Defect-Mediated Energy States in Brookite Nanorods: Implications for Photochemical Applications under Ultraviolet and Visible Light

The photochemical properties of brookite nanorods are systematically explored using light-induced electron-paramagnetic resonance (EPR) techniques at different wavelengths spanning the UV-vis region of the electromagnetic spectrum (355-650 nm). Under UV irradiation, electron-hole pairs are generated, leading to the stabilization of paramagnetic centers, primarily Ti3+ and O- species at the surface. Visible light irradiation at low temperature results in a unique pair of EPR signals, including electrons trapped at titanium cations and a distinct signal resonating at g = 2.004. The pair of signals disappears after annealing at room temperature, indicating that recombination pathways with trapped electrons are available. The chemical reactivity of the different photogenerated species is tested using electron and holes scavengers. While peculiar light-harvesting capabilities are observed for the brookite nanorods, experiments carried out in the presence of a hole scavenger indicate a limited potential for oxidative processes under visible light.