Nanocasting of Hierarchically porous Co3O4, Co, NiO, Ni, and Ag Monoliths: Impact of processing conditions on fidelity of replication

Hierarchically porous silica monoliths with a bicontinuous pore structure at the micrometer length scale and 3.4 nm surfactant templated mesopores were used as hard templates for preparing Co₃O₄, NiO, as well as cobalt, nickel and silver metal monoliths. Aqueous solutions of metal nitrates were repeatedly infiltrated into the porous silica templates after which the nitrate salts were thermally decomposed. Thermal decomposition in air or nitrogen produced porous NiO and Co₃O₄ oxide monoliths, while thermal decomposition under an atmosphere of 5% hydrogen in 95% nitrogen produced porous monoliths of nickel, cobalt and silver. Removal of the silica template using potassium hydroxide solutions produced free-standing hierarchically porous Co₃O₄, NiO, cobalt, nickel and silver monoliths. The surface areas of the materials determined by nitrogen adsorption were 141 m²/g for Co₃O₄, 42 m²/g for cobalt, 32 m²/g for NiO, 13.7 m²/g for nickel and 2.2 m²/g for silver. The impact of an additional annealing step was also assessed. The differences in the surface area, pore size distribution, morphology between the samples as well as the fidelity of the replication are discussed within the framework of a comprehensive model for nanocasting within porous monoliths. From this model, it is evident that the final replica structure is strongly dependent on the wettability of the metal oxide or metal on silica and the increased mobility at elevated temperatures, as the non-wetting metals typically displayed a higher migration out of the mesopores giving rise to a lower surface area compared to the metal oxides.