Variation of the surface stress-charge coefficient of platinum with electrolyte concentration

Operando Nanoscale Imaging of Electrochemically Induced Strain in a Locally Polarized Pt Grain

Developing new methods that reveal the structure of electrode materials under polarization is key to constructing robust structure-property relationships. However, many existing methods lack the spatial resolution in structural changes and fidelity to electrochemical operating conditions that are needed to probe catalytically relevant structures. Here, we combine a nanopipette electrochemical cell with three-dimensional X-ray Bragg coherent diffractive imaging to study how strain in a single Pt grain evolves in response to applied potential. During polarization, marked changes in surface strain arise from the Coulombic attraction between the surface charge on the electrode and the electrolyte ions in the electrochemical double layers, while the strain in the bulk of the crystal remains unchanged. The concurrent surface redox reactions have a strong influence on the magnitude and nature of the strain changes under polarization. Our studies provide a powerful blueprint to understand how structural evolution influences electrochemical performance at the nanoscale.

Compressive Stress and Charge Redistribution during CO Adsorption onto Pt

The change in surface stress associated with the adsorption and oxidative stripping of carbon monoxide (CO) on (111)-textured Pt is examined using the wafer curvature method in 0.1 mol/L KHCO3 electrolyte. The curvature of the Pt cantilever electrode was monitored as a function of potential in both CO-free and CO-saturated electrolytes. Although CO adsorbs as a neutral molecule, significant compressive stress, up to -1.3 N/m, is induced in the Pt. The magnitude of the stress change correlates directly with the CO coverage and, within the detection limits of the stress measurement, is elastically reversible. Density functional theory calculations of a CO-bound Pt surface indicate that charge redistribution from the first atomic layer of Pt to subsurface layers accounts for the observed compressive stress induced by the charge neutral adsorption of CO. A better understanding of adsorbate-induced surface stress is critical for the development of material platforms for sensing and catalysis.

Electronically integrated, mass-manufactured, microscopic robots

Fifty years of Moore's law scaling in microelectronics have brought remarkable opportunities for the rapidly evolving field of microscopic robotics^1-5. Electronic, magnetic and optical systems now offer an unprecedented combination of complexity, small size and low cost^6,7, and could be readily appropriated for robots that are smaller than the resolution limit of human vision (less than a hundred micrometres)^8-11. However, a major roadblock exists: there is no micrometre-scale actuator system that seamlessly integrates with semiconductor processing and responds to standard electronic control signals. Here we overcome this barrier by developing a new class of voltage-controllable electrochemical actuators that operate at low voltages (200 microvolts), low power (10 nanowatts) and are completely compatible with silicon processing. To demonstrate their potential, we develop lithographic fabrication-and-release protocols to prototype sub-hundred-micrometre walking robots. Every step in this process is performed in parallel, allowing us to produce over one million robots per four-inch wafer. These results are an important advance towards mass-manufactured, silicon-based, functional robots that are too small to be resolved by the naked eye.

Electrocapillary Coupling at Metal Surfaces from First Principles: On the Impact of Excess Charge on Surface Stress and Relaxation

We study the response of the surface stress to excess charge via ab initio simulation of metal surfaces in an external electric field. We focus on "simple" sp-bonded metals to gain insight into the mechanisms underlying electrocapillary coupling. Both the direct effect on the surface stress via charging of the bonds and the indirect effect resulting from the charge-induced relaxation are analyzed and discussed in relation to the trends of the coupling coefficients, which-owing to a Maxwell relation-are determined in terms of the response of the work function to strain. Al(111), Mg(0001), and Na(110) are investigated as prototypical sp-bonded metal surfaces with positive, vanishing, and negative coupling parameters, respectively. Mg(0001) and Al(111) exhibit an inward relaxation of the first atomic layer upon negative charging, whereas an outward relaxation occurs for Na(110). The indirect contribution of the relaxation to the coupling coefficient has the same sign as the total response and makes up about 30% of its magnitude for Al(111) and Na(110). Our study highlights that even the response behavior of the so-called simple metals is by no means readily captured within simple models.

Sign inversion of surface stress–charge response of bulk nanoporous nickel actuators with different surface states

Bulk nanoporous nickel exhibits different electrochemical actuation behaviors and associated stress–charge response in strongly (NaOH) and weakly (NaF) adsorbed electrolytes.

Sign-inverted response of aluminum work function to tangential strain

We have investigated the response of the work function, W, of low-index aluminum surfaces to tangential strain by using first-principles calculations based on density functional theory. This response parameter is a central quantity in electrocapillary coupling of metal electrodes relating to the performance of porous metal actuators and surface stress based sensing devices. We find that Al surfaces exhibit a positive response for all orientations considered. By contrast, previous studies reported negative-valued response parameters for clean surfaces of several transition metals. We discuss separately the response of W to different types of strain and the impact of the strain on the Fermi energy and the surface dipole. We argue that the reason for the abnormal positive sign of the Al response parameter lies in its high valence electron density.

