Nanoporous silver with controllable optical properties formed by chemical dealloying in supercritical CO₂

Nanoporous Ag was formed by selectively etching Cu from multiphase AgCu alloys using the oxidant hydrogen peroxide and the metal chelator hexafluoroacetylacetone (hfacH, 14 mM) dissolved in supercritical CO₂ (60 °C and 16 MPa). The Ag films were characterized using X-ray photoelectron spectroscopy (XPS), field emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), and ultraviolet-visible absorption spectroscopy. Sequential oxidation and chelation produced 50-100 nm Ag ligaments that exhibited an asymmetric localized surface plasmon resonance (LSPR) peak in the green range (500-570 nm). Simultaneous oxidation and chelation produced 100-200 nm faceted Ag particles that exhibited a broader, symmetric LSPR peak centered in the red range (600-700 nm). Dealloying of phase domains containing Cu concentrations as low as 24 at % was possible. These results demonstrate a means for large-scale and low environmental footprint preparation of nanoporous metals with controllable optical properties.