Evidence for Ti Interstitial Induced Extended Visible Absorption and Near Infrared Photoluminescence from Undoped TiO2 Nanoribbons: An In Situ Photoluminescence Study

Despite decades of research on the role of intrinsic defects in enhancing the performance of reduced TiO2 based materials, unambiguous identification of defects responsible for visible light absorption, and near-infrared (NIR) photoluminescence from undoped TiO2 has remained challenging. Herein, through in situ photoluminescence (PL) studies under a controlled environment, we investigated the origin of an extended visible absorption, visible and NIR PL emission from undoped TiO2 nanoribbons grown by a solvothermal route. Our studies reveal that oxygen vacancies, Ti3+, and F+ center in TiO2 are responsible for absorption in the violet and blue-green region and the PL emission in the visible region. On the other hand, absorption in the yellow-red to NIR region and PL emission in the NIR region at 1.47 and ∼1.30 eV are due to Ti4+ and Ti3+ interstitials, respectively, near the surface identified for the first time. The above conclusions are supported by electron paramagnetic resonance and X-ray photoelectron spectroscopy analyses. The development of such nanoporous undoped TiO2 nanoribbons with strong visible absorption and optical identification of Ti interstitial induced band gap states serves as an important milestone toward realizing improved visible light photocatalytic and photovoltaic applications of this novel material.