Nanoporous gold (NPG) is usually fabricated by the selective electrolytic dissolution (dealloying) of Ag from a binary Ag-Au alloy, or sometimes from a ternary alloy such as Ag-Au-Pt. The resulting ligament-pore structure is bicontinuous and has an extraordinarily high surface area-to-volume ratio, posing it as a promising candidate for catalyst applications [1-2]. The ligament size can be as small as 3-5 nm. While a lot of recent research has been focusing on the structure/property relationships of NPG as a functional material in its own right [2-5], the further use of NPG as a sophisticated template for fabricating complex nanostructured materials is worthy of attention.

Figure 1- SEM image of FIB (focused ion beam) cross-section showing the dealloyed layer after deposition of Cu. [5]

Recent work showing atomic-scale characterization of NPG by electrodeposition of a fully compact Cu support opens the door to implementing that newly developed method to the creation of finely tuned nanostructures using NPG as a template [5]. Fabrication of nanostructures/nanosurfaces pertaining to the interests of the catalysis community will be demonstrated through the electrodeposition of metals, such as Cu and Co, and metal oxides, such as Cr₂O₃ and MoO₂, on NPG. Furthermore, notable characteristics of NPG and the electrodeposition methodology that enables such compact deposits have been investigated experimentally and theoretically through reaction-transport modelling. A key aspect is the use of thin nanoporous layers, such that the characteristic diffusion time through the layer is short, in the ms range, but there are other subtleties. Electron diffraction and X-ray diffraction reveal the crystalline structure and orientation of the deposits and the associated growth mechanism. For metal deposition within a nanoporous material, not only ordinary thermodynamics have to be considered, but also the “subpotential” curvature-driven deposition described by Lee et al. [6].

References

[1] R C. Newman, “Dealloying,” in Shreir’s Corrosion (4th ed.), Elsevier, vol. 2, pp. 801–809, 2010.

[2] A.A. Vega, and R.C. Newman, “Nanoporous metals fabricated through electrochemical dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio,” Journal of the Electrochemical Society, vol. 161, pp. 1-10, 2014.

[3] M. Yan, T. Jin, Q. Chen, H. E. Ho, T. Fujita, L. Y. Chen, M. Bao, M. W. Chen, N. Asao, and Y. Yamamoto, “Unsupported nanoporous gold catalyst for highly selective hydrogenation of quinolines,” Organic Letters, vol. 15, pp. 1484–1487, 2013.

[4] A.A. El-Zoka, J. Howe, R.C. Newman, S. Dogel, M. Reynolds, H. Hosseinkhannazer, and D.D. Perovic, “Understanding the coarsening behaviors of nanoporous gold via in situ heating,” Microscopy and Microanalysis, vol. 22, pp. 1968-1969, 2016.

[5] A.A. El-Zoka, B. Langelier, G.A. Botton, and R.C. Newman, “Enhanced analysis of nanoporous gold by atom probe tomography,” Materials Characterization, In Press, Available online 10 March 2017.

[6] L. Lee, D. He, A.G. Carcea, and R.C. Newman, “Exploring the reactivity and nanoscale morphology of de-alloyed layers,” Corrosion Science, vol. 49, pp. 72-80, 2007.