Anisotropic d–d Transition in Rutile TiO₂

Abstract

The band gap state of TiO 2 , which is dominated by Ti 3+ 3d character, is of great relevance to light absorption, electron trapping, charge recombination, and conduction band structure. Despite the importance, the explanation of the excitation from this state is controversial. To this end, the electronic structures of TiO 2 (110) and TiO 2 ( 011)-(2 × 1) have been systematically measured with two-photon photoemission spectroscopy. The results reveal the anisotropic nature of the electronic structure in rutile TiO 2 at seemingly equivalent directions of [110] and [11̅ 0], the long axes of the TiO 6 blocking unit. Although the resonant energy of these two d−d transitions is identical, the energy levels are systematically shifted by 0.1 eV. We propose this anisotropy originates from the broken symmetry of the rutile TiO 2 crystals caused by the surface. The proposed asymmetry-caused electronic structure anisotropy could be generalized to other similar materials and may affect associated catalytic properties. This work provides an important benchmark for related calculations.

Valence Band of Rutile TiO2(110) Investigated by Polarized-Light-Based Angle-Resolved Photoelectron Spectroscopy

“…The parity symmetries of all the orbitals under mirror reflection with respect to the emission plane are listed in Table S4−S6, where octahedra with long axes along both [110] and [1−10] are considered. 23 The photoionization cross section of Ti 4s, Ti 3d, and O 2p orbitals are about 0.2, 3, and 0.8 Mb (1 Mb = 10 −27 cm 2 ) at hv = 42.5 eV respectively. 24 Assuming the electrons in the specific atomic orbitals distribute equally into the molecular orbitals they involve, the polarization dependent intensity for the molecular orbitals can be calculated.…”

TiO2 Polarons in the Time Domain: Implications for Photocatalysis

“…More recent work by Wang et al suggests that the difference between surface and bulk polaron excitation also contains an anisotropic component where polarons excited in the [110] direction are stabilized. 32 Enticingly, these works (as well as previous work by Zhang et al 26 and Mao et al 42 ) suggest that the interaction between light and polarons can be tuned depending on their local environment. This has become evident in 2PPE studies of more complex TiO 2 surface environments.…”

“…28,31 A recent publication by Wang et al discussed the merits of the computational methods used in each respective work. 32 Another noteworthy aspect of rutile TiO 2 (110) 2PPE spectra is the influence of water. It is well-established that water dissociates at O vac and forms bridging hydroxyls (OH b ) at the TiO 2 (110) surface.…”

“…To our knowledge, only one 2PPE study of Tuning polaron−light interactions may allow nonequilibrium dynamics to also be tuned. TiO 2 's minority facets of either rutile or anatase exists, 32 despite clear differences in the polaronic occupied energy. This information will be valuable as we build our picture of how surface structure influences polaron behavior.…”

Celebrating the 10th Anniversary of the Youth Innovation Promotion Association, Chinese Academy of Sciences: Emerging Young Scientists in Physical Chemistry

“…The Letter by Wang et al 15 reports the use of Te as the cathode in aqueous zinc batteries. The Letter by Wang et al 16 reveals the anisotropic d−d transition in rutile TiO 2 and describes a systematic energy shift of electronic structure due to symmetry broken at the surface. Apart from these mechanism studies, this Virtual Special Issue also contains recent advances on synthesis, assembly, and applications of advanced materials, such as Ag 2 Se thermoelectric material, 17 2D metal−organic frameworks (MOFs), 18 Published: January 20, 2022 25 porous aromatic frameworks (PAFs), 26 a SiO x -based memristor, 27 a graphitelike carbon nitride nanotube, 28 α-C:H film, 29 light-harvesting molecular wires, 30 a bioresponsive zeolitic imidazolate framework (ZIF) system, 31 an M-PM 2−x -H 2 -based aqueous Zn battery, 32 and a MnO x @PAA@HKUST-1-DSF@BSA-based theranostic nanoagent.…”