Mechanical-chemical coupling at surfaces: prediction and experiment

虽然材料科学家已经研究了固溶体中的外来原子或金属间化合物中的组分的部分原子体积，以及应力状态和化学势之间的耦合几十年，但到目前为止，表面的类似过程还没有得到太多的关注。该项目的主要目标是研究表面力学和吸附之间的耦合。基本现象是毛细力，即表面应力，与材料表面的状态变化相耦合。因此，来自气体的原子或来自溶液的离子的吸附，或表面处的电子电荷的富集/耗尽促使材料变形。这种现象最近在现代生物传感器中得到了应用，其微型化的杠杆在吸附原子或分子时可测量地变形。此外，已经表明，原子或离子的受控可逆吸附/脱附可以促使纳米多孔固体中的大应变，其有望用作具有大振幅和应变能量密度的金属致动器。对材料表面状态变化和应力状态耦合背后的微观机制的理解充其量仍然是初步的。一个理论，可以预测耦合强度为一个给定的材料表面和吸附物仍然缺乏。此外，可逆吸附和解吸的轻元素，这是特别相关的应用在驱动和传感的表面应力变化，仍然研究不足。在这方面，本提案的目的是1）。第一次从实验和理论两方面提供了毛细管力和吸附之间耦合的定量值，以及2.）深入了解潜在的微观机制。作为模型过程，我们计划研究吸附的H和O的Au，Pt和Ir表面。计算耦合强度的理论方法是密度泛函理论。为了有效和详尽的筛选配置空间，我们还采用集群扩展方法。在实验中，通过悬臂梁的弯曲，测定了吸附气体和电吸附时表面应力的变化。在不同的电吸附过程中作为应变的函数的电极电位的变化的补充测量用动态电化学机械分析（由申请人之一开发的技术）进行。

While the partial atomic volume of foreign atoms in solid solutions or of constituents in intermetallic compounds, and hence the coupling between the stress state and the chemical potential, have been studied already for many decades by materials scientists, up to now the analogous processes at surfaces have not received much attention. The key goal of this project is the investigation of the coupling between the mechanics of surfaces and adsorption. The basic phenomenon is that a capillary force, the surface stress, is coupled to changes of state at the materials surface. Thus, the adsorption of atoms from gas or ions from solution, or the enrichment/depletion of electronic charge at the surface prompts deformation of the material. This phenomenon has recently found application in modern biosensors, whose miniaturized cantilevers deform measurably upon adsorption of atoms or molecules. Furthermore, it has been shown that the controlled reversible adsorption/desorption of atoms or ions can prompt large strain in nanoporous solids that promise application as metallic actuators with large amplitude and strain energy density. Understanding of the microscopic mechanisms behind the coupling of changes of state at materials surfaces and the stress state is still rudimentary at best. A theory that could predict the coupling strength for a given materials surface and adsorbate is still missing. Moreover, the surface stress changes upon reversible adsorption and desorption of light elements, which are particularly relevant for applications in actuation and sensing, remain poorly studied. In this context, the present proposal aims at 1.) providing for the first time quantitative values for the coupling between capillary force and adsorption from both experiment and theory and 2.) insight into the underlying microscopic mechanims. As model processes we plan to study the adsorption of H and O on Au, Pt and Ir surfaces. The theoretical method for calculation of the coupling strength is density functional theory. For efficient and exhaustive screening of the configuration space we additionally employ cluster expansion methods. In experiment, changes of the surface stress upon adsorption from gas and electrosorption are determined from cantilever bending. Complementary measurements of the variation of the electrode potential as a function of strain during different electrosorption processes are carried out with Dynamic Electro-Chemo-Mechanical Analysis, a technique developed by one of the applicants.