Formation of Hierarchical Nanoporous Gold via Selective Dissolution and Dealloying of Ternary (Au–Ag)–Ge Two-Phase Hypereutectic Alloy

Nanoporous (np) metal foams exhibit unique three-dimensional bicontinuous structures characterized by nanometric pores and ligaments, remarkable surface area, catalytic activity, and mechanical robustness. Traditionally, np foams are fabricated by selective dissolution of less-noble elements from metal alloys, leaving behind noble metal frameworks. This study pioneers the use of a ternary Au-Ag-Ge eutectic alloy to synthesize hierarchically complex np-Au crystals. Through a two-step selective dissolution technique involving the removal of Ge followed by dealloying Ag, we achieved a dual-level porosity with controlled morphology. The resulting microstructures feature pore and ligament sizes spanning 10-50 nm to 100-350 nm. By varying the dissolution methods and durations, pore sizes were further refined to approximately 20 nm. High-resolution XRD and EDX confirm the dissolution of most of the Ge and Ag from the alloy, while cross-sectional SEM imaging reveals the complexity of the hierarchical architecture. The hierarchical np-Au crystals demonstrated a dramatic enhancement in catalytic activity, up to 10-fold compared to binary Au-Ge-derived np-Au and 2-fold relative to conventional np-Au. This advancement underscores the potential of ternary eutectic alloys for creating multifunctional nanostructures and offers a promising avenue for tuning the catalytic performance across various applications.