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Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3


Small polaron formation is known to limit ground-state mobilities in metal oxide photocatalysts. However, the role of small polaron formation in the photoexcited state and how this affects the photoconversion efficiency has yet to be determined. Here, transient femtosecond extreme-ultraviolet measurements suggest that small polaron localization is responsible for the ultrafast trapping of photoexcited carriers in haematite (α-Fe2O3). Small polaron formation is evidenced by a sub-100 fs splitting of the Fe 3p core orbitals in the Fe M2,3 edge. The small polaron formation kinetics reproduces the triple-exponential relaxation frequently attributed to trap states. However, the measured spectral signature resembles only the spectral predictions of a small polaron and not the pre-edge features expected for mid-gap trap states. The small polaron formation probability, hopping radius and lifetime varies with excitation wavelength, decreasing with increasing energy in the t2g conduction band. The excitation-wavelength-dependent localization of carriers by small polaron formation is potentially a limiting factor in haematite’s photoconversion efficiency.

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Figure 1: Change in core level transitions with photoexcitation.
Figure 2: Transient absorption after 480 nm excitation in Fe2O3.
Figure 3: Energy dependence of polaron formation and decay.
Figure 4: Effect of polaron formation across haematite’s absorption.


  1. 1

    Lin, Y., Yuan, G., Sheehan, S., Zhou, S. & Wang, D. Hematite-based solar water splitting: challenges and opportunities. Energy Environ. Sci. 4, 4862–4869 (2011).

    CAS  Article  Google Scholar 

  2. 2

    Sivula, K., Le Formal, F. & Grätzel, M. Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes. ChemSusChem 4, 432–449 (2011).

    CAS  Article  Google Scholar 

  3. 3

    Tamirat, A. G., Rick, J., Dubale, A. A., Su, W. N. & Hwang, B. J. Using hematite for photoelectrochemical water splitting: a review of current progress and challenges. Nanosc. Horiz. 1, 243–267 (2016).

    CAS  Article  Google Scholar 

  4. 4

    Cherepy, N. J., Liston, D. B., Lovejoy, J. A., Deng, H. & Zhang, J. Z. Ultrafast studies of photoexcited electron dynamics in γ- and α-Fe2O3 semiconductor nanoparticles. J. Phys. Chem. B 102, 770–776 (1998).

    CAS  Article  Google Scholar 

  5. 5

    Barroso, M., Pendlebury, S. R., Cowan, A. J. & Durrant, J. R. Charge carrier trapping, recombination and transfer in hematite (α-Fe2O3) water splitting photoanodes. Chem. Sci. 4, 2724–2734 (2013).

    CAS  Article  Google Scholar 

  6. 6

    Pendlebury, S. R. et al. Ultrafast charge carrier recombination and trapping in hematite photoanodes under applied bias. J. Am. Chem. Soc. 136, 9854–9857 (2014).

    CAS  Article  Google Scholar 

  7. 7

    Sorenson, S., Driscoll, E., Haghighat, S. & Dawlaty, J. M. Ultrafast carrier dynamics in hematite films: the role of photoexcited electrons in the transient optical response. J. Phys. Chem. C 118, 23621–23626 (2014).

    CAS  Article  Google Scholar 

  8. 8

    Joly, A. G. et al. Carrier dynamics in α-Fe2O3 (0001) thin films and single crystals probed by femtosecond transient absorption and reflectivity. J. Appl. Phys. 99, 053521 (2006).

    Article  Google Scholar 

  9. 9

    Fitzmorris, B. C. et al. Ultrafast transient absorption studies of hematite nanoparticles: the effect of particle shape on exciton dynamics. ChemSusChem 6, 1907–1914 (2013).

    CAS  Article  Google Scholar 

  10. 10

    Nadtochenko, V. A. et al. Femtosecond relaxation of photoexcited states in nanosized semiconductor particles of iron oxides. Russ. Chem. Bull. Int. Ed. 51, 457–461 (2002).

    CAS  Article  Google Scholar 

  11. 11

    LeFormal, F. et al. Back electron–hole recombination in hematite photoanodes for water splitting. J. Am. Chem. Soc. 136, 2564–2574 (2014).

    CAS  Article  Google Scholar 

  12. 12

    Braun, A. et al. Direct observation of two electron holes in a hematite photoanode during photoelectrochemical water splitting. J. Phys. Chem. C 116, 16870–16875 (2012).

    CAS  Article  Google Scholar 

  13. 13

    Zandi, O. & Hamann, T. W. The potential versus current state of water splitting with hematite. Phys. Chem. Chem. Phys. 17, 22485–22503 (2015).

    CAS  Article  Google Scholar 

  14. 14

    Gui, Q. et al. Enhanced photoelectrochemical water splitting performance of anodic TiO2 nanotube arrays by surface passivation. ACS Appl. Mater. Interfaces 6, 17053–17058 (2014).