Reversible Strain in Porous Metals Charged in Electrolytes

Nanoporous metal samples with millimetre size were prepared either by compacting nanocrystalline powders or by dealloying, the dissolution of the less noble metal(s) of an alloy. The samples were immersed in an electrolyte, and their length was measured as a function of the applied potential in-situ in a dilatometer. The results obtained for nanocrystalline platinum, nanoporous gold and for gold platinum alloys show that the length varies in dependence of the surface charge. The strain amplitude of nanocrystalline platinum was 0.15%, and even larger strains have been measured using an Au-Pt alloy. This strain is comparable to commercial piezoceramics, but it is achieved using smaller voltages.

The strain measured for nanoporous gold prepared by dealloying was smaller than that mainly due to the larger structure size (20 nm structure size compared to 6 nm Pt crystallite size), but in the case of gold, it was possible to prepare stable composite structures of a metal foil and of the nanoporous gold. If such a bimetallic foil is charged, it is found to bend. Due to the mechanical amplification of the contraction or expansion of the nanoporous part of the foil, it was possible to observe the effect of electric charges on the surface stress of metals directly with the naked eye for the first time.

These results demonstrate that nanoporous metals might be useful for actuator applications and for the study of surface strain effects. Furthermore, they are the first realization of a general concept that suggests that most of the properties of conducting nanomaterials can be tuned by controlling the surface charge.

Surface Chemistry in Nanoscale Materials

Although surfaces or, more precisely, the surface atomic and electronic structure, determine the way materials interact with their environment, the influence of surface chemistry on the bulk of the material is generally considered to be small. However, in the case of high surface area materials such as nanoporous solids, surface properties can start to dominate the overall material behavior. This allows one to create new materials with physical and chemical properties that are no longer determined by the bulk material, but by their nanoscale architectures. Here, we discuss several examples, ranging from nanoporous gold to surface engineered carbon aerogels that demonstrate the tuneability of nanoporous solids for sustainable energy applications.

Surface-chemistry-driven actuation in nanoporous gold

Although actuation in biological systems is exclusively powered by chemical energy, this concept has not been realized in man-made actuator technologies, as these rely on generating heat or electricity first. Here, we demonstrate that surface-chemistry-driven actuation can be realized in high-surface-area materials such as nanoporous gold. For example, we achieve reversible strain amplitudes of the order of a few tenths of a per cent by alternating exposure of nanoporous Au to ozone and carbon monoxide. The effect can be explained by adsorbate-induced changes of the surface stress, and can be used to convert chemical energy directly into a mechanical response, thus opening the door to surface-chemistry-driven actuator and sensor technologies.

Surface stress-charge response of a (111)-textured gold electrode under conditions of weak ion adsorption

We report a cantilever bending investigation into the variation of surface stress, f, with surface charge density, q, for (111)-textured thin films of gold in aqueous NaF and HClO 4. The graphs of f(q) are highly linear, and the surface stress-charge coefficients, d f/d q, are -1.95 V for 7 mM NaF and -2.0 V for 10 mM HClO 4 near the potential of zero charge. These values exceed some previously published experimental data by a factor of 2, but they agree with recent ab initio calculations of the surface stress-charge response of gold in vacuum.

Structural relaxation in charged metal surfaces and cluster ions

Space-charge layers can noticeably affect the properties of metal electrode surfaces, for instance by modifying the surface dielectric response or indirectly via the induced atomic relaxations. While there are efforts to exploit this concept for designing novel functional nanomaterials, the underlying microscopic processes are poorly understood. Here, we report on a density functional theory (DFT) study of atomic relaxation in Au cluster ions comprising up to 309 atoms. Suitable averages over atomic displacements respond to charging consistent with experimental observation on macroscopic Au single-crystal surfaces. Moreover, the overall DFT response is also consistent with predictions of a simple phenomenological model. Motivated by our observations, we propose a scenario in which the surface relaxation ("stretch") results from out-of-plane Hellman-Feynman forces exerted on the surface atoms by the excess charge, and where the in-plane surface stress represents essentially an elastic transverse contraction tendency of the surface layer in response to stretch.

Volume change during the formation of nanoporous gold by dealloying

We report a macroscopic shrinkage by up to 30 vol % during electrochemical dealloying of Ag-Au. Since the original crystal lattice is maintained during the process, we suggest that the formation of nanoporous gold in our experiments is accompanied by the creation of a large number of lattice defects and by local plastic deformation.