(in collaboration with Dr Andrew R. Mount)

Metal nitrides are technologically important materials that have a wide range of applications. For example, titanium nitride is used as hard coatings on cutting tools and engine components, as diffusion barriers for interconnects on silicon chips, as anti-reflective coatings, and as inert, hard-wearing layers on medical implants such as hip replacements.

Most existing routes to titanium nitride require the use of high temperatures (800 °C) and/or expensive high vacuum equipment. This is because the rate of reaction between titanium metal and nitrogen or nitrogen-containing compounds is normally very slow at ambient temperatures. Our strategy involves the use of electrochemical nitridation of titanium using liquid ammonia as a solvent at temperatures in the range -78 to +25 °C. This is exactly analogous to the anodisation of metals such as aluminium and titanium in aqueous solutions to make thicker layers of metal oxides - aluminium oxide coatings allow dyes to be adsorbed, whilst thin layers of titanium dioxide are responsible for the colours seen in titanium jewellery. Thus by making a piece of titanium metal the anode of cell containing a solution of suitable electrolyte in liquid ammonia, titanium is successfully converted into its nitride. Although other workers have attempted to use electrochemical methods to nitride titanium (including the use of liquid ammonia), invariably post-processing of the films or particles at high temperatures is required to generate titanium nitride, and frequently carbon-, oxygen-, or halogen-containing impurities are incorporated into the films.

Our method overcomes these disadvantages by the use of an electrochemical cleaning technique that disrupts and removes the native layer of titanium dioxide that exists on all titanium surfaces exposed to air, and second, by the use of a potassium amide as the electrolyte - this has the result of ensuring that only nitrogen-containing anions are attracted to the titanium anode. Our route produces pure titanium nitride without the need for post-processing. At low current densities we are able to generate films, the thickness of which can be controlled by the total amount of charge passed, but at higher current densities nano- particles of titanium nitride are produced.

Transmission electron microscopy image of TiN nanoparticles embedded in epoxy resin

Work is continuing in this area and is focussing on nitridation of other metals - we have successfully demonstrated its applicability for the preparation of nitrides of tantalum, molybdenum, and tungsten, and preliminary results suggest that aluminium and gallium can also be nitrided in this way, thus opening up new routes to nitrides with useful opto-electronic properties. We are also exploiting the enhanced reactivity of the TiN nano-particles for the development of low temperature routes to new ternary nitrides.

Low temperature electrochemical synthesis of titanium nitride
Lucy E. Griffiths, Martin R. Lee, Andrew R. Mount, Hiroshi Kondoh, Toshiaki Ohta, and Colin R. Pulham,
Chem. Commun., 2001, 579-580.

Mail to C.R.Pulham@ed.ac.uk