    CAS  Article  Google Scholar 

  15. 15

    Cesar, I., Sivula, K., Kay, A., Zboril, R. & Grätzel, M. Influence of feature size, film thickness, and silicon doping on the performance of nanostructured hematite photoanodes for solar water splitting. J. Phys. Chem. C 113, 772–782 (2008).

    Article  Google Scholar 

  16. 16

    Sivula, K. Metal oxide photoelectrodes for solar fuel production, surface traps, and catalysis. J. Phys. Chem. Lett. 4, 1624–1633 (2013).

    CAS  Article  Google Scholar 

  17. 17

    Ling, Y. & Li, Y. Review of Sn-doped hematite nanostructures for photoelectrochemical water splitting. Part. Part. Syst. Charact. 31, 1113–1121 (2014).

    CAS  Article  Google Scholar 

  18. 18

    Shen, S., Lindley, S. A., Chen, X. & Zhang, J. Z. Hematite heterostructures for photoelectrochemical water splitting: rational materials design and charge carrier dynamics. Energy Environ. Sci. 9, 2744–2775 (2016).

    CAS  Article  Google Scholar 

  19. 19

    Rettie, A. J., Chemelewski, W. D., Emin, D. & Mullins, C. B. Unravelling small-polaron transport in metal oxide photoelectrodes. J. Phys. Chem. Lett. 7, 471–479 (2016).

    CAS  Article  Google Scholar 

  20. 20

    Warren, S. C. Photoelectrochemical Hydrogen Production 293–316 (Springer, 2012).

    Book  Google Scholar 

  21. 21

    Emin, D. Condensed Matter Physics 16–34 (Springer, 1987).

    Book  Google Scholar 

  22. 22

    Ge, N. H. et al. Femtosecond dynamics of electron localization at interfaces. Science 279, 202–205 (1998).

    CAS  Article  Google Scholar 

  23. 23

    Wang, T., Caraiani, C., Burg, G. W. & Chan, W. L. From two-dimensional electron gas to localized charge: dynamics of polaron formation in organic semiconductors. Phys. Rev. B 91, 041201 (2015).

    Article  Google Scholar 

  24. 24

    Muntwiler, M. & Zhu, X. Y. Formation of two-dimensional polarons that are absent in three-dimensional crystals. Phys. Rev. Lett. 98, 246801 (2007).

    CAS  Article  Google Scholar 

  25. 25

    Katz, J. E. et al. Electron small polarons and their mobility in iron (oxyhydr) oxide nanoparticles. Science 337, 1200–1203 (2012).

    CAS  Article  Google Scholar 

  26. 26

    Vura-Weis, J. et al. Femtosecond M23-edge spectroscopy of transition-metal oxides: photoinduced oxidation state change in α-Fe2O3 . J. Phys. Chem. Lett. 4, 3667–3671 (2013).

    CAS  Article  Google Scholar 

  27. 27

    Bosman, A. J. & Van Daal, H. J. Small-polaron versus band conduction in some transition-metal oxides. Adv. Phys. 19, 1–117 (1970).

    CAS  Article  Google Scholar 

  28. 28

    De Groot, F. & Kotani, A. Core Level Spectroscopy of Solids 133–142 (CRC, 2008).

    Book  Google Scholar 

  29. 29

    Stavitski, E. & de Groot, F. M. F. The CTM4XAS program for EELS and XAS spectral shape analysis of transition metal L edges. Micron 41, 687–694 (2010).

    CAS  Article  Google Scholar 

  30. 30

    Wang, Z. & Bevan, K. H. Exploring the impact of semicore level electronic relaxation on polaron dynamics: an adiabatic ab initio study of FePO4 . Phys. Rev. B 93, 024303 (2016).

    Article  Google Scholar 

  31. 31

    Tränkle, G. et al. Dimensionality dependence of the band-gap renormalization in two- and three-dimensional electron–hole plasmas in GaAs. Phys. Rev. Lett. 58, 419–422 (1987).

    Article  Google Scholar 

  32. 32

    Rethfeld, B., Sokolowski-Tinten, K., Von Der Linde, D. & Anisimov, S. I. Timescales in the response of materials to femtosecond laser excitation. Appl. Phys. A 79, 767–769 (2004).

    CAS  Article  Google Scholar 

  33. 33

    Stampfli, P. & Bennemann, K. H. Theory for the laser-induced femtosecond phase transition of silicon and GaAs. Appl. Phys. A 60, 191–196 (1995).

    Article  Google Scholar 

  34. 34

    Bennett, B. R., Soref, R. A. & Del Alamo, J. A. Carrier-induced change in refractive index of InP, GaAs and InGaAsP. IEEE J. Quant. Electron. 26, 113–122 (1990).

