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Exciton-like trap states limit electron mobility in TiO2 nanotubes

Nature Nanotechnology volume 5, pages 769–772 (2010)Cite this article

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Abstract

Nanoparticle films have become a promising low-cost, high-surface-area electrode material for solar cells and solar fuel production1,2. Compared to sintered nanoparticle films, oriented polycrystalline titania nanotubes offer the advantage of directed electron transport, and are expected to have higher electron mobility3,4,5,6,7. However, macroscopic measurements have revealed their electron mobility to be as low as that of nanoparticle films8,9. Here, we show, through time-resolved terahertz spectroscopy10, that low mobility in polycrystalline TiO2 nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials11,12, but instead results from a single sharp resonance arising from exciton-like trap states. If the number of these states can be lowered, this could lead to improved electron transport in titania nanotubes and significantly better solar cell performance.

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Figure 1
Figure 2: Frequency-dependent photoconductivities.
Figure 3: Photoconductivity of a TiO2 nanotube sample.
Figure 4: Fitting parameter γ as a function of time after photoexcitation.

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References

  1. O'Regan, B. & Grätzel, M. A. Low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO2 films. Nature 353, 737–740 (1991).

    Article  CAS  Google Scholar 

  2. Gratzel, M. Photoelectrochemical cells. Nature 414, 338–344 (2001).

    Article  CAS  Google Scholar 

  3. Kopidakis, N. et al. Temperature dependence of the electron diffusion coefficient in electrolyte-filled TiO2 nanoparticle films: evidence against multiple trapping in exponential conduction-band tails. Phys. Rev. B 73, 045326 (2006).

    Article  Google Scholar 

  4. Varghese, O. K., Paulose, M. & Grimes, C. A. Long vertically aligned titania nanotubes on transparent conducting oxide for highly efficient solar cells. Nature Nanotech. 4, 592–597 (2009).

    Article  CAS  Google Scholar 

  5. Kuang, D. et al. Application of highly ordered TiO2 nanotube arrays in flexible dye-sensitized solar cells. ACS Nano 2, 1113–1116 (2008).

    Article  CAS  Google Scholar 

  6. Shankar, K. et al. Highly efficient solar cells using TiO2 nanotube arrays sensitized with a donor-antenna dye. Nano Lett. 8, 1654–1659 (2008).

    Article  CAS  Google Scholar 

  7. Kim, D., Ghicov, A., Albu, S. P. & Schmuki, P. Bamboo-type TiO2 nanotubes: improved conversion efficiency in dye-sensitized solar cells. J. Am. Chem. Soc. 130, 16454–16455 (2008).

    Article  CAS  Google Scholar 

  8. Zhu, K., Neale, N. R., Miedaner, A. & Frank, A. J. Enhanced charge-collection efficiencies and light scattering in dye-sensitized solar cells using oriented TiO2 nanotubes arrays. Nano Lett. 7, 69–74 (2007).

    Article  CAS  Google Scholar 

  9. Jennings, J. R., Ghicov, A., Peter, L. M., Schmuki, P. & Walker, A. B. Dye-sensitized solar cells based on oriented TiO2 nanotube arrays: transport, trapping and transfer of electrons. J. Am. Chem. Soc. 130, 13364–13372 (2008).

    Article  CAS  Google Scholar 

  10. Beard, M. C., Turner, G. M. & Schmuttenmaer, C. A. Transient photoconductivity in GaAs as measured by time-resolved terahertz spectroscopy. Phys. Rev. B 62, 15764–15777 (2000).

    Article  CAS  Google Scholar 

  11. Baxter, J. B. & Schmuttenmaer, C. A. Conductivity of ZnO nanowires, nanoparticles and thin films using time-resolved terahertz spectroscopy. J. Phys. Chem. B 110, 25229–25239 (2006).

    Article  CAS  Google Scholar 

  12. Turner, G. M., Beard, M. C. & Schmuttenmaer, C. A. Carrier localization and cooling in dye-sensitized nanocrystalline titanium dioxide. J. Phys. Chem. B 106, 11716–11719 (2002).

    Article  CAS  Google Scholar 

  13. Gong, D., Grimes, C. A. & Varghese, O. K. Titanium oxide nanotube arrays prepared by anodic oxidation. J. Mater. Res. 16, 3331–3334 (2001).

    Article  CAS  Google Scholar 

  14. Varghese, O. K., Paulose, M., LaTempa, T. J. & Grimes, C. A. High-rate solar photocatalytic conversion of CO2 and water vapor to hydrocarbon fuels. Nano Lett. 9, 731–737 (2009).

    Article  CAS  Google Scholar 

  15. Mor, G. K., Varghese, O. K., Paulose, M., Shankar, K. & Grimes, C. A. A review on highly ordered, vertically oriented TiO2 nanotube arrays: fabrication, material properties and solar. Sol. Energy Mater. Sol. Cells 90, 2011–2075 (2006).

    Article  CAS  Google Scholar 

  16. Shankar, K. et al. Recent advances in the use of TiO2 nanotube and nanowire arrays for oxidative photoelectrochemistry. J. Phys. Chem. C 113, 6327–6359 (2009).

    Article  CAS  Google Scholar 

  17. Richter, C. et al. Effect of potassium adsorption on the photochemical properties of titania nanotube arrays. J. Mater. Chem. 19, 2963–2967 (2009).

