Reconstructing a Nanoporous Metal in Three Dimensions: An Electron Tomography Study of Dealloyed Gold Leaf

Dealloying is a chemically or electrochemically driven corrosion process in which the less noble component of an alloy is selectively removed. 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, catalyst or catalyst support, or actuator material, 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. Furthermore, first data on mechanical performance are becoming available, testifying to astonishing properties on a nanoscale level. However, little quantitative information is available on the central microstructural and topological properties which characterize porous structures, such as specific surface area, distribution of poreand 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, and which has recently been extended beyond applications in life sciences towards the characterization of 3-D structures in materials science. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging is based on Z-contrast due to Rutherford scattering, where the intensity scales with the atomic number (Z ) and the projected sample thickness. Therefore HAADF-STEM imaging is well-suited for electron tomography of crystalline materials 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. 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 structures of dealloyed white gold leaf. As was pointed out recently by Erlebacher and coworkers, this material provides a convenient model material for studies of nanoporous gold synthesis and structure, and it has the added advantage that the roughly 100 nm thick leaf samples provide electron transparent nanoporous gold that can be investigated by TEM without further preparation. The 3-D reconstructions obtained by TEM are stereographically analysed to yield numbers for porosity and characteristic ligament and pore sizes.