    CAS  Article  Google Scholar 

  35. 35

    Rose-Petruck, C. et al. Picosecond–milliångström lattice dynamics measured by ultrafast X-ray diffraction. Nature 398, 310–312 (1999).

    CAS  Article  Google Scholar 

  36. 36

    Santomauro, F. G. et al. Femtosecond X-ray absorption study of electron localization in photoexcited anatase TiO2 . Sci. Rep. 5, 14834 (2015).

    CAS  Article  Google Scholar 

  37. 37

    Van Driel, H. M. Kinetics of high-density plasmas generated in Si by 1.06- and 0.53-μm picosecond laser pulses. Phys. Rev. B 35, 8166–8176 (1987).

    CAS  Article  Google Scholar 

  38. 38

    Becker, P. C. et al. Femtosecond intervalley scattering in GaAs. Appl. Phys. Lett. 53, 2089–2090 (1988).

    CAS  Article  Google Scholar 

  39. 39

    Lin, W. Z., Schoenlein, R. W., Fujimoto, J. G. & Ippen, E. P. Femtosecond absorption saturation studies of hot carriers in GaAs and AlGaAs. IEEE J. Quant. Electron. 24, 267–275 (1988).

    Article  Google Scholar 

  40. 40

    Fluegel, B. D. et al. Experimental and theoretical investigation of femtosecond carrier relaxation in CdSe. Solid State Commun. 83, 17–19 (1992).

    CAS  Article  Google Scholar 

  41. 41

    Shim, S.-H. & Duffy, T. S. Raman spectroscopy of Fe2O3 to 62 GPa. Am. Mineral. 87, 318–326 (2001).

    Article  Google Scholar 

  42. 42

    Jubb, A. M. & Allen, H. C. Vibrational spectroscopic characterization of hematite, maghemite, and magnetite thin films produced by vapor deposition. ACS Appl. Mater. Interfaces 2, 2804–2812 (2010).

    CAS  Article  Google Scholar 

  43. 43

    Holstein, T. Studies of polaron motion. Ann. Phys., NY 8, 325–342 (1959).

    CAS  Article  Google Scholar 

  44. 44

    Marcus, R. A. & Sutin, N. Electron transfers in chemistry and biology. BBA Rev. Bioenerg. 811, 265–322 (1985).

    CAS  Google Scholar 

  45. 45

    Peng, H. & Lany, S. Semiconducting transition-metal oxides based on d5 cations: theory for MnO and Fe2O3 . Phys. Rev. B 85, 201202 (2012).

    Article  Google Scholar 

  46. 46

    Sturman, B., Podivilov, E. & Gorkunov, M. Origin of stretched exponential relaxation for hopping-transport models. Phys. Rev. Lett. 91, 176602 (2003).

    CAS  Article  Google Scholar 

  47. 47

    Merschjann, C., Imlau, M., Brüning, H., Schoke, B. & Torbrügge, S. Nonexponential relaxation dynamics of localized carrier densities in oxide crystals without structural or energetic disorder. Phys. Rev. B 84, 052302 (2011).

    Article  Google Scholar 

  48. 48

    Lin, Y., Zhou, S., Sheehan, S. W. & Wang, D. Nanonet-based hematite heteronanostructures for efficient solar water splitting. J. Am. Chem. Soc. 133, 2398–2401 (2011).

    CAS  Article  Google Scholar 

  49. 49

    Gialanella, S. et al. On the goethite to hematite phase transformation. J. Therm. Anal. Calorim. 102, 867–873 (2010).

    CAS  Article  Google Scholar 

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This work was supported by the US Department of Energy, Office of Science, Office of Basic Energy Sciences, Materials Sciences and Engineering Division, under Contract No. DE-AC02-05-CH11231 within the Physical Chemistry of Inorganic Nanostructures Program (KC3103). S.K.C. acknowledges support by the Department of Energy, Office of Energy Efficiency and Renewable Energy (EERE) Postdoctoral Research Award under the EERE Solar Energy Technologies Office.

Author information




L.M.C., S.K.C. and S.R.L. designed the study. L.M.C. and S.K.C. performed the transient XUV measurements and data analysis. S.K.C. modelled the polaron spectral signature and dynamics. C.L. and Y.S. were responsible for sample fabrication and characterization. L.M.C., S.K.C., C.L., Y.S., P.Y., A.P.A. and S.R.L. wrote and revised the manuscript.

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Correspondence to Stephen R. Leone.

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The authors declare no competing financial interests.

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Carneiro, L., Cushing, S., Liu, C. et al. Excitation-wavelength-dependent small polaron trapping of photoexcited carriers in α-Fe2O3. Nature Mater 16, 819–825 (2017).

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