Synergistic effects of heteroatom engineering in N-doped TiO2 films probed by X-ray absorption and photoelectron spectroscopy

Abstract

The precise control of electronic structure in nitrogen-doped titanium dioxide (N-TiO₂) is crucial for optimizing its optoelectronic performance. In this study, N-TiO₂ films were prepared via an in-situ hydrothermal method by varying the urea concentrations. Surface-sensitive X-ray photoelectron (XPS) and high-brilliance synchrotron-based X-ray absorption (XAS) spectroscopy is employed to probe the electronic and local structural modifications. Despite an incremental change of urea concentration, the chemical state of Ti⁴⁺ state is preserved within the experimental conditions. This is possible since the stabilized oxygen vacancies is accessible by hosting electrons at the neighbouring Ti atoms, and altering the Ti–N/O bond geometries. The XAS further revealed distinct orbital features within the Ti L₃-edge in which t_2g and e_g peak splitting are highlighted the D_2d symmetry of anatase. The incorporation of nitrogen enhanced N 2p–O 2p hybridization, weakening Ti 3d–O 2p hybridization. The fluctuating trend in the area ratio of e_g (d_z²) orbital infers direct evidence of the electron redistribution within Ti 3d–O 2p orbitals along the z-axis. These findings were corroborated in a similar trend shown at the pre-edge features, indicating the unprecedented charge carrier dynamics in this doped TiO₂ system. These synchrotron-based measurements unveil the electronic modifications of N-TiO₂, paving the way for the rational design of tunable heteroatom-doped metal oxides materials.