Sponge-Like Porous Metal Surfaces from Anodization in Very Concentrated Acids

High surface area metals are of great importance for applications ranging from catalysts and electrodes to sensors or biomaterials. Many patents and scientific papers are devoted to a range of manufacturing approaches commonly involving multistep processing under harsh conditions, lacking general applicability and bearing the potential for contamination. Here we demonstrate the fabrication of porous metal layers by anodization at moderate voltages in highly concentrated acids. Porous metal layers were produced on copper, silver, iron and nickel using 99% phosphoric and sulfuric acids. The porous layer thickness can be tuned up to over one micrometer. Structures develop in 4 to 30 minutes independent of substrate purity or crystallographic features. The mechanism is believed to involve templated etching due to a near-stagnant bubble layer in a highly viscous electrolyte near the anode. It is therefore not dependent on any particular chemistry, as long as anodic oxygen bubbles are evolved at a sufficient rate. Since the principal processes of electropolishing are still operational, the surfaces remain flat at a larger scale, even though the optical properties (reflectivity, SERS activity) have changed significantly. Our method is reproducible, cheap, clean, fast and versatile, leading to a wider range of applications for porous metal surfaces.