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Fabrication and characterisation of solid titanium nitride and molybdenum nitride microelectrodes

Metal nitrides have gained interest due to their high melting point, mechanical resistance, thermal and electrical conductivity. To the best of our knowledge, titanium nitride (TiN) and molybdenum nitride (MoN) electrodes have always been prepared as thin films. However, thin film electrodes tend to delaminate or crack during preparation or usage which exposes the underlying substrate and increases their surface area. In addition, the vapour deposition techniques employed to prepare thin films can introduce contaminants to the samples. In this work we prepared solid metal nitride samples, characterised them with a range of physical methods and investigated their electrochemical properties. Our work demonstrates the feasibility of obtaining TiN and MoN from Ti and Mo foils and microwires via nitridation in a NH3 atmosphere. The process was confirmed using energy dispersive X-ray spectroscopy and X-ray diffraction (XRD). The XRD spectra also showed that hexagonal MoN and cubic Mo2N were obtained. This work also demonstrates the viability of fabricating solid TiN and MoN microelectrodes, microdisks and microbands, from the nitrided Ti and Mo samples. Hence a major objective of the project was to assess whether TiN and MoN could be used as alternatives to conventional microelectrode materials such as Pt, Au, and C. To the extent of our knowledge, MoN and TiN wires have never been used to construct microelectrodes. The electrochemical behaviour of the solid TiN and MoN microelectrodes is assessed using different redox mediators to cover a range of redox potentials. The cyclic voltammograms recorded with the untreated TiN microband electrodes showed that the redox processes at positive potentials were not electrochemically reversible. Yet, the electrochemical response was improved after etching the TiN surface with hydrofluoric acid vapour. In contrast, MoN microelectrodes exhibited sigmoidal shape cyclic voltammograms with a plateau region for all redox mediators even without surface treatment. The TiN and MoN microelectrodes exhibited good activity towards the oxygen reduction reaction recorded in pH 1, 7, 10, and 14. The TiN and MoN microelectrodes were also employed to assess their properties towards the reduction of peroxodisulfate, a very strong oxidising agent with a very complex reduction process. This study also employed bare Au, bare Pt, nanostructured Pt, and bismuth-adsorbed Pt microdisk electrodes to search for the electrode that produces a stable and preferably a diffusion-controlled current for the reduction of peroxodisulfate. Cyclic voltammograms with a plateau region were obtained with the nanostructured Pt, bismuth-modified Pt, HF-etched TiN, and MoN microelectrodes but not with the bare Au and bare Pt microelectrodes. However, only MoN microdisks demonstrated a stable steady-state current for the reduction of peroxodisulfate. To our knowledge, no group has observed cyclic voltammograms with a plateau region when employing bare electrodes for the reduction of peroxodisulfate. A linear relationship between the current and concentration was obtained with the MoN microdisk electrodes for concentrations above 0.1 mM. Similarly, the MoN microdisk electrode produced a diffusion-controlled current for scan rates between 5 and 50 V s-1 . Overall, the MoN microelectrodes produced more reliable amperometric results than the TiN microelectrode. Thus, the MoN microelectrodes could be exploited as an alternative to the conventional Pt, Au, and C microelectrodes.