The formation of curled architectures is a wide-spread phenomenon in biogenic materials, most prominently observed in the plant kingdom. As a common principle, bending, curling and coiling of film structures are based on a bilayer or gradient configuration/construction, such that heterogeneities with respect to composition or structure give rise to a non-uniform mechanical response of the material to external stimuli such as drying/swelling, heating or an electrochemical potential. The pronounced internal stress resulting from a misfit between the different layers of the film is ultimately released by bending, which can lead to complex folding patterns. While coiling mechanisms in organic matter are intensely studied, reports on self-rolling in inorganic systems remain extremely scarce due to the high curvature and bending strain which have to be accommodated.
We here explore a bio-inspired approach, in which a cobalt(II) hydroxide carbonate precursor with layered crystallographic structure is precipitated in the presence of synthetic polyelectrolytes, acting as mimics of the soluble structure-directing matrix associated with biological mineralization processes. We demonstrate that extended mineral sheets with µm-thickness can be formed at the air-solution interface when precipitation occurs via slow gas diffusion. Intriguingly, the film fragments isolated after drying characteristically show bent and even curled morphologies. In the presence of polymer additives this strain-induced spiralling effect is substantially more pronounced such that micro scrolls composed of a polymer/mineral hybrid material are obtained. This remarkable observation inspired us to systematically investigate film formation and curling behavior in interface-grown basic cobalt carbonates depending on the functionalization of the polymer additive as well as the interface geometry and composition.
Calcination then leads to a pseudomorphic transformation of the precursors into the functional cobalt(II,III) oxide phase, which 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. Therefore, our method endows us with the possibility to generate compact microelectrodes with spiral morphology and mesoscale channels promoting the transport of reagents and products.