Site-Selective Excitation of Ti³⁺ Ions in Rutile TiO₂ via Anisotropic Intra-Atomic 3d → 3d Transition

Abstract

Site-selective excitation (SSE), which is usually realized by tuning the wavelength of absorbed light, is an ideal way to study bond-selective chemistry, analyze the crystal structure, investigate protein conformation, etc., eventually leading to active manipulation of desired processes. Herein, SSE has been explored in (110)-, (100)-, and (011)-faced rutile TiO 2 , a prototypical material in both surface science and photocatalysis fields. Using ultraviolet photoelectron spectroscopy and photon energy-, substrate orientation-, and laser polarization-dependent two-photon photoemission spectroscopy (2PPE), intra-atomic 3d → 3d transition from the split Ti 3+ 3d orbitals, i.e., band gap states and excited states at ∼1.00 eV below and ∼2.40 eV above the Fermi level, respectively, has been proven for all of the samples, suggesting that it is a common property of this material. The distinct structure of rutile TiO 2 results in the anisotropic 3d → 3d transitions with the transition dipole moment along the long axes ([110] and [11̅ 0]) of TiO 6 blocking units. This anisotropy facilitates the selective excitation of Ti 3+ ions in the two types of TiO 6 , which cannot be realized by conventional wavelength tuning, via polarization alignment of the excitation source. Discovery in this work builds the foundation for future investigation of site-selective photophysical and photochemical processes and eventually possible active manipulation in this material at the atomic level.

Ultraviolet Photoelectron Spectroscopy Study of the Adsorption and Electronic Structure at the CO2/TiO2(110) Interface

“…Otherwise, ultraviolet and deep ultraviolet light are needed to realize photoexcited electron injection from the valence band. Due to the distinct electronic structure of TiO 2 , − excess electrons associated with the band gap states (about 1.00 eV below E F ) can be excited to empty Ti 3+ 3d orbitals at 2.50 ± 0.30 eV above E F via d-d transition. There is level match and orbital hybridization between Ti 3+ 3d excited states and CO 2 LUMO .…”