    Article  CAS  Google Scholar 

  18. Macak, J. M. et al. TiO2 nanotubes: self-organized electrochemical formation, properties and applications. Curr. Opin. Solid State Mater. Sci. 11, 3–18 (2007).

    Article  CAS  Google Scholar 

  19. Nemec, H., Kuzel, P. & Sundstrom, V. Far-infrared response of free charge carriers localized in semiconductor nanoparticles. Phys. Rev. B 79, 115309 (2009).

    Article  Google Scholar 

  20. Tiwana, P., Parkinson, P., Johnston, M. B., Snaith, H. J. & Herz, L. M. Ultrafast terahertz conductivity dynamics in mesoporous TiO2: influence of dye sensitization and surface treatment in solid-state dye-sensitized solar cells. J. Phys. Chem. C 114, 1365–1371 (2010).

    Article  CAS  Google Scholar 

  21. Smith, N. V. Drude theory and optical properties of liquid mercury. Phys. Lett. A 26, 126–127 (1968).

    Article  CAS  Google Scholar 

  22. Smith, N. V. Classical generalization of the Drude formula for the optical conductivity. Phys. Rev. B 64, 155106 (2001).

    Article  Google Scholar 

  23. Kaindl, R. A., Carnahan, M. A., Hagele, D., Lovenich, R. & Chemla, D. S. Ultrafast terahertz probes of transient conducting and insulating phases in an electron–hole gas. Nature 423, 734–738 (2003).

    Article  CAS  Google Scholar 

  24. Helgren, E., Armitage, N. P. & Grüner, G. Frequency-dependent conductivity of electron glasses. Phys. Rev. B 69, 014201 (2004).

    Article  Google Scholar 

  25. Vishwakarma, P. N. A.C. conductivity in boron doped amorphous carbon films. Solid State Commun. 149, 115–120 (2009).

    Article  CAS  Google Scholar 

  26. Sekiya, T. et al. Defects in anatase TiO2 single crystal controlled by heat treatments. J. Phys. Soc. Jpn 73, 703–710 (2004).

    Article  CAS  Google Scholar 

  27. Schwanitz, K., Weiler, U., Hunger, R., Mayer, T. & Jaegermann, W. Synchrotron-induced photoelectron spectroscopy of the dye-sensitized nanocrystalline TiO2/electrolyte interface: band gap states and their interaction with dye and solvent molecules. J. Phys. Chem. C 111, 849–854 (2007).

    Article  CAS  Google Scholar 

  28. Westermark, K. et al. Determination of the electronic density of states at a nanostructured TiO2/Ru-dye/electrolyte interface by means of photoelectron spectroscopy. Chem. Phys. 285, 157–165 (2002).

    Article  CAS  Google Scholar 

  29. Minato, T. et al. The electronic structure of oxygen atom vacancy and hydroxyl impurity defects on titanium dioxide (110) surface. J. Chem. Phys. 130, 124502 (2009).

    Article  Google Scholar 

  30. Jeong, B. S., Norton, D. P. & Budai, J. D. Conductivity in transparent anatase TiO2 films epitaxially grown by reactive sputtering deposition. Solid State Electron. 47, 2275–2278 (2003).

    Article  CAS  Google Scholar 

  31. Forro, L. et al. High-mobility N-type charge-carriers in large single-crystals of anatase (TiO2). J. Appl. Phys. 75, 633–635 (1994).

    Article  CAS  Google Scholar 

  32. Tang, H., Prasad, K., Sanjines, R., Schmid, P. E. & Levy, F. Electrical and optical properties of TiO2 anatase thin films. J. Appl. Phys. 75, 2042–2047 (1994).

    Article  CAS  Google Scholar 

  33. Enright, B. & Fitzmaurice, D. Spectroscopic determination of electron and mole effective masses in a nanocrystalline semiconductor film. J. Phys. Chem. 100, 1027–1035 (1996).

    Article  CAS  Google Scholar 

  34. Diebold, U. The surface science of titanium dioxide. Surf. Sci. Rep. 48, 53–229 (2003).

    Article  CAS  Google Scholar 

  35. Mahajan, V. K., Misra, M., Raja, K. S. & Mohapatra, S. K. Self-organized TiO2 nanotubular arrays for photoelectrochemical hydrogen generation: effect of crystallization and defect structures. J. Phys. D 41, 125307 (2008).

    Article  Google Scholar 

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Acknowledgements

The authors acknowledge support from the Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Science, US Department of Energy (DE-FG02-07ER15909) for partial support of this work.

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  1. Christiaan Richter

    Present address: Present address: Rochester Institute of Technology, Chemical and Biomedical Engineering, 160 Lomb Memorial Drive, Rochester, New York 14623-5603 USA,

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  1. Department of Chemistry, Yale University, 225 Prospect St., New Haven, 06520-8107, Connecticut, USA

    Christiaan Richter & Charles A. Schmuttenmaer

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Contributions

C.R. and C.A.S. conceived and designed the experiments, analysed the data and co-wrote the paper. C.R. performed the experiments.

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Correspondence to Charles A. Schmuttenmaer.

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Richter, C., Schmuttenmaer, C. Exciton-like trap states limit electron mobility in TiO2 nanotubes. Nature Nanotech 5, 769–772 (2010). https://doi.org/10.1038/nnano.2010.196

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