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Recently, Yang Xueming, Ph.D., Zhou Chuanyao, Ph.D., and Wang Zhiqiang, researchers of the State Key Laboratory of Molecular Reaction Dynamics, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, collaborated with Liu Limin, a researcher at Beijing Computational Science Research Center, and Annabella Selloni, a professor at Princeton University, to combine two-photon photoelectron spectroscopy. Two-photon photoemission spectroscopy (2PPE) and theoretical calculations revealed that the 3d orbitals of Ti3+ ions in titania were split into 1eV below Fermi level and 2.5eV above Fermi level due to John-Teller effect. The physical nature. Due to the band gap state and the width of the excited state itself, the absorption spectrum of TiO2 is extended to the visible region through the transition from the band gap state of the Ti3+ ion to the excited state (local d→d transition), successfully explaining the self-doping of Ti3+. Absorption spectrum and visible light catalytic activity. The relevant research results were published online in the American Chemical Society (DOI: 10.1021/jacs.5b04483) with the topic of Localized Excitation of Ti3+ Ions in the Photoabsorption and Photocatalytic Activity of Reduced Rutile TiO2.
As a model catalyst in many research fields such as photocatalysis and solar energy conversion, titanium dioxide is easily reduced to form Ti3+ with the appearance of a band gap state of Ti3d. Yang Xueming, Zhou Chuanyao, and Liu Limin have determined quantitative relationships between band gap states and Ti3+ concentrations in previous work (J. Phys. Chem. Lett., 2013, 4, 3839). The band gap state is the ground state electronic state of the d→d transition in TiO2 and is closely related to light absorption. For example, the reductive TiO2 is blue and the Ti3+ self-doping realizes visible light catalysis. Compared with the thorough study of the band gap state, the understanding of the excited state is very limited, one of the important reasons is the difficulty of experimental measurement.
The lack of experimental data is an important factor that leads to the slow development of theoretical calculation methods for excited electronic structures. 2PPE uses a 1+1 pump-probe method and is a powerful experimental method for studying excited electronic structures and ultrafast electron dynamics. In the past few years, Zhou Chuanyao used this method to study the photocatalytic dissociation of alcohols on TiO2 (110) surface (Chemical Science, 2010, 1, 575; Chemical Science, 2011, 2, 1980; Energy and Environmental Science, 2012, 5, 6833). In this work, Zhou Chuanyao and Wang Zhiqiang et al. found that the electronic excited state at the Fermi level above TiO2 (110) and 2.5±0.2 eV through the variable wavelength 2PPE is an intrinsic electronic state associated with Ti3+ instead of the previously reported empty of the adsorbate. Tracks (Science, 2005, 308, 1154; Science, 2006, 311, 1436). Liu Limin and Annabella Selloni applied the density functional theory calculations based on the hybrid functional (HSE06) to verify the experimental results, and defined the dxy properties of the gap states and the dxz/dyz/dz2 properties of the excited states.
On the one hand, this result clarifies the physical nature of the electronic excited state at 2.5±0.2 eV above the Fermi level of TiO2 (110), and on the other hand explains the reason why the self-doping of Ti3+ extends the absorption spectrum of TiO2 to achieve visible light catalysis. This provides an example for studying the ground state and the electronic structure of the metal oxide.
The study was funded by the National Natural Science Foundation of China, the 973 Program of the Ministry of Science and Technology, and the Chinese Academy of Sciences' major breakthrough in selecting and supporting projects.
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