Resonant Two-Photon Photoemission from Ti 3d Defect States of TiO2(110) Revisited

详细信息    查看全文

• 作者：Adam Argondizzo ; Shijing Tan ; Hrvoje Petek
• 刊名：Journal of Physical Chemistry C
• 出版年：2016
• 出版时间：June 23, 2016
• 年：2016
• 卷：120
• 期：24
• 页码：12959-12966
• 全文大小：435K
• 年卷期：0
• ISSN：1932-7455

文摘

Recently three groups reported on resonant two-photon photoemission (2PP) from Ti 3d defect states on rutile TiO₂(110) surface. The first publication (Argondizzo, A.; Cui, X.; Wang, C.; Sun, H.; Shang, H.; Zhao, J.; Petek, H. Phys. Rev. B 2015, 91, 155429) to appear assigned the resonance enhancement of the two-photon absorption processes to excitations from the nominally t_2g symmetry Ti³⁺ 3d defect states of reduced bulk TiO₂ through a one-photon resonance with nearly degenerate pair of e_g symmetry intermediate states, before absorbing another photon into the photoemission continuum. This resonance occurs for hν ∼ 3.6 eV, and has pronounced dependence on the alignment of the electric field vector of the excitation light with the in-plane crystalline axes of the TiO₂(110) surface. Subsequently, two other groups reported similar 2PP spectra of reduced TiO₂ surfaces, with one claiming a single d–d transition of OH terminated TiO₂ at a lower energy than in the primary report (Wang, Z.; Wen, B.; Hao, Q.; Liu, L.-M.; Zhou, C.; Mao, X.; Lang, X.; Yin, W.-J.; Dai, D.; Selloni, A.; Yang, X. J. Am. Chem. Soc. 2015, 137, 9146−9152), and the other attributing the resonance to wet electron states of low density surface OH, rather than the bulk d–d transitions (Zhang, Y.; Payne, D. T.; Pang, C. L.; Fielding, H. H.; Thornton, G. J. Phys. Chem. Lett. 2015, 6, 3391−3395). Both groups made bold claims that this UV resonance of reduced TiO₂ could confer photocatalytic activity in the visible region below the fundamental band gap at 3.0 eV. Here we reinvestigate the resonant 2PP spectra of reduced TiO₂(110) surface with wavelength tunable ultrafast laser pulse excitation for different surface preparations, temperatures, and modifications by molecular adsorbates. We conclude that the original assignment to predominantly bulk t_2g–e_g transitions entirely explains the new data, and the surface OH makes no contribution to the observed spectra. Moreover, the t_2g–e_g transition is inconsequential for sub-band-gap photocatalytic activity, but rather illuminates the nature of trapped carriers in TiO₂ and their interactions with chemisorbed molecules.