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.

Nanoporous Au-Pt alloys with pore- and ligament size down to few nanometers were fabricated by dealloying Ag-Au-Pt. Owing to the small structure size and large specific surface area, the surface stress and its variation give rise to significant stress and strain in the bulk of these materials. In fact, dilatometry experiments find electrochemical actuation with large reversible strain amplitude. The linear strain reaches approximately 1.3% and strain energy density is up to 6.0 MJ/m(3). The associated stresses may approach the elastic limit of the alloy.

Dealloying is a chemically or electrochemically driven corrosion process in which the less noble component of an alloy is selectively removed. [1][2][3][4] It has been used to produce monolithic metal bodies with a bi-continuous structure in which two phases, solid ligaments and pore space, interpenetrate at a characteristic length scale of a few nanometres. Ag-Au alloys form a model system for dealloying, since the continuous solid solubility and the very small difference in the atomic volumes of the constituents allow for an essentially coherent transformation from the master alloy to the nanoporous gold product structure. Recently, as nanoporous gold has become of interest for possible application as a low-temperature heat exchanger, [5] catalyst or catalyst support, [6] or actuator material, [7] studies of dealloying have turned from understanding corrosion towards learning how to make porous nanomaterials for functional or structural applications. Concurrent with this trend is an emerging need for detailed characterization of materials science aspects of nanoporous gold. The defect structure of the material is the subject of ongoing research. [8] Furthermore, first data on mechanical performance are becoming available, testifying to astonishing properties on a nanoscale level. [9,10] However, little quantitative information is available on the central microstructural and topological properties which characterize porous structures, such as specific surface area, distribution of pore-and ligament sizes, network connectivity, and structural anisotropy. Besides the freshness of the issue, the very smallness of the structures, often well below the 10 nm size range, has prevented detailed studies so far. Here, we present a study of the nanoporous gold microstructure using electron tomography in a transmission electron microscope (TEM).Electron tomography is a technique which provides three dimensional (3-D) information on a nanometre scale, [11,12] and which has recently been extended beyond applications in life sciences towards the characterization of 3-D structures in materials science. [13][14][15][16][17][18] High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging is based on Z-contrast due to Rutherford scattering, [12,17] where the intensity scales with the atomic number (Z ∼ 1.7 ) and the projected sample thickness. [12] Therefore HAADF-STEM imaging is well-suited for electron tomography of crystalline materials [12,19] since the contrast does not suffer from diffraction effects except when the observation area is imaged along a major zone axis. Since major zone axes are rarely encountered during the acquisition of tilt series, the overall effect on 3-D reconstructions is small. [12] However, limits to the resolution of the reconstructed volume are imposed by the restricted tilt range (missing wedge problem), which results from the pole piece/sample holder geometry.In this article we have applied HAADF-STEM imaging for electron tomography to characterize the 3-D structur...

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Recently, enhancement of the energy density of a supercapacitor is restricted by the inferior capacitance of negative electrodes, which impedes the commercial development of high-performance symmetric and asymmetric supercapacitors. This article introduces the in situ bulk-quantity synthesis of hydrophilic, porous, graphitic sulfur-doped carbon nano-onions (S-CNO) using a facile flame-pyrolysis technique and evaluated its potential applications as a high-performance supercapacitor electrode in a symmetric device configuration. The high-surface wettability in the as-prepared state enables the formation of highly suspended active conducting material S-CNO ink, which eliminates the routine use of binders for the electrode preparation. The as-prepared S-CNO displayed encouraging features for electrochemical energy storage applications with a high specific surface area (950 m2 g–1), ordered mesoporous structure (∼3.9 nm), high S-content (∼3.6 at. %), and substantial electronic conductivity, as indicated by the ∼80% sp2 graphitic carbon content. The in situ sulfur incorporation into the carbon framework of the CNO resulted in a high-polarized surface with well-distributed reversible pseudosites, increasing the electrode–electrolyte interaction and improving the overall conductivity. The S-CNOs showed a specific capacitance of 305 F g–1, an energy density of 10.6 W h kg–1, and a power density of 1004 W kg–1 at an applied current density of 2 A g–1 in a symmetrical two-electrode cell configuration, which is approximately three times higher than that of the pristine CNO-based device in a similar electrochemical testing environment. Even at 11 A g–1, the S-CNO||S-CNO device rendered an energy density (6.1 W h kg–1) at a deliverable power density of 5.5 kW kg–1, indicating a very good rate capability and power management during peak power delivery applications. Furthermore, it showed a high degree of electrochemical reversibility with excellent cycling stability, retaining ∼95% of its initial capacitance after more than 10 000 repetitive charge–discharge cycles at an applied current density of 5 A g–1.

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One step green synthesis of a silver–reduced graphene oxide nanocomposite (Ag–RGO) and its application to construct a nitrite (NO2−) sensor.

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