Nucleation and growth
The number of nucleation centres increases in a magnetic field; crystallites do not grow as big when the same charge is passed, and their shape changes. An explanation of the magnetic influence on nucleation may be sought in the electrical double layer.
Zinc crystallites grown on a PANI-covered electrode, showing the effect of no field (left) or a 5 T field (right) on the density of nucleation centres. The flowerlike shape of the crystallites changes with applied field.
Nanowires can be grown electrochemically in porous alumina templates. We make the templates by hard-pulse anodization, and the pore diameter can be varied from 30 – 300 nm. Techniques have been developed to modulate the pore diameter along a wire. Magnetic barcodes, composed of alternating segments of magnetic and nonmagnetic metal can be grown in the alumina pores. The barcodes are detected in a microfluidic channel with integrated spin valve sensors. The filled membranes can be used to create huge magnetic field gradients at the electrode surface.
Template-grown four-segment Co-Ni magnetic barcode nanowires with gold spacer segments (left). Modulated-diameter nanowires grown in closed pores are shown on the right.
Magnetic gradient effect on oxygen reduction
In a strong magnetic field gradient, the Kelvin force on paramagnetic species may drive convection which controls the thickness of the diffusion layer that governs mass transport. This effect can be used to enhance the transport of paramagnetic oxygen in solution. The field gradients can be produced by magnetic nanoparticles or nanowires.
Substrates modified with gold-covered microhemispheres made of (a) Zn, (b) Co, and (c) Fe showing the influence of magnetic field gradients on the oxygen-reduction reaction.
Contact Michael Coey and Anup Kumar
Patterning metallic electrodeposits with magnet arrays, P. Dunne and J. M. D. Coey, Phys. Rev. B 85 224411 (2012)
Nucleation and electrochemical growth of zinc crystals on polyanaline films, L. Monzon, L. Klodt and J. M. D. Coey, J. Phys. Chem C 116 18308-18317 (2012)
Electrosynthesis of iron, cobalt and zinc microcrystals and magnetic enhancement of the oxygen reduction reaction, L Monzon, K. Rode, M. Venkatesan and J. M. D. Coey, Chemistry of Materials 24 3880-3885 (2102)