Fabrication of hierarchically porous materials and nanowires through coffee ring effect

We report a versatile method for the fabrication of nanowires and hierarchical porous materials from a wide variety of ceramic materials such as CaCO₃, ZnO, CuO, Co₃O₄, Co-doped ZnO, and Ag₂O. The method consists of evaporation of CO₂-enriched water microdroplets (diameter ∼3 μm) deposited from an aerosol onto heated substrates (T = 120 °C). A variety of porous scaffolds with 1-3 μm sized pores can be generated by tuning the process conditions. Subsequent sintering of the scaffolds is shown to generate nanosized pores in the walls of the porous scaffold creating a dual hierarchy of pore sizes (∼50 nm and 1-3 μm). We propose a mechanism for the formation of scaffolds based on the coffee-ring effect during the evaporation of microdroplets. Ostwald-ripening of CaCO₃ scaffolds prepared without sintering yields scaffold structures consisting of two-dimensional crystals of CaCO₃ that are one unit cell thick. The favorable application of CaCO₃ scaffolds for the enhancement of bone healing around titanium implants with improved biocompatibility is also demonstrated.