Synthesis of transition metal nitrides and silicon based ternary nitrides

Abstract

Solution phase ammonolysis and sol-gel techniques that produce polymeric metal-amide precursors are of growing interest in the synthesis of nitride materials, which result from the thermal decomposition of the polymer. They can be used to control composition and to produce a large number of useful morphologies such as nanoparticles, films, monoliths, aerogels, and materials with large surface area such as foams. The synthesis of nitride materials using ammonolysis of metal amides and chlorides has so far largely focused on producing powders for applications such as catalysis, or thin films by chemical vapour deposition and related techniques.

In this thesis, formation of tantalum and molybdenum nitride nanoparticles and metal-silicon nitride based nanocomposites have been synthesised using non-oxide precursors by solution phase ammonolysis and sol-gel methods respectively. For tantalum nitride nanoparticles Ta(NMe₂)₅ in THF was ammonolysed with ammonia at - 78 °C and the polymeric precursor was pyrolysed at various temperatures under ammonia. Amorphous TaN was obtained at 700 °C and below, while Ta₃N₅ was obtained at 800 °C under ammonia and also by re-annealing the amorphous samples at 800 °C under nitrogen. Molybdenum nitride powders were obtained by solution phase ammonolysis of MoCl₅ or Mo(NMe₂)₄ and further annealing the polymeric precursors on different temperatures. The chloride precursor resulted in hexagonal MoN_x at 500 °C, or rock salt MoN_x at 700-1000 °C and mixture of both at 600 °C. The amide precursor resulted phase pure hexagonal MoN_x at 600 °C and rock salt mixed with hexagonal at 1000 °C. Samples produced at 600 °C consisted mainly of nanotubes. Some decomposition to molybdenum metal was found in MoN obtained at 1000 °C from either precursor source.

Co-ammonolysis of Ta(NMe₂)₅ or Mo(NMe₂)₄ with Si(NHMe)₄ was carried out using sol-gel technique. Polymeric metal-silicon amide precursors were annealed at 600 or 1000 °C. The Ta/Si precursor produced amorphous nanocomposites and no phase segregation was observed even after high temperature annealing. With molybdenum the products were nanocomposites of molybdenum nitride particles, including nanotubes, supported on a silicon nitride amorphous matrix.

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ORCID for Andrew L. Hector: orcid.org/0000-0002-9964-2163

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