Researchers Reveal the Origin of the Ti3+ Ions Related Electronic Structure in TiO2
Update: 2016-02-02 13:59:47      Author:

As a prototypical metal oxide in solar energy conversion, TiO2 is easily reduced, forming Ti3+ ions, for which a characteristic signature is the presence of a localized state in the band gap at ~ 1 eV below the Fermi level (EF). There is evidence that d→d transitions from the gap states contribute to the photoabsorption of reduced TiO2. In addition, due to the band gap narrowing and enhanced photo-absorption associated with the presence of the band gap state, visible light photocatalysis through Ti3+ self-doping has also been observed. While transitions from the band gap state are clearly important, little is known, however, about the character of the electronic excited states that are involved in these transitions, due to the greater difficulty to experimentally access the unoccupied states. A characterization of these states is not only scientifically interesting, but is also important for a better understanding and control of TiO2 photocatalysis.

A group at CSRC, led by Dr. Li-Min Liu, has worked together with a group at Dalian Institute of Chemical Physics (DICP) to study the electronic structure of the Ti3+ related state in TiO2.  Utilizing a combined two-photon photoemission spectroscopy (2PPE) and density functional theory (DFT) calculations, they successfully revealed that the John-Teller distortion-induced splitting of the 3d orbitals of Ti3+ ions of at TiO2(110) surface is the origin of the band gap states (~1 eV below EF) and excited states (~2.5 eV above EF) which are of dxy, dxz/dyz/dz2 character, respectively. Localized excitation of Ti3+ ions through d→d transition from the band gap states and excited states (Fig. 1) extends the photo-absorption into the visible region. They have identified the Ti3+ related intrinsic excited state which was thought to be an adsorbate induced feature previously (Science, 2005, 308, 1154), provided a thorough analysis of the photoabsorption of TiO2, explained the visible light photocatalysis associated with Ti3+ self-doping and also paved the way for the investigation of the electronic structure and photoabsorption of other metal oxides.


Fig. 1. Ti3+ states induced optical transition in TiO2.

For more information, please see the paper “Localized Excitation of Ti3+ Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2”, J. Amer. Chem. Soc., DOI: 10.1021/jacs.5b04483 (2015). 

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