Jan 19, 2012
Natural nanotubes direct deposition of near-atomic scale alloys
Alloy formation is an established way to tune many properties of metals, but how can this be achieved at the nanoscale? An elegant approach is to use electroless deposition from aqueous solutions that contain salts of the metals present in the desired alloy. Reporting their results in the journal Nanotechnology, researchers have found a way to scale this process down to the near-atomic scale. The scientists employ a template, which directs the deposition into a very thin wire-like shape. This template is a natural nanotube – the tobacco mosaic virus, which infects only plants.
Details of the process are hard to obtain as it takes place within minutes and inside a 4 nm wide channel. First, the Pd-based activator ("sensitizer") binds selectively to the inner wall of the tube. This is possible since the exterior surface of the virus particle is chemically quite different (it contains fewer carboxylate groups). After adding the so-called "bath", which contains Co, Fe and Ni salts, and a reducing agent based on borane, the metal ions are reduced to the zero-valent state. This requires catalytic Pd nuclei. It is quite likely that the wire grows from a single nucleus, which is by chance especially active. The metals then catalyse the reduction of more metal ions. The growing structure remains inside the viral tube.
One advantage of electroless deposition is that the composition of the alloy can be changed simply by removing or adding another salt. This is very unusual on a scale below 5 nm. In fact, it is not even a simple task to prove that the metals are mixed on the atomic scale. The team used energy-filtering electron microscopy, where the imaging electrons are analysed with respect to losses in kinetic energy. These losses stem from electronic excitations in the metal atoms, and hence their energy is characteristic for each element in the alloy.
Alloy nanowires, as small as the ones presented here, might be used for a variety of applications including high density data storage, imaging, sensing, and even drug delivery.
More information can be found in the journal Nanotechnology.
About the author
Alexander Bittner studied chemistry in Darmstadt, Germany, and in Norwich, UK. He received his PhD from Freie Universität Berlin, Germany, in 1996. He worked at the Fritz-Haber- Institute in Berlin, at ETH Lausanne, and at the MPI for Solid State Research in Stuttgart, before joining CIC nanoGUNE in San Sebastian, Spain, as Ikerbasque Res. Prof. in 2008. His research topics are solid/liquid interfaces, self-assembly with plant viruses, and electrospinning.