Investigation of the Electroless Deposition Process of Magnetic Nanostructures

Magnetic nanostructures can be formed via a variety of preparation methods. The most common of these is electrochemical deposition. However, recently there has been an increased interest in electroless deposition of magnetic nanostructures. Electroless deposition of nanotubes or nanowires requires a template, commonly a porous polycarbonate (PPC) or aluminium oxide membrane. Usually dimethylamine borane or hypophosphite is used to reduce ions such as Ni⁺², Co⁺², Fe⁺² and Cu⁺²to metals. The use of these reducing agents results in the simultaneous deposition of B or P in the magnetic nanostructures. Although magnetic structures have been deposited previously via electroless deposition a detailed investigation of the deposition process is lacking.

Previously we have prepared CoNiFe-B [1] and NiCu-B [2] nanostructures via electroless deposition and investigated their associated magnetic properties. In this work, we use electroless deposition of Ni-B as a model process to study the preparation of nanostructures on a PPC membrane. We obtained Ni-B nanostructures consisting of nanotubes with outer diameter 400 nm and length of 20 μm, connected together by a Ni-B film at both top and base. We have analysed the electroless deposition process using scanning electron microscopy (SEM) imaging at different stages of deposition as shown in insets (i) and (ii) in Fig. 1. The magnetic properties of these nanostructures have also been investigated as shown in Fig. 1.

Prior to electroless deposition the membrane is activated by deposition of catalytic palladium nuclei throughout the pores and surface. Initial electroless deposition of Ni-B occurs at these catalytic palladium sites. This results in the formation of Ni-B islands on the membrane surface and along the pore surface as shown in Fig. 1(i). Once initiated the deposition becomes self-sustaining. As deposition continues the islands coalesce together forming a continuous film on the membrane surface and nanotubes within the membrane pores. The membrane is then dissolved with only the Ni-B nanostructure remaining as shown in Fig. 1(ii).

We have observed that deposition on the membrane surface occurs at a faster rate than inside the membrane pores. We believe that the lower deposition rate inside the pores is associated with local changes of Ni concentration and pH. The midpoint of the tube axis has the lowest deposition rate and thus here, the tube wall is thinnest. Due to the higher deposition rate on the membrane surface, the nanotubes become closed at both ends before continuous nanowires are formed. If the rate of deposition on the surface is very large the nanotubes can become end-closed before the tube is fully formed.

In this study the nanostructure growth is analysed at different deposition rates by varying the concentration, temperature and pH of the deposition bath. By controlling each of these parameters the deposition rate at the surface and the within the pores can be controlled. Modelling of the deposition rate as a function of position along the tube axis is also carried out.

The authors would like to acknowledge funding provided by the Irish Research Council (RS/2011/270), New Foundations 2014 and Science Foundation Ireland (12/IP/1692).

[1] J.F. Rohan, D.P. Casey, B.M. Ahern, F.M.F Rhen, S. Roy, D. Fleming, S.E. Lawrence, Electrochem. Commun. 10,1419 (2008)

[2] D. Richardson, F.M.F. Rhen, IEEE Trans. Magn. Submitted (2014)

Figure. 1. Magnetic measurements carried out perpendicular and parallel to the tube axis, of end-closed Ni-B nanotubes after 15 minute electroless deposition at 65 °C at a pH of 9. Inset (i) SEM image of initial Ni-B island deposition at Pd nucleation sites and (ii) Ni-B nanotubes connected with a Ni-B thin film.