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 (TiO_xN_y) 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.