Excitons and Excess Electrons in Nanometer Size Molecular Polyoxotitanate Clusters: Electronic Spectra, Exciton Dynamics, and Surface States

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• 作者：Jianhua Bao ; Zhihao Yu ; Lars Gundlach ; Jason B. Benedict ; Philip Coppens ; Hung Cheng Chen ; John R. Miller ; Piotr Piotrowiak
• 刊名：The Journal of Physical Chemistry B
• 出版年：2013
• 出版时间：April 25, 2013
• 年：2013
• 卷：117
• 期：16
• 页码：4422-4430
• 全文大小：472K
• 年卷期：v.117,no.16(April 25, 2013)
• ISSN：1520-5207

文摘

The behavior of excitons and excess electrons in the confined space of a molecular polyoxotitanate cluster Ti₁₇(渭₄-O)₄(渭₃-O)₁₆(渭₂-O)₄(OPrⁱ)₂₀ (in short Ti17) was studied using femtosecond pump鈥損robe transient absorption, pulse radiolysis, and fluorescence spectroscopy. Due to pronounced quantum size effects, the electronic spectra of the exciton, Ti17*, and the excess electron carrying radical anion, Ti17^{鈥⑩€?/b>}, are blue-shifted in comparison with bulk TiO₂ and have maxima at 1.91 and 1.24 eV, respectively. The 0.7 eV difference in the position of the absorption maxima of Ti17* and Ti17^{鈥⑩€?/b>} indicates the presence of strong Coulomb interaction between the conduction band electron and the valence band hole in the 1 nm diameter cluster. Ground state Raman spectra and the vibronic structure of the fluorescence spectrum point to the importance of the interfacial ligand modes in the stabilization and localization of the fully relaxed exciton. Four pentacoordinate Ti sites near the surface of the cluster appear to play a special role in this regard. Solvent polarity has only a minor influence on the spectral behavior of Ti17*. Exciton recombination in Ti17 is faster than in anatase nanoparticles or mesoporous films. The kinetics exhibits three components, ranging from less than 1 ps to 100 ps, which are tentatively assigned to the geminate recombination within the core of the cluster and to the decay of the surface stabilized charge transfer exciton. A persistent long-lived component with 蟿 > 300 ps may indicate the involvement of intraband dark states, i.e., triplet excitons ³Ti17*.