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 similar to 3 mu m) deposited from an aerosol onto heated substrates (T = 120 degrees C). A variety of porous scaffolds with 13 mu m 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 (similar to 50 nm and 13 mu m). 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.