The effect of ammonolysis conditions on the structural properties and oxidation kinetics of cubic niobium oxynitride

Abstract

In recent years, transition metal oxynitrides have gained increasing attention due to offering attractive properties, such as high electronic and thermal conductivity, high melting points and hardness, as well as high catalytic activity. Niobium oxynitrides, especially, have been suggested for a diverse range of applications, including potential electrocatalysts (e.g., water splitting, nitrogen reduction, etc.), antibacterial agents, superconducting materials, and coatings, among others. One of the most promising ways to synthesize these materials is by ammonolysis. However, by this route the final composition can be heavily dependent on the ammonolysis conditions, with this factor currently understudied in the available literature. Thus, in this work, we carefully explore the impact of ammonolysis conditions on the crystalline phase formation of niobium oxynitride compounds. Several techniques were used to fully characterize the materials, including XRD, SEM-EDS, TGA/DSC, XPS, and chemical analysis. Depending on the ammonolysis temperature and time, different composition-dependent changes in crystallographic structure can be induced across δ-NbN_xO_y (cubic) → Nb₄(N,O)₅ (tetragonal) → ε-NbN (hexagonal) phases. Potential defect-chemistry models were proposed to support measured compositional variations. A detailed kinetic analysis was then performed on the cubic materials to understand how composition can influence the thermal oxidation behaviour within this structure-type. An F_n-type reaction was used to fit the experimental and calculated data. The selection of the Avrami model, which describes the crystallization kinetics, was supported by the identification of amorphous-crystalline phase transformations during thermal oxidation. This work demonstrates that samples with higher N-content are more resistive to oxidation and shows that the anion composition, cation/anion ratio, and the crystallographic structure can be tailored by careful control of ammonolysis conditions.