Visible Light Absorption and Hot Carrier Trapping in Anatase TiO2: The Role of Surface Oxygen Vacancies

Anatase TiO2 is an efficient water splitting photocatalyst using UV light, but solar energy harvesting requires the presence of midgap states to remain regarding the photophysics in O vacancy doped TiO2. By employing extreme ultraviolet reflection-absorption (XUV-RA) spectroscopy at the Ti M2,3-edge, spectral signatures of both large and small polaron states are identified, allowing ultrafast electron and hole dynamics in these states to be independently resolved. Results show that visible light absorption occurs via promotion of an electron from the small polaron state to the TiO2 conduction band. In contrast, absorption of UV light results in direct band gap excitation followed by carrier relaxation during which hot holes trap as small polarons in 45 +/- 42 fs, and hot electrons couple to polar optical phonons leading to vibrational coherence and large polaron formation in 945 +/- 92 fs.