Formation of Oxygen Vacancy Gradient TiO₂ Via Photoelectrochemical-Driven “Self-Purification” Process

Solar water splitting by oxide semiconductors is the most ideal strategy for clean and sustainable hydrogen production. However, the application of oxide semiconductors is limited by their poor charge separation efficiency, which results from the short diffusion length of minority carriers and bulk charge recombination. Oxygen vacancy (V o ) is a unique and crucial tool in the metal oxide semiconductor, which not only is one of the intrinsic defects but plays a significant role to modulate their optical, physical, and catalytic properties. For instance, the introduction of oxygen vacancy can suppress electron-hole recombination for efficient solar water splitting. However, it remains challenging to control oxygen vacancies due to their random formation and moving in the lattice of the nanostructure. In this work, we present a photoelectrochemical (PEC) driven “self-purification” approach for the formation of oxygen vacancy gradient (ΔV o ) by reconstructing the disorder overlayer from the TiO 2 nanowire photoanode under long-term PEC operation using “self-purification” mechanism. The presence of V o in oxygen vacancy gradient TiO 2 (ΔV o -TiO 2 ) was clearly confirmed by X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectra. Also, high-resolution transmission electron microscopy (HRTEM) and electron energy-loss spectroscopy (EELS) analysis showed that the gradient distribution of V o forms in ~9.5 nm-thickness range of ΔV o -TiO 2. After the “self-purification” process, the photocurrent density of ΔV o -TiO 2 doubles compared to that of pristine TiO 2 at the 1.23 V vs RHE. Also, we demonstrated that V o improved the charge separation in bulk region by charge carrier dynamics with time-correlated single-photon counting (TCSPC) analysis. Furthermore, ΔV o -TiO 2 exhibits ~95% charge transport efficiency, which is associated with bulk recombination. These results indicate the ~95% photon within the absorption range can be converted to charge. Lastly, we verified the electronic structure of ΔV o -TiO 2 by density functional theory (DFT) calculations. ΔV o -TiO 2 forms a cascade of energy gradients along the bulk region, which can polarize the charges to form a built-in electric field that thermodynamically boosts charge carrier migration and suppresses bulk charge recombination. In conclusion, we suppose that the key role of V o in the bulk region in a metal oxide photoanode is critical to enhancing charge separation in TiO 2 . Therefore, our demonstration can fully resolve the conventional efforts to establish the optimized V o for photocatalyst and offer a feasible approach for gradient doping semiconductors synthesis.