Nanoporous gold (NPG) is the paradigm of the novel class of nanostructured metals consisting of a randomly interconnected solid and void structure. Due to the high surface area of these materials, there are numerous promising fields of application in catalysis, sensors or electronics. The most common synthesis method of NPG is electrochemical dealloying of Ag-Au alloys [1].

To enlarge the surface area of NPG even further, a two-step dealloying process on a Pt-doped Ag-Au master alloy has been developed [2]. In the first step of the dealloying process, a fair amount of silver is removed by electrochemical corrosion, leading to a porous structure with very small ligaments of about 20 nm diameter. After gentle annealing, resulting in a porous structure with a feature size of about 100 nm (NPG), a second dealloying process is performed. The residual silver is removed, generating a porous structure (ligament size smaller than 10 nm) inside the initial formed ligaments of the porous structure, forming a nested-network nanoporous gold (N³PG) (Figure 1).

The retention of silver in the first dealloying step is presumed to be the result of a passivation of the master-alloy surface during dealloying by successive accumulation of platinum on the electrolyte-solid interface during the electrochemical removal of silver. During the subsequent annealing, the platinum diffuses from the surface into the Ag/Au Ligaments, which is consistent with thermodynamic calculations [3] and spectroscopic measurements [4], exposing a fresh surface of Ag/Au for the second dealloying step, resulting in the hierarchical structure of N³PG. The proposed accumulation of Pt is plausible because of the lower mobility of platinum compared to silver and gold and has been proven by cyclic voltammetry [5]. However, cyclic voltammetry and other spectroscopic methods only give an averaged analysis of the surface. To further investigate this theory, localized information of the surface composition of the NPG ligaments is needed.

In this work, we present the electron microscopic examination of NPG by the means of electron tomography in combination with EDX. The complex material structure with a high void fraction, high element numbers, and low concentration of minor constituents (Ag, Pt), however, aggravates the specimen preparation and data collection. To obtain reasonably thin, artifact-free specimen of NPG, the voids have to be filled with epoxy and then cut by ultramicrotomy. The image and elemental maps (Figure 2) collection is performed on a modern TEM optimized for electron tomography, equipped with four EDX-Detectors covering a large solid angle and a high brightness gun for high probe currents. For data analysis HyperSpy [6] is used.

[1] Z. Qi , J. Weissmüller, ACS Nano 2013, 7, 5948.

[2] Z. Qi, U. Vainio, A. Kornowski, M. Ritter, H. Weller, H. Jin, J. Weissmüller,
Advanced Functional Materials 03/2015; 25(17).

[3] P. A. Dowben, A. H. Miller, R. W. Vook, Gold Bull., 1987, 20, 3.

[4] J. A. Schwarz, R. S. Polizzotti, J. J. Burton, J. Vac. Sci. Technol. 1977, 14, 457.

[5] A. A. Vega , R. C. Newman , J. Electrochem. Soc. 2014, 161, C1.

[6] HyperSpy Home Page. www.hyperspy.org (accessed Mar 2016)

Figures:

Figure 1: Schematic drawing of structure generation during subsequent dealloying and annealing. Element concentration determined by EDS.

Figure 2: STEM-Image and elemental map of NPG after first dealloying and annealing step.

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