![]() In particular, low temperature (LT) EPR studies on powdered anatase TiO 2 under continuous UV irradiation, showed the appearance of two distinct traces attributed to electrons trapped at paramagnetic Ti 3+ sites, whose geometry remained unspecified 11. This was confirmed by scanning tunnelling microscopy/spectroscopy (STM/STS) studies of anatase TiO 2 at 6 K, showing that small polarons form near an O vac and never move 6.Īs far as photoinduced electron traps are concerned, trapping has been reported by steady-state methods, such as electron paramagnetic resonance (EPR) 9, 10, 11, photoluminescence 12 and O 2 photodesorption 13. The small polaron formation was attributed to pentacoordinated Ti 3+ centres, due to the presence of an O vac 4, 6, 7. However, for the (001) plane of anatase, no such signal was observed 5, 8 and a delocalized, “large polaron” (~40 meV below E F) was reported. For the anatase (101) surface, PE data always show a significant gap state ~1 eV below the Fermi level (E F), characteristic of a small polaron 4, 6, 7. Previous photoemission (PE) and electron energy loss spectroscopy (EELS) studies of the surface of low temperature (LT) single crystals of the rutile and anatase forms with a controllable concentration of Oxygen vacancies (O vac) 4, 5, 6, 7, showed that Ti 3+ centres occur in the form of polarons. The two excess electrons can therefore reduce two of the latter species. The former results from the removal of one neutral lattice oxygen atom that leaves two excess electrons at a point defect and three pentacoordinated Ti 4+ ions 3. There are two categories of trapped electrons in anatase TiO 2: those due to the presence of intrinsic Oxygen vacancies (O vac’s) and those resulting from photoexcitation. This competition is still debated, yet it is key to understanding the transport properties of the material 3. Electrons in the conduction band (CB) compete between delocalized (band-like) and localized states in the form of polarons. The Ti 4+ (3d 0) ions are hexacoordinated in predominantly octahedral symmetry sites having a weak D 2d distortion, with 4 equatorial and 2 distal Oxygen neighbour atoms at slightly different Ti-O distances. Stoichiometric anatase has an optical band gap (BG) of 3.2 eV. These applications are entirely based on the generation of charge carriers (electrons and holes) by absorption of light, their transport and eventually, their localization by the electron-phonon coupling and/or defects. The anatase form of Titanium dioxide (TiO 2) is among the most commonly used in solar energy conversion processes into electrical or chemical energy 1, 2. The present demonstration of fs hard X-ray absorption capabilities opens the way to a detailed description of the charge carrier dynamics in transition metal oxides. We conclude that electron localization is due to its trapping at pentacoordinated sites, mostly present in the surface shell region. We find that their localization at Titanium atoms occurs in <300 fs, forming Ti 3+ centres, in or near the unit cell where the electron is created. Here, we use fs Ti K-edge X-ray absorption spectroscopy (XAS) upon 3.49 eV (355 nm) excitation of aqueous colloidal anatase titanium dioxide nanoparticles to probe the trapping dynamics of photogenerated electrons. Identifying the latter’s dynamics at room temperature requires tools that combine elemental and structural sensitivity, with the atomic scale resolution of time (femtoseconds, fs). Transition metal oxides are among the most promising solar materials, whose properties rely on the generation, transport and trapping of charge carriers (electrons and holes). ![]()
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