Turning a new magnetic leaf

Researchers have used a fig leaf as a template to produce a complex, hierarchical structure from the magnetic and conductive material iron carbide. X-ray powder diffraction allowed them to reveal how the biological template was able to create an intricate vascular structure in a ceramic product.

Zoe Schnepp, Wen Yang, Markus Antonietti, and Cristina Giordano of the Max Planck Institute of Colloids and Interfaces Research Campus Golm, in Potsdam, Germany, explain that many biological microstructures display an unparalleled degree of complexity that could be exploited in many different areas of research. In the rapidly growing area of materials synthesis, for instance, such complex natural structures have been used to template the growth of metal oxides, and noble metals with enormous surface areas for catalytic applications.

"By incorporating features of the biological structure, such as porosity, high surface area, or even cellular function, the properties of the ceramic can be modified or enhanced," the team explain. Much work has already been done with oxides, the team says, but there is currently a dearth of publications outlining naturally inspired synthetic methods for metal carbide materials, although at least one team has used pre-carbonized biological structures and liquid or gaseous silicon to produce complex silicon carbide materials.

"Metal carbides offer a range of unique properties that are not available to their oxide counterparts," the team says. "As such, they have found applications as diverse as hard coatings, medical implants, and catalysis. Iron carbide in particular is more magnetic than iron oxide, chemically resistant, and owing to the fact that it is a metallic conductor, a favourable choice for electrode applications."

Schnepp and colleagues hoped to shift the focus to metal carbides using easy-to-handle aqueous iron salts. They have now used a leaf, with its complex vasculature to synthesise in a single step an intricate, hierarchical microstructure composed of magnetic iron carbide. Electrode studies with this magnetic "leaf" have demonstrated not only homogeneity of the material but also its potential for water splitting and electrodeposition of platinum. "Powder X-ray diffraction confirmed the presence of iron carbide as a primary crystalline phase in the sample," the team says, "with additional peaks corresponding to metallic iron and graphite."

"The structural network of veins of a leaf contains bundles of vessels and fibres that are responsible for mechanical strength and also the distribution of water and nutrients to the photosynthetic cells," the team explains. "These vascular bundles are particularly well-suited as a biotemplate for ceramic synthesis because they are rich in lignin...a biopolymer resistant to degradation."

The team points out that their approach to metal carbide structures side-steps the many challenges facing materials scientists in working with such materials, namely the common need for multiple -step syntheses, decomposition of complex starting materials, and very high temperatures. "In our method, the leaf skeleton acts as both a template and a carbon source for formation of the iron or iron carbide material by carbothermal reduction of iron(II) precursors," the team explains.

The magnetic leaves, and related structures could find applications as light-weight but relatively strong magnetic materials or electrodes. The improved transport pathways within such a network, which have essentially been optimized by millions of years of evolution, offer a maximal transport system through their hierarchical tubes, the team adds. "In the present context, it does not matter if these are metabolites or electrons," they explain. There are many possibilities for such structures in catalysis, reactions, electrochemistry, sensors, and leafy lab-on-a-chip devices perhaps.

The team describes in Angewandte Chemie how early investigations have shown that the precise crystalline composition of the product depend on reaction duration and temperature, and the precursor.