Hidden Complexity in the TiO2 to TiN Transformation Unveiled through Compositional Analysis

Heteroanionic materials such as oxynitrides offer a unique platform for tailoring electronic and structural properties. In this work, titanium dioxide (TiO2) was heated under ammonia (NH3) gas to create titanium oxynitride (TiOxNy) compounds. A combination of the indophenol blue method and thermogravimetric analysis (TGA) was used to quantitatively determine the composition of the bulk powder. These results were qualitatively confirmed by X-ray absorption spectroscopy (XAS), as the transition causes a reduction of Ti(IV) to Ti(III). The kinetics of transformation of TiO2 was compared between several morphologies of oxide nanoparticles. While larger particles transformed more slowly than smaller particles, as expected, in all morphologies the transformation occurred via a two-step process, a fast transformation accompanied by a phase transition from the rutile TiO2 to the halite TiN structure, followed by the gradual replacement of the remaining oxygen by nitrogen in a markedly slower process. Further insight into the mechanism of transformation was provided using extended X-ray absorption fine structure (EXAFS), which provides evidence that this transformation occurs through the presence of an amorphous intermediate phase. This work highlights the importance of accurately determining O/N ratios in bulk oxynitride samples for a deeper understanding of their chemistry and structure-property relationships.