Author ORCID Identifier:
Date of Graduation
7-2026
Document Type
Thesis
Degree Name
Master of Science in Materials Engineering (MS)
Degree Level
Graduate
Department
Materials Science & Engineering
Advisor/Mentor
Coridan, Robert H.
Committee Member
Hu, Han
Second Committee Member
Chen, Jingyi
Third Committee Member
Kohankek, Julia
Keywords
Urea Nitridation, Tantalum-based thin films, Oxynitride Synthesis, photoelectrochemical (PEC)
Abstract
Rising global energy consumption and the transition toward low-carbon technologies place new demands on functional materials. Particularly, semiconductors that convert sunlight directly into chemical fuels with high efficiency and long operational lifetimes. The synthesis of visible light active tantalum oxynitride (TaON) thin films for photoelectrochemical (PEC) water splitting typically requires ammonolysis with pressurized NH3 gas, which presents persistent barriers in safety and phase selectivity. In this work, a soft urea ammonolysis route was employed in which solid urea was employed as an in-situ nitrogen source inside a vacuum-sealed reactor. The finite urea mass establishes a self-limiting nitrogen chemical potential that intrinsically confines the synthesis trajectory within the β-TaON thermodynamic stability field, eliminating corrosive NH3 gas and the need for precise external flow-rate control. TaON thin films were obtained by nitridation of sputtered Ta and Ta2O5 on quartz and silicon substrates with urea in the 700- 900 ℃ temperature range. The resulting thin films were characterized to track the crystalline phase evolution of the film across the full synthesis matrix by X-ray diffraction (XRD), and surface oxidation states by XPS, which also confirmed the bulk nitrogen incorporation of nitrogen throughout the film. UV-Vis spectroscopy confirmed a pronounced red shift in the absorption edge following nitridation, where Ta2O5 (307-315 nm) transformed to β-TaON, exhibiting absorption edges of 552-609 nm, corresponding to an approximately 80-98% increase in absorption wavelength and reduction in bandgap from 4.0-3.9 eV to 2.0-2.3 eV, suggesting strong visible-light absorption. On the other hand, prolonged treatment (800 ℃, 12 h) induced partial re-oxidation, producing a 192 nm (approximately 32%) blue shift toward the Ta2O5 absorption edge. The small, estimated energy gap of TaON can be ascribed to the valence band structures consisting of N 2p orbitals. The approach was extended to the perovskite oxynitride by a two-step sequential method by using urea ammonolysis at 900 ℃ for 4 h. The results establish the soft urea route as a phase-selective, non-toxic, and scalable process for visible-light oxynitride thin films.
Citation
Srivastava, R. (2026). Soft Urea Nitridation of Tantalum-based Thin Films for Oxynitride Synthesis. Graduate Theses and Dissertations Retrieved from https://scholarworks.uark.edu/etd/6390
