], and [111] directions in octahedra. Once the phase-front is established, the interface propagates linearly with time and is rate-limited by surface-to-subsurface diffusion and/or the atomic rearrangements needed to accommodate lattice strain. Following nucleation, both particle morphologies take approximately the same time to reach equilibrium, hydrogenating at similar pressures and without equilibrium phase coexistence. Our results highlight the importance of low-coordination number sites and strain, more so than surface faceting, in governing solute-driven reactions.
纳米颗粒的表面刻面可深刻影响化学转化的速率和选择性。然而，表面终止的精确作用可能难以阐明，因为许多测量是在颗粒集合体上进行的，没有足够的空间分辨率来观察单个和亚颗粒水平的反应。在这里，我们研究了具有不同表面终止的单个氢化钯纳米颗粒中的溶质嵌入。通过结合环境透射电子显微镜（TEM）中的衍射、电子能量损失谱和暗场对比度，我们以约2纳米的空间分辨率比较了热力学，并直接可视化了40 - 70纳米的{100}终止立方体和{111}终止八面体的动力学。尽管它们的表面终止不同，但两种颗粒形态都在颗粒尖端成核新相。然而，在立方体中，氢化相前沿必须从[111]旋转到[100]才能传播，而在八面体中，相前沿可以沿[100]、[11̅0]和[111]方向传播。一旦建立相前沿，界面随时间线性传播，并且受表面到亚表面扩散和/或适应晶格应变所需的原子重排的速率限制。成核后，两种颗粒形态达到平衡所需的时间大致相同，在相似压力下氢化且不存在平衡相共存。我们的结果强调了低配位数位点和应变在控制溶质驱动反应中的重要性，比表面刻面更为重要。
