Primary photocatalytic water reduction and oxidation at an anatase TiO₂ and Pt-TiO₂ nanocrystalline electrode revealed by quantitative transient absorption studies

Quantitative assessments of electron and hole transfer dynamics with water on anatase nanocrystalline TiO₂ films were conducted by employing a series of transient absorption spectrometers. For water reduction reactions, both conduction band and trapped electrons decay with two different single exponential components with the difference in time scale of 6∼7 orders of magnitudes. The faster reaction occurs in 8–16 ns, while the slower component shows a lifetime of 0.1∼1.4 s. Pt nanoparticle deposition on the TiO₂ surface switches the slower single exponential reaction to a stretched exponential with an accelerated half lifetime of 2–7 ms, indicating that this slower reaction is limited by the electron trapping-detrapping movements inside the TiO₂. Water oxidation reactions occur multi-exponentially from 100 ns to 200 ms with a half lifetime of 10 μs. Pt on the TiO₂ surface catalyses water oxidation, occurring from 25 ns with a half lifetime of 8 μs.