Red-Ox Energy Partitioning of Light-Driven Electrons: From Laser Ablation to Plasmonics

In femtosecond-laser processing of titania in water, light can induce reduction and oxidation simultaneously. We follow this redox energy partitioning, in this perspective, from colloidal titania synthesis to hot-electron devices. Femtosecond ablation/fragmentation of an aqueous anatase suspension (515 nm, 230 fs, 5μJ, fluence F≈25.5 J cm^-2/pulse at clamped intensity ∼1013 W cm^-2) yields surface-reduced, Ti³⁺-rich bluish TiO_2-x, while the same optical breakdown generates reactive oxygen species (ROS), among them H₂O₂ and HO^• radicals, which compete by re-oxidising Ti³⁺. When the reduced titania is decorated with plasmonic nanoparticles (e.g., Au), an n-type plasmonic photo-electrode is realised: sp hot electrons are injected over the Schottky barrier, while the deep d-band supplies oxidising holes. The oxygen evolution reaction (OER) proceeds in stages at potentials well above the formal 1.23 V via the two-electron peroxide route (∼1.77 V) or, for sufficiently energetic holes, via the one-electron HO^• route (∼2.7 V). In a biased cell, H₂ evolves on Pt through the adsorbed (H₂⁺)_ad intermediate. The same Au/semiconductor physics on silicon enables sub-band-gap hot-electron photo-detection. Energy-level diagrams (flat-band and in-contact) and the sp- vs. d-band origin of the injected carriers are discussed.