Fabrication and characterization of niobium oxynitride thin films for energy and environmental applications

In this study, niobium oxynitride (NbO_xN_y) thin films were prepared using radio RF magnetron sputtering from an Nb₂O₅ target. Nitrogen gas was used as the precursor for N-doping due to its dry process and environment friendly. By X-ray diffraction and SEM cross-section analysis, NbO_xN_y thin film was in an amorphous state and its growth rate is ~50 nm/h. The cutoff wavelength of the absorption spectra of NbO_xN_y thin films is moreover shifted towards the visible region (red shift) with increasing film thickness, which is implied to dope N into Nb₂O₅. Similarly, the conductivity of the NbO_xN_y thin film is about 500 times larger than that of the Nb₂O₅ thin film. According to AFM analysis, NbO_xN_y thin films have smooth surfaces and grain sizes were slightly increased while film thickness increased. Furthermore, the co-presence of Nb-O, as well as Nb-O-N and Nb-N the film was characterized, and the N doping level of the sample was estimated at about 27.8 at. % from the XPS spectra. The electrochemical behaviors of NbOxNy thin films were characterized by capacity measurement and cycling performance. The volume capacity density of NbO_xN_y thin film is about 189 μAh/cm² μm, which is over twice as large as that of the amorphous Nb₂O₅ films, due to the formation of metal-rich niobium nitrides in the RF sputtering process. In addition, electrochemical characteristics of thin film rechargeable lithium-ion battery composed of LiMn₂O₄ thin films of ca. 720 nm, LiPON electrolyte thin films of ca. 1.0 μm and NbO_xN_y thin films of ca. 200 nm were also investigated. At a discharge current of 10 μAh/cm², the maximum discharge capacity of 19.3 μAh/cm² was obtained, and the coulombic efficiency was all over 90% except for the first few cycles. Furthermore, NbO_xN_y thin films demonstrated a remarkable degradation efficiency of 92.1 % within 480 min under white LED light irradiation, following a pseudo-first-order kinetic model equation with a rate constant of k = 5.5 x 10⁻³ min⁻¹, which is about twice as large as that of Nb₂O₅ thin films. These findings highlight the potential of the NbO_xN_y thin films as an efficient and sustainable material for energy and photocatalytic degradation.