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Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces

Abstract

Excess electrons from intrinsic defects, dopants and photoexcitation play a key role in many of the properties of TiO2. Understanding their behaviour is important for improving the performance of TiO2 in energy-related applications. We focus on anatase, the TiO2 polymorph most relevant in photocatalysis and solar energy conversion. Using first-principles simulations, we investigate the states and dynamics of excess electrons from different donors near the most common anatase (101) and (001) surfaces and aqueous interfaces. We find that the behaviour of excess electrons depends strongly on the exposed anatase surface, the environment and the character of the electron donor. Whereas no electron trapping is observed on the (101) surface in vacuo, an excess electron at the aqueous (101) interface can trigger water dissociation and become trapped into a stable surface Ti3+-bridging OH complex. By contrast, electrons avoid the (001) surface, indicating that oxidation reactions are favoured on this surface. Our results provide a bridge between surface science experiments and observations of crystal-face-dependent photocatalysis on anatase, and support the idea that optimization of the ratio between {101} and {001} facets could provide a way to enhance the photocatalytic activity of this material.

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Figure 1: Wulff shape of anatase and atomic structure of (101) and (001) surfaces.
Figure 2: Polaron formation energies at different Ti sites of a four-layer a-101 slab with respect to a delocalized state at the bottom of the conduction band.
Figure 3: FPMD simulations of hydroxylated a-101.
Figure 4: Polaron-induced water dissociation.
Figure 5: FPMD simulations of Nb-doped a-101.
Figure 6: FPMD simulation of hydroxylated a-001.

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Acknowledgements

This work was supported by DoE-BES, Division of Chemical Sciences, Geosciences and Biosciences under Award DE-FG02-12ER16286. We used resources of the National Energy Research Scientific Computing Center (DoE Contract No. DE-AC02-05CH11231). We also acknowledge use of the TIGRESS High Performance Computer Center at Princeton University.

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A.S. initiated and supervised this research project. Both authors designed the models and computational approaches. S.S. performed the first-principles calculations, analysed and visualized the data. Both authors contributed to discussions and writing the manuscript.

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Correspondence to Sencer Selcuk or Annabella Selloni.

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Selcuk, S., Selloni, A. Facet-dependent trapping and dynamics of excess electrons at anatase TiO2 surfaces and aqueous interfaces. Nature Mater 15, 1107–1112 (2016). https://doi.org/10.1038/nmat4672

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