Living organisms have developed elaborate mechanisms to construct intricately structured mineral architectures optimized at several hierarchical levels down to the molecular scale and can exert an astonishing degree of control over the size, shape, texture and even polymorph of minerals. While Nature’s dexterity in using organic matrices to generate bio-inorganic hybrid materials with internal interfaces on the nanometer level and a high degree of structural organization remains unmatched in synthetic systems, substantial progress has been made over the last years in translating some key concepts of biological mineralization into artificial materials. The implementation of such design principles holds enormous potential for the development of low-temperature routes to functional materials.
Our here presented research explores a bio-inspired approach, in which a cobalt(II) hydroxide carbonate precursor is precipitated in the presence of a range of contrasting synthetic water-soluble polymers, where the latter act as a mimic of the soluble structure-directing matrix associated with biological mineralization processes. Calcination leads to a pseudomorphic transformation of the precursors into the functional cobalt(II,III) oxide phase. Spinel-type Co3O4 finds applications in a wide range of technological fields, including gas sensing and clean energy conversion, where nanostructured Co3O4 may provide a cost-efficient alternative to Pt- and Ir-based catalysts for electrocatalytic water-splitting.
We demonstrate that extended mineral sheets with µm-thickness can be formed at the air-solution interface when precipitation occurs under slow diffusion conditions at room temperature. Intriguingly, the film fragments isolated after drying characteristically show bent and even curled morphologies. In the presence of polymer additives this effect is substantially more pronounced such that micro scrolls are obtained. This remarkable observation motivated us to systematically investigate the film formation and curling behavior of basic cobalt carbonates precipitated via ammonium carbonate diffusion depending on the functionalization and concentration of the polymer additive as well as the interface geometry provided by the reaction container. The nanostructure of the scrolls is studied by small-angle-x-ray scattering complemented by electron microscopy with the aim to elucidate the structural prerequisites for a mineral to accommodate such a high degree of bending.