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

Nature Materials volume 16, pages 819825 (2017) | Download Citation

Abstract

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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References

  1. 1.

    , , , & Hematite-based solar water splitting: challenges and opportunities. Energy Environ. Sci. 4, 4862–4869 (2011).

  2. 2.

    , & Solar water splitting: progress using hematite (α-Fe2O3) photoelectrodes. ChemSusChem 4, 432–449 (2011).

  3. 3.

    , , , & Using hematite for photoelectrochemical water splitting: a review of current progress and challenges. Nanosc. Horiz. 1, 243–267 (2016).

  4. 4.

    , , , & Ultrafast studies of photoexcited electron dynamics in γ- and α-Fe2O3 semiconductor nanoparticles. J. Phys. Chem. B 102, 770–776 (1998).

  5. 5.

    , , & Charge carrier trapping, recombination and transfer in hematite (α-Fe2O3) water splitting photoanodes. Chem. Sci. 4, 2724–2734 (2013).

  6. 6.

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

  7. 7.

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

  8. 8.

    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).

  9. 9.

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

  10. 10.

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

  11. 11.

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

  12. 12.

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

  13. 13.

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

  14. 14.

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

  15. 15.

    , , , & 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).

  16. 16.

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

  17. 17.

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

  18. 18.

    , , & Hematite heterostructures for photoelectrochemical water splitting: rational materials design and charge carrier dynamics. Energy Environ. Sci. 9, 2744–2775 (2016).

  19. 19.

    , , & Unravelling small-polaron transport in metal oxide photoelectrodes. J. Phys. Chem. Lett. 7, 471–479 (2016).

  20. 20.

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

  21. 21.

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

  22. 22.

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

  23. 23.

    , , & From two-dimensional electron gas to localized charge: dynamics of polaron formation in organic semiconductors. Phys. Rev. B 91, 041201 (2015).

  24. 24.

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

  25. 25.

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

  26. 26.

    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).

  27. 27.

    & Small-polaron versus band conduction in some transition-metal oxides. Adv. Phys. 19, 1–117 (1970).

  28. 28.

    & Core Level Spectroscopy of Solids 133–142 (CRC, 2008).

  29. 29.

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

  30. 30.

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

  31. 31.

    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).

  32. 32.

    , , & Timescales in the response of materials to femtosecond laser excitation. Appl. Phys. A 79, 767–769 (2004).

  33. 33.

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

  34. 34.

    , & Carrier-induced change in refractive index of InP, GaAs and InGaAsP. IEEE J. Quant. Electron. 26, 113–122 (1990).

  35. 35.

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

  36. 36.

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

  37. 37.

    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).

  38. 38.

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

  39. 39.

    , , & Femtosecond absorption saturation studies of hot carriers in GaAs and AlGaAs. IEEE J. Quant. Electron. 24, 267–275 (1988).

  40. 40.

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

  41. 41.

    & Raman spectroscopy of Fe2O3 to 62 GPa. Am. Mineral. 87, 318–326 (2001).

  42. 42.

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

  43. 43.

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

  44. 44.

    & Electron transfers in chemistry and biology. BBA Rev. Bioenerg. 811, 265–322 (1985).

  45. 45.

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

  46. 46.

    , & Origin of stretched exponential relaxation for hopping-transport models. Phys. Rev. Lett. 91, 176602 (2003).

  47. 47.

    , , , & Nonexponential relaxation dynamics of localized carrier densities in oxide crystals without structural or energetic disorder. Phys. Rev. B 84, 052302 (2011).

  48. 48.

    , , & Nanonet-based hematite heteronanostructures for efficient solar water splitting. J. Am. Chem. Soc. 133, 2398–2401 (2011).

  49. 49.

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

Download references

Acknowledgements

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

Author notes

    • Lucas M. Carneiro
    •  & Scott K. Cushing

    These authors contributed equally to this work.

    • Chong Liu

    Present address: Department of Chemistry and Chemical Biology, Harvard University, Cambridge, Massachusetts 02138, USA.

Affiliations

  1. Department of Chemistry, University of California, Berkeley, California 94720, USA

    • Lucas M. Carneiro
    • , Scott K. Cushing
    • , Chong Liu
    • , Yude Su
    • , Peidong Yang
    • , A. Paul Alivisatos
    •  & Stephen R. Leone
  2. Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Lucas M. Carneiro
    • , Scott K. Cushing
    •  & Stephen R. Leone
  3. Department of Materials Science and Engineering, University of California, Berkeley, California 94720, USA

    • Peidong Yang
    •  & A. Paul Alivisatos
  4. Material Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, USA

    • Peidong Yang
    •  & A. Paul Alivisatos
  5. Kavli Energy NanoScience Institute, Berkeley, California 94720, USA

    • Peidong Yang
    •  & A. Paul Alivisatos
  6. Department of Physics, University of California, Berkeley, California 94720, USA

    • Stephen R. Leone

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Contributions

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.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to Stephen R. Leone.

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DOI

https://doi.org/10.1038/nmat4936

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