Composition and structure of nanoporous Au dealloyed from AuAg and AuCu

FOR2213研究小组子项目“单金属和双金属纳米多孔泡沫的组成和结构”一期的主要成果之一是观察到由AuAg合金制备的npAu中存在富银团簇。在此之前，这一意想不到的发现引发了关于它们的形成、少量残余银浓度和表面组成的各种问题的理解。Ag团簇可能是npAu催化行为重现性较低的原因。因此，本项目的一部分有助于对上述影响进行系统的调查，并开发制备路线，以生产可再生催化剂的均匀组成分布。在第一阶段，在npAu韧带中广泛测量了相对于参考区域的晶格应变。发现部分圆柱形韧带的晶格沿轴向收缩，径向扩展，符合连续介质力学的理论预测。在第二阶段，我们的目标是通过测量的绝对应变与基于HAADF-STEM测量得出的几何模型的经验势能的能量松弛计算的应变进行定量匹配，从而巩固对应变的理解。在第二阶段，系统组合将扩展两倍：首先，将通过在起始合金中用Cu取代Ag来进一步研究低贵金属元素的作用。由于材料系统在低于380°C的温度下表现出各种有序的晶体相，并施加具有不同成分的区域，因此预计不那么贵重的元素的分布将发挥重要作用。由于Cu（Ag）和Au之间的晶格失配为11.5%(~0%)，因此预计应变效应明显更大。其次，多孔纳米颗粒将弥补最小长度仅为几纳米的纳米颗粒与韧带尺寸为几十纳米的npAu泡沫之间的差距。在这两种情况下，组成分布、应变状态和形貌将强烈依赖于除合金参数，如电位、电流和持续时间，并可能在催化过程中发生变化。为了实现对催化性能的深入了解，表征纳米多孔材料在其生命周期各个阶段的组成、应变和形态是必不可少的先决条件。

One of the main results in the first period of sub-project “Composition and structure of mono and bimetallic nanoporous foams” of the research unit FOR2213 was the observation of Ag rich clusters within npAu dealloyed from AuAg. This until then unexpected finding gave rise to a variety of questions concerning the understanding of their formation, their occurrence for small residual Ag concentrations and the composition of their surface. It is believed that Ag clusters are a possible reason for the low reproducibility of the catalytic behavior of npAu. Therefore, a part of the present project contributes to a systematic investigation of the aforementioned effects and the development of preparation routes producing a homogenous composition distribution for production of reproducible catalysts.Lattice strain relative to a reference region was measured extensively in npAu ligaments in the first period. It was found that the lattice in partially cylindrical ligaments is contracted along its axis and expanded in radial direction matching theoretical predictions by continuum mechanics. In the second period, we are aiming for a consolidated understanding of strain by quantitative matching of measured absolute strain with strain computed using energy relaxation with empirical potentials based on geometrical models derived from HAADF-STEM measurements. In the second period the portfolio of systems will be extended twofold: First, the role of the less noble element will be further investigated by replacing Ag by Cu in the starting alloys. The distribution of the less noble element is expected to play an important role, since the material system is exhibiting various ordered crystal phases for temperatures below 380°C that impose regions with different compositions. It is also expected that strain effects are significantly larger, since the lattice mismatch between Cu(Ag) and Au is 11.5% (~0%). Second, porous nanoparticles shall bridge the gap between nanoparticles representing smallest length scales of only few nm and npAu foams with ligament sizes of several tens of nanometers. In both cases the composition distribution, strain state and morphology will strongly depend on dealloying parameters such as potential, current and duration and might also change during catalysis. In order to achieve an in-depth understanding of catalytic properties, the characterization of composition, strain and morphology of the nanoporous materials at various stages of its life cycle is an indispensable prerequisite.