Deepak Francis Leonard, Casillas-Garcia Gilberto, Esparza Rodrigo, Barron H, Jose-Yacaman Miguel
Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, Texas 78249, USA.
J Cryst Growth. 2011 Jun 15;325(1):60-67. doi: 10.1016/j.jcrysgro.2011.04.026.
Bimetallic nanoparticles of Au-Pd find important applications in catalysis. Their catalytic performance is directly related to the structure, alloy formation and variation of composition in the structure. A standard idea is that bimetallic nanoparticles can be either an alloy or a core shell structure. Our group has investigated the structure and composition of Pd-Au nanoparticles by using aberration corrected high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). We reported previously that the nanoparticles are composed of an evenly alloyed inner core, an Au-rich intermediate layer, and a Pd-rich outer shell. The structure is more complicated than what simple models can predict. In this paper we report additional studies of this system wherein by carrying out spectral and chemical analysis (STEM*-EDAX, STEM-EELS) the interface structure can now be better identified and understood. Apart from the three-layered core-shell structures we have also been able to observe in some cases a four-layered core-shell structure as well. The entire core-shell structure is not rigid and there is indeed intercalation of Au-Pd into the other layers as well. In addition we have been able to locate stacking faults present in the nanoparticles. We also address the problem of the interface structure between the layers. By using nanodiffraction we have found that the whole structure of the nanoparticles becomes hcp in contrast to the bulk structure of Au or Pd.
金-钯双金属纳米颗粒在催化领域有着重要应用。它们的催化性能直接与结构、合金形成以及结构中成分的变化相关。一个标准观点认为双金属纳米颗粒可以是合金结构或核壳结构。我们团队通过使用像差校正高角度环形暗场扫描透射电子显微镜（HAADF-STEM）研究了钯-金纳米颗粒的结构和成分。我们之前报道过这些纳米颗粒由均匀合金化的内核、富金中间层和富钯外壳组成。其结构比简单模型所能预测的更为复杂。在本文中，我们报告了对该体系的进一步研究，通过进行光谱和化学分析（STEM*-EDAX、STEM-EELS），现在能够更好地识别和理解界面结构。除了三层核壳结构外，在某些情况下我们还观察到了四层核壳结构。整个核壳结构并非刚性的，实际上金-钯也会插入到其他层中。此外，我们已经能够确定纳米颗粒中存在的堆垛层错。我们还探讨了层间界面结构的问题。通过使用纳米衍射，我们发现与金或钯的体相结构相比，纳米颗粒的整体结构变为六方密堆积结构。
