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NanoCones - Magnetic-field assisted single-step electrodeposition of Co-Fe-Ni nano-structures

Subject Area Synthesis and Properties of Functional Materials
Solid State and Surface Chemistry, Material Synthesis
Physical Chemistry of Solids and Surfaces, Material Characterisation
Theoretical Chemistry: Molecules, Materials, Surfaces
Term from 2017 to 2021
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 381712986
 
Final Report Year 2022

Final Report Abstract

Micro- and nano-structured ferromagnetic layers are attractive for super-hydrophobic and electrocatalytic applications and can be effectively synthesized using electrodeposition. The project studied the electrodeposition of conical ferromagnetic nanostructures in magnetic fields. The Lorentz and the magnetic gradient force provide unique possibilities to control the mass transport of both electroactive ions and crystal modifier directly at the conical structures. The vision is to establish a novel and simple route toward ordered arrays of ferromagnetic alloy nano-cones with improved electro-catalytic properties. To approach this vision, systematic experimental and numerical investigations of all relevant electrochemical and hydrodynamic aspects were conducted on the length scale hierarchy: mm → μm→ nm. While the German team was in charge for the hydrodynamics of the electrodeposition process on the mm → μm scale, the Polish team was responsible for the characterization of the morphological and electro-catalytic properties achieved on the nanoscale. Complementary numerical simulations were performed by the German team. First numerical and experimental work by the German team have proven that magnetic fields promote the growth of mm-sized conical structures by alternatively generating a supportive local flow. A detailed understanding of the action of the magnetic forces, the local flow driven near conical elevations and the impact on the mass transfer was achieved. Furthermore, the quantification of the effect depending on the distance and the shape of the cones was elucidated, thereby finding agreement between simulations and measurements. To further explore the prospects of using magnetic fields to support the growth of smaller, micro-/nano-sized conical structures, numerical simulation were performed to study how the local electrolyte flow and the related inhomogeneous mass transfer change with shrinking cone size. Related scaling laws were derived, and stronger viscous friction along with smaller concentration changes inside the diffusion layer were found to limit the support of the magnetic field. At small sizes, the magnetic gradient force was found to dominate the effect compared to the Lorentz force. To enhance the structuring effect, pulsed electrodeposition and use of superconducting magnets were discussed. In parallel, at AGH systematic experiments on the template-free electrodeposition of nickel layers in magnetic fields of different orientations and intensities were performed. Regardless of the direction, strong fields were found to promote blunt-ended, shell-like structures, but no clear supportive tendency of stronger fields was observed. These results were finally discussed by the help of numerical simulations which additionally consider the global cell flow forced by the magnetic fields. Importantly, in the experiments performed global flow is found to dominate compared to local flow. We therefore propose improved electrode geometries for future research to clarify the prospects of strong magnetic fields. Ongoing research activities of the project partners further cover numerical studies of the impact of the hydrogen side reaction on the local mass transfer and measurements of the gas evolution and the catalytic activity of conically structured nickel deposits.

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