Modulating Optical Properties of TiN/TiNO Thin Films for Photocatalytic Applications*

Transition metal oxynitrides (TMON) can open new pathways to develop robust optoelectronic devices for use in photocatalysis and solar energy harvesting. TiN/TiNO thin films grown on sapphire substrates using a pulsed laser deposition (PLD) method in high vacuum conditions. However, some residual oxygen in the PLD chamber allowed both time-independent gas phase oxidation and time-dependent controlled surface oxidation of TiN to TiNO films. The time-dependent surface oxidation was controlled by deposition time, i.e., changing the number of laser pulses impinging on the polycrystalline TiN target. X-ray photoelectron spectroscopy (XPS) investigations revealed higher oxygen content in TiNO films prepared with a larger number of laser pulses (or longer deposition time), which was attributed to the surface diffusion of oxygen to the TiN film lattice. Higher oxygen content also increased the lattice constants of the TiNO films, as predicted by molecular dynamics (MD) simulations. The lattice constant increase was further explained based on a larger electrostatic repulsive force in the vicinity of Ti³⁺ vacancies and substitutional O atoms. UV-vis measurements demonstrated an asymmetric V-shape variation of the optical bandgap as a function of the number of pulses. The bandgap variations on the left and right arms of the V-curve were attributed to the quantum confinement effect and modification in the band structure due to the hybridization of O_2p and N_2p energy levels, respectively. Electrochemical catalytic activity experiments also demonstrated strong sensitivity to the number of pulses and oxidation state of the sample, e.g., the highest electrochemical activity was obtained for the lowest-oxidized TiNO sample (and lowest bandgap). This study suggests that the oxidation environment during fabrication and the resulting chemical state of TiN/TiNO thin films play a critical role in modulating the optical properties of the materials and could be tailored to design for applications in clean energy and water-splitting research.