Influence of processing conditions on structures of 3D ordered macroporous metals prepared by colloidal crystal templating

This paper presents general methods of preparing three-dimensionally ordered macroporous (3DOM) metals or alloys via templated precipitation and subsequent chemical conversion of metal salts (acetates, oxalates) within colloidal crystals of poly(methyl methacrylate) (PMMA) or polystyrene (PS) spheres. Three approaches are given to prepare 3DOM metallic Ni, Co, and Fe and the alloy Ni_1-xCo_x: (1) calcination of the metal oxalate/template composite in a nitrogen atmosphere, (2) formation of a 3DOM metal oxide followed by reduction in hydrogen, and (3) direct reduction of the metal oxalate/template composite in hydrogen. The 3DOM products obtained by these routes differ in size of the grains that compose the wall skeletons, in surface areas, and in compositions. Method 1 leads to very small grains and high surface areas but incomplete removal of carbon with graphitic layers surrounding metal grains. Method 2, as a two-step process, leads to relatively large metal grains, smaller surface areas, and carbon-free products. Method 3 strikes a compromise in these properties, with intermediate surface areas and small (<2%) amounts of remaining carbon. This paper presents synthetic details, discusses effects of the template choice (PMMA vs PS), and compares structural features of the macroporous metals and alloys, using the results of XRD, TGA, SEM, TEM, nitrogen adsorption data, and chemical analysis.