title: Localized Excitation of Ti(3+) Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2.
abstract: In reduced TiO2, electronic transitions originating from the Ti(3+)-induced states in the band gap are known to contribute to the photoabsorption, being in fact responsible for the material's blue color, but the excited states accessed by these transitions have not been characterized in detail. In this work we investigate the excited state electronic structure of the prototypical rutile TiO2(110) surface using two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations. Using 2PPE, an excited resonant state derived from Ti(3+) species is identified at 2.5 ± 0.2 eV above the Fermi level (EF) on both the reduced and hydroxylated surfaces. DFT calculations reveal that this excited state is closely related to the gap state at ∼1.0 eV below EF, as they both result from the Jahn-Teller induced splitting of the 3d orbitals of Ti(3+) ions in reduced TiO2. Localized excitation of Ti(3+) ions via 3d → 3d transitions from the gap state to this empty resonant state significantly increases the TiO2 photoabsorption and extends the absorbance to the visible region, consistent with the observed enhancement of the visible light induced photocatalytic activity of TiO2 through Ti(3+) self-doping. Our work reveals the physical origin of the Ti(3+) related photoabsorption and visible light photocatalytic activity in prototypical TiO2 and also paves the way for the investigation of the electronic structure and photoabsorption of other metal oxides.
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Zhiqiang
authors.initials: Z
authors.lastname: Wang
authors.affiliation: ‡Beijing Computational Science Research Center, Beijing, 100094, P. R. China.|∥International Center for Quantum Materials, Peking University, Beijing, 100871, P. R. China.
authors.forename: Bo
authors.initials: B
authors.lastname: Wen
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Qunqing
authors.initials: Q
authors.lastname: Hao
authors.affiliation: ‡Beijing Computational Science Research Center, Beijing, 100094, P. R. China.
authors.forename: Li-Min
authors.initials: LM
authors.lastname: Liu
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Chuanyao
authors.initials: C
authors.lastname: Zhou
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Xinchun
authors.initials: X
authors.lastname: Mao
authors.affiliation: ‡Beijing Computational Science Research Center, Beijing, 100094, P. R. China.
authors.forename: Xiufeng
authors.initials: X
authors.lastname: Lang
authors.affiliation: ‡Beijing Computational Science Research Center, Beijing, 100094, P. R. China.
authors.forename: Wen-Jin
authors.initials: WJ
authors.lastname: Yin
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Dongxu
authors.initials: D
authors.lastname: Dai
authors.affiliation: §Department of Chemistry, Princeton University, Princeton, New Jersey 08544, United States.
authors.forename: Annabella
authors.initials: A
authors.lastname: Selloni
authors.affiliation: †State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian, 116023, Liaoning, P. R. China.
authors.forename: Xueming
authors.initials: X
authors.lastname: Yang