A uniform magnetic field has a dramatic influence on the morphology of fractal electrodeposits grown in a thin flat cell. The effect of the Lorentz force has been modelled by adapting ‘diffusion limited aggregation’ (DLA) to take account of electric and magnetic fields, gravity and concentration.
Fractal electrodeposits These zinc electrodeposits are grown with no field (a), a perpendicular field upwards (b) or downwards (c) and a horizontal field (d).
Patterning with non-uniform fields
Metallic electrodeposits can be patterned with magnet arrays. The Kelvin force produced by the magnetic field gradient near the cathode modifies the thickness of the diffusion layer governing mass transport. Patterning depends on the susceptibility of the electroactive species relative to the nonelectroactive background. The patterning, which requires an orthogonal concentration gradient ∇c and field gradient ∇B2, appears after the diffusion layer has begun to form.
Patterned copper electrodeposits obtained with an array of small magnets behind the cathode, which faces (a) downwards, (b) horizontally or (c) upwards. The copper deposits where the field is strongest, but the forces are comparable to those due to gravity-induced convection
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.
|0 T||1 T||3 T||5 T|
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 eletrochemically 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 magntic 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.
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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)