Hierarchal 3D Structures of Inorganic Nanomaterials Using Calcareous Foraminiferal Shells as a Template

Nanostructure materials have witnessed tremendous progression toward their use in various applications due to their unique properties. However, their incorporation and fabrication into prototype devices remains a challenge mainly due to the limited ability to form hierarchical three dimensional (3D) structures in a large scale, low cost and facile process. Herein, we address this challenge and demonstrate the growth of unique hierarchical structures by coating calcareous foraminiferal shells with inorganic and organic materials. We formed fourteen different 3D materials using various calcareous foraminiferal shells as scaffolds, including the benthonic genus Sorites, Globigerinella siphonifera­, Loxostomina amygdaleformis, Calcarina baculatus, Calcarina hispida and Peneroplis planatus. Foraminifera are highly diverse and abundant marine unicellular protists that possess vastly complex, porous, hierarchical and large (from 0.1 mm to more than 200 mm) shells. Surprisingly, despite of their unique structures, they have not been used as templates to grow inorganic or organic materials. Coating the scaffold was simply conducted via thermal decomposition of single source precursors at moderate temperatures to form metal oxide and metal sulfide coatings, or reduction of metal salt directly on the surfaces of foraminiferal templates. These hierarchal structures were investigated in electrochemical water oxidation reaction and water purification processes of metal ions and dye molecules contaminates. For water oxidation, Sorites@Co and Sorites@NiO present a remarkable performance for oxygen evolution, 154.6 mA/cm² and 73.5 mA/cm² at 1 V vs. Ag/AgCl and an onset potential of ~0.55 V vs. Ag/AgCl, respectively. For water purification process, Sorites@Fe(OH)_x structures were used as an active material for removing both types of contaminates without using any external pressure. A superior performance of the prototype filter was achieved where the concentration of the contaminates after filtration, such as Pb²⁺, Cd²⁺, Cu²⁺, MB, Rh6G and RhB was reduced by 99.98%, 99.99 % and 99.99%, >99.7%, >98% and >88%, respectively.